This Week in Science

Science  22 Nov 1996:
Vol. 274, Issue 5291, pp. 1277
  1. Artificial molecules

    A quantum dot is a semiconductor structure in which electrons are confined within a small volume and have discrete energy levels resembling those of atoms. Livermore et al. (p. 1332) have constructed coupled quantum dots in which electrons can tunnel between dots, thus creating “artificial molecules.” The conductances observed in both the weak and strong tunneling limits agree with predictions from many-body theory.

  2. Glowing clusters

    Chemical reactions may be accompanied by emission of visible light, or chemiluminescence, if the energy created by the reaction is stored initially in an excited state that later decays. König et al. (p. 1353) report that formation of metal clusters can by accompanied by chemiluminescence. During cluster agglomeration in a noble gas matrix, formation of unstable intermediates is proposed to lead to emission of excited fragments, which decay while emitting visible light.

  3. Fresh air?

    Transport of air from the troposphere into and out of the stratosphere, and its residence time in the stratosphere, can determine the rates at which ozone-destroying compounds reach the ozone layer and the effects of aircraft emissions. However, details of the global transport of air are difficult to obtain. Boering et al. (p. 1340) measured various gases on board the NASA ER-2 aircraft at tropospheric and stratospheric altitudes between 1992 and 1996 and showed that air enters the stratosphere continuously throughout the year and is distributed rapidly. The measurement allowed the determination of the mean age of stratospheric air, which is related to the residence time of pollutants in the stratosphere.

  4. Not so stable

    Earth's lower mantle has generally been thought to be composed mostly of perovskite [(Mg,Fe)SiO3], and many geophysical models of the lower mantle are based on this assumption. Some earlier work had indicated, however, that perovskite containing some iron might not be stable throughout the pressure range of the lower mantle. Saxena et al. (p. 1357) performed synchrotron x-ray studies on high-pressure, high-temperature samples of the end-member MgSiO3. The results suggest that the perovskite broke down to MgO (periclase) and SiO2 (stishovite) at pressures of about 60 gigapascals.

  5. Aspirin and glutamate

    The neurotransmitter glutamate can actually be toxic to neurons if its levels are elevated for too long-as may occur during stroke. Grilli et al. (p. 1383) describe how aspirin, at doses already in frequent use for the treatment of arthritis, can help to protect rat neurons in primary culture and in hippocampal slices from glutamate-induced neurotoxicity.

  6. Tumor evasion

    Activated T cells are normally eliminated after an immune response is completed by expression of the Fas ligand (FasL, also called Apo-1 or CD95 ligand), and immune-privileged sites in the body such as the eye also express FasL. Hahne et al. (p. 1363; see the news story by Williams, p. 1302) show that unlike normal skin cells, malignant melanoma cells express FasL to avoid the immune response. Injection of mouse melanoma cells expressing FasL led to rapid tumorigenesis in normal mice but not in mice deficient in Fas.

  7. Restoring the liver

    Adult liver cells (hepatocytes) can replicate rapidly, thus allowing the liver to recover from toxic-induced damage and to regenerate in a few days after surgical procedures that remove most of its mass. Cressman et al. (p. 1379) have shown that interleukin-6 (IL-6) is a critical cytokine in this recovery. Mice lacking the gene for IL-6 were unable to regenerate liver tissue unless IL-6 was administered exogenously after tissue removal. The necessity of IL-6 is an important consideration in strategies that produce decreases in cytokine activity in order to control liver damage, such as those used in treating cirrhosis.

  8. Insulin Resistance, Diabetes, and Obesity

    Obesity can lead to insulin resistance and diabetes, and the two appeared inseparable in animal models. Hotamisligil et al. (p. 1377) now report that in mice lacking the gene encoding AP2, the fatty acid-binding protein from adipocytes, dietary obesity fails to cause insulin resistance or diabetes. Somehow, AP2 must be critical for the metabolic pathway that leads from obesity to insulin resistance. The results provide a focus possibly intervening in the process that causes abnormal glucose homeostasis and symptoms of diabetes as a consequence of obesity.

  9. Hats off

    There has been a recent explosion in knowledge about how the cell uses selective proteolysis to control a variety of functions. Tamura et al. (p. 1385; see the Perspective by Schneider, p. 1323) describe the discovery of a protease complex likely to form part of a multicatalytic complex. The protease subunits have a distinctive structure in electron micrographs and resemble a three-cornered (tricorn) hat.

  10. Stress signal

    When the nematode Caenorhabditis elegans finds its environment inhospitable, it develops into a dauer larva adapted for survival in adverse conditions. Ren et al. (p. 1389) show that response to the pheromone that induces the dauer phase includes alterations in transcription of a growth factor related to transforming growth factor-β. The transcriptional modulation occurs within certain chemosensory neurons of the larva.

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