Plasmodium and the apicoplast

The apicoplast is a plastid organelle, homologous to chloroplasts of plants or algae, that is found in apicomplexan parasites like Plasmodium or Toxoplasma. In hinsight it seems incredible that such an organelle could so long have concealed its identity in parasites that have received as much scientific attention as Plasmodium Chakraborty Arnish, Understanding the biology of the Plasmodium falciparum apicoplast; an excellent target for antimalarial drug development, Life Sciences (2016), doi: 10.1016/j.lfs.2016.06.030) Apicoplast function is necessary for both intraerythrocytic and intrahepatic development. Recently it was found that the apicoplast is also present in the gametocytogenesis, in the sexual stage of Plasmodium falciparum. But only the female macrogametes have an apicoplast, the male microgametes don’t. Okamoto N1, Spurck TP, Goodman CD, McFadden GI.Apicoplast and mitochondrion in gametocytogenesis of Plasmodium falciparum. Eukaryot Cell. 2009 Jan;8(1):128-32. doi: 10.1128/EC.00267-08. Epub 2008 Nov 7. Med Hypotheses. 2005;65(4):683-90. To compensate for this oxidative stress, Plasmodia have conserved the apicoplast. The apicoplast, located in close proximity to mitochondria, appears to synthesize the potent antioxidant lipoic acid, an organosulfur compound. Inhibition or alteration of the apicoplast leads to a curious phenomena known as "delayed death", whereby parasites die not in the present generation but in the ensuing one. Apicoplast inhibition may produce lipoic acid "starvation", increasing oxidative stress/mitochondrial injury during the subsequent asexual reproductive cycle Toler S. The plasmodial apicoplast was retained under evolutionary selective pressure to assuage blood stage oxidative stress. Med Hypotheses 2005, 65, 683-690 Plastids, including those of malaria parasites, have bacterial-type metabolic pathways all of which are vulnerable to antibacterial compounds. Indeed, many antibacterials kill malaria parasites by blocking essential processes in the plastid. It is logical that herbicides that target plastid metabolism of undesired plants are also parasitocidal, making them potential new leads for antimalarial drugs. The effects of anti-bacterials on the malaria parasite are well known Christopher Dean Goodman, Vanessa Su. The effects of anti-bacterials on the malaria parasite Plasmodium falciparum, Molecular and Biochemical Parasitology 152(2):181-91 · May 2007DOI: 10.1016/j.molbiopara.2007.01.005 · Some have an immediate effect: slowing parasite growth, retarding organellar growth and preventing nuclear division. Others don’t intervene in the first cycle, but lead to severe defects in the apicoplast in the second cycle and failure to complete this cycle. Inhibitors of apicoplast pathways can thus be explored as treatments for acute infections, as well as prophylactics with some degree of optimism. Antibiotics are used for the prevention of malaria. Already in 2007 it was shown that antibiotic treated parasites are unable to complete schizogony Dahl EL, Rosenthal PJ. Multiple antibiotics exert delayed effects against the Plasmodium falciparum apicoplast. Antimicrob Agents Chemother. 2007 Oct;51(10):3485-90. The authors relate this antimalarial effect to non-functional apicoplasts. Artemisia annua has strong bactericidal properties and as antibiotic can compete with penicillin or clamydin Yan LI , Hao-bin HU, Xu-dong ZHENG, Ji-hua ZHU and Li-ping LIU. Composition and antimicrobial activity of essential oil from the aerial of Artemisia annua.. Journal of Medicinal Plants Research. J. Med. Plants Res. Vol.5(16), pp. 3629-3633 , August 2011 Researchers from Heidelberg and Berlin have shown that if malaria infected mice are administered an antibiotic, no parasites appear in the blood and the mice are protected from this lifethreatening disease. The parasites which accumulate in the liver give the immune system sufficient stimulus to develop robust, long term immunity. A needle-free vaccination! Borrmann S, Adegnika AA, Moussavou F, Oyakhirome S, Esser G, Matsiegui PB Short-course regimens of artesunate-fosmidomycin in treatment of uncomplicated Plasmodium falciparum malaria., Antimicrob Agents Chemother. 2005 Sep;49(9):3749-54. Many herbicides also have an activity against the malaria protozoan. Stephen O. Duke, Herbicide and Pharmaceutical Relationships, Weed Science 58(3):334-339. 2010 A quote from this paper: “The Silent Spring image of pesticides is an anachronism that serves to obscure the pharmaceutical potential of herbicides. An opportunity missed!” Recently it was found that the inexpensive kitasamycin used for agricultural applications achieved an antimalarial IC50 in the 50 nM range by targeting the apicoplast Eric H. Ekland, Jessica Schneider and David A. Fidock, †Identifying apicoplast-targeting antimalarials using high-throughput compatible approaches. The FASEB Journal 2011, 25, 35833593. Other well known drugs like statins have a detrimental effect on the apicoplast. Inhibition of the development of Plasmodium falciparum by mevastatin is clearly stage-dependent, occurring only when parasites are treated before differentiation into schizonts. These findings indicate that the isoprenoid pathway may be an important regulatory target in the intra-erythrocytic stages. A S Couto, E A Kimura, V J Peres, M L Uhrig, and A M Katzin. Active isoprenoid pathway in the intra-erythrocytic stages of Plasmodium falciparum.Biochem J. 1999 Aug 1; 341(Pt 3): 629–637. Another drug, risedronate, a biphosphonate used in the treatment of oesteoporosis is a promising antiparasitic candidate. Recently it was demonstrated that lipophilic biphosphonates are effective against Plasmodium liver stages. Singh AP1, Zhang Y, No JH, Docampo R, Nussenzweig V, Oldfield E. Lipophilic bisphosphonates are potent inhibitors of Plasmodium liver-stage growth. Antimicrob Agents Chemother. 