Novel regulators of cardiac inflammation: Matricellular proteins expand their repertoire

Inhibition of SPARC signal by aerobic exercise to ameliorate atherosclerosis

BACKGROUND AND AIMS

Inflammation and atherosclerosis (AS) are closely associated to Secreted Protein Acidic and Rich in Cysteine (SPARC) and its related factors. This study attempted to define the role and the potential mechanism of SPARC and its related factors in ameliorating hyperlipidemia and AS by aerobic exercise intervention.

METHODS

The AS rat model was established with a high-fat diet plus vitamin D3 intraperitoneal injection. Treadmill exercises training (5 days/week at 14 m/min for 60 min/day) for 6 weeks was carried out for AS rat intervention method. Western blotting and qRT-PCR were used to analyze the mRNA and protein expression of SPARC and its related factors, respectively. H&E staining was applied to evaluate the morphological changes and inflammation damage. Von Kossa staining was used to measure the degree of vascular calcification. Fluorescence immunohistochemistry staining was used to detect the expression and distribution of SPARC signal molecules.

RESULTS

SPARC was highly expressed and co-localization with the smooth muscle marker α-SMC in the AS rat. And its downstream factors, NF-κB, Caspase-1, IL-1β and IL-18 were upregulated (P < 0.05 or P < 0.01), FNDC5 expression was downregulated in AS rat model. However, slight declined body weight, delayed AS progression, decreased hyperlipidemia and favorable morphology of skeletal muscle and blood vessels have been detected in AS rat with aerobic exercise intervention. Moreover, the expression of SPARC and its downstream factors were decreased (P < 0.05 or P < 0.01), while elevated the expression of FNDC5 (P < 0.01) was observed after aerobic exercise intervention.

CONCLUSIONS

This study suggested that aerobic exercise ameliorated hyperlipidemia and AS by effectively inhibiting SPARC signal, and vascular smooth muscle cells may contribute greatly to the protection of AS.

Don't go breakin' my heart: cardioprotective alterations to the mechanical and structural properties of reperfused myocardium during post-infarction inflammation

Myocardial infarctions (MIs) kickstart an intense inflammatory response resulting in extracellular matrix (ECM) degradation, wall thinning, and chamber dilation that leaves the heart susceptible to rupture. Reperfusion therapy is one of the most effective strategies for limiting adverse effects of MIs, but is a challenge to administer in a timely manner. Late reperfusion therapy (LRT; 3 + hours post-MI) does not limit infarct size, but does reduce incidences of post-MI rupture and improves long-term patient outcomes. Foundational studies employing LRT in the mid-twentieth century revealed beneficial reductions in infarct expansion, aneurysm formation, and left ventricle dysfunction. The mechanism by which LRT acts, however, is undefined. Structural analyses, relying largely on one-dimensional estimates of ECM composition, have found few differences in collagen content between LRT and permanently occluded animal models when using homogeneous samples from infarct cores. Uniaxial testing, on the other hand, revealed slight reductions in stiffness early in inflammation, followed soon after by an enhanced resistance to failure for cases of LRT. The use of one-dimensional estimates of ECM organization and gross mechanical function have resulted in a poor understanding of the infarct's spatially variable mechanical and structural anisotropy. To resolve these gaps in literature, future work employing full-field mechanical, structural, and cellular analyses is needed to better define the spatiotemporal post-MI alterations occurring during the inflammatory phase of healing and how they are impacted following reperfusion therapy. In turn, these studies may reveal how LRT affects the likelihood of rupture and inspire novel approaches to guide scar formation.

The matricellular protein SPARC induces inflammatory interferon-response in macrophages during aging

The risk of chronic diseases caused by aging is reduced by caloric restriction (CR)-induced immunometabolic adaptation. Here, we found that the matricellular protein, secreted protein acidic and rich in cysteine (SPARC), was inhibited by 2 years of 14% sustained CR in humans and elevated by obesity. SPARC converted anti-inflammatory macrophages into a pro-inflammatory phenotype with induction of interferon-stimulated gene (ISG) expression via the transcription factors IRF3/7. Mechanistically, SPARC-induced ISGs were dependent on toll-like receptor-4 (TLR4)-mediated TBK1, IRF3, IFN-β, and STAT1 signaling without engaging the Myd88 pathway. Metabolically, SPARC dampened mitochondrial respiration, and inhibition of glycolysis abrogated ISG induction by SPARC in macrophages. Furthermore, the N-terminal acidic domain of SPARC was required for ISG induction, while adipocyte-specific deletion of SPARC reduced inflammation and extended health span during aging. Collectively, SPARC, a CR-mimetic adipokine, is an immunometabolic checkpoint of inflammation and interferon response that may be targeted to delay age-related metabolic and functional decline.

