Dynamic change in the distribution of alpha5beta1 integrin on isolated ventral membrane: effect of divalent cation species

Non-muscle myosin II induces disassembly of actin stress fibres independently of myosin light chain dephosphorylation

Dynamic remodelling of actin stress fibres (SFs) allows non-muscle cells to adapt to applied forces such as uniaxial cell shortening. However, the mechanism underlying rapid and selective disassembly of SFs oriented in the direction of shortening remains to be elucidated. Here, we investigated how myosin crossbridge cycling induced by MgATP is associated with SF disassembly. Moderate concentrations of MgATP, or [MgATP], induced SF contraction. Meanwhile, at [MgATP] slightly higher than the physiological level, periodic actin patterns emerged along the length of SFs and dispersed within seconds. The actin fragments were diverse in length, but comparable to those in characteristic sarcomeric units of SFs. These results suggest that MgATP-bound non-muscle myosin II dissociates from the individual actin filaments that constitute the sarcomeric units, resulting in unbundling-induced disassembly rather than end-to-end actin depolymerization. This rapid SF disassembly occurred independent of dephosphorylation of myosin light chain. In terms of effects on actin-myosin interactions, a rise in [MgATP] is functionally equivalent to a temporal decrease in the total number of actin-myosin crossbridges. Actin-myosin crossbridges are known to be reduced by an assisting load on myosin. Thus, the present study suggests that reducing the number of actin-myosin crossbridges promotes rapid and orientation-dependent disassembly of SFs after cell shortening.

Integrin clustering is driven by mechanical resistance from the glycocalyx and the substrate

Integrins have emerged as key sensory molecules that translate chemical and physical cues from the extracellular matrix (ECM) into biochemical signals that regulate cell behavior. Integrins function by clustering into adhesion plaques, but the molecular mechanisms that drive integrin clustering in response to interaction with the ECM remain unclear. To explore how deformations in the cell-ECM interface influence integrin clustering, we developed a spatial-temporal simulation that integrates the micro-mechanics of the cell, glycocalyx, and ECM with a simple chemical model of integrin activation and ligand interaction. Due to mechanical coupling, we find that integrin-ligand interactions are highly cooperative, and this cooperativity is sufficient to drive integrin clustering even in the absence of cytoskeletal crosslinking or homotypic integrin-integrin interactions. The glycocalyx largely mediates this cooperativity and hence may be a key regulator of integrin function. Remarkably, integrin clustering in the model is naturally responsive to the chemical and physical properties of the ECM, including ligand density, matrix rigidity, and the chemical affinity of ligand for receptor. Consistent with experimental observations, we find that integrin clustering is robust on rigid substrates with high ligand density, but is impaired on substrates that are highly compliant or have low ligand density. We thus demonstrate how integrins themselves could function as sensory molecules that begin sensing matrix properties even before large multi-molecular adhesion complexes are assembled.

Critical cell decisions, including whether to live, proliferate, or assemble into tissue structures, are directed by cues from the extracellular matrix, the external protein scaffold that surrounds cells. Integrin receptors on the cell surface bind to the extracellular matrix and cluster into complexes that translate matrix cues into the set of instructions a cell follows. Using a newly developed model of the cell-matrix interface, in this work we detail a simple yet efficient mechanism by which integrins could “sense” important matrix properties, including chemical composition and mechanical stiffness, and cluster appropriately. This mechanism relies on mechanical resistance to integrin-matrix interaction provided by the glycocalyx, the slimy sugar and protein coating on the cell, as well as the stiffness of the matrix and the cell itself. In general, the resistance alters integrin-ligand reaction rates, such that integrin clustering is favored for many physiologically relevant conditions. Interestingly, the mechanical properties of the cell and ECM are altered in many prevalent diseases, such as cancer, and our work suggests how these mechanical perturbations might adversely influence integrin function.

Topical Application of Drugs Influencing Cytoskeleton and Cell Contractility Affects Alveolar Bone Loss in Rats

BACKGROUND

Several studies have shown that sectioning bundles of collagen fibers in the marginal gingiva during surgical procedures in animals is a distinct stimulus for alveolar bone resorption. Normally, gingival and periodontal fibroblasts, which reside on these collagen fibers, create physiological traction forces generated by the cytoskeleton. By splitting the fibers, traction forces are released, inducing changes in the cytoskeleton and cell shape. In this study, four drugs were selected, including cytochalasin D, EDTA, sodium orthovanadate, and H-7, all influencing the cytoskeleton-integrin-extracellular matrix (ECM) pathway, for their ability to reduce alveolar bone loss by local application.

METHODS

The drugs were applied locally only once at the site of mucoperiosteal flap surgery in a rat model. Cytochalasin D (1 microl/microl), EDTA (0.24 mg/microl), sodium orthovanadate (0.02 mg/microl), and H-7 (0.10 microl/microl), each separately, were carried by a protective paste and placed immediately after elevating the flap. The analysis of alveolar bone loss was performed 3 weeks after surgery by scanning the microradiographic films of the mandible cross-sections. The percentages of cross sections with no, moderate, or severe bone loss in treated in comparison to non-treated rats are presented.

RESULTS

EDTA, sodium orthovanadate, and H-7 were significantly effective in reducing alveolar bone loss. They were effective in reducing the amount of severe bone loss by 53%, 20%, and 58% while increasing the number of sections with no bone loss by 25%, 23%, and 35%, respectively. Cytochalasin D reduced alveolar bone loss insignificantly.

CONCLUSION

EDTA, sodium orthovanadate, and H-7 are effective in reducing alveolar bone loss in rats following mucoperiosteum surgery.

Mobility of integrin alpha5beta1 measured on the isolated ventral membranes of human skin fibroblasts

We have measured the lateral mobility of individual alpha5 integrin molecules in ventral plasma membranes of fibroblasts, which were prepared by removal of apical surfaces and nuclei followed by elimination of actin filaments with gelsolin, an actin-severing protein. The cytoplasmic domain of individual integrin molecules was tagged with 100 nm fluorescent polystyrene bead, and motion of the bead was observed and video-recorded. Position of the bead in each frame was determined from the centroid of the fluorescence image, from which plots of the mean-square displacement against time intervals were derived. Within short intervals of time (<100 ms) the mean-square displacement was proportional to the time interval, and the averaged translational diffusion coefficient of (5.3+/-4.4) x 10(-10) cm2/s was obtained with a broad distribution of (1.3-20) x 10(-10) cm2/s. The broad distribution might reflect the oligomerized state of integrin. The largest diffusion coefficient was comparable to that of lipid molecules previously measured in cells and probably represented the diffusion of a single integrin molecule in the presence of little interference of actin cytoskeleton or extracellular matrix. In longer time intervals (>100 ms) the motion of the bead was confined in an area, the average diameter of which was 410+/-160 nm. This was similar to the values described in previous reports, in which the motion of other membrane receptors labeled on their extracellular domain was measured in living cells.