Apicomplexa Cell Cycles: Something Old, Borrowed, Lost, and New

Abstract

Increased parasite burden is linked to the severity of clinical disease caused by Apicomplexa parasites such as Toxoplasma gondii, Plasmodium spp, and Cryptosporidium. Pathogenesis of apicomplexan infections is greatly affected by the growth rate of the parasite asexual stages. This review discusses recent advances in deciphering the mitotic structures and cell cycle regulatory factors required by Apicomplexa parasites to replicate. As the molecular details become clearer, it is evident that the highly unconventional cell cycles of these parasites is a blending of many ancient and borrowed elements, which were then adapted to enable apicomplexan proliferation in a wide variety of different animal hosts.

Keywords: Apicomplexa; Toxoplasma gondii; cell cycle; cyclin-dependent kinase; evolution; mitosis.

Figure 1.. Evolutionary History of Apicomplexa.

(A) Phylogenetic relationships of major eukaryotic groups are shown [2,73]. Two bikont groups acquired piastids, Archaeplastida by primary endosymbiosis (1°ES, green arrow) of ancestral cyanobacterium, and Chromalveolates by secondary endosymbiosis (2°ES, red arrow) of red algae [73]. Please note that schematics indicate the earliest predicted time of the secondary endosymbiosis, while the number and order of tertiary events are still a topic of discussion (for more details see recent reviews [74,75]). Common nuclear functions of modern eukaryotes were present in the last eukaryotic common ancestor (LECA) [5,73]. Adoption of multinuclear replication, such as schizogony, occurred in the proto-Apicomplexa ancestor (asterisks) prior to the switch to obligate parasitism [7,8]. Genetic reduction is the major feature of the Apicomplexa ancestor switch to intracellular parasitism [3]. Well-studied conventional eukaryotic cell cycles that obey ‘once only’ rule and complete cytokinesis following chromosome segregation are indicated in blue, while eukaryotic model systems (e.g., T. gondli), where unconventional cell cycles are utilized, are indicated in red. (B) Red alga and Apicomplexa spindle pole complexes have many morphogenetic similarities indicating a potential common evolutionary history. Schematic shows pre-prophase mitotic structures of T. gondii (Apicomplexa) and Apoglossum ruscifolium (Rhodophyta). The Toxoplasma centrocone is a composed image of the numerous electron microscopy studies. Note that the inner core was only detected by immunofluorescent microscopy [13,27]. The mitotic structure of the red algae was sketched from images 2 and 3 of the study by Dave and Godward, 1982 [30]. During mitosis, nuclear membrane protrusions (centrocone in the Apicomplexa) are initiated at cytoplasmic protein complexes (inner core versus polar organelle). Local fenestra then form at the nuclear protrusions (semi-closed mitosis) through which spindle microtubules extend into the nucleus and attach to segregating sister chromatids.

Figure 2.. Schematics of the Apicomplexan (A) Compared To Conventional (B) Cell Cycles.

In the conventional cell cycle, chromosome replication (S) and segregation (M, mitosis) phases are separated by two growth phases, G1 and G2. The diagram shows major checkpoints regulating somatic cell division that ensure successful completion of key events such as sufficient cell growth, complete replication of chromosomes, and the secure attachment of chromosomes to the spindle poles (see details in Box 2). The conventional cell cycle is governed by the ‘once only’ rule of chromosome replication that is functionally linked to centrosome duplication shown in the diagram as a centriolar pair. By contrast, the apicomplexan cell cycle is more complex and has different regulatory points. At the G1/S phase transition, parasites may choose one of two different chromosome cycles. They can enter either a nuclear cycle (blue arrows) where chromosome replication is not accompanied by budding or a budding cycle (black arrows) where chromosome replication and segregation is synchronized with the assembly of the daughter buds. Although signals that direct parasites into each type of cycle are largely unknown, recent studies of Toxoplasma gondii tachyzoites demonstrated that a bipartite centrosome plays an important role in the decision. Results indicate that an active outer core (green) favors the budding cycle route. Many apicomplexan parasites utilize the nuclear cycle (e.g., Plasmodium spp. schizogony) to significantly increase the number of the parasite progeny from a single infection event, including Toxoplasma merozoites (endopolygeny) in the cat life cycle. Note that mitosis of the nuclear cycle of the Sarcocystis neurona endopolygeny lacks telophase. Thus, S. neurona nucleus division occurs only in the budding cycle. Additional regulatory points evolved to regulate these complex parasite cell cycles (bolded letters), including reduplication of the centrosome in mitosis and assembly of the daughter bud cytoskeleton.

Figure 3.. Schematics of the Prototypical Eukaryotic Cell Cycle and Key Regulatory Mechanisms.

To highlight differences in the number and composition of controls, we superimposed the recent findings from apicomplexan Toxoplasma gondii onto the conventional cell cycle model of such extensively studied opisthokont organisms as animals and fungi. The schematic shows only key regulators of the complex network governing progression through specific cell cycle phases (color bars on the top). Toxoplasma factors orthologous to opisthokont regulators identified by pBLAST search as well as conserved processes are shown as filled shapes (‘old’). Missing factors indicated as open shapes (‘lost’) would be candidates for analogs (‘new’) or adapted factors (‘borrowed’). Identifiers of the central kinases and cyclins are shown in the corresponding tables below. Opisthokont regulators are listed along with factors detected in the apicomplexan ancestors, Chromerids (Chromera velia). The Apicomplexa phylum is represented by the well-studied T. gondii tachyzoite model. Our analysis revealed that novel apicomplexan central kinase/cyclin complexes also lack expected immediate regulators of opisthokont eukaryotes such as G1 Cdk inhibitors (INK, Kip, Cip), Cdc25 phosphatases, and Wee1 kinase [62], consistent with cell cycle mechanisms evolving as a unit. Apicomplexan mitotic Crks are missing cognitive cyclins. Note that, unlike opisthokont Cdks, multiple apicomplexan Crks are nonredundant, indicating that the true master regulator of the apicomplexan cell cycle is not yet identified.