Disruption of adenylyl cyclase type V does not rescue the phenotype of cardiac-specific overexpression of Galphaq protein-induced cardiomyopathy

Nanodomain cAMP signaling in cardiac pathophysiology: potential for developing targeted therapeutic interventions

The 3',5'-cyclic adenosine monophosphate (cAMP) mediates the effects of sympathetic stimulation on the rate and strength of cardiac contraction. Beyond this pivotal role, in cardiac myocytes cAMP also orchestrates a diverse array of reactions to various stimuli. To ensure specificity of response, the cAMP signaling pathway is intricately organized into multiple, spatially confined, subcellular domains, each governing a distinct cellular function. In this review, we describe the molecular components of the cAMP signaling pathway with a specific focus on adenylyl cyclases, A-kinase anchoring proteins, and phosphodiesterases. We discuss how they are organized inside the intracellular space and how they achieve exquisite regulation of signaling within nanometer-size domains. We delineate the key experimental findings that lead to the current model of compartmentalized cAMP signaling, and we offer an overview of our present understanding of how cAMP nanodomains are structured and regulated within cardiac myocytes. Furthermore, we discuss how compartmentalized cAMP signaling is affected in cardiac disease and consider the potential therapeutic opportunities arising from understanding such organization. By exploiting the nuances of compartmentalized cAMP signaling, novel and more effective therapeutic strategies for managing cardiac conditions may emerge. Finally, we highlight the unresolved questions and hurdles that must be addressed to translate these insights into interventions that may benefit patients.

Physiological roles of mammalian transmembrane adenylyl cyclase isoforms

Adenylyl cyclases (ACs) catalyze the conversion of ATP to the ubiquitous second messenger cAMP. Mammals possess nine isoforms of transmembrane ACs, dubbed AC1-9, that serve as major effector enzymes of G protein-coupled receptors (GPCRs). The transmembrane ACs display varying expression patterns across tissues, giving the potential for them to have a wide array of physiological roles. Cells express multiple AC isoforms, implying that ACs have redundant functions. Furthermore, all transmembrane ACs are activated by Gαs, so it was long assumed that all ACs are activated by Gαs-coupled GPCRs. AC isoforms partition to different microdomains of the plasma membrane and form prearranged signaling complexes with specific GPCRs that contribute to cAMP signaling compartments. This compartmentation allows for a diversity of cellular and physiological responses by enabling unique signaling events to be triggered by different pools of cAMP. Isoform-specific pharmacological activators or inhibitors are lacking for most ACs, making knockdown and overexpression the primary tools for examining the physiological roles of a given isoform. Much progress has been made in understanding the physiological effects mediated through individual transmembrane ACs. GPCR-AC-cAMP signaling pathways play significant roles in regulating functions of every cell and tissue, so understanding each AC isoform's role holds potential for uncovering new approaches for treating a vast array of pathophysiological conditions.

Knockout of adenylyl cyclase isoform 5 or 6 differentially modifies the β1-adrenoceptor-mediated inotropic response

Although only β₂-adrenergic receptors (βAR) dually couple with stimulatory G protein (G_s) and inhibitory G protein (G_i), inactivation of G_i enhances both β₁AR and β₂AR responsiveness. We hypothesize that G_i restrains spontaneous adenylyl cyclase (AC) activity independent of receptor activation. Subcellular localization of the AC5/6 subtypes varies contributing to the compartmentation of βAR signaling. The primary objectives were to determine: (1) if β₁AR-mediated inotropic responses were dependent upon either AC5 or AC6; (2) if intrinsic G_i inhibition is AC subtype selective and (3) the role of phosphodiesterases (PDE) 3/4 to regulate β₁AR responsiveness. β₁AR-mediated increases in contractile force and cAMP accumulation in cardiomyocytes were measured from wild type, AC5 and AC6 knockout (KO) mice, with or without pertussis toxin (PTX) pretreatment to inactivate G_i and/or after selective inhibition of PDEs 3/4. Noradrenaline potency at β₁ARs was increased in AC6 KO. PDE4 inhibition increased noradrenaline potency in wild type and AC5 KO, but not AC6 KO. PTX increased noradrenaline potency only in wild type but increased the maximal β₁AR response in all mouse strains. PDE3 inhibition increased noradrenaline potency only in AC5 KO that was treated prior with PTX. β₁AR-evoked cAMP accumulation was increased more by PDE4 inhibition than PDE3 inhibition in wild type and AC5 KO that was amplified by G_i inhibition. These data indicate that β₁AR-mediated inotropic responses are not dependent upon either AC5 or AC6 alone. Inactivation of G_i enhanced β₁AR-mediated inotropic responses despite not coupling to G_i, consistent with G_i exerting a tonic receptor independent inhibition upon AC5/6. PDE4 seems the primary regulator of β₁AR signaling through AC6 in wild type. AC6 KO results in a reorganization of β₁AR compartmentation characterized by signaling through AC5 regulated by G_i, PDE3 and PDE4 that maintains normal contractile function.

