MINIREVIEW
Malaria ) an overview
Renu Tuteja
Malaria Group, International Centre for Genetic Engineering and Biotechnology, New Delhi, India
Keywords
cerebral malaria; erythrocytes; malaria life
cycle; malaria parasite; mosquito; parasite
genome; parasite transcriptome;
pathogenesis; Plasmodium falciparum; red
blood cells
Correspondence
R. Tuteja, Malaria Group, International
Centre for Genetic Engineering and
Biotechnology, PO Box 10504, Aruna Asaf
Ali Marg, New Delhi 110067, India
Fax: +91 11 26742316
Tel: +91 11 26741358
E-mail: renu@icgeb.res.in
Malaria is caused by protozoan parasites of the genus Plasmodium and is a
major cause of mortality and morbidity worldwide. These parasites have a
complex life cycle in their mosquito vector and vertebrate hosts. The primary factors contributing to the resurgence of malaria are the appearance
of drug-resistant strains of the parasite, the spread of insecticide-resistant
strains of the mosquito and the lack of licensed malaria vaccines of proven
efficacy. This minireview includes a summary of the disease, the life cycle
of the parasite, information relating to the genome and proteome of the
species lethal to humans, Plasmodium falciparum, together with other recent
developments in the field.
(Received 30 April 2007, revised 26 June
2007, accepted 19 July 2007)
doi:10.1111/j.1742-4658.2007.05997.x
The term malaria is derived from the Italian ‘mal’aria’,
which means ‘bad air’, from the early association of
the disease with marshy areas. Towards the end of the
19th century, Charles Louis Alphonse Laveran, a
French army surgeon, noticed parasites in the blood of
a patient suffering from malaria, and Dr Ronald Ross,
a British medical officer in Hyderabad, India, discovered that mosquitoes transmitted malaria. The Italian
professor Giovanni Battista Grassi subsequently
showed that human malaria could only be transmitted
by Anopheles mosquitoes. Malaria affects a large number of countries and it has been reported that the incidence of the disease in 2004 was between 350 and
500 million cases. Over two billion people, representing
more than 40% of the world’s population, are at risk
of contracting malaria, and the number of malaria
deaths worldwide has been estimated at 1.1–1.3 million
per annum in World Health Organization (WHO)
reports 1999–2004. Malaria has a broad distribution in
both the subtropics and tropics, with many areas of
the tropics endemic for the disease. The countries of
sub-Saharan Africa account for the majority of all
malaria cases, with the remainder mostly clustered in
India, Brazil, Afghanistan, Sri Lanka, Thailand, Indonesia, Vietnam, Cambodia, and China [1,2]. Malaria is
estimated to cost Africa more than $12 billion annually and it accounts for about 25% of all deaths in
children under the age of five on that continent [3]. In
many temperate areas, such as western Europe and the
USA, public health measures and economic development have been successful in achieving near- or
complete elimination of the disease, other than cases
imported via international travel.
The parasites
Malaria is transmitted through the bite of an infected
female Anopheles mosquito. Of the approximately
Abbreviations
CSA, chondroitin sulfate A; IDC, intraerythrocytic developmental cycle; PfEMP1, Plasmodium falciparum erythrocyte membrane protein 1;
RBC, red blood cell.
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Malaria ) an overview
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400 species of Anopheles throughout the world, about
60 are malaria vectors under natural conditions, 30 of
which are of major importance. Malaria parasites are
eukaryotic single-celled microorganisms that belong to
the genus Plasmodium. More than 100 species of Plasmodium can infect numerous animal species such as
reptiles, birds and various mammals, but only four
species of parasite can infect humans under natural
conditions: Plasmodium falciparum, Plasmodium vivax,
Plasmodium ovale and Plasmodium malariae. These
four species differ morphologically, immunologically,
in their geographical distribution, in their relapse patterns and in their drug responses. P. falciparum is the
agent of severe, potentially fatal malaria and is the
principal cause of malaria deaths in young children in
Africa [3]. The least common malaria parasite is
P. ovale, which is restricted to West Africa, while
P. malariae is found worldwide, but also with relatively low frequency. The most widespread malaria
parasite is P. vivax but infections with this species are
rarely fatal. Although P. falciparum and P. vivax can
both cause severe blood loss (anemia), mild anemia is
more common in P. vivax infections, whereas severe
anemia in P. falciparum malaria is a major killer in
Africa. In addition, in the case of P. falciparum, the
infected erythrocytes can obstruct small blood vessels
and if this occurs in the brain, cerebral malaria results,
a complication that is often fatal, particularly in African infants. P. ovale and P. vivax have dormant liver
stages named hypnozoites that may remain in this
organ for weeks to many years before the onset of a
new round of pre-erythrocytic schizogony, resulting in
relapses of malaria infection. In some cases P. malariae
can produce long-lasting blood-stage infections, which,
if left untreated, can persist asymptomatically in the
human host for periods extending into several decades.
