Effect of inhibitors of the intracellular exocytic pathway on glycoprotein processing and maturation of Junin virus

The antimicrobial and immunomodulatory effects of Ionophores for the treatment of human infection

Ionophores are a diverse class of synthetic and naturally occurring ion transporter compounds which demonstrate both direct and in-direct antimicrobial properties against a broad panel of bacterial, fungal, viral and parasitic pathogens. In addition, ionophores can regulate the host-immune response during communicable and non-communicable disease states. Although the clinical use of ionophores such as Amphotericin B, Bedaquiline and Ivermectin highlight the utility of ionophores in modern medicine, for many other ionophore compounds issues surrounding toxicity, bioavailability or lack of in vivo efficacy studies have hindered clinical development. The antimicrobial and immunomodulating properties of a range of compounds with characteristics of ionophores remain largely unexplored. As such, ionophores remain a latent therapeutic avenue to address both the global burden of antimicrobial resistance, and the unmet clinical need for new antimicrobial therapies. This review will provide an overview of the broad-spectrum antimicrobial and immunomodulatory properties of ionophores, and their potential uses in clinical medicine for combatting infection.

How Do Enveloped Viruses Exploit the Secretory Proprotein Convertases to Regulate Infectivity and Spread?

Inhibition of the binding of enveloped viruses surface glycoproteins to host cell receptor(s) is a major target of vaccines and constitutes an efficient strategy to block viral entry and infection of various host cells and tissues. Cellular entry usually requires the fusion of the viral envelope with host plasma membranes. Such entry mechanism is often preceded by “priming” and/or “activation” steps requiring limited proteolysis of the viral surface glycoprotein to expose a fusogenic domain for efficient membrane juxtapositions. The 9-membered family of Proprotein Convertases related to Subtilisin/Kexin (PCSK) serine proteases (PC1, PC2, Furin, PC4, PC5, PACE4, PC7, SKI-1/S1P, and PCSK9) participate in post-translational cleavages and/or regulation of multiple secretory proteins. The type-I membrane-bound Furin and SKI-1/S1P are the major convertases responsible for the processing of surface glycoproteins of enveloped viruses. Stefan Kunz has considerably contributed to define the role of SKI-1/S1P in the activation of arenaviruses causing hemorrhagic fever. Furin was recently implicated in the activation of the spike S-protein of SARS-CoV-2 and Furin-inhibitors are being tested as antivirals in COVID-19. Other members of the PCSK-family are also implicated in some viral infections, such as PCSK9 in Dengue. Herein, we summarize the various functions of the PCSKs and present arguments whereby their inhibition could represent a powerful arsenal to limit viral infections causing the present and future pandemics.

Envelope glycoprotein of arenaviruses

Arenaviruses include lethal human pathogens which pose serious public health threats. So far, no FDA approved vaccines are available against arenavirus infections, and therapeutic options are limited, making the identification of novel drug targets for the development of efficacious therapeutics an urgent need. Arenaviruses are comprised of two RNA genome segments and four proteins, the polymerase L, the envelope glycoprotein GP, the matrix protein Z, and the nucleoprotein NP. A crucial step in the arenavirus life-cycle is the biosynthesis and maturation of the GP precursor (GPC) by cellular signal peptidases and the cellular enzyme Subtilisin Kexin Isozyme-1 (SKI-1)/Site-1 Protease (S1P) yielding a tripartite mature GP complex formed by GP1/GP2 and a stable signal peptide (SSP). GPC cleavage by SKI-1/S1P is crucial for fusion competence and incorporation of mature GP into nascent budding virion particles. In a first part of our review, we cover basic aspects and newer developments in the biosynthesis of arenavirus GP and its molecular interaction with SKI-1/S1P. A second part will then highlight the potential of SKI-1/S1P-mediated processing of arenavirus GPC as a novel target for therapeutic intervention to combat human pathogenic arenaviruses.

