We have extensively investigated since the inception of this grant in 1972 the initial formation of valvulogenic mesenchymal progenitor cells called cushion tissue. However, once formed, very little is known about cushions become valves. Hypotheses are lacking both as to the morphogenetic processes of cushion remodeling and the potential mechanisms regulating them. Yet, valvular defects are among the most common and serious of all congenital heart defects whose implications may extend into adult life, predisposing the valve to diseases or disorders that compromise their hemodynamic functions (e.g. stenosis, prolapse or calcification). The central questions to be addressed are (i) why cushion mesenchyme normally differentiates into the fibrous connective tissues of mature valve leaflets when it has the potential to form other mesodermal phenotypes, (ii) if cushion differentiation is linked functionally to changes in biomechanical properties (iii) if cushion cells inductively interact with their adjacent myocardium to promote morphogenetic remodeling processes required for leaflet delamination and formation of valve supportive structures. A focus will be on a candidate gene periostin-based on progress made during the current funding period that indicates periostin can have both signaling and scaffold roles in cushion morphogenesis. The Specific Aims are based on the valve phenotype of mice deficient in periostin: AIM 1: To test the hypotheses that post-EMT cushion mesenchymal cells promote their own differentiation into valvular interstitial fibroblasts by secreting periostin. We will also test this "pro-fibrogenic" hypothesis by asking if abnormal patterns of differentiation (e.g. chondrogenic, cardiomyogenic) are initiated if AIM 2: To determine if the organization of the extracellular matrix is regulated by periostin as reflected by changes in the biomechanical and functional properties of the cushions during development. A Key question to pursued will be whether any functional changes in the viscoelastic properties of developing cushions/valves can be mechanistically linked to periostin effects upon compaction of the ECM to form mature valve cusps. AIM 3: To test the hypothesis that periostin (secreted by cushion cells) and bigH3 (secreted by myocardial cells) induce changes in myocardial celhcell adhesion that promote separation of cushion tissue from the myocardium triggering a cascade of morphogenetic remodeling events that initiate delamination, loss of myocardial junctional continuity and formation of the tension supportive apparatus.