Isabelle Tardieux

The pharmacological targeting of apicomplexan myosins has emerged as a validated antiparasitic strategy, with selective inhibitors of MyoA demonstrating activity against parasite motility and invasion. However, apicomplexan parasites express multiple unconventional myosins with essential functions throughout their life cycles that remain unexplored pharmacologically. Here, we investigated pentachloropseudilin (PCIP), a reversible allosteric inhibitor characterized against class-I myosins, although canonical class-I myosins are absent from Apicomplexa. Structural modeling predicted PCIP-potentially compatible allosteric pockets in several parasite myosins, including MyoA, MyoF and MyoJ, suggesting interference with multiple myosin-dependent processes. To specifically resolve PCIP-induced phenotypes across the tachyzoite lytic cycle, we combined complementary high-content and high-resolution approaches, including real-time live-cell imaging, quantitative assays and ultrastructure expansion microscopy. This identified distinct steps at which parasite progression was impaired. PCIP rapidly and reversibly inhibited gliding motility and host-cell invasion in a dose-dependent manner within the submicromolar range. A pronounced effect was observed during intracellular development, where PCIP inhibited proliferation and arrested parasites near the onset of daughter-cell budding. Live imaging and expansion microscopy revealed failure to assemble daughter inner membrane complex scaffolds and subpellicular microtubule arrays, and to replicate apicoplast, consistent with disruption of MyoF-dependent processes. This arrest was not simply cytostatic, as even short PCIP exposure prevented a substantial proportion of intracellular tachyzoites from resuming replication following drug removal. At lower concentrations permitting daughter-cell formation, progeny displayed defective final maturation and frequently lost plasma-membrane integrity within an intact parasitophorous vacuole, preventing egress consistent with interference with MyoJ-dependent processes. Together, these findings reveal multiple points of PCIP-induced failure throughout the T. gondii lytic cycle and associate distinct phenotypes with MyoA-, MyoF- and MyoJ-dependent functions. They establish PCIP as a chemical probe for dissecting apicomplexan myosin-dependent processes and support multi-myosin interference as a promising antiparasitic strategy. Abstract Figure:

The French Society for Cell Biology (SBCF) gathers all researchers working in the broad field of cell biology and is actively involved in several missions. Indeed, from communicating about the latest breakthroughs to announcing upcoming events, the SBCF also identifies and nurtures emerging talents while financially supporting young researchers to help them attend scientific meetings. The society's mission is to promote the scientific area, to boost knowledge transmission, and be the reference for French research in cell biology on the international stage. In this context, the SBCF has woven networks of knowledge and expertise with global Societies of cell biology and continues to foster partnerships specifically through joint sessions at international meetings. In the same vein, since 2022, the SBCF has been organizing an annual themed international symposium called “Cell Biology of…” to spotlight specific topics of interest. Past editions have highlighted subjects such as the coronaviruses, neurons, and plants, showcasing the broad scope of the discipline. In addition to these scientific initiatives, the SBCF is also very much committed to supporting and regularly organizing a variety of events for raising the public awareness of science in general and cell biology in particular, and engaging it in the wonders of science.

Toxoplasma gondii is a protozoan apicomplexan parasite that uses an adhesion-dependent mode of motility termed gliding to access host cells and disseminate into tissues. Previous studies on Apicomplexa motile morphotypes, including the T. gondii tachyzoite, have identified a cortical actin–myosin motor system that drives the rearward translocation of transmembrane adhesins, thus powering forward movement. However, this model is currently questioned. Here, combining micropatterning and tunable surface chemistry (to edit parasite surface ligands) with flow force and live or super-resolution imaging, we show that tachyzoites build only one apical anchoring contact with the substrate, over which it slides. Furthermore, we show that glycosaminoglycan–parasite interactions are sufficient to promote such force-productive contact and find that the apicobasal flow is set up independent of adhesin release and surface interactions. These findings should enable further characterization of the molecular functions at the T. gondii–substrate mechanosensitive interface and their comparison across apicomplexans.

