ZG35CrMnSiRe热处理的试验研究

ZG35CrMnSiRe铸钢经不同的热处理,对其强度、硬度、塑性和冲击韧性等机械性能进行测定,实验结果表明其综合机械性能良好。     

The p66shc adaptor protein controls oxidative stress response and life span in mammals

Gene mutations in invertebrates have been identified that extend life span and enhance resistance to environmental stresses such as ultraviolet light or reactive oxygen species. In mammals, the mechanisms that regulate stress response are poorly understood and no genes are known to increase individual life span. Here we report that targeted mutation of the mouse p66shc gene induces stress resistance and prolongs life span. p66shc is a splice variant of p52shc/p46shc (ref. 2), a cytoplasmic signal transducer involved in the transmission of mitogenic signals from activated receptors to Ras. We show that: (1) p66shc is serine phosphorylated upon treatment with hydrogen peroxide (H2O2) or irradiation with ultraviolet light; (2) ablation of p66shc enhances cellular resistance to apoptosis induced by H2O2 or ultraviolet light; (3) a serine-phosphorylation defective mutant of p66shc cannot restore the normal stress response in p66shc-/- cells; (4) the p53 and p21 stress response is impaired in p66shc-/- cells; (5) p66shc-/- mice have increased resistance to paraquat and a 30% increase in life span. We propose that p66shc is part of a signal transduction pathway that regulates stress apoptotic responses and life span in mammals.     

Podocalyxin enhances the adherence of cells to platelets

Podocalyxin (PODXL) is a mucin protein of the CD34 family expressed in kidney glomerular podocytes, vascular endothelium, progenitor bone marrow and tumor cells. It is assumed that PODXL plays an anti-adherent role in kidney podocytes. CHO cells stably expressing human PODXL (CHO-PODXL) or human tumor cells (Tera-1) inherently expressing PODXL showed increased adherence to platelets. The adherence of cells was inhibited (70%) by blockers of platelet P-selectin, prevented by the soluble ectodomain of human PODXL (PODXL-Δ) or by the arginine-glycine-aspartate (RGDS) peptide and partially impeded by inhibition of integrin αVβ3/αVβ5, suggesting a coordinated action of P-selectin and integrins. Colocalization of platelet P-selectin and PODXL expressed on CHO cells was demonstrated by confocal immunofluorescence. No adherence to platelets was observed when PODXL was expressed in glycomutant CHO cells deficient in sialic acid. Received 14 August 2007; received after revision 12 September 2007; accepted 13 September 2007     

Germline gain-of-function mutations in SOS1 cause Noonan syndrome

Noonan syndrome, the most common single-gene cause of congenital heart disease, is characterized by short stature, characteristic facies, learning problems and leukemia predisposition. Gain-of-function mutations in PTPN11, encoding the tyrosine phosphatase SHP2, cause approximately 50% of Noonan syndrome cases. SHP2 is required for RAS-ERK MAP kinase (MAPK) cascade activation, and Noonan syndrome mutants enhance ERK activation ex vivo and in mice. KRAS mutations account for <5% of cases of Noonan syndrome, but the gene(s) responsible for the remainder are unknown. We identified missense mutations in SOS1, which encodes an essential RAS guanine nucleotide-exchange factor (RAS-GEF), in approximately 20% of cases of Noonan syndrome without PTPN11 mutation. The prevalence of specific cardiac defects differs in SOS1 mutation-associated Noonan syndrome. Noonan syndrome-associated SOS1 mutations are hypermorphs encoding products that enhance RAS and ERK activation. Our results identify SOS1 mutants as a major cause of Noonan syndrome, representing the first example of activating GEF mutations associated with human disease and providing new insights into RAS-GEF regulation.     

Polycomb-mediated methylation on Lys27 of histone H3 pre-marks genes for de novo methylation in cancer

Many genes associated with CpG islands undergo de novo methylation in cancer. Studies have suggested that the pattern of this modification may be partially determined by an instructive mechanism that recognizes specifically marked regions of the genome. Using chromatin immunoprecipitation analysis, here we show that genes methylated in cancer cells are specifically packaged with nucleosomes containing histone H3 trimethylated on Lys27. This chromatin mark is established on these unmethylated CpG island genes early in development and then maintained in differentiated cell types by the presence of an EZH2-containing Polycomb complex. In cancer cells, as opposed to normal cells, the presence of this complex brings about the recruitment of DNA methyl transferases, leading to de novo methylation. These results suggest that tumor-specific targeting of de novo methylation is pre-programmed by an established epigenetic system that normally has a role in marking embryonic genes for repression.     

Systematic pathway analysis using high-resolution fitness profiling of combinatorial gene deletions

Systematic genetic interaction studies have illuminated many cellular processes. Here we quantitatively examine genetic interactions among 26 Saccharomyces cerevisiae genes conferring resistance to the DNA-damaging agent methyl methanesulfonate (MMS), as determined by chemogenomic fitness profiling of pooled deletion strains. We constructed 650 double-deletion strains, corresponding to all pairings of these 26 deletions. The fitness of single- and double-deletion strains were measured in the presence and absence of MMS. Genetic interactions were defined by combining principles from both statistical and classical genetics. The resulting network predicts that the Mph1 helicase has a role in resolving homologous recombination-derived DNA intermediates that is similar to (but distinct from) that of the Sgs1 helicase. Our results emphasize the utility of small molecules and multifactorial deletion mutants in uncovering functional relationships and pathway order.