This Week in Science

Science  01 Jan 1999:
Vol. 283, Issue 5398, pp. 8
  1. Outer Planet Dynamos

    The magnetic fields of Neptune and Uranus may arise through electrical conductivity in thick layers of ice rich in water, methane, and ammonia that are thought to lie below their atmosphere. Cavazzoni et al. (p. 44) performed ab initio molecular dynamic simulations to determine the phase diagrams of water and ammonia at the extreme pressures (30 to 300 gigapascals) and temperatures (300 to 7000 kelvin) of these planetary interiors. Their simulations suggest that most of the thick ice layer would be composed of ionic fluids of water and ammonia, requiring a dynamo driven by proton mobility in the liquid. This proton mobility would increase with increasing pressure and thus depth in the ice layer. Near the boundary between the ice layer and the rocky core, water and ammonia transform to metallic liquids whose high electrical conductivity would also contribute strongly to the planetary dynamos.

  2. Creating Superconducting Condensates

    An important issue in understanding high-transition temperature superconductors is the energy scale of the states that form the superconducting condensate. For example, the interlayer tunneling (ILT) theory connects incoherent transport in the direction normal to the copper oxide planes with the formation of the superconducting condensate. Basov et al. (p. 49; see the Perspective by Klein and Blumberg) have measured infrared reflectivity for light polarized along the interplane direction of the single-layer cuprate Tl2Ba2CuO6+×and compared their results with previous measurements for the multiple-layer materials La2−×Sr×CuO4, and YBa2Cu3O6.6. They show that the states making up the condensate extend into the mid-infrared region, energies far greater than the superconducting gap energy. Analysis of the changes in infrared reflection in terms of optical conductivity sum rules can account for only part of the superfluid density formed. The authors suggest that this discrepancy may arise from a change in interlayer kinetic energy.

  3. Ozone Desertion

    Both chlorine and bromine species can catalyze ozone destruction, but whereas most chlorine sources are anthropogenic, most of the sources of bromine are natural. Sources of BrO seem to have been restricted to polar regions, but now Hebestreit et al. (p. 55) identify a potentially important source at lower latitudes: salt pans associated with inland lakes and seas. They monitored BrO and ozone concentrations and other atmospheric conditions at the Dead Sea, Israel, in spring 1997. Concentrations of BrO were exceptionally high and those of ozone levels low when winds blew over a large salt pan containing bromine-rich evaporite deposits.

  4. Microliquids on the Move

    The development of labs on chips will benefit from a better understanding of the behavior of liquids in microchannels and from new methods of pumping fluids through microchannels (see the Perspective by Grunze). Surfaces patterned with hydrophobic and hydrophilic regions can be used as microscopic channels for liquids. Gau et al. (p. 46) investigated the stability of aqueous structures in such microchannels. Beyond a certain adsorbed volume, the liquid structures filling the channels become unstable and change from a state with a spatially constant cross section to one with a single bulge. These instabilities can be pinned at corners in the channel, and used to form interconnections between the channels. Many approaches for moving fluids through channels, such as electrokinetic pumping, require high electric fields. Gallardo et al. (p. 57) show that fields of less than 1 volt can be used to pump fluids and create droplet patterns on nonpatterned surfaces. Electrochemical reactions convert an aqueous compound from a surfactant to a non-surfactant species. Concentration gradients of these molecules lead to local differences in surface tension that can push thin layers of fluids along channels and through T connections or that can selectively wet or dewet electrode surfaces.

  5. Valuing Diversity

    Developing CD4 T cells (thymocytes) are equipped with T cell antigen receptors (TCRs) specific to each T cell, but there is an enormous diversity of specificities over the population of thymocytes. How do the “right” T cells get selected to mature fully? Barton and Rudensky (p. 67) evaluated mice that were engineered to have 95% of their thymocytes expressing the same peptide for binding to the major histocompatibility complex (MHC). These mice had normal numbers of mature CD4 T cells, so it appeared that a single peptide could select millions of T cells. However, if the 5% of peptides that were different from the major peptide-MHC complex were not present, the number of mature CD4 T cells dropped. Thus, a diversity of low-abundance peptides is necessary to get a normal population of mature CD4 T cells.

