Molecular cloning and functional characterization of a rainbow trout liver Oatp

Cyanobacterial blooms have an impact on the aquatic ecosystem due to the production of toxins (e.g. microcystins, MCs), which constrain fish health or even cause fish death. However the toxicokinetics of the most abundant toxin, microcystin-LR (MC-LR), are not yet fully understood. To investigate the uptake mechanism, the novel Oatp1d1 in rainbow trout (rtOatp1d1) was cloned, identified and characterized. The cDNA isolated from a clone library consisted of 2772bp containing a 2115bp open reading frame coding for a 705 aa protein with an approximate molecular mass of 80kDa. This fish specific transporter belongs to the OATP1 family and has most likely evolved from a common ancestor of OATP1C1. Real time PCR analysis showed that rtOatp1d1 is predominantly expressed in the liver, followed by the brain while expression in other organs was not detectable. Transient transfection in HEK293 cells was used for further characterization. Like its human homologues OATP1A1, OATP1B1 and OATP1B3, rtOatp1d1 displayed multi-specific transport including endogenous and xenobiotic substrates. Kinetic analyses revealed a Km value of 13.9μM and 13.4μM for estrone-3-sulfate and methotrexate, respectively and a rather low affinity for taurocholate with a Km value of 103μM. Furthermore, it was confirmed that rtOatp1d1 is a MC-LR transporter and therefore most likely plays a key role in the susceptibility of rainbow trout to MC intoxications.