The role of P2X7 receptors in tissue fibrosis: a brief review

Ca2+ Signaling in Cardiovascular Fibroblasts

Fibrogenesis is a physiological process required for wound healing and tissue repair. It is induced by activation of quiescent fibroblasts, which first proliferate and then change their phenotype into migratory, contractile myofibroblasts. Myofibroblasts secrete extracellular matrix proteins, such as collagen, to form a scar. Once the healing process is terminated, most myofibroblasts undergo apoptosis. However, in some tissues, such as the heart, myofibroblasts remain active and sensitive to neurohumoral factors and inflammatory mediators, which lead eventually to excessive organ fibrosis. Many cellular processes involved in fibroblast activation, including cell proliferation, protein secretion and cell contraction, are highly regulated by intracellular Ca2+ signals. This review summarizes current research on Ca2+ signaling pathways underlying fibroblast activation. We present receptor- and ion channel-mediated Ca2+ signaling pathways, discuss how localized Ca2+ signals of the cell nucleus may be involved in fibroblast activation and present Ca2+-sensitive transcription pathways relevant for fibroblast biology. When investigated, we highlight how the function of Ca2+-handling proteins changes during cardiac and pulmonary fibrosis. Many aspects of Ca2+ signaling remain unexplored in different types of cardiovascular fibroblasts in relation to pathologies, and a better understanding of Ca2+ signaling in fibroblasts will help to design targeted therapies against fibrosis.

Purinergic signaling in liver disease: calcium signaling and induction of inflammation

Purinergic signaling regulates many metabolic functions and is implicated in liver physiology and pathophysiology. Liver functionality is modulated by ionotropic P2X and metabotropic P2Y receptors, specifically P2Y1, P2Y2, and P2Y6 subtypes, which physiologically exert their influence through calcium signaling, a key second messenger controlling glucose and fat metabolism in hepatocytes. Purinergic receptors, acting through calcium signaling, play an important role in a range of liver diseases. Ionotropic P2X receptors, such as the P2X7 subtype, and certain metabotropic P2Y receptors can induce aberrant intracellular calcium transients that impact normal hepatocyte function and initiate the activation of other liver cell types, including Kupffer and stellate cells. These P2Y- and P2X-dependent intracellular calcium increases are particularly relevant in hepatic disease states, where stellate and Kupffer cells respond with innate immune reactions to challenges, such as excess fat accumulation, chronic alcohol abuse, or infections, and can eventually lead to liver fibrosis. This review explores the consequences of excessive extracellular ATP accumulation, triggering calcium influx through P2X4 and P2X7 receptors, inflammasome activation, and programmed cell death. In addition, P2Y2 receptors contribute to hepatic steatosis and insulin resistance, while inhibiting the expression of P2Y6 receptors can alleviate alcoholic liver steatosis. Adenosine receptors may also contribute to fibrosis through extracellular matrix production by fibroblasts. Thus, pharmacological modulation of P1 and P2 receptors and downstream calcium signaling may open novel therapeutic avenues.

P2 X 7 receptor is a critical regulator of extracellular ATP-induced profibrotic genes expression in rat kidney: implication of transforming growth factor-β/Smad signaling pathway

This study was designed to investigate the potential of extracellular adenosine 5'-triphosphate (ATP) via the P2 X 7 receptor to activate the renal fibrotic processes in rats. The present study demonstrates that administration of ATP rapidly activated transforming growth factor-β (TGF-β) to induce phosphorylation of Smad-2/3. Renal connective tissue growth factor (CTGF) and tissue inhibitor of metalloproteinase-1 (TIMP-1) mRNA and protein expressions were also increased following ATP administration. A decrease in TGF-β amount in serum as well as renal Smad-2/3 phosphorylation was noticed in animals pre-treated with the specific antagonist of P2 X 7 receptor, A 438,079. In addition, a significant reduction in mRNA and protein expression of CTGF and TIMP-1were also observed in the kidneys of those animals. Collectively, the current findings demonstrate that ATP has the ability to augment TGF-β-mediated Smad-2/3 phosphorylation and enhance the expression of the pro-fibrotic genes, CTGF and TIMP-1, an effect that is largely mediated via P2 X 7 receptor.

P2X7 receptor is essential for ST36-attenuated cardiac fibrosis upon beta-adrenergic insult

P2X7 receptor (P2X7R) plays an important role in modulating inflammation and fibrosis, but information is limited whether Zusanli (ST36) can inhibit inflammation and fibrosis by regulating P2X7R. Isoprenaline at 5 mg/kg was subcutaneously injected to wild-type and P2X7R knockout mice for 7 days, while treatment groups received electroacupuncture (EA) stimulation at ST36 for 7 sessions. Following 7-session treatment, Masson's trichrome staining was performed to assess the fibrosis. Morphology, electrocardiogram, and echocardiography were carried out to evaluate the cardiac function and structure. Western blotting, hematoxylin and eosin staining, immunohistochemistry, and biochemical analysis of inflammatory cytokine and transmission electron microscopy were carried out to characterize the effect of ST36 on inflammation. P2X7R was overexpressed in ISO-treated mice. EA at ST36, but not at non-points, reduced ISO-induced cardiac fibrosis, increases in HW/BW, R+S wave relative to mice in ISO groups. In addition, EA at ST36 downregulated ISO-upregulated P2X7R and NLRP3 in ventricle. Moreover, EA reduced cytokines of IL-1β, IL-6, and IL-18 in serum, and inhibited foam cell gathering, inflammatory cell infiltration, and autophagy. However, EA at ST36 failed to attenuate the cardiac fibrosis and hypertrophy in P2X7R knockout mice. In conclusion, EA at ST36 attenuated ISO-induced fibrosis possibly via P2X7R.

