Role of hepcidin in oxidative stress and cell death of cultured mouse renal collecting duct cells: protection against iron and sensitization to cadmium

The liver hormone hepcidin regulates systemic iron homeostasis. Hepcidin is also expressed by the kidney, but exclusively in distal nephron segments. Several studies suggest hepcidin protects against kidney damage involving Fe2+ overload. The nephrotoxic non-essential metal ion Cd2+ can displace Fe2+ from cellular biomolecules, causing oxidative stress and cell death. The role of hepcidin in Fe2+ and Cd2+ toxicity was assessed in mouse renal cortical [mCCD(cl.1)] and inner medullary [mIMCD3] collecting duct cell lines. Cells were exposed to equipotent Cd2+ (0.5-5 μmol/l) and/or Fe2+ (50-100 μmol/l) for 4-24 h. Hepcidin (Hamp1) was transiently silenced by RNAi or overexpressed by plasmid transfection. Hepcidin or catalase expression were evaluated by RT-PCR, qPCR, immunoblotting or immunofluorescence microscopy, and cell fate by MTT, apoptosis and necrosis assays. Reactive oxygen species (ROS) were detected using CellROX™ Green and catalase activity by fluorometry. Hepcidin upregulation protected against Fe2+-induced mIMCD3 cell death by increasing catalase activity and reducing ROS, but exacerbated Cd2+-induced catalase dysfunction, increasing ROS and cell death. Opposite effects were observed with Hamp1 siRNA. Similar to Hamp1 silencing, increased intracellular Fe2+ prevented Cd2+ damage, ROS formation and catalase disruption whereas chelation of intracellular Fe2+ with desferrioxamine augmented Cd2+ damage, corresponding to hepcidin upregulation. Comparable effects were observed in mCCD(cl.1) cells, indicating equivalent functions of renal hepcidin in different collecting duct segments. In conclusion, hepcidin likely binds Fe2+, but not Cd2+. Because Fe2+ and Cd2+ compete for functional binding sites in proteins, hepcidin affects their free metal ion pools and differentially impacts downstream processes and cell fate.

A microneedle platform for buccal macromolecule delivery

Alternative means for drug delivery are needed to facilitate drug adherence and administration. Microneedles (MNs) have been previously investigated transdermally for drug delivery. To date, drug loading into MNs has been limited by drug solubility in the polymeric blend. We designed a highly drug-loaded MN patch to deliver macromolecules and applied it to the buccal area, which allows for faster delivery than the skin. We successfully delivered 1-mg payloads of human insulin and human growth hormone to the buccal cavity of swine within 30 s. In addition, we conducted a trial in 100 healthy volunteers to assess potential discomfort associated with MNs when applied in the oral cavity, identifying the hard palate as the preferred application site. We envisage that MN patches applied on buccal surfaces could increase medication adherence and facilitate the painless delivery of biologics and other drugs to many, especially for the pediatric and elderly populations.

A luminal unfolding microneedle injector for oral delivery of macromolecules

Insulin and other injectable biologic drugs have transformed the treatment of patients suffering from diabetes^1,2, yet patients and healthcare providers often prefer to use and prescribe less effective orally dosed medications^3-5. Compared with subcutaneously administered drugs, oral formulations create less patient discomfort⁴, show greater chemical stability at high temperatures⁶, and do not generate biohazardous needle waste⁷. An oral dosage form for biologic medications is ideal; however, macromolecule drugs are not readily absorbed into the bloodstream through the gastrointestinal tract⁸. We developed an ingestible capsule, termed the luminal unfolding microneedle injector, which allows for the oral delivery of biologic drugs by rapidly propelling dissolvable drug-loaded microneedles into intestinal tissue using a set of unfolding arms. During ex vivo human and in vivo swine studies, the device consistently delivered the microneedles to the tissue without causing complete thickness perforations. Using insulin as a model drug, we showed that, when actuated, the luminal unfolding microneedle injector provided a faster pharmacokinetic uptake profile and a systemic uptake >10% of that of a subcutaneous injection over a 4-h sampling period. With the ability to load a multitude of microneedle formulations, the device can serve as a platform to orally deliver therapeutic doses of macromolecule drugs.

