Pannexin1 contributes to pathophysiological ATP release in lipoapoptosis induced by saturated free fatty acids in liver cells

PANX1-mediated ATP release confers FAM3A's suppression effects on hepatic gluconeogenesis and lipogenesis

BACKGROUND

Extracellular adenosine triphosphate (ATP) is an important signal molecule. In previous studies, intensive research had revealed the crucial roles of family with sequence similarity 3 member A (FAM3A) in controlling hepatic glucolipid metabolism, islet β cell function, adipocyte differentiation, blood pressure, and other biological and pathophysiological processes. Although mitochondrial protein FAM3A plays crucial roles in the regulation of glucolipid metabolism via stimulating ATP release to activate P2 receptor pathways, its mechanism in promoting ATP release in hepatocytes remains unrevealed.

METHODS

db/db, high-fat diet (HFD)-fed, and global pannexin 1 (PANX1) knockout mice, as well as liver sections of individuals, were used in this study. Adenoviruses and adeno-associated viruses were utilized for in vivo gene overexpression or inhibition. To evaluate the metabolic status in mice, oral glucose tolerance test (OGTT), pyruvate tolerance test (PTT), insulin tolerance test (ITT), and magnetic resonance imaging (MRI) were conducted. Protein-protein interactions were determined by coimmunoprecipitation with mass spectrometry (MS) assays.

RESULTS

In livers of individuals and mice with steatosis, the expression of ATP-permeable channel PANX1 was increased (P < 0.01). Hepatic PANX1 overexpression ameliorated the dysregulated glucolipid metabolism in obese mice. Mice with hepatic PANX1 knockdown or global PANX1 knockout exhibited disturbed glucolipid metabolism. Restoration of hepatic PANX1 rescued the metabolic disorders of PANX1-deficient mice (P < 0.05). Mechanistically, ATP release is mediated by the PANX1-activated protein kinase B-forkhead box protein O1 (Akt-FOXO1) pathway to inhibit gluconeogenesis via P2Y receptors in hepatocytes. PANX1-mediated ATP release also activated calmodulin (CaM) (P < 0.01), which interacted with c-Jun N-terminal kinase (JNK) to inhibit its activity, thereby deactivating the transcription factor activator protein-1 (AP1) and repressing fatty acid synthase (FAS) expression and lipid synthesis (P < 0.05). FAM3A stimulated the expression of PANX1 via heat shock factor 1 (HSF1) in hepatocytes (P < 0.05). Notably, FAM3A overexpression failed to promote ATP release, inhibit the expression of gluconeogenic and lipogenic genes, and suppress gluconeogenesis and lipid deposition in PANX1-deficient hepatocytes and livers.

CONCLUSIONS

PANX1-mediated release of ATP plays a crucial role in maintaining hepatic glucolipid homeostasis, and it confers FAM3A's suppressive effects on hepatic gluconeogenesis and lipogenesis.

Pannexins in the musculoskeletal system: new targets for development and disease progression

During cell differentiation, growth, and development, cells can respond to extracellular stimuli through communication channels. Pannexin (Panx) family and connexin (Cx) family are two important types of channel-forming proteins. Panx family contains three members (Panx1-3) and is expressed widely in bone, cartilage and muscle. Although there is no sequence homology between Panx family and Cx family, they exhibit similar configurations and functions. Similar to Cxs, the key roles of Panxs in the maintenance of physiological functions of the musculoskeletal system and disease progression were gradually revealed later. Here, we seek to elucidate the structure of Panxs and their roles in regulating processes such as osteogenesis, chondrogenesis, and muscle growth. We also focus on the comparison between Cx and Panx. As a new key target, Panxs expression imbalance and dysfunction in muscle and the therapeutic potentials of Panxs in joint diseases are also discussed.

Targeting Pannexin-1 Channels: Addressing the 'Gap' in Chronic Pain

Chronic pain complicates many diseases and is notoriously difficult to treat. In search of new therapeutic targets, pannexin-1 (Panx1) channels have sparked intense interest as a key mechanism involved in a variety of chronic pain conditions. Panx1 channels are transmembrane proteins that release ions and small molecules, such as adenosine triphosphate (ATP). They are expressed along important nodes of the pain pathway, modulating activity of diverse cell types implicated in the development and progression of chronic pain caused by injury or pathology. This review highlights advances that have unlocked the core structure and machinery controlling Panx1 function with a focus on understanding and treating chronic pain.

Hepatic Macrophages as Targets for the MSC-Based Cell Therapy in Non-Alcoholic Steatohepatitis

Non-alcoholic steatohepatitis (NASH) is a serious public health issue associated with the obesity pandemic. Obesity is the main risk factor for the non-alcoholic fatty liver disease (NAFLD), which progresses to NASH and then to end-stage liver disease. Currently, there are no specific pharmacotherapies of NAFLD/NASH approved by the FDA or other national regulatory bodies and the treatment includes lifestyle adjustment and medicines for improving lipid metabolism, enhancing sensitivity to insulin, balancing oxidation, and counteracting fibrosis. Accordingly, further basic research and development of new therapeutic approaches are greatly needed. Mesenchymal stem cells (MSCs) and MSC-derived extracellular vesicles prevent induced hepatocyte death in vitro and attenuate NASH symptoms in animal models of the disease. They interact with hepatocytes directly, but also target other liver cells, including Kupffer cells and macrophages recruited from the blood flow. This review provides an update on the pathogenesis of NAFLD/NASH and the key role of macrophages in the development of the disease. We examine in detail the mechanisms of the cross-talk between the MSCs and the macrophages, which are likely to be among the key targets of MSCs and their derivatives in the course of NAFLD/NASH cell therapy.

Bovine Milk Proteome: Milk Fat Globule Membrane Protein Is the Most Sensitive Fraction in Response to High Somatic Cell Count

The impacts of high milk somatic cell count (SCC) on different milk fractions are not well understood. In this study, proteins in milk exosomes, milk fat globule membrane (MFGM), and whey from cows with low (<105 cells/mL, CG) and high SCC (>5 × 105 cells/mL, HSG) were identified using a tandem mass tag proteomic approach. In total, 1568, 2160, and 1002 proteins were identified, with 65, 552, and 98 proteins being altered by high SCC in exosomes, MFGM, and whey, respectively. With high SCC, the exosome marker (ACTB) was increased in the exosomes of HSG. The main MFGM proteins (BTN1A1, PLIN3, FABP3, and MFGE8) and functional proteins (MUC1, IGSF5, TLR5, and CD36/14) were decreased, while the lipid/energy metabolism-related proteins were increased in the MFGM of HSG. The glycolysis-related proteins were increased in the whey of HSG. Also, the host defense/inflammation-related proteins were changed in three fractions under high SCCs. MFGM was the most sensitive fraction to a high SCC, followed by whey. These findings provide guidance for the early detection of unhealthy mammary glands.

