Brefeldin A inhibits pestivirus release from infected cells, without affecting its assembly and infectivity

Delayed by Design: Role of Suboptimal Signal Peptidase Processing of Viral Structural Protein Precursors in Flaviviridae Virus Assembly

The Flaviviridae virus family is classified into four different genera, including flavivirus, hepacivirus, pegivirus, and pestivirus, which cause significant morbidity and mortality in humans and other mammals, including ruminants and pigs. These are enveloped, single-stranded RNA viruses sharing a similar genome organization and replication scheme with certain unique features that differentiate them. All viruses in this family express a single polyprotein that encodes structural and nonstructural proteins at the N- and C-terminal regions, respectively. In general, the host signal peptidase cleaves the structural protein junction sites, while virus-encoded proteases process the nonstructural polyprotein region. It is known that signal peptidase processing is a rapid, co-translational event. Interestingly, certain signal peptidase processing site(s) in different Flaviviridae viral structural protein precursors display suboptimal cleavage kinetics. This review focuses on the recent progress regarding the Flaviviridae virus genus-specific mechanisms to downregulate signal peptidase-mediated processing at particular viral polyprotein junction sites and the role of delayed processing at these sites in infectious virus particle assembly.

Rab1b-GBF1-ARFs mediated intracellular trafficking is required for classical swine fever virus replication in swine umbilical vein endothelial cells

Classical swine fever virus (CSFV), a plus-sense RNA virus, utilizes host intracellular membrane organelles for its replication. Our previous studies have shown that disruption of the intracellular membrane-trafficking events can inhibit CSFV replication. However, the underlying mechanism of this process in CSFV infection has not been elucidated. To determine the role of Golgi-associated anterograde and retrograde trafficking in CSFV replication, we revealed the effect of vesicular transport between Golgi and ER inhibitors Brefeldin A (BFA) and 2,2-methyl-N-(2,4,6,-trimethoxyphenyl) dodecanamide (CI-976), the GBF1 inhibitor golgicide A (GCA) on virus production. Our results showed that disruption of vesicular trafficking by BFA, CI-976, and GCA significantly inhibited CSFV infection. Subsequent experiments revealed that knockdown of Rab1b by lentiviruses and negative-mutant Rab1b-N121I transfection inhibited CSFV infection. Furthermore, we showed that the Rab1b downstream vesicular component effectors GBF1, and class I and class II ADP-ribosylation factors (ARFs) were also involved in virus replication. In addition, confocal microscopy assay showed that CSFV infection disrupted the Golgi apparatus resulting in extended Golgi distribution around the nucleus. We also showed that cell secretory pathway, measured using Gaussia luciferase flash assay, was blocked in CSFV infected cells. Taken together, these findings demonstrate that CSFV utilizes Rab1b-GBF1-ARFs mediated trafficking to promote its own replication. These findings also provide new insights into the intracellular trafficking pathways utilized for CSFV life cycle.

Mx Is Not Responsible for the Antiviral Activity of Interferon-α against Japanese Encephalitis Virus

Mx proteins are interferon (IFN)-induced dynamin-like GTPases that are present in all vertebrates and inhibit the replication of myriad viruses. However, the role Mx proteins play in IFN-mediated suppression of Japanese encephalitis virus (JEV) infection is unknown. In this study, we set out to investigate the effects of Mx1 and Mx2 expression on the interferon-α (IFNα) restriction of JEV replication. To evaluate whether the inhibitory activity of IFNα on JEV is dependent on Mx1 or Mx2, we knocked down Mx1 or Mx2 with siRNA in IFNα-treated PK-15 cells and BHK-21 cells, then challenged them with JEV; the production of progeny virus was assessed by plaque assay, RT-qPCR, and Western blotting. Our results demonstrated that depletion of Mx1 or Mx2 did not affect JEV restriction imposed by IFNα, although these two proteins were knocked down 66% and 79%, respectively. Accordingly, expression of exogenous Mx1 or Mx2 did not change the inhibitory activity of IFNα to JEV. In addition, even though virus-induced membranes were damaged by Brefeldin A (BFA), overexpressing porcine Mx1 or Mx2 did not inhibit JEV proliferation. We found that BFA inhibited JEV replication, not maturation, suggesting that BFA could be developed into a novel antiviral reagent. Collectively, our findings demonstrate that IFNα inhibits JEV infection by Mx-independent pathways.

