beta2-Adrenergic receptor regulation by GIT1, a G protein-coupled receptor kinase-associated ADP ribosylation factor GTPase-activating protein.

G protein-coupled receptor activation leads to the membrane recruitment and activation of G protein-coupled receptor kinases, which phosphorylate receptors and lead to their inactivation. We have identified a novel G protein-coupled receptor kinase-interacting protein, GIT1, that is a GTPase-activating protein (GAP) for the ADP ribosylation factor (ARF) family of small GTP-binding proteins. Overexpression of GIT1 leads to reduced beta2-adrenergic receptor signaling and increased receptor phosphorylation, which result from reduced receptor internalization and resensitization. These cellular effects of GIT1 require its intact ARF GAP activity and do not reflect regulation of GRK kinase activity. These results suggest an essential role for ARF proteins in regulating beta2-adrenergic receptor endocytosis. Moreover, they provide a mechanism for integration of receptor activation and endocytosis through regulation of ARF protein activation by GRK-mediated recruitment of the GIT1 ARF GAP to the plasma membrane.

Restoration of beta-adrenergic signaling in failing cardiac ventricular myocytes via adenoviral-mediated gene transfer.

Cardiovascular gene therapy is a novel approach to the treatment of diseases such as congestive heart failure (CHF). Gene transfer to the heart would allow for the replacement of defective or missing cellular proteins that may improve cardiac performance. Our laboratory has been focusing on the feasibility of restoring beta-adrenergic signaling deficiencies that are a characteristic of chronic CHF. We have now studied isolated ventricular myocytes from rabbits that have been chronically paced to produce hemodynamic failure. We document molecular beta-adrenergic signaling defects including down-regulation of myocardial beta-adrenergic receptors (beta-ARs), functional beta-AR uncoupling, and an up-regulation of the beta-AR kinase (betaARK1). Adenoviral-mediated gene transfer of the human beta2-AR or an inhibitor of betaARK1 to these failing myocytes led to the restoration of beta-AR signaling. These results demonstrate that defects present in this critical myocardial signaling pathway can be corrected in vitro using genetic modification and raise the possibility of novel inotropic therapies for CHF including the inhibition of betaARK1 activity in the heart.

Potentiation of beta-adrenergic signaling by adenoviral-mediated gene transfer in adult rabbit ventricular myocytes.

Our laboratory has been testing the hypothesis that genetic modulation of the beta-adrenergic signaling cascade can enhance cardiac function. We have previously shown that transgenic mice with cardiac overexpression of either the human beta2-adrenergic receptor (beta2AR) or an inhibitor of the beta-adrenergic receptor kinase (betaARK), an enzyme that phosphorylates and uncouples agonist-bound receptors, have increased myocardial inotropy. We now have created recombinant adenoviruses encoding either the beta2AR (Adeno-beta2AR) or a peptide betaARK inhibitor (consisting of the carboxyl terminus of betaARK1, Adeno-betaARKct) and tested their ability to potentiate beta-adrenergic signaling in cultured adult rabbit ventricular myocytes. As assessed by radioligand binding, Adeno-beta2AR infection led to approximately 20-fold overexpression of beta-adrenergic receptors. Protein immunoblots demonstrated the presence of the Adeno-betaARKct transgene. Both transgenes significantly increased isoproterenol-stimulated cAMP as compared to myocytes infected with an adenovirus encoding beta-galactosidase (Adeno-betaGal) but did not affect the sarcolemmal adenylyl cyclase response to Forskolin or NaF. beta-Adrenergic agonist-induced desensitization was significantly inhibited in Adeno-betaARKct-infected myocytes (16+/-2%) as compared to Adeno-betaGal-infected myocytes (37+/-1%, P < 0.001). We conclude that recombinant adenoviral gene transfer of the beta2AR or an inhibitor of betaARK-mediated desensitization can potentiate beta-adrenergic signaling.

Receptor and G betagamma isoform-specific interactions with G protein-coupled receptor kinases.

