SYMASYM

The vertebrate central nervous system (CNS) is a complex assembly of thousands of cell types organized in an exquisite manner to form functional neural circuits. The amazing diversity of neuronal and glial cell types develops from a limited pool of neuroepithelial progenitors. Precise coordination between growth and differentiation is paramount to produce the correct amount of cells “at the right place at the right time”. The progenitor pool is first amplified via proliferative symmetrical divisions (which produce two progenitors, thereafter called SYM divisions), progressively switches to neurogenic asymmetric divisions producing a progenitor and a committed progeny (ASYM), and finally to symmetrical terminal divisions producing two differentiating neurons (TERM). Asymmetric cell division is a fundamental mechanism to generate cell diversity. Much remains to be discovered about what controls the decision to enter an asymmetric rather than symmetric division, as well as the molecular and cellular mechanisms that execute the asymmetry during development of the vertebrate CNS. In this tissue, clonal analyses indicate that progenitors who have undergone an ASYM division do not normally re-enter the SYM mode, suggesting an irreversible switch in competence from SYM to ASYM. Indeed, several genes have been shown to be differentially expressed between SYM and ASYM progenitors, suggesting that the switch is under a specific genetic control. Our objective is to characterize and challenge the key regulatory circuits inducing this transition by exploring the chromatin and transcriptomic changes that occur before and during this switch. Our project is divided in three main tasks: Task1 aims at developing a new assay to produce a lineage restricted progenitor population in which the status (SYM or ASYM) can be identified thanks to the differential expression of specific markers. We will then rely on in vitro targeted motoneuron differentiation of ES cells to obtain highly standardized populations of both progenitor types. Task2 uses state of the art NGS techniques including single cell analyses to describe the transcriptome and chromatin state of these cells. We will develop and implement innovative analytical tools in order to reconstitute the dynamics of the transcriptional and chromatin states during this transition, and identify regulators of the SYM-ASYM transition and of ASYM execution. Task3 addresses the developmental relevance of candidates through a medium throughput functional screen of candidates in vitro followed by in vivo validation. Our ambitious program should identify key transcriptional regulations, signalling pathways and metabolic signatures that control the subtle transition from symmetric to asymmetric neurogenic divisions, which is central to brain development, evolution and disease. Exploring in parallel the transcriptional and epigenomic landscapes of pure SYM and ASYM populations will increase the predictive power of our data and thus give us access to an as yet unprecedented level of resolution in this type of analyses. In addition, by integrating data at the single cell resolution, we aim to order the sequence of events that drive the progression between the two modes of division, but also within each progenitor population. Overall, our project will answer fundamental questions in vertebrate neurogenesis and is therefore expected to improve our understanding of mechanisms whose dysfunction leads to human neurodevelopmental disorders. It should also provide more rational strategies to produce defined neuronal populations from stem cells, an extremely promising avenue for cell replacement therapies. Finally, it will contribute to decipher the fundamental mechanisms regulating asymmetric division and therefore the balance between proliferation and differentiation in stem cell populations, and should have an impact in various fields such as development, immunology, regeneration and cancer.