FBXW7 Confers Radiation Survival by Targeting p53 for Degradation

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Bag3-Induced Autophagy Is Associated with Degradation of JCV Oncoprotein, T-Ag

JC virus, JCV, is a human neurotropic polyomavirus whose replication in glial cells causes the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML). In addition, JCV possesses oncogenic activity and expression of its transforming protein, large T-antigen (T-Ag), in several experimental animals induces tumors of neural origin. Further, the presence of JCV DNA and T-Ag have been repeatedly observed in several human malignant tissues including primitive neuroectodermal tumors and glioblastomas. Earlier studies have demonstrated that Bag3, a member of the Bcl-2-associated athanogene (Bag) family of proteins, which is implicated in autophagy and apoptosis, is downregulated upon JCV infection of glial cells and that JCV T-Ag is responsible for suppressing the activity of the BAG3 promoter. Here, we investigated the possible impact of Bag3 on T-Ag expression in JCV-infected human primary glial cells as well as in cells derived from T-Ag-induced medulloblastoma in transgenic animals. Results from these studies revealed that overexpression of Bag3 drastically decreases the level of T-Ag expression by inducing the autophagic degradation of the viral protein. Interestingly, this event leads to the inhibition of JCV infection of glial cells, suggesting that the reduced levels of T-antigen seen upon the overexpression of Bag3 has a biological impact on the viral lytic cycle. Results from protein-protein interaction studies showed that T-Ag and Bag3 physically interact with each other through the zinc-finger of T-Ag and the proline rich domains of Bag3, and this interaction is important for the autophagic degradation of T-Ag. Our observations open a new avenue of research for better understanding of virus-host interaction by investigating the interplay between T-Ag and Bag3, and their impact on the development of JCV-associated diseases.     

Autophagy Contributes to Leaf Starch Degradation

Transitory starch, a major photosynthetic product in the leaves of land plants, accumulates in chloroplasts during the day and is hydrolyzed to maltose and Glc at night to support respiration and metabolism. Previous studies in Arabidopsis thaliana indicated that the degradation of transitory starch only occurs in the chloroplasts. Here, we report that autophagy, a nonplastidial process, participates in leaf starch degradation. Excessive starch accumulation was observed in Nicotiana benthamiana seedlings treated with an autophagy inhibitor and in autophagy-related (ATG) gene-silenced N. benthamiana and in Arabidopsis atg mutants. Autophagic activity in the leaves responded to the dynamic starch contents during the night. Microscopy showed that a type of small starch granule-like structure (SSGL) was localized outside the chloroplast and was sequestered by autophagic bodies. Moreover, an increased number of SSGLs was observed during starch depletion, and disruption of autophagy reduced the number of vacuole-localized SSGLs. These data suggest that autophagy contributes to transitory starch degradation by sequestering SSGLs to the vacuole for their subsequent breakdown.     

PrP Expression Level and Sensitivity to Prion Infection

Mice overexpressing the prion protein (PrP) sequence from various host species are widely used for measuring infectious titers in prion disease. However, the impact that the transgene expression level might have on the susceptibility to infection raises some concerns about the final biological relevance of these models. Here we report that endpoint titration of a sheep scrapie isolate in sheep and in mice overexpressing the ovine PrP results in similar estimates of the infectious titer.     

Rictor Undergoes Glycogen Synthase Kinase 3 (GSK3)-dependent,FBXW7-mediated Ubiquitination and Proteasomal Degradation

Rictor, an essential component of mTOR complex 2 (mTORC2), plays a pivotal role in regulating mTOR signaling and other biological functions. Posttranslational regulation of rictor (e.g. via degradation) and its underlying mechanism are largely undefined and thus are the focus of this study. Chemical inhibition of the proteasome increased rictor ubiquitination and levels. Consistently, inhibition of FBXW7 with various genetic means including knockdown, knock-out, and enforced expression of a dominant-negative mutant inhibited rictor ubiquitination and increased rictor levels, whereas enforced expression of FBXW7 decreased rictor stability and levels. Moreover, we detected an interaction between FBXW7 and rictor. Hence, rictor is degraded through an FBXW7-mediated ubiquitination/proteasome mechanism. We show that this process is dependent on glycogen synthase kinase 3 (GSK3): GSK3 was associated with rictor and directly phosphorylated the Thr-1695 site in a putative CDC4 phospho-degron motif of rictor; mutation of this site impaired the interaction between rictor and FBXW7, decreased rictor ubiquitination, and increased rictor stability. Finally, enforced activation of Akt enhanced rictor levels and increased mTORC2 activity as evidenced by increased formation of mTORC2 and elevated phosphorylation of Akt, SGK1, and PKCα. Hence we suggest that PI3K/Akt signaling may positively regulate mTORC2 signaling, likely through suppressing GSK3-dependent rictor degradation.     

