Pseudopod projection and cell spreading of passive leukocytes in response to fluid shear stress

Fluid shear stress-mediated mechanotransduction in circulating leukocytes and its defect in microvascular dysfunction

Leukocytes (neutrophils, monocytes) in the active circulation exhibit multiple phenotypic indicators for a low level of cellular activity, like lack of pseudopods and minimal amounts of activated, cell-adhesive integrins on their surfaces. In contrast, before these cells enter the circulation in the bone marrow or when they recross the endothelium into extravascular tissues of peripheral organs they are fully activated. We review here a multifaceted mechanism mediated by fluid shear stress that can serve to deactivate leukocytes in the circulation. The fluid shear stress controls pseudopod formation via the FPR receptor, the same receptor responsible for pseudopod projection by localized actin polymerization. The bioactivity of macromolecular factors in the blood plasma that interfere with receptor stimulation by fluid flow, such as proteolytic cleavage in the extracellular domain of the receptor or the membrane actions of cholesterol, leads to a defective ability to respond to fluid shear stress by actin depolymerization. The cell reaction to fluid shear involves CD18 integrins, nitric oxide, cGMP and Rho GTPases, is attenuated in the presence of inflammatory mediators and modified by glucocorticoids. The mechanism is abolished in disease models (genetic hypertension and hypercholesterolemia) leading to an increased number of activated leukocytes in the circulation with enhanced microvascular resistance and cell entrapment. In addition to their role in binding to biochemical agonists/antagonists, membrane receptors appear to play a second role: to monitor local fluid shear stress levels. The fluid shear stress control of many circulating cell types such as lymphocytes, stem cells, tumor cells remains to be elucidated.

Green-lipped mussel (Perna canaliculus) hemocytes: A flow cytometric study of sampling effects, sub-populations and immune-related functions

Green-lipped mussels (Perna canaliculus) are a commercially and culturally important bivalve species in New Zealand (NZ). As the highest value export aquaculture product in NZ, understanding and safeguarding the health of this species is imperative. The identification and characterization of hemocytes can provide useful information regarding the health of this species. Using flow cytometry (FCM), the present study assessed for the first time the use of different antiaggregant solutions and storage times on the immune-related parameters of hemocytes from cultured adult P. canaliculus. In addition, characterization of the immune-related functions of hemocyte sub-populations within the hemolymph were assessed. The two antiaggregant solutions tested (Modified Alserver's, MAS, A and B) maintained similar numbers of hemocytes in circulation over a 60 min period but, reduced the viability (MAS A) and increased the ROS production (MAS B) of the hemocytes compared to hemocytes diluted in cold filtered seawater (FSW). Hemocytes diluted in FSW and kept on ice showed significant aggregation after 2 h and a reduction in viability from 4 h. Three different hemocyte sub-populations were identified, discernible by their relative size and internal complexity: blast-like cells, hyalinocytes and granulocytes, which accounted for approximately 4, 67 and 29% of the total hemolymph population respectively. Granulocytes showed significantly higher reactive oxygen species production, phagocytic capabilities and neutral lipid content compared to hyalinocytes and blast-like cells. Results indicate that maintaining extracted hemolymph in cold FSW, completing analysis of fresh samples within 2 h of extraction and FCM assay incubation times of no longer than 30 min are best to obtain accurate results. Formalin fixation can also be used for future determination of hemocyte sub-populations and internal structures. Results from this study will allow effective future study of the effects of various stressors on P. canaliculus health and lead to improved management and production strategies in this species.

Effects of Transient Exposure to High Shear on Neutrophil Rolling Behavior

Introduction

Neutrophils display an array of behaviors ranging from rolling and migration to phagocytosis and granule secretion. Several of these behaviors are modulated by the local shear conditions. In the normal circulation, neutrophils experience shear rates from approximately 10-2,000 s^-1. However, neutrophils are also exposed to pathological shear levels in natural conditions such as severe stenosis and arteriosclerosis, as well as in blood-contacting devices such as ventricular assist devices (VADs) and hemodialysis machines. The effects of transiently (< 1 sec) exposing neutrophils to abnormally high shear rates (>3,000 s^-1) are not well understood.

Methods

We developed a set of microfluidic devices capable of exposing neutrophils to high shear rates for short durations (100-400 msec). Suspensions of isolated neutrophils were perfused through the devices and their rolling velocities on P-selectin were analyzed before and after shear exposure.

Results

We observed a significant increase in neutrophil rolling velocities on P-selectin coated regions following transient high shear exposure. The magnitude of the rolling velocity increase was dependent upon the duration of high shear exposure and became statistically significant for exposure times of 310 msec or longer. When polystyrene beads coated with a glycosulfopeptide that mimics the binding region of P-selectin glycoprotein ligand-1 (PSGL-1) were perfused through the devices, no change between the pre-shear and post-shear rolling velocities was observed.

Conclusions

These results suggest that high shear levels alter normal neutrophil rolling behavior and are important for understanding neutrophil biology in high shear conditions, as well as for improving medical device performance.

Ovine Leukocyte Microparticles Generated by the CentriMag Ventricular Assist Device In Vitro

Ventricular assist devices (VADs) are a life-saving form of mechanical circulatory support in heart failure patients. However, VADs have not yet reached their full potential due to the associated side effects (thrombosis, bleeding, infection) related to the activation and damage of blood cells and proteins caused by mechanical stress and foreign materials. Studies of the effects of VADs on leukocytes are limited, yet leukocyte activation and damage including microparticle generation can influence both thrombosis and infection rates. Therefore, the aim was to develop a multicolor flow cytometry assessment of leukocyte microparticles (LMPs) using ovine blood and the CentriMag VAD as a model for shear stress. Ovine blood was pumped for 6 h in the CentriMag and regular samples analyzed for hemolysis, complete blood counts and LMP by flow cytometry during three different pump operating conditions (low flow, standard, high speed). The high speed condition caused significant increases in plasma-free hemoglobin; decreases in total leukocytes, granulocytes, monocytes, and platelets; increases in CD45⁺ LMPs as well as two novel LMP populations: CD11b^bright /HLA-DR^- and CD11b^dull /HLA-DR⁺ , both of which were CD14^- /CD21^- . CD11b^bright /HLA-DR^- LMPs appeared to respond to an increase in shear magnitude whereas the CD11b^dull /HLA-DR⁺ LMPs significantly increased in all pumping conditions. We propose that these two populations are released from granulocytes and T cells, respectively, but further research is needed to better characterize these two populations.

