Systematic functional analysis of Rab GTPasesreveals limits of neuronal robustness in Drosophila

Rab GTPases are molecular switches that regulate membrane trafficking in all cells. Neurons have particular demands on membrane trafficking and express numerous Rab GTPases of unknown function. Here we report the generation and characterization of molecularly defined null mutants for all 26 rab genes in Drosophila. In addition, we created a transgenic fly collection for the acute, synchronous release system RUSH for all 26 Rabs. In flies, all rab genes are expressed in the nervous system where at least half exhibit particularly high levels compared to other tissues. Surprisingly, loss of any of these 13 nervous-system enriched Rabs yields viable and fertile flies without obvious morphological defects. However, 9 of these 13 affect either developmental timing when challenged with different temperatures, or neuronal function when challenged with continuous stimulation. These defects are non-lethal under laboratory conditions, but represent sensitized genetic backgrounds that reveal limits of developmental and functional robustness to environmental challenges. Interestingly, the neuronal rab26 was previously proposed to function in synaptic maintenance by linking autophagy and synaptic vesicle recycling and we identified rab26 as one of six rab mutants with reduced synaptic function under continuous stimulation conditions. However, we found no changes to autophagy or synaptic vesicle markers in the rab26 mutant, but instead a cell-specific role in membrane receptor turnover associated with cholinergic synapses in the fly visual system. Our systematic functional analyses suggest that several Rabs ensure robust development and function under varying environmental conditions. The mutant and transgenic fly collections generated in this study provide a basis for further studies of Rabs during development and homeostasis in vivo.