Soluble adenylyl cyclase (sAC) regulates calcium signaling in the vascular endothelium

The vascular endothelium acts as a selective barrier between the bloodstream and extravascular tissues. Intracellular [Ca²⁺]_i signaling is essential for vasoactive agonist-induced stimulation of endothelial cells (ECs), typically including Ca²⁺ release from the endoplasmic reticulum (ER). Although it is known that interactions of Ca²⁺ and cAMP as ubiquitous messengers are involved in this process, the individual contribution of cAMP-generating adenylyl cyclases (ACs), including the only soluble AC (sAC; ADCY10), remains less clear. Using life-cell microscopy and plate reader-based [Ca²⁺]_i measurements, we found that human immortalized ECs, primary aortic and cardiac microvascular ECs, and primary vascular smooth muscle cells treated with sAC-specific inhibitor KH7 or anti-sAC-small interfering RNA did not show endogenous or exogenous ATP-induced [Ca²⁺]_i elevation. Of note, a transmembrane AC (tmAC) inhibitor did not prevent ATP-induced [Ca²⁺]_i elevation in ECs. Moreover, l-phenylephrine-dependent constriction of ex vivo mouse aortic ring segments was also reduced by KH7. Analysis of the inositol-1,4,5-trisphosphate (IP₃) pathway revealed reduced IP₃ receptor phosphorylation after KH7 application, which also prevented [Ca²⁺]_i elevation induced by IP₃ receptor agonist adenophostin A. Our results suggest that sAC rather than tmAC controls the agonist-induced ER-dependent Ca²⁺ response in ECs and may represent a treatment target in arterial hypertension and heart failure.-Mewes, M., Lenders, M., Stappers, F., Scharnetzki, D., Nedele, J., Fels, J., Wedlich-Söldner, R., Brand, S.-M., Schmitz, B., Brand, E. Soluble adenylyl cyclase (sAC) regulates calcium signaling in the vascular endothelium.

An ingestible self-orienting system for oral delivery of macromolecules

Biomacromolecules have transformed our capacity to effectively treat diseases; however, their rapid degradation and poor absorption in the gastrointestinal (GI) tract generally limit their administration to parenteral routes. An oral biologic delivery system must aid in both localization and permeation to achieve systemic drug uptake. Inspired by the leopard tortoise's ability to passively reorient, we developed an ingestible self-orienting millimeter-scale applicator (SOMA) that autonomously positions itself to engage with GI tissue. It then deploys milliposts fabricated from active pharmaceutical ingredients directly through the gastric mucosa while avoiding perforation. We conducted in vivo studies in rats and swine that support the applicator's safety and, using insulin as a model drug, demonstrated that the SOMA delivers active pharmaceutical ingredient plasma levels comparable to those achieved with subcutaneous millipost administration.

Cadmium Complexed with β2-Microglubulin, Albumin and Lipocalin-2 rather than Metallothionein Cause Megalin:Cubilin Dependent Toxicity of the Renal Proximal Tubule

