Cell–tissue–tissue interaction is determined by specific short range forces between cell adhesion molecules (CAMs) and ligands of the tissue, long range repulsion forces mediated by cell surface ...grafted macromolecules and adhesion-induced elastic stresses in the cell envelope. This interplay of forces triggers the rapid random clustering of tightly coupled linkers. By coupling of actin gel patches to the intracellular domains of the CAMs, these clusters can grow in a secondary process resulting in the formation of functional adhesion microdomains (ADs). The ADs can act as biochemical steering centers by recruiting and activating functional proteins, such as GTPases and associated regulating proteins, through electrostatic–hydrophobic forces with cationic lipid domains that act as attractive centers.
First, I summarize physical concepts of cell adhesion revealed by studies of biomimetic systems. Then I describe the role of the adhesion domains as biochemical signaling platforms and force transmission centers promoting cellular protrusions, in terms of a shell string model of cells. Protrusion forces are generated by actin gelation triggered by molecular machines (focal adhesion kinase (FAK), Src-kinases and associated adaptors) which assemble around newly formed integrin clusters. They recruit and activate the GTPases Rac-1 and actin gelation promoters to charged membrane domains via electrostatic–hydrophobic forces. The cell front is pushed forward in a cyclic and stepwise manner and the step-width is determined by the dynamics antagonistic interplay between Rac-1 and RhoA. The global cell polarization in the direction of motion is mediated by the actin–microtubule (MT) crosstalk at adhesion domains. Supramolecular actin–MT assemblies at the front help to promote actin polymerization. At the rear they regulate the dismantling of the ADs through the Ca++-mediated activation of the protease calpain and trigger their disruption by RhoA mediated contraction via stress fibers. This article is part of a Special Issue entitled: Mechanobiology.
Directed cell locomotion is driven by cyclic progression of the front and retraction of the end regulated by actin microtubule (MT) crosstalk. Adhesion domains (ADs) formed about integrin clusters act as biochemical steering and force generating centers. Their function is controlled in a Ca++- and force-dependent way by quaternary complexes of actin, IQGAP, calmodulin, and microtubule. The rhythmic cell progression is determined by the dynamic antagonism between the GTPases Rac-1 and Rho-A. The retraction is mediated by protease (P)-activated by Ca++ release from ER stores which mediates the dismantling and retraction of rearmost ADs by activation of RhoA and inhibition of Rac-1 in the cell–tissue adhesion zone. Display omitted
•Cells migrate by concerted progression of the front and retraction of the end.•The stepwise process is guided by actin microtubule crosstalk at adhesion domains.•Actin gelation machines assemble about integrin clusters by electrostatic forces.•FAK/Src kinase tandem and strain sensitive Cas/Crk cascades trigger actin gelation.•Calcium sensitive IQGAP–calmodulin complexes act as Ca-sensitive on–off-switches.
Cell migration plays vital roles in many biologically relevant processes such as tissue morphogenesis and cancer metastasis, and it has fascinated biophysicists over the past several decades. ...However, despite an increasing number of studies highlighting the orchestration of proteins involved in different signaling pathways, the functional roles of lipid membranes have been essentially overlooked. Lipid membranes are generally considered to be a functionless two-dimensional matrix of proteins, although many proteins regulating cell migration gain functions only after they are recruited to the membrane surface and self-organize their functional domains. In this review, we summarize how the logistical recruitment and release of proteins to and from lipid membranes coordinates complex spatiotemporal molecular processes. As predicted from the classical framework of the Smoluchowski equation of diffusion, lipid/protein membranes serve as a 2D reaction hub that contributes to the effective and robust regulation of polarization and migration of cells involving several competing pathways.
The cellular cytoskeleton is a fascinating active network, in which Brownian motion is intercepted by distinct phases of active transport. We present a time-resolved statistical analysis dissecting ...phases of directed motion out of otherwise diffusive motion of tracer particles in living cells. The distribution of active lifetimes is found to decay exponentially with a characteristic time tauA = 0.65 s. The velocity distribution of active events exhibits several peaks, in agreement with a discrete number of motor proteins acting collectively.
In living cells, adhesion structures have the astonishing ability to grow and strengthen under force. Despite the rising evidence of the importance of this phenomenon, little is known about the ...underlying mechanism. Here, we show that force-induced adhesion-strengthening can occur purely because of the thermodynamic response to the elastic deformation of the membrane, even in the absence of the actively regulated cytoskeleton of the cell, which was hitherto deemed necessary. We impose pN-forces on two fluid membranes, locally pre-adhered by RGD-integrin binding. One of the binding partners is always mobile whereas the mobility of the other can be switched on or off. Immediate passive strengthening of adhesion structures occurs in both cases. When both binding partners are mobile, strengthening is aided by lateral movement of intact bonds as a transient response to force-induced membrane-deformation. By extending our microinterferometric technique to the suboptical regime, we show that the adhesion, as well as the resistance to force-induced de-adhesion, is greatly enhanced when both, rather than only one, of the binding partners are mobile. We formulate a theory that explains our observations by linking the macroscopic shape deformation with the microscopic formation of bonds, which further elucidates the importance of receptor mobility. We propose this fast passive response to be the first-recognition that triggers signaling events leading to mechanosensing in living cells.
