This Week in Science

Science  12 May 2006:
Vol. 312, Issue 5775, pp. 809
  1. A Cellular Fuel Sensor


    The brain plays a key role in body weight control. Within the hypothalamus, select populations of neurons sense changes in fuel availability and regulate food intake and metabolism, but the underlying signaling mechanisms have not been well understood. Cota et al. (p. 927; see the Perspective by Flier) implicate the atypical kinase mTOR (mammalian Target of Rapamycin) signaling pathway, which has been widely studied in other cell types where it regulates the rate of protein synthesis. In rodents, central administration of leucine, which increases mTOR signaling in nonneuronal cells, activated hypothalamic mTOR signaling and decreased food intake and body weight.

  2. Getting a Charge Out of Nanowires

    The protein coat of viruses has previously been used as templates for nanowires, and because some viruses can align in a liquid-crystalline phase, this approach can be used to form larger arrays of ordered nanoparticles. Nam et al. (p. 885, published online 6 April) exploit these properties to fabricate cobalt oxide nanowires for use as battery electrodes. Further modification of the virus allows for the formation of cobalt oxide-gold nanoparticle hybrid wires that enhance the charging capacity of the battery.

  3. Light on the Fast Track

    Photons travel at constant speed c, but in certain nonlinear optical media that exhibit anomalous dispersion, the speed of light pulses can appear to be faster than c, an effect called superluminal propagation. Theoretical results have suggested that the exiting pulse leaves before the entering pulse has entered the medium, and that the pulse peak propagates backward in the medium. Gehring et al. (p. 895) investigated both of these effects with a pumped erbium-doped fiber that exhibits a negative group velocity and they show that the underlying cause is the reshaping of the pulse in the gain medium. The peak of the exiting pulse is formed from the rising edge of the entering pulse, and the peak of the entering pulse becomes part of the trailing edge of the exiting pulse. Dolling et al. (p. 892) looked at the propagation of infrared femtosecond laser pulses through a negative-refractive-index metamaterial and directly measured the group and phase velocities (vgroup and vphase) by time-resolving the transmitted pulse using interferometry. This situation is the negative-index counterpart of experiments of Gehring et al. for positive index, where vphase > 0 and the induced vgroup < 0. They find conditions where vphase < 0 and vgroup < 0, and others where vphase < 0 and vgroup > 0. Together with the “usual” situation of vphase > 0 and vgroup > 0, all four sign combinations have now been observed in direct experiments, and in all cases, the Poynting vector is positive—energy flows in the forward direction.

  4. Not So Fast


    The strength of the Earth's magnetic field has decayed since accurate measurements began in 1840, and these changes have led to speculation that the field will disappear or reverse within this millennium. Extrapolating to earlier times has been difficult, in that direct measurements, which extend back another 250 years, recorded only direction, and there paleomagnetic data that has been extracted from rocks and archaeological artifacts is limited. Gubbins et al. (p. 900; see the Perspective by Kono) have devised a method to use paleointensity measurements in conjunction with directional information to extend the record of the Earth's magnetic field back to 1590. Contrary to the recent steep decline, they find that the dipole moment fell hardly at all until around 1800.

  5. Giving Metals the Push

    Crystalline metals can be thought to consist of nearly perfectly ordered grains separated by highly distorted walls. During plastic deformation, the grains will shrink and misalign, and new dislocations will form and take on ordered patterns, but it has been difficult to isolate the changes that occur to individual grains. Jakobsen et al. (p. 889; see the Perspective by Kubin) present an x-ray method that tracks the dynamics of individual grains deeply embedded within a crystal. They find some surprising behavior, including intermittent dynamics where the grains grow and shrink, and transient splitting of grains into subgrains.

  6. Lost in Translation

    Dyskeratosis congenita (DC) is a rare inherited disorder associated with bone marrow failure, skin defects, and an increased susceptibility to cancer. The X-linked form, X-DC, is caused by mutations in the DKC1 gene, which encodes a pseudouridine synthase that modifies ribosomal RNA. Yoon et al. (p. 902) show that disruption of DKC1 impairs translation of a select group of messenger RNAs (mRNAs) that initiate protein synthesis in an unusual way, through internal ribosome entry site (IRES) elements. Among the mRNAs affected were those encoding the tumor suppressor p27(Kip1) and two proteins that prevent cell death, Bcl-xL and XIAP (for X-linked Inhibitor of Apoptosis Protein). Loss of these protein functions may contribute to the pathogenesis of X-DC.

