HIBERNATION--A NEW APPROACH TO STROKE THERAPY

Neurological Disorders And Stroke

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Abstract

Efforts to develop effective measures for the treatment of stroke have generally been based on the implicit assumption that one, or at the most several, factors control the progressive brain injury that occurs during the early hours of focal brain ischemia. Postischemic progression of brain damage appears to be extremely multifactorial and may result from a constellation of minor causes. The quest for a dominant or controlling cause would then be ultimately futile. Unconventional approaches may be required to arrest cellular destruction in brain ischemia. One approach is to analyze states in which nature, with its vast reaches of time for trial and error, has solved this complex problem. One such state is hibernation with its metabolic, hematologic and cell membrane adjustments that permit animals to withstand extremely low blood flow in the brain for protracted periods with no cellular loss. Efforts to isolate and identify the mechanisms that regulate the controlled metabolic depression and tolerance of profound brain ischemia that forms the essence of natural hibernation are in progress. In hippocampal slices, hibernation confers robust resistance to hypoxia and glucose deprivation as compared to slices from non-hibernating ground squirrels and rats at 37 degrees C, 20 degrees C and 7 degrees C. This indicates that hibernation involves tolerance to an in vitro form of ischemic stress that is not strictly dependent on temperature. Protein synthesis in the hippocampal slices was found to be greatly depressed at the same incubation temperatures. Protein synthesis (PS), in vivo, was below the limit of autoradiographic detection in brain sections, and in brain extracts was determined to be 0.04% of the average rate from active squirrels. Further, it was threefold reduced in cell-free extracts from hibernating brain at 37 degrees C, eliminating hypothermia as the only cause for protein synthesis inhibition. PS suppression involved blocks of initiation and elongation and its onset coincided with the early transition phase into hibernation. An increased monosome peak with moderate ribosomal disaggregation in polysome profiles and the greatly increased phosphorylation of eIF2a are both consistent with an initiation block in hibernators. The elongation block was demonstrated by a threefold increase in ribosomal mean transit times in cell-free extracts from hibernators. Phosphorylation of eEF2 is increased, eEF2 kinase activity is increased and protein phosphatase 2A activity is decreased during hibernation which contribute to the elongation block. No abnormalities of ribosomal function or mRNA levels were detected. These findings implicate suppression of PS as a component of the regulated shutdown of cellular function that permits hibernating ground squirrels to tolerate "trickle" blood flow and reduced substrate and oxygen availability. Further study of the factors that control these phenomena may lead to identification of the molecular mechanisms that regulate this state. We have observed phase separation of lipids in cellular membranes in a variety of cells during hibernation. Cholesterol and high-freezing point lipids are segregated into gel-phase domains. Proteins are laterally displaced from these rigidified domains into the more fluid regions of the membrane that are enriched with low-freezing point lipids. When the animal arouses, and the temperature elevates, the gel-phase domains melt and the normal lipid distribution is restored. These changes seem to form a membrane fluidity buffer and participate in the cellular mechanism of temperature acclimation.