Cytoplasmic calcium response to fluid shear stress in cultured vascular endothelial cells

Endothelial calcium dynamics elicited by ATP release from red blood cells

Red blood cells (RBCs) exhibit an interesting response to hydrodynamic flow, releasing adenosine triphosphate (ATP). Subsequently, these liberated ATP molecules initiate a crucial interaction with endothelial cells (ECs), thereby setting off a cascade involving the release of calcium ions (Ca2 + ). Ca2 + exerts control over a plethora of cellular functions, and acts as a mediator for dilation and contraction of blood vessel walls. This study focuses on the relationship between RBC dynamics and Ca2 + dynamics, based on numerical simulations under Poiseuille flow within a linear two-dimensional channel. It is found that the concentration of ATP depends upon a variety of factors, including RBC density, channel width, and the vigor of the flow. The results of our investigation reveals several features. Firstly, the peak amplitude of Ca2 + per EC escalates in direct proportion to the augmentation of RBC concentration. Secondly, increasing the flow strength induces a reduction in the time taken to reach the peak of Ca2 + concentration, under the condition of a constant channel width. Additionally, when flow strength remains constant, an increase in channel width corresponds to an elevation in calcium peak amplitude, coupled with a decrease in peak time. This implies that Ca2 + signals should transition from relatively unconstrained channels to more confined pathways within real vascular networks. This notion gains support from our examination of calcium propagation in a linear channel. In this scenario, the localized Ca2 + release initiates a propagating wave that gradually encompasses the entire channel. Notably, our computed propagation speed agrees with observations.

Effects of Mechanical Stress on Endothelial Cells In Situ and In Vitro

Endothelial cells lining blood vessels are essential for maintaining vascular homeostasis and mediate several pathological and physiological processes. Mechanical stresses generated by blood flow and other biomechanical factors significantly affect endothelial cell activity. Here, we review how mechanical stresses, both in situ and in vitro, affect endothelial cells. We review the basic principles underlying the cellular response to mechanical stresses. We also consider the implications of these findings for understanding the mechanisms of mechanotransducer and mechano-signal transduction systems by cytoskeletal components.

Combining nitric oxide and calcium sensing for the detection of endothelial dysfunction

Cardiovascular diseases are the leading cause of death worldwide and are not typically diagnosed until the disease has manifested. Endothelial dysfunction is an early, reversible precursor in the irreversible development of cardiovascular diseases and is characterized by a decrease in nitric oxide production. We believe that more reliable and reproducible methods are necessary for the detection of endothelial dysfunction. Both nitric oxide and calcium play important roles in the endothelial function. Here we review different types of molecular sensors used in biological settings. Next, we review the current nitric oxide and calcium sensors available. Finally, we review methods for using both sensors for the detection of endothelial dysfunction.

Polarized Mechanosensitive Signaling Domains Protect Arterial Endothelial Cells Against Inflammation

Endothelial cells (ECs) in the descending aorta are exposed to high laminar shear stress, which supports an anti-inflammatory phenotype that protects them from atherosclerosis. High laminar shear stress also supports flow-aligned cell elongation and front-rear polarity, but whether this is required for athero-protective signaling is unclear. Here, we show that Caveolin-1-rich microdomains become polarized at the downstream end of ECs exposed to continuous high laminar flow. These microdomains are characterized by higher membrane rigidity, filamentous actin (F-actin) and lipid accumulation. Transient receptor potential vanilloid-type 4 (Trpv4) ion channels, while ubiquitously expressed, mediate localized Ca ²⁺ entry at these microdomains where they physically interact with clustered Caveolin-1. The resultant focal bursts in Ca ²⁺ activate the anti-inflammatory factor endothelial nitric oxide synthase (eNOS) within the confines of these domains. Importantly, we find that signaling at these domains requires both cell body elongation and sustained flow. Finally, Trpv4 signaling at these domains is necessary and sufficient to suppress inflammatory gene expression. Our work reveals a novel polarized mechanosensitive signaling hub that induces an anti-inflammatory response in arterial ECs exposed to high laminar shear stress.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Molecular tension in syndecan-1 is regulated by extracellular mechanical cues and fluidic shear stress

The endothelium plays a central role in regulating vascular homeostasis and is key in determining the response to materials implanted in the vascular system. Endothelial cells are uniquely sensitive to biophysical cues from applied forces and their local cellular microenvironment. The glycocalyx is a layer of proteoglycans, glycoproteins and glycosaminoglycans that lines the luminal surface of the vascular endothelium, interacting directly with the components of the blood and the forces of blood flow. In this work, we examined the changes in mechanical tension of syndecan-1, a cell surface proteoglycan that is an integral part of the glycocalyx, in response to substrate stiffness and fluidic shear stress. Our studies demonstrate that syndecan-1 has higher mechanical tension in regions of cell adhesion, on and in response to nanotopographical cues. In addition, we found that substrate stiffness also regulated the mechanical tension of syndecan-1 and altered its binding to actin, myosin iiB and signaling intermediates including Src, PKA and FAK. Application of fluidic shear stress created a gradient in tension in syndecan-1 and led to enhanced association with actin, Src, myosin IIb and other cytoskeleton related molecules. Overall, our studies support that syndecan-1 is responsive to the mechanical environment of the cells and alters its association with actin and signaling intermediates in response to mechanical stimuli.

3D-Bioprinting Strategies Based on In Situ Bone-Healing Mechanism for Vascularized Bone Tissue Engineering

Over the past decades, a number of bone tissue engineering (BTE) approaches have been developed to address substantial challenges in the management of critical size bone defects. Although the majority of BTE strategies developed in the laboratory have been limited due to lack of clinical relevance in translation, primary prerequisites for the construction of vascularized functional bone grafts have gained confidence owing to the accumulated knowledge of the osteogenic, osteoinductive, and osteoconductive properties of mesenchymal stem cells and bone-relevant biomaterials that reflect bone-healing mechanisms. In this review, we summarize the current knowledge of bone-healing mechanisms focusing on the details that should be embodied in the development of vascularized BTE, and discuss promising strategies based on 3D-bioprinting technologies that efficiently coalesce the abovementioned main features in bone-healing systems, which comprehensively interact during the bone regeneration processes.

