Manuel Winkler

Fibrosis is a hallmark of chronic disease. Although fibroblasts are involved, it is unclear to what extent endothelial cells also might contribute. We detected increased expression of the transcription factor Sox9 in endothelial cells in several different mouse fibrosis models. These models included systolic heart failure induced by pressure overload, diastolic heart failure induced by high-fat diet and nitric oxide synthase inhibition, pulmonary fibrosis induced by bleomycin treatment, and liver fibrosis due to a choline-deficient diet. We also observed up-regulation of endothelial SOX9 in cardiac tissue from patients with heart failure. To test whether SOX9 induction was sufficient to cause disease, we generated mice with endothelial cell–specific overexpression of Sox9 , which promoted fibrosis in multiple organs and resulted in signs of heart failure. Endothelial Sox9 deletion prevented fibrosis and organ dysfunction in the two mouse models of heart failure as well as in the lung and liver fibrosis mouse models. Bulk and single-cell RNA sequencing of mouse endothelial cells across multiple vascular beds revealed that SOX9 induced extracellular matrix, growth factor, and inflammatory gene expression, leading to matrix deposition by endothelial cells. Moreover, mouse endothelial cells activated neighboring fibroblasts that then migrated and deposited matrix in response to SOX9, a process partly mediated by the secreted growth factor CCN2, a direct SOX9 target; endothelial cell–specific Sox9 deletion reversed these changes. These findings suggest a role for endothelial SOX9 as a fibrosis-promoting factor in different mouse organs during disease and imply that endothelial cells are an important regulator of fibrosis.

Background and Aims: In hereditary hemorrhagic telangiectasia (HHT), severe liver vascular malformations are associated with mutations in the Activin A Receptor‐Like Type 1 ( ACVRL1 ) gene encoding ALK1, the receptor for bone morphogenetic protein (BMP) 9/BMP10, which regulates blood vessel development. Here, we established an HHT mouse model with exclusive liver involvement and adequate life expectancy to investigate ALK1 signaling in liver vessel formation and metabolic function. Approach and Results: Liver sinusoidal endothelial cell (LSEC)‐selective Cre deleter line, Stab2‐iCreF3 , was crossed with Acvrl1 ‐floxed mice to generate LSEC‐specific Acvrl1 ‐deficient mice ( Alk1 HEC‐KO ). Alk1 HEC‐KO mice revealed hepatic vascular malformations and increased posthepatic flow, causing right ventricular volume overload. Transcriptomic analyses demonstrated induction of proangiogenic/tip cell gene sets and arterialization of hepatic vessels at the expense of LSEC and central venous identities. Loss of LSEC angiokines Wnt2 , Wnt9b , and R‐spondin‐3 ( Rspo3 ) led to disruption of metabolic liver zonation in Alk1 HEC‐KO mice and in liver specimens of patients with HHT. Furthermore, prion‐like protein doppel ( Prnd ) and placental growth factor ( Pgf ) were upregulated in Alk1 HEC‐KO hepatic endothelial cells, representing candidates driving the organ‐specific pathogenesis of HHT. In LSEC in vitro , stimulation or inhibition of ALK1 signaling counter‐regulated Inhibitors of DNA binding (ID)1–3, known Alk1 transcriptional targets. Stimulation of ALK1 signaling and inhibition of ID1–3 function confirmed regulation of Wnt2 and Rspo3 by the BMP9/ALK1/ID axis. Conclusions: Hepatic endothelial ALK1 signaling protects from development of vascular malformations preserving organ‐specific endothelial differentiation and angiocrine signaling. The long‐term surviving Alk1 HEC‐KO HHT model offers opportunities to develop targeted therapies for this severe disease.