This Week in Science

Science  16 Oct 2009:
Vol. 326, Issue 5951, pp. 335
  1. Seeing the Brain's One, Two, Three

      CREDIT: NED T. SAHIN/UCSD, HARVARD UNIVERSITY

      Taking advantage of the rare opportunity to record neuronal activity in the human brain using intracranial electrodes, Sahin et al. (p. 445; see the Perspective by Hagoort and Levelt) document the spatial and temporal pattern of neuronal populations within Broca's area as patients thought of a single word, changed its tense (for verbs) or number (for nouns), and articulated the word silently. For these three stages, they detected activity at 200, 320, and 450 milliseconds, moving in a caudal to rostral direction. These data fit neatly within the roughly 600 milliseconds required for the onset of speech and map the distinct neural computations within an area of the brain, known for almost a century and a half, as important for the production of language.

    1. Confined Germanium Thermodynamics

        When a material is confined to nanoscale volumes, the very high proportion of surface to bulk can alter its thermodynamic properties. This has been studied using in situ electron microscopy, but in most cases the volume of the material is not constrained. Holmberg et al. (p. 405) studied the thermodynamics of a germanium nanowire attached to a gold seed and coated with a carbon shell to restrict its volume, measuring the reaction temperature, as well as the liquid composition without changes in volume throughout the heating cycle. This enabled monitoring of phase behavior while the germanium was being heated, and tracking solid-state diffusion across the confined interface.

      1. Balancing the Nitrogen Budget

          Setting the global budget for elements presents difficult challenges, such as accounting for possibly unknown sources or sinks. An unresolved imbalance in the oceanic nitrogen budget suggests that there may be additional sources of biological nitrogen fixation in the deep sea. Using high-resolution imaging techniques, Dekas et al. (p. 422; see the Perspective by Fulweiler) observed direct assimilation of isotopically labeled N2 by anaerobic methane-oxidizing archaea from deep marine sediment and the subsequent transfer of nitrogen to their sulfate-reducing bacterial symbionts. This slow and energetically costly conversion by archaea is dependent upon methane and requires physical contact with the associated bacterial partner. Such syntrophic consortia represent a potential source of nitrogen in the oceans and may help to balance the global nitrogen budget.

        1. Like Beads on a String

            The optimal packing of spheres, and the somewhat lower densities obtained in the compaction of granular materials are well studied problems. What is less clear is what happens when the spheres are connected, as in the case of polymeric materials—often represented by connected sphere models. Zou et al. (p. 408; see the Perspective by Reichhardt and Lopatina) examined the packing of chains of metal beads commonly used for securing bathroom drain plugs or for raising or lowering window blinds. Both the length of the chains, and whether they were linear or looped, influenced the overall packing density. Jamming the chains together captured the key physics of the glass transition of polymeric materials.

          1. Magnetic Monopoles

              Magnets come with a north and a south pole. Despite being predicted to exist, searches in astronomy and in high-energy particle physics experiments for magnetic monopoles (either north or south on their own) have defied observation. Theoretical work in condensed-matter systems has predicted that spin-ice structures may harbor such elusive particles (see the Perspective by Gingras). Fennell et al. (p. 415, published online 3 September) and Morris et al. (p. 411, published online 3 September) used polarized neutron scattering to probe the spin structure forming in two spin-ice compounds—Ho2Ti2O7 and Dy2Ti2O7—and present results in support of the presence of magnetic monopoles in both materials.

            1. Haploid Medaka Stem Cells

                Although diploid embryonic stem cells have been generated by various means, there would also be value in deriving haploid stem cells. In these cells, recessive mutations in essential genes would show phenotypes that would not be apparent in heterozygous animals. Yi et al. (p. 430) used the medaka fish model system to generate haploid stem cells that show stable growth and pluripotency. In addition, a fertile female medaka fish was produced by haploid embryonic stem cell nuclear transfer into a normal egg. This system has potential for analyzing recessive genes, for example, in disease phenotypes or in various cell lineages in culture.

              1. Diddy Dinosaurs

                  CREDIT: SERENO ET AL.

