ABSTRACT
Autosomal dominant mutations in GRIN2B are associated with severe encephalopathy, but little is known about the pathophysiological outcomes and any potential therapeutic interventions. Genetic studies have described the association between de novo mutations of genes encoding the subunits of the N-methyl-d-aspartate receptor (NMDAR) and severe neurological conditions. Here, we evaluated a missense mutation in GRIN2B, causing a proline-to-threonine switch (P553T) in the GluN2B subunit of NMDAR, which was found in a 5-year-old patient with Rett-like syndrome with severe encephalopathy. Structural molecular modeling predicted a reduced pore size of the mutant GluN2B-containing NMDARs. Electrophysiological recordings in a HEK-293T cell line expressing the mutated subunit confirmed this prediction and showed an associated reduced glutamate affinity. Moreover, GluN2B(P553T)-expressing primary murine hippocampal neurons showed decreased spine density, concomitant with reduced NMDA-evoked currents and impaired NMDAR-dependent insertion of the AMPA receptor subunit GluA1 at stimulated synapses. Furthermore, the naturally occurring coagonist d-serine restored function to GluN2B(P553T)-containing NMDARs. l-Serine dietary supplementation of the patient was hence initiated, resulting in the increased abundance of d-serine in the plasma and brain. The patient has shown notable improvements in motor and cognitive performance and communication after 11 and 17 months of l-serine dietary supplementation. Our data suggest that l-serine supplementation might ameliorate GRIN2B-related severe encephalopathy and other neurological conditions caused by glutamatergic signaling deficiency.
Subject(s)
Brain Diseases , Dietary Supplements , Loss of Function Mutation , Receptors, N-Methyl-D-Aspartate , Rett Syndrome , Serine , Animals , Brain Diseases/drug therapy , Brain Diseases/genetics , Brain Diseases/metabolism , Brain Diseases/pathology , Child , Cognition/drug effects , Humans , Male , Mice , Models, Molecular , Motor Activity/drug effects , Motor Activity/genetics , N-Methylaspartate/pharmacology , Receptors, N-Methyl-D-Aspartate/chemistry , Receptors, N-Methyl-D-Aspartate/genetics , Receptors, N-Methyl-D-Aspartate/metabolism , Rett Syndrome/drug therapy , Rett Syndrome/genetics , Rett Syndrome/metabolism , Rett Syndrome/pathology , Serine/administration & dosage , Serine/pharmacokineticsABSTRACT
Previous studies have shown that the skeletal dihydropyridine receptor (DHPR) pore subunit Ca(V)1.1 (alpha1S) physically interacts with ryanodine receptor type 1 (RyR1), and a molecular signal is transmitted from alpha1S to RyR1 to trigger excitation-contraction (EC) coupling. We show that the beta-subunit of the skeletal DHPR also binds RyR1 and participates in this signaling process. A novel binding site for the DHPR beta1a-subunit was mapped to the M(3201) to W(3661) region of RyR1. In vitro binding experiments showed that the strength of the interaction is controlled by K(3495)KKRR_ _R(3502), a cluster of positively charged residues. Phenotypic expression of skeletal-type EC coupling by RyR1 with mutations in the K(3495)KKRR_ _R(3502) cluster was evaluated in dyspedic myotubes. The results indicated that charge neutralization or deletion severely depressed the magnitude of RyR1-mediated Ca(2+) transients coupled to voltage-dependent activation of the DHPR. Meantime the Ca(2+) content of the sarcoplasmic reticulum was not affected, and the amplitude and activation kinetics of the DHPR Ca(2+) currents were slightly affected. The data show that the DHPR beta-subunit, like alpha1S, interacts directly with RyR1 and is critical for the generation of high-speed Ca(2+) signals coupled to membrane depolarization. These findings indicate that EC coupling in skeletal muscle involves the interplay of at least two subunits of the DHPR, namely alpha1S and beta1a, interacting with possibly different domains of RyR1.