Editors' Choice

Science  09 Jul 2010:
Vol. 329, Issue 5988, pp. 121
  1. Cell Biology

    Caught Off Balance

    1. Paula A. Kiberstis

    Otoconia: normal, abnormal (lower center), absent (lower right).

    CREDIT: MARIÑO ET AL., J. CLIN. INVEST. 120, 10.1172/JCI42601 (2010)

    An episode of dizziness, no matter how brief, reminds us that our body performs important physiological functions that we take for granted. Our sense of balance is dependent on small crystals in the inner ear called otoconia. These crystals are embedded within a fibrous extracellular matrix that couples the force of gravity to the cilia of sensory cells, which in turn send signals to the nervous system. The biosynthesis of otoconia occurs during fetal development when core proteins secreted by vestibular epithelial cells form a proteinaceous matrix that sequesters calcium carbonate. Little is known about the genes and cellular processes involved in otoconial assembly and maintenance.

    Mariño et al. have discovered that a degradative cellular process called autophagy is essential for otoconial biogenesis. Mice genetically deficient in a protein that plays a key role in autophagy, Atg4b, showed behaviors consistent with inner ear defects, such as head tilting, circling movements, and disorientation in swimming tests. These behaviors were accompanied by the absence of otoconia or by the presence of morphologically abnormal otoconia. Similar abnormalities were seen in mice deficient in Atg5, which like Atg4 appears to be important in the secretion and assembly of otoconial core proteins. Further mechanistic investigation of how these small but critical crystals are made and maintained throughout life may yield new treatments for balance-related disorders, which are common in the elderly and can also be a side effect of certain antibiotics.

    J. Clin. Invest. 120, 10.1172/JCI42601 (2010).

  2. Microbiology

    Restricting Promiscuity

    1. Caroline Ash

    Although it might seem that many species of bacteria swap DNA promiscuously, in reality, the transfer of genetic information is curtailed by sequence-specific restriction systems. Several such systems have been characterized; the core component is an endonuclease that cleaves unmodified DNA (the host's own DNA is protected against digestion, usually by methylation). Corvaglia et al. have discovered a new endonuclease in Staphylococcus aureus, which is a potentially pathogenic bacterium carried by up to half the human population. Methicillin-resistant strains of S. aureus (MRSA) were found to carry mutations in this restriction endonuclease, which has tentatively been assigned to the type III family; these mutations probably render these strains hypersusceptible to the uptake of antibiotic-resistance genes from gut bacteria.

    Proc. Natl. Acad. Sci. U.S.A. 107, 10.1073/pnas.1000489107 (2010).

  3. Chemistry

    Ironing Out Formic Acid

    1. Phil Szuromi

    Some biomass transformations, such as pathways that deoxygenate sugars, produce formic acid as a by-product, and the options for downstream use of large amounts of it have been limited. Boddien et al. now report a photocatalytic route for liberating hydrogen from formic acid. Previous catalysts for this reaction used noble metals, but after screening a range of more abundant transition metals, the authors discovered that effective catalysts formed in situ from an iron carbonyl cluster [Fe3(CO)12], a polydentate nitrogen-donating ligand, and triphenylphosphine. Under irradiation with the visible light from a 300-W xenon arc lamp, these catalysts generated hydrogen from formic acid solutions stabilized with triethylamine; the best examples had turnover frequencies up to 200 per hour and turnover numbers exceeding 100. Extensive spectroscopic studies implicated photogenerated iron hydrides as active species.

    J. Am. Chem. Soc. 132, 10.1021/ja100925n (2010).

  4. Physics

    Running in Circles

    1. Jelena Stajic
    CREDIT: GRASER ET AL., NAT. PHYS. 6, 10.1038/NPHYS1687 (2010)

    While the debate about the origin of high-temperature superconductivity in the cuprates has been raging since their discovery more than 20 years ago, many of these materials have found practical use, most commonly as superconducting magnets and wires. However, a serious limitation exists, especially pronounced in the yttrium barium copper oxide (YBCO) family: Grain boundaries (interfaces between mismatched neighboring crystal orientations), present in the technologically interesting polycrystals, limit the observed supercurrent to values considerably below that attainable in a perfect single crystal. Graser et al. use molecular dynamics simulations and an effective tight binding model to simulate the flow of a supercurrent through a grain boundary in YBCO. They find that, even away from the boundary, the flow is disrupted and can run in closed loops (see arrows in the figure) and even backwards (blue lines). The experimentally established exponential dependence of the supercurrent on the angle of misorientation is recovered; the authors identify the accumulation of charge at the boundary as the primary cause, with the d-wave symmetry of the superconducting gap having, surprisingly, little effect. There are indications that similar behavior might occur in other complex superconductors, such as the ferropnictides.

    Nat. Phys. 6, 10.1038/nphys1687 (2010).