Use of FLIPR membrane potential dyes for validation of high-throughput screening with the FLIPR and microARCS technologies: identification of ion channel modulators acting on the GABA(A) receptor

Fluorometric imaging plate reader (FLIPR) membrane potential dyes (FMP-Red-Dye and FMP-Blue-Dye) were evaluated for the detection of compounds acting either as positive allosteric modulators or agonists on the GABA(A) receptor (GABA(A)R). A stable HEK293 cell line with constitutive expression of the rat GABA(A)R alpha1, beta2, and gamma2 genes was used to establish a functional high-throughput screening (HTS) assay based on measurement of the membrane potential change in living cells. The assay was validated with the FLIPR technology for identification of agonists and positive allosteric modulators using GABA and diazepam as model compounds. The FMP-Red-Dye showed better performance than the FMP-Blue-Dye, and the effects induced by GABA and diazepam were comparable to electrophysiology data. Subsequently, the assay was also validated with an ultra-HTS approach known as microarrayed compound screening (microARCS). The LOPAC library was used in a test screen for an initial assessment of the technology. Finally, the FLIPR and microARCS technologies were tested with a larger screening campaign. A focused library of 3520 putative positive modulators was tested with the FLIPR assay, and a diverse subset of 84,480 compounds was selected for screening with the microARCS technology. All hits were subjected to verification using the FLIPR technology, and confirmed hits were subsequently evaluated by EC50 determination. Finally, selected hits were further confirmed with electrophysiology testing.     

An offline-addition format for identifying GPCR modulators by screening 384-well mixed compounds in the FLIPR

Although fluorescence imaging plate reader (FLIPR)-based assays have been widely used in high-throughput screening, improved efficiencies in throughput and fidelity continue to be investigated. This study presents an offline compound addition protocol coupled with a testing strategy using mixtures of compounds in a 384-well format to identify antagonists of the neurokinin-1 receptor expressed in the human astrocytoma cell line (U373 MG). Substance P evoked a concentration-dependent increase in intracellular cellular Ca(2+) with an EC(50) value of 0.30 +/- 0.17 nM, which was inhibited by neurokinin-1 (NK1) antagonists L-733,060 and L-703,606. Test compounds, as mixtures of 10 compounds/well, were added to the cells offline using an automated dispensing unit and incubated prior to performing the assay in the FLIPR. Using the offline protocol, a higher through put of ~200,000 compounds was achieved in an 8-h working day, and several novel structural classes of compounds were identified as antagonists for the NK1 receptor. These studies demonstrate that the offline compound addition format using a mixture of compounds in a 384-well FLIPR assay provides an efficient platform for screening and identifying modulators for G-protein-coupled receptors.     

Validation of a fluorescent imaging plate reader membrane potential assay for high-throughput screening of glycine transporter modulators

A fluorescent imaging plate reader (FLIPR) membrane potential (V(m)) assay was evaluated for pharmacological characterization and high-throughput screening (HTS) of rat glycine transporter type 2 (rGlyT(2)) in a stable rGlyT(2)-HEK cell line. Data show that glycine activation of rGlyT(2) consistently results in a concentration-dependent V(m) response on the FLIPR that is blocked by the potent and selective GlyT(2) antagonist 4-benzyloxy-3,5-dimethoxy-N-[1-dimethylamino-cyclopentyl)methyl]-benz-amide (Org-25543). Agonist and antagonist pharmacologies match those reported using conventional [(3)H]glycine uptake assays and electrophysiology. The glycine response is dependent on buffer ionic composition consistent with GlyT(2) physiology. Assay signal-to-background and coefficient of variation meets sufficient statistical criteria to conduct HTS. The results of a screen of the chemical inventory demonstrate that the assay is able to successfully identify and confirm GlyT(2) inhibitors. The advantages of this assay are its homogeneity, compatibility with both 96- and 384-well formats, and lack of radioactivity usage. Thus, the authors conclude that a fluorescence-based V(m) assay on FLIPR is a viable approach for identification and pharmacological profiling of small molecule modulators of the electrogenic transporter rGlyT(2).     

