Overexpression of insulin-like growth factor-1 in the heart is coupled with myocyte proliferation in transgenic mice

Signalling between microvascular endothelium and cardiomyocytes through neuregulin

Heterocellular communication in the heart is an important mechanism for matching circulatory demands with cardiac structure and function, and neuregulins (Nrgs) play an important role in transducing this signal between the hearts' vasculature and musculature. Here, we review the current knowledge regarding Nrgs, explaining their roles in transducing signals between the heart's microvasculature and cardiomyocytes. We highlight intriguing areas being investigated for developing new, Nrg-mediated strategies to heal the heart in acquired and congenital heart diseases, and note avenues for future research.

An analysis of the global expression of microRNAs in an experimental model of physiological left ventricular hypertrophy

Background

MicroRNAs (miRs) are a class of small non-coding RNAs that regulate gene expression. Studies of transgenic mouse models have indicated that deregulation of a single miR can induce pathological cardiac hypertrophy and cardiac failure. The roles of miRs in the genesis of physiological left ventricular hypertrophy (LVH), however, are not well understood.

Objective

To evaluate the global miR expression in an experimental model of exercise-induced LVH.

Methods

Male Balb/c mice were divided into sedentary (SED) and exercise (EXE) groups. Voluntary exercise was performed on an odometer-monitored metal wheels for 35 days. Various tests were performed after 7 and 35 days of training, including a transthoracic echocardiography, a maximal exercise test, a miR microarray (miRBase v.16) and qRT-PCR analysis.

Results

The ratio between the left ventricular weight and body weight was increased by 7% in the EXE group at day 7 (p<0.01) and by 11% at day 35 of training (p<0.001). After 7 days of training, the microarray identified 35 miRs that were differentially expressed between the two groups: 20 were up-regulated and 15 were down-regulated in the EXE group compared with the SED group (p = 0.01). At day 35 of training, 25 miRs were differentially expressed: 15 were up-regulated and 10 were decreased in the EXE animals compared with the SED animals (p<0.01). The qRT-PCR analysis demonstrated an increase in miR-150 levels after 35 days and a decrease in miR-26b, miR-27a and miR-143 after 7 days of voluntary exercise.

Conclusions

We have identified new miRs that can modulate physiological cardiac hypertrophy, particularly miR-26b, -150, -27a and -143. Our data also indicate that previously established regulatory gene pathways involved in pathological LVH are not changed in physiological LVH.

Recent advances in heart regeneration

Although cardiac stem cells (CSCs) and tissue engineering are very promising for cardiac regenerative medicine, studies with model organisms for heart regeneration will provide alternative therapeutic targets and opportunities. Here, we present a review on heart regeneration, with a particular focus on the most recent work in mouse and zebrafish. We attempt to summarize the recent progresses and bottlenecks of CSCs and tissue engineering for heart regeneration; and emphasize what we have learned from mouse and zebrafish regenerative models on discovering crucial genetic and epigenetic factors for stimulating heart regeneration; and speculate the potential application of these regenerative factors for heart failure. A brief perspective highlights several important and promising research directions in this exciting field.

Cardiac aging and insulin resistance: could insulin/insulin-like growth factor (IGF) signaling be used as a therapeutic target?

Intrinsic cardiac aging is an independent risk factor for cardiovascular disease and is associated with structural and functional changes that impede cardiac responses to stress and to cardio-protective mechanisms. Although systemic insulin resistance and the associated risk factors exacerbate cardiac aging, cardiac-specific insulin resistance without confounding systemic alterations, could prevent cardiac aging. Thus, strategies aimed to reduce insulin/insulin-like growth factor (IGF) signaling in the heart prevent cardiac aging in lower organisms and in mammals but the mechanisms underlying this protection are not fully understood. In this review, we describe the impact of aging on the cardiovascular system and discuss the mounting evidence that reduced insulin/IGF signaling in the heart could alleviate age-associated alterations and preserve cardiac performance.

Stimulating myocardial regeneration with periostin Peptide in large mammals improves function post-myocardial infarction but increases myocardial fibrosis

AIMS

Mammalian myocardium has a finite but limited capacity to regenerate. Experimentally stimulating proliferation of cardiomyocytes with extracellular regeneration factors like periostin enhances cardiac repair in rodents. The aim of this study was to develop a safe method for delivering regeneration factors to the heart and to test the functional and structural effects of periostin peptide treatment in a large animal model of myocardial infarction (MI).

METHODS AND RESULTS

We developed a controlled release system to deliver recombinant periostin peptide into the pericardial space. A single application of this method was performed two days after experimental MI in swine. Animals were randomly assigned to receive either saline or periostin peptide. Experimental groups were compared at baseline, day 2, 1 month and 3 months. Treatment with periostin peptide increased the EF from 31% to 41% and decreased by 22% the infarct size within 12 weeks. Periostin peptide-treated animals had newly formed myocardium strips within the infarct scar, leading to locally improved myocardial function. In addition the capillary density was increased in animals receiving periostin. However, periostin peptide treatment increased myocardial fibrosis in the remote region at one week and 12 weeks post-treatment.

CONCLUSION

Our study shows that myocardial regeneration through targeted peptides is possible. However, in the case of periostin the effects on cardiac fibrosis may limit its clinical application as a viable therapeutic strategy.

Acute myocardial infarction induced functional cardiomyocytes to re-enter the cell cycle

BACKGROUND

Loss of cardiomyocytes after myocardial infarction (MI) causes heart failure. In this study, we investigate whether the in situ cardiomyocytes can re-enter the cell cycle and to what extent cell division of cardiomyocytes occurs after acute MI (AMI) in rats.

METHODS

Sprague Dawley (SD) rats were used in this study; the left anterior descending coronary artery was ligated. At time points (3 days, 1 week, 2 weeks, 3 weeks, and 4 weeks) after the operation, five rats were euthanized, respectively. An additional five sham-operated rats serves as a control group and were euthanized at 3 days post-operation. The expressions of cyclin A2, Ki-67, phospho-histone H3 (H3P), and Aurora B in myocardial tissues were detected by Western blot and immunofluorescence.

RESULTS

The expression levels of cyclin A2 were significantly higher in all groups with AMI except the 4-week group than those found in the sham-operated group (P < 0.01). The percentage of Ki-67-positive nuclei in the border zones was significantly higher than the percentage in the distant normal myocardium (P < 0.01).

CONCLUSIONS

our results demonstrate that cardiomyocytes re-enter the cell cycle after AMI and that cyclin A2 is a reliable marker for the detection of cell cycle activity in cardiomyocytes.

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.