Hydrodynamics in Cell Studies

Hydrodynamic phenomena are ubiquitous in living organisms and can be used to manipulate cells or emulate physiological microenvironments experienced in vivo. Hydrodynamic effects influence multiple cellular properties and processes, including cell morphology, intracellular processes, cell–cell signaling cascades and reaction kinetics, and play an important role at the single-cell, multicellular, and organ level. Selected hydrodynamic effects can also be leveraged to control mechanical stresses, analyte transport, as well as local temperature within cellular microenvironments. With a better understanding of fluid mechanics at the micrometer-length scale and the advent of microfluidic technologies, a new generation of experimental tools that provide control over cellular microenvironments and emulate physiological conditions with exquisite accuracy is now emerging. Accordingly, we believe that it is timely to assess the concepts underlying hydrodynamic control of cellular microenvironments and their applications and provide some perspective on the future of such tools in in vitro cell-culture models. Generally, we describe the interplay between living cells, hydrodynamic stressors, and fluid flow-induced effects imposed on the cells. This interplay results in a broad range of chemical, biological, and physical phenomena in and around cells. More specifically, we describe and formulate the underlying physics of hydrodynamic phenomena affecting both adhered and suspended cells. Moreover, we provide an overview of representative studies that leverage hydrodynamic effects in the context of single-cell studies within microfluidic systems.

Go with the flow: GEF-H1 mediated shear stress mechanotransduction in neutrophils

Neutrophils in circulation experience significant shear forces due to blood flow when they tether to the vascular endothelium. Biochemical and biophysical responses of neutrophils to the physical force of flowing blood modulate their behavior and promote tissue recruitment under pro-inflammatory conditions. Neutrophil mechanotransduction responses occur through mechanisms that are not yet fully understood. In our recent work, we showed that GEF-H1, a RhoA specific guanine nucleotide exchange factor (GEF), is required to maintain neutrophil motility and migration in response to shear stress. GEF-H1 re-localizes to flottilin-rich uropods in neutrophils in response to fluid shear stress and promotes spreading and crawling on activated endothelial cells. GEF-H1 drives cellular contractility through myosin light chain (MLC) phosphorylation downstream of the Rho-ROCK signaling axis. We propose that GEF-H1-dependent cell spreading and crawling in shear stress-dependent neutrophil recruitment from the vasculature are due to the specific localization of Rho-induced contractility in the uropod.

Role of the Cytoskeleton in Myeloid Cell Function

During an innate immune response, myeloid cells undergo complex morphological adaptations in response to inflammatory cues, which allow them to exit the vasculature, enter the tissues, and destroy invading pathogens. The actin and microtubule cytoskeletons are central to many of the most essential cellular functions including cell division, cell morphology, migration, intracellular trafficking, and signaling. Cytoskeletal structure and regulation are crucial for many myeloid cell functions, which require rapid and dynamic responses to extracellular signals. In this chapter, we review the roles of the actin and microtubule cytoskeletons in myeloid cells, focusing primarily on their roles in chemotaxis and phagocytosis. The role of myeloid cell cytoskeletal defects in hematological disorders is highlighted throughout.

GEF-H1 is necessary for neutrophil shear stress-induced migration during inflammation

In their work, Fine et al. demonstrate that GEF-H1 is required for the spreading and crawling of neutrophils in response to intravascular blood flow. They uncover a novel mechanism that couples shear stress with Rho-dependent migratory behavior of neutrophils during inflammation.

Leukocyte crawling and transendothelial migration (TEM) are potentiated by shear stress caused by blood flow. The mechanism that couples shear stress to migration has not been fully elucidated. We found that mice lacking GEF-H1 (GEF-H1^−/−), a RhoA-specific guanine nucleotide exchange factor (GEF), displayed limited migration and recruitment of neutrophils into inflamed tissues. GEF-H1^−/− leukocytes were deficient in in vivo crawling and TEM in the postcapillary venules. We demonstrated that although GEF-H1 deficiency had little impact on the migratory properties of neutrophils under static conditions, shear stress triggered GEF-H1–dependent spreading and crawling of neutrophils and relocalization of GEF-H1 to flotillin-2–rich uropods. Our results identify GEF-H1 as a component of the shear stress response machinery in neutrophils required for a fully competent immune response to bacterial infection.