State-dependent compound inhibition of Nav1.2 sodium channels using the FLIPR Vm dye: on-target and off-target effects of diverse pharmacological agents

Voltage-gated sodium channels (NaChs) are relevant targets for pain, epilepsy, and a variety of neurological and cardiac disorders. Traditionally, it has been difficult to develop structure-activity relationships for NaCh inhibitors due to rapid channel kinetics and state-dependent compound interactions. Membrane potential (Vm) dyes in conjunction with a high-throughput fluorescence imaging plate reader (FLIPR) offer a satisfactory 1st-tier solution. Thus, the authors have developed a FLIPR Vm assay of rat Nav1.2 NaCh. Channels were opened by addition of veratridine, and Vm dye responses were measured. The IC50 values from various structural classes of compounds were compared to the resting state binding constant (Kr)and inactivated state binding constant (Ki)obtained using patch-clamp electrophysiology (EP). The FLIPR values correlated with Ki but not Kr. FLIPRIC50 values fell within 0.1-to 1.5-fold of EP Ki values, indicating that the assay generally reports use-dependent inhibition rather than resting state block. The Library of Pharmacologically Active Compounds (LOPAC, Sigma) was screened. Confirmed hits arose from diverse classes such as dopamine receptor antagonists, serotonin transport inhibitors, and kinase inhibitors. These data suggest that NaCh inhibition is inherent in a diverse set of biologically active molecules and may warrant counterscreening NaChs to avoid unwanted secondary pharmacology.     

Metabolic characterization of a hyper-productive state in an antibody producing NS0 myeloma cell line

Metabolic profiling or metabolomics is the analysis of a larger number of small metabolic compounds within cells. While this technique has been utilized to study microbial and yeast strains under different physiochemical conditions, very little has been reported regarding its application in mammalian cell culture. Here, the physiological and metabolic changes observed during the proliferation arrest of an antibody producing GS-NS0 mouse myeloma cell line were studied using conventional biochemical analysis and one-dimensional nuclear magnetic resonance (NMR)-based metabolic profiling. Proliferation-arrested cells had increased antibody productivity, enhanced normalized mitochondrial membrane potential, and showed changes in the consumption of several amino acids. Further investigation into these physiological changes was carried out by ¹H NMR profiling followed by principle component analysis (PCA). The resulting data showed a clear separation of the arrested and control spectra that related to the altered metabolic state of the arrested culture. Metabolites associated with phosphatidylcholine homeostasis, lipid and fatty acid metabolism, and ascorbate formation were found to be present in significant amount in these cultures. Taken together, the results suggested that there was a link between the metabolic alterations and the hyper-productive state, possibly relating to vesicle recycling and secretory functions, and mechanism to counteract against the generation of reactive oxygen species. While the use of metabolic profiling is still in its infancy, its potential to enhance the understanding of physiological processes in mammalian cell lines used for antibody production is certain.     

The use of the immobilization of whole living cells to increase stability of recombinant plasmids in escherichia coli

Immobilization of whole living cells was used as an experimental approach to enhance plasmid stability in cultured recombinant micro-organisms. pTG201 plasmid which is very unstable in continuous cultures with free cells, was found to be extremely stable in continuous cultures with immobilized cells.To elucidate the mechanism by which immobilization increases the plasmid stability, we analyzed the growth of pTG201-containing E. coli W3101 cells within the gel beads. We found that in immobilized continuous culture, plasmid-free segregants were not detected even after 240 generations. This appears to be due to the mechanical properties of the gel-bead system that allow only a limited number of cell divisions (10–16) to occur in each clone of cells before the clone escapes from the gel bead. This number of generations is not sufficient for the plasmid-free cells to appear within the cavities compared to what was observed in a free-system (plasmid-free segregants were detected after a lag period of approximately 25–30 generations). Even when they appear, they cannot overcome the culture. From the data described in this paper we conclude that cells released from the gel beads at any time during continuous culture are cells which are issued from cells grown in the cavities for only 10–16 generations.     

Design and characterization of a new bioreactor for continuous ultra-slow uniaxial distraction of a three-dimensional scaffold-free stem cell culture

A computer controlled dynamic bioreactor for continuous ultra-slow uniaxial distraction of a scaffold-free three-dimensional (3D) mesenchymal stem cell pellet culture was designed to investigate the influence of stepless tensile strain on behavior of distinct primary cells like osteoblasts, chondroblasts, or stem cells without the influence of an artificial culture matrix. The main advantages of this device include the following capabilities: (1) Application of uniaxial ultra-slow stepless distraction within a range of 0.5-250 μm/h and real-time control of the distraction distance with high accuracy (mean error -3.4%); (2) tension strain can be applied on a 3D cell culture within a standard CO(2) -incubator without use of an artificial culture matrix; (3) possibility of histological investigation without loss of distraction; (4) feasibility of molecular analysis on RNA and protein level. This is the first report on a distraction device capable of applying continuous tensile strain to a scaffold-free 3D cell culture within physiological ranges of motion comparable to distraction ostegenesis in vivo. We expect the newly designed microdistraction device to increase our understanding on the regulatory mechanisms of mechanical strains on the metabolism of stem cells.     