The Purinergic Nature of Pseudoxanthoma Elasticum

Pseudoxanthoma Elasticum (PXE) is an inherited disease characterized by elastic fiber calcification in the eyes, the skin and the cardiovascular system. PXE results from mutations in ABCC6 that encodes an ABC transporter primarily expressed in the liver and kidneys. It took nearly 15 years after identifying the gene to better understand the etiology of PXE. ABCC6 function facilitates the efflux of ATP, which is sequentially hydrolyzed by the ectonucleotidases ENPP1 and CD73 into pyrophosphate (PPi) and adenosine, both inhibitors of calcification. PXE, together with General Arterial Calcification of Infancy (GACI caused by ENPP1 mutations) as well as Calcification of Joints and Arteries (CALJA caused by NT5E/CD73 mutations), forms a disease continuum with overlapping phenotypes and shares steps of the same molecular pathway. The explanation of these phenotypes place ABCC6 as an upstream regulator of a purinergic pathway (ABCC6 → ENPP1 → CD73 → TNAP) that notably inhibits mineralization by maintaining a physiological Pi/PPi ratio in connective tissues. Based on a review of the literature and our recent experimental data, we suggest that PXE (and GACI/CALJA) be considered as an authentic "purinergic disease". In this article, we recapitulate the pathobiology of PXE and review molecular and physiological data showing that, beyond PPi deficiency and ectopic calcification, PXE is associated with wide and complex alterations of purinergic systems. Finally, we speculate on the future prospects regarding purinergic signaling and other aspects of this disease.

P2X7 Receptor Regulates Collagen Expression in Human Intestinal Fibroblasts: Relevance in Intestinal Fibrosis

Intestinal fibrosis is a common complication that affects more than 50% of Crohn´s Disease (CD) patients. There is no pharmacological treatment against this complication, with surgery being the only option. Due to the unknown role of P2X7 in intestinal fibrosis, we aim to analyze the relevance of this receptor in CD complications. Surgical resections from CD and non-Inflammatory Bowel Disease (IBD) patients were obtained. Intestinal fibrosis was induced with two different murine models: heterotopic transplant model and chronic-DSS colitis in wild-type and P2X7-/- mice. Human small intestine fibroblasts (HSIFs) were transfected with an siRNA against P2X7 and treated with TGF-β. A gene and protein expression of P2X7 receptor was significantly increased in CD compared to non-IBD patients. The lack of P2X7 in mice provoked an enhanced collagen deposition and increased expression of several profibrotic markers in both murine models of intestinal fibrosis. Furthermore, P2X7-/- mice exhibited a higher expression of proinflammatory cytokines and a lower expression of M2 macrophage markers. Moreover, the transient silencing of the P2X7 receptor in HSIFs significantly induced the expression of Col1a1 and potentiated the expression of Col4 and Col5a1 after TGF-β treatment. P2X7 regulates collagen expression in human intestinal fibroblasts, while the lack of this receptor aggravates intestinal fibrosis.

Ion channels as a therapeutic target for renal fibrosis

Renal ion channel transport and electrolyte disturbances play an important role in the process of functional impairment and fibrosis in the kidney. It is well known that there are limited effective drugs for the treatment of renal fibrosis, and since a large number of ion channels are involved in the renal fibrosis process, understanding the mechanisms of ion channel transport and the complex network of signaling cascades between them is essential to identify potential therapeutic approaches to slow down renal fibrosis. This review summarizes the current work of ion channels in renal fibrosis. We pay close attention to the effect of cystic fibrosis transmembrane conductance regulator (CFTR), transmembrane Member 16A (TMEM16A) and other Cl− channel mediated signaling pathways and ion concentrations on fibrosis, as well as the various complex mechanisms for the action of Ca2+ handling channels including Ca2+-release-activated Ca2+ channel (CRAC), purinergic receptor, and transient receptor potential (TRP) channels. Furthermore, we also focus on the contribution of Na+ transport such as epithelial sodium channel (ENaC), Na+, K+-ATPase, Na+-H+ exchangers, and K+ channels like Ca2+-activated K+ channels, voltage-dependent K+ channel, ATP-sensitive K+ channels on renal fibrosis. Proposed potential therapeutic approaches through further dissection of these mechanisms may provide new therapeutic opportunities to reduce the burden of chronic kidney disease.

Relaxin Inhibits the Cardiac Myofibroblast NLRP3 Inflammasome as Part of Its Anti-Fibrotic Actions via the Angiotensin Type 2 and ATP (P2X7) Receptors

Chronic NLRP3 inflammasome activation can promote fibrosis through its production of interleukin (IL)-1β and IL-18. Conversely, recombinant human relaxin (RLX) can inhibit the pro-fibrotic interactions between IL-1β, IL-18 and transforming growth factor (TGF)-β1. Here, the broader extent by which RLX targeted the myofibroblast NLRP3 inflammasome to mediate its anti-fibrotic effects was elucidated. Primary human cardiac fibroblasts (HCFs), stimulated with TGF-β1 (to promote myofibroblast (HCMF) differentiation), LPS (to prime the NLRP3 inflammasome) and ATP (to activate the NLRP3 inflammasome) (T+L+A) or benzoylbenzoyl-ATP (to activate the ATP receptor; P2X7R) (T+L+Bz), co-expressed relaxin family peptide receptor-1 (RXFP1), the angiotensin II type 2 receptor (AT2R) and P2X7R, and underwent increased protein expression of toll-like receptor (TLR)-4, NLRP3, caspase-1, IL-1β and IL-18. Whilst RLX co-administration to HCMFs significantly prevented the T+L+A- or T+L+Bz-stimulated increase in these end points, the inhibitory effects of RLX were annulled by the pharmacological antagonism of either RXFP1, AT2R, P2X7R, TLR-4, reactive oxygen species (ROS) or caspase-1. The RLX-induced amelioration of left ventricular inflammation, cardiomyocyte hypertrophy and fibrosis in isoproterenol (ISO)-injured mice, was also attenuated by P2X7R antagonism. Thus, the ability of RLX to ameliorate the myofibroblast NLRP3 inflammasome as part of its anti-fibrotic effects, appeared to involve RXFP1, AT2R, P2X7R and the inhibition of TLR-4, ROS and caspase-1.

