Mitotic spindle orients perpendicular to the forces imposed by dynamic shear

Confinement in fibrous environments positions and orients mitotic spindles

Accurate positioning of the mitotic spindle within the rounded cell body is critical to physiological maintenance. Adherent mitotic cells encounter confinement from neighboring cells or the extracellular matrix (ECM), which can cause rotation of mitotic spindles and, consequently, titling of the metaphase plate (MP). To understand the positioning and orientation of mitotic spindles under confinement by fibers (ECM-confinement), we use flexible ECM-mimicking nanofibers that allow natural rounding of the cell body while confining it to differing levels. Rounded mitotic bodies are anchored in place by actin retraction fibers (RFs) originating from adhesion clusters on the ECM-mimicking fibers. We discover the extent of ECM-confinement patterns RFs in 3D: triangular and band-like at low and high confinement, respectively. A stochastic Monte-Carlo simulation of the centrosome (CS), chromosome (CH), membrane interactions, and 3D arrangement of RFs on the mitotic body recovers MP tilting trends observed experimentally. Our mechanistic analysis reveals that the 3D shape of RFs is the primary driver of the MP rotation. Under high ECM-confinement, the fibers can mechanically pinch the cortex, causing the MP to have localized deformations at contact sites with fibers. Interestingly, high ECM-confinement leads to low and high MP tilts, which mechanistically depend upon the extent of cortical deformation, RF patterning, and MP position. We identify that cortical deformation and RFs work in tandem to limit MP tilt, while asymmetric positioning of MP leads to high tilts. Overall, we provide fundamental insights into how mitosis may proceed in fibrous ECM-confining microenvironments in vivo.

Mechanical stretching determines the orientation of osteoblast migration and cell division

Osteoblasts alignment and migration are involved in the directional formation of bone matrix and bone remodeling. Many studies have demonstrated that mechanical stretching controls osteoblast morphology and alignment. However, little is known about its effects on osteoblast migration. Here, we investigated changes in the morphology and migration of preosteoblastic MC3T3-E1 cells after the removal of continuous or cyclic stretching. Actin staining and time-lapse recording were performed after stretching removal. The continuous and cyclic groups showed parallel and perpendicular alignment to the stretch direction, respectively. A more elongated cell morphology was observed in the cyclic group than in the continuous group. In both stretch groups, the cells migrated in a direction roughly consistent with the cell alignment. Compared to the other groups, the cells in the cyclic group showed an increased migration velocity and were almost divided in the same direction as the alignment. To summarize, our study showed that mechanical stretching changed cell alignment and morphology in osteoblasts, which affected the direction of migration and cell division, and velocity of migration. These results suggest that mechanical stimulation may modulate the direction of bone tissue formation by inducing the directional migration and cell division of osteoblasts.

Isotropic myosin-generated tissue tension is required for the dynamic orientation of the mitotic spindle

The ability of cells to divide along their longest axis has been proposed to play an important role in maintaining epithelial tissue homeostasis in many systems. Because the division plane is largely set by the position of the anaphase spindle, it is important to understand how spindles become oriented. While several molecules have been identified that play key roles in spindle orientation across systems, most notably Mud/NuMA and cortical dynein, the precise mechanism by which spindles detect and align with the long cell axis remain poorly understood. Here, in exploring the dynamics of spindle orientation in mechanically distinct regions of the fly notum, we find that the ability of cells to properly reorient their divisions depends on local tissue tension. Thus, spindles reorient to align with the long cell axis in regions where isotropic tension is elevated, but fail to do so in elongated cells within the crowded midline, where tension is low, or in regions that have been mechanically isolated from the rest of the tissue via laser ablation. Importantly, these differences in spindle behavior outside and inside the midline can be recapitulated by corresponding changes in tension induced by perturbations that alter nonmuscle myosin II activity. These data lead us to propose that isotropic tension within an epithelium provides cells with a mechanically stable substrate upon which localized cortical motor complexes can act on astral microtubules to orient the spindle.

Force-generating mechanisms of anaphase in human cells

What forces drive chromosome segregation remains one of the most challenging questions in cell division. Even though the duration of anaphase is short, it is of utmost importance for genome fidelity that no mistakes are made. Seminal studies in model organisms have revealed different mechanisms operating during chromosome segregation in anaphase, but the translation of these mechanisms to human cells is not straightforward. Recent work has shown that kinetochore fiber depolymerization during anaphase A is largely motor independent, whereas spindle elongation during anaphase B is coupled to sliding of interpolar microtubules in human cells. In this Review, we discuss the current knowledge on the mechanisms of force generation by kinetochore, interpolar and astral microtubules. By combining results from numerous studies, we propose a comprehensive picture of the role of individual force-producing and -regulating proteins. Finally, by linking key concepts of anaphase to most recent data, we summarize the contribution of all proposed mechanisms to chromosome segregation and argue that sliding of interpolar microtubules and depolymerization at the kinetochore are the main drivers of chromosome segregation during early anaphase in human cells.

Closer to Nature Through Dynamic Culture Systems

Mechanics in the human body are required for normal cell function at a molecular level. It is now clear that mechanical stimulations play significant roles in cell growth, differentiation, and migration in normal and diseased cells. Recent studies have led to the discovery that normal and cancer cells have different mechanosensing properties. Here, we discuss the application and the physiological and pathological meaning of mechanical stimulations. To reveal the optimal conditions for mimicking an in vivo microenvironment, we must, therefore, discern the mechanotransduction occurring in cells.

