Modulation of phospholipase A2 activity generated by molecular evolution

Snake venom oligomeric neurotoxins offer several unique examples of modulation of phospholipase A2 (PLA2) activity generated by molecular evolution. This phenomenon was found in evolutionary younger snakes and is probably common for representatives of the genus Vipera. At present, the best-studied example is the heterodimeric neurotoxin vipoxin from the venom of the southeast European snake Vipera ammodytes meridionalis. It is a complex between a basic strongly toxic PLA2 and an acidic and catalytically inactive PLA2-like component (Inh). This is the first reported example of a high degree of structural homology (62%) between an enzyme and its natural protein inhibitor. The inhibitor is a product of the divergent evolution of the unstable PLA2 in order to stabilize it and to preserve the pharmacological activity/toxicity for a long time. Inh reduces both the catalytic activity and toxicity of PLA2. Vipoxin also illustrates evolution of the catalytic into a inhibitory function. Vipoxin analogues have been found in the venom of viperid snakes inhabiting diverse regions of the world. An attempt is made to explain modulation of the toxic function by the three-dimensional structure of vipoxin.     

Identification of human brain tumour initiating cells

The cancer stem cell (CSC) hypothesis suggests that neoplastic clones are maintained exclusively by a rare fraction of cells with stem cell properties. Although the existence of CSCs in human leukaemia is established, little evidence exists for CSCs in solid tumours, except for breast cancer. Recently, we prospectively isolated a CD133+ cell subpopulation from human brain tumours that exhibited stem cell properties in vitro. However, the true measures of CSCs are their capacity for self renewal and exact recapitulation of the original tumour. Here we report the development of a xenograft assay that identified human brain tumour initiating cells that initiate tumours in vivo. Only the CD133+ brain tumour fraction contains cells that are capable of tumour initiation in NOD-SCID (non-obese diabetic, severe combined immunodeficient) mouse brains. Injection of as few as 100 CD133+ cells produced a tumour that could be serially transplanted and was a phenocopy of the patient's original tumour, whereas injection of 10(5) CD133- cells engrafted but did not cause a tumour. Thus, the identification of brain tumour initiating cells provides insights into human brain tumour pathogenesis, giving strong support for the CSC hypothesis as the basis for many solid tumours, and establishes a previously unidentified cellular target for more effective cancer therapies.     

Crystal structure of a bacterial homologue of Na+/Cl--dependent neurotransmitter transporters

Na+/Cl--dependent transporters terminate synaptic transmission by using electrochemical gradients to drive the uptake of neurotransmitters, including the biogenic amines, from the synapse to the cytoplasm of neurons and glia. These transporters are the targets of therapeutic and illicit compounds, and their dysfunction has been implicated in multiple diseases of the nervous system. Here we present the crystal structure of a bacterial homologue of these transporters from Aquifex aeolicus, in complex with its substrate, leucine, and two sodium ions. The protein core consists of the first ten of twelve transmembrane segments, with segments 1-5 related to 6-10 by a pseudo-two-fold axis in the membrane plane. Leucine and the sodium ions are bound within the protein core, halfway across the membrane bilayer, in an occluded site devoid of water. The leucine and ion binding sites are defined by partially unwound transmembrane helices, with main-chain atoms and helix dipoles having key roles in substrate and ion binding. The structure reveals the architecture of this important class of transporter, illuminates the determinants of substrate binding and ion selectivity, and defines the external and internal gates.     

The genome of the protist parasite Entamoeba histolytica

Entamoeba histolytica is an intestinal parasite and the causative agent of amoebiasis, which is a significant source of morbidity and mortality in developing countries. Here we present the genome of E. histolytica, which reveals a variety of metabolic adaptations shared with two other amitochondrial protist pathogens: Giardia lamblia and Trichomonas vaginalis. These adaptations include reduction or elimination of most mitochondrial metabolic pathways and the use of oxidative stress enzymes generally associated with anaerobic prokaryotes. Phylogenomic analysis identifies evidence for lateral gene transfer of bacterial genes into the E. histolytica genome, the effects of which centre on expanding aspects of E. histolytica's metabolic repertoire. The presence of these genes and the potential for novel metabolic pathways in E. histolytica may allow for the development of new chemotherapeutic agents. The genome encodes a large number of novel receptor kinases and contains expansions of a variety of gene families, including those associated with virulence. Additional genome features include an abundance of tandemly repeated transfer-RNA-containing arrays, which may have a structural function in the genome. Analysis of the genome provides new insights into the workings and genome evolution of a major human pathogen.     

