Abstract 1061: Inositol 1,4,5-Trisphosphate Receptors Play a Redundant Role in Intracellular Calcium Signaling Essential for Cardiac Development
Intracellular calcium signaling is known to play essential roles in cardiac physiology and hypertrophy, however, its function in cardiac development remains to be elucidated. Inositol 1,4,5-trisphosphate receptor (IP3R) is an intracellular calcium release channel on the endo/sarcoplasmic reticulum. Three subtypes of IP3R (IP3R1, 2 and 3) have been identified and are expressed in many organs during embryogenesis as well as postnatal periods. We have investigated the expression patterns of all subtypes of IP3R during murine cardiogenesis and analyzed mouse embryos deficient for IP3Rs. Western blot analysis indicated that all subtypes of IP3R are expressed during cardiac development. The expression pattern of IP3R1 and 2 were partially overlapping: IP3R1 began to be expressed at E9.5, maintained throughout the heart at E10.5, and gradually downregulated as development proceeds; IP3R2 was expressed in the endocardium at first around E9.5, and later expanded from trabecular to compact layer of myocardium, suggesting their redundant function during cardiac development. Consistent with this notion, IP3R1 and 2 double knockout (DKO) mouse embryos demonstrated thin myocardial wall and died around E11.0 with signs of heart failure while each single knockout mouse showed normal cardiogenesis. Intriguingly, mesenchymal cells in the atrioventricular endocardial cushion were reduced in number, indicating that epithelial-mesenchymal transformation (EMT) was impaired in DKO embryonic hearts. Moreover, the EMT assay demonstrated that treatment of endocardial cushion explants with IP3R inhibitor resulted in impaired EMT. In the endocardium of DKO embryos, translocation of a transcription factor NFATc into the nucleus was inhibited at E9.5, and expression of NFATc was downregulated at E10.0. NFATc is known to be essential for valvuloseptal development and is a substrate of intracellular calcium-activated protein phosphatase, calcineurin, which stimulates NFATc nuclear localization. Taken together, our data show that IP3R1 and 2 have a redundant role in the myocardium and in the endocardial cushion, where they regulate calcineurin/NFAT signaling, and provided insight into a role of the intracellular calcium signaling during cardiac development.