Pannexin1: insight into inflammatory conditions and its potential involvement in multiple organ dysfunction syndrome

Sepsis represents a global health concern, and patients with severe sepsis are at risk of experiencing MODS (multiple organ dysfunction syndrome), which is associated with elevated mortality rates and a poorer prognosis. The development of sepsis involves hyperactive inflammation, immune disorder, and disrupted microcirculation. It is crucial to identify targets within these processes to develop therapeutic interventions. One such potential target is Panx1 (pannexin-1), a widely expressed transmembrane protein that facilitates the passage of molecules smaller than 1 KDa, such as ATP. Accumulating evidence has implicated the involvement of Panx1 in sepsis-associated MODS. It attracts immune cells via the purinergic signaling pathway, mediates immune responses via the Panx1-IL-33 axis, promotes immune cell apoptosis, regulates blood flow by modulating VSMCs' and vascular endothelial cells' tension, and disrupts microcirculation by elevating endothelial permeability and promoting microthrombosis. At the level of organs, Panx1 contributes to inflammatory injury in multiple organs. Panx1 primarily exacerbates injury and hinders recovery, making it a potential target for sepsis-induced MODS. While no drugs have been developed explicitly against Panx1, some compounds that inhibit Panx1 hemichannels have been used extensively in experiments. However, given that Panx1's role may vary during different phases of sepsis, more investigations are required before interventions against Panx1 can be applied in clinical. Overall, Panx1 may be a promising target for sepsis-induced MODS. Nevertheless, further research is needed to understand its complex role in different stages of sepsis fully and to develop suitable pharmaceutical interventions for clinical use.

Pannexin1 channels in the liver: an open enemy

Pannexin1 proteins form communication channels at the cell plasma membrane surface, which allow the transfer of small molecules and ions between the intracellular compartment and extracellular environment. In this way, pannexin1 channels play an important role in various cellular processes and diseases. Indeed, a plethora of human pathologies is associated with the activation of pannexin1 channels. The present paper reviews and summarizes the structure, life cycle, regulation and (patho)physiological roles of pannexin1 channels, with a particular focus on the relevance of pannexin1 channels in liver diseases.

Extracellular ATP: A powerful inflammatory mediator in the central nervous system

Nucleotides play a crucial role in extracellular signaling across species boundaries. All the three kingdoms of life (Bacteria, Archea and Eukariota) are responsive to extracellular ATP (eATP) and many release this and other nucleotides. Thus, eATP fulfills different functions, many related to danger-sensing or avoidance reactions. Basically all living organisms have evolved sensors for eATP and other nucleotides with very different affinity and selectivity, thus conferring a remarkable plasticity to this signaling system. Likewise, different intracellular transduction systems were associated during evolution to different receptors for eATP. In mammalian evolution, control of intracellular ATP (iATP) and eATP homeostasis has been closely intertwined with that of Ca²⁺, whether in the extracellular milieu or in the cytoplasm, establishing an inverse reciprocal relationship, i.e. high extracellular Ca²⁺ levels are associated to negligible eATP, while low intracellular Ca²⁺ levels are associated to high eATP concentrations. This inverse relationship is crucial for the messenger functions of both molecules. Extracellular ATP is sensed by specific plasma membrane receptors of widely different affinity named P2 receptors (P2Rs) of which 17 subtypes are known. This confers a remarkable plasticity to P2R signaling. The central nervous system (CNS) is a privileged site for purinergic signaling as all brain cell types express P2Rs. Accruing evidence suggests that eATP, in addition to participating in synaptic transmission, also plays a crucial homeostatic role by fine tuning microglia, astroglia and oligodendroglia responses. Drugs modulating the eATP concentration in the CNS are likely to be the new frontier in the therapy of neuroinflammation. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Early urate-lowering therapy in gouty arthritis with acute flares: a double-blind placebo controlled clinical trial

BACKGROUND

Gouty arthritis (GA) is a chronic systemic disease with recurrent acute monoarthritis. In a previous study, a higher incidence of acute flares was observed during the initial marked decrease in serum urate level. Our study evaluated the effect of early urate-lowering therapy in patients with acute GA flares.

METHODS

This study included 40 patients with acute GA; of them, 20 received colchicine 0.5 mg colchicine twice daily, while 20 received probenecid 500 mg and colchicine 0.5 mg twice daily. We evaluated GA severity and laboratory data for 2 weeks after the initial therapy. Medians and interquartile ranges (IQRs) were calculated to evaluate clinical presentations between these two groups.

RESULTS

Rapidly decreasing median serum uric acid levels was found in the patients treated with probenecid and colchicine compared with the patients treated with colchicine alone on day 8 (- 1.9 [IQR, - 3.7 to 0] vs 0.8 [IQR, - 0.1-2.2]; P < 0.001). However, the median decrease in visual analog scale score did not differ significantly between the two groups (- 5.5 [IQR, - 8.0 to - 3.0] vs - 3.5 [IQR, - 5.9 to - 2.0]; P = 0.080).

CONCLUSION

No significant increase was noted in acute gout flare severity or duration among GA patients treated with early aggressive control of hyperuricemia using probenecid plus colchicine.

Identification of MAP3K4 as a novel regulation factor of hepatic lipid metabolism in non-alcoholic fatty liver disease

Background

Non-alcoholic fatty liver disease (NAFLD) is a metabolic disorder with abnormal lipid metabolism. The present study was to identify regulatory genes related to lipid droplets (LDs) abnormal accumulation in NAFLD.

Methods

transcriptomic analysis and bioinformatics analysis (GEO database) were used to identify potential genes in abnormal lipid metabolism of NAFLD. A candidate gene MAP3K4 expression were detected by immunohistochemistry staining in NAFLD and controls. RNA interference and immunoblotting were used to verify the roles of MAP3K4 in the formation of hepatic LDs.

Results

A total of 134 candidate genes were screened, including 44 up-regulated genes and 90 down-regulated genes. 29 genes in the protein–protein interaction (PPI) were selected as hub genes, including MAP3K4. The expression levels of MAP3K4 were positively correlated with NAFLD activity score (r = 0.702, p = 0.002). Furthermore, we found a positive correlation of MAP3K4 expression with serum total cholesterol (r = 0.564, p = 0.023), uric acid levels (r = 0.520, p = 0.039), and body mass index (r = 0.574, p = 0.020). Downregulation of MAP3K4 decreased LDs accumulation in HepG2 cells and reduced the expression of CGI-58 and Plin-2 by imbibition of JNK and group IVA cytosolic phospholipase A2 (cPLA2) activation.

Conclusion

The study revealed a number of regulatory genes related to hepatic lipid metabolism of NAFLD, and demonstrated that MAP3K4 played a pivotal role in the hepatic lipogenesis of NAFLD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-022-03734-8.

Expression of connexins and pannexins in diseased human liver

Connexin proteins can form hexameric hemichannels and gap junctions that mediate paracrine and direct intercellular communication, respectively. Gap junction activity is crucial for the maintenance of hepatic homeostasis, while connexin hemichannels become particularly active in liver disease, such as hepatitis, fibrosis, cholestasis or even hepatocellular carcinoma. Channels consisting of connexin-like proteins named pannexins have been directly linked to liver inflammation and cell death. The goal of the present study was to characterize the expression and subcellular localization of connexins and pannexins in liver of patients suffering from various chronic and neoplastic liver diseases. Specifically, real-time quantitative reverse transcription polymerase chain reaction, immunoblotting and immunohistochemistry analyses were performed on human liver biopsies. It was found that pannexin1 and pannexin2 gene expression are correlated to a certain degree, as is pannexin1 protein expression with connexin32 and connexin43 protein expression. Furthermore, this study is the first to detect pannexin3 in human patient liver biopsies via both immunoblot and immunohistochemistry.

