By David N. Leff
Ask a bunch of kids: "What do bears do best?"
The majority answer: "Hibernate!"
"Many organisms hibernate," observed molecular geneticist Gary Ruvkun, of Harvard Medical School, in Boston. "Most of the insects in the world," he added, "go into hibernation when the weather is cold or dry. Many vertebrates do, like frogs in the desert. Nematodes hibernate. And like bears," he pointed out, "most of the animals change their metabolism drastically, depending on whether they hibernate or not."
Human beings, Ruvkun pointed out, are not exempt.
"Daily sleep might be the human equivalent of hibernation," he suggested, "but it's not clear. Scandinavians seem to hibernate. Seasonal affect disorder (SAD) might be related to that sort of thing. It's interesting," he noted, "that SAD is treated with bright light, with the view that light-dark cycles are playing into the endocrine system that controls emotional status."
Ruvkun's laboratory at Harvard focuses on such neuroendocrine phenomena in Caenorhabditis elegans, the lowly, backyard-variety nematode. Actually, to geneticists, this millimeter-long, transparent round worm is far from lowly. The 14,000 or so genes in its 100 million-base-pair genome express products that make up the 959 cells in its barely visible body. They are a happy hunting ground for genes with functional homologues in the human genome.
What motivates the C. elegans worm to hibernate?
"Normally," Ruvkun explained, "it's crowding that determines whether a worm switches to a fat-based metabolism or not. And that crowding is measured as the production of a pheromone, a small molecule, not a protein, that it detects with exposed neurons in its head. That unique input," he went on, "is to a very universal metabolism-regulating program that happens to use insulin."
He and his coworkers came across this finding last summer, "when we discovered that there is an insulin receptor in the worm's hibernation pathway. It's expressed by the daf-2 gene. It was only then," he added, "that it dawned on us that in fact we were dealing with the equivalent of metabolic control in mammals, specifically the pathway related to diabetes."
Ruvkun interprets this phenomenon as linked to the circumstance, namely crowding, that triggers hibernation in the worm. "We think it actually turns off the production of insulin. Just like us, when people with diabetes turn off insulin, the worms switch to a fat-based metabolism."
Sleep Ends; Sex Begins
As for what rouses C. elegans from its slumber — which can last for months — Ruvkun went on, "When they get out of the pheromone-signaled crowded situation, and when they have plenty of food, just like us, which induces insulin production, they exit hibernation."
That wake-up call "kicks in at worm puberty, if you will," he recounted. "They immediately start reproducing. They start living off the fat they've stored up, just like all the other critters that come out of hibernation and start reproducing."
It's been something like 800 million years since C. elegans and Homo sapiens branched out on the family tree of life. But nature is very parsimonious, and has conserved many DNA sequences through all those eons of time between both these phyla of the animal kingdom.
"One thing to interpret from all this," Ruvkun observed, "is that for all those 800 million years, the ancestors of worms and mammals, including humans, were using insulin to control their metabolism."
Ruvkun is senior author of a paper in this week's Nature, dated Oct. 30, 1997, which sums up his lab's research to date in this area. Its title: "The Fork head transcription factor DAF-16 transduces insulin-like metabolic and longevity signals in C. elegans."
He explained: "Fork head is a class of DNA binding proteins. It comes out of the Drosophila genome, where the original mutant was called fork head."
Ruvkun made the point that "the important thing in this paper is not so much that worms use insulin. It's that we identified a mutation that bypasses the need for insulin-receptor signaling, which means they no longer need insulin. In other words, the reason that the worm metabolism is screwed up in the absence of insulin is because of disregulation of this second gene, called daf-16."
From his Nature paper, the Harvard geneticist distilled three main points:
"That there are homologues of daf-16 in the human genome. We find them in databases, but never implicated in insulin signaling; we suggest that they are.
"We conclude that worms can live without insulin, if they are also lacking daf-16. If humans use homologues of these daf genes in C. elegans, then they are candidates for being the genetic cause of diabetes, which is a major mystery.
"The daf-16 gene encodes DNA binding proteins that regulate transcription — we think."
Designing Drugs In Worms
To which Ruvkun added a take-home point. "What we argue in this paper," he said, "is that there are two entry points for development of drugs to treat diabetes, both Type 1 and Type 2. These are the daf-16 and daf-3 binding proteins. If you inhibit the human homologs of daf-16 and daf-3 — some patients with one, some with the other, depending on where their disease is — that will be therapeutic." But, he cautioned: "We haven't proven this at all. We're going way out on a limb."
To develop such drugs, he proposes "to do it in C. elegans. You put the human gene into the worm, so you've got a humanized worm. Then its only good copy of daf-16 is the human copy. So now you give drugs, and if they inhibit that human copy, the worm will reproduce. If it does not, it will arrest, and hibernate."
He continued: "That's an avenue for getting candidate drugs. These worms grow in microtiter wells," he noted, "so you can screen for millions of compounds not too badly."
Ruvkund foresees that "we're six months away from showing how the human gene works in the worm, and three or four years away from screening for drugs in those humanized worms. Then you've got to test them in diabetic mouse models, and eventually in people."
Harvard has filed for "very broad patents" covering this area, Ruvkund said, "and wannabe licensees are already sniffin' around; that's for sure." *