Sp1-mediated downregulation of P2X4 receptor gene transcription in endothelial cells exposed to 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.

Novel Animal Model of Spontaneous Cerebral Petechial Hemorrhage Using Focused Ultrasound in Rats

Background and Objectives: Petechial cerebral hemorrhages can be caused by various factors, such as traumas, cerebral infarctions, and aging, and is related to the disruption of the blood–brain barrier or the cellular damage of blood vessels. However, there is no animal model that recapitulates cerebral petechial hemorrhages. Materials and Methods: Here, we implemented a petechial hemorrhage using a novel technology, i.e., microbubble-assisted focused ultrasound (MB + FUS). Results: This method increases the permeability of the blood–brain barrier by directly applying mechanical force to the vascular endothelial cells through cavitation of the microbubbles. Microbubble-enhanced cavitation has the advantage of controlling the degree and location of petechial hemorrhages. Conclusions: We thus generated a preclinical rat model using noninvasive focal MB + FUS. This method is histologically similar to actual petechial hemorrhages of the brain and allows the achievement of a physiologically resembling petechial hemorrhage. In the future, this method shall be considered as a useful animal model for studying the pathophysiology and treatment of petechial cerebral hemorrhages.

P2Y2 receptor modulates shear stress-induced cell alignment and actin stress fibers in human umbilical vein endothelial cells

Endothelial cells release ATP in response to fluid shear stress, which activates purinergic (P2) receptor-mediated signaling molecules including endothelial nitric oxide (eNOS), a regulator of vascular tone. While P2 receptor-mediated signaling in the vasculature is well studied, the role of P2Y₂ receptors in shear stress-associated endothelial cell alignment, cytoskeletal alterations, and wound repair remains ill defined. To address these aspects, human umbilical vein endothelial cell (HUVEC) monolayers were cultured on gelatin-coated dishes and subjected to a shear stress of 1 Pa. HUVECs exposed to either P2Y₂ receptor antagonists or siRNA showed impaired fluid shear stress-induced cell alignment, and actin stress fiber formation as early as 6 h. Similarly, when compared to cells expressing the P2Y₂ Arg-Gly-Asp (RGD) wild-type receptors, HUVECs transiently expressing the P2Y₂ Arg-Gly-Glu (RGE) mutant receptors showed reduced cell alignment and actin stress fiber formation in response to shear stress as well as to P2Y₂ receptor agonists in static cultures. Additionally, we observed reduced shear stress-induced phosphorylation of focal adhesion kinase (Y397), and cofilin-1 (S3) with receptor knockdown as well as in cells expressing the P2Y₂ RGE mutant receptors. Consistent with the role of P2Y₂ receptors in vasodilation, receptor knockdown and overexpression of P2Y₂ RGE mutant receptors reduced shear stress-induced phosphorylation of AKT (S473), and eNOS (S1177). Furthermore, in a scratched wound assay, shear stress-induced cell migration was reduced by both pharmacological inhibition and receptor knockdown. Together, our results suggest a novel role for P2Y₂ receptor in shear stress-induced cytoskeletal alterations in HUVECs.

Nucleotides in neuroregeneration and neuroprotection

Brain injury generates the release of a multitude of factors including extracellular nucleotides, which exhibit bi-functional properties and contribute to both detrimental actions in the acute phase and also protective and reparative actions in the later recovery phase to allow neuroregeneration. A promising strategy toward restoration of neuronal function is based on activation of endogenous adult neural stem/progenitor cells. The implication of purinergic signaling in stem cell biology, including regulation of proliferation, differentiation, and cell death has become evident in the last decade. In this regard, current strategies of acute transplantation of ependymal stem/progenitor cells after spinal cord injury restore altered expression of P2X4 and P2X7 receptors and improve functional locomotor recovery. The expression of both receptors is transcriptionally regulated by Sp1 factor, which plays a key role in the startup of the transcription machinery to induce regeneration-associated genes expression. Finally, general signaling pathways triggered by nucleotide receptors in neuronal populations converge on several intracellular kinases, such as PI3K/Akt, GSK3 and ERK1,2, as well as the Nrf-2/heme oxigenase-1 axis, which specifically link them to neuroprotection. In this regard, regulation of dual specificity protein phosphatases can become novel mechanism of actions for nucleotide receptors that associate them to cell homeostasis regulation. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Shear stress modulates endothelial KLF2 through activation of P2X4

