An endocytic-secretory cycle participates in Toxoplasma gondii in motility

Abstract

Apicomplexan parasites invade host cells in an active process involving their ability to move by gliding motility. While the acto-myosin system of the parasite plays a crucial role in the formation and release of attachment sites during this process, there are still open questions regarding the involvement of other mechanisms in parasite motility. In many eukaryotes, a secretory-endocytic cycle leads to the recycling of receptors (integrins), necessary to form attachment sites, regulation of surface area during motility, and generation of retrograde membrane flow. Here, we demonstrate that endocytosis operates during gliding motility in Toxoplasma gondii and appears to be crucial for the establishment of retrograde membrane flow, because inhibition of endocytosis blocks retrograde flow and motility. We demonstrate that extracellular parasites can efficiently incorporate exogenous material, such as labelled phospholipids, nanogold particles (NGPs), antibodies, and Concanavalin A (ConA). Using labelled phospholipids, we observed that the endocytic and secretory pathways of the parasite converge, and endocytosed lipids are subsequently secreted, demonstrating the operation of an endocytic-secretory cycle. Together our data consolidate previous findings, and we propose an additional model, working in parallel to the acto-myosin motor, that reconciles parasite motility with observations in other eukaryotes: an apicomplexan fountain-flow-model for parasite motility.

Rab11A regulates the constitutive secretory pathway duringToxoplasma gondiiinvasion of host cells and parasite replication

“…This may reflect a distinct timing of protein synthesis and vesicle release from the Golgi to the PM but it also suggests the existence of different regulatory pathways for the trafficking of protein localized at the PM versus proteins secreted into the vacuolar space. In particular, transmembrane proteins may be actively recycled during parasite division as suggested in a previous study on the retromer subunit TgVPS35 [37] and more recently during extracellular parasite motility [38]. Thus, our data indicate a broader role of Rab11A-mediated exocytosis for the delivery of proteins at the PM and for the release of DG proteins into the vacuolar space during parasite replication.…”

“…Therefore, one may envision that material internalized from the PM reaches this hydrid TGN/ELC compartment before being re-directed to other target membranes, such as the rhoptries, the PM and the degradative vacuole (VAC). Such a recycling process has been recently observed during extracellular parasite motility [38]. Recycling of mother material during daughter cell emergence may also follow this indirect secretory pathway, while de novo synthetized proteins may traffic via the direct TGN to PM route.…”

The Sec1/Munc18‐like proteins TgSec1 and TgVps45 play pivotal roles in assembly of the pellicle and sub‐pellicle network in Toxoplasma gondii

“…These findings demonstrate that anterograde transport pathways regulate organelle assembly during generation of daughter parasites in T. gondii. On the other hand, T. gondii endocytosis has been convincingly demonstrated recently (Gras et al, 2019). A parasite SNARE homolog of syntaxin-6 (TgStx6) and a retromer protein (TgVps35) are implicated in retrograde transport between the TGN and endosomal-like compartment (ELC) (Jackson et al, 2013;Sangaré et al, 2016).…”

Apicomplexan F‐actin is required for efficient nuclear entry during host cell invasion

“…According to the Glideosome model, the force generated for motility and invasion relies exclusively on F-actin polymerised at the apical tip of the parasite by the action of Formin-1 and translocated within the narrow space (~30 nm) between the IMC and PM of the parasite [11]. However, recent studies suggest that the parasite can also use other motility systems, such as a secretory-endocytic cycle that produces retrograde membrane flow [12], similar to the fountain flow model suggested for other eukaryotes [13,14].…”