Neurochemistry/physiology of proopiomelanocortin neurons

描述(申请人提供)：由2型糖尿病引起的肥胖及其并发症目前是全球主要的健康问题。过去十年的研究已经证实，下丘脑弓状核和孤束核(NTS)在中枢神经系统控制食欲和能量消耗中起着关键作用。我们和其他人已经证明，表达前阿片黑素皮质素(POMC)的肽能神经元位于这两个核团中，是调节能量稳态的神经回路的关键细胞元件。黑素皮质素多肽，包括α、β、伽马黑素细胞刺激激素和促肾上腺皮质激素，可抑制摄食，提高代谢率。神经黑素皮质素受体(MC4-R和MC3-4)或POMC的突变会导致人类和啮齿动物模型的肥胖，强调了这类神经肽递质的重要性。相比之下，POMC衍生的β-内啡肽刺激进食，并调节美味食物的行为强化特性。我们最近发现，相当大比例的POMC神经元还合成经典的快速抑制性神经递质伽马氨基丁酸(GABA)，这一发现挑战了人们普遍认为的POMC神经元将共同表达兴奋性氨基酸递质谷氨酸的假设。GABA以前被共同定位于弓状核中与食欲素、神经肽Y/刺鼠基因相关的转录神经元的相邻群体中，这些转录神经元通常与POMC神经元的功能相反。基于这些数据和其他研究表明POMC神经元对激素和突触信号的个体反应存在质的差异，我们假设POMC神经元不是同质细胞的同步群体，而是由在能量平衡中具有不同作用的亚群组成。此外，我们推测，GABA和肽调节剂从POMC神经元的协调释放可能是它们在自身受体以及其他神经元突触前和突触后位置作用机制的一个关键方面。因此，本项目的总体目标是进一步描述弓状核和NTS内POMC神经元亚群的神经化学基础，并将这些亚群与它们在调节代谢率和食物摄入方面的特定作用相匹配。其具体目的是：1)利用新建立的POMC选择性神经元缺陷的突变小鼠模型，确定不同的黑素皮质素受体配体在中枢神经系统控制能量稳态中的特定生理功能；2)定位GABA和POMC多肽在弓状核和POMC神经元下丘脑的共定位，并测试POMC神经元中GABA的释放如何与黑素皮质素和β-内啡肽的释放及其生理活性相结合对能量稳态的影响；以及3)利用以下基因技术探索弓状核和NTS的功能组织：a)通过转基因毒素对选定的POMC神经元群进行细胞特异性消融；以及b)聚合嵌合体分析，以并列大脑内野生型和POMC缺陷神经元的多个不同组合。这些研究的结果有望显著增加我们对参与体重调节的大脑区域和机制的基本了解，并导致预防和治疗肥胖症的新的治疗方法。

DESCRIPTION (provided by applicant): Obesity and its complications stemming from type 2 diabetes mellitus are now leading health concerns worldwide. Research in the past decade has established a critical role for the arcuate nucleus of the hypothalamus and the nucleus tractus solitarius (NTS) in the CNS control of appetite and energy expenditure. We and others have shown that peptidergic neurons expressing proopiomelanocortin (POMC), located within both of these nuclei, are key cellular elements of the neural circuits that mediate energy homeostasis. The melanocortin peptides, including alpha, beta, and gamma-melanocyte stimulating hormones and adrenocorticotrophic hormone inhibit feeding and increase metabolic rate. Mutations in either of the neural melanocortin receptors (MC4-R and MC3-4) or POMC cause obesity in humans and rodent models, emphasizing the importance of this class of neuropeptide transmitter. In contrast, POMC derived beta-endorphin stimulates feeding and mediates the behavioral reinforcing properties of palatable food. We have recently discovered that a substantial proportion of POMC neurons also synthesize the classical, fast inhibitory neurotransmitter gamma-amino butyric acid (GABA), a finding that challenges the widely held assumption that POMC neurons would coexpress the excitatory amino acid transmitter glutamate. GABA was previously co-localized in an adjacent population of orexigenic, neuropeptide Y/agouti gene-related transcript neurons in the arcuate nucleus, which generally oppose the function of POMC neurons. Based on these data and other studies showing qualitative differences among POMC neurons in their individual responses to hormonal and synaptic signals, we hypothesize that POMC neurons are not a synchronized population of homogenous cells, but instead composed of subpopulations with distinct roles in energy balance. Furthermore, we postulate that the coordinated release of GABA and peptide modulators from POMC neurons may be a key aspect of their mechanism of action at autoreceptors, and pre- and postsynaptic sites on other neurons. Therefore, the overall goal of this project is to further delineate the neurochemical basis of subpopulations of POMC neurons within the arcuate nucleus and NTS and to match these subpopulations with their specific actions in the regulation of metabolic rate and food intake. The specific aims are to: 1) Utilize a newly developed mutant mouse model with selective neuronal deficiency of POMC to determine the specific physiological functions of the different melanocortin receptor ligands in the central nervous system control of energy homeostasis; 2) Map the co-localization of GABA and POMC peptides within the arcuate nucleus and hypothalamic projections of POMC neurons and test how GABA release from POMC neurons is integrated with the release and physiological activity of melanocortins and beta-endorphin on energy homeostasis; and 3) Probe the functional organization of the arcuate nucleus and NTS using the genetic techniques of: a) Cell-specific ablation of selected groups of POMC neurons by transgene-directed toxin delivery; and b) Aggregation chimera analysis to juxtapose multiple different combinations of wild-type and POMC-deficient neurons within the brain. Results from these studies are expected to significantly increase our basic understanding of the brain areas and mechanisms involved in the regulation of body weight and lead to new therapeutic approaches for the prevention and treatment of obesity.