This Week in Science

Science  09 Nov 2012:
Vol. 338, Issue 6108, pp. 720
  1. Double-Edged Iron

    Hepcidin is a small peptide hormone discovered by three groups investigating iron-regulated liver genes and antimicrobial peptides. This new hormone turned out not only to regulate iron but also to have homology with peptides required for innate immune responses. Drakesmith and Prentice (p. 768) review the importance of hepcidin during infection, explaining how it is involved in withholding iron from microbial pathogens to curtail replication and how intracellular bacteria are able to thwart this host response. Recent work highlights the potential hazards of iron-supplementation in infection, particularly in malaria, whereby an overload of iron, meant to treat malaria-induced anemia, may negate the protective effects of hepcidin.

  2. Not Single After All

    Planetary nebulae form toward the end of the lives of sunlike stars. They appear after the star has shed its outer layers, and radiation from what is left of it ionizes the surrounding medium. Using the Very Large Telescope in Chile, Boffin et al. (p. 773) obtained spectra of the star at the center of Fleming 1, a point-symmetric planetary nebula with rotating bipolar jets. It has long been assumed that jets like these arose from an interacting binary system. Indeed, the data reveal that the central star in Fleming 1 has a companion in a very close orbit.

  3. A New Rock from Mars


    On 18 July 2011 a meteorite originating from Mars fell on the moroccan desert. Chennaoui Aoudjehane et al. (p. 785, published online 11 October) show that this meteorite was ejected from the surface of Mars 700,000 years ago and contains components derived from the interior, surface, and atmosphere of the red planet. Previous to this fall, only four other martian meteorites have been collected after being witnessed falling to Earth. All the other martian meteorites that are represented in collections around the world, have been found long after their arrival on Earth, and thus have suffered from exposure to the terrestrial environment.

  4. Keeping Coral Clean


    Seaweed overgrowth is a major problem for coral reefs and also seems to be a consequence of excessive harvesting of herbivorous fish. Dixson and Hay (p. 804) examined this effect on Fijian reefs. Species of small herbivorous gobies and coral-associated damselfish were compared for their effect on the toxic Chlorodesmis seaweed in experiments that required caging colonies of the branching coral Acropora nasuta and the associated fish species. Only the gobies actively removed algal fronds attached to the cages and only one species (itself toxic to predators) ate them; the damselfish simply defected from the arena when toxic algae were present. The hydrophobic toxins exuded in the algal mucus lysed coral polyps releasing cell constituents that, together with the algal toxin, attract the gobies, which then eat the algal fronds. Interestingly, the toxic goby became more toxic to predators after consumption of the seaweed, which may help to drive symbiosis with a coral colony.

  5. Thermally Transforming Thin Films

    Nanoscale features can be created by the phase separation that occurs in block copolymers that join together polymer segments with different wetting properties. For applications such as lithography, it is useful to generate small features and to orient them through simple processing steps. Top-layer coatings should be able to help drive alignment, but it is difficult to coat a layer that also has strong enough interactions to influence assembly. Bates et al. (p. 775) developed a water-soluble polymer that can top-coat lamellaforming block copolymers and that transforms during thermal annealing into a neutral wetting layer that helps drive the formation of vertically oriented lamellae.

  6. Putting Noise to Work

    Extracting energy from a noisy system (one that moves randomly) is possible if a mechanical system can induce periodicity on its motion, a phenomenon called stochastic resonance. Lotze et al. (p. 779; see the Perspective by Bockrath) created stochastic resonances between the cantilever attached to a scanning tunneling microscope tip and a hydrogen molecule adsorbed on a copper surface at cryogenic temperatures. The tunneling electrons at a particular bias voltage could excite the hydrogen molecules and couple their random motion to that of the cantilever, driving periodic oscillations.

  7. Maya and Climate

    Climate has affected the vitality of many different societies in the past, as shown by numerous records across the globe and throughout human history. One of the most obvious and spectacular examples of this is from the Classic Maya civilization, whose advanced culture left highly detailed records of all aspects of their existence between 300 and 1000 C.E. Kennett et al. (p. 788; see the cover) present a detailed climate record derived from a stalagmite collected from a cave in Belize, in the midst of the Classic Maya settlement. The fine resolution and precise dating of the record allows changes in precipitation to be related to the politics, war, and population fluctuations of the Mayans.

  8. Sensitivity Training

    Equilibrium climate sensitivity—the increase in global mean surface air temperature caused by the doubling of the concentration of atmospheric CO2—is needed to predict anthropogenic climate change. For decades, models have estimated its value to be between 1.5° and 4.5°C, but with too high an uncertainty to have strong predictive value. Fasullo and Trenberth (p. 792; see the Perspective by Shell) show that the seasonal variation of tropospheric relative humidity is related to climate sensitivity and could be used to constrain models and narrow the range of equilibrium climate sensitivity estimates, thereby allowing more accurate predictions of future warming.

  9. Wound Healing and Immunity

    Although wound healing and infection are often overlapping processes, whether the wound healing response modulates the immune response is not well understood. Doronin et al. (p. 795, published online 27 September; see the Perspective by Herzog and Ostrov) now show that coagulation factor X, an important component of the blood clotting cascade, helps to trigger antiviral immunity in response to adenovirus infection in mice. Factor X binds to human type C adenovirus with very high affinity. Structural analysis identified the critical binding residues between factor X and adenovirus, which, when mutated, inhibited binding. Despite being able to infect splenic macrophages in mice, transcriptional profiling of spleens from mice infected with a mutant adenovirus unable to bind to factor X revealed impaired activation of signaling cascades associated with innate immunity.

