During early embryogenesis in animals, cells are arranged into a species-specific pattern in a robust manner. Diverse cell arrangement patterns are observed, even among close relatives. In the ...present study, we evaluated the mechanisms by which the diversity and robustness of cell arrangements are achieved in developing embryos. We successfully reproduced various patterns of cell arrangements observed in various nematode species in
embryos by altering the eggshell shapes. The findings suggest that the observed diversity of cell arrangements can be explained by differences in the eggshell shape. Additionally, we found that the cell arrangement was robust against eggshell deformation. Computational modeling revealed that, in addition to repulsive forces, attractive forces are sufficient to achieve such robustness. The present model is also capable of simulating the effect of changing cell division orientation. Genetic perturbation experiments demonstrated that attractive forces derived from cell adhesion are necessary for the robustness. The proposed model accounts for both diversity and robustness of cell arrangements, and contributes to our understanding of how the diversity and robustness of cell arrangements are achieved in developing embryos.
The centrosome is generally maintained at the center of the cell. In animal cells, centrosome centration is powered by the pulling force of microtubules, which is dependent on cytoplasmic dynein. ...However, it is unclear how dynein brings the centrosome to the cell center, i.e., which structure inside the cell functions as a substrate to anchor dynein. Here, we provide evidence that a population of dynein, which is located on intracellular organelles and is responsible for organelle transport toward the centrosome, generates the force required for centrosome centration in Caenorhabditis elegans embryos. By using the database of full-genome RNAi in C. elegans, we identified dyrb-1, a dynein light chain subunit, as a potential subunit involved in dynein anchoring for centrosome centration. DYRB-1 is required for organelle movement toward the minus end of the microtubules. The temporal correlation between centrosome centration and the net movement of organelle transport was found to be significant. Centrosome centration was impaired when Rab7 and RILP, which mediate the association between organelles and dynein in mammalian cells, were knocked down. These results indicate that minus end-directed transport of intracellular organelles along the microtubules is required for centrosome centration in C. elegans embryos. On the basis of this finding, we propose a model in which the reaction forces of organelle transport generated along microtubules act as a driving force that pulls the centrosomes toward the cell center. This is the first model, to our knowledge, providing a mechanical basis for cytoplasmic pulling force for centrosome centration.
Abstract
Cytoplasmic dynein is responsible for various cellular processes during the cell cycle. The mechanism by which its activity is regulated spatially and temporarily inside the cell remains ...elusive. There are various regulatory proteins of dynein, including dynactin, NDEL1/NUD-2, and LIS1. Characterizing the spatiotemporal localization of regulatory proteins in vivo will aid understanding of the cellular regulation of dynein. Here, we focused on spindle formation in the
Caenorhabditis elegans
early embryo, wherein dynein and its regulatory proteins translocated from the cytoplasm to the spindle region upon nuclear envelope breakdown (NEBD). We found that (i) a limited set of dynein regulatory proteins accumulated in the spindle region, (ii) the spatial localization patterns were distinct among the regulators, and (iii) the regulatory proteins did not accumulate in the spindle region simultaneously but sequentially. Furthermore, the accumulation of NUD-2 was unique among the regulators. NUD-2 started to accumulate before NEBD (pre-NEBD accumulation), and exhibited the highest enrichment compared to the cytoplasmic concentration. Using a protein injection approach, we revealed that the C-terminal helix of NUD-2 was responsible for pre-NEBD accumulation. These findings suggest a fine temporal control of the subcellular localization of regulatory proteins.
Fission yeast undergoes premeiotic nuclear oscillation, which is dependent on microtubules and is driven by cytoplasmic dynein. Although the molecular mechanisms have been analyzed, how a robust ...oscillation is generated despite the dynamic behaviors of microtubules has yet to be elucidated. Here, we show that the oscillation exhibits cell length‐dependent frequency and requires a balance between microtubule and viscous drag forces, as well as proper microtubule dynamics. Comparison of the oscillations observed in living cells with a simulation model based on microtubule dynamic instability reveals that the period of oscillation correlates with cell length. Genetic alterations that reduce cargo size suggest that the nuclear movement depends on viscous drag forces. Deletion of a gene encoding Kinesin‐8 inhibits microtubule catastrophe at the cell cortex and results in perturbation of oscillation, indicating that nuclear movement also depends on microtubule dynamic instability. Our findings link numerical parameters from the simulation model with cellular functions required for generating the oscillation and provide a basis for understanding the physical properties of microtubule‐dependent nuclear movements.
Synopsis
In fission yeast, the meiotic nucleus exhibits oscillatory movement, which is driven by cytoplasmic dynein. A model that considers various microtubule properties, including pulling and pushing forces, recaptures robust nuclear oscillations.
A model considering microtubule pulling and pushing forces recapitulates the premeiotic nuclear oscillation in fission yeast.
Period of the spindle pole body (SPB) oscillation correlates with cell length.
Speed of the SPB movement is limited by viscous drag force associated with nuclear volume.
Kinesin‐8 family protein Klp6 contributes to efficient oscillation by controlling microtubule catastrophe.
In fission yeast, the meiotic nucleus exhibits oscillatory movement, which is driven by cytoplasmic dynein. A model that considers various microtubule properties, including pulling and pushing forces, recaptures robust nuclear oscillations.
