Significance: Despite the availability of an effective vaccine, mumps virus (MuV) still causes outbreaks even in highly vaccinated populations worldwide. MuV affects the salivary glands, testicles, ovaries, pancreas, meninges, and brain. This characteristic tropism to glandular tissues and the central nervous system remains to be explained. Here, using X-ray crystallography, computational energy calculations, isothermal titration calorimetry, glycan-binding assays, and MuV glycoprotein-mediated cell fusion assays, we show that the trisaccharide containing an α2,3-linked sialic acid on the cell surface acts as a receptor for MuV. In conjunction with elucidation of the distribution of structurally different sialylated glycans in tissues and organs, this finding will lead to a better understanding of MuV tropism and pathogenesis.

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.

Significance: Measles remains a major cause of childhood morbidity and mortality worldwide, and no licensed therapeutic agents are currently available. Measles virus (MeV) sometimes invades the central nervous system (CNS), causing neurodegenerative diseases several months/years after acute infection. Recently, MeV-induced encephalitis among human immunodeficiency virus (HIV)-infected children has become a great concern in high–HIV-prevalence countries. The tropism of MeV for the CNS is mediated by hyperfusogenic mutations in the MeV fusion (F) protein. Here we show the crystal structures of MeV-F alone and bound to inhibitors. The structures and accompanying cell-based fusion assays with inhibitors provide insight into the molecular mechanism for the inhibition of MeV-mediated fusion, which would help us conquer MeV-induced neurodegenerative diseases.

Annexins are a family of structurally related proteins that bind negatively charged membrane phospholipids in a Ca ²⁺ -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 Ca ²⁺ -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.

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 Lewis ^X 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.