Jingyuan Xu

Strain Hardening of Actin Filament Networks REGULATION BY THE DYNAMIC CROSS-LINKING PROTEIN α-ACTININ

Mechanical stresses applied to the plasma membrane of an adherent cell induces strain hardening of the cytoskeleton, i.e. the elasticity of the cytoskeleton increases with its deformation. Strain hardening is thought to mediate the transduction of mechanical signals across the plasma membrane through the cytoskeleton. Here, we describe the strain dependence of a model system consisting of actin filaments (F-actin), a major component of the cytoskeleton, and the F-actin cross-linking protein α-actinin, which localizes along contractile stress fibers and at focal adhesions. We show that the amplitude and rate of shear deformations regulate the resilience of F-actin networks. At low temperatures, for which the lifetime of binding of α-actinin to F-actin is long, F-actin/α-actinin networks exhibit strong strain hardening at short time scales and soften at long time scales. For F-actin networks in the absence of α-actinin or for F-actin/α-actinin networks at high temperatures, strain hardening appears only at very short time scales. We propose a model of strain hardening for F-actin networks, based on both the intrinsic rigidity of F-actin and dynamic topological constraints formed by the cross-linkers located at filaments entanglements. This model offers an explanation for the origin of strain hardening observed when shear stresses are applied against the cellular membrane.

Keratin Filament Suspensions Show Unique Micromechanical Properties

All epithelial cells feature a prominent keratin intermediate filament (IF) network in their cytoplasm. Studies in transgenic mice and in patients with inherited epithelial fragility syndromes showed that a major function of keratin IFs is to provide mechanical support to epithelial cell sheets. Yet the micromechanical properties of keratin IFs themselves remain unknown. We used rheological methods to assess the properties of suspensions of epidermal type I and type II keratin IFs and of vimentin, a type III IF polymer. We find that both types of IFs form gels with properties akin to visco-elastic solids. With increasing deformation they display strain hardening and yield relatively rapidly. Remarkably, both types of gels recover their preshear properties upon cessation of the deformation. Repeated imposition of small deformations gives rise to a progressively stiffer gel for keratin but not vimentin IFs. The visco-elastic moduli of both gels show a weak dependence upon the frequency of the input shear stress and the concentration of the polymer, suggesting that both steric and nonsteric interactions between individual polymers contribute to the observed mechanical properties. In support of this, the length of individual polymers contributes only modestly to the properties of IF gels. Collectively these properties render IFs unique among cytoskeletal polymers and have strong implications for their function in vivo.

Annealing Accounts for the Length of Actin Filaments Formed by Spontaneous Polymerization

We measured the lengths of actin filaments formed by spontaneous polymerization of highly purified actin monomers by fluorescence microscopy after labeling with rhodamine-phalloidin. The length distributions are exponential with a mean of approximately 7 microm (2600 subunits). This length is independent of the initial concentration of actin monomer, an observation inconsistent with a simple nucleation-elongation mechanism. However, with the addition of physically reasonable rates of filament annealing and fragmenting, a nucleation-elongation mechanism can reproduce the observed average length of filaments in two types of experiments: 1) filaments formed from a wide range of highly purified actin monomer concentrations, and 2) filaments formed from 24 microM actin over a range of CapZ concentrations.

Effect of capping protein, CapZ, on the length of actin filaments and mechanical properties of actin filament networks

We report on how physiological concentrations of capping protein shorten actin filaments and on the remarkably fluid nature of solutions of such short filaments even at the high concentrations that exist in cells. We measured the lengths of actin filaments formed by spontaneous polymerization of highly purified actin monomers by fluorescence microscopy after labeling with rhodamine-phalloidin. The length distributions are exponential with a mean of about 7 microm (2600 subunits). As observed previously with less quantitative assays, copolymerization with the actin capping protein, CapZ, reduces the length of the filaments. At cellular concentrations of capping protein, one filament forms for each molecule of capping protein and the population of filaments is uniformly short. Using CapZ to vary the length of actin filaments, we measured how their mechanical properties depend on length. The stiffness (elastic modulus) of actin filament networks depends steeply on the length, with long filaments contributing far out of proportion to their numbers to the stiffness. Even at physiological concentrations (300 microM), networks of filaments limited to lengths observed in cells with a 1 to 500 molar ratio of CapZ are more fluid and much less elastic than lower concentrations of longer actin filaments. Thus the high concentration of short actin filaments in cells must be crosslinked to produce the observed stiffness of the cortex.

Rheological Properties of Vital Wheat Gluten Suspensions

ABSTRACT Flour and doughs represent rheologically complex materials whose properties are dependent on many factors including processing conditions. To avoid some of the problems associated with the rheological characterization of dough, we have initiated a study focused on the rheological properties of one of the major components of dough, vital wheat gluten. Suspensions of vital wheat gluten were prepared with concentrations of 225–325 mg/mL.The moduli of the gluten suspensions was 0.2 Pa at 225 mg/mL to 37 Pa at 325 mg/mL. At 300 mg/mL, the gluten suspensions exhibited solidlike behavior. The crossover frequencies were independent of concentration and equal to 100 rad/sec. At 300 mg/mL, the high-frequency...

