The Effects of Combined Cyclic Stretch and Pressure on the Aortic Valve Interstitial Cell Phenotype

Abstract

Aortic valve interstitial cells (VIC) can exhibit phenotypic characteristics of fibroblasts, myofibroblasts, and smooth muscle cells. Others have proposed that valve cells become activated and exhibit myofibroblast or fibroblast characteristics during disease initiation and progression; however, the cues that modulate this phenotypic change remain unclear. We hypothesize that the mechanical forces experienced by the valve play a role in regulating the native phenotype of the valve and that altered mechanical forces result in an activated phenotype. Using a novel ex vivo cyclic stretch and pressure bioreactor, we subjected porcine aortic valve (AV) leaflets to combinations of normal and pathological stretch and pressure magnitudes. The myofibroblast markers α-SMA and Vimentin, along with the smooth muscle markers Calponin and Caldesmon, were analyzed using immunohistochemistry and immunoblotting. Tissue structure was analyzed using Movat’s pentachrome staining. We report that pathological stretch and pressure inhibited the contractile and possibly myofibroblast phenotypes as indicated by downregulation of the proteins α-SMA, Vimentin, and Calponin. In particular, Calponin downregulation implies depolymerization of actin filaments and possible conversion to a more synthetic (non-contractile) phenotype. This agreed well with the increase in spongiosa and fibrosa thickness observed under elevated pressure and stretch that are typically indicative of increased matrix synthesis. Our study therefore demonstrates how cyclic stretch and pressure may possibly act together to modulate the AVIC phenotype.

The Ski/Zeb2/Meox2 pathway provides a novel mechanism for regulation of the cardiac myofibroblast phenotype

“…Others have also identified stretch as being a modulator of myofibroblast phenotype (Thayer et al, 2011). Additional factors have been shown to affect myofibroblast phenotype, for example, Wnt3a signaling (Carthy et al, 2011).…”

Hemodynamic and Cellular Response Feedback in Calcific Aortic Valve Disease

“…44,45,106,108 These findings have potentially provocative implications linking pathological stretch with the deterioration in aortic valve tissue properties and function that occurs with disease. Indeed, Thayer et al 45 found that subjecting aortic valve leaflets to a combination of pathological stretch and pressure loads ex vivo caused a significant increase in the thickness of the fibrosa and spongiosa layers ( Figure 4C). …”

“…VIC phenotype and function are regulated by cues from their local environment, including paracrine signaling molecules (eg, endothelium-derived signals as discussed below), inflammatory cytokines (eg, TGF-β1), 19 the biochemical and biomechanical properties of their ECM, 11,22,23,[36][37][38][39][40] and mechanical stimuli induced by hemodynamic forces (eg, normal or pathological stretching of the valve tissue). [41][42][43][44][45] In addition, there appear to be interactions, and sometimes synergies, between each of these types of cues, with the mechanical environment playing a critical modulatory role. For example, increases in the transvalvular pressure gradient (TPG) lead to higher levels of valve tissue stretching, 46 likely resulting in mechanical activation of latent TGF-β1, 20 which in turn can potentiate VIC activation to the myofibroblast phenotype.…”

“…Multiple ex vivo and in vitro studies have demonstrated that pathological stretch of valve tissue or isolated VICs can elicit responses associated with valve disease development, most notably increased matrix synthesis and remodeling 45,[105][106][107] and VIC activation to the secretory myofibroblast phenotype. 44,45,106,108 These findings have potentially provocative implications linking pathological stretch with the deterioration in aortic valve tissue properties and function that occurs with disease.…”

Critical Role of Coaptive Strain in Aortic Valve Leaflet Homeostasis: Use of a Novel Flow Culture Bioreactor to Explore Heart Valve Mechanobiology

“…This well‐accepted role of flow is a conceptual extension from vascular endothelial cells (ECs) to VEC: Flow shear forces affect the biology of both VEC and vascular ECs with many similarities and some differences 16, 17, 18. However, other forces acting on leaflets also clearly influence valve cellular biology 19. Elevated transvalvular pressure (eg, in the setting of hypertension) is a recognized risk factor for commissural fusion during VAD therapy as well as valvular diseases presumably attributed to the increased static pressure on valve cells and the increased strain loading on the closed AV root 13, 20.…”