  10. Observing Sperm Factories


    The blood-testis barrier creates a permissive microenvironment for germ cell differentiation and protects meiotic germ cells from autoimmunity. To become sperm, large cysts of germline progenitors must cross the blood-testis barrier without causing damage. Smith and Braun (p. 798, published online 20 September) show that somatic Sertoli cells, the gatekeepers of the blood-testis barrier, manage this task by building a network of compartments bounded by old and new tight junctions. Intercellular bridges, which connect germ cells within cysts, span the transient compartments by passing through vertical strands at tricellular junctions. Only after the formation of a new blood-testis barrier are old tight junctions removed and germ cells released.

  11. Optogenetic Control

    Fluorescent proteins are widely used as optical sensors; however, optical control of protein activity remains challenging. Zhou et al. (p. 810) describe an approach that allows both sensing and control. Domains of the fluorescent protein Dronpa were designed to dimerize with one another and were fused to each of the termini of an enzyme domain. In the dark, the domains dimerized and inhibited the enzyme. However, exposure to light induced Dronpa dissociation and activated the enzyme, allowing optogenetic control.

  12. To Die For

    The unfolded protein response (UPR) adjusts the protein folding capacity of the endoplasmic reticulum (ER) to match demand. UPR signaling requires IRE1α, an ER transmembrane kinase-endoribonuclease (RNase) that becomes activated by unfolded protein accumulation within the ER and excises a segment in XBP1 messenger RNA (mRNA) to initiate production of the homeostatic transcription factor XBP1s. However, if ER stress is irremediable, sustained IRE1α RNase activity triggers cell death. Severe ER stress activates the protease Caspase-2 as an early apoptotic switch upstream of mitochondria. However, the molecular events leading from the detection of ER stress to Caspase-2 activation are unclear. Upton et al. (p. 818, published online 4 October) now report that IRE1α is the ER stress sensor that activates Caspase-2, and does so through a mechanism involving non-coding RNAs. Under irremediable ER stress, IRE1α's RNase triggers the rapid decay of select microRNAs that normally repress translation of Caspase-2 mRNA, rapidly increasing Caspase-2 levels as the first step in its activation.

  13. Bud, This Mitochondrion's for You

    How is organelle size adjusted to be appropriate for cell size? Rafelski et al. (p. 822) used a quantitative method for measuring mitochondria in living budding yeast cells and found that rather than using the apparently simplest mechanism of dividing the organelles equally among the mother and daughter cells, the cells adjusted the mitochondrial level in the bud, independent of the mother's own mitochondrial content, size, or age.

  14. An Organic Knot

    When people tie knots, they grasp both ends of a strand and loop them around each other. The task is rather more difficult at the molecular level, when there is no top-down organizing framework, and the strand needs to be coaxed into looping around itself. Recently, chemists have taken advantage of the tight geometrical restrictions of metal-ligand coordination to produce knot morphologies. Ponnuswamy et al. (p. 783; see the Perspective by Siegel) now demonstrate the spontaneous assembly of a trefoil knot from organic fragments without assistance from metal centers. The topology appears to be driven by hydrophobic interactions among aromatic segments that minimize exposure to surrounding water through their overlap.

  15. Sexual Versus Natural Selection

    In a 1948 experiment with Drosophila, Angus Bateman showed that sexual selection is strongest in males (versus females) because the number of mates they obtain is correlated with the number of offspring they produce. This relationship between mate number and offspring number has since become known as the Bateman slope, but there has been much debate concerning whether such a relationship can be expected in nature. Using a 30-year data set collected from a closed population of pronghorn antelope in Montana, Byers and Dunn (p. 802; see the Perspective by Wade) show that the relationship between mate number and offspring number indeed provides an opportunity for strong sexual selection in this polygynous and dimorphic species—however, environmental effects can make such selection null. In years when coyote predation on newborn fawns was high, sexual selection could not operate and reproductive success was instead determined by natural selection.

  16. Dissecting Phosphine Resistance

    Worldwide populations of pest insects—such as the lesser grain borer, Rhyzopertha dominica, and the rust-red flour beetle, Tribolium castaneum—have become highly resistant to the fumigant phosphine, providing a potential threat to global food security. The nematode, Caenorhabditis elegans is vulnerable to phosphine, but phosphine-resistant strains are known. Schlipalius et al. (p. 807) show that mutations in the delta-1-pyrroline-5-carboxylate dehydrogenase and dihydrolipoamide dehydrogenase (dld-1) genes both give rise to phosphine resistance in C. elegans. Phosphine resistance mutants in R. dominica, and T. castaneum also map to the dld-1 gene, which codes for a core metabolic enzyme. These mutants are, however, hypersensitive to arsenic, mimics of which might thus synergize with phosphine.

  17. Mitochondrial Lipid Trafficking

    Disturbances in cellular membrane lipid composition have severe functional consequences and are often associated with disease. How mitochondria—dynamic organelles that constantly fuse and divide—maintain their phospholipid composition and adjust it to physiological needs is unknown. Some phospholipids, like cardiolipin and phosphatidylethanolamine, are synthesized in the mitochondrial inner membrane from precursor molecules that are imported from the endoplasmic reticulum. Connerth et al. (p. 815) examined mechanisms determining the accumulation of cardiolipin in yeast mitochondria. A combination of quantitative lipidomics, yeast genetics, ultrastructural studies, and biochemical in vitro assays suggested that the protein Ups1 shuttles the precursor lipid phosphatidic acid between outer and inner mitochondrial membranes. Ups1 mediates lipid transport in complex with another mitochondrial protein, Mdm35, which stabilizes Ups1 in a transport-competent conformation and protects it against proteolysis. High cardiolipin concentrations inhibited the transport of phosphatidic acid by Ups1, which appears to provide a feedback control system that limits the accumulation of cardiolipin in mitochondrial membranes.