Multiplexed G-protein-coupled receptor Ca2+ flux assays for high-throughput screening

An early drug discovery approach focusing on gene families can benefit from strategies that exploit common signaling mechanisms to more effectively identify and characterize novel chemical lead structures. Multiplexing, defined as the screening of multiple targets within the same experiment, is an example of this strategy. Here, the authors describe a technique that allows multiplexing of a common assay type used to study G-protein-coupled receptors: changes in intracellular Ca2+ levels as measured by Molecular Device's fluorometric imaging plate reader (FLIPR). The multiplexed FLIPR assays showed the expected pharmacological properties of single assays, with good reproducibility and Z* factors. The authors used them to screen large compound libraries in 2 multiplexed assay designs. The 1st used a single-cell line expressing 2 different receptors and the 2nd a mixture of 2 cell lines of the same type each expressing distinct receptors. Screening using these multiplexed assays produced significant savings in reagents, time, and human resources and allowed the authors to quickly identify specific and selective hits.     

Generation and characterization of continuous lines of CD8+ suppressor T lymphocytes

The ability to grow normal T lymphocytes in long term culture has advanced our understanding of T cell biology. The growth of CD4+ cell lines allowed a further evaluation and appreciation of functional subtypes within this group. Cytotoxic CD8+ T cells have been characterized as well. The routine and continuous culture of Ag-nonspecific CD8+ Ts cells has been difficult to achieve. We have found that CD8+ T cells that suppress T cell proliferation and lack cytotoxic activity against T cells can be routinely obtained from PWM or PHA-stimulated PBMC. Continuous culture of T cell blasts from PWM or PHA-stimulated PBMC resulted in the growth of CD4+ and CD8+ T cells. These lines developed suppressor cell activity within 7 days after stimulation with PWM and 3 to 4 wk after stimulation with PHA. Concomitant with the development of suppressor activity was the loss of CD4+ T cells resulting in homogeneous lines of CD8+ suppressor cells. These cell lines have been maintained in continuous culture for greater than 6 mo by addition of rIL-2 twice weekly and restimulation with feeder cells and PHA every 2 wk. Activity of these cell lines was relatively resistant to irradiation or treatment with mitomycin C. Both cell lines suppressed proliferation of autologous or heterologous CD4+ T cells stimulated with PWM, OKT3, or tetanus toxoid but failed to suppress proliferation of CD4+ T cells in a mixed lymphocyte reaction. CD4+ T cells stimulated with PWM produced equivalent amounts of IL-2 in the presence or absence of Ts cells but failed to express the IL-2R (TAC) on their surface in the presence of Ts cells. By contrast, CD4+ T cell lines or cytotoxic CD8+ T cell lines failed to suppress proliferation of CD4+ T cells. With these results we describe methods for the generation and continuous culture of Ag-nonspecific CD8+ Ts cells and define some of their properties. These cells lines should be helpful in further elucidating the functional and phenotypic repertoire of CD8+ Ts cells.     

High-Resolution Functional Profiling of the Norovirus Genome

Human noroviruses (HuNoV) are a major cause of nonbacterial gastroenteritis worldwide, yet details of the life cycle and replication of HuNoV are relatively unknown due to the lack of an efficient cell culture system. Studies with murine norovirus (MNV), which can be propagated in permissive cells, have begun to probe different aspects of the norovirus life cycle; however, our understanding of the specific functions of the viral proteins lags far behind that of other RNA viruses. Genome-wide functional profiling by insertional mutagenesis can reveal protein domains essential for replication and can lead to generation of tagged viruses, which has not yet been achieved for noroviruses. Here, transposon-mediated insertional mutagenesis was used to create 5 libraries of mutagenized MNV infectious clones, each containing a 15-nucleotide sequence randomly inserted within a defined region of the genome. Infectious virus was recovered from each library and was subsequently passaged in cell culture to determine the effect of each insertion by insertion-specific fluorescent PCR profiling. Genome-wide profiling of over 2,000 insertions revealed essential protein domains and confirmed known functional motifs. As validation, several insertion sites were introduced into a wild-type clone, successfully allowing the recovery of infectious virus. Screening of a number of reporter proteins and epitope tags led to the generation of the first infectious epitope-tagged noroviruses carrying the FLAG epitope tag in either NS4 or VP2. Subsequent work confirmed that epitope-tagged fully infectious noroviruses may be of use in the dissection of the molecular interactions that occur within the viral replication complex.     

