stepwise-dosing-protocol-for-increased-throughput-label-free

Stepwise Dosing Protocol for Increased Throughput in Label-Free Impedance-Based GPCR Assays

Label-free impedance-based assays are increasingly used to non-invasively study ligand-induced GPCR activation in cell culture experiments. The approach ...

The formation of new blood vessels from pre-existing ones requires highly coordinated restructuring of endothelial cells (EC) and the surrounding extracellular matrix. Directed EC migration is a central step in this process and depends on cellular signaling cascades that initiate and control the structural rearrangements. On the basis of earlier findings that ERK5 deficiency in mouse EC results in massive defects in vessel architecture, we focused on the impact of the MEK5/ERK5 signaling pathway on EC migration. Using a retroviral gene transfer approach, we found that constitutive activation of MEK5/ERK5 signaling strongly inhibits EC migration and results in massive morphological changes. The area covered by spread EC was dramatically enlarged, accompanied by an increase in focal contacts and altered organization of actin filaments. Consequently, cells were more rigid and show reduced motility. This phenotype was most likely based on decreased focal contact turnover caused by reduced expression of p130Cas, a key player in directed cell migration. We demonstrate for the first time that ERK5 signaling not only is involved in EC survival and stress response but also controls migration and morphology of EC.

MEK5/ERK5 signaling modulates endothelial cell migration and focal contact turnover.

A nanogel for ratiometric fluorescent sensing of intracellular pH values.

Epithelial cell monolayers from rat kidney were imaged by scanning electrochemical microscopy (SECM) with sub-micrometer resolution in both lateral and vertical direction. Platinum disk ultra-microelectrodes (UMEs) with effective electrode radii between 200 and 600 nm were operated in the constant-height mode. The quality of the recorded SECM images compare favorably with those of phase contrast and confocal laser scanning microscopy. Besides the acquisition of SECM images, the UME was used to selectively attack a single living cell within the monolayer ensemble. Hydroxide ions were locally generated in the vicinity of a single target cell by the UME. The increase in pH induced cell necrosis that was subsequently imaged by SECM. It could be clearly demonstrated that the single target cell was selectively affected, whereas the adjacent reference cells remained unchanged.

Simultaneous imaging and chemical attack of a single living cell within a confluent cell monolayer by means of scanning electrochemical microscopy.

We report on the first dual nanosensors for imaging of pH values and oxygen partial pressure in cells. The sensors have a unique nanostructure in that a soft core structure is rigidized with a silane reagent, while poly(ethylene glycol) chains form an outer shell. Lipophilic oxygen-sensitive probes and reference dyes are encapsulated inside the hydrophobic core, while a pH-sensitive probe is covalently attached to the poly(ethylene glycol) end-group on the shell. The core/shell structure renders the nanosensors well dispersed and highly stable in various kinds of aqueous media. Their average size is 12 nm, and they respond to both pH and oxygen in the physiological range. They do not pass cell membranes, but can be internalized into the cellular cytosol by electroporation, upon which they enable sensing and imaging of pH values and oxygen with high spatial resolution. The nanosensor strategy shown here is expected to be applicable to the development of various other kinds of multiple nanosensors for in vivo studies.

Ultra-small, highly stable, and sensitive dual nanosensors for imaging intracellular oxygen and pH in cytosol.

Mutations of the LMX1B gene cause nail-patella syndrome, a rare autosomal-dominant disorder affecting the development of the limbs, eyes, brain, and kidneys. The characterization of conventional Lmx1b knockout mice has shown that LMX1B regulates the development of podocyte foot processes and slit diaphragms, but studies using podocyte-specific Lmx1b knockout mice have yielded conflicting results regarding the importance of LMX1B for maintaining podocyte structures. In order to address this question, we generated inducible podocyte-specific Lmx1b knockout mice. One week of Lmx1b inactivation in adult mice resulted in proteinuria with only minimal foot process effacement. Notably, expression levels of slit diaphragm and basement membrane proteins remained stable at this time point, and basement membrane charge properties also did not change, suggesting that alternative mechanisms mediate the development of proteinuria in these mice. Cell biological and biophysical experiments with primary podocytes isolated after 1 week of Lmx1b inactivation indicated dysregulation of actin cytoskeleton organization, and time-resolved DNA microarray analysis identified the genes encoding actin cytoskeleton-associated proteins, including Abra and Arl4c, as putative LMX1B targets. Chromatin immunoprecipitation experiments in conditionally immortalized human podocytes and gel shift assays showed that LMX1B recognizes AT-rich binding sites (FLAT elements) in the promoter regions of ABRA and ARL4C, and knockdown experiments in zebrafish support a model in which LMX1B and ABRA act in a common pathway during pronephros development. Our report establishes the importance of LMX1B in fully differentiated podocytes and argues that LMX1B is essential for the maintenance of an appropriately structured actin cytoskeleton in podocytes. 

