Two isoforms of G protein-coupled receptor kinase 4 identified by molecular cloning

Back-translating GWAS findings to animal models reveals a role for Hgfac and Slc39a8 in alcohol and nicotine consumption

Alcohol and tobacco are the most commonly used addictive substances, with high comorbidity rates between alcohol use disorder and tobacco use disorder. Risk for alcohol and nicotine addiction is highly heritable, and they share common genetic factors. A GWAS in over 1 million individuals has revealed 566 genetic variants in 406 loci associated with multiple stages of alcohol and tobacco use. Three novel genes-SLC39A8, GRK4 and HGFAC-within loci associated with altered alcoholic drinks per week (ADW) or cigarettes per day (CPD) were selected to further study their role in alcohol and tobacco use disorder. The role of these genes was assessed using the two-bottle choice addiction paradigm in transgenic mice for each of the genes. We found significant decreases in chronic alcohol consumption and preference in female Hgfac knockout (KO) mice, and decreased nicotine preference in male Hgfac KO compared with wild-type (WT) mice. Additionally, male Slc39a8 hypomorph mice showed greater overall nicotine preference compared with WT mice, while no differences were detected for Grk4 KO mice in alcohol or nicotine consumption and preference in either sex. Thus, this study implicates Hgfac and Slc39a8 in alcohol and tobacco use in a sex-specific manner.

The evolving impact of g protein-coupled receptor kinases in cardiac health and disease

G protein-coupled receptors (GPCRs) are important regulators of various cellular functions via activation of intracellular signaling events. Active GPCR signaling is shut down by GPCR kinases (GRKs) and subsequent β-arrestin-mediated mechanisms including phosphorylation, internalization, and either receptor degradation or resensitization. The seven-member GRK family varies in their structural composition, cellular localization, function, and mechanism of action (see sect. II). Here, we focus our attention on GRKs in particular canonical and novel roles of the GRKs found in the cardiovascular system (see sects. III and IV). Paramount to overall cardiac function is GPCR-mediated signaling provided by the adrenergic system. Overstimulation of the adrenergic system has been highly implicated in various etiologies of cardiovascular disease including hypertension and heart failure. GRKs acting downstream of heightened adrenergic signaling appear to be key players in cardiac homeostasis and disease progression, and herein we review the current data on GRKs related to cardiac disease and discuss their potential in the development of novel therapeutic strategies in cardiac diseases including heart failure.

Characterization of G protein-coupled receptor kinase 4 and measuring its constitutive activity in vivo

G protein-coupled receptor kinase 4 (GRK4) was originally identified in the brain and was initially thought to have a limited expression pattern and functionality; however, more recent studies have found that GRK4 is expressed in multiple tissues and cell types and that it contributes to cardiovascular disease. Additionally, human GRK4 exists as four splice variants and each variant can harbor at least three functionally relevant polymorphisms. The primary role of GRK4 is to phosphorylate G protein-coupled receptors (GPCR), which leads to desensitization of the G protein signaling mechanism while simultaneously recruiting β-arrestins and initializing the internalization of the receptor. Interestingly, GRK4 has been shown to be constitutively active in some, but not all, cases. A constitutive active GRK could lead to increased β-arrestin-mediated signaling while inhibiting traditional/canonical GPCR-mediated signaling mechanisms. Therefore, it is important to determine if GRK4 is constitutively active in a system. Measuring agonist-mediated activity of GRK4 is relatively straightforward since it inhibits second messenger signaling; however, only a few studies have directly examined the constitutive activity of GRK4 which requires techniques without an agonist. Since GRK4 has significant biological effects, identifying the mechanism underlying GRK4's constitutive activity and ligand-stimulated activity becomes increasingly important. Therefore, the methods provided here are designed to aid researchers in determining if GRK4 is expressed, and if so which GRK4 species is expressed, followed by procedures to identify if GRK4 is constitutively active in its model system. Last, procedures are explained for identifying if GRK4 is involved in its system in a nonconstitutive manner. The protocols described here are designed to be accessible to a wide range of scientists, which should allow for more laboratories to examine GRK4 constitutive activity as well as agonist-mediated activity.

