Bacterial swimming strategies and turbulence.
Most bacteria in the ocean can be motile. Chemotaxis allows bacteria to detect nutrient gradients, and hence motility is believed to serve as a method of approaching sources of food. This picture is well established in a stagnant environment. In the ocean a shear microenvironment is associated with turbulence. This shear flow prevents clustering of bacteria around local nutrient sources if they swim in the commonly assumed "run-and-tumble" strategy. Recent observations, however, indicate a "back-and-forth" swimming behavior for marine bacteria. In a theoretical study we compare the two bacterial swimming strategies in a realistic ocean environment. The "back-and-forth" strategy is found to enable the bacteria to stay close to a nutrient source even under high shear. Furthermore, rotational diffusion driven by thermal noise can significantly enhance the efficiency of this strategy. The superiority of the "back-and-forth" strategy suggests that bacterial motility has a control function rather than an approach function under turbulent conditions. (+info
In vitro characterization and micromechanics of tumor cell chemotactic protrusion, locomotion, and extravasation.
The objective of this paper is to introduce some novel in vitro applications in characterizing human melanoma cell protrusion and migration in response to soluble extracellular matrix protein stimulation. Specifically, we describe two assay systems: (1) dual-micropipette manipulation and (2) flow-migration chamber. Applications of the dual-micropipet technique provided kinetic measure of cell movement, cyclic pseudopod protrusion, and subsequent cell locomotion governed by chemotactic molecular transport dynamics. Chemotactic concentration gradient was found to influence significantly pseudopod protrusion frequency and locomotion speed, but not the protrusion extension. To further characterize active tumor cell extravasation, a process that involves dynamic tumor cell adhesion to vascular endothelium under flow conditions and subsequent transendothelial migration in response to chemotactic signals from the interstitial space, we developed a flow-migration chemotaxis system. This assay enabled characterization of tumor cell transcellular migration in terms of chemotactic signal gradients, shear forces, and cell-substrate adhesion. Results suggest that shear flow plays significant roles in tumor cell extravasation that is regulated by both tumor cell motility and tumor cell adhesion to endothelial molecules in a cooperative process. (+info
Fluid shear stress remodels expression and function of junctional proteins in cultured bone cells.
We tested the hypothesis that fluid shear stress (tau) modifies the expression, function, and distribution of junctional proteins [connexin (Cx)43, Cx45, and zona occludens (ZO)-1] in cultured bone cells. Cell lines with osteoblastic (MC3T3-E1 cells) and osteocytic (MLO-Y4 cells) phenotypes were exposed to tau-values of 5 or 20 dyn/cm(2) for 1-3 h. Immunostaining indicated that at 5 dyn/cm(2), the distribution of Cx43, Cx45, and ZO-1 was moderately disrupted at cell membranes; at 20 dyn/cm(2), disruption was more severe. Intercellular coupling was significantly decreased at both shear stress levels. Western blots showed the downregulation of membrane-bound Cx43 and ZO-1 and the upregulation of cytosolic Cx43 and Cx45 at different levels of shear stress. Similarly, Northern blots revealed that expression of Cx43, Cx45, and ZO-1 was selectively up- and downregulated in response to different shear stress levels. These results indicate that in cultured bone cells, fluid shear stress disrupts junctional communication, rearranges junctional proteins, and determines de novo synthesis of specific connexins to an extent that depends on the magnitude of the shear stress. Such disconnection from the bone cell network may provide part of the signal whereby the disconnected cells or the remaining network initiate focal bone remodeling. (+info
Shear bond strength of a new dental adhesive used to bond brackets to unetched enamel.
The aims of the present study were to measure the shear bond strength of a new multipurpose dental adhesive, IntegraCem, for direct bonding of stainless steel and ceramic brackets to unetched enamel, and to determine the mode of bond failure. Both stainless steel and ceramic brackets (GAC) were bonded with IntegraCem to unetched extracted human premolars. After bonding, the teeth were either stored in a water bath at 37 degrees C for 3 days or passed 2500 thermocycles from 6 to 60 degrees C. Debonding was then performed with a shearing force using an Instron universal testing machine. The force was recorded at bond failure. The results showed that the shear bond strength achieved was between 6.7 and 10.8 megapascals (MPa). Bond failure occurred at the enamel-adhesive interface, enabling more efficient enamel clean up. The shear bond strengths measured suggest that IntegraCem adhesive may be effectively used in orthodontic treatment. (+info
Shear properties of passive ventricular myocardium.
We examined the shear properties of passive ventricular myocardium in six pig hearts. Samples (3 x 3 x 3 mm) were cut from adjacent regions of the lateral left ventricular midwall, with sides aligned with the principal material axes. Four cycles of sinusoidal simple shear (maximum shear displacements of 0.1-0.5) were applied separately to each specimen in two orthogonal directions. Resulting forces along the three axes were measured. Three specimens from each heart were tested in different orientations to cover all six modes of simple shear deformation. Passive myocardium has nonlinear viscoelastic shear properties with reproducible, directionally dependent softening as strain is increased. Shear properties were clearly anisotropic with respect to the three principal material directions: passive ventricular myocardium is least resistant to simple shear displacements imposed in the plane of the myocardial layers and most resistant to shear deformations that produce extension of the myocyte axis. Comparison of results for the six different shear modes suggests that simple shear deformation is resisted by elastic elements aligned with the microstructural axes of the tissue. (+info
Comparative analysis of various platelet glycoprotein IIb/IIIa antagonists on shear-induced platelet activation and adhesion.
