Expression and analysis of green fluorescent proteins in human embryonic kidney cells by capillary electrophoresis

Two-Color Capillary Electrophoresis with On-Column Excitation and Wave-Guide Based Fluorescent Detection

An optical wave-guide based two-color capillary electrophoresis laser induced fluorescence (CE-LIF) instrument is described. The wave-guide based approach allows for on column excitation and detection with two-color discrimination. The instrument is designed to allow either electrokinetic or hydrodynamic injections. In its present configuration, the attainable limit of detection (LOD, S/N = 3) was 50 × 10−21 moles of fluorescein with a 488-nm excitation source. This study was designed to test the instrument design for applications in protein analyses. Fluorescent dyes with two different wavelengths were simultaneously separated and detected as were complexes formed by labeled antibodies to NF'B p65 and cdc2p34. Quantification of both proteins in THP-1 cell lysates performed using this approach illustrates a rapid screening application of this instrument.

Quantitative analysis of a ubiquitin-dependent substrate using capillary electrophoresis with dual laser-induced fluorescence

Protein degradation by the ubiquitin-proteasome system (UPS) affects many biological processes. Inhibition of the proteasome has emerged as a potential therapeutic target for cancer treatment. In this study, we developed a method for monitoring the degradation and accumulation of UPS-dependent substrates in cells using CE with dual LIF. We used a green fluorescent protein (GFP)-fusion of the ubiquitin substrate ribophorin 1 (GFP-RPN1) along with red fluorescent protein (RFP) as an internal control to normalize transfection efficiency. Determination of GFP-RPN1 and RFP in cell lysates were performed in an untreated capillary (75 μm × 50 cm) and 100 mM Tris-CHES buffer (pH 9.0) containing 10 mM SDS. GFP-RPN1 and RFP fluorescence were detected at excitation wavelengths of 488 and 635 nm, and emission wavelengths of 520 and 675 nm, respectively, without any interference or crosstalk. The intensity of GFP-RPN1 fluorescence was normalized to that of RFP. Additionally, the proposed approach was used successfully to detect the degradation of GFP-RPN1 and evaluate proteasome inhibitors. These results show that the developed method is effective and promising for rapid and quantitative monitoring of UPS-dependent substrates compared to the current common methods, such as immunoblotting and pulse chase assays.

Quantification of green fluorescent protein in cellular supernatant by capillary electrophoresis with laser-induced fluorescence detection for measurement of cell death

A common method for quantifying cell death is measuring the concentration of lactate dehydrogenase (LDH) released by cells as their membranes become unstable. In cells expressing green fluorescent protein (GFP), degradation of the cell membrane also results in the release of GFP into the surrounding supernatant. In this study, we used capillary electrophoresis with laser-induced fluorescence detection to measure the levels of GFP in supernatants of UBIGFP/BL6 primary macrophages that had been infected with Salmonella typhimurium, treated with staurosporine, or exposed to H(2)O(2), all known inducers of cell death. We also used a standard LDH assay to measure the release of LDH into supernatants. We observed the rate of cell death quantified by release of GFP and LDH into supernatant to be essentially identical, demonstrating that GFP release is at least as good as an indicator of macrophage cell death as the established LDH release method.

Single cell analytics: an overview

The research field of single cell analysis is rapidly expanding, driven by developments in flow cytometry, microscopy, lab-on-a-chip devices, and many other fields. The promises of these developments include deciphering cellular mechanisms and the quantification of cell-to-cell differences, ideally with spatio-temporal resolution. However, these promises are challenging as the analytical techniques have to cope with minute analyte amounts and concentrations. We formulate first these challenges and then present state-of-the-art analytical techniques available to investigate the different cellular hierarchies--from the genome to the phenome, i.e., the sum of all phenotypes.

