Oscillatory expression of Ascl1 in oligodendrogenesis

The proneural gene Ascl1 promotes formation of both neurons and oligodendrocytes from neural stem cells (NSCs), but it remains to be analyzed how its different functions are coordinated. It was previously shown that Ascl1 enhances proliferation of NSCs when its expression oscillates but induces differentiation into transit-amplifying precursor cells and neurons when its expression is up-regulated and sustained. By time-lapse imaging and immunohistological analyses, we found that Ascl1 expression oscillated in proliferating oligodendrocyte precursor cells (OPCs) at lower levels than in transit-amplifying precursor cells and was repressed when OPCs differentiated into mature oligodendrocytes. Induction of sustained overexpression of Ascl1 reduced oligodendrocyte differentiation and promoted neuronal differentiation. These results suggest that oscillatory expression of Ascl1 plays an important role in proliferating OPCs during oligodendrocyte formation.     

Proneural transcription factors regulate different steps of cortical neuron migration through Rnd-mediated inhibition of RhoA signaling

Little is known of the intracellular machinery that controls the motility of newborn neurons. We have previously shown that the proneural protein Neurog2 promotes the migration of nascent cortical neurons by inducing the expression of the atypical Rho GTPase Rnd2. Here, we show that another proneural factor, Ascl1, promotes neuronal migration in the cortex through direct regulation of a second Rnd family member, Rnd3. Both Rnd2 and Rnd3 promote neuronal migration by inhibiting RhoA signaling, but they control distinct steps of the migratory process, multipolar to bipolar transition in the intermediate zone and locomotion in the cortical plate, respectively. Interestingly, these divergent functions directly result from the distinct subcellular distributions of the two Rnd proteins. Because Rnd proteins also regulate progenitor divisions and neurite outgrowth, we propose that proneural factors, through spatiotemporal regulation of Rnd proteins, integrate the process of neuronal migration with other events in the neurogenic program.     

miR-34a repression in proneural malignant gliomas upregulates expression of its target PDGFRA and promotes tumorigenesis

Glioblastoma (GBM) and other malignant gliomas are aggressive primary neoplasms of the brain that exhibit notable refractivity to standard treatment regimens. Recent large-scale molecular profiling has revealed distinct disease subclasses within malignant gliomas whose defining genomic features highlight dysregulated molecular networks as potential targets for therapeutic development. The "proneural" designation represents the largest and most heterogeneous of these subclasses, and includes both a large fraction of GBMs along with most of their lower-grade astrocytic and oligodendroglial counterparts. The pathogenesis of proneural gliomas has been repeatedly associated with dysregulated PDGF signaling. Nevertheless, genomic amplification or activating mutations involving the PDGF receptor (PDGFRA) characterize only a subset of proneural GBMs, while the mechanisms driving dysregulated PDGF signaling and downstream oncogenic networks in remaining tumors are unclear. MicroRNAs (miRNAs) are a class of small, noncoding RNAs that regulate gene expression by binding loosely complimentary sequences in target mRNAs. The role of miRNA biology in numerous cancer variants is well established. In an analysis of miRNA involvement in the phenotypic expression and regulation of oncogenic PDGF signaling, we found that miR-34a is downregulated by PDGF pathway activation in vitro. Similarly, analysis of data from the Cancer Genome Atlas (TCGA) revealed that miR-34a expression is significantly lower in proneural gliomas compared to other tumor subtypes. Using primary GBM cells maintained under neurosphere conditions, we then demonstrated that miR-34a specifically affects growth of proneural glioma cells in vitro and in vivo. Further bioinformatic analysis identified PDGFRA as a direct target of miR-34a and this interaction was experimentally validated. Finally, we found that PDGF-driven miR-34a repression is unlikely to operate solely through a p53-dependent mechanism. Taken together, our data support the existence of reciprocal negative feedback regulation involving miR-34 and PDGFRA expression in proneural gliomas and, as such, identify a subtype specific therapeutic potential for miR-34a.     

Multiple roles of Notch signaling in the regulation of epidermal development

Recent studies have shown that Notch signaling plays an important role in epidermal development, but the underlying molecular mechanisms remain unclear. Here, by integrating loss- and gain-of-function studies of Notch receptors and Hes1, we describe molecular information about the role of Notch signaling in epidermal development. We show that Notch signaling determines spinous cell fate and induces terminal differentiation by a mechanism independent of Hes1, but Hes1 is required for maintenance of the immature state of spinous cells. Notch signaling induces Ascl2 expression to promote terminal differentiation, while simultaneously repressing Ascl2 through Hes1 to inhibit premature terminal differentiation. Despite the critical role of Hes1 in epidermal development, Hes1 null epidermis transplanted to adult mice showed no obvious defects, suggesting that this role of Hes1 may be restricted to developmental stages. Overall, we conclude that Notch signaling orchestrates the balance between differentiation and immature programs in suprabasal cells during epidermal development.