Key role of the postsynaptic density scaffold proteins Shank and Homer in the functional architecture of Ca2+ homeostasis at dendritic spines in hippocampal neurons

A key aspect of postsynaptic function, also important for plasticity, is the segregation within dendritic spines of Ca²⁺ rises attributable to release from intracellular stores. Previous studies have shown that overexpression in hippocampal neurons of two postsynaptic density (PSD) scaffold proteins, Shank1B and Homer1b, induces spine maturation, including translocation of the intracellular Ca²⁺ channel inositol trisphosphate receptor (IP₃R). The structural and functional significance of these processes remained undefined. Here, we show that in its relocation, IP₃R is accompanied by other endoplasmic reticulum (ER) proteins: the Ca²⁺ pump sarcoendoplasmic reticulum calcium ATPase, the lumenal Ca²⁺-binding protein calreticulin, the ER lumen-addressed green fluorescent protein, and, to a lesser extent, the membrane chaperone calbindin. The specificity of these translocations was demonstrated by their inhibition by both a Shank1 fragment and the dominant-negative Homer1a. Activation in Shank1B-transfected neurons of the metabotropic glutamatergic receptors 1/5 (mGluRs1/5), which induce IP₃ generation with ensuing Ca²⁺ release from the stores, triggered considerable increases in Ca²⁺-dependent responses: activation of the big K⁺ channel, which was revealed by patch clamping, and extracellular signal-regulated protein kinase (ERK) phosphorylation. The interaction of Shank1B and Homer1b appears as the molecular mechanism linking mGluRs1/5, strategically located in the spines, to IP₃R with the integration of entire ER cisternas in the PSD and with consequences on both local Ca ²⁺ homeostasis and overall neuronal signaling.