Reversal of cardiac hypertrophy in transgenic disease models by calcineurin inhibition

TongGuanWan Alleviates Doxorubicin- and Isoproterenol-Induced Cardiac Hypertrophy and Fibrosis by Modulating Apoptotic and Fibrotic Pathways

Heart failure, a major public health issue, often stems from prolonged stress or damage to the heart muscle, leading to cardiac hypertrophy. This can progress to heart failure and other cardiovascular problems. Doxorubicin (DOX), a common chemotherapy drug, and isoproterenol (ISO), a β-adrenergic agonist, both induce cardiac hypertrophy through different mechanisms. This study investigates TongGuanWan (TGW,), a traditional herbal remedy, for its effects on cardiac hypertrophy and fibrosis in DOX-induced H9c2 cells and ISO-induced mouse models. TGW was found to counteract DOX-induced increases in H9c2 cell surface area (n = 8, p < 0.01) and improve biomarkers like ANP (n = 3, p < 0.01)) and BNP (n = 3, p < 0.01). It inhibited the MAPK pathway (n = 4, p < 0.01) and GATA-4/calcineurin/NFAT-3 signaling, reduced inflammation by decreasing NF-κB p65 translocation, and enhanced apoptosis-related factors such as caspase-3 (n = 3, p < 0.01), caspase-9 (n = 3, p < 0.01), Bax (n = 3, p < 0.01), and Bcl-2 (n = 3, p < 0.01). Flow cytometry showed TGW reduced apoptotic cell populations. In vivo, TGW reduced heart (n = 8~10, p < 0.01), and left ventricle weights (n = 6~7), cardiac hypertrophy markers (n = 3, p < 0.01), and perivascular fibrosis in ISO-induced mice, with Western blot analysis confirming decreased levels of fibrosis-related factors like fibronectin, α-SMA (n = 3, p < 0.05), and collagen type I (n = 3, p < 0.05). These findings suggest TGW has potential as a therapeutic option for cardiac hypertrophy and fibrosis.

Cardiovascular toxicities by calcineurin inhibitors: Cellular mechanisms behind clinical manifestations

Calcineurin inhibitors (CNI), including cyclosporine A (CsA) and tacrolimus (TAC), are cornerstones of immunosuppressive therapy in solid organ transplant recipients. While extensively recognized for their capacity to induce nephrotoxicity, hypertension, and dyslipidemia, emerging reports suggest potential direct cardiovascular toxicities associated with CNI. Evidence from both in vitro and in vivo studies has demonstrated direct cardiotoxic impact of CNI, manifesting itself as induction of cardiomyocyte apoptosis, enhanced oxidative stress, inflammatory cell infiltration, and cardiac fibrosis. CNI enhances cellular apoptosis through CaSR via activation of the p38 MAPK pathway and deactivation of the ERK pathway, and enhancement of miR-377 axis. Although CNI could attenuate cardiac hypertrophy in certain animal models, CNI concurrently impaired systolic function, enhanced cardiac fibrosis, and increased the risk of heart failure. Evidence from in vivo studies demonstrated that CNI prolong the duration of action potentials through a decrease in potassium current. CNI also exerted direct effects on endothelial cell injury, inducing apoptosis and enhancing oxidative stress. CNI may induce vascular inflammation through TLR4 via MyD88 and TRIF pathways. In addition, CNI affects vascular function by impairing endothelial-dependent vasodilation and promoting vasoconstriction. Clinical studies in transplant patients also revealed an increased incidence of cardiac remodeling. However, the evidence is constrained by the limited number of participants and potential confounding factors. Several studies indicate differing cardiovascular toxicity profiles between CsA and TAC, and these could be potentially due to their different interactions with calcineurin subunits and calcineurin-independent effects. Further studies are needed to clarify these mechanisms to improve cardiovascular outcomes for transplant patients with CNI.

Calcineurin (PPP3CB) regulates angiotensin II-dependent vascular remodelling by potentiating EGFR signalling in mice

AIM

This study investigates the role of calcineurin for angiotensin II (AngII)-induced vascular remodelling with the help of a mouse model lacking the catalytic beta subunit of calcineurin (PPP3CB KO).

METHODS

Wildtype (WT) and PPP3CB KO mice were treated for 4 weeks with AngII followed by assessment of blood pressure, histological evaluation of aortas and mRNA analysis of aortic genes PPP3CB-dependently regulated by AngII. Primary murine vascular smooth muscle cells (VSMCs) were used for qPCR, ELISA and Western Blot experiments as well as wound healing and cell proliferation assays.

RESULTS

Upon AngII treatment, PPP3CB KO mice showed less aortic media thickening, lumen dilation and systolic blood pressure compared to WT mice. Next-generation sequencing data of aortic tissue indicated an increase in extracellular matrix components (EMCs), cell migration and cell proliferation. A PPP3CB-dependent increase in EMC was confirmed by qPCR in aorta and VSMCs. PPP3CB-dependent stimulation of VSMC migration could be verified by wound healing assays but markers of enhanced cell proliferation were only detectable in aortic tissue of WT mice but not in isolated WT or KO VSMCs. We could demonstrate in VSMCs with pharmacological inhibitors that PPP3CB leads to enhanced heparin-binding EGF-like growth factor (HB-EGF) secretion, epidermal growth factor receptor (EGFR) activation and consecutive stimulation of transforming growth factor β(TGFβ) and connective tissue growth factor (CTGF) signalling that enhances collagen expression.

CONCLUSION

AngII-induced vascular remodelling involves PPP3CB, which leads to enhanced EMC production, VSMC migration and sustained increase in systolic blood pressure via HBEGF/EGFR-TGFβ-CTGF signalling.

A Review of Calcineurin Biophysics with Implications for Cardiac Physiology

Calcineurin, also known as protein phosphatase 2B, is a heterodimeric serine threonine phosphatase involved in numerous signaling pathways. During the past 50 years, calcineurin has been the subject of extensive investigation. Many of its cellular and physiological functions have been described, and the underlying biophysical mechanisms are the subject of active investigation. With the abundance of techniques and experimental designs utilized to study calcineurin and its numerous substrates, it is difficult to reconcile the available information. There have been a plethora of reports describing the role of calcineurin in cardiac disease. However, a physiological role of calcineurin in healthy cardiomyocyte function requires clarification. Here, we review the seminal biophysical and structural details that are responsible for the molecular function and inhibition of calcineurin. We then focus on literature describing the roles of calcineurin in cardiomyocyte physiology and disease.

Calcium Signaling and Transcriptional Regulation in Cardiomyocytes

Calcium (Ca 2+ ) is a universal regulator of various cellular functions. In cardiomyocytes, Ca 2+ is the central element of excitation–contraction coupling, but also impacts diverse signaling cascades and influences the regulation of gene expression, referred to as excitation–transcription coupling. Disturbances in cellular Ca 2+ -handling and alterations in Ca 2+ -dependent gene expression patterns are pivotal characteristics of failing cardiomyocytes, with several excitation–transcription coupling pathways shown to be critically involved in structural and functional remodeling processes. Thus, targeting Ca 2+ -dependent transcriptional pathways might offer broad therapeutic potential. In this article, we (1) review cytosolic and nuclear Ca 2+ dynamics in cardiomyocytes with respect to their impact on Ca 2+ -dependent signaling, (2) give an overview on Ca 2+ -dependent transcriptional pathways in cardiomyocytes, and (3) discuss implications of excitation–transcription coupling in the diseased heart.

