A region of the third intracellular loop of the short form of the D2 dopamine receptor dictates Gi coupling specificity

The Gαi protein subclass selectivity to the dopamine D2 receptor is also decided by their location at the cell membrane

Background

G protein-coupled receptor (GPCR) signaling via heterotrimeric G proteins plays an important role in the cellular regulation of responses to external stimuli. Despite intensive structural research, the mechanism underlying the receptor–G protein coupling of closely related subtypes of Gαi remains unclear. In addition to the structural changes of interacting proteins, the interactions between lipids and proteins seem to be crucial in GPCR-dependent cell signaling due to their functional organization in specific membrane domains. In previous works, we found that Gαs and Gαi₃ subunits prefer distinct types of membrane-anchor lipid domains that also modulate the G protein trimer localization. In the present study, we investigated the functional selectivity of dopamine D₂ long receptor isoform (D₂R) toward the Gαi₁, Gαi₂, and Gαi₃ subunits, and analyzed whether the organization of Gαi heterotrimers at the plasma membrane affects the signal transduction.

Methods

We characterized the lateral diffusion and the receptor–G protein spatial distribution in living cells using two assays: fluorescence recovery after photobleaching microscopy and fluorescence resonance energy transfer detected by fluorescence-lifetime imaging microscopy. Depending on distribution of data differences between Gα subunits were investigated using parametric approach–unpaired T-test or nonparametric–Mann–Whitney U test.

Results

Despite the similarities between the examined subunits, the experiments conducted in the study revealed a significantly faster lateral diffusion of the Gαi₂ subunit and the singular distribution of the Gαi₁ subunit in the plasma membrane. The cell membrane partitioning of distinct Gαi heterotrimers with dopamine receptor correlated very well with the efficiency of D₂R-mediated inhibition the formation of cAMP.

Conclusions

This study showed that even closely related subunits of Gαi differ in their membrane-trafficking properties that impact on their signaling. The interactions between lipids and proteins seem to be crucial in GPCR-dependent cell signaling due to their functional organization in specific membrane domains, and should therefore be taken into account as one of the selectivity determinants of G protein coupling.

Critical Impact of Different Conserved Endoplasmic Retention Motifs and Dopamine Receptor Interacting Proteins (DRIPs) on Intracellular Localization and Trafficking of the D2 Dopamine Receptor (D2-R) Isoforms

The type 2 dopamine receptor D₂ (D₂-R), member of the G protein-coupled receptor (GPCR) superfamily, exists in two isoforms, short (D_2S-R) and long (D_2L-R). They differ by an additional 29 amino acids (AA) in the third cytoplasmic loop (ICL3) of the D_2L-R. These isoforms differ in their intracellular localization and trafficking functionality, as D_2L-R possesses a larger intracellular pool, mostly in the endoplasmic reticulum (ER). This review focuses on the evolutionarily conserved motifs in the ICL3 of the D₂-R and proteins interacting with the ICL3 of both isoforms, specifically with the 29 AA insert. These motifs might be involved in D₂-R exit from the ER and have an impact on cell-surface and intracellular localization and, therefore, also play a role in the function of dopamine receptor signaling, ligand binding and possible homo/heterodimerization. Our recent bioinformatic data on potential new interaction partners for the ICL3 of D₂-Rs are also presented. Both are highly relevant, and have clinical impacts on the pathophysiology of several diseases such as Parkinson’s disease, schizophrenia, Tourette’s syndrome, Huntington’s disease, manic depression, and others, as they are connected to a variety of essential motifs and differences in communication with interaction partners.

First identification of dopamine receptors in pikeperch, Sander lucioperca, during the pre-ovulatory period

Dopamine (DA) is a ubiquitous neurotransmitter exerting a range of pleiotropic actions through two DA receptor families, the D1 and the D2. To date in vertebrates, a maximum of four receptor subtypes have been identified within the D1 family, D₁ (former D_1A), D₅ (former D_1B), D₆ (former D_1C and D_1D) and D₇ (former D_1E), while the D2 family encloses five subtypes, D₂, D₃, D₄, D₈ (former D_2like or D_2l) and D₉ (former D_{4-related sequence} or D_4-rs). In teleosts, no study has investigated in parallel all the DA receptors to identify and localize the whole receptor repertoire from both families. In pikeperch, Sander lucioperca, a species of interest for aquaculture development, the existence, number and location of the DA receptors are totally unknown. To address these questions, RNA-seq with de novo transcriptome reconstruction, functional annotation and phylogenetic analysis were performed to characterize the transcript repertoire of DA receptors in the brain of female pikeperch at the pre-ovulatory period. Ten different cDNA were identified and showed to belong to the D1 family: two D₁, one D_5a, one D_6a and one D_6b and to the D2 family: two spliced variants of D₂, one D₃, one D₈ and one D_9. Unlike zebrafish, the subtypes D₄ and D₇ have not yet been isolated in pikeperch. As expected D₁, D₃, D₈ and D₉ are mostly expressed in brain parts except for the cerebellum (D₁ and D₃). The inter-species differences in the number of DA receptors and the inter-organ differences in the gene expression of all receptors support the complexity of the dopaminergic actions in vertebrate.

