Applications - Application Notes
Optimization of a Multi-Mode Detection Model for Measuring Real-time Cellular Respiration and Mitochondrial Function using Fluorophoric Biosensors
19-Jul-16Cytation 3, Synergy 2, Synergy H1, Synergy Neo2
Authors: Wendy Goodrich, Applications Department, BioTek Instruments, Inc., Winooski, VT, USA; Conn Carey, Luxcel Biosciences, Cork, Ireland
Cellular metabolism characterization is being aided by the development of new tools designed to provide ease-of-use, higher throughput, and multiplexed data markers for analysis. One of these tools is a simple mix-and-measure assay compatible with a variety of cellular matrices that utilizes fluorophoric probes to measure oxygen consumption rates (OCR), extracellular acidification (ECA), and intracellular oxygen levels useful to inform on the activity of the electron transport chain (ETC) and glycolytic flux. These probes can be detected using standard fluorescence, time-resolved fluorescence, or lifetime fluorescence with reduced background and increased signal dynamic range dependent on the detection mode. Optimization of biosensor recognition in all three fluorescent modes was done in microplate format using multiple cell lines and drug compound treatments. In particular, the lifetime timeresolved fluorescent mode is highlighted for generating drug compound dose response against OCR (µs/hr), presenting accurate comparisons of acidification rates converted to hydrogen ion scale (ECA[H+]/t), and converting lifetime detection to intracellular oxygen (% O2) in parallel with fluorescent imaging in live cell 2D monolayers.
Assay Overview and Detection Principle
The MitoXpress® Xtra - Extracellular Oxygen Consumption Assay [HS Method], pH-Xtra™ Glycolysis Assay, and MitoXpress® Intra – Intracellular O2 Assay are a family of fluorescent probes designed by Luxcel Biosciences to aide in the study of real-time analysis of mitochondrial function, metabolism and toxicity in a variety of biological matrices. The probes are chemically stable and inert, water-soluble, and can be multiplexed. The amount of fluorescent signal is an inverse relationship to intra- or extracellular O2 or proportional to extracellular H+ in the sample. Intracellular O2 levels (% O2), extracellular Oxygen Consumption Rate (OCR), and quantification of H+ levels are calculated from the changes in fluorescence signal over time. Probes can be optimized using a validation protocol in the kit insert for wet testing. Optimal detection parameters can then be applied to verification of cell lines and cell seeding densities using compound controls. Independent testing should be conducted to optimize each cell line individually.
Figure 1. Excitation and Emission spectra of the pH-Xtra™ Glycolysis Assay demonstrating normalized excitation (top left) and 3- to 6-fold increase in emission peak signals in response to increased acidification (top right). A 6-fold delta at 620 nm provides an optimal window for detecting signal change in response to pH levels. The MitoXpress® Xtra- and Intra-cellular probe show emission peaks at a normalized intensity of 380 nm (center left). The inverse relationship between probe signal and O2 levels is seen by a 4-fold increase of signal in deoxygenated conditions at emission peak 645 nm (center right). Principle of an optional dual time-resolved fluorescent lifetime (τ) detection mode that utilizes two reads at different times over the decay of the probe to increase stability and dynamic range in signal acquisition (bottom).
Cytation™ 3 Cell Imaging Multi-Mode Reader with Gas Controller combines automated digital microscopy and conventional microplate reading in one instrument. Its unique patent pending design is ideal for research and assay development applications in the field of cell biology. It was used in both imaging and filter based detection mode as shown by Table 1 at 37 °C using the O2 gas control module. The gas controller is also compatible with the Synergy™ H1 and Synergy™ Neo 2.
Synergy™ Neo 2 Hybrid Reader is a patented HTS multi-mode microplate reader with multiple parallel detectors for ultra-fast measurements and a dedicated filter-based optical system for live cell assays. Table 1 contains the optical parameters used for signal optimization and validation of Luxcel probes for live cell metabolic analysis. It is compatible with the gas controller and has onboard incubation.
