Dietschi Q, Assens A, Challet L, Carleton A, Rodriguez I

Department of Genetics and Evolution, School of Medicine, University of Geneva, Switzerland.

In the mouse, most members of the FPR receptor family are expressed by vomeronasal sensory neurons. The neural circuitry corresponding to this class of chemical sensors is unknown. Taking advantage of the presence of FPR-rs3 on both vomeronasal dendrites and axonal fibers, we visualized the distribution of sensory cells expressing this member of the FPR family, and their corresponding axonal projections in the olfactory bulb. We found a rostrocaudal gradient of receptor choice frequency in the vomeronasal sensory neuroepithelium, and observed a convergence of FPR-rs3 axons into multiple, linked and deeply located glomeruli. These were homogenously innervated, and spatially restricted to the basal portion of the rostral accessory olfactory bulb. This organization, reminiscent of the one that characterizes axonal projections of V1R-expressing neurons, supports a role played by these receptors in the perception of semiochemicals.

Dietschi Q., Challet L., Assens A., and Rodriguez I.

Department of Genetics and Evolution, University of Geneva

Brykczynska U, Tzika A C, Rodriguez I, Milinkovitch M C

Laboratory of Artificial & Natural Evolution (LANE), Department of Genetics & Evolution, University of Geneva, Sciences III, Geneva, Switzerland.

The vomeronasal organ (VNO) is an olfactory structure that detects pheromones and environmental cues. It consists of sensory neurons that express evolutionary unrelated groups of transmembrane chemoreceptors. The predominant V1R and V2R receptor repertoires are believed to detect airborne and water-soluble molecules, respectively. It has been suggested that the shift in habitat of early tetrapods from water to land is reflected by an increase in the ratio of V1R/V2R genes. Snakes, which have a very large VNO associated with a sophisticated tongue delivery system, are missing from this analysis. Here, we use RNA-seq and RNA in situ hybridization to study the diversity, evolution, and expression pattern of the corn snake vomeronasal receptor repertoires. Our analyses indicate that snakes and lizards retain an extremely limited number of V1R genes but exhibit a large number of V2R genes, including multiple lineages of reptile-specific and snake-specific expansions. We finally show that the peculiar bigenic pattern of V2R vomeronasal receptor gene transcription observed in mammals is conserved in squamate reptiles, hinting at an important but unknown functional role played by this expression strategy. Our results do not support the hypothesis that the shift to a vomeronasal receptor repertoire dominated by V1Rs in mammals reflects the evolutionary transition of early tetrapods from water to land. This study sheds light on the evolutionary dynamics of the vomeronasal receptor families in vertebrates and reveals how mammals and squamates differentially adapted the same ancestral vomeronasal repertoire to succeed in a terrestrial environment.

Matsuo T, Rossier D A, Kan C, Rodriguez I

Department of Genetics and Evolution and National Research Center Frontiers in Genetics, University of Geneva, 1211 Geneva 4, Switzerland.

The Grueneberg ganglion is a specialized olfactory sensor. In mice, its activation induces freezing behavior. The topographical map corresponding to the central projections of its sensory axons is poorly defined, as well as the guidance molecules involved in its establishment. We took a transgenic approach to label exclusively Grueneberg sensory neurons and their axonal projections. We observed that a stereotyped convergence map in a series of coalescent neuropil-rich structures is already present at birth. These structures are part of a peculiar and complex neuronal circuit, composed of a chain of glomeruli organized in a necklace pattern that entirely surrounds the trunk of the olfactory bulb. We found that the necklace chain is composed of two different sets of glomeruli: one exclusively innervated by Grueneberg ganglion neurons, the other by axonal inputs from the main olfactory neuroepithelium. Combining the transgenic Grueneberg reporter mouse with a conditional null genetic approach, we then show that the axonal wiring of Grueneberg neurons is dependent on neuropilin 1 expression. Neuropilin 1-deficient Grueneberg axonal projections lose their strict and characteristic avoidance of vomeronasal glomeruli, glomeruli that are innervated by secondary neurons expressing the repulsive guidance cue and main neuropilin 1 ligand Sema3a. Taken together, our observations represent a first step in the understanding of the circuitry and the coding strategy used by the Grueneberg system.

Groner A C, Tschopp P, Challet L, Dietrich J E, Verp S, Offner S, Barde I, Rodriguez I, Hiiragi T, Trono D

School of Life Sciences, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland.