Cell adhesion molecule close homolog of L1 binds to the dopamine receptor D2 and inhibits the internalization of its short isoform

Cell adhesion molecule close homolog of L1 (CHL1) and the dopamine receptor D2 (DRD2) are associated with psychiatric and mental disorders. We here show that DRD2 interacts with CHL1 in mouse brain, as evidenced by co-immunostaining, proximity ligation assay, co-immunoprecipitation, and pull-down assay with recombinant extracellular CHL1 domain fused to Fc (CHL1-Fc). Direct binding of CHL1-Fc to the first extracellular loop of DRD2 was shown by ELISA. Using HEK cells transfected to co-express CHL1 and the short (DRD2-S) or long (DRD2-L) DRD2 isoforms, co-localization of CHL1 and both isoforms was observed by immunostaining and proximity ligation assay. Moreover, CHL1 inhibited agonist-triggered internalization of DRD2-S. Proximity ligation assay showed close interaction between CHL1 and DRD2 in neurons expressing dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP32) or tyrosine hydroxylase (TH) in tissue sections of adult mouse striatum. In cultures of striatum or ventral midbrain, CHL1 was also closely associated with DRD2 in DARPP32- or TH-immunopositive cells, respectively. In the dorsal striatum of CHL1-deficient mice, lower levels of DRD2 and phosphorylated TH were observed, when compared to wild-type littermates. In the ventral striatum of CHL1-deficient mice, levels of phosphorylated DARPP32 were reduced. We propose that CHL1 regulates DRD2-dependent presynaptic and postsynaptic functions.

Preferential Coupling of Dopamine D2S and D2L Receptor Isoforms with Gi1 and Gi2 Proteins-In Silico Study

The dopamine D₂ receptor belongs to rhodopsin-like G protein-coupled receptors (GPCRs) and it is an important molecular target for the treatment of many disorders, including schizophrenia and Parkinson's disease. Here, computational methods were used to construct the full models of the dopamine D₂ receptor short (D_2S) and long (D_2L) isoforms (differing with 29 amino acids insertion in the third intracellular loop, ICL3) and to study their coupling with G_i1 and G_i2 proteins. It was found that the D_2L isoform preferentially couples with the G_i2 protein and D_2S isoform with the G_i1 protein, which is in accordance with experimental data. Our findings give mechanistic insight into the interplay between isoforms of dopamine D₂ receptors and G_i proteins subtypes, which is important to understand signaling by these receptors and their mediation by pharmaceuticals, in particular psychotic and antipsychotic agents.

Design, synthesis and evaluation of novel phenothiazine derivatives as inhibitors of breast cancer stem cells

A novel series of phenothiazine derivatives containing diethanolamine, methoxyethylamine, flavonoids, and a nitric oxide (NO) donor was designed and synthesized for the treatment of breast cancer. Phenothiazine derivatives (l) did not noticeably inhibit the growth of SUM159, MDA-MB-231, MCF-7, and SKBR-3 cells, whereas phenothiazine derivatives (ll) containing the NO donor were more potent or had comparable inhibitory activity to trifluoperazine (TFP) and thioridazine against SUM159, MDA-MB-231, MCF-7, and SKBR-3 cells. Compounds 20a-c and 21a-c showed the strongest activity in SUM159, MDA-MB-231, MCF-7, and SKBR-3 cells, and more potent inhibitory activity than TFP against KG1a cells (IC₅₀ = 1.63, 2.93, 1.14, 1.78, 2.20, and 1.20 vs. 4.58 μM). Compounds 20a and 21a had lower toxicity than compounds 20b-c and 21b-c, and inhibited colony formation in MCF-7 cells, decreased the formation of mammospheres in SUM159 cells, and inhibited the migration of MDA-MB-231 cells. Compounds 20a and 21a could inhibited pNF-κB-p65 as shown by dual-luciferase reporter assays and western blotting in MDA-MB-231 cells.

Monitoring Interactions Between S100B and the Dopamine D2 Receptor Using NMR Spectroscopy

S100B is a dimeric EF-hand protein that undergoes a calcium-induced conformational change and interacts with a wide range of proteins to modulate their functions. The dopamine D2 receptor is one potential S100B binding partner that may play a key role in neurological processing. In this chapter, we describe the use of NMR spectroscopy to examine the interaction between calcium-bound S100B and the third intracellular loop (IC3) from the dopamine D2 receptor. We provide details that allow the strength of the interaction (K _d) between the two proteins to be determined and the IC3 site of interaction on the structure of S100B to be identified. Both these characteristics can be identified from a single series of nondestructive experiments.

Evolution of dopamine receptors: phylogenetic evidence suggests a later origin of the DRD2l and DRD4rs dopamine receptor gene lineages

Dopamine receptors are integral membrane proteins whose endogenous ligand is dopamine. They play a fundamental role in the central nervous system and dysfunction of dopaminergic neurotransmission is responsible for the generation of a variety of neuropsychiatric disorders. From an evolutionary standpoint, phylogenetic relationships among the DRD₁ class of dopamine receptors are still a matter of debate as in the literature different tree topologies have been proposed. In contrast, phylogenetic relationships among the DRD₂ group of receptors are well understood. Understanding the time of origin of the different dopamine receptors is also an issue that needs further study, especially for the genes that have restricted phyletic distributions (e.g., DRD_2l and DRD_4rs). Thus, the goal of this study was to investigate the evolution of dopamine receptors, with emphasis on shedding light on the phylogenetic relationships among the D₁ class of dopamine receptors and the time of origin of the DRD_2l and DRD_4rs gene lineages. Our results recovered the monophyly of the two groups of dopamine receptors. Within the DRD₁ group the monophyly of each paralog was recovered with strong support, and phylogenetic relationships among them were well resolved. Within the DRD₁ class of dopamine receptors we recovered the sister group relationship between the DRD_1C and DRD_1E, and this clade was recovered sister to a cyclostome sequence. The DRD₁ clade was recovered sister to the aforementioned clade, and the group containing DRD₅ receptors was sister to all other DRD₁ paralogs. In agreement with the literature, among the DRD₂ class of receptors, DRD₂ was recovered sister to DRD₃, whereas DRD₄ was sister to the DRD₂/DRD₃ clade. According to our phylogenetic tree, the DRD_2l and DRD_4rs gene lineages would have originated in the ancestor of gnathostomes between 615 and 473 mya. Conservation of sequences required for dopaminergic neurotransmission and small changes in regulatory regions suggest a functional refinement of the dopaminergic pathways along evolution.

