Project Summary / Abstract Intraventricular hemorrhage (IVH) is a debilitating condition at any age but is especially common and devastating in premature infants as it is associated with substantial long-term morbidity (e.g., cerebral palsy, cognitive deficits) and mortality in this population. In premature infants, most IVH stems from hemorrhage in the germinal matrix (GM), a collection of highly vascularized neuronal and glial precursor located underneath the ventricular ependyma. Pericytes (PCs) and endothelial cells (ECs) are key components of the blood-brain barrier (BBB). In the GM, reduction in the numbers of PCs and levels of the ligand transforming growth factor (TGF)-β1 in comparison to the white matter and cortex has been implicated in the high propensity of the GM to hemorrhage. Indeed, we previously reported that deletion of the gene encoding activin receptor-like kinase 5 (ALK5), a type I TGFβ receptor, in PCs leads to gross GM hemorrhage (GMH)-IVH in embryonic mice (Dev Cell, 44:665) largely through effects on ECs. However, mechanisms underlying these non-cell autonomous effects on ECs are incompletely defined. To search for PC-derived factors that may signal to ECs, we conducted bulk RNA-sequencing of human brain PCs with and without both siRNA Alk5 knockdown and TGFβ1 treatment and identified the secreted factor angiopoietin (Angpt) 2. ANGPT2, which is broadly implicated in EC tube destabilization and/or remodeling, is predominantly expressed by ECs, but PC expression has not been reported. Moreover, the role ANGPT2, and certainly PC-derived ANGPT2, in GMH- IVH has not been studied. Follow-up studies confirmed that TGFβ1 treatment of human brain PCs rapidly and robustly down-regulates levels of Angpt2 mRNA (~65% and ~90% reduction in 3 h and 12 h, respectively) and protein in an ALK5-dependent manner. This rapid reduction in Angpt2 mRNA suggests agonist-induced transcript destabilization, which we will investigate in the proposed experiments. Furthermore, chromatin immunoprecipitation assays revealed that TGFβ1 treatment of PCs promotes epigenetic and transcriptional repression of Angpt2, increasing histone deacetylase-2 binding and reducing histone acetylation (H3K9ac, H3K27ac) and RNA polymerase II binding to the proximal promoter. Finally, in mice with Alk5 deletion in PCs, perivascular ANGPT2 expression is markedly increased. Thus, we hypothesize that when TGFβ-ALK5 signaling in PCs is disrupted, ANGPT2 levels are increased, destabilizing ECs and leading to disruption of the BBB and GMH-IVH. We will use cultured human brain vascular cells, transgenic mice, proteomic and 3’UTR/microRNA screens and de-identified human brain samples to test this hypothesis in two specific aims: 1) determine mechanisms of TGFβ-mediated regulation of Angpt2 in PCs and effects on ECs; and 2) elucidate role of PC ANGPT2 in GMH-IVH of mice and humans. This R21 will yield key insights into BBB formation and GMH-IVH pathogenesis and form the foundatio...