Notebook

The banded Gila monster's saliva may help provide a new treatment for type II diabetes mellitus. NEW TREATMENT FOR DIABETES II? A chemical in the saliva of the Gila monster Heloderma suspectum that helps digest gigantic meals may provide a new treatment for type II diabetes mellitus. The Gila monster is one of only two venomous lizards and hails from the Phoenix, Ariz., area, coincidentally the home of the Pima Indians, who have the world's highest incidence of type II diabetes." John Eng, a physician at the [Veterans Administration] Hospital in The Bronx, and his collaborators fished out peptides that they believed were part of the venom. Some of the peptides looked similar to the human hormone family that includes secretin, glucagon, and glucagon-like peptide 1 (GLP-1)," reports Andrew Young, vice president of research at Amylin Pharmaceuticals in San Diego. One peptide, exendin, had 50 percent homology with human GLP-1, known to have properties that could be useful in treating diabetes: stimulating insulin secretion, dampening appetite, slowing stomach emptying, and decreasing secretion of glucagon, the hormone that counters insulin action. But the human hormone has a vanishingly short half-life of mere minutes. Rather than trying to circumvent this limitation by synthesizing GLP-1 analogs resistant to degradation or by disabling the enzyme that breaks it down, Amylin is testing a 39-amino-acid synthetic peptide based on the Gila monster's peptide. The lizard produces exendin in addition to GLP-1, which it secretes in its gut, as humans do. Since exendin appears in the saliva shortly after the animal's thrice-yearly meal of various eggs and nestlings, it must aid digestion, rather than be part of the venom. "When they eat, they gorge, so they have to store nutrients. They do this in their tails, which can swell fourfold in volume. The lizard can go a year without eating. To store nutrients, it needs insulin. We don't know if exendin does the same thing in the Gila monster as it does in mammals, but it makes sense that it would provide insulin secretion and decrease appetite," says Young. Preclinical tests indicate that exendin is potent in tiny doses and deliverable across the mucous membranes of the mouth, nose, or lungs. Phase II clinical trials suggest that it is safe. --Ricki Lewis SECOND CHANCE IN CLONING A baby bull named Second Chance joins the slowly growing list of cloned mammals. Born in mid-August at Texas A&M University, the bull is the offspring of the beloved Chance, a Brahman bull who at the ripe old age of 21 is the oldest animal to be cloned. "This was a companion animal. He was his owners' friend and part of their livelihood," says Jonathan Hill, a veterinarian working on his Ph.D. with Mark Westhusin at the university. The same team is attempting to clone a dog. Chance, an unusually even-tempered bull, appeared on Late Night with David Letterman and starred in several television commercials. As in other cloning attempts, a liveborn result wasn't easy to come by. "It took 189 attempts to get six pregnancies. We removed two embryos at 30 days to study early development, and the rest, except for Second Chance, died by five months," Hill says. Chance died a few months ago, shortly after donating the skin fibroblasts from his belly that would provide the genetic instructions to create a biological replica. Since Chance apparently perished of old age, the state of telomere shrinkage in Second Chance will be of great interest. The work has not yet been accepted for publication. --Ricki Lewis SEX AND C. ELEGANS Genetically speaking, sex seems a bit unnecessary. Why didn't evolution let more species reproduce asexually, rely solely on their own genes, and avoid the "inconvenience" of sexual intercourse? It remains one of the big mysteries of modern biology. The possible key: understanding the evolutionary role of harmful genomic mutations. An asexually reproducing population that accrues a sufficiently large number of deleterious mutations might be at a disadvantage compared to a sexual population, with the latter eliminating harmful mutations at a higher rate. Backing up this theory by tracking the number of deleterious point mutations has been tricky. Oft-used so-called fitness tests count spontaneous mutations by looking for variability in traits based on phenotypes. As a result, they tend to miss many mutations that have small phenotypic effects. Using a novel system, a research team at the University of Edinburgh, Scotland, recently measured the fitness effects of previously overlooked deleterious point mutations in the asexual nematode Caenorhabditis elegans (E.K. Davies et al., "High frequency of cryptic deleterious mutations in Caenorhabditis elegans," Science, 285:1748-51, Sept. 10, 1999). "The magnitude of the effect of the mutations isn't the important factor for the deleterious mutation theory of the evolution of sex," says senior author Peter D. Keightley, a Royal Society research fellow at the university. "It's actually the number of mutations that occur in each individual, each generation that matters." Investigators used a mutagen to generate point mutations for which the number of DNA changes can be estimated. They concluded that a vast majority of these harmful mutations had "cryptic" or unidentifiable effects that would reduce fitness in the natural environment. In future work, Keightley's team will investigate whether more stressful environmental conditions reveal a higher fraction of mutations. They'll also investigate further the cause and effect relationship between numbers of mutations and fitness effects. --Eugene Russo Live specimen of Peloria , left, and normal Linaria (toadflax), right. The Linaria gene turns on early in floral development in the two top or dorsal