Reovirus μ1 Protein Affects Infectivity by Altering Virus-Receptor Interactions

The Reovirus σ1 Attachment Protein Influences the Stability of Its Entry Intermediate

Structural metastability of viral capsids is pivotal for viruses to survive in harsh environments and to undergo timely conformational changes required for cell entry. Mammalian orthoreovirus (reovirus) is a model to study capsid metastability. Following initial disassembly of the reovirus particle mediated by proteases, a metastable intermediate called the infectious subvirion particle (ISVP) is generated. Using a σ1 monoreassortant virus, we recently showed that σ1 properties affect its encapsidation on particles and the metastability of ISVPs. How metastability is impacted by σ1 and whether the lower encapsidation level of σ1 is connected to this property is unknown. To define a correlation between encapsidation of σ1 and ISVP stability, we generated mutant viruses with single amino acid polymorphisms in σ1 or those that contain chimeric σ1 molecules composed of σ1 portions from type 1 and type 3 reovirus strains. We found that under most conditions where σ1 encapsidation on the particle was lower, ISVPs displayed lower stability. Characterization of mutant viruses selected for enhanced stability via a forward genetic approach also revealed that in some cases, σ1 properties influence stability without influencing σ1 encapsidation. These data indicate that σ1 can also influence ISVP stability independent of its level of incorporation. Together, our work reveals an underappreciated effect of the σ1 attachment protein on the properties of the reovirus capsid. IMPORTANCE Reovirus particles are comprised of eight proteins. Among them, the reovirus σ1 protein functions engages cellular receptors. σ1 also influences the stability of an entry intermediate called ISVP. Here, we sought to define the basis of the link between σ1 properties and stability of ISVPs. Using variety of mutant strains, we determined that when virus preparations contain particles with a high amount of encapsidated σ1, ISVP stability is higher. Additionally, we identified portions of σ1 that impact its encapsidation and consequently the stability of ISVPs. We also determined that in some cases, σ1 properties alter stability of ISVPs without affecting encapsidation. This work highlights that proteins of these complex particles are arranged in an intricate, interconnected manner such that changing the properties of these proteins has a profound impact on the remainder of the particle.

Stability of enveloped and nonenveloped viruses in hydrolyzed gelatin liquid formulation

BACKGROUND

The thermal stability of viruses in gelatin liquid formulations for medical research and application is poorly understood and this study aimed to examine the thermal stability of 4 enveloped and nonenveloped DNA and RNA viruses in hydrolyzed gelatin liquid formulations.

METHODS

Bovine herpesvirus (BHV) was used as a model virus to examine the molecular weight (MW), concentration and gelatin type and to optimize virus stability in liquid formulations at 25 °C and 4 °C. Using the model virus liquid formulation, the stability of multiple enveloped and nonenveloped RNA and DNA viruses, including parainfluenza virus, reovirus (RV), BHV, and adenovirus (AdV), was monitored over up to a 30-week storage period.

RESULTS

The BHV model virus was considered stable after 3 weeks in hydrolyzed gelatin (MW: 4000) with a 0.8 LRV (log10 reduction value) at 25 °C or a 0.2 LRV at 4 °C, compared to the stabilities observed in higher MW gelatin (60,000 and 160,000) with an LRV above 1. Based on the gelatin type, BHV in alkaline-treated hydrolyzed gelatin samples were unexpectantly more stable than in acid-treated hydrolyzed gelatin sample. All four viruses exhibited stability at 4 °C for at least 8 weeks, BHV or AdV remained stable for over 30 weeks of storage, and at 25 °C, AdV and RV remained stable for 8 weeks.

CONCLUSION

The results demonstrated that 5% of 4000 MW hydrolyzed gelatin formulation can act as a relevant stabilizer for the thermal stability of viruses in medical research and application.

