Vascular signals coordinate cerebellar circuitry development.

Neural circuit assembly requires precise coordinated interactions between developing neurons and the vasculature; yet, the instructive signals provided by endothelial cells remain largely unknown. Here, we find a vascular-to-neural signaling axis that orchestrates postnatal cerebellar development. Endothelial-specific deletion of the adapter protein Dab1 (Dab1iΔEC) in mice disrupts vascular patterning and uncouples the growth of major cerebellar neuronal populations. We show that endothelial Dab1 in the cerebellum drives secretion of the morphogen Wnt5a, which acts through Frizzled-2 to restrain granule-cell progenitor proliferation and promotes Purkinje cell dendritic maturation. Endothelial Wnt5a deletion phenocopies Dab1iΔEC defects, whereas exogenous Wnt5a restores normal progenitor dynamics in Dab1iΔEC cerebellar slices, demonstrating pathway sufficiency. Functionally, loss of this vascular signal impairs Purkinje cell firing, diminishes parallel-fiber input, reduces synapse formation from both parallel and climbing fibers, and disrupts long-term plasticity. These findings indicate a key instructive role for blood vessels in shaping cerebellar architecture and establishing functional circuit connectivity.