This Week in Science

Science  28 Mar 1997:
Vol. 275, Issue 5308, pp. 1853
  1. Comet Hale-Bopp: The shape of things to come

    A series of reports focus on comet Hale-Bopp, including its extreme brightness, its increasing activity, and its flair for variable, dusty outbursts as it approaches the sun (see the Perspective by Cruikshank, p. 1895). Detailed images from the Hubble Space Telescope (Weaver et al.,p. 1900) indicate that the comet nucleus is very large (between 27 to 42 kilometers in diameter). Thermal infrared observations from the Earth-orbiting Infrared Satellite Observatory (Crovisier et al.,p. 1904) and ground-based telescopes (Hayward and Hanner, p. 1907) indicate that the dust grains coming off the nucleus as the comet moved inside of Mars' orbit are probably crystalline and amorphous silicates. Optical observations (Rauer et al., p. 1909), see cover, and Wagner and Schleicher, p. 1918) showed high rates of sublimation of icy, dust grains, dominated by water and cyanide even at large distances from the sun. Photometric observations (Schleicher et al., p. 1913) also indicated that the gas production rates are 20 to 100 times greater than any other comet at any other observed distance. Radio observations (Biver et al., p. 1915) tracked the rates and temporal changes in the gaseous release of molecules as Hale-Bopp approaches the sun and changes from sublimation of its dusty tail to sublimation of its nucleus. Cometary scientists have begun deciphering how this comet physically jets, spins, and chemically differentiates over an unusually long period of observations.

  2. Electron transport in nanotubes

    Single electrons represent the ultimate “bit” in computing, and much effort has been devoted to creating structures whose conductance varies for changes in charging of only one electron. Bockrath et al. (p. 1922; see the Perspective by Kouwenhoven,p. 1896) measured the transport properties of bundles, or “ropes,” of highly ordered single-walled carbon nanotubes that were lithographically attached to metal contacts separated by 200 to 500 nanometers. At temperatures at or below 10 kelvin, there is a region of suppressed conductance for low bias voltages. Large peaks are observed in the conductance as the number of electrons in the bundle changed with changes in a bias voltage. Resonant tunneling of single electrons through individual nanotubes appears to account for these results.

  3. Gut (immune) reaction

    The gut is not only the main site of exposure to pathogens, as a mucosal surface it is also a weak barrier to pathogen entry. Wang et al. (p. 1937; see the Perspective by Shanahan,p. 1897) add a new component to the known complexity and flexibility of intestinal immune defense. They describe a regulatory interaction between the intestinal epithelial cells and neighboring lymphocytes, mediated by neuroendocrine hormones of the hypothalamus-pituitary-thyroid axis. In the absence of a component of the network, the range of T cell types in the intestinal, but not peripheral, immune system is reduced. The network may be important in gut T cell development findings; it also suggests highly flexible and localized control of immune activity, an important property given the function of the gut.

  4. Brain cancer gene

    Glioblastoma is the most lethal form of human brain cancer, yet little is known about its molecular pathogenesis. Li et al. (p. 1943; see the news story by Pennisi, p. 1876) have identified a candidate tumor suppressor gene, PTEN, that is mutated in about one-third of the glioblastoma cell lines, xenografts, and primary tumors they tested. The PTEN protein has sequence homologies to protein tyrosine phosphatases and to tensin, a protein that binds actin filaments at sites of cell surface signaling. Mutations in PTEN were also detected at lower frequency in prostate and breast cancer samples.

  5. Getting a GRP

    Phosphoinositide (PI) 3-kinases are activated in response to many receptors on the cell surface. The enzyme catalyzes the formation of phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3], but subsequent steps in signaling through this pathway have been unclear. Klarlund et al. (p. 1927; see the Perspective by Hemmings, p. 1899) identified a protein they call GRP1 that binds to PtdIns(3,4,5)P3. GRP1 appears to interact with other proteins that mediate the cellular effects of PI 3-kinase. GRP1 may control cell adhesion through interaction with integrins, and interaction of GRP1 with the guanosine triphosphatase ARF may allow regulation of protein sorting and membrane trafficking.

  6. Out of the nucleus

    Gene transcription critical for the immune response is regulated by the transcription factor NF-AT. Calcium-dependent dephosphorylation of NF-AT leads to import of the transcription factor into the nucleus. Beals et al.(p. 1930) describe another layer of regulation of NF-AT. In this case, NF-AT is phosphorylated by the protein kinase glycogen synthase kinase-3 (GSK-3) and is exported. In this way, GSK-3 opposes the calcium-dependent activation of NF-AT. The effects of GSK-3 in control of pattern formation during development may result, at least in part, from regulation of NF-AT.

  7. Evolving rapidly

    How quickly can evolution occur in nature? Reznick et al. (p. 1934; see the news story by Morrell, p. 1880) transferred populations of guppies from high-predation environments to new locations, such as upstream of waterfalls, that were inaccessible to predators. Monitoring life history characters such as body size revealed rapid short-term changes, illustrating that evolution can happen quickly in nature, even outside the framework of human-induced environmental change.