Differential regulation of inotropy and lusitropy in overexpressed Gsalpha myocytes through cAMP and Ca2+ channel pathways

Effect of Gonadectomy and Angiotensin II Receptor Blockade in a Mouse Model of Isoproterenol-induced Cardiac Diastolic Dysfunction

BACKGROUND

Although heart failure (HF) with preserved ejection fraction (HFpEF) is more common in postmenopausal women than in men, the effect of sex hormones on cardiac diastolic function remains unclear. We examined the effect of gonadectomy with or without the angiotensin receptor blocker olmesartan (Olm) in an isoproterenol (ISO) -induced mouse model of left ventricular hypertrophy (LVH) and cardiac diastolic dysfunction.

METHODS

ISO or ISO with Olm were administered for 28 days in sham-operated male and female, castrated (CAS), and ovariectomized (OVX) mice. LV ejection fraction (EF) and E/A ratio were analyzed by echocardiography, and the LV and lung weight corrected by tibial length were used as indices of LVH and lung congestion, respectively.

RESULTS

On echocardiography, systolic function did not differ between the four groups. LV/tibial length (TL) and Lung/TL significantly increased in all groups. The LV/TL ratio was lower in castrated-ISO vs. Male-Sham-ISO but did not differ between Female-Sham-ISO and OVX-ISO. However, the Lung/TL ratio of OVX-ISO was greater than that of Female-Sham-ISO. Olm prevented LV hypertrophy in all groups. The decrease in E/A and increase in lung weight were improved by Olm in Male-Sham and OVX-ISO but not in the other groups.

CONCLUSION

These sex differences suggest that sex hormones play a pivotal role in modulating cardiac hypertrophy and diastolic dysfunction induced by chronic β-adrenoceptor stimulation, and thus affect the therapeutic potential of angiotensin receptor blockade.

Distinct physiological effects of β1- and β2-adrenoceptors in mouse ventricular myocytes: insights from a compartmentalized mathematical model

The β1- and β2-adrenergic signaling systems play different roles in the functioning of cardiac cells. Experimental data show that the activation of the β1-adrenergic signaling system produces significant inotropic, lusitropic, and chronotropic effects in the heart, whereas the effects of the β2-adrenergic signaling system is less apparent. In this paper, a comprehensive compartmentalized experimentally based mathematical model of the combined β1- and β2-adrenergic signaling systems in mouse ventricular myocytes is developed to simulate the experimental findings and make testable predictions of the behavior of the cardiac cells under different physiological conditions. Simulations describe the dynamics of major signaling molecules in different subcellular compartments; kinetics and magnitudes of phosphorylation of ion channels, transporters, and Ca2+ handling proteins; modifications of action potential shape and duration; and [Ca2+]i and [Na+]i dynamics upon stimulation of β1- and β2-adrenergic receptors (β1- and β2-ARs). The model reveals physiological conditions when β2-ARs do not produce significant physiological effects and when their effects can be measured experimentally. Simulations demonstrated that stimulation of β2-ARs with isoproterenol caused a marked increase in the magnitude of the L-type Ca2+ current, [Ca2+]i transient, and phosphorylation of phospholamban only upon additional application of pertussis toxin or inhibition of phosphodiesterases of type 3 and 4. The model also made testable predictions of the changes in magnitudes of [Ca2+]i and [Na+]i fluxes, the rate of decay of [Na+]i concentration upon both combined and separate stimulation of β1- and β2-ARs, and the contribution of phosphorylation of PKA targets to the changes in the action potential and [Ca2+]i transient.

Cardiac Effects of Attenuating Gsα - Dependent Signaling

Aims

Inhibition of β-adrenergic signalling plays a key role in treatment of heart failure. Gsα is essential for β-adrenergic signal transduction. In order to reduce side-effects of beta-adrenergic inhibition diminishing β-adrenergic signalling in the heart at the level of Gsα is a promising option.

Methods and Results

We analyzed the influence of Gsα on regulation of myocardial function and development of cardiac hypertrophy, using a transgenic mouse model (C57BL6/J mice) overexpressing a dominant negative Gsα-mutant under control of the α-MHC-promotor. Cardiac phenotype was characterized in vivo and in vitro and under acute and chronic β-adrenergic stimulation. At rest, Gsα-DN-mice showed bradycardia (602 ± 13 vs. 660 ± 17 bpm, p<0.05) and decreased dp/dt_max (5037 ± 546- vs. 6835 ± 505 mmHg/s, p = 0.02). No significant differences were found regarding ejection fraction, heart weight and cardiomyocyte size. β-blockade by propranolol revealed no baseline differences of hemodynamic parameters between wildtype and Gsα-DN-mice. Acute adrenergic stimulation resulted in decreased β-adrenergic responsiveness in Gsα-DN-mice. Under chronic adrenergic stimulation, wildtype mice developed myocardial hypertrophy associated with increase of LV/BW-ratio by 23% (4.4 ± 0.2 vs. 3.5 ± 0.1 mg/g, p<0.01) and cardiac myocyte size by 24% (14927 ± 442 px vs. 12013 ± 583 px, p<0.001). In contrast, both parameters were unchanged in Gsα-DN-mice after chronic isoproterenol stimulation.

Conclusion

Overexpression of a dominant negative mutant of Gsα leads to decreased β-adrenergic responsiveness and is protective against isoproterenol-induced hypertrophy. Thus, Gsα-DN-mice provide novel insights into β-adrenergic signal transduction and its modulation in myocardial overload and failure.