Fluid shear-induced cathepsin B release in the control of Mac1-dependent neutrophil adhesion

There is compelling evidence that circulatory hemodynamics prevent neutrophil activation, including adhesion to microvessels, in the microcirculation. However, the underlying mechanism or mechanisms by which that mechanoregulation occurs remain unresolved. Here, we report evidence that exposure to fluid shear stress (FSS) promotes neutrophils to release cathepsin B (ctsB) and that this autocrine regulatory event is antiadhesive for neutrophils on endothelial surfaces through Mac1-selective regulation. We used a combined cell-engineering and immunocytochemistry approach to find that ctsB was capable of cleaving Mac1 integrins on neutrophils and demonstrated that this proteolysis alters their adhesive functions. Under no-flow conditions, ctsB enhanced neutrophil migration though a putative effect on pseudopod retraction rates. We also established a flow-based cell detachment assay to verify the role of ctsB in the control of neutrophil adhesion by fluid flow stimulation. Fluid flow promoted neutrophil detachment from platelet and endothelial layers that required ctsB, consistent with its fluid shear stress-induced release. Notably, compared with leukocytes from wild-type mice, those from ctsB-deficient (ctsB ^-/- ) mice exhibited an impaired CD18 cleavage response to FSS, significantly elevated baseline levels of CD18 surface expression, and an enhanced adhesive capacity to mildly inflamed postcapillary venules. Taken together, the results of the present study support a role for ctsB in a hemodynamic control mechanism that is antiadhesive for leukocytes on endothelium. These results have implications in the pathogenesis of chronic inflammation, microvascular dysfunction, and cardiovascular diseases involving sustained neutrophil activation in the blood and microcirculation.

Modeling the Mechanosensitivity of Neutrophils Passing through a Narrow Channel

Recent experiments have found that neutrophils may be activated after passing through microfluidic channels and filters. Mechanical deformation causes disassembly of the cytoskeleton and a sudden drop of the elastic modulus of the neutrophil. This fluidization is followed by either activation of the neutrophil with protrusion of pseudopods or a uniform recovery of the cytoskeleton network with no pseudopod. The former occurs if the neutrophil traverses the narrow channel at a slower rate. We propose a chemo-mechanical model for the fluidization and activation processes. Fluidization is treated as mechanical destruction of the cytoskeleton by sufficiently rapid bending. Loss of the cytoskeleton removes a pathway by which cortical tension inhibits the Rac protein. As a result, Rac rises and polarizes through a wave-pinning mechanism if the chemical reaction rate is fast enough. This leads to recovery and reinforcement of the cytoskeleton at the front of the neutrophil, and hence protrusion and activation. Otherwise the Rac signal returns to a uniform pre-deformation state and no activation occurs. Thus, mechanically induced neutrophil activation is understood as the competition between two timescales: that of chemical reaction and that of mechanical deformation. The model captures the main features of the experimental observation.

Effect of Pseudopod Extensions on Neutrophil Hemodynamic Transport Near a Wall

During inflammation, circulating neutrophils roll on, and eventually tether to, the endothelial lining of blood vessels, allowing them to exit the bloodstream and enter the surrounding tissue to target pathogens. This process is mediated by the selectin family of adhesion proteins expressed by endothelial cells. Interestingly, only 10% of activated, migrating neutrophils transmigrate into the extravascular space; the other 90% detach from the wall and rejoin the blood flow. Neutrophils extrude pseudopods during the adhesion cascade; however, the transport behavior of this unique cell geometry has not been previously addressed. In this study, a three-dimensional computational model was applied to neutrophils with pseudopodial extensions to study the effect of cell shape on the hydrodynamic transport of neutrophils. The collision time, contact area, contact force, and collision frequency were analyzed as a function of pseudopod length. It was found that neutrophils experience more frequent collisions compared to prolate spheroids of equal volume and length. Longer pseudopods and lower shear rates increase the collision time integral contact area, a predictor of binding potential. Our results indicate that contact between the neutrophil and the vessel wall was found to be focused predominantly on the pseudopod tip.

Mechanotransduction in neutrophil activation and deactivation

Mechanotransduction refers to the processes through which cells sense mechanical stimuli by converting them to biochemical signals and, thus, eliciting specific cellular responses. Cells sense mechanical stimuli from their 3D environment, including the extracellular matrix, neighboring cells and other mechanical forces. Incidentally, the emerging concept of mechanical homeostasis,long term or chronic regulation of mechanical properties, seems to apply to neutrophils in a peculiar manner, owing to neutrophils' ability to dynamically switch between the activated/primed and deactivated/deprimed states. While neutrophil activation has been known for over a century, its deactivation is a relatively recent discovery. Even more intriguing is the reversibility of neutrophil activation and deactivation. We review and critically evaluate recent findings that suggest physiological roles for neutrophil activation and deactivation and discuss possible mechanisms by which mechanical stimuli can drive the oscillation of neutrophils between the activated and resting states. We highlight several molecules that have been identified in neutrophil mechanotransduction, including cell adhesion and transmembrane receptors, cytoskeletal and ion channel molecules. The physiological and pathophysiological implications of such mechanically induced signal transduction in neutrophils are highlighted as a basis for future work. This article is part of a Special Issue entitled: Mechanobiology.

Simulation and Analysis of Tethering Behavior of Neutrophils with Pseudopods

P-selectin and P-selectin glycoprotein ligand-1 (PSGL-1) play important roles in mediating the inflammatory cascade. Selectin kinetics, together with neutrophil hydrodynamics, regulate the fundamental adhesion cascade of cell tethering and rolling on the endothelium. The current study uses the Multiscale Adhesive Dynamics computational model to simulate, for the first time, the tethering and rolling behavior of pseudopod-containing neutrophils as mediated by P-selectin/PSGL-1 bonds. This paper looks at the effect of including P-selectin/PSGL-1 adhesion kinetics. The parameters examined included the shear rate, adhesion on-rate, initial neutrophil position, and receptor number sensitivity. The outcomes analyzed included types of adhesive behavior observed, tether rolling distance and time, number of bonds formed during an adhesive event, contact area, and contact time. In contrast to the hydrodynamic model, P-selectin/PSGL-1 binding slows the neutrophil’s translation in the direction of flow and causes the neutrophil to swing around perpendicular to flow. Several behaviors were observed during the simulations, including tethering without firm adhesion, tethering with downstream firm adhesion, and firm adhesion upon first contact with the endothelium. These behaviors were qualitatively consistent with in vivo data of murine neutrophils with pseudopods. In the simulations, increasing shear rate, receptor count, and bond formation rate increased the incidence of firm adhesion upon first contact with the endothelium. Tethering was conserved across a range of physiological shear rates and was resistant to fluctuations in the number of surface PSGL-1 molecules. In simulations where bonding occurred, interaction with the side of the pseudopod, rather than the tip, afforded more surface area and greater contact time with the endothelial wall.

