The P2X7 receptor and pannexin-1 are involved in glucose-induced autocrine regulation in β-cells

Could P2X7 receptor be a potencial target in neonatal sepsis?

The United Nations Inter-Agency Group for Child Mortality Estimation (UNIGME) estimates that every year 2.5 million neonates die in their first month of life, accounting for nearly one-half of deaths in children under 5 years of age. Neonatal sepsis is the third leading cause of neonatal mortality. The worldwide burden of bacterial sepsis is expected to increase in the next decades due to the lack of effective molecular therapies to replace the administration of antibiotics whose efficacy is compromised by the emergence of resistant strains. In addition, prolonged exposure to antibiotics can have negative effects by increasing the risk of infection by other organisms. With the global burden of sepsis increasing and no vaccine nor other therapeutic approaches proved efficient, the World Health Organization (WHO) stresses the need for new therapeutic targets for sepsis treatment and infection prevention (WHO, A73/32). In response to this unresolved clinical issue, the P2X7 receptor (P2X7R), a key component of the inflammatory cascade, has emerged as a potential target for treating inflammatory/infection diseases. Indeed numerous studies have demonstrated the relevance of the purinergic system as a pharmacological target in addressing immune-mediated inflammatory diseases by regulating immunity, inflammation, and organ function. In this review, we analyze key features of sepsis immunopathophysiology focusing in neonatal sepsis and on how the immunomodulatory role of P2X7R could be a potential pharmacological target for reducing the burden of neonatal sepsis.

Pharmacology of P2X Receptors and Their Possible Therapeutic Potential in Obesity and Diabetes

The role of P2X ionotropic receptors in the behavior of purinergic signaling on pathophysiological processes has been widely studied. In recent years, the important participation of P2X receptors in physiological and pathological processes, such as energy metabolism, characteristic inflammatory responses of the immune system, and nociceptive activity in response to pain stimuli, has been noted. Here, we explore the molecular characteristics of the P2X receptors and the use of the different agonist and antagonist agents recently described, focusing on their potential as new therapeutic targets in the treatment of diseases with emphasis on obesity, diabetes, and some of the complications derived from these pathologies.

Persistent organic pollutants disrupt the oxidant/antioxidant balance of INS-1E pancreatic β-cells causing their physiological dysfunctions

BACKGROUND

Persistent organic pollutants (POPs) have emerged as potent diabetogenic agents, but their mechanisms of action remain poorly identified.

OBJECTIVES

In this study, we aim to determine the mechanisms regulating the damaging effects of POPs in pancreatic β-cells, which have a central role in the development of diabetes.

METHODS

We treated INS-1E pancreatic β-cells with PCB-153, p,p'-DDE, PCB-126, or TCDD at doses ranging from 1 × 10-15to 5 × 10-6M. We measured insulin content and secretion, cell viability and assessed the mRNA expression of the xenobiotic nuclear receptors Nr1i2 and Nr1i3, and the aryl hydrocarbon receptor (Ahr). In addition, we evaluated the antioxidant defense and production of reactive oxygen species (ROS). Finally, we studied the ability of the antioxidant N-acetyl-L-cysteine (NAC) to counteract the effects of POPs in INS-1E cells.

RESULTS

When exposed to environmental POP levels, INS-1E cells had impaired production and secretion of insulin. These defects were observed for all tested POPs and were paralleled by reduced Ins1 and Ins2 mRNA expression. While POP treatment for 3 days did not affect INS-1E cell viability, longer treatment progressively killed the cells. Furthermore, we found that the xenobiotic detoxification machinery is poorly expressed in the INS-1E cells, as characterized by the absence of Nr1i2 and Nr1i3 and their respective downstream targets Cyp3a1/Cyp3a2 and Cyp2b1/Cyp2b3, and the weak functionality of the Ahr/Cyp1a1 signaling. Interestingly, POPs dysregulated key antioxidant enzymes such as glutathione peroxidases, peroxiredoxins, thioredoxins, and catalases. In parallel, the production of intracellular ROS, including superoxide anion (O2•-) and hydrogen peroxide (H2O2), was increased by POP exposure. Improving the oxidant scavenging capacity of INS-1E cells by NAC treatment restored the production and secretion of insulin.

