The new insights into the oyster antimicrobial defense: Cellular, molecular and genetic view

Abstract

Oysters are sessile filter feeders that live in close association with abundant and diverse communities of microorganisms that form the oyster microbiota. In such an association, cellular and molecular mechanisms have evolved to maintain oyster homeostasis upon stressful conditions including infection and changing environments. We give here cellular and molecular insights into the Crassostrea gigas antimicrobial defense system with focus on antimicrobial peptides and proteins (AMPs). This review highlights the central role of the hemocytes in the modulation and control of oyster antimicrobial response. As vehicles for AMPs and other antimicrobial effectors, including reactive oxygen species (ROS), and together with epithelia, hemocytes provide the oyster with local defense reactions instead of systemic humoral ones. These reactions are largely based on phagocytosis but also, as recently described, on the extracellular release of antimicrobial histones (ETosis) which is triggered by ROS. Thus, ROS can signal danger and activate cellular responses in the oyster. From the current literature, AMP production/release could serve similar functions. We provide also new lights on the oyster genetic background that underlies a great diversity of AMP sequences but also an extraordinary individual polymorphism of AMP gene expression. We discuss here how this polymorphism could generate new immune functions, new pathogen resistances or support individual adaptation to environmental stresses.

The paralytic shellfish toxin, saxitoxin, enters the cytoplasm and induces apoptosis of oyster immune cells through a caspase-dependent pathway

“…PST-induced programmed cell death may have important consequences on oyster defense from pathogens, as hemocytes play a key role in oyster immune response. These cell types are involved in a series of cellular immune reactions, such as phagocytosis of invading microorganisms, formation of nodules and capsules surrounding pathogens and their subsequent degradation through the release of host defense molecules (Bachere et al 2015). Hemocytes are able to migrate from the hemolymph to connective tissues and promote localized responses following injury or microorganism invasion.…”

“…Briefly, hemocytes freshly withdrawn from oyster adductor muscle were plated at a cell density sorted by size and granularity (blue) as described previously (Bachere et al 2015) and the percentage of TMRNEL-labeled cells (red) per population was determined using a BD LSR Fortessa™ flow cytometer analyzer.…”

Antimicrobial peptides in marine invertebrate health and disease

“…Some families appear to have evolved within specific phyla or species [31], whereas others are found in a diversity of phyla [32]. Most AMP families from molluscs, including defensins, big defensins and BPI (for recent review, see [33]), are also found in other protostomes (Ecdysozoa) and/ or in deuterostomes (Mammalia). By contrast, in cnidarians, taxonomically restricted AMPs have been described, such as arminins, which are among the most highly expressed genes in Hydra [34], or damicornin in the coral Pocillopora damicornis [43].…”

Infectious diseases of marine molluscs and host responses as revealed by genomic tools

“…A large number of immune effectors have been identified in molluscs including diverse antimicrobial peptides or proteins (AMPs), lysozymes, proteases, ribonucleases and oxidases [6,[129][130][131]. Diverse cysteine-rich AMPs have been reported in mussels including defensins, big defensins, mytilins, myticins, mytimacins, mytimycins, myticusins and mytiCRPs [129].…”