The G protein-coupled receptor (GPCR) kinases (GRKs) phosphorylate and desensitize agonist-occupied GPCRs. GRK2-mediated receptor phosphorylation is preceded by the agonist-dependent membrane association of this enzyme. Previous in vitro studies with purified proteins have suggested that this translocation may be mediated by the recruitment of GRK2 to the plasma membrane by its interaction with the free betagamma subunits of heterotrimeric G proteins (G betagamma). Here we demonstrate that this mechanism operates in intact cells and that specificity is imparted by the selective interaction of discrete pools of G betagamma with receptors and GRKs. Treatment of Cos-7 cells transiently overexpressing GRK2 with a beta-receptor agonist promotes a 3-fold increase in plasma membrane-associated GRK2. This translocation of GRK2 is inhibited by the carboxyl terminus of GRK2, a known G betagamma sequestrant. Furthermore, in cells overexpressing both GRK2 and G beta1 gamma2, activation of lysophosphatidic acid receptors leads to the rapid and transient formation of a GRK/G betagamma complex. That G betagamma specificity exists at the level of the GPCR and the GRK is indicated by the observation that a GRK2/G betagamma complex is formed after agonist occupancy of the lysophosphatidic acid and beta-adrenergic but not thrombin receptors. In contrast to GRK2, GRK3 forms a G betagamma complex after stimulation of all three GPCRs. This G betagamma binding specificity of the GRKs is also reflected at the level of the purified proteins. Thus the GRK2 carboxyl terminus binds G beta1 and G beta2 but not G beta3, while the GRK3 fusion protein binds all three G beta isoforms. This study provides a direct demonstration of a role for G betagamma in mediating the agonist-stimulated translocation of GRK2 and GRK3 in an intact cellular system and demonstrates isoform specificity in the interaction of these components.

Essential role of beta-adrenergic receptor kinase 1 in cardiac development and function.

The beta-adrenergic receptor kinase 1 (beta ARK1) is a member of the G protein-coupled receptor kinase (GRK) family that mediates the agonist-dependent phosphorylation and desensitization of G protein-coupled receptors. We have cloned and disrupted the beta ARK1 gene in mice by homologous recombination. No homozygote beta ARK1-/- embryos survive beyond gestational day 15.5. Prior to gestational day 15.5, beta ARK1-/- embryos display pronounced hypoplasia of the ventricular myocardium essentially identical to the "thin myocardium syndrome" observed upon gene inactivation of several transcription factors (RXR alpha, N-myc, TEF-1, WT-1). Lethality in beta ARK1-/- embryos is likely due to heart failure as they exhibit a > 70% decrease in cardiac ejection fraction determined by direct in utero intravital microscopy. These results along with the virtual absence of endogenous GRK activity in beta ARK1-/- embryos demonstrate that beta ARK1 appears to be the predominant GRK in early embryogenesis and that it plays a fundamental role in cardiac development.

Receptor-specific in vivo desensitization by the G protein-coupled receptor kinase-5 in transgenic mice.

Transgenic mice were generated with cardiac-specific overexpression of the G protein-coupled receptor kinase-5 (GRK5), a serine/threonine kinase most abundantly expressed in the heart compared with other tissues. Animals overexpressing GRK5 showed marked beta-adrenergic receptor desensitization in both the anesthetized and conscious state compared with nontransgenic control mice, while the contractile response to angiotensin II receptor stimulation was unchanged. In contrast, the angiotensin II-induced rise in contractility was significantly attenuated in transgenic mice overexpressing the beta-adrenergic receptor kinase-1, another member of the GRK family. These data suggest that myocardial overexpression of GRK5 results in selective uncoupling of G protein-coupled receptors and demonstrate that receptor specificity of the GRKs may be important in determining the physiological phenotype.

Monoclonal antibodies reveal receptor specificity among G-protein-coupled receptor kinases.

