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

Pannexin 3 deletion in mice results in knee osteoarthritis and intervertebral disc degeneration after forced treadmill running

Pannexin 3 (Panx3) is a glycoprotein that forms mechanosensitive channels expressed in chondrocytes and annulus fibrosus cells of the intervertebral disc (IVD). Evidence suggests Panx3 plays contrasting roles in traumatic versus aging osteoarthritis (OA) and intervertebral disc degeneration (IDD). However, whether its deletion influences the response of joint tissue to forced use is unknown. The purpose of this study was to determine if Panx3 deletion in mice causes increased knee joint OA and IDD after forced treadmill running. Male and female wildtype (WT) and Panx3 knockout (KO) mice were randomized to either a no-exercise group (sedentary; SED) or daily forced treadmill running (forced exercise; FEX) from 24 to 30 weeks of age. Knee cartilage and IVD histopathology were evaluated by histology, while tibial secondary ossification centers were analyzed using microcomputed tomography (µCT). Both male and female Panx3 KO mice developed larger superficial defects of the tibial cartilage after forced treadmill running compared with SED WT mice. Additionally, Panx3 KO mice developed reduced bone volume, and female PANX3 KO mice had lengthening of the lateral tubercle at the intercondylar eminence. In the lower lumbar spine, both male and female Panx3 KO mice developed histopathological features of IDD after running compared to SED WT mice. These findings suggest that the combination of deleting Panx3 and forced treadmill running induces OA and causes histopathological changes associated with the degeneration of the IVDs in mice.

Mechanisms Underlying the Rarity of Skeletal Muscle Cancers

Skeletal muscle (SKM), despite comprising ~40% of body mass, rarely manifests cancer. This review explores the mechanisms that help to explain this rarity, including unique SKM architecture and function, which prohibits the development of new cancer as well as negates potential metastasis to SKM. SKM also presents a unique immune environment that may magnify the anti-tumorigenic effect. Moreover, the SKM microenvironment manifests characteristics such as decreased extracellular matrix stiffness and altered lactic acid, pH, and oxygen levels that may interfere with tumor development. SKM also secretes anti-tumorigenic myokines and other molecules. Collectively, these mechanisms help account for the rarity of SKM cancer.

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.

Skin in the game: pannexin channels in healthy and cancerous skin

The skin is a highly organized tissue composed of multiple layers and cell types that require coordinated cell to cell communication to maintain tissue homeostasis. In skin cancer, this organized structure and communication is disrupted, prompting the malignant transformation of healthy cells into melanoma, basal cell carcinoma or squamous cell carcinoma tumours. One such family of channel proteins critical for cellular communication is pannexins (PANX1, PANX2, PANX3), all of which are present in the skin. These heptameric single-membrane channels act as conduits for small molecules and ions like ATP and Ca2+ but have also been shown to have channel-independent functions through their interacting partners or action in signalling pathways. Pannexins have diverse roles in the skin such as in skin development, aging, barrier function, keratinocyte differentiation, inflammation, and wound healing, which were discovered through work with pannexin knockout mice, organotypic epidermis models, primary cells, and immortalized cell lines. In the context of cutaneous cancer, PANX1 is present at high levels in melanoma tumours and functions in melanoma carcinogenesis, and both PANX1 and PANX3 expression is altered in non-melanoma skin cancer. PANX2 has thus far not been implicated in any skin cancer. This review will discuss pannexin isoforms, structure, trafficking, post-translational modifications, interactome, and channel activity. We will also outline the expression, localization, and function of pannexin channels within the diverse cell types of the epidermis, dermis, hypodermis, and adnexal structures of the skin, and how these properties are exploited or abrogated in instances of skin cancer.

Pannexin 3 channels regulate architecture, adhesion, barrier function and inflammation in the skin

The channel-forming glycoprotein Pannexin 3 (PANX3) functions in cutaneous wound healing and keratinocyte differentiation, but its role in skin homeostasis through aging is not yet understood. We found that PANX3 is absent in newborn skin but becomes upregulated with age. We characterized the skin of global Panx3 knockout mice (KO) and found that KO dorsal skin showed sex-differences at different ages, but generally had reduced dermal and hypodermal areas compared to aged-matched controls. Transcriptomic analysis of KO epidermis revealed reduced E-cadherin stabilization and Wnt signaling compared to WT, consistent with the inability of primary KO keratinocytes to adhere in culture, and diminished epidermal barrier function in KO mice. We also observed increased inflammatory signaling in KO epidermis and higher incidence of dermatitis in aged KO mice compared to wildtype controls. These findings suggest that during skin aging, PANX3 is critical in the maintenance of dorsal skin architecture, keratinocyte cell-cell and cell-matrix adhesion and inflammatory skin responses.

Potential Implications of Exercise Training on Pannexin Expression and Function

Pannexins (PANX1, 2, 3) are channel-forming glycoproteins that are expressed throughout the cardiovascular and musculoskeletal system. The canonical function of these proteins is to release nucleotides that act as purinergic signalling at the cell membrane or Ca2+ channels at the endoplasmic reticulum membrane. These two forms of signalling are essential for autocrine and paracrine signalling in health, and alterations in this signalling have been implicated in the pathogenesis of many diseases. Many musculoskeletal and cardiovascular diseases are largely the result of a lack of physical activity which causes altered gene expression. Considering exercise training has been shown to alter a wide array of gene expression in musculoskeletal tissues, understanding the interaction between exercise training, gene function and expression in relevant diseases is warranted. With regards to pannexins, multiple publications have shown that exercise training can influence pannexin expression and may influence the significance of its function in certain diseases. This review further discusses the potential interaction between exercise training and pannexin biology in relevant tissues and disease models. We propose that exercise training in relevant animal and human models will provide a more comprehensive understanding of the implications of pannexin biology in disease.

