Telomere analysis by fluorescence in situ hybridization and flow cytometry.

Determination of telomere length is traditionally performed by Southern blotting and densitometry, giving a mean telomere restriction fragment (TRF) value for the total cell population studied. Fluorescence in situ hybridization (FISH) of telomere repeats has been used to calculate telomere length, a method called quantitative (Q)-FISH. We here present a quantitative flow cytometric approach, Q-FISHFCM, for evaluation of telomere length distribution in individual cells based on in situ hybridization using a fluorescein-labeled peptide nucleic acid (PNA) (CCCTAA)3probe and DNA staining with propidium iodide. A simple and rapid protocol with results within 30 h was developed giving high reproducibility. One important feature of the protocol was the use of an internal cell line control, giving an automatic compensation for potential differences in the hybridization steps. This protocol was tested successfully on cell lines and clinical samples from bone marrow, blood, lymph nodes and tonsils. A significant correlation was found between Southern blotting and Q-FISHFCMtelomere length values ( P = 0.002). The mean sub-telomeric DNA length of the tested cell lines and clinical samples was estimated to be 3.2 kbp. With the Q-FISHFCMmethod the fluorescence signal could be determined in different cell cycle phases, indicating that in human cells the vast majority of telomeric DNA is replicated early in S phase.     

Cell kinetic analysis of mixed populations using three-color fluorescence flow cytometry.

The development of antibodies to DNA-incorporated thymidine analogs has in turn led to the development of flow cytometric techniques for rapidly measuring cell kinetics parameters. More recently, these techniques have been applied to clinical tumor material. One problem with such measurements has been the difficulty of distinguishing malignant cells from coexistent normal cells in the biopsy material. In the present study, the feasibility of selecting out the desired malignant cell population for kinetic analysis from a mixture of cells was tested in vitro. An anticytokeratin antibody was used to discriminate between a mixture of tumor cells (cytokeratin positive) and normal cells (cytokeratin negative). The cell lines chosen for the study, A549 human lung carcinoma cells and Chinese hamster ovary (CHO) cells, were pulse labeled with iododeoxyuridine (IdUrd) and sampled every hour up to 16 hours. Selecting out cells from the mixture required the application of three-color fluorescence flow cytometry, which was carried out using the fluorochromes FITC (fluorescein isothionate, green fluorescence, IdUrd-DNA antibody), PE (phycoerythrin, orange fluorescence, cytokeratin antibody), and PI (propidium iodide, red fluorescence, DNA). This allowed single laser excitation. The staining procedure involved incubation with the IdUrd antibodies (specific antibody plus FITC-conjugated second antibody) followed by the cytokeratin antibodies (specific antibody plus PE-conjugated second antibody) and lastly by the DNA stain containing RNase. Two analysis methods of the IdUrd/DNA cytograms were applied: a mid-S window analysis and a relative movement (RM) analysis. Results of the analyses for cells selected out of mixtures were compared with results of cells stained and analyzed separately. A clear separation of the two cell lines could be obtained on the basis of orange fluorescence (cytokeratin content) despite a large overlap of their DNA histograms. By gating on high or low orange fluorescence, almost pure populations of the individual cell types could be selected out for further kinetic analysis. Little difference was seen, with both the mid-S and RM analyses, between cells gated from mixtures or stained separately. It is concluded that this technique is feasible for use on clinical material, provided good cell suspensions can be obtained, leading to the hope of increasing the accuracy of kinetic measurements on human tumors.     

Nonparametric flow cytometry analysis.

A nonparametric statistical test for the analysis of flow cytometry derived histograms is presented. The method involves smoothing and translocation of data, area normalization, channel by channel determination of the mean and S.D., and use of Bayes' theorem for unknown histogram classification. With this statistical method, different sets of histograms from numerous biological systems can be compared.     

Double-beam autocompensation for fluorescence polarization measurements in flow cytometry.

The degree of depolarization of fluorescent light emitted from an organic dye, which is used as molecular probe, is a powerful tool in probing the microenvironment. By fluorescence depolarization the macromolecular structure can be investigated as well as the the mobility of the marker molecule itself or of the complex formed by the probe. Additional information such as energy transfer rates, donor-acceptor distances, and orientations are also measurable. These data are of particular interest if they can be measured from whole cells. Using flow cytometry, we can analyze a large number of cells with high statistical significance in a short period of time. We describe a newly developed double-beam epi-illumination arrangement for fluorescence polarization measurements that uses an autocompensation technique. This new technique permits the various depolarizing effects within the optical as well as the electronic components of the system to be continually compensated for on a cell by cell basis. Simultaneous measurements of other cell parameters for cell cycle analysis by total fluorescence intensity remains possible. The sensitivity of the system to measure polarization was determined as +/- 0.006 p (0 less than or equal to p less than or equal to 0.5 in isotropic media), which amounts to +/- 1.2% of the maximum p value. Polarization data for latex microspheres plotted in the histogram mode were measured with a standard deviation of 0.006, which proved the high resolution and the high performance of the system.     

