Abstract
BackgroundThe caspase family, which plays a central role in apoptosis in metazoans, has undergone an expansion in amphioxus, increasing to 45 members through domain recombination and shuffling.ResultsIn order to shed light on the conservation and uniqueness of this family in amphioxus, we cloned three representative caspase genes, designated as bbtCaspase-8, bbtCaspase-1/2 and bbtCaspase3-like, from the amphioxus Branchiostoma belcheri tsingtauense. We found that bbtCaspase-8 with conserved protein architecture is involved in the Fas-associated death domain-Caspase-8 mediated pro-apoptotic extrinsic pathway, while bbtCaspase3-like may mediate a nuclear apoptotic pathway in amphioxus. Also, bbtCaspase-1/2 can co-localize with bbtFADD2 in the nucleus, and be recruited to the cytoplasm by amphioxus apoptosis associated speck-like proteins containing a caspase recruitment domain, indicating that bbtCaspase-1/2 may serve as a switch between apoptosis and caspase-dependent innate immune response in invertebrates. Finally, amphioxus extrinsic apoptotic pathway related caspases played important roles in early embryogenesis.ConclusionsOur study not only demonstrates the conservation of bbtCaspase-8 in apoptosis, but also reveals the unique features of several amphioxus caspases with novel domain architectures arose some 500 million years ago.
Highlights
The caspase family, which plays a central role in apoptosis in metazoans, has undergone an expansion in amphioxus, increasing to 45 members through domain recombination and shuffling
From each of the known branches above we chose one caspase with a conserved domain and two with novel domain architecture for further analysis. These were bbtCaspase-8, which is the ortholog of caspase-8 with conserved protein architecture, bbtCaspase-1/2, which shares the greatest similarity with caspase-2, but its prodomain contains a death effector domain (DED) and a death domain (DD) instead of a caspase recruitment domain (CARD) in mammalian caspase-2 (Figure 1A) and bbtCaspase3-like, which is 33% homologous to the human caspase-3/7 caspase domain
Since the hepatic cecum and gut are considered as the first line of the amphioxus immune defense system, we proposed that the caspase family should play important roles in amphioxus immunity
Summary
The caspase family, which plays a central role in apoptosis in metazoans, has undergone an expansion in amphioxus, increasing to 45 members through domain recombination and shuffling. 11 members of this family have been identified in the human genome [4,5], caspase-1 to caspase-10, and caspase-14. They can be divided, with respect to function, into two major groups. The first, including the initiator caspases (caspase-2, -8, -9 and -10) and the effector caspases (caspase-3, -6 and -7), lead directly to apoptosis [6], while the others, including caspase-1, -4 and -5, are involved in the maturation of pro-inflammatory cytokines [7].
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