Roles of caspases in apoptosis, development, and cytokine maturation revealed by homozygous gene deficiencies
Jin Wang
0
Michael J. Lenardo
)
0
0
Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health
,
Building 10, Room 11N311, 10 Center Drive, MSC 1892, Bethesda, MD 20892-18892
,
USA
SUMMARY
Caspases are a group of cysteine proteases critical for
apoptosis of eukaryotic cells. Deletion of genes that encode
murine caspases suggests that caspases are involved not
only in apoptosis but also in cytokine maturation and cell
growth and differentiation. Among them, caspase-1 and
caspase-11 are primarily involved in the processing of
proinflammatory cytokines. Caspase-3 and caspase-9 are
essential for apoptosis during brain development.
Caspase8 is required for the development of heart muscle, cell
proliferation in the hematopoietic lineage and
deathreceptor-mediated apoptosis. These studies suggest that
caspases function in cell signaling events including
apoptosis, cell growth and differentiation.
INTRODUCTION
Caspases are a family of cysteine proteases that cleave target
proteins at specific aspartate residues (Alnemri et al., 1996;
Nicholson et al., 1995). In general, three amino acid residues
that lie upstream of the aspartate residue in the substrate
determine the specificity of recognition by individual caspases
(Thornberry et al., 1997). The roles of caspases in apoptosis
first became evident when a cell death-related gene, ced-3,
which is essential for apoptosis in Caenorhabditis elegans, was
found to be homologous to the mammalian caspases (Yuan et
al., 1993). It is now clear that caspases are essential effector
molecules for carrying out apoptosis in eukaryotic cells
(Nicholson and Thornberry, 1997; Salvesen and Dixit, 1997;
Thornberry and Lazebnik, 1998; Wang and Lenardo, 1997).
At least fourteen members of the caspase family have been
described in mammalian cells thus far (Cryns and Yuan, 1998).
They exist as zymogens containing a prodomain and a protease
domain (Cryns and Yuan, 1998; Thornberry and Lazebnik,
1998). Caspases which have long prodomains are believed to
be the upstream initiator caspases. Among them, caspase-8 and
caspase-10 contain two tandem repeats of the death effector
domains (DEDs) within their prodomains. Homotypic
interactions between the DEDs of caspase-8/10 and the DED
of an adaptor molecule, FAS-associating protein with a death
domain (FADD; also called Mort1), induce the recruitment of
these caspases to death receptors and lead to caspase activation
(Salvesen and Dixit, 1999). Another group of caspases that
have long prodomains includes caspase-1, caspase-2,
caspase4 and caspase-9, each of which contains a caspase-recruitment
domain (CARD). The CARDs of these caspases interact with
CARD-containing adaptor molecules, and these caspases
might also undergo similar adaptor-mediated aggregation and
self activation (Salvesen and Dixit, 1999). By contrast,
caspases with short prodomains, including caspase-3,
caspase6 and caspase-7, are believed to be downstream effector
caspases that depend on the upstream initiator caspases for
activation.
The protease domain of caspases contains a large subunit
and a small subunit. Activation of caspases possibly involves
the following two proteolytic steps (Thornberry and Lazebnik,
1998). First, the C-terminal, smaller subunit of the protease
domain is released. Then, the prodomain is removed from the
large subunit of the protease. Crystallography studies suggest
that active caspases are heterotetramers composed of two large
subunits and two small subunits (Rotonda et al., 1996; Walker
et al., 1994; Wilson et al., 1994).
CASPASE KNOCKOUT MICE
Transfection of caspases into cell lines suggests that
overexpression of each caspase can induce apoptosis in certain
cell lines. However, it is not clear whether apoptosis induced
by overexpressed caspases represents the physiological
Caspase-2 No gross abnormality
Caspase-3 Perinatal lethality
No IL-1b and IL-18 processing
Caspase-8 Embryonic lethality Fibroblasts resistant to Fas-,
Abnormal heart muscle and TNFR1- and DR-3-mediated
hemopoeitic cell development apoptosis
Caspase-9 Perinatal lethality
Abnormal brain development
No IL-1b and IL-18 processing
functions of these caspases in vivo. The use of tetrapeptides
resembling the recognition sites of caspases to block caspase
activities shows that caspases are required for most, if not all,
types of apoptosis (Nicholson and Thornberry, 1997; Salvesen
and Dixit, 1997). However, these caspase inhibitors are not
very specific, owing to the overlapping substrate specificities
of caspases. Moreover, it is not clear whether complete
inhibition of caspases can be achieved by these inhibitors in
cells undergoing apoptosis. Deletion of individual caspases
from cell lines or animals is therefore necessary for
understanding of unique functions of individual caspases.
Knockouts of caspases in mice by homologous
recombination reveal that caspases play essential roles in
development, immune regulation, and apopt (...truncated)