Phe27Cys Polymorphism Alters the Maturation and Subcellular Localization of the Human δ Opioid Receptor

Association of OPRD1 Gene Variants with Changes in Body Weight and Psychometric Indicators in Patients with Eating Disorders

Objectives: This study aimed to investigate whether genetic variations in the OPRD1 gene affect psychopathological symptoms and personality dimensions in eating disorders (ED) patients and/or contribute to ED risk. Methods: The study involved 221 female patients with anorexia nervosa (AN), 88 with bulimia nervosa (BN), and 396 controls. Sixteen tag-single nucleotide polymorphisms (SNPs) in OPRD1 were identified. Psychometric evaluations were conducted using the Symptom Checklist 90 Revised (SCL-90R) and the Eating Disorders Inventory Test-2 (EDI-2). p-values obtained by regression models were corrected for multiple testing by the False Discovery Rate (FDR) method. Results: In AN patients, genotypes rs204077TT and rs169450TT were linked to lower body-mass index (BMI) values (FDR-q = 0.035 and 0.017, respectively), as was rs2234918 in a log-additive model (BMI: 18.0 ± 0.28, 17.22 ± 0.18 and 16.59 ± 0.39 for TT, TC and CC carriers, FDR-q = 0.012). Additionally, AN patients carrying the rs72665504AA genotype had higher scores in interpersonal distrust (FDR-q = 0.030), whilst BN carriers of rs513269TT and rs2873795TT showed lower scores in ineffectiveness (FDR-q = 0.041 and FDR-q = 0.021). In the AN group, BMI correlated with variability in a distal haplotype (rs508448/rs204077/rs223491, FDR-q = 0.028), which was also associated with the global positive symptom total (PST) index of SCL-90R (FDR-q = 0.048). Associations were more noticeable in BN patients; again, the distal region of the gene was linked to EDI-2 total scores (FDR-q = 0.004-0.048 for the four last haplotypes) and two global SCL-90R indices (GSI: FDR-q = 0.011 and positive symptom distress index (PSDI): FDR-q = 0.003 for the last s204077/rs2234918/rs169450 combination). No associations with ED risk were observed. Conclusions: Genetic variation in the OPRD1 gene, particularly in its distal region, is associated with BMI and psychopathological comorbidities in ED patients.

GPR37 is processed in the N-terminal ectodomain by ADAM10 and furin

GPR37 is an orphan G protein-coupled receptor (GPCR) implicated in several neurological diseases and important physiological pathways in the brain. We previously reported that its long N-terminal ectodomain undergoes constitutive metalloprotease-mediated cleavage and shedding, which have been rarely described for class A GPCRs. Here, we demonstrate that the protease that cleaves GPR37 at Glu167↓Gln168 is a disintegrin and metalloprotease 10 (ADAM10). This was achieved by employing selective inhibition, RNAi-mediated downregulation, and genetic depletion of ADAM10 in cultured cells as well as in vitro cleavage of the purified receptor with recombinant ADAM10. In addition, the cleavage was restored in ADAM10 knockout cells by overexpression of the wild type but not the inactive mutant ADAM10. Finally, postnatal conditional depletion of ADAM10 in mouse neuronal cells was found to reduce cleavage of the endogenous receptor in the brain cortex and hippocampus, confirming the physiological relevance of ADAM10 as a GPR37 sheddase. Additionally, we discovered that the receptor is subject to another cleavage step in cultured cells. Using site-directed mutagenesis, the site (Arg54↓Asp55) was localized to a highly conserved region at the distal end of the ectodomain that contains a recognition site for the proprotein convertase furin. The cleavage by furin was confirmed by using furin-deficient human colon carcinoma LoVo cells and proprotein convertase inhibitors. GPR37 is thus the first multispanning membrane protein that has been validated as an ADAM10 substrate and the first GPCR that is processed by both furin and ADAM10. The unconventional N-terminal processing may represent an important regulatory element for GPR37.

Serotonin 5-HT2C Receptor Cys23Ser Single Nucleotide Polymorphism Associates with Receptor Function and Localization In Vitro

A non-synonymous single nucleotide polymorphism of the human serotonin 5-HT_2C receptor (5-HT_2CR) gene that converts a cysteine to a serine at amino acid codon 23 (Cys23Ser) appears to impact 5-HT_2CR pharmacology at a cellular and systems level. We hypothesized that the Cys23Ser alters 5-HT_2CR intracellular signaling via changes in subcellular localization in vitro. Using cell lines stably expressing the wild-type Cys23 or the Ser23 variant, we show that 5-HT evokes intracellular calcium release with decreased potency and peak response in the Ser23 versus the Cys23 cell lines. Biochemical analyses demonstrated lower Ser23 5-HT_2CR plasma membrane localization versus the Cys23 5-HT_2CR. Subcellular localization studies demonstrated O-linked glycosylation of the Ser23 variant, but not the wild-type Cys23, may be a post-translational mechanism which alters its localization within the Golgi apparatus. Further, both the Cys23 and Ser23 5-HT_2CR are present in the recycling pathway with the Ser23 variant having decreased colocalization with the early endosome versus the Cys23 allele. Agonism of the 5-HT_2CR causes the Ser23 variant to exit the recycling pathway with no effect on the Cys23 allele. Taken together, the Ser23 variant exhibits a distinct pharmacological and subcellular localization profile versus the wild-type Cys23 allele, which could impact aspects of receptor pharmacology in individuals expressing the Cys23Ser SNP.

