Myocardial aging and senescence: where have the stem cells gone?

IL-6 inhibitors effectively reverse post-infarction cardiac injury and ischemic myocardial remodeling via the TGF-β1/Smad3 signaling pathway

Approximately one in four myocardial infarctions occur in older patients. The majority of therapeutic advances are either not appropriate or not tested in elderly patients. The main reasons for deviating from the guidelines are justified concerns regarding the effectiveness of the recommended forms of therapy, fear of adverse drug reactions and ethical concerns. Targeting interleukin 6 (IL-6) for ventricular remodeling after cardiovascular damage is a feasible alternative to standard polypharmaceutics, but the underlying molecular mechanisms are not well understood. Continuous activation of the IL-6-associated cytokine receptor gp130 leads to cardiomyopathic hypertrophy. TGFβ1 is involved in forming fibrosis in various organs, and its overexpression can cause myocardial hypertrophy and fibrosis. Il-6 has been hypothesized to be indirectly involved in cardiac remodeling via the TGFβ1/Smad signaling transduction pathway. In the present study, a rat model of acute myocardial ischemia, IL-6 and IL-6 receptor blockers were injected directly into the necrotic myocardium. Changes in cardiac function, myocardial infarction area, myocardial collagen, necrotic myocardial fibrosis and levels of TGFβ1, IL-6 and MMP2/9 were quantified in myocardial tissue fibrosis by ELISA. The present study demonstrated that IL-6 stimulated myocardial fibrosis through the TGFβ1-Smad-MM2/9 signaling transduction pathway. Overall, this provided a solid foundation for understanding the relationship between IL-6 and ventricular remodeling.

Cardiac robustness regulated by reactive sulfur species

The human myocardium contains robust cells that constantly beat from birth to death without being replaced, even when exposed to various environmental stresses. Myocardial robustness is thought to depend primarily on the strength of the reducing power to protect the heart from oxidative stress. Myocardial antioxidant systems are controlled by redox reactions, primarily via the redox reaction of Cys sulfhydryl groups, such as found in thioredoxin and glutathione. However, the specific molecular entities that regulate myocardial reducing power have long been debated. Recently, reactive sulfide species, with excellent electron transfer ability, consisting of a series of multiple sulfur atoms, i.e., Cys persulfide and Cys polysulfides, have been found to play an essential role in maintaining mitochondrial quality and function, as well as myocardial robustness. This review presents the latest findings on the molecular mechanisms underlying mitochondrial energy metabolism and the maintenance of quality control by reactive sulfide species and provides a new insight for the prevention of chronic heart failure.

Opportunities and Challenges in Stem Cell Aging

Studying aging, as a physiological process that can cause various pathological phenotypes, has attracted lots of attention due to its increasing burden and prevalence. Therefore, understanding its mechanism to find novel therapeutic alternatives for age-related disorders such as neurodegenerative and cardiovascular diseases is essential. Stem cell senescence plays an important role in aging. In the context of the underlying pathways, mitochondrial dysfunction, epigenetic and genetic alterations, and other mechanisms have been studied and as a consequence, several rejuvenation strategies targeting these mechanisms like pharmaceutical interventions, genetic modification, and cellular reprogramming have been proposed. On the other hand, since stem cells have great potential for disease modeling, they have been useful for representing aging and its associated disorders. Accordingly, the main mechanisms of senescence in stem cells and promising ways of rejuvenation, along with some examples of stem cell models for aging are introduced and discussed. This review aims to prepare a comprehensive summary of the findings by focusing on the most recent ones to shine a light on this area of research.

Combinatorial intranasal delivery of bone marrow mesenchymal stem cells and insulin-like growth factor-1 improves neurovascularization and functional outcomes following focal cerebral ischemia in mice

Bone marrow mesenchymal stem cell (BMSC) transplantation is a promising treatment for ischemic stroke that carries a severe mortality and disability burden amongst the adult population globally. Thus far, BMSC transplantation has been insufficient for ameliorating neurological deficits resulting from cerebral ischemia. This shortcoming may be an outcome due to poor homing and viability of grafted cells in ischemic brain that limit the potential therapeutic benefits of BMSC transplantation. Insulin-like growth factor-1 (IGF-1), a potent anti-apoptotic agent, exerts neuroprotective effects in ischemic stroke as well as rescuing neuronal death in vitro. We hypothesized that IGF-1 could also protect BMSCs from apoptotic death, and examined whether the combination of BMSCs with IGF-1 can enhance functional recovery outcomes in mice following cerebral ischemia. Intranasal administration of BMSCs with IGF-1 was applied in a mouse focal ischemic stroke model. Our in vitro results indicated that BMSCs treated with IGF-1 exhibited less apoptotic death induced by oxygen-glucose deprivation (OGD), and an improved migratory capacity. At 14 days after ischemic insult, the combination of BMSCs with IGF-1 resulted in a larger number of NeuN/BrdU and Glut-1/BrdU co-labeled cells in the areas contiguous to the ischemic core than IGF-1 or BMSC treatment alone. Western blot assays demonstrated that the protein levels of BDNF, VEGF and Ang-1 were significantly upregulated in the peri-infarct region in the combination treatment group compared with single IGF- 1 or BMSC treatment. Co-administration of BMSCs and IGF-1 markedly increases local cerebral blood flow and promoted better functional behavior outcomes. These data suggest that intranasal delivery of BMSCs in conjunction with IGF-1 strengthened functional recovery following ischemia via increasing neurogenesis and angiogenesis, providing a novel optimized strategy for improving the therapeutic efficacy of BMSC transplantation for ischemia.

