Calpain and the glutamatergic synapse

Abstract

Calpain is a ubiquitous protease found in different tissue types and in many organisms including mammals. It generally does not destroy its large variety of substrates, but more commonly disrupts their function. In neurons, many of its substrates become dysregulated as a result of cleavage of their regulatory domain by this protease, leading to altered signaling between cells. In glutamatergic synaptic transmission, direct targets of calpain include all of the major glutamate receptors: NMDA receptors, AMPA receptors and mGluR. By cleaving these receptors and associated intracellular proteins, calpain may regulate the physiology at glutamatergic synapses. As a result, calpain-mediated cleavage in neurons might not only be involved in pathological events like excitotoxicity, but may also have neuroprotective effects and roles in physiological synaptic transmission.

Figure 1

A glutamatergic synapse showing the major glutamate receptors NMDAR, AMPAR, Kainate R and mGluR. NMDARs are mostly located on the postsynaptic end and are present at the synaptic and extrasynaptic locations, while AMPARs and mGluRs are located both pre- and postsynpatically. Calcium entry through synaptic NMDARs leads to activation of the protease calpain downstream of the receptor. When activated, calpain forms part of a feedback loop for controlling excessive calcium influx by cleaving most glutamate receptors and leading to a reduction in their function.

Figure 2

PSD-95 is one of the postsynaptic density proteins involved in clustering of NMDARs on the cell surface. It binds the C-terminus of the receptor at the ESDV motif, and protects against calpain-mediated cleavage of that end. The inset shows two Src family kinases (SFKs), Src and Fyn, that phosphorylate residues on the C-terminus of NR2B leading to differing regulation by calpain. Fyn phosphorylates Y1252, Y1336 and Y1472 and thus potentiates calpain-mediated cleavage, while Src phosphorylates Y1472 by binding PSD-95, and prevents cleavage by calpain.

Figure 3

a. Calpain cleavage and selected phosphorylation sites on different subunits of the NMDA, AMPA and mGlu receptors. The locations of the transmembrane domains are highly conserved among the ionotropic receptor subunits. The exact calpain cleavage sites on NR2B are not fully known, but it should be noted that many phosphorylation sites are located close to calpain cleavage sites, indicating the important role played by phosphorylation of C-terminal residues in controlling calpain-mediated cleavage. b. Effect of calpain-mediated cleavage on different glutamate receptor subunits and the proposed role of calpain in the fate of the cell.