Prostaglandin E2 receptor EP3alpha subtype: the role of N-glycosylation in ligand binding as revealed by site-directed mutagenesis

N-Linked Glycosylation of the p24 Family Protein p24δ5 Modulates Retrograde Golgi-to-ER Transport of K/HDEL Ligands in Arabidopsis

The K/HDEL receptor ERD2 mediates the transport of soluble endoplasmic reticulum (ER)-resident proteins containing a C-terminal K/HDEL signal from the Golgi apparatus back to the ER via COPI (COat Protein I)-coated vesicles. Sorting of ERD2 within COPI vesicles is facilitated by p24 proteins. In Arabidopsis, p24δ5 has been shown to interact directly with ERD2 via its luminal GOLD (GOLgi Dynamics) domain and with COPI proteins via its cytoplasmic C-terminal tail at the acidic pH of the Golgi apparatus. Several members of the p24 family in mammals and yeast have been shown to be glycosylated, but whether Arabidopsis p24 proteins are glycosylated and the role of the sugar moiety in p24 function remain unclear. Here, we show that Arabidopsis p24δ5 protein is N-glycosylated in its GOLD domain. Furthermore, we demonstrate that this post-translational modification is important for its coupled transport with p24β2 at the ER-Golgi interface, for its interaction with the K/HDEL receptor ERD2, and for retrograde transport of ERD2 and K/HDEL ligands from the Golgi apparatus back to the ER.

Multifactorial Regulation of G Protein-Coupled Receptor Endocytosis

Endocytosis is a process by which cells absorb extracellular materials via the inward budding of vesicles formed from the plasma membrane. Receptor-mediated endocytosis is a highly selective process where receptors with specific binding sites for extracellular molecules internalize via vesicles. G protein-coupled receptors (GPCRs) are the largest single family of plasma-membrane receptors with more than 1000 family members. But the molecular mechanisms involved in the regulation of GPCRs are believed to be highly conserved. For example, receptor phosphorylation in collaboration with β-arrestins plays major roles in desensitization and endocytosis of most GPCRs. Nevertheless, a number of subsequent studies showed that GPCR regulation, such as that by endocytosis, occurs through various pathways with a multitude of cellular components and processes. This review focused on i) functional interactions between homologous and heterologous pathways, ii) methodologies applied for determining receptor endocytosis, iii) experimental tools to determine specific endocytic routes, iv) roles of small guanosine triphosphate-binding proteins in GPCR endocytosis, and v) role of post-translational modification of the receptors in endocytosis.

Prostaglandin E2/cyclooxygenase pathway in human skeletal muscle: influence of muscle fiber type and age

Prostaglandin E2 (PGE2) produced by the cyclooxygenase (COX) pathway regulates skeletal muscle protein turnover and exercise training adaptations. The purpose of this study was twofold: 1) define the PGE2/COX pathway enzymes and receptors in human skeletal muscle, with a focus on type I and II muscle fibers; and 2) examine the influence of aging on this pathway. Muscle biopsies were obtained from the soleus (primarily type I fibers) and vastus lateralis (proportionally more type II fibers than soleus) of young men and women (n = 8; 26 ± 2 yr), and from the vastus lateralis of young (n = 8; 25 ± 1 yr) and old (n = 12; 79 ± 2 yr) men and women. PGE2/COX pathway proteins [COX enzymes (COX-1 and COX-2), PGE2 synthases (cPGES, mPGES-1, and mPGES-2), and PGE2 receptors (EP1, EP2, EP3, and EP4)] were quantified via Western blot. COX-1, cPGES, mPGES-2, and all four PGE2 receptors were detected in all skeletal muscle samples examined. COX-1 (P < 0.1) and mPGES-2 were ∼20% higher, while EP3 was 99% higher and EP4 57% lower in soleus compared with vastus lateralis (P < 0.05). Aging did not change the level of skeletal muscle COX-1, while cPGES increased 45% and EP1 (P < 0.1), EP3, and EP4 decreased ∼33% (P < 0.05). In summary, PGE2 production capacity and receptor levels are different in human skeletal muscles with markedly different type I and II muscle fiber composition. In aging skeletal muscle, PGE2 production capacity is elevated and receptor levels are downregulated. These findings have implications for understanding the regulation of skeletal muscle adaptations to exercise and aging by the PGE2/COX pathway and related inhibitors.

