Hypertension and albuminuria in chronic kidney disease mapped to a mouse chromosome 11 locus.

Chronic kidney disease (CKD) is a key cause of hypertension and a potent independent risk for cardiovascular disease. Epidemiological studies suggest a strong genetic component determining susceptibility for renal disease and, by inference, the associated cardiovascular risk. With a subtotal nephrectomy model of kidney disease, we found the 129S6 mouse strain to be very susceptible to the development of hypertension, albuminuria, and kidney injury, whereas the C57BL/6 strain is relatively resistant. Accordingly, we set out to map quantitative trait loci conferring susceptibility to hypertension and albuminuria using this model with F2 mice. We found significant linkage of the blood pressure trait to two loci. At D11Mit143, mice homozygous for the 129S6 allele had significantly higher systolic blood pressure than mice heterozygous or homozygous for the C57BL/6 allele. Similarly, at D1Mit308, there was an excellent correlation between genotype and the blood pressure phenotype. The effect of the chromosome 11 locus was verified with a separate cohort of F2 mice. For the albuminuria trait, a significant locus was found at D11Mit143, which overlaps the blood pressure trait locus. Our studies have identified a region spanning approximately 8 cM on mouse chromosome 11 that is associated with susceptibility to hypertension and albuminuria in CKD.

Defects in cardiomyocyte function: role of beta-adrenergic receptor dysfunction.

Heart failure is a common clinical syndrome characterized by increased levels of circulating catecholamines and extensive abnormalities in the beta-adrenergic receptor (betaAR) system. Interestingly, whether dampening of betaAR signals is beneficial or detrimental for the failing cardiomyocyte is still controversial. In this review we will discuss a number of studies addressing the role of betaAR dysfunction in the development and progression of cardiomyocyte failure, and novel possible strategies to ameliorate cardiomyocyte contractility in heart failure through the normalization of betaAR signaling.

Modifier genes and heart failure.

Recent progress in genomic applications have led to a better understanding of the relationship between genetic background and cardiovascular diseases such as heart failure. A considerable component of the variability in heart failure outcome is due to modifier genes, i.e. genes that are not involve in the genesis of a disease but modify the severity of the phenotypic expression once the disease has developed. The strategy most commonly used to identify modifier genes is based on association studies between the severity of the phenotype of the disease (morbidity and/or mortality) and the sequence variation(s) of selected candidate gene(s). This strategy has showed that several polymorphisms of the beta1 and beta2 adrenergic receptors genes and the angiotensin converting enzyme gene are correlated to the prognosis of patients with heart failure. Recently, we have applied an experimental strategy, known as genome mapping, for the identification of heart failure modifier genes. Genome mapping has previously been used with success to identify the genes involved in the development of both monogenic and multifactorial diseases. We have showed that the prognosis of heart failure mice, induced through overexpressing calsequestrin, is linked to 2 Quantitative Trait Loci (QTL) localized on chromosome 2 and 3. Using both strategies (candidate gene and genome mapping) should allow us to identify a number of modifier genes that may provide a more rational approach to identify patients at risk for disease and response to therapy.

Impact of genetic polymorphisms on heart failure prognosis.

Recent progress in genomic applications have led to a better understanding of the relationship between genetic background and cardiovascular diseases such as heart failure. The broad variability in heart failure patient outcome is in part secondary to modifier genes, i.e. genes that are not involved in the genesis of a disease but modify the severity of the phenotypic expression once the disease has developed. The strategy most commonly used to identify modifier genes is based on association studies between the severity of the phenotype and the sequence variation(s) of selected candidate gene(s). Using this strategy, several polymorphisms of the beta 1 and beta 2-adrenergic receptors genes and the angiotensin converting enzyme gene have been correlated to the prognosis of patients with heart failure. Recently, we have applied an experimental strategy, known as genome mapping, for the identification of heart failure modifier genes. Genome mapping has previously been used with success to identify the genes involved in the development of both monogenic and multifactorial diseases. We have shown that the prognosis of heart failure mice, induced through calsequestrin overexpression, is linked to two Quantitative Trait Loci localized on chromosomes 2 and 3. Using both strategies (candidate gene and genome mapping) should allow us to identify a number of modifier genes that may provide a more rational approach to identify patients with the worst prognosis and to predict their response to therapy.

