RGS domain in the amino-terminus of G protein-coupled receptor kinase 2 inhibits Gq-mediated signaling

Specific and behaviorally consequential astrocyte Gq GPCR signaling attenuation in vivo with iβARK

Astrocytes respond to neurotransmitters and neuromodulators using G-protein-coupled receptors (GPCRs) to mediate physiological responses. Despite their importance, there has been no method to genetically, specifically, and effectively attenuate astrocyte Gq GPCR pathways to explore consequences of this prevalent signaling mechanism in vivo. We report a 122-residue inhibitory peptide from β-adrenergic receptor kinase 1 (iβARK; and inactive D110A control) to attenuate astrocyte Gq GPCR signaling. iβARK significantly attenuated Gq GPCR Ca2+ signaling in brain slices and, in vivo, altered behavioral responses, spared other GPCR responses, and did not alter astrocyte spontaneous Ca2+ signals, morphology, electrophysiological properties, or gene expression in the striatum. Furthermore, brain-wide attenuation of astrocyte Gq GPCR signaling with iβARK using PHP.eB adeno-associated viruses (AAVs), when combined with c-Fos mapping, suggested nuclei-specific contributions to behavioral adaptation and spatial memory. iβARK extends the toolkit needed to explore functions of astrocyte Gq GPCR signaling within neural circuits in vivo.

The Regulator of G Protein Signaling Homologous Domain of G Protein-Coupled Receptor Kinase 2 Mediates Short-Term Desensitization of β3-Adrenergic Receptor

G protein coupled receptor (GPCR) kinases (GRKs) are key regulators of GPCR signaling. Canonical mechanism of GPCR desensitization involves receptor phosphorylation by GRKs followed by arrestin recruitment and uncoupling from heterotrimeric G protein. Although β3-adrenergic receptor (β3AR) lacks phosphorylation sites by GRKs, agonist treatment proved to induce β3AR desensitization in many cell types. Here we show that GRK2 mediates short-term desensitization of β3AR by a phosphorylation independent mechanism but mediated by its domain homologous to the regulator of G protein signaling (RGS). HEK293T cells overexpressing human β3AR presented a short-term desensitization of cAMP response stimulated by the β3AR agonist, BRL37344, and not by forskolin. We found that β3AR desensitization was higher in cells co-transfected with GRK2. Similarly, overexpression of the RGS homology domain but not kinase domain of GRK2 increased β3AR desensitization. Consistently, stimulation of β3AR increased interaction between GRK2 and Gαs subunit. Furthermore, in rat cardiomyocytes endogenously expressing β3AR, transfection with dominant negative mutant of RH domain of GRK2 (GRK2/D110A) increased cAMP response to BRL37344 and inhibited receptor desensitization. We expect our study to be a starting point for more sophisticated characterization of the consequences of GRK2 mediated desensitization of the β3AR in heart function and disease.

Noncanonical Roles of G Protein-coupled Receptor Kinases in Cardiovascular Signaling

G protein-coupled receptor kinases (GRKs) are classically known for their role in regulating the activity of the largest known class of membrane receptors, which influence diverse biological processes in every cell type in the human body. As researchers have tried to uncover how this family of kinases, containing only 7 members, achieves selective and coordinated control of receptors, they have uncovered a growing number of noncanonical activities for these kinases. These activities include phosphorylation of nonreceptor targets and kinase-independent molecular interactions. In particular, GRK2, GRK3, and GRK5 are the predominant members expressed in the heart. Their canonical and noncanonical actions within cardiac and other tissues have significant implications for cardiovascular function in healthy animals and for the development and progression of disease. This review summarizes what is currently known regarding the activity of these kinases, and particularly the role of GRK2 and GRK5 in the molecular alterations that occur during heart failure. This review further highlights areas of GRK regulation that remain poorly understood and how they may represent novel targets for therapeutic development.

A peptide of the RGS domain of GRK2 binds and inhibits Gα(q) to suppress pathological cardiac hypertrophy and dysfunction

G protein-coupled receptor (GPCR) kinases (GRKs) play a critical role in cardiac function by regulating GPCR activity. GRK2 suppresses GPCR signaling by phosphorylating and desensitizing active GPCRs, and through protein-protein interactions that uncouple GPCRs from their downstream effectors. Several GRK2 interacting partners, including Gα(q), promote maladaptive cardiac hypertrophy, which leads to heart failure, a leading cause of mortality worldwide. The regulator of G protein signaling (RGS) domain of GRK2 interacts with and inhibits Gα(q) in vitro. We generated TgβARKrgs mice with cardiac-specific expression of the RGS domain of GRK2 and subjected these mice to pressure overload to trigger adaptive changes that lead to heart failure. Unlike their nontransgenic littermate controls, the TgβARKrgs mice exhibited less hypertrophy as indicated by reduced left ventricular wall thickness, decreased expression of genes linked to cardiac hypertrophy, and less adverse structural remodeling. The βARKrgs peptide, but not endogenous GRK2, interacted with Gα(q) and interfered with signaling through this G protein. These data support the development of GRK2-based therapeutic approaches to prevent hypertrophy and heart failure.

