Three-dimensional forces exerted by leukocytes and vascular endothelial cells dynamically facilitate diapedesis.

Leukocyte transmigration across vessel walls is a critical step in the innate immune response. Upon their activation and firm adhesion to vascular endothelial cells (VECs), leukocytes preferentially extravasate across junctional gaps in the endothelial monolayer (paracellular diapedesis). It has been hypothesized that VECs facilitate paracellular diapedesis by opening their cell-cell junctions in response to the presence of an adhering leukocyte. However, it is unclear how leukocytes interact mechanically with VECs to open the VEC junctions and migrate across the endothelium. In this study, we measured the spatial and temporal evolution of the 3D traction stresses generated by the leukocytes and VECs to elucidate the sequence of mechanical events involved in paracellular diapedesis. Our measurements suggest that the contractile stresses exerted by the leukocytes and the VECs can separately perturb the junctional tensions of VECs to result in the opening of gaps before the initiation of leukocyte transmigration. Decoupling the stresses exerted by the transmigrating leukocytes and the VECs reveals that the leukocytes actively contract the VECs to open a junctional gap and then push themselves across the gap by generating strong stresses that push into the matrix. In addition, we found that diapedesis is facilitated when the tension fluctuations in the VEC monolayer were increased by proinflammatory thrombin treatment. Our findings demonstrate that diapedesis can be mechanically regulated by the transmigrating leukocytes and by proinflammatory signals that increase VEC contractility.

Sustained activation of XBP1 splicing leads to endothelial apoptosis and atherosclerosis development in response to disturbed flow.

X-box binding protein 1 (XBP1) is a key signal transducer in endoplasmic reticulum stress response, and its potential role in the atherosclerosis development is unknown. This study aims to explore the impact of XBP1 on maintaining endothelial integrity related to atherosclerosis and to delineate the underlying mechanism. We found that XBP1 was highly expressed at branch points and areas of atherosclerotic lesions in the arteries of ApoE(-/-) mice, which was related to the severity of lesion development. In vitro study using human umbilical vein endothelial cells (HUVECs) indicated that disturbed flow increased the activation of XBP1 expression and splicing. Overexpression of spliced XBP1 induced apoptosis of HUVECs and endothelial loss from blood vessels during ex vivo cultures because of caspase activation and down-regulation of VE-cadherin resulting from transcriptional suppression and matrix metalloproteinase-mediated degradation. Reconstitution of VE-cadherin by Ad-VEcad significantly increased Ad-XBP1s-infected HUVEC survival. Importantly, Ad-XBP1s gene transfer to the vessel wall of ApoE(-/-) mice resulted in development of atherosclerotic lesions after aorta isografting. These results indicate that XBP1 plays an important role in maintaining endothelial integrity and atherosclerosis development, which provides a potential therapeutic target to intervene in atherosclerosis.

Anisotropic rheology and directional mechanotransduction in vascular endothelial cells.

Adherent cells remodel their cytoskeleton, including its directionality, in response to directional mechanical stimuli with consequent redistribution of intracellular forces and modulation of cell function. We analyzed the temporal and spatial changes in magnitude and directionality of the cytoplasmic creep compliance (Gamma) in confluent cultures of bovine aortic endothelial cells subjected to continuous laminar flow shear stresses. We extended particle tracking microrheology to determine at each point in the cytoplasm the principal directions along which Gamma is maximal and minimal. Under static condition, the cells have polygonal shapes without specific alignment. Although Gamma of each cell exhibits directionality with varying principal directions, Gamma averaged over the whole cell population is isotropic. After continuous laminar flow shear stresses, all cells gradually elongate and the directions of maximal and minimal Gamma become, respectively, parallel and perpendicular to flow direction. This mechanical alignment is accompanied by a transition of the cytoplasm to be more fluid-like along the flow direction and more solid-like along the perpendicular direction; at the same time Gamma increases at the downstream part of the cells. The resulting directional anisotropy and spatial inhomogeneity of cytoplasmic rheology may play an important role in mechanotransduction in adherent cells by providing a means to sense the direction of mechanical stimuli.

Maintenance of HDACs and H3K9me3 Prevents Arterial Flow-Induced Venous Endothelial Damage.

