Flow patterns and endothelial cell morphology in a simplified model of an artificial ventricle

The aim of this study was to delineate the flow patterns in a non-unidirectional flow field inside a ventricle-shaped cell culture chamber, and examine the resulting morphology and integrity of the endothelium in select regions of the monolayer. The chamber was perfused by pulsatile flow, and the coherent motion of the fluid was studied using flow visualization aided by image analysis. Four distinct flow patterns were discerned and examined: central jet, flow impingement, flow separation, and recirculating eddies. The influence of these patterns on endothelial cell morphology was assessed after 20 h of exposure to flow. There were no signs of damage to the endothelium in the jet region nor was there evidence of cell alignment with the flow. Yet, there were changes in cell morphology and cytoskeletal architecture as compared to control. By contrast, within the eddies where the flow was highly disturbed, there was apparent damage to the endothelium. Thus, exposure of cells to random velocity fluctuations in regions of quasi-static flow compromises the integrity of the monolayer. Identification of such sites and acquisition of the knowledge necessary to protect the cells from denudation will be valuable for the endothelialization efforts of cardiac prostheses.     

Impact of plaques in the left coronary artery on wall shear stress and pressure gradient in coronary side branches

In this study, we investigate plaques located at the left coronary bifurcation. We focus on the effect that the resulting changes in wall shear stress (WSS) and wall pressure stress gradient (WPSG) have on atherosclerotic progress in coronary artery disease. Coronary plaques were simulated and placed at the left main stem and the left anterior descending to produce >50% narrowing of the coronary lumen. Computational fluid dynamics analysis was carried out, simulating realistic physiological conditions that show the in vivo cardiac haemodynamic. WSS and WPSG in the left coronary artery were calculated and compared in the left coronary models, with and without the presence of plaques during cardiac cycles. Our results showed that WSS decreased while WPSG was increased in coronary side branches due to the presence of plaques. There is a direct correlation between coronary plaques and subsequent WSS and WPSG variations based on the bifurcation plaques simulated in the realistic coronary models.     

Pulsatile flow through a constricted tube: effect of stenosis morphology on hemodynamic parameters

In this paper, we investigate pulsatile flow through a constricted tube with the aim of assessing the effect of stenosis morphology on hemodynamic parameters. The fluid-solid interaction of pulsatile flow through a compliant tube with elastic walls was simulated using an arbitrary Lagrangian-Eulerian (ALE) finite-element method. We consider blood flow through various mild stenoses of 25.8% severity in diameter with trapezoidal and bell-like morphologies at a fixed Womersley number of 7.75. The results show that the distribution of the time-averaged wall shear stress (TAWSS), which is the main factor affecting the hemodynamic parameters, strongly depends on the axial stretch of the stenosis; elongation of the stenotic region increases by 41.1% the maximum TAWSS for stenoses of trapezoidal morphology whereas the maximum TAWSS decreases by 14.8% for the corresponding stenoses of bell-like morphology. The present findings indicate that risk factors due to atherosclerosis may vary in a complicated manner as an atheromatous plaque gradually builds up and morphs with time.     

The effects of spatial inhomogeneities on flow through the endothelial surface layer

Flow through the endothelial surface layer (the glycocalyx and adsorbed plasma proteins) plays an important but poorly understood role in cell signaling through a process known as mechanotransduction. Characterizing the flow rates and shear stresses throughout this layer is critical for understanding how flow-induced ionic currents, deformations of transmembrane proteins, and the convection of extracellular molecules signal biochemical events within the cell, including cytoskeletal rearrangements, gene activation, and the release of vasodilators. Previous mathematical models of flow through the endothelial surface layer are based upon the assumptions that the layer is of constant hydraulic permeability and constant height. These models also assume that the layer is continuous across the endothelium and that the layer extends into only a small portion of the vessel lumen. Results of these models predict that fluid shear stress is dissipated through the surface layer and is thus negligible near endothelial cell membranes. In this paper, such assumptions are removed, and the resultant flow rates and shear stresses through the layer are described. The endothelial surface layer is modeled as clumps of a Brinkman medium immersed in a Newtonian fluid. The width and spacing of each clump, hydraulic permeability, and fraction of the vessel lumen occupied by the layer are varied. The two-dimensional Navier-Stokes equations with an additional Brinkman resistance term are solved using a projection method. Several fluid shear stress transitions in which the stress at the membrane shifts from low to high values are described. These transitions could be significant to cell signaling since the endothelial surface layer is likely dynamic in its composition, density, and height.     