Molecular Imaging of Opioid and Dopamine Systems: Insights Into the Pharmacogenetics of Opioid Use Disorders

Opioid use in the United States has steadily risen since the 1990s, along with staggering increases in addiction and overdose fatalities. With this surge in prescription and illicit opioid abuse, it is paramount to understand the genetic risk factors and neuropsychological effects of opioid use disorder (OUD). Polymorphisms disrupting the opioid and dopamine systems have been associated with increased risk for developing substance use disorders. Molecular imaging studies have revealed how these polymorphisms impact the brain and contribute to cognitive and behavioral differences across individuals. Here, we review the current molecular imaging literature to assess how genetic variations in the opioid and dopamine systems affect function in the brain's reward, cognition, and stress pathways, potentially resulting in vulnerabilities to OUD. Continued research of the functional consequences of genetic variants and corresponding alterations in neural mechanisms will inform prevention and treatment of OUD.

Site-specific O-glycosylation of N-terminal serine residues by polypeptide GalNAc-transferase 2 modulates human δ-opioid receptor turnover at the plasma membrane

G protein-coupled receptors (GPCRs) are an important protein family of signalling receptors that govern a wide variety of physiological functions. The capacity to transmit extracellular signals and the extent of cellular response are largely determined by the amount of functional receptors at the cell surface that is subject to complex and fine-tuned regulation. Here, we demonstrate that the cell surface expression level of an inhibitory GPCR, the human δ-opioid receptor (hδOR) involved in pain and mood regulation, is modulated by site-specific N-acetylgalactosamine (GalNAc) -type O-glycosylation. Importantly, we identified one out of the 20 polypeptide GalNAc-transferase isoforms, GalNAc-T2, as the specific regulator of O-glycosylation of Ser6, Ser25 and Ser29 in the N-terminal ectodomain of the receptor. This was demonstrated by in vitro glycosylation assays using peptides corresponding to the hδOR N-terminus, Vicia villosa lectin affinity purification of receptors expressed in HEK293 SimpleCells capable of synthesizing only truncated O-glycans, GalNAc-T edited cell line model systems, and site-directed mutagenesis of the putative O-glycosylation sites. Interestingly, a single-nucleotide polymorphism, at residue 27 (F27C), was found to alter O-glycosylation of the receptor in efficiency as well as in glycosite usage. Furthermore, flow cytometry and cell surface biotinylation assays using O-glycan deficient CHO-ldlD cells revealed that the absence of O-glycans results in decreased receptor levels at the plasma membrane due to enhanced turnover. In addition, mutation of the identified O-glycosylation sites led to a decrease in the number of ligand-binding competent receptors and impaired agonist-mediated inhibition of cyclic AMP accumulation in HEK293 cells. Thus, site-specific O-glycosylation by a selected GalNAc-T isoform can increase the stability of a GPCR, in a process that modulates the constitutive turnover and steady-state levels of functional receptors at the cell surface.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

OPRD1 Genetic Variation and Human Disease

The OPRD1 gene encodes the delta-opioid receptor, which has multiple functions including regulating reward pathways. The gene contains more than 2,000 verified genetic variants but only 2 currently have evidence for specific functions: rs1042114 disrupts maturation of the receptor and rs569356 affects OPRD1 expression. These polymorphisms and others in the gene have been found to be associated with human diseases. The most reproducible data are associations between opioid addiction and three variants in intron 1 (rs2236861, rs2236857, and rs3766951), which have been described in a number of independent populations. Several publications also point toward an association between anorexia and a haplotype block containing rs569356 and rs533123. Unfortunately the mechanisms underlying these two effects are currently unknown. In contrast, rs1042114 has been linked to Alzheimer's disease through an increasingly well-defined mechanism by which the variant allele reduces production of the beta-amyloid plaques associated with the disease. Additional studies of OPRD1 variants are necessary to replicate current findings and to delineate the functional roles of relevant polymorphisms.

Involvement of the coatomer protein complex I in the intracellular traffic of the delta opioid receptor

The delta opioid receptor (DOPr) is known to be mainly expressed in intracellular compartments. It remains unknown why DOPr is barely exported to the cell surface, but it seems that a substantial proportion of the immature receptor is trapped within the endoplasmic reticulum (ER) and the Golgi network. In the present study, we performed LC-MS/MS analysis to identify putative protein partners involved in the retention of DOPr. Analysis of the proteins co-immunoprecipitating with Flag-DOPr in transfected HEK293 cells revealed the presence of numerous subunits of the coatomer protein complex I (COPI), a vesicle-coating complex involved in recycling resident proteins from the Golgi back to the ER. Further analysis of the amino acid sequence of DOPr identified multiple consensus di-lysine and di-arginine motifs within the intracellular segments of DOPr. Using cell-surface ELISA and GST pulldown assays, we showed that DOPr interacts with COPI through its intracellular loops 2 and 3 (ICL2 and ICL3, respectively) and that the mutation of the K¹⁶⁴AK¹⁶⁶ (ICL2) or K²⁵⁰EK²⁵² (ICL3) putative COPI binding sites increased the cell-surface expression of DOPr in transfected cells. Altogether, our results indicate that COPI is a binding partner of DOPr and provide a putative mechanism to explain why DOPr is highly retained inside the cells.

A delta-opioid receptor genetic variant is associated with abstinence prior to and during cocaine dependence treatment

INTRODUCTION

An intronic polymorphism in the delta-opioid receptor gene (OPRD1) was previously associated with cocaine dependence in African-Americans. However, it is not known if the polymorphism (rs678849) is associated with dependence-related phenotypes within the cocaine dependent population.