Confocal laser scanning microscopy using a frequency doubled vertical external cavity surface emitting laser

We report on a frequency doubled 980 nm vertical external cavity surface emitting laser for applications in confocal laser scanning microscopy. The beam quality, wavelength flexibility, and low noise characteristics of this compact source make this prolific imaging technique an exemplary tool. Single pass frequency doubling via KNbO(3) was demonstrated, yielding 1.8 mW at 490 nm with a near diffraction limited beam quality. Detailed analysis and comparison of the laser performance with the current standard argon ion laser revealed clear advantages of the solid-state source for confocal imaging. Imaging of fluorescein and eGFP labeled biological samples using the attenuated solid-state source provided high-resolution images at lower cost and with improved reliability.

Chemical analysis of single cells

Chemical analysis of single cells requires methods for quickly and quantitatively detecting a diverse array of analytes from extremely small volumes (femtoliters to nanoliters) with very high sensitivity and selectivity. Microelectrophoretic separations, using both traditional capillary electrophoresis and emerging microfluidic methods, are well suited for handling the unique size of single cells and limited numbers of intracellular molecules. Numerous analytes, ranging from small molecules such as amino acids and neurotransmitters to large proteins and subcellular organelles, have been quantified in single cells using microelectrophoretic separation techniques. Microseparation techniques, coupled to varying detection schemes including absorbance and fluorescence detection, electrochemical detection, and mass spectrometry, have allowed researchers to examine a number of processes inside single cells. This review also touches on a promising direction in single cell cytometry: the development of microfluidics for integrated cellular manipulation, chemical processing, and separation of cellular contents.

Detection of Green Fluorescent Protein in a Single Bacterium by Capillary Electrophoresis with Laser-Induced Fluorescence

Green fluorescence protein (GFP) is a common reporter used to monitor protein expression in single cells. However, autofluorescence from endogenous components can mask the signal from GFP, particularly at low expression levels in prokaryotes. We employ capillary electrophoresis with laser-induced fluorescence for the analysis of the expression of green fluorescent protein in a single bacterium. Capillary electrophoresis separates GFP from native cellular autofluorescent components, reducing the background signal and improving detection limits. Our system provides 100 ymol (60 copies) limits of detection for GFP. To demonstrate the performance of this instrument, we employ a model system of Deinococcus radiodurans that has been engineered to express GFP under the control of the recA promoter. We report resolution and detection of GFP and autofluorescent components in a single D. radiodurans bacterium. This paper presents the first example of expression of GFP in D. radiodurans and the first detection of GFP in a single bacterium by capillary electrophoresis.

Microplate cell-retaining methodology for high-content analysis of individual non-adherent unanchored cells in a population

A high throughput Microtiter plate Cell Retainer (MCR) has been developed to enable, for the first time, high-content, time-dependent analysis of the same single non-adherent and non-anchored cells in a large cell population, while bio-manipulating the cells. The identity of each cell in the investigated population is secured, even during bio-manipulation, by cell retention in a specially designed concave microlens, acting as a picoliter well (PW). The MCR technique combines micro-optical features and microtiter plate methodology. The array of PWs serves as the bottom of a microtiter plate, fitted with a unique flow damper element. The latter enables rapid fluid exchange without dislodging the cells from their original PWs, thus maintaining the cells' identity. Loading cell suspensions and reagents into the MCR is performed by simple pouring, followed by gravitational sedimentation and settling of cells into the PWs. Cell viability and cell division within the MCR were shown to be similar to those obtained under similar conditions in a standard microtiter plate. The efficiency of single cell occupancy in the MCR exceeded 90%. No cell dislodging was observed when comparing images before and after bio-manipulations (rinsing, staining, etc.). The MCR permits the performance of kinetic measurements on an individual cell basis. Data acquisition is governed by software, controlling microscope performance, stage position and image acquisition and analysis. The PW's unique micro-optical features enable rapid, simultaneous signal analysis of each individual cell, bypassing lengthy image analysis.

