P2X7 receptor differentially couples to distinct release pathways for IL-1beta in mouse macrophage

Emerging role of P2X7 receptors in CNS health and disease

Purinergic signalling in the brain is becoming an important focus in the study of CNS health and disease. Various purinergic receptors are found to be present in different brain cells in varying extent, which get activated upon binding of ATP or its analogues. Conventionally, ATP was considered only as a major metabolic fuel of the cell but its recognition as a neurotransmitter in early 1970s, brought meaningful insights in neuron glia crosstalk, participating in various physiological functions in the brain. P2X7R, a member of ligand gated purinergic receptor (P2X) family, is gaining attention in the field of neuroscience because of its emerging role in broad spectrum of ageing and age related neurological disorders. The aim of this review is to provide an overview about the structure and function of P2X7R highlighting its unique features which distinguish it from the other members of its family. This review critically analyzes the literature mentioning the details about the agonist and antagonist of the P2X7R. It also emphasizes the advancements in understanding the dual role of P2X7R in brain development and disorders inviting meaningful insights about its involvement in Alzheimer's disease, Huntington's disease, Multiple Sclerosis, Neuropathic pain, Spinal Cord Injury and NeuroAIDS. Exploring the roles of P2X7R in detail is critical to identify its therapeutic potential in the treatment of acute and chronic neurodegenerative diseases. Moreover, this review also helps to raise more interest in the neurobiology of the purinergic receptors and thus providing new avenues for future research.

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.

Dichotomous roles for externalized cardiolipin in extracellular signaling: Promotion of phagocytosis and attenuation of innate immunity

Among the distinct molecular signatures present in the mitochondrion is the tetra-acylated anionic phospholipid cardiolipin, a lipid also present in primordial, single-cell bacterial ancestors of mitochondria and multiple bacterial species today. Cardiolipin is normally localized to the inner mitochondrial membrane; however, when cardiolipin becomes externalized to the surface of dysregulated mitochondria, it promotes inflammasome activation and stimulates the elimination of damaged or nonfunctional mitochondria by mitophagy. Given the immunogenicity of mitochondrial and bacterial membranes that are released during sterile and pathogen-induced trauma, we hypothesized that cardiolipins might function as "eat me" signals for professional phagocytes. In experiments with macrophage cell lines and primary macrophages, we found that membranes with mitochondrial or bacterial cardiolipins on their surface were engulfed through phagocytosis, which depended on the scavenger receptor CD36. Distinct from this process, the copresentation of cardiolipin with the Toll-like receptor 4 (TLR4) agonist lipopolysaccharide dampened TLR4-stimulated production of cytokines. These data suggest that externalized, extracellular cardiolipins play a dual role in host-host and host-pathogen interactions by promoting phagocytosis and attenuating inflammatory immune responses.

Purinergic mechanisms in neuroinflammation: An update from molecules to behavior

The principle functions of neuroinflammation are to limit tissue damage and promote tissue repair in response to pathogens or injury. While neuroinflammation has utility, pathophysiological inflammatory responses, to some extent, underlie almost all neuropathology. Understanding the mechanisms that control the three stages of inflammation (initiation, propagation and resolution) is therefore of critical importance for developing treatments for diseases of the central nervous system. The purinergic signaling system, involving adenosine, ATP and other purines, plus a host of P1 and P2 receptor subtypes, controls inflammatory responses in complex ways. Activation of the inflammasome, leading to release of pro-inflammatory cytokines, activation and migration of microglia and altered astroglial function are key regulators of the neuroinflammatory response. Here, we review the role of P1 and P2 receptors in mediating these processes and examine their contribution to disorders of the nervous system. Firstly, we give an overview of the concept of neuroinflammation. We then discuss the contribution of P2X, P2Y and P1 receptors to the underlying processes, including a discussion of cross-talk between these different pathways. Finally, we give an overview of the current understanding of purinergic contributions to neuroinflammation in the context of specific disorders of the central nervous system, with special emphasis on neuropsychiatric disorders, characterized by chronic low grade inflammation or maternal inflammation. An understanding of the important purinergic contribution to neuroinflammation underlying neuropathology is likely to be a necessary step towards the development of effective interventions. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

The NLRP3 Inflammasome and IL-1β Accelerate Immunologically Mediated Pathology in Experimental Viral Fulminant Hepatitis

Viral fulminant hepatitis (FH) is a severe disease with high mortality resulting from excessive inflammation in the infected liver. Clinical interventions have been inefficient due to the lack of knowledge for inflammatory pathogenesis in the virus-infected liver. We show that wild-type mice infected with murine hepatitis virus strain-3 (MHV-3), a model for viral FH, manifest with severe disease and high mortality in association with a significant elevation in IL-1β expression in the serum and liver. Whereas, the viral infection in IL-1β receptor-I deficient (IL-1R1^-/-) or IL-1R antagonist (IL-1Ra) treated mice, show reductions in virus replication, disease progress and mortality. IL-1R1 deficiency appears to debilitate the virus-induced fibrinogen-like protein-2 (FGL2) production in macrophages and CD45⁺Gr-1^high neutrophil infiltration in the liver. The quick release of reactive oxygen species (ROS) by the infected macrophages suggests a plausible viral initiation of NLRP3 inflammasome activation. Further experiments show that mice deficient of p47^phox, a nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunit that controls acute ROS production, present with reductions in NLRP3 inflammasome activation and subsequent IL-1β secretion during viral infection, which appears to be responsible for acquiring resilience to viral FH. Moreover, viral infected animals in deficiencies of NLRP3 and Caspase-1, two essential components of the inflammasome complex, also have reduced IL-1β induction along with ameliorated hepatitis. Our results demonstrate that the ROS/NLRP3/IL-1β axis institutes an essential signaling pathway, which is over activated and directly causes the severe liver disease during viral infection, which sheds light on development of efficient treatments for human viral FH and other severe inflammatory diseases.

The NLRP3 inflammasome and IL-1β play essential roles in mediating the primary inflammatory responses against pathogen invasions in the host. Hyperactivation of this signaling pathway can lead to life-threatening diseases under certain circumstances. However, it is not clear if NLRP3 inflammasome activation participates in the pathogenesis of viral fulminant hepatitis (FH), a clinical severe syndrome characterized by acute inflammation in the liver along with massive necrosis of hepatocytes and hepatic encephalopathy during viral infection. Using a mouse viral FH model by infection with murine hepatitis virus strain-3 (MHV-3), we observed a significant macrophage induction and the serum and liver massive accumulation of IL-1β. Conversely, interruption of IL-1β signals results in attenuation of the MHV-3-induced hepatitis and mortality. Blocking IL-1β activity reduces the virus-induced expression of fibrinogen-like protein-2 (FGL2) in macrophages, and limits the liver recruitment of CD45⁺Gr-1^high neutrophils upon the virus infection. We further show that proIL-1β is bioprocessed by NLRP3 inflammasome. Deletion of the components in the inflammasome complex, including NLRP3 and Caspase-1, leads to reduction in the virus-induced IL-1β production and lessening of disease progression. Further studies show that macrophages in deficiency of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunit p47^phox, a protein that controls acute ROS production, prevents NLRP3 inflammasome activation and IL-1β secretion, suggesting that the virus-induced ROS production can directly initiate NLRP3 inflammasome activation. Therefore, p47^phox-/- mice exhibited certain degrees of MHV-3 resistance. Taken together, these results demonstrate that ROS/NLRP3/IL-1β is the key pathway signaling exacerbated inflammatory responses that cause viral FH in mice, suggesting that mediation of this signal cascade may benefit on the disease treatment.