Epac1-dependent phospholamban phosphorylation mediates the cardiac response to stresses

PKA phosphorylates multiple molecules involved in calcium (Ca2+) handling in cardiac myocytes and is considered to be the predominant regulator of β-adrenergic receptor-mediated enhancement of cardiac contractility; however, recent identification of exchange protein activated by cAMP (EPAC), which is independently activated by cAMP, has challenged this paradigm. Mice lacking Epac1 (Epac1 KO) exhibited decreased cardiac contractility with reduced phospholamban (PLN) phosphorylation at serine-16, the major PKA-mediated phosphorylation site. In Epac1 KO mice, intracellular Ca2+ storage and the magnitude of Ca2+ movement were decreased; however, PKA expression remained unchanged, and activation of PKA with isoproterenol improved cardiac contractility. In contrast, direct activation of EPAC in cardiomyocytes led to increased PLN phosphorylation at serine-16, which was dependent on PLC and PKCε. Importantly, Epac1 deletion protected the heart from various stresses, while Epac2 deletion was not protective. Compared with WT mice, aortic banding induced a similar degree of cardiac hypertrophy in Epac1 KO; however, lack of Epac1 prevented subsequent cardiac dysfunction as a result of decreased cardiac myocyte apoptosis and fibrosis. Similarly, Epac1 KO animals showed resistance to isoproterenol- and aging-induced cardiomyopathy and attenuation of arrhythmogenic activity. These data support Epac1 as an important regulator of PKA-independent PLN phosphorylation and indicate that Epac1 regulates cardiac responsiveness to various stresses.

A compartmentalized mathematical model of the β1-adrenergic signaling system in mouse ventricular myocytes

The β₁-adrenergic signaling system plays an important role in the functioning of cardiac cells. Experimental data shows that the activation of this system produces inotropy, lusitropy, and chronotropy in the heart, such as increased magnitude and relaxation rates of [Ca²⁺]_i transients and contraction force, and increased heart rhythm. However, excessive stimulation of β₁-adrenergic receptors leads to heart dysfunction and heart failure. In this paper, a comprehensive, experimentally based mathematical model of the β₁-adrenergic signaling system for mouse ventricular myocytes is developed, which includes major subcellular functional compartments (caveolae, extracaveolae, and cytosol). The model describes biochemical reactions that occur during stimulation of β₁-adrenoceptors, changes in ionic currents, and modifications of Ca²⁺ handling system. Simulations describe the dynamics of major signaling molecules, such as cyclic AMP and protein kinase A, in different subcellular compartments; the effects of inhibition of phosphodiesterases on cAMP production; kinetics and magnitudes of phosphorylation of ion channels, transporters, and Ca²⁺ handling proteins; modifications of action potential shape and duration; magnitudes and relaxation rates of [Ca²⁺]_i transients; changes in intracellular and transmembrane Ca²⁺ fluxes; and [Na⁺]_i fluxes and dynamics. The model elucidates complex interactions of ionic currents upon activation of β₁-adrenoceptors at different stimulation frequencies, which ultimately lead to a relatively modest increase in action potential duration and significant increase in [Ca²⁺]_i transients. In particular, the model includes two subpopulations of the L-type Ca²⁺ channels, in caveolae and extracaveolae compartments, and their effects on the action potential and [Ca²⁺]_i transients are investigated. The presented model can be used by researchers for the interpretation of experimental data and for the developments of mathematical models for other species or for pathological conditions.

Mathematical modeling mechanisms of arrhythmias in transgenic mouse heart overexpressing TNF-α

Transgenic mice overexpressing tumor necrosis factor-α (TNF-α mice) possess many of the features of human heart failure, such as dilated cardiomyopathy, impaired Ca(2+) handling, arrhythmias, and decreased survival. Although TNF-α mice have been studied extensively with a number of experimental methods, the mechanisms of heart failure are not completely understood. We created a mathematical model that reproduced experimentally observed changes in the action potential (AP) and Ca(2+) handling of isolated TNF-α mice ventricular myocytes. To study the contribution of the differences in ion currents, AP, Ca(2+) handling, and intercellular coupling to the development of arrhythmias in TNF-α mice, we further created several multicellular model tissues with combinations of wild-type (WT)/reduced gap junction conductance, WT/prolonged AP, and WT/decreased Na(+) current (I(Na)) amplitude. All model tissues were examined for susceptibility to Ca(2+) alternans, AP propagation block, and reentry. Our modeling results demonstrated that, similar to experimental data in TNF-α mice, Ca(2+) alternans in TNF-α tissues developed at longer basic cycle lengths. The greater susceptibility to Ca(2+) alternans was attributed to the prolonged AP, resulting in larger inactivation of I(Na), and to the decreased SR Ca(2+) uptake and corresponding smaller SR Ca(2+) load. Simulations demonstrated that AP prolongation induces an increased susceptibility to AP propagation block. Programmed stimulation of the model tissues with a premature impulse showed that reduced gap junction conduction increased the vulnerable window for initiation reentry, supporting the idea that reduced intercellular coupling is the major factor for reentrant arrhythmias in TNF-α mice.

