Biology subjects

Salle, J.

Publications and source records attributed to Salle, J..

7 recordsLinked to original sources

bioRxiv2026

The Endoplasmic Reticulum as a Cytoplasmic Anchor for Microtubule-Driven Aster Centration

The proper positioning of microtubule (MT) asters is essential to regulate division positioning and multi-cellular morphogenesis, yet the mechanisms regulating this process remain poorly understood. Using sea urchin zygotes as a model, we combined high-resolution imaging, perturbation assays, and 3D modeling to investigate the nature of forces driving sperm aster centration after fertilization. We show that centering asters are highly anisotropic, featuring long microtubules extending at the aster front that may engage with organelles in bulk cytoplasm to generate pulling forces. While vesicular compartments, like acidic organelles, mitochondria or yolk, contributed minimally, we found that the endoplasmic reticulum (ER) undergoes a massive compaction around the centering aster and drags cytosol to produce a reactive centering force on the aster. Perturbing ER organization by cytoplasmic stratification or Atlastin-mediated fragmentation disrupted aster centration. A two-fluid model suggests that the frictional interaction between the compacting ER and cytosol fluid is sufficient to recapitulate key features of aster centration. These findings identify the ER as an important cytoplasmic anchor for MT force exertion in bulk cytoplasm, to guide aster centering motion and influence early embryonic division geometries. More broadly, our results highlight the role of cytoplasmic architecture in shaping the mechanics of organelle positioning during development.

cell biology↗

bioRxiv2026

A Composite Endomembrane Suspension Governs Cytoplasm Rheology

The cytoplasm of eukaryotic cells is populated by dense disordered suspensions of filamentous and granular endomembranes, yet how they contribute to the mechanical behavior of the cell interior remains unknown. We combined active micro-rheology, cell-like encapsulation and simulations to study the material properties of marine egg extract fractions enriched in distinct endomembrane components. We characterized the cytoplasm as a composite suspension made of yolk granules interspaced by sheets and tubules of endoplasmic reticulum (ER) bathed in cytosolic fluid, that occupies [~]38% of cell volume. Remarkably, while isolated cytosol, yolk or ER fractions had characteristics of Newtonian fluids, their combination yielded the emergence of viscoelasticity and glass-like dynamics closely resembling that of in vivo cytoplasm. Our data suggest that the ER acts as a sterically excluding backbone that drives the formation of load-bearing yolk flocculation structures to endow the cytoplasm with solid-like properties at volume fractions far below random close packing. This work establishes a generic framework to understand the material properties of composite endomembrane suspensions, and delineates a novel strategy by which eukaryotic cells may tune the physical state of their cytoplasm.

biophysics↗

bioRxiv2026

A Cell Size-Dependent Competition Between Geometry and Polarity Governs Nuclear and Spindle positioning in Early Embryos

Nuclei and mitotic spindles are actively positioned at defined locations within cells to regulate cell polarity, division and multicellular morphogenesis1-4. Forces generated by cytoskeleton networks regulate the positioning of these organelles and are commonly influenced by extrinsic cues such as cell geometry or polarity5-12. To date, however, most studies have investigated this problem in one given cell type, hampering our understanding for how mechanical systems that position nuclei and spindles may scale during multicellular development. We tracked the spatiotemporal behaviour of centrosomes, nuclei and spindles in early sea urchin embryos from the 1-cell to the [~]1000 cells blastula stage. We found that they are initially located at cell centers, but that they undergo a progressive decentration towards the embryo apical surface, as cells become smaller during development. This apical shift is mediated by microtubule (MTs) pulling forces which are influenced by both cell shapes and apical polarity domains. Using 3D mathematical models and embryo dissections, we propose that MT centering forces that derive from cell geometry decay in strength during development as a consequence of cell size reduction, allowing apical polarity decentering forces to take over. Our results support a self-organized scenario in which polarity cues progressively outcompete cell geometry, to modulate the overall balance of MT forces and pattern nuclear and spindle positioning throughout early embryo development.

cell biology↗

bioRxiv2023

Cell shape anisotropy enhances cytoplasm viscoelastic resistance to stabilize mitotic spindle position during early embryo development

