Drosophila Hrp48 is required for mushroom body axon growth, branching and guidance

RNA binding proteins assemble on mRNAs to control every single step of their life cycle, from nuclear splicing to cytoplasmic localization, stabilization or translation. Consistent with an essential role of RNA binding proteins in neuronal maturation and function, mutations in this class of proteins, in particular in members of the hnRNP family, have been associated with neurological diseases. To date, however, the physiological function of hnRNPs during in vivo neuronal development has remained poorly explored. Here, we have investigated the role of Drosophila Hrp48, a fly homologue of mammalian hnRNP A2/B1, during central nervous system development. Using a combination of mutant conditions, we showed that hrp48 is required for the formation, growth and guidance of axonal branches in Mushroom Body neurons. Furthermore, our results revealed that hrp48 inactivation induces an overextension of Mushroom Body dorsal axonal branches, with a significantly higher penetrance in females than in males. Finally, as demonstrated by immunolocalization studies, Hrp48 is confined to Mushroom Body neuron cell bodies, where it accumulates in the cytoplasm from larval stages to adulthood. Altogether, our data provide evidence for a crucial in vivo role of the hnRNP Hrp48 in multiple aspects of axon guidance and branching during nervous system development. They also indicate cryptic sex differences in the development of sexually non-dimorphic neuronal structures.