Apoptosis Involves a Proteolytic Cascade

APOPTOSIS INVOLVES A PROTEOLYTIC CASCADE

The first well-documented account of apoptosis was seen in the nematode Caenorhabditis elegans . This small worm is normally found in soil and eats soil bacteria. The worm is used as a model organism for developmental genetics because it is easy to maintain, has four developmental stages, and is transparent. Using a special microscope, the Nomarski or differential interference microscope , every cell of C. elegans can be seen ( Fig. 20.10 ).Using this technique, scientists have been able to trace each cell division from the singlecelledegg through the entire adult. During development, C. elegans generates 1090 cells, butthe final adult worm has only 959. The other 131 cells die via apoptosis.

Many mutant worms were identified in which the number of cells in the adult was more orless than 959, the wild-type number. Four genes involved in aberrant apoptosis have beencharacterized. Three of these are ced-3, ced-4 , and ced-9 (“ced” = cell death abnormal), and thefourth is egl-1 (for egg laying defective). When the ced-3, ced-4 , or egl-1 genes are defective, thereare more than 959 cells in the adult worm. Thus these genes initiate or execute the apoptoticprogram. When ced-9 is defective, there are fewer than 959 cells, indicating more apoptosis thannormal. Thus, CED-9 protein inhibits apoptosis.

The CED and EGL proteins work in a cascade that initiates cell death. That is, the action of one protein activates the next protein, which in turn activates further proteins. Genetic and biochemical experiments have given the following model for apoptosis in C. elegans ( Fig. 20.11 ). The three CED proteins form an inactive complex in the membrane of the mitochondrion. A signal from surrounding cells activates the synthesis of EGL-1 protein. EGL-1 binds to CED-9 and removes it from the complex. This activates CED-4, which is a protease that specifically cleaves a small inhibitory domain from the end of CED-3. Activated CED-3 forms a heterotetramer of two small and two large domains. This in turn digests various cellular proteins by cutting after aspartic acid residues. This type of enzyme is known as a caspase. Once CED-3 is active, it cleaves inhibitory domains off other proteases, nucleases, and other caspases, thus executing the apoptotic program.