Shandilya, R.;Childs, S.

Bradycardia occurs when the heart rate is lower than normal resulting in reduced cerebral blood flow and contributing to neurodegeneration in adults but how it affects embryonic cerebrovascular development is not well studied. We induce bradycardia by targeting the heart pacemaker channel Hcn4 via chemical (ivabradine) and genetic ( hcn4 mutant) methods. Bradycardia results in reduced brain vessel diameter and mural cell (pericyte and vascular smooth muscle cell) number. Endothelial cells are the first responders in sensing changes in blood flow, and we show that signalling through the canonical endothelial-autonomous mechanosensitive pathway (Piezo1, Mek5, Erk5, Klf2) is reduced in bradycardia. To identify the ligand-receptor combination that transmits signals to developing mural cells, we show that expression of the Notch ligand jagged2b is decreased in the brain of both hcn4 and klf2 mutants. jag2b knockdown reduces mural cell numbers in brain vessels. Restoring jag2b levels increases mural cell numbers in both wildtype and hcn4 mutants. Our work connects bradycardia, mechanosensitive signaling and mural cell recruitment demonstrating that mural cell numbers can be increased in bradycardia by restoring Notch signalling via upregulating endothelial Jag2b. Summary Bradycardia models show reduced blood flow, Piezo1- klf2-jag2b - notch3 mechanosensing and mural cell recruitment to developing brain vasculature. Restoration of jag2, an endogenous endothelial cell ligand, rescues mural cell numbers in bradycardia mutants.