Cerebral small vessel disease (CSVD) is a leading cause of stroke and vascular cognitive impairment and dementia. Studying monogenic CSVD can reveal pathways that are dysregulated in common sporadic forms of the disease and may represent therapeutic targets. Mutations in collagen type IV alpha 1 (COL4A1) and alpha 2 (COL4A2) cause highly penetrant CSVD as part of a multisystem disorder referred to as Gould syn-drome. COL4A1 and COL4A2 form heterotrimers [a1a1a2(IV)] that are fundamental constituents of base-ment membranes. However, their functions are poorly understood and the mechanism(s) by which COL4A1 and COL4A2 mutations cause CSVD are unknown. We used histological, molecular, genetic, pharmacologi-cal, and in vivo imaging approaches to characterize central nervous system (CNS) vascular pathologies in Col4a1 mutant mouse models of monogenic CSVD to provide insight into underlying pathogenic mecha-nisms. We describe developmental CNS angiogenesis abnormalities characterized by impaired retinal vascu-lar outgrowth and patterning, increased numbers of mural cells with abnormal morphologies, altered contractile protein expression in vascular smooth muscle cells (VSMCs) and age-related loss of arteriolar VSMCs in Col4a1 mutant mice. Importantly, we identified elevated TGF8 signaling as a pathogenic conse-quence of Col4a1 mutations and show that genetically suppressing TGF8 signaling ameliorated CNS vascu-lar pathologies, including partial rescue of retinal vascular patterning defects, prevention of VSMC loss, and significant reduction of intracerebral hemorrhages in Col4a1 mutant mice aged up to 8 months. This study identifies a novel biological role for collagen a1a1a2(IV) as a regulator of TGF8 signaling and demonstrates that elevated TGF8 signaling contributes to CNS vascular pathologies caused by Col4a1 mutations. Our find-ings suggest that pharmacologically suppressing TGF8 signaling could reduce the severity of CSVD, and potentially other manifestations associated with Gould syndrome and have important translational implica-tions that could extend to idiopathic forms of CSVD. (c) 2022 Elsevier B.V. All rights reserved.