Endothelium-dependent remote signaling in ischemia and reperfusion: Alterations in the cardiometabolic continuum

Intact endothelial function plays a fundamental role for the maintenance of cardiovascular (CV) health. The endothelium is also involved in remote signaling pathway-mediated protection against ischemia/reperfusion (I/R) injury. However, the transfer of these protective signals into clinical practice has been hampered by the complex metabolic alterations frequently observed in the cardiometabolic continuum, which affect redox balance and inflammatory pathways. Despite recent advances in determining the distinct roles of hyperglycemia, insulin resistance (InR), hyperinsulinemia, and ultimately diabetes mellitus (DM), which define the cardiometabolic continuum, our understanding of how these conditions modulate endothelial signaling remains challenging. It is widely accepted that endothelial cells (ECs) undergo functional changes within the cardiometabolic continuum. Beyond vascular tone and platelet-endothelium interaction, endothelial dysfunction may have profound negative effects on outcome during I/R. In this review, we summarize the current knowledge of the influence of hyperglycemia, InR, hyperinsulinemia, and DM on endothelial function and redox balance, their influence on remote protective signaling pathways, and their impact on potential therapeutic strategies to optimize protective heterocellular signaling.

Hemodynamic Influences on Vascular Endothelial Biology*

The vascular endothelium resides in a unique biomechanical stress environment resulting from the hemodynamics of the system. In vivo studies indicate that there are regional differences in endothelial biology and that this may be due to the influence of the local hemodynamic environment. To investigate this further, cell culture studies have been conducted using well-defined mechanical stress environments. To study flow effects, we have employed a parallel plate chamber in which endothelial cell monolayers are exposed to laminar flow. In such experiments and concomitant with changes in morphology, there are a variety of other alterations in cell function, including a decrease in the rate of cell proliferation for subconfluent monolayers. Changes in cell behavior due to the direct effect of pressure and in cultured cells which are in a cyclical stress field also have been observed. In the recognition/transduction of such a mechanical signal, the pathway may possibly include a membrane event linked to the control of intracellular calcium. It may be that the same signaling mechanisms are involved both in cytoskeletal/shape changes and in the control of the cell's growth program and, in exercising such an influence, hemodynamics may have an important role in the response of the arterial wall to injury and the resulting repair and/or disease processes.

Influence of Relative Residence Time on Side-Wall Aneurysm Inception

BACKGROUND

Relative residence time (RRT) is a marker of disturbed blood flow, marked by low magnitude and high oscillatory wall shear stress (WSS). The relation between solute residence time in proximity to the vascular endothelium and the atherosclerotic process is well appreciated in the literature.

OBJECTIVE

To assess the influence of RRT on side-wall aneurysm inception to better understand the role of atherosclerosis in aneurysm formation.

METHODS

Fourteen side-wall internal carotid artery aneurysms from the Aneurisk repository which met criteria for parent vessel reconstruction were reconstructed with Vascular Modeling Toolkit. Computational fluid dynamics analysis was carried out in Fluent. RRT was calculated in MATLAB (The MathWorks Inc, Natick, Massachusetts). We analyzed the results for correlations, defined as presence or absence of local elevations in RRT in specific regions of vasculature.

RESULTS

RRT was concluded to be negatively correlated with aneurysm inception in this study of side-wall internal carotid artery aneurysms, with 12/14 cases yielding the absence of local RRT elevations within or in close proximity of the removed ostium. Subsequent analysis of WSS showed that 11 of 14 aneurysms were formed in an atheroprotective environment, with only 1 of 14 formed in an atherogenic environment. Two models were found to be of indeterminate environment.

CONCLUSION

Atherogenesis and atherosclerosis have long been thought to be a major inciting factor responsible for the formation of aneurysms in the cerebral vasculature. We propose that inception of side-wall aneurysms occurs in hemodynamic environments that promote an atheroprotective endothelial phenotype and that the atheroprotective phenotype is therefore aneurysmogenic.

Lymphatic endothelial cell calcium pulses are sensitive to spatial gradients in wall shear stress

Cytosolic calcium (Ca²⁺) is a ubiquitous second messenger that influences numerous aspects of cellular function. In many cell types, cytosolic Ca²⁺ concentrations are characterized by periodic pulses, whose dynamics can influence downstream signal transduction. Here, we examine the general question of how cells use Ca²⁺ pulses to encode input stimuli in the context of the response of lymphatic endothelial cells (LECs) to fluid flow. Previous work shows that fluid flow regulates Ca²⁺ dynamics in LECs and that Ca²⁺-dependent signaling plays a key role in regulating lymphatic valve formation during embryonic development. However, how fluid flow might influence the Ca²⁺ pulse dynamics of individual LECs has remained, to our knowledge, little explored. We used live-cell imaging to characterize Ca²⁺ pulse dynamics in LECs exposed to fluid flow in an in vitro flow device that generates spatial gradients in wall shear stress (WSS), such as are found at sites of valve formation. We found that the frequency of Ca²⁺ pulses was sensitive to the magnitude of WSS, while the duration of individual Ca²⁺ pulses increased in the presence of spatial gradients in WSS. These observations provide an example of how cells can separately modulate Ca²⁺ pulse frequency and duration to encode distinct forms of information, a phenomenon that could extend to other cell types.

Biomechanical studies on biomaterial degradation and co-cultured cells: mechanisms, potential applications, challenges and prospects

This review provides a comprehensive overview of biomechanical studies on biomaterial degradation and co-cultured cells as well as valuable biomechanical ideas on how to design or optimize cell biomaterial co-culture system.

Non-muscle myosin II drives vesicle loss during human reticulocyte maturation

The process of maturation of reticulocytes into fully mature erythrocytes that occurs in the circulation is known to be characterized by a complex interplay between loss of cell surface area and volume, removal of remnant cell organelles and redundant proteins, and highly selective membrane and cytoskeletal remodeling. However, the mechanisms that underlie and drive these maturational processes in vivo are currently poorly understood and, at present, reticulocytes derived through in vitro culture fail to undergo the final transition to erythrocytes. Here, we used high-throughput proteomic methods to highlight differences between erythrocytes, cultured reticulocytes and endogenous reticulocytes. We identify a cytoskeletal protein, non-muscle myosin IIA (NMIIA) whose abundance and phosphorylation status differs between reticulocytes and erythrocytes and localized it in the proximity of autophagosomal vesicles. An ex vivo circulation system was developed to simulate the mechanical shear component of circulation and demonstrated that mechanical stimulus is necessary, but insufficient for reticulocyte maturation. Using this system in concurrence with non-muscle myosin II inhibition, we demonstrate the involvement of non-muscle myosin IIA in reticulocyte remodeling and propose a previously undescribed mechanism of shear stress-responsive vesicle clearance that is crucial for reticulocyte maturation.