Hypoxia-induced interaction of filamin with Drp1 causes mitochondrial hyperfission-associated myocardial senescence

Defective mitochondrial dynamics through aberrant interactions between mitochondria and actin cytoskeleton is increasingly recognized as a key determinant of cardiac fragility after myocardial infarction (MI). Dynamin-related protein 1 (Drp1), a mitochondrial fission-accelerating factor, is activated locally at the fission site through interactions with actin. Here, we report that the actin-binding protein filamin A acted as a guanine nucleotide exchange factor for Drp1 and mediated mitochondrial fission-associated myocardial senescence in mice after MI. In peri-infarct regions characterized by mitochondrial hyperfission and associated with myocardial senescence, filamin A colocalized with Drp1 around mitochondria. Hypoxic stress induced the interaction of filamin A with the GTPase domain of Drp1 and increased Drp1 activity in an actin-binding-dependent manner in rat cardiomyocytes. Expression of the A1545T filamin mutant, which potentiates actin aggregation, promoted mitochondrial hyperfission under normoxia. Furthermore, pharmacological perturbation of the Drp1-filamin A interaction by cilnidipine suppressed mitochondrial hyperfission-associated myocardial senescence and heart failure after MI. Together, these data demonstrate that Drp1 association with filamin and the actin cytoskeleton contributes to cardiac fragility after MI and suggests a potential repurposing of cilnidipine, as well as provides a starting point for innovative Drp1 inhibitor development.

Beyond cardiomyocyte loss: Role of Notch in cardiac aging

The knowledge of the cellular events occurring in the aging heart has dramatically expanded in the last decade and is expected to further grow in years to come. It is now clear that impaired function and loss of cardiomyocytes are major features of cardiac aging, but other events are likewise important. In particular, accumulating experimental evidence highlights the importance of fibroblast and cardiac progenitor cell (CPC) dysfunction. The Notch pathway regulates cardiomyocyte, fibroblast, and CPC activity and, thus, may be critically involved in heart disease associated with advanced age, especially heart failure. In a translational perspective, thorough investigation of the Notch system in the aging myocardium may lead to the identification of molecular targets for novel therapies for age-related cardiac disease.

Mitochondrial sirtuins in the heart

Sirtuins (SIRTs) are NAD(+)-dependent enzymes that catalyze deacylation of protein lysine residues. In mammals, seven sirtuins have been identified, SIRT1-7. SIRT3-5 are mainly or exclusively localized within mitochondria and mainly participate in the regulation of energy metabolic pathways. Since mitochondrial ATP regeneration is inevitably linked to the maintenance of cardiac pump function, it is not surprising that recent studies revealed a role for mitochondrial sirtuins in the regulation of myocardial energetics and function. In addition, mitochondrial sirtuins modulate the extent of myocardial ischemia reperfusion injury and the development of cardiac hypertrophy and failure. Thus, targeting mitochondrial sirtuins has been proposed as a novel approach to improve myocardial mitochondrial energetics, which is frequently impaired in cardiac disease and considered an important underlying cause contributing to several cardiac pathologies, including myocardial ischemia reperfusion injury and heart failure. In the current review, we present and discuss the available literature on mitochondrial sirtuins and their potential roles in cardiac physiology and disease.

Akt2 ablation prolongs life span and improves myocardial contractile function with adaptive cardiac remodeling: role of Sirt1-mediated autophagy regulation

Aging is accompanied with unfavorable geometric and functional changes in the heart involving dysregulation of Akt and autophagy. This study examined the impact of Akt2 ablation on life span and cardiac aging as well as the mechanisms involved with a focus on autophagy and mitochondrial integrity. Cardiac geometry, contractile, and intracellular Ca2+ properties were evaluated using echocardiography, IonOptix® edge-detection and fura-2 techniques. Levels of Sirt1, mitochondrial integrity, autophagy, and mitophagy markers were evaluated using Western blot. Our results revealed that Akt2 ablation prolonged life span (by 9.1%) and alleviated aging (24 months)-induced unfavorable changes in myocardial function and intracellular Ca2+ handling (SERCA2a oxidation) albeit with more pronounced cardiac hypertrophy (58.1%, 47.8%, and 14.5% rises in heart weight, wall thickness, and cardiomyocyte cross-sectional area). Aging downregulated levels of Sirt1, increased phosphorylation of Akt, and the nuclear transcriptional factor Foxo1, as well as facilitated acetylation of Foxo1, the effects of which (except Sirt1 and Foxo1 acetylation) were significantly attenuated or negated by Akt2 ablation. Advanced aging disturbed autophagy, mitophagy, and mitochondrial integrity as evidenced by increased p62, decreased levels of beclin-1, Atg7, LC3B, BNIP3, PTEN-induced putative kinase 1 (PINK1), Parkin, UCP-2, PGC-1α, and aconitase activity, the effects of which were reversed by Akt2 ablation. Aging-induced cardiomyocyte contractile dysfunction and loss of mitophagy were improved by rapamycin and the Sirt1 activator SRT1720. Activation of Akt using insulin or Parkin deficiency prevented SRT1720-induced beneficial effects against aging. In conclusion, our data indicate that Akt2 ablation protects against cardiac aging through restored Foxo1-related autophagy and mitochondrial integrity.

Aging and Autophagy in the Heart

The aging population is increasing in developed countries. Because the incidence of cardiac disease increases dramatically with age, it is important to understand the molecular mechanisms through which the heart becomes either more or less susceptible to stress. Cardiac aging is characterized by the presence of hypertrophy, fibrosis, and accumulation of misfolded proteins and dysfunctional mitochondria. Macroautophagy (hereafter referred to as autophagy) is a lysosome-dependent bulk degradation mechanism that is essential for intracellular protein and organelle quality control. Autophagy and autophagic flux are generally decreased in aging hearts, and murine autophagy loss-of-function models develop exacerbated cardiac dysfunction that is accompanied by the accumulation of misfolded proteins and dysfunctional organelles. On the contrary, stimulation of autophagy generally improves cardiac function in mouse models of protein aggregation by removing accumulated misfolded proteins, dysfunctional mitochondria, and damaged DNA, thereby improving the overall cellular environment and alleviating aging-associated pathology in the heart. Increasing lines of evidence suggest that autophagy is required for many mechanisms that mediate lifespan extension, such as caloric restriction, in various organisms. These results raise the exciting possibility that autophagy may play an important role in combating the adverse effects of aging in the heart. In this review, we discuss the role of autophagy in the heart during aging, how autophagy alleviates age-dependent changes in the heart, and how the level of autophagy in the aging heart can be restored.