Schisandrin B induces an Nrf2-mediated thioredoxin expression and suppresses the activation of inflammasome in vitro and in vivo

Reactive oxygen species (ROS)-mediated activation of inflammasome is involved in the development of a wide spectrum of diseases. We aimed to investigate whether (-)schisandrin B [(-)Sch B], a phytochemical that can induce cellular antioxidant response, can suppress the inflammasome activation. Results showed that (-)Sch B can induce an nuclear factor erythroid 2-related factor 2-driven thioredoxin expression in primary peritoneal macrophages and cultured RAW264.7 macrophages. A 4-h priming of peritoneal macrophages with LPS followed by a 30-min incubation with ATP caused the activation of caspase 1 and the release of IL-1β, indicative of inflammasome activation. Although LPS/ATP did not activate inflammasome in RAW264.7 macrophages, it caused the ROS-dependent c-Jun N-terminal kinase1/2 (JNK1/2) activation and an associated lactate dehydrogenase (LDH) release in RAW264.7 macrophages, an indication of cytotoxicity. (-)Sch B suppressed the LPS/ATP-induced activation of caspase 1 and release of IL-1β in peritoneal macrophages. (-)Sch B also attenuated the LPS/ATP-induced ROS production, JNK1/2 activation and LDH release in RAW264.7 macrophages. The ability of (-)Sch B to suppress LPS/ATP-mediated inflammation in vitro was further confirmed by an animal study, in which schisandrin B treatment (2 mmol/kg p.o.) ameliorated the Imject Alum-induced peritonitis, as indicated by suppressions of caspase1 activation and plasma IL-1β level. The ensemble of results suggests that (-)Sch B may offer a promising prospect for preventing the inflammasome-mediated disorders.

Sulforaphane inhibits multiple inflammasomes through an Nrf2-independent mechanism

The inflammasomes are intracellular complexes that have an important role in cytosolic innate immune sensing and pathogen defense. Inflammasome sensors detect a diversity of intracellular microbial ligands and endogenous danger signals and activate caspase-1, thus initiating maturation and release of the proinflammatory cytokines interleukin-1β and interleukin-18. These events, although crucial to the innate immune response, have also been linked to the pathology of several inflammatory and autoimmune disorders. The natural isothiocyanate sulforaphane, present in broccoli sprouts and available as a dietary supplement, has gained attention for its antioxidant, anti-inflammatory, and chemopreventive properties. We discovered that sulforaphane inhibits caspase-1 autoproteolytic activation and interleukin-1β maturation and secretion downstream of the nucleotide-binding oligomerization domain-like receptor leucine-rich repeat proteins NLRP1 and NLRP3, NLR family apoptosis inhibitory protein 5/NLR family caspase-1 recruitment domain-containing protein 4 (NAIP5/NLRC4), and absent in melanoma 2 (AIM2) inflammasome receptors. Sulforaphane does not inhibit the inflammasome by direct modification of active caspase-1 and its mechanism is not dependent on protein degradation by the proteasome or de novo protein synthesis. Furthermore, sulforaphane-mediated inhibition of the inflammasomes is independent of the transcription factor nuclear factor erythroid-derived 2-like factor 2 (Nrf2) and the antioxidant response-element pathway, to which many of the antioxidant and anti-inflammatory effects of sulforaphane have been attributed. Sulforaphane was also found to inhibit cell recruitment to the peritoneum and interleukin-1β secretion in an in vivo peritonitis model of acute gout and to reverse NLRP1-mediated murine resistance to Bacillus anthracis spore infection. These findings demonstrate that sulforaphane inhibits the inflammasomes through a novel mechanism and contributes to our understanding of the beneficial effects of sulforaphane.

Advances in the pharmacology of lGICs auxiliary subunits

Ligand-gated ion channels (LGICs) are cell surface integral proteins that mediate the fast neurotransmission in the nervous system. LGICs require auxiliary subunits for their trafficking, assembly and pharmacological modulation. Auxiliary subunits do not form functional homomeric receptors, but are reported to assemble with the principal subunits in order to modulate their pharmacological profiles. For example, nACh receptors are built at least by co-assemble of α and β subunits, and the neuronal auxiliary subunits β3 and α5 and muscle type β, δ, γ, and ϵ determine the agonist affinity of these receptors. Serotonergic 5-HT3B, 5-HT3C, 5-HT3D and 5-HT3E are reported to assemble with the 5-HT3A subunit to modulate its pharmacological profile. Functional studies evaluating the role of γ2 and δ auxiliary subunits of GABAA receptors have made important advances in the understanding of the action of benzodiazepines, ethanol and neurosteroids. Glycine receptors are composed principally by α1-3 subunits and the auxiliary subunit β determines their synaptic location and their pharmacological response to propofol and ethanol. NMDA receptors appear to be functional as heterotetrameric channels. So far, the existence of NMDA auxiliary subunits is controversial. On the other hand, Kainate receptors are modulated by NETO 1 and 2. AMPA receptors are modulated by TARPs, Shisa 9, CKAMP44, CNIH2-3 auxiliary proteins reported that controls their trafficking, conductance and gating of channels. P2X receptors are able to associate with auxiliary Pannexin-1 protein to modulate P2X7 receptors. Considering the pharmacological relevance of different LGICs auxiliary subunits in the present work we will highlight the therapeutic potential of these modulator proteins.

Connexins and pannexins in the immune system and lymphatic organs

Connexin43 and pannexin1 are found in immune cells. While gap junctional communication has been demonstrated between immune cells, hemichannels have been implicated in many cellular functions. Among the functions involved as being connexin dependent and pannexin dependent are cell migration, phagocytosis, antigen presentation, T-cell reactivity and B-cell responses. Surprisingly, many of these connexin-related and pannexin-related functions are not recapitulated in in vivo models. This is leading to a reevaluation of the role of these proteins in immune function.

Understanding the role of P2X7 in affective disorders-are glial cells the major players?

Pathophysiology associated with several psychiatric disorders has been linked to inflammatory biomarkers. This has generated a theory of major depressive disorders as an inflammatory disease. The idea of pro-inflammatory cytokines altering behavior is now well accepted however many questions remain. Microglia can produce a plethora of inflammatory cytokines and these cells appear to be critical in the link between inflammatory changes and depressive disorders. Microglia play a known role in sickness behavior which has many components of depressive-like behavior such as social withdrawal, sleep alterations, and anorexia. Numerous candidate genes have been identified for psychiatric disorders in the last decade. Single nucleotide polymorphisms (SNPs) in the human P2X7 gene have been linked to bipolar disorder, depression, and to the severity of depressive symptoms. P2X7 is a ligand-gated cation channel expressed on microglia with lower levels found on astrocytes and on some neuronal populations. In microglia P2X7 is a major regulator of pro-inflammatory cytokines of the interleukin-1 family. Genetic deletion of P2X7 in mice is protective for depressive behavior in addition to inflammatory responses. P2X7(-/-) mice have been shown to demonstrate anti-depressive-like behavior in forced swim and tail suspension behavioral tests and stressor-induced behavioral responses were blunted. Both neurochemical (norepinephrine, serotonin, and dopamine) and inflammatory changes have been observed in the brains of P2X7(-/-) mice. This review will discuss the recent evidence for involvement of P2X7 in the pathophysiology of depressive disorders and propose mechanisms by which altered signaling through this ion channel may affect the inflammatory state of the brain.

