Discriminating in vitro cell fusion from cell aggregation by flow cytometry combined with fluorescence resonance energy transfer

Cell-cell fusion in cancer: The next cancer hallmark?

In this review, we consider the role of cell-cell fusion in cancer development and progression through an evolutionary lens. We begin by summarizing the origins of fusion proteins (fusogens), of which there are many distinct classes that have evolved through convergent evolution. We then use an evolutionary framework to highlight how the persistence of fusion over generations and across different organisms can be attributed to traits that increase fitness secondary to fusion; these traits map well to the expanded hallmarks of cancer. By studying the tumor microenvironment, we can begin to identify the key selective pressures that may favor higher rates of fusion compared to healthy tissues. The paper concludes by discussing the increasing number of research questions surrounding fusion, recommendations for how to answer them, and the need for a greater interest in exploring cell fusion and evolutionary principles in oncology moving forward.

Precise Detection on Cell-Cell Fusion by a Facile Molecular Beacon-Based Method

Cell-cell fusion studies provide an experimental platform for evaluating disease progression and investigating cell infection. However, to realize sensitive and quantitative detection on cell-cell fusion is still a challenge. Herein, we report a facile molecular beacon (MB)-based method for precise detection on cell-cell fusion. By transfection of the spike protein (S protein) and enhanced green fluorescent protein (EGFP) in HEK 293 cells, the virus-mimicking fusogenic effector cells 293-S-EGFP cells were constructed to interact with target cells. Before mixing the effector cells with the target cells, the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) expression in 293-S-EGFP cells was silenced, and the MB for GAPDH mRNA detection was delivered into the GAPDH silenced 293-S-EGFP cells. Once cell-cell fusion occurred, MB migrated from the GAPDH silenced effector cells to the target cells and hybridized with GAPDH mRNA in the target cells to induce fluorescence emission. The cell-cell fusion can be easily visualized and quantitated by fluorescence microscopy and flow cytometry. The fluorescence intensity is strongly dependent on the number of fused target cells. This MB-based method can easily identify the differences in the cell fusions for various target cells with different angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2) expression levels, resulting in dramatically different fluorescence intensities in fused target cells. Our study provides a convenient and efficient quantitative detection approach to study cell-cell fusion.

Inhibition of HIV-1 envelope-dependent membrane fusion by serum antilymphocyte autoantibodies is associated with low plasma viral load

The HIV-1 envelope protein (Env) mediates the membrane fusion process allowing virus entry to target cells and the efficiency to induce membrane fusion is an important determinant of HIV-1 pathogenicity. In addition to virus receptors, other adhesion/signaling molecules on infected and target cells and virus particles can enhance fusion. The presence of antilymphocyte autoantibodies (ALA) in HIV patients' serum suggests that they may contribute to the inhibition of Env-mediated membrane fusion. Here, sera from 38 HIV-1 infected treatment-naïve men and 30 healthy donors were analyzed for the presence of IgG and IgM able to bind to CD4-negative Jurkat cells. The use of CD4-negative cells precluded the binding of virus-antibody immune complexes, and allowed detection of ALA different from anti-CD4 antibodies. IgG and IgM antibodies binding to Jurkat CD4-negative cells was detected in 74% and 84% of HIV-positive sera, respectively. Then, the activity of sera on fusion of CD4⁺ with HIV Env⁺ Jurkat cells was determined before and after their adsorption on CD4-negative Jurkat cells to remove ALA. Sera inhibited fusion at variable extents, and inhibitory activity decreased in 58% of serum samples after adsorption, indicating that ALA contributed to fusion inhibition in these sera (herein called fusion inhibitory ALA). The contribution of ALA to fusion inhibition in individual sera was highly variable, with an average of 33%. IgG purified from a pool of HIV⁺ sera inhibited fusion of primary CD4 T lymphocytes with Jurkat Env⁺, and adsorption of IgG on CD4-negative Jurkat cells diminished the fusion inhibitory activity. Thus, the inhibitory activity of sera was related to IgG ALA. Our observations suggest that fusion inhibitory ALA other than anti-CD4 antibodies may contribute significantly to the inhibition of Env-mediated cell-cell fusion. Fusion inhibitory ALA, but not total ALA levels, associated with low plasma viral loads, suggesting that specific ALA may participate in virus containment by inhibiting virus-cell fusion in a significant fraction of HIV-infected patients.

