Lysosome-Associated Membrane Proteins Support the Furin-Mediated Processing of the Mumps Virus Fusion Protein.

Mumps virus (MuV), an enveloped RNA virus of the Paramyxoviridae family and the causative agent of mumps, affects the salivary glands and other glandular tissues as well as the central nervous system. The virus enters the cell by inducing the fusion of its envelope with the plasma membrane of the target cell. Membrane fusion is mediated by MuV envelope proteins: the hemagglutinin-neuraminidase and fusion (F) protein. Cleavage of the MuV F protein (MuV-F) into two subunits by the cellular protease furin is a prerequisite for fusion and virus infectivity. Here, we show that 293T (a derivative of HEK293) cells do not produce syncytia upon expression of MuV envelope proteins or MuV infection. This failure is caused by the inefficient MuV-F cleavage despite the presence of functional furin in 293T cells. An expression cloning strategy revealed that overexpression of lysosome-associated membrane proteins (LAMPs) confers on 293T cells the ability to produce syncytia upon expression of MuV envelope proteins. The LAMP family comprises the ubiquitously expressed LAMP1 and LAMP2, the interferon-stimulated gene product LAMP3, and the cell type-specific proteins. The expression level of the LAMP3 gene, but not of LAMP1 and LAMP2 genes, differed markedly between 293T and HEK293 cells. Overexpression of LAMP1, LAMP2, or LAMP3 allowed 293T cells to process MuV-F efficiently. Furthermore, these LAMPs were found to interact with both MuV-F and furin. Our results indicate that LAMPs support the furin-mediated cleavage of MuV-F and that, among them, LAMP3 may be critical for the process, at least in certain cells.IMPORTANCE The cellular protease furin mediates proteolytic cleavage of many host and pathogen proteins and plays an important role in viral envelope glycoprotein maturation. MuV, an enveloped RNA virus of the Paramyxoviridae family and an important human pathogen, enters the cell through the fusion of its envelope with the plasma membrane of the target cell. Membrane fusion is mediated by the viral attachment protein and the F protein. Cleavage of MuV-F into two subunits by furin is a prerequisite for fusion and virus infectivity. Here, we show that LAMPs support the furin-mediated cleavage of MuV-F. Expression levels of LAMPs affect the processing of MuV-F and MuV-mediated membrane fusion. Among LAMPs, the interferon-stimulated gene product LAMP3 is most critical in certain cells. Our study provides potential targets for anti-MuV therapeutics.

Disruption of the Dimer-Dimer Interaction of the Mumps Virus Attachment Protein Head Domain, Aided by an Anion Located at the Interface, Compromises Membrane Fusion Triggering.

Mumps virus (MuV), an enveloped negative-strand RNA virus belonging to the family Paramyxoviridae, enters the host cell through membrane fusion mediated by two viral envelope proteins, an attachment protein hemagglutinin-neuraminidase (MuV-HN) and a fusion (F) protein. However, how the binding of MuV-HN to glycan receptors triggers membrane fusion is not well understood. The crystal structure of the MuV-HN head domain forms a tetramer (dimer of dimers) like other paramyxovirus attachment proteins. In the structure, a sulfate ion (SO42-) was found at the interface between two dimers, which may be replaced by a hydrogen phosphate ion (HPO42-) under physiological conditions. The anion is captured by the side chain of a positively charged arginine residue at position 139 of one monomer each from both dimers. Substitution of alanine or lysine for arginine at this position compromised the fusion support activity of MuV-HN without affecting its cell surface expression, glycan-receptor binding, and interaction with the F protein. Furthermore, the substitution appeared to affect the tetramer formation of the head domain as revealed by blue native-PAGE analysis. These results, together with our previous similar findings with the measles virus attachment protein head domain, suggest that the dimer-dimer interaction within the tetramer may play an important role in triggering membrane fusion during paramyxovirus entry.IMPORTANCE Despite the use of effective live vaccines, mumps outbreaks still occur worldwide. Mumps virus (MuV) infection typically causes flu-like symptoms and parotid gland swelling but sometimes leads to orchitis, oophoritis, and neurological complications, such as meningitis, encephalitis, and deafness. MuV enters the host cell through membrane fusion mediated by two viral proteins, a receptor-binding attachment protein, and a fusion protein, but its detailed mechanism is not fully understood. In this study, we show that the tetramer (dimer of dimers) formation of the MuV attachment protein head domain is supported by an anion located at the interface between two dimers and that the dimer-dimer interaction plays an important role in triggering the activation of the fusion protein and causing membrane fusion. These results not only further our understanding of MuV entry but provide useful information about a possible target for antiviral drugs.