Dopamine D3 and acetylcholine nicotinic receptor heteromerization in midbrain dopamine neurons: Relevance for neuroplasticity

Activation of nicotinic acetylcholine receptors (nAChR) promotes the morphological remodeling of cultured dopamine (DA) neurons, an effect requiring functional DA D3 receptors (D3R). The aim of this study was to investigate the mechanisms mediating D3R-nAChR cross-talk in the modulation of DA neuron structural plasticity. By using bioluminescence resonance energy transfer² (BRET²) and proximity ligation assay (PLA), evidence for the existence of D3R-nAChR heteromers has been obtained. In particular, BRET² showed that the D3R directly and specifically interacts with the β2 subunit of the nAChR. The D3R-nAChR complex was also identified in cultured DA neurons and in mouse Substantia Nigra/Ventral Tegmental Area by PLA. Cell permeable interfering peptides, containing highly charged amino acid sequences from the third intracellular loop of D3R (TAT-D3R) or the second intracellular loop of the β2 subunit (TAT-β2), were developed. Both peptides, but not their scrambled counterparts, significantly reduced the BRET² signal generated by D3R-GFP² and β2-Rluc. Similarly, the PLA signal was undetectable in DA neurons exposed to the interfering peptides. Moreover, interfering peptides abolished the neurotrophic effects of nicotine on DA neurons. Taken together these data first demonstrate that a D3R-nAChR heteromer is present in DA neurons and represents the functional unit mediating the neurotrophic effects of nicotine.

Constitutive activities and inverse agonism in dopamine receptors

The concept of activation in the absence of agonists has been demonstrated for many GPCRs and is now solidified as one of the principal aspects of GPCR signaling. In this chapter, we review how dopamine receptors demonstrate this ability. Although difficult to prove in vivo due to the presence of endogenous dopamine and lack of subtype-selective inverse agonists and "pure" antagonists (neutral ligands), in vitro assays such as measuring intracellular cAMP, [(35)S]GTPγS binding, and [(3)H]thymidine incorporation have uncovered the constitutive activation of D1- and D2-class receptors. Nevertheless, because of limited and inconsistent findings, the existence of constitutive activity for D2-class receptors is currently not well established. Mutagenesis studies have shown that basal signaling, notably by D1-class receptors, is governed by the collective contributions of transmembrane domains and extracellular/intracellular loops, such as the third extracellular loop, the third intracellular loop, and C-terminal tail. Furthermore, constitutive activities of D1-class receptors are subjected to regulation by kinases. Among the dopamine receptor family, the D5 receptor subtype exhibits a higher basal signaling and bears resemblance to constitutively active mutant forms of GPCRs. The presence of its constitutive activity in vivo and its pathophysiological relevance, with a brief mention of other subtypes, are also discussed.

A synergistic approach towards understanding the functional significance of dopamine receptor interactions

The importance of the neurotransmitter dopamine (DA) in the nervous system is underscored by its role in a wide variety of physiological and neural functions in both vertebrates and invertebrates. Binding of dopamine to its membrane receptors initiates precise signaling cascades that result in specific cellular responses. Dopamine receptors belong to a super-family of G-protein coupled receptors (GPCRs) that are characterized by seven trans-membrane domains. In mammals, five dopamine receptors have been identified which are grouped into two different categories D1- and D2-like receptors. The interactions of DA receptors with other proteins including specific Gα subunits are critical in deciding the fate of downstream molecular events carried out by effector proteins. In this mini-review we provide a synopsis of known protein-protein interactions of DA receptors and a perspective on the potential synergistic utility of Caenorhabditis elegans as a model eukaryote with a comparatively simpler nervous system to gain insight on the neuronal and behavioral consequences of the receptor interactions.

Differential signaling by splice variants of the human free fatty acid receptor GPR120

