NgR1 binding to reovirus reveals an unusual bivalent interaction and a new viral attachment protein

Nogo-66 receptor 1 (NgR1) binds a variety of structurally dissimilar ligands in the adult central nervous system to inhibit axon extension. Disruption of ligand binding to NgR1 and subsequent signaling can improve neuron outgrowth, making NgR1 an important therapeutic target for diverse neurological conditions such as spinal crush injuries and Alzheimer's disease. Human NgR1 serves as a receptor for mammalian orthoreovirus (reovirus), but the mechanism of virus-receptor engagement is unknown. To elucidate how NgR1 mediates cell binding and entry of reovirus, we defined the affinity of interaction between virus and receptor, determined the structure of the virus-receptor complex, and identified residues in the receptor required for virus binding and infection. These studies revealed that central NgR1 surfaces form a bridge between two copies of viral capsid protein σ3, establishing that σ3 serves as a receptor ligand for reovirus. This unusual binding interface produces high-avidity interactions between virus and receptor to prime early entry steps. These studies refine models of reovirus cell-attachment and highlight the evolution of viruses to engage multiple receptors using distinct capsid components.

Paired immunoglobulin-like receptor B is an entry receptor for mammalian orthoreovirus

Mammalian orthoreovirus (reovirus) infects most mammals and is associated with celiac disease in humans. In mice, reovirus infects the intestine and disseminates systemically to cause serotype-specific patterns of disease in the brain. To identify receptors conferring reovirus serotype-dependent neuropathogenesis, we conducted a genome-wide CRISPRa screen and identified paired immunoglobulin-like receptor B (PirB) as a receptor candidate. Ectopic expression of PirB allowed reovirus binding and infection. PirB extracelluar D3D4 region is required for reovirus attachment and infectivity. Reovirus binds to PirB with nM affinity as determined by single molecule force spectroscopy. Efficient reovirus endocytosis requires PirB signaling motifs. In inoculated mice, PirB is required for maximal replication in the brain and full neuropathogenicity of neurotropic serotype 3 (T3) reovirus. In primary cortical neurons, PirB expression contributes to T3 reovirus infectivity. Thus, PirB is an entry receptor for reovirus and contributes to T3 reovirus replication and pathogenesis in the murine brain.

The Murine Neuronal Receptor NgR1 Is Dispensable for Reovirus Pathogenesis

Engagement of host receptors is essential for viruses to enter target cells and initiate infection. Expression patterns of receptors in turn dictate host range, tissue tropism, and disease pathogenesis during infection. Mammalian orthoreovirus (reovirus) displays serotype-dependent patterns of tropism in the murine central nervous system (CNS) that are dictated by the viral attachment protein σ1. However, the receptor that mediates reovirus CNS tropism is unknown. Two proteinaceous receptors have been identified for reovirus, junctional adhesion molecule A (JAM-A) and Nogo-66 receptor 1 (NgR1). Engagement of JAM-A is required for reovirus hematogenous dissemination but is dispensable for neural spread and infection of the CNS. To determine whether NgR1 functions in reovirus neuropathogenesis, we compared virus replication and disease in wild-type (WT) and NgR1-/- mice. Genetic ablation of NgR1 did not alter reovirus replication in the intestine or transmission to the brain following peroral inoculation. Viral titers in neural tissues following intramuscular inoculation, which provides access to neural dissemination routes, also were comparable in WT and NgR1-/- mice, suggesting that NgR1 is dispensable for reovirus neural spread to the CNS. The absence of NgR1 also did not alter reovirus replication, neural tropism, and virulence following direct intracranial inoculation. In agreement with these findings, we found that the human but not the murine homolog of NgR1 functions as a receptor and confers efficient reovirus binding and infection of nonsusceptible cells in vitro. Thus, neither JAM-A nor NgR1 is required for reovirus CNS tropism in mice, suggesting that other unidentified receptors support this function. IMPORTANCE Viruses engage diverse molecules on host cell surfaces to navigate barriers, gain cell entry, and establish infection. Despite discovery of several reovirus receptors, host factors responsible for reovirus neurotropism are unknown. Human NgR1 functions as a reovirus receptor in vitro and is expressed in CNS neurons in a pattern overlapping reovirus tropism. We used mice lacking NgR1 to test whether NgR1 functions as a reovirus neural receptor. Following different routes of inoculation, we found that murine NgR1 is dispensable for reovirus dissemination to the CNS, tropism and replication in the brain, and resultant disease. Concordantly, expression of human but not murine NgR1 confers reovirus binding and infection of nonsusceptible cells in vitro. These results highlight species-specific use of alternate receptors by reovirus. A detailed understanding of species- and tissue-specific factors that dictate viral tropism will inform development of antiviral interventions and targeted gene delivery and therapeutic viral vectors.

