Meeting Koch's postulates for calcium signaling in cardiac hypertrophy

Effects of Long-Term Administration of Bovine Bone Gelatin Peptides on Myocardial Hypertrophy in Spontaneously Hypertensive Rats

The research purpose was to investigate the effects and the underlying molecular mechanisms of bovine bone gelatin peptides (BGP) on myocardial hypertrophy in spontaneously hypertensive rats (SHR). BGP relieved myocardial hypertrophy and fibrosis in SHR rats in a dose-dependent manner by reducing the left ventricular mass index, myocardial cell diameter, myocardial fibrosis area, and levels of myocardial hypertrophy markers (atrial natriuretic and brain natriuretic peptide). Label-free quantitative proteomics analysis showed that long-term administration of BGP changed the left ventricle proteomes of SHR. The 37 differentially expressed proteins in the high-dose BGP group participated in multiple signaling pathways associated with cardiac hypertrophy and fibrosis indicating that BGP could play a cardioprotective effect on SHR rats by targeting multiple signaling pathways. Further validation experiments showed that a high dose of BGP inhibited the expression of phosphoinositide 3-kinase (Pi3k), phosphorylated protein kinase B (p-Akt), and transforming growth factor-beta 1 (TGF-β1) in the myocardial tissue of SHR rats. Together, BGP could be an effective candidate for functional nutritional supplements to inhibit myocardial hypertrophy and fibrosis by negatively regulating the TGF-β1 and Pi3k/Akt signaling pathways.

BET bromodomain proteins regulate transcriptional reprogramming in genetic dilated cardiomyopathy

The bromodomain and extraterminal (BET) family comprises epigenetic reader proteins that are important regulators of inflammatory and hypertrophic gene expression in the heart. We previously identified the activation of proinflammatory gene networks as a key early driver of dilated cardiomyopathy (DCM) in transgenic mice expressing a mutant form of phospholamban (PLNR9C) - a genetic cause of DCM in humans. We hypothesized that BETs coactivate this inflammatory process, representing a critical node in the progression of DCM. To test this hypothesis, we treated PLNR9C or age-matched WT mice longitudinally with the small molecule BET bromodomain inhibitor JQ1 or vehicle. BET inhibition abrogated adverse cardiac remodeling, reduced cardiac fibrosis, and prolonged survival in PLNR9C mice by inhibiting expression of proinflammatory gene networks at all stages of disease. Specifically, JQ1 had profound effects on proinflammatory gene network expression in cardiac fibroblasts, while having little effect on gene expression in cardiomyocytes. Cardiac fibroblast proliferation was also substantially reduced by JQ1. Mechanistically, we demonstrated that BRD4 serves as a direct and essential regulator of NF-κB-mediated proinflammatory gene expression in cardiac fibroblasts. Suppressing proinflammatory gene expression via BET bromodomain inhibition could be a novel therapeutic strategy for chronic DCM in humans.

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.

Cerium nanoparticles synthesized using aqueous extract of Centella asiatica: characterization, determination of free radical scavenging activity and evaluation of efficacy against cardiomyoblast hypertrophy

Cerium nanoparticles synthesized usingCentella asiaticawere characterized, and tested for radical scavenging activities, cellular uptake, cytotoxicity and efficacy against cardiomyoblast hypertrophy and calcium overload.

Transient receptor potential channels contribute to pathological structural and functional remodeling after myocardial infarction

RATIONALE

The cellular and molecular basis for post-myocardial infarction (MI) structural and functional remodeling is not well understood.

OBJECTIVE

Our aim was to determine if Ca2+ influx through transient receptor potential canonical (TRPC) channels contributes to post-MI structural and functional remodeling.

METHODS AND RESULTS

TRPC1/3/4/6 channel mRNA increased after MI in mice and was associated with TRPC-mediated Ca2+ entry. Cardiac myocyte-specific expression of a dominant-negative (loss-of-function) TRPC4 channel increased basal myocyte contractility and reduced hypertrophy and cardiac structural and functional remodeling after MI while increasing survival in mice. We used adenovirus-mediated expression of TRPC3/4/6 channels in cultured adult feline myocytes to define mechanistic aspects of these TRPC-related effects. TRPC3/4/6 overexpression in adult feline myocytes induced calcineurin (Cn)-nuclear factor of activated T-cells (NFAT)-mediated hypertrophic signaling, which was reliant on caveolae targeting of TRPCs. TRPC3/4/6 expression in adult feline myocytes increased rested state contractions and increased spontaneous sarcoplasmic reticulum Ca2+ sparks mediated by enhanced phosphorylation of the ryanodine receptor. TRPC3/4/6 expression was associated with reduced contractility and response to catecholamines during steady-state pacing, likely because of enhanced sarcoplasmic reticulum Ca2+ leak.

CONCLUSIONS

Ca2+ influx through TRPC channels expressed after MI activates pathological cardiac hypertrophy and reduces contractility reserve. Blocking post-MI TRPC activity improved post-MI cardiac structure and function.

Differential metal content and gene expression in rat left ventricular hypertrophy due to hypertension and hyperactivity

The spontaneously hypertensive rat (SHR) has been studied extensively as a model of left ventricular hypertrophy (LVH) and associated cardiac dysfunction due to hypertension (HT). The SHR also possesses a hyperactive trait (HA). Crossbreeding SHR with Wistar-Kyoto (WKY) control rats, which are nonHT and nonHA, followed by selected inbreeding produced two additional homozygous strains: WKHT and WKHA, in which the traits of HT and HA, respectively, are expressed separately. WKHT, WKHA and SHR all display LVH, but only the SHR exhibits cardiac dysfunction. We hypothesized that cardiac dysfunction in the SHR is uniquely characterized by calcium overload. We measured total cardiac Ca, Cu, Fe, K, Mg and Zn in the four strains. We found elevated Ca and depressed Cu, Mg and Zn with HT, but not unique to SHR. We surmise that HT promotes aberrant regulation of cardiac Ca(2+), Cu(2+), Mg(2+) and Zn(2+), which does not necessarily result in cardiac dysfunction. Interestingly, Cu was elevated in HA strains compared to nonHA counterparts. We then analyzed gene expression as mRNA of Cu-containing proteins, most notably mitochondrial-Cox, Dbh, Lox, Loxl1, Loxl2, Sod1 and Tyr. The gene expression profiles of Lox, Loxl1, Loxl2 and Sod1 were found especially high in the WKHA, which if reflective of protein content could account for the high Cu content in the WKHA. The mRNA of other genes, notably Mb, Fxyd1, Maoa and Maob were also examined. We found that Maoa gene expression and monoamine oxidase-A (MAO-A) protein content were low in the SHR compared to the other strains. The finding that MAO-A protein is low in the SHR and normal in the WKHT and WKHA strains is most consistent with the idea that MAO-A protects against the development of cardiac dysfunction in LVH but not against LVH in these rats.

