β₁-adrenergic receptors (β₁ARs) mediate catecholamine actions in cardiomyocytes by coupling to both Gs/cAMP-dependent and Gs-independent/growth-regulatory pathways. Structural studies of the β₁AR define ligand-binding sites in the transmembrane helices and effector docking sites at the intracellular surface of the β₁AR, but the extracellular N-terminus, which is a target for post-translational modifications, typically is ignored. This study identifies β₁AR N-terminal O-glycosylation at Ser³⁷/Ser⁴¹ as a mechanism that prevents β₁AR N-terminal cleavage. We used an adenoviral overexpression strategy to show that both full-length/glycosylated β₁ARs and N-terminally truncated glycosylation-defective β₁ARs couple to cAMP and ERK-MAPK signaling pathways in cardiomyocytes. However, a glycosylation defect that results in N-terminal truncation stabilizes β₁ARs in a conformation that is biased toward the cAMP pathway. The identification of O-glycosylation and N-terminal cleavage as novel structural determinants of β₁AR responsiveness in cardiomyocytes could be exploited for therapeutic advantage.

FRET-based biosensor experiments in adult cardiomyocytes are a powerful way of dissecting the spatiotemporal dynamics of the complicated signaling networks that regulate cardiac health and disease. However, although much information has been gleaned from FRET studies on cardiomyocytes from larger species, experiments on adult cardiomyocytes from mice have been difficult at best. Thus the large variety of genetic mouse models cannot be easily used for this type of study. Here we develop cell culture conditions for adult mouse cardiomyocytes that permit robust expression of adenoviral FRET biosensors and reproducible FRET experimentation. We find that addition of 6.25 µM blebbistatin or 20 µM (S)-nitro-blebbistatin to a minimal essential medium containing 10 mM HEPES and 0.2% BSA maintains morphology of cardiomyocytes from physiological, pathological, and transgenic mouse models for up to 50 h after adenoviral infection. This provides a 10-15-h time window to perform reproducible FRET readings using a variety of CFP/YFP sensors between 30 and 50 h postinfection. The culture is applicable to cardiomyocytes isolated from transgenic mouse models as well as models with cardiac diseases. Therefore, this study helps scientists to disentangle complicated signaling networks important in health and disease of cardiomyocytes.

Background In murine heart failure models and in humans with diabetic-related heart hypertrophy, inhibition of phosphodiesterase 5 (PDE5) by sildenafil improves cardiac outcomes. However, the mechanism by which sildenafil improves cardiac function is unclear. We have observed a relationship between PDE5 and β2 adrenergic receptor (β2AR), which is characterized here as a novel mechanistic axis by which sildenafil improves symptoms of diabetic cardiomyopathy. Methods and Results Wild-type and β2AR knockout mice fed a high fat diet (HFD) were treated with sildenafil, and echocardiogram analysis was performed. Cardiomyocytes were isolated for excitation-contraction (E-C) coupling, fluorescence resonant energy transfer, and proximity ligation assays; while heart tissues were implemented for biochemical and histological analyses. PDE5 selectively associates with β2AR, but not β1 adrenergic receptor, and inhibition of PDE5 with sildenafil restores the impaired response to adrenergic stimulation in HFD mice and isolated ventriculomyocytes. Sildenafil enhances β adrenergic receptor (βAR)-stimulated cGMP and cAMP signals in HFD myocytes. Consequently, inhibition of PDE5 leads to protein kinase G-, and to a lesser extent, calcium/calmodulin-dependent kinase II-dependent improvements in adrenergically stimulated E-C coupling. Deletion of β2AR abolishes sildenafil's effect. Although the PDE5-β2AR association is not altered in HFD, phosphodiesterase 3 displays an increased association with the β2AR-PDE5 complex in HFD myocytes. Conclusions This study elucidates mechanisms by which the β2AR-PDE5 axis can be targeted for treating diabetic cardiomyopathy. Inhibition of PDE5 enhances β2AR stimulation of cGMP and cAMP signals, as well as protein kinase G-dependent E-C coupling in HFD myocytes.

Aims

β-blockers are widely used in therapy for heart failure and hypertension. β-blockers are also known to evoke additional diversified pharmacological and physiological effects in patients. We aim to characterize the underlying molecular signalling and effects on cardiac inotropy induced by β-blockers in animal hearts.

Methods and results

Wild-type mice fed high-fat diet (HFD) were treated with carvedilol, metoprolol, or vehicle and echocardiogram analysis was performed. Heart tissues were used for biochemical and histological analyses. Cardiomyocytes were isolated from normal and HFD mice and rats for analysis of adrenergic signalling, calcium handling, contraction, and western blot. Biosensors were used to measure β-blocker-induced cyclic guanosine monophosphate (cGMP) signal and protein kinase A activity in myocytes. Acute stimulation of myocytes with carvedilol promotes β1 adrenergic receptor (β1AR)- and protein kinase G (PKG)-dependent inotropic cardiac contractility with minimal increases in calcium amplitude. Carvedilol acts as a biased ligand to promote β1AR coupling to a Gi-PI3K-Akt-nitric oxide synthase 3 (NOS3) cascade and induces robust β1AR-cGMP-PKG signal. Deletion of NOS3 selectively blocks carvedilol, but not isoproterenol-induced β1AR-dependent cGMP signal and inotropic contractility. Moreover, therapy with carvedilol restores inotropic contractility and sensitizes cardiac adrenergic reserves in diabetic mice with minimal impact in calcium signal, as well as reduced cell apoptosis and hypertrophy in diabetic hearts.

