Cell Viability and Proliferation Assays

Introduction

Assays to measure cellular proliferation, cell viability, and cytotoxicity are commonly used to monitor the response and health of cells in culture after treatment with various stimuli. The proper choice of an assay method depends on the number and type of cells used as well as the expected outcome. Assays for cell proliferation may monitor the number of cells over time, the number of cellular divisions, metabolic activity, or DNA synthesis. Cell counting using viability dyes such as trypan blue or Calcein-AM can provide both the rate of proliferation as well as the percentage of viable cells.

Overview of Cell Viability and Proliferation Assays
 

Name Overview Detection Method Advantage Disadvantage
BrdU Assay BrdU incorporates into newly synthesized DNA and detected using anti-Brdu Antibody ICC, IHC, FACS, ELISA Cell Cycle Kinetics
Single Cell Resolution
Lengthy Protocol
Potential DNA Damage
EdU Assay Similar to BrdU technique but uses Click-Chemistry detection without antibodies ICC, IHC, FACS, ELISA Less Toxic than BrdU Faster Protocol No DNA Denaturation Expensive Reagents
MTT Assay MTT, a yellow tetrazole, is reduced to purple formazan in living cells Spectrophotometer Fast Protocol High Throughput Endpoint Assay Overestimation of Viability Final Solubilization Step
XTT Assay Actively respiring cells convert the XTT to a water-soluble, orange colored formazan product Spectrophotometer High Sensitivity Large Dynamic Range Water Soluble Endpoint Assay Overestimation of Viability
WST-1 Assay WST-1 is cleaved to a soluble formazan by a complex cellular mechanism that occurs primarily at the cell surface. Spectrophotometer Highest Sensitivity Faster Protocol Endpoint Assay Overestimation of Viability
Luminescent ATP Assay Firefly luciferase cell based assay for quantifying ATP in cell cultures used to measure cell viability Spectrophotometer Sensitive, Fast, High-Throughput Compatible Requires Cell Lysis
Ki67 Antibodies to Ki-67 nuclear protein can be used to measure cellular proliferation. ICC, IHC, WB In Vivo Applications Difficult to Quantify Requires Fixation
CFSE CFSE, a non-fluorescent cell permeable dye, is cleaved by intracellular esterases which results in the green fluorescence. ICC, FACS Live Cell Analysis Long Experiments Compatible with Lymphocytes Toxicity Green Channel Emission
Live/Dead Assays Simultaneous fluorescence staining of viable and dead cells using calcein-AM and propidium iodide (PI), which stain viable and dead cells, respectively ICC, FACS Live Cell Analysis Rapid Protocol Single Cell Resolution Background Autofluorecnce
Trypan Blue Dye exclusion test is based upon the concept that viable cells do not take up impermeable dyes but dead cells are permeable and take up the dye. Microscopy Low Cost Rapid Protocol Variability Inaccurate

DNA Synthesis Proliferation Assays

BrdU Cell Proliferation Assays

Cell proliferation may be studied by monitoring the incorporation of a radioisotope, [3H]-thymidine, into cellular DNA, followed by autoradiography. Alternatively, 5-bromo-2′-deoxy-uridine (BrdU assays) may be used instead of thymidine. Cells that have incorporated BrdU into DNA are easily detected using a monoclonal antibody against BrdU and an enzyme- or fluorochrome-conjugated second antibody.

BrdU Cell Proliferation Assays

Figure 1. A. Proliferating cells in the eye of E4 chick embryo using BrdU staining B. Anti-BrdU antibody validation using Camptothecin. By treating Jurkat cells with cell cycle arrest agent Camptothecin, circulating cells are trapped at the G1/S phase transition.

 

EdU Proliferation Assays

Baseclick EdU proliferation assays provide an efficient method for fluorescence detection of replicating DNA. The modified nucleoside EdU is added to live cells and is incorporated into replicating DNA. Cu-induced click chemistry allows rapid attachment of fluorescent probes to the EdU. This provides for a quantitative way to monitor cells that are proliferating. The assays are available in various formats for microscopy imaging, flow cytometry, high throughput screening, and for in vivo experiments. Four different fluorescent probes with excitation near 488, 555, 594, and 647 are available so that the readout can be multiplexed with other fluorescent probes.

Edu-Click cell proliferation kits

Figure 2. Edu-Click cell proliferation kits incorporates EdU (5-ethynyl-2’-deoxyuridine) into DNA during active DNA synthesis and is measured using a click-chemistry fluorescent detection method.

 

Metabolic Proliferation Assays

Assays that measure metabolic activity are suitable for analyzing proliferation, viability, and cytotoxicity. The reduction of tetrazolium salts such as MTT, XTT, WST-1 to colored formazan compounds or the bioreduction of resazurin only occurs in metabolically active cells. Actively proliferating cells increase their metabolic activity while cells exposed to toxins will have decreased activity.

MTT Cell Proliferation Assays

MTT (3-[4,5-dimethylthiazol-2-yl]-2,5- diphenyltetrazolium bromide; thiazolyl blue) is a water soluble tetrazolium salt yielding a yellowish solution when prepared in media or salt solutions lacking phenol red. Dissolved MTT is converted to an insoluble purple formazan by cleavage of the tetrazolium ring by dehydrogenase enzymes. This water insoluble formazan can be solubilized using isopropanol or other solvents and the dissolved material is measured spectrophotometrically yielding absorbance as a function of concentration of converted dye.

