Laboratory of regeneration and adult neurogenesis

Brigitte Galliot

Associate Professor

  • T: +41 22 379 67 74
  • office 4055B (Sciences III)

Keywords: hydra, cnidarian, regeneration, cellular remodelling, developmental plasticity, homeostasis, injury, wound healing, growth control, patterning, apoptosis-induced compensatory proliferation, signaling, evolution, neurogenesis

1. Summary

A fascinating, unanswered question in biology is how some organisms respond to injury by regenerating the missing body structure, whereas other have lost this potential. Here we propose to use the freshwater Hydra polyp as regeneration model organism to tackle this question. Indeed, Hydra is a simple animal that provides a powerful model system to understand how a highly dynamic homeostasis contributes to link wound healing to tissue repair and regeneration. The questions we address in our research are the following:

  • HOMEOSTASIS & REGENERATION: What are the cellular and molecular mechanisms that maintain homeostasis in Hydra and drive regeneration after bisection?
  • STEM CELLS: What are the respective functions of the stem cells and the differentiated cells in these processes?
  • MEMORY: What are the memory mechanisms that allow the regenerating tip to regrow the appropriate missing structure, i.e. a head on one side and a foot on the other side of the bisection?
  • ADULT NEUROGENESIS: What is the genetic circuitry that leads to de novo neurogenesis in cnidarians?
  • AGING: What are the mechanisms that keep the developmental program(s) such as head regeneration, foot regeneration and asexual reproduction (budding) accessible all along the hydra life?
  • EVOLUTION: Which of these mechanisms have been conserved along evolution?

To elucidate these questions, we want identify thanks to RNA interference (RNAi) the signaling cascades that govern cellular and developmental plasticity in Hydra. We recently showed that apoptosis-induced compensatory proliferation provides a mechanism to launch a complex regeneration program such as head regeneration. Interestingly apoptosis-induced compensatory proliferation seems to be also at work when Xenopus regenerates its tail, when Drosophila larvae regenerates their imaginal discs or even when rodents regenerate their skin or their liver (for recent reviews see [1, 2]). These results suggest that there might be some common paths to launch a regenerative response.

Figure 1: Schema depicting the position of Hydra in the metazoan tree. Among cnidarians, anthozoans (corals, sea anemones) live exclusively as polyps, whereas medusozoans (jellyfish) alternate between the medusa and polyp stages during their life cycle. Hydra is a freshwater polyp that lost the medusa stage.

2. The Hydra model system in few words

Hydra belongs to Cnidaria, a sister group to bilaterians (Fig.1). The anatomy of Hydra is rather simple: this is basically a bilayered tube that shows an apical-basal polarity, with at one extremity a head region that includes a mouth/anus opening and a ring of tentacles to actively catch the food (Hydra are carnivorous), and at the other extremity a basal disc. Its cellular organization is rather simple, with two cell layers, the ectoderm and the endoderm separated by an extracellular collagenoeous matrix, the mesogolea. Hydra differentiates all cell types required for neuro-muscular transmission, digestion, secretion and sexual reproduction. These different cell types differentiate from three distinct stem cell populations: myoepithelial cells located in the ectoderm, myoepithelial cells located in the endoderm and interstitial cells that are multipotent stem cells giving rise to neurons, mechano-sensory cells (nematocytes or cnidocytes), gland cells and gametes [3, 4].

Over the past 25 years, the genes encoding the signaling proteins that control and execute various cellular behaviors and developmental processes were shown to be present in all animals. The recent sequencing of the genome of two cnidarian species (Nematostella vectensis – sea anemone - and Hydra magnipapillata) indeed confirmed this high level of gene conservation from cnidarians to vertebrates. This definitively supports the relevance of Hydra as a model system to investigate complex biological questions [5, 6].

3. Functional analysis of the genetic circuitry supporting regeneration in Hydra

Gene silencing through RNAi by feeding identifies cellular phenotypes that are shared between Hydra and man

RNAi gene silencing obtained by feeding the animal on dsRNA-producing bacteria was first described in C. elegans and more recently adapted to planarians [7-9]. We adapted this strategy to Hydra (Fig. 2) and proved that this method that is harmless, stepwise and efficient, can induce gene-specific phenotypes [10-13]. In case of the protease inhibitor Kazal1that isspecifically expressed in gland cells of the gastrodermis, RNAi silencing induces a massive autophagy that extends to the digestive cells. This phenotype, which mimics the human SPINK1/mouse SPINK3 pancreatic phenotype, provides the first example of a conserved cellular mechanism from cnidarians to mammals, an example that definitely strengthen the paradigmatic value of this little animal.

This strategy offers the possibility of systematic RNAi screens in Hydra.

Gene silencing by RNA interference in Hydra

Figure 2: Gene silencing by RNA interference in Hydra. The dsRNA feeding strategy developed in nematodes [7, 8] and planarians [9] was adapted to Hydra polyps. Double-stranded RNAs are produced in bacteria and Hydra polyps are repeatedly fed with the bacteria-agarose mixture {Chera, 2006 #212; Buzgariu, 2008 #174}.

4. The apoptosis-dependent activation of the Wnt pathway in head-regenerating tips.

The canonical Wnt pathway is perfectly conserved in Hydra and required for the head to regenerate properly [14, 15]. We recently showed that an asymmetrical wave of apoptosis occurs immediately after mid-gastric bisection affecting 50% of the cells in head-regenerating tips but less than 7% in foot-regenerating ones [13]. Apoptotic cells actually transiently release the Wnt3 signal that activates b-catenin in the surrounding S-phase cells. Indeed progenitors migrate towards the wound, accumulate underneath the apoptotic zone and rapidly divide forming a proliferative zone. Upon inhibition of apoptosis by caspase inhibitors, or upon Wnt3 or b-catenin RNAi silencing, cell proliferation and head regeneration are abolished, whereas simply adding exogenous Wnt3 fully rescues these processes. Conversely
induction of apoptosis in foot-regenerating tips converts them to regenerate heads through the activation of the Wnt3/b-catenin pathway. Hence the level of apoptosis appears critical to trigger compensatory proliferation and head regeneration through the Wnt pathway.

Bi-headed Hydra resulting from the ectopic apoptosis-induced activation of the Wnt3-bcatenin pathway

Figure 3: Bi-headed Hydra resulting from the ectopic apoptosis-induced activation of the Wnt3-bcatenin pathway. Hydra polyps bisected at mid-gastric position regenerate a head from the lower half and a basal disc from the upper one. Here Hydra upper halves were forced to regenerate a head instead of a basal disc. This was obtained by inducing a high level of apoptosis in foot-regenerating tips by briefly heating them immediately after bisection. Ectopic apoptosis leads to the activation of the Wnt3-bcatenin pathway as observed in head-regenerating stumps and therefore converted foot regeneration to head regeneration (see in [1, 2]).

The immediate asymmetric regulation of the MAPK ➜ RSK ➜ CREB ➜ CBP pathway

The MAPK/CREB pathway seems to play a key role in the induction of the head regeneration process. Indeed the cAMP Response Element Binding (CREB) protein is a transcription factor that likely interacts with distinct partners in CRE-binding complexes immediately after bisection [16], and exhibits a series of immediate regulations in the head- but not in the foot-regenerating tips as an RSK-dependent phosphorylation [17] and a rapid up-regulation of the CREB gene expression [18]. In vertebrates, the phosphorylated form of CREB binds to the chromatin modifyer CBP in order to modulate gene expression. Similarly the Hydra CBP encodes a CREB-binding domain and silencing of either RSK or CREB or CBP prevents the immediate wave of apoptosis and the cellular reorganization normally observed in head-regenerating tips (Chera and Galliot, submitted). We are currently investigating how this signaling pathway can sense the stress of amputation to activate a head regeneration program.

The emergence of neurogenesis and apical patterning in metazoans

In Hydra neurogenesis takes place continuously in the adult polyp but the dense apical nervous system also forms de novo in budding and regenerating animals [19- 21]. In bilaterian animals whose ancestor already had a centralized nervous system and sensory organs, the homeobox genes of the ANTP and PAIRED class play a major role in the developing central nervous system [22-24]. Each class includes a large number of families that for most of them diversified quite early in animal evolution, preceding the divergence of Cnidaria [25-27]. This raises the question of the function of these gene families in cnidarians (that differentiate a sophisticated nervous system) and in poriferans (that have no nervous system)[21].

The PAIRED-class homeobox genes

Among evolutionary conserved gene families involved in the specification of brain and sensory organs in bilaterians, several are expressed in the nervous system of cnidarians [28-32]. Among these, the paired-class homeobox gene prdl-a, is expressed in apical nerve cells in adult polyps but after bisection, transiently in endodermal cells of the head-regenerating tip. This result suggested that prdl-a is involved in Hydra head organizer activity through inductive interactions from endoderm to the overlying ectoderm [19, 28]. This result was puzzling as some paired- like genes perform similar tasks at the time head organizer activity is established in mammals, suggesting that molecular mechanisms of anterior patterning can be traced back to cnidarians [33].

The Hox/ParaHox genes

Most ANTP-class gene families do have cnidarian counterparts although Hox-related families are not all present and less similar. In fact Hox-like genes were not found in poriferan, therefore, cnidarian Hox-like genes can be considered as representative of the proto-Hox genes [26, 34-36]. Out of them, cnox-2, theortholog of the paraHox Gsx gene, stimulates interest as its expression is restricted to the precursors of the neuronal and nematocyte cell lineages and cnox-2 is also upregulated during de novo apical neurogenesis at the time head is forming, during budding and in regenerating polyps [11, 26, 37]. When cnox-2 expression is silenced via RNAi, alterations of the apical nervous system and apical patterning process are observed. In vertebrates and Drosophila, Gsx is also involved in neurogenesis [38]. These data suggest an evolutionarily-conserved function for cnox-2/Gsx in neurogenesis and a possible functional link between neurogenesis and apical patterning established quite early in animal evolution [11, 20, 36].