Quantification of the asymmetric migration of the lipophilic dyes, DiO and DiD, in homotypic co-cultures of chondrosarcoma SW-1353 cells

DiO and DiD are lipophilic cell labelling dyes used in the staining of cells in vivo and in vitro. The aim of the present study was to quantify the asymmetrical distribution of dyes in co-cultured cells and to measure the intercellular transfer of DiO and DiD. DiO and DiD were applied separately to stain two identical populations of SW-1353 human chondrosarcoma cells that were subsequently co-cultured (homotypic co-culture). The intercellular migration of dyes in the co-cultured cells was measured by flow cytometry and recorded under a fluorescent microscope. DiD and DiO caused no effect on the proliferation of cells, the degradation rate of the two dyes was comparable and crossover effects between dyes were negligible. The results of the present study suggested that asymmetrical intercellular migration of DiD and DiO was responsible for the asymmetrical distribution of these dyes in co-cultured cells. To take advantage of the lipophilic dyes migration in the double-stained co-cultured cells we suggest to apply mixed-dyes controls prior to the flow cytometric analysis. These controls are performed by staining cells with a 1:1 mix of the two dyes and would enable the estimation of the intensity of intercellular contact in co-culture systems. A 1:1 premix of DiO and DiD was applied to estimate cellular effect on intercellular exchange of lipid dyes in co-cultures incubated with cycloheximide and cytochalasin B. The cellular effect contributed 6–7% of intercellular migration of the lipophilic dyes, DiO and DiD. The majority of the observed intercellular transfer of these dyes was due to non-cellular, passive transfer.

FRET in the Analysis of In Vitro Cell-Cell Fusion by Flow Cytometry

Cell-cell fusion is a frequent event in nature leading to modification of cell fate. In this chapter, we describe a flow cytometric procedure for the quantitative assessment of in vitro cell-cell fusion events that allows the discrimination of fused from aggregated cells. The assay is based on the differential labeling of fusion partners with lipophilic fluorescent probes DiI (red) and DiO (green). Double fluorescent fused cells can be detected after coculturing by means of a flow cytometer equipped with a 488 nm laser. Fusion events can be distinguished from cell aggregates by the enhancement of the DiI red fluorescence intensity due to resonance energy transfer between the two probes occurring in the fused but not in the aggregated cell population.

Enhanced bactericidal potency of nanoliposomes by modification of the fusion activity between liposomes and bacterium

Background

Pseudomonas aeruginosa represents a good model of antibiotic resistance. These organisms have an outer membrane with a low level of permeability to drugs that is often combined with multidrug efflux pumps, enzymatic inactivation of the drug, or alteration of its molecular target. The acute and growing problem of antibiotic resistance of Pseudomonas to conventional antibiotics made it imperative to develop new liposome formulations to overcome these mechanisms, and investigate the fusion between liposome and bacterium.

Methods

The rigidity, stability and charge properties of phospholipid vesicles were modified by varying the cholesterol, 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE), and negatively charged lipids 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol sodium salt (DMPG), 1,2-dimyristoyl-sn-glycero-3-phopho-L-serine sodium salt (DMPS), 1,2-dimyristoyl-sn-glycero-3-phosphate monosodium salt (DMPA), nature phosphatidylserine sodium salt from brain and nature phosphatidylinositol sodium salt from soybean concentrations in liposomes. Liposomal fusion with intact bacteria was monitored using a lipid-mixing assay.

Results

It was discovered that the fluid liposomes-bacterium fusion is not dependent on liposomal size and lamellarity. A similar degree of fusion was observed for liposomes with a particle size from 100 to 800 nm. The fluidity of liposomes is an essential pre-request for liposomes fusion with bacteria. Fusion was almost completely inhibited by incorporation of cholesterol into fluid liposomes. The increase in the amount of negative charges in fluid liposomes reduces fluid liposomes-bacteria fusion when tested without calcium cations due to electric repulsion, but addition of calcium cations brings the fusion level of fluid liposomes to similar or higher levels. Among the negative phospholipids examined, DMPA gave the highest degree of fusion, DMPS and DMPG had intermediate fusion levels, and PI resulted in the lowest degree of fusion. Furthermore, the fluid liposomal encapsulated tobramycin was prepared, and the bactericidal effect occurred more quickly when bacteria were cultured with liposomal encapsulated tobramycin.

Conclusion

The bactericidal potency of fluid liposomes is dramatically enhanced with respect to fusion ability when the fusogenic lipid, DOPE, is included. Regardless of changes in liposome composition, fluid liposomes-bacterium fusion is universally enhanced by calcium ions. The information obtained in this study will increase our understanding of fluid liposomal action mechanisms, and help in optimizing the new generation of fluid liposomal formulations for the treatment of pulmonary bacterial infections.