Transmural heterogeneity of repolarization and Ca2+ handling in a model of mouse ventricular tissue

Mouse hearts have a diversity of action potentials (APs) generated by the cardiac myocytes from different regions. Recent evidence shows that cells from the epicardial and endocardial regions of the mouse ventricle have a diversity in Ca(2+) handling properties as well as K(+) current expression. To examine the mechanisms of AP generation, propagation, and stability in transmurally heterogeneous tissue, we developed a comprehensive model of the mouse cardiac cells from the epicardial and endocardial regions of the heart. Our computer model simulates the following differences between epicardial and endocardial myocytes: 1) AP duration is longer in endocardial and shorter in epicardial myocytes, 2) diastolic and systolic intracellular Ca(2+) concentration and intracellular Ca(2+) concentration transients are higher in paced endocardial and lower in epicardial myocytes, 3) Ca(2+) release rate is about two times larger in endocardial than in epicardial myocytes, and 4) Na(+)/Ca(2+) exchanger rate is greater in epicardial than in endocardial myocytes. Isolated epicardial cells showed a higher threshold for stability of AP generation but more complex patterns of AP duration at fast pacing rates. AP propagation velocities in the model of two-dimensional tissue are close to those measured experimentally. Simulations show that heterogeneity of repolarization and Ca(2+) handling are sustained across the mouse ventricular wall. Stability analysis of AP propagation in the two-dimensional model showed the generation of Ca(2+) alternans and more complex transmurally heterogeneous irregular structures of repolarization and intracellular Ca(2+) transients at fast pacing rates.

Overexpression of Gsalpha compensates for myocyte loss in diabetic cardiomyopathy

The stimulatory G protein Gsalpha transmits signals from activated beta-adrenergic receptors via the cyclic AMP-PKA pathway, targeting the key regulatory protein phospholamban. We hypothesized that mice with intrinsic activation of cardiac Gsalpha are resistant to the development of the diabetic cardiomyopathy phenotype. Accordingly, streptozotocin (STZ)-diabetes mellitus was induced in genetically engineered mice with cardiac-specific Gsalpha overexpression and in nontransgenic (NTG) littermates. At 8 weeks, Gsalpha diabetic mice showed no impairment of LV contractility nor increase in myocyte apoptosis, whereas NTG diabetic mice showed a 30% decrease in +dP/dt and -dP/dt with sustained (3-fold) myocyte loss by apoptosis. To assess the level of myocardial reactive oxygen species, we measured malondialdehyde, a surrogate marker of oxidative stress, which was increased in the hearts of NTG and Gsalpha diabetic mice. In addition, chronic hyperglycemia also increased the activity of catalase and superoxide dismutase in the hearts of NTG and Gsalpha diabetic mice. Hearts of NTG diabetic mice, but not Gsalpha mice, showed increased expression of proapoptosis Bax, downregulation in Bcl2, and an increase in the Bax/Bcl2 ratio. Hearts of NTG diabetic mice showed 60% reduction in phosphorylation at the critical Ser16 residue of phospholamban, whereas phosphorylation at Ser16 was restored in hearts of Gsalpha-diabetic mice. We conclude that cardiac-specific overexpression of Gsalpha compensates for the loss of cardiac function in diabetes mellitus.

Characterization and mechanism of P2X receptor-mediated increase in cardiac myocyte contractility

Cardiac P2X purinergic receptors can mediate an increase in myocyte contractility and a potentially important role in the heart. The P2X(4) receptor (P2X(4)R) is an important subunit of native cardiac P2X receptors. With transgenic mice with cardiac-specific overexpression of P2X(4)R (Tg) used as a model, the objectives here were to characterize the P2X receptor-mediated cellular contractile and Ca(2+) transient effects and to determine the mechanism underlying the receptor-induced increase in myocyte contractility. In response to the agonist 2-methylthioATP (2-meSATP), Tg myocytes showed an increased intracellular Ca(2+) transient, as defined by fura 2 fluorescence ratio, and an enhanced contraction shortening that were unaccompanied by cAMP accumulation or L-type Ca(2+) channel activation. The increased Ca(2+) transient was not associated with any alteration in action potential duration, resting membrane potential, or diastolic fluorescence ratio or rates of rise and decline of the Ca(2+) transient. Simultaneous Ca(2+) transient and contraction measurements did not show any agonist-mediated change in myofilament Ca(2+) sensitivity. However, activation of the overexpressed P2X(4) receptor caused an enhanced SR Ca(2+) loading, as evidenced by a 2-meSATP-evoked increase in the caffeine-induced inward current and Ca(2+) transient. Similar data were obtained in wild-type mouse ventricular myocytes. Thus an increased SR Ca(2+) content, occurring in the absence of cAMP accumulation or L-type Ca(2+) channel activation, is the principal mechanism by which cardiac P2X receptor mediates a stimulatory effect on cardiac myocyte contractility.

Computer model of action potential of mouse ventricular myocytes

We have developed a mathematical model of the mouse ventricular myocyte action potential (AP) from voltage-clamp data of the underlying currents and Ca2+ transients. Wherever possible, we used Markov models to represent the molecular structure and function of ion channels. The model includes detailed intracellular Ca2+ dynamics, with simulations of localized events such as sarcoplasmic Ca2+ release into a small intracellular volume bounded by the sarcolemma and sarcoplasmic reticulum. Transporter-mediated Ca2+ fluxes from the bulk cytosol are closely matched to the experimentally reported values and predict stimulation rate-dependent changes in Ca2+ transients. Our model reproduces the properties of cardiac myocytes from two different regions of the heart: the apex and the septum. The septum has a relatively prolonged AP, which reflects a relatively small contribution from the rapid transient outward K+ current in the septum. The attribution of putative molecular bases for several of the component currents enables our mouse model to be used to simulate the behavior of genetically modified transgenic mice.

