Matthieu X. Moreau

Dans le cortex cérébral des mammifères, l'élaboration des fonctions cognitives complexes repose sur l'organisation de connexions établies entre des centaines de types de neurones distincts. Au cours de l'évolution, l'apparition de nouveaux neurones présentant des propriétés différentes à permis l'émergence de nouvelles structures cérébrales. Bien que la caractérisation des types cellulaires composant le cerveau des mammifères a révélé un niveau de diversité jusqu'ici insoupçonné, les processus à l'origine de l'invention de nouveaux types de neurones restent encore méconnus. Ce projet a visé à étudier la façon dont la modification des mécanismes moléculaires contrôlant la production des neurones peut entraîner la création d'une nouvelle identité au cours de l'évolution. En tirant profit des dernières avancées technologiques dans le séquençage de cellules uniques, nous avons retracé la succession des événements impliqués dans le contrôle de la production d'une population de neurones particulière, les cellules de Cajal-Retzius (CRs). Grâce à cette approche, nous avons montré qu'un module de gènes, contrôlé par Gmnc, est requis pour l'acquisition d'identité des CRs. Ce module était jusqu'à présent uniquement décrit pour son rôle dans le contrôle de l'amplification centriolaire des tissus multiciliés. Ces résultats ont permis de formuler l'hypothèse qu'une reconfiguration du réseau génétique contrôlé par Gmnc, et initialement impliqué dans la multiciliation du tissus adjacent au progéniteurs des CRs, les plexus choroïdes (CPx), est à l'origine de l'apparition de ces neurones au cours de l'évolution. Pour tester cette hypothèse nous avons caractérisé les conséquences de la perte de Gmnc sur la trajectoire de différentiation des CRs et des CPx par une approche de scRNAseq. En inférant puis en comparant les réseaux régulateurs contrôlés par Gmnc dans le contexte de spécification des CRs ou celui de multiciliation des CPx, nous avons mis en évidence que la réutilisation d'une réponse au stress réplicatif induite par Gmnc est impliqué dans l'acquisition de l'identité des CRs. Ce travail apportera un éclairage nouveau sur les mécanismes permettant l'émergence d'un nouveau type cellulaire au cours de l'évolution par le redéploiement d'un module génétique initialement impliqué dans un processus distinct.

Brain development requires correct tissue patterning and production of appropriate cell types. Transcription factors play essential roles in these processes, regulating the expression of target genes responsible for the specific features of neuronal subtypes. Cell adhesion molecules are key components of developmental processes that control cell sorting, migration, neurite outgrowth/guidance and synaptogenesis. To date, the link between transcription factors and cell adhesion molecules has been considered unidirectional. Here, we demonstrate that ectopic expression of Dbx1 leads to spatiotemporally restricted increased expression of Pcdh8 and cell aggregation, together with changes in neuronal identity. Surprisingly, ectopic Pcdh8 expression also induces Dbx1 expression, as well as a complete reorganisation of apico-basal polarity and dorso-ventral patterning via Notch signalling. Altogether, our work therefore points to cell adhesion molecules as unexpected, yet important, players in the regulation of cell identity and, in particular, Pcdh8 through its bidirectional interaction with the Dbx1 transcription factor.

Cajal-Retzius cells (CRs) are peculiar neurons in the developing mammalian cerebral cortex. They robustly secrete Reln, a glycoprotein essential for the establishment of cortical layers through the control of radial migration. We previously identified Gmnc as a crucial fate determinant for P73+ CR subtypes. In Gmnc−/− mutants, P73+ CRs are initially produced and cover the telencephalic vesicle but undergo massive apoptosis resulting in their complete depletion at mid-corticogenesis. Here, we investigated the consequences of such a CR depletion on dorsal cortex lamination and hippocampal morphogenesis. We found that preplate splitting normally occurs in Gmnc−/− mutants but is followed by defective radial migration arrest in the dorsal cortex, an altered cellular organization in the lateral cortex, aberrant hippocampal CA1 folding and lack of vasculature development in the hippocampal fissure. We then performed conditional Reln deletion in P73+ CRs to evaluate its relative contribution and found that only radial migration defects were recapitulated. We concluded that at mid-corticogenesis, CR-derived Reln is required for radial migration arrest and additionally identified Reln-independent functions for CRs in the control of hippocampal fissure formation and CA1 folding.

Cajal-Retzius cells (CRs) are a peculiar neuronal type within the developing mammalian cerebral cortex. One of their best documented feature is the robust secretion of Reln, a glycoprotein essential for the establishment of cortical layers through the control of radial migration of glutamatergic neurons. We previously identified Gmnc as a critical fate determinant for P73 ⁺ CRs subtypes from the hem, septum and thalamic eminence. In Gmnc ^-/- mutants, P73 ⁺ CRs are initially produced, cover the telencephalic vesicle but undergo massive apoptosis resulting in their complete depletion at mid-corticogenesis. Here we investigated the consequence of such a CRs depletion on dorsal cortex lamination and hippocampal morphogenesis. We found preplate splitting occurs normally in Gmnc ^-/- mutants but is followed by defective radial migration arrest in the dorsal cortex, altered cellular organization in the lateral cortex, aberrant hippocampal progenitor proliferation resulting in abnormal CA1 folding and lack of vasculature development in the hippocampal fissure. We then performed conditional Reln deletion in P73 ⁺ CRs to evaluate its relative contribution and found that only radial migration defects were recapitulated. We concluded that at mid-corticogenesis, CRs-derived Reln is required for radial migration arrest and additionally identified Reln-independent functions for CRs in the control of hippocampal progenitor proliferation and vessel remodelling.