Using capillary electrophoresis with laser-induced fluorescence to study the interaction of green fluorescent protein-labeled calmodulin with Ca2+- and calmodulin-binding protein

A separation using capillary electrophoresis with laser-induced fluorescence (CE-LIF) was applied to the study of green fluorescent protein tagged calmoldulin (GFP-CaM) that was expressed from Escherichia coli and purified with Ni(2+)-nitrilotriacetate (Ni-NTA) resin column. It was found that GFP-CaM not only has good fluorescence properties under various conditions similar to GFP, but also retains its calcium-binding ability as the native CaM. GFP-CaM was separated and detected by CE-LIF within 10 min with a limit-of-detection (LOD) of 2 x 10(-10) M for an injection volume of 3 nl, higher than that of common chemical fluorescent-tagged protein method. The results indicated that, as a fluorescence probe, GFP could overcome the drawback of inefficient derivatization of chemical fluorescence probes. The interaction between the GFP-CaM and Ca(2+) was studied in detail using affinity capillary electrophoresis with laser-induced fluorescence and the dissociation constant (K(d)) between GFP-CaM and Ca(2+) was determined to be 1.2 x 10(-5), which is in good agreement with the literature values of untagged CaM (10(-6) to 10(-5)M) obtained by conventional method. As a preliminary application, the interaction between GFP-CaM and OsCBK was also investigated. The method makes it possible to screen the trace amounts of target proteins in crude extracts interacting with CaM under physiological conditions.

Nuclear Localization of Aminoacyl-tRNA Synthetases Using Single-Cell Capillary Electrophoresis Laser-Induced Fluorescence Analysis

Aminoacyl-tRNA synthetases (aaRSs) are a family of enzymes whose function in specific aminoacylation of tRNAs is central to the process of protein translation, which occurs in the cytoplasm of all living cells. In addition to their well-established cytoplasmic localization, fluorescence microscopy studies and analysis of the aminoacylation state of nuclear tRNAs have revealed that synthetases are localized in the nuclei of cells from several species including Xenopus laevis and Saccharomyces cerevisiae. Whether nuclear localization of aaRSs is a general phenomenon that occurs in all eukaryotic cells is an open question. In the work described here, human methionyl-tRNA synthetase (MRS) and human lysyl-tRNA synthetase (KRS) were expressed in human-derived DeltaH2-1 osteosarcoma cells as enhanced green fluorescent protein (EGFP) fusion proteins. The subcellular localization of these EGFP-aaRSs was first probed by fluorescence microscopy using cells that coexpressed EGFP-aaRS and a nuclear marker fusion protein, nuDsRed. As expected, both aaRSs were present in the cytosol, while only EGFP-MRS was also clearly localized in the nucleus. To confirm these findings, and to investigate a potentially more sensitive, general method for nuclear localization studies, capillary electrophoresis with laser-induced fluorescence (CE-LIF) detection was used to analyze single DeltaH2-1 cells expressing both EGFP-aaRS and nuDsRed. While cytosolic EGFP signals were detected for both EGFP-MRS and EGFP-KRS, only EGFP-MRS was found in the nucleus, along with nuDsRed. The detection of EGFP-MRS in nuclei of DeltaH2-1 cells demonstrates the feasibility of using CE-LIF analysis in nuclear localization studies of proteins in mammalian cells.

Production of cell lines secreting TAT fusion proteins

Transduction of proteins and other macromolecules constitutes a potent technology to analyze cell functions and to achieve therapeutic interventions. In general, fusion proteins with protein transduction domains, such as TAT, are produced in a bacterial expression system. Here we describe the generation of a mammalian expression vector coding for TAT-EGFP fusion protein. Transfection of CHO-K1 cells by this vector and subsequent selection by Zeocin resulted in cell lines that express and secrete EGFP, a variant of the green fluorescent protein GFP. The ultimate cell line was produced by first cloning the stable integrants and subsequent selection of EGFP-expressing cells by flow cytometric sorting. In the resulting cell line approximately 98% of cells express EGFP. Using the same methodology, we generated cell lines that express DsRed fluorescent protein. The advantages of using such a mammalian expression system include the ease of generating TAT fusion proteins and the potential for sustained production of such proteins in vitro and, potentially, in vivo.

