Analysis of free intracellular calcium by flow cytometry: multiparameter and pharmacologic applications

Flow cytometry offers numerous advantages over traditional techniques for measuring intracellular Ca(2+) in lymphoid and nonlymphoid cells. In particular, the heterogeneity of cell responses can be defined by flow cytometry, and multiparameter analyses permit the determination of intracellular Ca(2+) in surface-marker-defined target cells as well as correlation of changes in Ca(2+) with other biochemical markers, including ligand binding. This article presents several established methods for measuring intracellular Ca(2+) by flow cytometry in lymphoid and nonlymphoid cells. Examples are provided for determination of Ca(2+) in human peripheral blood leukocytes and two human epithelial cell lines grown in monolayer. In addition, applications are reviewed or presented for correlating changes in intracellular Ca(2+) with other cell parameters, including cell cycle analysis, changes in cell membrane integrity, and the induction of apoptosis markers. Finally, a number of novel sample handling capabilities useful for performing kinetic analyses of Ca(2+) changes by flow cytometry are now available and one application is presented which is finding utility in pharmacologic studies.     

Flow cytometry: basic principles and applications

Flow cytometry is a sophisticated instrument measuring multiple physical characteristics of a single cell such as size and granularity simultaneously as the cell flows in suspension through a measuring device. Its working depends on the light scattering features of the cells under investigation, which may be derived from dyes or monoclonal antibodies targeting either extracellular molecules located on the surface or intracellular molecules inside the cell. This approach makes flow cytometry a powerful tool for detailed analysis of complex populations in a short period of time. This review covers the general principles and selected applications of flow cytometry such as immunophenotyping of peripheral blood cells, analysis of apoptosis and detection of cytokines. Additionally, this report provides a basic understanding of flow cytometry technology essential for all users as well as the methods used to analyze and interpret the data. Moreover, recent progresses in flow cytometry have been discussed in order to give an opinion about the future importance of this technology.     

Flow Cytometry Pulse Width Data Enables Rapid and Sensitive Estimation of Biomass Dry Weight in the Microalgae Chlamydomonas reinhardtii and Chlorella vulgaris

Dry weight biomass is an important parameter in algaculture. Direct measurement requires weighing milligram quantities of dried biomass, which is problematic for small volume systems containing few cells, such as laboratory studies and high throughput assays in microwell plates. In these cases indirect methods must be used, inducing measurement artefacts which vary in severity with the cell type and conditions employed. Here, we utilise flow cytometry pulse width data for the estimation of cell density and biomass, using Chlorella vulgaris and Chlamydomonas reinhardtii as model algae and compare it to optical density methods. Measurement of cell concentration by flow cytometry was shown to be more sensitive than optical density at 750 nm (OD₇₅₀) for monitoring culture growth. However, neither cell concentration nor optical density correlates well to biomass when growth conditions vary. Compared to the growth of C. vulgaris in TAP (tris-acetate-phosphate) medium, cells grown in TAP + glucose displayed a slowed cell division rate and a 2-fold increased dry biomass accumulation compared to growth without glucose. This was accompanied by increased cellular volume. Laser scattering characteristics during flow cytometry were used to estimate cell diameters and it was shown that an empirical but nonlinear relationship could be shown between flow cytometric pulse width and dry weight biomass per cell. This relationship could be linearised by the use of hypertonic conditions (1 M NaCl) to dehydrate the cells, as shown by density gradient centrifugation. Flow cytometry for biomass estimation is easy to perform, sensitive and offers more comprehensive information than optical density measurements. In addition, periodic flow cytometry measurements can be used to calibrate OD₇₅₀ measurements for both convenience and accuracy. This approach is particularly useful for small samples and where cellular characteristics, especially cell size, are expected to vary during growth.     

Cellometer image cytometry as a complementary tool to flow cytometry for verifying gated cell populations

Traditionally, many cell-based assays that analyze cell populations and functionalities have been performed using flow cytometry. However, flow cytometers remain relatively expensive and require highly trained operators for routine maintenance and data analysis. Recently, an image cytometry system has been developed by Nexcelom Bioscience (Lawrence, MA, USA) for automated cell concentration and viability measurement using bright-field and fluorescent imaging methods. Image cytometry is analogous to flow cytometry in that gating operations can be performed on the cell population based on size and fluorescent intensity. In addition, the image cytometer is capable of capturing bright-field and fluorescent images, allowing for the measurement of cellular size and fluorescence intensity data. In this study, we labeled a population of cells with an enzymatic vitality stain (calcein-AM) and a cell viability dye (propidium iodide) and compared the data generated by flow and image cytometry. We report that measuring vitality and viability using the image cytometer is as effective as flow cytometric assays and allows for visual confirmation of the sample to exclude cellular debris. Image cytometry offers a direct method for performing fluorescent cell-based assays but also may be used as a complementary tool to flow cytometers for aiding the analysis of more complex samples.     

