Cell cycle checkpoint activity in the malaria parasite Plasmodium falciparum

Plasmodium spp. have different modes of cell division from most eukaryotes. Little is known about how these are controlled, and cell cycle checkpoints are particularly poorly characterized. However, parasites can arrest their cell cycle when treated with the frontline antimalarial drug artemisinin, and artemisinin-resistant parasites can modulate their cell cycle progression, so it is important to understand these aspects of Plasmodium biology. Here, we show that P. falciparum displays hallmarks of an intra-S-phase checkpoint when exposed to DNA damage, including acute reduction of DNA replication and phosphorylation of a putative damage-marker histone. Compounds that inhibit human checkpoint kinases can inhibit this arrest of DNA replication and synergize with DNA damage in parasite killing. This suggests the existence of checkpoint kinase activity in P. falciparum, yet these kinases have no clear homologues in Plasmodium genomes. Their closest homologs are the phosphatidylinositol lipid kinases. We hypothesize that phosphatidylinositol 3-kinase-which is reportedly upregulated in artemisinin-resistant parasites-may moonlight in this role, and we characterize this essential kinase for the first time via expansion microscopy. Finally, we show that the cryptic checkpoint-kinase activity may also regulate the ring-stage survival phenotype after artemisinin damage, which resembles a G1/S checkpoint. Hence, we suggest that checkpoint kinase inhibitors are candidates for synergy with artemisinin.IMPORTANCEMalaria parasites infect red blood cells, wherein they replicate to produce many new parasites. This is unusual because most cells replicate simply by copying their genome and splitting in half (called binary fission), but malaria parasites make ~20 genome copies and then partition them simultaneously into 20 new cells (called schizogony). Here, we studied how schizogony is controlled: in particular, are there "checkpoints," i.e., pathways that can pause the cell cycle? We found that DNA damage did cause checkpoint hallmarks, yet the key proteins that enforce this in other cells are absent in malaria parasites. Furthermore, this checkpoint activity may be involved in the response to an antimalarial drug, in which parasites pause their cycle before active replication begins. This implies that inhibiting the checkpoint could exacerbate parasite killing by such drugs. Cancer therapies often work like this-by damaging DNA and also preventing the cancer cells from repairing it.