5. References

  1. Galliot, B., and Chera, S. (2010) The Hydra model: disclosing an apoptosis-driven generator of Wnt-based regeneration. Trends Cell Biol in press
  2. Galliot, B., and Ghila, L. (2010) Cell plasticity in homeostasis and regeneration. Mol Reprod Dev in press
  3. Steele, R.E. (2002) Developmental signaling in Hydra: what does it take to build a “simple” animal? Dev Biol 248, 199-219
  4. Galliot, B., Miljkovic-Licina, M., de Rosa, R., and Chera, S. (2006) Hydra, a niche for cell and developmental plasticity. Semin Cell Dev Biol 17, 492-502
  5. Putnam, N.H., Srivastava, M., Hellsten, U., Dirks, B., Chapman, J., Salamov, A., Terry, A., Shapiro, H., et al. (2007) Sea anemone genome reveals ancestral eumetazoan gene repertoire and genomic organization. Science 317, 86-94
  6. Chapman, J.A., Kirkness, E.F., Simakov, O., Hampson, S.E., Mitros, T., Weinmaier, T., Rattei, T., Balasubramanian, P.G., et al. (2010) The dynamic genome of Hydra. Nature 464, 592-596
  7. Timmons, L., Court, D.L., and Fire, A. (2001) Ingestion of bacterially expressed dsRNAs can produce specific and potent genetic interference in Caenorhabditis elegans. Gene 263, 103-112
  8. Kamath, R.S., Martinez-Campos, M., Zipperlen, P., Fraser, A.G., and Ahringer, J. (2001) Effectiveness of specific RNA-mediated interference through ingested double-stranded RNA in Caenorhabditis elegans. Genome Biol 2, RESEARCH0002
  9. Newmark, P.A., Reddien, P.W., Cebria, F., and Sanchez Alvarado, A. (2003) Ingestion of bacterially expressed double-stranded RNA inhibits gene expression in planarians. Proc Natl Acad Sci U S A 100 Suppl 1, 11861-11865
  10. Chera, S., de Rosa, R., Miljkovic-Licina, M., Dobretz, K., Ghila, L., Kaloulis, K., and Galliot, B. (2006)
    Silencing of the hydra serine protease inhibitor Kazal1 gene mimics the human SPINK1 pancreatic phenotype. J Cell Sci 119, 846-857
  11. Miljkovic-Licina, M., Chera, S., Ghila, L., and Galliot, B. (2007) Head regeneration in wild-type hydra requires de novo neurogenesis. Development 134, 1191-1201
  12. Buzgariu, W., Chera, S., and Galliot, B. (2008) Methods to investigate autophagy during starvation and regeneration in hydra. Methods Enzymol 451, 409-437
  13. Chera, S., Ghila, L., Dobretz, K., Wenger, Y., Bauer, C., Buzgariu, W., Martinou, J.C., and Galliot, B. (2009) Apoptotic cells provide an unexpected source of Wnt3 signaling to drive hydra head regeneration. Dev Cell 17, 279-289
  14. Hobmayer, B., Rentzsch, F., Kuhn, K., Happel, C.M., von Laue, C.C., Snyder, P., Rothbacher, U., and Holstein, T.W. (2000) WNT signalling molecules act in axis formation in the diploblastic metazoan Hydra. Nature 407, 186-189
  15. Lengfeld, T., Watanabe, H., Simakov, O., Lindgens, D., Gee, L., Law, L., Schmidt, H.A., Ozbek, S., et al. (2009) Multiple Wnts are involved in Hydra organizer formation and regeneration. Dev Biol 330, 186-199
  16. Galliot, B., Welschof, M., Schuckert, O., Hoffmeister, S., and Schaller, H.C. (1995) The cAMP response element binding protein is involved in hydra regeneration. Development 121, 1205-1216
  17. Kaloulis, K., Chera, S., Hassel, M., Gauchat, D., and Galliot, B. (2004) Reactivation of developmental programs: the cAMP-response element-binding protein pathway is involved in hydra head regeneration. Proc Natl Acad Sci U S A 101, 2363-2368
  18. Chera, S., Kaloulis, K., and Galliot, B. (2007) The cAMP response element binding protein (CREB) as an integrative HUB selector in metazoans: clues from the hydra model system. Biosystems 87, 191-203
  19. Miljkovic-Licina, M., Gauchat, D., and Galliot, B. (2004) Neuronal evolution: analysis of regulatory genes in a first-evolved nervous system, the hydra nervous system. Biosystems 76, 75-87
  20. Galliot, B., Quiquand, M., Ghila, L., de Rosa, R., Miljkovic-Licina, M., and Chera, S. (2009) Origins of neurogenesis, a cnidarian view. Dev Biol 332, 2-24
  21. Galliot, B. (2010) A Key Innovation in Evolution, the Emergence of Neurogenesis: Cellular and Molecular Cues from Cnidarian Nervous Systems. In Key Transitions in Animal Evolution (Schierwater, B., and De Salle, R., eds), 127-161, Science Publishers & CRC Press
  22. Pichaud, F., and Desplan, C. (2002) Pax genes and eye organogenesis. Curr Opin Genet Dev 12, 430-434
  23. Hirth, F., Kammermeier, L., Frei, E., Walldorf, U., Noll, M., and Reichert, H. (2003) An urbilaterian origin of the tripartite brain: developmental genetic insights from Drosophila. Development 130, 2365-2373
  24. Denes, A.S., Jekely, G., Steinmetz, P.R., Raible, F., Snyman, H., Prud'homme, B., Ferrier, D.E., Balavoine, G., et al. (2007) Molecular architecture of annelid nerve cord supports common origin of nervous system centralization in bilateria. Cell 129, 277-288
  25. Galliot, B., de Vargas, C., and Miller, D. (1999) Evolution of homeobox genes: Q50 Paired-like genes founded the Paired class. Dev Genes Evol 209, 186-197
  26. Gauchat, D., Mazet, F., Berney, C., Schummer, M., Kreger, S., Pawlowski, J., and Galliot, B. (2000)
    Evolution of Antp-class genes and differential expression of Hydra Hox/paraHox genes in anterior patterning. Proc Natl Acad Sci U S A 97, 4493-4498
  27. Ryan, J.F., Burton, P.M., Mazza, M.E., Kwong, G.K., Mullikin, J.C., and Finnerty, J.R. (2006) The cnidarian-bilaterian ancestor possessed at least 56 homeoboxes. Evidence from the starlet sea anemone, Nematostella vectensis. Genome Biol 7, R64
  28. Gauchat, D., Kreger, S., Holstein, T., and Galliot, B. (1998) prdl-a, a gene marker for hydra apical differentiation related to triploblastic paired-like head-specific genes. Development 125, 1637-1645
  29. Lindgens, D., Holstein, T.W., and Technau, U. (2004) Hyzic, the Hydra homolog of the zic/odd- paired gene, is involved in the early specification of the sensory nematocytes. Development 131, 191-201
  30. Gauchat, D., Escriva, H., Miljkovic-Licina, M., Chera, S., Langlois, M.C., Begue, A., Laudet, V., and Galliot, B. (2004) The orphan COUP-TF nuclear receptors are markers for neurogenesis from cnidarians to vertebrates. Dev Biol 275, 104-123
  31. Stierwald, M., Yanze, N., Bamert, R.P., Kammermeier, L., and Schmid, V. (2004) The Sine oculis/Six class family of homeobox genes in jellyfish with and without eyes: development and eye regeneration. Dev Biol 274, 70-81
  32. Marlow, H.Q., Srivastava, M., Matus, D.Q., Rokhsar, D., and Martindale, M.Q. (2009) Anatomy and development of the nervous system of Nematostella vectensis, an anthozoan cnidarian. Dev Neurobiol 69, 235-254
  33. Galliot, B., and Miller, D. (2000) Origin of anterior patterning. How old is our head? Trends Genet 16, 1-5
  34. Chourrout, D., Delsuc, F., Chourrout, P., Edvardsen, R.B., Rentzsch, F., Renfer, E., Jensen, M.F., Zhu, B., et al. (2006) Minimal ProtoHox cluster inferred from bilaterian and cnidarian Hox complements. Nature 442, 684-687
  35. Chiori, R., Jager, M., Denker, E., Wincker, P., Da Silva, C., Le Guyader, H., Manuel, M., and Queinnec, E. (2009) Are Hox genes ancestrally involved in axial patterning? Evidence from the hydrozoan Clytia hemisphaerica (Cnidaria). PLoS ONE 4, e4231
  36. Quiquand, M., Yanze, N., Schmich, J., Schmid, V., Galliot, B., and Piraino, S. (2009) More constraint on ParaHox than Hox gene families in early metazoan evolution. Dev Biol 328, 173-187
  37. Schummer, M., Scheurlen, I., Schaller, C., and Galliot, B. (1992) HOM/HOX homeobox genes are present in hydra (Chlorohydra viridissima) and are differentially expressed during regeneration. Embo J 11, 1815-1823
  38. Mieko Mizutani, C., and Bier, E. (2008) EvoDevo: the origins of BMP signalling in the neuroectoderm. Nat Rev Genet 9, 663-677
  • How Somatic Adult Tissues Develop Organizer Activity. Curr. Top. Dev. Biol. 2016 ;116():391-414. S0070-2153(15)00121-0. 10.1016/bs.ctdb.2015.11.002.

    Vogg MC, Wenger Y, Galliot B

    abstract

    The growth and patterning of anatomical structures from specific cellular fields in developing organisms relies on organizing centers that instruct surrounding cells to modify their behavior, namely migration, proliferation, and differentiation. We discuss here how organizers can form in adult organisms, a process of utmost interest for regenerative medicine. Animals like Hydra and planarians, which maintain their shape and fitness thanks to a highly dynamic homeostasis, offer a useful paradigm to study adult organizers in steady-state conditions. Beside the homeostatic context, these model systems also offer the possibility to study how organizers form de novo from somatic adult tissues. Both extracellular matrix remodeling and caspase activation play a key role in this transition, acting as promoters of organizer formation in the vicinity of the wound. Their respective roles and the crosstalk between them just start to be deciphered.

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  • The TALE face of Hox proteins in animal evolution. Front Genet 2015 ;6():267. 10.3389/fgene.2015.00267. PMC4539518.

    Merabet S, Galliot B

    abstract

    Hox genes are major regulators of embryonic development. One of their most conserved functions is to coordinate the formation of specific body structures along the anterior-posterior (AP) axis in Bilateria. This architectural role was at the basis of several morphological innovations across bilaterian evolution. In this review, we traced the origin of the Hox patterning system by considering the partnership with PBC and Meis proteins. PBC and Meis belong to the TALE-class of homeodomain-containing transcription factors and act as generic cofactors of Hox proteins for AP axis patterning in Bilateria. Recent data indicate that Hox proteins acquired the ability to interact with their TALE partners in the last common ancestor of Bilateria and Cnidaria. These interactions relied initially on a short peptide motif called hexapeptide (HX), which is present in Hox and non-Hox protein families. Remarkably, Hox proteins can also recruit the TALE cofactors by using specific PBC Interaction Motifs (SPIMs). We describe how a functional Hox/TALE patterning system emerged in eumetazoans through the acquisition of SPIMs. We anticipate that interaction flexibility could be found in other patterning systems, being at the heart of the astonishing morphological diversity observed in the animal kingdom.

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  • Multifunctionality and plasticity characterize epithelial cells in Hydra DOI:10.1080/21688370.2015.1068908

    W Buzgariu, S Al Haddad, S Tomczyk, Y Wenger, B Galliot

    abstract

    Epithelial sheets, a synapomorphy of all metazoans but porifers, are present as 2 layers in cnidarians, ectoderm and endoderm, joined at their basal side by an extra-cellular matrix named mesoglea. In the Hydra polyp, epithelial cells of the body column are unipotent stem cells that continuously self-renew and concomitantly express their epitheliomuscular features. These multifunctional contractile cells maintain homeostasis by providing a protective physical barrier, by digesting nutrients, by selecting a stable microbiota, and by rapidly closing wounds. In addition, epithelial cells are highly plastic, supporting the adaptation of Hydra to physiological and environmental changes, such as long starvation periods where survival relies on a highly dynamic autophagy flux. Epithelial cells also play key roles in developmental processes as evidenced by the organizer activity they develop to promote budding and regeneration. We propose here an integrative view of the homeostatic and developmental aspects of epithelial plasticity in Hydra.

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  • Hydra, a powerful model for aging studies. Invertebr. Reprod. Dev. 2015 Jan;59(sup1):11-16. 10.1080/07924259.2014.927805. 927805. PMC4463768. NIHMS685130.

