Defective myofibroblast formation from mesenchymal stem cells in the aging murine heart rescue by activation of the AMPK pathway

Heart Failure With Preserved Ejection Fraction in the Elderly Population: Basic Mechanisms and Clinical Considerations

Heart failure with preserved ejection fraction (HFpEF) refers to a clinical condition in which the signs of heart failure, such as pulmonary congestion, peripheral edema, and increased natriuretic peptide levels, are present despite normal ejection fractions and the absence of other causes (eg, pericardial disease). The ejection fraction cutoff for the definition of HFpEF has varied in the past, but recent society guidelines have settled on a consensus of 50%. HFpEF is particularly common in the elderly population. The aim of this narrative review is to summarize the available literature regarding HFpEF in elderly patients in terms of evidence for the age dependence, specific clinical features, and underlying mechanisms. In the clinical arena, we review the epidemiology, discuss distinct clinical phenotypes typically seen in elderly patients, the importance of frailty, the role of biomarkers, and the role of medical therapies (including sodium-glucose cotransport protein 2 inhibitors, renin-angiotensin-aldosterone system blockers, angiotensin receptor/neprilysin inhibitors, diuretics, and β-adrenergic receptor blockers). We then go on to discuss the basic mechanisms implicated in HFpEF, including cellular senescence, fibrosis, inflammation, mitochondrial dysfunction, enhanced production of reactive oxygen species, abnormal cellular calcium handling, changes in microRNA signalling, insulin resistance, and sex hormone changes. Finally, we review knowledge gaps and promising areas of future investigation. Improved understanding of the specific clinical manifestations of HFpEF in elderly individuals and of the fundamental mechanisms that contribute to the age-related risk of HFpEF promises to lead to novel diagnostic and treatment approaches that will improve outcomes for this common cardiac disorder in a vulnerable population.

The Senescent Heart-"Age Doth Wither Its Infinite Variety"

Cardiovascular diseases are a leading cause of morbidity and mortality world-wide. While many factors like smoking, hypertension, diabetes, dyslipidaemia, a sedentary lifestyle, and genetic factors can predispose to cardiovascular diseases, the natural process of aging is by itself a major determinant of the risk. Cardiac aging is marked by a conglomerate of cellular and molecular changes, exacerbated by age-driven decline in cardiac regeneration capacity. Although the phenotypes of cardiac aging are well characterised, the underlying molecular mechanisms are far less explored. Recent advances unequivocally link cardiovascular aging to the dysregulation of critical signalling pathways in cardiac fibroblasts, which compromises the critical role of these cells in maintaining the structural and functional integrity of the myocardium. Clearly, the identification of cardiac fibroblast-specific factors and mechanisms that regulate cardiac fibroblast function in the senescent myocardium is of immense importance. In this regard, recent studies show that Discoidin domain receptor 2 (DDR2), a collagen-activated receptor tyrosine kinase predominantly located in cardiac fibroblasts, has an obligate role in cardiac fibroblast function and cardiovascular fibrosis. Incisive studies on the molecular basis of cardiovascular aging and dysregulated fibroblast function in the senescent heart would pave the way for effective strategies to mitigate cardiovascular diseases in a rapidly growing elderly population.

A defective mechanosensing pathway affects fibroblast-to-myofibroblast transition in the old male mouse heart

The cardiac fibroblast interacts with an extracellular matrix (ECM), enabling myofibroblast maturation via a process called mechanosensing. Although in the aging male heart, ECM is stiffer than in the young mouse, myofibroblast development is impaired, as demonstrated in 2-D and 3-D experiments. In old male cardiac fibroblasts, we found a decrease in actin polymerization, α-smooth muscle actin (α-SMA), and Kindlin-2 expressions, the latter an effector of the mechanosensing. When Kindlin-2 levels were manipulated via siRNA interference, young fibroblasts developed an old-like fibroblast phenotype, whereas Kindlin-2 overexpression in old fibroblasts reversed the defective phenotype. Finally, inhibition of overactivated extracellular regulated kinases 1 and 2 (ERK1/2) in the old male fibroblasts rescued actin polymerization and α-SMA expression. Pathological ERK1/2 overactivation was also attenuated by Kindlin-2 overexpression. In contrast, old female cardiac fibroblasts retained an operant mechanosensing pathway. In conclusion, we identified defective components of the Kindlin/ERK/actin/α-SMA mechanosensing axis in aged male fibroblasts.

Senescent cardiac fibroblasts: A key role in cardiac fibrosis

Cardiac fibroblasts are a cell population that controls the homeostasis of the extracellular matrix and orchestrates a damage response to maintain cardiac architecture and performance. Due to these functions, fibroblasts play a central role in cardiac fibrosis development, and there are large differences in matrix protein secretion profiles between fibroblasts from aged versus young animals. Senescence is a multifactorial and complex process that has been associated with inflammatory and fibrotic responses. After damage, transient cellular senescence is usually beneficial, as these cells promote tissue repair. However, the persistent presence of senescent cells within a tissue is linked with fibrosis development and organ dysfunction, leading to aging-related diseases such as cardiovascular pathologies. In the heart, early cardiac fibroblast senescence after myocardial infarction seems to be protective to avoid excessive fibrosis; however, in non-infarcted models of cardiac fibrosis, cardiac fibroblast senescence has been shown to be deleterious. Today, two new classes of drugs, termed senolytics and senostatics, which eliminate senescent cells or modify senescence-associated secretory phenotype, respectively, arise as novel therapeutical strategies to treat aging-related pathologies. However, further studies will be needed to evaluate the extent of the utility of senotherapeutic drugs in cardiac diseases, in which pathological context and temporality of the intervention must be considered.

Compound 13 Promotes Epidermal Healing in Mouse Fetuses via Activation of AMPK

Unlike adults, early developing fetuses can completely regenerate tissue, and replicating this could lead to the development of treatments to reduce scarring. Mice epidermal structures, including wound healing patterns, are regenerated until embryonic day (E) 13, leaving visible scars thereafter. These patterns require actin cable formation at the epithelial wound margin through AMP-activated protein kinase (AMPK) activation. We aimed to investigate whether the administration of compound 13 (C13), a recently discovered AMPK activator, to the wound could reproduce this actin remodeling and skin regeneration pattern through its AMPK activating effect. The C13 administration resulted in partial formations of actin cables, which would normally result in scarring, and scar reduction during the healing of full-layer skin defects that occurred in E14 and E15 fetuses. Furthermore, C13 was found to cause AMPK activation in these embryonic mouse epidermal cells. Along with AMPK activation, Rac1 signaling, which is involved in leaflet pseudopodia formation and cell migration, was suppressed in C13-treated wounds, indicating that C13 inhibits epidermal cell migration. This suggests that actin may be mobilized by C13 for cable formation. Administration of C13 to wounds may achieve wound healing similar to regenerative wound healing patterns and may be a potential candidate for new treatments to heal scars.

