Salmonella exploits membrane reservoirs for invasion of host cells

Salmonella utilizes a type 3 secretion system to translocate virulence proteins (effectors) into host cells during infection1. The effectors modulate host cell machinery to drive uptake of the bacteria into vacuoles, where they can establish an intracellular replicative niche. A remarkable feature of Salmonella invasion is the formation of actin-rich protuberances (ruffles) on the host cell surface that contribute to bacterial uptake. However, the membrane source for ruffle formation and how these bacteria regulate membrane mobilization within host cells remains unclear. Here, we show that Salmonella exploits membrane reservoirs for the generation of invasion ruffles. The reservoirs are pre-existing tubular compartments associated with the plasma membrane (PM) and are formed through the activity of RAB10 GTPase. Under normal growth conditions, membrane reservoirs contribute to PM homeostasis and are preloaded with the exocyst subunit EXOC2. During Salmonella invasion, the bacterial effectors SipC, SopE2, and SopB recruit exocyst subunits from membrane reservoirs and other cellular compartments, thereby allowing exocyst complex assembly and membrane delivery required for bacterial uptake. Our findings reveal an important role for RAB10 in the establishment of membrane reservoirs and the mechanisms by which Salmonella can exploit these compartments during host cell invasion.

[Roles and mechanisms of m6A modification regulating RP11-426A6.5 in laryngeal squamous cell carcinoma]

Objective: To investigate the roles of N6-methyladenosine (m⁶A) modification in regulating RP11-426A6.5 in the development of laryngeal squamous cell carcinoma (LSCC). Methods: The methylation and expression levels of lncRNAs were identified and important lncRNAs were screened utilizing long non-coding RNA (lncRNA) m⁶A methylation microarray. Cancer and para cancer tissue samples were taken from 48 LSCC patients hospitalized to the Department of Otolaryngology-Head and Neck Surgery of the Second Affiliated Hospital of Harbin Medical University between January and September 2017. Expression profiling microarray was performed in 3 of 48 LSCC samples, and methylated RNA immunoprecipitation-quantitative PCR (MeRIP-qPCR) and quantitative real-time fluorescent PCR (qRT-PCR) were performed in the remaining 45 LSCC samples to verify the m⁶A modification and expression levels of RP11-426A6.5. Correlations between RP11-426A6.5 and clinical factors were anlysed. Laryngeal cancer cell line with low expression of RP11-426A6.5 was created in vitro using RNA interference (RNAi) technology. The 5-Ethynyl-2'-deoxyuridine (EdU) cell proliferation experiment, wound healing experiment, and transwell invasion experiment were used respectively to measure the proliferation, migration, and invasion of LSCC cells. The effect of RP11-426A6.5 down-regulation on the growth of transplanted tumors in vivo was verified by nude mice tumorigenesis assay. The Cancer Genome Atlas (TCGA) database and sequence-based RNA adenosine methylation site predictor (SRAMP) website were used to predict the enzymes and corresponding methylation sites. MazF digestion was chosen to validate the binding sites. RNAi technology was used to observe the changes in cell function after interfering with the expression of the corresponding genes of the modified enzymes. MeRIP-qPCR was used to detect the level of RP11-426A6.5 m⁶A cell line treated with actinomycin D was used to observe the stability of RP11-426A6.5. Results: RP11-426A6.5 methylation and expression levels were significantly higher in LSCC tissues than those in paracancerous tissues (methylation levels: 23.828±4.975 vs 20.280±3.607; expression levels: 1.197±0.314 vs 1.015±0.170, all P values<0.05). RP11-426A6.5 expression levels were closely correlated with T stage (T1-2: 1.081±0.298 vs T3-4: 1.306±0.292, χ²=5.35, P<0.05). The postoperative survival of patients with high RP11-426A6.5 expressions was significantly lower than that of patients with low RP11-426A6.5 expression (P=0.046). Assays in vitro and in vivo showed that the downregulation of RP11-426A6.5 significantly decreased the proliferation, migration, and invasion abilities of LSCC cells and the growth of transplanted tumors. The binding of methyltransferase-like 3 (METTL3), an m⁶A-modified enzyme, to the corresponding methylation site of RP11-426A6.5 enhanced its stability and mediated its regulation of malignant behaviors of LSCC cells. Conclusions: RP11-426A6.5 can regulate the malignant behaviors of LSCC cells, which is mediated by the m⁶A modification process involving in the methyltransferase METTL3.

Study on Lgr5 expression in pancreas tissues and organoids by lineage tracing

Leucine rich repeat containing G protein-coupled receptor 5(Lgr5) is widely expressed in multiple tissues and can be used as a stem cell marker in a variety of epithelial organs (including the small intestine, colon, stomach and hair follicles). In this study, we used Lgr5-Cre^ERT2+/- and Rosa26-mTmG hybridized transgenic mice to investigate the expression of Lgr5 in both ductal epithelial cells during pancreas development and in vitro cultured pancreatic duct organoids. After induction with Tamoxifen, the Lgr5 expression was analyzed by detecting the enhanced green fluorescence protein in the pancreatic tissue sections in adult animals and embryos at different developmental stages. The results showed that Lgr5 expression was detected neither in adult pancreatic duct epithelia nor in the embryonic pancreatic tissues at day 15.5 or in newborn mice. However, when 4-hydroxy-Tamoxifen was supplemented to the culture medium, EGFP could be detected in the primary pancreatic duct organoids from Lgr5-Cre ^ERT2+/-; Rosa26-mTmG mice. These results suggested that Lgr5 was not expressed in adult and embryonic pancreatic tissues; but could be expressed in the cultured pancreas ductal organoids. The research lays the foundation for exploring specific gene expression patterns in stem/progenitor cells during pancreatic development.

