Nuclear mechanics during cell migration (original) (raw)
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Philosophical Transactions of the Royal Society B: Biological Sciences
Directional cell migration in dense three-dimensional (3D) environments critically depends upon shape adaptation and is impeded depending on the size and rigidity of the nucleus. Accordingly, the nucleus is primarily understood as a physical obstacle; however, its pro-migratory functions by stepwise deformation and reshaping remain unclear. Using atomic force spectroscopy, time-lapse fluorescence microscopy and shape change analysis tools, we determined the nuclear size, deformability, morphology and shape change of HT1080 fibrosarcoma cells expressing the Fucci cell cycle indicator or being pre-treated with chromatin-decondensating agent TSA. We show oscillating peak accelerations during migration through 3D collagen matrices and microdevices that occur during shape reversion of deformed nuclei (recoil), and increase with confinement. During G1 cell-cycle phase, nucleus stiffness was increased and yielded further increased speed fluctuations together with sustained cell migration r...
Biophysical Journal, 2015
In cancer metastasis and other physiological processes, cells migrate through the three-dimensional (3D) extracellular matrix of connective tissue and must overcome the steric hindrance posed by pores that are smaller than the cells. It is currently assumed that low cell stiffness promotes cell migration through confined spaces, but other factors such as adhesion and traction forces may be equally important. To study 3D migration under confinement in a stiff (1.77 MPa) environment, we use soft lithography to fabricate polydimethylsiloxane (PDMS) devices consisting of linear channel segments with 20 mm length, 3.7 mm height, and a decreasing width from 11.2 to 1.7 mm. To study 3D migration in a soft (550 Pa) environment, we use self-assembled collagen networks with an average pore size of 3 mm. We then measure the ability of four different cancer cell lines to migrate through these 3D matrices, and correlate the results with cell physical properties including contractility, adhesiveness, cell stiffness, and nuclear volume. Furthermore, we alter cell adhesion by coating the channel walls with different amounts of adhesion proteins, and we increase cell stiffness by overexpression of the nuclear envelope protein lamin A. Although all cell lines are able to migrate through the smallest 1.7 mm channels, we find significant differences in the migration velocity. Cell migration is impeded in cell lines with larger nuclei, lower adhesiveness, and to a lesser degree also in cells with lower contractility and higher stiffness. Our data show that the ability to overcome the steric hindrance of the matrix cannot be attributed to a single cell property but instead arises from a combination of adhesiveness, nuclear volume, contractility, and cell stiffness.
2012
Cells often migrate in vivo in an extracellular matrix that is intrinsically three-dimensional (3D) and the role of actin filament architecture in 3D cell migration is less well understood. Here we show that, while recently identified linkers of nucleoskeleton to cytoskeleton (LINC) complexes play a minimal role in conventional 2D migration, they play a critical role in regulating the organization of a subset of actin filament bundlesthe perinuclear actin cap-connected to the nucleus through Nesprin2giant and Nesprin3 in cells in 3D collagen I matrix. Actin cap fibers prolong the nucleus and mediate the formation of pseudopodial protrusions, which drive matrix traction and 3D cell migration. Disruption of LINC complexes disorganizes the actin cap, which impairs 3D cell migration. A simple mechanical model explains why LINC complexes and the perinuclear actin cap are essential in 3D migration by providing mechanical support to the formation of pseudopodial protrusions. T he roles of actin filament dynamics and network organization in conventional cell migration and morphology on flat substrates have been studied extensively 1,2. However, fibroblasts often migrate in vivo in an extracellular matrix that is intrinsically three-dimensional (3D) and the role of actin filament architecture in 3D cell migration is less well-understood 3-5. In particular, whether recently discovered connections between nucleus and cytoskeleton mediated by Linkers of the Nucleoskeleton to the Cytoskeleton (LINC) complexes 6 play any role in cell shape, cell migration, and associated protrusion activity in 3D extracellular matrices is unknown 7. This question is important since protrusion activity plays a central role in 3D migration 8,9 , as pseudopodial protrusion processes allow cells to probe the pericellular matrix, locally attach to and pull on surrounding fibers, and detach from them dynamically 10. Corresponding local remodelling of the 3D matrix, which does not occur in conventional 2D migration, is required for effective cell migration inside a 3D matrix 11. This question is also important because cells on planar substrates display a flattened fan-like morphology, while cells completely embedded in a more physiological 3D matrix environment often adopt a spindle-like morphology well suited to negotiate tight matrices 11-15. LINC complexes are protein assemblies that span the nuclear envelope and mediate physical connections between the nuclear lamina and the cytoskeleton 6. These connections are mediated by interactions between SUN (Sad1/UNC-84) domain-containing proteins and KASH (Klarsicht/ANC-1/Syne-1 homology) domain-containing proteins at the outer nuclear membrane 16-21. The down-regulation of both Sun1 and Sun2 prevents the localization of Nesprin-2 giant at the nuclear envelope 6,22. The expression of either the recombinant SUN domain of Sun1 and Sun2 within the ER lumen or the KASH domain of Nesprins (nuclear envelope spectrins; also Syne) 1, 2, and 3 results in the displacement of all Nesprins from the NE to the ER 6,22,23. The KASH domain of Nesprins 1, 2, and 3 can interact promiscuously with either Sun1 or Sun2 23. Whether Nesprins and interactions between Nesprins and Sun proteins play a role in 3D cell migration are unknown. Here we use quantitative functional live-cell assays to show that LINC complexes play a critical role in regulating 3D actin architecture in cells in 3D matrix, and mediate protrusion dynamics, which in turn drive
Bulletin of Mathematical Biology
Recent biological experiments (Lämmermann et al. in Nature 453(7191):51–55, 2008; Reversat et al. in Nature 7813:582–585, 2020; Balzer et al. in ASEB J Off Publ Fed Am Soc Exp Biol 26(10):4045–4056, 2012) have shown that certain types of cells are able to move in structured and confined environments even without the activation of focal adhesion. Focusing on this particular phenomenon and based on previous works (Jankowiak et al. in Math Models Methods Appl Sci 30(03):513–537, 2020), we derive a novel two-dimensional mechanical model, which relies on the following physical ingredients: the asymmetrical renewal of the actin cortex supporting the membrane, resulting in a backward flow of material; the mechanical description of the nuclear membrane and the inner nuclear material; the microtubule network guiding nucleus location; the contact interactions between the cell and the external environment. The resulting fourth order system of partial differential equations is then solved numer...
