Intracellular trafficking of dense granule proteins in Toxoplasma gondii and experimental evidences for a regulated exocytosis

The dense granules of the intracellular protozoan Toxoplasma gondii are secretory vesicles that play a major role in the structural modifications of the parasitophorous vacuole (PV) in which the parasite develops. The biogenesis of dense granules as well as the regulatory mechanisms controlling their specific exocytosis are still poorly understood. In this paper, we analyzed the secretory pathway of dense granule proteins (GRA proteins) in extracellular T. gondii through the effects of brefeldin A (BFA). Ultrastructural studies of BFA-treated parasites showed disassembly of the Golgi apparatus and accumulation of GRA proteins in a dilated vacuolar system connected to the nuclear envelope. BFA reversibly blocked the intracellular transport of the newly synthesized GRA proteins in a dose-dependent manner (blockade of 95% at 1 μg/ml of BFA). By contrast, discharge of GRA proteins from preformed dense granules was unaffected by BFA over a course of 60 min incubation. GRA protein secretion was dependent on incubation temperature as it only occurred above 26°C and it could be stimulated by external factors. This stimulus might be provided by factor(s) present in the serum of the extracellular medium, as incubation of parasites in serum-free medium resulted in a dramatic decrease in protein secretion. Exocytosis can be restored in a dose-dependent fashion by serum addition (maximal stimulatory activity in the 30-200 kDa range) and was optimal at an extracellular pH of 6.5. Altogether, these results demonstrate that GRA proteins are exported through the Golgi apparatus via the classical secretory pathway and can be experimentally discharged from storage dense granules as regulated secretory proteins in response to specific stimulation, arguing in favor of a regulated component for dense granule exocytosis in T. gondii.