Before thinking seriously about that question, though, a little family history is useful. It’s helpful to know, first, that lemurs are primates, and so are we. But to say we can learn something about hibernation from a lemur is a little like saying I could pick up some running tips from Usain Bolt. Sure, we’re both human, but that’s pretty much where the similarity ends.
Just as humans and lemurs are both primates, you’d be hard-pressed to arrive at that conclusion just by looking at us. For instance, fat-tailed dwarf lemurs are about the size of a plush squeaky toy you’d toss to your dog. They are technically prosimians, a branch of the primate family that comes from the other side of the tracks. It’s obvious that somewhere during the growth of this particular family tree, they just splintered off. So there’s a whole lot of DNA distance between a lemur and a person.
And yet, no matter how much they may look like squirrels, they are much closer to us, genetically, than squirrels are. Closer than any other known hibernator is. And that’s important, because if we want to understand how hibernation works in a way that might someday help people, it pays to study hibernation in an animal that’s as close to us as possible. And that is what Klopfer’s lab is all about.
As Klopfer is showing me around his laboratory, he’s about to describe the procedures they use. With the squirrel experiment history fresh in my mind, I’m fearing the worst. But since lemurs are some of the most endangered animals in the world, Klopfer’s experiments are much more benign.
In all of the experiments that he and his colleagues do, there’s nothing that’s more invasive than what you or I would endure on a trip to an allergist’s clinic. The lemurs are poked and prodded, and very small electrodes are placed under their skin. But no one is cutting these guys open and taking blood from an aorta.
So what are they doing? I ask him about DADLE, and Klopfer rolls his eyes and then shakes his head.
“Hibernation,” he says slowly, “isn’t a conserved trait.” He pauses. “It’s convergent.” What he means, he explains, is that hibernation is something that species have evolved to do in very different ways. Hibernation in a thirteen-line ground squirrel might look like hibernation in a groundhog, a bear, or a Common Poorwill. In fact, to a researcher, the physiology—low temperature, slow metabolism, low blood pressure—looks identical.
However, the mechanisms that make it possible for each animal to hibernate are likely to be very different because these animals have very different genes and physiology. A ground squirrel might arrive at a state of hibernation with a dose of DADLE. Other animals might use other hormones or neuropeptides that we haven’t yet discovered. That is, they’ve all figured out how to hibernate in different ways. Same destination, different paths.
Klopfer makes this case based on the fact that the trait of hibernation is oddly scattered throughout the animal kingdom. There’s one bird (the Common Poorwill), and bears and groundhogs and squirrels. And now lemurs. They all hibernate, he says, but they’re equipped with very different anatomies and physiologies. It seems highly unlikely that they all hibernate in the same way.
So I ask him about the animals that scientists study in the laboratory. Squirrels are at the top of the list, but there are studies that have tried to induce hibernation-like changes in mice, sheep, and pigs. But Klopfer is, not surprisingly, a strong advocate for Team Lemur.
Klopfer admits that there is a bit of healthy rivalry between researchers who study lemur hibernation and what he describes as “the ground-squirrel team.” Each group believes its totem animal offers the best glimpse into how hibernation works.
“It’s a friendly rivalry,” he clarifies.
I’ll bet. The jury’s still out, scientifically speaking. The main sales pitch for Team Lemur, though, is humans’ proximity to lemurs on the evolutionary tree. If different species get to hibernation in different ways, depending on where they start, it stands to reason that we’ll learn most from the species that’s starting from a place that’s as close as possible to where we are.
Think about it this way: You’re standing by a beautiful mountain stream in Montana, holding a fishing rod. On the other side of the stream is someone else, who is also holding a fishing rod, but his fishing rod is connected to a fish. In fact, as long as you’ve been watching this guy, his fishing rod has been connected to countless fish. So, finally, you ask him which flies he’s using and he tells you, reluctantly, that he’s using a #10 caddis, a type of artificial fly that’s modeled on real insects in the order Trichoptera.
