Cardiomyogenesis in the aging and failing human heart

Metabolic dysfunction consistent with premature aging results from deletion of Pim kinases

RATIONALE

The senescent cardiac phenotype is accompanied by changes in mitochondrial function and biogenesis causing impairment in energy provision. The relationship between myocardial senescence and Pim kinases deserves attention because Pim-1 kinase is cardioprotective, in part, by preservation of mitochondrial integrity. Study of the pathological effects resulting from genetic deletion of all Pim kinase family members could provide important insight about cardiac mitochondrial biology and the aging phenotype.

OBJECTIVE

To demonstrate that myocardial senescence is promoted by loss of Pim leading to premature aging and aberrant mitochondrial function.

METHODS AND RESULTS

Cardiac myocyte senescence was evident at 3 months in Pim triple knockout mice, where all 3 isoforms of Pim kinase family members are genetically deleted. Cellular hypertrophic remodeling and fetal gene program activation were followed by heart failure at 6 months in Pim triple knockout mice. Metabolic dysfunction is an underlying cause of cardiac senescence and instigates a decline in cardiac function. Altered mitochondrial morphology is evident consequential to Pim deletion together with decreased ATP levels and increased phosphorylated AMP-activated protein kinase, exposing an energy deficiency in Pim triple knockout mice. Expression of the genes encoding master regulators of mitochondrial biogenesis, PPARγ (peroxisome proliferator-activated receptor gamma) coactivator-1 α and β, was diminished in Pim triple knockout hearts, as were downstream targets included in mitochondrial energy transduction, including fatty acid oxidation. Reversal of the dysregulated metabolic phenotype was observed by overexpressing c-Myc (Myc proto-oncogene protein), a downstream target of Pim kinases.

CONCLUSIONS

Pim kinases prevent premature cardiac aging and maintain a healthy pool of functional mitochondria leading to efficient cellular energetics.

Existing cardiomyocytes generate cardiomyocytes at a low rate after birth in mice

The mammalian heart has long been considered a postmitotic organ, implying that the total number of cardiomyocytes is set at birth. Analysis of cell division in the mammalian heart is complicated by cardiomyocyte binucleation shortly after birth, which makes it challenging to interpret traditional assays of cell turnover [Laflamme MA, Murray CE (2011) Nature 473(7347):326-335; Bergmann O, et al. (2009) Science 324(5923):98-102]. An elegant multi-isotope imaging-mass spectrometry technique recently calculated the low, discrete rate of cardiomyocyte generation in mice [Senyo SE, et al. (2013) Nature 493(7432):433-436], yet our cellular-level understanding of postnatal cardiomyogenesis remains limited. Herein, we provide a new line of evidence for the differentiated α-myosin heavy chain-expressing cardiomyocyte as the cell of origin of postnatal cardiomyogenesis using the "mosaic analysis with double markers" mouse model. We show limited, life-long, symmetric division of cardiomyocytes as a rare event that is evident in utero but significantly diminishes after the first month of life in mice; daughter cardiomyocytes divide very seldom, which this study is the first to demonstrate, to our knowledge. Furthermore, ligation of the left anterior descending coronary artery, which causes a myocardial infarction in the mosaic analysis with double-marker mice, did not increase the rate of cardiomyocyte division above the basal level for up to 4 wk after the injury. The clonal analysis described here provides direct evidence of postnatal mammalian cardiomyogenesis.

c-kit+ cells minimally contribute cardiomyocytes to the heart

If and how the heart regenerates after an injury event is highly debated. c-kit-expressing cardiac progenitor cells have been reported as the primary source for generation of new myocardium after injury. Here we generated two genetic approaches in mice to examine if endogenous c-kit⁺ cells contribute differentiated cardiomyocytes to the heart during development, with aging or after injury in adulthood. A cDNA encoding either Cre recombinase or a tamoxifen inducible MerCreMer chimeric protein was targeted to the Kit locus in mice and then bred with reporter lines to permanently mark cell lineage. Endogenous c-kit⁺ cells did produce new cardiomyocytes within the heart, although at a percentage of ≈0.03% or less, and if a preponderance towards cellular fusion is considered, the percentage falls below ≈0.008%. In contrast, c-kit⁺ cells amply generated cardiac endothelial cells. Thus, endogenous c-kit⁺ cells can generate cardiomyocytes within the heart, although likely at a functionally insignificant level.

Proliferative activity of cadriomyocytes in chronic hypercholesterolemia

We studied proliferative activity of cardiomyocytes (using proliferation marker Ki-67) and compared it with their total number in the heart under conditions of experimental chronic hypercholesterolemia and its combination with hypothyroidism. It was found that Ki-67-positive cells are primarily located in the subepicardial layer near the heart base in both intact and experimental animals. Replicative cardiomyocyte pool in intact rats constituted 1.67 ± 0.33‰ of the total cardiomyocyte population; after 68-day atherogenic diet with exogenous cholesterol alone, the replicative cardiomyocyte pool decreased by 16 % (to 1.40 ± 0.24‰). Treatment with mercazolil against the background of exogenous cholesterol increased this parameter by 40 % (to 2.33 ± 0.88‰). Changes in replicative activity of cardiomyocytes correlated with their total number in the heart and organ weight. We conclude that replicative cardiomyocyte pool primarily includes non-terminally differentiated cardiomyocytes (small mononuclear cardiomyocytes) and their proliferation maintains the total number of cardiomyocytes in the heart under conditions of cytopathic influences and provides the basis for physiological and reparative regeneration of the myocardium.

