G protein-coupled receptor kinases 3 and 6 use different pathways to desensitize the endogenous M3 muscarinic acetylcholine receptor in human SH-SY5Y cells

Determination of Region-Specific Roles of the M3 Muscarinic Acetylcholine Receptor in Gastrointestinal Motility

BACKGROUND

The specific role of the M₃ muscarinic acetylcholine receptor in gastrointestinal motility under physiological conditions is unclear, due to a lack of subtype-selective compounds.

AIMS

The objective of this study was to determine the region-specific role of the M₃ receptor in gastrointestinal motility.

METHODS

We developed a novel positive allosteric modulator (PAM) for the M₃ receptor, PAM-369. The effects of PAM-369 on the carbachol-induced contractile response of porcine esophageal smooth muscle and mouse colonic smooth muscle (ex vivo) and on the transit in mouse small intestine and rat colon (in vivo) were examined.

RESULTS

PAM-369 selectively potentiated the M₃ receptor under the stimulation of its orthosteric ligands without agonistic or antagonistic activity. Half-maximal effective concentrations of PAM activity for human, mouse, and rat M₃ receptors were 0.253, 0.345, and 0.127 μM, respectively. PAM-369 enhanced carbachol-induced contraction in porcine esophageal smooth muscle and mouse colonic smooth muscle without causing any contractile responses by itself. The oral administration of 30 mg/kg PAM-369 increased the small intestinal transit in both normal motility and loperamide-induced intestinal dysmotility mice but had no effects on the colonic transit, although the M₃ receptor mRNA expression is higher in the colon than in the small intestine.

CONCLUSIONS

This study provided the first direct evidence that the M₃ receptor has different region-specific roles in the motility function between the small intestine and colon in physiological and pathophysiological contexts. Selective PAMs designed for targeted subtypes of muscarinic receptors are useful for elucidating the subtype-specific function.

Beta-cell M3 muscarinic acetylcholine receptors as potential targets for novel antidiabetic drugs

A key feature of type 2 diabetes (T2D) is that beta-cells of the pancreatic islets fail to release sufficient amounts of insulin to overcome peripheral insulin resistance. Glucose-stimulated insulin secretion (GSIS) is regulated by the activity of numerous neurotransmitters, hormones and paracrine factors that act by stimulating specific G protein-coupled receptors expressed by pancreatic beta-cells. Studies with both mouse and human islets suggest that acetylcholine (ACh) acts on beta-cell M₃ muscarinic receptors (M3Rs) to promote GSIS. In mouse islets, beta-cell M3Rs are thought to be activated by ACh released from parasympathetic nerve endings. Interestingly, studies with human pancreatic islets suggest that ACh is synthesized, stored and released by alpha-cells, which, in human pancreatic islets, are intermingled with beta-cells. Independent of the source of pancreatic islet ACh, recent studies indicate that beta-cell M3Rs represent a potential target for drugs capable of promoting insulin release for therapeutic purposes. In this review, we will provide an overview about signaling pathways and molecules that regulate the activity of beta-cell M3Rs. We will also discuss a novel pharmacological strategy to stimulate the activity of these receptors to reduce the metabolic impairments associated with T2D.

Crosstalk between mAChRM3 and β2AR, via acetylcholine PI3/PKC/PBEP1/Raf-1 MEK1/2/ERK1/2 pathway activation, in human bronchial epithelial cells after long-term cigarette smoke exposure

BACKGROUND

Cigarette smoke extract (CSE) affects the expression of non-neuronal components of cholinergic system in bronchial epithelial cells and, as PEBP1/Raf-mediated MAPK1/2 and ERK1/2 pathway, promotes inflammation and oxidative stress.

AIMS

We studied whether Acetylcholine (ACh) is involved in the mechanism of crosstalk between mAChRM3 and β2Adrenergic receptors (β2AR) promoting, via PI3/PKC/PBEP1/Raf/MEK1/2/ERK1/2 activation, β2AR desensitization, inflammation and, oxidative stress in a bronchial epithelial cell line (16HBE) after long-term exposure to cigarette smoke extract (LECSE).

METHODS

We evaluated mAChRM3 and Choline Acetyltransferase (ChAT) expression, ACh production, PEBP1, ERk1/2, and β2AR phosphorylation, as well as NOX-4, ROS production and IL-8 release in 16HBE after LECSE. The inhibitory activity of Hemicholinium (HCh-3) (a potent choline uptake blocker), LY294002 (a highly selective inhibitor of PI3 kinase), Tiotropium (Spiriva®) (anticholinergic drug) and Olodaterol (β2AR agonist), were tested in 16HBE after LECSE.

RESULTS

mAChRM3, ChAT, ACh activity, pPEBP1, pβ2AR, pERK1/2, ROS, NOX-4 and IL-8 increased after LECSE in 16HBE LECSE compared to untreated cells. HCh-3 and LY294002 (alone or in combination) as well as Tiotropium (Spiriva®) or Olodaterol (alone or in combination) all reduced the levels of pPEBP1, pβ2AR, pERK1/2, ROS, NOX-4, and IL-8 in 16HBE LECSE compared to untreated cells.

CONCLUSIONS

LECSE promotes ACh production which enhances PI3/PKC/PEBP1/Raf-ERK1/2 pathway activation, heterologous β2AR desensitization, as well as release of inflammatory and oxidative mediators in bronchial epithelial cells. The use of anticholinergic drugs and long-acting β2-agonists, alone or in combination may be dampen these inflammatory mechanisms when used in combination in some epithelial cell types.

CK2 acts as a potent negative regulator of receptor-mediated insulin release in vitro and in vivo

G protein-coupled receptors (GPCRs) regulate virtually all physiological functions including the release of insulin from pancreatic β-cells. β-Cell M3 muscarinic receptors (M3Rs) are known to play an essential role in facilitating insulin release and maintaining proper whole-body glucose homeostasis. As is the case with other GPCRs, M3R activity is regulated by phosphorylation by various kinases, including GPCR kinases and casein kinase 2 (CK2). At present, it remains unknown which of these various kinases are physiologically relevant for the regulation of β-cell activity. In the present study, we demonstrate that inhibition of CK2 in pancreatic β-cells, knockdown of CK2α expression, or genetic deletion of CK2α in β-cells of mutant mice selectively augmented M3R-stimulated insulin release in vitro and in vivo. In vitro studies showed that this effect was associated with an M3R-mediated increase in intracellular calcium levels. Treatment of mouse pancreatic islets with CX4945, a highly selective CK2 inhibitor, greatly reduced agonist-induced phosphorylation of β-cell M3Rs, indicative of CK2-mediated M3R phosphorylation. We also showed that inhibition of CK2 greatly enhanced M3R-stimulated insulin secretion in human islets. Finally, CX4945 treatment protected mice against diet-induced hyperglycemia and glucose intolerance in an M3R-dependent fashion. Our data demonstrate, for the first time to our knowledge, the physiological relevance of CK2 phosphorylation of a GPCR and suggest the novel concept that kinases acting on β-cell GPCRs may represent novel therapeutic targets.

