Identification of a cis-acting DNA-protein interaction implicated in singular var gene choice in Plasmodium falciparum

The human malaria parasite Plasmodium falciparum can sense environmental changes and respond by antigenic switching

The primary antigenic and virulence determinant of the human malaria parasite Plasmodium falciparum is a variant surface protein called PfEMP1. Different forms of PfEMP1 are encoded by a multicopy gene family called var, and switching between active genes enables the parasites to evade the antibody response of their human hosts. var gene switching is key for the maintenance of chronic infections; however, what controls switching is unknown, although it has been suggested to occur at a constant frequency with little or no environmental influence. var gene transcription is controlled epigenetically through the activity of histone methyltransferases (HMTs). Studies in model systems have shown that metabolism and epigenetic control of gene expression are linked through the availability of intracellular S-adenosylmethionine (SAM), the principal methyl donor in biological methylation modifications, which can fluctuate based on nutrient availability. To determine whether environmental conditions and changes in metabolism can influence var gene expression, P. falciparum was cultured in media with altered concentrations of nutrients involved in SAM metabolism. We found that conditions that influence lipid metabolism induce var gene switching, indicating that parasites can respond to changes in their environment by altering var gene expression patterns. Genetic modifications that directly modified expression of the enzymes that control SAM levels similarly led to profound changes in var gene expression, confirming that changes in SAM availability modulate var gene switching. These observations directly challenge the paradigm that antigenic variation in P. falciparum follows an intrinsic, programed switching rate, which operates independently of any external stimuli.

Rapid activation of distinct members of multigene families in Plasmodium spp

The genomes of Plasmodium spp. encode a number of different multigene families that are thought to play a critical role for survival. However, with the exception of the P. falciparum var genes, very little is known about the biological roles of any of the other multigene families. Using the recently developed Selection Linked Integration method, we have been able to activate the expression of a single member of a multigene family of our choice in Plasmodium spp. from its endogenous promoter. We demonstrate the usefulness of this approach by activating the expression of a unique var, rifin and stevor in P. falciparum as well as yir in P. yoelii. Characterization of the selected parasites reveals differences between the different families in terms of mutual exclusive control, co-regulation, and host adaptation. Our results further support the application of the approach for the study of multigene families in Plasmodium and other organisms.

Omelianczyk, Loh et al. activate the expression of a single member of a multigene family in Plasmodium spp. from its endogenous promoter, identifying differences between the different families. This study supports the application of the Selection Linked Integration method for studying multigene families in Plasmodium.

The molecular machinery of translational control in malaria parasites

Translational control regulates the levels of protein synthesized from its transcript and is key for the rapid adjustment of gene expression in response to environmental stimuli. The regulation of translation is of special importance for malaria parasites, which pass through a complex life cycle that includes various replication phases in the different organs of the human and mosquito hosts and a sexual reproduction phase in the mosquito midgut. In particular, the quiescent transmission stages rely on translational control to rapidly adapt to the new environment, once they switch over from the human to the mosquito and vice versa. Three control mechanisms are currently proposed in Plasmodium, (1) global regulation that acts on the translation initiation complex; (2) mRNA-specific regulation, involving cis control elements, mRNA-binding proteins and translational repressors; and (3) induced mRNA decay by the Ccr4-Not and the RNA exosome complex. The main molecules controlling translation are highly conserved in malaria parasites and an increasing number of studies shed light on the interwoven pathways leading to the up or downregulation of protein synthesis in the diverse plasmodial stages. We here highlight recent findings on translational control during life cycle progression of Plasmodium and discuss the molecules involved in regulating protein synthesis.

Epigenetic Variation and Regulation in Malaria Parasites

Eukaryotic pathogens must survive in different hosts, respond to changing environments, and exploit specialized niches to propagate. Plasmodium parasites cause human malaria during bloodstream infections, where they must persist long enough to be transmitted. Parasites have evolved diverse strategies of variant gene expression that control critical biological processes of blood-stage infections, including antigenic variation, erythrocyte invasion, innate immune evasion, and nutrient acquisition, as well as life-cycle transitions. Epigenetic mechanisms within the parasite are being elucidated, with discovery of epigenomic marks associated with gene silencing and activation, and the identification of epigenetic regulators and chromatin proteins that are required for the switching and maintenance of gene expression. Here, we review the key epigenetic processes that facilitate transition through the parasite life cycle and epigenetic regulatory mechanisms utilized by Plasmodium parasites to survive changing environments and consider epigenetic switching in the context of the outcome of human infections.