A Dual Role for P2X7 Receptor during Porphyromonas gingivalis Infection

Emerging evidence suggests a role for purinergic signaling in the activation of multiprotein intracellular complexes called inflammasomes, which control the release of potent inflammatory cytokines, such as interleukin (IL) -1β and -18. Porphyromonas gingivalis is intimately associated with periodontitis and is currently considered one of the pathogens that can subvert the immune system by limiting the activation of the NLRP3 inflammasome. We recently showed that P. gingivalis can dampen eATP-induced IL-1β secretion by means of its fimbriae in a purinergic P2X7 receptor-dependent manner. Here, we further explore the role of this purinergic receptor during eATP-induced IL-1β processing and secretion by P. gingivalis-infected macrophages. We found that NLRP3 was necessary for eATP-induced IL-1β secretion as well as for caspase 1 activation irrespective of P. gingivalis fimbriae. Additionally, although the secretion of IL-1β from P. gingivalis-infected macrophages was dependent on NLRP3, its adaptor protein ASC, or caspase 1, the cleavage of intracellular pro-IL-1β to the mature form was found to occur independently of NLRP3, its adaptor protein ASC, or caspase 1. Our in vitro findings revealed that P2X7 receptor has a dual role, being critical not only for eATP-induced IL-1β secretion but also for intracellular pro-IL-1β processing. These results were relevant in vivo since P2X7 receptor expression was upregulated in a P. gingivalis oral infection model, and reduced IFN-γ and IL-17 were detected in draining lymph node cells from P2rx7(-/-) mice. Furthermore, we demonstrated that P2X7 receptor and NLRP3 transcription were modulated in human chronic periodontitis. Overall, we conclude that the P2X7 receptor has a role in periodontal immunopathogenesis and suggest that targeting of the P2X7/NLRP3 pathway should be considered in future therapeutic interventions in periodontitis.

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.

Silica-induced inflammasome activation in macrophages: role of ATP and P2X7 receptor

Silicosis is a fibrotic lung disease caused by the inhalation of silica particles, and is considered an occupational disease, given that these particles are present in the working environment of many mining and civil construction industries. NLRP3 inflammasome activation is an important mechanism during the inflammatory process of silicosis, and it promotes the production of cytokines, such as IL-1β and IL-18. ATP also plays an important role in silicosis. Specifically, extracellular ATP can activate P2X7 receptor, which then participates in the complete assembly of the NLRP3 inflammasome and its activation. Herein, we analyze the literature to provide a better understanding of the mechanisms underlying inflammasome activation and the role of P2X7 receptors in macrophages during silicosis.

Mechanisms of unconventional secretion of IL-1 family cytokines

One of the most poorly understood processes in cell biology is the peculiar ability of specific leaderless proteins to be secreted via ER/Golgi-independent mechanisms ('unconventional protein secretion'). One such leaderless protein is the major immune-activating cytokine, interleukin-1β (IL-1β). Unusual amongst cytokines, IL-1β is expressed in the cytosol as an inactive precursor protein. It requires maturation by the caspase-1 protease, which itself requires activation upon immune cell sensing of infection or cell stress. Despite 25 years of intensive research into IL-1β secretory mechanisms, how it exits the cell is still not well understood. Here we will review the various mechanisms by which macrophages have been proposed to secrete IL-1 family cytokines, and the potential involvement of caspase-1 therein. Since aberrant IL-1β production drives inherited and acquired human diseases (e.g. autoinflammatory diseases, arthritic diseases, gout, Alzheimer's disease), elucidation of the IL-1β secretory pathway may offer new therapeutic opportunities for treatment across this wide range of human conditions.

P2X3 receptors and sensitization of autonomic reflexes

A great deal of basic and applied physiology and pharmacology in sensory and autonomic neuroscience has teased apart mechanisms that drive normal perception of mechanical, thermal and chemical signals and convey them to CNS, the distinction of fiber types and receptors and channels that mediate them, and how they may become dysfunctional or maladaptive in disease. Likewise, regulation of efferent autonomic traffic to control organ reflexes has been well studied. In both afferent and efferent limbs, a wide array of potential therapeutic mechanisms has surfaced, some of which have progressed into clinic, if not full regrastration. One conversation that has been less well progressed relates to how the afferent limb and its sensitization shapes the efferent outputs, and where modulation may offer new therapeutic avenues, especially for poorly addressed and common signs and symptoms of disease. Therapeutics for CV disease (HF, hypertension), respiratory disease (asthma, COPD), urological disease (OAB), GI disease (IBS), and inter alia, have largely focused on the efferent control of effector cells to modulate movement, contraction and secretion; medicinal needs remain with limits to efficacy, AEs and treatment resistance being common. We now must turn, in the quest for improved therapeutics, to understand how sensation from these organs becomes maladapted and sensitized in disease, and what opportunities may arise for improved therapeutics given the abundance of targets, many pharmacologically untapped, on the afferent side. One might look at the treatment resistant hypertension and the emerging benefit of renal denervation; or urinary bladder overactivity / neurogenic bladder and the emergence of neuromodulation, capsaicin instillation or botox injections to attenuate sensitized reflexes, as examples of merely the start of such progress. This review examines this topic more deeply, as applies to four major organ systems all sharing a great need from unsatisfied patients.

Correlation of the composition of biominerals with their ability of stimulating intracellular DNA sensors and inflammatory cytokines

Biominerals (or microcalcification) deposited in soft tissues are associated with a number of pathogeneses and cancer progressions. Biominerals have also shown promise for DNA delivery and tissue engineering. Biominerals themselves may stimulate NALP3 inflammasomes, and DNA delivered by biominerals can potentially engage with intracellular DNA sensors, resulting in unwanted inflammatory responses. In this study, a library of biominerals doped with or without DNA is formed through surface-induced biomineralization. It is demonstrated that empty biominerals stimulate NALP3 inflammasomes and induce the production of IL-1β. They are also able to activate mouse embryonic fibroblasts (MEFs) and induce inflammatory cytokines, i.e. IL-6. DNA delivered by biominerals escapes the detection of TLR9, but activates DAI and inflammasomes. Furthermore, it is shown that the level of both IL-1β and IL-6 is correlated with the composition of biominerals, in particular the ratio of Mg(Sr) to Ca, and the pH sensitivity of biominerals. These results provide insights into the design of safe and effective DNA delivery systems and biocompatible implants as well as the understanding of the pathogeneses of biominerals deposited in soft tissues.

Carbon monoxide negatively regulates NLRP3 inflammasome activation in macrophages

Inflammasomes are cytosolic protein complexes that promote the cleavage of caspase-1, which leads to the maturation and secretion of proinflammatory cytokines, including interleukin-1β (IL-1β) and IL-18. Among the known inflammasomes, the nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3 (NLRP3)-dependent inflammasome is critically involved in the pathogenesis of various acute or chronic inflammatory diseases. Carbon monoxide (CO), a gaseous molecule physiologically produced in cells and tissues during heme catabolism, can act as an anti-inflammatory molecule and a potent negative regulator of Toll-like receptor signaling pathways. To date, the role of CO in inflammasome-mediated immune responses has not been fully investigated. Here, we demonstrated that CO inhibited caspase-1 activation and the secretion of IL-1β and IL-18 in response to lipopolysaccharide (LPS) and ATP treatment in bone marrow-derived macrophages. CO also inhibited IL-18 secretion in response to LPS and nigericin treatment, another NLRP3 inflammasome activation model. In contrast, CO did not suppress IL-18 secretion in response to LPS and poly(dA:dT), an absent in melanoma 2 (AIM2)-mediated inflammasome model. LPS and ATP stimulation induced the formation of complexes between NLRP3 and apoptosis-associated speck-like protein, or NLRP3 and caspase-1. CO treatment inhibited these molecular interactions that were induced by LPS and ATP. Furthermore, CO inhibited mitochondrial ROS generation and the decrease of mitochondrial membrane potential induced by LPS and ATP in macrophages. We also observed that the inhibitory effect of CO on the translocation of mitochondrial DNA into the cytosol was associated with suppression of cytokine secretion. Our results suggest that CO negatively regulates NLRP3 inflammasome activation by preventing mitochondrial dysfunction.

cGAS and Ifi204 cooperate to produce type I IFNs in response to Francisella infection

Type I IFN production is an important host immune response against viral and bacterial infections. However, little is known about the ligands and corresponding host receptors that trigger type I IFN production during bacterial infections. We used a model intracellular pathogen, Francisella novicida, to begin characterizing the type I IFN response to bacterial pathogens. F. novicida replicates in the cytosol of host cells and elicits a robust type I IFN response that is largely TLR independent, but is dependent on the adapter molecule STING, suggesting that the type I IFN stimulus during F. novicida infection is cytosolic. In this study, we report that the cytosolic DNA sensors, cyclic GMP-AMP synthase (cGAS) and Ifi204, are both required for the STING-dependent type I IFN response to F. novicida infection in both primary and immortalized murine macrophages. We created cGAS, Ifi204, and Sting functional knockouts in RAW264.7 macrophages and demonstrated that cGAS and Ifi204 cooperate to sense dsDNA and activate the STING-dependent type I IFN pathway. In addition, we show that dsDNA from F. novicida is an important type I IFN stimulating ligand. One outcome of cGAS-STING signaling is the activation of the absent in melanoma 2 inflammasome in response to F. novicida infection. Whereas the absent in melanoma 2 inflammasome is beneficial to the host during F. novicida infection, type I IFN signaling by STING and IFN regulatory factor 3 is detrimental to the host during F. novicida infection. Collectively, our studies indicate that cGAS and Ifi204 cooperate to sense cytosolic dsDNA and F. novicida infection to produce a strong type I IFN response.

