Post-transcriptional regulation of neural stem cell fate by the RNaseIII Drosha

Stem cells are found in several organs where they are committed to differentiate into tissue specific somatic cells. In the developing and adult mammalian brain neural stem cells (NSCs) have the ability to differentiate into different cell types, the neurons and glia. NSCs differentiation is tightly regulated in order to ensure e.g. the correct formation of a six-layered isocortex during embryogenesis or in the adult to contribute to cognition. A major role in controlling NSC maintenance and differentiation plays post-transcriptional regulation. The RNaseIII Drosha, which is involved in miRNA biogenesis, was recently shown to directly inhibit specific mRNAs in a non-canonical, miRNA-independent manner, thereby controlling stem cell maintenance. It remained elusive if the non-canonical function of Drosha is also involved in cell fate decisions. During my PhD I investigated the role and requirement of Drosha in embryonic and adult NSC fate decision.
During embryogenesis, cortical development is a temporal tightly organized process. First, deep-layer neurons are generated followed by upper-layer neurons. To study the role of Drosha in cortical development, I performed NSC-specific conditional knock-out (cKO) experiments. Drosha cKO at defined developmental stages revealed that early during development Drosha is involved in controlling the timing of deep- and upper-layer neuronal differentiation and NSC maintenance. My biochemical results suggest that Drosha regulates deep-layer specification by inhibiting the deep-layer specific transcription factor Ctip2 in a miRNA-independent manner.
Under physiological conditions, adult hippocampal NSCs are bi-potent, giving rise to neurons and astrocytes but not to oligodendrocytes. However, when we deleted Drosha in hippocampal NSCs, they activated an oligodendrogenesis pathway. We demonstrated that Drosha inhibits oligodendrogenesis by directly repressing the expression of the transcription factor NFIB in a miRNA-independent manner by cleaving and destabilizing its mRNA. These results demonstrate that adult hippocampal NSCs intrinsically are multipotent but Drosha restricts their fate.
In summary, the results of my PhD work show that Drosha plays a crucial role not only in NSC maintenance but also in NSC fate decision in the embryonic and adult brain. We find that during these processes, Drosha balances the expression of several differentiation factors thereby potentially fine-tuning the differentiation program. It will be of future interest to investigate how this specific miRNA-independent function of Drosha is targeted and if such a function is conserved in other stem cell populations.