Cadmium (Cd²⁺) in the environment is a significant health hazard. Chronic low Cd²⁺ exposure mainly results from food and tobacco smoking and causes kidney damage, predominantly in the proximal tubule. Blood Cd²⁺ binds to thiol-containing high (e.g., albumin, transferrin) and low molecular weight proteins (e.g., the high-affinity metal-binding protein metallothionein, β2-microglobulin, α1-microglobulin and lipocalin-2). These plasma proteins reach the glomerular filtrate and are endocytosed at the proximal tubule via the multiligand receptor complex megalin:cubilin. The current dogma of chronic Cd²⁺ nephrotoxicity claims that Cd²⁺-metallothionein endocytosed via megalin:cubilin causes renal damage. However, a thorough study of the literature strongly argues for revision of this model for various reasons, mainly: (i) It relied on studies with unusually high Cd²⁺-metallothionein concentrations; (ii) the K_D of megalin for metallothionein is ~10⁵-times higher than (Cd²⁺)-metallothionein plasma concentrations. Here we investigated the uptake and toxicity of ultrafiltrated Cd²⁺-binding protein ligands that are endocytosed via megalin:cubilin in the proximal tubule. Metallothionein, β2-microglobulin, α1-microglobulin, lipocalin-2, albumin and transferrin were investigated, both as apo- and Cd²⁺-protein complexes, in a rat proximal tubule cell line (WKPT-0293 Cl.2) expressing megalin:cubilin at low passage, but is lost at high passage. Uptake was determined by fluorescence microscopy and toxicity by MTT cell viability assay. Apo-proteins in low and high passage cells as well as Cd²⁺-protein complexes in megalin:cubilin deficient high passage cells did not affect cell viability. The data prove Cd²⁺-metallothionein is not toxic, even at >100-fold physiological metallothionein concentrations in the primary filtrate. Rather, Cd²⁺-β2-microglobulin, Cd²⁺-albumin and Cd²⁺-lipocalin-2 at concentrations present in the primary filtrate are taken up by low passage proximal tubule cells and cause toxicity. They are therefore likely candidates of Cd²⁺-protein complexes damaging the proximal tubule via megalin:cubilin at concentrations found in the ultrafiltrate.

Endothelial Nanomechanics in the Context of Endothelial (Dys)function and Inflammation

SIGNIFICANCE

Stiffness of endothelial cells is closely linked to the function of the vasculature as it regulates the release of vasoactive substances such as nitric oxide (NO) and reactive oxygen species. The outer layer of endothelial cells, consisting of the glycocalyx above and the cortical zone beneath the plasma membrane, is a vulnerable compartment able to adapt its nanomechanical properties to any changes of forces exerted by the adjacent blood stream. Sustained stiffening of this layer contributes to the development of endothelial dysfunction and vascular pathologies. Recent Advances: The development of specific techniques to quantify the mechanical properties of cells enables the detailed investigation of the mechanistic link between structure and function of cells.

CRITICAL ISSUES

Challenging the mechanical stiffness of cells, for instance, by inflammatory mediators can lead to the development of endothelial dysfunction. Prevention of sustained stiffening of the outer layer of endothelial cells in turn improves endothelial function.

FUTURE DIRECTIONS

The mechanical properties of cells can be used as critical marker and test system for the proper function of the vascular system. Pharmacological substances, which are able to improve endothelial nanomechanics and function, could take a new importance in the prevention and treatment of vascular diseases. Thus, detailed knowledge acquisition about the structure/function relationship of endothelial cells and the underlying signaling pathways should be promoted.

A two-phase response of endothelial cells to hydrostatic pressure

The vascular endothelium is exposed to three types of mechanical forces: blood flow-mediated shear stress, vessel-diameter dependent wall tension and hydrostatic pressure. Despite considerable variations of blood pressure in normal and pathological physiology, little is known about the acute molecular and cellular effects of hydrostatic pressure on endothelial cells. Here, we used a combination of quantitative fluorescence microscopy, atomic force microscopy and molecular perturbations to characterize the specific response of endothelial cells to pressure application. We identified a two-phase response of endothelial cells to acute (1 h) vs. chronic (24 h) pressure application (100 mmHg). While both regimes induce cortical stiffening, the acute response is linked to calcium-mediated myosin activation, whereas the chronic cell response is dominated by increased cortical actin density and a loss in endothelial barrier function. GsMTx-4 and amiloride inhibit the acute pressure response, which suggest the sodium channel ENaC as key player in endothelial pressure sensing. The described two-phase pressure response may participate in the differential effects of transient changes in blood pressure and hypertension.

