One-stop Guide to Cell Cycle Analysis: From PI/BrdU/EdU to Flow Cytometry/WB/IF

**One**

**Cell Cycle Basics**

During cell proliferation, normal cells undergo a cyclical transition through G1 phase → S phase → G2 phase → M phase in sequence, while quiescent cells enter the non-proliferative G0 phase. The characteristics of each phase and their core regulatory molecules are highly specific, forming the basis for the design of detection techniques.

**Image**

*Classic Cell Cycle Diagram*

**1. Key Characteristics of Each Phase**

-   **① G0 Phase**: Quiescent stage, non-proliferative state (e.g., differentiated mature neurons, hepatocytes).

- **（2） G1阶段**：早期间期，DNA含量2N。细胞生长，RNA/蛋白质合成。检查环境信号，决定是否进入S阶段（限制点）。

- **（3） S阶段**：DNA合成阶段。DNA从2N复制到4N。

- **（4） G2/M 阶段**：DNA 含量 4N。检查DNA复制完整性，进行有丝分裂（M期：前期、中期、后期、末期、细胞质分裂）。

**2.关键调控分子**

细胞周期的进展由Cyclin-CDK（细胞素依赖激酶）复合物精确调控，每个阶段的表达特征如下：

- ♬ 早期G1：周期蛋白D + CDK4/6

- ♬ G1/S转变：细胞周期蛋白E + CDK2

- ∭ S期：周期蛋白A + CDK2

- ⒦ G2/M转变：周期蛋白B + CDK1

- ∭ Ki-67：增殖标记，G0中表达低，G1-M表达高。

异常细胞周期常见于癌症（例如，由于Rb通路失活导致的G1失控）以及对化疗药物的反应（例如，紫杉醇诱导G2/M心脏骤停）。

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**二**

**细胞周期检测步骤**

无论采用何种具体检测方法，细胞周期实验的预处理步骤都高度一致。核心目标是在确保电池完整性的同时获得单细胞悬浮液。具体流程如下：

1.  **① Cell Culture and Treatment**: Culture cells in a 6-well plate to 60%-70% confluence. Add drugs, factors, etc., according to experimental requirements and continue culturing until the preset time point.

2.  **② Cell Collection**: Detach cells using trypsin, pipette to create a single-cell suspension. Centrifuge at 800rpm for 5 min to collect the cell pellet. Resuspend in pre-cooled PBS.

3.  **③ Cell Fixation**: Centrifuge the prepared single-cell suspension, remove the supernatant. Add 2-3 ml of pre-cooled 70% ethanol and fix at -20°C for at least 16 hours.

4.  **④ Cell Staining**: Centrifuge at 1000rpm for 5 min, discard supernatant. Wash twice with PBS (avoid DNase contamination). Add the corresponding cell cycle dyes/antibodies and incubate for 30 min at 37°C protected from light.

5.  **⑤ Filtration Before Analysis**: Filter the sample using a 200-mesh sieve to prevent cell clumping (especially important for PI staining, which is highly adhesive and prone to clumping).

6.  **⑥ Flow Cytometry Analysis**: Analyze samples at a low flow rate. It is recommended to acquire data for over 10,000 valid single cells.

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**Three**

**Principles and Methods of Cell Cycle Detection**

**(A) Flow Cytometry**

**图片**

**1. PI Single Staining**

-   **Principle**: PI (Propidium Iodide) intercalates into double-stranded DNA. Its fluorescence intensity is proportional to the DNA content. By detecting the fluorescence signal via flow cytometry, different phases can be distinguished: G0/G1 phase (2N, main peak), S phase (2N-4N, plateau), G2/M phase (4N, second peak). The Sub-G1 peak (<2N) represents apoptotic cells/debris.

-   **Advantages**: Low cost, quick operation, intuitive results.

-   **Disadvantages**: Cannot distinguish between G0 and G1 phases; can only detect fixed cells; requires adding RNase to eliminate RNA interference with the fluorescent signal.

-   **Standard Protocol**:

    1.  ① Wash cells after treatment, fix in 70% cold ethanol at 4°C overnight.

    2.  ② Wash twice with PBS. Add staining solution containing 50μg/mL RNase A + 50μg/mL PI. Incubate for 30 min at room temperature, protected from light.

    3.  ③ Use flow cytometer with 488nm excitation, detect in FL2/FL3 channel. Acquire 10,000-30,000 valid single cells.

**2. BrdU / EdU Incorporation**

-   **Principle**: Both BrdU and EdU are thymidine analogs that incorporate into newly synthesized DNA during S phase. Positive cells can be detected via fluorescent labeling. Combined with PI counterstaining, it allows precise identification of S phase cells. BrdU detection requires DNA denaturation (e.g., with HCl) followed by antibody staining. EdU is detected via a gentle Click chemistry reaction without denaturation.

-   **Advantages**: High specificity; precisely marks S phase cells with active DNA synthesis. EdU offers low background and high detection efficiency.

-   **Disadvantages**: Requires pulse labeling; high concentrations of labeling agents might be toxic and affect cell cycle progression.

-   **Standard Protocol (EdU example)**:

    1.  ① Incubate cells with 10μM EdU for 1-2 hours (pulse labeling) at 37°C.

    2.  ② Fix with 4% paraformaldehyde for 15 min, permeabilize with 0.5% Triton X-100 for 10 min.

    3.  ③ Perform Click reaction by adding Alexa 488-azide + CuSO₄. Incubate for 30 min, protected from light.

    4.  ④ Counterstain with PI. Analyze by flow cytometry: detect EdU in FL1 channel, PI in FL3 channel.

