Adenylyl cyclase 6 improves calcium uptake and left ventricular function in aged hearts

Adenylyl cyclase isoforms 5 and 6 in the cardiovascular system: complex regulation and divergent roles

Adenylyl cyclases (ACs) are crucial effector enzymes that transduce divergent signals from upstream receptor pathways and are responsible for catalyzing the conversion of ATP to cAMP. The ten AC isoforms are categorized into four main groups; the class III or calcium-inhibited family of ACs comprises AC5 and AC6. These enzymes are very closely related in structure and have a paucity of selective activators or inhibitors, making it difficult to distinguish them experimentally. AC5 and AC6 are highly expressed in the heart and vasculature, as well as the spinal cord and brain; AC6 is also abundant in the lungs, kidney, and liver. However, while AC5 and AC6 have similar expression patterns with some redundant functions, they have distinct physiological roles due to differing regulation and cAMP signaling compartmentation. AC5 is critical in cardiac and vascular function; AC6 is a key effector of vasodilatory pathways in vascular myocytes and is enriched in fetal/neonatal tissues. Expression of both AC5 and AC6 decreases in heart failure; however, AC5 disruption is cardio-protective, while overexpression of AC6 rescues cardiac function in cardiac injury. This is a comprehensive review of the complex regulation of AC5 and AC6 in the cardiovascular system, highlighting overexpression and knockout studies as well as transgenic models illuminating each enzyme and focusing on post-translational modifications that regulate their cellular localization and biological functions. We also describe pharmacological challenges in the design of isoform-selective activators or inhibitors for AC5 and AC6, which may be relevant to developing new therapeutic approaches for several cardiovascular diseases.

The early reduction of left ventricular mass after sleeve gastrectomy depends on the fall of branched-chain amino acid circulating levels

Introduction

Body-mass index is a major determinant of left-ventricular-mass (LVM). Bariatric-metabolic surgery (BMS) reduces cardiovascular mortality. Its mechanism of action, however, often encompasses a weight-dependent effect. In this translational study, we aimed at investigating the mechanisms by which BMS leads to LVM reduction and functional improvement.

Methods

Twenty patients (45.2 ± 8.5years) were studied with echocardiography at baseline and at 1,6,12 and 48 months after sleeve-gastrectomy (SG). Ten Wistar rats aged 10-weeks received high-fat diet ad libitum for 10 weeks before and 4 weeks after SG or sham-operation. An oral-glucose-tolerance-test was performed to measure whole-body insulin-sensitivity. Plasma metabolomics was analysed in both human and rodent samples. RNA quantitative Real-Time PCR and western blots were performed in rodent heart biopsies. The best-fitted partial-least-square discriminant-analysis model was used to explore the variable importance in the projection score of all metabolites.

Findings

Echocardiographic LVM (-12%,-23%,-28% and -43% at 1,6,12 and 48 months, respectively) and epicardial fat decreased overtime after SG in humans while insulin-sensitivity improved. In rats, SG significantly reduced LVM and epicardial fat, enhanced ejection-fraction and improved insulin-sensitivity compared to sham-operation. Metabolomics showed a progressive decline of plasma branched-chain amino-acids (BCAA), alanine, lactate, 3-OH-butyrate, acetoacetate, creatine and creatinine levels in both humans and rodents.

Hearts of SG rats had a more efficient BCAA, glucose and fatty-acid metabolism and insulin signaling than sham-operation. BCAAs in cardiomyocyte culture-medium stimulated lipogenic gene transcription and reduced mRNA levels of key mitochondrial β-oxidation enzymes promoting lipid droplet accumulation and glycolysis.

Interpretation

After SG a prompt and sustained decrease of the LVM, epicardial fat and insulin resistance was found. Animal and in vitro studies showed that SG improves cardiac BCAA metabolism with consequent amelioration of fat oxidation and insulin signaling translating into decreased intra-myocytic fat accumulation and reduced lipotoxicity.

miR‑351‑5p aggravates lipopolysaccharide‑induced acute lung injury via inhibiting AMPK