Therapeutic opportunities for senolysis in cardiovascular disease

Cellular senescence within the cardiovascular system has, until recently, been understudied and unappreciated as a factor in the development of age-related cardiovascular diseases such as heart failure, myocardial infarction and atherosclerosis. This is in part due to challenges with defining senescence within post-mitotic cells such as cardiomyocytes. However, recent evidence has demonstrated senescent-like changes, including a senescence-associated secretory phenotype (SASP), in cardiomyocytes in response to ageing and cell stress. Other replicating cells, including fibroblasts and vascular smooth muscle cells, within the cardiovascular system have also been shown to undergo senescence and contribute to disease pathogenesis. These findings coupled with the emergence of senolytic therapies, to target and eliminate senescent cells, have provided fascinating new avenues for management of several age-related cardiovascular diseases with high prevalence. In this review, we discuss the role of senescent cells within the cardiovascular system and highlight the contribution of senescence cells to common cardiovascular diseases. We discuss the emerging role for senolytics in cardiovascular disease management while highlighting important aspects of senescence biology which must be clarified before the potential of senolytics can be fully realized.

Transplanted allogeneic cardiac progenitor cells secrete GDF-15 and stimulate an active immune remodeling process in the ischemic myocardium

BACKGROUND

Despite promising results in clinical studies, the mechanism for the beneficial effects of allogenic cell-based therapies remains unclear. Macrophages are not only critical mediators of inflammation but also critical players in cardiac remodeling. We hypothesized that transplanted allogenic rat cardiac progenitor cells (rCPCs) augment T-regulatory cells which ultimately promote proliferation of M2 like macrophages by an as-yet undefined mechanism.

METHODS AND RESULTS

To test this hypothesis, we used crossover rat strains for exploring the mechanism of myocardial repair by allogenic CPCs. Human CPCs (hCPCs) were isolated from adult patients undergoing coronary artery bypass grafting, and rat CPCs (rCPCs) were isolated from male Wistar-Kyoto (WKY) rat hearts. Allogenic rCPCs suppressed the proliferation of T-cells observed in mixed lymphocyte reactions in vitro. Transplanted syngeneic or allogeneic rCPCs significantly increased cardiac function in a rat myocardial infarct (MI) model, whereas xenogeneic CPCs did not. Allogeneic rCPCs stimulated immunomodulatory responses by specifically increasing T-regulatory cells and M2 polarization, while maintaining their cardiac recovery potential and safety profile. Mechanistically, we confirmed the inactivation of NF-kB in Treg cells and increased M2 macrophages in the myocardium after MI by transplanted CPCs derived GDF15 and it's uptake by CD48 receptor on immune cells.

CONCLUSION

Collectively, these findings strongly support the active immunomodulatory properties and robust therapeutic potential of allogenic CPCs in post-MI cardiac dysfunction.

Fibroblast-mediated intercellular crosstalk in the healthy and diseased heart

Cardiac fibroblasts constitute a major cell population in the heart. They secrete extracellular matrix components and various other factors shaping the microenvironment of the heart. In silico analysis of intercellular communication based on single-cell RNA sequencing revealed that fibroblasts are the source of the majority of outgoing signals to other cell types. This observation suggests that fibroblasts play key roles in orchestrating cellular interactions that maintain organ homeostasis but that can also contribute to disease states. Here, we will review the current knowledge of fibroblast interactions in the healthy, diseased, and aging heart. We focus on the interactions that fibroblasts establish with other cells of the heart, specifically cardiomyocytes, endothelial cells and immune cells, and particularly those relying on paracrine, electrical, and exosomal communication modes.

Longevity genes, cardiac ageing, and the pathogenesis of cardiomyopathy: implications for understanding the effects of current and future treatments for heart failure

The two primary molecular regulators of lifespan are sirtuin-1 (SIRT1) and mammalian target of rapamycin complex 1 (mTORC1). Each plays a central role in two highly interconnected pathways that modulate the balance between cellular growth and survival. The activation of SIRT1 [along with peroxisome proliferator-activated receptor-gamma coactivator (PGC-1α) and adenosine monophosphate-activated protein kinase (AMPK)] and the suppression of mTORC1 (along with its upstream regulator, Akt) act to prolong organismal longevity and retard cardiac ageing. Both activation of SIRT1/PGC-1α and inhibition of mTORC1 shifts the balance of cellular priorities so as to promote cardiomyocyte survival over growth, leading to cardioprotective effects in experimental models. These benefits may be related to direct actions to modulate oxidative stress, organellar function, proinflammatory pathways, and maladaptive hypertrophy. In addition, a primary shared benefit of both SIRT1/PGC-1α/AMPK activation and Akt/mTORC1 inhibition is the enhancement of autophagy, a lysosome-dependent degradative pathway, which clears the cytosol of dysfunctional organelles and misfolded proteins that drive the ageing process by increasing oxidative and endoplasmic reticulum stress. Autophagy underlies the ability of SIRT1/PGC-1α/AMPK activation and Akt/mTORC1 suppression to extend lifespan, mitigate cardiac ageing, alleviate cellular stress, and ameliorate the development and progression of cardiomyopathy; silencing of autophagy genes abolishes these benefits. Loss of SIRT1/PGC-1α/AMPK function or hyperactivation of Akt/mTORC1 is a consistent feature of experimental cardiomyopathy, and reversal of these abnormalities mitigates the development of heart failure. Interestingly, most treatments that have been shown to be clinically effective in the treatment of chronic heart failure with a reduced ejection fraction have been reported experimentally to exert favourable effects to activate SIRT1/PGC-1α/AMPK and/or suppress Akt/mTORC1, and thereby, to promote autophagic flux. Therefore, the impairment of autophagy resulting from derangements in longevity gene signalling is likely to represent a seminal event in the evolution and progression of cardiomyopathy.

Treatment with a DC-SIGN ligand reduces macrophage polarization and diastolic dysfunction in the aging female but not male mouse hearts

Cardiac diastolic dysfunction in aging arises from increased ventricular stiffness caused by inflammation and interstitial fibrosis. The diastolic dysfunction contributes to heart failure with preserved ejection fraction (HFpEF), which in the aging population is more common in women. This report examines its progression over 12 weeks in aging C57BL/6J mice and correlates its development with changes in macrophage polarization and collagen deposition.Aged C57BL/6J mice were injected with dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) ligand 1 (DCSL1, an anti-inflammatory agent) or saline for 12 weeks. Echo and Doppler measurements were performed before and after 4 and 12 weeks of treatment. DCSL1 prevented the worsening of diastolic dysfunction over time in females but not in males. Cardiac single cell suspensions analyzed by flow cytometry revealed changes in the inflammatory infiltrate: (1) in males, there was an increased total number of leukocytes with an increased pro-inflammatory profile compared with females and they did not respond to DCSL1; (2) by contrast, DCSL1 treatment resulted in a shift in macrophage polarization to an anti-inflammatory phenotype in females. Notably, DCSL1 preferentially targeted tumor necrosis factor-α (TNFα+) pro-inflammatory macrophages. The reduction in pro-inflammatory macrophage polarization was accompanied by a decrease in collagen content in the heart.Age-associated diastolic dysfunction in mice is more severe in females and is associated with unique changes in macrophage polarization in cardiac tissue. Treatment with DCSL1 mitigates the changes in inflammation, cardiac function, and fibrosis. The characteristics of diastolic dysfunction in aging female mice mimic similar changes in aging women.