Life cycle of malaria parasites
The life cycle of malaria parasites is extremely complex
and requires specialized protein expression for survival
in both the invertebrate and vertebrate hosts. These
proteins are required for both intracellular and extracellular survival, for the invasion of a variety of cell types
and for the evasion of host immune responses. Once
injected into the human host, P. falciparum and P. malariae sporozoites trigger immediate schizogony, whereas
P. ovale and P. vivax sporozoites may either trigger
immediate schizogony or lead to delayed schizogony as
they pass through the hypnozoite stage mentioned
above. The life cycle of the malaria parasite is shown in
Fig. 1A and can be divided into several stages, starting
with sporozoite entry into the bloodstream.
Tissue schizogony (pre-erythrocytic schizogony)
Infective sporozoites from the salivary gland of the
Anopheles mosquito are injected into the human host
along with anticoagulant-containing saliva to ensure
an even-flowing blood meal. It was thought that sporozoites move rapidly away from the site of injection,
but a recent study using a rodent parasite species
(Plasmodium yoelii) as a model system indicates that,
at least in this case, the majority of infective sporozoites remain at the injection site for hours, with only
slow release into the circulation [4]. Once in the human
bloodstream, P. falciparum sporozoites reach the liver
and penetrate the liver cells (hepatocytes) where they
remain for 9–16 days and undergo asexual replication
known as exo-erythrocytic schizogony. The mechanism
of targeting and invading the hepatocytes is not yet
well understood, but studies have shown that sporozoite migration through several hepatocytes in the mammalian host is essential for completion of the life cycle
[5]. The receptors on sporozoites responsible for hepatocyte invasion are mainly the thrombospondin domains
on the circumsporozoite protein and on thrombospondin-related adhesive protein. These domains specifically
bind to heparan sulfate proteoglycans on the hepatocytes [6]. Each sporozoite gives rise to tens of
thousands of merozoites inside the hepatocyte and
each merozoite can invade a red blood cell (RBC) on
release from the liver. In an interesting study, also
using rodent malaria parasites (Plasmodium berghei), it
has been shown that liver-stage parasites manipulate
their host cells to guarantee the safe delivery of merozoites into the bloodstream [7]. Hepatocyte-derived
merosomes appear to act as shuttles that ensure the
protection of parasites from the host immune system
and the release of viable merozoites directly into the
circulation [7]. The time taken to complete the tissue
phase varies, depending on the infecting species; (8–25 days for P. falciparum, 8–27 days for
P. vivax, 9–17 days for P. ovale and 15–30 days for
P. malariae), and this interval is called the prepatent
period.