A CRISPR-Cas9 screen reveals a role for WD repeat-containing protein 81 (WDR81) in the entry of late penetrating viruses

Successful initiation of infection by many different viruses requires their uptake into the endosomal compartment. While some viruses exit this compartment early, others must reach the degradative, acidic environment of the late endosome. Mammalian orthoreovirus (reovirus) is one such late penetrating virus. To identify host factors that are important for reovirus infection, we performed a CRISPR-Cas9 knockout (KO) screen that targets over 20,000 genes in fibroblasts derived from the embryos of C57/BL6 mice. We identified seven genes (WDR81, WDR91, RAB7, CCZ1, CTSL, GNPTAB, and SLC35A1) that were required for the induction of cell death by reovirus. Notably, CRISPR-mediated KO of WD repeat-containing protein 81 (WDR81) rendered cells resistant to reovirus infection. Susceptibility to reovirus infection was restored by complementing KO cells with human WDR81. Although the absence of WDR81 did not affect viral attachment efficiency or uptake into the endosomal compartments for initial disassembly, it reduced viral gene expression and diminished infectious virus production. Consistent with the role of WDR81 in impacting the maturation of endosomes, WDR81-deficiency led to the accumulation of reovirus particles in dead-end compartments. Though WDR81 was dispensable for infection by VSV (vesicular stomatitis virus), which exits the endosomal system at an early stage, it was required for VSV-EBO GP (VSV that expresses the Ebolavirus glycoprotein), which must reach the late endosome to initiate infection. These results reveal a previously unappreciated role for WDR81 in promoting the replication of viruses that transit through late endosomes.

Viruses are obligate intracellular parasites that require the contributions of numerous host factors to complete the viral life cycle. Thus, the host-pathogen interaction can regulate cell death signaling and virus entry, replication, assembly, and egress. Functional genetic screens are useful tools to identify host factors that are important for establishing infection. Such information can also be used to understand cell biology. Notably, genome-scale CRISPR-Cas9 knockout screens are robust due to their specificity and the loss of host gene expression. Mammalian orthoreovirus (reovirus) is a tractable model system to investigate the pathogenesis of neurotropic and cardiotropic viruses. Using a CRISPR-Cas9 screen, we identified WD repeat-containing protein 81 (WDR81) as a host factor required for efficient reovirus infection of murine cells. Ablation of WDR81 blocked a late step in the viral entry pathway. Further, our work indicates that WDR81 is required for the entry of vesicular stomatitis virus that expresses the Ebolavirus glycoprotein.

The M2 Gene Is a Determinant of Reovirus-Induced Myocarditis

Although a broad range of viruses cause myocarditis, the mechanisms that underlie viral myocarditis are poorly understood. Here, we report that the M2 gene is a determinant of reovirus myocarditis. The M2 gene encodes outer capsid protein μ1, which mediates host membrane penetration during reovirus entry. We infected newborn C57BL/6 mice with reovirus strain type 1 Lang (T1L) or a reassortant reovirus in which the M2 gene from strain type 3 Dearing (T3D) was substituted into the T1L genetic background (T1L/T3DM2). T1L was non-lethal in wild-type mice, whereas greater than 90% of mice succumbed to T1L/T3DM2 infection. T1L/T3DM2 produced higher viral loads than T1L at the site of inoculation. In secondary organs, T1L/T3DM2 was detected with more rapid kinetics and reached higher peak titers than T1L. We found that hearts from T1L/T3DM2-infected mice were grossly abnormal, with large lesions indicative of substantial inflammatory infiltrate. Lesions in T1L/T3DM2-infected mice contained necrotic cardiomyocytes with pyknotic debris, and extensive lymphocyte and histiocyte infiltration. In contrast, T1L induced the formation of small purulent lesions in a small subset of animals, consistent with T1L being mildly myocarditic. Finally, more activated caspase-3-positive cells were observed in hearts from animals infected with T1L/T3DM2 compared to T1L. Together, our findings indicate that substitution of the T3D M2 allele into an otherwise T1L genetic background is sufficient to change a non-lethal infection into a lethal infection. Our results further indicate that T3D M2 enhances T1L replication and dissemination in vivo, which potentiates the capacity of reovirus to cause myocarditis. IMPORTANCE Reovirus is a non-enveloped virus with a segmented double-stranded RNA genome that serves as a model for studying viral myocarditis. The mechanisms by which reovirus drives myocarditis development are not fully elucidated. We found that substituting the M2 gene from strain type 3 Dearing (T3D) into an otherwise type 1 Lang (T1L) genetic background (T1L/T3DM2) was sufficient to convert the non-lethal T1L strain into a lethal infection in neonatal C57BL/6 mice. T1L/T3DM2 disseminated more efficiently and reached higher maximum titers than T1L in all organs tested, including the heart. T1L is mildly myocarditic and induced small areas of cardiac inflammation in a subset of mice. In contrast, hearts from mice infected with T1L/T3DM2 contained extensive cardiac inflammatory infiltration and more activated caspase-3-positive cells, which is indicative of apoptosis. Together, our findings identify the reovirus M2 gene as a new determinant of reovirus-induced myocarditis.

