The secreted neuronal signal Spock1 regulates the blood-brain barrier
The blood-brain barrier (BBB) is a unique set of properties of the brain vasculature which severely restricts its permeability to proteins and small molecules. Classic chick-quail chimera studies showed that these properties are not intrinsic to the brain vasculature but rather are induced by surrounding neural tissue. Here we identify Spock1 as a candidate neuronal signal for regulating BBB permeability in zebrafish and mice. Mosaic genetic analysis shows that neuronally-expressed Spock1 is cell non-autonomously required for a functional BBB. Leakage in spock1 mutants is associated with altered extracellular matrix (ECM), increased endothelial transcytosis, and altered pericyte-endothelial interactions. Furthermore, a single dose of recombinant SPOCK1  into spock1 mutants quenches gelatinase activity, restores vascular expression of BBB genes including mcamb,  and partially restores barrier function. These analyses support a model in which neuronally secreted Spock1 induces BBB properties by altering the ECM, thereby regulating pericyte-endothelial interactions and downstream vascular gene expression. Overall design: Bulk RNAseq (Libraries 66-71) of leaky mutant and wild type siblings to identify the genetic lesion responsible for the leaky phenotype. These analyses revealed linkage to chr14, and more specifically to the spock1 gene. scRNAseq (Library scDRBrain) of dissected spock1 mutant and wild type brains was then used to identify all cell-type specific changes in gene expression in the mutant background.