Endothelial permeability is controlled by spatially defined cytoskeletal mechanics: atomic force microscopy force mapping of pulmonary endothelial monolayer

AFM nanoindentation detection of the elastic modulus of tongue squamous carcinoma cells with different metastatic potentials

Although significant advances have been made in understanding the molecular mechanisms that influence tongue squamous cell carcinoma (TSCC) metastasis, less is known about the association between the cellular elastic modulus and TSCC metastasis. Atomic force microscopy (AFM) nanoindentation via the rate-jump method was used to detect the elastic modulus of TSCC cells from patients and cell lines with different metastatic potentials. TSCC cells with higher metastatic potential showed decreases in the elastic modulus compared to TSCC cells with lower metastatic potential. Moreover, the decrease in elastic modulus was accompanied with epithelial-mesenchymal transition (EMT), cytoskeleton (F-actin and β-tubulin) changes, small nucleus size and large nucleus/cytoplasm (N/C) ratio. The present findings demonstrate a close relationship between the cellular elastic modulus and the metastasis of TSCC. The elastic modulus detected by AFM nanoindentation via the rate-jump method can potentially be used to grade the metastatic potential of TSCC.

FROM THE CLINICAL EDITOR

This team of investigators report the use of an atomic force microscopy-based method to determine the elastic modulus of tongue squamous cell carcinoma cells, and demonstrate that such cells with higher metastatic potential show decreased elastic modulus compared to cells with lower metastatic potential.

Barrier-protective effects of activated protein C in human alveolar epithelial cells

Acute lung injury (ALI) is a clinical manifestation of respiratory failure, caused by lung inflammation and the disruption of the alveolar-capillary barrier. Preservation of the physical integrity of the alveolar epithelial monolayer is of critical importance to prevent alveolar edema. Barrier integrity depends largely on the balance between physical forces on cell-cell and cell-matrix contacts, and this balance might be affected by alterations in the coagulation cascade in patients with ALI. We aimed to study the effects of activated protein C (APC) on mechanical tension and barrier integrity in human alveolar epithelial cells (A549) exposed to thrombin. Cells were pretreated for 3 h with APC (50 µg/ml) or vehicle (control). Subsequently, thrombin (50 nM) or medium was added to the cell culture. APC significantly reduced thrombin-induced cell monolayer permeability, cell stiffening, and cell contraction, measured by electrical impedance, optical magnetic twisting cytometry, and traction microscopy, respectively, suggesting a barrier-protective response. The dynamics of the barrier integrity was also assessed by western blotting and immunofluorescence analysis of the tight junction ZO-1. Thrombin resulted in more elongated ZO-1 aggregates at cell-cell interface areas and induced an increase in ZO-1 membrane protein content. APC attenuated the length of these ZO-1 aggregates and reduced the ZO-1 membrane protein levels induced by thrombin. In conclusion, pretreatment with APC reduced the disruption of barrier integrity induced by thrombin, thus contributing to alveolar epithelial barrier protection.

Glyphosate-induced stiffening of HaCaT keratinocytes, a Peak Force Tapping study on living cells

The skin is the first physiological barrier, with a complex constitution, that provides defensive functions against multiple physical and chemical aggressions. Glyphosate is an extensively used herbicide that has been shown to increase the risk of cancer. Moreover there is increasing evidence suggesting that the mechanical phenotype plays an important role in malignant transformation. Atomic force microscopy (AFM) has emerged within the last decade as a powerful tool for providing a nanometer-scale resolution imaging of biological samples. Peak Force Tapping (PFT) is a newly released AFM-based investigation technique allowing extraction of chemical and mechanical properties from a wide range of samples at a relatively high speed and a high resolution. The present work uses the PFT technology to investigate HaCaT keratinocytes, a human epidermal cell line, and offers an original approach to study chemically-induced changes in the cellular mechanical properties under near-physiological conditions. These experiments indicate glyphosate induces cell membrane stiffening, and the appearance of cytoskeleton structures at a subcellular level, for low cytotoxic concentrations whereas cells exposed to IC50 (inhibitory concentration 50%) treatment exhibit control-like mechanical behavior despite obvious membrane damages. Quercetin, a well-known antioxidant, reverses the glyphosate-induced mechanical phenotype.