Guanine nucleotide-binding regulatory protein (G protein)-coupled receptor kinases (GRKs) constitute a family of serine/threonine kinases that play a major role in the agonist-induced phosphorylation and desensitization of G-protein-coupled receptors. Herein we describe the generation of monoclonal antibodies (mAbs) that specifically react with GRK2 and GRK3 or with GRK4, GRK5, and GRK6. They are used in several different receptor systems to identify the kinases that are responsible for receptor phosphorylation and desensitization. The ability of these reagents to inhibit GRK- mediated receptor phosphorylation is demonstrated in permeabilized 293 cells that overexpress individual GRKs and the type 1A angiotensin II receptor. We also use this approach to identify the endogenous GRKs that are responsible for the agonist-induced phosphorylation of epitope-tagged beta2- adrenergic receptors (beta2ARs) overexpressed in rabbit ventricular myocytes that are infected with a recombinant adenovirus. In these myocytes, anti-GRK2/3 mAbs inhibit isoproterenol-induced receptor phosphorylation by 77%, while GRK4-6-specific mAbs have no effect. Consistent with the operation of a betaAR kinase-mediated mechanism, GRK2 is identified by immunoblot analysis as well as in a functional assay as the predominant GRK expressed in these cells. Microinjection of GRK2/3-specific mAbs into chicken sensory neurons, which have been shown to express a GRK3-like protein, abolishes desensitization of the alpha2AR-mediated calcium current inhibition. The intracellular inhibition of endogenous GRKs by mAbs represents a novel approach to the study of receptor specificities among GRKs that should be widely applicable to many G-protein-coupled receptors.

Direct evidence that Gi-coupled receptor stimulation of mitogen-activated protein kinase is mediated by G beta gamma activation of p21ras.

Stimulation of Gi-coupled receptors leads to the activation of mitogen-activated protein kinases (MAP kinases). In several cell types, this appears to be dependent on the activation of p21ras (Ras). Which G-protein subunit(s) (G alpha or the G beta gamma complex) primarily is responsible for triggering this signaling pathway, however, is unclear. We have demonstrated previously that the carboxyl terminus of the beta-adrenergic receptor kinase, containing its G beta gamma-binding domain, is a cellular G beta gamma antagonist capable of specifically distinguishing G alpha- and G beta gamma-mediated processes. Using this G beta gamma inhibitor, we studied Ras and MAP kinase activation through endogenous Gi-coupled receptors in Rat-1 fibroblasts and through receptors expressed by transiently transfected COS-7 cells. We report here that both Ras and MAP kinase activation in response to lysophosphatidic acid is markedly attenuated in Rat-1 cells stably transfected with a plasmid encoding this G beta gamma antagonist. Likewise in COS-7 cells transfected with plasmids encoding Gi-coupled receptors (alpha 2-adrenergic and M2 muscarinic), the activation of Ras and MAP kinase was significantly reduced in the presence of the coexpressed G beta gamma antagonist. Ras-MAP kinase activation mediated through a Gq-coupled receptor (alpha 1-adrenergic) or the tyrosine kinase epidermal growth factor receptor was unaltered by this G beta gamma antagonist. These results identify G beta gamma as the primary mediator of Ras activation and subsequent signaling via MAP kinase in response to stimulation of Gi-coupled receptors.

Functionally active targeting domain of the beta-adrenergic receptor kinase: an inhibitor of G beta gamma-mediated stimulation of type II adenylyl cyclase.

The beta-adrenergic receptor kinase (beta ARK) phosphorylates its membrane-associated receptor substrates, such as the beta-adrenergic receptor, triggering events leading to receptor desensitization. beta ARK activity is markedly stimulated by the isoprenylated beta gamma subunit complex of heterotrimeric guanine nucleotide-binding proteins (G beta gamma), which translocates the kinase to the plasma membrane and thereby targets it to its receptor substrate. The amino-terminal two-thirds of beta ARK1 composes the receptor recognition and catalytic domains, while the carboxyl third contains the G beta gamma binding sequences, the targeting domain. We prepared this domain as a recombinant His6 fusion protein from Escherichia coli and found that it had both independent secondary structure and functional activity. We demonstrated the inhibitory properties of this domain against G beta gamma activation of type II adenylyl cyclase both in a reconstituted system utilizing Sf9 insect cell membranes and in a permeabilized 293 human embryonic kidney cell system. Gi alpha-mediated inhibition of adenylyl cyclase was not affected. These data suggest that this His6 fusion protein derived from the carboxyl terminus of beta ARK1 provides a specific probe for defining G beta gamma-mediated processes and for studying the structural features of a G beta gamma-binding domain.