Current Perspectives of Electrical Remodeling and Its Therapeutic Implications

Electrical remodeling involves alterations in the electrophysiologic milieu of myocardium in various disease states, such as ventricular hypertrophy, heart failure, atrial tachyarrhythmias, myocardial ischemia, and infarction that are associated with cardiac arrhythmias. Although research in this area dates back to early part of the 19th century, we still lack the exact knowledge of ionic remodeling, the role of various genes and channel proteins, and their relevance for the newer antiarrhythmic therapies. Structural remodeling may also have an impact on the electrical remodeling process, although differences in both structural and electrical remodeling are associated with different disease states. Various electrophysiologic, cellular, and structural alterations, including anisotropic conduction, increased intracellular calcium levels, and gap junction remodeling predispose to increased dispersion of action potential duration and refractoriness. This constitutes a favorable substrate for early and late afterdepolarizations and reentrant arrhythmias. Studying the role of ionic remodeling in the initiation and propagation of cardiac arrhythmias has significant relevance for developing newer antiarrhythmic therapies, for identifying patients at risk of developing fatal arrhythmias, and for implementing effective preventive measures. Further research is required to understand the specific effects of individual ion channel remodeling, to understand the signal transduction mechanisms, and to address whether detrimental effects of electrical remodeling can be altered.

Tropisetron inhibits high glucose-induced calcineurin/NFAT hypertrophic pathway in H9c2 myocardial cells

OBJECTIVES

Cardiomyocyte hypertrophy is an important structural feature of diabetic cardiomyopathy. Calcineurin/nuclear factor of activated T-cell (NFAT) pathway plays a central role in the pathogenesis of cardiac hypertrophy. The purpose of this study was to investigate the effects of tropisetron, a novel calcineurin inhibitor, on high glucose (HG)-induced cardiomyocyte hypertrophy and its underlying mechanism.

METHODS

H9c2 myocardial cells were treated with tropisetron or cyclosporine A 1 h before exposure to HG for 48 h.

KEY FINDINGS

Exposure to HG resulted in enhanced cell size, protein content and atrial natriuretic peptide (ANP) protein expression. HG significantly increased Ca(2+) level, calcineurin expression and nuclear translocation of NFATc4. Both tropisetron and cyclosporine A markedly prevented the hypertrophic characteristic features, calcineurin overexpression and nuclear localization of NFATc4 while intracellular Ca(2+) was not affected.

CONCLUSION

Our results showed that tropisetron may have protective effects against HG-induced cardiomyocyte hypertrophy. The mechanism responsible for this beneficial effect seems to be, at least in part, blockade of calcineurin/NFAT signalling pathway.

Potential for pharmacological manipulation of human embryonic stem cells

The therapeutic potential of human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) is vast, allowing disease modelling, drug discovery and testing and perhaps most importantly regenerative therapies. However, problems abound; techniques for cultivating self-renewing hESCs tend to give a heterogeneous population of self-renewing and partially differentiated cells and general include animal-derived products that can be cost-prohibitive for large-scale production, and effective lineage-specific differentiation protocols also still remain relatively undefined and are inefficient at producing large amounts of cells for therapeutic use. Furthermore, the mechanisms and signalling pathways that mediate pluripotency and differentiation are still to be fully appreciated. However, over the recent years, the development/discovery of a range of effective small molecule inhibitors/activators has had a huge impact in hESC biology. Large-scale screening techniques, coupled with greater knowledge of the pathways involved, have generated pharmacological agents that can boost hESC pluripotency/self-renewal and survival and has greatly increased the efficiency of various differentiation protocols, while also aiding the delineation of several important signalling pathways. Within this review, we hope to describe the current uses of small molecule inhibitors/activators in hESC biology and their potential uses in the future.

Isolation, culture, and functional characterization of adult mouse cardiomyoctyes

The use of primary cardiomyocytes (CMs) in culture has provided a powerful complement to murine models of heart disease in advancing our understanding of heart disease. In particular, the ability to study ion homeostasis, ion channel function, cellular excitability and excitation-contraction coupling and their alterations in diseased conditions and by disease-causing mutations have led to significant insights into cardiac diseases. Furthermore, the lack of an adequate immortalized cell line to mimic adult CMs, and the limitations of neonatal CMs (which lack many of the structural and functional biomechanics characteristic of adult CMs) in culture have hampered our understanding of the complex interplay between signaling pathways, ion channels and contractile properties in the adult heart strengthening the importance of studying adult isolated cardiomyocytes. Here, we present methods for the isolation, culture, manipulation of gene expression by adenoviral-expressed proteins, and subsequent functional analysis of cardiomyocytes from the adult mouse. The use of these techniques will help to develop mechanistic insight into signaling pathways that regulate cellular excitability, Ca(2+) dynamics and contractility and provide a much more physiologically relevant characterization of cardiovascular disease.

Physiologic functions of cyclophilin D and the mitochondrial permeability transition pore

This review focuses on the role of cyclophilin D (CypD) as a prominent mediator of the mitochondrial permeability transition pore (MPTP) and subsequent effects on cardiovascular physiology and pathology. Although a great number of reviews have been written on the MPTP and its effects on cell death, we focus on the biology surrounding CypD itself and the non-cell death physiologic functions of the MPTP. A greater understanding of the physiologic functions of the MPTP and its regulation by CypD will likely suggest novel therapeutic approaches for cardiovascular disease, both dependent and independent of programmed necrotic cell death mechanisms.  

Altered sarcoplasmic reticulum calcium cycling--targets for heart failure therapy

Cardiac myocyte function is dependent on the synchronized movements of Ca(2+) into and out of the cell, as well as between the cytosol and sarcoplasmic reticulum. These movements determine cardiac rhythm and regulate excitation-contraction coupling. Ca(2+) cycling is mediated by a number of critical Ca(2+)-handling proteins and transporters, such as L-type Ca(2+) channels (LTCCs) and sodium/calcium exchangers in the sarcolemma, and sarcoplasmic/endoplasmic reticulum calcium ATPase 2a (SERCA2a), ryanodine receptors, and cardiac phospholamban in the sarcoplasmic reticulum. The entry of Ca(2+) into the cytosol through LTCCs activates the release of Ca(2+) from the sarcoplasmic reticulum through ryanodine receptor channels and initiates myocyte contraction, whereas SERCA2a and cardiac phospholamban have a key role in sarcoplasmic reticulum Ca(2+) sequesteration and myocyte relaxation. Excitation-contraction coupling is regulated by phosphorylation of Ca(2+)-handling proteins. Abnormalities in sarcoplasmic reticulum Ca(2+) cycling are hallmarks of heart failure and contribute to the pathophysiology and progression of this disease. Correcting impaired intracellular Ca(2+) cycling is a promising new approach for the treatment of heart failure. Novel therapeutic strategies that enhance myocyte Ca(2+) homeostasis could prevent and reverse adverse cardiac remodeling and improve clinical outcomes in patients with heart failure.

FHL2 binds calcineurin and represses pathological cardiac growth

Stress-induced hypertrophic growth of the heart predisposes the heart to arrhythmia, contractile dysfunction, and clinical heart failure. FHL2 (four-and-a-half LIM domain protein 2) is expressed predominantly in the heart, and inactivation of the gene coding for FHL2 leads to exaggerated responsiveness to adrenergic stress. Activation of calcineurin occurs downstream of β-adrenergic signaling and is required for isoproterenol-induced myocardial hypertrophy. Based on these facts, we hypothesized that FHL2 suppresses stress-induced activation of calcineurin. FHL2 is upregulated in mouse hearts exposed to isoproterenol, a β-adrenergic agonist, and isoproterenol-induced increases in the NFAT target genes RCAN1.4 and BNP were amplified significantly in FHL2 knockout (FHL2(-/-)) mice compared with levels in wild-type (WT) mice. To determine whether the effect of FHL2 on NFAT target gene transcript levels occurred at the level of transcription, HEK 293 cells and neonatal rat ventricular myocytes (NRVMs) were transfected with a luciferase reporter construct harboring the NFAT-dependent promoters of either RCAN1 or interleukin 2 (IL-2). Consistent with the in vivo data, small interfering RNA (siRNA) knockdown of FHL2 led to increased activation of these promoters by constitutively active calcineurin or the calcium ionophore ionomycin. Importantly, activation of the RCAN1 promoter by ionomycin, in control and FHL2 knockdown cells, was abolished by the calcineurin inhibitor cyclosporine, confirming the calcineurin dependence of the response. Overexpression of FHL2 inhibited activation of both NFAT reporter constructs. Furthermore, NRVMs overexpressing FHL2 exhibited reduced hypertrophic growth in response to constitutively active calcineurin, as measured by cell cross-sectional area and fetal gene expression. Finally, immunostaining in isolated adult cardiomyocytes revealed colocalization of FHL2 and calcineurin predominantly at the sarcomere and activation of calcineurin by endothelin-1-facilitated interaction between FHL2 and calcineurin. FHL2 is an endogenous, agonist-dependent suppressor of calcineurin.