Although mechanisms that contribute to microtubule (MT) aster positioning have been extensively studied, still little is known on how asters move inside cells to faithfully target a cellular ...location. Here, we study sperm aster centration in sea urchin eggs, as a stereotypical large-scale aster movement with extreme constraints on centering speed and precision. By tracking three-dimensional aster centration dynamics in eggs with manipulated shapes, we show that aster geometry resulting from MT growth and interaction with cell boundaries dictates aster instantaneous directionality, yielding cell shape-dependent centering trajectories. Aster laser surgery and modeling suggest that dynein-dependent MT cytoplasmic pulling forces that scale to MT length function to convert aster geometry into directionality. In contrast, aster speed remains largely independent of aster size, shape, or absolute dynein activity, which suggests it may be predominantly determined by aster growth rate rather than MT force amplitude. These studies begin to define the geometrical principles that control aster movements.
Allometric relations between two observables are a widespread phenomenon in biology. The volume of nuclei, for example, has frequently been reported to scale linearly with cell volume, V_{N}∼V_{C}, ...but conflicting, sublinear power-law correlations have also been found. Given that nuclei are vital organelles that harbor and maintain the DNA of cells, an understanding of allometric nuclear volumes that ultimately define the concentration and accessibility of chromatin is of great interest. Using the model organism Caenorhabditis elegans, we show here that the allometry of nuclei is a dynamically adapting phenomenon; i.e., we find V_{N}∼V_{C}^{α} with a time-dependent scaling exponent α (“dynamic allometry”). This finding is due to relaxation growth of nuclear volumes at a rate that scales with cell size. If cell division stops the relaxation of nuclei in a premature stage, α<1 is observed, whereas completion of relaxation yields α=1 (“isometry”). Our experimental data are well captured by a simple and supposedly generic model in which nuclear size is determined by the available membrane area that can be integrated into the nuclear envelope to relax the expansion pressure from decondensed chromatin. Extrapolation of our results to growing and proliferating cells suggests that isometric scaling of cell and nuclear volumes is the generic case.
The structure, dynamics, and mechanics of mitotic and meiotic spindles have been progressively elucidated through the advancements in microscopic technology, identification of the genes involved, and ...construction of theoretical frameworks. Here, we review recent works that have utilized quantitative image analysis to advance our understanding of the complex spindle structure of animal cells. In particular, we discuss how microtubules (MTs) are nucleated and distributed inside the spindle. Accumulating evidence supports the presence of MT-dependent MT generation within the spindle. This mechanism would produce dense arrays of intraspindle MTs with various lengths, which may contribute to efficient spindle assembly and stabilize the metaphase spindle. RNA interference (RNAi) screens with quantitative image analysis led to the identification of the augmin complex that plays a key role in this MT generation process.
Cytoplasmic streaming is a type of intracellular transport widely seen in nature. Cytoplasmic streaming in Caenorhabditis elegans at the one-cell stage is bidirectional; the flow near the cortex ...("cortical flow") is oriented toward the anterior, whereas the flow in the central region ("cytoplasmic flow") is oriented toward the posterior. Both cortical flow and cytoplasmic flow depend on non-muscle-myosin II (NMY-2), which primarily localizes in the cortex. The manner in which NMY-2 proteins drive cytoplasmic flow in the opposite direction from remote locations has not been fully understood. In this study, we demonstrated that the hydrodynamic properties of the cytoplasm are sufficient to mediate the forces generated by the cortical myosin to drive bidirectional streaming throughout the cytoplasm. We quantified the flow velocities of cytoplasmic streaming using particle image velocimetry (PIV) and conducted a three-dimensional hydrodynamic simulation using the moving particle semiimplicit method. Our simulation quantitatively reconstructed the quantified flow velocity distribution resolved through PIV analysis. Furthermore, our PIV analyses detected microtubule-dependent flows during the pronuclear migration stage. These flows were reproduced via hydrodynamic interactions between moving pronuclei and the cytoplasm. The agreement of flow dynamics in vivo and in simulation indicates that the hydrodynamic properties of the cytoplasm are sufficient to mediate cytoplasmic streaming in C. elegans embryos.
The number of centrioles is tightly controlled to ensure bipolar spindle assembly, which is a prerequisite to maintain genome integrity. However, our understanding of the fundamental principle that ...governs the formation of a single procentriole per parental centriole is incomplete. Here, we show that the local restriction of Plk4, a master regulator of the procentriole formation, is achieved by a bimodal interaction of STIL with Plk4. We demonstrate that the conserved short coiled-coil region of STIL binds to and protects Plk4 from protein degradation at the site of procentriole formation. On the other hand, the conserved C-terminal region of STIL named truncated in microcephaly (TIM) domain promotes autophosphorylation and degradation of adjacent Plk4 by the direct interaction. Thus, we propose that positive and negative regulation based on the bimodal binding of Plk4 and STIL ensures the formation of a single procentriole per parental centriole.
Display omitted
•Plk4 asymmetry is achieved prior to procentriole formation•Autophosphorylation of Plk4 is critical for the restriction of Plk4 at a single site•Bimodal binding of STIL to Plk4 regulates the activation and degradation of Plk4•The TIM domain of STIL limits Plk4 at centrioles and the number of procentrioles
Ohta et al. show that Plk4 asymmetrically localizes around mother centrioles before the onset of procentriole formation. Furthermore, they reveal that bimodal binding of STIL to Plk4 restricts Plk4 localization at a single site and thus ensures formation of a single procentriole per mother centriole.
PDF
