Epigenetic control of the variable expression of a Plasmodium falciparum receptor protein for erythrocyte invasion

Heterochromatin formation in bistable chromatin domains controls the epigenetic repression of clonally variant Plasmodium falciparum genes linked to erythrocyte invasion

Clonally variant gene expression is a common survival strategy used by many pathogens, including the malaria parasite Plasmodium falciparum. Among the genes that show variant expression in this parasite are several members of small gene families linked to erythrocyte invasion, including the clag and eba families. The active or repressed state of these genes is clonally transmitted by epigenetic mechanisms. Here we characterized the promoters of clag3.1, clag3.2 and eba-140, and compared nuclease accessibility and post-translational histone modifications between their active and repressed states. Activity of these promoters in an episomal context is similar between parasite subclones characterized by different patterns of expression of the endogenous genes. Variant expression is controlled by the euchromatic or heterochromatic state of bistable chromatin domains. Repression is mediated by H3K9me3-based heterochromatin, whereas the active state is characterized by H3K9ac. These marks are maintained throughout the asexual blood cycle to transmit the epigenetic memory. Furthermore, eba-140 is organized in two distinct chromatin domains, probably separated by a barrier insulator located within its ORF. The 5' chromatin domain controls expression of the gene, whereas the 3' domain shares the chromatin conformation with the upstream region of the neighbouring phista family gene, which also shows variant expression.

Epigenetics in Plasmodium: what do we really know?

In the burgeoning field of Plasmodium gene expression, there are--to borrow some famous words from a former U.S. Secretary of Defense--"known knowns, known unknowns, and unknown unknowns." This is in itself an important achievement, since it is only in the past decade that facts have begun to move from the third category into the first. Nevertheless, much remains in the middle ground of known or suspected "unknowns." It is clear that the malaria parasite controls vital virulence processes such as host cell invasion and cytoadherence at least partly via epigenetic mechanisms, so a proper understanding of epigenetic transcriptional control in this organism should have great clinical relevance. Plasmodium, however, is an obligate intracellular parasite: it operates not in a vacuum but rather in the complicated context of its metazoan hosts. Therefore, as valuable data about the parasite's basic epigenetic machinery begin to emerge, it becomes increasingly important to relate in vitro studies to the situation in vivo. This review will focus upon the challenge of understanding Plasmodium epigenetics in an integrated manner, in the human and insect hosts as well as the petri dish.

Chromatin-mediated epigenetic regulation in the malaria parasite Plasmodium falciparum

Malaria is a major public health problem in many developing countries, with the malignant tertian parasite Plasmodium falciparum causing the most malaria-associated mortality. Extensive research, especially with the advancement of genomics and transfection tools, has highlighted the fundamental importance of chromatin-mediated gene regulation in the developmental program of this early-branching eukaryote. The Plasmodium parasite genomes reveal the existence of both canonical and variant histones that make up the nucleosomes, as well as a full collection of conserved enzymes for chromatin remodeling and histone posttranslational modifications (PTMs). Recent studies have identified a wide array of both conserved and novel histone PTMs in P. falciparum, indicating the presence of a complex and divergent "histone code." Genome-wide analysis has begun to decipher the nucleosome landscape and histone modifications associated with the dynamic organization of chromatin structures during the parasite's life cycle. Focused studies on malaria-specific phenomena such as antigenic variation and red cell invasion pathways shed further light on the involvement of epigenetic mechanisms in these processes. Here we review our current understanding of chromatin-mediated gene regulation in malaria parasites, with specific reference to exemplar studies on antigenic variation and host cell invasion.