Biology subjects

MINC, N.

Publications and source records attributed to MINC, N..

11 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

Mechanosensation Promotes Local Cell Wall Repair

Walled cells, such as those of fungi, plants and bacteria, must continuously survey the integrity of their protective cell wall (CW), as they grow, divide, or face mechanical challenges from their high internal turgor and environment. To date, however, how cells detect defects in their CW to activate local reinforcement or repair machineries remains unclear. We implemented a laser irradiation assay to locally wound the CW of model rod-shaped fission yeast cells. We found that laser irradiation causes a near instantaneous local thinning of the CW accompanied by the formation of a local bulge, a drop in turgor and growth arrest, followed by a progressive recovery of CW thickness over tens of minutes with cells eventually resuming growth. Remarkably, wounding the CW away from cell tips, caused the re-localization of canonical polarity regulators including the Rho GTPases Cdc42 and Rho1, that recruited actin assembly and vesicular trafficking of CW synthases to repair the CW. A candidate screen approach suggests that the transmembrane surface mechanosensors Wsc1 and Mlt2 detect local CW defects and promote the recruitment of the Rho GEFs Gef1 and Rgf1 that activate Cdc42 and Rho1. Therefore, these findings delineate a mechanochemical pathway, coupling surface mechanosensing to polarity regulation and CW synthesis, that defines how walled cells may detect and repair local defects in their CWs to safeguard surface integrity.

cell biology↗

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↗

bioRxiv2025

A CROSS-SPECIES ANALYSIS OF CELL WALL MECHANOSENSORS

The cell wall (CW) protects fungal cells from various mechanical challenges making its integrity essential for cell survival. CW integrity is monitored by transmembrane sensors that activate downstream effectors to promote CW synthesis in response to injuries. Sensors of the WSC family are found in most fungi, and share a conserved architecture, with a cytoplasmic tail, a single transmembrane domain and a long Serine Threonine Rich domain (STR) prolonged by a WSC domain, both embedded in the CW. In response to forces applied onto the CW, these extracellular domains promote force detection, sensor clustering and cell survival. Interestingly, Wsc sensors exhibit variations in domain sequence and size among different fungal species. To understand how these variations impact mechanosensing, we heterologously expressed Wsc sensors taken from S. cerevisiae and C. albicans, in the fission yeast S. pombe. Remarkably, we found that a subset of these foreign sensors could cluster at sites of CW compression, but that others failed, suggesting divergences in mechanosensing abilities. By swapping sensor domains, we demonstrate that both the cytoplasmic tail and STR influence mechanosensation. These findings reveal a high level of functional plasticity in fungal sensors, and identify tuneable modules that may regulate mechanosensing of various CWs. SIGNIFICANCE STATEMENTO_LICell Wall Mechanosensors of the WSC family are present in most fungi, but whether they can detect mechanical stimuli in a foreign cell wall of a distant fungal species is unknown. C_LIO_LIThe authors heterologously expressed Wsc sensors taken from S. cerevisiae and C. albicans in the fission yeast S. pombe and demonstrate that a subset of foreign sensors can probe mechanical stress in its Cell Wall. C_LIO_LIThis work highlights a remarkable plasticity in mechanosensors ability to detect mechanical stress in the Cell Wall and identifies domains involved in regulating mechanosensing. C_LI