Capillary Electrophoretic Separation of Nuclei Released from Single Cells

We report here the first capillary electrophoresis analysis of intact nuclei released on-column from single cells. Expression of the nuclear-targeted protein nuDsRed2 and the plasma membrane-bound farnesylated enhanced green fluorescent protein in cultured human DeltaH2-1 cells allowed fluorescent monitoring of the fate of these subcellular compartments upon injection of a single cell into the separation capillary. On-column treatment with digitonin allowed for the separation of the plasma membrane from the nucleus as indicated by their selective laser-induced fluorescence detection in two separate spectral regions. The data suggest that less than 0.1% of the plasma membrane remains bound to individually detected nuclei. In digitonin-treated cells, the electropherograms consisted of a prominent fluorescent peak attributed to nuDsRed2 localized to the nucleus and a collection of weakly fluorescent events (barely distinguishable from scattering) that seem to indicate additional localization of this protein to other subcellular regions. Taken together, this report points to the feasibility of studying intact organelles released from a single mammalian cell by capillary electrophoresis, which is a prerequisite to understanding the relevance of subcellular heterogeneity in biological systems.

Capillary sodium dodecyl sulfate-DALT electrophoresis with laser-induced fluorescence detection for size-based analysis of proteins in human colon cancer cells

Capillary sodium dodecyl sulfate (SDS)-DALT electrophoresis (SDS-DALT-CE) refers to CE separation of proteins based on their size; DALT is the abbreviation for Dalton, the unit used to describe molecular weight. In this work, seven proteins from 18 to 116 kDa were denatured by SDS, labeled by 3-(2-furoyl) quinoline-2-carboxaldehyde, separated by SDS-DALT-CE in polyethylene oxide sieving matrix, and detected by laser-induced fluorescence (LIF) in a sheath flow cuvette. This method was combined with detergent differential fractionation, which is a protein fractionation method using a series of detergent-containing buffers to sequentially extract protein fractions from cells, to analyze the proteins in HT29 human colon adenocarcinoma cells. In addition, on-column labeling was demonstrated for protein analysis by SDS-DALT-CE with LIF, and applied to analysis of proteins in a single HT29 cancer cell. Most proteins had molecular masses from 10 to 120 kDa. Similar protein profiles were obtained for single cells and protein extract of a large cell population.

Direct monitoring of the expression of the green fluorescent protein-extracellular signal-regulated kinase 2 fusion protein in transfected cells using capillary electrophoresis with laser-induced fluorescence detection

Proper subcellular localization of the extracellular signal-regulated kinases (ERKs) is important in regulating physiological functions such as proliferation and differentiation in the pheochromocytoma cell line (PC12 cells). Thus, a direct visualization method is necessary to observe ERK localization within the cell or in crude cellular extracts. In this paper, a determination method was established for the detection of ERK2 localization in PC12 cells using green fluorescent protein (GFP) and capillary electrophoresis with laser-induced fluorescence (LIF). GFP as a reporter or labeling tag for gene expression in biochemistry and cell biology was used for the detection of ERK2 localization in PC12 cells. PC12 cells were transfected with GFP-ERK2 plasmid construct that was inserted into a variant GFP gene (enhanced green fluorescent protein), and successfully expressed GFP-ERK2 fusion proteins. GFP-ERK2 fusion proteins were detected within 5 min by CE analysis using an uncoated fused-silica capillary with LIF. Optimum conditions for GFP-ERK2 fusion proteins detection were 100 mM 3-(cyclohexylamino)-1-propanesulfonic acid buffer containing 100 mM sodium dodecylsulfate, pH 11, running at 20 degrees C. This result offers new opportunity in screening for the determination of localization of intracellular components, protein-protein interactions and kinase activity within the cells.

