Reovirus infection is regulated by NPC1 and endosomal cholesterol homeostasis

Cholesterol-Dependent Membrane Deformation by Metastable Viral Capsids Facilitates Entry

Nonenveloped viruses employ unique entry mechanisms to breach and infect host cells. Understanding these mechanisms is crucial for developing antiviral strategies. Prevailing perspective suggests that nonenveloped viruses release membrane pore-forming peptides to breach host membranes. However, the precise involvement of the viral capsid in this entry remains elusive. Our study presents direct observations elucidating the dynamically distinctive steps through which metastable reovirus capsids disrupt host lipid membranes as they uncoat into partially hydrophobic intermediate particles. Using both live cells and model membrane systems, our key finding is that reovirus capsids actively deform and permeabilize lipid membranes in a cholesterol-dependent process. Unlike membrane pore-forming peptides, these metastable viral capsids induce more extensive membrane perturbations, including budding, bridging between adjacent membranes, and complete rupture. Notably, cholesterol enhances subviral particle adsorption, resulting in the formation of pores equivalent to the capsid size. This cholesterol dependence is attributed to the lipid condensing effect, particularly prominent at an intermediate cholesterol level. Furthermore, our results reveal a positive correlation between membrane disruption extent and efficiency of viral variants in establishing infection. This study unveils the crucial role of capsid-lipid interaction in nonenveloped virus entry, providing new insights into how cholesterol homeostasis influences virus infection dynamics.

NRP1 is a receptor for mammalian orthoreovirus engaged by distinct capsid subunits

Mammalian orthoreovirus (reovirus) is a nonenveloped virus that establishes primary infection in the intestine and disseminates to sites of secondary infection, including the CNS. Reovirus entry involves multiple engagement factors, but how the virus disseminates systemically and targets neurons remains unclear. In this study, we identified murine neuropilin 1 (mNRP1) as a receptor for reovirus. mNRP1 binds reovirus with nanomolar affinity using a unique mechanism of virus-receptor interaction, which is coordinated by multiple interactions between distinct reovirus capsid subunits and multiple NRP1 extracellular domains. By exchanging essential capsid protein-encoding gene segments, we determined that the multivalent interaction is mediated by outer-capsid protein σ3 and capsid turret protein λ2. Using capsid mutants incapable of binding NRP1, we found that NRP1 contributes to reovirus dissemination and neurovirulence in mice. Collectively, our results demonstrate that NRP1 is an entry receptor for reovirus and uncover mechanisms by which NRPs promote viral entry and pathogenesis.

Neuropilin invites reoviruses into neurons

In this issue of Cell Host & Microbe, Shang et al. identify murine neuropilin 1 as a host factor that binds reovirus particles, directing cell entry and contributing to viral dissemination and neurovirulence. This study highlights the reovirus model system to investigate host receptors and their significance in viral pathogenesis.

Lysosomal enzyme trafficking: from molecular mechanisms to human diseases

Lysosomes degrade and recycle macromolecules that are delivered through the biosynthetic, endocytic, and autophagic routes. Hydrolysis of the different classes of macromolecules is catalyzed by about 70 soluble enzymes that are transported from the Golgi apparatus to lysosomes in a mannose 6-phosphate (M6P)-dependent process. The molecular machinery that generates M6P tags for receptor-mediated targeting of lysosomal enzymes was thought to be understood in detail. However, recent studies on the M6P pathway have identified a previously uncharacterized core component, yielded structural insights in known components, and uncovered functions in various human diseases. Here we review molecular mechanisms of lysosomal enzyme trafficking and discuss its relevance for rare lysosomal disorders, cancer, and viral infection.

Viral regulation of organelle membrane contact sites

At the core of organelle functions lies their ability and need to form dynamic organelle-organelle networks that drive intracellular communication and coordination of cellular pathways. These networks are facilitated by membrane contact sites (MCSs) that promote both intra-organelle and inter-organelle communication. Given their multiple functions, MCSs and the proteins that form them are commonly co-opted by viruses during infection to promote viral replication. This Essay discusses mechanisms acquired by diverse human viruses to regulate MCS functions in either proviral processes or host defense. It also examines techniques used for examining MCSs in the context of viral infections.