Cardiac-specific over-expression of epidermal growth factor receptor 2 (ErbB2) induces pro-survival pathways and hypertrophic cardiomyopathy in mice

BACKGROUND

Emerging evidence shows that ErbB2 signaling has a critical role in cardiomyocyte physiology, based mainly on findings that blocking ErbB2 for cancer therapy is toxic to cardiac cells. However, consequences of high levels of ErbB2 activity in the heart have not been previously explored.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated consequences of cardiac-restricted over-expression of ErbB2 in two novel lines of transgenic mice. Both lines develop striking concentric cardiac hypertrophy, without heart failure or decreased life span. ErbB2 transgenic mice display electrocardiographic characteristics similar to those found in patients with Hypertrophic Cardiomyopathy, with susceptibility to adrenergic-induced arrhythmias. The hypertrophic hearts, which are 2-3 times larger than those of control littermates, express increased atrial natriuretic peptide and β-myosin heavy chain mRNA, consistent with a hypertrophic phenotype. Cardiomyocytes in these hearts are significantly larger than wild type cardiomyocytes, with enlarged nuclei and distinctive myocardial disarray. Interestingly, the over-expression of ErbB2 induces a concurrent up-regulation of multiple proteins associated with this signaling pathway, including EGFR, ErbB3, ErbB4, PI3K subunits p110 and p85, bcl-2 and multiple protective heat shock proteins. Additionally, ErbB2 up-regulation leads to an anti-apoptotic shift in the ratio of bcl-xS/xL in the heart. Finally, ErbB2 over-expression results in increased activation of the translation machinery involving S6, 4E-BP1 and eIF4E. The dependence of this hypertrophic phenotype on ErbB family signaling is confirmed by reduction in heart mass and cardiomyocyte size, and inactivation of pro-hypertrophic signaling in transgenic animals treated with the ErbB1/2 inhibitor, lapatinib.

CONCLUSIONS/SIGNIFICANCE

These studies are the first to demonstrate that increased ErbB2 over-expression in the heart can activate protective signaling pathways and induce a phenotype consistent with Hypertrophic Cardiomyopathy. Furthermore, our work suggests that in the situation where ErbB2 signaling contributes to cardiac hypertrophy, inhibition of this pathway may reverse this process.

IGF-IR signaling attenuates the age-related decline of diastolic cardiac function

Insulin-like growth factor (IGF-I) signaling has been implicated to play an important role in regulation of cardiac growth, hypertrophy, and contractile function and has been linked to the development of age-related congestive heart failure. Here, we address the question to what extent cardiomyocyte-specific IGF-I signaling is essential for maintenance of the structural and functional integrity of the adult murine heart. To investigate the effects of IGF-I signaling in the adult heart without confounding effects due to IGF-I overexpression or adaptation during embryonic and early postnatal development, we inactivated the IGF-I receptor (IGF-IR) by a 4-hydroxytamoxifen-inducible Cre recombinase in adult cardiac myocytes. Efficient inactivation of the IGF-IR (iCMIGF-IRKO) as assessed by Western analysis and real-time PCR went along with reduced IGF-I-dependent Akt and GSK3β phosphorylation. Functional analysis by conductance manometry and MRI revealed no functional alterations in young adult iCMIGF-IRKO mice (age 3 mo). However, when induced in aging mice (11 mo) diastolic cardiac function was depressed. To address the question whether insulin signaling might compensate for the defective IGF-IR signaling, we inactivated β-cells by streptozotocin. However, the diabetes-associated functional depression was similar in control and iCMIGF-IRKO mice. Similarly, analysis of the cardiac gene expression profile on 44K microarrays did not reveal activation of overt adaptive processes. Endogenous IGF-IR signaling is required for conservation of cardiac function of the aging heart, but not for the integrity of cardiac structure and function of young hearts.

The role of neuregulin/ErbB2/ErbB4 signaling in the heart with special focus on effects on cardiomyocyte proliferation

The signaling complex consisting of the growth factor neuregulin-1 (NRG1) and its tyrosine kinase receptors ErbB2 and ErbB4 has a critical role in cardiac development and homeostasis of the structure and function of the adult heart. Recent research results suggest that targeting this signaling complex may provide a viable strategy for treating heart failure. Clinical trials are currently evaluating the effectiveness and safety of intravenous administration of recombinant NRG1 formulations in heart failure patients. Endogenous as well as administered NRG1 has multiple possible activities in the adult heart, but how these are related is unknown. It has recently been demonstrated that NRG1 administration can stimulate proliferation of cardiomyocytes, which may contribute to repair failing hearts. This review summarizes the current knowledge of how NRG1 and its receptors control cardiac physiology and biology, with special emphasis on its role in cardiomyocyte proliferation during myocardial growth and regeneration.

Insulin-like growth factor 1 alleviates high-fat diet-induced myocardial contractile dysfunction: role of insulin signaling and mitochondrial function

Obesity is often associated with reduced plasma insulin-like growth factor 1 (IGF-1) levels, oxidative stress, mitochondrial damage, and cardiac dysfunction. This study was designed to evaluate the impact of IGF-1 on high-fat diet-induced oxidative, myocardial, geometric, and mitochondrial responses. FVB and cardiomyocyte-specific IGF-1 overexpression transgenic mice were fed a low- (10%) or high-fat (45%) diet to induce obesity. High-fat diet feeding led to glucose intolerance, elevated plasma levels of leptin, interleukin 6, insulin, and triglyceride, as well as reduced circulating IGF-1 levels. Echocardiography revealed reduced fractional shortening, increased end-systolic and end-diastolic diameter, increased wall thickness, and cardiac hypertrophy in high-fat-fed FVB mice. High-fat diet promoted reactive oxygen species generation, apoptosis, protein and mitochondrial damage, reduced ATP content, cardiomyocyte cross-sectional area, contractile and intracellular Ca(2+) dysregulation (including depressed peak shortening and maximal velocity of shortening/relengthening), prolonged duration of relengthening, and dampened intracellular Ca(2+) rise and clearance. Western blot analysis revealed disrupted phosphorylation of insulin receptor and postreceptor signaling molecules insulin receptor substrate 1 (tyrosine/serine phosphorylation), Akt, glycogen synthase kinase 3β, forkhead transcriptional factors, and mammalian target of rapamycin, as well as downregulated expression of mitochondrial proteins peroxisome proliferator-activated receptor-γ coactivator 1α and uncoupling protein 2. Intriguingly, IGF-1 mitigated high-fat-diet feeding-induced alterations in reactive oxygen species, protein and mitochondrial damage, ATP content, apoptosis, myocardial contraction, intracellular Ca(2+) handling, and insulin signaling but not whole body glucose intolerance and cardiac hypertrophy. Exogenous IGF-1 treatment also alleviated high-fat diet-induced cardiac dysfunction. Our data revealed that IGF-1 alleviates high-fat diet-induced cardiac dysfunction despite persistent cardiac remodeling, possibly because of preserved cell survival, mitochondrial function, and insulin signaling.