Cell cycle analysis of synchronized chinese hamster cells using bromodeoxyuridine labeling and flow cytometry

Summary Chinese hamster ovary cells were synchronized into purified populations of viable G1-, S-, G2-, and M-phase cells by a combination of methods, including growth arrest, aphidicolin block, cell cycle progression, mitotic shake-off, and centrifugal elutriation. The DNA content and bromodeoxyuridine (BrdUrd) labeling index were measured in each purified fraction by dual-parameter flow cytometry. The cell cycle distributions determined from the DNA measurements alone (single parameter) were compared with those calculated from both DNA and BrdUrd data (dual parameter). The results show that highly purified cells can be obtained using these methods, but the assessed purity depends on the method of cell cycle analysis. Using the single versus dual parameter measurement to determine cell cycle distributions gave similar results for most phases of the cell cycle, except for cells near the transition from G1- to S-phase and S- to G2-phase. There the BrdUrd labeling index determined by flow cytometry was more sensitive for detecting small amounts of DNA synthesis. As an alternative to flow cytometry, a simple method of measuring BrdUrd labeling index on cell smears was used and gave the same result as flow cytometry. Measuring both DNA content and DNA synthesis improves characterization of synchronized cell populations, especially at the transitions in and out of S-phase, when cells are undergoing dramatic shifts in biochemical activity.     

Binding kinetics of soluble ligands to transmembrane proteins: comparing an optical biosensor and dynamic flow cytometry

BACKGROUND: The kinetics of protein-protein interactions can be monitored with optical biosensors based on the principles of either surface plasmon resonance or mirror resonance. These methods are straightforward for soluble proteins, but not for proteins inserted in the plasma membrane. METHODS: We monitored with an IASys biosensor system, based on a resonant mirror: (1) the binding of cells to an immobilized ligand, (2) the binding of a soluble ligand to immobilized cells, and (3) the binding of a soluble ligand to immobilized plasma membrane vesicles. For comparison, the kinetics of fluorescent antibody binding to intact cells were measured by dynamic flow cytometry. RESULTS: With an optical biosensor, the useful configuration is the one based on immobilized plasma membrane vesicles. However, signals can be detected only for very abundant binding sites (>10(6) per cell). Dynamic flow cytometry allows the accurate determination of the k(on) and k(off) of antibody binding. The sensitivity of the method is two orders of magnitude better than with an optical biosensor. CONCLUSIONS: Although biosensors constitute a method of choice for measuring the interactions between soluble proteins, they are not well suited for measuring the interaction between soluble proteins and membrane-embedded proteins. On the contrary, flow cytometry is well suited for such an application, when it is used in a dynamic mode.     

A rapid method for flow cytometric cell counting and cell cycle analysis from monolayer cultures of tumor cells.

A rapid, simple, and reliable flow cytometric method using the histochemical fluorescent stain Hoechst 33342 in presence of the non-ionic detergent Triton X-100 has been reported. The processing of melanoma cell cultures to get nuclei stained with the fluorescent dye was accomplished in one step and within an hour permitted concurrent flow cytometric measurement of cell density and cell cycle analysis. The preparation is stable for more than three weeks at room temperature for flow cytometry. The histograms are reproducible and exhibit a coefficient of variation of less than 2.5% (G1 peak). The cell density measurements varied within +/- 5% limits.     

Single cell analysis using surface enhanced Raman scattering (SERS) tags

Fluorescence is a mainstay of bioanalytical methods, offering sensitive and quantitative reporting, often in multiplexed or multiparameter assays. Perhaps the best example of the latter is flow cytometry, where instruments equipped with multiple lasers and detectors allow measurement of 15 or more different fluorophores simultaneously, but increases beyond this number are limited by the relatively broad emission spectra. Surface enhanced Raman scattering (SERS) from metal nanoparticles can produce signal intensities that rival fluorescence, but with narrower spectral features that allow a greater degree of multiplexing. We are developing nanoparticle SERS tags as well as Raman flow cytometers for multiparameter single cell analysis of suspension or adherent cells. SERS tags are based on plasmonically active nanoparticles (gold nanorods) whose plasmon resonance can be tuned to give optimal SERS signals at a desired excitation wavelength. Raman resonant compounds are adsorbed on the nanoparticles to confer a unique spectral fingerprint on each SERS tag, which are then encapsulated in a polymer coating for conjugation to antibodies or other targeting molecules. Raman flow cytometry employs a high resolution spectral flow cytometer capable of measuring the complete SERS spectra, as well as conventional flow cytometry measurements, from thousands of individual cells per minute. Automated spectral unmixing algorithms extract the contributions of each SERS tag from each cell to generate high content, multiparameter single cell population data. SERS-based cytometry is a powerful complement to conventional fluorescence-based cytometry. The narrow spectral features of the SERS signal enables more distinct probes to be measured in a smaller region of the optical spectrum with a single laser and detector, allowing for higher levels of multiplexing and multiparameter analysis.     