Independent regulation of Plasmodium falciparum rif gene promoters

All Plasmodium species express variant antigens which may mediate immune escape in the vertebrate host. In Plasmodium falciparum, the rif gene family encodes variant antigens which are partly exposed on the infected red blood cell surface and may function as virulence factors. Not all rif genes are expressed at the same time and it is unclear what controls rif gene expression. In this work, we addressed global rif transcription using plasmid vectors with two drug resistance markers, one controlled by a rif 5′ upstream region and the second by a constitutively active promoter. After spontaneous integration into the genome of one construct, we observed that the resistance marker controlled by the rif 5′ upstream region was expressed dependent on the applied drug pressure. Then, the global transcription of rif genes in these transfectants was compared in the presence or absence of drugs. The relative transcript quantities of all rif loci did not change profoundly between strains grown with or without drug. We conclude that either there is no crosstalk between rif loci or that the elusive system of allelic exclusion of rif gene transcription is not controlled by their 5′ upstream region alone.

Chromatin Accessibility-Based Characterization of the Gene Regulatory Network Underlying Plasmodium falciparum Blood-Stage Development

Underlying the development of malaria parasites within erythrocytes and the resulting pathogenicity is a hardwired program that secures proper timing of gene transcription and production of functionally relevant proteins. How stage-specific gene expression is orchestrated in vivo remains unclear. Here, using the assay for transposase accessible chromatin sequencing (ATAC-seq), we identified ∼4,000 regulatory regions in P. falciparum intraerythrocytic stages. The vast majority of these sites are located within 2 kb upstream of transcribed genes and their chromatin accessibility pattern correlates positively with abundance of the respective mRNA transcript. Importantly, these regions are sufficient to drive stage-specific reporter gene expression and DNA motifs enriched in stage-specific sets of regulatory regions interact with members of the P. falciparum AP2 transcription factor family. Collectively, this study provides initial insights into the in vivo gene regulatory network of P. falciparum intraerythrocytic stages and should serve as a valuable resource for future studies.

Epigenetic Regulation of Virulence Gene Expression in Parasitic Protozoa

Protozoan parasites colonize numerous metazoan hosts and insect vectors through their life cycles, with the need to respond quickly and reversibly while encountering diverse and often hostile ecological niches. To succeed, parasites must also persist within individuals until transmission between hosts is achieved. Several parasitic protozoa cause a huge burden of disease in humans and livestock, and here we focus on the parasites that cause malaria and African trypanosomiasis. Efforts to understand how these pathogens adapt to survive in varied host environments, cause disease, and transmit between hosts have revealed a wealth of epigenetic phenomena. Epigenetic switching mechanisms appear to be ideally suited for the regulation of clonal antigenic variation underlying successful parasitism. We review the molecular players and complex mechanistic layers that mediate the epigenetic regulation of virulence gene expression. Understanding epigenetic processes will aid the development of antiparasitic therapeutics.

Knockdown of the Plasmodium falciparum SURFIN4.1 antigen leads to an increase of its cognate transcript

The genome of the malaria parasite Plasmodium falciparum contains the surf gene family which encodes large transmembrane proteins of unknown function. While some surf alleles appear to be expressed in sexual stages, others occur in asexual blood stage forms and may be associated to virulence-associated processes and undergo transcriptional switching. We accessed the transcription of surf genes along multiple invasions by real time PCR. Based on the observation of persistent expression of gene surf4.1, we created a parasite line which expresses a conditionally destabilized SURFIN4.1 protein. Upon destabilization of the protein, no interference of parasite growth or morphological changes were detected. However, we observed a strong increase in the transcript quantities of surf4.1 and sometimes of other surf genes in knocked-down parasites. While this effect was reversible when SURFIN4.1 was stabilized again after a few days of destabilization, longer destabilization periods resulted in a transcriptional switch away from surf4.1. When we tested if a longer transcript half-life was responsible for increased transcript detection in SURFIN4.1 knocked-down parasites, no alteration was found compared to control parasite lines. This suggests a specific feedback of the expressed SURFIN protein to its transcript pointing to a novel type of regulation, inedited in Plasmodium.