Calcineurin inhibitors and immunosuppression - a tale of two isoforms

Organ transplantation is the state of the art for treating end-stage organ failure. Over 25000 organ transplants are performed in the USA each year. Survival rates following transplantation are now approaching 90% for 1 year and 75% for 5 years. Central to this success was the introduction of drugs that suppress the immune system and prevent rejection. The most commonly used class of immunosuppressing drugs are calcineurin inhibitors (CNIs). Calcineurin is a ubiquitous enzyme that is important for T-cell function. With more people taking CNIs for longer and longer periods of time the consequences of calcineurin inhibition on other organ systems - particularly the kidney - have become a growing concern. Virtually all people who take a CNI will develop some degree of kidney toxicity and up to 10% will progress to kidney failure. In the past 15 years, research into calcineurin action has identified distinct actions of the two main isoforms of the catalytic subunit of the enzyme. The α-isoform is required for kidney function whereas the β-isoform has a predominant role in the immune system. This review will discuss the current state of knowledge about calcineurin isoforms and how these new insights may reshape post-transplant immunosuppression.

Calcineurin regulation of cytoskeleton organization: a new paradigm to analyse the effects of calcineurin inhibitors on the kidney

Calcineurin is a serine/threonine phosphatase originally involved in the immune response but is also known for its role as a central mediator in various non-immunological intracellular signals. The nuclear factor of activated T cell (NFAT) proteins are the most widely described substrates of calcineurin, but ongoing work has uncovered other substrates among which are the cytoskeleton organizing proteins (i.e. cofilin, synaptopodin, WAVE-1). Control over cytoskeletal proteins is of outmost interest because the phenotypic properties of cells are dependent on cytoskeleton architecture integrity, while rearrangements of the cytoskeleton are implicated in both physiological and pathological processes. Previous works investigating the role of calcineurin on the cytoskeleton have focused on neurite elongation, myocyte hypertrophic response and recently in kidney cells structure. Nuclear factor of activated T cell activation is expectedly identified in the signalling pathways for calcineurin-induced cytoskeleton organization, however new NFAT-independent pathways have also been uncovered. The aim of this review is to summarize the current knowledge on the effects of calcineurin on cytoskeletal proteins and related intracellular pathways. These newly described properties of calcineurin on cytoskeletal proteins may explain some of the beneficial or deleterious effects observed in kidney cells associated with the use of the calcineurin inhibitors, cyclosporine and tacrolimus.

Calcineurin A-β is required for hypertrophy but not matrix expansion in the diabetic kidney

Calcineurin is an important signalling protein that regulates a number of molecular and cellular processes. Previously, we found that inhibition of calcineurin with cyclosporine reduced renal hypertrophy and blocked glomerular matrix expansion in the diabetic kidney. Isoforms of the catalytic subunit of calcineurin are reported to have tissue specific expression and functions. In particular, the β isoform has been implicated in cardiac and skeletal muscle hypertrophy. Therefore, we examined the role of calcineurin β in diabetic renal hypertrophy and glomerular matrix expansion. Type I diabetes was induced in wild-type and β^−/− mice and then renal function, extracellular matrix expansion and hypertrophy were evaluated. The absence of β produced a significant decrease in total calcineurin activity in the inner medulla (IM) and reduced nuclear factor of activated T-cells (NFATc) activity. Loss of β did not alter diabetic renal dysfunction assessed by glomerular filtration rate, urine albumin excretion and blood urea nitrogen. Similarly, matrix expansion in the whole kidney and glomerulus was not different between diabetic wild-type and β^−/− mice. In contrast, whole kidney and glomerular hypertrophy were significantly reduced in diabetic β^−/− mice. Moreover, β^−/− renal fibroblasts demonstrated impaired phosphorylation of Erk1/Erk2, c-Jun N-terminal kinases (JNK) and mammalian target of rapamycin (mTOR) following stimulation with transforming growth factor-β and did not undergo hypertrophy with 48 hrs culture in high glucose. In conclusion, loss of the β isoform of calcineurin is sufficient to reproduce beneficial aspects of cyclosporine on diabetic renal hypertrophy but not matrix expansion. Therefore, while multiple signals appear to regulate matrix, calcineurin β appears to be a central mechanism involved in organ hypertrophy.

Tropomodulin capping of actin filaments in striated muscle development and physiology

Efficient striated muscle contraction requires precise assembly and regulation of diverse actin filament systems, most notably the sarcomeric thin filaments of the contractile apparatus. By capping the pointed ends of actin filaments, tropomodulins (Tmods) regulate actin filament assembly, lengths, and stability. Here, we explore the current understanding of the expression patterns, localizations, and functions of Tmods in both cardiac and skeletal muscle. We first describe the mechanisms by which Tmods regulate myofibril assembly and thin filament lengths, as well as the roles of closely related Tmod family variants, the leiomodins (Lmods), in these processes. We also discuss emerging functions for Tmods in the sarcoplasmic reticulum. This paper provides abundant evidence that Tmods are key structural regulators of striated muscle cytoarchitecture and physiology.

Molecular mechanisms underlying cardiac antihypertrophic and antifibrotic effects of natriuretic peptides

Natriuretic peptides (NPs) exert well-characterized protective effects on the cardiovascular system, such as vasorelaxation, natri- and diuresis, increase of endothelial permeability, and inhibition of renin-angiotensin-aldosterone system. It has been reported that they also possess antihypertrophic and antifibrotic properties and contribute actively to cardiac remodeling. As a consequence, they are involved in several aspects of cardiovascular diseases. Antihypertrophic and antifibrotic actions of NPs appear to be mediated by specific signaling pathways within a more complex cellular network. Elucidation of the molecular mechanisms underlying the effects of NPs on cardiac remodeling represents an important research objective in order to gain more insights on the complex network leading to cardiac hypertrophy, ventricular dysfunction, and transition to heart failure, and in the attempt to develop novel therapeutic agents. The aim of the present article is to review well-characterized molecular mechanisms underlying the antihypertrophic and antifibrotic effects of NPs in the heart that appear to be mainly mediated by guanylyl cyclase type A receptor. In particular, we discuss the calcineurin/NFAT, the sodium exchanger NHE-1, and the TGFβ1/Smad signaling pathways. The role of guanylyl cyclase type B receptor, along with the emerging functional significance of natriuretic peptide receptor type C as mediators of CNP antihypertrophic and antifibrotic actions in the heart are also considered.

Reversibility of adverse, calcineurin-dependent cardiac remodeling

RATIONALE

Studies to dissect the role of calcineurin in pathological cardiac remodeling have relied heavily on murine models, in which genetic gain- and loss-of-function manipulations are initiated at or before birth. However, the great majority of clinical cardiac pathology occurs in adults. Yet nothing is known about the effects of calcineurin when its activation commences in adulthood. Furthermore, despite the fact that ventricular hypertrophy is a well-established risk factor for heart failure, the relative pace and progression of these 2 major phenotypic features of heart disease are unknown. Finally, even though therapeutic interventions in adults are designed to slow, arrest, or reverse disease pathogenesis, little is known about the capacity for spontaneous reversibility of calcineurin-dependent pathological remodeling.

