Annexin V - FITC

Apoptosis, the physiological process of cell death, is a naturally occurring phenomenon. It is normally a tightly regulated process; however, if deregulated, tumor growth can result. Apoptotic processes can be measured in the laboratory easily and most effectively by using a FITC-labeled Annexin V protein to stain cells and analyze them via flow cytometry. It is a useful method to screen and analyze cells in a high throughput manner, resulting in a clearer understanding of the cellular response to various stimuli. When cells are undergoing cell death, phosphatidylserine (PS), a phospholipid which makes up a portion of the cell membrane, will translocate to the outside-facing side of the membrane. This translocation in the cell death mechanism exposes PS to the extracellular environment. Annexin V, a 35-36 kDa phospholipid-binding protein, has a high affinity for PS (KDa=7nmol/l). Once Annexin V is bound to the apoptotic cell it acts as a beacon for cells that have undergone apoptosis. Annexin V is often conjugated to FITC so the amount of signal can be measured and analyzed from labeled cells using a flow cytometer. The translocation of PS to the outside or exposed side of the membrane is an early event in the apoptotic process. Therefore, Annexin V staining is considered a marker for early stage apoptosis. Often times, Annexin V is paired with Propidium Iodide (PI) stain in order to analyze for late-apoptotic or dead cells. Early stage apoptotic cells will only take up the Annexin V stain but will remain PI negative, and late-stage apoptotic cells will be positive for both Annexin V and PI.