N-linked glycosylation of platelet P2Y12 ADP receptor is essential for signal transduction but not for ligand binding or cell surface expression

Proteome-wide analysis reveals G protein-coupled receptor-like proteins in rice (Oryza sativa)

G protein-coupled receptors (GPCRs) constitute the largest family of transmembrane proteins in metazoans that mediate the regulation of various physiological responses to discrete ligands through heterotrimeric G protein subunits. The existence of GPCRs in plant is contentious, but their comparable crucial role in various signaling pathways necessitates the identification of novel remote GPCR-like proteins that essentially interact with the plant G protein α subunit and facilitate the transduction of various stimuli. In this study, we identified three putative GPCR-like proteins (OsGPCRLPs) (LOC_Os06g09930.1, LOC_Os04g36630.1, and LOC_Os01g54784.1) in the rice proteome using a stringent bioinformatics workflow. The identified OsGPCRLPs exhibited a canonical GPCR 'type I' 7TM topology, patterns, and biologically significant sites for membrane anchorage and desensitization. Cluster-based interactome mapping revealed that the identified proteins interact with the G protein α subunit which is a characteristic feature of GPCRs. Computational results showing the interaction of identified GPCR-like proteins with G protein α subunit and its further validation by the membrane yeast-two-hybrid assay strongly suggest the presence of GPCR-like 7TM proteins in the rice proteome. The absence of a regulator of G protein signaling (RGS) box in the C- terminal domain, and the presence of signature motifs of canonical GPCR in the identified OsGPCRLPs strongly suggest that the rice proteome contains GPCR-like proteins that might be involved in signal transduction.

Role of N-linked glycosylation in porcine reproductive and respiratory syndrome virus (PRRSV) infection

Porcine reproductive and respiratory syndrome (PRRSV) is an enveloped single-stranded positive-sense RNA virus and one of the main pathogens that causes the most significant economical losses in the swine-producing countries. PRRSV is currently divided into two distinct species, PRRSV-1 and PRRSV-2. The PRRSV virion envelope is composed of four glycosylated membrane proteins and three non-glycosylated envelope proteins. Previous work has suggested that PRRSV-linked glycans are critical structural components for virus assembly. In addition, it has been proposed that PRRSV glycans are implicated in the interaction with host cells and critical for virus infection. In contrast, recent findings showed that removal of N-glycans from PRRSV does not influence virus infection of permissive cells. Thus, there are not sufficient evidences to indicate compellingly that N-glycans present in the PRRSV envelope play a direct function in viral infection. To gain insights into the role of N-glycosylation in PRRSV infection, we analysed the specific contribution of the envelope protein-linked N-glycans to infection of permissive cells. For this purpose, we used a novel strategy to modify envelope protein-linked N-glycans that consists of production of monoglycosylated PRRSV and viral glycoproteins with different glycan states. Our results showed that removal or alteration of N-glycans from PRRSV affected virus infection. Specifically, we found that complex N-glycans are required for an efficient infection in cell cultures. Furthermore, we found that presence of high mannose type glycans on PRRSV surface is the minimal requirement for a productive viral infection. Our findings also show that PRRSV-1 and PRRSV-2 have different requirements of N-glycan structure for an optimal infection. In addition, we demonstrated that removal of N-glycans from PRRSV does not affect viral attachment, suggesting that these carbohydrates played a major role in regulating viral entry. In agreement with these findings, by performing immunoprecipitation assays and colocalization experiments, we found that N-glycans present in the viral envelope glycoproteins are not required to bind to the essential viral receptor CD163. Finally, we found that the presence of N-glycans in CD163 is not required for PRRSV infection.

Desialylation by neuraminidases in platelets, kiss of death or bittersweet?

PURPOSE OF REVIEW

Loss of surface sialic acid by neuraminidases is known as 'desialylation'. Platelets are desialylated in bacterial or viral infections, during storage, senescence, various mutations, platelet auto antibodies, hemostasis and shear stress. In this review the recent literature on the different sialic acid capped glycan structures will be covered as well as platelet desialylation in inherited glycan disorders and induced by external neuraminidases.

RECENT FINDINGS

Neuraminidases are released from platelet intracellular stores and translocated to the platelet surface. Apart from clearance, loss of surface sialic acid by neuraminidases ('desialylation') affects platelet signaling including ligand binding and their procoagulant function. Platelets are also desialylated in infections, various mutations, presence of platelet auto antibodies.

SUMMARY

Since platelet desialylation occurs in various healthy and pathological conditions, measuring desialylation might be a new diagnostic tool.

Analysis of the Role of N-Linked Glycosylation in Cell Surface Expression, Function, and Binding Properties of SARS-CoV-2 Receptor ACE2

Human angiotensin I-converting enzyme 2 (hACE2) is a type I transmembrane glycoprotein that serves as the major cell entry receptor for SARS-CoV and SARS-CoV-2. The viral spike (S) protein is required for the attachment to ACE2 and subsequent virus-host cell membrane fusion. Previous work has demonstrated the presence of N-linked glycans in ACE2. N-glycosylation is implicated in many biological activities, including protein folding, protein activity, and cell surface expression of biomolecules. However, the contribution of N-glycosylation to ACE2 function is poorly understood. Here, we examined the role of N-glycosylation in the activity and localization of two species with different susceptibility to SARS-CoV-2 infection, porcine ACE2 (pACE2) and hACE2. The elimination of N-glycosylation by tunicamycin (TM) treatment, or mutagenesis, showed that N-glycosylation is critical for the proper cell surface expression of ACE2 but not for its carboxiprotease activity. Furthermore, nonglycosylable ACE2 was localized predominantly in the endoplasmic reticulum (ER) and not at the cell surface. Our data also revealed that binding of SARS-CoV or SARS-CoV-2 S protein to porcine or human ACE2 was not affected by deglycosylation of ACE2 or S proteins, suggesting that N-glycosylation does not play a role in the interaction between SARS coronaviruses and the ACE2 receptor. Impairment of hACE2 N-glycosylation decreased cell-to-cell fusion mediated by SARS-CoV S protein but not that mediated by SARS-CoV-2 S protein. Finally, we found that hACE2 N-glycosylation is required for an efficient viral entry of SARS-CoV/SARS-CoV-2 S pseudotyped viruses, which may be the result of low cell surface expression of the deglycosylated ACE2 receptor. IMPORTANCE Understanding the role of glycosylation in the virus-receptor interaction is important for developing approaches that disrupt infection. In this study, we showed that deglycosylation of both ACE2 and S had a minimal effect on the spike-ACE2 interaction. In addition, we found that the removal of N-glycans of ACE2 impaired its ability to support an efficient transduction of SARS-CoV and SARS-CoV-2 S pseudotyped viruses. Our data suggest that the role of deglycosylation of ACE2 on reducing infection is likely due to a reduced expression of the viral receptor on the cell surface. These findings offer insight into the glycan structure and function of ACE2 and potentially suggest that future antiviral therapies against coronaviruses and other coronavirus-related illnesses involving inhibition of ACE2 recruitment to the cell membrane could be developed.

