This Week in Science

Science  06 Aug 2004:
Vol. 305, Issue 5685, pp. 749
  1. Metamaterials in Review

    CREDIT: SMITH ET AL.

    The ability to design structured materials, or metamaterials, with desired magnetic and electric response properties not found naturally offers the possibility of exploiting new effects—some of which are counterintuitive. Smith et al. (p. 788) review recent theoretical and experimental developments of these metamaterials. Tuning of the magnetic and electric response can produce materials with artificial magnetism or an effective negative index of refraction. The latter materials are of interest in the development of “perfect lenses.”

  2. A Structured Route to Light Control

    The control of light on subwavelength length scales is an important goal for incorporating optics into nanotechnology. Plasmons, which are excitations on metal surfaces that arise from the interaction between the impinging light and the electrons in the metal, are a possible route for controlling and manipulating the light indirectly. However, finding materials with the right properties to create surface plasmon interactions is difficult. Pendry et al. (p. 847, published online 8 July 2004; see the Perspective by Barnes and Sambles) present a theoretical description of surface plasmons in conventional metals and in structured surfaces, and shows that there is a common link between the two. The ability to design structured surfaces should enable materials with the desired plasmonic properties to be fabricated quite readily.

  3. Stable Silicon Cation

    The highly reactive HSi+ species, which has one lone electron pair and two vacant orbitals on the silicon atom, is known from gas-phase studies and in solar spectra. Jutzi et al. (p. 849, published online 1 July 2004; see the Perspective by Bertrand) now report the synthesis of a “masked” silyliumylidene cation that is stable in the presence of air and moisture. The reaction of decamethylsilicocene, (Me5C5)2Si, where Me is a methyl group with a proton-transfer reagent, yields the salt (Me5C5)Si+B(C6F5)4. This compound opens up a variety of synthetic possibilities in silicon chemistry, and the authors have used it to synthesize a disilene compound.

  4. No Common Pharmacore

    CREDIT: NETTLES ET AL.

    Taxol has shown great success as an anticancer drug, and there is great interest in other microtubule-stabilizing drugs, such as epothilone, that may have enhanced pharmaceutical properties and overcome problems of drug resistance to taxol. Now Nettles et al. (p. 866), have used a combination of electron crystallography and nuclear magnetic resonance-based conformational analysis to determine the structure of epothilone A bound to zinc-stabilized tubulin sheets. Instead of the expected “common pharmacore” for epothilone and taxol, the two ligands show little overlap in their interaction with the tubulin binding pocket. This result explains the activity profile of epothilone derivatives and the different acquired-resistance mutations observed for taxol and epothilone.

  5. Growth and Decay

    The ratio of the activity of 234U to that of its precursor, 238U, varies by a factor of 3 in rivers, for reasons that are poorly understood, but this ratio is a commonly used quality-control indicator for coral dating. What determines its value in seawater, and how that value has varied over geological time, are questions that need to be answered if it is to be used as a geochemical proxy. Robinson et al. (p. 851) report results from New Zealand, where rainfall and weathering patterns vary systematically, and conclude that glaciers advanced there about 13,000 years ago because of rainfall increases rather than cooling. They also find that the 234U/238U ratio varies enough in seawater that the range of its values in corals commonly used to judge whether they have been contaminated by uranium in seawater is too restrictive.

  6. Room and Board

    Cowbirds, cuckoos, and other bird-brood parasites are not raised by their own parents but by a host bird. In some species, the parasitic nestlings do not always kill their host's offspring but sometimes tolerate their presence in the nest. Using brown-headed cowbirds, Kilner et al. (p. 877) tested whether brood parasites can derive any benefit from sharing the nest. Paradoxically, the cowbirds extract more resources from their foster parents when they restrained their selfishness toward their nestmates—the surviving host young actually assist the cowbird in soliciting a greater provisioning rate. Cowbirds reared with host young thus grow more quickly and attain a greater weight before fledging than those reared alone.

