UC Berkeley

White adipose tissue (WAT) plays a critical role in metabolic homeostasis by serving as the primary energy storage organ. Adipose tissue also secretes adipokines that control various biological processes such as appetite and insulin sensitivity. Dysregulation of WAT results in metabolic complications such as obesity, characterized by an excess accumulation of WAT, and lipodystrophy, characterized by a loss of adipose tissue. As a result, WAT is regulated, autonomously and by cross-talk with other type of cells and organs. With the world’s population aging rapidly, there is growing demand for understanding the mechanisms involved in aging-related metabolic diseases. Aging is a process that targets the whole organism including highly metabolically active organs, such as the brain. Obesity is associated with numerous diseases such as diabetes, cardiovascular diseases, cancer, as well as neurodegenerative diseases, such as Alzheimer’s disease. The aims of this dissertation work were to investigate changes in WAT as well as in brain during aging, which may serve as promising avenues to combat age-related health problems. Chapter 1 reviews the development of various adipose depots, particularly subcutaneous and visceral adipose. The two largest depots of white adipose tissue can be categorized into subcutaneous adipose tissue and visceral adipose tissue. While both depots share similar lipid accumulating functions, both subcutaneous and visceral adipose are developmentally and functionally distinct. Subcutaneous WAT develops perinatally while visceral WAT forms after birth. Importantly, while visceral adipose is associated with pathological conditions, such as insulin resistance and cardiovascular disease, subcutaneous adipose is protective against these diseases in humans. These differences have attracted much attention to elucidate the underlying developmental origins and cellular hierarchy. Lineage tracing has revealed multiple developmental origins of SAT and VAT, and single-cell transcriptomics has revealed population heterogeneity of adipose precursors. Some populations exist in both depots, such as DPP4+ precursors, ICAM1+ precursors, CP-A preadipocytes and Aregs, while others are depot-specific, such as FIPs in VAT. In aging, adipose undergoes diverse changes, including a decrease in adipose progenitor function, accumulation of senescent cells, and chronic inflammation. Importantly, in contrast to VAT mass that increases during aging, SAT mass decreases in aging, although its underlying mechanism is not understood. Chapter 2 describes the investigation of the subcutaneous-specific decrease in adipose during aging. single-cell RNA sequencing showed a unique anti-adipogenic population of aging-dependent regulatory cells (ARCs) that emerge only in subcutaneous adipose of aged mice and humans and that may contribute to the aging-associated loss of subcutaneous adipose tissue. ARCs are of fibroblast lineage and express adipose progenitor markers but lack adipogenic capacity. They secrete high levels of pro-inflammatory cytokines, such as CCL6, to inhibit the proliferation and differentiation of neighboring adipose precursors. PU.1 is the driving factor for ARC development. Overexpression of PU.1 in 3T3-L1 cells results in an ARC-like phenotype, and knockdown of PU.1 in isolated ARCs restores adipogenic capacity. Chapter 3 profiles work on the NADH oxidoreductase, Aifm3. Aifm3 is highly expressed in brain with enrichment in astrocytes but found at very low levelS in other tissues. I found Aifm3 to associate with mitochondria and increases NAD+/NADH, resulting in higher glycolytic rate and lactate production in C8-D1A astrocytes. [U-13C]-glucose stable isotope labeling and untargeted metabolomics showed increased metabolites relating to the glycolytic pathway, positively correlating with Aifm3 expression levels. Moreover, Aifm3 contains an iron-sulfur Rieske domain and revealed to respond to and protect astrocytes from oxidative stress. Astrocytes lacking the Rieske domain of Aifm3 are shown to insufficiently respond to oxidative stress compared to astrocytes with the full-length Aifm3. Overexpression of Aifm3 in astrocytes demonstrated higher glycolytic rates and lactate production whereas knockdown of Aifm3 in astrocytes decreased glycolytic rates and lactate production. Furthermore, Aifm3 expression decreased oxidative stress and expression of pro-inflammatory cytokines in astrocytes, and together with providing increased energy substrate for oxidative metabolism, supported the health and synaptic growth of neighboring neurons. Chapter 4 concludes and describes the importance of understanding the metabolic and cellular changes during aging and presents remaining questions and future directions.