LMX1B is essential for the maintenance of differentiated podocytes in adult kidneys.

Endothelial cells line the inner surface of all blood vessels and constitute a selective barrier between blood and tissue. Permeation of solutes across the endothelial cell monolayer occurs either paracellularly through specialized endothelial cell-cell junctions or transcellularly via special transport mechanisms including transcytosis, via the formation of transcellular channels, or by cell membrane transport proteins. Several in vitro assays have been developed in the past few decades to analyze the molecular mechanisms of transendothelial permeability. Measurement of the electrical resistance of the cell monolayer has proven to be particularly suitable for analyzing paracellular barrier function with high-time resolution over long time periods. We review the various permeability assays and focus on the electrical impedance analysis of endothelial cell monolayers. We also address current progress in the development of techniques used to investigate endothelial permeability with high-lateral resolution and under mechanical loads. 

Experimental tools to monitor the dynamics of endothelial barrier function: a survey of in vitro approaches.

DNA/RNA and protein microarrays have proven their outstanding bioanalytical performance throughout the past decades, given the unprecedented level of parallelization by which molecular recognition assays can be performed and analyzed. Cell microarrays (CMAs) make use of similar construction principles. They are applied to profile a given cell population with respect to the expression of specific molecular markers and also to measure functional cell responses to drugs and chemicals. This review focuses on the use of cell-based microarrays for assessing the cytotoxicity of drugs, toxins, or chemicals in general. It also summarizes CMA construction principles with respect to the cell types that are used for such microarrays, the readout parameters to assess toxicity, and the various formats that have been established and applied. The review ends with a critical comparison of CMAs and well-established microtiter plate (MTP) approaches. 

Cell-Based Microarrays for In Vitro Toxicology.

The most frequently used parameters to describe the barrier properties of endothelial cells (ECs) in vitro are (i) the macromolecular permeability, indicating the flux of a macromolecular tracer across the endothelium, and (ii) electrical impedance of ECs grown on gold-film electrodes reporting on the cell layer's tightness for ion flow. Due to the experimental differences between these approaches, inconsistent observations have been described. Here, we present the first direct comparison of these assays applied to one single cell type (human microvascular ECs) under the same experimental conditions. The impact of different pharmacological tools (histamine, forskolin, Y-27632, blebbistatin, TRAP) on endothelial barrier function was analyzed by Transwell(®) tracer assays and two commercial impedance devices (xCELLigence(®), ECIS(®)). The two impedance techniques provided very similar results for all compounds, whereas macromolecular permeability readings were found to be partly inconsistent with impedance. Possible reasons for these discrepancies are discussed. We conclude that the complementary combination of both approaches is highly recommended to overcome the restrictions of each assay. Since the nature of the growth support may contribute to the observed differences, structure-function relationships should be based on cells that are consistently grown on either permeable or impermeable growth supports in all experiments. 

Pitfalls in assessing microvascular endothelial barrier function: impedance-based devices versus the classic macromolecular tracer assay.