P2X7 Receptor and Heart Function in a Mouse Model of Systemic Inflammation Due to High Fat Diet

Purpose

Low-grade inflammation contributes to heart failure in obesity or type 2 diabetes mellitus. The P2X7 receptor (P2X7R) is a key regulator of several pro-inflammatory responses in multiple tissues and organs; however, its involvement in the onset of heart dysfunction remains unclear. The study evaluated the role of P2X7R as a cardiac function regulator in C57BL/6J wild-type (WT) and P2X7R knockout (KO) mice by inducing systemic inflammation with high fat diet (HFD).

Methods

Specific parameters of systolic and diastolic function and heart morphology were measured in vivo before animal sacrifice by high-frequency ultrasonographic analysis. Gene and protein expression of cardiac biomarkers associated with inflammatory-oxidative pathways were evaluated by real-time PCR and Western Blotting.

Results

P2X7R-mediated up-regulation of the NLRP3-caspase-1 complex, increased expression of key oxidative stress (NOS-2, TNFα), and chemotactic (MCP-1) mediators were revealed in WT-HFD animals. In KO-HFD mice, such inflammatory-oxidative pathway was silent. Nevertheless, HFD induced in vivo a clear alteration of diastolic pattern (E/A: p < 0.03 vs WT-HFD) and a cardiac morphologic remodelling (left ventricular mass: p < 0.05 vs WT-HFD) only in P2X7R KO animals. Surprisingly, the transcriptional and protein expression of IL-1β and IL-6, usually regulated through P2X7R activation, were significantly higher in KO-HFD than in WT-HFD mice (both p < 0.05). Furthermore, an up-regulation of miR-214 and a down-regulation of miR-126 in heart of HFD-KO mice were observed, suggesting a link between such epigenetic dysregulation and cytokine overexpression as a potential pathophysiologic mechanism concurring to the progressive cardiac dysfunction.

Conclusion

These findings seem to suggest a cardioprotective role of P2X7R toward this tissue-specific inflammatory damage, likely through tissue homeostasis and organ functionality preservation.

P2X7 Receptor Antagonist Reduces Fibrosis and Inflammation in a Mouse Model of Alpha-Sarcoglycan Muscular Dystrophy

Limb-girdle muscular dystrophy R3, a rare genetic disorder affecting the limb proximal muscles, is caused by mutations in the α-sarcoglycan gene (Sgca) and aggravated by an immune-mediated damage, finely modulated by the extracellular (e)ATP/purinoceptors axis. Currently, no specific drugs are available. The aim of this study was to evaluate the therapeutic effectiveness of a selective P2X7 purinoreceptor antagonist, A438079. Sgca knockout mice were treated with A438079 every two days at 3 mg/Kg for 24 weeks. The P2X7 antagonist improved clinical parameters by ameliorating mice motor function and decreasing serum creatine kinase levels. Histological analysis of muscle morphology indicated a significant reduction of the percentage of central nuclei, of fiber size variability and of the extent of local fibrosis and inflammation. A cytometric characterization of the muscle inflammatory infiltrates showed that A438079 significantly decreased innate immune cells and upregulated the immunosuppressive regulatory T cell subpopulation. In α-sarcoglycan null mice, the selective P2X7 antagonist A438079 has been shown to be effective to counteract the progression of the dystrophic phenotype and to reduce the inflammatory response. P2X7 antagonism via selective inhibitors could be included in the immunosuppressant strategies aimed to dampen the basal immune-mediated damage and to favor a better engraftment of gene-cell therapies.

Pathophysiological Role of Purinergic P2X Receptors in Digestive System Diseases

P2X receptors (P2XRs) are trimeric, non-selective cation channels activated by extracellular ATP and widely distributed in the digestive system. P2XRs have an important role in the physiological function of the digestive system, such as neurotransmission, ion transports, proliferation and apoptosis, muscle contraction, and relaxation. P2XRs can be involved in pain mechanisms both centrally and in the periphery and confirmed the association of P2XRs with visceral pain. In the periphery, ATP can be released as a result of tissue injury, visceral distension, or sympathetic activation and can excite nociceptive primary afferents by acting at homomeric P2X(3)R or heteromeric P2X(2/3)R. Thus, peripheral P2XRs, and homomeric P2X(3) and/or heteromeric P2X(2/3)R in particular, constitute attractive targets for analgesic drugs. Recently studies have shown that P2XRs have made significant advances in inflammation and cancer. P2X7R mediates NLRP3 inflammasome activation, cytokine and chemokine release, T lymphocyte survival and differentiation, transcription factor activation, and cell death. The P2X7R is a potent stimulant of inflammation and immunity and a promoter of cancer cell growth. This makes P2X7R an appealing target for anti-inflammatory and anti-cancer therapy. It is believed that with the further study of P2XRs and its subtypes, P2XRs and its specific antagonists will be expected to be widely used in the treatment of human digestive diseases in the future.

P2X7R antagonist protects against renal injury in mice with adriamycin nephropathy

