Caspases: evolutionary aspects of their functions in vertebrates

Abstract

Caspases (cysteine-dependent aspartyl-specific protease) belong to a family of cysteine proteases that mediate proteolytic events indispensable for biological phenomena such as cell death and inflammation. The first caspase was identified as an executioner of apoptotic cell death in the worm Caenorhabditis elegans. Additionally, a large number of caspases have been identified in various animals from sponges to vertebrates. Caspases are thought to play a pivotal role in apoptosis as an evolutionarily conserved function; however, the number of caspases that can be identified is distinct for each species. This indicates that species-specific functions or diversification of physiological roles has been cultivated through caspase evolution. Furthermore, recent studies suggest that caspases are also involved in inflammation and cellular differentiation in mammals. This review highlights vertebrate caspases in their universal and divergent functions and provides insight into the physiological roles of these molecules in animals.

Fig. 1

Schematic diagram of the human caspases. (a) The phylogenetic relationship of human caspases. A molecular phylogenetic tree of human caspases was generated based on the alignment of the amino acid sequences for the CASc protease domain by the maximum likelihood method. Numbers noted at the branches represent the bootstrap values obtained from 1000 replications. The gene identification numbers cited for the generation of the tree were listed in Table SI. (b) Protein structure. Procaspases carry a prodomain connected with a catalytic region (CASc) composed of large and small subunits. Caspases-3, -6, -7 and -14 contain a short prodomain (yellow), whereas the other caspases carry a long prodomain containing a caspase-recruitment domain (blue) or two death effector domains (red). (c) Substrate specificity. Preferred sequences in the substrates recognized and cleaved by each caspase were indicated as described previously (Earnshaw et al., 1999; Mikolajczyk et al., 2004;). (d) The physiological roles of caspases. Caspases are divided into three subfamilies in accordance with their physiological distinction between inflammatory, initiator and effector caspases. In contrast with other caspases, it is proposed that caspase-14 acts as a factor required for keratinocyte differentiation in the skin.

Fig. 2

Physical map of the genomic region including the Casp8 gene and its related genes in vertebrates. The bold arrows indicate the coding region and orientation of the gene. In humans and dogs, the CASP8, CASP10 and CFLAR genes form a cluster on the chromosome (chr.) 2 or 37. Rodents have lost the Casp10 gene. Opossums, chickens and frogs have the additional Casp8/Casp10homologous gene, Casp18, between the Casp8 and the Casp10 genes. In Danio rerio, the casp8, casp10 and cflar genes localize on different chromosomes and the card-casp8 gene exists upstream of the casp8 gene. Numbers indicate the starting point of the coding region in the Ensemble genome database. The gene identification numbers cited for the generation of the map were listed in Table SI. The figure was generated by combining the genomic data of dogs and opossums with the data published in a previous study (Sakata et al., 2007).

Fig. 3

Chromosomal analyses of the Casp1 and Casp14 genes and their orthologues. (a) Physical map of the region including the Casp1 gene and its related genes in vertebrates. The bold arrows indicate the coding region and orientation of the gene. In humans, the CASP1, CASP5, CASP4 and CASP12 genes form a cluster on the chromosome 11. The Casp1, Casp11(Casp5) and Casp12 genes and the Casp1, Casp13(Casp5) and Casp12-like genes form a cluster on the genome of the mouse and cow, respectively. Five Casp1-like genes were detectable in the opossum genome database. (b) Physical map of the region including the Casp14, Casp15 or Casp16 genes in vertebrates. The bold arrows indicate the coding region and orientation of the gene. In humans, dogs and opossums, three genes were identified in the genome, whereas rodents have lost the Casp15 gene. In the anole lizard Anolis carolinensis genome, the casp14-like gene was identified. Ccdc105, coiled-coil domain containing 105; Olfr, olfactory receptor; Slc25a38, solute carrier family 25, member 38; Rpsa, ribosomal protein SA; Zfp213, zinc finger protein 213. Asterisks indicate the presence of non-functional caspase sequences. Identification numbers of the genes examined were listed in Table SI.

Fig. 4

The phylogenetic relationship of vertebrate caspases. A molecular phylogenetic tree of vertebrate caspases (caspase-1, caspase-14 and their relatives) was generated based on the alignment of the amino acid sequences for the CASc protease domain by the maximum likelihood method. Human caspases-2 and -9 were used as the outgroup proteins for rooting the tree. Numbers noted at the branches represent the bootstrap values are shown only when >50%. Identification numbers of caspases examined were listed in Table SI.

Fig. 5

The phylogenetic relationship of fish caspase-8 and its orthologues. A molecular phylogenetic tree of fish caspases and cFlar was generated based on the alignment of the amino acid sequences for the CASc protease domain by the maximum likelihood method. Danio rerio caspases-2 and -9 were used as the outgroup proteins for rooting the tree. Identification numbers of molecules examined were listed in Table SI