Distinctive exercise-induced inflammatory response and exerkine induction in skeletal muscle of people with type 2 diabetes

Mechanistic insights into the molecular events by which exercise enhances the skeletal muscle phenotype are lacking, particularly in the context of type 2 diabetes. Here, we unravel a fundamental role for exercise-responsive cytokines (exerkines) on skeletal muscle development and growth in individuals with normal glucose tolerance or type 2 diabetes. Acute exercise triggered an inflammatory response in skeletal muscle, concomitant with an infiltration of immune cells. These exercise effects were potentiated in type 2 diabetes. In response to contraction or hypoxia, cytokines were mainly produced by endothelial cells and macrophages. The chemokine CXCL12 was induced by hypoxia in endothelial cells, as well as by conditioned medium from contracted myotubes in macrophages. We found that CXCL12 was associated with skeletal muscle remodeling after exercise and differentiation of cultured muscle. Collectively, acute aerobic exercise mounts a noncanonical inflammatory response, with an atypical production of exerkines, which is potentiated in type 2 diabetes.

Pannexin 3 deletion reduces fat accumulation and inflammation in a sex-specific manner

BACKGROUND

Pannexin 3 (PANX3) is a channel-forming glycoprotein that enables nutrient-induced inflammation in vitro, and genetic linkage data suggest that it regulates body mass index. Here, we characterized inflammatory and metabolic parameters in global Panx3 knockout (KO) mice in the context of forced treadmill running (FEX) and high-fat diet (HFD).

METHODS

C57BL/6N (WT) and KO mice were randomized to either a FEX running protocol or no running (SED) from 24 until 30 weeks of age. Body weight was measured biweekly, and body composition was measured at 24 and 30 weeks of age. Male WT and KO mice were fed a HFD from 12 to 28 weeks of age. Metabolic organs were analyzed for a panel of inflammatory markers and PANX3 expression.

RESULTS

In females there were no significant differences in body composition between genotypes, which could be due to the lack of PANX3 expression in female white adipose tissue, while male KOs fed a chow diet had lower body weight and lower fat mass at 24 and 30 weeks of age, which was reduced to the same extent as 6 weeks of FEX in WT mice. In addition, male KO mice exhibited significantly lower expression of multiple pro-inflammatory genes in white adipose tissue compared to WT mice. While on a HFD body weight differences were insignificant, multiple inflammatory genes were significantly different in quadriceps muscle and white adipose tissue resulting in a more anti-inflammatory phenotype in KO mice compared to WT. The lower fat mass in male KO mice may be due to significantly fewer adipocytes in their subcutaneous fat compared to WT mice. Mechanistically, adipose stromal cells (ASCs) cultured from KO mice grow significantly slower than WT ASCs.

CONCLUSION

PANX3 is expressed in male adult mouse adipose tissue and may regulate adipocyte numbers, influencing fat accumulation and inflammation.

Pannexin Channel Regulation of Cell Migration: Focus on Immune Cells

The role of Pannexin (PANX) channels during collective and single cell migration is increasingly recognized. Amongst many functions that are relevant to cell migration, here we focus on the role of PANX-mediated adenine nucleotide release and associated autocrine and paracrine signaling. We also summarize the contribution of PANXs with the cytoskeleton, which is also key regulator of cell migration. PANXs, as mechanosensitive ATP releasing channels, provide a unique link between cell migration and purinergic communication. The functional association with several purinergic receptors, together with a plethora of signals that modulate their opening, allows PANX channels to integrate physical and chemical cues during inflammation. Ubiquitously expressed in almost all immune cells, PANX1 opening has been reported in different immunological contexts. Immune activation is the epitome coordination between cell communication and migration, as leukocytes (i.e., T cells, dendritic cells) exchange information while migrating towards the injury site. In the current review, we summarized the contribution of PANX channels during immune cell migration and recruitment; although we also compile the available evidence for non-immune cells (including fibroblasts, keratinocytes, astrocytes, and cancer cells). Finally, we discuss the current evidence of PANX1 and PANX3 channels as a both positive and/or negative regulator in different inflammatory conditions, proposing a general mechanism of these channels contribution during cell migration.

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.

Pannexin biology and emerging linkages to cancer

Pannexins are a family of glycoproteins that comprises three members, PANX1, PANX2, and PANX3. The widely expressed and interrogated PANX1 forms heptameric membrane channels that primarily serve to connect the cytoplasm to the extracellular milieu by being selectively permeable to small signaling molecules when activated. Apart from notable exceptions, PANX1 in many tumor cells appears to facilitate tumor growth and metastasis, suggesting that pannexin-blocking therapeutics may have utility in cancer. Attenuation of PANX1 function must also consider the fact that PANX1 is found in stromal cells of the tumor microenvironment (TME), including immune cells. This review highlights the key discoveries of the past 5 years that suggest pannexins facilitate, or in some cases inhibit, tumor cell growth and metastasis via direct protein interactions and through the regulated efflux of signaling molecules.