Mechanosensing dysregulation in the fibroblast: A hallmark of the aging heart

The myofibroblast is a specialized fibroblast that expresses α-smooth muscle actin (α-SMA) and participates in wound contraction and fibrosis. The fibroblast to myofibroblast transition depends on chemical and mechanical signals. A fibroblast senses the changes in the environment (extracellular matrix (ECM)) and transduces these changes to the cytoskeleton and the nucleus, resulting in activation or inhibition of α-SMA transcription in a process called mechanosensing. A stiff matrix greatly facilitates the transition from fibroblast to myofibroblast, and although the aging heart is much stiffer than the young one, the aging fibroblast has difficulties in transitioning into the contractile phenotype. This suggests that the events occurring downstream of the matrix, such as activation or changes in expression levels of various proteins participating in mechanotransduction can negatively alter the ability of the aging fibroblast to become a myofibroblast. In this review, we will discuss in detail the changes in ECM, receptors (integrin or non-integrin), focal adhesions, cytoskeleton, and transcription factors involved in mechanosensing that occur with aging.

Differential Gender-Dependent Patterns of Cardiac Fibrosis and Fibroblast Phenotypes in Aging Mice

Aging is characterized by physiological changes within the heart leading to fibrosis and dysfunction even in individuals without underlying pathologies. Gender has been shown to influence the characteristics of cardiac aging; however, gender-dependent cardiac fibrosis occurring with age remains largely not elucidated. Thus, broadening our understanding of this phenomenon proves necessary in order to develop novel anti-fibrotic strategies in the elderly. In this study, we aim to characterize cardiac fibrosis and cardiac fibroblast (CF) populations in aged male and female mice. Echocardiography revealed eccentric hypertrophy with left ventricular dilatation in the aged male versus concentric hypertrophy with left posterior wall thickening in the female, with preserved cardiac function in both groups. Reactive fibrosis was evidenced in the myocardium and epicardium of the aged female mice hearts whereas perivascular and replacement ones where present in the male heart. Collagen I was predominant in the aged male heart whereas collagen III was the main component in the female heart. CFs in the aged male heart were mainly recruited from resident PDGFRα⁺ populations but not derived from epicardium as evidenced by the absence of epicardial progenitor transcription factors Tcf21, Tbx18 and Wt1. Our results present a paradigm for gender-dependent cardiac fibrosis and the origins of CFs with age. This sets forth to revisit cardiac anti-fibrotic management according to the gender in the elderly and to explore novel therapeutic targets.

Cardiac fibroblast diversity in health and disease

The cardiac stroma plays essential roles in health and following cardiac damage. The major player of the stroma with respect to extracellular matrix deposition, maintenance and remodeling is the poorly defined fibroblast. It has long been recognized that there is considerable variability to the fibroblast phenotype. With the advent of new, high throughput analytical methods our understanding and appreciation of this heterogeneity has grown dramatically. This review aims to explore the diversity of cardiac fibroblasts and highlights new insights into the diverse nature of these cells and their progenitors as revealed by single cell sequencing and fate mapping studies. We propose that at least in part the observed heterogeneity is related to the existence of a differentiation cascade within stromal cells. Beyond in-organ heterogeneity, we also discuss how the stromal response to damage differs between non-regenerating organs such as the heart and regenerating organs such as skeletal muscle. In exploring possible causes for these differences, we outline that although fibrogenic cells from different organs overlap in many properties, they still possess organ-specific transcriptional signatures and differentiation biases that make them functionally distinct.

Modelling cardiac fibrosis using three-dimensional cardiac microtissues derived from human embryonic stem cells

Background

Cardiac fibrosis is the most common pathway of many cardiac diseases. To date, there has been no suitable in vitro cardiac fibrosis model that could sufficiently mimic the complex environment of the human heart. Here, a three-dimensional (3D) cardiac sphere platform of contractile cardiac microtissue, composed of human embryonic stem cell (hESC)-derived cardiomyocytes (CMs) and mesenchymal stem cells (MSCs), is presented to better recapitulate the human heart.

Results

We hypothesized that MSCs would develop an in vitro fibrotic reaction in response to treatment with transforming growth factor-β1 (TGF-β1), a primary inducer of cardiac fibrosis. The addition of MSCs improved sarcomeric organization, electrophysiological properties, and the expression of cardiac-specific genes, suggesting their physiological relevance in the generation of human cardiac microtissue model in vitro. MSCs could also generate fibroblasts within 3D cardiac microtissues and, subsequently, these fibroblasts were transdifferentiated into myofibroblasts by the exogenous addition of TGF-β1. Cardiac microtissues displayed fibrotic features such as the deposition of collagen, the presence of numerous apoptotic CMs and the dissolution of mitochondrial networks. Furthermore, treatment with pro-fibrotic substances demonstrated that this model could reproduce key molecular and cellular fibrotic events.

Conclusions

This highlights the potential of our 3D cardiac microtissues as a valuable tool for manifesting and evaluating the pro-fibrotic effects of various agents, thereby representing an important step forward towards an in vitro system for the prediction of drug-induced cardiac fibrosis and the study of the pathological changes in human cardiac fibrosis.

Electronic supplementary material

The online version of this article (10.1186/s13036-019-0139-6) contains supplementary material, which is available to authorized users.

Changes in cardiac resident fibroblast physiology and phenotype in aging

The cardiac fibroblast plays a central role in tissue homeostasis and in repair after injury. With aging, dysregulated cardiac fibroblasts have a reduced capacity to activate a canonical transforming growth factor-β-Smad pathway and differentiate poorly into contractile myofibroblasts. That results in the formation of an insufficient scar after myocardial infarction. In contrast, in the uninjured aged heart, fibroblasts are activated and acquire a profibrotic phenotype that leads to interstitial fibrosis, ventricular stiffness, and diastolic dysfunction, all conditions that may lead to heart failure. There is an apparent paradox in aging, wherein reparative fibrosis is impaired but interstitial, adverse fibrosis is augmented. This could be explained by analyzing the effectiveness of signaling pathways in resident fibroblasts from young versus aged hearts. Whereas defective signaling by transforming growth factor-β leads to insufficient scar formation by myofibroblasts, enhanced activation of the ERK1/2 pathway may be responsible for interstitial fibrosis mediated by activated fibroblasts. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/fibroblast-phenotypic-changes-in-the-aging-heart/ .