Synergy™ H1 Multi-Mode Reader is a flexible monochromatorbased multi-mode microplate reader that can be turned into a high-performance Hybrid System with the addition of a filter-based optical module. The filter module is a completely independent add-on that includes its own light source, and a high performance dichroic-based wavelength selection system that was used as shown by Table 1. The reader can also be partnered with the gas controller.
Synergy™ 2 Multi-Mode Reader offers performance, speed and sensitivity. Based on BioTek’s popular Synergy™ HT platform, Synergy 2 has been further enhanced with improved sensitivity in Fluorescence Intensity by utilizing a dedicated optical element. Table 1 contains the detection configuration used for validation of Luxcel probes in standard RFU mode.
* A low well would be untreated cells seeded at 5-7 x 104 cells/well in 150 µL of fresh media w/probe in a 96-well microplate.
** A high well would be a GOx control without cells diluted in cell media containing probe.
Table 1. Detection settings for Luxcel probes on BioTek readers for results shown in this Application Note. The settings above cover a variety of instruments, optics positions, detection options, and read parameters that are recommended starting points for probe optimization when using other detection or configuration options available on BioTek readers and imagers. The assay wet test and cell line optimization protocols described in the kit inserts can be used to validate other configuration options.
Materials and Methods
- Luxcel Biosciences MitoXpress Xtra Oxygen Consumption Assay (HS Method) (Catalog No. MX-200)
- Luxcel Biosciences MitoXpress Intra - Intracellular O2 Assay (Cat No. MX-300)
- Luxcel Biosciences pH-Xtra - Glycolysis Assay (Cat No. PH-200)
- Glucose Oxidase (GOx) powder (e.g. Sigma Cat No. 49180) reconstituted in sterile water
- Respiration Buffer (1M Glucose+DMEM media to final 40 mM glucose concentration)
- Phosphate Buffered Saline (PBS e.g. Sigma P4417) at pH 7.25 and 6.2 adjusted w/ 0.1 mM NaOH or HCl
- Respiration Media (DMEM Sigma D5030, powder), 20mM HEPES, 1mM Sodium Pyruvate, 20 mM, Glucose, 10% FBS, 10% Pen-Strep)
- DMEM culture media (depending on cell type(s)) + additives (10% FBS, 10% P/S)
- DMEM culture media + additives + 20 mM glucose (wet testing media)
- Corning COSTAR 96-well microplate (#3904) and High Content Imaging Plate #4680
- Nunc™ MicroWell™ 96-Well Optical-Bottom Plates (Thermo Scientific p/n 165305)
- HEK293 cells stably transfected with antibiotic resistant marker (proprietary)
- HepG2 cells grown and cultured from stock
- Rotenone (2.5 µM final) vehicle DMSO
- FCCP in vehicle DMSO
- Antimycin A in vehicle DMSO
- Phenformin (50 µM final) in vehicle sterile water
Figure 2. 96-well Plate Map and reagent volumes for wet test signal optimization of Luxcel fluorescent probes on BioTek instrumentation. Wet testing of probes to validate detection parameters is recommended before implementing live cell analysis. Extracellular (-XTRA) probe tested with High Sensitivity (HS) oil provided with the kit.
Figure 3. Typical wet test results for the extracellular assay probe comparing signal:background (S/B) from top and bottom optics position on Synergy Neo2 (n=12) for each of the dual TR-F reads. The extracellular assay requires an oil overlay, but these results support that either optics position will produce similar signal response differentiation. This data suggests that the top optics position may result in less variability, and the bottom optics position in a wider assay window. Signal window can be further optimized by increasing measurements per well (mw) or adjusting the gain or plate height.
Figure 4. Example of BioTek's Gen5 Software protocol settings for wet test validation of Luxcel extracellular (-XTRA) probe in lifetime detection mode (left). The lifetime detection mode is implemented as two separate TR-F reads within a single kinetic loop at the fastest interval allowed. The first read is at TR-F 30_30 (delay_collection time), and the second immediately following using 70_30 (delay_collection time). In this example, gain is determined before the kinetic loop, with the value used in subsequent read steps. Following a blank step on each read, lifetime is calculated by using a custom transformation in the data reduction step of the protocol (right). Sample files are available on www.biotek.com for additional protocol definitions.
