DNA sequence and chromatin differentiate sequence-specific transcription factor binding in the human malaria parasite Plasmodium falciparum.

Development of the malaria parasite, Plasmodium falciparum, is regulated by a limited number of sequence-specific transcription factors (TFs). However, the mechanisms by which these TFs recognize genome-wide binding sites is largely unknown. To address TF specificity, we investigated the binding of two TF subsets that either bind CACACA or GTGCAC DNA sequence motifs and further characterized two additional ApiAP2 TFs, PfAP2-G and PfAP2-EXP, which bind unique DNA motifs (GTAC and TGCATGCA). We also interrogated the impact of DNA sequence and chromatin context on P. falciparum TF binding by integrating high-throughput in vitro and in vivo binding assays, DNA shape predictions, epigenetic post-translational modifications, and chromatin accessibility. We found that DNA sequence context minimally impacts binding site selection for paralogous CACACA-binding TFs, while chromatin accessibility, epigenetic patterns, co-factor recruitment, and dimerization correlate with differential binding. In contrast, GTGCAC-binding TFs prefer different DNA sequence context in addition to chromatin dynamics. Finally, we determined that TFs that preferentially bind divergent DNA motifs may bind overlapping genomic regions due to low-affinity binding to other sequence motifs. Our results demonstrate that TF binding site selection relies on a combination of DNA sequence and chromatin features, thereby contributing to the complexity of P. falciparum gene regulatory mechanisms.

H3K36 methylation reprograms gene expression to drive early gametocyte development in Plasmodium falciparum.

BACKGROUND: The Plasmodium sexual gametocyte stages are the only transmissible form of the malaria parasite and are thus responsible for the continued transmission of the disease. Gametocytes undergo extensive functional and morphological changes from commitment to maturity, directed by an equally extensive control program. However, the processes that drive the differentiation and development of the gametocyte post-commitment, remain largely unexplored. A previous study reported enrichment of H3K36 di- and tri-methylated (H3K36me2&3) histones in early-stage gametocytes. Using chromatin immunoprecipitation followed by high-throughput sequencing, we identify a stage-specific association between these repressive histone modifications and transcriptional reprogramming that define a stage II gametocyte transition point. RESULTS: Here, we show that H3K36me2 and H3K36me3 from stage II gametocytes are associated with repression of genes involved in asexual proliferation and sexual commitment, indicating that H3K36me2&3-mediated repression of such genes is essential to the transition from early gametocyte differentiation to intermediate development. Importantly, we show that the gene encoding the transcription factor AP2-G as commitment master regulator is enriched with H3K36me2&3 and actively repressed in stage II gametocytes, providing the first evidence of ap2-g gene repression in post-commitment gametocytes. Lastly, we associate the enhanced potency of the pan-selective Jumonji inhibitor JIB-04 in gametocytes with the inhibition of histone demethylation including H3K36me2&3 and a disruption of normal transcriptional programs. CONCLUSIONS: Taken together, our results provide the first description of an association between global gene expression reprogramming and histone post-translational modifications during P. falciparum early sexual development. The stage II gametocyte-specific abundance of H3K36me2&3 manifests predominantly as an independent regulatory mechanism targeted towards genes that are repressed post-commitment. H3K36me2&3-associated repression of genes is therefore involved in key transcriptional shifts that accompany the transition from early gametocyte differentiation to intermediate development.

Dissecting the role of PfAP2-G in malaria gametocytogenesis.

In the malaria parasite Plasmodium falciparum, the switch from asexual multiplication to sexual differentiation into gametocytes is essential for transmission to mosquitos. The transcription factor PfAP2-G is a key determinant of sexual commitment that orchestrates this crucial cell fate decision. Here we identify the direct targets of PfAP2-G and demonstrate that it dynamically binds hundreds of sites across the genome. We find that PfAP2-G is a transcriptional activator of early gametocyte genes, and identify differences in PfAP2-G occupancy between gametocytes derived via next-cycle and same-cycle conversion. Our data implicate PfAP2-G not only as a transcriptional activator of gametocyte genes, but also as a potential regulator of genes important for red blood cell invasion. We also find that regulation by PfAP2-G requires interaction with a second transcription factor, PfAP2-I. These results clarify the functional role of PfAP2-G during sexual commitment and early gametocytogenesis.

