Science Concentrates

Chemical cues give cell tissue its stripes

By supplying chemical cues, researchers have manipulated mammalian cells to form patterns in vitro [Proc. Natl. Acad. Sci. USA, 101, 9247 (2004)]. Cell patterning is crucial during embryogenesis because bone cells, for example, must cluster correctly as they set the stage for developing into ribs, fingers, or vertebrae. Using mathematical models based on Alan Turing's predictions that patterns form under the guidance of a fast-diffusing inhibitor and a slower-moving activator, Alan Garfinkel and his coworkers at UCLA coaxed vascular mesenchymal cells to cluster into circles (spots) or lines (stripes, shown; dark areas indicate high density). The trick was in finding the correct chemical stage managers. They guessed that the activator protein is well-known bone morphogenetic protein 2. And the inhibitor, they found, is an unusually small protein: matrix carboxyglutamic acid protein (MGP). MGP's small size allows it to diffuse quickly. Cell patterning is important not only in embryogenesis but also in pathogenesis. Cells are subject to the same type of pattern formation when growing atherosclerotic plaques in heart disease.

Comet dust is organic

The first results from the Stardust spacecraft, which grabbed samples from the comet 81P/Wild 2 while flying by, show that the chemistry of the comet's dust particles is largely organic. Though amino acids can form from predecessors in comet dust and liquid water, the mass spectrometer aboard the craft found no evidence of amino acids in the grains [Science, 304, 1774 (2004)]. Cometary scientist Jochen Kissel at the Max Planck Institute for Aeronomy in Katlenburg-Lindau, Germany, and colleagues also report that the chemical composition of Wild 2 is similar to that of the famous Halley's comet, even though Halley's is much older. Wild 2's dust particles don't contain much hydrogen or oxygen, presumably because those elements take the form of water vapor. However, the grains are nitrogen rich, likely containing nitriles and polymerization products of hydrocyanic acid. The researchers also found spectral evidence for organic sulfur.

Probing mRNA transport in the nucleus

Understanding the entire process of gene expression requires knowing what happens to messenger RNA following transcription. A team led by postdoc Yaron Shav-Tal and Robert H. Singer, professor of anatomy and structural biology at Albert Einstein College of Medicine, New York City, uses fluorescence imaging to track the movements of individual mRNA-protein complexes (mRNPs) tagged with yellow fluorescent protein [Science, 304, 1797 (2004)]. The researchers track the movements by taking sequential images of living cells with a low concentration of labeled mRNPs. They find that the mRNPs move through the nucleus following a simple diffusion model. The diffusion coefficient for the mRNPs is linearly related to temperature, arguing against an energy-driven mechanism. Although the transport is not energy driven, the diffusion is affected by cellular metabolism. "We find that the nuclear environment is highly [dependent] on energy and that depletion of energy sources results in a certain 'restructuring' of nuclear architecture," Shav-Tal says. "This structural change impedes the movements of mRNPs through the nucleoplasmic space."

On the genesis of homochirality

The L-proline-catalyzed reaction shown exhibits rate acceleration and amplification of product enantiomeric excess with time, according to a recent study. Donna G. Blackmond, a chemistry professor at Imperial College, London, and coworkers base this conclusion on continuous monitoring of reaction progress by reaction calorimetry [Angew. Chem. Int. Ed., 43, 3317 (2004)]. If their conclusion is confirmed, the reaction offers a model for the evolution of homochirality from simple organic molecules. Previous models for the genesis of homochirality involve organometallic reagents, including the frequently cited organozinc system of Kenso Soai at the Tokyo University of Science. Soai says the possibility that l-proline "participates in the evolution of homochirality" is "very interesting."

Tips for tetrapods and quantum rods

Until someone builds nano-alligator clips, scientists have to think of creative ways to wire nanostructures onto electrical circuitry. Connecting structures that are less than 100 nm long can be particularly tricky. To create contact points for CdSe semiconductor quantum rods and tetrapods, chemists at Hebrew University of Jerusalem devised a simple method for growing gold tips onto the tiny structures [Science, 304, 1787 (2004)]. The team, led by chemistry professor Uri Banin, reports that the gold tips provide natural anchor points for creating functional circuitry with DNA and for directed self-assembly with dithiols. Banin's group grows the gold-tipped structures with a simple solution-based method. By varying reagent concentrations, they can control the size of the gold tips. Remarkably, the gold grows selectively at the nanostructures' tips. The researchers speculate that this selectivity arises from an increased surface energy at the quantum rod and tetrapod tips. To demonstrate how the gold facilitates self-assembly, the team added hexane dithiol to a solution of the quantum rods. The thiol ligands bind strongly to the gold tips, linking the rods end-to-end into a chain.