Aicar treatment reduces interstitial fibrosis in aging mice: Suppression of the inflammatory fibroblast

In 2030, elderly people will represent 20% of the United States population. Even now, chronic cardiac diseases, especially heart failure with preserved systolic function (HFpEF), are the most expensive DRGs for Medicare. Progressive interstitial fibrosis in the aging heart is well recognized as an important component of HFpEF. Our recent studies suggested an important pathophysiologic role for reduced TGF-β receptor 1 (TGFβR1) signaling in mesenchymal stem cells (MSCs) and their mesenchymal fibroblast progeny in the development of interstitial fibrosis. This report arises from our previous studies, which suggest that an inflammatory phenotype exists in these mesenchymal fibroblasts as a result of a reduced TGF-β-Smad-dependent pathway but upregulated farnesyltransferase (FTase)-Ras-Erk signaling. In this report we provide evidence for a therapeutic approach that downregulates Erk activation through an adenosine monophosphate-activated kinase (AMPK) pathway. Aging C57BL/6J mice were treated with AICAR (an AMPK activator) for a 30-day period. This treatment suppressed excessive monocyte chemoattractant protein-1 (MCP-1) generation, which diminished leukocyte infiltration and in consequence suppressed the formation of macrophage-derived myeloid fibroblasts. Interestingly, the number of mesenchymal fibroblasts was also reduced. In addition, we observed changes in extracellular matrix (ECM) deposition, specifically that collagen type I and the alternatively spliced variant of fibronectin (EDA) expressions were reduced. These data suggest that the upregulation of AMPK activity is a potential therapeutic approach to fibrosis in the aging heart.

Dissecting the role of myeloid and mesenchymal fibroblasts in age-dependent cardiac fibrosis

Aging is associated with increased cardiac interstitial fibrosis and diastolic dysfunction. Our previous study has shown that mesenchymal fibroblasts in the C57BL/6J (B6J) aging mouse heart acquire an inflammatory phenotype and produce higher levels of chemokines. Monocyte chemoattractant protein-1 (MCP-1) secreted by these aged fibroblasts promotes leukocyte uptake into the heart. Some of the monocytes that migrate into the heart polarize into M2a macrophages/myeloid fibroblasts. The number of activated mesenchymal fibroblasts also increases with age, and consequently, both sources of fibroblasts contribute to fibrosis. Here, we further investigate mechanisms by which inflammation influences activation of myeloid and mesenchymal fibroblasts and their collagen synthesis. We examined cardiac fibrosis and heart function in three aged mouse strains; we compared C57BL/6J (B6J) with two other strains that have reduced inflammation via different mechanisms. Aged C57BL/6N (B6N) hearts are protected from oxidative stress and fibroblasts derived from them do not develop an inflammatory phenotype. Likewise, these mice have preserved diastolic function. Aged MCP-1 null mice on the B6J background (MCP-1KO) are protected from elevated leukocyte infiltration; they develop moderate but reduced fibrosis and diastolic dysfunction. Based on these studies, we further delineated the role of resident versus monocyte-derived M2a macrophages in myeloid-dependent fibrosis and found that the number of monocyte-derived M2a (but not resident) macrophages correlates with age-related fibrosis and diastolic dysfunction. In conclusion, we have found that ROS and inflammatory mediators are necessary for activation of fibroblasts of both developmental origins, and prevention of either led to better functional outcomes.

Knockdown of Plakophilin 2 Downregulates miR-184 Through CpG Hypermethylation and Suppression of the E2F1 Pathway and Leads to Enhanced Adipogenesis In Vitro

RATIONALE

PKP2, encoding plakophilin 2 (PKP2), is the most common causal gene for arrhythmogenic cardiomyopathy.

OBJECTIVE

To characterize miRNA expression profile in PKP2-deficient cells.

METHODS AND RESULTS

Control and PKP2-knockdown HL-1 (HL-1(Pkp2-shRNA)) cells were screened for 750 miRNAs using low-density microfluidic panels. Fifty-nine miRNAs were differentially expressed. MiR-184 was the most downregulated miRNA. Expression of miR-184 in the heart and cardiac myocyte was developmentally downregulated and was low in mature myocytes. MicroRNA-184 was predominantly expressed in cardiac mesenchymal progenitor cells. Knockdown of Pkp2 in cardiac mesenchymal progenitor cells also reduced miR-184 levels. Expression of miR-184 was transcriptionally regulated by the E2F1 pathway, which was suppressed in PKP2-deficient cells. Activation of E2F1, on overexpression of its activator CCND1 (cyclin D1) or knockdown of its inhibitor retinoblastoma 1, partially rescued miR-184 levels. In addition, DNA methyltransferase-1 was recruited to the promoter region of miR-184, and the CpG sites at the upstream region of miR-184 were hypermethylated. Treatment with 5-aza-2'-deoxycytidine, a demethylation agent, and knockdown of DNA methyltransferase-1 partially rescued miR-184 level. Pathway analysis of paired miR-184:mRNA targets identified cell proliferation, differentiation, and death as the main affected biological processes. Knockdown of miR-184 in HL-1 cells and mesenchymal progenitor cells induced and, conversely, its overexpression attenuated adipogenesis.

CONCLUSIONS

PKP2 deficiency leads to suppression of the E2F1 pathway and hypermethylation of the CpG sites at miR-184 promoter, resulting in downregulation of miR-184 levels. Suppression of miR-184 enhances and its activation attenuates adipogenesis in vitro. Thus, miR-184 contributes to the pathogenesis of adipogenesis in PKP2-deficient cells.

Mast cells promote proliferation and migration and inhibit differentiation of mesenchymal stem cells through PDGF

BACKGROUND

Mast cells (MCs) dynamically participate in wound healing after myocardial infarction (MI) by releasing cytokines. Indeed, MC-deficient mice undergo rapid left ventricular dilation post-MI. Mesenchymal stem cells (MSCs) are recruited to the injured region following an MI and have potential for cardiac repair. In the current study, we evaluate the effect of MCs on MSC proliferation and myogenic differentiation.

METHODS AND RESULTS

MCs were cultured from mouse bone marrow and MC granulate (MCG) was extracted from MCs via freeze-thaw cycles followed by filtration. α-SMA (smooth muscle actin) expression was examined as an indicator of myogenic differentiation. MSC/MC co-culture resulted in decreased MSC differentiation indicated by α-SMA suppression in MSCs. MCG also suppressed α-SMA expression and increased MSC migration and proliferation in a dose-dependent manner. Removal of MCG rescued α-SMA expression and MSC differentiation. Platelet derived growth factor (PDGF) receptor blockade using AG1296 also rescued MSC differentiation even after MCG treatment. Real-time PCR and Western blot showed that MCG exerted its effects on MSCs via downregulation of miR-145 and miR-143, downregulation of myocardin, upregulation of Klf4, and increased Erk and Elk1 phosphorylation.

CONCLUSIONS

MCs promote MSC proliferation and migration by suppressing their myogenic differentiation. MCs accomplish this via activation of the PDGF pathway, downregulation of miR-145/143, and modulation of the myocardin-Klf4 axis. These data suggest a potential role for MSC/MC interaction in the infarcted heart where MCs may inhibit MSCs from differentiation and promote their proliferation whereby increased cardiac MSC accumulation promotes eventual cardiac regeneration after MCs cease activity.