Function of Adenylyl Cyclase in Heart: the AKAP Connection

Cyclic adenosine monophosphate (cAMP), synthesized by adenylyl cyclase (AC), is a universal second messenger that regulates various aspects of cardiac physiology from contraction rate to the initiation of cardioprotective stress response pathways. Local pools of cAMP are maintained by macromolecular complexes formed by A-kinase anchoring proteins (AKAPs). AKAPs facilitate control by bringing together regulators of the cAMP pathway including G-protein-coupled receptors, ACs, and downstream effectors of cAMP to finely tune signaling. This review will summarize the distinct roles of AC isoforms in cardiac function and how interactions with AKAPs facilitate AC function, highlighting newly appreciated roles for lesser abundant AC isoforms.

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

Overexpression of adenylyl cyclase type 5 (AC5) confers a proarrhythmic substrate to the heart

Inhibition of β-adrenergic receptor (β-AR) signaling is one of the most common therapeutic approaches for patients with arrhythmias. Adenylyl cyclase (AC) is the key enzyme responsible for transducing β-AR stimulation to increases in cAMP. The two major AC isoforms in the heart are types 5 and 6. Therefore, it is surprising that prior studies on overexpression of AC5 and AC6 in transgenic (Tg) mice have not examined mediation of arrhythmogenesis. Our goal was to examine the proarrhythmic substrate in AC5Tg hearts. Intracellular calcium ion (Ca(2+) i) was imaged in fluo-4 AM-loaded ventricular myocytes. The sarcoplasmic reticulum (SR) Ca(2+) content, fractional Ca(2+) release, and twitch Ca(2+) transient were significantly higher in the AC5Tg vs. wild-type (WT) myocytes, indicating Ca(2+) overload in AC5Tg myocytes. Action potential (AP) duration was significantly longer in AC5Tg than in WT myocytes. Additionally, AC5Tg myocytes developed spontaneous Ca(2+) waves in a larger fraction compared with WT myocytes, particularly when cells were exposed to isoproterenol. The Ca(2+) waves further induced afterdepolarizations and triggered APs. AC5Tg hearts had increased level of SERCA2a, oxidized Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), and phosphorylation of ryanodine receptors (RyR) at the CaMKII site, especially after isoproterenol treatment. This was consistent with higher reactive oxygen species production in AC5Tg myocytes after isoproterenol treatment. Isoproterenol induced more severe arrhythmias in AC5Tg than in WT mice. We conclude that AC5 overexpression promotes arrhythmogenesis, by inducing SR Ca(2+) overload and hyperactivation of RyR (phosphorylation by CaMKII), which in turn induces spontaneous Ca(2+) waves and afterdepolarizations.

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.

Adenylyl cyclase type 5 in cardiac disease, metabolism, and aging

G protein-coupled receptor/adenylyl cyclase (AC)/cAMP signaling is crucial for all cellular responses to physiological and pathophysiological stimuli. There are nine isoforms of membrane-bound AC, with type 5 being one of the two major isoforms in the heart. Since the role of AC in the heart in regulating cAMP and acute changes in inotropic and chronotropic state are well known, this review will address our current understanding of the distinct regulatory role of the AC5 isoform in response to chronic stress. Transgenic overexpression of AC5 in cardiomyocytes of the heart (AC5-Tg) improves baseline cardiac function but impairs the ability of the heart to withstand stress. For example, chronic catecholamine stimulation induces cardiomyopathy, which is more severe in AC5-Tg mice, mediated through the AC5/sirtuin 1/forkhead box O3a pathway. Conversely, disrupting AC5, i.e., AC5 knockout, protects the heart from chronic catecholamine cardiomyopathy as well as the cardiomyopathies resulting from chronic pressure overload or aging. Moreover, AC5 knockout results in a 30% increase in a healthy life span, resembling the most widely studied model of longevity, i.e., calorie restriction. These two models of longevity share similar gene regulation in the heart, muscle, liver, and brain in that they are both protected against diabetes, obesity, and diabetic and aging cardiomyopathy. A pharmacological inhibitor of AC5 also provides protection against cardiac stress, diabetes, and obesity. Thus AC5 inhibition has novel, potential therapeutic applicability to several diseases not only in the heart but also in aging, diabetes, and obesity.

Adenylyl cyclase subtype-specific compartmentalization: differential regulation of L-type Ca2+ current in ventricular myocytes

RATIONALE

Adenylyl cyclase (AC) represents one of the principal molecules in the β-adrenergic receptor signaling pathway, responsible for the conversion of ATP to the second messenger, cAMP. AC types 5 (ACV) and 6 (ACVI) are the 2 main isoforms in the heart. Although highly homologous in sequence, these 2 proteins play different roles during the development of heart failure. Caveolin-3 is a scaffolding protein, integrating many intracellular signaling molecules in specialized areas called caveolae. In cardiomyocytes, caveolin is located predominantly along invaginations of the cell membrane known as t-tubules.