The cardiac stem cell compartment is indispensable for myocardial cell homeostasis, repair and regeneration in the adult

Resident cardiac stem cells in embryonic, neonatal and adult mammalian heart have been identified by different membrane markers and transcription factors. However, despite a flurry of publications no consensus has been reached on the identity and actual regenerative effects of the adult cardiac stem cells. Intensive research on the adult mammalian heart's capacity for self-renewal of its muscle cell mass has led to a consensus that new cardiomyocytes (CMs) are indeed formed throughout adult mammalian life albeit at a disputed frequency. The physiological significance of this renewal, the origin of the new CMs, and the rate of adult CM turnover are still highly debated. Myocyte replacement, particularly after injury, was originally attributed to differentiation of a stem cell compartment. More recently, it has been reported that CMs are mainly replaced by the division of pre-existing post-mitotic CMs. These latter results, if confirmed, would shift the target of regenerative therapy toward boosting mature CM cell-cycle re-entry. Despite this controversy, it is documented that the adult endogenous c-kit(pos) cardiac stem cells (c-kit(pos) eCSCs) participate in adaptations to myocardial stress, and, when transplanted into the myocardium, regenerate most cardiomyocytes and microvasculature lost in an infarct. Nevertheless, the in situ myogenic potential of adult c-kit(pos) cardiac cells has been questioned. To revisit the regenerative potential of c-kit(pos) eCSCs, we have recently employed experimental protocols of severe diffuse myocardial damage in combination with several genetic murine models and cell transplantation approaches showing that eCSCs are necessary and sufficient for CM regeneration, leading to complete cellular, anatomical, and functional myocardial recovery. Here we will review the available data on adult eCSC biology and their regenerative potential placing it in the context of the different claimed mechanisms of CM replacement. These data are in agreement with and have reinforced our view that most CMs are replaced by de novo CM formation through the activation, myogenic commitment and specification of the eCSC cohort.

Cell and gene therapy for severe heart failure patients: the time and place for Pim-1 kinase

Regenerative therapy in severe heart failure patients presents a challenging set of circumstances including a damaged myocardial environment that accelerates senescence in myocytes and cardiac progenitor cells. Failing myocardium suffers from deterioration of contractile function coupled with impaired regenerative potential that drives the heart toward decompensation. Efficacious regenerative cell therapy for severe heart failure requires disruption of this vicious circle that can be accomplished by alteration of the compromised myocyte phenotype and rejuvenation of progenitor cells. This review focuses upon potential for Pim-1 kinase to mitigate chronic heart failure by improving myocyte quality through preservation of mitochondrial integrity, prevention of hypertrophy and inhibition of apoptosis. In addition, cardiac progenitors engineered with Pim-1 possess enhanced regenerative potential, making Pim-1 an important player in future treatment of severe heart failure.

Human pluripotent stem cell-derived cardiomyocytes as research and therapeutic tools

Human pluripotent stem cells (hPSCs), namely, embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), with their ability of indefinite self-renewal and capability to differentiate into cell types derivatives of all three germ layers, represent a powerful research tool in developmental biology, for drug screening, disease modelling, and potentially cell replacement therapy. Efficient differentiation protocols that would result in the cell type of our interest are needed for maximal exploitation of these cells. In the present work, we aim at focusing on the protocols for differentiation of hPSCs into functional cardiomyocytes in vitro as well as achievements in the heart disease modelling and drug testing on the patient-specific iPSC-derived cardiomyocytes (iPSC-CMs).

Exosomes and cardiac repair after myocardial infarction

Myocardial infarction is a leading cause of death among all cardiovascular diseases. The analysis of molecular mechanisms by which the ischemic myocardium initiates repair and remodeling indicates that secreted soluble factors are key players in communication to local and distant tissues, such as bone marrow. Recently, actively secreted membrane vesicles, including exosomes, are being recognized as new candidates with important roles in intercellular and tissue-level communication. In this review, we critically examine the emerging role of exosomes in local and distant microcommunication mechanisms after myocardial infarction. A comprehensive understanding of the role of exosomes in cardiac repair after myocardial infarction could bridge a major gap in knowledge of the repair mechanism after myocardial injury.

Cell therapy for cardiac repair--lessons from clinical trials

The global impetus to identify curative therapies has been fuelled by the unmet needs of patients in the context of a growing heart failure pandemic. To date, regeneration trials in patients with cardiovascular disease have used stem-cell-based therapy in the period immediately after myocardial injury, in an attempt to halt progression towards ischaemic cardiomyopathy, or in the setting of congestive heart failure, to target the disease process and prevent organ decompensation. Worldwide, several thousand patients have now been treated using autologous cell-based therapy; the safety and feasibility of this approach has been established, pitfalls have been identified, and optimization procedures envisioned. Furthermore, the initiation of phase III trials to further validate the therapeutic value of cell-based regenerative medicine and address the barriers to successful clinical implementation has led to resurgence in the enthusiasm for such treatments among patients and health-care providers. In particular, poor definition of cell types used, diversity in cell-handling procedures, and functional variability intrinsic to autologously-derived cells have been identified as the main factors limiting adoption of cell-based therapies. In this Review, we summarize the experience obtained from trials of 'first-generation' cell-based therapy, and emphasize the advances in the purification and lineage specification of stem cells that have enabled the development of 'next-generation' stem-cell-based therapies targeting cardiovascular disease.

