Vascular smooth muscle overexpression of G protein-coupled receptor kinase 5 elevates blood pressure, which segregates with sex and is dependent on Gi-mediated signaling

G protein-coupled receptor kinases in hypertension: physiology, pathogenesis, and therapeutic targets

G protein-coupled receptors (GPCRs) mediate cellular responses to a myriad of hormones and neurotransmitters that play vital roles in the regulation of physiological processes such as blood pressure. In organs such as the artery and kidney, hormones or neurotransmitters, such as angiotensin II (Ang II), dopamine, epinephrine, and norepinephrine exert their functions via their receptors, with the ultimate effect of keeping normal vascular reactivity, normal body sodium, and normal blood pressure. GPCR kinases (GRKs) exert their biological functions, by mediating the regulation of agonist-occupied GPCRs, non-GPCRs, or non-receptor substrates. In particular, increasing number of studies show that aberrant expression and activity of GRKs in the cardiovascular system and kidney inhibit or stimulate GPCRs (e.g., dopamine receptors, Ang II receptors, and α- and β-adrenergic receptors), resulting in hypertension. Current studies focus on the effect of selective GRK inhibitors in cardiovascular diseases, including hypertension. Moreover, genetic studies show that GRK gene variants are associated with essential hypertension, blood pressure response to antihypertensive medicines, and adverse cardiovascular outcomes of antihypertensive treatment. In this review, we present a comprehensive overview of GRK-mediated regulation of blood pressure, role of GRKs in the pathogenesis of hypertension, and highlight potential strategies for the treatment of hypertension. Schematic representation of GPCR desensitization process. Activation of GPCRs begins with the binding of an agonist to its corresponding receptor. Then G proteins activate downstream effectors that are mediated by various signaling pathways. GPCR signaling is halted by GRK-mediated receptor phosphorylation, which causes receptor internalization through β-arrestin.

Guidelines on the use of sex and gender in cardiovascular research

In cardiovascular research, sex and gender have not typically been considered in research design and reporting until recently. This has resulted in clinical research findings from which not only all women, but also gender-diverse individuals have been excluded. The resulting dearth of data has led to a lack of sex- and gender-specific clinical guidelines and raises serious questions about evidence-based care. Basic research has also excluded considerations of sex. Including sex and/or gender as research variables not only has the potential to improve the health of society overall now, but it also provides a foundation of knowledge on which to build future advances. The goal of this guidelines article is to provide advice on best practices to include sex and gender considerations in study design, as well as data collection, analysis, and interpretation to optimally establish rigor and reproducibility needed to inform clinical decision-making and improve outcomes. In cardiovascular physiology, incorporating sex and gender is a necessary component when optimally designing and executing research plans. The guidelines serve as the first guidance on how to include sex and gender in cardiovascular research. We provide here a beginning path toward achieving this goal and improve the ability of the research community to interpret results through a sex and gender lens to enable comparison across studies and laboratories, resulting in better health for all.

Comprehensive insights in GRK4 and hypertension: From mechanisms to potential therapeutics

G protein-coupled receptors (GPCRs) mediate cellular responses to diverse extracellular stimuli that play vital roles in the regulation of biology, including behavior. Abnormal G protein-coupled receptor kinase (GRK)-mediated regulation of GPCR function is involved in the pathogenesis of hypertension. Among the seven GRK subtypes, GRK4 has attracted attention because of its constitutive activity and tissue-specific expression. Increasing number of studies show that GRK4 affects blood pressure by GPCR-mediated regulation of renal and arterial function. The target receptor of GRK4 is confined not only to GPCRs, but also to other blood pressure-regulating receptors, such as the adiponectin receptor. Genetic studies in humans show that in several ethnic groups, GRK4 gene variants (R65L, A142V, and A486V) are associated with salt-sensitive or salt-resistant essential hypertension and blood pressure responses to antihypertensive medicines. In this article, we present a comprehensive overview of GRK-mediated regulation of blood pressure, focusing on the latest research progress on GRK4 and hypertension and highlighting potential and novel strategies for the prevention and treatment of hypertension.

Mer-tyrosine kinase: a novel susceptibility gene for SLE related end-stage renal disease

OBJECTIVE

Lupus nephritis (LN) is a common and severe manifestation of SLE. The genetic risk for nephritis and progression to end-stage renal disease (ESRD) in patients with LN remains unclear. Herein, we aimed to identify novel genetic associations with LN, focusing on subphenotypes and ESRD.

METHODS

We analysed genomic data on 958 patients with SLE (discovery cohort: LN=338) with targeted sequencing data from 1832 immunological pathway genes. We used an independent multiethnic cohort comprising 1226 patients with SLE (LN=603) as a replication dataset. Detailed functional annotation and functional epigenomic enrichment analyses were applied to predict functional effects of the candidate variants.

RESULTS

A genetic variant (rs56097910) within the MERTK gene was associated with ESRD in both cohorts, meta-analysis OR=5.4 (2.8 to 10.6); p=1.0×10-6. We observed decreased methylation levels in peripheral blood cells from SLE patients with ESRD, compared with patients without renal SLE (p=2.7×10-4), at one CpG site (cg16333401) in close vicinity to the transcription start site of MERTK and located in a DNAse hypersensitivity region in T and B cells. Rs56097910 is linked to altered MERTK expression in kidney tissue in public eQTL databases. Two loci were replicated for association with proliferative LN: PRDM1 (rs6924535, pmeta=1.6×10-5, OR=0.58) and APOA1BP (NAXE) (rs942960, pmeta=1.2×10-5, OR=2.64).

CONCLUSION

We identified a novel genetic risk locus, MERTK, associated with SLE-ESRD using the data from two large SLE cohorts. Through DNA methylation analysis and functional annotation, we showed that the risk could be mediated through regulation of gene expression. Our results suggest that variants in the MERTK gene are important for the risk of developing SLE-ESRD and suggest a role for PRDM1 and APOA1BP in proliferative LN.

GRK5 Deficiency Causes Mild Cognitive Impairment due to Alzheimer's Disease

Prevention of Alzheimer's disease (AD) is a high priority mission while searching for a disease modifying therapy for AD, a devastating major public health crisis. Clinical observations have identified a prodromal stage of AD for which the patients have mild cognitive impairment (MCI) though do not yet meet AD diagnostic criteria. As an identifiable transitional stage before the onset of AD, MCI should become the high priority target for AD prevention, assuming successful prevention of MCI and/or its conversion to AD also prevents the subsequent AD. By pulling this string, one demonstrated cause of amnestic MCI appears to be the deficiency of G protein-coupled receptor-5 (GRK5). The most compelling evidence is that GRK5 knockout (GRK5KO) mice naturally develop into aMCI during aging. Moreover, GRK5 deficiency was reported to occur during prodromal stage of AD in CRND8 transgenic mice. When a GRK5KO mouse was crossbred with Tg2576 Swedish amyloid precursor protein transgenic mouse, the resulted double transgenic GAP mice displayed exaggerated behavioral and pathological changes across the spectrum of AD pathogenesis. Therefore, the GRK5 deficiency possesses unique features and advantage to serve as a prophylactic therapeutic target for MCI due to AD.

