This Week in Science

Science  22 Jan 2010:
Vol. 327, Issue 5964, pp. 391
  1. Making Connections


      Genetic interaction profiles highlight cross-connections between bioprocesses, providing a global view of cellular pleiotropy, and enable the prediction of genetic network hubs. Costanzo et al. (p. 425) performed a pairwise fitness screen covering approximately one-third of all potential genetic interactions in yeast, examining 5.4 million gene-gene pairs and generating quantitative profiles for ∼75% of the genome. Of the pairwise interactions tested, about 3% of the genes investigated interact under the conditions tested. On the basis of these data, a reference map for the yeast genetic network was created.

    1. Iapetus Revealed

        The striking appearance of Saturn's moon Iapetus—half black and half white—has puzzled astronomers for over three centuries. Now Spencer and Denk (p. 432, published online 10 December) present an explanation for this asymmetry: A thermally controlled runaway migration of water ice triggered by exogenic deposition of dark material on the moon's leading darker side, which faces the direction of motion of the moon in its orbit around Saturn. This mechanism is unique to Iapetus because it rotates slowly enough to allow large temperature variations to arise, it is small enough to allow long-range migration of water, and there is a source of dust to trigger the process. In a related paper, Denk et al. (p. 435, published online 10 December) present data derived from the Cassini Imaging Science Subsystem that reveal that both dark and bright materials on the leading side of Iapetus are redder than their trailing-side counterparts. This asymmetry results from the deposition of dust and debris from other moons in the saturnian system—the very same process that initiates the thermal segregation proposed above.

      1. Miniature Transport Engineers

          In its vegetative phase, the slime mold Physarum polycephalum “slimes” its way through the world seeking food. As it explores, it links previously found food sources with tubular structures. Tero et al. (p. 439; see the Perspective by Marwan) report that if food sources are deposited on a plate in a pattern corresponding in miniature to the positions of the cities that surround Tokyo, the resulting network of Physarum tubules that develops is rather similar in structure to the railroad network that connects the Japanese cities. A model was produced that describes the adaptive network development displayed by the slime mold. This biologically inspired model might provide insight into how to implement properties like resistance of transport systems to local failures into similar human-designed systems.

        1. Colloids as Models


            Colloids are often used as analogs for atoms in order to study crystallization kinetics or glassy dynamics using particles that are much easier to observe and that move on much slower time scales. Ganapathy et al. (p. 445; see the Perspective by Einstein and Stasevich) consider whether the analogous behavior extends to the growth of epitaxial films, a technique that is used in manufacturing. Controlling the rate of addition of the colloidal particles allowed the mapping of diffusional pathways during film nucleation and growth on a patterned substrate. The same relationships used to describe atomistic growth could be applied to the colloidal systems, but certain growth barriers such as those found at step edges and corners were controlled by diffusion rather than energetics.

          1. Dissecting Fermion Interactions

              Electrons, protons, and other building blocks of our universe belong to a class of particles we call fermions. Different interfermion interactions give rise to different forms of matter. In the strongly interacting resonant regime, however, fermionic systems have thermodynamic properties that depend only on the interparticle spacing and scaled temperature. Horikoshi et al. (p. 442) precisely characterize the thermodynamics in this universal regime for a system of ultracold fermionic lithium atoms. Analysis of a large number of trapped-gas density profiles confirms that the results depend neither on trap geometry nor the absolute temperature of the gas. The results are relevant to studies of all strongly interacting fermionic systems, including neutron stars and nuclear matter.

            1. How to Get a Date

                Radiometric dating relies on measuring the abundance of a radioactive isotope and/or its decay products. By knowing a decay rate and an isotopic starting abundance—both assumed to be constant—an age is determined. Using high-resolution mass spectrometry, Brennecka et al. (p. 449, published online 31 December; see the Perspective by Connelly) show that the known starting abundance of 238U and 235U isotopes in meteorites, which decay into 206Pb and 207Pb, respectively, is actually quite variable. Trace amounts of 247Cm in the early solar system may have unexpectedly contributed additional 235U, skewing the ratio. Pb-Pb dating, the method commonly used to date early solar system materials, may thus need a correction of up to 5 million years.

              1. Stormy Weather

                  One of the most active questions about the effects of global warming is whether, and how, it might affect the frequency and the strength of hurricanes. Some studies have suggested that warming will bring fewer, and less energetic, hurricanes, while others have claimed that we can expect more intense storms. Bender et al. (p. 454; see the news story by Kerr) explore the influence of global warming on hurricane dynamics over the Atlantic Ocean with a state-of-the-art hurricane prediction model. The model predicts that the annual total number of hurricanes in the 21st century will be less than now, but also that the number of the most intense storms per year will increase. The largest increase of the most intense hurricane frequency is predicted in the western Atlantic, which suggests that Hispaniola, the Bahamas, and the Southeast coast of the United States could be at greater risk.