Mitochondrial dynamics: Orchestrating the journey to advanced age

Aging is a degenerative process that unfortunately is an inevitable part of life and risk factor for cardiovascular disease including heart failure. Among the several theories purported to explain the effects of age on cardiac dysfunction, the mitochondrion has emerged a central regulator of this process. Hence, it is not surprising that abnormalities in mitochondrial quality control including biogenesis and turnover have such detrimental effects on cardiac function. In fact mitochondria serve as a conduit for biological signals for apoptosis, necrosis and autophagy respectively. The removal of damaged mitochondria by autophagy/mitophagy is essential for mitochondrial quality control and cardiac homeostasis. Defects in mitochondrial dynamism fission/fusion events have been linked to cardiac senescence and heart failure. In this review we discuss the impact of aging on mitochondrial dynamics and senescence on cardiovascular health. This article is part of a Special Issue entitled: CV Aging.

Cardiomyocyte Regeneration in the mdx Mouse Model of Nonischemic Cardiomyopathy

Endogenous regeneration has been demonstrated in the mammalian heart after ischemic injury. However, approximately one-third of cases of heart failure are secondary to nonischemic heart disease and cardiac regeneration in these cases remains relatively unexplored. We, therefore, aimed at quantifying the rate of new cardiomyocyte formation at different stages of nonischemic cardiomyopathy. Six-, 12-, 29-, and 44-week-old mdx mice received a 7 day pulse of BrdU. Quantification of isolated cardiomyocyte nuclei was undertaken using cytometric analysis to exclude nondiploid nuclei. Between 6–7 and 12–13 weeks, there was a statistically significant increase in the number of BrdU-labeled nuclei in the mdx hearts compared with wild-type controls. This difference was lost by the 29–30 week time point, and a significant decrease in cardiomyocyte generation was observed in both the control and mdx hearts by 44–45 weeks. Immunohistochemical analysis demonstrated BrdU-labeled nuclei exclusively in mononucleated cardiomyocytes. This study demonstrates cardiomyocyte regeneration in a nonischemic model of mammalian cardiomyopathy, controlling for changes in nuclear ploidy, which is lost with age, and confirms a decrease in baseline rates of cardiomyocyte regeneration with aging. While not attempting to address the cellular source of regeneration, it confirms the potential utility of innate regeneration as a therapeutic target.

Cardiac aging - Getting to the stem of the problem

Cardiac aging is a heterogeneous process caused by a combination of stochastic events which manifests as loss of structure and function in the heart, however several recent studies draw attention to aging being primarily a stem cell problem. This review summarizes findings in support of the "stem cell hypothesis of aging" and discusses the impact of age on cardiac stem cells and the niche. This article is part of a Special Issue entitled 'CV Aging'.

Clinical-scale in vitro expansion preserves biological characteristics of cardiac atrial appendage stem cells

OBJECTIVES

Cardiac atrial appendage stem cells (CASCs) have recently emerged as an attractive candidate for cardiac regeneration after myocardial infarction. As with other cardiac stem cells, CASCs have to be expanded ex vivo to obtain clinically relevant cell numbers. However, foetal calf serum (FCS), which is routinely used for cell culturing, is unsuitable for clinical purposes, and influence of long-term in vitro culture on CASC behaviour is unknown.

MATERIALS AND METHODS

We examined effects on CASC biology of prolonged expansion, and evaluated a culture protocol suitable for human use.

RESULTS

In FCS-supplemented medium, CASCs could be kept in culture for 55.75 ± 3.63 days, before reaching senescence. Despite a small reduction in numbers of proliferating CASCs (1.37 ± 0.52% per passage) and signs of progressive telomere shortening (0.04 ± 0.02 kb per passage), their immunophenotype and myocardial differentiation potential remained unaffected during the entire culture period. The cells were successfully expanded in human platelet plasma supernatant, while maintaining their biological properties.

CONCLUSIONS

We successfully developed a protocol for long-term culture, to obtain clinically relevant CASC numbers, while retaining their cardiogenic potential. These insights in CASC biology and optimization of a humanized platelet-based culture method are an important step towards clinical application of CASCs for cardiac regenerative medicine.

Myostatin regulates tissue potency and cardiac calcium-handling proteins

Attenuating myostatin enhances striated muscle growth, reduces adiposity, and improves cardiac contractility. To determine whether myostatin influences tissue potency in a manner that could control such pleiotropic actions, we generated label-retaining mice with wild-type and mstn(-/-) (Jekyll) backgrounds in which slow-cycling stem, transit-amplifying, and progenitor cells are preferentially labeled by histone 2B/green fluorescent protein. Jekyll mice were born with fewer label-retaining cells (LRCs) in muscle and heart, consistent with increased stem/progenitor cell contributions to embryonic growth of both tissues. Cardiac LRC recruitment from noncardiac sources occurred in both groups, but lasted longer in Jekyll hearts, whereas heightened β-adrenergic sensitivity of mstn(-/-) hearts was explained by elevated SERCA2a, phospholamban, and β2-adrenergic receptor levels. Jekyll mice were also born with more adipose LRCs despite significantly smaller tissue weights. Reduced adiposity in mstn(-/-) animals is therefore due to reduced lipid deposition as adipoprogenitor pools appear to be enhanced. By contrast, increased bone densities of mstn(-/-) mice are likely compensatory to hypermuscularity because LRC counts were similar in Jekyll and wild-type tibia. Myostatin therefore significantly influences the potency of different tissues, not just muscle, as well as cardiac Ca²⁺-handling proteins. Thus, the pleiotropic phenotype of mstn(-/-) animals may not be due to enhanced muscle development per se, but also to altered stem/progenitor cell pools that ultimately influence tissue potency.

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.

