ALK1 controls hepatic vessel formation, angiodiversity, and angiocrine functions in hereditary hemorrhagic telangiectasia of the liver

Abstract

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-KOmice 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-KOmice and in liver specimens of patients with HHT. Furthermore, prion-like protein doppel (Prnd) and placental growth factor (Pgf) were upregulated in Alk1 HEC-KOhepatic 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-KOHHT model offers opportunities to develop targeted therapies for this severe disease.