Persistent cellular motion control and trapping using mechanotactic signaling

Chemotactic signaling and the associated directed cell migration have been extensively studied owing to their importance in emergent processes of cellular aggregation. In contrast, mechanotactic signaling has been relatively overlooked despite its potential for unique ways to artificially signal cells with the aim to effectively gain control over their motile behavior. The possibility of mimicking cellular mechanotactic signals offers a fascinating novel strategy to achieve targeted cell delivery for in vitro tissue growth if proven to be effective with mammalian cells. Using (i) optimal level of extracellular calcium ([Ca²⁺ ]_ext mM) we found, (ii) controllable fluid shear stress of low magnitude (), and (iii) the ability to swiftly reverse flow direction (within one second), we are able to successfully signal Dictyostelium discoideum amoebae and trigger migratory responses with heretofore unreported control and precision. Specifically, we are able to systematically determine the mechanical input signal required to achieve any predetermined sequences of steps including straightforward motion, reversal and trapping. The mechanotactic cellular trapping is achieved for the first time and is associated with a stalling frequency of Hz for a reversing direction mechanostimulus, above which the cells are effectively trapped while maintaining a high level of directional sensing. The value of this frequency is very close to the stalling frequency recently reported for chemotactic cell trapping [Meier B, et al. (2011) Proc Natl Acad Sci USA 108:11417–11422], suggesting that the limiting factor may be the slowness of the internal chemically-based motility apparatus.

T lymphocytes orient against the direction of fluid flow during LFA-1-mediated migration

As they leave the blood stream and travel to lymph nodes or sites of inflammation, T lymphocytes are captured by the endothelium and migrate along the vascular wall to permissive sites of transmigration. These processes take place under the influence of hemodynamic shear stress; therefore, we investigated how migrational speed and directionality are influenced by variations in shear stress. We examined human effector T lymphocytes on intercellular adhesion molecule 1 (ICAM-1)-coated surfaces under the influence of shear stresses from 2 to 60 dyn.cm(-2). T lymphocytes were shown to respond to shear stress application by a rapid (30 s) and fully reversible orientation of their migration against the fluid flow without a change in migration speed. Primary T lymphocytes migrating on ICAM-1 in the presence of uniformly applied SDF-1α were also found to migrate against the direction of shear flow. In sharp contrast, neutrophils migrating in the presence of uniformly applied fMLP and leukemic HSB2 T lymphocytes migrating on ICAM-1 alone oriented their migration downstream, with the direction of fluid flow. Our findings suggest that, in addition to biochemical cues, shear stress is a contributing factor to leukocyte migration directionality.

Effects of high temperature and exposure to air on mussel (Mytilus galloprovincialis, Lmk 1819) hemocyte phagocytosis: modulation of spreading and oxidative response

Hemocytes are a critical component of the mussel defense system and the present study aims at investigating their spreading and oxidative properties during phagocytosis under in vivo experimental stress conditions. The spreading ability was measured by an automated cell analyzer on the basis of the circularity, a parameter corresponding to the hemocyte roundness. The oxidative activity was investigated by micromethod assay, measuring the respiratory burst as expression of the fluorescence generated by the oxidation of specific probe. Following the application of high temperature and exposure to air, there was evidence of negative modulation of spreading and oxidative response, as revealed by a cell roundness increase and fluorescence generation decrease. Therefore, the fall of respiratory burst appeared as matched with the inhibition of hemocyte morphological activation, suggesting a potential depression of the phagocytosis process and confirming the application of the circularity parameter as potential stress marker, both in experimental and field studies.

Shear-induced resistance to neutrophil activation via the formyl peptide receptor

The application of fluid shear stress on leukocytes is critical for physiological functions including initial adhesion to the endothelium, the formation of pseudopods, and migration into tissues. The formyl peptide receptor (FPR) on neutrophils, which binds to formyl-methionyl-leucyl-phenylalanine (fMLP) and plays a role in neutrophil chemotaxis, has been implicated as a fluid shear stress sensor that controls pseudopod formation. The role of shear forces on earlier indicators of neutrophil activation, such as L-selectin shedding and α(M)β(2) integrin activation, remains unclear. Here, human neutrophils exposed to uniform shear stress (0.1-4.0 dyn/cm(2)) in a cone-and-plate viscometer for 1-120 min showed a significant reduction in both α(M)β(2) integrin activation and L-selectin shedding after stimulation with 0.5 nM of fMLP. Neutrophil resistance to activation was directly linked to fluid shear stress, as the response increased in a shear stress force- and time-dependent manner. Significant shear-induced loss of FPR surface expression on neutrophils was observed, and high-resolution confocal microscopy revealed FPR internalized within neutrophils. These results suggest that physiological shear forces alter neutrophil activation via FPR by reducing L-selectin shedding and α(M)β(2) integrin activation in the presence of soluble ligand.

Forces on a wall-bound leukocyte in a small vessel due to red cells in the blood stream

As part of the inflammation response, white blood cells (leukocytes) are well known to bind nearly statically to the vessel walls, where they must resist the force exerted by the flowing blood. This force is particularly difficult to estimate due to the particulate character of blood, especially in small vessels where the red blood cells must substantially deform to pass an adhered leukocyte. An efficient simulation tool with realistically flexible red blood cells is used to estimate these forces. At these length scales, it is found that the red cells significantly augment the streamwise forces that must be resisted by the binding. However, interactions with the red cells are also found to cause an average wall-directed force, which can be anticipated to enhance binding. These forces increase significantly as hematocrit values approach 25% and decrease significantly as the leukocyte is made flatter on the wall. For a tube hematocrit of 25% and a spherical protrusion with a diameter three-quarters that of the vessel, the average forces are increased by ~40% and the local forces are more than double those estimated with an effective-viscosity-homogenized blood. Both the enhanced streamwise and wall-ward forces and their unsteady character are potentially important in regard to binding mechanisms.