Nucleoside reverse transcriptase inhibitors possess intrinsic anti-inflammatory activity

Nucleoside reverse transcriptase inhibitors (NRTIs) are mainstay therapeutics for HIV that block retrovirus replication. Alu (an endogenous retroelement that also requires reverse transcriptase for its life cycle)-derived RNAs activate P2X7 and the NLRP3 inflammasome to cause cell death of the retinal pigment epithelium in geographic atrophy, a type of age-related macular degeneration. We found that NRTIs inhibit P2X7-mediated NLRP3 inflammasome activation independent of reverse transcriptase inhibition. Multiple approved and clinically relevant NRTIs prevented caspase-1 activation, the effector of the NLRP3 inflammasome, induced by Alu RNA. NRTIs were efficacious in mouse models of geographic atrophy, choroidal neovascularization, graft-versus-host disease, and sterile liver inflammation. Our findings suggest that NRTIs are ripe for drug repurposing in P2X7-driven diseases.

Antibody landscapes after influenza virus infection or vaccination

We introduce the antibody landscape, a method for the quantitative analysis of antibody-mediated immunity to antigenically variable pathogens, achieved by accounting for antigenic variation among pathogen strains. We generated antibody landscapes to study immune profiles covering 43 years of influenza A/H3N2 virus evolution for 69 individuals monitored for infection over 6 years and for 225 individuals pre- and postvaccination. Upon infection and vaccination, titers increased broadly, including previously encountered viruses far beyond the extent of cross-reactivity observed after a primary infection. We explored implications for vaccination and found that the use of an antigenically advanced virus had the dual benefit of inducing antibodies against both advanced and previous antigenic clusters. These results indicate that preemptive vaccine updates may improve influenza vaccine efficacy in previously exposed individuals.

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

Mechanosensitive unpaired innexin channels in C. elegans touch neurons

Invertebrate innexin proteins share sequence homology with vertebrate pannexins and general membrane topology with both pannexins and connexins. While connexins form gap junctions that mediate intercellular communication, pannexins are thought to function exclusively as plasma membrane channels permeable to both ions and small molecules. Undoubtedly, certain innexins function as gap junction proteins. However, due to sequence similarity to pannexins, it was postulated that innexins also function as plasma membrane channels. Indeed, some of the leech innexins were found to mediate ATP release as unpaired membrane channels with shared pharmacology to pannexin channels. We show here that Caenorhabditis elegans touch-sensing neurons express a mechanically gated innexin channel with a conductance of ∼1 nS and voltage-dependent and K(+)-selective subconductance state. We also show that C. elegans touch neurons take up ethidium bromide through a mechanism that is activated and blocked by innexin activating stimuli and inhibitors, respectively. Finally, we present evidence that touch neurons' innexins are required for cell death induced by chemical ischemia. Our work demonstrates that innexins function as plasma membrane channels in native C. elegans neurons, where they may play a role in pathological cell death.

Pannexin 1 channels: new actors in the regulation of catecholamine release from adrenal chromaffin cells

Chromaffin cells of the adrenal gland medulla synthesize and store hormones and peptides, which are released into the blood circulation in response to stress. Among them, adrenaline is critical for the fight-or-flight response. This neurosecretory process is highly regulated and depends on cytosolic [Ca²⁺]. By forming channels at the plasma membrane, pannexin-1 (Panx1) is a protein involved in many physiological and pathological processes amplifying ATP release and/or Ca²⁺ signals. Here, we show that Panx1 is expressed in the adrenal gland where it plays a role by regulating the release of catecholamines. In fact, inhibitors of Panx1 channels, such as carbenoxolone (Cbx) and probenecid, reduced the secretory activity induced with the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP, 50 μM) in whole adrenal glands. A similar inhibitory effect was observed in single chromaffin cells using Cbx or ¹⁰Panx1 peptide, another Panx1 channel inhibitors. Given that the secretory response depends on cytosolic [Ca²⁺] and Panx1 channels are permeable to Ca²⁺, we studied the possible implication of Panx1 channels in the Ca²⁺ signaling occurring during the secretory process. In support of this possibility, Panx1 channel inhibitors significantly reduced the Ca²⁺ signals evoked by DMPP in single chromaffin cells. However, the Ca²⁺ signals induced by caffeine in the absence of extracellular Ca²⁺ was not affected by Panx1 channel inhibitors, suggesting that this mechanism does not involve Ca²⁺ release from the endoplasmic reticulum. Conversely, Panx1 inhibitors significantly blocked the DMPP-induce dye uptake, supporting the idea that Panx1 forms functional channels at the plasma membrane. These findings indicate that Panx1 channels participate in the control the Ca²⁺ signal that triggers the secretory response of adrenal chromaffin cells. This mechanism could have physiological implications during the response to stress.

CD39 is a negative regulator of P2X7-mediated inflammatory cell death in mast cells

Background

Mast cells (MCs) are major contributors to an inflammatory milieu. One of the most potent drivers of inflammation is the cytokine IL-1β, which is produced in the cytoplasm in response to danger signals like LPS. Several controlling mechanisms have been reported which limit the release of IL-1β. Central to this regulation is the NLRP3 inflammasome, activation of which requires a second danger signal with the capacity to subvert the homeostasis of lysosomes and mitochondria. High concentrations of extracellular ATP have the capability to perturb the plasma membrane by activation of P2X7 channels and serve as such a danger signal. In this study we investigate the role of P2X7 channels and the ecto-5´-nucleotidase CD39 in ATP-triggered release of IL-1β from LPS-treated mast cells.

Results

We report that in MCs CD39 sets an activation threshold for the P2X7-dependent inflammatory cell death and concomitant IL-1β release. Knock-out of CD39 or stimulation with non-hydrolysable ATP led to a lower activation threshold for P2X7-dependent responses. We found that stimulation of LPS-primed MCs with high doses of ATP readily induced inflammatory cell death. Yet, cell death-dependent release of IL-1β yielded only minute amounts of IL-1β. Intriguingly, stimulation with low ATP concentrations augmented the production of IL-1β in LPS-primed MCs in a P2X7-independent but caspase-1-dependent manner.

Conclusion

Our study demonstrates that the fine-tuned interplay between ATP and different surface molecules recognizing or modifying ATP can control inflammatory and cell death decisions.