Mechanisms of exercise-induced cardiac growth

Exercise is a well-established intervention for the prevention and treatment of cardiovascular disease. Increase in cardiomyocyte size is likely to be the central mechanism of exercise-induced cardiac growth, but recent research also supports a role for the generation of new cardiomyocytes as a contributor to physiological cardiac growth. Other cardiac cell types also respond to exercise. For example, endothelial cells are important for the regulation of large vessels and expansion of microvasculature in meeting demands of the growing heart. Cardiac fibroblasts are known to generate and respond to important signals from their environment, but their role in exercise is less well defined. Therefore, cardiac growth relies on complex, finely regulated and interdependent signaling pathways as well as cross-talk among cardiac cell types.

Recurrence of left ventricular dysfunction in patients with restored idiopathic dilated cardiomyopathy

BACKGROUND

In some patients with nonischemic idiopathic dilated cardiomyopathy (DCM), left ventricular (LV) dysfunction improves spontaneously but can recur. The factors predicting recurrence of LV dysfunction in recovered idiopathic DCM are poorly defined. We investigated the clinical, echocardiographic, and laboratory variables affecting recurrence of LV dysfunction in patients who recovered from DCM.

HYPOTHESIS

The recurrence of LV dysfunction in recovered idiopathic DCM is impacted by clinical, echocardiographic, and laboratory variables.

METHODS

The study comprised 85 consecutively enrolled patients (62 males, age 57 ± 16 years) with DCM who achieved a restoration of LV systolic function. Patients were followed up for 50 ± 33 months after recovery from LV dysfunction without discontinuation of standard medication for heart failure with depressed ejection fraction. Clinical, echocardiographic, and laboratory variables were analyzed to identify factors independently associated with recurrence of LV dysfunction.

RESULTS

LV dysfunction recurred in 33 patients (23 males, age 64 ± 12 years). Univariate analysis revealed that age, duration from initial presentation to recovery time, diabetes, and LV end-diastolic dimension (LVEDD) at initial presentation were associated with recurrence of LV dysfunction. Multivariate analysis revealed that only age, diabetes, and LVEDD at initial presentation were independent predictors in patients who recovered from LV dysfunction.

CONCLUSIONS

The recurrence of LV dysfunction was significantly correlated with age, presence of diabetes, and LVEDD at initial presentation. Clinicians should consider maintenance of intensive care to patients who recovered from DCM with these factors.

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.

Cardiac Hegemony of Senescence

Cardiac senescence and age-related disease development have gained general attention and recognition in the past decades due to increased accessibility and quality of health care. The advancement in global civilization is complementary to concerns regarding population aging and development of chronic degenerative diseases. Cardiac degeneration has been rigorously studied. The molecular mechanisms of cardiac senescence are on multiple cellular levels and hold a multilayer complexity level, thereby hampering development of unambiguous treatment protocols. In particular, the synergistic exchange of the senescence phenotype through a senescence secretome between myocytes and stem cells appears complicated and is of great future therapeutic value. The current review article will highlight hallmarks of senescence, cardiac myocyte and stem cell senescence, and the mutual exchange of senescent secretome. Future cardiac cell therapy approaches require a comprehensive understanding of myocardial senescence to improve therapeutic efficiency as well as efficacy.

Changes in the mitochondrial permeability transition pore in aging and age-associated diseases

Aging is a biological process associated with impairment of mitochondrial bioenergetic function, increased oxidative stress, attenuated ability to respond to stresses and increased risk in contracting age-associated diseases. When mitochondria are subjected to oxidative stress, accompanied by calcium overload and ATP depletion, they undergo "a permeability transition", characterized by sudden induced change of the inner mitochondrial membrane permeability for water as well as for low-molecular weight solutes (≤1.5kDa), resulting in membrane depolarization and uncoupling of oxidative phosphorylation. Research interest in the entity responsible for this phenomenon, the "mitochondrial permeability transition pore" (MPTP) has dramatically increased after demonstration that it plays a key role in the life and death decision in cells. The molecular structure and identity of MPTP is not yet known, although the pore is thought to exist as multiprotein complex. Some evidence indicate that the sensitivity of mitochondria to Ca(2+)-induced MPTP opening increases with aging; however the basis of this difference is unknown. Changes in MPTP structure and/or function may have important implications in the aging process and aged-associated diseases. This article examines data relevant to this issue. The important role of a principal lipidic counter-partner of the MPTP, cardiolipin, will also be discussed.

Mesenchymal stem cells conditioned with glucose depletion augments their ability to repair-infarcted myocardium

Mesenchymal stem cells (MSCs) are an attractive candidate for autologous cell therapy, but their ability to repair damaged myocardium is severely compromised with advanced age. Development of viable autologous cell therapy for treatment of heart failure in the elderly requires the need to address MSC ageing. In this study, MSCs from young (2 months) and aged (24 months) C57BL/6 mice were characterized for gene expression of IGF-1, FGF-2, VEGF, SIRT-1, AKT, p16(INK4a) , p21 and p53 along with measurements of population doubling (PD), superoxide dismutase (SOD) activity and apoptosis. Aged MSCs displayed senescent features compared with cells isolated from young animals and therefore were pre-conditioned with glucose depletion to enhance age affected function. Pre-conditioning of aged MSCs led to an increase in expression of IGF-1, AKT and SIRT-1 concomitant with enhanced viability, proliferation and delayed senescence. To determine the myocardial repair capability of pre-conditioned aged MSCs, myocardial infarction (MI) was induced in 24 months old C57BL/6 wild type mice and GFP expressing untreated and pre-conditioned aged MSCs were transplanted. Hearts transplanted with pre-conditioned aged MSCs showed increased expression of paracrine factors, such as IGF-1, FGF-2, VEGF and SDF-1α. This was associated with significantly improved cardiac performance as measured by dp/dt(max), dp/dt(min), LVEDP and LVDP, declined left ventricle (LV) fibrosis and apoptosis as measured by Masson's Trichrome and TUNEL assays, respectively, after 30 days of transplantation. In conclusion, pre-conditioning of aged MSCs with glucose depletion can enhance proliferation, delay senescence and restore the ability of aged cells to repair senescent infarcted myocardium.

