Pharmacological characterization of P2Y receptor subtypes - an update

Selective, Non-nucleotidic Radiotracer for P2Y12 Receptors: Design, Synthesis, Characterization, and Imaging of Brain Slices

The G protein-coupled, ADP-activated P2Y12 receptor (P2Y12R) expressed by microglial cells is involved in neuroinflammation constituting a promising biomarker. Here, we designed and characterized a potent and selective non-nucleotidic P2Y12-antagonist radioligand, [3H]PSB-22219 ([3H]18). The unlabeled compound was stable in rat liver microsomes and selective versus other ADP-activated receptors. [3H]18 displayed high-affinity binding to membrane preparations recombinantly expressing the human P2Y12R (KD = 4.57 nM), showing very low nonspecific binding. Radioligand binding assays were established and employed to characterize P2Y12Rs natively expressed in human platelet (KD = 2.53 nM), rat brain cortex (KD = 5.35 nM), and mouse microglial cell preparations (KD = 269 nM), with microglia showing extraordinarily high P2Y12R expression. Autoradiography studies allowed the visualization of human P2Y12R overexpression in the brain of a humanized rat model. The new radioligand is expected to become a useful pharmacological tool that will contribute to the development of therapeutics and radiodiagnostics targeting brain P2Y12Rs.

Structural insight into the self-activation and G-protein coupling of P2Y2 receptor

Purinergic P2Y2 receptor (P2Y2R) represents a typically extracellular ATP and UTP sensor for mediating purinergic signaling. Despite its importance as a pharmacological target, the molecular mechanisms underlying ligand recognition and G-protein coupling have remained elusive due to lack of structural information. In this study, we determined the cryo-electron microscopy (cryo-EM) structures of the apo P2Y2R in complex with Gq, ATP-bound P2Y2R in complex with Gq or Go, and UTP-bound P2Y4R in complex with Gq. These structures reveal the similarities and distinctions of ligand recognition within the P2Y receptor family. Furthermore, a comprehensive analysis of G-protein coupling reveals that P2Y2R exhibits promiscuity in coupling with both Gq and Go proteins. Combining molecular dynamics simulations and signaling assays, we elucidate the molecular mechanisms by which P2Y2R differentiates pathway-specific Gq or Go coupling through distinct structural components on the intracellular side. Strikingly, we identify a helix-like segment within the N-terminus that occupies the orthosteric ligand-binding pocket of P2Y2R, accounting for its self-activation. Taken together, these findings provide a molecular framework for understanding the activation mechanism of P2Y2R, encompassing ligand recognition, G-protein coupling, and a novel N-terminus-mediated self-activation mechanism.

Drug-like Antagonists of P2Y Receptor Subtypes: An Update

The hunt for drug-like P2YR antagonists continues, stimulated by ever-increasing pharmacological evidence for their clinical benefit and the astonishing array of biological functions which they orchestrate, including platelet aggregation, cancer proliferation, pain, neurodegenerative diseases, and immune regulation. Extensive research has identified modulators of P2Y receptors. However, only a limited number of small-molecule antagonists for the P2Y12 receptor have received approval for their clinical use. Recent pioneering discoveries of small-molecule ligand-bound X-ray crystal structures for the P2Y1 and P2Y12 receptors and homology modeling has stimulated research groups to explore orthosteric and allosteric receptor antagonists, aided in part by the discovery of fluorescent P2YR imaging tools and sensitive screening methods that allow the identification of low affinity P2Y receptor antagonists. This Perspective critically assesses P2Y receptor antagonists published since 2016, highlighting potential oral lead- or drug-like compounds that offer opportunities for the development of molecules for clinical evaluation.