Inflammation and oxidative stress have indispensable roles in the development of acute lung injury (ALI). MicroRNA (miRNA/miR)‑351‑5p was initially identified as a myogenesis‑associated miRNA; however, its role in lipopolysaccharide (LPS)‑induced ALI remains unclear. The aim of the present study was to investigate the role and potential mechanisms of miR‑351‑5p in ALI. ALI was induced through a single intratracheal injection of LPS for 12 h, and miR‑351‑5p agomir, antagomir or their corresponding negative controls were injected into the tail vein before LPS stimulation. Compound C, 2',5'‑dideoxyadenosine and H89 were used to inhibit AMP‑activated protein kinase (AMPK), adenylate cyclase and protein kinase A (PKA), respectively. miR‑351‑5p levels in the lungs were significantly increased in response to LPS injection. miR‑351‑5p antagomir alleviated, while miR‑351‑5p agomir aggravated LPS‑induced oxidative stress and inflammation in the lungs. The present results also demonstrated that miR‑351‑5p antagomir attenuated LPS‑induced ALI via activating AMPK, and that the cAMP/PKA axis was required for the activation of AMPK by the miR‑351‑5p antagomir. In conclusion, the present study indicated that miR‑351‑5p aggravated LPS‑induced ALI via inhibiting AMPK, suggesting that targeting miR‑351‑5p may help to develop efficient therapeutic approaches for treating ALI.

Cardiac adenylyl cyclase overexpression precipitates and aggravates age-related myocardial dysfunction

AIMS

Increase of cardiac cAMP bioavailability and PKA activity through adenylyl-cyclase 8 (AC8) overexpression enhances contractile function in young transgenic mice (AC8TG). Ageing is associated with decline of cardiac contraction partly by the desensitization of β-adrenergic/cAMP signalling. Our objective was to evaluate cardiac cAMP signalling as age increases between 2 months and 12 months and to explore whether increasing the bioavailability of cAMP by overexpression of AC8 could prevent cardiac dysfunction related to age.

METHODS AND RESULTS

Cardiac cAMP pathway and contractile function were evaluated in AC8TG and their non-transgenic littermates (NTG) at 2- and 12 months old. AC8TG demonstrated increased AC8, PDE1, 3B and 4D expression at both ages, resulting in increased phosphodiesterase and PKA activity, and increased phosphorylation of several PKA targets including sarco(endo)plasmic-reticulum-calcium-ATPase (SERCA2a) cofactor phospholamban (PLN) and GSK3α/β a main regulator of hypertrophic growth and ageing. Confocal immunofluorescence revealed that the major phospho-PKA substrates were co-localized with Z-line in 2-month-old NTG but with Z-line interspace in AC8TG, confirming the increase of PKA activity in the compartment of PLN/SERCA2a. In both 12-month-old NTG and AC8TG, PLN and GSK3α/β phosphorylation was increased together with main localization of phospho-PKA substrates in Z-line interspaces. Haemodynamics demonstrated an increased contractile function in 2- and 12-month-old AC8TG, but not in NTG. In contrast, echocardiography and tissue Doppler imaging (TDI) performed in conscious mice unmasked myocardial dysfunction with a decrease of systolic strain rate in both old AC8TG and NTG. In AC8TG TDI showed a reduced strain rate even in 2-month-old animals. Development of age-related cardiac dysfunction was accelerated in AC8TG, leading to heart failure (HF) and premature death. Histological analysis confirmed early cardiomyocyte hypertrophy and interstitial fibrosis in AC8TG when compared with NTG.

CONCLUSION

Our data demonstrated an early and accelerated cardiac remodelling in AC8TG mice, leading to the development of HF and reduced lifespan. Age-related reorganization of cAMP/PKA signalling can accelerate cardiac ageing, partly through GSK3α/β phosphorylation.

Echocardiographic Strain Analysis for the Early Detection of Left Ventricular Systolic/Diastolic Dysfunction and Dyssynchrony in a Mouse Model of Physiological Aging

Heart disease is the leading cause of hospitalization and death worldwide, severely affecting health care costs. Aging is a significant risk factor for heart disease, and the senescent heart is characterized by structural and functional changes including diastolic and systolic dysfunction as well as left ventricular (LV) dyssynchrony. Speckle tracking-based strain echocardiography (STE) has been shown as a noninvasive, reproducible, and highly sensitive methodology to evaluate LV function in both animal models and humans. Herein, we describe the efficiency of this technique as a comprehensive and sensitive method for the detection of age-related cardiac dysfunction in mice. Compared with conventional echocardiographic measurements, radial and longitudinal strain, and reverse longitudinal strain were able to detect subtle changes in systolic and diastolic cardiac function in mice at an earlier time point during aging. Additionally, the data show a gradual and consistent decrease with age in regional contractility throughout the entire LV, in both radial and longitudinal axes. Furthermore, we observed that LV segmental dyssynchrony in longitudinal axis reliably differentiated between aged and young mice. Therefore, we propose the use of echocardiographic strain as a highly sensitive and accurate technology enabling and evaluating the effect of new treatments to fight age-induced cardiac disease.

