翻譯 | 李言
Science, 03 APRIL 2020, VOL 368, ISSUE 6486
《科學》2020年4月3日，第368卷，6486期
物理學Physics
Critical differences in 3D atomic structure of individual ligand-protected nanocrystals in solution
溶液中單個配體保護的納米晶體3D原子結構的關鍵差異
▲ 作者：Byung Hyo Kim, Junyoung Heo, Sungin Kim, Cyril F. Reboul, Hoje Chun,et al.
▲ 鏈接：
https://science.sciencemag.org/content/368/6486/60
▲ 摘要：
準確地確定納米晶體的三維原子結構是瞭解和預測納米晶體物理性質的前提。來自同一合成批次的納米晶體通常被認為是微小的，但可能在尺寸、晶格畸變和缺陷方面有重要的差異，這隻能通過高空間三維分辨率的結構表徵來理解。
我們通過發展原子分辨率的三維液體細胞電子顯微鏡來解決單個膠體鉑納米晶體的結構，以揭示配體保護的鉑納米晶體在溶液中的關鍵內在異質性，包括結構簡併度、晶格參數偏差、內部缺陷和應變。
這些結構上的差異導致對自由能的大量貢獻，其結果足以使它們在任何關於納米晶體基本性質或應用的討論中都必須加以考慮。
▲ Abstract
Precise three-dimensional (3D) atomic structure determination of individual nanocrystals is a prerequisite for understanding and predicting their physical properties. Nanocrystals from the same synthesis batch display what are often presumed to be small but possibly important differences in size, lattice distortions, and defects, which can only be understood by structural characterization with high spatial 3D resolution. We solved the structures of individual colloidal platinum nanocrystals by developing atomic-resolution 3D liquid-cell electron microscopy to reveal critical intrinsic heterogeneity of ligand-protected platinum nanocrystals in solution, including structural degeneracies, lattice parameter deviations, internal defects, and strain. These differences in structure lead to substantial contributions to free energies, consequential enough that they must be considered in any discussion of fundamental nanocrystal properties or applications.
天文學Astronomy
An artificial impact on the asteroid (162173) Ryugu formed a crater in the gravity-dominated regime
對小行星（162173）龍宮的人為撞擊形成了一個以重力為主的隕石坑
▲ 作者：M. Arakawa, T. Saiki, K. Wada, K. Ogawa, T. Kadono, et al.
▲ 鏈接：
https://science.sciencemag.org/content/368/6486/67
▲ 摘要
隼鳥2號對小行星龍宮進行了探測，龍宮有一個碎石堆結構。我們描述了利用隼鳥2號的小型攜帶撞擊器在龍宮上進行的撞擊實驗。
這次撞擊產生了一個直徑大於10米的人工隕石坑，它有一個半圓形，邊緣凸起，中心有個坑。分離相機記錄了撞擊和產生的噴出物圖像，持續時間長於8分鐘，顯示了噴出物幕（噴出物的外邊緣）的生長和噴出物在表面的沉積。
噴出物幕是不對稱和不均勻的，它從來沒有完全脫離表面。隕石坑形成於重力主導的區域；換句話說，隕石坑的生長受到重力而不是表面的強度的限制。我們討論了龍宮地表年齡的影響。
▲ Abstract
The Hayabusa2 spacecraft investigated the small asteroid Ryugu, which has a rubble-pile structure. We describe an impact experiment on Ryugu using Hayabusa2’s Small Carry-on Impactor. The impact produced an artificial crater with a diameter>10 meters, which has a semicircular shape, an elevated rim, and a central pit. Images of the impact and resulting ejecta were recorded by the Deployable CAMera 3 for >8 minutes, showing the growth of an ejecta curtain (the outer edge of the ejecta) and deposition of ejecta onto the surface. The ejecta curtain was asymmetric and heterogeneous and it never fully detached from the surface. The crater formed in the gravity-dominated regime; in other words, crater growth was limited by gravity not surface strength. We discuss implications for Ryugu’s surface age.
材料科學Material Science
Extreme tensile strain states in La0.7Ca0.3MnO3 membranes
La0.7Ca0.3MnO3薄膜的極端拉伸應變狀態
▲ 作者：Seung Sae Hong, Mingqiang Gu, Manish Verma, et al.