塞内卡病毒通过激活自噬降解FBXW7拮抗宿主的抗病毒应答

【背景】塞内卡病毒(seneca valley virus, SVV)可引起猪严重的水疱性疾病,但目前仍无商业化疫苗,解析其与宿主抗病毒分子的相互作用具有重要意义。【目的】探究E3泛素连接酶(F-box and WD repeat domain containing 7, FBXW7)在抗SVV感染中的作用及其机制。【方法】利用小干扰技术敲低PK-15细胞FBXW7,结合RT-qPCR、组织半数感染量(tissue culture infective dose 50%,TCID50)和Western blotting分析FBXW7对SVV在PK-15细胞中复制及细胞因子的影响。【结果】Western blotting结果显示,SVV感染能够抑制PK-15细胞中FBXW7蛋白表达水平;敲低FBXW7后促进SVV复制,同时炎性细胞因子IL-6、IFN-β、TNF-αmRNA水平均显著降低;进一步研究显示,敲低ATG5抑制自噬后抑制FBXW7蛋白降解,SVV复制水平下降。【结论】本研究发现FBXW7对SVV复制具有抑制作用,SVV通过激活自噬介导FBXW7降解,为研发抗病毒靶点提供了新策略。     

Dysregulation of Autophagy in Murine Fibroblasts Resistant to HSV-1 Infection

The mouse L cell mutant, gro29, was selected for its ability to survive infection by herpes simplex virus type 1 (HSV-1). gro29 cells are fully susceptible to HSV-1 infection, however, they produce 2000-fold less infectious virus than parental L cells despite their capacity to synthesize late viral gene products and assemble virions. Because productive HSV-1 infection is presumed to result in the death of the host cell, we questioned how gro29 cells might survive infection. Using time-lapse video microscopy, we demonstrated that a fraction of infected gro29 cells survived infection and divided. Electron microscopy of infected gro29 cells, revealed large membranous vesicles that contained virions as well as cytoplasmic constituents. These structures were reminiscent of autophagosomes. Autophagy is an ancient cellular process that, under nutrient deprivation conditions, results in the degradation and catabolism of cytoplasmic components and organelles. We hypothesized that enhanced autophagy, and resultant degradation of virions, might explain the ability of gro29 to survive HSV-1 infection. Here we demonstrate that gro29 cells have enhanced basal autophagy as compared to parental L cells. Moreover, treatment of gro29 cells with 3-methyladenine, an inhibitor of autophagy, failed to prevent the formation of autophagosome-like organelles in gro29 cells indicating that autophagy was dysregulated in these cells. Additionally, we observed robust co-localization of the virion structural component, VP26, with the autophagosomal marker, GFP-LC3, in infected gro29 cells that was not seen in infected parental L cells. Collectively, these data support a model whereby gro29 cells prevent the release of infectious virus by directing intracellular virions to an autophagosome-like compartment. Importantly, induction of autophagy in parental L cells did not prevent HSV-1 production, indicating that the relationship between autophagy, virus replication, and survival of HSV-1 infection by gro29 cells is complex.     

