Molecular regulation of adipocyte progenitor quiescence and metabolic adaptation to obesity

点击翻译按钮获取中文摘要

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.