The group was trying to understand what gives a fit muscle its defining characteristics of burning fuel efficiently and looking ripped. OK, not the ripped part. They were actually looking at the types of muscle fibers in the mice. Slow twitch fibers are more prominent in endurance-trained muscle and fast twitch fibers give sprinters an edge.
They worked with two groups of mice, one called the couch potato mouse due to their general lethargy in the lab. The other was called the marathon mouse and here we have another CIRM connection because it was scientists down the road at Salk that created those mice. (Salk is also home to some outstanding CIRM-funded scientists.) Marathon mice happily run almost twice as far on a wheel compared with their labmates and naturally have a lot of slow twitch muscle fibers and burn fuel very efficiently.
I have spent a significant number of hours in the pursuit of becoming a marathon mouse, so am always interested in research involving them.
The scientists examined muscles from the two groups of mice and found that the marathon mice have more of tiny molecule called a microRNA that helps muscles switch from a fast twitch to slow twitch state. Essentially, the marathon mice naturally made the switch to have more endurance muscle fibers, for which I am rather jealous. Most people train long and hard to get their muscles to switch over.
Curious if their work applied only to mice, the group got muscle samples from couch potato and marathon humans. What they found is that people showed the same differences as the mice. Except that where the marathon mice were just born sleek and fit, the humans worked to get there. Sanford-Burnham quoted Daniel P. Kelly, M.D., who was senior author of the study:
“We’re now conducting additional human studies to further investigate the ERRγ-microRNA circuit as a potential avenue for improving fitness in people with chronic illness or injury. For example, next we want to know what happens to this circuit during exercise and what effect it has on the cardiovascular system.”When the time comes to induce a marathon mouse-like muscle status on humans, I hereby offer myself to medical research. Until then, I will have to persevere the old-fashioned way.
(Because this is a stem cell blog, here's a link to a few blog entries we've written about our grantees who are investigating muscle stem cells.)