Monitoring growth phase-related changes in phosphatidylcholine-specific phospholipase C production,adhesion properties and physiology of <Emphasis Type="Italic">Bacillus cereus</Emphasis> vegetative cells

The physiological status and metabolic heterogeneity of Bacillus cereus cells within a culture during an 8-h batch fermentation process was measured using flow cytometry (FCM). Concurrently, production of the toxin, PC-PLC, and the extent of cell adhesion of live and dead cells were monitored using novel fluorescent assays. Flow cytometry analysis detected growth phase-related changes in the physiological profiles of cells over the course of the fermentation, with variation in the percentage of cells displaying membrane damage and intracellular esterase and redox activities. As the exponential phase proceeded, populations became more uniform in terms of protein content as measured using FCM in tandem with a cell tracking dye, with the majority of cells becoming membrane intact, esterase positive and redox active. PC-PLC activity appeared strongly related to cell density. Permeabilisation of cells was accompanied by a loss in adherent properties, while 25–100% of cells with intracellular esterase activity possessed adhesion properties. Cells in late exponential phase appeared to have reduced adherence properties compared to cells in early exponential or lag phase. As well as demonstrating the utility of FCM for measuring heterogeneity in terms of cell physiological status throughout the course of batch cultures, the methods utilised in this study could be used to relate processes such as toxin production or cell adhesion to cell physiological state.     

Direct evidence of intercellular sharing of glutathione via metabolic cooperation

Intracellular glutathione (GSH) content and cell density are known to be two important determinants of cell sensitivity to free radicals and radiation. We have investigated intercellular sharing of GSH via metabolic cooperation (MC) by measuring the GSH content of Chinese hamster V79 cells under conditions that varied MC among cells. GSH was measured by flow cytometry with monochlorobimane, which becomes fluorescent after conjugation to GSH by GSH-S-transferase. High-performance liquid chromatography was used to confirm the accuracy of GSH measurements by flow cytometry. Several lines of evidence indicate sharing of GSH or its precursor gamma-glutamylcysteine via MC. These include a cell density-dependent heterogeneity in GSH content, reconstitution of GSH in GSH-depleted cells by coculture with nondepleted cells (except when the depleted cells were MC deficient), and decreased equilibration of GSH among GSH-depleted cells and nondepleted cells when an inhibitor of MC (phorbol myristate acetate) was present. The equilibration of GSH among GSH-depleted cells and nondepleted cells in coculture was not inhibitable by acivicin, suggesting that this form of intercellular sharing of GSH does not rely on gamma-glutamyltransferase-mediated extracellular transport of GSH.     

Cell cycle synchronization of animal cells and nuclei by centrifugal elutriation

Synchronization of cells and nuclei is a powerful technique for the exact study of regulatory mechanisms and for understanding cell cycle events. Counterflow centrifugal elutriation is a biophysical cell separation technique in which cell size and sedimentation density differences of living cells are exploited to isolate subpopulations in various stages of cell cycle. Here, a protocol is described for the separation of phase-enriched subpopulations from exponentially growing Chinese hamster ovary cells at high-resolution power of elutriation. The efficiency of elutriation is confirmed by measuring the DNA content fluorimetrically and by flow cytometry. The resolution power of elutriation is demonstrated by the ability to fractionate nuclei of murine pre-B cells. The installation and elutriation by collecting 16-30 synchronized fractions, including particle size analysis, can be achieved in 4-5 h.     

Scanning cytometry with a LEAP: laser-enabled analysis and processing of live cells in situ.