Aging increases stiffness of cardiac myocytes measured by atomic force microscopy nanoindentation

It is well established that the aging heart exhibits left ventricular (LV) diastolic dysfunction and changes in mechanical properties, which are thought to be due to alterations in the extracellular matrix. We tested the hypothesis that the mechanical properties of cardiac myocytes significantly change with aging, which could contribute to the global changes in LV diastolic dysfunction. We used atomic force microscopy (AFM), which determines cellular mechanical property changes at nanoscale resolution in myocytes, from young (4 mo) and old (30 mo) male Fischer 344 x Brown Norway F1 hybrid rats. A measure of stiffness, i.e., apparent elastic modulus, was determined by analyzing the relationship between AFM indentation force and depth with the classical infinitesimal strain theory and by modeling the AFM probe as a blunted conical indenter. This is the first study to demonstrate a significant increase (P < 0.01) in the apparent elastic modulus of single, aging cardiac myocytes (from 35.1 +/- 0.7, n = 53, to 42.5 +/- 1.0 kPa, n = 58), supporting the novel concept that the mechanism mediating LV diastolic dysfunction in aging hearts resides, in part, at the level of the myocyte.

Insights into cardioprotection obtained from study of cellular Ca2+ handling in myocardium of true hibernating mammals

Mammalian hibernators exhibit remarkable resistance to low body temperature, whereas non-hibernating (NHB) mammals develop ventricular dysfunction and arrhythmias. To investigate this adaptive change, we compared contractile and electrophysiological properties of left ventricular myocytes isolated from hibernating (HB) woodchucks (Marmota monax) and control NHB woodchucks. The major findings of this study were the following: 1) the action potential duration in HB myocytes was significantly shorter than in NHB myocytes, but the amplitude of peak contraction was unchanged; 2) HB myocytes had a 33% decreased L-type Ca2+ current (I(Ca)) density and twofold faster I(Ca) inactivation but no change in the current-voltage relationship; 3) there were no changes in the density of inward rectifier K+ current, transient outward K+ current, or Na+/Ca2+ exchange current, but HB myocytes had increased sarcoplasmic reticulum Ca2+ content as estimated from caffeine-induced Na+/Ca2+ exchange current values; 4) expression of the L-type Ca2+ channel alpha(1C)-subunit was decreased by 30% in HB hearts; and 5) mRNA and protein levels of sarco(endo)plasmic reticulum Ca2+-ATPase 2a (SERCA2a), phospholamban, and the Na+/Ca2+ exchanger showed a pattern that is consistent with functional measurements: SERCA2a was increased and phospholamban was decreased in HB relative to NHB hearts with no change in the Na+/Ca2+ exchanger. Thus reduced Ca2+ channel density and faster I(Ca) inactivation coupled to enhanced sarcoplasmic reticulum Ca2+ release may underlie shorter action potentials with sustained contractility in HB hearts. These changes may account for natural resistance to Ca2+ overload-related ventricular dysfunction and point to an important cardioprotective mechanism during true hibernation.

PDE3 inhibition in dilated cardiomyopathy: reasons to reconsider

BACKGROUND

PDE3 cyclic nucleotide phosphodiesterases have important roles in regulating cAMP- and cGMP-mediated signaling. Drugs that inhibit these enzymes raise cAMP and cGMP content in cardiac and vascular smooth muscle and increase the phosphorylation of proteins by cAMP- and cGMP-dependent protein kinases (PK-A and PK-G), thereby eliciting inotropic and vasodilatory responses.

METHODS

Although these actions are beneficial acutely in patients with dilated cardiomyopathy, long-term use of these agents was shown in several clinical trials to increase mortality. Several new clinical studies, however, suggest PDE3 inhibitors may be safe and effective when used in conjunction with beta-adrenergic receptor antagonists, whereas new studies at the cellular and molecular levels indicate that there are several isoforms of these enzymes in cardiac and vascular myocytes that are likely to regulate cAMP content in different intracellular compartments.

CONCLUSIONS

Both sets of observations suggest that PDE3 inhibition may be refined to allow more selective effects on phosphorylation of PK-A substrates, possibly allowing the beneficial effects of PDE3 inhibition to be separated from the adverse long-term consequences of their use.

Mechanism of enhanced cardiac function in mice with hypertrophy induced by overexpressed Akt