Erythrocytic schizogony
Merozoites enter erythrocytes by a complex invasion
process, which can be divided into four phases: (a) initial recognition and reversible attachment of the merozoite to the erythrocyte membrane; (b) reorientation
and junction formation between the apical end of the
merozoite (irreversible attachment) and the release of
substances from the rhoptry and microneme organelles, leading to formation of the parasitophorous
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A
Liver cell
Schizont
Mosquito takes a
blood meal
(injects sporozoites)
Exo-erythrocytic
cycle
Ruptured schizont
Rupturing
Oocyst
RBC
Erythrocytic cycle
Cycle in
mosquito
ring stage
Trophs
Oocyst
Ruptured
schizont
Gametocytes
Fuse & make Zygote
Male & female
gametocytes
B
Ring
Trophozoite
Schizont
vacuole; (c) movement of the junction and invagination of the erythrocyte membrane around the merozoite accompanied by removal of the merozoite’s surface
coat; and (d) resealing of the parasitophorous vacuole
and erythrocyte membranes after completion of merozoite invasion [8]. Because the invasion of erythrocytes
by P. falciparum requires a series of highly specific
molecular interactions, it is regarded as an attractive
target for the development of interventions to combat
malaria [6]. Asexual division starts inside the erythrocyte and the parasites develop through different stages
therein. The early trophozoite is often referred to as
the ‘ring form’, because of its characteristic morphology (Fig. 1). Trophozoite enlargement is accompanied
by highly active metabolism, which includes glycolysis
of large amounts of imported glucose, the ingestion of
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Fig. 1. (A) Life cycle of the malaria parasite
P. falciparum. The figure has been prepared
with the help of the information, artwork
and micrographs from the CDC’s websites
for parasite identification http://www.
dpd.cdc.gov/dpdx and http://www.itg.be.
(B) Different intraerythrocytic stages of
development of P. falciparum in culture.
host cytoplasm and the proteolysis of hemoglobin into
constituent amino acids. Malaria parasites cannot
degrade the heme by-product and free heme is potentially toxic to the parasite. Therefore, during hemoglobin degradation, most of the liberated heme is
polymerized into hemozoin (malaria pigment), a crystalline substance that is stored within the food vacuoles [8].
The end of this trophic stage is marked by multiple
rounds of nuclear division without cytokinesis resulting
in the formation of schizonts (Fig. 1). Each mature
schizont contains around 20 merozoites and these are
released after lysis of the RBC to invade further uninfected RBCs. This release coincides with the sharp
increases in body temperature during the progression
of the disease. This repetitive intraerythrocytic cycle of
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Malaria ) an overview
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invasion–multiplication–release–invasion
continues,
taking about 48 h in P. falciparum, P. ovale and
P. vivax infections and 72 h in P. malariae infection. It
occurs quite synchronously and the merozoites are
released at approximately the same time of the day.
The contents of the infected RBC that are released
upon its lysis stimulate the production of tumor necrosis factor and other cytokines, which are responsible
for the characteristic clinical manifestations of the disease.
A number of specific ligand–receptor interactions
have been identified as involved in invasion and it has
been reported that genetic disruption of any one of
these results in a shift to using other pathways [9,10].
The P. falciparum genome sequence, completed in
2002, indicates that several of the molecules involved
in invasion are members of larger gene families [11,12].
Merozoite surface proteins (MSP)1 to MSP)4) are an
important class of integral membrane proteins identified on the surface of developing and free merozoites.
These are involved in the initial recognition of the erythrocytes via interactions with sialic acid residues and
are likely to be important for invasion because antibodies directed against these proteins can block this
process [9]. Erythrocyte binding antigen 175 (EBA175) is a P. falciparum protein that binds the major
glycoprotein (glycophorin A) found on human erythrocytes during invasion [8]. The structure of EBA-175
has striking similarities with the Duffy antigen-binding
proteins of P. vivax that are essential for successful
invasion by this species. After invasion, the principal
parasite ligand known as P. falciparum erythrocyte
membrane protein 1 (PfEMP1), which is encoded by a
multigene family termed var, is expressed at the surface
of the infected RBC [13,14]. PfEMP1 has a pivotal role
in P. falciparum pathogenesis and several host receptors can be concurrently recognized by the numerous
adhesion domains located in the extracellular region of
PfEMP1 [15,16]. The extensive diversity in the var gene
family is mainly responsible for the evasion of specific
immune responses and many of these genes are
expressed in the parasite population, but at any given
time during an infection, parasites within infected cells
express only a single var gene [15–17]. In a recent
study, a specific epigenetic mark associated with the
silenced var genes has been identified and it has been
shown that the persistence of this mark provides
advantages to the parasite in pathogenesis and immune
evasion [18].