Label-free cell-based assays have been attracting growing attention in drug research. Optical approaches based on evanescent electric fields (e.g. EPIC, RWG/DMR, SPR) and electrochemical impedance analysis (ECIS, xCELLigence) are by far the most widespread techniques for such purposes. We compared three label-free approaches (ECIS, RWG/DMR and SPR) with respect to the activation of the human histamine H receptor (HR) expressed by U-373 MG glioblastoma and genetically engineered HEK 293T cells. HEK 293T cells were either expressing the hHR alone or in combination with the adhesion protein hMSR1. The β-adrenergic receptor (β-AR) expressed by bovine aortic endothelial cells (BAEC) served as a second cell model. Reduced cell adhesion to the surface of the sensing devices affected both, the optical and the impedance-based readout, but became much more obvious in case of RWG- or SPR-based assays. By contrast, the co-expression of hHR and hMSR1 in HEK 293T cells strongly enhanced the signal compared to hHR expression alone. As the sensitivity of the optical readouts is confined to a distance of 100-200nm from the surface, depending on the wavelength of the incident light, this observation is in accordance with tighter adhesion of the co-transfectants, inducing a shorter distance between the cell membrane and the substrate. Combining ECIS and SPR, allowing for simultaneous registration of both signals for a single cell population, provided a direct correlation of both readouts, when HR or β-AR stimulation was investigated for the same cell populations. Cell adhesion was found to have a critical impact on the results of label-free cell monitoring, in particular when techniques based on evanescent electric fields are applied.

Label-free analysis of GPCR-stimulation: The critical impact of cell adhesion.

The transepithelial or -endothelial electrical resistance (TEER) is a very common and routinely recorded parameter describing the expression of barrier-forming cell-cell contacts (tight junctions) in quantitative terms. To determine TEER, barrier-forming cell monolayers are cultured on porous filter supports that separate two fluid compartments. The frequency-dependent impedance of the cell layer is then recorded and analyzed by means of equivalent circuit modelling providing TEER and the cell layer capacitance. The latter serves as a quantitative indicator for membrane topography. When cells are co-cultured on opposite sides of such a porous support to model more complex biological barriers, TEER readings will integrate over both cell layers and the individual contributions are not assessable. This study describes the modification of commonly used porous filter inserts by coating their backside with a thin gold-film. When this gold-film is used as an additional electrode, both cell layers can be studied separately by impedance analysis. The electrical parameters of either cell layer are assessable independently by switching between different electrode combinations. The performance of this new approach is illustrated and documented by experiments that (i) follow the de novo formation of cell junctions between initially suspended cells and (ii) the manipulation of mature cell-cell junctions by cytoskeleton-active drugs. Both assays confirm that both cell layers are monitored entirely independently.

Independent impedimetric analysis of two cell populations co-cultured on opposite sides of a porous support.

Dynamic properties of cancer cells, most notably their ability to migrate, have been correlated successfully with their invasive nature in vivo. To establish a stronger experimental basis for such a correlation we subjected five different cancer cell lines of well-defined metastatic potential to a sequence of three independent assays reporting on three different aspects of cell dynamics, namely (1) the kinetics of cell spreading, (2) cell shape fluctuations, and (3) cell migration. The sequentially applied assays correspond to different measuring modes of the well-established ECIS technique that is based on non-invasive and label-free impedance readings of planar gold-film electrodes that serve as the growth substrate for the cells under study. Every individual assay returned a characteristic parameter describing the behavior of the cell lines in that particular assay quantitatively. The parameters of all three assays were ranked to establish individual profiles of cell dynamics for every cell line that correlate favorably with the cells' invasive properties. The sequence of impedance-based assays described here requires only small cell populations (< 10.000 cells), it is highly automated and easily adapted to 96-well formats. It provides an in-depth dynamic profile of adherent cells that might be useful in other areas besides cancer research as well.

Label-free profiling of cell dynamics: A sequence of impedance-based assays to estimate tumor cell invasiveness in vitro.

War against cancer constantly requires new affinity tools to selectively detect, localize, and quantify biomarkers for diagnosis or prognosis. Herein, carbon nanodots (CDs), an emerging class of fluorescent nanomaterials, coupled with molecularly imprinted polymers (MIPs), are employed as a biocompatible optical imaging tool for probing cancer biomarkers. First, N-doped CDs were prepared by hydrothermal synthesis using starch as carbon source and l-tryptophan as nitrogen atom provider to achieve a high quantum yield of 25.1 ± 2%. The CDs have a typical size of ∼3.2 nm and produce an intense fluorescence at 450 nm upon excitation with UV light. A MIP shell for specific recognition of glucuronic acid (GlcA) was then synthesized around the CDs, using the emission of the CDs as an internal light source for photopolymerization. GlcA is a substructure (epitope) of hyaluronan, a biomarker for certain cancers. The biotargeting and bioimaging of hyaluronan on fixated human cervical cancer cells using CD core-MIP shell nanocomposites is demonstrated. Human keratinocytes were used as noncancerous reference cells and indeed, less staining was observed by the CD-MIP.

