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

描述（由申请人提供）：本申请的长期目标是通过神经元中脂蛋白脂肪酶（LPL）介导的新发现途径揭示体重调节机制。LPL是一种限速酶，用于从循环中富含甘油三酯的脂蛋白的水解中摄取脂肪酸，用于外周组织的脂质储存和/或氧化。LPL也存在于整个神经系统，包括海马、下丘脑和其他大脑区域的神经元。神经元特异性缺失LPL的小鼠（NEXLPL-/-）在6个月时就会在饮食中肥胖，杂合小鼠（NEXLPL）也会在6到12个月时肥胖。在NEXLPL-/-和NEXLPL中，肥胖之前都有一段时间的贪食，随后代谢率显著降低。在NEXLPL-/-和NEXLPL小鼠中，下丘脑AgRP mRNA的大量增加发生在肥胖发生之前，并且在肥胖发生后持续升高，但升高幅度较小。初步数据表明，lpl依赖性缺失和下丘脑特定PUFAs的代谢导致AgRP基因表达上调。在Specific Aim #1中，将使用表征良好的下丘脑细胞系来测试LPL缺乏是否直接导致AgRP基因在体外的表达增加，以及这种调节是否取决于LPL的酶活性。机制研究旨在查明LPL是否通过调节脂肪酸摄取在下丘脑中起作用，以及这种调节是否也与这些细胞中的胰岛素作用有关。在Specific Aim #2中，将产生仅在产生agp的神经元中存在LPL缺陷的小鼠，以证明NEXLPL-/-小鼠体内产生agp的神经元是调节能量平衡和体重的主要部位。评估膳食脂肪酸如何调节表型的研究也在计划中。最后，尽管在12个月时出现严重肥胖，但NEXLPL-/-小鼠的葡萄糖耐量比同窝对照小鼠更好，并且下丘脑Mc3r基因表达增加，棕色脂肪组织（BAT）增生/肥大，BAT UCP-1基因表达增加。本实验旨在确定NEXLPL-/-小鼠在肥胖情况下改善葡萄糖代谢的机制。我们相信这项工作的临床意义不仅与脂蛋白携带的膳食脂质如何控制能量平衡和体重有关，还与BAT被刺激和葡萄糖耐量保持的机制有关。由于肥胖是一种流行病，因此需要更多地了解体重调节的生理学，包括肥胖与葡萄糖耐受不良的关系。此外，从这项工作中，通过生活方式和/或新药预防和/或治疗肥胖的新方法可能成为可能。总的来说，这是第一次观察到富tg脂蛋白及其在大脑中通过LPL代谢具有生理学相关性，我们希望通过一系列实验揭示富tg脂蛋白在大脑中的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.