Altered Glycan Expression on Breast Cancer Cells Facilitates Infection by T3 Seroptype Oncolytic Reovirus

Breast cancer is the most common cancer in women. Although current therapies have increased survival rates for some breast cancer types, other aggressive invasive breast cancers remain difficult to treat. As the onset of breast cancer is often associated with the appearance of extracellular markers, these could be used to better target therapeutic agents. Here, we demonstrated by nanobiophysical approaches that overexpression of α-sialylated glycans in breast cancer provides an opportunity to combat cancer cells with oncolytic reoviruses. Notably, a correlation between cellular glycan expression and the mechanical properties of reovirus attachment and infection is observed in a serotype-dependent manner. Furthermore, we enhance the infectivity of reoviruses in malignant cells by the coinjection of α-sialylated glycans. In conclusion, this study supports both the use of reoviruses as an oncolytic agent in nanomedicine and the role of α-sialylated glycans as adjuvants in oncolysis, offering new perspective in oncolytic cancer therapy.

Reovirus directly engages integrin to recruit clathrin for entry into host cells

Reovirus infection requires the concerted action of viral and host factors to promote cell entry. After interaction of reovirus attachment protein σ1 with cell-surface carbohydrates and proteinaceous receptors, additional host factors mediate virus internalization. In particular, β1 integrin is required for endocytosis of reovirus virions following junctional adhesion molecule A (JAM-A) binding. While integrin-binding motifs in the surface-exposed region of reovirus capsid protein λ2 are thought to mediate integrin interaction, evidence for direct β1 integrin-reovirus interactions and knowledge of how integrins function to mediate reovirus entry is lacking. Here, we use single-virus force spectroscopy and confocal microscopy to discover a direct interaction between reovirus and β1 integrins. Comparison of interactions between reovirus disassembly intermediates as well as mutants and β1 integrin show that λ2 is the integrin ligand. Finally, using fluidic force microscopy, we demonstrate a functional role for β1 integrin interaction in promoting clathrin recruitment to cell-bound reovirus. Our study demonstrates a direct interaction between reovirus and β1 integrins and offers insights into the mechanism of reovirus cell entry. These results provide new perspectives for the development of efficacious antiviral therapeutics and the engineering of improved viral gene delivery and oncolytic vectors.

Confocal Microscopy of Reovirus Transport in Living Dorsal Root Ganglion Neurons

Neurotropic reoviruses repurpose host machinery to traffic over long distances in neuronal processes and access distal replication sites. Understanding mechanisms of neuronal transmission is facilitated by using simplified in vitro primary neuronal culture models. Advances in the design of compartmentalized microfluidic devices lend robustness to neuronal culture models by enabling compartmentalization and manipulation of distinct neuronal processes. Here, we describe a streamlined methodology to culture sensory neurons dissociated from dorsal root ganglia of embryonic rats in microfluidic devices. We further describe protocols to exogenously label reovirus and image, track, and analyze transport of single reovirus particles in living neurons. These techniques can be adapted to study directed axonal transport of other neurotropic viruses and neuronal factors involved in signaling and pathology.

Reovirus uses macropinocytosis-mediated entry and fast axonal transport to infect neurons

Several barriers protect the central nervous system (CNS) from pathogen invasion. Yet viral infections of the CNS are common and often debilitating. Understanding how neurotropic viruses co-opt host machinery to overcome challenges to neuronal entry and transmission is important to combat these infections. Neurotropic reovirus disseminates through neural routes and invades the CNS to cause lethal encephalitis in newborn animals. To define mechanisms of reovirus neuronal entry and directional transport, we used primary neuron cultures, which reproduce in vivo infection patterns displayed by different reovirus serotypes. Treatment of neurons with small-molecule inhibitors of different endocytic uptake pathways allowed us to discover that the cellular machinery mediating macropinocytosis is required for reovirus neuronal entry. This mechanism of reovirus entry differs from clathrin-mediated endocytosis, which is used by reovirus to invade non-neuronal cells. Analysis of reovirus transport and release from isolated soma or axonal termini of neurons cultivated in microfluidic devices indicates that reovirus is capable of retrograde but only limited anterograde neuronal transmission. The dynamics of retrograde reovirus movement are consistent with fast axonal transport coordinated by dynein along microtubules. Further analysis of viral transport revealed that multiple virions are transported together in axons within non-acidified vesicles. Reovirus-containing vesicles acidify after reaching the soma, where disassembly of virions and release of the viral core into the cytoplasm initiates replication. These results define mechanisms of reovirus neuronal entry and transport and establish a foundation to identify common host factors used by neuroinvasive viruses. Furthermore, our findings emphasize consideration of cell type-specific entry mechanisms in the tailored design of neurotropic viruses as tracers, oncolytic agents, and delivery vectors.

