Functional importance of the carboxyl tail cysteine residues in the human D1 dopamine receptor

Associations of CALM1 and DRD1 polymorphisms, and their expression levels, with Taihang chicken egg-production traits

Egg production by hens is an important reproductive performance index in the poultry industry. To investigate the effects of the CALM1 and DRD1 genes on egg production in chicken, their mRNA expression and single nucleotide polymorphisms (SNP) levels were investigated, and bioinformatics and egg-production association analyses were performed. Three SNPs (g.44069941G > A and g.44069889A > G in CALM1 and g.10742639C > T in DRD1) were detected in the exons and introns of CALM1 and DRD1 in 400 Taihang chickens. Among them, g.44069941G > A was significantly associated with Taihang chicken egg production on the 500th day (p < 0.05), whereas g.10742639C > T was significantly associated with the 300th day (p < 0.05). The expression levels of CALM1 and DRD1 in ovarian tissues of a high-yielding Taihang group were greater than in a low-yielding group (p < 0.05). The bioinformatics analysis revealed that the mutations influenced the mRNA secondary structures of CALM1 and DRD1. This study provides new insights into the potential effects of CALM1 and DRD1 polymorphisms on chicken egg production. The two SNPs g.44069941G > A and g.10742639C > T are potential molecular markers for improving the reproductive traits of Taihang chicken.

Lipid rafts are required for effective renal D1 dopamine receptor function

Effective receptor signaling is anchored on the preferential localization of the receptor in lipid rafts, which are plasma membrane platforms replete with cholesterol and sphingolipids. We hypothesized that the dopamine D₁ receptor (D₁ R) contains structural features that allow it to reside in lipid rafts for its activity. Mutation of C347 palmitoylation site and Y218 of a newly identified Cholesterol Recognition Amino Acid Consensus motif resulted in the exclusion of D₁ R from lipid rafts, blunted cAMP response, impaired sodium transport, and increased oxidative stress in renal proximal tubule cells (RPTCs). Kidney-restricted silencing of Drd1 in C57BL/6J mice increased blood pressure (BP) that was normalized by renal tubule-restricted rescue with D₁ R-wild-type but not the mutant D₁ R 347A that lacks a palmitoylation site. Kidney-restricted disruption of lipid rafts by β-MCD jettisoned the D₁ R from the brush border, decreased sodium excretion, and increased oxidative stress and BP in C57BL/6J mice. Deletion of the PX domain of the novel D₁ R-binding partner sorting nexin 19 (SNX19) resulted in D₁ R partitioning solely to non-raft domains, while silencing of SNX19 impaired D₁ R function in RPTCs. Kidney-restricted silencing of Snx19 resulted in hypertension in C57BL/6J mice. Our results highlight the essential role of lipid rafts for effective D₁ R signaling.

Aminergic GPCR-Ligand Interactions: A Chemical and Structural Map of Receptor Mutation Data

The aminergic family of G protein-coupled receptors (GPCRs) plays an important role in various diseases and represents a major drug discovery target class. Structure determination of all major aminergic subfamilies has enabled structure-based ligand design for these receptors. Site-directed mutagenesis data provides an invaluable complementary source of information for elucidating the structural determinants of binding of different ligand chemotypes. The current study provides a comparative analysis of 6692 mutation data points on 34 aminergic GPCR subtypes, covering the chemical space of 540 unique ligands from mutagenesis experiments and information from experimentally determined structures of 52 distinct aminergic receptor-ligand complexes. The integrated analysis enables detailed investigation of structural receptor-ligand interactions and assessment of the transferability of combined binding mode and mutation data across ligand chemotypes and receptor subtypes. An overview is provided of the possibilities and limitations of using mutation data to guide the design of novel aminergic receptor ligands.

C-terminal di-leucine motif of dopamine D₁ receptor plays an important role in its plasma membrane trafficking

The dopamine D₁ receptor (D₁R), a G protein-coupled receptor, plays a critical role in regulating blood pressure through its actions on renal hemodynamics and epithelial ion transport, which are highly linked to its intracellular trafficking. In this study, we generated a series of C-terminal mutants of D₁R that were tagged with or without enhanced yellow fluorescent protein, and analyzed the consequences of these mutants on the plasma membrane trafficking of D₁R and cyclic AMP response to D₁R stimulation. D₁R with mutations within the endocytic recycling signal (amino acid residues 360–382) continued to be functional, albeit decreased relative to wild-type D₁R. Mutation of the palmitoylation site (347C>S) of D₁R did not impair its trafficking to the plasma membrane, but abolished its ability to increase cyclic AMP accumulation. In contrast, replacement of di-leucines (344–345L>A) by alanines resulted in the retention of D₁R in the early endosome, decreased its glycosylation, and prevented its targeting to the plasma membrane. Our studies suggest that di-L motif at the C-terminus of D₁R is critical for the glycosylation and cell surface targeting of D₁R.

