The binding site for the beta gamma subunits of heterotrimeric G proteins on the beta-adrenergic receptor kinase

Endothelial nitric-oxide synthase (eNOS) is activated through G-protein-coupled receptor kinase-interacting protein 1 (GIT1) tyrosine phosphorylation and Src protein

Nitric oxide (NO) is a critical regulator of vascular tone and plays an especially prominent role in liver by controlling portal blood flow and pressure within liver sinusoids. Synthesis of NO in sinusoidal endothelial cells by endothelial nitric-oxide synthase (eNOS) is regulated in response to activation of endothelial cells by vasoactive signals such as endothelins. The endothelin B (ETB) receptor is a G-protein-coupled receptor, but the mechanisms by which it regulates eNOS activity in sinusoidal endothelial cells are not well understood. In this study, we built on two previous strands of work, the first showing that G-protein βγ subunits mediated activation of phosphatidylinositol 3-kinase and Akt to regulate eNOS and the second showing that eNOS directly bound to the G-protein-coupled receptor kinase-interacting protein 1 (GIT1) scaffold protein, and this association stimulated NO production. Here we investigated the mechanisms by which the GIT1-eNOS complex is formed and regulated. GIT1 was phosphorylated on tyrosine by Src, and Y293F and Y554F mutations reduced GIT1 phosphorylation as well as the ability of GIT1 to bind to and activate eNOS. Akt phosphorylation activated eNOS (at Ser(1177)), and Akt also regulated the ability of Src to phosphorylate GIT1 as well as GIT1-eNOS association. These pathways were activated by endothelin-1 through the ETB receptor; inhibiting receptor-activated G-protein βγ subunits blocked activation of Akt, GIT1 tyrosine phosphorylation, and ET-1-stimulated GIT1-eNOS association but did not affect Src activation. These data suggest a model in which Src and Akt cooperate to regulate association of eNOS with the GIT1 scaffold to facilitate NO production.

Cardiovascular gene therapy for myocardial infarction

INTRODUCTION

Cardiovascular gene therapy is the third most popular application for gene therapy, representing 8.4% of all gene therapy trials as reported in 2012 estimates. Gene therapy in cardiovascular disease is aiming to treat heart failure from ischemic and non-ischemic causes, peripheral artery disease, venous ulcer, pulmonary hypertension, atherosclerosis and monogenic diseases, such as Fabry disease.

AREAS COVERED

In this review, we will focus on elucidating current molecular targets for the treatment of ventricular dysfunction following myocardial infarction (MI). In particular, we will focus on the treatment of i) the clinical consequences of it, such as heart failure and residual myocardial ischemia and ii) etiological causes of MI (coronary vessels atherosclerosis, bypass venous graft disease, in-stent restenosis).

EXPERT OPINION

We summarise the scheme of the review and the molecular targets either already at the gene therapy clinical trial phase or in the pipeline. These targets will be discussed below. Following this, we will focus on what we believe are the 4 prerequisites of success of any gene target therapy: safety, expression, specificity and efficacy (SESE).

β-Arrestin interacts with the beta/gamma subunits of trimeric G-proteins and dishevelled in the Wnt/Ca(2+) pathway in xenopus gastrulation

β-Catenin independent, non-canonical Wnt signaling pathways play a major role in the regulation of morphogenetic movements in vertebrates. The term non-canonical Wnt signaling comprises multiple, intracellularly divergent, Wnt-activated and β-Catenin independent signaling cascades including the Wnt/Planar Cell Polarity and the Wnt/Ca²⁺ cascades. Wnt/Planar Cell Polarity and Wnt/Ca²⁺ pathways share common effector proteins, including the Wnt ligand, Frizzled receptors and Dishevelled, with each other and with additional branches of Wnt signaling. Along with the aforementioned proteins, β-Arrestin has been identified as an essential effector protein in the Wnt/β-Catenin and the Wnt/Planar Cell Polarity pathway. Our results demonstrate that β-Arrestin is required in the Wnt/Ca²⁺ signaling cascade upstream of Protein Kinase C (PKC) and Ca²⁺/Calmodulin-dependent Protein Kinase II (CamKII). We have further characterized the role of β-Arrestin in this branch of non-canonical Wnt signaling by knock-down and rescue experiments in Xenopus embryo explants and analyzed protein-protein interactions in 293T cells. Functional interaction of β-Arrestin, the β subunit of trimeric G-proteins and Dishevelled is required to induce PKC activation and membrane translocation. In Xenopus gastrulation, β-Arrestin function in Wnt/Ca²⁺ signaling is essential for convergent extension movements. We further show that β-Arrestin physically interacts with the β subunit of trimeric G-proteins and Dishevelled, and that the interaction between β-Arrestin and Dishevelled is promoted by the beta/gamma subunits of trimeric G-proteins, indicating the formation of a multiprotein signaling complex.

Discovery of multiple interacting partners of gankyrin, a proteasomal chaperone and an oncoprotein--evidence for a common hot spot site at the interface and its functional relevance

Gankyrin, a non-ATPase component of the proteasome and a chaperone of proteasome assembly, is also an oncoprotein. Gankyrin regulates a variety of oncogenic signaling pathways in cancer cells and accelerates degradation of tumor suppressor proteins p53 and Rb. Therefore gankyrin may be a unique hub integrating signaling networks with the degradation pathway. To identify new interactions that may be crucial in consolidating its role as an oncogenic hub, crystal structure of gankyrin-proteasome ATPase complex was used to predict novel interacting partners. EEVD, a four amino acid linear sequence seems a hot spot site at this interface. By searching for EEVD in exposed regions of human proteins in PDB database, we predicted 34 novel interactions. Eight proteins were tested and seven of them were found to interact with gankyrin. Affinity of four interactions is high enough for endogenous detection. Others require gankyrin overexpression in HEK 293 cells or occur endogenously in breast cancer cell line- MDA-MB-435, reflecting lower affinity or presence of a deregulated network. Mutagenesis and peptide inhibition confirm that EEVD is the common hot spot site at these interfaces and therefore a potential polypharmacological drug target. In MDA-MB-231 cells in which the endogenous CLIC1 is silenced, trans-expression of Wt protein (CLIC1_EEVD) and not the hot spot site mutant (CLIC1_AAVA) resulted in significant rescue of the migratory potential. Our approach can be extended to identify novel functionally relevant protein-protein interactions, in expansion of oncogenic networks and in identifying potential therapeutic targets.

G protein-coupled receptor kinase 6 deficiency promotes angiogenesis, tumor progression, and metastasis

G protein-coupled receptor kinases (GRKs) phosphorylate the activated form of G protein-coupled receptors leading to receptor desensitization and downregulation. We have recently shown that the chemokine receptor, CXCR2, couples to GRK6 to regulate cellular responses including chemotaxis, angiogenesis, and wound healing. In this study, we investigate the role of GRK6 in tumorigenesis using murine models of human lung cancer. Mice deficient in GRK6 (GRK6(-/-)) exhibited a significant increase in Lewis lung cancer growth and metastasis relative to control littermates (GRK6(+/+)). GRK6 deletion had no effect on the expression of proangiogenic chemokine or vascular endothelial growth factor, but upregulated matrix metalloproteinase (MMP)-2 and MMP-9 release, tumor-infiltrating PMNs, and microvessel density. Because β-arrestin-2-deficient (βarr2(-/-)) mice exhibited increased Lewis lung cancer growth and metastasis similar to that of GRK6(-/-), we developed a double GRK6(-/-)/βarr2(-/-) mouse model. Surprisingly, GRK6(-/-)/βarr2(-/-) mice exhibited faster tumor growth relative to GRK6(-/-) or βarr2(-/-) mice. Treatment of the mice with anti-CXCR2 Ab inhibited tumor growth in both GRK6(-/-) and GRK6(-/-)/βarr2(-/-) animals. Altogether, the results indicate that CXCR2 couples to GRK6 to regulate angiogenesis, tumor progression, and metastasis. Deletion of GRK6 increases the activity of the host CXCR2, resulting in greater PMN infiltration and MMP release in the tumor microenvironment, thereby promoting angiogenesis and metastasis. Because GRK6(-/-)/βarr2(-/-) showed greater tumor growth relative to GRK6(-/-) or βarr2(-/-) mice, the data further suggest that CXCR2 couples to different mechanisms to mediate tumor progression and metastasis.

