The role of the cytoskeleton in the motility of coccidian sporozoites.

The sporozoites of Eimeria tenella and Eimeria acervulina show bending, pivoting and gliding motility. All these types of motility occur intermittently and with decreasing frequency during the life of a sporozoite. Gliding is the only locomotive action expressed by these sporozoites and is only seen when the sporozoites are in contact with the substratum. All gliding sporozoites adopt a set pattern of body 'attitudes', which suggests that locomotion involves a fixed body shape. The microtubule inhibitors, colchicine, griseofulvin, vinblastine sulphate and nocodazole, have no effect on sporozoite motility. Ultrastructural examination reveals, in addition, that they have no effect on the subpellicular microtubules. The microfilament inhibitor, cytochalasin B, completely, and reversibly, inhibits pivoting and gliding but bending is only slightly depressed by the drug. High magnesium ion concentration inhibits all motility completely. The cell membrane was readily labelled with fluorescein isothiocyanate-conjugated cationized ferritin, the label was rapidly capped and shed from the posterior of the sporozoite. This capping reaction takes place only during sporozoite locomotion. The membrane label was seen to 'move' backwards realtive to the sporozoite at the same rate as the sporozoite moved forwards relative to the substratum. The substratum and the leading edge of the cap remained static relative to each other. Both capping and locomotion are sensitive to low temperature and cytochalasin B. From these results a theory of sporozoite motility is postulated. The sporozoites adhere to the substratum by surface ligands. This ligand/substratum complex is then capped along the fixed spiral of the sporozoite body by a microfilament-based contractile system. This proposed model for motility of coccidia sporozoites is consistent with all current observations on cell invasion by the sporozoa and therefore suggests that locomotion is an integral component of host cell invasion in this group of parasites.