Induction cues for tegument formation during the transformation of Schistosoma mansoni cercariae

Adult schistosomes are parasitic blood flukes that have a continuous double lipid bilayered membrane surrounding the entire worm. This tegumental memb rane is synthesised during invasion of the vertebrate host by free-swimming infectious forms called cercariae. As cercariae invade their final hosts t hey lose their tails and encounter a changing environment that includes alt ered temperature. sugar concentration and osmolarity. We have identified a glucose transporter protein designated sGTP4 that is found exclusively in t he outer adult tegument and on membranous vesicles within the tegumental cy toplasm, By using immunofluorescence analysis to monitor the appearance and distribution of SGTP4 we can track the process of new tegumental membrane formation and examine the cues that trigger this developmental pathway. Cer cariae in water do not transform their tegument while those incubated in ri ch medium do so rapidly. We have examined which of the many constituents of rich medium are responsible for triggering this transformation. Incubation in a solution of moderate osmolarity (120 mOsM PBS) is sufficient by itsel f to trigger tegument transformation, albeit at a slower rate relative to i ncubation in rich medium. Adding either glucose (to 100 mM) to the solution or increasing the temperature of incubation (from 22 degrees C to 37 degre es C) further increased the rate of tegument biogenesis. The introduction o f glucose together with an increase in the incubation temperature further a ccelerated the process, suggesting that these factors act synergistically t o promote transformation rates. The critical nature of osmolarity in induci ng the process is highlighted by the fact that transformation proceeds as e fficiently in 360 mOsM alone as it does in rich medium. While the fatty aci ds linolenic acid (cis-9, cis-12, cis-15-octadecatrienoic acid at 1 mM) and capric acid (Decanoic acid, at 0.1 mM) have both been proposed to stimulat e tegumental transformation, we show that neither promotes the morphogenesi s of a normal schistosomulum tegument. The schistosomicide praziquantel (to 1 mM) has no detectable effect on new tegument formation. (C) 2000 Publish ed by Elsevier Science Ltd on behalf of the Australian Society for Parasito logy Inc. All rights reserved.