This Week in Science

Science  23 Oct 2015:
Vol. 350, Issue 6259, pp. 394
  1. Chloroplasts

    Quality control one chloroplast at a time

    1. Stella M. Hurtley

    False-color TEM image of chloroplasts (green) undergoing various levels of degradation

    PHOTO: SALK INSTITUTE

    How do plant cells get rid of chloroplasts that are not working as they should? Woodson et al. describe a chloroplast quality-control pathway that allows for the selective elimination of individual chloroplasts. Damage by reactive oxygen species during photosynthesis is recognized by a ubiquitin ligase, which marks out damaged chloroplasts for degradation. The findings reveal how cells balance inherently stressful energy production with organelle turnover.

    Science, this issue p. 450

  2. Innate Immunity

    Inflammasomes take the wheel

    1. Kristen L. Mueller

    Cells require microbial ligand binding to sense pathogens (see the Perspective by Liu and Xiao). Binding to the family of NOD-like receptors triggers the assembly of large protein signaling complexes called inflammasomes, leading infected cells to die and produce inflammatory mediators. Hu et al. and Zhang et al. use cryo–electron microscopy to uncover the structural and biochemical basis of two such receptors: NAIP2, which directly binds microbial ligands, and NLRC4, a protein functioning directly downstream. A self-propagating activation mechanism of downstream inflammasome signaling starts with one molecule of NAIP4 directly binding its microbial ligand. NAIP4 then catalyzes the activation of 10 to 12 NLRC4 molecules to form a wheel-like structure.

    Science, this issue p. 399, 404; see also p. 376

  3. Superconductivity

    A 3D approach to make 2D superconductors

    1. Jelena Stajic

    When the thickness of a superconducting film becomes comparable to the typical size of its electron pairs, its superconductivity enters a two-dimensional (2D) regime. Thinner films usually have higher amounts of disorder, making it difficult to isolate the 2D effects. To circumvent this limitation, Saito et al. induced charge carriers on the surface of the 3D insulator ZrNCl. This approach produced a clean superconducting layer thinner than the unit cell of the crystal. The superconducting state was extremely sensitive to the application of a perpendicular magnetic field, as expected for clean systems.

    Science, this issue p. 409

  4. Quantum Optics

    Probing the fluctuating vacuum

    1. Ian S. Osborne

    According to quantum mechanics, a vacuum is not empty space. A consequence of the uncertainly principle is that particles or energy can come into existence for a fleeting moment. Such vacuum or quantum fluctuations are known to exist, but evidence for them has been indirect. Riek et al. present an ultrafast optical based technique that probes the vacuum fluctuation of electromagnetic radiation directly.

    Science, this issue p. 420

  5. Microbial Metabolism

    Methane cycling gets more diverse

    1. Nicholas S. Wigginton

    The production and consumption of methane by microorganisms play a major role in the global carbon cycle. Although these processes can occur in a range of environments, from animal guts to the deep ocean, these metabolisms are confined to the Archaea. Evans et al. used metagenomics to assemble two nearly complete archaeal genomes from deep groundwater methanogens (see the Perspective by Lloyd). The two reconstructed genomes are members of the recently described Bathyarchaeota and not the phylum to which all previously known methane-metabolizing archaea belonged.

    Science, this issue p. 434, see also p. 384

  6. Reproductive Biology

    Mouse work may lead to male contraceptive

    1. Beverly A. Purnell

    Unintended pregnancies are a major health issue worldwide. Although oral contraceptives were developed decades ago for use in women, there are no male oral contraceptives. Miyata et al. show that genetic deletion or drug inhibition of sperm-specific calcineurin enzymes in mice cause male sterility (see the Perspective by Castaneda and Matzuk). Although calcineurin inhibitors resulted in male infertility within 2 weeks, fertility recovered 1 week after halting drug administration. Because the sperm-specific calcineuin complex is also found in humans, its inhibition may be a strategy for developing reversible male contraceptives.

