Dissecting the Role of G-Protein–Coupled Receptor Kinase 2 for Excitation-Contraction Coupling
2012; Lippincott Williams & Wilkins; Volume: 125; Issue: 17 Linguagem: Inglês
10.1161/circulationaha.112.109389
ISSN1524-4539
Autores Tópico(s)Phosphodiesterase function and regulation
ResumoHomeCirculationVol. 125, No. 17Dissecting the Role of G-Protein–Coupled Receptor Kinase 2 for Excitation-Contraction Coupling Free AccessEditorialPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessEditorialPDF/EPUBDissecting the Role of G-Protein–Coupled Receptor Kinase 2 for Excitation-Contraction Coupling Christoph Maack, MD Christoph MaackChristoph Maack From the Medizinische Klinik und Poliklinik, Innere Medizin III, Universitätsklinikum des Saarlandes, Homburg, Germany. Originally published10 Apr 2012https://doi.org/10.1161/CIRCULATIONAHA.112.109389Circulation. 2012;125:2054–2056Other version(s) of this articleYou are viewing the most recent version of this article. Previous versions: January 1, 2012: Previous Version 1 Chronic heart failure is the inability of the left ventricle (LV) to supply the body with sufficient amounts of blood. On the cellular level, this contractile dysfunction is caused by deterioration of excitation-contraction (EC) coupling in cardiac myocytes.1,2 In the normal heart, Ca2+ enters cardiac myocytes via L-type Ca2+ channels (LTCCs) and triggers an even greater Ca2+ release from the sarcoplasmic reticulum (SR). This Ca2+ binds to the myofilaments to induce contraction. During diastole, Ca2+ is taken back up into the SR by the SR Ca2+ ATPase (SERCA), and the amount of Ca2+ that entered the cell via LTCC is exported via the Na+/Ca2+ exchanger. In myocytes of failing hearts, SR Ca2+ load is reduced as a result of decreased SERCA activity and a leak of the Ca2+ release channels in the SR, the ryanodine receptors. Furthermore, expression and activity of the Na+/Ca2+ exchanger are elevated in failing myocardium, which further decreases SR Ca2+ load by removing Ca2+ from the cytosol during diastole.2Article see p 2108In response to decreased LV function, neurohormonal activation sustains cardiac output and blood pressure. In particular, sympathetic activation increases the local release of norepinephrine in the heart and of epinephrine from the adrenal glands. Norepinephrine binds to cardiac β-adrenergic receptors (β-ARs), which couple to the stimulatory G protein (Gs) and induce the dissociation of the α-subunit from the βγ-subunits of Gs.3 The former stimulates adenylyl cyclase to produce the second messenger cAMP, activating protein kinase A, which then phosphorylates LTCC, ryanodine receptors, phospholamban (inhibiting SERCA), and troponin I. This increases Ca2+ influx via LTCCs, Ca2+ uptake into the SR via SERCA, and Ca2+ release from the SR via ryanodine receptors, increasing the rate and amplitude of cytosolic Ca2+ transients and contraction of the cell.3 After dissociation from Gsα, the Gβγ-subunits bind to the G-protein–coupled receptor kinase 2 (GRK2), also known as β-AR kinase 1 (β-ARK1). GRK2 then associates with phosphoinositide 3-kinase γ and phosphorylates β-ARs, priming the binding of β-arrestin to the receptor, which inhibits the coupling of β-ARs to Gs. This uncoupling of β-ARs turns off the signal within seconds to minutes,4 a process called homologous desensitization.3 With continued β-AR stimulation, β-ARs are removed from the cell surface by internalization in clathrin-coated pits, which is called β-AR downregulation.3The positive inotropic, lusitropic, and chronotropic effects of acute β-AR activation mediate an increase in cardiac output, which is beneficial in the short term in the context of a physiological increase in workload (ie, during exercise). In response to reduced cardiac output in heart failure, however, the sympathetic nervous system is chronically activated, which desensitizes and downregulates cardiac β-ARs, rendering the heart refractory to catecholamine stimulation.5 Because the degree of sympathetic activation is related to an adverse outcome of patients with heart failure, it was perceived that β-AR downregulation was a major mechanism for reduced LV function in heart failure.6 However, the observation that cardiomyocyte-specific overexpression of β1-ARs7 induces dilated cardiomyopathy indicated that continuous β1-AR signaling (rather than the desensitization of this signaling pathway) accounts for maladaptive remodeling involving protein kinase A–dependent8 