Induced pluripotent stem cell-derived human neurons to model autoimmune encephalitis

Autoantibodies targeting proteins expressed on the cell surface of neurons can induce a group of diseases called autoimmune encephalitides, or more specifically, neuronal surface antibodymediated diseases (NSab-mediated diseases). These diseases vary in symptoms, outcome, and disease mechanisms based on which protein the antibodies target. Symptoms may be psychiatric, cognitive, or movement-related, and in severe cases, the autoantibodies can cause epileptic seizures and coma. Although patients of most subtypes generally recover well physically, long-term cognitive sequelae are common. Treatment strategies function by removing autoantibodies and stopping autoantibody production, but do not affect potential detrimental effects induced by autoantibodies on neurons. Autoantibodies against more than 20 different targets have been described to cause NSab-mediated disease, but the effects of autoantibody binding have only been studied in detail for a few types. An incomplete understanding of disease mechanisms halts the development of novel treatments.

This thesis aimed to map what is known about the molecular and cellular effects of autoantibody binding. We further aimed to evaluate the experimental evidence of pathogenicity for each type of autoantibody. We found no published evidence of four out of 21 described types of NSab-mediated disease-causative autoantibodies. For eight of the remaining 17 types, an independent research group had not verified the described pathophysiological effects of autoantibodies. For nine types of NSab-mediated disease-inducing autoantibodies, more than one independent research group had published comparable pathogenic effects, fulfilling our criteria for the highest class of experimental evidence of pathogenicity.

A further aim was to investigate the mechanisms of a type of NSab-mediated disease termed IgLON5 disease. Patients harbour antibodies against the cell adhesion molecule IgLON5 in this disease, and the outcome is particularly poor. Deposits of hyperphosphorylated Tau (pTau) have been found in the brains of these patients. We aimed to ascertain if the IgLON5 autoantibodies were causative for pTau accumulation and how IgLON5 autoantibodies affect neurons. Using a human induced pluripotent stem cell (hiPSC) model, we found that exposure to IgLON5 autoantibodies caused synaptic disruption, pTau accumulation, and increased cell death in a temporal manner. We studied the intracellular alterations caused by IgLON5 autoantibodies using large-scale mass spectrometry in combination with our stem cell model system. We found that exposure to IgLON5 autoantibodies for one day induced regulation of 111 proteins, including many proteasome subunits and mitochondrial proteins in hiPSC-derived neural cultures. From our results, we inferred that Glycogen synthase kinase 3ß (GSK-3ß) could be a central mediator of disease mechanisms. GSK-3ß phosphorylates Tau and, therefore, is a potential target for pharmaceutical intervention. By inhibiting GSK-3ß, we were able to prevent pTau accumulation and increased cell death in hiPSC-derived neural cultures exposed to IgLON5 autoantibodies.

Our results have cemented that our knowledge of NSab-mediated disease mechanisms is limited. Using IgLON5 autoantibodies, we have demonstrated that hiPSC model systems and large-scale proteomic analyses can contribute to our understanding of the disease pathways and mechanisms of NSab-mediated diseases.