OBJECTIVE

We take advantage of ACV and ACVI knockout mouse models to test the hypothesis that there is distinct compartmentalization of these isoforms in ventricular myocytes.

METHODS AND RESULTS

We demonstrate that ACV and ACVI isoforms exhibit distinct subcellular localization. The ACVI isoform is localized in the plasma membrane outside the t-tubular region and is responsible for β1-adrenergic receptor signaling-mediated enhancement of the L-type Ca(2+) current (ICa,L) in ventricular myocytes. In contrast, the ACV isoform is localized mainly in the t-tubular region where its influence on ICa,L is restricted by phosphodiesterase. We further demonstrate that the interaction between caveolin-3 with ACV and phosphodiesterase is responsible for the compartmentalization of ACV signaling.

CONCLUSIONS

Our results provide new insights into the compartmentalization of the 2 AC isoforms in the regulation of ICa,L in ventricular myocytes. Because caveolae are found in most mammalian cells, the mechanism of β- adrenergic receptor and AC compartmentalization may also be important for β-adrenergic receptor signaling in other cell types.

Type 5 adenylyl cyclase increases oxidative stress by transcriptional regulation of manganese superoxide dismutase via the SIRT1/FoxO3a pathway

BACKGROUND

For reasons that remain unclear, whether type 5 adenylyl cyclase (AC5), 1 of 2 major AC isoforms in heart, is protective or deleterious in response to cardiac stress is controversial. To reconcile this controversy we examined the cardiomyopathy induced by chronic isoproterenol in AC5 transgenic (Tg) mice and the signaling mechanisms involved.

METHODS AND RESULTS

Chronic isoproterenol increased oxidative stress and induced more severe cardiomyopathy in AC5 Tg, as left ventricular ejection fraction fell 1.9-fold more than wild type, along with greater left ventricular dilation and increased fibrosis, apoptosis, and hypertrophy. Oxidative stress induced by chronic isoproterenol, detected by 8-OhDG was 15% greater, P=0.007, in AC5 Tg hearts, whereas protein expression of manganese superoxide dismutase (MnSOD) was reduced by 38%, indicating that the susceptibility of AC5 Tg to cardiomyopathy may be attributable to decreased MnSOD expression. Consistent with this, susceptibility of the AC5 Tg to cardiomyopathy was suppressed by overexpression of MnSOD, whereas protection afforded by the AC5 knockout (KO) was lost in AC5 KO×MnSOD heterozyous KO mice. Elevation of MnSOD was eliminated by both sirtuin and MEK inhibitors, suggesting both the SIRT1/FoxO3a and MEK/ERK pathway are involved in MnSOD regulation by AC5.

CONCLUSIONS

Overexpression of AC5 exacerbates the cardiomyopathy induced by chronic catecholamine stress by altering regulation of SIRT1/FoxO3a, MEK/ERK, and MnSOD, resulting in oxidative stress intolerance, thereby shedding light on new approaches for treatment of heart failure.

Nuclear effects of G-protein receptor kinase 5 on histone deacetylase 5-regulated gene transcription in heart failure

BACKGROUND

G-protein receptor kinases (GRKs) modulate cardiac β-adrenergic signaling. GRK5 is upregulated in heart failure, and a gain-of-function polymorphism substituting leucine for wild-type glutamine at amino acid 41 (GRK5-Leu41) is associated with improved outcomes in heart failure and hypertension. GRK5 is distinguished by partial nuclear localization and class II histone deacetylases (HDAC) kinase activity that is postulated to regulate Gαq-stimulated cardiac gene expression.

METHODS AND RESULTS

We used in vitro tissue culture and in vivo mouse compound genetic models to examine the effects of GRK5 on HDAC phosphorylation, nucleo-cytoplasmic HDAC transport, and Gαq-dependent transcriptional regulation. In vitro, GRK5 stimulated HDAC5 nuclear export only in the context of Gαq signaling stimulated by angiotensin II. GRK5-Gln41 and Leu41 were similar inducers of HDAC5 nucleo-cytoplasmic shuttling. In vivo, GRK5-Gln41 and-Leu41 partitioned equally to nuclear and nonnuclear myocardial fractions. GRK5 increased cardiac HDAC5 phosphorylation and reversed the increase in nuclear HDAC5 content seen with cardiomyocyte-autonomous Gαq overexpression. Deep RNA sequencing showed few changes in gene expression induced by GRK5 overexpression or ablation alone, but GRK5 overexpression normalized steady-state expression levels of 48% (96 of 200) of all Gαq down-regulated mRNAs.

CONCLUSIONS

GRK5 is a transcriptional modifier of a subset of Gαq-downregulated genes, acting in opposition to the pathological effects of Gαq and normalizing levels of these transcripts. This transcriptional coregulator effect may act in concert with β-adrenergic receptor desensitization to protect against heart failure decompensation.