Regulation of insulin-like growth factor signaling by Yap governs cardiomyocyte proliferation and embryonic heart size

The Hippo signaling pathway regulates growth of the heart and other tissues. Hippo pathway kinases influence the activity of various targets, including the transcriptional coactivator Yap, but the specific role of Yap in heart growth has not been investigated. We show that Yap is necessary and sufficient for embryonic cardiac growth in mice. Deletion of Yap in the embryonic mouse heart impeded cardiomyocyte proliferation, causing myocardial hypoplasia and lethality at embryonic stage 10.5. Conversely, forced expression of a constitutively active form of Yap in the embryonic heart increased cardiomyocyte number and heart size. Yap activated the insulin-like growth factor (IGF) signaling pathway in cardiomyocytes, resulting in inactivation of glycogen synthase kinase 3β, which led to increased abundance of β-catenin, a positive regulator of cardiac growth. Our results point to Yap as a critical downstream effector of the Hippo pathway in the control of cardiomyocyte proliferation and a nexus for coupling the IGF, Wnt, and Hippo signaling pathways with the developmental program for heart growth.

Myocardial AKT: the omnipresent nexus

One of the greatest examples of integrated signal transduction is revealed by examination of effects mediated by AKT kinase in myocardial biology. Positioned at the intersection of multiple afferent and efferent signals, AKT exemplifies a molecular sensing node that coordinates dynamic responses of the cell in literally every aspect of biological responses. The balanced and nuanced nature of homeostatic signaling is particularly essential within the myocardial context, where regulation of survival, energy production, contractility, and response to pathological stress all flow through the nexus of AKT activation or repression. Equally important, the loss of regulated AKT activity is primarily the cause or consequence of pathological conditions leading to remodeling of the heart and eventual decompensation. This review presents an overview compendium of the complex world of myocardial AKT biology gleaned from more than a decade of research. Summarization of the widespread influence that AKT exerts upon myocardial responses leaves no doubt that the participation of AKT in molecular signaling will need to be reckoned with as a seemingly omnipresent regulator of myocardial molecular biological responses.

Hadp1, a newly identified pleckstrin homology domain protein, is required for cardiac contractility in zebrafish

The vertebrate heart is one of the first organs to form, and its early function and morphogenesis are crucial for continued embryonic development. Here we analyze the effects of loss of Heart adaptor protein 1 (Hadp1), which we show is required for normal function and morphogenesis of the embryonic zebrafish heart. Hadp1 is a pleckstrin homology (PH)-domain-containing protein whose expression is enriched in embryonic cardiomyocytes. Knockdown of hadp1 in zebrafish embryos reduced cardiac contractility and altered late myocyte differentiation. By using optical mapping and submaximal levels of hadp1 knockdown, we observed profound effects on Ca²⁺ handling and on action potential duration in the absence of morphological defects, suggesting that Hadp1 plays a major role in the regulation of intracellular Ca²⁺ handling in the heart. Hadp1 interacts with phosphatidylinositol 4-phosphate [PI4P; also known as PtdIns(4)P] derivatives via its PH domain, and its subcellular localization is dependent upon this motif. Pharmacological blockade of the synthesis of PI4P derivatives in vivo phenocopied the loss of hadp1 in zebrafish. Collectively, these results demonstrate that hadp1 is required for normal cardiac function and morphogenesis during embryogenesis, and suggest that hadp1 modulates Ca²⁺ handling in the heart through its interaction with phosphatidylinositols.

Insulin-like growth factor-1 overexpression in cardiomyocytes diminishes ex vivo heart functional recovery after acute ischemia

BACKGROUND

Acute insulin-like growth factor-1 administration has been shown to have beneficial effects in cardiac pathological conditions. The aim of the present study was to assess the structural and ex vivo functional impacts of long-term cardiomyocyte-specific insulin-like growth factor-1 overexpression in hearts of transgenic αMHC-IGF-1 Ea mice.

METHODS

Performance of isolated transgenic αMHC-IGF-1 Ea and littermate wild-type control hearts was compared under baseline conditions and in response to 20-min ischemic insult. Cardiac desmin and laminin expression patterns were determined histologically, and myocardial hydroxyproline was measured to assess collagen content.

RESULTS

Overexpression of insulin-like growth factor-1 did not modify expression patterns of desmin or laminin but was associated with a pronounced increase (∼30%) in cardiac collagen content (from ∼3.7 to 4.8 μg/mg). Baseline myocardial contractile function and coronary flow were unaltered by insulin-like growth factor-1 overexpression. In contrast to prior evidence of acute cardiac protection, insulin-like growth factor-1 overexpression was associated with significant impairment of acute functional response to ischemia-reperfusion. Insulin-like growth factor-1 overexpression did not modify ischemic contracture development, but postischemic diastolic dysfunction was aggravated (51±5 vs. 22±6 mmHg in nontransgenic littermates). Compared with wild-type control, recovery of pressure development and relaxation indices relative to baseline performance were significantly reduced in transgenic αMHC-IGF-1 Ea after 60-min reperfusion (34±7% vs. 62±7% recovery of +dP/dt; 35±11% vs. 57±8% recovery of -dP/dt).

CONCLUSIONS

Chronic insulin-like growth factor-1 overexpression is associated with reduced functional recovery after acute ischemic insult. Collagen deposition is elevated in transgenic αMHC-IGF-1 Ea hearts, but there is no change in expression of the myocardial structural proteins desmin and laminin. These findings suggest that sustained cardiac elevation of insulin-like growth factor-1 may not be beneficial in the setting of an acute ischemic insult.

Structural and molecular reorganization of cardiomyocytes in transposition of the main vessels

Studies of myocardial autopsy specimens from infants (0-12 months) with transposition of the main vessels showed the formation of a complex of compensatory adaptive, degenerative, and destructive changes, manifesting at a tissue level in cardiomyocyte heterogeneity and formation of cardiosclerosis zones. Cardiac myosin synthesis was replaced by synthesis of skeletal myosin, which was detected at the molecular level. Clinically it manifested in the progress of heart failure. Hyperplastic processes (intensive polyploid transformation of the nuclei) play an important role in heart remodeling in patients aged over 6 months. The findings of immunohistochemical and fluorescent studies seem to be prognostically important and provide more accurate data on the pathological processes in the myocardium at the initial stages of heart disease development starting from birth.

Uremic cardiomyopathy and insulin resistance: a critical role for akt?