The French Society for Cell Biology (SBCF) is actively involved in communicating the latest advances and organizing scientific events, as well as supporting young researchers, in this field. The SBCF also supports and organizes outreaching activities designed to raise public awareness of science in general and cell biology in particular. The Society, in its present form, was founded in 1984. To mark this milestone, we are organizing a memorable symposium hosted by the Académie des Sciences (https://sbcf.fr/en/event/symposium-des-40-ans-de-la-sbcf/) on September 10, 2024.

Toxoplasma gondii is a protozoan parasite that has evolved a developmental morphotype called tachyzoite that navigates between cells and moves in and out of them in a wide repertoire of homeothermic hosts. Relying on a uniquely constant apicobasal bipolarity coupled to an actomyosin-driven retrograde surface flow, the tachyzoite has elaborated a molecular machinery to assemble transient anchoring contacts with the environment, which support the traction force required to power a typical helical gliding motility. Combining micropatterning with live, reflection interference contrast and expansion microscopies, we bring first nanoscale evidence that the tachyzoite needs to build only one apical anchoring contact with the substrate, thus spatially defining a minimal force transmission platform over which it can slide. We uncover that the apicobasal driven surface flow is set up in response to extracellular biochemical cues independent of adhesin release and tachyzoite-surface interactions, hence prior to motile activity. Furthermore, to identify the minimal adhesion requirements for helical gliding at the level of individual molecular species, we combine biochemical and biophysical quantitative assays based on tunable surface chemistry and quartz crystal microbalance with dissipation monitoring. These approaches uncover that glycosaminoglycan (GAG)-parasite interactions are sufficient to promote a productive contact for helical gliding and pave the way for the characterization of the structure and density of the molecules functionally engaged at this essential parasite-substrate mechanosensitive interface.

The protozoa, Toxoplasma gondii and Plasmodium spp., are preeminent members of the Apicomplexa parasitic phylum in large part due to their public health and economic impact. Hence, they serve as model unicellular eukaryotes with which to explore the repertoire of molecular and cellular strategies that specific developmental morphotypes deploy to timely adjust to their host(s) in order to perpetuate. In particular, host tissue‐ and cell‐invasive morphotypes termed zoites alternate extracellular and intracellular lifestyles, thereby sensing and reacting to a wealth of host‐derived biomechanical cues over their partnership. In the recent years, biophysical tools especially related to real time force measurement have been introduced, teaching us how creative are these microbes to shape a unique motility system that powers fast gliding through a variety of extracellular matrices, across cellular barriers, in vascular systems or into host cells. Equally performant was this toolkit to start illuminating how parasites manipulate their hosting cell adhesive and rheological properties to their advantage. In this review, besides highlighting major discoveries along the way, we discuss the most promising development, synergy, and multimodal integration in active noninvasive force microscopy methods. These should in the near future unlock current limitations and allow capturing, from molecules to tissues, the many biomechanical and biophysical interplays over the dynamic host and microbe partnership.

T. gondii is a eukaryotic parasite that has evolved a stage called tachyzoite which multiplies in host cells by producing two daughter cells internally. These nascent tachyzoites bud off their mother and repeat the division process until the expanding progenies escape to settle and multiply in other host cells. Over these intra- and extra-cellular phases, the tachyzoite maintains an essential apicobasal polarity that emerges through a unique bidirectional budding process of the elongating cells. This process requires the assembly of several molecular complexes that, at the nascent pole, encompass structural and myosin motor elements. To characterize a recently identified basal pole marker named BCC7 with respect to the posterior myosin J and myosin C motors, we used conventional biochemistry as well as advanced proteomic and in silico analysis in conjunction with live and super resolution microscopy of transgenic fluorescent tachyzoites. We document that BCC7 forms a ribbed ring below which myosin C motor entities distribute regularly. In addition, we identified—among 13 BCC7 putative partners—two novel and five known members of the inner membrane complex (IMC) family which ends at the apical side of the ring. Therefore, BCC7 could assist the stabilization of the IMC plaques and contribute to the parasite biomechanical properties.