This Week in Science

Science  22 May 2020:
Vol. 368, Issue 6493, pp. 840
  1. Tropical Forests

    Thermal sensitivity of tropical trees

    1. Andrew M. Sugden

    Throughout the tropics, carbon stocks in forests, such as this one in Liberia, will be reduced in response to higher daytime temperatures.

    CREDIT: FABIAN PLOCK / ALAMY STOCK PHOTO

    A key uncertainty in climate change models is the thermal sensitivity of tropical forests and how this value might influence carbon fluxes. Sullivan et al. measured carbon stocks and fluxes in permanent forest plots distributed globally. This synthesis of plot networks across climatic and biogeographic gradients shows that forest thermal sensitivity is dominated by high daytime temperatures. This extreme condition depresses growth rates and shortens the time that carbon resides in the ecosystem by killing trees under hot, dry conditions. The effect of temperature is worse above 32°C, and a greater magnitude of climate change thus risks greater loss of tropical forest carbon stocks. Nevertheless, forest carbon stocks are likely to remain higher under moderate climate change if they are protected from direct impacts such as clearance, logging, or fires.

    Science, this issue p. 869

  2. Biofilms

    Bacteria maintain motile reserves

    1. Annalisa M. VanHook

    During biofilm formation, bacterial cells switch from a motile to a generally sessile, matrix-producing state. However, biofilms formed by Bacillus subtilis can spread to overtake and kill neighboring colonies of competitor species. Steinberg et al. found that a motile subpopulation of cells within B. subtilis biofilms was required for the biofilms to spread over foreign objects. This process required the matrix protein TasA, which stimulated a subset of cells within the biofilm to revert from a matrix-producing to a motile state, thus ensuring that the colony could spread.

    Sci. Signal. 13, eaaw8905 (2020).

  3. Device Technology

    Aligning dense carbon nanotube arrays

    1. Phil Szuromi

    Although semiconducting carbon nanotubes (CNTs) are promising candidates to replace silicon in transistors at extremely small dimensions, their purity, density, and alignment must be improved. Liu et al. combined a multiple dispersion sorting process, which improves purity, and a dimension-limited self-alignment process to produce well-aligned CNT arrays on a 10-centimeter silicon wafer. The density is sufficiently high (100 to 200 CNTs per micrometer) that large-scale integrated circuits could be fabricated. With ionic liquid gating, the performance metrics exceeded those of conventional silicon transistors with similar dimensions.

    Science, this issue p. 850

  4. Cell Biology

    Supramolecular attack particles

    1. Stella M. Hurtley

    Cytotoxic T cells (CTLs) are at the front lines against cancer and chronic infection. T cells kill by secreting caspase-activating granzymes and the pore-forming protein perforin from dense core granules. However, the structural basis of lethal hit delivery has remained unknown. Balint et al. enriched the synaptic output of CTLs to investigate the released form of perforin and granzyme B. They found that CTLs released perforin and granzymes in stable particles called supramolecular attack complexes or SMAPs. The SMAPs were composed of a core shell structure and were assembled in the CTL dense secretory granules before release. The released SMAPs showed an innate ability to kill target cells.

    Science, this issue p. 897

  5. Structural Biology

    Transport dependent on context

    1. Michael A. Funk

    Transporter proteins move substrates across a membrane, often coupling this activity to cellular ion concentration gradients. For neurotransmitter transporters, which reside in synaptic vesicles that fuse with the plasma membrane after an action potential, transport activity needs to be regulated so that they do not pump out neurotransmitters after vesicle fusion. Using cryo–electron microscopy, Li et al. determined the structure of a vesicular glutamate transporter from rat that unveils some of the distinctive features that enable it to function properly in two distinct cellular environments. An allosteric pH sensor, proposed to be a glutamate residue, gates binding of the substrate glutamate and simultaneously permits binding and counterflow of chloride ions. This molecular traffic light allows for a single ion channel to behave appropriately in different contexts.

    Science, this issue p. 893

  6. T Cell Memory

    Stepping down resident memory lane

    1. Anand Balasubramani

    The antigen-specific CD8+ T cell response to microbial infection includes the differentiation of a subset of CD8+ T cells into tissue-resident memory (TRM) cells that stop circulating and become confined within a nonlymphoid tissue. Kurd et al. used single-cell RNA sequencing of mouse CD8+ T cells at multiple time points during the first 90 days after viral infection to characterize how this differentiation process unfolds in the small intestine and to track the emergence of heterogeneity among TRM cells. They found evidence for TRM cell precursors in the intestine by 4 days after infection and identified several putative regulators of TRM cell differentiation. The results of this study provide a valuable transcriptomic atlas that will facilitate further investigation into the immune functions provided by TRM cells.