CONCLUSION

By promoting oxidative stress and impairing the ability of INS-1E cells to produce and secrete insulin, this study reveals how POPs can mechanistically act as diabetogenic agents, and provides new scientific evidence supporting the concept that POPs are fueling the diabetes epidemics.

Imaging of extracellular and intracellular ATP in pancreatic beta cells reveals correlation between glucose metabolism and purinergic signalling

Adenosine triphosphate (ATP) is a universal energy molecule and yet cells release it and extracellular ATP is an important signalling molecule between cells. Monitoring of ATP levels outside of cells is important for our understanding of physiological and pathophysiological processes in cells/tissues. Here, we focus on pancreatic beta cells (INS-1E) and test the hypothesis that there is an association between intra- and extracellular ATP levels which depends on glucose provision. We imaged real-time changes in extracellular ATP in pancreatic beta cells using two sensors tethered to extracellular aspects of the plasma membrane (eATeam3.10, iATPSnFR1.0). Increase in glucose induced fast micromolar ATP release to the cell surface, depending on glucose concentrations. Chronic pre-treatment with glucose increased the basal ATP signal. In addition, we co-expressed intracellular ATP sensors (ATeam1.30, PercevalHR) in the same cultures and showed that glucose induced fast increases in extracellular and intracellular ATP. Glucose and extracellular ATP stimulated glucose transport monitored by the glucose sensor (FLII12Pglu-700uDelta6). In conclusion, we propose that in beta cells there is a dynamic relation between intra- and extracellular ATP that depends on glucose transport and metabolism and these processes may be tuned by purinergic signalling. Future development of ATP sensors for imaging may aid development of novel approaches to target extracellular ATP in, for example, type 2 diabetes mellitus therapy.

Vitamin D3 mitigates type 2 diabetes induced by a high carbohydrate-high fat diet in rats: Role of the purinergic system

This study evaluated the effect of vitamin D3 (VIT D3) supplementation on the enzymatic activities and density of ectonucleoside triphosphate diphosphohydrolase (E-NTPDase), ecto-5-nucleotidase (E-5'-NT), adenosine deaminase (ADA), as well as the density of P2 × 7R, P2Y12R, A1R, A2AR receptors, IL-1β, and oxidative parameters in type 2 diabetic rats. Forty male Wistar rats were fed a high carbohydrate-high fat diet (HCHFD) and received an intraperitoneal injection containing a single dose of streptozotocin (STZ, 35 mg/kg). Animals were divided into four groups: 1) control; 2) control/VIT D3 12 µg/kg; 3) diabetic; and 4) diabetic/VIT D3 12 µg/kg. Results show that VIT D3 reduced blood glucose, ATP hydrolysis, ADA activity, P2Y12R density (platelets), as well as ATP, ADP, and AMP hydrolysis and ADA activity (synaptosomes). Moreover, VIT D3 increased insulin levels and AMP hydrolysis (platelets) and improved antioxidant defense. Therefore, we suggest that VIT D3 treatment modulates hyperglycemia-induced changes via purinergic enzymes and receptor expression, consequently attenuating insulin homeostasis dysregulation in the diabetic state.

Metabolic and Molecular Amplification of Insulin Secretion

The pancreatic β cells are at the hub of myriad signals to regulate the secretion of an adequate amount of insulin needed to re-establish postprandial euglycemia. The β cell possesses sophisticated metabolic enzymes and a variety of extracellular receptors and channels that amplify insulin secretion in response to autocrine, paracrine, and neurohormonal signals. Considerable research has been undertaken to decipher the mechanisms regulating insulin secretion. While the triggering pathway induced by glucose is needed to initiate the exocytosis process, multiple other stimuli modulate the insulin secretion response. This chapter will discuss the recent advances in understanding the role of the diverse glucose- and fatty acid-metabolic coupling factors in amplifying insulin secretion. It will also highlight the intracellular events linking the extracellular receptors and channels to insulin secretion amplification. Understanding these mechanisms provides new insights into learning more about the etiology of β-cell failure and paves the way for developing new therapeutic strategies for type 2 diabetes.