Guidelines for in vivo mouse models of myocardial infarction

Despite significant improvements in reperfusion strategies, acute coronary syndromes all too often culminate in a myocardial infarction (MI). The consequent MI can, in turn, lead to remodeling of the left ventricle (LV), the development of LV dysfunction, and ultimately progression to heart failure (HF). Accordingly, an improved understanding of the underlying mechanisms of MI remodeling and progression to HF is necessary. One common approach to examine MI pathology is with murine models that recapitulate components of the clinical context of acute coronary syndrome and subsequent MI. We evaluated the different approaches used to produce MI in mouse models and identified opportunities to consolidate methods, recognizing that reperfused and nonreperfused MI yield different responses. The overall goal in compiling this consensus statement is to unify best practices regarding mouse MI models to improve interpretation and allow comparative examination across studies and laboratories. These guidelines will help to establish rigor and reproducibility and provide increased potential for clinical translation.

Biased agonism at chemokine receptors

In the human chemokine system, interactions between the approximately 50 known endogenous chemokine ligands and 20 known chemokine receptors (CKRs) regulate a wide range of cellular functions and biological processes including immune cell activation and homeostasis, development, angiogenesis, and neuromodulation. CKRs are a family of G protein-coupled receptors (GPCR), which represent the most common and versatile class of receptors in the human genome and the targets of approximately one third of all Food and Drug Administration-approved drugs. Chemokines and CKRs bind with significant promiscuity, as most CKRs can be activated by multiple chemokines and most chemokines can activate multiple CKRs. While these ligand-receptor interactions were previously regarded as redundant, it is now appreciated that many chemokine:CKR interactions display biased agonism, the phenomenon in which different ligands binding to the same receptor signal through different pathways with different efficacies, leading to distinct biological effects. Notably, these biased responses can be modulated through changes in ligand, receptor, and or the specific cellular context (system). In this review, we explore the biochemical mechanisms, functional consequences, and therapeutic potential of biased agonism in the chemokine system. An enhanced understanding of biased agonism in the chemokine system may prove transformative in the understanding of the mechanisms and consequences of biased signaling across all GPCR subtypes and aid in the development of biased pharmaceuticals with increased therapeutic efficacy and safer side effect profiles.

Endothelin-1 Mediates the Systemic and Renal Hemodynamic Effects of GPR81 Activation

Supplemental Digital Content is available in the text.

GPR81 (G-protein-coupled receptor 81) is highly expressed in adipocytes, and activation by the endogenous ligand lactate inhibits lipolysis. GPR81 is also expressed in the heart, liver, and kidney, but roles in nonadipose tissues are poorly defined. GPR81 agonists, developed to improve blood lipid profile, might also provide insights into GPR81 physiology. Here, we assessed the blood pressure and renal hemodynamic responses to the GPR81 agonist, AZ′5538. In male wild-type mice, intravenous AZ′5538 infusion caused a rapid and sustained increase in systolic and diastolic blood pressure. Renal artery blood flow, intrarenal tissue perfusion, and glomerular filtration rate were all significantly reduced. AZ′5538 had no effect on blood pressure or renal hemodynamics in Gpr81^−/− mice. Gpr81 mRNA was expressed in renal artery vascular smooth muscle, in the afferent arteriole, in glomerular and medullary perivascular cells, and in pericyte-like cells isolated from kidney. Intravenous AZ′5538 increased plasma ET-1 (endothelin 1), and pretreatment with BQ123 (endothelin-A receptor antagonist) prevented the pressor effects of GPR81 activation, whereas BQ788 (endothelin-B receptor antagonist) did not. Renal ischemia-reperfusion injury, which increases renal extracellular lactate, increased the renal expression of genes encoding ET-1, KIM-1 (Kidney Injury Molecule 1), collagen type 1-α1, TNF-α (tumor necrosis factor-α), and F4/80 in wild-type mice but not in Gpr81^−/− mice. In summary, activation of GPR81 in vascular smooth muscle and perivascular cells regulates renal hemodynamics, mediated by release of the potent vasoconstrictor ET-1. This suggests that lactate may be a paracrine regulator of renal blood flow, particularly relevant when extracellular lactate is high as occurs during ischemic renal disease.

GRK2 and GRK5 as therapeutic targets and their role in maladaptive and pathological cardiac hypertrophy

INTRODUCTION

One in every four deaths in the United States is attributed to cardiovascular disease, hence the development and employment of novel and effective therapeutics are necessary to improve the quality of life and survival of affected patient. Pathological hypertrophy is a maladaptive response by the heart to relieve wall stress that could result from cardiovascular disease. Maladaptive hypertrophy can lead to further disease progression and complications such as heart failure; hence, efforts to target hypertrophy to prevent and treat further morbidity and mortality are necessary. Areas covered: This review summarizes the compelling literature that describes the mechanistic role of GRK2 and GRK5 in maladaptive cardiac hypertrophy; it examines the approaches to inhibit these kinases in hypertrophic animal models and furthermore, it assesses the potential of GRK2 and GRK5 as therapeutic targets for hypertrophy. Expert opinion: GRK2 and GRK5 are novel therapeutic targets for pathological hypertrophy and may have added benefits of ameliorating morbidity and mortality. Despite the lesser researched role of GRK2 in cardiac hypertrophy, it may be the advantageous strategy for treating cardiac hypertrophy because of its role in other maladaptive pathways. Anti-GRK2 therapy optimization and the discovery and development of specific GRK2 and GRK5 small-molecule inhibitors is necessary for the eventual application of successful, effective therapeutics.

