Biology Reference
In-Depth Information
the incredible divergence of these organisms with the associated wide open space for
fundamental discoveries and the fact that some of them are causing devastating disease
in humans and farm animals. The neglect is neatly illustrated by a PubMed search with
''microtubules and yeast'' retrieving 1871 articles, while only 38 hits were found for
''microtubules and malaria'' at the time of writing. Indeed, just the phylum Apicom-
plexa within the group of chromalveolates comprises an incredibly heterogeneous
group of intracellular parasites that includes species of relevance for humans due to
their health and/or economical effects (Tenter et al.
2002
). Some are: Plasmodium sp.,
the causative agents of malaria; Toxoplasma gondii, causing toxoplasmosis (a world-
wide distributed disease, especially important for pregnant women and immune-
compromised patients); Cystoisospora belli, causing isosporiasis and Cryptosporidi-
um that is responsible for cryptosporidiosis (the latter ones being intestinal parasites,
affecting especially immunodeficient people). Also, the livestock economy has to cope
with severe losses due to disease impact and prevention costs for some infectious
agents, like Eimeria (poultry), Babesia,andTheileria (cattle).
These protozoa present an intracellular organization that is superficially similar to
metazoan cells. They have a nucleus, an endoplasmic reticulum, a Golgi complex and
only one mitochondrion (Joiner and Roos
2002
). The group Apicomplexa is formed
by organisms that present a somewhat peculiar and exclusive structure-the apical
complex, composed of cytoskeletal components and secretory organelles (micro-
nemes and rhoptries) that are essential for invasion of the host cell (Morrissette and
Sibley
2002a
; Dubremetz
2007
; Carruthers and Tomley
2008
) and dense granules,
which modify the parasitophorous vacuole surrounding the intracellular stages of
these parasites (Cesbron-Delauw et al.
2008
). Curious here to note are the polar rings
and conoids that are present in some but not all apicomplexans (Fig.
19.1
a). The
conoid is formed by a unique open microtubule-like structure with the highest bend
observed so far for a tubulin ''tube'' (Hu et al.
2002
). This structure is important for
host cell invasion by Toxoplasma but curiously absent in parasites causing malaria.
The polar rings, appear to organize microtubules in a cage (Hu et al.
2002
). Only
recently, the first component of this structure, a protein with no homology to anything
outside the apicomplexans, and not even across all apicomplexans, was identified
(Tran et al.
2010
). In addition to the above features, apicomplexans contain a unique
organelle derived from a secondary endosymbiotic event, the apicoplast (Köhler
et al.
1997
; Ralph et al.
2004
). Curiously, this organelle was documented for decades
mainly by electron microscopy studies, but only in the late 1990s was it identified as a
unique organelle (Köhler et al.
1997
). The apicoplast is a non-photosynthetic plastid
responsible for the type II fatty acid synthesis, isoprenoids biosynthesis, and
carbohydrate metabolism (Gleeson
2000
; Mazumbar et al.
2006
; Fleige et al.
2007
).
Recently, Yeh and DeRisi (
2011
) demonstrated by adding isopentenyl pyrophos-
phate to parasites lacking the apicoplast that in blood stages of Plasmodium only the
isoprenoid biosynthesis pathway is essential. Intriguingly, Theileria lacks the fatty
acid biosynthesis enzymes, and Cryptosporidium may have lost the apicoplast along
its evolution (Goodman and McFadden
2007
), attesting the diversity of this group of
organisms. Apicomplexan parasites present other unique cytoplasmic inclusions
including enigmatic virus-like (Lemgruber and Lupetti
2011
).