GPR120 is a long-chain fatty acid receptor that stimulates incretin hormone release from colonic endocrine cells and is implicated in macrophage and adipocyte function. The functional consequences of long (L) and short (S) human GPR120 splice variants, which differ by insertion of 16 amino acids in the third intracellular loop, are currently unknown. Here we compare signaling and intracellular trafficking of GPR120S and GPR120L receptors, using calcium mobilization and dynamic mass redistribution (DMR) assays, together with quantitative imaging measurements of β-arrestin2 association and receptor internalization. FLAG- or SNAP-tagged GPR120S receptors elicited both intracellular calcium mobilization and DMR responses in human embryonic kidney 293 cells, when stimulated with oleic acid, myristic acid, or the agonist 4-[[(3-phenoxyphenyl)methyl]amino]benzenepropanoic acid (GW9508). Responses were insensitive to pertussis toxin, but increases in intracellular calcium were attenuated by 2-aminoethoxydiphenyl borate, an inhibitor of store inositol trisphosphate receptors. Despite equivalent cell surface expression of SNAP-tagged GPR120L receptors, no specific calcium or DMR responses were observed in cells transfected with this isoform. However, agonist-stimulated GPR120S and GPR120L receptors both recruited β-arrestin2 and underwent robust internalization, with similar agonist potencies in each case. After oleic acid-induced internalization, neither GPR120 isoform recycled rapidly to the cell surface. In both cases, confocal microscopy indicated receptor targeting to lysosomal compartments. Thus, the third intracellular loop insertion in GPR120L prevents G protein-dependent intracellular calcium and DMR responses, but this receptor isoform remains functionally coupled to the β-arrestin pathway, providing one of the first examples of a native β-arrestin-biased receptor.

Resistance to dopamine agonists in prolactinoma is correlated with reduction of dopamine D2 receptor long isoform mRNA levels

OBJECTIVE

Dopamine agonists normalize prolactin (PRL) levels and reduce tumour size in responsive prolactinoma. However, several cases have shown resistance to dopamine agonists upon initial treatment. Infrequently, prolactinoma initially responds, but then becomes refractory to prolonged treatment (secondary resistance). We investigated the possible mechanisms of resistance to dopamine agonists.

SUBJECTS AND METHODS

Twelve cases of prolactinoma were surgically resected and classified according to the responsiveness of PRL levels and tumour size to dopamine agonists: good responders (n = 5), poor responders (n = 5), or secondary resistance (n = 2). We examined the expression of dopamine D(2) receptor (D(2)R) isoform (short: D(2)S and long: D(2)L) mRNA and protein. We investigated DNA methylation patterns in the promoter region of the DRD2 gene.

RESULTS

The predominant D(2)R isoform expressed in prolactinoma was D(2)L. Levels of D(2)L mRNA were significantly lower in secondary resistance and poor responders than in good responders. Expression of D(2)R protein was variable among cases. Almost no CpG sites of the DRD2 gene promoter region were methylated.

CONCLUSION

Resistance of prolactinoma to dopamine agonists is correlated with a reduction in D(2)L isoform mRNA levels. Silencing of the DRD2 gene by methylation in the promoter region is unlikely to play a role in dopamine agonist resistance in prolactinoma.

The Caenorhabditis elegans D2-like dopamine receptor DOP-2 physically interacts with GPA-14, a Gαi subunit

Dopaminergic inputs are sensed on the cell surface by the seven-transmembrane dopamine receptors that belong to a superfamily of G-protein-coupled receptors (GPCRs). Dopamine receptors are classified as D1-like or D2-like receptors based on their homology and pharmacological profiles. In addition to well established G-protein coupled mechanism of dopamine receptors in mammalian system they can also interact with other signaling pathways. In C. elegans four dopamine receptors (dop-1, dop-2, dop-3 and dop-4) have been reported and they have been implicated in a wide array of behavioral and physiological processes. We performed this study to assign the signaling pathway for DOP-2, a D2-like dopamine receptor using a split-ubiquitin based yeast two-hybrid screening of a C. elegans cDNA library with a novel dop-2 variant (DOP-2XL) as bait. Our yeast two-hybrid screening resulted in identification of gpa-14, as one of the positively interacting partners. gpa-14 is a Gα coding sequence and shows expression overlap with dop-2 in C. elegans ADE deirid neurons. In-vitro pull down assays demonstrated physical coupling between dopamine receptor DOP-2XL and GPA-14. Further, we sought to determine the DOP-2 region necessary for GPA-14 coupling. We generated truncated DOP-2XL constructs and performed pair-wise yeast two-hybrid assay with GPA-14 followed by in-vitro interaction studies and here we report that the third intracellular loop is the key domain responsible for DOP-2 and GPA-14 coupling. Our results show that the extra-long C. elegans D2-like receptor is coupled to gpa-14 that has no mammalian homolog but shows close similarity to inhibitory G-proteins. Supplementing earlier investigations, our results demonstrate the importance of an invertebrate D2-like receptor's third intracellular loop in its G-protein interaction.

Polypyrimidine tract-binding protein 1 regulates the alternative splicing of dopamine receptor D2

Dopamine receptor D(2) (DRD2) has two splicing isoforms, a long form (D2L) and short form (D2S), which have distinct functions in the dopaminergic system. However, the regulatory mechanism of the alternative splicing of DRD2 is unknown. In this study, we examined which splicing factors regulate the expression of D2L and D2S by over-expressing several RNA-binding proteins in HEK293 cells. In a cellular splicing assay, the over-expression of polypyrimidine tract-binding protein 1 (PTBP1) reduced the expression of D2S, whereas the knockdown of PTBP1 increased the expression of D2S. We also identified the regions of DRD2 that are responsive to PTBP1 using heterologous minigenes and deletion mutants. Our results indicate that PTBP1 regulates the alternative splicing of DRD2. Considering that DRD2 inhibits cAMP-dependent protein kinase A, which modulates the intracellular localization of PTBP1, PTBP1 may contribute to the autoregulation of DRD2 by regulating the expression of its isoforms.