Commitment Isn't for Everyone.

The majority of malaria parasites during human infection are asexual and are unable to be transmitted to mosquitoes. Only sexually differentiated parasites (gametocytes) can be successfully transmitted to complete the lifecycle. In a recent study by Bancells et al. (Nat. Microbiol. 2019;4:144-154), a new route of sexual conversion is identified that does not require a prior round of replication.

Regulation of Sexual Commitment and Gametocytogenesis in Malaria Parasites.

Sexual differentiation of malaria parasites from the asexual blood stage into gametocytes is an essential part of the life cycle, as gametocytes are the form that is taken up by the mosquito host. Because of the essentiality of this process for transmission to the mosquito, gametocytogenesis is an extremely attractive target for therapeutic interventions. The subject of this review is the considerable progress that has been made in recent years in elucidating the molecular mechanisms governing this important differentiation process. In particular, a number of critical transcription factors and epigenetic regulators have emerged as crucial elements in the regulation of commitment. The identification of these factors has allowed us to understand better than ever before the events occurring prior to and during commitment to sexual development and offers potential for new therapeutic interventions.

Activation and clustering of a Plasmodium falciparum var gene are affected by subtelomeric sequences.

The Plasmodium falciparum var multigene family encodes the cytoadhesive, variant antigen PfEMP1. P. falciparum antigenic variation and cytoadhesion specificity are controlled by epigenetic switching between the single, or few, simultaneously expressed var genes. Most var genes are maintained in perinuclear clusters of heterochromatic telomeres. The active var gene(s) occupy a single, perinuclear var expression site. It is unresolved whether the var expression site forms in situ at a telomeric cluster or whether it is an extant compartment to which single chromosomes travel, thus controlling var switching. Here we show that transcription of a var gene did not require decreased colocalisation with clusters of telomeres, supporting var expression site formation in situ. However following recombination within adjacent subtelomeric sequences, the same var gene was persistently activated and did colocalise less with telomeric clusters. Thus, participation in stable, heterochromatic, telomere clusters and var switching are independent but are both affected by subtelomeric sequences. The var expression site colocalised with the euchromatic mark H3K27ac to a greater extent than it did with heterochromatic H3K9me3. H3K27ac was enriched within the active var gene promoter even when the var gene was transiently repressed in mature parasites and thus H3K27ac may contribute to var gene epigenetic memory.

Sexual development in Plasmodium parasites: knowing when it's time to commit.

Malaria is a devastating infectious disease that is caused by blood-borne apicomplexan parasites of the genus Plasmodium. These pathogens have a complex lifecycle, which includes development in the anopheline mosquito vector and in the liver and red blood cells of mammalian hosts, a process which takes days to weeks, depending on the Plasmodium species. Productive transmission between the mammalian host and the mosquito requires transitioning between asexual and sexual forms of the parasite. Blood- stage parasites replicate cyclically and are mostly asexual, although a small fraction of these convert into male and female sexual forms (gametocytes) in each reproductive cycle. Despite many years of investigation, the molecular processes that elicit sexual differentiation have remained largely unknown. In this Review, we highlight several important recent discoveries that have identified epigenetic factors and specific transcriptional regulators of gametocyte commitment and development, providing crucial insights into this obligate cellular differentiation process.

A Plasmodium Falciparum Bromodomain Protein Regulates Invasion Gene Expression.

During red-blood-cell-stage infection of Plasmodium falciparum, the parasite undergoes repeated rounds of replication, egress, and invasion. Erythrocyte invasion involves specific interactions between host cell receptors and parasite ligands and coordinated expression of genes specific to this step of the life cycle. We show that a parasite-specific bromodomain protein, PfBDP1, binds to chromatin at transcriptional start sites of invasion-related genes and directly controls their expression. Conditional PfBDP1 knockdown causes a dramatic defect in parasite invasion and growth and results in transcriptional downregulation of multiple invasion-related genes at a time point critical for invasion. Conversely, PfBDP1 overexpression enhances expression of these same invasion-related genes. PfBDP1 binds to acetylated histone H3 and a second bromodomain protein, PfBDP2, suggesting a potential mechanism for gene recognition and control. Collectively, these findings show that PfBDP1 critically coordinates expression of invasion genes and indicate that targeting PfBDP1 could be an invaluable tool in malaria eradication.

