Did you ever wonder why some Olympic runners are so good at short distances, while others excel at long-distance exercises? Much of this difference is accounted for by the types of muscle fibers that predominate in the individual: fast fibers for sprinters, and slow fibers for endurance athletes. How the ratio of different muscle fiber types is predetermined in the body has always been of interest to researchers.
Recently, scientists at the University of New Mexico, including Research Assistant Professor Anton Bryantsev, and Professor and Biology Department Chair Richard Cripps, used fruit flies to discover a mechanism that converts one muscle type into another, and to alter the function of the muscle.
The research, funded through a four-year, $800,000 grant from the National Institutes of Health and titled "Extradenticle and Homothorax Control Adult Muscle Fiber Identity in Drosophila," was published this week in the prestigious publication Developmental Cell.
Drosophila melanogaster, or fruit flies, have a long history of genetic research that has helped unravel processes of human development, and has helped to pinpoint various genetic problems. Bryantsev and Cripps were able to manipulate muscle fiber identity by using genes named Extradenticle and Homothorax to convert one muscle type into another. Their findings suggest that an evolutionarily conserved genetic pathway, also present in humans, determines muscle fiber differentiation.
"Our research shows that two different muscle types in fruit flies can be inter-converted, by changing the expression of two genes," said Cripps. "We show these two genes are at the top of a hierarchy of genes that control muscle fiber fate."
The scientists studied formation of two dramatically different muscles in the fly body, flight and jump muscles. The two-year project first involved identifying and cloning DNA elements that would report the proper development of each type of muscle.
"There are two molecular reporters we used," said Bryantsev. "One reporter tells if the flight muscle forms, and the other if the jump muscle forms."
The constructed reporters were then analyzed under conditions when various master control genes were experimentally turned off, or knocked down. Following the analysis of over 70 gene knockdowns, Bryantsev et al. found that selective knockdown of Extradenticle or Homothorax caused the flight muscle reporter to be active at much lower levels, while the jump muscle reporter dramatically increased its activity.
"This suggested to us that the flight muscles were turning into jump muscles." Bryantsev said. "When we looked inside the mutant flies, and saw that the morphology of the flight muscles was instead comprised of more jump-like muscles, we couldn't believe it. We did the tests again and again and still got the same result. We then did a crazy experiment, where we forced these two genes to be expressed in jump muscles, and it turned the jump muscle into a flight muscle. That was when we knew we had uncovered an important and basic mechanism for controlling muscle fiber identity."
"Muscle fibers are specialized to perform different tasks in the body, and this research has identified key genes that control this differentiation process during fly development," said Susan Haynes, Ph.D., of the National Institutes of Health's National Institute of General Medical Sciences, which partially funded the work. "Because these genes are also found in mammals, these findings may provide important insights into human muscle performance and the underlying causes of certain muscle diseases."
Several UNM researchers, including graduate student TyAnna Lovato and research technician Sandy Duong, played important roles in the discovery. The work also featured a collaboration with Juli Uhl and Brian Gebelein from Cincinnati Children's Hospital.
"I helped with developing the RNAi screen, generated many of the transgenic lines that were used, and carried out the in situ hybridization experiments." said Lovato.
Duong performed the extensive screening experiments that led to the discovery of the two genes' roles.
"I did a large portion of the RNAi screen as well as the sectioning and staining of the samples," said Duong. "I helped TyAnna with the in situs and Tonya (Brunetti, another graduate student) on the biochemical assays, by providing the sections."
The research also features contributions from Biology undergraduate majors, who spent long hours between classes in the research laboratory. Co-author and undergraduate student Cloyce Nelson said the research was one of the most exciting experiences she had as an undergraduate.
"Through this experience I was able to fully immerse myself in my major, as well as learn many aspects about being a biologist that are not taught in the classroom," said Nelson. "I was able to work on the beginning stages of this project, and it was an honor to assist many talented people to bring it to completion. Seeing our research published has demonstrated how my work can have a broader impact on scientific advancements. This has inspired me, and given me confidence that my academic efforts can lead to real world results."
While we won't be seeing fruit flies at the Olympics any time soon, the results of the research could play a key role in ongoing studies involving mammals. The same genes used in the Drosophila research are also found in humans.
Media contact: Steve Carr (505) 277-1821; email: firstname.lastname@example.org