Modeling Flow-Responsive Gene Regulation In Native Valve Tissue

AbstractA variety of approaches to heart valve tissue engineering are underway by many groups of investigators. We believe that a thorough understanding of the biophysical and biomolecular interactions that mediate normal leaflet homeostasis will inform improved approaches to the engineering of truly regenerative heart valve grafts. However, our understanding of the fundamental biology of normal valve tissue is still quite limited. This informational gap has led us to develop an ex vivo system for long-term valve culture in order to study the mechanobiology of native tissue. Using this system, we have cultured trileaflet rat heart valves with and without flow-induced valve cycling. We report here the preliminary findings about the effects of both ex vivo culture and of valve cycling on pulmonary leaflet cell gene expression following 7 days of flow culture. Expression of leaflet mRNA was queried using Affymetrix whole genome microarrays and the results analyzed using GeneSpring, EXPLAIN and MetaCore (GeneGO) software. Our analysis revealed 2147 genes whose expression was altered under flow conditions, and 1918 genes under static conditions. 1501 genes were common to both conditions. The effects of culture on gene expression affected multiple pathways, but the most active gene groups involved pathways for cytoskeletal remodeling, development, and cell adhesion. The genes involved in these pathways were altered in both culture conditions. Changes in cytoskeletal and ECM remodeling genes were more prominent in the static condition, while those involved in VEGF signaling were seen under flow condition. Our previously reported histological analyses of cultured rat pulmonary valve leaflet tissue revealed that flow promotes the maintenance of tissue integrity while the lack of flow leads to extracellular matrix remodeling and fibrinoid tissue formation. These gene expression analyses confirm the expected role of flow in maintaining leaflet tissue integrity as evidenced by its induction of VEGF signaling genes, while the lack of flow induces changes in genes involved in extracellular matrix and cytoskeletal remodeling.