The study of chromatin and its regulators is key to understanding and manipulating transcription. We previously exploited the Kruppel-associated box (KRAB) transcriptional repressor domain, present in hundreds of vertebrate-specific zinc finger proteins, to assess the effect of its binding to gene bodies. These experiments revealed that the ectopic and doxycycline (dox)-controlled tet repressor KRAB fusion protein (tTRKRAB) can induce reversible and long-range silencing of cellular promoters. Here, we extend this system to in vivo applications and use tTRKRAB to achieve externally controllable repression of an endogenous mouse locus. We employed lentiviral-mediated transgenesis with promoterless TetO-containing gene traps to engineer a mouse line where the endogenous kinesin family member 2A (Kif2A) promoter drives a YFP reporter gene. When these mice were crossed to animals expressing the TetO-binding tTRKRAB repressor, this regulator was recruited to the Kif2A locus, and YFP expression was reduced. This effect was reversed when dox was given to embryos or adult mice, demonstrating that the cellular Kif2A promoter was only silenced upon repressor binding. Molecular analyses confirmed that tTRKRAB induced transcriptional repression through the spread of H3K9me3-containing heterochromatin, without DNA methylation of the trapped Kif2A promoter. Therefore, we demonstrate that targeting of tTRKRAB to a gene body in vivo results in reversible transcriptional repression through the spreading of facultative heterochromatin. This finding not only sheds light on KRAB-mediated transcriptional processes, but also suggests approaches for the externally controllable and reversible modulation of chromatin and transcription in vivo.

Brechbuhl J, Luyet G, Moine F, Rodriguez I, Broillet M C

Department of Pharmacology and Toxicology, University of Lausanne.

Peter Karlson and Martin Luscher used the term pheromone for the first time in 1959 to describe chemicals used for intra-species communication. Pheromones are volatile or non-volatile short-lived molecules secreted and/or contained in biological fluids, such as urine, a liquid known to be a main source of pheromones. Pheromonal communication is implicated in a variety of key animal modalities such as kin interactions, hierarchical organisations and sexual interactions and are consequently directly correlated with the survival of a given species. In mice, the ability to detect pheromones is principally mediated by the vomeronasal organ (VNO), a paired structure located at the base of the nasal cavity, and enclosed in a cartilaginous capsule. Each VNO has a tubular shape with a lumen allowing the contact with the external chemical world. The sensory neuroepithelium is principally composed of vomeronasal bipolar sensory neurons (VSNs). Each VSN extends a single dendrite to the lumen ending in a large dendritic knob bearing up to 100 microvilli implicated in chemical detection. Numerous subpopulations of VSNs are present. They are differentiated by the chemoreceptor they express and thus possibly by the ligand(s) they recognize. Two main vomeronasal receptor families, V1Rs and V2Rs, are composed respectively by 240 and 120 members and are expressed in separate layers of the neuroepithelium. Olfactory receptors (ORs) and formyl peptide receptors (FPRs) are also expressed in VSNs. Whether or not these neuronal subpopulations use the same downstream signalling pathway for sensing pheromones is unknown. Despite a major role played by a calcium-permeable channel (TRPC2) present in the microvilli of mature neurons TRPC2 independent transduction channels have been suggested. Due to the high number of neuronal subpopulations and the peculiar morphology of the organ, pharmacological and physiological investigations of the signalling elements present in the VNO are complex. Here, we present an acute tissue slice preparation of the mouse VNO for performing calcium imaging investigations. This physiological approach allows observations, in the natural environment of a living tissue, of general or individual subpopulations of VSNs previously loaded with Fura-2AM, a calcium dye. This method is also convenient for studying any GFP-tagged pheromone receptor and is adaptable for the use of other fluorescent calcium probes. As an example, we use here a VG mouse line, in which the translation of the pheromone V1rb2 receptor is linked to the expression of GFP by a polycistronic strategy.

Sorce S, Bonnefont J, Julien S, Marq-Lin N, Rodriguez I, Dubois-Dauphin M, Krause K H

Department of Pathology and Immunology, University of Geneva, Switzerland.

BACKGROUND AND PURPOSE: The chemokine receptor CCR5 is well known for its function in immune cells; however, it is also expressed in the brain, where its specific role remains to be elucidated. Because genetic factors may influence the risk of developing cerebral ischaemia or affect its clinical outcome, we have analysed the role of CCR5 in experimental stroke. EXPERIMENTAL APPROACH: Permanent cerebral ischaemia was performed by occlusion of the middle cerebral artery in wild-type and CCR5-deficient mice. Locomotor behaviour, infarct size and histochemical alterations were analysed at different time points after occlusion. KEY RESULTS: The cerebral vasculature was comparable in wild-type and CCR5-deficient mice. However, the size of the infarct and the motor deficits after occlusion were markedly increased in CCR5-deficient mice as compared with wild type. No differences between wild-type and CCR5-deficient mice were elicited by occlusion with respect to the morphology and abundance of astrocytes and microglia. Seven days after occlusion the majority of CCR5-deficient mice displayed neutrophil invasion in the infarct region, which was not observed in wild type. As compared with wild type, the infarct regions of CCR5-deficient mice were characterized by increased neuronal death. CONCLUSIONS AND IMPLICATIONS: Lack of CCR5 increased the severity of brain injury following occlusion of the middle cerebral artery. This is of particular interest with respect to the relatively frequent occurrence of CCR5 deficiency in the human population (1-2% of the Caucasian population) and the advent of CCR5 inhibitors as novel drugs.