Isoform-specific uncoupling of the D2 dopamine receptors subtypes

Dopaminergic transmission is fundamental to many neural pathways of clinical interest. We have analyzed the alternatively-spliced isoforms of the D(2) dopamine receptor, D(2) long (D(2l)) and D(2) short (D(2s)), which differ only by a 29-amino acid insertion in the third cytoplasmic loop. Well-known determinants for GPCR signal transduction--the third intracellular loop regions--were co-expressed with the wild-type receptors to test for their ability to antagonize parent receptor function. We found that the D(2l)-mediated inhibition of forskolin-stimulated adenylyl cyclase was blocked by the co-expression of the third cytoplasmic loop of D(2l). However, expression of the third cytoplasmic loop of D(2s) did not inhibit D(2l)-mediated signal transduction. Conversely, expression of the D(2s) third cytoplasmic loop antagonized the D(2s) receptor's function and the D(2l) third cytoplasmic loop did not. In contrast, expression of the alternatively-spliced insert region had no effect when co-expressed with either wild-type receptor isoform. These results suggest that the third cytoplasmic loops of each receptor adopt unique conformations and that the primary sequence of the insert region is not the basis for differences in signaling between D(2s) and D(2l). These findings further support previous studies suggesting that the D2 receptor isoforms use distinct signal transduction mechanisms.

An adenylyl cyclase signaling pathway predicts direct dopaminergic input to vestibular hair cells

Adenylyl cyclase (AC) signaling pathways have been identified in a model hair cell preparation from the trout saccule, for which the hair cell is the only intact cell type. The use of degenerate primers targeting cDNA sequence conserved across AC isoforms, and reverse transcription-polymerase chain reaction (RT-PCR), coupled with cloning of amplification products, indicated expression of AC9, AC7 and AC5/6, with cloning efficiencies of 11:5:2. AC9 and AC5/6 are inhibited by Ca(2+), the former in conjunction with calcineurin, and message for calcineurin has also been identified in the trout saccular hair cell layer. AC7 is independent of Ca(2+). Given the lack of detection of calcium/calmodulin-activated isoforms previously suggested to mediate AC activation in the absence of Gαs in mammalian cochlear hair cells, the issue of hair-cell Gαs mRNA expression was re-examined in the teleost vestibular hair cell model. Two full-length coding sequences were obtained for Gαs/olf in the vestibular type II-like hair cells of the trout saccule. Two messages for Gαi have also been detected in the hair cell layer, one with homology to Gαi1 and the second with homology to Gαi3 of higher vertebrates. Both Gαs/olf protein and Gαi1/Gαi3 protein were immunolocalized to stereocilia and to the base of the hair cell, the latter consistent with sites of efferent input. Although a signaling event coupling to Gαs/olf and Gαi1/Gαi3 in the stereocilia is currently unknown, signaling with Gαs/olf, Gαi3, and AC5/6 at the base of the hair cell would be consistent with transduction pathways activated by dopaminergic efferent input. mRNA for dopamine receptors D1A4 and five forms of dopamine D2 were found to be expressed in the teleost saccular hair cell layer, representing information on vestibular hair cell expression not directly available for higher vertebrates. Dopamine D1A receptor would couple to Gαolf and activation of AC5/6. Co-expression with dopamine D2 receptor, which itself couples to Gαi3 and AC5/6, will down-modulate levels of cAMP, thus fine-tuning and gradating the hair-cell response to dopamine D1A. As predicted by the trout saccular hair cell model, evidence has been obtained for the first time that hair cells of mammalian otolithic vestibular end organs (rat/mouse saccule/utricle) express dopamine D1A and D2L receptors, and each receptor co-localizes with AC5/6, with a marked presence of all three proteins in subcuticular regions of type I vestibular hair cells. A putative efferent, presynaptic source of dopamine was identified in tyrosine hydroxylase-positive nerve fibers which passed from underlying connective tissue to the sensory epithelia, ending on type I and type II vestibular hair cells and on afferent calyces.

Alcoholism and alternative splicing of candidate genes

Gene expression studies have shown that expression patterns of several genes have changed during the development of alcoholism. Gene expression is regulated not only at the level of transcription but also through alternative splicing of pre-mRNA. In this review, we discuss some of the evidence suggesting that alternative splicing of candidate genes such as DRD2 (encoding dopamine D2 receptor) may form the basis of the mechanisms underlying the pathophysiology of alcoholism. These reports suggest that aberrant expression of splice variants affects alcohol sensitivities, and alcohol consumption also regulates alternative splicing. Thus, investigations of alternative splicing are essential for understanding the molecular events underlying the development of alcoholism.

Interactions between calmodulin, adenosine A2A, and dopamine D2 receptors

The Ca(2+)-binding protein calmodulin (CaM) has been shown to bind directly to cytoplasmic domains of some G protein-coupled receptors, including the dopamine D(2) receptor. CaM binds to the N-terminal portion of the long third intracellular loop of the D(2) receptor, within an Arg-rich epitope that is also involved in the binding to G(i/o) proteins and to the adenosine A(2A) receptor, with the formation of A(2A)-D(2) receptor heteromers. In the present work, by using proteomics and bioluminescence resonance energy transfer (BRET) techniques, we provide evidence for the binding of CaM to the A(2A) receptor. By using BRET and sequential resonance energy transfer techniques, evidence was obtained for CaM-A(2A)-D(2) receptor oligomerization. BRET competition experiments indicated that, in the A(2A)-D(2) receptor heteromer, CaM binds preferentially to a proximal C terminus epitope of the A(2A) receptor. Furthermore, Ca(2+) was found to induce conformational changes in the CaM-A(2A)-D(2) receptor oligomer and to selectively modulate A(2A) and D(2) receptor-mediated MAPK signaling in the A(2A)-D(2) receptor heteromer. These results may have implications for basal ganglia disorders, since A(2A)-D(2) receptor heteromers are being considered as a target for anti-parkinsonian agents.

