Cecile O. Mejean

Membrane Tension Maintains Cell Polarity by Confining Signals to the Leading Edge during Neutrophil Migration

Little is known about how neutrophils and other cells establish a single zone of actin assembly during migration. A widespread assumption is that the leading edge prevents formation of additional fronts by generating long-range diffusible inhibitors or by sequestering essential polarity components. We use morphological perturbations, cell-severing experiments, and computational simulations to show that diffusion-based mechanisms are not sufficient for long-range inhibition by the pseudopod. Instead, plasma membrane tension could serve as a long-range inhibitor in neutrophils. We find that membrane tension doubles during leading-edge protrusion, and increasing tension is sufficient for long-range inhibition of actin assembly and Rac activation. Furthermore, reducing membrane tension causes uniform actin assembly. We suggest that tension, rather than diffusible molecules generated or sequestered at the leading edge, is the dominant source of long-range inhibition that constrains the spread of the existing front and prevents the formation of secondary fronts.

Cell stimulation with optically manipulated microsources

Molecular gradients are important for various biological processes including the polarization of tissues and cells during embryogenesis and chemotaxis. Investigations of these phenomena require control over the chemical microenvironment of cells. We present a technique to set up molecular concentration patterns that are chemically, spatially and temporally flexible. Our strategy uses optically manipulated microsources, which steadily release molecules. Our technique enables the control of molecular concentrations over length scales down to about 1 μm and timescales from fractions of a second to an hour. We demonstrate this technique by manipulating the motility of single human neutrophils. We induced directed cell polarization and migration with microsources loaded with the chemoattractant formyl-methionine-leucine-phenylalanine. Furthermore, we triggered highly localized retraction of lamellipodia and redirection of polarization and migration with microsources releasing cytochalasin D, an inhibitor of actin polymerization.

Syndecan 4 is required for endothelial alignment in flow and atheroprotective signaling

Significance Atherosclerosis, the major cause of death and illness in industrialized nations, develops in regions of arteries in which fluid flow patterns are disturbed and endothelial cells fail to align in the direction of flow. In contrast, regions of laminar flow in which cells are aligned are protected. The current work shows that the transmembrane proteoglycan syndecan 4 is required for endothelial cell alignment in the direction of flow and for the protective effect of high laminar flow, yet other flow responses are intact. The data therefore identify a role for syndecan 4 in flow direction sensing, show that sensing flow direction is separable from sensing flow magnitude, and provide new support for the key role of cell alignment in atheroprotection. Atherosclerotic plaque localization correlates with regions of disturbed flow in which endothelial cells (ECs) align poorly, whereas sustained laminar flow correlates with cell alignment in the direction of flow and resistance to atherosclerosis. We now report that in hypercholesterolemic mice, deletion of syndecan 4 (S4−/−) drastically increased atherosclerotic plaque burden with the appearance of plaque in normally resistant locations. Strikingly, ECs from the thoracic aortas of S4−/− mice were poorly aligned in the direction of the flow. Depletion of S4 in human umbilical vein endothelial cells (HUVECs) using shRNA also inhibited flow-induced alignment in vitro, which was rescued by re-expression of S4. This effect was highly specific, as flow activation of VEGF receptor 2 and NF-κB was normal. S4-depleted ECs aligned in cyclic stretch and even elongated under flow, although nondirectionally. EC alignment was previously found to have a causal role in modulating activation of inflammatory versus antiinflammatory pathways by flow. Consistent with these results, S4-depleted HUVECs in long-term laminar flow showed increased activation of proinflammatory NF-κB and decreased induction of antiinflammatory kruppel-like factor (KLF) 2 and KLF4. Thus, S4 plays a critical role in sensing flow direction to promote cell alignment and inhibit atherosclerosis.

Vascular remodeling is governed by a VEGFR3-dependent fluid shear stress set point

Vascular remodeling under conditions of growth or exercise, or during recovery from arterial restriction or blockage is essential for health, but mechanisms are poorly understood. It has been proposed that endothelial cells have a preferred level of fluid shear stress, or ‘set point’, that determines remodeling. We show that human umbilical vein endothelial cells respond optimally within a range of fluid shear stress that approximate physiological shear. Lymphatic endothelial cells, which experience much lower flow in vivo, show similar effects but at lower value of shear stress. VEGFR3 levels, a component of a junctional mechanosensory complex, mediate these differences. Experiments in mice and zebrafish demonstrate that changing levels of VEGFR3/Flt4 modulates aortic lumen diameter consistent with flow-dependent remodeling. These data provide direct evidence for a fluid shear stress set point, identify a mechanism for varying the set point, and demonstrate its relevance to vessel remodeling in vivo. DOI: http://dx.doi.org/10.7554/eLife.04645.001

Multiplexed force measurements on live cells with holographic optical tweezers

We describe open-loop and closed-loop multiplexed force measurements using holographic optical tweezers. We quantify the performance of our novel video-based control system in a driven suspension of colloidal particles. We demonstrate our systems abilities with the measurement of the mechanical coupling between Aplysia bag cell growth cones and beads functionalized with the neuronal cell adhesion molecule, apCAM. We show that cells form linkages which couple beads to the underlying cytoskeleton. These linkages are intermittent, stochastic and heterogeneous across beads distributed near the leading edge of a single growth cone.

Elastic Coupling of Nascent apCAM Adhesions to Flowing Actin Networks

Adhesions are multi-molecular complexes that transmit forces generated by a cell’s acto-myosin networks to external substrates. While the physical properties of some of the individual components of adhesions have been carefully characterized, the mechanics of the coupling between the cytoskeleton and the adhesion site as a whole are just beginning to be revealed. We characterized the mechanics of nascent adhesions mediated by the immunoglobulin-family cell adhesion molecule apCAM, which is known to interact with actin filaments. Using simultaneous visualization of actin flow and quantification of forces transmitted to apCAM-coated beads restrained with an optical trap, we found that adhesions are dynamic structures capable of transmitting a wide range of forces. For forces in the picoNewton scale, the nascent adhesions’ mechanical properties are dominated by an elastic structure which can be reversibly deformed by up to 1 µm. Large reversible deformations rule out an interface between substrate and cytoskeleton that is dominated by a number of stiff molecular springs in parallel, and favor a compliant cross-linked network. Such a compliant structure may increase the lifetime of a nascent adhesion, facilitating signaling and reinforcement.

Abstract 33: Implementing CAD cells quantify the relationships between Ig-CAM-mediated adhesion, force transduction, and actin dynamics

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Several neuro-degenerative diseases and motor defects have been linked to malfunctions in the motility of neurons. This project is focused on the determining whether the roles of the transmembrane proteins NCAM and L1-CAM can be studied using the CAD cell line as a model. CAD cells are a cancer cell line that come from mice and can be stimulated to grow as either neurons or glial cells. In addition, this cell line has been shown to be an easily transfected line. In order to determine usefulness of this cell line several techniques were used. Western blots were implemented in order to determine if the NCAM and/or L1-CAM and proteins known to be involved in their signaling pathways were present in the cell line. Western blots were then confirmed by immunocytochemistry staining. Next, the CAD cell line was transfected with photoactivateable actin. Finally, immuno beads were attached to the membranes of these CAD cells and then holographic optical tweezers were used to determine whether or not thsee ligands coupled to the retrograde actin flow in the cells. The combination of these experiments should provide the evidence for whether or not the CAD cells would be a feasible model to study the nature of the NCAM and L1-CAM proteins and their relationship to the movement of actin in the cytoskeleton of neurons. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 33. doi:1538-7445.AM2012-33