Biologists at the University of California, San Diego have transformed ordinary laboratory mice into the rodent equivalent of Olympic endurance athletes by deleting a gene that allows mammalian muscles to switch from aerobic to anaerobic metabolism when oxygen levels in the muscle run low.
In a paper that will appear in the August 24 issue of the online journal PLoS Biology, the scientists say the inability of these genetically modified mice to generate energy through anaerobic metabolism, the biochemical process used for short sprints or bursts of power, provides them instead with an extraordinary capacity for longer, sustained aerobic endurance exercise.
But while these endurance-enhanced mice can run and swim to exhaustion in laboratory tests for far longer periods than their normal counterparts, the scientists discovered that their super-endurance capabilities appear to be only temporary and come at a high price. After four days of exercise tests, the gene-doped endurance mice exhibited significantly more muscle damage and were unable to run or swim as long as their normal counterparts.
"It's a double-edged sword," says Randall S. Johnson, a professor of biology at UCSD, who headed the study. "By changing the way skeletal muscles respond to low-oxygen levels, we've developed muscles that appear to be superiorly adapted or trained for long bouts of submaximal aerobic exercise. But these muscles also become damaged more easily than normal muscles during exercise and we don't know why."
The discovery not only has obvious importance for physiologists and others who study muscle metabolism to maximize human endurance. It should be of keen interest to medical researchers seeking treatments for human genetic disorders, such as McArdle's disease, whose sufferers tire quickly during exercise because they have an impaired ability to generate energy through anaerobic metabolism and experience soreness and pain after exercising because of the resulting muscle damage.
Most of our daily activities are performed aerobically, through biochemical mechanisms in our muscles that make full use of oxygen. But when the demands of our muscular system exceed its available supply of oxygen, as in sprinting for a bus or lifting a heavy object, a protein known as hypoxia inducible transcription factor-1, or HIF-1, is activated. This protein enables the muscle to switch to the more energetically explosive, but expensive anaerobic process, which does not use oxygen and generates lactic acid as its byproduct.
Two years ago, Johnson and his students discovered that "knocking out" the HIF-1 gene in the white blood cells of mice limited the tendency of the cells to rush en masse to areas of infection, a primary cause of inflammation. By limiting the cells' ability to guide themselves with the HIF-1 protein to the sites of infection by honing in on the low oxygen levels associated with infections, Johnson and his colleagues found a new method of reducing inflammation. They also noticed that these mice had pronounced endurance capabilities on a treadmill.
So the UCSD researchers, led by Johnson and Steven Mason, a graduate student in Johnson's laboratory and the first author of the study, set out to examine just how much better these super mice were, endurance-wise, when compared to normal mice. Using four-month-old mice that lacked the HIF-1 gene in their skeletal muscle, the scientists subjected groups of super mice and normal mice, with the HIF-1 gene, to a swimming endurance test and two treadmill running tests.
In the swimming test, the super mice swam on average 45 minutes longer than normal mice, which typically swam for 150 minutes before they were exhausted. In the two running tests, the scientists had the two groups of mice run on treadmills that were either tilted upwards by five degrees or downwards by 10 degrees. The treadmills were started at a speed of 10 meters per minute and increased in velocity every five minutes until the mice could no longer run.
The genetically modified super mice ran an average of 10 minutes longer on the uphill test than the normal mice, which averaged about 50 minutes. But those additional 10 minutes also included two velocity increases, demonstrating that the super mice had superior running as well as swimming endurance over normal mice, the UCSD scientists note in their paper.
When running downhill, however, the normal mice were able to sustain their pace for a longer period than the super mice without the HIF-1 gene. The researchers say this was to be expected because the eccentric contractions required by the leg muscles during downhill running depend to a greater degree on anaerobic, rather than aerobic metabolism.
The scientists confirmed that the superior endurance of the genetically modified mice resulted from their inability to generate energy anaerobically, because these super mice had very little lactic acid in their blood. The elevated activity of mitochondrial enzymes in the muscles of these mice also demonstrated that their ability to run and swim for long periods came from their greater dependence on aerobic metabolism, which produces high levels of such enzymes.
"Our studies demonstrate that exercise endurance in mice may be a model for genetic factors in exercise and endurance in humans," says Johnson. "The regulation of response to oxygen in muscle is clearly critical in regulating the sensation of exhaustion and is important for avoiding muscle damage during extended exercise."
The researchers note in their paper that high levels of mitochondrial enzymes and low amounts of lactate are typically observed in humans with phosphofructokinase deficiency and McArdle's disease. These individuals, who have an impaired ability to generate energy through anaerobic metabolism and suffer a high degree of muscle damage following exercise, also have a pronounced "second wind," the researchers state in their paper, "which allows them to exercise for extended periods of time at submaximal levels."
As a result, they add, mice lacking the HIF-1 gene could prove to be a useful tool to investigate ways of minimizing muscle damage and developing other treatments for such individuals as well as "an important model system to investigate the physiology of muscle response during work and oxygen depletion."
In addition to Johnson and Mason, other UCSD scientists who contributed to the study included Richard Howlett, Matthew Kim, Mark Olfert, Michael Hogan, Wayne McNulty and Peter Wagner. Reed Hickey and Fran Giordano from the Yale University medical school and C. Ronald Kahn from the Harvard University medical school were also coauthors of the study, which was supported by grants from the National Institutes of Health.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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