A small proportion of the merozoites in the red
blood cells eventually differentiate to produce microand macrogametocytes (male and female, respectively),
which have no further activity within the human host
(Fig. 1A). These gametocytes are essential for transmitting the infection to new hosts through female Anopheles mosquitoes. Normally, a variable number of cycles
of asexual erythrocytic schizogony occur before any
gametocytes are produced. In P. falciparum, erythrocytic schizogony takes 48 h and gametocytogenesis
takes 10–12 days. Gametocytes appear on the fifth day
of primary attack in P. vivax and P. ovale infections,
and thereafter become more numerous; they appear at
anything from 5 to 23 days after a primary attack by
P. malariae.
Sexual phase in the mosquito (sporogony)
A mosquito taking a blood meal on an infected individual may ingest these gametocytes into its midgut,
where macrogametocytes form macrogametes and
exflagellation of microgametocytes produces microgametes. These gametes fuse, undergo fertilization and
form a zygote. This transforms into an ookinete, which
penetrates the wall of a cell in the midgut and develops
into an oocyst (Fig. 1A). In a recent study, it has been
shown that gamete surface antigen Pfs230 mediates
human RBC binding to exflagellating male parasites to
form clusters termed exflagellation centers, from which
individual motile microgametes are released. This protein thus plays an important role in subsequent oocyst
development, which is a critical step in malaria transmission [19]. Sporogony within the oocyst produces
many sporozoites and when the oocyst ruptures, they
migrate to the salivary glands for onward transmission
into another host (Fig. 1A). This form of the parasite
is found in the salivary glands after 10–18 days
and thereafter the mosquito remains infective for
1–2 months. When an infected mosquito bites a susceptible host, the Plasmodium life cycle begins again.
Symptoms, diagnosis and treatment
The accumulation and sequestration of parasiteinfected RBCs in various organs such as the heart,
brain, lung, kidney, subcutaneous tissues and placenta
is a characteristic feature of infection with P. falciparum. Sequestration is a result of the interaction
between parasite-derived proteins, which are present
on the surface of infected RBCs, and a number of host
molecules expressed on the surface of uninfected
RBCs, endothelial cells and in some cases placental
cells [20]. In specific manifestations of malaria, some
receptors for parasite adhesion have been implicated,
such as hyaluronic acid and chondroitin sulfate A
(CSA) in placental infections and intercellular adhesion
molecule 1 (ICAM-1) in cerebral malaria [8,13,21].
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Malaria symptoms can develop as soon as 6–8 days
after being bitten by an infected mosquito, or as late
as several months after departure from a malarious
area. People infected with malaria parasites typically
experience fever, shivering, cough, respiratory distress,
pain in the joints, headache, watery diarrhea, vomiting
and convulsions [8]. Severe malaria is usually complex
and several key pathogenic processes such as jaundice,
kidney failure and severe anemia can combine to cause
serious and often fatal disease [8].
There are no life-threatening complications in most
cases of malaria, but what triggers the transition from
an uncomplicated to a serious infection is not well
understood [22]. Malaria is especially dangerous to
pregnant women and small children and in endemic
countries it is an important determinant of perinatal
mortality [23]. Parasite sequestration in the placenta is
a key feature of infection by P. falciparum during pregnancy and is associated with severe adverse outcomes
for both mother and baby, such as premature delivery,
low birthweight and increased mortality in the newborn [24]. PfEMP1, a ligand for CSA, is a major target
of antibodies associated with protective immunity and
P. falciparum isolates that sequester in the placenta
primarily bind CSA [25]. After repeated exposure to
malaria during pregnancy, women in areas of endemicity slowly develop immunity; thus multigravid women
are comparatively less susceptible to pregnancy-associated malaria than primagravid women.
Malaria is diagnosed using a combination of clinical
observations, case history and diagnostic tests, principally microscopic examination of blood [26]. Ideally,
blood should be collected when the patient’s temperature is rising, as that is when the greatest number of
parasites is likely to be found. Thick blood films are
used in routine diagnosis and as few as one parasite
per 200 lL blood can be detected. Rapid diagnostic
‘dipstick’ tests, which facilitate the detection of malaria
antigens in a finger-prick of blood in a few minutes
are easy to perform and do not require trained personnel or a special equipment [26]. However, they are
relatively expensive and although P. falciparum can be
diagnosed, P. ovale, P. malariae and P. vivax cannot
be distinguished from one another using this method.
Three consecutive days of tests that do not indicate
the presence of the parasite can rule out malaria.