Effect of LDL concentration polarization on the uptake of LDL by human endothelial cells and smooth muscle cells co-cultured

To substantiate our hypothesis that concentration polarization of low-density Upoprotein （LDL） plays an important role in the localization of atherogenesis, we investigated the effects of wall shear stress and water filtration rate （or perfusion pressure） on the luminal surface LDL concentration （cw） and the LDL uptake by human vascular endothelial cells and smooth muscle cells co-cultured on a permeable membrane using a parallel-plate flow chamber technique and a flow cyto-metry method. The results indicated that the uptake of fluorescent labeled LDL （DiI-LDL） by the co-cultured cells was positively correlated with Cw in a non-linear fashion. When cw was low, the uptake increased very sharply with increasing Cw. Then the increase became gradual and the uptake was seemingly leveled out when Cw reached beyond 160 μg/ml. The present study therefore has provided further experimental evidence that concentration polarization may occur in the arterial system and have a positive correlation with the uptake of LDLs by the arterial wall, which gives support to our hypothesis regarding the localization of atherogenesis.     

The regulation of UV-B responses by the circadian clock

The circadian clock modulates plant responses to environmental stimuli. In a recent study we showed that light and the circadian clock regulate daily changes in sensitivity to short treatments of high UV-B. Here we demonstrate that these time dependent changes in UV-B stress sensitivity are not mediated by the UV-B receptor UV RESISTANTCE LOCUS 8. We also discuss the potential mechanisms involved in this process and the role of the circadian clock in the acclimation to UV-B.     

Effects of flow on the synthesis and release of fibronectin by endothelial cells

Summary Human umbilical vein endothelial cells at confluence were subjected to steady shear flow. The effect of flow on the synthesis of fibronectin, its release into the medium, and incorporation into the extracellular matrix were investigated. The total content of fibronectin in endothelial cells exposed to flow was found to be lower than that in static controls after periods of 12 to 48 h. In the presence of cycloheximide there was no difference in the fibronectin content of sheared and unsheared cells. Our results suggest that the synthesis of fibronectin is inhibited by the flow-induced perturbation of endothelial cells. This work was supported by grant EET 8708692 from the National Science Foundation, Washington, DC; grant HL-40696 from the National Institutes of Health, Bethesda, MD; and a Research Initiation Grant from the Pennsylvania State University.     

Rac1 mediates laminar shear stress-induced vascular endothelial cell migration

The migration of endothelial cells (ECs) plays an important role in vascular remodeling and regeneration. ECs are constantly subjected to shear stress resulting from blood flow and are able to convert mechanical stimuli into intracellular signals that affect cellular behaviors and functions. The aim of this study is to elucidate the effects of Rac1, which is the member of small G protein family, on EC migration under different laminar shear stress (5.56, 10.02, and 15.27 dyn/cm²). The cell migration distance under laminar shear stress increased significantly than that under the static culture condition. Especially, under relative high shear stress (15.27 dyn/cm²) there was a higher difference at 8 h (P < 0.01) and 2 h (P < 0.05) compared with static controls. RT-PCR results further showed increasing mRNA expression of Rac1 in ECs exposed to laminar shear stress than that exposed to static culture. Using plasmids encoding the wild-type (WT), an activated mutant (Q61L), and a dominant-negative mutant (T17N), plasmids encoding Rac1 were transfected into EA.hy 926 cells. The average net migration distance of Rac1Q61L group increased significantly, while Rac1T17N group decreased significantly in comparison with the static controls. These results indicated that Rac1 mediated shear stress-induced EC migration. Our findings conduce to elucidate the molecular mechanisms of EC migration induced by shear stress, which is expected to understand the pathophysiological basis of wound healing in health and diseases.     

干细胞发育的生物力学调控

生物力学是采用力学方法对生物系统的结构和功能进行的研究,与生物化学信号一起是调节胚胎发育、干细胞发育分化和组织器官形成的重要因素。近年来,随着学科交叉的深入,生物力学因素越来越受到研究者的重视。目前的研究表明：在心血管和造血系统,血流产生的流体剪切力对于血管内皮和造血细胞的发育分化至关重要;此外,对于广泛研究的间充质干细胞,由细胞外基质物理特性诱导的细胞张力对于干细胞功能及其向不同子代细胞的分化也扮演了重要的角色;而在肝脏等上皮组织来源的器官,也有研究提示生物力学因素,如基质弹性等在疾病的发生发展过程中起到了不可忽视的作用。总之,在干细胞发育分化过程中,生物力学调控与生物化学信号通路怎样协同发挥作用将成为今后研究的重点。     

NK细胞和NKT细胞发育的转录调控

自然杀伤（natural killer,NK）细胞和自然杀伤T（natural killer T,NKT）细胞是参与机体抗病毒免疫和肿瘤免疫的两群淋巴细胞亚群,是介导先天性免疫（innate immunity）应答和调节适应性免疫（adaptive immunity）应答的重要效应细胞。近年来,随着对NK细胞和NKT细胞及其转录调控因子研究的不断深入,NK细胞和NKT细胞的发育机制逐步被阐明,这将为提高NK细胞和NKT细胞的抗病毒和肿瘤免疫疗效提供新的策略。