and/or –independent pathways.9 Accordingly, treatment of patients with heart failure with β-AR antagonists (β-blockers) improves LV ejection fraction, clinical symptoms, and survival in patients with heart failure.5 Although treatment with metoprolol or carvedilol led to similar improvements of prognosis,5 only treatment with metoprolol, but not carvedilol, increased the expression of cardiac β1-ARs in failing hearts,10 presumably related to differential binding characteristics of these β-blockers to cardiac β-ARs.11,12 In fact, β-blockade is more complete with long-term carvedilol compared with metoprolol treatment in patients with heart failure.13 These data support the notion that the resensitization of β-AR signaling per se may not be the key mechanism by which the biology of the failing heart is improved.5Although carvedilol does not resensitize cardiac β-ARs, it has in common with first- and second-generation β-blockers that it downregulates GRK2, whereas agonist stimulation of β-ARs upregulates GRK2,14 in agreement with upregulated GRK2 in failing human hearts.15 Considering the reciprocal association between GRK2 expression and the early perception that resensitization of β-AR signaling may be key to improve overall cardiac function in heart failure, Koch et al16 pioneered a transgenic strategy to inhibit elevated GRK2 activity by overexpressing a peptide corresponding to the Gβγ-interacting C-terminal domain of β-ARK1 called βARKct. Interestingly, this peptide not only resensitized β-AR signaling but also rescued several small- and large-animal models of heart failure.17,18 It is still controversial, however, whether the beneficial effects of βARKct are indeed related to resensitization of β-AR signaling by inhibiting GRK2 or to the scavenging of Gβγ subunits by βARKct that may exert other, GRK2-independent (maladaptive) structural or functional remodeling.17,19 In this respect, recent data from the Koch and Dorn laboratories supported the importance of GRK2 inhibition by improving cardiac function and survival after myocardial infarction by genetic postnatal deletion of cardiac GRK2.4,20Despite a great wealth of data on the effects of βARKct on β-adrenergic signaling and cardiac remodeling, surprisingly little information has been available on how βARKct or GRK2 deletion affects the actual underlying deficit in heart failure, ie, deteriorated EC coupling. In this issue of Circulation, Raake et al21 provide an elaborate mechanistic study that for the first time characterizes GRK2-deficient cardiac myocytes. Despite unchanged steady-state Ca2+ transients at baseline, the SR Ca2+ load is decreased in GRK2-deficient myocytes, whereas the fractional SR Ca2+ release during steady-state Ca2+ transients is increased. This increase in the fractional SR Ca2+ release can be explained by elevated open probability and currents of the LTCC secondary to constitutive protein kinase A–mediated phosphorylation of LTCC. The authors relate this to elevated constitutive activity of β-ARs in the absence of any GRK2-mediated phosphorylation. Because Ca2+ influx must match Ca2+ efflux, upregulation of Na+/Ca2+ exchanger protein and currents apparently counterbalances the increase in LTCC-mediated Ca2+ influx. On the other hand, decreased SR Ca2+ load was associated with decreased phospholamban phosphorylation in GRK-deficient myocytes, which could be reversed by the PDE4 inhibitor rolipram. This is in agreement with the observation that PDE4 associates with—and controls the activity of—SERCA but not LTCC in murine cardiac myocytes.22 Interestingly, besides the well-characterized activation of PDE4 by protein kinase A–mediated phosphorylation,23 SR-associated PDE4 is also under the control of phosphoinositide 3-kinase γ in cardiac myocytes,24 which suggests a close interplay between GRK2, phosphoinositide 3-kinase γ, and PDE4 to constitutively control local cAMP pools near the SR. The details of this complex interplay await further elucidation.Despite reduced baseline phosphorylation of phospholamban, β-AR agonist–induced phosphorylation of phospholamban was potentiated in GRK2-deficient myocytes, in line with the notion that in normal myocytes GRK2 rapidly uncouples β-ARs from Gs on agonist-induced receptor activation,4 reducing downstream β-AR signaling.3 Accordingly, SR Ca2+ load was potentiated after β-adrenergic stimulation in GRK2-deficient myocytes compared with controls. This is in line with the sensitized β-AR signaling