Constitutive inhibitory G protein activity upon adenylyl cyclase-dependent cardiac contractility is limited to adenylyl cyclase type 6

PURPOSE

We previously reported that inhibitory G protein (Gi) exerts intrinsic receptor-independent inhibitory activity upon adenylyl cyclase (AC) that regulates contractile force in rat ventricle. The two major subtypes of AC in the heart are AC5 and AC6. The aim of this study was to determine if this intrinsic Gi inhibition regulating contractile force is AC subtype selective.

METHODS

Wild-type (WT), AC5 knockout (AC5KO) and AC6 knockout (AC6KO) mice were injected with pertussis toxin (PTX) to inactivate Gi or saline (control).Three days after injection, we evaluated the effect of simultaneous inhibition of phosphodiesterases (PDE) 3 and 4 with cilostamide and rolipram respectively upon in vivo and ex vivo left ventricular (LV) contractile function. Also, changes in the level of cAMP were measured in left ventricular homogenates and at the membrane surface in cardiomyocytes obtained from the same mouse strains expressing the cAMP sensor pmEPAC1 using fluorescence resonance energy transfer (FRET).

RESULTS

Simultaneous PDE3 and PDE4 inhibition increased in vivo and ex vivo rate of LV contractility only in PTX-treated WT and AC5KO mice but not in saline-treated controls. Likewise, Simultaneous PDE3 and PDE4 inhibition elevated total cAMP levels in PTX-treated WT and AC5KO mice compared to saline-treated controls. In contrast, simultaneous PDE3 and PDE4 inhibition did not increase in vivo or ex vivo rate of LV contractility or cAMP levels in PTX-treated AC6KO mice compared to saline-treated controls. Using FRET analysis, an increase of cAMP level was detected at the membrane of cardiomyocytes after simultaneous PDE3 and PDE4 inhibition in WT and AC5KO but not AC6KO. These FRET data are consistent with the functional data indicating that AC6 activity and PTX inhibition of Gi is necessary for simultaneous inhibition of PDE3 and PDE4 to elicit an increase in contractility.

CONCLUSIONS

Together, these data suggest that AC6 is tightly regulated by intrinsic receptor-independent Gi activity, thus providing a mechanism for maintaining low basal cAMP levels in the functional compartment that regulates contractility.

New Insights in Cardiac β-Adrenergic Signaling During Heart Failure and Aging

Heart failure (HF) has become increasingly common within the elderly population, decreasing their survival and overall quality of life. In fact, despite the improvements in treatment, many elderly people suffer from cardiac dysfunction (HF, valvular diseases, arrhythmias or hypertension-induced cardiac hypertrophy) that are much more common in an older fragile heart. Since β-adrenergic receptor (β-AR) signaling is abnormal in failing as well as aged hearts, this pathway is an effective diagnostic and therapeutic target. Both HF and aging are characterized by activation/hyperactivity of various neurohormonal pathways, the most important of which is the sympathetic nervous system (SNS). SNS hyperactivity is initially a compensatory mechanism to stimulate contractility and maintain cardiac output. Unfortunately, this chronic stimulation becomes detrimental and causes decreased cardiac function as well as reduced inotropic reserve due to a decrease in cardiac β-ARs responsiveness. Therapies which (e.g., β-blockers and physical activity) restore β-ARs responsiveness can ameliorate cardiac performance and outcomes during HF, particularly in older patients. In this review, we will discuss physiological β-adrenergic signaling and its alterations in both HF and aging as well as the potential clinical application of targeting β-adrenergic signaling in these disease processes.