Variant Gene Expression and Antigenic Variation by Malaria Parasites

Malaria is a significant threat throughout the developing world. Among the most fascinating aspects of the protozoan parasites responsible for this disease are the methods they employ to avoid the immune system and perpetuate chronic infections. Key among these is antigenic variation: By systematically altering antigens that are displayed to the host's immune system, the parasite renders the adaptive immune response ineffective. For Plasmodium falciparum, the species responsible for the most severe form of human malaria, this process involves a complicated molecular mechanism that results in continuously changing patterns of variant-antigen-encoding gene expression. Although many features of this process remain obscure, significant progress has been made in recent years to decipher various molecular aspects of the regulatory cascade that causes chronic infection.

Malaria Epigenetics

Organisms with identical genome sequences can show substantial differences in their phenotypes owing to epigenetic changes that result in different use of their genes. Epigenetic regulation of gene expression plays a key role in the control of several fundamental processes in the biology of malaria parasites, including antigenic variation and sexual differentiation. Some of the histone modifications and chromatin-modifying enzymes that control the epigenetic states of malaria genes have been characterized, and their functions are beginning to be unraveled. The fundamental principles of epigenetic regulation of gene expression appear to be conserved between malaria parasites and model eukaryotes, but important peculiarities exist. Here, we review the current knowledge of malaria epigenetics and discuss how it can be exploited for the development of new molecular markers and new types of drugs that may contribute to malaria eradication efforts.

Recent advances in malaria genomics and epigenomics

Malaria continues to impose a significant disease burden on low- and middle-income countries in the tropics. However, revolutionary progress over the last 3 years in nucleic acid sequencing, reverse genetics, and post-genome analyses has generated step changes in our understanding of malaria parasite (Plasmodium spp.) biology and its interactions with its host and vector. Driven by the availability of vast amounts of genome sequence data from Plasmodium species strains, relevant human populations of different ethnicities, and mosquito vectors, researchers can consider any biological component of the malarial process in isolation or in the interactive setting that is infection. In particular, considerable progress has been made in the area of population genomics, with Plasmodium falciparum serving as a highly relevant model. Such studies have demonstrated that genome evolution under strong selective pressure can be detected. These data, combined with reverse genetics, have enabled the identification of the region of the P. falciparum genome that is under selective pressure and the confirmation of the functionality of the mutations in the kelch13 gene that accompany resistance to the major frontline antimalarial, artemisinin. Furthermore, the central role of epigenetic regulation of gene expression and antigenic variation and developmental fate in P. falciparum is becoming ever clearer. This review summarizes recent exciting discoveries that genome technologies have enabled in malaria research and highlights some of their applications to healthcare. The knowledge gained will help to develop surveillance approaches for the emergence or spread of drug resistance and to identify new targets for the development of antimalarial drugs and perhaps vaccines.

Trans-acting GC-rich non-coding RNA at var expression site modulates gene counting in malaria parasite

Monoallelic expression of the var multigene family enables immune evasion of the malaria parasite Plasmodium falciparum in its human host. At a given time only a single member of the 60-member var gene family is expressed at a discrete perinuclear region called the 'var expression site'. However, the mechanism of var gene counting remains ill-defined. We hypothesize that activation factors associating specifically with the expression site play a key role in this process. Here, we investigate the role of a GC-rich non-coding RNA (ncRNA) gene family composed of 15 highly homologous members. GC-rich genes are positioned adjacent to var genes in chromosome-central gene clusters but are absent near subtelomeric var genes. Fluorescence in situ hybridization demonstrates that GC-rich ncRNA localizes to the perinuclear expression site of central and subtelomeric var genes in trans. Importantly, overexpression of distinct GC-rich ncRNA members disrupts the gene counting process at the single cell level and results in activation of a specific subset of var genes in distinct clones. We identify the first trans-acting factor targeted to the elusive perinuclear var expression site and open up new avenues to investigate ncRNA function in antigenic variation of malaria and other protozoan pathogens.

