The Pannexin 1 Channel Activates the Inflammasome in Neurons and Astrocytes

Abstract

The inflammasome is a multiprotein complex involved in innate immunity. Activation of the inflammasome causes the processing and release of the cytokines interleukins 1␤ and 18. In primary macrophages, potassium ion flux and the membrane channel pannexin 1 have been suggested to play roles in inflammasome activation. However, the molecular mechanism(s) governing inflammasome signaling remains poorly defined, and it is undetermined whether these mechanisms apply to the central nervous system. Here we show that high extracellular potassium opens pannexin channels leading to caspase-1 activation in primary neurons and astrocytes. The effect of K ؉ on pannexin 1 channels was independent of membrane potential, suggesting that stimulation of inflammasome signaling was mediated by an allosteric effect. The activation of the inflammasome by K Pannexin 1 is a vertebrate ortholog of the invertebrate innexin gap junction proteins (1), but it does not appear to form functional gap junctions in vivo. Instead pannexin 1 acts as a membrane channel that carries ions and signaling molecules between the cytoplasm and the extracellular space (2, 3). As such, it is a candidate ATP release channel in various cell types, including erythrocytes, astrocytes, bronchial epithelial cells, and taste cells. Various functional roles have been ascribed to pannexin 1 including local vascular perfusion control and propagation of intercellular calcium waves (4 -6). Recently pannexin 1 was also shown to form the large pore of the P2X7 purinergic receptor (7, 8). P2X7 plays a major role in inflammation, and its activation by extracellular ATP results in release of interleukin (IL) 2 -1␤ from macrophages, probably involving pannexin 1 as a signaling molecule (7).IL-1␤ production and maturation are tightly regulated by caspase-1 incorporated into large protein complexes termed inflammasomes (9 -11). The molecular composition of the inflammasome depends on the identity of the NOD-like receptor (NLR) family member serving as a scaffold protein in the complex (12). The members of the cytosolic NLR family appear to recognize conserved microbial and viral components termed pathogen-associated molecular patterns in intracellular compartments (13). The bipartite adaptor protein apoptosis-associated speck-like protein containing a CARD (ASC) bridges the interaction between NLR proteins and inflammatory caspases and plays a central role in the assembly of inflammasomes and the activation of caspase-1 in response to a broad range of pathogen-associated molecular patterns and intracellular pathogens (14). In addition, the inflammasome can be activated by danger-associated molecular patterns, molecules endogenous to the organism that signal stress or injury, including extracellular ATP acting at ionotropic P2X7 receptors, fibronectin, or monosodium urate crystals (15,16). Moreover it has been suggested that a rapid K ϩ efflux through ATP-activated P2X7 receptors induces inflammasome assembly (17)(18)(19)(20).Despite the recent advances in the understanding of a...

Effects of Therapeutic Hypothermia on Inflammasome Signaling after Traumatic Brain Injury

“…In that in-vitro study, high extracellular K + activated caspase-1 through pannexin 1 channels in primary neurons and astrocytes. Thus, strategies that inhibit inflammasome formation or pannexin 1 channel function may be a potential therapy for ATPinduced cell death (Silverman et al, 2009).…”

Purinergic mechanisms in neuroinflammation: An update from molecules to behavior

“…It has been demonstrated that P2X7 receptor overexpression is both necessary and sufficient to drive these processes (Monif et al, 2009). Other studies (de Rivero Vaccari et al, 2012), however, revealed the importance of IL-1b release from neurons, while Silverman et al (Silverman et al, 2009) report that in neurons, high extracellular K þ can activate the inflammasome via Panx1-dependent efflux. These latter data indicate that neurons may be the first site of IL-1b release following an insult (Fig.…”

“…Reduced intracellular K þ concentration is the primary trigger of caspase-1 activation (Munoz-Planillo et al, 2013). While some reports indicate that Panx1 channel opening is an obligatory part of both NLRP1 and NLRP3 inflammasome activation (de Rivero Vaccari et al, 2008;Pelegrin and Surprenant, 2006;Silverman et al, 2009), other groups have reported P2X7 receptor-induced IL-1b release, independent of Panx1 (Pelegrin et al, 2008;Qu et al, 2011). A further study postulated that Panx1 is responsible for the release of ATP from dying cells, upstream of P2X7 activation in the signaling cascade (Dahl and Keane, 2012).…”

“…P2X7 receptor expression on different cell types, upregulated in response to CNS insult, combines to mediate a neuroinflammatory response. ATP-activated microglia are key regulators of the neuroinflammatory response, releasing IL-1b in response to insult (Ferrari et al, 1997;Silverman et al, 2009), adopting an activated morphology, proliferating and migrating to the site of insult, to form an inflammatory focus. It has been demonstrated that P2X7 receptor overexpression is both necessary and sufficient to drive these processes (Monif et al, 2009).…”

Evidence for sustained ATP release from liver cells that is not mediated by vesicular exocytosis

“…There is evidence for ATP release through ATPbinding cassette (ABC) transporters, connexin hemichannels and multiple Cl − channels [12][13][14]. More recent studies have found that pannexin1 hemichannels play a key role in ATP release from many different cells [15][16][17][18]. Vesicular exocytosis may also contribute to ATP release [19][20][21].…”