    Sci. Immunol. 5, eaaz6894 (2020).

  7. Coronavirus

    What happens next?

    1. Caroline Ash

    Four months into the severe acute respiratory syndrome–coronavirus 2 (SARS-CoV-2) outbreak, we still do not know enough about postrecovery immune protection and environmental and seasonal influences on transmission to predict transmission dynamics accurately. However, we do know that humans are seasonally afflicted by other, less severe coronaviruses. Kissler et al. used existing data to build a deterministic model of multiyear interactions between existing coronaviruses, with a focus on the United States, and used this to project the potential epidemic dynamics and pressures on critical care capacity over the next 5 years. The long-term dynamics of SARS-CoV-2 strongly depends on immune responses and immune cross-reactions between the coronaviruses, as well as the timing of introduction of the new virus into a population. One scenario is that a resurgence in SARS-CoV-2 could occur as far into the future as 2025.

    Science, this issue p. 860

  8. Social Environment

    Social animals need connection

    1. Sacha Vignieri

    Much research over the past decade or so has revealed that health and lifespan in humans, highly social animals, are reduced with social adversity. We humans are not the only animals that are social, however, and similar research has shown that other social mammals are similarly influenced by isolation and adversity. Snyder-Mackler et al. reviewed the relationships between social environment and many aspects of health and well-being across nonhuman mammals and investigated the similarities between these and patterns in humans. They found many of the same threats and responses across social mammals.

    Science, this issue p. eaax9553

  9. Medicine

    Correcting blindness

    1. Gemma Alderton

    Retinal diseases are a major cause of blindness but numerous developments offer hope that blindness can be reversed. In a Perspective, Dowling discusses advances in visual prostheses to detect light and transmit it to the brain, in using gene therapy to correct inherited blindness, and in applying regenerative approaches, including the implantation of stem cells, to restore visual circuitry. Many of these approaches are being tested in the clinical setting, and if they are successful, sight could be restored in a substantial number of patients blinded by retinal diseases.

    Science, this issue p. 827

  10. Coronavirus

    Drug repurposing

    1. Gemma Alderton

    Given the urgent need to find treatments for cases of severe coronavirus disease 2019 (COVID-19), repurposing of existing drugs known to be safe in humans could be a promising option. In a Perspective, Guy et al. discuss the underlying rationale for, and how to prioritize, testing of drugs developed for other applications. Considering the coronavirus replication cycle, certain drugs such as hydroxychloroquine, chloroquine, and remdesivir could have promising activity against COVID-19. However, the authors urge caution in relying on studies in cells and results from preliminary clinical trials involving small numbers of patients. Moreover, the side effects of some of these drugs should also be balanced against possible gains.

    Science, this issue p. 829

  11. Plant Science

    Fungal disease meets its match

    1. Pamela J. Hines

    Fusarium head blight (FHB), caused by a fungus, reduces wheat crop yield and introduces toxins into the harvest. From the assembly of the genome of Thinopyrum elongatum, a wild relative of wheat used in breeding programs to improve cultivated wheat, Wang et al. cloned a gene that can address both problems (see the Perspective by Wulff and Jones). The encoded glutathione S-transferase detoxifies the trichothecene toxin and, when expressed in wheat, confers resistance to FHB.

    Science, this issue p. eaba5435; see also p. 822

  12. Water Resources

    Dowsing for danger

    1. Michael A. Funk

    Arsenic is a metabolic poison that is present in minute quantities in most rock materials and, under certain natural conditions, can accumulate in aquifers and cause adverse health effects. Podgorski and Berg used measurements of arsenic in groundwater from ∼80 previous studies to train a machine-learning model with globally continuous predictor variables, including climate, soil, and topography (see the Perspective by Zheng). The output global map reveals the potential for hazard from arsenic contamination in groundwater, even in many places where there are sparse or no reported measurements. The highest-risk regions include areas of southern and central Asia and South America. Understanding arsenic hazard is especially essential in areas facing current or future water insecurity.

