Regulation of brain endothelial cell physiology by the TAM receptor tyrosine kinase Mer

The receptor tyrosine kinase Mer (gene name Mertk) acts in vascular endothelial cells (ECs) to tighten the blood-brain barrier (BBB) subsequent to viral infection. Correspondingly, we found that Mer regulates the expression and activity of a large cohort of cytoskeletal and BBB proteins, together with endothelial nitric oxide synthase, in brain ECs. We further found that Mer controls the expression of multiple angiogenic genes, and that EC-specific Mertk gene inactivation results in perturbed vascular sprouting and a compromised BBB after induced photothrombotic stroke. Unexpectedly, stroke lesions in the brain were also markedly reduced in the absence of EC Mer, which was linked to reduced plasma expression of fibrinogen, prothrombin, and other effectors of blood coagulation. Together, these results demonstrate that Mer is a central regulator of angiogenesis, BBB integrity, and blood coagulation in the mature vasculature. They may also account for disease severity following infection with the coronavirus SARS-CoV-2. Primary brain endothelial cells from WT and Mertk-/- mice were treated with the Mer inhibitor UNC2025 or vehicle alone (PBS) for 20h, after which RNA was harvested and used for bulk RNAseq