Glutamate receptor autoimmune diseases: Understanding the mechanisms, determining the epitopes, and developing better tests

Anti-NMDA receptor and anti-AMPA receptor encephalitis are two recently discovered autoimmune disorders in which patients develop antibodies to ionotropic glutamate receptors. These antibodies cause varied and severe neurologic and psychiatric symptoms. Both disorders are treatable with immunosuppression. Little is know about the mechanisms by which these antibodies cause the disorders or the sites to which antibodies bind on their target receptors. Here, we use a variety of molecular biological, biochemical, and electrophysiological techniques to study the binding site of patients\u27 antibodies and the effects of antibody binding, with the dual goals of determining how antibodies cause disease and developing better methods to test antibodies. Anti-NMDAR encephalitis antibodies bind to the bottom lobe of the amino terminal domain (ATD) of the GluN1 subunit of the NMDA receptor and show specific dependence on amino acid identity of a small region within the bottom lobe. Antibody binding is dependent on ATD conformation, and stabilizes the open conformation of the receptor. Within three hours of binding and continuing in severity over twenty-four hours, antibody causes a titer-dependent decrease in surface receptor expression. This decrease is mediated by proteasomal activity and partially mitigated by increased protein synthesis, and causes an increase in basal calcium concentration within neurons. Similarly, anti-AMPAR encephalitis cerebrospinal fluid (CSF) antibodies bind to the bottom lobe of the ATD of the GluA1 or GluA2 subunits. These antibodies appear to be less conformation sensitive, as denatured fusion proteins expressing AMPAR domains can be recognized by patient antibody on western blot. Fusion proteins also reveal significant variability between the antibody response of serum and CSF, and the serum antibody population appears to change over disease progression. The consequences of this serum variability are unknown; fusion proteins may represent an ideal tool to explore this question. Together, these results significantly increase our understanding of anti-NMDAR and anti-AMPAR encephalitis, although considerable work remains to fully understand these complex and evolving disorders