Jacques Pecreaux - Academia.edu (original) (raw)

Papers by Jacques Pecreaux

Code supporting Bouvrais et al. EMBO Reports 2021

Today, in optical microscopy, it becomes essential to increase the capacity of microscopes in ter... more Today, in optical microscopy, it becomes essential to increase the capacity of microscopes in terms of acquisition speed to suit the needs of different actors in this field, especially biologists. This ability allows to track dynamic events observed in live specimens or to increase the quantity of image flow coming from microscopes. Therefore, there is a challenge of optimizing the multidimensional image acquisition speed (time, Z, wavelength, XYZ positions...).

Asymmetric cell division is a complex process that is not yet fully understood. A very well-known... more Asymmetric cell division is a complex process that is not yet fully understood. A very well-known example of such a division is C. elegans embryos first division. To improve our understanding of this process, we used mathematical modelling to study C. elegans embryos first division, both on wild type cells and under a wide range of genetic perturbations. Asymmetry is clearly visible at the end of the anaphase, as the mitotic spindle is off-center. The study of the mitotic spindle dynamics is, thus, a useful tool to gain insights into the general mechanics of the system used by the cell to correctly achieve asymmetric division. The overall spindle behavior is led by the spindle poles behavior. We proposed a new dynamic model for the posterior spindle pole that explains the oscillatory behavior during anaphase and confirms some previous findings, such as the existence of a threshold number of active force-generator motors required for the onset of oscillations. We also confirmed that ...

EMBO reports, 2021

In the Caenorhabditis elegans zygote, astral microtubules generate forces, pushing against and pu... more In the Caenorhabditis elegans zygote, astral microtubules generate forces, pushing against and pulling from the cell periphery. They are essential to position the mitotic spindle. By measuring the dynamics of astral microtubules at the cortex, we revealed the presence of two populations, residing there for 0.4 s and 1.8 s, which correspond to the pulling and pushing events, respectively. Such an experiment offers a unique opportunity to monitor both forces that position the spindle under physiological conditions and study their variations along the anteroposterior axis (space) and the mitotic progression (time). By investigating pulling-force-generating events at the microscopic level, we showed that an anteroposterior asymmetry in dynein on-rate-encoding pulling-force imbalance-is sufficient to cause posterior spindle displacement. The regulation by spindle positionreflecting the number of microtubule contacts in the posterior-most region-reinforces this imbalance only in late-anaphase. Furthermore, we exhibited the first direct proof that the force-generator increasing persistence to pull (processivity) accounts for the temporal control of pulling force throughout mitosis. We thus propose a threefold control of pulling force, by the polarity, spindle position and mitotic progression. Focusing on pushing force, we discovered a correlation between its density and the stability of the spindle position during metaphase, which strongly suggests that the pushing force contributes to maintaining the spindle at the cell centre. This force remains constant and symmetric along the anteroposterior axis during the division. The pulling one increases in intensity and becomes dominant at anaphase. In conclusion, the two-population study enabled us to decipher the complex regulation of the spindle positioning during cell division. KEYWORDS Microtubule dynamics; spindle positioning forces; mitotic progression regulation; forcegenerator processivity increase; polarity encoding; pulling and pushing force coordination; asymmetric dynein on-rate; centring by pushing microtubules.

Artificial intelligence is nowadays used for cell detection and classification in optical microsc... more Artificial intelligence is nowadays used for cell detection and classification in optical microscopy, during post-acquisition analysis. The microscopes are now fully automated and next expected to be smart, to make acquisition decisions based on the images. It calls for analysing them on the fly. Biology further imposes training on a reduced dataset due to cost and time to prepare the samples and have the datasets annotated by experts. We propose here a real-time image processing, compliant with these specifications by balancing accurate detection and execution performance. We characterised the images using a generic, high-dimensional feature extractor. We then classified the images using machine learning for the sake of understanding the contribution of each feature in decision and execution time. We found that the non-linear-classifier random forests outperformed Fisher’s linear discriminant. More importantly, the most discriminant and time-consuming features could be excluded wit...

ABSTRACTIn theCaenorhabditis eleganszygote, astral microtubules generate forces, pushing against ... more ABSTRACTIn theCaenorhabditis eleganszygote, astral microtubules generate forces, pushing against and pulling from the cell periphery. They are essential to position the mitotic spindle. By measuring the dynamics of astral microtubules at the cortex, we revealed the presence of two populations, residing there for 0.4 s and 1.8 s, which correspond to the pulling and pushing events, respectively. Such an experiment offers a unique opportunity to monitor both forces that position the spindle under physiological conditions and study their variations along the anteroposterior axis (space) and the mitotic progression (time). By investigating pulling-force-generating events at the microscopic level, we showed that an anteroposterior asymmetry in dynein on-rate – encoding pulling-force imbalance – is sufficient to cause posterior spindle displacement. The regulation by spindle position – reflecting the number of microtubule contacts in the posterior-most region – reinforces this imbalance on...

