History of ectonucleotidases and their role in purinergic signaling

Coconut (Cocos nucifera (L.)) Water Improves Glucose Uptake with Concomitant Modulation of Antioxidant and Purinergic Activities in Isolated Rat Psoas Muscles

The present study investigated the effect of coconut water on glucose uptake and utilization, and metabolic activities linked to hyperglycemia in isolated rat psoas muscles. Coconut water was subjected to in vitro antioxidant and antidiabetic assays, which cover 2,2'-diphenyl-1-picrylhydrazyl (DPPH) scavenging activity, ferric reducing antioxidant power (FRAP), and inhibition of α-glucosidase and α-amylase activities. Psoas muscles were isolated from male Sprague Dawley rats and incubated with coconut water in the presence of glucose. Control consisted of muscles incubated with glucose only, while normal control consisted of muscles not incubated in coconut water and/or glucose. The standard antidiabetic drug was metformin. Incubation with coconut water led to a significant increase in muscle glucose uptake, with concomitant exacerbation of glutathione level, and SOD and catalase activities, while suppressing malondialdehyde level, and ATPase and E-NTDase activities. Coconut water showed significant scavenging activity against DPPH, and significantly inhibited α-glucosidase and α-amylase activities. LC-MS analysis of coconut water revealed the presence of ellagic acid, butin, quercetin, protocatechuic acid, baicalin, and silibinin. Molecular docking analysis revealed potent molecular interactions between the LC-MS-identified compounds, and AKT-2 serine and PI-3 kinase. These results indicate the potential of coconut water to enhance glucose uptake, while concomitantly improving antioxidative and purinergic activities. They also indicate the potential of coconut water to suppress postprandial hyperglycemia. These activities may be attributed to the synergistic effects of the LC-MS-identified compounds.

Extracellular nucleotides in smooth muscle contraction

Extracellular nucleotides and nucleosides are crucial signalling molecules, eliciting diverse biological responses in almost all organs and tissues. These molecules exert their effects by activating specific nucleotide receptors, which are finely regulated by ectonucleotidases that break down their ligands. In this comprehensive review, we aim to elucidate the relevance of extracellular nucleotides as signalling molecules in the context of smooth muscle contraction, considering the modulatory influence of ectonucleotidases on this intricate process. Specifically, we provide a detailed examination of the involvement of extracellular nucleotides in the contraction of non-vascular smooth muscles, including those found in the urinary bladder, the airways, the reproductive system, and the gastrointestinal tract. Furthermore, we present a broader overview of the role of extracellular nucleotides in vascular smooth muscle contraction.

Altered Expression of Astrocytic ATP Channels and Ectonucleotidases in the Cerebral Cortex and Hippocampus of Chronic Social Defeat Stress-Susceptible BALB/c Mice

The increasing number of patients with depressive disorder is a serious socioeconomic problem worldwide. Although several therapeutic agents have been developed and used clinically, their effectiveness is insufficient and thus discovery of novel therapeutic targets is desired. Here, focusing on dysregulation of neuronal purinergic signaling in depressive-like behavior, we examined the expression profiles of ATP channels and ectonucleotidases in astrocytes of cerebral cortex and hippocampus of chronic social defeat stress (CSDS)-susceptible BALB/c mice. Mice were exposed to 10-d CSDS, and their astrocytes were obtained using a commercially available kit based on magnetic activated cell sorting technology. In astrocytes derived from cerebral cortex of CSDS-susceptible mice, the expression levels of mRNAs for connexin 43, P2X7 receptors and maxi anion channels were increased, those for connexin 43 and P2X7 receptors being inversely correlated with mouse sociability, and the expression of mRNAs for ecto-nucleoside triphosphate diphosphohydrase 2 and ecto-5'nucleotidase was decreased and increased, respectively. On the other hand, the alteration profiles of ATP channels and ectonucleotidases in hippocampal astrocytes of CSDS-susceptible mice were different from in the case of cortical astrocytes, and there was no significant correlation between expression levels of their mRNAs and mouse sociability. These findings imply that increased expression of ATP channels in cerebral cortex might be involved in the development of reduced sociability in CSDS-subjected BALB/c mice. Together with recent findings, it is suggested that ATP channels expressed by cortical astrocytes might be potential therapeutic targets for depressive disorder.

Purinergic signaling in the battlefield of viral infections

Purinergic signaling has been associated with immune defenses against pathogens such as bacteria, protozoa, fungi, and viruses, acting as a sentinel system that signals to the cells when a threat is present. This review focuses on the roles of purinergic signaling and its therapeutic potential for viral infections. In this context, the purinergic system may play potent antiviral roles by boosting interferon signaling. In other cases, though, it can contribute to a hyperinflammatory response and disease severity, resulting in poor outcomes, such as during flu and potentially COVID-19. Lastly, a third situation may occur since viruses are obligatory intracellular parasites that hijack the host cell machinery for their infection and replication. Viruses such as HIV-1 use the purinergic system to favor their infection and persistence within the host cell. Therefore, understanding the particular nuances of purinergic signaling in each viral infection may contribute to designing proper therapeutic strategies to treat viral diseases.

Strategies for Drug Delivery into the Brain: A Review on Adenosine Receptors Modulation for Central Nervous System Diseases Therapy

The blood-brain barrier (BBB) is a biological barrier that protects the central nervous system (CNS) by ensuring an appropriate microenvironment. Brain microvascular endothelial cells (ECs) control the passage of molecules from blood to brain tissue and regulate their concentration-versus-time profiles to guarantee proper neuronal activity, angiogenesis and neurogenesis, as well as to prevent the entry of immune cells into the brain. However, the BBB also restricts the penetration of drugs, thus presenting a challenge in the development of therapeutics for CNS diseases. On the other hand, adenosine, an endogenous purine-based nucleoside that is expressed in most body tissues, regulates different body functions by acting through its G-protein-coupled receptors (A1, A2A, A2B and A3). Adenosine receptors (ARs) are thus considered potential drug targets for treating different metabolic, inflammatory and neurological diseases. In the CNS, A1 and A2A are expressed by astrocytes, oligodendrocytes, neurons, immune cells and ECs. Moreover, adenosine, by acting locally through its receptors A1 and/or A2A, may modulate BBB permeability, and this effect is potentiated when both receptors are simultaneously activated. This review showcases in vivo and in vitro evidence supporting AR signaling as a candidate for modifying endothelial barrier permeability in the treatment of CNS disorders.

