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Title: Human cardiac fibroblasts adaptive responses to controlled combined mechanical strain and oxygen changes in vitro}
Article:
Abstract:
Upon cardiac pathological conditions such as ischemia, microenvironmental changes instruct a series of cellular responses that trigger cardiac fibroblasts-mediated tissue adaptation and inflammation. A comprehensive model of how early environmental changes may induce cardiac fibroblasts( CF) pathological responses is far from being elucidated, partly due to the lack of approaches involving complex and simultaneous environmental stimulation. Here, we provide a first analysis of human primary CF behavior by means of a multi-stimulus microdevice for combined application of cyclic mechanical strain and controlled oxygen tension. Our findings elucidate differential human CFs responses to different combinations of the above stimuli. Individual stimuli cause proliferative effects( PHH3+ mitotic cells, YAP translocation, PDGF secretion) or increase collagen presence. Interestingly, only the combination of hypoxia and a simulated loss of contractility( 2% strain) is able to additionally induce increased CF release of inflammatory and pro-fibrotic cytokines and matrix metalloproteinases.
Introduction:
When supply of oxygen and nutrients to the myocardium is critically reduced( ischemia), a complex tissue response takes place: within hours tissue necrosis and death of contractile cardiac myocytes occurs in the infarcted area giving rise to an inflammatory phase that recruits immune cells and activates quiescent cardiac fibroblasts( CFs); within a few days a proliferative phase begins, where activated CFs invade the infarcted area and contribute to degrading and replacing the extra-cellular matrix with a collagen-based scar; within weeks the maturation of the fibrotic scar is completed( Frangogiannis, 2014; Heusch et al., 2014). Cellular and molecular events such as excessive proliferation of CFs, phenotypic switch of CFs, high levels of inflammatory cytokines and humoral factors, unbalanced synthesis of extracellular matrix( ECM) proteins and matrix metalloproteinases( MMP)-mediated degradation of ECM are generally regarded as hallmarks of early fibrotic tissue response( Fan et al., 2012; Krenning et al., 2010; Porter and Turner, 2009; Shinde and Frangogiannis, 2014). This essential process maintains tissue integrity, however, it often leads to excessive and adverse CFs remodeling of non-infarcted areas( Fan et al., 2012; Kania et al., 2009; Talman and Ruskoaho, 2016) associated with cardiac dysfunction and increased mortality( Okada et al., 2005). Controlled anti-fibrotic strategies still require deeper understanding and advanced models of cardiac fibrosis mechanisms( Leask, 2010; Roubille et al., 2014). In general, the onset of pathological myocardial conditions causes alterations of specific environmental cues at the cellular scale: mechanical strain decreases( loss of contractility); oxygen and nutrient levels dramatically decrease( ischemia); levels of inflammatory cytokines increase( post-injury inflammatory response). In an attempt to provide in vitro models of cardiac disease, CFs have been widely studied under relevant physico-chemical stimulation such as mechanical stress( Schroer and Merryman, 2015; Tomasek et al., 2002), oxygen deprivation( Clancy et al., 2007; Tamamori et al., 1997) and biochemical stimulation with pro-fibrotic cytokines( Edgley et al., 2012; Lijnen et al., 2000; Petrov et al., 2002). The application of mechanical stress has been shown to have the following effects on CFs: increased ECM protein synthesis( Carver et al., 1991); controversial proliferative behavior( Atance et al., 2004; Butt and Bishop, 1997; Dalla Costa et al., 2010; Liao et al., 2004), with recent reports of strain intensity-dependent effects( Ugolini et al., 2016); increased production of pro-fibrotic and inflammatory cytokines such as TGF-β( transforming growth factor-beta)( Leask, 2007) and TNF-α( tumor necrosis factor-alpha)( Yokoyama et al., 1999). While previous mechanical stress experiments were performed in a normoxic environment( NX, approximately 20% O2, the standard oxygen level of ambient air), CFs were shown to be sensitive to O2 level variations from physoxia( PX), defined as the physiologic oxygen level in living tissues( about 5–6% O2 in the myocardium[Gonschior et al., 1992; Roy et al., 2003; Sen et al., 2006; Winegrad et al., 1999]). Both hypoxia( HX, 1–3% O2) and NX induce a pro-inflammatory and fibrogenic phenotype in cultured CFs( Roy et al., 2003; Sen and Roy, 2010). These findings imply that normoxic oxygen levels are perceived by CFs as a state of hyperoxia and that a significant bias exists when culturing CFs in non-physiological oxygen environments. The exposure of CFs to HX has been shown to induce collagen production and proliferation of CFs( Gao et al., 2014; Tamamori et al., 1997), while studies suggest that MMP-based remodeling may not be triggered by HX alone( Riches et al., 2009). Given the complexity of in vivo pathological evolution, the elucidation of how environmental stimuli interplay and guide cellular responses is paramount. To date, no comprehensive model is able to recapitulate early cellular events taking place after acute myocardial injury and there is no report of cardiac cells subjected to simultaneous mechanical strain and controlled oxygen changes, two major environmental variables involved in cardiac injury and CF adaptive responses. This is mostly due to the lack of compact platforms enabling the controlled application of multiple stimuli. To address this need, we here report the improvement of a previously described multi-chamber microdevice dedicated to the application of cyclic strain to cell monolayers( Ugolini et al., 2016). By