Apicoplast division

To ensure perpetuation of their genomes, mitochondria and plastids must be present at all times, even if inactive. That means that mitochondria and plastids must divide and be partitioned during each cell cycle to provide organelles for the daughter cells. In unicellular eukaryotes, the events of organelle division tend to be coordinated with the cell cycle. This coordination is especially important since only a single mitochondrion and apicoplast are present in apicomplexaus, as opposed to the multiple chloroplasts and mitochondria found in each cell of multicellular eukaryotes.

The absence of a method to synchronize T. gondii complicates some aspects of analysis of coordination of events with the cell cycle. For example, it is unclear whether replication of plastid DNA is coordinated with nuclear DNA replication in T. gondii, as it is in P. falciparum (Shaw et al., 2001; Williamson et al., 2002). However, analysis of events in single T. gondii cells has produced insights into organelle division. The use of fluorescent reporters targeted to different cellular structures has expanded the tools available for cell-cycle analysis, permitting experiments to follow the partitioning of the apicoplast into daughter cells.

Division of the apicoplast is coordinated with the cell cycle. In an elegant study of apicoplast division in T. gondii (Striepen et al., 2000), a number of important observations were made (Figure 9.5). In apicomplexans, the centrosome is extranuclear and located close to the apicoplast, which is itself apical to the nucleus. During daughter cell formation, as the duplicated centrosomes move apart, the ends of the apicoplast follow. Apicoplast DNA is localized in a nucleoid and, as the organelle lengthens into a dumbbell and then a U-shape, two nucleoids can be seen, one at each end and each adjacent to a centrosome. Coordinately, the parasite nucleus is repositioned to the basal end of the cell and develops two arms that move toward the centrosomes in the forming daughter cells. Organelle division in many other organisms involves a division ring that constricts around the middle of an elongated organelle until it splits in two. However, genes for the responsible proteins, such as ftsZ, have not been detected in apicom-plexan genomes. Striepen et al. (2000) instead have posed a controversial hypothesis that centro-some movement elongates the apicoplast and that the newly forming inner membrane complex is responsible for the division.

Matsuzaki et al. (2001) present a very different picture. This group analyzed plastid division employing DAPI staining to follow DNA and transmission electron microscopy to assess structural changes during apicoplast division. They describe

FIGURE 9.5 Apicoplast division during endodyo-geny. A schematic depiction of endodyogeny is shown.

(A) Cell components are labeled: A, apicoplast; C, centrosome; Co, conoid; IMC, inner membrane complex; N, nucleus; Nu, nucleoid.

(B) Here, the nucleoid and centrosome have both been duplicated and the apicoplast lengthens as the centrosomes and nucleoids move outward.

(C) New conoids have formed and new IMC is developing. The nucleus and apicoplast are beginning to move into the forming daughter cells, both becoming U-shaped. The relative positions of the centrosomes and apicoplast have reversed.

(D) Daughter-cell formation is nearing completion. The organelles will return to the positions shown in (A) as cytokinesis is completed.

Steps are as described in Striepen et al. (2000).

similar changes to apicoplast shape during division as well as other features: thickening at the ends of the elongating apicoplast, a 'scratched' appearance at its central constriction, and a dark-stained structure associated with the constriction. They hypothesize that these may be, respectively, sites for attachments of centriole microtubules, a plastid division ring, and a structure involved in organelle scission. There are clearly many questions remaining to be answered before the mechanism of apicoplast division is resolved.

Apicoplast structure has also been examined in bradyzoites and in sexual-stage T. gondii (Dzierszinski et al., 2004; Ferguson et al., 2005). Tachyzoites are actively replicating stages that are principally responsible for the pathology of toxoplasmosis. Bradyzoites are encysted parasites that are only slowly replicative, but they can switch to the tachyzoite stage. It is bradyzoites that linger in the host and are responsible for recurrence of toxoplasmosis in previously infected individuals. Tachyzoites can be induced to switch to brady-zoites in culture; these share many characteristics with bradyzoites isolated from animals. Using fluorescent reporters localized to the apicoplast, Dzierszinski et al. (2004) found that 10-20 percent of in vitro-induced bradyzoites apparently lack plastids. Both mis-segregation and loss of signal without cell division were observed. In contrast, fluorescent tags showed no loss of mitochondria. Consistent with these observations, the authors noted parts of a mature in vivo bradyzoite cyst did not stain with antibodies to the apicoplast protein ACE Complementing this observation are data from a serendipitous mis-segregating apicoplast mutant in tachyzoites (He et al., 2001a). Analysis of its phenotype demonstrated that T. gondii without apicoplasts are viable as long as they are within the original vacuole. They are, however, unable to replicate in a new vacuole. Thus, any bradyzoites lacking an apicoplast would not be able to initiate a productive infection upon reactivation.

Ferguson et al. (2005) performed a comprehensive examination of the apicoplast in vivo, including bradyzoites, and the asexual and sexual forms in cat small intestinal villi. Electron micrographs and immunofluorescence assays were used to examine each stage. For bradyzoites, they report that there is an apicoplast adjacent to each nucleus in mature cysts, in contrast to the findings of Dzierszinski et al. (2004). Ferguson et al. (2005) attribute this discrepancy to possible differences between in vivo and in vitro cysts. The life-cycle stages of T. gondii that occur in the feline host include the coccidean phase, which has some similarities to the asexual erythrocytic cycle of Plasmodium. When resident in villi of the cat small intestine, the plastid has the expected small ovoid shape in trophozoites, but in schizonts it has elongated and developed branches. This is quite similar to changes observed in P. falciparum apicoplast morphology in the corresponding stages (Waller et al., 2000). The plastid in T.gondii microgametocytes, macrogametocytes, and macrogametes appears condensed and almost globular. In contrast, micro-gametes lack apicoplasts (Ferguson et al., 2005). This finding implies that the apicoplast is maternally inherited in T. gondii, as it is in P. falciparum (Vaidya et al., 1993; Creasey et al., 1994).

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

Get My Free Ebook

Post a comment