Endothelial Cell Reactive Oxygen Species and Ca2+ Signaling in Pulmonary Hypertension

Pulmonary hypertension (PH) refers to a disorder characterized by elevated pulmonary arterial pressure, leading to right ventricular overload and eventually right ventricular failure, which results in high morbidity and mortality. PH is associated with heterogeneous etiologies and distinct molecular mechanisms, including abnormal migration and proliferation of endothelial and smooth muscle cells. Although the exact details are not fully elucidated, reactive oxygen species (ROS) have been shown to play a key role in promoting abnormal function in pulmonary arterial smooth muscle and endothelial cells in PH. In endothelial cells, ROS can be generated from sources such as NADPH oxidase and mitochondria, which in turn can serve as signaling molecules in a wide variety of processes including posttranslational modification of proteins involved in Ca²⁺ homeostasis. In this chapter, we discuss the role of ROS in promoting abnormal vasoreactivity and endothelial migration and proliferation in various models of PH. Furthermore, we draw particular attention to the role of ROS-induced increases in intracellular Ca²⁺ concentration in the pathobiology of PH.

A PP2A-mediated feedback mechanism controls Ca2+-dependent NO synthesis under physiological oxygen

Intracellular O₂ is a key regulator of NO signaling, yet most in vitro studies are conducted in atmospheric O₂ levels, hyperoxic with respect to the physiologic milieu. We investigated NO signaling in endothelial cells cultured in physiologic (5%) O₂ and stimulated with histamine or shear stress. Culture of cells in 5% O₂ (>5 d) decreased histamine- but not shear stress–stimulated endothelial (e)NOS activity. Unlike cells adapted to a hypoxic environment (1% O₂), those cultured in 5% O₂ still mobilized sufficient Ca²⁺ to activate AMPK. Enhanced expression and membrane targeting of PP2A-C was observed in 5% O₂, resulting in greater interaction with eNOS in response to histamine. Moreover, increased dephosphorylation of eNOS in 5% O₂ was Ca²⁺-sensitive and reversed by okadaic acid or PP2A-C siRNA. The present findings establish that Ca²⁺ mobilization stimulates both NO synthesis and PP2A-mediated eNOS dephosphorylation, thus constituting a novel negative feedback mechanism regulating eNOS activity not present in response to shear stress. This, coupled with enhanced NO bioavailability, underpins differences in NO signaling induced by inflammatory and physiologic stimuli that are apparent only in physiologic O₂ levels. Furthermore, an explicit delineation between physiologic normoxia and genuine hypoxia is defined here, with implications for our understanding of pathophysiological hypoxia.—Keeley, T. P., Siow, R. C. M., Jacob, R., Mann, G. E. A PP2A-mediated feedback mechanism controls Ca²⁺-dependent NO synthesis under physiological oxygen.

Piezo1 Channels in Vascular Development and the Sensing of Shear Stress

A critical point in mammalian development occurs before mid-embryogenesis when the heart starts to beat, pushing blood into the nascent endothelial lattice. This pushing force is a signal, detected by endothelial cells as a frictional force (shear stress) to trigger cellular changes that underlie the essential processes of vascular remodeling and expansion required for embryonic growth. The processes are complex and multifactorial and Piezo1 became a recognized player only 2years ago, 4years after Piezo1's initial discovery as a functional membrane protein. Piezo1 is now known to be critical in murine embryonic development just at the time when the pushing force is first detected by endothelial cells. Murine Piezo1 gene disruption in endothelial cells is embryonic lethal and mutations in human PIEZO1 associate with severe disease phenotype due to abnormal lymphatic vascular development. Piezo1 proteins coassemble to form calcium-permeable nonselective cationic channels, most likely as trimers. They are large proteins with little if any resemblance to other proteins or ion channel subunits. The channels appear to sense mechanical force directly, including the force imposed on endothelial cells by physiological shear stress. Here, we review current knowledge of Piezo1 in the vascular setting and discuss hypotheses about how it might serve its vascular functions and integrate with other mechanisms. Piezo1 is a new important player for investigators in this field and promises much as a basis for better understanding of vascular physiology and pathophysiology and perhaps also discovery of new therapies.

A Y-Shaped Microfluidic Device to Study the Combined Effect of Wall Shear Stress and ATP Signals on Intracellular Calcium Dynamics in Vascular Endothelial Cells

The intracellular calcium dynamics in vascular endothelial cells (VECs) in response to wall shear stress (WSS) and/or adenosine triphosphate (ATP) have been commonly regarded as an important factor in regulating VEC function and behavior including proliferation, migration and apoptosis. However, the effects of time-varying ATP signals have been usually neglected in the past investigations in the field of VEC mechanobiology. In order to investigate the combined effects of WSS and dynamic ATP signals on the intracellular calcium dynamic in VECs, a Y-shaped microfluidic device, which can provide the cultured cells on the bottom of its mixing micro-channel with stimuli of WSS signal alone and different combinations of WSS and ATP signals in one single micro-channel, is proposed. Both numerical simulation and experimental studies verify the feasibility of its application. Cellular experimental results also suggest that a combination of WSS and ATP signals rather than a WSS signal alone might play a more significant role in VEC Ca²⁺ signal transduction induced by blood flow.

Ion Channels in Endothelial Responses to Fluid Shear Stress

Fluid shear stress is an important environmental cue that governs vascular physiology and pathology, but the molecular mechanisms that mediate endothelial responses to flow are only partially understood. Gating of ion channels by flow is one mechanism that may underlie many of the known responses. Here, we review the literature on endothelial ion channels whose activity is modulated by flow with an eye toward identifying important questions for future research.