XTT Cell Proliferation Assays

In contrast to MTT, the cleavage product of XTT is soluble in water; therefore, a solubilization step is not required. The tetrazolium salt XTT is cleaved to formazan by a complex cellular mechanism. This bioreduction occurs in viable cells only, and is related to NAD(P)H production through glycolysis. Therefore, the amount of formazan dye formed directly correlates to the number of metabolically active cells in the culture.

WST-1 Cell Proliferation Assays

The stable tetrazolium salt WST-1 is cleaved to a soluble formazan by a complex cellular mechanism that occurs primarily at the cell surface. This bioreduction is largely dependent on the glycolytic production of NAD(P)H in viable cells. Therefore, the amount of formazan dye formed directly correlates to the number of metabolically active cells in the culture.

The MTT assay is a colorimetric assay for assessing cell proliferation based on metabolic activity

                                                          Yellow                                                   Purple

 

Figure 3. The MTT assay is a colorimetric assay for assessing cell proliferation based on metabolic activity. NAD(P)H-dependent cellular oxidoreductase enzymes reflect the number of viable cells present. These enzymes are capable of reducing the yellow tetrazolium dye MTT 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide to its insoluble formazan, which has a purple color.

Luminescent Cell Viability Assays

Because ATP is an indicator of metabolically active cells, the number of viable cells can be assessed based on the amount of ATP available. The ATP Cell Viability Luciferase Assay offers a highly sensitive homogenous assay for quantifying ATP in cell cultures. This kit takes advantage of Firefly luciferase’s use of ATP to oxidize D-Luciferin and the resulting production of light in to assess the amount of ATP available in cell cultures. The sensitive assay procedure involves a single addition of ATP detection cocktail directly to cells cultured in a serum-supplemented medium. No cell washing, medium removal and multiple pipetting are required. The kit can be used to detect as little as a single cell or 0.01 picomoles of ATP. The signal produced is linear within 6 orders of magnitude. By relating the amount of ATP to the number of viable cells, the assay has wide applications, ranging from the determination of viable cell numbers to cell proliferation to cell cytotoxicity.

Measurement of Cell Viability

Measurement of Cell Viability using the ATP Luciferase Cell Viability Assay

Bioluminescent ATP luciferase cell viability assay

Figure 3. Bioluminescent ATP luciferase cell viability assay. Firefly luciferase’s use of ATP to oxidize D-Luciferin and the resulting production of light in order to assess the amount of ATP available that correlates to cell number and viability.

Fluorescent Dye Proliferation Assays

CFSE Labeling

5(6)-Carboxyfluorescein diacetate N-succinimidyl ester (CFSE) is a popular choice for measuring the number of cellular divisions a population has undergone. Upon entering the cell, CFSE is cleaved by intracellular esterases to form the fluorescent compound and the succinimidyl ester group covalently reacts with primary amines on intracellular proteins. Upon division, the fluorescence intensity of each daughter cell is halved which allows for the simple detection of the number of cell divisions by flow cytometry. CFSE has been widely used to measure the proliferation of lymphocytes and t-cells.

Live/Dead Cell Double Staining

Live/Dead Cell Double Staining can be utilized for simultaneous fluorescence staining of viable and dead cells. Calcein-AM is a highly lipophilic and cell membrane permeable dye. Though Calcein-AM itself is not a fluorescent molecule, the calcein generated from Calcein-AM by esterase in a viable cell emits a strong green fluorescence (λex 490 nm, λem 515 nm). Therefore, calcein-AM only stains viable cells. Alternatively, the nuclei staining dye Propidium Iodine cannot pass through a viable cell membrane. It reaches the nucleus by passing through disordered areas of dead cell membrane, and intercalates with the DNA double helix of the cell to emit red fluorescence (λex 535 nm, λem 617 nm). Since both Calcein and PI-DNA can be excited with 490 nm light, simultaneous monitoring of viable and dead cells is possible with a fluorescence microscope.

Trypan Blue Cell Counting

Trypan Blue Cell CountingTrypan Blue is one of several stains recommended for use in dye exclusion procedures for viable cell counting. This method is based on the principle that live (viable) cells do not take up the blue dye, whereas dead (non-viable) cells do. Cell viability can be calculated using the ratio of total live/total cells (live and dead). Staining also facilitates the visualization of overall cell morphology.

NOTE: Trypan Blue has a greater affinity for serum proteins than for cellular protein. If the background is too dark, cells should be pelleted and resuspended in protein-free medium or salt solution prior to counting.

Trypan Blue Protocol: Cell Counting Using a Hemocytometer and Trypan Blue

 

 

Materials

     

References

T Mosmann
Journal of Immunological Methods 1983-12-16
A tetrazolium salt has been used to develop a quantitative colorimetric assay for mammalian cell survival and proliferation. The assay detects living, but not dead cells and the signal generated is dependent on the degree of activation of the cells. This method can therefore be used to measure cytotoxicity, proliferation or acti...Read More
F Denizot, R Lang
Journal of Immunological Methods 1986-05-22
A convenient way to estimate the number of viable cells growing in microtitre tray wells is to use a colorimetric assay and an automatic microplate scanning spectrophotometer. One such assay, developed by Mosmann, depends on the reduction by living cells of tetrazolium salt, MTT, to form a blue formazan product. However the orig...Read More
J Carmichael, W G DeGraff, A F Gazdar, J D Minna, J B Mitchell
Cancer Research 1987-02-15
Drug sensitivity assays were performed using a variation of a colorimetric [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT)] assay on V79, CHO-AuxB1, CHRC5, NCI-H460, and NCI-H249 cell lines following optimization of experimental conditions for each cell line. Results from this assay were compared with data as...Read More