Humans may be able to live in a variety of climates, but we've discovered all sorts of creatures that can survive at temperatures that would kill us in short order. Genetic changes have allowed animals to adapt to temperatures that range from blazingly hot to right around freezing. In today's issue of Science, researchers describe how species of octopus that live in the frigid waters of the poles manage to keep their nerve cells working despite the chill. Instead of genetic changes, however, this adaptation relies on a process that edits the genetic information before it's made into a protein, a form of genetic editing that may be driven by the temperature difference itself.

It's not easy to survive at temperatures that hover at or below freezing, which will slow down many of the metabolic reactions that keep cells alive. But for multicellular organisms, the challenges are a bit more extensive, as they have to keep nerve cells firing at a reasonable clip. These nerve cells depend on a set of proteins, called voltage-gated channels, that we know change their behavior at low temperatures.

Nerves signals propagate as electrical pulses, caused by the shuffling of ions across the surface of the nerve cell. Voltage-gated ion channels are central to that process. These proteins sense the change in voltage caused by the nerve impulse reaching their neighborhood, and immediately open up, allowing ions to rush across the membrane. This causes a voltage spike that, in turn, causes the voltage-gated channels further down the nerve to open up. This process keeps the impulse moving down the nerve's body.

Temperature changes can completely change this dynamic, causing the channels to open more slowly or stay open longer. If these changes are large enough, it could completely shut down signaling across nerve cells. So, the authors predicted that there would be genetic changes in voltage-gated channels in species that are adapted to different temperatures.

To test this, they turned to the local waters of their home institute in Puerto Rico, where they collected an octopus that normally lives in 25°C waters. For a comparison, they obtained one from McMurdo Station in Antarctica, where the seawater is at equilibrium with the ice above it, about two degrees below freezing.

When they sequenced the genes for a voltage-gated potassium channel, however, they found that the two species made proteins that were nearly identical. And, when they expressed them in the lab, the proteins behaved almost identically, as well. Based on this behavior, the Antarctic octopus should make a protein that opens fourteen times more slowly and closes sixty times more slowly at its native temperature. This would do Bad Things to nerve cells.

How does the octopus avoid this? Despite what you probably learned in high school biology, not all genes are translated directly into proteins. Some genes are made into RNA that's then chemically edited, which changes some of the lettering, causing an A to be read as a G, for example. These edited RNAs are then translated into proteins that differ from the ones you'd predict based on the DNA sequence. When the authors checked, both the tropical and Antarctic octopus' RNA showed signs of having been edited. The edited version of the RNA encoded a protein that behaved very differently at low temperatures.

When they checked in other species from the Arctic and elsewhere, they found that the editing tended to correlate with the temperature at which the species lived. Thus, the editing seems to help the animals adapt to the cold.

So, why do animals edit their RNA differently in the cold? My initial response would be that the different editing would require some sort of genetic change, perhaps in the enzymes that perform the editing. But the authors point out that this might not be needed. Editing, it turns out, depends on the structure of the RNA being edited—how it folds back on itself and forms a three-dimensional object. Temperature itself will influence how an RNA folds, and can thus change the structure of the RNA in question. So, the cold itself may simply trigger a difference in the editing of the RNA in question.

Put more simply, an octopus may be able to adjust its nerves to the temperature without undergoing any genetic changes whatsoever.

Science, 2012. DOI: 10.1126/science.1212795 (About DOIs).