G protein-coupled receptor kinase 2: a link between myocardial contractile function and cardiac metabolism

Heart failure (HF) causes a tremendous burden on the worldwide healthcare system, affecting >23 million people. There are many cardiovascular disorders that contribute to the development of HF and multiple risk factors that accelerate its occurrence, but regardless of its underlying cause, HF is characterized by a marked decrease in myocardial contractility and loss of pump function. One biomarker molecule consistently shown to be upregulated in human HF and several animal models is G protein-coupled receptor kinase-2 (GRK2), a kinase originally discovered to be involved in G protein-coupled receptor desensitization, especially β-adrenergic receptors. Higher levels of GRK2 can impair β-adrenergic receptor-mediated inotropic reserve and its inhibition, or molecular reduction has shown to improve pump function in several animal models including a preclinical pig model of HF. Recently, nonclassical roles for GRK2 in cardiovascular disease have been described, including negative regulation of insulin signaling, a role in myocyte cell survival and apoptotic signaling, and it has been shown to be localized in/on mitochondria. These new roles of GRK2 suggest that GRK2 may be a nodal link in the myocyte, influencing both cardiac contractile function and cell metabolism and survival and contributing to HF independent of its canonical role in G protein-coupled receptor desensitization. In this review, classical and nonclassical roles for GRK2 will be discussed, focusing on recently discovered roles for GRK2 in cardiomyocyte metabolism and the effects that these roles may have on myocardial contractile function and HF development.

A new non-canonical pathway of Gα(q) protein regulating mitochondrial dynamics and bioenergetics

Contrary to previous assumptions, G proteins do not permanently reside on the plasma membrane, but are constantly monitoring the cytoplasmic surfaces of the plasma membrane and endomembranes. Here, we report that the Gαq and Gα11 proteins locate at the mitochondria and play a role in a complex signaling pathway that regulates mitochondrial dynamics. Our results provide evidence for the presence of the heteromeric G protein (Gαq/11βγ) at the outer mitochondrial membrane and for Gαq at the inner membrane. Both localizations are necessary to maintain the proper equilibrium between fusion and fission; which is achieved by altering the activity of mitofusin proteins, Drp1, OPA1 and the membrane potential at both the outer and inner mitochondrial membranes. As a result of the absence of Gαq/11, there is a decrease in mitochondrial fusion rates and a decrease in overall respiratory capacity, ATP production and OXPHOS-dependent growth. These findings demonstrate that the presence of Gαq proteins at the mitochondria serves as a physiological function: stabilizing elongated mitochondria and regulating energy production in Drp1 and Opa1 dependent mechanisms. This thereby links organelle dynamics and physiology.

G protein-coupled receptor kinases: more than just kinases and not only for GPCRs

G protein-coupled receptor (GPCR) kinases (GRKs) are best known for their role in homologous desensitization of GPCRs. GRKs phosphorylate activated receptors and promote high affinity binding of arrestins, which precludes G protein coupling. GRKs have a multidomain structure, with the kinase domain inserted into a loop of a regulator of G protein signaling homology domain. Unlike many other kinases, GRKs do not need to be phosphorylated in their activation loop to achieve an activated state. Instead, they are directly activated by docking with active GPCRs. In this manner they are able to selectively phosphorylate Ser/Thr residues on only the activated form of the receptor, unlike related kinases such as protein kinase A. GRKs also phosphorylate a variety of non-GPCR substrates and regulate several signaling pathways via direct interactions with other proteins in a phosphorylation-independent manner. Multiple GRK subtypes are present in virtually every animal cell, with the highest expression levels found in neurons, with their extensive and complex signal regulation. Insufficient or excessive GRK activity was implicated in a variety of human disorders, ranging from heart failure to depression to Parkinson's disease. As key regulators of GPCR-dependent and -independent signaling pathways, GRKs are emerging drug targets and promising molecular tools for therapy. Targeted modulation of expression and/or of activity of several GRK isoforms for therapeutic purposes was recently validated in cardiac disorders and Parkinson's disease.