Vascular endothelial cells that are in direct contact with blood flow are exposed to fluid shear stress and regulate vascular homeostasis. Studies report endothelial cells to release ATP in response to shear stress that in turn modulates cellular functions via P2 receptors with P2X4 mediating shear stress-induced calcium signaling and vasodilation. A recent study shows that a loss-of-function polymorphism in the human P2X4 resulting in a Tyr315>Cys variant is associated with increased pulse pressure and impaired endothelial vasodilation. Although the importance of shear stress-induced Krüppel-like factor 2 (KLF2) expression in atheroprotection is well studied, whether ATP regulates KLF2 remains unanswered and is the objective of this study. Using an in vitro model, we show that in human umbilical vein endothelial cells (HUVECs), apyrase decreased shear stress-induced KLF2, KLF4, and NOS3 expression but not that of NFE2L2. Exposure of HUVECs either to shear stress or ATPγS under static conditions increased KLF2 in a P2X4-dependent manner as was evident with both the receptor antagonist and siRNA knockdown. Furthermore, transient transfection of static cultures of human endothelial cells with the Tyr315>Cys mutant P2X4 construct blocked ATP-induced KLF2 expression. Also, P2X4 mediated the shear stress-induced phosphorylation of extracellular regulated kinase-5, a known regulator of KLF2. This study demonstrates a major physiological finding that the shear-induced effects on endothelial KLF2 axis are in part dependent on ATP release and P2X4, a previously unidentified mechanism.

Exposure of the thorax to a sublethal blast wave causes a hydrodynamic pulse that leads to perivenular inflammation in the brain

Traumatic brain injury (TBI) caused by an explosive blast (blast-TBI) is postulated to result, in part, from transvascular transmission to the brain of a hydrodynamic pulse (a.k.a., volumetric blood surge, ballistic pressure wave, hydrostatic shock, or hydraulic shock) induced in major intrathoracic blood vessels. This mechanism of blast-TBI has not been demonstrated directly. We tested the hypothesis that a blast wave impacting the thorax would induce a hydrodynamic pulse that would cause pathological changes in the brain. We constructed a Thorax-Only Blast Injury Apparatus (TOBIA) and a Jugular-Only Blast Injury Apparatus (JOBIA). TOBIA delivered a collimated blast wave to the right lateral thorax of a rat, precluding direct impact on the cranium. JOBIA delivered a blast wave to the fluid-filled port of an extracorporeal intravenous infusion device whose catheter was inserted retrograde into the jugular vein, precluding lung injury. Long Evans rats were subjected to sublethal injury by TOBIA or JOBIA. Blast injury induced by TOBIA was characterized by apnea and diffuse bilateral hemorrhagic injury to the lungs associated with a transient reduction in pulse oximetry signals. Immunolabeling 24 h after injury by TOBIA showed up-regulation of tumor necrosis factor alpha, ED-1, sulfonylurea receptor 1 (Sur1), and glial fibrillary acidic protein in veins or perivenular tissues and microvessels throughout the brain. The perivenular inflammatory effects induced by TOBIA were prevented by ligating the jugular vein and were reproduced using JOBIA. We conclude that blast injury to the thorax leads to perivenular inflammation, Sur1 up-regulation, and reactive astrocytosis resulting from the induction of a hydrodynamic pulse in the vasculature.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Mechanosensitive properties in the endothelium and their roles in the regulation of endothelial function

: Vascular endothelial cells (ECs) line the luminal surface of blood vessels, which are exposed constantly to mechanical stimuli, such as fluid shear stress, cyclic strain, and blood pressure. In recent years, more and more evidence indicates that ECs sense these mechanical stimuli and subsequently convert mechanical stimuli into intracellular signals. The properties of ECs that sense the mechanical stimuli are defined as mechanosensors. There are a variety of mechanosensors that have been identified in ECs. These mechanosensors play an important role in regulating the function of the endothelium and vascular function, including blood pressure. This review focuses on the mechanosensors that have been identified in ECs and on the roles that mechanosensors play in the regulation of endothelium function, and in the regulation of vascular function.

