This Week in Science

Science  01 Jun 2012:
Vol. 336, Issue 6085, pp. 1077
  1. Designer Hydrogels

    CREDIT: DROR SELIKTAR AND DVIR YELIN/TECHNION-ISRAEL INSTITUTE OF TECHNOLOGY

    Hydrogels, which consist of highly water swollen cross-linked polymer networks, can now be made with a range of chemistries and a combination of physical and chemical cross-links. They can also be designed to degrade gradually when exposed to chemical or biological signals and are thus finding use in a wide range of applications, including tissue engineering and drug delivery. Seliktar (p. 1124) reviews recent advances in tailoring hydrogels with specific properties and their applications to biology and medicine.

  2. Quantum Leap?

    Quantum computers are expected to be able to solve some of the most difficult problems in mathematics and physics. It is not known, however, whether quantum field theories (QFTs) can be simulated efficiently with a quantum computer. QFTs are used in particle and condensed matter physics and have an infinite number of degrees of freedom; discretization is necessary to simulate them digitally. Jordan et al. (p. 1130; see the Perspective by Hauke et al.) present an algorithm for the efficient simulation of a particular kind of QFT (with quartic interactions) and estimate the error caused by discretization. Even for the most difficult case of strong interactions, the run time of the algorithm was polynomial (rather than exponential) in parameters such as the number of particles, their energy, and the prescribed precision, making it much more efficient than the best classical algorithms.

  3. Unseen Planet

    The orbits of planetary bodies can be affected by gravitational interactions with other planets in the same system. If the planets can be seen to pass in front of their star (or to transit), the gravitational perturbations translate into variations in transit duration. Nesvorný et al. (p. 1133, published online 10 May; see the Perspective by Murray) inferred the presence of a far distant, previously unknown planet from the transit timing variations of a planet previously detected by the Kepler space telescope. The new planet is in a 57-day orbit and does not transit its host star. The analysis also revealed a third possible planet with a mass 1.7 times that of the Earth in a 6.8-day orbit. The radially spaced, roughly coplanar, and nearly circular orbits of the two confirmed planets mirror planetary orbits in our own solar system.

  4. Dissecting Cooper Pairs

    Angle-resolved photoemission spectroscopy (ARPES) is used in the study of the electronic structure of complex materials. Recently, time-resolved ARPES has become possible, where the state of the system is excited by a short “pump” pulse, and ARPES is performed using a second “probe” pulse applied after varying times. Smallwood et al. (p. 1137) used this technique to study the recombination of Cooper pairs—the fundamental charge carriers in superconductors—in a cuprate high-temperature superconductor.

  5. Updating the Triode with Graphene

    In early electronics, the triode—a vacuum device that combined a diode and an electrical grid—was used to control and amplify signals, but was replaced in most applications by solid-state silicon electronics. One characteristic of silicon-metal interfaces is that the Schottky barrier created—which acts as a diode—does not change with the work function of the metal—the Fermi level is pinned by the presence of surface states. Yang et al. (p. 1140, published online 17 May) now show that for a graphene-silicon interface, Fermi-level pinning can be overcome and a triode-type device with a variable barrier, a “barristor,” can be made and used to create devices such as inverters.

  6. Variation on a Theme

    Given the incredible diversity and complexity in social behavior and ecology that exists across animal taxa, revealing the evolution of neural mechanisms for behavior is a great challenge. O'Connell and Hofmann (p. 1154) examined the expression profiles of several genes involved in the social behavior network and the mesolimbic reward system in 88 species across five vertebrate lineages. A remarkable level of conservation was observed in brain regions linked to social behavior and decision-making, but flexibility seems to have been maintained through variability in neuroendocrine ligand expression across the brain.

  7. Going Up Against the Grain Boundaries

    CREDIT: TSEN ET AL.

    Exfoliated graphene sheets are single crystals that exhibit excellent electronic properties, but their fabrication is too slow for large-scale device fabrication. Growth methods such as chemical vapor deposition are faster, but create polycrystalline graphene sheets that contain grain boundaries that can scatter charge carriers and decrease performance. Tsen et al. (p. 1143) found that the presence of overlapping domains within polycrystalline graphene samples could increase conductivity of samples by an order of magnitude, allowing them to rival exfoliated samples.

  8. Untangling a Spectral Thicket

    Atoms and molecules absorb characteristic frequencies of ligh; however, assigning the spectra of certain molecules remains a challenge. For example, the loosely bound complex of CO with H2 produces a dense thicket of measured spectral lines that has resisted elucidation for over a decade. Now, Jankowski et al. (p. 1147) provide detailed theoretical calculations that match the observed pattern and thereby elucidate the vibrational and rotational states in play. The result may ultimately help to tease out the collisional dynamics of H2 and CO in the interstellar medium.

  9. See How They Grow

    Controlled assembly and disassembly of the actin cytoskeleton is essential for processes such as cell motility, cytokinesis, and tumor metastasis. The formation of new actin filaments appears to involve the protein formin paired with another actin assembly-promoting factor. Breitsprecher et al. (p. 1164) used triplecolor single-molecule fluorescence microscopy to visualize actin assembly promoted by the formin, mDia1, and the tumor-suppressor, adenomatous polyposis coli (APC). The two assembly factors interacted directly to initiate filament assembly, after which mDia1 moved with the growing barbed ends while APC remained at the site of nucleation.