METHODS

Cocaine and alcohol dependent subjects were randomized to either topiramate or placebo. Abstinence from cocaine use was confirmed by urine drug screens for benzoylecgonine three times per week. Cocaine withdrawal and craving were assessed at randomization using the Cocaine Selective Severity Assessment (CSSA) and Minnesota Cocaine Craving Scale (MCCS), respectively. Subjects were also interviewed using the Addiction Severity Index (ASI). Genotype at rs678849 was determined for 105 African-American subjects and compared to cocaine abstinence, as well as scores for CSSA, MCCS, and ASI.

RESULTS

African-American patients with the C/T or T/T genotypes (n=40) were more likely to be abstinent at the first urine drug screen and more likely to be abstinent for the week prior to randomization compared to patients with the C/C genotype (n=65). Subjects carrying the T allele were also more likely to have abstinent weeks over the course of the trial compared to those with the C/C genotype (RR=1.88, 95% CI=1.59-2.22, p=0.0035). No effects of rs678849 genotype on withdrawal, craving, or addiction severity were observed.

CONCLUSIONS

A polymorphism in OPRD1 appears to be associated with both cocaine dependence and cocaine use during treatment in African-Americans. Follow-up studies to confirm the effect on cocaine use are warranted.

Genetic Variation in δ-Opioid Receptor Associates with Increased β- and γ-Secretase Activity in the Late Stages of Alzheimer's Disease

The agonist-induced activation of human δ-opioid receptor (δOR) has been shown to increase β- (BACE1) and γ-secretase activities leading to increased production of amyloid-β (Aβ) peptide. We have recently shown that phenylalanine to cysteine substitution at amino acid 27 in δOR (δOR-Phe27Cys) increases amyloid-β protein precursor processing through altered endocytic trafficking. Also, a genetic meta-analysis of the δOR-Phe27Cys variation (rs1042114) in two independent Alzheimer's disease (AD) patient cohorts indicated that the heterozygosity of δOR-Phe27Cys increases the risk of AD. Here, we investigated α-, β-, and γ-secretase activities in human brain with respect to δOR-Phe27Cys variation in the temporal cortex of 71 subjects with varying degree of AD-related neurofibrillary pathology (Braak stages I-VI). As a result, a significant increase in β- (p = 0.03) and γ- (p = 0.01), but not α-secretase, activities was observed in late stage AD samples (Braak stages V-VI), which were heterozygous for δOR-Phe27Cys as compared to the δOR-Phe27 and δOR-Cys27 homozygotes. The augmented β-secretase activity was not associated with increased mRNA expression or protein levels of BACE1 in the late stage AD patients, who were heterozygous for the δOR-Phe27Cys variation. These findings suggest that δOR-Phe27Cys variation modulates β- and γ-secretase activity in the late stages of AD likely via post-translational mechanisms other than alterations in the mRNA or protein levels of BACE1, or, in the expression of γ-secretase complex components.

The Parkinson's-disease-associated receptor GPR37 undergoes metalloproteinase-mediated N-terminal cleavage and ectodomain shedding

The G-protein-coupled receptor 37 ( GPR37) has been implicated in the juvenile form of Parkinson's disease, in dopamine signalling and in the survival of dopaminergic cells in animal models. The structure and function of the receptor, however, have remained enigmatic. Here, we demonstrate that although GPR37 matures and is exported from the endoplasmic reticulum in a normal manner upon heterologous expression in HEK293 and SH-SY5Y cells, its long extracellular N-terminus is subject to metalloproteinase-mediated limited proteolysis between E167 and Q168. The proteolytic processing is a rapid and efficient process that occurs constitutively. Moreover, the GPR37 ectodomain is released from cells by shedding, a phenomenon rarely described for GPCRs. Immunofluorescence microscopy further established that although full-length receptors are present in the secretory pathway until the trans-Golgi network, GPR37 is expressed at the cell surface predominantly in the N-terminally truncated form. This notion was verified by flow cytometry and cell surface biotinylation assays. These new findings on the GPR37 N-terminal limited proteolysis may help us to understand the role of this GPCR in the pathophysiology of Parkinson's disease and in neuronal function in general.

N-Glycan-dependent and -independent quality control of human δ opioid receptor N-terminal variants

Quality control (QC) in the endoplasmic reticulum (ER) scrutinizes newly synthesized proteins and directs them either to ER export or ER-associated degradation (ERAD). Here, we demonstrate that the human δ-opioid receptor (hδOR) is subjected to ERQC in both N-glycan-dependent and -independent manners. This was shown by investigating the biosynthesis and trafficking of wild-type and non-N-glycosylated F27C variants in metabolic pulse-chase assays coupled with flow cytometry and cell surface biotinylation. Both QC mechanisms distinguished the minute one-amino acid difference between the variants, targeting a large fraction of hδOR-Cys(27) to ERAD. However, the N-glycan-independent QC was unable to compensate the N-glycan-dependent pathway, and some incompletely folded non-N-glycosylated hδOR-Cys(27) reached the cell surface in conformation incompatible with ligand binding. The turnover of receptors associating with the molecular chaperone calnexin (CNX) was significantly slower for the hδOR-Cys(27), pointing to an important role of CNX in the hδOR N-glycan-dependent QC. This was further supported by the fact that inhibiting the co-translational interaction of hδOR-Cys(27) precursors with CNX led to their ERAD. Opioid receptor pharmacological chaperones released the CNX-bound receptors to ER export and, furthermore, were able to rescue the Cys(27) variant from polyubiquitination and retrotranslocation to the cytosol whether carrying N-glycans or not. Taken together, the hδOR appears to rely primarily on the CNX-mediated N-glycan-dependent QC that has the capacity to assist in folding, whereas the N-glycan-independent mechanism constitutes an alternative, although less accurate, system for directing misfolded/incompletely folded receptors to ERAD, possibly in altered cellular conditions.