GRK5 - A Functional Bridge Between Cardiovascular and Neurodegenerative Disorders

Complex aging-triggered disorders are multifactorial programs that comprise a myriad of alterations in interconnected protein networks over a broad range of tissues. It is evident that rather than being randomly organized events, pathophysiologies that possess a strong aging component such as cardiovascular diseases (hypertensions, atherosclerosis, and vascular stiffening) and neurodegenerative conditions (dementia, Alzheimer's disease, mild cognitive impairment, Parkinson's disease), in essence represent a subtly modified version of the intricate molecular programs already in place for normal aging. To control such multidimensional activities there are layers of trophic protein control across these networks mediated by so-called "keystone" proteins. We propose that these "keystones" coordinate and interconnect multiple signaling pathways to control whole somatic activities such as aging-related disease etiology. Given its ability to control multiple receptor sensitivities and its broad protein-protein interactomic nature, we propose that G protein coupled receptor kinase 5 (GRK5) represents one of these key network controllers. Considerable data has emerged, suggesting that GRK5 acts as a bridging factor, allowing signaling regulation in pathophysiological settings to control the connectivity between both the cardiovascular and neurophysiological complications of aging.

N‑terminal truncated peroxisome proliferator‑activated receptor‑γ coactivator‑1α alleviates phenylephrine‑induced mitochondrial dysfunction and decreases lipid droplet accumulation in neonatal rat cardiomyocytes

N‑terminal truncated peroxisome proliferator‑activated receptor‑γ coactivator‑1α (NT‑PGC‑1α) is an alternative splice variant of PGC‑1α. NT‑PGC‑1α exhibits stronger anti‑obesity effects in adipose tissue than PGC‑1α; however, NT‑PGC‑1α has not yet been investigated in neonatal rat cardiomyocytes (NRCMs). The present study aimed to investigate the role of NT‑PGC‑1α in mitochondrial fatty acid metabolism and its possible regulatory mechanism in NRCMs. NRCMs were exposed to phenylephrine (PE) or angiotensin II (Ang II) to induce cardiac hypertrophy. Following this, NRCMs were infected with adenovirus expressing NT‑PGC‑1α, and adenosine 5'‑triphsophate (ATP) levels, reactive oxygen species (ROS) generation and mitochondrial membrane potential were subsequently detected. In addition, western blotting, lipid droplet staining and oxygen consumption assays were performed to examine the function of NT‑PGC‑1α in fatty acid metabolism. NT‑PGC‑1α was demonstrated to be primarily expressed in the cytoplasm, which differed from full‑length PGC‑1α, which was predominantly expressed in the nucleus. NT‑PGC‑1α overexpression alleviated mitochondrial function impairment, including ATP generation, ROS production and mitochondrial membrane potential integrity. Furthermore, NT‑PGC‑1α overexpression alleviated the PE‑induced suppression of fatty acid metabolism‑associated protein expression, increased extracellular oxygen consumption and decreased lipid droplet accumulation in NRCMs. Taken together, the present study demonstrated that NT‑PGC‑1α alleviated PE‑induced mitochondrial impairment and decreased lipid droplet accumulation in NRCMs, indicating that NT‑PGC‑1α may have ameliorated mitochondrial energy defects in NRCMs, and may be considered as a potential target for the treatment of heart failure.

Proinflammatory Cytokines Mediate GPCR Dysfunction

Proinflammatory reaction by the body occurs acutely in response to injury that is considered primarily beneficial. However, sustained proinflammatory cytokines observed with chronic pathologies such as metabolic syndrome, cancer, and arthritis are detrimental and in many cases is a major cardiovascular risk factor. Proinflammatory cytokines such as interleukin-1, interleukin-6, and tumor necrosis factor α (TNFα) have long been implicated in cardiovascular risk and considered to be a major underlying cause for heart failure (HF). The failure of the anti-TNFα therapy for HF indicates our elusive understanding on the dichotomous role of proinflammatory cytokines on acutely beneficial effects versus long-term deleterious effects. Despite these well-described observations, less is known about the mechanistic underpinnings of proinflammatory cytokines especially TNFα in pathogenesis of HF. Increasing evidence suggests the existence of an active cross-talk between the TNFα receptor signaling and G-protein-coupled receptors such as β-adrenergic receptor (βAR). Given that βARs are the key regulators of cardiac function, the review will discuss the current state of understanding on the role of proinflammatory cytokine TNFα in regulating βAR function.

G Protein-Coupled Receptor Kinases in the Inflammatory Response and Signaling

G protein-coupled receptor kinases (GRKs) are serine/threonine kinases that regulate a large and diverse class of G protein-coupled receptors (GPCRs). Through GRK phosphorylation and β-arrestin recruitment, GPCRs are desensitized and their signal terminated. Recent work on these kinases has expanded their role from canonical GPCR regulation to include noncanonical regulation of non-GPCR and nonreceptor substrates through phosphorylation as well as via scaffolding functions. Owing to these and other regulatory roles, GRKs have been shown to play a critical role in the outcome of a variety of physiological and pathophysiological processes including chemotaxis, signaling, migration, inflammatory gene expression, etc. This diverse set of functions for these proteins makes them popular targets for therapeutics. Role for these kinases in inflammation and inflammatory disease is an evolving area of research currently pursued in many laboratories. In this review, we describe the current state of knowledge on various GRKs pertaining to their role in inflammation and inflammatory diseases.

Secreted miR-27a Induced by Cyclic Stretch Modulates the Proliferation of Endothelial Cells in Hypertension via GRK6

Abnormal proliferation of endothelial cells (ECs) is important in vascular remodeling during hypertension, but the mechanisms are still unclear. In hypertensive rats caused by abdominal aortic coarctation, the expression of G-protein-coupled receptor kinase 6 (GRK6) in ECs at common carotid artery was repressed in vivo, and EC proliferation was increased. 15% cyclic stretch in vitro, which mimics the pathologically increased stretch in hypertension, repressed EC GRK6 expression via paracrine control by vascular smooth muscle cells (VSMCs). Furthermore, VSMC-derived microparticles (VSMC-MPs) were detected in the conditioned medium from VSMCs and in artery. VSMC-MPs from cells exposed to 15% cyclic stretch decreased GRK6 expression and increased EC proliferation. miR-27a was detected in VSMC-MPs and was upregulated by 15% cyclic stretch. miR-27a was transferred from VSMCs to ECs via VSMC-MPs and directly targeted on GRK6. Finally, a multi-point injection of antagomiR-27a around carotid artery decreased miR-27a expression in vivo, induced GRK6 expression, and reversed the abnormal EC proliferation. Pathologically elevated cyclic stretch increased the secretion of miR-27a, which was transferred from VSMCs to ECs via the VSMC-MPs, subsequently targeted GRK6, and induced EC proliferation. Locally decreasing miR-27a could be a novel therapeutic approach to attenuate the abnormal EC proliferation in hypertension.

