Subversion of CtBP1-controlled macropinocytosis by human adenovirus serotype 3.

Endocytosis supports cell communication, growth, and pathogen infection. The species B human adenovirus serotype 3 (Ad3) is associated with epidemic conjunctivitis, and fatal respiratory and systemic disease. Here we show that Ad3 uses dynamin-independent endocytosis for rapid infectious entry into epithelial and haematopoietic cells. Unlike Ad5, which uses dynamin-dependent endocytosis, Ad3 endocytosis spatially and temporally coincided with enhanced fluid-phase uptake. It was sensitive to macropinocytosis inhibitors targeting F-actin, protein kinase C, the sodium-proton exchanger, and Rac1 but not Cdc42. Infectious Ad3 macropinocytosis required viral activation of p21-activated kinase 1 (PAK1) and the C-terminal binding protein 1 of E1A (CtBP1), recruited to macropinosomes. These macropinosomes also contained the Ad3 receptors CD46 and alpha v integrins. CtBP1 is a phosphorylation target of PAK1, and is bifunctionally involved in membrane traffic and transcriptional repression of cell cycle, cancer, and innate immunity pathways. Phosphorylation-defective S147A-CtBP1 blocked Ad3 but not Ad5 infection, providing a direct link between PAK1 and CtBP1. The data show that viruses induce macropinocytosis for infectious entry, a pathway used in antigen presentation and cell migration.

Infectious adenovirus type 2 transport through early but not late endosomes.

Receptor-mediated endocytosis is a major gate for pathogens into cells. In this study, we analyzed the trafficking of human adenovirus type 2 and 5 (Ad2/5) and the escape-defective temperature-sensitive Ad2-ts1 mutant in epithelial cancer cells. Ad2/5 and Ad2-ts1 uptake into endosomes containing transferrin, major histocompatibility antigen 1 and the Rab5 effector early endosome antigen 1 (EEA1) involved dynamin, amphiphysin, clathrin and Eps15. Cointernalization experiments showed that most of the Ad2/5 and Ad2-ts1 visited the same EEA1-positive endosomes. In contrast to Ad2/5, Ad2-ts1 required functional Rab5 for endocytosis and lysosomal transport and was sensitive to the phosphatidyl-inositol-3 (PI3)-kinase inhibitor wortmannin or the ubiquitin-binding protein Hrs for sorting from early to late endosomes. Endosomal escape of Ad2 was not affected by incubation at 19 degrees C, which blocked membrane sorting in early endosomes and inhibited Ad2-ts1 transport to lysosomes. Unlike Semliki Forest Virus (SFV), sorting of Ad2-ts1 to late endosomes was independent of Rab7 and Ad2/5 infection independent of EEA1. The data indicate that Ad2/5 and Ad2-ts1 use an invariant machinery for clathrin-mediated uptake to early endosomes. We suggest that the infectious Ad2 particles are either directly released from early endosomes to the cytosol or sorted by a temperature-insensitive and PI3-kinase-independent mechanism to an escape compartment different from late endosomes or lysosomes.

Key role of splenic myeloid DCs in the IFN-alphabeta response to adenoviruses in vivo.

The early systemic production of interferon (IFN)-alphabeta is an essential component of the antiviral host defense mechanisms, but is also thought to contribute to the toxic side effects accompanying gene therapy with adenoviral vectors. Here we investigated the IFN-alphabeta response to human adenoviruses (Ads) in mice. By comparing the responses of normal, myeloid (m)DC- and plasmacytoid (p)DC-depleted mice and by measuring IFN-alphabeta mRNA expression in different organs and cells types, we show that in vivo, Ads elicit strong and rapid IFN-alphabeta production, almost exclusively in splenic mDCs. Using knockout mice, various strains of Ads (wild type, mutant and UV-inactivated) and MAP kinase inhibitors, we demonstrate that the Ad-induced IFN-alphabeta response does not require Toll-like receptors (TLR), known cytosolic sensors of RNA (RIG-I/MDA-5) and DNA (DAI) recognition and interferon regulatory factor (IRF)-3, but is dependent on viral endosomal escape, signaling via the MAP kinase SAPK/JNK and IRF-7. Furthermore, we show that Ads induce IFN-alphabeta and IL-6 in vivo by distinct pathways and confirm that IFN-alphabeta positively regulates the IL-6 response. Finally, by measuring TNF-alpha responses to LPS in Ad-infected wild type and IFN-alphabetaR(-/-) mice, we show that IFN-alphabeta is the key mediator of Ad-induced hypersensitivity to LPS. These findings indicate that, like endosomal TLR signaling in pDCs, TLR-independent virus recognition in splenic mDCs can also produce a robust early IFN-alphabeta response, which is responsible for the bulk of IFN-alphabeta production induced by adenovirus in vivo. The signaling requirements are different from known TLR-dependent or cytosolic IFN-alphabeta induction mechanisms and suggest a novel cytosolic viral induction pathway. The hypersensitivity to components of the microbial flora and invading pathogens may in part explain the toxic side effects of adenoviral gene therapy and contribute to the pathogenesis of adenoviral disease.

