Investigating Developmental Cues in Valvulogenesis
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Heart valve development proceeds through coordinated steps by which endocardial cushions (ECs) form thin, elongated, and stratified valves. Wnt signaling and its canonical effector β-catenin are proposed to contribute to endocardial-to-mesenchymal transformation (EMT) through postnatal steps of valvulogenesis. However, genetic redundancy and lethality have made it challenging to define specific roles of the canonical Wnt pathway at different stages of valve formation. We developed a transgenic mouse system that provides spatiotemporal inhibition of Wnt/β-catenin signaling by chemically-inducible overexpression of Dkk1. Unexpectedly, this approach indicates canonical Wnt signaling is required for EMT in the proximal outflow tract (pOFT) but not atrioventricular canal (AVC) cushions. Further, Wnt indirectly promotes pOFT EMT through its earlier activity in neighboring myocardial cells or their progenitors. Subsequently, Wnt/β-catenin signaling is activated in cushion mesenchymal cells where it supports FGF-driven expansion of ECs and then AVC valve extracellular matrix patterning. Mice lacking Axin2, a negative Wnt regulator, have larger valves, suggesting that accumulating Axin2 in maturing valves represents negative feedback that restrains tissue overgrowth rather than simply reporting Wnt activity. Disruption of these Wnt/β-catenin signaling roles that enable developmental transitions during valvulogenesis could account for common congenital valve defects. This work suggests that Wnt/β-catenin maintains a subpopulation of valve mesenchyme in a less-differentiated, progenitor-like state that allows these cells to respond to mitogens and morphogens. The coordinated interplay of signals with distinct effects on a “progenitor cell” pool is a common logic mechanism for balanced tissue growth and differentiation in many biological contexts. Inspired by epithelial organ homeostasis processes, we identified specific and dynamic expression of the well-established quiescent stem cell marker Lrig1 in the developing valves. Endocardial Lrig1 likely moderates ErbB2 levels and thereby signaling output to prevent excessive EMT and resulting pathologically enlarged valves. Finally, we use Cre-mediated lineage labeling to show that the cusps of the semilunar valves have differential mesenchymal origins and that the localization of said distinct mesenchyme may account for the discretely patterned extracellular matrix of mature valves. This dissertation includes previously published and unpublished coauthored material.