A Brief Introduction into the Renin-Angiotensin-Aldosterone System: New and Old Techniques

The renin-angiotensin-aldosterone system (RAAS) is a complex system of enzymes, receptors, and peptides that help to control blood pressure and fluid homeostasis. Techniques in studying the RAAS can be difficult due to such factors as peptide/enzyme stability and receptor localization. This paper gives a brief account of the different components of the RAAS and current methods in measuring each component. There is also a discussion of different methods in measuring stem and immune cells by flow cytometry, hypertension, atherosclerosis, oxidative stress, energy balance, and other RAAS-activated phenotypes. While studies on the RAAS have been performed for over 100 years, new techniques have allowed scientists to come up with new insights into this system. These techniques are detailed in this Methods in Molecular Biology Series and give students new to studying the RAAS the proper controls and technical details needed to perform each procedure.

Molecular genetics of essential hypertension

Hypertension is a major public health problem in the developing as well as in developed countries due to its high prevalence and its association with coronary heart disease, renal disease, stroke, peripheral vascular disease, and related disorders. Essential hypertension (EH) is the most common diagnosis in this disease, suggesting that a monocausal etiology has not been identified. However, a number of risk factors associated with EH have also been identified such as age, sex, demographic, environmental, genetic, and vascular factors. Recent advances in molecular biological research had achieved clarifying the molecular basis of Mendelian hypertensive disorders. Molecular genetic studies have now identified mutations in several genes that cause Mendelian forms of hypertension in humans. However, none of the single genetic variants has emerged from linkage or association analyses as consistently related to the blood pressure level in every sample and in all populations. Besides, a number of polymorphisms in candidate genes have been associated with differences in blood pressure. The most prominent candidate has been the polymorphisms in the renin-angiotensin-aldosterone system. In total, EH is likely to be a polygenic disorder that results from inheritance of a number of susceptibility genes and involves multiple environmental determinants. These determinants complicate the study of blood pressure variations in the general population. The complex nature of the hypertension phenotype makes large-scale studies indispensable, when screening of familial and genetic factors was intended. In this review, recent genetic studies exploring the molecular basis of EH, including different molecular pathways, are highlighted.

"Canonical and non-canonical actions of GRK5 in the heart"

As the average world-wide lifespan continues to increase, heart failure (HF) has dramatically increased in incidence leading to the highest degree of mortality and morbidity of any disease presently studied. G protein-coupled receptors (GPCRs) play a prominent role in regulation of cardiovascular function. GPCRs are effectively "turned off" by GPCR kinases (GRKs) in a process known as "desensitization". GRKs 2 and 5 are highly expressed in the heart, and known to be upregulated in HF. Over the last 20years, the role of GRK2 in HF has been widely studied. However, until recently, the role of GRK5 in cardiac pathophysiology had yet to be elucidated. In the present review, we will focus on GRK5's role in the myocardium in normal physiology, and its apparent critical role in the progression of HF. Further, we will also present potential therapeutic strategies (i.e. small molecule inhibition, gene therapy) that may have potential in combating the deleterious effects of GRK5 in HF.

Ubiquitin-specific Protease 20 Regulates the Reciprocal Functions of β-Arrestin2 in Toll-like Receptor 4-promoted Nuclear Factor κB (NFκB) Activation

Toll-like receptor 4 (TLR4) promotes vascular inflammatory disorders such as neointimal hyperplasia and atherosclerosis. TLR4 triggers NFκB signaling through the ubiquitin ligase TRAF6 (tumor necrosis factor receptor-associated factor 6). TRAF6 activity can be impeded by deubiquitinating enzymes like ubiquitin-specific protease 20 (USP20), which can reverse TRAF6 autoubiquitination, and by association with the multifunctional adaptor protein β-arrestin2. Although β-arrestin2 effects on TRAF6 suggest an anti-inflammatory role, physiologic β-arrestin2 promotes inflammation in atherosclerosis and neointimal hyperplasia. We hypothesized that anti- and proinflammatory dimensions of β-arrestin2 activity could be dictated by β-arrestin2's ubiquitination status, which has been linked with its ability to scaffold and localize activated ERK1/2 to signalosomes. With purified proteins and in intact cells, our protein interaction studies showed that TRAF6/USP20 association and subsequent USP20-mediated TRAF6 deubiquitination were β-arrestin2-dependent. Generation of transgenic mice with smooth muscle cell-specific expression of either USP20 or its catalytically inactive mutant revealed anti-inflammatory effects of USP20in vivoandin vitro Carotid endothelial denudation showed that antagonizing smooth muscle cell USP20 activity increased NFκB activation and neointimal hyperplasia. We found that β-arrestin2 ubiquitination was promoted by TLR4 and reversed by USP20. The association of USP20 with β-arrestin2 was augmented when β-arrestin2 ubiquitination was prevented and reduced when β-arrestin2 ubiquitination was rendered constitutive. Constitutive β-arrestin2 ubiquitination also augmented NFκB activation. We infer that pro- and anti-inflammatory activities of β-arrestin2 are determined by β-arrestin2 ubiquitination and that changes in USP20 expression and/or activity can therefore regulate inflammatory responses, at least in part, by defining the ubiquitination status of β-arrestin2.

G-protein-coupled receptor kinases in inflammation and disease

G-protein-coupled receptor kinases (GRKs) are serine/threonine protein kinases originally discovered for their role in G-protein-coupled receptor (GPCR) phosphorylation. Recent studies have demonstrated a much broader function for this kinase family including phosphorylation of cytosolic substrates involved in cell signaling pathways stimulated by GPCRs, as well as by non-GPCRs. In addition, GRKs modulate signaling via phosphorylation-independent functions. Because of these various biochemical functions, GRKs have been shown to affect critical physiological and pathophysiological processes, and thus are considered as drug targets in diseases such as heart failure. Role of GRKs in inflammation and inflammatory diseases is an evolving area of research and several studies including work from our lab in the recent years have demonstrated critical role of GRKs in the immune system. In this review, we discuss the classical and the newly emerging functions of GRKs in the immune system and their role in inflammation and disease processes.

Sex differences in the biomechanics and contractility of intramural coronary arteries in angiotensin II-induced hypertension

BACKGROUND

It is well known that sex differences occur in both the pathogenesis and therapy of hypertension. A deeper understanding of the underlying processes may be helpful when planning a personalized therapeutic strategy.

OBJECTIVE

In laboratory animal experiments, we studied the early mechanisms of vascular adaptation of the intramural small coronary arteries that play a fundamental role in the blood supply of the heart.

METHODS

In our study, an osmotic minipump was implanted into 10 male and 10 female Sprague-Dawley rats. The pump remained in situ for 4 weeks, infusing a dose of 100 ng/kg/min angiotensin II acetate. Four weeks later, the animals were killed, and the intramural coronary arteries from the left coronary branch, which are fundamentally responsible for the blood supply of the heart, were prepared. The pharmacologic reactivity and biomechanical properties of the prepared segments were studied in a tissue bath.

