A novel function for a TrpV channel in the Drosophila Malpighian tubule

Abstract

The Drosophila genome contains genes for two members of the vanilloid-receptor (trpV) subfamily of TRP channels: inactive (iav) and nanchung (nan). When expressed in heterologous cells, iav is reported to form an osmosensitive channel, and mutations in iav result in deafness, locomotor inactivity, and low levels of the biogenic amine tyramine (TA). Because TA is an important modulator of Malpighian tubule function in Drosophila, causing a rapid increase in transepithelial chloride conductance, and because the sensitivity of tubules to TA is modulated by osmolality, I have looked at the effect of iav mutations on tubule physiology. Electrophysiological recording from wild-type and iav1 tubules showed no difference in the response of the tubules to osmotic upshifts or downshifts: the changes in TA sensitivity and transepithelial potential were unaffected by mutation of iav. In contrast, mutation of iav profoundly inhibited the ability of the tubules to respond to tyrosine. In wild-type tubules, applied tyrosine is converted by the principal cells of the tubule into TA through the action of the enzyme tyrosine decarboxylase (TDC). Two different alleles of iav both caused tubules to be selectively insensitive to tyrosine but not TA, as if the tubules are unable to convert tyrosine to TA. The expression, localization, and activity of TDC, however, are unaffected by iav mutations, as determined by immunohistochemistry, enzyme assay, and real-time PCR of the tdc1 gene. While the exact function of iav in the conversion of tyrosine to TA remains unknown, the mutant phenotype suggests a novel role for trpV channels in epithelial function. Supported by Marquette University.