The Apoptotic Initiator Caspase-8: Its Functional Ubiquity and Genetic Diversity during Animal Evolution

Abstract

The caspases, a family of cysteine proteases, play multiple roles in apoptosis, inflammation, and cellular differentiation. Caspase-8 (Casp8), which was first identified in humans, functions as an initiator caspase in the apoptotic signaling mediated by cell-surface death receptors. To understand the evolution of function in the Casp8 protein family, casp8 orthologs were identified from a comprehensive range of vertebrates and invertebrates, including sponges and cnidarians, and characterized at both the gene and protein levels. Some introns have been conserved from cnidarians to mammals, but both losses and gains have also occurred; a new intron arose during teleost evolution, whereas in the ascidian Ciona intestinalis, the casp8 gene is intronless and is organized in an operon with a neighboring gene. Casp8 activities are near ubiquitous throughout the animal kingdom. Exogenous expression of a representative range of nonmammalian Casp8 proteins in cultured mammalian cells induced cell death, implying that these proteins possess proapoptotic activity. The cnidarian Casp8 proteins differ considerably from their bilaterian counterparts in terms of amino acid residues in the catalytic pocket, but display the same substrate specificity as human CASP8, highlighting the complexity of spatial structural interactions involved in enzymatic activity. Finally, it was confirmed that the interaction with an adaptor molecule, Fas-associated death domain protein, is also evolutionarily ancient. Thus, despite structural diversity and cooption to a variety of new functions, the ancient origins and near ubiquitous distribution of this activity across the animal kingdom emphasize the importance and utility of Casp8 as a central component of the metazoan molecular toolkit.

Rapid acclimation of juvenile corals to CO2‐mediated acidification by upregulation of heat shock protein and Bcl‐2 genes

“…For example, the Acropora protein Cluster002669p has caspase 8‐like specificity despite having caspase 3‐like residues at what are otherwise diagnostic positions (Sakamaki et al . ). Although phylogenetic analyses were equivocal, the presence of a CARD domain in one of the caspases (Cluster012253) up‐regulated during acute CO 2 stress suggests that it is likely to be an initiator caspase.…”

Evolutionary analyses of caspase‐8 and its paralogs: Deep origins of the apoptotic signaling pathways

“…In mammals, CASP8 indirectly interacts with canonical death receptors—the DED in CASP8 undergoes homotypic interactions with the FADD adaptor molecule, which in turn interacts homotypically with the intracellular DD of a death receptor. BLAST and HMM searches identified candidate Casp8 proteins in Acropora and Nematostella and, although these are atypical in terms of specific amino acid residues in the catalytic pocket previously considered diagnostic, the Acropora Casp8 protein displayed the same substrate specificity as its mammalian counterparts . A clear homolog of FADD has also been identified in Acropora , and co‐immunoprecipitation experiments demonstrate that the Acropora Casp8 and Fadd proteins are capable of interacting .…”

“…S1B) may reflect a degree of functional redundancy amongst these (CASP8, CASP10, CASP18) genes. Data from two clawed frogs provide some support for this idea; the CASP18 gene of Xenopus tropicalis encodes a protein that lacks a protease domain , and the transcript from Xenopus laevis with the highest level of similarity to the X. tropicalis cDNA likewise encodes only DED motifs . These data suggest that in amphibians the CASP18 gene may be losing its function, and perhaps this degeneration has been taken a step further in eutherians.…”

“…S3B) suggests that the platform to recruit and activate Casp8 was already in place before the Porifera diverged from the eumetazoan stem. To date, there is no convincing evidence for the presence of a canonical Casp8 gene in either placozoans or ctenophores, although caspases are present in representatives of both phyla . Broader phylogenomic surveys, including not only other “lower” animals, but also non‐metazoan holozoans, will be informative, but it is already clear that the apoptotic systems of mammals have much deeper origins in animal evolution than was previously assumed.…”

Viruses and the origin of microbiome selection and immunity

“…However, metazoan apoptosis appears to be unique through its use of TNF receptors (Quistad and Traylor-Knowles, 2016), which are activated by TNF ligands (Aggarwal, 2003). Many of the domains involved with apoptotic signaling via TNF receptors are also present and functional in cnidarians, considered to be among the oldest animal phyla (Lasi et al, 2010;Quistad et al, 2014;Sakamaki et al, 2014Sakamaki et al, , 2015Lu et al, 2016;Moya et al, 2016).…”