Actin polymerization stabilizes α4β1 integrin anchors that mediate monocyte adhesion

Rapid signaling and structural adaptations to the actin cytoskeleton enable leukocytes to stabilize α4 integrin–mediated adhesion and resist detachment from inflamed endothelium.

Leukocytes arrested on inflamed endothelium via integrins are subjected to force imparted by flowing blood. How leukocytes respond to this force and resist detachment is poorly understood. Live-cell imaging with Lifeact-transfected U937 cells revealed that force triggers actin polymerization at upstream α4β1 integrin adhesion sites and the adjacent cortical cytoskeleton. Scanning electron microscopy revealed that this culminates in the formation of structures that anchor monocyte adhesion. Inhibition of actin polymerization resulted in cell deformation, displacement, and detachment. Transfection of dominant-negative constructs and inhibition of function or expression revealed key signaling steps required for upstream actin polymerization and adhesion stabilization. These included activation of Rap1, phosphoinositide 3-kinase γ isoform, and Rac but not Cdc42. Thus, rapid signaling and structural adaptations enable leukocytes to stabilize adhesion and resist detachment forces.

Hydrodynamics of cell-cell mechanical signaling in the initial stages of aggregation

Mechanotactic cell motility has recently been shown to be a key player in the initial aggregation of crawling cells such as leukocytes and amoebae. The effects of mechanotactic signaling in the early aggregation of amoeboid cells are here investigated using a general mathematical model based on known biological evidence. We elucidate the hydrodynamic fundamentals of the direct guiding of a cell through mechanotaxis in the case where one cell transmits a mechanotactic signal through the fluid flow by changing its shape. It is found that any mechanosensing cells placed in the stimulus field of mechanical stress are able to determine the signal transmission direction with a certain angular dispersion which does not preclude the aggregation from happening. The ubiquitous presence of noise is accounted for by the model. Finally, the mesoscopic pattern of aggregation is obtained which constitutes the bridge between, on one hand, the microscopic world where the changes in the cell shape occur and, on the other hand, the cooperative behavior of the cells at the mesoscopic scale.

Internalization of Formyl Peptide Receptor in Leukocytes Subject to Fluid Stresses

Human leukocytes retract pseudopods under normal physiologic levels of fluid shear stress even in the absence of any other mediator. To gain more detailed understanding of the mechanisms that regulate this cell behavior, we exposed leukocytes to a steady state laminar shear field in a flow chamber and computed the fluid stresses distribution on the surface of individual cells with and without pseudopod. The surface fluid stress distribution on such cell is quite inhomogeneous. We hypothesized that the local fluid stresses on the cell surface serve to regulate pseudopod retraction by way of membrane receptors, especially the formyl peptide receptor (FPR). Comparison of the receptor distribution and the stress distribution over the surface of the cells indicates that the membrane fluid stress alone is not directly correlated with the extent of regional pseudopod retraction, giving further support to the hypothesis that membrane receptors are involved in the mechanotransduction of leukocytes. We observed that after exposure to fluid shear the FPR was internalized to a small intracellular compartment. This internalization appears to be independent of the original location of the receptor on the surface of the cell and the FPR appears to be more derived from multiple locations on the cell, with both higher and lower fluid stresses. The evidence suggests that FPR involvement in the pseudopod-retraction process is not limited to cell surface regions with the highest fluid shear stress, but rather a more global occurrence over the majority of the cell membrane.

A stochastic model of cell aggregation under planar flow in the dilute regime

Models of the adhesion of a population of cells in a plane flow are developed, considering the dilute regime. Cells considered as rigid punctual entities are virtually injected at regular times within a plane channel limited by two fixed planes. The pressure profile is supposed to be triangular (constant gradient), in accordance with the assumptions of a Poiseuille flow. The cell adherence to the channel wall is governed by the balance of forces, accounting for gravity, non-specific physical interactions, such as electrostatic effects (repulsive) and Van der Waals forces (attractive), specific adhesive forces representing the ligand-receptor interactions, and friction between cells and the fluid in the vicinity of the endothelium wall. The spatial distribution of the adhesion molecules along the wall is supposed to be a random event, accounted for by a stochastic spatial variability of the dipolar moments of those molecules, according to a Gaussian process. Experimental trends reported for the rate of aggregation of L-selectin mediated leukocytes under shear flow are in qualitative accordance with the evolution versus time of adhering cells obtained by the present simulations. The effect of the maximal injection pressure on those kinetics is assessed.

Critical stresses for cancer cell detachment in microchannels

We present experiments involving cancer cells adhering to microchannels, subjected to increasing shear stresses (0.1-30 Pa). Morphological studies were carried out at different shear stresses. Cells exhibit spreading patterns similar to those observed under static conditions, as long as the shear stress is not too high. At critical wall shear stresses (around 2-5 Pa), cell-substrate contact area decreases until detachment at the larger stresses. Critical shear stresses are found to be lower for higher confinements (i.e. smaller cell height to channel height ratio). Fluorescent techniques were used to locate focal adhesions (typically 1 lm(2) in size) under various shearing conditions, showing that cells increase the number of focal contacts in the region facing the flow. To analyze such data, we propose a model to determine the critical stress, resulting from the competition between hydrodynamic forces and the adhesive cell resistance. With this model, typical adhesive stresses exerted at each focal contact can be determined and are in agreement with previous works.

