New insights into the possible multiple roles of histidine-rich glycoprotein in blue mussels.

Histidine-rich Glycoprotein (HRG) is the most abundant protein in mussel haemolymph plasma. In this study, we determined by qRT-PCR and FISH analysis the tissues involved in HRG synthesis in Mytilus galloprovincialis. The relative HRG mRNA abundance in haemocytes, digestive gland, gills, gonads, posterior adductor muscle, and mantle edge was evaluated. Immunofluorescence analysis of HRG protein distribution in the whole mussel body was performed by a specific antibody. Our data showed the highest gene expression level of HRG in the mantle edge. In particular the outer fold of the mantle edge was shown to be the site that produced the highest amount of the protein. These data indicate a possible role of this Ca2++-binding protein in shell growth. HRG was also found in many other tissues and cells in contact with the haemolymph. This may be related to the immuno-responsive role of this protein. The presence of HRG in tissues related to the feeding pathways and mucous production could indicate the potential significance of this protein into mucus associated antimicrobial action. Overall, the results demonstrate that numerous mussel tissues are involved in HRG production, some of which can release the protein into the haemolymph and others into the extrapallial fluid. These data indicate that extrapallial (EP) protein and HRG are the same protein. An annual cycle survey showed a maximum HRG mRNA as well HRG protein production in mussel tissues in summer, a season in which the animals show the greatest growth, but are more likely to be exposed to microbial pathogens.