    Tomczyk S, Fischer K, Austad S, Galliot B

    abstract

    Cnidarian Hydra polyps escape senescence, most likely due to the robust activity of their three stem cell populations. These stem cells continuously self-renew in the body column and differentiate at the extremities following a tightly coordinated spatial pattern. Paul Brien showed in 1953 that in one particular species, Hydra oligactis, cold-dependent sexual differentiation leads to rapid aging and death. Here, we review the features of this inducible aging phenotype. These cellular alterations, detected several weeks after aging was induced, are characterized by a decreasing density of somatic interstitial cell derivatives, a disorganization of the apical nervous system, and a disorganization of myofibers of the epithelial cells. Consequently, tissue replacement required to maintain homeostasis, feeding behavior, and contractility of the animal are dramatically affected. Interestingly, this aging phenotype is not observed in all H. oligactis strains, thus providing a powerful experimental model for investigations of the genetic control of aging. Given the presence in the cnidarian genome of a large number of human orthologs that have been lost in ecdysozoans, such approaches might help uncover novel regulators of aging in vertebrates.

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  • A dynamic architecture of life. F1000Res 2015 ;4():1288. 10.12688/f1000research.7315.1. PMC4760269.

    Rubin BP, Brockes J, Galliot B, Grossniklaus U, Lobo D, Mainardi M, Mirouze M, Prochiantz A, Steger A

    abstract

    In recent decades, a profound conceptual transformation has occurred comprising different areas of biological research, leading to a novel understanding of life processes as much more dynamic and changeable. Discoveries in plants and animals, as well as novel experimental approaches, have prompted the research community to reconsider established concepts and paradigms. This development was taken as an incentive to organise a workshop in May 2014 at the Academia Nazionale dei Lincei in Rome. There, experts on epigenetics, regeneration, neuroplasticity, and computational biology, using different animal and plant models, presented their insights on important aspects of a dynamic architecture of life, which comprises all organisational levels of the organism. Their work demonstrates that a dynamic nature of life persists during the entire existence of the organism and permits animals and plants not only to fine-tune their response to particular environmental demands during development, but underlies their continuous capacity to do so. Here, a synthesis of the different findings and their relevance for biological thinking is presented.

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  • Injury-induced immune responses in Hydra. Semin. Immunol. 2014 Aug;26(4):277-94. S1044-5323(14)00061-X. 10.1016/j.smim.2014.06.004.

    Wenger Y, Buzgariu W, Reiter S, Galliot B

    abstract

    The impact of injury-induced immune responses on animal regenerative processes is highly variable, positive or negative depending on the context. This likely reflects the complexity of the innate immune system that behaves as a sentinel in the transition from injury to regeneration. Early-branching invertebrates with high regenerative potential as Hydra provide a unique framework to dissect how injury-induced immune responses impact regeneration. A series of early cellular events likely require an efficient immune response after amputation, as antimicrobial defence, epithelial cell stretching for wound closure, migration of interstitial progenitors toward the wound, cell death, phagocytosis of cell debris, or reconstruction of the extracellular matrix. The analysis of the injury-induced transcriptomic modulations of 2636 genes annotated as immune genes in Hydra identified 43 genes showing an immediate/early pulse regulation in all regenerative contexts examined. These regulations point to an enhanced cytoprotection via ROS signaling (Nrf, C/EBP, p62/SQSMT1-l2), TNFR and TLR signaling (TNFR16-like, TRAF2l, TRAF5l, jun, fos-related, SIK2, ATF1/CREB, LRRC28, LRRC40, LRRK2), proteasomal activity (p62/SQSMT1-l1, Ced6/Gulf, NEDD8-conjugating enzyme Ubc12), stress proteins (CRYAB1, CRYAB2, HSP16.2, DnaJB9, HSP90a1), all potentially regulating NF-κB activity. Other genes encoding immune-annotated proteins such as NPYR4, GTPases, Swap70, the antiproliferative BTG1, enzymes involved in lipid metabolism (5-lipoxygenase, ACSF4), secreted clotting factors, secreted peptidases are also pulse regulated upon bisection. By contrast, metalloproteinases and antimicrobial peptide genes largely follow a context-dependent regulation, whereas the protease inhibitor α2macroglobulin gene exhibits a sustained up-regulation. Hence a complex immune response to injury is linked to wound healing and regeneration in Hydra.

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  • Robust G2 pausing of adult stem cells in Hydra. Differentiation ;87(1-2):83-99. S0301-4681(14)00011-5. 10.1016/j.diff.2014.03.001.

    Buzgariu W, Crescenzi M, Galliot B

    abstract

    Hydra is a freshwater hydrozoan polyp that constantly renews its two tissue layers thanks to three distinct stem cell populations that cannot replace each other, epithelial ectodermal, epithelial endodermal, and multipotent interstitial. These adult stem cells, located in the central body column, exhibit different cycling paces, slow for the epithelial, fast for the interstitial. To monitor the changes in cell cycling in Hydra, we established a fast and efficient flow cytometry procedure, which we validated by confirming previous findings, as the Nocodazole-induced reversible arrest of cell cycling in G2/M, and the mitogenic signal provided by feeding. Then to dissect the cycling and differentiation behaviors of the interstitial stem cells, we used the AEP_cnnos1 and AEP_Icy1 transgenic lines that constitutively express GFP in this lineage. For the epithelial lineages we used the sf-1 strain that rapidly eliminates the fast cycling cells upon heat-shock and progressively becomes epithelial. This study evidences similar cycling patterns for the interstitial and epithelial stem cells, which all alternate between the G2 and S-phases traversing a minimal G1-phase. We also found interstitial progenitors with a shorter G2 that pause in G1/G0. At the animal extremities, most cells no longer cycle, the epithelial cells terminally differentiate in G2 and the interstitial progenitors in G1/G0. At the apical pole ~80% cells are post-mitotic differentiated cells, reflecting the higher density of neurons and nematocytes in this region. We discuss how the robust G2 pausing of stem cells, maintained over weeks of starvation, may contribute to regeneration.

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  • Mechanisms of regeneration. Preface. Curr. Top. Dev. Biol. 2014 ;108():xiii-xviii. B978-0-12-391498-9.10000-2. 10.1016/B978-0-12-391498-9.10000-2.

    Galliot B

    abstract

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  • Cell death: a program to regenerate. Curr. Top. Dev. Biol. 2014 ;108():121-51. B978-0-12-391498-9.00002-4. 10.1016/B978-0-12-391498-9.00002-4.

    Vriz S, Reiter S, Galliot B

    abstract

    Recent studies in Drosophila, Hydra, planarians, zebrafish, mice, indicate that cell death can open paths to regeneration in adult animals. Indeed injury can induce cell death, itself triggering regeneration following an immediate instructive mechanism, whereby the dying cells release signals that induce cellular responses over short and/or long-range distances. Cell death can also provoke a sustained derepressing response through the elimination of cells that suppress regeneration in homeostatic conditions. Whether common properties support what we name "regenerative cell death," is currently unclear. As key parameters, we review here the injury proapoptotic signals, the signals released by the dying cells, the cellular responses, and their respective timing. ROS appears as a common signal triggering cell death through MAPK and/or JNK pathway activation. But the modes of ROS production vary, from a brief pulse upon wounding, to repeated waves as observed in the zebrafish fin where ROS supports two peaks of cell death. Indeed regenerative cell death can be restricted to the injury phase, as in Hydra, Drosophila, or biphasic, immediate, and delayed, as in planarians and zebrafish. The dying cells release in a caspase-dependent manner a variety of signaling molecules, cytokines, growth factors, but also prostaglandins or ATP as recorded in Drosophila, Hydra, mice, and zebrafish, respectively. Interestingly, the ROS-producing cells often resist to cell death, implying a complex paracrine mode of signaling to launch regeneration, involving ROS-producing cells, ROS-sensing cells that release signaling molecules upon caspase activation, and effector cells that respond to these signals by proliferating, migrating, and/or differentiating.

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  • Regeneration in Hydra eLS, 3rd edition ed. John Wiley & Sons Ltd: Chichester, London. DOI: 10.1002/9780470015902.a0001096.pub3

    Galliot, B

    abstract

    Hydra freshwater polyps have a remarkable ability to regenerate after bisection or even after dissociation, and thus offer a unique model system to investigate the cellular and molecular basis of eumetazoan regeneration. From a single cut along the body column two different types of regeneration arise: foot regeneration from the apical part and head regeneration from the basal part. The high proportion of stem cells in the Hydra body column supports these fast and efficient processes. Grafting experiments proved that the gastric tissue in the head‐regenerating tip rapidly develops a de novo organising activity, as evidenced by the induction of an ectopic axis when transplanted onto a host. The molecular mechanisms involved in this transformation rely on the immediate activation of the mitogen activated protein kinase (MAPK) pathway and the subsequent activation of the canonical Wnt3 pathway. This early phase is followed by a patterning phase, when head regeneration requires de novo neurogenesis.

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  • Punctuated emergences of genetic and phenotypic innovations in eumetazoan, bilaterian, euteleostome, and hominidae ancestors. Genome Biol Evol 2013 ;5(10):1949-68. evt142. 10.1093/gbe/evt142. PMC3814200.

    Wenger Y, Galliot B

    abstract

    Phenotypic traits derive from the selective recruitment of genetic materials over macroevolutionary times, and protein-coding genes constitute an essential component of these materials. We took advantage of the recent production of genomic scale data from sponges and cnidarians, sister groups from eumetazoans and bilaterians, respectively, to date the emergence of human proteins and to infer the timing of acquisition of novel traits through metazoan evolution. Comparing the proteomes of 23 eukaryotes, we find that 33% human proteins have an ortholog in nonmetazoan species. This premetazoan proteome associates with 43% of all annotated human biological processes. Subsequently, four major waves of innovations can be inferred in the last common ancestors of eumetazoans, bilaterians, euteleostomi (bony vertebrates), and hominidae, largely specific to each epoch, whereas early branching deuterostome and chordate phyla show very few innovations. Interestingly, groups of proteins that act together in their modern human functions often originated concomitantly, although the corresponding human phenotypes frequently emerged later. For example, the three cnidarians Acropora, Nematostella, and Hydra express a highly similar protein inventory, and their protein innovations can be affiliated either to traits shared by all eumetazoans (gut differentiation, neurogenesis); or to bilaterian traits present in only some cnidarians (eyes, striated muscle); or to traits not identified yet in this phylum (mesodermal layer, endocrine glands). The variable correspondence between phenotypes predicted from protein enrichments and observed phenotypes suggests that a parallel mechanism repeatedly produce similar phenotypes, thanks to novel regulatory events that independently tie preexisting conserved genetic modules.

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  • RNAseq versus genome-predicted transcriptomes: a large population of novel transcripts identified in an Illumina-454 Hydra transcriptome. BMC Genomics 2013 ;14():204. 1471-2164-14-204. 10.1186/1471-2164-14-204. PMC3764976.

    Wenger Y, Galliot B

    abstract

    Evolutionary studies benefit from deep sequencing technologies that generate genomic and transcriptomic sequences from a variety of organisms. Genome sequencing and RNAseq have complementary strengths. In this study, we present the assembly of the most complete Hydra transcriptome to date along with a comparative analysis of the specific features of RNAseq and genome-predicted transcriptomes currently available in the freshwater hydrozoan Hydra vulgaris.