So now you’re set, right?
Yes, but only if you happen to have a #10 caddis in your box of flies. If you do, then you’re in business. If you don’t, then it’s a good bet you’re going to spend the rest of the afternoon standing by a stream, looking picturesque, perhaps, but holding a fishing rod that will remain annoyingly and persistently unconnected to a trout.
This is the challenge of interspecies research. If we find something that induces hibernation in another species that happens to fit our physiology, then there are enormous opportunities for clinical medicine. But if whatever provokes hibernation in another species doesn’t fit with how our bodies work, and isn’t even in our physiologic toolbox, then we’re back where we started. So what Klopfer and his colleagues are looking for is something in the lemur’s repertoire that we also have in ours. But what might that be?
Team Lemur doesn’t have a definitive answer yet. But one of the most promising candidates seems to be ghrelin, a 28-amino-acid peptide (a very small protein). Ghrelin is actually a downstream product that results from cleaving a larger, 117-amino-acid peptide (preproghrelin). Another product of that split is obestatin, which Klopfer and his colleagues also suspect is part of the lemur’s hibernation mechanism.
The most important thing to know about ghrelin is that it occurs in humans—the equivalent of that #10 caddis fishing fly. As Klopfer tells me this, I can begin to see doors opening to clinical medicine. Figure out how this peptide works in lemurs, he’s suggesting, and there’s at least a chance that it could work the same way in us.
OK, but how might ghrelin work? The dominant effect—at least as far as we know, Klopfer is careful to clarify—seems to be related to hunger. (The name ghrelin is derived from the Proto-Indo-European linguistic root ghre: “to grow.”) It has effects on the hypothalamus that induce hunger, for instance. It also reduces satiety, meaning that we eat more before we feel full.
Finally, and perhaps most interestingly, ghrelin seems to enhance reward mechanisms, increasing our responses to pleasurable stimuli, like food (or anything else), including, oddly, alcohol. There’s even very limited evidence that intravenous ghrelin can increase food intake in patients with anorexia. Studies like these are a long way from reaching the level of evidence needed for clinical medicine, but they do suggest, at least, that ghrelin is a very real force in human endocrine physiology.
As we walk back across the Lemur Center’s campus toward the visitor’s center building, I ask Klopfer how ghrelin works in lemurs. Surely it can’t just be about hunger regulation and reward behavior. There must be more, right?
Klopfer points out that in lemurs in the wild, the gene that is responsible for making preproghrelin seems to be turned off, or turned way down when days are shorter and temperatures are lower. That reduction, in turn, may reduce metabolism, reduce thyroid function, and may also increase REM sleep. All of that, he says, seems to be a setup for hibernation.
There is one problem, though. Ghrelin levels in humans vary throughout the day. They are also altered by diseases like Prader-Willi syndrome, a hereditary condition. Ghrelin levels may even be reduced after gastric bypass surgery. Despite this variation, though, and despite the fact that some people may have very low ghrelin levels, humans don’t hibernate. So if changes in ghrelin in lemurs are associated with hibernation, but similar fluctuations in humans aren’t, where does that leave us?
Klopfer isn’t sure. This is a new area of research in lemurs, and they just don’t have the data yet. They don’t even know if ghrelin is the trigger for hibernation. Maybe it’s just a side effect. Who knows?
Hormonal pathways are vastly complex, he says. It may be that there are pathways that are activated in lemurs that are dormant in humans, or vice versa. Nevertheless, a better understanding of lemurs, he points out, will at least give us a better sense of what those pathways might be in humans, and where they might be hiding.
HACKING THE HIBERNATION GENE: OF MICE AND MEN
“Don’t touch the mouse.”
Under normal circumstances, I’m fine with that strategy. Not touching rodents is a pretty good rule to live by. But today I confess that I do want to touch this mouse. Very much.
This particular mouse is lying on an upturned, gloved palm, and it looks about as dead as it’s possible for a mouse to look. This is why I want to touch it.
