Novel Paradigms Governing β1-Adrenergic Receptor Trafficking in Primary Adult Rat Cardiac Myocytes

Cytoskeletal disarray increases arrhythmogenic vulnerability during sympathetic stimulation in a model of hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) patients are advised to avoid strenuous exercise due to increased risk of arrhythmias. Mice expressing the human FHC-causing mutation R403Q in the myosin heavy chain gene (MYH6) recapitulate the human phenotype, including cytoskeletal disarray and increased arrhythmia susceptibility. Following in vivo administration of isoproterenol, mutant mice exhibited tachyarrhythmias, poor recovery and fatigue. Arrhythmias were attenuated with the β-blocker atenolol and protein kinase A inhibitor PKI. Mutant cardiac myocytes had significantly prolonged action potentials and triggered automaticity due to reduced repolarization reserve and connexin 43 expression. Isoproterenol shortened cycle length, and escalated electrical instability. Surprisingly isoproterenol did not increase CaV1.2 current. We found alterations in CaV1.2-β1 adrenergic receptor colocalization assessed using super-resolution nanoscopy, and increased CaV1.2 phosphorylation in mutant hearts. Our results reveal for the first time that altered ion channel expression, co-localization and β-adrenergic receptor signaling associated with myocyte disarray contribute to electrical instability in the R403Q mutant heart.

A Rab10-ACAP1-Arf6 GTPases cascade modulates M4 muscarinic acetylcholine receptor trafficking and signaling

Membrane trafficking processes regulate the G protein-coupled receptor activity. The muscarinic acetylcholine receptors (mAChRs) are highly pursued drug targets for neurological diseases, but the cellular machineries that control the trafficking of these receptors remain largely elusive. Here, we revealed the role of the small GTPase Rab10 as a negative regulator for the post-activation trafficking of M4 mAChR and the underlying mechanism. We show that constitutively active Rab10 arrests the receptor within Rab5-positive early endosomes and significantly hinders the resensitization of M4-mediated Ca2+ signaling. Mechanistically, M4 binds to Rab10-GTP, which requires the motif 386RKKRQMAA393 (R386-A393) within the third intracellular loop. Moreover, Rab10-GTP inactivates Arf6 by recruiting the Arf6 GTPase-activating protein, ACAP1. Strikingly, deletion of the motif R386-A393 causes M4 to bypass the control by Rab10 and switch to the Rab4-facilitated fast recycling pathway, thus reusing the receptor. Therefore, Rab10 couples the cargo sorting and membrane trafficking regulation through cycle between GTP-bound and GDP-bound state. Our findings suggest a model that Rab10 binds to the M4 like a molecular brake and controls the receptor's transport through endosomes, thus modulating the signaling, and this regulation is specific among the mAChR subtypes.

Involvement of SAP97 anchored multiprotein complexes in regulating cardiorenal signaling and trafficking networks

SAP97 is a member of the MAGUK family of proteins, but unlike other MAGUK proteins that are selectively expressed in the CNS, SAP97 is also expressed in peripheral organs, like the heart and kidneys. SAP97 has several protein binding cassettes, and this review will describe their involvement in creating SAP97-anchored multiprotein networks. SAP97-anchored networks localized at the inner leaflet of the cell membrane play a major role in trafficking and targeting of membrane G protein-coupled receptors (GPCR), channels, and structural proteins. SAP97 plays a major role in compartmentalizing voltage gated sodium and potassium channels to specific cellular compartments of heart cells. SAP97 undergoes extensive alternative splicing. These splice variants give rise to different SAP97 isoforms that alter its cellular localization, networking, signaling and trafficking effects. Regarding GPCR, SAP97 binds to the β₁-adrenergic receptor and recruits AKAP5/PKA and PDE4D8 to create a multiprotein complex that regulates trafficking and signaling of cardiac β₁-AR. In the kidneys, SAP97 anchored networks played a role in trafficking of aquaporin-2 water channels. Cardiac specific ablation of SAP97 (SAP97-cKO) resulted in cardiac hypertrophy and failure in aging mice. Similarly, instituting transverse aortic constriction (TAC) in young SAP97 c-KO mice exacerbated TAC-induced cardiac remodeling and dysfunction. These findings highlight a critical role for SAP97 in the pathophysiology of a number of cardiac and renal diseases, suggesting that SAP97 is a relevant target for drug discovery.