Size-and speed-dependent mechanical behavior in living mammalian cytoplasm
PNAS (Proceeding of the National Academy of Sciences of the United States of America), 2017
Active transport in the cytoplasm plays critical roles in living cell physiology. However, the mechanical resistance that intracellular compartments experience, which is governed by the cytoplasmic material property, remains elusive, especially its dependence on size and speed. Here we use optical tweezers to drag a bead in the cytoplasm and directly probe the mechanical resistance with varying size a and speed V. We introduce a method, combining the direct measurement and a simple scaling analysis, to reveal different origins of the size-and speed-dependent resistance in living mammalian cy-toplasm. We show that the cytoplasm exhibits size-independent vis-coelasticity as long as the effective strain rate V/a is maintained in a relatively low range (0.1 s −1 < V/a < 2 s −1) and exhibits size-dependent poroelasticity at a high effective strain rate regime (5 s −1 < V/a < 80 s −1). Moreover, the cytoplasmic modulus is found to be positively correlated with only V/a in the viscoelastic regime but also increases with the bead size at a constant V/a in the poroelastic regime. Based on our measurements, we obtain a full-scale state diagram of the living mammalian cytoplasm, which shows that the cytoplasm changes from a viscous fluid to an elastic solid, as well as from compressible material to incompressible material, with increases in the values of two dimensionless parameters, respectively. This state diagram is useful to understand the underlying mechanical nature of the cytoplasm in a variety of cellular processes over a broad range of speed and size scales. cell mechanics | poroelasticity | viscoelasticity | cytoplasmic state diagram T he cytoplasm of living mammalian cells is a crowded, yet dynamic , environment (1). There are continuous intracellular movements that are vital for cell physiology, such as transport of vesicles and other organelles. While biological motors and other enzymatic processes provide key driving forces for these activities, the mechanical properties of the cytoplasm are crucial for determining the mechanical resistance that cellular compartments experience. Indeed, both the active driving force and appropriate mechanical environment are critical for shaping the living cellular machinery. However, while the force that molecular motors generate both individually and collectively has been extensively studied (2, 3), the mechanical properties of the cytoplasmic environment remain elusive. In addition, the impact of object size and velocity on the mechanical resistance that active forces need to overcome to enable transport remains unclear. Such characterization is essential for understanding the physical environment and numerous key dynamic processes inside living cells. The cytoplasm is composed of cytoskeletal networks and many proteins, as well as organelles and vesicles. Materials with such complex microstructure are expected to display time-dependent or frequency-dependent properties (4). Indeed, it has been revealed by many experimental approaches that the mechanical properties of living cells exhibit clear frequency dependency (5, 6); the cell response follows a power-law rheology behavior within a broad frequency range (3, 7, 8). A common view is that cells are visco-elastic materials (9-13), and the observed mechanical properties depend on the timescale over which the deformation occurs during the measurement. One important feature of a viscoelastic material is that its mechanical property does not depend on any length scales of the observation (14). Interestingly, it has recently been demonstrated that living cells may also behave like a poroelastic gel at short timescales (15-17); the stress relaxation of cells can be entirely determined by migration of cytosol through cytoskeletal networks. In the framework of poroelasticity, the measured mechanical property strongly depends on the size of the probe, as it takes a longer time for cytosol to move over a longer distance, therefore the stress relaxes slower. The size-dependent poroelastic behavior is in direct contrast to viscoelasticity whose relaxation is a material property and is independent of probe size, set by the time-dependent response of the materials' macromolecular and supra-molecular constituents. More importantly, most of previous attempts to study cell mechanics probe cells from the exterior, such as by using atomic force microscopy or an optical stretcher, and thus the measurement depends more on the stiff actin-rich cell cortex rather than on the much softer cytoplasm (13, 15, 18). Therefore, it remains unclear if viscoelasticity or poroelasticity better describes the rate-dependent resistance of the cytoplasm of living mammalian cells, or if both are required. In this paper, we use optical tweezers to drag a plastic bead in the cytoplasm of a living mammalian cell and directly measure the force (denoted by F) and displacement (denoted by x) relationship, which reflects the mechanical behavior of the cytoplasm. Considering both viscoelasticity and poroelasticity, we identify two independent dimensional parameters in the experiments: V/a and Va, where V and a represent the speed and diameter of the probe bead, respectively. Using these two control parameters, and through a combination of experimental measurement and scaling analysis, we Significance Although the driving force generated by motor proteins to deliver intracellular cargos is widely studied, the mechanical nature of cytoplasm, which is also important for intracellular processes by providing mechanical resistance, remains unclear. We use optical tweezers to directly characterize the resistance to transport in living mammalian cytoplasm. Using scaling analysis, we successfully distinguish between the underlying mechanisms governing the resistance to mechanical deformation, that is, among viscosity, viscoelasticity, poroelasticity, or pure elasticity, depending on the speed and size of the probe. Moreover, a cytoplasmic state diagram is obtained to illustrate different mechanical behaviors as a function of two dimensionless parameters; with this, the underlying mechanics of various cellular processes over a broad range of speed and size scales is revealed.