Conclusion

These observations present a novel β1AR-NOS3 signalling pathway to promote cardiac inotropy in the heart, indicating that this signalling paradigm may be targeted in therapy of heart diseases with reduced ejection fraction.

The number and activity of Ca_v1.2 channels in the cardiomyocyte sarcolemma tunes the magnitude of Ca²⁺-induced Ca²⁺ release and myocardial contraction. β-Adrenergic receptor (βAR) activation stimulates sarcolemmal insertion of Ca_V1.2. This supplements the preexisting sarcolemmal Ca_V1.2 population, forming large "superclusters" wherein neighboring channels undergo enhanced cooperative-gating behavior, amplifying Ca²⁺ influx and myocardial contractility. Here, we determine this stimulated insertion is fueled by an internal reserve of early and recycling endosome-localized, presynthesized Ca_V1.2 channels. βAR-activation decreased Ca_V1.2/endosome colocalization in ventricular myocytes, as it triggered "emptying" of endosomal Ca_V1.2 cargo into the t-tubule sarcolemma. We examined the rapid dynamics of this stimulated insertion process with live-myocyte imaging of channel trafficking, and discovered that Ca_V1.2 are often inserted into the sarcolemma as preformed, multichannel clusters. Similarly, entire clusters were removed from the sarcolemma during endocytosis, while in other cases, a more incremental process suggested removal of individual channels. The amplitude of the stimulated insertion response was doubled by coexpression of constitutively active Rab4a, halved by coexpression of dominant-negative Rab11a, and abolished by coexpression of dominant-negative mutant Rab4a. In ventricular myocytes, βAR-stimulated recycling of Ca_V1.2 was diminished by both nocodazole and latrunculin-A, suggesting an essential role of the cytoskeleton in this process. Functionally, cytoskeletal disruptors prevented βAR-activated Ca²⁺ current augmentation. Moreover, βAR-regulation of Ca_V1.2 was abolished when recycling was halted by coapplication of nocodazole and latrunculin-A. These findings reveal that βAR-stimulation triggers an on-demand boost in sarcolemmal Ca_V1.2 abundance via targeted Rab4a- and Rab11a-dependent insertion of channels that is essential for βAR-regulation of cardiac Ca_V1.2.

Sinoatrial node (SAN) cells are the pacemakers of the heart. This study describes a method for culturing and infection of adult mouse SAN cells with FRET-based biosensors that can be exploited to examine signaling events. SAN cells cultured in media with blebbistatin or (S)-nitro-blebbistatin retain their morphology, protein distribution, action potential (AP) waveform, and cAMP dynamics for at least 40 h. SAN cells expressing targeted cAMP sensors show distinct β-adrenergic-mediated cAMP pools. Cyclic GMP, protein kinase A, Ca²⁺/CaM kinase II, and protein kinase D in SAN cells also show unique dynamics to different stimuli. Heart failure SAN cells show a decrease in cAMP and cGMP levels. In summary, a reliable method for maintaining adult mouse SAN cells in culture is presented, which facilitates studies of signaling networks and regulatory mechanisms during physiological and pathological conditions.

Rationale

In heart failure, myofilament proteins display abnormal phosphorylation, which contributes to contractile dysfunction. The mechanisms underlying the dysregulation of protein phosphorylation on myofilaments is not clear.

Objective

This study aims to understand the mechanisms underlying altered phosphorylation of myofilament proteins in heart failure.

Methods and results

We generate a novel genetically encoded protein kinase A (PKA) biosensor anchored onto the myofilaments in rabbit cardiac myocytes to examine PKA activity at the myofilaments in responses to adrenergic stimulation. We show that PKA activity is shifted from the sarcolemma to the myofilaments in hypertrophic failing rabbit myocytes. In particular, the increased PKA activity on the myofilaments is because of an enhanced β2 adrenergic receptor signal selectively directed to the myofilaments together with a reduced phosphodiesterase activity associated with the myofibrils. Mechanistically, the enhanced PKA activity on the myofilaments is associated with downregulation of caveolin-3 in the hypertrophic failing rabbit myocytes. Reintroduction of caveolin-3 in the failing myocytes is able to normalize the distribution of β2 adrenergic receptor signal by preventing PKA signal access to the myofilaments and to restore contractile response to adrenergic stimulation.

Conclusions

In hypertrophic rabbit myocytes, selectively enhanced β2 adrenergic receptor signaling toward the myofilaments contributes to elevated PKA activity and PKA phosphorylation of myofilament proteins. Reintroduction of caveolin-3 is able to confine β2 adrenergic receptor signaling and restore myocyte contractility in response to β adrenergic stimulation.

