This Week in Science

Science  23 Oct 2020:
Vol. 370, Issue 6515, pp. 418
  1. Animal Robots

    Trot on the wild side

    1. Michael M. Lee

    Integrating proprioceptive feedback allows a quadrupedal robot to navigate challenging terrain, such as a gravel-strewn alpine road.

    IMAGE: JOONHO LEE/ETH ZURICH

    Legged robots can access spaces that wheeled robots cannot. Lee et al. developed a robust locomotion controller that uses deep reinforcement learning to teach a quadrupedal robot how to navigate unseen and unstructured environments without the need for external sensors, relying solely on proprioception. The trained quadruped was deployed in various outdoor settings to demonstrate that it could traverse a range of challenging terrain: deformable surfaces such as mud and snow, dynamic footholds such as rubble, and impediments such as thick vegetation and flowing water.

    Sci. Robot. 5, eabc5986 (2020).

  2. Display Technology

    Metasurface-based microdisplays

    1. Ian S. Osborne

    Organic light-emitting diodes (OLEDs) have found wide application in high-resolution, large-area televisions and the handheld displays of smartphones and tablets. With the screen located some distance from the eye, the typical number of pixels per inch is in the region of hundreds. For near-eye microdisplays—for example, in virtual and augmented reality applications—the required pixel density runs to several thousand pixels per inch and cannot be met by present display technologies. Joo et al. developed a full-color, high-brightness OLED design based on an engineered metasurface as a tunable back-reflector. An ultrahigh density of 10,000 pixels per inch readily meets the requirements for the next-generation microdisplays that can be fabricated on glasses or contact lenses.

    Science, this issue p. 459

  3. Developmental Biology

    Origins of the pituitary gland

    1. Beverly A. Purnell

    Light microscopy image of a zebrafish embryo 24 hours postfertilization, at which point specialized neural structures have begun to form

    IMAGE: ISTOCK.COM/MICHALRENEE

    Placodes are specializations of the head ectoderm that are considered the source of many vertebrate novelties, including the nose, lens, ear, and hormone-producing portion of the pituitary. However, the presence of a pituitary-like structure in nonvertebrate chordates, derived instead from the endoderm, had suggested that the pituitary may predate placodes. Fabian et al. performed lineage tracing, time-lapse imaging, and single-cell messenger RNA sequencing to show that both endodermal and ectodermal cells can generate hormone-producing cells of the zebrafish pituitary. These experiments support that the vertebrate pituitary arises through interactions of an ancestral endodermal protopituitary with newly evolved placodal ectoderm.

    Science, this issue p. 463

  4. Neurodegeneration

    Detecting toxic protein

    1. Mattia Maroso

    Spinocerebellar ataxia type 3 (SCA3) is a neurodegenerative disorder caused by CAG trinucleotide repeat expansion in the ataxin-3 gene (ATXN3). Reducing the toxic polyglutamine ATXN3 protein might be an effective strategy for treating the disease, and identification of pharmacodynamic markers would facilitate the assessment of potential therapies. Prudencio et al. showed that the toxic protein could be detected in cerebrospinal fluid from patients, was associated with clinical features, and could be used to assess treatment response. Moreover, the authors identified a single-nucleotide polymorphism in the ATXN3 gene associated with CAG-expanded alleles. The results could improve the development of new therapies and the evaluation of treatment efficacy.

    Sci. Transl. Med. 12, eabb7086 (2020).

  5. Atmospheric Oxygen

    The iron did it

    1. H. Jesse Smith

    What factors controlled the accumulation of atmospheric oxygen gas (O2) early in the history of Earth? Heard et al. used high-precision iron isotopic measurements of Archean-Paleoproterozoic sediments, with ages between 3.8 billion and 2.3 billion years ago, and laboratory data about synthetic pyrites to show that pyrite, or iron sulfide, burial could have resulted in net O2 export. These reactions therefore may have contributed to early episodes of transient oxygenation before the Great Oxidation Event that began about 2.4 billion years ago.

