Selected tools to visualize membrane interactions.

In the past decade, we developed various fluorescence-based methods for monitoring membrane fusion, membrane docking, distances between membranes, and membrane curvature. These tools were mainly developed using liposomes as model systems, which allows for the dissection of specific interactions mediated by, for example, fusion proteins. Here, we provide an overview of these methods, including two-photon fluorescence cross-correlation spectroscopy and intramembrane Förster energy transfer, with asymmetric labelling of inner and outer membrane leaflets and the calibrated use of transmembrane energy transfer to determine membrane distances below 10 nm. We discuss their application range and their limitations using examples from our work on protein-mediated vesicle docking and fusion.

Screening and Functional Profiling of Small-Molecule HIV-1 Entry and Fusion Inhibitors.

HIV-1 entry and fusion with target cells is an important target for antiviral therapy. However, a few currently approved treatments are not effective as monotherapy due to the emergence of drug resistance. This consideration has fueled efforts to develop new bioavailable inhibitors targeting different steps of the HIV-1 entry process. Here, a high-throughput screen was performed of a large library of 100,000 small molecules for HIV-1 entry/fusion inhibitors, using a direct virus-cell fusion assay in a 384 half-well format. Positive hits were validated using a panel of functional assays, including HIV-1 specificity, cytotoxicity, and single-cycle infectivity assays. One compound-4-(2,5-dimethyl-pyrrol-1-yl)-2-hydroxy-benzoic acid (DPHB)-that selectively inhibited HIV-1 fusion was further characterized. Functional experiments revealed that DPHB caused irreversible inactivation of HIV-1 Env on cell-free virions and that this effect was related to binding to the third variable loop (V3) of the gp120 subunit of HIV-1 Env. Moreover, DPHB selectively inhibited HIV-1 strains that use CXCR4 or both CXCR4 and CCR5 co-receptors for entry, but not strains exclusively using CCR5. This selectivity was mapped to the gp120 V3 loop using chimeric Env glycoproteins. However, it was found that pure DPHB was not active against HIV-1 and that its degradation products (most likely polyanions) were responsible for inhibition of viral fusion. These findings highlight the importance of post-screening validation of positive hits and are in line with previous reports of the broad antiviral activity of polyanions.