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Identification of the PRC1-interacting protein, L3mbtl3 as a novel gatekeeper of neocortical neurogenesis and proneural gene function

ABSTRACT: The neocortex is comprised of six neuronal layers that are derived in an inside-out sequence. Neurog2, a proneural gene encoding a basic-helix-loop-helix transcription factor, is required and sufficient to specify the fate of only early-born, deep-layer neurons, despite being expressed throughout the cortical neurogenic period. To identify potential inhibitors of Neurog2 function, we used a TAP-tagging screen, identifying L3mbtl3, a histone methyl-lysine binding protein that interacts with Rnf2 in Polycomb Repressive Complex 1 (PRC1), as a novel Neurog2 interactor. We found that L3mbtl3 is co-expressed with Neurog2 and other PRC1 genes in cortical progenitors. In L3mbtl3 knock-out (KO) cortices, upper-layer neurons are generated in reduced numbers, and instead, the cortical progenitor pool is expanded. Transcriptomic analyses of L3mbtl3 KO cortices at early and mid neurogenesis revealed a striking upregulation of negative regulators of transcription, including Rest, a repressor of many neurogenic genes, and an associated downregulation of neuronal differentiation and maturation genes. Notably, transcriptomic changes occur in the absence of any changes to chromatin structure, suggesting that L3mbtl3 influences gene transcription directly and not via changes to the chromatin. Instead, L3mbtl3 represses Neurog2 transcriptional activity in vitro and blocks cortical progenitor cell maturation and neuronal migration in vivo by altering the expression of scaffold genes. L3mbtl3 is thus an essential negative regulator of cortical neurogenesis.

ORGANISM(S): Mus musculus

PROVIDER: GSE179377 | GEO | 2021/07/06

REPOSITORIES: GEO

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Project description:The proneural NEUROG2 is essential for neuronal commitment, cell cycle exit and neuronal differentiation. Characterizing genes networks regulated downstream of NEUROG2 is therefore of prime importance. To identify NEUROG2 early response genes, we combined gain of function in the neural tube with a global detection of modified transcripts using microarrays. We included in our study a mutant form of NEUROG2 (NEUROG2AQ) that cannot bind DNA and cannot trigger neurogenesis. Using this approach, we identified 942 genes modified at the onset of NEUROG2 activation. The global analysis of functions regulated by NEUROG2 allowed unmasking its rapid impact on cell cycle control. We found that NEUROG2 specifically represses a subset of cyclins acting at the G1 and S phases of the cell cycle, thereby impeding S phase re-entry. This repression occurs before modification of p27kip1, indicating that the decision to leave the cell cycle precedes the activation of this Cyclin-dependant Kinase Inhibitor. Moreover, NEUROG2 down-regulates only one of the D-type cyclins, cyclinD1, and maintaining cyclinD1 blocks the ability of the proneural to trigger cell cycle exit, without altering its capacity to trigger neuronal differentiation. The fact that NEUROG2 represses a subset but not all cell cycle regulators indicates that cell cycle exit is not an indirect consequence of neuronal differentiation but is precisely controlled by NEUROG2. Altogether our findings indicate that NEUROG2, by specifically repressing G1 and S cyclins, allows committed neuronal precursors to perform their last mitosis but blocks their re-entry in the cell cycle, thus favouring cell cycle exit. Stage HH10-11 embryos (11 to 15 somites) were electroporated with a control vector (pGIG-GFP), a NEUROG2-expressing vector (pCIGNEUROG2-GFP), or a NEUROG2AQ-expressing vector (pCIGNEUROG2AQ-GFP). For each biological replicate, neural tubes from 20 embryos were pooled for GFP+ cells collection. GFP+ cells were collected 6h later using FACS sorting (Epics Altra HSS cell sorter, Toulouse Rio platform) and processed for RNA probe preparation and hybridization on Affymetrix microarrays. For each experimental condition, four biological replicates were processed.

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Identification of the PRC1-interacting protein, L3mbtl3 as a novel gatekeeper of neocortical neurogenesis and proneural gene function

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