  10. Design and Build

    Self-assembling biomolecules are attractive building blocks in the development of functional materials. Sophisticated DNA-based materials have been developed; however, progress in designing protein-based materials has been slower. King et al. (p. 1171) describe a general computational method in which protein building blocks are first symmetrically docked onto a target architecture, and then binding interfaces that drive self-assembly of the building blocks are designed. As a proof of principle, trimeric building blocks were used to design self-assembling 12-subunit complexes with tetrahedral symmetry and 24-subunit complexes with octahedral symmetry. Lai et al. (p. 1129) were able to build a 12-subunit tetrahedral protein cage from fused oligomeric protein domains.

  11. Dissecting Chitin Binding

    CREDIT: LIU ET AL.

    The chitin in fungal cells walls serves as a trigger to initiate plant defenses against pathogenic fungi. Arabidopsis detects these signals through a cell surface chitin receptor whose intracellular kinase domain initiates a signaling cascade in response to chitin that activates the plant's response to infection. Liu et al. (p. 1160) have now solved the crystal structure of the Arabidopsis chitin receptor AtCERK1. The results show how chitin binds to the receptor and suggest that the biological response requires dimerisation of the receptor when it binds a chitin oligomer at least seven or eight subunits long.

  12. The Real McCoy

    Some secreted proteins are phosphorylated, the most prominent example being the milk protein casein, but the enzymes that catalyze such phosphorylation have not been identified. (The proteins known as “casein kinases” are in fact cytosolic proteins and do not mediate physiological phosphorylation of casein.) Tagliabracci et al. (p. 1150, published online 10 May) searched for a human protein with the characteristics expected of a secretory protein kinase and identified Fam20C. Mutations in the gene encoding Fam20C cause defects in bone formation. Furthermore, the consensus sequence for Fam20C phosphorylation was found in several secreted proteins that function in biomineralization. Thus, Fam20C appears to be the “real” casein kinase and to function in bone physiology.

  13. So Selective

    The key elements for long-lived antibody-mediated immunity—memory B cells and plasmablasts—are generated in germinal centers, where B cells expressing high-affinity antigen receptors are selected for survival and proliferation in a process called affinity maturation. Unexpectedly, Khalil et al. (p. 1178, published online 3 May; see the Perspective by Bannard and Cyster) found that, in contrast to naïve B cells and B cells outside the germinal center, proximal signaling events are impaired downstream of the antigen receptor in mouse germinal center B cells.

  14. Regaining Limb Movement

    Despite many years of intensive research, there is still an urgent need for novel treatments to help patients restore motor function after spinal cord injuries. van den Brand et al. (p. 1182) produced left and right hemisections at different levels of the rat thoracic spinal cord to cause complete hind limb paralysis mimicking the situation in humans with spinal cord injury. Systemic application of pharmacological agents, combined with a multisystem rehabilitation program including a robotic postural neuroprosthesis, restored voluntary movements of both hind limbs.

  15. Managing Trade-Offs

    Most organisms experience selection on a host of traits to determine their likelihood to succeed evolutionarily. However, specific traits may experience trade-offs in determining an organism's optimal phenotype. Shoval et al. (p. 1157; see the Perspective by Noor and Milo) relate physical traits to the task that they are optimizing using a Pareto curve, a power law probability distribution, to show that a single set of trait values optimizes performance at a given task and that performance decreases as an organism's phenotype moves away from this set of trait values. The results suggest how selection makes the best trade-offs for an arbitrary number of tasks and traits and may explain examples of evolutionary variation.

  16. Insights into Amyloidogenesis

    The amyloid-β (Aβ) peptides associated with Alzheimer's disease are generated by cleavage of the transmembrane C-terminal domain (C99) of the amyloid precursor protein by the enzyme γ-secretase. Barrett et al. (p. 1168) used nuclear magnetic resonance (NMR) and electron paramagnetic resonance spectroscopy to show that C99 contains surface-associated N- and C-terminal helices and a flexibly curved transmembrane helix that is well suited to processive cleavage by γ-secretase. Elevated cholesterol levels have been found to increase Aβ generation. NMR titration together with mutagenesis revealed a binding site for cholesterol within C99 that included a motif previously implicated in protein oligomerization.

  17. Predicting Catastrophic Collapse

    It is very difficult to predict the behavior of complex systems near tipping points, where a small change in conditions can lead to a large shift in the state of the system. Theory predicts that some generic statistical indicators can signal the approach of such tipping points, such as slower recoveries from perturbations of the system—a phenomenon called critical slowing down. Working with laboratory yeast populations, Dai et al. (p. 1175) provide an experimental measure for critical slowing down before catastrophic population collapse. This type of approach should help when trying to predict catastrophic thresholds in other complex systems, ranging from ecosystem transition to climate change, and even to crashes of financial markets.