A riboside hydrolase that salvages both nucleobases and nicotinamide in the auxotrophic parasite Trichomonas vaginalis.

Pathogenic parasites of the Trichomonas genus are causative agents of sexually transmitted diseases affecting millions of individuals worldwide and whose outcome may include stillbirths and enhanced cancer risks and susceptibility to HIV infection. Trichomonas vaginalis relies on imported purine and pyrimidine nucleosides and nucleobases for survival, since it lacks the enzymatic activities necessary for de novo biosynthesis. Here we show that T. vaginalis additionally lacks homologues of the bacterial or mammalian enzymes required for the synthesis of the nicotinamide ring, a crucial component in the redox cofactors NAD+ and NADP. Moreover, we show that a yet fully uncharacterized T. vaginalis protein homologous to bacterial and protozoan nucleoside hydrolases is active as a pyrimidine nucleosidase but shows the highest specificity toward the NAD+ metabolite nicotinamide riboside. Crystal structures of the trichomonal riboside hydrolase in different states reveals novel intermediates along the nucleoside hydrolase-catalyzed hydrolytic reaction, including an unexpected asymmetry in the homotetrameric assembly. The active site structure explains the broad specificity toward different ribosides and offers precise insights for the engineering of specific inhibitors that may simultaneously target different essential pathways in the parasite.

Combinatorial allosteric modulation of agonist response in a self-interacting G-protein coupled receptor.

The structural plasticity of G-protein coupled receptors (GPCRs) enables the long-range transmission of conformational changes induced by specific orthosteric site ligands and other pleiotropic factors. Here, we demonstrate that the ligand binding cavity in the sphingosine 1-phosphate receptor S1PR1, a class A GPCR, is in allosteric communication with both the ß-arrestin-binding C-terminal tail, and a receptor surface involved in oligomerization. We show that S1PR1 oligomers are required for full response to different agonists and ligand-specific association with arrestins, dictating the downstream signalling kinetics. We reveal that the active form of the immunomodulatory drug fingolimod, FTY720-P, selectively harnesses both these intramolecular networks to efficiently recruit ß-arrestins in a stable interaction with the receptor, promoting deep S1PR1 internalization and simultaneously abrogating ERK1/2 phosphorylation. Our results define a molecular basis for the efficacy of fingolimod for people with multiple sclerosis, and attest that GPCR signalling can be further fine-tuned by the oligomeric state.

The structure of N184K amyloidogenic variant of gelsolin highlights the role of the H-bond network for protein stability and aggregation properties.

Mutations in the gelsolin protein are responsible for a rare conformational disease known as AGel amyloidosis. Four of these mutations are hosted by the second domain of the protein (G2): D187N/Y, G167R and N184K. The impact of the latter has been so far evaluated only by studies on the isolated G2. Here we report the characterization of full-length gelsolin carrying the N184K mutation and compare the findings with those obtained on the wild type and the other variants. The crystallographic structure of the N184K variant in the Ca2+-free conformation shows remarkable similarities with the wild type protein. Only minimal local rearrangements can be observed and the mutant is as efficient as the wild type in severing filamentous actin. However, the thermal stability of the pathological variant is compromised in the Ca2+-free conditions. These data suggest that the N to K substitution causes a local disruption of the H-bond network in the core of the G2 domain. Such a subtle rearrangement of the connections does not lead to significant conformational changes but severely affects the stability of the protein.

RMCE-based insect cell platform to produce membrane proteins captured on HIV-1 Gag virus-like particles.

