Application of FITC-labeled Annexin V His-tag protein in apoptosis detection

Apoptosis, also known as programmed cell death, is an active death process initiated by cells to adapt to their survival environment. In the early stages of apoptosis, although cellular integrity is maintained, surface structures begin to undergo changes. One of the most significant changes is the translocation of phosphatidylserine from the inner to the outer leaflet of the cell membrane. In late apoptosis or necrosis, the cell membrane loses integrity, allowing dyes to enter the cell. During apoptosis, in addition to membrane changes, the cytoplasm and nucleus also exhibit a series of morphological alterations. The cell volume shrinks, but organelles within the cytoplasm remain intact. Chromatin structure in the nucleus undergoes changes, including condensation, margination, cytoplasmic compaction, and nuclear membrane folding, ultimately forming apoptotic bodies. These morphological features provide multiple entry points for apoptosis detection.

Annexin V is a calcium-dependent phospholipid-binding protein with a molecular weight of 35–36 kDa, exhibiting high affinity for phosphatidylserine. Under normal conditions, phosphatidylserine is located on the inner leaflet of the cell membrane but flips to the outer leaflet during early apoptosis. Thus, Annexin V selectively binds to early apoptotic cells. By conjugating Annexin V with fluorescein (FITC) and introducing a His-tag for purification and detection, FITC-labeled Annexin V His-tag protein is obtained. In the presence of calcium ions, this labeled protein specifically binds to phosphatidylserine on the surface of apoptotic cells, enabling quantitative detection via flow cytometry. The green fluorescence signal from FITC can be detected in the FL1 channel of a flow cytometer, facilitating the identification and enumeration of early apoptotic cells.

In apoptosis detection, Annexin V is often used in combination with viability dyes to distinguish between normal cells, early apoptotic cells, late apoptotic cells, and necrotic cells. Commonly used viability dyes include propidium iodide (PI) and 7-aminoactinomycin D (7-AAD). These dyes cannot penetrate intact cell membranes but enter cells with compromised membranes in late apoptosis or necrosis, binding to nuclear DNA and emitting red fluorescence. When FITC-labeled Annexin V His-tag protein is used alongside viability dyes, Annexin V detects phosphatidylserine on the outer membrane, while viability dyes assess membrane integrity. Flow cytometry analysis allows classification of cells into distinct populations based on the presence or absence of these two fluorescent signals.

In flow cytometry analysis, a scatter plot is generated with Annexin V fluorescence intensity on the x-axis and viability dye fluorescence intensity on the y-axis. Normal cells are Annexin V-negative and viability dye-negative, located in the lower-left quadrant. Early apoptotic cells are Annexin V-positive and viability dye-negative, appearing in the lower-right quadrant. Late apoptotic cells are Annexin V-positive and viability dye-positive, positioned in the upper-right quadrant. Necrotic cells are Annexin V-negative and viability dye-positive, found in the upper-left quadrant. Cells in the upper-left quadrant may include membrane-free cell debris or cells that died via non-apoptotic mechanisms and are typically excluded from apoptosis statistics. This analytical method enables accurate differentiation of cell populations and calculation of apoptosis rates.

During apoptosis, changes in cellular physical properties alter light scattering signals in flow cytometry. Forward scatter (FSC) correlates with cell size, while side scatter (SSC) reflects cellular granularity and complexity. In early apoptosis, cell shrinkage reduces FSC signals, while SSC signals may increase or shift due to chromatin condensation. Thus, gating strategy selection is critical for accurate apoptosis detection. In FSC vs. SSC plots, regions with low FSC signals should be included to avoid missing apoptotic cells. Restricting analysis to normal-sized cells may omit apoptotic populations, leading to experimental bias.

When using FITC-labeled Annexin V His-tag protein for apoptosis detection, note the following: Calcium ions are essential for Annexin V-phosphatidylserine binding, so detection buffers must contain appropriate calcium concentrations. Avoid excessive mechanical force during cell handling to prevent artificial membrane damage. Maintain consistent staining time and temperature for reproducibility. Analyze samples promptly after staining to prevent increased apoptosis due to prolonged storage. For adherent cells, use gentle digestion to preserve membrane integrity. Include appropriate controls (unstained, single-stained, and positive controls) to define fluorescence boundaries.

Nanjing UA-Bio Technology Co., Ltd. (UA-Bio) has independently developed "FITC-Labeled Annexin V His Tag Protein, Human" (Catalog No.: UA011315), a high-performance fluorescent reagent designed for flow cytometry and imaging analysis of apoptosis. This human-derived Annexin V protein, conjugated with FITC and His-tag, efficiently binds phosphatidylserine (PS) exposed on apoptotic cells, serving as a stable and reliable tool for quantitative flow cytometry, live-cell imaging, and drug screening.

Nanjing UA-Bio Technology Co., Ltd. is committed to delivering cutting-edge reagents for immunology, cell therapy, and drug discovery. For detailed specifications, validation data, or application guidance on "FITC-Labeled Annexin V His Tag Protein, Human" (Catalog No.: UA011315), please contact us.