Telomerase expression in the mammalian heart

While the mammalian heart has low, but functionally significant, levels of telomerase expression, the cellular population responsible remains incompletely characterized. This study aimed to identify the cell types responsible for cardiac telomerase activity in neonatal, adult, and cryoinjured adult hearts using transgenic mice expressing green fluorescent protein (GFP), driven by the promoter for murine telomerase reverse transcriptase (mTert), which is a necessary and rate-limiting component of telomerase. A rare population of mTert-GFP-expressing cells was identified that possessed all detectable cardiac telomerase RNA and telomerase activity. It was heterogeneous and included cells coexpressing markers of cardiomyocytic, endothelial, and mesenchymal lineages, putative cardiac stem cell markers, and, interestingly, cardiomyocytes with a differentiated phenotype. Quantification using both flow cytometry and immunofluorescence identified a significant decline in mTert-GFP cells in adult animals compared to neonates (∼9- and ∼20-fold, respectively). Cardiac injury resulted in a ∼6.45-fold expansion of this population (P<0.005) compared with sham-operated controls. This study identifies the cells responsible for cardiac telomerase activity, demonstrates a significant diminution with age but a marked response to injury, and, given the relationship between telomerase activity and stem cell populations, suggests that they represent a potential target for further investigation of cardiac regenerative potential.—Richardson, G. D., Breault, D., Horrocks, G., Cormack, S., Hole, N., Owens, W. A. Telomerase expression in the mammalian heart.

c-kit+ precursors support postinfarction myogenesis in the neonatal, but not adult, heart

We examined the myogenic response to infarction in neonatal and adult mice to determine the role of c-kit(+) cardiovascular precursor cells (CPC) that are known to be present in early heart development. Infarction of postnatal day 1-3 c-kit(BAC)-EGFP mouse hearts induced the localized expansion of (c-kit)EGFP(+) cells within the infarct, expression of the c-kit and Nkx2.5 mRNA, myogenesis, and partial regeneration of the infarction, with (c-kit)EGFP(+) cells adopting myogenic and vascular fates. Conversely, infarction of adult mice resulted in a modest induction of (c-kit)EGFP(+) cells within the infarct, which did not express Nkx2.5 or undergo myogenic differentiation, but adopted a vascular fate within the infarction, indicating a lack of authentic CPC. Explantation of infarcted neonatal and adult heart tissue to scid mice, and adoptive transfer of labeled bone marrow, confirmed the cardiac source of myogenic (neonate) and angiogenic (neonate and adult) cells. FACS-purified (c-kit)EGFP(+)/(αMHC)mCherry(-) (noncardiac) cells from microdissected infarcts within 6 h of infarction underwent cardiac differentiation, forming spontaneously beating myocytes in vitro; cre/LoxP fate mapping identified a noncardiac population of (c-kit)EGFP(+) myocytes within infarctions, indicating that the induction of undifferentiated precursors contributes to localized myogenesis. Thus, adult postinfarct myogenic failure is likely not due to a context-dependent restriction of precursor differentiation, and c-kit induction following injury of the adult heart does not define precursor status.

Higher frequencies of BCRP+ cardiac resident cells in ischaemic human myocardium

AIMS

Several cardiac resident progenitor cell types have been reported for the adult mammalian heart. Here we characterize their frequencies and distribution pattern in non-ischaemic human myocardial tissue and after ischaemic events.

METHODS AND RESULTS

We obtained 55 biopsy samples from human atria and ventricles and used immunohistological analysis to investigate two cardiac cell types, characterized by the expression of breast cancer resistance protein (BCRP)/ABCG2 [for side population (SP) cells] or c-kit. Highest frequencies of BCRP+ cells were detected in the ischaemic right atria with a median of 5.40% (range: 2.48-11.1%) vs. 4.40% (1.79-7.75%) in the non-ischaemic right atria (P = 0.47). Significantly higher amounts were identified in ischaemic compared with non-ischaemic ventricles, viz. 5.44% (3.24-9.30%) vs. 0.74% (0-5.23%) (P = 0.016). Few numbers of BCRP+ cells co-expressed the cardiac markers titin, sarcomeric α-actinin, or Nkx2.5; no co-expression of BCRP and progenitor cell marker Sca-1 or pluripotency markers Oct-3/4, SSEA-3, and SSEA-4 was detected. C-kit+ cells displayed higher frequencies in ischaemic (ratio: 1:25 000 ± 2500 of cell counts) vs. non-ischaemic myocardium (1:105 000 ± 43 000). Breast cancer resistance protein+/c-kit+ cells were not identified. Following in vitro differentiation, BCRP+ cells isolated from human heart biopsy samples (n = 6) showed expression of cardiac troponin T and α-myosin heavy-chain, but no full differentiation into functional beating cardiomyocytes was observed.

CONCLUSION

We were able to demonstrate that BCRP+/CD31- cells are more abundant in the heart than their c-kit+ counterparts. In the non-ischaemic hearts, they are preferentially located in the atria. Following ischaemia, their numbers are elevated significantly. Our data might provide a valuable snapshot at potential progenitor cells after acute ischaemia in vivo, and mapping of these easily accessible cells may influence future cell therapeutic strategies.

Autophagy and cardiovascular aging: lesson learned from rapamycin

The biological aging process is commonly associated with increased risk of cardiovascular diseases. Several theories have been put forward for aging-associated deterioration in ventricular function, including attenuation of growth hormone (insulin-like growth factors and insulin) signaling, loss of DNA replication and repair, histone acetylation and accumulation of reactive oxygen species. Recent evidence has depicted a rather unique role of autophagy as another important pathway in the regulation of longevity and senescence. Autophagy is a predominant cytoprotective (rather than self-destructive) process. It carries a prominent role in determination of lifespan. Reduced autophagy has been associated with aging, leading to accumulation of dysfunctional or damaged proteins and organelles. To the contrary, measures such as caloric restriction and exercise may promote autophagy to delay aging and associated comorbidities. Stimulation of autophagy using rapamycin may represent a novel strategy to prolong lifespan and combat aging-associated diseases. Rapamycin regulates autophagy through inhibition of the nutrient-sensing molecule mammalian target of rapamycin (mTOR). Inhibition of mTOR through rapamycin and caloric restriction promotes longevity. The purpose of this review is to recapitulate some of the recent advances in an effort to better understand the interplay between rapamycin-induced autophagy and decelerating cardiovascular aging.

