Malaria is caused by intraerythrocytic protozoan parasites belonging to Plasmodium spp. Plasmodium parasites have a complex life cycle that includes multiple stages in anopheline mosquito vectors and vertebrate hosts. During the life cycle, the parasites undergo several cycles of extreme population growth within a brief span, and this is critical for their continued transmission and a contributing factor for their pathogenesis in the host.
As with other eukaryotes, successful mitosis is an essential requirement for Plasmodium reproduction; however, some aspects of Plasmodium mitosis are quite distinct and not fully understood. In this review, we will discuss the current understanding of the architecture and key events of mitosis in Plasmodium falciparum Asexual life cycle of plasmodium sp related parasites and compare them with the traditional mitotic events described for other eukaryotes. There are four critical points in the life Asexual life cycle of plasmodium sp of Plasmodium parasites in which a small number of parasites rapidly multiply to generate much larger populations These life cycle stages are male gamete development 72sporozoite formation 513liver-stage development 68and blood-stage asexual reproduction 9 The first two of these processes occur within the mosquito vector, and the second two processes take place in the vertebrate host.
During each of these Plasmodium life cycle stages, the parasites Asexual life cycle of plasmodium sp their numbers by using serial rounds of mitosis to create multinuclear cells and then orchestrating mass cytokinesis events to release their progeny Mitosis is the process by which eukaryotic cells segregate their chromosomes in preparation for cell division 3347 To create male gametes in preparation for sexual reproduction, the parasite begins with a haploid 1n cell called a microgametocyte which is ingested by the mosquito during a blood meal 34 Over the next 3 min, these genomes separate from one another and eight new haploid 1n male gametes begin to assemble from the surface of the original cell 469 Within the mosquito midgut, a small number of the male gametes will fuse with female gametes that have also developed in this compartment, and this fusion will create diploid 2n zygotes These zygotes develop into motile ookinetes 4n 36 that ultimately become embedded in the basal lamina beneath the midgut epithelial wall as oocysts Over the course of several days, a single oocyst undergoes 10 to 11 rounds of DNA synthesis and mitosis to create a syncytial cell sporoblast with thousands of nuclei 6170 In a massive cytokinesis event, thousands of haploid 1n daughter sporozoites assemble from the surface of the mother cell 6167and these infective sporozoites then migrate to the mosquito salivary glands for transmission to the host.
Of the thousands of sporozoites that are produced from a sporoblast, only a few will be transmitted to the vertebrate host when the mosquito takes another blood meal, and an even smaller "Asexual life cycle of plasmodium sp" may reach the host liver for further development In liver-stage schizogony 45a single invading sporozoite grows as a trophozoite 1n to 2n within a liver cell.
The parasite then undergoes 13 to 14 rounds of DNA synthesis, mitosis, and nuclear division to produce a syncytial cell schizont with tens of thousands of nuclei.
From the surface of this syncytial parasite, tens of thousands 1668 of haploid 1n daughter liver-stage merozoites assemble and are eventually released into the bloodstream in parasite-filled vesicles called merosomes Once released into the bloodstream, merozoites invade red blood cells and continue to expand their numbers with blood-stage schizogony All of the clinical symptoms of malaria fever, anemia, and neurological pathologies are associated with the blood stage of the parasite life cycle In blood-stage schizogony, following invasion, a single invading merozoite begins within the red blood cell as a ring stage and progresses into a trophozoite to 2n and then undergoes three to four rounds of DNA synthesis, mitosis, and nuclear division to produce a syncytial schizont with 16 to 22 nuclei 61044 In a synchronous mass division step, approximately 22 haploid 1n daughter merozoites depending on the species assemble from the surface of the mother schizont, and with the rupture of the red blood cell, the new merozoites are released for more rounds of invasion and expansion 1044 In nonimmune hosts, blood-stage parasites may undergo uncontrolled growth unless they are restricted by innate and adaptive immune responses.
Splenic clearance is considered a major mechanism of this parasite growth regulation. However, the parasite has developed highly sophisticated mechanisms to evade immune-mediated clearance, such as expressing variant antigens on the surface of infected red cells to sequester them on endothelial cells in different organs.
Nonetheless, in both immune and nonimmune hosts, the parasite burden can be maintained by repeated cycles of asexual schizogony from only a few intraerythrocytic parasites. Thus, the malaria parasite builds large populations from a relatively small number of founding members in every major stage of its development, and each time, the parasite relies on serial mitosis to accomplish this growth. These growth periods enable parasite transmission, and as a by-product, the toxins released by rupturing schizonts help to fuel the pathogenic symptoms of the disease.
Similar reproductive strategies have been described for other apicomplexans, such as Sarcocystis 75 This paper highlights some of the distinctive features of mitosis in Plasmodium parasites with a special emphasis on the mitotic spindle and microtubule organizing centers MTOCsand we will compare them to some of the visual hallmarks of traditional mitosis that are often associated with higher eukaryotes. Historically, mitosis has been divided into stages that visually chronicle chromosome movement and nuclear membrane dynamics prophase, metaphase, anaphase, telophase ; however, it is clear that there is a rich diversity in the methods that eukaryotes use to accomplish mitosis 1833 Depending on the particular species and cell type, chromosomes may condense into discrete structures, or they may remain dispersed over large areas.