cell biology↗

bioRxiv2025

Inferring diffusion, reaction, and exchange parameters from imperfect FRAP

Fluorescence recovery after photobleaching (FRAP) is broadly used to investigate the dynamics of molecules in cells and tissues, notably to quantify diffusion coefficients. FRAP is based on the spatiotemporal imaging of fluorescent molecules following an initial bleaching of fluorescence in a region of the sample. Although a large number of methods have been developed to infer kinetic parameters from experiments, it is still a challenge to fully characterize molecular dynamics from noisy experiments in which diffusion is coupled to other molecular processes or in which the initial bleaching profile is not perfectly controlled. To address this challenge, we have developed HiFRAP to quantify the reaction-(or exchange-)diffusion kinetic parameters from FRAP under imperfect experimental conditions. HiFRAP is based on a low-rank approximation of a kernel related to the model Greens function and is implemented as an ImageJ/Python macro for (potentially curved) one-dimensional systems and for two-dimensional systems. To the best of our knowledge, HiFRAP offers features that have not been combined together: making no assumption on the initial bleaching profile, which does not need to be known; accounting for the limitation of the optical setup by diffraction; inferring several kinetic parameters from a single experiment; providing errors on parameter estimation; and testing model goodness. In the future, our approach could be applied to other dynamical processes described by linear partial differential equations, which could be useful beyond FRAP, in experiments where the concentration fields are monitored over space and time. SIGNIFICANCEFluorescence recovery after photobleaching (FRAP) is a microscopy approach that is widely used to investigate the diffusion and transport of molecules in life sciences and in material sciences. Numerous methods have been developed to derive kinetic parameters such as diffusion and binding coefficients. However, these methods suffer from limitations associated with experimental constraints, such as technical noise or an imperfectly known initial condition. To circumvent these limitations, we developed a comprehensive approach to estimate several kinetic parameters from a single experiment, to assess the precision of estimation, and to test whether the underlying model is well-suited. We implemented this approach in HiFRAP, an ImageJ/Python macro of broad applicability to one- and two-dimensional systems.

biophysics↗

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↗

bioRxiv2022

Length-limitation of astral microtubules orients cell divisions in intestinal crypts

Planar spindle orientation is critical for epithelial tissue organization, and generally instructed from the long cell shape axis or cortical polarity domains. We introduced mouse intestinal organoid crypts to study spindle orientation in a monolayered mammalian epithelium. Although spindles were planar in this tissue, mitotic cells remained elongated along the apico-basal axis and polarity complexes were segregated to basal poles, so that spindles oriented in an unconventional manner, orthogonal to both polarity and geometric cues. Using high-resolution 3D imaging, simulations, cell shape and cytoskeleton manipulations, we show that planar divisions resulted from a length-limitation in mitotic-phase astral microtubules which precludes them from interacting with basal polarity, and oriented spindles from the local geometry of apical domains. Accordingly, lengthening microtubules affected spindle planarity, cell positioning and crypt arrangement. We conclude that microtubule length regulation may serve as a key mechanism for spindles to sense local cell shapes and tissue forces to preserve mammalian epithelial architecture.

developmental biology↗

bioRxiv2022

Cell Wall Dynamics in a Filamentous Fungus

Hyphal tip growth allows filamentous fungi to colonize space, reproduce or infect. It features remarkable morphogenetic plasticity including unusually fast elongation rates, tip turning, branching or bulging. These shape changes are all driven from the expansion of a protective cell wall (CW) secreted from apical pools of exocytic vesicles. How CW secretion, remodeling and deformation are modulated in concert to support rapid tip growth and morphogenesis while ensuring surface integrity remains poorly understood. We implemented sub-resolution imaging to map the dynamics of CW thickness and secretory vesicles in Aspergillus nidulans. We found that tip growth is associated with balanced rates of CW secretion and expansion, which limit temporal fluctuations in CW thickness, elongation speed and vesicle amount, to less than 10-15%. Affecting this balance through modulations of growth or trafficking yield to near-immediate changes in CW thickness, mechanics and shape. We developed a model with mechanical feedback which accounts for steady states of hyphal growth as well as rapid adaptation of CW mechanics and vesicle recruitment to different perturbations. These data provide unprecedented details on how CW dynamics emerges from material secretion and expansion, to stabilize fungal tip growth as well as promote its morphogenetic plasticity.

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

DETECTION OF SURFACE FORCES BY A CELL WALL MECHANOSENSOR

Surface receptors of animal cells, such as integrins, promote mechanosensation by forming local clusters as signaling hubs that transduce tensile forces. Walled cells of plants and fungi also feature surface sensors, with long extracellular domains embedded in their cell wall (CW), thought to detect CW injuries and promote repair. How these sensors probe surface forces remains unknown. By studying the conserved CW sensor Wsc1 in fission yeast, we uncovered the formation of micrometer-sized clusters at sites of local force application onto the CW. These clusters form within minutes of CW compression, in dose-dependence with mechanical stress and dissolve upon stress relaxation. Our data support that Wsc1 senses CW mechanical stress and accumulates to local sites of enhanced stress through its CW-associated extracellular WSC domain, independently of canonical polarity, trafficking and downstream CW regulatory pathways. Wsc1 may represent an autonomous module to detect and transduce local surface forces onto the CW.

cell biology↗