Editors' Choice

Science  18 Aug 2006:
Vol. 313, Issue 5789, pp. 892


    1. Stella M. Hurtley

    The uptake of double-stranded RNA (dsRNA) from the medium is the mainstay of many an RNA silencing strategy, but what is the mechanism by which animal cells take up these macromolecules? It has been difficult to address this directly because in some cases, cells seem to take up dsRNA directly from the medium, yet in others there can be cell-to-cell transfer.

    Because Drosophila cells can take up dsRNA but do not transport it between cells, Saleh et al. used Drosophila tissue culture cells to characterize the uptake pathway. In a genome-wide screen for participants, components of the receptor-mediated endocytosis pathway were found to predominate. The receptors involved were members of the pattern-recognition receptor family, which is important in innate immunity and antimicrobial defense. Furthermore, similar mechanisms are likely to be widespread in evolution: Knockdown of orthologous endocytic players in nematodes also prevented RNA interference. How incoming dsRNA is diverted from the endocytic pathway so as to avoid degradation in lysosomes remains a mystery. — SMH

    Nat. Cell Biol. 8, 793 (2006).


    One In, One Out

    1. Ian S. Osborne

    The successful development of optical-based quantum information processing and quantum cryptography will require the ability to store and retrieve known numbers of photons in a medium of choice. Despite significant progress in techniques to store single photons within a cloud of rubidium or cesium atoms, the overall efficiency of the storage and retrieval process in such systems has been limited by low retrieval efficiencies and relatively high noise levels. Laurat et al. show that the retrieval efficiency of single excitations stored in an ensemble of cold cesium atoms can be increased by careful optimization of the experimental parameters. The authors found that by increasing the number of photons in each read pulse to approximately 107 and increasing the optical depth of the atomic ensemble, they could raise retrieval efficiency to ∼50%, with a concurrent order-of-magnitude reduction in two-photon emission events. They argue that such an improvement bodes well for long-distance quantum communication. — ISO

    Opt. Express 14, 6912 (2006).


    Relaxing Toward Rotaxanes

    1. Jake Yeston

    Traditional approaches to the chemical synthesis of complex molecular topologies, such as knots and interlocked rings, have focused on reducing kinetic barriers. More recently, an alternative approach has relied on the reversible assembly of comparatively simple building blocks that relax eventually into the desired topological conformation because it is the most favorable thermodynamic arrangement. Northrop et al. apply this second strategy to the preparation of [4]pseudorotaxanes, in which a Y-shaped core bears a ring on each of its three axes, and the rings in turn are linked to one another through either one or two central capping groups parallel to the planar core.

    The rings in this case are crown ether derivatives attracted to the core axes through hydrogen bonding to cationic ammonium groups. Capping is achieved by reversible imine bond formation between formyl groups appended to the ends of the rings and amine groups on the phenyl cap. The singly capped complex assembled within 2 hours of mixing the components in solution, whereas the doubly capped analog (in which the caps straddled the core) required 8 days to wend through assorted kinetic intermediates. Both complexes were characterized by nuclear magnetic resonance and mass spectrometry. — JSY

    Org. Lett. 8, 10.1021/ol061262u (2006).


    Pattern Formation in Mosquitos

    1. Stephen J. Simpson

    Like the innate immune systems of vertebrates, those of the insect world possess pattern recognition receptors that detect the broad signatures displayed by different classes of pathogens. In contrast, the narrow immune receptor specificity afforded by mechanisms of genetic recombination has been considered a feature unique to adaptive immunity in higher vertebrates. This view has recently undergone some revision, however, with the observation that lower vertebrates and invertebrates are also adept at manufacturing diverse immune receptors. For example, Drosophila use alternative splicing of transcripts from an immunoglobulin domain-containing locus—the Down syndrome cell adhesion molecule gene Dscam—to generate recognition receptors that assist in the phagocytosis of bacteria.

