Calcium transport proteins in the nonfailing and failing heart: gene expression and function

Role of Gi proteins in the regulation of blood pressure and vascular remodeling

Heterotrimeric guanine nucleotide regulatory proteins (G-proteins) through the activation of several signaling mechanisms including adenylyl cyclase/cAMP and phospholipase C (PLC)/phosphatidyl inositol (PI) turnover. regulate a variety of cellular functions, including vascular reactivity, proliferation and hypertrophy of VSMC. Activity of adenylyl cyclase is regulated by two G proteins, stimulatory (Gsα) and inhibitory (Giα). Gsα stimulates adenylyl cyclase activity and increases the levels of cAMP, whereas Giα inhibits the activity of adenylyl cyclase and results in the reduction of cAMP levels. Abnormalities in Giα protein expression and associated adenylyl cyclase\cAMP levels result in the impaired cellular functions and contribute to various pathological states including hypertension. The expression of Giα proteins is enhanced in various tissues including heart, kidney, aorta and vascular smooth muscle cells (VSMC) from genetic (spontaneously hypertensive rats (SHR)) and experimentally - induced hypertensive rats and contribute to the pathogenesis of hypertension. In addition, the enhanced expression of Giα proteins exhibited by VSMC from SHR is also implicated in the hyperproliferation and hypertrophy, the two key players contributing to vascular remodelling in hypertension. The enhanced levels of endogenous vasoactive peptides including angiotensin II (Ang II), endothelin-1 (ET-1) and growth factors contribute to the overexpression of Giα proteins in VSMC from SHR. In addition, enhanced oxidative stress, activation of c-Src, growth factor receptor transactivation and MAP kinase/PI3kinase signaling also contribute to the augmented expression of Giα proteins in VSMC from SHR. This review summarizes the role of Giα proteins, and the underlying molecular mechanisms implicated in the regulation of high blood pressure and vascular remodelling.

Exercise Training Attenuates Right Ventricular Remodeling in Rats with Pulmonary Arterial Stenosis

Introduction: Pulmonary arterial stenosis (PAS) is a congenital defect that causes outflow tract obstruction of the right ventricle (RV). Currently, negative issues are reported in the PAS management: not all patients may be eligible to surgeries; there is often the need for another surgery during passage to adulthood; patients with mild stenosis may have later cardiac adverse repercussions. Thus, the search for approaches to counteract the long-term PAS effects showed to be a current target. At the study herein, we evaluated the cardioprotective role of exercise training in rats submitted to PAS for 9 weeks.

Methods and Results: Exercise resulted in improved physical fitness and systolic RV function. Exercise also blunted concentric cavity changes, diastolic dysfunction, and fibrosis induced by PAS. Exercise additional benefits were also reported in a pro-survival signal, in which there were increased Akt₁ activity and normalized myocardial apoptosis. These findings were accompanied by microRNA-1 downregulation and microRNA-21 upregulation. Moreover, exercise was associated with a higher myocardial abundance of the sarcomeric protein α-MHC and proteins that modulate calcium handling—ryanodine receptor and Serca 2, supporting the potential role of exercise in improving myocardial performance.

Conclusion: Our results represent the first demonstration that exercise can attenuate the RV remodeling in an experimental PAS. The cardioprotective effects were associated with positive modulation of RV function, survival signaling pathway, apoptosis, and proteins involved in the regulation of myocardial contractility.

Matrix elasticity regulates the optimal cardiac myocyte shape for contractility

Concentric hypertrophy is characterized by ventricular wall thickening, fibrosis, and decreased myocyte length-to-width aspect ratio. Ventricular thickening is considered compensatory because it reduces wall stress, but the functional consequences of cell shape remodeling in this pathological setting are unknown. We hypothesized that decreases in myocyte aspect ratio allow myocytes to maximize contractility when the extracellular matrix becomes stiffer due to conditions such as fibrosis. To test this, we engineered neonatal rat ventricular myocytes into rectangles mimicking the 2-D profiles of healthy and hypertrophied myocytes on hydrogels with moderate (13 kPa) and high (90 kPa) elastic moduli. Actin alignment was unaffected by matrix elasticity, but sarcomere content was typically higher on stiff gels. Microtubule polymerization was higher on stiff gels, implying increased intracellular elastic modulus. On moderate gels, myocytes with moderate aspect ratios (∼7:1) generated the most peak systolic work compared with other cell shapes. However, on stiffer gels, low aspect ratios (∼2:1) generated the most peak systolic work. To compare the relative contributions of intracellular vs. extracellular elasticity to contractility, we developed an analytical model and used our experimental data to fit unknown parameters. Our model predicted that matrix elasticity dominates over intracellular elasticity, suggesting that the extracellular matrix may potentially be a more effective therapeutic target than microtubules. Our data and model suggest that myocytes with lower aspect ratios have a functional advantage when the elasticity of the extracellular matrix decreases due to conditions such as fibrosis, highlighting the role of the extracellular matrix in cardiac disease.

Persistent overexpression of SERCA2a affects bladder functions under physiological conditions, but not in bladder outlet obstruction-induced sub-acute pathological conditions

A functional impairment of the bladder and heart in a decompensated state caused by a pressure overload is accompanied by a decrease in the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2). The beneficial effects of SERCA2 overexpression in preserving cardiac functions have been previously reported. The aim of the present study was to investigate the effects of overexpressed SERCA2 on bladder functions under physiological and pathological conditions using partial bladder outlet obstruction (BOO) in SERCA2a transgenic Wistar rats (TG). Bladder cystometry and western blot analysis were performed using the wild-type Wistar rats (WT), TG, and BOO models (WTBOO and TGBOO). Persistent overexpression of SERCA2 induces reduced bladder compliance without hypertrophy in TG. BOO induces reduced bladder compliance and hypertrophy in WT and TG in the sub-acute phase, but persistent overexpression of SERCA2a in TG does not aggravate the bladder compliance and hypertrophy. In conclusion, SERCA2a overexpression affects bladder functions under physiological conditions, but not in BOO-induced sub-acute pathological conditions.