Platelets and Defective N-Glycosylation

N-glycans are covalently linked to an asparagine residue in a simple acceptor sequence of proteins, called a sequon. This modification is important for protein folding, enhancing thermodynamic stability, and decreasing abnormal protein aggregation within the endoplasmic reticulum (ER), for the lifetime and for the subcellular localization of proteins besides other functions. Hypoglycosylation is the hallmark of a group of rare genetic diseases called congenital disorders of glycosylation (CDG). These diseases are due to defects in glycan synthesis, processing, and attachment to proteins and lipids, thereby modifying signaling functions and metabolic pathways. Defects in N-glycosylation and O-glycosylation constitute the largest CDG groups. Clotting and anticlotting factor defects as well as a tendency to thrombosis or bleeding have been described in CDG patients. However, N-glycosylation of platelet proteins has been poorly investigated in CDG. In this review, we highlight normal and deficient N-glycosylation of platelet-derived molecules and discuss the involvement of platelets in the congenital disorders of N-glycosylation.

Identification and functional characterisation of N-linked glycosylation of the orphan G protein-coupled receptor Gpr176

G-protein-coupled receptors (GPCRs) are important drug targets with diverse therapeutic applications. However, there are still more than a hundred orphan GPCRs, whose protein functions and biochemical features remain unidentified. Gpr176 encodes a class-A orphan GPCR that has a role in circadian clock regulation in mouse hypothalamus and is also implicated in human breast cancer transcriptional response. Here we show that Gpr176 is N-glycosylated. Peptide-N-glycosidase treatment of mouse hypothalamus extracts revealed that endogenous Gpr176 undergoes N-glycosylation. Using a heterologous expression system, we show that N-glycosylation occurs at four conserved asparagine residues in the N-terminal region of Gpr176. Deficient N-glycosylation due to mutation of these residues reduced the protein expression of Gpr176. At the molecular function level, Gpr176 has constitutive, agonist-independent activity that leads to reduced cAMP synthesis. Although deficient N-glycosylation did not compromise this intrinsic activity, the resultant reduction in protein expression was accompanied by attenuation of cAMP-repressive activity in the cells. We also demonstrate that human GPR176 is N-glycosylated. Importantly, missense variations in the conserved N-glycosylation sites of human GPR176 (rs1473415441; rs761894953) affected N-glycosylation and thereby attenuated protein expression and cAMP-repressive activity in the cells. We show that N-glycosylation is a prerequisite for the efficient protein expression of functional Gpr176/GPR176.

P2Y receptors in neuropathic pain

This review summarizes and evaluates the relationship between neuropathic pain and P2Y receptors from inception to 2019. Purinergic receptors have been well studied in recent years using various molecular biological methods. The main research objective of this review is to determine the association of P2Y1, P2Y2, P2Y6, P2Y12 and P2Y13 receptors with neuropathic pain. This review includes the most comprehensive subtypes of P2Y that related to neuropathic pain and the current therapeutic method of neuropathic pain. G protein-coupled P2Y receptors are located on neurons, astrocytes, oligodendrocytes and microglial cells and regulate neurotransmission. Nerve injury is the prime reason for abnormal regulation of P2Y receptor mRNA expression, subsequently, inducing neuropathic pain. Neuropathic pain is a type of chronic pain that is divided into peripheral, central and mixed. Numerous studies demonstrated a positive correlation between the expression level of P2Y receptors and neuropathic pain generation. Also, several reports showed that P2Y short hairpin RNA (shRNA) and P2Y antagonist can be used as an analgesic to relieve neuropathic pain via decreasing P2Y receptor expression level and neural cell activation. However, the transformation process from basic experiments to clinical applications is a long process. Current deficiencies and future research directions are discussed at the end of this review.

Influence of ABO Locus on PFA-100 Collagen-ADP Closure Time Is Not Totally Dependent on the Von Willebrand Factor. Results of a GWAS on GAIT-2 Project Phenotypes

(1) Background: In a previous study, we found that two phenotypes related to platelet reactivity, measured with the PFA-100 system, were highly heritable. The aim of the present study was to identify genetic determinants that influence the variability of these phenotypes: closure time of collagen-ADP (Col-ADP) and of collagen-epinephrine (Col-Epi). (2) Methods: As part of the GAIT-2 (Genetic Analysis of Idiopathic Thrombophilia (2) Project, 935 individuals from 35 large Spanish families were studied. A genome-wide association study (GWAS) with ≈ 10 M single nucleotide polymorphisms (SNPs) was carried out with Col-ADP and Col-Epi phenotypes. (3) Results: The study yielded significant genetic signals that mapped to the ABO locus. After adjusting both phenotypes for the ABO genotype, these signals disappeared. After adjusting for von Willebrand factor (VWF) or for coagulation factor VIII (FVIII), the significant signals disappeared totally for Col-Epi phenotype but only partially for Col-ADP phenotype. (4) Conclusion: Our results suggest that the ABO locus exerts the main genetic influence on PFA-100 phenotypes. However, while the effect of the ABO locus on Col-Epi phenotype is mediated through VWF and/or FVIII, the effect of the ABO locus on Col-ADP phenotype is partly produced through VWF and/or FVIII, and partly through other mechanisms.

