Roles of G protein and β-arrestin in dopamine D 2 receptor-mediated ERK activation

Unveiling the intracellular dynamics of α4β2 nAChR-mediated ERK activation through the interplay of arrestin, Gβγ, and PKCβII

AIMS

In contrast to G protein-coupled receptors or receptor tyrosine kinases, the mechanism underlying ERK activation through nicotine acetylcholine receptors (nAChRs), members of the ligand-gated ion channel family, remains poorly elucidated. This study aimed to delineate the signaling pathway responsible for ERK activation by the α4β2 nAChR subtype, which is implicated in nicotine addiction and various mental disorders.

MATERIALS AND METHODS

Loss-of-function strategies and mutants of arrestin2/PKCβII with distinct functional characteristics were employed to identify the cellular components and processes involved in ERK activation.

KEY FINDINGS

ERK activation via α4β2 nAChR was observed within the nucleus and necessitated the nuclear translocation of arrestin2 and PKCβII, which exhibited mutual augmentation. Activation of PKCβII by α4β2 nAChR stimulation facilitated the nuclear translocation of arrestin2 by enhancing its interaction with importin β1. Apart from scaffolding ERK activation in the nucleus, arrestin2, in cooperation with GRK2, facilitated the activation of the Src/Syk/PKCβII signaling cascade, leading to the nuclear entry of PKCβII in a Gβγ-dependent manner. Upon nuclear localization, PKCβII underwent ubiquitination by Mdm2 and interacted with MEK1, resulting in ERK activation. In summary, α4β2 nAChR-mediated ERK activation in the nucleus involves the nuclear translocation of arrestin2 and PKCβII, which is reciprocally facilitated via positive feedback augmentation.

SIGNIFICANCE

As α4β2 nAChRs play a pivotal role in various cellular processes including drug addiction and mental disorders, our findings will offer insights into understanding the pathogenesis of α4β2 nAChR-related disorders and may facilitate the development of targeted therapeutic interventions.

Biased Dopamine D2 Receptors Exhibit Distinct Intracellular Trafficking Properties and ERK Activation in Different Subcellular Domains

Biased signaling or functional selectivity refers to the ability of an agonist or receptor to selectively activate a subset of transducers such as G protein and arrestin in the case of G protein-coupled receptors (GPCRs). Although signaling through arrestin has been reported from various GPCRs, only a few studies have examined side-by-side how it differs from signaling via G protein. In this study, two signaling pathways were compared using dopamine D2 receptor (D2R) mutants engineered via the evolutionary tracer method to selectively transduce signals through G protein or arrestin (D2G and D2Arr, respectively). D2G mediated the inhibition of cAMP production and ERK activation in the cytoplasm. D2Arr, in contrast, mediated receptor endocytosis accompanied by arrestin ubiquitination and ERK activation in the nucleus as well as in the cytoplasm. D2Arr-mediated ERK activation occurred in a manner dependent on arrestin3 but not arrestin2, accompanied by the nuclear translocation of arrestin3 via importin1. D2R-mediated ERK activation, which occurred in both the cytosol and nucleus, was limited to the cytosol when cellular arrestin3 was depleted. This finding supports the results obtained with D2Arr and D2G. Taken together, these observations indicate that biased signal transduction pathways activate distinct downstream mechanisms and that the subcellular regions in which they occur could be different when the same effectors are involved. These findings broaden our understanding on the relation between biased receptors and the corresponding downstream signaling, which is critical for elucidating the functional roles of biased pathways.

