Caspases from Scleractinian Coral Show Unique Regulatory Features

Abstract

Diseases affecting coral have led to massive decline and altered the community structure of reefs. In response to immune challenges, cnidaria activate apoptotic or autophagic pathways, and the particular pathway correlates with disease sensitivity (apoptosis) or resistance (autophagy).Although cnidaria contain complex apoptotic signaling pathways, similar to those in vertebrates, the mechanisms leading to cell death are largely unexplored. We identified and characterized two caspases each from Orbicella faveolata, a disease-sensitive stony coral, and Porites astreoides, a disease-resistant stony coral. The four caspases are predicted homologs of human caspases-3 and -7, but OfCasp3a and PaCasp7a contain an amino-terminal caspase activation and recruitment domain (CARD) similar to human initiator/inflammatory caspases. In contrast, OfCasp3b and PaCasp3 have short pro-domains, like human effector caspases. We show that OfCasp3a and PaCasp7a are DxxDases, like human caspases-3 and -7, while OfCasp3b and PaCasp3 are more similar to human caspase-6, with VxxDase activity. Our biochemical analyses suggest a mechanism in coral in which the CARD-containing DxxDase is activated on death platforms, but the protease does not directly activate the VxxDase. We also report the first X-ray crystal structure of a coral caspase, that of PaCasp7a determined at 1.57Å resolution. The structure reveals overall conservation of the caspase-hemoglobinase fold in coral as well as an N-terminal peptide bound near the active site that may serve as a regulatory exosite. The binding pocket has been observed in initiator caspases of other species, suggesting mechanisms for the evolution of substrate selection while maintaining common activation mechanisms of CARD-mediated dimerization.P. astreoides and O. faveolata contain VxxDases similar to HsCasp6. Our data show that the enzymes from both species have similar biochemical properties and are activated by similar mechanisms. Together, the data show that the role of the caspase cascade in disease resistance of P. astreoides or in disease sensitivity of O. faveolata may derive from differences in response mechanisms in the death receptor or the PIDDosome activation platforms, that is, signaling events upstream of the caspase cascade. Since the caspases in the two species exhibit similar biochemical properties and activation mechanisms, our data suggest that differences in the receptor-mediated activation of caspases as well as cross-talk between the autophagic and apoptotic pathways in the two coral species lead to the different physiological responses.

Disease resistance in coral is mediated by distinct adaptive and plastic gene expression profiles

“…Excessive levels of immune activation and inflammation can lead to apoptosis, which is further supported through the increased expression of caspase recruitment domains in disease-infected coral ( 18 ). Caspases are the effector proteins of apoptosis that are initiated through interactions with the caspase recruitment domain–containing proteins ( 57 , 58 ). All of these genes that contribute to apoptosis represent patterns of highly plastic expression that indicate that immune activation and inflammation could culminate in apoptosis for coral infected with white plague disease as seen in Acropora white syndrome ( 59 ).…”

Evolution of the folding landscape of effector caspases

“…Effector caspases-3, -6, and -7 have a significant role in apoptosis and serve overlapping but nonredundant functions (35), and multiple studies have examined enzyme specificity and regulation of extant caspases (21,33,36,37). In addition, we have previously examined evolutionary changes resulting in amino acid substitutions that affect enzyme specificity (15,38) and allosteric regulation (26,39,40), but there is a dearth of information regarding changes in the caspase folding landscape. To date, only human caspase-3 has been examined in detail (6,27,28,41), so it was not clear whether all effector caspases utilized the same folding landscape.…”

“…The oligomeric form of the zymogen and its activation mechanism is key to regulating apoptosis (13,15). While the amino acid sequence identity is low between caspase subfamilies (~40%) (15), the caspase-hemoglobinase fold is well-conserved (21) even from distantly related species of vertebrates and invertebrates, such as human, Danio rerio, Caenorhabditis elegans, Drosophila melanogaster, and Porites astreoides (22)(23)(24)(25)(26).The structure of the procaspase monomer is characterized by a 6-stranded -sheet core with several -helices on the surface (6). Each monomer of the procaspase homodimer consists of approximately 300 amino acids organized into an N-terminal prodomain followed by a protease domain.…”

Evolution of the folding landscape of effector caspases

“…Effector caspases-3, -6, and -7 have a significant role in apoptosis and serve overlapping but nonredundant functions (33), and multiple studies have examined enzyme specificity and regulation of extant caspases (20,31,34,35). In addition, we have previously examined evolutionary changes resulting in amino acid substitutions that affect enzyme specificity (15,36) and allosteric regulation (25,37,38), but there is a dearth of information regarding changes in the caspase folding landscape. To date, only human caspase-3 has been examined in detail (6,26,27,39), so it was not clear whether all effector caspases utilized the same folding landscape.…”

“…The oligomeric form of the zymogen and its activation mechanism is key to regulating apoptosis (13,15). While the amino acid sequence identity is low between caspase subfamilies (~40%) (15), the caspase-hemoglobinase fold is well-conserved (20) even from distantly related species of vertebrates and invertebrates, such as human, Danio rerio, Caenorhabditis elegans, Drosophila melanogaster, and Porites astreoides (21)(22)(23)(24)(25).The structure of the procaspase monomer is characterized by a 6stranded -sheet core with several -helices on the surface (6). Each monomer of the procaspase homodimer consists of approximately 300 amino acids organized into an Nterminal prodomain followed by a protease domain.…”