Biomechanics of Toxoplasma nucleus under constrained physiological motions – TOXONUC

In confined 3D microenvironments’ where metazoan cells move, their nuclear genome scaffolding and dynamic functions are preserved through the nucleus elastic properties conferred by components partitioning between the dense nucleus matrix and the cytoplasm-located skeletons. Specifically the intrinsic high viscosity of the nucleus as compared to other organelles is provided by the chromatin nuclear content together with the lamin-composed intermediate filament meshwork. These elements concur to structure the nuclear inner membrane and maintain the nuclear forces.
To cope with the physical constraints the nucleus senses during migration, cells have evolved distinct strategies: several leucocytes migrating through tight spaces show intrinsically flexible nuclei often characterized by their lower lamin content as compared to fibroblasts’ or cancer cells’ nuclei. Nucleus deformability however can also be increased through actin-based forces applied onto the nucleus envelope that promote transient rupture of the lamin shell preferentially at the highest curvature site. Inversely in some cancer cells, lamins A/C were shown to protect the nuclear envelope against curvature-induced rupture, hence preventing loss of DNA repair factors in response either to external probing forces or to contractile acto-myosin forces generated at adhesion sites.

The nucleus of Toxoplasma- a protozoan parasite that belongs to the Apicomplexa phylum- significantly deforms when sequentially (a) trafficking in extra-cellular confined matrices (b) then invading metazoan host cells on which relies its fitness. How the parasite proceeds within these various and non-uniform confined microenvironments and in particular how it prevents its nucleus from the threat of mechanical-induced injuries - a prerequisite to progeny production - remains fully elusive. Very little is actually known on the nucleus structure and function and there is yet no evidence of cytoskeleton element connecting the nucleus and the plasma membrane . While nucleus deformation is also prominent over the developmental program of the parasite in its obligate hosting cell. the concept of mechanotransduction has not yet been investigated in any Apicomplexa parasites.

The TOXONUC proposal primarily aims at filling key gaps of knowledge on the nuclear mechanics at work during physiologically relevant and confined motions of the protozoan Toxoplasma. To this end, the three member-consortium has been built around a true complementary expertise offering acute expertise in the molecular and cell biology – in particular high resolution and super resolution live imaging - of this single-celled eukaryote but also providing the advantage of combining with the long-standing expertise in plugin design dedicated to high-content 3 and 4D cell imaging – development of the ICY platform- . Adding biophysics concepts and tools with force microscopy in conjunction with nanotechnology and microfluidics at the heart of this program will contribute to give a multi-scale understanding of the integrated molecular structure of the nucleus within the cellular organization, in particular during sensing and response to cellular mechanical forces.
In conclusion actions taken by the consortium will allow decoding how the Toxoplasma nuclear genome scaffolding and functions could swiftly operate and be preserved over displacement and deformation but will also pave the way for better knowledge on the nuclear mechanotransduction mechanisms much beyond the field of Toxoplasma.