Investigating appearance and regulation of the MXR phenotype in early embryo stages of the Mediterranean mussel (Mytilus galloprovincialis)

Multixenobiotic resistance (MXR) efflux transporters constitute a broad-spectrum physiological defense system allowing marine bivalves to cope with environmental challenges. There is, however, scarce information on the type and role that different MXR transporters may have in embryos, which represent the most sensitive stages of bivalves to environmental stress. In this study regulation of MXR-related transporters was investigated in early developmental stages of the Mediterranean mussel (Mytilus galloprovincialis). In vitro fertilization experiments using gametes from naturally-spawning broodstocks were performed to follow embryo development from fertilized eggs (30 min post fertilization, pf) to fully developed D-shape veligers (48 h pf). Quantitative PCR analyses indicated that ABCB and ABCC transcripts encoding the MXR-related transporters P-glycoproteins (P-gp) and Multidrug resistance proteins (Mrp), respectively, were expressed soon after 30 min pf, with ABCC being more expressed than ABCB. Copy numbers of both transcripts were increased in trochophorae and D-veligers. MXR efflux activity assessed using the fluorescent substrate rhodamine 123 and selective P-gp or Mrp inhibitors showed that the P-gp mediated efflux was detected only in D-veligers, while a significant Mrp mediated efflux was detected soon after 30 min pf and remained almost unchanged in trochophorae and D-veligers. MXR modulation by propranolol and carbamazepine showed that the pharmaceuticals may act as transcriptional regulators and substrates. Results reported lead to hypothesize that while P-gp aids in xenobiotic efflux performing a prominent protective role, Mrp could be a dual-functioning transporter performing both protective and physiological functions in mussel development.