Mechanisms of Pannexin 1 (PANX1) Channel Mechanosensitivity and Its Pathological Roles

Pannexins (PANX) were cloned based on their sequence homology to innexins (Inx), invertebrate gap junction proteins. Although there is no sequence homology between PANX and connexins (Cx), these proteins exhibit similar configurations. The PANX family has three members, PANX1, PANX2 and PANX3. Among them, PANX1 has been the most extensively studied. The PANX1 channels are activated by many factors, including high extracellular K⁺ ([K⁺]_e), high intracellular Ca²⁺ ([Ca²⁺]_i), Src family kinase (SFK)-mediated phosphorylation, caspase cleavage and mechanical stimuli. However, the mechanisms mediating this mechanosensitivity of PANX1 remain unknown. Both force-from-lipids and force-from-filaments models are proposed to explain the gating mechanisms of PANX1 channel mechanosensitivity. Finally, both the physiological and pathological roles of mechanosensitive PANX1 are discussed.

Western Diet Decreases the Liver Mitochondrial Oxidative Flux of Succinate: Insight from a Murine NAFLD Model

Mitochondria play an essential role in the pathogenesis of nonalcoholic fatty liver disease (NAFLD). Previously, we found that succinate-activated respiration was the most affected mitochondrial parameter in mice with mild NAFLD. In this study, we focused on the role of succinate dehydrogenase (SDH) in NAFLD pathogenesis. To induce the progression of NAFLD to nonalcoholic steatohepatitis (NASH), C57BL/6J mice were fed a Western-style diet (WD) or control diet for 30 weeks. NAFLD severity was evaluated histologically and the expression of selected proteins and genes was assessed. Mitochondrial respiration was measured by high-resolution respirometry. Liver redox status was assessed using glutathione, malondialdehyde, and mitochondrial production of reactive oxygen species (ROS). Metabolomic analysis was performed by GC/MS. WD consumption for 30 weeks led to reduced succinate-activated respiration. We also observed decreased SDH activity, decreased expression of the SDH activator sirtuin 3, decreased gene expression of SDH subunits, and increased levels of hepatic succinate, an important signaling molecule. Succinate receptor 1 (SUCNR1) gene and protein expression were reduced in the livers of WD-fed mice. We did not observe signs of oxidative damage compared to the control group. The changes observed in WD-fed mice appear to be adaptive to prevent mitochondrial respiratory chain overload and massive ROS production.

Connexin hemichannel inhibitors with a focus on aminoglycosides

Connexins are membrane proteins involved directly in cell-to-cell communication through the formation of gap-junctional channels. These channels result from the head-to-head docking of two hemichannels, one from each of two adjacent cells. Undocked hemichannels are also present at the plasma membrane where they mediate the efflux of molecules that participate in autocrine and paracrine signaling, but abnormal increase in hemichannel activity can lead to cell damage in disorders such as cardiac infarct, stroke, deafness, cataracts, and skin diseases. For this reason, connexin hemichannels have emerged as a valid therapeutic target. Know small molecule hemichannel inhibitors are not ideal leads for the development of better drugs for clinical use because they are not specific and/or have toxic effects. Newer inhibitors are more selective and include connexin mimetic peptides, anti-connexin antibodies and drugs that reduce connexin expression such as antisense oligonucleotides. Re-purposed drugs and their derivatives are also promising because of the significant experience with their clinical use. Among these, aminoglycoside antibiotics have been identified as inhibitors of connexin hemichannels that do not inhibit gap-junctional channels. In this review, we discuss connexin hemichannels and their inhibitors, with a focus on aminoglycoside antibiotics and derivatives of kanamycin A that inhibit connexin hemichannels, but do not have antibiotic effect.

Extracellular ATP Increases Glucose Metabolism in Skeletal Muscle Cells in a P2 Receptor Dependent Manner but Does Not Contribute to Palmitate-Induced Insulin Resistance

Saturated fatty acids such as palmitate contribute to the development of Type 2 Diabetes by reducing insulin sensitivity, increasing inflammation and potentially contributing to anabolic resistance. We hypothesized that palmitate-induced ATP release from skeletal muscle cells may increase inflammatory cytokine production and contribute to insulin/anabolic resistance in an autocrine/paracrine manner. In C2C12 myotubes differentiated at physiological glucose concentrations (5.5 mM), palmitate treatment (16 h) at concentrations greater than 250 μM increased release of ATP and inflammatory cytokines IL-6 and MIF, significantly blunted insulin and amino acid-induced signaling and reduced mitochondrial function. In contrast to our hypothesis, degradation of extracellular ATP using apyrase, did not alter palmitate-induced insulin resistance nor alter release of cytokines. Moreover, treatment with ATPγS (16 h), a non-hydrolysable ATP analog, in the absence of palmitate, did not diminish insulin sensitivity. Acute treatment with ATPγS produced insulin mimetic roles; increased phosphorylation of PKB (aka AKT), S6K1 and ERK and enhanced GLUT4-mediated glucose uptake in the absence of exogenous insulin. The increases in PKB and S6K1 phosphorylation were completely prevented by pre-incubation with broad spectrum purinergic receptor (P2R) blockers PPADs and suramin but not by P2 × 4 or P2 × 7 blockers 5-BDBD or A-438079, respectively. Moreover, ATPγS increased IL-6 yet decreased MIF release, similar to the cytokine profile produced by exercise. Acute and chronic treatment with ATPγS increased glycolytic rate in a manner that was differentially inhibited by PPADs and suramin, suggesting heterogeneous P2R activation in the control of cellular metabolism. In summary, our data suggest that the palmitate-induced increase in ATP does not contribute to insulin/anabolic resistance in a cell autonomous manner.

Nonesterified free fatty acids enhance the inflammatory response in renal tubules by inducing extracellular ATP release

In proteinuric renal diseases, excessive plasma nonesterified free fatty acids bound to albumin can leak across damaged glomeruli to be reabsorbed by renal proximal tubular cells and cause inflammatory tubular cells damage by as yet unknown mechanisms. The present study was designed to investigate these mechanisms induced by palmitic acid (PA; one of the nonesterified free fatty acids) overload. Our results show that excess PA stimulates ATP release through the pannexin 1 channel in human renal tubule epithelial cells (HK-2), increasing extracellular ATP concentration approximately threefold compared with control. The ATP release is dependent on caspase-3/7 activation induced by mitochondrial reactive oxygen species. Furthermore, extracellular ATP aggravates PA-induced monocyte chemoattractant protein-1 secretion and monocyte infiltration of tubular cells, enlarging the inflammatory response in both macrophages and HK-2 cells via the purinergic P2X7 receptor-mammalian target of rapamycin-forkhead box O1-thioredoxin-interacting protein/NOD-like receptor protein 3 inflammasome pathway. Hence, PA increases mitochondrial reactive oxygen species-induced ATP release and inflammatory stress, which cause a "first hit," while ATP itself is a "second hit" in amplifying the renal tubular inflammatory response. Thus, inhibition of ATP release or the purinergic P2X7 receptor may be an approach to reduce renal inflammation and improve renal function.