Dual excitation multi- fluorescence flow cytometry for detailed analyses of viability and apoptotic cell transition

The discrimination of live/dead cells as well as the detection of apoptosis is a frequent need in many areas of experimental biology. Cell proliferation is linked to apoptosis and controlled by several genes. During the cell life, specific events can stimulate proliferation while others may trigger the apoptotic pathway. Very few methods (i.e. TUNEL) are now available for studies aimed at correlation between apoptosis and proliferation. Therefore, there is interest in developing new methodological approaches that are able to correlate apoptosis to the cell cycle phases. Recently new approaches have been proposed to detect and enumerate apoptotic cells by flow cytometry. Among these, the most established and applied are those based on the cell membrane modifications induced in the early phases of the apoptotic process. The dye pair Hoechst 33342 (HO) and Propidium Iodide (PI), thanks to their peculiar characteristics to be respectively permeable and impermeable to the intact cell membrane, seems to be very useful. Unfortunately the spectral interaction of these dyes generates a consistent "energy transfer" from HO to PI. The co-presence of the dyes in a nucleus results in a modification in the intensity of both the emitted fluorescences. In order to designate the damaged cells (red fluorescence) to the specific cell cycle phases (blue fluorescence), we have tested different staining protocols aimed to minimize the interference of these dyes as much as possible. In cell culture models, we are able to detect serum-starved apoptotic cells as well as to designate their exact location in the cell cycle phases using a very low PI concentration. Using a Partec PAS flow cytometer equipped with HBO lamp and argon ion laser, a double UV/blue excitation has been performed. This analytical approach is able to discriminate live blue cells from the damaged (blue-red) ones even at 0.05 micro g/mL PI. The same instrumental setting allows performing other multi-colour analyses including AnnexinV-FITC as well as the possibility to make a correlated analysis to phenotype markers.     

Attenuation of photobleaching in two-photon excitation fluorescence from green fluorescent protein with shaped excitation pulses

The two-photon excitation fluorescence (TPEF) process of an enhanced green fluorescent protein (EGFP) for fluorescence signals was adaptively controlled by the phase-modulation of femtosecond pulses. After the iteration of pulse shaping, a twofold increase in the ratio of the fluorescence signal to the laser peak power was achieved. Compared with conventional pulses optimized for peak power, phase-optimized laser pulses reduced the bleaching rate of EGFP by a factor of 4 while maintaining the same intensity of the fluorescence signal. Our method will provide a powerful solution to various problems confronting researchers, such as the photobleaching of dyes in two-photon excitation microscopy.     

Characterization of one- and two-photon excitation fluorescence resonance energy transfer microscopy

Advances in molecular biology provide various methods to define the structure and function of the individual proteins that form the component parts of subcellular structures. The ability to see the dynamic behavior of a specific protein inside the living cell became possible through the application of advanced fluorescence resonance energy transfer (FRET) microscope techniques. The fluorophore molecule used for FRET imaging has a characteristic absorption and emission spectrum that should be considered for characterizing the FRET signal. In this article we describe the system development for the image acquisition for one- and two-photon excitation FRET microscopy. We also describe the precision FRET (PFRET) data analysis algorithm that we developed to remove spectral bleed-through and variation in the fluorophore expression level (or concentration) for the donor and acceptor molecules. The acquired images have been processed using a PFRET algorithm to calculate the energy transfer efficiency and the distance between donor and acceptor molecules. We implemented the software correction to study the organization of the apical endosome in epithelial polarized MDCK cells and dimerization of the CAATT/enhancer binding protein alpha (C/EBPalpha). For these proteins, the results revealed that the extent of correction affects the conventionally calculated energy transfer efficiency (E) and the distance (r) between donor and acceptor molecules by 38 and 9%, respectively.     