This review deals with insights into complex cellular structures and processes obtained by measuring viscoelastic impedances of the cell envelope and the cytoplasm by colloidal bead microrheometry. I ...first introduce a mechanical cell model that allows us to understand their unique ability of mechanical self-stabilization by actin microtubule crosstalk. In the second part, I show how cell movements can be driven by pulsatile or propagating solitary actin gelatin waves (SAGW) that are generated on nascent adhesion domains by logistically controlled membrane recruitment of functional proteins by electrostatic-hydrophobic forces. The global polarization of cell migration is guided by actin-microtubule crosstalk that is mediated by the Ca++ and strain-sensitive supramolecular scaffolding protein IQGAP. In the third part, I introduce the traction force microscopy as a tool to measure the forces between somatic cells and the tissue ´Here I show, how absolute values of viscoelastic impedances of the composite cell envelope can be obtained by deformation field mapping techniques. In the fourth part, it is shown how the dynamic mechanical properties of the active viscoplastic cytoplasmic space can be evaluated using colloidal beads as phantom endosomes. Separate measurements of velocity distributions of directed and random motions of phantom endosomes, yield local values of transport forces, viscosities and life times of directed motion along microtubules. The last part deals with biomimetic experiments allowing us to quantitatively evaluate the mechanical properties of passive and active actin networks on the basis of the percolation theory of gelation.
We measured the viscoelastic properties of the cytoplasm of J774 macrophages with a recently developed microrheometer. Ferromagnetic beads (1.3
μm in diameter) were used to determine the local ...viscoelastic moduli. Step-force pulses were applied to the magnetic beads and the displacement was observed by single particle tracking. By analyzing the creep response curves in terms of a triphasic mechanical equivalent circuit, we measured the shear elastic modulus, the effective viscosities, and the strain relaxation time. The values of the shear modulus vary by more than an order of magnitude within the cell population (range, 20–735
Pa; average, 343
Pa) and by a factor of 2 within single cells. The effective viscosity of the cytoplasm exhibits a relatively sharp distribution about an average of
η
=
210
Pa s (± 143
Pa
s). We measured the displacement field generated by the local forces by observing the induced motion of nonmagnetic beads. Even at distances of the order of 1
μm, no induced motion was seen, suggesting that the cytoplasm is composed of clusters of densely packed and cross-linked filaments separated by soft regions. In another series of experiments we analyzed the magnetophoretic motion of the ferromagnetic beads at a constant magnetic force. Measuring the bead velocity parallel and perpendicular to the applied force showed that local active forces on the beads varied from 50 to 900
pN.
Cellular adhesion is a central element in tissue mechanics, biological cell–cell signaling, and cell motility. In this context, the cell–substrate distance has been investigated in the past by ...studying natural cells and biomimetic cell models adhering on solid substrates. The amount of water in the membrane substrate gap, however, is difficult to determine. Here, we present a neutron reflectivity (NR) structural study of confluent epithelial cell monolayers on silicon substrates. In order to ensure valid in vitro conditions, we developed a cell culture sample chamber allowing us to grow and cultivate cells under proper cell culture conditions while performing in vitro neutron reflectivity measurements. The cell chamber also enabled perfusion with cell medium and hence allowed for contrast variation in situ by sterile exchange of buffer with different H2O-to-D2O ratio. Contrast variation reduces the ambiguity of data modeling for determining the thickness and degree of hydration of the interfacial cleft between the adherent cells and the substrate. Our data suggest a three-layer interfacial organization. The first layer bound to the silicon surface interface is in agreement with a very dense protein film with a thickness of 9 ± 2 nm, followed by a highly hydrated 24 ± 4 nm thick layer, and a several tens of nanometers thick layer attributed to the composite membrane. Hence, the results provide clear evidence of a highly hydrated intermediate region between the composite cell membrane and the substrate, reminiscent of the basal lamina.
We demonstrate that composite envelopes of nucleated cells exhibit pronounced short wavelength (<or=0.5 microm) bending excitations of approximately 10 nm root mean square amplitudes at physiological ...temperatures, which impede strong adhesion due to entropic repulsion forces. Quantitative microinterferometric analysis of the dynamic cell surface roughness of macrophages in terms of the theory of statistical surfaces suggests that the membrane excitations are mainly thermally driven Brownian motions (although active driving forces may contribute substantially). We determine the effective bending modulus of the cell envelope (approximately 1000kBT), the cortical tension (approximately 10(-4) N m-1), and the work of adhesion (approximately 10(-5) J m-2).
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