BDNF and TrkB-containing neuronal circuits mediating energy balance

描述(申请人提供)：近三分之一的美国人口肥胖，超过60%的人超重。这是一个令人担忧的趋势，因为肥胖显著增加了患2型糖尿病、心血管疾病和其他医学疾病的易感性。越来越多的证据表明，脑源性神经营养因子(BDNF)/TrkB通路在能量平衡调节中起着关键作用，是新疗法的一个有前途的靶点。因此，小鼠和人类体内BDNF信号的减少会导致过度吞噬行为和严重肥胖。目前尚不清楚支持神经元存活、分化和突触可塑性的脑源性神经营养因子是作为成人大脑所需的饱腹感因子，还是作为喂养神经回路的发育促进剂。作为之前资助的项目的一部分，我们证明了BDNF在成年动物中起促进饱腹感的作用，并且下丘脑腹内侧核(VMH)是这种神经营养素的关键来源。支持的证据包括能量状态对VMH中BDNF和TrkB表达的强烈影响，以及选择性删除成年小鼠VMH中的BDNF引起的过度吞噬和肥胖。BDNF在VMH中引起厌食的细胞和分子机制尚不清楚。我们最近从中心性(BDNF2L/2LCk-cre)或VMH特异性缺失BDNF的小鼠的VMH中激光捕获的细胞转录组的分析显示，在最近的一项大规模人类研究中，a2d-1和另外两个与肥胖易感性相关的基因的表达减少。我们的初步研究表明，BDNF的厌食效应是通过A2d-1介导的，A2d-1是一个高电压门控钙通道亚单位，它增强钙电流并介导兴奋性突触发生。我们发现：1)在野生型VMH中长期注射加巴喷丁选择性地抑制a2d-1，增加了动物的摄食量和体重；ii)病毒介导的a2d-1导入BDNF2L/2LCk-cre突变体的VMH中，改善了它们的吞噬能力和体重增加。这项竞争性更新建议试图通过确定BDNF和a2D-1影响的细胞机制来建立在这些发现的基础上。由于BDNF突变体的VMH细胞钙电流是正常的，a2d-1的饱腹感可能与其以非钙依赖的方式诱导兴奋性突触发生有关。因此，目标1将利用解剖学和电生理学方法研究BDNF和a2d-1在营养线索诱导的厌食性POMC神经元VMH兴奋性驱动的动态变化中的作用。在目标2中，我们将研究BDNF和a2d-1是否也调节对厌食性VMH神经元的兴奋性驱动。在目标3中，我们将测试BDNF突变体VMH中另外两个候选基因的表达减少是否导致了过度吞噬行为和肥胖的出现，以及它们是否与VMH中的a2d-1共同作用。这些研究将从机制上理解BDNF的厌食作用，并揭示治疗肥胖症的新途径。

DESCRIPTION (provided by applicant): Close to a third of the population in the United States is obese and over 60% is overweight. This is an alarming trend as obesity significantly increases susceptibility to type 2 diabetes, cardiovascular disease and other medical disorders. Mounting evidence indicates that the brain-derived neurotrophic factor (BDNF)/TrkB pathway plays a critical part in energy balance regulation and is a promising target for novel therapies. Accordingly, reduced BDNF signaling in mice and humans results in hyperphagic behavior and dramatic obesity. It remained unclear whether BDNF, which supports neuronal survival, differentiation and synaptic plasticity, acted as a required satiety factor in the adult brain or as a developmental facilitator of feeding neural circuits. As part of the previously funded project, we showed that BDNF acts in the adult animal to promote satiety and that the ventromedial hypothalamus (VMH) is a critical source of this neurotrophin. Supportive evidence includes the robust effects of energy status on expression of BDNF and TrkB in the VMH and the hyperphagia and obesity elicited by selectively deleting BDNF in the VMH of adult mice. The cellular and molecular mechanisms underlying the anorexigenic effects of BDNF in the VMH remain to be elucidated. Our recent analysis of the transcriptome of cells laser-captured from the VMH of mice with central (BDNF2L/2LCk-cre) or VMH-specific depletion of BDNF revealed decreased expression of a2d-1 and of two other genes associated with obesity susceptibility in a recent large-scale human study. Our preliminary studies show that BDNF's anorexigenic effects are mediated by a2d-1, a high voltage-gated calcium channel subunit that enhances calcium currents and mediates excitatory synaptogenesis. We found that: i) selective a2d-1 inhibition by chronic gabapentin infusion into wild type VMH increased food intake and body weight and ii) viral-mediated a2d-1 delivery to the VMH of BDNF2L/2LCk-cre mutants ameliorated their hyperphagia and body weight gain. This competitive renewal proposal seeks to build on these findings by ascertaining cellular mechanisms underlying the effects of BDNF and a2d-1. Because calcium currents in VMH cells of BDNF mutants are normal, the satiety effects of a2d-1 might be related to its ability to induce excitatory synaptogenesis in a calcium-independent manner. Thus, Aim 1 will investigate the role of BDNF and a2d-1 in dynamic changes in VMH excitatory drive onto anorexigenic POMC neurons induced by nutritional cues using anatomical and electrophysiological approaches. In Aim 2, we will investigate whether BDNF and a2d-1 also regulate excitatory drive onto anorexigenic VMH neurons. In Aim 3, we will test whether the reduced expression of two other gene candidates in the BDNF mutant VMH contributes to the emergence of hyperphagic behavior and obesity and whether they act in common pathways with a2d-1 in the VMH. These studies will provide a mechanistic understanding of anorexigenic actions of BDNF and reveal novel avenues for the treatment of obesity.