    Science, this issue p. 446

  6. Neurodevelopment

    Spinal circuit development

    1. Pamela J. Hines

    Motor neuron circuits in the zebrafish spinal cord support both the rapid evasion response and the leisurely swimming response. Kishore et al. now follow the development of inhibitory interneurons as these circuits are assembled in the larva. Interneurons generated early in development drive different sorts of circuits and synapse onto different subcellular sections of the motor neurons than interneurons generated later in development. Thus, both rapid evasion and slower swimming are supported by the same cellular components assembled in different ways. The authors suggest that development follows an opportunistic rule in which interneurons synapse onto what is available to them at that moment in development.

    Science, this issue p. 431

  7. Nanomaterials

    Using curves to make twists

    1. Marc S. Lavine

    The growth of layered materials on flat substrates usually occurs in stacked layers, although defects or a lattice mismatch can induce strains that distort the shape of subsequent layers. However, these effects are usually small and can be uncontrolled. Zhao et al. now demonstrate the possibility of synthesizing multilayers of two-dimensional materials with certain twists between the layers induced by the presence of screw dislocations in combination with curved substrate surfaces. Different twist angles are achieved by varying the amount of nonplanarity and the character (conical or hyperbolic) of the surface.

    Science, this issue p. 442

  8. Structural Biology

    Saving a host cell from itself

    1. Valda Vinson

    A fundamental mammalian defense mechanism against pathogens and damaged cellular DNA is to recognize DNA fragments in the cytosol and trigger an inflammatory response. The cyclic guanosine monophosphate–adenosine monophosphate synthase (cGAS) that recognizes cytosolic DNA is also found in the nucleus, but here its activity is suppressed by tethering to chromatin. Two papers now report cryo–electron microscopy structures of cGAS bound to the nucleosome core particle (NCP). Kujirai et al. observed a structure with two cGAS molecules bridging two NCPs, whereas Boyer et al. observed cGAS bound to a single nucleosome. Together, these structures show how cGAS is prevented from autoreactivity toward host DNA.

    Science, this issue p. 455, p. 450

  9. Coronavirus

    Understanding epidemic spread

    1. Gemma Alderton

    The global spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has caused both quickly controlled outbreaks and large ongoing epidemics. These varied outcomes have prompted much investigation into how the virus is transmitted and what the key engines of viral spread are. In a Perspective, Lee et al. discuss the main pillars of SARS-CoV-2 spread, including household and residential settings, community and superspreading events, and interregional transmission. Understanding how to prevent transmission in these situations as well as the importance of these different settings in the pattern of epidemic spread should help to improve and focus mitigation measures and control the pandemic.

    Science, this issue p. 406

  10. Coronavirus

    Damaging the heart

    1. Gemma Alderton

    Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is largely considered a respiratory virus, but evidence is emerging that it can also affect the heart. In a Perspective, Topol discusses the indirect and direct effects that the virus can have on the heart. Direct effects range from mild injury to inflammation and shock, which can lead to arrhythmia and possibly cardiac arrest. SARS-CoV-2 also has vascular effects that can indirectly affect heart function, as can systemic inflammation. Heart damage does not seem to correlate with the severity of disease, so more assessment of heart function in people infected with SARS-CoV-2 is needed to understand the frequency and what determines whether someone will develop cardiac pathology.

    Science, this issue p. 408

  11. Archaeology

    New insights into Clovis-era archaeology

    1. Mark Aldenderfer

    Defined by a distinctive projectile point style, Clovis was once thought to be the earliest archaeological culture in North America. Although new finds throughout the Americas have overturned that hypothesis, questions remain about its origins, relationships to other cultures, and disappearance. Waters et al. obtained 32 high-precision radiocarbon dates from 10 Clovis archaeological sites and determined that the age of these sites spanned from 13,050 to 12,750 calibrated radiocarbon years before the present. These dates confirm that Clovis was a contemporary of at least three other distinctive archaeological cultures, a finding that complicates current models of the peopling of the Americas. The new dates also show that Clovis technology disappeared coincident with the extinction of large mammals such as mammoth, mastodon, and others.

    Sci. Adv. 10.1126/sciadv.aaz0455 (2020).

