Activin receptor-like kinase 1 (ALK1) is a receptor of the TGF-β family mainly expressed by endothelial cells and plays a crucial role in maintaining vascular quiescence. Loss-of-function mutations in the Alk1 receptor cause a rare vascular disease known as hereditary hemorrhagic telangiectasia (HHT), or Rendu-Osler disease. This disease is characterized by arteriovenous malformations (AVMs), particularly in the liver, brain, and lungs, severe epistaxis, and, in the most serious cases, heart failure requiring liver transplantation. Current treatments are limited to symptom management, with no curative solution. This disease worsens with age, reaching 100% penetrance around the age of 50.
The originality of my thesis lies in investigating the role of ALK1 in maintaining vascular quiescence in adult mice, in contrast to the majority of studies which have been conducted in neonatal mice. To do this, I used 2-month-old mice, in which I induced specific deletion of Alk1 in endothelial cells (Cre recombinase under the control of a specific promoter) by injection of tamoxifen (Alk1iECKO). These mice die quickly, eight days after Alk1 deletion, and exhibit severe pulmonary hemorrhages.
To identify the molecular mechanisms underlying these phenotypes, we performed single-cell RNA sequencing (scRNAseq) on the two primary organs affected in HHT, the liver and lungs, in WT and Alk1iECKO mice. I primarily focused on the liver, a central organ in metabolism and systemic regulation, notable for its uniquely organized vasculature and specialized functions.
My work demonstrates, for the first time that Alk1 deletion in adult mice, induces capillarization of LSECs and loss of their fenestrations, profoundly disrupting intercellular communication in the liver. Endothelial expression of Wnt genes targeting pericentral hepatocytes is reduced, leading to zonation defects, while expression of Pdgfb and Lrg1 genes targeting hepatic stellate cells are increased, promoting their activation and subsequent development of fibrosis. In parallel, decreased endothelial c-Kit expression accelerates vascular aging in the liver, impairing endothelial function. I also showed that loss of a single Alk1 allele in adult mice is sufficient to induce significant alterations in endothelial and hepatocyte zonation, that I was also able to observe in livers from HHT patients.
In conclusion, my results demonstrate that Alk1 deletion in adult endothelium induces severe hepatic phenotypes, affecting both autocrine and paracrine liver functions, and reveal, through scRNAseq analysis, novel potential therapeutic targets such as c-Kit and PDGFB, which may contribute to liver vascular malformations observed in HHT patients.

Supervision :

Sabine BAILLY

Nicolas RICARD​