Description: Annexins are a family of calcium-dependent phospholipid-binding proteins, which bind
to phosphatidylserine (PS) to identify apoptotic cells. In healthy cells, PS is predominantly
located along the cytosolic side of the plasma membrane. Upon initiation of apoptosis,
PS loses its asymmetric distribution in the phospholipid bilayer and translocates to
the extracellular membrane, which is detectable with fluorescently labeled Annexin
V. In early stages of apoptosis, the plasma membrane excludes viability dyes such as
propidium iodide (PI) and 7-AAD, therefore cells which display only Annexin V staining
(PI/7-AAD negative) are in early stages of apoptosis. During late-stage apoptosis, loss of
cell membrane integrity allows Annexin V binding to cytosolic PS, as well as cell uptake
of PI and 7-AAD. Annexin V staining, paired with 7-AAD or PI is widely used to identify
apoptotic stages by flow cytometry.