A beta-adrenergic receptor kinase-like enzyme is involved in olfactory signal termination.

We have previously shown that second-messenger-dependent kinases (cAMP-dependent kinase, protein kinase C) in the olfactory system are essential in terminating second-messenger signaling in response to odorants. We now document that subtype 2 of the beta-adrenergic receptor kinase (beta ARK) is also involved in this process. By using subtype-specific antibodies to beta ARK-1 and beta ARK-2, we show that beta ARK-2 is preferentially expressed in the olfactory epithelium in contrast to findings in most other tissues. Heparin, an inhibitor of beta ARK, as well as anti-beta ARK-2 antibodies, (i) completely prevents the rapid decline of second-messenger signals (desensitization) that follows odorant stimulation and (ii) strongly inhibits odorant-induced phosphorylation of olfactory ciliary proteins. In contrast, beta ARK-1 antibodies are without effect. Inhibitors of protein kinase A and protein kinase C also block odorant-induced desensitization and phosphorylation. These data suggest that a sequential interplay of second-messenger-dependent and receptor-specific kinases is functionally involved in olfactory desensitization.

The receptor kinase family: primary structure of rhodopsin kinase reveals similarities to the beta-adrenergic receptor kinase.

Light-dependent deactivation of rhodopsin as well as homologous desensitization of beta-adrenergic receptors involves receptor phosphorylation that is mediated by the highly specific protein kinases rhodopsin kinase (RK) and beta-adrenergic receptor kinase (beta ARK), respectively. We report here the cloning of a complementary DNA for RK. The deduced amino acid sequence shows a high degree of homology to beta ARK. In a phylogenetic tree constructed by comparing the catalytic domains of several protein kinases, RK and beta ARK are located on a branch close to, but separate from the cyclic nucleotide-dependent protein kinase and protein kinase C subfamilies. From the common structural features we conclude that both RK and beta ARK are members of a newly delineated gene family of guanine nucleotide-binding protein (G protein)-coupled receptor kinases that may function in diverse pathways to regulate the function of such receptors.

cAMP stimulates transcription of the beta 2-adrenergic receptor gene in response to short-term agonist exposure.

In addition to conveying cellular responses to an effector molecule, receptors are often themselves regulated by their effectors. We have demonstrated that epinephrine modulates both the rate of transcription of the beta 2-adrenergic receptor (beta 2AR) gene and the steady-state level of beta 2AR mRNA in DDT1MF-2 cells. Short-term (30 min) exposure to epinephrine (100 nM) stimulates the rate of beta 2AR gene transcription, resulting in a 3- to 4-fold increase in steady-state beta 2AR mRNA levels. These effects are mimicked by 1 mM N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate (Bt2cAMP) or foskolin but not by phorbol esters. The half-life of the beta 2AR mRNA after addition of actinomycin D (46.7 +/- 10.2 min; mean +/- SEM; n = 5) remained unchanged after 30 min of epinephrine treatment (46.8 +/- 10.6 min; mean +/- SEM; n = 4), indicating that a change in transcription rate is the predominant factor responsible for the increase of beta 2AR mRNA. Whereas brief exposure to epinephrine or Bt2cAMP does not significantly affect the total number of cellular beta 2ARs (assessed by ligand binding), continued exposure results in a gradual decline in beta 2AR number to approximately 20% (epinephrine) or approximately 45% (Bt2cAMP) of the levels in control cells by 24 hr. Similar decreases in agonist-stimulated adenylyl cyclase activity are observed. This loss of receptors with prolonged agonist exposure is accompanied by a 50% reduction in beta 2AR mRNA. Transfection of the beta 2AR promoter region cloned onto a reporter gene (bacterial chloramphenicol acetyltransferase) allowed demonstration of a 2- to 4-fold induction of transcription by agents that elevate cAMP levels, such as forskolin or phosphodiesterase inhibitors. These results establish the presence of elements within the proximal promoter region of the beta 2AR gene responsible for the transcriptional enhancing activity of cAMP and demonstrate that beta 2AR gene expression is regulated by a type of feedback mechanism involving the second messenger cAMP.