Ghrelin Inhibits Post-Operative Adhesions via Blockage of the TGF-β Signaling Pathway

Post-operative adhesions are a critical problem in pelvic and abdominal surgery despite a multitude of studies dedicated to finding modalities to prevent their occurrence. Ghrelin administration promotes an anti-fibrotic response in a surgical mouse model of adhesion-induction, but the mechanisms mediating this effect have not been established. In the current study, the molecular mechanisms that underlie the anti-adhesion effect of ghrelin were investigated. Post-surgical adhesions were experimentally created in C57BL/6 wild-type mice via a combination of ischemic peritoneal buttons and cecal multiple abrasions. Ghrelin or saline intraperitoneal injections were given twice daily from two days before surgery to selected time points post-surgically to assess the phenotypic and molecular effects of treatment (1 day (n = 20), 4 days (n = 20) and 20 days (n = 40) after surgery). Endpoints included the scoring of adhesions and gene and protein expression analysis of pro-fibrogenic factors conducted on peritoneal ischemic tissue by quantitative PCR and Western blot. Ghrelin administration significantly reduced post-surgical adhesions and down-regulated pro-inflammatory gene and protein expression, including Tgfb3 and Tgfbr2. The up-regulation of inhibitory proteins Smad6 and Smad7 confirmed the ghrelin-induced blockage of TGF-β signaling. Ghrelin is a candidate therapeutic drug for post-operative adhesion prevention, inhibiting inflammatory responses via blockage of the TGF-β signaling pathway at the onset of surgery before the occurrence of the granulation-remodeling phase.

Mesenchymal stem cell-derived inflammatory fibroblasts mediate interstitial fibrosis in the aging heart

Pathologic fibrosis in the aging mouse heart is associated with dysregulated resident mesenchymal stem cells (MSC) arising from reduced stemness and aberrant differentiation into dysfunctional inflammatory fibroblasts. Fibroblasts derived from aging MSC secrete higher levels of 1) collagen type 1 (Col1) that directly contributes to fibrosis, 2) monocyte chemoattractant protein-1 (MCP-1) that attracts leukocytes from the blood and 3) interleukin-6 (IL-6) that facilitates transition of monocytes into myeloid fibroblasts. The transcriptional activation of these proteins is controlled via the farnesyltransferase (FTase)-Ras-Erk pathway. The intrinsic change in the MSC phenotype acquired by advanced age is specific for the heart since MSC originating from bone wall (BW-MSC) or fibroblasts derived from them were free of these defects. The potential therapeutic interventions other than clinically approved strategies based on findings presented in this review are discussed as well. This article is a part of a Special Issue entitled "Fibrosis and Myocardial Remodeling".

Disruption of collagen homeostasis can reverse established age-related myocardial fibrosis

Heart failure, the leading cause of hospitalization of elderly patients, is correlated with myocardial fibrosis (ie, deposition of excess extracellular matrix proteins such as collagen). A key regulator of collagen homeostasis is lysyl oxidase (LOX), an enzyme responsible for cross-linking collagen fibers. Our objective was to ameliorate age-related myocardial fibrosis by disrupting collagen cross-linking through inhibition of LOX. The nonreversible LOX inhibitor β-aminopropionitrile (BAPN) was administered by osmotic minipump to 38-week-old C57BL/6J male mice for 2 weeks. Sirius Red staining of myocardial cross sections revealed a reduction in fibrosis, compared with age-matched controls (5.84 ± 0.30% versus 10.17 ± 1.34%) (P < 0.05), to a level similar to that of young mice at 8 weeks (4.9 ± 1.2%). BAPN significantly reduced COL1A1 mRNA, compared with age-matched mice (3.5 ± 0.3-fold versus 15.2 ± 4.9-fold) (P < 0.05), suggesting that LOX is involved in regulation of collagen synthesis. In accord, fibrotic factor mRNA expression was reduced after BAPN. There was also a novel increase in Ly6C expression by resident macrophages. By interrupting collagen cross-linking by LOX, the BAPN treatment reduced myocardial fibrosis. A novel observation is that BAPN treatment modulated the transforming growth factor-β pathway, collagen synthesis, and the resident macrophage population. This is especially valuable in terms of potential therapeutic targeting of collagen regulation and thereby age-related myocardial fibrosis.

Impact of AMP-Activated Protein Kinase α1 Deficiency on Tissue Injury following Unilateral Ureteral Obstruction

Background

AMP-activated protein kinase (Ampk) is a sensor of the cellular energy status and a powerful regulator of metabolism. Activation of Ampk was previously shown to participate in monocyte-to-fibroblast transition and matrix protein production in renal tissue. Thus, the present study explored whether the catalytic Ampkα1 isoform participates in the regulation of the renal fibrotic response following unilateral ureteral obstruction (UUO).

Methods

UUO was induced in gene-targeted mice lacking functional Ampkα1 (Ampkα1^-/-) and in corresponding wild-type mice (Ampkα1^+/+). In the obstructed kidney and, for comparison, in the non-obstructed control kidney, quantitative RT-PCR, Western blotting and immunostaining were employed to determine transcript levels and protein abundance, respectively.

Results

In Ampkα1^+/+ mice, UUO significantly up-regulated the protein abundance of the Ampkα1 isoform, but significantly down-regulated the Ampkα2 isoform in renal tissue. Phosphorylated Ampkα protein levels were significantly increased in obstructed kidney tissue of Ampkα1^+/+ mice but not of Ampkα1^-/- mice. Renal expression of α-smooth muscle actin was increased following UUO, an effect again less pronounced in Ampkα1^-/- mice than in Ampkα1^+/+ mice. Histological analysis did not reveal a profound effect of Ampkα1 deficiency on collagen 1 protein deposition. UUO significantly increased phosphorylated and total Tgf-ß-activated kinase 1 (Tak1) protein, as well as transcript levels of Tak1-downstream targets c-Fos, Il6, Pai1 and Snai1 in Ampkα1^+/+ mice, effects again significantly ameliorated in Ampkα1^-/- mice. Moreover, Ampkα1 deficiency inhibited the UUO-induced mRNA expression of Cd206, a marker of M2 macrophages and of Cxcl16, a pro-fibrotic chemokine associated with myeloid fibroblast formation. The effects of Ampkα1 deficiency during UUO were, however, paralleled by increased tubular injury and apoptosis.

Conclusions

Renal obstruction induces an isoform shift from Ampkα2 towards Ampkα1, which contributes to the signaling involved in cell survival and fibrosis.

Sirt7 Contributes to Myocardial Tissue Repair by Maintaining Transforming Growth Factor-β Signaling Pathway

BACKGROUND

Sirt7, 1 of the 7 members of the mammalian sirtuin family, promotes oncogenic transformation. Tumor growth and metastasis require fibrotic and angiogenic responses. Here, we investigated the role of Sirt7 in cardiovascular tissue repair process.

