ABSTRACT: Purpose: this study is to analyze the chromatin landscape of H3R2mes in Plasmodium falciparum. Methods: In this study, H3R2me2s enrichment in the chromatin of wildtype parasite at schizont stage was analyzed by Cleavage Under Targets and Tagmentation (CUT&Tag) coupled with next generation sequencing (CUT&Tag-seq). Synchronized WT 3D7 at 40–46 hpi schizonts were harvested and fixed with 1% formaldehyde for 1 min at room temperature, followed by lysis with 0.06% saponin. The parasite pellets were suspended in 100 μl nuclear extraction buffer (20 mM HEPES–KOH pH 7.9, 10 mM KCl, 0.1% Triton X-100, 20% Glycerol, 0.5 mM Spermidine, 1x EDTA-free Protease Inhibitor cocktail). Ten μl of activated Concanavalin A-coated magnetic beads (EpiCypher # 21-1401) were added to each sample (~0.5 × 106 nuclei) and incubated for 10 min. The bead-bound nuclei were resuspended in 50 μl Antibody150 buffer (20 mM HEPES, pH 7.5, 150 mM NaCl, 0.5 mM spermidine, 1× protease inhibitor cocktail, 0.01% digitonin, 2 mM EDTA) containing 0.5 μg of rabbit anti-H3R2me2s (Epigentek #A-3705-050) as the primary antibody or rabbit IgG (Cell Signaling Technology #2729S) serving as a control. The primary antibody was then removed, and nuclei were then incubated with 0.5 μg of the secondary antibodies in 50 μl of Digitonin150 buffer (Antibody150, no EDTA) at room temperature for 30 min. The secondary antibodies were mouse anti-rabbit IgG (Invitrogen #31213). The nuclei were washed thrice with 200 μl Digitonin150 Buffer for 5 min to remove unbound antibodies and finally resuspended in 50 μl Digitonin300 Buffer (Digitonin150 but with 300 mM NaCl). Then, 2.5 μl of CUTANA pAG-Tn5 (EpiCypher #15-1017) were added to each reaction at room temperature for 1 h. The nuclei were washed thrice for 5 min in 200 μl Digitonin300 Buffer to remove the unbound pA-Tn5. Next, the nuclei were resuspended in 50 μl Tagmentation buffer (10 mM MgCl2 in Digitonin300) and incubated at 37 °C for 1 h. The beads were washed with 50 µl TAPS buffer (10 mM TAPS, pH 8.5, 0.2 mM EDTA), mixed with 5 µL SDS release buffer (10 mM TAPS pH 8.5; 0.1% SDS) to quench tagmentation, and then incubated for 1 hr at 58ºC to release tagmented chromatin into solution. Finally, 15 µl of 0.67% Triton-X was added to each reaction to quench SDS. PCR with appropriate barcoded primers was performed using the CUTANA High Fidelity 2x PCR Master Mix (EpiCypher #15-1018) according to the manufacturer's recommendations. Amplified DNA libraries were captured by incubating with 1.3 × Kapa pure beads (Roche #KK8001) according to the manufacturer's recommendations, and next-generation sequencing was performed on the NextSeq 550 platform. The reads of CUT&Tag-seq from three biological replicates were mapped to the P. falciparum genome (PlasmoDB Release 39.0) by using BWA (Li and Durbin 2009) after FastQC quality control. The peak calling was finished by MACS2 version 2.2.7.1 with parameters ‘callpeak -f BAMPE’ and a q-value cutoff of 0.05. Results: The three replicates of CUT&Tag-seq using nuclei from the schizont stage showed high reproducibility, with correlation coefficients of ~0.8. Peak calling using criteria of presence in at least two of three replicates with at least 50% overlap of the peaks identified 1629 H3R2me2s signals, distributed among the 14 chromosomes. Among these peaks, 57% (927) are localized at 5’ UTR regions of 791 genes, 35% (577) at coding regions of 470 genes, and 8% (125) at 3’ UTR regions of 115 genes. The peaks are ~17.44 bp wide and ~1265 bp from the ATG sites. Comparing the transcriptome data showed genes with H3R2me2s enrichment in the 5’ UTRs showed significantly higher expression than other genes in the genome, suggesting that H3R2me2s is associated with gene activation. In addition, H3R2me2s peaks were highly co-localized with the active mark H3K9Ac, H2A.Z, and H3K4me3, further implying H3R2me2s as a euchromatin mark. Gene ontology (GO) enrichment analysis revealed a multitude of cellular pathways, including merozoite invasion, transcription, translation, stress response, cell cycle, schizogony, cell adhesion, exit from RBC, vesicle transport, signal transduction, DNA repair, and telomere organization, which are potentially regulated by this euchromatic mark. Conclusions: Collectively, H3R2me2s is a active chromatin mark and regulates invasion and other important processes in human malaria parasite.