  7. Programming Cell Death in Plants

    Programmed cell death serves many useful functions—it helps in remodeling tissues during development and when organisms respond to environmental challenges. In virally infected plants, infected cells are killed by programmed cell death during the hypersensitive response. However, the signature caspases that promote programmed cell death found in animals have not been found in plants. Hatsugai et al. (p. 855) now identify VPE, a vacuolar processing enzyme, as a critical component of programmed cell death in tobacco plants. It seems that each plant cell, confined within its rigid cell wall, contains within its own vacuole the seeds of its own destruction.

  8. Held at Arm's Length

    Mitochondria, the membrane-bound powerhouses of eukaryotic cells, continually undergo a process of fusion and fission. Koshiba et al. (p. 858) report the crystal structure of a portion of mitofusin, a mitochondrial outer membrane protein required for fusion. The C-terminal domain of mitofusin points outward and forms an anti parallel coiled coil with the corresponding domain of another mitofusin molecule. The authors suggest that mitofusin mediates the initial stage of fusion promoting a long-range tethering of two mitochondria at a distance of about 10 nanometers.

  9. A Rare Find for a Common Trait

    Many common traits and diseases in the general population arise from a combination of environmental and genetic factors. Current models favor the hypothesis that the genetic component reflects the cumulative contribution of many common DNA sequence variants, each of which has a small effect. Cohen et al. (p. 869) suggest that common traits can also arise from rare sequence variants with strong phenotypic effects. Studying individuals in the general population with exceptionally high or low plasma levels of high-density lipoprotein-cholesterol (HDL-C, or “good” cholesterol), they found that the “lows” were much more likely than the “highs” to carry deleterious mutations in the genes that cause extreme, familial forms of HDL-C deficiency.

  10. Natural Partners

    CREDIT: KHAKOO ET AL.

    Natural killer (NK) cells play an important role in clearing virally infected cells; an activity that is tightly regulated through a balance of signals transmitted by activation and inhibitory receptors on the NK cell surface. Haplotypes containing different combinations of NK receptor genes exist, and considerable variation exists between individuals carrying appropriate ligands for a given NK receptor. Thus, NK-dependent immunity may vary considerably within a population. Khakoo et al. (p. 872; see the Perspective by Parham) observe that a specific combination of killer cell immunoglobulin-like receptor (KIRs) and major histocompatibility complex (MHC) ligands associated strongly with the ability to spontaneously resolve infection with hepatitis C virus. The effect was probably the result of inefficient NK inhibition by this receptor-ligand combination, promoting NK activation.

  11. Whose Hand Is This?

    When people watch brush strokes on a rubber hand while simultaneously having their own (but out of sight) hand brushed, they feel that the rubber hand belongs to them. What brain circuits underlie this well-known “rubber hand” illusion? Using magnetic resonance brain imaging, Ehrsson et al. (p. 875, published online 1 July 2004; see the Perspective by Botvinick) identified several regions associated with this illusion, including premotor, intraparietal, and cerebellar areas. Only the premotor area correlated with the intensity of the illusion and showed a time course consistent with the evolution of the illusion. The conscious experience of body-part ownership can thus be attributed to this premotor area, whereas the other areas are probably involved in multimodal integration of sensory and proprioceptive signals.

  12. Repulsive Signaling

    Plexins are cell surface receptors for the semaphorin family of guidance molecules. Semaphorins were originally identified as repulsive axonal guidance molecules in the developing nervous system, but are also implicated in morphogenesis, angiogenesis, immune response, and tumor metastasis in a variety of tissues. Oinuma et al. (p. 862) describe a signal transduction pathway mediated by a semaphorin, the transmembrane receptor Plexin-B1. Plexin-B1 acts as a guanosine triphosphatase (GTPase)-activating protein toward the Ras family small GTPase R-Ras, a key regulator of cell adhesion, to promote repulsive signaling by Plexin-B1.

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