Molecular characterization of prostaglandin F receptor (FP) and E receptor subtype 3 (EP3) in chickens

Prostaglandin E and F regulate diverse physiological functions including gastrointestinal motility, fever induction and reproduction. This multitude of biological effects is mediated via their four E receptor subtypes (EP(1), EP(2), EP(3) and EP(4)) and F receptor (FP), respectively. Majority of these studies was performed in mammalian species, while investigations on their roles were impeded by inadequate information on their receptors in avian species. In present study, full-length cDNAs of chicken EP(3) (cEP(3)) and two isoforms of FP - cFPa and cFPb - were cloned from adult hen ovary. The putative cEP(3) and cFPa share high amino acid sequence identity with their respective orthologs, while the predicted cFPb is a novel middle-truncated splice variant which lacks 107 amino acids between transmembrane domains 4 and 6. RT-PCR showed that cEP(3), cFPa and cFPb are widely expressed in adult tissues examined, including ovary and oviduct. Using a pGL3-CRE luciferase reporter system, cEP(3)-expressing DF1 cells inhibited forskolin-induced luciferase activity (EC(50): <1.9 pM) upon PGE(2) treatment, suggesting that cEP(3) may functionally couple to Gi protein. Upon PGF(2α) addition, cFPa was shown to potentially couple to intracellular Ca(2+)-signaling pathway by pGL3-NFAT-RE reporter assay (EC(50): 2.9 nM), while cFPb showed no response. Using a pGL4-SRE reporter system, both cEP(3) and cFPa exhibited potential MAPK activation by PGE(2) and PGF(2α) at EC(50) 0.34 and 13 nM, respectively. Molecular characterization of these receptors paved the road to the better understanding of PGE(2) and PGF(2α) roles in avian physiology and comparative endocrinology studies.

Protein kinase C delta contributes to increase in EP3 agonist-induced contraction in mesenteric arteries from type 2 diabetic Goto-Kakizaki rats

Prostaglandin E(2) (PGE(2)), an important and ubiquitously present vasoactive eicosanoid, may either constrict or dilate systemic vascular beds. However, little is known about the vascular contractile responsiveness to and signaling pathways for PGE(2) at the chronic stage of type 2 diabetes. We hypothesized that PGE(2)-induced arterial contraction is augmented in type 2 diabetic Goto-Kakizaki (GK) rats via the protein kinase Cδ (PKCδ) pathway. Here, we investigated the vasoconstrictor effects of PGE(2) and of sulprostone (EP1-/EP3-receptor agonist) in rings cut from superior mesenteric arteries isolated from GK rats (37-44 weeks old). In arteries from GK rats (vs. those from age-matched Wistar rats), examined in the presence of a nitric oxide synthase inhibitor: 1) the PGE(2)- and sulprostone-induced vasocontractions (which were not blocked by the selective EP1 receptor antagonist sc19220) were enhanced, and these enhancements were suppressed by rottlerin (selective PKCδ inhibitor) but not by Gö6976 (selective PKCα/β inhibitor); 2) the sulprostone-stimulated phosphorylation of PKCδ (at Thr(505)), which yields an active form, was increased and 3) sulprostone-stimulated caldesmon phosphorylations, which are related to isometric force generation in smooth muscle, were increased. The protein expression of EP3 receptor in superior mesenteric arteries was similar between the two groups of rats. Our data suggest that the diabetes-related enhancement of EP3 receptor-mediated vasocontraction results from activation of the PKCδ pathway. Alterations in EP3 receptor-mediated vasocontraction may be important factors in the pathophysiological influences over arterial tone that are present in diabetic states.

The role of N-linked glycosylation in determining the surface expression, G protein interaction and effector coupling of the alpha (alpha) isoform of the human thromboxane A(2) receptor

In humans, thromboxane (TX) A(2) signals through two TXA(2) receptor (TP) isoforms, termed TPalpha and TPbeta, that diverge exclusively within the carboxyl terminal cytoplasmic domains. The amino terminal extracellular region of the TPs contains two highly conserved Asn (N)-linked glycosylation sites at Asn(4) and Asn(16). While it has been established that impairment of N-glycosylation of TPalpha significantly affects ligand binding/intracellular signalling, previous studies did not ascertain whether N-linked glycosylation was critical for ligand binding per se or whether it was required for the intracellular trafficking and the functional expression of TPalpha on the plasma membrane (PM). In the current study, we investigated the role of N-linked glycosylation in determining the functional expression of TPalpha, by assessment of its ligand binding, G protein coupling and intracellular signalling properties, correlating it with the level of antigenic TPalpha protein expressed on the PM and/or retained intracellularly. From our data, we conclude that N-glycosylation of either Asn(4) or Asn(16) is required and sufficient for expression of functionally active TPalpha on the PM while the fully non-glycosylated TPalpha(N4,N16-Q4,Q16) is almost completely retained within the endoplasmic reticulum (ER) and remains functionally inactive, failing to associate with its coupling G protein Galpha(q) and, in turn, failing to mediate phospholipase (PL) Cbeta activation.