observed in GRK2-deficient mouse hearts in vivo.4In a previous study,20 Raake et al observed that GRK2 deletion improved LV function and survival after myocardial infarction. Extending these results to the cellular levels, Raake et al21 here relate this improved LV function to maintained SR Ca2+ load, Na+/Ca2+ exchanger, and LTCC currents, preserving EC coupling compared with sham-operated GRK2-deficient animals. This contrasts with the maladaptive remodeling of EC coupling in wild-type animals after myocardial infarction, ie, reduced SR Ca2+ load, presumably as a result of upregulated Na+/Ca2+ exchanger activity, and downregulated LTCC current density, alterations that can also be found in the failing human heart.2 Together, these data suggest that GRK2 plays a key role in the maladaptive remodeling processes that deteriorate EC coupling on a cellular level; thus, targeting GRK2 may be an attractive approach to ameliorate the development of contractile dysfunction in heart failure.Despite these appealing novel results, a number of issues remain unresolved that can stimulate further research in this important and clinically relevant area. For instance, why does the phenotype of GRK2 deletion differ profoundly from the recently characterized EC coupling phenotype after βARKct overexpression25? In the latter, the only change of EC coupling that could be observed was βARKct-mediated relief of Gβγ-induced inhibition of LTCC, supporting the earlier idea that βARKct mediates its beneficial effects via Gβγ scavenging rather than GRK2 inhibition.19 One would have expected that βARKct should share some (if not all) effects of GRK2 deletion (or vice versa) plus the Gβγ-scavenging effects. Furthermore, in an earlier study on the same GRK2-deficient mice,4 constant β-adrenergic stimulation in vivo aggravated contractile dysfunction and maladaptive remodeling despite (or rather because of) a restoration of β-AR signaling, whereas after myocardial infarction, GRK2-deficient mice had ameliorated structural and functional remodeling.20,21 One way to address these issues is to combine βARKct expression with GRK2 deletion, which should give further insight into the complex interplay of receptor regulation and EC coupling processes revealed by the present study.In conclusion, the authors are to be congratulated on a rigorous and detailed characterization of GRK2-related regulation of EC coupling, which, in the face of the aching absence of data to this point, is a timely issue. From a clinical point of view, the study supports the notion that restoring (or maintaining) the physiological processes of EC coupling after cardiac stress (in this case, myocardial infarction) is an attractive approach to ameliorate the development of heart failure. In this context, it is important to note that also in β-blocker–treated patients, restoration of SERCA but not β1-AR expression was associated with a clinical improvement in these patients.26 The fact that in animal studies the beneficial effect of βARKct was additive to the effect of the β-blocker metoprolol encourages us to continue translational efforts, pioneered by the same group,18 to bring βARKct gene therapy eventually to the clinical arena.17 Finally, the encouraging data of the recent phase II Calcium Upregulation by Percutaneous Administration of Gene Therapy in Cardiac Disease (CUPID) trial27 with intracoronary gene therapy of SERCA lend further support to the concept of targeting defective EC coupling in patients with heart failure.Sources of FundingChristoph Maack is supported by the Deutsche Forschungsgemeinschaft (KFO-196, SFB 894 and Heisenberg Programm).DisclosuresNone.FootnotesThe opinions expressed in this article are not necessarily those of the editors or of the American Heart Association.Correspondence to Christoph Maack, MD, Klinik für Innere Medizin III, Universitätsklinikum des Saarlandes, 66421 Homburg, Germany. 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May 1, 2012Vol 125, Issue 17 Advertisement Article InformationMetrics © 2012 American Heart Association, Inc.https://doi.org/10.1161/CIRCULATIONAHA.112.109389PMID: 22496129 Originally publishedApril 10, 2012 Keywordsion channelsheart failurecalciumsarcoplasmic reticulumreceptors, adrenergic, betaadrenergic agentsEditorialsPDF download Advertisement SubjectsCalcium Cycling/Excitation-Contraction CouplingGenetically Altered and Transgenic ModelsPharmacology
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