Epac1 null mice have nephrogenic diabetes insipidus with deficient corticopapillary osmotic gradient and weaker collecting duct tight junctions.

AIM: The cAMP-mediator Epac1 (RapGef3) has high renal expression. Preliminary observations revealed increased diuresis in Epac1-/- mice. We hypothesized that Epac1 could restrict diuresis by promoting transcellular collecting duct (CD) water and urea transport or by stabilizing CD paracellular junctions to reduce osmolyte loss from the renal papillary interstitium. METHODS: In Epac1-/- and Wt C57BL/6J mice, renal papillae, dissected from snap-frozen kidneys, were assayed for the content of key osmolytes. Cell junctions were analysed by transmission electron microscopy. Urea transport integrity was evaluated by urea loading with 40% protein diet, endogenous vasopressin production was manipulated by intragastric water loading and moderate dehydration and vasopressin type 2 receptors were stimulated selectively by i.p.-injected desmopressin (dDAVP). Glomerular filtration rate (GFR) was estimated as [14 C]inulin clearance. The glomerular filtration barrier was evaluated by urinary albumin excretion and microvascular leakage by the renal content of time-spaced intravenously injected 125 I- and 131 I-labelled albumin. RESULTS: Epac1-/- mice had increased diuresis and increased free water clearance under antidiuretic conditions. They had shorter and less dense CD tight junction (TJs) and attenuated corticomedullary osmotic gradient. Epac1-/- mice had no increased protein diet-induced urea-dependent osmotic diuresis, and expressed Wt levels of aquaporin-2 (AQP-2) and urea transporter A1/3 (UT-A1/3). Epac1-/- mice had no urinary albumin leakage and unaltered renal microvascular albumin extravasation. Their GFR was moderately increased, unless when treated with furosemide. CONCLUSION: Our results conform to the hypothesis that Epac1-dependent mechanisms protect against diabetes insipidus by maintaining renal papillary osmolarity and the integrity of CD TJs.

Inducing fluorescence of uranyl acetate as a dual-purpose contrast agent for correlative light-electron microscopy with nanometre precision.

Correlative light-electron microscopy (CLEM) combines the high spatial resolution of transmission electron microscopy (TEM) with the capability of fluorescence light microscopy (FLM) to locate rare or transient cellular events within a large field of view. CLEM is therefore a powerful technique to study cellular processes. Aligning images derived from both imaging modalities is a prerequisite to correlate the two microscopy data sets, and poor alignment can limit interpretability of the data. Here, we describe how uranyl acetate, a commonly-used contrast agent for TEM, can be induced to fluoresce brightly at cryogenic temperatures (-195 °C) and imaged by cryoFLM using standard filter sets. This dual-purpose contrast agent can be used as a general tool for CLEM, whereby the equivalent staining allows direct correlation between fluorescence and TEM images. We demonstrate the potential of this approach by performing multi-colour CLEM of cells containing equine arteritis virus proteins tagged with either green- or red-fluorescent protein, and achieve high-precision localization of virus-induced intracellular membrane modifications. Using uranyl acetate as a dual-purpose contrast agent, we achieve an image alignment precision of ~30 nm, twice as accurate as when using fiducial beads, which will be essential for combining TEM with the evolving field of super-resolution light microscopy.

Biogenesis and architecture of arterivirus replication organelles.

All eukaryotic positive-stranded RNA (+RNA) viruses appropriate host cell membranes and transform them into replication organelles, specialized micro-environments that are thought to support viral RNA synthesis. Arteriviruses (order Nidovirales) belong to the subset of +RNA viruses that induce double-membrane vesicles (DMVs), similar to the structures induced by e.g. coronaviruses, picornaviruses and hepatitis C virus. In the last years, electron tomography has revealed substantial differences between the structures induced by these different virus groups. Arterivirus-induced DMVs appear to be closed compartments that are continuous with endoplasmic reticulum membranes, thus forming an extensive reticulovesicular network (RVN) of intriguing complexity. This RVN is remarkably similar to that described for the distantly related coronaviruses (also order Nidovirales) and sets them apart from other DMV-inducing viruses analysed to date. We review here the current knowledge and open questions on arterivirus replication organelles and discuss them in the light of the latest studies on other DMV-inducing viruses, particularly coronaviruses. Using the equine arteritis virus (EAV) model system and electron tomography, we present new data regarding the biogenesis of arterivirus-induced DMVs and uncover numerous putative intermediates in DMV formation. We generated cell lines that can be induced to express specific EAV replicase proteins and showed that DMVs induced by the transmembrane proteins nsp2 and nsp3 form an RVN and are comparable in topology and architecture to those formed during viral infection. Co-expression of the third EAV transmembrane protein (nsp5), expressed as part of a self-cleaving polypeptide that mimics viral polyprotein processing in infected cells, led to the formation of DMVs whose size was more homogenous and closer to what is observed upon EAV infection, suggesting a regulatory role for nsp5 in modulating membrane curvature and DMV formation.