β-Arrestin1 and distinct CXCR4 structures are required for stromal derived factor-1 to downregulate CXCR4 cell-surface levels in neuroblastoma

CXC chemokine receptor 4 (CXCR4) is a G protein-coupled receptor (GPCR) located on the cell surface that signals upon binding the chemokine stromal derived factor-1 (SDF-1; also called CXCL 12). CXCR4 promotes neuroblastoma proliferation and chemotaxis. CXCR4 expression negatively correlates with prognosis and drives neuroblastoma growth and metastasis in mouse models. All functions of CXCR4 require its expression on the cell surface, yet the molecular mechanisms that regulate CXCR4 cell-surface levels in neuroblastoma are poorly understood. We characterized CXCR4 cell-surface regulation in the related SH-SY5Y and SK-N-SH human neuroblastoma cell lines. SDF-1 treatment caused rapid down-modulation of CXCR4 in SH-SY5Y cells. Pharmacologic activation of protein kinase C similarly reduced CXCR4, but via a distinct mechanism. Analysis of CXCR4 mutants delineated two CXCR4 regions required for SDF-1 treatment to decrease cell-surface CXCR4 in neuroblastoma cells: the isoleucine-leucine motif at residues 328 and 329 and residues 343-352. In contrast, and unlike CXCR4 regulation in other cell types, serines 324, 325, 338, and 339 were not required. Arrestin proteins can bind and regulate GPCR cell-surface expression, often functioning together with kinases such as G protein-coupled receptor kinase 2 (GRK2). Using SK-N-SH cells which are naturally deficient in β-arrestin1, we showed that β-arrestin1 is required for the CXCR4 343-352 region to modulate CXCR4 cell-surface expression following treatment with SDF-1. Moreover, GRK2 overexpression enhanced CXCR4 internalization, via a mechanism requiring both β-arrestin1 expression and the 343-352 region. Together, these results characterize CXCR4 structural domains and β-arrestin1 as critical regulators of CXCR4 cell-surface expression in neuroblastoma. β-Arrestin1 levels may therefore influence the CXCR4-driven metastasis of neuroblastoma as well as prognosis.

Cyclic GMP regulates M₃AChR activity at plasma membranes from airway smooth muscle

Muscarinic acetylcholine receptors MAChRs from Bovine Tracheal Smooth Muscle (BTSM) plasma membranes are responsible for the cGMP rise and signal-amplitude peaks associated with smooth muscle contraction present in bronchial asthma. These MAChRs bind [(3)H]QNB and exhibit the classic G Protein Coupled-Receptor (GPCR) behavior towards muscarinic agonist and antagonists that is sensitive to sensitive to GTP analogs. Interestingly, the [(3)H]QNB binding activity was stimulated by cGMP and ATP, and was enhanced by IBMX and Zaprinast, inhibitors of cGMP-PDE. Cyclic GMP plus ATP affected the agonist-antagonist muscarinic binding activities. Thus, the high affinity agonist (Carbamylcholine) binding sites disappeared, whereas, 4-DAMP, a M3 selective antagonist displayed an additional high affinity-binding site. In contrast, non-selective (atropine) and M2-selective (methoctramine and gallamine) antagonists revealed one low binding site. Moreover, the 4-DAMP-mustard alkylation of the MAChRs blocked the cGMP effect indicating that the M3AChR is the main receptor target of cGMP. Interestingly, these cGMP effects were potentiated by an activator (Sp-8-pCPT-cGMPS), and diminished by an inhibitor (Rp-8-pCPT-CGMPS), of cGMP-dependent protein kinase (PKG-II), which was detected by Western blotting using specific PKG II antibodies. Finally, plasma membrane M3AChRs were phosphorylated in a cGMP-dependent manner and this novel post-translational reversible modification at M3AChRs may act as a feedback mechanism to terminate the cGMP dependent muscarinic signal transduction cascades at the sarcolema of BTSM.

Exploiting functional domains of GRK2/3 to alter the competitive balance of pro- and anticontractile signaling in airway smooth muscle

To clarify the potential utility of targeting GRK2/3-mediated desensitization as a means of manipulating airway smooth muscle (ASM) contractile state, we assessed the specificity of GRK2/3 regulation of procontractile and relaxant G-protein-coupled receptors in ASM. Functional domains of GRK2/3 were stably expressed, or siRNA-mediated GRK2/3 knockdown was performed, in human ASM cultures, and agonist-induced signaling was assessed. Regulation of contraction of murine tracheal rings expressing GRK2 C terminus was also assessed. GRK2/3 knockdown or expression of the GRK2 C terminus caused a significant (∼ 30-90%) increase in maximal β-agonist and histamine [phosphoinositide (PI) hydrolysis] signaling, without affecting the calculated EC50. GRK2 C-terminal expression did not affect signaling by methacholine, thrombin, or LTD4. Expression of the GRK2 N terminus or kinase-dead holo-GRK2 diminished (∼ 30-70%) both PI hydrolysis and Ca(2+) mobilization by every Gq-coupled receptor examined. Under conditions of GRK2 C-terminal expression, β-agonist inhibition of methacholine-stimulated PI hydrolysis was greater. Finally, transgenic expression of the GRK2 C terminus in murine ASM enabled ∼ 30-50% greater β-agonist-mediated relaxation of methacholine-induced contraction. Collectively these data demonstrate the relative selectivity of GRKs for the β2AR in ASM and the ability to exploit GRK2/3 functional domains to render ASM hyporesponsive to contractile agents while increasing responsiveness to bronchodilating β-agonist.

Arrestins differentially regulate histamine- and oxytocin-evoked phospholipase C and mitogen-activated protein kinase signalling in myometrial cells

BACKGROUND AND PURPOSE

The uterotonins oxytocin and histamine, mediate contractile signals through specific G protein-coupled receptors, a process which is tightly controlled during gestation to prevent preterm labour. We previously identified G protein-coupled receptor kinase (GRK)2 and GRK6 as respective cardinal negative regulators of histamine H(1) and oxytocin receptor signalling. GRK-mediated phosphorylation promotes arrestin recruitment, not only desensitizing receptors but activating an increasing number of diverse signalling pathways. Here we investigate potential roles that arrestins play in the regulation of myometrial oxytocin/histamine H(1) receptor signalling.