Photobiomodulation therapy's effects on cardiac fibrosis activation after experimental myocardial infarction

INTRODUCTION

Ischemic heart disease is the leading cause of death worldwide, and interventions to reduce myocardial infarction (MI) complications are widely researched. Photobiomodulation therapy (PBMT) has altered multiple biological processes in tissues and organs, including the heart.

OBJECTIVES

This study aimed to assess the temporal effects of PBMT on cardiac fibrosis activation after MI in rats. In this proof-of-concept study, we monitored the change in expression patterns over time of genes and microRNAs (miRNAs) involved in the formation of cardiac fibrosis post-MI submitted to PBMT.

MATERIALS AND METHODS

Experimental MI was induced, and PBMT was applied shortly after coronary artery ligation (laser light of wavelength 660 nm, 15 mW of power, energy density 22.5 J/cm² , 60 seconds of application, irradiated area 0.785 cm² , fluence 1.1 J/cm² ). Ventricular septal samples were collected at 30 minutes, 3, 6, 24 hours, and 3 days post-MI to determine temporal PBMT's effects on messenger RNA (mRNA) expression associated with cardiac fibrosis activation and miRNAs expression.

RESULTS

PBMT, when applied after ischemia, reversed the changes in mRNA expression of myocardial extracellular matrix genes induced by MI. Surprisingly, PBMT modified cardiac miRNAs expression related to fibrosis replacement in the myocardium. Expression correlations between myocardial mRNAs were assessed. The correlation coefficient between miRNAs and target mRNAs was also determined. A positive correlation was detected among miR-21 and transforming growth factor beta-1 mRNA. The miR-29a expression negatively correlated to Col1a1, Col3a1, and MMP-2 mRNA expressions. In addition, we observed that miR-133 and Col1a1 mRNA were negatively correlated.

CONCLUSION

The results suggest that PBMT, through the modulation of gene transcription and miRNA expressions, can interfere in cardiac fibrosis activation after MI, mainly reversing the signaling pathway of profibrotic genes.

Targeting Inflammation After Myocardial Infarction

PURPOSE OF REVIEW

Inflammation plays a key role in clearing cellular debris and recovery after acute myocardial infarction (AMI). Dysregulation of or prolonged inflammation may result in adverse cardiac remodeling and major adverse clinical events (MACE). Several pre-clinical studies and moderate sized clinical trials have investigated the role of immunomodulation in improving clinical outcomes in patients with AMI.

RECENT FINDINGS

Clinical data from the Canakinumab Atherothrombosis Outcome (CANTOS) and Colchicine Cardiovascular Outcomes Trial (COLCOT) have provided encouraging results among patients with AMI. Several other clinical and pre-clinical trials have brought about the prospect of modulating inflammation at various junctures of the inflammatory cascade including inhibition of complement cascade, interleukins, and matrix metalloproteinases. In patients with AMI, modulation of residual inflammation via various inflammatory pathways and mediators may hold promise for further reducing MACE. Learning from current data and understanding the nuances of immunomodulation in AMI are key for future trials and before widespread dissemination of such therapies.

Characterization of Matricellular Protein Expression Signatures in Mechanistically Diverse Mouse Models of Kidney Injury

Fibrosis is the most common pathophysiological manifestation of Chronic Kidney Disease (CKD). It is defined as excessive deposition of extracellular matrix (ECM) proteins. Embedded within the ECM are a family of proteins called Matricellular Proteins (MCPs), which are typically expressed during chronic pathologies for ECM processing. As such, identifying potential MCPs in the pathological secretome of a damaged kidney could serve as diagnostic/therapeutic targets of fibrosis. Using published RNA-Seq data from two kidney injury mouse models of different etiologies, Folic Acid (FA) and Unilateral Ureteral Obstruction (UUO), we compared and contrasted the expression profile of various members from well-known MCP families during the Acute and Fibrotic injury phases. As a result, we identified common and distinct MCP expression signatures between both injury models. Bioinformatic analysis of their differentially expressed MCP genes revealed similar top annotation clusters from Molecular Function and Biological Process networks, which are those commonly involved in fibrosis. Using kidney lysates from FA- and UUO-injured mice, we selected MCP genes from our candidate list to confirm mRNA expression by Western Blot, which correlated with injury progression. Understanding the expressions of MCPs will provide important insight into the processes of kidney repair, and may validate MCPs as biomarkers and/or therapeutic targets of CKD.