Oleic and Linoleic Acids Induce the Release of Neutrophil Extracellular Traps via Pannexin 1-Dependent ATP Release and P2X1 Receptor Activation

Non-esterified fatty acids (NEFAs) such as oleic acid (OA) and linoleic acid (LA) are associated with a higher incidence of infectious diseases such as metritis and mastitis during the bovine peripartum. Fatty acids can induce an increase in the release of ATP, and changes in the expression levels of purinergic receptors in bovine polymorphonuclears (PMN) during peripartum have also been reported. PMN respond to inflammatory processes with production of ROS, release of proteolytic and bactericidal proteins, and formation of neutrophil extracellular traps (NETs). NETs formation is known to require ATP production through glycolysis. Studies have shown that the above-mentioned metabolic changes alter innate immune responses, particularly in PMN. We hypothesized that NEFAs induce the formation of NETs through ATP release by Pannexin 1 and activation of purinergic receptors. In this study, we found that OA and LA induce NET formation and extracellular ATP release. Carbenoxolone, a pannexin-1 (PANX1) inhibitor, reduced OA- and LA-induced ATP release. We also found that P2X1, P2X4, P2X5, P2X7, and PANX1 were expressed at the mRNA level in bovine PMN. Additionally, NEFA-induced NET formation was completely abolished with exposure to NF449, a P2X1 antagonist, and partially inhibited by treatment with etomoxir, an inhibitor of fatty acid oxidation (FAO). Our results suggest that OA and LA induce NET formation and ATP release via PANX1 and activation of P2X1. These new data contribute to explaining the effects of NEFA high concentrations during the transition period of dairy cattle and further understanding of pro-inflammatory effects and outcome of postpartum diseases.

Cellular Interplay as a Consequence of Inflammatory Signals Leading to Liver Fibrosis Development

Inflammation has been known to be an important driver of fibrogenesis in the liver and onset of hepatic fibrosis. It starts off as a process meant to protect the liver from further damage, but it can become the main promoter of liver fibrosis. There are many inflammation-related pathways activated during liver fibrosis that lead to hepatic stellate cells (HSCs) activation and collagen-deposition in the liver. Such events are mostly modulated upstream of HSCs and involve signals from hepatocytes and innate immune cells. One particular event is represented by cell death during liver injury that generates multiple inflammatory signals that further trigger sterile inflammation and enhancement of inflammatory response. The assembly of inflammasome that responds to danger-associated molecular patterns (DAMPs) stimulates the release of pro-inflammatory cytokines and at the same time, initiates programmed cell death called pyroptosis. This review focuses on cellular and molecular mechanisms responsible for initiation and progress of inflammation in the liver.

In vitro effect of Pannexin 1 channel on the invasion and migration of I-10 testicular cancer cells via ERK1/2 signaling pathway

Pannexin (Panx) plays a crucial role in several cellular processes such as immune cell death, cell proliferation, invasion, and migration, apoptosis, and autophagy. However, the role of Panx in regulating cell migration and invasion in testicular cancer remains to be elucidated. In the present study, we determined the correlation between Panx-1 channel function and migration and invasion in I-10 testicular cancer cells. Transwell and wound healing assays showed that inhibition of Panx-1 by carbenoxolone (CBX) and probenecid (PBN) attenuated the migration and invasion of testicular cancer cells in vitro. Moreover, knockdown of Panx-1 with short hairpin RNA (shRNA) remarkably decreased the migration and invasion ability of I-10 cells. In shRNA-transfected cells, extracellular ATP (released through Panx channel) was also found to be decreased. Similarly, overexpression of Panx-1 with mPanx-1 increased the migration and invasion ability of I-10 cells. Moreover, we found that in mPanx-1-transfected cells treated with U0126 (inhibitor of p-ERK1/2), the migration and invasion of I-10 cells were remarkably attenuated. Overall, increased Panx-1 promotes migration and invasion in testicular cancer cells, and the effect is probably be related with ERK1/2 kinase activity. Thus, Panx-1 can serve as a potential therapeutic target for the treatment of testicular cancer.

Gap junctions in liver disease: Implications for pathogenesis and therapy

In the normal liver, cells interact closely through gap junctions. By providing a pathway for the trafficking of low molecular mass molecules, these channels contribute to tissue homeostasis and maintenance of hepatic function. Thus, dysfunction of gap junctions affects a wide variety of liver processes, such as differentiation, cell death, inflammation and fibrosis. In fact, dysfunctional gap junctions have been implicated, for more than a decade, in cholestatic disease, hepatic cancer and cirrhosis. Additionally, in recent years there is an increasing body of evidence that these channels are also involved in other relevant and prevalent liver pathological processes, such as non-alcoholic fatty liver disease, acute liver injury and portal hypertension. In parallel to these new clinical implications the available data include controversial observations. Thus, a comprehensive overview is required to better understand the functional complexity of these pores. This paper will review the most recent knowledge concerning gap junction dysfunction, with a special focus on the role of these channels in the pathogenesis of relevant clinical entities and on potential therapeutic targets that are amenable to modification by drugs.

Purinoceptor expression in hepatocellular virus (HCV)-induced and non-HCV hepatocellular carcinoma: an insight into the proviral role of the P2X4 receptor

The basic idea behind this study was to discover the association and prevalence of purinoceptors in hepatitis C virus (HCV) and non-HCV hepatocellular carcinoma (HCC). Immunohistochemistry was performed to study the expression of P2X4 and P2X7 receptors on ex-planted liver tissue samples that were collected from HCC patients. Antibodies specific for the P2X4 and P2X7 receptors were used to target the specific receptors and secondary antibody was used with 3,3'-diaminobenzidine (DAB) detection system to visualize the color change in case of any positive expression There was a substantial increase in P2X4 receptor expression in HCV induced HCC as compared to non-HCV HCC. Surprisingly, there was no increase in the P2X7 receptor expression in both HCV HCC and non-HCV HCC. We conclude that P2X4 receptor expression was significant in the presence of HCV HCC. This may confirms the potential role of P2X4 receptor in the presence of virus in liver pathology. However insignificant expression of P2X7 receptor may avert our attention towards understanding the role of this receptor in pro-inflammatory and immune responses.

Danger-Associated Molecular Patterns (DAMPs): Molecular Triggers for Sterile Inflammation in the Liver

Inflammatory liver diseases in the absence of pathogens such as intoxication by xenobiotics, cholestatic liver injury, hepatic ischemia-reperfusion injury (I/R), non-alcoholic steatohepatitis (NASH), or alcoholic liver disease (ALD) remain threatening conditions demanding specific therapeutic options. Caused by various different noxae, all these conditions have been recognized to be triggered by danger- or death-associated molecular patterns (DAMPs), discompartmentalized self-structures released by dying cells. These endogenous, ectopic molecules comprise proteins, nucleic acids, adenosine triphosphate (ATP), or mitochondrial compounds, among others. This review resumes the respective modes of their release—passively by necrotic hepatocytes or actively by viable or apoptotic parenchymal cells—and their particular roles in sterile liver pathology. It addresses their sensors and the initial inflammatory responses they provoke. It further addresses a resulting second wave of parenchymal death that might be of different mode, boosting the release of additional, second-line DAMPs. Thus, triggering a more complex and pronounced response. Initial and secondary inflammatory responses comprise the activation of Kupffer cells (KCs), the attraction and activation of monocytes and neutrophil granulocytes, and the induction of type I interferons (IFNs) and their effectors. A thorough understanding of pathophysiology is a prerequisite for identifying rational therapeutic targets.