Residues Surrounding the Glycosylphosphatidylinositol Anchor Attachment Site of PrP Modulate Prion Infection: Insight from the Resistance of Rabbits to Prion Disease

Prion diseases are a group of transmissible, invariably fatal neurodegenerative diseases that affect both humans and animals. According to the protein-only hypothesis, the infectious agent is a prion (proteinaceous infectious particle) that is composed primarily of PrP^Sc, the disease-associated isoform of the cellular prion protein, PrP. PrP^Sc arises from the conformational change of the normal, glycosylphosphatidylinositol (GPI)-anchored protein, PrP^C. The mechanism by which this process occurs, however, remains enigmatic. Rabbits are one of a small number of mammalian species reported to be resistant to prion infection. Sequence analysis of rabbit PrP revealed that its C-terminal amino acids differ from those of PrP from other mammals and may affect the anchoring of rabbit PrP through its GPI anchor. Using a cell culture model, this study investigated the effect of the rabbit PrP-specific C-terminal amino acids on the addition of the GPI anchor to PrP^C, PrP^C localization, and PrP^Sc formation. The incorporation of rabbit-specific C-terminal PrP residues into mouse PrP did not affect the addition of a GPI anchor or the localization of PrP. However, these residues did inhibit PrP^Sc formation, suggesting that these rabbit-specific residues interfere with a C-terminal PrP^Sc interaction site.Prion diseases, traditionally known as transmissible spongiform encephalopathies (TSE), are a group of invariably fatal neurodegenerative diseases that affect both humans and animals. According to the protein-only hypothesis, an abnormal isoform of the host-encoded prion protein (PrP^C), referred to as PrP^Sc, is the sole or major component of the infectious agent causing these diseases (33). These disorders affect a wide range of mammals and include diseases such as Creutzfeldt-Jakob disease (CJD), variant CJD, Gerstmann-Straüssler-Scheinker (GSS) syndrome, kuru, and fatal familial insomnia (FFI) in humans, scrapie in sheep and goats, chronic wasting disease (CWD) in cervids, and bovine spongiform encephalopathy (BSE) in cattle. The term “prion” was first used to describe the unique infectious agent and was derived from “proteinaceous infectious particle” to distinguish it from conventional pathogens such as bacteria and viruses (33).To date, rabbits are one of the few mammalian species reported to be resistant to prion infection. Rabbits do not develop clinical disease after inoculation with brain tissue from individuals affected by the human prion diseases CJD and kuru, or by a number of animal forms of the disease, including scrapie and transmissible mink encephalopathy (TME) (12). In addition, mouse neuroblastoma (MNB) cells overexpressing rabbit PrP are also resistant to prion infection (45). Evidence that rabbit cells per se have the correct cellular machinery to support prion propagation has come from studies using the rabbit kidney epithelial cell line RK13. Upon transfection with appropriate PrP-expressing transgenes, these cells are a highly efficient and robust model of prion infection (6, 25, 41, 43). RK13 cells do not have detectable levels of endogenous rabbit PrP^C and are therefore ideal for studying exogenous PrP^C and the propagation of prions from different species (6). Originally, it was shown that RK13 cells overexpressing ovine PrP became susceptible to infection with scrapie (43), and more recently, RK13 cells expressing rodent PrP^C, from either the mouse or the bank vole, were readily infected by prions adapted to and propagated in these two species (6, 41). RK13 cells expressing human PrP^C, however, were resistant to infection with human prions derived directly from a patient with sporadic CJD (25). Since RK13 cells overexpressing PrP are a well-established model of prion propagation, we can therefore conclude that while these cells apparently have the appropriate cellular machinery to support prion propagation, it may be a characteristic of the rabbit prion protein itself that results in the resistance of this species to prion infection. However, the loss of a cellular cofactor may also be a contributing factor.Analysis of the rabbit PrP amino acid sequence shows that it has all the features previously described for members of the PrP protein family, including an N-terminal signal peptide, an octapeptide repeat region, and a C-terminal signal sequence (26). While amino acid sequence comparison of both mouse and rabbit PrP species reveals 87% sequence homology, there are 22 amino acid differences between the two, and several of these reside in regions of PrP known to be important in PrP^Sc formation. In scrapie-infected MNB cells, the residues Gly99 and Met108 within the N terminus, Ser173 within the central region, and Ile214 within the C terminus of rabbit PrP were shown to inhibit PrP^Sc generation when incorporated into mouse PrP, suggesting that multiple amino acid residues in rabbit PrP inhibit PrP^Sc formation (45). Approximately one-third (9/33 residues in the immature sequence) of the amino acid difference between mouse and rabbit PrPs was shown to occur at the glycosylphosphatidylinositol (GPI) anchor attachment site (see Fig. S1 in the supplemental material). As yet, studies involving this region of rabbit PrP have not been performed. Therefore, this region of rabbit PrP may provide further insight into the resistance of rabbits to prion infection.GPI anchor addition occurs via a transamination reaction in the endoplasmic reticulum (ER) following cleavage of the C-terminal signal sequence (39). There is no consensus sequence with which to identify the C-terminal cleavage site, but there are three key C-terminal elements: (i) the cleavage site, or ω site, where the GPI anchor attaches to the COOH group of the ω amino acid; (ii) a hydrophilic spacer region of 8 to 12 amino acids (ω + 1 up to ω + 10); and (iii) a hydrophobic region of 10 to 20 amino acids (ω + 11 onwards) (9). Analysis of known GPI-anchored proteins has given rise to sequence motifs in the C-terminal signal peptide allowing the prediction of the ω site of proteins. Due to the complexity of experimentally determining the ω site of GPI-anchored proteins, relatively few of the many known GPI-anchored proteins have had their ω sites determined (36 of 340 proteins in 2008) (32) The ω site of hamster PrP was determined experimentally to be at amino acid 231 (34) and is predicted to be at the same site for PrPs from all mammals, based on amino acid sequence comparison. Amino acid substitutions near the ω site of mouse PrP revealed that mouse PrP has an ω site at residue 230 (17). It was also shown that single amino acid substitutions at and near the ω site of mouse PrP affect the anchoring and conversion efficiency of PrP (17). It is therefore possible that the amino acids at the C terminus and within the GPI anchor signal sequence of rabbit PrP lead to the resistance to prion infection.To date, no protein structures containing a GPI anchor have been determined by X-ray crystallography, and although the nuclear magnetic resonance (NMR) structures of mouse and rabbit PrP have been solved, they do not contain any structural information for the residues immediately preceding the GPI anchor. We therefore created a mutant mouse PrP model containing rabbit PrP-specific amino acids at the ω site to investigate whether these residues are involved in rabbit resistance to prion infection. Here we demonstrate that the GPI anchor attachment site is an important site that controls the ability of PrP to be converted into PrP^Sc and that residues ω and ω + 1 of PrP are important modulators of this pathogenic process.     