EXPERIMENTAL APPROACH

Endogenous arrestins2 and 3 were specifically depleted using RNA-interference in a human myometrial cell line and the consequences of this for G protein-coupled receptor-mediated signalling were assessed using Ca(2+) /inositol 1,4,5-trisphophate imaging and standard mitogen-activated protein kinase (MAPK) assays.

KEY RESULTS

Depletion of arrestin3, but not arrestin2 enhanced and prolonged H(1) receptor-stimulated Ca(2+) responses, whilst depletion of either arrestin increased oxytocin receptor responses. Arrestin3 depletion decreased H(1) receptor desensitization, whilst removal of either arrestin isoform was equally effective in preventing oxytocin receptor desensitization. Following arrestin3 depletion oxytocin-induced phospho-extracellular signal-regulated kinase1/2 signals were diminished and histamine-stimulated signals virtually absent, whereas depletion of arrestin2 augmented extracellular signal-regulated kinase1/2 responses to each agonist. Conversely, depletion of arrestin3 enhanced p38 signals to each agonist, whilst arrestin2 suppression increased oxytocin-, but not histamine-induced p38 MAPK responses.

CONCLUSIONS AND IMPLICATIONS

Arrestin proteins are key regulators of H(1) and oxytocin receptor desensitization, and play integral roles mediating uterotonin-stimulated MAPK-signalling. These data provide insights into the in situ regulation of these receptor subtypes and may inform pathophysiological functioning in preterm labour.

Approaches to study GPCR regulation in native systems

The ability to assess whether individual proteins are involved in the signalling or regulation of G -protein-coupled receptor signalling is highly dependent on the pharmacological tools available. In the absence of appropriate pharmacological agents, alternative molecular approaches have been developed to alter either protein function or expression. This has included the use of mutants, for example catalytically inactive (kinase-dead) enzymes, which when overexpressed function as dominant negatives to inhibit endogenous enzyme function, and more latterly small (21-23 bp) interfering RNA dsRNA oligos, whose antisense strand is designed complementary to the target protein mRNA and which can be used to deplete target protein expression. Critically, the success of these approaches depends on the transfection efficiency, and the chosen experimental assay in the cell type studied. Therefore, three transfection techniques and their merits and drawbacks are described. In addition, one method of examining G protein-coupled receptor (GPCR) regulation, combining siRNA-mediated GRK depletion and imaging of fluorescent GPCR -signalling reporter biosensors in difficult-to-transfect cells is briefly described.

Differential G-protein-coupled receptor phosphorylation provides evidence for a signaling bar code

G-protein-coupled receptors are hyper-phosphorylated in a process that controls receptor coupling to downstream signaling pathways. The pattern of receptor phosphorylation has been proposed to generate a “bar code” that can be varied in a tissue-specific manner to direct physiologically relevant receptor signaling. If such a mechanism existed, receptors would be expected to be phosphorylated in a cell/tissue-specific manner. Using tryptic phosphopeptide maps, mass spectrometry, and phospho-specific antibodies, it was determined here that the prototypical G_q/11-coupled M₃-muscarinic receptor was indeed differentially phosphorylated in various cell and tissue types supporting a role for differential receptor phosphorylation in directing tissue-specific signaling. Furthermore, the phosphorylation profile of the M₃-muscarinic receptor was also dependent on the stimulus. Full and partial agonists to the M₃-muscarinic receptor were observed to direct phosphorylation preferentially to specific sites. This hitherto unappreciated property of ligands raises the possibility that one mechanism underlying ligand bias/functional selectivity, a process where ligands direct receptors to preferred signaling pathways, may be centered on the capacity of ligands to promote receptor phosphorylation at specific sites.

Beneficial metabolic effects caused by persistent activation of beta-cell M3 muscarinic acetylcholine receptors in transgenic mice

Previous studies have shown that β-cell M(3) muscarinic acetylcholine receptors (M3Rs) play a key role in maintaining blood glucose homeostasis by enhancing glucose-dependent insulin release. In this study, we tested the hypothesis that long-term, persistent activation of β-cell M3Rs can improve glucose tolerance and ameliorate the metabolic deficits associated with the consumption of a high-fat diet. To achieve the selective and persistent activation of β-cell M3Rs in vivo, we generated transgenic mice that expressed the Q490L mutant M3R in their pancreatic β-cells (β-M3-Q490L Tg mice). The Q490L point mutation is known to render the M3R constitutively active. The metabolic phenotypes of the transgenic mice were examined in several in vitro and in vivo metabolic tests. In the presence of 15 mm glucose and the absence of M3R ligands, isolated perifused islets prepared from β-M3-Q490L Tg mice released considerably more insulin than wild-type control islets. This effect could be completely blocked by incubation of the transgenic islets with atropine (10 μm), an inverse muscarinic agonist, indicating that the Q490L mutant M3R exhibited ligand-independent signaling (constitutive activity) in mouse β-cells. In vivo studies showed that β-M3-Q490L Tg mice displayed greatly improved glucose tolerance and increased serum insulin levels as well as resistance to diet-induced glucose intolerance and hyperglycemia. These results suggest that chronic activation of β-cell M3Rs may represent a useful approach to boost insulin output in the long-term treatment of type 2 diabetes.

Regulation of oxytocin receptor responsiveness by G protein-coupled receptor kinase 6 in human myometrial smooth muscle

Oxytocin plays an important role in the progression, timing, and modulation of uterine contraction during labor and is widely used as an uterotonic agent. We investigated the mechanisms regulating oxytocin receptor (OTR) signaling in human primary myometrial smooth muscle cells and the ULTR cell-line. Oxytocin produced concentration-dependent increases in both total [(3)H]inositol phosphate accumulation and intracellular Ca(2+) concentration ([Ca(2+)](i)); however, responses were greater and more reproducible in the ULTR cell line. Assessment of phospholipase C activity in single cells revealed that the OTR desensitizes rapidly (within 5 min) in the presence of oxytocin (100 nm). To characterize OTR desensitization further, cells were stimulated with a maximally effective concentration of oxytocin (100 nm, 30 sec) followed by a variable washout period and a second identical application of oxytocin. This brief exposure to oxytocin caused a marked decrease (>70%) in OTR responsiveness to rechallenge and was fully reversed by increasing the time period between agonist challenges. To assess involvement of G protein-coupled receptor kinases (GRKs) in OTR desensitization, cells were transfected with small interfering RNAs to cause specific > or =75% knockdown of GRKs 2, 3, 5, or 6. In both primary myometrial and ULTR cells, knockdown of GRK6 largely prevented oxytocin-induced OTR desensitization; in contrast, selective depletion of GRKs 2, 3, or 5 was without effect. These data indicate that GRK6 recruitment is a cardinal effector of OTR responsiveness and provide mechanistic insight into the likely in vivo regulation of OTR signaling in uterine smooth muscle.