Uremic cardiomyopathy is a classic complication of chronic renal failure whose cause is unclear and treatment remains disappointing. Insulin resistance is an independent predictor of cardiovascular mortality in chronic renal failure. Underlying insulin resistance are defects in insulin signaling through the protein kinase, Akt. Akt acts as a nodal point in the control of both the metabolic and pleiotropic effects of insulin. Imbalance among these effects leads to cardiac hypertrophy, fibrosis, and apoptosis; less angiogenesis; metabolic remodeling; and altered calcium cycling, all key features of uremic cardiomyopathy. Here we consider the role of Akt in the development of uremic cardiomyopathy, drawing parallels from models of hypertrophic cardiac disease.

Cardiac-specific IGF-1 receptor transgenic expression protects against cardiac fibrosis and diastolic dysfunction in a mouse model of diabetic cardiomyopathy

OBJECTIVE

Compelling epidemiological and clinical evidence has identified a specific cardiomyopathy in diabetes, characterized by early diastolic dysfunction and adverse structural remodeling. Activation of the insulin-like growth factor 1 (IGF-1) receptor (IGF-1R) promotes physiological cardiac growth and enhances contractile function. The aim of the present study was to examine whether cardiac-specific overexpression of IGF-1R prevents diabetes-induced myocardial remodeling and dysfunction associated with a murine model of diabetes.

RESEARCH DESIGN AND METHODS

Type 1 diabetes was induced in 7-week-old male IGF-1R transgenic mice using streptozotocin and followed for 8 weeks. Diastolic and systolic function was assessed using Doppler and M-mode echocardiography, respectively, in addition to cardiac catheterization. Cardiac fibrosis and cardiomyocyte width, heart weight index, gene expression, Akt activity, and IGF-1R protein content were also assessed.

RESULTS

Nontransgenic (Ntg) diabetic mice had reduced initial (E)-to-second (A) blood flow velocity ratio (E:A ratio) and prolonged deceleration times on Doppler echocardiography compared with nondiabetic counterparts, indicative markers of diastolic dysfunction. Diabetes also increased cardiomyocyte width, collagen deposition, and prohypertrophic and profibrotic gene expression compared with Ntg nondiabetic littermates. Overexpression of the IGF-1R transgene markedly reduced collagen deposition, accompanied by a reduction in the incidence of diastolic dysfunction. Akt phosphorylation was elevated approximately 15-fold in IGF-1R nondiabetic mice compared with Ntg, and this was maintained in a setting of diabetes.

CONCLUSIONS

The current study suggests that cardiac overexpression of IGF-1R prevented diabetes-induced cardiac fibrosis and diastolic dysfunction. Targeting IGF-1R-Akt signaling may represent a therapeutic target for the treatment of diabetic cardiac disease.

Molecular distinction between physiological and pathological cardiac hypertrophy: experimental findings and therapeutic strategies

Cardiac hypertrophy can be defined as an increase in heart mass. Pathological cardiac hypertrophy (heart growth that occurs in settings of disease, e.g. hypertension) is a key risk factor for heart failure. Pathological hypertrophy is associated with increased interstitial fibrosis, cell death and cardiac dysfunction. In contrast, physiological cardiac hypertrophy (heart growth that occurs in response to chronic exercise training, i.e. the 'athlete's heart') is reversible and is characterized by normal cardiac morphology (i.e. no fibrosis or apoptosis) and normal or enhanced cardiac function. Given that there are clear functional, structural, metabolic and molecular differences between pathological and physiological hypertrophy, a key question in cardiovascular medicine is whether mechanisms responsible for enhancing function of the athlete's heart can be exploited to benefit patients with pathological hypertrophy and heart failure. This review summarizes key experimental findings that have contributed to our understanding of pathological and physiological heart growth. In particular, we focus on signaling pathways that play a causal role in the development of pathological and physiological hypertrophy. We discuss molecular mechanisms associated with features of cardiac hypertrophy, including protein synthesis, sarcomeric organization, fibrosis, cell death and energy metabolism and provide a summary of profiling studies that have examined genes, microRNAs and proteins that are differentially expressed in models of pathological and physiological hypertrophy. How gender and sex hormones affect cardiac hypertrophy is also discussed. Finally, we explore how knowledge of molecular mechanisms underlying pathological and physiological hypertrophy may influence therapeutic strategies for the treatment of cardiovascular disease and heart failure.

G-CSF promotes the proliferation of developing cardiomyocytes in vivo and in derivation from ESCs and iPSCs

During a screen for humoral factors that promote cardiomyocyte differentiation from embryonic stem cells (ESCs), we found marked elevation of granulocyte colony-stimulating factor receptor (G-CSFR) mRNA in developing cardiomyocytes. We confirmed that both G-CSFR and G-CSF were specifically expressed in embryonic mouse heart at the midgestational stage, and expression levels were maintained throughout embryogenesis. Intrauterine G-CSF administration induced embryonic cardiomyocyte proliferation and caused hyperplasia. In contrast, approximately 50% of csf3r(-/-) mice died during late embryogenesis because of the thinning of atrioventricular walls. ESC-derived developing cardiomyocytes also strongly expressed G-CSFR. When extrinsic G-CSF was administered to the ESC- and human iPSC-derived cardiomyocytes, it markedly augmented their proliferation. Moreover, G-CSF-neutralizing antibody inhibited their proliferation. These findings indicated that G-CSF is critically involved in cardiomyocyte proliferation during development, and may be used to boost the yield of cardiomyocytes from ESCs for their potential application to regenerative medicine.

Activation of proteasome by insulin-like growth factor-I may enhance clearance of oxidized proteins in the brain

The insulin-like growth factor type 1 (IGF-I) plays an important role in neuronal physiology. Reduced IGF-I levels are observed during aging and this decrease may be important to age-related changes in the brain. We studied the effects of IGF-I on total protein oxidation in brain tissues and in cell cultures. Our results indicate that in frontal cortex the level of oxidized proteins is significantly reduced in transgenic mice designed to overproduce IGF-I compared with wild-type animals. The frontal cortex of IGF-I-overproducing mice exhibited high chymotrypsin-like activity of the 20S and 26S proteasomes. The proteasome can also be activated in response to IGF-I in cell cultures. Kinetic studies revealed peak activation of the proteasome within 15 min following IGF-I stimulation. The effects of IGF-I on proteasome were not observed in R(-) cells lacking the IGF-I receptor. Experiments using specific kinase inhibitors suggested that activation of proteasome by IGF-I involves phosphatidyl inositol 3-kinase and mammalian target of rapamycin signaling. IGF-I also attenuated the increase in protein carbonyl content induced by proteasome inhibition. Thus, appropriate levels of IGF-I may be important for the elimination of oxidized proteins in the brain in a process mediated by activation of the proteasome.