Novel therapeutic targets for arenavirus hemorrhagic fevers

Several members of the family Arenaviridae can cause severe hemorrhagic fevers in humans, representing a serious public health problem in endemic areas of Africa and South America. The Lassa virus is the most prevalent and dangerous arenavirus, causing over 300,000 infections per year and several thousand deaths. Furthermore, pathogenic arenaviruses are considered as category A potential agents for bioterrorism. Based on the danger of arenaviruses for human health, the increased emergence of new viral species in recent years and the lack of effective tools for their control or prevention, the search for novel antiviral compounds effective against these pathogenic agents is a continuous demanding effort. This article focuses on novel strategies to identify inhibitors for arenavirus therapy, analyzing viral and host proteins essential for virus infection as potential targets for antiviral development.

Characterization of Junín arenavirus cell entry

Junín virus (JUNV) entry is conducted by receptor-mediated endocytosis. To explore the cellular entry mechanism of JUNV, inhibitory effects of drugs affecting the main endocytic pathways on JUNV entry into Vero cells were analysed. Compounds that impair clathrin-mediated endocytosis were shown to reduce virus internalization without affecting virion binding. In contrast, drugs that alter lipid-raft microdomains, impairing caveola-mediated endocytosis, were not able to block virus entry. To show direct evidence of JUNV entry, transmission electron microscopy was performed; it showed JUNV particles of about 60–100 nm in membrane depressions that had an electron-dense coating. In addition, JUNV particles were found within invaginations of the plasma membrane and vesicles that resembled those of pits and clathrin-coated vesicles. Taken together, these results demonstrate that clathrin-mediated endocytosis is the main JUNV entry pathway into Vero cells and represent an important contribution to the characterization of the arenavirus multiplication cycle.

Bitopic membrane topology of the stable signal peptide in the tripartite Junín virus GP-C envelope glycoprotein complex

The stable signal peptide (SSP) of the GP-C envelope glycoprotein of the Junín arenavirus plays a critical role in trafficking of the GP-C complex to the cell surface and in its membrane fusion activity. SSP therefore may function on both sides of the lipid membrane. In this study, we have investigated the membrane topology of SSP by confocal microscopy of cells treated with the detergent digitonin to selectively permeabilize the plasma membrane. By using an affinity tag to mark the termini of SSP in the properly assembled GP-C complex, we find that both the N and C termini reside in the cytosol. Thus, SSP adopts a bitopic topology in which the C terminus is translocated from the lumen of the endoplasmic reticulum to the cytoplasm. This model is supported by (i) the presence of two conserved hydrophobic regions in SSP (hphi1 and hphi2) and (ii) our previous demonstration that lysine-33 in the ectodomain loop is essential for pH-dependent membrane fusion. Moreover, we demonstrate that the introduction of a charged side chain or single amino acid deletion in the membrane-spanning hphi2 region significantly diminishes SSP association in the GP-C complex and abolishes membrane fusion activity. Taken together, our results suggest that bitopic membrane insertion of SSP is centrally important in the assembly and function of the tripartite GP-C complex.

Role of the stable signal peptide of Junín arenavirus envelope glycoprotein in pH-dependent membrane fusion

The envelope glycoprotein of the arenaviruses (GP-C) is unusual in that the mature complex retains the cleaved, 58-amino-acid signal peptide. Association of this stable signal peptide (SSP) has been shown to be essential for intracellular trafficking and proteolytic maturation of the GP-C complex. We identify here a specific and previously unrecognized role of SSP in pH-dependent membrane fusion. Amino acid substitutions that alter the positive charge at lysine K33 in SSP affect the ability of GP-C to mediate cell-cell fusion and the threshold pH at which membrane fusion is triggered. Based on the presumed location of K33 at or near the luminal domain of SSP, we postulate that SSP interacts with the membrane-proximal or transmembrane regions of the G2 fusion protein. This unique organization of the GP-C complex may suggest novel strategies for intervention in arenavirus infection.