Biophysical Journal, 2018

During asymmetric division of Caenorhabditis elegans zygote, to properly distribute cell fate det... more During asymmetric division of Caenorhabditis elegans zygote, to properly distribute cell fate determinants, the mitotic spindle is asymmetrically localized by a combination of centering and cortical pulling microtubule-mediated forces, the dynamics of the latter being regulated by mitotic progression. Here we show a novel and additional regulation of these forces by spindle position itself. For that, we observed the onset of transverse spindle oscillations which reflects the burst of anaphase pulling forces. After delaying anaphase onset, we found that the position at which the spindle starts to oscillate was unchanged compared to control embryos and uncorrelated to anaphase onset. In mapping the cortical microtubule dynamics, we measured a steep increase in microtubule contact density after the posterior centrosome reached the critical position of 70% of embryo length, strongly suggesting the presence of a positional switch for spindle oscillations. Expanding a previous model based on a forcegenerator temporal control, we implemented this positional switch and observed that the large increase in microtubule density accounted for the pulling force burst. Thus, we propose that the spindle position influences the cortical availability of microtubules, on which the active force generators, controlled by cell cycle progression, can pull. Importantly, we found that this positional control relies on the polarity-dependent LET-99 cortical band, whose boundary could be probed by microtubules. This dual positional and temporal control well accounted for our observation that the oscillation onset position resists changes in cellular geometry and moderate variations in the active force generator number. Finally, our model suggests that spindle position at mitosis end is more sensitive to the polarity factor LET-99, which restricts the region of active force generators to a posterior-most region, than to microtubule number or force generator number/activity. Overall, we show that robustness in spindle positioning originates in cell mechanics rather than biochemical networks.

Molecular Biology of the Cell, 2018

During asymmetric cell division, the molecular motor dynein generates cortical pulling forces tha... more During asymmetric cell division, the molecular motor dynein generates cortical pulling forces that position the spindle to reflect polarity and adequately distribute cell fate determinants. In Caenorhabditis elegans embryos, despite a measured anteroposterior force imbalance, antibody staining failed to reveal dynein enrichment at the posterior cortex, suggesting a transient localization there. Dynein accumulates at the microtubule plus ends, in an EBP-2EB–dependent manner. This accumulation, although not transporting dynein, contributes modestly to cortical forces. Most dyneins may instead diffuse to the cortex. Tracking of cortical dynein revealed two motions: one directed and the other diffusive-like, corresponding to force-generating events. Surprisingly, while dynein is not polarized at the plus ends or in the cytoplasm, diffusive-like tracks were more frequently found at the embryo posterior tip, where the forces are higher. This asymmetry depends on GPR-1/2LGNand LIN-5NuMA, w...

During the asymmetric division of theCaenorhabditis elegansnematode zygote, the polarity cues dis... more During the asymmetric division of theCaenorhabditis elegansnematode zygote, the polarity cues distribution and daughter cell fates depend on the correct positioning of the mitotic spindle, which results from both centering and cortical pulling forces. Revealed by anaphase spindle rocking, these pulling forces are regulated by the force generator dynamics, which are in turn consequent of mitotic progression. We found a novel, additional, regulation of these forces by the spindle position. It controls astral microtubule availability at the cortex, on which theactiveforce generators can pull. Importantly, this positional control relies on the polarity dependent LET-99 cortical band, which restricts or concentrates generators to a posterior crescent. After delaying anaphase onset, we detected this positional pulling force regulation inC. elegansas a precocious spindle rocking with respect to anaphase onset. We ascribed this control to the microtubule dynamics at the cortex. Indeed, in m...

During asymmetric cell divisions, cortical dyneins generate forces essential to position the spin... more During asymmetric cell divisions, cortical dyneins generate forces essential to position the spindle after polarity cues, prescribing daughter cells fate. In nematode zygote, cortical dynein pulls on microtubules transiently, raising the question of its targeting and dynamics. Tracking and fluorescence correlation spectroscopy revealed that in the cytoplasm, dynein spots displayed directed motions toward the cortex, localized at microtubule plus-ends through EBP-2/EB but are not actively transported. Surprisingly CLIP-1/CLIP170 is not involved. ebp-2(0) slightly reduced spindle rocking, thus most cortical forces remain suggesting a redundant mechanism. At the cortex, to relate dynein residency and forces generating, we tracked dynein and found two dynamically distinct populations. One of them would correspond to force generating events. Our experiments also indicated that an asymmetric (polarized) dynein-microbutule on-rate, causes the force imbalance positioning the spindle. GPR-1/...

Precise positioning of the mitotic spindle is important for specifying the plane of cell division... more Precise positioning of the mitotic spindle is important for specifying the plane of cell division, which in turn determines how the cytoplasmic contents of the mother cell are partitioned into the daughter cells, and how the daughters are positioned within the tissue. During metaphase in the earlyC. elegansembryo, the spindle is aligned and centered on the anterior-posterior axis by a microtubule-dependent machinery that exerts restoring forces when the spindle is displaced from the center. To investigate the accuracy and stability of centering, we tracked the position and orientation of the mitotic spindle during the first cell division with high temporal and spatial resolution. We found that the precision is remarkably high: the cell-to-cell variation in the transverse position of the center of the spindle during metaphase, as measured by the standard deviation, was only 1.5% of the length of the short axis of the cell. Spindle position is also very stable: the standard deviation ...