The insulin-like growth factor (IGF)-I E-peptides are required for isoform-specific gene expression and muscle hypertrophy after local IGF-I production

Insulin-like growth factor I (IGF-I) coordinates proliferation and differentiation in a wide variety of cell types. The igf1 gene not only produces IGF-I, but also generates multiple carboxy-terminal extensions, the E-peptides, through alternative splicing leading to different isoforms. It is not known if the IGF-I isoforms share a common pathway for their actions, or if there are specific actions of each protein. Viral administration of murine IGF-IA, IGF-IB, and mature IGF, which lacked an E-peptide extension, was utilized to identify IGF-I isoform-specific responsive genes in muscles of young growing mice. Microarray analysis revealed responses that were driven by increased IGF-I regardless of the presence of E-peptide, such as Bcl-XL. In contrast, distinct expression patterns were observed after viral delivery of IGF-IA or IGF-IB, which included matrix metalloproteinase 13 (MMP13). Expression of Bcl-XL was prevented when viral administration of the IGF-I isoforms was performed into muscles of MKR mice, which lack functional IGF-I receptors on the muscle fibers. However, MMP13 expression persisted under the same conditions after viral injection of IGF-IB. At 4 mo after viral delivery, expression of IGF-IA or IGF-IB promoted muscle hypertrophy, but viral delivery of mature IGF-I failed to increase muscle mass. These studies provide evidence that local production of IGF-I requires the E-peptides to drive hypertrophy in growing muscle and that both common and unique pathways exist for the IGF-I isoforms to promote biological effects.

Cardiac progenitor cell cycling stimulated by pim-1 kinase

RATIONALE

Cardioprotective effects of Pim-1 kinase have been previously reported but the underlying mechanistic basis may involve a combination of cellular and molecular mechanisms that remain unresolved. The elucidation of the mechanistic basis for Pim-1 mediated cardioprotection provides important insights for designing therapeutic interventional strategies to treat heart disease.

OBJECTIVE

Effects of cardiac-specific Pim-1 kinase expression on the cardiac progenitor cell (CPC) population were examined to determine whether Pim-1 mediates beneficial effects through augmenting CPC activity.

METHODS AND RESULTS

Transgenic mice created with cardiac-specific Pim-1 overexpression (Pim-wt) exhibit enhanced Pim-1 expression in both cardiomyocytes and CPCs, both of which show increased proliferative activity assessed using 5-bromodeoxyuridine (BrdU), Ki-67, and c-Myc relative to nontransgenic controls. However, the total number of CPCs was not increased in the Pim-wt hearts during normal postnatal growth or after infarction challenge. These results suggest that Pim-1 overexpression leads to asymmetric division resulting in maintenance of the CPC population. Localization and quantitation of cell fate determinants Numb and alpha-adaptin by confocal microscopy were used to assess frequency of asymmetric division in the CPC population. Polarization of Numb in mitotic phospho-histone positive cells demonstrates asymmetric division in 65% of the CPC population in hearts of Pim-wt mice versus 26% in nontransgenic hearts after infarction challenge. Similarly, Pim-wt hearts had fewer cells with uniform alpha-adaptin staining indicative of symmetrically dividing CPCs, with 36% of the CPCs versus 73% in nontransgenic sections.

CONCLUSIONS

These findings define a mechanistic basis for enhanced myocardial regeneration in transgenic mice overexpressing Pim-1 kinase.

Local IGF-1 isoform protects cardiomyocytes from hypertrophic and oxidative stresses via SirT1 activity

Oxidative and hypertrophic stresses contribute to the pathogenesis of heart failure. Insulin-like growth factor-1 (IGF-1) is a peptide hormone with a complex post-transcriptional regulation, generating distinct isoforms. Locally acting IGF-1 isoform (mIGF-1) helps the heart to recover from toxic injury and from infarct. In the murine heart, moderate overexpression of the NAD⁺-dependent deacetylase SirT1 was reported to mitigate oxidative stress. SirT1 is known to promote lifespan extension and to protect from metabolic challenges. Circulating IGF-1 and SirT1 play antagonizing biological roles and share molecular targets in the heart, in turn affecting cardiomyocyte physiology. However, how different IGF-1 isoforms may impact SirT1 and affect cardiomyocyte function is unknown. Here we show that locally acting mIGF-1 increases SirT1 expression/activity, whereas circulating IGF-1 isoform does not affect it, in cultured HL-1 and neonatal cardiomyocytes. mIGF-1-induced SirT1 activity exerts protection against angiotensin II (Ang II)-triggered hypertrophy and against paraquat (PQ) and Ang II-induced oxidative stress. Conversely, circulating IGF-1 triggered itself oxidative stress and cardiomyocyte hypertrophy. Interestingly, potent cardio-protective genes (adiponectin, UCP-1 and MT-2) were increased specifically in mIGF-1-overexpressing cardiomyocytes, in a SirT1-dependent fashion. Thus, mIGF-1 protects cardiomyocytes from oxidative and hypertrophic stresses via SirT1 activity, and may represent a promising cardiac therapeutic.

IGF-IR-dependent expression of Survivin is required for T-antigen-mediated protection from apoptosis and proliferation of neural progenitors

The Insulin-like Growth Factor-1 Receptor (IGF-IR) and the human polyomavirus JCV protein, T-Antigen cooperate in the transformation of neuronal precursors in the cerebellum, which may be a contributing factor in the development of brain tumors. Since it is not clear why T-Antigen requires IGF-IR for transformation, we investigated this process in neural progenitors from IGF-IR knockout embryos (ko-IGF-IR) and from their wild type non-transgenic littermates (wt-IGF-IR). In contrast to wt-IGF-IR, the brain and dorsal root ganglia of ko-IGF-IR embryos showed low levels of the anti-apoptotic protein Survivin, accompanied by elevated numbers of apoptotic neurons and an earlier differentiation phenotype. In wt-IGF-IR neural progenitors in vitro, induction of T-Antigen expression tripled the expression of Survivin, and accelerated cell proliferation. In ko-IGF-IR progenitors induction of T-Antigen failed to increase Survivin, resulting in massive apoptosis. Importantly, ectopic expression of Survivin protected ko-IGF-IR progenitor cells from apoptosis and siRNA inhibition of Survivin activated apoptosis in wt-IGF-IR progenitors expressing T-Antigen. Our results indicate that reactivation of the anti-apoptotic Survivin may be a critical step in JCV T-Antigen induced transformation, which in neural progenitors requires IGF-IR.

