Frontline Science: Rapid adipose tissue expansion triggers unique proliferation and lipid accumulation profiles in adipose tissue macrophages

Obesity-related changes in adipose tissue leukocytes, in particular adipose tissue macrophages (ATMs) and dendritic cells (ATDCs), are implicated in metabolic inflammation, insulin resistance, and altered regulation of adipocyte function. We evaluated stromal cell and white adipose tissue (WAT) expansion dynamics with high fat diet (HFD) feeding for 3-56 days, quantifying ATMs, ATDCs, endothelial cells (ECs), and preadipocytes (PAs) in visceral epididymal WAT and subcutaneous inguinal WAT. To better understand mechanisms of the early response to obesity, we evaluated ATM proliferation and lipid accumulation. ATMs, ATDCs, and ECs increased with rapid WAT expansion, with ATMs derived primarily from a CCR2-independent resident population. WAT expansion stimulated proliferation in resident ATMs and ECs, but not CD11c⁺ ATMs or ATDCs. ATM proliferation was unperturbed in Csf2- and Rag1-deficient mice with WAT expansion. Additionally, ATM apoptosis decreased with WAT expansion, and proliferation and apoptosis reverted to baseline with weight loss. Adipocytes reached maximal hypertrophy at 28 days of HFD, coinciding with a plateau in resident ATM accumulation and the appearance of lipid-laden CD11c⁺ ATMs in visceral epididymal WAT. ATM increases were proportional to tissue expansion and adipocyte hypertrophy, supporting adipocyte-mediated regulation of resident ATMs. The appearance of lipid-laden CD11c⁺ ATMs at peak adipocyte size supports a role in responding to ectopic lipid accumulation within adipose tissue. In contrast, ATDCs increase independently of proliferation and may be derived from circulating precursors. These changes precede and establish the setting in which large-scale adipose tissue infiltration of CD11c⁺ ATMs, inflammation, and adipose tissue dysfunction contributes to insulin resistance.

Imaging white adipose tissue with confocal microscopy

Adipose tissue is composed of a variety of cell types that include mature adipocytes, endothelial cells, fibroblasts, adipocyte progenitors, and a range of inflammatory leukocytes. These cells work in concert to promote nutrient storage in adipose tissue depots and vary widely based on location. In addition, overnutrition and obesity impart significant changes in the architecture of adipose tissue that are strongly associated with metabolic dysfunction. Recent studies have called attention to the importance of adipose tissue microenvironments in regulating adipocyte function and therefore require techniques that preserve cellular interactions and permit detailed analysis of three-dimensional structures in fat. This chapter summarizes our experience with the use of laser scanning confocal microscopy for imaging adipose tissue in rodents.

Macrophages and the regulation of adipose tissue remodeling

The ability of adipose tissue to adapt to a changing nutrient environment is critical to the maintenance of metabolic control. Nutrient excess and deficiency alter the shape of adipose tissue drastically and trigger many events that are collectively known as adipose tissue remodeling. Remodeling of adipose tissue involves more than adipocytes and is controlled by an extensive network of stromal cells and extracellular matrix proteins. Prominent players in this process are adipose tissue macrophages, which are a specialized leukocyte present in lean and obese states that contributes to adipose tissue inflammation. The interest in adipose tissue remodeling has been accelerated by the current epidemic of obesity and the chronic generation of signals that lead to expansion of adipose tissue. It is clear that evidence of dysfunctional remodeling events is a hallmark of obesity associated with metabolic disease. This review summarizes and highlights the recent work in this area and provides a framework in which to consider how adipose tissue macrophages contribute to the remodeling events in lean and obese states. Advancing our understanding of the involvement of macrophages in adipose tissue remodeling will promote one aspect of the new field of "immunometabolism," which connects control systems developed for regulation of immunity with those that control metabolism. It will also provide insight into how physiologic and pathophysiologic remodeling differs in adipose tissue and identify potential nodes for intervention to break the link between obesity and disease.