TREK-1 and Best1 channels mediate fast and slow glutamate release in astrocytes upon GPCR activation

Astrocytes release glutamate upon activation of various GPCRs to exert important roles in synaptic functions. However, the molecular mechanism of release has been controversial. Here, we report two kinetically distinct modes of nonvesicular, channel-mediated glutamate release. The fast mode requires activation of G(αi), dissociation of G(βγ), and subsequent opening of glutamate-permeable, two-pore domain potassium channel TREK-1 through direct interaction between G(βγ) and N terminus of TREK-1. The slow mode is Ca(2+) dependent and requires G(αq) activation and opening of glutamate-permeable, Ca(2+)-activated anion channel Best1. Ultrastructural analyses demonstrate that TREK-1 is preferentially localized at cell body and processes, whereas Best1 is mostly found in microdomains of astrocytes near synapses. Diffusion modeling predicts that the fast mode can target neuronal mGluR with peak glutamate concentration of 100 μM, whereas slow mode targets neuronal NMDA receptors at around 1 μM. Our results reveal two distinct sources of astrocytic glutamate that can differentially influence neighboring neurons.

Regulation of myocardial contractility: Insights from transgenic mice

Myocardial contraction occurs when calcium (Ca(2+)) is released from the sarcoplasmic reticulum, binding troponin C and allowing actin and myosin to cross link. Ca(2+) release and uptake is closely regulated by G protein-coupled β-adrenergic receptors through the action of the second messenger cAMP. An increase in cAMP level leads to phosphorylation of key regulatory proteins affecting intracellular Ca(2+) homeostasis. The β-adrenergic receptors themselves are regulated by a set of specific kinases, termed the G-protein-coupled receptor kinases (GRKs). The study of this complex system in vivo has recently been advanced by the development of transgenic and gene-targeted (knockout) mouse models. Combining transgenic technology with sophisticated physiological measurements of cardiac hemodynamics is an extremely powerful approach to the study of myocardial contractility and its regulation. This review focuses on several recent transgenic mouse models that have increased our understanding of the regulation of cardiac contractility.

Synthetic biology with surgical precision: targeted reengineering of signaling proteins

The complexity of living systems exceeds everything else studied by natural sciences. Sophisticated networks of intimately intertwined signaling pathways coordinate cellular functions. Clear understanding how the integration of multiple inputs produces coherent behavior is one of the major challenges of cell biology. Integration via perfectly timed highly regulated protein-protein interactions and precise targeting of the "output" proteins to particular substrates is emerging as a common theme of signaling regulation. This often involves specialized scaffolding proteins, whose key function is to ensure that correct partners come together in an appropriate place at the right time. Defective or faulty signaling underlies many congenital and acquired human disorders. Several pioneering studies showed that ectopic expression of existing proteins or their elements can restore functions destroyed by mutations or normalize the signaling pushed out of balance by disease and/or current small molecule-based therapy. Several recent studies show that proteins with new functional modalities can be generated by mixing and matching existing domains, or via functional recalibration and fine-tuning of existing proteins by precisely targeted mutations. Using arrestins as an example, we describe how manipulation of individual functions yields signaling-biased proteins. Creative protein redesign generates novel tools valuable for unraveling the intricacies of cell biology. Engineered proteins with specific functional changes also have huge therapeutic potential in disorders associated with inherited or acquired signaling errors.

Gβγ inhibits exocytosis via interaction with critical residues on soluble N-ethylmaleimide-sensitive factor attachment protein-25

Spatial and temporal regulation of neurotransmitter release is a complex process accomplished by the exocytotic machinery working in tandem with numerous regulatory proteins. G-protein βγ dimers regulate the core process of exocytosis by interacting with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins soluble N-ethylmaleimide-sensitive factor attachment protein-25 (SNAP-25), syntaxin 1A, and synaptobrevin. Gβγ binding to ternary SNAREs overlaps with calcium-dependent binding of synaptotagmin, inhibiting synaptotagmin-1 binding and fusion of the synaptic vesicle. To further explore the binding sites of Gβγ on SNAP-25, peptides based on the sequence of SNAP-25 were screened for Gβγ binding. Peptides that bound Gβγ were subjected to alanine scanning mutagenesis to determine their relevance to the Gβγ-SNAP-25 interaction. Peptides from this screen were tested in protein-protein interaction assays for their ability to modulate the interaction of Gβγ with SNAP-25. A peptide from the C terminus, residues 193 to 206, significantly inhibited the interaction. In addition, Ala mutants of SNAP-25 residues from the C terminus of SNAP-25, as well as from the amino-terminal region decreased binding to Gβ₁γ₁. When SNAP-25 with eight residues mutated to alanine was assembled with syntaxin 1A, there was significantly reduced affinity of this mutated t-SNARE for Gβγ, but it still interacted with synaptotagmin-1 in a Ca²⁺ -dependent manner and reconstituted evoked exocytosis in botulinum neurotoxin E-treated neurons. However, the mutant SNAP-25 could no longer support 5-hydroxytryptamine-mediated inhibition of exocytosis.

β(2)-Adrenoceptors increase translocation of GLUT4 via GPCR kinase sites in the receptor C-terminal tail

BACKGROUND AND PURPOSE

β-Adrenoceptor stimulation induces glucose uptake in several insulin-sensitive tissues by poorly understood mechanisms.

EXPERIMENTAL APPROACH

We used a model system in CHO-K1 cells expressing the human β(2)-adrenoceptor and glucose transporter 4 (GLUT4) to investigate the signalling mechanisms involved.

KEY RESULTS

In CHO-K1 cells, there was no response to β-adrenoceptor agonists. The introduction of β(2)-adrenoceptors and GLUT4 into these cells caused increased glucose uptake in response to β-adrenoceptor agonists. GLUT4 translocation occurred in response to insulin and β(2)-adrenoceptor stimulation, although the key insulin signalling intermediate PKB was not phosphorylated in response to β(2)-adrenoceptor stimulation. Truncation of the C-terminus of the β(2)-adrenoceptor at position 349 to remove known phosphorylation sites for GPCR kinases (GRKs) or at position 344 to remove an additional PKA site together with the GRK phosphorylation sites did not significantly affect cAMP accumulation but decreased β(2)-adrenoceptor-stimulated glucose uptake. Furthermore, inhibition of GRK by transfection of the βARKct construct inhibited β(2)-adrenoceptor-mediated glucose uptake and GLUT4 translocation, and overexpression of a kinase-dead GRK2 mutant (GRK2 K220R) also inhibited GLUT4 translocation. Introducing β(2)-adrenoceptors lacking phosphorylation sites for GRK or PKA demonstrated that the GRK sites, but not the PKA sites, were necessary for GLUT4 translocation.

CONCLUSIONS AND IMPLICATIONS

Glucose uptake in response to activation of β(2)-adrenoceptors involves translocation of GLUT4 in this model system. The mechanism is dependent on the C-terminus of the β(2)-adrenoceptor, requires GRK phosphorylation sites, and involves a signalling pathway distinct from that stimulated by insulin.

Regulation of GPCR activity, trafficking and localization by GPCR-interacting proteins

GPCRs represent the largest family of integral membrane proteins and were first identified as receptor proteins that couple via heterotrimeric G-proteins to regulate a vast variety of effector proteins to modulate cellular function. It is now recognized that GPCRs interact with a myriad of proteins that not only function to attenuate their signalling but also function to couple these receptors to heterotrimeric G-protein-independent signalling pathways. In addition, intracellular and transmembrane proteins associate with GPCRs and regulate their processing in the endoplasmic reticulum, trafficking to the cell surface, compartmentalization to plasma membrane microdomains, endocytosis and trafficking between intracellular membrane compartments. The present review will overview the functional consequence of β-arrestin, receptor activity-modifying proteins (RAMPS), regulators of G-protein signalling (RGS), GPCR-associated sorting proteins (GASPs), Homer, small GTPases, PSD95/Disc Large/Zona Occludens (PDZ), spinophilin, protein phosphatases, calmodulin, optineurin and Src homology 3 (SH3) containing protein interactions with GPCRs.