Transgenic mice with cardiac-specific overexpression of active Akt (TG) not only exhibit hypertrophy but also show enhanced left ventricular (LV) function. In 3-4-month-old TG, heart/body weight was increased by 60% and LV ejection fraction was elevated (84 +/- 2%, p < 0.01) compared with nontransgenic littermates (wild type (WT)) (73 +/- 1%). An increase in isolated ventricular myocyte contractile function (% contraction) in TG compared with WT (6.1 +/- 0.2 versus 3.5 +/- 0.2%, p < 0.01) was associated with increased Fura-2 Ca2+ transients (396 +/- 50 versus 250 +/- 24 nmol/liter, p < 0.05). The rate of relaxation (+dL/dt) was also enhanced in TG (214 +/- 15 versus 98 +/- 18 microm/s, p < 0.01). L-type Ca2+ current (ICa) density was increased in TG compared with WT (-9.0 +/- 0.3 versus 7.2 +/- 0.3 pA/pF, p < 0.01). Sarcoplasmic reticulum Ca2+ ATPase 2a (SERCA2a) protein levels were increased (p < 0.05) by 6.6-fold in TG, which could be recapitulated in vitro by adenovirus-mediated overexpression of Akt in cultured adult ventricular myocytes. Conversely, inhibiting SERCA with either ryanodine or thapsigargin affected myocyte contraction and relaxation and Ca2+ channel kinetics more in TG than in WT. Thus, myocytes from mice with overexpressed Akt demonstrated enhanced contractility and relaxation, Fura-2 Ca2+ transients, and Ca2+ channel currents. Furthermore, increased protein expression of SERCA2a plays an important role in mediating enhanced LV function by Akt. Up-regulation of SERCA2a expression and enhanced LV myocyte contraction and relaxation in Akt-induced hypertrophy is opposite to the down-regulation of SERCA2a and reduced contractile function observed in many other forms of LV hypertrophy.

Mesenteric Lymph From Rats With Thermal Injury Prolongs the Action Potential and Increases Ca2+ Transient in Rat Ventricular Myocytes

Although gut-derived mesenteric lymph from animals with thermal injury appears to lead to myocardial contractile dysfunction, the cellular mechanisms remain unclear. We examined the direct effects of intestinal lymph on excitation-contraction coupling in rat ventricular myocytes. Lymph from rats receiving burn injury (burn lymph), but not from sham-burned rats, rapidly enhanced myocyte contraction and the amplitude of Ca2+ transient; the average percentage of shortening was increased from 5.5 +/- 0.3% to 10.5 +/- 0.9%. 90% and the Ca2+ transients increased by 80% +/- 20%. Burn lymph had no effect on the amplitude of L-type Ca2+ current (ICa) or the inward rectifier K+ current, but the transient outward K+ currents (Ito) were reduced significantly by burn lymph. Inhibition of Ito was not altered by an alpha1-adrenergic receptor (AR) antagonist, prazosin, indicating that the block was not mediated via alpha1-AR signaling pathway. Action potential (AP) duration, measured at 50% and 90% repolarization, was prolonged by burn lymph. Stimulation of myocytes with AP voltage-clamp waveforms derived from prolonged AP induced by burn lymph revealed a 1.7-fold increase in Ca2+ influx via ICa compared with the Ca2+ influx induced by control AP. Blocking of Ito by 4-aminopyridine prolonged AP duration and increased Ca2+ transients, mimicking the effects of burn lymph. Burn lymph did not affect Na+/Ca2+ exchange currents or caffeine-induced SR Ca2+ release. Thus, acute exposure of normal cardiac myocytes to burn lymph increases Ca2+ transients by a prolongation of AP as a result of a reduction of Ito with no intrinsic change in ICa or exchanger. The electrophysiological changes are similar to those that occur during compensated cardiac hypertrophy, suggesting a common mechanistic link between burn lymph- and hypertrophy-induced cardiac dysfunction.

Activation of Mst1 causes dilated cardiomyopathy by stimulating apoptosis without compensatory ventricular myocyte hypertrophy

Activation of mammalian sterile 20-like kinase 1 (Mst1) by genotoxic compounds is known to stimulate apoptosis in some cell types. The importance of Mst1 in cell death caused by clinically relevant pathologic stimuli is unknown, however. In this study, we show that Mst1 is a prominent myelin basic protein kinase activated by proapoptotic stimuli in cardiac myocytes and that Mst1 causes cardiac myocyte apoptosis in vitro in a kinase activity-dependent manner. In vivo, cardiac-specific overexpression of Mst1 in transgenic mice results in activation of caspases, increased apoptosis, and dilated cardiomyopathy. Surprisingly, however, Mst1 prevents compensatory cardiac myocyte elongation or hypertrophy despite increased wall stress, thereby obscuring the use of the Frank-Starling mechanism, a fundamental mechanism by which the heart maintains cardiac output in response to increased mechanical load at the single myocyte level. Furthermore, Mst1 is activated by ischemia/reperfusion in the mouse heart in vivo. Suppression of endogenous Mst1 by cardiac-specific overexpression of dominant-negative Mst1 in transgenic mice prevents myocyte death by pathologic insults. These results show that Mst1 works as both an essential initiator of apoptosis and an inhibitor of hypertrophy in cardiac myocytes, resulting in a previously unrecognized form of cardiomyopathy.

Accelerated cardiomyopathy in mice with overexpression of cardiac G(s)alpha and a missense mutation in the alpha-myosin heavy chain

BACKGROUND

To understand further the pathogenesis of familial hypertrophic cardiomyopathy, we determined how the cardiomyopathy induced by an Arg403-->Gln missense mutation in the alpha-myosin heavy chain (403) is affected by chronically enhancing sympathetic drive by mating the mice with those overexpressing G(s)alpha (G(s)alpha x403).

