Changes in the receptor-G protein-adenylyl cyclase system in heart failure from various types of heart muscle disease

Gene expression and β-adrenergic signaling are altered in hypoplastic left heart syndrome

BACKGROUND

The purpose of the current study was to define the myocellular changes and adaptation of the β-adrenergic receptor (β-AR) system that occur in the systemic right ventricle (RV) of children with hypoplastic left heart syndrome (HLHS).

METHODS

Explanted hearts from children with HLHS and non-failing controls were used for this study. HLHS patients were divided into 2 groups: "compensated" (C-HLHS), infants listed for primary transplant with normal RV function and absence of heart failure symptoms, and "decompensated" (D-HLHS), patients listed for transplant after failed surgical palliation with RV failure and/or refractory protein-losing enteropathy or plastic bronchitis.

RESULTS

Compared with non-failing control RVs, the HLHS RV demonstrated decreased sarcoplasmic reticulum calcium-adenosine triphosphatase 2a and α-myosin heavy chain (MHC) gene expression, decreased total β-AR due to down-regulation of β1-AR, preserved cyclic adenosine monophosphate levels, and increased calcium/calmodulin-dependent protein kinase II (CaMKII) activity. There was increased atrial natriuretic peptide expression only in the C-HLHS group. Unique to those in the D-HLHS group was increased β-MHC and decreased α-MHC protein expression (MHC isoform switching), increased adenylyl cyclase 5 expression, and increased phosphorylation of the CaMK target site on phospholamban, threonine 17.

CONCLUSIONS

The HLHS RV has an abnormal myocardial gene expression pattern, downregulation of β1-AR, preserved cyclic adenosine monophosphate levels, and increased CaMKII activity compared with the non-failing control RV. There is MHC isoform switching, increased adenylyl cyclase 5, and increased phosphorylation of phospholamban threonine 17 only in the D-HLHS group. Although abnormal gene expression and changes in the β-AR system precede clinically evident ventricular failure in HLHS, additional unique adaptations occur in those with HLHS and failed surgical palliation.

Adenylyl cyclase regulation in heart failure due to myocardial infarction in rats

Cardiac adenylyl cyclase (AC) activity was described to be differentially regulated in left and right ventricles (LVs and RVs) of rats with heart failure (HF) due to LV myocardial infarction (MI) (Sethi et al. Am J Physiol 272:H884-H893, 1997). AC activities in LVs and RVs were increased and decreased respectively in rats 8 and 16 weeks post MI under basal and stimulatory conditions including AC activation via β-adrenergic receptors (β-ARs), stimulatory G protein (Gs), and direct AC activation with forskolin (FS). The current study aimed to detect alterations in rat heart AC activities in a comparable model of HF 9 weeks post LV MI. Therefore, cardiac AC activities were measured under basal and β-AR-, Gs-, or FS-stimulated conditions as well as under inhibition with various MANT [2'(3')-O-(N-methylanthraniloyl)]-nucleotide AC inhibitors and the P-site AC inhibitors NKY80 [2-amino-7-(2-furanyl)-7,8-dihydro-5(6H)-quinazolinone] and vidarabine (9-β-D-arabinosyladenine, AraAde). Basal and stimulated AC activities along with AC inhibition experiments did not reveal evidence for changes in AC activity in LVs and RVs from MI group animals despite the presence of congestive HF. However, our study is indeterminate. Further studies are required to identify the factors responsible for previously described changes in cardiac AC activity in MI induced HF and to elucidate the role of altered AC regulation in the pathophysiology of HF. In order to detect small changes in AC regulation, larger group sizes than the ones used in our present study are required.

Beta-adrenergic adaptation in paediatric idiopathic dilated cardiomyopathy

BACKGROUND

Although the pathophysiology and treatment of adult heart failure (HF) are well studied, HF in children remains poorly understood. In adults, adrenergic receptor (AR)-mediated adaptation plays a central role in cardiac abnormalities in HF, and these patients respond well to β-blocker (BB) therapy. However, in children with HF, there is a growing body of literature suggesting a lack of efficacy of adult HF therapies. Due to these unanticipated differences in response to therapy and the paucity of data regarding the molecular adaptation of the paediatric heart, we investigated the molecular characteristics of HF in children.