The P2X7 receptor channel: recent developments and the use of P2X7 antagonists in models of disease

The P2X7 receptor is a trimeric ATP-gated cation channel found predominantly, but not exclusively, on immune cells. P2X7 activation results in a number of downstream events, including the release of proinflammatory mediators and cell death and proliferation. As such, P2X7 plays important roles in various inflammatory, immune, neurologic and musculoskeletal disorders. This review focuses on the use of P2X7 antagonists in rodent models of neurologic disease and injury, inflammation, and musculoskeletal and other disorders. The cloning and characterization of human, rat, mouse, guinea pig, dog, and Rhesus macaque P2X7, as well as recent observations regarding the gating and permeability of P2X7, are discussed. Furthermore, this review discusses polymorphic and splice variants of P2X7, as well as the generation and use of P2X7 knockout mice. Recent evidence for emerging signaling pathways downstream of P2X7 activation and the growing list of negative and positive modulators of P2X7 activation and expression are also described. In addition, the use of P2X7 antagonists in numerous rodent models of disease is extensively summarized. Finally, the use of P2X7 antagonists in clinical trials in humans and future directions exploring P2X7 as a therapeutic target are described.

Immunogenicity of coiled-coil based drug-free macromolecular therapeutics

A two-component CD20 (non-internalizing) receptor crosslinking system based on the biorecognition of complementary coiled-coil forming peptides was evaluated. Exposure of B cells to Fab'-peptide1 conjugate decorates the cell surface with peptide1; further exposure of the decorated cells to P-(peptide2)x (P is the N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer backbone) results in the formation of coiled-coil heterodimers at the cell surface with concomitant induction of apoptosis. The aim of this study was to determine the potential immunogenicity of this therapeutic system that does not contain low molecular weight drugs. Enantiomeric peptides (L- and D-CCE and L- and D-CCK), HPMA copolymer-peptide conjugates, and Fab' fragment-peptide conjugates were synthesized and the immunological properties of peptide conjugates evaluated in vitro on RAW264.7 macrophages and in vivo on immunocompetent BALB/c mice. HPMA copolymer did not induce immune response in vitro and in vivo. Administration of P-peptide conjugates with strong adjuvant resulted in antibody response directed to the peptide. Fab' was responsible for macrophage activation of Fab'-peptide conjugates and a major factor in the antibody induction following i.v. administration of Fab'-conjugates. There was no substantial difference in the ability of conjugates of D-peptides and conjugates of L-peptides to induce Ab response.

The pathogenic mechanism of the Mycobacterium ulcerans virulence factor, mycolactone, depends on blockade of protein translocation into the ER

Infection with Mycobacterium ulcerans is characterised by tissue necrosis and immunosuppression due to mycolactone, the necessary and sufficient virulence factor for Buruli ulcer disease pathology. Many of its effects are known to involve down-regulation of specific proteins implicated in important cellular processes, such as immune responses and cell adhesion. We have previously shown mycolactone completely blocks the production of LPS-dependent proinflammatory mediators post-transcriptionally. Using polysome profiling we now demonstrate conclusively that mycolactone does not prevent translation of TNF, IL-6 and Cox-2 mRNAs in macrophages. Instead, it inhibits the production of these, along with nearly all other (induced and constitutive) proteins that transit through the ER. This is due to a blockade of protein translocation and subsequent degradation of aberrantly located protein. Several lines of evidence support this transformative explanation of mycolactone function. First, cellular TNF and Cox-2 can be once more detected if the action of the 26S proteasome is inhibited concurrently. Second, restored protein is found in the cytosol, indicating an inability to translocate. Third, in vitro translation assays show mycolactone prevents the translocation of TNF and other proteins into the ER. This is specific as the insertion of tail-anchored proteins into the ER is unaffected showing that the ER remains structurally intact. Fourth, metabolic labelling reveals a near-complete loss of glycosylated and secreted proteins from treated cells, whereas cytosolic proteins are unaffected. Notably, the profound lack of glycosylated and secreted protein production is apparent in a range of different disease-relevant cell types. These studies provide a new mechanism underlying mycolactone's observed pathological activities both in vitro and in vivo. Mycolactone-dependent inhibition of protein translocation into the ER not only explains the deficit of innate cytokines, but also the loss of membrane receptors, adhesion molecules and T-cell cytokines that drive the aetiology of Buruli ulcer.

Buruli ulcer is a progressive necrotic skin lesion caused by infection with the human pathogen Mycobacterium ulcerans. Mycolactone, a small compound produced by the mycobacterium, is the root cause of the disease pathology, but until now there has been no unifying mechanism explaining why. We have been using a model system to investigate the reason for the selective loss of protein that is a common feature of mycolactone exposure. Specifically, this involves identifying the point at which it stops immune cells making inflammatory mediators. In this work, we demonstrate that mycolactone inhibits production of such proteins by blocking the first step of protein export: translocation into a cellular compartment called the endoplasmic reticulum (ER). Proteins due for export are instead made in the cell cytosol where they are recognised as being in the wrong place and are rapidly degraded, causing a general cessation of the production of proteins that have to travel through the ER, including almost all secreted and surface proteins. This has a profound effect on basic cell functions such as growth, adhesion and survival. Therefore, we have identified the molecular basis underlying the key features of Buruli ulcer, and this will transform our understanding of disease progression.

The pathogen Candida albicans hijacks pyroptosis for escape from macrophages

The fungal pathogen Candida albicans causes macrophage death and escapes, but the molecular mechanisms remained unknown. Here we used live-cell imaging to monitor the interaction of C. albicans with macrophages and show that C. albicans kills macrophages in two temporally and mechanistically distinct phases. Early upon phagocytosis, C. albicans triggers pyroptosis, a proinflammatory macrophage death. Pyroptosis is controlled by the developmental yeast-to-hypha transition of Candida. When pyroptosis is inactivated, wild-type C. albicans hyphae cause significantly less macrophage killing for up to 8 h postphagocytosis. After the first 8 h, a second macrophage-killing phase is initiated. This second phase depends on robust hyphal formation but is mechanistically distinct from pyroptosis. The transcriptional regulator Mediator is necessary for morphogenesis of C. albicans in macrophages and the establishment of the wild-type surface architecture of hyphae that together mediate activation of macrophage cell death. Our data suggest that the defects of the Mediator mutants in causing macrophage death are caused, at least in part, by reduced activation of pyroptosis. A Mediator mutant that forms hyphae of apparently wild-type morphology but is defective in triggering early macrophage death shows a breakdown of cell surface architecture and reduced exposed 1,3 β-glucan in hyphae. Our report shows how Candida uses host and pathogen pathways for macrophage killing. The current model of mechanical piercing of macrophages by C. albicans hyphae should be revised to include activation of pyroptosis by hyphae as an important mechanism mediating macrophage cell death upon C. albicans infection.

Upon phagocytosis by macrophages, Candida albicans can transition to the hyphal form, which causes macrophage death and enables fungal escape. The current model is that the highly polarized growth of hyphae results in macrophage piercing. This model is challenged by recent reports of C. albicans mutants that form hyphae of wild-type morphology but are defective in killing macrophages. We show that C. albicans causes macrophage cell death by at least two mechanisms. Phase 1 killing (first 6 to 8 h) depends on the activation of the pyroptotic programmed host cell death by fungal hyphae. Phase 2 (up to 24 h) is rapid and depends on robust hyphal formation but is independent of pyroptosis. Our data provide a new model for how the interplay between fungal morphogenesis and activation of a host cell death pathway mediates macrophage killing by C. albicans hyphae.

Extracellular ATP drives systemic inflammation, tissue damage and mortality

Systemic inflammatory response syndromes (SIRS) may be caused by both infectious and sterile insults, such as trauma, ischemia-reperfusion or burns. They are characterized by early excessive inflammatory cytokine production and the endogenous release of several toxic and damaging molecules. These are necessary to fight and resolve the cause of SIRS, but often end up progressively damaging cells and tissues, leading to life-threatening multiple organ dysfunction syndrome (MODS). As inflammasome-dependent cytokines such as interleukin-1β are critically involved in the development of MODS and death in SIRS, and ATP is an essential activator of inflammasomes in vitro, we decided to analyze the ability of ATP removal to prevent excessive tissue damage and mortality in a murine LPS-induced inflammation model. Our results indeed indicate an important pro-inflammatory role for extracellular ATP. However, the effect of ATP is not restricted to inflammasome activation at all. Removing extracellular ATP with systemic apyrase treatment not only prevented IL-1β accumulation but also the production of inflammasome-independent cytokines such as TNF and IL-10. In addition, ATP removal also prevented systemic evidence of cellular disintegration, mitochondrial damage, apoptosis, intestinal barrier disruption and even mortality. Although blocking ATP receptors with the broad-spectrum P2 purinergic receptor antagonist suramin imitated certain beneficial effects of apyrase treatment, it could not prevent morbidity or mortality at all. We conclude that removal of systemic extracellular ATP could be a valuable strategy to dampen systemic inflammatory damage and toxicity in SIRS.

