Adipose tissue plays an active role in metabolism and overall health. In mice, this tissue performs various functions beyond energy storage, including hormone secretion and temperature regulation. Mouse fat serves as an important subject in biological research, especially for understanding metabolic processes and diseases.
Types of Fat in Mice
Mice possess three types of adipose tissue: White Adipose Tissue (WAT), Brown Adipose Tissue (BAT), and Beige (or Brite) Adipose Tissue. WAT is the most abundant type, characterized by large, single lipid droplets, specializing in long-term energy storage. It is found in various anatomical locations, including subcutaneous and visceral depots.
Brown Adipose Tissue, or BAT, appears distinctively brown due to its high concentration of mitochondria and multiple smaller lipid droplets. It is located in specific depots like the interscapular region. Beige adipose tissue, sometimes called brite fat, shares characteristics of both white and brown fat. These cells can emerge within WAT depots and contain multiple lipid droplets and more mitochondria than typical white adipocytes.
Functions of Mouse Fat
Each type of mouse fat performs specialized biological functions. White adipose tissue serves as an energy reservoir, storing excess calories as triglycerides in lipid droplets. Beyond storage, WAT also acts as an endocrine organ, secreting hormones and cytokines like adiponectin and leptin that regulate appetite, energy expenditure, and glucose and lipid metabolism.
Brown adipose tissue is highly specialized for thermogenesis, producing heat. This “non-shivering thermogenesis” is achieved through uncoupling protein 1 (UCP1) in its mitochondria, which dissipates energy as heat rather than storing it as ATP. Beige fat also possesses thermogenic properties, expressing UCP1 and increasing heat production, particularly in response to cold exposure or other stimuli. These beige cells can switch between energy storage and energy dissipation, offering a flexible metabolic response.
Why Mice are Used in Fat Research
Mice are widely used in fat research due to advantages for studying metabolism, obesity, and related diseases. Their genetic makeup is well-characterized, with many genetically defined strains and a fully sequenced genome, allowing for precise genetic manipulation, such as gene knock-outs or overexpression. This genetic tractability enables researchers to investigate the function of specific genes related to fat metabolism and obesity.
Mice also have rapid reproduction rates and relatively short lifespans, which facilitates studies on long-term effects of interventions or genetic modifications across generations. Researchers can maintain mice in controlled environments, which allows for standardized experimental conditions that minimize external variables affecting fat accumulation and metabolism. Mice exhibit physiological similarities to humans in terms of fat storage and metabolic pathways, making them relevant models for understanding human health conditions like insulin resistance and dyslipidemia.
Distinctions Between Mouse and Human Fat
While mice are valuable models, understanding the differences between mouse and human adipose tissue is important for accurate translation of findings. Fat distribution varies between species; for example, humans have significant omental visceral fat, while mice have a larger perigonadal visceral fat depot. Human subcutaneous fat expansion often has neutral or beneficial metabolic effects, whereas excess visceral fat correlates with metabolic and cardiovascular risks.
Differences also exist in specific metabolic pathways and responses to stimuli. Human adipocytes express lower levels of the beta3 adrenergic receptor, relying more on beta1 and beta2 receptors for catecholamine-induced lipolysis, unlike mice where beta3 is more prominent. Browning of white adipose tissue, while observed in both, may not respond identically to stimuli like exercise in humans compared to mice. These distinctions emphasize that while mouse models provide significant insights, direct extrapolation to human physiology requires careful consideration of species-specific fat biology.