This Week in Science

Science  15 Apr 2011:
Vol. 332, Issue 6027, pp. 281
  1. Mixing It Up


    The action of winds on the surface of the ocean and processes that change the density of ocean water masses, such as cooling, freezing, and evaporation, create currents and transfer energy to the ocean. The ocean does not continue to circulate faster and faster, however, because this energy is dissipated through the creation of turbulence. D'Asaro et al. (p. 318, published online 10 March; see the Perspective by Ferrari) present observations from an ocean front within a strong ocean current (the Kuroshio) that show that energy can be transferred from the large to the small scale of turbulence at fronts and dissipated at a significant rate. These findings will help in the generation of accurate simulations of ocean circulation and its effects on climate.

  2. Adenosine Receptor Activation

    G protein–coupled receptors (GPCRs) are transmembrane proteins that act as key gatekeepers between external signals and cellular responses. GPCRs that are activated by diffusible ligands are dynamic, but in structural studies GPCRs have generally been cocrystallized with antagonists to stabilize them in their inactive state. Xu et al. (p. 322, published online 10 March) now report a high-resolution crystal structure of human A2A adenosine receptor bound to a potent agonist, which provides insight into the structural basis of activation.

  3. Tightly Wound Waves

    In circularly polarized light, the field vectors rotate while the wave propagates through space. The resultant helical pattern is chiral, and, consequently, the field interacts differently with a single chiral molecule than with its mirror image isomer (enantiomer). For most small molecules, however, the light polarization varies on a spatial scale much larger than the atomic dimensions, and so differential absorption by enantiomers is relatively weak. Tang and Cohen (p. 333) have now engineered a more sharply rotating polarization pattern, termed superchiral light, that shows a greater than 10-fold enhancement in discriminating enantiomers relative to ordinary circular polarization.

  4. The Flavor of Graphene

    Graphene, a single-atom-thick layer of graphite, exhibits many phenomena characteristic of two-dimensional systems, such as the quantum Hall effect. But in addition to charge degrees of freedom, graphene also has the so-called spin and valley (jointly called flavor) degrees of freedom. Abanin et al. (p. 328; see the Perspective by Castro Neto) measured the nonlocal response of graphene samples close to the Dirac or neutrality point, where the charge carrier density tends to zero. A significant voltage drop on one end of the device was observed in response to current flowing through the other end in the presence of a perpendicular magnetic field, consistent with the appearance of long-range flavor currents.

  5. A Few-Atom Trap

    Atoms consist of a relatively small number of protons, neutrons, and electrons, which all belong to a class of particles known as fermions. Much is known about these few-fermion systems from the study of naturally occurring elements; however, in order to be able to tune their properties, man-made systems are more desirable. Serwane et al. (p. 336) created a system of up to 10 fermionic cold atoms in a precisely defined quantum state of an optical dipole trap. Magnetic fields allowed the tuning of the interactions between the fermions and observation of an interaction shift for a system of two fermions.

  6. Bicycle Stability Revisited

    It takes practice to learn how to ride a bicycle, but experiments have shown that a riderless bicycle will self-correct its steering against falling. Although bicycles are familiar machines, the mechanics that describes their motion is quite complex, and approximations are needed to understand what effects underlie stability. Kooijman et al. (p. 339) reexamined bicycle steering by performing linear stability calculations and by building a bicycle that canceled gyroscopic effects with counterspinning wheels and that set the steering wheel with its pivot point behind its contact to the ground. Such a bicycle could still move stably. Other factors, such as the mass distribution along the bicycle, also played key roles in steering and stability.

  7. Capturing Bottles in DNA

    Several methods can exploit DNA base pairing to create complex shapes. Han et al. (p. 342; see the cover) describe a method that can create arbitrary closed shapes by first decomposing the shape into circular contour lines and then designing DNA double helices that can bend and follow these contours. Suitable crossover positions link DNA along adjacent contours. Complex shapes, including a 70-nanometer-high nanoflask, were constructed. This design approach is based on simple geometric considerations, and the structures formed in high yield despite their inherent strain.