                  Tyrannosaurs were the dominant large dinosaur predator during the Late Cretaceous. They have several distinct specialized features, including an oversized skull, huge hindlimbs, and tiny arms, that have been thought to have evolved in concert with their large size and carnivorous diet. Sereno et al. (p. 418, published online 17 September; see the Perspective by Clark) now describe an earlier, diminutive tyrannosaur from China that also has these common specializations. Thus, these features were not a result of size increase but appear to have been required for feeding efficiency at all sizes.

                1. Coupling Clocks and Metabolism

                    Circadian clocks in mammals coordinate behavior and physiology with daily light-dark cycles by driving rhythmic transcription of thousands of genes. The master clock in the brain is set by light, but clocks in peripheral tissues, such as the liver, are set by daily feeding. Such coupling should allow tissues to “anticipate” food consumption and optimize the timing of metabolic processes, but how nutritional status is communicated to peripheral clocks is unclear. Studying cell culture models and mice, Lamia et al. (p. 437; see the Perspective by Suter and Schibler) show that the nutrient-responsive signaling molecule AMPK (AMP-activated protein kinase) provides metabolic information to circadian clocks by triggering phosphorylation and subsequent degradation of the clock component cryptochrome-1. Thus—cryptochromes, which originally evolved as blue-light photoreceptors in plants, act as chemical energy sensors in mammals.

                  1. The Taming of the Silkworm

                      Silkworms, Bombyx mori, represent one of the few domesticated insects, having been domesticated over 10,000 years ago. Xia et al. (p. 433, published online 27 August) sequenced 29 domestic and 11 wild silkworm lines and identified genes that were most likely to be selected during domestication. These genes represent those that enhance silk production, reproduction, and growth. Furthermore, silkworms were probably only domesticated once from a large progenitor population, rather than on multiple occasions, as has been observed for other domesticated animals.

                    1. Teaching Teacher

                        High-school science teachers rarely have an opportunity to participate in scientific research. A program from Columbia University studies what happens when these teachers do get laboratory experience. After a summer program spanning 2 years, including professional development programs, as well as laboratory research, Silverstein et al. (p. 440) show that the teachers' students benefit. Students of teachers who participated in this summer research experience showed greater pass rates on the tough New York State Regents exams than did students of teachers who did not participate in the program.

                      1. A Fix to the Heart

                          CREDIT: DOMIAN ET AL.

                          Regenerative cardiovascular medicine is a promising avenue for therapeutic application in advanced heart failure. Although clinical trials have suggested some limited benefits in cell transplantation therapy, robust cardiac muscle formation is lacking. Domian et al. (p. 426) examined the developmental processes in normal mature cardiac muscle. A two-color murine reporter system was used to isolate committed ventricular progenitors, which were then used to build functional force-generating cardiac tissue. Such combinations of tissue engineering and stem cell biology may eventually lead to cardiac regenerative therapy.

                        1. Gee Fizz

                            The next time you enjoy a carbonated beverage, you can do so with an enhanced understanding of the molecular mechanism that provides its distinctive flavor sensation. Chandrashekar et al. (p. 443) genetically ablated specific sets of taste cells in mice and found that the sensation of CO2 was lost in animals lacking taste cells that sense sour flavors. A screen for genes specifically expressed in these cells revealed the gene encoding carbonic anhydrase 4, which catalyzes hydration of CO2 to form bicarbonate and free protons. Knockout animals not expressing the carbonic anhydrase 4 gene also showed diminished sensation of CO2. The protons produced by the enzyme appear to be the actual molecules sensed by the sour-sensitive cells. This process, combined with tactile sensations, appears to be the source of the popular fizzy sensation.

                          1. Subcortical Network Regulation

                              Subcortical neuromodulatory centers dominate the motivational and emotional state–dependent control of cortical functions. Control of cortical circuits has been thought to involve a slow, diffuse neuromodulation that affects the excitability of large numbers of neurons relatively indiscriminately. Varga et al. (p. 449) describe a form of subcortical control of cortical information processing whereby strong, spatiotemporally precise excitatory input from midbrain serotonergic neurons produces a robust activation of hippocampal interneurons. This effect is mediated by a synaptic release of both serotonin and glutamate and impacts network activity patterns.