Hippo signaling impedes adult heart regeneration

Heart failure due to cardiomyocyte loss after ischemic heart disease is the leading cause of death in the United States in large part because heart muscle regenerates poorly. The endogenous mechanisms preventing mammalian cardiomyocyte regeneration are poorly understood. Hippo signaling, an ancient organ size control pathway, is a kinase cascade that inhibits developing cardiomyocyte proliferation but it has not been studied postnatally or in fully mature adult cardiomyocytes. Here, we investigated Hippo signaling in adult cardiomyocyte renewal and regeneration. We found that unstressed Hippo-deficient adult mouse cardiomyocytes re-enter the cell cycle and undergo cytokinesis. Moreover, Hippo deficiency enhances cardiomyocyte regeneration with functional recovery after adult myocardial infarction as well as after postnatal day eight (P8) cardiac apex resection and P8 myocardial infarction. In damaged hearts, Hippo mutant cardiomyocytes also have elevated proliferation. Our findings reveal that Hippo signaling is an endogenous repressor of adult cardiomyocyte renewal and regeneration. Targeting the Hippo pathway in human disease might be beneficial for the treatment of heart disease.

Murine cardiosphere-derived cells are impaired by age but not by cardiac dystrophic dysfunction

To be clinically relevant as a therapy for heart failure, endogenous progenitor cells must be isolated and expanded from aged and/or diseased tissue. Here, we investigated the effect of age and cardiac impairment resulting from lack of dystrophin on murine cardiosphere-derived cells (CDCs). CDCs were isolated and expanded from atrial biopsies from wild-type mice aged 1.5, 6, 18, and 24 months and from mdx mice aged 6 and 18 months. Cardiac function was measured in mdx mice and age-matched wild-type mice using high-resolution cine magnetic resonance imaging. CDCs could be isolated and expanded from all mice, however, the number of cells obtained, and their regenerative potential, decreased with age, as demonstrated by decreased expression of stem cell markers, c-kit and Sca-1, and decreased cell proliferation, migration, clonogenicity, and differentiation. Six-month-old mdx mice showed right ventricular (RV) dilation and reduced RV ejection fraction (EF) in comparison to wild-type mice. Older mdx mice displayed significant RV and left ventricular dilation and decreased EF in both ventricles, compared with age-matched wild-type mice. Mdx mouse hearts contained significantly more fibrotic tissue than age-matched wild-type mouse hearts. However, CDCs isolated from mice aged 6 and 18 months had the same number and regenerative potential from mdx mice and age-matched wild-type mice. Thus, the cardiac progenitor cell population is impaired by age but is not substantially altered by the progressive deterioration in function of the dystrophic heart.

The role of bioactive lipids in stem cell mobilization and homing: novel therapeutics for myocardial ischemia

Despite significant advances in medical therapy and interventional strategies, the prognosis of millions of patients with acute myocardial infarction (AMI) and ischemic heart disease (IHD) remains poor. Currently, short of heart transplantation with all of its inherit limitations, there are no available treatment strategies that replace the infarcted myocardium. It is now well established that cardiomyocytes undergo continuous renewal, with contribution from bone marrow (BM)-derived stem/progenitor cells (SPCs). This phenomenon is upregulated during AMI by initiating multiple innate reparatory mechanisms through which BMSPCs are mobilized towards the ischemic myocardium and contribute to myocardial regeneration. While a role for the SDF-1/CXCR4 axis in retention of BMSPCs in bone marrow is undisputed, its exclusive role in their mobilization and homing to a highly proteolytic microenvironment, such as the ischemic/infarcted myocardium, is currently being challenged. Recent evidence suggests a pivotal role for bioactive lipids in the mobilization of BMSPCs at the early stages following AMI and their homing towards ischemic myocardium. This review highlights the recent advances in our understanding of the mechanisms of stem cell mobilization, provides newer evidence implicating bioactive lipids in BMSPC mobilization and differentiation, and discusses their potential as therapeutic agents in the treatment of IHD.

CENP-A is essential for cardiac progenitor cell proliferation

Centromere protein A (CENP-A) is a homolog of histone H3 that epigenetically marks the heterochromatin of chromosomes. CENP-A is a critical component of the cell cycle machinery that is necessary for proper assembly of the mitotic spindle. However, the role of CENP-A in the heart and cardiac progenitor cells (CPCs) has not been previously studied. This study shows that CENP-A is expressed in CPCs and declines with age. Silencing CENP-A results in a decreased CPC growth rate, reduced cell number in phase G₂/M of the cell cycle, and increased senescence associated β-galactosidase activity. Lineage commitment is not affected by CENP-A silencing, suggesting that cell cycle arrest induced by loss of CENP-A is a consequence of senescence and not differentiation. CENP-A knockdown does not exacerbate cell death in undifferentiated CPCs, but increases apoptosis upon lineage commitment. Taken together, these results indicate that CPCs maintain relatively high levels of CENP-A early in life, which is necessary for sustaining proliferation, inhibiting senescence, and promoting survival following differentiation of CPCs.