Role of reactive oxygen and nitrogen species in the vascular responses to inflammation

Inflammation is a complex and potentially life-threatening condition that involves the participation of a variety of chemical mediators, signaling pathways, and cell types. The microcirculation, which is critical for the initiation and perpetuation of an inflammatory response, exhibits several characteristic functional and structural changes in response to inflammation. These include vasomotor dysfunction (impaired vessel dilation and constriction), the adhesion and transendothelial migration of leukocytes, endothelial barrier dysfunction (increased vascular permeability), blood vessel proliferation (angiogenesis), and enhanced thrombus formation. These diverse responses of the microvasculature largely reflect the endothelial cell dysfunction that accompanies inflammation and the central role of these cells in modulating processes as varied as blood flow regulation, angiogenesis, and thrombogenesis. The importance of endothelial cells in inflammation-induced vascular dysfunction is also predicated on the ability of these cells to produce and respond to reactive oxygen and nitrogen species. Inflammation seems to upset the balance between nitric oxide and superoxide within (and surrounding) endothelial cells, which is necessary for normal vessel function. This review is focused on defining the molecular targets in the vessel wall that interact with reactive oxygen species and nitric oxide to produce the characteristic functional and structural changes that occur in response to inflammation. This analysis of the literature is consistent with the view that reactive oxygen and nitrogen species contribute significantly to the diverse vascular responses in inflammation and supports efforts that are directed at targeting these highly reactive species to maintain normal vascular health in pathological conditions that are associated with acute or chronic inflammation.

Age-related intimal stiffening enhances endothelial permeability and leukocyte transmigration

Age is the most significant risk factor for atherosclerosis; however, the link between age and atherosclerosis is poorly understood. During both aging and atherosclerosis progression, the blood vessel wall stiffens owing to alterations in the extracellular matrix. Using in vitro and ex vivo models of vessel wall stiffness and aging, we show that stiffening of extracellular matrix within the intima promotes endothelial cell permeability--a hallmark of atherogenesis. When cultured on hydrogels fabricated to match the elasticity of young and aging intima, endothelial monolayers exhibit increased permeability and disrupted cell-cell junctions on stiffer matrices. In parallel experiments, we showed a corresponding increase in cell-cell junction width with age in ex vivo aortas from young (10 weeks) and old (21 to 25 months) healthy mice. To investigate the mechanism by which matrix stiffening alters monolayer integrity, we found that cell contractility increases with increased matrix stiffness, mechanically destabilizing cell-cell junctions. This increase in endothelial permeability results in increased leukocyte extravasation, which is a critical step in atherosclerotic plaque formation. Mild inhibition of Rho-dependent cell contractility using Y-27632, an inhibitor of Rho-associated kinase, or small interfering RNA restored monolayer integrity in vitro and in vivo. Our results suggest that extracellular matrix stiffening alone, which occurs during aging, can lead to endothelial monolayer disruption and atherosclerosis pathogenesis. Because previous therapeutics designed to decrease vascular stiffness have been met with limited success, our findings could be the basis for the design of therapeutics that target the Rho-dependent cellular contractile response to matrix stiffening, rather than stiffness itself, to more effectively prevent atherosclerosis progression.

Role of FAK in S1P-regulated endothelial permeability

The vascular endothelium serves as a semi-selective barrier between the circulating contents of the blood and the tissues through which they flow. Disruption of this barrier results in significant organ dysfunction during devastating inflammatory syndromes such as sepsis and acute lung injury (ALI). Sphingosine 1-phosphate (S1P) is an endogenous lipid regulator of endothelial permeability that produces potent barrier enhancement via actin and junctional protein rearrangement and resultant cytoskeletal changes. A key effector protein in this S1P response is focal adhesion kinase (FAK), a highly conserved cytoplasmic tyrosine kinase involved in the engagement of integrins and assembly of focal adhesions (FA) through the catalysis of multiple downstream signals. After stimulation by S1P, endothelial FAK undergoes specific tyrosine phosphorylation that results in activation of the kinase and dynamic interactions with other effector molecules to improve the endothelial barrier. FAK participates in peripheral actin cytoskeletal rearrangement as well as cell-matrix (FA) and cell-cell (adherens junction) junctional complex strengthening that combine to decrease vascular permeability. This review summarizes the current knowledge of the role of FAK in mediating enhanced endothelial barrier function by S1P.

