Biology Reference
In-Depth Information
being assembled, they are prone to some drugs that depolymerize microtubules.
For example, treatment with oryzalin or colchicine at lower concentrations in
Toxoplasma does not interrupt the division process but the forming cells do not
present subpellicular microtubules and are not capable to infect new host cells
(Morrissette and Sibley
2002b
). Oryzalin belongs to a group of compounds that
selectively disrupt microtubules in several protozoa and in plants, with little effect
on host cell microtubules. Generation of oryzalin-resistant Toxoplasma lines
showed that the drug specifically targets a-tubulin (Morrissette et al.
2004
;
Ma et al.
2007
,
2010
). Also in Plasmodium erythrocytic stages several drugs were
shown to affect microtubules (Pouvelle et al.
1994
; Schrével et al.
1994
, Sinou
et al.
1996
,
1998
; Fowler et al.
1998
; Fennell et al.
2006
).
Most of the studies on the replication process performed so far focused on the
more prominent members of apicomplexa, Toxoplasma, and Plasmodium, where
molecular tools are available to study the cell division process, e.g. by gene
replacement. In others organisms, like Eimeria and Theileria, most of the studies
were limited to observation by light or transmission electron microscopy. Never-
theless, these studies show that the apicomplexa are a diverse group of organisms,
each one presenting an intriguing variation of unique cell biological processes.
Here, we summarize what is known about the division process of Toxoplasma,
Plasmodium, and Theileria without being exhaustive in the hope to encourage
more readers to work on these intriguing and understudied organisms.
19.2.1 Plasmodium
Malaria is an infectious disease caused by members of the genus Plasmodium and
transmitted by Anopheles mosquitoes. In developing countries it represents an
important health and economical problem. Despite declining disease incidence,
there are still approximately 225 million annual infections, resulting in nearly one
million deaths, mostly children (WHO report
2010
). The parasite undergoes a
complex life cycle between its mosquito vector and vertebrate host with hundreds
of species being transmitted by different species of Anopheles to vertebrates as
different as lizards, birds, and humans. The clinical symptoms of malaria in
humans are caused by the blood stages, when parasites infect and undergo asexual
replication in red blood cells.
Parasite transmission occurs when an infected female mosquito bites a host in
order to probe for blood (Fig.
19.3
a). Prior to sucking blood, the mosquito injects
saliva into the skin and with the saliva a small number of parasites. These start to
migrate immediately in the skin to ultimately enter the blood stream (Vanderberg
and Frevert
2004
; Amino et al.
2006
). Through the blood circulatory system the
parasites reach the liver, where they leave the blood stream to infect a hepatocyte
(Prudêncio et al.
2006
). Within the hepatocyte, the sporozoite forms a parasi-
tophorous vacuole (PV), where it transforms into a stage that undergoes a
sequential cell division process, resulting in the formation of thousands of progeny