Vav1 is essential for mechanotactic crawling and migration of neutrophils out of the inflamed microvasculature

Mac-1-dependent crawling is a new step in the leukocyte recruitment cascade that follows LFA-1-dependent adhesion and precedes emigration. Neutrophil adhesion via LFA-1 has been shown to induce cytoskeletal reorganization through Vav1-dependent signaling, and the current study investigates the role of Vav1 in the leukocyte recruitment process in vivo with particular attention to the events immediately downstream of LFA-1-dependent adhesion. Intravital and spinning-disk-confocal microscopy was used to investigate intravascular crawling in relation to endothelial junctions in vivo in wild-type and Vav1(-/-) mice. Adherent wild-type neutrophils almost immediately began crawling perpendicular to blood flow via Mac-1 until they reached an endothelial junction where they often changed direction. This pattern of perpendicular, mechanotactic crawling was recapitulated in vitro when shear was applied. In sharp contrast, the movement of Vav1(-/-) neutrophils was always in the direction of flow and appeared more passive as if the cells were dragged in the direction of flow in vivo and in vitro. More than 80% of Vav1(-/-) neutrophils moved independent of Mac-1 and could be detached with LFA-1 Abs. An inability to release the uropod was frequently noted for Vav1(-/-) neutrophils, leading to greatly elongated tails. The Vav1(-/-) neutrophils failed to stop or follow junctions and ultimately detached, leading to fewer emigrated neutrophils. The Vav1(-/-) phenotype resulted in fewer neutrophils recruited in a relevant model of infectious peritonitis. Clearly, Vav1 is critical for the complex interplay between LFA-1 and Mac-1 that underlies the programmed intravascular crawling of neutrophils.

Leucocyte recruitment under fluid shear: mechanical and molecular regulation within the inflammatory synapse

1. Nature has evolved an exquisite system for regulation of leucocyte recruitment at sites of tissue inflammation. Mechanical energy translated to the red and white blood cells transports them from large arteries down to the microcirculation. 2. Neutrophils overcome the drag forces of blood flow by forming selectin and integrin adhesive bonds with the endothelium that coats the vessel wall. Leucocyte adhesion receptors have evolved unique mechanical and chemical properties that optimize for sequential binding and uptake of traction forces. 3. In the present brief review, we address how dispersive forces acting on a neutrophil in shear flow function to stabilize and synchronize bond formation within a macromolecular membrane complex we denote the inflammatory synapse.

Mechanical forces induced by the transendothelial migration of human neutrophils

The mechanisms regulating neutrophil transmigration of vascular endothelium are not fully elucidated, but involve neutrophil firm attachment and passage through endothelial cell-cell junctions. The goal of this study was to characterize the tangential forces exerted by neutrophils during transendothelial migration at cell-cell junctions using an in vitro laminar shear flow model in which confluent activated endothelium is grown on a microfabricated pillar substrate. The tangential forces are deduced from the measurement of pillar deflection beneath the endothelial cell-cell junction as neutrophils transmigrate. The force diagram displays an initial force increase, which coincides with neutrophil penetration into the intercellular space and formation of a gap in VE-cadherin staining. This is followed by a rapid and large increase of traction forces exerted by endothelial cells on the substrate in response to the transmigration process and the disruption of cell-cell contacts. The average maximum force exerted by an actively transmigrating neutrophil is three times higher than the force generated by an adherent neutrophil that does not transmigrate. Furthermore, we show that substrate rigidity can modify the mechanical forces induced by the transmigration of a neutrophil through the endothelium. Our data suggest that the force induced by neutrophil transmigration plays a key role in the disruption of endothelial adherens junctions.

Modeling cell interactions under flow

In this review, we summarize the current state of understanding of the processes by which leukocytes, and other cells, such as tumor cells interact with the endothelium under various blood flow conditions. It is shown that the interactions are influenced by cell-cell adhesion properties, shear stresses due to the flow field and can also be modified by the cells microrheological properties. Different adhesion proteins are known to be involved leading to particular mechanisms by which interactions take place during inflammation or metastasis. Cell rolling, spreading, migration are discussed, as well as the effect of flow conditions on these mechanisms, including microfluidic effects. Several mathematical models proposed in recent years capturing the essential features of such interaction mechanisms are reviewed. Finally, we present a recent model in which the adhesion is given by a kinetics theory based model and the cell itself is modeled as a viscoelastic drop. Qualitative agreement is found between the predictions of this model and in vitro experiments.

Fluid stresses on the membrane of migrating leukocytes

We recently demonstrated that migrating human leukocytes respond to normal physiologic fluid stresses (approximately 1 dyn/cm(2)) by active control of local cytoplasmic extensions (pseudopods). To better understand the governing mechanisms of this response, we determined the fluid stress distributions on individual migrating leukocytes whose shapes were reconstructed with serial confocal microscopy. The flow over adherent leukocytes was computed by solution of the Stokes equation for plasma motion over the cell membrane. The fluid stresses are highest at the top of the cell and lowest in the substrate contact region. Pseudopods experience enhanced shear stresses but at lower values than at the top. Interestingly, leukocytes retract pseudopods in all regions and not only at sites with maximum fluid stresses. Therefore we hypothesized that sub-micron membrane folds (microvilli) serve to locally enhance the fluid stress on the cell. Using a separate model, we found that tips of microvilli experience greatly increased levels of stresses while the troughs between microvilli are shielded from fluid shear. This evidence suggests that the highly irregular shape of active leukocytes leads to fluid stresses that may stimulate local mechanosensory responses at many sites on the plasma membrane, even if they are located close to the cell-substrate contact region.