RESULTS

The relative heart mass and vessel wall thickness were greater in females than males (0.387 [0.009] g/100 g vs 0.306 [0.006] g/100 g body weight; 41.9 [4.09] μm vs 33.45 [3.37] μm on 50 mm Hg). The vessel tone and vasoconstriction in response to thromboxane agonists were, however, significantly more pronounced in males. The extent of relaxation in response to bradykinin was also greater in females. Although we observed inward eutrophic remodeling in females, an increase in wall stress and elastic modulus dominated in males.

CONCLUSION

The early steps of angiotensin II-dependent hypertension evoked very different adaptation mechanisms in males and females.

MARCH2 promotes endocytosis and lysosomal sorting of carvedilol-bound β(2)-adrenergic receptors

The β₂-adrenergic receptor antagonist carvedilol recruits MARCH2, a unique E3 ubiquitin ligase, to promote receptor endocytosis and lysosomal trafficking.

Lysosomal degradation of ubiquitinated β₂-adrenergic receptors (β₂ARs) serves as a major mechanism of long-term desensitization in response to prolonged agonist stimulation. Surprisingly, the βAR antagonist carvedilol also induced ubiquitination and lysosomal trafficking of both endogenously expressed β₂ARs in vascular smooth muscle cells (VSMCs) and overexpressed Flag-β₂ARs in HEK-293 cells. Carvedilol prevented β₂AR recycling, blocked recruitment of Nedd4 E3 ligase, and promoted the dissociation of the deubiquitinases USP20 and USP33. Using proteomics approaches (liquid chromatography–tandem mass spectrometry), we identified that the E3 ligase MARCH2 interacted with carvedilol-bound β₂AR. The association of MARCH2 with internalized β₂ARs was stabilized by carvedilol and did not involve β-arrestin. Small interfering RNA–mediated down-regulation of MARCH2 ablated carvedilol-induced ubiquitination, endocytosis, and degradation of endogenous β₂ARs in VSMCs. These findings strongly suggest that specific ligands recruit distinct E3 ligase machineries to activated cell surface receptors and direct their intracellular itinerary. In response to β blocker therapy with carvedilol, MARCH2 E3 ligase activity regulates cell surface β₂AR expression and, consequently, its signaling.

G protein-coupled receptor kinases in cardiovascular disease: why "where" matters

Cardiac function is mainly controlled by β-adrenergic receptors (β-ARs), members of the G protein-coupled receptor (GPCR) family. GPCR signaling and expression are tightly controlled by G protein-coupled receptor kinases (GRKs), which induce GPCR internalization and signal termination through phosphorylation. Reduced β-AR density and activity associated with elevated cardiac GRK expression and activity have been described in various cardiovascular diseases. Moreover, alterations in extracardiac GRKs have been observed in blood vessels, adrenal glands, kidneys, and fat cells. The broad tissue distribution of GPCRs and GRKs suggests that a keen appreciation of integrative physiology may drive future therapeutic development. In this review, we provide a brief summary of GRK isoforms, subcellular localization, and interacting partners that impinge directly or indirectly on the cardiovascular system. We also discuss GRK/GPCR interactions and their implications in cardiovascular pathophysiology.

G protein receptor kinase 4 polymorphisms: β-blocker pharmacogenetics and treatment-related outcomes in hypertension

G protein-coupled receptor kinases (GRKs) are important regulatory proteins for many G protein-coupled receptors, but little is known about GRK4 pharmacogenetics. We hypothesized that 3 nonsynonymous GRK4 single-nucleotide polymorphisms, R65L (rs2960306), A142V (rs1024323), and A486V (rs1801058), would be associated with blood pressure response to atenolol, but not hydrochlorothiazide, and would be associated with long-term cardiovascular outcomes (all-cause death, nonfatal myocardial infarction, nonfatal stroke) in participants treated with an atenolol-based versus verapamil-SR-based antihypertensive strategy. GRK4 single-nucleotide polymorphisms were genotyped in 768 hypertensive participants from the Pharmacogenomic Evaluation of Antihypertensive Responses (PEAR) trial. In whites and blacks, increasing copies of the variant 65L-142V haplotype were associated with significantly reduced atenolol-induced diastolic blood pressure lowering (-9.1±6.8 versus -6.8±7.1 versus -5.3±6.4 mm Hg in participants with 0, 1, and 2 copies of 65L-142V, respectively; P=0.0088). One thousand four hundred sixty participants with hypertension and coronary artery disease from the INternational VErapamil SR/Trandolapril STudy (INVEST) were genotyped, and variant alleles of all 3 GRK4 single-nucleotide polymorphisms were associated with increased risk for adverse cardiovascular outcomes in an additive fashion, with 486V homozygotes reaching statistical significance (odds ratio, 2.29 [1.48-3.55]; P=0.0002). These effects on adverse cardiovascular outcomes were independent of antihypertensive treatment. These results suggest that the presence of GRK4 variant alleles may be important determinants of blood pressure response to atenolol and risk for adverse cardiovascular events. The associations with GRK4 variant alleles were stronger in patients who were also ADRB1 389R homozygotes, suggesting a potential interaction between these 2 genes.

Abnormalities in renal dopamine signaling and hypertension: the role of GRK4

PURPOSE OF REVIEW

This review will highlight the recent findings concerning the role of the intrarenal dopaminergic system in hypertension, especially the role of alterations in G-protein receptor kinase 4 (GRK4) activity.

RECENT FINDINGS

Recent studies highlight the importance of the intrarenal dopaminergic system in blood pressure regulation and how defects in dopamine signaling are involved in the development of hypertension. There are recent experimental models that definitively demonstrate that abnormalities in intrarenal dopamine production or receptor signaling can predispose to salt-sensitive hypertension and a dysregulated renin-angiotensin system. Furthermore, studies in experimental animal models and in humans with salt-sensitive hypertension implicate abnormalities in dopamine receptor regulation because of receptor desensitization resulting from increased GRK4 activity. Functional polymorphisms that predispose to increased basal GRK4 activity both decrease dopamine receptor activity and increase angiotensin II AT1 receptor activity and are associated with essential hypertension in a number of different human cohorts.

SUMMARY

The ongoing elucidation of this important regulatory pathway further emphasizes the importance of the kidney in maintenance of blood pressure control and may help to delineate the underlying mechanisms predisposing individuals or populations to increased risk for development of hypertension.