G protein-coupled receptor kinase 4gamma interacts with inactive Galpha(s) and Galpha13

G protein-coupled receptors (GPCRs) are regulated by multiple families of kinases including GPCR kinases (GRKs). GRK4 is constitutively active towards GPCRs, and polymorphisms of GRK4gamma are linked to hypertension. We examined, through co-immunoprecipitation, the interactions between GRK4gamma and the Galpha and Gbeta subunits of heterotrimeric G proteins. Because GRK4 has been shown to inhibit Galpha(s)-coupled GPCR signaling and lacks a PH domain, we hypothesized that GRK4gamma would interact with active Galpha(s), but not Gbeta. Surprisingly, GRK4gamma preferentially interacts with inactive Galpha(s) and Gbeta to a greater extent than active Galpha(s). GRK4gamma also interacts with inactive Galpha(13) and Gbeta. Functional studies demonstrate that wild-type GRK4gamma, but not kinase-dead GRK4gamma, ablates isoproterenol-mediated cAMP production indicating that the kinase domain is responsible for GPCR regulation. This evidence suggests that binding to inactive Galpha(s) and Gbeta may explain the constitutive activity of GRK4gamma towards Galpha(s)-coupled receptors.

Pathophysiological roles of G-protein-coupled receptor kinases

G-protein-coupled receptor kinases (GRKs) interact with the agonist-activated form of G-protein-coupled receptors (GPCRs) to effect receptor phosphorylation and to initiate profound impairment of receptor signalling, or desensitization. GPCRs form the largest family of cell surface receptors known and defects in GRK function have the potential consequence to affect GPCR-stimulated biological responses in many pathological situations. This review focuses on the physiological role of GRKs revealed by genetically modified animals but also develops the involvement of GRKs in human diseases as, Oguchi disease, heart failure, hypertension or rhumatoid arthritis. Furthermore, the regulation of GRK levels in opiate addiction, cancers, psychiatric diseases, cystic fibrosis and cardiac diseases is discussed. Both transgenic mice and human pathologies have demonstrated the importance of GRKs in the signalling pathways of rhodopsin, beta-adrenergic and dopamine-1 receptors. The modulation of GRK activity in animal models of cardiac diseases can be effective to restore cardiac function in heart failure and opens a novel therapeutic strategy in diseases with GPCR dysregulation.

Differential expression of GRK isoforms in nonmalignant and malignant human granulosa cells

Granulosa cell tumors are serious ovarian neoplasms that can occur in women of all ages. While there have been numerous attempts to understand the cause of these malignancies, the pathogenesis of granulosa cell tumors (GCTs) still remains largely unknown. G-protein coupled receptor kinases (GRKs) are important regulators of signal transduction through the process of receptor desensitization and internalization. Receptors that are regulated by GRKs are members of the large family of seven-transmembrane receptors and include the follicle stimulating hormone receptor (FSHR). In granulosa cells, the FSH signaling system is responsible for cell proliferation, differentiation, and steroidogenesis. In the studies presented, we examined GRK mRNA and protein expression in nonmalignant human granulosa cells, in KGN cells, a human GCT cell line, and in a panel of human GCT samples. The KGN tumor cells express significantly less GRK4 alpha/beta protein and higher levels of GRK2 and GRK4 gamma/delta protein as compared to nonmalignant human granulosa cells. In human GCT samples, GRK4 alpha/beta protein was detected in 3 of the 13 tumor samples, whereas gamma/delta proteins expression was detected in all samples. These findings suggest that GRK protein expression is altered in GCTs and may be involved in the pathogenesis of these tumors.