Loss of FBXW7 is related to the susceptibility and poor prognosis of cervical squamous carcinoma

Objective: To investigate the relationship between F box/WD-40 domain protein 7 (FBXW7) and cervical squamous cancer.

Methods: We investigated the FBXW7 expression in 136 cervical squamous carcinoma cases through immunohistochemistry and Western-blot analysis to evaluate the clinical significance of FBXW7 and to elucidate the relationship of FBXW7 expression with progression-free survival (PFS) and overall survival (OS).

Results: Low FBXW7 expression was associated with high histologic grade, lymphovascular space invasion and lymph node metastasis, among other parameters. Patients with low FBXW7 expression exhibited poor OS and PFS.

Conclusions: FBXW7 is related to the susceptibility and prognosis of cervical squamous carcinoma, indicating FBXW7 may be a potentially important target for the prediction of prognosis.     

Listeriolysin O Is Necessary and Sufficient to Induce Autophagy during Listeria monocytogenes Infection

Background

Recent studies have suggested that autophagy is utilized by cells as a protective mechanism against Listeria monocytogenes infection.

Methodology/Principal Findings

However we find autophagy has no measurable role in vacuolar escape and intracellular growth in primary cultured bone marrow derived macrophages (BMDMs) deficient for autophagy (atg5^−/−). Nevertheless, we provide evidence that the pore forming activity of the cholesterol-dependent cytolysin listeriolysin O (LLO) can induce autophagy subsequent to infection by L. monocytogenes. Infection of BMDMs with L. monocytogenes induced microtubule-associated protein light chain 3 (LC3) lipidation, consistent with autophagy activation, whereas a mutant lacking LLO did not. Infection of BMDMs that express LC3-GFP demonstrated that wild-type L. monocytogenes was encapsulated by LC3-GFP, consistent with autophagy activation, whereas a mutant lacking LLO was not. Bacillus subtilis expressing either LLO or a related cytolysin, perfringolysin O (PFO), induced LC3 colocalization and LC3 lipidation. Further, LLO-containing liposomes also recruited LC3-GFP, indicating that LLO was sufficient to induce targeted autophagy in the absence of infection. The role of autophagy had variable effects depending on the cell type assayed. In atg5^−/− mouse embryonic fibroblasts, L. monocytogenes had a primary vacuole escape defect. However, the bacteria escaped and grew normally in atg5^−/− BMDMs.