Rejuvenation: an integrated approach to regenerative medicine

The word "rejuvenate" found in the Merriam-Webster dictionary is (1) to make young or youthful again: give new vigor to, and (2) to restore to an original or new state. Regenerative medicine is the process of creating living, functional tissues to repair or replace tissue or organ function lost due to age, disease, damage, or congenital defects. To accomplish this, approaches including transplantation, tissue engineering, cell therapy, and gene therapy are brought into action. These all use exogenously prepared materials to forcefully mend the failed organ. The adaptation of the materials in the host and their integration into the organ are all uncertain. It is a common sense that tissue injury in the younger is easily repaired and the acute injury is healed better and faster. Why does the elder have a diminished capacity of self-repairing, or why does chronic injury cause the loss of the self-repairing capacity? There must be some critical elements that are involved in the repair process, but are suppressed in the elder or under the chronic injury condition. Rejuvenation of the self-repair mechanism would be an ideal solution for functional recovery of the failed organ. To achieve this, it would involve renewal of the injury signaling, reestablishment of the communication and transportation system, recruitment of the materials for repairing, regeneration of the failed organ, and rehabilitation of the renewed organ. It thus would require a comprehensive understanding of developmental biology and a development of new approaches to activate the critical players to rejuvenate the self-repair mechanism in the elder or under chronic injury condition. Efforts focusing on rejuvenation would expect an alternative, if not a better, accomplishment in the regenerative medicine.

The cardiac hypoxic niche: emerging role of hypoxic microenvironment in cardiac progenitors

Resident stem cells persist throughout the entire lifetime of an organism where they replenishing damaged cells. Numerous types of resident stem cells are housed in a low-oxygen tension (hypoxic) microenvironment, or niches, which seem to be critical for survival and maintenance of stem cells. Recently our group has identified the adult mammalian epicardium and subepicardium as a hypoxic niche for cardiac progenitor cells. Similar to hematopoietic stem cells (LT-HSCs), progenitor cells in the hypoxic epicardial niche utilize cytoplasmic glycolysis instead of mitochondrial oxidative phosphorylation, where hypoxia inducible factor 1α (Hif-1α) maintains them in glycolytic undifferentiated state. In this review we summarize the relationship between hypoxic signaling and stem cell function, and discuss potential roles of several cardiac stem/progenitor cells in cardiac homeostasis and regeneration.

Human heart failure: is cell therapy a valid option?

The concept of the heart as a terminally differentiated organ incapable of replacing damaged myocytes has been at the center of cardiovascular research and therapeutic development for the past 50 years. The progressive decline in myocyte number with aging and the formation of scarred tissue following myocardial infarction have been interpreted as irrefutable proofs of the post-mitotic characteristics of the adult heart. However, emerging evidence supports a more dynamic view of the myocardium in which cell death and cell restoration are vital components of the remodeling process that governs organ homeostasis, aging and disease. The identification of dividing myocytes throughout the life span of the organisms and the recognition that undifferentiated primitive cells regulate myocyte turnover and tissue regeneration indicate that the heart is a self-renewing organ controlled by a compartment of resident stem cells. Moreover, exogenous progenitors of bone marrow origin transdifferentiate and acquire the cardiomyocyte and vascular lineages. This new reality constitutes the foundation of the numerous cell-based clinical trials that have been conducted in the last decade for the treatment of ischemic and non-ischemic cardiomyopathies.

Adult c-kit(pos) cardiac stem cells are necessary and sufficient for functional cardiac regeneration and repair

The epidemic of heart failure has stimulated interest in understanding cardiac regeneration. Evidence has been reported supporting regeneration via transplantation of multiple cell types, as well as replication of postmitotic cardiomyocytes. In addition, the adult myocardium harbors endogenous c-kit(pos) cardiac stem cells (eCSCs), whose relevance for regeneration is controversial. Here, using different rodent models of diffuse myocardial damage causing acute heart failure, we show that eCSCs restore cardiac function by regenerating lost cardiomyocytes. Ablation of the eCSC abolishes regeneration and functional recovery. The regenerative process is completely restored by replacing the ablated eCSCs with the progeny of one eCSC. eCSCs recovered from the host and recloned retain their regenerative potential in vivo and in vitro. After regeneration, selective suicide of these exogenous CSCs and their progeny abolishes regeneration, severely impairing ventricular performance. These data show that c-kit(pos) eCSCs are necessary and sufficient for the regeneration and repair of myocardial damage.

Cardiomyocyte proliferation vs progenitor cells in myocardial regeneration: The debate continues

In recent years, several landmark studies have provided compelling evidence that cardiomyogenesis occurs in the adult mammalian heart. However, the rate of new cardiomyocyte formation is inadequate for complete restoration of the normal mass of myocardial tissue, should a significant myocardial injury occur, such as myocardial infarction. The cellular origin of postnatal cardiomyogenesis in mammals remains a controversial issue and two mechanisms seem to be participating, proliferation of pre-existing cardiomyocytes and myogenic differentiation of progenitor cells. We will discuss the relative importance of these two processes in different settings, such as normal ageing and post-myocardial injury, as well as the strengths and limitations of the existing experimental methodologies used in the relevant studies. Further clarification of the mechanisms underlying cardiomyogenesis in mammals will open the way for their therapeutic exploitation in the clinical field, with the scope of myocardial regeneration.

Pathological ventricular remodeling: mechanisms: part 1 of 2

Despite declines in heart failure morbidity and mortality with current therapies, rehospitalization rates remain distressingly high, substantially affecting individuals, society, and the economy. As a result, the need for new therapeutic advances and novel medical devices is urgent. Disease-related left ventricular remodeling is a complex process involving cardiac myocyte growth and death, vascular rarefaction, fibrosis, inflammation, and electrophysiological remodeling. Because these events are highly interrelated, targeting a single molecule or process may not be sufficient. Here, we review molecular and cellular mechanisms governing pathological ventricular remodeling.

