Cohen, S. M. | bioRxiv Science

bioRxiv2019

Promoter proximal pausing limits Yki-induced tumorous growth in Drosophila

Promoter proximal pausing (PPP) of RNA Polymerase II has emerged as a crucial rate-limiting-step in the regulation of gene expression. Regulation of PPP is brought about by complexes 7SK snRNP, P-TEFb (Cdk9/cycT) and the Negative Elongation Factor (NELF) which are highly conserved from Drosophila to humans. Here we show that RNAi-mediated depletion of bin3 or Hexim of the 7SK snRNP complex or depletion of individual components of the NELF complex enhance Yki-driven growth leading to neoplastic transformation of Drosophila wing imaginal discs. We also show that increased CDK9 expression cooperates with Yki, in driving neoplastic growth. Interestingly, over-expression of CDK9 on its own or in the background of depletion of one of the components of 7SK snRNP or the NELF complex necessarily and specifically needed Yki over-expression to cause tumorous growth. Genome-wide gene expression analyses suggested that deregulation of protein homeostasis is associated with tumorous growth of wing imaginal discs. As both Fat/Hippo/Yki pathway and PPP are highly conserved, our observations may provide insights into mechanisms of oncogenic function of YAP, the orthologue of Yki in human.

cancer biology↗

bioRxiv2019

The Eya1 phosphatase mediates Shh-driven symmetric cell division of cerebellar granule cell precursors

During neural development, stem and precursor cells can divide either symmetrically or asymmetrically. The transition between symmetric and asymmetric cell divisions is a major determinant of precursor cell expansion and neural differentiation, but the underlying mechanisms that regulate this transition are not well understood. Here, we identify the Sonic hedgehog (Shh) pathway as a critical determinant regulating the mode of division of cerebellar granule cell precursors (GCPs). Using partial gain and loss of function mutations within the Shh pathway, we show that pathway activation determines spindle orientation of GCPs, and that mitotic spindle orientation directly correlates with the mode of division. Mechanistically, we show that the phosphatase Eya1 is essential for implementing Shh-dependent GCP spindle orientation. We identify atypical protein kinase C (aPKC) as a direct target of Eya1 activity and show that Eya1 dephosphorylates Threonine (T410) in the activation loop of this polarity complex component. Thus, Eya1 inactivates the cell polarity complex, resulting in reduced phosphorylation of Numb and other components that regulate the mode of division. This Eya1-dependent cascade is critical in linking spindle orientation, cell cycle exit and terminal differentiation. Together these findings demonstrate that a Shh-Eya1 regulatory axis selectively promotes symmetric cell divisions during cerebellar development by coordinating spindle orientation and cell fate determinants.\n\nSummary statementBiological responses to Shh signaling are specified by the magnitude of pathway activation and the cellular context. This study shows that potent Shh signaling regulates mitotic orientation and symmetric division of cerebellar granule cell precursors in a process that requires the phosphatase Eya1 and unequal distribution of cell fate determinants to daughter cells.

neuroscience↗