All methods used pre-warmed plates, media, and buffers. Probes are reconstituted in 1mL sterile water and assayed at RT. Compounds are kept at -20 °C and brought to RT. Partial plate analysis was done by only reading assay wells of the 96-well plate (Figure 2). Full plate analysis was done by reading all wells of the 96-well plate.
Signal Optimization, H+ Quantitation, Cellular Metabolic Analysis
MitoXpress Xtra: For signal optimization - assay in volumes and locations shown in Figure 2 (top). Commence kinetic read (fastest interval) at 30 °C using detection mode and parameters defined by Table 1. Figures 5 and 6 show data in kinetic average. Top and bottom optics position S/B comparative wet test results (Figure 3) were generated from Synergy Neo2 on 2 different plates using the same lot of media and probe by dispensing 12 replicates of 10 µL probe in 140 µL 20 mM glucose media to wells A1 – A12 (PC), and 12 replicates of 140 µL 20 mM glucose media to wells C1-C12 (BLK) followed by a dispense of 100 µL of high sensitivity oil to all wells. For cellular metabolic analysis – assay following the kit insert procedure using compound treatments of Antimycin (1 µM final, or as a 1:2 dilution from a start concentration of 1 µM); Rotenone (1 µM final); Phenformin (50 µM final); and FCCP (1 µM final, or as a 1:2 dilution from a start concentration of 20 µM) on Day 2 of the protocol. Blanks (150 µL media + 100 µL oil overlay only), Signal Control (15x probe in media), and PC (10 µL 15x probe, 10 µL 15x GOx (solubilized 1mg/mL in 1 mL sterile water) to 130 µL media per well) were run on each plate. Data shown by Figures 8 and 10.
pH-Xtra: For signal optimization and pH scale/ H+ conversion - assay in volumes and locations shown in Figure 2 (bottom). Commence 90 minute kinetic read (fastest interval) on H1 at 30 °C in full and partial plate read modes using lifetime detection parameters defined by Table 1. Data shown by Figure 7.
MitoXpress Intra: For cellular metabolic analysis - HepG2 cells were plated in 200 µL culture media and incubated ON at 37 °C 5% CO2 . On Day 2 probe was reconstituted 1:11 in culture media. Spent cell media was aspirated and replaced with 100 µL/well of Intra Probe in media stock then incubated ON at 37 °C 5% CO2 . On Day 3 spent media was aspirated and cells were washed twice with Respiration Media. After the final aspiration 150 µL of fresh Respiration media was added to the wells. Controls were run on each plate as described for the Xtra probe. 1 µL of test compounds were then added. Commence kinetic read at 37 oC using either Synergy Neo 2 or Cytation 3 with parameters shown by Table 1. Data shown by Figures 9 and 11.
Figure 5. Assay window results on average signal for ambient (baseline ~19% O2 ), 0% O2 (glucose depletion via GOx), and blank in standard TR-F mode for MitoXpress Xtra. The kinetic interval for a full plate was 50 seconds between reads over 45 mins.
Figure 6. Correlation of lifetime signal stability for partial and full plate reads for MitoXpress Xtra. The full plate interval results in a 0.0274/sec change in assay window compared with 0.14385/sec in partial plate mode.
Figure 7. (Left) Lifetime signal reproducibility between full and partial plate reads for pH-Xtra probe on the Synergy H1 at 30 °C. Using a default conversion function, pH is calibrated from lifetime values and H+ is quantified from pH. ECAR is calculated from H+ on average pH over the 45 min kinetic read. Δ pH are all <1.6% of origin although slightly higher than within error range. Delta pH can be improved by performing instrument specific pH calibration to adjust calculation variables. (Right top) Lifetime profiles of GOx diluted 1:10 in respiration buffer from GOx1 converted to pH scale (bottom) demonstrates the analogous relationship between probe signal change and acidification rate (greater acidification= increased signal change), where the highest [GOx] (GOx1) yields the greatest acidification / signal increase (red profiles GOx1).