    Science, this issue p. 442, see also p. 385

  7. Asteroseismology

    Stellar oscillations as magnetic probes

    1. Keith T. Smith
    Artistic representation of a magnetized red giant starPHOTO: RAFAEL A. GARCÍA/SAP CEA, JIM FULLER/CALTECH, GABRIEL PÉREZ/SMM/IAC, KYLE AUGUSTSON/HA, SOLAR IMAGE FROM AIA/SDO

    Unlike magnetic fields on the surfaces of stars, those within a star have so far remained difficult to observe. Fuller et al. have developed a method of delving into the magnetic depths by exploiting the oscillations of red giant stars. A high magnetic field can cause sound waves to become trapped within the central regions of the star, damping certain vibration modes. Using seismological techniques, this suppression can help infer the core magnetic field for several red giants.

    Science, this issue p. 423

  8. Vascular Biology

    Stopping aneurysms before they start

    1. Wei Wong

    The smooth muscle cells in aortas are connected to the extracellular matrix. Mutations in components of the extracellular matrix, such as fibulin-4, can lead to the enlargement of the aortic lumen, otherwise known as an aneurysm. Yamashiro et al. found that mice lacking fibulin-4 in smooth muscle cells had disrupted connections with the extracellular matrix. The mice also had abnormal increases in mechanosensitive proteins and enhanced activity of an actin cytoskeleton–remodeling enzyme called cofilin. Inhibiting the activity of cofilin or its upstream activators could therefore prevent the development of aneurysms.

    Sci. Signal. 8, ra105 (2015).

  9. Bioengineering

    Drugs ride waves across tissue barriers

    1. Megan Frisk

    Drugs that travel through the gastrointestinal (GI) tract meet tissue barriers that limit their uptake and dilute potency. Schoellhammer et al. used a common handheld ultrasonic probe to temporarily disrupt the barriers, allowing drugs to pass. The ultrasonic waves drove drugs into pig colonic tissue faster than natural absorption, without damage to the tissue. The approach also encouraged drug delivery into mouse colon tissue, leading to the healing of acute colitis. Ultrasound-mediated drug delivery may offer a versatile solution to barrier challenge in the GI tract.

    Sci. Transl. Med. 7, 310ra168 (2015).

  10. Clathrin Adaptors

    HIV proteins exploit clathrin coats

    1. Stella M. Hurtley

    Clathrin-coated vesicles are involved in the sorting of membrane and cargo at the trans-Golgi network. Clathrin coats exploit adaptor proteins, including AP-1 and AP-2, in selecting their cargoes, assisted by the small membrane-trafficking associated GTPase Arf1. Shen et al. were interested in how the HIV-1 Nef protein affected this process. They leveraged a structure solved in the presence of Nef to show that the AP-1:clathrin coat is far more intricately organized than previously thought. Furthermore, Arf1 played a central structural role in these coated vesicles. It seems that HIV-1 Nef hijacks the clathrin pathway to its own ends through very sophisticated structural perturbations.

    Science, this issue p. 10.1126/science.aac5137

  11. Magnetic Resonance

    EPR, one atom at a time

    1. Phil Szuromi

    Electron paramagnetic resonance (EPR) usually detects atoms with unpaired electrons as ensemble averages. Baumann et al. used a spin-polarized scanning tunneling microscope tip to measure EPR spectra of single iron atoms adsorbed on a magnesium oxide surface at cryogenic temperatures. The measurement depends on the atomic orbital symmetry; no signal was observed for cobalt atoms under the same conditions

    Science, this issue p. 417

  12. Environment

    Detectives of the nanoscale

    1. Julia Fahrenkamp-Uppenbrink

    Many consumer products contain nanometer-sized components. Are these nanomaterials harmful when they are released into the environment? As Valsami-Jones and Lynch explain in their Perspective, answers to this question are not easy to come by. Nanoparticles undergo drastic changes that are difficult to predict and that affect their uptake by biological systems, including the human body. A lack of consensus on laboratory protocols further impedes progress. Application of state-of-the-art techniques holds promise for clearer answers to emerge on whether engineered nanomaterials are safe to use.

