Abstract
Transcellular calcium transport in the kidney, pancreas, small intestine, and placenta is partly mediated by transient receptor potential (TRP) channels. The highly selective TRPV6 calcium channel protein is most likely important for the calcium transfer in different specialized epithelial cells. In the human placenta the protein is expressed in trophoblast tissue, where it is implicated in the transepithelial calcium transfer from mother to the fetus. We enriched the TRPV6 channel protein endogenously expressed in placenta together with annexin A2 and cyclophilin B (CypB), which is a member of the huge immunophilin family. In the human placenta TRPV6 and CypB are mainly located intracellularly in the syncytiotrophoblast layer, but a small amount of the mature glycosylated TRPV6 channel protein and CypB is also expressed in microvilli apical membranes, the fetomaternal barrier. To understand the role of CypB on the TRPV6 channel function, we evaluated the effect of CypB co-expression on TRPV6-mediated calcium uptake into Xenopus laevis oocytes expressing TRPV6. A significant increase of TRPV6-mediated calcium uptake was observed after CypB/TRPV6 co-expression. This stimulatory effect of CypB was reversed by the immunosuppressive drug cyclosporin A, which inhibits the enzymatic activity of CypB. Cyclosporin A had no significant effect on TRPV6 and CypB protein expression levels in the oocytes. In summary, our results establish CypB as a new TRPV6 accessory protein with potential involvement in TRPV6 channel activation through its peptidyl-prolyl cis/trans isomerase activity.
Highlights
Functional transient receptor potential (TRP) channels require oligomerization of apparently four pore-forming units, which co-assemble with additional membrane-associated proteins
We show that the TRPV6 protein is expressed in human placenta, and we demonstrate for the first time that the immunophilin cyclophilin B (CypB) co-purifies together with the TRPV6 proteins from human placenta microsomal membranes
Both proteins are co-localized in the syncytiotrophoblast layer of the placenta, and the TRPV6-evoked calcium influx in Xenopus laevis oocytes is markedly increased with CypB co-expression
Summary
Functional TRP channels require oligomerization of apparently four pore-forming units, which co-assemble with additional membrane-associated proteins. Both proteins are co-localized in the syncytiotrophoblast layer of the placenta, and the TRPV6-evoked calcium influx in Xenopus laevis oocytes is markedly increased with CypB co-expression. As we have shown previously [46], the C terminus of TRPV6 binds calmodulin in a Ca2ϩ-dependent manner; we used calcium/calmodulin chromatography to enrich TRPV6 (Fig. 1, upper blot) from solubilized human placenta microsomal proteins.
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