Comparative study of the molecular mechanisms underlying the G protein and β-arrestin-dependent pathways that lead to ERKs activation upon stimulation by dopamine D2 receptor

Dopamine D2 receptor (D2 R) has been shown to activate extracellular signal-regulated kinases (ERKs) via distinct pathways dependent on either G-protein or β-arrestin. However, there has not been a systematic study of the regulatory process of D2 R-mediated ERKs activation by G protein- versus β-arrestin-dependent signaling since D2 R stimulation of ERKs reflects the simultaneous action of both pathways. Here, we investigated that differential regulation of D2 R-mediated ERKs activation via these two pathways. Our results showed that G protein-dependent ERKs activation was transient, rapid, reached maximum level at around 2 min, and importantly, the activated ERKs were entirely confined to the cytoplasm. In contrast, β-arrestin-dependent ERKs activation was more sustained, slower, reached maximum level at around 10 min, and phosphorylated ERKs translocated into the nucleus. Src was found to be commonly involved in both the G protein- and β-arrestin-dependent pathway-mediated ERKs activation. Pertussis toxin Gi/o inhibitor, GRK2-CT, AG1478 epidermal growth factor receptor inhibitor, and wortmannin phosphoinositide 3-kinase inhibitor all blocked G protein-dependent ERKs activation. In contrast, GRK2 and β-Arr2 played a main role in β-arrestin-dependent ERKs activation. Receptor endocytosis showed minimal effect on the activation of ERKs mediated by both pathways. Furthermore, we found that the formation of a complex composed of phospho-ERKs, β-Arr2, and importinβ1 promoted the nuclear translocation of activated ERKs. The differential regulation of various cellular components, as well as temporal and spatial patterns of ERKs activation via these two pathways, suggest the existence of distinct physiological outcomes.

Unveiling the Differences in Signaling and Regulatory Mechanisms between Dopamine D2 and D3 Receptors and Their Impact on Behavioral Sensitization

Dopamine receptors are classified into five subtypes, with D2R and D3R playing a crucial role in regulating mood, motivation, reward, and movement. Whereas D2R are distributed widely across the brain, including regions responsible for motor functions, D3R are primarily found in specific areas related to cognitive and emotional functions, such as the nucleus accumbens, limbic system, and prefrontal cortex. Despite their high sequence homology and similar signaling pathways, D2R and D3R have distinct regulatory properties involving desensitization, endocytosis, posttranslational modification, and interactions with other cellular components. In vivo, D3R is closely associated with behavioral sensitization, which leads to increased dopaminergic responses. Behavioral sensitization is believed to result from D3R desensitization, which removes the inhibitory effect of D3R on related behaviors. Whereas D2R maintains continuous signal transduction through agonist-induced receptor phosphorylation, arrestin recruitment, and endocytosis, which recycle and resensitize desensitized receptors, D3R rarely undergoes agonist-induced endocytosis and instead is desensitized after repeated agonist exposure. In addition, D3R undergoes more extensive posttranslational modifications, such as glycosylation and palmitoylation, which are needed for its desensitization. Overall, a series of biochemical settings more closely related to D3R could be linked to D3R-mediated behavioral sensitization.

The complex molecular pharmacology of the dopamine D2 receptor: Implications for pramipexole, ropinirole, and rotigotine

With L-DOPA, dopamine agonists such as pramipexole, ropinirole and rotigotine constitute key therapeutic options for the management of motor symptoms of Parkinson's disease. These compounds exert their beneficial effect on motor behaviours by activating dopamine D₂-class receptors and thereby compensating for the declining dopaminergic transmission in the dorsal striatum. Despite a strong similarity in their mechanism of action, these three dopamine agonists present distinct clinical profiles, putatively underpinned by differences in their pharmacological properties. In this context, this review aims at contributing to close the gap between clinical observations and data from molecular neuropharmacology by exploring the properties of pramipexole, ropinirole and rotigotine from both the clinical and molecular perspectives. Indeed, this review first summarizes and compares the clinical features of these three dopamine agonists, and then explores their binding profiles at the different dopamine receptor subtypes. Moreover, the signalling profiles of pramipexole, ropinirole and rotigotine at the D₂ receptor are recapitulated, with a focus on biased signalling and the potential therapeutic implications. Overall, this review aims at providing a unifying framework of interpretation for both clinicians and fundamental pharmacologists interested in a deep understanding of the pharmacological properties of pramipexole, ropinirole and rotigotine.

