team
CellDiv
team manager: Tournier Sylvie & Gachet Yannick
Presentation
In mitosis, the coordination between chromosome segregation and the axis of cell division must be carefully regulated to prevent chromosome instability, to control cell fate and to shape tissues. This control is essential during embryonic development since it can lead to severe genetic diseases such as Cri-du-Chat, Down, Klinefelter, Turner, William or Bloom syndrome. To avoid chromosome loss during mitosis, fidelity is key. Our team uses interdisciplinary approaches (live cell imaging, modeling, simulation) to study the fundamental mechanisms controlling mitotic robustness in a renowned system model of cell division, the fission yeast Schizosaccharomyces pombe. We are exploring several aspects of mitotic control (project 1) including chromosome capture, positioning, oscillation, bi-orientation (Gachet et al. 2008; Courtheoux et al. 2009; Gay et al. 2012; Goldstone et al. 2010; Mary et al. 2015; Li et al. 2017; Li et al. 2024) as well as chromosome arm and telomere separation (Reyes et al. 2015; Gachet Y. et al. 2015; Berthezene et al. 2020; Colin et al. 2023). Since individual cells or cells within tissues are constantly exposed to physical stress, we have also recently been investigating the robustness of cell division in the presence of mechanical stress (project 2).
Project 1
From yeasts to humans, accurate chromosome segregation is driven by sister-centromere segregation towards opposite spindle poles upon cohesin cleavage at anaphase onset. Using the fission yeast S. pombe as a model system, we revealed that an additional pathway operates at telomeres to achieve full sister-chromatid separation. We coined this pathway the TDP for “Telomere Dissociation Pathway”. We have shown that the TDP involves the mitotic kinase Aurora-B (Ark1 in fission yeast) and the condensin complex (Reyes et al. 2015; Gachet et al. 2015; Berthezene et al. 2020). When Ark1 or condensin is impaired, centromeres reach the spindle poles but sister-TELs fail to disjoin in anaphase, forming large chromatin bridges. Condensin is best known for shaping mitotic chromosomes by folding chromatin into loops, most likely through the ATP-dependent extrusion of DNA. Aurora-B promotes condensin binding to chromatin upon mitotic entry. More recently, we revealed that condensin is enriched at telomeres in mitosis and takes part in sister-telomere disjunction in-cis and independently of DNA decatenation by topoisomerase II (Colin et al. 2023). Condensin enrichment at telomeres necessitates Taz1TRF1-2, a core component of the shelterin complex that ensures telomeric functions. At the mechanistic level, we further showed that inactivating either Ark1 or condensin increases the association of cohesin with chromosome ends, suggesting that the TDP might control cohesin-mediated cohesion at telomeres. Until our recent work, the role played by telomeres in chromosome segregation has remain overlooked. Our discovery of the TDP illustrates the need for further investigations of the processes underlying chromosome segregation. Using the exploratory power of the fission yeast model, we ambition to reveal with high spatial, temporal and mechanistic resolution, how the integrated functioning of Ark1, condensin, cohesin and shelterin governs the timely dissociation of sister-TELs in anaphase. This will pave the way for future studies linking chromosome segregation to telomere biology. Our discovery is reminiscent of telomere separation in human cells that specifically relies on the activity of the poly(ADP-ribose) polymerase tankyrase 1, and suggest therefore that the existence of a dedicated pathway for sister-telomere disjunction is a conserved feature of eukaryotic cells.