Genetic ablation of pannexin1 counteracts liver fibrosis in a chemical, but not in a surgical mouse model

Liver fibrosis is the final common pathway for almost all causes of chronic liver injury. This chronic disease is characterized by excessive deposition of extracellular matrix components mainly due to transdifferentiation of quiescent hepatic stellate cell into myofibroblasts-like cells, which in turn is driven by cell death and inflammation. In the last few years, paracrine signaling through pannexin1 channels has emerged as a key player in the latter processes. The current study was set up to investigate the role of pannexin1 signaling in liver fibrosis. Wild-type and whole body pannexin1 knock-out mice were treated with carbon tetrachloride or subjected to bile duct ligation. Evaluation of the effects of pannexin1 deletion was based on a number of clinically relevant read-outs, including markers of liver damage, histopathological analysis, oxidative stress, inflammation and regenerative capacity. In parallel, to elucidate the molecular pathways affected by pannexin1 deletion as well as to mechanistically anchor the clinical observations, whole transcriptome analysis of liver tissue was performed. While pannexin1 knock-out mice treated with carbon tetrachloride displayed reduced collagen content, hepatic stellate cell activation, inflammation and hepatic regeneration, bile duct ligated counterparts showed increased hepatocellular injury and antioxidant enzyme activity with a predominant immune response. Gene expression profiling revealed a downregulation of fibrotic and immune responses in pannexin1 knock-out mice treated with carbon tetrachloride, whereas bile duct ligated pannexin1-deficient animals showed a pronounced inflammatory profile. This study shows for the first time an etiology-dependent role for pannexin1 signaling in experimental liver fibrosis.

Macrophage P2X4 receptors augment bacterial killing and protect against sepsis

The macrophage is a major phagocytic cell type, and its impaired function is a primary cause of immune paralysis, organ injury, and death in sepsis. An incomplete understanding of the endogenous molecules that regulate macrophage bactericidal activity is a major barrier for developing effective therapies for sepsis. Using an in vitro killing assay, we report here that the endogenous purine ATP augments the killing of sepsis-causing bacteria by macrophages through P2X4 receptors (P2X4Rs). Using newly developed transgenic mice expressing a bioluminescent ATP probe on the cell surface, we found that extracellular ATP levels increase during sepsis, indicating that ATP may contribute to bacterial killing in vivo. Studies with P2X4R-deficient mice subjected to sepsis confirm the role of extracellular ATP acting on P2X4Rs in killing bacteria and protecting against organ injury and death. Results with adoptive transfer of macrophages, myeloid-specific P2X4R-deficient mice, and P2rx4 tdTomato reporter mice indicate that macrophages are essential for the antibacterial, antiinflammatory, and organ protective effects of P2X4Rs in sepsis. Pharmacological targeting of P2X4Rs with the allosteric activator ivermectin protects against bacterial dissemination and mortality in sepsis. We propose that P2X4Rs represent a promising target for drug development to control bacterial growth in sepsis and other infections.

A high concentration of fatty acids induces TNF-α as well as NO release mediated by the P2X4 receptor, and the protective effects of puerarin in RAW264.7 cells

Circulating levels of free fatty acids (FFAs) are often found to be increased in patients with type 2 diabetes mellitus (T2DM) and metabolic syndrome (MS). High plasma FFA levels may give rise to maladaptive macrophage activation and promote inflammatory responses, which has been proposed as a potential mechanism for the development of DM and MS. P2X4 receptor (P2X4R), a ligand-gated cation channel activated by extracellular adenosine triphosphate (ATP), plays a primary role in the regulation of inflammatory responses. Puerarin has been reported to possess potential anti-inflammatory activity. However, the anti-inflammatory activity of puerarin and the underlying molecular mechanisms in a setting of a high concentration of FFAs remain unknown. In this study, we found that a high concentration of FFAs increased the expression of P2X4R, cytosolic Ca²⁺ concentration and the phosphorylation of extracellular signal-regulated kinase (ERK) and induced the expression of tumor necrosis factor (TNF)-α and inducible nitric oxide synthase (iNOS) mRNA and the release of TNF-α and nitric oxide (NO) in RAW264.7 macrophages. Such a high concentration FFA-induced inflammation may be reversed by the P2X4R selective antagonist 5-BDBD, which manifests the important role of P2X4R in the TNF-α and NO release caused by the high concentration of FFAs in RAW264.7 cells. Molecular docking data showed that puerarin could interfere with the activation of P2X4R by forming hydrogen bonding towards residue Arg267, an important residue essential for the canonical activation of P2X4R. Treatment with puerarin dose-dependently reduced high concentration FFA-elevated P2X4R expression and inhibited P2X4R-mediated inflammatory signalling, including high concentration FFA-evoked [Ca²⁺]_i, ERK phosphorylation, expression of TNF-α and iNOS mRNA and release of TNF-α and NO. Our findings emphasize the critical role of P2X4R in high concentration FFA-induced TNF-α and NO release of RAW264.7 macrophages. Puerarin notably counteracts these high concentration FFA-induced adverse effects through its inhibition of P2X4R expression and P2X4R-mediated inflammatory signalling.

Transport of charged small molecules after electropermeabilization - drift and diffusion

Background

Applications of electric-field-induced permeabilization of cells range from cancer therapy to wastewater treatment. A unified understanding of the underlying mechanisms of membrane electropermeabilization, however, has not been achieved. Protocols are empirical, and models are descriptive rather than predictive, which hampers the optimization and expansion of electroporation-based technologies. A common feature of existing models is the assumption that the permeabilized membrane is passive, and that transport through it is entirely diffusive. To demonstrate the necessity to go beyond that assumption, we present here a quantitative analysis of the post-permeabilization transport of three small molecules commonly used in electroporation research — YO-PRO-1, propidium, and calcein — after exposure of cells to minimally perturbing, 6 ns electric pulses.

Results

Influx of YO-PRO-1 from the external medium into the cell exceeds that of propidium, consistent with many published studies. Both are much greater than the influx of calcein. In contrast, the normalized molar efflux of calcein from pre-loaded cells into the medium after electropermeabilization is roughly equivalent to the influx of YO-PRO-1 and propidium. These relative transport rates are correlated not with molecular size or cross-section, but rather with molecular charge polarity.

Conclusions

This comparison of the kinetics of molecular transport of three small, charged molecules across electropermeabilized cell membranes reveals a component of the mechanism of electroporation that is customarily taken into account only for the time during electric pulse delivery. The large differences between the influx rates of propidium and YO-PRO-1 (cations) and calcein (anion), and between the influx and efflux of calcein, suggest a significant role for the post-pulse transmembrane potential in the migration of ions and charged small molecules across permeabilized cell membranes, which has been largely neglected in models of electroporation.

Electronic supplementary material

The online version of this article (10.1186/s13628-018-0044-2) contains supplementary material, which is available to authorized users.

