Mythical Myokines

Irisin, Bigfoot and the Self-Correcting Nature of Science
March 29, 2015 
 
Let’s start with some headlines. 
 
“Irisin: The ‘Exercise Hormone’ has Powerful Health Benefits,” appeared in a Psychology Today blog about a year ago. Men’s Fitness Magazine published a short piece called “Miracle Weight Loss Pill Irisin Allows for Easy Workouts.” Even The Wall Street Journal got in on the action, topping their January 19th research report with “Exercise Hormone May Offer Breast Cancer Protection.”
 
I could go on but you get the point. Since its discovery in 2012, the “exercise hormone” irisin has reached the status of a natural wonder as more than 170 studies have investigated its role in curbing obesity, diabetes and even cancer.
 
Trouble is, according to a new study, scientists have not proven irisin does any of those things. The “exercise hormone” may not even exist.
 
Harold P. Erickson is cell biology and biochemistry professor at the Duke School of Medicine and a coauthor of the study. He and the other scientists assert that the methods used to measure the levels of irisin in the body are faulty to the point that we cannot be sure that irisin even exists in our blood.
 
Irisin, as originally reported in 2012, is what physiologists call a myokine. Myokines are amino-acid based – often called peptide – signalers built by the muscles to cause chemical changes in other parts of the body.Irisin Peptide
 
Irisin in particular, begins as the tail of a muscle protein called FNDC5. When you exercise – or get cold – FNDC5 docks its tail and irisin goes off in search of fat tissue. Once there, irisin converts normal beer-gut white fat to something more like the brown fat that babies and hibernating bears use to keep warm. Brown fat takes calories from white fat and burns them, which in theory made irisin a promising method of combating obesity.
 
The Harvard scientists who discovered irisin billed the myokine as a molecule that “could be beneficial for human metabolic disease and other disorders that are improved with exercise,” citing diabetes specifically.
 
They published their study in the prestigious journal Nature and it took off. The study has been cited more than 100 times in the past three years as justifiably excited scientists probed irisin for further benefits and, as the headlines above indicate, found them.
 
The issue Erickson and his colleagues found with many of these studies was how they detected irisin in the blood. Detecting blood peptides involves staining them with antibodies. The antibodies attack peptides with a specific shape or substituent and dye them a certain color. Based on how bright the color is in their sample, scientists can determine whether their target peptide is present and how much there is.
 
Many of the irisin studies used a collection of antibodies called ELISA to measure how much irisin was present in their samples. ELISA is often used as a single pot method, meaning that scientists will add ELISA to their sample and it will sift through all the peptides present and light up when they find the shape or component they are looking for.
 
Single-pot methods are great because they work without the scientists having to separate out the peptides. The issue is there are only so many shapes a peptide can take and only so many things they can be made of. Theoretically, a completely different peptide with a similar shape to irisin could set off the antibodies and give a false positive. Scientists call this cross-reaction.
 
Western Blot TestErickson thought ELISA might be cross-reacting with peptides other than irisin so he teamed up with scientists from Germany, Norway and Switzerland to test for irisin with a different method, called Western blot. The Western blot is the old school method that involves first separating the peptides by size before subjecting them to antibody tests. It takes longer, but minimizes cross-reaction.
 
According to earlier irisin studies, irisin is a small peptide that should be found in most mammal species. When Erickson and his colleagues took blood samples from humans, mice and horses and separated their peptides based on size, the ELISA antibodies did turn up a single peptide in irisin’s size. However, the ELISA antibodies lit up peptides in other size ranges too.
 
As a final kicker, Erickson synthesized irisin in the lab and added it into the blood samples, both in its plain form and wrapped in sugar, as it would be found in the body. He separated the peptides out by size again and this time, ELISA lit up in the size range where irisin should be. 
 
To summarize, Erickson showed that ELISA lights up in the presence of irisin, but in actual blood samples, it did not light up for anything in irisin’s size. Further, ELISA lit up for peptides too big to be irisin, suggesting that many irisin blood tests have given false positives. This calls into question many studies of irisin and its effects, suggesting that despite all the buzz around the myokine scientists might have gotten it wrong.
 
Irisin was a captivating find, and captivating finds with the right publicity can deeply embed themselves in the public consciousness in a heartbeat. You can probably rattle a dozen examples of this phenomenon without having to think about it, but I personally find one to be particularly apropos. 
 
Think big. Think Bigfoot.
 
Let me first clarify that I don’t level this comparison as a criticism of the scientists who discovered and studied irisin. On the contrary — they had something that all their data suggested could fundamentally improve human health and they did exactly what scientists are supposed to do in opening their work to review and further investigating their findings. Rather, this comparison is meant to illustrate how publicity and discovery interplay to help science evolve.
 
Bigfoot legends exist in cultures around the world in many different forms: Yetis, Sasquatch, the Skunk-Ape, etc. In the 1950s, however, a few photographs of Yeti footprints made the rounds of local news outlets in the Pacific BigfootNorthwest, picking up the name “Bigfoot.” The Associated Press finally picked up the story in 1958 and reported Bigfoot sightings rose from one or two per year to four or five. In 1967, the famous “Patterson-Gimiln Film” — showing Bigfoot walking across a streambed — came out and the yearly number of sightings jumped into the five to 10 range and slowly rose until about 2005
 
The idea of big hairy ape-man wandering the woods is fascinating and there are several groups that actively research Bigfoot to this day, including one prominently featured on Animal Planet’s Finding Bigfoot. But as more analysis of Bigfoot evidence comes out and more hoaxes are discovered, fewer and fewer people actually believe Bigfoot is out there doing whatever it is that Bigfeet do.
 
Such is the self-correcting nature of science. Scientists postulate their theories and present their evidence for the world to see and other scientists do their absolute best to tear those theories asunder. When evidence against a theory comes out, scientists in support of that theory can fire back with evidence of their own. Through this data-driven back and forth, theories are sometimes trashed, but more often refined and perfected.
 
This is the beauty of science. Hoaxes like Fleischmann-Pons cold fusion can be debunked. Plainly incorrect theories like the world being flat can be disproven. But seemingly correct theories by history's most famous scientists are regularly modified and improved upon. Newton’s theory of gravity was a definitive answer to the workings of the universe for hundreds of years before Einstein used general relativity to show Newton did not have it all figured out. Fourteen years later, Edwin Hubble and others showed that Einstein's picture of a static universe was flawed as well — as the universe expands. That, however, does not render the earlier theories useless. We still use Newtonian gravity for all sorts of small-scale problems, and the Hubble's addition to relativity only knocked one term out of Einstein's equations. The additions and corrections advance our understanding beyond the original theories but we needed the original theories to get to that point. 
 
Erickson and his colleagues’ work represents a sizable blow to irisin research, but it will most likely not be the last word. Rather, the science surrounding it has officially begun to correct itself. Scientists will re-examine their work and develop new explanations for what they found, and the debate will continue until the correct answer is found.  Erickson’s study appeared in the journal Scientific Reports.

— Daniel Lane

Daniel Lane covers science, engineering, medicine and the environment in North Carolina.


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