Possible role of pannexin 1 channels and purinergic receptors in the pathogenesis and mechanism of action of SARS-CoV-2 and therapeutic potential of targeting them in COVID-19

Identifying signaling pathways and molecules involved in SARS-CoV-2 pathogenesis is pivotal for developing new effective therapeutic or preventive strategies for COVID-19. Pannexins (PANX) are ATP-release channels in the plasma membrane essential in many physiological and immune responses. Activation of pannexin channels and downstream purinergic receptors play dual roles in viral infection, either by facilitating viral replication and infection or inducing host antiviral defense. The current review provides a hypothesis demonstrating the possible contribution of the PANX1 channel and purinergic receptors in SARS-CoV-2 pathogenesis and mechanism of action. Moreover, we discuss whether targeting these signaling pathways may provide promising preventative therapies and treatments for patients with progressive COVID-19 resulting from excessive pro-inflammatory cytokines and chemokines production. Several inhibitors of this pathway have been developed for the treatment of other viral infections and pathological consequences. Specific PANX1 inhibitors could be potentially included as part of the COVID-19 treatment regimen if, in future, studies demonstrate the role of PANX1 in COVID-19 pathogenesis. Of note, any ATP therapeutic modulation for COVID-19 should be carefully designed and monitored because of the complex role of extracellular ATP in cellular physiology.

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.

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 role of P2X7 receptor in infection and metabolism: Based on inflammation and immunity

The P2X7 receptor (P2X7R) is a ligand-gated receptor belonging to the P2 receptor family. It is distributed in various tissues of the human body and is involved in regulating the physiological functions of tissues and cells to affect the occurrence and development of diseases. Unlike all other P2 receptors, the P2X7 receptor is mainly expressed in immune cells and can be activated not only by extracellular nucleotides but also by non-nucleotide substances which act as positive allosteric modulators. In this review, we comprehensively describe the role of the P2X7 receptor in infection and metabolism based on its role as an important regulator of inflammation and immunity, and briefly introduce the structure and general function of the P2X7 receptor. These provide a clear knowledge framework for the study of the P2X7 receptor in human health. Targeting the P2X7 receptor may be an effective method for the treatment of inflammatory and immune diseases. And its role in microbial infection and metabolism may be the main direction for in-depth research on the P2X7 receptor in the future.

The P2X7 Receptor Stimulates IL-6 Release from Pancreatic Stellate Cells and Tocilizumab Prevents Activation of STAT3 in Pancreatic Cancer Cells

Pancreatic stellate cells (PSCs) are important pancreatic fibrogenic cells that interact with pancreatic cancer cells to promote the progression of pancreatic ductal adenocarcinoma (PDAC). In the tumor microenvironment (TME), several factors such as cytokines and nucleotides contribute to this interplay. Our aim was to investigate whether there is an interaction between IL-6 and nucleotide signaling, in particular, that mediated by the ATP-sensing P2X7 receptor (P2X7R). Using human cell lines of PSCs and cancer cells, as well as primary PSCs from mice, we show that ATP is released from both PSCs and cancer cells in response to mechanical and metabolic cues that may occur in the TME, and thus activate the P2X7R. Functional studies using P2X7R agonists and inhibitors show that the receptor is involved in PSC proliferation, collagen secretion and IL-6 secretion and it promotes cancer cell migration in a human PSC-cancer cell co-culture. Moreover, conditioned media from P2X7R-stimulated PSCs activated the JAK/STAT3 signaling pathway in cancer cells. The monoclonal antibody inhibiting the IL-6 receptor, Tocilizumab, inhibited this signaling. In conclusion, we show an important mechanism between PSC-cancer cell interaction involving ATP and IL-6, activating P2X7 and IL-6 receptors, respectively, both potential therapeutic targets in PDAC.

Synthesis, In-vitro evaluation and molecular docking studies of oxoindolin phenylhydrazine carboxamides as potent and selective inhibitors of ectonucleoside triphosphate diphosphohydrolase (NTPDase)

