Activation of Yap-Directed Transcription by Knockdown of Conserved Cellular Functions

The Yap-Hippo pathway has a significant role in regulating cell proliferation and growth, thus controlling organ size and regeneration. The Hippo pathway regulates two highly conserved, transcription coactivators, YAP and TAZ. The upstream regulators of the Yap-Hippo pathway have not been fully characterized. The aim of this study was to use a siRNA screen, in a liver biliary cell line, to identify regulators of the Yap-Hippo pathway that allow activation of the YAP transcription coactivator at high cell density. Activation of the YAP transcription coactivator was monitored using a high-content, image-based assay that measured the intracellular localization of native YAP protein. Active siRNAs were identified and further validated by quantification of CYR61 mRNA levels (a known YAP target gene). The effect of compounds targeting the putative gene targets identified as hits was also used for further validation. A number of validated hits reveal basic aspects of Yap-Hippo biology, such as components of the nuclear pore, by which YAP cytoplasmic-nuclear shuttling occurs, or how proteasomal degradation regulates intracellular YAP concentrations, which then alter YAP localization and transcription. Such results highlight how targeting conserved cellular functions can lead to validated activity in phenotypic assays.

Constitutive Notch2 signaling in neural stem cells promotes tumorigenic features and astroglial lineage entry

Recent studies identified a highly tumorigenic subpopulation of glioma stem cells (GSCs) within malignant gliomas. GSCs are proposed to originate from transformed neural stem cells (NSCs). Several pathways active in NSCs, including the Notch pathway, were shown to promote proliferation and tumorigenesis in GSCs. Notch2 is highly expressed in glioblastoma multiforme (GBM), a highly malignant astrocytoma. It is therefore conceivable that increased Notch2 signaling in NSCs contributes to the formation of GBM. Here, we demonstrate that mice constitutively expressing the activated intracellular domain of Notch2 in NSCs display a hyperplasia of the neurogenic niche and reduced neuronal lineage entry. Neurospheres derived from these mice show increased proliferation, survival and resistance to apoptosis. Moreover, they preferentially differentiate into astrocytes, which are the characteristic cellular population of astrocytoma. Likewise, we show that Notch2 signaling increases proliferation and resistance to apoptosis in human GBM cell lines. Gene expression profiling of GBM patient tumor samples reveals a positive correlation of Notch2 transcripts with gene transcripts controlling anti-apoptotic processes, stemness and astrocyte fate, and a negative correlation with gene transcripts controlling proapoptotic processes and oligodendrocyte fate. Our data show that Notch2 signaling in NSCs produces features of GSCs and induces astrocytic lineage entry, consistent with a possible role in astrocytoma formation.