Dopamine: Functions, Signaling, and Association with Neurological Diseases

The dopaminergic system plays important roles in neuromodulation, such as motor control, motivation, reward, cognitive function, maternal, and reproductive behaviors. Dopamine is a neurotransmitter, synthesized in both central nervous system and the periphery, that exerts its actions upon binding to G protein-coupled receptors. Dopamine receptors are widely expressed in the body and function in both the peripheral and the central nervous systems. Dopaminergic signaling pathways are crucial to the maintenance of physiological processes and an unbalanced activity may lead to dysfunctions that are related to neurodegenerative diseases. Unveiling the neurobiology and the molecular mechanisms that underlie these illnesses may contribute to the development of new therapies that could promote a better quality of life for patients worldwide. In this review, we summarize the aspects of dopamine as a catecholaminergic neurotransmitter and discuss dopamine signaling pathways elicited through dopamine receptor activation in normal brain function. Furthermore, we describe the potential involvement of these signaling pathways in evoking the onset and progression of some diseases in the nervous system, such as Parkinson's, Schizophrenia, Huntington's, Attention Deficit and Hyperactivity Disorder, and Addiction. A brief description of new dopaminergic drugs recently approved and under development treatments for these ailments is also provided.

Molecular targets of atypical antipsychotics: From mechanism of action to clinical differences

The introduction of atypical antipsychotics (AAPs) since the discovery of its prototypical drug clozapine has been a revolutionary pharmacological step for treating psychotic patients as these allow a significant recovery not only in terms of hospitalization and reduction in symptoms severity, but also in terms of safety, socialization and better rehabilitation in the society. Regarding the mechanism of action, AAPs are weak D₂ receptor antagonists and they act beyond D₂ antagonism, involving other receptor targets which regulate dopamine and other neurotransmitters. Consequently, AAPs present a significant reduction of deleterious side effects like parkinsonism, hyperprolactinemia, apathy and anhedonia, which are all linked to the strong blockade of D₂ receptors. This review revisits previous and current findings within the class of AAPs and highlights the differences in terms of receptor properties and clinical activities among them. Furthermore, we propose a continuum spectrum of "atypia" that begins with risperidone (the least atypical) to clozapine (the most atypical), while all the other AAPs fall within the extremes of this spectrum. Clozapine is still considered the gold standard in refractory schizophrenia and in psychoses present in Parkinson's disease, though it has been associated with adverse effects like agranulocytosis (0.7%) and weight gain, pushing the scientific community to find new drugs as effective as clozapine, but devoid of its side effects. To achieve this, it is therefore imperative to characterize and compare in depth the very complex molecular profile of AAPs. We also introduce relatively new concepts like biased agonism, receptor dimerization and neurogenesis to identify better the old and new hallmarks of "atypia". Finally, a detailed confrontation of clinical differences among the AAPs is presented, especially in relation to their molecular targets, and new means like therapeutic drug monitoring are also proposed to improve the effectiveness of AAPs in clinical practice.

Effect of Stimulation of Neurotransmitter Systems on Heart Rate Variability and β-Adrenergic Responsiveness of Erythrocytes in Outbred Rats

We studied heart rate variability and β-adrenergic responsiveness of erythrocytes and changes in these parameters in response to single administration of β-adrenoblocker propranolol (2 mg/kg) in outbred male rats against the background of activation of the noradrenergic, serotonergic, and dopaminergic neurotransmitter systems achieved by 4-fold injections maprotiline (10 mg/kg), 5-hydroxytryptophan (50 mg/kg) combined with fluoxetine (3 mg/kg), and L-DOPA (20 mg/kg) with amantadine (20 mg/kg), respectively. Stimulation of the noradrenergic system moderately enhanced the heart rhythm rigidity and β-adrenergic responsiveness of erythrocytes. In addition, it markedly augmented the moderating effect of subsequently administered propranolol on LF and VLF components in the heart rate variability and reversed the effect of propranolol on β-adrenergic responsiveness of erythrocytes. Stimulation of the serotonergic system dramatically decreased all components in the heart rate variability and pronouncedly enhanced β-adrenergic responsiveness of erythrocytes. Subsequent injection of propranolol slightly restored all components in the heart rate variability and decreased β-adrenergic responsiveness of erythrocytes to the control level. Stimulation of the dopaminergic system made the heart rate more rigid due to decrease of all components in the heart rate variability; in addition, it slightly but significantly enhanced β-adrenergic responsiveness of erythrocytes. Subsequent injection of propranolol produced no significant effects on all components in the heart rate variability and on β-adrenergic responsiveness of erythrocytes. Stimulation of noradrenergic, serotonergic, and dopaminergic neurotransmitter systems produced unidirectional and consorted effects on heart rate variability and β-adrenergic responsiveness of erythrocytes, although the magnitudes of these effects were different. Probably, the changes in the heart rate variability in rats with stimulated neurotransmitter systems results from modification of the cellular sensitivity in peripheral organs to adrenergic influences. However, the differences in the reactions to β-adrenoblocker attest to specificity of the mechanisms underlying the changes in membrane reception and adrenergic pathways in every experimental model employed in this study.

