An impedimetric microelectrode-based array sensor for label-free detection of tau hyperphosphorylation in human cells

Optical chemosensors for the detection of proximally phosphorylated peptides and proteins

Proximal multi-site phosphorylation is a critical post-translational modification in protein biology. The additive effects of multiple phosphosite clusters in close spatial proximity triggers integrative and cooperative effects on protein conformation and activity. Proximal phosphorylation has been shown to modulate signal transduction pathways and gene expression, and as a result, is implicated in a broad range of disease states through altered protein function and/or localization including enzyme overactivation or protein aggregation. The role of proximal multi-phosphorylation events is becoming increasingly recognized as mechanistically important, although breakthroughs are limited due to a lack of detection technologies. To date, there is a limited selection of facile and robust sensing tools for proximal phosphorylation. Nonetheless, there have been considerable efforts in developing optical chemosensors for the detection of proximal phosphorylation motifs on peptides and proteins in recent years. This review provides a comprehensive overview of optical chemosensors for proximal phosphorylation, with the majority of work being reported in the past two decades. Optical sensors, in the form of fluorescent and luminescent chemosensors, hybrid biosensors, and inorganic nanoparticles, are described. Emphasis is placed on the rationale behind sensor scaffolds, relevant protein motifs, and applications in protein biology.

Optically transparent electrodes to study living cells: A mini review

The use of electrochemical methods to study living systems, including cells, has been of interest to researchers for a long time. Thus, controlling the polarization of the electrode contacting living cells, one can influence, for example, their proliferation or the synthesis of specific proteins. Moreover, the electrochemical approach formed the basis of the biocompatibility improvement of the materials contacting with body tissues that use in carbon hemosorbents and implants development. It became possible to reach a fundamentally new level in the study of cell activity with the introduction of optically transparent electrodes in this area. The advantage of the using of optically transparent electrodes is the possibility of simultaneous analysis of living cells by electrochemical and microscopic methods. The use of such materials allowed approaching to the study of the influence of the electrode potential on adhesion activity and morphology of the different cell types (HeLa cells, endothelial cell, etc.) more detailed. There are a negligible number of publications in this area despite the advantages of the usage of optically transparent electrodes to study living cells. This mini-review is devoted to some aspects of the interaction of living cells with conductive materials and current advances in the use of optically transparent electrodes for the study of living cells, as well as the prospects for their use in cellular technologies.

Impedance-based Real-time Monitoring of Neural Stem Cell Differentiation

We present here the first impedance-based characterization of the differentiation process of two human mesencephalic fetal neural stem lines. The two dopaminergic neural stem cell lines used in this study, Lund human mesencephalic (LUHMES) and human ventral mesencephalic (hVM1 Bcl-X_L), have been developed for the study of Parkinsonian pathogenesis and its treatment using cell replacement therapy. We show that if only relying on impedance magnitude analysis, which is by far the most usual approach in, e.g., cytotoxicity evaluation and drug screening applications, one may not be able to distinguish whether the neural stem cells in a population are proliferating or differentiating. However, the presented results highlight that equivalent circuit analysis can provide detailed information on cellular behavior, e.g. simultaneous changes in cell morphology, cell-cell contacts, and cell adhesion during formation of neural projections, which are the fundamental behavioral differences between proliferating and differentiating neural stem cells. Moreover, our work also demonstrates the sensitivity of impedance-based monitoring with capability to provide information on changes in cellular behavior in relation to proliferation and differentiation. For both of the studied cell lines, in already two days (one day after induction of differentiation) equivalent circuit analysis was able to show distinction between proliferation and differentiation conditions, which is significantly earlier than by microscopic imaging. This study demonstrates the potential of impedance-based monitoring as a technique of choice in the study of stem cell behavior, laying the foundation for screening assays to characterize stem cell lines and testing the efficacy epigenetic control.

