排序方式: 共有151条查询结果,搜索用时 0 毫秒
81.
Seibert MM Ekeberg T Maia FR Svenda M Andreasson J Jönsson O Odić D Iwan B Rocker A Westphal D Hantke M DePonte DP Barty A Schulz J Gumprecht L Coppola N Aquila A Liang M White TA Martin A Caleman C Stern S Abergel C Seltzer V Claverie JM Bostedt C Bozek JD Boutet S Miahnahri AA Messerschmidt M Krzywinski J Williams G Hodgson KO Bogan MJ Hampton CY Sierra RG Starodub D Andersson I Bajt S Barthelmess M Spence JC Fromme P Weierstall U Kirian R Hunter M Doak RB Marchesini S Hau-Riege SP Frank M 《Nature》2011,470(7332):78-81
X-ray lasers offer new capabilities in understanding the structure of biological systems, complex materials and matter under extreme conditions. Very short and extremely bright, coherent X-ray pulses can be used to outrun key damage processes and obtain a single diffraction pattern from a large macromolecule, a virus or a cell before the sample explodes and turns into plasma. The continuous diffraction pattern of non-crystalline objects permits oversampling and direct phase retrieval. Here we show that high-quality diffraction data can be obtained with a single X-ray pulse from a non-crystalline biological sample, a single mimivirus particle, which was injected into the pulsed beam of a hard-X-ray free-electron laser, the Linac Coherent Light Source. Calculations indicate that the energy deposited into the virus by the pulse heated the particle to over 100,000?K after the pulse had left the sample. The reconstructed exit wavefront (image) yielded 32-nm full-period resolution in a single exposure and showed no measurable damage. The reconstruction indicates inhomogeneous arrangement of dense material inside the virion. We expect that significantly higher resolutions will be achieved in such experiments with shorter and brighter photon pulses focused to a smaller area. The resolution in such experiments can be further extended for samples available in multiple identical copies. 相似文献
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Harris TH Banigan EJ Christian DA Konradt C Tait Wojno ED Norose K Wilson EH John B Weninger W Luster AD Liu AJ Hunter CA 《Nature》2012,486(7404):545-548
Chemokines have a central role in regulating processes essential to the immune function of T cells, such as their migration within lymphoid tissues and targeting of pathogens in sites of inflammation. Here we track T cells using multi-photon microscopy to demonstrate that the chemokine CXCL10 enhances the ability of CD8+ T cells to control the pathogen Toxoplasma gondii in the brains of chronically infected mice. This chemokine boosts T-cell function in two different ways: it maintains the effector T-cell population in the brain and speeds up the average migration speed without changing the nature of the walk statistics. Notably, these statistics are not Brownian; rather, CD8+ T-cell motility in the brain is well described by a generalized Lévy walk. According to our model, this unexpected feature enables T cells to find rare targets with more than an order of magnitude more efficiency than Brownian random walkers. Thus, CD8+ T-cell behaviour is similar to Lévy strategies reported in organisms ranging from mussels to marine predators and monkeys, and CXCL10 aids T cells in shortening the average time taken to find rare targets. 相似文献
84.
A fundamental aspect of all biological systems is cooperation. Cooperative interactions are required for many levels of biological organization ranging from single cells to groups of animals. Human society is based to a large extent on mechanisms that promote cooperation. It is well known that in unstructured populations, natural selection favours defectors over cooperators. There is much current interest, however, in studying evolutionary games in structured populations and on graphs. These efforts recognize the fact that who-meets-whom is not random, but determined by spatial relationships or social networks. Here we describe a surprisingly simple rule that is a good approximation for all graphs that we have analysed, including cycles, spatial lattices, random regular graphs, random graphs and scale-free networks: natural selection favours cooperation, if the benefit of the altruistic act, b, divided by the cost, c, exceeds the average number of neighbours, k, which means b/c > k. In this case, cooperation can evolve as a consequence of 'social viscosity' even in the absence of reputation effects or strategic complexity. 相似文献
85.