Pannexin-1 as a potentiator of ligand-gated receptor signaling

Pannexins are a class of plasma membrane spanning proteins that presumably form a hexameric, non-selective ion channel. Although similar in secondary structure to the connexins, pannexins notably do not form endogenous gap junctions and act as bona fide ion channels. The pannexins have been primarily studied as ATP-release channels, but the overall diversity of their functions is still being elucidated. There is an intriguing theme with pannexins that has begun to develop. In this review we analyze several recent reports that converge on the idea that pannexin channels (namely Panx1) can potentiate ligand-gated receptor signaling. Although the literature remains sparse, this emerging concept appears consistent between both ionotropic and metabotropic receptors of several ligand families.

The role of pannexin hemichannels in inflammation and regeneration

Tissue injury involves coordinated systemic responses including inflammatory response, targeted cell migration, cell-cell communication, stem cell activation and proliferation, and tissue inflammation and regeneration. The inflammatory response is an important prerequisite for regeneration. Multiple studies suggest that extensive cell-cell communication during tissue regeneration is coordinated by purinergic signaling via extracellular adenosine triphosphate (ATP). Most recent data indicates that ATP release for such communication is mediated by hemichannels formed by connexins and pannexins. The Pannexin family consists of three vertebrate proteins (Panx 1, 2, and 3) that have low sequence homology with other gap junction proteins and were shown to form predominantly non-junctional plasma membrane hemichannels. Pannexin-1 (Panx1) channels function as an integral component of the P2X/P2Y purinergic signaling pathway and is arguably the major contributor to pathophysiological ATP release. Panx1 is expressed in many tissues, with highest levels detected in developing brain, retina and skeletal muscles. Panx1 channel expression and activity is reported to increase significantly following injury/inflammation and during regeneration and differentiation. Recent studies also report that pharmacological blockade of the Panx1 channel or genetic ablation of the Panx1 gene cause significant disruption of progenitor cell migration, proliferation, and tissue regeneration. These findings suggest that pannexins play important roles in activation of both post-injury inflammatory response and the subsequent process of tissue regeneration. Due to wide expression in multiple tissues and involvement in diverse signaling pathways, pannexins and connexins are currently being considered as therapeutic targets for traumatic brain or spinal cord injuries, ischemic stroke and cancer. The precise role of pannexins and connexins in the balance between tissue inflammation and regeneration needs to be further understood.

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.

Molecular pathways of pannexin1-mediated neurotoxicity

Pannexin1 (Panx1) forms non-selective membrane channels, structurally similar to gap junction hemichannels, and are permeable to ions, nucleotides, and other small molecules below 900 Da. Panx1 activity has been implicated in paracrine signaling and inflammasome regulation. Recent studies in different animal models showed that overactivation of Panx1 correlates with a selective demise of several types of neurons, including retinal ganglion cells, brain pyramidal, and enteric neurons. The list of Panx1 activators includes extracellular ATP, glutamate, high K(+), Zn(2+), fibroblast growth factors (FGFs),pro-inflammatory cytokines, and elevation of intracellular Ca(2+). Most of these molecules are released following mechanical, ischemic, or inflammatory injury of the CNS, and rapidly activate the Panx1 channel. Prolonged opening of Panx1 channel induced by these "danger signals" triggers a cascade of neurotoxic events capable of killing cells. The most vulnerable cell type are neurons that express high levels of Panx1. Experimental evidence suggests that Panx1 channels mediate at least two distinct neurotoxic processes: increased permeability of the plasma membrane and activation of the inflammasome in neurons and glia. Importantly, both pharmacological and genetic inactivation of Panx1 suppresses both these processes, providing a marked protection in several disease and injury models. These findings indicate that external danger signals generated after diverse types of injuries converge to activate Panx1. In this review we discuss molecular mechanisms associated with Panx1 toxicity and the crosstalk between different pathways.

P2X7-dependent, but differentially regulated release of IL-6, CCL2, and TNF-α in cultured mouse microglia

ATP is an important regulator of microglia and its effects on microglial cytokine release are currently discussed as important contributors in a variety of brain diseases. We here analyzed the effects of ATP on the production of six inflammatory mediators (IL-6, IL-10, CCL2, IFN-γ, TNF-α, and IL-12p70) in cultured mouse primary microglia. Stimulation of P2X7 receptor by ATP (1 mM) or BzATP (500 µM) evoked the mRNA expression and release of proinflammatory cytokines IL-6, TNF-α, and the chemokine CCL2 in WT cells but not in P2X7(-/-) cells. The effects of ATP and BzATP were inhibited by the nonselective P2 receptor antagonists PPADs and suramin. Various selective P2X7 receptor antagonists blocked the P2X7-dependent release of IL-6 and CCL2, but, surprisingly, had no effect on BzATP-induced release of TNF-α in microglia. Calcium measurements confirmed that P2X7 is the main purine receptor activated by BzATP in microglia and showed that all P2X7 antagonists were functional. It is also presented that pannexin-1 hemichannel function and potential P2X4/P2X7 heterodimers are not involved in P2X7-dependent release of IL-6, CCL2, and TNF-α in microglia. How P2X7-specific antagonists only affect P2X7-dependent IL-6 and CCL2 release, but not TNF-α release is at the moment unclear, but indicates that the P2X7-dependent release of cytokines in microglia is differentially regulated.

Activation of inflammasomes in adipose tissue of women with gestational diabetes

Gestational diabetes mellitus (GDM) is characterised by maternal peripheral insulin resistance, increased inflammation, and increasing levels of circulating free fatty acids (FFAs) and advanced glycation endproducts (AGEs). Caspase-1 is a key component of the inflammasome, which is activated upon cellular infection or stress to trigger the maturation IL-1β, a pro-inflammatory cytokine that mediated insulin resistance. The aim of this study was to determine whether the inflammasome is activated in adipose tissue from women with gestational diabetes mellitus (GDM) and if it interferes with the insulin signalling pathway leading to the insulin resistance that is evident in GDM. Protein expression of active caspase-1 and mature IL-1β secretion was increased in adipose tissue of women with GDM. Treatment of adipose tissue with IL-1β decreased insulin-stimulated phosphorylation of IRS-1, GLUT-4 expression and glucose uptake. Low-grade inflammation (induced by LPS), the FFA palmitate and AGE conjugated to BSA (AGE-BSA), induced IL-1β secretion via inflammasome activation. In conclusion, the present findings describe an important role for adipose tissue inflammasome activation in the development of insulin resistance associated in pregnancies complicated by GDM.

The P2X7 receptor: a key player in immune-mediated bone loss?

Inflammatory diseases are often multiorganic diseases with manifestations not related directly to the primary affected organ. They are often complicated by a generalized bone loss that subsequently leads to osteoporosis and bone fractures. The exact mechanism for the accompanying bone loss is not understood in full detail, but factors such as glucocorticoid treatment, immobilization, malnutrition, and insufficient intake of vitamin D play a role. However, it has become evident that the inflammatory process itself is involved and the resulting bone loss is termed immune-mediated bone loss. It stems from an increase in bone resorption and the pro-inflammatory cytokines tumor necrosis factor alpha and interleukin 1 beta and has been shown to not only mediate the inflammatory response but also to strongly stimulate bone degradation. The purinergic P2X7 receptor is central in the processing of these two cytokines and in the initiation of the inflammatory response, and it is a key molecule in the regulation of both bone formation and bone resorption. The aim of this review is therefore to provide evidence-based novel hypotheses of the role of ATP-mediated purinergic signalling via the P2X7 receptor in immune-mediated bone loss and -osteoporosis.

