This Week in Science

Science  04 Feb 2000:
Vol. 287, Issue 5454, pp. 765
  1. Stress in the West

    The coast of California is dominated by strike-slip faults from the shearing of the Pacific Plate past the North American Plate. Further east in Nevada, the Basin and Range is characterized by extensional structures, and other large regions of the western United States show no significant crustal deformation. Flesch et al. (p. 834; see the Perspective by Houseman) have developed a self-consistent model of the lithosphere of the western United States that explains these different stress regimes. Their model indicates that plate boundary forces and buoyancy forces contribute to the style of deformation across the west. From their model, they can estimate the stronger regions of the crust, such as the Basin and Range, from the weaker regions of the crust, such as the coast of California.

  2. Better in the Stretch

    Small devices that can convert electrical energy into mechanical motion are desirable for applications ranging from acoustic speakers to miniature robots. Although more exotic materials such as piezoelectric ceramics, carbon nanotubes, and conducting polymers have been used in such actuators, ordinary polymers with a simple dielectric response to an applied electric field can produce large motions (strains) if sandwiched as thin films between compliant electrodes that can change shape. Pelrine et al. (p. 836; see the news story by Cho) now show that actuators containing certain commercial silicone or polyurethane polymers that were prestretched can expand by a factor of 2 or greater. The response is fast, and these actuators can transfer large amounts of mechanical power relative to other actuators.

  3. Cooler Equatorially

    One of the most controversial issues in paleoclimatology is how much cooler sea surface and continental temperatures were during the last glacial maximum (LGM) compared to the present, especially at low latitudes. Weyhenmeyer et al. (p. 842) have looked at the noble gas composition of groundwaters from northern Oman and found that the mean annual temperature there was 6.5°C cooler 15,000 to 20,000 years ago. They also found from the relation between the isotopic compositions of the hydrogen and oxygen in water that the dominant source of moisture at that time was the Indian Ocean, unlike present conditions where the Mediterranean is dominant. These results lend strong support to the notion of widespread and significantly cooler temperatures at low latitudes during the LGM, and suggest that important changes in atmospheric circulation patterns have occurred since then.

  4. Secretion on Demand

    Specialized secretory cells regulate the release of important bioregulatory molecules such as insulin and ensure their availability at appropriate times. Rivera et al. (p. 826; see the Perspective by Aridor and Balch) demonstrate that a non-secretory cell can behave like a secretory cell if the protein of interest is engineered to accumulate within its secretory pathway. Insulin and growth hormone were linked to domains designed to aggregate. The resulting protein aggregates that formed were too large to transit through the cell's endoplasmic reticulum, but secretion was efficiently induced with a cell-permeant small molecule drug that disrupted the aggregates. Drug administration permitted the release of unmodified protein, as demonstrated in a mouse model for insulin release. This strategy holds potential therapeutic value in that secretion of physiologically important molecules may be controlled directly.

  5. Measuring a Beeline

    How do bees measure the distance flown between the hive and food source and then communicate that information to their sisters by their dances? By comparing waggle dances performed after flying through a narrow tunnel with those performed after flying freely outside, Srinivasan et al. (p. 851; see the cover and the Perspective by Collett) show that flying through a tunnel is equivalent—in the bees' perception—to flying much longer distances. Because the width of the tunnel is known, it has been possible to measure the relation between image movement and the waggle dance and show that optic flow is an important component of distance measurement.

  6. Bigger Dipper Populations?

    A central question in ecology is the relative contribution of density-dependent effects and stochastic variation to population fluctuations, and a major contemporary concern is to predict how organisms will react to an expected global warming. Sæther et al. (p. 854; see the news story by Wuethrich) address both these questions in a long-term study of a small temperate songbird, the dipper, in Norway. By using new methods of estimating and modeling stochasticities in population models, they demonstrate quantitatively that global warming will strongly influence the population dynamics of the dipper. In particular, they show that warming should have the effect of increasing average population density by reducing the frequency of environmental events that limit population size.

