Psi controls signalling from the Drosophila cortex glial niche to regulate neural stem cell fate

Abstract

Glioma patients experience high rates of morbidity and invariable fatality, yet no new, effective treatments have emerged in more than 35 years. Progress in therapeutic development is hindered by a limited understanding of the biology of these heterogeneous brain tumours, which are driven by glioma stem cells supported by a glial niche. Genomic sequencing of the low-grade brain tumour oligodendroglioma identified frequent loss-of-function mutations in the single-stranded DNA/RNA binding protein FUBP1, which are predicted to drive these tumours. However, functional studies investigating FUBP1 function in mammalian systems are complicated by compensatory effects from paralogues within the FUBP protein family, which share common gene binding targets. In Drosophila, the three mammalian FUBP proteins are represented by a single orthologue, Psi, which shares high structural and functional similarity to FUBP1. By taking advantage of this reduced functional redundancy, and the powerful tractability of Drosophila genetics, we dissect FUBP1/Psi function in the cortex glial niche, which functions analogously to the oligodendroglioma niche by providing the structural support and secreted signals which regulate stem cell proliferation and differentiation.

Here we demonstrate Psi knockdown specifically in the cortex glial niche of the Drosophila larval brain reduces cortex glial growth and proliferation. Moreover, cortex glial-specific Psi depletion cell non-autonomously drives the proliferation and expansion of otherwise wild-type NSCs. Thus, we present the first evidence that an FUBP family protein functions in a cell non-autonomous manner to control NSC fate. To determine the molecular basis for Psi's capacity to control NSC renewal and differentiation from the supporting cortex glial niche, we used Targeted DamID (TaDa) to identify direct, genome-wide Psi binding targets in cortex glia. We further identified differentially expressed and/or spliced targets via RNA sequencing of Psi-depleted cortex glia and associated NSCs compared with isogenic controls. Intersection of Psi binding targets and transcriptionally altered genes in Psi-depleted cortex glia revealed Psi directly regulates components of major developmental signalling pathways, Hippo and MAPK, among other networks essential for niche function. Our functional genetic studies indicate Psi represses upstream Hippo pathway components in the niche to maintain normal cortex glia growth and proliferation. Furthermore, Psi knockdown directly upregulates EGFR ligand spitz (spi) in cortex glia, which functions in the niche to cell-autonomously regulate cortex glia growth and proliferation. Psi depletion also upregulates another EGFR ligand, gurken (grk), which, in contrast to spi, is required to enable expansion and proliferation of neighbouring NSCs driven cell non-autonomously by loss of Psi. Together, our data demonstrate Psi functions cell non-autonomously in the glial niche to orchestrate signalling networks required to prevent excessive NSC renewal. Thus, given the high degree of functional homology with Psi, we predict FUBP1 loss-of-function drives oligodendroglioma tumourigenesis, at least in part, by dysregulating intra-tumour interactions between the glial niche and glioma stem cells. Show more