This Week in Science

Science  04 Jun 2010:
Vol. 328, Issue 5983, pp. 1203
  1. Making Menorahs


      Highly branched neuronal processes are typical of numerous neurons. The mechanisms controlling the formation, maintenance and regeneration of neuronal trees are poorly understood. Oren-Suissa et al. (p. 1285, published online 6 May) observed the development of extensively arborized neurite trees—menorahs—in two highly branched mechanosensory neurons in Caenorhabditis elegans. EFF-1, a fusion protein known to be important for cell fusion, played a key role in arborization.

    1. All Together Now

        An atom, when excited, will typically decay with a characteristic decay time. An ensemble of atoms, collectively coupled together with just one of the atoms excited will conspire to decay much faster than the single atom case. This enhancement of light-matter interaction is known as superradiance. Röhlsberger et al. (p. 1248, published online 13 May; see the cover; see the Perspective by Scully and Svidzinsky) present the realization of an artificial superradiant system comprising resonant iron atoms embedded in a semiconductor cavity and excited by synchrotron radiation and report the signature collective Lamb shift expected from the cooperative interaction and enhanced decay rate. The availability of such a controlled system to look closer at this effect should shed light on its role in natural and complex light-harvesting systems, and possibly allow the production of more efficient solar cells.

      1. Selectively Spun

          Biaryl compounds, in which two phenyl rings are linked by a single bond, exhibit an interesting sort of chirality, termed atropisomerism. If bulky substituents block the mutual rotation of the rings about the linking bond, then two isomers can be isolated that differ only in the direction one ring has swiveled away from the plane of the other. This feature is useful in ligand design for asymmetric catalysis and also appears in a number of polycyclic natural products. However, selective synthesis of a single isomer is difficult. Gustafson et al. (p. 1251) now show that a simple tripeptide derivative acts as an efficient catalyst for this purpose, trapping a freely rotating precursor in one orientation through selective bromination; the large bromine substituents then inhibit further swiveling.

        1. Not So Random Walk

            In rotaxanes, a molecular ring can shuttle back and forth between docking sites along an axle. Panman et al. (p. 1255) traced the intricacies of this shuttling motion using vibrational spectroscopy. The kinetics were dominated by the slow scission of hydrogen bonds tying the ring to its starting site. Varying the length of the axle allowed the extraction of relative rates for forward and backward motion once the ring was free: Somewhat surprisingly, forward motion toward the destination site was slightly hindered relative to regression toward the starting place.

          1. Down the Drain

              CREDIT: YU ET AL.

              A pervasive cooling event affected much of the Northern Hemisphere approximately 9300 years ago. This event was accompanied by changes in ocean circulation in the North Atlantic, forced presumably by a large injection of fresh water produced by melting of the Laurentide Ice Sheet, but the source, magnitude, and routing of the meltwater remain unknown. Yu et al. (p. 1262, published online 29 April) present evidence that the trigger for this cooling episode was an outburst flood from Lake Superior. Reconstructing lake-level changes in the Superior basin suggests that a rapid fall of lake level of about 45 meters occurred 9300 years ago, possibly due to the sudden failure of a drift dam. Rapid drainage through the North Bay–Ottawa River–St. Lawrence River valleys into the North Atlantic should have been sufficient to disturb ocean circulation in line with the geologic record.

            1. The Past Looks Hazy

                During the first couple of billion years of Earth's history, the Sun is thought to have been 30% less luminous than today, yet the surface of the planet was warm enough to prevent glacier formation and for early life to become established. Why were temperatures so high, despite the lower flux of solar energy? Wolf and Toon (p. 1266; see the Perspective by Chyba) propose that the presence of a photochemical haze with a fractal size distribution was the reason. Such a haze, unlike one composed of spherical particles assumed in previous models, could have been opaque enough to block ultraviolet radiation that would have hindered or prevented life from arising, but transparent enough in the shorter wavelengths to keep the atmosphere warm.

              1. Insects in the Wild

                  Insects are of fundamental importance to terrestrial ecosystems and provide laboratory model systems for the study of physiology and genetics. Studies examining how natural and sexual selection operate to drive evolution in wild populations have often neglected invertebrates, resulting in a chasm between our understandings of how things work in the lab versus the natural environment. Rodríguez-Muñoz et al. (p. 1269; see the Perspective by Zuk) bridge this gap by comprehensively monitoring the life histories, behavior, and reproductive success of an entire population of field crickets. Adding genetic data allowed evaluation of how behavior impacts reproductive success and confirmed that male reproductive success varies more than that of females.

                1. Influenza Escape Tricks

                    Tamiflu, or oseltamivir, has been extensively stockpiled by several governments in anticipation of a dangerous influenza pandemic. So far, its large-scale use has not been required, but, despite this, resistance has emerged in seasonal strains mediated by a single point mutation of histidine to tyrosine in the 274 residue (H274Y) of neuraminidase. When the resistant virus was first discovered in 1998, it grew poorly, but by 2008 the virus was reinvigorated and the mutation had spread worldwide in seasonal influenza. So what happened that improved viral fitness so radically? Bloom et al. (p. 1272; see the Perspective by Holmes) show that the H274Y mutation hinders the folding of the neuraminidase enzyme. In the more vigorous recent oseltamivir-resistant isolates, other mutations compensate for the deleterious effect of H274Y and restore fitness to the virus.

                  1. Seeing REDD

                      CREDIT: INSTITUTO CENTRO VIDA

                      Reducing emissions from deforestation and degradation (REDD) has become a key economic mechanism in the global strategy to reduce the rate of anthropogenic climate change. For Brazilian Amazonia, Aragão and Shimabukuro (p. 1275) point out that the efficacy of REDD could depend not only on avoiding deforestation but also on tackling fire usage. A satellite-based time-series of Amazonian deforestation rates and fire incidence suggests an increased trend in fire incidence across most of the region, despite the overall decrease in deforestation rates. The survey also suggests that the introduction of managed agriculture, instead of traditional slash-and-burn land use in already deforested areas, may promote a reduction of in fire incidence in the Amazon.