Cardiomyogenesis Modeling Using Pluripotent Stem Cells: The Role of Microenvironmental Signaling

Pluripotent stem cells (PSC) can be used as a model to study cardiomyogenic differentiation. In vitro modeling can reproduce cardiac development through modulation of some key signaling pathways. Therefore, many studies make use of this strategy to better understand cardiomyogenesis complexity and to determine possible ways to modulate cell fate. However, challenges remain regarding efficiency of differentiation protocols, cardiomyocyte (CM) maturation and therapeutic applications. Considering that the extracellular milieu is crucial for cellular behavior control, cardiac niche studies, such as those identifying secreted molecules from adult or neonatal tissues, allow the identification of extracellular factors that may contribute to CM differentiation and maturation. This review will focus on cardiomyogenesis modeling using PSC and the elements involved in cardiac microenvironmental signaling (the secretome - extracellular vesicles, extracellular matrix and soluble factors) that may contribute to CM specification and maturation.

The Influence of the Extracellular Matrix in Inflammation: Findings from the SPARC-Null Mouse

Matricellular proteins are secreted proteins that, among other functions, can contribute to extracellular matrix (ECM) assembly including modulation of cell:ECM interactions. Recent discoveries have indicated a fundamental role for the ECM in the regulation of inflammatory responses including cell extravasation and recruitment, immune cell differentiation, polarization, activation, and retention in tissues. Secreted protein acidic and rich in cysteine (SPARC) is a matricellular collagen-binding protein implicated in fibrillar collagen assembly in the ECM of connective tissue as well as in basal lamina organization. Functions of SPARC in modulating cell adhesion events are also reported. Studies of phenotypic responses observed in SPARC-null mice to a variety of injury models have yielded interesting insight into the functional importance of SPARC production and aberrations in ECM structure that occur in the absence of SPARC that influence immune cell behavior and inflammatory pathways. In this review, we will discuss several examples from different tissues in which SPARC-null mice exhibited an inflammatory response distinct from those of SPARC expressing mice and provide insight into novel ECM-dependent mechanisms that influence these responses. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc.

Inflammation and fibrosis in murine models of heart failure

Heart failure is a consequence of various cardiovascular diseases and associated with poor prognosis. Despite progress in the treatment of heart failure in the past decades, prevalence and hospitalisation rates are still increasing. Heart failure is typically associated with cardiac remodelling. Here, inflammation and fibrosis are thought to play crucial roles. During cardiac inflammation, immune cells invade the cardiac tissue and modulate tissue-damaging responses. Cardiac fibrosis, however, is characterised by an increased amount and a disrupted composition of extracellular matrix proteins. As evidence exists that cardiac inflammation and fibrosis are potentially reversible in experimental and clinical set ups, they are interesting targets for innovative heart failure treatments. In this context, animal models are important as they mimic clinical conditions of heart failure patients. The advantages of mice in this respect are short generation times and genetic modifications. As numerous murine models of heart failure exist, the selection of a proper disease model for a distinct research question is demanding. To facilitate this selection, this review aims to provide an overview about the current understanding of the pathogenesis of cardiac inflammation and fibrosis in six frequently used murine models of heart failure. Hence, it compares the models of myocardial infarction with or without reperfusion, transverse aortic constriction, chronic subjection to angiotensin II or deoxycorticosterone acetate, and coxsackievirus B3-induced viral myocarditis in this context. It furthermore provides information about the clinical relevance and the limitations of each model, and, if applicable, about the recent advancements in their methodological proceedings.