Figure 8. Synergy 2 RFU detection of Extracellular O2 probe in HEK293 cells at 5 cell densities under 2 drug treatments compared to basal response. OCR becomes non-linear at 100K cells/well in basal and FCCP treated cells. 1 µM of Antimycin inhibits oxygen rates regardless of cell density (left). HEK293 cells plated at 6 x 104/well were used to validate OCR inhibition compared to both basal cell response and signal baseline of extracellular probe (right). RFU is converted to OCR (MeanV = RFU/ min) from a 40 minute kinetic read with results illustrating 10 fold rate inhibition from compound dose shown.
Figure 9. Dose dependent oxygen consumption rate (OCR) in HepG2 cells (70K/well) as calculated from Synergy Neo2 lifetime measurement from the linear portion (9:16 -130 mins) of a 3 hour kinetic run. (Bottom) AntiMycin dose response maintains basal cell oxygen consumption rates at 0.0075 µM, but results in complete inhibition of the mitochondrial ETC (Electron Transport Chain) at 1 μM. (Top) FCCP stimulation of maximal respiration is indicated at 2.5 µM followed by inhibition of oxygen depletion to below basal levels at higher concentrations.
Figure 10. Lifetime detection of extracellular oxygen consumption in HepG2 cells detected on Cytation 3. Consistent, stable lifetime measurements are shown over the full time course at a kinetic interval of 2:19 mins (top). Slope is calculated from the linear portion (10-80 mins) of the 2 hour kinetic run. Predicted and well differentiated cellular OCR response of basal (UT) and agonist/ antagonist treatment is shown bottom.
Figure 11. (Top) Intracellular response in respiring and non-respiring cells to progressively lower applied O2 is measured in parallel by fluorescent imaging (4x) and lifetime detection of HepG2 cells using the Cytation 3 and gas controller over a 3 hour kinetic time course. Cells were plated at 7 x 104 cells/well and read on Day 3. Oxygen levels were decreased at intervals shown and images were captured at the end of each applied O2 level without interrupting the read. Mean RFU values calculated from images at 3 intervals of decreased O2 levels in Antimycin treated cells illustrates the principle detection MoA of the Luxcel probes. (Bottom) Lifetime conversion to Intracellular O2 (% O2) is done using a First Order Exponential function with the default calibration constantsin Gen5 Software. A protocol file that includes read parameters and calculations for performing this conversion is available from the Sample Files tab under Gen5 at http://www.biotek.com/resources/app_notes.html. It can be adapted to independent experimental conditions using Gen5 version 2.07 or higher. Using the Same first Order Experimental Fit Function in combination with the gas controller, a user specific calibration function with new constants can be generated.
- Luxcel MitoXpress –Xtra, -Intra, and pH-Xtra probes can be detected in RFU, standard TR-F, and lifetime detection modes using a variety of BioTek readers to inform cellular metabolic analysis.
- Signal acquisition of Luxcel probes using the BioTek lifetime detection algorithm is stable for kinetic interval times from 20 sec to 2:19 minutes. Standard TR-F and RFU detection modes can be done on a full plate in kinetic intervals <= 50 seconds.
- Detecting pH-Xtra probe in lifetime mode allows direct conversion of signal to pH scale and H+ quantification using a default conversion function.
- BioTek readers and Luxcel probes are compatible for analyzing extracellular oxygen consumption rates and quantifying intracellular O2 levels in whole cells, using the optimal dual-read TR-F measurement mode and subsequent conversions.
- The MitoXpress-Intra probe is conducive to fluorescent imaging, facilitating increased data analysis options - demonstrated here paired with lifetime detection and a gas controller, intracellular O2 response to progressive reduction in oxygen can be applied to yield a user generated calibration function using a First Order Exponential Fit in Gen5 software.
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