Contribution of Extra-Cardiac Cells in Murine Heart Valves is Age-Dependent.

BACKGROUND: Heart valves are dynamic structures that open and close over 100 000 times a day to maintain unidirectional blood flow during the cardiac cycle. Function is largely achieved by highly organized layers of extracellular matrix that provide the necessary biomechanical properties. Homeostasis of valve extracellular matrix is mediated by valve endothelial and interstitial cell populations, and although the embryonic origins of these cells are known, it is not clear how they are maintained after birth. The goal of this study is to examine the contribution of extracardiac cells to the aortic valve structure with aging using lineage tracing and bone marrow transplantation approaches. METHODS AND RESULTS: Immunohistochemistry and fate mapping studies using CD45-Cre mice show that the contribution of hematopoietic-derived cells to heart valve structures begins during embryogenesis and increases with age. Short-term (6 weeks), CD45-derived cells maintain CD45 expression and the majority coexpress monocyte markers (CD11b), whereas coexpression with valve endothelial (CD31) and interstitial (Vimentin) cell markers were infrequent. Similar molecular phenotypes are observed in heart valves of irradiated donor mice following transplantation of whole bone marrow cells, and engraftment efficiency in this tissue is age-dependent. CONCLUSIONS: Findings from this study demonstrate that the percentage of CD45-positive extracardiac cells reside within endothelial and interstitial regions of heart valve structures increases with age. In addition, bone transplantation studies show that engraftment is dependent on the age of the donor and age of the tissue environment of the recipient. These studies create a foundation for further work defining the role of extracardiac cells in homeostatic and diseased heart valves.

Valve Endothelial Cell-Derived Tgfß1 Signaling Promotes Nuclear Localization of Sox9 in Interstitial Cells Associated With Attenuated Calcification.

OBJECTIVE: Aortic valve disease, including calcification, affects >2% of the human population and is caused by complex interactions between multiple risk factors, including genetic mutations, the environment, and biomechanics. At present, there are no effective treatments other than surgery, and this is because of the limited understanding of the mechanisms that underlie the condition. Previous work has shown that valve interstitial cells within the aortic valve cusps differentiate toward an osteoblast-like cell and deposit bone-like matrix that leads to leaflet stiffening and calcific aortic valve stenosis. However, the mechanisms that promote pathological phenotypes in valve interstitial cells are unknown. APPROACH AND RESULTS: Using a combination of in vitro and in vivo tools with mouse, porcine, and human tissue, we show that in valve interstitial cells, reduced Sox9 expression and nuclear localization precedes the onset of calcification. In vitro, Sox9 nuclear export and calcific nodule formation is prevented by valve endothelial cells. However, in vivo, loss of Tgfß1 in the endothelium leads to reduced Sox9 expression and calcific aortic valve disease. CONCLUSIONS: Together, these findings suggest that reduced nuclear localization of Sox9 in valve interstitial cells is an early indicator of calcification, and therefore, pharmacological targeting to prevent nuclear export could serve as a novel therapeutic tool in the prevention of calcification and stenosis.

Dynamic Heterogeneity of the Heart Valve Interstitial Cell Population in Mitral Valve Health and Disease.

The heart valve interstitial cell (VIC) population is dynamic and thought to mediate lay down and maintenance of the tri-laminar extracellular matrix (ECM) structure within the developing and mature valve throughout life. Disturbances in the contribution and distribution of valve ECM components are detrimental to biomechanical function and associated with disease. This pathological process is associated with activation of resident VICs that in the absence of disease reside as quiescent cells. While these paradigms have been long standing, characterization of this abundant and ever-changing valve cell population is incomplete. Here we examine the expression pattern of Smooth muscle α-actin, Periostin, Twist1 and Vimentin in cultured VICs, heart valves from healthy embryonic, postnatal and adult mice, as well as mature valves from human patients and established mouse models of disease. We show that the VIC population is highly heterogeneous and phenotypes are dependent on age, species, location, and disease state. Furthermore, we identify phenotypic diversity across common models of mitral valve disease. These studies significantly contribute to characterizing the VIC population in health and disease and provide insights into the cellular dynamics that maintain valve structure in healthy adults and mediate pathologic remodeling in disease states.