Malaria is a curable disease if treated adequately
and promptly. Quinine from the bark of the Andean
Cinchona tree was the first widely used antimalarial
treatment and was discovered long before the causes
of malaria were known. However, the parasite can rapidly develop resistance to common antimalarial drugs.
In many parts of the world P. falciparum has become
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resistant to Fansidar and chloroquine, which are the
two most commonly used and most affordable antimalarial drugs [27,28]. To overcome this problem and to
prolong the useful life of current drugs, combination
therapy is being increasingly employed. Artemisinin,
which is obtained from the plant Artemisia annua, is
an extremely effective antimalarial, and this drug, or
its derivatives such as artesunate or artemether, are
being used in mainly pairwise combinations with several other drugs such as Fansidar [29] and mefloquine
[30], the latter an important and still highly efficacious
drug against which resistance, especially in southeast
Asia is, however, of increasing concern. The inexorable
spread of drug resistance is a major problem in
malaria control, especially as there are no clinically
approved malaria vaccines available to date, even
though a number are in development and testing.
Recent reports have described state-of-the-art malaria
vaccine development and selected malaria vaccines in
current clinical development [31,32].
Several major international initiatives have been
launched to tackle malaria (Table 1) [33]. These
include the WHO’s Roll Back Malaria program, the
Multilateral Initiative in Malaria [34], the Medicines
for Malaria Venture , the Malaria Vaccine Initiative,
and the Global Fund to Fight AIDS, TB and Malaria,
which supports the implementation of prevention and
treatment programs. There are a number of ways to
decrease malaria transmission but none currently offers
a complete block, therefore new methods are urgently
required [35]. The three combined strategies of drug
treatment, vaccination and vector control will ultimately be required to significantly reduce malaria
transmission [29,36].
With respect to the last of these, another potential
option for reducing malaria is by the use of genetically
modified mosquitoes that are refractory to transmission of the pathogen [37]. Recently, important technical advances, which include germ-line transformation
of mosquitoes, the characterization of tissue-specific
promoters and the identification of effector molecules
that interfere with parasite development, have resulted
in the production of transgenic mosquitoes incapable
of spreading the malaria parasite [37]. However, in
order for Plasmodium-refractory mosquitoes to be
effective, they need to be able to thrive in the wild and
compete successfully with their wild-type counterparts.
One major concern about the use of these engineered
mosquitoes is whether the modification would be stable long-term [37]. Even though the possibility of
genetically modifying mosquito vector competence has
been well studied in the laboratory, much work is still
needed to develop strategies for the release and
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Table 1. Important websites.
No.
Description
Website
1
2
3
4
5
6
7
8
9
10
11
12
13
WHO Roll Back Malaria program
Multilateral Initiative in Malaria
Medicines for Malaria Venture
Malaria Vaccine Initiative
Global Fund to Fight AIDS, TB and Malaria
Plasmodium genome database PlasmoDB
Plasmodium falciparum Gene database
Malaria Parasite Metabolic Pathways
Malaria Transcriptome database
Plasmodium falciparum genome ⁄ pathway database
Malaria Research and Reference Reagent Resource Center
Understanding higher-order function from genome information
Detection of enzyme-encoding genes in P. falciparum genome
http://www.rbm.who.int
http://www.mim.su.se
http://www.mmv.org
http://www.malariavaccine.org
http://www.theglobal fund.org
http://www.plasmodb.org/
http://www..genedb.org/genedb/malaria/
http://sites.huji.ac.il/malaria/
http://malaria.ucsf.edu/comparison
http://plasmocyc.stanford.edu/
http://www.mr4.org/
http://www.genome.ad.jp/kegg/
http://bioinformatics.leeds.ac.uk/shark/
survival of these engineered mosquito populations in
the field. In a recent study, it was reported that when
fed on Plasmodium-infected blood, transgenic malariaresistant mosquitoes had a significant fitness advantage
over wild-type mosquitoes [38].
The genome, proteome and
transcriptome
The genome of P. falciparum clone 3D7 was the first
to be sequenced and annotation of the predicted genes
is at an advanced stage [12]. The availability of the
P. falciparum genome sequence has the potential to
reveal a large number of possible new drug targets and
genes important for parasite biology and pathogenesis.
