DISRUPTION OF THE RNAI PATHWAY ABOLISHES SURFACE ANTIGEN VARIATION IN THE INTESTINAL PARASITE Giardia

PRUCCA CG; SLAVIN I; QUIROGA, R; ELIAS VE; RIVERO, FD; SAURA, A; CARRANZA, P; LUJÁN, HD

Carlos Paz, Argentina.

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

SAIB

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.