Purine and purinergic receptors in health and disease

Purines and purinergic receptors are widely distributed throughout the human body. Purine molecules within cells play crucial roles in regulating energy metabolism and other cellular processes, while extracellular purines transmit signals through specific purinergic receptors. The ubiquitous purinergic signaling maintains normal neural excitability, digestion and absorption, respiratory movement, and other complex physiological activities, and participates in cell proliferation, differentiation, migration, and death. Pathological dysregulation of purinergic signaling can result in the development of various diseases, including neurodegeneration, inflammatory reactions, and malignant tumors. The dysregulation or dysfunction of purines and purinergic receptors has been demonstrated to be closely associated with tumor progression. Compared with other subtypes of purinergic receptors, the P2X7 receptor (P2X7R) exhibits distinct characteristics (i.e., a low affinity for ATP, dual functionality upon activation, the mediation of ion channels, and nonselective pores formation) and is considered a promising target for antitumor therapy, particularly in patients with poor response to immunotherapy This review summarizes the physiological and pathological significance of purinergic signaling and purinergic receptors, analyzes their complex relationship with tumors, and proposes potential antitumor immunotherapy strategies from tumor P2X7R inhibition, tumor P2X7R overactivation, and host P2X7R activation. This review provides a reference for clinical immunotherapy and mechanism investigation.

Drugs targeting adenosine signaling pathways: A current view

Adenosine is an endogenous nucleoside that regulates many physiological and pathological processes. It is derived from either the intracellular or extracellular dephosphorylation of adenosine triphosphate and interacts with cell-surface G-protein-coupled receptors. Adenosine plays a substantial role in protecting against cell damage in areas of increased tissue metabolism and preventing organ dysfunction in pathological states. Targeting adenosine metabolism and receptor signaling may be an effective therapeutic approach for human diseases, including cardiovascular and central nervous system disorders, rheumatoid arthritis, asthma, renal diseases, and cancer. Several lines of evidence have shown that many drugs exert their beneficial effects by modulating adenosine signaling pathways but this knowledge urgently needs to be summarized, and most importantly, actualized. The present review collects pharmaceuticals and pharmacological or diagnostic tools that target adenosine signaling in their primary or secondary mode of action. We overviewed FDA-approved drugs as well as those currently being studied in clinical trials. Among them are already used in clinic A2A adenosine receptor modulators like istradefylline or regadenoson, but also plenty of anti-platelet, anti-inflammatory, or immunosuppressive, and anti-cancer drugs. On the other hand, we investigated dozens of specific adenosine pathway regulators that are tested in clinical trials to treat human infectious and noninfectious diseases. In conclusion, targeting purinergic signaling represents a great therapeutic challenge. The actual knowledge of the involvement of adenosinergic signaling as part of the mechanism of action of old drugs has open a path not only for drug-repurposing but also for new therapeutic strategies.

Heparins are potent inhibitors of ectonucleotide pyrophosphatase/phospho-diesterase-1 (NPP1) - a promising target for the immunotherapy of cancer

Introduction

Heparins, naturally occurring glycosaminoglycans, are widely used for thrombosis prevention. Upon application as anticoagulants in cancer patients, heparins were found to possess additional antitumor activities. Ectonucleotidases have recently been proposed as novel targets for cancer immunotherapy.

Methods and results

In the present study, we discovered that heparin and its derivatives act as potent, selective, allosteric inhibitors of the poorly investigated ectonucleotidase NPP1 (nucleotide pyrophosphatase/phosphodiesterase-1, CD203a). Structure-activity relationships indicated that NPP1 inhibition could be separated from the compounds' antithrombotic effect. Moreover, unfractionated heparin (UFH) and different low molecular weight heparins (LMWHs) inhibited extracellular adenosine production by the NPP1-expressing glioma cell line U87 at therapeutically relevant concentrations. As a consequence, heparins inhibited the ability of U87 cell supernatants to induce CD4+ T cell differentiation into immunosuppressive Treg cells.

Discussion

NPP1 inhibition likely contributes to the anti-cancer effects of heparins, and their specific optimization may lead to improved therapeutics for the immunotherapy of cancer.

Molecular Dynamics Simulations of the Human Ecto-5'-Nucleotidase (h-ecto-5'-NT, CD73): Insights into Protein Flexibility and Binding Site Dynamics

Human ecto-5'-nucleotidase (h-ecto-5'-NT, CD73) is a homodimeric Zn2+-binding metallophosphoesterase that hydrolyzes adenosine 5'-monophosphate (5'-AMP) to adenosine and phosphate. h-Ecto-5'-NT is a key enzyme in purinergic signaling pathways and has been recognized as a promising biological target for several diseases, including cancer and inflammatory, infectious, and autoimmune diseases. Despite its importance as a biological target, little is known about h-ecto-5'-NT dynamics, which poses a considerable challenge to the design of inhibitors of this target enzyme. Here, to explore h-ecto-5'-NT flexibility, all-atom unbiased molecular dynamics (MD) simulations were performed. Remarkable differences in the dynamics of the open (catalytically inactive) and closed (catalytically active) conformations of the apo-h-ecto-5'-NT were observed during the simulations, and the nucleotide analogue inhibitor AMPCP was shown to stabilize the protein structure in the closed conformation. Our results suggest that the large and complex domain motion that enables the h-ecto-5'-NT open/closed conformational switch is slow, and therefore, it could not be completely captured within the time scale of our simulations. Nonetheless, we were able to explore the faster dynamics of the h-ecto-5'-NT substrate binding site, which is mainly located at the C-terminal domain and well conserved among the protein's open and closed conformations. Using the TRAPP ("Transient Pockets in Proteins") approach, we identified transient subpockets close to the substrate binding site. Finally, conformational states of the substrate binding site with higher druggability scores than the crystal structure were identified. In summary, our study provides valuable insights into h-ecto-5'-NT structural flexibility, which can guide the structure-based design of novel h-ecto-5'-NT inhibitors.