New Insights in Cardiac β-Adrenergic Signaling During Heart Failure and Aging

Heart failure (HF) has become increasingly common within the elderly population, decreasing their survival and overall quality of life. In fact, despite the improvements in treatment, many elderly people suffer from cardiac dysfunction (HF, valvular diseases, arrhythmias or hypertension-induced cardiac hypertrophy) that are much more common in an older fragile heart. Since β-adrenergic receptor (β-AR) signaling is abnormal in failing as well as aged hearts, this pathway is an effective diagnostic and therapeutic target. Both HF and aging are characterized by activation/hyperactivity of various neurohormonal pathways, the most important of which is the sympathetic nervous system (SNS). SNS hyperactivity is initially a compensatory mechanism to stimulate contractility and maintain cardiac output. Unfortunately, this chronic stimulation becomes detrimental and causes decreased cardiac function as well as reduced inotropic reserve due to a decrease in cardiac β-ARs responsiveness. Therapies which (e.g., β-blockers and physical activity) restore β-ARs responsiveness can ameliorate cardiac performance and outcomes during HF, particularly in older patients. In this review, we will discuss physiological β-adrenergic signaling and its alterations in both HF and aging as well as the potential clinical application of targeting β-adrenergic signaling in these disease processes.

The type VI adenylyl cyclase protects cardiomyocytes from β-adrenergic stress by a PKA/STAT3-dependent pathway

BACKGROUND

The type VI adenylyl cyclase (AC6) is a main contributor of cAMP production in the heart. The amino acid (aa) sequence of AC6 is highly homologous to that of another major cardiac adenylyl cyclase, AC5, except for its N-terminus (AC6-N, aa 1-86). Activation of AC6, rather than AC5, produces cardioprotective effects against heart failure, while the underlying mechanism remains to be unveiled. Using an AC6-null (AC6^-/-) mouse and a knockin mouse with AC6-N deletion (AC6 ^ΔN/ΔN), we aimed to investigate the cardioprotective mechanism of AC6 in the heart.

METHODS

Western blot analysis and immunofluorescence staining were performed to determine the intracellular distribution of AC6, AC6-ΔN (a truncated AC6 lacking the first 86 amino acids), and STAT3 activation. Activities of AC6 and AC6-ΔN in the heart were assessed by cAMP assay. Apoptosis of cardiomyocytes were evaluated by the TUNEL assay and a propidium iodine-based survival assay. Fibrosis was examined by collagen staining.

RESULTS

Immunofluorescence staining revealed that cardiac AC6 was mainly anchored on the sarcolemmal membranes, while AC6-ΔN was redistributed to the sarcoplasmic reticulum. AC6^ΔN/ΔN and AC6^-/- mice had more apoptotic myocytes and cardiac remodeling than WT mice in experimental models of isoproterenol (ISO)-induced myocardial injury. Adult cardiomyocytes isolated from AC6^ΔN/ΔN or AC6^-/- mice survived poorly after exposure to ISO, which produced no effect on WT cardiomyocytes under the condition tested. Importantly, ISO treatment induced cardiac STAT3 phosphorylation/activation in WT mice, but not in AC6^ΔN/ΔN and AC6^-/- mice. Pharmacological blockage of PKA-, Src-, or STAT3- pathway markedly reduced the survival of WT myocytes in the presence of ISO, but did not affect those of AC6^ΔN/ΔN and AC6^-/- myocytes, suggesting an important role of AC6 in mediating cardioprotective action through the activation of PKA-Src-STAT3-signaling.

CONCLUSIONS

Collectively, AC6-N controls the anchorage of cardiac AC6 on the sarcolemmal membrane, which enables the coupling of AC6 with the pro-survival PKA-STAT3 pathway. Our findings may facilitate the development of novel therapies for heart failure.

Noninvasive evaluation of left ventricular elastance according to pressure-volume curves modeling in arterial hypertension

