Improved methodologies for the study of adipose biology: insights gained and opportunities ahead

Abstract

Adipocyte differentiation and function have become areas of intense focus in the field of energy metabolism; however, understanding the role of specific genes in the establishment and maintenance of fat cell function can be challenging and complex. In this review, we offer practical guidelines for the study of adipocyte development and function. We discuss improved cellular and genetic systems for the study of adipose biology and highlight recent insights gained from these new approaches.

Keywords: adipogenesis; energy metabolism; obesity.

Fig. 1.

Tracking adipogenesis in vivo using the AdipoChaser mouse. A: The AdipoChaser mouse. AdipoChaser mice are derived from interbreeding three transgenic strains: 1) transgenic mice expressing the “tet-on” transcription factor rtTA under the control of the adiponectin gene promoter (Adn-rtTA); 2) a tet-responsive CRE (TRE-Cre) line that can be activated with rtTA in the presence of doxycycline (Dox); and 3) Rosa26 LacZ reporter mice expressing a reporter gene from the Rosa26 locus in a Cre-dependent manner (Rosa26-loxP-STOP-loxP-LacZ). In the absence of doxycycline, rtTA remains inactive and there is no Cre expression. Upon treatment with doxycycline, rtTA activates the TRE promoter to induce Cre expression, and Cre protein will subsequently eliminate the floxed transcriptional stop cassette and permanently turn on reporter gene expression in every mature adipocyte present during doxycycline exposure. B: Tracking cold-induced beige adipogenesis in inguinal WAT. Prior to doxycycline treatment, inguinal adipocytes are devoid of LacZ expression. Upon doxycycline treatment at room temperature, all adipocytes become LacZ positive (pulse labeling). After doxycycline withdrawal, mice were switched exposed to 4°C for 3 days; accumulated multilocular adipocytes are observed as LacZ negative, indicating that they were formed from cells originally unlabeled rather than preexisting mature adipocytes.

Fig. 2.

Lean or lipodystrophic? A commonly observed phenotype in animal models is a change in adiposity. In recent years, more and more mouse models exhibiting protection from diet-induced obesity have been described. Ultimately, the level of adiposity is dictated by the demand for energy storage. An engineered mouse with reduced adiposity may exhibit reduced fat mass simply because of reduced food intake or because of elevated energy expenditure. Such a mouse can be considered “lean” low adiposity is a normal consequence of a negative energy balance. A lean mouse (e.g., C57BL/6 mice on chow diet) exhibits greater glucose tolerance when compared with metabolically unhealthy obese animals (e.g., C57BL/6 mice on high-fat diet). A true adipose deficiency, or “lipodystrophy”, occurs when energy storage in adipose tissue cannot meet the demand for retention of fatty acids. This occurs when there is a primary defect in adipose tissue function or development (e.g., Fat-ATTAC, A-ZIP, or aP2-SREBP-1c transgenic animals; various Pparγ knockout models) (220). Lipodystrophic animals exhibit a metabolically unhealthy phenotype, similar (or worse than) that observed in unhealthy obese animals. This includes severe insulin resistance, glucose intolerance, elevated TGs, and ectopic lipid deposition and ceramide accumulation in the liver, pancreas, and/or skeletal muscle. The residual adipose tissue, though lacking adipocytes, actually exhibits features of pathological adipose tissue seen in unhealthy obese animals. This includes increased fibrosis and inflammation. Thus, it is not necessarily obesity per se that leads to metabolic disease. In fact, mice can be engineered (e.g., aP2-Mitoneet, aP2-Glut4, aP2-adiponectin transgenic animals) (87, 88, 221) to exhibit a “healthy-obese” phenotype. This obese phenotype includes preferential expansion of subcutaneous WAT, and insulin sensitivity, glucose tolerance, and adiponectin levels seen in normal BMI individuals. Importantly, the adipose tissue appears histologically similar to WAT of lean individuals; adipose expansion occurs through adipocyte hyperplasia and limited fibrosis occurs. Furthermore, the “healthy-obese” phenotype correlates with low ectopic lipid deposition. The image details the adipose and metabolic phenotypes commonly observed in rodent models and in humans. sWAT, subcutaneous adipose tissue; gWAT, gonadal adipose tissue.