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  • Guidelines for the use and interpretation of assays for monitoring autophagy. Autophagy 2012 Apr;8(4):445-544. PMC3404883.

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Bourquin JP, Boya P, Boyer-Guittaut M, Bozhkov PV, Brady NR, Brancolini C, Brech A, Brenman JE, Brennand A, Bresnick EH, Brest P, Bridges D, Bristol ML, Brookes PS, Brown EJ, Brumell JH, Brunetti-Pierri N, Brunk UT, Bulman DE, Bultman SJ, Bultynck G, Burbulla LF, Bursch W, Butchar JP, Buzgariu W, Bydlowski SP, Cadwell K, Cahová M, Cai D, Cai J, Cai Q, Calabretta B, Calvo-Garrido J, Camougrand N, Campanella M, Campos-Salinas J, Candi E, Cao L, Caplan AB, Carding SR, Cardoso SM, Carew JS, Carlin CR, Carmignac V, Carneiro LA, Carra S, Caruso RA, Casari G, Casas C, Castino R, Cebollero E, Cecconi F, Celli J, Chaachouay H, Chae HJ, Chai CY, Chan DC, Chan EY, Chang RC, Che CM, Chen CC, Chen GC, Chen GQ, Chen M, Chen Q, Chen SS, Chen W, Chen X, Chen X, Chen X, Chen YG, Chen Y, Chen Y, Chen YJ, Chen Z, Cheng A, Cheng CH, Cheng Y, Cheong H, Cheong JH, Cherry S, Chess-Williams R, Cheung ZH, Chevet E, Chiang HL, Chiarelli R, Chiba T, Chin LS, Chiou SH, Chisari FV, Cho CH, Cho DH, Choi AM, Choi D, 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TH, Hupp TR, Hur GM, Hurley JB, Hussain SN, Hussey PJ, Hwang JJ, Hwang S, Ichihara A, Ilkhanizadeh S, Inoki K, Into T, Iovane V, Iovanna JL, Ip NY, Isaka Y, Ishida H, Isidoro C, Isobe K, Iwasaki A, Izquierdo M, Izumi Y, Jaakkola PM, Jäättelä M, Jackson GR, Jackson WT, Janji B, Jendrach M, Jeon JH, Jeung EB, Jiang H, Jiang H, Jiang JX, Jiang M, Jiang Q, Jiang X, Jiang X, Jiménez A, Jin M, Jin S, Joe CO, Johansen T, Johnson DE, Johnson GV, Jones NL, Joseph B, Joseph SK, Joubert AM, Juhász G, Juillerat-Jeanneret L, Jung CH, Jung YK, Kaarniranta K, Kaasik A, Kabuta T, Kadowaki M, Kagedal K, Kamada Y, Kaminskyy VO, Kampinga HH, Kanamori H, Kang C, Kang KB, Kang KI, Kang R, Kang YA, Kanki T, Kanneganti TD, Kanno H, Kanthasamy AG, Kanthasamy A, Karantza V, Kaushal GP, Kaushik S, Kawazoe Y, Ke PY, Kehrl JH, Kelekar A, Kerkhoff C, Kessel DH, Khalil H, Kiel JA, Kiger AA, Kihara A, Kim DR, Kim DH, Kim DH, Kim EK, Kim HR, Kim JS, Kim JH, Kim JC, Kim JK, Kim PK, Kim SW, Kim YS, Kim Y, Kimchi A, Kimmelman AC, King JS, Kinsella TJ, Kirkin V, Kirshenbaum LA, Kitamoto K, Kitazato K, Klein L, Klimecki WT, Klucken J, Knecht E, Ko BC, Koch JC, Koga H, Koh JY, Koh YH, Koike M, Komatsu M, Kominami E, Kong HJ, Kong WJ, Korolchuk VI, Kotake Y, Koukourakis MI, Kouri Flores JB, Kovács AL, Kraft C, Krainc D, Krämer H, Kretz-Remy C, Krichevsky AM, Kroemer G, Krüger R, Krut O, Ktistakis NT, Kuan CY, Kucharczyk R, Kumar A, Kumar R, Kumar S, Kundu M, Kung HJ, Kurz T, Kwon HJ, La Spada AR, Lafont F, Lamark T, Landry J, Lane JD, Lapaquette P, Laporte JF, László L, Lavandero S, Lavoie JN, Layfield R, Lazo PA, Le W, Le Cam L, Ledbetter DJ, Lee AJ, Lee BW, Lee GM, Lee J, Lee JH, Lee M, Lee MS, Lee SH, Leeuwenburgh C, Legembre P, Legouis R, Lehmann M, Lei HY, Lei QY, Leib DA, Leiro J, Lemasters JJ, Lemoine A, Lesniak MS, Lev D, Levenson VV, Levine B, Levy E, Li F, Li JL, Li L, Li S, Li W, Li XJ, Li YB, Li YP, Liang C, Liang Q, Liao YF, Liberski PP, Lieberman A, Lim HJ, Lim KL, Lim K, Lin CF, Lin FC, 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    abstract

    In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process vs. those that measure flux through the autophagy pathway (i.e., the complete process); thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from stimuli that result in increased autophagic activity, defined as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (in most higher eukaryotes and some protists such as Dictyostelium) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the field understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field.

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  • Hydra, a fruitful model system for 270 years. Int. J. Dev. Biol. 2012 ;56(6-8):411-23. 120086bg. 10.1387/ijdb.120086bg.

    Galliot B

    abstract

    The discovery of Hydra regeneration by Abraham Trembley in 1744 promoted much scientific curiosity thanks to his clever design of experimental strategies away from the natural environment. Since then, this little freshwater cnidarian polyp flourished as a potent and fruitful model system. Here, we review some general biological questions that benefitted from Hydra research, such as the nature of embryogenesis, neurogenesis, induction by organizers, sex reversal, symbiosis, aging, feeding behavior, light regulation, multipotency of somatic stem cells, temperature-induced cell death, neuronal transdifferentiation, to cite only a few. To understand how phenotypes arise, theoricists also chose Hydra to model patterning and morphogenetic events, providing helpful concepts such as reaction-diffusion, positional information, and autocatalysis combined with lateral inhibition. Indeed, throughout these past 270 years, scientists used transplantation and grafting experiments, together with tissue, cell and molecular labelings, as well as biochemical procedures, in order to establish the solid foundations of cell and developmental biology. Nowadays, thanks to transgenic, genomic and proteomic tools, Hydra remains a promising model for these fields, but also for addressing novel questions such as evolutionary mechanisms, maintenance of dynamic homeostasis, regulation of stemness, functions of autophagy, cell death, stress response, innate immunity, bioactive compounds in ecosystems, ecotoxicant sensing and science communication.

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  • Preface: the hydra model system. Int. J. Dev. Biol. 2012 ;56(6-8):407-9. 120094bg. 10.1387/ijdb.120094bg.

    Galliot B

    abstract

    The freshwater Hydra polyp emerged as a model system in 1741 when Abraham Trembley not only discovered its amazing regenerative potential, but also demonstrated that experimental manipulations pave the way to research in biology. Since then, Hydra flourished as a potent and fruitful model system to help answer questions linked to cell and developmental biology, as such as the setting up of an organizer to regenerate a complex missing structure, the establishment and maintainance of polarity in a multicellular organism, the development of mathematical models to explain the robust developmental rules observed in this animal, the maintainance of stemness and multipotency in a highly dynamic environment, the plasticity of differentiated cells, to name but a few. However the Hydra model system is not restricted to cell and developmental biology; during the past 270 years it has also been heavily used to investigate the relationships between Hydra and its environment, opening new horizons concerning neurophysiology, innate immunity, ecosystems, ecotoxicology, symbiosis...

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  • Injury-induced asymmetric cell death as a driving force for head regeneration in Hydra. Dev. Genes Evol. 2013 Mar;223(1-2):39-52. 10.1007/s00427-012-0411-y.

    Galliot B

    abstract

    The freshwater Hydra polyp provides a unique model system to decipher the mechanisms underlying adult regeneration. Indeed, a single cut initiates two distinct regenerative processes, foot regeneration on one side and head regeneration on the other side, the latter relying on the rapid formation of a local head organizer. Two aspects are discussed here: the asymmetric cellular remodeling induced by mid-gastric bisection and the signaling events that trigger head organizer formation. In head-regenerating tips (but not in foot ones), a wave of cell death takes place immediately, leading the apoptotic cells to transiently release Wnt3 and activate the β-catenin pathway in the neighboring cycling cells to push them through mitosis. This process, which mimics the apoptosis-induced compensatory proliferation process deciphered in Drosophila larvae regenerating their discs, likely corresponds to an evolutionarily conserved mechanism, also at work in Xenopus tadpoles regenerating their tail or mice regenerating their skin or liver. How is this process generated in Hydra? Several studies pointed to the necessary activation of the extracellular signal-regulated kinase (ERK) 1-2 and mitogen-activated protein kinase (MAPK) pathways during early head regeneration. Indeed inhibition of ERK 1-2 or knockdown of RSK, cAMP response element-binding protein (CREB), and CREB-binding protein (CBP) prevent injury-induced apoptosis and head regeneration. The current scenario involves an asymmetric activation of the MAPK/CREB pathway to trigger injury-induced apoptosis in the interstitial cells and in the epithelial cells a CREB/CBP-dependent transcriptional activation of early genes essential for head-organizing activity as wnt3, HyBra1, and prdl-a. The question now is how bisection in the rather uniform central region of the polyp can generate this immediately asymmetric signaling.

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  • How to use Hydra as a model system to teach biology in the classroom. Int. J. Dev. Biol. 2012 ;56(6-8):637-52. 123523pb. 10.1387/ijdb.123523pb.

    Bossert P, Galliot B

    abstract

    As scientists it is our duty to fight against obscurantism and loss of rational thinking if we want politicians and citizens to freely make the most intelligent choices for the future generations. With that aim, the scientific education and training of young students is an obvious and urgent necessity. We claim here that Hydra provides a highly versatile but cheap model organism to study biology at any age. Teachers of biology have the unenviable task of motivating young people, who with many other motivations that are quite valid, nevertheless must be guided along a path congruent with a 'syllabus' or a 'curriculum'. The biology of Hydra spans the history of biology as an experimental science from Trembley's first manipulations designed to determine if the green polyp he found was plant or animal to the dissection of the molecular cascades underpinning, regeneration, wound healing, stemness, aging and cancer. It is described here in terms designed to elicit its wider use in classrooms. Simple lessons are outlined in sufficient detail for beginners to enter the world of 'Hydra biology'. Protocols start with the simplest observations to experiments that have been pretested with students in the USA and in Europe. The lessons are practical and can be used to bring 'life', but also rational thinking into the study of life for the teachers of students from elementary school through early university.

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  • Hydra, a versatile model to study the homeostatic and developmental functions of cell death. Int. J. Dev. Biol. 2012 ;56(6-8):593-604. 123499sr. 10.1387/ijdb.123499sr.

