Increase in multidrug transport activity is associated with oocyte maturation in sea stars

In this study, we report on the presence of efflux transporter activity before oocyte maturation in sea stars and its upregulation after maturation. This activity is similar to the multidrug resistance (MDR) activity mediated by ATP binding cassette (ABC) efflux transporters. In sea star oocytes the efflux activity, as measured by exclusion of calcein-am, increased two-fold 3 h post-maturation. Experiments using specific and non-specific dyes and inhibitors demonstrated that the increase in transporter activity involves an ABCB protein, P-glycoprotein (P-gp), and an ABCC protein similar to the MDR-associated protein (MRP)-like transporters. Western blots using an antibody directed against mammalian P-gp recognized a 45 kDa protein in sea star oocytes that increased in abundance during maturation. An antibody directed against sea urchin ABCC proteins (MRP) recognized three proteins in immature oocytes and two in mature oocytes. Experiments using inhibitors suggest that translation and microtubule function are both required for post-maturation increases in transporter activity. Immunolabeling revealed translocation of stored ABCB proteins to the plasma cell membrane during maturation, and this translocation coincided with increased transport activity. These MDR transporters serve protective roles in oocytes and eggs, as demonstrated by sensitization of the oocytes to the maturation inhibitor, vinblastine, by MRP and PGP-specific transporter inhibitors.