Iothalamate measured by capillary electrophoresis is a suitable alternative to radiolabeled inulin in renal micropuncture

BACKGROUND

Inulin remains the gold standard for measurements of fluid reabsorption (Jv) and single nephron glomerular filtration rate (SNGFR) in micropuncture experiments. However, the method used to measure cold inulin in nanoliter samples is time-consuming, while the use of radiolabeled inulin is disadvantaged by possible radioactive contamination, disposal of radioactive material and cost of the isotope. It has been reported that non-radiolabeled iothalamate may be a suitable alternative for estimation of whole kidney GFR. The present study tested whether iothalamate can be used to measure Jv in microperfusion and free-flow micropuncture experiments.

METHODS

Superficial loops of Henle (LOH) were perfused from late proximal to early distal tubules with an end-like proximal solution. In the first set of experiments, the perfusate contained both iothalamate (1.9 mmol/L) and 3H-methoxy-inulin (50 microCi. mL(-1)). To test if iothalamate was able to detect changes in Jv, two additional sets of experiments were performed: (1) mannitol (61 mmol/L) was added to the perfusate to partially replace NaCl, a condition known to inhibit Jv; (2) LOH of remnant kidneys were perfused, which in previous experiments we showed to have a higher Jv. Lastly, free-flow micropuncture experiments were performed by infusing iothalamate IV at 18.3 mg. h(-1). Iothalamate analysis in nanoliter samples of renal tubular fluid obtained in vivo was performed by capillary electrophoresis (CE).

RESULTS

In the first set of experiments, liquid scintillation counting of 3H-methoxy-inulin versus iothalamate analysis with CE resulted in almost identical calculated perfusion rates (20.4 +/- 0.6 vs. 20.6 +/- 0.7 nL. min(-1), N = 20) and tubular fluid/perfusate ratios (TF/P; 1.35 +/- 0.04 vs. 1.36 +/- 0.04) and thus also Jv (5.17 +/- 0.50 vs. 5.38 +/- 0.59 nL. min(-1)). In the mannitol experiments, iothalamate measurements showed that the addition of mannitol significantly reduced Jv from 4.98 +/- 0.40 (N = 19) to 0.72 +/- 0.58 nL. min(-1) (N = 33; P < 0.0001). Iothalamate determinations by CE were able to detect a significant increase in Jv in LOH of remnant rats perfused at 40 nL. min(-1)[from to 8.40 +/- 0.73 (N = 20) in sham-operated to 17.8 +/- 2.9 nL. min(-1) (N = 6) in remnant animals; P < 0.0001]. In free flow micropuncture experiments the ratio of tubular fluid to plasma iothalamate (TF/P) along the proximal tubule was 1.62 +/- 0.10 (N = 15).

CONCLUSIONS

These data demonstrate that iothalamate can replace inulin to measure Jv in microperfusion and free-flow micropuncture experiments. Since iothalamate analysis by CE technique is a fast, easy and highly reproducible technique, it may become the gold standard method for the detection of fluid reabsorption in microperfused nephron segments.

Optical imaging fiber-based single live cell arrays: a high-density cell assay platform

A high-density, ordered array containing thousands of microwells is fabricated on an optical imaging fiber. Each individually addressable microwell is used to accommodate a single living cell. A charged coupled device (CCD) detector is employed to monitor and spatially resolve the fluorescence signals obtained from each individual cell, allowing simultaneous monitoring of cellular responses of all the cells in the array using reporter genes (lacZ, EGFP, ECFP, DsRed) or fluorescent indicators. Yeast and bacteria cell arrays were fabricated and used to perform multiplexed cell assays with resolution at the single-cell level. Monitoring gene expression in single yeast cells carrying a two-hybrid system was used to detect in vivo protein-protein interactions. The single-cell array technology provides a new platform for monitoring the unique multiple responses of large populations of individual cells from different strains or cell lines. The rich data acquired by the cell array has the potential to be employed as a new tool for cell biology research as well as to improve cell-based high-throughput screening (HTS) applications, such as the validation of new disease-associated cellular targets and the early-stage evaluation of potential drug candidates.

