This Week in Science

Science  27 May 2016:
Vol. 352, Issue 6289, pp. 1071
  1. Martian Climate

    Evidence for ice ages on Mars

    1. Keith T. Smith

    Contours are clearly visible on the martian polar ice cap.


    Models predict that Mars should have undergone ice ages in the past, but evidence has been scant. Smith et al. used radar measurements from NASA's Mars Reconnaissance Orbiter to probe the planet's polar ice caps. Clear layers within the ice allowed them to calculate how much ice was deposited at different times. The results provide evidence for a recent ice age on Mars. Understanding the martian climate will help determine when the planet was habitable in the past, and how that changed, and may inform studies of climate change on Earth.

    Science, this issue p. 1075

  2. Quantum Physics

    Quantum cats here and there

    1. Ian S. Osborne

    The story of Schrödinger's cat being hidden away in a box and being both dead and alive is often invoked to illustrate the how peculiar the quantum world can be. On a twist of the dead/alive behavior, Wang et al. now show that the cat can be in two separate locations at the same time. Constructing their cat from coherent microwave photons, they show that the state of the “electromagnetic cat” can be shared by two separated cavities. Going beyond common-sense absurdities of the classical world, the ability to share quantum states in different locations could be a powerful resource for quantum information processing.

    Science, this issue p. 1087

  3. Cardiovascular Disease

    Rhythm remodeling traced to tiny RNA

    1. Megan Frisk

    Despite treatment, atrial fibrillation (AF)—a type of irregular heart rhythm—is able to recur, by “remodeling” the heart tissue electrically and structurally to maintain its unsteady beat. Reilly et al. discovered a noncoding RNA, miR-31, that is responsible for such remodeling. An increase in miR-31 reduced neuronal nitric oxide synthase (nNOS) and dystrophin (which binds nNOS in muscle cells) in the fibrillating heart tissue of humans and goats. Because up-regulation of miR-31 and the resulting loss of dystrophin and nNOS are specific to the atrium, interventions targeted to this remodeling pathway could potentially provide a safer therapeutic option for AF patients than ablation and ion channel blockers.

    Sci. Transl. Med. 8, 340a74 (2016).

  4. Pharmacology

    Designing better estrogens

    1. Wei Wong

    Estrogen and its synthetic versions can enhance blood vessel repair after injury or improve metabolism in the liver and adipose tissue. But they can also cause breast or uterine cancer because they stimulate cell proliferation in these tissues. Madak-Erdogan et al. designed estrogen-like molecules that did not bind to receptors as strongly as estrogen and did not increase mammary gland growth or uterine weight in ovariectomized mice. These estrogens provided vascular and metabolic benefits and could perhaps be further developed as postmenopausal hormone replacement therapies.

    Sci. Signal. 9, ra53 (2016).

  5. Plant Science

    Calcium signals the making of symbiosis

    1. Pamela J. Hines

    Plant cell nuclei respond to signals from symbiotic nitrogenfixing rhizobial bacteria or arbuscular mycorrhizal fungi with oscillating Ca2+ release. Charpentier et al. identified a trio of responsible Ca2+ channels in a legume. These channels contain nuclear localization signals and are expressed in root cell nuclear envelopes. The channels function early in the establishment of symbiosis to produce oscillations in Ca2+ release from nuclear stores.

    Science, this issue p. 1102

  6. Genetic Mapping

    Narrowing down genetic loci of interest

    1. Laura M. Zahn

    Often it can be difficult to identify a gene or genetic region that underlies a specific trait. Traditional mapping relies on naturally occurring recombination events but can be limited in resolution by the natural recombination frequency. Sadhu et al. used CRISPR technology to more easily map genomic sites of interest in yeast. The method systematically introduces local recombination events, allowing for the fine mapping of trait variants. They used their method to identify the causative mutation responsible for altered manganese sensitivity in yeast.

    Science, this issue p. 1113

  7. Mucosal Immunology

    Genes and microbes converge in colitis

    1. Kristen L. Mueller

    Both host genetics and intestinal microbes probably contribute to a person's overall susceptibility to inflammatory bowel disease (IBD). The human gut microbe Bacteroides fragilis produces immunomodulatory molecules that it releases via outer membrane vesicles (OMVs). These molecules can protect mice from experimentally induced colitis. Chu et al. now find that OMV-mediated protection from colitis requires Atg16l1 and Nod2 genes whose human orthologs are associated with an increased risk for developing IBD. OMVs trigger an ATG16L1 and NOD2–dependent noncanonical autophagy pathway in dendritic cells (DCs). OMV-primed DCs, in turn, induce regulatory T cells in the intestine that protect against colitis.

    Intestinal structures are disrupted in colitis.


    Science, this issue p. 1116

  8. Cell Division

    Forcing the spindle to the cell center

    1. Beverly A. Purnell

    As a cell divides, the mitotic spindle segregates chromosomes to daughter cells. It does so while maintaining a centralized position; however, the mechanism for spindle placement is unclear. Garzon-Coral et al. used magnetic tweezers to show that the dynamical properties of the astral microtubules act as a force-generating machinery to keep the spindle at the cell center. The stiffness of the spring-like force increases during anaphase and also with decreasing cell size. This machinery is strong enough to quench thermal fluctuations to ensure precise localization of the spindle, but soft enough to allow molecular force generators to fine-tune the position of the mitotic spindle.

    Science, this issue p. 1124

  9. Atmospheric Particles

    From neutral to new

    1. H. Jesse Smith

    Many of the particles in the troposphere are formed in situ, but what fraction of all tropospheric particles do they constitute and how exactly are they made? Bianchi et al. report results from a high-altitude research station. Roughly half of the particles were newly formed by the condensation of highly oxygenated multifunctional compounds. A combination of laboratory results, field measurements, and model calculations revealed that neutral nucleation is more than 10 times faster than ion-induced nucleation, that particle growth rates are size-dependent, and that new particle formation occurs during a limited time window.