Inborn Errors of Purine Salvage and Catabolism

Cellular purine nucleotides derive mainly from de novo synthesis or nucleic acid turnover and, only marginally, from dietary intake. They are subjected to catabolism, eventually forming uric acid in humans, while bases and nucleosides may be converted back to nucleotides through the salvage pathways. Inborn errors of the purine salvage pathway and catabolism have been described by several researchers and are usually referred to as rare diseases. Since purine compounds play a fundamental role, it is not surprising that their dysmetabolism is accompanied by devastating symptoms. Nevertheless, some of these manifestations are unexpected and, so far, have no explanation or therapy. Herein, we describe several known inborn errors of purine metabolism, highlighting their unexplained pathological aspects. Our intent is to offer new points of view on this topic and suggest diagnostic tools that may possibly indicate to clinicians that the inborn errors of purine metabolism may not be very rare diseases after all.

The Role of P2X7 Purinoceptors in the Pathogenesis and Treatment of Muscular Dystrophies

Muscular dystrophies are inherited neuromuscular diseases, resulting in progressive disability and often affecting life expectancy. The most severe, common types are Duchenne muscular dystrophy (DMD) and Limb-girdle sarcoglycanopathy, which cause advancing muscle weakness and wasting. These diseases share a common pathomechanism where, due to the loss of the anchoring dystrophin (DMD, dystrophinopathy) or due to mutations in sarcoglycan-encoding genes (LGMDR3 to LGMDR6), the α-sarcoglycan ecto-ATPase activity is lost. This disturbs important purinergic signaling: An acute muscle injury causes the release of large quantities of ATP, which acts as a damage-associated molecular pattern (DAMP). DAMPs trigger inflammation that clears dead tissues and initiates regeneration that eventually restores normal muscle function. However, in DMD and LGMD, the loss of ecto-ATPase activity, that normally curtails this extracellular ATP (eATP)-evoked stimulation, causes exceedingly high eATP levels. Thus, in dystrophic muscles, the acute inflammation becomes chronic and damaging. The very high eATP over-activates P2X7 purinoceptors, not only maintaining the inflammation but also tuning the potentially compensatory P2X7 up-regulation in dystrophic muscle cells into a cell-damaging mechanism exacerbating the pathology. Thus, the P2X7 receptor in dystrophic muscles is a specific therapeutic target. Accordingly, the P2X7 blockade alleviated dystrophic damage in mouse models of dystrophinopathy and sarcoglycanopathy. Therefore, the existing P2X7 blockers should be considered for the treatment of these highly debilitating diseases. This review aims to present the current understanding of the eATP-P2X7 purinoceptor axis in the pathogenesis and treatment of muscular dystrophies.

CD73: Friend or Foe in Lung Injury

Ecto-5′-nucleotidase (CD73) plays a strategic role in calibrating the magnitude and chemical nature of purinergic signals that are delivered to immune cells. Its primary function is to convert extracellular ATP to adenosine in concert with ectonucleoside triphosphate diphosphohydrolase-1 (CD39) in normal tissues to limit an excessive immune response in many pathophysiological events, such as lung injury induced by a variety of contributing factors. Multiple lines of evidence suggest that the location of CD73, in proximity to adenosine receptor subtypes, indirectly determines its positive or negative effect in a variety of organs and tissues and that its action is affected by the transfer of nucleoside to subtype-specific adenosine receptors. Nonetheless, the bidirectional nature of CD73 as an emerging immune checkpoint in the pathogenesis of lung injury is still unknown. In this review, we explore the relationship between CD73 and the onset and progression of lung injury, highlighting the potential value of this molecule as a drug target for the treatment of pulmonary disease.

The P2X7 receptor in mood disorders: Emerging target in immunopsychiatry, from bench to bedside

Psychiatric disorders are among the most burdensome disorders worldwide. Though therapies have evolved over the last decades, treatment resistance still affects many patients. Recently, neuroimmune systems have been identified as important factors of mood disorder biology. The underlying dysregulation in neuroimmune cross-talk is driven by genetic risk factors and accumulating adverse environmental influences like chronic psychosocial stress. These result in a cluster of proinflammatory cytokines and quantitative and functional changes of immune cell populations (e.g., microglia, monocytes, T cells), varying by disease entity and state. Among the emerging immune targets, purinergic signalling revolving around the membranous and ATP specific P2X7 receptor (P2X7R) has gained wider attention and clinical studies making use of antagonistic drugs are on-going. Still, no clinically meaningful applications have been identified so far. A major problem is the often overly simplified approach taken to translate findings from bench to bedside. Therefore, the present review focuses on purinergic signalling via P2X7R in the context of recent advances in immunopsychiatric mood disorder research. Our aim is to provide an overview of the current P2X7R-related findings, from bench to bedside. First, we summarize the characteristics of purinergic signalling and P2X7R, followed by a depiction of genetic and clinical data connecting P2X7R to mood disorders. We close with our perspective on current developments and discuss changes necessary to translate the evident potential of P2X7R signalling modulation into meaningful clinical application. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Effect of Thiazolidin-4-one Against Lipopolysaccharide-Induced Oxidative Damage, and Alterations in Adenine Nucleotide Hydrolysis and Acetylcholinesterase Activity in Cultured Astrocytes

Astrocytes play multiple important roles in brain physiology. However, depending on the stimuli, astrocytes may exacerbate inflammatory reactions, contributing to the development and progression of neurological diseases. Therefore, therapies targeting astrocytes represent a promising area for the development of new brain drugs. Thiazolidinones are heterocyclic compounds that have a sulfur and nitrogen atom and a carbonyl group in the ring and represent a class of compounds of great scientific interest due to their pharmacological properties. The aim of this study was to investigate the effect of 3-(3-(diethylamino)propyl)-2-(4-(methylthio)phenyl)thiazolidin-4-one (DS27) on cell proliferation and morphology, oxidative stress parameters, activity of the enzymes ectonucleotidases and acetylcholinesterase (AChE) and interleukin 6 (IL-6) levels in primary astrocyte cultures treated with lipopolysaccharide (LPS), to model neuroinflammation. The astrocyte culture was exposed to LPS (10 μg/ml) for 3 h and subsequently treated with compound DS27 for 24 and 48 h (concentrations ranging to 10-100 μM). LPS induced an increase in astrocyte proliferation, AChE activity, IL-6 levels, oxidative damage, ATP and ADP and a reduction in AMP hydrolysis in rat primary astrocyte cultures. DS27 treatment was effective in reversing these alterations induced by LPS. Our findings demonstrated that DS27 is able to modulate cholinergic and purinergic signaling, redox status, and the levels of pro-inflammatory cytokines in LPS-induced astrocyte damage. These glioprotective effects of DS27 may be very important for improving neuroinflammation, which is associated with many brain diseases.

