This Week in Science

Science  02 Oct 2015:
Vol. 350, Issue 6256, pp. 52
  1. Clouds

    Cloud mixing and droplet evolution

    1. H. Jesse Smith

    Cumulonimbus rain cloud

    CREDIT: GEORG GERSTER/SCIENCE SOURCE

    When clouds mix with drier air because of atmospheric turbulence, some of the cloud droplets evaporate. Beals et al. developed an airborne holographic imaging system to look at changes in the spatial structure and sizes of these droplets (see the Perspective by Bodenschatz). Turbulent mixing of clouds with clear air caused dramatic fluctuations in the number density of water droplets but left their mean diameter essentially unchanged. This finding should help models better represent these hard-to-parameterize cloud processes.

    Science, this issue p. 87; see also p. 40

  2. Earth History

    Making an impact in more ways than one

    1. Brent Grocholski

    Massive eruptions formed the Deccan Traps flood basalts in India at around the same time as the mass extinction event 65 million years ago. Renne et al. precisely dated the massive volcanic field, which suggests a simultaneous increase in volcanism associated with the famous Chicxulub impact. Strong ecologic recovery may have been impossible until the volcanism slowed down 500,000 years later.

    Science, this issue p. 76

  3. Ocean Acidification

    Resilience of the oceans revealed

    1. Shahid Naeem

    Earth will witness a massive change in ocean acidification in this century. Ocean acidification adversely affects marine biodiversity and biogeochemistry. Dissolved organic matter (DOM) is especially important for its role in storing roughly 700 Gt of carbon. Zark et al. used large mesocosms suspended in a Swedish fjord to measure DOM response to acidification likely to be experienced in the year 2100. Encouragingly, despite enormously complex system dynamics, DOM in acidified mesocosms rarely differed from that in controls. We're not necessarily off the hook, but at least carbon stored in DOM may withstand the insult of acidification that our excessive generation of CO2 will create in this century.

    Sci Adv. 10.1126.sciadv.1500531 (2015).

  4. Paleoecology

    No precedent

    1. Sacha Vignieri

    Human activities are creating a mass extinction event. The intensity of this event is unprecedented during human times, but there have been several comparable events during Earth's history. Roopnarine and Angielczyk examined one of the largest, the Permian-Triassic Extinction (see the Perspective by Marshall). The structure and diversity of communities were key predictors of stability through the event. Furthermore, extinctions were not random, with smaller-bodied species being more prone to extinction. This pattern is in direct contradiction to the patterns seen in our current extinction. Thus, the current anthropogenically driven extinction is fundamentally different from previous catastrophic extinctions.

    Science, this issue p. 90; see also p. 38

    The large ancient mammal Lystrosauros survived the Permiani-Triassic mass extinction

    CREDIT: MARLENE DONNELLY
  5. Injury Recovery

    Motivation helps reverse neuronal damage

    1. Peter Stern

    Motivation plays a fundamental role in mediating recovery after neurological injuries. Sawada et al. evaluated the potential contribution of the nucleus accumbens, a brain motivation center, to movement control after spinal cord injury in monkeys. The activity of the nucleus accumbens during the early phase after injury was important for the recovery of motor performance.

    Science, this issue p. 98

  6. Structural Biology

    Building a gate to the nucleus

    1. Valda Vinson

    Nuclear pore complexes form a gateway between the cytoplasm and the nucleus (see the Perspective by Ullman and Powers). Stuwe et al. combined structural, biochemical, and functional analyses to elucidate the architecture of a six-protein complex that makes up the inner ring of the fungal nuclear pore. This includes a central trimeric complex homologous to the Nup62 complex found in metazoans that is incorporated into the nuclear pore inner-ring complex. Chug et al. report the structure of the metazoan trimeric Nup62 complex. Neither study supports a model in which the pore can dilate and constrict. Instead they suggest a rigid pore in which flexible domains called FG repeats fill the channel and form a barrier that can be traversed by receptors that carry cargos across.

    Science, this issue pp. 56 and 106; see also p. 33

  7. Planetary Science

    An exoplanet extracted from the bright

    1. Brent Grocholski

    Direct imaging of Jupiter-like exoplanets around young stars provides a glimpse into how our solar system formed. The brightness of young stars requires the use of next-generation devices such as the Gemini Planet Imager (GPI). Using the GPI, Macintosh et al. discovered a Jupiter-like planet orbiting a young star, 51 Eridani (see the Perspective by Mawet). The planet, 51 Eri b, has a methane signature and is probably the smallest exoplanet that has been directly imaged. These findings open the door to understanding solar system origins and herald the dawn of a new era in next-generation planetary imaging.

    Science, this issue p. 64; see also p. 39

  8. Genome Editing

    Improving gene editing in human T cells

    1. Katrina L. Kelner

    Gene editing holds promise for correcting human disease, but has been hampered by low efficiencies. To address this, Sather et al. devised a more effective way to disrupt and replace the CCR5 locus in human T cells, a procedure known to improve HIV clearance. Serotype 6 of an adeno-associated viral vector worked particularly well for the delivery of nucleases and repair templates to primary human T cells, achieving efficient gene editing rates and little toxicity. The nuclease they used (megaTAL) generated homology-directed repair (rather than nonhomologous end-joining repair) and so could both delete and replace the target locus. When inserted into the locus, chimeric antigen receptors and an HIV fusion inhibitor ameliorated HIV infection in mice.