SUMMARY: Brain development requires correct tissue patterning and production of appropriate cell types. Transcription factors (TFs) play essential roles in these processes, regulating the expression of target genes responsible for neuronal subtypes specific features. Cell adhesion molecules are key components of developmental processes that control cell sorting, migration, neurite outgrowth/guidance and synaptogenesis. To date, the link between TFs and cell adhesion molecules is considered to be unidirectional. Here, we demonstrate that ectopic expression of Dbx1 leads to spatio-temporally restricted increased expression of Pcdh8 and cell aggregation, together with changes in neuronal identity. Surprisingly, Pcdh8 overexpression also induces Dbx1 expression as well as a complete reorganisation of apico-basal polarity and dorso-ventral patterning via Notch signalling. Altogether, our work therefore points to cell adhesion molecules as unexpected, yet important, players in the regulation of cell identity and, in particular, Pcdh8 through its bidirectional interaction with the Dbx1 transcription factor.

In vertebrates, the embryonic olfactory epithelium contains progenitors that will give rise to distinct classes of neurons, including olfactory sensory neurons (OSNs; involved in odor detection), vomeronasal sensory neurons (VSNs; responsible for pheromone sensing), and gonadotropin-releasing hormone (GnRH) neurons that control the hypothalamic-pituitary-gonadal axis. Currently, these three neuronal lineages are usually believed to emerge from uniform pools of progenitors. Here, we found that the homeodomain transcription factor Dbx1 is expressed by neurogenic progenitors in the developing and adult mouse olfactory epithelium. We demonstrate that Dbx1 itself is dispensable for neuronal fate specification and global organization of the olfactory sensory system. Using lineage tracing, we characterize the contribution of Dbx1 lineages to OSN, VSN, and GnRH neuron populations and reveal an unexpected degree of diversity. Furthermore, we demonstrate that Dbx1-expressing progenitors remain neurogenic in the absence of the proneural gene Ascl1. Our work therefore points to the existence of distinct neurogenic programs in Dbx1-derived and other olfactory lineages.

SUMMARY: Cajal-Retzius (CR) neurons are key players of cortical development that display a very unique transcriptomic identity. However, little is known about the mechanisms involved in their fate specification. Here we use scRNAseq to reconstruct the differentiation trajectory of hem-derived CR cells (CRs) and unravel the transient expression of a complete gene module previously known to control the cellular process of multiciliogenesis. However, we find that CRs do not undergo centriole amplification or multiciliation. We show that upon genetic disruption of Gmnc , the master regulator of the multiciliation cascade, CRs are initially produced but fail to reach their normal identity and lean towards an aberrant fate resulting in their massive apoptosis. We further dissect the contribution of multiciliation effector genes and identify Trp73 as a key determinant. Finally, we use in utero electroporation to demonstrate that the intrinsic competence of hem progenitors as well as the heterochronic expression of Gmnc prevent centriole amplification in the CR lineage. Our work exemplifies how the co-option of a complete gene module, repurposed to control a completely distinct process, may contribute to the emergence of novel cell identities.

In vertebrates, the embryonic olfactory epithelium contains progenitors that will give rise to distinct classes of neurons, including olfactory sensory neurons (OSN, involved in odor detection), vomeronasal sensory neurons (VSN, responsible for pheromone sensing) and GnRH neurons that control the hypothalamic-pituitary-gonadal axis. Currently, these three neuronal lineages are usually believed to emerge from uniform pools of progenitors. Here we found that the homeodomain transcription factor Dbx1 is expressed by neurogenic progenitors in the developing and adult mouse olfactory epithelium. We demonstrate that Dbx1 itself is dispensable for neuronal fate specification and global organization of the olfactory sensory system. Using lineage tracing we characterize the contribution of Dbx1 lineages to OSN, VSN and GnRH neuron populations and reveal an unexpected degree of diversity. Furthermore, we demonstrate that Dbx1 -expressing progenitors remain neurogenic in the absence of the proneural gene Ascl1 . Our work therefore points to the existence of distinct neurogenic programs in Dbx1-derived and other olfactory lineages.

In the developing cerebral cortex, how progenitors that seemingly display limited diversity end up producing a vast array of neurons remains a puzzling question. The prevailing model suggests that temporal maturation of progenitors is a key driver in the diversification of the neuronal output. However, temporal constraints are unlikely to account for all diversity, especially in the ventral and lateral pallium where neuronal types significantly differ from their dorsal neocortical counterparts born at the same time. In this study, we implemented single-cell RNAseq to sample the diversity of progenitors and neurons along the dorso-ventral axis of the early developing pallium. We first identified neuronal types, mapped them on the tissue and determined their origin through genetic tracing. We characterised progenitor diversity and disentangled the gene modules underlying temporal versus spatial regulations of neuronal specification. Finally, we reconstructed the developmental trajectories followed by ventral and dorsal pallial neurons to identify lineage-specific gene waves. Our data suggest a model by which discrete neuronal fate acquisition from a continuous gradient of progenitors results from the superimposition of spatial information and temporal maturation.