The differential expression of multiple isoenzyme forms during stage conversion of Toxoplasma gondii: an adaptive developmental strategy

The apicomplexan parasite Toxoplasma gondii has the ability to switch betwe en a rapidly replicating tachyzoite and a slowly dividing encysted bradyzoi te within its intermediate hosts such as humans or other warm-blooded verte brates. It is likely that in vivo, the tachyzoites differentiate into encys ted bradyzoites in response to the immune system attack during disease prog ression. As part of a developmental strategy and, in order to survive withi n infected hosts, T. gondii tachyzoites undergo profound metabolic and morp hological changes by differentiating into encysted bradyzoites. Bradyzoites are characterised by their resistance to both the immune system and chemot herapy. The stimulus that triggers Toxoplasma encystation and the molecular mechanisms triggering the switch from tachyzoite to bradyzoite remain unkn own. It is very important to elucidate these mechanisms since bradyzoites w ithin tissue cysts are not only the source of infection transmitted from do mestic animals to humans, but can also be converted into tachyzoites that a re the cause of fatal toxoplasmic encephalitis in acquired immunodeficiency syndrome patients. In this review, I focus on recent efforts towards the c haracterisation of genes that encode several stage-specific isoenzymes. The picture emerging from these studies is that stage-specific expression of i soenyzmes having different biochemical properties accompanies the interconv ersion of tachyzoite into bradyzoite, and vice versa. It can be hypothesise d that the difference found between these enzymatic activities may be instr umental in maintaining some major parasitic metabolisms such as glycolysis in pace with the stage-specific requirements of carbohydrate or polysacchar ide biosynthesis. (C) 2001 Australian Society for Parasitology Inc. Publish ed by Elsevier Science Ltd. All rights reserved.