Human prostacyclin receptor

Prostacyclin, a member of the eicosanoid family of lipid mediators, is the major product of arachidonic acid metabolism formed in the marcovascular endothelium. It is a potent vasodilator, antithrombotic, and antiplatelet agent that mediates it effects through a membrane-associated receptor termed the IP. Cloning of the cDNA for IP, from human and other species, indicated its membership of the G protein-coupled receptor superfamily and has allowed detailed examination of the signaling and regulatory pathways utilized by this receptor. This article examines the current state of knowledge of the IP, its signaling and regulation, and its biological role in vivo and examines the possible existence of multiple PGI2 receptor sites.

Molecular cloning and functional characterization of the canine prostaglandin E2 receptor EP4 subtype

Prostaglandin E2 (PGE2) is an important mediator of diverse biologic functions in many tissues and binds with high affinity to four cell surface, seven-transmembrane domain, G protein-coupled receptors (EP1-EP4). The EP4 receptor subtype has a long intracellular carboxy-terminal region and is functionally coupled to adenylate cyclase, resulting in elevated intracellular cyclic adenosine 5' monophosphate (cAMP) levels upon activation. To further study EP4 receptor subtype function, a canine kidney cDNA library was screened and three clones were isolated and sequenced. The longest clone was 3,103 bp and contained a single open reading frame of 1,476 bp, potentially encoding a protein of 492 amino acids with a predicted molecular weight of 53.4 kDa. Sequence analysis of this open reading frame reveals 89% identity to the human EP4 protein coding region at the nucleotide level and 90% identity when the putative canine and human protein sequences are compared. Northern blot analysis showed relatively high levels of canine EP4 expression in heart, lung and kidney, while Southern blot analysis of canine genomic DNA suggests the presence of a single copy gene. Following transfection of canine EP4 into CHO-KI cells, Scatchard analysis revealed a dissociation constant of 24 nM for PGE, while competition binding studies using 3H-PGE2 as ligand demonstrated specific displacement by PGE2 prostaglandin E, (PGE1), and prostaglandin A3 (PGA3). Treatment with PGE2 also resulted in increased levels of cAMP in transfected, but not in parental, CHO-KI cells. In contrast, butaprost, an EP2 selective ligand, and sulprostone, an EP1/EP3 selective ligand, did not bind to this receptor at the maximal concentration used (320 nM). To further investigate secondary signaling, the canine EP4 cDNA was truncated to produce an 1,117 bp fragment encoding a 356 amino acid protein lacking the intracellular carboxy-terminus. When transfected, this truncated cDNA produced a protein with a dissociation constant of 11 nM for PGE2 and a binding and cAMP accumulation profile similar to that of the full-length protein. Both full-length and truncated canine EP4 underwent short term PGE2-induced desensitization as shown by a lack of continuing cAMP accumulation after the initial PGE2 stimulation, suggesting no involvement of the C-terminal intracellular tail. This result is in contrast to that reported for the human EP4 receptor, where residues within the C-terminal intracellular tail were shown to mediate short term, ligand induced desensitization.

Immunohistochemical localization of prostaglandin EP3 receptor in the rat nervous system

The prostaglandin EP3 receptor (EP3R) subtype is believed to mediate large portions of diverse physiologic actions of prostaglandin E2 in the nervous system. However, the distribution of EP3R protein has not yet been unveiled in the peripheral or central nervous systems. The authors raised a polyclonal antibody against an amino-terminal portion of rat EP3R that recognized specifically the receptor protein. In this study, immunoblotting analysis with this antibody showed several immunoreactive bands with different molecular weights in rat brain extracts and in membrane fractions of recombinant EP3R-expressing culture cells, and treatment with N-glycosidase shifted those immunoreactive bands to an apparently single band with a lower molecular weight, suggesting that EP3R proteins are modified posttranslationally with carbohydrate moieties of various sizes. The authors performed immunohistochemical investigation of EP3R in the rat brain, spinal cord, and peripheral ganglia by using the antibody. EP3R-like immunoreactivity was observed in many and discrete regions of the rostrocaudal axis of the nervous system. The signals were particularly strong in the anterior, intralaminar, and midline thalamic nuclear groups; the median preoptic nucleus; the medial mammillary nucleus; the superior colliculus; the periaqueductal gray; the lateral parabrachial nucleus; the nucleus of the solitary tract; and laminae I and II of the medullary and spinal dorsal horns. Sensory ganglia, such as the trigeminal, dorsal root, and nodose ganglia, contained many immunopositive neurons. Neuronal cells in the locus coeruleus and raphe nuclei exhibited EP3R-like immunoreactivity. This suggests that EP3R plays regulatory roles in the noradrenergic and serotonergic monoamine systems. Autonomic preganglionic nuclei, such as the dorsal motor nucleus of the vagus nerve, the spinal intermediolateral nucleus, and the sacral parasympathetic nucleus, also contained neuronal cell bodies with the immunoreactivity, implying modulatory functions of EP3R in the central autonomic nervous system. The characteristic distribution of EP3R provides valuable information on the mechanisms for various physiologic actions of prostaglandin E2 in the central and peripheral nervous systems.