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Mytilus shows great immune resistance to various bacteria from the living waters, indicating a complex immune recognition mechanism against various microbes. Peptidoglycan recognition proteins (PGRPs) play an important role in the defense against invading microbes via the recognition of the immunogenic substance peptidoglycan (PGN). Therefore, eight PGRPs were identified from the gill transcriptome of Mytilus coruscus. The sequence features, expression pattern in various organs and larval development stages, and microbes induced expression profiles of these Mytilus PGRPs were determined. Our data revealed the constitutive expression of PGRPs in various organs with relative higher expression level in immune-related organs. The expression of PGRPs is developmentally regulated, and most PGRPs are undetectable in larvae stages. The expression level of most PGRPs was significantly increased with in vivo microbial challenges, showing strong response to Gram-positive strain in gill and digestive gland, strong response to Gram-negative strain in hemocytes, and relative weaker response to fungus in the three tested organs. In addition, the function analysis of the representative recombinant expressed PGRP (rMcPGRP-2) confirmed the antimicrobial and agglutination activities, showing the immune-related importance of PGRP in Mytilus. Our work suggests that Mytilus PGRPs can act as pattern recognition receptors to recognize the invading microorganisms and the antimicrobial effectors during the innate immune response of Mytilus. • Eight PGRP genes were identified from the gill transcriptome of M. coruscus. • The expression profiles of the eight Mytilus PGRP genes were determined. • McPGRPs showed different responses to different in vivo microbial challenges. • Recombinant McPGRP-2 showed classical PGRP functions. [ABSTRACT FROM AUTHOR]

In this study, seven transcripts representing a novel antimicrobial peptide (AMP) family with structural features similar to those of arthropod defensins were identified from Mytilus coruscus. These novel defensins from the Mytilus AMP family were named myticofensins. To explore the possible immune-related functions of these myticofensins, we examined their expression profiles in different tissues and larval stages, as well as in three immune-related tissues under the threat of different microbes. Our data revealed that the seven myticofensins had relatively high expression levels in immune-related tissues. Most myticofensins were undetectable, or had low expression levels, in different larval mussel stages. Additionally, in vivo microbial challenges significantly increased the expression levels of myticofensins in M. coruscus hemocytes, gills, and digestive glands, showing different immune response patterns under challenges from different microbes. Our data indicates that different myticofensins may have different immune functions in different tissues. Furthermore, peptide sequences corresponding to the beta-hairpin, alpha-helix, and N-terminal loop of myticofensin were synthesized and the antimicrobial activities of these peptide fragments were tested. Our data confirms the diversity of defensins in Mytilus and reports the complex regulation of these defensins in the mussel immune response to different microbes in immune-related tissues. The immune system of Mytilus has been studied for years as they are a species with strong environmental adaptations. Our data can be regarded as a step forward in the study of the adaptation of Mytilus spp. to an evolving microbial world. • Seven novel antimicrobial peptides named myticofensin were identified from M. coruscus. • The expression profiles of the myticofensin genes were determined. • Myticofensins showed different responses to different microbial challenges in vivo. • The possible active sites were verified for several myticofensins. [ABSTRACT FROM AUTHOR]

Histones and their N-terminal or C-terminal derived peptides have been studied in vertebrates and presented as potential antimicrobial agents playing important roles in the innate immune defenses. Although histones and their derived peptides had been reported as components of innate immunity in invertebrates, the knowledge about the histone derived antimicrobial peptides (HDAPs) in invertebrates are still limited. Using a peptidomic technique, a set of peptide fragments derived from the histones was identified in this study from the serum of microbes challenged Mytilus coruscus. Among the 85 identified histone-derived-peptides with high confidence, 5 HDAPs were chemically synthesized and the antimicrobial activities were verified, showing strong growth inhibition against Gram-positive bacteria, Gram-negative bacteria, and fungus. The gene expression level of the precursor histones matched by representative HDAPs were further tested using q-PCR, and the results showed a significant upregulation of the histone gene expression levels in hemocytes, gill, and mantle of the mussel after immune stress. In addition, three identified HDAPs were selected for preparation of specific antibodies, and the corresponding histones and their derived C-terminal fragments were detected by Western blotting in the blood cell and serum of immune challenged mussel, respectively, indicating the existence of HDAPs in M. coruscus. Our findings revealed the immune function of histones in Mytilus , and confirmed the existence of HDAPs in the mussel. The identified Mytilus HDAPs represent a new source of immune effector with antimicrobial function in the innate immune system, and thus provide promising candidates for the treatment of microbial infections in aquaculture and medicine. • Abundant HDAPs were identified from Mytilus coruscus serum. • Synthesized Mytilus HDAPs showed strong growth inhibition against bacteria and fungus. • The histone gene expression level of Mytilus histones showed significant upregulation in tissues of challenged mussel. • Western blotting with specific antibody confirmed the existence of HDAPs in M. coruscus serum. [ABSTRACT FROM AUTHOR]