Activation of the purinergic P2X7 receptor (P2X7R) has been associated with the development of experimental nephritis. Therefore, the current study aimed to explore the mechanism of P2X7R in renal injured mice with adriamycin (ADR) nephropathy. The protective effect of a P2X7R antagonist on the kidneys of mice with ADR nephropathy was also evaluated. Nephropathy was induced by a single intravenous injection of ADR (10.5 mg/kg). A total of 6 h before the model was established, the P2X7R antagonist A438079 (100, 200 and 300 µmol/kg) was injected into the mice, which was subsequently administered daily for 1 week by intraperitoneal injection. Subsequently, all mice were sacrificed, after which blood, 24 h-urine and the kidneys were collected. The levels of albumin (ALB) and total cholesterol (TC) in the serum, along with urine protein content at 24 h were determined using an automatic biochemical analyzer. The levels of IL-1β and IL-18 were additionally detected in the renal tissues by ELISA. Moreover, the expression of P2X7R, oxidized (ox)-low density lipoprotein (LDL), C-X-C motif chemokine ligand 16 (CXCL16), Bax, caspase-3 and NLRP3 in renal tissues was detected by immunohistochemistry. Apoptosis in the renal tissues was observed using the TUNEL assay. The results demonstrated that compared with the control group, decreased weight, increased proteinuria, decreased serum ALB and increased serum TC was observed in the ADR group. The expression of IL-1β, IL-18, P2X7R, ox-LDL, CXCL16, Bax, caspase-3 and NLRP3, as well as cellular apoptosis in the renal tissues of the ADR group, was significantly increased in the ADR group compared with the control. However, compared with the ADR group, the changes in all indices in the ADR + A438079 groups were attenuated. Overall, P2X7R, ox-LDL and CXCL16 may be associated with ADR nephropathy, while inhibition of P2X7R may reduce the expression of ox-LDL by downregulating the CXCL16 pathway to alleviate kidney injury in mice with ADR nephropathy. Furthermore, activated P2X7R may promote the release of inflammatory cytokines IL-1β and IL-18 through the downstream P2X7R/NLRP3 pathway and upregulate the expression of Bax and caspase-3 to promote apoptosis, which participates in the process of ADR nephropathy. Inhibiting P2X7R may also reduce the release of IL-1β and IL-18 by downregulating the P2X7R/NLRP3 pathway, downregulating the expression of Bax and caspase-3, and reducing apoptosis, thereby alleviating kidney injury in mice with ADR nephropathy.

The P2X7 purinergic receptor in intervertebral disc degeneration

Mechanisms involved in the development of intervertebral disc (IVD) degeneration are only partially known, thus making the implementation of effective therapies very difficult. In this study, we investigated P2X7 purinergic receptor (P2X7R), NLRP3 inflammasome, and interleukin (IL)-1β expression in IVD specimens at different stages of disease progression, and during the in vitro dedifferentiation process of the primary cells derived thereof. We found that P2X7R, NLRP3, and IL-1β expression was higher in the IVD samples at a more advanced stage of degeneration and in the expanded IVD cells in culture which partially recapitulated the in vivo degeneration process. In IVD cells, the P2X7R showed a striking nuclear localization, while NLRP3 was mainly cytoplasmic. Stimulation with the semiselective P2X7R agonist benzoyl ATP together with lipopolysaccharide treatment triggered P2X7R transfer to the cytoplasm and P2X7R/NLRP3 colocalization. Taken together, these findings support pathophysiological evidence that the degenerated disc is a highly inflamed microenvironment and highlight the P2X7R/NLRP3 axis as a suitable therapeutic target. The immunohistochemical analysis and the assessment of subcellular localization revealed a substantial expression of P2X7R also in normal disc tissue. This gives us the opportunity to contribute to the few studies performed in natively expressed human P2X7R so far, and to understand the possible physiological ATP-mediated P2X7R homeostasis signaling. Therefore, collectively, our findings may offer a new perspective and pave the way for the exploration of a role of P2X7R-mediated purinergic signaling in IVD metabolism that goes beyond its involvement in inflammation.

Purinergic signalling in the urinary bladder - When function becomes dysfunction

Knowledge of the participation of ATP and related purines in urinary tract physiology has been established over the last five decades through the work of many independent groups, inspired by, and building on the pioneering studies of Professor Geoffrey Burnstock and his coworkers. As part of a series of reviews in this tribute edition, the present article summarises our current understanding of purines and purinergic signalling in modulating and regulating urinary tract function. Purinergic mechanisms underlying the origin of bladder pain; sensations of bladder filling and urinary tract motility; and regulation of detrusor smooth muscle contraction are described, encompassing the relevant history of discovery and consolidation of knowledge as methodologies and pharmacological tools have developed. We consider normal physiology, including development and ageing and then move to pathophysiology, discussing the causal and consequential contribution of purinergic signalling mechanism and their constituent components (receptors, signal transduction, effector molecules) to bladder dysfunction.

P2X7 Receptor as a Potential Target for Major Depressive Disorder

Major depressive disorder (MDD) is a common mental disorder. Although the genetic, biochemical, and psychological factors have been related to the development of MDD, it is generally believed that a series of pathological changes in the brain caused by chronic stress is the main cause of MDD. However, the specific mechanisms underlying chronic stress-induced MDD are largely undermined. Recent investigations have found that increased pro-inflammatory cytokines and changes in the inflammatory pathway in the microglia cells in the brain are the potential pathophysiological mechanism of MDD. P2X7 receptor (P2X7R) and its mediated signaling pathway play a key role in microglia activation. The present review aimed to present and discuss the accumulating data on the role of P2X7R in MDD. Firstly, we summarized the research progress in the correlation between P2X7R and MDD. Subsequently, we presented the P2X7R mediated microglia activation in MDD and the role of P2X7R in increased blood-brain barrier (BBB) permeability caused by chronic stress. Lastly, we also discussed the potential mechanism underlying-P2X7R expression changes after chronic stress. In conclusion, P2X7R is a key molecule regulating the activation of microglia. Chronic stress activates microglia in the hippocampus by secreting interleukin- 1β (IL-1β) and other inflammatory cytokines, and increasing the BBB permeability, thus promoting the occurrence and development of MDD, which indicated that P2X7R might be a promising therapeutic target for MDD.

Cardiac Connexin-43 Hemichannels and Pannexin1 Channels: Provocative Antiarrhythmic Targets

Cardiac connexin-43 (Cx43) creates gap junction channels (GJCs) at intercellular contacts and hemi-channels (HCs) at the peri-junctional plasma membrane and sarcolemmal caveolae/rafts compartments. GJCs are fundamental for the direct cardiac cell-to-cell transmission of electrical and molecular signals which ensures synchronous myocardial contraction. The HCs and structurally similar pannexin1 (Panx1) channels are active in stressful conditions. These channels are essential for paracrine and autocrine communication through the release of ions and signaling molecules to the extracellular environment, or for uptake from it. The HCs and Panx1 channel-opening profoundly affects intracellular ionic homeostasis and redox status and facilitates via purinergic signaling pro-inflammatory and pro-fibrotic processes. These conditions promote cardiac arrhythmogenesis due to the impairment of the GJCs and selective ion channel function. Crosstalk between GJCs and HCs/Panx1 channels could be crucial in the development of arrhythmogenic substrates, including fibrosis. Despite the knowledge gap in the regulation of these channels, current evidence indicates that HCs and Panx1 channel activation can enhance the risk of cardiac arrhythmias. It is extremely challenging to target HCs and Panx1 channels by inhibitory agents to hamper development of cardiac rhythm disorders. Progress in this field may contribute to novel therapeutic approaches for patients prone to develop atrial or ventricular fibrillation.