Biophysical Journal, 2016

Journal of Cell Science, 2015

In higher eukaryotes, efficient chromosome congression relies, among other players, on the activi... more In higher eukaryotes, efficient chromosome congression relies, among other players, on the activity of chromokinesins. Here, we provide a quantitative analysis of kinetochore oscillations and positioning in S. Pombe, a model organism lacking chromokinesins. In wild type cells, chromosomes align during prophase and while oscillating, maintain this alignment throughout metaphase. Chromosome oscillations are dispensable both for kinetochore congression and stable kinetochore alignment during metaphase. In higher eukaryotes, Kinesin-8 controls chromosome congression by regulating their oscillations. Oppositely, we demonstrate that fission yeast Kinesin-8 controls chromosome congression by an alternative mechanism. We propose that Kinesin-8 aligns chromosomes by controlling pulling forces in a length dependent manner. A coarse grained model of chromosome segregation implemented with a length-dependent process that controls the force at kinetochores is necessary and sufficient to mimic ki...

Development, 2015

E-cadherin (E-cad) is the main component of epithelial junctions in multicellular organisms, wher... more E-cadherin (E-cad) is the main component of epithelial junctions in multicellular organisms, where it is essential for cell-cell adhesion. The localisation of E-cad is often strongly polarised in the apico-basal axis. However, the mechanisms required for its polarised distribution are still largely unknown. We performed a systematic RNAi screen in vivo to identify genes required for the strict E-cad apical localisation in C. elegans epithelial epidermal cells. We found that the loss of clathrin, its adaptor AP-1 and the AP-1 interactor SOAP-1 induced a basolateral localisation of E-cad without affecting the apico-basal diffusion barrier. We further found that SOAP-1 controls AP-1 localisation, and that AP-1 is required for clathrin recruitment. Finally, we also show that AP-1 controls E-cad apical delivery and actin organisation during embryonic elongation, the final morphogenetic step of embryogenesis. We therefore propose that a molecular pathway, containing SOAP-1, AP-1 and clath...

2006 International Conference on Image Processing, 2006

Automatic segmentation and tracking of biological objects from dynamic microscopy data is of grea... more Automatic segmentation and tracking of biological objects from dynamic microscopy data is of great interest for quantitative biology. A successful framework for this task are active contours, curves that iteratively minimize a cost function, which contains both dataattachment terms and regularization constraints reflecting prior knowledge on the contour geometry. However the choice of these latter terms and of their weights is largely arbitrary, thus requiring timeconsuming empirical parameter tuning and leading to sub-optimal results. Here, we report on a first attempt to use regularization terms based on known biophysical properties of cellular membranes. The present study is restricted to 2D images and cells with a simple cytoskeletal cortex underlying the membrane. We describe our new active contour model and its implementation, and show a first application to real biological images. The obtained segmentation is slightly better than standard active contours, however the main advantage lies in the self-consistent and automated determination of the weights of regularization terms. This encouraging result will lead us to extend the approach to 3D and more complex cells.

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Journal of Cell Biology, 2013

During the first embryonic division in Caenorhabditis elegans, the mitotic spindle is pulled towa... more During the first embryonic division in Caenorhabditis elegans, the mitotic spindle is pulled toward the posterior pole of the cell and undergoes vigorous transverse oscillations. We identified variations in spindle trajectories by analyzing the outwardly similar one-cell stage embryo of its close relative Caenorhabditis briggsae. Compared with C. elegans, C. briggsae embryos exhibit an anterior shifting of nuclei in prophase and reduced anaphase spindle oscillations. By combining physical perturbations and mutant analysis in both species, we show that differences can be explained by interspecies changes in the regulation of the cortical Gα–GPR–LIN-5 complex. However, we found that in both species (1) a conserved positional switch controls the onset of spindle oscillations, (2) GPR posterior localization may set this positional switch, and (3) the maximum amplitude of spindle oscillations is determined by the time spent in the oscillating phase. By investigating microevolution of a s...

PLoS ONE, 2010

Asymmetric positioning of the mitotic spindle in C. elegans embryos is mediated by force-generati... more Asymmetric positioning of the mitotic spindle in C. elegans embryos is mediated by force-generating complexes that are anchored at the plasma membrane and that pull on microtubules growing out from the spindle poles. Although asymmetric distribution of the force generators is thought to underlie asymmetric positioning of the spindle, the number and location of the force generators has not been well defined. In particular, it has not been possible to visualize individual force generating events at the cortex. We discovered that perturbation of the acto-myosin cortex leads to the formation of long membrane invaginations that are pulled from the plasma membrane toward the spindle poles. Several lines of evidence show that the invaginations, which also occur in unperturbed embryos though at lower frequency, are pulled by the same force generators responsible for spindle positioning. Thus, the invaginations serve as a tool to localize the sites of force generation at the cortex and allow us to estimate a lower limit on the number of cortical force generators within the cell.