Deciphering the Roles of N-Glycans on Collagen-Platelet Interactions

Collagen is a potent agonist for platelet activation, presenting itself as a key contributor to coagulation via interactions with platelet glycoproteins. The fine details dictating platelet-collagen interactions are poorly understood. In particular, glycosylation could be a key determinant in the platelet-collagen interaction. Here, we report an affinity purification coupled to a mass spectrometry-based approach to elucidate the function of N-glycans in dictating platelet-collagen interactions. By integrative proteomic and glycoproteomic analysis of collagen-platelet interactive proteins with N-glycan manipulation, we demonstrate that the interaction of platelet adhesive receptors with collagen is highly N-glycan regulated, with glycans on many receptors playing positive roles in collagen binding, with glycans on other platelet glycoproteins exhibiting inhibitory roles on the binding to collagen. Our results significantly enhance our understanding of the details of glycans influencing the platelet-collagen interaction.

Desialylation of Platelets by Pneumococcal Neuraminidase A Induces ADP-Dependent Platelet Hyperreactivity

Platelets are increasingly recognized to play a role in the complications of Streptococcus pneumoniae infections. S. pneumoniae expresses neuraminidases, which may alter glycans on the platelet surface. In the present study, we investigated the capability of pneumococcal neuraminidase A (NanA) to remove sialic acid (desialylation) from the platelet surface, the consequences for the platelet activation status and reactivity, and the ability of neuraminidase inhibitors to prevent these effects. Our results show that soluble NanA induces platelet desialylation. Whereas desialylation itself did not induce platelet activation (P-selectin expression and platelet fibrinogen binding), platelets became hyperreactive to ex vivo stimulation by ADP and cross-linked collagen-related peptide (CRP-XL). Platelet aggregation with leukocytes also increased. These processes were dependent on the ADP pathway, as inhibitors of the pathway (apyrase and ticagrelor) abrogated platelet hyperreactivity. Inhibition of NanA-induced platelet desialylation by neuraminidase inhibitors (e.g., oseltamivir acid) also prevented the platelet effects of NanA. Collectively, our findings show that soluble NanA can desialylate platelets, leading to platelet hyperreactivity, which can be prevented by neuraminidase inhibitors.

Dynamic States of the Ligand-Free Class A G Protein-Coupled Receptor Extracellular Side

G protein-coupled receptors (GPCRs) make up the largest family of drug targets. The second extracellular loop (ECL2) and extracellular end of the third transmembrane helix (TM3) are basic structural elements of the GPCR ligand binding site. Currently, the disulfide bond between the two conserved cysteines in the ECL2 and TM3 is considered to be a basic GPCR structural feature. This disulfide bond has a significant effect on receptor dynamics and ligand binding. Here, molecular dynamics simulations and experimental results show that the two cysteines are distant from one another in the highest-population conformational state of ligand-free class A GPCRs and do not form a disulfide bond, indicating that the dynamics of the GPCR extracellular side are different from our conventional understanding. These surprising dynamics should have important effects on the drug binding process. On the basis of the two distinct ligand-free states, we suggest two kinetic processes for binding of ligands to GPCRs. These results challenge our commonly held beliefs regarding both GPCR structural features and ligand binding.

ASK1 facilitates tumor metastasis through phosphorylation of an ADP receptor P2Y12 in platelets

Tumor metastasis is the major cause of deaths in cancer patients and is modulated by intertwined stress-responsive signaling cascades. Here we demonstrate that deletion of stress-responsive apoptosis signal-regulating kinase 1 (Ask1) in platelets results in unstable hemostasis and drastic attenuation of tumor lung metastasis, both of which are attributable to platelet dysfunction. Platelet-specific deletion of Ask1 in mice leads to defects in ADP-dependent platelet aggregation, unstable hemostasis and subsequent attenuation of tumor metastasis. We also revealed that activating phosphorylation of Akt is attenuated in Ask1-deficient platelets, contrary to the previous reports suggesting that Akt is negatively regulated by ASK1. Mechanistically, ASK1-JNK/p38 axis phosphorylates an ADP receptor P2Y₁₂ at Thr345, which is required for the ADP-dependent sustained Akt activity that is vital to normal platelet functions. Our findings offer insight into positive regulation of Akt signaling through P2Y₁₂ phosphorylation as well as MAPK signaling in platelets by ASK1 and suggest that ASK1-JNK/p38 axis provides a new therapeutic opportunity for tumor metastasis.

Protease-activated receptor-4 and purinergic receptor P2Y12 dimerize, co-internalize, and activate Akt signaling via endosomal recruitment of β-arrestin

Vascular inflammation and thrombosis require the concerted actions of several different agonists, many of which act on G protein-coupled receptors (GPCRs). GPCR dimerization is a well-established phenomenon that can alter protomer function. In platelets and other cell types, protease-activated receptor-4 (PAR4) has been shown to dimerize with the purinergic receptor P2Y12 to coordinate β-arrestin-mediated Akt signaling, an important mediator of integrin activation. However, the mechanism by which the PAR4-P2Y12 dimer controls β-arrestin-dependent Akt signaling is not known. We now report that PAR4 and P2Y12 heterodimer internalization is required for β-arrestin recruitment to endosomes and Akt signaling. Using bioluminescence resonance energy transfer, immunofluorescence microscopy, and co-immunoprecipitation in cells expressing receptors exogenously and endogenously, we demonstrate that PAR4 and P2Y12 specifically interact and form dimers expressed at the cell surface. We also found that activation of PAR4 but not of P2Y12 drives internalization of the PAR4-P2Y12 heterodimer. Remarkably, activated PAR4 internalization was required for recruitment of β-arrestin to endocytic vesicles, which was dependent on co-expression of P2Y12. Interestingly, stimulation of the PAR4-P2Y12 heterodimer promotes β-arrestin and Akt co-localization to intracellular vesicles. Moreover, activated PAR4-P2Y12 internalization is required for sustained Akt activation. Thus, internalization of the PAR4-P2Y12 heterodimer is necessary for β-arrestin recruitment to endosomes and Akt signaling and lays the foundation for examining whether blockade of PAR4 internalization reduces integrin and platelet activation.