Fetal myocardium in the kidney capsule: an in vivo model of repopulation of myocytes by bone marrow cells

Debate surrounds the question of whether the heart is a post-mitotic organ in part due to the lack of an in vivo model in which myocytes are able to actively regenerate. The current study describes the first such mouse model — a fetal myocardial environment grafted into the adult kidney capsule. Here it is used to test whether cells descended from bone marrow can regenerate cardiac myocytes. One week after receiving the fetal heart grafts, recipients were lethally irradiated and transplanted with marrow from green fluorescent protein (GFP)-expressing C57Bl/6J (B6) donors using normal B6 recipients and fetal donors. Levels of myocyte regeneration from GFP marrow within both fetal myocardium and adult hearts of recipients were evaluated histologically. Fetal myocardium transplants had rich neovascularization and beat regularly after 2 weeks, continuing at checkpoints of 1, 2, 4, 6, 8 and12 months after transplantation. At each time point, GFP-expressing rod-shaped myocytes were found in the fetal myocardium, but only a few were found in the adult hearts. The average count of repopulated myocardium with green rod-shaped myocytes was 996.8 cells per gram of fetal myocardial tissue, and 28.7 cells per adult heart tissue, representing a thirty-five fold increase in fetal myocardium compared to the adult heart at 12 months (when numbers of green rod-shaped myocytes were normalized to per gram of myocardial tissue). Thus, bone marrow cells can differentiate to myocytes in the fetal myocardial environment. The novel in vivo model of fetal myocardium in the kidney capsule appears to be valuable for testing repopulating abilities of potential cardiac progenitors.

Rb1 protects endothelial cells from hydrogen peroxide-induced cell senescence by modulating redox status

Senescence of endothelial cells has been proposed to play an important role in endothelial dysfunction and atherogenesis. In the present study we aimed to investigate whether ginsenoside Rb1, a major constituent of ginseng, protects endothelial cells from H(2)O(2)-induced endothelial senescence. While H(2)O(2) induced premature senescent-like phenotype of human umbilical vein endothelial cells (HUVECs), as judged by increased senescence-associated β-galactosidase (SA-β-gal) activity, enlarged, flattened cell morphology and sustained growth arrest, our results demonstrated that Rb1 protected endothelial cells from oxidative stress induced senescence. Mechanistically, we found that Rb1 could markedly increase intracellular superoxide dismutase (Cu/Zn SOD/SOD1) activity and decrease the malondialdehyde (MDA) level in H(2)O(2)-treated HUVECs, and suppress the generation of intracellular reactive oxygen species (ROS). Consistent with these findings, Rb1 could effectively restore the protein expression of Cu/Zn SOD, which was down-regulated in H(2)O(2) treated cells. Taken together, our data demonstrate that Rb1 exhibits antioxidant effects and antagonizes H(2)O(2)-induced cellular senescence.

Chronic Akt activation accentuates aging-induced cardiac hypertrophy and myocardial contractile dysfunction: role of autophagy

Aging is often accompanied with geometric and functional changes in the heart, although the underlying mechanisms remain unclear. Recent evidence has described a potential role of Akt and autophagy in aging-associated organ deterioration. This study was to examine the impact of cardiac-specific Akt activation on aging-induced cardiac geometric and functional changes and underlying mechanisms involved. Cardiac geometry, contractile and intracellular Ca(2+) properties were evaluated using echocardiography, edge-detection and fura-2 techniques. Level of insulin signaling and autophagy was evaluated by western blot. Our results revealed cardiac hypertrophy (enlarged chamber size, wall thickness, myocyte cross-sectional area), fibrosis, decreased cardiac contractility, prolonged relengthening along with compromised intracellular Ca(2+) release and clearance in aged (24-26 month-old) mice compared with young (3-4 month-old) mice, the effects of which were accentuated by chronic Akt activation. Aging enhanced Akt and mTOR phosphorylation while reducing that of PTEN, AMPK and ACC with a more pronounced response in Akt transgenic mice. GSK3β phosphorylation and eNOS levels were unaffected by aging or Akt overexpression. Levels of beclin-1, Atg5 and LC3-II-to-LC3-I ratio were decreased in aged hearts, the effect of which with the exception of Atg 5 was exacerbated by Akt overactivation. Levels of p62 were significantly enhanced in aged mice with a more pronounced increase in Akt mice. Neither aging nor Akt altered β-glucuronidase activity and cathepsin B although aging reduced LAMP1 level. In addition, rapamycin reduced aging-induced cardiomyocyte contractile and intracellular Ca(2+) dysfunction while Akt activation suppressed autophagy in young but not aged cardiomyocytes. In conclusion, our data suggest that Akt may accentuate aging-induced cardiac geometric and contractile defects through a loss of autophagic regulation.

Caloric restriction does not alter effects of aging in cardiac side population cells

The aged heart displays a loss of cardiomyocyte number and function, possibly due to the senescence and decreased regenerative potential that has been observed in some cardiac progenitor cells. An important cardiac progenitor that has not been studied in the context of aging is the cardiac side population (CSP) cell. To address this, flow cytometry-assisted cell sorting was used to isolate CSP cells from adult (6-10 months old) and aged (24-32 months old) C57Bl/6 mice that were fed either a control diet or an anti-aging diet (caloric restriction, CR). Aging caused a 2.3-fold increase in the total number of CSP cells and a 3.2-fold increase in the cardiomyogenic sca1(+)/CD31(-) subpopulation. Aging did not affect markers of proliferation or senescence, including telomerase activity and expression of cell cycle genes, in sca1(+)/CD31(-) CSP cells. In contrast, the aged cells had reduced expression of genes associated with differentiation, including smooth muscle actin and cardiac muscle actin (5.1- and 3.2-fold, respectively). None of these age effects were altered by CR diet. Therefore, it appears that the manner in which CSP cells age is distinct from the aging of post-mitotic tissue (and perhaps other progenitor cells) that can often be attenuated by CR.

