Hydrodynamic forces regulate ECM protein expression and deposition during cardiac valve development

Abstract

The primordia of atrioventicular (AV) valves are cushions that attenuate, elongate, and stiffen into fibrous tissues as they mature. It has been postulated that increasing hydrodynamic forces during development contribute to valve morphogenesis. To investigate the molecular mechanisms of flow‐generated force on valve maturation, we have utilized a novel 3D in‐vitro culture system. AV cushion explants were cultured inside a tubular collagen scaffold attached to a pulsatile pump. Estimated in‐vivo flows were applied to developing valve tissues. Data from RT‐PCR, confocal, and Western blot analyses show that flow regulates the expression and localization of ECM proteins in developing AV cushions. Further, the RhoA signaling pathway has been shown to be an important mediator of mechanical signals in endothelial cells. Our data show that RhoA mRNA levels in AV cushion explants are enhanced under flow conditioning. Additionally, compared to controls, cushion explants undergoing flow forces had increased expression of ECM proteins when treated with RhoA activator lysophosphatidic acid, while Y‐27632 Rho‐kinase inhibitor suppresses ECM protein expression. AFM studies confirm that flow and RhoA also regulate the material properties of developing valves. Our data indicate that hydrodynamic forces can drive expression and deposition of fibrous ECM proteins that are critical for proper valve morphogenesis.