Functional expression of dopamine D2 receptor is regulated by tetraspanin 7-mediated postendocytic trafficking

The dopaminergic system plays an essential role in various functions of the brain, including locomotion, memory, and reward, and the deregulation of dopaminergic signaling as a result of altered functionality of dopamine D2 receptor (DRD2) is implicated in multiple neurologic and psychiatric disorders. Tetraspanin-7 (TSPAN7) is expressed to variable degrees in different tissues, with the highest level in the brain, and multiple mutations in TSPAN7 have been implicated in intellectual disability. Here, we tested the hypothesis that TSPAN7 may be a binding partner of DRD2 that is involved in the regulation of its functional activity. Our results showed that TSPAN7 was associated with DRD2 and reduced its surface expression by enhancing DRD2 internalization. Immunocytochemical analysis revealed that TSPAN7 that resides in the plasma membrane and early and late endosomes promoted internalization of DRD2 and its localization to endosomal compartments of the endocytic pathway. Furthermore, we observed that TSPAN7 deficiency increased surface localization of DRD2 concurrent with the decrease of its endocytosis, regardless of dopamine treatment. Finally, TSPAN7 negatively affects DRD2-mediated signaling. These results disclosed a previously uncharacterized role of TSPAN7 in the regulation of the expression and functional activity of DRD2 by postendocytic trafficking.-Lee, S.-A., Suh, Y., Lee, S., Jeong, J., Kim, S. J., Kim, S. J., Park, S. K. Functional expression of dopamine D2 receptor is regulated by tetraspanin 7-mediated postendocytic trafficking.

In silico analyses and global transcriptional profiling reveal novel putative targets for Pea3 transcription factor related to its function in neurons

Pea3 transcription factor belongs to the PEA3 subfamily within the ETS domain transcription factor superfamily, and has been largely studied in relation to its role in breast cancer metastasis. Nonetheless, Pea3 plays a role not only in breast tumor, but also in other tissues with branching morphogenesis, including kidneys, blood vasculature, bronchi and the developing nervous system. Identification of Pea3 target promoters in these systems are important for a thorough understanding of how Pea3 functions. Present study particularly focuses on the identification of novel neuronal targets of Pea3 in a combinatorial approach, through curation, computational analysis and microarray studies in a neuronal model system, SH-SY5Y neuroblastoma cells. We not only show that quite a number of genes in cancer, immune system and cell cycle pathways, among many others, are either up- or down-regulated by Pea3, but also identify novel targets including ephrins and ephrin receptors, semaphorins, cell adhesion molecules, as well as metalloproteases such as kallikreins, to be among potential target promoters in neuronal systems. Our overall results indicate that rather than early stages of neurite extension and axonal guidance, Pea3 is more involved in target identification and synaptic maturation.

Differential Effects of Oxycodone, Hydrocodone, and Morphine on Activation Levels of Signaling Molecules

BACKGROUND

Opioids alter the responses of D2-like dopamine receptors (D2DRs), known to be involved in the pathology of addiction and other mental illnesses. Importantly, our recent results demonstrated that various opioids differentially modulate the behavioral responses of D2DRs.

