Functional role of the calmodulin- and inositol 1,4,5-trisphosphate receptor-binding (CIRB) site of TRPC6 in human platelet activation.

BACKGROUND: All identified mammalian TRPC channels show a C-terminal calmodulin (CaM)- and inositol 1,4,5-trisphosphate receptors (IP(3)Rs)-binding (CIRB) site involved in the regulation of TRPC channel function.

OBJECTIVES: To assess the basis of CaM/IP(3)Rs binding to the CIRB site of TRPC6 and its role in platelet physiology.

METHODS: Protein association was detected by co-immunoprecipitation and Western blotting, Ca(2+) mobilization was measured by fluorimetric techniques and platelet function was analyzed by aggregometry.

RESULTS: Co-immunoprecipitation of TRPC6 with CaM or the IP(3)Rs at different cytosolic free Ca(2+) concentrations ([Ca(2+)](c)) indicates that the association between these proteins is finely regulated by cytosolic Ca(2+) via association of CaM and displacement of the IP(3)Rs at high [Ca(2+)](c). Thrombin-stimulated association of TRPC6 with CaM or the IP(3)Rs was sensitive to 2-APB and partially inhibited by dimethyl BAPTA loading, thus suggesting that the association between these proteins occurs through both Ca(2+)-dependent and -independent mechanisms. Incorporation of an anti-TRPC6 C-terminal antibody, whose epitope overlaps the CIRB region, impaired the dynamics of the association of TRPC6 with CaM and the IP(3)Rs, which lead to both inhibition and enhancement of thrombin- and thapsigargin-evoked Ca(2+) entry in the presence of low or high, respectively, extracellular Ca(2+) concentrations, as well as altered thrombin-evoked platelet aggregation.

CONCLUSIONS: Our results indicate that the CIRB site of TRPC6 plays an important functional role in platelets both modulating Ca(2+) entry and aggregation through its interaction with CaM and IP(3)Rs.