Scleraxis Regulates Arterial Smooth Muscle Phenotype to Alter Vascular Stiffness

Background: Arterial stiffness impacts vessel function, and may change due to alterations in wall components such as extracellular matrix (ECM) or vascular smooth muscle cells (VSMC). Stiffening is associated with multiple cardiovascular diseases, thus understanding its regulation is crucial for attenuating vessel pathology and function. We previously showed that scleraxis controls cardiac fibrosis-related gene expression, but its vascular role is unknown. Methods: Using a smooth muscle-specific scleraxis over-expression mouse model and angiotensin II (AngII) minipump infusion, we measured mesenteric artery physical characteristics using pressure myography and assessed vessel structure by histology. We assessed the impact of scleraxis over-expression in human aortic smooth muscle cells (HAOSMC). Results: In vivo scleraxis over-expression stimulated VSMC proliferation and increased mesenteric artery stiffness via a leftward shift of the stress vs strain curve. In vitro, scleraxis over-expression induced HAOSMC proliferation. Scleraxis knockout rightward shifted the mesentery stress vs strain curve. AngII markedly increased scleraxis expression in coronary arteries. Scleraxis over-expression with AngII infusion increased measures of mesentery stiffness, increased VSMC contractility and impaired relaxation, while unexpectedly reducing ECM gene expression and increasing contractile gene expression and media/lumen ratio, suggesting arterial remodeling. Similarly, HAOSMCs infected with scleraxis and treated with AngII demonstrated decreased proliferation, and an increased contractile phenotype. Conclusion: Scleraxis expression in VSMCs is a novel regulator of vascular stiffness via altered proliferation, with its effect varying with AngII presence. These findings reveal the first evidence of a role for scleraxis in vascular structure and function, highlighting its complex and context-dependent impact on vessel characteristics.