Zebrafish Oatp-mediated transport of microcystin congeners

Microcystins (MC), representing >100 congeners being produced by cyanobacteria, are a hazard for aquatic species. As MC congeners vary in their toxicity, the congener composition of a bloom primarily dictates the severity of adverse effects and appears primarily to be governed by toxicokinetics, i.e., whether transport of MCs occurs via organic anion-transporting polypeptides (Oatps). Differences in observed MC toxicity in various fish species suggest differential expression of Oatp subtypes leading to varying tissue distribution of the very same MC congener within different species. The objectives of this study were the functional characterization and analysis of the tissue distribution of Oatp subtypes in zebrafish (Danio rerio) as a surrogate model for cyprinid fish. Zebrafish Oatps (zfOatps) were cloned, and the organ distribution was determined at the mRNA level. zfOatps were transiently expressed in HEK293 cells for functional characterization using the Oatp substrates estrone-3-sulfate, taurocholate and methotrexate and specific MC congeners (MC-LR, MC-RR, MC-LF and MC-LW). Novel zfOatp isoforms were isolated. Among these isoforms, the organ-specific expression of zfOatp1d1 and of members of the zfOatp1f subfamily was identified. At the functional level, zfOatp1d1, zfOatp1f2, zfOatp1f3 and zfOatp1f4 transported at least one of the Oatp substrates, and zfOatp1d1, zfOatp1f2 and zfOatp1f4 were shown to transport MC congeners. MC-LF and MC-LW were generally transported faster than MC-LR and MC-RR. The subtype-specific expression of zfOatp1d1 and of members of the zfOatp1f subfamily as well as differences in the transport of MC congeners could explain the MC congener-dependent differences in toxicity in cyprinids.