Insight into SUCNR1 (GPR91) structure and function

SUCNR1 (or GPR91) belongs to the family of G protein-coupled receptors (GPCR), which represents the largest group of membrane proteins in human genome. The majority of marketed drugs targets GPCRs, directly or indirectly. SUCNR1 has been classified as an orphan receptor until a landmark study paired it with succinate, a citric acid cycle intermediate. According to the current paradigm, succinate triggers SUCNR1 signaling pathways to indicate local stress that may affect cellular metabolism. SUCNR1 implication has been well documented in renin-induced hypertension, ischemia/reperfusion injury, inflammation and immune response, platelet aggregation and retinal angiogenesis. In addition, the SUCNR1-induced increase of blood pressure may contribute to diabetic nephropathy or cardiac hypertrophy. The understanding of SUCNR1 activation, signaling pathways and functions remains largely elusive, which calls for deeper investigations. SUCNR1 shows a high potential as an innovative drug target and is probably an important regulator of basic physiology. In order to achieve the full characterization of this receptor, more specific pharmacological tools such as small-molecules modulators will represent an important asset. In this review, we describe the structural features of SUCNR1, its current ligands and putative binding pocket. We give an exhaustive overview of the known and hypothetical signaling partners of the receptor in different in vitro and in vivo systems. The link between SUCNR1 intracellular pathways and its pathophysiological roles are also extensively discussed.

ADP-induced bladder contractility is mediated by P2Y12 receptor and temporally regulated by ectonucleotidases and adenosine signaling

Purinergic signaling comprises one key pathway in modulating bladder smooth muscle (BSM) contractility, disorders of which become highly prevalent with aging. ADP was first observed to modulate BSM contractility >40 yr ago, yet the underlying molecular mechanism still remains unclear. Here, we demonstrate, using myography, that ADP and ADPβS dose-dependently induce mouse BSM contraction, and ADP-induced BSM contraction is blocked by a selective P2Y12 receptor (P2Y12R) antagonist, PSB 0739 (25 μM), but is unaffected by P2Y1 and P2Y13 receptor antagonists. P2Y12R in BSM exhibits distinct pharmacological properties that are different from P2Y12R in platelets. After an immediate contraction, prolonged exposure to ADP causes BSM to become refractory to further ADP-mediated contraction. However, in mice lacking ectonucleotidases Entpd1 (ATP→ADP→AMP) or Nt5e (AMP→adenosine), or by inhibiting adenosine signaling, the refractory response was altered, resulting in repeated BSM contractions in response to repeated ADP (0.1-1 mM) stimulation. Our data indicate that P2Y12R undergoes slow desensitization; ADP-P2Y12 signaling is tightly regulated by Entpd1/Nt5e activity and adenosine receptors; and ADP-adenosine signaling play an important role in modulating P2X-mediated BSM contraction. The identification of P2Y12R in BSM, and the current clinical availability of P2Y12R inhibitors, such as clopidogrel, offers potentially novel treatment strategies for bladder contractility disorders.

Molecular mechanisms of platelet P2Y(12) receptor regulation

Platelets are critical for haemostasis, however inappropriate activation can lead to the development of arterial thrombosis, which can result in heart attack and stroke. ADP is a key platelet agonist that exerts its actions via stimulation of two surface GPCRs (G-protein-coupled receptors), P2Y(1) and P2Y(12). Similar to most GPCRs, P2Y receptor activity is tightly regulated by a number of complex mechanisms including receptor desensitization, internalization and recycling. In the present article, we review the molecular mechanisms that underlie P2Y(1) and P2Y(12) receptor regulation, with particular emphasis on the structural motifs within the P2Y(12) receptor, which are required to maintain regulatory protein interaction. The implications of these findings for platelet responsiveness are also discussed.

Expression and localization of purinergic P2Y(12) receptor in human nasal mucosa

BACKGROUND

Extracellular nucleotides such as ATP and UTP are released from essentially all cells, and they interact with cell surface P2 receptors to produce a broad range of physiological responses. P2Y12 receptor is the major platelet receptor that mediates ADP-induced aggregation, P2Y12 receptor inhibitors such as clopidogrel and prasugrel inhibit platelet aggregation, and thus, they are used in the treatment and prevention of coronary artery disease. Recently, studies have focused on the P2Y12 receptor as a receptor for leukotriene E4 (LTE4), because this receptor is required for LTE4-mediated pulmonary inflammation. To establish the presence of P2Y12 receptor in human nasal mucosa, we investigated the expression and the localization of the P2Y12 receptor in human nasal mucosa.

METHODS

Human turbinates were obtained by turbinectomy from 12 patients with nasal obstruction refractory to medical therapy. The expression of P2Y12 receptor was evaluated by RT-PCR, western blotting, and immunohistochemical analysis.

RESULTS

RT-PCR analysis of total RNA extracted from human nasal turbinate, primary cultured human nasal epithelial cells and nasal vascular endothelial cells demonstrated the expression of P2Y12 receptor mRNA. A band of approximately 55 kDa was detected in human turbinates by western blot analysis using anti-P2Y12 receptor antibody. We could not find any differences between P2Y12 receptor levels in allergic and non-allergic nasal mucosa. An immunohistochemical study revealed that epithelial cells, submucosal glands and vascular endothelial cells showed intense immunoreactivity for the P2Y12 receptor.

CONCLUSIONS

The results may have important clinical implications for understanding the role of P2Y12 receptor in upper airway diseases such as allergic rhinitis and non-allergic rhinitis.