Disruption of CREB function in brain leads to neurodegeneration 总被引:24,自引:0,他引:24
Mantamadiotis T Lemberger T Bleckmann SC Kern H Kretz O Martin Villalba A Tronche F Kellendonk C Gau D Kapfhammer J Otto C Schmid W Schütz G 《Nature genetics》2002,31(1):47-54
Control of cellular survival and proliferation is dependent on extracellular signals and is a prerequisite for ordered tissue development and maintenance. Activation of the cAMP responsive element binding protein (CREB) by phosphorylation has been implicated in the survival of mammalian cells. To define its roles in the mouse central nervous system, we disrupted Creb1 in brain of developing and adult mice using the Cre/loxP system. Mice with a Crem(-/-) background and lacking Creb in the central nervous system during development show extensive apoptosis of postmitotic neurons. By contrast, mice in which both Creb1 and Crem are disrupted in the postnatal forebrain show progressive neurodegeneration in the hippocampus and in the dorsolateral striatum. The striatal phenotype is reminiscent of Huntington disease and is consistent with the postulated role of CREB-mediated signaling in polyglutamine-triggered diseases. 相似文献
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Realization of the Cirac-Zoller controlled-NOT quantum gate 总被引:4,自引:0,他引:4
Schmidt-Kaler F Häffner H Riebe M Gulde S Lancaster GP Deuschle T Becher C Roos CF Eschner J Blatt R 《Nature》2003,422(6930):408-411
Quantum computers have the potential to perform certain computational tasks more efficiently than their classical counterparts. The Cirac-Zoller proposal for a scalable quantum computer is based on a string of trapped ions whose electronic states represent the quantum bits of information (or qubits). In this scheme, quantum logical gates involving any subset of ions are realized by coupling the ions through their collective quantized motion. The main experimental step towards realizing the scheme is to implement the controlled-NOT (CNOT) gate operation between two individual ions. The CNOT quantum logical gate corresponds to the XOR gate operation of classical logic that flips the state of a target bit conditioned on the state of a control bit. Here we implement a CNOT quantum gate according to the Cirac-Zoller proposal. In our experiment, two 40Ca+ ions are held in a linear Paul trap and are individually addressed using focused laser beams; the qubits are represented by superpositions of two long-lived electronic states. Our work relies on recently developed precise control of atomic phases and the application of composite pulse sequences adapted from nuclear magnetic resonance techniques. 相似文献
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JNK phosphorylates paxillin and regulates cell migration 总被引:1,自引:0,他引:1
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Enhanced synaptic plasticity in newly generated granule cells of the adult hippocampus 总被引:1,自引:0,他引:1
Neural stem cells in various regions of the vertebrate brain continuously generate neurons throughout life. In the mammalian hippocampus, a region important for spatial and episodic memory, thousands of new granule cells are produced per day, with the exact number depending on environmental conditions and physical exercise. The survival of these neurons is improved by learning and conversely learning may be promoted by neurogenesis. Although it has been suggested that newly generated neurons may have specific properties to facilitate learning, the cellular and synaptic mechanisms of plasticity in these neurons are largely unknown. Here we show that young granule cells in the adult hippocampus differ substantially from mature granule cells in both active and passive membrane properties. In young neurons, T-type Ca2+ channels can generate isolated Ca2+ spikes and boost fast Na+ action potentials, contributing to the induction of synaptic plasticity. Associative long-term potentiation can be induced more easily in young neurons than in mature neurons under identical conditions. Thus, newly generated neurons express unique mechanisms to facilitate synaptic plasticity, which may be important for the formation of new memories. 相似文献
90.
Understanding the emergence of cooperation is a fundamental problem in evolutionary biology. Evolutionary game theory has become a powerful framework with which to investigate this problem. Two simple games have attracted most attention in theoretical and experimental studies: the Prisoner's Dilemma and the snowdrift game (also known as the hawk-dove or chicken game). In the Prisoner's Dilemma, the non-cooperative state is evolutionarily stable, which has inspired numerous investigations of suitable extensions that enable cooperative behaviour to persist. In particular, on the basis of spatial extensions of the Prisoner's Dilemma, it is widely accepted that spatial structure promotes the evolution of cooperation. Here we show that no such general predictions can be made for the effects of spatial structure in the snowdrift game. In unstructured snowdrift games, intermediate levels of cooperation persist. Unexpectedly, spatial structure reduces the proportion of cooperators for a wide range of parameters. In particular, spatial structure eliminates cooperation if the cost-to-benefit ratio of cooperation is high. Our results caution against the common belief that spatial structure is necessarily beneficial for cooperative behaviour. 相似文献