Viral infections of the central nervous system (CNS) cause a significant health burden globally and compel a better mechanistic understanding of neural invasion by viruses to develop effective interventions. Neurotropic reovirus disseminates through neural routes to infect the CNS and serves as a tractable model to study neural invasion by viruses. Despite knowledge of reovirus neurotropism for decades, mechanisms mediating reovirus neuronal infection remain undefined. We used primary neurons cultured in microfluidic devices to study entry and directional transport of reovirus. We discovered that reovirus uses macropinocytosis for neuronal entry as opposed to the use of a clathrin-mediated pathway in non-neuronal cells. We are unaware of another virus using macropinocytosis to enter neurons. Following internalization, reovirus spreads in the retrograde direction using dynein-mediated fast axonal transport but exhibits limited anterograde spread. We further demonstrate that reovirus disassembly and replication occur in the neuronal soma subsequent to axonal transport. Remarkably, these entry and transport mechanisms mirror those used by misfolded proteins implicated in neurodegenerative diseases. Our findings establish the mechanics of reovirus neuronal uptake and spread and provide clues about therapeutic targets to limit neuropathology inflicted by pathogens and misfolded proteins.

Glycan-mediated enhancement of reovirus receptor binding

Viral infection is an intricate process that requires the concerted action of both viral and host cell components. Entry of viruses into cells is initiated by interactions between viral proteins and their cell surface receptors. Despite recent progress, the molecular mechanisms underlying the multistep reovirus entry process are poorly understood. Using atomic force microscopy, we investigated how the reovirus σ1 attachment protein binds to both α-linked sialic acid (α-SA) and JAM-A cell-surface receptors. We discovered that initial σ1 binding to α-SA favors a strong multivalent anchorage to JAM-A. The enhanced JAM-A binding by virions following α-SA engagement is comparable to JAM-A binding by infectious subvirion particles (ISVPs) in the absence of α-SA. Since ISVPs have an extended σ1 conformer, this finding suggests that α-SA binding triggers a conformational change in σ1. These results provide new insights into the function of viral attachment proteins in the initiation of infection and open new avenues for the use of reoviruses as oncolytic agents.

Reovirus infection triggers inflammatory responses to dietary antigens and development of celiac disease

Viral infections have been proposed to elicit pathological processes leading to the initiation of T helper 1 (T_H1) immunity against dietary gluten and celiac disease (CeD). To test this hypothesis and gain insights into mechanisms underlying virus-induced loss of tolerance to dietary antigens, we developed a viral infection model that makes use of two reovirus strains that infect the intestine but differ in their immunopathological outcomes. Reovirus is an avirulent pathogen that elicits protective immunity, but we discovered that it can nonetheless disrupt intestinal immune homeostasis at inductive and effector sites of oral tolerance by suppressing peripheral regulatory T cell (pT_reg) conversion and promoting T_H1 immunity to dietary antigen. Initiation of T_H1 immunity to dietary antigen was dependent on interferon regulatory factor 1 and dissociated from suppression of pT_reg conversion, which was mediated by type-1 interferon. Last, our study in humans supports a role for infection with reovirus, a seemingly innocuous virus, in triggering the development of CeD.

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.

Dual mechanisms regulate the recruitment of spindle assembly checkpoint proteins to the budding yeast kinetochore

Quantitative knowledge of the recruitment of spindle assembly checkpoint (SAC) proteins by the kinetochore is essential to understanding the mechanisms that regulate protein recruitment and hence the strength of the SAC. Here this recruitment is quantified, and novel mechanisms are identified that strongly modulate SAC protein recruitment by the kinetochore.