Calcium-mediated actin reset (CaAR) mediates acute cell adaptations

Actin has well established functions in cellular morphogenesis. However, it is not well understood how the various actin assemblies in a cell are kept in a dynamic equilibrium, in particular when cells have to respond to acute signals. Here, we characterize a rapid and transient actin reset in response to increased intracellular calcium levels. Within seconds of calcium influx, the formin INF2 stimulates filament polymerization at the endoplasmic reticulum (ER), while cortical actin is disassembled. The reaction is then reversed within a few minutes. This Calcium-mediated actin reset (CaAR) occurs in a wide range of mammalian cell types and in response to many physiological cues. CaAR leads to transient immobilization of organelles, drives reorganization of actin during cell cortex repair, cell spreading and wound healing, and induces long-lasting changes in gene expression. Our findings suggest that CaAR acts as fundamental facilitator of cellular adaptations in response to acute signals and stress.

DOI:http://dx.doi.org/10.7554/eLife.19850.001

Our skeleton plays a vital role in giving shape and structure to our body, it also allows us to make coordinated movements. Similarly, each cell contains a microscopic network of structures and supports called the cytoskeleton that helps cells to adopt specific shapes and is crucial for them to move around. Unlike our skeleton, which is relatively unchanging, the cytoskeleton of each cell constantly changes and adapts to the specific needs of the cell.

One part of the cytoskeleton is a dense, flexible meshwork of fibers called the cortex that lies just beneath the surface of the cell. The cortex is constructed using a protein called actin, and many of these proteins join together to form each fiber. When cells need to adapt rapidly to an injury or other sudden changes in their environment they activate a so-called stress response. This response often begins with a rapid increase in the amount of calcium ions inside a cell, which can then trigger changes in actin organization. However, it is not clear how cells under stress are able to globally remodel their actin cytoskeleton without compromising stability and integrity of the cortex.

Wales, Schuberth, Aufschnaiter et al. used a range of mammalian cells to investigate how actin responds to stress signals. All cells responded to the resulting influx of calcium ions by deconstructing large parts of the actin cortex and simultaneously forming actin filaments near the center of the cell. Wales, Schuberth, Aufschnaiter et al. termed this response calcium-mediated actin reset (CaAR), as it lasted for only a few minutes before the actin cortex reformed. The experiments show that a protein called INF2 controls CaAR by rapidly removing actin from the cortex and forming new filaments near a cell compartment called the endoplasmic reticulum.

CaAR allows cells to rapidly and drastically alter the cortex in response to stress. The experiments also show that this sudden shift in actin can change the activity of certain genes, leading to longer-term effects on the cell. The findings of Wales, Schuberth, Aufschnaiter et al. suggest that calcium ions globally regulate the actin cytoskeleton and hence cell shape and movement under stress. This could be relevant for many important processes and conditions such as wound healing, inflammation and cancer. A future challenge will be to understand the role of CaAR in these processes.

DOI:http://dx.doi.org/10.7554/eLife.19850.002

Feeling for Filaments: Quantification of the Cortical Actin Web in Live Vascular Endothelium

Contact-mode atomic force microscopy (AFM) has been shown to reveal cortical actin structures. Using live endothelial cells, we visualized cortical actin dynamics simultaneously by AFM and confocal fluorescence microscopy. We present a method that quantifies dynamic changes in the mechanical ultrastructure of the cortical actin web. We argue that the commonly used, so-called error signal imaging in AFM allows a qualitative, but not quantitative, analysis of cortical actin dynamics. The approach we used comprises fast force-curve-based topography imaging and subsequent image processing that enhances local height differences. Dynamic changes in the organization of the cytoskeleton network can be observed and quantified by surface roughness calculations and automated morphometrics. Upon treatment with low concentrations of the actin-destabilizing agent cytochalasin D, the cortical cytoskeleton network is thinned out and the average mesh size increases. In contrast, jasplakinolide, a drug that enhances actin polymerization, consolidates the cytoskeleton network and reduces the average mesh area. In conclusion, cortical actin dynamics can be quantified in live cells. To our knowledge, this opens a new pathway for conducting quantitative structure-function analyses of the endothelial actin web just beneath the apical plasma membrane.

