This Week in Science

Science  21 Jul 2017:
Vol. 357, Issue 6348, pp. 263
  1. Economics

    Ecosystem protection payments pay off

    1. Gilbert Chin

    Paying to protect Ugandan forests is worth the cost.

    PHOTO: ANDAMANSE/ISTOCK

    Trees take up a lot of CO2, so one approach to reducing the rate of increase in atmospheric CO2 levels is to reduce the cutting down of trees. Jayachandran et al. evaluated a program in which forest owners in Uganda were paid to not cut down their trees. Encouragingly, payments did reduce deforestation, and owners did not compensate by cutting down trees in neighboring forests. Furthermore, even in a scenario where cutting resumed after payments ceased, the beneficial delay in CO2 release from cut trees, as quantified by the social cost of carbon, outweighed the monetary cost.

    Science, this issue p. 267

  2. Geochemistry

    Volcanoes find a new carbon platform

    1. Brent Grocholski

    The geological carbon cycle assumes that carbon is emitted by volcanic eruptions and removed through various forms of burial. Mason et al. found that not all volcanic eruptions have the same source for carbon in their volcanic gas. Arc volcanic activity appears to harvest carbon from old carbonate platforms, which results in a massive difference in the isotopic signature of the carbon emitted during eruption. This discovery requires revision of the global carbon cycle, decreasing the amount of organic carbon believed to be being buried.

    Science, this issue p. 290

  3. Mechanochemistry

    Pulling on bonds counterintuitively

    1. Jake Yeston

    Experimental mechanochemistry has largely focused on the application of force along chemical bonds to accelerate their cleavage. Akbulatov et al. now demonstrate that force can also play a more subtle role. They generated strained macrocyclic rings photochemically and then studied the influence of that strain on the rates of reactions that cleaved either phosphorus-oxygen or silicon-oxygen bonds. P–O cleavage was accelerated by force orthogonal to the bond axis, whereas the Si–O cleavage was inhibited by force along the bond. Both results were consistent with the respective transition states predicted by theory.

    Science, this issue p. 299

  4. Animal Physiology

    Hydraulic fins

    1. Sacha Vignieri

    The lymphatic system in fish has much the same function as it does in mammals—immune response and homeostasis. Pavlov et al. show, however, that in the scromboid (tuna and mackerel) family of fish, this fluid homeostasis function has been co-opted to help facilitate dorsal fin rigidity and movement (see the Perspective by Triantafyllou). In bluefin tuna, a series of lymphatic vessels are integrated with muscles that allow the fish to raise and stiffen their dorsal fin. This provides extra stability during swimming.

    Science, this issue p. 310; see also p. 251

  5. DNA Replication

    Getting loaded—make mine a double!

    1. Stella M. Hurtley

    Chromosomal DNA replication initiates bidirectionally by loading two ring-shaped helicases onto DNA in opposite orientations. How this symmetry is achieved has been puzzling because replication initiation sites contain only one essential binding site for the initiator, the origin recognition complex (ORC). Coster and Diffley now show that both helicases are loaded by a similar mechanism. Efficient loading requires binding of two ORC complexes to two ORC binding sites in opposite orientations. Natural origins were found to be partially symmetrical, containing functionally relevant secondary ORC sites. Sites can be flexibly spaced, but introducing an intervening “roadblock” prevented loading, suggesting that individual helicases translocate toward each other on DNA to form a stable double ring.

    Science, this issue p. 314

  6. Soft Robots

    Reaching out as a way to grow

    1. Marc S. Lavine

    Humans normally think of motion as a physical shifting from one location to another. But for some plants, such as creeping vines, and even some cells, such as neurons, motion is accomplished by growth from a present location toward a desired one. Leveraging the flexibility that is possible with soft materials, Hawkes et al. used pneumatics to lengthen the tip of a robot over an extended distance. They inflated chambers on only one side of the robot to make it turn. An onboard camera connected to a feedback loop allowed the robot to track toward a distant light source.

