Endothelial <i>TIE2</i> Mutation Induced Contraction Deficiency of Vascular Smooth Muscle Cells via Phenotypic Transition Regulation in Venous Malformations

<b>Introduction:</b> Venous malformations (VMs) are congenital vascular malformations characterized by venous cavity enlargement and malformation. Although <i>TIE2</i> mutation is a recognized genetic landscape in VMs, the regulatory role of <i>TIE2</i> in vascular smooth muscle cell (VSMC) contraction remains unclear. <b>Materials and Methods:</b> We generated <i>Tie2-R848W^fl/fl;Tie2^Cre+</i> and <i>Tie2-R848W^fl/fl;Apln^ER+</i> mice through specific expression of <i>Tie2-R848W</i>, a typical mutation in inherited VM, in endothelial cells (ECs). Histological and transcriptome sequencing analyses were performed on vascular abnormalities in the mutant mouse model. Postnatal vascular development <i>in vivo</i> was studied through morphometric analysis of the retinal vasculature. Under <i>in vitro</i> coculture conditions, the functional abnormality of VSMCs was studied using transwell analysis, proliferation analysis, a cell contraction assay and transcriptome sequencing analysis. Markers related to the VSMC phenotypic transition were analyzed via western blotting and quantitative RT‑PCR. <b>Results:</b> <i>Tie2-R848W^fl/fl;Tie2^Cre+</i> mice developed spontaneous pulmonary vascular malformations displaying internal hemorrhage and increased vasculature with α-SMA+ enveloped VSMCs. In <i>Tie2-R848W^fl/fl;Apln^ER+</i> mice, <i>Tie2-R848W</i> mutation also induced postnatal retinal vascular malformations (higher vascular density and coverage of α-SMA+ VSMCs). According to phenotypes and molecular markers (Acta2, Cnn1, Sm22a and Opn), dysregulated phenotypic transition of VSMCs might be the pathogenic basis. Under <i>in vitro</i> coculture condition, the decreased contractile ability of synthetic VSMCs was significant in the mutant group, while downregulated ion transmembrane transport and TNFSF10 may play substantial roles in initiating this process. <b>Conclusion</b>: Endothelial <i>TIE2</i> mutation might induce an abnormal EC-VSMC regulatory relationship strongly associated with phenotypic transition of VSMCs. Weakened contractility and abnormal proliferation induce chronic cavity expansion and thickening of the muscle layer, which may be potential mechanism basis of VMs.