Sharpening the edges of understanding the structure/function of the LPA1 receptor: expression in cancer and mechanisms of regulation

Since the molecular cloning of the vzg-1/Edg-2/LPA1 gene, studies have attempted to characterize LPA1 receptor functionality into a single categorical role, different from the other Edg-family LPA receptors. The desire to categorize LPA1 function has highlighted its complexity and demonstrated that the LPA1 receptor does not have one absolute function throughout every system. The central nervous system is highly enriched in the LPA1 receptor, suggesting an integral role in neuronal processes. Metastatic and invasive breast cancer also appears to have LPA-mediated LPA1 receptor functions that enhance phenotypes associated with tumorigenesis. LPA1 possesses a number of motifs conserved among G protein-coupled receptors (GPCRs): a DRY-like motif, a PDZ domain, Ser/Thr predicted sites of phosphorylation, a di-leucine motif, double cysteines in the tail and conserved residues that stabilize structure and determine ligand binding. The third intracellular loop of the LPA1 receptor may be the crux of receptor signaling and attenuation with phosphorylation of Thr-236 potentially a key determinant of basal LPA1 signaling. Mutagenesis data supports the notion that Thr-236 regulates this process since mutating Thr-236 to Ala-236 increased basal and LPA-mediated serum response factor (SRF) signaling activity and Lys-236 further increased this basal signaling. Here we describe progress on defining the major functions of the LPA1 receptor, discuss a context dependent dualistic role as both a negative regulator in cancer and a proto-oncogene, outline its structural components at the molecular amino acid level and present mutagenesis data on the third intracellular loop of the receptor.

An 80-amino acid deletion in the third intracellular loop of a naturally occurring human histamine H3 isoform confers pharmacological differences and constitutive activity

In this article, we pharmacologically characterized two naturally occurring human histamine H3 receptor (hH3R) isoforms, hH3R(445) and hH3R(365). These abundantly expressed splice variants differ by a deletion of 80 amino acids in the intracellular loop 3. In this report, we show that the hH3R(365) is differentially expressed compared with the hH3R(445) and has a higher affinity and potency for H3R agonists and conversely a lower potency and affinity for H3R inverse agonists. Furthermore, we show a higher constitutive signaling of the hH3R(365) compared with the hH3R(445) in both guanosine-5'-O-(3-[35S]thio) triphosphate binding and cAMP assays, likely explaining the observed differences in hH3R pharmacology of the two isoforms. Because H3R ligands are beneficial in animal models of obesity, epilepsy, and cognitive diseases such as Alzheimer's disease and attention deficit hyperactivity disorder and currently entered clinical trails, these differences in H3R pharmacology of these two isoforms are of great importance for a detailed understanding of the action of H3R ligands.

Role of a Galphai2 protein splice variant in the formation of an intracellular dopamine D2 receptor pool

Treatment of D2-receptor-expressing cells with specific drugs upregulates the receptor number at the cell surface independently of protein synthesis, leading to the concept of an intracellular receptor pool. However, how this pool is operating is still an enigma. Here, we report that a splice variant of the Galphai2 protein, protein sGalphai2, plays a crucial role in the maintenance of this D2-receptor pool. Co-expression of sGi2 with D2 receptor reduced receptor localization to cell surface by one-third. This effect is associated with specific intracellular protein-protein interaction and the formation of a sGi2-D2-receptor complex. It has been suggested that the formation of this complex serves to prevent D2 receptors from reaching the cell membrane. Treatment of D2-receptor-expressing cells with agonists increased the number of cell surface D2 receptors and coincided with a reduction in these receptors from intracellular complexes, suggesting that agonist treatment released D2 receptors from the complex allowing them to localize to the cell membrane. Thus, in addition to elucidating how the intracellular pool of D2 receptor functions, our findings uncover a novel mechanism regulating the density of cell surface D2 receptors.

Novel insights in dopamine receptor physiology

The dopaminergic system has a pivotal role in the central nervous system but also plays important roles in the periphery, mainly in the endocrine system. Dopamine exerts its functions via five different receptors, named D(1)-D(5), belonging to the category of G protein coupled membrane receptors. Dopamine receptors are heterogeneously expressed in different cells, tissues and organs, where they stimulate or inhibit different functions, including neurotransmission and hormone synthesis and secretion. In particular, the dopamineric system has a pivotal role in the physiological regulation of the hypothalamus-pituitary-adrenal axis. Recent data have demonstrated the expression and function of dopamine receptors not only in endocrine organs but also in endocrine tumors, mainly those belonging to the hypothalamus-pituitary-adrenal axis, and also in the so-called 'neuroendocrine' tumors. These data confirm the important role of the dopaminergic system in this endocrine axis, as well as in the neuroendocrine system. This review summarizes the main structural and functional characteristics of dopamine receptors, emphasizing the most recent novelties, and focused on the physiological and pathological regulation of the hypothalamus-pituitary-adrenal axis by the dopaminergic system. In addition, the recent findings on the relationship between dopamine receptors and neuroendocrine tumors are summarized.