In vivo spatiotemporal patterns of oligodendrocyte and myelin damage at the neural electrode interface

Intracortical microelectrodes with the ability to detect intrinsic electrical signals and/or deliver electrical stimulation into local brain regions have been a powerful tool to understand brain circuitry and for therapeutic applications to neurological disorders. However, the chronic stability and sensitivity of these intracortical microelectrodes are challenged by overwhelming biological responses, including severe neuronal loss and thick glial encapsulation. Unlike microglia and astrocytes whose activity have been extensively examined, oligodendrocytes and their myelin processes remain poorly studied within the neural interface field. Oligodendrocytes have been widely recognized to modulate electrical signal conductance along axons through insulating myelin segments. Emerging evidence offers an alternative perspective on neuron-oligodendrocyte coupling where oligodendrocytes provide metabolic and neurotrophic support to neurons through cytoplasmic myelin channels and monocarboxylate transporters. This study uses in vivo multi-photon microscopy to gain insights into the dynamics of oligodendrocyte soma and myelin processes in response to chronic device implantation injury over 4 weeks. We observe that implantation induces acute oligodendrocyte injury including initial deformation and substantial myelinosome formation, an early sign of myelin injury. Over chronic implantation periods, myelin and oligodendrocyte soma suffer severe degeneration proximal to the interface. Interestingly, wound healing attempts such as oligodendrogenesis are initiated over time, however they are hampered by continued degeneration near the implant. Nevertheless, this detailed characterization of oligodendrocyte spatiotemporal dynamics during microelectrode-induced inflammation may provide insights for novel intervention targets to facilitate oligodendrogenesis, enhance the integration of neural-electrode interfaces, and improve long-term functional performance.

Recent Advances in Monitoring Cell Behavior Using Cell-Based Impedance Spectroscopy

Cell-based impedance spectroscopy (CBI) is a powerful tool that uses the principles of electrochemical impedance spectroscopy (EIS) by measuring changes in electrical impedance relative to a voltage applied to a cell layer. CBI provides a promising platform for the detection of several properties of cells including the adhesion, motility, proliferation, viability and metabolism of a cell culture. This review gives a brief overview of the theory, instrumentation, and detection principles of CBI. The recent applications of the technique are given in detail for research into cancer, neurodegenerative diseases, toxicology as well as its application to 2D and 3D in vitro cell cultures. CBI has been established as a biophysical marker to provide quantitative cellular information, which can readily be adapted for single-cell analysis to complement the existing biomarkers for clinical research on disease progression.

Brain-derived neurotrophic factor (BDNF) promotes molecular polarization and differentiation of immature neuroblastoma cells into definitive neurons

Throughout development, neuronal progenitors undergo complex transformation into polarized nerve cells, warranting the directional flow of information in the neural grid. The majority of neuronal polarization studies have been carried out on rodent-derived precursor cells, programmed to develop into neurons. Unlike rodent neuronal cells, SH-SY5Y cells derived from human bone marrow present a sub-clone of neuroblastoma line, with their transformation into neuron-like cells showing a range of highly instructive neurobiological characteristics. We applied two-step retinoic acid (RA) and brain-derived neurotrophic factor (BDNF) protocol to monitor the conversion of undifferentiated SH-SY5Y into neuron-like cells with distinctly polarized axon-dendritic morphology and formation of bona fide synaptic connections. We show that BDNF is a key driver and regulator of the expression of axonal marker tau and dendritic microtubule-associated protein-2 (MAP2), with their sorting to distinct cellular compartments. Using selective kinase inhibitors downregulating BDNF-TrkB signaling, we demonstrate that constitutive activation of TrkB receptor is essential for the maintenance of established polarization morphology. Importantly, the proximity ligation assay applied in our preparation demonstrates that differentiating neuron-like cells develop elaborate synaptic connections enriched with hallmark pre- and postsynaptic proteins. Described herein findings highlight several fundamental processes related to neuronal polarization and synaptogenesis in human-derived cells, which are of major relevance to neurobiology and translational neuroscience.

Electrochemical live monitoring of tumor cell migration out of micro-tumors on an innovative multiwell high-dense microelectrode array

Understanding of cell migration and spreading out of tumor tissue is of great interest concerning the mechanism and causes of tumor malignancy and metastases. Although there are methods available for studying cell migration on monolayer cell cultures like transwell assays, novel techniques for monitoring cell spreading out of 3D organoids or tumor tissue samples are highly required. In this context, we developed an innovative high-dense microelectrode array for impedimetric monitoring of cell migration from 3D tumor cultures. For a proof of concept, a strongly migrating breast cancer cell line (MDA-MB-231) and two malignant melanoma cell lines (T30.6.9, T12.8.10ZII) were used for generating viable micro-tumor models. The migration propensity was determined by impedimetric monitoring over 144 hours, correlated by microscopy and validated by transwell assays. The impedimetric analysis of covered electrodes and the relative impedance maximum values revealed extended information regarding the contribution of proliferative effects. More strikingly, using reference populations of mitomycin C treated spheroids where proliferation was suppressed, distinction of proliferation and migration was possible. Therefore, our high-dense microelectrode array based impedimetric migration monitoring has the capability for an automated quantitative analysis system that can be easily scaled up as well as integrated in lab on chip devices.