Sulfonylurea receptor 1 in central nervous system injury: a focused review

The sulfonylurea receptor 1 (Sur1)-regulated NC(Ca-ATP) channel is a nonselective cation channel that is regulated by intracellular calcium and adenosine triphosphate. The channel is not constitutively expressed, but is transcriptionally upregulated de novo in all cells of the neurovascular unit, in many forms of central nervous system (CNS) injury, including cerebral ischemia, traumatic brain injury (TBI), spinal cord injury (SCI), and subarachnoid hemorrhage (SAH). The channel is linked to microvascular dysfunction that manifests as edema formation and delayed secondary hemorrhage. Also implicated in oncotic cell swelling and oncotic (necrotic) cell death, the channel is a major molecular mechanism of 'accidental necrotic cell death' in the CNS. In animal models of SCI, pharmacological inhibition of Sur1 by glibenclamide, as well as gene suppression of Abcc8, prevents delayed capillary fragmentation and tissue necrosis. In models of stroke and TBI, glibenclamide ameliorates edema, secondary hemorrhage, and tissue damage. In a model of SAH, glibenclamide attenuates the inflammatory response due to extravasated blood. Clinical trials of an intravenous formulation of glibenclamide in TBI and stroke underscore the importance of recent advances in understanding the role of the Sur1-regulated NC(Ca-ATP) channel in acute ischemic, traumatic, and inflammatory injury to the CNS.

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.

Hemorrhagic progression of a contusion after traumatic brain injury: a review

The magnitude of damage to cerebral tissues following head trauma is determined by the primary injury, caused by the kinetic energy delivered at the time of impact, plus numerous secondary injury responses that almost inevitably worsen the primary injury. When head trauma results in a cerebral contusion, the hemorrhagic lesion often progresses during the first several hours after impact, either expanding or developing new, non-contiguous hemorrhagic lesions, a phenomenon termed hemorrhagic progression of a contusion (HPC). Because a hemorrhagic contusion marks tissues with essentially total unrecoverable loss of function, and because blood is one of the most toxic substances to which the brain can be exposed, HPC is one of the most severe types of secondary injury encountered following traumatic brain injury (TBI). Historically, HPC has been attributed to continued bleeding of microvessels fractured at the time of primary injury. This concept has given rise to the notion that continued bleeding might be due to overt or latent coagulopathy, prompting attempts to normalize coagulation with agents such as recombinant factor VIIa. Recently, a novel mechanism was postulated to account for HPC that involves delayed, progressive microvascular failure initiated by the impact. Here we review the topic of HPC, we examine data relevant to the concept of a coagulopathy, and we detail emerging data elucidating the mechanism of progressive microvascular failure that predisposes to HPC after head trauma.

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.

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.

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.

Shear stress-induced transcriptional regulation via hybrid promoters as a potential tool for promoting angiogenesis

Among the key effects of fluid shear stress on vascular endothelial cells is modulation of gene expression. Promoter sequences termed shear stress response elements (SSREs) mediate the responsiveness of endothelial genes to shear stress. While previous studies showed that shear stress responsiveness is mediated by a single SSRE, these endogenous promoters often encode for multiple SSREs. Moreover, hybrid promoters encoding a single SSRE rarely respond to shear stress at the same magnitude as the endogenous promoter. Thus, to better understand the interplay between the various SSREs, and between SSREs and endothelial-specific sequences (ESS), we generated a series of constructs regulated by SSREs cassettes alone, or in combination with ESS, and tested their response to shear stress and endothelial specific expression. Among these constructs, the most responsive promoter (NR1/2) encoded a combination of two GAGACC/SSREs, the Sp1/Egr1 sequence, as well as a TPA response element (TRE). This construct was four- to five-fold more responsive to shear stress than a promoter encoding a single SSRE. The expression of constructs containing other SSRE combinations was unaffected or suppressed by shear stress. Addition of ESS derived from the Tie2 promoter, either 5' or 3' to NR1/2 resulted in shear stress transcriptional suppression, yet retained endothelial specific expression. Thus, the combination and localization order of the various SSREs in a single promoter is crucial in determining the pattern and degree of shear stress responsiveness. These shear stress responsive cassettes may prove beneficial in our attempt to time the expression of an endothelial transgene in the vasculature.