Morphology and Molecular Composition of Purified Bovine Viral Diarrhea Virus Envelope

The family Flaviviridae includes viruses that have different virion structures and morphogenesis mechanisms. Most cellular and molecular studies have been so far performed with viruses of the Hepacivirus and Flavivirus genera. Here, we studied bovine viral diarrhea virus (BVDV), a member of the Pestivirus genus. We set up a method to purify BVDV virions and analyzed their morphology by electron microscopy and their protein and lipid composition by mass spectrometry. Cryo-electron microscopy showed near spherical viral particles displaying an electron-dense capsid surrounded by a phospholipid bilayer with no visible spikes. Most particles had a diameter of 50 nm and about 2% were larger with a diameter of up to 65 nm, suggesting some size flexibility during BVDV morphogenesis. Morphological and biochemical data suggested a low envelope glycoprotein content of BVDV particles, E1 and E2 being apparently less abundant than E^rns. Lipid content of BVDV particles displayed a ~2.3 to 3.5-fold enrichment in cholesterol, sphingomyelin and hexosyl-ceramide, concomitant with a 1.5 to 5-fold reduction of all glycerophospholipid classes, as compared to lipid content of MDBK cells. Although BVDV buds in the endoplasmic reticulum, its lipid content differs from a typical endoplasmic reticulum membrane composition. This suggests that BVDV morphogenesis includes a mechanism of lipid sorting. Functional analyses confirmed the importance of cholesterol and sphingomyelin for BVDV entry. Surprisingly, despite a high cholesterol and sphingolipid content of BVDV envelope, E2 was not found in detergent-resistant membranes. Our results indicate that there are differences between the structure and molecular composition of viral particles of Flaviviruses, Pestiviruses and Hepaciviruses within the Flaviviridae family.

Bovine viral diarrhea virus (BVDV) is the etiologic agent of mucosal disease and bovine viral diarrhea, two economically important diseases of the livestock. BVDV is a member of the Pestivirus genus in the Flaviviridae family, which also includes Hepacivirus and Flavivirus genera. Members of this family share similar genome organization and replication strategies, but differ about their mode of transmission and particle structure. Whereas most studies have been so far performed on viruses of the Hepacivirus and Flavivirus genera, little is known about infectious particles of pestiviruses. In this study, we set up a novel purification method of BVDV infectious particles and analyzed their morphology by cryo-electron microscopy and their molecular composition by mass spectrometry. Our results provide new insights into the structure and biochemical composition of a pestivirus infectious particle, and have implications for research on molecular mechanisms of their morphogenesis and entry.

Exosomes from HIV-1-infected Cells Stimulate Production of Pro-inflammatory Cytokines through Trans-activating Response (TAR) RNA

HIV-1 infection results in a chronic illness because long-term highly active antiretroviral therapy can lower viral titers to an undetectable level. However, discontinuation of therapy rapidly increases virus burden. Moreover, patients under highly active antiretroviral therapy frequently develop various metabolic disorders, neurocognitive abnormalities, and cardiovascular diseases. We have previously shown that exosomes containing trans-activating response (TAR) element RNA enhance susceptibility of undifferentiated naive cells to HIV-1 infection. This study indicates that exosomes from HIV-1-infected primary cells are highly abundant with TAR RNA as detected by RT-real time PCR. Interestingly, up to a million copies of TAR RNA/μl were also detected in the serum from HIV-1-infected humanized mice suggesting that TAR RNA may be stable in vivo. Incubation of exosomes from HIV-1-infected cells with primary macrophages resulted in a dramatic increase of proinflammatory cytokines, IL-6 and TNF-β, indicating that exosomes containing TAR RNA could play a direct role in control of cytokine gene expression. The intact TAR molecule was able to bind to PKR and TLR3 effectively, whereas the 5' and 3' stems (TAR microRNAs) bound best to TLR7 and -8 and none to PKR. Binding of TAR to PKR did not result in its phosphorylation, and therefore, TAR may be a dominant negative decoy molecule in cells. The TLR binding through either TAR RNA or TAR microRNA potentially can activate the NF-κB pathway and regulate cytokine expression. Collectively, these results imply that exosomes containing TAR RNA could directly affect the proinflammatory cytokine gene expression and may explain a possible mechanism of inflammation observed in HIV-1-infected patients under cART.

The Molecular Biology of Pestiviruses

Pestiviruses are among the economically most important pathogens of livestock. The biology of these viruses is characterized by unique and interesting features that are both crucial for their success as pathogens and challenging from a scientific point of view. Elucidation of these features at the molecular level has made striking progress during recent years. The analyses revealed that major aspects of pestivirus biology show significant similarity to the biology of human hepatitis C virus (HCV). The detailed molecular analyses conducted for pestiviruses and HCV supported and complemented each other during the last three decades resulting in elucidation of the functions of viral proteins and RNA elements in replication and virus-host interaction. For pestiviruses, the analyses also helped to shed light on the molecular basis of persistent infection, a special strategy these viruses have evolved to be maintained within their host population. The results of these investigations are summarized in this chapter.

