Physiological roles of G-protein-coupled receptor kinases revealed by gene-targeting technology

Lithium treatment attenuates muscarinic M(1) receptor dysfunction

OBJECTIVE

Altered muscarinic acetylcholine receptor levels and receptor-coupled signaling processes have been reported in mood disorders. M(1) , one of five muscarinic receptor subtypes, couples to the phospholipase C/protein kinase C and extracellular signal-regulated kinase (ERK) pathways. Mood stabilizers regulate these pathways. MicroRNAs (miRNAs) are small noncoding RNAs that suppress translation in a sequence-selective manner. Lithium downregulates several miRNAs, including let-7b and let-7c. One predicted target of let-7b and let-7c is the M(1) receptor. We hypothesized that miRNAs regulate M(1) receptor translation, and that disrupted M(1) expression leads to aberrant behaviors and disrupted downstream signaling pathways that are rescued by lithium treatment.

METHODS

The effects of miRNAs and chronic treatment with mood stabilizers on M(1) levels were tested in primary cultures and in rat frontal cortex. Effects of M(1) ablation and chronic treatment with mood stabilizers on several signaling cascades and M(1) -modulated behaviors were examined in wild-type and M(1) knockout mice.

RESULTS

  Let-7b, but not let-7c, negatively regulated M(1) levels. Chronic treatment with lithium, but not valproate, increased M(1) levels in the rat cortex. M(1) knockout mice exhibit ERK pathway deficits and behavioral hyperactivity; chronic treatment with lithium attenuated these deficits and hyperactivity.

CONCLUSIONS

Lithium treatment can affect M(1) receptor function through intracellular signaling enhancement and, in situations without M(1) ablation, concomitant receptor upregulation via mechanisms involving miRNAs. Muscarinic dysfunction may contribute to mood disorders, while M(1) receptors and the downstream ERK pathway may serve as potential therapeutic targets for alleviating manic symptoms such as psychomotor hyperactivity.

Antagonism of muscarinic M1 receptors by dicyclomine inhibits the consolidation of morphine-associated contextual memory

M1 muscarinic receptor has been shown to be involved in cognitive functions of the brain. Conditioned place preference (CPP) paradigm involves memory for the association between environmental stimuli and the rewarding properties produced by a treatment. Using a balanced CPP design, we studied the possible involvement of M1 muscarinic receptors on the acquisition, expression and consolidation of morphine place conditioning in male mice. Subcutaneous administration of morphine sulphate-induced CPP in a dose-dependent manner. Using a 6-day schedule of conditioning, it was found that dicyclomine, an M1 muscarinic antagonist, significantly reduced the time spent by mice in the morphine compartment when given immediately, but not 6h, after each conditioning session (consolidation). It had no effect when administered 30 min before each conditioning session during CPP training period (acquisition) or 30 min before testing for place preference in the absence of morphine (expression). It is concluded that M1 muscarinic receptors may play a time-dependent role in the consolidation of reward-related memory of morphine.

Behavioral effects of morphine and cocaine in M1 muscarinic acetylcholine receptor-deficient mice

RATIONALE

Muscarinic acetylcholine receptors (M1-M5) modulate the activity of the central nervous system and an array of physiological functions. Recent evidence has also implicated muscarinic receptors in behavioral effects of drugs of abuse such as morphine and cocaine. However, the genetic similarity between muscarinic receptors and the coexpression of multiple subtypes in most cells has impeded the development of selective antagonists and the determination of the role of each muscarinic receptor subtype in morphine's and cocaine's behavioral effects.

OBJECTIVE

The present studies employ mice deficient in the M1 receptor subtype (M1 KO) to assess morphine antinociception (2.5, 5.0, 10, or 20 mg/kg) and the conditioned rewarding effects of morphine and cocaine (2.5, 5.0, or 10 mg/kg).

METHODS

M1 KO and their wild-type (WT) littermates were tested using a 56 degrees C hotplate assay and a conditioned place preference procedure. Parallel studies using the M1 receptor antagonist, pirenzepine, were also conducted in the background strain C57BL6 mice.

RESULTS

The results demonstrate that M1 KO mice display a greater antinociceptive effect of morphine in the hotplate assay; however, the effects of morphine as well as cocaine were attenuated in the conditioned place preference procedure. Comparable results were obtained with the pharmacological antagonism of the M1 receptor by pirenzepine.

CONCLUSIONS

These results suggest a modulatory role of the M1 muscarinic receptor in opioid antinociception and conditioned drug reward, and demonstrate the utility of M1 receptor knockout models for the determination of the role of the M1 subtype in the behavioral effects of morphine and cocaine.

