Regulation of HSP70 gene expression during the life cycle of the parasitic helminth Schistosoma mansoni

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Abstract
Analyses of RNA from different developmental stages of Schistosoma mansoni showed stage-specific expression of heat-shock protein 70 (hsp70), which is regulated by a developmental program and by stress. The developmental program, common to hsp70 and other genes (e. g. paramyosin), refers to constitutive expression in miracidia sporocyst and adult worm but not in cercariae, and to the termination of hsp70 gene transcription during sporocyst/cercaria transformation. Stress induction, specific to hsp70, refers to transient accumulation of high levels of hsp70 mRNA during cercariae/schistosomula transformation and in adult worms after heat shock (42 °C). Cercariae/schistosomula transformation can be visualized as a physiological stress involving shifts in temperature (23–37 °C) and in salt concentration (from water to isotonic medium), as well as removal of tails from cercariae to yield isolated bodies that transform into schistosomula. It was found that temperature is an important factor, but not sufficient for strong induction of the hsp70 genes of schistosomula. Tail removal is an obligatory step for full induction of the hsp70 genes of schistosomula, in response to a temperature shift from 23–37 °C. The hsp70 genes in cercariae and isolated tails do not respond to stimuli (salt and temperature increases) that strongly activate the genes in isolated bodies (i. e., schistosomula). We speculate that the hsp70 genes in intact cercariae are not inducible because the tails can produce inhibitory signals that diffuse to the bodies and suppress their hsp70 genes. This hypothesis is useful to explain the termination of hsp70 gene transcription during sporocyst/cercaria transformation by the inhibitory effect of the growing tail.