New drugs vs. old concepts: a fresh look at antiarrhythmics

Common arrhythmias, particularly atrial fibrillation (AF) and ventricular tachycardia/fibrillation (VT/VF) are a major public health concern. Classic antiarrhythmic (AA) drugs for AF are of limited effectiveness, and pose the risk of life-threatening VT/VF. For VT/VF, implantable cardiac defibrillators appear to be the unique, yet unsatisfactory, solution. Very few AA drugs have been successful in the last few decades, due to safety concerns or limited benefits in comparison to existing therapy. The Vaughan-Williams classification (one drug for one molecular target) appears too restrictive in light of current knowledge of molecular and cellular mechanisms. New AA drugs such as atrial-specific and/or multichannel blockers, upstream therapy and anti-remodeling drugs, are emerging. We focus on the cellular mechanisms related to abnormal Na⁺ and Ca²⁺ handling in AF, heart failure, and inherited arrhythmias, and on novel strategies aimed at normalizing ionic homeostasis. Drugs that prevent excessive Na⁺ entry (ranolazine) and aberrant diastolic Ca²⁺ release via the ryanodine receptor RyR2 (rycals, dantrolene, and flecainide) exhibit very interesting antiarrhythmic properties. These drugs act by normalizing, rather than blocking, channel activity. Ranolazine preferentially blocks abnormal persistent (vs. normal peak) Na⁺ currents, with minimal effects on normal channel function (cell excitability, and conduction). A similar "normalization" concept also applies to RyR2 stabilizers, which only prevent aberrant opening and diastolic Ca²⁺ leakage in diseased tissues, with no effect on normal function during systole. The different mechanisms of action of AA drugs may increase the therapeutic options available for the safe treatment of arrhythmias in a wide variety of pathophysiological situations.

Modulation of Gi Proteins in Hypertension: Role of Angiotensin II and Oxidative Stress

Guanine nucleotide regulatory proteins (G-proteins) play a key role in the regulation of various signal transduction systems including adenylyl cyclase/cAMP and phospholipase C (PLC)/phosphatidyl inositol turnover (PI). These are implicated in the modulation of a variety of physiological functions such as platelet functions, cardiovascular functions, including arterial tone and reactivity. Several abnormalities in adenylyl cyclase activity, cAMP levels and G proteins have shown to be responsible for the altered cardiac performance and vascular functions observed in cardiovascular disease states. The enhanced or unaltered levels of inhibitory G-proteins (Giα-2 and Giα-3) and mRNA have been reported in different models of hypertension, whereas Gsα levels were shown to be unaltered. These changes in G-protein expression were associated with Gi functions. The enhanced levels of Giα proteins precede the development of blood pressure and suggest that overexpression of Gi proteins may be one of the contributing factors for the pathogenesis of hypertension. The augmented levels of vasoactive peptides, including angiotensin II (AngII), were shown to contribute to enhanced expression of Giα proteins and associated adenylyl cyclase signaling and thereby increased blood pressure. In addition, enhanced oxidative stress in hypertension due to Ang II may also be responsible for the enhanced expression of Giα proteins observed in hypertension. The mechanism by which oxidative stress enhances the expression of Gi proteins appears to be through the activation of mitogen activated protein (MAP) kinase activity.

Calciomics: prediction and analysis of EF-hand calcium binding proteins by protein engineering

Ca²⁺ plays a pivotal role in the physiology and biochemistry of prokaryotic and mammalian organisms. Viruses also utilize the universal Ca²⁺ signal to create a specific cellular environment to achieve coexistence with the host, and to propagate. In this paper we first describe our development of a grafting approach to understand site-specific Ca²⁺ binding properties of EF-hand proteins with a helix-loop-helix Ca²⁺ binding motif, then summarize our prediction and identification of EF-hand Ca²⁺ binding sites on a genome-wide scale in bacteria and virus, and next report the application of the grafting approach to probe the metal binding capability of predicted EF-hand motifs within the streptococcal hemoprotein receptor (Shr) of Streptococcus pyrogenes and the nonstructural protein 1 (nsP1) of Sindbis virus. When methods such as the grafting approach are developed in conjunction with prediction algorithms we are better able to probe continuous Ca²⁺-binding sites that have been previously underrepresented due to the limitation of conventional methodology.

The non-excitable smooth muscle: calcium signaling and phenotypic switching during vascular disease

Calcium (Ca(2+)) is a highly versatile second messenger that controls vascular smooth muscle cell (VSMC) contraction, proliferation, and migration. By means of Ca(2+) permeable channels, Ca(2+) pumps and channels conducting other ions such as potassium and chloride, VSMC keep intracellular Ca(2+) levels under tight control. In healthy quiescent contractile VSMC, two important components of the Ca(2+) signaling pathways that regulate VSMC contraction are the plasma membrane voltage-operated Ca(2+) channel of the high voltage-activated type (L-type) and the sarcoplasmic reticulum Ca(2+) release channel, Ryanodine Receptor (RyR). Injury to the vessel wall is accompanied by VSMC phenotype switch from a contractile quiescent to a proliferative motile phenotype (synthetic phenotype) and by alteration of many components of VSMC Ca(2+) signaling pathways. Specifically, this switch that culminates in a VSMC phenotype reminiscent of a non-excitable cell is characterized by loss of L-type channels expression and increased expression of the low voltage-activated (T-type) Ca(2+) channels and the canonical transient receptor potential (TRPC) channels. The expression levels of intracellular Ca(2+) release channels, pumps and Ca(2+)-activated proteins are also altered: the proliferative VSMC lose the RyR3 and the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase isoform 2a pump and reciprocally regulate isoforms of the ca(2+)/calmodulin-dependent protein kinase II. This review focuses on the changes in expression of Ca(2+) signaling proteins associated with VSMC proliferation both in vitro and in vivo. The physiological implications of the altered expression of these Ca(2+) signaling molecules, their contribution to VSMC dysfunction during vascular disease and their potential as targets for drug therapy will be discussed.