Members of the ectonucleoside triphosphate diphosphohydrolases (NTPDases) constitute the major family of enzymes responsible for the maintenance of extracellular levels of nucleotides and nucleosides by catalyzing the hydrolysis of nucleoside triphosphate (NTP) and nucleoside diphosphates (NDP) to nucleoside monophosphate (NMP). Although, NTPDase inhibitors can act as potential drug candidates for the treatment of various diseases, there is lack of potent as well as selective inhibitors of NTPDases. The current study describes the synthesis of a number of carboxamide derivatives that were tested on recombinant human (h) NTPDases. The most promising inhibitors were 2h (h-NTPDase1, IC₅₀: 0.12 ± 0.03 µM), 2d (h-NTPDase2, IC₅₀: 0.15 ± 0.01 µM) and 2a (h-NTPDase3, IC₅₀: 0.30 ± 0.04 µM; h-NTPDase8, IC₅₀: 0.16 ± 0.02 µM). Four compounds (2e, 2f, 2g and 2h) were associated with the selective inhibition of h-NTPDase1 while 2b was identified as a selective h-NTPDase3 inhibitor. Considering the importance of NTPDase3 in the regulation of insulin release, the NTPDase3 inhibitors were further investigated to elucidate their role in the insulin release. The obtained data suggested that compound 2a was actively participating in regulating the insulin release without producing any effect on NTPDase3 mRNA. Moreover, the most potent inhibitors were docked within the active site of respective enzyme and the observed interactions were in compliance with in vitro results. Hence, these compounds can be used as pharmacological tool to further investigate the role of NTPDase3 coupled to insulin release.

Functionalized Oxoindolin Hydrazine Carbothioamide Derivatives as Highly Potent Inhibitors of Nucleoside Triphosphate Diphosphohydrolases

Ectonucleoside triphosphate diphosphohydrolases (NTPDases) are ectoenzymes that play an important role in the hydrolysis of nucleoside triphosphate and diphosphate to nucleoside monophosphate. NTPDase1, -2, -3 and -8 are the membrane bound members of this enzyme family that are responsible for regulating the levels of nucleotides in extracellular environment. However, the pathophysiological functions of these enzymes are not fully understood due to lack of potent and selective NTPDase inhibitors. Herein, a series of oxoindolin hydrazine carbothioamide derivatives is synthesized and screened for NTPDase inhibitory activity. Four compounds were identified as selective inhibitors of h-NTPDase1 having IC₅₀ values in lower micromolar range, these include compounds 8b (IC₅₀ = 0.29 ± 0.02 µM), 8e (IC₅₀ = 0.15 ± 0.009 µM), 8f (IC₅₀ = 0.24 ± 0.01 µM) and 8l (IC₅₀ = 0.30 ± 0.03 µM). Similarly, compound 8k (IC₅₀ = 0.16 ± 0.01 µM) was found to be a selective h-NTPDase2 inhibitor. In case of h-NTPDase3, most potent inhibitors were compounds 8c (IC₅₀ = 0.19 ± 0.02 µM) and 8m (IC₅₀ = 0.38 ± 0.03 µM). Since NTPDase3 has been reported to be associated with the regulation of insulin secretion, we evaluated our synthesized NTPDase3 inhibitors for their ability to stimulate insulin secretion in isolated mice islets. Promising results were obtained showing that compound 8m potently stimulated insulin secretion without affecting the NTPDase3 gene expression. Molecular docking studies of the most potent compounds were also carried out to rationalize binding site interactions. Hence, these compounds are useful tools to study the role of NTPDase3 in insulin secretion.

The interplay between α7 nicotinic acetylcholine receptors, pannexin-1 channels and P2X7 receptors elicit exocytosis in chromaffin cells

Pannexin-1 (Panx1) forms plasma membrane channels that allow the exchange of small molecules between the intracellular and extracellular compartments, and are involved in diverse physiological and pathological responses in the nervous system. However, the signaling mechanisms that induce their opening still remain elusive. Here, we propose a new mechanism for Panx1 channel activation through a functional crosstalk with the highly Ca²⁺ permeable α7 nicotinic acetylcholine receptor (nAChR). Consistent with this hypothesis, we found that activation of α7 nAChRs induces Panx1-mediated dye uptake and ATP release in the neuroblastoma cell line SH-SY5Y-α7. Using membrane permeant Ca²⁺ chelators, total internal reflection fluorescence microscopy in SH-SY5Y-α7 cells expressing a membrane-tethered GCAMP3, and Src kinase inhibitors, we further demonstrated that Panx1 channel opening depends on Ca²⁺ signals localized in submembrane areas, as well as on Src kinases. In turn, Panx1 channels amplify cytosolic Ca²⁺ signals induced by the activation of α7 nAChRs, by a mechanism that seems to involve ATP release and P2X7 receptor activation, as hydrolysis of extracellular ATP with apyrase or blockage of P2X7 receptors with oxidized ATP significantly reduces the α7 nAChR-Ca²⁺ signal. The physiological relevance of this crosstalk was also demonstrated in neuroendocrine chromaffin cells, wherein Panx1 channels and P2X7 receptors contribute to the exocytotic release of catecholamines triggered by α7 nAChRs, as measured by amperometry. Together these findings point to a functional coupling between α7 nAChRs, Panx1 channels and P2X7 receptors with physiological relevance in neurosecretion.

