Infections cause house finch mitochondria to make less ATP Free

Getting sick can sap your energy, leaving you exhausted and wondering when you'll finally feel normal again. That's because your immune system uses a lot of energy fighting off the infection. When you're sick, your mitochondria work overtime to provide more energy to your body, in the form of ATP. Recently, scientists showed that mitochondria not only power your cells, but power your immune system as well. So, it came as a real surprise to Yufeng Zhang of the University of Memphis, USA, when studying wild house finches (Haemorhous mexicanus), that the birds that were infected with a bacteria called Mycoplasma gallisepticum (MG) had mitochondria that weren't producing as much ATP as they normally would in healthy birds. However, these were wild birds, and Zhang had no idea how long they'd been infected for, so his team of Chidambaram Ramanathan and Elina Thomas joined forces with Amberleigh Henschen and James Adelman, also of the University of Memphis, USA, to see if the mitochondria in recently sick birds were acting any differently from the mitochondria in birds that had been sick for a while.

The MG epidemic in finches started on the east coast of the United States ∼30 years ago and spread all the way to the west coast 15–20 years later, meaning that animals from the east have had a bit more time to evolve along with the bacteria. With this in mind, the scientists collected wild finches near Auburn, Alabama and Davis, California, to see if the finches from the east had an advantage in this evolutionary arms race between themselves and the infectious agent. After transporting the birds back to their lab in Tennessee, the team infected some of the finches with MG and checked to see how their mitochondria were doing after 3 days. The team didn't see a difference between birds from Alabama or California, but the mitochondria of the infected finches already produced less energy than those of healthy birds. Importantly, this decrease in energy was caused by a particular process, called complex II, in the mitochondria not working properly, which also affects their immune system. ‘Recent research suggests that mitochondria serve not only as the powerhouse of the cell, but also as the powerhouse of organism's immunity,’ explains Zhang, meaning the birds have less energy, and their immune system isn't working to fight off the infection as well as it should. Could this trend continue until the infection is gone?

Ramanathan and colleagues checked the mitochondria of the finches 34 days after they were infected and saw that, not only was complex II still producing less energy than it was in healthy birds, but now complex I was also producing less energy. This suggests that, later on during their infection, the birds were struggling even more to make enough energy to fight off their infection. So, why might bacteria want to reduce the amount of energy the mitochondria are pumping out? ‘During early infection, MG may target mitochondrial complex II to suppress the immune system. At later stages, it appears to hamper overall mitochondrial respiration, suppressing the birds’ metabolism. Both mechanisms could delay recovery and extend infectious periods,’ states Zhang. While the way the bacteria interferes with the birds’ mitochondria is unknown, it certainly has an ingenious way of ensuring that it sticks around, giving it ample time to infect more birds along the way.