Transient receptor potential vanilloid 2-mediated shear-stress responses in C2C12 myoblasts are regulated by serum and extracellular matrix

The developmental sensitivity of skeletal muscle to mechanical forces is unparalleled in other tissues. Calcium entry via reputedly mechanosensitive transient receptor potential (TRP) channel classes has been shown to play an essential role in both the early proliferative stage and subsequent differentiation of skeletal muscle myoblasts, particularly TRP canonical (TRPC) 1 and TRP vanilloid (TRPV) 2. Here we show that C2C12 murine myoblasts respond to fluid flow-induced shear stress with increments in cytosolic calcium that are largely initiated by the mechanosensitive opening of TRPV2 channels. Response to fluid flow was augmented by growth in low extracellular serum concentration (5 vs. 20% fetal bovine serum) by greater than 9-fold and at 18 h in culture, coincident with the greatest TRPV2 channel expression under identical conditions (P < 0.02). Fluid flow responses were also enhanced by substrate functionalization with laminin, rather than with fibronectin, agreeing with previous findings that the gating of TRPV2 is facilitated by laminin. Fluid flow-induced calcium increments were blocked by ruthenium red (27%) and SKF-96365 (38%), whereas they were unaltered by 2-aminoethoxydiphenyl borate, further corroborating that TRPV2 channels play a predominant role in fluid flow mechanosensitivity over that of TRPC1 and TRP melastatin (TRPM) 7.

Shear stress regulated gene expression and angiogenesis in vascular endothelium

The behavior of vascular EC is greatly altered in sites of pathological angiogenesis, such as a developing tumor or atherosclerotic plaque. Until recently it was thought that this was largely due to abnormal chemical signaling, i.e., endothelial cell chemo transduction, at these sites. However, we now demonstrate that the shear stress intensity encountered by EC can have a profound impact on their gene expression and behavior. We review the growing body of evidence suggesting that mechanotransduction, too, is a major regulator of pathological angiogenesis. This fits with the evolving story of physiological angiogenesis, where a combination of metabolic and mechanical signaling is emerging as the probable mechanism by which tight feedback regulation of angiogenesis is achieved in vivo.

Flow regulation of endothelin-1 production in the inner medullary collecting duct

Collecting duct-derived endothelin (ET)-1 is an autocrine inhibitor of Na(+) and water reabsorption; its deficiency causes hypertension and water retention. Extracellular fluid volume expansion increases collecting duct ET-1, thereby promoting natriuresis and diuresis; however, how this coupling between volume expansion and collecting duct ET-1 occurs is incompletely understood. One possibility is that volume expansion increases tubular fluid flow. To investigate this, cultured IMCD3 cells were subjected to static or flow conditions. Exposure to a shear stress of 2 dyn/cm(2) for 2 h increased ET-1 mRNA content by ∼2.3-fold. Absence of perfusate Ca(2+), chelation of intracellular Ca(2+), or inhibition of Ca(2+) signaling (calmodulin, Ca(2+)/calmodulin-dependent kinase, calcineurin, PKC, or phospholipase C) prevented the flow response. Evaluation of possible flow-activated Ca(2+) entry pathways revealed no role for transient receptor potential (TRP)C3, TRPC6, and TRPV4; however, cells with TRPP2 (polycystin-2) knockdown had no ET-1 flow response. Flow increased intracellular Ca(2+) was blunted in TRPP2 knockdown cells. Nonspecific blockade of P2 receptors, as well as specific inhibition of P2X7 and P2Y2 receptors, prevented the ET-1 flow response. The ET-1 flow response was not affected by inhibition of either epithelial Na(+) channels or the mitochondrial Na(+)/Ca(2+) exchanger. Taken together, these findings provide evidence that in IMCD3 cells, flow, via polycystin-2 and P2 receptors, engages Ca(2+)-dependent signaling pathways that stimulate ET-1 synthesis.

Modeling of TRPV₄-C₁ -mediated calcium signaling in vascular endothelial cells induced by fluid shear stress and ATP

The calcium signaling plays a vital role in flow-dependent vascular endothelial cell (VEC) physiology. Variations in fluid shear stress and ATP concentration in blood vessels can activate dynamic responses of cytosolic-free [Formula: see text] through various calcium channels on the plasma membrane. In this paper, a novel dynamic model has been proposed for transient receptor potential vanilloid 4 [Formula: see text]-mediated intracellular calcium dynamics in VECs induced by fluid shear stress and ATP. Our model includes [Formula: see text] signaling pathways through P2Y receptors and [Formula: see text] channels (indirect mechanism) and captures the roles of the [Formula: see text] compound channels in VEC [Formula: see text] signaling in response to fluid shear stress (direct mechanism). In particular, it takes into account that the [Formula: see text] compound channels are regulated by intracellular [Formula: see text] and [Formula: see text] concentrations. The simulation studies have demonstrated that the dynamic responses of calcium concentration produced by the proposed model correlate well with the existing experimental observations. We also conclude from the simulation studies that endogenously released ATP may play an insignificant role in the process of intracellular [Formula: see text] response to shear stress.

The primary cilium as sensor of fluid flow: new building blocks to the model. A review in the theme: cell signaling: proteins, pathways and mechanisms

The primary cilium is an extraordinary organelle. For many years, it had the full attention of only a few dedicated scientists fascinated by its uniqueness. Unexpectedly, after decades of obscurity, it has moved very quickly into the limelight with the increasing evidence of its central role in the many genetic variations that lead to what are now known as ciliopathies. These studies implicated unique biological functions of the primary cilium, which are not completely straightforward. In parallel, and initially completely unrelated to the ciliopathies, the primary cilium was characterized functionally as an organelle that makes cells more susceptible to changes in fluid flow. Thus the primary cilium was suggested to function as a flow-sensing device. This characterization has been substantiated for many epithelial cell types over the years. Nevertheless, part of the central mechanism of signal transduction has not been explained, largely because of the substantial technical challenges of working with this delicate organelle. The current review considers the recent advances that allow us to fill some of the holes in the model of signal transduction in cilium-mediated responses to fluid flow and to pursue the physiological implications of this peculiar organelle.