OBJECTIVE

We set out to address these 3 questions by studying mice engineered to harbor in cardiomyocytes a constitutively active calcineurin transgene driven by a tetracycline-responsive promoter element.

METHODS AND RESULTS

Expression of the mutant calcineurin transgene was initiated for variable lengths of time to determine the natural history of disease pathogenesis, and to determine when, if ever, these events are reversible. Activation of the calcineurin transgene in adult mice triggered rapid and robust cardiac growth with features characteristic of pathological hypertrophy. Concentric hypertrophy preceded the development of systolic dysfunction, fetal gene activation, fibrosis, and clinical heart failure. Furthermore, cardiac hypertrophy reversed spontaneously when calcineurin activity was turned off, and expression of fetal genes reverted to baseline. Fibrosis, a prominent feature of pathological cardiac remodeling, manifested partial reversibility.

CONCLUSIONS

Together, these data establish and define the deleterious effects of calcineurin signaling in the adult heart and reveal that calcineurin-dependent hypertrophy with concentric geometry precedes systolic dysfunction and heart failure. Furthermore, these findings demonstrate that during much of the disease process, calcineurin-dependent remodeling remains reversible.

Opposing roles of FoxP1 and Nfat3 in transcriptional control of cardiomyocyte hypertrophy

Cardiac homeostasis is maintained by a balance of growth-promoting and growth-modulating factors. Sustained elevation of calcium signaling can induce cardiac hypertrophy through activation of Nfat family transcription factors. FoxP family transcription factors are known to interact with Nfat proteins and to modulate their transcriptional activities in lymphocytes. We investigated FoxP1 interaction with Nfat3 (Nfatc4) and their effects on transcription of hypertrophy-associated genes in neonatal rat cardiomyocytes. FoxP1-Nfat3 complexes were visualized using bimolecular fluorescence complementation (BiFC) analysis. Calcineurin activation induced FoxP1-Nfat3 BiFC complex formation. Amino acid substitutions in the predicted interaction interface inhibited it. FoxP1 repressed hypertrophy-associated genes (Myh7, Rcan1, Cx43, Anf, and Bnp) and counteracted their activation by constitutively nuclear Nfat3 (cnNfat3). In contrast, FoxP1 activated genes that maintain normal heart functions (Myh6 and p57Kip2) and cnNfat3 counteracted their activation by FoxP1. Amino acid substitutions in FoxP1 or cnNfat3 that inhibited their interaction abrogated the activation of hypertrophy-associated gene transcription by cnNfat3 and the repression of these genes by FoxP1. FoxP1 and Nfat3 co-occupied the promoter regions of hypertrophy-associated genes in neonatal and adult heart tissue. FoxP1 counteracted hypertrophic cardiomyocyte growth, and connexin 43 mislocalization caused by cnNfat3 expression. These data suggest that the opposing transcriptional activities of FoxP1 and Nfat3 maintain cardiomyocyte homeostasis.

The development of familial hypertrophic cardiomyopathy: from mutation to bedside

Hypertrophic cardiomyopathy (HCM) is a familial disorder characterized by left ventricular hypertrophy in the absence of other cardiac or systemic disease likely to cause this hypertrophy. HCM is considered a disease of the sarcomere as most causal mutations are identified in genes encoding sarcomeric proteins, although several other disorders have also been linked to the HCM phenotype. The clinical course of HCM is characterized by a large inter- and intrafamilial variability, ranging from severe symptomatic HCM to asymptomatic individuals. The general picture emerges that the underlying pathophysiology of HCM is complex and still scarcely clarified. Recent findings indicated that both functional and morphological (macroscopic and microscopic) changes of the HCM muscle are present before the occurrence of HCM phenotype. This review aims to provide an overview of the myocardial alterations that occur during the gradual process of wall thickening in HCM on a myofilament level, as well as the structural and functional abnormalities that can be observed in genetically affected individuals prior to the development of HCM with state of the art imaging techniques, such as tissue Doppler echocardiography and cardiovascular magnetic resonance imaging. Additionally, present and future therapeutic options will be briefly discussed.

What causes a broken heart--molecular insights into heart failure

Our understanding of the molecular processes which regulate cardiac function has grown immeasurably in recent years. Even with the advent of β-blockers, angiotensin inhibitors and calcium modulating agents, heart failure (HF) still remains a seriously debilitating and life-threatening condition. Here, we review the molecular changes which occur in the heart in response to increased load and the pathways which control cardiac hypertrophy, calcium homeostasis, and immune activation during HF. These can occur as a result of genetic mutation in the case of hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) or as a result of ischemic or hypertensive heart disease. In the majority of cases, calcineurin and CaMK respond to dysregulated calcium signaling and adrenergic drive is increased, each of which has a role to play in controlling blood pressure, heart rate, and left ventricular function. Many major pathways for pathological remodeling converge on a set of transcriptional regulators such as myocyte enhancer factor 2 (MEF2), nuclear factors of activated T cells (NFAT), and GATA4 and these are opposed by the action of the natriuretic peptides ANP and BNP. Epigenetic modification has emerged in recent years as a major influence cardiac physiology and histone acetyl transferases (HATs) and histone deacetylases (HDACs) are now known to both induce and antagonize hypertrophic growth. The newly emerging roles of microRNAs in regulating left ventricular dysfunction and fibrosis also has great potential for novel therapeutic intervention. Finally, we discuss the role of the immune system in mediating left ventricular dysfunction and fibrosis and ways this can be targeted in the setting of viral myocarditis.

Experimental therapies in hypertrophic cardiomyopathy

The quintessential clinical diagnostic phenotype of human hypertrophic cardiomyopathy (HCM) is primary cardiac hypertrophy. Cardiac hypertrophy is also a major determinant of mortality and morbidity including the risk of sudden cardiac death (SCD) in patients with HCM. Reversal and attenuation of cardiac hypertrophy and its accompanying fibrosis is expected to improve morbidity as well as decrease the risk of SCD in patients with HCM.The conventionally used pharmacological agents in treatment of patients with HCM have not been shown to reverse or attenuate established cardiac hypertrophy and fibrosis. An effective treatment of HCM has to target the molecular mechanisms that are involved in the pathogenesis of the phenotype. Mechanistic studies suggest that cardiac hypertrophy in HCM is secondary to activation of various hypertrophic signaling molecules and, hence, is potentially reversible. The hypothesis is supported by the results of genetic and pharmacological interventions in animal models. The results have shown potential beneficial effects of angiotensin II receptor blocker losartan, mineralocorticoid receptor blocker spironolactone, 3-hydroxy-3-methyglutaryl-coenzyme A reductase inhibitors simvastatin and atorvastatin, and most recently, N-acetylcysteine (NAC) on reversal or prevention of hypertrophy and fibrosis in HCM. The most promising results have been obtained with NAC, which through multiple thiol-responsive mechanisms completely reversed established cardiac hypertrophy and fibrosis in three independent studies. Pilot studies with losartan and statins in humans have established the feasibility of such studies. The results in animal models have firmly established the reversibility of established cardiac hypertrophy and fibrosis in HCM and have set the stage for advancing the findings in the animal models to human patients with HCM through conducting large-scale efficacy studies.