Metoprolol Mitigates Ischemic Heart Remodeling and Fibrosis by Increasing the Expression of AKAP5 in Ischemic Heart

The harm of heart failure mainly causes patients to develop dyspnea, fatigue, fluid retention, and other symptoms, which impair patients' activity tolerance and lead to a dramatic decrease in patients' quality of life. The purpose of this study was to verify whether metoprolol regulates AKAP5 expression and test the role of AKAP5 postinjury in mitigating cardiac infarction-associated tissue remodeling and fibrosis. Sprague-Dawley (SD) rats underwent coronary artery ligation (CAL), which was followed immediately with metoprolol daily. And western blot and coimmunoprecipitation experiments were performed to detect the expression of related proteins in the sham-operated group, model group, and drug-treated group. HW/BW ratio and cardiac expression of COL1 and COL3 were increased in rats following CAL compared with shams. Treatment with metoprolol postinjury was associated with a decrease in HW/BW ratio and COL1/COL3 expression compared to uncontrolled rats. CAL resulted in decreased cardiac AKAP5 expression compared to the control group, while metoprolol treatment restored levels compared to baseline shams. Cardiac expression levels of NFATc3/p-NFATc3 and GATA4 were modest at baseline and increased with injury, whereas metoprolol suppressed gene expression to below injury-associated changes. Immunoprecipitation indicated that AKAP5 could bind and regulate PP2B. In summary, we know that metoprolol alleviates ischemic cardiac remodeling and fibrosis, and the mechanism of alleviating remodeling may improve cardiac AKAP5 expression and AKAP5-PP2B interaction.

Subcellular β-Adrenergic Receptor Signaling in Cardiac Physiology and Disease

ABSTRACT

Adrenergic receptors are critical regulators of cardiac function with profound effects on cardiac output during sympathetic stimulation. Chronic stimulation of the adrenergic system of the heart under conditions of cardiac stress leads to cardiac dysfunction, hypertrophy, and ultimately failure. Emerging data have revealed that G protein-coupled receptors in intracellular compartments are functionally active and regulate distinct cellular processes from those at the cell surface. β2 adrenergic receptors internalize onto endosomes in various cell types where they have recently been shown to continue to stimulate cAMP production to selectively regulate gene expression. Other studies have identified β1 adrenergic receptors at the nuclear envelope and the Golgi apparatus. Here, we discuss data on signaling by β1 and β2 adrenergic receptors in the heart and the possible influence of their subcellular locations on their divergent physiological functions in cardiac myocytes and in cardiac pathology. Understanding the relative roles of these receptors at these locations could have a significant impact on pharmacological targeting of these receptors for the treatment of heart failure and cardiac diseases.

Visualization of β-adrenergic receptor dynamics and differential localization in cardiomyocytes

A key question in receptor signaling is how specificity is realized, particularly when different receptors trigger the same biochemical pathway(s). A notable case is the two β-adrenergic receptor (β-AR) subtypes, β1 and β2, in cardiomyocytes. They are both coupled to stimulatory Gs proteins, mediate an increase in cyclic adenosine monophosphate (cAMP), and stimulate cardiac contractility; however, other effects, such as changes in gene transcription leading to cardiac hypertrophy, are prominent only for β1-AR but not for β2-AR. Here, we employ highly sensitive fluorescence spectroscopy approaches, in combination with a fluorescent β-AR antagonist, to determine the presence and dynamics of the endogenous receptors on the outer plasma membrane as well as on the T-tubular network of intact adult cardiomyocytes. These techniques allow us to visualize that the β2-AR is confined to and diffuses within the T-tubular network, as opposed to the β1-AR, which is found to diffuse both on the outer plasma membrane as well as on the T-tubules. Upon overexpression of the β2-AR, this compartmentalization is lost, and the receptors are also seen on the cell surface. Such receptor segregation depends on the development of the T-tubular network in adult cardiomyocytes since both the cardiomyoblast cell line H9c2 and the cardiomyocyte-differentiated human-induced pluripotent stem cells express the β2-AR on the outer plasma membrane. These data support the notion that specific cell surface targeting of receptor subtypes can be the basis for distinct signaling and functional effects.

β-blockers Reverse Agonist-Induced β2-AR Downregulation Regardless of Their Signaling Profile

Altered β-adrenergic receptor (β-AR) density has been reported in cells, animals, and humans receiving β-blocker treatment. In some cases, β-AR density is upregulated, but in others, it is unaffected or even reduced. Collectively, these results would imply that changes in β-AR density and β-blockade are not related. However, it has still not been clarified whether the effects of β-blockers on receptor density are related to their ability to activate different β-AR signaling pathways. To this aim, five clinically relevant β-blockers endowed with inverse, partial or biased agonism at the β₂-AR were evaluated for their effects on β₂-AR density in both human embryonic kidney 293 (HEK293) cells expressing exogenous FLAG-tagged human β₂-ARs and human lymphocytes expressing endogenous β₂-ARs. Cell surface β₂-AR density was measured by enzyme-linked immunosorbent assay (ELISA) and flow cytometry. Treatment with propranolol, carvedilol, pindolol, sotalol, or timolol did not induce any significant change in surface β₂-AR density in both HEK293 cells and human lymphocytes. On the contrary, treatment with the β-AR agonist isoproterenol reduced the number of cell surface β₂-ARs in the tested cell types without affecting β₂-AR-mRNA levels. Isoproterenol-induced effects on receptor density were completely antagonized by β-blocker treatment. In conclusion, the agonistic activity of β-blockers does not exert an important effect on short-term regulation of β₂-AR density.