                1. Late-Running Clock Components

                    Many mammalian cells contain a well-characterized biological clock with a 24-hour cycle. In the latter part of the day, transcription mediated by one of the clock components, the transcription factor made up of the CLOCK and BMAL1 proteins, is inhibited, but the mechanism of inhibition has been unclear. Robles et al. (p. 463) used mass spectrometry to identify proteins that RACK1 (receptor for activated C kinase–1), a scaffold protein that brings protein kinase C–α (PKCα ) into contact with its substrates, caused to be associated with BMAL1 at the time of day when its transcription-activating function was inhibited. Further studies implicated PKCα and RACK1 as integral components of the clock, without which the clock's free-running period was shortened.

                  1. Space and Spikelets

                      CREDIT: EPSZTEIN ET AL.

                      In neurons, spikelets are voltage fluctuations of small amplitude with a spike-like waveform. Spikelets are difficult to detect with extracellular techniques traditionally used to record neuronal activity in freely moving animals. Epsztein et al. (p. 474) used head-anchored whole-cell recordings to analyze spikelet activity during spatial exploration in freely moving rats. A high incidence of spikelets was often followed by action potentials. Like action potentials, spikelets were all-or-none, but had different kinetics and amplitude, and were clearly distinct from excitatory postsynaptic potentials and occurred to a different extent in different cells. In cells with clear place fields, spikelets had similar spatial firing preferences, as did regular action potentials. Thus, spatially modulated spikelets may be involved in information processing in cortical neuronal networks.

                    1. Garnering Information on Granulomas

                        In tuberculosis, the tuberculous granuloma has been viewed traditionally as a host-protective structure that serves to “wall off” mycobacteria. However, recent work in the zebrafish embryo showed that mycobacteria convert the nascent granuloma into a vehicle for bacterial expansion and dissemination. Thus, intercepting granuloma formation could provide a strategy for treating tuberculosis, an urgent public health goal in light of the epidemic of extensively drug-resistant tuberculosis. Now Volkman et al. (p. 466, published online 10 December; see the Perspective by Agarwal and Bishai) present the molecular pathway by which mycobacteria induce granulomas in zebrafish. Inhibition of this pathway attenuates infection by reducing granuloma formation, suggesting a therapeutic target for tuberculosis treatment.

                      1. MRSA, Close and Personal

                          Methods for differentiating pathogen isolates are essential for understanding their evolution and spread, as well as for the formulation of effective clinical strategies. Current typing methods for bacterial pathogens focus on a limited set of characteristics providing data with limited resolving power. Harris et al. (p. 469) used a high-throughput genome sequencing approach to show that isolates of methicillin-resistant Staphylococcus aureus (MRSA) are precisely differentiated into a global geographic structure. The findings suggest that intercontinental transmission has occurred for nearly four decades. The method could also detect individual person-to-person transmission events of MRSA within a hospital environment.

                        1. The Long and Short of It

                            Semi-arid forests cover close to 18% of Earth's land surface. If climate change were to stimulate carbon accumulation in these areas, resulting changes in the forests could both promote climate cooling and warming: On one hand, forest growth would draw CO2 from the atmosphere, providing a cooling effect on climate; on the other, as forests grew and became more dense, their albedo would decrease, which would warm climate. Rotenberg and Yakir (p. 451; see the Perspective by Schimel) now report that a shift in peak photosynthetic activities from summer to early spring would, indeed, cause carbon accumulation by the forests, but that a suppression of reflected longwave radiation effect would complement the better-known (shortwave) albedo effect, doubling the amount of potential warming. Several decades of carbon accumulation would thus be necessary to counteract these radiative changes.

                          1. Of Wnt, Prorenin Receptor, and V-ATPase

                              The Wnt protein binds to receptors at the cell surface and regulates signaling pathways that control a wide range of critical biological processes from stem cell differentiation to generation of cancer. In a screen for components required for Wnt signaling, Cruciat et al. (p. 459) discovered an unexpected partner, the prorenin receptor (PRR). PRR bound to the Wnt receptor proteins Fz8 and LRP6. The PRR protein could interact with the receptors and promote Wnt signaling without its cytoplasmic domain through which it initiates signaling in response to the prorenin protein. Its role in Wnt signaling appears to be rather different. The PRR binds to the vacuolar H+–adenosine triphosphatase (V-ATPase), a proton pump that can influence endocytosis by acidification of vesicles. The V-ATPase was also necessary for phosphorylation of LRP6 and Wnt. Thus, PRR may link the V-ATPase to the Wnt receptor protein LRP6, allowing acidification in the vicinity of the activated receptor, which appears to be necessary for phosphorylation of LRP6 and subsequent signaling.