Brown fat, or brown adipose tissue (BAT), is particularly abundant in babies and in small mammals, and plays a key role in generating body heat and burning excess energy. In adults, brown fat activation is linked with lower body weight, and better glucose and lipid regulation, which makes it a potential therapeutic target for metabolic diseases such as diabetes and obesity.
Studies in rodents by a team at Beth Israel Deaconess Medical Center (BIDMC) now indicate that brown fat can also act to regulate skeletal muscle function. The results showed that deleting a key gene known as IRF4 in the animals’ brown fat led to reduced exercise performance and changes to muscle structure, whereas overexpressing the brown fat gene boosted muscle exercise capacity.
“We knew that muscles could regulate brown fat – exercising increases brown fat – but it was unknown whether brown fat affected muscle function,” says research lead Evan Rosen, M.D., Ph.D., chief of the division of endocrinology, diabetes and metabolism at BIDMC. “In this new study, we closed the loop and demonstrated that the loss of IRF4 in brown fat tissue reduces exercise capacity in rodents, affecting cellular function and causing physiological abnormalities in the muscle tissue itself.” As well as providing new insights into the functions of brown fat, the BIDMC studies could ultimately help develop new treatment approaches to specific metabolic and muscular diseases. The findings are reported in Cell Metabolism, in a paper titled, “Brown adipose tissue controls skeletal muscle function via the secretion of myostatin.”
Dr. Rosen’s research has focused largely on the transcription factor IRF4. The protein is generally linked with immune system regulation, but within the last decade, the Beth Israel Deaconess team has found that IRF4 is critical for adipose tissue function, and is a key regulator of thermogenesis in brown fat. Their studies showed that mice lacking IRF4 in BAT (BAT14KO mice) are sensitive to cold exposure, lay down more adipose fat, and have reduced insulin resistance when fed a high-fat diet.
There has been a lot of recent interest in the observation that exercise can promote browning of the body’s less beneficial white adipose tissue (WAT), but what isn’t known is what purpose this serves in an exercising animal, the Cell Metabolism authors write. “Intrigued by these data, we asked whether the skeletal muscle/BAT axis could also communicate in the reverse direction; that is, could skeletal muscle activity be under control of factors from BAT?”
To investigate this in more detail, the BIDMC team and colleagues, including first author Xingxing Kong, Ph.D., previously a postdoctoral fellow in Dr. Rosen’s lab, and now a faculty member at the David Geffen School of Medicine at the University of California, Los Angeles, studied the exercise capacity of mice in the LRF4-knockout BATI4K0 mice, or in animals engineered to overexpress IRF4 in BAT (BATI4OE mice). In contrast with BAT14KO mice, BAT140E animals showed increased thermogenesis, are lean and insulin-sensitive on a high-fat diet. “The BAT14KO and BAT14OE are thus excellent models of deficient and hyperefficient BAT function, generally,” the researchers state.
Their exercise test results showed that compared with wild-type control animals, the IRF4 knockout mice performed 14% worse on a moderate intensity, slow-speed treadmill exercise test, and 38% worse at higher intensity exercise. Mice lacking IRF4 in their BAT also showed abnormalities in specific muscle tissues. “Even more strikingly,” the authors add, electron microscopy showed that the white vastus thigh muscle tissue in BAT14KO mice developed tubular aggregates, and structural features akin to some muscular disorders.
Speculating that the IRF4-deficient BAT might be secreting some sort of myopathic factor, the team carried out RNA sequencing of brown fat tissue from both control, and BAT14KO mice. The results suggested that more than 500 genes were differentially regulated in the wild-type and BAT14KO mice, with many of the upregulated genes relating to myocyte differentiation and muscle function. One of the upregulated genes, Mstn, encodes myostatin, a protein that is known to hold back skeletal muscle enlargement, partly by blocking mTOR signaling. Further analyses of the engineered mice showed that levels of Mstn protein were increased in the BAT of BAT14KO animals, but not in muscle, while myostatin levels were increased in the serum of BAT14KO mice.
Interestingly, exercise capacity dropped by 15% in control mice given a single injection of myostatin. Myostatin administration was also linked with a similar gene expression profile in the thigh muscle of these animals as that observed in the BAT14KO mice. Conversely, chemically inhibiting myostatin in BAT14KO animals transiently restored their running capacity.
The authors note that while myostatin is believed to be produced primarily by muscle, the results from both the BAT14KO and BAT14OE mouse studies suggest that “BAT also contributes significantly to serum myostatin levels.” Although, as they point out, “What portion of human serum myostatin derives from BAT, and under what conditions that may rise and fall, is unknown.”
BAT14OE mice that overexpress IRF4 showed that these animals demonstrated completely opposite features to those of the BAT13KO that lacked LRF4 in BAT. BAT14OE animals exhibited 24% higher exercise capacity than control animals, higher mitochondrial respiration rates and mitochondrial DNA in the thigh muscle, and decreased serum levels of myostatin, and levels of the Mstn protein in BAT.
The team next carried out a series of experiments to put their findings into physiological context. Wild-type control mice housed at 30ºC, a temperature that significantly reduces IRF4 gene expression, demonstrated a phenotype that was “remarkably similar” to that of the BAT14KO mice, including a 14.5% reduced exercise capacity, and increased tissue and serum myostatin levels, as well as a gene expression profile in the thigh muscle that was similar to that of the IRF4 knockout mice, and reduced mTOR signalling.
“Taken together, our data provide evidence of an unsuspected lever of inter-organ cross-talk between BAT and skeletal muscle, involving the transcription factor IRF4 and the secreted protein myostatin,” the authors conclude. “It left the muscle cells unable to process energy well, but the key finding here is that by altering brown fat tissue, we altered the muscle inadvertently,” Dr. Rosen notes. The findings also support the notion held by many athletes that training in the cold can improve performance. “Intriguingly, there are data showing that cold exposure can be used as a “cross-training” modality in sparrows,” the authors point out, “such that cold-exposed birds show enhanced muscle function and exercise performance …”