Urinary ATP Levels Are Controlled by Nucleotidases Released from the Urothelium in a Regulated Manner

Adenosine 5′-triphosphate (ATP) is released in the bladder lumen during filling. Urothelial ATP is presumed to regulate bladder excitability. Urinary ATP is suggested as a urinary biomarker of bladder dysfunctions since ATP is increased in the urine of patients with overactive bladder, interstitial cystitis or bladder pain syndrome. Altered urinary ATP might also be associated with voiding dysfunctions linked to disease states associated with metabolic syndrome. Extracellular ATP levels are determined by ATP release and ATP hydrolysis by membrane-bound and soluble nucleotidases (s-NTDs). It is currently unknown whether s-NTDs regulate urinary ATP. Using etheno-ATP substrate and HPLC-FLD detection techniques, we found that s-NTDs are released in the lumen of ex vivo mouse detrusor-free bladders. Capillary immunoelectrophoresis by ProteinSimple Wes determined that intraluminal solutions (ILS) collected at the end of filling contain ENTPD3 > ENPP1 > ENPP3 ≥ ENTPD2 = NT5E = ALPL/TNAP. Activation of adenylyl cyclase with forskolin increased luminal s-NTDs release whereas the AC inhibitor SQ22536 had no effect. In contrast, forskolin reduced and SQ22536 increased s-NTDs release in the lamina propria. Adenosine enhanced s-NTDs release and accelerated ATP hydrolysis in ILS and lamina propria. Therefore, there is a regulated release of s-NTDs in the bladder lumen during filling. Aberrant release or functions of urothelial s-NTDs might cause elevated urinary ATP in conditions with abnormal bladder excitability.

Purinergic Signalling in the Cochlea

The mammalian cochlea is the sensory organ of hearing with a delicate, highly organised structure that supports unique operating mechanisms. ATP release from the secretory tissues of the cochlear lateral wall (stria vascularis) triggers numerous physiological responses by activating P2 receptors in sensory, supporting and neural tissues. Two families of P2 receptors, ATP-gated ion channels (P2X receptors) and G protein-coupled P2Y receptors, activate intracellular signalling pathways that regulate cochlear development, homeostasis, sensory transduction, auditory neurotransmission and response to stress. Of particular interest is a purinergic hearing adaptation, which reflects the critical role of the P2X₂ receptor in adaptive cochlear response to elevated sound levels. Other P2 receptors are involved in the maturation of neural processes and frequency selectivity refinement in the developing cochlea. Extracellular ATP signalling is regulated by a family of surface-located enzymes collectively known as "ectonucleotidases" that hydrolyse ATP to adenosine. Adenosine is a constitutive cell metabolite with an established role in tissue protection and regeneration. The differential activation of A₁ and A_2A adenosine receptors defines the cochlear response to injury caused by oxidative stress, inflammation, and activation of apoptotic pathways. A₁ receptor agonism, A_2A receptor antagonism, and increasing adenosine levels in cochlear fluids all represent promising therapeutic tools for cochlear rescue from injury and prevention of hearing loss.

Purinergic Signalling and Inflammation-Related Diseases

While acute inflammation is widely accepted as an important response mechanism of cells against tissue injury, sustained inflammatory processes are increasingly recognized as one of the main contributors to numerous diseases, including central-nervous system (CNS)-related and non-CNS-related diseases such as depression, neurodegenerative diseases, type 2 diabetes, hypertension, cardiovascular diseases, chronic kidney disease, osteoporosis, and cancer [...].

Altered purinergic receptor expression in the frontal cortex in schizophrenia

ATP functions as a neurotransmitter, acting on the ubiquitously expressed family of purinergic P2 receptors. In schizophrenia (SCZ), the pathways that modulate extracellular ATP and its catabolism to adenosine are dysregulated. However, the effects of altered ATP availability on P2 receptor expression in the brain in SCZ have not been assessed. We assayed P2 receptor mRNA and protein expression in the DLPFC and ACC in subjects diagnosed with SCZ and matched, non-psychiatrically ill controls (n = 20-22/group). P2RX7, P2RX4 and male P2RX5 mRNA expression were significantly increased (p < 0.05) in the DLPFC in SCZ. Expression of P2RX7 protein isoform was also significantly increased (p < 0.05) in the DLPFC in SCZ. Significant increases in P2RX4 and male P2RX5 mRNA expression may be associated with antipsychotic medication effects. We found that P2RX4 and P2RX7 mRNA are significantly correlated with the inflammatory marker SERPINA3, and may suggest an association between upregulated P2XR and neuroinflammation in SCZ. These findings lend support for brain-region dependent dysregulation of the purinergic system in SCZ.

Transcriptomic and Histological Analysis of the Greentail Prawn (Metapenaeus bennettae) Following Light Crude Oil Exposure

Oil spills pose a significant threat to marine biodiversity. Crude oil can partition into sediments where it may be persistent, placing benthic species such as decapods at particular risk of exposure. Transcriptomic and histological tools are often used to investigate the effects of hydrocarbon exposure on marine organisms following oil spill events, allowing for the identification of metabolic pathways impacted by oil exposure. However, there is limited information available for decapod crustaceans, many of which carry significant economic value. In the present study, we assess the sublethal impacts of crude oil exposure in the commercially important Australian greentail prawn (Metapenaeus bennettae) using transcriptomic and histological analyses. Prawns exposed to light, unweathered crude oil "spiked" sediments for 90 h were transferred to clean sediments for a further 72 h to assess recovery. Chemical analyses indicated that polycyclic aromatic hydrocarbons increased by approximately 65% and 91% in prawn muscle following 24 and 90 h of exposure, respectively, and significantly decreased during 24- and 72-h recovery periods. Transcriptomic responses followed an exposure and recovery pattern with innate immunity and nutrient metabolism transcripts significantly lowered in abundance after 24 h of exposure and were higher in abundance after 72 h of recovery. In addition, transcription/translation, cellular responses, and DNA repair pathways were significantly impacted after 24 h of exposure and recovered after 72 h of recovery. However, histological alterations such as tubule atrophy indicated an increase in severity after 24 and 72 h of recovery. The present study provides new insights into the sublethal impacts of crude oil exposure in greentail prawns and identifies molecular pathways altered by exposure. We expect these findings to inform future management associated with oil extraction activity and spills. Environ Toxicol Chem 2022;41:2162-2180. © 2022 John Wiley & Sons Ltd. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Protein kinase inhibitor ceritinib blocks ectonucleotidase CD39 - a promising target for cancer immunotherapy

Background

An important mechanism, by which cancer cells achieve immune escape, is the release of extracellular adenosine into their microenvironment. Adenosine activates adenosine A_2A and A_2B receptors on immune cells constituting one of the strongest immunosuppressive mediators. In addition, extracellular adenosine promotes angiogenesis, tumor cell proliferation, and metastasis. Cancer cells upregulate ectonucleotidases, most importantly CD39 and CD73, which catalyze the hydrolysis of extracellular ATP to AMP (CD39) and further to adenosine (CD73). Inhibition of CD39 is thus expected to be an effective strategy for the (immuno)therapy of cancer. However, suitable small molecule inhibitors for CD39 are not available. Our aim was to identify drug-like CD39 inhibitors and evaluate them in vitro.

