Characterization of a serotonin transporter in the parasitic flatworm, Schistosoma mansoni: cloning, expression and functional analysis.

The biogenic amine serotonin (5-hydroxytryptamine: 5HT) is a widely distributed neuroactive substance of vertebrates and invertebrates. Among parasitic flatworms, in particularly the bloodfluke, Schistosoma mansoni, 5HT is an important modulator of neuromuscular function and metabolism. Previous work has shown that schistosomes take up 5HT from host blood via a carrier mediated mechanism. This transport is thought to contribute to the control of schistosome motility in the bloodstream and is essential for survival of the parasite. Here we provide the first molecular evidence for the existence of a 5HT transporter in S. mansoni. A cDNA showing high homology with plasma membrane serotonin transporters (SERT) from other species was cloned and characterized by heterologous expression in cultured HEK293 cells. Functional studies showed that the recombinant schistosome transporter (SmSERT) mediates specific and saturable [(3)H]-5HT transport with a K(t)=1.30+/-0.05 microM. The heterologously expressed protein was inhibited by classic SERT blockers (clomipramine, fluoxetine, citalopram) and the same drugs also inhibited [(3)H]-5HT uptake by intact schistosomula in culture, suggesting that SmSERT may be responsible for this transport. Conventional (end-point) and real-time quantitative RT-PCR analyses determined that SmSERT is expressed both in the free-living stage (cercaria) and parasitic forms of S. mansoni but the expression level is significantly higher in the parasites. These results suggest that SmSERT is upregulated following cercarial transformation, possibly to mediate the recruitment of exogenous 5HT from the host.