Contribution of Toxoplasma gondii variant histones in chromatin modulation

ANGEL, SO; DALMASSO, MC; SULLIVAN, WJ JR.

Mar del Plata

Congreso; IX Congreso Argentino de Protozoología y Enfermedades Parasitaria; 2011

Sociedad Argentina de Protozoologia

Toxoplasma gondii is a coccidian protozoan parasite that belongs to the phylum Apicomplexa. It is estimated that toxoplasmosis exists as a chronic asymptomatic form in 5 hundred million to 1 billion of the world human population (Tenter et al., 2000). Although infection with T. gondii is usually asymptomatic in most individuals, it is of great medical significance for pregnant women and immunocompromised patients. In human, T. gondii infection is characterized by two stages, the rapidly growing tachyzoites, and the latent bradyzoite tissue cysts. Tachyzoites are responsible for acute illness and congenital birth defects. Bradyzoites form latent cysts for many years but still capable of converting in to the destructive tachyzoite form if host immunity wanes. These two developmental stages are essential for disease propagation and causation. Tachyzoite to bradyzoite conversion, and vice-versa, includes high number of gene expression modifications (Radke et al., 2005). It is believed that the epigenetic control of gene regulation is crucial for parasite development, a process that relay at the post-translational modification (PTM) of histones and histone variants exchange (Sullivan et al., 2006). T. gondii possess the four canonical histones H2A, H2B, H3 and H4 and variant histones of H3 and H2A families (Dalmasso et al., 2011). Concerning H2A family, the parasite has H2AZ and H2AX variants, a feature that is not shared by other Apicomplexas which H2AX is not present. In higher eukaryotes, the role of H2AZ is associated to transcriptional regulation, genome stability, and blocking the spread of heterochromatin, whereas H2AX is involved in DNA repair being recruited at double strand break (DSB) site, a process that require the phsophorylation of the serine present at C-terminal motif SQEY/F. H2A and H2B form dimmers that pair with an H3-H4 tetramer to form the core nucleosome particle. H2B and H4 do not present typical variants in higher eukaryotas. Interestingly, T. gondii has a variant of H2B, named H2Bv (Dalmasso et al., 2009). This aspect also contributes to convert T. gondii in a unique model to gain understanding regarding the role of histone variants in chromatin modulation in the nature. In T. gondii, H2Bv forms dimmers mainly with H2AZ, but not with H2AX, which forms dimmers with canonical H2B. H2AZ and H2AX are not present in the same nucleosome. Acetylated H3 (H3ac), a mark of active transcription, can be present in the same nucleosome with H2Bv and H2AZ but is poorly in H2AX-H2B nucleosomes. These findings reveal that nuclesosmal arrangements are not random in protozoa, highlighting their relevance in chromatin composition and regulation. In fact, the different nucleosomes are associated to different genome regions. Regions upstream of active genes are enriched in H3ac, H2AZ and H2Bv, whereas regions upstream of inactive genes and silent genome are enriched in H4 mono-methylated at lysine 20 (H4K20met), a marker of heterochromatin, H2AX. During bradyzoite conversion in vitro, the h2ax gene showed a significant increase of its expression in comparison with tachyzoite stage. In contrast, h2az levels were stable during bradyzoite development, inferring that the effect on the h2ax gene is due to bradyzoite development signals. Since the gene expression decrease in bradyzoite stage, it is tempting to speculate that the increase of H2AX is necessary to spread chromatin repression during the latent bradyzoite stage. These observations suggest that H2AX may have other novel functions beside DNA repair. As mentioned above, the phosphorylation of the serine present at SQEY/F (gammaH2AX) is essential to be recruited at DSB sites. In fact, the level of gammaH2AX increases significantly in parasites under oxidative stress that produce DNA damage. However this phosphorylation is not relevant to dimmerize with canonical H2B. In conclusion, T. gondii presents a novel nucleosome composition that would have important implicances in chromatin modulation and gene regulation (Figure 1).