Atypical organization of the malaria parasite division machinery

Every year about 400 000 people, mainly children, die from malaria. The disease is caused by the proliferation of a small unicellular eukaryote of the Plasmodium spp. in human red blood cells. In extreme cases this can lead to the accumulation of half a kilo of parasite biomass. The cell division mechanisms underlying this rapid multiplication remain poorly studied but display some marked differences when compared to model organisms. Notably, the parasite delays cytokinesis and undergoes multiple rounds of asynchronous and closed nuclear division prior to daughter cell formation.

Centrosomes are key regulatory hubs of nuclear division, they coordinate assembly of the mitotic spindle apparatus, and can come in various shapes and forms. The centrosome of proliferative blood-stage parasites stands out by the absence of any clear structural features, such as centrioles or spindle pole bodies. Our knowledge about the organization of this divergent organelle has been limited to a few decades-old electron micrographs.

More detailed investigation of this small-scale structure in proliferative blood-stage parasites was hindered by the unavailability of state-of-the-art imaging technologies. This study showcases the adaptation of microtubule live-cell imaging, correlative light and electron microscopy, STED nanoscopy and a recent expansion microscopy protocol allowing 3D multicolor super-resolution imaging. Here, the HFSP Career Development Award was particularly conducive to enable a significant investment into technology development prior to harvesting the first results.

These approaches demonstrate a bipartite organization of the centrosome into a centrin-containing extranuclear compartment and uncovered a novel, chromatin-free, but protein-dense intranuclear compartment harboring the spindle microtubule nucleation sites. Electron tomography reveals the organization of an exceptionally small mitotic spindle, which seems detached from a nuclear membrane-bound complex. The resulting working model sets the cell biological framework to subsequently investigate assembly, composition, and duplication of the parasite centrosome. Thereby, we hope to reveal new targets for intervention within this divergent cell division machinery.

Three-dimensional organization of microtubules and centriolar plaques in a multinucleated malaria parasite using ultrastructure expansion microscopy. 3D-rendering of Plasmodium falciparum blood-stage schizont parasite labeled with anti-centrin (green), anti-tubulin (magenta), and stained with Hoechst (blue) after isotropic expansion by a factor of 4.5.