Extracellular adenosine 5'-triphosphate in pulmonary disorders

Adenosine 5'-triphosphate (ATP) is found in every cell of the human body where it plays a critical role in cellular energetics and metabolism. ATP is released from cells under physiologic and pathophysiologic condition; extracellular ATP is rapidly degraded to adenosine 5'-diphosphate (ADP) and adenosine by ecto-enzymes (mainly, CD39 and CD73). Before its degradation, ATP acts as an autocrine and paracrine agent exerting its effects on targeted cells by activating cell surface receptors named P2 Purinergic receptors. The latter are expressed by different cell types in the lungs, the activation of which is involved in multiple pulmonary disorders. This succinct review summarizes the role of ATP in inflammation processes associated with these disorders including bronchoconstriction, cough, mechanical ventilation-induced lung injury and idiopathic pulmonary fibrosis. All of these disorders still constitute unmet clinical needs. Therefore, the various ATP-signaling pathways in pulmonary inflammation constitute attractive targets for novel drug-candidates that would improve the management of patients with multiple pulmonary diseases.

The P2X7 Receptor and NLRP3 Axis in Non-Alcoholic Fatty Liver Disease: A Brief Review

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide, and its prevalence is reaching epidemic characteristics both in adults and in children. The increase of NAFLD prevalence parallels that of obesity, now representing the major cause of liver inflammation, increasing the risk of cirrhosis and hepatocarcinoma. Furthermore, NAFLD is a risk factor for cardiovascular diseases and type 2 diabetes, two of the major leading causes of morbidity and mortality in western countries. Thus a significant amount of studies have dealt with the evaluation of the possible molecular mechanisms leading to NAFLD and its inflammatory consequences within the liver, the non-alcoholic steatohepatitis, and cirrhosis. The inflammasome is a key player in the inflammation and fibrogenic responses in many different tissues, including the liver. The activation of the NLRP3 inflammasome requires the activation by extracellular adenosine tri-phosphate (ATP) of a specific purinergic receptor named P2X7 located in the target cells, although other pathways have been described. To this regard, extracellular ATP acts as an internal danger signal coming from damaged cells participating in the activation of the inflammatory process, a signaling pathway common to many different tissues. Here, we briefly review the involvement of the P2X7 receptor/inflammasome NLRP3 axis in the pathophysiological events leading to NAFLD and its inflammatory and fibrotic evolutions, reporting the possible therapeutical strategies targeting the P2X7 receptor/NLRP3 inflammasome.

Polyhexamethylene Guanidine Phosphate Damages Tight Junctions and the F-Actin Architecture by Activating Calpain-1 via the P2RX7/Ca2+ Signaling Pathway

Polyhexamethylene guanidine phosphate (PHMG-p), a member of the polymeric guanidine family, has strong antimicrobial activity and may increase the risk of inflammation-associated pulmonary fibrosis. However, the effect of PHMG-p on the barrier function of the bronchial epithelium is unknown. Epithelial barrier functioning is maintained by tight junctions (TJs); damage to these TJs is the major cause of epithelial barrier breakdown during lung inflammation. The present study showed that, in BEAS-2B human bronchial epithelial cells, exposure to PHMG-p reduced the number of TJs and the E-cadherin level and impaired the integrity of the F-actin architecture. Furthermore, exposure to PHMG-p stimulated the calcium-dependent protease calpain-1, which breaks down TJs. However, treatment with the calpain-1 inhibitor, ALLN, reversed the PHMG-p-mediated impairment of TJs and the F-actin architecture. Furthermore, exposure to PHMG-p increased the intracellular Ca²⁺ level via P2X purinoreceptor 7 (P2RX7) and inhibition of P2RX7 abolished the PHMG-p-induced calpain-1 activity and protein degradation and increased the intracellular Ca²⁺ level. Although exposure to PHMG-p increased the extracellular ATP level, hydrolysis of extracellular ATP by apyrase did not influence its detrimental effect on bronchial epithelial cells. These results implicate the impairment of TJs and the F-actin architecture in the pathogenesis of pulmonary diseases.

Shaping of Innate Immune Response by Fatty Acid Metabolite Palmitate

Innate immune cells monitor invading pathogens and pose the first-line inflammatory response to coordinate with adaptive immunity for infection removal. Innate immunity also plays pivotal roles in injury-induced tissue remodeling and the maintenance of tissue homeostasis in physiological and pathological conditions. Lipid metabolites are emerging as the key players in the regulation of innate immune responses, and recent work has highlighted the importance of the lipid metabolite palmitate as an essential component in this regulation. Palmitate modulates innate immunity not only by regulating the activation of pattern recognition receptors in local innate immune cells, but also via coordinating immunological activity in inflammatory tissues. Moreover, protein palmitoylation controls various cellular physiological processes. Herein, we review the updated evidence that palmitate catabolism contributes to innate immune cell-mediated inflammatory processes that result in immunometabolic disorders.