Cardiomyocyte-restricted inhibition of G protein-coupled receptor kinase-3 attenuates cardiac dysfunction after chronic pressure overload

Transgenic mice with cardiac-specific expression of a peptide inhibitor of G protein-coupled receptor kinase (GRK)3 [transgenic COOH-terminal GRK3 (GRK3ct) mice] display myocardial hypercontractility without hypertrophy and enhanced α(1)-adrenergic receptor signaling. A role for GRK3 in the pathogenesis of heart failure (HF) has not been investigated, but inhibition of its isozyme, GRK2, has been beneficial in several HF models. Here, we tested whether inhibition of GRK3 modulated evolving cardiac hypertrophy and dysfunction after pressure overload. Weight-matched male GRK3ct transgenic and nontransgenic littermate control (NLC) mice subjected to chronic pressure overload by abdominal aortic banding (AB) were compared with sham-operated (SH) mice. At 6 wk after AB, a significant increase of cardiac mass consistent with induction of hypertrophy was found, but no differences between GRK3ct-AB and NLC-AB mice were discerned. Simultaneous left ventricular (LV) pressure-volume analysis of electrically paced, ex vivo perfused working hearts revealed substantially reduced systolic and diastolic function in NLC-AB mice (n = 7), which was completely preserved in GRK3ct-AB mice (n = 7). An additional cohort was subjected to in vivo cardiac catheterization and LV pressure-volume analysis at 12 wk after AB. NLC-AB mice (n = 11) displayed elevated end-diastolic pressure (8.5 ± 3.1 vs. 2.9 ± 1.2 mmHg, P < 0.05), reduced cardiac output (3,448 ± 323 vs. 4,488 ± 342 μl/min, P < 0.05), and reduced dP/dt(max) and dP/dt(min) (both P < 0.05) compared with GRK3ct-AB mice (n = 16), corroborating the preserved cardiac structure and function observed in GRK3ct-AB hearts assessed ex vivo. Increased cardiac mass and myocardial mRNA expression of β-myosin heavy chain confirmed the similar induction of cardiac hypertrophy in both AB groups, but only NLC-AB hearts displayed significantly elevated mRNA levels of brain natriuretic peptide and myocardial collagen contents as well as reduced β(1)-adrenergic receptor responsiveness to isoproterenol, indicating increased LV wall stress and the transition to HF. Inhibition of cardiac GRK3 in mice does not alter the hypertrophic response but attenuates cardiac dysfunction and HF after chronic pressure overload.

Structural domains required for Caenorhabditis elegans G protein-coupled receptor kinase 2 (GRK-2) function in vivo

G protein-coupled receptor kinases (GRKs) are key regulators of signal transduction that specifically phosphorylate activated G protein-coupled receptors (GPCRs) to terminate signaling. Biochemical and crystallographic studies have provided great insight into mammalian GRK2/3 interactions and structure. However, despite extensive in vitro characterization, little is known about the in vivo contribution of these described GRK structural domains and interactions to proper GRK function in signal regulation. We took advantage of the disrupted chemosensory behavior characteristic of Caenorhabditis elegans grk-2 mutants to discern the interactions required for proper in vivo Ce-GRK-2 function. Informed by mammalian crystallographic and biochemical data, we introduced amino acid substitutions into the Ce-grk-2 coding sequence that are predicted to selectively disrupt GPCR phosphorylation, Gα(q/11) binding, Gβγ binding, or phospholipid binding. Changing the most amino-terminal residues, which have been shown in mammalian systems to be required specifically for GPCR phosphorylation but not phosphorylation of alternative substrates or recruitment to activated GPCRs, eliminated the ability of Ce-GRK-2 to restore chemosensory signaling. Disrupting interaction between the predicted Ce-GRK-2 amino-terminal α-helix and kinase domain, posited to stabilize GRKs in their active ATP- and GPCR-bound conformation, also eliminated Ce-GRK-2 chemosensory function. Finally, although changing residues within the RH domain, predicted to disrupt interaction with Gα(q/11), did not affect Ce-GRK-2 chemosensory function, disruption of the predicted PH domain-mediated interactions with Gβγ and phospholipids revealed that both contribute to Ce-GRK-2 function in vivo. Combined, we have demonstrated functional roles for broadly conserved GRK2/3 structural domains in the in vivo regulation of organismal behavior.

The connection between GRKs and various signaling pathways involved in diabetic nephropathy

Diabetic nephropathy (DN) is a known microvascular complication in patients with diabetes mellitus. DN has become one of the main causes of death in diabetic patients. The occurrence and development of DN results from the comprehensive action of multi-factors, though the exact mechanism is not very clear. Recently, a study found that numerous pathways are activated during the course of the disease, including the PGE2-EP-G protein system, the renin-angiotensin system, protein kinase C, MAPK and oxidative stress, and transforming growth factor-β. G protein-coupled receptor kinases (GRKs), specifically recognize and phosphorylate agonist-activated G protein-coupled receptors, which play a major role in the above-mentioned pathways. The purpose of this paper is to review current information concerning the connection between GRKs and various signaling pathways involved in DN.

Targeting GRK2 by gene therapy for heart failure: benefits above β-blockade

Heart failure (HF) is a common pathological end point for several cardiac diseases. Despite reasonable achievements in pharmacological, electrophysiological and surgical treatments, prognosis for chronic HF remains poor. Modern therapies are generally symptom oriented and do not currently address specific intracellular molecular signaling abnormalities. Therefore, new and innovative therapeutic approaches are warranted and, ideally, these could at least complement established therapeutic options if not replace them. Gene therapy has potential to serve in this regard in HF as vectors can be directed toward diseased myocytes and directly target intracellular signaling abnormalities. Within this review, we will dissect the adrenergic system contributing to HF development and progression with special emphasis on G-protein-coupled receptor kinase 2 (GRK2). The levels and activity of GRK2 are increased in HF and we and others have demonstrated that this kinase is a major molecular culprit in HF. We will cover the evidence supporting gene therapy directed against myocardial as well as adrenal GRK2 to improve the function and structure of the failing heart and how these strategies may offer complementary and synergistic effects with the existing HF mainstay therapy of β-adrenergic receptor antagonism.

G protein-coupled Receptor Kinase 2 as a Therapeutic Target for Heart Failure

An ever-increasing number of people world-wide are developing and suffering from heart failure, and existing therapies, although improved are not ideal. Therefore, innovative treatment strategies are urgently needed. As our understanding of the underlying dysfunction of the myocardium increases, so does our ability to target the mechanisms responsible for heart failure progression. In this review we discuss novel molecular therapies and approaches for the treatment of heart failure. We will focus on the G protein-coupled receptor kinase GRK2, an increasing target for heart failure therapy, based on its important role in disease progression and the therapeutic potential of GRK2 inhibitors.

G protein coupled receptor kinases as therapeutic targets in cardiovascular disease

G protein-coupled receptors (GPCRs) represent the largest family of membrane receptors and are responsible for regulating a wide variety of physiological processes. This is accomplished via ligand binding to GPCRs, activating associated heterotrimeric G proteins and intracellular signaling pathways. G protein-coupled receptor kinases (GRKs), in concert with β-arrestins, classically desensitize receptor signal transduction, thus preventing hyperactivation of GPCR second-messenger cascades. As changes in GRK expression have featured prominently in many cardiovascular pathologies, including heart failure, myocardial infarction, hypertension, and cardiac hypertrophy, GRKs have been intensively studied as potential diagnostic or therapeutic targets. Herein, we review our evolving understanding of the role of GRKs in cardiovascular pathophysiology.

