Regulated secretion of multi-lamellar vesicles leads to formation of a tubulo-vesicular network in host-cell vacuoles occupied by <i>Toxoplasma gondii</i>

TLDR

Toxoplasma gondii invades nucleated cells and survives within a parasitophorous vacuole that it remodels by secreting proteins and forming a network of membranous tubules connected to the vacuolar membrane. The study aimed to investigate how the dense granule proteins GRA1 and GRA2 contribute to the formation of the intravacuolar network. Immunolabeling and cell fractionation were employed to track the secretion of GRA1 and GRA2 from dense granules. GRA1 is secreted as a soluble protein that associates with the network, while GRA2 is released within multi‑lamellar vesicles that assemble into the network, demonstrating a regulated exocytosis pathway that modifies the vacuole through both soluble and membrane secretion.

Abstract

ABSTRACT Toxoplasma gondii is an obligate intracellular parasite that actively invades virtually all types of nucleated cells, surviving within a specialized vacuole called the para-sitophorous vacuole. Shortly after invasion, the parasite modifies this vacuole by secreting a variety of proteins from electron-dense storage granules. Additionally, the parasite forms a network of membranous tubules within the lumen of the vacuole and connecting with the vacuolar membrane. We have used immunolabeling and cell fractionation to examine the secretion of two dense granule proteins, GRA1 and GRA2, which are involved in formation of the intravacuolar network. Following host-cell invasion, GRA1 was secreted into the lumen of the vacuole as a soluble protein that subsequently became peripherally associated with the network. In addition to being secreted as a soluble protein from dense granules, GRA2 was secreted within multi-lamellar vesicles released from a specialized posterior invagination of the parasite. The multi-lamellar vesicles assemble to form the intravacuolar network, which contains an integral membrane form of GRA2. These findings indicate that Toxoplasma has a highly developed regulated exocytosis pathway that modifies the para-sitophorous vacuole by secretion of soluble proteins and by a novel process of membrane secretion.