Objectives: Altered Wnt/beta-catenin signaling has been implicated in human heart valve disease; however, a causal relation has not been established. This study is aimed at determining the role of Wnt/beta-catenin signaling in adult valve homeostasis and disease. In endochondral bone development, Wnt/beta-catenin signaling inhibits early chondrogenesis and promotes osteogenesis. Therefore, we hypothesize that loss of Wnt/beta-catenin signaling promotes chondrogenic differentiation of valvular interstitial cells (VICs) resembling myxomatous valve disease.
Methods & Results: Periostin (Postn)Cre was used for conditional loss of beta-catenin (Ctnnb1) specifically in VICs in mice. PostnCre is active in valve mesenchymal progenitors by embryonic day (E)13.5, therefore bypassing requirements for beta-catenin in endothelial-to-mesenchymal transition and endocardial cushion formation. Histology of adult heart valves was analyzed by Masson’s Trichrome and Movat’s Pentachrome staining, and the expression of extracellular matrix (ECM) genes was examined at both mRNA and protein level. Heart valve development appears normal at both fetal and neonatal stages with loss of beta-catenin. However proteoglycan-rich nodules form in adult aortic valves (AoVs) lacking beta-catenin at 3 months with 100% penetrance. These nodular cells are enlarged and express nuclear Sox9 and chondrogenic ECM genes, including Aggrecan, Collagen type 2 (Col1) and Col10, reminiscent of hypertrophic cartilage. Fibrillar collagens 1 and 3, as well as Versican, are excluded from the nodules, accompanied by abnormal ECM deposition throughout the AoV leaflets. Likewise, treatment of cultured porcine aortic VICs with the Wnt pathway inhibitor XAV also induces Aggrecan and Col2 gene expression in vitro. This work demonstrates that beta-catenin is required for the maintenance of normal ECM deposition and composition in adult mice, and, therefore, is indispensible for adult heart valve homeostasis.
Conclusions: We have demonstrated that beta-catenin is a key factor in maintaining normal valve ECM organization in adult AoV. In the absence of beta-catenin, VICs are susceptible to chondrogenic differentiation related to myxomatous valve disease.