Methods

We pursued a repurposing approach by screening a self-compiled collection of approved, mostly ATP-competitive protein kinase inhibitors, on human CD39. The best hit compound was further characterized and evaluated in various orthogonal assays and enzyme preparations, and on human immune and cancer cells.

Results

The tyrosine kinase inhibitor ceritinib, a potent anticancer drug used for the treatment of anaplastic lymphoma kinase (ALK)-positive metastatic non-small cell lung cancer, was found to strongly inhibit CD39 showing selectivity versus other ectonucleotidases. The drug displays a non-competitive, allosteric mechanism of CD39 inhibition exhibiting potency in the low micromolar range, which is independent of substrate (ATP) concentration. We could show that ceritinib inhibits ATP dephosphorylation in peripheral blood mononuclear cells in a dose-dependent manner, resulting in a significant increase in ATP concentrations and preventing adenosine formation from ATP. Importantly, ceritinib (1–10 µM) substantially inhibited ATP hydrolysis in triple negative breast cancer and melanoma cells with high native expression of CD39.

Conclusions

CD39 inhibition might contribute to the effects of the powerful anticancer drug ceritinib. Ceritinib is a novel CD39 inhibitor with high metabolic stability and optimized physicochemical properties; according to our knowledge, it is the first brain-permeant CD39 inhibitor. Our discovery will provide the basis (i) to develop more potent and balanced dual CD39/ALK inhibitors, and (ii) to optimize the ceritinib scaffold towards interaction with CD39 to obtain potent and selective drug-like CD39 inhibitors for future in vivo studies.

Mechanosensitive Hydrolysis of ATP and ADP in Lamina Propria of the Murine Bladder by Membrane-Bound and Soluble Nucleotidases

Prior studies suggest that urothelium-released adenosine 5′-triphosphate (ATP) has a prominent role in bladder mechanotransduction. Urothelial ATP regulates the micturition cycle through activation of purinergic receptors that are expressed in many cell types in the lamina propria (LP), including afferent neurons, and might also be important for direct mechanosensitive signaling between urothelium and detrusor. The excitatory action of ATP is terminated by enzymatic hydrolysis, which subsequently produces bioactive metabolites. We examined possible mechanosensitive mechanisms of ATP hydrolysis in the LP by determining the degradation of 1,N⁶-etheno-ATP (eATP) at the anti-luminal side of nondistended (empty) or distended (full) murine (C57BL/6J) detrusor-free bladder model, using HPLC. The hydrolysis of eATP and eADP was greater in contact with LP of distended than of nondistended bladders whereas the hydrolysis of eAMP remained unchanged during filling, suggesting that some steps of eATP hydrolysis in the LP are mechanosensitive. eATP and eADP were also catabolized in extraluminal solutions (ELS) that were in contact with the LP of detrusor-free bladders, but removed from the organ chambers prior to addition of substrate. The degradation of both purines was greater in ELS from distended than from nondistended preparations, suggesting the presence of mechanosensitive release of soluble nucleotidases in the LP. The released enzyme activities were affected differently by Ca²⁺ and Mg²⁺. The common nucleotidase inhibitors ARL67156, POM-1, PSB06126, and ENPP1 Inhibitor C, but not the alkaline phosphatase inhibitor (-)-p-bromotetramisole oxalate, inhibited the enzymes released during bladder distention. Membrane-bound nucleotidases were identified in tissue homogenates and in concentrated ELS from distended preparations by Wes immunodetection. The relative distribution of nucleotidases was ENTPD1 >> ENPP1 > ENTPD2 = ENTPD3 > ENPP3 = NT5E >> ENTPD8 = TNAP in urothelium and ENTPD1 >> ENTPD3 >> ENPP3 > ENPP1 = ENTPD2 = NT5E >> ENTPD8 = TNAP in concentrated ELS, suggesting that regulated ectodomain shedding of membrane-bound nucleotidases possibly occurs in the LP during bladder filling. Mechanosensitive degradation of ATP and ADP by membrane-bound and soluble nucleotidases in the LP diminishes the availability of excitatory purines in the LP at the end of bladder filling. This might be a safeguard mechanism to prevent over-excitability of the bladder. Proper proportions of excitatory and inhibitory purines in the bladder wall are determined by distention-associated purine release and purine metabolism.

The purinergic signalling and inflammation in the pathogenesis and progression of diabetes: key factors and therapeutic targets

Type 2 diabetes mellitus (T2DM) is an important chronic disease around the world, and according to the World Health Organization, it is the 9th principal cause of global death. This pathology is characterized by high levels of circulating glucose as a result of insulin resistance, and it is well stated that inflammation related to obesity is directly associated with the development of the disease. The purinergic signalling is involved in both pancreatic destruction, which impairs insulin secretion, and the cytokine production that favors insulin resistance in T2DM. In this review, the purinergic signalling aspects will be discussed, showing the impact of the enzymes, nucleotides, nucleosides, and receptors of this system and the cytokines that result in inflammation, in the development and progression of T2DM, besides, pointing the purinergic receptors as a possible therapeutic approach.