Augmentation of cardiac contractility mediated by the human beta(3)-adrenergic receptor overexpressed in the hearts of transgenic mice.

BACKGROUND: Stimulation of beta(1)- and beta(2)-adrenergic receptors (ARs) in the heart results in positive inotropy. In contrast, it has been reported that the beta(3)AR is also expressed in the human heart and that its stimulation leads to negative inotropic effects. METHODS AND RESULTS: To better understand the role of beta(3)ARs in cardiac function, we generated transgenic mice with cardiac-specific overexpression of 330 fmol/mg protein of the human beta(3)AR (TGbeta(3) mice). Hemodynamic characterization was performed by cardiac catheterization in closed-chest anesthetized mice, by pressure-volume-loop analysis, and by echocardiography in conscious mice. After propranolol blockade of endogenous beta(1)- and beta(2)ARs, isoproterenol resulted in an increase in contractility in the TGbeta(3) mice (30%), with no effect in wild-type mice. Similarly, stimulation with the selective human beta(3)AR agonist L-755,507 significantly increased contractility in the TGbeta(3) mice (160%), with no effect in wild-type mice, as determined by hemodynamic measurements and by end-systolic pressure-volume relations. The underlying mechanism of the positive inotropy incurred with L-755,507 in the TGbeta(3) mice was investigated in terms of beta(3)AR-G-protein coupling and adenylyl cyclase activation. Stimulation of cardiac membranes from TGbeta(3) mice with L-755,507 resulted in a pertussis toxin-insensitive 1.33-fold increase in [(35)S]GTPgammaS loading and a 1.6-fold increase in adenylyl cyclase activity. CONCLUSIONS: Cardiac overexpression of human beta(3)ARs results in positive inotropy only on stimulation with a beta(3)AR agonist. Overexpressed beta(3)ARs couple to G(s) and activate adenylyl cyclase on agonist stimulation.

Beta-adrenergic axis and heart disease.

Beta-adrenergic receptors (beta-ARs) belong to a large family of G-protein-coupled receptors (GPCRs) that form the interface between the sympathetic nervous system and the cardiovascular system. The beta-AR signal system is one of the most powerful regulators of cardiac function, mediated by the effects of the sympathetic transmitters epinephrine and norepinephrine. In a number of cardiac diseases, however, the biology of beta-AR signaling pathways is altered dramatically. Here we discuss the role of beta-AR signaling in the normal and abnormal heart and how the use of genetically engineered mouse models has helped in our understanding of the pathophysiology of cardiac disease.

G protein-coupled receptor signalling in in vivo cardiac overload.

Cardiac myocytes respond to biomechanical stress by initiating cellular processes that lead to hypertrophy. Although cardiac hypertrophy is a response to increased stress on the heart, it is associated with elevated plasma catecholamine levels and an increase in cardiac morbidity and mortality. Understanding the cellular signals that initiate the hypertrophic response will be of critical importance to identify pathways that mediate the maladaptive deterioration of the hypertrophic heart to one of cardiac failure. This review will focus on the role of G protein-coupled receptors in the activation of signalling pathways in the heart, such as the mitogen activated protein kinase and phosphoinositide-3 kinase pathways.

Important role of endogenous norepinephrine and epinephrine in the development of in vivo pressure-overload cardiac hypertrophy.

OBJECTIVES: We sought to define the role of norepinephrine and epinephrine in the development of cardiac hypertrophy and to determine whether the absence of circulating catecholamines alters the activation of downstream myocardial signaling pathways. BACKGROUND: Cardiac hypertrophy is associated with elevated plasma catecholamine levels and an increase in cardiac morbidity and mortality. Although considerable evidence suggests that G-protein-coupled receptors are involved in the hypertrophic response, it remains controversial whether catecholamines are required for the development of in vivo cardiac hypertrophy. METHODS: We performed transverse aortic constriction (TAC) in dopamine beta-hydroxylase knockout mice (Dbh(-/-), genetically altered mice that are completely devoid of endogenous norepinephrine and epinephrine) and littermate control mice. After induction of cardiac hypertrophy, the mitogen-activated protein kinase (MAPK) signaling pathways were measured in pressure-overloaded/wild-type and Dbh(-/-) hearts. RESULTS: Compared with the control animals, cardiac hypertrophy was significantly blunted in Dbh(-/-) mice, which was not associated with altered cardiac function, as assessed by transthoracic echocardiography in conscious mice. The extracellularly regulated kinase (ERK 1/2), c-jun-NH(2)-terminal kinase (JNK) and p38 MAPK pathways were all activated by two- to threefold after TAC in the control animals. In contrast, induction of the three pathways (ERK 1/2, JNK and p38) was completely abolished in Dbh(-/-) mice. CONCLUSIONS: These data demonstrate a nearly complete requirement of endogenous norepinephrine and epinephrine for the induction of in vivo pressure-overload cardiac hypertrophy and for the activation of hypertrophic signaling pathways.