Conformationally complex membrane proteins (MPs) are therapeutic targets in many diseases, but drug discovery has been slowed down by the lack of efficient production tools. Co-expression of MPs with matrix proteins from enveloped viruses is a promising approach to obtain correctly folded proteins at the surface of virus-like particles (VLPs), preserving their native lipidic environment. Here, we implemented a site-specific recombinase-mediated cassette exchange (RMCE) strategy to establish a reusable HIV-1 Gag-expressing insect cell line for fast production of target MPs on the surface of Gag-VLPs. The Sf9 cell line was initially tagged with a Gag-GFP-expressing cassette incorporating two flipase recognition target sites (FRTs), one within the fusion linker of Gag-GFP. The GFP cassette was afterwards replaced by a Cherry cassette via flipase (Flp) recombination. The fusion of Gag to fluorescent proteins enabled high-throughput screening of cells with higher Gag expression and Flp-mediated cassette exchange ability, while keeping the functionality of the VLP scaffold unaltered. The best cell clone was then Flp-recombinated to produce Gag-VLPs decorated with a human ß2-adrenergic receptor (ß2AR). Release of a fluorescently labeled ß2AR into the culture supernatant was confirmed by immunoblotting, and its co-localization with Gag-VLPs was visualized by confocal microscopy. Furthermore, the differential avidity of ß2AR-dsplaying Gag-VLPs versus "naked" Gag-VLPs to an anti-ß2AR antibody measured by ELISA corroborated the presence of ß2AR at the surface of the Gag-VLPs. In conclusion, this novel insect cell line represents a valuable platform for fast production of MPs in their native conformation, which can accelerate small-molecule and antibody drug discovery programs.

IL-27, but not IL-35, inhibits neuroinflammation through modulating GM-CSF expression.

IL-27 and IL-35 are heterodimeric cytokines, members of the IL-12 family and considered to have immunomodulatory properties. Their role during neuroinflammation had been investigated using mutant mice devoid of either one of their subunits or lacking components of their receptors, yielding conflicting results. We sought to understand the therapeutic potential of IL-27 and IL-35 delivered by gene therapy in neuroinflammation. We constructed lentiviral vectors expressing IL-27 and IL-35 from a single polypeptide chain, and we validated in vitro their biological activity. We injected IL-27 and IL-35-expressing lentiviral vectors into the cerebrospinal fluid (CSF) of mice affected by experimental neuroinflammation (EAE), and performed clinical, neuropathological and immunological analyses. Both cytokines interfere with neuroinflammation, but only IL-27 significantly modulates disease development, both clinically and neuropathologically. IL-27 protects from autoimmune inflammation by inhibiting granulocyte macrophages colony-stimulating factor (GM-CSF) expression in CD4+ T cells and by inducing program death-ligand 1 (PD-L1) expression in both CNS-resident and CNS-infiltrating myeloid cells. We demonstrate here that IL-27 holds therapeutic potential during neuroinflammation and that IL-27 inhibits GM-CSF and induces pd-l1 mRNA in vivo.

Palmitoylation Strengthens Cholesterol-dependent Multimerization and Fusion Activity of Human Cytomegalovirus Glycoprotein B (gB).

Herpesviruses are a large order of animal enveloped viruses displaying a virion fusion mechanism of unusual complexity. Their multipartite machinery has a conserved core made of the gH/gL ancillary complexes and the homo-trimeric fusion protein glycoprotein B (gB). Despite its essential role in starting the viral infection, gB interaction with membrane lipids is still poorly understood. Here, evidence is provided demonstrating that human cytomegalovirus (HCMV) gB depends on the S-palmitoylation of its endodomain for an efficient interaction with cholesterol-rich membrane patches. We found that, unique among herpesviral gB proteins, the HCMV fusion factor has a Cys residue in the C-terminal region that is palmitoylated and mediates methyl-ß-cyclodextrin-sensitive self-association of purified gB. A cholesterol-dependent virus-like particle trap assay, based on co-expression of the HIV Gag protein, confirmed that this post-translational modification is functional in the context of cellular membranes. Mutation of the palmitoylated Cys residue to Ala or inhibition of protein palmitoylation decreased HCMV gB export via Gag particles. Moreover, purified gBC777A showed an increased kinetic sensitivity in a cholesterol depletion test, demonstrating that palmitoyl-gB limits outward cholesterol diffusion. Finally, gB palmitoylation was required for full fusogenic activity in human epithelial cells. Altogether, these results uncover the palmitoylation of HCMV gB and its role in gB multimerization and activity.

Enhanced expression of full-length human cytomegalovirus fusion protein in non-swelling baculovirus-infected cells with a minimal fed-batch strategy.

