UC Berkeley

While white adipose tissue (WAT) stores excess energy as triglycerides, brown adipose tissue (BAT) dissipates heat through uncoupling protein 1 (UCP1) by utilizing fuels such as fatty acids and glucose. A third type of adipocytes are beige adipocytes, which arise in WAT depots upon certain stimuli and are capable of thermogenesis, whose process is called browning or beiging. Since the recent discovery of active BAT or BAT-like tissue in human adults, targeting BAT has been proposed as an attractive strategy to fight against obesity and its associated diseases as there are currently no effective treatments other than lifestyle changes. Regardless, the mechanisms underlying BAT differentiation and thermogenesis are not fully understood. The aim of my dissertation work was to identify and characterize novel regulators of BAT differentiation and thermogenesis.Chapter 1 reviews the current knowledge on signaling pathways that regulate thermogenesis in adipocytes. Upon cold exposure, norepinephrine is released and activates β3-adrenergic signaling in BAT. This causes activation of UCP1, upregulation of thermogenic genes, and increase in substrate supply for thermogenesis. Additionally, insulin/IGF-1 signaling is also important for BAT development as well as browning of WAT. The involvement of other signaling pathways such as those for thyroid hormone and TGFβ superfamily are also discussed. Chapter 2 is to identify and characterize novel thermogenic genes in brown adipocytes. By utilizing CRISPR-Cas9 screening along with a temperature-sensitive dye, I identified Apold1, which is enriched in BAT and responsive to β3-adrenergic stimuli. Through in vitro gain-of-function and loss-of-function experiments, I confirmed that Apold1 indeed promotes thermogenesis. Unexpectedly, Apold1 global knockout mice showed lower body weight due to reduced food intake. Thus, the tissue-specific role of Apold1 was examined by viral injection of Apold1 into BAT, which increased body temperature and oxygen consumption in mice. Chapter 3 describes another screening effort to identify novel thermogenic regulators. Using a different dye as a readout for thermogenesis, I found Rasl12 to promote thermogenesis in brown adipocytes. The role of Rasl12 on thermogenesis was verified by in vitro experiments. Interestingly, co-immunoprecipitation combined with mass spectrometry revealed the possible interaction of Rasl12 with nuclear transporters and RNA processing factors, which is an unexplored aspect of thermogenesis regulation. Chapter 4 profiles yet another screening effort to identify and characterize novel regulators for brown adipocyte differentiation. I found Zc2hc1c, which suppressed brown adipocyte differentiation when overexpressed. RNA-sequencing and RT-qPCR revealed that Zc2hc1c KO cells had increased expression of PPARγ, a master regulator of adipocyte differentiation, suggesting the inhibitory effect of Zc2hc1c on differentiation. Lastly, chapter 5 concludes my work describing the importance of novel genes identified in BAT thermogenesis and differentiation, future directions and remaining questions. All together, these studies have identified novel regulators of thermogenesis and differentiation in brown adipocytes, which will provide a better understanding of BAT biology and advance the field to combat obesity.