Nuclear membranes may completely disperse, partially disperse, or appear to Asexual life cycle of plasmodium sp completely intact throughout mitosis Microtubule organizing centers Asexual life cycle of plasmodium spthe complex structures that nucleate and help organize microtubule arrays, may assume a wide variety of intricate conformations that can persist in the cytoplasm or may be found embedded within membranes. These variations can be particularly striking in fungi and in protozoan species such as Plasmodium spp.
At the onset of traditional mitosis prophasechromosomes begin to condense into discrete structures within the nucleus and the MTOC duplicates within the cytoplasm Fig. The mammalian MTOCs, called centrosomes, are complex structures consisting of a central pair of cylindrical structures centrioles that are tethered to one another within a protein matrix in the cytoplasm 8. Since these MTOCs and their microtubules are usually cytoplasmic, the nuclear membrane must disassemble before the microtubules can contact the chromosomes.
Depending on the cell type, the nuclear membrane may disperse partially, or it may disperse completely Fig. The main points of contact between the chromosomes and the microtubules are protein structures on the chromosomes called kinetochores. Comparison of traditional views of mitosis with current knowledge of mitosis Asexual life cycle of plasmodium sp blood-stage Plasmodium parasites. A schematic cartoon of mitosis indicating plasma membranes black linesmicrotubule organizing centers MTOCs, red circlesmicrotubules green lineskinetochores tan ovalsnuclear membranes dark blue linescondensed chromosomes light blueand uncondensed chromosomes light blue with stipple pattern.
In early traditional mitosis prophasechromosomes begin to condense within the nuclear membrane, and cytoplasmic microtubules are nucleated by "Asexual life cycle of plasmodium sp" cytoplasmic MTOCs.
In early Plasmodium mitosis, chromosomes remain uncondensed, the two MTOCs are embedded in the nuclear membrane, and microtubules begin to form inside the nucleus. In traditional mitosis, the nuclear membrane has disassembled, condensed chromosomes are attached to the bipolar Asexual life cycle of plasmodium sp microtubules through the kinetochores, and the kinetochores are aligned at the metaphase plate. In Plasmodium mitosis, the nuclear membrane remains intact, the chromosomes remain uncondensed, and the kinetochores are captured by the bipolar mitotic spindle forming inside the nucleus.
In traditional mitosis, sister chromatids and their associated kinetochores separate and begin to move to opposite poles of the spindle. In Plasmodiumsister kinetochores separate and migrate to opposite spindle poles while the sister chromatids remain uncondensed. In traditional views, nuclear membranes assemble around each daughter genome, chromosomes begin to decondense, and the cell begins to divide.
In Plasmodium blood-stage schizogony, the nuclear membrane divides to separate daughter genomes and the cell does not divide until several cycles of mitosis have produced a multinuclear cell. These ordered events of mitosis are controlled by complex networks of regulatory proteins, structural scaffolds, motor proteins, and proteases 84751 Some of these networks, called checkpoints, can delay the progression of the cell cycle until the critical elements in a particular step have been accomplished.
Checkpoints help control the timing of the start of mitosis, the assembly of the spindle, and its coordination with DNA replication, and by doing so they serve important quality control functions for the cell 48 Since mitotic events can take place in various forms in different cells, the components of these regulatory networks can also serve as precise benchmarks for cell cycle progression, provided that these components are identified and their functions defined The uncondensed chromosomes of Plasmodium spp.
The mitotic MTOCs of Plasmodium parasites also known as kinetic centers, centriolar plaques, or centrosome equivalents are electron-dense plaques that appear to be embedded in pores of the nuclear membrane with one face exposed to the nuclear interior and the other face exposed to the cytoplasm 11 The spindle microtubules that form inside the parasite nucleus are anchored to the nuclear face of these MTOCs Fig.
Thus, an early hallmark of mitosis in Plasmodium spp. However, it is not clear whether the parasite kinetochores align in a single midpoint plane of the Asexual life cycle of plasmodium sp, and so far, it is not clear whether the parasites use a spindle assembly to control the timing of mitosis exit similarly to mammalian cells.
In the related coccidian parasites Toxoplasma and Eimeriathe mitotic spindle forms through a cone-shaped extension of the nuclear membrane called the centrocone 223075 ; however, the nuclear membrane also remains largely intact in these organisms.
Thus, at the conclusion of mitosis, organisms that retain their nuclear envelope closed mitosissuch as Plasmodium spp. Interpretation of hemispindle and full mitotic spindle development in Plasmodium from TEM. Images of mitosis during sporogony in P. A A single hemispindle. C Formation of a full mitotic spindle. Microtubules F and F. Reprinted from reference 66 with permission of the publisher.