▲ 鏈接：
https://science.sciencemag.org/content/368/6486/71
▲ 摘要
複合氧化物中呈展現象的一個顯著特徵是基態之間的競爭與合作。在錳氧化物中，金屬相和絕緣相之間的平衡可以通過晶格來調節；擴展點陣控制的範圍將增強訪問其他相的能力。
我們在納米級La0.7Ca0.3MnO3 薄膜中穩定了均勻的極端拉伸應變，單軸超過8%，雙軸超過5%。單軸和雙軸應變抑制了明顯不同的應變值的鐵磁金屬，使其變成可以被磁場轉換的絕緣體。
電子結構計算表明，絕緣子由電荷有序Mn4+和Mn3+構成，並在平面內具有交錯應變增強的姜—泰勒畸變。這種高度可調的應變膜方法為設計和處理相關電子態提供了機會。
▲ Abstract
A defining feature of emergent phenomena in complex oxides is the competition and cooperation between ground states. In manganites, the balance between metallic and insulating phases can be tuned by the lattice; extending the range of lattice control would enhance the ability to access other phases. We stabilized uniform extreme tensile strain in nanoscaleLa0.7Ca0.3MnO3 membranes, exceeding 8% uniaxially and 5% biaxially. Uniaxial and biaxialstrain suppresses the ferromagnetic metal at distinctly different strain values, inducing an insulator that can be extinguished by a magnetic field. Electronic structure calculations indicate that the insulator consists of charge-ordered Mn4+ and Mn3+ with staggered strain-enhanced Jahn-Teller distortions within the plane. This highly tunable strained membrane approach provides a broad opportunity to design and manipulate correlated electron states.
地理學Geography
A slip law for glaciers on deformable beds
冰川在變形床層上的滑動定律
▲ 作者：Lucas K. Zoet, Neal R. Iverson
▲ 鏈接：
https://science.sciencemag.org/content/368/6486/76
▲ 摘要
南極西部冰蓋導致海平面上升，大部分是由於海洋終端冰流在可變形冰磧床上的滑動造成的。其他地方的冰原和傾斜層狀冰川的流動模型需要一條與床層的滑動阻力、滑動速度和水壓相關的定律。
本文介紹了加壓冰在飽和水床上滑動的實驗結果。由於冰在床層上的滑動，穩態的滑動阻力隨滑動速度的增加而增大，但大於臨界速度，直到達到與速度無關的庫侖強度。
這些結果為結合了硬層滑動和床層變形過程的冰川流模型提供了一個廣義滑動定律。
▲ Abstract
Slip of marine-terminating ice streams over beds of deformable till is responsible for most of the contribution of the West Antarctic Ice Sheet to sea level rise. Flow models of the ice sheet and till-bedded glaciers elsewhere require a law that relates slip resistance, slip velocity, and water pressure at the bed. We present results of experiments in which pressurized ice at its melting temperature is slid over a water-saturated till bed. Steady-state slip resistance increases with slip velocity owing to sliding of ice across the bed, but above a threshold velocity, till shears at its rate-independent Coulomb strength. These results motivate a generalized slip law for glacier-flow models that combines processes of hard-bedded sliding and bed deformation.
古人類學Paleoanthropology
Contemporaneity of Australopithecus, Paranthropus, and earlyHomo erectus in South Africa
南非南方古猿、傍人和早期直立人的同時期性
▲ 作者：Andy I. R. Herries, Jesse M. Martin, A. B. Leece, et al.
▲ 鏈接：
https://science.sciencemag.org/content/368/6486/eaaw7293
▲ 摘要
對南非南方古猿的滅絕和傍人及人屬起源的瞭解一直受到人們所認為的古人類化石的複雜地質環境、較差的年代分辨率以及缺乏保存完好的早期人屬標本的阻礙。
我們描述了在南非的Drimolen Main Quarry發現兩個人頭顱的過程，並確定了它們的年代。DNH 152誕生約204 ~ 195萬年前，是最早出現的羅百氏傍人，而DNH 134則是最早出現的與直立人有明顯親緣關係的頭蓋骨。
這些化石還表明，直立人、傍人和南方古猿在大約200萬年前是同時期的。這種高度的分類學多樣性也反映在非人族的物種中，併為在氣候變化時期特有的進化和擴散提供了證據。
▲ Abstract
Understanding the extinction of Australopithecus and origins of Paranthropusand Homo in South Africa has been hampered by the perceived complex geological context of hominin fossils, poor chronological resolution, and a lack of well-preserved early Homo specimens. We describe, date, and contextualize the discovery of two hominin crania from Drimolen Main Quarry in South Africa. At ~2.04 million to 1.95 million years old, DNH 152 represents the earliest definitive occurrence of Paranthropus robustus, and DNH 134 represents the earliest occurrence of a cranium with clear affinities to Homo erectus. These crania also show that Homo, Paranthropus, andAustralopithecus were contemporaneous at ~2 million years ago. This high taxonomic diversity is also reflected in non-hominin species and provides evidence of endemic evolution and dispersal during a period of climatic variability.