Agonist-Dependent Coupling of the Promiscuous Adenosine A2B Receptor to Gα Protein Subunits

The adenosine A_2B receptor (A_2BAR) belongs to the rhodopsin-like G protein-coupled receptor (GPCR) family. It is upregulated under hypoxic conditions, in inflammation and cancer. Previous studies indicated the coupling of the A_2BAR to different G proteins, mainly G_s, but in some cases G_q/11 or G_i, depending on the cell type. We have now utilized novel technologies, (i) heterologous expression of individual members of the Gα_q/11 protein family (Gα_q, Gα₁₁, Gα₁₄, and Gα₁₅) in Gα_q/11 knockout cells, and (ii) the TRUPATH platform, allowing the direct observation of Gα protein activation for each of the Gα subunits by bioluminescence resonance energy transfer (BRET) measurements. Three structurally diverse A_2BAR agonists were studied: the cognate agonist adenosine, its metabolically stable analog NECA, and the non-nucleosidic partial agonist BAY 60-6583. Adenosine and NECA activated most members of all four Gα protein families (Gα_s, Gα_q/11, Gα_i, and Gα_12/13). Significant differences in potencies and efficacies were observed; the highest efficacies were determined at the Gα₁₅, Gα_s, and Gα₁₂ proteins, and for NECA additionally at the Gα_i2 protein. In contrast, the partial agonist BAY 60-6583 only activated Gα₁₅, Gα_s, and Gα₁₂ proteins. Adenosine deaminase, an allosteric modulator of ARs, selectively increased the potency and efficacy of NECA and BAY 60-6583 at the Gα₁₅ protein, while it had no effect or decreased efficacy at the other Gα proteins. We conclude that the A_2BAR is preferably coupled to the Gα₁₅, Gα_s, and Gα₁₂ proteins. Upon upregulation of receptor or Gα protein expression, coupling to further Gα proteins likely occurs. Importantly, different agonists can display different activation profiles.

Ectonucleotidases as bridge between the ATP and adenosine world: reflections on Geoffrey Burnstock

Historically, mainly by the work of Robert Berne, extracellular adenosine was the first purine compound recognized as an important signaling molecule linking energy metabolism to function by acting on membrane bound receptors. Geoffrey Burnstock by his vision and endurance pioneered the idea that cells release ATP that also acts as an extracellular signaling molecule under many physiological and pathophysiological circumstances. Only later, it was appreciated that extracellular ATP and adenosine are metabolically linked by the activity of several ectoenzymes which critically determine the concentrations of these purines at their respective receptors. In this brief review, I will report some personal recollections on Geoffrey Burnstock and his impressive personality. In addition, I will give a brief overview on our present knowledge of extracellular purine metabolism and its control and will address some still open issues.

Extracellular cAMP-Adenosine Pathway Signaling: A Potential Therapeutic Target in Chronic Inflammatory Airway Diseases

Adenosine is a purine nucleoside that, via activation of distinct G protein-coupled receptors, modulates inflammation and immune responses. Under pathological conditions and in response to inflammatory stimuli, extracellular ATP is released from damaged cells and is metabolized to extracellular adenosine. However, studies over the past 30 years provide strong evidence for another source of extracellular adenosine, namely the “cAMP-adenosine pathway.” The cAMP-adenosine pathway is a biochemical mechanism mediated by ATP-binding cassette transporters that facilitate cAMP efflux and by specific ectoenzymes that convert cAMP to AMP (ecto-PDEs) and AMP to adenosine (ecto-nucleotidases such as CD73). Importantly, the cAMP-adenosine pathway is operative in many cell types, including those of the airways. In airways, β₂-adrenoceptor agonists, which are used as bronchodilators for treatment of asthma and chronic respiratory diseases, stimulate cAMP efflux and thus trigger the extracellular cAMP-adenosine pathway leading to increased concentrations of extracellular adenosine in airways. In the airways, extracellular adenosine exerts pro-inflammatory effects and induces bronchoconstriction in patients with asthma and chronic obstructive pulmonary diseases. These considerations lead to the hypothesis that the cAMP-adenosine pathway attenuates the efficacy of β₂-adrenoceptor agonists. Indeed, our recent findings support this view. In this mini-review, we will highlight the potential role of the extracellular cAMP-adenosine pathway in chronic respiratory inflammatory disorders, and we will explore how extracellular cAMP could interfere with the regulatory effects of intracellular cAMP on airway smooth muscle and innate immune cell function. Finally, we will discuss therapeutic possibilities targeting the extracellular cAMP-adenosine pathway for treatment of these respiratory diseases.

The Purinergic Landscape of Type 2 Diabetes Mellitus

Adenosine triphosphate (ATP) is the key energy intermediate of cellular metabolic processes and a ubiquitous extracellular messenger. As an extracellular messenger, ATP acts at plasma membrane P2 receptors (P2Rs). The levels of extracellular ATP (eATP) are set by both passive and active release mechanisms and degradation processes. Under physiological conditions, eATP concentration is in the low nanomolar range but can rise to tens or even hundreds of micromoles/L at inflammatory sites. A dysregulated eATP homeostasis is a pathogenic factor in several chronic inflammatory diseases, including type 2 diabetes mellitus (T2DM). T2DM is characterized by peripheral insulin resistance and impairment of insulin production from pancreatic β-cells in a landscape of systemic inflammation. Although various hypoglycemic drugs are currently available, an effective treatment for T2DM and its complications is not available. However, counteracting systemic inflammation is anticipated to be beneficial. The postulated eATP increase in T2DM is understood to be a driver of inflammation via P2X7 receptor (P2X7R) activation and the release of inflammatory cytokines. Furthermore, P2X7R stimulation is thought to trigger apoptosis of pancreatic β-cells, thus further aggravating hyperglycemia. Targeting eATP and the P2X7R might be an appealing novel approach to T2DM therapy.

P2-type purinergic signaling in the regulation of pancreatic β-cell functional plasticity as a promising novel therapeutic approach for the treatment of type 2 diabetes?

Diabetes Mellitus is a metabolic disorder characterized by a chronic hyperglycemia due to an impaired insulin secretion and a decreased in peripheral insulin sensitivity. This disease is a major public health problem due to it sharp prevalence. Therefore, it is crucial to readapt therapeutic approaches for the treatment of this pathology. One of the strategies would be through P2-type purinergic receptors pathway via ATP binding. In addition to its well-known role as an intracellular energy intermediary in numerous biochemical and physiological processes, ATP is also an important extracellular signaling molecule. ATP mediates its effects by binding and activating two classes of P2 purinoreceptors: P2X receptors that are ligand-gated ion channel receptors, existing in seven isoforms (P2X 1 to 7) and P2Y receptors that are G-protein coupled receptors, existing in eight isoforms (P2Y 1/2/4/6/11/12/13/14). These receptors are ubiquitously distributed and involved in numerous physiological processes in several tissues. The concept of purinergic signaling, originally formulated by Geoffrey Burnstock (1929-2020), was also found to mediate various responses in the pancreas. Several studies have shown that P2 receptors are expressed in the endocrine pancreas, notably in β cells, where ATP could modulate their function but also their plasticity and thus play a physiological role in stimulating insulin secretion to face some metabolic demands. In this review, we provide a historical perspective and summarize current knowledge on P2-type purinergic signaling in the regulation of pancreatic β-cell functional plasticity, which would be a promising novel therapeutic approach for the treatment of type 2 diabetes.

