We calculate the membrane-induced interaction between inclusions, in terms of the membrane stretching and bending moduli and the spontaneous curvature. We find that the membrane-induced interaction between inclusions varies nonmonotonically as a function of the inclusion spacing. The location of the energy minimum depends on the spontaneous curvature and the membrane perturbation decay length, where the latter is set by the membrane moduli. The membrane perturbation energy increases with the inclusion radius. The Ornstein-Zernike theory, with the Percus-Yevick closure, is used to calculate the radial distribution function of inclusions. We find that when the spontaneous curvature is zero, the interaction between inclusions due to the membrane deformation is qualitatively similar to the hard-core interaction. However, in the case of finite spontaneous curvature, the effective interaction is dramatically modified.

A vast amount of work has been dedicated to the effects of shear flow and cytokines on leukocyte transmigration. However, no studies have explored the effects of substrate stiffness on transmigration. Here, we investigated important aspects of endothelial cell contractionmediated neutrophil transmigration using an in vitro model of the vascular endothelium. We modeled blood vessels of varying mechanical properties using fibronectin-coated polyacrylamide gels of varying physiologic stiffness, plated with human umbilical vein endothelial cell (HUVEC) monolayers, which were activated with tumor necrosis factor-␣. Interestingly, neutrophil transmigration increased with increasing substrate stiffness below the endothelium. HUVEC intercellular adhesion molecule-1 expression, stiffness, cytoskeletal arrangement, morphology, and cell-substrate adhesion could not account for the dependence of transmigration on HUVEC substrate stiffness. We also explored the role of cell contraction and observed that large holes formed in endothelium on stiff substrates several minutes after neutrophil transmigration reached a maximum. Further, suppression of contraction through inhibition of myosin light chain kinase normalized the effects of substrate stiffness by reducing transmigration and eliminating hole formation in HUVECs on stiff substrates. These results provide strong evidence that neutrophil transmigration is regulated by myosin light chain kinase-mediated endothelial cell contraction and that this event depends on subendothelial cell matrix stiffness. (Blood. 2011;118(6):1632-1640) IntroductionLeukocyte transmigration through the vascular endothelium is a crucial step in the normal immune response. However, it is a complicated biologic process that involves many proteins and requires a coordinated effort between the leukocytes and endothelial cells (ECs). The biophysical aspects of leukocyte transmigration are also important, 1 as mechanical force transmission is an essential regulator of vascular homeostasis. It is probable that the mechanical properties of the vasculature depend on both vessel size (large vessels vs microvasculature) and location (soft brain vs stiffer muscle or tumor). Further, in the cardiovascular disease of atherosclerosis, the arteries stiffen 2-5 as an increased number of leukocytes penetrate the endothelium and tumor vasculature is also stiffer. 6 However, it is unknown how changes in vessel stiffness affect the behavior of the ECs lining the blood vessel, or the behavior of the leukocytes migrating along and transmigrating through the endothelium. Interestingly, polymorphonuclear neutrophils are capable of sensing differences in both substrate stiffness [7][8][9] and surface-bound adhesion proteins. 8 Therefore, we would expect neutrophils to be capable of sensing similar changes that may occur in their physiologic substrate, the endothelium.The mechanical properties of ECs are affected by a number of physiologic factors, including shear stress, 10 cholesterol content, 11,12 and oxidized low-density...

This study has investigated the effect of cellular cholesterol on membrane deformability of bovine aortic endothelial cells. Cellular cholesterol content was depleted by exposing the cells to methyl-beta-cyclodextrin or enriched by exposing the cells to methyl-beta-cyclodextrin saturated with cholesterol. Control cells were treated with methyl-beta-cyclodextrin-cholesterol at a molar ratio that had no effect on the level of cellular cholesterol. Mechanical properties of the cells with different cholesterol contents were compared by measuring the degree of membrane deformation in response to a step in negative pressure applied to the membrane by a micropipette. The experiments were performed on substrate-attached cells that maintained normal morphology. The data were analyzed using a standard linear elastic half-space model to calculate Young elastic modulus. Our observations show that, in contrast to the known effect of cholesterol on membrane stiffness of lipid bilayers, cholesterol depletion of bovine aortic endothelial cells resulted in a significant decrease in membrane deformability and a corresponding increase in the value of the elastic coefficient of the membrane, indicating that cholesterol-depleted cells are stiffer than control cells. Repleting the cells with cholesterol reversed the effect. An increase in cellular cholesterol to a level higher than that of normal cells, however, had no effect on the elastic properties of bovine aortic endothelial cells. We also show that although cholesterol depletion had no apparent effect on the intensity of F-actin-specific fluorescence, disrupting F-actin with latrunculin A abrogated the stiffening effect. We suggest that cholesterol depletion increases the stiffness of the membrane by altering the properties of the submembrane F-actin and/or its attachment to the membrane.

Numerous RNAs are enriched within cellular protrusions, but the underlying mechanisms are largely unknown. We had shown that the APC (adenomatous polyposis coli) protein controls localization of some RNAs at protrusions. Here, using protrusion-isolation schemes and RNA-Seq, we find that RNAs localized in protrusions of migrating fibroblasts can be distinguished in two groups, which are differentially enriched in distinct types of protrusions, and are additionally differentially dependent on APC. APC-dependent RNAs become enriched in high-contractility protrusions and, accordingly, their localization is promoted by increasing stiffness of the extracellular matrix. Dissecting the underlying mechanism, we show that actomyosin contractility activates a RhoA-mDia1 signaling pathway that leads to formation of a detyrosinated-microtubule network, which in turn is required for localization of APC-dependent RNAs. Importantly, a competition-based approach to specifically mislocalize APC-dependent RNAs suggests that localization of the APC-dependent RNA subgroup is functionally important for cell migration.

Neutrophils are one type of migrating cell in the body's innate immune system and are the first line of defense against inflammation or infection. While extensive work exists on the effect of adhesive proteins on neutrophil motility, little is known about how neutrophil motility is affected by the mechanical properties of their physical environment. This study investigated the effects of substrate stiffness on the morphology, random motility coefficient, track speed (v), spreading area, and distribution of turning angles of neutrophils during chemokinesis. Human neutrophils were plated onto polyacrylamide gels of varying stiffness, ranging from 3 to 13 kPa, and coated with the extracellular matrix protein fibronectin, and timelapse images were taken with phase contrast microscopy. Our results show a biphasic behavior between neutrophil motility and substrate stiffness, with the optimum stiffness for motility depending on the concentration of fibronectin on the surface of the gel. On 100 microg/mL fibronectin, the optimum stiffness is 4 kPa (v = 6.9 +/- 0.6 microm/min) while on 10 microg/mL fibronectin, the optimum stiffness increases to 7 kPa (v = 4.5 +/- 2.0 microm/min). This biphasic behavior most likely arises because neutrophils on soft gels are less adherent, preventing production of traction forces, while neutrophils on stiff gels adhere strongly, resulting in decreased migration. At intermediate stiffness, however, neutrophils can attain optimal motility as a function of extracellular matrix coating.