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

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

Selective regulation of H1 histamine receptor signaling by G protein-coupled receptor kinase 2 in uterine smooth muscle cells

Histamine stimulates uterine contraction; however, little is known regarding the mechanism or regulation of uterine histamine receptor signaling. Here we investigated the regulation of Galpha(q/11)-coupled histamine receptor signaling in human myometrial smooth muscle cells using the inositol 1,4,5-trisphosphate biosensor pleckstrin homology domain of phospholipase-delta1 tagged to enhanced green fluorescent protein and the Ca(2+)-sensitive dye Fluo-4. Histamine addition caused concentration-dependent increases in inositol 1,4,5-trisphosphate and [Ca(2+)](i) in the ULTR human uterine smooth muscle cell line and primary human myometrial cells. These effects were completely inhibited by the H(1) histamine receptor antagonist, diphenhydramine, and were unaffected by the H(2) histamine receptor antagonist, cimetidine. ULTR and primary myometrial cells were transfected with either dominant-negative G protein-coupled receptor kinases (GRKs) or small interfering RNAs targeting specific GRKs to assess the roles of this protein kinase family in H(1) histamine receptor desensitization. Dominant-negative GRK2, but not GRK5 or GRK6, prevented H(1) histamine receptor desensitization. Similarly, transfection with short interfering RNAs (that each caused >70% depletion of the targeted GRK) for GRK2, but not GRK3 or GRK6, also prevented H(1) histamine receptor desensitization. Our data suggest that histamine stimulates phospholipase C-signaling in myometrial smooth muscle cells through H(1) histamine receptors and that GRK2 recruitment is a key mechanism in the regulation of H(1) histamine receptor signaling in human uterine smooth muscle. These data provide insights into the in situ regulation of this receptor subtype and may inform pathophysiological functioning in preterm labor and other conditions involving uterine smooth muscle dysregulation.

M3 muscarinic acetylcholine receptor-mediated signaling is regulated by distinct mechanisms

We have used RNA interference previously to demonstrate that G protein-coupled receptor kinase 2 (GRK2) regulates endogenously expressed H1 histamine receptor in human embryonic kidney 293 cells. In this report, we investigate the regulation of endogenously expressed M(3) muscarinic acetylcholine receptor (M(3) mAChR). We show that knockdown of GRK2, GRK3, or GRK6, but not GRK5, significantly increased carbachol-mediated calcium mobilization. Stable expression of wild-type GRK2 or a kinase-dead mutant (GRK2-K220R) reduced calcium mobilization after receptor activation, whereas GRK2 mutants defective in Galpha(q) binding (GRK2-D110A, GRK2-R106A, and GRK2-R106A/K220R) had no effect on calcium signaling, suggesting that GRK2 primarily regulates G(q) after M(3) mAChR activation. The knockdown of arrestin-2 or arrestin-3 also significantly increased carbachol-mediated calcium mobilization. Knockdown of GRK2 and the arrestins also significantly enhanced carbachol-mediated activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), whereas prolonged ERK1/2 activation was only observed with GRK2 or arrestin-3 knockdown. We also investigated the role of casein kinase-1alpha (CK1alpha) and found that knockdown of CK1alpha increased calcium mobilization but not ERK activation. In summary, our data suggest that multiple proteins dynamically regulate M(3) mAChR-mediated calcium signaling, whereas GRK2 and arrestin-3 play the primary role in regulating ERK activation.

Oxidative stress reduces renal dopamine D1 receptor-Gq/11alpha G protein-phospholipase C signaling involving G protein-coupled receptor kinase 2

The dopamine D1 receptors (D1R), expressed in renal proximal tubules, participate in the regulation of sodium transport. A defect in the coupling of the D1R to its G protein/effector complex in renal tubules has been reported in various conditions associated with oxidative stress. Because G protein-coupled receptor kinases (GRKs) are known to play an important role in D1R desensitization, we tested the hypothesis that increased oxidative stress in obese Zucker rats may cause GRK2 upregulation and, subsequently, D1R dysfunction. Lean and obese rats were given normal diet or diet supplemented with antioxidant lipoic acid for 2 wk. Compared with lean rats, obese rats exhibited oxidative stress, D1R were uncoupled from G(q/11)alpha at basal level, and SKF-38393 failed to elicit D1R-G protein coupling, stimulate phospholipase C (PLC), and inhibit Na-K-ATPase activity. These animals showed increased basal protein kinase C (PKC) activity and membranous translocation of GRK2 and increased GKR2-G(q/11)alpha interaction and D1R serine phosphorylation. Enzymatic dephosphorylation of D1R restored SKF-38393-induced adenylyl cyclase stimulation but not PLC activation. Treatment of obese rats with lipoic acid restored D1R-G protein coupling and SKF-38393-induced PLC stimulation and Na-K-ATPase inhibition. Lipoic acid treatment also normalized PKC activity, GRK2 sequestration, and GKR2-G(q/11)alpha interaction. In conclusion, these data show that oxidative stress increases PKC activity causing GRK2 membranous translocation. GRK2 interacts with G(q/11)alpha and acts, at least in part, as a regulator of G protein signaling leading to the D1R-G(q/11)alpha uncoupling, causing inability of SKF-38393 to stimulate PLC and inhibit Na/K-ATPase. Lipoic acid, while reducing oxidative stress, normalized PKC activity and restored D1R-G(q/11)alpha-PLC signaling and the ability of SKF-38393 to inhibit Na-K-ATPase activity.

PKC-dependent regulation of the receptor locus dominates functional consequences of cysteinyl leukotriene type 1 receptor activation

Leukotrienes are important lipid mediators of asthma that contribute to airway inflammation and bronchoconstriction. Critical mechanisms for physiological regulation of the main G protein-coupled receptor (GPCR) mediating the leukotriene responses in asthma, cysteinyl leukotriene type 1 receptor (CysLT1R), have not been delineated. Although desensitization of GPCRs is a well-established phenomenon, studies demonstrating its physiological relevance are lacking. Here, we demonstrate that relief of PKC-mediated desensitization of endogenous CysLT1Rs augments multiple LTD4-stimulated cellular functions, with associated increases in intracellular signaling events. In analyses of airway smooth muscle contraction ex vivo, PKC inhibition augmented LTD4-stimulated contraction, and increased phosphoinositide hydrolysis and calcium flux in both murine and human airway smooth muscle cells. Similarly, for human monocytes, PKC inhibition augmented LTD4-stimulated calcium flux and cell migration assessed in transwell chamber experiments and also augmented LTD4-induced production of monocyte chemotactic protein assessed by ELISA. In contrast, PKC inhibition had no effect or slightly attenuated these cell functions and signaling events promoted by other receptor agonists, suggesting that despite antithetical effects on downstream events, desensitization of the CysLT1R is the principal mechanism by which PKC regulates the functional consequences of CysLT1R activation.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Regulation of bladder muscarinic receptor subtypes by experimental pathologies

