No abstract

Abstract-The pathogenesis of cardiac valve disease correlates with the emergence of muscle-like fibroblasts (myofibroblasts). These cells display prominent stress fibers containing ␣-smooth muscle actin (␣-SMA) and are believed to differentiate from valvular interstitial cells (VICs). However, the biological factors that initiate myofibroblast differentiation and activation in valves remain unidentified. We show that transforming growth factor-␤1 (TGF-␤1) mediates differentiation of VICs into active myofibroblasts in vitro in a dose-dependent manner, as determined by a significant increase in ␣-SMA and the dramatic augmentation of stress fiber formation and alignment. Additionally, TGF-␤1 and increased mechanical stress function synergistically to enhance contractility. In turn, contractile valve myofibroblasts exert tension on the extracellular matrix, resulting in a dramatic realignment of extracellular fibronectin fibrils. TGF-␤1 also inhibits valve myofibroblast proliferation without enhancing apoptosis. Our results are consistent with activation of a highly contractile myofibroblast phenotype by TGF-␤1 and are the first to connect valve myofibroblast contractility with pathological valve matrix remodeling. We suggest that the activation of contractile myofibroblasts by TGF-␤1 may be a significant first step in promoting alterations to the valve matrix architecture that are evident in valvular heart disease.

Hydrogels formed from the photoinitiated, solution polymerization of macromolecular monomers present distinct advantages as cell delivery materials and are enabling researchers to threedimensionally encapsulate cells within diverse materials that mimic the extracellular matrix and support cellular viability. Approaches to synthesize gels with biophysically and biochemically controlled microenvironments are becoming increasingly important, and require strategies to control gel properties (e.g., degradation rate and mechanism) on multiple time and size scales. Furthermore, biological responses of gel-encapsulated cells can be promoted by hydrogel degradation products, as well as by the release of tethered biologically relevant molecules.

Valvular interstitial cells (VICs) possess many properties that make them attractive for use in the construction of a tissue-engineered valve; however, we have found that the surfaces to which VICs will adhere and spread are limited. For example, VICs adhere and spread on collagen and laminin-coated surfaces, but display altered morphology and do not proliferate. Interestingly, fibronectin (FN) was one adhesion protein that facilitated VIC adhesion and proliferation. Yet VICs did not spread on surfaces modified with RGD, a ubiquitous cell-adhesive peptide, nor with other FN-specific peptide sequences such as EILDV and PHSRN. Hyaluronic acid (HA) is a highly elastic polysaccharide that is involved in natural valve morphogenesis and possesses binding interactions with FN. Hyaluronic acid was modified to form photopolymerizable hydrogels, and VICs were found to spread and proliferate on HA-based gels, forming a confluent monolayer on the gels within 4 days. Modified HA retained its ability to specifically bind FN, allowing for the formation of gels containing both HA and FN. Valvular interstital cells cultured on HA surfaces displayed significantly increased production of extracellular matrix proteins, indicating that HA-based scaffolds may provide useful biological cues to stimulate heart valve tissue formation.