Phosphatidylinositol-4-phosphate signaling regulates dense granule biogenesis and maturation in Toxoplasma gondii

Phosphoinositide metabolism defines the foundation of a major signaling pathway that is conserved throughout the eukaryotic kingdom. Although the 4-OH phosphorylated phosphoinositides phosphatidylinositol-4-phosphate (PtdIns4P) and phosphatidylinositol-4,5-bisphosphate are each individually required for the viability of all eukaryotic cells studied thus far, their activities in parasite biology are less well understood. Using intracellular tachyzoites of the apicomplexan parasite Toxoplasma gondii as model for studying PtdIns4P signaling in a protozoan, we demonstrate the presence of PtdIns4P pools in Golgi/trans-Golgi (TGN) system and in post-TGN compartments of the parasite. Moreover, using a combination of super-resolution confocal microscopy and correlative light electron microscopy, we show that deficits in PtdIns4P signaling result in structural perturbation of compartments that house dense granule cargo with accompanying deficits in dense granule exocytosis. Taken together, the data report a direct role for PtdIns4P in dense granule biogenesis and exocytosis. The data further suggest that the biogenic pathway for secretion-competent dense granule formation in T. gondii is more complex than simple budding of fully matured dense granules from the TGN.