Phenotypic modulation of human cardiospheres between stemness and paracrine activity, and implications for combined transplantation in cardiovascular regeneration

As the search for new cell types for cardiovascular regeneration continues, it has become increasingly important to optimize ex vivo cell processing. We aimed to develop an optimal processing strategy for human cardiac progenitor cells. We hypothesized that enhancing the stemness potential and promoting the secretory activity for paracrine effects are mutually exclusive routes. Therefore, we investigated the two divergent cell processing methods to enhance cellular potency and humoral activity, respectively. We obtained human right ventricular tissues and sequentially generated primary cardiosphere (CS), primary CS-derived cells (PCDC) and secondary CSs. During secondary CS formation, inhibiting the ERK pathway, using selective RTK1 and TGF-β inhibitors, Oct4 increased 20 fold and VEGF was decreased. When the ERK pathway was stimulated by addition of EGF and TGF-β, VEGF expression was upregulated and Oct4 was downregulated, indicating that the ERK pathway serves a directional role for cellular potency versus paracrine capacity. Transplantation of PCDCs or secondary CSs into the infarcted heart of immunocompromised mouse showed significant angiogenic effects compared with PBS injection. Interestingly, combined transplantation of the two differently-processed, dual-purpose secondary CSs resulted in an additional increase in neovascularization. Human VEGF was primarily produced from secondary CSs under ERK stimulating conditions. Cardiomyocyte-like cells were produced from secondary CSs under ERK inhibitory conditions. These findings indicate that combined transplantation of specifically-processed human secondary CSs enhances infarct repair through the complementary enhancement of cardiopoietic regenerative and paracrine protective effect. Furthermore, these results underscore the fact that optimal cell processing methods have the potential to maximize the therapeutic benefits.

Inositol 1, 4, 5-trisphosphate receptors and human left ventricular myocytes

BACKGROUND

Little is known about the function of inositol 1,4,5-trisphosphate receptors (IP3Rs) in the adult heart experimentally. Moreover, whether these Ca(2+) release channels are present and play a critical role in human cardiomyocytes remains to be defined. IP3Rs may be activated after Gαq-protein-coupled receptor stimulation, affecting Ca(2+) cycling, enhancing myocyte performance, and potentially favoring an increase in the incidence of arrhythmias.

METHODS AND RESULTS

IP3R function was determined in human left ventricular myocytes, and this analysis was integrated with assays in mouse myocytes to identify the mechanisms by which IP3Rs influence the electric and mechanical properties of the myocardium. We report that IP3Rs are expressed and operative in human left ventricular myocytes. After Gαq-protein-coupled receptor activation, Ca(2+) mobilized from the sarcoplasmic reticulum via IP3Rs contributes to the decrease in resting membrane potential, prolongation of the action potential, and occurrence of early afterdepolarizations. Ca(2+) transient amplitude and cell shortening are enhanced, and extrasystolic and dysregulated Ca(2+) elevations and contractions become apparent. These alterations in the electromechanical behavior of human cardiomyocytes are coupled with increased isometric twitch of the myocardium and arrhythmic events, suggesting that Gαq-protein-coupled receptor activation provides inotropic reserve, which is hampered by electric instability and contractile abnormalities. Additionally, our findings support the notion that increases in Ca(2+) load by IP3Rs promote Ca(2+) extrusion by forward-mode Na(+)/Ca(2+) exchange, an important mechanism of arrhythmic events.

CONCLUSIONS

The Gαq-protein/coupled receptor/IP3R axis modulates the electromechanical properties of the human myocardium and its propensity to develop arrhythmias.

CXCR4+ and FLK-1+ identify circulating cells associated with improved cardiac function in patients following myocardial infarction

The biomarkers CXCR4/FLK-1 select cardiac progenitors from a stem cell pool in experimental models. However, the translational value of these cells in human ischemic heart disease is unknown. Here, flow-cytometry identified CD45(-)/CXCR4(+)/FLK-1(+) cells in 30 individuals without ischemic heart disease and 33 first-time acute myocardial infarction (AMI) patients. AMI patients had higher CD45(-)/CXCR4(+)/FLK-1(+) cell-load at 48-h and 3- and 6-months post-AMI (p = 0.003,0.04,0.04, respectively) than controls. Cardiovascular risk factors and left ventricular (LV) ejection fraction were not associated with cell-load. 2D-speckle-tracking strain echocardiography assessment of LV systolic function showed improvement in longitudinal strain and dyssynchrony during follow-up associated with longitudinal increases in and higher 48-h post-AMI CD45(-)/CXCR4(+)/FLK-1(+) cell-load (r = -0.525, p = 0.025; r = -0.457, p = 0.029, respectively). In conclusion, CD45(-)/CXCR4(+)/FLK-1(+) cells are present in adult human circulation, increased in AMI and associated with improved LV systolic function. Thus, CD45(-)/CXCR4(+)/FLK-1(+) cells may provide a diagnostic tool to follow cardiac regenerative capacity and potentially serve as a prognostic marker in AMI.