Common α2A and α2C adrenergic receptor polymorphisms do not affect plasma membrane trafficking

Various naturally occurring polymorphic forms of human G protein-coupled receptors (GPCRs) have been identified and linked to diverse pathological diseases, including receptors for vasopressin type 2 (nephrogenic diabetes insipidus) and gonadotropin releasing hormone (hypogonadotropic hypogonadism). In most cases, polymorphic amino acid mutations disrupt protein folding, altering receptor function as well as plasma membrane expression. Other pathological GPCR variants have been found that do not alter receptor function, but instead affect only plasma membrane trafficking (e.g., delta opiate and histamine type 1 receptors). Thus, altered membrane trafficking with retained receptor function may be another mechanism causing polymorphic GPCR dysfunction. Two common human α2A and α2C adrenergic receptor (AR) variants have been identified (α2A N251K and α2C Δ322-325 ARs), but pharmacological analysis of ligand binding and second messenger signaling has not consistently demonstrated altered receptor function. However, possible alterations in plasma membrane trafficking have not been investigated. We utilized a systematic approach previously developed for the study of GPCR trafficking motifs and accessory proteins to assess whether these α2 AR variants affected intracellular trafficking or plasma membrane expression. By combining immunofluorescent microscopy, glycosidic processing analysis, and quantitative fluorescent-activated cell sorting (FACS), we demonstrate that neither variant receptor had altered intracellular localization, glycosylation, nor plasma membrane expression compared to wild-type α2 ARs. Therefore, pathopharmacological properties of α2A N251K and α2C Δ322-325 ARs do not appear to be due to altered receptor pharmacology or plasma membrane trafficking, but may involve interactions with other intracellular signaling cascades or proteins.

Pharmacological chaperoning: a primer on mechanism and pharmacology

Approximately forty percent of diseases are attributable to protein misfolding, including those for which genetic mutation produces misfolding mutants. Intriguingly, many of these mutants are not terminally misfolded since native-like folding, and subsequent trafficking to functional locations, can be induced by target-specific, small molecules variably termed pharmacological chaperones, pharmacoperones, or pharmacochaperones (PCs). PC targets include enzymes, receptors, transporters, and ion channels, revealing the breadth of proteins that can be engaged by ligand-assisted folding. The purpose of this review is to provide an integrated primer of the diverse mechanisms and pharmacology of PCs. In this regard, we examine the structural mechanisms that underlie PC rescue of misfolding mutants, including the ability of PCs to act as surrogates for defective intramolecular interactions and, at the intermolecular level, overcome oligomerization deficiencies and dominant negative effects, as well as influence the subunit stoichiometry of heteropentameric receptors. Not surprisingly, PC-mediated structural correction of misfolding mutants normalizes interactions with molecular chaperones that participate in protein quality control and forward-trafficking. A variety of small molecules have proven to be efficacious PCs and the advantages and disadvantages of employing orthostatic antagonists, active-site inhibitors, orthostatic agonists, and allosteric modulator PCs are considered. Also examined is the possibility that several therapeutic agents may have unrecognized activity as PCs, and this chaperoning activity may mediate/contribute to therapeutic action and/or account for adverse effects. Lastly, we explore evidence that pharmacological chaperoning exploits intrinsic ligand-assisted folding mechanisms. Given the widespread applicability of PC rescue of mutants associated with protein folding disorders, both in vitro and in vivo, the therapeutic potential of PCs is vast. This is most evident in the treatment of lysosomal storage disorders, cystic fibrosis, and nephrogenic diabetes insipidus, for which proof of principle in humans has been demonstrated.

Targeting opioid receptors with pharmacological chaperones

G protein-coupled receptors (GPCRs) are polytopic membrane proteins that have a pivotal role in cellular signaling. Like other membrane proteins, they fold in the endoplasmic reticulum (ER) before they are transported to the plasma membrane. The ER quality control monitors the folding process and misfolded proteins and slowly folding intermediates are targeted to degradation in the cytosol via the ubiquitin-proteasome pathway. The high efficiency of the quality control machinery may lead to the disposal of potentially functional receptors. This is the major underlying course for loss-of-function conformational diseases, such as retinitis pigmentosa, nephrogenic diabetes insipidus and early onset obesity, which involve mutant GPCRs. During the past decade, it has become increasingly evident that small-molecular lipophilic and pharmacologically selective receptor ligands, called pharmacological chaperones (PCs), can rescue these mutant receptors from degradation by stabilizing newly synthesized receptors in the ER and enhancing their transport to the cell surface. This has raised the interesting prospect that PCs might have therapeutic value for the treatment of conformational diseases. At the same time, accumulating evidence has indicated that wild-type receptors might also be targeted by PCs, widening their therapeutic potential. This review focuses on one GPCR subfamily, opioid receptors that have been useful models to unravel the mechanism of action of PCs. In contrast to most other GPCRs, compounds that act as PCs for opioid receptors, including widely used opioid drugs, target wild-type receptors and their common natural variants.