Short Stat5-interacting peptide derived from phospholipase C-β3 inhibits hematopoietic cell proliferation and myeloid differentiation

Constitutive activation of the transcription factor Stat5 in hematopoietic stem/progenitor cells leads to various hematopoietic malignancies including myeloproliferative neoplasm (MPN). Our recent study found that phospholipase C (PLC)-β3 is a novel tumor suppressor involved in MPN, lymphoma and other tumors. Stat5 activity is negatively regulated by the SH2 domain-containing protein phosphatase SHP-1 in a PLC-β3-dependent manner. PLC-β3 can form the multimolecular SPS complex together with SHP-1 and Stat5. The close physical proximity of SHP-1 and Stat5 brought about by interacting with the C-terminal segment of PLC-β3 (PLC-β3-CT) accelerates SHP-1-mediated dephosphorylation of Stat5. Here we identify the minimal sequences within PLC-β3-CT required for its tumor suppressor function. Two of the three Stat5-binding noncontiguous regions, one of which also binds SHP-1, substantially inhibited in vitro proliferation of Ba/F3 cells. Surprisingly, an 11-residue Stat5-binding peptide (residues 988-998) suppressed Stat5 activity in Ba/F3 cells and in vivo proliferation and myeloid differentiation of hematopoietic stem/progenitor cells. Therefore, this study further defines PLC-β3-CT as the Stat5- and SHP-1-binding domain by identifying minimal functional sequences of PLC-β3 for its tumor suppressor function and implies their potential utility in the control of hematopoietic malignancies.

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.

Modulation of beta-adrenergic receptor signaling in heart failure and longevity: targeting adenylyl cyclase type 5

Despite remarkable advances in therapy, heart failure remains a leading cause of morbidity and mortality. Although enhanced beta-adrenergic receptor stimulation is part of normal physiologic adaptation to either the increase in physiologic demand or decrease in cardiac function, chronic beta-adrenergic stimulation has been associated with increased mortality and morbidity in both animal models and humans. For example, overexpression of cardiac Gsalpha or beta-adrenergic receptors in transgenic mice results in enhanced cardiac function in young animals, but with prolonged overstimulation of this pathway, cardiomyopathy develops in these mice as they age. Similarly, chronic sympathomimetic amine therapy increases morbidity and mortality in patients with heart failure. Conversely, the use of beta-blockade has proven to be of benefit and is currently part of the standard of care for heart failure. It is conceivable that interrupting distal mechanisms in the beta-adrenergic receptor-G protein-adenylyl cyclase pathway may also provide targets for future therapeutic modalities for heart failure. Interestingly, there are two major isoforms of adenylyl cyclase (AC) in the heart (type 5 and type 6), which may exert opposite effects on the heart, i.e., cardiac overexpression of AC6 appears to be protective, whereas disruption of type 5 AC prolongs longevity and protects against cardiac stress. The goal of this review is to summarize the paradigm shift in the treatment of heart failure over the past 50 years from administering sympathomimetic amine agonists to administering beta-adrenergic receptor antagonists, and to explore the basis for a novel therapy of inhibiting type 5 AC.

Signaling pathways of isoproterenol-induced ERK1/2 phosphorylation in primary cultures of astrocytes are concentration-dependent

Stimulation of β-adrenoceptors activates the canonical adenylate cyclase pathway (via G(s) protein) but can also evoke phosphorylation of extracellular-regulated kinases 1 and 2 (ERK(1/2) ) via G(s)/G(i) switching or β-arrestin-mediated recruitment of Src. In primary cultures of mouse astrocytes, activation of the former of these pathways required micromolar concentrations of the β(1)/β(2) -adrenergic agonist isoproterenol, that acted on β(1)-adrenoceptors, whereas the latter was activated already by nanomolar concentrations, acting on β(2) receptors. Protein kinase A activity was required for G(s)/G(i) switching, which was followed by Ca(2+) release from intracellular stores and G(iα)- and metalloproteinase-dependent transactivation of the epidermal growth factor receptor (EGFR; at its Y1173 phophorylation site), via its receptor-tyrosine kinase, β-arrestin 1/2 recruitment, and MAPK/ERK kinase-dependent ERK(1/2) phosphorylation. ERK(1/2) phosphorylation by Src activation depended on β-arrestin 2, but not β-arrestin 1, was accompanied by Src/EGFR co-precipitation and phosphorylation of the EGFR at the Src-phosphorylated Y845 site and the Y1045 autophosphorylation site; it was independent of transactivation but dependent on MAPK/ERK kinase activity, suggesting EGFR phosphorylation independently of the receptor-tyrosine kinase or activation of Ras or Raf directly from Src. Most astrocytic consequences of activating either pathway (or both) are unknown, but morphological differentiation and increase in glial fibrillary acidic protein in response to dibutyryl cAMP-mediated increase in cAMP depend on G(s)/G(i) switching and transactivation.

betaARKct: a therapeutic approach for improved adrenergic signaling and function in heart disease

One of the most powerful regulators of cardiovascular function is catecholamine-stimulated adrenergic receptor (AR) signaling. The failing heart is characterized by desensitization and impaired beta-AR responsiveness as a result of upregulated G protein-coupled receptor kinase-2 (GRK2) present in injured myocardium. Deterioration of cardiac function is progressively enhanced by chronic adrenergic over-stimulation due to increased levels of circulating catecholamines. Increased GRK2 activity contributes to this pathological cycle of over-stimulation but lowered responsiveness. Over the past two decades the GRK2 inhibitory peptide betaARKct has been identified as a potential therapy that is able to break this vicious cycle of self-perpetuating deregulation of the beta-AR system and subsequent myocardial malfunction, thus halting development of cardiac failure. The betaARKct has been shown to interfere with GRK2 binding to the betagamma subunits of the heterotrimeric G protein, therefore inhibiting its recruitment to the plasma membrane that normally leads to phosphorylation and internalization of the receptor. In this article we summarize the current data on the therapeutic effects of betaARKct in cardiovascular disease and report on recent and ongoing studies that may pave the way for this peptide towards therapeutic application in heart failure and other states of cardiovascular disease.

Internalization of the human nicotinic acid receptor GPR109A is regulated by G(i), GRK2, and arrestin3

Nicotinic acid (niacin) has been widely used as a favorable lipid-lowering drug for several decades, and the orphan G protein-coupled receptor GPR109A has been identified to be a receptor for niacin. Mechanistic investigations have shown that as a G(i)-coupled receptor, GPR109A inhibits adenylate cyclase activity upon niacin activation, thereby inhibiting free fatty acid liberation. However, the underlying molecular mechanisms that regulate signaling and internalization of GPR109A remain largely unknown. To further characterize GPR109A internalization, we made a construct to express GPR109A fused with enhanced green fluorescent protein (EGFP) at its carboxyl-terminal end. In stable GPR109A-EGFP-expressing HEK-293 cells, GPR109A-EGFP was mainly localized at the plasma membrane and was rapidly internalized in a dose- and time-dependent manner upon agonist stimulation. GPR109A internalization was completely blocked by hypertonic sucrose, indicating that GPR109A internalizes via the clathrin-coated pit pathway. Further investigation demonstrated that internalized GPR109A was recycled to the cell surface after the removal of agonist, and recycling of the internalized receptors was not blocked by treatment with acidotropic agents, NH(4)Cl and monensin. Pertussis toxin pretreatment not only inhibited forskolin-induced cAMP accumulation and intracellular Ca(2+) mobilization; it also significantly attenuated agonist-promoted GPR109A internalization. Moreover, RNA interference experiments showed that knockdown of GRK2 (G protein-coupled receptor kinase 2) and arrestin3 expression significantly impaired receptor internalization. Taken together, these results indicate that the agonist-induced internalization of GPR109A receptors is regulated by GRK2 and arrestin3 in a pertussis toxin-sensitive manner and that internalized receptor recycling is independent of endosomal acidification.