Deletion of one SERCA2 allele confers protection against bladder wall hypertrophy in a murine model of partial bladder outlet obstruction

The sarco(endo)plasmic reticulum Ca2+-ATPase2 (SERCA2) is downregulated in cardiac hypertrophy with decompensation. We sought to determine whether mice heterozygous for the SERCA2 allele would develop greater bladder hypertrophy and decompensation than their wild-type littermates following partial bladder outlet obstruction (pBOO). We found that following 4 wk of surgically created pBOO, SERCA2 heterozygous murine bladders showed significantly less hypertrophy, improved in vitro cystometry performance, diminished expression of the slow myosin isoform A analyzed by RT-PCR, a significant drop in nuclear translocation of nuclear factor of activated T cells by EMSA, and decreased cell proliferation within the smooth muscle layer following 5-bromo-2′-deoxyuridine labeling compared with their wild-type littermates. Thus, in contrast to cardiac muscle, deletion of a SERCA2 allele confers protection against bladder hypertrophy in a murine model of pBOO. Compensatory mechanisms in heterozygous mice seem to be related to the calcineurin pathway. Further studies are underway to better define the molecular basis of this observation, which has potential clinical applications.

Intracellular Redistribution of Dihydropyridine Receptor in the Rat Heart During the Progression of Sepsis

BACKGROUND

Dihydropyridine receptor (DHPR) regulates the rate and force of cardiac muscle contraction. This study examined the alteration in the intracellular redistribution of DHPR and its association with the development of the two distinct cardiodynamic states in the rat heart during the progression of sepsis.

MATERIAL AND METHODS

Sepsis was induced by cecal ligation and puncture (CLP). DHPRs were assayed using [(3)H]PN200-100 binding and photoaffinity labeling with [(3)H]azidopine followed by polyacrylamide gel electrophoresis.

RESULTS

[(3)H]PN200-110 binding shows that during the early hyperdynamic phase of sepsis (9 h post-CLP), the Bmax was increased by 27% in sarcolemma while decreased by 24% in light vesicle. During the late hypodynamic phase of sepsis (18 h post-CLP), the Bmax was decreased by 39% in sarcolemma but increased by 59% in light vesicle. The sum of the Bmax for both membrane fractions was increased by 16% during early sepsis while decreased by 17% during late sepsis. Photoaffinity labeling shows that the incorporation of [(3)H]azidopine into 165 kDa peptides during early sepsis was increased by 28% in sarcolemma whereas decreased by 23% in light vesicle. During late sepsis, the incorporation was decreased by 38% in sarcolemma but increased by 46% in light vesicle. The sum of the 165 kDa peptides for both membrane fractions was increased by 13% during early while decreased by 13% during late sepsis.

CONCLUSIONS

These data indicate that DHPRs in the rat heart were externalized from light vesicles to sarcolemma during the early hyperdynamic phase whereas they were internalized from surface membranes to intracellular sites during the late hypodynamic phase of sepsis. Furthermore, DHPRs were overexpressed during early sepsis while they were underexpressed during late sepsis. Alterations in the expression and intracellular redistribution of DHPRs may contribute to the development of the biphasic cardiodynamic states during the progression of sepsis.

The effects of overexpression of the Na+/Ca2+ exchanger on calcium regulation in hypertrophied mouse cardiac myocytes

In cardiac hypertrophy and failure it has been shown that the amount of Na/Ca exchanger protein can increase. Several studies have investigated this modification in overt heart failure. However, the role of Na/Ca exchanger overexpression during the development of hypertrophy is unknown. To address this question we investigated Ca2+ regulation in an early stage of cardiac hypertrophy before signs of heart failure occurred and evaluated the role of Na/Ca exchanger overexpression. Cardiac hypertrophy was induced by a constant infusion of angiotensin II (Ang, 1 microg/min/kg) via an osmotic pump for 14 days. Thereafter, ventricular myocytes from either wild type (NON) or transgenic mice overexpressing the Na/Ca exchanger (TR) were isolated. Myocytes were loaded with indo-1 AM or fluo-4 AM to monitor cytoplasmic [Ca2+] with all experiments performed at 37 degrees C. In myocytes exposed to Ang there was an increase in cell capacitance of more than 20% indicating cellular hypertrophy. Ca2+ transients were prolonged in hypertrophied NON myocytes but not in TR myocytes. Action potentials had a less negative plateau in TR myocytes. Sarcoplasmic reticulum (SR) Ca2+ content, measured using rapid caffeine application, was greater in TR myocytes but unaffected by hypertrophy. Ca2+ spark frequency was significantly greater in TR. Na/Ca exchanger overexpression prevented the prolongation of the Ca2+ transient observed in hypertrophy and maintained a similar SR Ca2+ leak suggesting a compensatory role in Ca2+ regulation in hypertrophied cardiac myocytes from transgenic mice. We suggest this compensatory effect is mediated by increased SR Ca2+ content and faster Ca2+ removal via the Na/Ca exchanger.

Phenotypes of SERCA and PMCA knockout mice

P-type Ca2+-ATPases of the sarco(endo)plasmic reticulum (SERCAs) and plasma membrane (PMCAs) are responsible for maintaining the Ca2+ gradients across cellular membranes that are required for regulation of Ca2+-mediated signaling and other biological processes. Gene-targeting studies of SERCA isoforms 1, 2, and 3 and PMCA isoforms 1, 2, and 4 have confirmed some of the general functions proposed for these pumps, such as a major role in excitation-contraction coupling for SERCA1 and SERCA2 and housekeeping functions for PMCA1 and SERCA2, but have also revealed some unexpected phenotypes. These include squamous cell cancer and plasticity in the regulation of Ca2+-mediated exocytosis in SERCA2 heterozygous mutant mice, modulation of Ca2+ signaling in SERCA3-deficient mice, deafness and balance disorders in PMCA2 null mice, and male infertility in PMCA4 null mice. These unique phenotypes provide new information about the cellular functions of these pumps, the requirement of their activities for higher order physiological processes, and the pathophysiological consequences of pump dysfunction.

