arXiv subjects

Sylvain Pastezeur

Publications and source records attributed to Sylvain Pastezeur.

5 recordsLinked to original sources

arXiv2026

Semi supervised GAN for smart microscopy, fast and data efficient cell cycle classification

Modern optical microscopes are fully motorised; however, transforming them into truly smart systems requires real-time adjustment of acquisition settings in response to detected objects and dynamic biological events. At the core are classification algorithms that commonly depend on customised software and are generally designed for narrowly-defined biological applications. In addition, they often require substantial annotated datasets for effective training. We introduce a semi-supervised generative adversarial network (SGAN) for robust cell-cycle stage classification under low-resource conditions, adaptable to diverse cellular structures. The framework combines unlabelled microscopy images with synthetically generated samples to mitigate limited annotation, while preserving stable performance even when the unlabelled subset is class-imbalanced. Tested on the Mitocheck dataset, which features five mitosis classes, the model achieved $93 \pm 2\%$ accuracy using only 80 labelled per class and 600 unlabelled images. The proposed algorithm is generic and can be readily adapted to new labeling schemes, classification targets, cell lines, or microscopy modalities through transfer learning. SGAN is well suited for integration into automated microscopes, enabling efficient and adaptable image analysis across diverse biological and microscopy applications.

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arXiv2025

Kinesin-12 KLP-18 contributes to the kinetochore-microtubule poleward flux during the metaphase of C. elegans one-cell embryo

The mitotic spindle partitions chromosomes during cell division by connecting the poles to kinetochores through microtubules (MTs). Their plus-ends, facing the chromosomes, exhibit dynamic instability, which is critical for proper attachment. The poleward flux implicates the displacement of Mts towards the spindle poles, while plus-ends polymerise. It may result from minus-end depolymerisation (treadmilling), sliding by kinesins (e.g., Kinesin-5), or pushing by chromokinesins. Intriguingly, such flux had not been reported in the C. elegans zygote, despite homologs of flux-associated proteins being present. To investigate this, we fluorescently labelled Mts and used photobleaching. We observed no global flux; instead, the bleached zone's edges moved inward. The centrosome-facing front reflected MT dynamic instability, but the chromosome-facing front showed faster recovery, suggesting an additional mechanism. This extra velocity was spatially restricted to the vicinity of chromosomes, suggesting that only the kinetochore Mts may undergo flux. Supporting this, flux required key kinetochore regulators: NDC-80, $\text{CLS-2}^\text{CLASP}$, and $\text{ZYG-9}^\text{XMAP215}$. Flux declined as metaphase progressed, correlating with the attachment maturation from lateral to end-on, and was suppressed by SKA-1 recruitment. Classic treadmilling was unlikely, as most kinetochore MTs in C. $elegans$ do not reach spindle poles. Instead, depleting $\text{KLP-18}^\text{KIF15}$, a kinesin that cross-links and organises Mts during meiosis, reduced front movement. We propose that only kinetochore Mts undergo flux, sliding along the spindle Mts, likely powered by KLP-18. This localised sliding contrasts with global flux seen in other systems, and aligns with observations in human cells showing flux reduction as chromosome-to-pole distance increases.

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arXiv2018

LET-99-dependent spatial restriction of active force generators makes spindle's position robust

During the asymmetric division of the Caenorhabditis elegans nematode 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 the active force 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. We ascribed this control to the microtubule dynamics at the cortex. Indeed, in mapping the cortical contacts, we found a correlation between the centrosome-cortex distance and the microtubule contact density. In turn, it modulates pulling force generator activity. We modelled this control, predicting and experimentally validating that the posterior crescent extent controlled where the anaphase oscillations started, in addition to mitotic progression. Finally, we propose that spatially restricting force generator to a posterior crescent sets the spindle's final position, reflecting polarity through the LET-99 dependent restriction of force generators to a posterior crescent. This regulation superimposes that of force generator processivity. This novel control confers a low dependence on microtubule and active force generator exact numbers or dynamics, provided that they exceed the threshold needed for posterior displacement. Interestingly, this robustness originates in cell mechanics rather than biochemical networks.

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arXiv2017

Dynein dynamics at the microtubule plus-ends and cortex during division in the C. $\textit{elegans}$ zygote

During asymmetric cell division, dynein generates forces, which position the spindle to reflect polarity and ensure correct daughter cell fates. The transient cortical localization of dynein raises the question of its targeting. We found that it accumulates at the microtubule plus-ends like in budding yeast, indirectly hitch-hiking on $\text{EBP-2}^{\text{EB1}}$ likely via dynactin. Importantly, this mechanism, which modestly accounts for cortical forces, does not transport dynein, which displays the same binding/unbinding dynamics as $\text{EBP-2}^{\text{EB1}}$. At the cortex, dynein tracks can be classified as having either directed or diffusive-like motion. Diffusive-like tracks reveal force-generating dyneins. Their densities are higher on the posterior tip of the embryos, where $\text{GPR-1/2}^{\text{LGN}}$ concentrate, but their durations are symmetric. Since dynein flows to the cortex are non-polarized, we suggest that this posterior enrichment increases dynein binding, thus accounts for the force imbalance reflecting polarity, and supplements the regulation of mitotic progression via the non-polarized detachment rate.

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arXiv2016

The mitotic spindle in the one-cell C. elegans embryo is positioned with high precision and stability

Precise positioning of the mitotic spindle is important for specifying the plane of cell division, which in turn determines how the cytoplasmic contents are partitioned into the daughter cells, and how the daughters are positioned within the tissue. During metaphase in the early C. elegans embryo, 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 of the fluctuations in transverse spindle position during metaphase was only 0.5% of the short axis of the cell. Assuming that stability is limited by fluctuations in the number of independent motor elements such as microtubules or dyneins underlying the centering machinery, we infer that the number is on the order of one thousand, consistent with the several thousand of astral microtubules in these cells. Astral microtubules grow out from the two spindle poles, make contact with the cell cortex, and then shrink back shortly thereafter. The high stability of centering can be accounted for quantitatively if, while making contact with the cortex, the astral microtubules buckle as they exert compressive, pushing forces. We thus propose that the large number of microtubules in the asters provides a highly precise mechanism for positioning the spindle during metaphase while assembly is completed prior to the onset of anaphase.

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