Molecular regulation of adipocyte progenitor quiescence and metabolic adaptation to obesity

脂肪组织扩张通过新脂肪细胞形成（导致增生性生长）和现有脂肪细胞肥大的组合发生，其相对贡献可能影响全身代谢健康。由于脂肪细胞发育失败导致的先天性脂肪营养不良与严重的胰岛素抵抗有关。由发育基因中的遗传变异引起的脂肪形成的更微妙的损伤也易患胰岛素抵抗和2型糖尿病。这些例子反映了一个更大的数据主体，为代谢健康依赖于功能性脂肪细胞群体的假设提供了概念基础。通过利用稳定同位素示踪剂和高精度质谱方法，我们现在还发现了脂肪生成的年龄依赖性下降。因此，脂肪祖细胞-新脂肪细胞的细胞来源-必须在成熟脂肪组织中处于相对静止的状态。尽管脂肪形成的下游驱动因素已被充分描述，但维持脂肪祖细胞静止或其在体内向增殖和分化的近端转变的机制在很大程度上仍是未知的。为了解决这一知识缺口，我们利用转录组学数据以及新鲜分离的原代脂肪细胞祖细胞中可接近的染色质的基因组规模图来模拟负责静止状态的转录因子网络。我们的数据暗示孤儿核受体NR 4A 1是一个关键的调控节点。在体外和体内功能基因组学研究提供了额外的支持，我们的NR 4A 1为中心的假设：NR 4A 1调节转录程序，建立了代谢有害状态的静止脂肪祖细胞。这些数据还为两个相互关联的具体目标提供了依据。在目标1中，我们将测试NR 4A 1是脂肪祖细胞静止的核心转录调节因子的假设。我们将使用报告基因分析，CRISPRi和CRISPR/Cas9来功能性地询问关键脂肪形成转录因子处的NR 4A 1-DNA调控节点。在目标2中，我们将在基因组规模上对脂肪祖细胞中NR 4A 1依赖的染色质调控景观进行公正的询问。我们将绘制NR 4A 1-DNA相互作用的图谱，并将NR 4A 1与典型的顺式调控标记结合。然后，我们将使用低输入基因组学方法结合NR 4A 1功能的获得或丧失来绘制NR 4A 1依赖的染色质调控景观和转录输出。总的来说，通过有针对性的功能基因组学实验和无偏的基因组规模分析，我们将测试我们的模型NR 4A 1作为一个主调节基因程序控制祖细胞静止。定义基本的分子机制，使居民脂肪祖细胞静止持有的承诺，以确定分子障碍，脂肪生成和代谢健康的脂肪。

Adipose tissue expansion occurs by a combination of new adipocyte formation (resulting in hyperplastic growth) and hypertrophy of existing adipocytes, the relative contributions of which may impact systemic metabolic health. Congenital lipodystrophy due to failed adipocyte development is associated with severe insulin resistance. More subtle impairments in adipogenesis arising from genetic variants in developmental genes also predispose to insulin resistance and Type 2 Diabetes Mellitus. These examples reflect a larger body of data providing a conceptual basis for the hypothesis that metabolic health is dependent on a functional adipocyte population. By leveraging stable isotope tracers with high precision mass spectrometry methods, we have now also identified an age-dependent decline in adipogenesis. Therefore, adipocyte progenitors—the cellular source of new adipocytes—must reside in a state of relative quiescence in mature adipose tissue. Although downstream drivers of adipogenesis are well described, the mechanisms that maintain quiescence of adipocyte progenitors or their proximal transition to proliferate and differentiate, in vivo, remain largely unknown. To address this knowledge gap, we have leveraged transcriptomic data together with genome scale maps of accessible chromatin in freshly isolated primary adipocyte progenitors to model the network of transcription factors responsible for the quiescent state. Our data implicate the orphan nuclear receptor, NR4A1, as a key regulatory node. In vitro and in vivo functional genomics studies have provided additional support for our NR4A1-centric hypothesis: that NR4A1 regulates a transcriptional program that establishes a metabolically deleterious state of quiescence in adipocyte progenitors. These data also provide rationale for two interrelated Specific Aims. In Aim 1, we will test the hypothesis that NR4A1 is a core transcriptional regulator of adipocyte progenitor quiescence. We will use reporter assays, CRISPRi and CRISPR/Cas9 to functionally interrogate NR4A1-DNA regulatory nodes at key adipogenic transcription factors. In Aim 2, we will perform an unbiased interrogation of the NR4A1-depedent chromatin regulatory landscape in adipocyte progenitor cells at genome scale. We will map NR4A1-DNA interactions and co-localize NR4A1 binding with canonical cis-regulatory marks. We will then use low-input genomics methods coupled with NR4A1 gain or loss of function to map the NR4A1 dependent chromatin regulatory landscape and transcriptional output. Collectively, through targeted functional genomics experiments and unbiased genome scale analyses we will test our model of NR4A1 as a master regulator of a gene program controlling progenitor quiescence. Defining fundamental molecular mechanisms that render resident adipocyte progenitor cells quiescent holds promise to identify molecular barriers to adipogenesis and metabolically healthy fat.