End-systolic left ventricular (LV) elastance (E_es) has been previously calculated and validated invasively using LV pressure-volume (P-V) loops. Noninvasive methods have been proposed, but clinical application remains complex. The aims of the present study were to 1) estimate E_es according to modeling of the LV P-V curve during ejection ("ejection P-V curve" method) and validate our method with existing published LV P-V loop data and 2) test the clinical applicability of noninvasively detecting a difference in E_es between normotensive and hypertensive subjects. On the basis of the ejection P-V curve and a linear relationship between elastance and time during ejection, we used a nonlinear least-squares method to fit the pressure waveform. We then computed the slope and intercept of time-varying elastance as well as the volume intercept (V₀). As a validation, 22 P-V loops obtained from previous invasive studies were digitized and analyzed using the ejection P-V curve method. To test clinical applicability, ejection P-V curves were obtained from 33 hypertensive subjects and 32 normotensive subjects with carotid tonometry and real-time three-dimensional echocardiography during the same procedure. A good univariate relationship (r² = 0.92, P < 0.005) and good limits of agreement were found between the invasive calculation of E_es and our new proposed ejection P-V curve method. In hypertensive patients, an increase in arterial elastance (E_a) was compensated by a parallel increase in E_es without change in E_a/E_es In addition, the clinical reproducibility of our method was similar to that of another noninvasive method. In conclusion, E_es and V₀ can be estimated noninvasively from modeling of the P-V curve during ejection. This approach was found to be reproducible and sensitive enough to detect an expected increase in LV contractility in hypertensive patients. Because of its noninvasive nature, this methodology may have clinical implications in various disease states.NEW & NOTEWORTHY The use of real-time three-dimensional echocardiography-derived left ventricular volumes in conjunction with carotid tonometry was found to be reproducible and sensitive enough to detect expected differences in left ventricular elastance in arterial hypertension. Because of its noninvasive nature, this methodology may have clinical implications in various disease states.

Cardiac-Directed Expression of Adenylyl Cyclase Catalytic Domain Reverses Cardiac Dysfunction Caused by Sustained Beta-Adrenergic Receptor Stimulation

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Cardiac-targeted expression of C1C2 reduces cAMP production yet mice maintain normal cardiac function through increased Ca²⁺ handling.

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Sustained isoproterenol infusion reduces heart function in normal mice, but improves heart function in mice with increased cardiac C1C2 expression.

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Reduced cardiac cAMP generation and resistance to catecholamine cardiomyopathy are attractive features of this potential heart failure therapeutic.

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Removing the large transmembrane domains of AC6 and fusing the two intracellular domains provides a small molecule, C1C2, that replicates many of the beneficial effects of AC6, but is sufficiently small to be expressed in an AAV vector for gene transfer.

Transgenic mice with cardiac-directed C1C2, a fusion protein of the intracellular C1 and C2 segments of adenylyl cyclase type 6, had normal left ventricular (LV) function, but diminished cAMP generation. Cardiac myocytes from C1C2 mice showed increased Ca²⁺ release. Mice underwent continuous isoproterenol infusion to stress the heart. In C1C2 mice, sustained isoproterenol infusion increased rather than decreased LV function. LV SERCA2a and Ca²⁺ release were increased. Reduced cAMP generation and resistance to catecholamine cardiomyopathy are attractive features of this potential heart failure therapeutic.

Role of HSF1-upregulated AC6 in ameliorating heart failure in mice

PURPOSE

Our previous studies discovered that Heat shock factor 1(HSF1) can alleviate pressure overload induced heart failure in mice. However, its molecular mechanisms are yet to be further explained. Many studies have already verified that Adenylyl Cyclase 6 (AC6) can ameliorate heart failure, but it is still unknown whether or not the pathway HSF1 is involved in the process. Our preliminary experiment showed that the expression level of AC6 is positively associated with HSF1. Therefore, in the present study, we aimed to explore whether HSF1 can play its role in ameliorating heart failure by regulating AC6, and how the potential internal mechanisms work.

METHODS

We applied the Transverse Aortic Constriction (TAC) for 4 weeks to develop the C57BL/6 mice pressure overload induced heart failure model. First, the mice were divided into TAC group and SHAM group. Changes in the cardiac function and morphology of the mice were observed by an ultrasonic device and Masson staining slices, expressions of AC6 mRNA were observed by RT-QPCR, expressions of HSF1 and proteinkinase A (PKA) were examined by Western Blotting, and the levels of cyclic adenosine monophosphate (cAMP) from aortic blood were measured by ELISA. Second, the TAC group were further divided into subgroups of HSF1 transgene mice, HSF1 knockout mice and wild type mice, followed by the aforesaid observations.

RESULTS

In the SHAM group, no obvious variations of cardiac function, AC6 mRNAHSF1, PKA, cAMP and other test results were found among each of the subgroups. Compared to the SHAM group, the TAC group presented clearly weakened heart functions, while, expressions of AC6 mRNA, HSF1, PKA and cAMP all recorded obvious increases. In the TAC group, compared to the WT subgroup, the HSF1 KO subgroup presented decreases in expressions of AC6 mRNA, HSF1, PKA and cAMP, and at the same time, the heart functions were weaker, while, the HSF1 TG subgroup recorded the contrary results.