Epigenetic regulation of the Plasmodium falciparum genome.

Recent research has highlighted some unique aspects of chromatin biology in the malaria parasite Plasmodium falciparum. During its erythrocytic lifecycle P. falciparum maintains its genome primarily as unstructured euchromatin. Indeed there is no clear role for chromatin-mediated silencing of the majority of the developmentally expressed genes in P. falciparum. However discontinuous stretches of heterochromatin are critical for variegated expression of contingency genes that mediate key pathogenic processes in malaria. These range from invasion of erythrocytes and antigenic variation to solute transport and growth adaptation in response to environmental changes. Despite lack of structure within euchromatin the nucleus maintains functional compartments that regulate expression of many genes at the nuclear periphery, particularly genes with clonally variant expression. The typical components of the chromatin regulatory machinery are present in P. falciparum; however, some of these appear to have evolved novel species-specific functions, e.g. the dynamic regulation of histone variants at virulence gene promoters. The parasite also appears to have repeatedly acquired chromatin regulatory proteins through lateral transfer from endosymbionts and from the host. P. falciparum chromatin regulators have been successfully targeted with multiple drugs in laboratory studies; hopefully their functional divergence from human counterparts will allow the development of parasite-specific inhibitors.

The role of chromatin in Plasmodium gene expression.

The malaria parasite Plasmodium falciparum dynamically regulates transcription of the majority of its genes during its intraerythrocytic developmental cycle. Chromatin is an important contributor to this tight regulation of gene expression. P. falciparum appears to utilize most of the mechanisms of chromatin creation and modification found in other eukaryotes, although it occasionally uses them in surprising ways. Much of the P. falciparum genome is maintained in a euchromatic state, potentially permissive for transcription and heterochromatin appears to have a specialized role limited to silencing islands of genes involved in redundant host-parasite interactions. P. falciparum histones share canonical modifications with other eukaryotes but also have unique modifications of unknown function including hyperacetylations of two alternative histones possibly involved in gene regulation. Much of our knowledge of chromatin regulation of gene expression in P. falciparum derives from the study of virulence genes that are subject to chromatin regulatory mechanisms ranging from histone modifications and nucleosomal occupancy to non-protein-coding RNAs and subnuclear architecture. These mechanisms will be discussed along with other characterized components of P. falciparum chromatin.

PfSET10, a Plasmodium falciparum methyltransferase, maintains the active var gene in a poised state during parasite division.

A major virulence factor of the malaria parasite Plasmodium falciparum is erythrocyte membrane protein 1 (PfEMP1), a variant protein expressed on the infected erythrocyte surface. PfEMP1 is responsible for adherence of infected erythrocytes to the endothelium and plays an important role in pathogenesis. Mutually exclusive transcription and switched expression of one of 60 var genes encoding PfEMP1 in each parasite genome provides a mechanism for antigenic variation. We report the identification of a parasite protein, designated PfSET10, which localizes exclusively to the perinuclear active var gene expression site. PfSET10 is a histone 3 lysine 4 methyltransferase required to maintain the active var gene in a poised state during division for reactivation in daughter parasites, and as such is required for P. falciparum antigenic variation. PfSET10 likely maintains the transcriptionally permissive chromatin environment of the active var promoter and thus retains memory for heritable transmission of epigenetic information during parasite division.

The role of bromodomain proteins in regulating gene expression.

Histone modifications are important in regulating gene expression in eukaryotes. Of the numerous histone modifications which have been identified, acetylation is one of the best characterised and is generally associated with active genes. Histone acetylation can directly affect chromatin structure by neutralising charges on the histone tail, and can also function as a binding site for proteins which can directly or indirectly regulate transcription. Bromodomains specifically bind to acetylated lysine residues on histone tails, and bromodomain proteins play an important role in anchoring the complexes of which they are a part to acetylated chromatin. Bromodomain proteins are involved in a diverse range of functions, such as acetylating histones, remodeling chromatin, and recruiting other factors necessary for transcription. These proteins thus play a critical role in the regulation of transcription.