Noncanonical Cell Death Induction by Reassortant Reovirus

Triple-negative breast cancer (TNBC) constitutes 10 to 15% of all breast cancer and is associated with worse prognosis than other subtypes of breast cancer. Current therapies are limited to cytotoxic chemotherapy, radiation, and surgery, leaving a need for targeted therapeutics to improve outcomes for TNBC patients. Mammalian orthoreovirus (reovirus) is a nonenveloped, segmented, double-stranded RNA virus in the Reoviridae family. Reovirus preferentially kills transformed cells and is in clinical trials to assess its efficacy against several types of cancer. We previously engineered a reassortant reovirus, r2Reovirus, that infects TNBC cells more efficiently and induces cell death with faster kinetics than parental reoviruses. In this study, we sought to understand the mechanisms by which r2Reovirus induces cell death in TNBC cells. We show that r2Reovirus infection of TNBC cells of a mesenchymal stem-like (MSL) lineage downregulates the mitogen-activated protein kinase/extracellular signal-related kinase pathway and induces nonconventional cell death that is caspase-dependent but caspase 3-independent. Infection of different MSL lineage TNBC cells with r2Reovirus results in caspase 3-dependent cell death. We map the enhanced oncolytic properties of r2Reovirus in TNBC to epistatic interactions between the type 3 Dearing M2 gene segment and type 1 Lang genes. These findings suggest that the genetic composition of the host cell impacts the mechanism of reovirus-induced cell death in TNBC. Together, our data show that understanding host and virus determinants of cell death can identify novel properties and interactions between host and viral gene products that can be exploited for the development of improved viral oncolytics.IMPORTANCE TNBC is unresponsive to hormone therapies, leaving patients afflicted with this disease with limited treatment options. We previously engineered an oncolytic reovirus (r2Reovirus) with enhanced infective and cytotoxic properties in TNBC cells. However, how r2Reovirus promotes TNBC cell death is not known. In this study, we show that reassortant r2Reovirus can promote nonconventional caspase-dependent but caspase 3-independent cell death and that the mechanism of cell death depends on the genetic composition of the host cell. We also map the enhanced oncolytic properties of r2Reovirus in TNBC to interactions between a type 3 M2 gene segment and type 1 genes. Our data show that understanding the interplay between the host cell environment and the genetic composition of oncolytic viruses is crucial for the development of efficacious viral oncolytics.

Quantitative Proteome Responses to Oncolytic Reovirus in GM-CSF- and M-CSF-Differentiated Bone Marrow-Derived Cells

The efficacy of oncolytic viruses (OVs), such as reovirus, is dictated by host immune responses, including those mediated by the pro- versus anti-inflammatory macrophages. As such, a detailed understanding of the interaction between reovirus and different macrophage types is critical for therapeutic efficacy. To explore reovirus-macrophage interactions, we performed tandem mass tag (TMT)-based quantitative temporal proteomics on mouse bone marrow-derived macrophages (BMMs) generated with two cytokines, macrophage colony stimulating factor (M-CSF) and granulocytic-macrophage colony stimulating factor (GM-CSF), representing anti- and proinflammatory macrophages, respectively. We quantified 6863 proteins across five time points in duplicate, comparing M-CSF (M-BMM) and GM-CSF (GM-BMM) in response to OV. We find that GM-BMMs have lower expression of key intrinsic proteins that facilitate an antiviral immune response, express higher levels of reovirus receptor protein JAM-A, and are more susceptible to oncolytic reovirus infection compared to M-BMMs. Interestingly, although M-BMMs are less susceptible to reovirus infection and subsequent cell death, they initiate an antireovirus adaptive T cell immune response comparable to that of GM-BMMs. Taken together, these data describe distinct proteome differences between these two macrophage populations in terms of their ability to mount antiviral immune responses.