The role of liver-derived insulin-like growth factor-I

IGF-I is expressed in virtually every tissue of the body, but with much higher expression in the liver than in any other tissue. Studies using mice with liver-specific IGF-I knockout have demonstrated that liver-derived IGF-I, constituting a major part of circulating IGF-I, is an important endocrine factor involved in a variety of physiological and pathological processes. Detailed studies comparing the impact of liver-derived IGF-I and local bone-derived IGF-I demonstrate that both sources of IGF-I can stimulate longitudinal bone growth. We propose here that liver-derived circulating IGF-I and local bone-derived IGF-I to some extent have overlapping growth-promoting effects and might have the capacity to replace each other (= redundancy) in the maintenance of normal longitudinal bone growth. Importantly, and in contrast to the regulation of longitudinal bone growth, locally derived IGF-I cannot replace (= lack of redundancy) liver-derived IGF-I for the regulation of a large number of other parameters including GH secretion, cortical bone mass, kidney size, prostate size, peripheral vascular resistance, spatial memory, sodium retention, insulin sensitivity, liver size, sexually dimorphic liver functions, and progression of some tumors. It is clear that a major role of liver-derived IGF-I is to regulate GH secretion and that some, but not all, of the phenotypes in the liver-specific IGF-I knockout mice are indirect, mediated via the elevated GH levels. All of the described multiple endocrine effects of liver-derived IGF-I should be considered in the development of possible novel treatment strategies aimed at increasing or reducing endocrine IGF-I activity.

Cardiac-specific overexpression of insulin-like growth factor I (IGF-1) rescues lipopolysaccharide-induced cardiac dysfunction and activation of stress signaling in murine cardiomyocytes

Lipopolysaccharide (LPS), a component of the outer membrane of Gram-negative bacteria, plays a key role in cardiac dysfunction in sepsis. Low circulating levels of insulin-like growth factor 1 (IGF-1) are found in sepsis, although the influence of IGF-1 on septic cardiac defect is unknown. This study was designed to examine the impact of IGF-1 on LPS-induced cardiac contractile and intracellular Ca2+ dysfunction, activation of stress signal and endoplasmic reticulum (ER) stress. Mechanical and intracellular Ca2+ properties were examined in cardiomyocytes from Fast Violet B and cardiac-specific IGF-1 overexpression mice treated with or without LPS (4 mg kg(-1), 6 h). Reactive oxygen species (ROS), protein carbonyl formation and apoptosis were measured. Activation of mitogen-activated protein kinase pathways (p38, c-jun N-terminal kinase [JNK] and extracellular signal-related kinase [ERK]), ER stress and apoptotic markers were evaluated using Western blot analysis. Our results revealed decreased peak shortening and maximal velocity of shortening/relengthening and prolonged duration of relengthening in LPS-treated Fast Violet B cardiomyocytes associated with reduced intracellular Ca2+ decay. Accumulation of ROS protein carbonyl and apoptosis were elevated after LPS treatment. Western blot analysis revealed activated p38 and JNK, up-regulated Bax, and the ER stress markers GRP78 and Gadd153 in LPS-treated mouse hearts without any change in ERK and Bcl-2. Total protein expression of p38, JNK, and ERK was unaffected by either LPS or IGF-1. Interestingly, these LPS-induced changes in mechanical and intracellular Ca2+ properties, ROS, protein carbonyl, apoptosis, stress signal activation, and ER stress markers were effectively ablated by IGF-1. In vitro LPS exposure (1 microg mL(-1)) produced cardiomyocyte mechanical dysfunction reminiscent of the in vivo setting, which was alleviated by exogenous IGF-1 (50 nM). These data collectively suggested a beneficial of IGF-1 in the management of cardiac dysfunction under sepsis.

eNOS activation by physical forces: from short-term regulation of contraction to chronic remodeling of cardiovascular tissues

Nitric oxide production in response to flow-dependent shear forces applied on the surface of endothelial cells is a fundamental mechanism of regulation of vascular tone, peripheral resistance, and tissue perfusion. This implicates the concerted action of multiple upstream "mechanosensing" molecules reversibly assembled in signalosomes recruiting endothelial nitric oxide synthase (eNOS) in specific subcellular locales, e.g., plasmalemmal caveolae. Subsequent short- and long-term increases in activity and expression of eNOS translate this mechanical stimulus into enhanced NO production and bioactivity through a complex transcriptional and posttranslational regulation of the enzyme, including by shear-stress responsive transcription factors, oxidant stress-dependent regulation of transcript stability, eNOS regulatory phosphorylations, and protein-protein interactions. Notably, eNOS expressed in cardiac myocytes is amenable to a similar regulation in response to stretching of cardiac muscle cells and in part mediates the length-dependent increase in cardiac contraction force. In addition to short-term regulation of contractile tone, eNOS mediates key aspects of cardiac and vascular remodeling, e.g., by orchestrating the mobilization, recruitment, migration, and differentiation of cardiac and vascular progenitor cells, in part by regulating the stabilization and transcriptional activity of hypoxia inducible factor in normoxia and hypoxia. The continuum of the influence of eNOS in cardiovascular biology explains its growing implication in mechanosensitive aspects of integrated physiology, such as the control of blood pressure variability or the modulation of cardiac remodeling in situations of hemodynamic overload.

A heart full of stem cells: the spectrum of myocardial progenitor cells in the postnatal heart

Influencing cellular regeneration processes in the heart has been a long-standing goal in cardiovascular medicine. To some extent, this has been successful in terms of vascular regeneration as well as intercellular connective tissue remodeling processes. Several components of today's routine heart failure medication influence endothelial progenitor cell behavior and support collateral vessel growth in the heart, or have been shown to prevent or reverse fibrosis processes. Cardiomyocyte regeneration, however, has so far escaped therapeutic manipulation strategies. Delivery of exogenous cells of bone marrow origin to the human myocardium may improve heart function, but is not associated with relevant neomyogenesis. However, accumulating evidence indicates that the myocardium contains resident cardiac progenitor cells (CPC) that may be therapeutically useful. This notion indeed represents a paradigm shift but is still controversial. The purpose of this review is to summarize the rapidly expanding current knowledge on CPC, and to assess whether it may be translated into solid therapeutic concepts.

Expression of a transgene encoding mutant p193/CUL7 preserves cardiac function and limits infarct expansion after myocardial infarction

BACKGROUND

Transgenic mice expressing the dominant interfering p193 protein in cardiomyocytes (MHC-1152stop mice) exhibit an induction of cell cycle activity and altered remodelling after experimental myocardial infarction (MI).

OBJECTIVE

To determine whether the altered remodelling results in improved cardiac function in the MHC-1152stop mice after MI, as compared with non-transgenic mice.

METHODS

MHC-1152stop mice and non-transgenic littermates were subjected to experimental MI via permanent occlusion of the coronary artery. Infarct size was determined at 24 h and at 4 weeks after MI, and left ventricular pressure-volume measurements were performed at 4 weeks after MI in infarcted and sham-operated animals.