Video-rate scanning two-photon excitation fluorescence microscopy and ratio imaging with cameleons

A video-rate (30 frames/s) scanning two-photon excitation microscope has been successfully tested. The microscope, based on a Nikon RCM 8000, incorporates a femtosecond pulsed laser with wavelength tunable from 690 to 1050 nm, prechirper optics for laser pulse-width compression, resonant galvanometer for video-rate point scanning, and a pair of nonconfocal detectors for fast emission ratioing. An increase in fluorescent emission of 1.75-fold is consistently obtained with the use of the prechirper optics. The nonconfocal detectors provide another 2.25-fold increase in detection efficiency. Ratio imaging and optical sectioning can therefore be performed more efficiently without confocal optics. Faster frame rates, at 60, 120, and 240 frames/s, can be achieved with proportionally reduced scan lines per frame. Useful two-photon images can be acquired at video rate with a laser power as low as 2.7 mW at specimen with the genetically modified green fluorescent proteins. Preliminary results obtained using this system confirm that the yellow "cameleons" exhibit similar optical properties as under one-photon excitation conditions. Dynamic two-photon images of cardiac myocytes and ratio images of yellow cameleon-2.1, -3.1, and -3.1nu are also presented.     

Molecular dynamics in living cells observed by fluorescence correlation spectroscopy with one- and two-photon excitation.

Multiphoton excitation (MPE) of fluorescent probes has become an attractive alternative in biological applications of laser scanning microscopy because many problems encountered in spectroscopic measurements of living tissue such as light scattering, autofluorescence, and photodamage can be reduced. The present study investigates the characteristics of two-photon excitation (2PE) in comparison with confocal one-photon excitation (1PE) for intracellular applications of fluorescence correlation spectroscopy (FCS). FCS is an attractive method of measuring molecular concentrations, mobility parameters, chemical kinetics, and fluorescence photophysics. Several FCS applications in mammalian and plant cells are outlined, to illustrate the capabilities of both 1PE and 2PE. Photophysical properties of fluorophores required for quantitative FCS in tissues are analyzed. Measurements in live cells and on cell membranes are feasible with reasonable signal-to-noise ratios, even with fluorophore concentrations as low as the single-molecule level in the sampling volume. Molecular mobilities can be measured over a wide range of characteristic time constants from approximately 10(-3) to 10(3) ms. While both excitation alternatives work well for intracellular FCS in thin preparations, 2PE can substantially improve signal quality in turbid preparations like plant cells and deep cell layers in tissue. At comparable signal levels, 2PE minimizes photobleaching in spatially restrictive cellular compartments, thereby preserving long-term signal acquisition.     

Applying two-photon excitation fluorescence lifetime imaging microscopy to study photosynthesis in plant leaves

This study investigates to which extent two-photon excitation (TPE) fluorescence lifetime imaging microscopy can be applied to study picosecond fluorescence kinetics of individual chloroplasts in leaves. Using femtosecond 860 nm excitation pulses, fluorescence lifetimes can be measured in leaves of Arabidopsis thaliana and Alocasia wentii under excitation-annihilation free conditions, both for the F ₀- and the F _m-state. The corresponding average lifetimes are ~250 ps and ~1.5 ns, respectively, similar to those of isolated chloroplasts. These values appear to be the same for chloroplasts in the top, middle, and bottom layer of the leaves. With the spatial resolution of ~500 nm in the focal (xy) plane and 2 μm in the z direction, it appears to be impossible to fully resolve the grana stacks and stroma lamellae, but variations in the fluorescence lifetimes, and thus of the composition on a pixel-to-pixel base can be observed.     

The two-photon laser-induced fluorescence of the tumor-localizing photosensitizer hematoporphyrin derivative. Resonance-enhanced 750 nm two-photon excitation into the near-UV Soret band

The tumor-localizing photosensitizer hematoporphyrin derivative (HPD) is shown to undergo a simultaneous two-photon excitation into the near-ultraviolet Soret band system upon intense laser irradiation at 750 nm, a spectral region where there is no significant HPD one-photon absorbance in aqueous solution. Subsequent to this excitation, internal conversion and vibrational relaxation occur, resulting in the population of the vibrationless level of the first electronically excited singlet state. This state relaxes by two channels, the emission of fluorescence in the spectral region 600-700 nm and intersystem crossing into the triplet manifold, followed by near-resonant electronic energy transfer with surrounding oxygen to result in the generation of highly reactive singlet molecular oxygen (1 delta g). Evidence for the two-photon excitation consists in the observation both of the HPD fluorescence spectrum in the region of 615 nm as a result of 750 nm excitation and the quadratic dependence of this fluorescence emission intensity upon the excitation laser intensity. Since, in general, the penetration depth of ultraviolet and visible light into tissue varies directly with wavelength (red penetrating more deeply than blue), these studies suggest the possibility that two-photon-induced localization of tumor-bound HPD might facilitate the detection of deeper lying tumors than allowed by the current one-photon photolocalization method.     