Pannexin 3 channels in health and disease

Pannexin 3 (PANX3) is a member of the pannexin family of single membrane channel-forming glycoproteins. Originally thought to have a limited localization in cartilage, bone, and skin, PANX3 has now been detected in a variety of other tissues including skeletal muscle, mammary glands, the male reproductive tract, the cochlea, blood vessels, small intestines, teeth, and the vomeronasal organ. In many cell types of the musculoskeletal system, such as osteoblasts, chondrocytes, and odontoblasts, PANX3 has been shown to regulate the balance of proliferation and differentiation. PANX3 can be induced during progenitor cell differentiation, functioning at the cell surface as a conduit for ATP and/or in the endoplasmic reticulum as a calcium leak channel. Evidence in osteoblasts and monocytes also highlight a role for PANX3 in purinergic signalling through its function as an ATP release channel. PANX3 is critical in the development and ageing of bone and cartilage, with its levels temporally regulated in other tissues such as skeletal muscle, skin, and the cochlea. In diseases such as osteoarthritis and intervertebral disc degeneration, PANX3 can have either protective or detrimental roles depending on if the disease is age-related or injury-induced. This review will discuss PANX3 function in tissue growth and regeneration, its role in cellular differentiation, and how it becomes dysregulated in disease conditions such as obesity, Duchenne's muscular dystrophy, osteosarcoma, and non-melanoma skin cancer, where most of the findings on PANX3 function can be attributed to the characterization of Panx3 KO mouse models.

High extracellular ATP levels released through pannexin-1 channels mediate inflammation and insulin resistance in skeletal muscle fibres of diet-induced obese mice

AIMS/HYPOTHESIS

Skeletal muscle is a key target organ for insulin's actions and is the main regulator of blood glucose. In obese individuals and animal models, there is a chronic low-grade inflammatory state affecting highly metabolic organs, leading to insulin resistance. We have described that adult skeletal muscle fibres can release ATP to the extracellular medium through pannexin-1 (PANX1) channels. Besides, it is known that high extracellular ATP concentrations can act as an inflammatory signal. Here, we propose that skeletal muscle fibres from obese mice release high levels of ATP, through PANX1 channels, promoting inflammation and insulin resistance in muscle cells.

METHODS

C57BL/6J mice were fed with normal control diet (NCD) or high-fat diet (HFD) for 8 weeks. Muscle fibres were isolated from flexor digitorum brevis (FDB) muscle. PANX1-knockdown FDB fibres were obtained by in vivo electroporation of a short hairpin RNA Panx1 plasmid. We analysed extracellular ATP levels in a luciferin/luciferase assay. Gene expression was studied with quantitative real-time PCR (qPCR). Protein levels were evaluated by immunoblots, ELISA and immunofluorescence. Insulin sensitivity was analysed in a 2-NBDG (fluorescent glucose analogue) uptake assay, immunoblots and IPGTT.

RESULTS

HFD-fed mice showed significant weight gain and insulin resistance compared with NCD-fed mice. IL-6, IL-1β and TNF-α protein levels were increased in FDB muscle from obese mice. We observed high levels of extracellular ATP in muscle fibres from obese mice (197 ± 55 pmol ATP/μg RNA) compared with controls (32 ± 10 pmol ATP/μg RNA). ATP release in obese mice fibres was reduced by application of 100 μmol/l oleamide (OLE) and 5 μmol/l carbenoxolone (CBX), both PANX1 blockers. mRNA levels of genes linked to inflammation were reduced using OLE, CBX or 2 U/ml ATPase apyrase in muscle fibres from HFD-fed mice. In fibres from mice with pannexin-1 knockdown, we observed diminished extracellular ATP levels (78 ± 10 pmol ATP/μg RNA vs 252 ± 37 pmol ATP/μg RNA in control mice) and a lower expression of inflammatory markers. Moreover, a single pulse of 300 μmol/l ATP to fibres from control mice reduced insulin-mediated 2-NBDG uptake and promoted an elevation in mRNA levels of inflammatory markers. PANX-1 protein levels were increased two- to threefold in skeletal muscle from obese mice compared with control mice. Incubation with CBX increased Akt activation and 2-NBDG uptake in HFD fibres after insulin stimulation, rescuing the insulin resistance condition. Finally, in vivo treatment of HFD-fed mice with CBX (i.p. injection of 10 mg/kg each day) for 14 days, compared with PBS, reduced extracellular ATP levels in skeletal muscle fibres (51 ± 10 pmol ATP/μg RNA vs 222 ± 28 pmol ATP/μg RNA in PBS-treated mice), diminished inflammation and improved glycaemic management.

CONCLUSIONS/INTERPRETATION

In this work, we propose a novel mechanism for the development of inflammation and insulin resistance in the skeletal muscle of obese mice. We found that high extracellular ATP levels, released by overexpressed PANX1 channels, lead to an inflammatory state and insulin resistance in skeletal muscle fibres of obese mice.