New method to disaggregate and analyze single isolated helminthes cells using flow cytometry: proof of concept

In parasitology, particularly in helminthes studies, several methods have been used to look for the expression of specific molecules, such as RT-PCR, western blot, 2D-electrophoresis, and microscopy, among others. However, these methods require homogenization of the whole helminth parasite, preventing evaluation of individual cells or specific cell types in a given parasite tissue or organ. Also, the extremely high interaction between helminthes and host cells (particularly immune cells) is an important point to be considered. It is really hard to obtain fresh parasites without host cell contamination. Then, it becomes crucial to determine that the analyzed proteins are exclusively from parasitic origin, and not a consequence of host cell contamination. Flow cytometry is a fluorescence-based technique used to evaluate the expression of extra-and intracellular proteins in different type cells, including protozoan parasites. It also allows the isolation and recovery of single-cell populations. Here, we describe a method to isolate and obtain purified helminthes cells.

Decreased CD4 and wide-ranging expression of other immune receptors after HIV-envelope-mediated formation of syncytia in vitro

In human HIV infection, multinucleated cells (syncytia) are formed by fusion of HIV-infected cells with CD4+ cells. In order to examine possible functional implications of syncytia formation for the immune response, the expression of important surface molecules by T-cell syncytia and surrounding cells that remain unfused (bystander cells) was analyzed in cocultures of HIV-Env- and CD4-expressing E6 Jurkat T cells. Fusion partners were differentially labeled with lipophilic probes, and syncytia and bystander cells were identified by flow cytometry. The cellular phenotype and response to activation stimulus after fusion were analyzed with antibodies coupled to third-party fluorochromes. Cocultured unfused E6 cells showed a marked decrease in CD4 expression, suggesting the selective recruitment of cells strongly expressing CD4 into syncytia. However, the incorporated CD4 was not detected in the syncytia, whereas the range of expression of CD28, ICAM-1, CXCR4 and CD3 was wider than that of unfused cells. Limited expression of CD4 in the bystander unfused population, as well as in the newly formed syncytia, would result in limitation of further viral entry and a failure to identify these cells, and it could partially contribute to functional impairment and a decrease in the number of CD4+ T cells in AIDS. Most of the syncytia were viable and expressed CD25 and IL-2 in response to activation by phorbol myristate acetate (PMA) and ionomicyn. Thus, syncytia populations harboring widely heterogeneous levels of receptors would constitute a potential source of anomalous immune function.

Quantitative assays for cell fusion

Cell fusion would seem to be obviously recognizable upon visual inspection, and many studies employ a simple microscopic fusion index to quantify the rate and extent of fusion in cell culture. However, when cells are not in monolayers or when there is a large background of multinucleation through failed cytokinesis, cell-cell fusion can only be proven by mixing of cell contents. Furthermore, determination of the microscopic fusion index must generally be carried out manually, creating opportunities for unintended observer bias and limiting the numbers of cells assayed and therefore the statistical power of the assay. Strategies for making assays dependent on fusion and independent of visual observation are critical to increasing the accuracy and throughput of screens for molecules that control cell fusion. A variety of in vitro biochemical and nonbiochemical techniques have been developed to assay and monitor fusion events in cultured cells. In this chapter, we briefly discuss several in vitro fusion assays, nearly all based on systems of two components that interact to create a novel assayable signal only after cells fuse. We provide details for the use of one example of such a system, intracistronic complementation of beta-galactosidase activity by mutants of Escherichia coli lacZ, which allows for either cell-by-cell microscopic assay of cell fusion or quantitative and kinetic detection of cell fusions in whole populations. In addition, we describe a combination of gene knock-down protocols with this assay to study factors required for myoblast fusion.

In vitro cell fusion between CD4+ and HIV-1 Env+ T cells generates a diversity of syncytia varying in total number, size and cellular content

Syncytia formation in HIV infections is driven by the virus fusion-active molecules (Env) interacting with membrane components of hosts cells. HIV-syncytia are usually interpreted as pathogenic entities and although they may potentially vary in size, numbers and types of constituent cells, little is known about the extent and significance of their diversity. Here, we describe numerically the cell population dynamics and the diversity of syncytia produced in the in vitro cell-fusion between two Jurkat T cell lines, one CD4(+) and the other Env(+). Cell-fusion partners were differentially stained with the lipophilic DiI and DiO, or with the cytoplasmic CMFDA and CMTMR tracers and syncytia showing double fluorescence were counted in a flow cytometer. The total number of syncytia formed, their size, cellular complexity and ratio of CD4(+)/Env(+) cells recruited, varied significantly in relation with time of reaction and initial proportions of fusion partners. The considerable structural diversity of syncytia formed, in so limited an in vitro cell fusion reaction, suggests that a greater heterogeneity may be formed in the natural course of disease. Identification of the main determinants of syncytia diversity allows for a detailed study of the relation between the syncytia structure and function.