Comparative analysis of surface membrane immunoglobulin determination by flow cytometry and fluorescence microscopy

The analysis of membrane surface immunoglobulin (SmIg) on B lymphocytes was carried out in 59 normal individuals and nine patients with B-cell non-Hodgkin's lymphomas by conventional immunofluorescence microscopy and flow cytometry. Five channel settings of a cytofluorograph were evaluated (100, 150, 200, 250, 300) and the mean and standard deviation of the percent positive cells were calculated and compared to the mean and standard deviation of the microscope reading. On the basis of the relative fluorescence reactivity, we were able to determine a fluorescence intensity at which the results of flow cytometry and fluorescence microscopy were comparable. In normal individuals, for cells expressing surface Ia, the channel giving similar results to that of fluorescence microscopy was 150; for kappa and lambda chains, channel 200; for Fab'PV, channel 200; and for IgM, channel 250. In patients with B-cell non-Hodgkin's lymphomas, for cells expressing surface Ia the channel giving similar results to that of fluorescence microscopy was 100; for kappa, channel 100; for lambda, channel 200; for Fab'FV, channel 150; and for IgM, channel 150. Flow cytometric analysis of SmIg appears to be superior to fluorescence microscopy in efficiency, and has the added advantages of being a rapid, sensitive, and objectively quantitative methodology.     

Photobleaching in two-photon excitation microscopy

The intensity-squared dependence of two-photon excitation in laser scanning microscopy restricts excitation to the focal plane and leads to decreased photobleaching in thick samples. However, the high photon flux used in these experiments can potentially lead to higher-order photon interactions within the focal volume. The excitation power dependence of the fluorescence intensity and the photobleaching rate of thin fluorescence samples ( approximately 1 microm) were examined under one- and two-photon excitation. As expected, log-log plots of excitation power versus the fluorescence intensity and photobleaching rate for one-photon excitation of fluorescein increased with a slope of approximately 1. A similar plot of the fluorescence intensity versus two-photon excitation power increased with a slope of approximately 2. However, the two-photon photobleaching rate increased with a slope > or =3, indicating the presence of higher-order photon interactions. Similar experiments on Indo-1, NADH, and aminocoumarin produced similar results and suggest that this higher-order photobleaching is common in two-photon excitation microscopy. As a consequence, the use of multi-photon excitation microscopy to study thin samples may be limited by increased photobleaching.     

Ca2+ fluorescence imaging with pico- and femtosecond two-photon excitation: signal and photodamage.

The signal and limitations of calcium florescence imaging using nonresonant multiphoton absorption of near-infrared femto- and picosecond laser pulses were examined. The fluorescence changes of various Ca(2+)-indicators induced by transient increases of the intradendritic calcium concentration were evaluated by evoking physiological activity in neocortical neurons in rat brain slices. Photodamage was noticeable as irreversible changes in the parameters describing the calcium fluorescence transients. At higher two-photon excitation rates, a great variety of irregular functional and structural alterations occurred. Thus, signal and observation time were limited by phototoxic effects. At lower excitation rates, photodamage accumulated linearly with exposure time. Femtosecond and picosecond laser pulses were directly compared with respect to this cumulative photodamage. The variation of the pulse length at a constant two-photon excitation rate indicated that a two-photon excitation mechanism is mainly responsible for the cumulative photodamage within the investigated window of 75 fs to 3.2 ps. As a direct consequence, at low excitation rates, the same image quality is achieved irrespective of whether two-photon Ca(2+)-imaging is carried out with femto- or picosecond laser pulses.     

DNA analysis by flow cytometry

Accurate quantification of DNA from cells of several species is possible with flow cytometry. When one species is used as a reference, cytometric readings from two or more different species can be compared to obtain relative percent DNA or DNA indices. Differences in DNA from the male and female of the same species also can be measured. The method allows rapid screening of chromosomal abnormalities among large clinical populations, and evaluation of errors of sex determination such as XY sex reversal.     