E-NTPDases: Possible Roles on Host-Parasite Interactions and Therapeutic Opportunities

Belonging to the GDA1/CD39 protein superfamily, nucleoside triphosphate diphosphohydrolases (NTPDases) catalyze the hydrolysis of ATP and ADP to the monophosphate form (AMP) and inorganic phosphate (Pi). Several NTPDase isoforms have been described in different cells, from pathogenic organisms to animals and plants. Biochemical characterization of nucleotidases/NTPDases has revealed the existence of isoforms with different specificities regarding divalent cations (such as calcium and magnesium) and substrates. In mammals, NTPDases have been implicated in the regulation of thrombosis and inflammation. In parasites, such as Trichomonas vaginalis, Trypanosoma spp., Leishmania spp., Schistosoma spp. and Toxoplasma gondii, NTPDases were found on the surface of the cell, and important processes like growth, infectivity, and virulence seem to depend on their activity. For instance, experimental evidence has indicated that parasite NTPDases can regulate the levels of ATP and Adenosine (Ado) of the host cell, leading to the modulation of the host immune response. In this work, we provide a comprehensive review showing the involvement of the nucleotidases/NTPDases in parasites infectivity and virulence, and how inhibition of NTPDases contributes to parasite clearance and the development of new antiparasitic drugs.

Expanding the repertoire of methanocarba nucleosides from purinergic signaling to diverse targets

Nucleoside derivatives are well represented as pharmaceuticals due to their druglike physicochemical properties, and some nucleoside drugs are designed to act on receptors. The purinergic signaling pathways for extracellular nucleosides and nucleotides, consisting of adenosine receptors, P2Y/P2X receptors for nucleotides, and enzymes such as adenosine (ribo)kinase, have been extensively studied. A general modification, i.e. a constrained, bicyclic ring system (bicyclo[3.1.0]hexane, also called methanocarba) substituted in place of a furanose ring, can increase nucleoside/nucleotide potency and/or selectivity at purinergic and antiviral targets and in interactions at diverse and unconventional targets. Compared to other common drug discovery scaffolds containing planar rings, methanocarba nucleosides display greater sp3 character (i.e. more favorable as drug-like molecules) and can manifest as sterically-constrained North (N) or South (S) conformations. Initially weak, off-target interactions of (N)-methanocarba adenosine derivatives were detected as leads that were structurally optimized to enhance activity and selectivity toward target proteins that normally do not recognize nucleosides. By this approach, novel modulators for 5HT2 serotonin and κ-opioid receptors, dopamine (DAT) and ATP-binding cassette (ABC) transporters were found, and previously undetected antiviral activities were revealed. Thus, through methanocarba nucleoside synthesis, structure-activity relationships, and multi-target pharmacology, a robust purinergic receptor scaffold has been repurposed to satisfy the pharmacophoric requirements of various GPCRs, enzymes and transporters.

A novel definition and treatment of hyperinflammation in COVID-19 based on purinergic signalling

Hyperinflammation plays an important role in severe and critical COVID-19. Using inconsistent criteria, many researchers define hyperinflammation as a form of very severe inflammation with cytokine storm. Therefore, COVID-19 patients are treated with anti-inflammatory drugs. These drugs appear to be less efficacious than expected and are sometimes accompanied by serious adverse effects. SARS-CoV-2 promotes cellular ATP release. Increased levels of extracellular ATP activate the purinergic receptors of the immune cells initiating the physiologic pro-inflammatory immune response. Persisting viral infection drives the ATP release even further leading to the activation of the P2X7 purinergic receptors (P2X7Rs) and a severe yet physiologic inflammation. Disease progression promotes prolonged vigorous activation of the P2X7R causing cell death and uncontrolled ATP release leading to cytokine storm and desensitisation of all other purinergic receptors of the immune cells. This results in immune paralysis with co-infections or secondary infections. We refer to this pathologic condition as hyperinflammation. The readily available and affordable P2X7R antagonist lidocaine can abrogate hyperinflammation and restore the normal immune function. The issue is that the half-maximal effective concentration for P2X7R inhibition of lidocaine is much higher than the maximal tolerable plasma concentration where adverse effects start to develop. To overcome this, we selectively inhibit the P2X7Rs of the immune cells of the lymphatic system inducing clonal expansion of Tregs in local lymph nodes. Subsequently, these Tregs migrate throughout the body exerting anti-inflammatory activities suppressing systemic and (distant) local hyperinflammation. We illustrate this with six critically ill COVID-19 patients treated with lidocaine.

The elegant complexity of mammalian ecto-5'-nucleotidase (CD73)

Purinergic signaling is a fundamental mechanism used by all cells to control their internal activities and interact with the environment. A key component of the purinergic system, the enzyme ecto-5'-nucleotidase (CD73) catalyzes the last step in the extracellular metabolism of ATP to form adenosine. Efforts to harness the therapeutic potential of endogenous adenosine in cancer have culminated in the ongoing clinical development of multiple CD73-targeting antibodies and small-molecule inhibitors. However, recent studies are painting an increasingly complex picture of CD73 mRNA and protein regulation and function in cellular homeostasis, physiological adaptation, and disease development. This review discusses the latest conceptual and methodological advances that are helping to unravel the complexity of this important enzyme that was identified nearly 90 years ago.