Rationale

β₁ARs (β₁-adrenoceptors) exist at intracellular membranes and OCT3 (organic cation transporter 3) mediates norepinephrine entry into cardiomyocytes. However, the functional role of intracellular β₁AR in cardiac contractility remains to be elucidated.

Objective

Test localization and function of intracellular β₁AR on cardiac contractility.

Methods and results

Membrane fractionation, super-resolution imaging, proximity ligation, coimmunoprecipitation, and single-molecule pull-down demonstrated a pool of β₁ARs in mouse hearts that were associated with sarco/endoplasmic reticulum Ca²⁺-ATPase at the sarcoplasmic reticulum (SR). Local PKA (protein kinase A) activation was measured using a PKA biosensor targeted at either the plasma membrane (PM) or SR. Compared with wild-type, myocytes lacking OCT3 (OCT3-KO [OCT3 knockout]) responded identically to the membrane-permeant βAR agonist isoproterenol in PKA activation at both PM and SR. The same was true at the PM for membrane-impermeant norepinephrine, but the SR response to norepinephrine was suppressed in OCT3-KO myocytes. This differential effect was recapitulated in phosphorylation of the SR-pump regulator phospholamban. Similarly, OCT3-KO selectively suppressed calcium transients and contraction responses to norepinephrine but not isoproterenol. Furthermore, sotalol, a membrane-impermeant βAR-blocker, suppressed isoproterenol-induced PKA activation at the PM but permitted PKA activation at the SR, phospholamban phosphorylation, and contractility. Moreover, pretreatment with sotalol in OCT3-KO myocytes prevented norepinephrine-induced PKA activation at both PM and the SR and contractility.

Conclusions

Functional β₁ARs exists at the SR and is critical for PKA-mediated phosphorylation of phospholamban and cardiac contractility upon catecholamine stimulation. Activation of these intracellular β₁ARs requires catecholamine transport via OCT3.

Elevated blood glucose (hyperglycemia) is a hallmark metabolic abnormality in diabetes. Hyperglycemia is associated with protein kinase A (PKA)-mediated stimulation of L-type Ca2+ channels in arterial myocytes resulting in increased vasoconstriction. However, the mechanisms by which glucose activates PKA remain unclear. Here, we showed that elevating extracellular glucose stimulates cAMP production in arterial myocytes, and that this was specifically dependent on adenylyl cyclase 5 (AC5) activity. Super-resolution imaging suggested nanometer proximity between subpopulations of AC5 and the L-type Ca2+ channel pore-forming subunit CaV1.2. In vitro, in silico, ex vivo and in vivo experiments revealed that this close association is critical for stimulation of L-type Ca2+ channels in arterial myocytes and increased myogenic tone upon acute hyperglycemia. This pathway supported the increase in L-type Ca2+ channel activity and myogenic tone in two animal models of diabetes. Our collective findings demonstrate a unique role for AC5 in PKA-dependent modulation of L-type Ca2+ channel activity and vascular reactivity during acute hyperglycemia and diabetes.

Rationale

Cardiotoxic β₁ adrenergic receptor (β₁AR)-CaMKII (calmodulin-dependent kinase II) signaling is a major and critical feature associated with development of heart failure. SAP97 (synapse-associated protein 97) is a multifunctional scaffold protein that binds directly to the C-terminus of β₁AR and organizes a receptor signalosome.

Objective

We aim to elucidate the dynamics of β₁AR-SAP97 signalosome and its potential role in chronic cardiotoxic β₁AR-CaMKII signaling that contributes to development of heart failure.

Methods and results

The integrity of cardiac β₁AR-SAP97 complex was examined in heart failure. Cardiac-specific deletion of SAP97 was developed to examine β₁AR signaling in aging mice, after chronic adrenergic stimulation, and in pressure overload hypertrophic heart failure. We show that the β₁AR-SAP97 signaling complex is reduced in heart failure. Cardiac-specific deletion of SAP97 yields an aging-dependent cardiomyopathy and exacerbates cardiac dysfunction induced by chronic adrenergic stimulation and pressure overload, which are associated with elevated CaMKII activity. Loss of SAP97 promotes PKA (protein kinase A)-dependent association of β₁AR with arrestin2 and CaMKII and turns on an Epac (exchange protein directly activated by cAMP)-dependent activation of CaMKII, which drives detrimental functional and structural remodeling in myocardium. Moreover, we have identified that GRK5 (G-protein receptor kinase-5) is necessary to promote agonist-induced dissociation of SAP97 from β₁AR. Cardiac deletion of GRK5 prevents adrenergic-induced dissociation of β₁AR-SAP97 complex and increases in CaMKII activity in hearts.

Conclusions

These data reveal a critical role of SAP97 in maintaining the integrity of cardiac β₁AR signaling and a detrimental cardiac GRK5-CaMKII axis that can be potentially targeted in heart failure therapy. Graphical Abstract: A graphical abstract is available for this article.