Oxygen is the key to life for every species. But some creatures have evolved to get by with virtually none for short periods, thanks to a suite of specially developed physiological mechanisms known as the hypoxia response. One of the main regulators of the hypoxia response is a transcription factor protein known as hypoxia-inducible factor (HIF), which controls survival mechanisms, such as the upregulation of metabolic enzymes, when oxygen is scarce. When oxygen levels are high, one of the HIF subunits, HIF1a, is degraded, disrupting the transcription factor and inactivating the protective responses. But when oxygen levels fall, HIF1a degradation stops and the active transcription factor bursts into action. When Mikko Nikinmaa identified the first HIF in a cold-blooded fish, it occurred to him that the hypoxia response must be fine tuned for different environmental conditions; the physiological challenges of an hypoxic encounter might be very different at low and high temperatures. Could temperature regulate the hypoxia response? Working with Eeva Rissanen and Hanna Tranberg, Nikinmaa began testing whether temperature regulates HIF in a champion of hypoxia tolerance, the crucian carp(p. 994).
First the team had to catch some fish. Tranberg headed out to a local pond,well known for its crucian carp population, trapping several hundred fish ready to analyse the fish's response. Back in the lab, Rissanen and Tranberg acclimated the fish to 26, 18 and 8°C for several weeks, before she and Tranberg exposed them to hypoxia and measured HIF mRNA levels, protein levels and the protein's DNA binding activity.
First the team analysed the effects of temperature alone on the transcription factor, and the results were intriguing. As the temperature fell from 26°C, the normoxic fish's mRNA levels didn't vary, but their HIF protein levels rose significantly. And when the team tested the protein's activity level it had increased by as much as threefold when the temperature dropped from 26°C to 8°C. The transcription factor's behaviour changed as the fish got colder.
Turning to the effect of temperature on the fish's hypoxia response, the team were surprised to see that the mRNA levels did rise a little, even at 26°C, even though it is known that HIF mRNA levels remain unchanged in hypoxic mammalian cells. And when the team measured the fish's HIF protein levels at all temperatures, they saw them rise too. However, the protein's DNA binding activity increased only in the cold fish. The transcription factor's response to hypoxia had been modified by the cold.
Rissanen explains that when the temperature falls, chilly enzymes become more sluggish. But if the fish can synthesize higher levels of metabolic enzymes by activating HIF to compensate for the activity loss, they can maintain higher levels of metabolism than expected in the cool conditions. The team suspect that HIF could play a major role in temperature acclimation in cold-blooded creatures.