adding a system for controlling oxygen changes, we significantly expanded the experimental complexity and mimicking capabilities of the device. We here finely controlled the applied mechanical strain and oxygen regimes sensed by CFs to model early environmental changes in cardiac injury and provide insights into individual or synergistic contributions of the environmental signals in the activation of early CF responses relevant to cardiac fibrotic disease. Shortly after an ischemic myocardial insult( e. g., acute coronary artery occlusion) tissue oxygen levels drop from approximately 5–6% O2( Gonschior et al., 1992; Sen et al., 2006; Winegrad et al., 1999) to near-zero( Roy et al., 2003), rapidly inducing loss of cardiac myocytes contractility. In terms of in vitro model parameters, we thus selected 5% O2( PX) as a physiological oxygen level and 1% O2( HX) as an oxygen level characteristic of ischemic myocardial injury. Dramatic alterations of injured myocardial tissue also take place in early timeframes: tissue bulging and dilations are commonly observed together with loss of contractile function( Eek et al., 2010; Pfeffer and Braunwald, 1990; Picard et al., 1990; Tennant and Wiggers, 1935). Cardiac imaging studies agree in the interpretation that global strains are abruptly reduced shortly after myocardial insult( Flachskampf et al., 2011; Hoit, 2011; Mollema et al., 2010). Quantitatively, strain values recorded vary significantly throughout the heart. Although absolute values may depend on imaging algorithms, strains in ischemic/injured regions have been reported to be 2–4 fold lower than in healthy myocardial regions( e. g. less than 3% in ischemic regions versus approximately 10% in healthy regions[Dandel et al., 2009; Vartdal et al., 2007]). Within this range and in line with previous in vitro observations of CFs behavior( Ugolini et al., 2016), we selected 2% strain as indicative of reduced contractility and 8% strain as representative of full myocardial contractility. These environmental changes and the following initial cellular responses happen in a timeframe of hours, as shown in vitro( van Nieuwenhoven et al., 2013; Turner et al., 2007, 2009) as well as in vivo( Guillén et al., 1995; Morishita et al., 2015). We thus performed experiments lasting 24 hr and evaluated the main early aspects that govern fibrotic responses in the injured myocardium: ECM remodeling, with stainings for collagen I and quantifications of MMP expression; proliferation of CFs, with analyses of mitotic cells, Hippo pathway signaling and mitogenic PDGF( platelet-derived growth factor) expression; secretion of inflammatory and pro-fibrotic cytokines in CF supernatants; myofibroblast differentiation, through stainings for αSMA( α-smooth muscle actin). Results of our investigation provide insights into how the combination of environmental stimuli may act synergistically or independently to drive in vitro adaptive early CF responses.
Discussion:
In summary, we propose an innovative model that recreates a pathological environment for understanding CF responses during cardiac injury. Replicating for the first time the combination of oxygen changes and mechanical cues, we revealed how these environmental stimulations combine to trigger CF pathological responses and highlighted emerging adaptive cellular mechanisms. We found that mimicking the combination of HX and reduced contractility proved crucial in eliciting inflammatory and fibrotic remodeling responses of CFs, while individual environmental stimuli only regulated proliferative and collagen-related responses. These insights have impact on future studies of pathological myocardial remodeling and the in vitro model here described provides a tool for better understanding pathological mechanisms and tailoring reparative strategies.
When the supply of oxygen to the heart is reduced , its cells start to die within hours , the heart muscle becomes less able to contract , and the area becomes inflamed . This inflammation is accompanied by an influx of immune cells . It also activates other cells known as cardiac fibroblasts that help to break down the framework of molecules that supported the damaged heart tissue and replace it with a scar . This response is part of the normal repair process , but it can lead to the formation of scar tissue in non-damaged areas of the heart . Excess scar tissue makes the heart muscle less able to contract and increases the affected individual’s chance of dying . Understanding how this repair process works is an important step in developing strategies to minimise the damage caused by coronary artery disease or heart attacks . However , existing laboratory models are only partly able to recreate the conditions seen in real heart tissue . To properly understand the response at the level of living cells , a more complete model is needed . Ugolini et al . now report improvements to a small device , referred to as a lab-on-chip , that can subject cells to mechanical strain . The improvements mean the device could also recreate other conditions seen early on in damaged heart tissue , specifically the reduced supply of oxygen . Replicating combinations of mechanical changes and oxygen supplies meant that the impact of these conditions on human cardiac fibroblasts could be directly observed in the laboratory for the first time . Ugolini et al . found that a lack of contraction and low oxygen levels triggered the cardiac fibroblasts to produce inflammatory molecules and molecules associated with the formation of scar tissue . This resembles the response seen in living hearts . The next step is to improve the lab-on-chip device further by adding other cell types , including heart muscle cells and immune cells . A more complete model may aid future research into how our hearts operate in both health and disease .