Materials and Reagents

Falcon^® 5 ml round bottom polystyrene test tube (Corning, Falcon^®, catalog number: 352058 )
Dulbecco’s phosphate buffered saline, no calcium, no magnesium (DPBS) (Thermo Fisher Scientific, Thermo Scientific^TM, catalog number: 14200166 )
Alexa Fluor^® 647 rat anti-phospho-Histone H3 (pS28) (BD, BD Bioscience, catalog number: 558217 )
Sodium citrate (Na₃C₆H₅O₇·2H₂O) (Sigma-Aldrich, catalog number: 1613859 )
Triton^TM X-100 (Sigma-Aldrich, catalog number: T8787 )
Propidium iodide (PI) (C₂₇H₃₄I₂N₄) (Sigma-Aldrich, catalog number: P4170 )
Hypotonic lysis/PI buffer (see Recipes)

Equipment

Centrifuge (Eppendorf, model: 5424 R )
Flow cytometer (ACEA BIO, model: NovoCyte Flow Cytometer , 488-nm laser line, for excitation)

Software

FlowJo_V10 is used for analyzing the flow cytometry data

Procedure

Note: All the procedures are done at room temperature.

Suspend cells at 0.4 x 10⁶cells in 1 ml of DPBS in Falcon^® 5 ml tubes.
Centrifuge at 200 x g for 5 min.
Aspirate off the DPBS.
Re-suspend the cell pellet in 100 μl of hypotonic buffer, add 0.5 μl Alexa Fluor^® 647 anti-phospho-Histone H3 per sample (Alexa Fluor 647 can be detected at APC channel), then gently mix the sample.
Note: Since the hypotonic shock removes the majority of the RNA, RNase treatment is not required.
Place the tubes in the dark at room temperature, before flow cytometry analysis, for at least 20 min but no longer than 2 h.
Note: Including an additional washing step before measurement does not change the result but may cause cell loss.
Measure fluorescence (detector 585/40 nm at PE channel for PI and 675/30 nm at APC channel for phospho-Histone H3 staining) by flow cytometer. Collect at least 20,000 total events, gate-out residual debris and measure diploid and tetraploid DNA peaks (Figures 1A-1D).

Figure 1. Illustration of the gating strategy in analyzing cell cycle profile of HeLa cell. A. Exclusion of cell debris by forward scatter (FSC) and side scatter (SSC); B. Exclusion of clumps and doublets by forward scatter width (FSC-W) and height (FSC-H); C. Identification of G₁, S and G₂/M phase by PI; D. Identification of M-phase by mitosis-specific antibody (phospho-Histone H3-APC). The final percentage (%) of cells for each phase is about G₁: 61.6%, S: 13.8%, G₂/M: 22.5, M: 1.59% according to the analysis.

Data analysis

As shown in Figure 1, the histogram of DNA content could categorize cells into three groups, G₁, S and G₂/M phase (Figure 1C). Mitosis-specific antibody (phospho-Histone H3-APC) is applied to distinguish G₂from M cells (Figure 1D).

Notes

For applying this protocol to other cell lines, the careful titration on cell number is recommended.
Leaving the cell in hypotonic buffer for longer than 2 h may significantly increase the cell debris.
Hypotonic buffer is not compatible with concurrent examination of intracellular fluorescent (GFP, RFP etc.) fusion proteins.

Recipes

Hypotonic lysis/PI buffer
Deionized/distilled water
0.1% sodium citrate (wt/v)
0.1% Triton X-100 (v/v)
50 μg/ml PI
Notes:
1. Solution can be kept at 4 °C for months.
2. PI is a suspected carcinogen and should be handled with care. The dye must be disposed of safely and in accordance with applicable local regulations.

Acknowledgments

This work was supported by R21AI117547 and 1R01AI114581 from National Institute of Health, V2014-001 from the V-Foundation and 128436-RSG-15-180-01-LIB from the American Cancer Society (R.W.).

References

Liu, L., Lu, Y., Martinez, J., Bi, Y., Lian, G., Wang, T., Milasta, S., Wang, J., Yang, M., Liu, G., Green, D. R. and Wang, R. (2016). Proinflammatory signal suppresses proliferation and shifts macrophage metabolism from Myc-dependent to HIF1α-dependent. Proc Natl Acad Sci U S A 113(6): 1564-1569.
Riccardi, C. and Nicoletti, I. (2006). Analysis of apoptosis by propidium iodide staining and flow cytometry. Nat Protoc 1(3): 1458-1461.