Ca²⁺ signaling in microdomains: Homer1 mediates the interaction between RyR2 and Cav1.2 to regulate excitation-contraction coupling

Excitation-contraction (E-C) coupling and Ca²⁺-induced Ca ²⁺ release in smooth and cardiac muscles is mediated by the L-type Ca²⁺ channel isoform Ca_v1.2 and the ryanodine receptor isoform RyR2. Although physical coupling between Ca_v1.1 and RyR1 in skeletal muscle is well established, it is generally assumed that Ca _v1.2 and RyR2 do not directly communicate either passively or dynamically during E-C coupling. In the present work, we re-examined this assumption by studying E-C coupling in the detrusor muscle of wild type and Homer1^-/- mice and by demonstrating a Homer1-mediated dynamic interaction between Ca_v1.2 and RyR2 using the split green fluorescent protein technique. Deletion of Homer1 in mice (but not of Homer2 or Homer3) resulted in impaired urinary bladder function, which was associated with higher sensitivity of the detrusor muscle to muscarinic stimulation and membrane depolarization. This was not due to an altered expression or function of RyR2 and Ca_v1.2. Most notably, expression of Ca_v1.2 and RyR2 tagged with the complementary C- and N-terminal halves of green fluorescent protein and in the presence and absence of Homer1 isoforms revealed that H1a and H1b/c reciprocally modulates a dynamic interaction between Ca_v1.2 and RyR2 to regulate the intensity of Ca²⁺-induced Ca²⁺ release and its dependence on membrane depolarization. These findings define the molecular basis of a "two-state" model of E-C coupling by Ca _v1.2 and RyR2. In one state, Ca_v1.2 couples to RyR2 by H1b/c, which results in reduced responsiveness to membrane depolarization and in the other state H1a uncouples Ca_v1.2 and RyR2 to enhance responsiveness to membrane depolarization. These findings reveal an unexpected and novel mode of interaction and communication between Ca_v1.2 and RyR2 with important implications for the regulation of smooth and possibly cardiac muscle E-C coupling.