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

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 function on neuronal membrane, causing neurological disorders including epilepsy and intellectual disability. Here, we investigated the proteostasis maintenance of NMDARs containing epilepsy-associated variations in the GluN2A subunit, including M705V and A727T. In the transfected HEK293T cells, we showed that the two variants were targeted to the proteasome for degradation and had 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), dose-dependently enhanced the functional surface expression of the M705V and A727T variants in HEK293T cells. Moreover, BIX (10 µM) increased the surface protein levels of the M705V variant in human iPSC-derived neurons. We revealed that BIX promoted folding, inhibited degradation, and enhanced anterograde trafficking of the M705V variant by modest activation of the IRE1 pathway of the unfolded protein response. Our results suggest 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.

Pharmacological chaperones restore proteostasis of epilepsy-associated GABAA receptor variants.

Recent advances in genetic diagnosis identified variants in genes encoding GABAA receptors as causative for genetic epilepsy. Here, we selected eight disease-associated variants in the α1 subunit of GABAA receptors causing mild to severe clinical phenotypes and showed that they are loss of function, mainly by reducing the folding and surface trafficking of the α1 protein. Furthermore, we sought client protein-specific pharmacological chaperones to restore the function of pathogenic receptors. Applications of positive allosteric modulators, including Hispidulin and TP003, increase the functional surface expression of the α1 variants. Mechanism of action study demonstrated that they enhance the folding and assembly and reduce the degradation of GABAA variants without activating the unfolded protein response in HEK293T cells and human iPSC-derived neurons. Since these compounds cross the blood-brain barrier, such a pharmacological chaperoning strategy holds great promise to treat genetic epilepsy in a GABAA receptor-specific manner.

An epilepsy-associated ACTL6B variant captures neuronal hyperexcitability in a human induced pluripotent stem cell model.

ACTL6B is a component of the neuronal BRG1/brm-associated factor (nBAF) complex, which is required for chromatin remodeling in postmitotic neurons. We recently reported biallelic pathogenic variants in ACTL6B in patients diagnosed with early infantile epileptic encephalopathy, subtype 76 (EIEE-76), presenting with severe, global developmental delay, epileptic encephalopathy, cerebral atrophy, and abnormal central nervous system myelination. However, the pathophysiological mechanisms underlying their phenotype is unknown. Here, we investigate the molecular pathogenesis of ACTL6B p.(Val421_Cys425del) using in silico 3D protein modeling predictions and patient-specific induced pluripotent stem cell-derived neurons. We found neurons derived from EIEE-76 patients showed impaired accumulation of ACTL6B compared to unaffected relatives, caused by reduced protein stability. Furthermore, EIEE-76 patient-derived neurons had dysregulated nBAF target gene expression, including genes important for neuronal development and disease. Multielectrode array system analysis unveiled elevated electrophysiological activity of EIEE-76 patients-derived neurons, consistent with the patient phenotype. Taken together, our findings validate a new model for EIEE-76 and reveal how reduced ACTL6B expression affects neuronal function.