Mechanisms of Obesity in Mice with Neuron-Specific Lipoprotein Lipase Deficiency

描述(申请人提供)：本申请的长期目标是通过新发现的由神经元中的脂蛋白脂酶(LPL)介导的途径来揭示体重调节的机制。LPL是从富含甘油三酯的循环脂蛋白的水解物中摄取脂肪酸的限速酶，用于外周组织中的脂肪储存和/或氧化。LPL也存在于神经系统，包括海马体、下丘脑和其他脑区的神经元。神经元特异性LPL缺失的小鼠(NEXLPL-/-)在饮食6个月后肥胖，杂合子小鼠(NEXLPL)也在6至12个月变胖。在NEXLPL-/-和NEXLPL中，肥胖之前都有一段时间的吞噬过度，随后代谢率显着下降。在NEXLPL-/-和NEXLPL小鼠中，下丘脑AgRP mRNA的显著增加发生在肥胖开始之前，较低的升高但在肥胖发生后持续存在。初步数据表明，下丘脑中特定多不饱和脂肪酸的LPL依赖的缺乏和代谢导致了AgRP基因表达的上调。在特定的目标#1中，我们将使用特征良好的下丘脑细胞系来测试LPL缺乏是否直接导致体外AgRP基因表达增加，以及这种调节是否依赖于LPL的酶活性。机械性研究旨在明确LPL是否通过调节脂肪酸摄取在下丘脑发挥作用，以及这种调节是否也与这些细胞中的胰岛素作用有关。为了证明NEXLPL-/-小鼠体内产生AgRP的神经元是调节能量平衡和体重的主要部位，在特定的目的#2中，将产生仅产生AgRP的神经元的LPL缺乏。评估饮食脂肪酸如何调节表型的研究也在计划中。最后，尽管在12个月时存在严重肥胖，NEXLPL-/-小鼠的糖耐量比仔鼠对照组更好，而且还显示出下丘脑Mc3r基因表达增加，明显的棕色脂肪组织(BAT)增生/肥大，BAT UCP-1基因表达增加。这一目标的实验将旨在确定尽管NEXLPL-/-小鼠肥胖，但糖代谢改善的机制。我们认为，这项工作的临床相关性不仅与脂蛋白中携带的饮食脂质如何控制能量平衡和体重有关，还与刺激BAT和保持糖耐量的机制有关。由于肥胖是一种流行病，因此需要更深入地了解体重调节的生理学，包括肥胖与糖耐量异常的关系。此外，通过这项工作，通过生活方式和/或新药预防和/或治疗肥胖症的新方法可能成为可能。综上所述，这是首次观察到富含甘油三酯的脂蛋白及其在大脑中的LPL代谢具有生理学相关性，并希望通过一系列实验来揭示大脑中富含甘油三酯的脂蛋白感知和代谢如何影响能量平衡和体重调节的机制。

DESCRIPTION (provided by applicant): The long-term objective of this application is to uncover mechanisms of body weight regulation through a newly discovered pathway mediated by the enzyme lipoprotein lipase (LPL) in the neuron. LPL is the rate limiting enzyme for the uptake of fatty acids from the hydrolysis of circulating triglyceride-rich lipoproteins for lipid storage and/or oxidation in peripheral tissues. LPL is also present throughout the nervous system including neurons in the hippocampus, hypothalamus, and other brain regions. Mice with neuron-specific deletion of LPL (NEXLPL-/-) are obese on dietary chow by 6 months, and heterozygous mice (NEXLPL) also become obese between 6 and 12 mo. In both NEXLPL-/- and NEXLPL a period of hyperphagia precedes obesity followed by marked reductions in metabolic rate ensue. In both NEXLPL-/- and NEXLPL mice, substantial increases in AgRP mRNA in the hypothalamus occur before the onset of obesity and are less elevated but persist after obesity develops. Preliminary data suggest that the LPL-dependent deficiency and metabolism of specific PUFAs in the hypothalamus leads to this up-regulation of AgRP gene expression. In Specific Aim #1 well characterized hypothalamic cell lines will be used to test whether LPL deficiency directly leads to increased AgRP gene expression in vitro and whether such regulation depends on the enzyme activity of LPL. Mechanistic studies are designed to pinpoint whether LPL functions in the hypothalamus by regulating fatty acid uptake and whether such regulation also relates to insulin action in these cells. In Specific Aim #2 mice deficient in LPL only in AgRP-producing neurons will be generated to demonstrate that AgRP-producing neurons in NEXLPL-/- mice are the major site of regulation of energy balance and body weight. Studies to evaluate how dietary fatty acids regulate the phenotype are also planned. Finally, despite the presence of severe obesity at 12 mo, NEXLPL-/- mice have better glucose tolerance than littermate controls, and also demonstrate increases in Mc3r gene expression in the hypothalamus, marked brown adipose tissue (BAT) hyperplasia/hypertrophy and increased BAT UCP-1 gene expression. Experiments in this aim will be directed to determining the mechanism of improved glucose metabolism despite obesity in NEXLPL-/- mice. We believe the clinical relevance of this work relates not only to how dietary lipids carried in lipoproteins control energy balance and body weight, but to mechanisms by which BAT is stimulated and glucose tolerance is preserved. Because obesity is epidemic, more insight into the physiology of body weight regulation including how obesity relates to glucose intolerance is needed. Moreover, from this work novel approaches to the prevention and or treatment of obesity by lifestyle and/or new drugs may be possible. Overall, this is the first observation that TG-rich lipoproteins and their metabolism by LPL in the brain has physiologic relevance, and through a series of experiments we hope to reveal mechanisms of how TG-rich lipoprotein sensing and metabolism in the brain impact energy balance and body weight regulation.