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

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

The G-protein-coupled receptor kinase GRK4 mediates homologous desensitization of metabotropic glutamate receptor 1

G-protein-coupled receptor kinases (GRKs) are involved in the regulation of many G-protein-coupled receptors. As opposed to the other GRKs, such as rhodopsin kinase (GRK1) or beta-adrenergic receptor kinase (beta ARK, GRK2), no receptor substrate for GRK4 has been so far identified. Here we show that GRK4 is expressed in cerebellar Purkinje cells, where it regulates mGlu(1) metabotropic glutamate receptors, as indicated by the following: 1) When coexpressed in heterologous cells (HEK293), mGlu(1) receptor signaling was desensitized by GRK4 in an agonist-dependent manner (homologous desensitization). 2) In transfected HEK293 and in cultured Purkinje cells, the exposure to glutamate agonists induced internalization of the receptor and redistribution of GRK4. There was a substantial colocalization of the receptor and kinase both under basal condition and after internalization. 3) Kinase activity was necessary for desensitizing mGlu(1a) receptor and agonist-dependent phosphorylation of this receptor was also documented. 4) Antisense treatment of cultured Purkinje cells, which significantly reduced the levels of GRK4 expression, induced a marked modification of the mGlu(1)-mediated functional response, consistent with an impaired receptor desensitization. The critical role for GRK4 in regulating mGlu(1) receptors implicates a major involvement of this kinase in the physiology of Purkinje cell and in motor learning.

G protein-coupled receptor kinases

G protein-coupled receptor kinases (GRKs) constitute a family of six mammalian serine/threonine protein kinases that phosphorylate agonist-bound, or activated, G protein-coupled receptors (GPCRs) as their primary substrates. GRK-mediated receptor phosphorylation rapidly initiates profound impairment of receptor signaling, or desensitization. This review focuses on the regulation of GRK activity by a variety of allosteric and other factors: agonist-stimulated GPCRs, beta gamma subunits of heterotrimeric GTP-binding proteins, phospholipid cofactors, the calcium-binding proteins calmodulin and recoverin, posttranslational isoprenylation and palmitoylation, autophosphorylation, and protein kinase C-mediated GRK phosphorylation. Studies employing recombinant, purified proteins, cell culture, and transgenic animal models attest to the general importance of GRKs in regulating a vast array of GPCRs both in vitro and in vivo.

Rat G protein-coupled receptor kinase GRK4: identification, functional expression, and differential tissue distribution of two splice variants

G protein-coupled receptor kinases (GRKs) specifically phosphorylate the agonist-occupied form of G protein-coupled receptors, leading to the homologous mode of desensitization. We report here on the cloning of complementary DNAs that encode two rat GRK4 variants. Rat GRK4A (575 amino acids) displays 76% identity with the long human GRK4 splice variant. Rat GRK4B (545 amino acids) delineates a new variant that is identical to GRK4A except for a 31-amino acid deletion in the N-terminal domain, corresponding to exon VI in the human GRK4 gene. GRKs4A and B are likely produced by alternative splicing from a single gene, the partial characterization of which revealed a structural organization similar to that of the human GRK4 gene. GRK4A messenger RNA (mRNA) is abundant only in testis. A combination of in situ hybridization and quantitative RT-PCR studies demonstrated that GRK4A mRNA level increases during testicular development and predominates in leptotene to late pachytene primary spermatocytes and round spermatids. GRK4B mRNA is poorly expressed in testis and most rat tissues but is heterogeneously distributed in the kidney, with 20-fold enrichment in the outer medulla. GRKs4A and B are both functional protein kinases, as demonstrated in a rhodopsin phosphorylation assay. The differential tissue distribution of GRKA4 and GRK4B suggests that individual GRK4 variants may serve distinct physiological functions.