G alpha(q)-mediated activation of GRK2 by mechanical stretch in cardiac myocytes: the role of protein kinase C

G protein-coupled receptor kinase-2 (GRK2) is a critical regulator of beta-adrenergic receptor (beta-AR) signaling and cardiac function. We studied the effects of mechanical stretch, a potent stimulus for cardiac myocyte hypertrophy, on GRK2 activity and beta-AR signaling. To eliminate neurohormonal influences, neonatal rat ventricular myocytes were subjected to cyclical equi-biaxial stretch. A hypertrophic response was confirmed by "fetal" gene up-regulation. GRK2 activity in cardiac myocytes was increased 4.2-fold at 48 h of stretch versus unstretched controls. Adenylyl cyclase activity was blunted in sarcolemmal membranes after stretch, demonstrating beta-AR desensitization. The hypertrophic response to mechanical stretch is mediated primarily through the G alpha(q)-coupled angiotensin II AT(1) receptor leading to activation of protein kinase C (PKC). PKC is known to phosphorylate GRK2 at the N-terminal serine 29 residue, leading to kinase activation. Overexpression of a mini-gene that inhibits receptor-G alpha(q) coupling blunted stretch-induced hypertrophy and GRK2 activation. Short hairpin RNA-mediated knockdown of PKC alpha also significantly attenuated stretch-induced GRK2 activation. Overexpression of a GRK2 mutant (S29A) in cardiac myocytes inhibited phosphorylation of GRK2 by PKC, abolished stretch-induced GRK2 activation, and restored adenylyl cyclase activity. Cardiac-specific activation of PKC alpha in transgenic mice led to impaired beta-agonist-stimulated ventricular function, blunted cyclase activity, and increased GRK2 phosphorylation and activity. Phosphorylation of GRK2 by PKC appears to be the primary mechanism of increased GRK2 activity and impaired beta-AR signaling after mechanical stretch. Cross-talk between hypertrophic signaling at the level of PKC and beta-AR signaling regulated by GRK2 may be an important mechanism in the transition from compensatory ventricular hypertrophy to heart failure.

Reversal of impaired myocardial beta-adrenergic receptor signaling by continuous-flow left ventricular assist device support

BACKGROUND

Myocardial beta-adrenergic receptor (beta-AR) signaling is severely impaired in chronic heart failure (HF). This study was conducted to determine if left ventricular (LV) beta-AR signaling could be restored after continuous-flow LV assist device (LVAD) support.

METHODS

Twelve patients received LVADs as a bridge to transplant. Paired LV biopsy specimens were obtained at the time of LVAD implant (HF group) and transplant (LVAD group). The mean duration of LVAD support was 152 +/- 34 days. Myocardial beta-AR signaling was assessed by measuring adenylyl cyclase (AC) activity, total beta-AR density (B(max)), and G protein-coupled receptor kinase-2 (GRK2) expression and activity. LV specimens from 8 non-failing hearts (NF) were used as controls.

RESULTS

Basal and isoproterenol-stimulated AC activity was significantly lower in HF vs NF, indicative of beta-AR uncoupling. Continuous-flow LVAD support restored basal and isoproterenol-stimulated AC activity to levels similar to NF. B(max) was decreased in HF vs NF and increased to nearly normal in the LVAD group. GRK2 expression was increased 2.6-fold in HF vs NF and was similar to NF after LVAD support. GRK2 activity was 3.2-fold greater in HF vs NF and decreased to NF levels in the LVAD group.

CONCLUSIONS

Myocardial beta-AR signaling can be restored to nearly normal after continuous-flow LVAD support. This is similar to previous data for volume-displacement pulsatile LVADs. Decreased GRK2 activity is an important mechanism and indicates that normalization of the neurohormonal milieu associated with HF is similar between continuous-flow and pulsatile LVADs. This may have important implications for myocardial recovery.

Regulation of G-protein signaling by RKTG via sequestration of the G betagamma subunit to the Golgi apparatus

Upon ligand binding, G-protein-coupled receptors (GPCRs) impart the signal to heterotrimeric G proteins composed of alpha, beta, and gamma subunits, leading to dissociation of the G alpha subunit from the G betagamma subunit. While the G alpha subunit is imperative for downstream signaling, the G betagamma subunit, in its own right, mediates a variety of cellular responses such as GPCR desensitization via recruiting GRK to the plasma membrane and AKT stimulation. Here we report a mode of spatial regulation of the G betagamma subunit through alteration in subcellular compartmentation. RKTG (Raf kinase trapping to Golgi apparatus) is a newly characterized membrane protein specifically localized at the Golgi apparatus. The N terminus of RKTG interacts with G beta and tethers G betagamma to the Golgi apparatus. Overexpression of RKTG impedes the interaction of G betagamma with GRK2, abrogates the ligand-induced change of subcellular distribution of GRK2, reduces isoproterenol-stimulated phosphorylation of the beta2-adrenergic receptor (beta 2AR), and alters beta 2AR desensitization. In addition, RKTG inhibits G betagamma- and ligand-mediated AKT phosphorylation that is enhanced in cells with downregulation of RKTG. Silencing of RKTG also alters GRK2 internalization and compromises ligand-induced G beta translocation to the Golgi apparatus. Taken together, our results reveal that RKTG can modulate GPCR signaling through sequestering G betagamma to the Golgi apparatus and thereby attenuating the functions of G betagamma.

Blockade of ethanol-induced potentiation of glycine receptors by a peptide that interferes with Gbetagamma binding

The large intracellular loop (IL) of the glycine receptor (GlyR) interacts with various signaling proteins and plays a fundamental role in trafficking and regulation of several receptor properties, including a direct interaction with Gbetagamma. In the present study, we found that mutation of basic residues in the N-terminal region of the IL reduced the binding of Gbetagamma to 21 +/- 10% of control. Two basic residues in the C-terminal region, on the other hand, contributed to a smaller extent to Gbetagamma binding. Using docking analysis, we found that both basic regions of the IL bind in nearby regions to the Gbetagamma dimer, within an area of high density of amino acids having an electronegative character. Thereafter, we generated a 17-amino acid peptide with the N-terminal sequence of the wild-type IL (RQH) that was able to inhibit the in vitro binding of Gbetagamma to GlyRs to 57 +/- 5% of control in glutathione S-transferase pull-down assays using purified proteins. More interestingly, when the peptide was intracellularly applied to human embryonic kidney 293 cells, it inhibited the Gbetagamma-mediated modulations of G protein-coupled inwardly rectifying potassium channel by baclofen (24 +/- 14% of control) and attenuated the GlyR potentiation by ethanol (51 +/- 10% versus 10 +/- 3%).

Future g protein-coupled receptor targets for treatment of heart failure

Heart failure (HF) still poses an enormous clinical challenge, as its incidence, morbidity, and mortality rates are continuously rising. G protein-coupled receptors (GPCRs) constitute the most ubiquitous superfamily of plasma membrane receptors and represent the single most important type of therapeutic drug target. Because there is overstimulation of the failing heart by various endogenous ligands, such as catecholamines and angiotensin II--which by activating their cognate GPCRs in cardiac muscle induce detrimental effects--therapeutic targeting of these receptors has been pursued. This research has led to the development of successful and useful drug classes, such as angiotensin-converting enzyme inhibitors and beta-adrenergic receptor blockers. However, there still is a need to develop innovative treatments that might be more effective at reversing compromised myocyte function. Over the past several years, much evidence has accumulated indicating that a single GPCR, activated by the same endogenous ligand, can elicit several different signaling pathways with quite different, and often opposite, cellular effects. Because the aforementioned ligands, currently used for HF, target these receptors on their extracellular interface, thus merely preventing the endogenous agonists from binding the receptor, they inhibit all the signaling pathways elicited by the receptor indiscriminately. Importantly, several of these pathways emanating from the same GPCR can actually be beneficial for therapy, so their enhancement rather than their blockade is desirable for HF therapy. This highlights the need for selective targeting of GPCR-induced signaling pathways on the intracellular interface of the receptor, which might produce new and innovative therapies for cardiovascular disease.

Inhibition of WNT signaling by G protein-coupled receptor (GPCR) kinase 2 (GRK2)

Activation of Wnt signaling pathways causes release and stabilization of the transcription regulator beta-catenin from a destruction complex composed of axin and the adenomatous polyposis coli (APC) protein (canonical signaling pathway). Assembly of this complex is facilitated by a protein-protein interaction between APC and a regulator of G protein signaling (RGS) domain in axin. Because G protein-coupled receptor kinase 2 (GRK2) has a RGS domain that is closely related to the RGS domain in axin, we determined whether GRK2 regulated canonical signaling. We found that GRK2 inhibited Wnt1-induced activation of a reporter construct as well as reduced Wnt3a-dependent stabilization and nuclear translocation of beta-catenin. GRK2 enzymatic activity was required for this negative regulatory effect, and depletion of endogenous GRK2 using small interfering RNA enhanced canonical signaling. GRK2-dependent inhibition of canonical signaling is relevant to osteoblast (OB) biology because overexpression of GRK2 attenuated Wnt/beta-catenin signaling in calvarial OBs. Coimmunoprecipitation studies found that: 1) GRK2 bound APC; 2) The GRK2-APC interaction was promoted by GRK2 enzymatic activity; and 3) Deletion of the RGS domain in GRK2 prevented both the GRK2-APC interaction and GRK2-dependent inhibition of canonical signaling. These data suggest that: 1) GRK2 negatively regulates Wnt signaling; 2) GRK2-dependent inhibition of canonical signaling requires a protein-protein interaction between the RGS domain in GRK2 and APC; and 3) Enzymatic activity promotes the GRK2-APC interaction and is required for the negative regulatory effect on canonical signaling. We speculate that inhibiting GRK2 activity in bone-forming OBs might be a useful therapeutic strategy for increasing bone mass.