METHODS AND RESULTS

In wild-type mice, Sirt7 expression increased in response to acute cardiovascular injury, including myocardial infarction and hind-limb ischemia, particularly at the active wound healing site. Compared with wild-type mice, homozygous Sirt7-deficient (Sirt7(-/-)) mice showed susceptibility to cardiac rupture after myocardial infarction, delayed blood flow recovery after hind-limb ischemia, and impaired wound healing after skin injury. Histological analysis showed reduced fibrosis, fibroblast differentiation, and inflammatory cell infiltration in the border zone of infarction in Sirt7(-/-) mice. In vitro, Sirt7(-/-) mouse-derived or Sirt7 siRNA-treated cardiac fibroblasts showed reduced transforming growth factor-β signal activation and low expression levels of fibrosis-related genes compared with wild-type mice-derived or control siRNA-treated cells. These changes were accompanied by reduction in transforming growth factor receptor I protein. Loss of Sirt7 activated autophagy in cardiac fibroblasts. Transforming growth factor-β receptor I downregulation induced by loss of Sirt7 was blocked by autophagy inhibitor, and interaction of Sirt7 with protein interacting with protein kinase-Cα was involved in this process.

CONCLUSION

Sirt7 maintains transforming growth factor receptor I by modulating autophagy and is involved in the tissue repair process.

Fibrosis Related Inflammatory Mediators: Role of the IL-10 Cytokine Family

Importance of chronic fibroproliferative diseases (FDs) including pulmonary fibrosis, chronic kidney diseases, inflammatory bowel disease, and cardiovascular or liver fibrosis is rapidly increasing and they have become a major public health problem. According to some estimates about 45% of all deaths are attributed to FDs in the developed world. Independently of their etiology the common hallmark of FDs is chronic inflammation. Infiltrating immune cells, endothelial, epithelial, and other resident cells of the injured organ release an orchestra of inflammatory mediators, which stimulate the proliferation and excessive extracellular matrix (ECM) production of myofibroblasts, the effector cells of organ fibrosis. Abnormal amount of ECM disturbs the original organ architecture leading to the decline of function. Although our knowledge is rapidly expanding, we still have neither a diagnostic tool to detect nor a drug to specifically target fibrosis. Therefore, there is an urgent need for the more comprehensive understanding of the pathomechanism of fibrosis and development of novel diagnostic and therapeutic strategies. In the present review we provide an overview of the common key mediators of organ fibrosis highlighting the role of interleukin-10 (IL-10) cytokine family members (IL-10, IL-19, IL-20, IL-22, IL-24, and IL-26), which recently came into focus as tissue remodeling-related inflammatory cytokines.

Multipotent stem cells of the heart-do they have therapeutic promise?

The last decade has brought a comprehensive change in our view of cardiac remodeling processes under both physiological and pathological conditions, and cardiac stem cells have become important new players in the general mainframe of cardiac homeostasis. Different types of cardiac stem cells show different capacities for differentiation into the three major cardiac lineages: myocytes, endothelial cells and smooth muscle cells. Physiologically, cardiac stem cells contribute to cardiac homeostasis through continual cellular turnover. Pathologically, these cells exhibit a high level of proliferative activity in an apparent attempt to repair acute cardiac injury, indicating that these cells possess (albeit limited) regenerative potential. In addition to cardiac stem cells, mesenchymal stem cells represent another multipotent cell population in the heart; these cells are located in regions near pericytes and exhibit regenerative, angiogenic, antiapoptotic, and immunosuppressive properties. The discovery of these resident cardiac stem cells was followed by a number of experimental studies in animal models of cardiomyopathies, in which cardiac stem cells were tested as a therapeutic option to overcome the limited transdifferentiating potential of hematopoietic or mesenchymal stem cells derived from bone marrow. The promising results of these studies prompted clinical studies of the role of these cells, which have demonstrated the safety and practicability of cellular therapies for the treatment of heart disease. However, questions remain regarding this new therapeutic approach. Thus, the aim of the present review was to discuss the multitude of different cardiac stem cells that have been identified, their possible functional roles in the cardiac regenerative process, and their potential therapeutic uses in treating cardiac diseases.

Mesenchymal stem cell-derived inflammatory fibroblasts promote monocyte transition into myeloid fibroblasts via an IL-6-dependent mechanism in the aging mouse heart

Fibrosis in the old mouse heart arises partly as a result of aberrant mesenchymal fibroblast activation. We have previously shown that endogenous mesenchymal stem cells (MSCs) in the aged heart are markedly resistant to TGF-β signaling. Fibroblasts originating from these MSCs retain their TGF-β unresponsiveness and become inflammatory. In current studies, we found that these inflammatory fibroblasts secreted higher levels of IL-6 (3-fold increase, P < 0.05) when compared with fibroblasts derived from the young hearts. Elevated IL-6 levels in fibroblasts derived from old hearts arose from up-regulated expression of Ras protein-specific guanine nucleotide releasing factor 1 (RasGrf1), a Ras activator (5-fold, P < 0.01). Knockdown of RasGrf1 by gene silencing or pharmacologic inhibition of farnesyltransferase (FTase) or ERK caused reduction of IL-6 mRNA (more than 65%, P < 0.01) and decreased levels of secreted IL-6 (by 44%, P < 0.01). In vitro, IL-6 markedly increased monocyte chemoattractant protein-1-driven monocyte-to-myeloid fibroblast formation after transendothelial migration (TEM; 3-fold, P < 0.01). In conclusion, abnormal expression of RasGrf1 promoted production of IL-6 by mesenchymal fibroblasts in the old heart. Secreted IL-6 supported conversion of monocyte into myeloid fibroblasts. This process promotes fibrosis and contributes to the diastolic dysfunction in the aging heart.

Gingival wound healing: an essential response disturbed by aging?

Gingival wound healing comprises a series of sequential responses that allow the closure of breaches in the masticatory mucosa. This process is of critical importance to prevent the invasion of microbes or other agents into tissues, avoiding the establishment of a chronic infection. Wound healing may also play an important role during cell and tissue reaction to long-term injury, as it may occur during inflammatory responses and cancer. Recent experimental data have shown that gingival wound healing is severely affected by the aging process. These defects may alter distinct phases of the wound-healing process, including epithelial migration, granulation tissue formation, and tissue remodeling. The cellular and molecular defects that may explain these deficiencies include several biological responses such as an increased inflammatory response, altered integrin signaling, reduced growth factor activity, decreased cell proliferation, diminished angiogenesis, reduced collagen synthesis, augmented collagen remodeling, and deterioration of the proliferative and differentiation potential of stem cells. In this review, we explore the cellular and molecular basis of these defects and their possible clinical implications.

The Murphy Roths Large (MRL) mouse strain is naturally resistant to high fat diet-induced hyperglycemia

OBJECTIVE

Due to their previously identified naturally and chronically increased levels of skeletal muscle pAMPK we hypothesized and now investigated whether the MRL/MpJ (MRL) mice would be resistant to high fat diet (HFD)-induced metabolic changes.

MATERIALS/METHODS

Three-week old male MRL and control C57Bl/6 (B6) mice were randomly assigned to 12weeks of high fat diets (HFD) or control diets (CD). Weekly animal masses and fasting blood glucose measurements were acquired. During the last week of diet intervention, fasted animals were subjected to glucose and insulin tolerance tests. At harvest, tissues were dissected for immunoblots and serum was collected for ELISA assays.

