Progression of experimental autoimmune encephalomyelitis in mice and neutrophil-mediated blood-brain barrier dysfunction requires non-muscle myosin light chain kinase

Blood-brain barrier (BBB) dysfunction occurs in numerous central nervous system disorders. Unfortunately, a limited understanding of the mechanisms governing barrier function hinders the identification and assessment of BBB-targeted therapies. Previously, we found that non-muscle myosin light chain kinase (nmMLCK) negatively regulates the tight junction protein claudin-5 in brain microvascular endothelial cells (BMVECs) under inflammatory conditions. Here, we used complementary animal and primary cell co-culture models to further investigate nmMLCK and claudin-5 during neuroinflammation. We found that <i>nmMLCK</i>-knockout mice resisted experimental autoimmune encephalomyelitis (EAE), including paralysis, demyelination, neutrophil infiltration, and BBB dysfunction. However, transiently silencing claudin-5 culminated in a fulminant disease course. In parallel, we found that neutrophil-secreted factors triggered a biphasic loss in the barrier quality of wild-type BMVEC monolayers, plus pronounced neutrophil migration during the second phase. Conversely, <i>nmMLCK</i>-knockout monolayers resisted barrier dysfunction and neutrophil migration. Lastly, we found an inverse relationship between claudin-5 expression in BMVECs and neutrophil migration. Overall, our findings support a pathogenic role for nmMLCK in BMVECs during EAE that includes BBB dysfunction and neutrophil infiltration, reveal that claudin-5 contributes to the immune barrier properties of BMVECs, and underscore the harmful effects of claudin-5 loss during neuroinflammation.