Functional desensitization of the isolated beta-adrenergic receptor by the beta-adrenergic receptor kinase: potential role of an analog of the retinal protein arrestin (48-kDa protein).

The beta-adrenergic receptor kinase is an enzyme, possibly analogous to rhodopsin kinase, that multiply phosphorylates the beta-adrenergic receptor only when it is occupied by stimulatory agonists. Since this kinase may play an important role in mediating the process of homologous, or agonist-specific, desensitization, we investigated the functional consequences of receptor phosphorylation by the kinase and possible analogies with the mechanism of action of rhodopsin kinase. Pure hamster lung beta 2-adrenergic receptor, reconstituted in phospholipid vesicles, was assessed for its ability to mediate agonist-promoted stimulation of the GTPase activity of coreconstituted stimulatory guanine nucleotide-binding regulatory protein. When the receptor was phosphorylated by partially (approximately 350-fold) purified preparations of beta-adrenergic receptor kinase, as much as 80% inactivation of its functional activity was observed. However, the use of more highly purified enzyme preparations led to a dramatic decrease in the ability of phosphorylation to inactivate the receptor such that pure enzyme preparations (approximately 20,000-fold purified) caused only minimal (approximately 1off/- 7%) inactivation. Addition of pure retinal arrestin (48-kDa protein or S antigen), which is involved in enhancing the inactivating effect of rhodopsin phosphorylation by rhodopsin kinase, led to partial restoration of the functional effect of beta-adrenergic receptor kinase-promoted phosphorylation (41 +/- 3% inactivation). These results suggest the possibility that a protein analogous to retinal arrestin may exist in other tissues and function in concert with beta-adrenergic receptor kinase to regulate the activity of adenylate cyclase-coupled receptors.

The role of GRK6 in animal models of Parkinson's disease and L-DOPA treatment.

G protein-coupled Receptor Kinase 6 (GRK6) belongs to a family of kinases that phosphorylate GPCRs. GRK6 levels were found to be altered in Parkinson's Disease (PD) and D(2) dopamine receptors are supersensitive in mice lacking GRK6 (GRK6-KO mice). To understand how GRK6 modulates the behavioral manifestations of dopamine deficiency and responses to L-DOPA, we used three approaches to model PD in GRK6-KO mice: 1) the cataleptic response to haloperidol; 2) introducing GRK6 mutation to an acute model of absolute dopamine deficiency, DDD mice; 3) hemiparkinsonian 6-OHDA model. Furthermore, dopamine-related striatal signaling was analyzed by assessing the phosphorylation of AKT/GSK3β and ERK1/2. GRK6 deficiency reduced cataleptic behavior, potentiated the acute effect of L-DOPA in DDD mice, reduced rotational behavior in hemi-parkinsonian mice, and reduced abnormal involuntary movements induced by chronic L-DOPA. These data indicate that approaches to regulate GRK6 activity could be useful in modulating both therapeutic and side-effects of L-DOPA.

Unveiling Protein Kinase A Targets in Cryptococcus neoformans Capsule Formation.

The protein kinase A (PKA) signal transduction pathway has been associated with pathogenesis in many fungal species. Geddes and colleagues [mBio 7(1):e01862-15, 2016, doi:10.1128/mBio.01862-15] used quantitative proteomics approaches to define proteins with altered abundance during protein kinase A (PKA) activation and repression in the opportunistic human fungal pathogen Cryptococcus neoformans. They observed an association between microbial PKA signaling and ubiquitin-proteasome regulation of protein homeostasis. Additionally, they correlated these processes with expression of polysaccharide capsule on the fungal cell surface, the main virulence-associated phenotype in this organism. Not only are their findings important for microbial pathogenesis, but they also support similar associations between human PKA signaling and ubiquitinated protein accumulation in neurodegenerative diseases.