    Dong et al. observe that in the mosquito Anopheles gambiae (the vector for malaria), the large number of exons in AgDscam could yield as many as 31,000 alternatively spliced products, a range similar to that calculated for Drosophila. Challenging mosquito cell lines with different pathogens resulted in a varied representation of these exons via alternative splicing and AgDscam molecules with distinct specificities. Evidence for alternative splicing of AgDscam was also demonstrated in adult mosquitos, and RNA interference-mediated silencing decreased the resistance of mosquitos to bacterial infection and to oocytes of the malaria parasite carried in the insect midgut. As in Drosophila, the AgDscam forms appeared to enhance phagocytosis of bacteria by hemocytes, although it is likely that the mechanism of Dscam action extends to other modes of immune defense. A further series of experiments revealed that the repertoires of AgDscam molecules could be tailored, in terms of binding affinity, to the infecting pathogens, underscoring the degree to which specificity provided by the Dscam system might help refine pathogen pattern recognition in insects. — SJS

    PLoS Biol. 4, e229 (2006).


    One of Everything

    1. Gilbert J. Chin

    Recent molecular analyses of marine microbes (see, for example, DeLong et al. Reports, 27 January 2006, p. 496) have documented how the environmental pressures of living in the ocean at depths down to several kilometers are reflected in the corresponding genomic complements. Derelle et al. provide the genome sequence of Ostreococcus tauri, a green alga of extraordinarily small size (about 1 μm in diameter) and remarkably high gene density. This picoeukaryote achieves the feat of packing over 8000 genes into less than 13 Mb by making the average gene just slightly longer than 1.2 kb and reducing the intergene distance to 0.2 kb. Nevertheless, it still contains entire plantlike metabolic pathways, such as the enzymes for C4 photosynthesis (an evolutionary adaptation to low CO2 levels) and for storing glucose as one large starch granule within the single chloroplast. Also appearing in only one copy each are the mitochondrion, a Golgi body, and the nuclear pore, which presumably reflect the physical advantages of small intracellular distances and a high surface-to-volume ratio. — GJC

    Proc. Natl. Acad. Sci. U.S.A. 103, 11647 (2006).


    Advantages of Neutrality

    1. Phil D. Szuromi

    Electron beam lithography, often used to pattern the smallest features on semiconducting silicon substrates, can also modify insulating substrates. However, at typical beam energies, the insulating surface builds up negative charge that deflects the beam and so distorts the desired pattern. Several approaches have been developed to overcome this problem, but they require additional sample processing steps or complex gas-handling and vacuum equipment. Joo et al. note that at lower energies, electron beams can instead induce positive charging of insulating surfaces; therefore, a critical energy exists for which the surface will remain neutral. For 65-nm-thick poly(methyl methacrylate) films on glass, they determine a critical energy value of 1.3 keV. By tuning the incident beam to this energy, they successfully create features finer than 100 nm on this substrate. A 5-keV beam, in contrast, produces distortions that are clearly evident in scanning electron micrographs. — PDS

    Nano Lett. 6, 10.1021/nl061211q (2006).

  7. STKE

    Moving PIP3 About

    1. Nancy Gough

    Phosphatidylinositol-(3,4,5)-phosphate (PIP3), the product of phosphatidylinositol 3-kinase (PI3K), is important in the establishment of cell polarity. Horiguchi et al. provide evidence that PIP3 is produced not only at the plasma membrane by local activation of PI3K, but also at internal membranes that are then transported as PIP3-containing vesicles on microtubules to the growing tips of neuronal projections. First, they determined that GAKIN (guanylate kinase-associated kinesin) interacted with PIP3 binding protein (PIP3 BP); in vitro, GAKIN and PIP3 BP mediated the movement of PIP3 liposomes on microtubules. In PC12 cells and in cultured hippocampal neurons, tagged GAKIN, tagged PIP3 BP, and a marker for PIP3 were colocalized at the tips of neurites, and in hippocampal cells, these three molecules were most abundant in the longest neurite, the axon. Overexpression of a dominant-negative form of GAKIN (with the kinesin motor domain deleted) in PC12 cells decreased the abundance of PIP3 at neurite tips. In hippocampal neurons, overexpression of wild-type GAKIN or dominant-negative GAKIN disrupted the formation of the morphologically distinct axon-dendrite structure and produced cells with multiple, highly branched neurites. The authors suggest that PIP3 produced at internal membranes or PIP3 produced at the cell body may contribute to cell polarity. — NRG

    J. Cell Biol. 174, 425 (2006).