Cell geometry is a key parameter for the regulation of mitotic spindle positioning during early embryo development and tissue morphogenesis. To date, however, we still lack an understanding for how intracellular forces that position, orient or hold mitotic spindles depend on cell geometry. Here, we used in vivo magnetic tweezers to directly measure the forces that maintain the mitotic spindle in the center of sea urchin cells that adopt different shapes during early embryo development. We found that spindles are held by viscoelastic forces that progressively increase in amplitude as cells become more elongated during early development. By coupling direct cell shape manipulations and in vivo force measurements, we establish how spindle associated forces increase in dose dependence with cell shape anisotropy. Cytoplasm flow analysis and hydrodynamic simulations suggest that this geometry-dependent mechanical enhancement results from a stronger hydrodynamic coupling between the spindle and cell boundaries, which dampens cytoplasm flows and spindle mobility as cells become more elongated. These findings establish how cell shape affects spindle associated forces, and suggest a novel mechanism for shape-sensing and division positioning mediated by intracellular hydrodynamics with functional implications for early embryo morphogenesis.

cell biology↗

bioRxiv2021

Contribution of cytoplasm viscoelastic properties to mitotic spindle positioning

Cells are filled with macromolecules and polymer networks that set scale-dependent viscous and elastic properties to the cytoplasm. Although the role of these parameters in molecular diffusion, reaction kinetics and cellular biochemistry is being increasingly recognized, their contributions to the motion and positioning of larger organelles, such as mitotic spindles for cell division remain unknown. Here, using magnetic tweezers to displace and rotate mitotic spindles in living embryos, we uncovered that the cytoplasm can impart viscoelastic reactive forces that move spindles, or passive objects with similar size, back to their original position. These forces are independent of cytoskeletal force generators, yet reach hundreds of piconewtons and scale with cytoplasm crowding. Spindle motion shears and fluidizes the cytoplasm, dissipating elastic energy and limiting spindle recoils with functional implications for asymmetric and oriented divisions. These findings suggest that bulk cytoplasm material properties may constitute important control elements for the regulation of division positioning and cellular organization. Significance StatementThe regulation of mitotic spindle positioning is a key process for tissue architecture, embryo development and stem cells. To date, most models have assumed that spindles are positioned by forces exerted by polar cytoskeleton networks, like microtubule asters or acto-myosin bundles. Here, using in situ magnetic tweezers to apply calibrated forces and torques to mitotic spindles in live dividing sea urchin cells, we found that the viscoelastic properties of the cytoplasm medium in which spindles are embedded can hold spindles in place, and move them back if their original position is perturbed. These viscoelastic forces are large and may significantly participate in the force balance that position and orient mitotic spindles in many cell types.

cell biology↗

bioRxiv2020

IN VITRO RECONSTITUTION OF DYNEIN FORCE EXERTION IN BULK CYTOPLASM

The forces generated by Microtubules (MTs) and their associated motors orchestrate essential cellular processes ranging from vesicular trafficking to centrosome positioning [1, 2]. To date, most studies have focused on force exertion from motors anchored on a static surface, such as the cell cortex in vivo or glass surfaces in vitro [2-4]. However, motors also transport large cargos and endomembrane networks, whose hydrodynamic interactions with the viscous cytoplasm should generate sizable forces in bulk. Such forces may contribute to MT aster centration, organization and orientation [5-14], but have yet to be evidenced and studied in a minimal reconstituted system. By developing a bulk motility assay, based on stabilized MTs and dynein-coated beads freely floating in a viscous medium away from any surface, we demonstrate that the motion of a cargo exerts a pulling force on the MT and propels it in opposite direction. Quantification of resulting MT movements for different motors, motor velocities, over a range of cargo size and medium viscosities, shows that the efficiency of this mechanism is primarily determined by cargo size and MT length. Forces exerted by cargos are additive, allowing us to recapitulate tug-of-war situations, or bi-dimensional motions of minimal asters. These data also reveal unappreciated effects of the nature of viscous crowders and hydrodynamic interactions between cargos and MTs, likely relevant to understand this mode of force exertion in living cells. This study places endomembrane transport as a significant mode of MT force exertion with far-reaching consequences for cellular organization.

biophysics↗

bioRxiv2019

The perinuclear ER scales nuclear size independently of cell size in early embryos

SUMMARYNuclear size plays pivotal roles in gene expression, embryo development, and disease. A central hypothesis in organisms ranging from yeast to vertebrates is that nuclear size scales to cell size. This implies that nuclei may reach steady state sizes set by limiting cytoplasmic pools of size-regulating components. By monitoring nuclear dynamics in early sea urchin embryos, we found that nuclei undergo substantial growth in each interphase, reaching a maximal size prior to mitosis that declined steadily over the course of development. Manipulations of cytoplasmic volume through multiple chemical and physical means ruled out cell size as a major determinant of nuclear size and growth. Rather, our data suggest that the perinuclear endoplasmic reticulum, accumulated through dynein activity, serves as a limiting membrane pool that sets nuclear surface growth rate. Partitioning of this local pool at each cell division modulates nuclear growth kinetics and dictates size scaling throughout early development.

cell biology↗