Eosinophil depletion suppresses radiation-induced small intestinal fibrosis

Radiation-induced intestinal fibrosis (RIF) is a serious complication after abdominal radiotherapy for pelvic tumor or peritoneal metastasis. Herein, we show that RIF is mediated by eosinophil interactions with α-smooth muscle actin-positive (α-SMA⁺) stromal cells. Abdominal irradiation caused RIF especially in the submucosa (SM) of the small intestine, which was associated with the excessive accumulation of eosinophils in both human and mouse. Eosinophil-deficient mice showed markedly ameliorated RIF, suggesting the importance of eosinophils. After abdominal irradiation, chronic crypt cell death caused elevation of extracellular adenosine triphosphate, which in turn activated expression of C-C motif chemokine 11 (CCL11) by pericryptal α-SMA⁺ cells in the SM to attract eosinophils in mice. Inhibition of C-C chemokine receptor 3 (CCR3) by genetic deficiency or neutralizing antibody (Ab) treatment suppressed eosinophil accumulation in the SM after irradiation in mice, suggesting a critical role of the CCL11/CCR3 axis in the eosinophil recruitment. Activated α-SMA⁺ cells also expressed granulocyte-macrophage colony-stimulating factor (GM-CSF) to activate eosinophils. Transforming growth factor-β1 from GM-CSF-stimulated eosinophils promoted collagen expression by α-SMA⁺ cells. In translational studies, treatment with a newly developed interleukin-5 receptor α-targeting Ab, analogous to the human agent benralizumab, depleted intestinal eosinophils and suppressed RIF in mice. Collectively, we identified eosinophils as a crucial factor in the pathogenesis of RIF and showed potential therapeutic strategies for RIF by targeting eosinophils.

Ca2+ Signaling in Cardiac Fibroblasts and Fibrosis-Associated Heart Diseases

Cardiac fibrosis is the excessive deposition of extracellular matrix proteins by cardiac fibroblasts and myofibroblasts, and is a hallmark feature of most heart diseases, including arrhythmia, hypertrophy, and heart failure. This maladaptive process occurs in response to a variety of stimuli, including myocardial injury, inflammation, and mechanical overload. There are multiple signaling pathways and various cell types that influence the fibrogenesis cascade. Fibroblasts and myofibroblasts are central effectors. Although it is clear that Ca2+ signaling plays a vital role in this pathological process, what contributes to Ca2+ signaling in fibroblasts and myofibroblasts is still not wholly understood, chiefly because of the large and diverse number of receptors, transporters, and ion channels that influence intracellular Ca2+ signaling. Intracellular Ca2+ signals are generated by Ca2+ release from intracellular Ca2+ stores and by Ca2+ entry through a multitude of Ca2+-permeable ion channels in the plasma membrane. Over the past decade, the transient receptor potential (TRP) channels have emerged as one of the most important families of ion channels mediating Ca2+ signaling in cardiac fibroblasts. TRP channels are a superfamily of non-voltage-gated, Ca2+-permeable non-selective cation channels. Their ability to respond to various stimulating cues makes TRP channels effective sensors of the many different pathophysiological events that stimulate cardiac fibrogenesis. This review focuses on the mechanisms of Ca2+ signaling in fibroblast differentiation and fibrosis-associated heart diseases and will highlight recent advances in the understanding of the roles that TRP and other Ca2+-permeable channels play in cardiac fibrosis.

P2X7 receptor in cardiovascular disease: The heart side

The P2X7 receptor is a ligand-gated purinergic receptor activated by extracellular ATP. The receptor is highly expressed in immune cells and in the brain, and, upon activation, the P2X7 receptor allows a cation flux, leading to the distinct activation of intracellular signalling pathways as the secretion of pro-inflammatory cytokines, and modulation of cell survival. Through these molecular mechanisms, P2X7 is known to play important roles in physiology and pathophysiology of a wide spectrum of diseases, including cancer, inflammatory diseases, neurological, respiratory and more recently cardiovascular diseases. Recent studies demonstrated that the P2X7 could modulate the assembly of the NLRP3 inflammasome, leading to the secretion of pro-inflammatory factors and worsen the cardiac disease phenotypes. This review discusses the critical molecular function of P2X7 in the modulation of the onset, progression and resolution of cardiovascular diseases and analyses the putative future use of P2X7-based therapies that modulate the IL-1β secretion arm and direct P2X7 antagonists.

The P2X4 purinergic receptor regulates hepatic myofibroblast activation during liver fibrogenesis

BACKGROUND & AIMS

Liver fibrosis is characterized by the accumulation of extracellular matrix produced by hepatic myofibroblasts (hMF), the activation of which is critical to the fibrogenic process. Extracellular ATP, released by dying or stressed cells, and its purinergic receptors, constitute a powerful signaling network after injury. Although the purinergic receptor P2X4 (P2RX4) is highly expressed in the liver, its functions in hMF had never been investigated during liver fibrogenesis.

METHODS

In vivo, bile duct ligation was performed and methionine- and choline-deficient diet administered in wild-type and P2x4 knock-out (P2x4-KO) mice. In vitro, hMF were isolated from mouse (wild-type and P2x4-KO) and human liver. P2X4 pharmacological inhibition (in vitro and in vivo) and P2X4 siRNAs (in vitro) were used. Histological, biochemical and cell culture analysis allowed us to study P2X4 expression and its involvement in the regulation of fibrogenic and fibrolytic factors, as well as of hMF activation markers and properties.

RESULTS

P2X4 genetic invalidation or pharmacological inhibition protected mice from liver fibrosis and hMF accumulation after bile duct ligation or methionine- and choline-deficient diet. Human and mouse hMFs expressed P2X4, mainly in lysosomes. Invalidation of P2X4 in human and mouse hMFs blunted their activation marker expression and their fibrogenic properties. Finally, we showed that P2X4 regulates calcium entry and lysosomal exocytosis in hMF, impacting on ATP release, profibrogenic secretory profile, and transcription factor activation.

CONCLUSION

P2X4 expression and activation is critical for hMF to sustain their activated and fibrogenic phenotype. Therefore, the inactivation of P2X4 may be of therapeutic interest during liver fibrotic diseases.

LAY SUMMARY

During chronic injury, the liver often repairs with fibrotic tissue, which impairs liver function, and for which there is currently no treatment. We found that a previously unexplored pathway involving the purinergic receptor P2X4, can modulate fibrotic liver repair. Therefore, this receptor could be of interest in the development of novel therapies for fibrotic liver diseases.