A Biophysical Model for Curvature-Guided Cell Migration

The latest experiments have shown that adherent cells can migrate according to cell-scale curvature variations via a process called curvotaxis. Despite identification of key cellular factors, a clear understanding of the mechanism is lacking. We employ a mechanical model featuring a detailed description of the cytoskeleton filament networks, the viscous cytosol, the cell adhesion dynamics, and the nucleus. We simulate cell adhesion and migration on sinusoidal substrates. We show that cell adhesion on three-dimensional curvatures induces a gradient of pressure inside the cell that triggers the internal motion of the nucleus. We propose that the resulting out-of-equilibrium position of the nucleus alters cell migration directionality, leading to cell motility toward concave regions of the substrate, resulting in lower potential energy states. Altogether, we propose a simple mechanism explaining how intracellular mechanics enable the cells to react to substratum curvature, induce a deterministic cell polarization, and break down cells basic persistent random walk, which correlates with latest experimental evidences.

Adaptive responses of murine osteoblasts subjected to coupled mechanical stimuli

Restitution of the natural organization and orientation of cells is imperative for the construction of functional tissue scaffolds. While numerous studies have exploited mechanical methods to engineer tissues with the desired cellular architecture, fundamental knowledge is still lacking in understanding the manner in which morphological features can be modulated through coupled mechanical cues. To address this knowledge gap, the adhesion and alignment response of murine osteoblast cells under the synergistic effects of matrix rigidity and cyclic mechanical loading was investigated. This was accomplished by applying cyclic mechanical strain (1% at 0.05Hz) to MC3T3-E1 cells seeded on PDMS substrates of different elastic moduli (1.22, 1.70 and 2.04MPa). Results demonstrate that the overall cell density and expression of inactive vinculin increased on substrates subjected to cyclic stimulus in comparison to substrates under static loading. Conversely, in terms of the adhesion response, osteoblasts exhibited an increased growth of focal adhesion complexes under static substrates. Interestingly, results also elucidate that substrates of a stiffer matrix exposed to cyclic stimulus, had a significantly higher percentage of osteoblasts aligned parallel to the direction of the applied strain, as well as a higher degree of internal order with respect to the strain axis, in comparison to both cells seeded on substrates of lower stiffness under cyclic loading or under static conditions. These findings suggest the role of cyclic mechanical strain coupled with matrix rigidity in eliciting mechanosensitive adaptations in cell functions that allow for the reconstitution of the spatial and orientational assembly of cells in vivo for tissue engineering.

Colloquium: Mechanical formalisms for tissue dynamics

The understanding of morphogenesis in living organisms has been renewed by tremendous progress in experimental techniques that provide access to cell scale, quantitative information both on the shapes of cells within tissues and on the genes being expressed. This information suggests that our understanding of the respective contributions of gene expression and mechanics, and of their crucial entanglement, will soon leap forward. Biomechanics increasingly benefits from models, which assist the design and interpretation of experiments, point out the main ingredients and assumptions, and ultimately lead to predictions. The newly accessible local information thus calls for a reflection on how to select suitable classes of mechanical models. We review both mechanical ingredients suggested by the current knowledge of tissue behaviour, and modelling methods that can help generate a rheological diagram or a constitutive equation. We distinguish cell scale ("intra-cell") and tissue scale ("inter-cell") contributions. We recall the mathematical framework developed for continuum materials and explain how to transform a constitutive equation into a set of partial differential equations amenable to numerical resolution. We show that when plastic behaviour is relevant, the dissipation function formalism appears appropriate to generate constitutive equations; its variational nature facilitates numerical implementation, and we discuss adaptations needed in the case of large deformations. The present article gathers theoretical methods that can readily enhance the significance of the data to be extracted from recent or future high throughput biomechanical experiments.

Force and the spindle: mechanical cues in mitotic spindle orientation

The mechanical environment of a cell has a profound effect on its behaviour, from dictating cell shape to driving the transcription of specific genes. Recent studies have demonstrated that mechanical forces play a key role in orienting the mitotic spindle, and therefore cell division, in both single cells and tissues. Whilst the molecular machinery that mediates the link between external force and the mitotic spindle remains largely unknown, it is becoming increasingly clear that this is a widely used mechanism which could prove vital for coordinating cell division orientation across tissues in a variety of contexts.

The mechanics behind cell division

It is now well established that the orientation of the plane of cell division highly depends on cell geometry in plants. However, the related molecular mechanism remains largely unknown. Recent data in animal systems highlight the role of the cytoskeleton response to mechanical stress in this process. Interestingly, these results are consistent with some data obtained in parallel in plants. Here we review and confront these studies, across kingdoms, and we explore the possibility that the intrinsic mechanical properties of the cytoskeleton play a key role in the nexus between cell division and mechanical stress. This opens many avenues for future research that are also discussed in this review.

Automated tracking of mitotic spindle pole positions shows that LGN is required for spindle rotation but not orientation maintenance

Spindle orientation defines the plane of cell division and, thereby, the spatial position of all daughter cells. Here, we develop a live cell microscopy-based methodology to extract spindle movements in human epithelial cell lines and study how spindles are brought to a pre-defined orientation. We show that spindles undergo two distinct regimes of movements. Spindles are first actively rotated toward the cells' long-axis and then maintained along this pre-defined axis. By quantifying spindle movements in cells depleted of LGN, we show that the first regime of rotational movements requires LGN that recruits cortical dynein. In contrast, the second regime of movements that maintains spindle orientation does not require LGN, but is sensitive to 2ME2 that suppresses microtubule dynamics. Our study sheds first insight into spatially defined spindle movement regimes in human cells, and supports the presence of LGN and dynein independent cortical anchors for astral microtubules.