Antigenic Variation in Plasmodium falciparum

Plasmodium falciparum is the protozoan parasite that causes most malaria-associated morbidity and mortality in humans with over 500,000 deaths annually. The disease symptoms are associated with repeated cycles of invasion and asexual multiplication inside red blood cells of the parasite. Partial, non-sterile immunity to P. falciparum malaria develops only after repeated infections and continuous exposure. The successful evasion of the human immune system relies on the large repertoire of antigenically diverse parasite proteins displayed on the red blood cell surface and on the merozoite membrane where they are exposed to the human immune system. Expression switching of these polymorphic proteins between asexual parasite generations provides an efficient mechanism to adapt to the changing environment in the host and to maintain chronic infection. This chapter discusses antigenic diversity and variation in the malaria parasite and our current understanding of the molecular mechanisms that direct the expression of these proteins.

Dual regulatory effects of non-coding GC-rich elements on the expression of virulence genes in malaria parasites

As the primary virulence factor of falciparum malaria, var genes harboring mutually exclusive expression pattern lead to antigenic variation and immune evasion of this pathogen in human host. Although various mechanisms contribute to silence of var genes, little is known of transcriptional activation pathways of a single var gene and maintenance of its active state with other silent var loci. Here, we report a monoallelic expression pattern of the non-coding GC-elements flanking chromosomal internal var genes, and transcript from the active one was required for activation of the var gene in the same array. Meanwhile, GFP reporter assays revealed a repressive effect on the adjacent gene induced by DNA motifs of the insulator-like GC-element, which was linked to heterochromatin subnuclear localization. Taken together, these data for the first time provide experimental evidence of the dual cis- and trans-acting regulatory functions of the GC-elements in both silence and activation of var genes, which would advance our understanding of the complex regulatory network of the virulence gene family in P. falciparum.

Deciphering the principles that govern mutually exclusive expression of Plasmodium falciparum clag3 genes

The product of the Plasmodium falciparum genes clag3.1 and clag3.2 plays a fundamental role in malaria parasite biology by determining solute transport into infected erythrocytes. Expression of the two clag3 genes is mutually exclusive, such that a single parasite expresses only one of the two genes at a time. Here we investigated the properties and mechanisms of clag3 mutual exclusion using transgenic parasite lines with extra copies of clag3 promoters located either in stable episomes or integrated in the parasite genome. We found that the additional clag3 promoters in these transgenic lines are silenced by default, but under strong selective pressure parasites with more than one clag3 promoter simultaneously active are observed, demonstrating that clag3 mutual exclusion is strongly favored but it is not strict. We show that silencing of clag3 genes is associated with the repressive histone mark H3K9me3 even in parasites with unusual clag3 expression patterns, and we provide direct evidence for heterochromatin spreading in P. falciparum. We also found that expression of a neighbor ncRNA correlates with clag3.1 expression. Altogether, our results reveal a scenario where fitness costs and non-deterministic molecular processes that favor mutual exclusion shape the expression patterns of this important gene family.

Heterochromatin protein 1 secures survival and transmission of malaria parasites

Clonally variant expression of surface antigens allows the malaria parasite Plasmodium falciparum to evade immune recognition during blood stage infection and secure malaria transmission. We demonstrate that heterochromatin protein 1 (HP1), an evolutionary conserved regulator of heritable gene silencing, controls expression of numerous P. falciparum virulence genes as well as differentiation into the sexual forms that transmit to mosquitoes. Conditional depletion of P. falciparum HP1 (PfHP1) prevents mitotic proliferation of blood stage parasites and disrupts mutually exclusive expression and antigenic variation of the major virulence factor PfEMP1. Additionally, PfHP1-dependent regulation of PfAP2-G, a transcription factor required for gametocyte conversion, controls the switch from asexual proliferation to sexual differentiation, providing insight into the epigenetic mechanisms underlying gametocyte commitment. These findings show that PfHP1 is centrally involved in clonally variant gene expression and sexual differentiation in P. falciparum and have major implications for developing antidisease and transmission-blocking interventions against malaria.