Matricellular Proteins: Functional Insights From Non-mammalian Animal Models

The extracellular matrix (ECM) has central roles in tissue integrity and remodeling throughout the life span of animals. While collagens are the most abundant structural components of ECM in most tissues, tissue-specific molecular complexity is contributed by ECM glycoproteins. The matricellular glycoproteins are categorized primarily according to functional criteria and represented predominantly by the thrombospondin, tenascin, SPARC/osteonectin, and CCN families. These proteins do not self-assemble into ECM fibrils; nevertheless, they shape ECM properties through interactions with structural ECM proteins, growth factors, and cells. Matricellular proteins also promote cell migration or morphological changes through adhesion-modulating or counter-adhesive actions on cell-ECM adhesions, intracellular signaling, and the actin cytoskeleton. Typically, matricellular proteins are most highly expressed during embryonic development. In adult tissues, expression is more limited unless activated by cues for dynamic tissue remodeling and cell motility, such as occur during inflammatory response and wound repair. Many insights in the complex roles of matricellular proteins have been obtained from studies of gene knockout mice. However, with the exception of chordate-specific tenascins, these are highly conserved proteins that are encoded in many animal phyla. This review will consider the increasing body of research on matricellular proteins in nonmammalian animal models. These models provide better access to the very earliest stages of embryonic development and opportunities to study biological processes such as limb and organ regeneration. In aggregate, this research is expanding concepts of the functions and mechanisms of action of matricellular proteins.

The innate immune system in chronic cardiomyopathy: a European Society of Cardiology (ESC) scientific statement from the Working Group on Myocardial Function of the ESC

Activation of the immune system in heart failure (HF) has been recognized for over 20 years. Initially, experimental studies demonstrated a maladaptive role of the immune system. However, several phase III trials failed to show beneficial effects in HF with therapies directed against an immune activation. Preclinical studies today describe positive and negative effects of immune activation in HF. These different effects depend on timing and aetiology of HF. Therefore, herein we give a detailed review on immune mechanisms and their importance for the development of HF with a special focus on commonalities and differences between different forms of cardiomyopathies. The role of the immune system in ischaemic, hypertensive, diabetic, toxic, viral, genetic, peripartum, and autoimmune cardiomyopathy is discussed in depth. Overall, initial damage to the heart leads to disease specific activation of the immune system whereas in the chronic phase of HF overlapping mechanisms occur in different aetiologies.

Sanggenon C protects against cardiomyocyte hypoxia injury by increasing autophagy

Sanggenon C is isolated from Morus alba, a plant that has been used for anti-inflammatory purposes in Oriental medicine. Little is known about the effect of Sanggenon C on cardiomyocyte hypoxia injury. This study, using H9c2 rat cardiomyoblasts, was designed to determine the effects of Sanggenon C on cardiomyocyte hypoxia injury. Inflammatory cytokine levels were measured by reverse transcription-polymerase chain reaction, reactive oxygen species were measured by 2′,7′-dichlorofluorescin diacetate fluorescent probe, autophagy was detected using the LC3II/I ratio and cell apoptosis was detected by TUNEL staining. The molecular mechanisms underlying Sanggenon C-induced cyto-protection were also determined by western blotting, especially the possible involvement of autophagy and AMP-activated protein kinase (AMPK). Results indicated that samples pretreated with different concentrations of Sanggenon C (1, 10 and 100 µM) reduced the expression levels of pro-inflammatory cytokines, including tumor necrosis factor α, interleukin (IL)-1 and IL-6, under hypoxia. The beneficial effects of Sanggenon C were also associated with reduced levels of reactive oxygen species generation and increased levels of antioxidant nitric oxide and superoxide dismutase. Sanggenon C enhanced hypoxia-induced autophagy as evidenced by the increased expression levels of autophagy-associated proteins Beclin and autophagy related 5 as well as the decreased the accumulation of p62, and increased the LC3II/I ratio. Sanggenon C also reduced hypoxia-induced apoptosis as detected by TUNEL staining and the expression of Bcl-2 proteins. The beneficial effects of Sanggenon C were associated with enhanced activation level of AMPKα and suppressed hypoxia-induced mechanistic target of rapamycin (mTOR) and forkhead box O3a (FOXO3a) phosphorylation. The AMPK inhibitor Compound C (CpC) was used, and the anti-apoptotic and pro-autophagy effects of Sanggenon C in response to hypoxia were abolished by CpC. In conclusion, the current study demonstrated that Sanggenon C possessed direct cytoprotective effects against hypoxia injury in cardiac cells via signaling mechanisms involving the activation of AMPK and concomitant inhibition of mTOR and FOXO3a.