Genetic inhibition of calcineurin induces diastolic dysfunction in mice with chronic pressure overload

Calcineurin is a Ca(2+)/calmodulin-dependent protein phosphatase that induces myocardial growth in response to several physiological and pathological stimuli. Calcineurin inhibition, induced either via cyclosporine or genetically, can decrease myocardial hypertrophy secondary to pressure overload without affecting left ventricular (LV) systolic function. Since hypertrophy can also affect LV diastolic function, the goal of this study was to examine the effects of chronic pressure overload (2 wk aortic banding) in transgenic (Tg) mice overexpressing Zaki-4beta (TgZ), a specific endogenous inhibitor of calcineurin, on LV diastolic function. As expected, in the TgZ mice with calcineurin inhibitor overexpression, aortic banding reduced the degree of LV hypertrophy, as assessed by LV weight-to-body weight ratio (3.5 + or - 0.1) compared with that in non-Tg mice (4.6 + or - 0.2). LV systolic function remained compensated in both groups with pressure overload. However, the LV end-diastolic stress-to-LV end-diastolic dimension ratio, an index of diastolic stiffness and LV pressure half-time and isovolumic relaxation time, two indexes of isovolumic relaxation, increased significantly more in TgZ mice with aortic banding. Protein levels of phosphorylated phospholamban (PS16), sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a, phosphorylated ryanodine receptor, and the Na(+)/Ca(2+) exchanger were also reduced significantly (P < 0.05) in the banded TgZ mice. As expected, genetic calcineurin inhibition inhibited the development of LV hypertrophy with chronic pressure overload but also induced LV diastolic dysfunction, as reflected by both impaired isovolumic relaxation and increased myocardial stiffness. Thus genetic calcineurin inhibition reveals a new mechanism regulating LV diastolic function.

MicroRNA-1 negatively regulates expression of the hypertrophy-associated calmodulin and Mef2a genes

Calcium signaling is a central regulator of cardiomyocyte growth and function. Calmodulin is a critical mediator of calcium signals. Because the amount of calmodulin within cardiomyocytes is limiting, the precise control of calmodulin expression is important for the regulation of calcium signaling. In this study, we show for the first time that calmodulin levels are regulated posttranscriptionally in heart failure. The cardiomyocyte-restricted microRNA miR-1 inhibited the translation of calmodulin-encoding mRNAs via highly conserved target sites within their 3' untranslated regions. In keeping with its effect on calmodulin expression, miR-1 downregulated calcium-calmodulin signaling through calcineurin to NFAT. miR-1 also negatively regulated the expression of Mef2a and Gata4, key transcription factors that mediate calcium-dependent changes in gene expression. Consistent with the downregulation of these hypertrophy-associated genes, miR-1 attenuated cardiomyocyte hypertrophy in cultured neonatal rat cardiomyocytes and in the intact adult heart. Our data indicate that miR-1 regulates cardiomyocyte growth responses by negatively regulating the calcium signaling components calmodulin, Mef2a, and Gata4.

Drugs that interact with cardiac electro-mechanics: old and new targets for treatment

The concept of mechano-electrical feedback was derived from the observation that a short stretch applied to the beating heart can invoke an electrical response in the form of an afterdepolarization or a premature ventricular beat. More recent work has identified stretch-activated channels whose specific inhibition might help to treat atrial fibrillation in the near future. But the interaction between electrical and mechanical function of the heart is a continuum from short-term (within milliseconds) to long-term (within weeks or months) effects. The long-term effects of pressure overload have been well-described on the molecular and cellular level, and substances that interact with these processes are used in clinical routine in the care of patients with cardiac hypertrophy and heart failure. These treatments help to prevent lethal arrhythmias (sudden death) and potentially atrial fibrillation. The intermediate interaction between mechanical and electrical function of the heart is less well-understood. Several recently identified regulatory mechanisms may provide novel antiarrhythmic targets associated with the "intermediate" response of the myocardium to stretch.

Calcineurin-induced energy wasting in a transgenic mouse model of heart failure

Overexpression of calcineurin (CLN) in the mouse heart induces severe hypertrophy that progresses to heart failure, providing an opportunity to define the relationship between energetics and contractile performance in the severely failing mouse heart. Contractile performance was studied in isolated hearts at different pacing frequencies and during dobutamine challenge. Energetics were assessed by 31P-NMR spectroscopy as ATP and phosphocreatine concentrations ([ATP] and [PCr]) and free energy of ATP hydrolysis (|Delta G( approximately ATP)|). Mitochondrial and glycolytic enzyme activities, myocardial O2 consumption, and myocyte ultrastructure were determined. In transgenic (TG) hearts at all levels of work, indexes of systolic performance were reduced and [ATP] and capacity for ATP synthesis were lower than in non-TG hearts. This is the first report showing that myocardial [ATP] is lower in a TG mouse model of heart failure. [PCr] was also lower, despite an unexpected increase in the total creatine pool. Because Pi concentration remained low, despite lower [ATP] and [PCr], |Delta G( approximately ATP)| was normal; however, chemical energy did not translate to systolic performance. This was most apparent with beta-adrenergic stimulation of TG hearts, during which, for similar changes in |Delta G( approximately ATP)|, systolic pressure decreased, rather than increased. Structural abnormalities observed for sarcomeres and mitochondria likely contribute to decreased contractile performance. On the basis of the increases in enzyme activities of proteins important for ATP supply observed after treatment with the CLN inhibitor cyclosporin A, we also conclude that CLN directed inhibition of ATP-producing pathways in non-TG and TG hearts.

Overexpression of pyruvate dehydrogenase kinase 4 in heart perturbs metabolism and exacerbates calcineurin-induced cardiomyopathy

The heart adapts to changes in nutritional status and energy demands by adjusting its relative metabolism of carbohydrates and fatty acids. Loss of this metabolic flexibility such as occurs in diabetes mellitus is associated with cardiovascular disease and heart failure. To study the long-term consequences of impaired metabolic flexibility, we have generated mice that overexpress pyruvate dehydrogenase kinase (PDK)4 selectively in the heart. Hearts from PDK4 transgenic mice have a marked decrease in glucose oxidation and a corresponding increase in fatty acid catabolism. Although no overt cardiomyopathy was observed in the PDK4 transgenic mice, introduction of the PDK4 transgene into mice expressing a constitutively active form of the phosphatase calcineurin, which causes cardiac hypertrophy, caused cardiomyocyte fibrosis and a striking increase in mortality. These results demonstrate that cardiac-specific overexpression of PDK4 is sufficient to cause a loss of metabolic flexibility that exacerbates cardiomyopathy caused by the calcineurin stress-activated pathway.

Cardiac hypertrophy: mechanisms and therapeutic opportunities

Cardiac hypertrophy and heart failure are major causes of morbidity and mortality in Western societies. Many factors have been implicated in cardiac remodeling, including alterations in gene expression in myocytes, cardiomyocytes apoptosis, cytokines and growth factors that influence cardiac dynamics, and deficits in energy metabolism as well as alterations in cardiac extracellular matrix composition. Many therapeutic means have been shown to prevent or reverse cardiac hypertrophy. New concepts for characterizing the pathophysiology of cardiac hypertrophy have been drawn from various aspects, including medical therapy and gene therapy, or use of stem cells for tissue regeneration. In this review, we focus on various types of cardiac hypertrophy, defining the causes of hypertrophy, describing available animal models of hypertrophy, discussing the mechanisms for development of hypertrophy and its transition to heart failure, and presenting the potential use of novel promising therapeutic strategies derived from new advances in basic scientific research.

Loss of the alpha-isoform of calcineurin is sufficient to induce nephrotoxicity and altered expression of transforming growth factor-beta

BACKGROUND

Use of calcineurin inhibitors is frequently limited by fibrosis, closely linked with increased transforming growth factor (TGF)-beta. However, mechanisms of extracellular matrix expansion and TGFbeta regulation following calcineurin inhibition are unknown. Mice lacking specific calcineurin catalytic subunit isoforms may offer important insight into this pathway.