1 The M3 muscarinic receptor subtype is widely accepted as the receptor on smooth muscle cells that mediates cholinergic contraction of the normal urinary bladder and other smooth muscle tissues, however, we have found that the M2 receptor participates in contraction under certain abnormal conditions. The aim of this study was to determine the effects of various experimental pathologies on the muscarinic receptor subtype mediating urinary bladder contraction. 2 Experimental pathologies resulting in bladder hypertrophy (denervation and outlet obstruction) result in an up-regulation of bladder M2 receptors and a change in the receptor subtype mediating contraction from M3 towards M2. Preventing the denervation-induced bladder hypertrophy by urinary diversion prevents this shift in contractile phenotype indicating that hypertrophy is responsible as opposed to denervation per se. 3 The hypertrophy-induced increase in M2 receptor density and contractile response is accompanied by an increase in the tissue concentrations of mRNA coding for the M2 receptor subtype, however, M3 receptor protein density does not correlate with changes in M3 receptor tissue mRNA concentrations across different experimental pathologies. 4 This shift in contractile phenotype from M3 towards M2 subtype is also observed in aged male Sprague-Dawley rats but not females or either sex of the Fisher344 strain of rats. 5 Four repeated, sequential agonist concentration response curves also cause this shift in contractile phenotype in normal rat bladder strips in vitro, as evidenced by a decrease in the affinity of the M3 selective antagonist p-fluoro-hexahydro-sila-diphenidol (p-F-HHSiD). 6 A similar decrease in the contractile affinity of M3 selective antagonists (darifenacin and p-F-HHSiD) is also observed in bladder specimens from patients with neurogenic bladder as well as certain organ transplant donors. 7 It is concluded that although the M3 receptor subtype predominantly mediates contraction under normal circumstances, the M2 receptor subtype can take over a contractile role when the M3 subtype becomes inactivated by, for example, repeated agonist exposures or bladder hypertrophy. This finding has substantial implications for the clinical treatment of abnormal bladder contractions.

M2 and M3 muscarinic receptor activation of urinary bladder contractile signal transduction. I. Normal rat bladder

The muscarinic receptor subtype-activated signal transduction mechanisms mediating rat urinary bladder contraction are incompletely understood. M(3) mediates normal rat bladder contractions; however, the M(2) receptor subtype has a more dominant role in contractions of the hypertrophied bladder. Normal bladder muscle strips were exposed to inhibitors of enzymes thought to be involved in signal transduction in vitro followed by a single cumulative concentration-response curve to the muscarinic receptor agonist carbachol. The outcome measures were the maximal contraction, the potency of carbachol, and the affinity of the M(3) -selective antimuscarinic agent darifenacin for inhibition of contraction. Inhibition of phosphoinositide-specific phospholipase C (PI-PLC) with 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphorylcholine (ET-18-OCH(3)) reduces carbachol potency and reduces darifenacin affinity, whereas inhibition of phosphatidyl choline-specific phospholipase C (PC-PLC) with O-tricyclo[5.2.1.02,6]dec-9-yl dithiocarbonate potassium salt (D609) attenuates the carbachol maximal contraction. Inhibition of rho kinase with (R)-(+)-trans-4-(1-aminoethyl)-N-(4-pyridyl)cyclohexanecarboxamide dihydrochloride (Y-27632) reduces carbachol potency and increases darifenacin affinity. Inhibition of rho kinase, protein kinase A (PKA), and protein kinase G (PKG) with 1-(5-isoquinolinesulfonyl)-homopiperazine.HCl (HA-1077) reduces the carbachol maximal contraction, carbachol potency, and darifenacin affinity. Inhibition of protein kinase C (PKC) with chelerythrine increases darifenacin affinity, whereas inhibition of rho kinase, PKA, PKG, and PKC with 1-(5-isoquinolinesulfonyl)-2-methylpiperazine.2HCl (H7) reduces the carbachol maximum and carbachol potency while increasing darifenacin affinity. Inhibition of rho kinase, PKA, and PKG with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide.2HCl (H89) reduces carbachol maximum and carbachol potency. Both the M(2) and the M(3) receptor subtype are involved in normal rat bladder contractions. The M(3)subtype seems to mediate contraction by activation of PI-PLC, PC-PLC, and PKA, whereas the M(2) signal transduction cascade may include activation of rho kinase, PKC, and an additional contractile signal transduction mechanism independent of rho kinase or PKC.

Roles of phosphorylation-dependent and -independent mechanisms in the regulation of M1 muscarinic acetylcholine receptors by G protein-coupled receptor kinase 2 in hippocampal neurons

When co-expressed with the inositol 1,4,5-trisphosphate biosensor eGFP-PH(PLC delta), G protein-coupled receptor kinase 2 (GRK2) can suppress M1 muscarinic acetylcholine (mACh) receptor-mediated phospholipase C signaling in hippocampal neurons through a phosphorylation-independent mechanism, most likely involving the direct binding of the RGS homology domain of GRK2 to G alpha(q/11). To define the importance of this mechanism in comparison with classical, phosphorylation-dependent receptor regulation by GRKs, we have examined M1 mACh receptor signaling in hippocampal neurons following depletion of GRK2 and also in the presence of non-G alpha(q/11)-binding GRK2 mutants. Depletion of neuronal GRK2 using an antisense strategy almost completely inhibited M1 mACh receptor desensitization without enhancing acute agonist-stimulated phospholipase C activity. By stimulating neurons with a submaximal agonist concentration before (R1) and after (R2) a period of exposure to a maximal agonist concentration, an index (R2/R1) of agonist-induced desensitization of signaling could be obtained. Co-transfection of neurons with either a non-G alpha(q/11)-binding (D110A) GRK2 mutant or the catalytically inactive (D110A,K220R)GRK2 did not suppress acute M1 mACh receptor-stimulated inositol 1,4,5-trisphosphate production. However, using the desensitization (R2/R1) protocol, it could be shown that expression of (D110A)GRK2 enhanced, whereas (D110A,K220R)GRK2 inhibited, agonist-induced M1 mACh receptor desensitization. In Chinese hamster ovary cells, the loss of G alpha(q/11) binding did not affect the ability of the (D110A)GRK2 mutant to phosphorylate M1 mACh receptors, whereas expression of (D110A,K220R)GRK2 had no effect on receptor phosphorylation. These data indicate that in hippocampal neurons endogenous GRK2 is a key regulator of M1 mACh receptor signaling and that the regulatory process involves both phosphorylation-dependent and -independent mechanisms.

