The voltage sensing phosphatase (VSP) localizes to the apical membrane of kidney tubule epithelial cells

Voltage-sensing phosphatases (VSPs) are transmembrane proteins that couple changes in membrane potential to hydrolysis of inositol signaling lipids. VSPs catalyze the dephosphorylation of phosphatidylinositol phosphates (PIPs) that regulate diverse aspects of cell membrane physiology including cell division, growth and migration. VSPs are highly conserved among chordates, and their RNA transcripts have been detected in the adult and embryonic stages of frogs, fish, chickens, mice and humans. However, the subcellular localization and biological function of VSP remains unknown. Using reverse transcriptase-PCR (RT-PCR), we show that both Xenopus laevis VSPs (XI-VSP1 and XI-VSP2) mRNAs are expressed in early embryos, suggesting that both XI-VSPs are involved in early tadpole development. To understand which embryonic tissues express XI-VSP mRNA, we used in situ hybridization (ISH) and found XI-VSP mRNA in both the brain and kidney of NF stage 32-36 embryos. By Western blot analysis with a VSP antibody, we show increasing levels of XI-VSP protein in the developing embryo, and by immunohistochemistry (IHC), we demonstrate that XI-VSP protein is specifically localized to the apical membrane of both embryonic and adult kidney tubules. We further characterized the catalytic activity of both XI-VSP homologs and found that while XI-VSP1 catalyzes 3- and 5-phosphate removal, XI-VSP2 is a less efficient 3phosphatase with different substrate specificity. Our results suggest that XI-VSP1 and XI-VSP2 serve different functional roles and that VSPs are an integral component of voltage-dependent PIP signaling pathways during vertebrate kidney tubule development and function.