Applications - Application Notes
Use of Phase Contrast Imaging to Track Morphological Cellular Changes due to Apoptotic Activity
Authors: Brad Larson and Peter Banks, Applications Department, BioTek Instruments, Inc., Winooski, VT
Phenotypic screening, or the determination of the effects (phenotypes) that a molecule has on a cell, tissue, or whole organism, dates back to earliest drug discovery efforts. Due to advances in molecular biology and biochemistry in the 1990s, this approach was de-emphasized in favor of a more “reductionist” target-based approach1. Through mounting evidence, however, the shift appears not only to have failed in accelerating discovery of new first-in-class medicines, but has also led to higher attrition rates of new lead molecules2. Therefore, a more balanced “holistic” approach, which incorporates both discovery methods is now being implemented.
One of the most important and widely studied phenotypic responses is apoptosis; particularly in oncology research. Understanding apoptosis as it relates to a particular disease helps in understanding its pathogenesis, as well as how it can be treated. In cancer, the normal course pursued by a cell towards death via specific stimuli is lost, leading to uncontrolled cell division. This represents a major causative factor in the development and progression of the disease3. A variety of methods exist to track apoptotic activity in whole cells or specific organelles, including antibodies, fluorescent stains, and proluminescent substrates. These methods, while easy to use, result in changes to the original state of the cell by means of foreign material introduction, in addition to lysis or fixation and permeabilization, and can also diminish the ability to perform multiplexed analyses. Through the incorporation of cellular imaging, morphological changes that are the hallmark of apoptosis, including shrinkage of the cell and fragmentation into membrane-bound apoptotic bodies4, can be monitored in a label-free manner.
Here we demonstrate a method to incorporate automated, digital widefield phase contrast microscopy to monitor potential apoptotic effects of lead molecules. The ability to control conditions within the imaging chamber at 37 °C/5% CO2 enabled kinetic images to be captured on an hourly basis throughout the entire incubation period. Gen5™ cellular analysis also allowed calculation of rounded apoptotic cells. Determination of apoptotic activity was also performed with a fluorescent live cell assay. Results confirmed the validity of the image-based method to provide accurate analysis of apoptotic induction.
Materials and Methods
MDA-MB-231 cells (Catalog No. 92020424) were purchased from Sigma-Aldrich (Saint Louis, MO). The MDA-MB-231 cells were propagated in Advanced DMEM Medium (Catalog No. 12491-015) plus Fetal Bovine Serum (FBS), 10% (Catalog No. 10437-028) and Pen- Strep-Glutamine, 1x (Catalog No. 10378-016) each from Life Technologies (Carlsbad, CA).
Kinetic Apoptosis Kit (Catalog No. ab129817) was purchased from Abcam (Cambridge, MA). Oridonin (Catalog No. O9639) and Hoechst 33342 (Catalog No. 14533) were purchased from Sigma- Aldrich.
Cytation 5 combines automated digital microscopy and conventional multi-mode microplate detection providing rich phenotypic cellular information and well-based quantitative data. With special emphasis on live-cell assays, Cytation 5 features temperature control to 65 °C, CO2/O2 gas control and dual injectors for kinetic assays. The phase contrast and GFP imaging channels were used to monitor morphological, as well as changes in fluorescence from the kinetic apoptosis reagent, respectively.
Gen5 software controls the operation of the Cytation™ 5 for both automated digital microscopy and PMT-based microplate reading. Image analysis and subpopulation calculations allow for counting of apoptotic cells meeting pre-determined signal threshold and circularity requirements.
Cell Preparation and Dispensing into Microplates
Cells were harvested and diluted to a concentration of 5.0x104 cells/mL. A volume of 100 μL was then dispensed to appropriate wells of a 96 well clear bottom, black TCtreated microplate (Catalog No. 3904) from Corning Life Sciences (Corning, NY).
Component Preparation and Addition
Hoechst 33342 was diluted to a final 1X concentration of 5 μM in medium. Following removal of the plating medium, 100 μL of the diluted dye was added per well and incubated for 15 minutes at 37 °C/5% CO2. The pSIVA-IANBD apoptosis reagent was then diluted into medium at a concentration of 10 μL/mL, followed by an oridonin dilution into medium containing apoptosis reagent or medium alone to a concentration of 100 μM. Serial 2.5x dilutions were carried out to create two 12-point titration curves (including a no compound control), with concentrations ranging from 100-0.01 μM. Following the 15 minute incubation, medium was again removed and replaced with 100 μL of compound with or without apoptosis reagent.
Kinetic Image-Based Monitoring of Apoptotic Induction
The 96 well plate containing cells and compound was immediately placed into the Cytation 5, with temperature and gas control having been previously set to 37 °C/5% CO2. Imaging of each well was completed using 4x and 20x objectives. The phase contrast imaging channel was used to capture images from wells containing nonlabeled cells, while fluorescent signal from the pSIVAIANBD reagent was imaged using the GFP channel. A Discontinuous Kinetic Procedure was chosen where imaging was carried out with each designated well once every hour over a 24 hour incubation period.
Gen5 Cellular Analysis
Cellular analysis was performed using Gen5 software on the 4x phase contrast and GFP images captured. The number of apoptotic cells per image was counted morphologically through changes in contrast, and increases in circularity exhibited by this cell type, and confirmed through fluorescent tracking of external phosphatidylserine exposure. Tables 1 and 2 describe the parameters used to count cells with the phase contrast and GFP channels.