Identification of determinants required for agonistic and inverse agonistic ligand properties at the ADP receptor P2Y12

The ADP receptor P2Y(12) belongs to the superfamily of G protein-coupled receptors (GPCRs), and its activation triggers platelet aggregation. Therefore, potent antagonists, such as clopidogrel, are of high clinical relevance in prophylaxis and treatment of thromboembolic events. P2Y(12) displays an elevated basal activity in vitro, and as such, inverse agonists may be therapeutically beneficial compared with antagonists. Only a few inverse agonists of P2Y(12) have been described. To expand this limited chemical space and improve understanding of structural determinants of inverse agonist-receptor interaction, this study screened a purine compound library for lead structures using wild-type (WT) human P2Y(12) and 28 constitutively active mutants. Results showed that ATP and ATP derivatives are agonists at P2Y(12). The potency at P2Y(12) was 2-(methylthio)-ADP > 2-(methylthio)-ATP > ADP > ATP. Determinants required for agonistic ligand activity were identified. Molecular docking studies revealed a binding pocket for the ATP derivatives that is bordered by transmembrane helices 3, 5, 6, and 7 in human P2Y(12,) with Y(105), E(188), R(256), Y(259), and K(280) playing a particularly important role in ligand interaction. N-Methyl-anthraniloyl modification at the 3'-OH of the 2'-deoxyribose leads to ligands (mant-deoxy-ATP [dATP], mant-deoxy-ADP) with inverse agonist activity. Inverse agonist activity of mant-dATP was found at the WT human P2Y(12) and half of the constitutive active P2Y(12) mutants. This study showed that, in addition to ADP and ATP, other ATP derivatives are not only ligands of P2Y(12) but also agonists. Modification of the ribose within ATP can result in inverse activity of ATP-derived ligands.

P2Y12 receptor: platelet thrombus formation and medical interventions

Platelets express a wide range of receptors and proteins that play essential roles in thrombus formation. Among these, the P2Y(12) receptor, a member of the G protein-coupled receptor family, has attracted a significant amount of attention. Stimulation of the P2Y(12) receptor by ADP results in activation of various signaling pathways involved in amplification of platelet activation and aggregation. There have been extensive attempts to design an ideal antithrombotic agent to block P2Y(12), which shows selective expression, as an intervention for cardiovascular disease. Current inhibitors of the P2Y(12) receptor include indirect inhibitor members of the thienopyridine family (ticlopidine, clopidogrel, and prasugrel), and direct P2Y(12) inhibitors (ticagrelor, cangrelor and elinogrel). Of these, clopidogrel is the most commonly prescribed P2Y(12) blocker; however, this product does not fulfill the ideal therapeutic requirements. The main limitations of clopidogrel administration include slow onset, prevention of recovery of platelet functions, and interindividual variability. Hence, advanced studies have been carried out to achieve more efficient and safer P2Y(12) blockade. In this review, we provide a brief but comprehensive report on P2Y(12), its role on platelet thrombus formation, and the targeting of this receptor as an intervention for cardiovascular disease, for the benefit of basic science and clinical researchers.

P2Y(12) receptors in platelets and other hematopoietic and non-hematopoietic cells

The P2Y(12) receptor is a Gi-coupled ADP receptor first described in blood platelets where it plays a central role in the complex processes of activation and aggregation. Platelet granules store important amounts of ADP which are released upon stimulation by interaction of platelets with the damaged vessel wall. Therefore, the P2Y(12) receptor is a key player in primary hemostasis and in arterial thrombosis and is an established target of antithrombotic drugs like the thienopyridine compounds ticlopidine, clopidogrel, and prasugrel or the direct, reversible antagonists ticagrelor and cangrelor. Beyond the platelet physiology and pharmacology, recent studies have revealed the expression of the P2Y(12) receptor in other hematopoietic cells including leukocyte subtypes and microglia in the central nervous system as well as in vascular smooth muscle cells. These studies indicate putative roles of the P2Y(12) receptor in inflammatory states and diseases of the brain, lung, and blood vessels. The selective role of P2Y(12) among other P2 receptors as well as the possible impact of P2Y(12) targeting drugs in these processes remain to be evaluated.

Regulation of GIP and GLP1 receptor cell surface expression by N-glycosylation and receptor heteromerization

In response to a meal, Glucose-dependent Insulinotropic Polypeptide (GIP) and Glucagon-like Peptide-1 (GLP-1) are released from gut endocrine cells into the circulation and interact with their cognate G-protein coupled receptors (GPCRs). Receptor activation results in tissue-selective pleiotropic responses that include augmentation of glucose-induced insulin secretion from pancreatic beta cells. N-glycosylation and receptor oligomerization are co-translational processes that are thought to regulate the exit of functional GPCRs from the ER and their maintenance at the plasma membrane. Despite the importance of these regulatory processes, their impact on functional expression of GIP and GLP-1 receptors has not been well studied. Like many family B GPCRs, both the GIP and GLP-1 receptors possess a large extracellular N-terminus with multiple consensus sites for Asn-linked (N)-glycosylation. Here, we show that each of these Asn residues is glycosylated when either human receptor is expressed in Chinese hamster ovary cells. N-glycosylation enhances cell surface expression and function in parallel but exerts stronger control over the GIP receptor than the GLP-1 receptor. N-glycosylation mainly lengthens receptor half-life by reducing degradation in the endoplasmic reticulum. N-glycosylation is also required for expression of the GIP receptor at the plasma membrane and efficient GIP potentiation of glucose-induced insulin secretion from the INS-1 pancreatic beta cell line. Functional expression of a GIP receptor mutant lacking N-glycosylation is rescued by co-expressed wild type GLP1 receptor, which, together with data obtained using Bioluminescence Resonance Energy Transfer, suggests formation of a GIP-GLP1 receptor heteromer.

Molecular pharmacology, physiology, and structure of the P2Y receptors

The P2Y receptors are a widely expressed group of eight nucleotide-activated G protein-coupled receptors (GPCRs). The P2Y(1)(ADP), P2Y(2)(ATP/UTP), P2Y(4)(UTP), P2Y(6)(UDP), and P2Y(11)(ATP) receptors activate G(q) and therefore robustly promote inositol lipid signaling responses. The P2Y(12)(ADP), P2Y(13)(ADP), and P2Y(14)(UDP/UDP-glucose) receptors activate G(i) leading to inhibition of adenylyl cyclase and to Gβγ-mediated activation of a range of effector proteins including phosphoinositide 3-kinase-γ, inward rectifying K(+) (GIRK) channels, phospholipase C-β2 and -β3, and G protein-receptor kinases 2 and 3. A broad range of physiological responses occur downstream of activation of these receptors ranging from Cl(-) secretion by epithelia to aggregation of platelets to neurotransmission. Useful structural models of the P2Y receptors have evolved from extensive genetic analyses coupled with molecular modeling based on three-dimensional structures obtained for rhodopsin and several other GPCRs. Selective ligands have been synthesized for most of the P2Y receptors with the most prominent successes attained with highly selective agonist and antagonist molecules for the ADP-activated P2Y(1) and P2Y(12) receptors. The widely prescribed drug, clopidogrel, which results in irreversible blockade of the platelet P2Y(12) receptor, is the most important therapeutic agent that targets a P2Y receptor.