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

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

Molecular changes in the early phase of renin-dependent cardiac hypertrophy in hypertensive cyp1a1ren-2 transgenic rats

An early response to high arterial pressure is the development of cardiac hypertrophy. Functional and transcriptional regulation of ion channels and Ca(2+) handling proteins are involved in this process but the relative contribution of each is unclear. In this study, we investigated the expression of genes involved in action potential generation and Ca(2+) homeostasis of cardiomyocytes in hypertensive cyp1a1ren-2 transgenic rats. In this model, the transgene prorenin was induced by indole-3-carbinol for 2 weeks allowing the induction of hypertension. Electrophysiological recordings from cardiomyocytes of hypertensive rats revealed a slight increase in membrane capacitance consistent with cellular hypertrophy. L-type calcium current density was reduced by 30%. Left ventricles of hypertensive rats showed a significant increase in transcript and protein levels of the cation channel TRPC6 and FK506-binding protein, whereas levels of SERCA2 and voltage-dependent potassium channels K(v)4.2 and K(v)4.3 were found to be decreased. Further, a marked nuclear localization of the transcription factors GATA4 and NFATC4 was observed in cardiac tissue of hypertensive rats. The cyp1a1ren-2 transgenic rat thus appears to be a valid model to investigate early changes in cardiac hypertrophy. This study points to roles for TRPC6, FK506BP, SERCA2, K(v)4.2, and K(v)4.3 in the development of cardiac hypertrophy.

Calcineurin mediates bladder wall remodeling secondary to partial outlet obstruction

We hypothesized that the calcineurin-nuclear factor of activated T-cells (NFAT) pathway is activated following partial bladder outlet obstruction (pBOO), which would allow for pharmacologic treatment to prevent the ensuing bladder wall hypertrophy. Using a model of pBOO in male mice, we were able to demonstrate increased nuclear importation of the transcription factors NFAT and myocyte enhanching factor 2 both of which are under control of calcineurin in both the whole bladder wall as well as the urothelium. We further confirmed that this pathway was activated using transgenic mice containing an NFAT-luciferase reporter construct. Mice were randomized following pBOO to treatment with or without cyclosporine A (CsA), a known inhibitor of calcineurin. The bladder-to-body mass ratio (mg bladder wt/g body wt) of 0.95 ± 0.03 in shams increased to 3.1 ± 0.35 following pBOO, and it dropped back to 1.7 ± 0.22 in the CsA+ group (P < 0.001). Luciferase values (RLU) of 1,130 ± 133 in shams increased to 2,010 ± 474 following pBOO and were suppressed to 562 ± 177 in the CsA+ group (P < 0.05). The myosin heavy chain mRNA (A/B) isoform ratio of 0.07 ± 0.03 in shams increased to 1.04 ± 0.19 following pBOO but it diminished to 0.24 ± 0.1 in the CsA+ group (P < 0.001). In vitro whole organ physiology studies demonstrated improved responses in those bladders from mice treated with CsA. The mRNAs for all four known calcineurin-responsive NFAT isoforms are expressed in the bladder wall, although NFATc(3) and NFATc(4) predominate. Both NFATc3 and NFATc4 are expressed in urothelial as well as smooth muscle cells. We conclude that pBOO activates the calcineurin-NFAT pathway and that CsA treatment decreased bladder hypertrophy, shifted the pattern of myosin isoform mRNA expression back toward that seen in normal controls, and resulted in improved in vitro whole organ performance.

Endoplasmic reticulum-mitochondria crosstalk in NIX-mediated murine cell death

Transcriptional upregulation of the proapoptotic BCL2 family protein NIX limits red blood cell formation and can cause heart failure by inducing cell death, but the requisite molecular events are poorly defined. Here, we show complementary mechanisms for NIX-mediated cell death involving direct and ER/sarcoplasmic reticulum-mediated (ER/SR-mediated) mitochondria disruption. Endogenous cardiac NIX and recombinant NIX localize both to the mitochondria and to the ER/SR. In genetic mouse models, cardiomyocyte ER/SR calcium stores are proportional to the level of expressed NIX. Whereas Nix ablation was protective in a mouse model of apoptotic cardiomyopathy, genetic correction of the decreased SR calcium content of Nix-null mice restored sensitivity to cell death and reestablished cardiomyopathy. Nix mutants specific to ER/SR or mitochondria activated caspases and were equally lethal, but only ER/SR-Nix caused loss of the mitochondrial membrane potential. These results establish a new function for NIX as an integrator of transcriptional and calcium-mediated signals for programmed cell death.

Histone deacetylase inhibition in the treatment of heart disease

Recent work has demonstrated the importance of chromatin remodeling, especially histone acetylation, in the control of gene expression in the heart. Studies in preclinical models suggest that inhibition of histone deacetylase (HDAC) activity - using compounds that show promise in ongoing oncology trials - blunts pathologic growth of cardiac myocytes. Indeed, small-molecule inhibitors of HDACs are members of an evolving class of pharmacologic agents in development for the treatment of several diseases. If proved effective in the treatment of heart disease, HDAC inhibitors could have a significant impact on public health, as cardiovascular disease remains the leading cause of death in the US. This paper reviews understanding of the mechanisms of action of HDAC inhibitors in the heart and summarizes emerging data regarding their effects on disease-related cardiac remodeling and function.

The Ca2+ ATPase of cardiac sarcoplasmic reticulum: Physiological role and relevance to diseases

The Ca(2+) ATPase of sarcoplasmic reticulum has a prominent role in excitation/contraction coupling of cardiac muscle, as it induces relaxation by sequestering Ca(2+) from the cytoplasm. The stored Ca(2+) is in turn released to trigger contraction. We review here experiments demonstrating that in cardiac myocytes Ca(2+) signaling and contractile activation are strongly altered by pharmacological inhibition or transcriptional down-regulation of SERCA. On the other hand, kinetics, and intensity of Ca(2+) signaling are improved by SERCA overexpression following delivery of exogenous cDNA by adenovirus vectors. Experiments on adrenergic hypertrophy demonstrate SERCA down-regulation, consistent with its pathogenetic involvement in cardiac hypertrophy and failure, as also shown in other experimental models and clinical studies. Compensation by alternate Ca(2+) signaling proteins, including functional activation and increased expression of Na(+)/Ca(2+) exchanger and TRPC proteins has been observed. These compensatory mechanisms, including calcineurin activation, remain to be clarified and are a most important subject of current studies.