P2X7 Receptor in the Management of Energy Homeostasis: Implications for Obesity, Dyslipidemia, and Insulin Resistance

Whole-body energy metabolism entails the highly regulated balance between food intake, nutrient breakdown, energy generation (ATP), and energy storage for the preservation of vital functions and body mass. Purinergic signaling has attracted increasing attention in the regulatory mechanisms not only for the reverse processes of white adipose tissue lipogenesis and lipolysis, but also for brown adipocyte-dependent thermogenesis and leptin production. This regulatory role has remarkable implications in the handling of body's energy expenditure and energy reservoir. Hence, selected purinergic receptors can play a relevant function in lipid metabolism, endocrine activity, glucose uptake, ATP-dependent increased expression of uncoupling protein 1, and browning of adipose tissue. Indeed, purinergic P2 receptors regulate adipogenesis and lipid metabolism and are involved in adipogenic differentiation. In particular, the ionotropic ATP-activated P2X7 subtype is involved in fat distribution, as well as in the modulation of inflammatory pathways in white adipose tissue. Within this context, very recent evidence has established a direct function of P2X7 in energy metabolism. Specifically, either genetic deletion (P2X7 knockout mice) or subchronic pharmacological inhibition of the receptor produces a decrease of whole-body energy expenditure and, concurrently, an increase of carbohydrate oxidation. As further evidence, lipid accumulation, increased fat mass distribution, and weight gain are reported in P2X7-depleted mice. Conversely, the stimulation of P2X7 enhances energy expenditure. Altogether, this knowledge supports the role of P2X7 signaling in the fight against obesity and insulin resistance, as well as in the promotion of adaptive thermogenesis.

Update of P2Y receptor pharmacology: IUPHAR Review 27

Eight G protein-coupled P2Y receptor subtypes respond to extracellular adenine and uracil mononucleotides and dinucleotides. P2Y receptors belong to the δ group of rhodopsin-like GPCRs and contain two structurally distinct subfamilies: P2Y₁ , P2Y₂ , P2Y₄ , P2Y₆ , and P2Y₁₁ (principally G_q protein-coupled P2Y₁ -like) and P2Y_12-14 (principally G_i protein-coupled P2Y₁₂ -like) receptors. Brain P2Y receptors occur in neurons, glial cells, and vasculature. Endothelial P2Y₁ , P2Y₂ , P2Y₄ , and P2Y₆ receptors induce vasodilation, while smooth muscle P2Y₂ , P2Y₄ , and P2Y₆ receptor activation leads to vasoconstriction. Pancreatic P2Y₁ and P2Y₆ receptors stimulate while P2Y₁₃ receptors inhibits insulin secretion. Antagonists of P2Y₁₂ receptors, and potentially P2Y₁ receptors, are anti-thrombotic agents, and a P2Y₂ /P2Y₄ receptor agonist treats dry eye syndrome in Asia. P2Y receptor agonists are generally pro-inflammatory, and antagonists may eventually treat inflammatory conditions. This article reviews recent developments in P2Y receptor pharmacology (using synthetic agonists and antagonists), structure and biophysical properties (using X-ray crystallography, mutagenesis and modelling), physiological and pathophysiological roles, and present and potentially future therapeutic targeting.