An overview of the role of Niemann-pick C1 (NPC1) in viral infections and inhibition of viral infections through NPC1 inhibitor

Viruses communicate with their hosts through interactions with proteins, lipids, and carbohydrate moieties on the plasma membrane (PM), often resulting in viral absorption via receptor-mediated endocytosis. Many viruses cannot multiply unless the host's cholesterol level remains steady. The large endo/lysosomal membrane protein (MP) Niemann-Pick C1 (NPC1), which is involved in cellular cholesterol transport, is a crucial intracellular receptor for viral infection. NPC1 is a ubiquitous housekeeping protein essential for the controlled cholesterol efflux from lysosomes. Its human absence results in Niemann-Pick type C disease, a deadly lysosomal storage disorder. NPC1 is a crucial viral receptor and an essential host component for filovirus entrance, infection, and pathogenesis. For filovirus entrance, NPC1's cellular function is unnecessary. Furthermore, blocking NPC1 limits the entry and replication of the African swine fever virus by disrupting cholesterol homeostasis. Cell entrance of quasi-enveloped variants of hepatitis A virus and hepatitis E virus has also been linked to NPC1. By controlling cholesterol levels, NPC1 is also necessary for the effective release of reovirus cores into the cytoplasm. Drugs that limit NPC1's activity are effective against several viruses, including SARS-CoV and Type I Feline Coronavirus (F-CoV). These findings reveal NPC1 as a potential therapeutic target for treating viral illnesses and demonstrate its significance for several viral infections. This article provides a synopsis of NPC1's function in viral infections and a review of NPC1 inhibitors that may be used to counteract viral infections. Video Abstract.

Chemical synthesis and biochemical properties of cholestane-5α,6β-diol-3-sulfonate: A non-hydrolysable analogue of cholestane-5α,6β-diol-3β-sulfate

Cholestane-3β,5α,6β-triol (CT) is a primary metabolite of 5,6-epoxycholesterols (5,6-EC) that is catalyzed by the cholesterol-5,6-epoxide hydrolase (ChEH). CT is a well-known biomarker for Niemann-Pick disease type C (NP-C), a progressive inherited neurodegenerative disease. On the other hand, CT is known to be metabolized by the 11β-hydroxysteroid-dehydrogenase of type 2 (11β-HSD2) into a tumor promoter named oncosterone that stimulates the growth of breast cancer tumors. Sulfation is a major metabolic transformation leading to the production of sulfated oxysterols. The production of cholestane-5α,6β-diol-3β-O-sulfate (CDS) has been reported in breast cancer cells. However, no data related to CDS biological properties have been reported so far. These studies have been hampered because sulfate esters of sterols and steroids are rapidly hydrolyzed by steroid sulfatase to give free steroids and sterols. In order to get insight into the biological properties of CDS, we report herein the synthesis and the characterization of cholestane-5α,6β-diol-3β-sulfonate (CDSN), a non-hydrolysable analogue of CDS. We show that CDSN is a potent inhibitor of 11β-HSD2 that blocks oncosterone production on cell lysate. The inhibition of oncosterone biosynthesis of a whole cell assay was observed but results from the blockage by CDSN of the uptake of CT in MCF-7 cells. While CDSN inhibits MCF-7 cell proliferation, we found that it potentiates the cytotoxic activity of post-lanosterol cholesterol biosynthesis inhibitors such as tamoxifen and PBPE. This effect was associated with an increase of free sterols accumulation and the appearance of giant multilamellar bodies, a structural feature reminiscent of Type C Niemann-Pick disease cells and consistent with a possible inhibition by CDSN of NPC1. Altogether, our data showed that CDSN is biologically active and that it is a valuable tool to study the biological properties of CDS and more specifically its impact on immunity and viral infection.

Proteasome activity is required for reovirus entry into cells

IMPORTANCE

Due to their limited genetic capacity, viruses are reliant on multiple host systems to replicate successfully. Mammalian orthoreovirus (reovirus) is commonly used as a model system for understanding host-virus interactions. In this study, we identify that the proteasome system, which is critical for cellular protein turnover, affects reovirus entry. Inhibition of the proteasome using a chemical inhibitor blocks reovirus uncoating. Blocking these events reduces subsequent replication of the virus. This work identifies that additional host factors control reovirus entry.

