First Fruit Flies, Then Mice, Now Zebrafish, Chart Genetics OfEmbryonic Development

By David N. Leff

Editor's note: Science Scan is a round-up of recently publishedbiotechnology-related research.

Geneticists don't shop for fruitflies (Drosophila melanogaster) in apet store. But that's where they first found the zebrafish(Brachydanio rerio) in the early 1980s.

This tiny (inch-long), smartly striped freshwater mini-fish has a lotgoing for it as a vertebrate model for studying embryogenesis andgenetics in higher forms of life, including Homo sapiens. For onething, its embryos are transparent; you can watch their hearts beat 24hours after fertilization, and eye-ball their cells, one at a time.

For another, zebrafish hatch in 48 hours, go from egg to breeding in11 weeks. And they lay 400 eggs a week. (See BioWorld Today,Aug. 1, 1994, p. 1.)

So what began as a scientific cult at the University of Oregon some15 years ago is now a worldwide network of academic B. rerioaficionados, with inputs from industry. The den mother of thisresearch movement is developmental biologist Christiane Nsslein-Volhard, who won a Nobel prize last year for half a lifetime ofcharting genes and their mutations in fruitflies.

Now, in the `90s, Nsslein-Volhard, at the Max-Planck Institute ofDevelopmental Biology in Tubingen, Germany, has shifted her focusfrom D. melanogaster to B. rerio. The December issue of the Britishmonthly journal Development carries 36 papers, which launch asystematic genetic analysis of how the vertebrate embryo is formed.Its model is the zebrafish.

The international consortium is co-chaired by Nsslein-Volhard andher former student, Wolfgang Driever, now at Boston'sMassachusetts General Hospital. Among the group's prime movers isDavid Grunwald, at the University of Utah, one of the early settlerson zebrafish territory.

Grunwald has a brief run-down on the endeavor in Science datedDec. 6, 1996. Its title: "A fin-de-siecle achievement: Charting newwaters in vertebrate biology."

So far, Grunwald reported, "the groups have amassed and begun toanalyze a collection of more than 1,800 developmental mutantsrepresenting defects in about 500 different genes that contribute tothe form and function of the zebrafish embryo. All told, theyexamined some 6,600 different zebrafish mutants."

Grunwald called the timing of this effort "superb," because of recentrecognition as to how much "both the molecules and the logicunderpinning embryonic development are conserved throughout thevertebrates."

He anticipates that studying the zebrafish mutants will not onlyelucidate embryogenesis but "when altered, result in a variety ofinherited diseases in humans. Several of the cardiovascular andhematopoietic mutants," he added, "clearly phenocopy known humandisorders."

These mutants, he observed, are natural rather than induced. Theyaffect phenomena ranging from cell division and nuclear replicationto body pattern formation and defects in specific organs and tissues.Two sets of the aberrant genes focus on locomotor behavior andretinal innervation.

One compelling reason for choosing the zebrafish to mount thismutation survey, Grunwald pointed out, "is the extreme ease withwhich cells can be transplanted between mutant and wild-typezebrafish embryos, which allows for the rapid identification of theparticular cells that normally provide the gene function missing in amutant.

"In sum," he concluded, "the zebrafish mutant collection has uniquevirtues as a tool for the study of vertebrate development, and willprovide a much-needed complement to the mouse developmentmutants that are generated by targeting selected genes formutagenesis."

Developmental biologist William Matthews, at Genentech Inc., ofEmeryville, Calif., told BioWorld Today: "We think that thezebrafish is an excellent developmental organism as a genetic modelfor studding vertebrate organogenesis."

Matthews added: "Genentech actually has an interaction now withMark Fishman at Massachusetts General Hospital, and we're goingforward. We're funding efforts to improve the maps that are availablefor positional cloning in zebrafish, and that's a collaboration withMark Fishman as well, as we're interested in further screens to revealmutants."

Princeton, Transcell, Develop Drug DiscoveryStrategy Based On Carbohydrate Adhesion

A tropical vine, Bauhinia purpurea, bears gorgeous, deep-purple,orchid-like flowers. A native of India, B. purpurea does well inFlorida gardens.

Its seeds contain lectin, a protein with unusual binding specificity. Itsnumerous carbohydrate binding sites latch on to carbohydratesstudding the surface of red blood cells, causing them to clump. Thisproperty suggested to Princeton University chemist Daniel Kahne,that the lectin "makes a good model system for cell adhesion proteinsthat recognize cell surface carbohydrates."

Kahne is senior author of an article in Science dated Nov. 29, 1996,titled: "Parallel synthesis and screening of a solid phase carbohydratelibrary." It describes a strategy that identifies carbohydrate-basedtarget molecules for any receptor.

The screen, Kahne pointed out, "may prove especially valuable fordiscovering new compounds that bind to proteins participating in celladhesion." In particular, such drug development would target chronicinflammation, viral and bacterial infection, primary and metastatictumors.

The Princeton group tackled the low yields, lack of specificity andlong production times that have bedeviled carbohydrate chemists formore than 20 years, in their attempts to construct compound librariesinvolving synthesis of the molecules on a solid support. Several yearsago, Kahne discovered a method of glycosylation at lowtemperatures, permitting "nearly quantitative yields on the solidphase."

The carbohydrate library he designed contains 1,300 di- andtrisaccharides, including two derivatives of the B. purpurea ligand.He and his co-authors incubated some 9,000 resin beads _ six copiesof the 1,300-member library _ with biotin-labeled lectin, thenexposed them to avidin-linked alkaline phosphatase.

Over a 20-minute period, "a total of 25 dark purple beads werepicked out of the library and decoded," Kahne reported. His groupultimately synthesized two oligosaccharides that bound to the lectinmore tightly than to a known compound.

Kahne is scientific founder of Transcell Technologies Inc., inMonmouth Junction, N.J. Princeton has licensed his combinatorialchemistry technology to the company, for use in accelerating drugdiscovery. n

(c) 1997 American Health Consultants. All rights reserved.