As climate change threatens various species, researchers are particularly interested in how animals can deal with heat, particularly coldblooded (ectothermic) critters whose temperatures fluctuate with the environment. One of the many damaging effects of toasty temperatures starts in the cell. As conditions get hotter, proteins and other molecules in the cell can unfurl, become sticky and form hazardous protein bundles that can damage the rest of the cell. Thankfully, animals have a built-in protective mechanism that can overcome these perils. As damage occurs, animals can activate autophagy – where cell components consume these menacing protein clumps, broken small structures and old proteins to clean up – along with other protective mechanisms. To figure out whether autophagy can also help insects handle the heat, Quentin Willot and colleagues from Stellenbosch University in South Africa were interested in seeing whether an increase in autophagy led to an increase in the fruit flies’ heat tolerance.
The team's first step was chemically turning on the cell's self-cleaning system in the flies. To do this, Willot and colleagues fed the fruit flies rapamycin, a chemical that triggers autophagy. They also knew that rapamycin limits growth, so they could tell whether they had successfully activated autophagy because the flies whose self-cleaning process had been triggered would grow more slowly. Additionally, they predicted that there would be an increase in the number of lysosomes – structures within the cell that break down damaged or old cellular components. Sure enough, the researchers found that flies fed rapamycin took 4–5 days longer to develop than flies that were not fed the drug. They also found that flies fed rapamycin had more lysosomes containing damaged cell parts in midgut cells. The researchers had successfully chemically activated the cell's autophagic self-cleaning process.
To determine the relationship between the cell's self-cleaning system and heat stress, Willot and the team then measured the ability of flies that had been fed rapamycin for 2 days to withstand heat in two different scenarios. First, they put the flies into glass vials and immersed these vials in a hot water bath at 37°C. Every 15 minutes, the team gently shook the vials, looked for flies that had fallen and could not get up or move, and recorded the time it took them to lose this balance. In a second set of experiments, the team measured how long the rapamycin-fed flies took to recover after they lost balance as a result of the stressful effects of the heat. In this case, the team immersed the flies in a hot water bath at 41°C until they fell and could not get up after a gentle shake. Then, they were immediately put at room temperature and the team monitored how long it took the flies to stand on their legs without toppling or falling over after gentle shaking.
Impressively, the team found that the fruit flies that had been fed rapamycin took longer to fall and lose control as a result of heat stress compared with those not fed rapamycin. This suggests that activating autophagy protects the flies, allowing them to handle higher temperatures without losing coordination. Moreover, the researchers found that when the flies were fed rapamycin, they recovered from the ill effects of heat faster, showing that the cells’ self-cleaning defended them from the heat. This is extremely important as it highlights autophagy as a vital process for surviving future environmental stresses, as insects have little control over their internal temperatures.