  7. Reassessing Marine Dispersal

    Are there barriers to dispersal of larval organisms in the sea? It has commonly been assumed that larvae are widely dispersed between oceanic islands, and that therefore ecological communities are “open” or connected with one another. Using a modeling approach, Cowen et al. (p. 857) show that such connectivity—at least in the Caribbean islands—has probably been overestimated a billionfold. It appears, instead, that populations are very largely sustained by local reproduction and larval retention. These findings are likely to have direct impact on conservation and management strategy for coastal marine habitats: If local retention is the rule, then local management initiatives might have a greater chance of producing local results.

  8. Manipulating T Cell Types

    T cells can help the immune response generate either a cell-mediated (type-1) or an allergic (type-2) response. The cytokines that heavily influence one pathway or another are interleukin-12 (IL-12) and IL-10, respectively. Ashkar et al. (p. 860) now find that the cytokine Eta-1 (also known as osteopondin) is critical for directing the increased production of IL-12, through binding to an integrin, and the decreased production of IL-10, through binding to CD44. Through the use of mice genetically deficient in Eta-1, IL-12, or IL-10, the authors found that Eta-1 was necessary to protect mice from Listeria infections and to aid in granuloma formation. The effects of Eta-1 on the IL-12:IL-10 ratio provide another possible therapeutic target for manipulation of the immune response.

  9. Almost Assembling the Brain

    The development of an organ as complex as the brain depends on a sequence of precisely controlled events. Is synaptic communication between neurons a requirement for the proper assembly of the brain? Verhage et al. (p. 864) describe a new mouse mutant, with a deletion of the munc 18-1 gene, where neurotransmitter secretion is completely blocked. The brains of these mice are nevertheless normally assembled, major pathways are present, and structural elements of synapses form. Neurotransmission seems thus not to be necessary for synapses to form and connections to be established initially. At later stages, however, neurons die by apoptosis, tremendous cell loss occurs, and brain integrity is compromised.

  10. Following the Effects of Yeast Pheromone

    Pheromone communication between yeast cells induces arrest in the G1 phase of the cell cycle and causes mating projections to form. Roberts et al. (p. 873) used DNA microarray analysis to observe global patterns of gene expression in yeast cells associated with alterations in signaling through the pheromone response pathway. Pheromone receptors activate a mitogen-activated protein (MAP) kinase pathway that leads to alterations in gene expression. The results show the global patterns of regulation mediated by this pathway and related pathways that use MAP kinase signaling modules for other biological responses. They found that partial activation of the pheromone response pathway produces a gene expression pattern reminiscent of that in cells undergoing haploid invasive growth. Such regulation was previously thought to be under the contol of a distinct MAP kinase pathway. Data from 383 highly regulated genes over 46 sets of experimental conditions begin to show the predictive power of such large data sets when presented as a two-dimensional hierarchical matrix.

  11. Nucleic Acid Partners

    The character of specific interactions of macromolecules is central to biochemistry. Hermann and Patel (p. 820) review structural studies of ligand-nucleic acid complexes, where the ligands include amino acids, small aromatic molecules, and oligosaccharides. Nucleic acids function primarily as carriers of information, encoding the DNA sequences of genes, and also as purveyors of that information in the personages of messenger, transfer, and ribosomal RNAs. More recent work, however, has revealed the importance of interaction specificity of nucleic acids in accomplishing these functions and for realizing their potential in biotechnology.

  12. Redfield Ratio Trends

    If any quantity could be called the Holy Grail of biogeochemical oceanography, it is probably the relative proportions of the elements carbon, nitrogen, and phosphorus (known as the Redfield ratio) that marine organisms use to make soft tissue. The Redfield ratio has varied over geological time scales but generally is believed to be temporally constant at any given location in modern deep water. Pahlow and Riebesell (p. 831) have reexamined deep-water nutrient concentration measurements made throughout the world ocean during the past 50 years, and concluded that the Redfield ratio has changed during this period in the Northern Hemisphere. They suggest that this could be due to intensified export production there, possibly linked to enhanced bioavailability of aeolian iron in a marine carbon cycle that is not operating at steady state.