Association analysis of the dopamine receptor D2 (DRD2) SNP rs1076560 in alcoholic patients

The dopamine system plays a well-established role in alcoholism. In this study, we examined the association between the single-nucleotide polymorphism (SNP) rs1076560 of the dopamine receptor D2 (DRD2) gene and susceptibility to alcoholism. SNP rs1076560 (C/A) is located in intron 6 of DRD2, where it is 1.4 kb downstream from alternative exon 6 and 83 bp upstream from exon 7. A total of 248 alcoholic patients and 322 healthy controls, all Japanese males, were genotyped for rs1076560 polymorphism by direct sequencing and allele-specific PCR. Data were analyzed using standard chi(2) statistics and a backwards logistic regression approach to adjust for the contribution of aldehyde dehydrogenase-2 (ALDH2) genotype status. The DRD2 risk allele A was more prevalent in the alcoholic patients (40.1%) than in the healthy controls (34.0%) (P=0.034, odds ratio=1.300, 95% confidence interval=1.020-1.657). These data identify SNP rs1076560 as a potentially important variable in the development of alcoholism.

Advances in the treatment of prolactinomas

Prolactinomas account for approximately 40% of all pituitary adenomas and are an important cause of hypogonadism and infertility. The ultimate goal of therapy for prolactinomas is restoration or achievement of eugonadism through the normalization of hyperprolactinemia and control of tumor mass. Medical therapy with dopamine agonists is highly effective in the majority of cases and represents the mainstay of therapy. Recent data indicating successful withdrawal of these agents in a subset of patients challenge the previously held concept that medical therapy is a lifelong requirement. Complicated situations, such as those encountered in resistance to dopamine agonists, pregnancy, and giant or malignant prolactinomas, may require multimodal therapy involving surgery, radiotherapy, or both. Progress in elucidating the mechanisms underlying the pathogenesis of prolactinomas may enable future development of novel molecular therapies for treatment-resistant cases. This review provides a critical analysis of the efficacy and safety of the various modes of therapy available for the treatment of patients with prolactinomas with an emphasis on challenging situations, a discussion of the data regarding withdrawal of medical therapy, and a foreshadowing of novel approaches to therapy that may become available in the future.

Protein-protein coupling/uncoupling enables dopamine D2 receptor regulation of AMPA receptor-mediated excitotoxicity

here is considerable evidence that dopamine D2 receptors can modulate AMPA receptor-mediated neurotoxicity. However, the molecular mechanism underlying this process remains essentially unclear. Here we report that D2 receptors inhibit AMPA-mediated neurotoxicity through two pathways: the activation of phosphoinositide-3 kinase (PI-3K) and downregulation of AMPA receptor plasma membrane expression, both involving a series of protein-protein coupling/uncoupling events. Agonist stimulation of D2 receptors promotes the formation of the direct protein-protein interaction between the third intracellular loop of the D2 receptor and the ATPase N-ethylmaleimide-sensitive factor (NSF) while uncoupling the NSF interaction with the carboxyl tail (CT) of the glutamate receptor GluR2 subunit of AMPA receptors. Previous studies have shown that full-length NSF directly couples to the GluR2CT and facilitates AMPA receptor plasma membrane expression. Furthermore, the CT region of GluR2 subunit is also responsible for several other intracellular protein couplings, including p85 subunit of PI-3K. Therefore, the direct coupling of D2-NSF and concomitant decrease in the NSF-GluR2 interaction results in a decrease of AMPA receptor membrane expression and an increase in the interaction between GluR2 and the p85 and subsequent activation of PI-3K. Disruption of the D2-NSF interaction abolished the ability of D2 receptor to attenuate AMPA-mediated neurotoxicity by blocking the D2 activation-induced changes in PI-3K activity and AMPA receptor plasma membrane expression. Furthermore, the D2-NSF-GluR2-p85 interactions are also responsible for the D2 inhibition of ischemia-induced cell death. These data may provide a new avenue to identify specific targets for therapeutics to modulate glutamate receptor-governed diseases, such as stroke.

Investigation of the alternatively spliced insert region of the D2L dopamine receptor by epitope substitution

Alternatively spliced variants of the D2 dopamine receptor have distinct neuronal function and localization. The long isoform (D2L) of this heptahelical transmembrane receptor differs from the short form only by the presence of a 29-amino acid insert in the third intracellular loop-a region known to be important for G protein coupling. Short and long isoforms have been shown to have distinct Galphai/o protein coupling specificities. However, the exact role of the alternatively spliced insert region in D2 dopamine receptor function needs a more comprehensive examination. One way to address this is to substitute the entire insert region with an equivalent length, yet nonhomologous protein sequence. This report demonstrates the feasibility of replacing the 29-amino acid insert with a hemagglutinin double epitope tag with no recognizable functional consequences. The D2L mutant is indistinguishable from the wild type D2L receptor in terms of its ligand binding characteristics, as well as two effector responses: the agonist-mediated inhibition of forskolin-stimulated cAMP production, and agonist-stimulated MAPK phosphorylation. These data demonstrate that the epitope substitution generates a functional receptor, and that the alternatively spliced insert region, itself, does not appear to play a direct role in signal transduction. The epitope substitution permits dissection of sequence-mediated effects from structural effects due to the presence of the alternatively spliced insert region. Thus, this new construct could be a valuable tool for the study of D2 receptor function.