The genetic effects of the dopamine D1 receptor gene on chicken egg production and broodiness traits

Background

The elevation of egg production and the inhibition of incubation behavior are the aims of modern poultry production. Prolactin (PRL) gene is confirmed to be critical for the onset and maintenance of these reproductive behaviors in birds. Through PRL, dopamine D1 receptor (DRD1) was also involved in the regulation of chicken reproductive behavior. However, the genetic effects of this gene on chicken egg production and broodiness have not been studied extensively. The objective of this research was to evaluate the genetic effects of the DRD1 gene on chicken egg production and broodiness traits.

Results

In this study, the chicken DRD1 gene was screened for the polymorphisms by cloning and sequencing and 29 variations were identified in 3,342 bp length of this gene. Seven single nucleotide polymorphism (SNPs) among these variations, including a non-synonymous mutation (A+505G, Ser169Gly), were located in the coding region and were chosen to analyze their association with chicken egg production and broodiness traits in 644 Ningdu Sanhuang individuals. Two SNPs, G+123A and C+1107T, were significantly associated with chicken broody frequency (P < 0.05). Significant association was also found between the G+1065A - C+1107T haplotypes and chicken broody frequency (P < 0.05). In addition, the haplotypes of G+123A and T+198C were significantly associated with weight of first egg (EW) (P = 0.03). On the other hand, the distribution of the DRD1 mRNA was observed and the expression difference was compared between broodiness and non-broodiness chickens. The DRD1 mRNA was predominantly expressed in subcutaneous fat and abdominal fat of non-broodiness chicken, and then in heart, kidney, oviduct, glandular stomach, hypothalamus, and pituitary. In subcutaneous fat and abdominal fat, the level of non-broodiness was 26 to 28 times higher than that of broodiness. In pituitary, it was 5-fold higher. In heart, oviduct, and kidney, a 2-3 times decrease from non-broodiness to broodiness was displayed. In glandular stomach and hypothalamus, the level seen in non-broodiness and broodiness was almost the same.

Conclusion

The polymorphisms of the DRD1 gene and their haplotypes were associated with chicken broody frequency and some egg production traits. The mRNA distribution was significant different between broodiness and non-broodiness chickens.

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.

The cytoplasmic tail of the D1A receptor subtype: identification of specific domains controlling dopamine cellular responsiveness

In this study the rat D1A receptor (wild-type, WT) and truncation mutants thereof, are utilized to delineate specific cytoplasmic tail (CT) domains responsible for regulating ligand binding and receptor-mediated adenylyl cyclase activation. In human embryonic kidney (HEK) cells, all truncation mutants of the D1A receptor (Delta425, Delta379, Delta351) display cell surface localization and express at high but different receptor numbers. Binding studies suggest that residues located between Cys(351) and Asp(425) may serve to restrain the agonist binding conformation of the D1A receptor. This contention is supported by the observation that the constitutive activation of Delta351 is significantly increased in comparison with WT, Delta425 and Delta379. Furthermore, we demonstrate that the extent of dopamine-mediated maximal activation of adenylyl cyclase is significantly augmented in cells expressing Delta351 when compared with WT or mutants harboring shorter truncations. These results suggest that in addition to restraining receptor conformation, determinants located downstream of Cys(351) may act as negative regulators of the G protein coupling efficiency and adenylyl cyclase activation. Interestingly, all truncated receptors used in the present study display a decrease in dopamine potency when compared with WT. We show that inhibition of protein kinase A (PKA) activity leads also to a reduction in dopamine potency in cells expressing WT but not Delta351 receptors. These results hint at a potential previously unanticipated role for PKA in facilitating D1A receptor coupling efficiency in HEK cells. Overall, the present study has uncovered specific CT domains involved in regulating discrete aspects of the D1A receptor signaling.

