A Genetic-Neuroanatomic Dissection of MC4R function

描述（由申请人提供）：谷氨酸能神经回路基础MC 4 R行动大脑控制的能量平衡，防止肥胖。这种中枢调节的一个重要组成部分是黑皮质素系统，它通过黑皮质素-4受体（MC 4 R）起作用，促进体重减轻。事实上，MC 4 R的缺乏会导致明显的食欲过盛和大量肥胖。尽管关于MC 4 R的重要性是确定的，但关于潜在的神经回路的信息相对缺乏。我们研究的目的是了解MC 4 R介导的能量平衡调节的神经基础。我们已经发现Sim 1+（可能是室旁核- PVN）和Sim 1-神经元（见下文）上的MC 4 R控制食物摄入。有趣的是，这两类神经元（Sim 1+和Sim 1-）上的MC 4 R在功能上是冗余的，这表明它们是相互连接的。在一组平行的研究中，我们还发现调节食物摄入和体重的MC 4 R仅位于兴奋性神经元（标记为VGLUT 2）上。基于这些发现，以及其他人的工作，我们提出了一种新的，相互关联的“神经元网络”来解释MC 4 R的作用。在该模型中，控制食物摄入的MC 4 Rs位于三组兴奋性神经元上。两组是Sim 1-，位于后脑，构成了一条线性的、上行的、将饱腹感信号从肠道传递到前脑的交感神经能通路（迷走神经传入-> NTS ->臂旁外侧核（L-PBN））。第三组是Sim 1+，位于PVN中，并将下行兴奋性投射发送到上行通路（在NTS和L-PBN）。AgRP和POMC神经元在这三个位点中的每一个都投射到并接合MC 4 R-承载的神经元。通过将表达MC 4 R的神经元置于调节饱腹感的互连途径中，这种黑皮质素作用的分布式模型解释了Sim 1+与Sim 1-神经元上MC 4 R的冗余性。三个目标将探索这种模式。在目标1中，我们将测试模型所基于的基本前提（在Sim 1+神经元、VGLUT 2+神经元以及PVN、NTS和L-PBN中的神经元上MC 4 R的充分性与必要性）。在目标2中，我们将a）鉴定PVN内的关键Sim 1 +/VGLUT 2+神经元，B）使用光遗传学来测试连接性（PVN -> NTS和PVN -> L-PBN），然后c）使用DREADD技术来远程、急性和可逆地调节体内功能。在目标3中，我们将关注NTS和L-PBN中的Sim 1-/VGLUT 2+神经元。总的来说，对MC 4 R作用的这种分布式、相互关联的神经元模型的研究应该为调节食物摄入和能量平衡的神经回路提供新的线索。

DESCRIPTION (provided by applicant): Glutamatergic Neurocircuitry Underlying MC4R Action Brain control of energy balance prevents obesity. An important component of this central regulation is the melanocortin system, which, working through melanocortin-4 receptors (MC4Rs), promotes weight loss. Indeed, absence of MC4Rs causes marked hyperphagia and massive obesity. Despite certainty regarding the importance of MC4Rs, there is a comparative lack of information regarding the underlying neurocircuitry. The goal of our studies is to understand the neural basis for MC4R-mediated regulation of energy balance. We have discovered that MC4Rs on both Sim1+ (likely the paraventricular nucleus - PVN) and Sim1- neurons (see below) control food intake. Of interest, MC4Rs on these two classes of neurons (Sim1+ and Sim1-) are functionally redundant, suggesting that they are interconnected. In a parallel set of studies, we have also discovered that the food intake- and body weight-regulating MC4Rs are located exclusively on glutamatergic (excitatory) neurons (marked by VGLUT2). Based upon these findings, and the work of others, we propose a novel, interconnected "glutamatergic network" to account for MC4R action. In this model, MC4Rs controlling food intake is on three groups of glutamatergic (excitatory) neurons. Two groups are Sim1-, are in the hindbrain, and constitute a linear, ascending, glutamatergic pathway that relays satiety signals from the gut to the forebrain (vagal afferents -> NTS -> lateral parabrachial nucleus (L-PBN)). The third group is Sim1+, is in the PVN, and sends descending, excitatory projections to the ascending pathway (at the NTS and L-PBN). AgRP and POMC neurons project to and engage MC4R-bearing glutamatergic neurons at each of these three sites. By placing MC4R-expressing neurons into an interconnecting pathway regulating satiety, this distributed model of melanocortin action accounts for the redundancy of MC4Rs on Sim1+ versus Sim1- neurons. Three Aims will probe this model. In Aim 1, we will test the underlying premises upon which the model is based (sufficiency versus necessity of MC4Rs on Sim1+ neurons, VGLUT2+ neurons, as well as on neurons in the PVN, NTS and L-PBN). In Aim 2, we will a) identify the key Sim1+/VGLUT2+ neurons within the PVN, b) use optogenetics to test connectivity (PVN -> NTS and PVN -> L-PBN), and then c) use DREADD technology to remotely, acutely and reversibly modulate function in vivo. In Aim 3, we will focus on the Sim1-/VGLUT2+ neurons in the NTS and L-PBN. In total, investigation of this distributed, interconnected, glutamatergic model of MC4R action should shed new light on neural circuits regulating food intake and energy balance.