Fluid shear stress induces arterial differentiation of endothelial progenitor cells

Endothelial progenitor cells (EPCs) are mobilized from bone marrow to peripheral blood and contribute to angiogenesis in tissues. In the process, EPCs are exposed to the shear stress generated by blood flow and tissue fluid flow. Our previous study showed that shear stress promotes differentiation of EPCs into mature endothelial cells. In this study, we investigated whether EPCs differentiate into arterial or venous endothelial cells in response to shear stress. When cultured EPCs derived from human peripheral blood were exposed to controlled levels of shear stress in a flow-loading device, the mRNA levels of the arterial endothelial cell markers ephrinB2, Notch1/3, Hey1/2, and activin receptor-like kinase 1 increased, but the mRNA levels of the venous endothelial cell markers EphB4 and neuropilin-2 decreased. Both the ephrinB2 increase and the EphB4 decrease were shear stress dependent rather than shear rate dependent. EphrinB2 protein was increased in shear-stressed EPCs, and the increase in ephrinB2 expression was due to activated transcription and not mRNA stabilization. Deletion analysis of the ephrinB2 promoter indicated that the cis-element (shear stress response element) is present within 106 bp 5' upstream from the transcription initiation site. This region contains the Sp1 consensus sequence, and a mutation in its sequence decreased the basal level of transcription and abolished shear stress-induced ephrinB2 transcription. Electrophoretic mobility shift assays and chromatin immunoprecipitation assays showed that shear stress markedly increased binding of Sp1 to its consensus sequence. These results indicate that shear stress induces differentiation of EPCs into arterial endothelial cells by increasing ephrinB2 expression in EPCs through Sp1 activation.

Fluid flow induces mechanosensitive ATP release, calcium signalling and Cl- transport in biliary epithelial cells through a PKCzeta-dependent pathway

ATP in bile is a potent secretogogue, stimulating cholangiocyte Cl- and fluid secretion via binding to membrane P2 receptors, though the physiological stimuli involved in biliary ATP release are unknown. The goal of the present studies was to determine the potential role of fluid flow in biliary ATP release and secretion. In both human Mz-Cha-1 biliary cells and normal rat cholangiocyte monolayers, exposure to flow increased relative ATP release which was proportional to the shear stress. In parallel studies, shear was associated with an increase in [Ca2+]i and membrane Cl- permeability, which were both dependent on extracellular ATP and P2 receptor stimulation. Flow-stimulated ATP release was dependent on [Ca2+]i, exhibited desensitization with repetitive stimulation, and was regulated by PKCzeta. In conclusion, both human and rat biliary cells exhibit flow-stimulated, PKCzeta-dependent, ATP release, increases in [Ca2+]i and Cl- secretion. The finding that fluid flow can regulate membrane transport suggests that mechanosensitive ATP release may be a key regulator of biliary secretion and an important target to modulate bile flow in the treatment of cholestatic liver diseases.

