Despite the fact that apoptosis has been described in metazoans cells and hasbeen associated with multicellularity, it had been suggested that unicellulareukaryotes also undergo apoptosis and to death with typical features ofmammalian programmed cell death. It has been described in several speciesincluding the protozoans 19. Recentfindings have been described programmed cell death in eukaryotic microorganismsincluding yeast 20,21, kinetoplastids;Trypanosoma brucei rhodesiensis, Trypanosoma cruzi, Leishmania amazonensis,Leishmania donovani and Leishmania major22-26 and apicomplexans 27-28 and even in protozoa which do not present anevident mitochondrial system, as in Trichomonas 29-31, Giardia and Entamoeba 32. Similar alterations as multicellular organismlike mitochondrial membrane potential and nuclear DNA fragmentation areobserved in parasitic protozoa submitted to different stress conditions such ascontact with chemotherapeutic agents 33,34 and different temperatures 35.
The phosphotidylserine externalizationinduces the phagocytosis of apoptotic cells by macrophages and this mechanismresults in an anti-inflammatory effect and suppression of pro-inflammatorymediators 36, 37. Two of the major characteristic features of mammalian cellapoptosis, that occur with protozoa and distinguishes it from the passive andchaotic destruction process of necrosis, are cell shrinkage and maintenance ofplasma membrane integrity 26. Studies carried out with T. brucei and T. cruzi haveidentified genes coding for metacaspase 38-41 which belongs to cysteinprotease family as caspase in multicellular organisms. Metacaspase enzymes werealso reported in Plasmodium and itexhibits a calcium-dependent, arginine-specific protease activity which inducedcell death in malaria parasite 42. Overviewof PCD in Plasmodium Considering the importance of programmed cell death inmetazoan, it is reasonable to ask if such a predetermined behaviour mechanismor operation exists in Plasmodium. First time in 1997, Picot S and hiscolleagues reported PCD in Plasmodium who observed oligonucleosomal DNAfragmentation during drug pressure of chloroquine in asexual stages ofchloroquine sensitive strain of P.
falciparumby using agrose gel electrophoresis 28 and suggested apoptotic cell death inmalaria parasite. Only single apoptotic marker, taken alone, is not proof ofapoptosis, it is justified to classify a process as apoptosis-like cell deathwhen several apoptotic markers can be reproducibly shown to occur in a malariaparasite. That is why researchers need to study other parameters of PCD. Inthis direction, TUNEL Assay (terminal deoxynucleotidyl transferase-mediateddUTP-fluorescein nick end-labeling) was done to evaluate DNA fragmentation in asexualstages of P.
falciparum after exposure ofelevated temperature. In this study parasite crisis forms were observed. Crisisform is related to apoptotic bodies, retain their staining properties andappear to be condensed, which are considered as a final hallmark of apoptosisin Plasmodium 43.
In a review article, Deponte and Becker have also reportedTUNEL activity in P. falciparum blood-stageschizonts treated with antimalarial drugs and H2O2 44. In addition Meslin et al have done work by the use of apoptosis induceretoposide and antimalarial chloroquine and found evident of apoptosis celldeath in blood stage Plasmodium by the observation of DNA fragmentation anddisruption of mitochondrial membrane potential.
They also identified P. falciparum metacaspase protein PfMCA1 which belongs to cysteine protease family ascaspase in metazoans 45. At the same time apoptosis related protein (PfARP) (Gene ID- PF10450C) was alsocharacterized by other group and they found stage specific expression of PfARP; highest expression in trophozoite and minimum inschizont stage 46.
Recently it was reported that mefloquine induce ROSmediated apoptosis in asexual stage of Plasmodium 47. Whileusing P.berghei, Al-olyan and his colleaguesexamined the mode of parasite death during sexual stages within the midgutlumen and obtained an additional process close to cell suicide which isinvolved in regulating parasite numbers in Anopheles stephensi, a mosquitovector of human malaria that will also transmit P.
berghei. In addition, the presence of caspase-like activityin the cytoplasm of the ookinete was observed suggesting once again thepresence of a cellular mechanism, similar to those found in mammalian cells48. Several other researchers could also be able to found apoptosis like celldeath in sexual stages of Plasmodium life cycle. Whereas few groups failed tofind apoptotic characteristics in P. falciparum after treatment with ceramide, artemisinin, mefloquine 49, etoposide 50, S-nitroso-N-acetyl-penicillamine (SNAP), staurosporineand CQ 51,52 but they suggest someother type of programmed cell death occur in parasite like autophagy andsecondary necrosis. Nyakeriga et al. suggested that although P.
falciparum die by PCD, this might take place without typicalfeatures of apoptosis. It was reported that P. falciparum die by autophagy after the exposer of CQ, SNAP andstaurosporine, considering the lack of apoptotic characteristics and thepresence of parasites with autophagic vacuolization in cytoplasm 51. RecentlyMary Luz López showed morphological and physiological evidences associated withsome apoptotic-like and autophagic-like cell death characteristics in P.
falciparum asexual blood stages treated with compound isolatedfrom Solanum nudum Dunal (Solanaceae) 53. These differing observations mightbe based on several experimental pitfalls. Autophagosome formation is thecentral event of autophagy process and is governed by autophagy-related (Atg)proteins, which were originally identified in yeast 54, 8. Further Noboru andhis colleagues reported biochemical properties and subcellular localization ofAtg8 protein of human malaria parasite Plasmodium falciparum (PfAtg8). Multiple markers of apoptosis have beenobserved in stages of Plasmodium life cycle that occur in the vector(ookinetes) and the host (liver schizont and blood stages). Thesereports make it plain that there is no consensus amongst malaria researchersabout which process of PCD takes place in malaria parasites during theirexposure to antimalarials, resulting in their subsequent death, and if just oneor a combination of PCD processes occurs to bring about their death.