If you’ve ever had a high temperature, you’ll know how ill you felt, but shore dwelling snails often suffer temperatures that make a raging human fever seem like a mild cold. Yet they survive. Their well-developed cellular safety systems protect them even when the temperature goes critical. There are five protein components in the heat shock system: a transcription factor called hsf1, and four heat shock proteins (hsp): hsp40, hsp70 (comprising two isoforms, hsp72 and hsp74) and hsp90. Hsf1 activates transcription of the hsps. During heat shock, hsps have two roles: they protect the cell by stabilising threatened or denatured proteins, and they form a macromolecular complex with hsf1 to prevent it from activating an unprovoked heat shock response.
Over the last few years George Somero and Lars Tomanek have worked to understand how this molecular safety system protects threatened organisms and whether it could help to identify snails that find temperatures on their Monterey Bay beach stressful. In This Issue of the J. Exp. Biol. they explain how two key components of the heat shock machinery could be the best warning signal for animals that can’t stand the heat anymore.
Somero’s lab at Hopkins Marine Station sits above a beautiful Californian beach, which is home to three members of the Tugula family of snails. T. funebralis lives above the low tide mark, and T. montereyi and T. brunnea both live beneath the low tide line, and are rarely exposed to the atmosphere.
T. funebralis experiences the greatest diurnal range of temperatures of the three species, and survives. But in 1999 Tomanek discovered that T.funebralis’ couldn’t adapt to warmer temperatures, no matter how hard it tried. The snail’s thermostat had ‘maxed out’. Meanwhile T.montereyi and T.brunnea easily adapted to warmer waters by raising the temperature that activated their heat shock response (called Ton). T.funebralis’ inability to respond to increasing temperatures means that the snail would have to find cooler waters or face death if the beach’s temperature rose. But it wasn’t clear how the other two species reset the thermostat, and how T.funebralis’ thermometer had reached its limit.
Tomanek and Somero looked at the molecular switch components to see how they varied between the three species in their different thermal worlds. Hsp70 levels had previously been believed to give the best indication of a thermally stressed animal, so they checked the snails’ hsp70 levels. At 13°C and 18°C, all three snails had the same total levels of hsp70. However, when the snails were acclimated to 23°C, T.montereyi and T.brunnea sent their hsp70 levels rocketing, while T.funebralis’ hsp70 levels didn’t change at all as it acclimated to a higher temperature.
Although the total levels of hsp70 didn’t differ between the three species, the ratio of the two isomers (hsp72/74) did; stressed T.funebralis always showed the biggest variation. Tomanek thinks that this ratio is probably the best indicator of how ecologically stressed a species is.
Tomanek was also surprised when he realised that T.funbralis also had unusually high levels of hsf1. No one had ever expected hsf1 levels to vary between closely related animals. It was also believed that high hsf1 levels would result in a reduced Ton, but T.funebralis’ had stayed high, which probably means that controlling the heat shock response is even more complex than had been thought before.
The planet could well experience its own ‘heat shock’ in the not too distant future. Identifying species that are already living at their safety limits, before they vanish, is probably the only hope for species like T. funebralis.