Regulation of P2X4 receptors by lysosomal targeting, glycan protection and exocytosis

The P2X4 receptor has a widespread distribution in the central nervous system and the periphery, and plays an important role in the function of immune cells and the vascular system. Its upregulation in microglia contributes to neuropathic pain following nerve injury. The mechanisms involved in its regulation are not well understood, although we have previously shown that it is constitutively retrieved from the plasma membrane and resides predominantly within intracellular compartments. Here, we show that the endogenous P2X4 receptors in cultured rat microglia, vascular endothelial cells and freshly isolated peritoneal macrophages are localized predominantly to lysosomes. Lysosomal targeting was mediated through a dileucine-type motif within the N-terminus, together with a previously characterized tyrosine-based endocytic motif within the C-terminus. P2X4 receptors remained stable within the proteolytic environment of the lysosome and resisted degradation by virtue of their N-linked glycans. Stimulation of phagocytosis triggered the accumulation of P2X4 receptors at the phagosome membrane. Stimulating lysosome exocytosis, either by incubating with the Ca2+ ionophore ionomycin, for normal rat kidney (NRK) cells and cultured rat microglia, or the weak base methylamine, for peritoneal macrophages, caused an upregulation of both P2X4 receptors and the lysosomal protein LAMP-1 at the cell surface. Lysosome exocytosis in macrophages potentiated ATP-evoked P2X4 receptor currents across the plasma membrane. Taken together, our data suggest that the P2X4 receptor retains its function within the degradative environment of the lysosome and can subsequently traffic out of lysosomes to upregulate its exposure at the cell surface and phagosome.

Adequate design of customized cDNA macroarray for convenient multiple gene expression analysis

To establish a convenient, cost-effective, and reasonably reliable method for monitoring multiple gene expression using customized membrane-based macroarray, we constructed a cDNA macroarray with multiple probes for 13 human vascular endothelial genes and assessed the accuracy of the macroarray measurements. For each gene, two cDNA probes (450-550 bp) were designed from different regions (coding region and 3'-untranslated region [3'-UTR], respectively) on the basis of simple criteria concerning length and sequence specificity and spotted on the macroarray. In addition, unmodified oligonucleotide probes (80 mer) targeted to a unique sequence from the coding region of each gene were spotted on the same macroarray. Using this macroarray, shear stress-induced mRNA expression changes were analyzed in human coronary artery endothelial cells. Comparison of the expression ratios obtained with those measured using quantitative real-time polymerase chain reaction (PCR) as a reference method revealed that cDNA probes designed from a sequence within the coding region provided a highly accurate expression profile, whereas results obtained from oligonucleotide probes showed no correlation with real-time PCR data, which might be caused by inadequate immobilization of oligonucletotide probes on the nylon membrane. In addition, we observed that cDNA probes targeting different regions of a gene yielded different signal intensities. Most cDNA probes designed from a sequence within the coding region showed detectable signals, whereas few cDNA probes designed from 3'-UTR did.

Shear stress induces hepatocyte PAI-1 gene expression through cooperative Sp1/Ets-1 activation of transcription

Partial hepatectomy causes hemodynamic changes that increase portal blood flow in the remaining lobe, where the expression of immediate-early genes, including plasminogen activator inhibitor-1 (PAI-1), is induced. We hypothesized that a hyperdynamic circulatory state occurring in the remaining lobe induces immediate-early gene expression. In this study, we investigated whether the mechanical force generated by flowing blood, shear stress, induces PAI-1 expression in hepatocytes. When cultured rat hepatocytes were exposed to flow, PAI-1 mRNA levels began to increase within 3 h, peaked at levels significantly higher than the static control levels, and then gradually decreased. The flow-induced PAI-1 expression was shear stress dependent rather than shear rate dependent and accompanied by increased hepatocyte production of PAI-1 protein. Shear stress increased PAI-1 transcription but did not affect PAI-1 mRNA stability. Functional analysis of the 2.1-kb PAI-1 5'-promoter indicated that a 278-bp segment containing transcription factor Sp1 and Ets-1 consensus sequences was critical to the shear stress-dependent increase of PAI-1 transcription. Mutations of both the Sp1 and Ets-1 consensus sequences, but not of either one alone, markedly prevented basal PAI-1 transcription and abolished the response of the PAI-1 promoter to shear stress. EMSA and chromatin immunoprecipitation assays showed binding of Sp1 and Ets-1 to each consensus sequence under static conditions, which increased in response to shear stress. In conclusion, hepatocyte PAI-1 expression is flow sensitive and transcriptionally regulated by shear stress via cooperative interactions between Sp1 and Ets-1.

