This Week in Science

Science  26 Mar 2010:
Vol. 327, Issue 5973, pp. 1552
  1. Trophic Trade-Offs

      CREDIT: ELLEN CAITILIN WOODS

      There have been many attempts to document and explain the effects of predators on plant biomass in so-called “trophic cascades.” Theory suggests that fast-growing plants are relatively undefended and suffer more from herbivory, which implies a functional trade-off between investment in traits relating to growth and defensive strategies. Mooney et al. (p. 1642; see the Perspective by Hambäck) compared responses to fertilization and aphid predators in 16 milkweed species. As predicted, interspecific variation in the strength of top-down control in terms of a tradeoff with growth was observed.

    1. Platinum-Free Diesel

        The efficiency advantages inherent in diesel-based combustion engines are counterbalanced by the production of pollutants such as nitrogen oxides (NOx). Currently, expensive precious metals, such as platinum, are required to remove these pollutants. Kim et al. (p. 1624; see the Perspective by Parks) show that a strontium-doped perovskite catalyst, prepared from more abundant (and cheaper) elements, may help to lower the cost of NOx treatments and thus ultimately make diesel a more cost-effective automotive fuel. Under conditions realistically simulating exhaust streams, the catalyst rivaled platinum in accelerating NOx decomposition.

      1. Iron Exposure

          The macrocyclic heme motif coordinates iron ions in proteins and plays a widespread role in biochemical oxidative catalysis. Bezzu et al. (p. 1627) prepared crystals in which analogous iron-centered macrocycles were aligned in pairs. The outer faces of the pairs exposed the iron ions to vacant cavities, where ligand exchange could take place; the inner faces were bound together by rigid bridging ligands lending the crystals structural integrity. The stability and high porosity of these crystals lend themselves to potential catalytic applications.

        1. Preventing Radiation Damage

            Inside a nuclear reactor, long-term exposure to radiation causes structural damage and limits the lifetimes of the reactor components. Bai et al. (p. 1631; see the Perspective by Ackland) now show, using three simulation methods able to cover a wide range of time and length scales, that grain boundaries in copper can act as sinks for radiation-induced defects. The boundaries are able to store up defects, in the form of interstitials, which subsequently annihilate with vacancies in the bulk. This recombination mechanism has a lower energy barrier than the bulk equivalent, and so provides a lower-cost route for the copper to self-heal.

          1. Fermion Behavior in an Optical Lattice

              CREDIT: HACKERMÜLLER ET AL.

              Due to their extreme tunability, optical lattices loaded with fermions and bosons are expected to act as quantum simulators, answering complicated many-body physics questions beyond the reach of theory and computation. Some of these many-body states, such as the Mott insulator and the superfluid, have been achieved in bosonic optical lattices by simply changing the characteristic depth of the lattice potential wells. Now, Hackermüller et al. (p. 1621) describe an unusual effect in an optical lattice loaded with fermions: When the strength of the attraction between the fermions is increased adiabatically, instead of contracting, the gas expands in order to preserve entropy.

            1. Perfect Mismatch

                Heteroepitaxy, or the overgrowth of one crystalline material onto a second crystalline material, is a key fabrication method for making thin films and nanoparticles. But if the lattice mismatch between the two materials is too large or messy, fractured interfaces result. Zhang et al. (p. 1634) describe a synthesis strategy to obtain spherical nanoparticles with a core-shell architecture that does not depend on heteroepitaxy. Silver was deposited and converted to various semiconductors through a series of chemical transformations to yield structurally perfect single-crystal semiconductor shells on a gold core, despite mismatches approaching 50%.

              1. Lipid Kinase Revealed

                  The lipid kinase, Vps34, makes the key signaling lipid phosphatidylinositol 3-phosphate [PI(3)P] and has essential roles in autophagy, membrane trafficking, and cell signaling. It is a class III PI 3-kinase, a class against which there is currently no specific inhibitor. Miller et al. (p. 1638) now describe the crystal structure of Vps34. Modeling substrate binding and combining structural data with mutagenesis suggests a mechanism in which Vps34 is auto-inhibited in solution, but adopts a catalytically active conformation on membranes. Structures of Vps34 with existing inhibitors might allow for the generation of inhibitors with high affinity and specificity.

                1. Mosquito Double Act

                    Peroxidase/dual oxidase (duox) systems act in concert to catalyze the nonspecific formation of dityrosine bonds, which cross-link a variety of proteins. Knowing that these reactions are involved in fine-tuning insect immune responses, Kumar et al. (p. 1644, published online 11 March) investigated how the peroxidase/duox system in malaria-vector mosquitoes protects the gut flora by modulating midgut antibacterial responses. Generating immune reactions resulted in a loss of mosquito egg viability, but modulating host responses allowed malaria parasites to persist among the surviving commensal flora. The peroxidase/duox system appears to promote dityrosine bond formation between proteins across the surface of midgut epithelial cells to form a layer that inhibits immune recognition and mediator release. Interference with the formation of this layer might provide a target for mosquito and malaria control.