Innate regeneration in the aging heart: healing from within

The concept of the heart as a terminally differentiated organ incapable of replacing damaged myocytes has been at the center of cardiovascular research and therapeutic development for the past 50 years. The progressive decline in myocyte number as a function of age and the formation of scarred tissue after myocardial infarction have been interpreted as irrefutable proofs of the postmitotic characteristic of the heart. However, emerging evidence supports a more dynamic view of the heart in which cell death and renewal are vital components of the remodeling process that governs cardiac homeostasis, aging, and disease. The identification of dividing myocytes in the adult and senescent heart raises the important question concerning the origin of these newly formed cells. In vitro and in vivo findings strongly suggest that replicating myocytes derive from lineage determination of resident primitive cells, supporting the notion that cardiomyogenesis is controlled by activation and differentiation of a stem cell compartment. It is the current view that the myocardium is an organ permissive of tissue regeneration mediated by exogenous and endogenous progenitor cells.

Sustained-release delivery of prostacyclin analogue enhances bone marrow-cell recruitment and yields functional benefits for acute myocardial infarction in mice

Background

A prostacyclin analogue, ONO-1301, is reported to upregulate beneficial proteins, including stromal cell derived factor-1 (SDF-1). We hypothesized that the sustained-release delivery of ONO-1301 would enhance SDF-1 expression in the acute myocardial infarction (MI) heart and induce bone marrow cells (BMCs) to home to the myocardium, leading to improved cardiac function in mice.

Methods and Results

ONO-1301 significantly upregulated SDF-1 secretion by fibroblasts. BMC migration was greater to ONO-1301-stimulated than unstimulated conditioned medium. This increase was diminished by treating the BMCs with a CXCR4-neutralizing antibody or CXCR4 antagonist (AMD3100). Atelocollagen sheets containing a sustained-release form of ONO-1301 (n = 33) or ONO-1301-free vehicle (n = 48) were implanted on the left ventricular (LV) anterior wall immediately after permanent left-anterior descending artery occlusion in C57BL6/N mice (male, 8-weeks-old). The SDF-1 expression in the infarct border zone was significantly elevated for 1 month in the ONO-1301-treated group. BMC accumulation in the infarcted hearts, detected by in vivo imaging after intravenous injection of labeled BMCs, was enhanced in the ONO-1301-treated hearts. This increase was inhibited by AMD3100. The accumulated BMCs differentiated into capillary structures. The survival rates and cardiac function were significantly improved in the ONO-1301-treated group (fractional area change 23±1%; n = 22) compared to the vehicle group (19±1%; n = 20; P = 0.004). LV anterior wall thinning, expansion of infarction, and fibrosis were lower in the ONO-1301-treated group.

Conclusions

Sustained-release delivery of ONO-1301 promoted BMC recruitment to the acute MI heart via SDF-1/CXCR4 signaling and restored cardiac performance, suggesting a novel mechanism for ONO-1301-mediated acute-MI heart repair.

Therapeutic application of cardiac stem cells and other cell types

Various researches on regenerative medicine were carried out experimentally, and selected modalities have been introduced to the clinical arena. Meanwhile, the presence of resident stem cells in the heart and their role in physiological cell turnover were demonstrated. So far skeletal myoblasts, bone marrow-derived cells, mesenchymal stromal cells, and resident cardiac cells have been applied for therapeutic myocardial regeneration. Among them, autologous transplantation of c-kit-positive cardiac stem cells in congestive heart failure patients resulted in an outstanding outcome, with long-lasting beneficial effects without major adverse events. By reviewing these clinical trials, an endeavor was made to seek for an ideal cellular therapy for cardiovascular diseases.

Cardiac stem cell therapy and the promise of heart regeneration

Stem cell therapy for cardiac disease is an exciting but highly controversial research area. Strategies such as cell transplantation and reprogramming have demonstrated both intriguing and sobering results. Yet as clinical trials proceed, our incomplete understanding of stem cell behavior is made evident by numerous unresolved matters, such as the mechanisms of cardiomyocyte turnover or the optimal therapeutic strategies to achieve clinical efficacy. In this Perspective, we consider how cardiac stem cell biology has led us into clinical trials, and we suggest that achieving true cardiac regeneration in patients may ultimately require resolution of critical controversies in experimental cardiac regeneration.

Stem cell stimulation of endogenous myocyte regeneration

Cell-based therapy has emerged as a promising approach to combat the myocyte loss and cardiac remodelling that characterize the progression of left ventricular dysfunction to heart failure. Several clinical trials conducted over the past decade have shown that a variety of autologous bone-marrow- and peripheral-blood-derived stem and progenitor cell populations can be safely administered to patients with ischaemic heart disease and yield modest improvements in cardiac function. Concurrently, rapid progress has been made at the pre-clinical level to identify novel therapeutic cell populations, delineate the mechanisms underlying cell-mediated cardiac repair and optimize cell-based approaches for clinical use. The following review summarizes the progress that has been made in this rapidly evolving field over the past decade and examines how our current understanding of the mechanisms involved in successful cardiac regeneration should direct future investigation in this area. Particular emphasis is placed on discussion of the general hypothesis that the benefits of cell therapy primarily result from stimulation of endogenous cardiac repair processes that have only recently been identified in the adult mammalian heart, rather than direct differentiation of exogenous cells. Continued scientific investigation in this area will guide the optimization of cell-based approaches for myocardial regeneration, with the ultimate goal of clinical implementation and substantial improvement in our ability to restore cardiac function in ischaemic heart disease patients.