BACKGROUND: Scanning cytometry now has many of the features (and power) of multiparameter flow cytometry while keeping its own advantages as an imaging technology. Modern instruments combine capabilities of scanning cytometry with the ability to manipulate cells. A new technology, called LEAP (laser-enabled analysis and processing), offers a unique combination of capabilities in cell purification and selective macromolecule delivery (optoinjection). METHODS: LEAP-mediated cell purification and optoinjection effects were assessed in model experiments using adherent and suspension cell types and cell mixtures plated and processed at different densities. Optoinjection effects were visualized by delivering fluorescent dextrans into cells. Results were analyzed using the LEAP instrument's own imaging system as well as by fluorescence and confocal microscopy. RESULTS: Live cell samples (adherent and suspension) could be purified to 90-100% purity with 50-90% yield, causing minimal cell damage depending on the cell type and plating density. Nearly one hundred percent of the targeted cells of all cell types examined could be successfully optoinjected with dextrans of 3-70 kDa, causing no visual damage to the cells. Indirect optoinjection effects were observed on untargeted cells within 5-60 microm to targeted areas under conditions used here. CONCLUSIONS: LEAP provides solutions in cell purification and targeted macromolecule delivery for traditional and challenging applications where other methods fall short.     

Flow cytometric analysis of electronic nuclear volume and DNA content in normal mouse tissues

Light scatter is used in flow cytometry for identification of cells based on their size and/or granularity. However, forward light scatter is not an accurate measure of cell size. The measurement of Electronic Volume (EV) by Coulter principle is more accurate. However, EV cannot be measured on most of the commercially available flow cytometers. We have described the development and applications of a flow cytometer that can simultaneously measure Electronic Nuclear Volume (ENV) and DNA content. In the present study we have used a commercially available NPE Quanta for measuring EV and DNA content of different normal mice tissues. Fresh/frozen or formalin fixed-paraffin embedded tissues from mice were processed for isolation of nuclei, which were then analyzed for EV versus DNA content. By using these two parameters, distinct sub-populations were identified in liver, thymus, small intestine and bone marrow. Dual parametric analysis of EV versus DNA content can be a valuable technique for identification of sub-populations in heterogeneous cell mixtures such as those of complex tissues like bone marrow, intestine and tumors. The methods established are rapid and can provide valuable data for identification and characterization of sub-populations for cell cycle analysis by flow cytometry.     

Flow Cytometric Analysis of Electronic Nuclear Volume and DNA Content in Normal Mouse Tissues

Light scatter is used in flow cytometry for identification of cells based on their size and/or granularity. However, forward light scatter is not an accurate measure of cell size. The measurement of Electronic Volume (EV) by Coulter principle is more accurate. However, EV cannot be measured on most of the commercially available flow cytometers. We have described the development and applications of a flow cytometer that can simultaneously measure Electronic Nuclear Volume (ENV) and DNA content. In the present study we have used a commercially available NPE QuantaTm for measuring EV and DNA content of different normal mice tissues.Fresh/frozen or formalin fixed-paraffin embedded tissues from mice were processed for isolation of nuclei, which were then analyzed for EV versus DNA content. By using these two parameters, distinct sub-populations were identified in liver, thymus, small intestine and bone marrow. Dual parametric analysis of EV versus DNA content can be a valuable technique for identification of sub-populations in heterogeneous cell mixtures such as those of complex tissues like bone marrow, intestine and tumors. The methods established are rapid and can provide valuable data for identification and characterization of sub-populations for cell cycle analysis by flow cytometry.     

Cell biologists sort things out: Analysis and purification of intracellular organelles by flow cytometry

Flow cytometry was established originally for measuring DNA content and for the analysis of cell-surface markers in combination with cell sorting. During the past two decades, it has added new dimensions to various areas of immunology and medicine. Increased sensitivity and precision of flow cytometers, accompanied by the development of new fluorescent dyes and probes, has led to new applications in molecular cell biology and genetics. This article focuses on applications of flow cytometry in analysis and sorting of intracellular organelles.     

Quantification of cell hybridoma yields with confocal microscopy and flow cytometry

The fusion of antigen presenting and cancer cells leads to the formation of hybrid cells, which are considered a potential vaccine for treating cancer. The quality assessment of hybrid cell vaccines is crucial for the introduction of this new treatment. Flow cytometry was the method used recently, since it is faster in comparison to classical microscopy. Here we describe a rapid confocal microscopy based approach to quantify hybrid cell yields. The extent of fusion rate was determined by confocal microscopy by counting dual fluorescent cells and by measuring the area of co-localized pixels. Results of both methods showed high degree of correlation. The same samples were also analyzed by flow cytometry. Fusion rates determined with both techniques showed significant correlation. In conclusion, using confocal microscopy we developed a sensitive and a rapid method to assess the yield of hybridomas in a large number of electrofused cells.     