Structures of the prefusion form of measles virus fusion protein in complex with inhibitors.

Measles virus (MeV), a major cause of childhood morbidity and mortality, is highly immunotropic and one of the most contagious pathogens. MeV may establish, albeit rarely, persistent infection in the central nervous system, causing fatal and intractable neurodegenerative diseases such as subacute sclerosing panencephalitis and measles inclusion body encephalitis. Recent studies have suggested that particular substitutions in the MeV fusion (F) protein are involved in the pathogenesis by destabilizing the F protein and endowing it with hyperfusogenicity. Here we show the crystal structures of the prefusion MeV-F alone and in complex with the small compound AS-48 or a fusion inhibitor peptide. Notably, these independently developed inhibitors bind the same hydrophobic pocket located at the region connecting the head and stalk of MeV-F, where a number of substitutions in MeV isolates from neurodegenerative diseases are also localized. Since these inhibitors could suppress membrane fusion mediated by most of the hyperfusogenic MeV-F mutants, the development of more effective inhibitors based on the structures may be warranted to treat MeV-induced neurodegenerative diseases.

Molecular Mechanism of the Flexible Glycan Receptor Recognition by Mumps Virus.

Mumps virus (MuV) is an important aerosol-transmitted human pathogen causing epidemic parotitis, meningitis, encephalitis, and deafness. MuV preferentially uses a trisaccharide containing α2,3-linked sialic acid as a receptor. However, given the MuV tropism toward glandular tissues and the central nervous system, an additional glycan motif(s) may also serve as a receptor. Here, we performed a large-scale glycan array screen with MuV hemagglutinin-neuraminidase (MuV-HN) attachment proteins by using 600 types of glycans from The Consortium for Functional Glycomics Protein-Glycan Interaction Core in an effort to find new glycan receptor motif(s). According to the results of the glycan array, we successfully determined the crystal structures of MuV-HN proteins bound to newly identified glycan motifs, sialyl LewisX (SLeX) and the oligosaccharide portion of the GM2 ganglioside (GM2-glycan). Interestingly, the complex structures showed that SLeX and GM2-glycan share the same configuration with the reported trisaccharide motif, 3'-sialyllactose (3'-SL), at the binding site of MuV-HN, while SLeX and GM2-glycan have several unique interactions compared with those of 3'-SL. Thus, MuV-HN protein can allow an additional spatial modification in GM2-glycan and SLeX at the second and third carbohydrates from the nonreducing terminus of the core trisaccharide structure, respectively. Importantly, MuV entry was efficiently inhibited in the presence of 3'-SL, SLeX, or GM2-glycan derivatives, which indicates that these motifs can serve as MuV receptors. The α2,3-sialylated oligosaccharides, such as SLeX and 3'-sialyllactosamine, are broadly expressed in various tissues, and GM2 exists mainly in neural tissues and the adrenal gland. The distribution of these glycan motifs in human tissues/organs may have bearing on MuV tropism.IMPORTANCE Mumps virus (MuV) infection is characterized by parotid gland swelling and can cause pancreatitis, orchitis, meningitis, and encephalitis. MuV-related hearing loss is also a serious complication because it is usually irreversible. MuV outbreaks have been reported in many countries, even in high-vaccine-coverage areas. MuV has tropism toward glandular tissues and the central nervous system. To understand the unique MuV tropism, revealing the mechanism of receptor recognition by MuV is very important. Here, using a large-scale glycan array and X-ray crystallography, we show that MuV recognizes sialyl LewisX and GM2 ganglioside as receptors, in addition to a previously reported MuV receptor, a trisaccharide containing an α2,3-linked sialic acid. The flexible recognition of these glycan receptors by MuV may explain the unique tropism and pathogenesis of MuV. Structures will also provide a template for the development of effective entry inhibitors targeting the receptor-binding site of MuV.

Annexin A2 Mediates the Localization of Measles Virus Matrix Protein at the Plasma Membrane.