Rho-kinase as a therapeutic target in vascular diseases: striking nitric oxide signaling

Rho GTPases are a globular, monomeric group of small signaling G-protein molecules. Rho-associated protein kinase/Rho-kinase (ROCK) is a downstream effector protein of the Rho GTPase. Rho-kinases are the potential therapeutic targets in the treatment of cardiovascular diseases. Here, we have primarily discussed the intriguing roles of ROCK in cardiovascular health in relation to nitric oxide signaling. Further, we highlighted the biphasic effects of Y-27632, a ROCK inhibitor under shear stress, which acts as an agonist of nitric oxide production in endothelial cells. The biphasic effects of this inhibitor raised the question of safety of the drug usage in treating cardiovascular diseases.

A microfluidic shear device that accommodates parallel high and low stress zones within the same culturing chamber

Fluid shear stress (FSS) plays a critical role in regulating endothelium function and maintaining vascular homeostasis. Current microfluidic devices for studying FSS effects on cells either separate high shear stress zone and low shear stress zone into different culturing chambers, or arranging the zones serially along the flow direction, which complicates subsequent data interpretation. In this paper, we report a diamond shaped microfluidic shear device where the high shear stress zone and the low shear stress zone are arranged in parallel within one culturing chamber. Since the zones with different shear stress magnitudes are aligned normal to the flow direction, the cells in one stress group are not substantially affected by the flow-induced cytokine/chemokine releases by cells in the other group. Cell loading experiments using human umbilical vein endothelial cells show that the device is able to reveal stress magnitude-dependent and loading duration-dependent cell responses. The co-existence of shear stress zones with varied magnitudes within the same culturing chamber not only ensures that all the cells are subject to the identical culturing conditions, but also allows the resemblance of the differential shear stress pattern in natural arterial conditions. The device is expected to provide a new solution for studying the effects of heterogeneous hemodynamic patterns in the onset and progression of various vascular diseases.

Biomechanical force in blood development: extrinsic physical cues drive pro-hematopoietic signaling

The hematopoietic system is dynamic during development and in adulthood, undergoing countless spatial and temporal transitions during the course of one's life. Microenvironmental cues in the many unique hematopoietic niches differ, characterized by distinct soluble molecules, membrane-bound factors, and biophysical features that meet the changing needs of the blood system. Research from the last decade has revealed the importance of substrate elasticity and biomechanical force in determination of stem cell fate. Our understanding of the role of these factors in hematopoiesis is still relatively poor; however, the developmental origin of blood cells from the endothelium provides a model for comparison. Many endothelial mechanical sensors and second messenger systems may also determine hematopoietic stem cell fate, self renewal, and homing behaviors. Further, the intimate contact of hematopoietic cells with mechanosensitive cell types, including osteoblasts, endothelial cells, mesenchymal stem cells, and pericytes, places them in close proximity to paracrine signaling downstream of mechanical signals. The objective of this review is to present an overview of the sensors and intracellular signaling pathways activated by mechanical cues and highlight the role of mechanotransductive pathways in hematopoiesis.

Effects of shear stress on germ lineage specification of embryonic stem cells

Mechanobiology to date has focused on differentiated cells or progenitors, yet the effects of mechanical forces on early differentiation of pluripotent stem cells are still largely unknown. To study the effects of cellular deformation, we utilize a fluid flow bioreactor to apply steady laminar shear stress to mouse embryonic stem cells (ESCs) cultured on a two dimensional surface. Shear stress was found to affect pluripotency, as well as germ specification to the mesodermal, endodermal, and ectodermal lineages, as indicated by gene expression of OCT4, T-BRACHY, AFP, and NES, respectively. The ectodermal and mesodermal response to shear stress was dependent on stress magnitude (ranging from 1.5 to 15 dynes cm(-2)). Furthermore, increasing the duration from one to four days resulted in a sustained increase in T-BRACHY and a marked suppression of AFP. These changes in differentiation occurred concurrently with the activation of Wnt and estrogen pathways, as determined by PCR arrays for signalling molecules. Together these studies show that the mechanical microenvironment may be an important regulator during early differentiation events, including gastrulation. This insight furthers understanding of normal and pathological events during development, as well as facilitates strategies for scale up production of stem cells for clinical therapies.

Mechano-sensing by actin filaments and focal adhesion proteins

Mechanosensitive ion channels have long been the only established molecular class of cell mechanosensors with known molecular entities. However, recent advances in the state-of-the-art techniques, including single-molecule manipulation and imaging, have enabled an investigation of non-channel type cell mechanosensors and the underlying biophysical mechanisms of their activation. To date, two focal adhesion proteins, talin and p130Cas, have been postulated to act as putative mechanosensors, acting through mechano-induced unfolding of their particular soft domain(s) susceptible to phosphorylation. More recently, the actin filament has been demonstrated to act as a mechanosensor in the presence of the soluble actin-severing protein, cofilin. The cofilin severing activity negatively depends on the tension in the actin filament through tension-dependent binding/unbinding of cofilin to/from the actin filament. As a result, relaxed actin filaments are severed, while tensed ones are either not severed or severed after a long delay. Here we review the latest progress in the mechanosensing by non-channel type proteins and discuss the possible physiological roles of the mechanosensing performed by actin filaments in the course of cell migration.

Gαq/11-mediated intracellular calcium responses to retrograde flow in endothelial cells

Disturbed flow patterns, including reversal in flow direction, are key factors in the development of dysfunctional endothelial cells (ECs) and atherosclerotic lesions. An almost immediate response of ECs to fluid shear stress is the increase in cytosolic calcium concentration ([Ca(2+)](i)). Whether the source of [Ca(2+)](i) is extracellular, released from Ca(2+) intracellular stores, or both is still undefined, though it is likely dependent on the nature of forces involved. We have previously shown that a change in flow direction (retrograde flow) on a flow-adapted endothelial monolayer induces the remodeling of the cell-cell junction along with a dramatic [Ca(2+)](i) burst compared with cells exposed to unidirectional or orthograde flow. The heterotrimeric G protein-α q and 11 subunit (Gα(q/11)) is a likely candidate in effecting shear-induced increases in [Ca(2+)](i) since its expression is enriched at the junction and has been previously shown to be activated within seconds after onset of flow. In flow-adapted human ECs, we have investigated to what extent the Gα(q/11) pathway mediates calcium dynamics after reversal in flow direction. We observed that the elapsed time to peak [Ca(2+)](i) response to a 10 dyn/cm(2) retrograde shear stress was increased by 11 s in cells silenced with small interfering RNA directed against Gα(q/11). A similar lag in [Ca(2+)](i) transient was observed after cells were treated with the phospholipase C (PLC)-βγ inhibitor, U-73122, or the phosphatidylinositol-specific PLC inhibitor, edelfosine, compared with controls. Lower levels of inositol 1,4,5-trisphosphate accumulation seconds after the onset of flow correlated with the increased lag in [Ca(2+)](i) responses observed with the different treatments. In addition, inhibition of the inositol 1,4,5-trisphosphate receptor entirely abrogated flow-induced [Ca(2+)](i). Taken together, our results identify the Gα(q/11)-PLC pathway as the initial trigger for retrograde flow-induced endoplasmic reticulum calcium store release, thereby offering a novel approach to regulating EC dysfunctions in regions subjected to the reversal of blood flow.

