Correlation of structural development and differential expression of invasion-related molecules in schizonts of Plasmodium falciparum.

During asexual development Plasmodium schizonts undergo a series of complex biochemical and structural changes. Using tightly synchronized cultures of 2 P. falciparum lines (clone C10 and strain ITO4) for light microscopy and fluorescence imaging we monitored the timing and sequence of expression of proteins associated with invasion-related organelles. Antibodies to rhoptry, micronemal and dense granule proteins (Rhoptry Associated Protein 1, Apical Membrane Antigen 1, Erythrocyte Binding Antigen 175, Ring-infected Erythrocyte Surface Antigen) and to pellicle-associated proteins (Merozoite Surface Protein 1, PfMyosin-A) were used. Clone C10 developed faster than ITO4; this difference was also found in the timing of protein expression seen by immunofluorescence. Light microscopic data were combined with transmission electron microscopic analysis using serial sectioning of ITO4 schizonts to determine nuclear number and organellar development. Thus a timetable of schizont structural maturation was established. Generally, the timing of organelle-specific antigen expression correlates well with the ultrastructural data. Rhoptries are formed mainly between second and fourth nuclear divisions, micronemes between the end of the fourth nuclear division and merozoite separation from the residual body, while dense granules are generated mainly after the micronemes. PfAMA-1 appears in micronemes before EBA-175, suggesting micronemal heterogeneity.