P2Y1 and P2Y12 Receptors Mediate Aggregation of Dog and Cat Platelets: A Comparison to Human Platelets

Thrombosis is one of the most prevalent and serious health issues amongst humans. A key component of thrombotic events is the activation and aggregation of platelets, of which the P2Y1 and P2Y12 receptors play a crucial role in this process. Despite a breadth of knowledge on thrombosis and its mechanisms and treatment in various disorders in humans, there is less of an understanding of the expression and exact role of these receptors in companion animals such as dogs and cats. Therefore, this study aimed to investigate P2Y1 and P2Y12 receptors on dog and cat platelets in platelet-rich plasma and compare them to human platelets. Immunoblotting revealed the presence of P2Y1 and P2Y12 receptor proteins on dog and cat platelets, although relative amounts of each receptor appeared to contrast those of human platelets, with increased amounts of P2Y1 compared to P2Y12 receptors in dogs and cats. Using a modified 384-well plate aggregation assay, designed for use with small volumes, the human P2Y1 and P2Y12 receptor agonists adenosine 5'-diphosphate and 2-methylthio-adenosine 5'-diphosphate caused aggregation of dog and cat platelets. This aggregation was near-completely inhibited by the selective P2Y12 antagonist ticagrelor. Aggregation of dog and cat platelets was partly inhibited by the human P2Y1 receptor antagonist MRS2179. The agonist and antagonist responses in dog and cat platelets were like those of human platelets. In contrast, the aggregation of dog platelets in the absence of added nucleotides was two-fold greater than that of cats and humans. This study indicates that platelets of cats and dogs possess functional P2Y1 and P2Y12 receptors that can be inhibited by human antagonists. The data presented suggest differing roles or responses of the platelet P2Y receptors in dogs and cats compared to humans but also highlight the potential of using currently available P2Y1 or P2Y12 antiplatelet drugs such as ticagrelor for the treatment of thrombosis in these companion animals.

Adenosine diphosphate stimulates VEGF-independent choroidal endothelial cell proliferation: A potential escape from anti-VEGF therapy

We hypothesized that a strategy employing tissue-specific endothelial cells (EC) might facilitate the identification of tissue- or organ-specific vascular functions of ubiquitous metabolites. An unbiased approach was employed to identify water-soluble small molecules with mitogenic activity on choroidal EC. We identified adenosine diphosphate (ADP) as a candidate, following biochemical purification from mouse EL4 lymphoma extracts. ADP stimulated the growth of bovine choroidal EC (BCEC) and other bovine or human eye-derived EC. ADP induced rapid phosphorylation of extracellular signal-regulated kinase in a dose- and time-dependent manner. ADP-induced BCEC proliferation could be blocked by pretreatment with specific antagonists of the purinergic receptor P2Y1 but not with a vascular endothelial growth factor (VEGF) inhibitor, indicating that the EC mitogenic effects of ADP are not mediated by stimulation of the VEGF pathway. Intravitreal administration of ADP expanded the neovascular area in a mouse model of choroidal neovascularization. Single-cell transcriptomics from human choroidal datasets show the expression of P2RY1, but not other ADP receptors, in EC with a pattern similar to VEGFR2. Although ADP has been reported to be a growth inhibitor for vascular EC, here we describe its growth-stimulating effects for BCEC and other eye-derived EC.

Mechanosensitive release of ATP in the urinary bladder mucosa

The urinary bladder mucosa (urothelium and suburothelium/lamina propria) functions as a barrier between the content of the urine and the underlying bladder tissue. The bladder mucosa is also a mechanosensitive tissue that releases signaling molecules that affect functions of cells in the bladder wall interconnecting the mucosa with the detrusor muscle and the CNS. Adenosine 5'-triphosphate (ATP) is a primary mechanotransduction signal that is released from cells in the bladder mucosa in response to bladder wall distention and activates cell membrane-localized P2X and P2Y purine receptors on urothelial cells, sensory and efferent neurons, interstitial cells, and detrusor smooth muscle cells. The amounts of ATP at active receptor sites depend significantly on the amounts of extracellularly released ATP. Spontaneous and distention-induced release of ATP appear to be under differential control. This review is focused on mechanisms underlying urothelial release of ATP in response to mechanical stimulation. First, we present a brief overview of studies that report mechanosensitive ATP release in bladder cells or tissues. Then, we discuss experimental evidence for mechanosensitive release of urothelial ATP by vesicular and non-vesicular mechanisms and roles of the stretch-activated channels PIEZO channels, transient receptor potential vanilloid type 4, and pannexin 1. This is followed by brief discussion of possible involvement of calcium homeostasis modulator 1, acid-sensing channels, and connexins in the release of urothelial ATP. We conclude with brief discussion of limitations of current research and of needs for further studies to increase our understanding of mechanotransduction in the bladder wall and of purinergic regulation of bladder function.