Molecular mechanism of the intracellular segments of the melanocortin-4 receptor for NDP-MSH signaling

Mutations of the human melanocortin-4 receptor (hMC4R) have been previously identified to be the most common cause of monogenic human obesity. Specifically, mutations of the intracellular C terminus and the third intracellular loop of hMC4R have been reported to play an important role in human obesity. However, the molecular basis of these hMC4R intracellular segments in receptor function remains unclear. In this study, we utilized deletions and mutations of specific portions of the hMC4R to determine the molecular mechanism of both the C terminus and the third intracellular loop in receptor signaling. Our results indicate that deletions of the distal 25 (the entire C terminus), 22, 18, 17, 16, and 15 amino acids of the C terminus result in the complete loss of both [Nle(4)-d-Phe(7)]-alpha-melanocyte stimulating hormone (NDP-MSH) binding and NDP-MSH-mediated cAMP production. Deletion of the distal 14 amino acids of the C terminus significantly decreases both NDP-MSH binding affinity and potency, but deletion of the distal 13 amino acids of the C terminus does not affect NDP-MSH activity. Further analysis revealed that the proximal 12 amino acids of the C terminus are not only important for receptor signaling but also important for ligand binding. Our results also indicate that the third intracellular loop of the hMC4R is important for receptor signaling but not ligand binding. In summary, our findings suggest that the proximal region of the melanocortin-4 receptor (MC4R) C terminus is crucial not only for receptor signaling but also for ligand binding, while the third intracellular loop is important mainly for receptor signaling.

D3 dopamine receptor activates phospholipase D through a pertussis toxin-insensitive pathway

Within the dopamine receptor family, the D(3) dopamine receptor's function remains inadequately described. The D(3) receptor has been shown to couple to inhibition of adenylyl cyclase, stimulation of mitogenesis, and regulation of K(+) and Ca(2+) currents, all in a pertussis toxin (PTX)-sensitive manner. Here we report D(3) receptor activation of the phospholipase D (PLD) enzyme in HEK 293 cells heterologously expressing the human D(3) receptor. Activation by agonist is dose dependent and displays the pharmacology expected of the D(3) receptor. The D(3) receptor specific antagonists AJ-76 and U99194A ablated the increase in activity by the preferring D(3) agonist (+) 7-OH DPAT. In addition, the D(3) receptor-mediated activation of PLD is not mediated by G-proteins of the G(i)/G(o) family, as pretreatment with PTX had no effect. PLD activation is a novel finding for the D(3) receptor, and is the first example of an effector system where D(3) signals without G(i)/G(o) protein intermediates.

Inhibition of ATP-sensitive potassium channels by haloperidol

Chronic haloperidol treatment has been associated with an increased incidence of glucose intolerance and type-II diabetes mellitus. We studied the effects of haloperidol on native ATP-sensitive potassium (K(ATP)) channels in mouse pancreatic beta cells and on cloned Kir6.2/SUR1 channels expressed in HEK293 cells. The inhibitory effect of haloperidol on the K(ATP) channel was not mediated via the D2 receptor signaling pathway, as both D2 agonists and antagonists blocked the channel. K(ATP) currents were studied using the patch-clamp technique in whole-cell and outside-out patch configurations. Addition of haloperidol to the extracellular solution inhibited the K(ATP) conductance immediately, in a reversible and voltage-independent manner. Haloperidol did not block the channel when applied intracellularly in whole-cell recordings. Haloperidol blocked cloned Kir6.2/SUR1 and Kir6.2DeltaC36 K(ATP) channels expressed in HEK cells. This suggests that the drug interacts with the Kir6.2 subunit of the channel. The IC(50) for inhibition of the K(ATP) current by haloperidol was 1.6 microM in 2 mM extracellular K(+) concentration ([K(+)](o)) and increased to 23.9 microM in 150 mM [K(+)](o). The Hill coefficient was close to unity, suggesting that the binding of a single molecule of haloperidol is sufficient to close the channel. Haloperidol block of K(ATP) channels may contribute to the side effects of this drug when used therapeutically.

An Important Functional Role of the N Terminus Domain of Type VI Adenylyl Cyclase in Gαi-mediated Inhibition

We show herein that removal of the first 86 amino acids (aa) of the N terminus (designated N) of type VI adenylyl cyclase (ACVI) caused the resultant ACVI mutant (ACVI-DeltaA87) to be more greatly inhibited by a Galpha(i)-coupled receptor or activated Galpha(i) protein. Moreover, in vitro binding of the full-length N and C1a domain (designated C1a), which interacts with Galpha(i), was detected. A truncated N terminus (aa 1-86) also interacted with C1a, suggesting that the C1a-interacting region is located within aa 1-86. Mutation analyses further revealed that N might interact with C1a in the region (aa 434-505) where Galpha(i) is bound. Mutations of two residues (Leu-472 and Val-476) located in this N-binding region of C1a suppressed the interaction between recombinant N and C1a and markedly reduced Galpha(i)-mediated inhibition of ACVI-DeltaA87. Further biochemical analyses of the effect of internal mutations of Leu-472/Val-476 on Galpha(i)-mediated inhibition of wild-type ACVI and ACVI-DeltaA87 suggested that N modulates the Galpha(i)-mediated inhibition of ACVI via binding to C1a when the level of Galpha(i) is low (i.e. around the IC(50) value) and that a more complicated interfering mode results when the level of Galpha(i) is high (i.e. approximately 10- to 20-fold of the IC(50) value). Collectively, data presented herein suggest a novel function of the N terminus of ACVI in Galpha(i)-mediated regulation.