Decreased GRK3 but not GRK2 expression in frontal cortex from bipolar disorder patients

Overactivation of G-protein-mediated functions and altered G-protein regulation have been reported in bipolar disorder (BD) brain. Further, drugs effective in treating BD are reported to up-regulate expression of G-protein receptor kinase (GRK) 3 in rat frontal cortex. We therefore hypothesized that some G-protein subunits and GRK levels would be reduced in the brain of BD patients. We determined protein and mRNA levels of G-protein beta and gamma subunits, GRK2, and GRK3 in post-mortem frontal cortex from 10 BD patients and 10 age-matched controls by using immunoblots and real-time RT-PCR. There were statistically significant decreases in protein and mRNA levels of G-protein subunits beta and gamma and of GRK3 in BD brain but not a significant difference in the GRK2 level. Decreased expression of G-protein subunits and of GRK3 may alter neurotransmission, leading to disturbed cognition and behaviour in BD.

Loss of pleckstrin defines a novel pathway for PKC-mediated exocytosis

Pleckstrin, the platelet and leukocyte C kinase substrate, is a prominent substrate of PKC in platelets, monocytes, macrophages, lymphocytes, and granulocytes. Pleckstrin accounts for 1% of the total protein in these cells, but it is best known for containing the 2 prototypic Pleckstrin homology, or PH, domains. Overexpressed pleckstrin can affect polyphosphoinositide second messenger-based signaling events; however, its true in vivo role has been unknown. Here, we describe mice containing a null mutation within the pleckstrin gene. Platelets lacking pleckstrin exhibit a marked defect in exocytosis of delta and alpha granules, alphaIIbbeta3 activation, actin assembly, and aggregation after exposure to the PKC stimulant, PMA. Pleckstrin-null platelets aggregate normally in response to thrombin, but they fail to aggregate in response to thrombin in the presence of PI3K inhibitors, suggesting that a PI3K-dependent signaling pathway compensates for the loss of pleckstrin. Although pleckstrin-null platelets merged their granules in response to stimulation of PKC, they failed to empty their contents into the open canalicular system. This might be attributable to impaired actin assembly present in cells lacking pleckstrin. These data show that pleckstrin regulates the fusion of granules to the cell membrane and is an essential component of PKC-mediated exocytosis.

Beta-adrenergic receptor mediated protection against doxorubicin-induced apoptosis in cardiomyocytes: the impact of high ambient glucose

Recent studies have demonstrated that the beta2-adrenergic receptor (beta2AR)-Galphai signaling pathway exerts a cardiac antiapoptotic effect. The goals of this study were to determine the intracellular signaling factors involved in beta2AR-mediated protection against doxorubicin-induced apoptosis in H9c2 cardiomyocyte and explore the impact of high ambient glucose on the antiapoptotic effect. Under physiological glucose environment (100 mg/dl), beta2AR stimulation prevented doxorubicin-induced apoptosis, which was attenuated by cotreatment with wortmannin, a phosphoinositide 3-kinase (PI3K) inhibitor, or transfection of a dominant-negative Akt. Inhibition of Src kinase with 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d] pyrimidine or cSrc small interfering RNA 32 also attenuated the antiapoptotic effect. Inhibition of platelet-derived growth factor receptor (PDGFR) with AG1296 reversed the beta2AR-induced antiapoptotic effect. Transfection of an active Src cDNA (Y529F) alone was sufficient to render the cells resistant to apoptosis, and the resistance was blocked by wortmannin. Transfection of an active PI3K minigene (iSH2-p110) alone also induced resistance to apoptosis, and the resistance was reversed by an Akt-inhibitor but not by AG1296. High ambient glucose (450 mg/dl) caused two major effects: 1) it significantly reduced betaAR-induced PDGFR phosphorylation, Src kinase activity, and activation of PI3K signaling pathway; and 2) it partially attenuated beta2AR-induced antiapoptotic effect. These data provide in vitro evidence supporting a signaling cascade by which beta2AR exerts a protective effect against doxorubicin-induced apoptosis via sequential involvement of Galphai, Gbetagamma, Src, PDGFR, PI3K, and Akt. High ambient glucose significantly attenuates beta2AR-mediated cardioprotection by suppressing factors involved in this cascade including PDGFR, Src, and PI3K/Akt.

Inhibition of vascular smooth muscle G protein-coupled receptor kinase 2 enhances alpha1D-adrenergic receptor constriction

G protein-coupled receptor kinase 2 (GRK2) is a serine/theorinine kinase that phosphorylates and desensitizes agonist-bound G protein-coupled receptors. GRK2 is increased in expression and activity in lymphocytes and vascular smooth muscle (VSM) in human hypertension and animal models of the disease. Inhibition of GRK2 using the carboxyl-terminal portion of the protein (GRK2ct) has been an effective tool to restore compromised beta-adrenergic receptor (AR) function in heart failure and improve outcome. A well-characterized dysfunction in hypertension is attenuation of betaAR-mediated vasodilation. Therefore, we tested the role of inhibition of GRK2 using GRK2ct or VSM-selective GRK2 gene ablation in a renal artery stenosis model of elevated blood pressure (BP) [the two-kidney, one-clip (2K1C) model]. Use of the 2K1C model resulted in a 30% increase in conscious BP, a threefold increase in plasma norepinephrine levels, and a 50% increase in VSM GRK2 mRNA levels. BP remained increased despite VSM-specific GRK2 inhibition by either GRK2 knockout (GRK2KO) or peptide inhibition (GRK2ct). Although betaAR-mediated dilation in vivo and in situ was enhanced, alpha(1)AR-mediated vasoconstriction was also increased. Further pharmacological experiments using alpha(1)AR antagonists revealed that GRK2 inhibition of expression (GRK2KO) or activity (GRK2ct) enhanced alpha(1D)AR vasoconstriction. This is the first study to suggest that VSM alpha(1D)ARs are a GRK2 substrate in vivo.

The monomeric G proteins AGS1 and Rhes selectively influence Galphai-dependent signaling to modulate N-type (CaV2.2) calcium channels

Activator of G protein Signaling 1 (AGS1) and Ras homologue enriched in striatum (Rhes) define a new group of Ras-like monomeric G proteins whose signaling properties and physiological roles are just beginning to be understood. Previous results suggest that AGS1 and Rhes exhibit distinct preferences for heterotrimeric G proteins, with AGS1 selectively influencing Galphai and Rhes selectively influencing Galphas. Here, we demonstrate that AGS1 and Rhes trigger nearly identical modulation of N-type Ca(2+) channels (Ca(V)2.2) by selectively altering Galphai-dependent signaling. Whole-cell currents were recorded from HEK293 cells expressing Ca(V)2.2 and Galphai- or Galphas-coupled receptors. AGS1 and Rhes reduced basal current densities and triggered tonic voltage-dependent (VD) inhibition of Ca(V)2.2. Additionally, each protein attenuated agonist-initiated channel inhibition through Galphai-coupled receptors without reducing channel inhibition through a Galphas-coupled receptor. The above effects of AGS1 and Rhes were blocked by pertussis toxin (PTX) or by expression of a Gbetagamma-sequestering peptide (masGRK3ct). Transfection with HRas, KRas2, Rap1A-G12V, Rap2B, Rheb2, or Gem failed to duplicate the effects of AGS1 and Rhes on Ca(V)2.2. Our data provide the first demonstration that AGS1 and Rhes exhibit similar if not identical signaling properties since both trigger tonic Gbetagamma signaling and both attenuate receptor-initiated signaling by the Gbetagamma subunits of PTX-sensitive G proteins. These results are consistent with the possibility that AGS1 and Rhes modulate Ca(2+) influx through Ca(V)2.2 channels under more physiological conditions and thereby influence Ca(2+)-dependent events such as neurosecretion.

