Loss of chromatin modulator Dpy30 compromises proliferation and differentiation of postnatal neural stem cells

Abstract

Epigenetic regulation via chromatin modulation plays pivotal roles in regulating neural stem cells (NSCs) both during embryonic development and in adult neurogenesis (Yao et al., 2016). One classic epigenetic mechanism is covalent post-translational modifications to histone proteins, including methylation, phosphorylation, acetylation, ubiquitination, and sumoylation. In particular, methylation of histone H3 at K4 and K27 positions act antagonistically to maintain active and repressed gene expression states, respectively. Although it is established that gene expression regulated by H3K27 methylation is one of the major determinants of the capacity of NSCs for either self-renewal or lineage differentiation, little is known about the role of H3K4 methylation in NSC regulation (Albert and Huttner, 2018). In the current issue, Shah et al. (2020) show that Dpy30 and H3K4 methylation are essential for proliferation and differentiation of postnatal NSCs. H3K4 methylation, catalyzed by methyltransferases, is a conserved feature of active genes. In mammalian cells, there are six H3K4 methyltransferases that share a highly conserved catalytic SET domain (SET1A, SET1B, MLL1, MLL2, MLL3, and MLL4). SET1/MLL enzymes function towards H3K4 methylation in the context of a multi-subunit complex, which contains WDR5, RbBP5, Ash2l, and Dpy30 as integral core subunits (Hyun et al., 2017). A previous study explored whether H3K4 methyltransferases regulate neurogenesis by deleting MLL1 in NSCs at embryonic day 13.5 (E13.5) and found compromised neuronal differentiation in the postnatal subventricular zone (SVZ) due to dysregulation of Dlx2 expression, a key regulator of SVZ neurogenesis (Lim et al., 2009). Surprisingly, instead of reducing H3K4 tri-methylation, MLL1 deletion resulted in increased deposition of H3K27 tri-methylation at the Dlx2 promoter, which converted the state of Dlx2 gene from ‘active’ to ‘repressed’. Therefore, defective postnatal neurogenesis caused by MLL1 deficiency seems to be irrelevant to H3K4 methylation. In the current study, Shah et al. (2020) silenced Dpy30 expression in NSCs from E13.5 using a Cre-loxP system. Dpy30 knockout (KO) pups exhibited growth retardation and ataxia and died during postnatal days 20–27 (P20–27). Moreover, the loss of Dpy30 resulted in enlarged lateral ventricles, deformed dentate gyrus (DG), and a reduction in cerebellar folia, accompanied by global H3K4 methylation reduction (Figure 1). Similar to the SVZ, DG is also a neurogenic region in the mammalian postnatal brain (Ming and Song, 2011). Using DG and SVZ tissues dissected from P12 KO mice, transcriptome analysis revealed downregulation of neuronal markers in the DG and astrocytic markers in the SVZ of KO mice. These results suggest perturbed neurogenesis and gliogenesis selectively in KO DG and SVZ, respectively, although we cannot rule out the probability that potential apoptosis induced by the loss of Dpy30 contributes to these changes. To further explore the impact of Dpy30 loss on neurogenesis and gliogenesis, Shah et al. (2020) performed immunostaining and found a reduced number of NSCs in the postnatal KO DG and SVZ. The astrocytic population decreased in both KO DG and SVZ, and the neuronal decrease was found specifically in KO DG. Furthermore, Shah et al. (2020) applied an in vitro serial replating assay to investigate the proliferative capacity of NSCs collected from SVZ. KO NSCs exhibited a reduced ability to form neurospheres compared to control NSCs, and no viable KO cells were observed after several passages, supporting the model that Dpy30 is required for NSC self-renewal in a cellautonomous manner. Finally, using short hairpin RNA (shRNA) to knock down Dpy30 in mouse and human neural progenitor cell (NPC) lines and then inducing them to differentiate,