On a thousand-mile road trip from Ontario to Mississippi, exercise physiologist Jay McFarlan developed a newfound respect for white-throated sparrows. Each autumn these tiny birds leave their breeding grounds, the boreal forests of northern Ontario, Canada, to migrate as far south as the Gulf of Mexico. `During our 17-hour drive, it struck me as an amazing feat that these birds travel this entire distance on their own wing power,'McFarlan says. So how do they accomplish it?
Migrating birds must rely on their `spare tyre' to fuel their flight,McFarlan explains, because fat is the most weight-efficient fuel for long journeys. But from what we know about mammals, this shouldn't be possible:during high-intensity endurance exercise, mammals burn carbohydrates rather than fat. If mammals can't burn fat during high-intensity exercise, how do birds manage it? To fuel flight, fatty acids from a bird's fat stores must pass across the muscle cell membrane to reach the mitochondria – the cell's powerhouses. Christopher Guglielmo of the University of Western Ontario, Canada, suspected that migrating birds must ramp up the production of proteins that shuttle fatty acids across the muscle cell membrane(p. 2934).
Two such transport proteins, fatty acid translocase and plasma membrane fatty acid-binding protein, are known to exist in mammals – but do they also exist in birds, and are they more abundantly expressed during migration?To find out, McFarlan and Guglielmo teamed up with Arend Bonen of the University of Guelph, Canada, one of the first researchers to characterise these transport proteins in the muscles of exercising mammals. First, the team caught wintering white-throated sparrows in Mississippi and migrating birds in Ontario during spring and autumn, and took samples of their chest muscles. Using the online chicken genome to locate avian DNA sequences homologous to the mammalian transport proteins, they were able to identify these proteins in white-throated sparrow muscle tissue. `This is the first evidence that these proteins exist in non-mammals,' says McFarlan. `The fact that these genes are so highly conserved across species speaks to their important physiological function.'
Next, the team investigated whether sparrows express more of these genes during migratory seasons. Sure enough, mRNA expression of these genes in birds caught during spring and autumn was 70% to 1000% higher than in those caught during the winter, while protein expression of one of the genes was 110%higher during migration than during the winter. So not only are these genes expressed in birds, but they are particularly highly expressed during migratory seasons, enabling faster fat transport to the muscles' mitochondria. To make use of this increased fuel influx, migrating birds also need to boost their ability to metabolise fatty acids. To test whether birds have this ability, the team measured the maximal activity of three key fat-oxidising enzymes, and found that enzyme activity also increased dramatically in the sparrows' muscles during migration. `All the steps in the chain are neatly matched,' McFarlan says.
To cope with the gruelling demands of migration, it appears that birds crank up fat transport to their muscles and bolster their ability to burn fat.`Our results suggest that simple biochemical changes can have a big impact on an animal's performance,' McFarlan concludes.