Genetic reconstitution of the human adenovirus type 2 temperature-sensitive 1 mutant defective in endosomal escape.

Human adenoviruses infect the upper and lower respiratory tracts, the urinary and digestive tracts, lymphoid systems and heart, and give rise to epidemic conjunctivitis. More than 51 human serotypes have been identified to-date, and classified into 6 species A-F. The species C adenoviruses Ad2 and Ad5 (Ad2/5) cause upper and lower respiratory disease, but how viral structure relates to the selection of particular infectious uptake pathways is not known. An adenovirus mutant, Ad2-ts1 had been isolated upon chemical mutagenesis in the past, and shown to have unprocessed capsid proteins. Ad2-ts1 fails to package the viral protease L3/p23, and Ad2-ts1 virions do not efficiently escape from endosomes. It had been suggested that the C22187T point mutation leading to the substitution of the conserved proline 137 to leucine (P137L) in the L3/p23 protease was at least in part responsible for this phenotype. To clarify if the C22187T mutation is necessary and sufficient for the Ad2-ts1 phenotype, we sequenced the genes encoding the structural proteins of Ad2-ts1, and confirmed that the Ad2-ts1 DNA carries the point mutation C22187T. Introduction of C22187T to the wild-type Ad2 genome in a bacterial artificial chromosome (Ad2-BAC) gave Ad2-BAC46 virions with the full Ad2-ts1 phenotype. Reversion of Ad2-BAC46 gave wild-type Ad2 particles indicating that P137L is necessary and sufficient for the Ad2-ts1 phenotype. The kinetics of Ad2-ts1 uptake into cells were comparable to Ad2 suggesting similar endocytic uptake mechanisms. Surprisingly, infectious Ad2 or Ad5 but not Ad2-ts1 uptake required CALM (clathrin assembly lymphoid myeloid protein), which controls clathrin-mediated endocytosis and membrane transport between endosomes and the trans-Golgi-network. The data show that no other mutations than P137L in the viral protease are necessary to give rise to particles that are defective in capsid processing and endosomal escape. This provides a basis for genetic analyses of distinct host requirements for Ad endocytosis and escape from endosomes.

Cholesterol is required for endocytosis and endosomal escape of adenovirus type 2.

The species C adenovirus type 2 (Ad2) and Ad5 bind the coxsackievirus B Ad receptor and alphav integrin coreceptors and enter epithelial cells by clathrin-mediated endocytosis. This pathway is rapid and efficient. It leads to cell activation and the cholesterol-dependent formation of macropinosomes. Macropinosomes are triggered to release their contents when incoming Ad2 escapes from endosomes. Here, we show that cholesterol extraction of epithelial cells by methyl-beta-cyclodextrin (mbetaCD) treatment reduced Ad5-mediated luciferase expression approximately 4-fold. The addition of cholesterol to normal cells increased gene expression in a dose-dependent manner up to threefold, but it did not restore gene expression in mbetaCD-treated cells. mbetaCD had no effect in the presence of excess cholesterol, indicating that the inhibition of gene expression was due specifically to cholesterol depletion. Cholesterol depletion inhibited rapid Ad2 endocytosis, endosomal escape, and nuclear targeting, consistent with the notion that clathrin-dependent endocytosis of Ad2 is cholesterol dependent. In cholesterol-reduced cells, Ad2 internalized at a low rate, suggestive of an alternative, clathrin-independent, low-capacity entry pathway. While exogenous cholesterol completely restored rapid Ad2 endocytosis, macropinocytosis, and macropinosome disruption, it did not, surprisingly, restore viral escape from endosomes. Our results indicate that macropinosome disruption and endosomal escape of Ad2 are independent events in cells depleted of and then refilled with cholesterol, suggesting that viral escape from endosomes requires lipid-controlled membrane homeostasis, trafficking, or signaling.