  12. Natural Killer Cells

    Liver NK cells with antigen specificity

    1. Ifor Williams

    Natural killer (NK) cells are heterogeneous innate effector cells, with some NK subsets displaying features of adaptive immunity, including memory and antigen specificity. Because liver NK cells are enriched for adaptive NK cells, Stary et al. used RNA sequencing and flow cytometry of human liver NK cells to search for correlations between NK cell phenotypes and their capacity to carry out adaptive effector functions. A distinct subset of liver NK cells expressed a cytotoxicity-associated gene program and exhibited antigen-specific killing of autologous target cells pulsed with viral antigens or metal allergens. Identification of this human hepatic NK subset is an advance in the ongoing quest to understand the molecular basis for antigen-specific recognition by adaptive NK cells.

    Sci. Immunol. 5, eaba6232 (2020).

  13. Structural Biology

    Protected by dimerization

    1. Wei Wong

    Ubiquitination is critical for mitotic exit and requires the E2 ubiquitin-conjugating enzyme UBE2S, which can autoubiquitinate and promote its own turnover. Liess et al. found that dimerization of UBE2S prevented its autoubiquitination and kept this protein in an inactive state (see the Focus by Bremm). Human cells that expressed wild-type UBE2S were able to exit from drug-induced mitotic arrest, unlike those expressing the dimerization-defective form of UBE2S. Thus, UBE2S may dimerize to prevent its turnover in noncycling cells and ensure its availability for future mitotic cycles.

    Sci. Signal. 13, eaba8208, eabd9892 (2020).

  14. Coronavirus

    The genetics underlying severe COVID-19

    1. Laura M. Zahn

    The immune system is complex and involves many genes, including those that encode cytokines known as interferons (IFNs). Individuals that lack specific IFNs can be more susceptible to infectious diseases. Furthermore, the autoantibody system dampens IFN response to prevent damage from pathogen-induced inflammation. Two studies now examine the likelihood that genetics affects the risk of severe coronavirus disease 2019 (COVID-19) through components of this system (see the Perspective by Beck and Aksentijevich). Q. Zhang et al. used a candidate gene approach and identified patients with severe COVID-19 who have mutations in genes involved in the regulation of type I and III IFN immunity. They found enrichment of these genes in patients and conclude that genetics may determine the clinical course of the infection. Bastard et al. identified individuals with high titers of neutralizing autoantibodies against type I IFN-α2 and IFN-ω in about 10% of patients with severe COVID-19 pneumonia. These autoantibodies were not found either in infected people who were asymptomatic or had milder phenotype or in healthy individuals. Together, these studies identify a means by which individuals at highest risk of life-threatening COVID-19 can be identified.

    Science, this issue p. eabd4570, p. eabd4585; see also p. 404

  15. Framework Materials

    Higher-valency ligands for COFs

    1. Phil Szuromi

    Metal-organic frameworks (MOFs) have exhibited more extensive connectivity (valency) and topological diversity than covalent organic frameworks (COFs), mainly because MOF linkers can connect from 3 to 24 discrete units or even infinity for one-dimensional rods. For COFs, linkers generally have a valency of 3 or 4 that reflect the valency of organic carbon. Gropp et al. created cubane-like linkers from 1,4-boronophenylphosphonic acid that could condense to make COFs with a valency of 8 or, by adding acid, could form large, single crystals with an infinite-rod topology.

    Science, this issue p. eabd6406

  16. Geochemistry

    Getting rid of fool's gold

    1. Brent Grocholski

    Pyrite, also called fool's gold, is an iron sulfide mineral that is very commonly found in rock but is almost nonexistent in sediments today. Pyrite oxidizes quickly and is a major source of sulfur to the ocean, but it is also a proxy for the oxygen content historically in Earth's atmosphere. Gu et al. conducted a set of detailed observations of the pyrite oxidation process in a shale unit. The authors found that erosion tied to fracturing is just as important as the oxygen content for the dissolution process. They developed a model that helps determine the conditions in Earth's past for which pyrite might have been stable and the role of microorganisms in the oxidation process.