C5orf51 is a component of the MON1-CCZ1 complex and controls RAB7A localization and stability during mitophagy

Depolarized mitochondria can be degraded via mitophagy, a selective form of autophagy. The RAB GTPase RAB7A was recently shown to play a key role in this process. RAB7A regulates late endocytic trafficking under normal growth conditions but is translocated to the mitochondrial surface following depolarization. However, how RAB7A activity is regulated during mitophagy is not understood. Here, using a proximity-dependent biotinylation approach (miniTurbo), we identified C5orf51 as a specific interactor of GDP-locked RAB7A. C5orf51 also interacts with the RAB7A guanine nucleotide exchange factor (GEF) complex members MON1 and CCZ1. In the absence of C5orf51, localization of RAB7A on depolarized mitochondria is compromised and the protein is degraded by the proteasome. Furthermore, depletion of C5orf51 also inhibited ATG9A recruitment to depolarized mitochondria. Together, these results indicate that C5orf51 is a positive regulator of RAB7A in its shuttling between late endosomes and mitochondria to enable mitophagy.

Abbreviations: ATG9A: autophagy related 9A; Baf A₁: bafilomycin A₁; BioID: proximity-dependent biotin identification; CCCP: carbonyl cyanide m-chlorophenylhydrazone; CCZ1: CCZ1 homolog, vacuolar protein trafficking and biogenesis associated; DQ-BSA: dye quenched-bovine serum albumin; FYCO1: FYVE and coiled-coil domain autophagy adaptor 1; GAP: GTPase activating protein; GEF: guanine nucleotide exchange factor; KO: knockout; LRPPRC: leucine rich pentatricopeptide repeat containing; MG132: carbobenzoxy-Leu-Leu-leucinal; MON1: MON1 homolog, secretory trafficking associated; mtDNA: mitochondrial DNA; PINK1: PTEN induced kinase 1; PRKN/PARKIN: parkin RBR E3 ubiquitin protein ligase; RMC1: regulator of MON1-CCZ1; TBC1D15: TBC1 domain family member 15; TBC1D17: TBC1 domain family member 17; TOMM20: translocase of outer mitochondrial membrane 20; WDR91: WD repeat domain 91; WT: wild type.

PKACs attenuate innate antiviral response by phosphorylating VISA and priming it for MARCH5-mediated degradation

Sensing of viral RNA by RIG-I-like receptors initiates innate antiviral response, which is mediated by the central adaptor VISA. How the RIG-I-VISA-mediated antiviral response is terminated at the late phase of infection is enigmatic. Here we identified the protein kinase A catalytic (PKAC) subunits α and β as negative regulators of RNA virus-triggered signaling in a redundant manner. Viral infection up-regulated cellular cAMP levels and activated PKACs, which then phosphorylated VISA at T54. This phosphorylation abrogated virus-induced aggregation of VISA and primed it for K48-linked polyubiquitination and degradation by the E3 ligase MARCH5, leading to attenuation of virus-triggered induction of downstream antiviral genes. PKACs-deficiency or inactivation by the inhibitor H89 potentiated innate immunity to RNA viruses in cells and mice. Our findings reveal a critical mechanism of attenuating innate immune response to avoid host damage at the late phase of viral infection by the house-keeping PKA kinase.

VISA is a central adaptor protein required for innate immune response to RNA virus. Phosphorylation of VISA by protein kinase A leads to its polyubiquitination and degradation by the E3 ligase MARCH5 at the late phase of viral infection, which provides a critical control mechanism for the host to avoid excessive and harmful immune response.

The RNA-binding protein Mex3B is a coreceptor of Toll-like receptor 3 in innate antiviral response

Recognition of viral dsRNA by Toll-like receptor 3 (TLR3) leads to induction of interferons (IFNs) and proinflammatory cytokines, and innate antiviral response. Here we identified the RNA-binding protein Mex3B as a positive regulator of TLR3-mediated signaling by expression cloning screens. Cells from Mex3b^−/− mice exhibited reduced production of IFN-β in response to the dsRNA analog poly(I:C) but not infection with RNA viruses. Mex3b^−/− mice injected with poly(I:C) was more resistant to poly(I:C)-induced death. Mex3B was associated with TLR3 in the endosomes. It bound to dsRNA and increased the dsRNA-binding activity of TLR3. Mex3B also promoted the proteolytic processing of TLR3, which is critical for its activation. Mutants of Mex3B lacking its RNA-binding activity inhibited TLR3-mediated IFN-β induction. These findings suggest that Mex3B acts as a coreceptor of TLR3 in innate antiviral response.

Cumulative risk assessment for plasticizer-contaminated food using the hazard index approach

Phthalates strongly and adversely affect reproduction, development and liver function. We did a cumulative risk assessment for simultaneous exposure to nine phthalates using the hazard index (HI) and the levels of nine phthalates in 1200 foodstuff samples. DEHP (di-2-ethylhexyl phthalate) present the highest level (mean: 0.443 mg/kg) in 1200 samples, and the highest average daily dose (ADD) was found in DEHP, ΣDBP(i + n) (the sum of dibutyl phthalate [DBP] isomers [DnBP + DiBP]) posed the highest risk potential of all the phthalates. In seven phthalates, the 95th percentiles of the ADDs for ΣDBP(i + n) in 0-6-yr-old children accounted for 91% (79-107%) of the tolerable daily intake, and the 95th percentiles of the HIs for the anti-androgenic effects of five phthalates in 0-3-yr-old children and 4-6-yr-old girls were >1. We conclude that the health of younger Taiwanese may be adversely affected by overexposure of phthalate-contaminated foods.