Abstract
Previous research has shown that an antimicrobial peptide (AMP) of the myticin class C (Myt C) is the most abundantly expressed gene in cDNA and suppressive subtractive hybridization (SSH) libraries after immune stimulation of mussel Mytilus galloprovincialis. However, to date, the expression pattern, the antimicrobial activities and the immunomodulatory properties of the Myt C peptide have not been determined. In contrast, it is known that Myt C mRNA presents an unusual and high level of polymorphism of unidentified biological significance. Therefore, to provide a better understanding of the features of this interesting molecule, we have investigated its function using four different cloned and expressed variants of Myt C cDNA and polyclonal anti-Myt C sera. The in vivo results suggest that this AMP, mainly present in hemocytes, could be acting as an immune system modulator molecule because its overexpression was able to alter the expression of mussel immune-related genes (as the antimicrobial peptides Myticin B and Mytilin B, the C1q domain-containing protein MgC1q, and lysozyme). Moreover, the in vitro results indicate that Myt C peptides have antimicrobial and chemotactic properties. Their recombinant expression in a fish cell line conferred protection against two different fish viruses (enveloped and non-enveloped). Cell extracts from Myt C expressing fish cells were also able to attract hemocytes. All together, these results suggest that Myt C should be considered not only as an AMP but also as the first chemokine/cytokine-like molecule identified in bivalves and one of the few examples in all of the invertebrates.
Highlights
Antimicrobial peptides (AMPs) are small, gene-encoded cationic peptides that constitute important innate immune effectors from organisms spanning most of the phylogenetic spectrum [1,2]
All of the cells were not marked by the probe, which indicates that some hemocytes do not express myticin class C (Myt C)
Consistent with the Myt C RNA expression pattern, immunocytochemical and immunohistochemical analysis using the sera produced against the two Myt C partial sequence peptides showed that the hemocytes were the main source of mussel Myt C peptide (Figure 2 and Figure S1, respectively)
Summary
Antimicrobial peptides (AMPs) are small, gene-encoded cationic peptides that constitute important innate immune effectors from organisms spanning most of the phylogenetic spectrum [1,2]. Constitutive and induced production of AMPs has been reported, with various expression patterns depending on the species, tissue and cell type and infection or inflammation state. These natural antibiotics (41000 AMPs have been estimated in multicellular organisms) exemplify the complexity and heterogeneity of the innate immune responses because they can directly kill microbes and act as modifiers of innate and even adaptive immune responses [7]. In marine invertebrates, which live in environments with an abundance of potentially pathogenic microorganisms, AMPs are the leading elements of the immune response. In Mediterranean mussels (Mytilus galloprovincialis), which in comparison with other bivalves have almost no massive mortality records, several types of AMPs have been described, among them defensins, mytilins and myticins [8,9,10]
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