Concise review: heart regeneration and the role of cardiac stem cells

Acute myocardial infarction leads to irreversible loss of cardiac myocytes, thereby diminishing the pump function of the heart. As a result, the strenuous workload imposed on the remaining cardiac myocytes often gives rise to subsequent cell loss until the vicious circle ends in chronic heart failure (CHF). Thus, we are in need of a therapy that could ameliorate or even reverse the disease progression of CHF. Endogenous regeneration of the mammalian heart has been shown in the neonatal heart, and the discovery that it may still persist in adulthood sparked hope for novel cardioregenerative therapies. As the basis for cardiomyocyte renewal, multipotent cardiac stem/progenitor cells (CSCs) that reside in the heart have been shown to differentiate into cardiac myocytes, smooth muscle cells, and vascular endothelial cells. These CSCs do have the potential to actively regenerate the heart but clearly fail to do so after abundant and segmental loss of cells, such as what occurs with myocardial infarction. Therefore, it is vital to continue research for the most optimal therapy based on the use or in situ stimulation of these CSCs. In this review, we discuss the current status of the cardioregenerative field. In particular, we summarize the current knowledge of CSCs as the regenerative substrate in the adult heart and their use in preclinical and clinical studies to repair the injured myocardium.

The existence of myocardial repair: mechanistic insights and enhancements

The lack of myocardial repair after myocardial infarction and the heart failure that eventually ensues was thought of as proof that myocardial cell regeneration and myocardial repair mechanisms do not exist. Recently, growing experimental and clinical evidence has proven this concept wrong. Cardiac stem cells and endogenous myocardial repair mechanisms do exist; however, they do not produce significant myocardial repair. Similarly, the preliminary results of stem cell therapy for myocardial repair have shown early promise but modest results. Preclinical studies are the key to understanding stem cell senescence and lack of cellular contact and vasculature in the infarcted region. Additional laboratory studies are sure to unlock the therapeutic mechanisms that will be required for significant myocardial repair.

Hypoinnervation is an early event in experimental myocardial remodelling induced by pressure overload

Structural and functional remodelling of cardiomyocytes, capillaries and cardiac innervation occurs in left ventricular hypertrophy (LVH) and heart failure (HF) in response to pressure-induced overload. However, the onset, time course and the extent of these morphological alterations remain controversial. In the present study, we tested the hypothesis that the progression from hypertrophy to HF is accompanied by changes in the innervation (hyper- or hypoinnervation). Left ventricles of wild-type murine hearts subjected to pressure overload-induced hypertrophy by transverse aortic constriction (TAC) were investigated by morphometric and design-based stereological methods at 1 and 4 weeks after TAC and compared with sham-operated mice. Mice developed compensated LVH at 1 week and typical signs of HF, such as left ventricular dilation, reduced ejection fraction and increased relative lung weight at 4 weeks post-TAC. At the (sub-)cellular level, cardiomyocyte myofibrillar and mitochondrial volume increased progressively in response to mechanical overload. The total length of capillaries was not significantly increased after TAC, indicating a misrelationship between the cardiomyocyte and the capillary compartment. The myocardial innervation decreased already during the development of LVH and did not significantly decrease further during the progression to HF. In conclusion, our study suggests that early loss of myocardial innervation density and increased heterogeneity occur during pressure overload-induced hypertrophy, and that these changes appear to be independent of cardiomyocyte and capillary remodelling.

Adipose stromal cells primed with hypoxia and inflammation enhance cardiomyocyte proliferation rate in vitro through STAT3 and Erk1/2

Background

Experimental clinical stem cell therapy has been used for more than a decade to alleviate the adverse aftermath of acute myocardial infarction (aMI). The post-infarcted myocardial microenvironment is characterized by cardiomyocyte death, caused by ischemia and inflammation. These conditions may negatively affect administered stem cells. As postnatal cardiomyocytes have a poor proliferation rate, while induction of proliferation seems even more rare. Thus stimulation of their proliferation rate is essential after aMI. In metaplastic disease, the pro-inflammatory cytokine interleukin-6 (IL-6) has been identified as potent mediators of the proliferation rate. We hypothesized that IL-6 could augment the proliferation rate of (slow-)dividing cardiomyocytes.

Methods

To mimic the behavior of therapeutic cells in the post-infarct cardiac microenvironment, human Adipose Derived Stromal Cells (ADSC) were cultured under hypoxic (2% O₂) and pro-inflammatory conditions (IL-1β) for 24h. Serum-free conditioned medium from ADSC primed with hypoxia and/or IL-1β was added to rat neonatal cardiomyocytes and adult cardiomyocytes (HL-1) to assess paracrine-driven changes in cardiomyocyte proliferation rate and induction of myogenic signaling pathways.

Results

We demonstrate that ADSC enhance the proliferation rate of rat neonatal cardiomyocytes and adult HL-1 cardiomyocytes in a paracrine fashion. ADSC under hypoxia and inflammation in vitro had increased the interleukin-6 (IL-6) gene and protein expression. Similar to conditioned medium of ADSC, treatment of rat neonatal cardiomyocytes and HL-1 with recombinant IL-6 alone also stimulated their proliferation rate. This was corroborated by a strong decrease of cardiomyocyte proliferation after addition of IL-6 neutralizing antibody to conditioned medium of ADSC. The stimulatory effect of ADSC conditioned media or IL-6 was accomplished through activation of both Janus Kinase-Signal Transducer and Activator of Transcription (JAK/STAT) and Mitogen-Activated Protein (MAP) kinases (MAPK) mitogenic signaling pathways.

Conclusion

ADSC are promising therapeutic cells for cardiac stem cell therapy. The inflammatory and hypoxic host post-MI microenvironment enhances the regenerative potential of ADSC to promote the proliferation rate of cardiomyocytes. This was achieved in paracrine manner, which warrants the development of ADSC conditioned medium as an “of-the-shelf” product for treatment of post-myocardial infarction complications.