Age-related β-adrenergic receptor-mediated vasorelaxation is changed by altering G protein receptor kinase 2 expression

Beta-adrenergic receptor- (β-AR) mediated vasorelaxation declines with age. This change is likely related to receptor desensitization, rather than down regulation. One kinase responsible for desensitization is G protein receptor kinase 2 (GRK2). We have shown that GRK expression and activity increases with age in Fischer 344 rat aorta. In this study we validated that carotid arteries have similar age-related changes in the β-AR signaling axis as aorta. This finding allowed use of in vivo infection and delivery of two adenovirus vectors to carotid arteries of 2-month-old (2M) and 12-month-old (12M) male Fischer 344 rats. Adeno-GRK2 was used to overexpress GRK2, and adeno-β-ARK-ct was used to inhibit GRK2 function. Following a five-day infection, vessels were collected and ex vivo tissue bath was used to evaluate vasoreactivity. We used KCl contracted segments, and determined that overexpression of GRK2 significantly impaired isoproterenol (ISO)-mediated vasorelaxation in both age groups. Maximum relaxation (MAX) to ISO in vessels from 2M decreased from 44% to 21%. MAX to ISO in vessels from 12M decreased from 12% to 6%. Sensitivity (ED₅₀) in vessels from 2M and 12M was also impaired 57%, and 30% respectively. We also determined that expression of adeno-β-ARK-ct significantly improved ISO-mediated vasorelaxation in both age groups. MAX in vessels from 2M increased from 44% to 58%. MAX in vessels from 12M increased from 15% to 69%. ED₅₀ in vessels from 2M and 12M was also improved 46%, and 50% respectively. These findings further implicate age-related increases in GRK2 expression as an important regulator of the age-related decline in β-AR-mediated vasorelaxation.

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.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Renal actions of RGS2 control blood pressure

G protein-coupled receptors (GPCRs) have key roles in cardiovascular regulation and are important targets for the treatment of hypertension. GTPase-activating proteins, such as RGS2, modulate downstream signaling by GPCRs. RGS2 displays regulatory selectivity for the Gαq subclass of G proteins, and mice lacking RGS2 develop hypertension through incompletely understood mechanisms. Using total body RGS2-deficient mice, we used a kidney crosstransplantation strategy to examine separately the contributions of RGS2 actions in the kidney from those in extrarenal tissues with regard to BP regulation. Loss of renal RGS2 was sufficient to cause hypertension, whereas the absence of RGS2 from all extrarenal tissues including the peripheral vasculature did not significantly alter BP. Accordingly, these results suggest that RGS2 acts within the kidney to modulate BP and prevent hypertension. These data support a critical role for the renal epithelium and/or vasculature as the final determinants of the intra-arterial pressure in hypertension.

Is overall blockade superior to selective blockade of adrenergic receptor subtypes in suppressing left ventricular remodeling in spontaneously hypertensive rats?

To test the hypothesis that nonselective blockade of adrenergic receptor (AR) subtypes is superior to selective blockade of AR subtypes in suppressing left ventricular (LV) remodeling induced by hypertension. Sixty-four spontaneously hypertensive rats (SHR) were randomly divided into four groups: bisoprolol-treated, propranolol-treated, carvedilol-treated and no treatment groups (n=16, each). Sixteen Wistar-Kyoto (WKY) rats served as a control group. Echocardiography and cardiac catheterization were carried out to record the mitral flow velocity ratio of E wave to A wave (E/A), LV mass index (LVMI), maximal rising (dp/dt(max)) and falling (-dp/dt(max)) rate of the LV pressure and LV relaxation time constant (τ). The mRNA and protein expression levels of AR, protein kinase(PK) and G-protein subtypes, intracellular free calcium (Ca) concentration and cardiocyte apoptoisis rate were determined. Three drug-treated groups showed higher velocity ratio of E wave to A wave (E/A) and -dp/dt(max) and lower systolic blood pressure (SBP), LVMI, τ, apoptosis rate and intracellular free Ca(2+) concentration than the no treatment group. The mRNA expression levels of AR-α(1B) in the carvedilol group were significantly lower than the other two drug-treated groups. The mRNA expression levels of AR-β(1), AR-β(2) and Gsα were significantly higher in the three drug-treated groups than in the no treatment group, with the expression levels of AR-β(2) being the highest in the carvedilol-treated group. The protein expression levels of PKA and PKC subtype α and δ were lower in the three drug-treated groups than in the no treatment group. Overall blockade of AR subtypes is not superior to selective blockade of AR subtypes in suppressing LV remodeling in SHR. Although carvedilol is the most effective in attenuating cardiocyte apoptosis, normalizing AR-α(1B) and Gsα expression and increasing AR-β(2) expression.

Evidence for sex differences in cardiovascular aging and adaptive responses to physical activity

There are considerable data addressing sex-related differences in cardiovascular system aging and disease risk/progression. Sex differences in cardiovascular aging are evident during resting conditions, exercise, and other acute physiological challenges (e.g., orthostasis). In conjunction with these sex-related differences-or perhaps even as an underlying cause-the impact of cardiorespiratory fitness and/or physical activity on the aging cardiovascular system also appears to be sex-specific. Potential mechanisms contributing to sex-related differences in cardiovascular aging and adaptability include changes in sex hormones with age as well as sex differences in baseline fitness and the dose of activity needed to elicit cardiovascular adaptations. The purpose of the present paper is thus to review the primary research regarding sex-specific plasticity of the cardiovascular system to fitness and physical activity in older adults. Specifically, the paper will (1) briefly review known sex differences in cardiovascular aging, (2) detail emerging evidence regarding observed cardiovascular outcomes in investigations of exercise and physical activity in older men versus women, (3) explore mechanisms underlying the differing adaptations to exercise and habitual activity in men versus women, and (4) discuss implications of these findings with respect to chronic disease risk and exercise prescription.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.

Regulation of GPCR signaling in hypertension

Hypertension represents a complex, multifactorial disease and contributes to the major causes of morbidity and mortality in industrialized countries: ischemic and hypertensive heart disease, stroke, peripheral atherosclerosis and renal failure. Current pharmacological therapy of essential hypertension focuses on the regulation of vascular resistance by inhibition of hormones such as catecholamines and angiotensin II, blocking them from receptor activation. Interaction of G-protein coupled receptor kinases (GRKs) and regulator of G-protein signaling (RGS) proteins with activated G-protein coupled receptors (GPCRs) effect the phosphorylation state of the receptor leading to desensitization and can profoundly impair signaling. Defects in GPCR regulation via these modulators have severe consequences affecting GPCR-stimulated biological responses in pathological situations such as hypertension, since they fine-tune and balance the major transmitters of vessel constriction versus dilatation, thus representing valuable new targets for anti-hypertensive therapeutic strategies. Elevated levels of GRKs are associated with human hypertensive disease and are relevant modulators of blood pressure in animal models of hypertension. This implies therapeutic perspective in a disease that has a prevalence of 65million in the United States while being directly correlated with occurrence of major adverse cardiac and vascular events. Therefore, therapeutic approaches using the inhibition of GRKs to regulate GPCRs are intriguing novel targets for treatment of hypertension and heart failure.

