Muscles are not islands; to function harmoniously within the body, they must communicate with one another and with the brain. But precisely which molecules are involved in this communication has proven difficult to determine, as they’re often crowded out by other, more abundant biomolecules. Now, a group of scientists has figured out a faster and easier way to identify these molecules, according to a study published Friday (January 20) in Cell Metabolism. The new method could help researchers figure out how muscles talk and zero in on proteins that could be used in medicines.
“It’s remarkably simple,” says Christopher Newgard, a Duke University Medical Center molecular physiologist who wasn’t involved in the research. “Relative to other approaches to this in the past, it almost seems impossibly simple.”
Bruce Spiegelman, a cell biologist at Harvard’s Dana-Farber Cancer Institute and senior author of the paper, has been working with colleagues for more than a decade to understand which hormones muscles and fat release during exercise, and how those hormones interact with the rest of the body. But when he used mass spectrometry to examine entire muscle and fat tissues, other, more dominant proteins such as albumin (which is an abundant component in blood) crowded out the hormones he was looking for. Spiegelman knew, though, that, these hormones build up in the extracellular fluid surrounding tissues once they were secreted, and he figured that he would be able to examine the hormones more closely and potentially identify new ones by isolating this extracellular fluid.
Spiegelman had seen colleagues at Harvard use centrifuges at very slow speeds to separate out small metabolites, so he decided to try a similar method to isolate molecules from extracellular fluid. Adding some muscle tissue to a centrifuge, he began experimenting. Getting the right centrifuge speed turned out to be more difficult than expected: The centrifuge needed to spin fast enough to separate out the fluid but not so fast as to rip apart the muscle tissue. Eventually, Spiegelman and the rest of the team nailed down a centrifugal force of around 600 times the force of gravity. At this speed, they were able to separate a pink-yellowish liquid from the muscle. After running the fluid through the centrifuge again to remove as much of the remaining albumin and immunoglobulins as they could, the researchers put the resulting liquid through gel electrophoresis and then examined it using mass spectrometry.
“The computer analysis of the mass spec told us that it didn’t look like muscle and it didn’t look like blood,” Spiegelman says. “It was something different, which is what we were hoping for.”
With putative extracellular fluid in hand, the team then set to the task of finding novel biomolecules. After some searching, they discovered a muscle and fat-secreted hormone they named prosaposin. They found the newly identified protein assists in thermogenesis in mice, which could make it useful in burning fat and therefore in treatments for obesity. “The fact that we found a bona fide neurotrophic factor coming out of the muscle, I think, is pretty exciting,” Spiegelman says. “And we couldn’t have done it any other way.”
Going forward, Spiegelman hopes to use this method to continue investigating the protein makeup of muscle and fat tissues, furthering research into the intercellular communication problems that arise with neurodegenerative diseases and cancers. However, Newgard cautions that there is still some work left to be done. As mentioned in the paper, the analysis of the centrifuged extracellular fluid revealed a few proteins that are normally found in plasma, which could mean that plasma is somehow leaking into the supposedly isolated fluid. Still, he is hopeful that the study could become a resource for other scientists in their future research.
“There are some questions remaining,” he says. “But I would call it a very brave first attempt.”
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