CONCLUSION

In the pressure overload heart failure model, HSF1 can ameliorate heart failure by positively regulating the pathway of AC6/cAMP/PKA.

Disruption of Epac1 protects the heart from adenylyl cyclase type 5-mediated cardiac dysfunction

Type 5 adenylyl cyclase (AC5) plays an important role in the development of chronic catecholamine stress-induced heart failure and arrhythmia in mice. Epac (exchange protein activated by cAMP), which is directly activated by cAMP independent of protein kinase A, has been recently identified as a novel mediator of cAMP signaling in the heart. However, the role of Epac in AC5-mediated cardiac dysfunction and arrhythmias remains poorly understood. We therefore generated AC5 transgenic mice (AC5TG) with selective disruption of the Epac1 gene (AC5TG-Epac1KO), and compared their phenotypes with those of AC5TG after chronic isoproterenol (ISO) infusion. Decreased cardiac function as well as increased susceptibility to pacing-induced atrial fibrillation (AF) in response to ISO were significantly attenuated in AC5TG-Epac1KO mice, compared to AC5TG mice. Increased cardiac apoptosis and cardiac fibrosis were also concomitantly attenuated in AC5TG-Epac1KO mice compared to AC5TG mice. These findings indicate that Epac1 plays an important role in AC5-mediated cardiac dysfunction and AF susceptibility.

Type VI adenylyl cyclase negatively regulates GluN2B-mediated LTD and spatial reversal learning

The calcium-sensitive type VI adenylyl cyclase (AC6) is a membrane-bound adenylyl cyclase (AC) that converts ATP to cAMP under stimulation. It is a calcium-inhibited AC and integrates negative inputs from Ca²⁺ and multiple other signals to regulate the intracellular cAMP level. In the present study, we demonstrate that AC6 functions upstream of CREB and negatively controls neuronal plasticity in the hippocampus. Genetic removal of AC6 leads to cyclase-independent and N-terminus of AC6 (AC6N)-dependent elevation of CREB expression, and enhances the expression of GluN2B-containing NMDA receptors in hippocampal neurons. Consequently, GluN2B-dependent calcium signaling and excitatory postsynaptic current, long-term depression, and spatial reversal learning are enhanced in the hippocampus of AC6^−/− mice without altering the gross anatomy of the brain. Together, our results suggest that AC6 negatively regulates neuronal plasticity by modulating the levels of CREB and GluN2B in the hippocampus.

Gene Therapy in Heart Failure

Heart failure is a significant burden to the global healthcare system and represents an underserved market for new pharmacologic strategies, especially therapies which can address root cause myocyte dysfunction. Modern drugs, surgeries, and state-of-the-art interventions are costly and do not improve survival outcome measures. Gene therapy is an attractive strategy, whereby selected gene targets and their associated regulatory mechanisms can be permanently managed therapeutically in a single treatment. This in theory could be sustainable for the patient's life. Despite the promise, however, gene therapy has numerous challenges that must be addressed together as a treatment plan comprising these key elements: myocyte physiologic target validation, gene target manipulation strategy, vector selection for the correct level of manipulation, and carefully utilizing an efficient delivery route that can be implemented in the clinic to efficiently transfer the therapy within safety limits. This chapter summarizes the key developments in cardiac gene therapy from the perspective of understanding each of these components of the treatment plan. The latest pharmacologic gene targets, gene therapy vectors, delivery routes, and strategies are reviewed.

How cardiomyocyte excitation, calcium release and contraction become altered with age

Cardiovascular disease is the main cause of death globally, accounting for over 17 million deaths each year. As the incidence of cardiovascular disease rises markedly with age, the overall risk of cardiovascular disease is expected to increase dramatically with the aging of the population such that by 2030 it could account for over 23 million deaths per year. It is therefore vitally important to understand how the heart remodels in response to normal aging for at least two reasons: i) to understand why the aged heart is increasingly susceptible to disease; and ii) since it may be possible to modify treatment of disease in older adults if the underlying substrate upon which the disease first develops is fully understood. It is well known that age modulates cardiac function at the level of the individual cardiomyocyte. Generally, in males, aging reduces cell shortening, which is associated with a decrease in the amplitude of the systolic Ca(2+) transient. This may arise due to a decrease in peak L-type Ca(2+) current. Sarcoplasmic reticulum (SR) Ca(2+) load appears to be maintained during normal aging but evidence suggests that SR function is disrupted, such that the rate of sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA)-mediated Ca(2+) removal is reduced and the properties of SR Ca(2+) release in terms of Ca(2+) sparks are altered. Interestingly, Ca(2+) handling is modulated by age to a lesser degree in females. Here we review how cellular contraction is altered as a result of the aging process by considering expression levels and functional properties of key proteins involved in controlling intracellular Ca(2+). We consider how changes in both electrical properties and intracellular Ca(2+) handling may interact to modulate cardiomyocyte contraction. We also reflect on why cardiovascular risk may differ between the sexes by highlighting sex-specific variation in the age-associated remodeling process. This article is part of a Special Issue entitled CV Aging.

