This Week in Science

Science  11 Jan 2013:
Vol. 339, Issue 6116, pp. 117
  1. Mini Mighty Muscle


    Actuators—or artificial muscles—take electrical or chemical energy and convert it into mechanical force. Typically, actuators made from polymers can show large deformations, but cannot generate a lot of force. Ma et al. (p. 186; see the Perspective by Kim and Kwon) describe a polymer composite based on a modified polypyrrole that expands in response to water absorption. The composite was able to generate large stresses and forces, and offered a high work density approaching those of the best conducting polymer electrochemical actuators. Magnetic nanoparticles incorporated into polymer films were used to control the locomotion of the actuator.

  2. Macrophage JNK in Metabolic Disease

    Inflammation is thought to be an important driver of diet-induced obesity and insulin resistance. Proinflammatory, M1 phenotype macrophages and the c-jun NH2 terminal kinases (JNK) are central players in this process. But whether JNK expression is specifically required inside macrophages is unclear. In mice containing a macrophage-specific deletion in both Jnk1 and Jnk2, Han et al. (p. 218, published online 6 December; see the Perspective by Ferrante Jr.) found that the mice were protected against many of the diet-induced metabolic changes, including insulin resistance, despite similar weight gain as control mice on a high-fat diet. This protection was associated with a decrease in the presence of M1 macrophages in adipose tissue.

  3. Tracking Quantum Evolution

    The actual process of measuring a quantum system has an effect on the result making the outcome unpredictable. Using a superconducting qubit placed in a microwave cavity, Hatridge et al. (p. 178) found that a series of partial measurements on a quantum system left the system in a pure state. Looking at the record of the actual measurements allowed the final state of a superconducting-based quantum system to be determined accurately. Such control is crucial for achieving full feedback control of a general quantum system.

  4. Ribosomal Rotaxane?

    The ribosome is an extraordinarily sophisticated molecular machine, assembling amino acids into proteins based on the precise sequence dictated by messenger RNA. Lewandowski et al. (p. 189) have now taken a step toward the preparation of a stripped-down synthetic ribosome analog. Their machine comprises a rotaxane—a ring threaded on a rod—in which the ring bears a pendant thiol that can pluck amino acids off the rod; the terminal nitrogen then wraps around to form a peptide bond and liberate the thiol for further reaction. The system was able to link three amino acids in order from the preassembled rod.

  5. Pupfish Speciation

    Evolution moves along phenotypic trajectories that can be visualized as a topographic landscape of multiple peaks of relatively high-fitness and low-fitness valleys. Martin and Wainwright (p. 208) examined the adaptive landscape of three species of Cyprinodon pupfishes. These species represent a recent adaptive radiation, each having moved into a difference niche within their specialized environment. Examining replicate hybrid transplants relative to parental types in high- and low-density enclosures, the authors recovered the specialist parental phenotypes and observed higher survival and growth. Thus, high density can drive multiple fitness peaks during the early stages of adaptive radiation.

  6. A Varied Bouquet

    Pollinators display innate attractions to odor, but can also learn to associate odor with a nectar reward. Riffell et al. (p. 200, published online 6 December; see the Perspective by Knaden and Hansson) characterized the odor profile for flowers to which hawkmoths are innately attracted and found that the majority contain a distinct chemical profile, which is uniquely represented on their olfactory lobe. The moths could also be trained to associate nonattractive odors with a reward and thus learn novel odor attractions. Though learning altered neurons within the antennal lobe, the innate preferences were not changed.

  7. Optimizing Carbon Nanotubes

    Shorter carbon nanotubes are easier to make, but, when assembled into fibers, the resulting fiber properties are much poorer than might be predicted by theory. Conversely, longer carbon nanotubes have much better properties but are harder to process. Behabtu et al. (p. 182) combined the best of both worlds through scalable wet spinning method, in which they dissolved longer carbon nanotubes and then spun them into fibers that showed excellent strength, stiffness, and thermal conductivity.

  8. Stress Protector

    During prolonged fasting, the oxidation of fatty acids leads to increased accumulation of d-β-hydroxybutyrate (βOHB) in the bloodstream. Such increased concentrations of βOHB inhibit class I histone deacetylases. Histone acetylation in turn influences transcriptional activity at various genes. Shimazu et al. (p. 211, published online 6 December; see the Perspective by Sassone-Corsi) found that among the genes showing increased transcription in animals treated with high concentrations of βOHB were two genes implicated in cellular responses to oxidative stress. When treated ahead of time with βOHB, mice were protected from the toxic effects of the oxidative stress causing poison paraquat.