METHODS AND RESULTS

Heart rate in 3-month-old conscious mice was elevated similarly (P<0.05) in mice overexpressing G(s)alpha (G(s)alpha mice; 746 +/- 14 bpm) and G(s)alpha x403 mice (718+/- 19 bpm) compared with littermate wild-type mice (WT; 623+/- 18 bpm) and 403 mice (594+/- 16 bpm). Left ventricular ejection fraction (LVEF), as determined by echocardiography, was enhanced in G(s)alpha x403 mice (88+/- 1%, P<0.001) compared with WT (69+/- 1%), 403 (75+/- 1%), and G(s)alpha (69 +/- 2%) mice. Isolated cardiomyocytes from G(s)alpha x403 mice also exhibited higher (P<0.001) baseline percent contraction (11.9+/- 0.5%) than WT (7.0+/- 0.5%), 403 (5.5+/- 0.5%), and G(s)alpha (7.8+/- 0.3%) cardiomyocytes. Relaxation of myocytes was impaired in 403 mice compared with WT but enhanced in G(s)alpha and normalized in G(s)alpha x403 mice. This was also observed in vivo. In vivo isoproterenol (0.1 microgram . kg(-1) . min(-1)) increased LVEF to maximal levels in G(s)alpha x403 and G(s)alpha, whereas in 403, the response was attenuated compared with WT. At 10 months of age, G(s)alpha x403 had significantly depressed LVEF (57 +/- 4%). Histopathological examination demonstrated that myocyte hypertrophy and fibrosis were already present in young G(s)alpha x403 mice and that old animals had severe cardiomyopathy. By 15 months of age, the survival of G(s)alpha x403 was 0% compared with 100% for WT, 71% for G(s)alpha, and 100% for 403 mice (P<0.05).

CONCLUSIONS

These results show that the cardiomyopathy developed by G(s)alpha x403 mice is synergistic rather than additive, most likely owing to the elevated baseline function combined with enhanced responsiveness to sympathetic stimulation.

Enhanced iNOS function in myocytes one day after brief ischemic episode

There is increasing evidence that nitric oxide (NO) produced by inducible NO synthase (iNOS) plays a key role in cadioprotection during the "second window of protection" (SWOP). The goals of this study were to determine 1) whether a transient ischemic episode [10-min coronary artery occlusion (CAO), followed by full reperfusion] enhances NOS function in cardiac myocytes, 2) which specific NOS isoform is responsible for the enhanced NOS function in myocytes, and 3) to localize iNOS expression during SWOP. To address these questions, 10 dogs were instrumented to measure aortic and left ventricular pressures and wall thickness. At 1-2 wk after recovery, myocardial ischemia was induced regionally by a 10-min left circumflex CAO. After 24-h reperfusion, cardiac myocytes were isolated from the previously ischemic and nonischemic regions (n = 6). Myocyte contractile function was assessed using a video motion detector at 1 Hz (35 +/- 2 degrees C). At baseline, myocyte contractile function (% contraction) was similar in the two regions (ischemic 7.8 +/- 0.5% vs. nonischemic 7.8 +/- 0.2%). L-Arginine (1 mM) significantly reduced (P < 0.05) myocyte contraction in the ischemic (-34 +/- 3%, P < 0.05) but not (-7 +/- 4%) nonischemic regions; these responses were abolished by N(G)-nitro-L-arginine (1 mM), a nonspecific NOS inhibitor, as well as 2-amino-5,6-dihydro-6-methy-4H-1,3,thiazine (1 mM), a specific iNOS inhibitor. Immunohistochemistry also revealed enhanced iNOS expression in the myocardium and in particular the interstitial spaces in the ischemic zone. These results indicate that a brief ischemic episode upregulates iNOS function in myocytes as well as in the interstitial space between blood vessels and myocytes, strategically where it can regulate both vascular and myocyte function during the SWOP.

Altered excitation-contraction coupling in myocytes from remodeled myocardium after chronic myocardial infarction

Following myocardial infarction (MI), the left ventricle undergoes progressive dilatation and eccentric hypertrophy, i.e., remodeling, which is greater in the adjacent than the remote region. The cellular mechanisms underlying these regional differences were studied. One (n=5) and 8 weeks (n=8) after anteroapical MI in sheep, cardiac myocytes were isolated from the adjacent and remote regions. At 8 weeks after MI, myocyte function in the remote region was not different from values either in sham controls (n=3) or animals 1 week after MI. At 8 weeks after MI, myocyte contractile function (% contraction) was decreased, P<0.01, in the adjacent region (6.4+/-0.4%), as compared with the remote region (8.8+/-0.5%) and was associated with decreased amplitude of Ca(2+)transients (adjacent, 0.69+/-0.09 v remote, 1.08+/-0.20, P<0.05) and L-type Ca(2+)current density (adjacent, 3.6+/-0.2 v remote, 4.8+/-0.2 pA/pF, P<0.05). Relaxation was also impaired significantly in myocytes from the adjacent region, associated with decreased protein levels of SERCA2a. The myocytes were hypertrophied more in the adjacent region than the remote region. Furthermore, focal areas of central myofibrillar lysis and increased glycogen deposition were observed in the adjacent region. These results indicate that impaired excitation-contraction coupling underlies dysfunction of myocytes from the adjacent non-infarcted myocardium after chronic MI, even in the absence of heart failure. Hypertrophy is implicated as the mechanism, since these changes were noted at 8 weeks, but not at 1 week after MI.