β-Adrenergic agonists mediate enhancement of β1-adrenergic receptor N-terminal cleavage and stabilization in vivo and in vitro

The β(1)-adrenergic receptor (β(1)AR) is the predominant βAR in the heart and is the main target for β-adrenergic antagonists, widely used in the treatment of cardiovascular diseases. Previously, we have shown that the human (h) β(1)AR is cleaved in its N terminus by a metalloproteinase, both constitutively and in a receptor activation-dependent manner. In this study, we investigated the specific events involved in β(1)AR regulation, focusing on the effects of long-term treatment with β-adrenergic ligands on receptor processing in stably transfected human embryonic kidney 293(i) cells. The key findings were verified using the transiently transfected hβ(1)AR and the endogenously expressed receptor in neonatal rat cardiomyocytes. By using flow cytometry and Western blotting, we demonstrated that isoproterenol, S-propranolol, CGP-12177 [4-[3-[(1,1-dimethylethyl)amino]2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one], pindolol, and timolol, which displayed agonistic properties toward the β(1)AR in either the adenylyl cyclase or the mitogen-activated protein kinase signaling pathways, induced cleavage of the mature cell-surface receptor. In contrast, metoprolol, bisoprolol, and CGP-20712 [1-[2-((3-carbamoyl-4-hydroxy)phenoxy)ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl)phenoxy]-2-propanol], which showed no agonistic activity, had only a marginal or no effect. Importantly, the agonists also stabilized intracellular receptor precursors, possibly via their pharmacological chaperone action, and they stabilized the receptor in vitro. The opposing effects on the two receptor forms thus led to an increase in the amount of cleaved receptor fragments at the plasma membrane. The results underscore the pluridimensionality of β-adrenergic ligands and extend this property from receptor activation and signaling to the regulation of β(1)AR levels. This phenomenon may contribute to the exceptional resistance of β(1)ARs to downregulation and tendency toward upregulation following long-term ligand treatments.

A novel dominant hyperekplexia mutation Y705C alters trafficking and biochemical properties of the presynaptic glycine transporter GlyT2

Hyperekplexia or startle disease is characterized by an exaggerated startle response, evoked by tactile or auditory stimuli, producing hypertonia and apnea episodes. Although rare, this orphan disorder can have serious consequences, including sudden infant death. Dominant and recessive mutations in the human glycine receptor (GlyR) α1 gene (GLRA1) are the major cause of this disorder. However, recessive mutations in the presynaptic Na(+)/Cl(-)-dependent glycine transporter GlyT2 gene (SLC6A5) are rapidly emerging as a second major cause of startle disease. In this study, systematic DNA sequencing of SLC6A5 revealed a new dominant GlyT2 mutation: pY705C (c.2114A→G) in transmembrane domain 11, in eight individuals from Spain and the United Kingdom. Curiously, individuals harboring this mutation show significant variation in clinical presentation. In addition to classical hyperekplexia symptoms, some individuals had abnormal respiration, facial dysmorphism, delayed motor development, or intellectual disability. We functionally characterized this mutation using molecular modeling, electrophysiology, [(3)H]glycine transport, cell surface expression, and cysteine labeling assays. We found that the introduced cysteine interacts with the cysteine pair Cys-311-Cys-320 in the second external loop of GlyT2. This interaction impairs transporter maturation through the secretory pathway, reduces surface expression, and inhibits transport function. Additionally, Y705C presents altered H(+) and Zn(2+) dependence of glycine transport that may affect the function of glycinergic neurotransmission in vivo.

The genetics of the opioid system and specific drug addictions

Addiction to drugs is a chronic, relapsing brain disease that has major medical, social, and economic complications. It has been established that genetic factors contribute to the vulnerability to develop drug addiction and to the effectiveness of its treatment. Identification of these factors may increase our understanding of the disorders, help in the development of new treatments and advance personalized medicine. In this review, we will describe the genetics of the major genes of the opioid system (opioid receptors and their endogenous ligands) in connection to addiction to opioids, cocaine, alcohol and methamphetamines. Particular emphasis is given to association and functional studies of specific variants. We will provide information on the sample populations and the size of each study, as well as a list of the variants implicated in association with addiction-related phenotypes, and with the effectiveness of pharmacotherapy for addiction.

Opioid system and Alzheimer's disease

The opioid system may be involved in the pathogenesis of AD, including cognitive impairment, hyperphosphorylated tau, Aβ production, and neuroinflammation. Opioid receptors influence the regulation of neurotransmitters such as acetylcholine, norepinephrine, GABA, glutamate, and serotonin which have been implicated in the pathogenesis of AD. Opioid system has a close relation with Aβ generation since dysfunction of opioid receptors retards the endocytosis and degradation of BACE1 and γ-secretase and upregulates BACE1 and γ-secretase, and subsequently, the production of Aβ. Conversely, activation of opioid receptors increases the endocytosis of BACE1 and γ-secretase and downregulates BACE1 and γ-secretase, limiting the production of Aβ. The dysfunction of opioid system (opioid receptors and opioid peptides) may contribute to hyperphosphorylation of tau and neuroinflammation, and accounts for the degeneration of cholinergic neurons and cognitive impairment. Thus, the opioid system is potentially related to AD pathology and may be a very attractive drug target for novel pharmacotherapies of AD.

New roles of carboxypeptidase E in endocrine and neural function and cancer

Carboxypeptidase E (CPE) or carboxypeptidase H was first discovered in 1982 as an enkephalin-convertase that cleaved a C-terminal basic residue from enkephalin precursors to generate enkephalin. Since then, CPE has been shown to be a multifunctional protein that subserves many essential nonenzymatic roles in the endocrine and nervous systems. Here, we review the phylogeny, structure, and function of CPE in hormone and neuropeptide sorting and vesicle transport for secretion, alternative splicing of the CPE transcript, and single nucleotide polymorphisms in humans. With this and the analysis of mutant and knockout mice, the data collectively support important roles for CPE in the modulation of metabolic and glucose homeostasis, bone remodeling, obesity, fertility, neuroprotection, stress, sexual behavior, mood and emotional responses, learning, and memory. Recently, a splice variant form of CPE has been found to be an inducer of tumor growth and metastasis and a prognostic biomarker for metastasis in endocrine and nonendocrine tumors.

