Single-cell quantitative bioimaging of P. berghei liver stage translation

IMPORTANCE

Plasmodium parasites cause malaria in humans. New multistage active antimalarial drugs are needed, and a promising class of drugs targets the core cellular process of translation, which has many potential molecular targets. During the obligate liver stage, Plasmodium parasites grow in metabolically active hepatocytes, making it challenging to study core cellular processes common to both host cells and parasites, as the signal from the host typically overwhelms that of the parasite. Here, we present and validate a flexible assay to quantify Plasmodium liver stage translation using a technique to fluorescently label the newly synthesized proteins of both host and parasite followed by computational separation of their respective nascent proteomes in confocal image sets. We use the assay to determine whether a test set of known compounds are direct or indirect liver stage translation inhibitors and show that the assay can also predict the mode of action for novel antimalarial compounds.

Atypical B cells consist of subsets with distinct functional profiles

Atypical B cells are a population of activated B cells that are commonly enriched in individuals with chronic immune activation but are also part of a normal immune response to infection or vaccination. To better define the role of atypical B cells in the human adaptive immune response, we performed single-cell sequencing of transcriptomes, cell surface markers, and B cell receptors in individuals with chronic exposure to the malaria parasite Plasmodium falciparum, a condition known to lead to accumulation of circulating atypical B cells. We identified three previously uncharacterized populations of atypical B cells with distinct transcriptional and functional profiles and observed marked differences among these three subsets in their ability to produce immunoglobulin G upon T-cell-dependent activation. Our findings help explain the conflicting observations in prior studies regarding the function of atypical B cells and highlight their different roles in the adaptive immune response in chronic inflammatory conditions.

Single-cell quantitative bioimaging of P. berghei liver stage translation

Plasmodium parasite resistance to existing antimalarial drugs poses a devastating threat to the lives of many who depend on their efficacy. New antimalarial drugs and novel drug targets are in critical need, along with novel assays to accelerate their identification. Given the essentiality of protein synthesis throughout the complex parasite lifecycle, translation inhibitors are a promising drug class, capable of targeting the disease-causing blood stage of infection, as well as the asymptomatic liver stage, a crucial target for prophylaxis. To identify compounds capable of inhibiting liver stage parasite translation, we developed an assay to visualize and quantify translation in the P. berghei-HepG2 infection model. After labeling infected monolayers with o-propargyl puromycin (OPP), a functionalized analog of puromycin permitting subsequent bioorthogonal addition of a fluorophore to each OPP-terminated nascent polypetide, we use automated confocal feedback microscopy followed by batch image segmentation and feature extraction to visualize and quantify the nascent proteome in individual P. berghei liver stage parasites and host cells simultaneously. After validation, we demonstrate specific, concentration-dependent liver stage translation inhibition by both parasite-selective and pan-eukaryotic active compounds, and further show that acute pre-treatment and competition modes of the OPP assay can distinguish between direct and indirect translation inhibitors. We identify a Malaria Box compound, MMV019266, as a direct translation inhibitor in P. berghei liver stages and confirm this potential mode of action in P. falciparum asexual blood stages.

Histone modification analysis reveals common regulators of gene expression in liver and blood stage merozoites of Plasmodium parasites

Gene expression in malaria parasites is subject to various layers of regulation, including histone post-translational modifications (PTMs). Gene regulatory mechanisms have been extensively studied during the main developmental stages of Plasmodium parasites inside erythrocytes, from the ring stage following invasion to the schizont stage leading up to egress. However, gene regulation in merozoites that mediate the transition from one host cell to the next is an understudied area of parasite biology. Here, we sought to characterize gene expression and the corresponding histone PTM landscape during this stage of the parasite lifecycle through RNA-seq and ChIP-seq on P. falciparum blood stage schizonts, merozoites, and rings, as well as P. berghei liver stage merozoites. In both hepatic and erythrocytic merozoites, we identified a subset of genes with a unique histone PTM profile characterized by a region of H3K4me3 depletion in their promoter. These genes were upregulated in hepatic and erythrocytic merozoites and rings, had roles in protein export, translation, and host cell remodeling, and shared a DNA motif. These results indicate that similar regulatory mechanisms may underlie merozoite formation in the liver and blood stages. We also observed that H3K4me2 was deposited in gene bodies of gene families encoding variant surface antigens in erythrocytic merozoites, which may facilitate switching of gene expression between different members of these families. Finally, H3K18me and H2K27me were uncoupled from gene expression and were enriched around the centromeres in erythrocytic schizonts and merozoites, suggesting potential roles in the maintenance of chromosomal organization during schizogony. Together, our results demonstrate that extensive changes in gene expression and histone landscape occur during the schizont-to-ring transition to facilitate productive erythrocyte infection. The dynamic remodeling of the transcriptional program in hepatic and erythrocytic merozoites makes this stage attractive as a target for novel anti-malarial drugs that may have activity against both the liver and blood stages.