Rodriguez I

Department of Zoology and Animal Biology, National Research Center Frontiers in Genetics, University of Geneva, Geneva, Switzerland. ivan.rodriguez@unige.ch

Rodents exhibit an innate fear-like behavior when they sense the chemical traces of predators. In this issue, Papes et al. (2010) report that the major urinary proteins (Mups) released by predators are detected by sensory neurons in the mouse vomeronasal organ (which also detects pheromones involved in aggression), triggering a fear response.

Selected

Riviere S, Challet L, Fluegge D, Spehr M, Rodriguez I

Department of Zoology and Animal Biology, and National Center of Competence Frontiers in Genetics, University of Geneva, 1205 Geneva, Switzerland.

Mammals rely heavily on olfaction to interact adequately with each other and with their environment. They make use of seven-transmembrane G-protein-coupled receptors to identify odorants and pheromones. These receptors are present on dendrites of olfactory sensory neurons located in the main olfactory or vomeronasal sensory epithelia, and pertain to the odorant, trace amine-associated receptor and vomeronasal type 1 (ref. 4) or 2 (refs 5-7) receptor superfamilies. Whether these four sensor classes represent the complete olfactory molecular repertoire used by mammals to make sense of the outside world is unknown. Here we report the expression of formyl peptide receptor-related genes by vomeronasal sensory neurons, in multiple mammalian species. Similar to the four known olfactory receptor gene classes, these genes encode seven-transmembrane proteins, and are characterized by monogenic transcription and a punctate expression pattern in the sensory neuroepithelium. In vitro expression of mouse formyl peptide receptor-like 1, 3, 4, 6 and 7 provides sensitivity to disease/inflammation-related ligands. Establishing an in situ approach that combines whole-mount vomeronasal preparations with dendritic calcium imaging in the intact neuroepithelium, we show neuronal responses to the same molecules, which therefore represent a new class of vomeronasal agonists. Taken together, these results suggest that formyl peptide receptor-like proteins have an olfactory function associated with the identification of pathogens, or of pathogenic states.

Capello L, Roppolo D, Jungo V P, Feinstein P, Rodriguez I

Department of Zoology and Animal Biology, and NCCR Frontiers in Genetics, University of Geneva, Geneva, Switzerland.

Sensory coding strategies within vertebrates involve the expression of a limited number of receptor types per sensory cell. In mice, each vomeronasal sensory neuron transcribes monoallelically a single V1R pheromone receptor gene, chosen from a large V1R repertoire. The nature of the signals leading to this strict receptor expression is unknown, but is apparently based on a negative feedback mechanism initiated by the transcription of the first randomly chosen functional V1R gene. We show, in vivo, that the genetic replacement of the V1rb2 pheromone receptor coding sequence by an unrelated one from the odorant receptor gene M71 maintains gene exclusion. The expression of this exogenous odorant receptor in vomeronasal neurons does not trigger the transcription of odorant receptor-associated signalling molecules. These results strongly suggest that despite the different odorant and vomeronasal receptor expression sites, function and transduction cascades, a common mechanism is used by these chemoreceptors to regulate their transcription.

Rodriguez I, Boehm U

Department of Zoology and Animal Biology, University of Geneva, 30 Quai Ernest Ansermet, Geneva, 1211, Switzerland. Ivan.Rodriguez@zoo.unige.ch

Beginning with the neuroepithelium of the vomeronasal organ, the accessory olfactory system in rodents runs parallel to the main olfactory system and is specialized in the detection of pheromones. Only a small number of vomeronasal agonists carrying pheromonal information have been identified this far. These structurally diverse classes of chemicals include peptides secreted by exocrine glands and range from small volatile molecules to proteins and fragments thereof present in urine. Most pheromones activate both vomeronasal and main olfactory sensory neurons, making the identification of functionally relevant populations of sensory neurons difficult. Analyses of gene-targeted mice selectively affecting either vomeronasal or main olfactory signaling have attempted to elucidate the functional contribution of the different chemosensory epithelia to pheromone sensing in mice. These mouse models suggest that both the main and the accessory olfactory systems can converge and synergize to express the complex array of stereotyped behaviors and hormonal changes triggered by pheromones.

Rodriguez I

Department of Zoology and Animal Biology, and NCCR Frontiers in Genetics, University of Geneva, Geneva, Switzerland. ivan.rodriguez@zoo.unige.ch

The largest mammalian gene family codes for odorant receptors and is exclusively devoted to the perception of the outside world. Its expression is very peculiar, since olfactory sensory neurons are only allowed to express a single of its numerous members, from a single parental allele. How this is achieved is unknown, but recent work points to multiple regulatory mechanisms, possibly shared by pheromone receptor genes, acting at (a) a general level, via the expression of the chemoreceptor itself and (b) a more restricted level, defined by activator elements.