Enhanced Killing of Triple-Negative Breast Cancer Cells by Reassortant Reovirus and Topoisomerase Inhibitors

Breast cancer is the second leading cause of cancer-related deaths in women in the United States. Triple-negative breast cancer constitutes a subset of breast cancer that is associated with higher rates of relapse, decreased survival, and limited therapeutic options for patients afflicted with this type of breast cancer. Mammalian orthoreovirus (reovirus) selectively infects and kills transformed cells, and a serotype 3 reovirus is in clinical trials to assess its efficacy as an oncolytic agent against several cancers. It is unclear if reovirus serotypes differentially infect and kill triple-negative breast cancer cells and if reovirus-induced cytotoxicity of breast cancer cells can be enhanced by modulating the activity of host molecules and pathways. Here, we generated reassortant reoviruses by forward genetics with enhanced infective and cytotoxic properties in triple-negative breast cancer cells. From a high-throughput screen of small-molecule inhibitors, we identified topoisomerase inhibitors as a class of drugs that enhance reovirus infectivity and cytotoxicity of triple-negative breast cancer cells. Treatment of triple-negative breast cancer cells with topoisomerase inhibitors activates DNA damage response pathways, and reovirus infection induces robust production of type III, but not type I, interferon (IFN). Although type I and type III IFNs can activate STAT1 and STAT2, triple-negative breast cancer cellular proliferation is only negatively affected by type I IFN. Together, these data show that reassortant viruses with a novel genetic composition generated by forward genetics in combination with topoisomerase inhibitors more efficiently infect and kill triple-negative breast cancer cells.IMPORTANCE Patients afflicted by triple-negative breast cancer have decreased survival and limited therapeutic options. Reovirus infection results in cell death of a variety of cancers, but it is unknown if different reovirus types lead to triple-negative breast cancer cell death. In this study, we generated two novel reoviruses that more efficiently infect and kill triple-negative breast cancer cells. We show that infection in the presence of DNA-damaging agents enhances infection and triple-negative breast cancer cell killing by reovirus. These data suggest that a combination of a genetically engineered oncolytic reovirus and topoisomerase inhibitors may provide a potent therapeutic option for patients afflicted with triple-negative breast cancer.

Selection and Characterization of a Reovirus Mutant with Increased Thermostability

The environment represents a significant barrier to infection. Physical stressors (heat) or chemical agents (ethanol) can render virions noninfectious. As such, discrete proteins are necessary to stabilize the dual-layered structure of mammalian orthoreovirus (reovirus). The outer capsid participates in cell entry: (i) σ3 is degraded to generate the infectious subviral particle, and (ii) μ1 facilitates membrane penetration and subsequent core delivery. μ1-σ3 interactions also prevent inactivation; however, this activity is not fully characterized. Using forward and reverse genetic approaches, we identified two mutations (μ1 M258I and σ3 S344P) within heat-resistant strains. σ3 S344P was sufficient to enhance capsid integrity and to reduce protease sensitivity. Moreover, these changes impaired replicative fitness in a reassortant background. This work reveals new details regarding the determinants of reovirus stability.IMPORTANCE Nonenveloped viruses rely on protein-protein interactions to shield their genomes from the environment. The capsid, or protective shell, must also disassemble during cell entry. In this work, we identified a determinant within mammalian orthoreovirus that regulates heat resistance, disassembly kinetics, and replicative fitness. Together, these findings show capsid function is balanced for optimal replication and for spread to a new host.

Components of the Reovirus Capsid Differentially Contribute to Stability

The mammalian orthoreovirus (reovirus) outer capsid is composed of 200 μ1-σ3 heterohexamers and a maximum of 12 σ1 trimers. During cell entry, σ3 is degraded by luminal or intracellular proteases to generate the infectious subviral particle (ISVP). When ISVP formation is prevented, reovirus fails to establish a productive infection, suggesting proteolytic priming is required for entry. ISVPs are then converted to ISVP*s, which is accompanied by μ1 rearrangements. The μ1 and σ3 proteins confer resistance to inactivating agents; however, neither the impact on capsid properties nor the mechanism (or basis) of inactivation is fully understood. Here, we utilized T1L/T3D M2 and T3D/T1L S4 to investigate the determinants of reovirus stability. Both reassortants encode mismatched subunits. When μ1-σ3 were derived from different strains, virions resembled wild-type particles in structure and protease sensitivity. T1L/T3D M2 and T3D/T1L S4 ISVPs were less thermostable than wild-type ISVPs. In contrast, virions were equally susceptible to heating. Virion associated μ1 adopted an ISVP*-like conformation concurrent with inactivation; σ3 preserves infectivity by preventing μ1 rearrangements. Moreover, thermostability was enhanced by a hyperstable variant of μ1. Unlike the outer capsid, the inner capsid (core) was highly resistant to elevated temperatures. The dual layered architecture allowed for differential sensitivity to inactivating agents.IMPORTANCE Nonenveloped and enveloped viruses are exposed to the environment during transmission to a new host. Protein-protein and/or protein-lipid interactions stabilize the particle and protect the viral genome. Mammalian orthoreovirus (reovirus) is composed of two concentric, protein shells. The μ1 and σ3 proteins form the outer capsid; contacts between neighboring subunits are thought to confer resistance to inactivating agents. We further investigated the determinants of reovirus stability. The outer capsid was disrupted concurrent with the loss of infectivity; virion associated μ1 rearranged into an altered conformation. Heat sensitivity was controlled by σ3; however, particle integrity was enhanced by a single μ1 mutation. In contrast, the inner capsid (core) displayed superior resistance to heating. These findings reveal structural components that differentially contribute to reovirus stability.