Palmitoylation-dependent control of degradation, life span, and membrane expression of the CCR5 receptor

We have shown that the chemokine and HIV receptor CCR5 is palmitoylated on a cluster of cysteine residues located at the boundary between the seventh transmembrane region and the cytoplasmic tail. Single or combined substitutions of the three cysteines (Cys-321, Cys-323, and Cys-324) or incubation of wild-type CCR5-transfected cells with the palmitic acid analog 2-bromopalmitate prevented palmitoylation of the receptor. Moreover, failure of CCR5 to be palmitoylated resulted in both accumulation in intracellular stores and a profound decrease of membrane expression of the receptor. Upon metabolic labeling, kinetic experiments showed that the half-life of palmitoylation-deficient CCR5 is profoundly decreased. Bafilomycin A1, but not a specific proteasome inhibitor, prevented early degradation of palmitoylation-deficient CCR5 and promoted its accumulation in lysosomal compartments. Although membrane expression of the CCR5 mutant was diminished, the molecules reaching the membrane were still able to interact efficiently with the chemokine ligand MIP1 beta and remained able to function as HIV co-receptors. Thus we conclude that palmitoylation controls CCR5 expression through regulation of the life span of this receptor.

The palmitoylation state of the beta(2)-adrenergic receptor regulates the synergistic action of cyclic AMP-dependent protein kinase and beta-adrenergic receptor kinase involved in its phosphorylation and desensitization

Although palmitoylation of the beta(2)-adrenergic receptor (beta(2)AR), as well as its phosphorylation by the cyclic AMP-dependant protein kinase (PKA) and the beta-adrenergic receptor kinase (beta ARK), are known to play important roles in agonist-promoted desensitization, their relative contribution and mutual regulatory influences are still poorly understood. In this study, we investigated the role that the carboxyl tail PKA site (Ser(345,346)) of the beta(2)AR plays in its rapid agonist-promoted phosphorylation and desensitization. Mutation of this site (Ala(345,346)beta(2)AR) significantly reduced the rate and extent of the rapid desensitization promoted by sustained treatment with the agonist isoproterenol. The direct contribution of Ser(345,346) in desensitization was then studied by mutating all other putative PKA and beta ARK phosphorylation sites (Ala(261,262)beta ARK(-)beta(2)AR). We found this mutant receptor to be phosphorylated upon receptor activation but not following direct activation of PKA, suggesting a role in receptor-specific (homologous) but not heterologous phosphorylation. However, despite its phosphorylated state, Ala(261,262)beta ARK(-)beta(2)AR did not undergo rapid desensitization upon agonist treatment, indicating that phosphorylation of Ser(345,346) alone is not sufficient to promote desensitization. Taken with the observation that mutation of either Ser(345,346) or of the beta ARK phosphorylation sites prevented both the hyper-phosphorylation and constitutive desensitization of a palmitoylation-less mutant (Gly(341)beta(2)AR), our data suggest a concerted/synergistic action of the two kinases that depends on the palmitoylation state of the receptor. Consistent with this notion, in vitro phosphorylation of Gly(341)beta(2)AR by the catalytic subunit of PKA facilitated further phosphorylation of the receptor by purified beta ARK. Our study therefore allows us to propose a coordinated mechanism by which sequential depalmitoylation, and phosphorylation by PKA and beta ARK lead to the functional uncoupling and desensitization of the ss(2)AR.

Palmitoylation occurs at cysteine 347 and cysteine 351 of the dopamine D(1) receptor

To determine the palmitoylation sites in the human dopamine D(1) receptor, we expressed wild type and mutant receptors in which candidate cysteines in the carboxyl tail were substituted by alanines both individually (A347, A351) and together (AA). Our results showed that palmitoylation levels of A347 and A351 were reduced substantially and that AA had no detectable signal of palmitoylation. These data indicate that cysteines 347 and 351 were both palmitoylated and that they were the only sites of palmitoylation. We introduced a cAMP-dependent protein kinase site encompassing the position 351. We predicted that a functional cAMP-dependent protein kinase site would impair receptor-G protein coupling if it is not occluded by palmitoylation. Our results demonstrated that indeed, the introduction of the cAMP-dependent protein kinase site caused reduced potency of dopamine stimulation of adenylyl cyclase, and thus confirmed that when unoccluded, the cAMP-dependent protein kinase site introduced to position 351 of dopamine D(1) receptor could confer constitutive desensitization.