Molecular cloning of two rat GRK6 splice variants

Desensitization of G protein-coupled receptors is frequently triggered by G protein-coupled receptor kinases (GRKs) that preferentially phosphorylate agonist-occupied receptors. In this study, two GRK6 splice variants were cloned from the rat kidney. One isoform (GRK6a) encodes a 576-amino acid protein that is virtually identical (98% identity) to human GRK6. The second isoform is similar except for a 2-base pair insert that constitutes part of an intron interrupting the 3'-end coding region. This new isoform (GRK6b, 589 amino acids) has therefore a specific COOH-terminal region. A reverse transcription-polymerase chain reaction assay designed to discriminate GRK6 splice variants demonstrated that GRK6b mRNA is widely distributed and expressed at much higher levels than GRK6a mRNA in most peripheral tissues. In contrast, GRK6a predominates in brain. Functional studies, performed with cytosol extracts from transfected Chinese hamster ovary cells, indicated that GRK6a and GRK6b both phosphorylate light-activated rhodopsin as well as a synthetic peptide. The identification of GRK6b extends the family of GRKs. Further studies will be required to establish the tissue and subcellular distribution of this protein and to delineate its physiological role.

Regulation of G protein-coupled receptor kinases by calmodulin and localization of the calmodulin binding domain

G protein-coupled receptor kinases (GRKs) specifically phosphorylate and regulate the activated form of multiple G protein-coupled receptors. Recent studies have revealed that GRKs are also subject to regulation. In this regard, GRK2 and GRK5 can be phosphorylated and either activated or inhibited, respectively, by protein kinase C. Here we demonstrate that calmodulin, another mediator of calcium signaling, is a potent inhibitor of GRK activity with a selectivity for GRK5 (IC50 approximately 50 nM) > GRK6 >> GRK2 (IC50 approximately 2 microM) >> GRK1. Calmodulin inhibition of GRK5 is mediated via a reduced ability of the kinase to bind to both receptor and phospholipid. Interestingly, calmodulin also activates autophosphorylation of GRK5 at sites distinct from the two major autophosphorylation sites on GRK5. Moreover, calmodulin-stimulated autophosphorylation directly inhibits GRK5 interaction with receptor even in the absence of calmodulin. Using glutathione S-transferase-GRK5 fusion proteins either to inhibit calmodulin-stimulated autophosphorylation or to bind directly to calmodulin, we determined that an amino-terminal domain of GRK5 (amino acids 20-39) is sufficient for calmodulin binding. This domain is abundant in basic and hydrophobic residues, characteristics typical of calmodulin binding sites, and is highly conserved in GRK4, GRK5, and GRK6. These studies suggest that calmodulin may serve a general role in mediating calcium-dependent regulation of GRK activity.

G protein-coupled receptor kinase GRK4. Molecular analysis of the four isoforms and ultrastructural localization in spermatozoa and germinal cells

G protein-coupled receptor kinase 4 (GRK4) presents some peculiar characteristics that make it a unique member within the GRK multigene family. For example, this is the only GRK for which four splice variants (GRK4alpha, -beta, -gamma, -delta) have been identified. We developed a simple assay to study kinase activity, and we found that GRK4alpha, but not GRK4beta, -gamma, and -delta, was able to phosphorylate rhodopsin in an agonist-dependent manner. GRK4alpha kinase activity was inhibited by Ca2+/calmodulin (CaM) (IC50 = 80 nM), and a direct interaction between GRK4alpha and Ca2+/CaM was revealed using CaM-conjugated Sepharose 4B. The other three GRK4 isoforms did not interact with CaM in parallel experiments. The present investigation also aimed to define cellular and ultrastructural localization of GRK4. A substantial expression of GRK4 mRNA was only found in testis and in the spermatogonia cell line GC-1 spg. Specific GRK4 immunoreactivity was only found on sperm membranes, and immunochemical and ultrastructural analyses showed that it is associated to the acrosomal membranes and to the outer mitochondrial membranes. GRK4gamma was the only detectable isoform in human sperm. We concluded that: i) only GRK4alpha can phosphorylate rhodopsin and that this activity is inhibited by CaM; ii) the other three isoforms do not phosphorylate rhodopsin and do not interact with CaM; and iii) the association of GRK4 with highly specialized sperm organelles, which are essential for fertilization, strongly indicates that this kinase is involved in this process.