RESULTS

The MRL mouse strain is known for its ability to regenerate ear punch wounds, cardiac cryoinjury, and skeletal muscle disease. Despite gaining weight and increasing their fat deposits the MRL mice were resistant to all other indicators of HFD-induced metabolic alterations assayed. Only the HFD-B6 mice displayed fasting hyperglycemia, hyperinsulinemia and hypersensitivity to glucose challenge. HFD-MRL mice were indistinguishable from their CD-MRL counterparts in these metrics. Skeletal muscles from the HFD-MRL contained heightened levels of pAMPK, even above their CD counterparts.

CONCLUSIONS

The MRL mouse strain is the first naturally occurring mouse strain that we are aware of that is resistant to HFD-induced metabolic changes. Furthermore, the increased pAMPK suggests a proximal mechanism for these beneficial metabolic differences. We further hypothesize that these metabolic differences and plasticity provide the basis for the MRL mouse strain's super healing characteristics. This project's ultimate aim is to identify novel therapeutic targets, which specifically increase pAMPK.

AMPK Activators as a Drug for Diabetes, Cancer and Cardiovascular Disease

The cellular mechanisms of AMP-Activated Protein Kinase (AMPK) activators in the treatment and prevention of diabetes, cancer, and cardiovascular disease.

Defective Wound-healing in Aging Gingival Tissue

Aging may negatively affect gingival wound-healing. However, little is known about the mechanisms underlying this phenomenon. The present study examined the cellular responses associated with gingival wound-healing in aging. Primary cultures of human gingival fibroblasts were obtained from healthy young and aged donors for the analysis of cell proliferation, cell invasion, myofibroblastic differentiation, and collagen gel remodeling. Serum from young and old rats was used to stimulate cell migration. Gingival repair was evaluated in Sprague-Dawley rats of different ages. Data were analyzed by the Mann-Whitney and Kruskal-Wallis tests, with a p value of .05. Fibroblasts from aged donors showed a significant decrease in cell proliferation, migration, Rac activation, and collagen remodeling when compared with young fibroblasts. Serum from young rats induced higher cell migration when compared with serum from old rats. After TGF-beta1 stimulation, both young and old fibroblasts demonstrated increased levels of alpha-SMA. However, alpha-SMA was incorporated into actin stress fibers in young but not in old fibroblasts. After 7 days of repair, a significant delay in gingival wound-healing was observed in old rats. The present study suggests that cell migration, myofibroblastic differentiation, collagen gel remodeling, and proliferation are decreased in aged fibroblasts. In addition, altered cell migration in wound-healing may be attributable not only to cellular defects but also to changes in serum factors associated with the senescence process.

Human mesenchymal stem cells express a myofibroblastic phenotype in vitro: comparison to human cardiac myofibroblasts

Cardiac fibrosis accompanies a variety of myocardial disorders, and is induced by myofibroblasts. These cells may be composed of a heterogeneous population of parent cells, including interstitial fibroblasts and circulating progenitor cells. Direct comparison of human bone marrow-derived mesenchymal stem cells (BM-MSCs) and cardiac myofibroblasts (CMyfbs) has not been previously reported. We hypothesized that BM-MSCs readily adopt a myofibroblastic phenotype in culture. Human primary BM-MSCs and human CMyfbs were isolated from patients undergoing open heart surgery and expanded under standard culture conditions. We assessed and compared their phenotypic and functional characteristics by examining their gene expression profile, their ability to contract collagen gels and synthesize collagen type I. In addition, we examined the role of non-muscle myosin II (NMMII) in modulating MSC myogenic function using NMMII siRNA knockdown and blebbistatin, a specific small molecule inhibitor of NMMII. We report that, while human BM-MSCs retain pluripotency, they adopt a myofibroblastic phenotype in culture and stain positive for the myofibroblast markers α-SMA, vimentin, NMMIIB, ED-A fibronectin, and collagen type 1 at each passage. In addition, they contract collagen gels in response to TGF-β1 and synthesize collagen similar to human CMyfbs. Moreover, inhibition of NMMII activity with blebbistatin completely attenuates gel contractility without affecting cell viability. Thus, human BM-MSCs share and exhibit similar physiological and functional characteristics as human CMyfbs in vitro, and their propensity to adopt a myofibroblast phenotype in culture may contribute to cardiac fibrosis.

Role of adenosine A2B receptor signaling in contribution of cardiac mesenchymal stem-like cells to myocardial scar formation

Adenosine levels increase in ischemic hearts and contribute to the modulation of that pathological environment. We previously showed that A2B adenosine receptors on mouse cardiac Sca1(+)CD31(-) mesenchymal stromal cells upregulate secretion of paracrine factors that may contribute to the improvement in cardiac recovery seen when these cells are transplanted in infarcted hearts. In this study, we tested the hypothesis that A2B receptor signaling regulates the transition of Sca1(+)CD31(-) cells, which occurs after myocardial injury, into a myofibroblast phenotype that promotes myocardial repair and remodeling. In vitro, TGFβ1 induced the expression of the myofibroblast marker α-smooth muscle actin (αSMA) and increased collagen I generation in Sca1(+)CD31(-) cells. Stimulation of A2B receptors attenuated TGFβ1-induced collagen I secretion but had no effect on αSMA expression. In vivo, myocardial infarction resulted in a rapid increase in the numbers of αSMA-positive cardiac stromal cells by day 5 followed by a gradual decline. Genetic deletion of A2B receptors had no effect on the initial accumulation of αSMA-expressing stromal cells but hastened their subsequent decline; the numbers of αSMA-positive cells including Sca1(+)CD31(-) cells remained significantly higher in wild type compared with A2B knockout hearts. Thus, our study revealed a significant contribution of cardiac Sca1(+)CD31(-) cells to the accumulation of αSMA-expressing cells after infarction and implicated A2B receptor signaling in regulation of myocardial repair and remodeling by delaying deactivation of these cells. It is plausible that this phenomenon may contribute to the beneficial effects of transplantation of these cells to the injured heart.

Adverse fibrosis in the aging heart depends on signaling between myeloid and mesenchymal cells; role of inflammatory fibroblasts

Aging has been associated with adverse fibrosis. Here we formulate a new hypothesis and present new evidence that unresponsiveness of mesenchymal stem cells (MSC) and fibroblasts to transforming growth factor beta (TGF-β), due to reduced expression of TGF-β receptor I (TβRI), provides a foundation for cardiac fibrosis in the aging heart via two mechanisms. 1) TGF-β promotes expression of Nanog, a transcription factor that retains MSC in a primitive state. In MSC derived from the aging heart, Nanog expression is reduced and therefore MSC gradually differentiate and the number of mesenchymal fibroblasts expressing collagen increases. 2) As TGF-β signaling pathway components negatively regulate transcription of monocyte chemoattractant protein-1 (MCP-1), a reduced expression of TβRI prevents aging mesenchymal cells from shutting down their own MCP-1 expression. Elevated MCP-1 levels that originated from MSC attract transendothelial migration of mononuclear leukocytes from blood to the tissue. MCP-1 expressed by mesenchymal fibroblasts promotes further migration of monocytes and T lymphocytes away from the endothelial barrier and supports the monocyte transition into macrophages and finally into myeloid fibroblasts. Both myeloid and mesenchymal fibroblasts contribute to fibrosis in the aging heart via collagen synthesis. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium ".