A novel mechanism for myocardial stunning involving impaired Ca(2+) handling

The mechanism of myocardial stunning has been studied extensively in rodents and is thought to involve a decrease in Ca(2+) responsiveness of the myofilaments, degradation of Troponin I (TnI), and no change in Ca(2+) handling. We studied the mechanism of stunning in isolated myocytes from chronically instrumented pigs. Myocytes were isolated from the ischemic (stunned) and nonischemic (normal) regions after 90-minute coronary stenosis followed by 60-minute reperfusion. Baseline myocyte contraction was reduced, P<0.01, in stunned myocytes (6.3+/-0.4%) compared with normal myocytes (8.8+/-0.4%). The time for 70% relaxation was prolonged, P<0.01, in stunned myocytes (131+/-8 ms) compared with normal myocytes (105+/-5 ms). The impaired contractile function was associated with decreased Ca(2+) transients (stunned, 0.33+/-0.04 versus normal, 0.49+/-0.05, P<0.01). Action potential measurements in stunned myocytes demonstrated a decrease in plateau potential without a change in resting membrane potential. These changes were associated with decreased L-type Ca(2+)-current density (stunned, -4.8+/-0.4 versus normal, -6.6+/-0.4 pA/pF, P<0.01). There were no differences in TnI, sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a), and phospholamban protein quantities. However, the fraction of phosphorylated phospholamban monomer was reduced in stunned myocardium. In rats, stunned myocytes demonstrated reduced systolic contraction but actually accelerated relaxation and no change in Ca(2+) transients. Thus, mechanisms of stunning in the pig are radically different from the widely held concepts derived from studies in rodents and involve impaired Ca(2+) handling and dephosphorylation of phospholamban, but not TnI degradation.

Gene program for cardiac cell survival induced by transient ischemia in conscious pigs

Therapy for ischemic heart disease has been directed traditionally at limiting cell necrosis. We determined by genome profiling whether ischemic myocardium can trigger a genetic program promoting cardiac cell survival, which would be a novel and potentially equally important mechanism of salvage. Although cardiac genomics is usually performed in rodents, we used a swine model of ischemia/reperfusion followed by ventricular dysfunction (stunning), which more closely resembles clinical conditions. Gene expression profiles were compared by subtractive hybridization between ischemic and normal tissue of the same hearts. About one-third (23/74) of the nuclear-encoded genes that were up-regulated in ischemic myocardium participate in survival mechanisms (inhibition of apoptosis, cytoprotection, cell growth, and stimulation of translation). The specificity of this response was confirmed by Northern blot and quantitative PCR. Unexpectedly, this program also included genes not previously described in cardiomyocytes. Up-regulation of survival genes was more profound in subendocardium over subepicardium, reflecting that this response in stunned myocardium was proportional to the severity of the ischemic insult. Thus, in a swine model that recapitulates human heart disease, nonlethal ischemia activates a genomic program of cell survival that relates to the time course of myocardial stunning and differs transmurally in relation to ischemic stress, which induced the stunning. Understanding the genes up-regulated during myocardial stunning, including those not previously described in the heart, and developing strategies that activate this program may open new avenues for therapy in ischemic heart disease.

Interactions between phospholamban and beta-adrenergic drive may lead to cardiomyopathy and early mortality

BACKGROUND

Relieving the inhibition of sarcoplasmic reticular function by phospholamban is a major target of beta-adrenergic stimulation. Chronic beta-adrenergic receptor activity has been suggested to be detrimental, on the basis of transgenic overexpression of the receptor or its signaling effectors. However, it is not known whether physiological levels of sympathetic tone, in the absence of preexisting heart failure, are similarly detrimental.

METHODS AND RESULTS

Transgenic mice overexpressing phospholamban at 4-fold normal levels were generated, and at 3 months, they exhibited mildly depressed ventricular contractility without heart failure. As expected, transgenic cardiomyocyte mechanics and calcium kinetics were depressed, but isoproterenol reversed the inhibitory effects of phospholamban on these parameters. In vivo cardiac function was substantially depressed by propranolol administration, suggesting enhanced sympathetic tone. Indeed, plasma norepinephrine levels and the phosphorylation status of phospholamban were elevated, reflecting increased adrenergic drive in transgenic hearts. On aging, the chronic enhancement of adrenergic tone was associated with a desensitization of adenylyl cyclase (which intensified the inhibitory effects of phospholamban), the development of overt heart failure, and a premature mortality.

CONCLUSIONS

The unique interaction between phospholamban and increased adrenergic drive, elucidated herein, provides the first evidence that compensatory increases in catecholamine stimulation can, even in the absence of preexisting heart failure, be a primary causative factor in the development of cardiomyopathy and early mortality.

Maximal inhibition of SERCA2 Ca(2+) affinity by phospholamban in transgenic hearts overexpressing a non-phosphorylatable form of phospholamban

Phospholamban is a phosphoprotein in the cardiac sarcoplasmic reticulum (SR) which regulates the apparent Ca(2+) affinity of the SR Ca(2+)-ATPase (SERCA2). To determine the levels of phospholamban which are associated with maximal inhibition of SERCA2, several lines of transgenic mice were generated which expressed increasing levels of a non-phosphorylatable form of phospholamban (S16A,T17A) specifically in the heart. This mutant form of phospholamban was chosen to prevent phosphorylation as a compensatory mechanism in vivo. Quantitative immunoblotting revealed increased phospholamban protein levels of 1.8-, 2.6-, 3.7-, and 4.7-fold in transgenic hearts compared with wild types. There were no changes in the expression levels of SERCA2, calsequestrin, calreticulin, and ryanodine receptor. Assessment of SR Ca(2+) uptake in hearts of transgenic mice indicated increases in the inhibition of the affinity of SERCA2 for Ca(2+) with increased phospholamban expression. Maximal inhibition was obtained at phospholamban expression levels of 2.6-fold or higher. Transgenic hearts with functional saturation in phospholamban:SERCA2 (>/=2.6:1) exhibited increases in beta-myosin heavy chain expression, associated with cardiac hypertrophy. These findings demonstrate that overexpression of a non-phosphorylatable form of phospholamban in transgenic mouse hearts resulted in saturation of the functional phospholamban:SERCA2 ratio at 2.6:1 and suggest that approximately 40% of the SR Ca(2+) pumps are functionally regulated by phospholamban in vivo.

