Out of one, many: Regulation of cytokinesis in Plasmodium falciparum, the human malaria parasite

Abstract

Plasmodium parasites cause malaria, a devastating disease that, despite significant public health interventions, continues to cause approximately 200 million cases and 400,000 deaths each year. T¬¬he symptoms of malaria are driven by parasite growth and exponential expansion within human red blood cells (RBCs). Importantly, during this stage Plasmodium parasites produce 20-36 daughter cells per 48-hour round of replication, vastly increasing the burden of infection each cycle. There is a dearth of knowledge on how this unique process of cell division is governed. Here, we investigate protein and ultrastructural features important for Plasmodium cytokinesis. First, we describe a novel member of the Plasmodium falciparum contractile ring machinery, PfCINCH. To produce dozens of daughter cells, or merozoites, per replication cycle, the parasite first undergoes several rounds of nuclear replication and organellar expansion without cytokinesis. Then, in a final round of budding, the parasite drives membranes inward to partition individual merozoites and their cognate organelles. In Plasmodium, this process was hypothesized to require a specialized contractile ring, the basal complex, but this had not been functionally evaluated. Here, we identify a conserved Plasmodium protein of previously unknown function, PF3D7_0407800, which we named PfCINCH, as a novel member of the basal complex and show that PfCINCH is required for daughter cell individualization. Additionally, we identify a suite of novel basal complex members through PfCINCH immunoprecipitation. This work provides a foundation from which the mechanistic drivers of basal complex formation can be explored. Next, we examine cytokinesis in Plasmodium falciparum using the volume electron microscopy technique FIB-SEM. FIB-SEM allows visualization of parasite ultrastructure with lateral and axial resolution in the single-digit nanoscale. Previously, it was hypothesized that the final round of nuclear division in schizogony occurs synchronously and simultaneously with cytokinesis. Here, we show that this round occurs asynchronously – nearly all cells at mid-division contain both 2n and 1n nuclei. Furthermore, we provide the most detailed evidence yet that daughter cells are produced in multiples of two and that the parasite can produce 20-36 daughters per mother. Taken together, this work provides a more detailed understanding of the unique process of Plasmodium cell division.