Recoil and stiffening by adherent leukocytes in response to fluid shear

Prolonged exposure to fluid shear stress alters leukocyte functions associated with the immune response. We examined the initial response of freshly isolated human leukocytes to fluid shear stress under high magnification. Adherent leukocytes exhibit a rapid biomechanical response to physiological levels of fluid shear stress. After passive displacement in the direction of a constant fluid shear stress, adherent leukocytes actively recoil back in the opposite direction of the fluid flow. Recoil is observed within seconds of the applied fluid shear stress. Simultaneously, fluid shear stress induces a stiffening of the cell. The immediate cell displacement in response to a step increase in fluid shear stress is greatly attenuated in subsequent steps compared to the initial fluid shear stress step. Recoil is not mediated by actin polymerization-dependent mechanisms, as cytochalasin D had no effect on this early response. However, stiffening was determined in part by an intact actin cytoskeleton. Inhibiting myosin force generation with ML-7 abolished the recoil and stiffening responses, implicating force generation by myosin as an important contributor to the early leukocyte response to fluid shear stress. This initial shear stress response may be particularly important in facilitating leukocyte attachment under sustained fluid shear stress by the flowing blood in the microcirculation.

Shear-induced capping of L-selectin on the neutrophil surface during centrifugation

L-selectin on leukocytes is critical in leukocyte tethering and adhesion to inflamed endothelium and lymphocyte homing to lymphoid organs. The spatial distribution of L-selectin on leukocytes controls cellular adhesive function in hydrodynamic shear. How L-selectin changes its position on the cell membrane remains an open question, but a possible candidate is shear stress encountered on the cell surface. Here we demonstrate shear-induced L-selectin polarization on the membrane during the process of centrifugation of resting neutrophils via immunofluorescent microscopy. It was found that randomly distributed L-selectin on neutrophils moves to a polar cap at one end of the cell after centrifugation (300 x g for 2 min) without inflammatory stimuli. This L-selectin redistribution under shear was predicted by Monte Carlo simulations that show how convection dominates over diffusion, leading to L-selectin cap formation during centrifugation at 280 x g or during leukocyte adhesion to the endothelial wall at 1 dyn/cm(2). Those results point to a role for shear stress in the modulation of L-selectin distribution, and suggest a possible alternate mechanism and reinterpretation of previous in vitro studies of L-selectin mediated adhesion of neutrophils isolated via centrifugation.

Chronotropic response of cultured neonatal rat ventricular myocytes to short-term fluid shear

Ventricular myocytes are continuously exposed to fluid shear in vivo by relative movement of laminar sheets and adjacent cells. Preliminary observations have shown that neonatal myocytes respond to fluid shear by increasing their beating rate, which could have an arrhythmogenic effect under elevated shear conditions. The objective of this study is to investigate the characteristics of the fluid shear response in cultured myocytes and to study selected potential mechanisms. Cultured neonatal rat ventricular myocytes that were spontaneously beating were subjected to low shear rates (5-50/s) in a fluid flow chamber using standard culture medium. The beating rate was measured from digital microscopic recordings. The myocytes reacted to low shear rates by a graded and reversible increase in their spontaneous beating rate of up to 500%. The response to shear was substantially attenuated in the presence of the beta-adrenergic agonist isoproterenol (by 86+/-8%), as well as after incubation with integrin-blocking RGD peptides (by 92+/-8%). The results suggest that the beta-adrenergic signaling pathway and integrin activation, which are known to interact, may play an important role in the response mechanism.

Analysis of inflammation

In the past, inflammation has been associated with infections and with the immune system. But more recent evidence suggests that a much broader range of diseases have telltale markers for inflammation. Inflammation is the basic mechanism available for repair of tissue after an injury and consists of a cascade of cellular and microvascular reactions that serve to remove damaged and generate new tissue. The cascade includes elevated permeability in microvessels, attachment of circulating cells to the vessels in the vicinity of the injury site, migration of several cell types, cell apoptosis, and growth of new tissue and blood vessels. This review provides a summary of the major microvascular, cellular, and molecular mechanisms that regulate elements of the inflammatory cascade. The analysis is largely focused on the identification of the major participants, notably signaling and adhesion molecules, and their mode of action in the inflammatory cascade. We present a new hypothesis for the generation of inflammatory mediators in plasma that are derived from the digestive pancreatic enzymes responsible for digestion. The inflammatory cascade offers a large number of opportunities for development of quantitative models that describe various aspects of human diseases.

Morphological analysis of tumor cell/endothelial cell interactions under shear flow

In the process of hematogenous cancer metastasis, tumor cells (TCs) must shed into the blood stream, survive in the blood circulation, migrate through the vascular endothelium (extravasation) and proliferate in the target organs. However, the precise mechanisms by which TCs penetrate the endothelial cell (EC) junctions remain one of the least understood aspects of TC extravasation. This question has generally been addressed under static conditions, despite the important role of flow induced mechanical stress on the circulating cell-endothelium interactions. Moreover, flow studies were generally focused on transient or firm adhesion steps of TC-EC interactions and did not consider TCs spreading or extravasation. In this paper, we used a parallel-plate flow chamber to investigate TC-EC interactions under flow conditions. An EC monolayer was cultured on the lower plate of the flow chamber to model the endothelial barrier. Circulating TCs were introduced into the flow channel under a well-defined flow field and TC cell shape changes on the EC monolayer were followed in vitro with live phase contrast and fluorescence microscopy. Two spreading patterns were observed: radial spreading which corresponds to TC extravasation, and axial spreading where TCs formed a mosaic TC-EC monolayer. By investigating the changes in area and minor/major aspect ratio, we have established a simple quantitative basis for comparing spreading modes under various shear stresses. Contrary to radial spreading, the extent of axial spreading was increased by shear stress.

