Genetics of Apoptosis

apoptosis in response to diverse death stimuli (Earnshaw et al., 1999). As such,
caspases represent the fundamental executioners of cell death.

3.2

Activation of caspases by death receptors

Caspases are synthesized as inactive zymogens that comprise an N-terminal
prodomain and two other domains, p20 and p10, which form the active mature
enzyme upon cleavage between the p20 and p10 domains as well as between the p20
domain and the prodomain (Earnshaw et al., 1999). Since these Asp-X cleavage sites
correspond to caspase substrate motifs, procaspases can be activated by either
previously activated upstream caspases or by autocatalytic processing (Thornberry et
al., 1997).
Members of the death receptor family share the same fundamental mechanism(s)
of activating caspases and amplifying this enzymatic cascade (Figure 2).
These sequential steps and signaling pathways are dissected below:

Formation of the death-inducing signaling complex—activation

of the initiator caspase-8/FLICE

Death receptors are type I transmembrane proteins containing cytoplasmic sequences
(DDs) that are essential for transduction of the apoptotic signal (Itoh and Nagata,
1993; Tartaglia et al., 1993). The oligomerization of death receptors by engagement
of their cognate ligands results in the rapid assembly of a membrane-bound death-
inducing signaling complex (DISC) (Kischkel et al.,1995). Ligand-induced
trimerization of the CD95/Fas/Apol receptor facilitates binding of the adapter
protein, FADD (Fas-associated DD protein; also known as mediator of receptor-
induced toxicity [MORT1]) (Boldin et al., 1995; Chinnaiyan et al., 1995) through
homotypic interactions between their DD (Brakebusch et al., 1992; Itoh and Nagata,
1993; Tartaglia et al., 1993). Receptor-bound FADD molecules form higher-order
oligomers, or ‘fibers’ (Siegel et al., 1998). FADD also carries a so-called death-effector
domain (DED), which, in turn, interacts with the analogous DED motifs found in
the N-terminal region of the zymogen form of caspase-8 (procaspase-8; also called
FLICE or MACH) (Boldin et al., 1996; Muzio et al., 1996; Medema et al., 1997).
FADD-dependent recruitment and aggregation of multiple procaspase-8 molecules
to the receptor/FADD scaffold results in autocatalytic cleavage and cross-activation
by induced proximity, thereby releasing active caspase-8 into the cytoplasm (Martin
et al., 1998; Muzio et al., 1998; Yang et al., 1998a).
Other death receptors activate caspase-8 in a fashion analogous to that of CD95.
While death receptors for Apo2L/TRAIL (TRAIL-R1/DR4 and TRAIL-R2/DR5)
directly bind FADD, TNF-α-bound TNFR1 binds the adapter molecule TRADD
(TNFR1-associated DD protein), which, in turn, recruits FADD to the receptor
complex (Hsu et al., 1996). Experiments with FADD gene-knockout mice (Zhang,
L. et al., 1998) or transgenic mice expressing a dominant-negative mutant of FADD

4 GENETICS OF APOPTOSIS