Determinants of the cardiomyopathic phenotype in chimeric mice overexpressing cardiac Gsalpha

Mice with overexpressed cardiac Gsalpha develop cardiomyopathy, characterized by myocyte hypertrophy and extensive myocardial fibrosis. The cardiomyopathy likely involves chronically enhanced beta-adrenergic signaling, because it can be blocked with long-term propranolol treatment. It remains unknown whether the genotype of the myocyte is solely responsible for the progressive pathological changes. A chimeric population in the heart should answer this question. Accordingly, we developed a chimeric animal, which combined cells from a transgenic overexpressed Gsalpha parent and a Rosa mouse containing the LacZ reporter gene, facilitating identification of the non-Gsalpha cells, which express a blue color with exposure to beta-galactosidase. We studied these animals at 14 to 17 months of age (when cardiomyopathy should have been present), with the proportion of Gsalpha cells in the myocardium ranging from 5% to 88%. beta-Galactosidase staining of the hearts demonstrated Gsalpha and Rosa cells, exhibiting a mosaic pattern. The fibrosis and hypertrophy, characteristic of the cardiomyopathy, were not distributed randomly. There was a direct correlation (r=0.85) between the extent of myocyte hypertrophy (determined by computer imaging) and the quantity of Gsalpha cells. The fibrosis, determined by picric acid Sirius red, was also more prominent in areas with the greatest Gsalpha cell density, with a correlation of r=0.88. Thus, the overexpressed Gsalpha can exert its action over the life of the animal, resulting in a local picture of cardiomyopathic damage in discrete regions of the heart, where clusters of the overexpressed Gsalpha cells reside, sparing the clusters of normal cells derived from the normal Rosa parent.

Beta-adrenergic receptor blockade arrests myocyte damage and preserves cardiac function in the transgenic G(salpha) mouse

Transgenic (TG) mice with cardiac G(salpha) overexpression exhibit enhanced inotropic and chronotropic responses to sympathetic stimulation, but develop cardiomyopathy with age. We tested the hypothesis that cardiomyopathy in TG mice with G(salpha) overexpression could be averted with chronic beta-adrenergic receptor (beta-AR) blockade. TG mice and age-matched wild-type littermates were treated with the beta-AR blocker propranolol for 6-7 months, starting at a time when the cardiomyopathy was developing but was not yet severe enough to induce significant cardiac depression (9.5 months of age), and ending at a time when cardiac depression and cardiomyopathy would have been clearly manifest (16 months of age). Propranolol treatment, which can induce cardiac depression in the normal heart, actually prevented cardiac dilation and the depressed left ventricular function characteristic of older TG mice, and abolished premature mortality. Propranolol also prevented the increase in myocyte cross-sectional area and myocardial fibrosis. Myocyte apoptosis, already apparent in 9-month-old TG mice, was actually eliminated by chronic propranolol. This study indicates that chronic sympathetic stimulation over an extended period is deleterious and results in cardiomyopathy. Conversely, beta-AR blockade is salutary in this situation and can prevent the development of cardiomyopathy.

Beta-adrenergic receptor-G protein-adenylyl cyclase signal transduction in the failing heart

The beta-adrenergic receptor signal transduction pathway is critical for rapid adjustments to increased cardiovascular demand (e.g., during exercise). In the face of chronic stimulation of this pathway, as occurs in the pathogenesis of heart failure, beta-adrenergic receptor stimulation may become maladaptive. Under these conditions, elevation of circulating catecholamines and depletion of cardiac tissue stores of norepinephrine occur in the failing heart, resulting in desensitization. Whether or not stimulation or inhibition of the beta-adrenergic receptor signaling pathway is beneficial in heart failure is controversial. One approach to address this question is to specifically overexpress a component of the beta-adrenergic receptor signaling pathway in a transgenic mouse heart. We have characterized young and old adult mice with overexpressed cardiac G(s alpha) which couples the beta-adrenergic receptor to adenylyl cyclase. In younger animals, beta-adrenergic receptor stimulation results in an augmented heart rate and cardiac contractility. Over the life of the animal, however, a picture of cardiomyopathy develops. The result is a dilated heart with a large amount of fibrosis and myocyte hypertrophy, degeneration atrophy, and apoptosis. Conversely, chronic beta-adrenergic receptor blockade prevents the development of cardiomyopathy. These experiments support the point of view that chronic beta-adrenergic stimulation during the development of heart failure is deleterious and that protecting the heart with chronic beta-adrenergic receptor blockade is salutary, conceptually consistent with results of recent clinical trials examining the effects of beta-adrenergic receptor blockers in patients with heart failure.