Short tandem repeat profiling: part of an overall strategy for reducing the frequency of cell misidentification

The role of cell authentication in biomedical science has received considerable attention, especially within the past decade. This quality control attribute is now beginning to be given the emphasis it deserves by granting agencies and by scientific journals. Short tandem repeat (STR) profiling, one of a few DNA profiling technologies now available, is being proposed for routine identification (authentication) of human cell lines, stem cells, and tissues. The advantage of this technique over methods such as isoenzyme analysis, karyotyping, human leukocyte antigen typing, etc., is that STR profiling can establish identity to the individual level, provided that the appropriate number and types of loci are evaluated. To best employ this technology, a standardized protocol and a data-driven, quality-controlled, and publically searchable database will be necessary. This public STR database (currently under development) will enable investigators to rapidly authenticate human-based cultures to the individual from whom the cells were sourced. Use of similar approaches for non-human animal cells will require developing other suitable loci sets. While implementing STR analysis on a more routine basis should significantly reduce the frequency of cell misidentification, additional technologies may be needed as part of an overall authentication paradigm. For instance, isoenzyme analysis, PCR-based DNA amplification, and sequence-based barcoding methods enable rapid confirmation of a cell line’s species of origin while screening against cross-contaminations, especially when the cells present are not recognized by the species-specific STR method. Karyotyping may also be needed as a supporting tool during establishment of an STR database. Finally, good cell culture practices must always remain a major component of any effort to reduce the frequency of cell misidentification.     

Evaluation of no-wash calcium assay kits: enabling tools for calcium mobilization

The no-wash calcium assay kits developed by Molecular Devices Corporation have greatly enhanced the throughput of cell-based calcium mobilization high-throughput screening (HTS) assays and enabled screening using nonadherent cells. The fluorescent imaging plate reader (FLIPR) Calcium 3 Assay Kit, optimal for targets that have proteins or peptides as agonists, has 2 potential drawbacks: 1) a significant downward spike in fluorescence signal upon liquid transfer that can be the same magnitude as the agonist response, making data analysis difficult; and 2) medium removal is required for some targets, which essentially reintroduces a wash step. Several no-wash products were introduced in 2005. The authors compare the Fluo-4 NW Calcium Assay Kit and the BD Calcium Assay Kit with the FLIPR Calcium 3 Assay Kit using human native rhabdomyosarcoma cells expressing the urotensin-II receptor (UT). The BDtrade mark Calcium Assay Kit gives the best performance in the true no-wash mode, in which both agonist and antagonist activity are easily quantified. Although these new products provide additional options for measuring calcium mobilization, the different results observed with each kit, using the UT receptor as an example, suggest that one should characterize all dyes against each target in a systematic way prior to choosing one for HTS.     

Long-term stroma-dependent cultures are a consistent source of immunostimulatory dendritic cells.

A long-term culture (LTC) system has been established that supports the continuous production of dendritic cells (DC) from haemopoietic cells present in the culture. The production of cells depends on the presence of an intact stromal cell layer containing a mixture of fibroblasts and endothelial cells. Cells are shed from foci of dividing cells in contact with the stromal cell matrix. They resemble DC in terms of morphology and cell surface marker expression. The LTC can be derived from different lymphoid tissues, but most success has been achieved with murine spleen. Different LTC vary in capacity to produce immunostimulatory DC. Some LTC produce DC that are very effective APC and can stimulate both mixed lymphocyte and antigen-specific T cell responses. The DC produced in others are weak APC. Different LTC appear to produce DC reflecting different stages of maturation or development, reflected by different phenotypic and functional characteristics. The production of cells within LTC occurs independently of added cytokines and is dependent on maintenance of the stromal cell layer and the presence of a subset of smaller progenitor cells. Long-term cultures remain a valuable source of cells for study of DC development and function.     

