Coupling Polar Adhesion with Traction, Spring, and Torque Forces Allows High-Speed Helical Migration of the Protozoan Parasite Toxoplasma

Abstract

Among the eukaryotic cells that navigate through fully developed metazoan tissues, protozoans from the Apicomplexa phylum have evolved motile developmental stages that move much faster than the fastest crawling cells owing to a peculiar substrate-dependent type of motility, known as gliding. Best-studied models are the Plasmodium sporozoite and the Toxoplasma tachyzoite polarized cells for which motility is vital to achieve their developmental programs in the metazoan hosts. The gliding machinery is shared between the two parasites and is largely characterized. Localized beneath the cell surface, it includes actin filaments, unconventional myosin motors housed within a multimember glideosome unit, and apically secreted transmembrane adhesins. In contrast, less is known about the force mechanisms powering cell movement. Pioneered biophysical studies on the sporozoite and phenotypic analysis of tachyzoite actin-related mutants have added complexity to the general view that force production for parasite forward movement directly results from the myosin-driven rearward motion of the actin-coupled adhesion sites. Here, we have interrogated how forces and substrate adhesion−de-adhesion cycles operate and coordinate to allow the typical left-handed helical gliding mode of the tachyzoite. By combining quantitative traction force and reflection interference microscopy with micropatterning and expansion microscopy, we unveil at the millisecond and nanometer scales the integration of a critical apical anchoring adhesion with specific traction and spring-like forces. We propose that the acto-myoA motor directs the traction force which allows transient energy storage by the microtubule cytoskeleton and therefore sets the thrust force required for T. gondii tachyzoite vital helical gliding capacity.

Blocking palmitoylation ofToxoplasma gondiimyosin light chain 1 disrupts glideosome composition but has little impact on parasite motility

“…The data also show clearly that the inhibitory effects of the palmitoylation inhibitor 2-bromopalmitate on parasite motility are not due to changes in palmitoylation of TgMLC1, as previously hypothesized, although changes in the palmitoylation of other glideosome components could be involved (64). While the data presented here do not, by themselves, disprove the linear motor model of motility, they add to a growing list of evidence (24)(25)(26)(27)(28)(29)33) suggesting that the mechanisms underlying apicomplexan parasite motility are more complicated than what is currently encapsulated by the linear motor model. How the different motility-associated proteins of the parasite interact and work together to generate the forces necessary to drive parasite movement and whether more than one underlying mechanism exists remain important open questions for future study.…”

“…While the importance of TgMyoA, TgMLC1 and the other glideosome components in motility is well established, recent data have called into question whether they are organized and function as described by the linear motor model and/or whether alternative motility mechanisms exist (24)(25)(26)(27)(28)(29). For example, the ability of apicomplexan parasites to rock back and forth on a substrate along their anterior to posterior axis (30)(31)(32)(33)(34)(35)(36) is hard to reconcile with the linear motor model, as is the ability of parasites engineered to lack key components of the glideosome to continue moving ( (24)(25)(26); see also (37)).…”

The Riveting Cellular Structures of Apicomplexan Parasites

“…Hence, gliding of sporozoites proceeds in a stick-slip manner that sees brief high instantaneous speed peaks followed by periods of lower speed [70]. Similar peaks of instantaneous speed have been observed in T. gondii tachyzoites [72], supporting a similar mechanism between the species. Recently, our understanding of T. gondii motility was also extended through an elegant combination of live-cell imaging and detailed analysis of traction forces.…”

“…Recently, our understanding of T. gondii motility was also extended through an elegant combination of live-cell imaging and detailed analysis of traction forces. It was shown that tachyzoites can use the actinmyosin motor complex to induce increased curvature of the cortical microtubules [72]. This generates a spring-like force which powers forward motility by the parasite.…”

“…The stability of parasite microtubules appears to depend on multiple factors, including post-translational modifications of tubulin, the presence of microtubule-binding proteins (MAPs), including proteins localized within the microtubule lumen, and the anchoring of the tubules to the alveoli [26,93,96,97]. However, while the microtubules are highly stable, recently it was shown that extended contraction by the parasite's motility machinery can cause them to deform and break [72], highlighting the forces involved in parasite motility.…”