OBJECTIVE

To examine the effect of various opioids on striatal activation levels of Akt and ERK1/2, as well as the signaling responses of D2DRs following opioid exposure.

METHODS

Mice were pre-treated with 20 mg/kg morphine, hydrocodone, oxycodone, or saline for 6 days. Twenty-four hours later, mice were injected with vehicle or a D2/D3 receptor agonist, quinpirole. Thirty minutes later, dorsal striatum was collected and analyzed using Western blot.

RESULTS

In morphine-pretreated animals, baseline Akt activation level was unchanged, but was reduced in response to quinpirole. In contrast, baseline Akt activation levels were reduced in mice pretreated with hydrocodone and oxycodone, but were unchanged in response to quinpirole. In mice pretreated with all opioids, baseline ERK2 activation levels were unchanged and increased in response to quinpirole. However, quinpirole-induced ERK2 activation was significantly higher than drug naïve animals only in the morphine-pretreated mice.

CONCLUSIONS

Various opioids differentially modulate the baseline activation levels of signaling molecules, which in turn results in ligand-selective effects on the responses to a D2/D3 dopamine receptor agonist. This demonstrates a complex interplay between opioid receptors and D2DRs, and supports the notion that various opioids carry differential risks to the dopamine reward system. This information should be considered when prescribing opioid pain medication, to balance effectiveness with minimal risk.

Bias analyses of preclinical and clinical D2 dopamine ligands: studies with immediate and complex signaling pathways

G protein-coupled receptors (GPCRs) often activate multiple signaling pathways, and ligands may evoke functional responses through individual pathways. These unique responses provide opportunities for biased or functionally selective ligands to preferentially modulate one signaling pathway over another. Studies with several GPCRs have suggested that selective activation of signaling pathways downstream of a GPCR may lead to safer and more effective drug therapies. The dopamine D2 receptor (D2R) is one of the main drug targets in the therapies for Parkinson's disease and schizophrenia. Recent studies suggest that selective modulation of individual signaling pathways downstream of the D2R may lead to safer antipsychotic drugs. In the present study, immediate effectors of the D2R (i.e., Gαi/o, Gβγ, β-arrestin recruitment) and more complex signaling pathways (i.e., extracellular signal-regulated kinase phosphorylation, heterologous sensitization, and dynamic mass redistribution) were examined in response to a series of D2R ligands. This was accomplished using Chinese hamster ovary cells stably expressing the human D2L dopamine receptor in the PathHunter β-Arrestin GPCR Assay Platform. The use of a uniform cellular background was designed to eliminate potential confounds associated with cell-to-cell variability, including expression levels of receptor as well as other components of signal transduction, including G protein subunits. Several well characterized and clinically relevant D2R ligands were evaluated across each signaling pathway in this cellular model. The most commonly used methods to measure ligand bias were compared. Functional selectivity analyses were also used as tools to explore the relative contribution of immediate D2R effectors for the activation of more complex signaling pathways.

Implication of the ERK/MAPK pathway in antipsychotics-induced dopamine D2 receptor upregulation and in the preventive effects of (±)-α-lipoic acid in SH-SY5Y neuroblastoma cells

Chronic administration of antipsychotics (APs) has been associated with dopamine D2 receptor (D2R) upregulation and tardive dyskinesia. We previously showed that haloperidol, a first-generation AP, exerted a more robust increase in D2R expression than amisulpride, a second-generation AP and that (±)-α-lipoic acid pre-treatment reversed the AP-induced D2R upregulation. We also demonstrated that the Akt/GSK-3β/β-catenin pathway is involved in the control of D2R expression levels, but is unlikely implicated in the preventive effects of (±)-α-lipoic acid since co-treatment with haloperidol and (±)-α-lipoic acid exerts synergistic effects on Akt/GSK-3β activation. These findings led us to examine whether the ERK/MAPK signaling pathway may be involved in D2R upregulation elicited by APs, and in its reversal by (±)-α-lipoic acid, in SH-SY5Y human neuroblastoma cells. Our results revealed that haloperidol, in parallel with an elevation in D2R mRNA levels, induced a larger increase of ERK (p42/p44) phosphorylation than amisulpride. Pre-treatment with the selective ERK inhibitor U0126 attenuated haloperidol-induced increase in D2R upregulation. Furthermore, (±)-α-lipoic acid prevented AP-induced ERK activation. These results show that (1) the ERK/MAPK pathway is involved in haloperidol-induced D2R upregulation; (2) the preventive effect of (±)-α-lipoic acid on haloperidol-induced D2R upregulation is in part mediated by an ERK/MAPK-dependent signaling cascade. Taken together, our data suggest that (±)-α-lipoic acid exerts synergistic effects with haloperidol on the Akt/GSK-3β pathway, potentially involved in the therapeutic effects of APs, and antagonism of ERK activation and D2R upregulation, potentially involved in tardive dyskinesia and treatment resistance.