Regulation of myocardial betaARK1 expression in catecholamine-induced cardiac hypertrophy in transgenic mice overexpressing alpha1B-adrenergic receptors.

OBJECTIVES: Using a transgenic mouse model of myocardial-targeted overexpression of the wild-type alpha1B adrenergic receptor (AR) (Tg alpha43), we studied the role of the betaAR kinase (betaARK1) in the evolution of myocardial hypertrophy and its transition to heart failure (HF). BACKGROUND: Increased myocardial expression of betaARK1 has been shown to be associated with HF and certain models of hypertrophy. METHODS: Tg alpha43 mice and their nontransgenic littermate controls were treated with the alpha1AR agonist phenylephrine (PE) for 3, 7 or 14 days to characterize the cardiac consequences. RESULTS: Nontransgenic littermate control mice treated for 14 days with PE display cardiac hypertrophy with no increase in betaARK1 expression. However, Tg alpha43 animals show a reduced tolerance to 14-day PE treatment, demonstrated by reduced survival and severe cardiac hypertrophy. Moreover, PE treatment for three and seven days in Tg alpha43 mice resulted in an exaggerated hypertrophic response accompanied by significant cardiac biochemical abnormalities that are normally associated with HF, including fetal gene expression, reduced betaAR density and enhanced betaARK1 expression. We also found reduced myocardial stores of the sympathetic neurotransmitter neuropeptide Y. CONCLUSIONS: These data suggest that PE-treated Tg alpha43 mice have chronic activation of the cardiac sympathetic nervous system, which may be responsible for the appearance of apparent maladaptive hypertrophy with an evolution towards HF and sudden death. Thus, the cardiac phenotypes found in these mice are not the direct result of enhanced alpha1B AR signaling and suggest that betaARK1 is a key molecule in the transition of myocardial hypertrophy to HF.

Cardiac beta ARK1 inhibition prolongs survival and augments beta blocker therapy in a mouse model of severe heart failure.

Chronic human heart failure is characterized by abnormalities in beta-adrenergic receptor (betaAR) signaling, including increased levels of betaAR kinase 1 (betaARK1), which seems critical to the pathogenesis of the disease. To determine whether inhibition of betaARK1 is sufficient to rescue a model of severe heart failure, we mated transgenic mice overexpressing a peptide inhibitor of betaARK1 (betaARKct) with transgenic mice overexpressing the sarcoplasmic reticulum Ca(2+)-binding protein, calsequestrin (CSQ). CSQ mice have a severe cardiomyopathy and markedly shortened survival (9 +/- 1 weeks). In contrast, CSQ/betaARKct mice exhibited a significant increase in mean survival age (15 +/- 1 weeks; P < 0.0001) and showed less cardiac dilation, and cardiac function was significantly improved (CSQ vs. CSQ/betaARKct, left ventricular end diastolic dimension 5.60 +/- 0.17 mm vs. 4.19 +/- 0.09 mm, P < 0.005; % fractional shortening, 15 +/- 2 vs. 36 +/- 2, P < 0.005). The enhancement of the survival rate in CSQ/betaARKct mice was substantially potentiated by chronic treatment with the betaAR antagonist metoprolol (CSQ/betaARKct nontreated vs. CSQ/betaARKct metoprolol treated, 15 +/- 1 weeks vs. 25 +/- 2 weeks, P < 0.0001). Thus, overexpression of the betaARKct resulted in a marked prolongation in survival and improved cardiac function in a mouse model of severe cardiomyopathy that can be potentiated with beta-blocker therapy. These data demonstrate a significant synergy between an established heart-failure treatment and the strategy of betaARK1 inhibition.