                    1. Germline Quality Control

                        The p53 tumor suppressor protein plays a key role in protecting organisms from aberrant cancer cells. But during evolution, animals would rarely, if ever, have lived long enough to develop cancer and so need such a function of p53. What, then, were the original functions for which p53 was selected? Lu et al. (p. 1278) observed a pulse of p53 activation during Drosophila development in the female germ line. In cancer, p53 is activated in response to DNA damage. Similarly, in this study, breaks in DNA that occur normally during meiosis also caused p53 activation. In animals in which resolution of DNA breaks during crossing over was inhibited, activation of p53 was prolonged; furthermore, when p53 was also lacking oogenesis was abnormal. Exactly how activation of p53 contributes to the process of chromosome recombination during meiosis remains unclear, but it may provide quality control, only allowing survival of gametes that possess intact DNA.

                      1. Pharmacological Forgetting

                          Extinction of conditioned fear forms a new memory in the infralimbic medial prefrontal cortex that is critical for subsequent retrieval of the extinction memory. Understanding the mechanisms that underlie this extinction-related plasticity could help in the treatment of anxiety disorders. By infusing BDNF (brain-derived neurotrophic factor) into the infralimbic cortex, Peters et al. (p. 1288) caused the extinction of conditioned fear, even without an extinction trial. In fact, BDNF infusion seemed to act as if an extinction training session had been given. Thus, the hippocampus is a likely source of the BDNF input to the infralimbic cortex, and individual differences in extinction memory may reflect variations in hippocampal BDNF content.

                        1. Keeping Tabs on Second-Messenger Localization

                            The prokaryotic second-messenger cyclic diguanosine monophosphate (c-di-GMP) is a global bacterial signaling molecule that controls the switch between motile or planktonic life-styles and sedentary or adhesive life-styles. Regulation by this second messenger has been associated with virulence traits, biofilm formation, and antibiotic tolerance. Christen et al. (p. 1295) engineered fluorescence resonance energy transfer–based sensors that enabled the visualization of c-di-GMP fluctuations in individual bacterial cells. Using the sensor, c-di-GMP distribution was found to change during the cell cycle in Caulobacter crescentus and Pseudomonas aeruginosa cells.

                          1. Quantum Spin Liquid

                              In antiferromagnets, the lowest-energy state is reached when neighboring spins on the underlying lattice point in opposite directions. Because of geometric constraints on some lattices (such as the two-dimensional triangular lattice), this magnetic ordering cannot be achieved even at temperatures close to absolute zero, and these compounds are predicted to be in a quantum spin liquid state. Yamashita et al. (p. 1246) measured the thermal conductivity of a recently identified candidate quantum spin liquid, the organic compound EtMe3Sb[Pd(dmit)2]2, and characterized its lowest-lying excitations. Two types of excitations were observed: ballistically propagating gapless excitations and excitations associated with a finite spin gap. These results contribute to our understanding of this unusual state of matter, which is potentially relevant to other two-dimensional quantum systems.

                            1. The Weakness of HO3

                                OH radicals play a critical role in the chemistry of Earth's atmosphere. Understanding atmospheric reaction networks thus requires an accurate knowledge of OH sources and sinks. One vexing question has been whether or not a significant pool of OH binds temporarily with oxygen to form HO3. Le Picard et al. (p. 1258) have succeeded in measuring the equilibrium constant for this reaction using sensitive fluorescent tracking of OH in a laboratory apparatus. This measurement was then used to quantify the strength of the O2–OH bond, which was found to be too weak for the complexation to play a major role in the atmosphere.

                              1. Initiator for Coated Pit Assembly

                                  During clathrin-mediated endocytosis, it has been thought that the sensing and binding of the clathrin adaptor protein AP2 to cargo and lipids leads to the recruitment of clathrin, nucleating the formation of a clathrin-coated pit. Henne et al. (p. 1281, published online 6 May) have now found that this process of AP2 binding may not in fact represent either the first or the nucleation event of endocytosis. Instead, ubiquitous proteins called FCHo1/2 (F-BAR proteins) bind to the plasma membrane and define the sites of endocytosis independently of AP2. The F-BAR protein can generate very low curvature and, at higher concentration, generates higher curvature like those required at the neck of budding vesicles. The C terminus of the protein has a mu-homology domain (with homology to the mu domain of the AP2 complex) that interacts with Eps15 and intersectin, and via these proteins recruits AP2, which further recruits clathrin. Thus, a curvature-inducing protein can act to nucleate clathrin-coated pit assembly during endocytosis.

                                1. Sepsis Protection

                                    Sepsis is a serious medical condition characterized by an uncontrolled inflammatory response to infection. Sepsis often results in organ failure and/or death, and current treatments are not very effective. Puneet et al. (p. 1290) now show that the enzyme sphingosine kinase 1 (SphK1) may represent an important therapeutic target for the treatment of sepsis. SphK1 expression increased on human phagocytes in response to bacterial products and was also highly expressed on phagocytes from septic patients. Inhibition of SphK1 reduced the production of inflammatory mediators in vitro by human phagocytes stimulated with bacterial products. In vivo, pretreatment with small interfering RNA against SphK1 or a specific SphK1 inhibitor protected mice from death in two lethal models of sepsis. Protection was also seen when mice were treated with the SphK1 inhibitor up to 8 hours after sepsis induction, and this protection was enhanced if mice were given a broad-spectrum antibiotic.