Skatebro:Plasmodium falciparum

Skatebro:Notes_Other

Plasmodium falciparum

• 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. what special about falciparum?
• 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 68/460 and only anopheles?
• Malaria in humans develops via two phases: an exoerythrocytic (hepatic) and an erythrocytic (blood) phase.
• The mosquito is an obligatory vector for malaria transmission. weak link of life cycle?

Life Cycle

1. Prior to transmission, Plasmodium falciparum resides within the salivary gland 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 (<30) before infecting hepatocytes (human liver cells).
3. Liver Stage (hepatic):
   • At this point, the parasite loses its apical complex and surface coat, and transforms into a trophozoite (activated form)
   • Undergoes schizogonic development: multiply asexually and asymptomatically for a period of 6–15 days.
   • After segmentation, the parasite cells are differentiated into 1000s of merozoites which, following rupture of their host cells, enter the bloodstream and infect red blood cells, thus beginning the erythrocytic stage of the life cycle.
   • The parasite escapes from the liver undetected by wrapping itself in the cell membrane of the infected host liver cell.
4. Blood Stage (erythrocytic): called the clinically relevant stage, why exactly?
   • Merozoite:
     • Once in the blood, the merozoites 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 trophozoite. The young trophozoite grows substantially before undergoing schizogonic division.
   • Schizont:
     • At the schizont stage, the parasite asexually 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.
     • Classical descriptions of waves of fever arise from simultaneous waves of merozoites escaping and infecting red blood cells.
     • 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 (PfEMP1) proteins being present on the red blood cell membrane, and it is these proteins that bind to receptors on the walls of small blood vessels.
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.
     • 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.
   • Sexual 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 2 membranes: the peritrophic membrane of the mosquito midgut and 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 matures and divides to form multiple haploid sporozoites. Immature sporozoites break through the oocyst wall into the haemolymph. The sporozoites then migrate to the salivary glands and complete their differentiation.
     • Mature sporozoites can infect a human host during a mosquito bite.

Apicoplast

1. 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.
2. 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.
3. As humans do not harbor apicoplasts, this organelle and its constituents are seen as a possible target for antimalarial drugs.

Biosynthesis

1. Hemoglobin metabolism:
   • During the erythrocytic stage of the parasite's life cycle, it uses intracellular hemoglobin as a food source. The protein is broken down into peptides, and the heme group is released and detoxified in the form of hemazoin.
   • On a molecular level, the parasite damages red blood cells using plasmepsin enzymes - aspartic acid proteases which degrade hemoglobin.
   • Heme biosynthesis by the parasite has been reported.
2. Carbohydrate metabolism:
   • During erythrocytic stages, the parasite produces its energy mainly through anaerobic glycolysis, with pyruvate being converted into lactate.
   • Genes encoding the TCA cycle enzymes as well as genes for nearly all of the pentose phosphate pathway enzymes are present in the genome
3. Protein metabolism:
   • It has been hypothesized that the parasite obtains all, or nearly all, of its amino acids by salvaging from the host or through the degradation of hemoglobin. This is supported by the fact that genomic analysis has found no enzymes necessary for amino acid biosynthesis, except for glycine-serine, cysteine-alanine, aspartate-asparagine, proline-ornithine, and glutamine-glutamate interconversions
4. Nucleotide biosynthesis:
   • The parasite is unable to biosynthesize purines, but is able to transport and interconvert host purines. The parasite can produce pyrimidines de novo using glutamine, bicarbonate, and aspartate

Human Immune System Evasion

• The parasite is relatively protected from attack by the body's immune system because for most of its human life cycle it resides within the liver and blood cells and is relatively invisible to immune surveillance.
• However, circulating infected blood cells are destroyed in the spleen. To avoid this fate, the P. falciparum parasite displays adhesive proteins on the surface of the infected blood cells, causing the blood cells to stick to the walls of small blood vessels, thereby sequestering the parasite from passage through the general circulation and the spleen.
  • This "stickiness" is the main factor giving rise to hemorrhagic complications of malaria.
• Although the red blood cell surface adhesive proteins (called PfEMP1, for Plasmodium falciparum erythrocyte membrane protein 1) are exposed to the immune system they do not serve as good immune targets because of their extreme diversity; there are at least 60 variations of the protein within a single parasite and perhaps limitless versions within parasite populations.

Variable Genes

1. var family:
   • The var genes encode for the P. falciparum erythrocyte membrane protein 1 (PfEMP1) proteins. The genes are found in the subtelomeric regions of the chromosomes. There exist an estimated 59 var genes within the genome.
   • The proteins encoded by the var genes are ultimately transported to the erythrocyte membrane and cause the infected erythrocytes to adhere to host endothelial receptors. Due to transcriptional switching between var genes, antigenic variation occurs which enables immune evasion by the parasite.
2. rif family:
   • The rif genes encode for repetitive interspersed family (rifin) proteins. The genes are found in the subtelomeric regions of the chromosomes. There exist an estimated 149 rif genes within the genome.
   • Rifin protein are ultimately transported to the erythrocyte membrane. The function of these proteins is currently unknown.
3. stevor family:
   • The stevor genes encode for the sub-telomeric variable open reading frame (stevor) proteins. The genes are found in the subtelomeric regions of the chromosomes. There exist an estimated 28 stevor genes within the genome.
   • The function of the stevor proteins is currently unknown.

Genome

1. The genome of Plasmodium falciparum (clone 3D7) was fully sequenced in 2002.
2. The parasite has a 23 megabase genome, divided into 14 chromosomes.
3. The genome codes for approximately 5,300 genes.
4. It is estimated 52.6% of the genome is a coding region, with 53.9% of the putative genes containing at least one intron.
5. 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%.

Proteome

1. About 60% of the putative proteins have little or no similarity to proteins in other organisms, and thus currently have no functional assignment.
2. The parasite has different subsets of its proteome expressed during various stages of its developmental cycle

Transcriptome

1. A transcriptome analysis has been conducted on the intraerythrocytic development cycle of P. falciparum. Roughly 60% of the genome is transcriptionally active during this portion of the parasite's life cycle. Whereas many genes appear to have stable mRNA levels throughout the cycle, many of the genes are transcriptionally regulated in a continuous cascade.
2. Closely adjacent genes along the chromosome do not exhibit common transcription characteristics. Thus, genes are likely individually regulated along the parasite chromosome.
3. Conversely, the apicoplast genome is polycistronic and most of its genes are coexpressed during the intraerythrocytic development cycle.