Biophysical and genetic cues regulating lipid droplet packaging and alterations in obesity

总结 肥胖是一种主要的流行病，在美国，每三个人中就有一个人受到肥胖的影响。广泛 脂肪组织的重塑和脂滴（LD）的生物发生在恒定的 营养过剩来储存额外的脂肪。控制脂肪的生物物理和遗传机制 扩张还有待充分理解。胆汁酸（BA）-身体的主要天然表面活性剂- 不仅促进脂肪消化，而且作为信号分子调节脂肪代谢， 激活其受体。我们的初步数据表明，在响应BA，LD分离自 棕色脂肪组织（BAT，含有小的多房LD）保持相同的大小， 白色（WAT，含有大的单室LD）脂肪LD在其核心捕获中整合BA 可以在液滴之间快速穿梭的TAG。我们将检验LD成熟， 在BA表面活性剂的调节下，LD生长的主导机制是聚结。我们 还发现脂肪在LD中是分层堆积的。另外，我们发现细胞外基质 (ECM)BAT的优势比WAT的ECM更明显。我们将研究 脂肪和细胞外基质之间的关系在肥胖症中发生了改变。最后，我们开发了一个巨大的单层 囊泡（GUV），模仿内质网LD生物合成，以梳理这些脂肪包装 差异我们的初步实验表明，TAG在GUV膜中均匀分布， 聚集在新生的TAG冷凝物中。我们将研究LD的一个新假设 除了膜张力之外，还受液-液相分离控制的生物发生。 我们在脂肪组织中检测到几种BA的存在，发现脂肪细胞特异性 敲除法尼醇X受体（Fxr）-一种BA受体-导致更大的LD大小， 下调控制脂质代谢的基因的表达。这些惊人的结果表明 LD重塑可能通过BA-Fxr轴进行转录调控。我们还 发现了一个关键的BA合成基因Cyp 27 a1， 前脂肪细胞中Cyp 27 a1的缺失损害了生长，这表明 Cyp 27 a1在脂肪形成中重要作用除了有针对性的方法外，我们还将 研究脂肪重塑过程中转录组和表观基因组的变化， 肥胖症的发展。我们令人兴奋的初步数据强调了BA角色的重要性 （表面活性剂和信号传导）在调节LD大小和膨胀中的作用。该提案将（1）确定 饮食和BA调节如何影响LD结构和生物发生;（2）描述遗传 肥胖期间调节LD扩张的机制。总的来说，这个项目将揭示 控制肥胖症LD动力学的基本原则。

Summary Obesity is a major epidemic, which affects one in three individuals in the United States. Extensive remodeling of the adipose tissue and biogenesis of lipid droplets (LDs) occur in constant overnutrition to store the additional fat. The biophysical and genetic mechanisms controlling fat expansion are yet to be fully understood. Bile acids (BAs)—the body’s major natural surfactants— not only facilitate fat digestion but also act as signaling molecules to regulate fat metabolism by activating its receptors. Our preliminary data suggests that in response to BAs, LDs isolated from brown adipose tissue (BAT, containing small multilocular LDs) remain the same size whereas white (WAT, containing large unilocular LDs) adipose LDs integrate BAs in their core capturing TAGs that can rapidly shuttle from droplet to droplet. We will test the hypothesis that LD ripening, regulated by BA surfactants, is a dominant mechanism of LD growth instead of coalescence. We also found that fat is packed in layers in LDs. Additionally, we found that the extracellular matrix (ECM) of BAT is significantly more aligned that WAT’s ECM. We will investigate how the interplay between fat and ECM packing is altered in obesity. Finally, we developed a giant unilamellar vesicle (GUV) that mimics the endoplasmic reticulum LD biogenesis to tease these fat packaging differences. Our pilot experiments show that TAG distributes evenly in the GUV membrane before accumulating in a nascent TAG condensate. We will investigate a novel hypothesis for LD biogenesis that, in addition to membrane-tension, is controlled by liquid-liquid phase separation. We detected the presence of several BAs in adipose tissue and discovered that adipocyte-specific knockout of Farnesoid X Receptor (Fxr)—a BA receptor—resulted in a larger LD size and downregulated expression of genes controlling lipid metabolism. These surprising results indicate that LD remodeling may be transcriptionally regulated through the BA-Fxr axis. Further, we uncovered that a critical BA synthesis gene, Cyp27a1, was specifically induced upon adipogenesis and that its deletion of Cyp27a1 in preadipocytes impaired growth, indicating an important role of Cyp27a1 in adipogenesis. In addition to the focused approaches, we will also investigate the alterations in the transcriptome and epigenome of adipose remodeling during the development of obesity. Our exciting preliminary data underscores the importance of BAs' role (surfactant and signaling) in regulating LD size and expansion. This proposal will (1) determine how diet and BA regulation affect LD structure and biogenesis and (2) delineate the genetic mechanisms that regulate LD expansion during obesity. Overall, this project will uncover fundamental principles that govern LD dynamics in obesity.