Annexins are a family of structurally related proteins that bind negatively charged membrane phospholipids in a Ca2+-dependent manner. Annexin A2 (AnxA2), a member of this family, has been implicated in a variety of cellular functions, including the organization of membrane domains, vesicular trafficking, and cell-cell adhesion. AnxA2 generally forms a heterotetrameric complex with a small Ca2+-binding protein, S100A10. Measles virus (MV), a member of the family Paramyxoviridae, is an enveloped virus with a nonsegmented negative-strand RNA genome. Knockdown of AnxA2 greatly reduced MV growth in cells without affecting its entry and viral RNA production. In MV-infected, AnxA2 knockdown cells, the expression level of the matrix (M) protein, but not other viral proteins, was reduced compared with that in control cells, and the distribution of the M protein at the plasma membrane was decreased. The M protein lines the inner surface of the envelope and plays an important role in virus assembly by connecting the nucleocapsid to the envelope proteins. The M protein bound to AnxA2 independently of AnxA2's phosphorylation or its association with S100A10 and was colocalized with AnxA2 within cells. Truncation of the N-terminal 10 amino acid residues, but not the N-terminal 5 residues, compromised the ability of the M protein to interact with AnxA2 and localize at the plasma membrane. These results indicate that AnxA2 mediates the localization of the MV M protein at the plasma membrane by interacting with its N-terminal region (especially residues at positions 6 to 10), thereby aiding in MV assembly.IMPORTANCE MV is an important human pathogen, still claiming ∼100,000 lives per year despite the presence of effective vaccines, and it causes occasional outbreaks even in developed countries. Replication of viruses largely relies on the functions of host cells. Our study revealed that the reduction of the host protein annexin A2 compromises the replication of MV within the cell. Further studies demonstrated that annexin A2 interacts with the MV M protein and mediates the localization of the M protein at the plasma membrane where MV particles are formed. The M protein lines the inner surface of the MV envelope membrane and plays a role in MV particle formation. Our results provide useful information for the understanding of the MV replication process and potential development of antiviral agents.

Trisaccharide containing α2,3-linked sialic acid is a receptor for mumps virus.

Mumps virus (MuV) remains an important pathogen worldwide, causing epidemic parotitis, orchitis, meningitis, and encephalitis. Here we show that MuV preferentially uses a trisaccharide containing α2,3-linked sialic acid in unbranched sugar chains as a receptor. Crystal structures of the MuV attachment protein hemagglutinin-neuraminidase (MuV-HN) alone and in complex with the α2,3-sialylated trisaccharide revealed that in addition to the interaction between the MuV-HN active site residues and sialic acid, other residues, including an aromatic residue, stabilize the third sugar of the trisaccharide. The importance of the aromatic residue and the third sugar in the MuV-HN-receptor interaction was confirmed by computational energy calculations, isothermal titration calorimetry studies, and glycan-binding assays. Furthermore, MuV-HN was found to bind more efficiently to unbranched α2,3-sialylated sugar chains compared with branched ones. Importantly, the strategically located aromatic residue is conserved among the HN proteins of sialic acid-using paramyxoviruses, and alanine substitution compromised their ability to support cell-cell fusion. These results suggest that not only the terminal sialic acid but also the adjacent sugar moiety contribute to receptor function for mumps and these paramyxoviruses. The distribution of structurally different sialylated glycans in tissues and organs may explain in part MuV's distinct tropism to glandular tissues and the central nervous system. In the crystal structure, the epitopes for neutralizing antibodies are located around the α-helices of MuV-HN that are not well conserved in amino acid sequences among different genotypes of MuV. This may explain the fact that MuV reinfection sometimes occurs.

Invasive myoepithelial carcinoma ex pleomorphic adenoma of the major salivary gland: two case reports.

BACKGROUND: Myoepithelial carcinoma (MEC) is a rare salivary gland tumor. Its long-term prognosis remains unknown because of the paucity of reported cases with long-term follow-up. Although some case series exist, the clinical features of MEC vary considerably depending on the site of origin. Therefore, accumulation of these rare cases is important. CASE PRESENTATION: Case 1: An 89-year-old man presented with a 10-year history of a mass originating from the right parotid gland and involving the neck. The mass grew rapidly for 3 months, reaching approximately 8 cm. There was no facial paralysis. MEC ex pleomorphic adenoma (PA) was suspected. Superficial parotid gland resection was performed in 2013; the tumor grade was pT3N0M0, and the resection margins were free of carcinoma. Because of several high-risk factors for metastasis (i.e., invasive carcinoma ex PA, high MIB1 index, and mutant p53 protein positivity), radiotherapy and chemotherapy were recommended as adjuvant therapy. Although the patient refused adjuvant therapy, he was recurrence-free at 36 months after surgery. Case 2: A 54-year-old woman presented with a >10-year history of a right submandibular mass, which grew rapidly for 1 year, reaching approximately 6 cm. Preoperative diagnosis was PA of the right submandibular gland. Submandibular gland resection was performed in 2013. Pathological analysis revealed invasive MEC ex PA, pT3N0M0; in addition, the carcinoma portion had an extra capsule and had invaded the platysma muscle close to the margin. An MIB1 index of 40 % and mutant p53 protein positivity indicated a high risk for metastasis. Additional resection and right neck dissection revealed no residual carcinoma. The patient refused adjuvant chemotherapy. One year after surgery, metastasis to the right pulmonary hilar node and both lungs were detected. Chemotherapy prevented recurrent growth of the lesion and extended survival. The patient was alive with cancer 30 months after the first surgery. CONCLUSIONS: High expression of the Ki67 labeling index might reflect prognosis of these cases. Chemotherapy for distant metastasis was effective, as expected. Further accumulation of cases and long follow-up data are needed to elucidate the pathophysiology and prognosis of MEC ex PA.

