Hypothalamic mechanisms in caloric restriction and aging
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The long-term objective of the proposed studies is to assess the role of neuroendocrine responses to caloric restriction in mediating effects of caloric restriction on age- related impairments and life span. The role of neuroendocrine systems in mediating effects of caloric restriction on life span may entail two distinct mechanisms. One possible mechanism, termed "hysteretic", is that nutritional stimulation cumulatively damages essential nutrition-stimulated hypothalamic neurons (especially including neurons that produce POMC). Since nutrition-stimulated hypothalamic neurons produce catabolic effects, erosion of these neurons would lead to the enhanced anabolic tone observed with age, with the consequent deleterious metabolic syndrome, including hyperinsulinemia. An alternate mechanism, which may be viewed as homeostatic, is that caloric restriction produces neuroendocrine responses, such as elevated glucocorticoids and reduced growth hormone, that effectively protect the organism, leading to increased life span. In this case the anabolic tone developed by the aging neuroendocrine system, possibly due to impaired sensitivity to nutritional factors, might actually be protective. The present proposal will address these distinct mechanisms. (1) Why does expression of hypothalamic POMC decrease with age? Degeneration vs. insensitivity. If nutritional stimulation cumulatively damages nutrition-simulated hypothalamic neurons, then expression of nutritionally stimulated hypothalamic genes should preferentially decrease with age. Alternatively, expression of POMC amay decrease due to decreased sensitivity to nutritional sensitivity. To assess these predictions, the number of neurons in the nutrition-stimulated hypothalamic field, especially neurons expressing POMC in 6-, 15-, and 24-month-old mice will be counted using stereological methods. Electrophysiological responsiveness of hypothalamic neurons to glucose, leptin, and insulin at the same ages will also be assessed. Finally, the prediction that nutrition-stimulated hypothalamic mRNAs are specifically susceptible to aging will be assessed using DNA array analysis. (2-4) What are the roles of neuroendocrine responses dependent on POMC, leptin, and glucose in mediating effects of caloric restriction on age-related impairments? If neuroendocrine responses mediate effects of caloric restriction on age-related impairments, then blocking those responses should block those effects. To assess this prediction, transgenic mice have been produced that express POMC, leptin, or glucokinase under control of the neuron-specific enolase promoter; it is anticipated that these transgenes will block these neuroendocrine responses to caloric restriction that depend on POMC, leptin, or glucose, respectively. Effects of these transgenes on age-related impairments and longevity will be assessed in pair-fed and calorically restricted mice. These studies should clarify mechanisms mediating effects of caloric restriction on age- related pathologies and longevity.
