Disruption of the RNAi pathway abolishes surface antigen variation in the intestinal parasite Giardia.

CESAR G PRUCCA; ILEANA SLAVIN; RODRIGO QUIROGA; ELIANA V. ELIAS; FERNANDO D RIVERO; ALICIA SAURA; PEDRO G CARRANZA; HUGO D LUJAN

Villa Carlos Paz

Congreso; XLIV Reunión anual Sociedad Argentina de Investigación en Bioquímica y Biología Molecular; 2008

Giardia is one of the most common parasites of humans and a major cause of diarrhoea worldwide. To evade the immune response, Giardia undergoes antigenic variation; a process that allows the parasite to develop chronic and recurrent infections. From a repertoire of 190 variant-specific surface proteins (VSPs) coding genes, Giardia expresses only one VSP on the surface of each trophozoite at a particular time, but switches to the expression of a different VSP by unknown mechanisms. Here we show that regulation of VSP expression involves a system comprising homologues of RNA-dependent RNA-polymerase (RdRP), Dicer, and Argonaute (Ago), known components of the RNA interference (RNAi) machinery. In Giardia, clones expressing a single surface antigen efficiently transcribe several other vsps but only accumulate transcripts encoding thce actual VSP. Detection of antisense RNAs corresponding to the silenced vsps and cleavage of dsRNAs into 22- 25 nt siRNA from the silenced but not for the expressed vsp implicate the RNAi pathway on Giardia?s antigenic variation. Remarkably, knock-down of Dicer and RdRP leads to a change from single to multiple VSP expression in individual trophozoites, while loss of Ago shows more drastic effect on Giardia viability. Our results suggest the involvement of a PTGS mechanism in regulating the expression of surface antigens in this important human pathogen.