Editors' Choice

Science  19 Sep 2003:
Vol. 301, Issue 5640, pp. 1631

    Leaf Signals

    1. Andrew M. Sugden

    There are many potential explanations for the adaptive significance of autumn colors in the senescing leaves of temperate deciduous trees. One idea is that coloration is a signal to insects about the tree's investment in chemical defenses against herbivore attack. Hagen et al. investigated this possibility over three growing seasons in mountain birch trees in northern Norway. The degree of autumn coloration in September was negatively correlated with the amount of insect damage to the following season's leaf crop, and with physiological indices of stress and reproductive investment. The commonest leaf-chewing insects in the study area are geometrid moths that lay their eggs in the autumn. Though the causal link is yet to be established, the results are consistent with the hypothesis that the insects prefer less discolored leaves. — AMS

    Ecol. Lett. 6, 807 (2003).


    Direct Dating of Pots

    1. Phil D. Szuromi

    Pottery is often dated indirectly by relying on carbon-14 analysis of proximal bone or seed or through dendrochronology. Such materials are not always available, or their own histories are confused, so there is a need to date such materials based on the organic material in or on the surface of the pottery itself. Stott et al. adapted a compound-specific method that isolates particular lipids and dates pottery on the basis of materials that would have been processed in vessels, such as animal fats. Lipid extracts were subjected to preparative capillary gas chromatography to extract individual fractions of n-hexadecanoic (C16) and n-octadecadonic (C18) acids with high purity (>95%) and in sufficient quantity (>200 mg) for accelerator mass spectrometric analysis of 14C. Analyses were performed relative to analytical blanks for vessels from the medieval, Roman, and Neolithic periods. The best correlations with other associated ages were seen for the C18 acids; the C16 acids tended to give slightly younger ages that could be explained by the greater mobility of the shorter chain acid. — PDS

    Anal. Chem. 10.1021/ac020743y (2003).


    Understanding Adaptors

    1. Stella M. Hurtley

    Clathrin-coated pits mediate the internalization of clustered receptors from the cell surface in a process termed receptor-mediated endocytosis. Between the clathrin and the membrane-bound receptors is a layer of proteins known as adaptors. The AP-2 adaptor complex has been thought of as a key structural and functional component of plasma membrane-derived coated pits and vesicles, and it is found in stoichiometric amounts in isolated clathrin-coated vesicles. Now two groups, Motley et al. and Conner and Schmid, provide compelling evidence that challenges the assumption that AP-2 adaptors are necessary components of endocytic clathrin-coated vesicles. Motley et al. abrogated the expression of AP-2 in tissue culture cells using RNA interference. Clathrin-coated pits at the plasma membrane were reduced 12-fold; however, only receptor-mediated endocytosis of the transferrin receptor was severely compromised. Epidermal growth factor receptor-mediated uptake and low-density lipoprotein receptor-mediated uptake were unaffected. Conner and Schmid overexpressed an accessory kinase that inhibits AP-2 function and similarly found selective effects on the uptake of different receptors from the cell surface. Thus AP-2 is not essential for clathrin-coated vesicle formation at the plasma membrane, and AP-2 can be viewed simply as one adaptor among many involved in the recruitment of different receptors to clathrin-coated pits. — SMH

    J. Cell Biol. 162, 909; 773 (2003).


    In Flew Influenza

    1. Caroline Ash

    Population-level antibody responses to the hemaglutinin and neuraminidase antigens of the influenza virus are watched closely for signs that gradual antigenic drift is giving way to sudden antigenic shift that presages a pandemic. By contrast, Gog et al. have chosen to model the population-level dynamics of cellular immunity to influenza A. This virus generates cytotoxic T lymphocyte (CTL) escape mutants by point mutation, particularly in CTL epitopes of the nucleoprotein gene. Theoretically, the benefits of HLA escape mutation are small for the virus, yet mutations spread to fixation very quickly. Modeling of entire annual cycles of the virus suggests that the answer lies in the advantage the mutant gains in being able to persist undetected by immune responses during the summer, ready to found a new epidemic as the next winter flu season approaches. — CA

    Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1833677100 (2003).


    Ultrafast Petawatt Lasers

    1. Ian S. Osborne

    High-powered lasers with output powers in the petawatt region are currently being developed for a whole host of fundamental applications ranging from fusion ignition systems and high-energy particle physics to medical applications involving radioactive isotope generation. However, these Nd: glass-based lasers are typically large-scale dedicated facilities where each several-hundred-femtosecond pulse can take a day or so of cranking up the system before being released. For practical applications, however, there is a desire for the pulses to be delivered more frequently. One possibility presented by Aoyama et al. is an improvement in the performance of the Ti: sapphire laser, a much smaller laser. Chirped pulse amplification, where the femtosecond pulses are stretched, amplified, and then compressed again, was used in conjunction with improved optical components to deliver 0.85-petawatt pulses. Such lab-sized systems, which replicate the capability of much larger and much more expensive systems, should open the field for further advances and applications. — ISO

    Opt. Lett. 28, 1594 (2003).


    Beneficial Defects

    1. Marc S. Lavine

    Although bone is composed of small inorganic crystals, consisting primarily of hydroxyapatite (HA), it is constantly remodeled through processes of dissolution and redeposition. Pure HA has been used as a bone graft, but it has poor rates of reactivity and integration with existing bone when compared with bioactive glasses and glass ceramics. Previous studies had shown that incorporation of Si atoms into the HA increased the bioactivity, and that dissolution processes are enhanced at grain boundaries. Porter et al. studied the dissolution of HA and Si-substituted HA that was implanted into sheep using high-resolution transmission electron microscopy. Pure HA implants showed no change after 6 weeks, with some small voids appearing after 12 weeks, probably at the sites of dislocations. In contrast, samples with both 0.8 and 1.5 weight % Si showed extensive dissolution after 6 weeks. The material loss was found to initiate at grain boundaries and triple junctions (where three grain boundaries meet) and was far more extensive in the more highly substituted HA. In addition to creating more grain boundaries, the Si also leads to the formation of smaller crystals, which dissolve more readily. This in turn leads to an enhanced concentration of ions at the bone-graft interface, thus enhancing the rate of the incorporation of the graft though the precipitation of biological apatite. — MSL

    Biomaterials 24, 4609 (2003).

  7. STKE

    Easing the Pain

    1. Lisa D. Chong

    Drugs currently used to treat neuropathic pain caused by damage in the nervous system typically produce side effects such as drowsiness and dizziness. Drugs that are directed at targets outside the central nervous system (CNS) could avoid such problems. Ibrahim et al. suggest that activating the CB2 cannabinoid receptor with a selective agonist could be an alternative treament. The CB2 receptor is not found in the CNS but is expressed primarily on mast and immune cells. An aminoalkylindole called AM1241 was shown to be a selective CB2 agonist. Administration of the drug reversed sensory hypersensitivity observed in a rat model of neuropathic pain. In this rat model, spinal nerve ligation increases sensitivity to tactile and thermal stimuli, two characteristics of human neuropathic pain that can result from injury or disease of primary afferent neurons. In mice lacking the CB1 receptor, AM1241 was still able to block pain, indicating the specificity of the effect through CB2 receptors. AM1241 could have an antiinflammatory effect on mast and immune cells that would otherwise release mediators that sensitize primary afferent neurons. — LDC

    Proc. Natl. Acad. Sci. U.S.A. 100, 10529 (2003).