醫學Medical Science
Maintenance of neural stem cell positional identity by mixed-lineage leukemia 1
MLL1對神經幹細胞定位識別的維持
▲ 作者：Ryan N. Delgado, Benjamin Mansky, Sajad Hamid Ahanger, et al.
▲ 鏈接：
https://science.sciencemag.org/content/368/6486/48
▲ 摘要
發育中的大腦和出生後的大腦中的神經幹細胞（NSCs）具有不同的位置特徵，這些特徵決定了它們產生的神經元的類型。
雖然形態形成學最初在神經管中建立了NSC的位置識別，但隨著前腦越來越大、解剖結構越來越複雜，這種差異是如何維持的還不清楚。
我們發現，維持NSC在小鼠大腦中的定位需要MLL1依賴的表觀遺傳記憶系統。在音蝟因子建立後，腹側的NSC獨立於這一形態。即使是短暫的MLL1抑制也會導致持續性的腹側特徵喪失，導致體內產生具有背側NSCs特徵的神經元。
因此，形態形成所提供的空間信息可以轉化為表觀遺傳機制，在前腦中維持區域上不同的發育程序。
▲ Abstract
Neural stem cells (NSCs) in the developing and postnatal brain have distinct positional identities that dictate the types of neurons they generate. Although morphogens initially establish NSC positional identity in the neural tube, it is unclear how such regional differences are maintained as the forebrain grows much largerand more anatomically complex. We found that the maintenance of NSC positional identity in the murine brain requires a mixed-lineage leukemia 1 (Mll1)–dependent epigenetic memory system. After establishment by sonic hedgehog, ventral NSC identity became independent of this morphogen. Even transient MLL1 inhibition caused a durable loss of ventral identity, resulting in the generation of neurons with the characteristics of dorsal NSCs in vivo. Thus, spatial information provided by morphogens can be transitioned to epigenetic mechanisms that maintain regionally distinct developmental programs in the forebrain.
Cysteine depletion induces pancreatic tumor ferroptosis in mice
半胱氨酸缺失誘導小鼠胰腺腫瘤鐵死亡
▲ 作者：Michael A. Badgley, Daniel M. Kremer, H. Carlo Maurer, et al.
▲ 鏈接：
https://science.sciencemag.org/content/368/6486/85
▲ 摘要
鐵死亡是細胞死亡的一種形式，是由脂質活性氧（ROS）的災難性積累引起的。致癌信號提高了許多腫瘤類型的脂質ROS的生成，並被來自氨基酸半胱氨酸的代謝物抵消。
在這項研究中，我們發現氧化半胱氨酸（cystine）通過xC- 系統輸入是胰腺導管腺癌（PDAC）的一個重要依賴項，是導致癌症死亡的主要原因。PDAC細胞利用半胱氨酸合成谷胱甘肽和輔酶A，兩者共同下調鐵死亡。
通過對基因改造小鼠的研究，我們發現xC- 亞基Slc7a11的缺失，誘導了腫瘤選擇性鐵死亡，抑制了PDAC的生長。
▲ Abstract
Ferroptosis is a form of cell death that results from the catastrophic accumulation of lipid reactive oxygen species (ROS). Oncogenic signaling elevates lipid ROS production in many tumor types and is counteracted by metabolites that are derived from the amino acid cysteine. In this work, we show that the import of oxidized cysteine (cystine) via system xC- is a critical dependency of pancreatic ductal adenocarcinoma (PDAC), which is a leading cause of cancer mortality. PDAC cells used cysteine to synthesize glutathione and coenzyme A, which, together, down-regulated ferroptosis. Studying genetically engineered mice, we found that the deletion of a system xC-subunit, Slc7a11, induced tumor-selective ferroptosis and inhibited PDAC growth.
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