    Sci. Transl. Med. 7, 307ra156 (2015).

  9. Polymer Chemistry

    Transient polymer links are better

    1. Marc S. Lavine

    Very long polymer molecules elongate in shearing flows. This ordering of the chains lowers the viscosity of small-molecule solvents. The chains also reduce the dangers of misting during explosive spreading of the solvents. However, the long polymer chains degrade during normal fuel handling and clog pumping equipment. Wei et al. used telechelic polymers—short chains with reactive end groups—to form extremely long chains in organic solvents (see the Perspective by Jaffe and Allam). These reformable polymers offer the potential for better fuel safety without the drawbacks of covalently bonded long-chain polymers.

    Science, this issue p. 72; see also p. 32

  10. Gut Microbiota

    Diet shapes host and gut microbe fitness

    1. Caroline Ash

    The human gut microbiota is hugely diverse, with many strain variants having a multiplicity of effects on host metabolism and immunity. To define some of these functions, Wu et al. made libraries of mutants of Bacteroides species known for their capacity to process otherwise intractable dietary fiber. Germ-free mice colonized with defined gut microbiota communities containing the mutants were fed specific diets containing different ratios of fat and fiber. Genes, strains, and species were identified that were associated with specific metabolic pathways. The community responses to dietary shifts were manipulated in an attempt to characterize species for their probiotic or therapeutic potential.

    Science, this issue p. 10.1126/science.aac5992.

  11. Water Structure

    How well does water share its protons?

    1. Jake Yeston

    Chemists have spent centuries trying to understand what acids look like at the molecular level. It's clear now that water molecules in the liquid accommodate extra protons. Less clear is whether the protons piggyback on individual water molecules (Eigen structure) or find shared accommodation between two at a time (Zundel structure). Thämer et al. acquired time-resolved vibrational spectra across an unusually broad span of the mid-infrared, allowing them to monitor stretches and bends at the same time. Their results imply a more prominent role for the Zundel structure than previously anticipated.

    Science, this issue p. 78

  12. Neurodevelopment

    Individualized neuronal mutations in the human brain

    1. Pamela J. Hines

    The neurons of the human brain can last for decades, carrying out computational and signaling functions. Lodato et al. analyzed the DNA of individual neurons sampled from postmortem human brains and found that individual neurons acquired somatic mutations (see the Perspective by Linnarsson). The mechanism of mutation involved gene transcription rather than DNA replication. Thus, postmitotic neurons would seem to be their own worst enemy: Genes used for neuronal function are the very genes put most at risk of somatic mutation.

    Science, this issue p. 94; see also p. 37

  13. Memory Mechanisms

    Memory consolidation by gene suppression

    1. Peter Stern

    Storing a persistent memory in the brain involves dynamic gene regulation. However, our knowledge of the target genes controlled during memory formation is limited. Cho et al. used RNA sequencing and ribosome profiling to compare transcription and translational levels in the mouse hippocampus before and after memory formation. Under basal conditions, there was an unexpected translational repression of ribosomal protein-coding genes. Early after learning, specific genes were translationally repressed. Later, suppression of a group of genes resulted from the inhibition of estrogen receptor alpha signaling. Thus, suppression mechanisms in the hippocampus appear to play a major role during memory consolidation.

    Science, this issue p. 82

  14. Neurotransmission

    An alternative way of making GABA

    1. Peter Stern

    Midbrain dopaminergic neurons release both the inhibitory neurotransmitter GABA and dopamine. In the central nervous system, GABA synthesis is usually mediated by two glutamate decarboxylases (GAD65 and GAD67). Kim et al. found that midbrain dopaminergic neurons use a different, evolutionary conserved GABA synthesis pathway that is independent of GAD65 and GAD67. These cells synthesize GABA from putrescine via the enzymes diamine oxidase and aldehyde dehydrogenase 1a1. GABA synthesized by this pathway accounts for approximately 70% of co-released GABA.

    Science, this issue p. 102

  15. Cell Biology

    Increasing the response to TGF-β signals

    1. Wei Wong

    TGF-β is a cytokine that binds to cell surface receptors and triggers the epithelial-to-mesenchyme transition. This process promotes pathological scarring known as fibrosis and contributes to cancer progression and metastasis. Budi et al. found that insulin, which is released in response to high glucose and used to treat diabetes, enhanced the trafficking of TGF-β receptors to the surface of mouse embryonic fibroblasts and epithelial cells. These results may help to explain why diabetics are prone to fibrosis and how hyperglycemia and insulin could enhance cancer progression and metastasis.

    Sci. Signal. 8, ra96 (2015).

  16. Nanoelectronics

    Making better small contacts

    1. Phil Szuromi

    Semiconducting single-walled carbon nanotubes have potential size and conductivity advantages over silicon for making smaller transistors. However, as metal electrical contacts decrease in size, the associated resistance increases to impractical values. Cao et al. reacted molybdenum films with semiconducting carbon nanotubes to create a carbide contact. The resistance of these contacts remained low even for 10-nm-scale contacts.

    Science, this issue p. 68

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