Role of sodium channel deglycosylation in the genesis of cardiac arrhythmias in heart failure.

We investigated the cellular and molecular mechanisms underlying arrhythmias in heart failure. A genetically engineered mouse lacking the expression of the muscle LIM protein (MLP-/-) was used in this study as a model of heart failure. We used electrocardiography and patch clamp techniques to examine the electrophysiological properties of MLP-/- hearts. We found that MLP-/- myocytes had smaller Na+ currents with altered voltage dependencies of activation and inactivation and slower rates of inactivation than control myocytes. These changes in Na+ currents contributed to longer action potentials and to a higher probability of early afterdepolarizations in MLP-/- than in control myocytes. Western blot analysis suggested that the smaller Na+ current in MLP-/- myocytes resulted from a reduction in Na+ channel protein. Interestingly, the blots also revealed that the alpha-subunit of the Na+ channel from the MLP-/- heart had a lower average molecular weight than in the control heart. Treating control myocytes with the sialidase neuraminidase mimicked the changes in voltage dependence and rate of inactivation of Na+ currents observed in MLP-/- myocytes. Neuraminidase had no effect on MLP-/- cells thus suggesting that Na+ channels in these cells were sialic acid-deficient. We conclude that deficient glycosylation of Na+ channel contributes to Na+ current-dependent arrhythmogenesis in heart failure.

Pressure-independent enhancement of cardiac hypertrophy in natriuretic peptide receptor A-deficient mice.

Mice lacking natriuretic peptide receptor A (NPRA) have marked cardiac hypertrophy and chamber dilatation disproportionate to their increased blood pressure (BP), suggesting, in support of previous in vitro data, that the NPRA system moderates the cardiac response to hypertrophic stimuli. Here, we have followed the changes in cardiac function in response to altered mechanical load on the heart of NPRA-null mice (Npr1-/-). Chronic treatment with either enalapril, furosemide, hydralazine, or losartan were all effective in reducing and maintaining BP at normal levels without affecting heart weight/body weight. In the reverse direction, we used transverse aortic constriction (TAC) to induce pressure overload. In the Npr1-/- mice, TAC resulted in a 15-fold increase in atrial natriuretic peptide (ANP) expression, a 55% increase in left ventricular weight/body weight (LV/BW), dilatation of the LV, and significant decline in cardiac function. In contrast, banded Npr1+/+ mice showed only a threefold increase in ANP expression, an 11% increase in LV/BW, a 0.2 mm decrease in LV end diastolic dimension, and no change in fractional shortening. The activation of mitogen-activated protein kinases that occurs in response to TAC did not differ in the Npr1+/+ and Npr1-/- mice. Taken together, these results suggest that the NPRA system has direct antihypertrophic actions in the heart, independent of its role in BP control.

Cardiac overexpression of a G(q) inhibitor blocks induction of extracellular signal-regulated kinase and c-Jun NH(2)-terminal kinase activity in in vivo pressure overload.

BACKGROUND: Understanding the cellular signals that initiate cardiac hypertrophy is of critical importance in identifying the pathways that mediate heart failure. The family of mitogen-activated protein kinases (MAPKs), including the extracellular signal-regulated kinases (ERKs), c-Jun NH(2)-terminal kinase (JNK), and p38 MAPKs, may play specific roles in myocardial growth and function. METHODS AND RESULTS: To determine the mechanism of activation of MAPK pathways during the development of cardiac hypertrophy, we evaluated the induction of MAPK activity after aortic constriction in wild-type and in 2 types of cardiac gene-targeted mice: one overexpressing a carboxyl-terminal peptide of Galpha(q) that inhibits G(q)-mediated signaling (TG GqI mouse) and another overexpressing a carboxyl-terminal peptide of beta-adrenergic receptor kinase-1 that inhibits Gbetagamma signaling (TG betaARKct mouse). Wild-type mice with pressure overload showed an acute induction of JNK, followed by the induction of p38/p38beta at 3 days and ERK at 7 days. Both JNK and p38 activity remained elevated at 7 days after banding. In TG GqI mice, hypertrophy was significantly attenuated, and induction of ERK and JNK activity was abolished, whereas the induction of p38 and p38beta was robust, but delayed. By contrast, all 3 MAPK pathways were activated by aortic constriction in the TG betaARKct hearts, suggesting a role for Galpha(q), but not Gbetagamma. CONCLUSIONS: Taken together, these data show that the induction of ERK and JNK activity in in vivo pressure-overload hypertrophy is mediated through the stimulation of G(q)-coupled receptors and that non-G(q)-mediated pathways are recruited to activate p38 and p38beta.

