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Immunology and Infection

A High-throughput Cre-Lox Activated Viral Membrane Fusion Assay to Identify Inhibitors of HIV-1 Viral Membrane Fusion

Published: August 14th, 2018

DOI:

10.3791/58074

1Division of Infectious Diseases, Department of Medicine, Icahn School of Medicine at Mount Sinai, Immunology Institute, 2Department of Biology, New Jersey City University

We describe a cell-based assay to report on HIV-1 fusion via the expression of green fluorescent protein detectable by flow cytometry or fluorescence microscopy. It can be used to test inhibitors of viral entry (specifically at the fusion step) in cell-free and cell-to-cell infection systems.

This assay is designed to specifically report on HIV-1 fusion via the expression of green fluorescent protein (GFP) detectable by flow cytometry or fluorescence microscopy. An HIV-1 reporter virus (HIV-1 Gag-iCre) is generated by inserting Cre recombinase into the HIV-1 genome between the matrix and the capsid proteins of the Gag polyprotein. This results in a packaging of Cre recombinase into virus particles, which is then released into a target cell line stably expressing a Cre recombinase-activated red fluorescent protein (RFP) to GFP switch cassette. In the basal state, this cassette expresses RFP only. Following the delivery of Cre recombinase into the target cell, the RFP, flanked by loxP sites, excises, resulting in GFP expression. This assay can be used to test any inhibitors of viral entry (specifically at the fusion step) in cell-free and cell-to-cell infection systems and has been used to identify a class of purinergic receptor antagonists as novel inhibitors of HIV-1 viral membrane fusion.

The need for novel antiretroviral therapies has prompted the developement of high-throughput screens for inhibitors of HIV-1 entry. The Gag-iCre reporter assay is developed to identify inhibitors of viral entry at the fusion step in a cell-to-cell infection system by specifically measuring viral membrane fusion with the host cell membrane1. An assay was developed to screen for novel inhibitors that acted specifically at the early stages of HIV-1 infection up to the point of viral membrane fusion. One challenge to measuring cell-cell infection is that the initial inoculum contains infected donor cells and ininfected target ce....

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1. Generation of Target Cell Lines

NOTE: This step is optional; if using an existing RG target cell line, start at step 2.

  1. Cotransfect a 70% confluent 10 cm plate of 293T cells11 [in 10 mL of Dulbecco's modified Eagle's medium (DMEM) with 10% fetal bovine serum (FBS)] with pMSCV-loxp-dsRed-loxp-eGFP-Puro-WPRE10 and pCL-10A112 (packaging plasmid) in a 1:1 ratio (20 µg total) using calcium-based tran.......

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Uninfected RG Jurkat Cells exhibit a low level of background GFP signal (0.3%) with a very strong RFP signal (Figure 2A, uninfected column). The infection with Gag-iCre causes an increase in GFP signal (24.9%,) while the presence of the HIV-1 fusion inhibitor AMD3100 (20 µM) inhibits the development of this signal, bringing it down to uninfected background levels (0.34%). When an inhibitor of a post-fusion event such as the reverse transcription inhibito.......

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The Gag-iCre assay has proven to be very useful for screening drug candidates which may inhibit the fusion step of viral replication. When performing this assay, the most important steps to get a good signal are similar to most viral infection assays. The first critical step is producing high titers of good-quality virus. This step requires that the 293T cells are passaged frequently (at least 1x every 48 h) so they do not become overconfluent and clump together. Additionally, it may be worth experimenting with different.......

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This research was supported by grants NIH/NIAID AI112423 and NIH/NIGMS GM113885 to Benjamin K. Chen and NIH/NIAID K08-AI120806 to Talia H. Swartz. We would like to thank the Icahn School of Medicine at Mount Sinai Dean's flow Cytometry CORE.

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Name Company Catalog Number Comments
Dulbecco's Modified Eagle's Medium (DMEM) Sigma Aldrich D5546 Media for 293 Cells
RPMI-1640 Sigma Aldrich R0883 Media for Jurkats
Fetal Bovine Serum Albumin Gibco 16140-071 Serum for 293 Cells
Cosmic Calf Serum Hyclone SH30087.03 Serum for Jurkat Cells
Hyclone Pennecillin Streptomycin solution GE Healthcare Life sciences SV30010 Penn/Strep used in both media
T75 Flasks Corning 3073 Used for Culture of Jurkat Cells
10cm Tissue Culture Plates Corning 430167 Used for Culture of 293 Cells
96 Well Plates (tissue culture Treated) Corning 3595 Used for fusion assay
Polyjet Transfection Reagent Signagen SL100688 Used to transfect 293 Cells
Dulbecco's Phosphate Buffered Saline Sigma Aldrich D8537-100ML Used in wash steps
Hyclone Trypsin Protease GE Healthcare Life sciences SH30042.01 For Trypsinization of 293 cells
Amaxa Cell Line Nucleofector Kit V Lonza VACA-1003 For Nucleofection of Jurkats
Ficoll-Paque plus GE Healthcare Life sciences 17144002 For Nucleofection of Jurkats
Serological Pipettes Fisher Brand 13-678-11E For all tissue culture
Pipettor Tips Denville Scientific P3020-CPS For all tissue culture and liquid handling steps
Millex Syringe Filter (0.45 micron) Millipore SLHA033 For filtration of virus
BD Slip Tip Sterile syringes BD Diagnostics 309656 For filtration of virus
Amaxa Nucleofector Lonza 2b for Nucleofection of Jurkats (various models available)
BD Fortessa Flow Cytometer BD Biosciences for flow cytometry analyss of samples
Tissue Culture Hood Various models Fortessa 2
pMSCV-loxp-dsRed-loxp-eGFP-Puro-W PRE Addgene 32702 Koo BK et al. Controlled gene expression in primary Lgr5 organoid cultures. Nature Methods 9:81-83 (2011).
pCL10a1 Novus Bio NBP2-29542 Naviaux, RK, Costanzi, E, Haas, M and Verma, I. The pCL vector system: Rapid production of helper-free, high titer, recombinant retroviruses. Journal of Virology 70: 5701-5705 (1996)
Gag-iCre Benjamin Chen Lab Esposito AM et al. A high throughput Cre-lox activated viral membrane fusion assay identifies pharmacological inhibitors of HIV entry. Virology 490:6-16 (2016).

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  10. Koo, B. K., et al. Controlled gene expression in primary Lgr5 organoid cultures. Nature Methods. 9, 81-83 (2011).
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  12. Naviaux, R. K., Costanzi, E., Haas, M., Verma, I. The pCL vector system: Rapid production of helper-free, high titer, recombinant retroviruses. Journal of Virology. 70, 5701-5705 (1996).
  13. Kingston, R. E., Chen, C. A., Okayama, H. Calcium Phosphate Transfection. Current Protocols in Immunology. 10, 10-13 (2001).

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