Imaging of muscarinic acetylcholine receptor signaling in hippocampal neurons: evidence for phosphorylation-dependent and -independent regulation by G-protein-coupled receptor kinases

We used the inositol 1,4,5-trisphosphate (IP3) biosensor, the pleckstrin homology (PH) domain of PLCdelta1 (phospholipase C) tagged with enhanced green fluorescent protein (eGFP-PH(PLCdelta)), to examine muscarinic acetylcholine (mACh) receptor regulation of phospholipase C/IP3 signaling in intact single hippocampal neurons in "real time." Initial experiments produced a pharmacological profile consistent with the presence of a predominant M1 mACh receptor population coupled to the IP3 response. To investigate M1 mACh receptor regulation, neurons were stimulated with approximate EC50 concentrations of the mACh receptor agonist methacholine before (R1) and after (R2) a short (60 sec) exposure to a high concentration of agonist. This resulted in a marked attenuation in the R2 relative to R1 response. Inhibition of endogenous GRK6 (G-protein-coupled receptor kinase) activity, by the introduction of catalytically inactive (K215R)GRK6, partially reversed the attenuation of agonist-induced responsiveness, whereas overexpression of wild-type GRK6 increased receptor desensitization. Manipulation of endogenous GRK2 activity through introduction of either wild-type or catalytically inactive GRK2 ((K220R)GRK2) almost completely inhibited agonist-stimulated IP3 production, implying a phosphorylation-independent regulation of M1 mACh receptor signaling, most probably mediated by a GRK2 N-terminal RGS-like (regulator of G-protein signaling) domain interaction with GTP-bound Galpha(q/11). Together, our data suggest a role for both phosphorylation-dependent and -independent regulation of M1 mACh receptors in hippocampal neurons.

Clathrin-mediated endocytosis of m3 muscarinic receptors. Roles for Gbetagamma and tubulin

Receptors as well as some G protein subunits internalize after agonist stimulation. It is not clear whether Galpha(q) or Gbetagamma undergo such regulated translocation. Recent studies demonstrate that m3 muscarinic receptor activation in SK-N-SH neuroblastoma cells causes recruitment of tubulin to the plasma membrane. This subsequently transactivates Galpha(q) and activates phospholipase Cbeta1. Interaction of tubulin-GDP with Gbetagamma at the offset of phospholipase Cbeta1 signaling appears involved in translocation of tubulin and Gbetagamma to vesicle-like structures in the cytosol (Popova, J. S., and Rasenick, M. M. (2003) J. Biol. Chem. 278, 34299-34308). The relationship of this internalization to the clathrin-mediated endocytosis of the activated m3 muscarinic receptors or Galpha(q) involvement in this process has not been clarified. To test this, SK-N-SH cells were treated with carbachol, and localization of Galpha(q), Gbetagamma, tubulin, clathrin, and m3 receptors were analyzed by both cellular imaging and biochemical techniques. Upon agonist stimulation both tubulin and clathrin translocated to the plasma membrane and co-localized with receptors, Galpha(q) and Gbetagamma. Fifteen minutes later receptors, Gbetagamma and tubulin, but not Galpha(q), internalized with the clathrin-coated vesicles. Coimmunoprecipitation of m3 receptors with Gbetagamma, tubulin, and clathrin from the cytosol of carbachol-treated cells was readily observed. These data suggested that Gbetagamma subunits might organize the formation of a multiprotein complex linking m3 receptors to tubulin since they interacted with both proteins. Such protein assemblies might explain the dynamin-dependent but beta-arrestin-independent endocytosis of m3 muscarinic receptors since tubulin interaction with dynamin might guide or insert the complex into clathrin-coated pits. This novel mechanism of internalization might prove important for other beta-arrestin-independent endocytic pathways. It also suggests cross-regulation between G protein-mediated signaling and the dynamics of the microtubule cytoskeleton.

Characterization of GRK2 RH domain-dependent regulation of GPCR coupling to heterotrimeric G proteins

Heterotrimeric guanine nucleotide (G)-coupled receptors (GPCRs) form the largest family of integral membrane proteins. GPCR activation by an agonist promotes the exchange of GDP for GTP on the Galpha subunit of the heterotrimeric G protein. The dissociated Galpha and Gbetagamma subunits subsequently modulate the activity of a diverse assortment of effector systems. GPCR signaling via heterotrimeric G proteins is attenuated rapidly by the engagement of protein kinases. The canonical model for GPCR desensitization involves G protein-coupled receptor kinase (GRK)-dependent receptor phosphorylation to promote the binding of arrestin proteins that function to sterically block receptor:G-protein interactions. GRK2 and GRK3 have been shown to interact with Galphaq via the regulator of G-protein signaling (RGS) homology (RH) domain localized within their amino-terminal domains. It now appears that the G-protein uncoupling of many GPCRs linked to Galphaq, in particularly metabotropic glutamate receptors, may be mediated by the GRK2 RH domain via a phosphorylation-independent mechanism. This article reviews much of the background and methodology required for the characterization of the GRK2 phosphorylation-independent attenuation of GPCR signaling.

Non-visual GRKs: are we seeing the whole picture?

G-protein-coupled receptor kinases (GRKs) comprise a family of seven mammalian serine/threonine protein kinases that phosphorylate and regulate agonist-occupied or constitutively active G-protein-coupled receptors (GPCRs). Studies of the details and consequences of these mechanisms have focused heavily on the original beta-adrenoceptor kinase (beta-ARK) family (GRK2 and GRK3) and, in particular, on phosphorylation-dependent recruitment of adaptor proteins such as the beta-arrestins. However, recent work has indicated roles for the other, non-visual GRKs (GRK4, GRK5 and GRK6) and has revealed potential phosphorylation-independent regulation of GPCRs by GRK2 and GRK3. In this article, we review this newer information and attempt to put it into context with GRKs as physiological regulators that could be appropriate targets for future pharmacological intervention.