Conclusions/Significance

We propose that membrane damage, such as that caused by LLO, triggers bacterial-targeted autophagy, although autophagy does not affect the fate of wild-type intracellular L. monocytogenes in primary BMDMs.     

The Role of Autophagy in Chloroplast Degradation and Chlorophagy in Immune Defenses during Pst DC3000 (AvrRps4) Infection

Background

Chlorosis of leaf tissue normally observed during pathogen infection may result from the degradation of chloroplasts. There is a growing evidence to suggest that the chloroplast plays a significant role during pathogen infection. Although most degradation of the organelles and cellular structures in plants is mediated by autophagy, its role in chloroplast catabolism during pathogen infection is largely unknown.

Results

In this study, we investigated the function of autophagy in chloroplast degradation during avirulent Pst DC3000 (AvrRps4) infection. We examined the expression of defensive marker genes and suppression of bacterial growth using the electrolyte leakage assay in normal light (N) and low light (L) growing environments of wild-type and atg5-1 plants during pathogen treatment. Stroma-targeted GFP proteins (CT-GFP) were observed with LysoTracker Red (LTR) staining of autophagosome-like structures in the vacuole. The results showed that Arabidopsis expressed a significant number of small GFP-labeled bodies when infected with avirulent Pst DC3000 (AvrRps4). While barely detectable, there were small GFP-labeled bodies in plants with the CT-GFP expressing atg5-1 mutation. The results showed that chloroplast degradation depends on autophagy and this may play an important role in inhibiting pathogen growth.

Conclusion

Autophagy plays a role in chloroplast degradation in Arabidopsis during avirulent Pst DC3000 (AvrRps4) infection. Autophagy dependent chloroplast degradation may be the primary source of reactive oxygen species (ROS) as well as the pathogen-response signaling molecules that induce the defense response.     

miR-17-5p靶向自噬相关蛋白ATG7调控巨噬细胞抗结核分枝杆菌感染作用的研究

目的:通过研究miR-17-5p对自噬相关基因ATG7的靶向调控机制和对细胞自噬的作用,探究miR-17-5p在结核分枝杆菌介导的自噬途径中的作用及其机制。方法:生物信息学分析得到miR17-5p的靶基因ATG7,通过成功构建载体ATG7野生型(p Mir GLO-ATG7-3'UTR-WT)和突变型,利用双萤光素酶报告系统、Western blot验证miR-17-5p和ATG7的靶向关系,同时构建结核分枝杆菌(H37Ra)感染的人源性THP-1巨噬细胞模型,将做不同处理的细胞分为三组:miR-17-5p mimics、miR-17-5p inhibitor、miR-17-5p nc。通过实时荧光定量PCR(quantitative real-time PCR,qRT-PCR)检测H37Ra感染对miR-17-5p表达量的影响,并且进一步通过Western blot、免疫荧光观察检测LC3蛋白的表达量和自噬小体的数量。结果:MTB感染能够引起miR-17-5p的下调,随着感染复数的增加有明显的降低。而生物信息学预测结果显示miR-17-5p与ATG7具有靶向性,双萤光素酶报告实验、Western blot验证miR-17-5p能够和ATG7靶向结合,并对其进行负调控。进一步通过Western blot、免疫荧光观察发现miR-17-5p mimics组LC3Ⅱ的表达下调,自噬小体表达降低,而miR-17-5p inhibitor组相反。其中对H37Ra感染组与未感染组之间比较,ATG7和LC3Ⅱ蛋白表达明显增强。结论:miR-17-5p直接靶向结合ATG7 3'UTR抑制自噬,在巨噬细胞抗MTB过程中发挥作用。     

Effect of Persistent Fibroma Virus Infection on Susceptibility of Cells to Other Viruses