A switch in metabolism precedes increased mitochondrial biogenesis in respiratory chain-deficient mouse hearts

We performed global gene expression analyses in mouse hearts with progressive respiratory chain deficiency and found a metabolic switch at an early disease stage. The tissue-specific mitochondrial transcription factor A (Tfam) knockout mice of this study displayed a progressive heart phenotype with depletion of mtDNA and an accompanying severe decline of respiratory chain enzyme activities along with a decreased mitochondrial ATP production rate. These characteristics were observed after 2 weeks of age and became gradually more severe until the terminal stage occurred at 10-12 weeks of age. Global gene expression analyses with microarrays showed that a metabolic switch occurred early in the progression of cardiac mitochondrial dysfunction. A large number of genes encoding critical enzymes in fatty acid oxidation showed decreased expression whereas several genes encoding glycolytic enzymes showed increased expression. These alterations are consistent with activation of a fetal gene expression program, a well-documented phenomenon in cardiac disease. An increase in mitochondrial mass was not observed until the disease had reached an advanced stage. In contrast to what we have earlier observed in respiratory chain-deficient skeletal muscle, the increased mitochondrial biogenesis in respiratory chain-deficient heart muscle did not increase the overall mitochondrial ATP production rate. The observed switch in metabolism is unlikely to benefit energy homeostasis in the respiratory chain-deficient hearts and therefore likely aggravates the disease. It can thus be concluded that at least some of the secondary gene expression alterations in mitochondrial cardiomyopathy do not compensate but rather directly contribute to heart failure progression.

Decreased mAKAP, ryanodine receptor, and SERCA2a gene expression in mdx hearts

Duchenne muscular dystrophy (DMD) is a common genetic disease resulting from mutations in the dystrophin gene. The lack of dystrophin function as a cytoskeletal protein leads to abnormal intracellular Ca(2+) homeostasis, the actual source and functional consequences of which remain obscure. The mdx mouse, a mouse model of DMD, revealed alterations in contractile properties that are likely due to defective Ca(2+) handling. However, the exact mechanisms of the Ca(2+) handling defect are unclear. We performed suppressive subtractive hybridization to isolate differentially expressed genes between 5-month-old mdx and control mice. We observed a decrease in muscle A-kinase anchoring protein (mAKAP) in the mdx hearts. We noticed not only down-regulation of mAKAP mRNA but also decreased mRNA level of the molecules involved in Ca(2+) handling and excitation-contraction (E-C) coupling in the sarcoplasmic reticulum (SR), the cardiac ryanodine receptor, and the sarcoplasmic reticulum Ca(2+) ATPase. Therefore, dystrophin deficiency may cause an impairment of SR Ca(2+) homeostasis and E-C coupling in mdx hearts, in part, by decreased gene expression of molecules involved in SR Ca(2+) handling.

Effects of IgM from rheumatic fever patients on intracellular calcium levels of neonatal rat cardiac myocytes

Rheumatic fever (RF), a potential sequela of Streptococcus pyogenes pharyngitis, sometimes results in myocarditis and heart failure. Antibodies have been implicated in the pathogenesis of RF and anti-cardiac myosin antibody levels are elevated in RF patients. Since myocarditis is associated with altered cardiomyocyte calcium transients it was of interest to determine the direct effects of RF patient antibodies on calcium transients in cultured myocytes. RF patient polyclonal IgM treatment caused increased calcium retention by neonatal rat heart cells in vitro as determined with isotopically labeled calcium. Therefore, to further characterize this finding, calcium transients were evaluated by real time fluorescence spectroscopy and deconvolution imaging. RF patient polyclonal IgM produced increased calcium retention during the relaxation stage of the contraction cycle leading to a slowing of contraction rate, disorganized calcium transients, and eventual tetany. In contrast, calcium transient studies of cardiomyocytes following treatment with monoclonal anti-myosin antibodies revealed declining intracellular calcium levels, accompanied by disorganized transients and tetany. Treatment with both antibodies led to myocyte dysfunction and these novel findings suggest a role for antibodies in the pathogenesis of the myocarditis associated with rheumatic carditis.

Quantitation and distribution of beta-tubulin in human cardiac myocytes

Increasing evidence suggests that derangements of cytoskeletal proteins contribute to alterations in intracellular signaling, myocyte function, and the coupling of myocytes to the extracellular matrix during cardiac hypertrophy and failure. Data from animal studies have shown an increased density of beta-tubulin protein in the right or left ventricle subjected to pressure overload, and have demonstrated that interfering with excess polymerization of beta-tubulin improves contractility. We tested the hypothesis that beta-tubulin is increased in human left ventricular hypertrophy and end-stage heart failure. Confocal microscopy of fluorescently labeled beta-tubulin protein revealed an increased density of the beta-tubulin network in cardiomyocytes from both hypertrophied and failing human hearts as compared to cells from nonfailing hearts. Western blot analysis on total heart homogenate showed no change in beta-tubulin when data were normalized to either actin or calsequestrin, although there was a significant increase in failing human hearts when data were normalized only for a constant amount of protein per heart. The mRNA for beta-tubulin was not changed in hypertrophied hearts, but was significantly decreased in failing human hearts. Thus, similar to animal models, we have shown that the density of the microtubular network within the cardiomyocyte is increased in end-stage failing human hearts. We have also shown for the first time that beta-tubulin density is increased in cells from hypertrophied human hearts. Although the functional implications of this finding in the human heart remain to be explored, data from animal studies suggest that increased beta-tubulin protein contributes to cardiac dysfunction.

Platelet and cardiac function in Darier's disease

ATP2A2, the gene that is abnormal in Darier's disease, encodes SERCA2, a calcium pump that is expressed in many tissues. The wide expression of SERCA2 might suggest that ATP2A2 mutations would cause a multisystem disease. There is however, no evidence of consistent extracutaneous manifestations of Darier's disease. We have conducted preliminary studies in patients with Darier's disease, in two extracutaneous systems in which SERCA2 is known to be important, in order to investigate whether subtle defects have been overlooked. We found no evidence for altered cardiac function in 10 patients using two-dimensional, colour and Doppler echocardiography. There were no consistent defects in platelet function in 12 patients, using bleeding time and aggregation studies. We conclude that the skin is sensitive to defects in SERCA2 function to which other systems appear robust.