Intrinsic properties and regulation of Pannexin 1 channel

Pannexin 1 (Panx1) channels are generally represented as non-selective, large-pore channels that release ATP. Emerging roles have been described for Panx1 in mediating purinergic signaling in the normal nervous, cardiovascular, and immune systems, where they may be activated by mechanical stress, ionotropic and metabotropic receptor signaling, and via proteolytic cleavage of the Panx1 C-terminus. Panx1 channels are widely expressed in various cell types, and it is now thought that targeting these channels therapeutically may be beneficial in a number of pathophysiological contexts, such as asthma, atherosclerosis, hypertension, and ischemic-induced seizures. Even as interest in Panx1 channels is burgeoning, some of their basic properties, mechanisms of modulation, and proposed functions remain controversial, with recent reports challenging some long-held views regarding Panx1 channels. In this brief review, we summarize some well-established features of Panx1 channels; we then address some current confounding issues surrounding Panx1 channels, especially with respect to intrinsic channel properties, in order to raise awareness of these unsettled issues for future research.

Visfatin contributes to the differentiation of monocytes into macrophages through the differential regulation of inflammatory cytokines in THP-1 cells

Visfatin is a novel multifunctional adipocytokine with inflammatory properties. Although a link between visfatin and atherosclerosis has recently been suggested, its actions in the development of atherosclerosis remain unknown. Therefore, we investigated a potential role and underlying mechanism(s) of visfatin in monocytes/macrophages differentiation, a critical early step in atherogenesis, using phorbol-12-myristate-13-acetate (PMA)-stimulated THP-1 cell models. The co-incubation of PMA with visfatin-induced CD36 expression with a concomitant increase in the phagocytosis of latex beads compared with PMA alone treatment. Moreover, visfatin markedly increased interleukin (IL)-1β secretion by enhancing IL-1β mRNA stability in a short-term incubation. Visfatin also significantly elevated the secretion of IL-6 as well as IL-1β in a longer incubation period, which was partially suppressed by nuclear factor-κB (NF-κB) inhibitor, BAY11-7082, and c-Jun-N-terminal kinase (JNK) inhibitor, SP600125. Furthermore, silencing IL-1β successfully blocked IL-6 secretion, CD36 expression, and NF-κB activation in response to visfatin. Collectively, these results suggest that visfatin enhances the IL-1β-dependent induction of IL-6 and CD36 via distinct signaling pathways mediated by JNK and NF-κB, respectively, and consequently, leading to the acceleration of monocytes/macrophages differentiation.

Responses of macrophages against Salmonella infection compared with phagocytosis

To explore the responses of host cell after infection with live Salmonella compared with phagocytosis to dead bacteria, the responses of mouse macrophage after infection with Salmonella enteritidis C50041 and the fixed C50041 (C50041-d) were analyzed. Results indicated that the cytotoxicity induced by C50041 was stronger than C50041-d. Similar changing trends of mitochondrial membrane potential, intracellular concentration of calcium ions, reactive oxygen species and nitric oxide were found between C50041 and C50041-d infection. But the cell responses against C50041 were earlier and stronger than C50041-d. LC3 expression of macrophage induced by C50041 was lower than C50041-d. C50041 significantly inhibited the production of tumor necrosis factor and interleukin (IL)-6. Whereas intracellular caspase-1 activation and IL-1β release induced by C50041 were stronger than C50041-d, caspase-1 activation and IL-1β release are the innate defense responses of macrophage. Therefore, it will be beneficial to explore the use of this pathway in the control of Salmonella infection.

Inflammasomes and metabolic disease

Innate immune response pathways and metabolic pathways are evolutionarily conserved throughout species and are fundamental to survival. As such, the regulation of whole-body and cellular metabolism is intimately integrated with immune responses. However, the introduction of new variables to this delicate evolutionarily conserved physiological interaction can lead to deleterious consequences for organisms as a result of inappropriate immune responses. In recent decades, the prevalence and incidence of metabolic diseases associated with obesity have dramatically increased worldwide. As a recently acquired human characteristic, obesity has exposed the critical role of innate immune pathways in multiple metabolic pathophysiological processes. Here, we review recent evidence that highlights inflammasomes as critical sensors of metabolic perturbations in multiple tissues and their role in the progression of highly prevalent metabolic diseases.

Caspase-1 activation is increased with human labour in foetal membranes and myometrium and mediates infection-induced interleukin-1β secretion

PROBLEM

Interleukin-1β (IL-1β) is a pro-inflammatory cytokine that is involved in human parturition, especially in the context of infection-induced preterm birth. Caspase-1 is a key component of inflammasomes, which are activated upon infection to trigger the maturation of IL-1β.

METHOD OF STUDY

To determine the effect of human labour on caspase-1 activation in human foetal membranes and myometrium. In addition, the mechanisms by which inflammasome activation regulates IL-1β production were also be assessed.

RESULTS

Higher caspase-1 gene and protein expression were detected in foetal membranes myometrium obtained from term labouring women when compared with samples taken from non labouring women. Lipopolysaccharide induced the transcription and secretion of IL-1β from foetal membranes and myometrium; both events were dependent on nuclear factor kappa B (NF-κB). However, levels of extracellular IL-1β were greatly increased by subsequent treatment with the potassium-proton ionophore Adenosine triphosphate (ATP) or nigericin; an effect that was dependent on active caspase-1. Additionally, ATP induced IL-1β secretion via the purinergic P2X7 receptor, whereas the pannexin-1 channel was required for nigericin induced IL-1β secretion.

CONCLUSION

Taken together, these results demonstrate that caspase-1 activation is increased with human labour in foetal membranes and myometrium, and is required for infection-induced IL-1β secretion.

P2X7 receptor-Pannexin1 interaction mediates stress-induced interleukin-1 beta expression in human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Pannexin 1 (Panx1) has been found to form nonjunctional hemichannels. It is also proposed to combine with the P2X7 receptor, forming a complex involved in adenosine triphosphate (ATP)-induced interleukin-1beta (IL-1β) release in macrophages. Previously, we reported that mechanical stress induced IL-1β expression via the ATP/P2X7 receptor-dependent pathway in human periodontal ligament (HPDL) cells and that ATP was released through the connexin 43 (Cx43) hemichannel. In the present work, we examined the role of Panx1 in stress-induced IL-1β induction in HPDL cells.

MATERIAL AND METHODS

Cultured HPDL cells were treated with compressive loading or ATP to stimulate IL-1β expression. Inhibitors, antagonists and the small interfering RNA technique were used to investigate the involvement of Panx1 in IL-1β induction. Co-immunoprecipitation (Co-IP) and immunostaining were used to determine the association of Panx1 with the P2X7 receptor. The IL-1β release mechanism was analyzed using inhibitors.

RESULTS

Blocking Panx1 significantly decreased ATP release, as well as IL-1β up-regulation, upon stimulation with stress or ATP. Co-IP revealed the association of Panx1 and the P2X7 receptor in HPDL cells, which was increased in response to mechanical loading. Pretreatment with vesicular trafficking inhibitors significantly reduced the amount of IL-1β released from stimulated cells, suggesting that IL-1β might be released through vesicles.

CONCLUSION

We clearly illustrated the contribution of Panx1 in ATP release, as well as in IL-1β induction in HPDL cells. The association of Panx1 and the P2X7 receptor might be required for IL-1β induction, and their possible novel role in IL-1β vesicular release was indicated.

