Strokes are the third-leading cause of death in the United States killing about 275,000 people a year. One of the major mediators of neuronal death due to ischemic stroke is the elevated level of glutamate in the extracellular synaptic space which leads to excitotoxicity and neuronal cell death. Astrocytes, the most numerous cells in the brain, are essential for neuronal viability and stimulation of key astrocytic functions such as glutamate and potassium homeostasis in ischemic or post-ischemic brain could potentially contribute to neuroprotection. Potassium channels in astrocytes are predominantly responsible for maintaining the hyperpolarized membrane potential of astrocytes which allows these cells to effectively take up glutamate and K+. One type of K+ channel localized in astrocytes that is likely to be pertinent during ischemic conditions is the TREK-2 tandem-pore domain K+ channel. TREK-2 channels are activated by polyunsaturated fatty acids, intracellular acidosis in the physiological range and by mechanical stretch. During ischemia, activation of phospholipases promotes liberation and accumulation of arachidonic acid, the intracellular pH of astrocytes becomes acidic and astrocytes swell. All of these changes will activate TREK-2 channels in astrocytes to help maintain extracellular glutamate and K+ concentrations low during pathological events such as anoxia, ischemia, hypoxia, hypoglycemia and/or spreading depression. Our working hypothesis is that TREK-2 potassium channels play a major role in potassium buffering and glutamate clearance during ischemia and this hypothesis is supported by our preliminary data demonstrating functional upregulation of TREK-2 channels in astrocytes after experimental ischemia. Using a combination of techniques (such as RNAi, whole cell voltage clamp, glutamate clearance assays and an in vitro model of ischemia), we propose to directly examine the role of TREK-2 channels in maintaining the membrane potential of astrocytes and in buffering glutamate and K+ during normal and pathological conditions (glutamate excitoxicity, anoxia and hypoglycemia), as well as their ability to protect neurons during ischemic insults such as stroke.
Although strokes are the third-leading cause of death in the United States, there are about 5.4 million stroke survivors in the United States today. Despite the severity of the insult, many cells are not irreversibly damaged within the first few hours and can be rescued by early restoration of blood flow or other interventions. Astrocytes, the most numerous cells in the brain, normally perform many functions that are essential for neuronal viability and stimulation of astrocytic functions such as potassium buffering and glutamate clearance in ischemic or post-ischemic brain could potentially contribute to neuroprotection. The significance of the proposed experiments is that they represent a comprehensive effort to mechanistically elucidate the role of TREK-2 channels in astroctyes in the maintenance of neuronal function during pathophysiological conditions such as ischemia due to stroke.
