An official website of the United States government
Oriented cell division is fundamental to development and tissue organization, requiring precise control of both spindle positioning and orientation. While cortical pulling forces mediated by dynein motor proteins are well-established drivers of spindle dynamics, the contribution of microtubule polymerization-based pushing forces remains unclear. We developed a generalizable computational biophysical model that integrates both pulling and pushing mechanisms to investigate spindle behavior across diverse cell types and geometries. This model successfully recapitulates experimental observations in three well-studied models:Drosophilafollicular epithelial cells,Drosophilaneuroblasts, and the earlyC. elegansembryo. Systematic analysis reveals that while pulling forces are the primary drivers of directed spindle orientation, pushing forces play crucial supporting roles by preventing spindle stalling and promoting alignment dynamics, particularly at high initial misalignment angles. We further applied our model to irregularly shaped zebrafish endothelial cells, which present unique challenges due to their non-spherical morphology and dynamic shape changes during mitosis. Our results demonstrate that asymmetric cortical force generator distributions, potentially localized at cell-cell junctions, can account for the observed off-center spindle positioning in these cells. This work provides a unified framework for understanding how the interplay between cell geometry, molecular polarity cues, and competing physical forces determines spindle dynamics, offering new insights into both canonical and non-canonical division orientations across cell types.
more »
« less
Laser ablation and fluid flows reveal the mechanism behind spindle and centrosome positioning
Few techniques are available for studying the nature of forces that drive subcellular dynamics. Here we develop two complementary ones. The first is femtosecond stereotactic laser ablation, which rapidly creates complex cuts of subcellular structures and enables precise dissection of when, where and in what direction forces are generated. The second is an assessment of subcellular fluid flows by comparison of direct flow measurements using microinjected fluorescent nanodiamonds with large-scale fluid-structure simulations of different force transduction models. We apply these techniques to study spindle and centrosome positioning in early Caenorhabditis elegans embryos and to probe the contributions of microtubule pushing, cytoplasmic pulling and cortical pulling upon centrosomal microtubules. Based on our results, we construct a biophysical model to explain the dynamics of centrosomes. We demonstrate that cortical pulling forces provide a general explanation for many behaviours mediated by centrosomes, including pronuclear migration and centration, rotation, metaphase spindle positioning, asymmetric spindle elongation and spindle oscillations. This work establishes methodologies for disentangling the forces responsible for cell biological phenomena.
more »
« less
- Award ID(s):
- 1944156
- PAR ID:
- 10318639
- Publisher / Repository:
- Nature
- Date Published:
- Journal Name:
- Nature physics
- ISSN:
- 1745-2481
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)The spindle shows remarkable diversity, and changes in an integrated fashion, as cells vary over evolution. Here, we provide a mechanistic explanation for variations in the first mitotic spindle in nematodes. We used a combination of quantitative genetics and biophysics to rule out broad classes of models of the regulation of spindle length and dynamics, and to establish the importance of a balance of cortical pulling forces acting in different directions. These experiments led us to construct a model of cortical pulling forces in which the stoichiometric interactions of microtubules and force generators (each force generator can bind only one microtubule), is key to explaining the dynamics of spindle positioning and elongation, and spindle final length and scaling with cell size. This model accounts for variations in all the spindle traits we studied here, both within species and across nematode species spanning over 100 million years of evolution.more » « less
-
The centrosomal aster is a mobile and adaptable cellular organelle that exerts and transmits forces necessary for tasks such as nuclear migration and spindle positioning. Recent experimental and theoretical studies of nematode and human cells demonstrate that pulling forces on asters by cortically anchored force generators are dominant during such processes. Here, we present a comprehensive investigation of the S-model (S for stoichiometry) of aster dynamics based solely on such forces. The model evolves the astral centrosome position, a probability field of cell-surface motor occupancy by centrosomal microtubules (under an assumption of stoichiometric binding), and free boundaries of unattached, growing microtubules. We show how cell shape affects the stability of centering of the aster, and its transition to oscillations with increasing motor number. Seeking to understand observations in single-cell nematode embryos, we use highly accurate simulations to examine the nonlinear structures of the bifurcations, and demonstrate the importance of binding domain overlap to interpreting genetic perturbation experiments. We find a generally rich dynamical landscape, dependent upon cell shape, such as internal constant-velocity equatorial orbits of asters that can be seen as traveling wave solutions. Finally, we study the interactions of multiple asters which we demonstrate an effective mutual repulsion due to their competition for surface force generators. We find, amazingly, that centrosomes can relax onto the vertices of platonic and nonplatonic solids, very closely mirroring the results of the classical Thomson problem for energy-minimizing configurations of electrons constrained to a sphere and interacting via repulsive Coulomb potentials. Our findings both explain experimental observations, providing insights into the mechanisms governing spindle positioning and cell division dynamics, and show the possibility of new nonlinear phenomena in cell biology. Published by the American Physical Society2025more » « less
-
Abstract The orientation of the mitotic spindle at metaphase determines the placement of the daughter cells. Spindle orientation in animals typically relies on an evolutionarily conserved biological machine comprised of at least four proteins – called Pins, Gαi, Mud, and Dynein in flies – that exerts a pulling force on astral microtubules and reels the spindle into alignment. The canonical model for spindle orientation holds that the direction of pulling is determined by asymmetric placement of this machinery at the cell cortex. In most cell types, this placement is thought to be mediated by Pins, and a substantial body of literature is therefore devoted to identifying polarized cues that govern localized cortical enrichment of Pins. In this study we revisit the canonical model and find that it is incomplete. Spindle orientation in theDrosophilafollicular epithelium and embryonic ectoderm requires not only Pins localization but also direct interaction between Pins and the multifunctional protein Discs large. This requirement can be over‐ridden by interaction with another Pins interacting protein, Inscuteable.more » « less
-
RNA regulation plays a critical role in mitosis, yet the mechanisms remain unclear. Our lab recently identified that the conserved RNA-Binding Protein (RBP), ATX-2, regulates cytokinesis by regulating the targeting of ZEN-4 to the spindle midzone through a conserved translation regulator, PAR-5/14-3-3sigma (Gnazzo et al., 2016). While co-depletion of ATX-2 and PAR-5 restored ZEN-4 targeting to the spindle midzone, it did not rescue cell division. To identify factors that may work in concert with ATX-2 to regulate cell division, we conducted a two-part, candidate RNAi suppressor and visual screen to identify factors that are important for cell division and also mediate the targeting of ATX-2 to the centrosomes and the spindle midzone. Using this approach, we identified ten genes that suppress the embryonic lethality defect observed in atx-2 mutant embryos. These ten genes, including act-2, cgh-1, cki-1, hum-6, par-2, rnp-4, vab-3, vhl-1, vps-24, and wve-1, all have some role regulating RNA or the cell cycle. Five of these genes (cgh-1, cki-1, vab-3, vhl-1, vps-24) fail to target ATX-2 to the centrosomes and midzone when depleted. The strongest suppressor of the atx-2 phenotype is the DEAD-box RNA helicase CGH-1/DDX6, which has been implicated in cell division, RNA processing and translation, and neuronal function. Loss of CGH-1 rescued the cytokinesis defect and also restored ZEN-4 localization to the spindle midzone. ATX-2 and CGH-1 are mutually required for their localization to centrosomes and the spindle midzone. Our findings provide the first functional evidence that CGH-1/DDX6 regulates ATX-2 function during mitosis to target ZEN-4 to the spindle midzone via PAR-5/14-3-3sigma. We suggest that RNA machinery is necessary for the completion of cytokinesis.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- The DOI is not currently available.