METHODS AND RESULTS

Explanted hearts from adults and children with idiopathic dilated cardiomyopathy and non-failing controls were used in the study. Our results show that the molecular characteristics of paediatric HF are strikingly different from their adult counterparts. These differences include: (i) down-regulation of β1- and β2-AR in children, whereas β2-AR expression is maintained in adults; (ii) up-regulation of connexin43 in children, whereas down-regulation is observed in adults; (iii) no differences in phosphatase expression, whereas up-regulation is observed in adults; (iv) no decrease in the phosphorylation of phospholamban at the Ser16 or Thr17 sites in children, which are known characteristics of adult HF.

CONCLUSION

There is a different adaptation of β-AR and adrenergic signalling pathways in children with HF compared with adults. Our results begin to address the disparities in cardiovascular research specific to children and suggest that age-related differences in adaptation could influence the response to therapy. These findings could lead to a paradigm shift in the contemporary management of children with HF.

β-Adrenergic receptor stimulation and activation of protein kinase A protect against α1-adrenergic-mediated phosphorylation of protein kinase D and histone deacetylase 5

INTRODUCTION

Chronic activation of β(1)-adrenergic receptor (β(1)-AR) signaling can have deleterious effects on the heart, and animal models overexpressing β(1)-ARs develop a dilated cardiomyopathy and heart failure. In the classic β-AR pathway, receptor occupancy by an agonist results in increased cyclic adenosine monophosphate (cAMP) levels and activation of protein kinase A (PKA). However, the role of PKA-dependent signaling in the development and progression of cardiomyopathies and heart failure is controversial, because β-AR signal transduction is generally desensitized in the failing heart and PKA activity is not increased.

METHODS AND RESULTS

Neonatal rat ventricular myocytes were acutely (15 minutes) or chronically (48 hours) treated with isoproterenol, and phosphorylation of protein kinase D (PKD) and histone deacetylase 5 (HDAC5) was measured. Acute β(1)-AR stimulation or expression of constitutively active (CA) PKA reduced α(1)-adrenergic-mediated phosphorylation of HDAC5 and PKD by activation of a phosphatase. Overexpression of CA-PKA also reduced α(1)-adrenergic-mediated increased expression of contractile protein fetal isoforms and promoted repression of adult isoforms, but had no effect on α(1)-adrenergic-mediated cellular hypertrophy.

CONCLUSIONS

These data indicate that the PKA-dependent arm of β-AR signaling can be antihypertrophic and presumably beneficial, through dephosphorylation of PKD and HDAC5 and reduction of hypertrophic fetal isoform gene expression.

Dependence of changes in β-adrenoceptor signal transduction on type and stage of cardiac hypertrophy

To examine whether cardiac hypertrophy is associated with changes in β-adrenoceptor signal transduction mechanisms, pressure overload (PO) was induced by occlusion of the abdominal aorta and volume overload (VO) by creation of an aortocaval shunt for 4 and 24 wk in rats. After hemodynamic assessment of the animals, the left ventricular (LV) particulate fraction was isolated for measurement of β1-adrenoceptors and adenylyl cyclase activity, and cardiomyocytes were isolated for monitoring of the intracellular Ca2+ concentration. Although PO and VO produced cardiac hypertrophy and increased LV end-diastolic pressure at 4 wk, cardiac function was increased in animals subjected to PO but remained unaltered in animals subjected to VO. Cardiac hypertrophy and increased LV end-diastolic pressure were associated with depressed cardiac function at 24 wk of PO or VO, but clinical signs of congestive heart failure were evident only in animals subjected to VO. Isoproterenol-induced increases in cardiac function, activation of adenylyl cyclase activity, and increase in intracellular Ca2+ concentration, as well as β1-adrenoceptor density, were unaltered by PO at 4 wk, augmented by VO at 4 wk, and attenuated by PO and VO at 24 wk. These results suggest that alterations in β1-adrenoceptor signal transduction are dependent on the type and stage of cardiac hypertrophy.

