OXIDATIVE STRESS, AGING, AND BRAIN Ca2+ TRANSPORT SYSTEMS

University Of Kansas Lawrence, Lawrence KS

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Abstract

The process of aging in the mammalian central nervous system is complex and highly individualized. However, some observations regarding molecular and cellular events occurring in the aging brain do appear to be consistent across many experimental paradigms. One such observation that aged neurons differ from young neurons in regulation of free intraneuronal Ca2+ ([Ca2+]i, the most ubiquitous mediator of signal transduction. Disruptions in [Ca2+]i homeostasis ready lead to neuronal cell death, but the less conspicuous alterations noted in aging neurons suggest modest differences in Ca2+ activity in critical processes underlying cognitive function. We have been testing the hypothesis the hypotheses that aging leads to oxidative changes in plasma membrane Ca2+ transporting proteins and thereby contributes t to the altered regulation of free [Ca2+]i observed in aged brain neurons. We found the activity of (Ca2+ + Mg2+)-ATPase (PMCA) to be significantly reduce din brain synaptic membranes (SPMs) from rats of increasing age. In addition, PMCA activity is highly sensitive to free radical-induced inactivation, suggesting that age-dependent oxidative stress may contribute to the loss of PMCA. Neither cellular events underlying the age-related decrease in PMCA activity nor the functional consequences for the cell of such a reduction in Ca2+ transport activity are yet known. The hypothesis for future investigations has been divided into two components: (1) The age-related decrease in PMCA activity and levels in brain SPMs is due to oxidative stress leading either to structural changes and enhanced degradation of the protein or to decreased PMCA expression; and (2) a decrease in PMCA activity leads to alterations in regulation of [Ca2+]i similar to those observed with aging and enhances the sensitivity of neurons to various types of metabolic stresses. Strategies for testing the hypothesis involve the use of brain tissue from animals of various ages to determine whether oxidative modification is the likely mechanism underlying loss of PMCA activity and primary neuronal cultures in which cellular and molecular processes can be investigated in greater depth. Brains from aging rats will be analyzed for changes in PMCA transcription and signals of oxidative modifications in the purified PMCA protein. Studies with primary neurons will provide insights into the (1) effects of oxidative stress on PMCA turnover and expression, (2) contributions of the mitochondria, ER, and PMCA to the reduced Ca2+ buffering in aging neurons; and (3) effects of reduced PMCA expression on Ca2+ regulation and the effects of altered Ca2+ and oxidative stress on PMCA expression. Oxidative damage is often involved as a cause for age-dependent loss of function, but the demonstration of such damage in specific macromolecules is exceedingly limited. Our studies may provide evidence that oxidative damage to a critical Ca2+-regulating protein does indeed occur in the aging brain.