METHODS

We compared mice lacking the alpha or beta isoform to a model of cyclosporin nephrotoxicity. Histological features common with cyclosporin nephrotoxicity including matrix expansion, arteriole hyalinization, and inflammation were assessed. Next, regulation specifically of fibronectin and TGFbeta was examined in vivo and in vitro. Finally, the role of TGFbeta in upregulation of fibronectin with loss of calcineurin activity was examined.

RESULTS

Loss of the alpha isoform results in histologic features and matrix expansion similar to cyclosporin, whereas loss of the beta does not. Fibronectin and TGFbeta are increased and renal function is impaired in alpha-null and aged alpha+/-. In primary alpha-/- renal fibroblasts, nuclear translocation of the calcineurin substrate NFATc is normal but regulation is lost in beta-null fibroblasts, confirming that the isoforms have distinct functions. Consistent with in vivo findings, alpha-null cells have increased fibronectin and TGFbeta. However, neutralizing TGFbeta antibody did not reduce fibronectin accumulation.

CONCLUSIONS

Our data show that calcineurin-alpha is key to regulation of fibrosis and TGFbeta and loss of this isoform reproduces features of cyclosporine nephrotoxicity in vivo and in vitro. In addition, we show that upregulation of TGFbeta and fibronectin likely result from a shared mechanism, but changes in fibronectin expression are independent of TGFbeta in renal fibroblasts.

Association of calcineurin with mitochondrial proteins

Using mouse hearts from Swiss Webster mice, calcineurin was immunoprecipitated using commercially available anti-calcineurin antibody and the resulting complex analyzed by using sodium dodecyl sulfate-gel electrophoresis with silver staining. Distinct proteins were observed and subjected to in situ trypsin digestion followed by extraction of the resulting peptides. Peptides from each protein band were loaded onto a target for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and analyzed. The resulting peptide mass spectrum was compared with the Mascot and Protein Prospector databases and resulted in the specific identification of heart mitochondrial proteins, specifically Mn-superoxide dismutase (SOD), aconitase (ACN), and malate dehydrogenase (MDH). Each of the three mitochondrial enzymes was identified with approximately 15-25% sequence coverage and all with statistical significance (P < 0.05) according to the Mascot database search engine. Tandem mass spectrometry analysis of the peptide fragmentation spectra confirmed the identification of these protein partners and also yielded the identification of mitochondrial isocitrate dehydrogenase (ICDH) as another protein in the immunoprecipitated complex. Using antibody preparations against Mn-SOD, ACN, and ICDH showed the presence of calcineurin and each of the three proteins in the immunoprecipitated complex by Western slot blotting. The activity of ACN, but not MDH or ICDH, was enhanced after incubation with calcineurin indicating one possible regulatory function for the complex. The mitochondrial forms of Mn-SOD, ACN, MDH, and ICDH were identified as partner proteins of calcineurin with all the proteins present in a single multiprotein complex.

Specific and sustained down-regulation of genes involved in fatty acid metabolism is not a hallmark of progression to cardiac failure in mice

Preferential and specific down-regulation of genes involved in fatty acid (FA) uptake and metabolism is considered a hallmark of severe hypertrophic remodeling and progression to cardiac failure. Therefore, we investigated the time course of changes in cardiac metabolic gene expression (1) in mice subjected to regional myocardial infarction (MI) for 4 days, 1 month, or 3 months and (2) in mice overexpressing calcineurin (Cn) which initially develop concentric hypertrophy progressing after the age of 4 weeks to dilated cardiomyopathy and failure. In both models, hypertrophy was characterized by increased expression of beta-myosin heavy chain protein and atrial natriuretic factor mRNA, indicative of marked structural remodeling. Fractional shortening progressively decreased from 31% to 15.1% and 3.7% 1 and 3 months after MI, respectively. One month post-MI, the expression of several metabolic genes, i.e., acyl-CoA synthetase (-50%), muscle-type carnitine palmitoyl transferase 1 (-37%) and citrate synthase (-28%), was significantly reduced in the surviving myocardium. Despite overt signs of cardiac failure 3 months post-MI, the expression of these genes had returned to normal levels. In hearts of both 4- and 6-week-old Cn mice, genes involved in both FA and glucose metabolism and mitochondrial citrate synthase were down-regulated, reflecting an overall decline in metabolic gene expression, rather than a specific and preferential down-regulation of genes involved in FA uptake and metabolism. These findings challenge the concept that specific and sustained down-regulation of genes involved in FA uptake and metabolism represents a hallmark of the development of cardiac hypertrophy and progression to failure.

Inhibition of calcineurin by polyunsaturated lipids

From earlier studies on calcineurin, the presence of multiple double bonds in putative inhibitors was hypothesized as critical features for effective inhibition. Polyunsaturated fatty acids were tested as inhibitors of calcineurin and found to inhibit the phosphatase activity of calcineurin although effective inhibition was observed only in the absence of calmodulin. Calmodulin and fatty acids seemed to compete for the enzyme with the activation curve of calmodulin shifted approximately 100-fold in the presence of 50 microM eicosa-11Z,14Z-dienoic acid (20:2, n-6) or 50 microM eicosa-8Z,11Z,14Z-trienoic acid (20:3, n-6). Leukotriene B4 and derivatives also were screened as inhibitors. The most effective inhibition was caused by the 6-trans,12-epi-leukotriene B4 with an IC50 of 16.4 microM for the inhibition of calcineurin with pNPP as the substrate. Lipoxins A4 and B4 likewise caused inhibition in the presence of calmodulin with an IC50 of 42.7 microM for lipoxin B4. There was no protection by calmodulin, as found with the inhibition by the fatty acids. These data support the hypothesis that effective inhibition is bolstered by the presence of conjugated double bonds in the inhibitor. Consideration of cis- and trans-orientation of the double bonds suggests that presentation of the delocalized electron density is also a factor in effective inhibition of calcineurin.

Calcium-calcineurin signaling in the regulation of cardiac hypertrophy

Cardiac hypertrophy is a leading predicator of progressive heart disease that often leads to heart failure and a loss of cardiac contractile performance associated with profound alterations in intracellular calcium handling. Recent investigation has centered on identifying the molecular signaling pathways that regulate cardiac myocyte hypertrophy, as well as the mechanisms whereby alterations in calcium handling are associated with progressive heart failure. One potential focal regulator of cardiomyocyte hypertrophy that also responds to altered calcium handling is the calmodulin-activated serine/threonine protein phosphatase calcineurin (PP2B). Once activated by increases in calcium, calcineurin mediates the hypertrophic response through its downstream transcriptional effector nuclear factor of activated T cells (NFAT), which is directly dephosphorylated by calcineurin resulting in nuclear translocation. While previous studies have convincingly demonstrated the sufficiency of calcineurin to mediate cardiac hypertrophy and progressive heart failure, its necessity remains an area of ongoing investigation. Here we weigh an increasing body of literature that suggests a causal link between calcineurin signaling and the cardiac hypertrophic response and heart failure through the use of pharmacologic inhibitors (cyclosporine A and FK506) and genetic approaches. We will also discuss the manner in which calcineurin-NFAT signaling is negatively regulated in the heart through a diverse array of kinases and inhibitory proteins. Finally, we will discuss emerging theories as to the mechanisms whereby alterations in intracellular calcium handling might stimulate calcineurin within the context of a contractile cell continually experiencing calcium flux.

