Activation of calpain may alter the postsynaptic density structure and modulate anchoring of NMDA receptors

Elevation of calcium during sustained synaptic activity may lead to the act ivation of the postsynaptic calcium-dependent protease calpain and thus cou ld alter the integrity and localization of endogenous proteins. The distrib ution of anchoring proteins for neuroreceptors is an important determinant of the efficacy of neuronal transmission. Many of these anchoring proteins are concentrated within the postsynaptic density (PSD). In the present stud y, we examined the effects of calpain II on isolated PSDs using biochemical and electron microscopic techniques. Biochemical analysis reveals that PSD -95, a clustering molecule which anchors NMDA receptors by interaction with their NR2 subunits, as well as the NR2 subunits themselves, are cleaved by calpain. On the other hand, under conditions where all the PSD-95 protein is cleaved, actin and alpha -actinin-a protein thought to anchor NMDA recep tors to actin filaments-remain intact. For analysis by electron microscopy, PSDs were adsorbed on glass, immunogold-labeled with an antibody to PSD-95 , slam frozen, freeze dried, and rotary shadowed. Electron micrographs of r eplicas indicate that PSDs are disc-shaped and are composed of a lattice-li ke structure which labels with PSD-95 immunogold. After calpain treatment, PSDs adsorbed on glass become thinner overall and the lattice becomes fragm ented. Altogether, these results suggest that calpain activity could produc e changes in the organization of the PSD and, by cleaving PSD-95 associated with the PSD lattice, could modify the anchoring of NMDA receptors. Synaps e 40:302-309, 2001. (C) 2001 Wiley-Liss, Inc.dagger