Regulation by Calcium of the TRP channel Polycystin-2 (TRPP2).

MR CANTERO ; HF CANTIELLO.

Congreso; 53rd Annual Meeting Biophysical Society.; 2009

Polycystin-2 (PC2, TRPP2) is a member of the TRP (transient receptor potential) superfamily of cation channels. Like other members of this superfamily, PC2 permeates Ca2+, which is involved in both signal transduction, and Ca2+ entry. Previously, we showed that PC2 is normally active at intracellularly high Ca2+ concentrations (10-15 ìM). Little is known, however, about the role intracellular Ca2+ plays in PC2 channel function. Here, we explored the role of physiological concentrations of intracellular Ca2+ in PC2-mediated channel function in reconstituted apical membranes from term human syncytiotrophoblast (hST). Addition of either EGTA (1 mM) or BAPTA (2 mM) to reach low intracellular Ca2+ (<5 nM) at the cytoplasmic side, elicited a complete PC2 channel inhibition. A dose response elicited by addition of increasing cytoplasmic Ca2+ showed that Ca2+ activated PC2 with an apparent half activating concentration of 4.78 nM and a Hill coefficient of 5. Conversely, extracellular Ca2+ concentrations, between 0.5 mM and 5 mM, had a stimulatory effect on PC2 channel activity while higher external concentrations (10-90 mM) were inhibitory. Further, this activating mechanism was not intrinsic to the PC2 channel but instead seems to be mediated by PC2-associated proteins. Channel function of the in vitro translated PC2 protein with Ca2+ concentrations of 10-15 mM was non-responsive to lowering cytoplasmic Ca2+ with either EGTA or BAPTA. Our data are consistent with a regulatory role of both cytoplasmic and external Ca2+ in PC2 channel function, which does not involve putative Ca2+-binding sites on the channel protein, but instead external sites to the channel protein.