Phenylephrine hypertrophy, Ca2+-ATPase (SERCA2), and Ca2+ signaling in neonatal rat cardiac myocytes

We endeavored to use a basic and well-controlled experimental system to characterize the extent and time sequence of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) involvement in the development of cardiac hypertrophy, including transcription, protein expression, Ca(2+) transport, and cytoplasmic Ca(2+) signaling. To this end, hypertrophy of neonatal rat cardiac myocytes in culture was obtained after adrenergic activation with phenylephrine (PE). Micrographic assessment of myocyte size, rise of [(14)C]phenylalanine incorporation and total protein expression, and increased transcription of atrial natriuretic factor demonstrated unambiguously the occurrence of hypertrophy. An early and prominent feature of hypertrophy was a reduction of the SERCA2 transcript, as determined by RT-PCR with reference to a stable marker such as glyceraldehyde-3-phosphate dehydrogenase. Reduction of Ca(2+)-ATPase protein levels and Ca(2+) transport activity to approximately 50% of control values followed with some delay, evidently as a consequence of a primary effect on transcription. Cytosolic Ca(2+) signaling kinetics, measured with a Ca(2+)-sensitive dye after electrical stimuli, were significantly altered in hypertrophic myocytes. However, the effect of PE hypertrophy on cytosolic Ca(2+) signaling kinetics was less prominent than observed in myocytes subjected to drastic SERCA2 downregulation with small interfering RNA or inhibition with thapsigargin (10 nM). We conclude that SERCA2 undergoes significant downregulation after hypertrophic stimuli, possibly due to lack of SERCA gene involvement by the hypertrophy transcriptional program. The consequence of SERCA2 downregulation on Ca(2+) signaling is partially compensated by alternate Ca(2+) transport mechanisms. These alterations may contribute to a gradual onset of functional failure in long-term hypertrophy.

CaMKII, an emerging molecular driver for calcium homeostasis, arrhythmias, and cardiac dysfunction

Maintenance of cytoplasmic calcium homeostasis is critical for all cells. An exciting field has emerged in elucidating the multiple roles that Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) plays in regulating Ca(2+) cycling in normal cardiac myocytes and in pathophysiological states. Moreover, CaMKII was recently identified as a potential drug target in cardiac disease. This work has given us a closer view of the complexity and therapeutic possibilities of CaMKII regulation of Ca(2+) signaling in cardiac myocytes.

Changes of expression of stretch-activated potassium channel TREK-1 mRNA and protein in hypertrophic myocardium

The expression of stretch-activated potassium channel TREK-1 mRNA and protein of hypertrophic myocardium was measured. Using a model of hypertrophy induced by coarctation of abdominal aorta in male Wistar rats, the expression of TREK-1 mRNA and protein was detected by using semi-quantitative RT PCR and Western blot respectively. At 4th and 8th week after constriction of the abdominal aorta, rats developed significant left ventricular hypertrophy. As compared to sham-operated group, stretch-activated potassium channel TREK-1 mRNA was strongly expressed and protein was up-regulated in operation groups (P < 0.05). It was concluded that the expression of TREK-1 was up-regulated in hypertrophic myocardium induced by chronic pressure overload in Wistar rats.

Overexpression of calmodulin induces cardiac hypertrophy by a calcineurin-dependent pathway

The possible role of calcineurin in cardiac hypertrophy induced by calmodulin (CaM) overexpression in the heart was investigated. CaM transgenic (CaM-TG) mice developed marked cardiac hypertrophy and exhibited up-regulation of atrial natriuretic factor (ANF) and beta-myosin heavy chain gene expression in the heart during the first 2 weeks after birth. The activity of calcineurin in the heart was also significantly increased in CaM-TG mice compared with wild-type littermates. Treatment of CaM-TG mice with the calcineurin inhibitor FK506 (1mg/kg per day) prevented the increase in the heart-to-body weight ratio as well as that in cardiomyocyte width. FK506 also inhibited the induction of fetal-type cardiac gene expression in CaM-TG mice. Overexpression of CaM in cultured rat cardiomyocytes activated the ANF gene promoter in a manner sensitive to FK506. Activation of a calcineurin-dependent pathway thus contributes to the development of cardiac hypertrophy induced by CaM overexpression in the heart.

Preeclampsia: a renal perspective

Preeclampsia is a syndrome that affects 5% of all pregnancies, producing substantial maternal and perinatal morbidity and mortality. The aim of this review is to summarize our current understanding of the pathogenesis of preeclampsia with special emphasis on the recent discovery that circulating anti-angiogenic proteins of placental origin may play an important role in the pathogenesis of proteinuria and hypertension of preeclampsia.

Does load-induced ventricular hypertrophy progress to systolic heart failure?

Ventricular hypertrophy develops in response to numerous forms of cardiac stress, including pressure or volume overload, loss of contractile mass from prior infarction, neuroendocrine activation, and mutations in genes encoding sarcomeric proteins. Hypertrophic growth is believed to have a compensatory role that diminishes wall stress and oxygen consumption, but Framingham and other studies established ventricular hypertrophy as a marker for increased risk of developing chronic heart failure, suggesting that hypertrophy may have maladaptive features. However, the relative contribution of comorbid disease to hypertrophy-associated systolic failure is unknown. For instance, coronary artery disease is induced by many of the same risk factors that cause hypertrophy and can itself lead to systolic dysfunction. It is uncertain, therefore, whether ventricular hypertrophy commonly progresses to systolic dysfunction without the contribution of intervening ischemia or infarction. In this review, we summarize findings from epidemiologic studies, preclinical experiments in animals, and clinical trials to lay out what is known—and not known—about this important question.

