Some thoughts on the road to hibernation in space

Have you ever dreamed of visiting a planet circling another star? The trip might take so long that you’d need to undergo some form of hibernation. Someday, humans might travel to planets around other stars. But such a trip might take longer than a human lifetime.

One solution might be to put astronauts into cold hibernation — so that they age more slowly. But there’s a catch. When we get very cold, our immune systems shut down and we can’t fight off bacterial infections.

Jim Christiansen is a biologist at Drake University in Des Moines, Iowa. He says turtles might provide clues about how to solve this problem. Both humans and turtles have special cells that help clean bacteria from the blood. Christiansen chilled these cells in his lab to near freezing and then added bacteria. The turtle cells were able to consume the bacteria — but the human cells couldn’t.

Jim Christiansen: So here seems to be a mechanism whereby reptiles have solved a problem that we warm blooded mammals have lost the ability to solve . . . And it’s quite possible that the human genome even contains the genes for particular proteins that might make this possible. So then the question becomes, hypothetically, if we want to awaken this ability in our selves, we only have to find those genes and figure out how to activate them.

Christiansen is now looking for those genes.

Excerpts from interview with Dr. Christiansen:

ES: So you study aging and hibernation?

Well hibernation has some links to aging, in that perhaps when an animal is hibernating — it’s rate of cell division is of course slowed down — and it may not advance in age as rapidly as another animal that was not hibernating. That of course was a hypothesis that people have played around with for a long time for space travel. Because if we really want to get anywhere worthwhile, it’s more than a human lifetime to get there. But maybe we could somehow slow the process of aging by going into something close to hibernation. And if we were able to do that for long enough, one person might live the equivalent of many normal life spans and still be functional at the end. Kind of on the edge of science fiction there.

We’re looking specifically at one problem — and that is, how do they control infection when their body is cooled to near freezing temperatures, through a five month period of hibernation. The problem with mammals is that if mammals allow their body temperature to drop a large amount, they will usually wake up, warm up for a period and then go back and cool off. And that may be a stimulus to get our immune system functioning. For the most part, our immune system turns off when we get very cold. Of course, bacterial production slows down a great deal when they get cold, but they still do reproduce. So the worry is that if we were to cool a human off to near freezing temperatures, that bacteria would slowly take over the body and cause irreversible damage.

So we’re looking at how reptiles might be controlling bacterial infection while their body temperatures are very low. And we’ve stumbled on a cell that may be a key in that battle.

ES: Please tell me more about these special cells.

Well the cell has been known for a long time. I think it was first described in fish. It’s a black macrophage [MAK ro faj] — a macrophage being one of the white blood cells that crawls around the body and cleans up dead cells and bacteria and other things that accumulate in the body. So it’s a very important part of the immune system. It even picks up foreign substances such as bacteria and presents them to other cells in the immune system to alert them to organize an attack against this invading organism. So they do a lot of interesting things. In fish and amphibians and reptiles, there’s a macrophage that we’ve found throughout the bodies of turtles that is black and it’s black because it produces melanin. It may also consume melanin, but it clearly produces melanin. We don’t know for sure why it does that, although we have some ideas. But one aspect of this cell is that it concentrates in the liver and spleen so that all the blood for example that comes from the intestines in the animals has to flow by a gauntlet of these cells before it can go on to the rest of the body. And they then have an opportunity to take bacteria and other things in the blood before it can pass on to the rest of the body. So it’s a very good place to set up a station to filter bacteria out of the bloodstream.

And so we looked at these cells running as controls mammalian macrophages, laid them side by side in experimental cultures and cultured them at different temperatures and fed to them bacteria that were fluorescence labeled so we could see the bacteria with a fluorescent microscope, whether they were inside the cell or outside the cell. What we learned form these experiments is that melanomacrophages from the turtles that we’re studying are able to phagosize bacteria — phagositize bacteria — that means to consume them — at temperatures within about two degrees of freezing. And no mammalian cell that we’ve run with these can come anywhere near close to that.

So here seems to be a mechanism whereby reptiles have solved a problem that we warm blooded mammals have lost the ability to solve. Now if we came from reptiles, then it suggests that we had that ability once. And it’s quite possible that the human genome even contains the genes for particular proteins that might make this possible. So then the question becomes, hypothetically, if we want to awaken this ability in our selves, we only have to find those genes and figure out how to activate them.

ES: So what’s next?

So our steps now involve such things as learning what it is in these cells that enables them to function at low temperatures and one thing that we’ve been looking at there is cold shock versions of heat shock proteins. And we have found some heat shock proteins that are present in mammalian cells. We’re looking now for cold shock versions of these that might aid in keeping proteins from changing their nature and becoming nonfunctional when the temperature drops. And this is a difficult search and it’s one of the things we’re involved with, along with many other cellular process that we’re examining to learn exactly what these cells are capable of doing and how temperature affects what they’re capable of doing.

What you have in your mind?