Human cytomegalovirus congenital infection represents an unmet medical issue and attempts are ongoing to develop an effective vaccine. The virion fusion players of this enveloped virus are the natural targets to achieve this goal and to develop novel anti-viral therapies. The secreted ectodomain of the viral fusion factor glycoprotein B (gB) has been exploited so far as an alternative to the cumbersome expression of the wild type trans-membrane protein. In the soluble form, gB showed encouraging but limited potential as antigen candidate calling for further efforts. Here, the exhaustive evaluation of the Baculovirus/insect cell expression system has been coupled to an orthogonal screening for expression additives to produce full-length gB. In detail, rapamycin was found to prolong gB intracellular accumulation while inhibiting the infection-induced cell swelling. Not obvious to predict, this inhibition did not affect Baculovirus growth, revealing that the virus-induced cell size increase is a dispensable side phenotype. In parallel, a feeding strategy for the limiting nutrient cysteine has been set up which improved gB stability. This multi-modal scheme allowed the production of full-length, mutation-free gB in the milligram scale. The recombinant full-length gB obtained was embedded into a stable mono-dispersed particle substantially larger than the protein trimer itself, according to the reported association of this protein with detergent-resistant lipid domains.

Production of rotavirus core-like particles in Sf9 cells using recombinase-mediated cassette exchange.

A flexible Sf9 insect cell line was recently developed leveraging the recombinase-mediated cassette exchange (RMCE) technology, which competes with the popular baculovirus expression vector system (BEVS) in terms of speed to produce new proteins. Herein, the ability of this cell platform to produce complex proteins, such as rotavirus core-like particles, was evaluated. A gene construct coding for a VP2-GFP fusion protein was targeted to a pre-characterized high recombination efficiency locus flanked by flipase (Flp) recognition target sites and, after three weeks in selection, an isogenic cell population was obtained. Despite the lower cell specific productivities with respect to those obtained by baculovirus infection, the titers of VP2-GFP reached in shake flask batch cultures were comparable as a result of higher cell densities. To further improve the VP2-GFP levels from stable expression, analysis of exhausted medium was undertaken to design feeding strategies enabling higher cell densities as well as increased culture duration. The implementation of the best strategy allowed reaching 20 million cells per ml in bioreactor cultures; the integrity of the rotavirus core-like particles could be confirmed by electron microscopy. Overall, we show that this Sf9-Flp cell platform represents a valuable alternative to the BEVS for producing complex recombinant proteins, such as rotavirus core-like particles.

A cell sorting protocol for selecting high-producing sub-populations of Sf9 and High Five™ cells.

Insect cell lines such as Sf9 and High Five™ have been widely used to produce recombinant proteins mostly by the lytic baculovirus vector system. We have recently established an expression platform in Sf9 cells using a fluorescence-based recombinase mediated cassette exchange (RMCE) strategy which has similar development timelines but avoids baculovirus infection. To expedite cell engineering efforts, a robust fluorescence-activated cell sorting (FACS) protocol optimized for insect cells was developed here. The standard sorting conditions used for mammalian cells proved to be unsuitable, resulting in post-sorting viabilities below 10% for both cell lines. We found that the extreme sensitivity to the shear stress displayed by Sf9 and High Five™ cells was the limiting factor, and using Pluronic F-68 in the cell suspension could increase post-sorting viabilities in a dose dependent manner. The newly developed protocol was then used to sort stable populations of both cell lines tagged with a DsRed-expressing cassette. Before sorting, the average fluorescence intensity of the Sf9 cell population was 3-fold higher than that of the High Five™ cell population. By enriching with the 10% strongest DsRed-fluorescent cells, the productivity of both cell populations could be successfully improved. The established sorting protocol potentiates the use of RMCE technology for recombinant protein production in insect cells.

Protein pUL128 of human cytomegalovirus is necessary for monocyte infection and blocking of migration.