Molecular mechanisms of opioid receptor-dependent signaling and behavior

Opioid receptors have been targeted for the treatment of pain and related disorders for thousands of years and remain the most widely used analgesics in the clinic. Mu (μ), kappa (κ), and delta (δ) opioid receptors represent the originally classified receptor subtypes, with opioid receptor like-1 (ORL1) being the least characterized. All four receptors are G-protein coupled and activate inhibitory G proteins. These receptors form homo- and heterodimeric complexes and signal to kinase cascades and scaffold a variety of proteins.The authors discuss classic mechanisms and developments in understanding opioid tolerance and opioid receptor signaling and highlight advances in opioid molecular pharmacology, behavioral pharmacology, and human genetics. The authors put into context how opioid receptor signaling leads to the modulation of behavior with the potential for therapeutic intervention. Finally, the authors conclude there is a continued need for more translational work on opioid receptors in vivo.

Role of chaperones in G protein coupled receptor signaling complex assembly

G protein coupled receptors are involved in highly efficient and specific activation of signaling pathways. Yet, we do not fully understand the processes required to assemble the different partners of the GPCR signaling complex. In order to address this issue, we need to understand how receptors and their signaling -partners are synthesized, folded and regulated during quality control steps in order to generate functional proteins. Several molecular chaperones are involved in this process for most proteins, including GPCRs. Several membrane proteins require the assembly of different subunits to be functional. In recent years, GPCRs have been shown to form oligomers, which could be interpreted as subunits of a larger complex. Yet, those oligomers would not be functional without the association of other signaling partners; thus, there is a requirement for the specific assembly of the -different partners. In this chapter, we will cover some aspects of the current knowledge about how chaperones are involved in both the formation of GPCR oligomers and in the assembly of the receptors with their signaling complex components.

Cys-27 variant of human δ-opioid receptor modulates maturation and cell surface delivery of Phe-27 variant via heteromerization

The important role of G protein-coupled receptor homo/heteromerization in receptor folding, maturation, trafficking, and cell surface expression has become increasingly evident. Here we investigated whether the human δ-opioid receptor (hδOR) Cys-27 variant that shows inherent compromised maturation has an effect on the behavior of the more common Phe-27 variant in the early secretory pathway. We demonstrate that hδOR-Cys-27 acts in a dominant negative manner and impairs cell surface delivery of the co-expressed hδOR-Phe-27 and impairs conversion of precursors to the mature form. This was demonstrated by metabolic labeling, Western blotting, flow cytometry, and confocal microscopy in HEK293 and human SH-SY5Y neuroblastoma cells using differentially epitope-tagged variants. The hδOR-Phe-27 precursors that were redirected to the endoplasmic reticulum-associated degradation were, however, rescued by a pharmacological chaperone, the opioid antagonist naltrexone. Co-immunoprecipitation of metabolically labeled variants revealed that both endoplasmic reticulum-localized precursors and mature receptors exist as homo/heteromers. The existence of homo/heteromers was confirmed in living cells by bioluminescence resonance energy transfer measurements, showing that the variants have a similar propensity to form homo/heteromers. By forming both homomers and heteromers, the hδOR-Cys-27 variant may thus regulate the levels of receptors at the cell surface, possibly leading to altered responsiveness to opioid ligands in individuals carrying the Cys-27 variant.

Delta opioid receptor analgesia: recent contributions from pharmacology and molecular approaches

Delta opioid receptors represent a promising target for the development of novel analgesics. A number of tools have been developed recently that have significantly improved our knowledge of δ receptor function in pain control. These include several novel δ agonists with potent analgesic properties, and genetic mouse models with targeted mutations in the δ opioid receptor gene. Also, recent findings have further documented the regulation of δ receptor function at cellular level, which impacts on the pain-reducing activity of the receptor. These regulatory mechanisms occur at transcriptional and post-translational levels, along agonist-induced receptor activation, signaling and trafficking, or in interaction with other receptors and neuromodulatory systems. All these tools for in-vivo research, and proposed mechanisms at molecular level, have tremendously increased our understanding of δ receptor physiology, and contribute to designing innovative strategies for the treatment of chronic pain and other diseases such as mood disorders.

Cysteine 27 variant of the delta-opioid receptor affects amyloid precursor protein processing through altered endocytic trafficking

Agonist-induced activation of the δ-opioid receptor (δOR) was recently shown to augment β- and γ-secretase activities, which increased the production of β-amyloid peptide (Aβ), known to accumulate in the brain tissues of Alzheimer's disease (AD) patients. Previously, the δOR variant with a phenylalanine at position 27 (δOR-Phe27) exhibited more efficient receptor maturation and higher stability at the cell surface than did the less common cysteine (δOR-Cys27) variant. For this study, we expressed these variants in human SH-SY5Y and HEK293 cells expressing exogenous or endogenous amyloid precursor protein (APP) and assessed the effects on APP processing. Expression of δOR-Cys27, but not δOR-Phe27, resulted in a robust accumulation of the APP C83 C-terminal fragment and the APP intracellular domain, while the total soluble APP and, particularly, the β-amyloid 40 levels were decreased. These changes upon δOR-Cys27 expression coincided with decreased localization of APP C-terminal fragments in late endosomes and lysosomes. Importantly, a long-term treatment with a subset of δOR-specific ligands or a c-Src tyrosine kinase inhibitor suppressed the δOR-Cys27-induced APP phenotype. These data suggest that an increased constitutive internalization and/or concurrent signaling of the δOR-Cys27 variant affects APP processing through altered endocytic trafficking of APP.