RESULTS

Infarct size in MHC-1152stop mice and non-transgenic littermates was not statistically different at 24 h after MI, as measured by tetrazolium staining. Morphometric analysis showed that infarct scar expansion at 4 weeks after MI was reduced by 10% in the MHC-1152stop mice (p<0.05). No differences in cardiac function were detected between sham-operated MHC-1152stop mice and their non-transgenic littermates. However, at 4 weeks after MI, the ventricular isovolumic relaxation time constant (tau) was decreased by 19% (p<0.05), and the slope of the dP/dt(max)-EDV relationship was increased 99% (p<0.05), in infarcted MHC-1152stop mice as compared with infarcted non-transgenic littermates.

CONCLUSION

Expression of the dominant interfering p193 transgene results in a decrease in infarct scar expansion and preservation of myocardial function at 4 weeks after MI. Antagonism of p193 activity may represent an important strategy for the treatment of MI.

Cardioprotection by vanadium compounds targeting Akt-mediated signaling

Treatment with inorganic and organic compounds of vanadium has been shown to exert a wide range of cardioprotective effects in myocardial ischemia/reperfusion-induced injury, myocardial hypertrophy, hypertension, and vascular diseases. Furthermore, administration of vanadium compounds improves cardiac performance and smooth muscle cell contractility and modulates blood pressure in various models of hypertension. Like other vanadium compounds, we documented bis(1-oxy-2-pyridinethiolato) oxovanadium (IV) [VO(OPT)] as a potent cardioprotective agent to elicit cardiac functional recovery in myocardial infarction and pressure overload-induced hypertrophy. Vanadium compounds activate Akt signaling through inhibition of protein tyrosine phosphatases, thereby eliciting cardioprotection in myocardial ischemia/reperfusion-induced injury and myocardial hypertrophy. Vanadium compounds also promote cardiac functional recovery by stimulation of glucose transport in diabetic heart. We here discuss the current understanding of mechanisms underlying vanadium compound-induced cardioprotection and propose a novel therapeutic strategy targeting for Akt signaling to rescue cardiomyocytes from heart failure.

IGF-1 expression in infarcted myocardium and MGF E peptide actions in rat cardiomyocytes in vitro

Insulinlike growth factor-1 (IGF-1) expression is implicated in myocardial pathophysiology, and two IGF-1 mRNA splice variants have been detected in rodents, IGF-1Ea and mechano-growth factor (MGF). We investigated the expression pattern of IGF-1 gene transcripts in rat myocardium from 1 h up to 8 wks after myocardial infarction induced by left anterior descending coronary artery ligation. In addition, we characterized IGF-1 and MGF E peptide action and their respective signaling in H9C2 myocardial-like cells in vitro. IGF-1Ea and MGF expression were significantly increased, both at transcriptional and translational levels, during the late postinfarction period (4 and 8 wks) in infarcted rat myocardium. Measurements of serum IGF-1 levels in infarcted rats were initially decreased (24 h up to 1 wk) but remained unaltered throughout the late experimental phase (4 to 8 wks) compared with sham-operated rats. Furthermore, specific anti-IGF-1R neutralizing antibody failed to block the synthetic MGF E peptide action, whereas it completely blocked IGF-1 action on the proliferation of H9C2 cells. Moreover, this synthetic MGF E peptide did not activate Akt phosphorylation, whereas it activated ERK1/2 in H9C2 rat myocardial cells. These data support the role of IGF-1 expression in the myocardial repair process and suggest that synthetic MGF E peptide actions may be mediated via an IGF-1R independent pathway in rat myocardial cells, as suggested by our in vitro experiments.

Effects of intrafetal IGF-I on growth of cardiac myocytes in late-gestation fetal sheep

Intrafetal insulin-like growth factor (IGF)-I promotes cardiac hypertrophy in the late-gestation fetal sheep; whether these effects are sustained is unknown. IGF-I was infused for 4 days at 80 microg/h from 121 to 125 days of gestation, and its effects at 128 days, 3 days after the infusion stopped, were determined by comparison with untreated fetal sheep. After IGF-I treatment, fetal weights were similar to those in control fetuses but kidney weights were bigger (P < 0.05), as were spleen weights of male fetuses (P < 0.05). Cardiac myocytes were larger in female than male fetal sheep (P < 0.001). IGF-I increased male (P < 0.001) but not female myocyte volumes. IGF-I did not alter the proportions of uni- or binucleated right or left ventricular myocytes. Female fetal sheep had a greater proportion of binucleated cardiac myocytes than males (P < 0.05). IGF-I-treated fetuses had a slightly greater proportion of right ventricular nuclei in cell cycle phase G(2)/M and a reduced proportion of G(0)/G(1) phase nuclei (P < 0.1). Therefore, evidence for IGF-I-stimulated cardiac cell hyperplasia in fetal sheep in late gestation was limited. In conclusion, the greater sizes and larger proportion of binucleated cardiac myocytes in female fetal sheep suggest that myocyte maturation may occur earlier in females than in males. This may explain in part the male sex-specific responsiveness of cardiac hypertrophy to IGF-I in late gestation. If IGF-I-stimulated cardiomyocyte growth is accompanied by maturation of contractile function, IGF-I may be a potential therapeutic agent for maintaining cardiac output in preterm males.

Genetic manipulation of periostin expression in the heart does not affect myocyte content, cell cycle activity, or cardiac repair

Following a pathological insult, the adult mammalian heart undergoes hypertrophic growth and remodeling of the extracellular matrix. Although a small subpopulation of cardiomyocytes can reenter the cell cycle following cardiac injury, the myocardium is largely thought to be incapable of significant regeneration. Periostin, an extracellular matrix protein, has recently been proposed to induce reentry of differentiated cardiomyocytes back into the cell cycle and promote meaningful repair following myocardial infarction. Here, we show that although periostin is induced in the heart following injury, it does not stimulate DNA synthesis, mitosis, or cytokinesis of cardiomyocytes in vitro or in vivo. Mice lacking the gene encoding periostin and mice with inducible overexpression of full-length periostin were analyzed at baseline and after myocardial infarction. There was no difference in heart size or a change in cardiomyocyte number in either periostin transgenic or gene-targeted mice at baseline. Quantification of proliferating myocytes in the periinfarct area showed no difference between periostin-overexpressing and -null mice compared with strain-matched controls. In support of these observations, neither overexpression of periostin in cell culture, via an adenoviral vector, nor stimulation with recombinant protein induced DNA synthesis, mitosis, or cytokinesis. Periostin is a regulator of cardiac remodeling and hypertrophy and may be a reasonable pharmacological target to mitigate heart failure, but manipulation of this protein appears to have no obvious effect on myocardial regeneration.