    Science, this issue p. 388

  13. Autoimmunity

    An Aluring new autoantibody target

    1. Kristen L. Mueller

    Autoimmunity is the immune system's ultimate act of betrayal. Cells designed to protect against invading microbes suddenly target the host instead. In the autoimmune disease systemic lupus erythematosus, antibodies target DNA and host proteins, including the RNA binding protein Ro60. Hung et al. discovered that Ro60 bound to endogenous Alu retroelements. They detected antibody-Ro60-Alu RNA immune complexes in the blood of individuals with lupus and an enrichment of Alu transcripts. Ro60 bound to Alu probably primes RNA-binding innate immune receptors within B cells, leading these cells to make antibodies that target Ro60-Alu RNA and drive disease-causing inflammation.

    Science, this issue p. 455

  14. Topological Matter

    Breaking chiral symmetry in a solid

    1. Jelena Stajic

    Dirac semimetals have graphene-like electronic structure, albeit in three rather than two dimensions. In a magnetic field, their Dirac cones split into two halves, one supporting left-handed and the other right-handed fermions. If an electric field is applied parallel to the magnetic field, this “chiral” symmetry may break: a phenomenon called the chiral anomaly. Xiong et al. observed this anomaly in the Dirac semimetal Na3Bi (see the Perspective by Burkov). Transport measurements lead to the detection of the predicted large negative magnetoresistance, which appeared only when the two fields were nearly parallel to each other.

    Science, this issue p. 413, see also p. 378

  15. Plant Science

    Metered rehydration in pollen grains

    1. Pamela J. Hines

    When a desiccated pollen grain lands on fertile territory, it rehydrates on the way to activating its growth and metabolic processes. Studying the small plant Arabidopsis, Hamilton et al. have identified a mechanosensory ion channel that responds to the distention of the plasma membrane as the pollen grain rehydrates. With this channel damaged or absent, the pollen grains germinated overenthusiastically but then showed a tendency to burst.

    Science, this issue p. 438

  16. Root Development

    Multifunctional root regulators

    1. Pamela J. Hines

    The growing plant root undergoes a variety of developmental steps that determine thickness and branching as the roots elaborate. Moreno-Risueno et al. identify a suite of transcription factors, some of which mobilize between cells, that regulate shifting fates during root growth. The same set of transcription factors governs identity and proliferation of the stem cells as well as the fates of daughter cells.

    Science, this issue p. 426

  17. Alzheimer's Disease

    Early signs of dementia

    1. Peter Stern

    There is currently no cure for Alzheimer's disease. One of the reasons could be that interventions start too late, when there is already irreversible damage to the brain. Developing a biomarker that would help to effectively start therapy at very early stages of the disease is thus of high interest. Kunz et al. studied neural correlates of spatial navigation in the entorhinal cortex in control study participants and individuals at risk of developing Alzheimer's. The at-risk group showed a different brain signal many decades before the onset of the disease, and they navigated differently in a virtual environment.

    Science, this issue p. 430

  18. Protein Dynamics

    Observing ultrafast myoglobin dynamics

    1. Valda Vinson

    The oxygen-storage protein myoglobin was the first to have its three-dimensional structure determined and remains a workhorse for understanding how protein structure relates to function. Barends et al. used x-ray free-electron lasers with femtosecond short pulses to directly observe motions that occur within half a picosecond of CO dissociation (see the Perspective by Neutze). Combining the experiments with simulations shows that ultrafast motions of the heme couple to subpicosecond protein motions, which in turn couple to large-scale motions.

    Science, this issue p. 445, see also p. 381

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