Proteases for cell suicide: functions and regulation of caspases

Abstract

Caspases are a large family of evolutionarily conserved proteases found from Caenorhabditis elegans to humans. Although the first caspase was identified as a processing enzyme for interleukin-1beta, genetic and biochemical data have converged to reveal that many caspases are key mediators of apoptosis, the intrinsic cell suicide program essential for development and tissue homeostasis. Each caspase is a cysteine aspartase; it employs a nucleophilic cysteine in its active site to cleave aspartic acid peptide bonds within proteins. Caspases are synthesized as inactive precursors termed procaspases; proteolytic processing of procaspase generates the tetrameric active caspase enzyme, composed of two repeating heterotypic subunits. Based on kinetic data, substrate specificity, and procaspase structure, caspases have been conceptually divided into initiators and effectors. Initiator caspases activate effector caspases in response to specific cell death signals, and effector caspases cleave various cellular proteins to trigger apoptosis. Adapter protein-mediated oligomerization of procaspases is now recognized as a universal mechanism of initiator caspase activation and underlies the control of both cell surface death receptor and mitochondrial cytochrome c-Apaf-1 apoptosis pathways. Caspase substrates have bene identified that induce each of the classic features of apoptosis, including membrane blebbing, cell body shrinkage, and DNA fragmentation. Mice deficient for caspase genes have highlighted tissue- and signal-specific pathways for apoptosis and demonstrated an independent function for caspase-1 and -11 in cytokine processing. Dysregulation of caspases features prominently in many human diseases, including cancer, autoimmunity, and neurodegenerative disorders, and increasing evidence shows that altering caspase activity can confer therapeutic benefits.

FIG. 1

Mammalian caspase family and C. elegans caspase CED-3. All mammalian caspases are of human origin except for murine caspase-11 and -12, for which no human counterparts have been identified yet. Phylogenetic relationships are based on sequence similarity among the protease domains. Alternative names are listed in parentheses after each caspase. Dotted box, DED domains; wavy boxe, CARD domain. Substrate preferences at the P1 to P4 positions are indicated. Based on the substrate specificity, caspases are divided into three groups (indicated in parentheses) (212).

FIG. 2

Structure of the caspase-3 tetramer in complex with Ac-DEVD-CHO. The p17 subunits are shown in blue and green, the p12 subunits in red and pink, and the bound inhibitors in yellow. The C termini of p17 and N termini of p12 are indicated. Reproduced with permission from J. Rotonda et al., Nature Structure Biology 3:619–625, 1996.

FIG. 3

Activation of the caspase cascade. Apoptotic signals trigger oligomerization of death adapter proteins (e.g., CED-4, Apaf-1, and FADD). Death adapter oligomers in turn oligomerize procaspases, which leads to their autoproteolytic activation. Active initiator caspases then process and activate effector procaspases. Effector caspases cleave various death substrates to induce apoptosis.

FIG. 4

(A) C. elegans apoptosis pathway and mammalian homologues of C. elegans cell death proteins. (B) CED-4 oligomerization as a unifying mechanism in the C. elegans cell death pathway.

FIG. 5

Three Drosophila apoptotic pathways converge at caspase activation. Reaper, Grim, and Hid activate caspases through inhibition of DIAP-1. Caspases can also be activated by an Apaf-1-like pathway (Dapaf-1/HAC-1/Dark) and a FADD-like pathway (dFADD).

FIG. 6

Mammalian apoptosis pathways. Procaspase-8 and -9 are activated by oligomerized FADD and Apaf-1, respectively. Upon activation, these two caspases cleave effector caspases such as caspase-3 (see Fig. 3). The red circles indicate cytochrome c.