Recruitment of spindle assembly checkpoint (SAC) proteins by an unattached kinetochore leads to SAC activation. This recruitment is licensed by the Mps1 kinase, which phosphorylates the kinetochore protein Spc105 at one or more of its six MELT repeats. Spc105 then recruits the Bub3-Bub1 and Mad1-Mad2 complexes, which produce the inhibitory signal that arrests cell division. The strength of this signal depends, in part, on the number of Bub3-Bub1 and Mad1-Mad2 molecules that Spc105 recruits. Therefore regulation of this recruitment will influence SAC signaling. To understand this regulation, we established the physiological binding curves that describe the binding of Bub3-Bub1 and Mad1-Mad2 to the budding yeast kinetochore. We find that the binding of both follows the mass action law. Mps1 likely phosphorylates all six MELT repeats of Spc105. However, two mechanisms prevent Spc105 from recruiting six Bub3-Bub1 molecules: low Bub1 abundance and hindrance in the binding of more than one Bub3-Bub1 molecule to the same Spc105. Surprisingly, the kinetochore recruits two Mad1-Mad2 heterotetramers for every Bub3-Bub1 molecule. Finally, at least three MELT repeats per Spc105 are needed for accurate chromosome segregation. These data reveal that kinetochore-intrinsic and -extrinsic mechanisms influence the physiological operation of SAC signaling, potentially to maximize chromosome segregation accuracy.

Plasticity and epigenetic inheritance of centromere-specific histone H3 (CENP-A)-containing nucleosome positioning in the fission yeast

Nucleosomes containing the specific histone H3 variant CENP-A mark the centromere locus on each chromatin and initiate kinetochore assembly. For the common type of regional centromeres, little is known in molecular detail of centromeric chromatin organization, its propagation through cell division, and how distinct organization patterns may facilitate kinetochore assembly. Here, we show that in the fission yeast S. pombe, a relatively small number of CENP-A/Cnp1 nucleosomes are found within the centromeric core and that their positioning relative to underlying DNA varies among genetically homogenous cells. Consistent with the flexible positioning of Cnp1 nucleosomes, a large portion of the endogenous centromere is dispensable for its essential activity in mediating chromosome segregation. We present biochemical evidence that Cnp1 occupancy directly correlates with silencing of the underlying reporter genes. Furthermore, using a newly developed pedigree analysis assay, we demonstrated the epigenetic inheritance of Cnp1 positioning and quantified the rate of occasional repositioning of Cnp1 nucleosomes throughout cell generations. Together, our results reveal the plasticity and the epigenetically inheritable nature of centromeric chromatin organization.

The budding yeast point centromere associates with two Cse4 molecules during mitosis

The centromere is defined by the incorporation of the centromere-specific histone H3 variant centromere protein A (CENP-A). Like histone H3, CENP-A can form CENP-A-H4 heterotetramers in vitro. However, the in vivo conformation of CENP-A chromatin has been proposed by different studies as hemisomes, canonical, or heterotypic nucleosomes. A clear understanding of the in vivo architecture of CENP-A chromatin is important, because it influences the molecular mechanisms of the assembly and maintenance of the centromere and its function in kinetochore nucleation. A key determinant of this architecture is the number of CENP-A molecules bound to the centromere. Accurate measurement of this number can limit possible centromere architectures. The genetically defined point centromere in the budding yeast Saccharomyces cerevisiae provides a unique opportunity to define this number accurately, as this 120-bp-long centromere can at the most form one nucleosome or hemisome. Using novel live-cell fluorescence microscopy assays, we demonstrate that the budding yeast centromere recruits two Cse4 (ScCENP-A) molecules. These molecules are deposited during S phase and they remain stably bound through late anaphase. Our studies suggest that the budding yeast centromere incorporates a Cse4-H4 tetramer.

Conformational dynamics of single pre-mRNA molecules during in vitro splicing

The spliceosome is a complex small nuclear RNA (snRNA)-protein machine that removes introns from pre-mRNAs via two successive phosphoryl transfer reactions. The chemical steps are isoenergetic, yet splicing requires at least eight RNA-dependent ATPases responsible for substantial conformational rearrangements. To comprehensively monitor pre-mRNA conformational dynamics, we developed a strategy for single-molecule FRET (smFRET) that uses a small, efficiently spliced yeast pre-mRNA, Ubc4, in which donor and acceptor fluorophores are placed in the exons adjacent to the 5' and 3' splice sites. During splicing in vitro, we observed a multitude of generally reversible time- and ATP-dependent conformational transitions of individual pre-mRNAs. The conformational dynamics of branchpoint and 3'-splice site mutants differ from one another and from wild type. Because all transitions are reversible, spliceosome assembly appears to be occurring close to thermal equilibrium.