  9. Size Affects Shape

    Porous molecular framework materials can adopt a different phase when guest molecules absorb and uniformly distort the framework. Usually the framework returns to its original shape when the guests desorb. Sakata et al. (p. 193) noted that because surface stress drives this process, it might be avoided in smaller crystals. Indeed, a flexible porous coordination polymer, [Cu2(dicarboxylate)2(amine)]n, could retain the structure induced by guest molecules such as methanol if crystallites were made sufficiently small (submicrometer scale) and did so to a greater degree as the crystallite dimensions decreased.

  10. The Adult Astrocyte Is Different

    The concept of the tripartite synapse, whereby astrocytes actively modulate the communication between the pre- and postsynaptic site, is widely accepted. The release of gliotransmitters has been linked to release of Ca from intracellular stores via the activation of astrocytic metabotropic glutamate receptor 5 (mGluR5) by glutamate spillover from synapses. However, nearly all studies on the tripartite synapse have used brain tissue collected from young individuals. Many receptors undergo changes in expression level during development. Sun et al. (p. 197; see the Perspective by Grosche and Reichenbach) applied genomic analysis, electron microscopy, and calcium imaging in slices and in vivo to assess the presence and the functionality of mGluR5 and mGluR3 receptors during postnatal development in human and mouse astrocytes. Astrocytic expression of mGluR5 was lost by the third postnatal week in mice and was not present in human cortical astrocytes, which calls into question the viability of the tripartite synapse model for adult synapses.

  11. Destination Specificity

    During brain development, some types of neurons migrate from where they are born to their final functional locations. Some neurons migrate radially, from the inside to the outside, while others migrate tangentially. Di Meglio et al. (p. 204) analyzed the migration of a group of tangentially migrating neurons in the hindbrain. Although these neurons all entered the same migratory stream, they each retained positional information such that their relative organization in the destination site reflected their original organization. Interactions between epigenetic signals and the genes encoding Hox transcription factors encoded the positional information and fine-tuned migration.

  12. Repair and Recombination

    In most sexually reproducing organisms, the haploid gametes are produced by meiosis, a specialized cell-division during which recombination between the parental chromosomes ensures proper chromosome segregation. Double-strand breaks (DSB) generated in the DNA drive this recombination and are linked to recombination hotspots in the chromatin where histone H3 is methylated on lysine residue 4 (H3K4me) by the Set1C/COMPASS enzyme complex. Acquaviva et al. (p. 215, published online 15 November) show that the Mer2/Mei4/Rec114 complex of the DSB generating machinery is linked to the Set1C/COMPASS enzyme complex through the Spp1 subunit, which contains a PHD-finger capable of binding to H3K4me. The Spp1 subunit recruits the Mer2 protein to sites where recombination will occur, probably by binding to both the H3K4me-marked chromatin and other factors.

  13. SAM, Histones, and Stem Cells

    Mouse embryonic stem cells require threonine for growth and express large amounts of the enzyme that catalyzes the first step in threonine metabolism. To find out what is so important about threonine in these cells, Shyh-Chang et al. (p. 222, published online 1 November; see the Perspective by Sassone-Corsi) monitored changes in metabolism by mass spectrometry in induced pluripotent stem cells. Threonine was required to maintain cellular concentrations of S-adenosylmethionine (SAM), a substrate used for protein methylation. Restriction of threonine inhibited methylation of histones, which is characteristic of chromatin in embryonic stem cells. Thus, altered metabolism of threonine and methionine in stem cells may be linked to epigenetic changes that influence genetic reprogramming and decisions of stem cells to proliferate or differentiate.

  14. Diffraction Before Destruction

    A bottleneck in x-ray crystallography is the growth of well-ordered crystals large enough to obtain high-resolution diffraction data within an exposure that limits radiation damage. Serial femtosecond crystallography promises to overcome these constraints by using short intense pulses that out-run radiation damage. A stream of crystals is flowed across the free-electron beam and for each pulse, diffraction data is recorded from a single crystal before it is destroyed. Redecke et al. (p. 227, published online 29 November; see the Perspective by Helliwell) used this technique to determine the structure of an enzyme from Trypanosoma brucei, the parasite that causes sleeping sickness, from micron-sized crystals grown within insect cells. The structure shows how this enzyme, which is involved in degradation of host proteins, is natively inhibited prior to activation, which could help in the development of parasite-specific inhibitors.