Regulatory mechanisms that modulate signalling by G-protein-coupled receptors

The large and functionally diverse group of G-protein-coupled receptors includes receptors for many different signalling molecules, including peptide and non-peptide hormones and neuro-transmitters, chemokines, prostanoids and proteinases. Their principal function is to transmit information about the extracellular environment to the interior of the cell by interacting with the heterotrimeric G-proteins, and they thereby participate in many aspects of regulation. Cellular responses to agonists of these receptors are usually rapidly attenuated. Mechanisms of signal attenuation include removal of agonists from the extracellular fluid, receptor desensitization, endocytosis and down-regulation. Agonists are removed by dilution, uptake by transporters and enzymic degradation. Receptor desensitization is mediated by receptor phosphorylation by G-protein receptor kinases and second-messenger kinases, interaction of phosphorylated receptors with arrestins and receptor uncoupling from G-proteins. Agonist-induced receptor endocytosis also contributes to desensitization by depleting the cell surface of high-affinity receptors, and recycling of internalized receptors contributes to resensitization of cellular responses. Receptor down-regulation is a form of desensitization that occurs during continuous, long-term exposure of cells to receptor agonists. Down-regulation, which may occur during the development of drug tolerance, is characterized by depletion of the cellular receptor content, and is probably mediated by alterations in the rates of receptor degradation and synthesis. These regulatory mechanisms are important, as they govern the ability of cells to respond to agonists. A greater understanding of the mechanisms that modulate signalling may lead to the development of new therapies and may help to explain the mechanism of drug tolerance.

Identification and chromosomal localization of a processed pseudogene of human GRK6

G-protein-coupled receptor kinases (GRKs) phosphorylate agonist-occupied G-protein-coupled receptors, resulting in desensitization of receptor signaling. To date, 6 mammalian GRKs have been identified by molecular cloning. Several lines of evidence indicate that a homologue of GRK6, the most recently described GRK, is present in the human genome. Northern analysis identifies two transcripts which hybridize to GRK6, and genomic Southern analysis indicates that GRK6 is localized to chromosome 5, with a second GRK6-like locus on chromosome 13. To identify the GRK6 homologue on chromsome 13, several sets of closely-spaced primers were designed based on the GRK6 cDNA sequence and then used to amplify human genomic DNA by PCR. Two products were identified, the larger of which is a fragment of the GRK6 gene which contains introns, while the smaller fragment is 94% homologous to GRK6 and contains no introns. In order to further characterize this GRK6 homologue, primers from the 5' and 3' coding regions of GRK6 were used to amplify a product of 1458 base pairs from human genomic DNA. This 1458 base pair PCR fragment displays 94% homology to GRK6 and contains multiple nucleotide insertions and deletions compared to GRK6, including a C to T mutation at base pair 202 which creates a predicted in-frame stop codon. In an effort to determine whether this gene is transcriptionally active, primers designed to preferentially amplify either GRK6 or the homologue were used in reverse transcription PCR. In contrast to the GRK6-specific primers, primers which selectively amplify the GRK6 homologue fail to produce a PCR product in any RNA tested, indicating that this gene is most likely transcriptionally inactive. PCR amplification of rodent/human hybrid cell lines using these same primers confirms the previously established chromosome 5 localization of GRK6, and localizes this homologue to chromosome 13. Northern analysis indicates that the two GRK6-hybridizing species seen in RNA differ by approximately 500 base pairs in the 3' untranslated region, indicating that both transcripts likely arise from differential processing of a single gene. Taken together, these data indicate that the GRK6-hybridizing species on chromosome 13 is a transcriptionally inactive processed pseudogene of GRK6, while the two GRK6 transcripts differ in the 3' untranslated region.

G protein-coupled receptors: heterologous regulation of homologous desensitization and its implications

Two patterns of rapid desensitization have been characterized for G protein-coupled receptors: homologous desensitization, which mainly involves G protein-coupled receptor kinases and arrestins, and heterologous desensitization, which mainly involves protein kinases A (PKA) and C (PKC). In this review, Tsu Tshen Chuang and colleagues discuss evidence to show that PKA and PKC can modify the functional state of the G protein-coupled receptor kinases/arrestin homologous desensitization machinery, providing a novel level of cross-talk in signal transduction. Studies on regulation of G protein-coupled receptor kinases and arrestins confirm that the functional state of this machinery may have important consequences for cellular responsiveness and may represent new targets for therapeutic strategies.

