Cell-Cell Contact Formation Governs Ca ²⁺ Signaling by TRPC4 in the Vascular Endothelium : EVIDENCE FOR A REGULATORY TRPC4-β-CATENIN INTERACTION ^*

Abstract

TRPC4 is well recognized as a prominent cation channel in the vascular endothelium, but its contribution to agonist-induced endothelial Ca ²⁺ entry is still a matter of controversy. Here we report that the cellular targeting and Ca ²⁺ signaling function of TRPC4 is determined by the state of cell-cell adhesions during endothelial phenotype transitions. TRPC4 surface expression in human microvascular endothelial cells (HMEC-1) increased with the formation of cell-cell contacts. Epidermal growth factor recruited TRPC4 into the plasma membrane of proliferating cells but initiated retrieval of TRPC4 from the plasma membrane in quiescent, barrier-forming cells. Epidermal growth factor-induced Ca ²⁺ entry was strongly promoted by the formation of cell-cell contacts, and both siRNA and dominant negative knockdown experiments revealed that TRPC4 mediates stimulated Ca ²⁺ entry exclusively in proliferating clusters that form immature cell-cell contacts. TRPC4 co-precipitated with the junctional proteins β-catenin and VE-cadherin. Analysis of cellular localization of fluorescent fusion proteins provided further evidence for recruitment of TRPC4 into junctional complexes. Analysis of TRPC4 function in the HEK293 expression system identified β-catenin as a signaling molecule that enables cell-cell contact-dependent promotion of TRPC4 function. Our results place TRPC4 as a Ca ²⁺ entry channel that is regulated by cell-cell contact formation and interaction with β-catenin. TRPC4 is suggested to serve stimulated Ca ²⁺ entry in a specific endothelial state during the transition from a proliferating to a quiescent phenotype. Thus, TRPC4 may adopt divergent, as yet unappreciated functions in endothelial Ca ²⁺ homeostasis and emerges as a potential key player in endothelial phenotype switching and tuning of cellular growth factor signaling.