Plastid segregation and cell division in the apicomplexan parasiteSarcocystis neurona

Abstract

Apicomplexan parasites harbor a secondary plastid that is essential to their survival. Several metabolic pathways confined to this organelle have emerged as promising parasite-specific drug targets. The maintenance of the organelle and its genome is an equally valuable target. We have studied the replication and segregation of this important organelle using the parasite Sarcocystis neurona as a cell biological model. This model system makes it possible to differentiate and dissect organellar growth, fission and segregation over time, because of the parasite's peculiar mode of cell division. S. neurona undergoes five cycles of chromosomal replication without nuclear division, thus yielding a cell with a 32N nucleus. This nucleus undergoes a sixth replication cycle concurrent with nuclear division and cell budding to give rise to 64 haploid daughter cells. Interestingly, intranuclear spindles persist throughout the cell cycle, thereby providing a potential mechanism to organize chromosomes and organelles in an organism that undergoes dramatic changes in ploidy. The development of the plastid mirrors that of the nucleus, a continuous organelle, which grows throughout the parasite's development and shows association with all centrosomes. Pharmacological ablation of the parasite's multiple spindles demonstrates their essential role in the organization and faithful segregation of the plastid. By using several molecular markers we have timed organelle fission to the last replication cycle and tied it to daughter cell budding. Finally, plastids were labeled by fluorescent protein expression using a newly developedS. neurona transfection system. With these transgenic parasites we have tested our model in living cells employing laser bleaching experiments.

Toxoplasma gondii sequesters centromeres to a specific nuclear region throughout the cell cycle

“…6 D and E) in which the centrocone, a unique subcompartment of the nucleus, serves as a master organizer of chromosome location not only during mitosis but throughout the intracellular development of the parasite. Persistence of kinetochores and spindle microtubules could provide the mechanism for this association through a constant spindle (2,8). Alternatively, centromeric heterochromatin could be anchored through protein components of the nuclear membranes (40,41).…”

“…An important unsolved question is how apicomplexan daughter cells emerge with the complete set of chromosomes (∼10 depending on species) after passing through multinucleated and polyploid stages. In Sarcocystis neurona the mitotic spindle persists throughout the cell cycle, and small monopolar spindles housed in a specialized elaboration of the nuclear envelope, the centrocone, are evident throughout interphase (8). More recent work in T. gondii identified a molecular marker for the centrocone, the repeat protein membrane occupation and recognition nexus protein 1 (MORN1) (9, 10) that is found in a variety of apicomplexan species representing all three cell-division modes (11).…”

GFP‐targeting allows visualization of the apicoplast throughout the life cycle of live malaria parasites

“…Sarcocystis parasites, which share the feature of having nuclear replication separated from cytokinesis, differ, however, in their mechanism of apicoplast replication. Rather than forming a branched apicoplast, Sarcocystis parasites have a single elongated apicoplast with no branches, which stretches around the large polyploid nucleus that forms following five rounds of nuclear replication without nuclear division (reviewed in Vaishnava et al, 2005). It will be interesting to find out how the Sarcocystis apicoplast is divided, as in some regards the relationship between the apicoplast and nuclei of the Sarcocystis parasite resembles the relationship seen between these organelles in a single 'lobe' of the cytomere stage of the exo-erythrocytic Plasmodium parasite (shown in the model in Figure 6C).…”

“…It could be predicted that such formation of constrictions occurs prior to apicoplast concertinaing, with folds in the apicoplast possibly occurring at constrictions due to them being points of weakness in the structure. However, Ftsz proteins appear to be absent from apicomplexa, as are homologues of other proteins involved in downstream events of plastid fission following constriction formation, as discussed in detail by Vaishnava et al (2005). In mammalian cells, fission of the mitochondrion (reviewed in Westermann, 2008) does not involve Ftsz.…”

Organellar dynamics during the cell cycle of Toxoplasma gondii

“…Fluorescent markers greatly facilitate the study of organellar dynamics in living parasites Striepen et al, 2000;He et al, 2001;Hu et al, 2002;Pelletier et al, 2002;Dzierszinski et al, 2004;Vaishnava et al, 2005;van Dooren et al, 2005;Hartmann et al, 2006;Hu et al, 2006). For example, the parasite Golgi has been shown to elongate laterally before dividing and segregating between the two daughter cells (Pelletier et al, 2002).…”