Platelet accretion into arterial thrombus in stenotic arterial vessels involves shear-induced platelet activation and adhesion. The Cone and Plate(let) Analyzer (CPA) is designed to simulate such conditions in vitro under a rotating high shear rate in whole blood. In the present study, we evaluated various experimental conditions (including aspirin, temperature, and calcium concentration) and investigated the effects of small molecules along with peptide glycoprotein IIb/IIIa antagonists on platelet adhesion using the CPA system. Concentration-dependent effect of glycoprotein IIb/IIIa antagonists on shear-induced platelet adhesion showed marked differences in potencies: IC50 = 34, 35, 91, 438, and 606 nM for DPC802 (a specific glycoprotein IIb/IIIa antagonist), roxifiban, sibrafiban, lotrafiban, and orbofiban (free acid forms), respectively, and IC50 values of 43, 430, and 5781 nM for abciximab, tirofiban, and eptifibatide, respectively. Parallel study was also conducted to evaluate the effect of glycoprotein IIb/IIIa inhibitors using optical aggregometry. The potency of fibans in blocking shear-induced platelet adhesion correlated well with their binding affinity to the resting and activated glycoprotein IIb/IIIa receptors, as well as their "off-rates". Nevertheless, none of these fibans was able to effectively block shear-induced platelet adhesion at targeted clinical dosing regimens except for abciximab. These data suggest that glycoprotein IIb/IIIa antagonists that show similar efficacy in the inhibition of platelet aggregation in a static in vitro assay may differ substantially in a shear-based system of platelet adhesion. The clinical significance of this phenomenon awaits further investigation. (+info
Shear stress magnitude and directionality modulate growth factor gene expression in preconditioned vascular endothelial cells.
OBJECTIVE: The purpose of this study was to simultaneously monitor the transcriptional levels of 12 endothelial growth factor genes in response to alterations in wall shear stress (WSS) under conditions relevant to the development of intimal hyperplasia, a major cause of arterial bypass graft failure. METHODS: Human umbilical vein endothelial cells were preconditioned in vitro under steady flow (WSS, 15 dynes/cm(2)) for 24 hours before being subjected to WSS at 25 (Delta = +10), 15 (Delta = 0), 5 (Delta = -10), 2.5 (Delta = -12.5), and 0 (Delta = -15) dynes/cm(2) or low magnitude WSS reversal (-2.5 dynes/cm(2)) for 6 hours. A focused complementary DNA array was used to simultaneously measure messenger RNA expression levels for END1, endothelial nitric oxide synthase (NOS3), platelet-derived growth factor A, platelet-derived growth factor B (PDGFB), acidic fibroblast growth factor, basic fibroblast growth factor, transforming growth factor-alpha, transforming growth factor-beta, vascular endothelial growth factor, insulin-like growth factor-1, epidermal growth factor, and angiotensin converting enzyme. RESULTS: Preconditioning significantly (P <.05) increased the fold expression of NOS3 (4.1 +/- 1.4), basic fibroblast growth factor (3.90 +/- 1.16), vascular endothelial growth factor (3.39 +/- 1.04), and insulin-like growth factor-1 (2.8 +/- 0.7) but decreased END1 (0.47 +/- 0.05) and PDGFB (0.70 +/- 0.04) messenger RNA expression levels relative to no-flow controls, an effect that was sustained on removal from flow for 6 hours. Notably, the ratio of END1/NOS3 expression was diminished (0.11 +/- 0.03) relative to that of cells maintained in static culture. Although few differences in gene expression from baseline (15 dynes/cm(2)) were measured in cells exposed to either constant (Delta = 0) or step decreases (Delta = -10, -12.5, or -15 dynes/cm(2)) in WSS, marked changes were seen in the group exposed to a step increase in WSS (Delta = +10) or to WSS reversal. Low magnitude retrograde WSS evoked significant (P <.05) transcriptional changes in multiple genes, including elevated END1 (4.1 +/- 0.5), platelet-derived growth factor A (1.5 +/- 0.2), PDGFB (2.3 +/- 0.3), and transforming growth factor-beta (1.5 +/- 0.2) levels, but depressed NOS3 (0.60 +/- 0.17) levels, and a marked increase in END1/NOS3 (6.7 +/- 1.6) when compared with equal magnitude antegrade WSS (2.5 dynes/cm(2)). CONCLUSION: These results support the implementation of a preconditioning phase for in vitro WSS studies to establish a physiologic baseline. Our findings complement previous macroscale findings and are consistent with a cellular mechanism involving increased END1 and PDGFB levels, but decreased NOS3 levels, leading to intimal hyperplasia at regions of low magnitude reversing WSS. (+info
Microrheology of human lung epithelial cells measured by atomic force microscopy.
Lung epithelial cells are subjected to large cyclic forces from breathing. However, their response to dynamic stresses is poorly defined. We measured the complex shear modulus (G(*)(omega)) of human alveolar (A549) and bronchial (BEAS-2B) epithelial cells over three frequency decades (0.1-100 Hz) and at different loading forces (0.1-0.9 nN) with atomic force microscopy. G(*)(omega) was computed by correcting force-indentation oscillatory data for the tip-cell contact geometry and for the hydrodynamic viscous drag. Both cell types displayed similar viscoelastic properties. The storage modulus G'(omega) increased with frequency following a power law with exponent approximately 0.2. The loss modulus G"(omega) was approximately 2/3 lower and increased similarly to G'(omega) up to approximately 10 Hz, but exhibited a steeper rise at higher frequencies. The cells showed a weak force dependence of G'(omega) and G"(omega). G(*)(omega) conformed to the power-law model with a structural damping coefficient of approximately 0.3, indicating a coupling of elastic and dissipative processes within the cell. Power-law behavior implies a continuum distribution of stress relaxation time constants. This complex dynamics is consistent with the rheology of soft glassy materials close to a glass transition, thereby suggesting that structural disorder and metastability may be fundamental features of cell architecture. (+info