Flow-dependent mass transfer may trigger endothelial signaling cascades

It is well known that fluid mechanical forces directly impact endothelial signaling pathways. But while this general observation is clear, less apparent are the underlying mechanisms that initiate these critical signaling processes. This is because fluid mechanical forces can offer a direct mechanical input to possible mechanotransducers as well as alter critical mass transport characteristics (i.e., concentration gradients) of a host of chemical stimuli present in the blood stream. However, it has recently been accepted that mechanotransduction (direct mechanical force input), and not mass transfer, is the fundamental mechanism for many hemodynamic force-modulated endothelial signaling pathways and their downstream gene products. This conclusion has been largely based, indirectly, on accepted criteria that correlate signaling behavior and shear rate and shear stress, relative to changes in viscosity. However, in this work, we investigate the negative control for these criteria. Here we computationally and experimentally subject mass-transfer limited systems, independent of mechanotransduction, to the purported criteria. The results showed that the negative control (mass-transfer limited system) produced the same trends that have been used to identify mechanotransduction-dominant systems. Thus, the widely used viscosity-related shear stress and shear rate criteria are insufficient in determining mechanotransduction-dominant systems. Thus, research should continue to consider the importance of mass transfer in triggering signaling cascades.

Mechanotransduction of shear in the endothelium: basic studies and clinical implications

The endothelium plays an integral role in the development and progression of atherosclerosis. Hemodynamic forces, particularly shear stress, have a powerful influence on endothelial phenotype and function; however, there is no clear consensus on how endothelial cells sense shear. Nevertheless, multiple endothelial cell signal transduction pathways are activated when exposed to shear stress in vitro. The type of shear, laminar or oscillatory, impacts which signal transduction pathways are initiated as well as which subsequent genes are up- or down-regulated, thereby influencing endothelial phenotype and function. Recently, human studies have examined the impact of shear stress and different shear patterns at rest and during exercise on endothelial function. Current evidence supports the theory that augmented exercise-induced shear stress contributes to improved endothelial function following acute exercise and exercise training, whereas retrograde shear initiates vascular dysfunction. The purpose of this review is to examine the current theories on how endothelial cells sense shear stress, to provide an overview on shear stress-induced signal transduction pathways and subsequent gene expression, and to review the current literature pertaining to shear stress and shear patterns at rest as well as during exercise in humans and the related effects on endothelial function.

Calcium and endothelium-mediated vasodilator signaling

Vascular tone refers to the balance between arterial constrictor and dilator activity. The mechanisms that underlie tone are critical for the control of haemodynamics and matching circulatory needs with metabolism, and thus alterations in tone are a primary factor for vascular disease etiology. The dynamic spatiotemporal control of intracellular Ca(2+) levels in arterial endothelial and smooth muscle cells facilitates the modulation of multiple vascular signaling pathways. Thus, control of Ca(2+) levels in these cells is integral for the maintenance of tone and blood flow, and intimately associated with both physiological and pathophysiological states. Hence, understanding the mechanisms that underlie the modulation of vascular Ca(2+) activity is critical for both fundamental knowledge of artery function, and for the development of targeted therapies. This brief review highlights the role of Ca(2+) signaling in vascular endothelial function, with a focus on contact-mediated vasodilator mechanisms associated with endothelium-derived hyperpolarization and the longitudinal conduction of responses over distance.

Acute exercise activates AMPK and eNOS in the mouse aorta

Exercise can prevent endothelial cell (EC) dysfunction and atherosclerosis even in the absence of improvements in plasma lipids. However, the mechanisms responsible for these effects are incompletely understood. In this study we examined in mice whether an acute bout of exercise activates enzymes that could prevent EC dysfunction, such as AMP-activated protein kinase (AMPK) and endothelial nitric oxide synthase (eNOS). We also examined whether exercise alters known regulators of these enzymes. C57BL/6 mice underwent a single bout of exhaustive treadmill exercise after which their aortas were analyzed for activation of AMPK, AMPK regulatory proteins, eNOS, and various enzymes that, like AMPK, activate eNOS. We found that such exercise acutely activates both AMPK and eNOS in the whole aorta and that the magnitude of these effects correlated with both the distance run and activation of the AMPK regulatory proteins silent information regulator-1 (SIRT1)-LKB1 and CaMKKβ. In contrast, Akt, PKA, PKG, and Src, other kinases known to activate eNOS, were unaffected. Immunohistochemical analysis revealed that AMPK and eNOS were both activated in the ECs of the aorta. This study provides the first evidence that an acute bout of exercise activates AMPK and eNOS in the endothelium of the aorta. The results also suggest that AMPK likely is the principal activator of eNOS in this setting and that its own activation may be mediated by both SIRT1-LKB1 and CaMKKβ.