Cyclic AMP synthesis and hydrolysis in the normal and failing heart

Cyclic AMP regulates a multitude of cellular responses and orchestrates a network of intracellular events. In the heart, cAMP is the main second messenger of the β-adrenergic receptor (β-AR) pathway producing positive chronotropic, inotropic, and lusitropic effects during sympathetic stimulation. Whereas short-term stimulation of β-AR/cAMP is beneficial for the heart, chronic activation of this pathway triggers pathological cardiac remodeling, which may ultimately lead to heart failure (HF). Cyclic AMP is controlled by two families of enzymes with opposite actions: adenylyl cyclases, which control cAMP production and phosphodiesterases, which control its degradation. The large number of families and isoforms of these enzymes, their different localization within the cell, and their organization in macromolecular complexes leads to a high level of compartmentation, both in space and time, of cAMP signaling in cardiac myocytes. Here, we review the expression level, molecular characteristics, functional properties, and roles of the different adenylyl cyclase and phosphodiesterase families expressed in heart muscle and the changes that occur in cardiac hypertrophy and failure.

AAV-mediated gene therapy for heart failure: enhancing contractility and calcium handling

Heart failure is a progressive, debilitating disease that is characterized by inadequate contractility of the heart. With an aging population, the incidence and economic burden of managing heart failure are anticipated to increase substantially. Drugs for heart failure only slow its progression and offer no cure. However, results of recent clinical trials using recombinant adeno-associated virus (AAV) gene delivery offer the promise, for the first time, that heart failure can be reversed. The strategy is to improve contractility of cardiac muscle cells by enhancing their ability to store calcium through increased expression of the sarco(endo)plasmic reticulum Ca(2+)-ATPase pump (SERCA2a). Preclinical trials have also identified other proteins involved in calcium cycling in cardiac muscle that are promising targets for gene therapy in heart failure, including the following: protein phosphatase 1, adenylyl cyclase 6, G-protein-coupled receptor kinase 2, phospholamban, SUMO1, and S100A1. These preclinical and clinical trials represent a "quiet revolution" that may end up being one of the most significant and remarkable breakthroughs in modern medical practice. Of course, a number of uncertainties remain, including the long-term utility and wisdom of improving the contractile performance of "sick" muscle cells. In this regard, gene therapy may turn out to be a way of buying additional time for actual cardiac regeneration to occur using cardiac stem cells or induced pluripotent stem cells.

Cardiac remodeling and subcellular defects in heart failure due to myocardial infarction and aging

Although several risk factors including hypertension, cardiac hypertrophy, coronary artery disease, and diabetes are known to result in heart failure, elderly subjects are more susceptible to myocardial infarction and more likely to develop heart failure. This article is intended to discuss that cardiac dysfunction in hearts failing due to myocardial infarction and aging is associated with cardiac remodeling and defects in the subcellular organelles such as sarcolemma (SL), sarcoplasmic reticulum (SR), and myofibrils. Despite some differences in the pattern of heart failure due to myocardial infarction and aging with respect to their etiology and sequence of events, evidence has been presented to show that subcellular remodeling plays a critical role in the occurrence of intracellular Ca(2+)-overload and development of cardiac dysfunction in both types of failing heart. In particular, alterations in gene expression for SL and SR proteins induce Ca(2+)-handling abnormalities in cardiomyocytes, whereas those for myofibrillar proteins impair the interaction of Ca(2+) with myofibrils in hearts failing due to myocardial infarction and aging. In addition, different phosphorylation mechanisms, which regulate the activities of Ca(2+)-cycling proteins in SL and SR membranes as well as Ca(2+)-binding proteins in myofibrils, become defective in the failing heart. Accordingly, it is suggested that subcellular remodeling involving defects in Ca(2+)-handling and Ca(2+)-binding proteins as well as their regulatory mechanisms is intimately associated with cardiac remodeling and heart failure due to myocardial infarction and aging.