  6. Neuronal Receptor Structure

    The gene encoding a receptor for γ-aminobutyric acid (GABA) that is important in fine-tuning of synaptic transmission has recently been identified. This gene encodes a protein known as GBR1 (GABAB receptor 1) that possesses several of the functional properties of GABAB receptors. Kuner et al. (p. 74; see the news story by Wickelgren) have now identified a second receptor subunit, GBR2, that forms a heteromeric receptor with GBR1. The heteromeric receptor can interact with each of the known signal transduction pathways involved in physiological GABAB receptor function.

  7. Learning the Rules

    Is language acquired by training a neural network through trial and error, or are there abstract rules applied to determine what constitutes correct usage? It has been shown that infants can learn transitional probabilities of syllables, that is, how likely “re” will follow “do.” Marcus et al. (p. 77; see the Perspective by Pinker) present results indicating that infants can also learn rules, for instance, that “do-re-re” falls into the same pattern as “mi-fa-fa,” suggesting that language acquisition may rely on both types of capabilities.

  8. Fibroblasts and Wound Healing

    DNA microarray technology has made it possible to reexamine old questions with a new dimension of genomic information. Iyer et al. (p. 83; see the news story by Pennisi) have used a complementary DNA microarray system to look at temporal changes in gene expression when quiescent human fibroblasts are exposed to serum. They find clustering of genes that suggests clues to function and a possible recapitulation of many of the steps of wound repair.

  9. Drug Production in Vivo

    Gene therapy will be most useful if the expression of the therapeutic proteins can be controlled and regulated. Ye et al. (p. 88) used an adeno-associated virus system in which erythropoietin expression is under the control of two chimeric proteins that are reconstituted in vivo into an inducible transcription factor complex. Stable, expression was observed in immune-competent mice for 6 months and in rhesus monkeys for 3 months.

  10. Notch on the Move

    Throughout development, the Notch signaling pathway functions in directing cell fate, patterning, and morphogenesis. Previous work indicated that the Notch cell surface receptor is activated by direct cell contact with a ligand-expressing cell, with Notch forming a complex with transmembrane ligands such as Delta and Serrate. Qi et al. (p. 91) used biochemical and genetic analyses to show that the metalloprotease Kuzbanian cleaves the Notch ligand Delta. This work is contrary to previous models showing that Notch ligands act exclusively as membrane-bound molecules. Instead, the ligand can be found as an active diffusable ligand, possibly permitting Notch signaling that is not restricted to adjacent cells.

  11. Passing RNA Between Plant Cells

    Certain viruses spread through their infected host plants with the aid of virally encoded proteins that shepherd nucleic acids from one cell to the next through the plasmodesmata, channels connecting neighboring cells. Xoconostle-Cázares et al. (p. 94; see the cover and the news story by Strauss) show that the pumpkin plant encodes its own protein with similarities to these viral movement proteins. Thus, the marked mobility of viral components between cells may be a reflection of physiological processes already present, but perhaps regulated with more discrimination, in the normal plant. These processes may allow RNA molecules to be carried far from the cell that synthesized them.

  12. Nanotube Waves

    In carbon nanotubes, conduction electrons are confined such that they can only travel in one dimension, along the tube axis. As a result, energy level quantization has been observed for nanotubes of limited length, but further detailed experimental characterization is needed to fully understand their electronic properties. Venema et al. p. 52 have used scanning tunneling microscopy to map the wave functions of single molecular orbitals in a carbon nanotube. Discrete electronic wave functions have a different periodicity than the underlying atomic lattice constant, and the measured wavelengths are in good agreement with theoretical predictions.