Shope fibroma virus establishes a persistent cytoplasmic infection in primary (RK) and serially cultivated (DRK(3)) rabbit kidney cells which is accompanied by a morphological alteration of the cells. The response of such cells to superinfection by other viruses was compared with that of control cells by determining plaque production and virus yield of superinfecting viruses. It was found that the growth of other poxviruses, myxoma and vaccinia, was greatly inhibited in the fibroma virus-infected cells, but that of pseudorabies and herpes simplex viruses, which are unrelated deoxyribonucleic acid viruses, was virtually unaffected. The ribonucleic acid (RNA) viruses, poliovirus 1 and coxsackievirus B1, did not produce plaques on either RK or fibroma virus-infected (F-RK) monolayers. However, the growth of several other RNA viruses, vesicular stomatitis virus, encephalomyocarditis virus, Sindbis virus, and Newcastle disease virus, was enhanced in F-RK cells. None of these latter RNA viruses produced any infectious progeny in DRK(3) cells, but they all plaqued on and produced good yields in DRK(3) cells persistently infected with fibroma virus. This phenomenon is termed facilitation. Facilitation results from the infection of DRK(3) cells by fibroma virus. Neither interference nor facilitation were due to changes in the adsorption or eclipse of the superinfecting virus.     

Protein Kinase R Degradation Is Essential for Rift Valley Fever Virus Infection and Is Regulated by SKP1-CUL1-F-box (SCF)FBXW11-NSs E3 Ligase

Activated protein kinase R (PKR) plays a vital role in antiviral defense primarily by inhibiting protein synthesis and augmenting interferon responses. Many viral proteins have adopted unique strategies to counteract the deleterious effects of PKR. The NSs (Non-structural s) protein which is encoded by Rift Valley fever virus (RVFV) promotes early PKR proteasomal degradation through a previously undefined mechanism. In this study, we demonstrate that NSs carries out this activity by assembling the SCF (SKP1-CUL1-F-box)^FBXW11 E3 ligase. NSs binds to the F-box protein, FBXW11, via the six amino acid sequence DDGFVE called the degron sequence and recruits PKR through an alternate binding site to the SCF^FBXW11 E3 ligase. We further show that disrupting the assembly of the SCF^FBXW11-NSs E3 ligase with MLN4924 (a small molecule inhibitor of SCF E3 ligase activity) or NSs degron viral mutants or siRNA knockdown of FBXW11 can block PKR degradation. Surprisingly, under these conditions when PKR degradation was blocked, NSs was essential and sufficient to activate PKR causing potent inhibition of RVFV infection by suppressing viral protein synthesis. These antiviral effects were antagonized by the loss of PKR expression or with a NSs deleted mutant virus. Therefore, early PKR activation by disassembly of SCF^FBXW11-NSs E3 ligase is sufficient to inhibit RVFV infection. Furthermore, FBXW11 and BTRC are the two homologues of the βTrCP (Beta-transducin repeat containing protein) gene that were previously described to be functionally redundant. However, in RVFV infection, among the two homologues of βTrCP, FBXW11 plays a dominant role in PKR degradation and is the limiting factor in the assembly of the SCF^FBXW11 complex. Thus, FBXW11 serves as a master regulator of RVFV infection by promoting PKR degradation. Overall these findings provide new insights into NSs regulation of PKR activity and offer potential opportunities for therapeutic intervention of RVFV infection.     

Degradation of Escherichia coli B Deoxyribonucleic Acid After Infection with Deoxyribonucleic Acid-Defective Amber Mutants of Bacteriophage T7

The degradation of bacterial deoxyribonucleic acid (DNA) was studied after infection of Escherichia coli B with DNA-negative amber mutants of bacteriophage T7. Degradation occurred in three stages. (i) Release of the DNA from a rapidly sedimenting cellular structure occurred between 5 and 6 min after infection. (ii) The DNA was cleaved endonucleolytically to fragments having a molecular weight of about 2 x 10(6) between 6 and 10 min after infection. (iii) These fragments of DNA were reduced to acid-soluble products between 7.5 and 15 min after infection. Stage 1 did not occur in the absence of the gene 1 product (ribonucleic acid polymerase sigma factor), stage 2 did not occur in the absence of the gene 3 product (phage T7-induced endonuclease), and stage 3 did not occur in the absence of the gene 6 product.