Neuropeptide Y induces cardiomyocyte hypertrophy via calcineurin signaling in rats

Neuropeptide Y (NPY) has been shown to participate in cardiac hypertrophy. However, the mechanisms by which NPY induces cardiomyocyte hypertrophy are poorly understood. This study tested the hypothesis that NPY induces cardiomyocyte hypertrophy through Ca2+/CaM-dependent calcineurin (CaN) pathway in cultured neonatal rat cardiomyocytes. After 24-h treatment, NPY (100 nM) significantly increased 3H-leucine incorporation and c-Jun mRNA expression, concomitant with augment of CaN activity and protein level in cardiomyocytes compared to those cells without NPY treatment. The enhancement of 3H-leucine incorporation and c-Jun mRNA expression in cardiomyocytes treated with NPY were markedly inhibited by cyclosporine A (CsA), a selective inhibitor of CaN. We also investigated the effect of NPY on intracellular Ca2+ level in cardiomyocytes. There were no obvious changes in intracellular Ca2+ level of cytoplasm and nucleus in cardiomyocytes treated with NPY (100 nM) for 10 min. However, NPY significantly increased intracellular Ca2+ level of cytoplasm and nucleus in cardiomyocytes after 24-h treatment. The result suggested that NPY could induce hypertrophy of cardiomyocytes via Ca2+/CaM-dependent CaN signal pathway. The enhancement of [Ca2+]i caused by NPY may activate CaN signal pathways to mediate cardiac hypertrophy.

Genetic manipulation of calcium-handling proteins in cardiac myocytes. I. Experimental studies

Two genetic experimental approaches, de novo expression of parvalbumin (Parv) and overexpression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2a), have been shown to increase relaxation rates in myocardial tissue. However, the relative effect of Parv and SERCA2a on systolic function and on β-adrenergic responsiveness at varied pacing rates is unknown. We used gene transfer in isolated rat adult cardiac myocytes to gain a fuller understanding of Parv/SERCA2a function. As demonstrated previously, when Parv is expressed in elevated concentration (>0.1 mM), the transduced myocytes showed a reduction in sarcomere-shortening amplitude: 129 ± 17, 81 ± 8, and 149 ± 14 nm for control, Parv, and SERCA2a, respectively. At physiological temperature, shortening amplitude responses of Parv and SERCA2a myocytes to the β-adrenergic agonist isoproterenol (Iso) were not statistically different from that of control myocytes. However, in SERCA2a myocytes, in which baseline was slightly elevated and the Iso-stimulated value was slightly lower, the increase in shortening was slightly less than in Parv or control myocytes: 108 ± 14, 169 ± 39, and 34 ± 12% for control, Parv, and SERCA2a, respectively. In another test set, Parv myocytes had the strongest early postrest potentiation among all groups studied (rest time = 2–10 s), and SERCA2a myocytes were the least sensitive to variations in stimulation rhythm. To replicate the deficient Ca2+ removal observed in heart failure, we used 150 nM thapsigargin. Under these conditions, control myocytes exhibited slowed relaxation, whereas Parv myocytes retained their rapid kinetics, showing that Parv is still able to control relaxation, even when SERCA2a function is impaired.

Ca2+ Controls Functional Expression of the Cardiac K+ Transient Outward Current via the Calcineurin Pathway

The transient outward K+ current (Ito) modulates transmembrane Ca2+ influx into cardiomyocytes, which, in turn, might act on Ito. Here, we investigated whether Ca2+ modifies functional expression of Ito. Whole-cell Ito were recorded using the patch clamp technique in single right ventricular myocytes isolated from adult rats and incubated for 24 h at 37 degrees C in a serum-free medium containing various Ca2+ concentrations ([Ca2+]o). Increasing the [Ca2+]o from 0.5 to 1.0 and 2.5 mM produced a gradual decrease in Ito density without change in current kinetics. Quantitativereverse transcriptase-PCR showed that a decrease of the Kv4.2 mRNA could account for this decrease. In the acetoxymethyl ester form of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM)-loaded myocytes (a permeant Ca2+ chelator), Ito density increased significantly when cells were exposed for 24 h to either 1 or 2.5 mM [Ca2+]o. Moreover, 24-h exposure to the Ca2+ channel agonist, Bay K8644, in 1 mM [Ca2+]o induced a decrease in Ito density, whereas the Ca2+ channel antagonist, nifedipine, blunted Ito decrease in 2.5 mM [Ca2+]o. The decrease of Ito in 2.5 mM [Ca2+]o was also prevented by co-incubation with either the calmodulin inhibitor W7 or the calcineurin inhibitors FK506 or cyclosporin A. Furthermore, in myocytes incubated for 24 h with 2.5 mM [Ca2+]o, calcineurin activity was significantly increased compared with 1 mM [Ca2+]o. Our data suggest that modulation of [Ca2+]i via L-type Ca2+ channels, which appears to involve the Ca2+/calmodulin-regulated protein phosphatase calcineurin, down-regulates the functional expression of Ito. This effect might be involved in many physiological and pathological modulations of Ito channel expression in cardiac cells, as well other cell types.

Genetic manipulation of calcium-handling proteins in cardiac myocytes. II. Mathematical modeling studies

We developed a mathematical model specific to rat ventricular myocytes that includes electrophysiological representation, ionic homeostasis, force production, and sarcomere movement. We used this model to interpret, analyze, and compare two genetic manipulations that have been shown to increase myocyte relaxation rates, parvalbumin (Parv) de novo expression, and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) overexpression. The model was used to seek mechanistic insights into 1) the relative contribution of two mechanisms by which SERCA2a overexpression modifies Ca2+ sequestration, i.e., more pumps and an increase in the SERCA2a-to-phospholamban ratio, 2) the mechanisms behind postrest potentiation and how Parv and SERCA2a influence this response, and 3) why Parv myocytes retain their fast kinetics when endogenous SERCA2a is partially impaired by thapsigargin (a condition used to mimic diastolic dysfunction). The model was also utilized to predict whether Parv metal-binding characteristics might be modified to improve diastolic and systolic functions and whether Parv or SERCA2a might affect diastolic Ca2+ levels and myocyte energetics. One outcome of the model was to demonstrate a higher peak and total ATP consumption in SERCA2a myocytes and more even distribution of ATP throughout the cardiac cycle in Parv myocytes. This may have implications for failing hearts that are energetically compromised.

Cardiac expression of Gal4 causes cardiomyopathy in a dose-dependent manner

Cardiac expression of a transgene is a common approach for determining the role of gene products in the processes underlying cardiomyopathy and heart failure (HF). We have generated transgenic mice that express the 'harmless' yeast transcription factor Gal4 in the heart under control of the alpha-myosin heavy chain promoter and found that expression of this gene causes cardiomyopathy and HF, the severity of which correlated with the number of copies of the transgene integrated into the genome and with the expression level. A line with a single copy of the transgene targeted to the hprt locus correctly expressed the transgene but did not develop cardiomyopathy. Our results indicate that expression of a transgene in the heart may non-specifically cause HF in a dose-dependent manner.

