Tài liệu Báo cáo khoa học: Malaria ) an overview ppt

The parasites Malaria is transmitted through the bite of an infected female Anopheles mosquito.. Of the approximately Keywords cerebral malaria; erythrocytes; malaria life cycle; malaria

Malaria ) an overview

Renu Tuteja

Malaria Group, International Centre for Genetic Engineering and Biotechnology, New Delhi, India

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,

discov-ered 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

num-ber of countries and it has been reported that the

inci-dence 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, Indo-nesia, Vietnam, Cambodia, and China [1,2] Malaria is estimated to cost Africa more than $12 billion annu-ally 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 develop-ment 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

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

(Received 30 April 2007, revised 26 June

2007, accepted 19 July 2007)

doi:10.1111/j.1742-4658.2007.05997.x

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 pri-mary 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

Abbreviations

CSA, chondroitin sulfate A; IDC, intraerythrocytic developmental cycle; PfEMP1, Plasmodium falciparum erythrocyte membrane protein 1; RBC, red blood cell.

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

Plas-modium 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

pat-terns 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

rela-tively 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

Afri-can 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

extracel-lular 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

mal-ariaesporozoites 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 spor-ozoites 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 sporo-zoites 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 sporozo-ite migration through several hepatocytes in the mam-malian host is essential for completion of the life cycle [5] The receptors on sporozoites responsible for hepato-cyte invasion are mainly the thrombospondin domains

on the circumsporozoite protein and on thrombospon-din-related adhesive protein These domains specifically bind to heparan sulfate proteoglycans on the hepato-cytes [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 mer-ozoites 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 spe-cies; (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) ini-tial recognition and reversible attachment of the mero-zoite 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 organ-elles, leading to formation of the parasitophorous

vacuole; (c) movement of the junction and

invagina-tion of the erythrocyte membrane around the

merozo-ite accompanied by removal of the merozomerozo-ite’s surface

coat; and (d) resealing of the parasitophorous vacuole

and erythrocyte membranes after completion of

mero-zoite 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

erythro-cyte and the parasites develop through different stages

therein The early trophozoite is often referred to as

the ‘ring form’, because of its characteristic

morphol-ogy (Fig 1) Trophozoite enlargement is accompanied

by highly active metabolism, which includes glycolysis

of large amounts of imported glucose, the ingestion of

host cytoplasm and the proteolysis of hemoglobin into constituent amino acids Malaria parasites cannot degrade the heme by-product and free heme is poten-tially toxic to the parasite Therefore, during hemo-globin degradation, most of the liberated heme is polymerized into hemozoin (malaria pigment), a crys-talline substance that is stored within the food vacu-oles [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 un-infected RBCs This release coincides with the sharp increases in body temperature during the progression

of the disease This repetitive intraerythrocytic cycle of

Fuse & make Zyg ote

Oocyst

Cycle in

mo squito

Rupturing

Oocyst

Liver cell

Exo-erythrocytic cycle

Schizont

Ruptured schizont

RB C

ring stage

Trophs

Ga me tocytes

Male & fe ma le

ga me tocytes

Ruptured schizont

Erythrocytic cycle

Mosquito takes a

A

B

blood m eal

(injects sporozoites)

Trophozoite Schizont Ring

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.

invasion–multiplication–release–invasion continues,

taking about 48 h in P falciparum, P ovale and

P vivaxinfections 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

necro-sis factor and other cytokines, which are responsible

for the characteristic clinical manifestations of the

dis-ease

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

identi-fied on the surface of developing and free merozoites

These are involved in the initial recognition of the

ery-throcytes via interactions with sialic acid residues and

are likely to be important for invasion because

anti-bodies directed against these proteins can block this

process [9] Erythrocyte binding antigen 175

(EBA-175) is a P falciparum protein that binds the major

glycoprotein (glycophorin A) found on human

erythro-cytes 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

recep-tors 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

micro-and macrogametocytes (male micro-and female, respectively),

which have no further activity within the human host

(Fig 1A) These gametocytes are essential for transmit-ting the infection to new hosts through female Anophe-les mosquitoes Normally, a variable number of cycles

of asexual erythrocytic schizogony occur before any gametocytes are produced In P falciparum, erythro-cytic 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 indi-vidual may ingest these gametocytes into its midgut, where macrogametocytes form macrogametes and exflagellation of microgametocytes produces microga-metes 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 pro-tein thus plays an important role in subsequent oocyst development, which is a critical step in malaria trans-mission [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 sus-ceptible host, the Plasmodium life cycle begins again