    Reiter S, Crescenzi M, Galliot B, Buzgariu W

    abstract

    In the freshwater cnidarian polyp Hydra, cell death takes place in multiple contexts. Indeed apoptosis occurs during oogenesis and spermatogenesis, during starvation, and in early head regenerating tips, promoting local compensatory proliferation at the boundary between heterografts. Apoptosis can also be induced upon exposure to pro-apoptotic agents (colchicine, wortmannin), upon heat-shock in the thermosensitive sf-1 mutant, and upon wounding. In all these contexts, the cells that undergo cell death belong predominantly to the interstitial cell lineage, whereas the epithelial cells, which are rather resistant to pro-apoptotic signals, engulf the apoptotic bodies. Beside this clear difference between the interstitial and the epithelial cell lineages, the different interstitial cell derivatives also show noticeable variations in their respective apoptotic sensitivity, with the precursor cells appearing as the most sensitive to pro-apoptotic signals. The apoptotic machinery has been well conserved across evolution. However, its specific role and regulation in each context are not known yet. Tools that help characterize apoptotic activity in Hydra have recently been developed. Among them, the aposensor Apoliner initially designed in Drosophila reliably measures wortmannin-induced apoptotic activity in a biochemical assay. Also, flow cytometry and TUNEL analyses help identify distinctive features between wortmannin-induced and heat-shock induced apoptosis in the sf-1 strain. Thanks to the live imaging tools already available, Hydra now offers a model system with which the functions of the apoptotic machinery to maintain long-term homeostasis, stem cell renewal, germ cell production, active developmental processes and non-self response can be deciphered.

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  • A two-step process in the emergence of neurogenesis. Eur. J. Neurosci. 2011 Sep;34(6):847-62. 10.1111/j.1460-9568.2011.07829.x.

    Galliot B, Quiquand M

    abstract

    Cnidarians belong to the first phylum differentiating a nervous system, thus providing suitable model systems to trace the origins of neurogenesis. Indeed corals, sea anemones, jellyfish and hydra contract, swim and catch their food thanks to sophisticated nervous systems that share with bilaterians common neurophysiological mechanisms. However, cnidarian neuroanatomies are quite diverse, and reconstructing the urcnidarian nervous system is ambiguous. At least a series of characters recognized in all classes appear plesiomorphic: (1) the three cell types that build cnidarian nervous systems (sensory-motor cells, ganglionic neurons and mechanosensory cells called nematocytes or cnidocytes); (2) an organization of nerve nets and nerve rings [those working as annular central nervous system (CNS)]; (3) a neuronal conduction via neurotransmitters; (4) a larval anterior sensory organ required for metamorphosis; (5) a persisting neurogenesis in adulthood. By contrast, the origin of the larval and adult neural stem cells differs between hydrozoans and other cnidarians; the sensory organs (ocelli, lens-eyes, statocysts) are present in medusae but absent in anthozoans; the electrical neuroid conduction is restricted to hydrozoans. Evo-devo approaches might help reconstruct the neurogenic status of the last common cnidarian ancestor. In fact, recent genomic analyses show that if most components of the postsynaptic density predate metazoan origin, the bilaterian neurogenic gene families originated later, in basal metazoans or as eumetazoan novelties. Striking examples are the ParaHox Gsx, Pax, Six, COUP-TF and Twist-type regulators, which seemingly exert neurogenic functions in cnidarians, including eye differentiation, and support the view of a two-step process in the emergence of neurogenesis.

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  • [Between homeostasis and development, which strategies to regenerate?]. Biol Aujourdhui 2011 ;205(2):125-37. 10.1051/jbio/2011011. jbio2011011.

    Galliot B

    abstract

    The Hydra model system is well suited to decipher the mechanisms underlying adult regeneration, specifically those that were robust enough to be maintained across evolution. After mid-gastric bissection head regeneration in Hydra relies on apoptosis-induced compensatory proliferation via the release of Wnt3 by the apoptotic interstitial cells and activation of the β-catenin pathway in the surrounding cycling interstitial cells. As apoptosis-induced compensatory proliferation is also at work in Drosophila regenerating imaginal discs, Xenopus tadpole regenerating their tail and mice regenerating their skin or their liver, this mechanism might represent an evolutionarily-conserved way to launch a regenerative response. However after decapitation, the analysis of the activation of the canonical Wnt pathway in decapitated Hydra showed that apoptosis-induced compensatory proliferation does not take place in this context. Given that the proportion of interstitial stem cells is significantly higher in the middle part than in the upper part of the body column, this suggested that the route taken to regenerate a structure as complex as the head dramatically varies according to the homeostatic status of the tissue at the time of injury. From these observations, we propose a tri-modular model for animal regeneration where the first module or "wound healing module" is followed by a transient module named "inducing module of regeneration" that allows the recruitment of the third module named "re-development module", necessary for repatterning the missing structure. We claim that among these three modules, the inducing module of regeneration is the most drastically constrained by the homeostatic conditions of any given tissue or organ at the time of injury and therefore the most variable.

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  • Injury-induced activation of the MAPK/CREB pathway triggers apoptosis-induced compensatory proliferation in hydra head regeneration. Dev. Growth Differ. 2011 Feb;53(2):186-201. 10.1111/j.1440-169X.2011.01250.x.

    Chera S, Ghila L, Wenger Y, Galliot B

    abstract

    After bisection, Hydra polyps regenerate their head from the lower half thanks to a head-organizer activity that is rapidly established at the tip. Head regeneration is also highly plastic as both the wild-type and the epithelial Hydra (that lack the interstitial cell lineage) can regenerate their head. In the wild-type context, we previously showed that after mid-gastric bisection, a large subset of the interstitial cells undergo apoptosis, inducing compensatory proliferation of the surrounding progenitors. This asymmetric process is necessary and sufficient to launch head regeneration. The apoptotic cells transiently release Wnt3, which promotes the formation of a proliferative zone by activating the beta-catenin pathway in the adjacent cycling cells. However the injury-induced signaling that triggers apoptosis is unknown. We previously reported an asymmetric immediate activation of the mitogen-activated protein kinase/ribosomal S6 kinase/cAMP response element binding protein (MAPK/RSK/CREB) pathway in head-regenerating tips after mid-gastric bisection. We show here that pharmacological inhibition of the MAPK/ERK pathway or RNAi knockdown of the RSK, CREB, CREB binding protein (CBP) genes prevents apoptosis, compensatory proliferation and blocks head regeneration. As the activation of the MAPK pathway upon injury plays an essential role in regenerating bilaterian species, these results suggest that the MAPK-dependent activation of apoptosis-induced compensatory proliferation represents an evolutionary-conserved mechanism to launch a regenerative process.

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  • Information processing in cells and tissues (IPCAT'2009): from small scale dynamics to understanding systems behavior. BioSystems 2010 Oct;102(1):1-2. S0303-2647(10)00123-1. 10.1016/j.biosystems.2010.07.015.

    Egri-Nagy A, Galliot B, Naef F, Nehaniv CL

    abstract

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  • The Hydra model: disclosing an apoptosis-driven generator of Wnt-based regeneration. Trends Cell Biol. 2010 Sep;20(9):514-23. S0962-8924(10)00099-1. 10.1016/j.tcb.2010.05.006.

    Galliot B, Chera S

    abstract

    The Hydra model system is well suited for the eludication of the mechanisms underlying regeneration in the adult, and an understanding of the core mechanisms is likely to cast light on pathways conserved in other species. Recent detailed analyses of the activation of the Wnt-beta-catenin pathway in bisected Hydra shows that the route taken to regenerate a structure as complex as the head varies dramatically according to the level of the amputation. When decapitation induces direct re-development due to Wnt3 signaling from epithelial cells, head regeneration after mid-gastric section relies first on Wnt3 signaling from interstitial cells, that undergo apoptosis-induced compensatory proliferation, and subsequently on activation of Wnt3 signaling in the epithelial cells. The relative distribution between stem cells and head progenitor cells is strikingly different in these two contexts, indicating that the pre-amputation homeostatic conditions define and constrain the route that bridges wound-healing to the re-development program of the missing structure.

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  • Cell plasticity in homeostasis and regeneration. Mol. Reprod. Dev. 2010 Oct;77(10):837-55. 10.1002/mrd.21206.

    Galliot B, Ghila L

    abstract

    Over the past decades, genetic analyses performed in vertebrate and invertebrate organisms deciphered numerous cellular and molecular mechanisms deployed during sexual development and identified genetic circuitries largely shared among bilaterians. In contrast, the functional analysis of the mechanisms that support regenerative processes in species randomly scattered among the animal kingdom, were limited by the lack of genetic tools. Consequently, unifying principles explaining how stress and injury can lead to the reactivation of a complete developmental program with restoration of original shape and function remained beyond reach of understanding. Recent data on cell plasticity suggest that beside the classical developmental approach, the analysis of homeostasis and asexual reproduction in adult organisms provides novel entry points to dissect the regenerative potential of a given species, a given organ or a given tissue. As a clue, both tissue homeostasis and regeneration dynamics rely on the availability of stem cells and/or on the plasticity of differentiated cells to replenish the missing structure. The freshwater Hydra polyp provides us with a unique model system to study the intricate relationships between the mechanisms that regulate the maintenance of homeostasis, even in extreme conditions (starvation and overfeeding) and the reactivation of developmental programs after bisection or during budding. Interestingly head regeneration in Hydra can follow several routes according to the level of amputation, suggesting that indeed the homeostatic background dramatically influences the route taken to bridge injury and regeneration.

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  • A Key Innovation in Animal Evolution, the Emergence of Neurogenesis: Cellular and Molecular Cues from Cnidarian Nervous Systems BOOK CHAPTER: In Key Transitions in Animal Evolution, Edited by B. Schierwater, and R. De Salle. ISBN 978-1-57808-695-5

    Brigitte GALLIOT

    abstract

    The emergence of neurogenesis led to the acquisition of an efficient neuromuscular transmission in eumetazoans, as shown by cnidarians that use evolutionarily-conserved neurophysiological principles to crumple, feed, swim. However, the cnidarian neuroanatomies are quite diverse and reconstructing the urcnidarian nervous system is not an easy task. Three types of characters shared by anthozoans and medusozoans appear plesiomorphic: 1) three cell types that all cnidarians differentiate, neurosensory cells, ganglionic neurons and nematocytes (cnidocytes) that combine mechano-chemosensation and venom secretion; 2) a chemical conduction through nerve nets and nerve rings, those being considered as annular central nervous systems; 3) a larval apical sensory organ that initiates metamorphosis. Other characters receive a disputed origin: 1) the neural stem cell(s), multipotent interstitial stem cell in hydrozoans, not identified in other classes; 2) the electrical conduction through neurons and epithelial cells present only in hydrozoans; 3) the embryonic origin of the nervous system; 4) the medusa sensory organs, ocelli or lens-eyes for light, statocysts for pressure, lacking in anthozoans. Nevertheless numerous gene families that regulate bilaterian neurogenesis are expressed during cnidarian neurogenesis, e.g. cnidarian eyes express Pax, Six and opsin, supporting a common origin for vision. However data establishing a clear picture of the cnidarian neurogenic circuitry are currently missing. Finally many “neurogenic” gene families likely arose and evolved in the absence of neurogenesis, as exemplified by Porifera that express them but lack synaptic transmission. Therefore some eumetazoan-specific families, missing in Porifera as ParaHox/Hox-like and Otx-like genes, might have contributed to the emergence of neurogenesis.

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  • Apoptotic cells provide an unexpected source of Wnt3 signaling to drive hydra head regeneration. Dev. Cell 2009 Aug;17(2):279-89. S1534-5807(09)00298-6. 10.1016/j.devcel.2009.07.014.