The platelet P2Y₁₂ receptor for adenosine diphosphate: congenital and drug-induced defects

P2Y₁₂, the G(i)-coupled platelet receptor for adenosine diphosphate (ADP), plays a central role in platelet function. Patients with congenital P2Y₁₂ defects display a mild to moderate bleeding diathesis, characterized by mucocutaneous bleedings and excessive post-surgical and post-traumatic blood loss. Defects of P2Y₁₂ should be suspected when ADP, even at high concentrations (≥ 10 μM), is unable to induce full, irreversible platelet aggregation. Tests that evaluate the degree of inhibition of adenylyl cyclase by ADP should be used to confirm the diagnosis. Drugs that inhibit P2Y₁₂ are potent antithrombotic drugs, attesting the central role played by P2Y₁₂ in platelet thrombus formation. Clopidogrel, the most widely used drug that inhibits P2Y₁₂, is effective both in monotherapy and in combination with acetylsalicylic acid. The most important drawback of clopidogrel is its inability to inhibit adequately P2Y₁₂-dependent platelet function in approximately one-third of patients who are therefore not protected from major cardiovascular events. New drugs, such as prasugrel and ticagrelor, which effectively inhibit P2Y₁₂ in the majority of patients, proved to be more efficacious than clopdidogrel in preventing major cardiovascular events. Although they increase the incidence of major bleedings, the net clinical benefit is in favor of the new P2Y₁₂ inhibitors.

Involvement of basic amino acid residues in transmembrane regions 6 and 7 in agonist and antagonist recognition of the human platelet P2Y(12)-receptor

The P2Y(12)-receptor plays a prominent role in ADP-induced platelet aggregation. In the present study, we searched for amino acid residues involved in ligand recognition of the human P2Y(12)-receptor. Wild-type or mutated receptors were expressed in 1321N1 astrocytoma cells and Chinese hamster ovary (CHO) cells. There were no major differences in cellular expression of the constructs. Cellular cAMP production and cAMP response element (CRE)-dependent luciferase expression was increased by isoproterenol (astrocytoma cells) or forskolin (CHO cells). In cells expressing wild-type receptors, R256K or S101A mutant constructs, 2-methylthio-ADP inhibited the induced cAMP production with IC(50) concentrations of about 0.3nM. In cells expressing R256A constructs, the IC(50) concentration amounted to 25nM. In cells expressing H253A/R256A, Y259D and K280A constructs, 2-methylthio-ADP failed to affect the cellular cAMP production. Moreover, in cells expressing Y259D and K280A constructs, 2-methylthio-ADP did also not change the forskolin-induced CRE-dependent luciferase expression and caused only small increases in the serum response element-dependent luciferase expression. The antagonist cangrelor had similar potencies at wild-type receptors and R256A constructs (apparent pK(B)-value at wild-type receptors: 9.2). In contrast, reactive blue-2 had a lower potency at the R256A construct (apparent pK(B)-value at wild-type receptors: 7.6). In summary, the data indicate the involvement of Arg256, Tyr259 and, possibly, H253 (transmembrane region TM6) as well as Lys280 (TM7) in the function of the human P2Y(12)-receptor. Arg256 appears to play a role in the recognition of nucleotide agonists and the non-nucleotide antagonist reactive blue-2, but no role in the recognition of the nucleotide antagonist cangrelor.

The P2Y14 receptor activity in glioma C6 cells

In this study, we demonstrated the presence and the activity of the P2Y14 receptor in glioma C6 cells. We found that P2Y14 could exist in two forms, highly predominating glycosylated and non-glycosylated. Binding of UDP-glucose evoked two responses: calcium signal and adenylate cyclase inhibition, both pertussis toxin-sensitive. Separate glycosylation pattern and functional profile of these two receptor forms were observed in non-starved and serum-starved cells. During long-term serum deprivation (96 h), the level of glycosylated form strongly decreased, while non-glycosylated increased, what was correlated with the decrease of calcium signaling activity and stronger adenylate cyclase inhibition, suggesting that receptor N-glycosylation may modulate its functional activity.

N-Glycosylation of the human kappa opioid receptor enhances its stability but slows its trafficking along the biosynthesis pathway

We examined glycosylation of FLAG-hKOR expressed in CHO cells and determined its functional significance. FLAG-hKOR was resolved as a broad and diffuse 55-kDa band and a less diffuse 45-kDa band by immunoblotting, indicating that the receptor is glycosylated. Endoglycosidase H cleaved the 45-kDa band to approximately 38 kDa but did not change the 55-kDa band, demonstrating that the 45-kDa band is N-glycosylated with high-mannose or hybrid-type glycan. Peptide-N-glycosidase F digestion of solubilized hKOR or incubation of cells with tunicamycin resulted in two species of 43 and 38 kDa, suggesting that the 43-kDa band is O-glycosylated. FLAG-hKOR was reduced to lower Mr bands by neuraminidase and O-glycosidase, indicating that the hKOR contains O-linked glycan. Mutation of Asn25 or Asn39 to Gln in the N-terminal domain reduced the Mr by approximately 5 kDa, indicating that both residues were glycosylated. The double mutant hKOR-N25/39Q was resolved as a 43-kDa (mature form) and a 38-kDa (intermediate form) band. When transiently expressed, hKOR-N25/39Q had a lower expression level than the wild type. In CHO cells stably expressing the hKOR-N25/39Q, pulse-chase experiments revealed that the turnover rate constants (ke) of the intermediate and mature forms were approximately 3 times those of the wild type. In addition, the maturation rate constant (ka) of the 43-kDa form of hKOR-N25/39Q was 6 times that of the mature form of the wild type. The hKOR-N25/39Q mutant showed increased agonist-induced receptor phosphorylation, desensitization, internalization, and downregulation, without changing ligand binding affinity or receptor-G protein coupling. Thus, N-glycosylation of the hKOR plays important roles in stability and trafficking along the biosynthesis pathway of the receptor protein as well as agonist-induced receptor regulation.

