This Week in Science

Science  03 Aug 2007:
Vol. 317, Issue 5838, pp. 568
  1. Stress and Wrinkles


    Under most circumstances, when a stiff thin film is placed under either a compressive or tensile stress, the film buckles, rather than stretches or shrinks. Huang et al. (p. 650; see the Perspective by Miller) placed a drop of water on a free-floating polymer film and show that the stresses caused by the surface tension between the droplet and the film cause the film to wrinkle. The periodic wrinkling pattern that formed confirms recent theoretical results, and the authors could relate the wrinkling characteristic features to the elasticity and thickness of the film. This system provides a simple, bench-top method for the study of the viscoelastic response of thin films.

  2. Quantized Transport in Graphene Sheets

    The manipulation of the carrier density and carrier sign over an entire graphene sheet (a single layer of graphite) by electrostatic gating has been demonstrated. Efforts are now being directed to local gating so that transport properties of different areas can be simultaneously manipulated in order to make device-like structures with advanced functionality. Williams et al. (p. 638, published online 28 June) show experimentally that a global bottom gate combined with a patterned top gate provides the ability to controllably form bipolar p-n junctions in the graphene layer (as well as unipolar p-p and n-n junctions). Subsequent magnetotransport measurements revealed quantum Hall effect behavior, the details of which depend on whether the junctions are bipolar or unipolar. Abanin and Levitov (p. 641, published online 28 June) theoretically explain these results in terms of quantum Hall edge-state mixing at the interface region.

  3. An Arc Anchoring Saturn's G Ring

    Saturn's G ring is a faint and narrow ring lying beyond the main set of rings. Despite being well defined, it has been unclear why it exists at this precise location; in particular, it is not flanked by moons that might shepherd it and carve it out or infuse it with vapor. Hedman et al. (p. 653), using the Imaging Science Subsystem aboard the Cassini spacecraft, show that the G ring contains a bright arc of material made up of centimeter- to meter-sized icy bodies held in a 7:6 corotation resonance with the major moon Mimas. Dust from this concentration of particles trails out to enscribe the ring.

  4. Manipulating Micelle Formation


    In solution, block copolymers can form a variety of thermodynamically controlled structures, including micelles (see the Perspective by Hillmyer). Polyferrocenylsilane-based block copolymers have an unusual propensity to form cylindrical micelles that are stable over a range of polymer concentrations. Wang et al. (p. 644) show that the shape and composition of the micelles can be further controlled in a process analogous to the synthesis of polymers through living polymerization. The addition of more polymer to the solution leads to its epitaxial attachment and to the growth of the micelles with narrower size distributions. With the addition of a different block copolymer, comicelles consisting of the two different block copolymers are formed. Cui et al. (p. 647) describe a general strategy for the preparation of complex one-dimensional nanostructures from block copolymers. Unlike classical block copolymer self-assembly, in which the structure is dictated by the lengths of the two polymer blocks, assembly was controlled through electrostatic interactions and solvation. The process involves the formation in water of polymer micelle structures, which flatten upon the rapid addition of tetrahydrofuran. The resulting flattened micelles subsequently grow in an anisotropic fashion to yield unusual architectures. When the diamine additives were replaced with amine-coated gold particles, even more complex structures are obtained.

  5. Genome Transplant

    In the 1970s, it was a revolutionary advance to be able to cut and paste small sequences of DNA and to transfer an individual gene into a cell at will. Lartigue et al. (p. 632, published online 28 June) now describe the replacement of one entire bacterial genome (Mycoplasma capricolum) with another (Mycoplasma mycoides), which allows the production of a colony of cells that produced the protein products encoded by the donor genome.

  6. Match-Making in Plants

    In plants, as in animals, fertilization requires male and female gametes selectively and exclusively to meet and fuse. In plants, unlike in animals, it is not the actual gametes that participate in this dance, but rather the gametophytes, which carry immotile gametes. Escobar-Restrepo et al. (p. 656; see the Perspective by McCormick) have now identified one of the molecules by which the female gametophyte (the embryo sac) recognizes the male gametophyte (the pollen). A receptor kinase, FER, is situated at cell surfaces within the female gametophyte. As the pollen tube approaches, a signaling cascade is initiated in the embryo sac that closes the door to late-arriving pollen tubes and halts further growth of the successful pollen tube, which then releases the male gamete. A good match between FER and its presumed ligand from the pollen tube appears to form the basis of reproductive compatibility.

  7. Insulin Signaling Up-Close and Personal

    Novel quantitative methods are allowing the analysis of gene and protein expression in unprecedented detail. Dong et al. (p. 660, see the Perspective by Kim) profiled protein abundance changes in response to perturbations of insulin and insulin-like growth factor-1 (IGF-1) signaling in Caenorhabditis elegans and identified 86 targets. More than half of the targets identified by this quantitative proteomics approach were not previously identified and may provide new insights into the mechanisms of diabetes and aging.

  8. Monitoring Cellular Mechanical Stress

    Cells exert and respond to mechanical forces, but investigating how these signals are transduced is a challenge. By measuring differential labeling of cysteines in stressed and relaxed cells, Johnson et al. (p. 663) identify proteins that change their structure in response to stress. Using mass spectroscopy, they determined the specific Cys residues that experience structural changes. In red blood cells, spectrin unfolds as cells are stressed, and in mesenchymal stem cells, both myosin IIA and vimentin show differential labeling in tensed versus drug-relaxed cells.

  9. Leukocytes on Border Patrol


    Immune cells move between the circulation system and tissues in response to infection. Upon detection of inflammation, leukocytes initiate a highly orchestrated sequence of events, whereby they attach and roll along on the surface of the endothelium before squeezing through to the underlying tissue. Auffray et al. (p. 666) reveal a quite distinct type of behavior displayed by a population of monocytes that remain attached to the endothelium in the absence of inflammation. The resident cells appeared to survey the surface of postcapillary venules, veins, and arteries, depending on specific chemokine and integrin signals. Upon detection of inflammatory cues, the cells moved into infected sites, where they undergo differentiation into macrophages. Mueller et al. (p. 670) reveal how the down-regulation of chemokines in the lymph node prevents T lymphocytes from entering during an ongoing immune response. This delay may help to optimize the lymph node environment to produce the most effective immune response.

  10. Keeping Tabs on a TLR Response

    Toll-like receptors (TLRs) exert powerful proinflammatory responses to microbial pathogens, and TLR responses are stringently regulated during infection so that the chronic exposure of cells to microbial products can ultimately lead to a state of hyporesponsiveness. Carmody et al. (p. 675) identify an essential role for the proto-oncogene protein B cell leukemia (Bcl)-3 in negatively regulating TLR signaling in this context. Bcl-3 blocks ubiquitination of the nuclear factor-κB subunit p50, which prevents its degradation and allows it to maintain its inhibition of gene transcription in response to TLR signals. This pathway offers a means by which microbial signals can be prevented from overpowering the immune response.

  11. Slimy Cooperative Requires Housekeeper

    Phagocytes are innate immune cells found in animals that specialize in scavenging bacteria and other foreign matter. Chen et al. (p. 678; see the news story by Leslie) show that a similar waste-disposal system helps maintain well-being in slime molds; the aggregates formed by the social amoebae Dictyostelium discoideum. Specialized sentinel cells engulfed bacteria and removed toxins, and were themselves ejected from the colony as it migrated. A protein related to those found in animal innate immune systems was required for this function, as well as in allowing individual amoebae to feed on their diet of bacteria when not part of a colony.