Table 1. 4x Phase Contrast Image Cellular Analysis Parameters.
Table 2. 4x GFP Image Cellular Analysis Parameters.
Results and Discussion
Analysis of Apoptotic Induction using Phase Contrast Imaging
From a visual analysis of the phase contrast images captured using a 20x objective (Figure 1), numerous characteristic morphological features of apoptotic cells can be identified. Cell rounding due to shrinkage and cytoplasm condensation, indicators of early apoptosis4, can be seen in Figure 1B. Separation into apoptotic bodies, or blebs, a marker of later-stage apoptosis4, is also witnessed in Figures 1C and D.
Figure 1. Phase Contrast Image-Based Monitoring of Apoptosis Induction. 20x phase contrast images of MDA-MB-231 cells after a seven hour incubation with (A) 0, (B) 1, (C) 40, or (D) 100 μM oridonin.
Label-free quantification of apoptotic activity can also be performed using a 4x objective to sample a larger portion of the total cell population within the well (Figure 2A), and Gen5 Data Analysis Software. Using the primary cellular analysis parameters previously described in Table 1, object masks are drawn around cells within the image (Figure 2B). However, not all cells within the image are apoptotic. Therefore a sub-population criteria is also applied which takes advantage of the round appearance of apoptotic cells (Figure 2C). Healthy cells remain spread out on the bottom of the well in a noncircular manner. This secondary criteria allows for a more accurate count of cells undergoing apoptosis.
Figure 2. Apoptotic Cell Determination. (A) 4x phase contrast image of MDA-MB-231 cells following 40 μM oridonin incubation. (B) Total object count using initial cellular analysis parameters. (C) Apoptotic cell count following sub-population parameter application.
Determination of the apoptotic effect of a test molecule at various endpoints can then be performed by comparing the number of cells identified from treated vs. untreated wells (Figure 3).
Figure 3. Quantification of Apoptosis Induction. Ratio of cell counts from wells containing 10 to 100,000 nM oridonin compared to untreated wells after 7, 12, and 24 hour incubation periods.
Kinetic Apoptotic Analysis
As a result of a label-free analysis method being used, which does not require the addition of an endpoint reagent, kinetic analysis of apoptotic induction can also be performed through the incorporation of temperature and gas control within the imaging chamber (Figure 4A-C). This enables a more definitive investigation of all concentrations being examined.
Figure 4. Kinetic Monitoring of Apoptosis Induction. 20x phase contrast images of MDA-MB-231 cells captured after (A) 0, (B) 7, and (C) 24 hour incubation with 40 μM oridonin. (D) Cell counts from kinetic 4x images of wells containing cells treated with 0, 40, and 100 μM oridonin. (E) Cell number ratios from treated versus untreated wells.
The cell count and subsequent cell ratio graphs (Figure 4D-E) illustrate that apoptosis is induced at a faster rate with higher oridonin concentrations. However the level of induction does not reach that seen with lower concentrations. This is most likely due to the higher level of overt toxicity within the cells causing necrosis, rather than apoptosis.
Fluorescent Live Cell Assay Validation
Validation of the data generated using phase contrast imaging was completed by performing cellular analysis on oridonin treated and untreated wells, having been pre-incubated with Hoechst 33342 and the kinetic apoptosis reagent (Figure 5). Table 2 outlines the parameters used for the fluorescence-based analysis.
Figure 5. Fluorescent Apoptotic Activity Monitoring. Kinetic 4x images taken of MDA-MB-231 cells stained with Hoechst 33342 (blue) and kinetic apoptosis reagent (green). Images captured after (A) 0, (B) 7, and (C) 24 hour incubation with 40 μM oridonin.
External exposure of phosphatidylserine (PS) is a seen as a hallmark of early apoptosis. However, increasing knowledge supports the idea that transient exposure occurs as the cell proceeds further along in the cell death process5,6. This phenomenon is also witnessed during oridonin treatment (Figure 5). Little to no signal is seen initially from the green pSIVA reagent. After seven hours of incubation with higher concentrations of the molecule, high levels of fluorescence are exhibited from binding of the reagent to PS (Figure 5B). The signal then wanes after extended exposure (Figure 5C), indicating later stages of apoptosis.
Figure 6. Cellular Analysis Method Comparison. Cell count ratios calculated using 4x phase contrast or fluorescent images following a (A) 7, (B) 12, or (C) 24 hour oridonin incubation.
A comparison of ratios from treated versus untreated wells, calculated using phase contrast or fluorescence imaging, demonstrates that equivalent results can be seen when tracking changes to cellular morphology, as opposed to fluctuations in the signal from a fluorescent live cell reagent (Figure 6).
The use of changes in morphology is a desired method to track apoptotic activity in a target cell type, due to the process being label-free and multiplexing possibilities. Morphological changes which occur during this process are well characterized, and appear across all cell types, keeping analysis consistent. Any potential bias that might be seen when introducing an optical probe or other substrate is also eliminated. Through the incorporation of phase contrast imaging, this technique can be accomplished with the Cytation™ 5. Higher magnification images taken with a 20x objective provide qualitative assessment, while the use of lower magnification, such as 4x, in addition to Gen5™ Data Analysis Software, allow for quantitative, statistical determinations to be made. Temperature and gas control within the imaging chamber also enable automated, kinetic evaluations. The combination provides a simple, yet powerful method to study this important mechanism of cell death.
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