Regulators of G-protein signaling and their Gα substrates: promises and challenges in their use as drug discovery targets

Because G-protein coupled receptors (GPCRs) continue to represent excellent targets for the discovery and development of small-molecule therapeutics, it is posited that additional protein components of the signal transduction pathways emanating from activated GPCRs themselves are attractive as drug discovery targets. This review considers the drug discovery potential of two such components: members of the "regulators of G-protein signaling" (RGS protein) superfamily, as well as their substrates, the heterotrimeric G-protein α subunits. Highlighted are recent advances, stemming from mouse knockout studies and the use of "RGS-insensitivity" and fast-hydrolysis mutations to Gα, in our understanding of how RGS proteins selectively act in (patho)physiologic conditions controlled by GPCR signaling and how they act on the nucleotide cycling of heterotrimeric G-proteins in shaping the kinetics and sensitivity of GPCR signaling. Progress is documented regarding recent activities along the path to devising screening assays and chemical probes for the RGS protein target, not only in pursuits of inhibitors of RGS domain-mediated acceleration of Gα GTP hydrolysis but also to embrace the potential of finding allosteric activators of this RGS protein action. The review concludes in considering the Gα subunit itself as a drug target, as brought to focus by recent reports of activating mutations to GNAQ and GNA11 in ocular (uveal) melanoma. We consider the likelihood of several strategies for antagonizing the function of these oncogene alleles and their gene products, including the use of RGS proteins with Gα(q) selectivity.

The GRK2 Overexpression Is a Primary Hallmark of Mitochondrial Lesions during Early Alzheimer Disease

Increasing evidence points to vascular damage as an early contributor to the development of two leading causes of age-associated dementia, namely Alzheimer disease (AD) and AD-like pathology such as stroke. This review focuses on the role of G protein-coupled receptor kinases (GRKs) as they relate to dementia and how the cardio and cerebrovasculature is involved in AD pathogenesis. The exploration of GRKs in AD pathogenesis may help bridge gaps in our understanding of the heart-brain connection in relation to neurovisceral damage and vascular complications of AD. The a priori basis for this inquiry stems from the fact that kinases of this family regulate numerous receptor functions in the brain, myocardium and elsewhere. The aim of this review is to discuss the finding of GRK2 overexpression in the context of early AD pathogenesis. Also, we consider the consequences for this overexpression as a loss of G-protein coupled receptor (GPCR) regulation, as well as suggest a potential role for GPCRs and GRKs in a unifying theory of AD pathogenesis through the cerebrovasculature. Finally, we synthesize this newer information in an attempt to put it into context with GRKs as regulators of cellular function, which makes these proteins potential diagnostic and therapeutic targets for future pharmacological intervention.

Once and future signaling: G protein-coupled receptor kinase control of neuronal sensitivity

G protein-coupled receptors (GPCRs) are the most numerous class of cell surface receptor, and substances acting through GPCRs mediate many critical signaling events and physiological processes. GPCR sensitivity and signaling is dynamic, responding rapidly to adjust to changes in the ambient level of stimulation of target cells. One important mediator of such receptor sensitivity is the family of GPCR kinases (GRKs). Like heterotrimeric G proteins, GRKs recognize agonist-bound, activated receptors, and this recognition promotes catalytic activation of GRKs, resulting in the preferential phosphorylation of activated receptors. GRK-phosphorylated receptors are then targeted by arrestin proteins, which bind to phosphorylated receptors. Arrestin-bound receptors are uncoupled from heterotrimeric G proteins, resulting in decreased sensitivity to further receptor stimulation (desensitization). Arrestin-bound receptors are also accelerated into internalization pathways and linked to distinct arrestin-mediated signaling pathways. GRKs thus serve as gatekeepers for receptors, terminating some signaling pathways and initiating others. One major outstanding question concerning GRKs understanding the mechanisms by which any particular receptor subtype (of the 800 or so in the body) is regulated by a specific GRK(s), and the consequences of this specificity. An understanding of this regulatory specificity could allow targeting of GRK function to ameliorate diseases involving GPCR dysregulation.

