The long term goal of this project is to identify blood-brain barrier (BBB) ion transporters that mediate ischemia-induced brain edema. During the early hours of ischemic stroke, edema forms in the presence of an intact BBB by a process involving BBB transport of Na and Cl from blood into brain. Our studies have shown that Na-K-Cl cotransport, present in the luminal BBB membrane, is stimulated by ischemic factors, including hypoxia, aglycemia, vasopressin (AVP) and that inhibiting the cotransporter reduces edema in a rat model of stroke. Thus, the BBB Na-K-Cl cotransporter appears to be a major contributor to ischemia-induced edema. We have now found in preliminary studies that a BBB Na/H exchanger also appears to participate in ischemia- induced edema formation which suggests the exciting possibility of an additional BBB target for reduction of edema during the early hours of stroke. Our hypothesis is that, in addition to the cotransporter, a luminal BBB Na/H exchanger is stimulated during ischemia to increase transport of Na from blood into brain.
The first aim i s to determine whether Na/H exchange is present at the luminal BBB membrane and is stimulated by ischemic factors. We will use immunoelectron microscopy to evaluate BBB Na/H exchange protein in situ and microspectrofluorometry to assess ischemic factor effects on CMEC Na/H exchange activity.
The second aim i s to determine whether inhibition of the BBB Na/H exchanger attenuates ischemia-induced edema. Here, we will examine the effect of BBB Na/H exchange inhibition on ischemia-induced changes in rat brain Na and water, using nuclear magnetic resonance methods. We will also assess the efficacy of Na/H exchange and Na-K-Cl cotransport inhibitors for reduction of cerebral edema when administered after the onset of ischemia.
The third aim of this project is to evaluate the signaling pathways by which ischemia stimulates BBB Na-K-Cl cotransporter and Na/H exchanger activities. We will start by evaluating the roles of AMP kinase, p38 MAP kinase and intracellular [Ca] in hypoxia, aglycemia and AVP-induced stimulation of the cotransporter and exchanger for reasons detailed in the application . However, our studies will also include an initial assessment of protein kinase C, ERK1/2 MAP kinase and JNK MAP kinase. For these studies we will use cerebral microvascular endothelial cells and perfusion-fixed rat brain to evaluate ischemia-induced activation of the kinases by Western blot and confocal immunofluorescence, respectively. We will also evaluate the effects of kinase inhibitors on ischemia (hypoxia, aglycemia and AVP) stimulated cotransporter activity (radioisotope flux) and Na/H exchanger activity (spectrofluorometry and radioisotope flux).
Ischemia-induced edema is a major cause of brain damage in stroke, a leading cause of death in the U.S. The proposed studies will reveal whether therapeutic approaches aimed at preventing ischemia stimulation of BBB Na-K-Cl cotransporter and/or Na/H exchange activity may be of value for attenuating stroke-induced brain edema. The fact that luminal-facing BBB Na transporters appear to contribute to edema formation and are readily accessible to intravenously administered drugs makes them a therapeutic target that we must investigate.
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