Symptoms, diagnosis and treatment

The accumulation and sequestration of parasite-infected RBCs in various organs such as the heart, brain, lung, kidney, subcutaneous tissues and placenta

is a characteristic feature of infection with P falcipa-rum 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]

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

preg-nancy and is associated with severe adverse outcomes

for both mother and baby, such as premature delivery,

low birthweight and increased mortality in the

new-born [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

endemic-ity slowly develop immunendemic-ity; thus multigravid women

are comparatively less susceptible to

pregnancy-associ-ated malaria than primagravid women

Malaria is diagnosed using a combination of clinical

observations, case history and diagnostic tests,

princi-pally microscopic examination of blood [26] Ideally,

blood should be collected when the patient’s

tempera-ture 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

person-nel 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

rap-idly develop resistance to common antimalarial drugs

In many parts of the world P falciparum has become

resistant to Fansidar and chloroquine, which are the two most commonly used and most affordable antima-larial 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 sev-eral 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 ulti-mately 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 transmis-sion of the pathogen [37] Recently, important techni-cal 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 sta-ble 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

survival of these engineered mosquito populations in

the field In a recent study, it was reported that when

fed on Plasmodium-infected blood, transgenic

malaria-resistant 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

mito-chondrial 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

some-times 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

expres-sion 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 path-ways 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 meta-bolic pathways; for example, the synthesis of a major-ity of the 20 amino acids, synthesis of purines and the salvage of pyrimidines, as well as two protein compo-nents of ATP synthase (a mitochondrial ATP-pro-ducing enzyme) and components of a conventional NADH dehydrogenase complex [12] It has also been proposed that the regulation of protein levels is con-trolled through mRNA processing and translation, in addition to the level of gene transcription [12] Molec-ular transfection technology, together with the ability

to introduce fluorescent reporter proteins, is a rela-tively 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 des-tined 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 replica-tion of the apicoplast genome [12,50] The organellar genome sequence also identified molecules within the

Table 1 Important websites.

10 Plasmodium falciparum genome ⁄ pathway database http://plasmocyc.stanford.edu/

11 Malaria Research and Reference Reagent Resource Center http://www.mr4.org/

12 Understanding higher-order function from genome information http://www.genome.ad.jp/kegg/

13 Detection of enzyme-encoding genes in P falciparum genome http://bioinformatics.leeds.ac.uk/shark/

apicoplast that, in other systems, are the targets of

sev-eral existing drugs, such as antibiotics, and there are

now experimental data showing that such compounds

can also inhibit the growth of P falciparum by

target-ing 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

gameto-cytes, have been profiled using multidimensional

pro-tein 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

propor-tion of the total Of the proteins found in multiple

stages, the most common were mainly housekeeping

proteins such as ribosomal proteins, transcription

fac-tors, 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

interac-tions These interesting proteins with no homology to

sequences in other organisms represent potential

Plas-modium-specific molecules that might provide targets

for new drug and vaccine development [53] In a

simi-lar 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-spe-cific inserts in the sequences of P falciparum proteins

is uncertain [55]

Using ORF-specific DNA microarrays, the

expres-sion 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

transcrip-tionally 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

Plasmodium-specific functionalities, such as genes involved in

erythrocyte invasion [39] Recently, the same group

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 con-served among all three clones but there are some dif-ferences 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 biolog-ical, physiological and biochemical roles of a particular gene, a website summarizing malaria parasite metabolic pathways as maps has been constructed and is continu-ously 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 replica-tion, 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 pack-age, 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 demo-nstrated 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

of the world There is an urgent need to design new

drugs and⁄ or vaccines that can substantially and

con-sistently interrupt the life cycle of this complex

para-site A wealth of information has been generated from

genome-wide studies of the transcriptome and

prote-ome of the parasite and now it is a real challenge to

use this information efficiently to determine the

appro-priate therapeutic targets for developing and testing

new formulations Malaria vaccine development is

cur-rently 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

The author is grateful to Professor John Hyde

(Uni-versity of Manchester, UK) and Dr C Chitnis

(IC-GEB, 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

lab-oratory is supported by grants from Department of

Biotechnology, Defence Research and Development

Organization and Department of Science and

Technol-ogy Infrastructural support from the Department of

Biotechnology, Government of India is gratefully

acknowledged

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