By revealing that ALS/FTD mutations drive cell-autonomous effects in the endothelium and that specific targeting of TDP-43 in brain endothelial cells in vivo results in rapid increases in BBB permeability, our data implicates this pathway in BBB disruption in ALS, FTD and perhaps also the many other situations in which both TDP-43 and the BBB are disrupted, including traumatic brain injury, Alzheimer’s disease and Huntington’s disease. Here, TARDBP is linked to early-onset autosomal dominant Alzheimer disease.