Specificity of g protein-coupled receptor kinase 6-mediated phosphorylation and regulation of single-cell m3 muscarinic acetylcholine receptor signaling

Previously we have shown that G protein-coupled receptor kinase (GRK) 6 plays a major role in the regulation of the human M3 muscarinic acetylcholine receptor (M3 mAChR) in the human neuroblastoma SH-SY5Y. However, 30-fold overexpression of the catalytically inactive, dominant-negative K215RGRK6 produced only a 50% suppression of M3 mAChR phosphorylation and desensitization. Here, we have attempted to determine whether other endogenous kinases play a role in the regulation of M3 mAChR signaling. In contrast to the clear attenuating effect of K215RGRK6 expression on M3 mAChR regulation, dominant-negative forms of GRKs (K220RGRK2, K220RGRK3, K215RGRK5) and casein kinase 1alpha (K46RCK1alpha) were without effect. In addition, inhibition of a variety of second-messenger-regulated kinases and the tyrosine kinase Src also had no effect upon agonist-stimulated M3 mAChR regulation. To investigate further the desensitization process we have followed changes in inositol 1,4,5-trisphosphate in single SHSY5Y cells using the pleckstrin homology domain of PLCdelta1 tagged with green fluorescent protein (eGFP-PHPLCdelta1). Stimulation of cells with approximate EC50 concentrations of agonist before and after a desensitizing period of agonist exposure resulted in a marked attenuation of the latter response. Altered GRK6 activity, through overexpression of wild-type GRK6 or K215RGRK6, enhanced or reduced the degree of M3 mAChR desensitization, respectively. Taken together, our data indicate that M3 mAChR desensitization is mediated by GRK6 in human SH-SY5Y cells, and we show that receptor desensitization of phospholipase C signaling can be monitored in 'real-time' in single, living cells.

Regulation of muscarinic receptor function in developing oligodendrocytes by agonist exposure

1 Oligodendrocytes, the myelin forming cells in the CNS, express muscarinic acetylcholine receptors (mAChR), primarily M3, coupled to various signal transduction pathways. 2 In the present study we have investigated whether mAChR undergo functional agonist-induced regulation in cultured oligodendrocyte progenitors and differentiated oligodendrocytes. 3 The muscarinic agonist, carbachol (CCh) caused a time-dependent desensitization of phosphoinositide (PI) hydrolysis, and the internalization and down-regulation of receptors. Short-time desensitization (5 min) of PI hydrolysis occurred without receptor internalization and reached 54% by 1 h. The same treatment decreased cell surface receptors labelled with the non-permeable ligand [(3)H]-NMS by 47%, while total receptor density ([(3)H]-scopolamine binding) decreased by 30%. Longer CCh treatment down-regulated receptors by 70% and desensitized the PI response by 80%. 4 Although protein kinase C (PKC) activation desensitized mAChR, CCh-mediated desensitization was independent of PKC. 5 Inhibition of receptor endocytosis by low temperature during the pre-stimulation period or in the presence of hyperosmotic sucrose (0.5 M) blocked desensitization, receptor internalization and down-regulation. 6 Recovery of surface mAChR and their functional activity following down-regulation was slow, returning to control levels by 24 h after agonist removal. In progenitor cells, dose-response curves for CCh-mediated PI hydrolysis and c-fos mRNA expression showed that newly synthesized mAChR were supersensitive after recovery. 7 Overall, the present results provide evidence of functional agonist-mediated mAChR regulation in brain oligodendroglial cells.

Alteration of lipids, G proteins, and PKC in cell membranes of elderly hypertensives

In this study, we quantified the levels of lipids and signaling proteins in erythrocyte membranes from elderly normotensive and hypertensive subjects. In hypertensive subjects, the cholesterol/phospholipid ratio increased significantly in erythrocyte membranes, owing to the reduction of phospholipid levels concomitant with a rise in the levels of cholesterol. In addition, differences were also found in the amount of fatty acids in both phospholipid and cholesterol esters. Erythrocyte membranes from hypertensive subjects contained higher levels of monounsaturated and lower levels of polyunsaturated fatty acids. On the other hand, signaling proteins such as G proteins and protein kinase C have been implicated in the control of blood pressure. Previous studies have shown that the cellular localization and the activity of these proteins are modulated by the type and the abundance of membrane lipids. For this reason, we assessed the levels of these signaling molecules in the membrane. We found that the levels of membrane-associated (active/preactive) G proteins (Galpha(i), Galpha(o), and Gbeta) and protein kinase C were significantly reduced in hypertensive subjects. We believe that these alterations could be related to the etiopathology of hypertension in elderly subjects or alternatively may correspond to adaptive compensatory mechanisms.

Measurement of agonist-dependent and -independent signal initiation of alpha(1b)-adrenoceptor mutants by direct analysis of guanine nucleotide exchange on the G protein galpha(11)

Immunoprecipitation of a fusion protein between the alpha(1b)-adrenoceptor and Galpha(11) following a [(35)S]GTPgammaS [guanosine-5'-O-(3-thio)triphosphate] binding assay resulted in incorporation of low levels of nucleotide. The agonist phenylephrine increased incorporation some 30-fold. Agonist-induced binding represented 1.0 mol of [(35)S]GTPgammaS/mol of fusion protein. This was to the G protein linked to the receptor rather than endogenous Galpha(q)/Galpha(11) as a fusion protein containing the alpha(1b)-adrenoceptor and a form of Galpha(11) (G(208)A) unable to exchange guanine nucleotides effectively, bound [(35)S]GTPgammaS very poorly. Fusion proteins between A(293)E, D(142)A, and 3CAM mutants of the alpha(1b)-adrenoceptor and Galpha(11) bound substantially greater levels of [(35)S]GTPgammaS in the absence of agonist than the fusion incorporating the wild-type receptor. Constitutive binding of the nucleotide induced by these mutants was only 20% of the level achieved by phenylephrine. These mutant receptors thus do not provide an accurate mimic of the agonist-occupied state. Phentolamine reduced the binding of [(35)S]GTPgammaS and acted as a partial inverse agonist for each of the constitutively active mutants. [(35)S]GTPgammaS binding to Galpha(11) was elevated by phenylephrine in both wild-type and constitutively active mutant forms of the fusion proteins, but agonist potency and binding affinity were 50 times higher for the fusions containing the mutated receptors. These studies provide the first direct demonstration of the capacity of constitutively active mutants of a receptor to stimulate guanine nucleotide exchange on the alpha subunit of a G(q) family G protein and defines a strategy potentially suitable for any receptor that couples to these G proteins.