    Chera S, Ghila L, Dobretz K, Wenger Y, Bauer C, Buzgariu W, Martinou JC, Galliot B

    abstract

    Decapitated Hydra regenerate their heads via morphallaxis, i.e., without significant contributions made by cell proliferation or interstitial stem cells. Indeed, Hydra depleted of interstitial stem cells regenerate robustly, and Wnt3 from epithelial cells triggers head regeneration. However, we find a different mechanism controlling regeneration after midgastric bisection in animals equipped with both epithelial and interstitial cell lineages. In this context, we see rapid induction of apoptosis and Wnt3 secretion among interstitial cells at the head- (but not foot-) regenerating site. Apoptosis is both necessary and sufficient to induce Wnt3 production and head regeneration, even at ectopic sites. Further, we identify a zone of proliferation beneath the apoptotic zone, reminiscent of proliferative blastemas in regenerating limbs and of compensatory proliferation induced by dying cells in Drosophila imaginal discs. We propose that different types of injuries induce distinct cellular modes of Hydra head regeneration, which nonetheless converge on a central effector, Wnt3.

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  • Origins of neurogenesis, a cnidarian view. Dev. Biol. 2009 Aug;332(1):2-24. S0012-1606(09)00875-6. 10.1016/j.ydbio.2009.05.563.

    Galliot B, Quiquand M, Ghila L, de Rosa R, Miljkovic-Licina M, Chera S

    abstract

    New perspectives on the origin of neurogenesis emerged with the identification of genes encoding post-synaptic proteins as well as many "neurogenic" regulators as the NK, Six, Pax, bHLH proteins in the Demosponge genome, a species that might differentiate sensory cells but no neurons. However, poriferans seem to miss some key regulators of the neurogenic circuitry as the Hox/paraHox and Otx-like gene families. Moreover as a general feature, many gene families encoding evolutionarily-conserved signaling proteins and transcription factors were submitted to a wave of gene duplication in the last common eumetazoan ancestor, after Porifera divergence. In contrast gene duplications in the last common bilaterian ancestor, Urbilateria, are limited, except for the bHLH Atonal-class. Hence Cnidaria share with Bilateria a large number of genetic tools. The expression and functional analyses currently available suggest a neurogenic function for numerous orthologs in developing or adult cnidarians where neurogenesis takes place continuously. As an example, in the Hydra polyp, the Clytia medusa and the Acropora coral, the Gsx/cnox2/Anthox-2 ParaHox gene likely supports neurogenesis. Also neurons and nematocytes (mechanosensory cells) share in hydrozoans a common stem cell and several regulatory genes indicating that they can be considered as sister cells. Performed in anthozoan and medusozoan species, these studies should tell us more about the way(s) evolution hazards achieved the transition from epithelial to neuronal cell fate, and about the robustness of the genetic circuitry that allowed neuromuscular transmission to arise and be maintained across evolution.

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  • More constraint on ParaHox than Hox gene families in early metazoan evolution. Dev. Biol. 2009 Apr;328(2):173-87. S0012-1606(09)00062-1. 10.1016/j.ydbio.2009.01.022.

    Quiquand M, Yanze N, Schmich J, Schmid V, Galliot B, Piraino S

    abstract

    Hox and ParaHox (H/P) genes belong to evolutionary-sister clusters that arose through duplication of a ProtoHOX cluster early in animal evolution. In contrast to bilaterians, cnidarians express, beside PG1, PG2 and Gsx orthologs, numerous Hox-related genes with unclear origin. We characterized from marine hydrozoans three novel Hox-related genes expressed at medusa and polyp stages, which include a Pdx/Xlox ParaHox ortholog induced 1 day later than Gsx during embryonic development. To reconstruct H/P genes' early evolution, we performed multiple systematic comparative phylogenetic analyses, which identified derived sequences that blur the phylogenetic picture, recorded dramatically different evolutionary rates between ParaHox and Hox in cnidarians and showed the unexpected grouping of [Gsx-Pdx/Xlox-PG2-PG3] families in a single metagroup distinct from PG1. We propose a novel more parsimonious evolutionary scenario whereby H/P genes originated from a [Gsx-Pdx/Xlox-PG2-PG3]-related ProtoHox gene, the "posterior" and "anterior" H/P genes appearing secondarily. The ProtoHOX cluster would have contained the three Gsx/PG2, Pdx/PG3, Cdx/PG9 paralogs and produced through tandem duplication the primordial HOX and ParaHOX clusters in the Cnidaria-Bilateria ancestor. The stronger constraint on cnidarian ParaHox genes suggests that the primary function of pre-bilaterian H/P genes was to drive cellular evolutionary novelties such as neurogenesis rather than axis specification.

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  • Autophagy in Hydra: a response to starvation and stress in early animal evolution. Biochim. Biophys. Acta 2009 Sep;1793(9):1432-43. S0167-4889(09)00082-2. 10.1016/j.bbamcr.2009.03.010.

    Chera S, Buzgariu W, Ghila L, Galliot B

    abstract

    The Hydra polyp provides a powerful model system to investigate the regulation of cell survival and cell death in homeostasis and regeneration as Hydra survive weeks without feeding and regenerates any missing part after bisection. Induction of autophagy during starvation is the main surviving strategy in Hydra as autophagic vacuoles form in most myoepithelial cells after several days. When the autophagic process is inhibited, animal survival is actually rapidly jeopardized. An appropriate regulation of autophagy is also essential during regeneration as Hydra RNAi knocked-down for the serine protease inhibitor Kazal-type (SPINK) gene Kazal1, exhibit a massive autophagy after amputation that rapidly compromises cell and animal survival. This excessive autophagy phenotype actually mimics that observed in the mammalian pancreas when SPINK genes are mutated, highlighting the paradigmatic value of the Hydra model system for deciphering pathological processes. Interestingly autophagy during starvation predominantly affects ectodermal epithelial cells and lead to cell survival whereas Kazal1(RNAi)-induced autophagy is restricted to endodermal digestive cells that rapidly undergo cell death. This indicates that distinct regulations that remain to be identified, are at work in these two contexts. Cnidarian express orthologs for most components of the autophagy and TOR pathways suggesting evolutionarily-conserved roles during starvation.

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  • Methods to investigate autophagy during starvation and regeneration in hydra. Meth. Enzymol. 2008 ;451():409-37. S0076-6879(08)03226-6. 10.1016/S0076-6879(08)03226-6.

    Buzgariu W, Chera S, Galliot B

    abstract

    In hydra, the regulation of the balance between cell death and cell survival is essential to maintain homeostasis across the animal and promote animal survival during starvation. Moreover, this balance also appears to play a key role during regeneration of the apical head region. The recent finding that autophagy is a crucial component of this balance strengthens the value of the Hydra model system to analyze the implications of autophagy in starvation, stress response and regeneration. We describe here how we adapted to Hydra some established tools to monitor steady-state autophagy. The ATG8/LC3 marker used in biochemical and immunohistochemical analyses showed a significant increase in autophagosome formation in digestive cells after 11 days of starvation. Moreover, the maceration procedure that keeps intact the morphology of the various cell types allows the quantification of the autophagosomes and autolysosomes in any cell type, thanks to the detection of the MitoFluor or LysoTracker dyes combined with the anti-LC3, anti-LBPA, and/or anti-RSK (ribosomal S6 kinase) immunostaining. The classical activator (rapamycin) and inhibitors (wortmannin, bafilomycin A(1)) of autophagy also appear to be valuable tools to modulate autophagy in hydra, as daily-fed and starved hydra display slightly different responses. Finally, we show that the genetic circuitry underlying autophagy can be qualitatively and quantitatively tested through RNA interference in hydra repeatedly exposed to double-stranded RNAs.

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  • Triggering the regeneration and tissue repair programs. Development 2009 Feb;136(3):349-53. 136/3/349. 10.1242/dev.031682.

    Tanaka E, Galliot B

    abstract

    In early October 2008, researchers from diverse backgrounds gathered at an EMBO conference entitled 'The Molecular and Cellular Basis of Regeneration and Tissue Repair' to discuss the basic biology of regeneration. Topics included cell plasticity in regenerative and developmental contexts, and the link between wound healing and regeneration. The meeting also highlighted the progress made in identifying the molecular networks that underlie regeneration in a variety of model systems.

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  • Guidelines for the use and interpretation of assays for monitoring autophagy in higher eukaryotes. Autophagy 2008 Feb;4(2):151-75. 5338. PMC2654259. NIHMS96070.

    Klionsky DJ, Abeliovich H, Agostinis P, Agrawal DK, Aliev G, Askew DS, Baba M, Baehrecke EH, Bahr BA, Ballabio A, Bamber BA, Bassham DC, Bergamini E, Bi X, Biard-Piechaczyk M, Blum JS, Bredesen DE, Brodsky JL, Brumell JH, Brunk UT, Bursch W, Camougrand N, Cebollero E, Cecconi F, Chen Y, Chin LS, Choi A, Chu CT, Chung J, Clarke PG, Clark RS, Clarke SG, Clavé C, Cleveland JL, Codogno P, Colombo MI, Coto-Montes A, Cregg JM, Cuervo AM, Debnath J, Demarchi F, Dennis PB, Dennis PA, Deretic V, Devenish RJ, Di Sano F, Dice JF, Difiglia M, Dinesh-Kumar S, Distelhorst CW, Djavaheri-Mergny M, Dorsey FC, Dröge W, Dron M, Dunn WA, Duszenko M, Eissa NT, Elazar Z, Esclatine A, Eskelinen EL, Fésüs L, Finley KD, Fuentes JM, Fueyo J, Fujisaki K, Galliot B, Gao FB, Gewirtz DA, Gibson SB, Gohla A, Goldberg AL, Gonzalez R, González-Estévez C, Gorski S, Gottlieb RA, Häussinger D, He YW, Heidenreich K, Hill JA, Høyer-Hansen M, Hu X, Huang WP, Iwasaki A, Jäättelä M, Jackson WT, Jiang X, Jin S, Johansen T, Jung JU, Kadowaki M, Kang C, Kelekar A, Kessel DH, Kiel JA, Kim HP, Kimchi A, Kinsella TJ, Kiselyov K, Kitamoto K, Knecht E, Komatsu M, Kominami E, Kondo S, Kovács AL, Kroemer G, Kuan CY, Kumar R, Kundu M, Landry J, Laporte M, Le W, Lei HY, Lenardo MJ, Levine B, Lieberman A, Lim KL, Lin FC, Liou W, Liu LF, Lopez-Berestein G, López-Otín C, Lu B, Macleod KF, Malorni W, Martinet W, Matsuoka K, Mautner J, Meijer AJ, Meléndez A, Michels P, Miotto G, Mistiaen WP, Mizushima N, Mograbi B, Monastyrska I, Moore MN, Moreira PI, Moriyasu Y, Motyl T, Münz C, Murphy LO, Naqvi NI, Neufeld TP, Nishino I, Nixon RA, Noda T, Nürnberg B, Ogawa M, Oleinick NL, Olsen LJ, Ozpolat B, Paglin S, Palmer GE, Papassideri I, Parkes M, Perlmutter DH, Perry G, Piacentini M, Pinkas-Kramarski R, Prescott M, Proikas-Cezanne T, Raben N, Rami A, Reggiori F, Rohrer B, Rubinsztein DC, Ryan KM, Sadoshima J, Sakagami H, Sakai Y, Sandri M, Sasakawa C, Sass M, Schneider C, Seglen PO, Seleverstov O, Settleman J, Shacka JJ, Shapiro IM, Sibirny A, Silva-Zacarin EC, Simon HU, Simone C, Simonsen A, Smith MA, Spanel-Borowski K, Srinivas V, Steeves M, Stenmark H, Stromhaug PE, Subauste CS, Sugimoto S, Sulzer D, Suzuki T, Swanson MS, Tabas I, Takeshita F, Talbot NJ, Tallóczy Z, Tanaka K, Tanaka K, Tanida I, Taylor GS, Taylor JP, Terman A, Tettamanti G, Thompson CB, Thumm M, Tolkovsky AM, Tooze SA, Truant R, Tumanovska LV, Uchiyama Y, Ueno T, Uzcátegui NL, van der Klei I, Vaquero EC, Vellai T, Vogel MW, Wang HG, Webster P, Wiley JW, Xi Z, Xiao G, Yahalom J, Yang JM, Yap G, Yin XM, Yoshimori T, Yu L, Yue Z, Yuzaki M, Zabirnyk O, Zheng X, Zhu X, Deter RL