Transcription of the var genes from a freshly-obtained field isolate of Plasmodium falciparum shows more variable switching patterns than long laboratory-adapted isolates

Background

Antigenic variation in Plasmodium falciparum involves switching among multicopy var gene family and is responsible for immune evasion and the maintenance of chronic infections. Current understanding of var gene expression and switching patterns comes from experiments conducted on long laboratory-adapted strains, with little known about their wild counterparts.

Methods

Genome sequencing was used to obtain 50 var genes from a parasite isolated from the China-Myanmar border. Four clones with different dominant var genes were cultured in vitro in replicates for 50 generations. Transcription of the individual var gene was detected by real-time PCR and then the switching process was analysed.

Results

The expression of multicopy var genes is mutually exclusive in clones of a wild P. falciparum isolate. The activation of distinct primary dominant var genes leads to different and favoured switching patterns in the four clones. The on/off rates of individual var genes are variable and the choice of subsequent dominant var genes are random, which results in the different switching patterns among replicates of each clonal wild P. falciparum isolate with near identical initial transcription profiles.

Conclusions

This study suggests that the switching patterns of var genes are abundant, which consist of both conserved and random parts.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0565-y) contains supplementary material, which is available to authorized users.

A var gene upstream element controls protein synthesis at the level of translation initiation in Plasmodium falciparum

Clonally variant protein expression in the malaria parasite Plasmodium falciparum generates phenotypic variability and allows isogenic populations to adapt to environmental changes encountered during blood stage infection. The underlying regulatory mechanisms are best studied for the major virulence factor P. falciparum erythrocyte membrane protein 1 (PfEMP1). PfEMP1 is encoded by the multicopy var gene family and only a single variant is expressed in individual parasites, a concept known as mutual exclusion or singular gene choice. var gene activation occurs in situ and is achieved through the escape of one locus from epigenetic silencing. Singular gene choice is controlled at the level of transcription initiation and var 5' upstream (ups) sequences harbour regulatory information essential for mutually exclusive transcription as well as for the trans-generational inheritance of the var activity profile. An additional level of control has recently been identified for the var2csa gene, where an mRNA element in the 5' untranslated region (5' UTR) is involved in the reversible inhibition of translation of var2csa transcripts. Here, we extend the knowledge on post-transcriptional var gene regulation to the common upsC type. We identified a 5' UTR sequence that inhibits translation of upsC-derived mRNAs. Importantly, this 5' UTR element efficiently inhibits translation even in the context of a heterologous upstream region. Further, we found var 5' UTRs to be significantly enriched in uAUGs which are known to impair the efficiency of protein translation in other eukaryotes. Our findings suggest that regulation at the post-transcriptional level is a common feature in the control of PfEMP1 expression in P. falciparum.

Epigenetic memory takes center stage in the survival strategy of malaria parasites

Malaria parasites run through a complex life cycle in the vertebrate host and mosquito vector. This not only requires tightly controlled mechanisms to govern stage-specific gene expression but also necessitates effective strategies for survival under changing environmental conditions. In recent years, the combination of different -omics approaches and targeted functional studies highlighted that Plasmodium falciparum blood stage parasites use heterochromatin-based gene silencing as a unifying strategy for clonally variant expression of hundreds of genes. In this article, we describe the epigenetic control mechanisms that mediate alternative expression states of genes involved in antigenic variation, nutrient uptake and sexual conversion and discuss the relevance of this strategy for the survival and transmission of malaria parasites.

Functional analysis of the 5' untranslated region of the phosphoglutamase 2 transcript in Plasmodium falciparum

Plasmodium falciparum transcripts contain long untranslated regions (UTR), with some of the longest in any eukaryote that uses monocistronic transcription. Owing to the extreme AT nucleotide bias within the intergenic regions that encode these UTR, attempts to characterise how they are apportioned over genes and to describe their contribution to the absolute and temporal control of gene expression have been limited. Here we describe a study using a typical house-keeping gene that encodes phosphoglutamase 2 (PFD0660w), whose expression is subject to developmentally linked control during intraerythrocytic development. We show that deletion of a significant proportion (80%) of the predicted 5' UTR has no apparent effect on the developmentally linked expression of a luciferase reporter cassette. Further, serial deletions reveal that whilst the absolute level of transcription is unaffected when up to 50% of the predicted 5' UTR is removed, the subsequent efficiency of translation is affected. These data provide key insights into the interplay of transcriptional and post-transcriptional mechanisms in the control of gene expression in this important human pathogen.