Phosphorylation-independent regulation of metabotropic glutamate receptor signaling by G protein-coupled receptor kinase 2

The accepted paradigm for G protein-coupled receptor kinase (GRK)-mediated desensitization of G protein-coupled receptors involves GRK-mediated receptor phosphorylation followed by the binding of arrestin proteins. Although GRKs contribute to metabotropic glutamate receptor 1 (mGluR1) inactivation, beta-arrestins do not appear to be required for mGluR1 G protein uncoupling. Therefore, we investigated whether the phosphorylation of serine and threonine residues localized within the C terminus of mGluR1a is sufficient to allow GRK2-mediated attenuation of mGluR1a signaling. We find that the truncation of the mGluR1a C-terminal tail prevents mGluR1a phosphorylation and that GRK2 does not contribute to the phosphorylation of an mGluR1 splice variant (mGluR1b). However, mGluR1a-866Delta- and mGluR1b-stimulated inositol phosphate formation is attenuated following GRK2 expression. The expression of the GRK2 C-terminal domain to block membrane translocation of endogenous GRK2 increases mGluR1a-866Delta- and mGluR1b-stimulated inositol phosphate formation, presumably by blocking membrane translocation of GRK2. In contrast, expression of the kinase-deficient GRK2-K220R mutant inhibits inositol phosphate formation by these unphosphorylated receptors. Expression of the GRK2 N-terminal domain (residues 45-185) also attenuates both constitutive and agonist-stimulated mGluR1a, mGluR1a-866Delta, and mGluR1b signaling, and the GRK2 N terminus co-precipitates with mGluR1a. Taken together, our observations indicate that attenuation of mGluR1 signaling by GRK2 is phosphorylation-independent and that the interaction of the N-terminal domain of GRK2 with mGluR1 contributes to the regulation of mGluR1 G protein coupling.

Effective information transfer from the alpha 1b-adrenoceptor to Galpha 11 requires both beta/gamma interactions and an aromatic group four amino acids from the C terminus of the G protein

Co-expression of the alpha(1b)-adrenoreceptor and Galpha(11) in cells derived from a Galpha(q)/Galpha(11) knock-out mouse allows agonist-mediated elevation of intracellular Ca(2+) levels that is transduced by beta/gamma released from the G protein alpha subunit. Mutation of Tyr(356) of Galpha(11) to Phe, within a receptor contact domain, had little effect on function but this was reduced greatly by alteration to Ser and virtually eliminated by conversion to Asp. This pattern was replicated following incorporation of each form of Galpha(11) into fusion proteins with the alpha(1b)-adrenoreceptor. Following a [(35)S]guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding assay, immunoprecipitation of the wild type alpha(1b)-adrenoreceptor-Galpha(11) fusion protein indicated that the agonist phenylephrine stimulated guanine nucleotide exchange on Galpha(11) more than 30-fold. Information transfer by agonist was controlled in residue 356 Galpha(11) mutants with rank order Tyr > Phe > Trp > Ile > Ala = Gln = Arg > Ser > Asp, although these alterations did not alter the binding affinity of either phenylephrine or an antagonist ligand. Mutation of a beta/gamma contact interface in the alpha(1b)-adrenoreceptor-Tyr(356) Galpha(11) fusion protein did not alter ligand binding affinity but did reduce greatly beta/gamma binding and phenylephrine stimulation of [(35)S]GTPgammaS binding. It also prevented agonist elevation of intracellular Ca(2+) levels, as did a mutation in Galpha(11) that prevents G protein subunit dissociation. These results indicate that a bulky aromatic group is required four amino acids from the C terminus of Galpha(11) to maximize information transfer from an agonist-occupied receptor and disprove the hypothesis that tyrosine phosphorylation of this residue is required for G protein activation (Umemori, H., Inoue, T., Kume, S., Sekiyama, N., Nagao, M., Itoh, H., Nakanishi, S., Mikoshiba, K., and Yamamoto, T. (1997) Science 276, 1878-1881). This is distinct from Galpha(i1), where hydrophobicity of the amino acid is the key determinant at this location. They also further demonstrate a key role for the beta/gamma complex in enhancing receptor to G protein alpha subunit information transfer.

Are we beta-ARKing up the wrong tree? Casein kinase 1 alpha provides an additional pathway for GPCR phosphorylation

Although originally linked to receptor desensitization, G-protein-coupled receptor (GPCR) phosphorylation has now been implicated in coupling receptors to specific signalling pathways. Generally, this phosphorylation event is thought to be mediated by one of the members of the GPCR kinase (GRK) family. However, recent studies have indicated that protein kinases distinct from the GRK family might also be involved in agonist-mediated GPCR phosphorylation. This review analyses the approaches employed to investigate the nature of GPCR phosphorylation and discusses recent developments implicating other kinases, particularly casein kinase 1 alpha, in the phosphorylation of GPCRs.

Interactions between opioid and chemokine receptors: heterologous desensitization

The opioid and chemokine receptors are both members of the seven transmembrane G protein-coupled receptor (GPCR) superfamily. Desensitization is believed to be a major element of the regulation of the function of these receptors, and recent findings suggest that both agonist-dependent (homologous) desensitization and heterologous desensitization can control receptor activity. The cross-desensitization between opioid and chemokine receptors has significant implications for our understanding of both the regulation of leukocyte trafficking, as well as the regulation of chemokine receptor function in inflammatory disease states. We also review findings which suggest that pro-inflammatory chemokine receptor-induced heterologous desensitization of opioid receptors has important implications for the regulation of opioid receptor function in the nervous system.

Endogenous G protein-coupled receptor kinase 6 Regulates M3 muscarinic acetylcholine receptor phosphorylation and desensitization in human SH-SY5Y neuroblastoma cells

We have previously shown that overexpression of G protein-coupled receptor kinase 6 (GRK6) enhanced the phosphorylation and desensitization of the endogenously expressed M(3) muscarinic acetylcholine (mACh) receptor in human SH-SY5Y neuroblastoma cells. In this study we have examined the potential role of endogenous GRK6 in the regulation of M(3) mACh receptor by blocking its action through the introduction of a kinase-dead, dominant-negative GRK6 ((K215R)GRK6). (K215R)GRK6 expression inhibited methacholine-stimulated M(3) mACh receptor phosphorylation by 50% compared with plasmid transfected control cells. Guanosine-5'-O-(3-[(35)S]thio)triphosphate binding and immunoprecipitation studies, conducted after agonist pretreatment (3 min), indicated that M(3) mACh receptor-G alpha(q/11) uncoupling was attenuated by 50% in cells expressing (K215R)GRK6 when compared with control cells. In contrast, expression of the related dominant-negative kinase (K215R)GRK5 had no effect on M(3) mACh receptor phosphorylation or uncoupling. Time course studies also showed that agonist-stimulated [(3)H]inositol phosphate accumulations were more sustained in cells expressing (K215R)GRK6 compared with control and (K215R)GRK5-expressing cells, whereas (K215R)GRK6 expression had no effect on the phospholipase C response to direct stimulation of G proteins with AlF(4)(-). The ability of (K215R)GRK6 to inhibit agonist-mediated M(3) mACh receptor phosphorylation and G protein uncoupling suggests that endogenous GRK6 mediates, at least in part, M(3) mACh receptor desensitization in the SH-SY5Y cell line.