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The APOBEC-AID family of cytidine deaminase prefers single-stranded nucleic acids for cytidine-to-uridine deamination. Single-stranded nucleic acids are commonly involved in the DNA repair system for breaks generated by CRISPR-Cas9. Here, we show in human cells that APOBEC3 can trigger cytidine deamination of single-stranded oligodeoxynucleotides, which ultimately results in base substitution mutations in genomic DNA through homology-directed repair (HDR) of Cas9-generated double-strand breaks. In addition, the APOBEC3-catalyzed deamination in genomic single-stranded DNA formed during the repair of Cas9 nickase-generated single-strand breaks in human cells can be further processed to yield mutations mainly involving insertions or deletions (indels). Both APOBEC3-mediated deamination and DNA-repair proteins play important roles in the generation of these indels. Therefore, optimizing conditions for the repair of CRISPR-Cas9-generated DNA breaks, such as using double-stranded donors in HDR or temporarily suppressing endogenous APOBEC3s, can repress these unwanted mutations in genomic DNA.

APOBECs (apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like) are a family of cytidine deaminases that prefer single-stranded nucleic acids as substrates. Besides their physiological functions, APOBEC family members have been found to cause hypermutations of cancer genomes, which could be correlated with cancer development and poor prognosis. Recently, APOBEC family members have been combined with the versatile CRISPR/Cas9 system to perform targeted base editing or induce hypermutagenesis. This combination improved the CRISPR/Cas9-mediated gene editing at single-base precision, greatly enhancing its usefulness. Here, we review the physiological functions and structural characteristics of APOBEC family members and their roles as endogenous mutators that contribute to hypermutations during carcinogenesis. We also review the various iterations of the APOBEC-CRISPR/Cas9 gene-editing tools, pointing out their features and limitations as well as the possibilities for future developments.

In this paper, W6Mo5Cr4V2 steel gear tooth flank was firstly melted by laser irradiation, and then treated by microshot peening (MSP) with shot particles of different materials and peening time. The materials of shot particles were high-carbon cast steels and ceramics. The peening time was from 20 s to 100 s. The fatigue behavior of gear was tested using Forschungsstelle fur Zahnrader und Getriebebau (FZG) back-to-back spur gear test machine. The experimental results have showed that the microhardness of gears increased, and the surface roughness of gears decreased after microshot peening treatment. In addition, the residual stress state in the near-surface layer of microshot peened gears was changed from tensile stress to compressive stress. The microshot peening treatment has improved the fatigue strength of gears in comparison with gears undergoing the laser surface melted (LSM). The material of microshot particle influenced the fatigue behavior of gears going through laser surface melting and microshot peening (LSMMSP). The fatigue properties of LSMMSP gears did not enhance with an increase in shot peening time. The effects of the relaxation mechanism of the compressive residual stress on the fatigue performance of LSMMSP gears have also been discussed. Retained austenite has played a positive role in improving the fatigue limit of LSMMSP gears.

In this paper, the effect of severe shot peening combined with laser melting (LSMSSP for short) on the fatigue resistance of 20CrMiTi steel gears is investigated in comparison with the effect of traditional shot peening on the fatigue resistance of the laser surface melted (LSMTSP for short) 20CrMiTi steel gear. The surface characteristics of the gear have been analyzed by a scanning electron microscope (SEM) and an X-ray diffractometer (XRD). The Forschungsstelle fur Zahnrader und Getriebebau (FZG) back-to-back spur gear test rig was used for fatigue experiments. Experimental results showed that the residual stresses, full width at half maximum (FWHM), microhardness and retained austenite of the LSMSSP gears and LSMTSP gear were entirely different. Although the LSMSSP gears had higher surface roughness than the LSMTSP gear, the LSMSSP gears still had better fatigue resistance than the LSMTSP gear and laser surface melted gear. The nanocrystallized surface layer on the gear tooth flank created by severe shot peening might be a very important factor for improving the fatigue property of the LSMSSP gears.

The effect of combined laser surface melting and shot peening (LSMSP for short) on fatigue resistance of 20CrMnTi steel gear is investigated in this paper. After laser surface melting (LSM) treatment to gear tooth flanks, gear was treated by shot peening (SP) with different peening intensities. The residual stresses, full width at half maximum (FWHM), microhardness, retained austenite volume fraction and surface roughness of the LSMSP gear were measured. The Forschungsstelle fur Zahnrader und Getriebebau (FZG) back-to-back spur gear test rig was used for fatigue experiments. Experimental results showed that compressive residual stress in the LSMSP gear tooth firstly increased to a maximum value and then decreased to stress state of the substrate. With improving strength of shot peened specimen, maximum values and depths of compressive residual stresses increased in the LSMSP gear. Owing to grain refinement, FWHM of the LSMSP gear tooth was broadened obviously along the direction of depth. Through LSMSP treatment, the retained austenite volume fraction of the LSMSP gear was lower than 5% in the LSMSP surface layer. The microhardness of the LSMSP gear tooth has been raised above 20%. The fatigue and wear resistance of the LSMSP gear was much better than that of the LSM and SP gears.

Shell was the simplest biological mineral material protecting the soft tissue. In this paper, the effects of the cross bedded structure of Limaria basilanica shell and organic matter on the three-point bending behavior of Limaria basilanica shell were studied. Experimental results showed that the mechanical properties of shell were closely related to its microstructure, and both of them had anisotropy. The mechanical properties and density of the shell decreased evidently with the increase of temperature. It was indicated that the organic matter in the shell decreased with the increase of heating temperature, which lead to a marked decrease in mechanical properties of Limaria basilanica. The bearing capacity of the shell in the vertical direction was higher than that of the shell in the horizontal direction. When the organic matter in the shell was removed, the three-point bending strength of the shell was only one- twenty or one-thirty of its original strength. The main toughening mechanisms of Limaria basilanica shell were crack deflection, fiber pull out, and viscoelastic properties of organic matter. The bionic study on the toughening mechanism of shell material provided important information about design of the composite materials.

In this paper, the friction and wear properties of Scapharca broughtonii were investigated under low-speed (5-45 mm/s) and low-load (1.5N-6N) conditions by using a micro-tribological tester. The stereoscopic microscope and computer imaging analysis system was employed to test the abrasion slide size and calculate the wear volume of Scapharca broughtonii. The worn surface morphology of Scapharca broughtonii was observed by a scanning electron microscope (SEM), and the wear mechanism of Scapharca broughtonii was obtained. Experimental results show that the friction coefficient decreased rapidly with an increase in sliding speed under low-load and low-speed dry sliding conditions. When the load was below 6N, the influence of microstructure on the friction coefficient was very small. The wear rate of Scapharca broughtonii increased with an increase in sliding speed when the load was greater than 6N. When the speed increased, the influence of load on the friction coefficient occupied a dominant position. At the same time, the wear rate was increased with an increase in load. The main wear mechanisms of Scapharca broughtonii were stress fatigue wear and three-body abrasive wear, and mild adhesive wear and abrasion.