Neurogenesis in mammals
The mammalian brain has a remarkable capacity for plasticity, critical for learning and memory and compensating for damage. However, the brains of mammals regenerate poorly, failing to generate appreciable numbers of new neurons. This was thought to be due to a lack of stem and progenitors cells in the postnatal brain, including in humans. It is accepted that the adult brain contains neural stem cells (NSCs) and in some species continue to generate neurons. Newborn adult neurons in the lateral forebrain and in the hippocampus contribute to olfaction and specific forms of memory, respectively. Using conditional mouse genetics and cell culture we are trying to understand the molecular mechanisms controlling NSC activity and fate during development and adulthood. We are also trying to elucidate why active NSCs are lost in infant humans and during aging.
NSCs in the adult forebrain are confined to niches in the subventricular zone (SVZ) and hippocampus. Most NSCs are quiescent, proliferate sporadically, and produce committed neurogenic progeny. The SVZ and hippocampus retain a remarkable capacity for repair indicating the importance of NSCs in the regenerative NSCs. Loss of Notch 1, one member of the four-strong Notch family, results in a selective loss of activated neurogenic NSCs. In contrast, dormant NSCs are Notch1-insensitive until stimulated by a lesion to the SVZ. Hence, Notch1 is a key component of the adult SVZ niche promoting maintenance of neurogenic and activated NSC. Using genetic markers and lineage tracing we addressed NSC heterogeneity in the adult brain. We identified subpopulations of adult SVZ NSCs (type 1-3) and found that activated NSCs express brain lipid binding protein (BLBP, FABP7) and epidermal growth factor (EGF) receptor. They proliferate in response to EGF and are a major clonogenic popluation in the SVZ. We found a similar population of BLBP-expressing mitotic progenitors in the postnatal human brain and these activated NSCs are diminished in aged rodents and humans leaving only dormant stem cells.
We also identified morphologically distinct NSCs in the hippocampus of adult mice that can shuttle between mitotic activity and quiescence. Radial and horizontal NSCs respond selectively to neurogenic and pathophysiological stimuli including physical exercise and epileptic seizures. We found that the age-related reduction in neurogenesis in the hippo-campus correlates with a loss of active horizontal NSCs and their transition to a quiescent NSCs can be reactivated to rejuvenate hippocampal neurogenesis in aged mice. The selective response of NSC populations and reversible quiescence has important implications for adaptive learning for regenerative therapy.
Notch in neurogenesis
We and others have demonstrated the importance of Notch signaling in regulating NSC maintenance and cell fate during development. Notch controls the Expression of a cascade of transcription factors critical for progenitor maintenance and differentiation. Although transcriptional regulation of target genes is pivotal, we have addressed other mechanisms controlled by Notch signaling and which contribute to neurogenesis. We performed genome-wide studies of NSC transcriptomes following ablation of Notch. We study a cluster of RNA-binding proteins and components of the microRNA pathway that are regulated downstream of Notch in NSCs. We showed that the RNAselll Drosha and DGCR8/Pasha, key components of the microRNA microprocessor, play a central role in neurogenesis in the embryonic mouse forebrain. Drosha negatively regulates Expression of the proneurogenic transcription factors Neurogenin2 and NeuroD1 through binding to and cleaving hairpin structures in their mRNAs to destabilize the transcripts. We continue to study the role on mRNA destabilisation to expand our understanding of the targets of the Drosha/DGCR8 complex in NSCs of the mammalian brain.
Drosha in neurogenesis
Hypostable transcription factors of the Hes and proneural gene families regulate NSC maintenance and neurogenesis. An intrinsic negative feedback regulation of Hes factors on their own promoters and their forward repression of proneural gene expression are key in setting-up oscillatory expression of these genes. The short half-lives of the proteins and mRNAs regulates NSCs entry into cell cycle and differentiation. We have shown that Drosha and the RBP DGCR8 have important functions in mouse neurogenesis. Drosha maintains forebrain NSCs and progenitors and prevents precocious differentiation through a miRNA and Dicer-independent regulation of proneural gene expression. Drosha negatively regulates expression of the transcription factors Neurog2 and NeuroD1 by binding to and cleaving evolutionarily conserved hairpins in their mRNAs. Multilineage neuronal, astrocytic and oligodendryoctic potential is considered a NSC trait. However, adult DG NSCs are fated to become granule neurons and astrocytes but not oligodendrocytes. How this is regulated was unknown. We recently showed that Drosha regulates NSC maintenance and differentiation in the adult mouse DG of the hippocampus. Drosha silences Nuclear Factor I/B (NFIB) in hippocampal NSCs, targeting double-stranded hairpins in the mRNA of NFIB, thereby repressing its expression in a miRNA and Dicer independent manner.