The role and regulation of pannexin ion channels during cytotoxic edema

A primary and often fatal consequence of stroke, traumatic brain injury, and other brain insults is edema: an increase in brain tissue water content. Cytotoxic edema is a component of this process and occurs when excess ions and water enter across the neuronal plasma membrane -the semi-permeable barrier separating the intra- and extracellular space. This increase in cell volume causes membrane swelling and ultimately results in cell death.    

Presently, the cascade of events by which neuronal swelling triggers cell death remains obscure. Preliminary evidence from Dr. Brian MacVicar's lab (the host) indicates that swelling triggers cell death by activating pannexins- a class of large transmembrane ion channels. Following activation, pannexins form large pores in the membrane and allow ions and small molecules to diffuse between the intra- and extracellular compartments. Consequently, pannexins can initiate cell death by collapsing the transmembrane electrochemical gradient and/or promoting the loss of essential cellular components. The precise mechanism by which swelling triggers the opening of pannexins is unknown. Interestingly, these ion channels can be mechanically activated by membrane stretch. Moreover, membrane stretch also leads to the production of reactive oxygen species (ROS)-a group of harmful chemical agents that can directly activate pannexins.    

For the present proposal, we will test the hypothesis that pannexin activation is a crucial step underlying cell death following cytotoxic edema. Furthermore, we hypothesize that pannexins are activated by neuronal swelling through direct mechanical stimulation and/or the production of ROS.   

These hypotheses will be tested in acutely prepared rat brain slices using advanced microscopy/imaging and electrophysiology techniques. As there are few effective treatments for edema, this research could reveal new avenues for therapeutic intervention following a variety of brain insults. Considering the implications of this project for basic biomedical and clinical research, it will be essential to diffuse and disseminate our knowledge to a variety of communities. This will be done largely through symposiums/presentations at the Society for Neuroscience as well as publication in peer-reviewed scientific journals.

A primary and often fatal consequence of brain insults such as stroke and traumatic injury is edema: an increase in brain tissue water content. Cytotoxic edema is a component of this process, which occurs at the level of individual brain cells, or neurons. The cells swell up as excess ions and water enter, causing them to die. This project will build on earlier work carried out under the project supervisor, which suggests that cytotoxic edema is caused by the action of pannexins.

Pannexins are activated through unknown mechanisms when the cell membrane is caused to stretch, either chemically through the production of reactive oxygen species or mechanically. Following activation, pannexins form large pores in the cell membrane that allow ions and small molecules to pass through. They are believed to cause cell death by collapsing the transmembrane electrochemical gradient and/or by promoting the loss of essential cellular components.

We will study tissue samples using advanced imaging and electrophysiology techniques to test the hypotheses that:

1. Pannexin activation is a crucial step underlying neuronal cell death in the brain following cytotoxic edema
2. Pannexins are activated by neuronal swelling through direct mechanical stimulation and/or the production of reactive oxygen species

New lines of research for therapies for damage to the brain are greatly needed and some could arise from this work.