We have previously shown that only endotheliotropic strains of human cytomegalovirus (HCMV), such as TB40E, infect monocytes and impair their chemokine-driven migration. The proteins encoded by the UL128-131A region (UL128, UL130, and UL131A) of the HCMV genome, which assemble into a pentameric gH-gL-UL128-UL130-UL131A envelope complex, have been recognized as determinants for HCMV endothelial cell tropism. The genes for these proteins are typically inactivated by mutations in all fibroblast-adapted strains that have lost the diversified tropism of clinical isolates. By using mutant HCMV reconstituted from TB40E-derived bacterial artificial chromosomes (BAC) encoding a wild-type (wt) or mutated form of UL128, we show here that UL128-131A products are essential determinants of infection in monocytes and that pUL128, in particular, can block chemokine-driven motility. The virus BAC4, encoding wt UL128, established infection in monocytes, induced the intracellular retention of several chemokine receptors, and rendered monocytes unresponsive to different chemokines. In contrast, the virus BAC1, encoding a mutated UL128, failed to infect monocytes and to downregulate chemokine receptors. BAC1-exposed monocytes did not express immediate-early (IE) products, retained virions in cytoplasmic vesicles, and exhibited normal chemokine responsiveness. A potential role of second-site mutations in the observed phenotype was excluded by using the revertant viruses BAC1rep and BAC4mut. By incubating noninfected monocytes with soluble recombinant pUL128, we observed both the block of migration and the chemokine receptor internalization. We propose that among the gH-gL-UL128-UL130-UL131A complex subunits, the UL128 protein is the one that triggers monocyte paralysis.

Human cytomegalovirus serum neutralizing antibodies block virus infection of endothelial/epithelial cells, but not fibroblasts, early during primary infection.

A panel of human sera exhibited a >or=128-fold higher neutralizing potency against a human cytomegalovirus (HCMV) clinical isolate propagated and tested in endothelial (or epithelial) cells than against the same virus infecting human fibroblasts. In a group of 18 primary infections, the reverse geometric mean titre was in the range of 10-15 in human fibroblasts within the first 3 months after the onset of infection, whereas the endothelial cell infection-neutralizing activity was already present within the first 10 days, reaching median levels of 122, 320 and 545 at respectively 30, 60 and 90 days after onset, then declining slowly. This difference was also confirmed in the majority of reactivated and remote HCMV infections, as well as in a hyperimmune globulin preparation. The antibody response to HCMV pUL131A, pUL130 and pUL128 locus products, which are required for endothelial/epithelial cell infection, provided a potential molecular basis for such a differential neutralizing activity. In addition, monoclonal/monospecific antibodies raised against the pUL131A, pUL130 and pUL128 proteins were found to display an inhibitory activity on HCMV plaque formation and HCMV leukocyte transfer from HCMV-infected cells. Hence, conventional determination of the neutralizing activity of human sera in fibroblasts is misleading. Antibodies to pUL131A, pUL130 and pUL128 appear to display a major HCMV-neutralizing and dissemination-inhibiting activity.

Cytomegalovirus UL131-128 products promote gB conformational transition and gB-gH interaction during entry into endothelial cells.

Herpesviruses use gB and gH-gL glycoproteins to execute fusion. Other virus-specific glycoproteins are required for receptor binding and fusion activation. The human cytomegalovirus (HCMV) UL131-128 proteins are essential for the infection of leukocytes, endothelial cells (ECs), and many epithelial cell lines. Here we show that UL131-128 play a role in a chain of events involving gB and gH during HCMV entry into ECs. An HCMV strain bearing the wild-type (wt) UL131-128 locus exhibited a gB transition from a protease-resistant to protease-sensitive form, a conformational change that was suppressed by a thiourea inhibitor of fusion (WY1768); in contrast, gH was susceptible to proteolysis throughout entry. Moreover, gB and gH transiently interacted, and a lipid mixing assay showed that the wt strain had carried out fusion by 60 min postinfection. However, these events were greatly altered when UL131-128-defective strains were used for infection or when there was an excess of soluble pUL128 during wt infection: the gB conformational change became WY1768 resistant, the gB-gH complex was no longer observed, and fusion was prevented. Both gB and gH in this case showed late protease resistance, related to their endocytic uptake. Our data point to the involvement of UL131-128 proteins in driving gB through a WY1768-sensitive fold transition, thus promoting a short-lived gB-gH complex and fusion; they also suggest that HCMV fuses with the EC plasma membrane and that endocytosis ensues only when the virus cannot trigger UL131-128-dependent steps.

