Vascular mural cells protect the adult brain from haemorrhage but do not control the blood-brain barrier in developing zebrafish

The blood-brain barrier (BBB) protects the brain from circulating metabolites and plays central roles in neurological diseases. Endothelial cells (ECs) of the BBB are enwrapped by mural cells including pericytes and vascular smooth muscle cells (vSMCs) that regulate angiogenesis, vessel stability and barrier function. To explore mural cell control of the BBB, we investigated neurovascular phenotypes in zebrafishpdgfrbmutants that lack brain pericytes and vSMCs. As expected, mutants showed an altered cerebrovascular network with mis-patterned capillaries. Unexpectedly, mutants displayed no BBB leakage from larval through to young adult stages. This demonstrates that pericytes and vSMCs do not control BBB function in young zebrafish. Instead, we observed adult BBB disruption occurring at "hotspot" focal haemorrhages at major vessel aneurysms. ECs at leakage hotspots showed induction of caveolae on abluminal surfaces and major structural defects including basement membrane thickening and disruption. Our work suggests that capillary pericytes regulate cerebrovascular patterning in development and vSMCs of major arteries protect from haemorrhage and BBB breakdown in older zebrafish. The fact that young zebrafish can have a conserved BBB with intact barrier function in the absence of mural cells, warrants renewed interrogation of the paradigm of mural cell control of the BBB.