P2X7 receptor mediates NLRP3 inflammasome activation in depression and diabetes

The increasing prevalence of depression and diabetes mellitus has become a major public health problem worldwide. Studies have shown that people with diabetes are at a high risk of being diagnosed with depression, and diabetes complicates depression treatment by promoting the deterioration of glycemic control, reducing self-care ability and quality of life, and causing severe functional disability and early mortality. Moreover, health deterioration dramatically increases the financial cost of social and health care system. Thus, how to treat depression, diabetes, and diabetes complicated by depression has become one of the world’s urgent concerns. The activation of nod-like receptor family pyrin domain containing 3 (NLRP3) is closely related to mental illness. This finding provides a new perspective for studying depression. NLRP3 plays an important role in the development of diabetes. In this review, we elaborate the definition and epidemiology of depression, diabetes, and diabetic depression and introduce the functional characteristics of an NLRP3 inflammasome and upstream P2X7 receptor. Moreover, related research on NLRP3 inflammasomes and P2X7 receptors is summarized and used as a reference for confirming that the excessive activation of P2X7- NLRP3 leads to the increased release of inflammatory cytokines, such as IL-1β, in depression and diabetes. We provide insights into the P2X7–NLRP3–IL-1β pathway as an important pathological mechanism and novel therapeutic target in diabetes and depression. Given that the P2X7–NLRP3–IL-1β pathway may play an important role in diabetes confounded by comorbid depression, the possibility of intervention with baicalin is proposed.

Pannexin-1 mediated ATP release in adipocytes is sensitive to glucose and insulin and modulates lipolysis and macrophage migration

AIM

Extracellular ATP signalling is involved in many physiological and pathophysiological processes in several tissues, including adipose tissue. Adipocytes have crucial functions in lipid and glucose metabolism and they express purinergic receptors. However, the sources of extracellular ATP in adipose tissue are not well characterized. In the present study, we investigated the mechanism and regulation of ATP release in white adipocytes, and evaluated the role of extracellular ATP as potential autocrine and paracrine signal.

METHODS

Online ATP release was monitored in C3H10T1/2 cells and freshly isolated murine adipocytes. The ATP release mechanism and its regulation were tested in cells exposed to adrenergic agonists, insulin, glucose load and pharmacological inhibitors. Cell metabolism was monitored using Seahorse respirometry and expression analysis of pannexin-1 was performed on pre- and mature adipocytes. The ATP signalling was evaluated in live cell imaging (Ca²⁺ , pore formation), glycerol release and its effect on macrophages was tested in co-culture and migration assays.

RESULTS

Here, we show that upon adrenergic stimulation white murine adipocytes release ATP through the pannexin-1 pore that is regulated by a cAMP-PKA-dependent pathway. The ATP release correlates with increased cell metabolism and is sensitive to glucose. Extracellular ATP induces Ca²⁺ signalling and lipolysis in adipocytes and promotes macrophage migration. Importantly, ATP release is markedly inhibited by insulin, which operates via the activation of phosphodiesterase 3.

CONCLUSIONS

Our findings reveal an insulin-pannexin-1-purinergic signalling crosstalk in adipose tissue and we propose that deregulation of this signalling may contribute to adipose tissue inflammation and type 2 diabetes.

Differential Effect of Glucose on ER-Mitochondria Ca2+ Exchange Participates in Insulin Secretion and Glucotoxicity-Mediated Dysfunction of β-Cells

Glucotoxicity-induced β-cell dysfunction in type 2 diabetes is associated with alterations of mitochondria and the endoplasmic reticulum (ER). Both organelles interact at contact sites, defined as mitochondria-associated membranes (MAMs), which were recently implicated in the regulation of glucose homeostasis. The role of MAMs in β-cells is still largely unknown, and their implication in glucotoxicity-associated β-cell dysfunction remains to be defined. Here, we report that acute glucose treatment stimulated ER-mitochondria interactions and calcium (Ca²⁺) exchange in INS-1E cells, whereas disruption of MAMs altered glucose-stimulated insulin secretion (GSIS). Conversely, chronic incubations with high glucose of either INS-1E cells or human pancreatic islets altered GSIS and concomitantly reduced ER Ca²⁺ store, increased basal mitochondrial Ca²⁺, and reduced ATP-stimulated ER-mitochondria Ca²⁺ exchanges, despite an increase of organelle interactions. Furthermore, glucotoxicity-induced perturbations of Ca²⁺ signaling are associated with ER stress, altered mitochondrial respiration, and mitochondria fragmentation, and these organelle stresses may participate in increased organelle tethering as a protective mechanism. Last, sustained induction of ER-mitochondria interactions using a linker reduced organelle Ca²⁺ exchange, induced mitochondrial fission, and altered GSIS. Therefore, dynamic organelle coupling participates in GSIS in β-cells, and over time, disruption of organelle Ca²⁺ exchange might be a novel mechanism contributing to glucotoxicity-induced β-cell dysfunction.