Flow in the early embryonic human heart: a numerical study

Computational fluid dynamic (CFD) experimentation provides a unique medium for detailed examination of flow through complex embryonic heart structures. The purpose of this investigation was to demonstrate that streaming blood flow patterns exist in the early embryonic heart and that fluid surface stresses change significantly with anomalous alterations in fetal heart lumen shape. Stages 10 and 11 early human embryo hearts were digitized as calibrated two-dimensional (2D) cross-sectional sequential images. A 3D surface was constructed from the stacking of these 2D images. CFD flow solutions were obtained (steady and pulsatile flow). Particle traces were placed in the inlet and outlet portions of these two stages. Sections of the embryonic heart were artificially reshaped. CFD flow solutions were obtained and surface stress changes analyzed. Streaming was shown to exist, with particles released on one or the other side of the cardiac lumen tending not to cross over and mix with particles released from the opposite side of the cardiac lumen. Shear stress changes (stage 10) occur in the altered lumens. Streaming exists in steady and pulsatile flow scenarios in the embryonic heart models. There are differences in local shear stress distributions with surface shape anomalies of the fetal heart lumen. These observations may help shed light on the potential role of fluid dynamic factors in determining patterns of abnormal heart development.

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.

Alterations in mitochondrial function in a mouse model of hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease characterized by varying degrees of ventricular hypertrophy and myofibrillar disarray. Mutations in cardiac contractile proteins cause HCM. However, there is an unexplained wide variability in the clinical phenotype, and it is likely that there are multiple contributing factors. Because mitochondrial dysfunction has been described in heart disease, we tested the hypothesis that mitochondrial dysfunction contributes to the varying HCM phenotypes. Mitochondrial function was assessed in two transgenic models of HCM: mice with a mutant myosin heavy chain gene (MyHC) or with a mutant cardiac troponin T (R92Q) gene. Despite mitochondrial ultrastructural abnormalities in both models, the rate of state 3 respiration was significantly decreased only in the mutant MyHC mice by approximately 23%. Notably, this decrease in state 3 respiration preceded hemodynamic dysfunction. The maximum activity of alpha-ketogutarate dehydrogenase as assayed in isolated disrupted mitochondria was decreased by 28% compared with isolated control mitochondria. In addition, complexes I and IV were decreased in mutant MyHC transgenic mice. Inhibition of beta-adrenergic receptor kinase, which is elevated in mutant MyHC mouse hearts, can prevent mitochondrial respiratory impairment in mutant MyHC mice. Thus our results suggest that mitochondria may contribute to the hemodynamic dysfunction seen in some forms of HCM and offer a plausible mechanism responsible for some of the heterogeneity of the disease phenotypes.

[Ion transporters and cardiovascular diseases: pH control or modulation of intracellular calcium concentration]

The regulation of the intracellular pH is under tight control by several ion transport systems including the sodium-proton exchanger, the sodium-bicarbonate cotransporter and the chlore-bicarbonate anion exchanger. While the activation of the anion exchange induces a cellular acidification, both the sodium-proton exchanger and the sodium-bicarbonate cotransporter are responsible for a protection against acidosis by extruding protons or importing bicarbonate. These transporters are transmembrane proteins whose activity is regulated by several mechanisms including phosphorylation, calcium binding and which are involved in several pathophysiologic processes such as ischemia, hypertrophy and arrhythmias. Recent studies suggest that the activation of these transporters during various diseases induces an increase in intracellular calcium concentration. Therefore, inhibiting these transporters could represent novel therapeutic strategies for the treatment of cardiovascular diseases.

Activation of nuclear factor-kappaB is necessary for myotrophin-induced cardiac hypertrophy

The transcription factor nuclear factor-kappaB (NF-kappaB) regulates expression of a variety of genes involved in immune responses, inflammation, proliferation, and programmed cell death (apoptosis). Here, we show that in rat neonatal ventricular cardiomyocytes, activation of NF-kappaB is involved in the hypertrophic response induced by myotrophin, a hypertrophic activator identified from spontaneously hypertensive rat heart and cardiomyopathic human hearts. Myotrophin treatment stimulated NF-kappaB nuclear translocation and transcriptional activity, accompanied by IkappaB-alpha phosphorylation and degradation. Consistently, myotrophin-induced NF-kappaB activation was enhanced by wild-type IkappaB kinase (IKK) beta and abolished by the dominant-negative IKKbeta or a general PKC inhibitor, calphostin C. Importantly, myotrophin-induced expression of two hypertrophic genes (atrial natriuretic factor [ANF] and c-myc) and also enhanced protein synthesis were partially inhibited by a potent NF-kappaB inhibitor, pyrrolidine dithio-carbamate (PDTC), and calphostin C. Expression of the dominant-negative form of IkappaB-alpha or IKKbeta also partially inhibited the transcriptional activity of ANF induced by myotrophin. These findings suggest that the PKC-IKK-NF-kappaB pathway may play a critical role in mediating the myotrophin-induced hypertrophic response in cardiomyocytes.

Hyperglycemia inhibits capacitative calcium entry and hypertrophy in neonatal cardiomyocytes

Hyperglycemia alters cardiac function and often leads to diabetic cardiomyopathy as cardiomyocyte apoptosis causes a hypertrophied heart to deteriorate to dilation and failure. Paradoxically, many short-term animal models of hyperglycemia protect against ischemia-induced damage, including apoptosis, by limiting Ca(2+) overload. We have determined that, like nonexcitable cells, both neonatal and adult cardiomyocytes respond to depletion of sarcoplasmic/endoplasmic reticulum Ca(2+) stores with an influx of extracellular Ca(2+) through channels distinct from voltage-gated Ca(2+) channels, a process termed capacitative Ca(2+) entry (CCE). Here, we demonstrate that in neonatal rat cardiomyocytes, hyperglycemia decreased CCE induced by angiotensin II or the Ca(2+)ATPase inhibitor thapsigargin. Hyperglycemia also significantly blunted Ca(2+)-dependent hypertrophic responses by approximately 60%, as well as the Ca(2+)-sensitive nuclear translocation of a chimeric protein bearing the nuclear localization signal of a nuclear factor of activated T-cells transcription factor. The attenuation of CCE by hyperglycemia was prevented by azaserine, an inhibitor of hexosamine biosynthesis, and partially by inhibitors of oxidative stress. This complements previous work showing that increasing hexosamine metabolites in neonatal cardiomyocytes also inhibited CCE. The inhibition of CCE by hyperglycemia thus provides a likely explanation for the transition to diabetic cardiomyopathy as well as to the protection afforded to injury after ischemia/reperfusion in diabetic models.