A homogeneous single-label time-resolved fluorescence cAMP assay

G-protein-coupled receptors (GPCRs) are an important class of pharmaceutical drug targets. Functional high-throughput GPCR assays are needed to test an increasing number of synthesized novel drug compounds and their function in signal transduction processes. Measurement of changes in the cyclic adenosine monophosphate (cAMP) concentration is a widely used method to verify GPCR activation in the adenylyl cyclase pathway. Here, a single-label time-resolved fluorescence and high-throughput screening (HTS)-feasible method was developed to measure changes in cAMP levels in HEK293(i) cells overexpressing either β(2)-adrenergic or δ-opioid receptors. In the quenching resonance energy transfer (QRET) technique, soluble quenchers reduce the signal of unbound europium(III)-labeled cAMP in solution, whereas the antibody-bound fraction is fluorescent. The feasibility of this homogeneous competitive assay was proven by agonist-mediated stimulation of receptors coupled to either the stimulatory G(s) or inhibitory G(i) proteins. The reproducibility of the assays was excellent, and Z' values exceeded 0.7. The dynamic range, signal-to-background ratio, and detection limit were compared with a commercial time-resolved fluorescence resonance energy transfer (TR-FRET) assay. In both homogeneous assays, similar assay parameters were obtained when adenylyl cyclase was stimulated directly by forskolin or via agonist-mediated activation of the G(s)-coupled β(2)AR. The advantage of using the single-label approach relates to the cost-effectiveness of the QRET system compared with the two-label TR-FRET assay as there is no need for labeling of two binding partners leading to reduced requirements for assay optimization.

Phe27Cys polymorphism of the human delta opioid receptor predisposes cells to compromised calcium signaling

A quarter of the human population with European background carries at least one allele of the OPRD1 gene that encodes the delta opioid receptor with cysteine at the amino acid position 27 (hδOR(Cys27)) instead of the evolutionary conserved phenylalanine (hδOR(Phe27)). The two variants have indistinguishable pharmacological properties but, importantly, hδOR(Cys27) differs from hδOR(Phe27) in having low maturation efficiency, lower stability at the cell surface and pronounced intracellular location. Both variants were previously shown to interact with the Sarco(endo)plasmic reticulum Ca²+ ATPase (SERCA) 2b in the early phase of their biosynthesis. We analyzed by pulse-chase assays, whether cellular signaling can affect hδOR(Cys27) maturation. Neither activation of the receptor by a δOR-specific agonist Leu-enkephalin, induction of intracellular calcium (Ca²+) release by ATP nor the direct stimulation of SERCA 2b by protein kinase C activation affected receptor maturation in HEK-293 cells. No signaling-mediated regulation of receptor maturation could therefore be demonstrated. Instead, we found by using single cell Ca²+ measurements that over-expression of hδOR(Cys27), but not hδOR(Phe27), compromised ATP-induced intracellular Ca²+-signaling. Furthermore, hδOR(Cys27) precursors showed slower dissociation from SERCA2b and hδOR(Cys27) expression caused down-regulation of the homocysteine-inducible endoplasmic reticulum-resident ubiquitin domain-like member 1 protein (HERP). We suggest that aging individuals with at least one hδOR(Cys27) encoding allele might have lowered threshold for Ca²+ dysregulation in neurons expressing hδOR.

Human beta1-adrenergic receptor is subject to constitutive and regulated N-terminal cleavage

The beta(1)-adrenergic receptor (beta(1)AR) is the predominant betaAR in the heart, mediating the catecholamine-stimulated increase in cardiac rate and force of contraction. Regulation of this important G protein-coupled receptor is nevertheless poorly understood. We describe here the biosynthetic profile of the human beta(1)AR and reveal novel features relevant to its regulation using an inducible heterologous expression system in HEK293(i) cells. Metabolic pulse-chase labeling and cell surface biotinylation assays showed that the synthesized receptors are efficiently and rapidly transported to the cell surface. The N terminus of the mature receptor is extensively modified by sialylated mucin-type O-glycosylation in addition to one N-glycan attached to Asn(15). Furthermore, the N terminus was found to be subject to limited proteolysis, resulting in two membrane-bound C-terminal fragments. N-terminal sequencing of the fragments identified two cleavage sites between Arg(31) and Leu(32) and Pro(52) and Leu(53), which were confirmed by cleavage site and truncation mutants. Metalloproteinase inhibitors were able to inhibit the cleavage, suggesting that it is mediated by a matrix metalloproteinase or a disintegrin and metalloproteinase (ADAM) family member. Most importantly, the N-terminal cleavage was found to occur not only in vitro but also in vivo. Receptor activation mediated by the betaAR agonist isoproterenol enhanced the cleavage in a concentration- and time-dependent manner, and it was also enhanced by direct stimulation of protein kinase C and adenylyl cyclase. Mutation of the Arg(31)-Leu(32) cleavage site stabilized the mature receptor. We hypothesize that the N-terminal cleavage represents a novel regulatory mechanism of cell surface beta(1)ARs.

