Abstract
BackgroundIn the past decades, many studies focused on the cell motility of apicomplexan invasive stages as they represent a potential target for chemotherapeutic intervention. Gregarines (Conoidasida, Gregarinasina) are a heterogeneous group that parasitize invertebrates and urochordates, and are thought to be an early branching lineage of Apicomplexa. As characteristic of apicomplexan zoites, gregarines are covered by a complicated pellicle, consisting of the plasma membrane and the closely apposed inner membrane complex, which is associated with a number of cytoskeletal elements. The cell cortex of eugregarines, the epicyte, is more complicated than that of other apicomplexans, as it forms various superficial structures.ResultsThe epicyte of the eugregarines, Gregarina cuneata, G. polymorpha and G. steini, analysed in the present study is organised in longitudinal folds covering the entire cell. In mature trophozoites and gamonts, each epicytic fold exhibits similar ectoplasmic structures and is built up from the plasma membrane, inner membrane complex, 12-nm filaments, rippled dense structures and basal lamina. In addition, rib-like myonemes and an ectoplasmic network are frequently observed. Under experimental conditions, eugregarines showed varied speeds and paths of simple linear gliding. In all three species, actin and myosin were associated with the pellicle, and this actomyosin complex appeared to be restricted to the lateral parts of the epicytic folds. Treatment of living gamonts with jasplakinolide and cytochalasin D confirmed that actin actively participates in gregarine gliding. Contributions to gliding of specific subcellular components are discussed.ConclusionsCell motility in gregarines and other apicomplexans share features in common, i.e. a three-layered pellicle, an actomyosin complex, and the polymerisation of actin during gliding. Although the general architecture and supramolecular organisation of the pellicle is not correlated with gliding rates of eugregarines, an increase in cytoplasmic mucus concentration is correlated. Furthermore, our data suggest that gregarines utilize several mechanisms of cell motility and that this is influenced by environmental conditions.
Highlights
Apicomplexans are one of the most successful and diverse groups of eukaryotic unicellular parasites that exhibit unique adaptations to life in a wide spectrum of vertebrate and invertebrate hosts
Infective stages of Apicomplexa are characterised by an apical complex of organelles as well as a complicated cell cortex consisting of cortical alveoli, i.e., flattened vesicles limited by a membrane and packed into a continuous layer, underlying the plasma membrane
The protomerite of G. steini did not show any changes during gliding; only a slight bending of the gamont deutomerite, usually in its posterior half, was observable when turning to the side
Summary
Apicomplexans are one of the most successful and diverse groups of eukaryotic unicellular parasites that exhibit unique adaptations to life in a wide spectrum of vertebrate and invertebrate hosts. Infective stages of Apicomplexa are characterised by an apical complex of organelles as well as a complicated cell cortex consisting of cortical alveoli, i.e., flattened vesicles limited by a membrane and packed into a continuous layer (inner membrane complex), underlying the plasma membrane. The invasive zoites of Apicomplexa are motile and use actinbased gliding for host invasion and tissue traversal. This gliding mechanism called ‘glideosome’ was first described for Toxoplasma [4] and has been extended as a concept to sporozoites of Plasmodium [5] and other apicomplexans [6]. As characteristic of apicomplexan zoites, gregarines are covered by a complicated pellicle, consisting of the plasma membrane and the closely apposed inner membrane complex, which is associated with a number of cytoskeletal elements. The cell cortex of eugregarines, the epicyte, is more complicated than that of other apicomplexans, as it forms various superficial structures
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