Orb2 RNA-binding activity promotes neural stem cell development and brain growth in Drosophila larvae

Neurodevelopment requires precise translational control, the disruption of which is implicated in neurological disorders, developmental delays, and microcephaly. We identify a role for the Drosophila CPEB-family protein Orb2, a translational regulator, in controlling brain size in a dose-dependent manner. Loss of orb2 results in larval brain hypotrophy, whereas orb2 overexpression causes brain overgrowth. We demonstrate that orb2 is required for neural stem cell (NSC) development from embryonic through larval neurogenesis. Structure-function analysis reveals that Orb2 RNA-binding activity promotes brain growth, while its poly-Q and ZZ domains restrain overgrowth. We identify a genetic and biochemical association between Orb2 and Brain tumor (Brat), a repressor of NSC differentiation and brain hypertrophy. Loss of orb2 diminishes Brat levels, while restoring Brat activity is sufficient to rescue orb2-dependent microcephaly. We propose that Orb2 promotes Brat translation directly and through the co-regulation of shared RNA targets, including Myc mRNA, to support Drosophila neurodevelopment.