Effects on product formation in Lactococcus lactis 65.1 in continuous culture with complete cell recycling

Lactococcus lactis 65.1 was cultivated in a batch culture, which underwent starvation for 3 days, continuous culture and continuous culture with complete cell recycling. The objective was to study the product formation and intracellular protein pattern. Changes from homofermentative to heterofermentative metabolism were observed in continuous culture at the lower dilution rates as well as continuous cultures with complete cell recycling at a fixed dilution rate (D=0.4 h^–1). Changes in intracellular protein pattern were observed when starving the cells in a batch culture and also when recycling the cells in a continuous culture. Some changes were the same in these two cases. The data collected from these experiments show how the fermentation technique can affect the products of the microorganism being cultured and gives some interesting information on the complete cell recycling technique, which is of great interest in fermentation processes.     

A modular suite of hardware enabling spaceflight cell culture research

BioServe Space Technologies, a NASA Research Partnership Center (RPC), has developed and operated various middeck payloads launched on 23 shuttle missions since 1991 in support of commercial space biotechnology projects. Modular cell culture systems are contained within the Commercial Generic Bioprocessing Apparatus (CGBA) suite of flight-qualified hardware, compatible with Space Shuttle, SPACEHAB, Spacelab and International Space Station (ISS) EXPRESS Rack interfaces. As part of the CGBA family, the Isothermal Containment Module (ICM) incubator provides thermal control, data acquisition and experiment manipulation capabilities, including accelerometer launch detection for automated activation and thermal profiling for culture incubation and sample preservation. The ICM can accommodate up to 8 individually controlled temperature zones. Command and telemetry capabilities allow real-time downlink of data and video permitting remote payload operation and ground control synchronization. Individual cell culture experiments can be accommodated in a variety of devices ranging from 'microgravity test tubes' or standard 100 mm Petri dishes, to complex, fed-batch bioreactors with automated culture feeding, waste removal and multiple sample draws. Up to 3 levels of containment can be achieved for chemical fixative addition, and passive gas exchange can be provided through hydrophobic membranes. Many additional options exist for designing customized hardware depending on specific science requirements.     

The chronic effects of db-cAMP on cell cycle parameters and cell size in asynchronous culture of Chinese hamster ovary cells

N⁶,O^2′-dibutyryl adenosine 3′,5′-cyclic-phosphate (db-cAMP) has been shown to convert Chinese hamster cells of ovarian origin (CHO-K₁) from compact, randomly oriented cells growing in multilayers to elongated fibroblast-like cells which grow in monolayers. This compound also has been reported to have a variety of effects on the cell cycle. Most such studies have employed synchronized cells to determine cell cycle effects, and consequently have been limited to the short-term effects of the compound. We have looked for chronic effects on the cell cycle in cultures exposed continuously to db-cAMP from the initiation of the cultures until they had reached or approached the plateau phase. This was done by combined autoradiography and Feulgen microspectrophotometry plus measurements of the protein content of mitotic cells to detect any influence on cell size. The overall results were that continuous exposure to db-cAMP had at most only minor effects on the cell cycle and cell size when the culture medium was renewed daily. Somewhat greater effects were found on plateau-phase cells in cultures in which the medium was not renewed. In this case fewer cells appeared to remain in the cell cycle in the cultures with db-cAMP. Comparison with our earlier results with Chinese hamster V79 cells led to the conclusions that cell cycle parameters and cell size at mitosis were less altered during culture growth in CHO cells, but that CHO cells seemed to be less able to maintain cells in the cell cycle in crowded cultures.     

Whole genome expression profiling using DNA microarray for determining biocompatibility of polymeric surfaces

There is an ever increasing need to find surfaces that are biocompatible for applications like medical implants and microfluidics-based cell culture systems. The biocompatibility of five different surfaces with different hydrophobicity was determined using gene expression profiling as well as more conventional methods to determine biocompatibility such as cellular growth rate, morphology and the hydrophobicity of the surfaces. HeLa cells grown on polymethylmethacrylate (PMMA) or a SU-8 surface treated with HNO3-ceric ammonium nitrate (HNO3-CAN) and ethanolamine showed no differences in growth rate, morphology or gene expression profiles as compared to HeLa cells grown in cell culture flasks. Cells grown on SU-8 treated with only HNO3-CAN showed almost the same growth rate (36 +/- 1 h) and similar morphology as cells grown in cell culture flasks (32 +/- 1 h), indicating good biocompatibility. However, more than 200 genes showed different expression levels in cells grown on SU-8 treated with HNO3-CAN compared to cells grown in cell culture flasks. This shows that gene expression profiling is a simple and precise method for determining differences in cells grown on different surfaces that are otherwise difficult to find using conventional methods. It is particularly noteworthy that no correlation was found between surface hydrophobicity and biocompatibility.