Human delta opioid receptor biogenesis is regulated via interactions with SERCA2b and calnexin

Sarco(endo)plasmic reticulum calcium ATPase (SERCA)2b maintains the cellular Ca(2+) homeostasis by transferring Ca(2+) from the cytosol to the lumen of the endoplasmic reticulum (ER). Recently, SERCA2b has also been shown to be involved in the biosynthesis of secreted and membrane proteins via direct protein-protein interactions, involving components of the ER folding and quality-control machinery, as well as newly synthesized G protein-coupled receptors. Here we demonstrate that the human delta opioid receptor (hdeltaOR) exists in a ternary complex with SERCA2b and the ER molecular chaperone calnexin. The interaction between SERCA2b and hdeltaOR in vivo did not require calnexin as it was independent of the C-terminal calnexin-interacting domain of SERCA2b. However, the receptor was able to mediate co-immunoprecipitation of calnexin with the C-terminally truncated SERCA2b. The association of SERCA2b with hdeltaOR was regulated in vitro by Ca(2+) and ATP in a manner that was opposite to the calnexin-hdeltaOR interaction. Importantly, co-expression of the catalytically inactive SERCA2b(D351A) or calnexin binding-compromised SERCA2bDeltaC mutants with the receptor decreased the expression of mature receptors in a manner that did not directly relate to changes in the ER Ca(2+) concentration. We conclude that dynamic interactions among SERCA2b, calnexin and the hdeltaOR precursor orchestrate receptor biogenesis and are regulated by Ca(2+) and ATP. We further hypothesize that the primary role of SERCA2b in this process is to act as a Ca(2+) sensor in the vicinity of active translocons, integrating protein folding with local fluctuations of ER Ca(2+) levels.

Delta and kappa opioid receptors as suitable drug targets for pain

Similar to mu opioid receptors, kappa and delta opioid receptors reside in the periphery, the dorsal root ganglion, the spinal cord, and in supraspinal regions associated with pain modulation. Both delta and kappa opioid agonists have been shown to activate pain inhibitory pathways in the central nervous system. Yet, currently there are only a few pharmacologic agents that target kappa receptors, and none that target delta receptors. Spurred by the need for an efficacious analgesic without the unwanted side effects associated with the typical clinical profile of mu opioid agonists, new research has provided insight into why the development of effective kappa and delta opioid receptor agonists has remained elusive thus far, and importantly, how these obstacles may be overcome. For example, for delta opioid agonists to be effective, a state of inflammation may be required as this induces delta opioid receptors to migrate to the surface of neuronal cells and thereby become accessible to delta opioid agonists. Studies have shown that delta opioid agonists can provide relief of inflammatory pain and malignant bone pain. Meanwhile, peripherally restricted kappa opioid agonists have been developed to target kappa opioid receptors located on visceral and somatic afferent nerves for relief of inflammatory, visceral, and neuropathic chronic pain. The recently shown efficacy of these analgesics combined with a possible lower abuse potential and side effect burden than mu opioid receptor agonists makes delta and peripherally restricted kappa opioid receptor agonists promising targets for treating pain.

Differential use of an in-frame translation initiation codon regulates human mu opioid receptor (OPRM1)

The pharmacological effects of morphine and morphine-like drugs are mediated primarily through the micro opioid receptor. Here we show that differential use of an in-frame translational start codon in the 5'-untranslated region of the OPRM1 generates different translational products in vivo and in vitro. The 5'-end of the OPRM1 gene is necessary for initiating the alternate form and for subsequent degradation of the protein. Initiation of OPRM1 at the upstream site decreases the initiation at the main AUG site. However, alternative initiation of the long form of OPRM1 produces a protein with a short half-life, resulting from degradation mediated by the ubiquitin-proteasome pathway. Reporter and degradation assays showed that mutations of this long form at the second and third lysines reduce ubiquitin-dependent proteasome degradation, stabilizing the protein. The data suggest that MOP expression is controlled in part by initiation of the long form of MOP at the alternate site.

Delta receptors are required for full inhibitory coupling of mu-receptors to voltage-dependent Ca(2+) channels in dorsal root ganglion neurons

Recombinant micro and delta opioid receptors expressed in cell lines can form heterodimers with distinctive properties and trafficking. However, a role for opioid receptor heterodimerization in neurons has yet to be identified. The inhibitory coupling of opioid receptors to voltage-dependent Ca(2+) channels (VDCCs) is a relatively inefficient process and therefore provides a sensitive assay of altered opioid receptor function and expression. We examined micro-receptor coupling to VDCCs in dorsal root ganglion neurons of delta(+/+), delta(+/-), and delta(-/-) mice. Neurons deficient in delta receptors exhibited reduced inhibition of VDCCs by morphine and [D-Ala(2),Phe(4),Gly(5)-ol]-enkephalin (DAMGO). An absence of delta receptors caused reduced efficacy of DAMGO without affecting potency. An absence of delta receptors reduced neither the density of VDCCs nor their inhibition by either the GABA(B) receptor agonist baclofen or intracellular guanosine 5'-O-(3-thio)triphosphate. Flow cytometry revealed a reduction in micro-receptor surface expression in delta(-/-) neurons without altered DAMGO-induced internalization. There was no change in micro-receptor mRNA levels. D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)-sensitive mu-receptor-coupling efficacy was fully restored to delta(+/+) levels in delta(-/-) neurons by expression of recombinant delta receptors. However, the dimerization-deficient delta-15 construct expressed in delta(-/-) neurons failed to fully restore the inhibitory coupling of micro-receptors compared with that seen in delta(+/+) neurons, suggesting that, although not essential for micro-receptor function, micro-delta receptor dimerization contributes to full micro-agonist efficacy. Because DAMGO exhibited a similar potency in delta(+/+) and delta(-/-) neurons and caused similar levels of internalization, the role for heterodimerization is probably at the level of receptor biosynthesis.