Protein Mismatches Caused by Reassortment Influence Functions of the Reovirus Capsid

Following attachment to host receptors via σ1, reovirus particles are endocytosed and disassembled to generate infectious subvirion particles (ISVPs). ISVPs undergo conformational changes to form ISVP*, releasing σ1 and membrane-targeting peptides from the viral μ1 protein. ISVP* formation is required for delivery of the viral core into the cytoplasm for replication. We characterized the properties of T3D^F/T3D^CS1, an S1 gene monoreassortant between two laboratory isolates of prototype reovirus strain T3D: T3D^F and T3D^C T3D^F/T3D^CS1 is poorly infectious. This deficiency is a consequence of inefficient encapsidation of S1-encoded σ1 on T3D^F/T3D^CS1 virions. Additionally, compared to T3D^F, T3D^F/T3D^CS1 undergoes ISVP-to-ISVP* conversion more readily, revealing an unexpected role for σ1 in regulating ISVP* formation. The σ1 protein is held within turrets formed by the λ2 protein. To test if the altered properties of T3D^F/T3D^CS1 are due to a mismatch between σ1 and λ2 proteins from T3D^F and T3D^C, properties of T3D^F/T3D^CL2 and T3D^F/T3D^CS1L2, which express a T3D^C-derived λ2, were compared. The presence of T3D^C λ2 allowed more efficient σ1 incorporation, producing particles that exhibit T3D^F-like infectivity. Compared to T3D^F, T3D^F/T3D^CL2 prematurely converts to ISVP*, uncovering a role for λ2 in regulating ISVP* formation. Importantly, a virus with matching σ1 and λ2 displayed a more regulated conversion to ISVP* than either T3D^F/T3D^CS1 or T3D^F/T3D^CL2. In addition to identifying new regulators of ISVP* formation, our results highlight that protein mismatches produced by reassortment can alter virus assembly and thereby influence subsequent functions of the virus capsid.IMPORTANCE Cells coinfected with viruses that possess a multipartite or segmented genome reassort to produce progeny viruses that contain a combination of gene segments from each parent. Reassortment places new pairs of genes together, generating viruses in which mismatched proteins must function together. To test if such forced pairing of proteins that form the virus shell or capsid alters the function of the particle, we investigated properties of reovirus variants in which the σ1 attachment protein and the λ2 protein that anchors σ1 on the particle are mismatched. Our studies demonstrate that a σ1-λ2 mismatch produces particles with lower levels of encapsidated σ1, consequently decreasing virus attachment and infectivity. The mismatch between σ1 and λ2 also altered the capacity of the viral capsid to undergo conformational changes required for cell entry. These studies reveal new functions of reovirus capsid proteins and illuminate both predictable and novel implications of reassortment.

Cleavage of the C-Terminal Fragment of Reovirus μ1 Is Required for Optimal Infectivity

