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 developed S. neurona transfection system. With these transgenic parasites we have tested our model in living cells employing laser bleaching experiments.
Highlights
The phylum Apicomplexa represents a large and diverse group of protozoan parasites
Recent cell biological work on Apicomplexa has been focused on the cellular structures and molecules involved in their ability to invade mammalian cells, their peculiar cell cycle and mechanisms of cell division, and the presence of several unique organelles
After invasion of the host cell, the parasite develops over 2-3 days during which the nucleus seems to grow continuously until it fragments into multiple nuclei, which are packaged into daughter cells (Speer and Dubey, 1999; Speer and Dubey, 2001)
Summary
The phylum Apicomplexa represents a large and diverse group of protozoan parasites. Among these are the human pathogens that cause malaria, AIDS-related encephalitis (Toxoplasma) and severe enteritis (Cryptosporidium and Cyclospora). The phylum contains many parasites of substantial veterinary importance such as Theileria, Babesia, Eimeria and Sarcocystis. Aside from their obvious medical and economic importance protozoan parasites have long fascinated cell biologists as model organisms. Recent cell biological work on Apicomplexa has been focused on the cellular structures and molecules involved in their ability to invade mammalian cells, their peculiar cell cycle and mechanisms of cell division, and the presence of several unique organelles. One of these organelles, which has received particular attention, is the apicomplexan plastid or apicoplast
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