Influence of nucleus deformability on cell entry into cylindrical structures
Biomechanics and Modeling in Mechanobiology, 2013
Mechanical properties of cell nucleus have been demonstrated to play a fundamental role in cell movement across extracellular networks and micro-channels. In this work, we focus on the mathematical description of a cell entering a cylindrical channel composed of extracellular matrix. An energetic approach is derived in order to obtain a necessary condition for which cells enter cylindrical structures. The nucleus of the cell is treated either (i) as an elastic membrane surrounding a liquid droplet or (ii) as an incompressible elastic material with Neo-Hookean constitutive equation. The results obtained highlight the importance of the interplay between mechanical deformability of the nucleus and the capability of the cell to establish adhesive bonds.
2009
Cellular traction forces, resulting in cell-substrate physical interactions, are generated by actin-myosin complexes and transmitted to the extracellular matrix through focal adhesions. These processes are highly dynamic under physiological conditions and modulate cell migration. To better understand the precise dynamics of cell migration, we measured the spatiotemporal redistribution of cellular traction stresses (force per area) during fibroblast migration at a submicron level and correlated it with nuclear translocation, an indicator of cell migration, on a physiologically relevant extracellular matrix mimic. We found that nuclear translocation occurred in pulses whose magnitude was larger on the low ligand density surfaces than on the high ligand density surfaces. Large nuclear translocations only occurred on low ligand density surfaces when the rear traction stresses completely relocated to a posterior nuclear location, whereas such relocation took much longer time on high ligand density surfaces, probably due to the greater magnitude of traction stresses. Nuclear distortion was also observed as the traction stresses redistributed. Our results suggest that the reinforcement of the traction stresses around the nucleus as well as the relaxation of nuclear deformation are critical steps during fibroblast migration, serving as a speed regulator, which must be considered in any dynamic molecular reconstruction model of tissue cell migration. A traction gradient foreshortening model was proposed to explain how the relocation of rear traction stresses leads to pulsed fibroblast migration.
2020
During cell migration in confinement, the nucleus has to deform for a cell to pass through small constrictions. Such nuclear deformations require significant forces. A direct experimental measure of the deformation force field is extremely challenging. However, experimental images of nuclear shape are relatively easy to obtain. Therefore, here we present a method to calculate predictions of the deformation force field based purely on analysis of experimental images of nuclei before and after deformation. Such an inverse calculation is technically non-trivial and relies on a mechanical model for the nucleus. Here we compare two simple continuum elastic models of a cell nucleus undergoing deformation. In the first, we treat the nucleus as a homogeneous elastic solid and, in the second, as an elastic shell. For each of these models we calculate the force field required to produce the deformation given by experimental images of nuclei in dendritic cells migrating in microchannels with c...
A Chemomechanical Model for Nuclear Morphology and Stresses during Cell Transendothelial Migration
Biophysical journal, 2016
It is now evident that the cell nucleus undergoes dramatic shape changes during important cellular processes such as cell transmigration through extracellular matrix and endothelium. Recent experimental data suggest that during cell transmigration the deformability of the nucleus could be a limiting factor, and the morphological and structural alterations that the nucleus encounters can perturb genomic organization that in turn influences cellular behavior. Despite its importance, a biophysical model that connects the experimentally observed nuclear morphological changes to the underlying biophysical factors during transmigration through small constrictions is still lacking. Here, we developed a universal chemomechanical model that describes nuclear strains and shapes and predicts thresholds for the rupture of the nuclear envelope and for nuclear plastic deformation during transmigration through small constrictions. The model includes actin contraction and cytosolic back pressure th...