Exogenous peptide and protein expression levels using retroviral vectors in human cells

Isolation of a peptide inhibitor of human rhinovirus

Cell culture-based transdominant genetic techniques provide new methods for discovering peptide/RNA modulators of cellular pathways. We applied this technology to isolate a peptide inhibitor of human rhinovirus. A green fluorescent protein (GFP)-scaffolded library of cDNA fragments was expressed in HeLa cells from a retroviral vector and screened for inhibitors of rhinovirus-mediated cell killing. A DNA clone, I421, increased cell survival in an HRV14 challenge assay from less than 0.5% to greater than 60%. It encodes a 53-amino-acid C-terminal extension of the GFP scaffold. Particular subclones of Hela cells expressing I421 (exemplified by I421dp3) show a delay in virus production and a 50-fold decrease in viral RNA levels at 6-8 h postinfection. HRV2, HRV14, and HRV16 show a dramatic decrease in plaque-forming ability on I421dp3 while Coxsackievirus B3 showed a small reduction. Levels of ICAM-1, the receptor for the main rhinovirus serotype, are not altered in I421dp3.

Genetic selection for modulators of a retinoic-acid-responsive reporter in human cells

We used a genetic screening methodology, a human cell line bearing a retinoic-acid-responsive enhanced GFP reporter, and a flow sorter to recover dominant modulators of reporter expression. Four inducers and three suppressors that were fused to the C terminus of a protein scaffold for stability were isolated and their mechanisms of action studied. Mutagenesis experiments indicated that six of these dominant agents exerted their effects at the protein level. The single cDNA coding fragment that was isolated comprised the central 64-amino-acid section of human cyclophilin B, which contained its peptidyl-prolyl isomerase domain; this cyclophilin fragment repressed expression of the retinoic-acid-responsive reporter. The remaining clones encoded peptides shorter than 30 amino acids unrelated to known gene open reading frames. Genetic epistasis studies between the strongest inducer, R3, and a dominant-negative mutant of RARalpha suggest that the two factors function in the same pathway. Transcript microarray analyses suggest that R3 induced a subset of the retinoid-responsive genes in melanoma cells. Finally, yeast two-hybrid assays and co-immunoprecipitation studies of human cell extracts identified PAT1 as a protein that interacts with R3.

Enhanced expression of exogenous genes from retroviruses using HIV2/Tat

The capacity to produce large amounts of protein in mammalian cells is important in several contexts, including large-scale generation of biologically useful proteins, gene therapy, and transdominant genetics in cultured cells. For transdominant genetics, retroviral vectors are especially useful for delivery of expression libraries. However, even the potent CMV promoter is often unable to stimulate single-copy production of protein beyond the 1 microM level. We have adapted the HIV2/Tat expression system to retroviral vectors to boost expression above levels attainable with CMV promoters. We show that the system produces protein levels in four cell types tested which exceed levels attained by wild-type CMV or modified CMV promoters. In one cell line, the increase is 10-fold above CMV. Coupled with a stable expressed protein, levels of about 4 microM can be produced from presumptive single-copy retroviral transductants, and 30 microM from multicopy transductants.

Rapid screening for high-titer retroviral packaging cell lines using an in situ fluorescence assay

The production of high-titer recombinant retrovirus is a major determinant of the efficiency of target cell transduction. Titer assessment for producer clones that contain vectors encoding proteins that can be detected using fluorescence is typically performed by flow cytometry. However, this method is both costly and labor intensive, severely limiting the number of clones that can be screened for each construct. In this report we describe a rapid, high-throughput screening method for viral quantitation of producer clone supernatant on target cells using a 96-well format. Plates were assayed using a multichannel fluorescent reader to determine the percentage of target cells expressing green (EGFP), cyan (ECFP), yellow (EYFP) or red (DsRed) fluorescent reporter genes, or their combinations. The relative fluorescence counts of target cells incubated with viral supernatant from each packaging cell clone correlated with the level of transduction, and hence, viral titer. Correlation of cell fluorescence between the fluorescent plate reader assay and flow cytometric assessment was high (r(2) = 0.96). Independent detection of different fluorescent reporters enabled multiplex assays to be performed. Simultaneous cell density analysis using alamarBlue fluorescence was proportional to cell number per well (r(2) = 1.0). In situ titer assessment of 66 FLYRD packaging cells encoding the EGFP reporter gene identified clones (>10(7) colony forming units per milliliter [CFU/ml]) that provided titers up to sevenfold over the parent population. The application of this rapid, high-throughput screening method overcomes many limitations imposed by the current flow cytometric screening method. This robust assay maximizes the chance of identifying rare high-titer packaging clones and offers a further opportunity to optimize gene transfer protocols.

An automated system for screening retroviral expression constructs in microplate format

Retroviruses are useful for genetics studies to deliver genes that express proteins, peptides, and RNAs. Several steps, including DNA preparation, transfection, packaging, transduction, and assay, are required to execute screens using retroviral constructs. Unlike screens with purified components, whole-cell assays using retroviral constructs need a large number of steps with microplate manipulations. The nature of these steps, especially the involvement of cultured mammalian cells, limits the throughput of such screens. To improve the efficiency of genetic experiments with retroviral expression vectors, an automated system for retroviral screening in microplates was devised and tested. The system, called Somata, provides high throughputs and robust, reproducible performance.

Stable expression of Anthozoa fluorescent proteins in mammalian cells

BACKGROUND

Fluorescent proteins have become invaluable reporters in many areas of cellular and developmental biology. An enhanced version of the Aequorea victoria green fluorescent protein (AvEGFP) is the most widely used fluorescent protein. For a variety of reasons, it is useful to have alternative fluorescent proteins to AvEGFP.

METHODS

The cDNA sequences for enhanced variants of the Anemonia cyan fluorescent protein (AmCyan1), as well as the Zoanthus green (ZsGreen1) and yellow (ZsYellow1) fluorescent proteins, were cloned downstream of a constitutive cytomegalovirus (CMV) promoter within a retroviral expression vector. NIH3T3, HEK293, SW620, and WM35 cells were transduced with recombinant retroviruses at a low multiplicity of infection (MOI) to bias for single-copy integration. Both unselected and stably selected cells transduced with the retroviral expression constructs were characterized. Expression of each fluorescent protein in cells was detected using flow cytometry and fluorescence microscopy with filter sets typically used for AvEGFP/fluorescein isothiocyanate (FITC) detection and was compared with the expression of AvEGFP. In addition, a fluorescence plate reader with several excitation and emission filter sets was used for detection.

RESULTS

Expression of each protein was observable by fluorescence microscopy. Under given conditions of flow cytometry, the ZsGreen1 mean fluorescence was approximately 3-fold, 10-fold, and 50-fold greater than that of AvEGFP, ZsYellow1, and AmCyan1, respectively. AmCyan1, ZsGreen1, and AvEGFP were detected by a fluorescence plate reader.

CONCLUSION

We determined that fluorescent proteins from Anthozoa species are detectable using a standard flow cytometer and fluorescence microscope. All of the mammalian cell lines tested expressed detectable levels of fluorescent proteins from stable integrated provirus. In cell lines where the AvEGFP protein is toxic or poorly expressed, these Anthozoa fluorescent proteins may serve as alternative fluorescent reporters.