Matching material and cellular timescales maximizes cell spreading on viscoelastic substrates

Recent evidence has shown that, in addition to rigidity, the viscous response of the extracellular matrix (ECM) significantly affects the behavior and function of cells. However, the mechanism behind such mechanosensitivity toward viscoelasticity remains unclear. In this study, we systematically examined the dynamics of motor clutches (i.e., focal adhesions) formed between the cell and a viscoelastic substrate using analytical methods and direct Monte Carlo simulation. Interestingly, we observe that, for low ECM rigidity, maximum cell spreading is achieved at an optimal level of viscosity in which the substrate relaxation time falls between the timescale for clutch binding and its characteristic binding lifetime. That is, viscosity serves to stiffen soft substrates on a timescale faster than the clutch off-rate, which enhances cell−ECM adhesion and cell spreading. On the other hand, for substrates that are stiff, our model predicts that viscosity will not influence cell spreading, since the bound clutches are saturated by the elevated stiffness. The model was tested and validated using experimental measurements on three different material systems and explained the different observed effects of viscosity on each substrate. By capturing the mechanism by which substrate viscoelasticity affects cell spreading across a wide range of material parameters, our analytical model provides a useful tool for designing biomaterials that optimize cellular adhesion and mechanosensing.

Spectrin Organization and Dynamics: New Insights

Spectrin is the major constituent protein of the erythrocyte cytoskeleton which forms a filamentous network on the cytoplasmic face of the membrane by providing a scaffold for a variety of proteins. In this review, several aspects of spectrin organization are highlighted, particularly with respect to its ability to bind hydrophobic ligands and its interaction with membrane surfaces. The characteristic binding of the fluorescent hydrophobic probes Prodan and pyrene to spectrin, which allows an estimation of the polarity of the hydrophobic probe binding site, is illustrated. In addition, the contribution of uniquely localized and conserved tryptophan residues in the ‘spectrin repeats’ in these processes is discussed. A functional implication of the presence of hydrophobic binding sites in spectrin is its recently discovered chaperone-like activity. Interestingly, spectrin exhibits residual structural integrity even after denaturation which could be considered as a hallmark of cytoskeletal proteins. Future research could provide useful information about the possible role played by spectrin in cellular physiology in healthy and diseased states.

Entrapment of helium ions at (100) and (110) tungsten surfaces

Two (100) and two (110) tungsten surfaces have been bombarded with helium ions of selected energies between 5 and 2000 eV. The helium thermal desorption spectra obtained when the targets were subsequently heated at 40 °K/s were used to deduce the fraction of the incident particles trapped in the crystal, and their binding energies. For ion energies ≤400 eV, the fractions trapped were small (~ 1 × 10−3 on the (100) surfaces and ~ 1 × 10−2 on the (110) surfaces) and almost all the trapping occurred in surface-related sites with binding energies ≤ 2.1 eV. For ion energies >500 eV, trapping occurred predominantly in sites characteristic of the bulk material, the sites being created by the incoming ions as soon as their energy was sufficient to produce atomic displacements in the lattice (480 eV). At doses <1 × 1013 ions/cm2, four characteristic binding energies were observed: 2.65, 3.05, 3.35, and 4.15 eV. At higher doses, additional binding states were observed with energies up to 5.4 eV.The results suggest that helium diffuses rapidly in tungsten at room temperature and that it is trapped within the crystals only if it encounters lattice defects. Estimates of the ion penetration depth lead to the tentative conclusion that in the annealed crystals (ion energies < 400 eV) the fractional trap concentration is <1 × 10−9 and the helium incident at a few hundred eV energy becomes trapped at depths up to at least 1 μ.

A study of the interaction of copper(II). zinc(II), and cadmium(II) with transfusion gelatin

The binding of copper, zinc, and cadmium with transfusion gelatin has been measured indirectly by observing the displacement of hydrogen ions from the acidic groups of transfusion gelatin. It is seen that the binding data obtained from pH measurements agree well with those obtained from equilibrium dialysis. Copper, zinc, and cadmium show the characteristic binding with the carboxyl groups in the pH range 3-5.5 for which the log K values are found to be 2.18, 1.87, and 1.96 respectively. Potentiometric titration data also indicate that the interaction is a competitive one, i.e. the metal ions compete with the hydrogen ions for common sites. The extent of copper and zinc binding increases with increase in pH, whereas cadmium binding remains unaffected. It is, therefore, concluded that copper and zinc combine with the imidazole groups also (log K values are 3.40 and 2.91 respectively), but cadmium fails to react with the imidazole groups.