Regulation of Secreted Frizzled-related Protein-1 by Heparin

Secreted Frizzled-related protein-1 (sFRP-1) belongs to a class of extracellular antagonists that modulate Wnt signaling pathways by preventing ligand-receptor interactions among Wnts and Frizzled membrane receptor complexes. sFRP-1 and Wnts are heparin-binding proteins, and their interaction can be stabilized by heparin in vitro. Here we report that heparin can specifically enhance recombinant sFRP-1 accumulation in a cell type-specific manner. The effect requires O-sulfation in heparin, and involves fibroblast growth factor-2 as well as fibroblast growth factor receptor-1. Interestingly, further investigation uncovers that heparin can also affect the post-translational modification of sFRP-1. We demonstrate that sFRP-1 is post-translationally modified by tyrosine sulfation at tyrosines 34 and 36, which is inhibited by the treatment of heparin. The results suggest that accumulation of sFRP-1 induced by heparin is in part due to the relative stabilization of unsulfated sFRP-1 and the direct stabilization by heparin. The study has revealed a multifaceted regulation on sFRP-1 protein by heparin.

Distinct roles for protein kinase C isoforms in regulating platelet purinergic receptor function

ADP is a critical regulator of platelet activation, mediating its actions through two G protein-coupled receptors (GPCRs), P2Y1 and P2Y12. We have shown previously that the receptors are functionally desensitized, in a homologous manner, by distinct kinase-dependent mechanisms in which P2Y1 is regulated by protein kinase C (PKC) and P2Y12 by G protein-coupled receptor kinases. In this study, we addressed whether different PKC isoforms play different roles in regulating the trafficking and activity of these two GPCRs. Expression of PKCalpha and PKCdelta dominant-negative mutants in 1321N1 cells revealed that both isoforms regulated P2Y1 receptor signaling and trafficking, although only PKCdelta was capable of regulating P2Y12, in experiments in which PKC was directly activated by the phorbol ester phorbol 12-myristate 13-acetate (PMA). These results were paralleled in human platelets, in which PMA reduced subsequent ADP-induced P2Y1 and P2Y12 receptor signaling. PKC isoform-selective inhibitors revealed that novel, but not conventional, isoforms of PKC regulate P2Y12 function, whereas both novel and classic isoforms regulate P2Y1 activity. It is also noteworthy that we studied receptor internalization in platelets by a radioligand binding approach showing that both receptors internalize rapidly in these cells. ADP-induced P2Y1 receptor internalization is attenuated by PKC inhibitors, whereas that of the P2Y12 receptor is unaffected. Both P2Y1 and P2Y12 receptors can also undergo PMA-stimulated internalization, and here again, novel but not classic PKCs regulate P2Y12, whereas both novel and classic isoforms regulate P2Y1 internalization. This study therefore is the first to reveal distinct roles for PKC isoforms in the regulation of platelet P2Y receptor function and trafficking.

Regulation of platelet functions by P2 receptors

The main role of blood platelets is to ensure primary hemostasis, which is the maintenance of vessel integrity and cessation of bleeding upon injury. While playing a major part in acute arterial thrombosis, platelets are also involved in inflammation, atherosclerosis, and angiogenesis. ADP and ATP play a crucial role in platelet activation, and their receptors are potential targets for antithrombotic drugs. The ATP-gated cation channel P2X(1) and the two G protein-coupled ADP receptors, P2Y(1) and P2Y(12), selectively contribute to platelet aggregation and formation of a thrombus. Owing to its central role in the growth and stabilization of a thrombus, the P2Y(12) receptor is an established target of antithrombotic drugs such as clopidogrel. Studies in P2Y(1) and P2X(1) knockout mice and selective P2Y(1) and P2X(1) antagonists have shown that these receptors are also attractive targets for new antithrombotic compounds. The potential role of platelet P(2) receptors in the involvement of platelets in inflammatory processes is also discussed.

Postnatal development of P2 receptors in the murine gastrointestinal tract

The actions of purine and pyrimidine compounds on isolated segments of the mouse intestine were investigated during postnatal development. The localization of P2Y(1), P2Y(2), P2Y(4), P2X(1,) P2X(2) and P2X(3) receptors were examined immunohistochemically, and levels of expression of P2Y(1), P2X(1) and P2X(2) were studied by Western immunoblot. From day 12 onwards, the order of potency for relaxation of longitudinal muscle of all regions was 2-MeSADP>or=alpha,beta-meATP>or=ATP=UTP=adenosine, suggesting P2Y(1) receptors. This was supported by the sensitivity of responses to 2-MeSADP to the selective antagonist MRS 2179 and P2Y(1) receptor immunoreactivity on longitudinal muscle and a subpopulation of myenteric neurons. A further alpha,beta-meATP-sensitive P2Y receptor subtype was also indicated. ATP and UTP were equipotent suggesting a P2Y(2) and/or P2Y(4) receptor. Adenosine relaxed the longitudinal muscle in all regions via P1 receptors. The efficacy of all agonists to induce relaxation of raised tone preparations increased with age, being comparable to adult by day 20, the weaning age. During postnatal development the contractile response of the ileum and colon was via P2Y(1) receptors, while the relaxant response mediated by P2Y(1) receptors gradually appeared along the mouse gastrointestinal tract, being detectable in the stomach from day 3 and in the duodenum from day 6. In the ileum and colon relaxant responses to 2-MeSADP were not detected until days 8 and 12, respectively. 2-MeSADP induced contractions on basal tone preparations from day 3, but decreased significantly at day 12 and disappeared by day 20. At day 8, contractions of colonic longitudinal muscle to ATP showed no desensitisation suggesting the involvement of P2X(2) receptors. Immunoreactivity to P2X(2) receptors only was observed on the longitudinal muscle of the colon and ileum from day 1 and on a subpopulation of myenteric neurons from day 3. These data suggest that P2Y(1) receptors undergo postnatal developmental changes in the mouse gut, with a shift from contraction to relaxation. Such changes occur 1 week before weaning and may contribute to the changes that take place in the gut when the food composition changes from maternal milk to solid food.