Isotropic actomyosin dynamics promote organization of the apical cell cortex in epithelial cells

Apical membrane organization of nonconfluent epithelial cells is driven by a dynamic network of actin and myosin II filaments.

Although cortical actin plays an important role in cellular mechanics and morphogenesis, there is surprisingly little information on cortex organization at the apical surface of cells. In this paper, we characterize organization and dynamics of microvilli (MV) and a previously unappreciated actomyosin network at the apical surface of Madin–Darby canine kidney cells. In contrast to short and static MV in confluent cells, the apical surfaces of nonconfluent epithelial cells (ECs) form highly dynamic protrusions, which are often oriented along the plane of the membrane. These dynamic MV exhibit complex and spatially correlated reorganization, which is dependent on myosin II activity. Surprisingly, myosin II is organized into an extensive network of filaments spanning the entire apical membrane in nonconfluent ECs. Dynamic MV, myosin filaments, and their associated actin filaments form an interconnected, prestressed network. Interestingly, this network regulates lateral mobility of apical membrane probes such as integrins or epidermal growth factor receptors, suggesting that coordinated actomyosin dynamics contributes to apical cell membrane organization.

Fixed endothelial cells exhibit physiologically relevant nanomechanics of the cortical actin web

It has been unknown whether cells retain their mechanical properties after fixation. Therefore, this study was designed to compare the stiffness properties of the cell cortex (the 50-100 nm thick zone below the plasma membrane) before and after fixation. Atomic force microscopy was used to acquire force indentation curves from which the nanomechanical cell properties were derived. Cells were pretreated with different concentrations of actin destabilizing agent cytochalasin D, which results in a gradual softening of the cell cortex. Then cells were studied 'alive' or 'fixed'. We show that the cortical stiffness of fixed endothelial cells still reports functional properties of the actin web qualitatively comparable to those of living cells. Myosin motor protein activity, tested by blebbistatin inhibition, can only be detected, in terms of cortical mechanics, in living but not in fixed cells. We conclude that fixation interferes with motor proteins while maintaining a functional cortical actin web. Thus, fixation of cells opens up the prospect of differentially studying the actions of cellular myosin and actin.

Epithelial sodium channel stiffens the vascular endothelium in vitro and in Liddle mice

Liddle syndrome, an inherited form of hypertension, is caused by gain-of-function mutations in the epithelial Na(+) channel (ENaC), the principal mediator of Na(+) reabsorption in the kidney. Accordingly, the disease pathology was ascribed to a primary renal mechanism. Whether this is the sole responsible mechanism, however, remains uncertain as dysregulation of ENaC in other tissues may also be involved. Previous work indicates that ENaC in the vascular endothelium is crucial for the regulation of cellular mechanics and thus vascular function. The hormone aldosterone has been shown to concomitantly increase ENaC surface expression and stiffness of the cell cortex in vascular endothelial cells. The latter entails a reduced release of the vasodilator nitric oxide, which eventually leads to an increase in vascular tone and blood pressure. Using atomic force microscopy, we have found a direct correlation between ENaC surface expression and the formation of cortical stiffness in endothelial cells. Stable knockdown of αENaC in endothelial cells evoked a reduced channel surface density and a lower cortical stiffness compared with the mock control. In turn, an increased αENaC expression induced an elevated cortical stiffness. More importantly, using ex vivo preparations from a mouse model for Liddle syndrome, we show that this disorder evokes enhanced ENaC expression and increased cortical stiffness in vascular endothelial cells in situ. We conclude that ENaC in the vascular endothelium determines cellular mechanics and hence might participate in the control of vascular function.