Capillary electrophoresis of proteins 1999-2001

This review article with 223 references describes recent developments in capillary electrophoresis (CE) of proteins and covers papers published during last two years, from the previous review (V. Dolnik, Electrophoresis 1999, 20, 3106-3115) through Spring 2001. It describes the topics related to CE of proteins including modeling of the electrophoretic properties of proteins, sample pretreatment, wall coatings, improving selectivity, detection, special electrophoretic techniques, and applications.

Capillary sodium dodecyl sulfate-DALT electrophoresis of proteins in a single human cancer cell

Capillary Sodium dodlecyl sulfate (SDS)-DALT an (abbreviation for Dalton) electrophoresis was applied to analysis of proteins in single HT29 human colon adenocarcinoma cells. A vacuum pulse was employed to introduce a single cell into the coated capillary. Once the cell was lysed, proteins were denatured with SDS, fluorescantly labeled with 3-(2-furoyl)-quinoline-2-carboxaldehyde (FQ), and then separated by using 8% pullulan as the sieving matrix. This method offers a few advantages for single-cell protein analysis. First, it provides reproducible separation of single-cell proteins according to their size. Based on comparison with the migration time of standard proteins, most components from a single HT29 cancer cell have molecular masses within the range of 10-100 kDa. Second, as a one-dimensional separation method, it gives fairly good resolution for proteins. Typically, around 30 protein components of a single HT29 cell were resolved, indicating that this method has similar peak capacity to SDS-polyacrylamide gel electrophoresis (PAGE). Third, this method shows high detection sensitivity and wide dynamic range, which is important because of the wide range of protein expression in living systems. Detection limits for standard proteins ranged from 10(-10) to 10(-11) M. Finally, this method provides much higher speed than classical gel electrophoresis methods, and it provides automated anlysis of cellular proteins at the single-cell level; the separation is complete in 30 min and the entire analysis takes approximately 45 min.

Characterization of dynamically prepared phospholipid-modified reversed-phase columns

We have modified a reversed-phase (RP8) column by passing through it an aqueous solution of phosphatidylcholine-based liposomes. The phospholipids from the liposomes adsorb onto the octyl chain of the stationary phase, thus altering the nature of the stationary phase and of the chromatographic interactions. The properties of the phospholipid-modified column were investigated using solutes belonging to several chemical classes. We found that the retention factors of negatively and positively charged solutes decreased as the amount of phospholipid in the column was increased. For the solutes studied here the extent of the decrease was smaller for the positive solutes. With neutral solutes, the retention factors of some (benzenediols) increased markedly while with others (ketones) the retention factors decreased. The selectivities between the various solutes on the phospholipid-modified column were different than on the original reversed-phase column. The retention behavior of the solutes can be explained in terms of (1) effective shielding of the hydrophobic part of the stationary phase by the polar head groups of the phospholipids and (2) hydrogen bond formation between the solutes and the carbonyl oxygens as well as the non-ester phosphate oxygens in the polar head groups of the phospholipids.

Two-wavelength fluorescence assay for DNA repair

A simple and reliable quantitative assay for measuring cellular DNA repair capacity has been developed. It is based on the host cell reactivation of the UV-irradiated plasmid pEGFP carrying the marker gene for the enhanced green fluorescent protein (EGFP). As a reference we used the plasmid pEYFP carrying the gene for a red-shifted fluorescent protein (EYFP). Both proteins can be excited by visible light with a maximum at 488 nm, but EGFP emits with a maximum at 509 nm, while EYFP emits with a maximum at 527 nm. This makes it possible to monitor the expression of the two genes simultaneously by measuring the fluorescence at two wavelengths. HEK293 cells were cotransfected with a mixture of UV-irradiated pEGFP and undamaged pEYFP. At different time intervals after transfection the fluorescence of EGFP was determined relative to the fluorescence of EYFP to compensate for any differences in the transfection efficiency or other experimental variables. It was used to calculate the number of UV lesions in DNA and hence the repair capacity of the host cells. It was found that HEK293 cells were able to repair approximately 1.4 UV lesions per 1000 nucleotides DNA for 12 h on the average.