Hemodynamics regulate spatiotemporal artery muscularization in the developing circle of Willis.

Brain arteries are wrapped by vascular smooth muscle cells (VSMCs). Fully differentiated VSMCs are important for brain artery homeostasis, and they are lost in several cerebrovascular diseases. How healthy VSMCs differentiate on different brain arteries during development is unclear. Such knowledge will help regenerate lost VSMCs in brain arteriopathy. To answer this question, we studied the developmental muscularization of the zebrafish circle of Willis (CW) arteries, the major arterial loop that supplies blood to the brain in all vertebrates. We found that artery specification of CW endothelial cells (ECs) happens after they migrate from primitive veins to form CW arteries. VSMCs differentiate from pdgfrb+ common vascular mural cell progenitors at the time when embryo circulation starts and progress temporally and spatially from anterior to posterior CW arteries. Computational fluid dynamic simulation confirms that earlier VSMC differentiation coincide with higher pulsatile flow hemodynamics in anterior CW arteries. Pulsatile blood flow induces the differentiation of human brain pdgfrb+ progenitors into VSMCs and reducing pulsatile blood flow by blocking the zebrafish embryo heartbeat after pdgfrb+ recruitment but before VSMC differentiation limits the number of mature VSMCs. Congruently, the flow responsive transcription factor klf2a is activated in ECs before VSMC differentiation and knockdown delays VSMC differentiation on CW arteries. Overall, our data place hemodynamic activation of endothelial klf2a signaling as key determinant of spatiotemporal VSMC differentiation on CW arteries.