Effect of jasplakinolide and cytochalasin D on cortical elements involved in the gliding motility of the eugregarine Gregarina garnhami (Apicomplexa)

Ultrastructure of Myzocytosis and Cyst Formation, and the Role of Actin in Tubular Tether Formation in Colpodella sp. (ATCC 50594)

Free-living relatives of the Apicomplexa such as Colpodella species, Alphamonas species, and Voromonas pontica are predators that prey on ciliate, bodonid, and algal prey using the process of myzocytosis. During myzocytosis, the pseudoconoid is used to attach to the prey leading to aspiration of cytoplasmic contents of the prey into a posterior food vacuole formed in the predator, aided by secretions from the apical complex organelles. The conoid and associated proteins are conserved among the apicomplexa. However, the organization and function of the pseudoconoid during myzocytosis are not well understood. In this study, we investigated the morphology and ultrastructure of Colpodella sp. (ATCC 50594) during the stages of myzocytosis and cyst formation in the life cycle using light microscopy and transmission electron microscopy (TEM) in order to identify the organization of the tubular tether involved in nutrient aspiration by Colpodella sp. Tubular tethers of varying lengths were identified by light microscopy. We report that initial contact by Colpodella sp. trophozoites with Parabodo caudatus prey is by an area posterior to the apical tip of the rostrum that engulfs the membrane of the prey pulling it into the cytoplasm of the predator. The tubular tether that forms contains membranes of both predator and prey and is facilitated by microtubule organization and the cytoskeleton at the point of contact. Cytochalasin D treatment of diprotist cultures resulted in morphological distortions of trophozoites and the tubular tether suggesting a role of actin in the formation of the tubular tether. This mechanism of predation may provide insight into the mode of invasion observed in pathogenic apicomplexan zoites during host cell entry.

Nutrient Acquisition and Attachment Strategies in Basal Lineages: A Tough Nut to Crack in the Evolutionary Puzzle of Apicomplexa

Apicomplexa are unicellular eukaryotes that parasitise a wide spectrum of invertebrates and vertebrates, including humans. In their hosts, they occupy a variety of niches, from extracellular cavities (intestine, coelom) to epicellular and intracellular locations, depending on the species and/or developmental stages. During their evolution, Apicomplexa thus developed an exceptionally wide range of unique features to reach these diversified parasitic niches and to survive there, at least long enough to ensure their own transmission or that of their progeny. This review summarises the current state of knowledge on the attachment/invasive and nutrient uptake strategies displayed by apicomplexan parasites, focusing on trophozoite stages of their so far poorly studied basal representatives, which mostly parasitise invertebrate hosts. We describe their most important morphofunctional features, and where applicable, discuss existing major similarities and/or differences in the corresponding mechanisms, incomparably better described at the molecular level in the more advanced Apicomplexa species, of medical and veterinary significance, which mainly occupy intracellular niches in vertebrate hosts.

Motility and cytoskeletal organisation in the archigregarine Selenidium pygospionis (Apicomplexa): observations on native and experimentally affected parasites

Representatives of Apicomplexa perform various kinds of movements that are linked to the different stages of their life cycle. Ancestral apicomplexan lineages, including gregarines, represent organisms suitable for research into the evolution and diversification of motility within the group. The vermiform trophozoites and gamonts of the archigregarine Selenidium pygospionis perform a very active type of bending motility. Experimental assays and subsequent light, electron, and confocal microscopic analyses demonstrated the fundamental role of the cytoskeletal proteins actin and tubulin in S. pygospionis motility and allowed us to compare the mechanism of its movement to the gliding machinery (the so-called glideosome concept) described in apicomplexan zoites. Actin-modifying drugs caused a reduction in the movement speed (cytochalasin D) or stopped the motility of archigregarines completely (jasplakinolide). Microtubule-disrupting drugs (oryzalin and colchicine) had an even more noticeable effect on archigregarine motility. The fading and disappearance of microtubules were documented in ultrathin sections, along with the formation of α-tubulin clusters visible after the immunofluorescent labelling of drug-treated archigregarines. The obtained data indicate that subpellicular microtubules most likely constitute the main motor structure involved in S. pygospionis bending motility, while actin has rather a supportive function.