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.

eATP/P2X7R Axis: An Orchestrated Pathway Triggering Inflammasome Activation in Muscle Diseases

In muscle ATP is primarily known for its function as an energy source and as a mediator of the "excitation-transcription" process, which guarantees muscle plasticity in response to environmental stimuli. When quickly released in massive concentrations in the extracellular space as in presence of muscle membrane damage, ATP acts as a damage-associated molecular pattern molecule (DAMP). In experimental murine models of muscular dystrophies characterized by membrane instability, blockade of eATP/P2X7 receptor (R) purinergic signaling delayed the progression of the dystrophic phenotype dampening the local inflammatory response and inducing Foxp3⁺ T Regulatory lymphocytes. These discoveries highlighted the relevance of ATP as a harbinger of immune-tissue damage in muscular genetic diseases. Given the interactions between the immune system and muscle regeneration, the comprehension of ATP/purinerigic pathway articulated organization in muscle cells has become of extreme interest. This review explores ATP release, metabolism, feedback control and cross-talk with members of muscle inflammasome in the context of muscular dystrophies.

Nucleotides released from palmitate-activated murine macrophages attract neutrophils

Obesity and elevation of circulating free fatty acids are associated with an accumulation and proinflammatory polarization of macrophages within metabolically active tissues, such as adipose tissue, muscle, liver, and pancreas. Beyond macrophages, neutrophils also accumulate in adipose and muscle tissues during high-fat diets and contribute to a state of local inflammation and insulin resistance. However, the mechanisms by which neutrophils are recruited to these tissues are largely unknown. Here we used a cell culture system as proof of concept to show that, upon exposure to a saturated fatty acid, palmitate, macrophages release nucleotides that attract neutrophils. Moreover, we found that palmitate up-regulates pannexin-1 channels in macrophages that mediate the attraction of neutrophils, shown previously to allow transfer of nucleotides across membranes. These findings suggest that proinflammatory macrophages release nucleotides through pannexin-1, a process that may facilitate neutrophil recruitment into metabolic tissues during obesity.

Central and Peripheral Inflammation Link Metabolic Syndrome and Major Depressive Disorder

Metabolic syndrome and major depression are two of the most common and debilitating disorders worldwide, occurring with significant rates of comorbidity. Recent studies have uncovered that each of these conditions is associated with chronic, low-grade inflammation. This is characterized by increased circulating pro-inflammatory cytokines, altered leukocyte population frequencies in blood, accumulation of immune cells in tissues including the brain, and activation of these immune cells. Cytokines that become elevated during obesity can contribute to the progression of metabolic syndrome by directly causing insulin resistance. During chronic stress, there is evidence that these cytokines promote depression-like behavior by disrupting neurotransmitter synthesis and signal transduction. Animal models of obesity and depression have revealed a bi-directional relationship whereby high-fat feeding and chronic stress synergize and exacerbate metabolic dysregulation and behavioral abnormalities. Although far from conclusive, emerging evidence suggests that inflammation in the central and peripheral immune system may link metabolic syndrome to major depressive disorder. In this review, we will synthesize available data supporting this view and identify critical areas for future investigation.

Sphingolipid changes do not underlie fatty acid-evoked GLUT4 insulin resistance nor inflammation signals in muscle cells

Ceramides contribute to obesity-linked insulin resistance and inflammation in vivo, but whether this is a cell-autonomous phenomenon is debated, particularly in muscle, which dictates whole-body glucose uptake. We comprehensively analyzed lipid species produced in response to fatty acids and examined the consequence to insulin resistance and pro-inflammatory pathways. L6 myotubes were incubated with BSA-adsorbed palmitate or palmitoleate in the presence of myriocin, fenretinide, or fumonisin B1. Lipid species were determined by lipidomic analysis. Insulin sensitivity was scored by Akt phosphorylation and glucose transporter 4 (GLUT4) translocation, while pro-inflammatory indices were estimated by IκBα degradation and cytokine expression. Palmitate, but not palmitoleate, had mild effects on Akt phosphorylation but significantly inhibited insulin-stimulated GLUT4 translocation and increased expression of pro-inflammatory cytokines Il6 and Ccl2 Ceramides, hexosylceramides, and sphingosine-1-phosphate significantly heightened by palmitate correlated negatively with insulin sensitivity and positively with pro-inflammatory indices. Inhibition of sphingolipid pathways led to marked changes in cellular lipids, but did not prevent palmitate-induced impairment of insulin-stimulated GLUT4 translocation, suggesting that palmitate-induced accumulation of deleterious lipids and insulin resistance are correlated but independent events in myotubes. We propose that muscle cell-endogenous ceramide production does not evoke insulin resistance and that deleterious effects of ceramides in vivo may arise through ancillary cell communication.

Expression of Pannexin 1 and Pannexin 3 during skeletal muscle development, regeneration, and Duchenne muscular dystrophy

Pannexin 1 (Panx1) and Pannexin 3 (Panx3) are single membrane channels recently implicated in myogenic commitment, as well as myoblast proliferation and differentiation in vitro. However, their expression patterns during skeletal muscle development and regeneration had yet to be investigated. Here, we show that Panx1 levels increase during skeletal muscle development becoming highly expressed together with Panx3 in adult skeletal muscle. In adult mice, Panx1 and Panx3 were differentially expressed in fast- and slow-twitch muscles. We also report that Panx1/PANX1 and Panx3/PANX3 are co-expressed in mouse and human satellite cells, which play crucial roles in skeletal muscle regeneration. Interestingly, Panx1 and Panx3 levels were modulated in muscle degeneration/regeneration, similar to the pattern seen during skeletal muscle development. As Duchenne muscular dystrophy is characterized by skeletal muscle degeneration and impaired regeneration, we next used mild and severe mouse models of this disease and found a significant dysregulation of Panx1 and Panx3 levels in dystrophic skeletal muscles. Together, our results are the first demonstration that Panx1 and Panx3 are differentially expressed amongst skeletal muscle types with their levels being highly modulated during skeletal muscle development, regeneration, and dystrophy. These findings suggest that Panx1 and Panx3 channels may play important and distinct roles in healthy and diseased skeletal muscles.