GPCR signalling in hypertension: role of GRKs

Hypertension is a prevalent condition in the developed world and disease severity is directly correlated with additional cardiovascular complications. It is estimated that 30% of the adult population in the United States has hypertension, which is classified as a systolic blood pressure > or =140 mmHg and/or a diastolic blood pressure > or =90 mmHg. A prolonged increase in afterload ultimately leads to congestive heart failure in the majority of cases. Currently, medication designed to treat hypertension is inadequate, thus new therapies need to be explored. Blood pressure is tightly regulated by blood vessel radius, which is established by hormones and/or peptides binding to GPCRs (G-protein-coupled receptors). Catecholamines and peptide hormones, such as AngII (angiotensin II), are elevated in hypertension and, therefore, signalling by these GPCRs is increased. Their signalling is tightly controlled by a class of proteins, the GRKs (GPCR kinases). Elevated levels of either GRK2 or GRK5 in both the lymphocytes and VSM (vascular smooth muscle) are associated with human hypertension and animal models of the disease. The focus of the present review is on the role GRKs, and their regulation of GPCRs, play in high blood pressure.

A GRK5 polymorphism that inhibits beta-adrenergic receptor signaling is protective in heart failure

Beta-adrenergic receptor (betaAR) blockade is a standard therapy for cardiac failure and ischemia. G protein-coupled receptor kinases (GRKs) desensitize betaARs, suggesting that genetic GRK variants might modify outcomes in these syndromes. Re-sequencing of GRK2 and GRK5 revealed a nonsynonymous polymorphism of GRK5, common in African Americans, in which leucine is substituted for glutamine at position 41. GRK5-Leu41 uncoupled isoproterenol-stimulated responses more effectively than did GRK5-Gln41 in transfected cells and transgenic mice, and, like pharmacological betaAR blockade, GRK5-Leu41 protected against experimental catecholamine-induced cardiomyopathy. Human association studies showed a pharmacogenomic interaction between GRK5-Leu41 and beta-blocker treatment, in which the presence of the GRK5-Leu41 polymorphism was associated with decreased mortality in African Americans with heart failure or cardiac ischemia. In 375 prospectively followed African-American subjects with heart failure, GRK5-Leu41 protected against death or cardiac transplantation. Enhanced betaAR desensitization of excessive catecholamine signaling by GRK5-Leu41 provides a 'genetic beta-blockade' that improves survival in African Americans with heart failure, suggesting a reason for conflicting results of beta-blocker clinical trials in this population.

Cardiac-restricted expression of the carboxyl-terminal fragment of GRK3 Uncovers Distinct Functions of GRK3 in regulation of cardiac contractility and growth: GRK3 controls cardiac alpha1-adrenergic receptor responsiveness

G protein-coupled receptor kinase-2 and -3 (GRK2 and GRK3) in cardiac myocytes catalyze phosphorylation and desensitization of different G protein-coupled receptors through specificity controlled by their carboxyl-terminal pleckstrin homology domain. Although GRK2 has been extensively investigated, the function of cardiac GRK3 remains unknown. Thus, in this study cardiac function of GRK3 was investigated in transgenic (Tg) mice with cardiac-restricted expression of a competitive inhibitor of GRK3, i.e. the carboxyl-terminal plasma membrane targeting domain of GRK3 (GRK3ct). Cardiac myocytes from Tg-GRK3ct mice displayed significantly enhanced agonist-stimulated alpha(1)-adrenergic receptor-mediated activation of ERK1/2 versus cardiac myocytes from nontransgenic littermate control (NLC) mice consistent with inhibition of GRK3. Tg-GRK3ct mice did not display alterations of cardiac mass or left ventricular dimensions compared with NLC mice. Tail-cuff plethysmography of 3- and 9-month-old mice revealed elevated systolic blood pressure in Tg-GRK3ct mice versus control mice (3-month-old mice, 136.8 +/- 3.6 versus 118.3 +/- 4.7 mm Hg, p < 0.001), an observation confirmed by radiotelemetric recording of blood pressure of conscious, unrestrained mice. Simultaneous recording of left ventricular pressure and volume in vivo by miniaturized conductance micromanometry revealed increased systolic performance with significantly higher stroke volume and stroke work in Tg-GRK3ct mice than in NLC mice. This phenotype was corroborated in electrically paced ex vivo perfused working hearts. However, analysis of left ventricular function ex vivo as a function of increasing filling pressure disclosed significantly reduced (dP/dt)(min) and prolonged time constant of relaxation (tau) in Tg-GRK3ct hearts at elevated supraphysiological filling pressure compared with control hearts. Thus, inhibition of GRK3 apparently reduces tolerance to elevation of preload. In conclusion, inhibition of cardiac GRK3 causes hypertension because of hyperkinetic myocardium and increased cardiac output relying at least partially on cardiac myocyte alpha(1)-adrenergic receptor hyper-responsiveness. The reduced tolerance to elevation of preload may cause impaired ability to withstand pathophysiological mechanisms of heart failure.

Targeting myocardial beta-adrenergic receptor signaling and calcium cycling for heart failure gene therapy

Heart failure (HF) is a leading cause of morbidity and mortality in Western countries and projections reveal that HF incidence in the coming years will rise significantly because of an aging population. Pharmacologic therapy has considerably improved HF treatment during the last 2 decades, but fails to rescue failing myocardium and to increase global cardiac function. Therefore, novel therapeutic approaches to target the underlying molecular defects of ventricular dysfunction and to increase the outcome of patients in HF are needed. Failing myocardium generally exhibits distinct changes in beta-adrenergic receptor (betaAR) signaling and intracellular Ca2+-handling providing opportunities for research. Recent advances in transgenic and gene therapy techniques have presented novel therapeutic strategies to alter myocardial function and to target both betaAR signaling and Ca2+-cycling. In this review, we will discuss functional alterations of the betaAR system and Ca2+-handling in HF as well as corresponding therapeutic strategies. We will then focus on recent in vivo gene therapy strategies using the targeted inhibition of the betaAR kinase (betaARK1 or GRK2) and the restoration of S100A1 protein expression to support the injured heart and to reverse or prevent HF.

Substrate specificities of g protein-coupled receptor kinase-2 and -3 at cardiac myocyte receptors provide basis for distinct roles in regulation of myocardial function

The closely related G protein-coupled receptor kinases GRK2 and GRK3 are both expressed in cardiac myocytes. Although GRK2 has been extensively investigated in terms of regulation of cardiac beta-adrenergic receptors, the substrate specificities of the two GRK isoforms at G protein-coupled receptors (GPCR) are poorly understood. In this study, the substrate specificities of GRK2 and GRK3 at GPCRs that control cardiac myocyte function were determined in fully differentiated adult cardiac myocytes. Concentration-effect relationships of GRK2, GRK3, and their respective competitive inhibitors, GRK2ct and GRK3ct, at endogenous endothelin, alpha(1)-adrenergic, and beta(1)-adrenergic receptor-generated responses in cardiac myocytes were achieved by adenovirus gene transduction. GRK3 and GRK3ct were highly potent and efficient at the endothelin receptors (IC(50) for GRK3, 5 +/- 0.7 pmol/mg of protein; EC(50) for GRK3ct, 2 +/- 0.2 pmol/mg of protein). The alpha(1)-adrenergic receptor was also a preferred substrate of GRK3 (IC(50),7 +/- 0.4 pmol/mg of protein). GRK2 lacked efficacy at both endothelin and alpha(1)-adrenergic receptors despite massive overexpression. On the contrary, both GRK2ct and GRK3ct enhanced beta(1)-adrenergic receptor-induced cAMP production with comparable potencies. However, the potency of GRK3ct at beta(1)-adrenergic receptors was at least 20-fold lower than that at endothelin receptors. In conclusion, this study demonstrates distinct substrate specificities of GRK2 and GRK3 at different GPCRs in fully differentiated adult cardiac myocytes. As inferred from the above findings, GRK2 may play its primary role in regulation of cardiac contractility and chronotropy by controlling beta(1)-adrenergic receptors, whereas GRK3 may play important roles in regulation of cardiac growth and hypertrophy by selectively controlling endothelin and alpha(1)-adrenergic receptors.