Prevention of hypertrophy by overexpression of Kv4.2 in cultured neonatal cardiomyocytes

BACKGROUND

Prolonged action potentials (APs) and decreased transient outward K+ currents (I(to)) are consistent findings in hypertrophic myocardium. However, the connection of these changes with cardiac hypertrophy is unknown. The present study investigated the effects of changes in I(to) and the associated alterations in AP on myocyte hypertrophy induced by phenylephrine.

METHODS AND RESULTS

Chronic incubation of cultured neonatal ventricular rat myocytes (NVRMs) with phenylephrine (PE) reduced I(to) density and prolonged AP duration, leading to a 2-fold increase in the net Ca2+ influx per beat and a 1.4-fold increase in Ca2+-transient amplitude. PE treatment of chronically paced (2-Hz) NVRM also induced increases in cell size, protein/DNA ratio, atrial natriuretic factor mRNA expression, as well as beta/alpha myosin mRNA ratio. These hypertrophic changes were associated with a 2.4-fold increase in activation of nuclear factor of activated T-cells (NFAT), indicating increased activity of the Ca2+-dependent phosphatase calcineurin. Overexpression of Kv4.2 channels using adenovirus prevented the AP duration prolongation as well as the increases in Ca2+ influx and Ca2+-transient amplitude induced by PE. Kv4.2 overexpression also prohibited the PE-induced increases in cell size, protein/DNA ratio, atrial natriuretic factor expression, beta/alpha myosin mRNA ratio, and NFAT activation.

CONCLUSIONS

Our results demonstrate that PE-mediated hypertrophy in NRVMs seems to require I(to) reductions and AP prolongation associated with increased Ca2+ influx and Ca2+ transients as well as calcineurin activation. The clinical implications of these studies and the possible involvement of other signaling pathways are discussed.

Capacitative calcium entry contributes to nuclear factor of activated T-cells nuclear translocation and hypertrophy in cardiomyocytes

In nonexcitable cells, depletion of endoplasmic reticulum Ca(2+) stores leads to activation of plasma membrane Ca(2+) channels, a process termed capacitative Ca(2+) entry. Here, we demonstrate that this pathway functions in cells that also contain voltage-gated Ca(2+) channels, neonatal rat ventricular myocytes. The depletion of sarcoplasmic reticulum Ca(2+) stores elicited a prolonged increase in cytoplasmic Ca(2+) dependent on extracellular Ca(2+). Inhibitors of store-operated channels but not L-type channels diminished this response. The importance of this pathway to cardiac hypertrophy, which often is dependent on Ca(2+)/calmodulin-dependent transcription factors, was also assessed in this model. Hypertrophy and atrial natriuretic factor expression induced by angiotensin II or phenylephrine was more effectively attenuated by inhibitors of capacitative entry than of L-type channels. Additionally, cardiomyocytes were transfected with a construct encoding a fluorescent nuclear factor of activated T-cells chimeric protein to follow nuclear localization in response to thapsigargin, angiotensin II, and phenylephrine. This translocation was completely prevented by inhibitors of capacitative Ca(2+) entry and only partially abrogated by inhibitors of L-type channels. In contrast, a hypertrophic response induced by overexpression of the transcription factor MEK1 was unaffected by inhibitors of capacitative entry. Together, these data suggest a role for CCE in cardiomyocyte physiology and, in particular, in Ca(2+)-mediated cardiac hypertrophy.

Activated glycogen synthase-3 beta suppresses cardiac hypertrophy in vivo

The adult myocardium responds to a variety of pathologic stimuli by hypertrophic growth that frequently progresses to heart failure. The calcium/calmodulin-dependent protein phosphatase calcineurin is a potent transducer of hypertrophic stimuli. Calcineurin dephosphorylates members of the nuclear factor of activated T cell (NFAT) family of transcription factors, which results in their translocation to the nucleus and activation of calcium-dependent genes. Glycogen synthase kinase-3 (GSK-3) phosphorylates NFAT proteins and antagonizes the actions of calcineurin by stimulating NFAT nuclear export. To determine whether activated GSK-3 can act as an antagonist of hypertrophic signaling in the adult heart in vivo, we generated transgenic mice that express a constitutively active form of GSK-3 beta under control of a cardiac-specific promoter. These mice were physiologically normal under nonstressed conditions, but their ability to mount a hypertrophic response to calcineurin activation was severely impaired. Similarly, cardiac-specific expression of activated GSK-3 beta diminished hypertrophy in response to chronic beta-adrenergic stimulation and pressure overload. These findings reveal a role for GSK-3 beta as an inhibitor of hypertrophic signaling in the intact myocardium and suggest that elevation of cardiac GSK-3 beta activity may provide clinical benefit in the treatment of pathologic hypertrophy and heart failure.

Compensatory changes in Ca(2+) and myocardial O(2) consumption in beta-tropomyosin transgenic hearts

Transgenic mice overexpressing beta-tropomyosin have increased myofilament Ca(2+) sensitivity that we hypothesized would result in altered relationships among pressure and heart rates, intracellular Ca(2+), and myocardial O(2) consumption. In perfused hearts from transgenic mice there was a marked negative force-frequency response between 6 and 10 Hz with a 30 +/- 3% reduction in peak-positive first derivative of pressure development over time (dP/dt) compared with 14 +/- 2% in wild-type mice (P < 0.001). At 8 Hz systolic pressures were normal, though peak systolic intracellular Ca(2+) was significantly reduced in transgenic mice versus wild type (726 +/- 61 vs. 936 +/- 67 nM, P < 0.05) indicating an alteration in the pressure-Ca(2+) relationship. Over a wide range of positive and negative inotropic interventions there were normal developed pressures, though marked elevations in myocardial O(2) consumption (15-54%). Because pressures are normal and intracellular Ca(2+) decreased and myocardial O(2) consumption increased, this suggests that these abnormalities are at least in part compensatory mechanisms to the altered myofilament function.

Electrical remodeling in pressure-overload cardiac hypertrophy: role of calcineurin

BACKGROUND

Myocyte hypertrophy accompanies many forms of heart disease, but its contribution to electrical remodeling is unknown.