IFN-γ and TNF-α Pre-licensing Protects Mesenchymal Stromal Cells from the Pro-inflammatory Effects of Palmitate

The use of mesenchymal stromal cell (MSC) therapy for the treatment of type 2 diabetes (T2D) and T2D complications is promising; however, the investigation of MSC function in the setting of T2D has not been thoroughly explored. In our current study, we investigated the phenotype and function of MSCs in a simulated in vitro T2D environment. We show that palmitate, but not glucose, exposure impairs MSC metabolic activity with moderate increases in apoptosis, while drastically affecting proliferation and morphology. In co-culture with peripheral blood mononuclear cells (PBMCs), we found that MSCs not only lose their normal suppressive ability in high levels of palmitate, but actively support and enhance inflammation, resulting in elevated PBMC proliferation and pro-inflammatory cytokine release. The pro-inflammatory effect of MSCs in palmitate was partially reversed via palmitate removal and fully reversed through pre-licensing MSCs with interferon-gamma and tumor necrosis factor alpha. Thus, palmitate, a specific metabolic factor enriched within the T2D environment, is a potent modulator of MSC immunosuppressive function, which may in part explain the depressed potency observed in MSCs isolated from T2D patients. Importantly, we have also identified a robust and durable pre-licensing regimen that protects MSC immunosuppressive function in the setting of T2D.

Regulation of Skeletal Muscle Myoblast Differentiation and Proliferation by Pannexins

Pannexins are newly discovered channels that are now recognized as mediators of adenosine triphosphate release from several cell types allowing communication with the extracellular environment. Pannexins have been associated with various physiological and pathological processes including apoptosis, inflammation, and cancer. However, it is only recently that our work has unveiled a role for Pannexin 1 and Pannexin 3 as novel regulators of skeletal muscle myoblast proliferation and differentiation. Myoblast differentiation is an ordered multistep process that includes withdrawal from the cell cycle and the expression of key myogenic factors leading to myoblast differentiation and fusion into multinucleated myotubes. Eventually, myotubes will give rise to the diverse muscle fiber types that build the complex skeletal muscle architecture essential for body movement, postural behavior, and breathing. Skeletal muscle cell proliferation and differentiation are crucial processes required for proper skeletal muscle development during embryogenesis, as well as for the postnatal skeletal muscle regeneration that is necessary for muscle repair after injury or exercise. However, defects in skeletal muscle cell differentiation and/or deregulation of cell proliferation are involved in various skeletal muscle pathologies. In this review, we will discuss the expression of pannexins and their post-translational modifications in skeletal muscle, their known functions in various steps of myogenesis, including myoblast proliferation and differentiation, as well as their possible roles in skeletal muscle development, regeneration, and diseases such as Duchenne muscular dystrophy.

Cluster of Differentiation 36 Deficiency Aggravates Macrophage Infiltration and Hepatic Inflammation by Upregulating Monocyte Chemotactic Protein-1 Expression of Hepatocytes Through Histone Deacetylase 2-Dependent Pathway

AIMS

Cluster of differentiation 36 (CD36) is involved in the development of nonalcoholic steatohepatitis (NASH). Excess CD36 facilitates liver cells taking fatty acid and activates inflammatory signals to promote hepatic steatosis and inflammation. However, CD36 deficiency paradoxically promotes nonalcoholic fatty liver disease by unknown mechanisms. We explored the probable molecular mechanism of hepatic inflammation induced by CD36 deficiency.

RESULTS

CD36 deletion in mice (CD36^-/- mice) specifically increased monocyte chemotactic protein-1 (MCP-1) in hepatocytes, promoted macrophage migration to the liver, and aggravated hepatic inflammatory response and fibrosis. The nuclear expression of histone deacetylase 2 (HDAC2), which highly expresses in wild-type hepatocytes and has an inhibitory effect on acetyl histone 3 (H3), was reduced in CD36-deficient hepatocytes. Consequently, the level of acetyl H3 binding to MCP-1 promoters was increased in CD36-deficient hepatocytes, causing hepatic-specific MCP-1 transcriptional activation. Reduction of nuclear HDAC2 in both CD36^-/- mice liver and cultured hepatocytes was due to reduction of intracellular reactive oxygen species (ROS) level, while supplement of low-concentration hydrogen peroxide (H₂O₂) overcame the suppression of HDAC2 caused by CD36 deficiency, decreasing MCP-1 gene transcription and microphage migration.

INNOVATION

Our results provide first evidence that decreased ROS production by CD36 deletion was also harmful for livers. The fine balance of CD36 plays an important role in maintaining balances of hepatic ROS and nuclear HDAC2, which could be a potential new therapeutic strategy for the prevention of NASH development.