Characterization of the G protein-coupled receptor kinase GRK4. Identification of four splice variants

A novel human G protein-coupled receptor kinase was recently identified by positional cloning in the search for the Huntington's disease locus (Ambrose, C., James, M., Barnes, G., Lin, C., Bates, G., Altherr, M., Duyao, M., Groot, N., Church, D., Wasmuth, J. J., Lehrach, H., Housman, D., Buckler, A., Gusella, J. F., and MacDonald, M. E. (1993) Hum. Mol. Genet. 1, 697-703). Comparison of the deduced amino acid sequence of GRK4 with those of the closely related GRK5 and GRK6 suggested the apparent loss of 32 codons in the amino-terminal domain and 46 codons in the carboxyl-terminal domain of GRK4. These two regions undergo alternative splicing in the GRK4 mRNA, resulting from the presence or absence of exons filling one or both of these apparent gaps. Each inserted sequence maintains the open reading frame, and the deduced amino acid sequences are similar to corresponding regions of GRK5 and GRK6. Thus, the GRK4 mRNA and the GRK4 protein can exist as four distinct variant forms. The human GRK4 gene is composed of 16 exons extending over 75 kilobase pairs of DNA. The two alternatively spliced exons correspond to exons II and XV. The genomic organization of the GRK4 gene is completely distinct from that of the human GRK2 gene, highlighting the evolutionary distance since the divergence of these two genes. Human GRK4 mRNA is expressed highly only in testis, and both alternative exons are abundant in testis mRNA. The four GRK4 proteins have been expressed, and all incorporate [3H]palmitate. GRK4 is capable of augmenting the desensitization of the rat luteinizing hormone/chorionic gonadotropin receptor upon coexpression in HEK293 cells and of phosphorylating the agonist-occupied, purified beta2-adrenergic receptor, indicating that GRK4 is a functional protein kinase.

Regulation of G protein-coupled receptor kinase subtypes in activated T lymphocytes. Selective increase of beta-adrenergic receptor kinase 1 and 2

Beta-adrenergic receptor kinase (beta ARK) is a serine-threonine kinase involved in the process of homologous desensitization of G-coupled receptors. beta ARK is a member of a multigene family, consisting of six known subtypes, also named G protein-coupled receptor kinases (GRK 1-6). In this study we investigated the expression of GRKs during the process of T cell activation, which is of fundamental importance in regulating immune responses. T cell activation was induced by exposing mononuclear leukocytes (MNL) to PHA and confirmed by tritiated thymidine incorporation measurement. A substantial increase of GRK activity (as measured by in vitro phosphorylation of rhodopsin) was found after 48 h (331 +/- 80% of controls) and 72 h (347 +/- 86% of controls) of exposure to PHA. A threefold increase of beta ARK1 immunoreactivity was found in MNL exposed to PHA for 72 h. Persistent activation of protein kinase C (PKC) by 10 nM 12-O-tetradecanoylphorbol-13-acetate (TPA) was able to increase beta ARK activity to the same extent as PHA, suggesting a PKC-mediated mechanism. The kinetic of beta-adrenergic-stimulated cAMP production was substantially modified in TPA and PHA-activated cells, indicating that the increased GRK activity resulted in an increased beta-adrenergic homologous desensitization. A three- to fourfold increase in GRK activity was also observed in a population of T cell blasts (> 97% CD3+) exposed to PHA for 48-72 h. A significant increase in beta ARK1 and beta ARK2 mRNA expression was observed 48 h after mitogen stimulation, while mRNA expression of GRK5 and GRK6 was not changed. In conclusion our data show that the expression of GRK subtypes is actively and selectively modulated according to the functional state of T lymphocytes.