Homologous desensitization of signalling by the beta (β) isoform of the human thromboxane A2 receptor

Thromboxane (TX) A(2) is a potent stimulator of platelet activation/aggregation and smooth muscle contraction and contributes to a variety of pathologies within the vasculature. In this study, we investigated the mechanism whereby the cellular responses to TXA(2) mediated through the TPbeta isoform of the human TXA(2) receptor (TP) are dynamically regulated by examining the mechanism of agonist-induced desensitization of intracellular signalling and second messenger generation by TPbeta. It was established that TPbeta is subject to profound agonist-induced homologous desensitization of signalling (intracellular calcium mobilization and inositol 1,3,5 trisphosphate generation) in response to stimulation with the TXA(2) mimetic U46619 and this occurs through two key mechanisms: TPbeta undergoes partial agonist-induced desensitization that occurs through a GF 109203X-sensitive, protein kinase (PK)C mechanism whereby Ser(145) within intracellular domain (IC)(2) has been identified as the key phospho-target. In addition, TPbeta also undergoes more profound and sustained agonist-induced desensitization involving G protein-coupled receptor kinase (GRK)2/3-phosphorylation of both Ser(239) and Ser(357) within its IC(3) and carboxyl-terminal C-tail domains, respectively. Inhibition of phosphorylation of either Ser(239) or Ser(357), through site directed mutagenesis, impaired desensitization while mutation of both Ser(239) and Ser(357) almost completely abolished desensitization of signalling, GRK phosphorylation and beta-arrestin association, thereby blocking TPbeta internalization. These data suggest a model whereby agonist-induced PKC phosphorylation of Ser(145) partially impairs. TPbeta signalling while GRK2/3 phosphorylation at both Ser(239) and Ser(357) within its IC(3) and C-tail domains, respectively, sterically inhibits G-protein coupling, profoundly desensitizing signalling, and promotes beta-arrestin association and, in turn, facilitates TPbeta internalization. Thromboxane (TX) A(2) is a potent stimulator of platelet aggregation and smooth muscle contraction and contributes to a variety of vascular pathologies. Herein the mechanism whereby the cellular responses to TXA(2) mediated through the TPbeta isoform of the human TXA(2) receptor (TP) are dynamically regulated was investigated by examining the mechanism of its agonist-induced desensitization of intracellular signalling and second messenger generation. TPbeta is subject to profound agonist-induced homologous desensitization of signalling (intracellular calcium mobilization and inositol 1,3,5 trisphosphate generation) in response to stimulation with the TXA(2) mimetic U46619 and this occurs through two key mechanisms: TPbeta undergoes partial agonist-induced desensitization that occurs through a GF 109203X-sensitive, protein kinase (PK)C mechanism whereby Ser(145) within intracellular domain (IC)(2) was identified as the key phospho-target. In addition, TPbeta also undergoes more profound and sustained agonist-induced desensitization involving G protein-coupled receptor kinase (GRK)2/3-phosphorylation of both Ser(239) and Ser(357) within its IC(3) and carboxyl-terminal C-tail domains, respectively. Inhibition of phosphorylation of either Ser(239) or Ser(357), through site directed mutagenesis, impaired desensitization while mutation of both Ser(239) and Ser(357) almost completely abolished desensitization of signalling, GRK phosphorylation and beta-arrestin association, thereby blocking TPbeta internalization. These data suggest a model whereby agonist-induced PKC phosphorylation of Ser(145) partially impairs TPbeta signalling while GRK2/3 phosphorylation at both Ser(239) and Ser(357) within its IC(3) and C-tail domains, respectively, sterically inhibits G-protein coupling, profoundly desensitizing signalling, and promotes beta-arrestin association and, in turn, facilitates TPbeta internalization.

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.

Desensitization of G protein-coupled receptors and neuronal functions

G protein-coupled receptors (GPCRs) have proven to be the most highly favorable class of drug targets in modern pharmacology. Over 90% of nonsensory GPCRs are expressed in the brain, where they play important roles in numerous neuronal functions. GPCRs can be desensitized following activation by agonists by becoming phosphorylated by members of the family of G protein-coupled receptor kinases (GRKs). Phosphorylated receptors are then bound by arrestins, which prevent further stimulation of G proteins and downstream signaling pathways. Discussed in this review are recent progress in understanding basics of GPCR desensitization, novel functional roles, patterns of brain expression, and receptor specificity of GRKs and beta arrestins in major brain functions. In particular, screening of genetically modified mice lacking individual GRKs or beta arrestins for alterations in behavioral and biochemical responses to cocaine and morphine has revealed a functional specificity in dopamine and mu-opioid receptor regulation of locomotion and analgesia. An important and specific role of GRKs and beta arrestins in regulating physiological responsiveness to psychostimulants and morphine suggests potential involvement of these molecules in certain brain disorders, such as addiction, Parkinson's disease, mood disorders, and schizophrenia. Furthermore, the utility of a pharmacological strategy aimed at targeting this GPCR desensitization machinery to regulate brain functions can be envisaged.