CONCLUSION

CD36 deficiency promoted the development of NASH by facilitating the transcription of MCP-1 in hepatocytes due to the reduction of ROS and nuclear HDAC2. Antioxid. Redox Signal. 00, 000-000.

Pannexin hemichannels: A novel promising therapy target for oxidative stress related diseases

Pannexins, which contain three subtypes: pannexin-1, -2, and -3, are vertebrate glycoproteins that form non-junctional plasma membrane intracellular hemichannels via oligomerization. Oxidative stress refers to an imbalance of the generation and elimination of reactive oxygen species (ROS). Studies have shown that elevated ROS levels are pivotal in the development of a variety of diseases. Recent studies indicate that the occurrence of these oxidative stress related diseases is associated with pannexin hemichannels. It is also reported that pannexins regulate the production of ROS which in turn may increase the opening of pannexin hemichannels. In this paper, we review recent researches about the important role of pannexin hemichannels in oxidative stress related diseases. Thus, pannexin hemichannels, novel therapeutic targets, hold promise in managing oxidative stress related diseases such as the tumor, inflammatory bowel diseases (IBD), pulmonary fibrosis, chronic obstructive pulmonary disease (COPD), cardiovascular disease, insulin resistance (IR), and neural degeneration diseases.

Deconstructing metabolic inflammation using cellular systems

Over the past years, we have embarked in a systematic analysis of the effect of obesity or fatty acids on circulating monocytes, microvascular endothelial cells, macrophages, and skeletal muscle cells. With the use of cell culture strategies, we have deconstructed complex physiological systems and then reconstructed "partial equations" to better understand cell-to-cell communication. Through these approaches, we identified that in high saturated fat environments, cell-autonomous proinflammatory pathways are activated in monocytes and endothelial cells, promoting monocyte adhesion and transmigration. We think of this as a paradigm of the conditions promoting immune cell infiltration into tissues during obesity. In concert, it is possible that muscle and adipose tissue secrete immune cell chemoattractants, and indeed, our tissue culture reconstructions reveal that myotubes treated with the saturated fatty acid palmitate, but not the unsaturated fatty acid palmitoleate, release nucleotides that attract monocytes and other compounds that promote proinflammatory classically activated "(M1)-like" polarization in macrophages. In addition, palmitate directly triggers an M1-like macrophage phenotype, and secretions from these activated macrophages confer insulin resistance to target muscle cells. Together, these studies suggest that in pathophysiological conditions of excess fat, the muscle, endothelial and immune cells engage in a synergistic crosstalk that exacerbates tissue inflammation, leukocyte infiltration, polarization, and consequent insulin resistance.

Innate immune receptors in skeletal muscle metabolism

Recent decades have seen increasing evidence for a role for both innate and adaptive immunity in response to changes in and in the modulation of metabolic status. This new field of immunometabolism builds on evidence for activation of immune-derived signals in metabolically relevant tissues such as adipose tissue, liver, hypothalamus and skeletal muscle. Skeletal muscle is the primary site of dietary glucose disposal and therefore a key player in the development of diabetes, but studies on the role of inflammation in modulating skeletal muscle metabolism and its possible impact on whole body insulin sensitivity are scarce. This review describes the baseline mRNA expression of innate immune receptors (Toll- and NOD-like receptors) in human skeletal muscle and summarizes studies on putative role of these receptors in skeletal muscle in the context of diabetes, obesity and whole body metabolism.

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.

Saturated fatty acids activate caspase-4/5 in human monocytes, triggering IL-1β and IL-18 release

Obesity is associated with metabolic tissue infiltration by monocyte-derived macrophages. Saturated fatty acids contribute to proinflammatory gene induction in tissue-embedded immune cells. However, it is unknown how circulating monocytes, the macrophage precursors, react to high-fat environments. In macrophages, saturated fatty acids activate inflammatory pathways and, notably, prime caspase-associated inflammasomes. Inflammasome-activated IL-1β contributes to type 2 diabetes. We hypothesized that 1) human monocytes from obese patients show caspase activation, and 2) fatty acids trigger this response and consequent release of IL-1β/IL-18. Human peripheral blood monocytes were sorted by flow cytometry, and caspase activity was measured with a FLICA dye-based assay. Blood monocytes from obese individuals exhibited elevated caspase activity. To explore the nature and consequence of this activity, human THP1 monocytes were exposed to saturated or unsaturated fatty acids. Caspase activity was revealed by isoform-specific cleavage and enzymatic activity; cytokine expression/release was measured by qPCR and ELISA. Palmitate, but not palmitoleate, increased caspase activity in parallel to the release of IL-1β and IL-18. Palmitate induced eventual monocyte cell death with features of pyroptosis (an inflammation-linked cell death program involving caspase-4/5), scored through LDH release, vital dye influx, cell volume changes, and nuclear morphology. Notably, selective gene silencing or inhibition of caspase-4/5 reduced palmitate-induced release of IL-1β and IL-18. In summary, monocytes from obese individuals present elevated caspase activity. Mechanistically, palmitate activates a pyroptotic program in monocytes through caspase-4/5, causing inflammatory cytokine release, additional to inflammasomes. These caspases represent potential, novel, therapeutic targets to taper obesity-associated inflammation.

