Structural basis and functional analysis of NMDA receptor regulation by calmodulin

TL;DR

This paper explores how calmodulin interacts with NMDA receptors to regulate calcium-dependent channel desensitization, using structural and functional experiments.

Contribution

The study reveals novel structural interactions between calmodulin and specific NMDA receptor subunit regions that are crucial for channel desensitization.

Findings

GluN1-C0 binds to both N-lobe and C-lobe of Ca2+-CaM, while GluN2A-C0 binds only to the C-lobe.

Mutations in GluN1 and GluN2A residues disrupt calmodulin binding and reduce calcium-dependent desensitization.

A structural model suggests four calmodulin molecules bind per NMDA receptor tetramer to induce desensitization.

Abstract

The synaptic plasticity mechanisms that are thought to underlie learning and memory require Ca2+ influx mediated by N-methyl-D-aspartate receptors composed of glycine-binding GluN1 and glutamate-binding GluN2 subunits. Calmodulin (CaM) binding to the cytosolic regions in both GluN1 (residues 841–865, called GluN1-C0) and GluN2A (residues 1004–1023, called GluN2A-C0) may be important for Ca2+-dependent channel desensitization (CDD). Here, we report NMR, ITC and electrophysiological experiments to probe the structure and functional role of Ca2+-bound CaM (Ca2+-CaM) binding to both GluN1 and GluN2A subunits. Our ITC studies show that the GluN1-C0 peptide binds to both the N-lobe and C-lobe of Ca2+-CaM, whereas the GluN2A-C0 peptide binds to only the Ca2+-CaM C-lobe. Our NMR analysis reveals GluN2A residues (W1014 and V1018) interact with exposed hydrophobic residues in the Ca2+-CaM C-lobe. The NMR structure of Ca2+-CaM bound to the GluN1-C0 peptide indicates the two CaM lobes bind to opposite sides of the GluN1-C0 helix (C-lobe contacts M848, F852, A853 and N-lobe contacts A854, V855, W858). The GluN1 mutant F852E and the GluN2A mutant W1014E both perturbed CaM binding in ITC studies, and also diminished electrophysiologically-measured CDD, suggesting CaM interaction with these residues contributes to CDD. We propose a structural mechanism of CDD wherein channel desensitization is caused by the binding of four CaM per N-methyl-D-aspartate receptor subunit tetramer.

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