Homologous regulation of the heptahelical D1A receptor responsiveness: specific cytoplasmic tail regions mediate dopamine-induced phosphorylation, desensitization and endocytosis

In the present study, we investigate the role of specific cytoplasmic tail (CT) regions of the D1A receptor in mediating dopamine (DA)-induced phosphorylation, desensitization and endocytosis. Results obtained in human embryonic kidney (HEK) cells expressing the wild-type (WT) or truncation forms (Delta425, Delta379 and Delta351) of the D1A receptor show that sequences located downstream of Gly379 regulate DA-mediated phosphorylation-dependent desensitization of D1A receptors. However, the longer truncation mutant Delta351 failed to undergo detectable DA-induced phosphorylation while exhibiting DA-induced desensitization features similar to the shorter truncation mutant Delta379. These data potentially suggest a novel role for a receptor phosphorylation-independent process in the DA-promoted D1A subtype desensitization. Our immunofluorescence data also suggest that sequences located between Cys351 and Gly379 play an important role in DA-mediated receptor endocytosis. Additionally, time-course studies were done in intact cells expressing WT or truncation receptors to measure the observed rate constant for adenylyl cyclase (AC) activation or k(obs), a parameter linked to the receptor-G protein coupling status. In agreement with the desensitization data, Delta425- and Delta379-expressing cells exhibit an increase of kobs in comparison with WT-expressing cells. Nevertheless, Delta351-expressing cells, which harbor similar desensitization features of Delta379-expressing cells, display no change in k(obs) when compared with WT-expressing cells. Our results suggest that a defective DA-induced endocytosis may hamper Delta351 resensitization and concomitant increase in k(obs). Thus, our study showing that specific D1A receptor CT sequences regulate DA-induced phosphorylation, desensitization, and endocytosis highlights the underlying molecular complexity of signaling at dopaminergic synapses.

G protein-coupled receptor kinases, beta-arrestin-2 and associated regulatory proteins in the human brain: postmortem changes, effect of age and subcellular distribution

G protein-coupled receptor kinases (GRKs) and beta-arrestin-2 play a crucial role in the regulation of neurotransmitter receptors in brain. In this study, GRK2, GRK6, beta-arrestin-2 and associated regulatory proteins (Gbeta proteins and protein phosphatase (PP)-2A) were quantitated in human brains (immunodensity with specific antibodies) to assess for postmortem changes (pattern of protein degradation) and to investigate the effect of aging on these regulatory proteins as well as their subcellular distribution (cytosol and membrane fractions). In brain (prefrontal cortex, total homogenate) of healthy subjects (n=14) the immunodensities of GRK2 (r=-0.76), GRK6 (r=-0.64), beta-arrestin-2 (r=-0.57), Gbeta proteins (r=-0.59) and neurofilament (NF)-L (r=-0.64), but not PP-2A, declined markedly with the length of postmortem delay (PMD, 3-81 h). With these linear decay models, the average decreases per 12 h of PMD (from 12 to 72 h) were 7-11% for the various proteins. The immunodensities of GRK2 (r=-0.71), GRK6 (r=-0.61), and beta-arrestin-2 (r=-0.54) in human brain (n=12) also declined with aging (16 to 87 years) and the average decreases per decade (from 20 to 80 years) were 3-5%. In contrast, the immunodensities of PP-2A, Gbeta and NF-L in brain did not correlate significantly with the age of the subject at death (16-87 years). The immunodensities of GRK2/6 and beta-arrestin-2 showed marked individual variations and were strongly reduced after several freeze/thaw cycles. In the prefrontal cortex the subcellular distribution (cytosol/membrane) of the two GRKs differed markedly (GRK2: 60%/40%; GRK6: 5%/95%), and that of beta-arrestin-2 was as expected for a soluble protein (60%/40%). In brains of healthy subjects, the immunodensities of cytosolic GRK2 and beta-arrestin-2 correlated, respectively, with those of membrane-associated GRK2 (r=0.67, P=0.049, n=9) and membrane-associated beta-arrestin-2 (r=0.77, P=0.01, n=9). The results of this study emphasize the importance of examining relevant variables (PMD, age) and potential artifacts (individual variation, freeze-thawing effect) when designing signal transduction studies in neuropsychiatric disorders using the postmortem human brain.