    Science, this issue p. 845; see also p. 818

  13. Topological Optics

    Topological insulators go nonlinear

    1. Ian S. Osborne

    Whereas solid-state insulators tend to be fixed by material properties, photonic topological insulators can be designed at will to mimic a variety of scenarios and complex interactions. Mukherjee and Rechtsman go beyond the linear optical regime that has been studied to date and show that photonic topological insulators can also exhibit nonlinear optical features (see the Perspective by Ablowitz and Cole). Their array of laser-written waveguides can support solitons, which are also found to exhibit topological features, performing cyclotron-like orbits associated with the topology of the lattice. The nonlinear properties provide a rich playground for further exploration, with the possibility of mimicking other interacting bosonic systems.

    Science, this issue p. 856; see also p. 821

  14. Metrology

    Good timing for microwave technology

    1. Ian S. Osborne

    Timing standards around the world define the second using atomic clocks, specifically the microwave frequencies emitted from trapped atoms. Optical clocks, which are based on optical transitions of atoms, operate at much higher frequency and have been shown to exhibit better stability. Nakamura et al. demonstrate a framework that carries the improved stability of the optical domain over to microwaves (see the Perspective by Curtis). In addition to contributing to the eventual redefinition of the second based on optical clocks, this work will also lead to improvements in microwave-based technologies such as astronomical imaging and geodesy through very long baseline interferometry, radar, communications, and navigation systems.

    Science, this issue p. 889; see also p. 825

  15. Device Technology

    DNA bricks build nanotube transistors

    1. Phil Szuromi

    Semiconducting carbon nanotubes (CNTs) are an attractive platform for field-effect transistors (FETs) because they potentially can outperform silicon as dimensions shrink. Challenges to achieving superior performance include creating highly aligned and dense arrays of nanotubes as well as removing coatings that increase contact resistance. Sun et al. aligned CNTs by wrapping them with single-stranded DNA handles and binding them into DNA origami bricks that formed an array of channels with precise intertube pitches as small as 10.4 nanometers. Zhao et al. then constructed single and multichannel FETs by attaching the arrays to a polymer-templated silicon wafer. After adding metal contacts across the CNTs to fix them to the substrate, they washed away all of the DNA and then deposited electrodes and gate dielectrics. The FETs showed high on-state performance and fast on-off switching.

    Science, this issue p. 874, p. 878

  16. Chemical Physics

    Electronic and nuclear dynamics in one

    1. Yury Suleymanov

    Because of the complex, ultrafast interplay between nuclear and electronic degrees of freedom, probing both nuclear and electronic dynamics in excited electronic states within a single time-resolved experiment is a great challenge. Yang et al. used ultrafast electron diffraction in combination with ab initio nonadiabatic molecular dynamics and diffraction simulations to study the relaxation dynamics of isolated pyridine molecules after photoexcitation to the S1 state (see the Perspective by Domcke and Sobolewski). They showed that electronic state evolution and molecular structural changes can be recorded simultaneously and independently by tracing a transient signal in small-angle inelastic scattering and large-angle elastic diffraction, respectively.

    Science, this issue p. 885; see also p. 820

  17. Pollinators

    Bumble bee gardeners

    1. Sacha Vignieri

    Bumble bees rely heavily on pollen resources for essential nutrients as they build their summer colonies. Therefore, we might expect that annual differences in the availability of these resources must simply be tolerated, but Pashalidou et al. made observations suggesting that bees may have strategies to cope with irregular seasonal flowering (see the Perspective by Chittka). When faced with a shortage of pollen, bumble bees actively damaged plant leaves in a characteristic way, and this behavior resulted in earlier flowering by as much as 30 days. Experimenters were not able to fully replicate the results with their own damage, suggesting that there is a distinct method that the bees use to stimulate earlier flowering.

    Science, this issue p. 881; see also p. 824

  18. Bone

    Screening for side-effect susceptibility

    1. Caitlin Czajka

    Osteoporosis is typically treated with nitrogen-containing bisphosphonates (N-BPs) to inhibit bone resorption; however, N-BPs can cause serious side effects, including osteonecrosis and fracture. To understand susceptibility to these side effects, Surface et al. investigated the role of the gene ATRAID in mediating response to N-BPs. Mice deficient in this gene had weaker bone and did not respond to N-BP treatment in models of osteoporosis. Patients with coding variants in ATRAID taking N-BPs presented with fractures and osteonecrosis of the jaw. The authors determined that ATRAID is necessary for inhibition of osteoclast function by N-BPs. It may thus be prudent to screen patients for variants in ATRAID to avoid N-BP side effects.

    Sci. Transl. Med. 12, eaav9166 (2020).

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