Emerging themes of cAMP regulation of the pulmonary endothelial barrier

The presence of excess fluid in the interstitium and air spaces of the lung presents severe restrictions to gas exchange. The pulmonary endothelial barrier regulates the flux of fluid and plasma proteins from the vascular space into the underlying tissue. The integrity of this endothelial barrier is dynamically regulated by transitions in cAMP (3',5'-cyclic adenosine monophosphate), which are synthesized in discrete subcellular compartments. Cyclic AMP generated in the subplasma membrane compartment acts through PKA and Epac (exchange protein directly activated by cAMP) to tighten cell adhesions, strengthen cortical actin, reduce actomyosin contraction, and decrease permeability. Confining cAMP within the subplasma membrane space is critical to its barrier-protective properties. When cAMP escapes the near membrane compartment and gains access to the cytosolic compartment, or when soluble adenylyl cyclases generate cAMP within the cytosolic compartment, this second messenger activates established cytosolic cAMP signaling cascades to perturb the endothelial barrier through PKA-mediated disruption of microtubules. Thus the concept of cAMP compartmentalization in endothelial barrier regulation is gaining momentum and new possibilities are being unveiled for cytosolic cAMP signaling with the emergence of the bicarbonate-regulated mammalian soluble adenylyl cyclase (sAC or AC10).

Atomic force microscopy of biological samples

The ability to evaluate structural-functional relationships in real time has allowed scanning probe microscopy (SPM) to assume a prominent role in post genomic biological research. In this mini-review, we highlight the development of imaging and ancillary techniques that have allowed SPM to permeate many key areas of contemporary research. We begin by examining the invention of the scanning tunneling microscope (STM) by Binnig and Rohrer in 1982 and discuss how it served to team biologists with physicists to integrate high-resolution microscopy into biological science. We point to the problems of imaging nonconductive biological samples with the STM and relate how this led to the evolution of the atomic force microscope (AFM) developed by Binnig, Quate, and Gerber, in 1986. Commercialization in the late 1980s established SPM as a powerful research tool in the biological research community. Contact mode AFM imaging was soon complemented by the development of non-contact imaging modes. These non-contact modes eventually became the primary focus for further new applications including the development of fast scanning methods. The extreme sensitivity of the AFM cantilever was recognized and has been developed into applications for measuring forces required for indenting biological surfaces and breaking bonds between biomolecules. Further functional augmentation to the cantilever tip allowed development of new and emerging techniques including scanning ion-conductance microscopy (SICM), scanning electrochemical microscope (SECM), Kelvin force microscopy (KFM) and scanning near field ultrasonic holography (SNFUH).

Cathelicidin LL-37 peptide regulates endothelial cell stiffness and endothelial barrier permeability

LL-37 peptide is a multifunctional host defense molecule essential for normal immune responses to infection or tissue injury. In this study we assess the impact of LL-37 on endothelial stiffness and barrier permeability. Fluorescence microscopy reveals membrane localization of LL-37 after its incubation with human umbilical vein endothelial cells (HUVECs). A concentration-dependent increase in stiffness was observed in HUVECs, bovine aortic endothelial cells (BAECs), human pulmonary microvascular endothelial cells, and mouse aorta upon LL-37 (0.5-5 μM) addition. Stiffening of BAECs by LL-37 was blocked by P2X7 receptor antagonists and by the intracellular Ca²(+) chelator BAPTA-AM. Increased cellular stiffness correlated with a decrease in permeability of HUVEC cell monolayers after LL-37 addition compared with nontreated cells, which was similar to the effect observed upon treatment with sphingosine 1-phosphate, and both treatments increased F-actin content in the cortical region of the cells. These results suggest that the antiinflammatory effect of LL-37 at the site of infection or injury involves an LL-37-mediated increase in cell stiffening that prevents increased pericellular permeability. Such a mechanism may help to maintain tissue fluid homeostasis.

Combined atomic force microscopy and fluorescence microscopy

The atomic force microscope (AFM) is a high-resolution scanning-probe instrument which has become an important tool for cellular and molecular biophysics in recent years, but lacks the time resolution and functional specificities offered by fluorescence microscopic techniques. The advantages of both methods may be exploited by combining and synchronizing them. In this paper, the biological applications of AFM, fluorescence, and their combinations are briefly reviewed, and the assembly and utilization of a spatially and temporally synchronized AFM and total internal reflection fluorescence microscope are described. The application of the method is demonstrated on a fluorescently labeled cell culture.