The transition of flow microenvironments from veins to arteries in vein graft surgery induces "peel-off" of venous endothelial cells (vECs) and results in restenosis. Recently, arterial laminar shear stress (ALS) and oscillatory shear stress (OS) have been shown to affect the cell cycle and inflammation through epigenetic controls such as histone deacetylation by histone deacetylases (HDACs) and trimethylation on lysine 9 of histone 3 (H3K9me3) in arterial ECs. However, the roles of H3K9me3 and HDAC in vEC damage under ALS are not known. We hypothesized that the different responses of HDACs and H3K9me3 might cause vEC damage under the transition of venous flow to arterial flow. We found that arterial ECs showed high expression of H3K9me3 protein and were retained in the G0 phase of the cell cycle after being subjected to ALS. vECs became round under ALS with a decrease in the expression of H3K9me3, HDAC3, and HDAC5, and an increase in the expression of vascular cell adhesion molecule 1 (VCAM-1). Inhibition of HDACs activity by a specific inhibitor, phenylbutyrate, in arterial ECs caused similar ALS-induced inflammation and cell loss as observed in vECs. Activation of HDACs and H3K9me3 by ITSA-1, an HDAC activator, could prevent ALS-induced peel-off and reduced VCAM-1 expression in vECs. Moreover, shear stress modulates EC morphology by the regulation of focal adhesion kinase (FAK) expression. ITSA-1 or EGF could increase phosphorylated (p)-FAK expression in vECs under ALS. We found that perturbation of the activity of p-FAK and increase in p-FAK expression restored ALS-induced H3K9me3 expression in vECs. Hence, the abnormal mechanoresponses of H3K9me3 and HDAC in vECs after being subjected to ALS could be reversed by ITSA-1 or EGF treatment: this offers a strategy to prevent vein graft failure.

MiR-145 mediates cell morphology-regulated mesenchymal stem cell differentiation to smooth muscle cells.

The use of biochemical signaling to derive smooth muscle cells (SMCs) from mesenchymal stem cells (MSCs) has been explored, but the induction of a fully functional SMC phenotype remains to be a major challenge. Cell morphology has been shown to regulate MSC differentiation into various lineages, including SMCs. We engineered substrates with microgrooves to induce cell elongation to study the mechanism underlying the MSC shape modulation in SMC differentiation. In comparison to those on flat substrates, MSCs cultured on engineered substrates were elongated with increased aspect ratios for both cell body and nucleus, as well as augmented cytoskeletal tensions. Biochemical studies indicated that the microgroove-elongated cells expressed significantly higher levels of SMC markers. MicroRNA analyses showed that up-regulation of miR-145 and the consequent repression of KLF4 in these elongated cells promoted MSC-to-SMC differentiation. Rho/ROCK inhibitions, which impair cytoskeletal tension, attenuated cell and nuclear elongations and disrupted the miR-145/KLF4 regulation for SMC differentiation. Furthermore, cell traction force measurements showed that miR-145 is essential for the functional contractility in the microgroove-induced SMC differentiation. Collectively, our findings demonstrate that, through a Rho-ROCK/miR-145/KLF4 pathway, the elongated cell shape serves as a decisive geometric cue to direct MSC differentiation into functional SMCs.

Human mesenchymal stem cells suppress the stretch-induced inflammatory miR-155 and cytokines in bronchial epithelial cells.

Current research in pulmonary pathology has focused on inflammatory reactions initiated by immunological responses to allergens and irritants. In addition to these biochemical stimuli, physical forces also play an important role in regulating the structure, function, and metabolism of the lung. Hyperstretch of lung tissues can contribute to the inflammatory responses in asthma, but the mechanisms of mechanically induced inflammation in the lung remain unclear. Our results demonstrate that excessive stretch increased the secretion of inflammatory cytokines by human bronchial epithelial cells (hBECs), including IL-8. This increase of IL-8 secretion was due to an elevated microRNA-155 (miR-155) expression, which caused the suppression of Src homology 2 domain-containing inositol 5-phosphatase 1 (SHIP1) production and the subsequent activation of JNK signaling. In vivo studies in our asthmatic mouse model also showed such changes in miR-155, IL-8, and SHIP1 expressions that reflect inflammatory responses. Co-culture with human mesenchymal stem cells (hMSCs) reversed the stretch-induced hBEC inflammatory responses as a result of IL-10 secretion by hMSCs to down-regulate miR-155 expression in hBECs. In summary, we have demonstrated that mechanical stretch modulates the homeostasis of the hBEC secretome involving miR-155 and that hMSCs can be used as a potential therapeutic approach to reverse bronchial epithelial inflammation in asthma.