AICAR-dependent AMPK activation improves scar formation in the aged heart in a murine model of reperfused myocardial infarction

We have demonstrated that scar formation after myocardial infarction (MI) is associated with an endogenous pool of CD44(pos)CD45(neg) multipotential mesenchymal stem cells (MSC). MSC differentiate into fibroblasts secreting collagen that forms a scar and mature into myofibroblasts that express alpha smooth muscle actin (α-SMA) that stabilizes the scar. In the aging mouse, cardiac repair after MI is associated with impaired differentiation of MSC; MSC derived from the aged hearts form dysfunctional fibroblasts that deposit less collagen in response to transforming growth factor beta-1 (TGF-β1) and poorly mature into myofibroblasts. We found in vitro that the defect in myofibroblast maturation can be remedied by AICAR, which activates non-canonical TGF-β signaling through AMP-activated protein kinase (AMPK). In the present study, we injected aged mice with AICAR and subjected them to 1h occlusion of the left anterior descending artery (LAD) and then reperfusion for up to 30days. AICAR-dependent AMPK signaling led to mobilization of an endogenous CD44(pos)CD45(neg) MSC and its differentiation towards fibroblasts and myofibroblasts in the infarct. This was accompanied by enhanced collagen deposition and collagen fiber maturation in the scar. The AICAR-treated group has demonstrated reduced adverse remodeling as indicated by improved apical end diastolic dimension but no changes in ejection fraction and cardiac output were observed. We concluded that these data indicate the novel, previously not described role of AMPK in the post-MI scar formation. These findings can potentially lead to a new therapeutic strategy for prevention of adverse remodeling in the aging heart.

Hepatocyte growth factor inhibits TGF-β1-induced myofibroblast differentiation in tendon fibroblasts: role of AMPK signaling pathway

The transforming growth factor-β1 (TGF-β1)-induced myofibroblastic differentiation in tendon fibroblasts was thought to be one of the most important features of scar fibrosis formation, which is associated with occurrence of re-rupture. Previously, we reported that hepatocyte growth factor (HGF) inhibited TGF-β1-induced myofibroblast differentiation and extracellular matrix deposition in the Achilles tendon of rats. Here, we investigated the potential molecular mechanisms underlying the inhibitory effect of HGF on TGF-β1-induced myofibroblast differentiation. We found that treatment with HGF (10, 20, and 40 ng/ml) increased phosphorylation of adenosine monophosphate kinase (AMPK) and acetyl-CoA carboxylase (ACC) in tendon fibroblasts. Pharmacological inhibition of the AMPK signaling pathway using compound C, a specific blocker of AMPK signaling, remarkably attenuated the inhibitory effect of HGF on TGF-β1-induced myofibroblastic differentiation in tendon fibroblasts. Moreover, small interfering RNA (siRNA)-mediated knockdown of AMPKα1 subunit decreased the inhibitory effect of HGF on TGF-β1-induced myofibroblastic differentiation in tendon fibroblasts. Finally, overexpression of constitutively active AMPKα1, which led to constitutive activation of the AMPK signaling pathway in tendon fibroblasts, mimicked the inhibitory effect of HGF on the TGF-β1-induced myofibroblastic differentiation. Our study therefore suggests that HGF inhibits TGF-β1-induced myofibroblastic differentiation via an AMPK signaling pathway-dependent manner in tendon fibroblasts.

Aberrant differentiation of fibroblast progenitors contributes to fibrosis in the aged murine heart: role of elevated circulating insulin levels

With age, the collagen content of the heart increases, leading to interstitial fibrosis. We have shown that CD44(pos) fibroblasts derived from aged murine hearts display reduced responsiveness to TGF-β but, paradoxically, have increased collagen expression in vivo and in vitro. We postulated that this phenomenon was due to the defect in mesenchymal stem cell (MSC) differentiation in a setting of elevated circulating insulin levels and production that we observed in aging mice. We discovered that cultured fibroblasts derived from aged but not young cardiac MSCs of nonhematopoietic lineage displayed increased basal and insulin-induced (1 nM) collagen expression (2-fold), accompanied by increased farnesyltransferase (FTase) and Erk activities. In a quest for a possible mechanism, we found that a chronic pathophysiologic insulin concentration (1 nM) caused abnormal fibroblast differentiation of MSCs isolated from young hearts. Fibroblasts derived from these MSCs responded to insulin by elevating collagen expression as seen in untreated aged fibroblast cultures, suggesting a causal link between increased insulin levels and defective MSC responses. Here we report an insulin-dependent pathway that specifically targets collagen type I transcriptional activation leading to a unique mechanism of fibrosis that is TGF-β and inflammation-independent in the aged heart.

Origin of developmental precursors dictates the pathophysiologic role of cardiac fibroblasts

Fibroblasts in the heart play a critical function in the secretion and modulation of extracellular matrix critical for optimal cellular architecture and mechanical stability required for its mechanical function. Fibroblasts are also intimately involved in both adaptive and nonadaptive responses to cardiac injury. Fibroblasts provide the elaboration of extracellular matrix and, as myofibroblasts, are responsible for cross-linking this matrix to form a mechanically stable scar after myocardial infarction. By contrast, during heart failure, fibroblasts secrete extracellular matrix, which manifests itself as excessive interstitial fibrosis that may mechanically limit cardiac function and distort cardiac architecture (adverse remodeling). This review examines the hypothesis that fibroblasts mediating scar formation and fibroblasts mediating interstitial fibrosis arise from different cellular precursors and in response to different autocoidal signaling cascades. We demonstrate that fibroblasts which generate scars arise from endogenous mesenchymal stem cells, whereas those mediating adverse remodeling are of myeloid origin and represent immunoinflammatory dysregulation.

Bone marrow-derived cells contribute to cell turnover in aging murine hearts

The paradigm that cardiac myocytes are non-proliferating, terminally differentiated cells was recently challenged by studies reporting the ability of bone marrow-derived cells (BMCs) to differentiate into cardiomyocytes after myocardial damage. However, little knowledge exists about the role of BMCs in the heart during physiological aging. Twelve-week-old mice (n=36) were sublethally irradiated and bone marrow from littermates transgenic for enhanced green fluorescent protein (eGFP) was transplanted. After 4 weeks, 18 mice were sacrificed at the age of 4 months and served as controls (group A); the remaining mice were sacrificed at the age of 18 months (group B). Group A did not exhibit a significant number of eGFP+ cells, whereas 9.4±2.8 eGFP+ cells/mm2 was documented in group B. In total, only five eGFP+ cardiomyocytes were detected in 20 examined hearts, excluding a functional role of BM differentiation in cardiomyocytes. Similarly, a relevant differentiation of BMCs in endothelial or smooth muscle cells was excluded. In contrast, numerous BM-derived fibroblasts and myofibroblasts were observed in group B, but none were detected in group  A. The present study demonstrates that BMCs transdifferentiate into fibroblasts and myofibroblasts in the aging murine myocardium, suggesting their contribution to the preservation of the structural integrity of the myocardium, while they do not account for regenerative processes of the heart.