Loss of flow responsive Tie1 results in Impaired Aortic valve remodeling

The mechanisms regulating endothelial cell response to hemodynamic forces required for heart valve develop- ment, especially valve remodeling, remain elusive. Tie1, an endothelial specific receptor tyrosine kinase, is up- regulated by oscillating shear stress and is required for lymphatic valve development. In this study, we demon- strate that valvular endothelial Tie1 is differentially expressed in a dynamic pattern predicted by disturbed flow during valve remodeling. Following valvular endocardial specific deletion of Tie1 in mice, we observed enlarged aortic valve leaflets, decreased valve stiffness and valvular insufficiency. Valve abnormalities were only detected in late gestation and early postnatal mutant animals and worsened with age. The mutant mice developed per- turbed extracellular matrix (ECM) deposition and remodeling characterized by increased glycosaminoglycan and decreased collagen content, as well as increased valve interstitial cell expression of Sox9, a transcription factor essential for normal ECM maturation during heart valve development. This study provides the first evidence that Tie1 is involved in modulation of late valve remodeling and suggests that an important Tie1-Sox9 signaling axis exists through which disturbed flows are converted by endocardial cells to paracrine Sox9 signals to modulate normal matrix remodeling of the aortic valve. 1.