Organellar dynamics during the cell cycle of <i>Toxoplasma gondii</i>

TLDR

Apicomplexa comprises ~5000 obligate intracellular parasites, such as malaria and toxoplasmosis agents, that replicate not by binary fission but by assembling daughter cells de novo within the cytoplasm. The study investigates how Toxoplasma gondii tachyzoites package a complete set of organelles during replication to preserve the polarized organization required for host cell invasion. Time‑lapse fluorescence microscopy of organelle‑specific markers in tachyzoites was employed to track organelle dynamics during cell division. Golgi division and apicoplast elongation occur first, followed by cytoskeletal scaffold growth that sequentially partitions the Golgi, apicoplast, nucleus, ER, and mitochondrion—entering only late—while micronemes and rhoptries form de novo, revealing that organelle segregation occupies ~75 % of the cycle and encompasses S phase, suggesting atypical cell‑cycle control.

Abstract

The protozoan phylum Apicomplexa encompasses ∼5000 species of obligate intracellular parasites, including those responsible for malaria and toxoplasmosis. Rather than dividing by binary fission, apicomplexans use a remarkable mechanism for replication, assembling daughters de novo within the cytoplasm. Here, we exploit time-lapse microscopy of fluorescent markers targeted to various subcellular structures in Toxoplasma gondii tachyzoites to determine how these unicellular eukaryotes efficiently package a complete set of organelles, maintaining the highly polarized organization necessary for host cell invasion and pathogenesis. Golgi division and elongation of the apicoplast are among the first morphologically observable events, associated with an unusual pattern of centriolar migration. Daughter parasites are assembled on cytoskeletal scaffolding, whose growth proceeds from the apical end, first encapsulating the divided Golgi. Further extension of the cytoskeletal scaffold results in partitioning of the apicoplast, nucleus, endoplasmic reticulum, and finally the mitochondrion, which enters the developing daughters rapidly, but only very late during the division cycle. The specialized secretory organelles (micronemes and rhoptries) form de novo. This distinctive pattern of replication – in which organellar segregation spans ∼75% of the cell cycle, completely encompassing S phase – suggests an unusual mechanism of cell cycle regulation.