Mechanism and function of the capa/capaR in the desiccation stress response in Drosophila. Selim Terhzaz, Pablo Cabrero, Louise Henderson, Julian A.T. Dow, Shireen-A. Davies. Institute of Molecular Cell and Systems Biology, University of Glasgow, Glasgow, United Kingdom.

   Drosophila species occur in a wide range of habitats, including hot and dry conditions. Their ability to survive desiccation stress has been studied and the enhanced desiccation resistance in Drosophila is the result of reduced rates of water loss. Although the major routes for water loss are being through the cuticle and the spiracles, the excretory water loss involving the Malpighian renal tubules and hindgut makes a significant contribution to the total water loss in desiccated fruit flies. Fluid secretion by the Malpighian tubules of insects is under elaborate neuropeptide control, which modulates appropriate cell signaling and ion transport pathways. The endogenous D. melanogaster capa neuropeptides (Drm-capa-1 and -2) increase fluid transport by adult Malpighian tubules of Drosophila. Capa-1 and capa-2 act via elevation of intracellular calcium and nitric oxide/cGMP signaling, in tubule principal cells. We recently demonstrated the kinetics of capa-1-induced activation and desensitisation of its cognate G-protein coupled receptor, capaR. CapaR is highly expressed in tubules and plays a role in desiccation stress resistance for the whole fly. CapaR gene expression in tubules is reduced under desiccation stress, whilst tubules from desiccated flies show markedly inhibited basal and capa-1-stimulated rates of fluid transport. Capa peptide amounts in capa-expressing peptidergic Va neurons are increased in response to desiccation; and capa gene expression is increased by exposure of flies to desiccation or to high salt. Precise spatial targeting of capa RNAi to the Va neurons caused increased survival of whole flies to water stress, both for desiccation (water deficiency) and osmotic (high salt) stress but did not reveal a phenotype in response to starvation, oxidative or immune stress. Taken together, the capa/capaR signalling acts in the key fluid-transporting tissue to regulate responses to desiccation stress in the fly.