Modulation of A₂a receptor antagonist on D₂ receptor internalization and ERK phosphorylation

AIM

To explore the effects of heterodimerization of D2 receptor/A2a receptor (D2R/A2aR) on D2R internalization and D2R downstream signaling in primary cultured striatal neurons and HEK293 cells co-expressing A2aR and D2R in vitro.

METHODS

Primary cultured rat striatal neurons and HEK293 cells co-expressing A2aR and D2R were treated with A2aR- or D2R-specific agonists. D2R internalization was detected using a biotinylation assay and confocal microscopy. ERK, Src kinase and β-arrestin were measured using Western blotting. The interaction between A2aR and D2R was detected using bioluminescence resonance energy transfer (BRET) and immunoprecipitation.

RESULTS

D2R and A2aR were co-localized and formed complexes in striatal neurons, while both the receptors formed heterodimers in the HEK293 cells. In striatal neurons and the HEK293 cells, the D2R agonist quinpirole (1 μmol/L) marked increased Src phosphorylation and β-arrestin recruitment, thereby D2R internalization. Co-treatment with the A2aR antagonist ZM241385 (100 nmol/L) significantly attenuated these D2R-mediated changes. Furthermore, both ZM241385 (100 nmol/L) and the specific Src kinase inhibitor PP2 (5 μmol/L) blocked D2R-mediated ERK phosphorylation. Moreover, expression of the mutant β-arrestin (319-418) significantly attenuated D2R-mediated ERK phosphorylation in HEK293 cells expressing both D2R and A2aR, but not in those expressing D2R alone.

CONCLUSION

A2aR antagonist ZM241385 significantly attenuates D2R internalization and D2R-mediated ERK phosphorylation in striatal neurons, involving Src kinase and β-arrestin. Thus, A2aR/D2R heterodimerization plays important roles in D2R downstream signaling.

Activated human hydroxy-carboxylic acid receptor-3 signals to MAP kinase cascades via the PLC-dependent PKC and MMP-mediated EGFR pathways

BACKGROUND AND PURPOSE 3-Hydroxy-octanoate, recently identified as a ligand for, the orphan GPCR, HCA(3), is of particular interest given its ability to treat lipid disorders and atherosclerosis. Here we demonstrate the pathway of HCA(3)-mediated activation of ERK1/2. EXPERIMENTAL APPROACH Using CHO-K1 cells stably expressing HCA(3) receptors and A431 cells, a human epidermoid cell line with high levels of endogenous expression of functional HCA(3) receptors, HCA(3)-mediated activation of ERK1/2 was measured by Western blot. KEY RESULTS HCA(3)-mediated activation of ERK1/2 was rapid, peaking at 5 min, and was Pertussis toxin sensitive. Our data, obtained by time course analyses in combination with different kinase inhibitors, demonstrated that on agonist stimulation, HCA(3) receptors evoked ERK1/2 activation via two distinct pathways, the PLC/PKC pathway at early time points (≤ 2 min) and the MMP/ epidermal growth factor receptor (EGFR) transactivation pathway with a maximum response at 5 min. Furthermore, our present results also indicated that the βγ-subunits of the G(i) protein play a critical role in HCA(3)-activated ERK1/2 phosphorylation, whereas β-arrestins and Src were not required for ERK1/2 activation. CONCLUSIONS AND IMPLICATIONS We have described the molecular mechanisms underlying the coupling of human HCA(3) receptors to the ERK1/2 MAP kinase pathway in CHO-K1 and A431 cells, which implicate the G(i) protein-initiated, PLC/PKC -and platelet-derived growth factor receptor/EGFR transactivation-dependent pathways. These observations may provide new insights into the pharmacological effects and the physiological functions modulated by the HCA(3)-mediated activation of ERK1/2.