Crosstalk Among Glial Cells in the Blood-Brain Barrier Injury After Ischemic Stroke

Blood-brain barrier (BBB) is comprised of brain microvascular endothelial cells (ECs), astrocytes, perivascular microglia, pericytes, neuronal processes, and the basal lamina. As a complex and dynamic interface between the blood and the central nervous system (CNS), BBB is responsible for transporting nutrients essential for the normal metabolism of brain cells and hinders many toxic compounds entering into the CNS. The loss of BBB integrity following stroke induces tissue damage, inflammation, edema, and neural dysfunction. Thus, BBB disruption is an important pathophysiological process of acute ischemic stroke. Understanding the mechanism underlying BBB disruption can uncover more promising biological targets for developing treatments for ischemic stroke. Ischemic stroke-induced activation of microglia and astrocytes leads to increased production of inflammatory mediators, containing chemokines, cytokines, matrix metalloproteinases (MMPs), etc., which are important factors in the pathological process of BBB breakdown. In this review, we discussed the current knowledges about the vital and dual roles of astrocytes and microglia on the BBB breakdown during ischemic stroke. Specifically, we provided an updated overview of phenotypic transformation of microglia and astrocytes, as well as uncovered the crosstalk among astrocyte, microglia, and oligodendrocyte in the BBB disruption following ischemic stroke.

Purinergic Signaling in Non-Parenchymal Liver Cells

Purinergic signaling has emerged as an important paracrine-autocrine intercellular system that regulates physiological and pathological processes in practically all organs of the body. Although this system has been thoroughly defined since the nineties, recent research has made substantial advances regarding its role in aspects of liver physiology. However, most studies have mainly targeted the entire organ, 70% of which is made up of parenchymal cells or hepatocytes. Because of its physiological role, the liver is exposed to toxic metabolites, such as xenobiotics, drugs, and fatty acids, as well as to pathogens such as viruses and bacteria. Under injury conditions, all cell types within the liver undergo adaptive changes. In this context, the concentration of extracellular ATP has the potential to increase dramatically. Indeed, this purinergic response has not been studied in sufficient detail in non-parenchymal liver cells. In the present review, we systematize the physiopathological adaptations related to the purinergic system in chronic liver diseases of non-parenchymal liver cells, such as hepatic stellate cells, Kupffer cells, sinusoidal endothelial cells, and cholangiocytes. The role played by non-parenchymal liver cells in these circumstances will undoubtedly be strategic in understanding the regenerative activities that support the viability of this organ under stressful conditions.

Microglial purinergic signaling in Alzheimer's disease

Alzheimer's disease (AD) is a progressive and fatal neurodegenerative disease. The prevalent features of AD pathogenesis are the appearance of β-amyloid (Aβ) plaques and neurofibrillary tangles, which cause microglial activation, synaptic deficiency, and neuronal loss. Microglia accompanies AD pathological processes and is also linked to cognitive deficits. Purinergic signaling has been shown to play a complex and tight interplay with the chemotaxis, phagocytosis, and production of pro-inflammatory factors in microglia, which is an important mechanism for regulating microglia activation. Here, we review recent evidence for interactions between AD, microglia, and purinergic signaling and find that the purinergic P2 receptors pertinently expressed on microglia are the ionotropic receptors P2X4 and P2X7, and the subtypes of P2YRs expressed by microglia are metabotropic receptors P2Y2, P2Y6, P2Y12, and P2Y13. The adenosine P1 receptors expressed in microglia include A1R, A2AR, and A2BR. Among them, the activation of P2X4, P2X7, and adenosine A1, A2A receptors expressed in microglia can aggravate the pathological process of AD, whereas P2Y2, P2Y6, P2Y12, and P2Y13 receptors expressed by microglia can induce neuroprotective effects. However, A1R activation also has a strong neuroprotective effect and has a significant anti-inflammatory effect in chronic neuroinflammation. These receptors regulate a variety of pathophysiological processes in AD, including APP processing, Aβ production, tau phosphorylation, neuroinflammation, synaptic dysfunction, and mitochondrial dysfunction. This review also provides key pharmacological advances in purinergic signaling receptors.