Beta-adrenergic receptors and calcium cycling proteins in non-failing, hypertrophied and failing human hearts: transition from hypertrophy to failure

Left ventricular hypertrophy may lead to heart failure. The transition between hypertrophy and heart failure is, however, incompletely understood. On the cellular level, human heart failure is characterized by alterations in Ca(2+)-cycling proteins and beta-adrenergic receptor density, but the hypertrophied human heart remains largely under studied. In this investigation, 21 donor hearts which could not be used for transplantation were studied. Ten of these hearts came from organ donors with documented left ventricular hypertrophy and normal cardiac function. Eleven of the hearts were non-failing, obtained from individuals with no evidence of cardiac disease. Nine failing hearts from transplant recipients were also studied. beta-adrenergic receptor density was determined by radioligand binding. mRNA for atrial natriuretic factor, calsequestrin, sarcoplasmic reticulum Ca(2+)-ATPase, and phospholamban was measured by Northern blot. Actin, calsequestrin, sarcoplasmic reticulum Ca(2+)-ATPase, and phospholamban proteins were quantified by Western blot. In both hypertrophied and failing ventricles, mRNA for atrial natriuretic factor was expressed, as compared to no expression in non-failing hearts. In failing hearts, beta -adrenergic receptor density and both mRNA and protein levels of the Ca(2+)-ATPase were significantly decreased v non-failing hearts. By comparison, hypertrophied hearts showed a reduction in mRNA expression for both the Ca(2+)-ATPase and phospholamban with no change in the corresponding protein levels, and no change in beta-receptors. These data suggest that the previously demonstrated reduction in beta-adrenergic receptors and Ca(2+)-cycling proteins in the failing human heart may be features of the decompensated state, but are not found in human hearts with left ventricular hypertrophy and preserved systolic function.

Increased in vivo apoptosis in cells lacking mitochondrial DNA gene expression

We have attempted to determine whether loss of mtDNA and respiratory chain function result in apoptosis in vivo. Apoptosis was studied in embryos with homozygous disruption of the mitochondrial transcription factor A gene (Tfam) and tissue-specific Tfam knockout animals with severe respiratory chain deficiency in the heart. We found massive apoptosis in Tfam knockout embryos at embryonic day (E) 9.5 and increased apoptosis in the heart of the tissue-specific Tfam knockouts. Furthermore, mtDNA-less (rho(0)) cell lines were susceptible to apoptosis induced by different stimuli in vitro. The data presented here provide in vivo evidence that respiratory chain deficiency predisposes cells to apoptosis, contrary to previous assumptions based on in vitro studies of cultured cells. These results suggest that increased apoptosis is a pathogenic event in human mtDNA mutation disorders. The finding that respiratory chain deficiency is associated with increased in vivo apoptosis may have important therapeutic implications for human disease. Respiratory chain deficiency and cell loss and/or apoptosis have been associated with neurodegeneration, heart failure, diabetes mellitus, and aging. Furthermore, chemotherapy and radiation treatment of cancer are intended to induce apoptosis in tumor cells. It would therefore be of interest to determine whether manipulation of respiratory chain function can be used to inhibit or enhance apoptosis in these conditions.

Early expression of natriuretic peptides and SERCA in mild heart failure: association with severity of the disease

BACKGROUND

We investigated changes in genetic expression of atrial and brain natriuretic peptides (ANP and BNP) and sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) in patients with stable mild to moderate chronic heart failure (CHF), since data on this topic were primarily obtained in end-stage CHF.

METHODS

We studied tissue from 25 patients with idiopathic dilated cardiomyopathy (IDC) in New York Heart Association (NYHA) class II (n=12) and III-IV (n=13). Myocardial tissue from normal hearts (n=10) served as controls. Messenger RNA (mRNA) expression of ANP, BNP, and SERCA was isolated, and correlated with severity of CHF, left ventricular function (LVEF), peak oxygen uptake (peak VO(2)), and wedge pressure.

RESULTS

A significant trend for gradual changes in mRNA expression according to increasing NYHA class was found for ANP, BNP (P<0.0001) and SERCA (P=0.04), with a marked increase in patients with more advanced CHF (ANP and BNP: P<0.01 vs. controls; SERCA: NS) and less pronounced changes in patients with mild CHF. mRNA of ANP and BNP correlated strongly with LVEF (-0.621 and -0.621, respectively, both P<0.01) and peak VO(2) (-0.625 and -0.555, respectively, both P<0.01) and, to a lesser extent, with wedge pressure (0.440 and 0.488, respectively, both P<0.05). SERCA correlated most strongly with wedge pressure (-0.623, P<0.01), and weak, non-significant correlations with LVEF and peak VO(2) were found.

CONCLUSIONS

Genetic expression of ANP, BNP, and SERCA is progressively altered in proportion to the severity of CHF, although this is more marked for ANP and to a lesser extent BNP, than for SERCA. These changes support the concept that already early in CHF, genetic expression is affected, which has implications for the understanding of the pathophysiology of CHF.

Overexpression of the Na(+)/Ca(2+) exchanger and inhibition of the sarcoplasmic reticulum Ca(2+)-ATPase in ventricular myocytes from transgenic mice

BACKGROUND

Myocytes from failing hearts produce slower and smaller Ca(2+) transients associated with reduction in expression of sarcoplasmic reticulum (SR) Ca(2+) ATPase and an overexpression of Na(+)/Ca(2+) exchanger. Since the physiological role of both these proteins is competing for, and removing, Ca(2+) from the cytoplasm, overexpression of the exchanger may compensate for less effective SR Ca(2+) uptake. This study demonstrates this compensatory effect and provides a quantitative description of the results.

METHODS

Ventricular myocytes from transgenic mice overexpressing the Na(+)/Ca(2+) exchanger (TR) and nontransgenic littermates (NON) were used. Cell shortening, cytoplasmic [Ca] (using indo-1 AM) and electrophysiological parameters were monitored.

RESULTS

TR myocytes displayed faster Ca(2+) transients and twitches compared with NON myocytes. Superfusion with thapsigargin prolonged the time-course of Ca(2+) transients of TR myocytes until these were equal to the ones measured in NON myocytes. The amount of SR Ca(2+)-ATPase (SERCA) inhibition needed to obtain such transients was calculated as a function of V(max) for the Ca(2+) flux via SERCA and found to be 28%. In TR myocytes V(max) for the Ca(2+) flux via Na(+)/Ca(2+)exchange was 240% of NON myocytes. When Ca(2+) transients in TR myocytes were slowed by thapsigargin to similar values to the ones recorded in NON myocytes, SR Ca(2+) content was also correspondingly reduced.