Human cytomegalovirus UL130 protein promotes endothelial cell infection through a producer cell modification of the virion.

Human cytomegalovirus (HCMV) growth in endothelial cells (EC) requires the expression of the UL131A-128 locus proteins. In this study, the UL130 protein (pUL130), the product of the largest gene of the locus, is shown to be a luminal glycoprotein that is inefficiently secreted from infected cells but is incorporated into the virion envelope as a Golgi-matured form. To investigate the mechanism of the UL130-mediated promotion of viral growth in EC, we performed a complementation analysis of a UL130 mutant strain. To provide UL130 in trans to viral infections, we constructed human embryonic lung fibroblast (HELF) and human umbilical vein endothelial cell (HUVEC) derivative cell lines that express UL130 via a retroviral vector. When the UL130-negative virus was grown in UL130-complementing HELF, the infectivity of progeny virions for HUVEC was restored to the wild-type level. In contrast, the infectivity of the UL130-negative virus for UL130-complementing HUVEC was low and similar to that of the same virus infecting control noncomplementing HUVEC. The UL130-negative virus, regardless of whether or not it had been complemented in the prior cycle, could form plaques only on UL130-complementing HUVEC, not control HUVEC. Because (i) both wild-type and UL130-transcomplemented virions maintained their infectivity for HUVEC after purification, (ii) UL130 failed to complement in trans the UL130-negative virus when it was synthesized in a cell separate from the one that produced the virions, and (iii) pUL130 is a virion protein, models are favored in which pUL130 acquisition in the producer cell renders HCMV virions competent for a subsequent infection of EC.

Human cytomegalovirus UL131-128 genes are indispensable for virus growth in endothelial cells and virus transfer to leukocytes.

Human cytomegalovirus (HCMV), a ubiquitous human pathogen, is the leading cause of birth defects and morbidity in immunocompromised patients and a potential trigger for vascular disease. HCMV replicates in vascular endothelial cells and drives leukocyte-mediated viral dissemination through close endothelium- leukocyte interaction. However, the genetic basis of HCMV growth in endothelial cells and transfer to leukocytes is unknown. We show here that the UL131-128 gene locus of HCMV is indispensable for both productive infection of endothelial cells and transmission to leukocytes. The experimental evidence for this is based on both the loss-of-function phenotype in knockout mutants and natural variants and the gain-of-function phenotype by trans-complementation with individual UL131, UL130, and UL128 genes. Our findings suggest that a common mechanism of virus transfer may be involved in both endothelial cell tropism and leukocyte transfer and shed light on a crucial step in the pathogenesis of HCMV infection.

The human cytomegalovirus UL45 gene product is a late, virion-associated protein and influences virus growth at low multiplicities of infection.

Human cytomegalovirus (HCMV) encodes a protein related to the large (R1) subunit of ribonucleotide reductase (RR), but does not encode the corresponding small (R2) subunit. The R1 homologue, UL45, lacks many catalytic residues, and its impact on deoxyribonucleotide (dNTP) production remains unknown. Here, UL45 is shown to accumulate at late stages of infection and to be a virion tegument protein. To study UL45 function in its genome context, UL45 was disrupted by transposon insertion. The UL45-knockout (UL45-KO) mutant exhibited a growth defect in fibroblasts at a low m.o.i. and also a cell-to-cell spread defect. This did not result from a reduced dNTP supply because dNTP pools were unchanged in resting cells infected with the mutant virus. Irrespective of UL45 expression, all cellular RR subunits - S-phase RR subunits, and the p53-dependent p53R2 - were induced by infection. p53R2 was targeted to the infected cell nucleus, suggesting that HCMV diverts a mechanism normally activated by DNA damage response. Cells infected with the UL45-KO mutant were moderately sensitized to Fas-induced apoptosis relative to those infected with the parental virus. Together with the report on the UL45-KO endotheliotropic HCMV mutant (Hahn et al., J Virol 76, 9551-9555, 2002), these data suggest that UL45 does not share the prominent antiapototic role attributed to the mouse cytomegalovirus homologue M45 (Brune et al., Science 291, 303-305, 2001).