  13. Rethinking Thermal Motion in NMR

    Fewer peaks may be seen for a molecule in nuclear magnetic resonance (NMR) spectroscopy as temperature is increased because thermal motion “averages” the chemical environment of spins. Most calculations that try to relate the distinct peaks of low-temperature measurements to the averaged peaks at higher temperatures usually average over the spatial movement with the assumption that this procedure has no effect on the spin states. Mueller and Weitekamp p.61 now show that dynamical terms relating spin and spatial degrees of freedom must be included to account for their measurements of equilibrium constants based on carbon-13 NMR spectra of methylcyclohexane.

  14. Matched Parts for Assembly

    Electrostatic interactions have been used to guide the supramolecular assembly of polymers in solution. Harada and Kataoka p.65 synthesized block copolymers that had a polyethylene glycol block and either a polycation (polylysine) or polyanion (polyaspartate) block of 18 or 78 repeat units. When one of these polymers was mixed with both types of oppositely charged polymers, they selectively formed pairs with the type that had the repeat unit of the same length and then formed large aggregates with very narrow molecular weight distributions. When the polymers were forced to mix with unmatched repeat units, they formed 4:1 complexes that did not assemble further.

  15. Walking a Tightrope

    During early development of the brain, neurons undergo a phase of hypersensitivity toward glutamate, the major excitatory neurotransmitter in the central nervous system. This hypersensitivity has been well established to be mediated by N-methyl-D-aspartate (NMDA) receptors. Thus, while too much NMDA receptor excitation can have an adverse effect on neurons, Ikonomidou et al. p.70 now show that the opposite can also be true. Blockade of NMDA receptors, even transiently, can trigger waves of apoptotic neurodegeneration in the developing brain. There appears to be a critical window for NMDA receptor activation for normal development.

  16. Packaging the HIV Genome

    Viruses display a remarkable propensity for assembling regular geometric structures from hundreds or even thousands of identical subunits. Within the human immunodeficiency virus (HIV), the RNA genome lies packaged inside a pine cone-shaped structure made of two proteins called CA and NC. Ganser et al. p.80 describe how geometrical considerations dictate that a total of 12 pentagons are needed to convert a tube composed of hexagons into a structure closed at both ends. They go on to show that the five possible pine cone-like shapes are all observed when a CA-NC fusion protein and genomic RNA are incubated in vitro. Furthermore, the predominant shape from reassembly matches what is seen in an intact virus.

  17. Gene Targeting in Human Cells Without Isogenic DNA

    “Because gene targeting in human somatic cells is rapidly gaining acceptance,” J. M. Sedivy et al. “have compiled gene targeting data available to date” to assess whether the procedure's success depends on the use of isogenic DNA, which would be a significant technical impediment. They found “numerous examples of high-efficiency gene targeting using nonisogenic DNA,” and “thus envision the rapid emergence of a library of tested and optimized gene targeting vectors that will be available for widespread gene analysis in the large number of human experimental cell systems. “The full text of this comment can be seen at www.sciencemag.org/cgi/content/full/283/5398/9a

  18. Endocranial Capacity of Early Hominids

    G. C. Conroy et al. (Reports, 12 June, p. 1730) used transaxial computed tomography (CT) scans to measure the endocranial capacity of an early hominid skull (Stw 505) from Sterkfontein, South Africa, tenatively assigned Australopithecus africanus. The capacity was estimated to be about 515 cubic centimeters, “markedly less than anecdotal reports of endocranial capacity exceeding 600 cubic centimeters.”

    C. A. Lockwood and W. H. Kimbel comment that not all “sources of postmortem distortion have been taken into account” and that the reported measure underestimates the actual capacity “perhaps by as much as 10 to 15%.” J. Hawks and M. H. Wolpoff estimate endocranial volume “by stepwise multiple regression” with the use of “seven linear measurements” to arrive at a capacity of 598 cubic centimeters.

    In response, Conroy et al. discuss how they accounted for “the obvious displacement of the left parietal-temporal bones” in their study. They state that several of the seven linear measurements used by Hawks and Wolpoff may well be “values that have been overestimated in the paloeanthropological literature and are themselves in need of reassessment” (see related Letters to the Editor, p. 34). The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/283/5398/9b

Navigate This Article