Adapting the endoplasmic reticulum proteostasis rescues epilepsy-associated NMDA receptor variants

Abstract

The GRIN genes encoding N-methyl-D-aspartate receptor (NMDAR) subunits are remarkably intolerant to variation. Many pathogenic NMDAR variants result in their protein misfolding, inefficient assembly, reduced surface expression, and impaired functionality at the plasma membrane, causing neurological disorders including epilepsy and intellectual disability. Here, we concentrate on the proteostasis maintenance of NMDARs containing epilepsy-associated variations in the GluN2A (or NR2A) subunit, including M705V and A727T. We showed that these two variants are targeted to the proteasome for degradation and have reduced functional surface expression. We demonstrated that the application of BIX, a known small molecule activator of an HSP70 family chaperone BiP (Binding immunoglobulin Protein) in the endoplasmic reticulum (ER), significantly increases total and surface protein levels, and thus the function of the M705V and A727T variants in HEK293T cells. Mechanistic studies revealed that BIX promotes folding, inhibits degradation, and enhances anterograde trafficking of the M705V variant by modest activation of the IRE1 pathway of the unfolded protein response. Our results showed that adapting the ER proteostasis network restores the folding, trafficking, and function of pathogenic NMDAR variants, representing a potential treatment for neurological disorders resulting from NMDAR dysfunction.