Structural basis for the coiled-coil architecture of human CtIP

The DNA repair factor CtIP has a critical function in double-strand break (DSB) repair by homologous recombination, promoting the assembly of the repair apparatus at DNA ends and participating in DNA-end resection. However, the molecular mechanisms of CtIP function in DSB repair remain unclear. Here, we present an atomic model for the three-dimensional architecture of human CtIP, derived from a multi-disciplinary approach that includes X-ray crystallography, small-angle X-ray scattering (SAXS) and diffracted X-ray tracking (DXT). Our data show that CtIP adopts an extended dimer-of-dimers structure, in agreement with a role in bridging distant sites on chromosomal DNA during the recombinational repair. The zinc-binding motif in the CtIP N-terminus alters dynamically the coiled-coil structure, with functional implications for the long-range interactions of CtIP with DNA. Our results provide a structural basis for the three-dimensional arrangement of chains in the CtIP tetramer, a key aspect of CtIP function in DNA DSB repair.     

Molecules involved in cell–cell adhesion during development

A peculiarity of nitrosamines is the high degree of cell and organ specificity in inducing tumors. There is substantial evidence that the initiation of the carcinogenesis process by carcinogens of this group is linked to the metabolic competence of the target tissue or cell to convert these carcinogens into mutagenic metabolites and to the binding of those metabolites to cellular DNA. Alkylation occurs in the DNA at the N-1, N-3, and N-7 positions of adenine; the N-3, N-7, and O⁶ of guanine; the N-3, and O² of cytosine; and the N-3, O⁴, and O² of thymine; and the phosphate groups. The initial proportion of each DNA adduct depends upon the alkylating agent used. The various DNA adducts are lost to a variable extent from DNA in vivo by spontaneous release of bases and Or by specific DNA repair processes. Studies conducted in vitro and in vivo indicate that alkylation at the oxygen atoms of DNA bases is more critical than alkylation at other positions in the mutagenesis and carcinogenesis induced by N-nitroso compounds. In particular, tissues in which tumors occur more frequently after a pulse dose of nitrosamine are those in which O⁶-alkylguanine persists longest in DNA, presumably resulting in an increased probability that a miscoding event (mutation) will take place during DNA synthesis. The more rapid removal of O⁶-methylguanine from the DNA of liver (as compared with cxtrahepatic tissues) of rats has been associated with the absence of tumor production in this organ by a single dose of dimethylnitrosamine; however, a significant incidence of liver tumors is observed if the same dose is given 24 hr after partial hepatectomy, and tumors arc induced by such a dose of dimethyl-nitrosamine in the liver of hamsters, which has a low capacity to remove O⁶-methylguanine from its DNA. These data also indicate that the rate of disappearance of 7-methylguanine from the liver or cxtrahepatic tissues is independent of the dose of dimethylnitrosamine; whereas O⁶-methylguanine is lost from DNA more rapidly after a low dose of this nitrosamine. It has been shown that in liver the removal of O⁶-methylguanine, but not of other DNA adducts, from DNA can be affected by pretreating the animals with N-nitroso compounds. The modulation of DNA repair processes observed after a single dose and after chronic treatment with nitrosamines is discussed in relation to the tissue-specific carcinogenic effect of this group of carcinogens.     

Effects of nicotinamide on NAD and poly (ADP-ribose) metabolism in DNA-damaged human lymphocytes

The effect of nicotinamide on unscheduled DNA synthesis was studied in resting human lymphocytes. In cells treated with UV irradiation or with MNNG, nicotinamide caused a two-fold stimulation of unscheduled DNA synthesis and retarded the rate of NAD⁺ lowering caused by these treatments. Nicotinamide also reduced the burst of poly(ADP-ribose) synthesis caused by MNNG treat-ment. Thus under conditions that it enhances unscheduled DNA synthesis, nicotinamide causes marked effects on the metabolism of NAD⁺ and poly(ADP-ribose). The effect of nicotinamide on unscheduled DNA synthesis was shown to be independent of protein or polyamine synthesis.     