Reverse remodelling and myocardial recovery in heart failure

Advances in medical and device therapies have demonstrated the capacity of the heart to reverse the failing phenotype. The development of normative changes to ventricular size and function led to the concept of reverse remodelling. Among heart failure therapies, durable mechanical circulatory support is most consistently associated with the largest degree of reverse remodelling. Accordingly, research to analyse human tissue after a period of mechanical circulatory support continues to yield a wealth of information. In this Review, we summarize the latest findings on reverse remodelling and myocardial recovery. Accumulating evidence shows that the molecular changes associated with heart failure, in particular in the transcriptome, metabalome, and extracellular matrix, persist in the reverse-remodelled myocardium despite apparent normalization of macrolevel properties. Therefore, reverse remodelling should be distinguished from true myocardial recovery, in which a failing heart regains both normal function and molecular makeup. These findings have implications for future research to develop therapies to repair fully the failing myocardium. Meanwhile, recognition by society guidelines of this new clinical phenotype, which is coming to be known as a state of heart failure remission, underscores the need to accurately define and identify reverse modelled myocardium for the establishment of appropriate therapies.

Transgenic mice overexpressing desmocollin-2 (DSC2) develop cardiomyopathy associated with myocardial inflammation and fibrotic remodeling

Background

Arrhythmogenic cardiomyopathy is an inherited heart muscle disorder leading to ventricular arrhythmias and heart failure, mainly as a result of mutations in cardiac desmosomal genes. Desmosomes are cell-cell junctions mediating adhesion of cardiomyocytes; however, the molecular and cellular mechanisms underlying the disease remain widely unknown. Desmocollin-2 is a desmosomal cadherin serving as an anchor molecule required to reconstitute homeostatic intercellular adhesion with desmoglein-2. Cardiac specific lack of desmoglein-2 leads to severe cardiomyopathy, whereas overexpression does not. In contrast, the corresponding data for desmocollin-2 are incomplete, in particular from the view of protein overexpression. Therefore, we developed a mouse model overexpressing desmocollin-2 to determine its potential contribution to cardiomyopathy and intercellular adhesion pathology.

Methods and results

We generated transgenic mice overexpressing DSC2 in cardiac myocytes. Transgenic mice developed a severe cardiac dysfunction over 5 to 13 weeks as indicated by 2D-echocardiography measurements. Corresponding histology and immunohistochemistry demonstrated fibrosis, necrosis and calcification which were mainly localized in patches near the epi- and endocardium of both ventricles. Expressions of endogenous desmosomal proteins were markedly reduced in fibrotic areas but appear to be unchanged in non-fibrotic areas. Furthermore, gene expression data indicate an early up-regulation of inflammatory and fibrotic remodeling pathways between 2 to 3.5 weeks of age.

Conclusion

Cardiac specific overexpression of desmocollin-2 induces necrosis, acute inflammation and patchy cardiac fibrotic remodeling leading to fulminant biventricular cardiomyopathy.

The Quest for New Approaches in Myocarditis and Inflammatory Cardiomyopathy

Myocarditis is a diverse group of heart-specific immune processes classified by clinical and histopathological manifestations. Up to 40% of dilated cardiomyopathy is associated with inflammation or viral infection. Recent experimental studies revealed complex regulatory roles for several microribonucleic acids and T-cell and macrophage subtypes. Although the prevalence of myocarditis remained stable between 1990 and 2013 at about 22 per 100,000 people, overall mortality from cardiomyopathy and myocarditis has decreased since 2005. The diagnostic and prognostic value of cardiac magnetic resonance has increased with new, higher-sensitivity sequences. Positron emission tomography has emerged as a useful tool for diagnosis of cardiac sarcoidosis. The sensitivity of endomyocardial biopsy may be increased, especially in suspected sarcoidosis, by the use of electrogram guidance to target regions of abnormal signal. Investigational treatments on the basis of mechanistic advances are entering clinical trials. Revised management recommendations regarding athletic participation after acute myocarditis have heightened the importance of early diagnosis.

Altered protein levels in the isolated extracellular matrix of failing human hearts with dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is associated with extensive pathological cardiac remodeling and involves numerous changes in the protein expression profile of the extracellular matrix of the heart. We obtained seven human, end-stage, failing hearts with DCM (DCM-failing) and nine human, nonfailing donor hearts and compared their extracellular matrix protein profiles. We first showed that the DCM-failing hearts had indeed undergone extensive remodeling of the left ventricle myocardium relative to nonfailing hearts. We then isolated the extracellular matrix from a subset of these hearts and performed a proteomic analysis on the isolated matrices. We found that the levels of 26 structural proteins were altered in the DCM-failing isolated cardiac extracellular matrix compared to nonfailing isolated cardiac extracellular matrix. Overall, most of the extracellular matrix proteins showed reduced levels in the DCM-failing hearts, while all of the contractile proteins showed increased levels. There was a mixture of increased and decreased levels of cytoskeletal and nuclear transport proteins. Using immunoprobing, we verified that collagen IV (α₂ and α₆ isoforms), zyxin, and myomesin protein levels were reduced in the DCM-failing hearts. We expect that these data will add to the understanding of the pathology associated with heart failure with DCM.

