N-methyl-D-aspartate receptor-induced proteolytic conversion of postsynaptic class C L-type calcium channels in hippocampal neurons

Ca²⁺ influx controls multiple neuronal functions including neurotransmitter release, protein phosphorylation, gene expression, and synaptic plasticity. Brain L-type Ca²⁺ channels, which contain either α(1C) or α(1D) as their pore-forming subunits, are an important source of calcium entry into neurons. α(1C) exists in long and short forms, which are differentially phosphorylated, and C-terminal truncation of α(1C) increases its activity ≃4-fold in heterologous expression systems. Although most L- type calcium channels in brain are localized in the cell body and proximal dendrites, α(1C) subunits in the hippocampus are also present in clusters along the dendrites of neurons. Examination by electron microscopy shows that these clusters of α(1C) are localized in the postsynaptic membrane of excitatory synapses, which are known to contain glutamate receptors. Activation of N-methyl-D-aspartate (NMDA)-specific glutamate receptors induced the conversion of the long form of α(1C) into the short form by proteolytic removal of the C terminus. Other classes of Ca²⁺ channel α₁ subunits were unaffected. This proteolytic processing reaction required extracellular calcium and was blocked by inhibitors of the calcium-activated protease calpain, indicating that calcium entry through NMDA receptors activated proteolysis of α(1C) by calpain. Purified calpain catalyzed conversion of the long form of immunopurified α(1C) to the short form in vitro, consistent with the hypothesis that calpain is responsible for processing of α(1C) in hippocampal neurons. Our results suggest that NMDA receptor-induced processing of the postsynaptic class C L-type Ca²⁺ channel may persistently increase Ca²⁺ influx following intense synaptic activity and may influence Ca²⁺-dependent processes such as protein phosphorylation, synaptic plasticity, and gene expression.