    Science, this issue p. 1109

  10. Inflammation

    Unwinding DNA and unleasing inflammation

    1. Kristen L. Mueller

    Fighting infections often comes with collateral damage, which sometimes can be deadly. For instance, in septic shock, the overwhelming release of inflammatory mediators drives multi-organ failure. Rialdi et al. now report a potential new therapeutic target for controlling excessive inflammation: the DNA unwinding enzyme topoisomerase I (Top1) (see the Perspective by Pope and Medzhitov). Upon infection, Top1 specifically localizes to the promoters of pathogen-induced genes and promotes their transcription by helping to recruit RNA polymerase II. Pharmacological inhibition of Top1 in a therapeutic setting increased survival in several mouse models of severe microbially induced inflammation.

    Science, this issue p. 10.1126/science.aad7993; see also p. 1058

  11. Organic Chemistry

    A radical route to ophiobolin rings

    1. Jake Yeston

    Chemical ring-closing cascades resemble molecular yoga. One reactive site on a linear precursor can pull the whole molecule into a remarkably complex polycyclic arrangement. Cyclase enzymes rely on substantial internal scaffolding to guide this process during the biosynthesis of the ophiobolin sesterterpene frameworks, which comprise two pentagons sharing edges with an octagon. Brill et al. now show that the same motif is accessible abiotically by tweaking the cascade mechanism to rely on neutral radical intermediates in place of the positively charged activated sites in the biosynthetic pathways.

    Science, this issue p. 1078

  12. Polymer Chemistry

    Precise control from a metal-free catalyst

    1. Jake Yeston

    Polymerization can be a rather dangerous free for all, with molecules joining randomly in chains at a chaotic pace. One of modern chemistry's great accomplishments has been the development of methods to assemble polymers in steady, orderly steps. However, order comes at a price, and often it's the need for metal catalysts that are hard to remove from the plastic product. Theriot et al. used theory to guide the design of a metal-free light-activated catalyst that offers precise control in atom transfer radical polymerization, alleviating concerns about residual metal contamination (see the Perspective by Shanmugam and Boyer).

    Science, this issue p. 1082; see also p. 1053

  13. Quantum Simulation

    Cold atoms do geometry

    1. Jelena Stajic

    Electrons in solids populate energy bands, which can be simulated in cold atom systems using optical lattices. The geometry of the corresponding wave functions determines the topological properties of the system, but getting a direct look is tricky. Fläschner et al. and Li et al. measured the detailed structure of the band wave functions in hexagonal optical lattices, one resembling a boron-nitride and the other a graphene lattice. These techniques will make it possible to explore more complex situations that include the effects of interactions.

    Science, this issue pp. 1091 and 1094

  14. Biochemistry

    The fuel for fungal enzymes

    1. Nicolas S. Wigginton

    Many microorganisms have specialized enzymes to target and break down plant biomass. In fungi, these enzymes, called lytic polysaccharide monooxygenases (LPMOs), partner with electron transfer partners to oxidatively cleave the polysaccharide backbone of lignocellulosic polymers. Kracher et al. examined several potential extracellular electron transfer partners for LPMO, including other enzymes and small redoxactive metabolites (see the Perspective by Martínez). All three were able to donate electrons to the single-copper active site. Such versatility helps these fungi adapt to a range of redox conditions and potentially use other extracellular electron donors to fuel biomass degradation.

    Science, this issue p. 1098; see also p. 1050

  15. Metal Acquisition

    A new metal scavenger for bacteria

    1. Nicolas S. Wigginton

    All cells must find a way to acquire trace metals. Bacteria and plants scavenge iron, for instance, by synthesizing and releasing iron-chelating compounds called siderophores. Ghssein et al. describe three enzymes in Staphylococcus aureus that are responsible for the biosynthesis of another type of metallophore (see the Perspective by Nolan). Metabolomics and a range of biochemical assays show that this compound, named staphylopine, is involved in the uptake of a range of metals, depending on the growth environment. The genes required for staphylopine biosynthesis are conserved across a number of pathogenic bacteria and are similar to those for a broad-spectrum metallophore produced by plants.

    Science, this issue p. 1105; see also p. 1055

  16. Cell Cycle

    Phosphorylation cues exit from mitosis

    1. L. Bryan Ray

    The entry and exit from the cell cycle are controlled by waves of protein phosphorylation and degradation events. Fujimitsu et al. describe the precise mechanism by which the cell cycle machinery controls exit from mitosis. The critical event is activation of a ubiquitin ligase, the anaphase-promoting complex or cyclosome (APC/C). The authors used purified components and the Xenopus egg extract system to show that two subunits of APC/C were directly phosphorylated by cyclin-dependent kinase 1 (CDK1). Phosphorylation of one subunit helped recruit CDK1 for further phosphorylation of another subunit. The second subunit interacted with the APC/C activator and target of anticancer therapy known as Cdc20.

    Science, this issue p. 1121

  17. Economics

    The promise of digital markets

    1. Julia Fahrenkamp-Uppenbrink

    Digital platforms are increasingly brokering matches between consumers and service providers. The highest-profile example of such “matching markets” is Uber, the taxi company that uses location tracking, computer navigation, and dynamic pricing to connect riders with drivers. In a Perspective, Azevedo and Weyl also highlight other examples, such as advertising, which used to be broadcast widely across populations but is now highly targeted thanks to digital tracking. The authors call for more collaborations between academia and industry to make these technologies more efficient and to help ensure that regulation is put in place to protect the public interest.

    Science, this issue p. 1056

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