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391.
In the developing central nervous system (CNS), the control of synapse number and function is critical to the formation of neural circuits. We previously demonstrated that astrocyte-secreted factors powerfully induce the formation of functional excitatory synapses between CNS neurons. Astrocyte-secreted thrombospondins induce the formation of structural synapses, but these synapses are postsynaptically silent. Here we use biochemical fractionation of astrocyte-conditioned medium to identify glypican 4 (Gpc4) and glypican 6 (Gpc6) as astrocyte-secreted signals sufficient to induce functional synapses between purified retinal ganglion cell neurons, and show that depletion of these molecules from astrocyte-conditioned medium significantly reduces its ability to induce postsynaptic activity. Application of Gpc4 to purified neurons is sufficient to increase the frequency and amplitude of glutamatergic synaptic events. This is achieved by increasing the surface level and clustering, but not overall cellular protein level, of the GluA1 subunit of the AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) glutamate receptor (AMPAR). Gpc4 and Gpc6 are expressed by astrocytes in vivo in the developing CNS, with Gpc4 expression enriched in the hippocampus and Gpc6 enriched in the cerebellum. Finally, we demonstrate that Gpc4-deficient mice have defective synapse formation, with decreased amplitude of excitatory synaptic currents in the developing hippocampus and reduced recruitment of AMPARs to synapses. These data identify glypicans as a family of novel astrocyte-derived molecules that are necessary and sufficient to promote glutamate receptor clustering and receptivity and to induce the formation of postsynaptically functioning CNS synapses. 相似文献
392.
Undercooled Fe75Ni25 melts were phase-seeded by a high purity iron. It was found that above a critical undercooling,△Tc = 135 K, a metastable δ phase (b.c.c) solidifies from the iron-seeded melts instead of a thermodynamically stable γ phase (f.c.c). While undercooling of the melt (△T ) is below △Tc, solidification of the γ phase prevails in the iron-seeded melts. For the undercooled melts subjected to spontaneous nucleation, the γ phase always solidifies. After
solidification, the as-solidified γ phase transforms completely to martensite; whereas the as-solidified metastable δ phase partially transforms to the γ phase, and then to martensite. The
untransformed δ phase retains as a -ferrite particles in the microstructures. Based on the classical nucleation theory, catalytic nucleation of the metastable δ phase in the phase-seeded undercooledFe75Ni25 melts was analyzed. It was quantitatively demonstrated that when△T>△Tc, the formation of the δ phase can be ascribed to a better catalytic effect of the iron on its nucleation than that on the nucleation of the γphase. 相似文献
393.
A facile and green preparation of high surface area activated carbons with mixed
microporosity and mesoporosity from durian shell waste is reported in this work. The pore
structure and surface chemistry of the parent carbon were modified by the combination of
ultrasonication and microwave irradiation techniques. The effects of temperature and time in the
ultrasonication treatment and power output and time in the microwave irradiation were studied.
The electrochemical performance of carbon electrodes for supercapacitors was tested by cyclic
voltammeter (CV) and galvanostatic charge–discharge. The results show that the capacitive energy
storage of electrodes is critically dependent on the microporosity and surface chemistry of activated
carbons. The highest electrode capacitance in this work was 103.6 F/g that prepared from activated
carbon modified at an ultrasonication temperature of 323.15 K for 10 min and microwave power
output of 900 W for 10 min. 相似文献
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Crystal structure of an HIV-binding recombinant fragment of human CD4 总被引:68,自引:0,他引:68
S E Ryu P D Kwong A Truneh T G Porter J Arthos M Rosenberg X P Dai N H Xuong R Axel R W Sweet 《Nature》1990,348(6300):419-426
CD4 glycoprotein on the surface of T cells helps in the immune response and is the receptor for HIV infection. The structure of a soluble fragment of CD4 determined at 2.3 A resolution reveals that the molecule has two intimately associated immunoglobulin-like domains. Residues implicated in HIV recognition by analysis of mutants and antibody binding are salient features in domain D1. Domain D2 is distinguished by a variation on the beta-strand topologies of antibody domains and by an intra-sheet disulphide bridge. 相似文献
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399.
Novel regulation and function of Src tyrosine kinase 总被引:4,自引:0,他引:4
Src tyrosine kinase is a critical signal transducer that modulates a wide variety of cellular functions. Misregulation of
Src leads to cell transformation and cancer. Heterotrimeric guanine-nucleotide-binding proteins (G proteins) are another group
of signaling molecules that transduce signals from cell-surface receptors to generate physiological responses. Recently, it
was discovered that Gαs and Gαi could directly stimulate Src family tyrosine kinase activity. This novel regulation of Src
tyrosine kinase by G proteins provides insights into the adenylyl cyclase-independent signaling mechanisms involved in ligand-induced
receptor desensitization, internalization and other physiological processes.
Received 17 August 2001; received after revision 22 October 2001; accepted 24 October 2001 相似文献
400.