Cortical actin nanodynamics determines nitric oxide release in vascular endothelium

The release of the main vasodilator nitric oxide (NO) by the endothelial NO synthase (eNOS) is a hallmark of endothelial function. We aim at elucidating the underlying mechanism how eNOS activity depends on cortical stiffness (К(cortex)) of living endothelial cells. It is hypothesized that cortical actin dynamics determines К(cortex) and directly influences eNOS activity. By combined atomic force microscopy and fluorescence imaging we generated mechanical and optical sections of single living cells. This approach allows the discrimination between К(cortex) and bulk cell stiffness (К(bulk)) and, additionally, the simultaneous analysis of submembranous actin web dynamics. We show that К(cortex) softens when cortical F-actin depolymerizes and that this shift from a gel-like stiff cortex to a soft G-actin rich layer, triggers the stiffness-sensitive eNOS activity. The results implicate that stiffness changes in the ∼100 nm phase of the submembranous actin web, without affecting К(bulk), regulate NO release and thus determines endothelial function.

Membrane potential depolarization decreases the stiffness of vascular endothelial cells

The stiffness of vascular endothelial cells is crucial to mechanically withstand blood flow and, at the same time, to control deformation-dependent nitric oxide release. However, the regulation of mechanical stiffness is not yet understood. There is evidence that a possible regulator is the electrical plasma membrane potential difference. Using a novel technique that combines fluorescence-based membrane potential recordings with atomic force microscopy (AFM)-based stiffness measurements, the present study shows that membrane depolarization is associated with a decrease in the stiffness of endothelial cells. Three different depolarization protocols were applied, all of which led to a similar and significant decrease in cell stiffness, independently of changes in cell volume. Moreover, experiments using the actin-destabilizing agent cytochalasin D indicated that depolarization acts by affecting the cortical actin cytoskeleton. A model is proposed whereby a change of the electrical field across the plasma membrane is directly sensed by the submembranous actin network, regulating the actin polymerization:depolymerization ratio and thus cell stiffness. This depolarization-induced decrease in the stiffness of endothelial cells could play a role in flow-mediated nitric-oxide-dependent vasodilation.

The epithelial sodium channel (ENaC): Mediator of the aldosterone response in the vascular endothelium?

In the kidney the epithelial sodium channel (ENaC) is regulated by the mineralocorticoid hormone aldosterone, which is essential for long-term blood pressure control. Evidence has accumulated showing that ENaC is expressed in endothelial cells. Moreover, its activity modifies the biomechanical properties of the endothelium. Therefore, the vascular system is also an important target for aldosterone and responds to the hormone with an increase in cell volume, surface area, and mechanical stiffness. These changes occur in a concerted fashion from minutes to hours and can be prevented by the specific sodium channel blocker amiloride and the mineralocorticoid receptor (MR) blocker spironolactone. Aldosterone acts on cells of the vascular system via genomic and non-genomic pathways. There is evidence that the classical cytosolic MR could mediate both types of response. Using a nanosensor covalently linked to aldosterone, binding sites at the plasma membrane were identified by atomic force microscopy. The interaction of aldosterone and this newly identified surface receptor could precede the slow classic genomic aldosterone response resulting in fast activation of endothelial ENaC. Recent data suggest that aldosterone-induced ENaC activation initiates a sequence of cellular events leading to a reduced release of vasodilating nitric oxide. We propose a model in which ENaC is the key mediator of aldosterone-dependent blood pressure control in the vascular endothelium.