The mammalian orthoreovirus (reovirus) outer capsid, which is composed of 200 μ1/σ3 heterohexamers and a maximum of 12 σ1 trimers, contains all of the proteins that are necessary for attaching to and entering host cells. Following attachment, reovirus is internalized by receptor-mediated endocytosis and acid-dependent cathepsin proteases degrade the σ3 protein. This process generates a metastable intermediate, called infectious subviral particle (ISVP), in which the μ1 membrane penetration protein is exposed. ISVPs undergo a second structural rearrangement to deposit the genome-containing core into the host cytoplasm. The conformationally altered particle is called ISVP*. ISVP-to-ISVP* conversion culminates in the release of μ1 N- and C-terminal fragments, μ1N and Φ, respectively. Released μ1N is thought to facilitate core delivery by generating size-selective pores within the endosomal membrane, whereas the precise role of Φ, particularly in the context of viral entry, is undefined. In this report, we characterize a recombinant reovirus that fails to cleave Φ from μ1 in vitro Φ cleavage, which is not required for ISVP-to-ISVP* conversion, enhances the disruption of liposomal membranes and facilitates the recruitment of ISVP*s to the site of pore formation. Moreover, the Φ cleavage-deficient strain initiates infection of host cells less efficiently than the parental strain. These results indicate that μ1N and Φ contribute to reovirus pore forming activity.IMPORTANCE Host membranes represent a physical barrier that prevents infection. To overcome this barrier, viruses utilize diverse strategies, such as membrane fusion or membrane disruption, to access internal components of the cell. These strategies are characterized by discrete protein-protein and protein-lipid interactions. The mammalian orthoreovirus (reovirus) outer capsid undergoes a series of well-defined conformational changes, which conclude with pore formation and delivery of the viral genetic material. In this report, we characterize the role of the small, reovirus-derived Φ peptide in pore formation. Φ cleavage from the outer capsid enhances membrane disruption and facilitates the recruitment of virions to membrane-associated pores. Moreover, Φ cleavage promotes the initiation of infection. Together, these results reveal an additional component of the reovirus pore forming apparatus and highlight a strategy for penetrating host membranes.

Bacteria and bacterial envelope components enhance mammalian reovirus thermostability

Enteric viruses encounter diverse environments as they migrate through the gastrointestinal tract to infect their hosts. The interaction of eukaryotic viruses with members of the host microbiota can greatly impact various aspects of virus biology, including the efficiency with which viruses can infect their hosts. Mammalian orthoreovirus, a human enteric virus that infects most humans during childhood, is negatively affected by antibiotic treatment prior to infection. However, it is not known how components of the host microbiota affect reovirus infectivity. In this study, we show that reovirus virions directly interact with Gram positive and Gram negative bacteria. Reovirus interaction with bacterial cells conveys enhanced virion thermostability that translates into enhanced attachment and infection of cells following an environmental insult. Enhanced virion thermostability was also conveyed by bacterial envelope components lipopolysaccharide (LPS) and peptidoglycan (PG). Lipoteichoic acid and N-acetylglucosamine-containing polysaccharides enhanced virion stability in a serotype-dependent manner. LPS and PG also enhanced the thermostability of an intermediate reovirus particle (ISVP) that is associated with primary infection in the gut. Although LPS and PG alter reovirus thermostability, these bacterial envelope components did not affect reovirus utilization of its proteinaceous cellular receptor junctional adhesion molecule-A or cell entry kinetics. LPS and PG also did not affect the overall number of reovirus capsid proteins σ1 and σ3, suggesting their effect on virion thermostability is not mediated through altering the overall number of major capsid proteins on the virus. Incubation of reovirus with LPS and PG did not significantly affect the neutralizing efficiency of reovirus-specific antibodies. These data suggest that bacteria enhance reovirus infection of the intestinal tract by enhancing the thermal stability of the reovirus particle at a variety of temperatures through interactions between the viral particle and bacterial envelope components.

An Orchestra of Reovirus Receptors: Still Searching for the Conductor

Viruses are constantly engaged in a molecular arms race with the host, where efficient and tactical use of cellular receptors benefits critical steps in infection. Receptor use dictates initiation, establishment, and spread of viral infection to new tissues and hosts. Mammalian orthoreoviruses (reoviruses) are pervasive pathogens that use multiple receptors to overcome protective host barriers to disseminate from sites of initial infection and cause disease in young mammals. In particular, reovirus invades the central nervous system (CNS) with serotype-dependent tropism and disease. A single viral gene, encoding the attachment protein σ1, segregates with distinct patterns of CNS injury. Despite the identification and characterization of several reovirus receptors, host factors that dictate tropism via interaction with σ1 remain undefined. Here, we summarize the state of the reovirus receptor field and discuss open questions toward understanding how the reovirus attachment protein dictates CNS tropism.

