NMDA receptor-dependent recruitment of calnexin to the neuronal plasma membrane.

Calnexin is a molecular chaperone that resides in the endoplasmic reticulum and participates in the folding and assembly of nascent proteins. In the present study, calnexin was found in both synaptic and non-synaptic membrane components of rat brain tissue. Immunohistochemical staining of mouse hippocampal sections revealed the presence of calnexin in the neuronal cell soma, as well as dendrite-enriched regions. Staining of permeabilized cultured rat hippocampal neurons with anti-calnexin antibody produced intense staining throughout the cytoplasm of the cell body and dendrites. In non-permeabilized cells, calnexin was found on the surface of the cell body and dendrites. To further confirm the surface localization of calnexin, cell surface proteins were selectively labeled with a membrane-impermeable biotinylation reagent. Calnexin and other plasma membrane proteins including N-methyl-D-aspartate (NMDA) receptor and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor were biotinylated, and the amount of calnexin on the plasma membrane markedly increased after NMDA receptor activation. These results suggest that a significant fraction of calnexin localizes to the neuronal cell membrane, and that this recruitment is regulated in an NMDA receptor-dependent manner. Moreover, immunoisolation of vesicles revealed co-localization of the AMPA receptor subunit, GluA2, and calnexin in post-endoplasmic reticulum intracellular membrane components. These findings provide support for the hypothesis that calnexin may play a role in NMDA receptor-dependent neuronal functions.