Protective effect of genetic deletion of pannexin1 in experimental mouse models of acute and chronic liver disease

Pannexins are transmembrane proteins that form communication channels connecting the cytosol of an individual cell with its extracellular environment. A number of studies have documented the presence of pannexin1 in liver as well as its involvement in inflammatory responses. In this study, it was investigated whether pannexin1 plays a role in acute liver failure and non-alcoholic steatohepatitis, being prototypical acute and chronic liver pathologies, respectively, both featured by liver damage, oxidative stress and inflammation. To this end, wild-type and pannexin1^-/- mice were overdosed with acetaminophen for 1, 6, 24 or 48h or were fed a choline-deficient high-fat diet for 8weeks. Evaluation of the effects of genetic pannexin1 deletion was based on a number of clinically relevant read-outs, including markers of liver damage, histopathological analysis, lipid accumulation, protein adduct formation, oxidative stress and inflammation. In parallel, in order to elucidate molecular pathways affected by pannexin1 deletion as well as to mechanistically anchor the clinical observations, whole transcriptome analysis of liver tissue was performed. The results of this study show that pannexin1^-/- diseased mice present less liver damage and oxidative stress, while inflammation was only decreased in pannexin1^-/- mice in which non-alcoholic steatohepatitis was induced. A multitude of genes related to inflammation, oxidative stress and xenobiotic metabolism were differentially modulated in both liver disease models in wild-type and in pannexin1^-/- mice. Overall, the results of this study suggest that pannexin1 may play a role in the pathogenesis of liver disease.

Revisiting multimodal activation and channel properties of Pannexin 1

Pannexin 1 (Panx1) forms plasma membrane ion channels that are widely expressed throughout the body. Panx1 activation results in the release of nucleotides such as adenosine triphosphate and uridine triphosphate. Thus, these channels have been implicated in diverse physiological and pathological functions associated with purinergic signaling, such as apoptotic cell clearance, blood pressure regulation, neuropathic pain, and excitotoxicity. In light of this, substantial attention has been directed to understanding the mechanisms that regulate Panx1 channel expression and activation. Here we review accumulated evidence for the various activation mechanisms described for Panx1 channels and, where possible, the unitary channel properties associated with those forms of activation. We also emphasize current limitations in studying Panx1 channel function and propose potential directions to clarify the exciting and expanding roles of Panx1 channels.

Inhibitors of connexin and pannexin channels as potential therapeutics

While gap junctions support the exchange of a number of molecules between neighboring cells, connexin hemichannels provide communication between the cytosol and the extracellular environment of an individual cell. The latter equally holds true for channels composed of pannexin proteins, which display an architecture reminiscent of connexin hemichannels. In physiological conditions, gap junctions are usually open, while connexin hemichannels and, to a lesser extent, pannexin channels are typically closed, yet they can be activated by a number of pathological triggers. Several agents are available to inhibit channels built up by connexin and pannexin proteins, including alcoholic substances, glycyrrhetinic acid, anesthetics and fatty acids. These compounds not always strictly distinguish between gap junctions, connexin hemichannels and pannexin channels, and may have effects on other targets as well. An exception lies with mimetic peptides, which reproduce specific amino acid sequences in connexin or pannexin primary protein structure. In this paper, a state-of-the-art overview is provided on inhibitors of cellular channels consisting of connexins and pannexins with specific focus on their mode-of-action and therapeutic potential.

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.

Role of Pannexin1 channels in the resistance of I-10 testicular cancer cells to cisplatin mediated by ATP/IP3 pathway

Cisplatin (DDP) is the most commonly used drug in testicular cancer. However, drug resistance severely limits its clinical use and the underlying mechanisms need to be further clarified. The aim of present study was to investigate the role of ATP/IP₃ pathway mediated by pannexin1 (Panx-1) channels on DDP-induced apoptosis and to reveal the potential mechanisms of DDP-resistance in testicular cancer. We found that the expression of Panx-1 in I-10/DDP cells (DDP-resistance) was decreased compared with parental I-10 cells determined by western blotting and immunofluorescence assay. To further clarify the role of Panx-1 in DDP resistance, Panx-1 function was modulated by overexpression and knockdown of Panx-1 expression. Panx-1 overexpression increased DDP-induced apoptosis, ATP release and IP₃ levels. On the contrary, Panx-1 silencing decreased DDP-induced apoptosis, ATP release and IP₃ levels. Apyrase (hydrolyzing extracellular ATP) or xestospongin C (antagonizing IP₃ receptor) also decreased DDP-induced apoptosis. Our findings demonstrate that Panx-1 is involved in DDP-resistance and ATP/IP₃ pathway mediated by Panx-1 channels participates in DDP-induced apoptosis in testicular cancer. Panx-1 modulation may be interesting to amplify the clinical effect of DDP and reverse the resistance of testicular cancer cells to DDP.

Inhibition of pannexin1 channels alleviates acetaminophen-induced hepatotoxicity

Pannexins constitute a relatively new family of transmembrane proteins that form channels linking the cytoplasmic compartment with the extracellular environment. The presence of pannexin1 in the liver has been documented previously, where it underlies inflammatory responses, such as those occurring upon ischemia-reperfusion injury. In the present study, we investigated whether pannexin1 plays a role in acute drug-induced liver toxicity. Hepatic expression of pannexin1 was characterized in a mouse model of acetaminophen-induced hepatotoxicity. Subsequently, mice were overdosed with acetaminophen followed by treatment with the pannexin1 channel inhibitor 10Panx1. Sampling was performed 1, 3, 6, 24 and 48 h after acetaminophen administration. Evaluation of the effects of pannexin1 channel inhibition was based on a number of clinically relevant readouts, including protein adduct formation, measurement of aminotransferase activity and histopathological examination of liver tissue as well as on a series of markers of inflammation, oxidative stress and regeneration. Although no significant differences were found in histopathological analysis, pannexin1 channel inhibition reduced serum levels of alanine and aspartate aminotransferase. This was paralleled by a reduced amount of neutrophils recruited to the liver. Furthermore, alterations in the oxidized status were noticed with upregulation of glutathione levels upon suppression of pannexin1 channel opening. Concomitant promotion of regenerative activity was detected as judged on increased proliferating cell nuclear antigen protein quantities in 10Panx1-treated mice. Pannexin1 channels are important actors in liver injury triggered by acetaminophen. Inhibition of pannexin1 channel opening could represent a novel approach for the treatment of drug-induced hepatotoxicity.

Quantitative Limits on Small Molecule Transport via the Electropermeome - Measuring and Modeling Single Nanosecond Perturbations

The detailed molecular mechanisms underlying the permeabilization of cell membranes by pulsed electric fields (electroporation) remain obscure despite decades of investigative effort. To advance beyond descriptive schematics to the development of robust, predictive models, empirical parameters in existing models must be replaced with physics- and biology-based terms anchored in experimental observations. We report here absolute values for the uptake of YO-PRO-1, a small-molecule fluorescent indicator of membrane integrity, into cells after a single electric pulse lasting only 6 ns. We correlate these measured values, based on fluorescence microphotometry of hundreds of individual cells, with a diffusion-based geometric analysis of pore-mediated transport and with molecular simulations of transport across electropores in a phospholipid bilayer. The results challenge the “drift and diffusion through a pore” model that dominates conventional explanatory schemes for the electroporative transfer of small molecules into cells and point to the necessity for a more complex model.