Comparison of methods for assessing integrity of equine sperm membranes

Sperm membrane integrity (SMI) is thought to be an important measure of stallion sperm quality. The objective was to compare three methods for evaluating SMI: flow cytometry using SYBR-14/propidium iodide (PI) stain; an automated cell counting device using PI stain; and eosin-nigrosin stain. Raw equine semen was subjected to various treatments containing 20 to 80% seminal plasma in extender, with differing sperm concentrations, to simulate spontaneous loss of SMI. The SMI was assessed immediately, and after 1 and 2 d of cooled storage. Agreement between methods was determined according to Bland-Altman methodology. Eosin-nigrosin staining yielded higher (2%) overall mean values for SMI than did flow cytometry. Flow cytometry yielded higher (6%) overall mean values for SMI than did the automated cell counter. As percentage of membrane-damaged sperm increased, agreement of SMI measurement between methods decreased. When semen contained 50-79% membrane-intact sperm, the 95% limits of agreement between SMI determined by flow cytometry and eosin-nigrosin staining were greater (range = −26.9 to 24.3%; i.e., a 51.2% span) than for SMI determined by flow cytometry and the automated cell counter (range = −3.1 to 17.0%; 20.1% span). When sperm populations contained <50% membrane-intact sperm, the 95% limits of agreement between SMI determined by flow cytometry and eosin-nigrosin staining were greater (range = −35.9 to 19.0%; 54.9% span) than for SMI determined by flow cytometry and the automated cell counter (range = −11.6 to 28.7%; 40.3% span). We concluded that eosin-nigrosin staining assessments of percent membrane-intact sperm agreed less with flow cytometry when <80% of sperm had intact membranes, whereas automated cell counter assessments of percent membrane-intact sperm agreed less with flow cytometry when <30% of sperm had intact membranes.     

Sorting cells for basal and induced autophagic flux by quantitative ratiometric flow cytometry

We detail here a protocol using tandem-tagged mCherry-EGFP-LC3 (C-G-LC3) to quantify autophagic flux in single cells by ratiometric flow cytometry and to isolate subpopulations of cells based on their relative levels of autophagic flux. This robust and sensitive method measures autophagic flux rather than autophagosome number and is an important addition to the autophagy researcher’s array of tools for measuring autophagy. Two crucial steps in this protocol are i) generate cells constitutively expressing C-G-LC3 with low to medium fluorescence and low fluorescence variability, and ii) correctly set up gates and voltage/gain on a properly equipped flow cytometer. We have used this method to measure autophagic flux in a variety of cell types and experimental systems using many different autophagy stimuli. On a sorting flow cytometer, this technique can be used to isolate cells with different levels of basal autophagic flux, or cells with variable induction of flux in response to a given stimulus for further analysis or experimentation. We have also combined quantification of autophagic flux with methods to measure apoptosis and cell surface proteins, demonstrating the usefulness of this protocol in combination with other flow cytometry labels and markers.     

Detection of B cell lymphoma involvement in T cell-rich serous fluid by immunoglobulin gene rearrangement: a report of 2 cases

BACKGROUND: Cytology, flow cytometry and gene rearrangement studies are methods to detect lymphoma involvement of body fluid. The cytology of the effusion is equivocal for lymphoma involvement. CASES: We report 2 cases of diffuse large B cell lymphoma with massive lymphocyte predominant serous fluid, ascites in one case and pleural effusion in the other. Immunophenotypic analysis by flow cytometry showed that lymphoid cells express only T cell markers. Lymphoma involvement of effusion was confirmed by immunoglobulin gene rearrangement detected by polymerase chain reaction. CONCLUSION: Our case demonstrated the value of gene rearrangement study in establishing diagnosis of equivocal cases.     

Isolation and Flow Cytometric Analysis of Immune Cells from the Ischemic Mouse Brain

Ischemic stroke initiates a robust inflammatory response that starts in the intravascular compartment and involves rapid activation of brain resident cells. A key mechanism of this inflammatory response is the migration of circulating immune cells to the ischemic brain facilitated by chemokine release and increased endothelial adhesion molecule expression. Brain-invading leukocytes are well-known contributing to early-stage secondary ischemic injury, but their significance for the termination of inflammation and later brain repair has only recently been noticed.Here, a simple protocol for the efficient isolation of immune cells from the ischemic mouse brain is provided. After transcardial perfusion, brain hemispheres are dissected and mechanically dissociated. Enzymatic digestion with Liberase is followed by density gradient (such as Percoll) centrifugation to remove myelin and cell debris. One major advantage of this protocol is the single-layer density gradient procedure which does not require time-consuming preparation of gradients and can be reliably performed. The approach yields highly reproducible cell counts per brain hemisphere and allows for measuring several flow cytometry panels in one biological replicate. Phenotypic characterization and quantification of brain-invading leukocytes after experimental stroke may contribute to a better understanding of their multifaceted roles in ischemic injury and repair.