Biorheology - Volume 48, issue 1

Authors: Sundd, Prithu | Pospieszalska, Maria K. | Cheung, Luthur Siu-Lun | Konstantopoulos, Konstantinos | Ley, Klaus

Article Type: Review Article

Abstract: Leukocyte rolling on endothelial cells and other P-selectin substrates is mediated by P-selectin binding to P-selectin glycoprotein ligand-1 expressed on the tips of leukocyte microvilli. Leukocyte rolling is a result of rapid, yet balanced formation and dissociation of selectin–ligand bonds in the presence of hydrodynamic shear forces. The hydrodynamic forces acting on the bonds may either increase (catch bonds) or decrease (slip bonds) their lifetimes. The force-dependent ‘catch–slip’ bond kinetics are explained using the ‘two pathway model’ for bond dissociation. Both the ‘sliding–rebinding’ and the ‘allosteric’ mechanisms attribute ‘catch–slip’ bond behavior to the force-induced conformational changes in the lectin–EGF domain …hinge of selectins. Below a threshold shear stress, selectins cannot mediate rolling. This ‘shear-threshold’ phenomenon is a consequence of shear-enhanced tethering and catch bond-enhanced rolling. Quantitative dynamic footprinting microscopy has revealed that leukocytes rolling at venular shear stresses (>0.6 Pa) undergo cellular deformation (large footprint) and form long tethers. The hydrodynamic shear force and torque acting on the rolling cell are thought to be synergistically balanced by the forces acting on tethers and stressed microvilli, however, their relative contribution remains to be determined. Thus, improvement beyond the current understanding requires in silico models that can predict both cellular and microvillus deformation and experiments that allow measurement of forces acting on individual microvilli and tethers. Show more

Keywords: Quantitative dynamic footprinting, footprint, P-selectin, event tracking model of adhesion, immersed boundary method

DOI: 10.3233/BIR-2011-0579

Citation: Biorheology, vol. 48, no. 1, pp. 1-35, 2011

Authors: Rath, B. | Nam, J. | Deschner, J. | Schaumburger, J. | Tingart, M. | Grässel, S. | Grifka, J. | Agarwal, S.

Article Type: Research Article

Abstract: Osteoblasts are mechanosensitive cells, which respond to biomechanical stimuli to regulate the bone structure through anabolic and catabolic gene regulation. To examine the effects of mechanical forces on the osteogenic responses through the SMAD signaling in osteoblasts, the cells were cultured in well-characterized mechanoresponsive 3-D scaffolds and exposed to 10% dynamic compressive strain (Cmp) at 1 Hz. Subsequently, SMAD phosphorylation and osteogenic gene induction was examined. Osteoblasts cultured in 3-D scaffolds exhibited increased constitutive SMAD 1/5/8 phosphorylation, as compared to monolayers cultures. This SMAD 1/5/8 phosphorylation was further upregulated after 10, 30 and 60 min in response to Cmp, exhibiting …a peak activation at 30 min. No significant changes in SMAD2 phosphorylation were observed, suggesting signals generated by Cmp may not activate the Transforming Growth Factor-β signaling cascade. Subsequently, biomechanical stimulation-induced SMAD 1/5/8 phosphorylation upregulated the expression of osteogenic genes such as Osteoprotegrin, Msx2 and Runx2. Dorsomorphin, a selective inhibitor of the bone morphogenetic protein (BMP) receptor type 1 (BMPR1), blocked Cmp-induced SMAD 1/5/8 phosphorylation, as well as Osteoprotegrin, Msx2 and Runx2 gene expression. Collectively, the present findings demonstrate that biomechanical stimulation of osteoblasts activates SMAD 1/5/8 in the BMP signaling pathway through BMPR1 and may enhance osteogenesis by upregulating SMAD-dependent osteogenic genes. Show more

Keywords: Scaffold, mechanical signals, BMP, SMAD signaling, dorsomorphin

DOI: 10.3233/BIR-2011-0580

Citation: Biorheology, vol. 48, no. 1, pp. 37-48, 2011

Authors: Furlow, M. | Diamond, S.L.

Article Type: Research Article

Abstract: Using microfluidic assays at a 100 s−1 wall shear rate, we examined the effects of ethanol on cholesterol-loaded neutrophils with respect to: (1) collision efficiency and membrane tethering to P-selectin-coated microbeads, (2) rolling on P-selectin-coated surfaces, and (3) primary and secondary interactions with neutrophils pre-adhered to intercellular adhesion molecule-1 (ICAM-1). Using methyl-β-cyclodextrin:cholesterol complexes, membrane cholesterol was increased over control by 4.6-fold (no ethanol), 3.6-fold (0.3% ethanol pre-loading), and 1.6-fold (0.3% ethanol post-loading). These treatments did not alter CD11b expression; however, PSGL-1 and L-selectin were lowered by cholesterol enrichment (±ethanol). Cholesterol enrichment enhanced microbead collision efficiency, which was abrogated by …ethanol. Ethanol had no effect on elevation of tethering fraction by cholesterol enrichment. Incubation of cholesterol-loaded neutrophils with ethanol resulted in significantly longer membrane tethers, due to tether lifetime enhancement. On P-selectin-coated surfaces, cholesterol-enriched neutrophils exposed to ethanol rolled faster and with more variability than cholesterol-enriched neutrophils. Ethanol reduced homotypic collision efficiency of cholesterol-loaded neutrophils without effect on tethering fraction or secondary collision efficiency. Tether length during cholesterol-loaded neutrophil homotypic collisions was enhanced by ethanol, in part due to increased L-selectin/PSGL-1 bond tether lifetime. Overall, ethanol attenuated cholesterol-induced adhesion increases while increasing membrane fluidity as indicated by tether length. Show more

Keywords: Neutrophil membrane, membrane fluidity, cell adhesion, atherosclerosis, bead collision assay

DOI: 10.3233/BIR-2011-0583

Citation: Biorheology, vol. 48, no. 1, pp. 49-64, 2011

Authors: Morgan, Jessica M. | Wong, Alice | Genetos, Damian C. | Yellowley, Clare E.

Article Type: Research Article

Abstract: The ability of bone cells to detect and transduce mechanical signals is central to the mechanism whereby bone adapts to mechanical load and maintains healthy bone mass. Src, a non-receptor tyrosine kinase, is located in focal adhesions, highly specialized and localized sites of attachment, that are thought to be a primary site of mechanotransduction. While Src is activated by mechanical loads in other cell types, its role in osteoblast mechanotransduction is unclear. In this study we examined whether oscillatory fluid flow influenced Src phosphorylation, and Src's role in the flow-induced osteopontin response. Additionally, we investigated the effect of constitutively active …Src on osteopontin expression. Oscillatory fluid flow induced a statistically significant increase in phosphorylation of Src at tyrosine residue 416 after a 15 min exposure. Transfection with constitutively-active Src resulted in an increase in Src-Y416 phosphorylation and an increase in osteopontin mRNA transcript under static conditions. However, inhibition of Src activity had no effect on oscillatory fluid flow-stimulated osteopontin expression or ERK1/2 phosphorylation. These data suggest that although Src activity regulates osteopontin expression under static conditions, and is induced under conditions of shear stress, it is not required for load-induced osteopontin expression. Show more

Keywords: Mechanotransduction, oscillatory fluid flow, bone, ERK, fluid shear stress

DOI: 10.3233/BIR-2011-0582

Citation: Biorheology, vol. 48, no. 1, pp. 65-74, 2011