Lysosomal involvement in apoptosis

and morphologically distinct way of eliminating cells. Apoptosis occurs without induction of the inflammatory response that typically accompanies necrotic cell death with its attendant release of toxic and chemotactic material from decomposing cells. Morphological hallmarks of apoptosis include cellular shrinkage, cytoplasmic vacuoles, plasma and nuclear membrane blebbing, nuclear pycnosis and fragmentation. These changes are usually accompanied by leakage to the cytosol of mitochondrial cytochrome c and other apoptosis activating factors with ensuing caspase activation – all within intact plasma membranes. Necrosis, on the contrary, is characterized by cellular swelling, nuclear disintegration, and plasma membrane rupture. A final important distinction between these two modes of cell death is that apoptosis is an active energy requiring process, while necrosis is passive.1–4 Apoptosis is a necessary process during embryonic development and organ remodeling, allowing superfluous cells to form apoptotic bodies which are easily phagocytosed by neighboring cells. This phagocytosis is facilitated by externalization of phosphatidylserine, which normally resides almost exclusively in the inner leaflet of the cell membrane. The exposed phosphatidylserine, either directly or through the opsonic function of annexins, serves as a ligand for scavenger receptors present on the surface of most cells. In this way, apoptotic cells are eliminated in an efficient and orderly manner. Furthermore, there is a certain economy in this process in as much as the apoptotic cells may contain nutrients for neighboring normal cells.5–9 Dysfunction of apoptosis has been invoked in a variety of pathological conditions, such as developmental disorders, cancer, AIDS, atherosclerosis, aging, and autoimmune and neurodegenerative diseases. This explains why the study of apoptosis is today such an important and rapidly growing area of research. A variety of agonists may induce apoptosis. These range from blunt instruments, such as oxidative stress, to more subtle triggers of apoptosis which include agonists that bind to receptors belonging to the TNF-receptor superfamily, such as those for tumor necrosis factor-α, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), and the Fas-ligand. In these latter cases, ligation activates binding of pro-caspase-8 to intracellular ‘death domains’, resulting in caspase-8 activation.3,10,11 Through a cascade of events, effector caspases, such as caspases 3 or 7, are ultimately activated and apoptosis ensues. Another pathway of apoptosis involving mitochondria is thought to proceed via a changed balance between antiand pro-apoptotic factors, such as Bcl-2, Bcl-xL, Bax, and Bid, which causes a drop in the mitochondrial transmembrane potential and opens mitochondrial pores allowing cytochrome c and other pro-apoptotic factors, such as apoptosis-inducing factor, to diffuse into the cytosol. Released cytochrome c binds to cytosolic apoptosis activating factor (apaf-1) and pro-caspase-9 to form an apoptosome. This leads to activation of caspase-9 which in turn activates the effector caspases 3 and 7. Granzyme B, which is used by NK cells to kill targets, proteolytically activates effector caspases and/or the pro-apoptotic protein Bid.3,12–14 Most contemporary research on apoptosis focuses on the above mechanisms. Since the pioneering work of Kerr et al. during the early 1970s,1,2 details of this series of events have become clearer and there is now abundant experimental support for most of these mechanisms. Nonetheless, there are aspects of apoptosis that remain mysterious. For a long time, apoptosis and necrosis were considered as fundamentally different as suicide and homicide, each with its own distinct initiating mechanisms. Only recently was it pointed out that one agonist alone may induce either apoptosis or necrosis depending on the magnitude of the insult. This is especially obvious for oxidative stress which in low doses may induce cell proliferation, in