The State of the Art of Nanobioscience in Japan

This paper reviews a part of the state of the art of nanobioscience in Japan. The importance of combination and integration of interdisciplinary principles is emphasized for the development of nanobioscience. Biomagnetics, biomechanics, nanomachining, self-replicating cell model, neuronal network, drug delivery system, and tissue engineering are discussed.

Differential regulation of urokinase-type plasminogen activator expression by fluid shear stress in human coronary artery endothelial cells

Atherosclerotic plaques preferentially localize at arterial regions exposed to turbulent low-shear flow. Urokinase-type plasminogen activator (uPA) plays a role in vascular remodeling by facilitating smooth muscle cell migration and proliferation in addition to the proteolysis of extracellular matrix, and the expression of uPA is elevated in atherosclerotic lesions. In this study, we analyzed the effects of laminar and turbulent shear stress on uPA expression in cultured human coronary artery endothelial cells. The application of laminar shear stress (1.5 or 15 dyn/cm2) significantly decreased the amount of uPA mRNA as well as the secretion of uPA protein. In contrast, turbulent shear stress (average intensity, 1.5 dyn/cm2) markedly increased uPA gene expression and protein secretion. Laminar shear stress downregulated uPA gene expression transcriptionally and posttranscriptionally; laminar shear stress activated transcription factor GATA6, which binds to a GATA consensus element located between -692 and -687 bp in the uPA promoter, thereby inhibiting uPA gene transcription. Laminar shear stress also accelerated the degradation of uPA mRNA; the half-life of uPA mRNA decreased to about half of the static control's half-life. Although turbulent shear stress had no effect on the transcription of uPA, it significantly increased uPA mRNA stability; the half-life of uPA mRNA increased by about two times the static control's half-life. Our results suggest that endothelial uPA expression is flow sensitive and differentially regulated by laminar and turbulent shear stress in vitro. We speculate that this effect may contribute to the local nature of atherosclerosis.

Fluid shear stress and the vascular endothelium: for better and for worse

As blood flows, the vascular wall is constantly subjected to physical forces, which regulate important physiological blood vessel responses, as well as being implicated in the development of arterial wall pathologies. Changes in blood flow, thus generating altered hemodynamic forces are responsible for acute vessel tone regulation, the development of blood vessel structure during embryogenesis and early growth, as well as chronic remodeling and generation of adult blood vessels. The complex interaction of biomechanical forces, and more specifically shear stress, derived by the flow of blood and the vascular endothelium raise many yet to be answered questions:How are mechanical forces transduced by endothelial cells into a biological response, and is there a "shear stress receptor"?Are "mechanical receptors" and the final signaling pathways they evoke similar to other stimulus-response transduction systems?How do vascular endothelial cells differ in their response to physiological or pathological shear stresses?Can shear stress receptors or shear stress responsive genes serve as novel targets for the design of diagnostic and therapeutic modalities for cardiovascular pathologies?The current review attempts to bring together recent findings on the in vivo and in vitro responses of the vascular endothelium to shear stress and to address some of the questions raised above.

P2 receptor expression profiles in human vascular smooth muscle and endothelial cells

