WAYS OF FORMATION AND DIVERSITY OF PARASITOPHOROUS VACUOLES IN PARASITIC PROTOZOA.
THE COCCIDIA (SPOROZOA, APICOMOPLEXA)
T. V. Beyer, N. V. Svezhova, A. I. Radchenko, N. V. Sidorenko
Institute of Cytology RAS, St. Petersburg;
e-mail: beyer@cit.ras.spb.ru
Data on parasitophorous vacuole (PV) formation in host cells (HC) harbouring different intracellular protozoan parasites have been reviewed and critically
analysed, with special reference to the main representatives of the Coccidia. The vacuole membrane (PVM) is the interface between host and parasite, playing
a role in nutrient acquisition by the parasite from the HC. The PV phenomenon is regarded as a generalized HC response to the introduction of alien bodies
(microorganisms), which eventually reflects the evolutionary established host-parasite relationships at cellular, subcellular and molecular levels. Special attention
has been paid to the existing morpho-functional diversity of the PVs within the same genera and species of parasites, and even at different stages of the parasite
life cycle. The PVM is generally considered to derive from the HC plasmalemma, whose biochemical composition undergoes significant changes as the intravacuolar
parasite grows. The original HC proteins are selectively excluded from the PVM, while those of the parasite are incorporated. As the result, the changed PVM
becomes not fusigenic for HC lysosomes. For Toxoplasma gondii and other cyst-forming coccidia (Isospora, Sarcocystis), a definite correlation
has been noticed between the extent of rhoptry and dense granule secrets released by a zoite during HC internalization, on the one hand, and the pattern of the PV
that forms, on the other one. In T. gondii, tachyzoites, known to discharge abundant secrets, commonly force the development of PVs limited with a single
unit membrane and equipped with a tubulovesicular network in the lumen. Unlike, bradyzoites known to be deficient in secretory materials trigger the formation of
PVs with a three-membrane lining composed of the changed invaginated plasmalemma in addition to two membranes of endoplasmic reticulum. The two different
types of PV harbour, respectively, exoenteric and enteric stages of T. gondii, the latter being confined to the cat intestine only. Unlike, all endogenous stages
of the classic intestinal coccidia (Eimeria spp.) develop within PVs limited with a single membrane, with some invaginations extending into the PV lumen.
Unusual PV patterns are characteristic of the extracytoplasmic eimerian coccidia (Cryptosporidium, Epieimeria) and adeleid haemogreagarines
(Karyolysus). In cyst-forming coccidia, the PVM is actively involved in tissue cyst wall formation, thus protecting the encysted parasites from recognition
by the host immune system. All this strongly suggests that the PV is far from being an indifferent membraneous vesicle containing a parasite, but represents a
metabolically active compartment in infected cells. Since all the coccidia are obligate intracellular parasites, the mode of their intimate interaction with the HC,
largely accomplished via the PV and its membrane, is vital for their survival as biological species.
Key words: parasitophorous vacuole, intracellular protozoan parasites, Toxoplasma gondii, Sarcocystis,
Eimeria, Cryptosporidium , Karyolysus
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