Molecular characterisation of neonatal cardiac hypertrophy and its regression

Neonatal cardiac hypertrophy associated with diabetic pregnancy is transient and regresses naturally, but is associated with increased morbidity and mortality. This study was undertaken to analyse the changes in expression of 5 cardiac genes, including atrial natriuretic peptide, alpha- and beta-myosin heavy chain, and cardiac and skeletal alpha-actin genes, using a rat neonatal model, in which cardiac hypertrophy was induced via maternal diabetes. In the hypertrophied left ventricle of neonates from diabetic mothers, the levels of mRNA from all the above genes except skeletal alpha-actin were increased by between 1.8- and 12-fold compared with the controls at birth (p < 0.05). In the first 28 days, the level of mRNA for alpha-myosin heavy chain increased slightly, while that for atrial natriuretic peptide and beta-myosin heavy chain decreased continuously similar to the controls, but at a significantly faster rate. No significant difference between the two groups of neonates was observed in all 5 genes after 1 month, indicating complete regression. Expression of 5 cardiac genes in the neonatal cardiac hypertrophy was characterised in both hypertrophic and regressive phases. Hypertrophic regression provides a unique model for the testing of new drugs or genetic modifying factors in cardiac hypertrophy.

Activation of calcineurin expression in ischemia-reperfused rat heart and in human ischemic myocardium

Calcineurin (CaN) has been reported as a critical mediator for cardiac hypertrophy and cardiac myocyte apoptosis. In the present study, we investigated the activity and expression of CaN and the effect of calpain in rat heart after ischemia and reperfusion. Rat ischemic heart showed significant increase in CaN activity. Western blot analysis of normal rat heart extract with a polyclonal antibody raised against bovine CaN indicated a prominent immunoreactive band of 60 kDa (CaN A). In ischemic-reperfused hearts, the expression of CaN A was significantly low and immunoreactivity was observed in proteolytic bands of 46 kDa. This may be due to the proteolytic degradation of CaN A in ischemic tissues by m-calpain. We also noticed in vitro proteolysis of bovine cardiac CaN A by m-calpain. Immunohistochemical studies showed strong staining of immunoreactivity in rat hearts that had gone under 30 min ischemia followed by 30 min reperfusion similar to that found in human ischemic heart. Ischemia is associated with multiple alterations in the extracellular and intracellular signaling of cardiomyocytes and may act as an inducer of apoptosis. The increase in CaN activity and strong immunostaining observed in ischemic/perfused rat heart may be due to the calpain-mediated proteolysis of this phosphatase.

Roles of calcineurin and calcium/calmodulin-dependent protein kinase II in pressure overload-induced cardiac hypertrophy

Calcineurin and calcium/calmodulin-dependent protein kinase (CaMK) II have been suggested to be the signaling molecules in cardiac hypertrophy. It was not known, however, whether these mechanisms are involved in cardiac hypertrophy induced by pressure overload without the influences of blood-derived humoral factors, such as angiotensin II. To elucidate the roles of calcineurin and CaMK II in this situation, we examined the effects of calcineurin and CaMK II inhibitors on pressure overload-induced expression of c-fos, an immediate-early gene, and protein synthesis using heart perfusion model. The hearts isolated from Sprague-Dawley rats were perfused according to the Langendorff technique, and then subjected to the acute pressure overload by raising the perfusion pressure. The activation of calcineurin was evaluated by its complex formation with calmodulin and by its R-II phosphopeptide dephosphorylation. CaMK II activation was evaluated by its autophosphorylation. Expression of c-fos mRNA and rates of protein synthesis were measured by northern blot analysis and by 14C-phenylalanine incorporation, respectively. Acute pressure overload significantly increased calcineurin activity, CaMK II activity, c-fos expression and protein synthesis. Cyclosporin A and FK506, the calcineurin inhibitors, significantly inhibited the increases in both c-fos expression and protein synthesis. KN62, a CaMK II inhibitor, also significantly prevented the increase in protein synthesis, whereas it failed to affect the expression of c-fos. These results suggest that both calcineurin and CaMK II pathways are critical in the pressure overload-induced acceleration of protein synthesis, and that transcription of c-fos gene is regulated by calcineurin pathway but not by CaMK II pathway.

Is depressed myocyte contractility centrally involved in heart failure?

This review examines the evidence for and against the hypothesis that abnormalities in cardiac contractility initiate the heart failure syndrome and drive its progression. There is substantial evidence that the contractility of failing human hearts is depressed and that abnormalities of basal Ca2+ regulation and adrenergic regulation of Ca2+ signaling are responsible. The cellular and molecular defects that cause depressed myocyte contractility are not well established but seem to culminate in abnormal sarcoplasmic reticulum uptake, storage, and release. There are also strong links between Ca2+ regulation, Ca2+ signaling pathways, hypertrophy, and heart failure that need to be more clearly delineated. There is not substantial direct evidence for a causative role for depressed contractility in the initiation and progression of human heart failure, and some studies show that heart failure can occur without depressed myocyte contractility. Stronger support for a causal role for depressed contractility in the initiation of heart failure comes from animal studies where maintaining or improving contractility can prevent heart failure. Recent clinical studies in humans also support the idea that beneficial heart failure treatments, such as beta-adrenergic antagonists, involve improved contractility. Current or previously used heart failure treatments that increase contractility, primarily by increasing cAMP, have generally increased mortality. Novel heart failure therapies that increase or maintain contractility or adrenergic signaling by selectively modulating specific molecules have produced promising results in animal experiments. How to reliably implement these potentially beneficial inotropic therapies in humans without introducing negative side effects is the major unanswered question in this field.

Time-dependent systolic and diastolic function in mice overexpressing calcineurin

Echocardiograms have been assessed only at 56 days in mice overexpressing calcineurin (CN mice). Age-dependent echocardiographic changes were evaluated because the development of sudden death is time dependent. Because cyclosporin A (CsA) reverses hypertrophy in CN mice, its effects on the time course of the development of sudden death and cardiac dysfunction were assessed. In wild-type (WT) mice, the left ventricular (LV) internal end-diastolic dimension (LVIDd) increased and the LV mass index (LVMI) decreased with age. In CN mice, two distinct phases of pathophysiology were found. After 14 days, in CN mice, the LVIDd and LVMI were significantly increased, but sudden death had not occurred. After 28 days, in CN mice, relative dilation of the left ventricle occurred, whereas the LVMI decreased. Sudden death developed during progressive dilation associated with systolic and diastolic dysfunction. CsA treatment reversed hypertrophy in CN mice but did not reverse systolic and diastolic dysfunction and exaggerated sudden death. Sudden cardiac death was associated with systolic and diastolic dysfunction but was not related to isolated cardiac hypertrophy in CN mice.

Calcineurin is activated in diabetes and is required for glomerular hypertrophy and ECM accumulation

Diabetic nephropathy is characterized by the rapid onset of hypertrophy and ECM expansion. Previously, we showed that calcineurin phosphatase is required for hypertrophy and ECM synthesis in cultured mesangial cells. Therefore, we examined the effect of calcineurin inhibition on renal hypertrophy and ECM accumulation in streptozotocin-induced diabetic rats. After 2 wk of diabetes, calcineurin protein was increased in whole cortex and glomeruli in conjunction with increased phosphatase activity. Daily administration of cyclosporin A blocked accumulation of both calcineurin protein and calcineurin activity. Also associated with calcineurin upregulation was nuclear localization of the calcineurin substrate NFATc1. Inhibition of calcineurin reduced whole kidney hypertrophy and abolished glomerular hypertrophy in diabetic rats. Furthermore, calcineurin inhibition substantially reduced ECM accumulation in diabetic glomeruli but not in cortical tissue, suggesting a differential effect of calcineurin inhibition in glomerular vs. extraglomerular tissue. Corresponding increases in fibronectin mRNA and transforming growth factor-beta mRNA were observed in tubulointerstitium but not in glomeruli. In summary, calcineurin plays an important role in glomerular hypertrophy and ECM accumulation in diabetic nephropathy.

Effects of the calcineurin dependent signaling pathway inhibition by cyclosporin A on early and late cardiac remodeling following myocardial infarction

BACKGROUND

The calcineurin-mediated signaling pathway has been implicated as one of the crucial pathways in cardiac hypertrophy. However, the role of calcineurin pathway on cardiac remodeling after myocardial infarction (MI) has not been well defined.