Monocyte and macrophage contributions to cardiac remodeling

The mammalian heart contains a population of resident macrophages that expands in response to myocardial infarction and hemodynamic stress. This expansion occurs likely through both local macrophage proliferation and monocyte recruitment. Given the role of macrophages in tissue remodeling, their contribution to adaptive processes in the heart is conceivable but currently poorly understood. In this review, we discuss monocyte and macrophage heterogeneity associated with cardiac stress, the cell's potential contribution to the pathogenesis of cardiac fibrosis, and describe different tools to study and characterize these innate immune cells. Finally, we highlight their potential role as therapeutic targets.

Extracellular matrix-mediated cellular communication in the heart

The extracellular matrix (ECM) is a complex and dynamic scaffold that maintains tissue structure and dynamics. However, the view of the ECM as an inert architectural support has been increasingly challenged. The ECM is a vibrant meshwork, a crucial organizer of cellular microenvironments. It plays a direct role in cellular interactions regulating cell growth, survival, spreading, proliferation, differentiation and migration through the intricate relationship among cellular and acellular tissue components. This complex interrelationship preserves cardiac function during homeostasis; however it is also responsible for pathologic remodeling following myocardial injury. Therefore, enhancing our understanding of this cross-talk may provide mechanistic insights into the pathogenesis of heart failure and suggest new approaches to novel, targeted pharmacologic therapies. This review explores the implications of ECM-cell interactions in myocardial cell behavior and cardiac function at baseline and following myocardial injury.

Modifying the mechanics of healing infarcts: Is better the enemy of good?

Myocardial infarction (MI) is a major source of morbidity and mortality worldwide, with over 7 million people suffering infarctions each year. Heart muscle damaged during MI is replaced by a collagenous scar over a period of several weeks, and the mechanical properties of that scar tissue are a key determinant of serious post-MI complications such as infarct rupture, depression of heart function, and progression to heart failure. Thus, there is increasing interest in developing therapies that modify the structure and mechanics of healing infarct scar. Yet most prior attempts at therapeutic scar modification have failed, some catastrophically. This article reviews available information about the mechanics of healing infarct scar and the functional impact of scar mechanical properties, and attempts to infer principles that can better guide future attempts to modify scar. One important conclusion is that collagen structure, mechanics, and remodeling of healing infarct scar vary so widely among experimental models that any novel therapy should be tested across a range of species, infarct locations, and reperfusion protocols. Another lesson from past work is that the biology and mechanics of healing infarcts are sufficiently complex that the effects of interventions are often counterintuitive; for example, increasing infarct stiffness has little effect on heart function, and inhibition of matrix metalloproteases (MMPs) has little effect on scar collagen content. Computational models can help explain such counterintuitive results, and are becoming an increasingly important tool for integrating known information to better identify promising therapies and design experiments to test them. Moving forward, potentially exciting new opportunities for therapeutic modification of infarct mechanics include modulating anisotropy and promoting scar compaction.

Mechanical control of cardiac myofibroblasts

Fibroblasts produce and turn over collagenous extracellular matrix as part of the normal adaptive response to increased mechanical load in the heart, e.g. during prolonged exercise. However, chronic overload as a consequence of hypertension or myocardial injury trigger a repair program that culminates in the formation of myofibroblasts. Myofibroblasts are opportunistically activated from various precursor cells that all acquire a phenotype promoting excessive collagen secretion and contraction of the neo-matrix into stiff scar tissue. Stiff fibrotic tissue reduces heart distensibility, impedes pumping and valve function, contributes to diastolic and systolic dysfunction, and affects myocardial electrical transmission, potentially leading to arrhythmia and heart failure. Here, we discuss how mechanical factors, such as matrix stiffness and strain, are feeding back and cooperate with cytokine signals to drive myofibroblast activation. We elaborate on the importance of considering the mechanical boundary conditions in the heart to generate better cell culture models for mechanistic studies of cardiac fibroblast function. Elements of the force transmission and mechanoperception apparatus acting in myofibroblasts are presented as potential therapeutic targets to treat fibrosis.