Air bubble contact with endothelial cells in vitro induces calcium influx and IP3-dependent release of calcium stores

Gas embolism is a serious complication of decompression events and clinical procedures, but the mechanism of resulting injury remains unclear. Previous work has demonstrated that contact between air microbubbles and endothelial cells causes a rapid intracellular calcium transient and can lead to cell death. Here we examined the mechanism responsible for the calcium rise. Single air microbubbles (50-150 μm), trapped at the tip of a micropipette, were micromanipulated into contact with individual human umbilical vein endothelial cells (HUVECs) loaded with Fluo-4 (a fluorescent calcium indicator). Changes in intracellular calcium were then recorded via epifluorescence microscopy. First, we confirmed that HUVECs rapidly respond to air bubble contact with a calcium transient. Next, we examined the involvement of extracellular calcium influx by conducting experiments in low calcium buffer, which markedly attenuated the response, or by pretreating cells with stretch-activated channel blockers (gadolinium chloride or ruthenium red), which abolished the response. Finally, we tested the role of intracellular calcium release by pretreating cells with an inositol 1,4,5-trisphosphate (IP3) receptor blocker (xestospongin C) or phospholipase C inhibitor (neomycin sulfate), which eliminated the response in 64% and 67% of cases, respectively. Collectively, our results lead us to conclude that air bubble contact with endothelial cells causes an influx of calcium through a stretch-activated channel, such as a transient receptor potential vanilloid family member, triggering the release of calcium from intracellular stores via the IP3 pathway.

Effects of shear stress and stretch on endothelial function

Vascular endothelial cells (ECs) play a central role in the control of blood vessel function and circulatory system homeostasis. It is well known that that EC functions are regulated by chemical mediators, including hormones, cytokines, and neurotransmitters, but it has recently become apparent that EC functions are also controlled by hemodynamic forces such as shear stress and stretch (cyclic strain). ECs recognize shear stress and cyclic strain as mechanical stimuli, and transmit the signal into the interior of the cells, thereby triggering a variety of cellular responses that involve alterations in cell morphology, cell function, and gene expression. Impaired EC responses to shear stress and cyclic strain lead to vascular diseases, including hypertension, thrombosis, and atherosclerosis. A great deal of research has already been conducted on the mechanotransduction of shear stress and cyclic strain, and its molecular mechanisms are gradually coming to be understood. However, much remains unclear, and further studies of mechanotransduction should increase our understanding of the molecular basis of the hemodynamic-force-mediated control of vascular functions.

Non-muscle myosin II induces disassembly of actin stress fibres independently of myosin light chain dephosphorylation

Dynamic remodelling of actin stress fibres (SFs) allows non-muscle cells to adapt to applied forces such as uniaxial cell shortening. However, the mechanism underlying rapid and selective disassembly of SFs oriented in the direction of shortening remains to be elucidated. Here, we investigated how myosin crossbridge cycling induced by MgATP is associated with SF disassembly. Moderate concentrations of MgATP, or [MgATP], induced SF contraction. Meanwhile, at [MgATP] slightly higher than the physiological level, periodic actin patterns emerged along the length of SFs and dispersed within seconds. The actin fragments were diverse in length, but comparable to those in characteristic sarcomeric units of SFs. These results suggest that MgATP-bound non-muscle myosin II dissociates from the individual actin filaments that constitute the sarcomeric units, resulting in unbundling-induced disassembly rather than end-to-end actin depolymerization. This rapid SF disassembly occurred independent of dephosphorylation of myosin light chain. In terms of effects on actin-myosin interactions, a rise in [MgATP] is functionally equivalent to a temporal decrease in the total number of actin-myosin crossbridges. Actin-myosin crossbridges are known to be reduced by an assisting load on myosin. Thus, the present study suggests that reducing the number of actin-myosin crossbridges promotes rapid and orientation-dependent disassembly of SFs after cell shortening.

New molecular mechanisms for cardiovascular disease:blood flow sensing mechanism in vascular endothelial cells

Endothelial cells (ECs) lining blood vessels have a variety of functions and play a critical role in the homeostasis of the circulatory system. It has become clear that biomechanical forces generated by blood flow regulate EC functions. ECs are in direct contact with blood flow and exposed to shear stress, a frictional force generated by flowing blood. A number of recent studies have revealed that ECs recognize changes in shear stress and transmit signals to the interior of the cell, which leads to cell responses that involve changes in cell morphology, cell function, and gene expression. These EC responses to shear stress are thought to play important roles in blood flow-dependent phenomena such as vascular tone control, angiogenesis, vascular remodeling, and atherogenesis. Much research has been done on shear stress sensing and signal transduction, and their molecular mechanisms are gradually becoming understood. However, much remains uncertain, and many candidates have been proposed for shear stress sensors. More extensive studies of vascular mechanobiology should increase our understanding of the molecular basis of the blood flow-mediated control of vascular functions.

Lysophosphatidic acid induces shear stress-dependent Ca2+ influx in mouse aortic endothelial cells in situ

Using real-time two-photon laser scanning microscopy, we have demonstrated that lysophosphatidic acid (LPA), a bioactive lipid mediator, causes shear stress-dependent oscillatory local increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in fluo-4-loaded endothelial cells of isolated mouse aortic strips in situ. The increase in [Ca(2+)](i) occurred independently in the individual endothelial cells in a stepwise manner or repetitively during constant flow. The percentage of cells that responded and the averaged level of increase in [Ca(2+)](i) were dependent on both the concentration of LPA (0.3-10 μm) and the shear stress (10-80 dyn cm(-2)). The response was inhibited by removing extracellular Ca(2+), but not by thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase. The spatiotemporal properties of the [Ca(2+)](i) response were completely different from those of a Ca(2+) wave induced by ATP, a Ca(2+)-mobilizing agonist. These results were almost the same as those in the previous investigation using cultured bovine aortic endothelial cells, and suggest that LPA enhanced the shear stress-induced oscillatory Ca(2+) influx, termed 'Ca(2+) spot', in endothelial cells via activation of elementary Ca(2+) influx. In conclusion, the present study demonstrates, for the first time, that LPA functions as an endogenous sensitizer for mechanotransduction in endothelial cells in shear conditions in aortic strips in situ as well as in cultured cells. This indicates an important role for LPA as an endogenous factor in fluid flow-induced endothelial function.

Effects of smoking and hypertension on wall shear stress and oscillatory shear index at the site of intracranial aneurysm formation

OBJECTIVE

The mechanisms by which smoking and hypertension lead to increased incidence of intracranial aneurysm (IA) formation remain poorly understood. The current study investigates the effects of these risk factors on wall shear stress (WSS) and oscillatory shear index (OSI) at the site of IA initiation.

