Basal body structure and composition in the apicomplexans Toxoplasma and Plasmodium

Abstract

The phylum Apicomplexa encompasses numerous important human and animal disease-causing parasites, including the Plasmodium species, and Toxoplasma gondii, causative agents of malaria and toxoplasmosis, respectively. Apicomplexans proliferate by asexual replication and can also undergo sexual recombination. Most life cycle stages of the parasite lack flagella; these structures only appear on male gametes. Although male gametes (microgametes) assemble a typical 9+2 axoneme, the structure of the templating basal body is poorly defined. Moreover, the relationship between asexual stage centrioles and microgamete basal bodies remains unclear. While asexual stages of Plasmodium lack defined centriole structures, the asexual stages of Toxoplasma and closely related coccidian apicomplexans contain centrioles that consist of nine singlet microtubules and a central tubule. There are relatively few ultra-structural images of Toxoplasma microgametes, which only develop in cat intestinal epithelium. Only a subset of these include sections through the basal body: to date, none have unambiguously captured organization of the basal body structure. Moreover, it is unclear whether this basal body is derived from pre-existing asexual stage centrioles or is synthesized de novo. Basal bodies in Plasmodium microgametes are thought to be synthesized de novo, and their assembly remains ill-defined. Apicomplexan genomes harbor genes encoding δ- and ε-tubulin homologs, potentially enabling these parasites to assemble a typical triplet basal body structure. Moreover, the UNIMOD components (SAS6, SAS4/CPAP, and BLD10/CEP135) are conserved in these organisms. However, other widely conserved basal body and flagellar biogenesis elements are missing from apicomplexan genomes. These differences may indicate variations in flagellar biogenesis pathways and in basal body arrangement within the phylum. As apicomplexan basal bodies are distinct from their metazoan counterparts, it may be possible to selectively target parasite structures in order to inhibit microgamete motility which drives generation of genetic diversity in Toxoplasma and transmission for Plasmodium.

Keywords: Centriole; Coccidia; Flagellum; Malaria; Microgamete; Microtubule organizing center.

Fig. 1

Life cycle and microtubule-based structures of apicomplexa. a–e Simplified schematic of the life cycle of Apicomplexa in their different hosts. Apicomplexa replicate either sexually or asexually. Differentiation into gametes and sexual replication occur within definitive hosts. Definitive hosts vary among apicomplexan species; T. gondii replicates sexually within felines, while Plasmodium species do so in mosquitoes. Flagellated forms of Apicomplexa are only found in definitive hosts, where they differentiate into male (micro) and female (macro) gametes. Fusion of gametes gives rise to a zygote which further differentiates into oocysts able to sporulate. Microgametes of different Apicomplexa vary in their number of flagella. T. gondii microgametes, represented here, have two protruding flagella. Plasmodium spp. microgametes emerge with a single flagellum upon terminal differentiation, and are assembled entirely within the cytoplasm of the undifferentiated originating cell. a, b In intermediate hosts, such as humans, apicomplexans grow vegetatively. Distinct replication modes among Apicomplexa allow them to adapt to different host niches. However, they all generate new infective zoites by assembly of daughter cells within the mother cell´s cytosol or at the mother cell surface, and undergo closed mitosis of the nuclear content. c Infective forms of Apicomplexa organize microtubules using functionally and physically distinct MTOCs. subpellicular microtubules, which impart shape and polarity to the cells, are organized by an MTOC localized at the apex, known as the APR. In addition, coccidian species in the phylum contain a specialized tubulin-based structure known as the conoid which has been evolutionarily linked to basal bodies of related flagellated alveolates [51, 52, 56]. Nuclear division occurs by closed mitosis. Chromosomes are organized by an intra-nuclear spindle nucleated by a cytosolic centrosome. Apicomplexa centriole-based centrosomes contain two centrioles of 9+1 singlet microtubule structure, oriented parallel to each other. Malaria-causing parasites (Plasmodium spp.) do not have canonical centrosomes, and organize their mitotic spindle from a “centriolar plaque” which can be identified using anti-centrin antibodies. The centriolar plaque is embedded in the nuclear envelope (not shown). d, e Microgamete flagella and basal body structures. Apicomplexa flagellar axonemes are composed of 9 doublet microtubules and a central pair [–18]. d Basal bodies in malaria are better characterized, and consist of nine single A-tubules with no central tube, embedded in an electron-dense mass [16]. e Basal body structures are not well characterized in T. gondii. A small number of ultra-structural studies have led researchers to propose multiple alternative microtubule arrangements; a nine singlet microtubules, and a central tubule [20], atypical 9+0 and 9+2 arrangements, or a typical triplet microtubule structure with ninefold symmetry [, , –26]

Fig. 2

Basal Body and axoneme structures in Plasmodium, Toxoplasma, and Sarcocystis. a Serial transverse sections of the developing axoneme of a P. falciparum microgamete. From left to right: a basal body made of singlet microtubules, and embedded in an electron-dense mass and the distal flagellar region made of a 9+2 microtubule arrangement, can be observed. Reproduced from Fig. 1f in [16] with permission. b Transverse section through the pole (P) of a developing T. gondii microgamete. Several organelles are visible including a basal body (B) and the nucleus (N). c Longitudinal section through a T. gondii microgamete where one flagellum and its originating basal body (B) are visible. d Longitudinal section through a T. gondii microgamete where two flagella, and their originating basal bodies (B), are visible. b–d Reproduced from Fig. 2a, b, and d in [17], respectively. e Longitudinal section through the anterior portion of a T. gondii microgamete showing the position of the basal body (B) and flagella (FL) enclosed by a unit membrane (UM). Reproduced from Fig. 25 in [21]. f Transverse section through a basal body of a Sarcocystis suihominis microgamete, a coccidian parasite closely related to T. gondii, showing some microtubules doublets and triplets (arrow, B). g Transverse section through two flagellar axonemes (arrow, F) of S. suihominis. f, g Reproduced from Figs. 9 and 10 from [23], respectively