Expression of metallothionein mRNAs by in situ hybridization in the gills of Mytilus galloprovincialis, from natural polluted environments.

Metallothioneins (MTs), metal-inducible proteins, are crucial proteins for the regulation of essential metals, and are transcriptionally induced in all organisms by certain heavy metals, oxidative stress and inflammation. The gills represent an organ of uptake and loss of metals in which different mechanisms are present controlling the functions directly involved in the maintenance of homeostasis. In this study, the morphological and histomorphological aspects of branchial epithelium in Mytilus galloprovincialis from polluted environment (Faro swamp, Messina, Italy) have been investigated. The reverse transcriptase-polymerase chain reaction (PCR) has been used to isolate complementary DNA of both MT isoforms present from RNA extracted from mussel gills. The respective mRNAs on histological sections have been visualized by in situ hybridization. These methods showed that MT-10 mRNA is expressed at the basal level. In contrast, the MT-20 expression level was very low under basal conditions, while its mRNA increased dramatically in individuals collected in Faro. The presence of acid mucocytes and MTs in the gills may be considered a further defensive mechanism also related to the significantly higher concentration of Cd, Pb and Cr found in gills of M. galloprovincialis from Faro than specimens from the reference site (Goro). The results obtained show that, in stressed mussels, the defensive processes increase to maintain the normal functions of the organs more exposed to the action of polluted substances.