Chronic fructose renders pancreatic β-cells hyper-responsive to glucose-stimulated insulin secretion through extracellular ATP signaling

Fructose is widely used as a sweetener in processed food and is also associated with metabolic disorders, such as obesity. However, the underlying cellular mechanisms remain unclear, in particular, regarding the pancreatic β-cell. Here, we investigated the effects of chronic exposure to fructose on the function of insulinoma cells and isolated mouse and human pancreatic islets. Although fructose per se did not acutely stimulate insulin exocytosis, our data show that chronic fructose rendered rodent and human β-cells hyper-responsive to intermediate physiological glucose concentrations. Fructose exposure reduced intracellular ATP levels without affecting mitochondrial function, induced AMP-activated protein kinase activation, and favored ATP release from the β-cells upon acute glucose stimulation. The resulting increase in extracellular ATP, mediated by pannexin1 (Panx1) channels, activated the calcium-mobilizer P2Y purinergic receptors. Immunodetection revealed the presence of both Panx1 channels and P2Y1 receptors in β-cells. Addition of an ectonucleotidase inhibitor or P2Y1 agonists to naïve β-cells potentiated insulin secretion stimulated by intermediate glucose, mimicking the fructose treatment. Conversely, the P2Y1 antagonist and Panx1 inhibitor reversed the effects of fructose, as confirmed using Panx1-null islets and by the clearance of extracellular ATP by apyrase. These results reveal an important function of ATP signaling in pancreatic β-cells mediating fructose-induced hyper-responsiveness.

ATP release into ADPKD cysts via pannexin-1/P2X7 channels decreases ENaC activity

Genetic predisposition is necessary for polycystic kidney disease (PKD) initiation, although there are other, incompletely identified downstream processes that are required for cyst growth. Their characterization may provide a unique opportunity for clinical interventions. One of the poorly studied phenomena in PKD is high ATP content in cysts. Unfortunately, neither origins of uncontrolled ATP release, nor consequences of abnormal purinergic signaling in relation to epithelial transport are well explored in the polycystic kidney. We tested the distribution of pannexin-1 (Panx1) and P2X7, two proteins potentially involved in ATP release, in the kidneys of the Pkd1RC/RC mice, a model of autosomal dominant PKD (ADPKD). Abundances of both proteins were abnormally increased in the cyst lining cells compared to non-dilated collecting ducts. To establish if pannexin-1 contributes to ATP release in the collecting ducts (CD), we measured luminal accumulation of ATP in M1 cell renal CD monolayers, and found that treatment with probenecid, a Panx1 blocker, prevents ATP release. Single channel patch clamp analysis of polarized M1 cells revealed that apical stimulation of P2X receptors with αβ-MeATP acutely reduces ENaC activity. We conclude that in ADPKD progression, an abnormal hyperexpression of both PANX1 and P2RX7 occurs in the cyst lining epithelial cells. High abundance of both proteins is not typical for non-dilated CDs but, when it happens in cysts, pannexin1/P2X7 cooperation elevates ATP release into the luminal space. High ATP level is a pathogenic factor facilitating cystogenesis by reducing ENaC-mediated reabsorption from the lumen.

Role of the P2X7 receptor in the pathogenesis of type 2 diabetes and its microvascular complications

P2X7 receptors can be found in many tissues and organs, where they mediate several biological functions. This review summarizes the current knowledge about the role of this receptor in the pathogenesis of type 2 diabetes, in which the key clinical features are impaired insulin secretion and sensitivity, hyperglycemia, coexistence of other cardiovascular risk factors such as dyslipidemia and hypertension, and subclinical inflammation. The receptor modulates crucial pathways in the pancreatic islets (where it can either exert a trophic or detrimental action on β cells), and in the liver, in the adipose tissue and in the skeletal muscle, which are main sites of insulin resistance. P2X7 receptors also modulate a series of inflammatory responses that participate in the development of the microvascular complications of the disease. Potent and selective P2X7R blockers are available to be tested in Phase I/II clinical studies for the treatment of several chronic diseases, and it might be worthwhile to consider inclusion of patients with type 2 diabetes and its complications.