    Sci Robot. 10.1126/scirobotics.aan3028 (2017).

  7. Neurorehabilitation

    Greater gait with gravity

    1. Caitlin Czajka

    Often taken for granted, gravity—the force that keeps you on the ground—becomes a notable challenge during rehabilitation from injury. Mignardot et al. “harnessed” gravity, using a robotic body weight–supportive device to supply upward and forward forces to the torso to assist with locomotion. Patients recovering from stroke or spinal cord injury demonstrated improved gait performance with the robotic harness. An algorithm was developed to adjust the forces provided by the robotic harness according to the patient's needs. Nonambulatory patients were able to walk naturally with the harness, and ambulatory patients showed improved balance, limb coordination, foot placement, and steering. A clinical trial using this robot-assistive rehabilitation approach for patients with spinal cord injury is now under way.

    Sci. Transl. Med. 9, eaah3621 (2017).

  8. Batteries

    A stretchy binder protects the silicon

    1. Marc S. Lavine

    Disintegrated silicon microparticles

    A challenge in using silicon particles for lithium batteries is that the large volume changes during charge-discharge cycling cause the particles to fracture, which builds up an insulating interface layer. Choi et al. show that traditional binder materials used to cushion the silicon particles can be improved by adding small amounts of polyrotaxanes (see the Perspective by Ryu and Park). The molecules consist of multiple rings that are strung along a linear segment and stoppered at each end. Some of the rings are anchored to the polymer binder, whereas others float freely, yielding a highly mobile but connected network that anchors the binder, and thus the silicon particles, together.

    Science, this issue p. 279; see also p. 250

  9. Cancer

    Cancer epigenetics in the driver's seat

    1. Paula A. Kiberstis

    Recent cancer genome projects unexpectedly highlighted the role of epigenetic alterations in cancer development. About half of human cancers were found to harbor mutations in chromatin proteins. In a Review, Flavahan et al. propose that chromatin and epigenetic aberrations have the potential to confer on cells the full range of oncogenic properties represented in the classic “hallmarks” depiction of cancer. They suggest that genetic, environmental, and metabolic factors can make chromatin aberrantly permissive or restrictive. Permissive chromatin creates a state of “epigenetic plasticity,” which can activate oncogene expression or cell fate changes that drive cancer development.

    Science, this issue p. eaal2380

  10. Structural Biology

    Untangling aggregates one step at a time

    1. Stella M. Hurtley

    Conserved AAA+ protein complexes exploit adenosine triphosphate hydrolysis to unfold and disaggregate their substrates in response to cell stress, but exactly how they do this has been unclear. Gates et al. determined high-resolution cryo–electron microscopy structures of the Hsp104 disaggregase bound to an unfolded polypeptide substrate in its channel. The structures reveal substrate interactions and two different translocation states. Hsp104 undergoes conformational changes that drive movement along the substrate by two-amino-acid steps. These states help explain how this molecular machine can solubilize protein aggregates and amyloids.

    Science, this issue p. 273

  11. Organic Chemistry

    Lighting the way to carbon borylation

    1. Jake Yeston

    Boron substituents provide versatile reactivity, and their utility has been emerging in pharmaceutical contexts. Fawcett et al. show that visible light can induce replacement of carboxylic acid groups with boronate esters, which will ease their introduction into a wide variety of compounds. Once the acids are activated with phthalimide substituents, they can react with catecholborane dimers under illumination in amide solvents, with no need for catalysts or other additives. The reaction appears to proceed by radical chain propagation after photoinitiation.

    Science, this issue p. 283

  12. Topological Matter

    A propagating Majorana mode

    1. Jelena Stajic

    Although Majorana fermions remain elusive as elementary particles, their solid-state analogs have been observed in hybrid semiconductor-superconductor nanowires. In a nanowire setting, the Majorana states are localized at the ends of the wire. He et al. built a two-dimensional heterostructure in which a one-dimensional Majorana mode is predicted to run along the sample edge (see the Perspective by Pribiag). The heterostructure consisted of a quantum anomalous Hall insulator (QAHI) bar contacted by a superconductor. The authors used an external magnetic field as a “knob” to tune into a regime where a Majorana mode was propagating along the edge of the QAHI bar covered by the superconductor. A signature of this propagation—half-quantized conductance—was then observed in transport experiments.