Cholesterol 25-hydroxylase suppresses avian reovirus replication by its enzymatic product 25-hydroxycholesterol

Avian reovirus (ARV) causing viral arthritis/tenosynovitis and viral enteritis in domestic fowl has significantly threatened on the poultry industry worldwide. ARV is a non-enveloped fusogenic virus that belongs to the Reoviridae family. Previous research revealed that cellular cholesterol in lipid rafts is essential for ARV replication. It has been reported that cholesterol 25-hydroxylase (CH25H) and its product 25-hydroxycholesterol (25HC) have antiviral activities against enveloped viruses. However, few studies characterized the association of non-enveloped viruses with CH25H and the role of CH25H in the regulation of ARV replication. In this study, the expression of chicken CH25H (chCH25H) was found to be upregulated in ARV-infected cells at the early stage of infection. The results of overexpression and knockdown assays revealed that chCH25H has a significant antiviral effect against ARV infection. Furthermore, a 25HC treatment significantly inhibited ARV replication in a dose-dependent manner at both the entry and post-entry stages, and a chCH25H mutant lacking hydroxylase activity failed to inhibit ARV infection. These results indicate that CH25H, depending on its enzyme activity, exerts the antiviral effect against ARV via the synthesis of 25HC. In addition, we revealed that 25HC produced by CH25H inhibits viral entry by delaying the kinetics of ARV uncoating, and CH25H blocks cell-cell membrane fusion induced by the p10 protein of ARV. Altogether, our findings showed that CH25H, as a natural host restriction factor, possessed antiviral activity against ARV targeting viral entry and syncytium formation, through an enzyme activity-dependent way. This study may provide new insights into the development of broad-spectrum antiviral therapies.

Potential use of the cholesterol transfer inhibitor U18666A as an antiviral drug for research on various viral infections

Cholesterol plays critical functions in arranging the biophysical attributes of proteins and lipids in the plasma membrane. For various viruses, an association with cholesterol for virus entrance and/or morphogenesis has been demonstrated. Therefore, the lipid metabolic pathways and the combination of membranes could be targeted to selectively suppress the virus replication steps as a basis for antiviral treatment. U18666A is a cationic amphiphilic drug (CAD) that affects intracellular transport and cholesterol production. A robust tool for investigating lysosomal cholesterol transfer and Ebola virus infection is an androstenolone derived termed U18666A that suppresses three enzymes in the cholesterol biosynthesis mechanism. In addition, U18666A inhibited low-density lipoprotein (LDL)-induced downregulation of LDL receptor and triggered lysosomal aggregation of cholesterol. According to reports, U18666A inhibits the reproduction of baculoviruses, filoviruses, hepatitis, coronaviruses, pseudorabies, HIV, influenza, and flaviviruses, as well as chikungunya and flaviviruses. U18666A-treated viral infections may act as a novel in vitro model system to elucidate the cholesterol mechanism of several viral infections. In this article, we discuss the mechanism and function of U18666A as a potent tool for studying cholesterol mechanisms in various viral infections.

Characterization of Sialic Acid-Independent Simian Rotavirus Mutants in Viral Infection and Pathogenesis

Rotaviruses (RVs) are nonenveloped viruses that cause gastroenteritis in infants and young children. Sialic acid is an initial receptor, especially for animal RVs, including rhesus RV. Sialic acid binds to the VP8* subunit, a part of the outer capsid protein VP4 of RV. Although interactions between virus and glycan receptors influence tissue and host tropism and viral pathogenicity, research has long been limited to biochemical and structural studies due to the unavailability of an RV reverse genetics system. Here, we examined the importance of sialic acid in RV infections using recombinant RVs harboring mutations in sialic acid-binding sites in VP4 via a simian RV strain SA11-based reverse genetics system. RV VP4 mutants that could not bind to sialic acid had replicated to decreased viral titer in MA104 cells. Wild-type virus infectivity was reduced, while that of VP4 mutants was not affected in sialic acid-deficient cells. Unexpectedly, in vivo experiments demonstrated that VP4 mutants suppressed mouse pups' weight gain and exacerbated diarrhea symptoms compared to wild-type viruses. Intestinal contents enhanced VP4 mutants' infectivity. Thus, possibly via interactions with other unknown receptors and/or intestinal contents, VP4 mutants are more likely than wild-type viruses to proliferate in the murine intestine, causing diarrhea and weight loss. These results suggest that RVs binding sialic acid notably affect viral infection in vitro and viral pathogenesis in vivo. IMPORTANCE Various studies have been conducted on the binding of VP8* and glycans, and the direct interaction between purified VP8* and glycans has been investigated by crystalline structure analyses. Here, we used a reverse genetics system to generate rotaviruses (RVs) with various VP4 mutants. The generated mutant strains clarified the importance of glycan binding in vitro and in vivo. Moreover, even when VP4 mutants could not bind to sialic acid, they were able to bind to an unknown receptor. As RVs evolve, pathogenicity can also be modified by easily altering the glycans to which VP4 binds.