Characterization of the GRK2 binding site of Galphaq

Heterotrimeric guanine nucleotide-binding proteins (G proteins) transmit signals from membrane bound G protein-coupled receptors (GPCRs) to intracellular effector proteins. The G(q) subfamily of Galpha subunits couples GPCR activation to the enzymatic activity of phospholipase C-beta (PLC-beta). Regulators of G protein signaling (RGS) proteins bind to activated Galpha subunits, including Galpha(q), and regulate Galpha signaling by acting as GTPase activating proteins (GAPs), increasing the rate of the intrinsic GTPase activity, or by acting as effector antagonists for Galpha subunits. GPCR kinases (GRKs) phosphorylate agonist-bound receptors in the first step of receptor desensitization. The amino termini of all GRKs contain an RGS homology (RH) domain, and binding of the GRK2 RH domain to Galpha(q) attenuates PLC-beta activity. The RH domain of GRK2 interacts with Galpha(q/11) through a novel Galpha binding surface termed the "C" site. Here, molecular modeling of the Galpha(q).GRK2 complex and site-directed mutagenesis of Galpha(q) were used to identify residues in Galpha(q) that interact with GRK2. The model identifies Pro(185) in Switch I of Galpha(q) as being at the crux of the interface, and mutation of this residue to lysine disrupts Galpha(q) binding to the GRK2-RH domain. Switch III also appears to play a role in GRK2 binding because the mutations Galpha(q)-V240A, Galpha(q)-D243A, both residues within Switch III, and Galpha(q)-Q152A, a residue that structurally supports Switch III, are defective in binding GRK2. Furthermore, GRK2-mediated inhibition of Galpha(q)-Q152A-R183C-stimulated inositol phosphate release is reduced in comparison to Galpha(q)-R183C. Interestingly, the model also predicts that residues in the helical domain of Galpha(q) interact with GRK2. In fact, the mutants Galpha(q)-K77A, Galpha(q)-L78D, Galpha(q)-Q81A, and Galpha(q)-R92A have reduced binding to the GRK2-RH domain. Finally, although the mutant Galpha(q)-T187K has greatly reduced binding to RGS2 and RGS4, it has little to no effect on binding to GRK2. Thus the RH domain A and C sites for Galpha(q) interaction rely on contacts with distinct regions and different Switch I residues in Galpha(q).

GRK2 is an endogenous protein inhibitor of the insulin signaling pathway for glucose transport stimulation

G protein-coupled receptor kinases (GRKs) represent a class of proteins that classically phosphorylate agonist-activated G protein-coupled receptors, leading to uncoupling of the receptor from further G protein activation. Recently, we have reported that the heterotrimeric G protein alpha-subunit, Galphaq/11, can mediate insulin-stimulated glucose transport. GRK2 contains a regulator of G protein signaling (RGS) domain with specificity for Galphaq/11. Therefore, we postulated that GRK2 could be an inhibitor of the insulin signaling cascade leading to glucose transport in 3T3-L1 adipocytes. In this study, we demonstrate that microinjection of anti-GRK2 antibody or siRNA against GRK2 increased insulin-stimulated insulin-responsive glucose transporter 4 (GLUT4) translocation, while adenovirus-mediated overexpression of wild-type or kinase-deficient GRK2 inhibited insulin-stimulated GLUT4 translocation as well as 2-deoxyglucose uptake. Importantly, a mutant GRK2 lacking the RGS domain was without effect. Taken together, these results indicate that through its RGS domain endogenous GRK2 functions as a negative regulator of insulin-stimulated glucose transport by interfering with Galphaq/11 signaling to GLUT4 translocation. Furthermore, inhibitors of GRK2 can lead to enhanced insulin sensitivity.

Phosphorylation-independent desensitization of GABA(B) receptor by GRK4

Agonist-promoted desensitization of the heterodimeric metabotropic GABA(B) receptor was investigated. Whereas no desensitization was observed in HEK293 cells heterologously expressing the receptor, GABA and the synthetic agonist baclofen induced a robust desensitization in cerebellar granule cells endogenously expressing the receptor. Taking advantage of this cell-specific desensitization phenotype, we identified GRK4 as the kinase involved in the neuronal desensitization. Transfection of small interference RNA directed against GRK4 significantly reduced GRK4 levels in cerebellar granule cells and strongly inhibited the agonist-promoted desensitization. Reciprocally, transfection of GRK4 in HEK293 cells restored agonist-promoted desensitization, confirming that this kinase is sufficient to support desensitization. Surprisingly, this desensitization occurred in the absence of ligand-induced receptor phosphorylation and could be promoted by GRK4 mutants deleted of their kinase domain. Taken together, these results suggest that GRK4 plays a central role in the agonist-promoted desensitization of GABA(B) receptor and that it does so through an atypical mechanism that challenges the generally accepted model linking the kinase activity of GRKs to their role in receptor desensitization.