  7. Manipulating Microtubule Motion

    For small fluidic and reactor systems, one solution for controlling the transport of reagents and products would be to incorporate biological motors. Previous studies have shown that microtubules can be chemically modified to carry cargo, but controlling their motion is still a challenge. Van den Heuvel et al. (p. 910; see the Perspective by Hess) studied the behavior of microtubules in constant electric fields. With detailed experiments and theory, they show that individual microtubules driven by the motor protein kinesin across the surface of micrometer-sized fluidic channels can be driven in a desired direction and that the sorting occurs with high efficiency.

  8. PINning Down Auxin Flow

    The plant hormone auxin regulates a variety of growth and developmental responses and must be transported within the plant in an organized fashion. Petrášek et al. (p. 914, published online 6 April; see the Brevia by Wiśniewska et al. and the Perspective by Sieberer and Leyser) now show, by using inducible overexpression in plant cells and expression in human and yeast cells, that the protein PIN is responsible for the direction in which auxin flows out of the cell.

  9. Hold and Cut


    In nuclear transfer RNA and archeal RNA, introns must be removed from folded precursors to produce functional RNA. Xue et al. (p. 906) present the structure of a dimeric splicing endonuclease from Archaeglobus fulgidus bound to a bulge-helix-bulge RNA containing a precleaved and a cleaved splice site at 2.85 angstrom resolution. The cleavage sites are within the bulges, and an arginine pair from each catalytic domain sandwiches a flipped-out base from the bulge cleaved by the other catalytic domain. This motif leads to cooperativity in binding and cleavage of the two splice sites. Interactions between the RNA and the endonuclease at the active sites are consistent with the idea that three conserved residues form a catalytic triad.

  10. Charting Oceanic Microbial Abundance

    Prochlorococcus may represent the most abundant photosynthetic organism on Earth. Bouman et al. (p. 918) present a circumglobal sampling effort in the Southern Hemisphere of Prochlorococcus, its pigments, and the distribution of its specific genetic variants (such as ecotypes), across the Southern Pacific, Atlantic, and Indian oceans. The distribution of phylotypes and ecological types among the three ocean basins reflects the gradients of light and nutrients and oceanographic characteristics of the three basins.

  11. Endocytosis and Developmental Patterning

    During animal patterning in development, morphogens such as Wnt form gradients that control local developmental responses. While searching for factors involved in Caenorhabditis elegans larval cell migration, Coudreuse et al. (p. 921, published online 27 April) found a role for components of the conserved endocytic retromer complex. The retromer complex is required in cells that produce the Wnt ortholog EGL-20 and is needed to establish the EGL-20 concentration gradient as well as for long-range signaling. Experiments with mammalian cell lines and Xenopus suggest a conserved function for the retromer complex in Wnt signaling, possibly by recycling the Wnt cargo-receptor from the endosome to the Golgi.

  12. Stroke, Ischemia, and Ion Flux

    The rapid decrease of oxygen and glucose in brain tissue after an acute stroke can trigger necrotic neuronal cell death within minutes. The main underlying cause is the dysregulation of major intracellular ion concentrations, but it has been unclear which particular ion channels are activated by ischemic conditions in pyramidal neurons. Pannexin 1 (Px1) is a member of a family of gap junction proteins that are highly expressed in pyramidal neurons. In acutely isolated neurons and brain slices, Thompson et al. (p. 924) found that Px1 hemichannel opening was activated by ischemic stress. Thus, hemichannel activation by ischemia during stroke could be responsible for the profound ionic dysregulation contributing to excitotoxicity.

  13. Testing Potential Drivers of Diversity

    The fossil record shows a large number of different groups have diversified during the Phanerozoic (since 540 million years ago). One hypothesis for this macroevolution has been that the ecological interactions, such as between predators and prey or other ecologically related groups, cause both groups to diversify. Madin et al. (p. 897) tested these large-scale trends in Phanerozoic benthic marine invertebrates using the Paleobiology Database. The analysis shows the general diversification of both carnivores and noncarnivores. However, the relation between the groups is not correlated, which implies separate diversification trends that were not driven by ecological interactions.

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