The role of α-smooth muscle actin in fibroblast-mediated matrix contraction and remodeling

Abstract

Cardiac myofibroblasts play an important role in myocardial remodeling. Although α-smooth muscle actin (α-SMA) expression is the hallmark of mature myofibroblasts, its role in regulating fibroblast function remains poorly understood. We explore the effects of the matrix environment in modulating cardiac fibroblast phenotype, and we investigate the role of α-SMA in fibroblast function using loss- and gain-of-function approaches. In murine myocardial infarction, infiltration of the infarct border zone with abundant α-SMA-positive myofibroblasts was associated with scar contraction. Isolated cardiac fibroblasts cultured in plates showed high α-SMA expression localized in stress fibers, exhibited activation of focal adhesion kinase (FAK), and synthesized large amounts of extracellular matrix proteins. In contrast, when these cells were cultured in collagen lattices, they exhibited marked reduction of α-SMA expression, negligible FAK activation, attenuated collagen synthesis, and increased transcription of genes associated with matrix metabolism. Transforming Growth Factor-βl-mediated contraction of fibroblast-populated collagen pads was associated with accentuated α-SMA synthesis. In contrast, serum- and basic Fibroblast Growth Factor-induced collagen pad contraction was associated with reduced α-SMA expression. α-SMA siRNA knockdown attenuated contraction of collagen pads populated with serum-stimulated cells. Surprisingly, α-SMA overexpression also reduced collagen pad contraction, suggesting that α-SMA is not sufficient to promote contraction of the matrix. Reduced contraction by α-SMA-overexpressing cells was associated with attenuated proliferative activity, in the absence of any effects on apoptosis. α-SMA may be implicated in contraction and remodeling of the extracellular matrix, but is not sufficient to induce contraction. α-SMA expression may modulate cellular functions, beyond its effects on contractility.

Mechanisms of Fibroblast Activation in the Remodeling Myocardium

“…As the neutrophil infiltrate is cleared by macrophages, fibroblasts expand and undergo myofibroblast transdifferentiation [13], expressing contractile proteins, such as α-smooth muscle actin (αSMA) (Figure 1), and secreting large amounts of extracellular matrix proteins [27],[28]. Activated fibroblasts play a critical role in preservation of the structural integrity of the infarcted ventricle [10]; however excessive or prolonged activation of fibroblast populations may reduce ventricular compliance, promote adverse remodeling and precipitate heart failure [29].…”

Pitx2c inhibition increases atrial fibroblast activity: Implications in atrial arrhythmogenesis

“…α-SMA is a major differentiation marker of myofibroblasts, which are more contractible and synthesize higher amounts of extracellular matrix proteins. 23 α-SMA-containing stress fibres have an arrhythmic effect on cardiomyocytes. 24 These effects may contribute to the increasing risk of AF in patients with Pitx2c deficiency with its known electrophysiological effects in the atrium and enhanced genesis of pulmonary vein myocardial sleeves.…”

“…In this study, for the first time, we found that Pitx2c deficiency increased atrial fibroblast activity with increasing migration and higher expression of α‐SMA. α‐SMA is a major differentiation marker of myofibroblasts, which are more contractible and synthesize higher amounts of extracellular matrix proteins . α‐SMA‐containing stress fibres have an arrhythmic effect on cardiomyocytes .…”

Endothelial cells enhance adipose mesenchymal stromal cell‐mediated matrix contraction via ALK receptors and reduced follistatin: Potential role of endothelial cells in skin fibrosis

“…This suggests that either a longer time in co‐culture with EC or additional factors from, for example, keratinocytes are required for ASC to fully differentiate into myofibroblasts. It has also been shown that MSC are able to contract tissue in an α‐SMA independent manner, via microfilaments (Dawes et al, ; Shinde, Humeres, & Frangogiannis, ).…”