Sim1 Neurons and Body Weight Regulation

描述（由申请人提供）：编码单一思维1 （Sim1）和黑素皮质素受体4 （MC4Rs）的基因突变导致人类严重肥胖。与此一致的是，Sim1单倍不全或PVH病变的小鼠是肥胖的。重要的是，Sim1神经元中MC4R的特异性遗传恢复可使MC4R缺失小鼠的肥胖减少60-70%，这表明Sim1神经元中的MC4R在体重调节中起着关键作用。然而，尽管Sim1神经元在人类和啮齿类动物中的重要性已经得到了很好的确立，但我们对Sim1神经通路的理解仍然局限于Sim1神经元的水平。在Sim1神经元的下游，介导其作用的神经递质身份尚不清楚。研究主要集中在Sim1神经元释放的神经肽（如促肾上腺皮质激素释放激素、促甲状腺激素释放激素和催产素）的作用上。然而，敲除这些神经肽中的每一种只产生很少或有限的摄食或体重缺陷，这表明额外的神经递质的作用。Sim1表达区含有丰富的泡状谷氨酸转运蛋白2 （Vglut2，突触前谷氨酸释放所必需的），表明大多数Sim1神经元是谷氨酸能的。在Sim1神经元的上游，Sim1神经元的活动是如何被调控的，这在很大程度上仍然是未知的。除了黑素皮质素输入外，突触输入的功能尚不清楚。电生理数据提示gaba能输入PVH Sim1神经元的重要作用。然而，gaba能输入的生理意义尚未得到证实。目的1将通过特异性丧失Sim1神经元谷氨酸释放的小鼠，验证谷氨酸释放介导Sim1神经元在体重调节中的作用。Aim 2将在MC4R零背景下，利用MC4Rs同时重新表达和谷氨酸释放仅限于Sim1神经元的小鼠，验证谷氨酸释放介导Sim1神经元中表达的MC4Rs在体重调节中的作用。目的3将通过小鼠模型确定成人Sim1神经元的gaba能输入对体重调节的作用，在该模型中，只有Sim1神经元中的GABA-A受体对唑吡坦（一种特异性苯二氮卓类药物模拟GABA-A激动剂）敏感，而所有其他脑神经元中的GABA-A受体则不敏感。所有神经元中的GABA- a受体都会对GABA做出正常反应，因此这些小鼠除了唑吡坦作用外行为正常。gaba能输入的特异性激活对体重、食物摄入和能量消耗的影响将通过以暂时可控和可逆的方式给予唑吡坦来检查。这些研究将揭示Sim1神经通路重要的下游调节因子谷氨酸释放和上游调节因子gaba能输入的功能，从而为我们了解脑神经回路对体重调节的作用迈出了重要的一步。

DESCRIPTION (provided by applicant): Mutations in the genes encoding single-minded 1 (Sim1) and the melanocortin receptors 4 (MC4Rs) cause severe obesity in humans. In agreement, mice with Sim1 haploinsufficiency or PVH lesion are obese. Importantly, specific genetic restoration of MC4R in Sim1 neurons reduces obesity resulted from MC4R null mice by 60-70%, demonstrating a critical role for MC4Rs in Sim1 neurons in body weight regulation. However, despite the well established importance of Sim1 neurons in both humans and rodents, our understanding of Sim1 neural pathway is still limited to the level of Sim1 neurons. Downstream of Sim1 neurons, the identity of the neurotransmitters that mediate their action is not clear. Studies have been focused on the roles of neuropeptides released from Sim1 neurons (e.g. corticotrophin-releasing hormone, thyrotrophin-releasing hormone and oxytocin). However, knockout of each of these neuropeptides produced little or limited defects in feeding or body weight, suggesting a role for additional neurotransmitters. Sim1-expressing regions contain abundant vesicular glutamate transporter 2 (Vglut2, required for presynaptic glutamate release), indicating that the majority of Sim1 neurons are glutamatergic. Upstream of Sim1 neurons, how the activity of Sim1 neuron is regulated remains largely unknown. With the exception of the melanocortin input, the function of synaptic inputs is unknown. Electrophysiological data have suggested an important role for GABAergic input to PVH Sim1 neurons. However, the physiological significance of the GABAergic input is yet to be demonstrated. Aim 1 will test the hypothesis that glutamate release mediates the action of Sim1 neurons in body weight regulation using mice with specific loss of glutamate release from Sim1 neurons. Aim 2 will test the hypothesis that glutamate release mediates the action of MC4Rs expressed in Sim1 neurons in body weight regulation using mice with concurrent re-expression of MC4Rs and disruption of glutamate release restricted to Sim1 neurons on MC4R null background. Aim 3 will determine the function of GABAergic input to adult Sim1 neurons on body weight regulation using a mouse model in which only GABA-A receptors in Sim1 neurons will be sensitive to Zolpidem (a specific benzodiazepine mimicking GABA-A agonist), but those in all other brain neurons will not. GABA-A receptors in all neurons will respond normally to GABA, thus these mice will behave normally except for Zolpidem action. Effects of specific activation of GABAergic input on body weight, food intake and energy expenditure will be examined by administering Zolpidem in a temporally controlled and reversible fashion. These studies will reveal the function of glutamate release, an important downstream mediator, and GABAergic input, an important upstream regulator, of Sim1 neural pathway, thus representing a significant step in our understanding of brain neurocircuitry on body weight regulation. PUBLIC HEALTH RELEVANCE: Obesity is one of the greatest and the most challenging problems in medicine and is an important risk factor for type II diabetes mellitus, hypertension, and coronary heart disease. Mutations in Sim1 gene are one of important monogenic causes of obesity in both humans and rodents. Our study based on rodent models will reveal Sim1 neural pathway in body weight regulation, thus providing a framework for using Sim1 neural pathway as a therapeutic target against the current obesity epidemic.