Project 2
It is currently accepted that the control of spindle formation or chromosome attachment is essential for mitotic robustness, but how these processes are controlled under stress conditions, such as the solid stresses that occur under spatial confinement, remains poorly understood. While theoretical, modelling and experimental efforts have been devoted to understanding chromosome segregation fidelity, most studies fail to consider the realistic cellular microenvironment in terms of the mechanical challenges faced by cells. In fact, cells are stretched, deformed or even compressed in their natural habitat. This applies to both higher and lower eukaryotes. Yeast and bacteria proliferate naturally in the form of biofilms where they grow in an elastic matrix, such as in dough, but are also subjected to pressure during fermentation. S. pombe cells growing in such an environment can be subjected to mechanical stress and even buckle. This situation is similar to what happens when tumours grow in a confined space. During normal development, cells subjected to such constraints are robust and divide faithfully, whereas in a pathological context, mechanical stress during mitosis could lead to aneuploidy. How the dynamics and robustness of mitosis depend on external mechanical stimuli and how cells cope with a harsh physical environment is elusive and needs to be better understood. Therefore, the overall goal of this project is to unravel the underlying molecular mechanisms of mitotic robustness using a combination of live cell microscopy and modelling. In collaboration with M. Delarue (LAAS, Toulouse), we have developed complementary methods that allow tracking multiple mitotic parameters (Li et al. 2017; Li et al. 2024), while simultaneously measuring cytoplasmic fluidity (providing information on medium rheology) in fission yeast cells subjected to well-defined mechanical conditions (advanced microfluidic devices). In parallel, to address the complexity of the mitotic spindle and understand mitotic robustness, we have developed a mitotic simulation for fission yeast chromosome alignment, segregation and correction of chromosome attachment defects, which has already demonstrated its predictive value (Gay et al. 2012; Mary et al. 2015). We use these coarse-grained models and simulations, in which the extensive details at the molecular level are reduced to a smaller number of objects with few parameters, to understand mitotic robustness. Our fundamental research project will provide the basis for understanding how mechanical stress affects the cell cycle and, in particular, chromosome segregation. Mechanical stress has been implicated in a variety of heart, brain and inflammatory diseases, as well as cancer. Understanding robustness to mechanical stress is of paramount importance to prevent or even modulate mitosis-related developmental pathologies.
– Li T., Gachet Y., Tournier S. MAARS software for automatic and quantitative analysis of mitotic progression. Methods in Molecular Biology 2024 Book: Cell Cycle Control. DOI 10.1007/978-1-0716-3557-5.
– Colin L., Reyes C., Berthezene J., Maestroni L., Modolo L., Toselli E., Chanard N., Schaak S., Cuvier O., Gachet Y., Coulon S., Bernard P., Tournier S. Condensin positioning at telomeres by Shelterin proteins promotes telomere disjunction in anaphase. E.life 2023 doi: https://doi.org/10.7554/eLife.89812.1
– Berthezene J., Reyes C., Li T., Coulon S., Bernard P., Gachet Y., Tournier S. Aurora B and condensin are dispensable for chromosome arm and telomere separation during meiosis II. Mol Biol Cell. 2020 Apr 15;31(9):889-905.
– Maestroni L., Reyes C., Vaurs M, Gachet Y, Tournier S., Géli V., Coulon S. Nuclear envelope attachment of telomeres limits TERRA and telomeric rearrangements in quiescent fission yeast cells. Nucleic Acids Res. 2020 Apr 6;48(6):3029-3041.
– Li T., Mary H., Grosjean M., Fouchard J., Cabello S., Reyes C., Tournier S., Gachet Y. (2017) MAARS: a novel high-content acquisition software for the analysis of mitotic defects in fission yeast. Mol Biol Cell. Jun 15;28(12):1601-1611.
– Mary H., Fouchard J., Gay G., Reyes C., Gauthier T., Gruget C., Pécréaux J., Tournier S., Gachet Y. (2015) Fission yeast kinesin-8 controls chromosome congression independently of oscillations. J Cell Sci. 2015 Oct 15;128(20):3720-30.
– Reyes C., Serrurier C., Gauthier T., Gachet Y., Tournier S. (2015) Aurora B prevents chromosome arm separation defects by promoting telomere dispersion and disjunction J Cell Biol Mar 16;208(6):713-27.
– Gay G., Courtheoux T., Reyes C., Tournier S., Gachet Y. A stochastic model of kinetochore–microtubule attachment accurately describes chromosome segregation. J Cell Biol. Mar 19;196(6):757-74. Epub 2012 Mar 12.
– Courtheoux T., Gay G., Gachet Y., Tournier S. (2009). Ase1/Prc1-dependent spindle elongation corrects merotely during anaphase in fission yeast. J Cell Biol 187, 399-412.
Affiliation