The reason I really want to touch this dead-appearing mouse is because the hand on which it’s resting is connected to a researcher named Dr. Cheng Chi Lee. Cheng is a respected scientist at the University of Texas Health Science Center at Houston who is studying suspended animation, or hibernation, or torpor, or, to use the phrase that he’s come to rely on as being least likely to incite riots of science-fiction fans: hypometabolism.
Cheng is a short, balding, middle-aged biochemist who has been in the research game ever since he immigrated to the United States in 1986. He has the weathered face, wire-rim glasses, and frumpy clothes of a farmer, or a hardware store clerk, or, I suppose, of a biochemist. He looks eminently trustworthy. And that’s a very useful attribute to have if you’re trying to convince the world that you just may have stumbled on the secret of suspended animation (sorry, I mean hypometabolism).
“This is really neat,” Cheng says about the apparently dead mouse.
It’s at this point in our brief conversation that I wonder whether, perhaps, Cheng’s definition of “neat” is a little different from mine.
When we first walked into Cheng’s inner sanctum of hypometabolism research, a hermetically sealed and temperature-controlled room, the mouse that is now lying on Cheng’s outstretched hand was scampering around his cage. Then Cheng’s graduate student Tre held it carefully by the scruff of its neck and gave it an injection of a very small amount of a clear liquid. Then my little mouse friend was placed in a sealed box in a dark, cool cabinet that allowed us to monitor his activity, temperature, and—most important—his metabolism, from a computer just outside.
The mouse’s moniker is #0011, and for the past forty minutes, we’ve been watching #0011’s vital statistics flicker down the screen. The most important of these is his VO2, a measurement of the milliliters of oxygen per kilogram that he uses per hour. Essentially, VO2 is a measure of how fast his metabolism is working. Initially, #0011 had a VO2 of 4,410. That was right after Tre’s injection.
For the first minute or so, nothing happened. Then things began to change. Over the next ten minutes, his VO2 dropped precipitously to around 1,000. Then it dipped into the 900s. Then to 694. Then to 420. Meanwhile, #0011’s body temperature went from a healthy 37 to 23.1 degrees Celsius.
Now he’s lying on an outstretched palm. And he’s dead. Or is he?
I look more closely. As I do, I notice two things. First, he’s breathing. Slowly, but he’s breathing. So funeral arrangements would be premature.
My second observation takes me a little longer. This little mouse is idling at room temperature, but he’s not shivering. And there’s no piloerection, meaning the mouse’s fur is not standing up to provide increased insulation the way it would if a normal mouse found itself at this temperature.
Is he sleeping? No, that doesn’t cover the low body temperature and hypometabolism. Hibernating? No, because mice don’t hibernate. None of those terms really fits. He is a mouse that has been injected with . . . something.
I turn to Cheng, who is grinning. He’s been waiting for this moment. OK, I ask him. What the hell was in that injection?
A few minutes later, we’re in Cheng’s office, a Spartan, light-filled space, strewn with papers and a few lonely plants in beakers.
To understand what was in that injection, Cheng says, I need to hear the whole story. It seems like he also wants to give me a sense of how science works and how lucky some of his discoveries have been.
Ten years ago, Cheng was interested in circadian rhythms, the way that our physiology changes throughout a twenty-four-hour cycle, and particularly changes in response to light and dark. In a routine experiment he realized that a co-enzyme called colipase was expressed in tissue where it shouldn’t been found. But he found it there only under constant dark conditions. So constant darkness seems to create what he describes to me as a “wonderful process of nature” that hadn’t previously been identified.
That unexpected finding prompted Cheng to embark on a hunt for the molecules that might be involved in regulating a gene in response to light. To do that, he used a common technique called high-pressure liquid chromatography. This is a way of dividing a substance into its constituent molecules that can be categorized based on their size and behavior. The result is a little like the “deconstructed” meals you might find on your plate in a restaurant that specializes in molecular gastronomy, in which a common dish is often presented as its component parts.
