Cyclin-Dependent Kinase-Dependent Phosphorylation of Sox2 at Serine 39 Regulates Neurogenesis.

Sox2 is known to be important for neuron formation, but the precise mechanism through which it activates a neurogenic program and how this differs from its well-established function in self-renewal of stem cells remain elusive. In this study, we identified a highly conserved cyclin-dependent kinase (Cdk) phosphorylation site on serine 39 (S39) in Sox2. In neural stem cells (NSCs), phosphorylation of S39 enhances the ability of Sox2 to negatively regulate neuronal differentiation, while loss of phosphorylation is necessary for chromatin retention of a truncated form of Sox2 generated during neurogenesis. We further demonstrated that nonphosphorylated cleaved Sox2 specifically induces the expression of proneural genes and promotes neurogenic commitment in vivo Our present study sheds light on how the level of Cdk kinase activity directly regulates Sox2 to tip the balance between self-renewal and differentiation in NSCs.

Long non-coding RNAs in corticogenesis: deciphering the non-coding code of the brain.

Evidence on the role of long non-coding (lnc) RNAs has been accumulating over decades, but it has been only recently that advances in sequencing technologies have allowed the field to fully appreciate their abundance and diversity. Despite this, only a handful of lncRNAs have been phenotypically or mechanistically studied. Moreover, novel lncRNAs and new classes of RNAs are being discovered at growing pace, suggesting that this class of molecules may have functions as diverse as protein-coding genes. Interestingly, the brain is the organ where lncRNAs have the most peculiar features including the highest number of lncRNAs that are expressed, proportion of tissue-specific lncRNAs and highest signals of evolutionary conservation. In this work, we critically review the current knowledge about the steps that have led to the identification of the non-coding transcriptome including the general features of lncRNAs in different contexts in terms of both their genomic organisation, evolutionary origin, patterns of expression, and function in the developing and adult mammalian brain.

Identification and expression patterns of novel long non-coding RNAs in neural progenitors of the developing mammalian cortex.

Long non-coding (lnc)RNAs play key roles in many biological processes. Elucidating the function of lncRNAs in cell type specification during organ development requires knowledge about their expression in individual progenitor types rather than in whole tissues. To achieve this during cortical development, we used a dual-reporter mouse line to isolate coexisting proliferating neural stem cells, differentiating neurogenic progenitors and newborn neurons and assessed the expression of lncRNAs by paired-end, high-throughput sequencing. We identified 379 genomic loci encoding novel lncRNAs and performed a comprehensive assessment of cell-specific expression patterns for all, annotated and novel, lncRNAs described to date. Our study provides a powerful new resource for studying these elusive transcripts during stem cell commitment and neurogenesis.

Generation and characterization of Neurod1-CreER(T2) mouse lines for the study of embryonic and adult neurogenesis.

Neurod1 is a transcription factor involved in several developmental programs of the gastrointestinal tract, pancreas, neurosensory, and central nervous system. In the brain, Neurod1 has been shown to be essential for neurogenesis as well as migration, maturation, and survival of newborn neurons during development and adulthood. Interestingly, Neurod1 expression is maintained in a subset of fully mature neurons where its function remains unclear. To study the role of Neurod1, systems are required that allow the temporal and spatial genetic manipulation of Neurod1-expressing cells. To this aim, we have generated four Neurod1-CreER(T2) mouse lines in which CreER(T2) expression, although at different levels, is restricted within areas of physiological Neurod1 expression and Neurod1 positive cells. In particular, the different levels of CreER(T2) expression in different mouse lines offers the opportunity to select the one that is more suited for a given experimental approach. Hence, our Neurod1-CreER(T2) lines provide valuable new tools for the manipulation of newborn neurons during development and adulthood as well as for studying the subpopulation of mature neurons that retain Neurod1 expression throughout life. In this context, we here report that Neurod1 is not only expressed in immature newborn neurons of the adult hippocampus, as already described, but also in fully mature granule cells of the dentate gyrus.

MicroRNAs establish robustness and adaptability of a critical gene network to regulate progenitor fate decisions during cortical neurogenesis.

Over the course of cortical neurogenesis, the transition of progenitors from proliferation to differentiation requires a precise regulation of involved gene networks under varying environmental conditions. In order to identify such regulatory mechanisms, we analyzed microRNA (miRNA) target networks in progenitors during early and late stages of neurogenesis. We found that cyclin D1 is a network hub whose expression is miRNA-dosage sensitive. Experimental validation revealed a feedback regulation between cyclin D1 and its regulating miRNAs miR-20a, miR-20b, and miR-23a. Cyclin D1 induces expression of miR-20a and miR-20b, whereas it represses miR-23a. Inhibition of any of these miRNAs increases the developmental stage-specific mean and dynamic expression range (variance) of cyclin D1 protein in progenitors, leading to reduced neuronal differentiation. Thus, miRNAs establish robustness and stage-specific adaptability to a critical dosage-sensitive gene network during cortical neurogenesis. Understanding such network regulatory mechanisms for key developmental events can provide insights into individual susceptibilities for genetically complex neuropsychiatric disorders.

Transcriptome sequencing during mouse brain development identifies long non-coding RNAs functionally involved in neurogenic commitment.

Transcriptome analysis of somatic stem cells and their progeny is fundamental to identify new factors controlling proliferation versus differentiation during tissue formation. Here, we generated a combinatorial, fluorescent reporter mouse line to isolate proliferating neural stem cells, differentiating progenitors and newborn neurons that coexist as intermingled cell populations during brain development. Transcriptome sequencing revealed numerous novel long non-coding (lnc)RNAs and uncharacterized protein-coding transcripts identifying the signature of neurogenic commitment. Importantly, most lncRNAs overlapped neurogenic genes and shared with them a nearly identical expression pattern suggesting that lncRNAs control corticogenesis by tuning the expression of nearby cell fate determinants. We assessed the power of our approach by manipulating lncRNAs and protein-coding transcripts with no function in corticogenesis reported to date. This led to several evident phenotypes in neurogenic commitment and neuronal survival, indicating that our study provides a remarkably high number of uncharacterized transcripts with hitherto unsuspected roles in brain development. Finally, we focussed on one lncRNA, Miat, whose manipulation was found to trigger pleiotropic effects on brain development and aberrant splicing of Wnt7b. Hence, our study suggests that lncRNA-mediated alternative splicing of cell fate determinants controls stem-cell commitment during neurogenesis.

Bioelectric state and cell cycle control of Mammalian neural stem cells.

The concerted action of ion channels and pumps establishing a resting membrane potential has been most thoroughly studied in the context of excitable cells, most notably neurons, but emerging evidences indicate that they are also involved in controlling proliferation and differentiation of nonexcitable somatic stem cells. The importance of understanding stem cell contribution to tissue formation during embryonic development, adult homeostasis, and regeneration in disease has prompted many groups to study and manipulate the membrane potential of stem cells in a variety of systems. In this paper we aimed at summarizing the current knowledge on the role of ion channels and pumps in the context of mammalian corticogenesis with particular emphasis on their contribution to the switch of neural stem cells from proliferation to differentiation and generation of more committed progenitors and neurons, whose lineage during brain development has been recently elucidated.