Agonist-dependent recruitment of phosphoinositide 3-kinase to the membrane by beta-adrenergic receptor kinase 1. A role in receptor sequestration.

Agonist-dependent desensitization of the beta-adrenergic receptor requires translocation and activation of the beta-adrenergic receptor kinase1 by liberated Gbetagamma subunits. Subsequent internalization of agonist-occupied receptors occurs as a result of the binding of beta-arrestin to the phosphorylated receptor followed by interaction with the AP2 adaptor and clathrin proteins. Receptor internalization is known to require D-3 phosphoinositides that are generated by the action of phosphoinositide 3-kinase. Phosphoinositide 3-kinases form a family of lipid kinases that couple signals via receptor tyrosine kinases and G-protein-coupled receptors. The molecular mechanism by which phosphoinositide 3-kinase acts to promote beta-adrenergic receptor internalization is not well understood. In the present investigation we demonstrate a novel finding that beta-adrenergic receptor kinase 1 and phosphoinositide 3-kinase form a cytosolic complex, which leads to beta-adrenergic receptor kinase 1-mediated translocation of phosphoinositide 3-kinase to the membrane in an agonist-dependent manner. Furthermore, agonist-induced translocation of phosphoinositide 3-kinase results in rapid interaction with the receptor, which is of functional importance, since inhibition of phosphoinositide 3-kinase activity attenuates beta-adrenergic receptor sequestration. Therefore, agonist-dependent recruitment of phosphoinositide 3-kinase to the membrane is an important step in the process of receptor sequestration and links phosphoinositide 3-kinase to G-protein-coupled receptor activation and sequestration.

Physiological induction of a beta-adrenergic receptor kinase inhibitor transgene preserves ss-adrenergic responsiveness in pressure-overload cardiac hypertrophy.

BACKGROUND: Transgenic mice with constitutive myocardium-targeted expression of a peptide inhibitor of the ss-adrenergic receptor kinase (ssARKct) have increased in vivo cardiac function and enhanced ss-adrenergic receptor (ssAR) responsiveness. METHODS AND RESULTS: In the present study, we created transgenic mice with myocardium-targeted ssARKct transgene expression under control of the CARP (cardiac ankyrin repeat protein) promoter, which is active during cardiac development and inactive in the normal adult mouse heart. Consistent with this, adult CARP-ssARKct transgenic mice have normal in vivo cardiac contractility and ssAR responsiveness indistinguishable from their nontransgenic littermates (NLCs). However, because CARP is in a group of fetal genes activated in the adult ventricle during hypertrophy, we subjected animals to transverse aortic constriction (TAC) to induce pressure overload. Seven days after TAC, CARP-ssARKct hearts had elevations in left ventricular mass similar to those in NLCs; however, TAC did induce demonstrable ssARKct expression in the transgenic hearts. TAC in NLC mice resulted in an upregulation of myocardial ssARK1 and a loss of ssAR-mediated inotropic reserve. Importantly, although ssARK1 was increased in the hypertrophic CARP-ssARKct mice, the in vivo loss of ssAR responsiveness was not seen after induced ssARKct expression. CONCLUSIONS: These results demonstrate that acute ssARK1 inhibition can restore lost myocardial ssAR responsiveness and inotropic reserve in vivo. Furthermore, these mice demonstrate the novel utility of the CARP promoter as an inducible element responsive to pathophysiological conditions in the adult heart.

Cellular and functional defects in a mouse model of heart failure.