[A case of hypopharyngeal ulceration due to cytomegarovirus infection].

Cytomegalovirus (CMV) is a common opportunistic infection in immunosuppressive patients. One of the organs often involved is the gastrointestinal tract, but the pharynx, especially hypopharyngeal involvement, is rare. In this report, we present a case of a 61-year-old male compromised host, admitted to the hospital for the treatment of dermatomyositis, who presented with hypopharyngeal ulceration due to cytomegalovirus infection. Multiple deep ulcerative lesions with thick belaque, or with a protruding bony edge were observed in the adjacent areas in/around the hypopharynx, which repeatedly demonstrated exacerbation and remission in a relatively short period. The lesions developed acute bleeding and required emergency operative hemostasis. The ulcerations completely disappeared after the treatment with gancyclovir. We reviewed our case as well as other cases reported so far, and extracted the points we should be aware of when we encounter a case of hypopharyngeal ulceration based on cytomegalovirus infection; 1. acute bleeding, 2. paralysis of the vocal cords and subsequent breathing difficulty, 3. pharyngeal perforation.

Murine cochlear cell sorting and cell-type-specific organoid culture.

Neonatal mouse cochlear duct cells can proliferate and grow in vitro into inner ear organoids. Distinctive cochlear duct cell types have different organoid formation capacities. Here, we provide a flow cytometric cell-sorting method that allows the subsequent culture of individual cochlear cell populations. For the efficient culture of the sorted cells, we provide protocols for growing free-floating inner ear organoids, the adherence of organoids to a substrate, and the expansion of organoid-derived inner ear colonies. For complete details on the use and execution of this protocol, please refer to Kubota et al. (2021).

Unique Tropism and Entry Mechanism of Mumps Virus.

Mumps virus (MuV) is an important human pathogen that causes parotitis, orchitis, oophoritis, meningitis, encephalitis, and sensorineural hearing loss. Although mumps is a vaccine-preventable disease, sporadic outbreaks have occurred worldwide, even in highly vaccinated populations. MuV not only causes systemic infection but also has a unique tropism to glandular tissues and the central nervous system. In general, tropism can be defined by multiple factors in the viral life cycle, including its entry, interaction with host factors, and host-cell immune responses. Although the underlying mechanisms of MuV tropism remain to be fully understood, recent studies on virus-host interactions have provided insights into viral pathogenesis. This review was aimed at summarizing the entry process of MuV by focusing on the glycan receptors, particularly the recently identified receptors with a trisaccharide core motif, and their interactions with the viral attachment proteins. Here, we describe the receptor structures, their distribution in the human body, and the recently identified host factors for MuV and analyze their relationship with MuV tropism.

Greater epithelial ridge cells are the principal organoid-forming progenitors of the mouse cochlea.

In mammals, hearing loss is irreversible due to the lack of regenerative potential of non-sensory cochlear cells. Neonatal cochlear cells, however, can grow into organoids that harbor sensory epithelial cells, including hair cells and supporting cells. Here, we purify different cochlear cell types from neonatal mice, validate the composition of the different groups with single-cell RNA sequencing (RNA-seq), and assess the various groups' potential to grow into inner ear organoids. We find that the greater epithelial ridge (GER), a transient cell population that disappears during post-natal cochlear maturation, harbors the most potent organoid-forming cells. We identified three distinct GER cell groups that correlate with a specific spatial distribution of marker genes. Organoid formation was synergistically enhanced when the cells were cultured at increasing density. This effect is not due to diffusible signals but requires direct cell-to-cell contact. Our findings improve the development of cell-based assays to study culture-generated inner ear cell types.

Characterization of Natural and Synthetic Sialoglycans Targeting the Hemagglutinin-Neuraminidase of Mumps Virus.