Antitumoral Action of Resveratrol Through Adenosinergic Signaling in C6 Glioma Cells

Gliomas are the most common and aggressive primary tumors in the central nervous system. The nucleoside adenosine is considered to be one major constituent within the tumor microenvironment. The adenosine level mainly depends on two enzymatic activities: 5′-nucleotidase (5′NT or CD73) that synthesizes adenosine from AMP, and adenosine deaminase (ADA) that converts adenosine into inosine. Adenosine activates specific G-protein coupled receptors named A₁, A_2A, A_2B, and A₃ receptors. Resveratrol, a natural polyphenol present in grapes, peanuts, and berries, shows several healthy effects, including protection against cardiovascular, endocrine, and neurodegenerative diseases and cancer. However, the molecular mechanisms of resveratrol actions are not well known. Recently, we demonstrated that resveratrol acts as an agonist for adenosine receptors in rat C6 glioma cells. The present work aimed to investigate the involvement of adenosine metabolism and adenosine receptors in the molecular mechanisms underlying the antitumoral action of resveratrol. Results presented herein show that resveratrol was able to decrease cell numbers and viability and to reduce CD73 and ADA activities, leading to the increase of extracellular adenosine levels. Some resveratrol effects were reduced by the blockade of A₁ or A₃ receptors by DPCPX or MRS1220, respectively. These results suggest that reduced CD73 activity located in the plasma membrane in addition to a fine-tuned modulatory role of adenosine receptors could be involved, at least in part, in the antiproliferative action of resveratrol in C6 glioma cells.

Structure-activity relationships of pyrimidine nucleotides containing a 5'-α,β-methylene diphosphonate at the P2Y6 receptor

The Gq-coupled P2Y6 receptor (P2Y6R) is a component of the purinergic signaling system and functions in inflammatory, cardiovascular and metabolic processes. UDP, the native P2Y6R agonist and P2Y14R partial agonist, is subject to hydrolysis by ectonucleotidases. Therefore, we have synthesized UDP/CDP analogues containing a stabilizing α,β-methylene bridge as P2Y6R agonists and identified compatible affinity-enhancing pyrimidine modifications. A distal binding region on the receptor was explored with 4-benzyloxyimino cytidine 5'-diphosphate analogues and their potency determined in a calcium mobilization assay. A 4-trifluoromethyl-benzyloxyimino substituent in 25 provided the highest human P2Y6R potency (MRS4554, 0.57 µM), and a 5-fluoro substitution of the cytosine ring in 28 similarly enhanced potency, with >175- and 39-fold selectivity over human P2Y14R, respectively. However, 3-alkyl (31-33, 37, 38), β-d-arabinofuranose (39) and 6-aza (40) substitution prevented P2Y6R activation. Thus, we have identified new α,β-methylene bridged N4-extended CDP analogues as P2Y6R agonists that are highly selective over the P2Y14R.

Extracellular metabolism of 3',5'-cyclic AMP as a source of interstitial adenosine in the rat airways

In the respiratory tract, intracellular 3',5'-cAMP mediates smooth muscle relaxation triggered by the β₂-adrenoceptor/Gs protein/adenylyl cyclase axis. More recently, we have shown that β₂-adrenoceptor agonists also increase extracellular 3',5'-cAMP levels in isolated rat trachea, which leads to contraction of airway smooth muscle. In many other tissues, extracellular 3',5'-cAMP is metabolized by ectoenzymes to extracellular adenosine, a catabolic pathway that has never been addressed in airways. In order to evaluate the possible extracellular degradation of 3',5'-cAMP into 5'-AMP and adenosine in the airways, isolated rat tracheas were incubated with exogenous 3',5'-cAMP and the amount of 5'-AMP, adenosine and inosine (adenosine metabolite) produced was evaluated using ultraperformance liquid chromatography-tandem mass spectrometry. Incubation of tracheal tissue with 3',5'-cAMP induced a time- and concentration-dependent increase in 5'-AMP, adenosine and inosine in the medium. Importantly, IBMX (non-selective phosphodiesterase (PDE) inhibitor) and DPSPX (selective ecto-PDE inhibitor) reduced the extracellular conversion of 3',5'-cAMP to 5'-AMP. In addition, incubation of 3',5'-cAMP in the presence of AMPCP (inhibitor of ecto-5'-nucleotidase) increased extracellular levels of 5'-AMP while drastically reducing extracellular levels of adenosine and inosine. These results indicate that airways express an extracellular enzymatic system (ecto-phosphodiesterase, ecto-5'-nucleotidase and adenosine deaminase) that sequentially converts 3',5'-cAMP into 5'-AMP, adenosine and inosine. The observation that extracellular 3',5'-cAMP is a source of interstitial adenosine supports the idea that the extrusion and extracellular metabolism of 3',5'-cAMP has a role in respiratory physiology and pathophysiology.

Extracellular ATP hydrolysis in Caco-2 human intestinal cell line

Extracellular nucleotides and nucleosides activate signaling pathways that play major roles in the physiology and pathophysiology of the gastrointestinal tract. Ectonucleotidases hydrolyze extracellular nucleotides and thus regulate ligand exposure to purinergic receptors. In this study, we investigated the expression, localization and activities of ectonucleotidases using Caco-2 cells, a model of human intestinal epithelial cells. In addition, by studying ATP release and the rates of extracellular ATP (eATP) hydrolysis, we analyzed the contribution of these processes to the regulation of eATP in these cells. Results show that Caco-2 cells regulate the metabolism of eATP and by-products by ecto-nucleoside triphosphate diphosphohydrolase-1 and -2, a neutral ecto-phosphatase and ecto-5'-nucleotidase. All these ectoenzymes were kinetically characterized using intact cells, and their presence confirmed by denatured and native gels, western blot and cytoimmunofluorescence techniques. In addition, regulation of eATP was studied by monitoring the dynamic balance between intracellular ATP release and ectoATPase activity. Following mechanical and hypotonic stimuli, Caco-2 cells triggered a strong but transient release of intracellular ATP, with almost no energy cost, leading to a steep increase of eATP concentration, which was later reduced by ectoATPase activity. A data-driven algorithm allowed quantifying and predicting the rates of ATP release and ATP consumption contributing to the dynamic accumulation of ATP at the cell surface.

TNAP as a New Player in Chronic Inflammatory Conditions and Metabolism

This review summarizes important information on the ectoenzyme tissue-nonspecific alkaline phosphatase (TNAP) and gives a brief insight into the symptoms, diagnostics, and treatment of the rare disease Hypophosphatasia (HPP), which is resulting from mutations in the TNAP encoding ALPL gene. We emphasize the role of TNAP beyond its well-known contribution to mineralization processes. Therefore, above all, the impact of the enzyme on central molecular processes in the nervous system and on inflammation is presented here.