Myticin C

Little is known about the antiviral response in mollusks. As in other invertebrates, the interferon signaling pathways have not been identified, and in fact, there is a debate about whether invertebrates possess antiviral immunity similar to that of vertebrates. In marine bivalves, due to their filtering activity, interaction with putative pathogens, including viruses, is very high, suggesting that they should have mechanisms to address these infections. In this study, we confirmed that constitutively expressed molecules in naive mussels confer resistance in oysters to ostreid herpesvirus 1 (OsHV-1) when oyster hemocytes are incubated with mussel hemolymph. Using a proteomic approach, myticin C peptides were identified in both mussel hemolymph and hemocytes. Myticins, antimicrobial peptides that have been previously characterized, were constitutively expressed in a fraction of mussel hemocytes and showed antiviral activity against OsHV-1, suggesting that these molecules could be responsible for the antiviral activity of mussel hemolymph. For the first time, a molecule from a bivalve has shown antiviral activity against a virus affecting mollusks. Moreover, myticin C peptides showed antiviral activity against human herpes simplex viruses 1 (HSV-1) and 2 (HSV-2). In summary, our work sheds light on the invertebrate antiviral immune response with the identification of a molecule with potential biotechnological applications. Importance: Several bioactive molecules that have potential pharmaceutical or industrial applications have been identified and isolated from marine invertebrates. Myticin C, an antimicrobial peptide from the Mediterranean mussel (Mytilus galloprovincialis) that was identified by proteomic techniques in both mussel hemolymph and hemocytes, showed potential as an antiviral agent against ostreid herpesvirus 1 (OsHV-1), which represents a major threat to the oyster-farming sector. Both hemolymph from mussels and a myticin C peptide inhibited OsHV-1 replication in oyster hemocytes. Additionally, a modified peptide derived from myticin C or the nanoencapsulated normal peptide also showed antiviral activity against the human herpesviruses HSV-1 and HSV-2. Therefore, myticin C is an example of the biotechnological and therapeutic potential of mollusks.

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.

Myticin C is the most studied antimicrobial peptide in the marine mussel Mytilusgalloprovincialis. Although it is constitutively expressed in mussel hemocytes and displays antibacterial, antiviral, and chemotactic functions, recent work has suggested that this molecule is mainly activated after tissue injury. Therefore, the main objective of this work was to characterize the hemocytes' transcriptomic response after a myticin C treatment, in order to understand the molecular changes induced by this cytokine-like molecule. The transcriptome analysis revealed the modulation of genes related to cellular movement, such as myosin, transgelin, and calponin-like proteins, in agreement with results of functional assays, where an implication of myticin C in the in vitro activation of hemocytes and migration was evidenced. This was also observed in vivo after a tissue injury, when hemocytes, with high concentrations of myticin C, migrated to the damaged area to heal the wound. All these properties allowed us to think about the biotechnological application of these molecules as wound healers. Human keratinocytes and larvae zebrafish models were used to confirm this hypothesis. Accelerated regeneration after a wound or tail fin amputation was observed after treatment with the myticin C peptide, supporting the chemotactic and healing activity of myticin C.