P2 receptors mediate the actions of the extracellular nucleotides ATP, ADP, UTP, and UDP, regulating several physiologic responses including cardiac function, vascular tone, smooth muscle cell (SMC) proliferation, platelet aggregation, and the release of endothelial factors. P2 receptor characterization has been hampered by the lack of selective antagonists. The aim of the current study was to investigate the mRNA and protein expression of P2X and P2Y receptors in human SMC and in endothelial cells (EC). Smooth muscle cells were obtained from human mammary artery and EC from human umbilical vein. Using real-time PCR, the authors established quantitative mRNA assays. Protein expression was studied using Western blotting with recently developed antibodies. The P2X1 receptor was highly specific for human SMC, while the P2X4 was the highest expressed receptor in EC. The P2Y2 receptor was present in both SMC and EC. UTP-mediated effects in these cells are likely to be mediated by P2Y2 and not P2Y4 receptors since the latter had considerably lower expression. The P2Y6 receptor was expressed in both SMC and EC. The P2Y1 and surprisingly the P2Y11 receptors were the most abundantly expressed P2Y receptors in the endothelium. Overall, Western blotting confirmed the mRNA findings in most aspects, and most interestingly, indicated oligomerization of the P2Y1 receptor that may be important for its function. In conclusion, P2X1, P2Y2, and P2Y6 are the most expressed P2 receptors in SMC and are thus probably mediating the contractile and mitogenic actions of extracellular nucleotides. The P2X4, P2Y11, P2Y1, and P2Y2 are the most expressed P2 receptors in EC, and are most likely mediating release of nitric oxide, endothelium-dependent hyperpolarizing factor (EDHF), and t-PA induced by extracellular nucleotides. These findings will help to direct future cardiovascular drug development against the large P2 receptor family.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

The involvement of smooth muscle P2X receptors in the prolonged vasorelaxation response to purine nucleotides in the rat mesenteric arterial bed

1. ATP and adenine dinucleotides can elicit three different types of vasomotor response in the rat mesenteric arterial bed; vasocontraction, rapid relaxation (which may be masked by contraction) and slow and prolonged vasorelaxation. Contraction is mediated by smooth muscle P2X receptors and rapid relaxation by endothelial P2Y receptors. The mechanism of prolonged relaxation is, however, controversial. 2. In the present study, bolus injection of doses of alpha,beta-methylene ATP (alpha,beta-meATP; 5 pmol - 0.5 micromol; P2X receptor agonist) in methoxamine-preconstricted rat isolated mesenteric arterial beds, mimicked the action of ATP, causing contraction (R(max) 76+/-9 mmHg) followed by prolonged relaxation (78+/-11%; t(1/2) 14.6+/-1.5 min). KCl also elicited a biphasic response (R(max) contraction 73+/-8 mmHg; R(max) prolonged relaxation 70+/-6%; t(1/2) 7.7+/-1.9 min). 3. P2X receptor desensitization caused by perfusion with alpha,beta-meATP (10 microM) abolished contraction and prolonged relaxation to doses of alpha,beta-meATP (50 nmol). Rapid relaxation (32+/-7%; t(1/2) 32+/-2 s) was revealed, which was abolished by removal of the endothelium using distilled water. 4. Sodium deoxycholate treatment blocked contractile and prolonged relaxation responses to alpha,beta-meATP, ATP and KCl, whilst distilled water treatment had no significant effect on either phase of the biphasic responses. 5. These data indicate that smooth muscle P2X receptors are involved in both phases of the biphasic response (contraction followed by prolonged relaxation) to purine nucleotides in the rat isolated mesenteric arterial bed. Caution should be applied when using sodium deoxycholate to remove the endothelium because of possible damage caused by the detergent to receptors and/or the vascular smooth muscle.

P2 receptors: new potential players in atherosclerosis

Atherosclerosis is a focal inflammatory disease of the arterial wall. It starts with the formation of fatty streaks on the arterial wall that evolve to form a raised plaque made of smooth muscle cells (SMCs), and infiltrating leukocytes surrounding a necrotic core. The pathogenesis of the atherosclerotic lesion is incompletely understood, but it is clear that a dysfunction of the endothelium, recruitment and activation of inflammatory cells and SMC proliferation have a pivotal role. Over recent years receptors for extracellular nucleotides, the P2 receptors, have been recognized as fundamental modulators of leukocytes, platelets, SMCs and endothelial cells. P2 receptors mediate chemotaxis, cytokine secretion, NO generation, platelet aggregation and cell proliferation in response to accumulation of nucleotides into the extracellular milieu. Clinical trials have shown the benefit of antagonists of the ADP platelet receptor(s) in the prevention of vascular accidents in patients with atherosclerosis. Therefore, we anticipate that a deeper understanding of the involvement of P2 receptors in atheroma formation will open new avenues for drug design and therapeutic intervention.