Benzoxazinone derivatives: new fluorescent probes for two-color flow cytometry analysis using one excitation wavelength

A new class of fluorescent dye which upon excitation at 488 nm turns red is shown to be probe-suitable for using in flow cytometry alone or in conjunction with fluorescein derivatives. 7-dimethylamino 3-(p-formylstyryl) 1,4 benzoxazin 2-one is suitable for rendering microorganisms, such as Plasmodium merozoites and cells detectable by flow cytometry, allowing a dual fluorescence analysis when the cells are labelled with suitable fluoresceinylated ligands such as fluorescein labeled neoglycoproteins or antibodies. The synthesis of the new benzoxazinone derivatives is described: p-[beta-(7-dimethylamino 1,4 benzoxazin 2-one 3-yl)-vinyl]-phenylpropenoic acid can be easily activated as a hydroxysuccinimide derivative and linked to amino groups of polypeptides. Hydrophilic polypeptides such as poly-L-lysine or glycosylated polymers combined with this new fluorescent dye are shown to be helpful in analyzing cell surface receptors, in dual fluorescence flow cytometry analysis, using a single excitation wavelength and two sets of compounds labeled with the new benzoxazinone derivative and with fluorescein isothiocyanate, respectively. The new benzoxazinone derivative has a high molar absorbance, a good quantum yield fluorescence when it is bound to hydrophilic polypeptides and its fluorescence intensity is not dependent on pH in the physiological pH range.     

Practical time-gated luminescence flow cytometry. II: experimental evaluation using UV LED excitation.

In the previous article [Part 1 (8)], we have modelled alternative approaches to design of practical time-gated luminescence (TGL) flow cytometry and examined the feasibility of employing a UV LED as the excitation source for the gated detection of europium dye labelled target in rapid flow stream. The continuous flow-section approach is well suited for rare-event cell counting in applications with a large number of nontarget autofluorescent particles. This article presents details of construction, operation and evaluation of a TGL flow cytometer using a UV LED excitation and a gated high-gain channel photomultiplier tube (CPMT) for detection. The compact prototype TGL flow cytometer was constructed and optimised to operate at a TGL cycle rate of 6 kHz, with each cycle consisting of 100 micros LED pulsed excitation and approximately 60 micros delay-gated detection. The performance of the TGL flow cytometer was evaluated by enumerating 5.7 microm Eu(3+) luminescence beads (having comparable intensity to europium-chelate-labeled Giardia cysts) in both autofluorescence-rich environmental water concentrates and Sulforhodamine 101 (S101) solutions (broadband red fluorescence covering the spectral band of target signals), respectively.The prototype TGL flow cytometer was able to distinguish the target beads, and a maximum signal to background ratio of 38:1 was observed. Neither the environmental water concentrates nor S101 solution contributed to the background in the TGL detection phase. The counting efficiency of the TGL flow cytometer was typically >93% of values determined using conventional counting methods.     

Cellular characterization of adenylate kinase and its isoform: two-photon excitation fluorescence imaging and fluorescence correlation spectroscopy

Adenylate kinase (AK) is a ubiquitous enzyme that regulates the homeostasis of adenine nucleotides in the cell. AK1beta (long form) from murine cells shares the same protein sequence as AK1 (short form) except for the addition of 18 amino acid residues at its N-terminus. It is hypothesized that these residues serve as a signal for protein lipid modification and targeting of the protein to the plasma membrane. To better understand the cellular function of these AK isoforms, we have used several modern fluorescence techniques to characterize these two isoforms of AK enzyme. We fused cytosolic adenylate kinase (AK1) and its isoform (AK1beta) with enhanced green fluorescence protein (EGFP) and expressed the chimera proteins in HeLa cells. Using two-photon excitation scanning fluorescence imaging, we were able to directly visualize the localization of AK1-EGFP and AK1beta-EGFP in live cells. AK1beta-EGFP mainly localized on the plasma membrane, whereas AK1-EGFP distributed throughout the cell except for trace amounts in the nuclear membrane and some vesicles. We performed fluorescence correlation spectroscopy measurements and photon-counting histogram analysis in specific domains of live cells. For AK1-EGFP, we observed only one diffusion component in the cytoplasm. For AK1beta-EGFP, we observed two distinct diffusion components on the plasma membrane. One corresponded to the free diffusing protein, whereas the other represented the membrane-bound AK1beta-EGFP. The diffusion rate of AK1-EGFP was slowed by a factor of 1.8 with respect to that of EGFP, which was 50% more than what we would expect for a free diffusing AK1-EGFP. To rule out the possibility of oligomer formation, we performed photon-counting histogram analysis to direct analyze the brightness difference between AK1-EGFP and EGFP. From our analysis, we concluded that cytoplasmic AK1-EGFP is monomeric. fluorescence correlation spectroscopy proved to be a powerful technique for quantitatively studying the mobility of the target protein in live cells. This technology offers advantages in studying protein interactions and function in the cell.