Hypothalamic mechanisms in caloric restriction and aging

拟议研究的长期目标是评估神经内分泌对卡路里限制的反应在卡路里限制对年龄相关损害和寿命的影响中的作用。神经内分泌系统在调节卡路里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，被称为“滞后”，是营养刺激累积损害基本的营养刺激的下丘脑神经元(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年龄增长的合成代谢张力增强，从而导致有害的代谢综合征，包括高胰岛素血症。另一种可能被视为动态平衡的机制是，限制卡路里会产生神经内分泌反应，如糖皮质激素升高和生长激素减少，从而有效地保护机体，导致寿命延长。在这种情况下，老化的神经内分泌系统产生的合成代谢张力，可能是由于对营养因素的敏感性受损，实际上可能具有保护作用。本提案将涉及这些不同的机制。(1)为什么下丘脑POMC的表达随着年龄的增长而减少？堕落与麻木不仁。如果营养刺激累积损害了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该优先随着年龄的增长而减少。或者，由于对营养敏感性的降低，POMC amc的表达可能会减少。为了评估这些预测，将使用体视学方法计算营养刺激下丘脑区域的神经元数量，特别是6、15和24月龄小鼠中表达POMC的神经元的数量。还将评估相同年龄的下丘脑神经元对葡萄糖、瘦素和胰岛素的电生理反应性。最后，将使用DNA阵列分析来评估营养刺激的下丘脑mRNAs对衰老特别敏感的预测。(2-4)依赖POMC、瘦素和葡萄糖的神经内分泌反应在热量限制对年龄相关损伤的中介效应中起什么作用？如果神经内分泌反应在卡路里限制对年龄相关损伤的影响中起到中介作用，那么阻止这些反应应该会阻止这些影响。为了评估这一预测，已经产生了在神经元特异性烯醇化酶启动子的控制下表达POMC、瘦素或葡糖激酶的转基因小鼠；预计这些转基因将分别阻断依赖POMC、瘦素或葡萄糖的热量限制的神经内分泌反应。这些转基因对年龄相关损伤和寿命的影响将在配对喂养和卡路里限制的小鼠中进行评估。这些研究应该阐明卡路里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.