NO OXYGEN? SHUT DOWN YOUR F1FO-ATPASE

Abstract

Mitochondria are our bodies' powerhouses, and to efficiently produce energy, via aerobic respiration, they use the enzymatic activities of four different protein complexes (I to IV) to pump hydrogen ions across the inner mitochondrial membrane. The H+ ions will then flow back into the mitochondria, down their concentration gradient, via a fifth complex, which drives ATP production, where the H+ ions are then accepted by oxygen to make water. ‘Without oxygen, animals have to move to anaerobic respiration, which yields a much lower level of ATP. So their balance between ATP supply and demand is greatly affected’, explains Gina Galli, from the University of Manchester, UK. The absence of oxygen (anoxia) can be disastrous, as can be seen in pathologies such as stroke or heart attacks, but for some animals, such as the red-eared slider turtle, anoxic periods are just a normal part of life. So, as part of her postdoc in Jeffrey Richards' lab at the University of British Columbia, Canada, Galli decided to find out what we can learn from anoxia tolerant turtles (p. 3283).Galli already knew some of the tricks turtles employ during periods of anoxia: ‘They have the ability to shut down all the cellular processes in their bodies that consume a large amount of ATP, but we wanted to see if that was also the case at the level of the mitochondria.’ To investigate this possibility, she divided 26 turtles into two groups. One group was kept in normal, normoxic water conditions while the other group was submerged in water devoid of oxygen for 2 weeks. Galli then isolated cardiac fibres from the turtle's hearts and placed individual fibres into tiny respirometer chambers, equipped with oxygen-sensing electrodes. Galli could then measure the fibres' aerobic respiration capacity by how much oxygen was depleted. To measure the capacity of the individual complexes I–IV, Galli used specific drugs to inhibit each complex. Overall, she found that the aerobic capacity at complexes I–IV was reduced in the cardiac fibres from anoxic turtles.Next, Galli wondered whether this reduction in aerobic capacity was caused by a reduction in the enzymatic activity of each complex. To test this, Galli supplied tissue samples with substrates that were unique to each complex, one by one, and measured how much product was produced. To her surprise, Galli found that each of the complexes from anoxic turtles was working at the same rate as complexes from normoxic turtles. Perplexed, Galli wondered whether the apparent reduction in aerobic capacity was perhaps just due to a decrease in the total number of mitochondria in cardiac cells from anoxic turtles. However, anoxic and normoxic cardiac fibres were able to produce the same amount of citrate (via the unique mitochondrial enzyme citrate synthase), suggesting that total mitochondrial levels were the same in the two conditions.So, Galli decided to test the enzymatic activity of the fifth and final complex, also known as the F1FO-ATPase, and found its enzymatic activity was indeed shut down. Suddenly, everything fell into place – shutting down this complex would reduce flow through complexes I–IV but not their enzymatic activities. In addition, shutting down this complex has clear benefits, as Galli explains: ‘When there's no oxygen, the F1FO-ATP synthase will start consuming ATP to keep that H+ gradient going. It starts to chew up all of the available ATP; it will eventually lead to ATP depletion and that's one of the biggest contributors to cellular breakdown.’ Recent studies have shown that, in mammals, drugs inhibiting this final complex do protect tissues against anoxia, but it seems that turtles have known the secret all along!