GILT is a critical host factor for Listeria monocytogenes infection

Listeria monocytogenes is a gram-positive, intracellular, food-borne pathogen that can cause severe illness in humans and animals. On infection, it is actively phagocytosed by macrophages; it then escapes from the phagosome, replicates in the cytosol, and subsequently spreads from cell to cell by a non-lytic mechanism driven by actin polymerization. Penetration of the phagosomal membrane is initiated by the secreted haemolysin listeriolysin O (LLO), which is essential for vacuolar escape in vitro and for virulence in animal models of infection. Reduction is required to activate the lytic activity of LLO in vitro, and we show here that reduction by the enzyme gamma-interferon-inducible lysosomal thiol reductase (GILT, also called Ifi30) is responsible for the activation of LLO in vivo. GILT is a soluble thiol reductase expressed constitutively within the lysosomes of antigen-presenting cells, and it accumulates in macrophage phagosomes as they mature into phagolysosomes. The enzyme is delivered by a mannose-6-phosphate receptor-dependent mechanism to the endocytic pathway, where amino- and carboxy-terminal pro-peptides are cleaved to generate a 30-kDa mature enzyme. The active site of GILT contains two cysteine residues in a CXXC motif that catalyses the reduction of disulphide bonds. Mice lacking GILT are deficient in generating major histocompatibility complex class-II-restricted CD4(+) T-cell responses to protein antigens that contain disulphide bonds. Here we show that these mice are resistant to L. monocytogenes infection. Replication of the organism in GILT-negative macrophages, or macrophages expressing an enzymatically inactive GILT mutant, is impaired because of delayed escape from the phagosome. GILT activates LLO within the phagosome by the thiol reductase mechanism shared by members of the thioredoxin family. In addition, purified GILT activates recombinant LLO, facilitating membrane permeabilization and red blood cell lysis. The data show that GILT is a critical host factor that facilitates L. monocytogenes infection.     

Effects of gadolinium addition on the microstructure and mechanical properties of Mg-9Al alloy

This research aims to study the significance of Gd addition (0wt%-2wt%) on the microstructure and mechanical properties of Mg-9Al alloy. The effect of Gd addition on the microstructure was investigated via X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The Mg-9Al alloy contained two phases, α-Mg and β-Mg17Al12. Alloying with Gd led to the emergence of a new rectangular-shaped phase, Al₂Gd. The grain size also decreased marginally upon Gd addition. The ultimate tensile strength and microhardness of Mg-9Al alloy increased by 23% and 19%, respectively, upon 1.5wt% Gd addition. We observed that, although Mg-9Al-2.0Gd alloy exhibited the smallest grain size (181 μm) and the highest dislocation density (5.1×1010 m-2) among the investigated compositions, the Mg-9Al-1.5Gd alloy displayed the best mechanical properties. This anomalous behavior was observed because the Al₂Gd phase was uniformly distributed and present in abundance in Mg-9Al-1.5Gd alloy, whereas it was coarsened and asymmetrically conglomerated in Mg-9Al-2.0Gd.     

Microstructures and impact toughness behavior of Al 5083 alloy processed by cryorolling and afterwards annealing

The influence of rolling at liquid nitrogen temperature and annealing on the microstructure and mechanical properties of Al 5083 alloy was studied in this paper. Cryorolled samples of Al 5083 show significant improvements in strength and hardness. The ultimate tensile strength increases up to 340 MPa and 390 MPa for the 30% and 50% cryorolled samples, respectively. The cryorolled samples, with 30% and 50% reduction, were subjected to Charpy impact testing at various temperatures from ?190℃ to 100℃. It is observed that increasing the percentage of reduction of samples during cryorolling has significant effect on decreasing impact toughness at all temperatures by increasing yield strength and decreasing ductility. Annealing of samples after cryorolling shows remarkable increment in impact toughness through recovery and recrystallization. The average grain size of the 50% cryorolled sample (14 μm) after annealing at 350℃ for 1 h is found to be finer than that of the 30% cryorolled sample (25 μm). The scanning electron microscopy (SEM) analysis of fractured surfaces shows a large-size dimpled morphology, resembling the ductile fracture mechanism in the starting material and fibrous structure with very fine dimples in cryorolled samples corresponding to the brittle fracture mechanism.     

印度

与其他自2000年以来科研发展几乎停滞不前的国家相比,印度近年的科研水平可以说是突飞猛进。然而,众多的观察家注意到印度的发展轨迹是一条上升的弹道曲线。数据分析表明,印度在科研发展初始阶段的低谷仅仅是蛰伏,随后的快速发展展露出印度沉睡巨人的本质。