Gene transfer for congestive heart failure: update 2013

Congestive heart failure is a major cause of morbidity and mortality with increasing social and economic costs. There have been no new high impact therapeutic agents for this devastating disease for more than a decade. However, many pivotal regulators of cardiac function have been identified using cardiac-directed transgene expression and gene deletion in preclinical studies. Some of these increase function of the failing heart. Altering the expression of these pivotal regulators using gene transfer is now either being tested in clinical gene transfer trials, or soon will be. In this review, we summarize recent progress in cardiac gene transfer for clinical congestive heart failure.

H-89 decreases the gain of excitation-contraction coupling and attenuates calcium sparks in the absence of beta-adrenergic stimulation

This study used the selective protein kinase A (PKA) inhibitor H-89 (N-[2-(p-Bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide) to determine the role of basal PKA activity in modulating cardiac excitation-contraction coupling in the absence of β-adrenergic stimulation. Basal intracellular cyclic AMP (cAMP) levels measured in isolated murine ventricular myocytes with an enzyme immunoassay were increased upon adenylyl cyclase activation (forskolin; 1 and 10 μM) or phosphodiesterase inhibition (3-isobutyl-1-methylxanthine, IBMX; 300 μM). Forskolin and IBMX also caused concentration-dependent increases in peak Ca(2+) transients (fura-2) and cell shortening (edge-detector) measured simultaneously in field-stimulated myocytes (37 °C). Similar effects were seen upon application of dibutyryl cAMP. In voltage-clamped myocytes, H-89 (2 μM) decreased basal Ca(2+) transients, contractions and underlying Ca(2+) currents. H-89 also decreased diastolic Ca(2+) and the gain of excitation-contraction coupling (Ca(2+) release/Ca(2+) current), especially at negative membrane potentials. This was independent of alterations in sarcoplasmic reticulum (SR) Ca(2+) loading, as SR stores were unchanged by PKA inhibition. H-89 also decreased the frequency, amplitude and width of spontaneous Ca(2+) sparks measured in quiescent myocytes (loaded with fluo-4), but increased time-to-peak. Thus, H-89 suppressed SR Ca(2+) release by decreasing Ca(2+) current and by reducing the gain of excitation-contraction coupling, in part by decreasing the size of individual Ca(2+) release units. These data suggest that basal PKA activity enhances SR Ca(2+) release in the absence of ß-adrenergic stimulation. This may depress contractile function in models such as aging, where the cAMP/PKA pathway is altered due to low basal cAMP levels.

Molecular targets in heart failure gene therapy: current controversies and translational perspectives

Use of gene therapy for heart failure is gaining momentum as a result of the recent successful completion of phase II of the Calcium Upregulation by Percutaneous Administration of Gene Therapy in Cardiac Disease (CUPID) trial, which showed clinical safety and efficacy of an adeno-associated viral vector expressing sarco-endoplasmic reticulum calcium ATPase (SERCA2a). Resorting to gene therapy allows the manipulation of molecular targets not presently amenable to pharmacologic modulation. This short review focuses on the molecular targets of heart failure gene therapy that have demonstrated translational potential. At present, most of these targets are related to calcium handling in the cardiomyocyte. They include SERCA2a, phospholamban, S100A1, ryanodine receptor, and the inhibitor of the protein phosphatase 1. Other targets related to cAMP signaling are reviewed, such as adenylyl cyclase. MicroRNAs are emerging as novel therapeutic targets and convenient vectors for gene therapy, particularly in heart disease. We propose a discussion of recent advances and controversies in key molecular targets of heart failure gene therapy.

Prospects for gene transfer for clinical heart failure

Congestive heart failure is an inexorable disease associated with unacceptably high morbidity and mortality. Preclinical results indicate that gene transfer using various proteins is a safe and effective approach for increasing function of the failing heart. In the current review, we provide a summary of cardiac gene transfer in general and summarize findings using adenylyl cyclase 6 as therapeutic gene in the failing heart. We also discuss the potential usefulness of a new treatment for congestive heart failure, paracrine-based gene transfer.