Molecular Control of Brown Adipose Cell Fate and Energy Metabolism

描述（由申请人提供）：当能量摄入长期超过总能量消耗时，肥胖症就会发生。目前，大多数抗肥胖药物通过抑制食欲或抑制肠道脂肪吸收来抑制能量摄入。然而，由于副作用，包括抑郁症，油性排便和脂肪漏，迫切需要替代方法。由于棕色脂肪组织（BAT）耗散能量产生热量以抵御寒冷和肥胖，改变分子途径以增加BAT的数量或产热活性可能导致替代和有效的治疗干预，以对抗人类肥胖和代谢紊乱。我们的长期目标是了解调节棕色脂肪细胞命运规范的分子电路，并研究它们在能量稳态中的生理作用。我们之前已经证明，棕色脂肪细胞是通过转录因子PRDM16的作用，从一个皮肤细胞前体亚群中产生的；然而，目前尚不清楚PRDM16在肌母细胞到棕色脂肪转换中的作用是如何被调节的。我们发现赖氨酸甲基转移酶EHMT1是PRDM16转录复合体的关键组成部分。EHMT1在BAT中表达最高，受PRDM16的高度诱导。值得注意的是，EHMT1似乎作为棕色脂肪细胞与肌细胞的发育开关。重要的是，EHMT1基因的缺失与小鼠和人类的肥胖有关；然而，其潜在的机制仍然完全未知。因此，我们目前的目标是研究EHMT1在体内控制棕色脂肪细胞命运的生理功能和机制。基于我们的初步数据，我们将验证EHMT1在能量稳态中发挥关键作用的假设，作为一个发育开关，通过调节PRDM16复合物的功能来控制棕色脂肪细胞的命运。为了验证这一假设，我们将追求以下具体目标：在Aim1中，我们将在体外和体内确定EHMT1在棕色脂肪细胞命运规范和维持中的遗传需求。在Aim2中，我们将分析脂肪特异性EHMT1敲除小鼠和EHMT1杂合缺失小鼠的代谢表型，并批判性地表征EHMT1在体内控制能量消耗和葡萄糖稳态中的生理作用。在aims中，我们将进行生化分析并使用培养细胞来阐明EHMT1作为棕色脂肪谱系发育开关的机制。这些研究的预期结果是表征一个全新的上游调控途径棕色脂肪细胞命运规范。我们的发现将产生重大影响，因为据我们所知，这项研究将描述第一种控制棕色脂肪和骨骼肌之间细胞命运转换的酶。确定的机制将允许我们通过药理学方法操纵这一发育途径，这可能提供一个可能的治疗靶点。

DESCRIPTION (provided by applicant): Obesity develops when energy intake chronically exceeds total energy expenditure. Currently, most anti- obesity medications act to repress energy intake, either by suppressing appetite or by inhibiting intestinal fat absorption. However, due to side effects including depression, oily bowel movements and steatorrhea, there is an urgent need for alternative approaches. Because brown adipose tissue (BAT) dissipates energy to produce heat as a defense against cold and obesity, altering the molecular pathway to increase the amount or thermogenic activity of BAT may lead to an alternative and effective therapeutic intervention to counteract human obesity and metabolic disorders. Our long-term goals are to understand the molecular circuits that regulate the fate specification of brown adipose cells and to investigate their physiological roles in energy homeostasis. We have previously shown that brown adipocytes arise from a subset of dermomytomal precursors through the action of a transcription factor, PRDM16; however, it remains unclear how the PRDM16 action in the myoblast-to-brown fat switch is regulated. We identified a lysine methyltransferase, EHMT1 as a critical component of the PRDM16 transcriptional complex. EHMT1 is expressed at its highest in BAT and is highly induced by PRDM16. Notably, EHMT1 appears to act as a developmental switch of brown adipocytes versus myocytes. Importantly, loss of the EHMT1 gene is associated with obesity in mice and in humans; however, its underlying mechanism remains completely unknown. Our current objective is thus to investigate the physiological function and mechanism of EHMT1 that controls brown adipose cell fate in vivo. Based on our preliminary data, we will test the hypothesis that EHMT1 plays a pivotal role in energy homeostasis as a developmental switch that controls brown adipose cell fate through modulating the function of the PRDM16 complex. To test this hypothesis, we will pursue the following specific aims: In Aim1, we will determine the genetic requirement of EHMT1 in the fate specification and maintenance of brown adipose cells in vitro and in vivo. In Aim2, we will analyze the metabolic phenotypes of adipose-specific EHMT1 knockout mice and EHMT1 heterozygous null mice and critically characterize EHMT1's physiological role in controlling energy expenditure and glucose homeostasis in vivo. In Aim3, we will conduct biochemical analyses and use cultured cells to elucidate the mechanism by which EHMT1 acts as a developmental switch of brown fat lineage. The expected outcome of these studies is to characterize a completely novel upstream regulatory pathway of brown adipose cell fate specification. Our findings will have a significant impact, because, to our knowledge, this study will characterize the first enzyme that controls the cell fate switch between brown adipose versus skeletal muscle. The identified mechanism will allow us to manipulate this developmental pathway by pharmacological approaches, which may provide a possible therapeutic target. PUBLIC HEALTH RELEVANCE: Obesity and its metabolic consequences continue to be among the most important biomedical challenges in the USA today. Because brown fat dissipates energy to produce heat as a defense against cold and obesity, understanding the molecular control of brown fat development and function will provide new and promising therapeutic strategies for human obesity. Hence, the proposed research is closely aligned with the part of NIH's mission.