    abstract

    Research in autophagy continues to accelerate,(1) and as a result many new scientists are entering the field. Accordingly, it is important to establish a standard set of criteria for monitoring macroautophagy in different organisms. Recent reviews have described the range of assays that have been used for this purpose.(2,3) There are many useful and convenient methods that can be used to monitor macroautophagy in yeast, but relatively few in other model systems, and there is much confusion regarding acceptable methods to measure macroautophagy in higher eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers of autophagosomes versus those that measure flux through the autophagy pathway; thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from fully functional autophagy that includes delivery to, and degradation within, lysosomes (in most higher eukaryotes) or the vacuole (in plants and fungi). Here, we present a set of guidelines for the selection and interpretation of the methods that can be used by investigators who are attempting to examine macroautophagy and related processes, as well as by reviewers who need to provide realistic and reasonable critiques of papers that investigate these processes. This set of guidelines is not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to verify an autophagic response.

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  • RNAi gene silencing affects cell and developmental plasticity in hydra. C. R. Biol. ;330(6-7):491-7. S1631-0691(07)00047-9. 10.1016/j.crvi.2007.01.008.

    Galliot B, Miljkovic-Licina M, Ghila L, Chera S

    abstract

    The recent establishment of gene silencing through RNA interference upon feeding opens avenues to decipher the genetic control of regeneration in hydra. Following that approach, we identified three main stages for head regeneration. Immediately post-amputation, the serine protease inhibitor Kazal1 gene produced by the gland cells prevents from an excessive autophagy in regenerating tips. This cytoprotective function, or self-preservation, is similar to that played by Kazal-type proteins in the mammalian exocrine pancreas, in homeostatic or post-injury conditions, likely reflecting an evolutionarily conserved mechanism linking cell survival to tissue repair. Indeed, in wild-type hydra, within the first hours following mid-gastric section, an extensive cellular remodelling is taking place, including phenotypic cellular transitions and cell proliferation. The activation of the MAPK pathway, which leads to the RSK-dependent CREB phosphorylation, is required for these early cellular events. Later, at the early-late stage, the expression of the Gsx/cnox-2 ParaHox gene in proliferating apical neuronal progenitors is required for the de novo neurogenesis that precedes the emergence of the tentacle rudiments. Hence, head regeneration in wild-type hydra relies on spatially restricted and timely orchestrated cellular modifications, which display similarities with those reported during vertebrate epimorphic regeneration. These results suggest some conservation across evolution of the mechanisms driving the post-amputation reactivation of developmental programs.

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  • Head regeneration in wild-type hydra requires de novo neurogenesis. Development 2007 Mar;134(6):1191-201. dev.02804. 10.1242/dev.02804.

    Miljkovic-Licina M, Chera S, Ghila L, Galliot B

    abstract

    Because head regeneration occurs in nerve-free hydra mutants, neurogenesis was regarded as dispensable for this process. Here, in wild-type hydra, we tested the function of the ParaHox gsx homolog gene, cnox-2, which is a specific marker for bipotent neuronal progenitors, expressed in cycling interstitial cells that give rise to apical neurons and gastric nematoblasts (i.e. sensory mechanoreceptor precursors). cnox-2 RNAi silencing leads to a dramatic downregulation of hyZic, prdl-a, gsc and cnASH, whereas hyCOUP-TF is upregulated. cnox-2 indeed acts as an upstream regulator of the neuronal and nematocyte differentiation pathways, as cnox-2(-) hydra display a drastic reduction in apical neurons and gastric nematoblasts, a disorganized apical nervous system and a decreased body size. During head regeneration, the locally restricted de novo neurogenesis that precedes head formation is cnox-2 dependent: cnox-2 expression is induced in neuronal precursors and differentiating neurons that appear in the regenerating tip; cnox-2 RNAi silencing reduces this de novo neurogenesis and delays head formation. Similarly, the disappearance of cnox-2(+) cells in sf-1 mutants also correlates with head regeneration blockade. Hence in wild-type hydra, head regeneration requires the cnox-2 neurogenic function. When neurogenesis is missing, an alternative, slower and less efficient, head developmental program is possibly activated.

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  • The cAMP response element binding protein (CREB) as an integrative HUB selector in metazoans: clues from the hydra model system. BioSystems 2007 Feb;87(2-3):191-203. S0303-2647(06)00166-3. 10.1016/j.biosystems.2006.09.014.

    Chera S, Kaloulis K, Galliot B

    abstract

    In eukaryotic cells, a multiplicity of extra-cellular signals can activate a unique signal transduction system that at the nuclear level will turn on a variety of target genes, eliciting thus diverse responses adapted to the initial signal. How distinct signals can converge on a unique signalling pathway that will nevertheless produce signal-specific responses provides a theoretical paradox that can be traced back early in evolution. In bilaterians, the CREB pathway connects diverse extra-cellular signals via cytoplasmic kinases to the CREB transcription factor and the CBP co-activator, regulating according to the context, cell survival, cell proliferation, cell differentiation, pro-apoptosis, long-term memory, hence achieving a "hub" function for cellular and developmental processes. In hydra, the CREB pathway is highly conserved and activated during early head regeneration through RSK-dependent CREB phosphorylation. We show here that the CREB transcription factor and the RSK kinase are co-expressed in all three hydra cell lineages including dividing interstitial stem cells, proliferating nematoblasts, proliferating spermatogonia and spermatocytes, differentiating and mature neurons as well as ectodermal and endodermal myoepithelial cells. In addition, CREB gene expression is specifically up-regulated during early regeneration and early budding. When the CREB function was chemically prevented, the early post-amputation induction of the HyBraI gene was no longer observed and head regeneration was stacked. Thus, in hydra, the CREB pathway appears already involved in multiple tasks, such as reactivation of developmental programs in an adult context, self-renewal of stem cells, proliferation of progenitors and neurogenesis. Consequently, the hub function played by the CREB pathway was established early in animal evolution and might have contributed to the formation of an efficient oral pole through the integration of the neurogenic and patterning functions.

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  • Autophagy and self-preservation: a step ahead from cell plasticity? Autophagy ;2(3):231-3. 2706.

    Galliot B

    abstract

    Silencing the SPINK-related gene Kazal1 in hydra gland cells induces an excessive autophagy of both gland and digestive cells, leading to animal death. Moreover, during regeneration, autophagosomes are immediately detected in regenerating tips, where Kazal1 expression is lowered. When Kazal1 is completely silenced, hydra no longer survive the amputation stress (Chera S, de Rosa R, Miljkovic-Licina M, Dobretz K, Ghila L, Kaloulis K, Galliot B. Silencing of the hydra serine protease inhibitor Kazal1 gene mimics the human Spink1 pancreatic phenotype. J Cell Sci 2006; 119:846-57). These results highlight the essential digestive and cytoprotective functions played by Kazal1 in hydra. In mammals, autophagy of exocrine pancreatic cells is also induced upon SPINK1/Spink3 inactivation, whereas Spink3 is activated in injured pancreatic cells. Hence SPINKs, by preventing an excessive autophagy, appear to act as key players of the stress-induced self-preservation program. In hydra, this program is a prerequisite to the early cellular transition, whereby digestive cells of the regenerating tips transform into a head-organizer center. Enhancing the self-preservation program in injured tissues might therefore be the condition for unmasking their potential cell and/or developmental plasticity.

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  • Hydra, a niche for cell and developmental plasticity. Semin. Cell Dev. Biol. 2006 Aug;17(4):492-502. S1084-9521(06)00054-1. 10.1016/j.semcdb.2006.05.005.

    Galliot B, Miljkovic-Licina M, de Rosa R, Chera S

    abstract

    The silencing of genes whose expression is restricted to specific cell types and/or specific regeneration stages opens avenues to decipher the molecular control of the cellular plasticity underlying head regeneration in hydra. In this review, we highlight recent studies that identified genes involved in the immediate cytoprotective function played by gland cells after amputation; the early dedifferentiation of digestive cells into blastema-like cells during head regeneration, and the early late proliferation of neuronal progenitors required for head patterning. Hence, developmental plasticity in hydra relies on spatially restricted and timely orchestrated cellular modifications, where the functions played by stem cells remain to be characterized.

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  • Silencing of the hydra serine protease inhibitor Kazal1 gene mimics the human SPINK1 pancreatic phenotype. J. Cell. Sci. 2006 Mar;119(Pt 5):846-57. jcs.02807. 10.1242/jcs.02807.

    Chera S, de Rosa R, Miljkovic-Licina M, Dobretz K, Ghila L, Kaloulis K, Galliot B

    abstract

    In hydra, the endodermal epithelial cells carry out the digestive function together with the gland cells that produce zymogens and express the evolutionarily conserved gene Kazal1. To assess the hydra Kazal1 function, we silenced gene expression through double-stranded RNA feeding. A progressive Kazal1 silencing affected homeostatic conditions as evidenced by the low budding rate and the induced animal death. Concomitantly, a dramatic disorganization followed by a massive death of gland cells was observed, whereas the cytoplasm of digestive cells became highly vacuolated. The presence of mitochondria and late endosomes within those vacuoles assigned them as autophagosomes. The enhanced Kazal1 expression in regenerating tips was strongly diminished in Kazal1(-) hydra, and the amputation stress led to an immediate disorganization of the gland cells, vacuolization of the digestive cells and death after prolonged silencing. This first cellular phenotype resulting from a gene knock-down in cnidarians suggests that the Kazal1 serine-protease-inhibitor activity is required to prevent excessive autophagy in intact hydra and to exert a cytoprotective function to survive the amputation stress. Interestingly, these functions parallel the pancreatic autophagy phenotype observed upon mutation within the Kazal domain of the SPINK1 and SPINK3 genes in human and mice, respectively.

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  • The orphan COUP-TF nuclear receptors are markers for neurogenesis from cnidarians to vertebrates. Dev. Biol. 2004 Nov;275(1):104-23. S0012-1606(04)00510-X. 10.1016/j.ydbio.2004.07.037.