METHODS AND RESULTS

We studied mouse hearts subjected to pressure overload by surgical thoracic aortic banding. In unbanded control hearts, action potential duration (APD) was significantly longer in subendocardial myocytes compared with subepicardial myocytes. Hypertrophy-associated APD prolongation was significantly greater in subendocardial myocytes compared with subepicardial myocytes, indicating stress-induced amplification of repolarization dispersion. To investigate the underlying basis, we performed voltage-clamp recordings on dissociated myocytes. Under control unoperated conditions, subendocardial myocytes exhibited significantly less transient outward current (I(to)) than did subepicardial cells. Hypertrophy was not associated with significant changes in I(to), sustained current, or inward rectifier current densities, but peak L-type Ca(2+) current density (I(Ca,L)) increased 26% (P<0.05). Recovery from I(Ca,L) inactivation was accelerated in hypertrophied myocytes. Inhibition of calcineurin with cyclosporin A prevented increases in heart mass and myocyte size but was associated with an intermediate APD. The hypertrophy-associated increase in I(Ca,L) and the accelerated recovery from inactivation were blocked by cyclosporin A.

CONCLUSIONS

These data reveal regional variation in the electrophysiological response within the left ventricle by way of a mechanism involving upregulated Ca(2+) current and calcineurin. Furthermore, these results reveal partial uncoupling of electrophysiological and structural remodeling in hypertrophy.

The decompensated detrusor V: molecular correlates of bladder function after reversal of experimental outlet obstruction

PURPOSE

Calcium ion homeostasis has a significant role in smooth muscle function. Its regulation requires complex storage and release mechanisms via ion pumps and channels located within intracellular storage sites (sarcoplasmic reticulum) and at the plasma membrane. We have previously reported a dramatic loss of the 2 major sarcoplasmic reticulum proteins sarcoplasmic endoplasmic reticulum calcium magnesium adenosine triphosphatase (SERCA2) and the ryanodine sensitive ion channel, also called the ryanodine receptor, after outlet obstruction. In our current study we investigated the correlation of the expression of these 2 major sarcoplasmic reticulum components with bladder function recovery after the reversal of outlet obstruction.

METHODS AND METHODS

Standard partial bladder outlet obstruction was created in adult New Zealand White rabbits. Voiding patterns were monitored 2 and 4 weeks postoperatively, and rabbits were selected for outlet obstruction reversal based on a voiding pattern consistent with a decompensated state, as indicated by a frequency of greater than 30 voids daily and an average voided volume of less than 4 cc. Bladder biopsy was done when outlet obstruction was reversed. Voiding performance was monitored postoperatively and the animals were sacrificed 2 weeks later. Voiding patterns and muscle strip studies enabled us to define 2 functional outcome categories after reversal, namely normal versus minimally improved. Microsomal membrane protein fractions were prepared from the same bladder tissues before and after reversal, and probed by Western blot analysis for SERCA2 and ryanodine receptor expression.

RESULTS

Western blot analysis revealed a major loss of SERCA2 and ryanodine receptor expression at the time of reversal and biopsy. In 65% of bladders obstruction reversal resulted in a normalized voiding pattern with a recovery of ryanodine receptor expression that was 15% to 65% of control values. In contrast, in the 35% of bladders with persistent voiding symptoms there was minimal recovery of ryanodine receptor expression. SERCA2 expression increased slightly in each group after reversal but did not differ in bladders with normalized versus improved function.

CONCLUSIONS

Bladder decompensation is highly associated with a loss of sarcoplasmic reticulum function. Furthermore, the decompensated detrusor recovers function after obstruction reversal, which is associated with the recovery of these sarcoplasmic reticulum components.

Superinhibition of sarcoplasmic reticulum function by phospholamban induces cardiac contractile failure

To determine whether selective impairment of cardiac sarcoplasmic reticulum (SR) Ca(2+) transport may drive the progressive functional deterioration leading to heart failure, transgenic mice, overexpressing a phospholamban Val(49) --> Gly mutant (2-fold), which is a superinhibitor of SR Ca(2+)-ATPase affinity for Ca(2+), were generated, and their cardiac phenotype was examined longitudinally. At 3 months of age, the increased EC(50) level of SR Ca(2+) uptake for Ca(2+) (0.67 +/- 0.09 microm) resulted in significantly higher depression of cardiomyocyte rates of shortening (57%), relengthening (31%), and prolongation of the Ca(2+) signal decay time (165%) than overexpression (2-fold) of wild type phospholamban (68%, 64%, and 125%, respectively), compared with controls (100%). Echocardiography also revealed significantly depressed function and impaired beta-adrenergic responses in mutant hearts. The depressed contractile parameters were associated with left ventricular remodeling, recapitulation of fetal gene expression, and hypertrophy, which progressed to dilated cardiomyopathy with interstitial tissue fibrosis and death by 6 months in males. Females also had ventricular hypertrophy at 3 months but exhibited normal systolic function up to 12 months of age. These results suggest a causal relationship between defective SR Ca(2+) cycling and cardiac remodeling leading to heart failure, with a gender-dependent influence on the time course of these alterations.

Activation of NF-kappa B is required for hypertrophic growth of primary rat neonatal ventricular cardiomyocytes

The transcription factor NF-kappaB regulates expression of genes that are involved in inflammation, immune response, viral infection, cell survival, and division. However, the role of NF-kappaB in hypertrophic growth of terminally differentiated cardiomyocytes is unknown. Here we report that NF-kappaB activation is required for hypertrophic growth of cardiomyocytes. In cultured rat primary neonatal ventricular cardiomyocytes, the nuclear translocation of NF-kappaB and its transcriptional activity were stimulated by several hypertrophic agonists, including phenylephrine, endothelin-1, and angiotensin II. The activation of NF-kappaB was inhibited by expression of a "supersuppressor" IkappaBalpha mutant that is resistant to stimulation-induced degradation and a dominant negative IkappaB kinase (IKKbeta) mutant that can no longer be activated by phosphorylation. Furthermore, treatment with phenylephrine induced IkappaBalpha degradation in an IKK-dependent manner, suggesting that NF-kappaB is a downstream target of the hypertrophic agonists. Importantly, expression of the supersuppressor IkappaBalpha mutant or the dominant negative IKKbeta mutant blocked the hypertrophic agonist-induced expression of the embryonic gene atrial natriuretic factor and enlargement of cardiomyocytes. Conversely, overexpression of NF-kappaB itself induced atrial natriuretic factor expression and cardiomyocyte enlargement. These findings suggest that NF-kappaB plays a critical role in the hypertrophic growth of cardiomyocytes and may serve as a potential target for the intervention of heart disease.