Activation of the beta(2)-adrenergic receptor-Galpha(s) complex leads to rapid depalmitoylation and inhibition of repalmitoylation of both the receptor and Galpha(s)

Palmitoylation is unique among lipid modifications in that it is reversible. In recent years, dynamic palmitoylation of G protein alpha subunits and of their cognate receptors has attracted considerable attention. However, very little is known concerning the acylation/deacylation cycle of the proteins in relation to their activity status. In particular, the relative contribution of the activation and desensitization of the signaling unit to the regulation of the receptors and G proteins palmitoylation state is unknown. To address this issue, we took advantage of the fact that a fusion protein composed of the stimulatory alpha subunit of trimeric G protein (Galpha(s)) covalently attached to the beta(2)-adrenergic receptor (beta(2)AR) as a carboxyl-terminal extension (beta(2)AR-Galpha(s)) can be stimulated by agonists but does not undergo rapid inactivation, desensitization, or internalization. When expressed in Sf9 cells, both the receptor and the Galpha(s) moieties of the fusion protein were found to be palmitoylated via thioester linkage. Stimulation with the beta-adrenergic agonist isoproterenol led to a rapid depalmitoylation of both the beta(2)AR and Galpha(s) and inhibited repalmitoylation. The extent of depalmitoylation induced by a series of agonists was correlated (0.99) with their intrinsic efficacy to stimulate the adenylyl cyclase activity. However, forskolin-stimulated cAMP production did not affect the palmitoylation state of beta(2)AR-Galpha(s), indicating that the agonist-promoted depalmitoylation is linked to conformational changes and not to second messenger generation. Given that, upon activation, the fusion protein mimics the activated receptor-G protein complex but cannot undergo desensitization, the data demonstrate that early steps in the activation process lead to the depalmitoylation of both receptor and G protein and that repalmitoylation requires later events that cannot be accommodated by the activated fusion protein.

Elimination of palmitoylation sites in the human dopamine D1 receptor does not affect receptor-G protein interaction

We have eliminated putative palmitoylation sites in the carboxyl tail of the human dopamine D1 receptor by replacing the two cysteine residues with alanines either separately or together. The wild type and the three mutated dopamine D1 receptors were stably expressed in baby hamster kidney cells and characterized to detect any resulting alterations in receptor-G protein interactions. The three mutant dopamine D1 receptors retained the same proportion of high affinity state for agonists as wild type receptors and also no difference was observed in the stimulation of adenylyl cyclase activity. Our results are in contrast to those observed with the beta 2-adrenoceptor and consistent with similar studies of luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptors, alpha 2-adrenoceptors, muscarinic M2 receptors and thyrotropin releasing hormone (TRH) receptors. Thus, we suggest that palmitoylation appears to play a unique role in the beta 2-adrenoceptors, and appears not to be essential in G protein coupling for the dopamine D1 receptors.

Early emergence of three dopamine D1 receptor subtypes in vertebrates. Molecular phylogenetic, pharmacological, and functional criteria defining D1A, D1B, and D1C receptors in European eel Anguilla anguilla

The existence of dopamine D1C and D1D receptors in Xenopus and chicken, respectively, challenged the established duality (D1A and D1B) of the dopamine D1 receptor class in vertebrates. To ascertain the molecular diversity of this gene family in early diverging vertebrates, we isolated four receptor-encoding sequences from the European eel Anguilla anguilla. Molecular phylogeny assigned two receptor sequences (D1A1 and D1A2) to the D1A subtype, and a third receptor to the D1B subtype. Additional sequence was orthologous to the Xenopus D1C receptor and to several other previously unclassified fish D1-like receptors. When expressed in COS-7 cells, eel D1A and D1B receptors display affinity profiles for dopaminergic ligands similar to those of other known vertebrate homologues. The D1C receptor exhibits pharmacological characteristics virtually identical to its Xenopus homologue. Functionally, while all eel D1 receptors stimulate adenylate cyclase, the eel D1B receptor exhibits greater constitutive activity than either D1A or D1C receptors. Semiquantitative reverse transcription-polymerase chain reaction reveals the differential distribution of D1A1, D1A2, D1B, and D1C receptor mRNA within the hypothalamic-pituitary axis of the eel brain. Taken together, these data suggest that the D1A, D1B, and D1C receptors arose prior to the evolutionary divergence of fish and tetrapods and exhibit molecular, pharmacological, and functional attributes that unambiguously allow for their classification as distinct D1 receptor subtypes in the vertebrate phylum.

