Transient receptor potential-like channels are essential for calcium signaling and fluid transport in a Drosophila epithelium

Calcium signaling is an important mediator of neuropeptide-stimulated fluid transport by Drosophila Malpighian (renal) tubules. We demonstrate the first epithelial role, in vivo, for members of the TRP family of calcium channels. RT-PCR revealed expression of trp, trpl, and trpγ in tubules. Use of antipeptide polyclonal antibodies for TRP, TRPL, and TRPγ showed expression of all three channels in type 1 (principal) cells in the tubule main segment. Neuropeptide (CAP_2b)-stimulated fluid transport rates were significantly reduced in tubules from the trpl³⁰² mutant and the trpl;trp double mutant, trpl³⁰²;trp³⁴³. However, a trp null, trp³⁴³, had no impact on stimulated fluid transport. Measurement of cytosolic calcium concentrations ([Ca²⁺]_i) in tubule principal cells using an aequorin transgene in trp and trpl mutants showed a reduction in calcium responses in trpl³⁰². Western blotting of tubule preparations from trp and trpl mutants revealed a correlation between TRPL levels and CAP_2b-stimulated fluid transport and calcium signaling. Rescue of trpl³⁰² with a trpl transgene under heat-shock control resulted in a stimulated fluid transport phenotype that was indistinguishable from wild-type tubules. Furthermore, restoration of normal stimulated rates of fluid transport by rescue of trpl³⁰² was not compromised by introduction of the trp null, trp³⁴³. Thus, in an epithelial context, TRPL is sufficient for wild-type responses. Finally, a scaffolding component of the TRPL/TRP-signaling complex, INAD, is not expressed in tubules, suggesting that inaD is not essential for TRPL/TRP function in Drosophila tubules.