Pannexin3 inhibits TNF-α-induced inflammatory response by suppressing NF-κB signalling pathway in human dental pulp cells

Human dental pulp cells (HDPCs) play a crucial role in dental pulp inflammation. Pannexin 3 (Panx3), a member of Panxs (Pannexins), has been recently found to be involved in inflammation. However, the mechanism of Panx3 in human dental pulp inflammation remains unclear. In this study, the role of Panx3 in inflammatory response was firstly explored, and its potential mechanism was proposed. Immunohistochemical staining showed that Panx3 levels were diminished in inflamed human and rat dental pulp tissues. In vitro, Panx3 expression was significantly down‐regulated in HDPCs following a TNF‐α challenge in a concentration‐dependent way, which reached the lowest level at 10 ng/ml of TNF‐α. Such decrease could be reversed by MG132, a proteasome inhibitor. Unlike MG132, BAY 11‐7082, a NF‐κB inhibitor, even reinforced the inhibitory effect of TNF‐α. Quantitative real‐time PCR (qRT‐PCR) and enzyme‐linked immunosorbent assay (ELISA) were used to investigate the role of Panx3 in inflammatory response of HDPCs. TNF‐α‐induced pro‐inflammatory cytokines, interleukin (IL)‐1β and IL‐6, were significantly lessened when Panx3 was overexpressed in HDPCs. Conversely, Panx3 knockdown exacerbated the expression of pro‐inflammatory cytokines. Moreover, Western blot, dual‐luciferase reporter assay, immunofluorescence staining, qRT‐PCR and ELISA results showed that Panx3 participated in dental pulp inflammation in a NF‐κB‐dependent manner. These findings suggested that Panx3 has a defensive role in dental pulp inflammation, serving as a potential target to be exploited for the intervention of human dental pulp inflammation.

Panx3 links body mass index and tumorigenesis in a genetically heterogeneous mouse model of carcinogen-induced cancer

Background

Body mass index (BMI) has been implicated as a primary factor influencing cancer development. However, understanding the relationship between these two complex traits has been confounded by both environmental and genetic heterogeneity.

Methods

In order to gain insight into the genetic factors linking BMI and cancer, we performed chemical carcinogenesis on a genetically heterogeneous cohort of interspecific backcross mice ((Mus Spretus × FVB/N) F1 × FVB/N). Using this cohort, we performed quantitative trait loci (QTL) analysis to identify regions linked to BMI. We then performed an integrated analysis incorporating gene expression, sequence comparison between strains, and gene expression network analysis to identify candidate genes influencing both tumor development and BMI.

Results

Analysis of QTL linked to tumorigenesis and BMI identified several loci associated with both phenotypes. Exploring these loci in greater detail revealed a novel relationship between the Pannexin 3 gene (Panx3) and both BMI and tumorigenesis. Panx3 is positively associated with BMI and is strongly tied to a lipid metabolism gene expression network. Pre-treatment Panx3 gene expression levels in normal skin are associated with tumor susceptibility and inhibition of Panx function strongly influences inflammation.

Conclusions

These studies have identified several genetic loci that influence both BMI and carcinogenesis and implicate Panx3 as a candidate gene that links these phenotypes through its effects on inflammation and lipid metabolism.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-016-0334-8) contains supplementary material, which is available to authorized users.

Regulation of Pannexin-1 channel activity

Pannexin-1 (Panx1) forms anion-selective channels with a permeability up to 1 kDa and represents a pathway for the release of cytosolic ATP. Several structurally similar connexin (Cx) proteins have been identified in platelets and shown to play roles in haemostasis and thrombosis. More recently, functional Panx1 channels have been demonstrated on the surface of human platelets [Taylor et al. (2014) J. Thromb. Haemost. 12, 987-998]. Since their identification in the year 2000, several mechanisms have been reported to activate Panx1 channels, including mechanical stimulation, oxygen-glucose deprivation, a rise of [Ca2+]i, caspase cleavage and phosphorylation. Within this review, the regulation of Panx1 channels is discussed, with a focus on how they may contribute to platelet function.

Contracting C2C12 myotubes release CCL2 in an NF-κB-dependent manner to induce monocyte chemoattraction

Muscle inflammation following exercise is characterized by expression of inflammatory cytokines and chemokines. Exercise also increases muscle macrophages derived from circulating monocytes. However, it is unknown whether muscle cells themselves attract circulating monocytes, or what is the underlying mechanism. We used an in vitro system of electrical stimulation (ES) causing C2C12 myotube contraction to explore whether monocyte chemoattraction ensues and investigated the mediating chemoattractants. Conditioned medium from ES-contracted myotubes caused robust chemoattraction of THP-1 monocytes across Boyden chambers. Following ES, expression of several known monocyte chemokines [C-C motif ligand 2 (CCL2) and C-X-C motif ligand (CXCL)1, -2, and -5] was elevated, but of these, only recombinant CCL2 effectively reproduced monocyte migration. Electrically stimulated myotubes secreted CCL2, and neutralization of CCL2 in conditioned medium or antagonizing the CCL2 receptor (CCR2) in THP-1 monocytes inhibited ES-induced monocyte migration. N-benzyl-p-toluene sulfonamide (BTS), a myosin II-ATPase inhibitor, prevented ES-induced myotube contraction but not CCL2 gene expression and secretion. The membrane-permeant calcium chelator BAPTA-AM reduced ES-induced CCL2 secretion. Hence, electrical depolarization, rather than mechanical contraction, drives the rise in CCL2, with partial calcium input. ES activated the NF-κB pathway; NF-κB inhibitors reduced ES-induced CCL2 gene expression and secretion and repressed ES-induced THP-1 chemoattraction. Thus, electrically stimulated myotubes chemoattract monocytes through NF-κB-regulated CCL2 secretion.