Deletion of Fn14 receptor protects from right heart fibrosis and dysfunction

Pulmonary arterial hypertension (PAH) is a fatal disease for which no cure is yet available. The leading cause of death in PAH is right ventricular (RV) failure. Previously, the TNF receptor superfamily member fibroblast growth factor-inducible molecule 14 (Fn14) has been associated with different fibrotic diseases. However, so far there is no study demonstrating a causal role for endogenous Fn14 signaling in RV or LV heart disease. The purpose of this study was to determine whether global ablation of Fn14 prevents RV fibrosis and remodeling improving heart function. Here, we provide evidence for a causative role of Fn14 in pulmonary artery banding (PAB)-induced RV fibrosis and dysfunction in mice. Fn14 expression was increased in the RV after PAB. Mice lacking Fn14 (Fn14^−/−) displayed substantially reduced RV fibrosis and dysfunction following PAB compared to wild-type littermates. Cell culture experiments demonstrated that activation of Fn14 induces collagen expression via RhoA-dependent nuclear translocation of myocardin-related transcription factor-A (MRTF-A)/MAL. Furthermore, activation of Fn14 in vitro caused fibroblast proliferation and myofibroblast differentiation, which corresponds to suppression of PAB-induced RV fibrosis in Fn14^−/− mice. Moreover, our findings suggest that Fn14 expression is regulated by endothelin-1 (ET-1) in cardiac fibroblasts. We conclude that Fn14 is an endogenous key regulator in cardiac fibrosis and suggest this receptor as potential new target for therapeutic interventions in heart failure.

Cardiomyocyte proliferation contributes to heart growth in young humans

The human heart is believed to grow by enlargement but not proliferation of cardiomyocytes (heart muscle cells) during postnatal development. However, recent studies have shown that cardiomyocyte proliferation is a mechanism of cardiac growth and regeneration in animals. Combined with evidence for cardiomyocyte turnover in adult humans, this suggests that cardiomyocyte proliferation may play an unrecognized role during the period of developmental heart growth between birth and adolescence. We tested this hypothesis by examining the cellular growth mechanisms of the left ventricle on a set of healthy hearts from humans aged 0-59 y (n = 36). The percentages of cardiomyocytes in mitosis and cytokinesis were highest in infants, decreasing to low levels by 20 y. Although cardiomyocyte mitosis was detectable throughout life, cardiomyocyte cytokinesis was not evident after 20 y. Between the first year and 20 y of life, the number of cardiomyocytes in the left ventricle increased 3.4-fold, which was consistent with our predictions based on measured cardiomyocyte cell cycle activity. Our findings show that cardiomyocyte proliferation contributes to developmental heart growth in young humans. This suggests that children and adolescents may be able to regenerate myocardium, that abnormal cardiomyocyte proliferation may be involved in myocardial diseases that affect this population, and that these diseases might be treatable through stimulation of cardiomyocyte proliferation.

The pathobiology of acute coronary syndromes: clinical implications and central role of the mitochondria

Ongoing investigation has provided new insights into the pathobiology of myocardial ischemic injury. These include an improved understanding of the roles of the major modes of cell injury and death, including oncosis, apoptosis, and unregulated autophagy, as well as the central role of the mitochondria in the progression of myocardial ischemic injury, reperfusion injury, and myocardial conditioning. This understanding is providing insights for developing new pathophysiologic, pharmacologic, and cell-based therapies, alone or in combination with percutaneous coronary interventions, for better preservation of myocardium and reduction of morbidity and mortality rates from ischemic heart disease.

Molecular basis of physiological heart growth: fundamental concepts and new players

The heart hypertrophies in response to developmental signals as well as increased workload. Although adult-onset hypertrophy can ultimately lead to disease, cardiac hypertrophy is not necessarily maladaptive and can even be beneficial. Progress has been made in our understanding of the structural and molecular characteristics of physiological cardiac hypertrophy, as well as of the endocrine effectors and associated signalling pathways that regulate it. Physiological hypertrophy is initiated by finite signals, which include growth hormones (such as thyroid hormone, insulin, insulin-like growth factor 1 and vascular endothelial growth factor) and mechanical forces that converge on a limited number of intracellular signalling pathways (such as PI3K, AKT, AMP-activated protein kinase and mTOR) to affect gene transcription, protein translation and metabolism. Harnessing adaptive signalling mediators to reinvigorate the diseased heart could have important medical ramifications.

Molecular basis of physiological heart growth: fundamental concepts and new players

The heart hypertrophies in response to developmental signals as well as increased workload. Although adult-onset hypertrophy can ultimately lead to disease, cardiac hypertrophy is not necessarily maladaptive and can even be beneficial. Progress has been made in our understanding of the structural and molecular characteristics of physiological cardiac hypertrophy, as well as of the endocrine effectors and associated signalling pathways that regulate it. Physiological hypertrophy is initiated by finite signals, which include growth hormones (such as thyroid hormone, insulin, insulin-like growth factor 1 and vascular endothelial growth factor) and mechanical forces that converge on a limited number of intracellular signalling pathways (such as PI3K, AKT, AMP-activated protein kinase and mTOR) to affect gene transcription, protein translation and metabolism. Harnessing adaptive signalling mediators to reinvigorate the diseased heart could have important medical ramifications.