Tracking Hyaluronan: Molecularly Imprinted Polymer Coated Carbon Dots for Cancer Cell Targeting and Imaging.

Measuring the Permeability of Endothelial Cell Monolayers: Teaching New Tricks to an Old Dog.

Oxidative stress-induced damage of the retinal pigment epithelium (RPE) and chronic inflammation have been suggested as major contributors to a range of retinal diseases. Here, we examined the effects of oxidative stress on endogenous complement components and proinflammatory and angiogenic responses in RPE cells. ARPE-19 cells exposed for 1-48 h to HO had reduced cell-cell contact and increased markers for epithelial-mesenchymal transition but showed insignificant cell death. Stressed ARPE-19 cells increased the expression of complement receptors CR3 (subunit CD11b) and C5aR1. CD11b was colocalized with cell-derived complement protein C3, which was present in its activated form in ARPE-19 cells. C3, as well as its regulators complement factor H (CFH) and properdin, accumulated in the ARPE-19 cells after oxidative stress independently of external complement sources. This cell-associated complement accumulation was accompanied by increased  and  expression and the subsequently enhanced secretion of proinflammatory and proangiogenic factors. The complement-associated ARPE-19 reaction to oxidative stress, which was independent of exogenous complement sources, was further augmented by the poly(ADP-ribose) polymerase (PARP) inhibitor olaparib. Our results indicate that ARPE-19 cell-derived complement proteins and receptors are involved in ARPE-19 cell homeostasis following oxidative stress and should be considered as targets for treatment development for retinal degeneration.

Oxidative Stress Increases Endogenous Complement-Dependent Inflammatory and Angiogenic Responses in Retinal Pigment Epithelial Cells Independently of Exogenous Complement Sources.

Article	Year
MEK5/ERK5 signaling modulates endothelial cell migration and focal contact turnover. The Journal of biological chemistry\| PubMed ID: 19605361	2009
A nanogel for ratiometric fluorescent sensing of intracellular pH values. Angewandte Chemie (International ed. in English)\| PubMed ID: 20446280	2010
Simultaneous imaging and chemical attack of a single living cell within a confluent cell monolayer by means of scanning electrochemical microscopy. Analytical chemistry\| PubMed ID: 21138288	2011
Ultra-small, highly stable, and sensitive dual nanosensors for imaging intracellular oxygen and pH in cytosol. Journal of the American Chemical Society\| PubMed ID: 23017056	2012
LMX1B is essential for the maintenance of differentiated podocytes in adult kidneys. Journal of the American Society of Nephrology : JASN\| PubMed ID: 23990680	2013
Experimental tools to monitor the dynamics of endothelial barrier function: a survey of in vitro approaches. Cell and tissue research\| PubMed ID: 24585359	2014
Cell-Based Microarrays for In Vitro Toxicology. Annual review of analytical chemistry (Palo Alto, Calif.)\| PubMed ID: 26077916	2015
Pitfalls in assessing microvascular endothelial barrier function: impedance-based devices versus the classic macromolecular tracer assay. Scientific reports\| PubMed ID: 27025965	2016
Label-free analysis of GPCR-stimulation: The critical impact of cell adhesion. Pharmacological research\| PubMed ID: 27133553	2016
Independent impedimetric analysis of two cell populations co-cultured on opposite sides of a porous support. Experimental cell research\| PubMed ID: 28087264	2017
Label-free profiling of cell dynamics: A sequence of impedance-based assays to estimate tumor cell invasiveness in vitro. Experimental cell research\| PubMed ID: 28734866	2017
Tracking Hyaluronan: Molecularly Imprinted Polymer Coated Carbon Dots for Cancer Cell Targeting and Imaging. ACS applied materials & interfaces\| PubMed ID: 29299913	2018
Measuring the Permeability of Endothelial Cell Monolayers: Teaching New Tricks to an Old Dog. Biophysical journal\| PubMed ID: 30975456	2019
Oxidative Stress Increases Endogenous Complement-Dependent Inflammatory and Angiogenic Responses in Retinal Pigment Epithelial Cells Independently of Exogenous Complement Sources. Antioxidants (Basel, Switzerland)\| PubMed ID: 31766295	2019

Joachim Wegener

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