Antiviral Innate Immune Response Interferes with the Formation of Replication-Associated Membrane Structures Induced by a Positive-Strand RNA Virus.

Infection with nidoviruses like corona- and arteriviruses induces a reticulovesicular network of interconnected endoplasmic reticulum (ER)-derived double-membrane vesicles (DMVs) and other membrane structures. This network is thought to accommodate the viral replication machinery and protect it from innate immune detection. We hypothesized that the innate immune response has tools to counteract the formation of these virus-induced replication organelles in order to inhibit virus replication. Here we have investigated the effect of type I interferon (IFN) treatment on the formation of arterivirus-induced membrane structures. Our approach involved ectopic expression of arterivirus nonstructural proteins nsp2 and nsp3, which induce DMV formation in the absence of other viral triggers of the interferon response, such as replicating viral RNA. Thus, this setup can be used to identify immune effectors that specifically target the (formation of) virus-induced membrane structures. Using large-scale electron microscopy mosaic maps, we found that IFN-ß treatment significantly reduced the formation of the membrane structures. Strikingly, we also observed abundant stretches of double-membrane sheets (a proposed intermediate of DMV formation) in IFN-ß-treated samples, suggesting the disruption of DMV biogenesis. Three interferon-stimulated gene products, two of which have been reported to target the hepatitis C virus replication structures, were tested for their possible involvement, but none of them affected membrane structure formation. Our study reveals the existence of a previously unknown innate immune mechanism that antagonizes the viral hijacking of host membranes. It also provides a solid basis for further research into the poorly understood interactions between the innate immune system and virus-induced replication structures. IMPORTANCE: Viruses with a positive-strand RNA genome establish a membrane-associated replication organelle by hijacking and remodeling intracellular host membranes, a process deemed essential for their efficient replication. It is unknown whether the cellular innate immune system can detect and/or inhibit the formation of these membrane structures, which could be an effective mechanism to delay viral RNA replication. In this study, using an expression system that closely mimics the formation of arterivirus replication structures, we show for the first time that IFN-ß treatment clearly reduces the amount of induced membrane structures. Moreover, drastic morphological changes were observed among the remaining structures, suggesting that their biogenesis was impaired. Follow-up experiments suggested that host cells contain a hitherto unknown innate antiviral mechanism, which targets this common feature of positive-strand RNA virus replication. Our study provides a strong basis for further research into the interaction of the innate immune system with membranous viral replication organelles.

Optimisations and Challenges Involved in the Creation of Various Bioluminescent and Fluorescent Influenza A Virus Strains for In Vitro and In Vivo Applications.

Bioluminescent and fluorescent influenza A viruses offer new opportunities to study influenza virus replication, tropism and pathogenesis. To date, several influenza A reporter viruses have been described. These strategies typically focused on a single reporter gene (either bioluminescent or fluorescent) in a single virus backbone. However, whilst bioluminescence is suited to in vivo imaging, fluorescent viruses are more appropriate for microscopy. Therefore, the idea l reporter virus varies depending on the experiment in question, and it is important that any reporter virus strategy can be adapted accordingly. Herein, a strategy was developed to create five different reporter viruses in a single virus backbone. Specifically, enhanced green fluorescent protein (eGFP), far-red fluorescent protein (fRFP), near-infrared fluorescent protein (iRFP), Gaussia luciferase (gLUC) and firefly luciferase (fLUC) were inserted into the PA gene segment of A/PR/8/34 (H1N1). This study provides a comprehensive characterisation of the effects of different reporter genes on influenza virus replication and reporter activity. In vivo reporter gene expression, in lung tissues, was only detected for eGFP, fRFP and gLUC expressing viruses. In vitro, the eGFP-expressing virus displayed the best reporter stability and could be used for correlative light electron microscopy (CLEM). This strategy was then used to create eGFP-expressing viruses consisting entirely of pandemic H1N1, highly pathogenic avian influenza (HPAI) H5N1 and H7N9. The HPAI H5N1 eGFP-expressing virus infected mice and reporter gene expression was detected, in lung tissues, in vivo. Thus, this study provides new tools and insights for the creation of bioluminescent and fluorescent influenza A reporter viruses.

Schistosome-derived omega-1 drives Th2 polarization by suppressing protein synthesis following internalization by the mannose receptor.

Omega-1, a glycosylated T2 ribonuclease (RNase) secreted by Schistosoma mansoni eggs and abundantly present in soluble egg antigen, has recently been shown to condition dendritic cells (DCs) to prime Th2 responses. However, the molecular mechanisms underlying this effect remain unknown. We show in this study by site-directed mutagenesis of omega-1 that both the glycosylation and the RNase activity are essential to condition DCs for Th2 polarization. Mechanistically, we demonstrate that omega-1 is bound and internalized via its glycans by the mannose receptor (MR) and subsequently impairs protein synthesis by degrading both ribosomal and messenger RNA. These experiments reveal an unrecognized pathway involving MR and interference with protein synthesis that conditions DCs for Th2 priming.