Liver kinase B1/AMP-activated protein kinase-mediated regulation by gentiopicroside ameliorates P2X7 receptor-dependent alcoholic hepatosteatosis

BACKGROUND AND PURPOSE

Regulating P2X7 receptor-mediated activation of NLRP3 inflammasomes could be a therapeutic strategy to treat alcoholic hepatosteatosis. We investigated whether this process was modulated by gentiopicroside, the main active secoiridoid glycoside from Gentiana manshurica Kitagawa.

EXPERIMENTAL APPROACH

In vivo models of acute and chronic alcoholic hepatosteatosis were established by intragastrically administered ethanol or using chronic plus binge ethanol feeding of Lieber-DeCarli liquid diet to male C57BL/6 mice. In vitro, HepG2 cells were treated with ethanol. RAW 264.7 macrophages and murine bone marrow-derived macrophages (BMDMs) were stimulated with LPS and ATP.

KEY RESULTS

In both the acute and chronic alcohol-induced mouse hepatosteatosis models, gentiopicroside decreased serum aminotransferases and triglyceride accumulation. Up-regulated SREBP1, down-regulated PPARα and phosphorylated acetyl-CoA carboxylase caused by acute and chronic alcohol feeding were modulated by gentiopicroside, through the elevation of LKB1 and AMPK. Suppression of P2X7 receptor-NLRP3 activation by gentiopicroside inhibited IL-1β production. In ethanol-exposed HepG2 cells, gentiopicroside reduced lipogenesis and promoted lipid oxidation via activation of P2X7 receptor-NLRP3 inflammasomes. Genetic or pharmacological blockade of P2X7 receptors enhanced AMPK activity and reduced SREBP1 expression in ethanol-treated HepG2 cells. Gentiopicroside down-regulated P2X7 receptor-mediated inflammatory responses in LPS/ATP-stimulated RAW 264.7 macrophages and BMDMs. IL-1β from macrophages accelerated lipid accumulation in hepatocytes. Depleting macrophages by clodronate liposomes ameliorated alcoholic hepatosteatosis, and it was further alleviated by gentiopicroside.

CONCLUSIONS AND IMPLICATIONS

Activation of LKB1/AMPK signalling by gentiopicroside was mediated by the P2X7 receptor-NLRP3 inflammasome, suggesting the therapeutic value of blocking P2X7 receptors in the treatment of alcoholic hepatosteatosis.

The potential of P2X7 receptors as a therapeutic target, including inflammation and tumour progression

Seven P2X ion channel nucleotide receptor subtypes have been cloned and characterised. P2X7 receptors (P2X7R) are unusual in that there are extra amino acids in the intracellular C terminus. Low concentrations of ATP open cation channels sometimes leading to cell proliferation, whereas high concentrations of ATP open large pores that release inflammatory cytokines and can lead to apoptotic cell death. Since many diseases involve inflammation and immune responses, and the P2X7R regulates inflammation, there has been recent interest in the pathophysiological roles of P2X7R and the potential of P2X7R antagonists to treat a variety of diseases. These include neurodegenerative diseases, psychiatric disorders, epilepsy and a number of diseases of peripheral organs, including the cardiovascular, airways, kidney, liver, bladder, skin and musculoskeletal. The potential of P2X7R drugs to treat tumour progression is discussed.

The P2X7 Receptor in Inflammatory Diseases: Angel or Demon?

Under physiological conditions, adenosine triphosphate (ATP) is present at low levels in the extracellular milieu, being massively released by stressed or dying cells. Once outside the cells, ATP and related nucleotides/nucleoside generated by ectonucleotidases mediate a high evolutionary conserved signaling system: the purinergic signaling, which is involved in a variety of pathological conditions, including inflammatory diseases. Extracellular ATP has been considered an endogenous adjuvant that can initiate inflammation by acting as a danger signal through the activation of purinergic type 2 receptors-P2 receptors (P2Y G-protein coupled receptors and P2X ligand-gated ion channels). Among the P2 receptors, the P2X7 receptor is the most extensively studied from an immunological perspective, being involved in both innate and adaptive immune responses. P2X7 receptor activation induces large-scale ATP release via its intrinsic ability to form a membrane pore or in association with pannexin hemichannels, boosting purinergic signaling. ATP acting via P2X7 receptor is the second signal to the inflammasome activation, inducing both maturation and release of pro-inflammatory cytokines, such as IL-1β and IL-18, and the production of reactive nitrogen and oxygen species. Furthermore, the P2X7 receptor is involved in caspases activation, as well as in apoptosis induction. During adaptive immune response, P2X7 receptor modulates the balance between the generation of T helper type 17 (Th17) and T regulatory (Treg) lymphocytes. Therefore, this receptor is involved in several inflammatory pathological conditions. In infectious diseases and cancer, P2X7 receptor can have different and contrasting effects, being an angel or a demon depending on its level of activation, cell studied, type of pathogen, and severity of infection. In neuroinflammatory and neurodegenerative diseases, P2X7 upregulation and function appears to contribute to disease progression. In this review, we deeply discuss P2X7 receptor dual function and its pharmacological modulation in the context of different pathologies, and we also highlight the P2X7 receptor as a potential target to treat inflammatory related diseases.

Modulation of the TGF-β1-induced epithelial to mesenchymal transition (EMT) mediated by P1 and P2 purine receptors in MDCK cells

Epithelial to mesenchymal transition (EMT) occurs during embryogenesis or under pathological conditions such as hypoxia, injury, chronic inflammation, or tissue fibrosis. In renal tubular epithelial cells (MDCK), TGF-β1 induces EMT by reducing or increasing epithelial or mesenchymal marker expression, respectively. In this study, we confirmed that the cAMP analogues, 8-CPT-cAMP or N6-Ph-cAMP, inhibited the TGF-β1-driven overexpression of the mesenchymal markers ZEB-1, Slug, Fibronectin, and α-SMA. Furthermore, we showed that A1, A2A, P2Y1, P2Y11, and P2X7 purine receptor agonists modulated the TGF-β1-induced EMT through the involvement of PKA and/or MAPK/ERK signaling. The stimulation of A2A receptor reduced the overexpression of the EMT-related markers, mainly through the cAMP-dependent PKA pathway, as confirmed by cell pre-treatment with Myr-PKI. Both A1 and P2Y1 receptor stimulation exacerbated the TGF-β1-driven effects, which were reduced by cell pre-treatment with the MAPK inhibitor PD98059, according to the increased ERK1/2 phosphorylation upon receptor activation. The effects induced by P2Y11 receptor activation were oppositely modulated by PKA or MAPK inhibition, in line with the dual nature of the Gs- and Gq-coupled receptor. Differently, P2X7 receptor induced, per se, similar and not additive effects compared to TGF-β1, after prolonged cell exposure to BzATP. These results suggest a putative role of purine receptors as target for anti-fibrotic agents.