CXCR4 modulates contractility in adult cardiac myocytes

The inflammatory response is critical to the development and progression of heart failure. Chemokines and their receptors are a distinct class of inflammatory modulators that may play a role in mediating myocardial dysfunction in heart failure. Levels of the chemokine CXCL12, also known as stromal cell-derived factor (SDF), and its receptor, CXCR4, are elevated in patients with heart failure, and we undertook this study to determine whether this chemokine system can directly affect cardiac function in the absence of leukocytes. Murine papillary muscles and adult rat cardiac myocytes treated with CXCL12, the only identified ligand of CXCR4, demonstrate blunted inotropic responses to physiologic concentrations of calcium. The negative inotropic effects on cardiac myocytes are accompanied by a proportional diminution of calcium transients. The effects are abrogated by AMD3100, a specific CXCR4 inhibitor. Overexpression of the receptor through adenoviral infection with a CXCR4 construct accentuates the negative inotropic effects of CXCL12 on cardiac myocytes during calcium stimulation. CXCR4 activation also attenuates beta-adrenergic-mediated increases in calcium mobilization and fractional shortening in cardiac myocytes. In electrophysiologic studies, CXCL12 decreases forskolin- and isoproterenol-induced voltage-gated L-type calcium channel activation. These studies demonstrate that activation of CXCR4 results in a direct negative inotropic modulation of cardiac myocyte function. The specific mechanism of action involves alterations of calcium channel activity on the membrane. The presence of functional CXCR4 on cardiac myocytes introduces a new target for treating cardiac dysfunction.

Alterations of adenylyl cyclase and G proteins in aortocaval shunt-induced heart failure

Unlike most other experimental models of congestive heart failure, the volume overload model induced by aortocaval shunt (AVS) in rats was found to exhibit enhanced β-adrenoceptor (β-AR) signaling. To study whether the adenylyl cyclase (AC)-G protein system is involved in such a change, we examined cardiac AC activity and protein content as well as Gsα and Giα activities, protein contents, and mRNA levels in both left (LV) and right (RV) ventricles at the failing stage (16 wk after surgery). Basal and forskolin-stimulated AC activities were significantly increased in both LV and RV from the failing hearts; this change was associated with an upregulation of type V/VI AC protein. In contrast to 5′-guanylyl imidodiphosphate and NaF, the stimulatory effect of isoproterenol on AC was increased in the failing heart. Although Gsα and Giα protein contents in the failing hearts were not altered, the mRNA level for Gsα was decreased by 20% and that for Giα was increased by 20%. In addition, the activity of Gsα, but not Giα, as assessed by toxin-catalyzed ADP ribosylation, was significantly decreased in the failing heart. Losartan and imidapril treatments improved cardiac function and attenuated alterations in mRNA levels for Gsα and Giα proteins, as well as Gsα activity, without affecting changes in AC protein content or activities in heart failure due to volume overload. These data suggest that increased AC activity may contribute to the enhanced β-AR signaling in the AVS model of heart failure, whereas alterations in gene expression for G proteins may be of an adaptive nature at this stage of heart failure.

Modification of beta-adrenoceptor signal transduction pathway by genetic manipulation and heart failure

The beta-adrenoceptor (beta-AR) mediated signal transduction pathway in cardiomyocytes is known to involve beta1- and beta2-ARs, stimulatory (Gs) and inhibitory (Gi) guanine nucleotide binding proteins, adenylyl cyclase (AC) and cAMP-dependent protein kinase (PKA). The activation of beta1- and beta2-ARs has been shown to increase heart function by increasing Ca2+ -movements across the sarcolemmal membrane and sarcoplasmic reticulum through the stimulation of Gs-proteins, activation of AC and PKA enzymes and phosphorylation of the target sites. The activation of PKA has also been reported to increase phosphorylation of some myofibrillar proteins (for promoting cardiac relaxation) and nuclear proteins (for cardiac hypertrophy). The activation of beta2-AR has also been shown to affect Gi-proteins, stimulate mitogen activated protein kinase and increase protein synthesis by enhancing gene expression. Beta1- and beta2-ARs as well as AC are considered to be regulated by PKA- and protein kinase C (PKC)-mediated phosphorylations directly; both PKA and PKC also regulate beta-AR indirectly through the involvement of beta-AR kinase (betaARK), beta-arrestins and Gbeta gamma-protein subunits. Genetic manipulation of different components and regulators of beta-AR signal transduction pathway by employing transgenic and knockout mouse models has provided insight into their functional and regulatory characteristics in cardiomyocytes. The genetic studies have also helped in understanding the pathophysiological role of PARK in heart dysfunction and therapeutic role of betaARK inhibitors in the treatment of heart failure. Varying degrees of defects in the beta-AR signal transduction system have been identified in different types of heart failure to explain the attenuated response of the failing heart to sympathetic stimulation or catecholamine infusion. A decrease in beta1-AR density, an increase in the level of G1-proteins and overexpression of betaARK are usually associated with heart failure; however, these attenuations have been shown to be dependent upon the type and stage of heart failure as well as region of the heart. Both local and circulating renin-angiotensin systems, sympathetic nervous system and endothelial cell function appears to regulate the status of beta-AR signal transduction pathway in the failing heart. Thus different components and regulators of the beta-AR signal transduction pathway appears to represent important targets for the development of therapeutic interventions for the treatment of heart failure.

