Figure 1. Cellular electrical impedance as a measure of viral induced cell fusion.

(A) Assay schematic. Cells are grown in microtitre plates (96- or 384-well) with embedded gold microelectrodes. Viral fusion proteins expressed on the surface of infected or transfected cells are triggered to undergo a conformational change following receptor binding or exposure to low pH depending on viral fusion class. This conformational change initially involves the formation of a short-lived, elongated intermediate that collapses into a trimeric hairpin, which drives the merger of neighboring cell membranes (dengue virus E protein mediated membrane fusion is schematically shown in the lower inset). (B) SEM of cells growing on microelectrodes. Cells were fixed and imaged when infected cells showed maximum cell impedance. The large syncytia that accompany cell fusion are clearly visible in both DENV and RSV infected cells (C6/36 insect cells and BEAS2B human bronchial airway epithelial cells, respectively). (C) Maximum cell index, CI (bars represent SE; N = 3), following infection of cells by RSV and DENV or transient expression of recombinant VSV G. show significant membrane fusion for all three fusion classes. (D–F) Raw, real-time output of impedance measurements over the time course of the fusion assay reveals quantitative fusion detection that correlates with live cell microscopy (lower panels) for all three classes of viral fusion proteins. Microscopy based assays were performed with C6/36 cells for DENV and imaged in bright field, while RSV and VSV fusion were visualized using fluorescent microcopy of a mixed population of stable COS-7 cells expressing GFP and mCherry, fusion is visualised as an overlap of the GFP and mCherry signals. Measurements were performed on infected (RSV and DENV) cells and cells transiently expressing recombinant VSV-G. Maximal cell impedance for DENV and VSV G was only observed following treatment of cells with low pH.