Hypothalamic mechanisms in caloric restriction and aging

拟议研究的长期目标是评估对热量限制的神经内分泌反应在介导热量限制对年龄相关损伤和寿命的影响中的作用。 神经内分泌系统在调节热量限制对寿命的影响中的作用可能涉及两种不同的机制。 一种可能的机制，称为“滞后”，是营养刺激累积损害必需的营养刺激下丘脑神经元（特别是包括产生POMC的神经元）。 由于营养刺激的下丘脑神经元产生分解代谢效应，这些神经元的侵蚀将导致随着年龄的增长观察到的合成代谢张力增强，随之而来的是有害的代谢综合征，包括高胰岛素血症。 另一种可能被视为体内平衡的机制是热量限制产生神经内分泌反应，如糖皮质激素升高和生长激素降低，有效保护生物体，从而延长寿命。 在这种情况下，可能由于对营养因素的敏感性受损，衰老的神经内分泌系统产生的合成代谢张力实际上可能是保护性的。 本提案将涉及这些不同的机制。 (1)为什么下丘脑POMC的表达随年龄增长而减少？ 退化与不敏感。 如果营养刺激累积损伤营养模拟下丘脑神经元，那么营养刺激下丘脑基因的表达应优先随年龄而降低。 或者，POMC的表达可能由于对营养敏感性的敏感性降低而降低。为了评估这些预测，将使用体视学方法对营养刺激的下丘脑区域中的神经元数量，特别是6月龄、15月龄和24月龄小鼠中表达POMC的神经元数量进行计数。 还将评估相同年龄下丘脑神经元对葡萄糖、瘦素和胰岛素的电生理反应。 最后，预测营养刺激下丘脑mRNA特别容易老化将使用DNA阵列分析进行评估。(2-4)依赖于POMC、瘦素和葡萄糖的神经内分泌反应在介导热量限制对年龄相关性损伤的影响中起什么作用？ 如果神经内分泌反应介导了热量限制对年龄相关性损伤的影响，那么阻断这些反应就应该阻断这些影响。 为了评估这一预测，转基因小鼠已产生的表达POMC，瘦素，或葡萄糖激酶的控制下的神经元特异性烯醇化酶启动子，预计这些转基因将阻止这些神经内分泌反应的热量限制，依赖于POMC，瘦素，或葡萄糖，分别。 这些转基因对年龄相关性损伤和寿命的影响将在成对喂养和热量限制的小鼠中进行评估。 这些研究应该澄清调节热量限制对年龄相关的病理和寿命的影响的机制。

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.