  13. Chained Together

    The electronic excitations of conjugated polymers, such as those used in organic light-emitting diodes, are normally thought to move up and down the same chain. Österbacka et al. (p. 839) present spectroscopic evidence, which shows that such one-dimensional excitations are the norm in isolated or disordered chains, but that hopping between chains occurs in highly ordered nanostructures of polymers that are regioregular (that is, having the same head-to-tail arrangement throughout the chain). Thus, in highly regular, conjugated structures, the excitations move in two dimensions rather than in one.

  14. Mitochondrial Tumor Suppressor

    Hereditary paraganglioma (PGL) is a rare disorder characterized by the development of benign, vascularized tumors in the head and neck. The most common tumor site is the carotid body (CB), a small organ in the carotid artery that senses and responds to oxygen levels in the blood. Baysal et al. (p. 848) show that individuals with PGL have germ line mutations in the SDHD gene and their tumors show selective loss of the wild-type allele. The SDHD gene is nuclear, but it encodes a mitochondrial respiratory chain protein—the small subunit of cytochrome b in succinate-ubiquinone oxidoreductase. This finding indicates that mitochondria play an important role in the pathogenesis of certain tumors, and it may lead to a better understanding of normal CB physiology.

  15. Multiple Infections in Malaria

    In regions of the world where malaria is prevalent, many people are infected from birth with more than one species or genotype of Plasmodium. Interactions between these different organisms could have important consequences for treatments and vaccine development. Bruce et al. (p. 845) examined 34 children from Papua New Guinea who were infected but asymptomatic—most of them harbored multiple Plasmodium infections. Those children with an increased number of positive smear tests tended to have a greater number of infecting species or genotypes, and the children exhibiting the highest proportion of positive smear tests exhibited a relatively constant parasite density despite underlying changes in the density of individual species. This evidence for density-dependent regulation of population suggests that vaccines targeted against only one species, such as P. falciparum, may simply lead to increased densities in infected individuals of other Plasmodium species.

  16. Ras Proteins and I

    The IκB proteins are inhibitors of the NF-κB transcription factor, which has major functions in gene regulation in response to inflammatory stimuli of the immune system. In this system, transcription is controlled though regulated proteolysis of the IκB inhibitory proteins. In screening proteins that interact with IκB, Fenwick et al. (p. 864) detected a protein they call κB-Ras1 because of its similarity to the small guanosine triphosphatase Ras. The κB-Ras1 protein is unusual in that it has mutations similar to those found in oncogenic mutants of Ras. The κB-Ras1 and IκB proteins were shown to interact in intact cells. Overexpression of κB-Ras1 decreased degradation of IκB proteins in cells and consequently inhibited NF-κB-dependent transcription. The κB-Ras1 protein appears to associate specifically with complexes of NF-κB with IκBβ, which may explain why IκBβ is degraded more slowly than is IκBβ.

  17. Photoemission Experiments and Pseudogaps

    Joynt (Reports, 30 Apr., p. 777) argued that the “pseudogaps” observed in energy spectra from angle-resolved photoemission (ARPES) experiments may, for very low conductivity materials such as certain colossal magnetoresistance (CMR) manganates, stem not from intrinsic material properties but from ohmic energy losses after electron ejection. In a comment, Dessau and Saitoh, noting that these extrinsic losses should be momentum- or angle-independent, cite data from the layered CMR material La1.2Sr1.8Mn2O7 that show strongly anisotropic pseudogap behavior in momentum space. This anisotropy, they suggest, points to an intrinsic origin for the pseudogap. Because this CMR system's resistivity is three orders of magnitude larger than the threshold at which Joynt expected extrinsic losses to become relevant, Dessau and Saitoh conclude that such effects may not be a major experimental concern for conductivities of that order or better.

    Joynt responds that, while Dessau and Saitoh are correct for the system they studied, extending conclusions from that system to all poorly conducting solids may be misleading, in part because the loss function does not depend solely on conductivity. Accurately assessing pseudogaps in poorly conducting solids, Joynt maintains, requires “both optical and photoemission measurements, followed by a careful consideration of the characteristic frequencies” for the system in question. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/287/5454/767a

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