Heart failure and dilated cardiomyopathy develop in mice that lack the muscle LIM protein (MLP) gene (MLP(-/-)). The character and extent of the heart failure that occurs in MLP(-/-) mice were investigated using echocardiography and in vivo pressure-volume (P-V) loop measurements. P-V loop data were obtained with a new method for mice (sonomicrometry) using two pairs of orthogonal piezoelectric crystals implanted in the endocardial wall. Sonomicrometry revealed right-shifted P-V loops in MLP(-/-) mice, depressed systolic contractility, and additional evidence of heart failure. Cellular changes in MLP(-/-) mice were examined in isolated single cells using patch-clamp and confocal Ca(2+) concentration ([Ca(2+)]) imaging techniques. This cellular investigation revealed unchanged Ca(2+) currents and Ca(2+) spark characteristics but decreased intracellular [Ca(2+)] transients and contractile responses and a defect in excitation-contraction coupling. Normal cellular and whole heart function was restored in MLP(-/-) mice that express a cardiac-targeted transgene, which blocks the function of beta-adrenergic receptor (beta-AR) kinase-1 (betaARK1). These data suggest that, despite the persistent stimulus to develop heart failure in MLP(-/-) mice (i.e., loss of the structural protein MLP), downregulation and desensitization of the beta-ARs may play a pivotal role in the pathogenesis. Furthermore, this work suggests that the inhibition of betaARK1 action may prove an effective therapy for heart failure.

Dilated cardiomyopathy in transgenic mice expressing a mutant A subunit of protein phosphatase 2A.

The protein phosphatase 2A (PP2A) holoenzyme consists of a catalytic subunit, C, and two regulatory subunits, A and B. The PP2A core enzyme is composed of subunits A and C. Both the holoenzyme and the core enzyme are similarly abundant in heart tissue. Transgenic mice were generated expressing high levels of a dominant negative mutant of the A subunit (A delta 5) in the heart, skeletal muscle, and smooth muscle that competes with the endogenous A subunit for binding the C subunit but does not bind B subunits. We found that the ratio of core enzyme to holoenzyme was increased in A delta 5-expressing hearts. Importantly, already at day 1 after birth, A delta 5-transgenic mice had an increased heart weight-to-body weight ratio that persisted throughout life. Echocardiographic analysis of A delta 5-transgenic hearts revealed increased end-diastolic and end-systolic dimensions and decreased fractional shortening. In addition, the thickness of the septum and of the left ventricular posterior wall was significantly reduced. On the basis of these findings, we consider the heart phenotype of A delta 5-transgenic mice to be a form of dilated cardiomyopathy that frequently leads to premature death.

Functional consequences of altering myocardial adrenergic receptor signaling.

From the ability to successfully manipulate the mouse genome has come important transgenic and gene-targeted knockout models that impact many areas of biomedical research. Genetically engineered mouse models geared toward the study of cardiovascular regulation have recently been described and provide powerful tools to study normal and compromised cardiac physiology. The genetic manipulation of the adrenergic receptor (AR) signaling system in the heart, including its regulation by desensitizing kinases, has shed light on the role of this signaling pathway in the regulation of cardiac contractility. One major finding, supported by several mouse models, is that in vivo contractility can be enhanced via alteration of myocardial AR signaling. Thus genetic manipulation of this critical receptor system in the heart represents a novel therapeutic approach for improving function of the failing heart.

Enhanced cardiac function in transgenic mice expressing a Ca(2+)-stimulated adenylyl cyclase.

The predominant functional adenylyl cyclases normally expressed in cardiac tissue and coupled to beta-adrenergic receptors are inhibited by micromolar Ca(2+) concentration. To modify the overall balance of activities, we have generated transgenic mice expressing the Ca(2+)-stimulatable adenylyl cyclase type 8 (AC8) specifically in the heart. AC activity is increased by at least 7-fold in heart membranes from transgenic animals and is stimulated by Ca(2+) in the same range of concentration that inhibits the endogenous activity. Moreover, the in vivo basal protein kinase A activity was augmented 4-fold. Overexpression of AC8 in the heart has no detrimental consequences on global cardiac function. Basal heart rate and contractile function, measured by noninvasive echocardiography, were unchanged. In contrast, on release of parasympathetic tone, the intrinsic contractility is heightened and unresponsive to further beta-adrenergic receptor stimulation. AC8 transgenic mice thus represent an original model to investigate the relative influence of Ca(2+) and cAMP on cardiac function within a phenotype of enhanced cardiac contractility and relaxation.