Regulation of pannexin and connexin channels and their functional role in skeletal muscles

Myogenic precursor cells express connexins (Cx) and pannexins (Panx), proteins that form different membrane channels involved in cell-cell communication. Cx channels connect either the cytoplasm of adjacent cells, called gap junction channels (GJC), or link the cytoplasm with the extracellular space, termed hemichannels (HC), while Panx channels only support the latter. In myoblasts, Panx1 HCs play a critical role in myogenic differentiation, and Cx GJCs and possibly Cx HCs coordinate metabolic responses during later steps of myogenesis. After innervation, myofibers do not express Cxs, but still express Panx1. In myotubes and innervated myofibers, Panx1 HCs allow release of adenosine triphosphate and thus they might be involved in skeletal muscle plasticity. In addition, Panx1 HCs present in adult myofibers mediate adenosine triphosphate release and glucose uptake required for potentiation of muscle contraction. Under pathological conditions, such as upon denervation and spinal cord injury, levels of Panx1 are upregulated. However, Panx1(-/-) mice show similar degree of atrophy as denervated wild-type muscles. Skeletal muscles also express Cx HCs in the sarcolemma after denervation or spinal cord injury, plus other non-selective membrane channels, including purinergic P2X7 receptors and transient receptor potential type V2 channels. The absence of Cx43 and Cx45 is sufficient to drastically reduce denervation atrophy. Moreover, inflammatory cytokines also induce the expression of Cxs in myofibers, suggesting the expression of these Cxs as a common factor for myofiber degeneration under diverse pathological conditions. Inhibitors of skeletal muscle Cx HCs could be promising tools to prevent muscle wasting induced by conditions associated with synaptic dysfunction and inflammation.

Palmitate-induced inflammatory pathways in human adipose microvascular endothelial cells promote monocyte adhesion and impair insulin transcytosis

Obesity is associated with inflammation and immune cell recruitment to adipose tissue, muscle and intima of atherosclerotic blood vessels. Obesity and hyperlipidemia are also associated with tissue insulin resistance and can compromise insulin delivery to muscle. The muscle/fat microvascular endothelium mediates insulin delivery and facilitates monocyte transmigration, yet its contribution to the consequences of hyperlipidemia is poorly understood. Using primary endothelial cells from human adipose tissue microvasculature (HAMEC), we investigated the effects of physiological levels of fatty acids on endothelial inflammation and function. Expression of cytokines and adhesion molecules was measured by RT-qPCR. Signaling pathways were evaluated by pharmacological manipulation and immunoblotting. Surface expression of adhesion molecules was determined by immunohistochemistry. THP1 monocyte interaction with HAMEC was measured by cell adhesion and migration across transwells. Insulin transcytosis was measured by total internal reflection fluorescence microscopy. Palmitate, but not palmitoleate, elevated the expression of IL-6, IL-8, TLR2 (Toll-like receptor 2), and intercellular adhesion molecule 1 (ICAM-1). HAMEC had markedly low fatty acid uptake and oxidation, and CD36 inhibition did not reverse the palmitate-induced expression of adhesion molecules, suggesting that inflammation did not arise from palmitate uptake/metabolism. Instead, inhibition of TLR4 to NF-κB signaling blunted palmitate-induced ICAM-1 expression. Importantly, palmitate-induced surface expression of ICAM-1 promoted monocyte binding and transmigration. Conversely, palmitate reduced insulin transcytosis, an effect reversed by TLR4 inhibition. In summary, palmitate activates inflammatory pathways in primary microvascular endothelial cells, impairing insulin transport and increasing monocyte transmigration. This behavior may contribute in vivo to reduced tissue insulin action and enhanced tissue infiltration by immune cells.

Pannexin channels and ischaemia

An ischaemic stroke occurs during loss of blood flow in the brain from the occlusion of a blood vessel. The ischaemia itself comprises a complex array of insults, including oxygen and glucose deprivation (OGD), glutamate excitotoxicity, acidification/hypercapnia, and loss of sheer forces. A substantial amount of knowledge has accumulated that define the excitotoxic cascade downstream of N-methyl-d-aspartate receptors (NMDARs). While the NMDAR can influence numerous downstream elements, one critical target during ischaemia is the ion channel, pannexin-1 (Panx1). The C-terminal region of Panx1 appears critical for its regulation under a host of physiological and pathological stimuli. We have shown using hippocampal brain slices that Panx1 is activated by NMDARs through Src family kinases. However, it is not yet certain if this involves direct phosphorylation of Panx1 or an allosteric interaction between the channel's C-terminal tail and Src. Interestingly, Panx1 opening during ischaemia and NMDAR over-activation is antagonized by an interfering peptide that comprises amino acids 305-318 of Panx1. Thus, targeting the activation of Panx1 by NMDARs and Src kinases is an attractive mechanism to reduce anoxic depolarizations and neuronal death.