Effects of physical exercise on myocardial telomere-regulating proteins, survival pathways, and apoptosis

OBJECTIVES

The purpose of this study was to study the underlying molecular mechanisms of the protective cardiac effects of physical exercise.

BACKGROUND

Telomere-regulating proteins affect cellular senescence, survival, and regeneration.

METHODS

C57/Bl6 wild-type, endothelial nitric oxide synthase (eNOS)-deficient and telomerase reverse transcriptase (TERT)-deficient mice were randomized to voluntary running or no running wheel conditions (n = 8 to 12 per group).

RESULTS

Short-term running (21 days) up-regulated cardiac telomerase activity to >2-fold of sedentary controls, increased protein expression of TERT and telomere repeat binding factor (TRF) 2, and reduced expression of the proapoptotic mediators cell-cycle-checkpoint kinase 2 (Chk2), p53, and p16. Myocardial and leukocyte telomere length did not differ between 3-week- and 6-month-old sedentary or running mice, but telomerase activity, TRF2 and TERT expression were persistently increased after 6 months and the expression of Chk2, p53, and p16 remained down-regulated. The exercise-induced changes were absent in both TERT(-/-) and eNOS(-/-) mice. Running increased cardiac expression of insulin-like growth factor (IGF)-1. Treatment with IGF-1 up-regulated myocardial telomerase activity >14-fold and increased the expression of phosphorylated Akt protein kinase and phosphorylated eNOS. To test the physiologic relevance of these exercise-mediated prosurvival pathways, apoptotic cardiomyopathy was induced by treatment with doxorubicin. Up-regulation of telomere-stabilizing proteins by physical exercise in mice reduced doxorubicin-induced p53 expression and potently prevented cardiomyocyte apoptosis in wild-type, but not in TERT(-/-) mice.

CONCLUSIONS

Long- and short-term voluntary physical exercise up-regulates cardiac telomere-stabilizing proteins and thereby induces antisenescent and protective effects, for example, to prevent doxorubicin-induced cardiomyopathy. These beneficial cardiac effects are mediated by TERT, eNOS, and IGF-1.

Stemming heart failure with cardiac- or reprogrammed-stem cells

Despite extensive efforts to control myocyte growth by genetic targeting of the cell cycle machinery and small molecules for cardiac repair, adult myocytes themselves appeared to divide a limited number of times in response to a variety of cardiac muscle stresses. Rare tissue-resident stem cells are thought to exist in many adult organs that are capable of self-renewal and differentiation and possess a range of actions that are potentially therapeutic. Recent studies suggest that a population of cardiac stem cells (CSCs) is maintained after cardiac development in the adult heart in mammals including human beings; however, homeostatic cardiomyocyte replacement might be stem cell-dependent, and functional myocardial regeneration after cardiac muscle damage is not yet considered as sufficient to fully maintain or reconstitute the cardiovascular system and function. Although it is clear that adult CSCs have limitations in their capabilities to proliferate extensively and differentiate in response to injury in vivo for replenishing mature car-diomyocytes and potentially function as resident stem cells. Transplantation of CSCs expanded ex vivo seems to require an integrated strategy of cell growth-enhancing factor(s) and tissue engineering technologies to support the donor cell survival and subsequent proliferation and differentiation in the host microenvironment. There has been substantial interest regarding the evidence that mammalian fibroblasts can be genetically reprogrammed to induced pluripotent stem (iPS) cells, which closely resemble embryonic stem (ES) cell properties capable of differentiating into functional cardiomyocytes, and these cells may provide an alternative cell source for generating patient-specific CSCs for therapeutic applications.

DUSP6 (MKP3) null mice show enhanced ERK1/2 phosphorylation at baseline and increased myocyte proliferation in the heart affecting disease susceptibility

The strength and duration of mitogen-activated protein kinase signaling is regulated through phosphorylation and dephosphorylation by dedicated dual-specificity kinases and phosphatases, respectively. Here we investigated the physiological role that extracellular signal-regulated kinases 1/2 (ERK1/2) dephosphorylation plays in vivo through targeted disruption of the gene encoding dual-specificity phosphatase 6 (Dusp6) in the mouse. Dusp6(-/-) mice, which were viable, fertile, and otherwise overtly normal, showed an increase in basal ERK1/2 phosphorylation in the heart, spleen, kidney, brain, and fibroblasts, but no change in ERK5, p38, or c-Jun N-terminal kinases activation. However, loss of Dusp6 did not increase or prolong ERK1/2 activation after stimulation, suggesting that its function is more dedicated to basal ERK1/2 signaling tone. In-depth analysis of the physiological effect associated with increased baseline ERK1/2 signaling was performed in cultured mouse embryonic fibroblasts (MEFs) and the heart. Interestingly, mice lacking Dusp6 had larger hearts at every age examined, which was associated with greater rates of myocyte proliferation during embryonic development and in the early postnatal period, resulting in cardiac hypercellularity. This increase in myocyte content in the heart was protective against decompensation and hypertrophic cardiomyopathy following long term pressure overload and myocardial infarction injury in adult mice. Dusp6(-/-) MEFs also showed reduced apoptosis rates compared with wild-type MEFs. These results demonstrate that ERK1/2 signaling is physiologically restrained by DUSP6 in coordinating cellular development and survival characteristics, directly impacting disease-responsiveness in adulthood.

Early origins of cardiac hypertrophy: does cardiomyocyte attrition programme for pathological 'catch-up' growth of the heart?

1. Epidemiological and experimental evidence suggests that adult development of cardiovascular disease is influenced by events of prenatal and early postnatal life. Cardiac hypertrophy is recognized as an important predictor of cardiovascular morbidity and mortality, but the developmental origins of this condition are not well understood. 2. In the heart, a switch from hyperplastic to hypertrophic cellular growth occurs during late prenatal or early postnatal life. Postnatal growth of the heart is almost entirely reliant on hypertrophy of individual cardiomyocytes, and damage to heart muscle in adulthood is typically not reparable by cell replacement. Therefore, a reduced number of cardiomyocytes may render the heart more vulnerable in situations where an increased workload is required. 3. A number of different animal models have been used to study fetal programming of adult diseases, including nutritional, hypoxic, maternal/neonatal endocrine stress and genetic models. Although studies investigating the cellular basis of myocardial disease in growth-restricted models are limited, a reduction in cardiomyocyte number through either reduced cellular proliferation or increased apoptosis appears to be a central feature. 4. The mechanisms responsible for the programming of adult cardiovascular disease are poorly understood. We hypothesize that cardiac hypertrophy can have a developmental origin in excess cardiomyocyte attrition during a critical perinatal growth window. Findings that have directly assessed the impact of fetal growth restriction on the myocardium are considered and cellular and molecular mechanisms involved in the potential pathological 'catch-up' growth of the heart during later maturation are identified.