Thrombospondin 1 mediates high-fat diet-induced muscle fibrosis and insulin resistance in male mice

Thrombospondin 1 (THBS1 or TSP-1) is a circulating glycoprotein highly expressed in hypertrophic visceral adipose tissues of humans and mice. High-fat diet (HFD) feeding induces the robust increase of circulating THBS1 in the early stages of HFD challenge. The loss of Thbs1 protects male mice from diet-induced weight gain and adipocyte hypertrophy. Hyperinsulinemic euglycemic clamp study has demonstrated that Thbs1-null mice are protected from HFD-induced insulin resistance. Tissue-specific glucose uptake study has revealed that the insulin-sensitive phenotype of Thbs1-null mice is mostly mediated by skeletal muscles. Further assessments of the muscle phenotype using RNA sequencing, quantitative PCR, and histological studies have demonstrated that Thbs1-null skeletal muscles are protected from the HFD-dependent induction of Col3a1 and Col6a1, coupled with a new collagen deposition. At the same time, the Thbs1-null mice display a better circadian rhythm and higher amplitude of energy expenditure with a browning phenotype in sc adipose tissues. These results suggest that THBS1, which circulates in response to a HFD, may induce insulin resistance and fibrotic tissue damage in skeletal muscles as well as the de-browning of sc adipose tissues in the early stages of a HFD challenge. Our study may shed new light on the pathogenic role played by a circulating extracellular matrix protein in the cross talk between adipose tissues and skeletal muscles during obesity progression.

Wnt6, Wnt10a and Wnt10b inhibit adipogenesis and stimulate osteoblastogenesis through a β-catenin-dependent mechanism

Wnt10b is an established regulator of mesenchymal stem cell (MSC) fate that inhibits adipogenesis and stimulates osteoblastogenesis, thereby impacting bone mass in vivo. However, downstream mechanisms through which Wnt10b exerts these effects are poorly understood. Moreover, whether other endogenous Wnt ligands also modulate MSC fate remains to be fully addressed. In this study, we identify Wnt6 and Wnt10a as additional Wnt family members that, like Wnt10b, are downregulated during development of white adipocytes in vivo and in vitro, suggesting that Wnt6 and/or Wnt10a may also inhibit adipogenesis. To assess the relative activities of Wnt6, Wnt10a and Wnt10b to regulate mesenchymal cell fate, we used gain- and loss-of function approaches in bipotential ST2 cells and in 3T3-L1 preadipocytes. Enforced expression of Wnt10a stabilizes β-catenin, suppresses adipogenesis and stimulates osteoblastogenesis to a similar extent as Wnt10b, whereas stable expression of Wnt6 has a weaker effect on these processes than Wnt10a or Wnt10b. In contrast, knockdown of endogenous Wnt6 is associated with greater preadipocyte differentiation and impaired osteoblastogenesis than knockdown of Wnt10a or Wnt10b, suggesting that, among these Wnt ligands, Wnt6 is the most potent endogenous regulator of MSC fate. Finally, we show that knockdown of β-catenin completely prevents the inhibition of adipogenesis and stimulation of osteoblast differentiation by Wnt6, Wnt10a or Wnt10b. Potential mechanisms whereby Wnts regulate fate of MSCs downstream of β-catenin are also investigated. In conclusion, this study identifies Wnt10a and Wnt6 as additional regulators of MSC fate and demonstrates that mechanisms downstream of β-catenin are required for Wnt6, Wnt10a and Wnt10b to influence differentiation of mesenchymal precursors.

An up-close look at the pre-mRNA 3'-end processing complex

Polyadenylation is a nearly universal mRNA processing step in eukaryotic gene expression and it takes place in a macromolecular machinery termed the mRNA 3' processing complex. In a recent study, we reported the purification and comprehensive characterization of functional mammalian mRNA 3' processing complexes. Our study defined the protein composition of this machinery, revealed new potential links between mRNA 3' processing and other cellular processes, and characterized basic structural features of the "core" mRNA 3' processing complex. These results provided new insights into the mechanism of mRNA 3'-end formation.