**3. Ki-67 Staining**

-   **Principle**: Utilizes the expression pattern of Ki-67 combined with PI DNA content analysis to distinguish G0 phase from G1 phase: Ki-67 low expression + 2N DNA = G0 phase (quiescent cells); Ki-67 high expression + 2N DNA = G1 phase (proliferating cells). It also quantifies the proportion of cells in S/G2/M phases (proliferating fraction).

-   **Advantages**: The only mainstream method capable of distinguishing G0 from G1 phase; accurately assesses proliferative activity and quiescent fraction.

-   **Disadvantages**: Requires fixation/permeabilization; results reflect the average expression level of the cell population; cannot analyze real-time dynamics in living cells.

**(B) Western Blot**

**Image**

-   **Principle**: By lysing cells and extracting total protein, the expression levels of key regulators like Cyclins (D/E/A/B), CDKs (4/2/1), and CDK inhibitors (p21/p27) are detected. Changes in their expression reflect the regulatory state of the cell cycle. For example, upregulated Cyclin D suggests G1 phase progression; upregulated p21/p27 suggests G1 phase arrest.

-   **Advantages**: Explains cell cycle changes at the molecular mechanism level; results can validate flow cytometry findings; good experimental reproducibility.

-   **Disadvantages**: Detects the average expression level of the cell population; not real-time; cannot analyze the cell cycle status of individual cells.

-   **Standard Protocol**:

    1.  ① After treatment, lyse cells with RIPA buffer containing protease inhibitors. Perform protein quantification using BCA assay.

    2.  ② Perform SDS-PAGE electrophoresis, transfer to membrane, block with 5% non-fat milk for 1 hour.

    3.  ③ Incubate with primary antibody (e.g., anti-Cyclin D/E/A/B, p21, CDK4/2, diluted 1:1000) overnight at 4°C. Incubate with secondary antibody for 1 hour at room temperature.

    4.  ④ Develop using ECL chemiluminescence. Quantify relative protein expression using software like ImageJ (normalize to loading control like β-actin/GAPDH).

**(C) Immunofluorescence (IF)**

**Image**

-   **Core Principle**: Combines EdU/BrdU incorporation or Ki-67 staining with immunofluorescence techniques to label cell cycle-related targets in situ. Cell status is observed via fluorescence microscopy, providing直观 results on single-cell cycle status and spatial distribution. EdU + DAPI co-staining is commonly used to mark S phase cells.

-   **Advantages**: In situ detection provides直观 results; allows correlation with cell morphology; suitable for observing proliferation characteristics in localized tissues/cell clusters.

-   **Disadvantages**: Low quantification efficiency; suitable for qualitative or semi-quantitative analysis; not ideal for high-throughput sample screening.

-   **Standard Protocol (EdU + DAPI)** :

    1.  ① Culture cells on coverslips. Incubate with 10μM EdU for 1-2 hours (pulse labeling).

    2.  ② Fix with 4% paraformaldehyde, permeabilize with 0.5% Triton X-100. Perform Click reaction.

    3.  ③ Counterstain nuclei with DAPI. Observe under a fluorescence microscope; EdU-positive nuclei indicate S phase cells.

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**Four**

**Common Problems and Solutions**

1.  **(1) Do I need to set up controls?** Unstained samples (blank control) are not strictly necessary, but you must design appropriate negative controls based on your experiment (e.g., isotype control for antibody staining).

2.  **(2) What precautions are needed during nucleic acid dye incubation?** Turn off lights during the staining process. Place samples in a dark container (light-proof box) for incubation.

3.  **(3) How to choose the fixative?** Common fixatives include formaldehyde, methanol, and ethanol. Formaldehyde can reduce dye binding by inducing chromatin cross-linking. High concentrations of methanol can cause cell clumping, making it difficult to resuspend into single cells. Ethanol is a precipitant and currently considered one of the best fixatives for preparing cell cycle samples. **Crucially, prepare 70% ethanol using pure water.** PBS contains salts that can precipitate upon ethanol addition, causing physical damage to cells.

4.  **(4) How long should fixation last?** Minimum of 4 hours; fixation overnight is recommended.

5.  **(5) How to avoid cell debris?** Handle cells gently. Avoid excessive or forceful pipetting to minimize cell fragmentation.

6.  **(6) What is the appropriate cell seeding density?** Maintain a moderate seeding density. Overconfluent cultures can lead to contact inhibition, causing cells to arrest in G0/G1 phase. Perform pilot experiments to determine optimal density.

7.  **(7) What are the key points for sample acquisition on the flow cytometer?** For cell cycle analysis, samples must be acquired at a **low flow rate**. A low rate ensures minimal variation between cells as they pass through the laser, resulting in higher resolution (better CV values).

8.  **(8) How many cells are needed for flow cytometry analysis?** Typically, acquire data for 20,000 to 30,000 single cells. Cell cycle analysis software uses mathematical models to fit the data, requiring a sufficient number of events for accurate modeling. If cell numbers are low, minimize centrifugation steps to reduce cell loss. If drug treatment causes excessive cell death, consider lowering the drug concentration.

9.  **(9) How to exclude doublets?** Doublets (cell aggregates) can be easily confused with G2/M phase cells, leading to false positive results. We can reduce their interference through the following 3 methods: [Content seems cut off in original]

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**Five**

**Summary**

Various techniques are available for cell cycle analysis, each with its own focus. Flow cytometry is the preferred method for quantitative analysis of cycle distribution. PI single staining is suitable for rapid screening. EdU incorporation allows precise detection of S phase cells. Ki-67 + PI co-staining is ideal for distinguishing G0 from G1 phase. Western Blot is suitable for validating cycle regulation at the molecular mechanism level, corroborating flow cytometry results. Immunofluorescence is ideal for in situ observation of single-cell cycle status, suitable for studies combining morphology and proliferation.

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