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991.
Lappe-Siefke C Goebbels S Gravel M Nicksch E Lee J Braun PE Griffiths IR Nave KA 《Nature genetics》2003,33(3):366-374
Myelination of axons by oligodendrocytes enables rapid impulse propagation in the central nervous system. But long-term interactions between axons and their myelin sheaths are poorly understood. Here we show that Cnp1, which encodes 2',3'-cyclic nucleotide phosphodiesterase in oligodendrocytes, is essential for axonal survival but not for myelin assembly. In the absence of glial cyclic nucleotide phosphodiesterase, mice developed axonal swellings and neurodegeneration throughout the brain, leading to hydrocephalus and premature death. But, in contrast to previously studied myelin mutants, the ultrastructure, periodicity and physical stability of myelin were not altered in these mice. Genetically, the chief function of glia in supporting axonal integrity can thus be completely uncoupled from its function in maintaining compact myelin. Oligodendrocyte dysfunction, such as that in multiple sclerosis lesions, may suffice to cause secondary axonal loss. 相似文献
992.
Loss of neuropathy target esterase in mice links organophosphate exposure to hyperactivity 总被引:13,自引:0,他引:13
Neuropathy target esterase (NTE) is involved in neural development and is the target for neurodegeneration induced by selected organophosphorus pesticides and chemical warfare agents. We generated mice with disruptions in Nte, the gene encoding NTE. Nte(-/-) mice die after embryonic day 8, and Nte(+/-) mice have lower activity of Nte in the brain and higher mortality when exposed to the Nte-inhibiting compound ethyl octylphosphonofluoridate (EOPF) than do wild-type mice. Nte(+/-) and wild-type mice treated with 1 mg per kg of body weight of EOPF have elevated motor activity, showing that even minor reduction of Nte activity leads to hyperactivity. These studies show that genetic or chemical reduction of Nte activity results in a neurological phenotype of hyperactivity in mammals and indicate that EOPF toxicity occurs directly through inhibition of Nte without the requirement for Nte gain of function or aging. 相似文献
993.
Inada N Oguri M Pindor B Hennawi JF Chiu K Zheng W Ichikawa S Gregg MD Becker RH Suto Y Strauss MA Turner EL Keeton CR Annis J Castander FJ Eisenstein DJ Frieman JA Fukugita M Gunn JE Johnston DE Kent SM Nichol RC Richards GT Rix HW Sheldon ES Bahcall NA Brinkmann J Ivezić Z Lamb DQ McKay TA Schneider DP York DG 《Nature》2003,426(6968):810-812
Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures (that is, clusters of galaxies and even larger assemblies) predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004 + 4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model. 相似文献
994.
Recent ice ages on Mars 总被引:1,自引:0,他引:1
A key pacemaker of ice ages on the Earth is climatic forcing due to variations in planetary orbital parameters. Recent Mars exploration has revealed dusty, water-ice-rich mantling deposits that are layered, metres thick and latitude dependent, occurring in both hemispheres from mid-latitudes to the poles. Here we show evidence that these deposits formed during a geologically recent ice age that occurred from about 2.1 to 0.4 Myr ago. The deposits were emplaced symmetrically down to latitudes of approximately 30 degrees--equivalent to Saudi Arabia and the southern United States on the Earth--in response to the changing stability of water ice and dust during variations in obliquity (the angle between Mars' pole of rotation and the ecliptic plane) reaching 30-35 degrees. Mars is at present in an 'interglacial' period, and the ice-rich deposits are undergoing reworking, degradation and retreat in response to the current instability of near-surface ice. Unlike the Earth, martian ice ages are characterized by warmer polar climates and enhanced equatorward transport of atmospheric water and dust to produce widespread smooth deposits down to mid-latitudes. 相似文献
995.
In many aquatic ecosystems, most microbes live in matrix-enclosed biofilms and contribute substantially to energy flow and nutrient cycling. Little is known, however, about the coupling of structure and dynamics of these biofilms to ecosystem function. Here we show that microbial biofilms changed the physical and chemical microhabitat and contributed to ecosystem processes in 30-m-long stream mesocosms. Biofilm growth increased hydrodynamic transient storage-streamwater detained in quiescent zones, which is a major physical template for ecological processes in streams-by 300% and the retention of suspended particles by 120%. In addition, by enhancing the relative uptake of organic molecules of lower bioavailability, the interplay of biofilm microarchitecture and mass transfer changed their downstream linkage. As living zones of transient storage, biofilms bring hydrodynamic retention and biochemical processing into close spatial proximity and influence biogeochemical processes and patterns in streams. Thus, biofilms are highly efficient and successful ecological communities that may also contribute to the influence that headwater streams have on rivers, estuaries and even oceans through longitudinal linkages of local biogeochemical and hydrodynamic processes. 相似文献
996.