That analysis pointed to an unexpected nueleotide that Cheng eventually identified as adenosine monophosphate, or AMP. AMP can be broken down to adenosine. Or it can be built up with phosphate molecules to form adenosine diphosphate or adenosine triphosphate, the molecules we met in chapter 3 that are the currency that mitochondria use to create and store energy.
Soon Cheng was able to show that you can induce mouse livers to produce colipase by injecting them with AMP. And it was in one of those experiments that a graduate student told Cheng that mice that had been injected “felt cold.” Skeptical at first, but increasingly intrigued, Cheng began to investigate the effects of AMP on physiology in mice, pigs, and dogs.
At this point, Cheng leans back in his chair as if we’re done. The answer, he’s saying, is AMP. And in fact, the mouse we’d just met—mouse #0011—had received just such an injection before it entered a state of hypometabolism that fooled me into thinking that it was dead.
So AMP causes suspended animation?
Cheng looks at me with the expression of someone who is tempted to say, If you think that you can summarize ten years of my work with a simple sentence, remind me of why I’m wasting a day talking to you?
Instead, he switches on his laptop and we huddle in close. He shows me a graph with two lines that travel from left to right above a border that is divided into ten-minute increments. Both lines belong to a mouse after it’s injected with AMP and placed in a cold room, as #0011 has been. One line, in black, is the mouse’s VO2. It begins at a plateau and then takes a deep dive almost straight down, so it looks like this mouse’s VO2 dropped by more than 75 percent in fifteen minutes.
The second line, in red, tracks the mouse’s temperature. It seems to hover above the black line, following it down, but slowly and hesitantly. The dive in temperature is not nearly as steep, or as deep, as the dive in VO2 is.
As Cheng takes me through that graph and subsequent slides, there are two things he wants to point out. The first is that we shouldn’t attribute too much importance to AMP. It doesn’t “cause” hypothermia, he says. At most, it’s an initiator of hypometabolism. It sets the stage, but hypothermia is caused by heat loss.
The second point that Cheng wants to make, though, is the most interesting I’ve heard so far. He points at the graph of the mouse’s VO2, and then at the line of temperature that’s lagging behind.
“You noticed?” he asks.
I did. At least, I think I did. They don’t coincide. They’re not even parallel. VO2 decrea
ses and then temperature decreases.
Cheng nods enthusiastically. What that means, he explains, is that AMP is causing a drop in VO2 that’s far ahead of—and seemingly out of proportion to—the drop in temperature. VO2 is decreased, and temperature is decreased, but these decreases seem to be happening almost independently.
That gets my attention, because it reminds me of a study I’d just read. It didn’t seem very important at the time, but now I remember it. Cheng’s graph has me thinking, improbably, of Yogi Bear.
YOGI BEAR’S TENUOUS MEMBERSHIP IN THE HIBERNATION CLUB
Everyone knows that bears hibernate. Yogi Bear? Jellystone Park? In every other episode, it seems, Yogi is attempting to enter a much-coveted state of hibernation while various malefactors are trying to keep him awake. So you’d think that bears’ claim to hibernation is unassailable. But you’d be wrong.
Some of the first doubts about whether bears hibernate were raised by an intrepid naturalist (and bear hunter) named Henry Clapp. He pointed out that when hibernating bears exhale, steam rises from their nostrils. How Clapp got close enough to determine this is a little unclear. However, his vivid descriptions of the insides of freshly killed hibernating bears will reassure concerned readers that he didn’t wander into these bear dens unarmed.
Based on his observation of steam and nostrils, Clapp—and many scientists who came after him—concluded that hibernating bears must stay warm. Too warm, they thought, to have the sort of decreased metabolism that is associated with hibernation. Ergo, they don’t really hibernate.
It wasn’t until more than a hundred years later that it occurred to anyone that even relatively warm bears might still have a reduced metabolism. Asking that question, though, required using a method that was more scientific—and fortunately less dangerous to the scientist—than crawling into bed with them and watching them breathe. But eventually someone did.
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