CONCLUSIONS

The results suggest that in pathophysiological conditions where there is a reduction in SERCA function, overexpression of Na(+)/Ca(2+) exchanger can compensate and allow normal Ca(2+) homeostasis to be maintained. In mouse ventricular myocytes a 2.4-fold increase in Na(+)/Ca(2+) exchange activity compensates for a reduction in SERCA function by 28% so maintaining the duration of the Ca(2+) transient.

Alterations in the inotropic responses to forskolin and Ca2+ and reduced gene expressions of Ca2+-signaling proteins induced by chronic volume overload in rabbits

Volume overload results in eccentric cardiac hypertrophy, but it is still unknown how this mechanical overload modulates the inotropic response to exogenous Ca2+ or adenylyl cyclase stimulation. Inotropic responsiveness in vivo and the levels of gene expression of Ca2+ signaling proteins were studied in rabbit hearts hypertrophied as a result of volume overload at 4 and 12 weeks after arteriovenous shunt formation. In sham-operated control rabbits, left ventricular (LV)+dP/dt was augmented in response to graded doses of CaCl2. Dose-related changes of LV+dP/dt to CaCl2 were attenuated significantly in shunt rabbits with volume overload. Forskolin dose-dependently augmented LV+dP/dt in sham rabbits, which was also attenuated significantly in rabbits with volume overload. The mRNA levels of dihydropyridine receptor, Na+/Ca2+ exchanger, sarcoplasmic reticulum Ca2+-ATPase, and ryanodine receptor decreased significantly at 4 and 12 weeks in the volume-overload rabbits compared with the sham rabbits, but the mRNA levels of phospholamban and calsequestrin remained unchanged. Chronic volume overload alters contractile responsiveness to Ca2+ or adenylyl cyclase stimulation, and downregulation of steady state mRNA levels of Ca2+ signaling proteins might be, at least in part, related to this pathologic process.

Sarco(endo)plasmic reticulum calcium pumps: recent advances in our understanding of structure/function and biology (review)

This review examines the structure and function of the sarco(endo)plasmic reticulum calcium pump (SERCA1a) in the light of the recent publication of the 2.6 A resolution structure of this protein, and looks at the increasing awareness of the key role played by SERCAs in calcium signalling. The roles played by the calcium pump isoforms, SERCA1a/b, SERCA2a/b and SERCA3a/b/c in cellular function are discussed, and the modulation of SERCA activity by phospholamban, sarcolipin and other modulatory influences is examined. The recent discoveries of human SERCA mutations leading to disease states is reviewed, and the insights into SERCA function using transgenic approaches are outlined.

Gene knockout studies of Ca2+-transporting ATPases

The biochemical functions of intracellular and plasma membrane Ca2+-transporting ATPases in the control of cytosolic and organellar Ca2+ levels are well established, but the physiological roles of specific isoforms are less well understood. There appear to be three different types of Ca2+ pumps in mammalian tissues: the sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs), which sequester Ca2+ within the endoplasmic or sarcoplasmic reticulum, the plasma membrane Ca2+-ATPases (PMCAs), which extrude Ca2+ from the cell, and the putative secretory pathway Ca2+-ATPase (SPCA), the function of which is poorly understood. This review describes the results of recent analyses of mouse models with null mutations in the genes encoding SERCA and PMCA isoforms and genetic studies of SERCA and SPCA dysfunction in both humans and model organisms. These studies are yielding important insights regarding the physiological functions of individual Ca2+-transporting ATPases in vivo.

Heterotrimeric G proteins in heart disease

Guanine nucleotide binding proteins (G proteins) are largely grouped into three classes: heterotrimeric G proteins, ras-like or small molecular weight GTP binding proteins, and others like Gh. In the heart G proteins transduce signals from a variety of membrane receptors to generate diverse effects on contractility, heart rate, and myocyte growth. This central position of G proteins forming a switchboard between extracellular signals and intracellular effectors makes them candidates possibly involved in the pathogenesis of cardiac hypertrophy, heart failure, and arrhythmia. This review focuses primarily on discoveries of heterotrimeric G protein alterations in heart diseases that help us to understand the pathogenesis and pathophysiology. We also discuss the underlying molecular mechanisms of heterotrimeric G protein signalling.Key words: G proteins, signal transduction, adrenergic system, heart failure, hypertrophy.

Comparison of ventricular pressure relaxation assessments in human heart failure: quantitative influence on load and drug sensitivity analysis

OBJECTIVES

We contrasted various methods for assessing ventricular pressure decay time constants to test whether sensitivity to slight data instability or disparities between model-assumed and real decay are systematically altered by cardiac failure. We hypothesized that such discrepancies could result in apparent increased relaxation sensitivity to load and drug stimulation.

BACKGROUND

Deviation of relaxation behavior from model-assumed waveforms may be worsened by failure, enhancing instability and apparent load and drug sensitivity of commonly used indexes.

METHODS

Pressure-volume relations were measured in patients with normal (n = 14), hypertrophic (hypertrophic cardiomyopathy [HCM], n = 15) and dilated-myopathic (dilated cardiomyopathy [DCM], n = 37) hearts before and during preload reduction or inotropic stimulation. Relaxation parameters (monoexponential [ME] model assuming zero-T(in) or non-zero-T(D), T(F) asymptote:, hybrid logistic-T(L), linear-T(LR), and pressure halftime-T(1/2)) were contrasted regarding sensitivity to slight data range manipulation and loading or drug changes.

RESULTS

In DCM, T(D) and T(F) prolonged 15% to 25% (p < 0.0001) by deletion of only 1-2 data points, whereas this had minimal effect on controls or HCM. This stemmed from systematic deviation of relaxation from an ME decay in DCM. T(1/2) and T(in) were highly sensitive to pure pressure offsets, whereas T(L) was most stable to both manipulations in all hearts. As a result, T(D) and T(F) appeared to be much more sensitive to systolic load in DCM than T(1/2) or T(L) and disproportionately sensitive to increased cyclic adenosine monophosphate (cAMP).

CONCLUSIONS

Relaxation consistently deviates from an ME decay in DCM resulting in instability and amplified relaxation systolic load or drug dependence of ME-based indexes in failing versus control (or HCM) hearts. The hybrid-logistic method improves quantitative analyses by providing more consistent data fits with all three heart types.

