Human metapneumovirus fusion protein triggering: Increasing complexities by analysis of new HMPV fusion proteins.

The human metapneumovirus (HMPV) fusion protein (F) mediates fusion of the viral envelope and cellular membranes to establish infection. HMPV F from some, but not all, viral strains promotes fusion only after exposure to low pH. Previous studies have identified several key residues involved in low pH triggering, including H435 and a proposed requirement for glycine at position 294. We analyzed the different levels of fusion activity, protein expression and cleavage of three HMPV F proteins not previously examined. Interestingly, low pH-triggered fusion in the absence of G294 was identified in one F protein, while a novel histidine residue (H434) was identified that enhanced low pH promoted fusion in another. The third F protein failed to promote cell-to-cell fusion, suggesting other requirements for F protein triggering. Our results demonstrate HMPV F triggering is more complex than previously described and suggest a more intricate mechanism for fusion protein function and activation.

Inhibition of Human Metapneumovirus Binding to Heparan Sulfate Blocks Infection in Human Lung Cells and Airway Tissues.

UNLABELLED: Human metapneumovirus (HMPV), a recently discovered paramyxovirus, infects nearly 100% of the world population and causes severe respiratory disease in infants, the elderly, and immunocompromised patients. We previously showed that HMPV binds heparan sulfate proteoglycans (HSPGs) and that HMPV binding requires only the viral fusion (F) protein. To characterize the features of this interaction critical for HMPV binding and the role of this interaction in infection in relevant models, we utilized sulfated polysaccharides, heparan sulfate mimetics, and occluding compounds. Iota-carrageenan demonstrated potent anti-HMPV activity by inhibiting binding to lung cells mediated by the F protein. Furthermore, analysis of a minilibrary of variably sulfated derivatives of Escherichia coli K5 polysaccharide mimicking the HS structure revealed that the highly O-sulfated K5 polysaccharides inhibited HMPV infection, identifying a potential feature of HS critical for HMPV binding. The peptide dendrimer SB105-A10, which binds HS, reduced binding and infection in an F-dependent manner, suggesting that occlusion of HS at the target cell surface is sufficient to prevent infection. HMPV infection was also inhibited by these compounds during apical infection of polarized airway tissues, suggesting that these interactions take place during HMPV infection in a physiologically relevant model. These results reveal key features of the interaction between HMPV and HS, supporting the hypothesis that apical HS in the airway serves as a binding factor during infection, and HS modulating compounds may serve as a platform for potential antiviral development. IMPORTANCE: Human metapneumovirus (HMPV) is a paramyxovirus that causes respiratory disease worldwide. It has been previously shown that HMPV requires binding to heparan sulfate on the surfaces of target cells for attachment and infection. In this study, we characterize the key features of this binding interaction using heparan sulfate mimetics, identify an important sulfate modification, and demonstrate that these interactions occur at the apical surface of polarized airway tissues. These findings provide insights into the initial binding step of HMPV infection that has potential for antiviral development.