Antiviral activity of antimicrobial cationic peptides against Junin virus and herpes simplex virus

The in vitro antiviral activity of antimicrobial cationic peptides: cecropin A, melittin, magainin I and II and indolicidin against the arenavirus Junin virus (JV), and herpes simplex virus type 1 (HSV-1) and 2 (HSV-2) was evaluated. Cecropin A effectively inhibited JV multiplication and failed to affect HSV replication whereas melittin impeded the multiplication of JV and HSV, but was highly toxic for the host cell. Magainins I and II exhibited inhibitory action toward HSV-1 and HSV-2 but were inactive against JV. Only indolicidin showed a direct inactivation effect on cell-free virus stocks. Besides its inhibitory effect on JV replication cecropin A also was active against the arenaviruses Tacaribe and Pichinde, mainly affecting late events of arenavirus multiplication cycle by preventing viral morphogenesis and egress from infected cells.

Intermediate filament integrity is required for Junin virus replication

The role of the cytoskeletal framework in Junin virus (JUNV) replication has already been demonstrated with compounds interfering with the microfilament (MF) and microtubule (MT) networks. In this work, we evaluated the role of intermediate filaments (IF) during JUNV infection. We tested the effect of acrylamide, a compound that selectively disrupts IF, in culture of three different cell types: Vero cells, murine astrocytes and human foreskin fibroblasts. Perturbation of intermediate filaments had an inhibitory effect on JUNV production within a range of acrylamide concentration of 0.5-3mM in a dose-dependent manner, without cell viability modification. Recovery experiments showed that viral production was partially increased when medium containing acrylamide was replaced by normal maintenance medium (MM). The adsorption and internalization steps were not affected by IF disruption. The expression of JUNV proteins was highly reduced in the presence of 2mM acrylamide while immunofluorescence staining of IF showed network disruption with the formation of cytoplasmic aggregates containing vimentin or glial fibrillary acidic protein (GFAP). We conclude that the IF network may play a role in the early step of JUNV multiplication, subsequent to virus entry and that its integrity is a necessary condition for the normal replication of JUNV in neural and fibroblast cells as well as in the Vero cell line.

Identification of a novel consensus sequence at the cleavage site of the Lassa virus glycoprotein

The Lassa virus glycoprotein consists of an amino-terminal and a carboxy-terminal cleavage fragment designated GP-1 and GP-2, respectively, that are derived by proteolysis from the precursor GP-C. The membrane-anchored GP-2 obtained from purified virions of the Josiah strain revealed the N-terminal tripeptide GTF(262) when analyzed by Edman degradation. Upstream of this site, GP-C contains the tetrapeptide sequence RRLL(259), which is conserved in all Lassa virus isolates published to date. Systematic mutational analysis of vector-expressed GP-C revealed that the motif R-X (L/I/V)-L(259) (where X stands for L, I, or V) is essential for cleavage of the peptide bond between leucine(259) and glycine(260). This cleavage motif is homologous to the consensus sequence recognized by a novel class of cellular endoproteases which have so far not been implicated in the processing of viral glycoproteins.

Myristic acid analogs are inhibitors of Junin virus replication

The effects of two myristic acid analogs on Junin virus (JV) replication were investigated. The compounds chosen for the study were DL-2-hydroxymyristic acid (2OHM), an inhibitor of N-myristoyltransferase (NMT), which binds the enzyme and blocks protein myristoylation, and 13-oxamyristic acid (13OM), a competitive inhibitor of NMT which incorporates into the protein instead of myristic acid. Both types of analogs achieved dose-dependent inhibition of viral multiplication at concentrations not affecting cell viability. The 50% inhibitory concentration values determined by a virus-yield inhibition assay for different strains of JV, including a human pathogenic strain, and for the related arenavirus, Tacaribe, were in the range 1.6 to 20.1 microM, with 13OM as the most active compound. From time of addition and removal experiments, it can be concluded that both analogs inhibit a late stage in the JV replicative cycle, and their effect was partially reversible. The cytoplasmic and surface expression of JV glycoproteins was not affected in the presence of the compounds, as revealed by immunofluorescence staining, suggesting that JV glycoprotein myristoylation would not be essential for the intracellular transport of the envelope proteins, but it may have an important role in their interaction with the plasma membrane during virus budding.