G-protein-coupled receptor kinases in cardiovascular conditions: focus on G-protein-coupled receptor kinase 2, a gain in translational medicine

With increasing knowledge of the regulatory mechanisms of G-protein-coupled receptor signaling in heart physiology, many studies have focused on the role of this system in cardiovascular disease. In recent years, scientists have moved their attention from the receptors to their regulatory proteins: the G-protein-coupled receptor kinases. This class of protein is indispensable for terminating signaling of G-protein-coupled receptors through receptor desensitization and downregulation. This article attempts to assemble the currently available information regarding G-protein-coupled receptor kinases and their role in cardiovascular disease and, in particular, the potential employment of G-protein-coupled receptor kinase 2 as biomarker of cardiac dysfunction.

G protein-coupled receptor kinase 2 expression and activity are associated with blood pressure in black Americans

Hypertension occurs with higher prevalence and morbidity in black Americans compared with other groups. Alterations in the signal transduction pathways of 7-transmembrane spanning receptors are found in hypertensive patients. G protein-coupled receptor kinases (GRKs) play an important role in regulating this receptor signaling. The 2 most abundantly expressed GRKs in the cardiovascular system are GRK2 and GRK5, and each has unique substrates. Understanding changes in expression may give us insight into activated receptors in the pathophysiological progression of hypertension. In heart failure and white hypertensives, increased GRK2 expression arises because of neurohormonal stimulation of particular receptors. GRK2 subsequently desensitizes specific receptors, including beta-adrenergic receptors. In blood pressure control, beta-adrenergic receptor desensitization could lead to increased blood pressure. GRK2 and GRK5 mRNA were evaluated in lymphocytes of black Americans via quantitative real-time PCR. GRK2 mRNA expression directly correlated with systolic blood pressure and norepinephrine levels. GRK2 was elevated >30% among those with systolic blood pressure > or =130 mm Hg. No significant correlation between GRK5 mRNA expression and blood pressure or catecholamines was observed. Diabetic status, age, sex, and body mass index were also compared with GRK2 expression using univariate and multivariate analyses. GRK2 protein expression was elevated 2-fold in subjects with higher blood pressure, and GRK activity was increased >40%. Our data suggest that GRK2, but not GRK5, is correlated with increasing blood pressure in black Americans. Understanding the receptors stimulated by increased neurohormonal activation may give insight into the pathophysiology of hypertension in this at-risk population.

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.

Inhibition of angiotensin II Gq signaling augments beta-adrenergic receptor mediated effects in a renal artery stenosis model of high blood pressure

Chronic ventricular pressure overload states, such as hypertension, and elevated levels of neurohormones (norepinephrine, angiotensin II, endothelin-1) initiate cardiac hypertrophy and dysfunction and share the property of being able to bind to Gq-coupled 7-transmembrane receptors. The goal of the current study was to determine the role of endogenous cardiac myocyte Gq signaling and its role in cardiac hypertrophy and dysfunction during high blood pressure (BP). We induced renal artery stenosis for 8 weeks in control mice and mice expressing a peptide inhibitor of Gq signaling (GqI) using a 2 kidney, 1 clip renal artery stenosis model. 8 weeks following chronic high BP, control mice had cardiac hypertrophy and depressed function. Inhibition of cardiomyocyte Gq signaling did not reverse cardiac hypertrophy but attenuated increases in a profile of cardiac profibrotic genes and genes associated with remodeling. Inhibition of Gq signaling also attenuated the loss of cardiac function. We determined that Gq signaling downstream of angiotensin II receptor stimulation negatively impacted beta-adrenergic receptor (AR) responses and inhibition of Gq signaling was sufficient to restore betaAR-mediated responses. Therefore, in this study we found that Gq signaling negatively impacts cardiac function during high BP. Specifically, we found that inhibition of AT1-Gq signaling augmented betaAR mediated effects in a renal artery stenosis model of hypertension. These observations may underlie additional, beneficial effects of angiotensinogen converting enzyme (ACE) inhibitors and angiotensin receptor antagonists observed during times of hemodynamic stress.

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.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.

Rgs5 targeting leads to chronic low blood pressure and a lean body habitus

RGS5 is a potent GTPase-activating protein for G(ialpha) and G(qalpha) that is expressed strongly in pericytes and is present in vascular smooth muscle cells. To study the role of RGS5 in blood vessel physiology, we generated Rgs5-deficient mice. The Rgs5(-/-) mice developed normally, without obvious defects in cardiovascular development or function. Surprisingly, Rgs5(-/-) mice had persistently low blood pressure, lower in female mice than in male mice, without concomitant cardiac dysfunction, and a lean body habitus. The examination of the major blood vessels revealed that the aortas of Rgs5(-/-) mice were dilated compared to those of control mice, without altered wall thickness. Isolated aortic smooth muscle cells from the Rgs5(-/-) mice exhibited exaggerated levels of phosphorylation of vasodilator-stimulated phosphoprotein and extracellular signal-regulated kinase in response to stimulation with either sodium nitroprusside or sphingosine 1-phosphate. The results of this study, along with those of previous studies demonstrating that RGS5 stability is under the control of nitric oxide via the N-end rule pathway, suggest that RGS5 may balance vascular tone by attenuating vasodilatory signaling in vivo in opposition to RGS2, another RGS (regulator of G protein signaling) family member known to inhibit G protein-coupled receptor-mediated vasoconstrictor signaling. Blocking the function or the expression of RGS5 may provide an alternative approach to treat hypertension.