Human T-lymphotropic virus type 1-infected cells secrete exosomes that contain Tax protein

Human T-lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. The HTLV-1 transactivator protein Tax controls many critical cellular pathways, including host cell DNA damage response mechanisms, cell cycle progression, and apoptosis. Extracellular vesicles called exosomes play critical roles during pathogenic viral infections as delivery vehicles for host and viral components, including proteins, mRNA, and microRNA. We hypothesized that exosomes derived from HTLV-1-infected cells contain unique host and viral proteins that may contribute to HTLV-1-induced pathogenesis. We found exosomes derived from infected cells to contain Tax protein and proinflammatory mediators as well as viral mRNA transcripts, including Tax, HBZ, and Env. Furthermore, we observed that exosomes released from HTLV-1-infected Tax-expressing cells contributed to enhanced survival of exosome-recipient cells when treated with Fas antibody. This survival was cFLIP-dependent, with Tax showing induction of NF-κB in exosome-recipient cells. Finally, IL-2-dependent CTLL-2 cells that received Tax-containing exosomes were protected from apoptosis through activation of AKT. Similar experiments with primary cultures showed protection and survival of peripheral blood mononuclear cells even in the absence of phytohemagglutinin/IL-2. Surviving cells contained more phosphorylated Rb, consistent with the role of Tax in regulation of the cell cycle. Collectively, these results suggest that exosomes may play an important role in extracellular delivery of functional HTLV-1 proteins and mRNA to recipient cells.

Correlative scanning-transmission electron microscopy reveals that a chimeric flavivirus is released as individual particles in secretory vesicles

The intracellular morphogenesis of flaviviruses has been well described, but flavivirus release from the host cell remains poorly documented. We took advantage of the optimized production of an attenuated chimeric yellow fever/dengue virus for vaccine purposes to study this phenomenon by microscopic approaches. Scanning electron microscopy (SEM) showed the release of numerous viral particles at the cell surface through a short-lived process. For transmission electron microscopy (TEM) studies of the intracellular ultrastructure of the small number of cells releasing viral particles at a given time, we developed a new correlative microscopy method: CSEMTEM (for correlative scanning electron microscopy - transmission electron microscopy). CSEMTEM analysis suggested that chimeric flavivirus particles were released as individual particles, in small exocytosis vesicles, via a regulated secretory pathway. Our morphological findings provide new insight into interactions between flaviviruses and cells and demonstrate that CSEMTEM is a useful new method, complementary to SEM observations of biological events by intracellular TEM investigations.

Morphogenesis of pestiviruses: new insights from ultrastructural studies of strain Giraffe-1

Knowledge on the morphogenesis of pestiviruses is limited due to low virus production in infected cells. In order to localize virion morphogenesis and replication sites of pestiviruses and to examine intracellular virion transport, a cell culture model was established to facilitate ultrastructural studies. Based on results of virus growth kinetic analysis and quantification of viral RNA, pestivirus strain Giraffe-1 turned out to be a suitable candidate for studies on virion generation and export from culture cells. Using conventional transmission electron microscopy and single-tilt electron tomography, we found virions located predominately in the lumen of the endoplasmic reticulum (ER) in infected cells and were able to depict the budding process of virions at ER membranes. Colocalization of the viral core protein and the envelope glycoprotein E2 with the ER marker protein disulfide isomerase (PDI) was demonstrated by immunogold labeling of cryosections. Moreover, pestivirions could be shown in transport vesicles and the Golgi complex and during exocytosis. Interestingly, viral capsid protein and double-stranded RNA (dsRNA) were detected in multivesicular bodies (MVBs), which implies that the endosomal compartment plays a role in pestiviral replication. Significant cellular membrane alterations such as those described for members of the Flavivirus and Hepacivirus genera were not found. Based on the gained morphological data, we present a consistent model of pestivirus morphogenesis.

