The functional role of a serotonin transporter in Schistosoma mansoni elucidated through immunolocalization and RNA interference (RNAi)

Serotonin is an important neurotransmitter in both vertebrates and invertebrates. In the parasitic flatworm, Schistosoma mansoni, serotonin stimulates worm movement and potentiates muscle contraction. A specific serotonin transporter (SmSERT) was previously cloned from S. mansoni and characterized in vitro. Here we conduct a first investigation of the native protein in the worm so as to elucidate the biological role of SmSERT and to assess its drug targeting potential. Confocal immunofluorescence studies using specific antibodies determined that SmSERT is expressed predominantly in the nervous system both in adult worms and larvae (schistosomula). SmSERT immunoreactivity was detected in the main nerve cords of the central nervous system and the peripheral nerve plexus of the body wall in adult males and females, in apparent nerve endings of the male tubercles and possibly the male tegument. In the larvae, SmSERT localized mainly to the peripheral nerve plexus of the body wall. Co-localization experiments showed that the pattern of SmSERT expression coincides with that of serotonin itself, suggesting that SmSERT is present in serotonergic neurons. To test whether SmSERT is involved in the motor effects of serotonin, we treated S. mansoni schistosomula with SmSERT blockers or SmSERT-specific short-interfering RNAs (siRNAs) and then recorded larval motility, using a quantitative imaging assay. In both cases, the treatment produced a strongly hyperactive phenotype, corresponding to a ~3-fold increase in larval motility, roughly the same effect as treatment with an excess of exogenous serotonin. The siRNA effect correlated with a ≈50% decrease in expression of the SmSERT when tested by real-time qPCR. To test if SmSERT mediates transport of exogenous serotonin across the tegument, uptake assays were also performed in intact schistosomula treated with SmSERT siRNAs or an irrelevant siRNA. We found a significant but modest decrease (~25%) in serotonin uptake in the siRNA-suppressed larvae when compared to the negative controls. These results suggest that the SmSERT's function is primarily neuromuscular and may also play a secondary role in the uptake of exogenous (host-derived) serotonin.

Classical transmitters and their receptors in flatworms

The flatworm nervous system employs a wide repertoire of neuroactive substances, including small chemical messengers, the so called classical transmitters, and several types of neuropeptides. A large body of research accumulated over four decades has provided a wealth of information on the tissue localization and effects of these substances, their biochemistry and, recently, their molecular modes of action in all major classes of flatworms. This evidence will be reviewed, with particular emphasis on the small (classical) transmitters and the receptors that mediate their effects. One of the themes that will emerge from this discussion is that classical transmitters regulate core activities such as movement, metabolism and transport, and thus are essential for survival of the organism. In addition, the evidence shows that flatworms have multiple neurotransmitter receptors, many with unusual pharmacological features, which make them particularly attractive as drug targets. Understanding the molecular basis of these distinctive properties, and developing new, more specific receptor agonists and antagonists will undoubtedly become a major challenge in future research.

SEROTONIN-INDUCED MUSCULAR ACTIVITY IN SCHISTOSOMA MANSONI LARVAL STAGES: IMPORTANCE OF 5-HT TRANSPORT AND ROLE IN DAUGHTER SPOROCYST PRODUCTION

Mother sporocysts of Schistosoma mansoni transport exogenously supplied serotonin (5-hydroxytrypamine; 5-HT), and respond to it with increases in motility. In the present study, we investigated the importance of 5-HT transporter activity in the manifestation of these 5-HT-induced motility changes, and further examined the role of 5-HT in the development of daughter sporocysts in vitro. Serotonin-induced motility of in vitro-derived sporocysts is not inhibited by antidepressant compounds, e.g., fluoxetine, that block 5-HT transport, suggesting that the receptors responsible for motility responses to 5-HT are surface exposed. Using a sporocyst in vitro culture system, we show that depletion of larval stores of 5-HT reduces production of daughter sporocysts, the second intramolluscan larval stage. Moreover, we demonstrate a strong correlation between endogenous 5-HT levels and basal mother sporocyst muscle activity. Overall, these data suggest that larval stages of S. mansoni can detect exogenous 5-HT via surface-exposed receptors, and they are consistent with the hypothesis that endogenous stores of 5-HT are important for the proper regulation of muscular contractions in mother sporocysts, and for the successful emergence of daughter sporocysts.

Localization of a sodium-dependent high-affinity serotonin transporter and recruitment of exogenous serotonin by the cestode Hymenolepis diminuta: an autoradiographic and immunohistochemical study

Uptake of serotonin by tissues of intact and hemitransected Hymenolepis diminuta was studied by autoradiography and immunohistochemistry. Hemitransected worms were incubated in balanced saline containing 10 µM [3H]serotonin and washed extensively. The density of silver grains over the serotonin-immunoreactive longitudinal nerve cords and commissural rings, male reproductive system, tissues surrounding the genital pouch, and deep longitudinal muscles was significantly greater than that over the parenchyma. The presence of serotonin in spermatozoa suggested a role for this amine in spermatozoon activity. In contrast, uptake of 10 µM [3H]serotonin in sodium-free saline was significantly reduced compared with that in balanced saline in all tissues examined except the parenchyma. Analysis of the data revealed that the sodium-dependent high-affinity serotonin transport system is localized primarily in the serotonergic-like neurons of H. diminuta, which suggests possible recycling of neuronally released serotonin. Following incubation of intact worms in vitro for 12 h in 5 µM [3H]serotonin, the density of silver grains was significantly higher over the serotonin-immunoreactive nerves, elements of the male reproductive system, tissues surrounding the genital pouch, and deep longitudinal muscles than over the parenchyma. These results demonstrate recruitment of exogenous serotonin by intact H. diminuta and suggest sequestration and concentration by the serotonin-immunoreactive neurons via the sodium-dependent high-affinity transporter. These data further suggest that although H. diminuta can synthesize serotonin, it may obtain serotonin from the host. Nonetheless, the amount of serotonin recruited by H. diminuta from the host in vivo compared with that which they synthesize is not known.