Viscoelastic properties of oat β-glucan-rich aqueous dispersions.

C-trim is a healthy food product containing soluble dietary fibre β-glucan. The dispersion of C-trim in water is a hydrocolloid biopolymer. The linear and non-linear rheological properties of dispersions of C-trim biopolymers were investigated. The linear viscoelastic behaviours for C-trim dispersions were dependent on the β-glucan that C-trim contained. The C-trim20 and C-trim30, which have about 20% and 30% β-glucan, respectively, exhibited more fluid-like behaviours. The C-trim50 and C-trim95, which contain about 50% and 95% β-glucan, respectively, showed solid viscoelastic properties. The power law model fitting, as well as spectra, for the linear dynamic frequency sweep and stress relaxation of C-trim dispersions, suggested that the C-trim dispersions were composed of physical entanglement networks instead of chemical cross ones. The non-linear steady shearing studies for C-trim dispersions indicated that all four of the C-trim dispersions exhibited shear-thinning behaviours, which could be best described by the power law model.

Rheological and Pasting Properties of Buckwheat (Fagopyrum esculentum Möench) Flours With and Without Jet-Cooking1

ABSTRACT Pasting, rheological, and water-holding properties of buckwheat (Fagopyrum esculentum) flour obtained from whole achenes separated into three particle sizes, and three commercial flours (Fancy, Supreme, and Farinetta) were measured with or without jet-cooking. Fancy had instantaneous paste viscosity (measured using RVA) after jet-cooking that was not observed for Supreme or Farinetta, and paste viscosity was lower for the latter two flours. Supreme jet-cooked flour exhibited higher peak viscosity than flour without jet-cooking, and paste exhibited high shear-thinning. Fancy exhibited strongest viscoelastic properties (measured using a rheometer). Jet-cooking damaged buckwheat flour structure, thereby reducing viscoelasticity. Buckwheat flour pastes experienced shear-thinning over a wide range of shear rates. Jet-cooking greatly enhanced water-holding capacity. Buckwheat flour particle size did not greatly influence paste viscosity. Study showed buckwheat flours have unique pasting and rheological...

Preparation of starch-poly-glutamic acid graft copolymers by microwave irradiation and the characterization of their properties.

Graft copolymers of waxy maize starch and poly-γ-glutamic acid (PGA) were produced in an aqueous solution using microwave irradiation. The microwave reaction conditions were optimized with regard to temperature and pH. The temperature of 180°C and pH7.0 were the best reaction conditions resulting in a PGA graft of 0.45% based on nitrogen analysis. The average graft content and graft efficiency for the starch-PGA graft copolymer prepared at 180°C and pH7.0 were 4.20% and 2.73%, respectively. The starch-PGA graft copolymer produced at 180°C and pH7.0 could absorb more than 20 times its own weight amount of water and form a gel. The preliminary rheology study revealed that the starch-PGA graft copolymer gel exhibited viscoelastic solid behavior while the control sample of waxy starch showed viscoelastic liquid behavior.

High-frequency dynamics and microrheology of macromolecular solutions probed by diffusing wave spectroscopy: the case of concentrated solutions of F-actin

The linear viscoelastic properties of macromolecular solutions and networks are traditionally investigated by probing their mechanical response upon applied shear. However, a small-amplitude strain may be insufficient to reach the torque resolution of a mechanical rheometer, especially for weak hydrogels. Such materials may require large deformations, which can orient and even rupture the constitutive polymers. This paper reviews a recently-developed approach to probe the mechanical properties of macromolecular networks and solutions non-invasively in the linear regime, and over an extended frequency range. This approach consists in measuring the thermally-induced displacement of microspheres imbedded within the solution, from which equilibrium, linear viscoelastic moduli can be extracted. The displacement of the probing microspheres is measured by diffusing wave spectroscopy (DWS). The extended bandwidth of DWS provides for new insight into the short-time and short-length scales motion of individual polymers in solution and offers a more stringent test of models of polymer dynamics. Here, we illustrate the use of DWS on a semi-dilute solution of model semi-flexible polymers: a concentrated actin-filament network.

Multiple-Particle Tracking Study of Microheterogeneity of Nutrim-10 Suspensions

ABSTRACT Nutrim is a newly developed food product containing dietary soluble fiber β-glucan. The microstructural heterogeneities of Nutrim-10 suspensions were investigated by monitoring the thermally driven displacements of well-dispersed microspheres through video fluorescence microscopy. By comparing the distribution of the time-dependent mean-square displacement (MSD) of polystyrene microspheres embedded in three concentrations of Nutrim-10 suspensions, we found that the degree of heterogeneity of suspensions increased dramatically within a narrow range of Nutrim-10 concentrations. The ensemble-averaged MSD of 5.5% Nutrim-10 suspension exhibited a power-law behavior scaling linearly with time, which was similar to the behavior for a homogeneous aqueous glycerol solution. But the MSD distribution was wider and more asymmetric than for glycerol. Increasing Nutrim-10 concentration rendered the MSD distribution much more asymmetric and skewed. This study provided a quantitative method to characterize the o...