Endothelial beta2 adrenergic signaling to AKT: role of Gi and SRC

We have recently demonstrated that endothelial beta(2) adrenergic receptors (beta(2)AR) regulate eNOS activity and consequently vascular tone, through means of PKB/AKT. In this work we explored the signal transduction pathway leading to AKT/eNOS activation in endothelial cells (EC). Using pharmacological and molecular inhibitors both in cultured EC cells and in ex vivo rat carotid preparations, we found that G(i) coupling of the beta(2)AR is needed for AKT activation and vasorelaxation. Since endothelial activation is sensitive to pertussis toxin but not to G(ibetagamma) inhibition by betaARKct, we conclude that G(alphai) mediates betaAR induced AKT activation. Downstream, betaAR signalling requires the soluble tyrosine kinase SRC, as both in cultured EC and rat carotid, the mutant dominant negative of SRC prevent beta(2)AR induced endothelial activation and vasodilation. In EC, G(alphai) directly interacts with SRC and this interaction leads to SRC activation and phosphorylation in a manner that is regulated by beta(2)AR stimulation. We propose a novel signal transduction pathway for beta(2)AR stimulation trough G(alphai) and SRC, leading to activation of AKT.

The cell biology of Smo signalling and its relationships with GPCRs

The Smoothened (Smo) signalling pathway participates in many developmental processes, contributing to the regulation of gene expression by controlling the activity of transcription factors belonging to the Gli family. The key elements of the pathway were identified by means of genetic screens carried out in Drosophila, and subsequent analysis in other model organisms revealed a high degree of conservation in both the proteins involved and in their molecular interactions. Recent analysis of the pathway, using a combination of biochemical and cell biological approaches, is uncovering the intricacies of Smo signalling, placing its elements in particular cellular compartments and qualifying the molecular processes involved. These include the synthesis, secretion and diffusion of the ligand, the activation of the receptor and the modifications in the activity of nuclear effectors. In this review we discuss recent advances in understanding biochemical and cellular aspects of Smo signalling, with particular focus in the similarities in the mechanism of signal transduction between Smo and other transmembrane proteins belonging to the G-Protein coupled receptors superfamily (GPCR).

Lymphocyte G-protein-coupled receptor kinase-2 is upregulated in patients with Alzheimer's disease

Alterations in signal transduction pathway of G-protein-coupled receptors (GPCRs) have been found in the cerebrocortex and in the peripheral cultured tissues of patients with Alzheimer's disease (AD). The G-protein-coupled receptor kinase-2 (GRK2) plays an important role in regulating the GPCRs signaling: its increased expression is associated with receptor desensitization. The aim of this study was to explore GRK2 levels in peripheral lymphocytes of AD patients and to establish a correlation between lymphocyte protein concentrations and the degree of cognitive impairment. GRK2 mRNA and protein expression were evaluated in the lymphocytes of AD patients with mild or moderate/severe cognitive impairment and in age-matched healthy subjects. Both GRK2 mRNA and protein expression were higher in AD patients lymphocytes compared to controls. Furthermore, lymphocyte GRK2 levels were significantly correlated to the degree of cognitive decline. Our preliminary data suggest that GRK2 is involved in GPCRs coupling dysfunction observed in AD patients. Further studies are needed in order to verify whether the lymphocyte GRK2 might be utilized as a novel biomarker in AD diagnosis and clinical monitoring.

Chronic treatment with mood stabilizers increases membrane GRK3 in rat frontal cortex

BACKGROUND

G-protein receptor kinases (GRKs) are a family of serine/threonine kinases involved in the homologous desensitization of agonist activated G-protein coupled receptors (GPCRs). G-protein coupled receptor supersensitivity, possibly as a result of decreased GRK, has been suggested in affective disorders.

METHODS

We used immunobloting to determine if chronic, therapeutically relevant doses of lithium (Li+), carbamazepine (CBZ), and valproate (VPA), would increase GRK2/3 protein levels in rat frontal cortex.

RESULTS

Chronic Li+ (24%) and CBZ (44%) significantly increased GRK3 in the membrane but not cytosol fractions. Chronic VPA had no effect on GRK3. G-protein receptor kinase 2 protein levels were unchanged by all treatments. The GRK3 membrane to cytosol ratio was increased significantly in Li+ and CBZ treated rats.

CONCLUSIONS

These results show that chronically administered Li+ and CBZ, but not VPA, increase the translocation of GRK3 from cytosol to membrane, possibly correcting supersensitivity of GPCRs in bipolar disorder.

G-protein coupled receptor kinase-like immunoreactivity in the snail, Helix pomatia, neurons

GPRCs are regulated via phosphorylation by different protein kinases including GRKs and PKA and PKC. The purpose of this study was to determine the presence and physiological role of GRKs in the tissues of the snail, Helix pomatia. Here we report that immunoblotting of brain homogenate with anti-GRK2/3 antibody prepared from mammalian tissue can be detected in snail GRK-like immunoreactivity. The GRK2/3 immunoreactivity was found at approximately 80 kDa in a variety of cells, including salivary duct, salivary gland and eye. Intracellular injection of the anti-GRK2/3 prevented the neuron from desensitization and agonist-induced activation augmented the phosphorylated GRKs in the membrane fraction suggesting that GRKs may have a functional role in the neuropeptide receptor desensitization in snail.

The G protein-coupled receptor kinase (GRK) interactome: role of GRKs in GPCR regulation and signaling

G protein-coupled receptor kinases (GRKs) and arrestins are key participants in the canonical pathways leading to phosphorylation-dependent GPCR desensitization, endocytosis, intracellular trafficking and resensitization as well as in the modulation of important intracellular signaling cascades by GPCR. Novel studies have revealed a phosphorylation-independent desensitization mechanism operating through their RGS-homology (RH) domain and the recent determination of the crystal structures of GRK2 and GRK6 has uncovered interesting details on the structure-function relationships of these kinases. Emerging evidence indicates that the activity of GRKs is tightly modulated by mechanisms including phosphorylation by different kinases and interaction with several cellular proteins such as calmodulin, caveolin or RKIP. In addition, GRKs are involved in multiple interactions with non-receptor proteins (PI3K, Akt, GIT or MEK) that point to novel GRK cellular roles. In this article, our purpose is to describe the ever increasing map of functional interactions for GRK proteins as a basis to better understand its contribution to cellular processes.

Regulation of receptor tyrosine kinase signaling by GRKs and beta-arrestins

Receptor tyrosine kinases (RTKs) are a unique family of cell surface receptors, each containing a common intracellular domain that has tyrosine kinase activity. However, RTKs share many signaling molecules with another unique family of cell surface receptors, the seven-transmembrane receptors (7TMRs), and these receptor families can activate similar signaling cascades. In this review of RTK signaling, we describe the role of cross talk between RTKs and 7TMRs, focusing specifically on the role played in this process by beta-arrestins and by G proteins.

The structure of G protein-coupled receptor kinase (GRK)-6 defines a second lineage of GRKs

We describe the 2.6-A crystal structure of human G protein-coupled receptor kinase (GRK)-6, a key regulator of dopaminergic signaling and lymphocyte chemotaxis. GRK6 is a member of the GRK4 subfamily of GRKs, which is represented in most, if not all, metazoans. Comparison of GRK6 with GRK2 confirms that the catalytic core of all GRKs consists of intimately associated kinase and regulator of G protein signaling (RGS) homology domains. Despite being in complex with an ATP analog, the kinase domain of GRK6 remains in an open, presumably inactive conformation, suggesting that G protein-coupled receptors activate GRKs by inducing kinase domain closure. The structure reveals a putative phospholipid-binding site near the N terminus of GRK6 and structural elements within the kinase substrate channel that likely influence G protein-coupled receptor access and specificity. The crystalline GRK6 RGS homology domain forms an extensive dimer interface using conserved hydrophobic residues distinct from those in GRK2 that bind Galpha(q), although dimerization does not appear to occur in solution and is not required for receptor phosphorylation.