petals and one of the immature stamens, which eventually arrests. HELICAL SCIENCE Discoveries raise interesting questions that often take new methods and many years to answer. More than a century after Gregor Mendel found the basic principles of heredity in peas, biologists identified the genes behind his wrinkled seed and dwarf traits. Molecular methods recently confirmed Charles Darwin's insights into speciation of Galapagos finches. Now it's Carl Linnaeus' turn. The 18th century father of modern classification described a variant of Linaria vulgaris (toadflax) in which flower symmetry goes from bilateral to radial. A team led by Enrico Coen , research group leader at the John Innes Centre, Norwich, England, traced the peloric defect to a gene called cycloidea, first identified in Linaria's cousin snapdragon where it controls dorsoventral asymmetry (P. Cubas et al., "An epigenetic mutation responsible for natural variation in floral symmetry," Nature, 401:157-61, Sept. 9, 1999.) The Linaria gene, or Lcyc, turns on early in floral development in the two top or dorsal petals and one of the immature stamens, which eventually arrests. With inactive Lcyc, the dorsal and side petals look just like the lowest or ventral petal, which has an orange lip and a nectary spur. All five stamens are normal. Coen's team showed that peloric arises from an inherited, gene-silencing methylation of Lcyc, not an alteration in its DNA sequence. The change is unstable, and Lcyc sometimes reverts in new branches. When it demethylates, normal flowers develop. According to Coen, heritable "epigenetic" mutations are unusual. In fact, they're unreported in animals, probably because the germ line is segregated early in development. In plants, where flowers form later in life, mutations in shoot meristems can reach gametes. "My view is that many genes might be methylated sporadically by 'mistake' in meristematic cells," Coen hypothesizes. "In the case of Lcyc, the gene inactivation was not detrimental to the vegetative propagation of the plant and therefore survives as a rare form." --Barry A. Palevitz RESEARCH VS. RIGHTS The proprietary concerns of the genomics industry and the curiosity-driven world of fundamental science may have found themselves at cross purposes once again, following the discovery of a way to improve the accuracy of DNA sequencing (Y. Li et al., "Structure-based design of Taq DNA polymerases with improved properties of dideoxynucleotide incorporation," Proceedings of the National Academy of Sciences, 96:9491-6, Aug. 17, 1999). The discovery, by associate professor of biochemistry and molecular biophysics Gabriel Waksman and colleagues at Washington University School of Medicine, came from X-ray studies of the structure of Taq, the enzyme most commonly used to amplify template DNA for genome sequencing. Several years ago, a mutation to the enzyme was introduced that improved its role in determining the order of nucleotides in a piece of DNA. But the mutant enzyme still had a shortcoming: It generated sequencing patterns that had gaps, or variable band intensity. Waksman's team identified a structural anomaly that led them to substitute an amino acid at a specific position with another amino acid, resulting in much better band intensity. However, he doesn't know if his group's finding will ever be applied commercially. "You can't use the double mutation without having licensing for the first one," he explains. Moreover, the companies that do sequencing have now told him that the addition of another enzyme to the reaction mixture will yield the same band intensity as his doubly mutated one, without the patent considerations. "My view is, I'm not in that business," he shrugs. His group already is studying another part of Taq. --Steve Bunk METALS ON THE BRAIN Several studies done in fish and rodents since the mid-1980s have shown that the normal process of axonal transport, essential for ferrying neuronal nutrients, waste, and structural components, can also facilitate the unwanted entry of toxic metals such as cadmium, manganese, nickel, and mercury. With the help of axonal transport, these metals can enter the animal brains via the olfactory system, circumventing the blood-brain barrier meant to protect the brain from toxins. The most recent such report suggests a novel mode of entry for mercury into the brains of brown and rainbow trout (C. Rouleau et al., "Accumulation of waterborne mercury (II) in specific areas of fish brains," Environmental Science and Technology, 33:3384-9, Oct.1,1999). "We found that not only [is] the olfactory nerve a way of entry for mercury to the brain, but also it appears that all other water-exposed sensory nerves are a route of entry for mercury," explains lead author Claude Rouleau, a research scientist at Environment Canada's National Water Research Institute in Ontario and a member of a team based at the Swedish University of Agricultural Sciences in Uppsala, Sweden. By injecting mercury into some fish intravenously to verify that the blood-brain barrier is mercury-tight, investigators were able to conclude that waterborne mercury found in the brain did not come from the blood itself, but was accumulated via the nerve. The findings don't represent any immediate danger to humans since people don't generally eat fish brains. But they do suggest a possible danger to fish populations and further evidence that toxic metals could enter human brains via the olfactory system, a hypothesis not yet well studied. The important next step, says Rouleau, is to evaluate the toxicological significance of the recent findings by studying fish taken from their natural setting, a project he plans to do with pike next summer. --Eugene Russo

Notebook

Sep 26, 1999

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