The inhibition of surface viral glycoproteins offers great potential to hamper the attachment of viruses to the host cells surface and the spreading of viral infection. Mumps virus (MuV) is the etiological agent of the mumps infectious disease and causes a wide spectrum of mild to severe symptoms due to the inflammation of the salivary glands. Here we focus our attention on the hemagglutinin-neuraminidase (HN) isolated from MuV SBL-1 strain. We describe the molecular features of host sialoglycans recognition by HN protein by means of NMR, fluorescence assays and computational studies. Furthermore, we also describe the synthesis of a N-acetylneuraminic acid-derived thiotrisaccharide targeting the viral protein, and the corresponding 3D-complex. Our results provide the basis to improve the design and synthesis of potent viral hemagglutinin-neuraminidase inhibitors.

Large-Scale Expression and Purification of Mumps Virus Hemagglutinin-Neuraminidase for Structural Analyses and Glycan-Binding Assays.

Many viruses utilize cell-surface glycans as receptors for host cell entry. Viral surface glycoproteins specifically interact with glycan motifs, which strongly contributes to viral tropism. Recently, the interactions between host cell glycan receptors and the mumps virus (MuV) hemagglutinin-neuraminidase (MuV-HN) protein were characterized by determining the co-crystal structure of MuV-HN in complex with glycan receptors. Here, we describe protocols for large-scale expression, purification and crystallization of MuV-HN proteins for structural analyses and glycan-binding assays with the overarching goal of investigating glycan-protein interactions.

Structural basis for Glycan-receptor binding by mumps virus hemagglutinin-neuraminidase.

Mumps virus is one of the main cause of respiratory illnesses in humans, especially children. Among the viral surface glycoproteins, the hemagglutinin - neuraminidase, MuV-HN, plays key roles in virus entry into host cells and infectivity, thus representing an ideal target for the design of novel inhibitors. Here we report the detailed analysis of the molecular recognition of host cell surface sialylated glycans by the viral glycoprotein MuV-HN. By a combined use of NMR, docking, molecular modelling and CORCEMA-ST, the structural features of sialoglycans/MuV-HN complexes were revealed. Evidence for a different enzyme activity toward longer and complex substrates compared to unbranched ligands was also examined by an accurate NMR kinetic analysis. Our results provide the basis for the structure-based design of effective drugs against mumps-induced diseases.

Development of conductive hearing loss due to posterior semicircular canal dehiscence.

We herein report a case of posterior semicircular canal dehiscence (SCD) syndrome who had been audiologically followed up for eight years. The patient originally had sensorineural hearing loss. The audiogram had gradually transformed to pure conductive hearing loss. The posterior SCD was identified in CT scan. The reported case showed the possibility to distinguish the mechanism at play underlying the typical conductive hearing loss in SCD patients by tracing the transition of the hearing loss pattern. This information is of much help to predict the hearing outcomes if surgical intervention were chosen for the treatment.

Crystallization strategy for the glycoprotein-receptor complex between measles virus hemagglutinin and its cellular receptor SLAM.

Measles virus (MV), one of the most contagious agents, infects immune cells using the signaling lymphocyte activation molecule (SLAM) on the cell surface. A complex of SLAM and the attachment protein, hemagglutinin (MVH), has remained elusive due to the intrinsic handling difficulty including glycosylation. Furthermore, crystals obtained of this complex are either nondiffracting or poorly-diffracting. To solve this problem, we designed a systematic approach using a combination of the following techniques; (1) a transient expression system in HEK293SGnTI(-) cells, (2) lysine methylation, (3) structure-guided mutagenesis directed at better crystal packing, (4) Endo H treatment, (5) single-chain formation for stable complex, and (6) floating-drop vapor diffusion. Using our approach, the receptor-binding head domain of MV-H covalently fused with SLAM was successfully crystallized and diffraction was improved from 4.5 Å to a final resolution of 3.15 Å . These combinational methods would be useful as crystallization strategies for complexes of glycoproteins and their receptors.

Structure of the measles virus hemagglutinin bound to its cellular receptor SLAM.

Measles virus, a major cause of childhood morbidity and mortality worldwide, predominantly infects immune cells using signaling lymphocyte activation molecule (SLAM) as a cellular receptor. Here we present crystal structures of measles virus hemagglutinin (MV-H), the receptor-binding glycoprotein, in complex with SLAM. The MV-H head domain binds to a ß-sheet of the membrane-distal ectodomain of SLAM using the side of its ß-propeller fold. This is distinct from attachment proteins of other paramyxoviruses that bind receptors using the top of their ß-propeller. The structure provides templates for antiviral drug design, an explanation for the effectiveness of the measles virus vaccine, and a model of the homophilic SLAM-SLAM interaction involved in immune modulations. Notably, the crystal structures obtained show two forms of the MV-H-SLAM tetrameric assembly (dimer of dimers), which may have implications for the mechanism of fusion triggering.