Muscarinic receptor knockout mice: role of muscarinic acetylcholine receptors M(2), M(3), and M(4) in carbamylcholine-induced gallbladder contractility

Muscarinic receptors play a major role in gallbladder function, although the muscarinic receptor(s) mediating smooth muscle contractility is unclear. This study compared smooth muscle contractile responses to carbamylcholine (10(-7)-10(-3) M) in isolated gallbladder from wild-type and M(2), M(3), and M(4) receptor knockout mice. Carbamylcholine-induced contraction in gallbladder was associated with tachyphylaxis and the release of a cyclooxygenase product because indomethacin (10(-6) M) inhibited carbamylcholine-induced contraction. The M(3) receptor was the major muscarinic receptor involved in contraction because carbamylcholine-induced contractility was inhibited in gallbladder from M(3) receptor knockout mice. Furthermore, the muscarinic receptor antagonists 11-[[[2-diethylamino-O-methyl]-1-piperidinyl]acetyl]-5,11-dihydrol-6H-pyridol[2,3-b][1,4]benzodiazepine-6-one (AF-DX 116) and pirenzepine dextrally shifted contraction to carbamylcholine in gallbladder from wild-type, M(2), and M(4) receptor knockout mice, with affinities consistent with M(3) receptor interaction. In addition, maximal contraction to carbamylcholine was reduced in gallbladder from M(2) receptor knockout mice and affinities for AF-DX 116 and pirenzepine in gallbladder from M(3) receptor knockout mice were consistent with their affinities at M(2) receptors. In M(4) receptor knockout mice, contraction to carbamylcholine was dextrally shifted, although the affinities for AF-DX 116 and pirenzepine in gallbladder from M(2) or M(3) knockout mice were not similar to their affinities at M(4) receptors. The M(4) receptor may serve as an accessory protein necessary for optimal potency of M(2) and M(3) receptor-mediated responses. Thus, muscarinic receptor knockout mice provided direct and unambiguous evidence that M(3), and to a lesser extent, M(2) receptors are the predominant muscarinic receptors mediating gallbladder contractility, and M(4) receptors appear necessary for optimal potency of carbamylcholine in gallbladder contraction.

G protein-coupled receptor kinases and beta arrestins are relocalized and attenuate cyclic 3',5'-adenosine monophosphate response to follicle-stimulating hormone in rat primary Sertoli cells

The FSH receptor (FSH-R) is a member of the rhodopsin-like subfamily of G protein-coupled receptors that undergoes homologous desensitization upon agonist stimulation. In immortalized cell lines overexpressing the FSH-R, G protein-coupled receptor kinases (GRKs) and beta-arrestins are involved in the phosphorylation, uncoupling, and internalization of this receptor. In an effort to appreciate the physiological relevance of GRK/beta-arrestin actions in natural FSH-R-bearing cells, we used primary rat Sertoli cells as a model. GRK2, -3, -5, -6a, and -6b and beta-arrestins 1 and 2 were expressed in primary rat Sertoli cells. Overexpression of these different GRKs and beta-arrestins in primary rat Sertoli cells significantly attenuated the FSH-induced cAMP response, and FSH rapidly triggered a relocalization of endogenously expressed GRK2, -3, -5, and -6 and beta-arrestins 1 and 2 from the cytosol to the membranes. These results highlight the relationship existing between the GRK/beta-arrestin regulatory system and the FSH-R signaling machinery in a physiological model.

Distribution of G-protein-coupled receptor kinase (GRK) isoforms 2, 3, 5 and 6 mRNA in the rat brain

There is limited knowledge about the distribution of the different G-protein-coupled receptor kinases (GRKs) in the rat brain, especially for the recently cloned isoforms GRK5 and GRK6. In this work an overview will be given of the mRNA expression patterns of four G-protein-coupled receptor kinases, GRK2 (betaARK1), GRK3 (betaARK2), GRK5 and GRK6 in the rat brain. As now shown by us and recently by others GRK2 and GRK3 are widely distributed in rat brain with nearly the same expression pattern. But GRK3, in general, appeared to be weaker expressed than GRK2 in most brain areas. Exceptions were the islands of Calleja, the compact part of the substantia nigra and the locus coeruleus. GRK3 mRNA was very low expressed or absent in the striatum and in some hypothalamic and thalamic nuclei. The expression pattern of GRK6 was also similar to GRK2. In the caudate putamen GRK6 yielded the strongest hybridization signal of all GRK types. GRK5 took a special position. The message for this form was not expressed ubiquitously in the brain but was mainly localized in limbic brain regions with a very prominent expression in the lateral septal area. GRK5 may therefore be involved in reward and addiction. Accordingly, a higher expression level of GRK5 mRNA was found in the lateral septum of cocaine-sensitized rats as compared to controls.