    Gauchat D, Escriva H, Miljkovic-Licina M, Chera S, Langlois MC, Begue A, Laudet V, Galliot B

    abstract

    In bilaterians, COUP-TF nuclear receptors participate in neurogenesis and/or CNS patterning. In hydra, the nervous system is formed of sensory mechanoreceptor cells (nematocytes) and neuronal cells, both lineages deriving from a common stem cell. The hydra COUP-TF gene, hyCOUP-TF, which encodes highly conserved DNA-binding and ligand-binding domains, belongs to the monophyletic COUP-TFs orphan receptor family (NR2F). In adult polyps, hyCOUP-TF is expressed in nematoblasts and a subset of neuronal cells. Comparative BrDU labeling analyses performed on cells expressing either hyCOUP-TF or the paired-like gene prdl-b showed that prdl-b expression corresponded to early stages of proliferation, while hyCOUP-TF was detected slightly later. HyCOUP-TF and prdl-b expressing cells disappeared in sf-1 mutants becoming "nerve-free". Moreover hyCOUP-TF and prdl-b expression was excluded from regions undergoing developmental processes. These data suggest that hyCOUP-TF and prdl-b belong to a genetic network that appeared together with neurogenesis during early metazoan evolution. The hyCOUP-TF protein specifically bound onto the evolutionarily conserved DR1 and DR5 response elements, and repressed transactivation induced by RAR:RXR nuclear receptors in a dose-dependent manner when expressed in mammalian cells. Hence, a cnidarian transcription factor can be active in vertebrate cells, implying that functional interactions between COUP-TF and other nuclear receptors were evolutionarily conserved.

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  • Neuronal evolution: analysis of regulatory genes in a first-evolved nervous system, the hydra nervous system. BioSystems ;76(1-3):75-87. 10.1016/j.biosystems.2004.05.030. S0303264704000693.

    Miljkovic-Licina M, Gauchat D, Galliot B

    abstract

    Cnidarians represent the first animal phylum with an organized nervous system and a complex active behavior. The hydra nervous system is formed of sensory-motoneurons, ganglia neurons and mechanoreceptor cells named nematocytes, which all differentiate from a common stem cell. The neurons are organized as a nerve net and a subset of neurons participate in a more complex structure, the nerve ring that was identified in most cnidarian species at the base of the tentacles. In order to better understand the genetic control of this neuronal network, we analysed the expression of evolutionarily conserved regulatory genes in the hydra nervous system. The Prd-class homeogene prdl-b and the nuclear orphan receptor hyCOUP-TF are expressed at strong levels in proliferating nematoblasts, a lineage where they were found repressed during patterning and morphogenesis, and at low levels in distinct subsets of neurons. Interestingly, Prd-class homeobox and COUP-TF genes are also expressed during neurogenesis in bilaterians, suggesting that mechanoreceptor and neuronal cells derive from a common ancestral cell. Moreover, the Prd-class homeobox gene prdl-a, the Antp-class homeobox gene msh, and the thrombospondin-related gene TSP1, which are expressed in distinct subset of neurons in the adult polyp, are also expressed during early budding and/or head regeneration. These data strengthen the fact that two distinct regulations, one for neurogenesis and another for patterning, already apply to these regulatory genes, a feature also identified in bilaterian related genes.

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  • Reactivation of developmental programs: the cAMP-response element-binding protein pathway is involved in hydra head regeneration. Proc. Natl. Acad. Sci. U.S.A. 2004 Feb;101(8):2363-8. 101/8/2363. PMC356956.

    Kaloulis K, Chera S, Hassel M, Gauchat D, Galliot B

    abstract

    Hydra regenerate throughout their life. We previously described early modulations in cAMP-response element-binding protein (CREB) DNA-binding activity during regeneration. We now show that the Ser-67 residue located in the P-box is a target for post-translational regulation. The antihydra CREB antiserum detected CREB-positive nuclei distributed in endoderm and ectoderm, whereas the phosphoSer133-CREB antibody detected phospho-CREB-positive nuclei exclusively in endodermal cells. During early regeneration, we observed a dramatic increase in the number of phospho-CREB-positive nuclei in head-regenerating tips, exceeding 80% of the endodermal cells. We identified among CREB-binding kinases the p80 kinase, which showed an enhanced activity and a hyperphosphorylated status during head but not foot regeneration. According to biochemical and immunological evidence, this p80 kinase belongs to the Ribosomal protein S6 kinase family. Exposure to the U0126 mitogen-activated protein kinase kinase inhibitor inhibited head but not foot regeneration, abolished CREB phosphorylation and activation of the early gene HyBra1 in head-regenerating tips. These data support a role for the mitogen-activated protein kinase/ribosomal protein S6 kinase/CREB pathway in hydra head organizer activity.

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  • Cnidarian and bilaterian promoters can direct GFP expression in transfected hydra. Dev. Biol. 2002 Jun;246(2):377-90. 10.1006/dbio.2002.0676. S0012160602906767.

    Miljkovic M, Mazet F, Galliot B

    abstract

    Complete sexual development is not easily amenable to experimentation in hydra. Therefore, the analysis of gene function and gene regulation requires the introduction of exogenous DNA in a large number of cells of the hydra polyps and the significant expression of reporter constructs in these cells. We present here the procedure whereby we coupled DNA injection into the gastric cavity to electroporation of the whole animal in order to efficiently transfect hydra polyps. We could detect GFP fluorescence in both endodermal and ectodermal cell layers of live animals and in epithelial as well as interstitial cell types of dissociated hydra. In addition, we could confirm GFP protein expression by showing colocalisation between GFP fluorescence and anti-GFP immunofluorescence. Finally, when a FLAG epitope was inserted in-frame with the GFP coding sequence, GFP fluorescence also colocalised with anti-FLAG immunofluorescence. This GFP expression in hydra cells was directed by various promoters, either homologous, like the hydra homeobox cnox-2 gene promoter, or heterologous, like the two nematode ribosomal protein S5 and L28 gene promoters, and the chicken beta-actin gene promoter. This strategy provides new tools for dissecting developmental molecular mechanisms in hydra; more specifically, the genetic regulations that take place in endodermal cells at the time budding or regeneration is initiated.

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  • Cnidarians as a model system for understanding evolution and regeneration. Int. J. Dev. Biol. 2002 Jan;46(1):39-48.

    Galliot B, Schmid V

    abstract

    Hydra and Podocolyne are two cnidarian animals which provide complementary advantages for analysing developmental mechanisms possibly reflecting the basic developmental processes shared by most bilaterians. Interestingly, these mechanisms remain accessible all along the life of these animals, which bud and regenerate, whatever their age. The Hydra polyp permits a direct study of the molecular cascades linking amputation to regeneration. Podocoryne displays a complete life cycle, polyp and medusa stages with a fast and inducible sexual cycle and an unparalleled In vitro transdifferentiation potential. In both cases, a large number of evolutionarily conserved molecular markers are available, and analysis of their regulation highlights the molecular mechanisms which underly pattern formation in these two species.

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Highlights in General Media about our Research (selection)

2015

Jeunesse éternelle. Simples Immortels. Documentary movie by Romain Miranda for TSR (TV Swiss channel), “Mise au Point” Oct. 25, 2015. http://www.rts.ch/emissions/mise-au-point/7086228-conseil-federal-peages-urbains-jeunesse-eternelle.html

Interview on the channel RTS (Radio Télévision Suisse Romande), CQFD program 2015. Journalist: Bastien Confino.
L’hydre modifie son programme génétique

  • http://www.rts.ch/la-1ere/programmes/cqfd/7237869-l-hydre-modifie-son-programme-genetique-23-11-2015.html 
  • interview accessible on the site Avis d’Experts: http://avisdexperts.ch/videos/view/4613/10 (10 minutes)
    • Hydra can modify its genetic program. Science Daily, 23.11.2015
    • Las hidras pueden modificar su programa genético. Noticias de la Ciencia y la Tecnologia, 26.11.2015
    • Immortal Hydra Is Able To Genetically Modify Itself. IFL Science, 26.11.2015
    • Hydra genetically reprograms skin cells after losing its nerve. The Guardian, 27.11.2015
    • Comment survivre sans neurones. Pour la Science, 03.12.2015
    • L’hydre peut vivre sans cerveau. Elle n’est pas la seule. Le Matin, 04.12.2015
    • Ein leuchtender Jungbrunnen.  faz.net / Frankfurter Allgemeine Zeitung Online, 28.11.2015

2012      

Interview on the channel RTS (Radio Télévision Suisse Romande), CQFD program 2015. Journalist: Bastien Confino.
L'hydre, du mythe à la science : http://www.rts.ch/la-1ere/programmes/cqfd/4469156-l-hydre-du-mythe-a-la-science-13-12-2012.html

2009

Report on the Evening News at the Swiss TV of the discovery of cell death and regeneration. DÉCOUVERTE SUR LA FACULTÉ DE RÉGÉNÉRATION DE L'HYDRE
This interview is accessible on the site Avis d’Experts: http://www.avisdexperts.ch/videos/view/925  (2’03 minutes)


 

Movies 

1999     

Galliot B. L’hydre, un embryon immortel.  (8 min) Université de Genève

2003     

Regeneration and Stem Cells (52 min) Documentary movie directed by Jean-Marie Cornuel produced by Télé-Images-Nature. www.teleimages.com/newsletter.htm

2007

L’immortalité? (70 min). 36°9, produced and directed by Mario Fossatti & Isabelle Moncada. Documentary movie produced by the TSR (Télévision Suisse Romande). (French)
http://www.rts.ch/play/tv/36-9/video/le-secret-de-lhydre?id=57661

2008    

Quand la science va à la plage!   Documentary movie directed by Claude-Julie Parisot, produced by KAMI products for ARTE channel.
http://boutique.arte.tv/f2355-quandlasciencevaalaplage

2009   

Einstein: Der Polyp mit dem Ewigen Leben. (3.30 min) Documentary movie produced by SF (Schweizer Fernsehen).

2014     

Immortels. (52 min) Directed by Sarah Lainé. Coproduction : DOCLAND YARD, AB PRODUCTIONS  for Encyclo (Science et Vie TV), a channel dedicated to discovery and scientific knowledge (French)
http://www.gedeonprogrammes.com/wp-content/uploads/2014/01/CATALOGUE-GEDEON_2_VF_BD2.pdf


 

Publications as Guest Editor of special issues

Galliot, B, Tanaka, E.M. Simon A. guest editors of “Cellular and Molecular Basis of Regeneration and Tissue Repair”. Cellular and Molecular Life Sciencesvol. 65, issue 1 (9 articles).

Galliot B. guest editor of « The Hydra model system » International Journal of Developmental Biology, vol 56, issues 6/7/8 (22 articles).

Galliot B. guest editor of “Mechanisms of Regeneration” (11 articles) Current Topics in Developmental Biology, 108,  http://www.sciencedirect.com/science/bookseries/00702153


 

Articles on Science and Society

2003     

Galliot B. A lire avant de vous lancer dans une carrière de chercheuse. Médecine & Hygiène, 2459, 2303-2306.

2007     

Galliot B. La culture scientifique pour une honnête femme du 21ème siècle, est-ce encore possible ? édité par J-J Forney, Fondation Culture & Rencontre, Genève.

2008     

Gehring W, Galliot B and Piraino S. In memoriam: Volker Schmid. Int J Dev Biol, 52, 1013 – 1014.

2016    

Lievens P, Galliot B, Floors F et al. Fundamental Research in HORIZON2020. LERU paper, in press.