2010 Jul;54(7):2987-93. In a very efficient way, and no merozoites reach the red blood cells for several weeks. Ca++ is critical for the development of blood stages of Plasmodium falciparum and is a promising target for antimalarial drug development. Ca++ is 20 times higher in infected RBCs than in uninfected ones. The apicoplast plays a key role in this process : like in plants it helps in generating proteins which are essential, but which cannot be found in the human body. During the erythrocyte stage, the rapid growth of the parasite is fueled by other precursors and the synthesis of structural and regulatory phospholipids. The enzymes involved in these biosynthetic pathways are either absent from humans, or markedly different from their human counterparts, and thus represent other important drug targets. Isoprenoids, sometimes called terpenoids, play an important role in herbal remedies. Well known terpenoids include menthol, camphor, citral, 1.8-cineole. The human body cannot generate them. In Plasmodium the apicoplast is the only site of isoprenoid precursor synthesis. It even seems to have as only true essential function the production of one chemical, the isoprenoid isopentenyl pyrophosphate (IPP) which pays an essential role in protecting the Plasmodium against aggression by other molecules. Ellen Yeh , Joseph L. DeRisi . Chemical Rescue of Malaria Parasites Lacking an Apicoplast Defines Organelle Function in Blood-Stage Plasmodium falciparum, PLOS August 30, 2011http://dx.doi.org/10.1371/journal.pbio.1001138 These authors treated cells with antibiotics, which destroy the entire apicoplast, and then with IPP. The cells, usually rendered dead after a treatment with antibiotics, stayed alive, demonstrating the importance of IPP to malaria. Limonene is known as inhibitor of isoprenylation of proteins in Plasmodium falciparum and arrests parasite development. Ivan Cruz Moura, Gerhard Wunderlich and Emı́lia A. Kimura, Limonene Arrests Parasite Development and Inhibits Isoprenylation of Proteins in Plasmodium falciparum, Antimicrob. Agents Chemother. September 2001 vol. 45 no. 9 2553-2558 This becomes very evident 48 hours after the treatment. The in vitro IC50 against Plasmodium in these trials was found to be 2.27 mM, significantly below the value of 15.5 mM which had been found previously in in vivo clinical trials in patients with advanced cancer. The pharmacokinetics are favourable: limonene and its metabolites stay at least for 48 hours in the plasma. This is important for the elimination of gametes and malaria transmission. The monoterpene limonene is a by-product of the orange juice industry, has a very favourable toxicity profile and is easily available at low prices. Limonene is also present in Artemisia annua at concentrations of 1 mg/kg R Chougouo, Thèse, Université des Montagnes, Cameroon, 2011). So far studies have concentrated on this particular monoterpene, but it is not excluded that others present in the essential oil of Artemisia plants might have a similar detrimental action on the apicoplast, like eucalyptol Su V, King D, Woodrow. Plasmodium falciparum growth is arrested by monoterpenes from eucalyptus oil. Flavour Fragr J 2008; 23:315-18. And that the saponins which are present in Artemisia annua may facilitate the penetration of limonene and other drugs into the infected erythrocyte. This has been studied and documented H Florian et al., Synergistic action of saponins and monoterpenes in HeLa cells and in erythrocytes, Int J Phytotherapy, Oct 2011. Nerolidol which is found in Artemisia herba alba, in Inula viscosa and in lemongrass also is able to arrest development of the intraerythrocytic stages of the parasite. It has an IC50 of 760 nM compared to that of 1.220.000 nM for limonene. Herbert Rodrigues Goulart, Emília A. Kimura, Valnice J. Peres1 Alicia S. Couto, Fulgencio A. Aquino Duarte, and Alejandro M. Katzin. Terpenes Arrest Parasite Development and Inhibit Biosynthesis of Isoprenoids in Plasmodium falciparum. Antimicrob Agents Chemother. 2004 Jul; 48(7): 2502–2509. It interfers with the isoprenoid biosynthesis. Indians of the Amazon basin in Brazil use the vapors of the leaves of Viola surinamensis. The sesquiterpene nerolidol was identified as the active constituent leading to 100 % growth inhibition at the schizont stage. Luteolin from Artemisia afra has similar properties. Erika Piccirillo, Alberto Malvezzi , Leandro de Rezende, Alejandro M. Katzin , Renata Tonhosolo and Antonia T-do AmaralHomology model and docking studies on Plasmodium falciparum . 5th Brazilian Symposium on Medicinal Chemistry – BrazMedChem2010 Supply of the isoprenoid building blocks isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) is the essential metabolic function of the apicoplast for isoprenoid biosynthesis, particularly during gametocytogenesis. When IPP supplementation was removed early in gametocytogenesis, developmental defects were observed, supporting the essential role of isoprenoids for normal gametocytogenesis. Furthermore, mosquitoes infected with gametocytes lacking the apicoplast developed fewer and smaller oocysts that failed to produce sporozoites. This finding further supports the essential role of the apicoplast in establishing a successful infection in the mosquito vector. Jessica D. Wiley, Emilio F. Merino, Priscilla M. Krai, Kyle J. McLean, Abhai K. Tripathi, Joel Vega-Rodríguez, Marcelo Jacobs-Loren, Michael Klemba and Maria B. Cassera. Isoprenoid Precursor Biosynthesis Is the Essential Metabolic Role of the Apicoplast during Gametocytogenesis in Plasmodium falciparum. Eukaryotic Cell 2015, 14, 128-133)