METHODS

Two (n=2) IAs from two patients with history of smoking and hypertension were artificially removed with the help of software @neuFuse (Supercomputing Solutions, Bologna, Italy) and the vessel geometry reconstructed to mimic the condition prior to IA formation. Two computational fluid dynamics (CFD) analyses were performed on each data-set by using in turn the normal physiological values of blood viscosity (BV), and high BV values specific to smoking and hypertension, obtained from literature.

RESULTS

At normal BV, high WSS (>15 Pa) was observed at the site of IA initiation in both patients. When BV values specific to smoking and hypertension were used, both the areas affected by high WSS (>15 Pa) and the maximum WSS were increased whilst the magnitude and distribution of OSI showed no significant change.

CONCLUSIONS

Long-term exposure to high WSS may result in an increased risk of IA development. An incremental increase in areas of high WSS observed secondary to smoking and hypertension may indicate a further increase in the risk of IA initiation. Interestingly, the relationship between BV and the area of increased WSS was not linear, reflecting the need for patient-specific CFD analysis.

Vascular mechanobiology: endothelial cell responses to fluid shear stress

Endothelial cells (ECs) lining blood vessel walls respond to shear stress, a fluid mechanical force generated by flowing blood, and the EC responses play an important role in the homeostasis of the circulatory system. Abnormal EC responses to shear stress impair various vascular functions and lead to vascular diseases, including hypertension, thrombosis, and atherosclerosis. Bioengineering approaches in which cultured ECs are subjected to shear stress in fluid-dynamically designed flow-loading devices have been widely used to analyze EC responses at the cellular and molecular levels. Remarkable progress has been made, and the results have shown that ECs alter their morphology, function, and gene expression in response to shear stress. Shear stress affects immature cells, as well as mature ECs, and promotes differentiation of bone-marrow-derived endothelial progenitor cells and embryonic stem cells into ECs. Much research has been done on shear stress sensing and signal transduction, and their molecular mechanisms are gradually coming to be understood. However, much remains uncertain, and many candidates have been proposed for shear stress sensors. More extensive studies of vascular mechanobiology should increase our understanding of the molecular basis of the blood-flow-mediated control of vascular functions.

Integrated model of endothelial NO regulation and systemic circulation for the comparison between pulsatile and continuous perfusion

Many experimental studies concerning nitric oxide (NO) release from endothelium and its vasodilative action are available in the literature, but no analytical description or modeling of these phenomena can be found. On the contrary, a large modeling literature is available concerning the other cardiovascular control mechanisms, such as the myogenic and metabolic control. In order to analytically describe these phenomena, a model of the endothelial control (defined in the Laplace domain and based on experimental data) was implemented and integrated with a lumped-parameter model of the systemic circulation, consisting of large artery segments and peripheral networks. The endothelial regulation model was based on the hypothesis proposed by Kuchan and Frangos, considering that NO release from the endothelium is generated by two parallel paths. The whole model was then applied to study the different vascular constriction or dilation under continuous or pulsatile perfusion, in order to better understand the clinical evidences of a poor organ perfusion in the presence of continuous with respect to pulsatile cardiopulmonary bypass. According to the experimental evidences, the main results obtained from the model revealed a widespread vascular constriction under continuous perfusion with respect to pulsatile. This result remains constant in the presence of different conditions of blood parameters and flow waveform.

Nitric oxide generation directly responds to ultrasound exposure

Recently, several reports have been published on ultrasonic vascular dilation produced with relatively low-frequency ultrasound. It has been speculated that nitric oxide (NO) is an important factor for this ultrasonic vascular dilation. However, a quantitative relationship between the ultrasound intensity and NO generation was not clarified in these reports. We investigated the quantity of NO generated by various ultrasonic intensities by means of real-time measurement of NO concentration in the adductor muscles of the thigh of New Zealand white rabbits exposed to a continuous-wave ultrasound (490 kHz). In the quantitative relationship between NO generation and ultrasonic intensity, the percent increase in NO concentration was 1.25% +/- 1.25%, 10.6% +/- 2.9% and 20.1% +/- 3.5%, with the maximum muscle temperature increase 0.5 +/- 0.2 degrees C, 0.7 +/- 0.2 degrees C, and 0.8 +/- 0.3 degrees C at the ultrasonic intensity (SPTA) of 0.21, 0.35 and 0.48 W/cm(2), respectively. The effect of ultrasound on NO generation was intensity-dependent with a progressive increase from 0.21 W/cm(2) to 0.48 W/cm(2) without significant thermal effect. Ultrasonic NO generation was partially reduced by NOS inhibitor of L-NMMA, clarifying that ultrasound can activate both NOS-dependent and NOS-independent NO generation. These new findings provided scientific basis for ultrasonic vasodilatation and support the potentiality of a new ultrasonic technology for the treatment and prevention of the ischemic tissue based on the new concept of NO generated angiogenesis. (E-mail: furuhata@jikei.ac.jp).

Difference in vulnerability of cerebral arterial bifurcation and straight portion for development of endothelial damage

Cerebral atherosclerotic changes develop most commonly on the lateral side of the bifurcation, one of the sites where the wall shear stress is lowest in the cerebral arteries. Endothelial cells exposed to the lower shear stress appear to be more vulnerable to stimuli such as free radicals. We hypothesized that the difference in endothelial vulnerability on cerebral arteries may be involved in the local preference of atherogenesis. To clarify this hypothesis, the present study was carried out by using a laser-dye technique that causes cell damage by heat and free radicals. A helium-neon laser in the presence of circulating Evans blue was used to illuminate three sites of the rat middle cerebral artery; the straight portion, the apex of the bifurcation, and the lateral side of the bifurcation. The magnitude of endothelial damage was morphologically estimated with the electron microscope. After the laser irradiation, the straight portion and the lateral side of the bifurcation developed severe endothelial damage. However, the apex of the bifurcation developed no appreciable damage, showing significantly milder changes compared with other sites. The results suggest that endothelial cells are more vulnerable to stimuli by free radicals at the straight portion and the lateral side of the bifurcation than at the apex. We conclude that the imbalance between the strength of stress stimuli, such as free radicals, and the vulnerability of endothelium is likely to be one of the key requirements for the development of cerebral atherosclerotic changes.