[Genetically modified animal models in cardiovascular research]

It is a basic tenet of molecular and clinical medicine that specific protein complements underlie cell and organ function. Since cellular and ultimately organ function depend upon the polypeptides that are present, it is not surprising that when function is altered changes in the protein pools occur. In the heart, numerous examples of contractile protein changes correlate with functional alterations, both during normal development and during the development of numerous pathologies. Similarly, different congenital heart diseases are characterized by certain shifts in the motor proteins. To understand these relationships, and to establish models in which the pathogenic processes can be studied longitudinally, it is necessary to direct the heart to stably synthesize, in the absence of other peliotropic changes, the candidate protein. Subsequently, one can determine if the protein's presence causes the effects directly or indirectly with the goal being to define potential therapeutic targets. By affecting the heart's protein complement in a defined manner, one has the means to establish both mechanism and the function of the different mutated proteins of protein isoforms. Gene targeting and transgenesis in the mouse provides a means to modify the mammalian genome and the cardiac motor protein complement. By directing expression of an engineered protein to the heart, one is now able to effectively remodel the cardiac protein profile and study the consequences of a single genetic manipulation at the molecular, biochemical, cytological and physiologic levels, both under normal and stress stimuli.

Na+-Ca2+ exchanger remodeling in pressure overload cardiac hypertrophy

Perturbations of Ca(2+) metabolism are central to the pathogenesis of cardiac hypertrophy. The electrogenic Na(+)-Ca(2+) exchanger mediates a substantial component of transmembrane Ca(2+) movement in cardiac myocytes and is up-regulated in heart failure. However, the role of the exchanger in the pathogenesis of cardiac hypertrophy is poorly understood. Thoracic aortic banding in mice induced 50-60% increases in heart mass and cardiomyocyte size. Despite the absence of myocardial dysfunction, steady-state NCX1 transcript and protein levels were increased to an extent similar to that reported in heart failure. As recent studies indicate that calcineurin is critical to the expression of Na(+)-Ca(2+) exchanger genes, we inhibited calcineurin with cyclosporin. Calcineurin inhibition blunted the increases in NCX1 transcript and protein levels and eliminated the increases in heart mass and cell volume normally associated with pressure overload. To examine the functional significance of these changes, we measured Na(+)-Ca(2+) exchanger current in two independent ways. Surprisingly, exchanger current density was decreased in hypertrophied myocytes, and this down-regulation was eliminated by calcineurin inhibition. Together, these data reveal a role for Na(+)-Ca(2+) exchanger current in the electrical remodeling of hypertrophy and implicate calcineurin signaling therein. In addition, these data suggest the Na(+)-Ca(2+) exchanger is functionally regulated in hypertrophy.

[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.

The left ventricular stress-velocity relation in transgenic mice expressing a dominant negative CREB transgene in the heart

OBJECTIVE

CREB(A133) transgenic mice that express a dominant negative CREB transcription factor in cardiomyocytes develop a dilated cardiomyopathy that is anatomically, physiologically, and clinically similar to human idiopathic dilated cardiomyopathy. The goals of this study were to quantitate left ventricular (LV) contractility and measure cardiac reserve in CREB(A133) mice by using the relation of end-systolic wall stress to the velocity of fiber shortening.

METHODS

A total of 37 adult CD-1 mice (including both nontransgenic and CREB(A133) transgenic mice) were studied with simultaneously acquired high-fidelity instantaneous aortic pressures and 2-dimensionally targeted M-mode echocardiograms.

RESULTS

CREB(A133) mice displayed significantly lower values of LV fiber shortening velocities over a wide range of afterloads, and they displayed smaller dobutamine-induced shifts from baseline contractility relations. Counterbalancing effects of differences in LV geometry and aortic pressures resulted in comparable levels of LV wall stress during ejection in both groups.

CONCLUSION

These results demonstrate directly that CREB(A133) mice display reduced LV contractility at baseline and decreased cardiac reserve.

Independent signals control expression of the calcineurin inhibitory proteins MCIP1 and MCIP2 in striated muscles

Calcineurin, a calcium/calmodulin-regulated protein phosphatase, modulates gene expression in cardiac and skeletal muscles during development and in remodeling responses such as cardiac hypertrophy that are evoked by environmental stresses or disease. Recently, we identified two genes encoding proteins (MCIP1 and MCIP2) that are enriched in striated muscles and that interact with calcineurin to inhibit its enzymatic activity. In the present study, we show that expression of MCIP1 is regulated by calcineurin activity in hearts of mice with cardiac hypertrophy, as well as in cultured skeletal myotubes. In contrast, expression of MCIP2 in the heart is not altered by activated calcineurin but responds to thyroid hormone, which has no effect on MCIP1. A approximately 900-bp intragenic segment located between exons 3 and 4 of the MCIP1 gene functions as an alternative promoter that responds to calcineurin. This region includes a dense cluster of 15 consensus binding sites for NF-AT transcription factors. Because MCIP proteins can inhibit calcineurin, these results suggest that MCIP1 participates in a negative feedback circuit to diminish potentially deleterious effects of unrestrained calcineurin activity in cardiac and skeletal myocytes. Inhibitory effects of MCIP2 on calcineurin activity may be pertinent to gene switching events driven by thyroid hormone in striated muscles. The full text of this article is available at http://www. circresaha.org.

An abnormal Ca(2+) response in mutant sarcomere protein-mediated familial hypertrophic cardiomyopathy

Dominant-negative sarcomere protein gene mutations cause familial hypertrophic cardiomyopathy (FHC), a disease characterized by left-ventricular hypertrophy, angina, and dyspnea that can result in sudden death. We report here that a murine model of FHC bearing a cardiac myosin heavy-chain gene missense mutation (alphaMHC(403/+)), when treated with calcineurin inhibitors or a K(+)-channel agonist, developed accentuated hypertrophy, worsened histopathology, and was at risk for early death. Despite distinct pharmacologic targets, each agent augmented diastolic Ca(2+) concentrations in wild-type cardiac myocytes; alphaMHC(403/+) myocytes failed to respond. Pretreatment with a Ca(2+)-channel antagonist abrogated diastolic Ca(2+) changes in wild-type myocytes and prevented the exaggerated hypertrophic response of treated alphaMHC(403/+) mice. We conclude that FHC-causing sarcomere protein gene mutations cause abnormal Ca(2+) responses that initiate a hypertrophic response. These data define an important Ca(2+)-dependent step in the pathway by which mutant sarcomere proteins trigger myocyte growth and remodel the heart, provide definitive evidence that environment influences progression of FHC, and suggest a rational therapeutic approach to this prevalent human disease.