                  1. The Making of the Males

                      CREDIT: SAUMITOU-LAPRADE ET AL.

                      Most plants have a hermaphroditic mating system with flowers with both male and female function. However, in some cases, species are invaded by a sex-specific sterility factor. When female sterility factors invade a population, it results in a mating system called androdioecy. Theoretically, these female sterile (male) individuals should occur at low frequencies because of their reduced reproductive capacity. However, some species in the olive family have a greater than expected frequency of males. Saumitou-Laprade et al. (p. 1648) show that, for one species, males were able to reach high frequencies because of the retention of a self-incompatibility factor within hermaphroditic individuals. In this case, hermaphroditic individuals can only mate with individuals outside of their incompatibility type, reducing their available mating partners, whereas males are able to mate with all hermaphrodites. This explains how, contrary to theory, high frequencies of males can exist within populations.

                    1. A Pathway to Leukemia

                        Leukemia is initiated and maintained by a small number of self-renewing cells called leukemia stem cells (LSCs), which share properties with hematopoietic stem cells (HSCs), the self-renewing cells that produce healthy blood cells. Wang et al. (p. 1650) studied mouse models of acute myelogenous leukemia (AML), a disease that is often refractory to existing therapies. Activation of the Wnt/β-catenin signaling pathway was required for efficient oncogene-mediated conversion of HSCs into LSCs. This pathway is among the most well studied signaling pathways in cell biology, setting the stage for testing of β-catenin signaling antagonists in preclinical models of AML.

                      1. Heart Cell Signaling in 3D

                          A healthy heart relies on the proper transduction of cellular signals through the β1- and β2-adrenergic receptors (βARs), which are located on the surface of the heart's muscle cells (cardiomyocytes). The surface of these cells resembles a highly organized series of hills and valleys and it has been unclear whether this topography plays a role in the βAR signaling events that are critical to cell function. Nikolaev et al. (p. 1653, published online 25 February; see Perspective by Dorn) monitored the cyclic adenosine monophosphate (cAMP) signals generated by the βARs in living cardiomyocytes. In cells from healthy rats and from rats with heart failure, the β1ARs were localized across the entire cell surface. In contrast, the spatial localization of the β2ARs differed in healthy and failing cells. In healthy cardiomyocytes, the β2ARs resided exclusively within surface invaginations called transverse tubules, thereby producing spatially confined cAMP signals, whereas in failing cardiomyocytes, the β2ARs redistributed to other cell surface areas, thereby producing diffuse cAMP signals. Thus, changes in the spatial localization of β2AR-induced cAMP signaling may contribute to heart failure.

                        1. Shelterin the Ends

                            The ends of linear chromosomes suffer two problems: They cannot be replicated to their termini, resulting in loss of terminal sequences; and they can be mistakenly sensed as DNA double-strand breaks, activating DNA repair pathways that can result in serious genome derangement. These problems are solved by the addition of telomeres, repeat sequences at the ends of chromosomes, which are shielded by a protein complex called shelterin. Sfeir et al. (p. 1657) show that the mouse Rap1 protein, which is part of the shelterin complex and which binds to a second shelterin protein called TRF2, helps prevent telomeres undergoing unscheduled homologous recombination. Such recombination could threaten telomere integrity by generating sequence exchanges between sister telomeres resulting in critically shortened telomeres.

                          1. “Spontaneous” Release Trigger

                              Synaptic vesicle release occurs in different phases that can be tightly coupled to action potentials (synchronous), immediately following action potentials (asynchronous), or as stochastic events not triggered by action potentials (spontaneous). The vesicle protein synaptotagmin is thought to act as the Ca2+ sensor in the synchronous phase, but for the other two phases, Ca2+ sensors have not been identified. Groffen et al. (p. 1614, published online 11 February) now show that cytoplasmic proteins known as Doc2 (double C2 domain) proteins are required for spontaneous release. Doc2 proteins promote membrane fusion in response to exceptionally low increases in Ca2+, and are several orders of magnitude more sensitive to Ca2+ than synaptotagmin. Doc2 and synaptotagmin compete for SNARE-complex binding during membrane fusion. A mutation that abolishes the Ca2+ dependence of Doc2b also abolishes the Ca2+ dependence of spontaneous release. Thus, Doc2 is a high-affinity Ca2+ sensor for spontaneous release that competes with synaptotagmin for SNARE complex binding.

                            1. News from the Dark Side?

                                Dark matter is thought to represent 85% of all matter in the universe and to have been responsible for the formation of structure in the early universe, but its nature is still a mystery. Ahmed et al. (p. 1619, published online 11 February; see the Perspective by Lang) describe the results from the completed Cryogenic Dark Matter Search (CDMS II) experiment, which searched for dark matter in the form of weakly interacting massive particles (WIMP). Two candidate signals were observed, whereas only one background event was expected. The probability of having two or more events from the background would have been 23%. The results of this analysis cannot be interpreted with confidence as evidence for WIMP interactions, but, at the same time, neither event can be ruled out as representing signal.