Endocytosis at the micropore of Toxoplasma gondii.

Ultrastructural studies were undertaken to reexamine the structure and function of the micropore of Toxoplasma gondii. By incubating tachyzoites with the tracer horseradish peroxidase (HRP), we showed for the first time cytochemically that an extracellular tracer was internalized into vacuoles at the micropore. Our morphological observations also demonstrated that the base of the micropore in both tachyzoites and bradyzoites was sometimes covered by a clathrin-like bristle coat. A coated vesicle was observed in continuity with a bradyzoite micropore, and large (150-nm) coated vesicles were occasionally present just beneath the micropore. These results suggest that receptor-mediated endocytosis occurs at the micropore. In other micrographs, however, the micropore appeared uncoated. In some bradyzoites, the uncoated micropore was greatly dilated, and it contained vesicles like those found in the cyst matrix associated with debris from disintegrated parasites. We had previously observed such debris from fragmented organisms in cysts prepared in vivo. These results indicate that residues from dead bradyzoites may provide nutrients for younger, developing parasites in the same cysts. Our observations also suggest that either receptor-mediated or bulk endocytosis can occur at the micropore, perhaps depending upon the availability of specific ligands. Investigation of a receptor-mediated pathway may reveal a means for targeting therapy selectively to the parasites to benefit patients with disseminated toxoplasmosis.