News and Commentary Apoptosis Timeline

This timeline of cell death (Figure 1), illustrates how independent strands of research coalesced in the field known as apoptosis ± currently the hottest field of biological research. Although the fact that cells die during normal development was recognized over 150 years ago, this was forgotten, only to be re-discovered several times until the influential review by Glucksmann in 1951. Even after this time, up until the late 1980's, study of physiological cell death processes, in which an organism's cells activate intrinsic mechanisms for the purpose of killing themselves, remained relatively obscure, usually with less than 10 papers published each year. Initially, analysis of cell death was mainly morphological, and between the late 1800's and 1960's elegant figures were published illustrating the light (see reviews by Clarke and Clarke and Lockshin) and electron microscopic features of cell death, such as cell shrinkage, chromatin condensation, break-up of the cell and its engulfment. Even well after the proposal of the term `apoptosis' for cell death in 1972, interest remained low. The `modern' era of cell death research, and the explosion of interest in the field, came with the identification of the biochemical and genetic processes that implement it, beginning with recognition of the first component of the cell death system, Bcl-2, in 1988. Since then, growth of the field has been exponential, and currently over 200 publications appear every week that refer to `apoptosis'. A genetic understanding of cell death has primarily come from study of C. elegans, in which 131 of the 1090 somatic cells formed in the hermaphrodite are fated to die during development. This started with the recognition of cell death in the worm in 1976, and generation of the first ced (cell death abnormal) mutants in 1983. In 1982, in a journal that unfortunately folded soon after, a paper appeared providing evidence that cell deaths in the worm were caused by a process that was specific for cell death, and had no other role, indicating that cell death in the worm is an active process whose only purpose is to remove unwanted cells. Similar conclusions were reached earlier in vertebrate systems, such as when Tata showed that cell deaths during tadpole metamorphosis could be inhibited by cycloheximide, and therefore required the cell's own proteins. At this time, the term most commonly used for the study of these cell death was `programmed cell death', first used in 1965 to describe developmental cell deaths in insect systems by Lockshin and Williams. The term `apoptosis' was proposed in 1972 by Kerr and colleagues, who realized that the morphology of cells dying due to toxins or hormones resembled that described for developmental cell death by Glucksmann. For Kerr, this did not mark the beginning of apoptosis research, because he had been studying it continuously since his first publication on cell death in 1965; rather, it marked the adoption of a new terminology, because until then he had used the terms such as `shrinkage necrosis'. The first marker of physiological cell death that did not rely on morphology came with the recognition that cell death is usually accompanied by rapid activation of endonucleases. Subsequently, `ladders' seen after electrophoresis of cleaved DNA were specifically associated with apoptosis. It took a further 17 years to identify the major endonuclease responsible (DFF/CAD). The observation that phosphatidyl serine is exposed on dying cells provided another convenient marker of apoptosis, and also gave an early lead into how dead cells are recognized prior to their engulfment. Although genetic analysis of cell death progressed most rapidly in the worm, with identification of more and more ced mutant lines, biochemical analysis of cell death was faster in mammals. While Bcl-2 was cloned in 1986, and its role in cell death was established in 1988, the first ced gene to be cloned and sequences was ced-4 in 1992. Comparisons of the morphological and anatomical features of developmental cell deaths in invertebrates and vertebrates have been made since 1969, but unification of the molecular processes of cell death did not occur until 1992, when it was shown that the human bcl-2 gene could inhibit developmental cell death in the worm. This united `apoptosis' in vertebrates with `programmed cell death' in invertebrates, showing them to be the same, evolutionarily conserved process, and it meant that discoveries based on genetics in C. elegans could be applied to analysis of apoptosis in mammalian cells. While Bcl-2 was the first component of the apoptosis mechanism to be recognized, it had been cloned not because it was a cell death gene, but because it is translocated in follicular lymphoma, one of the most common cancers of blood cells in humans. Initially, it was assumed that bcl-2 may be like other oncogenes involved in translocations, such as abl and c-myc, and be a promoter of cell proliferation, but it turned out that when bcl-2 was over-expressed, it did not stimulate cell division, but prevented cells from dying when growth factor was removed. These experiments therefore not only identified Bcl-2 as a component of the apoptosis mechanism, but showed that inhibition of cell death could ultimately lead to Cell Death and Differentiation (2002) 9, 349 ± 354 ã 2002 Nature Publishing Group All rights reserved 1350-9047/02 $25.00