HMGB1 induces human lung endothelial cell cytoskeletal rearrangement and barrier disruption

Acute lung injury (ALI) results from loss of alveolar-capillary barrier integrity and the evolution of high-permeability pulmonary edema resulting in alveolar flooding and significant morbidity and mortality. HMGB1 is a late mediator of sepsis which uniquely participates in the evolution of sepsis and sepsis-induced ALI. The molecular events by which HMGB1 contributes to ALI remain poorly characterized. We characterized the role of HMGB1 in endothelial cell (EC) cytoskeletal rearrangement and vascular permeability, events essential to paracellular gap formation and barrier dysfunction characteristic of ALI. Initial experiments demonstrated HMGB1-mediated dose-dependent (5-20 μg/ml) decreases in transendothelial cell electrical resistance (TER) in the human pulmonary artery EC, a reflection of loss of barrier integrity. Furthermore, HMGB1 produced dose-dependent increases in paracellular gap formation in concert with loss of peripheral organized actin fibers, dissociation of cell-cell junctional cadherins, and the development of central stress fibers, a phenotypic change associated with increased contractile activity and increased EC permeability. Using siRNA strategies directed against known HMGB1 receptors (RAGE, TLR2, TLR4), we systematically determined that the receptor for advanced glycation end products (RAGE) is the primary receptor signaling HMGB1-induced TER decreases and paracellular gap formation via p38 MAP kinase activation and phosphorylation of the actin-binding protein, Hsp27. These studies add to the understanding of HMGB1-induced inflammatory events and vascular barrier disruption and offer the potential for clinical intervention in sepsis-induced ALI.

Septin-2 mediates airway epithelial barrier function in physiologic and pathologic conditions

Epithelial cells have the ability to regulate paracellular permeability dynamically in response to extracellular stimuli. With every respiratory effort, airway epithelial cells are exposed to both physiologic as well as pathologic stimuli, and regulation of the epithelial barrier in response to these stimuli is crucial to respiratory function. We report that increased membrane septin-2 localization mediates decreases in paracellular permeability by altering cortical actin arrangement in human airway epithelial cells. This phenomenon occurs in response to both physiologic levels of shear stress and a pathologic stimulus, particular matter exposure. The resulting changes in barrier function in response to septin-2 redistribution have a significant impact on the ability of the apical ligand, epidermal growth factor, to interact with its receptor, epidermal growth factor receptor, which is segregated to the basolateral side in airway epithelial cells. This suggests that the dynamic regulation of the epithelial barrier function is essential in regulating signaling responses to extracellular stimuli. These findings indicate that septin-2 plays a fundamental role in regulating barrier function by altering cortical actin expression.

A biophysical view of the interplay between mechanical forces and signaling pathways during transendothelial cell migration

The vascular endothelium is exposed to an array of physical forces, including shear stress via blood flow, contact with other cells such as neighboring endothelial cells and leukocytes, and contact with the basement membrane. Endothelial cell morphology, protein expression, stiffness and cytoskeletal arrangement are all influenced by these mechanochemical forces. There are many biophysical tools that are useful in studying how forces are transmitted in endothelial cells, and these tools are also beginning to be used to investigate biophysical aspects of leukocyte transmigration, which is a ubiquitous mechanosensitive process. In particular, the stiffness of the substrate has been shown to have a significant impact on cellular behavior, and this is true for both endothelial cells and leukocytes. Thus, the stiffness of the basement membrane as an endothelial substrate, as well as the stiffness of the endothelium as a leukocyte substrate, is relevant to the process of transmigration. In this review, we discuss recent work that has related the biophysical aspects of endothelial cell interactions and leukocyte transmigration to the biochemical pathways and molecular interactions that take place during this process. Further use of biophysical tools to investigate the biological process of leukocyte transmigration will have implications for tissue engineering, as well as atherosclerosis, stroke and immune system disease research.

Effect of neutrophil adhesion on the mechanical properties of lung microvascular endothelial cells

Neutrophil adhesion to pulmonary microvascular endothelial cells (ECs) initiates intracellular signaling, resulting in remodeling of F-actin cytoskeletal structure of ECs. The present study determined the mechanical properties of ECs and the changes induced by neutrophil adhesion by atomic force microscopy. The elastic moduli of ECs were compared before neutrophils were present, as soon as neutrophil adhesion was detected, and 1 minute later. ECs that were adjacent to those with adherent neutrophils were also evaluated. Neutrophil adhesion induced a decrease in the elastic moduli in the 6.25-μm rim of ECs surrounding adherent neutrophils as soon as firmly adherent neutrophils were detected, which was transient and lasted less than 1 minute. Adjacent ECs developed an increase in stiffness that was significant in the central regions of these cells. Intercellular adhesion molecule-1 crosslinking did not induce significant changes in the elastic modulus of ECs in either region, suggesting that crosslinking intercellular adhesion molecule-1 is not sufficient to induce the observed changes. Our results demonstrate that neutrophil adhesion induces regional changes in the stiffness of ECs.