Stem cell therapy for ischemic heart disease

Stem cell transplantation has emerged as a novel treatment option for ischemic heart disease. Different cell types have been utilized and the recent development of induced pluripotent stem cells has generated tremendous excitement in the regenerative field. Bone marrow-derived multipotent progenitor cell transplantation in preclinical large animal models of postinfarction left ventricular remodeling has demonstrated long-term functional and bioenergetic improvement. These beneficial effects are observed despite no significant engraftment of bone marrow cells in the myocardium and even lower differentiation of these cells into cardiomyocytes. It is thought to be related to the paracrine effect of these stem cells, which secrete factors that lead to long-term gene expression changes in the host myocardium, thereby promoting neovascularization, inhibiting apoptosis, and stimulating resident cardiac progenitor cells. Future studies are warranted to examine the changes in the recipient myocardium after stem cell transplantation and to investigate the signaling pathways involved in these effects.

Endothelial and cardiac progenitors: boosting, conditioning and (re)programming for cardiovascular repair

Preclinical studies performed in cell culture and animal systems have shown the outstanding ability of stem cells to repair ischemic heart and lower limbs by promoting the formation of new blood vessels and new myocytes. In contrast, clinical studies of stem cell administration in patients with myocardial ischemia have revealed only modest, although promising, results. Basic investigations have shown the feasibility of adult cells reprogramming into pluripotent cells by defined factors, thus opening the way to the devise of protocols to ex vivo derive virtually unexhausted cellular pools. In contrast, cellular and molecular studies have indicated that risk factors limit adult-derived stem cell survival, proliferation and engraftment in ischemic tissues. The use of fully reprogrammed cells raises safety concerns; therefore, adult cells remain a primary option for clinicians interested in therapeutic cardiovascular repair. Pharmacologic approaches have been devised to restore the cardiovascular repair ability of failing progenitors from patients at risk. In the present contribution, the most advanced pharmacologic approaches to (re)program, boost, and condition endothelial and cardiac progenitor cells to enhance cardiovascular regeneration are discussed.

Repair of senescent myocardium by mesenchymal stem cells is dependent on the age of donor mice

Myocardial infarction is one of the leading causes of mortality in aged people. Whether age of donors of mesenchymal stem cells (MSCs) affects its ability to repair the senescent heart tissue is unknown. In the present study, MSCs from young (2 months) and aged (18 months) green fluorescent protein expressing C57BL/6 mice were characterized with p16(INK4a) and β-gal associated senescence. Myocardial infarction was produced in 18-month-old wild-type C57BL/6 mice transplanted with MSCs from young and aged animals in the border of the infarct region. Expression of p16(INK4a) in MSCs from aged animals was significantly higher (21.5%± 1.2, P < 0.05) as compared to those from young animals (9.2%± 2.8). A decline in the tube-forming ability on Matrigel was also observed in aged MSCs as well as down-regulation of insulin-like growth factor-1, fibroblast growth factor (FGF-2), vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF) compared to young cells. Mice transplanted with young MSCs exhibited significant improvement in their left ventricle (LV) systolic and diastolic function as demonstrated by dp/dt(max) , dp/dt(min) , P(max) . Reduction in the LV fibrotic area was concomitant with neovascularization as demonstrated by CD31 and smooth muscle actin (SMA) expression. Real-time RT-PCR analysis for VEGF, stromal cell derived factor (SDF-1α) and GATA binding factor 4 (GATA-4) genes further confirmed the effect of age on MSC differentiation towards cardiac lineages and enhanced angiogenesis. These studies lead to the conclusion that repair potential of MSCs is dependent on the age of donors and the repair of senescent infarcted myocardium requires young healthy MSCs.

Cellular mechanisms of cardioprotection by calorie restriction: state of the science and future perspectives

Evidence from animal models and preliminary studies in humans indicates that calorie restriction (CR) delays cardiac aging and can prevent cardiovascular disease. These effects are mediated by a wide spectrum of biochemical and cellular adaptations, including redox homeostasis, mitochondrial function, inflammation, apoptosis, and autophagy. Despite the beneficial effects of CR, its large-scale implementation is challenged by applicability issues as well as health concerns. However, preclinical studies indicate that specific compounds, such as resveratrol, may mimic many of the effects of CR, thus potentially obviating the need for drastic food intake reductions. Results from ongoing clinical trials will reveal whether the intriguing alternative of CR mimetics represents a safe and effective strategy to promote cardiovascular health and delay cardiac aging in humans.

Effects of myostatin deletion in aging mice

Inhibitors of myostatin, a negative regulator of skeletal muscle mass, are being developed to mitigate aging-related muscle loss. Knock-out (KO) mouse studies suggest myostatin also affects adiposity, glucose handling and cardiac growth. However, the cardiac consequences of inhibiting myostatin remain unclear. Myostatin inhibition can potentiate cardiac growth in specific settings (Morissette et al., 2006), a concern because of cardiac hypertrophy is associated with adverse clinical outcomes. Therefore, we examined the systemic and cardiac effects of myostatin deletion in aged mice (27-30 months old). Heart mass increased comparably in both wild-type (WT) and KO mice. Aged KO mice maintained twice as much quadriceps mass as aged WT; however, both groups lost the same percentage (36%) of adult muscle mass. Dual-energy X-ray absorptiometry revealed increased bone density, mineral content, and area in aged KO vs. aged WT mice. Serum insulin and glucose levels were lower in KO mice. Echocardiography showed preserved cardiac function with better fractional shortening (58.1% vs. 49.4%, P = 0.002) and smaller left ventricular diastolic diameters (3.41 vs. 2.71, P = 0.012) in KO vs. WT mice. Phospholamban phosphorylation was increased 3.3-fold in KO hearts (P < 0.05), without changes in total phospholamban, sarco(endo)plasmic reticulum calcium ATPase 2a or calsequestrin. Aged KO hearts showed less fibrosis by Masson's Trichrome staining. Thus, myostatin deletion does not affect aging-related increases in cardiac mass and appears beneficial for bone density, insulin sensitivity and heart function in senescent mice. These results suggest that clinical interventions designed to inhibit skeletal muscle mass loss with aging could have beneficial effects on other organ systems as well.