Laminin acts via beta 1 integrin signalling to alter cholinergic regulation of L-type Ca(2+) current in cat atrial myocytes

A perforated patch recording method was used to determine how plating cells on laminin (20 microg ml(-1); >2 h) alters cholinergic regulation of L-type Ca(2+) current (I(Ca,L)) in atrial myocytes. Acetylcholine (ACh; 1 microm)-induced inhibition of basal I(Ca,L) was not different between cells on glass and laminin. However, stimulation of I(Ca,L) elicited by ACh withdrawal was significantly smaller in cells on laminin (10 +/- 2 %) than on glass (48 +/- 5 %) (P < 0.001). Stimulation of I(Ca,L) induced by either spermine-NO (200 microm), milrinone (10 microm), IBMX (100 microm) or forskolin (1 microm) was significantly smaller in cells plated on laminin than on glass. However, stimulation of I(Ca,L) by 100 microm 8-CPT-cAMP or intracellular dialysis with 50 microM cAMP was not different between cells plated on laminin or glass. Basal, forskolin- and IBMX-stimulated cAMP content was significantly smaller in cells plated on laminin than on glass. Stimulation of I(Ca,L) by ACh withdrawal was significantly smaller in cells plated on an alpha beta 1-integrin antibody (10 +/- 4 %) than on glass (3 +/- 6 %; P < 0.001). In cells on laminin, prior exposure to 100 microg ml-1 YIGSR, a laminin receptor-binding peptide, restored ACh-induced stimulation of I(Ca,L) (58 +/- 14 %)laminin alone (7 +/- 2 %; P < 0. 05). Addition of 20 microm cytochalasin D or 1 microM latrunculin A, agents that prevent actin polymerization, to cells on laminin restored ACh-induced stimulation of I(Ca,L). We conclude that laminin binding to beta 1 integrins acts in association with the actin-based cytoskeleton to attenuate adenylate cyclase activity. As a result, laminin inhibits NO-mediated stimulation of I(Ca,L) elicited by ACh withdrawal. Laminin-integrin signalling may be relevant to changes in autonomic regulation that occur during cardiac development and/or disease.

Regional patterns of myocardial sympathetic denervation in dilated cardiomyopathy: an analysis using carbon-11 hydroxyephedrine and positron emission tomography

OBJECTIVE

To assess presynaptic function of cardiac autonomic innervation in patients with advanced congestive heart failure using positron emission tomography (PET) and the recently developed radiolabelled catecholamine analogue carbon-11 hydroxyephedrine (HED) as a marker for neuronal catecholamine uptake function.

DESIGN AND PATIENTS

29 patients suffering from dilated cardiomyopathy with moderate to severe heart failure were compared with eight healthy controls. Perfusion scan was followed by HED dynamic PET imaging of cardiac sympathetic innervation. The scintigraphic results were compared with markers of disease severity and the degree of sympathetic dysfunction assessed by means of heart rate variability.

RESULTS

In contrast to nearly normal perfusions, mean (SD) HED retention in dilated cardiomyopathy patients was abnormal in 64 (32)% of the left ventricle. Absolute myocardial HED retention was 10.7 (1.0)%/min in controls v 6.2 (1.6)%/min in dilated cardiomyopathy patients (p < 0.001). Moreover, significant regional reduction of HED retention was demonstrated in apical and inferoapical segments. HED retention was significantly correlated with New York Heart Association functional class (r = -0.55, p = 0. 002) and ejection fraction (r = 0.63, p < 0.001), but not, however, with plasma noradrenaline concentrations as well as parameters of heart rate variability.