Price PA Fox DW Kulkarni SR Peterson BA Schmidt BP Soderberg AM Yost SA Berger E Djorgovski SG Frail DA Harrison FA Sari R Blain AW Chapman SC 《Nature》2003,423(6942):844-847
Past studies of cosmological gamma-ray bursts (GRBs) have been hampered by their extreme distances, resulting in faint afterglows. A nearby GRB could potentially shed much light on the origin of these events, but GRBs with a redshift z 相似文献
997.
CUL-4 ubiquitin ligase maintains genome stability by restraining DNA-replication licensing 总被引:20,自引:0,他引:20
To maintain genome stability, DNA replication is strictly regulated to occur only once per cell cycle. In eukaryotes, the presence of 'licensing proteins' at replication origins during the G1 cell-cycle phase allows the formation of the pre-replicative complex. The removal of licensing proteins from chromatin during the S phase ensures that origins fire only once per cell cycle. Here we show that the CUL-4 ubiquitin ligase temporally restricts DNA-replication licensing in Caenorhabditis elegans. Inactivation of CUL-4 causes massive DNA re-replication, producing cells with up to 100C DNA content. The C. elegans orthologue of the replication-licensing factor Cdt1 (refs 2, 3) is required for DNA replication. C. elegans CDT-1 is present in G1-phase nuclei but disappears as cells enter S phase. In cells lacking CUL-4, CDT-1 levels fail to decrease during S phase and instead remain constant in the re-replicating cells. Removal of one genomic copy of cdt-1 suppresses the cul-4 re-replication phenotype. We propose that CUL-4 prevents aberrant re-initiation of DNA replication, at least in part, by facilitating the degradation of CDT-1. 相似文献
998.
Little is known about the origins of globular clusters, which contain hundreds of thousands of stars in a volume only a few light years across. Radiation pressure and winds from luminous young stars should disperse the star-forming gas and disrupt the formation of the cluster. Globular clusters in our Galaxy cannot provide answers; they are billions of years old. Here we report the measurement of infrared hydrogen recombination lines from a young, forming super star cluster in the dwarf galaxy NGC5253. The lines arise in gas heated by a cluster of about one million stars, including 4,000-6,000 massive, hot 'O' stars. It is so young that it is still enshrouded in gas and dust, hidden from optical view. The gases within the cluster seem bound by gravity, which may explain why the windy and luminous O stars have not yet blown away those gases. Young clusters in 'starbursting' galaxies in the local and distant Universe may also be gravitationally confined and cloaked from view. 相似文献
999.
SHISuhua DUYaqing D.E.Boufford GONGXun HUANGYelin HEHanghang ZHONGYang 《科学通报(英文版)》2003,48(15):1576-1580
The chloroplast gene matK and the internal transcribed spacers (ITS) of the nuclear ribosomal DNA from Schnabelia, a genus endemic to China, and 6 genera of Verbenaceae and 13 genera of Lamiaceae were sequenced. The phylogenetic signal and validity outgroups were measured and evaluated by means of the relatively apparent synapomorphy analysis (RASA). Independent and combined phylogenetic analyses for the matK and ITS sequences were performed using the maximum parsimony (MP), neighbor-joining (NJ) and maximum likelihood (ML) methods, indicating that Schnabelia oligophylla and Caryopteris terniflora form a sister-group relationship. The Caryopteris complex is not shown to be a monophyly because Trichostema, C. paniculata and C. forrestii are paraphyletic to the clade containing the remaining members of the complex. A monophyly of Ajugoideae proposed by Cantino et al., including 8 genera in this study, is strongly supported and the closest relatives of Schnabelia are in the Ajugoideae (Lamiaceae), especially near Caryopteris terniflora. The polygenetic analyses also showed that the genera of Lamiaceae and Verbenaceae sampled in this tudy are phylogenetically mixed and the genus Avicennia is distant to other genera of Verbenaceae. RASA and combined analysis can be used as effective approaches to determining the relationships among phylogenetically complex groups. 相似文献
1000.
Robert E. Butts 《Foundations of Science》2003,8(2):213-214