Reduced myocardial sarcoplasmic reticulum Ca(2+)-ATPase protein expression in compensated primary and secondary human cardiac hypertrophy

Pathological intracellular calcium handling has been proposed to underlie the alterations of contractile behavior in hypertrophied myocardium. However, the myocardial protein expression of intracellular calcium transport proteins in compensated human left ventricular hypertrophy has not yet been studied. We investigated septal myocardial specimens of patients suffering from hypertrophic obstructive cardiomyopathy (n=14) or from acquired aortic valve stenosis (n=11) undergoing myectomy or aortic valve replacement, respectively. For comparison, we studied non-hypertrophied myocardium of six non-failing hearts which could not be transplanted for technical reasons. The myocardial density of the calcium release channel of the sarcoplasmic reticulum (SR) was determined by(3)H-ryanodine binding. Myocardial contents of SR Ca(2+)-ATPase, phospholamban, calsequestrin and Na(+)/Ca(2+)-exchanger were analysed by Western blot analysis. The myocardial SR calcium release channel density was not significantly different in hypertrophied and non-failing human myocardium. In both hypertrophic obstructive cardiomyopathy and in aortic valve stenosis, SR Ca(2+)-ATPase expression was reduced by about 30% compared to non-failing myocardium (P<0.05), whereas the expression of phospholamban, calsequestrin, and the Na(+)/Ca(2+)-exchanger was unchanged. The decrease of SR Ca(2+)-ATPase expression was still observable when related to its regulatory protein phospholamban or to the myosin content of the homogenates (P<0.05). Furthermore, the SR Ca(2+)-ATPase expression was inversely correlated to the septum thickness assessed by echocardiography, but not to age, cardiac index or outflow tract gradient. In primary as well as in secondary hypertrophied human myocardium, the expression of SR Ca(2+)-ATPase is reduced and inversely related to the degree of the hypertrophy. The diminished SR Ca(2+)-ATPase expression might result in reduced Ca(2+)reuptake into the SR and might contribute to altered contractile behavior in hypertrophied human myocardium.

Measurement of sarcoplasmic reticulum Ca2+-ATPase activity and E-type Mg2+-ATPase activity in rat heart homogenates

The presence of a high and nonlinear Ca2+-independent (or basal) ATPase activity in rat heart preparations makes difficult the reliable measurement of sarcoplasmic reticulum (SR) Ca2+-ATPase activity by usual methods. A spectrophotometric assay for the accurate determination of SR Ca2+-ATPase activity in unfractionated homogenates from rat heart is described. The procedure is based on that reported by Simonides and van Hardeveld (1990, Anal. Biochem. 191, 321-331) for skeletal muscle homogenates. To avoid overestimation of the Ca2+-ATPase activity of cardiac homogenates that occurs when sequential measurements of total and basal ATPase activities are performed, two parallel and independent assays are required: one with low (micromolar) and other high (millimolar) calcium concentration. Addition of thapsigargin (0.2 microM) blocked totally the activity considered as Ca2+-ATPase activity. Using this method, the rat heart homogenate Ca2+-ATPase activity was 10.5 +/- 2.0 micromol. min-1 x g-1 tissue wet weight (n = 8). Likewise, a spectrophotometric assay for measuring E-type Mg2+-ATPase activity in cardiac total homogenates has been developed, comparing the following characteristics of the enzymatic activity in homogenate and a membrane-enriched fraction: first-order rate constant for ATP-dependent inactivation, Km for ATP, and effects of concanavalin A, Triton X-100, and specific inhibitors.

Impaired cardiac performance in heterozygous mice with a null mutation in the sarco(endo)plasmic reticulum Ca2+-ATPase isoform 2 (SERCA2) gene

The sarco(endo)plasmic reticulum Ca2+-ATPase isoform 2 (SERCA2) gene encodes both SERCA2a, the cardiac sarcoplasmic reticulum Ca2+ pump, and SERCA2b, which is expressed in all tissues. To gain a better understanding of the physiological functions of SERCA2, we used gene targeting to develop a mouse in which the promoter and 5' end of the gene were eliminated. Mating of heterozygous mutant mice yielded wild-type and heterozygous offspring; homozygous mutants were not observed. RNase protection, Western blotting, and biochemical analysis of heart samples showed that SERCA2 mRNA was reduced by approximately 45% in heterozygous mutant hearts and that SERCA2 protein and maximal velocity of Ca2+ uptake into the sarcoplasmic reticulum were reduced by approximately 35%. Measurements of cardiovascular performance via transducers in the left ventricle and right femoral artery of the anesthetized mouse revealed reductions in mean arterial pressure, systolic ventricular pressure, and the absolute values of both positive and negative dP/dt in heterozygous mutants. These results demonstrate that two functional copies of the SERCA2 gene are required to maintain normal levels of SERCA2 mRNA, protein, and Ca2+ sequestering activity, and that the deficit in Ca2+ sequestering activity due to the loss of one copy of the SERCA2 gene impairs cardiac contractility and relaxation.

Cardiac-specific overexpression of mouse cardiac calsequestrin is associated with depressed cardiovascular function and hypertrophy in transgenic mice

Calsequestrin is a high capacity Ca2+-binding protein in the sarcoplasmic reticulum (SR) lumen. To elucidate the functional role of calsequestrin in vivo, transgenic mice were generated that overexpressed mouse cardiac calsequestrin in the heart. Overexpression (20-fold) of calsequestrin was associated with cardiac hypertrophy and induction of a fetal gene expression program. Isolated transgenic cardiomyocytes exhibited diminished shortening fraction (46%), shortening rate (60%), and relengthening rate (60%). The Ca2+ transient amplitude was also depressed (45%), although the SR Ca2+ storage capacity was augmented, as suggested by caffeine application studies. These alterations were associated with a decrease in L-type Ca2+ current density and prolongation of this channel's inactivation kinetics without changes in Na+-Ca2+ exchanger current density. Furthermore, there were increases in protein levels of SR Ca2+-ATPase, phospholamban, and calreticulin and decreases in FKBP12, without alterations in ryanodine receptor, junctin, and triadin levels in transgenic hearts. Left ventricular function analysis in Langendorff perfused hearts and closed-chest anesthetized mice also indicated depressed rates of contraction and relaxation of transgenic hearts. These findings suggest that calsequestrin overexpression is associated with increases in SR Ca2+ capacity, but decreases in Ca2+-induced SR Ca2+ release, leading to depressed contractility in the mammalian heart.

