Investigating competitive interactions between normal and transformed neural stem cells

Abstract

Glioblastoma is the most aggressive subtype of malignant glioma and, despite intensive research efforts, has an extremely poor prognosis with a median survival of just 15 months. It has long been speculated that adult neural stem cells (NSCs) that reside in the subventricular zone (SVZ) of the brain could be the cell of origin in glioblastoma. Indeed, it has been demonstrated that murine SVZ NSCs, which have lost the Ink4a/Arf tumour suppressor locus and express a constitutively active form of the epidermal growth factor receptor, EGFRvIII, are able to form aggressive tumours in vivo that recapitulate many of the features of glioblastoma. In this project, I sought to investigate the interactions between these transformed NSCs and their wild-type counterparts to determine whether a selective advantage could be aiding their expansion in the niche. I found that, when these cells were co-cultured, transformed NSCs induced wild-type NSCs to dramatically reduce their proliferation rate and adopt a quiescent-like state. This could be observed through a reduction in cell number and a decreased proportion of BrdU-positive wild-type cells in co-culture compared to homogenous culture. Transcriptional profiling also revealed downregulated expression of genes associated with cell cycle progression in co-cultured wild-type NSCs. Importantly, this phenotype was contact dependent, as no reduction in proliferation was observed when the two cell types were physically separated. To test the molecular mechanism underlying this cell cycle repression, I inhibited Notch signalling, a cell-to-cell signalling pathway known to have a role in sustaining NSC proliferation. Interestingly, I found that both treatment with γ-secretase inhibitors and genetic deletion of key Notch signalling genes rescued the proliferation of wild-type NSCs in co-culture. Together these results indicate that NSCs that acquire oncogenic mutations expand in the niche in part by suppressing the growth of surrounding NSCs in a Notch-dependent manner.