Connexin-based signaling and drug-induced hepatotoxicity

Being critical mediators of liver homeostasis, connexins and their channels are frequently involved in liver toxicity. In the current paper, specific attention is paid to actions of hepatotoxic drugs on these communicative structures. In a first part, an overview is provided on the structural, regulatory and functional properties of connexin-based channels in the liver. In the second part, documented effects of acetaminophen, hypolipidemic drugs, phenobarbital and methapyriline on connexin signaling are discussed. Furthermore, the relevance of this subject for the fields of clinical and in vitro toxicology is demonstrated.

Relevance for patients: The role of connexin signaling in drug-induced hepatotoxicity may be of high clinical relevance, as it offers perspectives for the therapeutic treatment of such insults by interfering with connexin channel opening.

Regulation of Connexin-Based Channels by Fatty Acids

In this mini-review, we briefly summarize the current knowledge about the effects of fatty acids (FAs) on connexin-based channels, as well as discuss the limited information about the impact FAs may have on pannexins (Panxs). FAs regulate diverse cellular functions, some of which are explained by changes in the activity of channels constituted by connexins (Cxs) or Panxs, which are known to play critical roles in maintaining the functional integrity of diverse organs and tissues. Cxs are transmembrane proteins that oligomerize into hexamers to form hemichannels (HCs), which in turn can assemble into dodecamers to form gap junction channels (GJCs). While GJCs communicate the cytoplasm of contacting cells, HCs serve as pathways for the exchange of ions and small molecules between the intra and extracellular milieu. Panxs, as well as Cx HCs, form channels at the plasma membrane that enable the interchange of molecules between the intra and extracellular spaces. Both Cx- and Panx-based channels are controlled by several post-translational modifications. However, the mechanism of action of FAs on these channels has not been described in detail. It has been shown however that FAs frequently decrease GJC-mediated cell-cell communication. The opposite effect also has been described for HC or Panx-dependent intercellular communication, where, the acute FA effect can be reversed upon washout. Additionally, changes in GJCs mediated by FAs have been associated with post-translational modifications (e.g., phosphorylation), and seem to be directly related to chemical properties of FAs (e.g., length of carbon chain and/or degree of saturation), but this possible link remains poorly understood.

Structure, Regulation and Function of Gap Junctions in Liver

Gap junctions are a specialized group of cell-to-cell junctions that mediate direct intercellular communication between cells. They arise from the interaction of two hemichannels of adjacent cells, which in turn are composed of six connexin proteins. In liver, gap junctions are predominantly found in hepatocytes and play critical roles in virtually all phases of the hepatic life cycle, including cell growth, differentiation, liver-specific functionality and cell death. Liver gap junctions are directed through a broad variety of mechanisms ranging from epigenetic control of connexin expression to post-translational regulation of gap junction activity. This paper reviews established and novel aspects regarding the architecture, control and functional relevance of liver gap junctions.

Connexins and pannexins in liver damage

Connexins and pannexins are key players in the control of cellular communication and thus in the maintenance of tissue homeostasis. Inherent to this function these proteins are frequently involved in pathological processes. The present paper reviews the role of connexins and pannexins in liver toxicity and disease. As they act both as sensors and effectors in these deleterious events connexins and pannexins could represent a set of novel clinical diagnostic biomarkers and drug targets.

Connexins, Pannexins, and Their Channels in Fibroproliferative Diseases

Cellular and molecular mechanisms of wound healing, tissue repair, and fibrogenesis are established in different organs and are essential for the maintenance of function and tissue integrity after cell injury. These mechanisms are also involved in a plethora of fibroproliferative diseases or organ-specific fibrotic disorders, all of which are associated with the excessive deposition of extracellular matrix components. Fibroblasts, which are key cells in tissue repair and fibrogenesis, rely on communicative cellular networks to ensure efficient control of these processes and to prevent abnormal accumulation of extracellular matrix into the tissue. Despite the significant impact on human health, and thus the epidemiologic relevance, there is still no effective treatment for most fibrosis-related diseases. This paper provides an overview of current concepts and mechanisms involved in the participation of cellular communication via connexin-based pores as well as pannexin-based channels in the processes of tissue repair and fibrogenesis in chronic diseases. Understanding these mechanisms may contribute to the development of new therapeutic strategies to clinically manage fibroproliferative diseases and organ-specific fibrotic disorders.

A novel innexin2 forming membrane hemichannel exhibits immune responses and cell apoptosis in Scylla paramamosain

Innexins are a class of transmembrane proteins that are important for embryonic development, morphogenesis and electrical synapse formation. In the present study, a novel innexin2 gene from Scylla paramamosain was named Sp-inx2 and characterized. The complete cDNA and genomic DNA sequences of Sp-inx2 were revealed. Sp-inx2 mRNA transcripts were distributed in various tissues of S. paramamosain and were most abundant in the hemocytes. The Sp-inx2 was significantly upregulated in hemocyte, gill and hepatopancreas tissues with the challenge of either Vibrio alginolyticus, Vibrio parahaemolyticus or lipopolysaccharides (LPSs) when analyzed at 3 and 6 h using quantitative real-time PCR, suggesting that it could activate an immune response against the challenge of LPSs or Vibrio species. Using the chemical inhibitors carbenoxolone and probenecid, the absorption of the fluorescent dye Lucifer yellow decreased in the primary cultured hemocytes of crabs, thus confirming that hemichannels composed of Sp-inx2 existed in the crab hemocytes. With LPS stimulation, the level of mRNA transcripts and protein expression of Sp-inx2 in the same cultured hemocytes gradually increased from 6 to 48 h, while the activity of hemichannels was down-regulated at 6 and 12 h, demonstrating that LPSs could modulate the absorption activity of hemichannels in addition to its upregulation of Sp-inx2 gene expression. Furthermore, the dye uptake rate in HeLa cells in which Sp-inx2 was ectopically expressed increased dramatically but the increase was significantly down-regulated with the addition of 50 μg mL(-1) LPS, suggesting that the LPS stimulation could effectively reduce the activity of hemichannels. Interestingly, with the ectopic expression of Sp-inx2 in HeLa and EPC cells, apoptosis spontaneously occurred in both cultured cell lines when detected using TUNEL assay. In summary, a new Sp-inx2 gene was first characterized in a marine animal S. paramamosain and it had a function associated with immune response and cell apoptosis.

Connexin and pannexin signaling in gastrointestinal and liver disease

Gap junctions, which mediate intercellular communication, are key players in digestive homeostasis. They are also frequently involved in gastrointestinal and liver pathology. This equally holds true for connexin (Cx) hemichannels, the structural precursors of gap junctions, and pannexin (Panx) channels, Cx-like proteins assembled in a hemichannel configuration. Both Cx hemichannels and Panx channels facilitate extracellular communication and drive a number of deteriorative processes, such as cell death and inflammation. Cxs, Panxs, and their channels underlie a wide spectrum of gastrointestinal and liver diseases, including gastritis and peptic ulcer disease, inflammatory intestinal conditions, acute liver failure, cholestasis, hepatitis and steatosis, liver fibrosis and cirrhosis, infectious gastrointestinal pathologies, and gastrointestinal and liver cancer. This could open promising perspectives for the characterization of new targets and biomarkers for therapeutic and diagnostic clinical purposes in the area of gastroenterology and hepatology.

