Characterizing cell adhesion by using micropipette aspiration

International audienceWe have developed a technique to directly quantify cell-substrate adhesion force using micropipette aspiration. The micropipette is positioned perpendicular to the surface of an adherent cell and a constant-rate aspiration pressure is applied. Since the micropipette diameter and the aspiration pressure are our control parameters, we have direct knowledge of the aspiration force, whereas the cell behavior is monitored either in brightfield or interference reflection microscopy. This setup thus allows us to explore a range of geometric parameters, such as projected cell area, adhesion area, or pipette size, as well as dynamical parameters such as the loading rate. We find that cell detachment is a well-defined event occurring at a critical aspiration pressure, and that the detachment force scales with the cell adhesion area (for a given micropipette diameter and loading rate), which defines a critical stress. Taking into account the cell adhesion area, intrinsic parameters of the adhesion bonds, and the loading rate, a minimal model provides an expression for the critical stress that helps rationalize our experimental results

Assessment of the permeability of a microvessel-on-chip to small and large molecules

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Focal adhesion clustering drives endothelial cell morphology on patterned surfaces

International audienceIn many cell types, shape and function are intertwined. In vivo, vascular endothelial cells (ECs) are typically elongated and aligned in the direction of blood flow; however, near branches and bifurcations where atherosclerosis develops, ECs are often cuboidal and have no preferred orientation. Thus, understanding the factors that regulate EC shape and alignment is important. In vitro, EC morphology and orientation are exquisitely sensitive to the composition and topography of the substrate on which the cells are cultured; however, the underlying mechanisms remain poorly understood. Different strategies of substrate patterning for regulating EC shape and orientation have been reported including adhesive motifs on planar surfaces and micro- or nano-scale gratings that provide substrate topography. Here, we explore how ECs perceive planar bio-adhesive versus microgrooved topographic surfaces having identical feature dimensions. We show that while the two types of patterned surfaces are equally effective in guiding and directing EC orientation, the cells are considerably more elongated on the planar patterned surfaces than on the microgrooved surfaces. We also demonstrate that the key factor that regulates cellular morphology is focal adhesion clustering which subsequently drives cytoskeletal organization. The present results promise to inform design strategies of novel surfaces for the improved performance of implantable cardiovascular devices

The influence of hemodynamic forces and intercellular interactions on endothelial cell migration

International audienceEndothelial cell (EC) migration plays a fundamental role in a number of vascular scenarios including angiogenesis, wound healing, and re-endothelialization of vascular grafts. Hemodynamic forces from blood flow are known to mechanically regulate the migration of ECs by applying shear stresses to their apical surfaces. Much research has shown that cell migration is correlated to the level of applied shear stress, yet little has been studied on the specific effects of shear rate in modulating cell mobility. It was the aim of this study to characterize the individual contribution of these two factors on subconfluent bovine aortic ECs under steady laminar flow. Shear stress and shear rate were independently controlled by adjusting the viscosity of the culture medium, and resulting cell velocities and overall net displacements were observed. We demonstrate that cell mobility is not only modulated by shear stress but is rather a result of a combination of hemodynamic factors. More specifically, shear stress tends to regulate cell velocity, whereas shear rate guides cell movement in the direction of flow. Implications for this research are vast, as the failure of the endothelium to adapt to flow can lead to atherosclerosis or abnormal vessel repair. To understand if these trends hold true in a more physiologically-relevant environment, we also performed flow experiments on EC-smooth muscle cell co-cultures and elucidated the interplay of these cell-cell interactions in regulating flow-induced EC migration. Taken together, these findings provide insight into the contributions of the mechanical environment on vascular function and dysfunctio

Substrate regulation of vascular endothelial cell morphology and alignment

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Dynamic monitoring of cell mechanical properties using profile microindentation.

International audienceWe have developed a simple and relatively inexpensive system to visualize adherent cells in profile while measuring their mechanical properties using microindentation. The setup allows simultaneous control of cell microenvironment by introducing a micropipette for the delivery of soluble factors or other cell types. We validate this technique against atomic force microscopy measurements and, as a proof of concept, measure the viscoelastic properties of vascular endothelial cells in terms of an apparent stiffness and a dimensionless parameter that describes stress relaxation. Furthermore, we use this technique to monitor the time evolution of these mechanical properties as the cells' actin is depolymerized using cytochalasin-D

Luminal flow actuation generates coupled shear and strain in a microvessel-on-chip

International audienceIn the microvasculature, blood flow-derived forces are key regulators of vascular structure and function. Consequently, the development of hydrogel-based microvessel-on-chip systems that strive to mimic the in vivo cellular organization and mechanical environment has received great attention in recent years. However, despite intensive efforts, current microvessel-on-chip systems suffer from several limitations, most notably failure to produce physiologically relevant wall strain levels. In this study, a novel microvessel-on-chip based on the templating technique and using luminal flow actuation to generate physiologically relevant levels of wall shear stress and circumferential stretch is presented. Normal forces induced by the luminal pressure compress the surrounding soft collagen hydrogel, dilate the channel, and create large circumferential strain. The fluid pressure gradient in the system drives flow forward and generates realistic pulsatile wall shear stresses. Rigorous characterization of the system reveals the crucial role played by the poroelastic behavior of the hydrogel in determining the magnitudes of the wall shear stress and strain. The experimental measurements are combined with an analytical model of flow in both the lumen and the porous hydrogel to provide an exceptionally versatile user manual for an application-based choice of parameters in microvessels-on-chip. This unique strategy of flow actuation adds a dimension to the capabilities of microvessel-on-chip systems and provides a more general framework for improving hydrogel-based in vitro engineered platforms