Phylogenetic analysis of the caspase family in bivalves: implications for programmed cell death, immune response and development

Abstract

Background: Apoptosis is an important process for an organism’s innate immune system to respond to pathogens, while also allowing for cell differentiation and other essential life functions. Caspases are one of the key protease enzymes involved in the apoptotic process, however there is currently a very limited understanding of bivalve caspase diversity and functions. Results: In this work, we investigated the presence of caspase homologs using a combination of bioinformatics and phylogenetic analyses. We blasted the Crassostrea gigas genome for caspase homologs and identified thirty-five potential homologs in the addition to the already cloned twenty-three bivalve caspases. As such, we present information about the phylogenetic relationship of all identified bivalve caspases in relation to their homology to well-established vertebrate and invertebrate caspases. Our results reveal unexpected novelty and complexity in the bivalve caspase family. Notably, we were unable to identify direct homologues to the initiator caspase-9, a key-caspase in the vertebrate apoptotic pathway, inflammatory caspases (caspase-1,-4 or -5) or executioner caspases-3, -6, -7. We also explored the fact that bivalves appear to possess several unique homologs to the initiator caspase groups -2 and -8. Large expansions of caspase-3 like homologues (caspase-3A-C), caspase-3/7 group and caspase-3/7-like homologues were also identified, suggesting unusual roles of caspases with direct implications for our understanding of immune response in relation to common bivalve diseases. Furthermore, we assessed the gene expression of two initiator (Cg2A, Cg8B) and four executioner caspases (Cg3A, Cg3B, Cg3C, Cg3/7) in C. gigas late-larval development and during metamorphosis, indicating that caspase expressions vary across the different developmental stages.Conclusion: Our analysis provides the first overview of caspases across different bivalve species with essential new insight into caspase diversity, knowledge that can be used for further investigations into response to pathogens or regulation of developmental processes.

Effects of HMGA2 gene silencing on cell cycle and apoptosis in the metastatic renal carcinoma cell line ACHN

“…41,42,44 HMGA2 can affect apoptosis through the Bcl-2-caspase-3-caspase-9 signalling pathway and promote the occurrence and development of renal cancer cells. 18,30,41,43,47 The findings of the current study showed that low levels of HMGA2 mRNA and protein reduced the levels of Bcl-2 mRNA and protein in HMGA2-siRNA transfected ACHN cells, which activated the downstream genes for caspase-3 and caspase-9, initiating the caspase cascade reaction. The inverse correlation between the mRNA and protein levels of Bcl-2 and the two caspases demonstrated in the current study appears to promote the apoptosis of renal cancer cells.…”

“…40,41 The caspase family of proteins is a group of proteases with similar structures that exist in the cytoplasm and are involved in the regulation of cell growth, differentiation and apoptosis. 42,43 Among them, caspase-3 and caspase-9 are the key enzymes involved in apoptosis. 44–46 Once the signal transduction pathway is activated, caspase-3 and caspase-9 are activated.…”

The expanded inhibitor of apoptosis gene family in oysters possesses novel domain architectures and may play diverse roles in apoptosis following immune challenge

“…Recent whole genome sequencing of marine invertebrates has revealed large scale expansions of immune gene families, including several related to regulated cell death [ 3 , 4 , 6 , 8 , 57 – 62 ]. Functional diversification of expanded immune gene repertoires may contribute to the remarkable ability of invertebrates to mount specific responses to immune challenge in the absence of traditional adaptive immunity [ 3 , 6 , 37 , 61 ].…”

Intrinsic apoptosis is evolutionarily divergent among metazoans

“…Like other protostomes, molluscs are devoid of caspase-9 but caspase-2 has been identified in bivalves and was suspected to function in "a caspase-9-like manner" 72 . Hence, caspase-2 is responsible for apoptotic processes in molluscs, but more surprisingly, despite the absence of caspase-9 and Apaf-1 (and thus, of a mammalian-like apoptosome), this peculiar pathway is surprisingly associated with cytochrome c release 35,38,[73][74][75][76] .…”

“…Hence, caspase-2 is responsible for apoptotic processes in molluscs, but more surprisingly, despite the absence of caspase-9 and Apaf-1 (and thus, of a mammalian-like apoptosome), this peculiar pathway is surprisingly associated with cytochrome c release 35,38,[73][74][75][76] . The complexity of intrinsic apoptosis in molluscs seems to be important, but divergent from what was observed in ecdysozoans or vertebrates, with a putative expansion of caspases that participate both in immunity, stress responses, and apoptosis (Figure 2) 38,72,[77][78][79][80] .…”