Mobilization of intracellular calcium stimulates microneme discharge in Toxoplasma gondii

Apicomplexan parasites, including Toxoplasma gondii, apically attach to their host cells before invasion. Recent studies have implicated the contents of micronemes, which are small secretory organelles confined to the apical region of the parasite, in the process of host cell attachment. Here, we demonstrate that microneme discharge is regulated by parasite cytoplasmic free Ca2⁺ and that the micronemal contents, including the MIC2 adhesin, are released through the extreme apical tip of the parasite. Microneme secretion was triggered by Ca2⁺ ionophores in both the presence and the absence of external Ca2⁺, while chelation of intracellular Ca2⁺ prevented release. Mobilization of intracellular calcium with thapsagargin or NH₄Cl also triggered microneme secretion, indicating that intracellular calcium stores are sufficient to stimulate release. Following activation of secretion by the Ca2⁺ ionophore A23187, MIC2 initially occupied the apical surface of the parasite, but was then rapidly treadmilled to the posterior end and released into the culture supernatant. This capping and release of MIC2 by ionophore-stimulated tachyzoites mimics the redistribution of MIC2 that occurs during attachment and penetration of host cells, and both events are dependent on the actin-myosin cytoskeleton of the parasite. These studies indicate that microneme release is a stimulus-coupled secretion system responsible for releasing adhesins involved in cell attachment.