Identification of a prototypical single-stranded uracil DNA glycosylase from Listeria innocua

A recent phylogenetic study on UDG superfamily estimated a new clade of family 3 enzymes (SMUG1-like), which shares a lower homology with canonic SMUG1 enzymes. The enzymatic properties of the newly found putative DNA glycosylase are unknown. To test the potential UDG activity and evaluate phylogenetic classification, we isolated one SMUG1-like glycosylase representative from Listeria innocua (Lin). A biochemical screening of DNA glycosylase activity in vitro indicates that Lin SMUG1-like glycosylase is a single-strand selective uracil DNA glycosylase. The UDG activity on DNA bubble structures provides clue to its physiological significance in vivo. Mutagenesis and molecular modeling analyses reveal that Lin SMUG1-like glycosylase has similar functional motifs with SMUG1 enzymes; however, it contains a distinct catalytic doublet S67-S68 in motif 1 that is not found in any families in the UDG superfamily. Experimental investigation shows that the S67M-S68N double mutant is catalytically more active than either S67M or S68N single mutant. Coupled with mutual information analysis, the results indicate a high degree of correlation in the evolution of SMUG1-like enzymes. This study underscores the functional and catalytic diversity in the evolution of enzymes in UDG superfamily.     

Apparent gene conversion in an Escherichia coli rec+ strain is explained by multiple rounds of reciprocal crossing-over

Summary Gene conversion, the non-reciprocal transfer of sequence information between homologous DNA sequences, has been reported in lower eukaryotes, mammals and in Escherichia coli. In an E. coli rec ⁺ strain, we established a plasmid carrying two different deleted neo genes (neoDL and neoDR) in an inverted orientation and then selected for homologous recombination events that had reconstructed an intact neo ⁺ gene. We found some plasmids that had apparently experienced intramolecular gene conversion. Further evidence, however, suggests that they are products of multiple rounds of reciprocal crossing-over,apparently involving two plasmid molecules. First, most of the Neo⁺ clones contained multiple types of Neo⁺ plasmids, although the frequency of producing the neo ⁺ clones was low. Second, all the neo ⁺ clones also contained, as a minority, one particular form of dimer, which can be formed by reciprocal crossing-over between neoDL of one plasmid molecule and neoDR of another plasmid molecule. Third, in reconstruction experiments, we cloned and purified this dimer and transferred it back into the rec ⁺ cells. The dimer gave rise to clones containing multiple types of neo ⁺ recombinant monomers, including those apparent gene conversion types, and containing only few molecules of this dimer plasmid.     

间充质干细胞在2019新型冠状病毒肺炎(COVID-19)中的治疗潜能、临床研究与应用前景*

当前因SARS-CoV-2感染而引起的2019新型冠状病毒肺炎(COVID-19)肆虐全球,严重危害人类健康。SARS-CoV-2感染性强,危重症患者死亡率高,尽管各种各样的治疗正在进行临床试验,但目前尚无有效的治疗方法。间充质干细胞(mesenchymal stem cell,MSC)在临床前试验中对多种疾病有良好的治疗效果,因而受到了广泛地关注。MSC可能利用分化潜能诱导分化成功能性肺样细胞、免疫调节与免疫细胞互作、抑制炎症来降低促炎细胞因子分泌、迁移和归巢靶向损伤肺部、抗病毒作用来减少肺上皮细胞中的病毒复制、产生细胞外囊泡来修复受损的组织,进而使COVID-19患者肺功能逐渐恢复正常,缓解并达到治疗COVID-19的目的。综合讨论了COVID-19的基本特征和当前主要治疗手段,同时总结了MSC在COVID-19中的临床研究和当前面临的挑战,探讨了MSC治疗COVID-19的应用前景,为MSC在COVID-19中的治疗提供了理论基础和现实依据。