METHODS

Infarcted rats (n = 45) were randomized into calcineurin inhibitor, cyclosporin A (CsA) or vehicle groups, 3 days after MI and treated for 2 weeks (early post-MI cardiac remodeling stage), or randomized 17 days after MI and treated for 2 weeks (late remodeling stage).

RESULTS

Calcineurin pathway inhibition during the early cardiac remodeling stage attenuated the myocardial hypertrophy after MI (P < 0.05). However, left ventricular dimensions were further increased and fractional shortening deteriorated with calcineurin inhibition during this stage (P < 0.05, each). During late remodeling stage, CsA treatment did not affect myocardial hypertrophy and cardiac dilation following MI.

CONCLUSION

Our results strongly support the hypothesis that calcineurin pathway mediates compensatory myocardial hypertrophy during the early remodeling stage after MI. However, the calcineurin pathway does not seem to affect the late remodeling after MI.

Calcineurin and cardiac hypertrophy: where have we been? Where are we going?

The heart is a dynamic organ capable of adapting its size and architecture in response to alterations in workload associated with developmental maturation, physiological stimulation and pathological diseases. Such alterations in heart size typically result from the hypertrophic growth of individual myocytes, but not myocyte cellular proliferation. In recent years, a great deal of investigation has gone toward elucidating the molecular signalling machinery that underlies the hypertrophic response and manner in which increased cardiac load promotes alterations in gene expression. To this end, the Ca(2+)-calmodulin-activated phosphatase calcineurin has been proposed as a necessary component of the multi-pathway hypertrophy program in the heart. Despite initial controversy over this hypothesis due to disparate results from pharmacological inhibitory studies in animal models of hypertrophy, compelling data from genetic models with calcineurin inhibition now exist. This review will summarize many of these studies and will attempt to address a number of unanswered issues. In particular, specific downstream mediators of calcineurin signalling will be discussed, as well as the need to identify calcineurin's temporal activation profile, transcriptional targets and cross-communication with other reactive signalling pathways in the heart. Finally, we will present evidence suggesting that calcineurin, as a Ca(2+)-responsive enzyme, may function as an internal load sensor in cardiac myocytes, matching output demands to hypertrophic growth.

Calcineurin and hypertrophic heart disease: novel insights and remaining questions

In the past 2 years, an emerging body of research has focused on a novel transcriptional pathway involved in the cardiac hypertrophic response. Ever since its introduction, the significance of the calcineurin-NFAT module has been subject of controversy. The aim of this review is to provide both an update on the current status of knowledge and discuss the remaining issues regarding the involvement of calcineurin in hypertrophic heart disease. To this end, the molecular biology of calcineurin and its direct downstream transcriptional effector NFAT are discussed in the context of the genetic studies that established the existence of this signaling paradigm in the heart. The pharmacological mode-of-action and specificity of the calcineurin inhibitors cyclosporine A (CsA) and FK506 is discussed, as well as their inherent limitations to study the biology of calcineurin. A critical interpretation is given on studies aimed at analyzing the role of calcineurin in cardiac hypertrophy using systemic immunosuppression. To eliminate the controversy surrounding CsA/FK506 usage, recent studies employed genetic inhibitory strategies for calcineurin, which confirm the pivotal role for this signal transduction pathway in the ventricular hypertrophy response. Finally, unresolved issues concerning the role of calcineurin in cardiac pathobiology are discussed based upon the information available, including its controversial role in cardiomyocyte viability, the reciprocal relationship between myocyte Ca(2+) homeostasis and calcineurin activity and the relative importance of calcineurin in relation to other hypertrophic signaling cascades.

Angiotensin II induced cardiac hypertrophy in vivo is inhibited by cyclosporin A in adult rats

Recently, the calcium-calmodulin-dependent calcineurin pathway has been defined as a central pathway for the induction of cardiac hypertrophy. The purpose of this study was to determine if cardiac hypertrophy in animals chronically treated with angiotensin II (AngII), could be prevented by blocking this pathway with cyclosporin A (CsA). Female Wistar rats were treated with AngII by subcutaneous infusion and injected twice a day with CsA (25 mg/kg) for 7 days. In the AngII treated group there was a 30% increase in the heart/body weight ratio (p < 0.05 vs. control). The increase in heart weight was blocked with CsA. Substantial increases in ANF and betaMHC gene expression were detected in the AngII treated animals, which were either attenuated or blocked with CsA treatment. Thus, this study demonstrates that CsA does prevent the development of cardiac hypertrophy in AngII treated rats, suggesting that the calcium-calmodulin-dependent calcineurin pathway is associated with angiotensin II induced hypertrophy in vivo.

Ultrasound imaging: principles and applications in rodent research

Ultrasound imaging utilizes the interaction of sound waves with living tissue to produce an image of the tissue or, in Doppler-based modes, determine the velocity of a moving tissue, primarily blood. These dynamic, real time images can be analyzed to obtain quantitative structural and functional information from the target organ. This versatile, noninvasive diagnostic tool is widely used and accepted in human and veterinary medicine. Until recently its application as a research tool was limited primarily to larger, nonrodent species. Due to advances in ultrasound imaging technology, commercially available ultrasound systems now have the spatial and temporal resolution to obtain accurate images of rat and mouse hearts, kidneys, and other target tissues including tumor masses. As a result, ultrasound imaging is being used more frequently as a research tool to image rats and mice, and particularly to evaluate cardiac structure and function. The developing technology of ultrasound biomicroscopy has even greater spatial resolution and has been used to evaluate developing mouse embryos and guide site-specific injections into mouse embryos. Additional ultrasound imaging technologies, including contrast-enhanced imaging and intravascular ultrasound transducers adapted for transesophageal use, have been utilized in rats and mice. This paper provides an overview of basic ultrasound principles, equipment, and research applications. The use of noninvasive ultrasound imaging in research represents both a significant refinement as a potential replacement for more invasive techniques and a significant advancement in research techniques to study rats and mice.

Cytoplasmic signaling pathways that regulate cardiac hypertrophy

This review discusses the rapidly progressing field of cardiomyocyte signal transduction and the regulation of the hypertrophic response. When stimulated by a wide array of neurohumoral factors or when faced with an increase in ventricular-wall tension, individual cardiomyocytes undergo hypertrophic growth as an adaptive response. However, sustained cardiac hypertrophy is a leading predictor of future heart failure. A growing number of intracellular signaling pathways have been characterized as important transducers of the hypertrophic response, including specific G protein isoforms, low-molecular-weight GTPases (Ras, RhoA, and Rac), mitogen-activated protein kinase cascades, protein kinase C, calcineurin, gp130-signal transducer and activator of transcription, insulin-like growth factor I receptor pathway, fibroblast growth factor and transforming growth factor beta receptor pathways, and many others. Each of these signaling pathways has been implicated as a hypertrophic transducer, which collectively suggests an emerging paradigm whereby multiple pathways operate in concert to orchestrate a hypertrophic response

[Genetics and molecular biology in cardiology (IV). Myocardial response to biomechanical stress]

Biomechanical stress of the myocardium is the situation resulting from hypoxia, hypertension, and other forms of myocardial injury, that invariably lead to increased demands for cardiac work and/or loss of functional myocardium. As a consequence of biomechanical stress a number of responses develop involving all the myocardial cells, namely cardiomyocytes. As a result some myocardial phenotypic changes develop that are initially compensatory (i.e., hypertrophy) but which may mediate the eventual decline in myocardial function that occurs with the transition from hypertrophy to failure in conditions of persistent stress (i.e., apoptosis and fibrosis). This review focuses on the steps involved in the response of the myocardium to biomechanical stress and highlights the most recent developments in the molecular mechanisms involved in the development of heart failure.