    Science, this issue p. 294; see also p. 252

  13. Topological Matter

    Making a large-gap topological insulator

    1. Jelena Stajic

    Although of interest to basic research, topological insulators (TIs) have not yet lived up to their technological potential. This is partly because their protected surface-edge state usually lives within a narrow energy gap, with its exotic transport properties overwhelmed by the ordinary bulk material. Reis et al. show that a judicious choice of materials can make the gap wide enough for the topological properties to be apparent at room temperature. Numerical calculations indicate that a monolayer of Bismuth grown on SiC(0001) is a two-dimensional TI with a large energy gap. The researchers fabricated such a heterostructure and characterized it using scanning tunneling spectroscopy. The size of the experimentally measured gap was consistent with the calculations.

    Science, this issue p. 287

  14. Nonlinear Optics

    Hitting the highs in solid state

    1. Ian S. Osborne

    The ability to generate high harmonics of optical frequencies through the nonlinear interaction between intense light pulses and gas atoms has opened up the area of ultrafast optics and spectroscopy. Sivis et al. now show that high harmonics can also be generated with a solid-state sample. They used nanofabricated structured targets of ZnO and varied the chemical composition of the sample to demonstrate that (modest) high harmonics can be generated as the light interacts with the target materials. The results present the possibility of developing solid-state ultrafast optical devices.

    Science, this issue p. 303

  15. Piezoelectrics

    Finding a more flexible mechanical sensor

    1. Brent Grocholski

    Piezoelectric materials allow conversion between electricity and mechanical stresses. The most efficient piezoelectric materials are ceramics such as BaTiO3 or PbZrO3, which are also extremely stiff. You et al. identified an organic perovskite structured piezoelectric material that is far more pliable yet has a piezoelectric response similar to that of traditional ceramics. This material may be a better option to use as a mechanical sensor for flexible devices, soft robotics, biomedical devices, and other micromechanical applications that benefit from a less stiff piezoelectric material.

    Science, this issue p. 306

  16. Geoengineering

    Geoengineering for temperature control

    1. Julia Fahrenkamp-Uppenbrink

    As global temperatures continue to rise, potential large-scale interventions in Earth's climate system to reduce global warming are receiving increasing attention (see the Editorial by Pasztor et al.). In a Perspective, Niemeier and Tilmes explore one such geoengineering approach, stratospheric aerosol modification. This approach would involve injecting vast amounts of sulfur or other aerosols into the stratosphere to reduce the amount of solar light reaching Earth's surface. In a second Perspective, Lohmann and Gasparini explain a less known geoengineering approach, cirrus cloud seeding. This approach aims to increase the amount of longwave radiation leaving Earth's atmosphere. Although these approaches might reduce warming, they have many as yet unpredictable side effects and would not address problems such as ocean acidification.

    Science, this issue p. 231, p. 246; see also p. 248

  17. Neurodegeneration

    The amyloid connection in Parkinson's

    1. Leslie K. Ferrarelli

    Parkinson's disease is a progressive neurodegenerative disorder that is commonly associated with activating mutations in the kinase LRRK2. The disease results in dementia owing to loss of dopaminergic neurons. Chen et al. discovered why activating LRRK2 mutations are toxic to neurons. When phosphorylated by mutant LRRK2, a cleavage product of amyloid precursor protein (APP) translocated to the nucleus and was more transcriptionally active, which killed the dopaminergic neurons. These findings connect the pathology of Parkinson's to that of Alzheimer's disease, in which dementia is associated with another APP cleavage product, β-amyloid.

    Sci. Signal. 10, eaam6790 (2017).