Over the past several decades, much has been revealed about the nature of the host innate immune response to microorganisms, with the identification of pattern recognition receptors (PRRs) and pathogen-associated molecular patterns, which are the conserved microbial motifs sensed by these receptors. It is now apparent that these same PRRs can also be activated by non-microbial signals, many of which are considered as damage-associated molecular patterns. The sterile inflammation that ensues either resolves the initial insult or leads to disease. Here, we review the triggers and receptor pathways that result in sterile inflammation and its impact on human health.

Sterile inflammation: sensing and reacting to damage

Inflammasomes are key signalling platforms that detect pathogenic microorganisms and sterile stressors, and that activate the highly pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18. In this Review, we discuss the complex regulatory mechanisms that facilitate a balanced but effective inflammasome-mediated immune response, and we highlight the similarities to another molecular signalling platform - the apoptosome - that monitors cellular health. Extracellular regulatory mechanisms are discussed, as well as the intracellular control of inflammasome assembly, for example, via ion fluxes, free radicals and autophagy.

/pdf/activation-and-regulation-of-the-inflammasomes-3jj26fqvmx.pdf

Activation and regulation of the inflammasomes

P2X 7 receptors are ATP‐gated cation channels; their activation in macrophage also leads to rapid opening of a membrane pore permeable to dyes such as ethidium, and to release of the pro‐inflammatory cytokine, interleukin‐1β (IL‐1β). It has not been known what this dye‐uptake path is, or whether it is involved in downstream signalling to IL‐1β release. Here, we identify pannexin‐1, a recently described mammalian protein that functions as a hemichannel when ectopically expressed, as this dye‐uptake pathway and show that signalling through pannexin‐1 is required for processing of caspase‐1 and release of mature IL‐1β induced by P2X 7 receptor activation.

/pdf/pannexin-1-mediates-large-pore-formation-and-interleukin-1b-4xozu6i4bz.pdf

Pannexin-1 mediates large pore formation and interleukin-1β release by the ATP-gated P2X7 receptor

Apoptotic cells were shown recently to discharge the nucleotides ATP and UTP to act as 'find-me' signals for phagocytes that engulf dying cells before they release potentially harmful cellular contents. This paper shows that the release of ATP and UTP is through the plasma membrane channel pannexin 1, which is specifically opened by caspase activity. The discovery of a role for pannexin 1 in phagocyte chemoattraction could have implications for human diseases that arise from defective clearance of dying cells. Apoptotic cells discharge ATP and UTP, which act as 'find-me' signals for phagocytes that in turn engulf dying cells before potentially harmful cellular contents are released. These authors show that the release of ATP and UTP is exclusively by means of the plasma membrane channel pannexin 1, which is opened specifically by caspase activity. Apoptotic cells release ‘find-me’ signals at the earliest stages of death to recruit phagocytes1. The nucleotides ATP and UTP represent one class of find-me signals2, but their mechanism of release is not known. Here, we identify the plasma membrane channel pannexin 1 (PANX1) as a mediator of find-me signal/nucleotide release from apoptotic cells. Pharmacological inhibition and siRNA-mediated knockdown of PANX1 led to decreased nucleotide release and monocyte recruitment by apoptotic cells. Conversely, PANX1 overexpression enhanced nucleotide release from apoptotic cells and phagocyte recruitment. Patch-clamp recordings showed that PANX1 was basally inactive, and that induction of PANX1 currents occurred only during apoptosis. Mechanistically, PANX1 itself was a target of effector caspases (caspases 3 and 7), and a specific caspase-cleavage site within PANX1 was essential for PANX1 function during apoptosis. Expression of truncated PANX1 (at the putative caspase cleavage site) resulted in a constitutively open channel. PANX1 was also important for the ‘selective’ plasma membrane permeability of early apoptotic cells to specific dyes3. Collectively, these data identify PANX1 as a plasma membrane channel mediating the regulated release of find-me signals and selective plasma membrane permeability during apoptosis, and a new mechanism of PANX1 activation by caspases.

/pdf/pannexin-1-channels-mediate-find-me-signal-release-and-ulfonl8ucb.pdf

Pannexin 1 channels mediate ‘find-me’ signal release and membrane permeability during apoptosis

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of astroglia

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/j.febslet.2005.12.004

Activation of pannexin 1 channels by ATP through P2Y receptors and by cytoplasmic calcium

Connexin mimetic peptides are widely used to assess the contribution of nonjunctional connexin channels in several processes, including ATP release. These peptides are derived from various connexin sequences and have been shown to attenuate processes downstream of the putative channel activity. Yet so far, no documentation of effects of peptides on connexin channels has been presented. We tested several connexin and pannexin mimetic peptides and observed attenuation of channel currents that is not compatible with sequence specific actions of the peptides. Connexin mimetic peptides inhibited pannexin channel currents but not the currents of the channel formed by connexins from which the sequence was derived. Pannexin mimetic peptides did inhibit pannexin channel currents but also the channels formed by connexin 46. The same pattern of effects was observed for dye transfer, except that the inhibition levels were more pronounced than for the currents. The channel inhibition by peptides shares commonalities with channel effects of polyethylene glycol (PEG), suggesting a steric block as a mechanism. PEG accessibility is in the size range expected for the pore of innexin gap junction channels, consistent with a functional relatedness of innexin and pannexin channels.

Modulation of membrane channel currents by gap junction protein mimetic peptides: size matters

Pannexin1 and Pannexin2 Channels Show Quaternary Similarities to Connexons and Different Oligomerization Numbers from Each Other

Pannexins (Panx1, 2, and 3) comprise a group of proteins expressed in vertebrates that share weak yet significant sequence homology with the invertebrate gap junction proteins, the innexins. In contrast to the other vertebrate gap junction protein family (connexin), pannexins do not form intercellular channels, but at least Panx1 forms nonjunctional plasma membrane channels. Panx1 is ubiquitously expressed and has been shown to form large conductance (500 pS) channels that are voltage dependent, mechanosensitive, and permeable to relatively large molecules such as ATP. Pharmacological and knockdown approaches have indicated that Panx1 is the molecular substrate for the so-called "hemichannel" originally attributed to connexin43 and that Panx1 is the pore-forming unit of the P2X(7) receptor. Here, we describe, for the first time, conductance and permeability properties of Panx1-null astrocytes. The electrophysiological and fluorescence imaging analyses performed on these cells fully support our previous pharmacological and Panx1 knockdown studies that showed profoundly lower dye uptake and ATP release than wild-type untreated astrocytes. As a consequence of decreased ATP paracrine signaling, intercellular calcium wave spread is altered in Panx1-null astrocytes. Moreover, we found that in astrocytes as in Panx1-expressing oocytes, elevated extracellular K(+) activates Panx1 channels independently of membrane potential. Thus, on the basis of our present findings and our previous report, we propose that Panx1 channels serve as K(+) sensors for changes in the extracellular milieu such as those occurring under pathological conditions.

/pdf/atp-signaling-is-deficient-in-cultured-pannexin1-null-mouse-2nqspfmrpt.pdf

Junjie Wang

Papers

Activation of pannexin 1 channels by ATP through P2Y receptors and by cytoplasmic calcium

Modulation of membrane channel currents by gap junction protein mimetic peptides: size matters

Pannexin1 and Pannexin2 Channels Show Quaternary Similarities to Connexons and Different Oligomerization Numbers from Each Other

ATP signaling is deficient in cultured Pannexin1-null mouse astrocytes.

The bizarre pharmacology of the ATP release channel pannexin1