Mechanisms of Diabetic Hyperphagia and Insulin Resistance

描述（由申请人提供）：不受控制的胰岛素缺乏性糖尿病（uDM）的行为和代谢后果部分源于下丘脑弓状核（ARC）和下丘脑腹内侧核（VMN）等关键脑区对血浆环境变化的反应，包括胰岛素和瘦素循环水平的显著降低和胃饥饿素水平的升高。因此，uDM的啮齿动物模型构成了研究这些神经内分泌控制系统的独特而有价值的工具。在uDM中激活的ARC神经元亚群中，有那些表达促食肽(或“食物摄取刺激”肽，如神经肽Y （NPY）和agouti相关肽（AgRP）的神经元，而邻近的厌氧、产生黑素皮质素（或“POMC”）神经元则受到抑制，这些反应的组合与uDM的食物摄入特征显著增加（称为“糖尿病性贪食”）有关。在细胞水平上，通过胰岛素受体底物-磷脂酰肌醇-3激酶（IRS-PI3K）途径的信号传导在外周组织的胰岛素作用中起着关键作用，虽然该途径对中枢神经系统中瘦素和胰岛素的作用也很重要，但具体涉及的神经元亚群仍有待确定。PI3K下游的信号分子包括蛋白激酶B （PKB）和哺乳动物雷帕霉素靶蛋白（mTOR），两者都与下丘脑对食物摄入的控制有关。越来越多的证据表明，下丘脑神经回路感知来自胰岛素、瘦素和胃饥饿素的输入参与外周组织胰岛素敏感性的控制，我们最近的研究表明，在b细胞毒素链脲佐菌素（STZ）诱导的uDM大鼠的进行性胰岛素抵抗中，这些神经回路的功能障碍是由PI3K信号减少引发的。基于这些观察结果，我们在Specific Aim 1中提出采用stz诱导的uDM小鼠模型，使我们能够识别通过IRS-PI3K-PKB途径信号转导调节食物摄入和葡萄糖代谢的特定脑区和神经元亚群。具体来说，我们将使用Cre-loxP遗传和腺病毒基因治疗技术的组合来增加NPY/Agrp神经元、POMC神经元、表达瘦素受体的神经元或VMN神经元（表达转录因子SF- 1）在stz诱导的uDM小鼠中的PKB。通过这种方式，我们将确定ARC和VMN中的神经元亚群，其中PKB信号的减少有助于uDM的摄食和代谢后果。同样，Aim 2试图描述下丘脑mTOR信号减少在大鼠对uDM的行为和代谢反应中的作用。在Aim 3中，我们研究了uDM中血浆胃饥饿素水平升高的机制，并确定了这种升高对这种情况下贪食和胰岛素抵抗的贡献。总之，这些信息将揭示控制食物摄入和胰岛素敏感性的神经内分泌机制，并有助于阐明这些系统的功能障碍如何导致肥胖和糖尿病患者进食行为紊乱和葡萄糖代谢的发病机制。

DESCRIPTION (provided by applicant): Both behavioral and metabolic consequences of uncontrolled, insulin-deficient diabetes mellitus (uDM) arise in part from the response of key brain areas such as the hypothalamic arcuate nucleus (ARC) and ventromedial hypothalamic nucleus (VMN) to changes in the humoral milieu, including marked decreases in the circulating levels of both insulin and leptin, and elevated levels of ghrelin. Rodent models of uDM therefore constitute a unique and valuable tool with which to study these neuroendocrine control systems. Among ARC neuronal subsets activated in uDM are those that express orexigenic (or 'food intake-stimulatory') peptides such as neuropeptide Y (NPY) and agouti-related peptide (AgRP), whereas adjacent anorexigenic, melanocortin-producing (or 'POMC') neurons are inhibited, a combination of responses implicated in the pronounced increase of food intake characteristic of uDM (termed "diabetic hyperphagia"). At the cellular level, signaling via the insulin receptor substrate-phosphotidylinositol-3 kinase (IRS-PI3K) pathway plays a critical role in insulin action in peripheral tissues and while this pathway is also critical for both leptin and insulin action in the CNS, the specific neuronal subsets involved remain to be determined. Signaling molecules downstream of PI3K include protein kinase B (PKB) and mammalian target of rapamycin (mTOR), both of which are also implicated in hypothalamic control of food intake. Growing evidence also suggests that hypothalamic neurocircuits that sense input from insulin, leptin and ghrelin participate in the control of insulin sensitivity in peripheral tissues, and our recent work implicates dysfunction of these neurocircuits, triggered by reduced PI3K signaling, in the progressive insulin resistance seen in rats with uDM induced by the b-cell toxin, streptozotocin (STZ). Based on these observations, we propose in Specific Aim 1 to employ mouse models of STZ-induced uDM that enable us to identify the specific brain areas and neuronal subsets in which signal transduction via the IRS-PI3K-PKB pathway regulates food intake and glucose metabolism. Specifically, we will use a combination of Cre-loxP genetic and adenoviral gene therapy techniques to increase PKB specifically in NPY/Agrp neurons, POMC neurons, neurons that express leptin receptors, or VMN neurons (that express the transcription factor SF- 1) in mice with STZ-induced uDM. In this way, we will identify neuronal subsets in the ARC and VMN in which reduced PKB signaling contributes to feeding and metabolic consequences of uDM. Similarly, Aim 2 seeks to delineate the role of reduced hypothalamic mTOR signaling in behavioral and metabolic responses to uDM in rats. In Aim 3, we investigate mechanisms underlying increased plasma ghrelin levels in uDM and determine the contribution made by this increase to hyperphagia and insulin resistance in this setting. Together, this information will shed new light on neuroendocrine mechanisms controlling food intake and insulin sensitivity and help to clarify how dysfunction within these systems contributes to the pathogenesis of disordered feeding behavior and glucose metabolism in obesity and diabetes.