Roles of connexins and pannexins in digestive homeostasis

Connexin proteins are abundantly present in the digestive system. They primarily form gap junctions, which control the intercellular exchange of critical homeostasis regulators. By doing so, gap junctions drive a plethora of gastrointestinal and hepatic functional features, including gastric and gut motility, gastric acid secretion, intestinal innate immune defense, xenobiotic biotransformation, glycogenolysis, bile secretion, ammonia detoxification and plasma protein synthesis. In the last decade, it has become clear that connexin hemichannels, which are the structural precursors of gap junctions, also provide a pathway for cellular communication, namely between the cytosol and the extracellular environment. Although merely pathological functions have been described, some physiological roles have been attributed to connexin hemichannels, in particular in the modulation of colonic motility. This equally holds true for cellular channels composed of pannexins, connexin-like proteins recently identified in the intestine and the liver, which have become acknowledged key players in inflammatory processes and that have been proposed to control colonic motility, secretion and blood flow.

Lipoapoptosis induced by saturated free fatty acids stimulates monocyte migration: a novel role for Pannexin1 in liver cells

Recruitment of monocytes in the liver is a key pathogenic feature of hepatic inflammation in nonalcoholic steatohepatitis (NASH), but the mechanisms involved are poorly understood. Here, we studied migration of human monocytes in response to supernatants obtained from liver cells after inducing lipoapoptosis with saturated free fatty acids (FFA). Lipoapoptotic supernatants stimulated monocyte migration with the magnitude similar to a monocyte chemoattractant protein, CCL2 (MCP-1). Inhibition of c-Jun NH2-terminal kinase (JNK) in liver cells with SP600125 blocked migration of monocytes in a dose-dependent manner, indicating that JNK stimulates release of chemoattractants in lipoapoptosis. Notably, treatment of supernatants with Apyrase to remove ATP potently inhibited migration of THP-1 monocytes and partially blocked migration of primary human monocytes. Inhibition of the CCL2 receptor (CCR2) on THP-1 monocytes with RS102895, a specific CCR2 inhibitor, did not block migration induced by lipoapoptotic supernatants. Consistent with these findings, lipoapoptosis stimulated pathophysiological extracellular ATP (eATP) release that increased supernatant eATP concentration from 5 to ~60 nM. Importantly, inhibition of Panx1 expression in liver cells with short hairpin RNA (shRNA) decreased supernatant eATP concentration and inhibited monocyte migration, indicating that monocyte migration is mediated in part by Panx1-dependent eATP release. Moreover, JNK inhibition decreased supernatant eATP concentration and inhibited Pannexin1 activation, as determined by YoPro-1 uptake in liver cells in a dose-dependent manner. These results suggest that JNK regulates activation of Panx1 channels, and provide evidence that Pannexin1-dependent pathophysiological eATP release in lipoapoptosis is capable of stimulating migration of human monocytes, and may participate in the recruitment of monocytes in chronic liver injury induced by saturated FFA.

ATP as a multi-target danger signal in the brain

ATP is released in an activity-dependent manner from different cell types in the brain, fulfilling different roles as a neurotransmitter, neuromodulator, in astrocyte-to-neuron communication, propagating astrocytic responses and formatting microglia responses. This involves the activation of different ATP P2 receptors (P2R) as well as adenosine receptors upon extracellular ATP catabolism by ecto-nucleotidases. Notably, brain noxious stimuli trigger a sustained increase of extracellular ATP, which plays a key role as danger signal in the brain. This involves a combined action of extracellular ATP in different cell types, namely increasing the susceptibility of neurons to damage, promoting astrogliosis and recruiting and formatting microglia to mount neuroinflammatory responses. Such actions involve the activation of different receptors, as heralded by neuroprotective effects resulting from blockade mainly of P2X7R, P2Y1R and adenosine A_2A receptors (A_2AR), which hierarchy, cooperation and/or redundancy is still not resolved. These pleiotropic functions of ATP as a danger signal in brain damage prompt a therapeutic interest to multi-target different purinergic receptors to provide maximal opportunities for neuroprotection.

Nucleotides released from palmitate-challenged muscle cells through pannexin-3 attract monocytes

Obesity-associated low-grade inflammation in metabolically relevant tissues contributes to insulin resistance. We recently reported monocyte/macrophage infiltration in mouse and human skeletal muscles. However, the molecular triggers of this infiltration are unknown, and the role of muscle cells in this context is poorly understood. Animal studies are not amenable to the specific investigation of this vectorial cellular communication. Using cell cultures, we investigated the crosstalk between myotubes and monocytes exposed to physiological levels of saturated and unsaturated fatty acids. Media from L6 myotubes treated with palmitate-but not palmitoleate-induced THP1 monocyte migration across transwells. Palmitate activated the Toll-like receptor 4 (TLR4)/nuclear factor-κB (NF-κB) pathway in myotubes and elevated cytokine expression, but the monocyte chemoattracting agent was not a polypeptide. Instead, nucleotide degradation eliminated the chemoattracting properties of the myotube-conditioned media. Moreover, palmitate-induced expression and activity of pannexin-3 channels in myotubes were mediated by TLR4-NF-κB, and TLR4-NF-κB inhibition or pannexin-3 knockdown prevented monocyte chemoattraction. In mice, the expression of pannexin channels increased in adipose tissue and skeletal muscle in response to high-fat feeding. These findings identify pannexins as new targets of saturated fatty acid-induced inflammation in myotubes, and point to nucleotides as possible mediators of immune cell chemoattraction toward muscle in the context of obesity.

Possible role of hemichannels in cancer

In humans, connexins (Cxs) and pannexins (Panxs) are the building blocks of hemichannels. These proteins are frequently altered in neoplastic cells and have traditionally been considered as tumor suppressors. Alteration of Cxs and Panxs in cancer cells can be due to genetic, epigenetic and post-transcriptional/post-translational events. Activated hemichannels mediate the diffusional membrane transport of ions and small signaling molecules. In the last decade hemichannels have been shown to participate in diverse cell processes including the modulation of cell proliferation and survival. However, their possible role in tumor growth and expansion remains largely unexplored. Herein, we hypothesize about the possible role of hemichannels in carcinogenesis and tumor progression. To support this theory, we summarize the evidence regarding the involvement of hemichannels in cell proliferation and migration, as well as their possible role in the anti-tumor immune responses. In addition, we discuss the evidence linking hemichannels with cancer in diverse models and comment on the current technical limitations for their study.

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

The pannexins: past and present

The pannexins (Panxs) are a family of chordate proteins homologous to the invertebrate gap junction forming proteins named innexins. Three distinct Panx paralogs (Panx1, Panx2, and Panx3) are shared among the major vertebrate phyla, but they appear to have suppressed (or even lost) their ability to directly couple adjacent cells. Connecting the intracellular and extracellular compartments is now widely accepted as Panx's primary function, facilitating the passive movement of ions and small molecules along electrochemical gradients. The tissue distribution of the Panxs ranges from pervasive to very restricted, depending on the paralog, and are often cell type-specific and/or developmentally regulated within any given tissue. In recent years, Panxs have been implicated in an assortment of physiological and pathophysiological processes, particularly with respect to ATP signaling and inflammation, and they are now considered to be a major player in extracellular purinergic communication. The following is a comprehensive review of the Panx literature, exploring the historical events leading up to their discovery, outlining our current understanding of their biochemistry, and describing the importance of these proteins in health and disease.