Genome information for P. falciparum and other
species of Plasmodium is freely available at http://
www.plasmodb.org, and it has been shown that the
P. falciparum genome covers 23 megabase pairs of
DNA, split into 14 chromosomes. P. falciparum also
has a circular plastid-like genome and a linear mitochondrial genome [39]. The nuclear genome is the most
(A+T)-rich genome sequenced to date, with an overall
(A+T) composition of 81%, which increases to
90% in intergenic regions and introns [12]. About
5300 genes have been predicted from the genome
sequence, of which only a few have been identified to
date as encoding enzymes. The regions near the ends
of each chromosome are interesting; the genes residing
here encode surface proteins or antigens that are sometimes recognized by the human immune system to
stimulate immune responses. However, exchange of
material between chromosome ends gives the parasite
a considerable capacity for changes in antigen expression and thereby immune evasion. The genome
sequence of P. falciparum has also revealed new gene
families encoding proteins responsible for mediating
erythrocyte invasion [9]. It is interesting to note that,
although the homologs of genes involved in basic pathways such as translation initiation, DNA replication,
repair and recombination are present in the genome of
the parasite [12,40], it appears to lack some key metabolic pathways; for example, the synthesis of a majority of the 20 amino acids, synthesis of purines and the
salvage of pyrimidines, as well as two protein components of ATP synthase (a mitochondrial ATP-producing enzyme) and components of a conventional
NADH dehydrogenase complex [12]. It has also been
proposed that the regulation of protein levels is controlled through mRNA processing and translation, in
addition to the level of gene transcription [12]. Molecular transfection technology, together with the ability
to introduce fluorescent reporter proteins, is a relatively recent development that is facilitating a greater
understanding of many other aspects of the parasite’s
cell biology [41].
It is noteworthy that components of some anabolic
pathways for the synthesis of fatty acids, isoprenoid
precursors, heme and iron sulfur complexes seem to be
localized in the apicoplast, a structure within the cell
related to the plastids of plant species that has its own
genome [12,42–46], as mentioned above. Studies have
shown that the apicoplast is essential for survival of
the parasite [47,48]. Its genome is 35 kb and encodes
only 57 proteins but it is estimated that around 10%
of the proteins encoded by the nucleus may be destined for this structure [49]. Such proteins are targeted
into the organelle by the use of a bipartite-targeting
signal [49]. One protein in this class is encoded by an
unusual gene on chromosome 14 specifying contiguous
DNA polymerase, DNA primase and DNA helicase
activities and thought to play a key role in the replication of the apicoplast genome [12,50]. The organellar
genome sequence also identified molecules within the
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apicoplast that, in other systems, are the targets of several existing drugs, such as antibiotics, and there are
now experimental data showing that such compounds
can also inhibit the growth of P. falciparum by targeting this bacterium-derived endosymbiotic organelle
[51,52].
At the proteomics level, the proteins from four
stages of the life cycle of P. falciparum (clone 3D7),
i.e. sporozoites, merozoites, trophozoites and gametocytes, have been profiled using multidimensional protein identification technology and MS analysis [53]. It
has been reported that the sporozoite proteome is
markedly different from the other stages and about
half of the sporozoite proteins are unique to this stage.
In contrast, trophozoites, merozoites and gametocytes
have fewer unique proteins, sharing a greater proportion of the total. Of the proteins found in multiple
stages, the most common were mainly housekeeping
proteins such as ribosomal proteins, transcription factors, histones and cytoskeletal proteins [53]. The results
also suggested that the P. falciparum genome encodes
a large number of unique proteins, many of which
might be required for specific host–parasite interactions. These interesting proteins with no homology to
sequences in other organisms represent potential Plasmodium-specific molecules that might provide targets
for new drug and vaccine development [53]. In a similar study the proteomic analysis of selected stages of
P. falciparum (NF54 isolate) by high-accuracy MS
revealed 1289 proteins, of which 645 were identified in
gametes, 931 in gametocytes and 714 in asexual blood
stages, respectively [54]. Previous studies have shown
that in many cases, the proteins from P. falciparum are
consistently bigger than their homologous counterparts
from other species, but the role of these parasite-specific inserts in the sequences of P. falciparum proteins
is uncertain [55].