From mechanosensitivity to inflammatory responses: new players in the pathology of glaucoma

PURPOSE OF THE STUDY

Many blinding diseases of the inner retina are associated with degeneration and loss of retinal ganglion cells (RGCs). Recent evidence implicates several new signaling mechanisms as causal agents associated with RGC injury and remodeling of the optic nerve head. Ion channels such as Transient receptor potential vanilloid isoform 4 (TRPV4), pannexin-1 (Panx1) and P2X7 receptor are localized to RGCs and act as potential sensors and effectors of mechanical strain, ischemia and inflammatory responses. Under normal conditions, TRPV4 may function as an osmosensor and a polymodal molecular integrator of diverse mechanical and chemical stimuli, whereas P2X7R and Panx1 respond to stretch- and/or swelling-induced adenosine triphosphate release from neurons and glia. Ca(2+) influx, induced by stimulation of mechanosensitive ion channels in glaucoma, is proposed to influence dendritic and axonal remodeling that may lead to RGC death while (at least initially) sparing other classes of retinal neuron. The secondary phase of the retinal glaucoma response is associated with microglial activation and an inflammatory response involving Toll-like receptors (TLRs), cluster of differentiation 3 (CD3) immune recognition molecules associated with the T-cell antigen receptor, complement molecules and cell type-specific release of neuroactive cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). The retinal response to mechanical stress thus involves a diversity of signaling pathways that sense and transduce mechanical strain and orchestrate both protective and destructive secondary responses.

CONCLUSIONS

Mechanistic understanding of the interaction between pressure-dependent and independent pathways is only beginning to emerge. This review focuses on the molecular basis of mechanical strain transduction as a primary mechanism that can damage RGCs. The damage occurs through Ca(2+)-dependent cellular remodeling and is associated with parallel activation of secondary ischemic and inflammatory signaling pathways. Molecules that mediate these mechanosensory and immune responses represent plausible targets for protecting ganglion cells in glaucoma, optic neuritis and retinal ischemia.

Cucurbitacin D is a new inflammasome activator in macrophages

We previously reported that cucurbitacin D isolated from Trichosanthes kirilowii has anti-tumor roles to leukemia cells. However, the effect of cucurbitacin D on immune cells is not fully understood although there is no toxic activity to normal cells. In this study, immunomodulating activities of cucurbitacin D were investigated in macrophages. Cucurbitacin D could increase LPS-induced interleukin (IL)-1β production in culture supernatant of THP-1 cells, peritoneal exudate cells (PECs), bone marrow derived macrophages (BMDMs), and RAW264 cells. At the transcriptional level, cucurbitacin D enhanced LPS-induced IL-1β mRNA expression through activation of ERK1/2 mitogen-activated protein kinases (MAPKs). At the posttranscriptional level, the activation of caspase-1 induced by cucurbitacin D has also been demonstrated following treatment with a caspase-1 inhibitor and siRNA. Importantly, cucurbitacin D has further been shown to induce inflammasome activation independent of ERK1/2 activation. Western blotting showed interaction of NOD-like receptor family, pyrin domain containing 3 (NALP3) and apoptosis-associated speck-like protein containing a caspase-activating and recruitment domain (ASC), suggesting activation of the inflammasome and a possible reason for activation of caspase-1. Taken together, these results suggest that cucurbitacin D could initiate immunomodulating activity in macrophages to lead to inflammasome activation as well as enhancement of LPS signaling.

Proapoptotic chemotherapeutic drugs induce noncanonical processing and release of IL-1β via caspase-8 in dendritic cells

The identification of noncanonical (caspase-1-independent) pathways for IL-1β production has unveiled an intricate interplay between inflammatory and death-inducing signaling platforms. We found a heretofore unappreciated role for caspase-8 as a major pathway for IL-1β processing and release in murine bone marrow-derived dendritic cells (BMDC) costimulated with TLR4 agonists and proapoptotic chemotherapeutic agents such as doxorubicin (Dox) or staurosporine (STS). The ability of Dox to stimulate release of mature (17-kDa) IL-1β was nearly equivalent in wild-type (WT) BMDC, Casp1(-/-)Casp11(-/-) BMDC, WT BMDC treated with the caspase-1 inhibitor YVAD, and BMDC lacking the inflammasome regulators ASC, NLRP3, or NLRC4. Notably, Dox-induced production of mature IL-1β was temporally correlated with caspase-8 activation in WT cells and greatly suppressed in Casp8(-/-)Rip3(-/-) or Trif(-/-) BMDC, as well as in WT BMDC treated with the caspase-8 inhibitor, IETD. Similarly, STS stimulated robust IL-1β processing and release in Casp1(-/-)Casp11(-/-) BMDC that was IETD sensitive. These data suggest that TLR4 induces assembly of caspase-8-based signaling complexes that become licensed as IL-1β-converting enzymes in response to Dox and STS. The responses were temporally correlated with downregulation of cellular inhibitor of apoptosis protein 1, suggesting suppressive roles for this and likely other inhibitor of apoptosis proteins on the stability and/or proteolytic activity of the caspase-8 platforms. Thus, proapoptotic chemotherapeutic agents stimulate the caspase-8-mediated processing and release of IL-1β, implicating direct effects of such drugs on a noncanonical inflammatory cascade that may modulate immune responses in tumor microenvironments.

Purinergic signaling in inflammatory cells: P2 receptor expression, functional effects, and modulation of inflammatory responses

Extracellular ATP and related nucleotides promote a wide range of pathophysiological responses via activation of cell surface purinergic P2 receptors. Almost every cell type expresses P2 receptors and/or exhibit regulated release of ATP. In this review, we focus on the purinergic receptor distribution in inflammatory cells and their implication in diverse immune responses by providing an overview of the current knowledge in the literature related to purinergic signaling in neutrophils, macrophages, dendritic cells, lymphocytes, eosinophils, and mast cells. The pathophysiological role of purinergic signaling in these cells include among others calcium mobilization, actin polymerization, chemotaxis, release of mediators, cell maturation, cytotoxicity, and cell death. We finally discuss the therapeutic potential of P2 receptor subtype selective drugs in inflammatory conditions.

A combined omics study on activated macrophages--enhanced role of STATs in apoptosis, immunity and lipid metabolism

BACKGROUND

Macrophage activation by lipopolysaccharide and adenosine triphosphate (ATP) has been studied extensively because this model system mimics the physiological context of bacterial infection and subsequent inflammatory responses. Previous studies on macrophages elucidated the biological roles of caspase-1 in post-translational activation of interleukin-1β and interleukin-18 in inflammation and apoptosis. However, the results from these studies focused only on a small number of factors. To better understand the host response, we have performed a high-throughput study of Kdo2-lipid A (KLA)-primed macrophages stimulated with ATP.

RESULTS

The study suggests that treating mouse bone marrow-derived macrophages with KLA and ATP produces 'synergistic' effects that are not seen with treatment of KLA or ATP alone. The synergistic regulation of genes related to immunity, apoptosis and lipid metabolism is observed in a time-dependent manner. The synergistic effects are produced by nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) and activator protein (AP)-1 through regulation of their target cytokines. The synergistically regulated cytokines then activate signal transducer and activator of transcription (STAT) factors that result in enhanced immunity, apoptosis and lipid metabolism; STAT1 enhances immunity by promoting anti-microbial factors; and STAT3 contributes to downregulation of cell cycle and upregulation of apoptosis. STAT1 and STAT3 also regulate glycerolipid and eicosanoid metabolism, respectively. Further, western blot analysis for STAT1 and STAT3 showed that the changes in transcriptomic levels were consistent with their proteomic levels. In summary, this study shows the synergistic interaction between the toll-like receptor and purinergic receptor signaling during macrophage activation on bacterial infection.

AVAILABILITY

Time-course data of transcriptomics and lipidomics can be queried or downloaded from http://www.lipidmaps.org.

CONTACT

shankar@ucsd.edu.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.