Regulation of hepatitis B virus infection by Rab5, Rab7, and the endolysosomal compartment

Despite important progress toward deciphering human hepatitis B virus (HBV) entry into host cells, many aspects of the early steps of the life cycle remained completely obscure. Following endocytosis, HBV must travel through the complex network of the endocytic pathway to reach the cell nucleus and initiate replication. In addition to guiding the viral particles to the replication site, the endosomal vesicles may play a crucial role in infection, providing the appropriate environment for virus uncoating and nucleocapsid release. In this work, we investigated the trafficking of HBV particles internalized in permissive cells. Expression of key Rab proteins, involved in specific pathways leading to different intracellular locations, was modulated in HepaRG cells, using a stable and inducible short hairpin RNA (shRNA) expression system. The trafficking properties of the newly developed cells were demonstrated by confocal microscopy and flow cytometry using specific markers. The results showed that HBV infection strongly depends on Rab5 and Rab7 expression, indicating that HBV transport from early to mature endosomes is required for a step in the viral life cycle. This may involve reduction of disulfide bond-linked envelope proteins, as alteration of the redox potential of the endocytic pathway resulted in inhibition of infection. Subcellular fractionation of HBV-infected cells showed that viral particles are further transported to lysosomes. Intriguingly, infection was not dependent on the lysosomal activity, suggesting that trafficking to this compartment is a "dead-end" route. Together, these data add to our understanding of the HBV-host cell interactions controlling the early stages of infection.

The endoplasmic reticulum protein folding factory and its chaperones: new targets for drug discovery?

Cytosolic heat shock proteins have received significant attention as emerging therapeutic targets. Much of this excitement has been triggered by the discovery that HSP90 plays a central role in the maintenance and stability of multifarious oncogenic membrane receptors and their resultant tyrosine kinase activity. Numerous studies have dealt with the effects of small molecules on chaperone- and stress-related pathways of the endoplasmic reticulum (ER). However, unlike cytosolic chaperones, relatively little emphasis has been placed upon translational avenues towards targeting of the ER for inhibition of folding/secretion of disease-promoting proteins. Here, we summarise existing small molecule inhibitors and potential future targets of ER chaperone-mediated inhibition. Client proteins of translational relevance in disease treatment are outlined, alongside putative future disease treatment modalities based on ER-centric targeted therapies. Particular attention is paid to cancer and autoimmune disorders via the effects of the GRP94 inhibitor geldanamycin and its population of client proteins, overloading of the unfolded protein response, and inhibition of members of the IL-12 family of cytokines by celecoxib and non-coxib analogues.

Molecular characterization of RNA and protein synthesis during a one-step growth curve of bovine viral diarrhoea virus in ovine (SFT-R) cells

The aim of this study was to determine the kinetics of noncytopathic bovine viral diarrhoea virus (BVDV) multiplication and synthesis of BVDV specific RNA and proteins in ovine cells (SFT-R) during a one-step growth curve. The virus titre and RNA level were determined by focus-forming assay and real time RT-PCR. The RNA synthesis was detected by Northern blot while synthesis of E2 and NS3 proteins was assayed by immunohistochemistry and Western blot. The results showed that synthesis of viral RNA is initiated at 4h, NS3 and E2 proteins are detectable at 6-7h and the replication cycle is complete at 10-12h. Additionally, we provide evidence that NS2-3 protein was cleaved in ovine cells early during infection and in proliferated leukocytes of acutely infected sheep. This study showed that synthesis of BVDV RNA and proteins in ovine cells occurs at similar times as found in bovine cells.

Architects of assembly: roles of Flaviviridae non-structural proteins in virion morphogenesis

Viruses of the Flaviviridae family, including hepatitis C, dengue and bovine viral diarrhoea, are responsible for considerable morbidity and mortality worldwide. Recent advances in our understanding of virion assembly have uncovered commonalities among distantly related members of this family. We discuss the emerging hypothesis that physical virion components are not alone in forming the infectious particle, but that non-structural proteins are intimately involved in orchestrating morphogenesis. Pinpointing the roles of Flaviviridae proteins in virion production could reveal new avenues for antiviral therapeutics.

Cellular determinants of hepatitis C virus assembly, maturation, degradation, and secretion

Intracellular infectious hepatitis C virus (HCV) particles display a distinctly higher buoyant density than do secreted virus particles, suggesting that the characteristic low density of extracellular HCV particles is acquired during viral egress. We took advantage of this difference to examine the determinants of assembly, maturation, degradation, and egress of infectious HCV particles. The results demonstrate that HCV assembly and maturation occur in the endoplasmic reticulum (ER) and post-ER compartments, respectively, and that both depend on microsomal transfer protein and apolipoprotein B, in a manner that parallels the formation of very-low-density lipoproteins (VLDL). In addition, they illustrate that only low-density particles are efficiently secreted and that immature particles are actively degraded, in a proteasome-independent manner, in a post-ER compartment of the cell. These results suggest that by coopting the VLDL assembly, maturation, degradation, and secretory machinery of the cell, HCV acquires its hepatocyte tropism and, by mimicry, its tendency to persist.