The Loop Formed by Residues 340 to 343 of Reovirus μ1 Controls Entry-Related Conformational Changes

Reovirus particles are covered with 200 μ1/σ3 heterohexamers. Following attachment to cell surface receptors, reovirus is internalized by receptor-mediated endocytosis. Within the endosome, particles undergo a series of stepwise disassembly events. First, the σ3 protector protein is degraded by cellular proteases to generate infectious subviral particles (ISVPs). Second, the μ1 protein rearranges into a protease-sensitive conformation to generate ISVP*s and releases two virus-encoded peptides, μ1N and Φ. The released peptides promote delivery of the genome-containing core by perforating the endosomal membrane. Thus, to establish a productive infection, virions must be stable in the environment but flexible to disassemble in response to the appropriate cellular cue. The reovirus outer capsid is stabilized by μ1 intratrimer, intertrimer, and trimer-core interactions. As a consequence of ISVP-to-ISVP* conversion, neighboring μ1 trimers unwind and separate. Located within the μ1 jelly roll β barrel domain, which is a known regulator of ISVP* formation, residues 340 to 343 form a loop and have been proposed to facilitate viral entry. To test this idea, we generated recombinant reoviruses that encoded deletions within this loop (Δ341 and Δ342). Both deletions destabilized the outer capsid. Notably, Δ342 impaired the viral life cycle; however, replicative fitness was restored by an additional change (V403A) within the μ1 jelly roll β barrel domain. In the Δ341 and Δ342 backgrounds, V403A also rescued defects in ISVP-to-ISVP* conversion. Together, these findings reveal a new region that regulates reovirus disassembly and how perturbing a metastable capsid can compromise replicative fitness.IMPORTANCE Capsids of nonenveloped viruses are composed of protein complexes that encapsulate, or form a shell around, nucleic acid. The protein-protein interactions that form this shell must be stable to protect the viral genome but also sufficiently flexible to disassemble during cell entry. Thus, capsids adopt conformations that undergo rapid disassembly in response to a specific cellular cue. In this work, we identify a new region within the mammalian orthoreovirus outer capsid that regulates particle stability. Amino acid deletions that destabilize this region impair the viral replication cycle. Nonetheless, replicative fitness is restored by a compensatory mutation that restores particle stability. Together, this work demonstrates the critical balance between assembling virions that are stable and maintaining conformational flexibility. Any factor that perturbs this balance has the potential to block a productive infection.

The Orchestra of Reovirus Cell Entry

Purpose of Review

The ability of viruses to infect host cells is dependent on several factors including the availability of cell-surface receptors, antiviral state of cells, and presence of host factors needed for viral replication. Here, we review findings from in vitro and in vivo studies using mammalian orthoreovirus (reovirus) that have identified an intricate group of molecules and mechanisms used by the virus to attach and enter cells.

Recent Findings

Recent findings provide an improved mechanistic understanding of reovirus cell entry. Of special note is the identification of a cellular mediator of cell entry in neuronal and non-neuronal cells, the effect of cell entry on the outcome of infection and cytopathic effects on the host cell, and an improved understanding of the components that promote viral penetration of cellular membranes.

Summary

A mechanistic understanding of the interplay between host and viral factors has enhanced our view of how viruses usurp cellular processes during infection.

Viral Protein Kinetics of Piscine Orthoreovirus Infection in Atlantic Salmon Blood Cells

Piscine orthoreovirus (PRV) is ubiquitous in farmed Atlantic salmon (Salmo salar) and the cause of heart and skeletal muscle inflammation. Erythrocytes are important target cells for PRV. We have investigated the kinetics of PRV infection in salmon blood cells. The findings indicate that PRV causes an acute infection of blood cells lasting 1-2 weeks, before it subsides into persistence. A high production of viral proteins occurred initially in the acute phase which significantly correlated with antiviral gene transcription. Globular viral factories organized by the non-structural protein µNS were also observed initially, but were not evident at later stages. Interactions between µNS and the PRV structural proteins λ1, µ1, σ1 and σ3 were demonstrated. Different size variants of µNS and the outer capsid protein µ1 appeared at specific time points during infection. Maximal viral protein load was observed five weeks post cohabitant challenge and was undetectable from seven weeks post challenge. In contrast, viral RNA at a high level could be detected throughout the eight-week trial. A proteolytic cleavage fragment of the µ1 protein was the only viral protein detectable after seven weeks post challenge, indicating that this µ1 fragment may be involved in the mechanisms of persistent infection.