Multiple signaling routes involved in the regulation of adenylyl cyclase and extracellular regulated kinase by dopamine D(2) and D(3) receptors

Most G protein coupled receptors (GPCR) regulate multiple cellular processes by coupling to more than one kind of G protein. Furthermore, recent studies have reported G protein-independent/β-arrestin-dependent signaling pathway for some GPCRs. Dopamine D(2) and D(3) receptors (D(2)R, D(3)R), the major targets of currently used antipsychotic drugs, are co-expressed in some of the same dopaminergic neurons and regulate the same overlapping effectors. However, the specific subunits of G proteins that regulate each signaling pathway are not clearly identified. In addition, the existence of β-arrestin-dependent/G protein-independent signaling is not clear for these receptors. In this study, we determined the G protein subtypes and β-arrestin dependency involved in the signaling of D(2)R and D(3)R, which was measured by inhibition of adenylyl cyclase and extracellular signal-regulated kinase (ERK) activation. For the inhibition of cAMP production in HEK-293 cells, D(2)R used the Gαo subunit but D(3)R used the βγ subunit of Gi family proteins. For the regulation of ERK activation, D(2)R used the α subunits of Gi/o proteins both in HEK-293 cells and COS-7 cells, but D(3)R used Gαo and Gβγ in HEK-293 cells and COS-7 cells, respectively. β-Arrestin-dependent/G protein-independent ERK activation was not observed for both D(2)R and D(3)R. Agonist-induced β-arrestin translocation was observed with D(2)R but not with D(3)R, and β-arrestins exerted inhibitory influences on G protein-dependent ERK activation by D(2)R, but not D(3)R. These results show that the D(2)R and D(3)R, which have overlapping cellular expressions and functional roles, employ distinct G protein subunits depending on the cell types and the effectors they control.

Dopamine D2 receptor-mediated Akt/PKB signalling: initiation by the D2S receptor and role in quinpirole-induced behavioural activation

The short and long isoforms of the dopamine D2 receptor (D2S and D2L respectively) are highly expressed in the striatum. Functional D2 receptors activate an intracellular signalling pathway that includes a cAMP-independent route involving Akt/GSK3 (glycogen synthase kinase 3). To investigate the Akt/GSK3 response to the seldom-studied D2S receptor, we established a rat D2S receptor-expressing cell line [HEK (human embryonic kidney)-293/rD2S]. We found that in HEK-293/rD2S cells, the D2/D3 agonists bromocriptine and quinpirole significantly induced Akt and GSK3 phosphorylation, as well as ERK1/2 (extracellular-signal-regulated kinase 1/2) activation. The D2S receptor-induced Akt signals were profoundly inhibited by the internalization blockers monodansyl cadaverine and concanavalin A. Activation of the D2S receptor in HEK-293/rD2S cells appeared to trigger Akt/phospho-Akt translocation to the cell membrane. In addition to our cell culture experiments, we studied D2 receptor-dependent Akt in vivo by systemic administration of the D2/D3 agonist quinpirole. The results show that quinpirole evoked Akt-Ser⁴⁷³ phosphorylation in the ventral striatum. Furthermore, intra-accumbens administration of wortmannin, a PI3K (phosphoinositide 3-kinase) inhibitor, significantly suppressed the quinpirole-evoked behavioural activation. Overall, we demonstrate that activation of the dopamine D2S receptor stimulates Akt/GSK3 signalling. In addition, in vivo Akt activity in the ventral striatum appears to play an important role in systemic D2/D3 agonist-induced behavioural activation.