Analysis of the spatial and temporal arrangement of transcripts over intergenic regions in the human malarial parasite Plasmodium falciparum

Background

The ability of the human malarial parasite Plasmodium falciparum to invade, colonise and multiply within diverse host environments, as well as to manifest its virulence within the human host, are activities tightly linked to the temporal and spatial control of gene expression. Yet, despite the wealth of high throughput transcriptomic data available for this organism there is very little information regarding the location of key transcriptional landmarks or their associated cis-acting regulatory elements. Here we provide a systematic exploration of the size and organisation of transcripts within intergenic regions to yield surrogate information regarding transcriptional landmarks, and to also explore the spatial and temporal organisation of transcripts over these poorly characterised genomic regions.

Results

Utilising the transcript data for a cohort of 105 genes we demonstrate that the untranscribed regions of mRNA are large and apportioned predominantly to the 5′ end of the open reading frame. Given the relatively compact size of the P. falciparum genome, we suggest that whilst transcriptional units are likely to spatially overlap, temporal co-transcription of adjacent transcriptional units is actually limited. Critically, the size of intergenic regions is directly dependent on the orientation of the two transcriptional units arrayed over them, an observation we extend to an analysis of the complete sequences of twelve additional organisms that share moderately compact genomes.

Conclusions

Our study provides a theoretical framework that extends our current understanding of the transcriptional landscape across the P. falciparum genome. Demonstration of a consensus gene-spacing rule that is shared between P. falciparum and ten other moderately compact genomes of apicomplexan parasites reveals the potential for our findings to have a wider impact across a phylum that contains many organisms important to human and veterinary health.

Silence, activate, poise and switch! Mechanisms of antigenic variation in Plasmodium falciparum

Phenotypic variation in genetically identical malaria parasites is an emerging topic. Although antigenic variation is only part of a more global parasite strategy to create adaptation through epigenetically controlled transcriptional variability, it is the central mechanism enabling immune evasion and promoting pathogenesis. The var gene family is the best-studied example in a wide range of clonally variant gene families in Plasmodium falciparum. It is unique in its strict selection of a single member for activation, a process termed monoallelic expression. The conceptual advances that have emerged from studying var genes show striking common epigenetic features with many other clonally variant gene families or even single-copy genes that show a variegated expression in parasite populations. However, major mechanistic questions, such as the existence of a potential expression site and the identity of transcription factors or genetic elements driving singular gene choice, are still unanswered. In this review we discuss the recent findings in the molecular processes essential for clonal variation, namely silencing, activation, poising and switching. Integrating findings about all clonally variant gene families and other mutually exclusive expression systems will hopefully drive mechanistic understanding of antigenic variation.

Insulator-like pairing elements regulate silencing and mutually exclusive expression in the malaria parasite Plasmodium falciparum

Plasmodium falciparum causes the deadliest form of human malaria. Its virulence is attributed to its ability to modify the infected RBC and to evade human immune attack through antigenic variation. Antigenic variation is achieved through tight regulation of antigenic switches between variable surface antigens named "P. falciparum erythrocyte membrane protein-1" encoded by the var multicopy gene family. Individual parasites express only a single var gene at a time, maintaining the remaining var genes in a transcriptionally silent state. Strict pairing between var gene promoters and a second promoter within an intron found in each var gene is required for silencing and counting of var genes by the mechanism that controls mutually exclusive expression. We have identified and characterized insulator-like DNA elements that are required for pairing var promoters and introns and thus are essential for regulating silencing and mutually exclusive expression. These elements, found in the regulatory regions of each var gene, are bound by distinct nuclear protein complexes. Any alteration in the specific, paired structure of these elements by either deletion or insertion of additional elements results in an unregulated var gene. We propose a model by which silencing and mutually exclusive expression of var genes is regulated by the precise arrangement of insulator-like DNA pairing elements.