Mpp4 is required for proper localization of plasma membrane calcium ATPases and maintenance of calcium homeostasis at the rod photoreceptor synaptic terminals

Abstract

Membrane palmitoylated protein 4 (Mpp4) is a member of the membrane-associated guanylate kinase family. We show that Mpp4 localizes specifically to the plasma membrane of photoreceptor synaptic terminals. Plasma membrane Ca²⁺ ATPases (PMCAs), the Ca²⁺ extrusion pumps, interact with an Mpp4-dependent presynaptic membrane protein complex that includes Veli3 and PSD95. In mice lacking Mpp4, PMCAs were lost from rod photoreceptor presynaptic membranes. Synaptic ribbons were enlarged, a phenomenon known to correlate with higher Ca²⁺. SERCA2 (sarcoplasmic-endoplasmic reticulum Ca²⁺ ATPase, type 2), which pumps cytosolic Ca²⁺ into intracellular Ca²⁺ stores and localizes next to the ribbons, was increased. The distribution of IP₃RII (InsP₃ receptor, type 2), which releases Ca²⁺ from the stores, was shifted away from the synaptic terminals. Synaptic transmission to second-order neurons was maintained but was reduced in amplitude. These data suggest that loss of Mpp4 disrupts a Ca²⁺ extrusion mechanism at the presynaptic membranes, with ensuing adaptive responses by the photoreceptor to restore Ca²⁺ homeostasis. We propose that Mpp4 organizes a presynaptic protein complex that includes PMCAs and has a role in modulating Ca²⁺ homeostasis and synaptic transmission in rod photoreceptors.