Distinct kinetic and spatial patterns of protein kinase C (PKC)- and epidermal growth factor receptor (EGFR)-dependent activation of extracellular signal-regulated kinases 1 and 2 by human nicotinic acid receptor GPR109A

Nicotinic acid (niacin) has been widely used as a lipid-lowering drug for several decades, and recently, orphan G protein-coupled receptor GPR109A has been identified as a receptor for niacin. Mechanistic investigations have shown that, upon niacin activation, GPR109A couples to a G(i) protein and inhibits adenylate cyclase activity, leading to inhibition of liberation of free fatty acid. However, the underlying molecular mechanisms for GPR109A signaling remain largely unknown. Using CHO-K1 cells stably expressing GPR109A and A431 cells, which are a human epidermoid cell line with high levels of endogenous expression of functional GPR109A receptors, we found that activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) by niacin was rapid, peaking at 5 min, and was significantly blocked by pertussis toxin. Furthermore, time course experiments with different kinase inhibitors demonstrated that GPR109A induced ERK1/2 activation via the matrix metalloproteinase/epidermal growth factor receptor transactivation pathway at both early and later time points (2-5 min); this pathway was distinct from the PKC pathway-mediated ERK1/2 phosphorylation that occurs at early time points (≤2 min) in response to niacin. Overexpression of Gβγ subunit scavengers βARK1-CT and the Gα subunit of transducin led to a significant reduction of ERK1/2 phosphorylation, suggesting a critical role for βγ subunits in GPR109A-activated ERK1/2 phosphorylation. Using arrestin-2/3-specific siRNA and an internalization-deficient GPR109A mutant, we found that arrestin-2 and arrestin-3 were not involved in GPR109A-mediated ERK1/2 activation. In conclusion, our findings demonstrate that upon binding to niacin GPR109A receptors initially activate G(i), leading to dissociation of the Gβγ subunit from activated G(i), and subsequently induce ERK1/2 activation via two distinct pathways, one PKC-dependent pathway occurring at a peak time of ≤2 min and the other matrix metalloproteinase-dependent growth factor receptor transactivation occurring at both early and later time points (2-5 min).

GPR54 regulates ERK1/2 activity and hypothalamic gene expression in a Gα(q/11) and β-arrestin-dependent manner

G protein-coupled receptor 54 (GPR54) is a G_q/11-coupled 7 transmembrane-spanning receptor (7TMR). Activation of GPR54 by kisspeptin (Kp) stimulates PIP₂ hydrolysis, Ca²⁺ mobilization and ERK1/2 MAPK phosphorylation. Kp and GPR54 are established regulators of the hypothalamic-pituitary-gonadal (HPG) axis and loss-of-function mutations in GPR54 are associated with an absence of puberty and hypogonadotropic hypogonadism, thus defining an important role of the Kp/GPR54 signaling system in reproductive function. Given the tremendous physiological and clinical importance of the Kp/GPR54 signaling system, we explored the contributions of the GPR54-coupled G_q/11 and β-arrestin pathways on the activation of a major downstream signaling molecule, ERK, using G_q/11 and β-arrestin knockout mouse embryonic fibroblasts. Our study revealed that GPR54 employs the G_q/11 and β-arrestin-2 pathways in a co-dependent and temporally overlapping manner to positively regulate ERK activity and pERK nuclear localization. We also show that while β-arrestin-2 potentiates GPR54 signaling to ERK, β-arrestin-1 inhibits it. Our data also revealed that diminished β-arrestin-1 and -2 expression in the GT1-7 GnRH hypothalamic neuronal cell line triggered distinct patterns of gene expression following Kp-10 treatment. Thus, β-arrestin-1 and -2 also regulate distinct downstream responses in gene expression. Finally, we showed that GPR54, when uncoupled from the G_q/11 pathway, as is the case for several naturally occurring GPR54 mutants associated with hypogonadotropic hypogonadism, continues to regulate gene expression in a G protein-independent manner. These new and exciting findings add significantly to our mechanistic understanding of how this important receptor signals intracellularly in response to kisspeptin stimulation.

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.