Molecular networks and epigenomic mechanisms underpinning white fat browning capacity

当能量摄入长期超过能量消耗时，肥胖就会发生。生热脂肪细胞(棕色和棕褐色/米黄色)具有释放化学能量的能力，因此增加能量消耗为对抗肥胖提供了巨大的潜力。在人类棕色脂肪数量有限的情况下，增加白色脂肪中Brite细胞的相对丰度(Wat棕色)提供了增加生热Brite脂肪组织质量的机会，同时减少了Wat的数量，从而将一个储能器官转变为一个能量耗散器官。然而，对调节Wat褐变的调控机制仍然缺乏全面的了解。近交系小鼠品系之间白色脂肪褐变倾向的差异提供了一个独特的机会，可以放大不同基因型别之间褐化程序的核心调控因素。在之前的研究中，我结合了比较转录学、基于扰动的分析和基因网络分析，揭示了新的调控因子以及与Brite脂肪形成有关的核心调控网络。尽管这些结果构成了定义白色脂肪褐变背后的调控网络的最全面的尝试，但还需要进一步努力来深入表征关键因素，完善网络的组织，并绘制该系统中基因表达可变性来源的更完整的画布。此外，褐变倾向存在更复杂的应变变异来源，例如C57BL/6J和129S6/SvEvTac之间的顺式驱动，而C57BL/6J和FVB/NJ之间的反式驱动。我未来的工作目标是对转录因子结合、表观遗传状态和基因表达进行系统分析，以确定影响褐化能力的自然遗传变异。具体地说，我假设不同的遗传变异会影响转录因子的DNA结合，这会导致组蛋白的可变组蛋白修饰，并共同影响基因的表达和褐化倾向(目标A)，并通过高度连接的针对褐化能力的基因细胞网络发挥作用(目标B)。检验这些假说将导致发现新的功能调控元件、候选基因、表观遗传学基础和导致白色脂肪褐变的分子网络。结合进一步的体内外验证(目标C)，通过获得和失去功能的实验，以及通过CRISPR-dCas9-激活子系统同时激活多个基因，将对新的关键转录因子(S)以及控制褐变能力的分子网络有深入的了解。我们的目标是共同设计一个全面的分子网络来调节白色脂肪的褐变。从长远来看，充分了解Brite细胞募集的分子结构可以促进对抗代谢失衡的治疗方法的发展。

Obesity develops when energy intake chronically exceeds energy expenditure. The capacity for thermogenic adipocytes (brown and brite/beige) to dissipate chemical energy therefore increasing energy expenditure offers great potential to combat obesity. With limited amount of human brown fat, increasing the relative abundance of brite cells in white fat (WAT browning) offers an opportunity to increase the mass of thermogenic brite adipose tissues and meanwhile decrease the amount of WAT, thereby turning an energy-storing organ into an energy-dissipating one. However, a comprehensive understanding of the regulatory mechanisms mediating WAT browning is still lacking. Variation in white fat browning propensity among inbred mouse strains provides a unique opportunity to zoom in on the core regulators of the browning program across different genotypes. In previous studies, combining comparative transcriptomics, perturbation-based assays and gene network analyses, I revealed novel regulators as well as a core regulatory network that contributes to brite adipogenesis. Although these results constitute the most comprehensive attempt to define the regulatory network underlying white fat browning, further efforts are needed to perform in-depth characterization of key factors, refine the organization of the network and draw a more complete canvas of the sources of gene expression variability in this system. Moreover, there is a more complex source of strain variation in browning propensity, exemplified by cis-driven between C57BL/6J and 129S6/SvEvTac, while trans-driven between C57BL/6J and FVB/NJ. My future work now aims at the systematic analyses of transcription factor binding, epigenetic state and gene expression to identify natural genetic variants that affect the browning capacity. Specifically, I hypothesize that distinct genetic variants affect DNA-binding of transcription factors, which lead to variable histone modifications and collectively influence gene expression and browning propensity (Objective A) and operating through highly connected cellular networks of genes targeting browning capacity (Objective B). Testing these hypotheses will lead to the discovery of novel functional regulatory elements, candidate genes, epigenetic basis and molecular networks underlying white fat browning. Combined with further extensive validation both in vitro and in vivo (Objective C) through gain- and loss-of-function experiments as well as simultaneous activation of multiple genes with the CRISPR–dCas9-activator system, an in-depth understanding of novel key transcription factor(s) as well as molecular networks in the control of browning capability will be obtained. Together, we aim to devise a comprehensive molecular network that regulates white fat browning. In the long run, a full understanding of the molecular architecture underlying brite cell recruitment could facilitate the development of therapeutic approaches for combating metabolic imbalance.