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Malaria and Plasmodium falciparum (source: wikipedia)

1. Plasmodium falciparum is a protozoan parasite, one of the species of Plasmodium that cause malaria in humans.
   • It accounts for 80% of all human malarial infections and 90% of the deaths.
   • P. falciparum is transmitted to humans only by the females of the Anopheles species of mosquito. There are about 460 species of Anopheles mosquito, but only 68 transmit malaria.
     • why only females, certain species?

Life Cycle

1. Prior to transmission, Plasmodium falciparum resides within the salivary glad of the mosquito. The parasite is in its sporozoite stage at this point.
2. Human Infection:
   • Each infected bite contains 5-200 sporozoites. Once in the human bloodstream, sporozoites only circulate for a matter of minutes before infecting hepatocytes (human liver cells).
3. Liver Stage:
   • At this point, the parasite loses its apical complex and surface coat, and transforms into a trophozoite (activated form)
   • Within hepatocyte, undergoes schizogonic development for a minimum of 5.5 days: nucleus divides multiple times with a concomitant increase in cell size, but without cell segmentation.
   • After segmentation, the parasite cells are differentiated into merozoites that after maturation are released from the hepatocytes -- (note that these cells do not reinfect hepatocytes but, rather, infect erythroyctes/red blood cells).
4. Erythrocytic Stage:
   • Merozoite:
     • After release from the hepatocytes, the merozoites enter the bloodstream and use their apicomplexan invasion organelles (apical complex, pellicle and surface coat) to recognize and enter the host erythrocyte.
     • The parasite first binds to the erythrocyte in a random orientation and then reorients such that a tight junction is formed between the parasite and erythrocyte.
     • As it enters the red blood cell, the parasite forms a parasitophorous vesicle, to allow for its development inside the erythrocyte.
   • Trophozoite:
     • After invading the erythrocyte, the parasite loses its specific invasion organelles (apical complex and surface coat) and de-differentiates into a round trophozoite located within a parasitophorous vacuole in the red blood cell cytoplasm. The young trophozoite grows substantially before undergoing schizogonic division.
   • Schizont:
     • At the schizont stage, the parasite replicates its DNA multiple times without cellular segmentation. These schizonts then undergo cellular segmentation and differentiation to form roughly 16-18 merozoite cells in the erythrocyte. The merozoites burst from the the red blood cell, and proceed to infect other erythrocytes. The parasite is in the bloodstream for roughly 60 seconds before it has entered another erythrocyte.
     • This infection cycle occurs in a highly synchronous fashion, with roughly all of the parasites throughout the blood in the same stage of development. This precise clocking mechanism has been shown to be dependent on the human host's own circadian rhythm. Specifically, human body temperature changes, as a result of the circadian rhythm, seem to play a role in the development of P. falciparum within the erythrocytic stage.
     • Also, within the red blood cell, the parasite metabolism depends greatly on the digestion of hemoglobin.
     • Infected erythrocytes are often sequestered in various human tissues or organs, such as the heart, liver and brain. This is caused by parasite-derived cell surface proteins being present on the red blood cell membrane, and it is these proteins that bind to receptors on human cells.
       • Sequestration in the brain causes cerebral malaria, a very severe form of the disease, which increases the victim's likelihood of death.
     • The parasite can also alter the morphology of the red blood cell, causing knobs on the erythrocyte membrane.
5. Gametocyte Differentiation:
   • During the erythrocytic stage, some meriozoites develop into male and female gametocytes. This process is called gametocytogenesis. These gametocytes take roughly 8-10 days to reach full maturity. Note that the gametocytes remain within the erythrocytes until taken up by the mosquito host.
6. Mosquito Stage:
   • Gametogenesis:
     • Upon being taken up by the mosquito, the gametocytes leave the erythrocyte shell and differentiate into gametes. The female gamete maturation process entails slight morphological changes, as it becomes enlarged and spherical. On the other hand, the male game maturation involves significant morphological development. The male gamete's DNA divides three times to form eight nuclei. Concurrently, eight flagella are formed. Each flagella pairs with a nucleus to form a microgamete, which separates from the parasite cell. This process is referred to as exflagellation.
     • Gametogenesis has been shown to be caused by: 1) a sudden drop in temperature upon leaving the human host, 2) a rise in pH within the mosquito, and 3) xanthurenic acid within the mosquito.
   • Fertilization:
     • Fertilization of the female gamete by the male gamete occurs rapidly after gametogenesis. The fertilization event produces a zygote. The zygote then develops into an ookinete. The zygote and ookinete are the only diploid stages of P. falciparum.
   • Ookinete:
     • The diploid ookinete is an invasive form of P. falciparum within the mosquito. It traverses the peritrophic membrane of the mosquito midgut and cross the midgut epithelium. Once through the epithelium, the ookinete enters the basil lamina, and forms an oocyst.
     • During the ookinete stage, genetic recombination can occur. This takes place if the ookinete was formed from male and female gametes derived from different populations. This can occur if the human host contained multiple populations of the parasite, or if the mosquito fed of multiple infected individuals within a short time-frame.
   • Sporogony:
     • Over the period of a 1-3 weeks, the oocyst grows to a size of tens to hundreds of microns. During this time, multiple nuclear divisions occur. After oocyst maturation is complete, the oocyst divides to form multiple haploid sporozoites. Immature sporozoites break through the oocyst wall into the haemolymph. The sporozoites then migrate to the salivary glands an complete their differentiation.
     • Once mature, the sporozoites can proceed to infect a human host during a subsequent mosquito bite.

Cell Biology

1. Apicoplast:
   • The function of the apicoplast remains to be fully determined, but it appears to be involved in the metabolism of fatty acids, isoprenoids, and heme.
   • The apicoplast contains a 35-kb genome, which encodes for 30 proteins. Other, nuclear-encoded, proteins are transported into the apicoplast using a specific signal peptide. It is estimated that 551, or roughly 10%, of the predicted nuclear-encoded proteins are targeted to the apicoplast.
   • As humans do not harbor apicoplasts, this organelle and its constituents are seen as a possible target for antimalarial drugs.

Genome

1. The genome of Plasmodium falciparum (clone 3D7) was fully sequenced in 2002.
   • The parasite has a 23 megabase genome, divided into 14 chromosomes.
   • The genome codes for approximately 5,300 genes.
   • About 60% of the putative proteins have little or no similarity to proteins in other organisms, and thus currently have no functional assignment.
   • It is estimated 52.6% of the genome is a coding region, with 53.9% of the putative genes containing at least one intron.
   • The genome has an AT content of roughly 80.6%. Within the intron and intergenic regions, this AT composition rises to roughly 90%. The putative exons contain an AT content of 76.3%.