Enhanced sterol response element-binding protein in postintervention restenotic blood vessels plays an important role in vascular smooth muscle proliferation

Postintervention restenosis (PIRS) after balloon angioplasty or stent implantation is a limitation for these interventional procedures even with the advent of new drug-eluting stents. Sterol regulatory element-binding proteins (SREBP) are transcription factors governing cellular lipid biosynthesis and thus critical in the regulation of the lipid-rich cell membranes. PIRS following injury results partially from newly proliferating cells expressing vascular smooth muscle cell (VSMC) markers. Platelet-derived growth factor (PDGF), lysophosphatidic acid (LPA) and alpha(1)-adrenergic receptor stimulation are well recognized diverse mitogens for VSMC activation in PIRS. We examined whether PDGF, LPA and alpha(1)-adrenergic receptor stimulation with phenylephrine (PE) regulate SREBP expression and subsequently, VSMC proliferation. Our results show that PDGF, LPA and PE upregulate SREBP-1 in a time- and dose-dependent manner. PDGF, LPA and PE-mediated proliferation is dependent on SREBP since inhibition of SREBP expression using targeted knockdown of the SREBP precursor SREBP activating protein (SCAP) by siRNA led to an attenuation of SREBP expression and decreased PDGF, LPA and PE induced proliferation. In two different in vivo PIRS models we found that SREBP-1 was enhanced in the injured blood vessel wall, especially within the neointima and co-localized with alpha-smooth muscle actin positive cells. Thus, SREBP is enhanced in the vessel wall following PIRS and is important in the regulation of pro-hyperplasia molecular signaling. SREBP inhibition may be a powerful tool to limit PIRS.

Vascular smooth muscle Gqsignaling is involved in high blood pressure in both induced renal and genetic vascular smooth muscle-derived models of hypertension

More than 30% of the US population has high blood pressure (BP), and less than a third of people treated for hypertension have it controlled. In addition, the etiology of most high BP is not known. Having a better understanding of the mechanisms underlying hypertension could potentially increase the effectiveness of treatment. Because Gqsignaling mediates vasoconstriction and vascular function can cause BP abnormalities, we were interested in determining the role of vascular smooth muscle (VSM) Gqsignaling in two divergent models of hypertension: a renovascular model of hypertension through renal artery stenosis and a genetic model of hypertension using mice with VSM-derived high BP. Inhibition of VSM Gqsignaling attenuated BP increases induced by renal artery stenosis to a similar extent as losartan, an ANG II receptor blocker and current antihypertensive therapy. Inhibition of Gqsignaling also attenuated high BP in our genetic VSM-derived hypertensive model. In contrast, BP remained elevated 25% following treatment with losartan, and prazosin, an α1-adrenergic receptor antagonist, only decreased BP by 35%. Inhibition of Gqsignaling attenuated VSM reactivity to ANG II and resulted in a 2.4-fold rightward shift in EC50. We also determined that inhibition of Gqsignaling was able to reverse VSM hypertrophy in the genetic VSM-derived hypertensive model. These results suggest that Gqsignaling is an important signaling pathway in two divergent models of hypertension and, perhaps, optimization of antihypertensive therapy could occur with the identification of particular Gq-coupled receptors involved.

Physiological Roles of G Protein–Coupled Receptor Kinases and Arrestins

Heterotrimeric G protein-coupled receptors (GPCRs) are found on the surface of all cells of multicellular organisms and are major mediators of intercellular communication. More than 800 distinct GPCRs are present in the human genome, and individual receptor subtypes respond to hormones, neurotransmitters, chemokines, odorants, or tastants. GPCRs represent the most widely targeted pharmacological protein class. Because drugs that target GPCRs often engage receptor regulatory mechanisms that limit drug effectiveness, particularly in chronic treatment, there is great interest in understanding how GPCRs are regulated, as a basis for designing therapeutic drugs that evade this regulation. The major GPCR regulatory pathway involves phosphorylation of activated receptors by G protein-coupled receptor kinases (GRKs), followed by binding of arrestin proteins, which prevent receptors from activating downstream heterotrimeric G protein pathways while allowing activation of arrestin-dependent signaling pathways. Although the general mechanisms of GRK-arrestin regulation have been well explored in model cell systems and with purified proteins, much less is known about the role of GRK-arrestin regulation of receptors in physiological and pathophysiological settings. This review focuses on the physiological functions and potential pathophysiological roles of GRKs and arrestins in human disorders as well as on recent studies using knockout and transgenic mice to explore the role of GRK-arrestin regulation of GPCRs in vivo.

Genetics of arterial hypertension and hypotension

Human hypertension affects affects more than 20% of the adult population in industrialized countries, and it is implicated in millions of deaths worldwide each year from stroke, heart failure and ischemic heart disease. Available evidence suggests a major genetic impact on blood pressure regulation. Studies in monogenic hypertension revealed that renal salt and volume regulation systems are predominantly involved in the genesis of these disorders. Mutations here affect the synthesis of mineralocorticoids, the function of the mineralocorticoid receptor, epithelial sodium channels and their regulation by a new class of kinases, termed WNK kinases. It has been learned from monogenic hypotension that almost all ion transporters involved in the renal uptake of Na(+) have a major impact on blood pressure regulation. For essential hypertension as a complex disease, many candidate genes have been analysed. These include components of the renin-angiotensin-aldosterone system, adducin, beta-adrenoceptors, G protein subunits, regulators of G protein signalling (RGS) proteins, Rho kinases and G protein receptor kinases. At present, the individual impact of common polymorphisms in these genes on the observed blood pressure variation, on risk for stroke and as predictors of antihypertensive responses remains small and clinically irrelevant. Nevertheless, these studies have greatly augmented our knowledge on the regulation of renal functions, cellular signal transduction and the integration of both. Together, this provides the basis for the identification of novel drug targets and, hopefully, innovative antihypertensive drugs.

Mechanisms of disease: the role of GRK4 in the etiology of essential hypertension and salt sensitivity

Hypertension and salt sensitivity of blood pressure are two conditions the etiologies of which are still elusive because of the complex influences of genes, environment, and behavior. Recent understanding of the molecular mechanisms that govern sodium homeostasis is shedding new light on how genes, their protein products, and interacting metabolic pathways contribute to disease. Sodium transport is increased in the proximal tubule and thick ascending limb of Henle of the kidney in human essential hypertension. This Review focuses on the counter-regulation between the dopaminergic and renin-angiotensin systems in the renal proximal tubule, which is the site of about 70% of total renal sodium reabsorption. The inhibitory effect of dopamine is most evident under conditions of moderate sodium excess, whereas the stimulatory effect of angiotensin II is most evident under conditions of sodium deficit. Dopamine and angiotensin II exert their actions via G protein-coupled receptors, which are in turn regulated by G protein-coupled receptor kinases (GRKs). Polymorphisms that lead to aberrant action of GRKs cause a number of conditions, including hypertension and salt sensitivity. Polymorphisms in one particular member of this family-GRK4-have been shown to cause hyperphosphorylation, desensitization and internalization of a member of the dopamine receptor family, the dopamine 1 receptor, while increasing the expression of a key receptor of the renin-angiotensin system, the angiotensin II type 1 receptor. Novel diagnostic and therapeutic approaches for identifying at-risk subjects, followed by selective treatment of hypertension and salt sensitivity, might center on restoring normal receptor function through blocking the effects of GRK4 polymorphisms.