The Role of Connexin and Pannexin Channels in Perinatal Brain Injury and Inflammation

Abstract

Perinatal brain injury remains a major cause of death and life-long disability. Perinatal brain injury is typically associated with hypoxia-ischemia and/or infection/inflammation. Both hypoxia-ischemia and infection trigger an inflammatory response in the brain. The inflammatory response can contribute to brain cell loss and chronic neuroinflammation leading to neurological impairments. It is now well-established that brain injury evolves over time, and shows a striking spread from injured to previously uninjured regions of the brain. There is increasing evidence that this spread is related to opening of connexin hemichannels and pannexin channels, both of which are large conductance membrane channels found in almost all cell types in the brain. Blocking connexin hemichannels within the first 3 h after hypoxia-ischemia has been shown to improve outcomes in term equivalent fetal sheep but it is important to also understand the downstream pathways linking membrane channel opening with the development of injury in order to identify new therapeutic targets. Open membrane channels release adenosine triphosphate (ATP), and other neuroactive molecules, into the extracellular space. ATP has an important physiological role, but has also been reported to act as a damage-associated molecular pattern (DAMP) signal mediated through specific purinergic receptors and so act as a primary signal 1 in the innate immune system inflammasome pathway. More crucially, extracellular ATP is a key inflammasome signal 2 activator, with purinergic receptor binding triggering the assembly of the multi-protein inflammasome complex. The inflammasome pathway and complex formation contribute to activation of inflammatory caspases, and the release of inflammatory cytokines, including interleukin (IL)-1β, tumor necrosis factor (TNF)-α, IL-18, and vascular endothelial growth factor (VEGF). We propose that the NOD-like receptor protein-3 (NLRP3) inflammasome, which has been linked to inflammatory responses in models of ischemic stroke and various inflammatory diseases, may be one mechanism by which connexin hemichannel opening especially mediates perinatal brain injury.

Keywords: ATP; connexin; hemichannel; inflammasome; inflammation; ischemia; pannexin.

Figure 1

Schematic depicting the Connexin 43 (Cx43) gap junction and hemichannel and pannexin 1 (Px1) channel. A connexon consists of six connexin subunits. A Cx43 gap junction is made up of two opposing connexons, each contributed from two adjacent cells. A Cx43 hemichannel is an unopposed connexon. A Px1 channel is formed by six pannexin subunits. Px1 does not form gap junctions due to N-linked glycosylation on the extracellular loop.

Figure 2

Schematic diagram of the potential involvement of connexin 43 hemichannels and pannexin 1 channels and purinergic receptors P2X4 and P2X7 in the activation of microglia and NLRP3 inflammasome. Cx43 hemichannels and Px1 channels may open after exposure to HI or infection/inflammation. The opening of these channels leads to the release of ATP into the extracellular space. Extracellular ATP can activate purinergic receptors P2X4R and P2X7R. The activation of P2X4R and P2X7R can lead to the activation of microglia. Signal 1 of NLRP3 inflammasome activation is provided by PAMPs or DAMPs, which leads to the upregulation of NLRP3, pro-IL-1β, and pro-IL-18. Signal 2 is mediated by ATP activation of P2X7R and can lead to the assembly of NLRP3, ASC, and pro-caspase 1, forming the NLRP3 inflammasome. Pro-caspase 1 is activated into caspase 1, which cleaves pro-IL-1β and pro-IL-18 into their mature forms. Mature IL-1β and IL-18 is secreted.