Overexpression of the Na(+)-Ca2+ exchanger leads to enhanced inotropic responsiveness to Na(+)-channel agonist without sarcoplasmic reticulum protein changes in transgenic mice

The present study investigated the influence of over-expression of the cardiac Na(+)-Ca2+ exchanger on myocardial force of contraction and alterations in sarcoplasmic reticulum (SR) Ca(2+)-handling proteins in a transgenic mouse model. Inotropic effects of Na+ channel agonist BDF 9148 and isoprenaline were determined in isolated electrically driven atria. Protein levels of key SR Ca(2+)-handling proteins were determined by Western blot analysis. Transgenic animals had no myocardial hypertrophy or failure. The positive inotropic effect of BDF 9148 was significantly more pronounced in myocardium for transgenic animals, whereas the inotropic response to isoprenaline was similar in both groups. Strong immunoreactivity of the transgene Na(+)-Ca2+ exchanger was detected in myocardium of transgenic animals. Protein levels of SR Ca(2+)-ATPase, phospholamban, and calsequestrin were unchanged. In conclusion, transgenic overexpression of the Na(+)-Ca2+ exchanger is accompanied by increased force development following Na+ channel agonist administration, even though Ca2+ proteins of the SR are unchanged.

The Raf-MEK-ERK cascade represents a common pathway for alteration of intracellular calcium by Ras and protein kinase C in cardiac myocytes

Ras and protein kinase C (PKC), which regulate the Raf-MEK-ERK cascade, may participate in the development of cardiac hypertrophy, a condition characterized by diminished and prolonged contractile calcium transients. To directly examine the influence of this pathway on intracellular calcium ([Ca2+]i), cardiac myocytes were cotransfected with effectors of this pathway and with green fluorescent protein, which allowed the living transfected myocytes to be identified and examined for [Ca2+]i via indo-1. Transfection with constitutively active Ras (Ha-RasV12) increased cell size, decreased expression of the myofibrils and the calcium-regulatory enzyme SERCA2, and reduced the magnitude and prolonged the decay phase of the contractile [Ca2+]i transients. Similar effects on [Ca2+]i were obtained with Ha-RasV12S35, a Ras mutant that selectively couples to Raf, and with constitutively active Raf. In contrast, Ha-RasV12C40, a Ras mutant that activates the phosphatidylinositol 3-kinase pathway, had a lesser effect. The PKC-activating phorbol ester, phorbol 12-myristate 13-acetate, also prolonged the contractile [Ca2+]i transients. Cotransfection with dnMEK inhibited the effects of Ha-RasV12, Raf, and phorbol 12-myristate 13-acetate on [Ca2+]i. The effects of Ha-RasV12 and Raf on [Ca2+]i were also counteracted by SERCA2 overexpression. Both Ras and PKC may thus regulate cardiac [Ca2+]i via the Raf-MEK-ERK cascade, and this pathway may represent a critical determinant of cardiac physiological function.

Excitation-contraction coupling in the day 15 embryonic chick heart with persistent truncus arteriosus

Ca2+ transients were examined in embryonic chick hearts with an experimentally induced cardiac neural crest-related outflow tract defect known as persistent truncus arteriosus (PTA). In all of the animal models of neural crest-related heart defects, prenatal mortality is too high to be attributed to structural defects of the heart alone, suggesting that there is altered development of the myocardium. Earlier reports indicating reduced L-type Ca2+ current in hearts with PTA suggest that poor viability may be related to impairment of cardiac excitation-contraction coupling. To test this hypothesis, direct measurements of the systolic Ca2+ transient in fura-2-loaded myocytes from normal hearts and hearts with PTA were carried out. We found that Ca2+ transients were severely depressed in hearts with PTA and difficult to measure above background noise unless signal averaged or treated with isoproterenol (ISO). We confirmed that the reduced Ca2+ transients were due, at least partly, to a reduction in L-type Ca2+ current. In addition we found that although ISO could raise the L-type current in hearts with PTA to the level found in normal hearts in the absence of ISO, it could not fully restore the Ca2+ transient. Furthermore, caffeine-stimulated Ca2+ transients were diminished in size and the time-to-peak and the decaying phase were significantly slowed. Interestingly, these observations were not accompanied by a reduction in the number of Ca2+ release channels. These results indicated an impairment of SR function in addition to the reduction in L-type Ca2+ current. These results strongly support our hypothesis that the poor viability of embryos with PTA is due to impaired cardiac excitation-contraction coupling.

The sarco(endo)plasmic reticulum Ca(2+)-ATPase gene is regulated at the transcriptional level during compensated left ventricular hypertrophy in the rat

In mammalian myocardium, relaxation is mainly triggered by the reuptake of calcium from the cytosol to the lumen of the sarcoplasmic reticulum (SR) through the cardiac isoform of the sarco(endo)plasmic reticulum calcium ATPase, SERCA2a. Relaxation abnormalities related to deficient SR Ca(2+)-uptake have been identified in human heart failure and in animal models of cardiac hypertrophy and failure. These alterations have been associated with a reduction in SERCA2a activity and in steady-state SERCA2a protein and mRNA levels. As a first step in the analysis of the mechanisms responsible for this reduction, we have studied a possible down-regulation of the SERCA2 gene transcription during left ventricular hypertrophy (LVH) induced by constriction of the ascending aorta in the rat. Quantifications of the mRNA levels demonstrated no alteration, compared to sham-operated rats, at 5 d after imposition of the pressure overload, whereas a significant decrease was observed at 11 d. Transcription in-vitro experiments (cardiac nuclear run-on assays) performed in isolated cardiomyocytes nuclei showed no changes at 5 d and a 37% reduction of the SERCA2 gene transcription at 11 d. These results strongly suggest that SERCA2 gene expression down-regulation during cardiac hypertrophy occurs, at least in part, at the level of the transcription.