P2X7R Blockade Prevents NLRP3 Inflammasome Activation and Pancreatic Fibrosis in a Mouse Model of Chronic Pancreatitis

OBJECTIVES

The aim of this study was to investigate the role of P2X7R (purinergic 2X7 receptor) and NLRP3 (NACHT, LRR, and PYD domains-containing protein 3) inflammasome activation in the process of pancreatic fibrosis in a mouse model of chronic pancreatitis (CP).

METHODS

Chronic pancreatitis was induced by repeated intraperitoneal injections of 50 μg/kg cerulein for 6 weeks in mice. P2X7R antagonist oxidized ATP (OxATP) or brilliant blue G (BBG) was administered after the last cerulein injection for 2 weeks. Pancreatic chronic inflammation and fibrosis were evaluated by histological score, Sirius red staining, and alpha-smooth muscle actin immunohistochemical staining. We further determined pancreatic P2X7R, NLRP3, and caspase-1 expressions in gene and protein levels and the pancreatic concentrations of caspase-1, interleukin 1β (IL-1β), and IL-18.

RESULTS

The pancreatic P2X7R, NLRP3, and caspase-1 expressions in gene and protein levels and the pancreatic concentrations of caspase-1, IL-1β, and IL-18 were all reduced significantly in both the OxATP and BBG groups (P < 0.05). The pancreatic chronic inflammation and the fibrosis indices were all remarkably attenuated (P < 0.05).

CONCLUSIONS

P2X7R antagonist OxATP and BBG significantly decreased pancreatic chronic inflammation and fibrosis in a mouse CP model and suggested that blockade of P2X7R-NLRP3 inflammasome signaling pathway may represent a novel therapeutic strategy for CP and its fibrotic process.

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

Searching Novel Therapeutic Targets for Scleroderma: P2X7-Receptor Is Up-regulated and Promotes a Fibrogenic Phenotype in Systemic Sclerosis Fibroblasts

Objectives: Systemic sclerosis (SSc) is a connective tissue disorder presenting fibrosis of the skin and internal organs, for which no effective treatments are currently available. Increasing evidence indicates that the P2X7 receptor (P2X7R), a nucleotide-gated ionotropic channel primarily involved in the inflammatory response, may also have a key role in the development of tissue fibrosis in different body districts. This study was aimed at investigating P2X7R expression and function in promoting a fibrogenic phenotype in dermal fibroblasts from SSc patients, also analyzing putative underlying mechanistic pathways. Methods: Fibroblasts were isolated by skin biopsy from 9 SSc patients and 8 healthy controls. P2X7R expression, and function (cytosolic free Ca²⁺ fluxes, α-smooth muscle actin [α-SMA] expression, cell migration, and collagen release) were studied. Moreover, the role of cytokine (interleukin-1β, interleukin-6) and connective tissue growth factor (CTGF) production, and extracellular signal-regulated kinases (ERK) activation in mediating P2X7R-dependent pro-fibrotic effects in SSc fibroblasts was evaluated. Results: P2X7R expression and Ca²⁺ permeability induced by the selective P2X7R agonist 2'-3'-O-(4-benzoylbenzoyl)ATP (BzATP) were markedly higher in SSc than control fibroblasts. Moreover, increased αSMA expression, cell migration, CTGF, and collagen release were observed in lipopolysaccharides-primed SSc fibroblasts after BzATP stimulation. While P2X7-induced cytokine changes did not affect collagen production, it was completely abrogated by inhibition of the ERK pathway. Conclusion: In SSc fibroblasts, P2X7R is overexpressed and its stimulation induces Ca²⁺-signaling activation and a fibrogenic phenotype characterized by increased migration and collagen production. These data point to the P2X7R as a potential, novel therapeutic target for controlling exaggerated collagen deposition and tissue fibrosis in patients with SSc.

Regulation of P2X Purinergic Receptor Signaling by Cholesterol

P2X receptors are cation-selective channels that are activated by the binding of extracellular ATP. They have a high permeability to Ca²⁺, Na⁺, and K⁺ and are expressed widely throughout the nervous, immune, cardiovascular, skeletal, gastrointestinal, respiratory, and endocrine systems. Seven mammalian subtypes of P2X receptor subunits have been identified, P2X1-7, and those that function as homotrimeric receptors (P2X1, 2, 3, 4, and 7) are targeted to lipid rafts, although they show limited resistance to solubilization by Triton X-100. Recent crystal structures of P2X3 and P2X4 receptors have provided considerable high-resolution information about the architecture of this family of receptors and yet the molecular details of how they are regulated by cholesterol are unknown. Currents mediated by the P2X1-4 receptors are either inhibited or relatively insensitive to cholesterol depletion, but there is no clear evidence to support the direct binding of cholesterol to these receptors. In contrast, the activity of the low-affinity, proinflammatory P2X7 receptor is potentiated by cholesterol depletion and regions within the proximal C-terminus play an important role in coupling changes in cholesterol to the gating of the pore. Based upon our understanding of the lipid signaling events that are triggered downstream of P2X7 receptor activation, a change in the levels of cholesterol may contribute to the sensitization of receptor currents and the dilation of the pore that occurs following prolonged, high-level stimulation. This chapter focuses on the regulation of P2X7 receptor signaling by cholesterol and our current understanding of the mechanisms that underlie this.