CONCLUSIONS

In this study, using PET in combination with HED in patients with dilated cardiomyopathy, not only global reduction but also regional abnormalities of cardiac sympathetic tracer uptake were demonstrated. The degree of abnormality was positively correlated to markers of severity of heart failure. The pathogenetic mechanisms leading to the regional differences of neuronal damage as well as the prognostic significance of these findings remain to be defined.

Myocyte recovery after mechanical circulatory support in humans with end-stage heart failure

BACKGROUND

The failing myocardium is characterized by decreased force production, slowed relaxation, and depressed responses to beta-adrenergic stimulation. In some heart failure patients, heart function is so poor that a left ventricular assist device (LVAD) is inserted as a bridge to transplantation. In the present research, we investigated whether circulatory support with an LVAD influenced the functional properties of myocytes from the failing heart.

METHODS AND RESULTS

Myocytes were isolated from human explanted failing hearts (HF-myocytes) and failing hearts with antecedent LVAD support (HF-LVAD-myocytes). Studies of myocyte function indicated that the magnitude of contraction was greater (9.6+/-0.7% versus 6.9+/-0.5% shortening), the time to peak contraction was significantly abbreviated (0.37+/-0.01 versus 0.75+/-0.04 seconds), and the time to 50% relaxation was reduced (0.55+/-0.02 versus 1.45+/-0.11 seconds) in the HF-LVAD-myocytes compared with the HF-myocytes (P<0.05). The HF-LVAD-myocytes had larger contractions than the HF-myocytes at all frequencies of stimulation tested. The negative force-frequency relationship of the HF-myocytes was improved in HF-LVAD-myocytes but was not reversed. Responses to beta-adrenergic stimulation (by isoproterenol) were greater in HF-LVAD-myocytes versus HF-myocytes.

CONCLUSIONS

The results of the study strongly support the idea that circulatory support with an LVAD improves myocyte contractile properties and increases beta-adrenergic responsiveness.

Developmental changes in beta-adrenergic modulation of L-type Ca2+ channels in embryonic mouse heart

In the adult mammalian myocardium, cellular Ca2+ entry is regulated by the sympathetic nervous system. L-type Ca2+ channel currents are markedly increased by beta-adrenergic (beta-A) agonists, which contribute to changes in pacing and contractile activity of the heart. In the developing mammalian heart, the regulation of Ca2+ entry by this enzyme cascade has not been clearly established, because changes in receptor density and coupling to downstream elements of the signaling cascade are known to occur during embryogenesis. In this study, we systematically investigated the regulation of L-type Ca2+ channel currents during development of the murine embryonic heart. We used conventional whole-cell and perforated-patch-clamp procedures to study modulation of L- type Ca2+ channel currents and to assay functional activity of distinct steps in the beta-A signaling cascade in murine embryonic myocytes at different stages of gestation. Our data indicate that the L-type Ca2+ channels in early-stage (day-11 to -13) myocytes are unresponsive to either isoproterenol or cAMP. L-type Ca2+ channels in late-stage (day-17 to -19) murine myocytes, however, exhibit responses to isoproterenol and cAMP similar to responses in adult cells, providing evidence that the beta-A cascade becomes functionally active during this period of embryonic development. We found that L-type Ca2+ channel activity in early-stage cells is increased by cell dialysis with the catalytic subunit of cAMP-dependent protein kinase (cA-PK) and that dialysis of early-stage cells with the holoenzyme of cA-PK restores functional responses to forskolin and cAMP, but not to isoproterenol. Our results provide strong evidence that a key factor in the early-stage insensitivity of L-type Ca2+ channels to cAMP is the absence, or low expression level, of the holoenzyme of cA-PK but that in addition, another element in the signaling cascade upstream from adenylate cyclase is expressed at a nonfunctional level or is uncoupled from the cascade and thus contributes to L-type Ca2+ channel insensitivity to beta-A agonists in early stages of the developing murine heart.