Genotoxic stress abrogates renewal of melanocyte stem cells by triggering their differentiation

Somatic stem cell depletion due to the accumulation of DNA damage has been implicated in the appearance of aging-related phenotypes. Hair graying, a typical sign of aging in mammals, is caused by the incomplete maintenance of melanocyte stem cells (MSCs) with age. Here, we report that irreparable DNA damage, as caused by ionizing radiation, abrogates renewal of MSCs in mice. Surprisingly, the DNA-damage response triggers MSC differentiation into mature melanocytes in the niche, rather than inducing their apoptosis or senescence. The resulting MSC depletion leads to irreversible hair graying. Furthermore, deficiency of Ataxia-telangiectasia mutated (ATM), a central transducer kinase of the DNA-damage response, sensitizes MSCs to ectopic differentiation, demonstrating that the kinase protects MSCs from their premature differentiation by functioning as a "stemness checkpoint" to maintain the stem cell quality and quantity.

Inhibition of ref-1 stimulates the production of reactive oxygen species and induces differentiation in adult cardiac stem cells

Redox effector protein-1 (Ref-1) plays an essential role in DNA repair and redox regulation of several transcription factors. In the present study, we examined the role of Ref-1 in maintaining the redox status and survivability of adult cardiac stem cells challenged with a subtoxic level of H2O2 under inhibition of Ref-1 by RNA interference. Treatment of cardiac stem cells with a low concentration of H2O2 induced Ref-1-mediated survival signaling through phosphorylation of Akt. However, Ref-1 inhibition followed by H2O2 treatment extensively induced the level of intracellular reactive oxygen species (ROS) through activation of the components of NADPH oxidase, like p22( phox ), p47( phox ), and Nox4. Cardiac differentiation markers (Nkx2.5, MEF2C, and GATA4), and cell death by apoptosis were significantly elevated in Ref-1 siRNA followed by H2O2-treated stem cells. Further, inhibition of Ref-1 increased the level of p53 but decreased the phosphorylation of Akt, a molecule involved in survival signaling. Treatment with ROS scavenger N-acetyl-L-cysteine attenuated Ref-1 siRNA-mediated activation of NADPH oxidase and cardiac differentiation. Taken together, these results indicate that Ref-1 plays an important role in maintaining the redox status of cardiac stem cells and protects them from oxidative injury-mediated cell death and differentiation.

Pim-1 kinase antagonizes aspects of myocardial hypertrophy and compensation to pathological pressure overload

Pim-1 kinase exerts potent cardioprotective effects in the myocardium downstream of AKT, but the participation of Pim-1 in cardiac hypertrophy requires investigation. Cardiac-specific expression of Pim-1 (Pim-WT) or the dominant-negative mutant of Pim-1 (Pim-DN) in transgenic mice together with adenoviral-mediated overexpression of these Pim-1 constructs was used to delineate the role of Pim-1 in hypertrophy. Transgenic overexpression of Pim-1 protects mice from pressure-overload-induced hypertrophy relative to wild-type controls as evidenced by improved hemodynamic function, decreased apoptosis, increases in antihypertrophic proteins, smaller myocyte size, and inhibition of hypertrophic signaling after challenge. Similarly, Pim-1 overexpression in neonatal rat cardiomyocyte cultures inhibits hypertrophy induced by endothelin-1. On the cellular level, hearts of Pim-WT mice show enhanced incorporation of BrdU into myocytes and a hypercellular phenotype compared to wild-type controls after hypertrophic challenge. In comparison, transgenic overexpression of Pim-DN leads to dilated cardiomyopathy characterized by increased apoptosis, fibrosis, and severely depressed cardiac function. Furthermore, overexpression of Pim-DN leads to reduced contractility as evidenced by reduced Ca(2+) transient amplitude and decreased percentage of cell shortening in isolated myocytes. These data support a pivotal role for Pim-1 in modulation of hypertrophy by impacting responses on molecular, cellular, and organ levels.

Cardiac stem cells and myocardial disease

Recent data indicate that the heart is a self-renewing organ and contains a pool of progenitor cells (PCs). According to the new paradigm, this resident population of multipotent undifferentiated cells gives rise to myocytes, endothelial cells, smooth muscle cells and fibroblasts. Understanding the function of cardiac PCs is critical for the implementation of these cells in the treatment of the diseased human heart. However, cardiac repair is an extremely complex phenomenon. Efficient myocardial regeneration requires restoration of segmental and focal areas of myocardial scarring, replacement of damaged coronary arteries, arterioles and capillaries, and substitution of hypertrophied poorly contracting myocytes with smaller better functioning parenchymal cells. To achieve these goals, the acquisition of a more profound knowledge of the biology of cardiac PCs cells and their fate following pathologic insults represents an essential need.

Isolation of ckit-positive cardiosphere-forming cells from human atrial biopsy

There is increasing interest in developing cell-based therapies to regenerate functional muscle and blood vessels in infarcted dysfunctional myocardium, using stem cells resident in the adult heart. The objective of our study was to identify an easy and cost-effective method for the isolation and expansion of human adult cardiac-resident stem cells. The cells were isolated from right atrial biopsy samples obtained from patients with ischemic heart disease, who were undergoing coronary artery bypass grafting. Two different isolation methods, enzymatic and nonenzymatic, were employed. The cell yield and cluster formation were not significantly different with either of the techniques used for cell isolation. The nonenzymatic method is recommended because of its simplicity and lower cost compared to the enzymatic method.