Distribution of P2Y and P2X purinergic receptor expression within the intestine

Nucleotides are potent extracellular signaling molecules during homeostasis, infection, and injury due to their ability to activate purinergic receptors. The nucleotide ATP activates P2X receptors (P2RXs), whereas the nucleotides ADP, ATP, UTP, and UDP-glucose selectively activate different P2Y receptors (P2RYs). Several studies have established crucial roles for P2 receptors during intestinal inflammatory and infectious diseases, yet the most extensive characterization of purinergic signaling has focused on immune cells and the central and enteric nervous systems. As epithelial cells serve as the first barrier against irritants and infection, we hypothesized that the gut epithelium may express multiple purinergic receptors that respond to extracellular nucleotide signals. Using the Human Protein Atlas and Gut Cell Survey, we queried single-cell RNA sequencing (RNAseq) data for the P2 purinergic receptors in the small and large intestines. In silico analysis reveals robust mRNA expression of P2RY1, P2RY2, P2RY11, and P2RX4 throughout the gastrointestinal tract. Human intestinal organoids exhibited a similar expression pattern with a prominent expression of P2RY1, P2RY2, and P2RX4, but this purinergic receptor repertoire was not conserved in T84, Caco2, and HT29 intestinal epithelial cell lines. Finally, P2YR1 and P2YR2 agonists elicited robust calcium responses in human intestinal organoids, but calcium responses were weaker or absent in the cell lines. These findings suggest that the gastrointestinal epithelia respond to extracellular purinergic signaling via P2RY1, P2RY2, P2RY11, and P2RX4 receptors and highlight the benefit of using intestinal organoids as a model of intestinal purinergic signaling.NEW & NOTEWORTHY Several studies have revealed crucial roles for P2 receptors during inflammatory and infectious diseases, however, these have largely been demonstrated in immune cells and the enteric nervous system. Although epithelial cells serve as the first barrier against infection and inflammation, the role of purinergic signaling within the gastrointestinal tract remains largely unknown. This work expands our knowledge of purinergic receptor distribution and relative expression along the intestine.

Pharmacological Nature of the Purinergic P2Y Receptor Subtypes That Participate in the Blood Pressure Changes Produced by ADPβS in Rats

Purine nucleosides (adenosine) and nucleotides such as adenosine mono/di/triphosphate (AMP/ADP/ATP) may produce complex cardiovascular responses. For example, adenosine-5'-(β-thio)-diphosphate (ADPβS; a stable synthetic analogue of ADP) can induce vasodilatation/vasodepressor responses by endothelium-dependent and independent mechanisms involving purinergic P2Y receptors; however, the specific subtypes participating in these responses remain unknown. Therefore, this study investigated the receptor subtypes mediating the blood pressure changes induced by intravenous bolus of ADPβS in male Wistar rats in the absence and presence of central mechanisms with the antagonists MRS2500 (P2Y1), PSB0739 (P2Y12), and MRS2211 (P2Y13). For this purpose, 120 rats were divided into 60 anaesthetised rats and 60 pithed rats, and further subdivided into four groups (n = 30 each), namely: (a) anaesthetised rats, (b) anaesthetised rats with bilateral vagotomy, (c) pithed rats, and (d) pithed rats continuously infused (intravenously) with methoxamine (an α1-adrenergic agonist that restores systemic vascular tone). We observed, in all four groups, that the immediate decreases in diastolic blood pressure produced by ADPβS were exclusively mediated by peripheral activation of P2Y1 receptors. Nevertheless, the subsequent increases in systolic blood pressure elicited by ADPβS in pithed rats infused with methoxamine probably involved peripheral activation of P2Y1, P2Y12, and P2Y13 receptors.