Broadly Applicable, Virus-Free Dual Reporter Assay to Identify Compounds Interfering with Membrane Fusion: Performance for HSV-1 and SARS-CoV-2

Membrane fusion constitutes an essential step in the replication cycle of numerous viral pathogens, hence it represents an important druggable target. In the present study, we established a virus-free, stable reporter fusion inhibition assay (SRFIA) specifically designed to identify compounds interfering with virus-induced membrane fusion. The dual reporter assay is based on two stable Vero cell lines harboring the third-generation tetracycline (Tet3G) transactivator and a bicistronic reporter gene cassette under the control of the tetracycline responsive element (TRE3G), respectively. Cell–cell fusion by the transient transfection of viral fusogens in the presence of doxycycline results in the expression of the reporter enzyme secreted alkaline phosphatase (SEAP) and the fluorescent nuclear localization marker EYFPNuc. A constitutively expressed, secreted form of nanoluciferase (secNLuc) functioned as the internal control. The performance of the SRFIA was tested for the quantification of SARS-CoV-2- and HSV-1-induced cell–cell fusion, respectively, showing high sensitivity and specificity, as well as the reliable identification of known fusion inhibitors. Parallel quantification of secNLuc enabled the detection of cytotoxic compounds or insufficient transfection efficacy. In conclusion, the SRFIA reported here is well suited for high-throughput screening for new antiviral agents and essentially will be applicable to all viral fusogens causing cell–cell fusion in Vero cells.