De-Activation of Neutrophils in Suspension by Fluid Shear Stress: A Requirement for Erythrocytes

Leukocyte de-activation in response to a mechanical stimulus may be an important mechanism to reduce inflammation in the circulation and cardiovascular complications. We examine here a specific form of leukocyte activation in the form of pseudopod projection, a process that is important during cell spreading and migration, but if it occurs in circulating leukocytes, may also lead to their entrapment in the microvascular network. Fresh neutrophils were activated with fMLP, suspended without adhesion to endothelium, and sheared in a cone-and-plate device while both shear stress and shear rate were measured. A fraction of the activated neutrophils retracted their pseudopods under the influence of fluid shear and returned to round shape. Pseudopod retraction was observed only in the presence of erythrocytes (at shear stresses up to approximately 25 dyn/cm(2)). At a constant hematocrit and increasing plasma viscosities with addition of macromolecules, the number of de-activated neutrophils scaled with shear stress and less so with shear rate. We examined a biochemical and rheological role of erythrocytes during shear de-activation of neutrophils. Addition of superoxide dismutase (SOD) in phosphate buffer served to enhance neutrophil de-activation by fluid shear. Replacement of erythrocytes by solid microspheres (5.4 mum) to simulate the particle properties of the erythrocytes, did not serve to enhance neutrophil de-activation unless in the presence of SOD. At higher shear stresses without erythrocytes (38-77 dyn/cm(2)), we also observed neutrophil de-activation but only in the presence of SOD. These results suggest that erythrocytes play an important role in neutrophil de-activation by reducing the superoxide level in plasma. Shear stress, rather than shear rate, is the key determinant that regulates neutrophil de-activation.

Cytoskeletal remodeling and cellular activation during deformation of neutrophils into narrow channels

Neutrophils are subjected to mechanical stimulation as they deform into the narrow capillary segments of the pulmonary microcirculation. The present study seeks to understand the changes in the cytoskeletal structure and the extent of biological activation as a result of this process. Neutrophils were passed through narrow polycarbonate filter pores under physiological driving pressures, fixed, and stained downstream to visualize the F-actin content and distribution. Below a threshold capillary size, the cell remodeled its cytoskeleton through initial F-actin depolymerization, followed by recovery and increase in F-actin content associated with formation of pseudopods. This rapid depolymerization and subsequent recovery of F-actin was consistent with our previous observation of an immediate reduction in moduli with eventual recovery when the cells were subjected to deformation. Results also show that neutrophils must be retained in their elongated shape for an extended period of time for pseudopod formation, suggesting that a combination of low driving pressures and small capillary diameters promotes cellular activation. These observations show that mechanical deformation of neutrophils into narrow pulmonary capillaries have the ability to influence cytoskeletal structure, the degree of cellular activation, and migrational tendencies of the cells.

Control of neutrophil pseudopods by fluid shear: role of Rho family GTPases

Blood vessels and blood cells are under continuous fluid shear. Studies on vascular endothelium and smooth muscle cells have shown the importance of this mechanical stress in cell signal transduction, gene expression, vascular remodeling, and cell survival. However, in circulating leukocytes, shear-induced signal transduction has not been investigated. Here we examine in vivo and in vitro the control of pseudopods in leukocytes under the influence of fluid shear stress and the role of the Rho family small GTPases. We used a combination of HL-60 cells differentiated into neutrophils (1.4% dimethyl sulfoxide for 5 days) and fresh leukocytes from Rac knockout mice. The cells responded to shear stress (5 dyn/cm2) with retraction of pseudopods and reduction of their projected cell area. The Rac1 and Rac2 activities were decreased by fluid shear in a time- and magnitude-dependent manner, whereas the Cdc42 activity remained unchanged (up to 5 dyn/cm2). The Rho activity was transiently increased and recovered to static levels after 10 min of shear exposure (5 dyn/cm2). Inhibition of either Rac1 or Rac2 slightly but significantly diminished the fluid shear response. Transfection with Rac1-positive mutant enhanced the pseudopod formation during shear. Leukocytes from Rac1-null and Rac2-null mice had an ability to form pseudopods in response to platelet-activating factor but did not respond to fluid shear in vitro. Leukocytes in wild-type mice retracted pseudopods after physiological shear exposure, whereas cells in Rac1-null mice showed no retraction during equal shear. On leukocytes from Rac2-null mice, however, fluid shear exerted a biphasic effect. Leukocytes with extended pseudopods slightly decreased in length, whereas initially round cells increased in length after shear application. The disruption of Rac activity made leukocytes nonresponsive to fluid shear, induced cell adhesion and microvascular stasis, and decreased microvascular density. These results suggest that deactivation of Rac activity by fluid shear plays an important role in stable circulation of leukocytes.

Mechanical deformation of neutrophils into narrow channels induces pseudopod projection and changes in biomechanical properties

Neutrophils traversing the pulmonary microcirculation are subjected to mechanical stimulation during their deformation into narrow capillaries. To better understand the time-dependant changes caused by this mechanical stimulus, neutrophils were caused to flow into a microchannel, which allowed simultaneous visualization of cell morphology and passive rheological measurement by tracking the Brownian motion of endogenous granules. Above a threshold stimulus, mechanical deformation resulted in neutrophil activation with pseudopod projection. The activation time was inversely correlated to the rate of mechanical deformation experienced by the neutrophils. A reduction in shear moduli was observed within seconds after the onset of the mechanical stimulus, suggesting a sudden disruption of the neutrophil cytoskeleton when subjected to mechanical deformation. However, the magnitude of the reduction in moduli was independent of the degree of deformation. Recovery to nearly the initial values of viscoelastic moduli occurred within 1 min. These observations confirm that mechanical deformation of neutrophils, similar to conditions encountered in the pulmonary capillaries, is not a passive event; rather, it is capable of activating the neutrophils and enhancing their migratory tendencies.