    Science, this issue p. eabb8092

  17. Coronavirus

    Miniproteins against SARS-CoV-2

    1. Valda Vinson

    Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is decorated with spikes, and viral entry into cells is initiated when these spikes bind to the host angiotensin-converting enzyme 2 (ACE2) receptor. Many monoclonal antibody therapies in development target the spike proteins. Cao et al. designed small, stable proteins that bind tightly to the spike and block it from binding to ACE2. The best designs bind with very high affinity and prevent SARS-CoV-2 infection of mammalian Vero E6 cells. Cryo–electron microscopy shows that the structures of the two most potent inhibitors are nearly identical to the computational models. Unlike antibodies, the miniproteins do not require expression in mammalian cells, and their small size and high stability may allow formulation for direct delivery to the nasal or respiratory system.

    Science, this issue p. 426

  18. Polymer Chemistry

    A new future for polyethylene

    1. Jake Yeston

    Most current plastic recycling involves chopping up the waste and repurposing it in materials with less stringent engineering requirements than the original application. Chemical decomposition at the molecular level could, in principle, lead to higher-value products. However, the carbon-carbon bonds in polyethylene, the most common plastic, tend to resist such approaches without exposure to high-pressure hydrogen. F. Zhang et al. now report that a platinum/alumina catalyst can transform waste polyethylene directly into long-chain alkylbenzenes, a feedstock for detergent manufacture, with no need for external hydrogen (see the Perspective by Weckhuysen).

    Science, this issue p. 437; see also p. 400

  19. Adaptation

    Microbial selection drives adaptation

    1. Laura M. Zahn

    Many legumes have a host-symbiote relationship with nitrogen-fixing bacteria, or rhizobia, that provides a benefit to both the plant and the microbe. Batstone et al. experimentally evolved the association between five legume accessions and different bacterial isolates. Rather than observe selection by the host for bacterial associations (host choice), mutations accumulated within a bacterial plasmid and increased the strength of the mutualism. Thus, local and recent associations between bacterial strains and plant genotypes are due to selection for bacterial adaptation.

    Science, this issue p. 476

  20. Oxygen Sensing

    Origins and evolution of hypoxia response

    1. Sacha Vignieri

    In our current oxygen-rich atmosphere, the ability of eukaryotic cells to sense variation in oxygen concentrations is essential for adapting to low-oxygen conditions. However, Earth's atmosphere has not always contained such high oxygen concentrations. Hammarlund et al. discuss oxygen-sensing systems across both plants and animals and argue that the systems are functionally convergent and that their emergence in an initially hypoxic environment shaped how they operate today.

    Science, this issue p. eaba3512

  21. Microbiota

    So much more to mucus

    1. Caroline Ash

    Mammals accommodate a dense community of metabolically active microorganisms in their gut. This is not a passive relationship, and host and microbe have antagonistic as well as mutualistic responses to each other. Using a whole-colon imaging method in mice, Bergstrom et al. looked at the role of colonic mucus in segregating the microbiota from host cells during elimination of feces (see the Perspective by Birchenough and Johansson). Host goblet cells synthesize two forms of mucin that differ in branched chain O-glycosylation and the site of production in the colon. A “thick” mucus in the proximal, ascending colon wraps the microbiota to form fecal pellets. Transit along the distal, descending colon is lubricated by “thin” mucus that transiently links with the thick mucus. Normal mucus encapsulation prevents inflammation and hyperplasia and thus is important for maintenance of a healthy gut.

    Science, this issue p. 467; see also p. 402

  22. Sociality

    Old chimp friends

    1. Sacha Vignieri

    As humans age, we prioritize established positive friendships over the new, but risky, socializing we do when we are young. It has been hypothesized that this shift may come as our own sense of mortality kicks in. Rosati et al. analyzed a rare, long-term dataset on social bonds among male chimpanzees and found a very similar focus on old and positive friendships (see the Perspective by Silk). Though there is evidence of some sense of time among nonhuman animals, it seems unlikely that they have the same impending sense of mortality that we experience; thus, these results suggest that a different, and deeper, mechanism may be at play.

    Science, this issue p. 473; see also p. 403

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