Ménage à trois: aldosterone, sodium and nitric oxide in vascular endothelium

Aldosterone, a mineralocorticoid hormone mainly synthesized in the adrenal cortex, has been recognized to be a regulator of cell mechanics. Recent data from a number of laboratories implicate that, besides kidney, the cardiovascular system is an important target for aldosterone. In the endothelium, it promotes the expression of epithelial sodium channels (ENaC) and modifies the morphology of cells in terms of mechanical stiffness, surface area and volume. Additionally, it renders the cells highly sensitive to small changes in extracellular sodium and potassium. In this context, the time course of aldosterone action is pivotal. In the fast (seconds to minutes), non-genomic signalling pathway vascular endothelial cells respond to aldosterone with transient swelling, softening and insertion of ENaC in the apical plasma membrane. In parallel, nitric oxide (NO) is released from the cells. In the long-term (hours), aldosterone has opposite effects: The mechanical stiffness increases, the cells shrink and NO production decreases. This leads to the conclusion that both the physiology and pathophysiology of aldosterone action in the vascular endothelium are closely related. Aldosterone, at concentrations in the physiological range and over limited time periods can stabilize blood pressure and regulate tissue perfusion while chronically high concentrations of this hormone over extended time periods impair sodium homeostasis promoting endothelial dysfunction and the development of tissue fibrosis.

The hydrogel nature of mammalian cytoplasm contributes to osmosensing and extracellular pH sensing

Cytoplasm is thought to have many hydrogel-like characteristics, including the ability to absorb large amounts of water and change volume in response to alterations in external environment, as well as having limited leakage of ions and proteins. Some gel-like behaviors have not been rigorously confirmed in mammalian cells, and others should be examined under conditions where gel volume can be accurately monitored. Thus, possible contributions of cytoplasm hydrogel properties to cellular processes such as volume sensing and regulation remain unclear. We used three-dimensional imaging to measure volume of single substrate-attached cells after permeabilization of their plasma membrane. Permeabilized cells swelled or shrinked reversibly in response to variations of external osmolality. Volume changes were 3.7-fold greater than observed with intact cells, consistent with cytoplasm's high water-absorbing capacity. Volume was maximal at neutral pH and shrunk at acidic or alkaline pH, consistent with pH-dependent changes of protein charge density and repulsive forces within cellular matrix. Volume shrunk with increased Mg(2+) concentration, as expected for increased charge screening and ionic crosslinking effects. Findings demonstrate that mammalian cytoplasm resembles hydrogel and functions as a highly sensitive osmosensor and extracellular pH sensor. Its high water-absorbing capacity may allow rapid modulation of local fluidity, macromolecular crowding, and activity of intracellular environment.

Simultaneous mechanical stiffness and electrical potential measurements of living vascular endothelial cells using combined atomic force and epifluorescence microscopy

The degree of mechanical stiffness of vascular endothelial cells determines the endogenous production of the vasodilating gas nitric oxide (NO). However, the underlying mechanisms are not yet understood. Experiments on vascular endothelial cells suggest that the electrical plasma membrane potential is involved in this regulatory process. To test this hypothesis we developed a technique that simultaneously measures the electrical membrane potential and stiffness of vascular endothelial cells (GM7373 cell line derived from bovine aortic endothelium) under continuous perfusion with physiological electrolyte solution. The cellular stiffness was determined by nano-indentation using an atomic force microscope (AFM) while the electrical membrane potential was measured with bis-oxonol, a voltage-reporting fluorescent dye. These two methods were combined using an AFM attached to an epifluorescence microscope. The electrical membrane potential and mechanical stiffness of the same cell were continuously recorded for a time span of 5 min. Fast fluctuations (in the range of seconds) of both the electrical membrane potential and mechanical stiffness could be observed that were not related to each other. In contrast, slow cell depolarizations (in the range of minutes) were paralleled by significant increases in mechanical stiffness. In conclusion, using the combined AFM-fluorescence technique we monitored for the first time simultaneously the electrical plasma membrane potential and mechanical stiffness in a living cell. Vascular endothelial cells exhibit oscillatory non-synchronized waves of electrical potential and mechanical stiffness. The sustained membrane depolarization, however, is paralleled by a concomitant increase of cell stiffness. The described method is applicable for any fluorophore, which opens new perspectives in biomedical research.