Dr Madlaina Kuby Boillat

The claustro-insular region is an evolutionarily conserved and extensively interconnected brain area, critical for functions such as attention, cognitive flexibility, interoception, and affective processing. Despite its importance, its cellular composition and organization remain poorly characterized, hindering a comprehensive understanding of the mechanisms underlying its diverse functions. By combining single-cell RNA sequencing and spatial transcriptomics, we create a high-resolution atlas of this region in mice, uncovering distinct neuronal subtypes and unexpected complexity. Leveraging this atlas, we investigate the role of NR4A2, a neuropsychiatric risk factor expressed in several claustro-insular neuronal subtypes. In an Nr4a2 haploinsufficiency model, we find that only claustrum neurons exhibited shifts in molecular identity. This identity shift, which involves the activation of a transcription factor cascade, is associated with alterations in neuronal firing activity. Our findings provide new insights into the cellular architecture of the claustro-insular region and highlights Nr4a2 as a key regulator of its component's identities.

The claustro-insular region is an evolutionarily conserved and extensively interconnected brain area, critical for functions such as attention, cognitive flexibility, interoception, and affective processing. Despite its importance, its cellular composition and organization remain poorly characterized, hindering a comprehensive understanding of the mechanisms underlying its diverse functions. By combining single-cell RNA sequencing and spatial transcriptomics, we created a high-resolution atlas of this region in mice, uncovering distinct neuronal subtypes and unexpected complexity. Leveraging this atlas, we investigated the role of NR4A2, a neuropsychiatric risk factor expressed in several claustro-insular neuronal subtypes. In an Nr4a2 haploinsufficiency model, we found that only claustrum neurons exhibited shifts in molecular identity. This identity shift, which involved the activation of a transcription factor cascade, was associated with alterations in neuronal firing activity. Our findings provide new insights into the cellular architecture of the claustro-insular region and highlights Nr4a2 as a master regulator of its component’s identities.

The claustrum (CLA), a subcortical nucleus in mammals, essentially composed of excitatory projection neurons and known for its extensive connections with the neocortex, has recently been associated with a variety of functions ranging from consciousness to impulse control. However, research on the CLA has been challenging due to difficulties in specifically and comprehensively targeting its neuronal populations. In various cases, this limitation has led to inconsistent findings and a lack of reliable data. In the present work, we describe the expression profile of the Smim32 gene, which is almost exclusively transcribed in excitatory neurons of the CLA and the endopiriform nucleus, as well as in inhibitory neurons of the thalamic reticular nucleus. Leveraging this unique expression pattern, we developed a series of Cre- and Flippase-expressing knockin and BAC transgenic mouse lines with different expression profiles. With these novel tools in hand, we propose new standards for the interrogation of CLA function.

Rodents perceive pheromones via vomeronasal receptors encoded by highly evolutionarily dynamic Vr and Fpr gene superfamilies. We report here that high numbers of V1r pseudogenes are scattered in mammalian genomes, contrasting with the clustered organization of functional V1r and Fpr genes. We also found that V1r pseudogenes are more likely to be expressed when located in a functional V1r gene cluster than when isolated. To explore the potential regulatory role played by the association of functional vomeronasal receptor genes with their clusters, we dissociated the mouse from its native cluster via transgenesis. Singular and specific transgenic transcription was observed in young vomeronasal neurons but was only transient. Our study of natural and artificial dispersed gene duplications uncovers the existence of transcription-stabilizing elements not coupled to vomeronasal gene units but rather associated with vomeronasal gene clusters and thus explains the evolutionary conserved clustered organization of functional vomeronasal genes.

In mammals, chemoperception relies on a diverse set of neuronal sensors able to detect chemicals present in the environment, and to adapt to various levels of stimulation. The contribution of endogenous and external factors to these neuronal identities remains to be determined. Taking advantage of the parallel coding lines present in the olfactory system, we explored the potential variations of neuronal identities before and after olfactory experience. We found that at rest, the transcriptomic profiles of mouse olfactory sensory neuron populations are already divergent, specific to the olfactory receptor they express, and are associated with the sequence of these latter. These divergent profiles further evolve in response to the environment, as odorant exposure leads to reprogramming via the modulation of transcription. These findings highlight a broad range of sensory neuron identities that are present at rest and that adapt to the experience of the individual, thus adding to the complexity and flexibility of sensory coding.

Variations in gene expression patterns represent a powerful source of evolutionary innovation. In a rodent living about 70 million years ago, a genomic accident led an immune formyl peptide receptor (FPR) gene to hijack a vomeronasal receptor regulatory sequence. This gene shuffling event forced an immune pathogen sensor to transition into an olfactory chemoreceptor, which thus moved from sensing the internal world to probing the outside world. We here discuss the evolution of the FPR gene family, the events that led to their neofunctionalization in the vomeronasal organ and the functions of immune and vomeronasal FPRs.

Reports indicate an association between COVID-19 and anosmia, as well as the presence of SARS-CoV-2 virions in the olfactory bulb. To test whether the olfactory neuroepithelium may represent a target of the virus, we generated RNA-seq libraries from human olfactory neuroepithelia, in which we found substantial expression of the genes coding for the virus receptor angiotensin-converting enzyme-2 (ACE2), and for the virus internalization enhancer TMPRSS2. We analyzed a human olfactory single-cell RNA-seq dataset and determined that sustentacular cells, which maintain the integrity of olfactory sensory neurons, express and . ACE2 protein was highly expressed in a subset of sustentacular cells in human and mouse olfactory tissues. Finally, we found transcripts in specific brain cell types, both in mice and humans. Sustentacular cells thus represent a potential entry door for SARS-CoV-2 in a neuronal sensory system that is in direct connection with the brain.

In rodents, the decrease of felid aversion induced by Toxoplasma gondii, a phenomenon termed fatal attraction, is interpreted as an adaptive manipulation by the neurotropic protozoan parasite. With the aim of understanding how the parasite induces such specific behavioral modifications, we performed a multiparametric analysis of T. gondii-induced changes on host behavior, physiology, and brain transcriptome as well as parasite cyst load and distribution. Using a set of complementary behavioral tests, we provide strong evidence that T. gondii lowers general anxiety in infected mice, increases explorative behaviors, and surprisingly alters predator aversion without selectivity toward felids. Furthermore, we show a positive correlation between the severity of the behavioral alterations and the cyst load, which indirectly reflects the level of inflammation during brain colonization. Taken together, these findings refute the myth of a selective loss of cat fear in T. gondii-infected mice and point toward widespread immune-related alterations of behaviors.

Although sociability offers many advantages, a major drawback is the increased risk of exposure to contagious pathogens, like parasites, viruses, or bacteria. Social species have evolved various behavioral strategies reducing the probability of pathogen exposure. In rodents, sick conspecific avoidance can be induced by olfactory cues emitted by parasitized or infected conspecifics. The neural circuits involved in this behavior remain largely unknown. We observed that olfactory cues present in bodily products of mice in an acute inflammatory state or infected with a viral pathogen are aversive to conspecifics. We found that these chemical signals trigger neural activity in the vomeronasal system, an olfactory subsystem controlling various innate behaviors. Supporting the functional relevance of these observations, we show that preference toward healthy individuals is abolished in mice with impaired vomeronasal function. These findings reveal a novel function played by the vomeronasal system.