Apoptosis in transfusion medicine:of death and dying—is that all there is?

Death is a universal constant. The process is part of the natural continuum of life. Death on the cellular level can occur either as necrosis or apoptosis. Necrosis occurs after severe cellular injury that often is due to trauma or hypoxia. Swelling is a classical feature of necrosis, due to loss of membrane control of ion movement with resulting fluid influx. Necrosis is further characterized by inflammation with migration of macrophages and neutrophils into the area where they phagocytize of the remnants of the dying cell. In this process, however, cytokines and inflammatory response modifiers are released that can affect the surrounding tissue, often causing substantial damage to neighboring cells.1 Apoptosis presents a completely different scenario. Initially described by Kerr et al.2 in 1972, the concept of apoptosis (also called programmed cell death) has over the past several years been more widely studied and appreciated.3,4 Triggered by either extracellular (e.g., tumor necrosis factor α [TNFα] ) or intracellular (e.g., DNA damage) signals, apoptosis usually results in an intact cell corpse without the leakage of intracellular substances and associated inflammation. The dying cells upregulate surface expression of phosphatidylserine and are efficiently removed by phagocytosis. An increasingly complicated series of reactions has been described that links the triggering event(s) to the final removal of the apoptotic cell. These may be generally grouped into three families of molecules: cell death receptors, executioners of apoptosis, and regulators of apoptosis. The cell death receptor family includes the hematopoietic growth factor receptors, the Fas receptor, the TNFα receptor p55, and numerous other membrane receptors that bind their respective ligands and promote or prevent apoptosis. Upon binding a ligand, such as TNFα, a series of signal transduction events occurs that includes activation of the executioners of apoptosis and/or regulators of apoptosis. The caspase family is the executioner of apoptosis and consists of over 14 enzymatic proteins carrying cysteine protease activity that cleave their substrates after the appearance of aspartate residues.5-8 Each caspase enzyme can be further classified as either a primary or secondary caspase. Primary caspases are those activated by death receptors and subsequently regulate downstream caspase activity. The best example of this subfamily is caspase 8, which can be activated by the binding of TNFα to its receptor and then is released into the cytoplasm, where it cleaves downstream caspases. Secondary caspases are those activated by primary caspases and that subsequently act on noncaspase substrates. Caspase 3 is a well-known representative of this subfamily. Upon activation by caspase 8, procaspase 3 is transformed into an active heterodimer. The cellular substrates of active caspase 3 range widely from nuclear proteins, such as enzymatic regulators for DNA repair, to cytoplasmic protein, such as gelsolin,9 a cytoskeletal regulatory protein. For example, caspase 3 activation directly results in DNA cleavage, giving rise to the classic DNA ladder pattern10 as well as other apoptotic morphologic changes, including plasma membrane shrinkage and blebbing11 and the appearance of phosphatidylserine residues on the cell surface. Although activated by death receptors and other caspases, caspases may also be activated directly by the release of cytochrome C from mitochondria, which serves to amplify the caspase "signal. " The apoptosis process is regulated in part by two families of molecules, the Bcl-2 family and the IAP (inhibitors of apoptosis) family. The Bcl-2 family includes members that suppress apoptosis (Bcl-2 and Bcl-X) and those that stimulate it (Bax, Bad, Bid). These proteins are often bound to the outer mitochondrial membrane by a hydrophobic tail, and they may form heterodimeric complexes with each other to modify their net effect on apoptosis. They do not seem to bind the caspases directly, but may influence caspase activity indirectly via effects on cytochrome C release from mitochondria. In contrast, the IAP family binds to caspases or procaspases and directly inhibits them. Although the cascades of apoptotic signaling and execution occur in the cytoplasm, the nucleus is an important apoptotic target. It undergoes classic morphologic changes that are associated with the appearance of characteristic DNA cleavage products. However, the role of the nucleus in the programmed cell death process is still not fully understood. Early work on tadpole tail development showed that cell death could be blocked by the inhibition of gene expression,12 which implies that the regulation of gene expression is required for the completion of apoptosis. However, depleApoptosis in transfusion medicine: of death and dying—is that all there is?