Homer 2 tunes G protein-coupled receptors stimulus intensity by regulating RGS proteins and PLCβ GAP activities

Homers are scaffolding proteins that bind G protein-coupled receptors (GPCRs), inositol 1,4,5-triphosphate (IP3) receptors (IP3Rs), ryanodine receptors, and TRP channels. However, their role in Ca²⁺ signaling in vivo is not known. Characterization of Ca²⁺ signaling in pancreatic acinar cells from Homer2^-/- and Homer3^-/- mice showed that Homer 3 has no discernible role in Ca²⁺ signaling in these cells. In contrast, we found that Homer 2 tunes intensity of Ca²⁺ signaling by GPCRs to regulate the frequency of [Ca²⁺]_i oscillations. Thus, deletion of Homer 2 increased stimulus intensity by increasing the potency for agonists acting on various GPCRs to activate PLCβ and evoke Ca²⁺ release and oscillations. This was not due to aberrant localization of IP3Rs in cellular microdomains or IP3R channel activity. Rather, deletion of Homer 2 reduced the effectiveness of exogenous regulators of G proteins signaling proteins (RGS) to inhibit Ca²⁺ signaling in vivo. Moreover, Homer 2 preferentially bound to PLCβ in pancreatic acini and brain extracts and stimulated GAP activity of RGS4 and of PLCβ in an in vitro reconstitution system, with minimal effect on PLCβ-mediated PIP2 hydrolysis. These findings describe a novel, unexpected function of Homer proteins, demonstrate that RGS proteins and PLCβ GAP activities are regulated functions, and provide a molecular mechanism for tuning signal intensity generated by GPCRs and, thus, the characteristics of [Ca²⁺]_i oscillations.