Homer proteins accelerate Ca ²⁺ clearance mediated by the plasma membrane Ca ²⁺ pump in hippocampal neurons

The plasma membrane Ca ²⁺ ATPase (PMCA) is responsible for maintaining basal intracellular Ca ²⁺ concentration ([Ca ²⁺] _i) and returning small increases in [Ca ²⁺] _i back to resting levels. The carboxyl terminus of some PMCA splice variants bind Homer proteins; how binding affects PMCA function is unknown. Here, we examined the effects of altered expression of Homer proteins on PMCA-mediated Ca ²⁺ clearance from rat hippocampal neurons in culture. The kinetics of PMCA-mediated recovery from the [Ca ²⁺] _i increase evoked by a brief train of action potentials was determined in the soma of single neurons using indo-1-based photometry. Exogenous expression of Homer 1a, Homer 1c or Homer 2a did not affect PMCA function. However, shRNA mediated knockdown of Homer 1 slowed PMCA mediated Ca ²⁺ clearance by 28% relative to cells expressing non-silencing shRNA. The slowed recovery rate in cells expressing Homer 1 shRNA was reversed by expression of a short Homer 2 truncation mutant. These results indicate that constitutively expressed Homer proteins tonically stimulate PMCA function in hippocampal neurons. We propose a model in which binding of short or long Homer proteins to the carboxyl terminus of the PMCA stimulates Ca ²⁺ clearance rate. PMCA-mediated Ca ²⁺ clearance may be stimulated following incorporation of the pump into Homer organized signaling domains and following induction of the Homer 1a immediate early gene.