Description
Apoptosis is a well-documented phenomenon in many cellular systems.1 It plays a key role in tissue and organ development during embryogenesis as well as in regulating cell turnover in adult tissues. In the immune system, apoptosis provides the mechanism for deletion of autoreactive T cells in the thymus 2, low responsive B lymphocytes in germinal centers 3, and of target cells susceptible to attack by cytotoxic T lymphocytes and natural killer cells.1, 4 Structurally, apoptosis is characterized by chromatin condensation, reduction in cell size, and endonuclease cleavage of DNA into nucleosomal size fragments.1,4,5 In addition to these changes in cell morphology that occur during apoptosis, a major event is the loss of membrane phospholipid asymmetry, with translocation of phosphatidylserine (PS) from the inner leaflet of the phospholipid bilayer to the cell surface.6 While the function of PS externalization is unclear, it may serve as a "signal" for recognition by phagocytic cells which respond by engulfing the apoptotic cell before loss of plasma membrane integrity. Exposure of PS on the cell surface provides a simple means for detecting cells undergoing apoptosis.
Annexin V (Mr 36-kDa), a member of the annexin family of calcium-dependent phospholipid binding proteins, has a high affinity for PS-containing phospholipid bilayers.7,8 When PS is exposed on the extracellular face of a cell membrane, annexin V binds with an affinity of approximately 7 nM.9 Fluorochrome conjugates of annexin V can be used to monitor changes in cell membrane phospholipid asymmetry 10-13, thereby providing a convenient tool for detection of apoptotic cells (Figure 1). Since externalization of PS occurs earlier than the nuclear changes associated with apoptosis, the ApoScreen Annexin V Apoptosis Kit can be used for detection of cells earlier in the apoptotic pathway than do DNA-based assays.14 Labeled annexin V can also be used to analyze changes in membrane asymmetry in platelets 10 and erythrocytes.13 For individual ApoScreenä labeled annexin V reagents see product nos. 10040-02 and 10040-09