Cell-based membrane fusion assays with viral fusion proteins for identification of entry inhibitors

The fusion of viral and cellular membranes is an important step in infections caused by enveloped viruses. Safe and rapid cell-based assay systems have been developed to identify inhibitors of various infections caused by enveloped viruses. In this review, we have described a cell-based membrane fusion assay and its application in drug screening. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and Middle East respiratory syndrome coronavirus (MERS-CoV) induce membrane fusion by cleavage of the N-terminal region of the fusion peptide in the Spike protein by cellular proteases. To quantify the membrane fusion mediated by SARS-CoV-2 and MERS-CoV, effector cells expressing the Spike protein and target cells expressing the receptor and host protease TMRPSS2 were used. Cell fusion caused by co-culture was measured by monitoring the signals from dual split proteins (DSPs) as the reporter. Using this assay system, nafamostat was identified as a potential inhibitor of the membrane fusion caused by MERS-CoV and SARS-CoV-2. Clinical trials of nafamostat are underway. Cell-based membrane fusion assays using DSPs are expected to be useful for the discovery of viral infection inhibitors and for the evaluation of neutralizing antibodies.