Pharmacological profiles of cloned mammalian P2Y-receptor subtypes

Membrane-bound P2-receptors mediate the actions of extracellular nucleotides in cell-to-cell signalling. P2X-receptors are ligand-gated ion channels, whereas P2Y-receptors belong to the superfamily of G-protein-coupled receptors (GPCRs). So far, the P2Y family is composed out of 8 human subtypes that have been cloned and functionally defined; species orthologues have been found in many vertebrates. P2Y1-, P2Y2-, P2Y4-, P2Y6-, and P2Y11-receptors all couple to stimulation of phospholipase C. The P2Y11-receptor mediates in addition a stimulation of adenylate cyclase. In contrast, activation of the P2Y12-, P2Y13-, and P2Y14-receptors causes an inhibition of adenylate cyclase activity. The expression of P2Y1-receptors is widespread. The receptor is involved in blood platelet aggregation, vasodilatation and neuromodulation. It is activated by ADP and ADP analogues including 2-methylthio-ADP (2-MeSADP). 2'-Deoxy-N6-methyladenosine-3',5'-bisphosphate (MRS2179) and 2-chloro-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate (MRS2279) are potent and selective antagonists. P2Y2 transcripts are abundantly distributed. One important example for its functional role is the control of chloride ion fluxes in airway epithelia. The P2Y2-receptor is activated by UTP and ATP and blocked by suramin. The P2Y2-agonist diquafosol is used for the treatment of the dry eye disease. P2Y4-receptors are expressed in the placenta and in epithelia. The human P2Y4-receptor has a strong preference for UTP as agonist, whereas the rat P2Y4-receptor is activated about equally by UTP and ATP. The P2Y4-receptor is not blocked by suramin. The P2Y6-receptor has a widespread distribution including heart, blood vessels, and brain. The receptor prefers UDP as agonist and is selectively blocked by 1,2-di-(4-isothiocyanatophenyl)ethane (MRS2567). The P2Y11-receptor may play a role in the differentiation of immunocytes. The human P2Y11-receptor is activated by ATP as naturally occurring agonist and it is blocked by suramin and reactive blue 2 (RB2). The P2Y12-receptor plays a crucial role in platelet aggregation as well as in inhibition of neuronal cells. It is activated by ADP and very potently by 2-methylthio-ADP. Nucleotide antagonists including N6-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP (=cangrelor; AR-C69931MX), the nucleoside analogue AZD6140, as well as active metabolites of the thienopyridine compounds clopidogrel and prasugrel block the receptor. These P2Y12-antagonists are used in pharmacotherapy to inhibit platelet aggregation. The P2Y13-receptor is expressed in immunocytes and neuronal cells and is again activated by ADP and 2-methylthio-ADP. The 2-chloro-5-nitro pyridoxal-phosphate analogue 6-(2'-chloro-5'-nitro-azophenyl)-pyridoxal-alpha5-phosphate (MRS2211) is a selective antagonist. mRNA encoding for the human P2Y14-receptor is found in many tissues. However, a physiological role of the receptor has not yet been established. UDP-glucose and related analogues act as agonists; antagonists are not known. Finally, UDP has been reported to act on receptors for cysteinyl leukotrienes as an additional agonist--indicating a dual agonist specificity of these receptors.

The influence of N-linked glycosylation on the function of platelet glycoprotein VI

Using recombinant human glycoprotein VI (GPVI), we evaluated the effect of N-linked glycosylation at the consensus site Asparagine92-Glycine-Serine94 (N92GS94) on binding of this platelet-specific receptor to its ligands, human type I collagen, collagen-related peptide (CRP), and the snake venom C-type lectin convulxin (CVX). In COS-7 cells transiently transfected with GPVI, deglycosylation with peptide-N-glycosidase F (PNGase F; specific for complex N-linked glycans) or tunicamycin decreases the molecular weight of GPVI and reduces transfected COS-7 cell binding to both CRP and CVX. In stably transfected Dami cells, the substitutions N92A or S94A, but not L95H, resulted in a 30% to 40% decrease in adhesion to CVX, but a 90% or greater decrease in adhesion to CRP and a 65% to 70% decrease in adhesion to type I collagen. Treatment with PNGase F, but not Endoglycosidase H (Endo H) (specific for high-mannose N-linked glycans), produced an equivalent decrease in molecular weight. Neither N92A nor S94A affected the expression of GPVI, based on the direct binding of murine anti-human GPVI monoclonal antibody 204-11 to transfected Dami cells. These findings indicate that N-linked glycosylation at N92 in human GPVI is not required for surface expression, but contributes to maximal adhesion to type I collagen, CRP and, to a lesser extent, CVX.

N-linked glycosylation facilitates processing and cell surface expression of rat luteinizing hormone receptor

The extracellular domain of the luteinizing hormone (LH) receptor has six potential N-linked glycosylation sites. Although previous studies have shown that mutation of the first three sites results in decreased ligand binding at the cell surface, the role of glycosylation in LH receptor processing is not understood. In the present study, we examined whether mutation of the first three sites has any affect on receptor synthesis, processing, and degradation of the mutant receptors. The data show that mutation of N77, N152, or N173 did not affect receptor synthesis, but did significantly reduce processing of the receptor precursor to the mature, cell surface form. Furthermore, defective processing was due to increased degradation of the precursor rather than increased turnover of cell surface receptors. Thus, lack of glycosylation decreases LH receptor processing and targets the receptors for degradation thereby leading to decreased cell surface expression. These results show that glycosylation of the LH receptor plays an important role in receptor processing and cell surface expression.

The role of glycosylation in the function of a 48-kDa glycoprotein from carrot

Carrot extracellular dermal glycoprotein (EDGP) may play an important role in plant defense systems and in signal transduction. Our experiments show that differences in pI values of EDGP isoforms are caused by differences in amino acid sequence and not by heterogeneity in phosphorylation. The binding affinity of native EDGP for a 4-kDa hormone-like peptide from soybean was approximately one-third that of deglycosylated EDGP, and deglycosylation of EDGP caused complete loss of its ability to inhibit xyloglucan-specific endo-beta-1,4-glucanase. Experiments using tunicamycin-treated carrot cell cultures showed that glycosylation is essential for correct EDGP folding and secretion, and that tunicamycin does not affect EDGP gene transcription.