Email : helene [dot] bouvrais [at] univ-rennes1 [dot] fr
Phone : +33 (0)2 23 23 40 08
IN A FEW WORDS
After a PhD in Biophysics in 2011 as part of a co-supervision between the Danish laboratory "MEMPHYS: Center for Biomembrane Physics" and the University of Rennes 1, and supervised by John Hjort Ipsen and Philippe Meléard, I completed a first post-doc between two Danish laboratories, "MEMPHYS: Center for Biomembrane Physics" and the Department of "Bioscience and Terrestrial Ecology" (Silkeborg), during which I became interested in the study of membrane mechanical properties of giant vesicles: either with simple lipid bilayers aiming to understand how they are affected in the presence of different molecules (eg fluorophores, sodium salts, magainin, sodium and potassium pump, ...), or with vesicles reconstituted from membrane extracts to study how the rigidity of these membranes reacts to different stresses (temperature, presence of ethanol, ...).
In 2012, I joined the laboratory of Jacques Pécréaux at the IGDR as part of a post-doctorate, during which I focused my interest on the asymmetric division of the C. elegans embryo, and more precisely on the robustness of the positioning of the mitotic spindle by a biophysical approach. I was then recruited as a researcher at the CNRS in 2015 and since then I continue my research project in the team of J. Pécréaux to understand how the regulation of microtubule mechanics and dynamics leads to a robust positioning of the mitotic spindle.
Previous research work
The robustness of the mitotic spindle positioning arises from microtubule dynamics and from the restriction of force generators in a posterior-most region
We have revealed a positional control of the pulling forces, which depends on the microtubule dynamics and the presence of an active region where the active force generators are located, this region being probed by the astral microtubules (Bouvrais et al., 2018). This positional control associated with the temporal control of the cell cycle, proposed elsewhere, fully explains the regulation of pulling forces at anaphase onset. Indeed, the mitotic spindle position influences the cortical availability of microtubules on which the active force generators, themselves controlled by the mitotic progression, can pull.
This dual control provides robustness to changes in cell geometry and to moderate variations in the number of active force generators. The final position of the spindle is in turn dependent on the LET-99 band, which restricts the region of the active force generators to a posterior-most crescent, while the number of microtubules or the number and activity of force generators have little influence.
The three-fold control of pulling forces
We have developed the DiLiPop tool, “Distinct Lifetime subPopulation” (which enables to identify multiple dynamical behaviours within a sample) and showed that there are two populations within the microtubules contacting the cortex: a short-lived one (0.4 s) reflecting pulling events and a long lived one (1.5 s) showing pushing events.
Having direct readouts of the mitotic-spindle-positioning forces, with their evolution within the embryo and during mitosis, we have revealed their regulations and their coordination (Bouvrais et al., 2021).
- Spatial control of the pulling forces through a higher on-rate of dynein to microtubules at the posterior-most region (probably regulated by GPR-1/2 proteins) from early metaphase to late anaphase. This asymmetry in the dynein on-rate is sufficient to correctly recapitulate the final position of the mitotic spindle.
- Temporal control of the pulling forces by increasing the processivity of dynein motors during anaphase, thus reflecting mitotic progression.
- Independence between the controls of the pulling forces by the spindle position and by the polarity, which is itself independent of the regulation by the mitotic progression.
Finally, we have measured at the microscopic scale that the pushing forces, which dominate during metaphase, allow the precise and stable maintenance of the spindle in the cell centre. During anaphase, the pulling forces become dominant and thus allow the posterior displacement.
Current and future research projects
The DiLiPop tool allows us to have a reading of the cortical pulling and pushing forces with fine spatial and temporal resolutions, and therefore opens the door to new studies. In particular, we aim to study how forces external to the mitotic spindle can be affected by changes within the mitotic spindle (e.g. chromosomal defects).
Our previous work has revealed the major role of microtubule dynamics in the positioning of the mitotic spindle (Bouvrais et al., 2018; Bouvrais et al., 2021). What about the mechanical properties of microtubules, such as the bending stiffness ? Our preliminary results suggest that these properties are regulated in vivo. This can be done in various ways: formation of microtubule bundles, coupling of protofilaments leading to stiffening of the microtubules, mechanical coupling with the elastic environment, post-translational modifications, presence of holes or defects along the microtubules. We will examine these different possible mechanisms in the C. elegans embryo.
H. Bouvrais, L. Chesneau, Y. Le Cunff, D. Fairbrass, N. Soler, S. Pastezeur, T. Pécot, C. Kervrann, J. Pécréaux, "The coordination of spindle-positioning forces during the asymmetric division of the C. elegans zygote is revealed by distinct microtubule dynamics at the cortex", EMBO Reports, 2021.
H. Bouvrais, L. Chesneau, S. Pastezeur, M. Delattre, J. Pécréaux, “Microtubule Feedback and LET-99-Dependent Control of Pulling Forces Ensure Robust Spindle Position”, Biophysical Journal, 115(11), 2189-2205, 2018.
M. Holmstrup, H. Bouvrais, P. Westh, C. Wang, S. Slotsbo, D. Waagner, K. Enggrob, J.H. Ipsen, “Lipophilic contaminants influence cold tolerance of invertebrates through changes in cell membrane fluidity”, Environ. Sci. Technol., 48(16): 9797–9803, 2014.
H. Bouvrais, L. Duelund, J.H. Ipsen, “Buffers affect the bending rigidity of model lipid membranes”, Langmuir, 30(1): 13-16, 2014.
H. Bouvrais, P. Westh, M. Holmstrup, J.H. Ipsen, “Analysis of the shape fluctuations of reconstituted membranes using GUVs made from lipid extracts of invertebrates”, Biology Open, 2:373-378, 2013.
H. Bouvrais, F. Cornelius, J.H. Ipsen, O.G. Mouritsen, “Intrinsic reaction-cycle time scale of Na+,K+-ATPase manifests itself in the lipid–protein interactions of nonequilibrium membranes”, PNAS, 109(45): 18442-18446, 2012.
H. Bouvrais, T. Pott, L.A. Bagatolli, J.H. Ipsen, P. Méléard, “Impact of membrane-anchored fluorescent probes on the mechanical properties of lipid bilayers”, Biophys. Biochim. Acta, 1798(7): 1333-1337, 2010.
H. Bouvrais, P. Méléard, T. Pott, K.J. Jensen, J. Brask, J.H. Ipsen, “Softening of POPC membranes by magainin”, Biophysical chemistry, 137: 7-12, 2008.
T. Pott, H. Bouvrais, P. Méléard, “Giant unilamellar vesicle formation under physiologically relevant conditions”, Chem. Phys. Lip., 154 : 115-119, 2008.