Using ORF-specific DNA microarrays, the expression profile across 48 individual 1-h time points from
the complete asexual intraerythrocytic developmental
cycle (IDC) of the HB3 clone of P. falciparum has
been examined [39,56]. This transcriptome analysis
revealed that at least 60% of the genome is transcriptionally active during this stage and that > 75% of
these expressed genes are activated only once during
the IDC [39]. These interesting data demonstrate that
P. falciparum exhibits an unusual and quite specialized
mode of transcriptional regulation, which produces a
continuous cascade of gene expression, starting with
genes corresponding to general cellular processes, such
as protein synthesis, and ending with Plasmodiumspecific functionalities, such as genes involved in
erythrocyte invasion [39]. Recently, the same group
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determined the transcriptome of the IDC for two more
clones of P. falciparum, 3D7 and Dd2, with different
geographical origins from HB3 [57]. Their results
revealed that the transcriptome is remarkably well conserved among all three clones but there are some differences in the expression of genes coding for surface
antigens involved in host–parasite interactions [57]. All
of these strain-specific data are publicly available at
both http://malaria.ucsf.edu/comparison/ and http://
www.plasmoDB.org.
Table 1 is a compilation of important websites that
have been created to organize and exploit data arising
from postgenomic studies of P. falciparum and its
related species. For a better understanding of the biological, physiological and biochemical roles of a particular
gene, a website summarizing malaria parasite metabolic
pathways as maps has been constructed and is continuously being expanded [58] (http://sites.huji.ac.il/malaria/).
In addition to classical biochemical pathways, this
website contains maps dealing with biological processes
such as cell–cell interactions, protein trafficking and
transport, and fundamental pathways including replication, transcription and translation [58]. PlasmoCyc is
another genome ⁄ pathway database that specifically
developed for P. falciparum (http://plasmocyc.stanford.
edu/). In this database, the metabolic pathways are
displayed with detailed information about individual
enzymatic reactions with the chemical structures of the
substrates and reactants. The database also contains
information about antimalarial drugs and their targets,
as well as an overview of all the metabolic pathways and
tools for comparing pathways between organisms.
Another important website, Kyoto Encyclopedia of
Genes and Genomics (KEGG) at (http://www.genome.
ad.jp/kegg/), can also be used for exploring higher-order
functional aspects of parasite biology from its genome
information [59]. A new fully automated software package, the metashark can be used for the detection of
enzyme-encoding genes within unannotated genome
data from organisms such as P. falciparum and their
visualization in the context of the relevant metabolic
network(s) [60]. The sharkhunt package can be
downloaded from the metashark website at (http://
bioinformatics.leeds.ac.uk/shark/). This search method
was successfully used to detect the experimentally demonstrated but unannotated pantothenate to coenzyme A
pathway encoded in the P. falciparum genome [60].
Conclusions
Malaria caused by the mosquito-transmitted parasite
P. falciparum is the cause of an enormous number of
deaths every year in the tropical and subtropical areas
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of the world. There is an urgent need to design new
drugs and ⁄ or vaccines that can substantially and consistently interrupt the life cycle of this complex parasite. A wealth of information has been generated from
genome-wide studies of the transcriptome and proteome of the parasite and now it is a real challenge to
use this information efficiently to determine the appropriate therapeutic targets for developing and testing
new formulations. Malaria vaccine development is currently at an encouraging stage and it is critical that the
momentum achieved to date be maintained in the
future. A combination of new antimalarial drugs and
vaccines with efficient vector control measures will be
required to halt the transmission of malaria in the
affected areas of the world.
Acknowledgements
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The author is grateful to Professor John Hyde (University of Manchester, UK) and Dr C. Chitnis (ICGEB, New Delhi) for critical reading and corrections
on the manuscript and the referees for constructive
suggestions. The author thanks Arun Pradhan for help
in the preparation of figure. The work in author’s laboratory is supported by grants from Department of
Biotechnology, Defence Research and Development
Organization and Department of Science and Technology. Infrastructural support from the Department of
Biotechnology, Government of India is gratefully
acknowledged.
14
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