Palmitoylated cysteine 341 modulates phosphorylation of the beta2-adrenergic receptor by the cAMP-dependent protein kinase

We previously showed that substitution of a glycine residue for the palmitoylated cysteine 341 of the human beta2-adrenergic receptor (Gly341beta2AR), increases the basal level of the receptor phosphorylation and reduces its ability to functionally interact with Gs. In the present study, we show that additional mutation of serines 345 and 346 (Ala345,346Gly341beta2AR) restored normal phosphorylation and receptor-Gs coupling, thus suggesting that the increased phosphorylation of this site, rather than the lack of palmitoylation per se, is responsible for the poor coupling of the unpalmitoylated receptor. This is supported by the observation that chemical depalmitoylation of purified beta2AR did not affect the ability of the receptor to stimulate adenylyl cyclase in reconstitution assays. Furthermore, mutation of Ser345,346 in a wild type receptor background (Ala345,346beta2AR) significantly decreased the rate of agonist-promoted desensitization of the receptor-stimulated adenylyl cyclase activity, supporting a role for this phosphorylation site in regulating the functional coupling of the receptor. Since serines 345 and 346 are located in a putative cyclic AMP-dependent protein kinase (PKA) phosphorylation site immediately downstream of the palmitoylated cysteine 341, the hypothesis that the accessibility of this site may be regulated by the receptor palmitoylation state was further assessed in vitro. In membrane phosphorylation assays, Gly341beta2AR was found to be a better substrate for PKA than the wild type receptor, thus supporting the notion that palmitoylation restrains access of the phosphorylation site to the enzyme. Taken together, the data demonstrate that palmitoylation of cysteine 341 controls the phosphorylation state of the PKA site located in the carboxyl tail of the beta2AR and by doing so modulates the responsiveness of the receptor.

Cytoplasmic tail length influences fatty acid selection for acylation of viral glycoproteins

We report remarkable differences in the fatty acid content of thioester-type acylated glycoproteins of enveloped viruses from mammalian cells. The E2 glycoprotein of Semliki Forest virus contains mainly palmitic acid like most other palmitoylated proteins analysed so far. However, the other glycoprotein (E1) of the same virus, as well as the HEF (haemagglutinin esterase fusion) glycoprotein of influenza C virus, are unique in this respect because they are acylated primarily with stearic acid. Comparative radiolabelling of uninfected cells with different fatty acids suggests that stearate may also be the prevailing fatty acid in some cellular acylproteins. To look for further differences between palmitoylated and stearoylated glycoproteins we characterized stearoylation in more detail. We identified the acylation site of HEF as a cysteine residue located at the boundary between the transmembrane region and the cytoplasmic tail. The attachment of stearate to HEF and E1 occurs post-translationally in a pre-Golgi compartment. Thus, stearoylated and palmitoylated proteins cannot be discriminated on the basis of the fatty acid linkage site or the intracellular compartment, where acylation occurs. However, stearoylated acylproteins contain a very short, positively charged cytoplasmic tail, whereas in palmitoylated proteins this molecular region is longer. Replacing the short cytoplasmic tail of stearoylated HEF with the long influenza A virus haemagglutinin (HA) tail in an HEF-HA chimera, and subsequent vaccinia T7 expression in CV-1 cells, yielded proteins with largely palmitic acid bound. The reverse chimera, HA-HEF with a short cytoplasmic tail was not fatty acylated at all during expression, indicating that conformational or topological constraints control fatty acid transfer.

Palmitoylation: a post-translational modification that regulates signalling from G-protein coupled receptors

Protein acylation is a post-translational modification that has seized much attention in the last few years. Depending on the nature of the fatty acid added, protein acylation can take the form of palmitoylation, myristoylation, or prenylation. Palmitoylation has been implicated in the modification of several different proteins and is particularly prevalent in G-protein coupled receptors and their cognate G-proteins, where it is thought to have an important regulatory function. Given that palmitoylation of these proteins is a dynamic phenomenon in which turnover rate is modulated by agonist activation, it is thought to be implicated in processes such as receptor phosphorylation and desensitization as well as in G-protein membrane translocation. A better understanding of the regulation of signal transduction mediated by G-protein coupled receptors will require the identification and characterization of those enzymes implicated in the palmitoylation and depalmitoylation process of this large class of receptors and their signalling allies.