  8. Teleporting Cats


    The strong correlations observed between entangled states and the ability for states to be in a super-position (for example, Schrödinger's cat being dead and alive) are well-known features of quantum mechanics. Both play a role in the development of protocols for quantum information processing, being able to transport information securely and instantaneously. Lee et al. (p. 330; see the Perspective by Grangier) combine squeezing, photon subtraction, entanglement, and homodyne detection to form a teleporter of nonclassical wave packets of light. The control and manipulation of these “cat” states should prove useful in generating more complex and robust protocols for quantum information science.

  9. The Origin of Language

    Human populations are thought to have gone through a series of genetic bottlenecks as they expanded out of Africa, with small segments of established populations moving on to found new populations with diminished genetic variation. Atkinson (p. 346) analyzed what happened to languages through these population bottlenecks by studying the number of phonemes—perceptually distinct units of sound that differentiate words—in languages across the world. The largest phoneme inventories are found in Africa and the smallest in South America and Oceania. Extrapolating back from the geographic variation in phoneme diversity suggests that language originated in central and southern Africa.

  10. Queens vs. Jesters

    Are species-species interactions or the interactions of a species with its environment more important drivers of speciation and extinction? Ezard et al. (p. 349) investigated macroevolutionary dynamics of planktonic foraminifera by examining their fossil records, characterizing their ecologies, and dissecting the interplay between species' ecology and their physical environment. Species with distinct ecologies had significantly different speciation and extinction rates, and the dominance of particular ecologies fluctuated with the changing environment. Extinction appears to be more strongly shaped by environmental change than by diversity-dependence, whereas the reverse may be true for speciation.

  11. Ready, Discharge, Go


    In crayfish brains, one can record an increased neuronal spike discharge rate a few seconds before the onset of walking and a subsequent decrease once walking is initiated. However, what the synaptic mechanisms of this so-called readiness discharge are and what types of neurons are involved in the production of this spike activity are unclear. Kagaya and Takahata (p. 365) identified a class of cells in the crayfish brain that discharged in advance of electromyographic activity in the walking legs at the beginning of a bout of voluntary walking, but not when walking was triggered by an external stimulus. Other classes of neurons were also identified, including some that were active throughout a bout of walking and some that fired primarily at the end of walking.

  12. Assessing Filopodia Form and Function

    During development, Drosophila cells form filopodia that protrude in the direction of other cells that produce morphogens that influence their development. Roy et al. (p. 354; see the Perspective by Affolter and Basler) examined images of cells of developing Drosophila embryos and suggest that particular growth factor receptors are sorted into distinct filopodia and cytoplasmic vesicles. Under certain conditions, cells overexpressing fluorescently labeled growth-factor receptors had filopodia that were oriented in a specific direction and appeared to segregate particular receptor types into distinct filopodia. Further investigation of the nature of the receptor-sorting mechanism and its possible function in signaling may elucidate signaling mechanisms controlling development.

  13. Seeking the Right Pathway

    Activation of transforming growth factor–β (TGF-β) signaling promotes the development of aortic aneurysms in the connective tissue disorder Marfan syndrome (MFS). Losartan, a drug that inhibits TGF-β signaling, is in clinical trials for this disorder. Like many cytokines, TGF-β activates multiple intracellular signaling pathways. In the context of aortic disease, TGF-β has been assumed to act through the “canonical” Smad pathway. Holm et al. (p. 358) and Habashi et al. (p. 361) now show that the “noncanonical” TGF-β pathway, which involves the signaling proteins ERK1/2, is the prominent driver of aortic disease in MFS mice and that it is this pathway through which losartan exerts its beneficial effects. Analysis of ERK1/2 activation status in MFS patients may help optimize losartan dosage, and drugs specifically targeting the noncanonical pathway may merit exploration as possible therapies for aortic aneurysms.

  14. Controlled Access

    Formate is a key metabolite and regulatory molecule in a wide range of bacterial species. At neutral and higher pH, formate is passively exported, but at low pH it is imported in what has been proposed to be an active transport process. Both import and export are mediated by the integral membrane protein FocA. Previously determined structures of FocA at high pH revealed a pentamer with an open substrate channel in each monomer. Now, et al. (p. 352) report the structure of FocA at pH 4.0. The N termini of the monomers adopt three conformations, with one conformation gating the channel. Coordinated cycling of the monomers through the three conformations could provide a mechanism for active uptake.

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