Real-Time Tau Protein Detection by Sandwich-Based Piezoelectric Biosensing: Exploring Tubulin as a Mass Enhancer

Human tau protein is one of the most advanced and accepted biomarkers for AD and tauopathies diagnosis in general. In this work, a quartz crystal balance (QCM) immunosensor was developed for the detection of human tau protein in buffer and artificial cerebrospinal fluid (aCSF), through both direct and sandwich assays. Starting from a conventional immuno-based sandwich strategy, two monoclonal antibodies recognizing different epitopes of tau protein were used, achieving a detection limit for the direct assay in nanomolar range both in HBES-EP and aCSF. Afterward, for exploring alternative specific receptors as secondary recognition elements for tau protein biosensing, we tested tubulin and compared its behavior to a conventional secondary antibody in the sandwich assay. Tau–tubulin binding has shown an extended working range coupled to a signal improvement in comparison with the conventional secondary antibody-based approach, showing a dose–response trend at lower tau concentration than is usually investigated and closer to the physiological levels in the reference matrix for protein tau biomarker. Our results open up new and encouraging perspectives for the use of tubulin as an alternative receptor for tau protein with interesting features due to the possibility of taking advantage of its polymerization and reversible binding to this key hallmark of Alzheimer’s disease.

A novel microfluidic microelectrode chip for a significantly enhanced monitoring of NPY-receptor activation in live mode

Lab-on-a-chip devices that combine, e.g. chemical synthesis with integrated on-chip analytics and multi-compartment organ-on-a-chip approaches, are a fast and attractive evolving research area. While integration of appropriate cell models in microfluidic setups for monitoring the biological activity of synthesis products or test compounds is already in focus, the integration of label-free bioelectronic analysis techniques is still poorly realized. In this context, we investigated the capabilities of impedance spectroscopy as a non-destructive real-time monitoring technique for adherent cell models in a microfluidic setup. While an initial adaptation of a microelectrode array (MEA) layout from a static setup revealed clear restrictions in the application of impedance spectroscopy in a microfluidic chip, we could demonstrate the advantage of a FEM simulation based rational MEA layout optimization for an optimum electrical field distribution within microfluidic structures. Furthermore, FEM simulation based analysis of shear stress and time-dependent test compound distribution led to identification of an optimal flow rate. Based on the simulation derived optimized microfluidic MEA, comparable impedance spectra characteristics were achieved for HEK293A cells cultured under microfluidic and static conditions. Furthermore, HEK293A cells expressing Y1 receptors were used to successfully demonstrate the capabilities of impedimetric monitoring of cellular alterations in the microfluidic setup. More strikingly, the maximum impedimetric signal for the receptor activation was significantly increased by a factor of 2.8. Detailed investigations of cell morphology and motility led to the conclusion that cultivation under microfluidic conditions could lead to an extended and stabilized cell-electrode interface.

Plasmonic Imaging of Electrochemical Impedance

Electrochemical impedance spectroscopy (EIS) measures the frequency spectrum of an electrochemical interface to resist an alternating current. This method allows label-free and noninvasive studies on interfacial adsorption and molecular interactions and has applications in biosensing and drug screening. Although powerful, traditional EIS lacks spatial resolution or imaging capability, hindering the study of heterogeneous electrochemical processes on electrodes. We have recently developed a plasmonics-based electrochemical impedance technique to image local electrochemical impedance with a submicron spatial resolution and a submillisecond temporal resolution. In this review, we provide a systematic description of the theory, instrumentation, and data analysis of this technique. To illustrate its present and future applications, we further describe several selected samples analyzed with this method, including protein microarrays, two-dimensional materials, and single cells. We conclude by summarizing the technique's unique features and discussing the remaining challenges and new directions of its application.

Adapting tissue-engineered in vitro CNS models for high-throughput study of neurodegeneration

Neurodegenerative conditions remain difficult to treat, with the continuing failure to see therapeutic research successfully advance to clinical trials. One of the obstacles that must be overcome is to develop enhanced models of disease. Tissue engineering techniques enable us to create organised artificial central nervous system tissue that has the potential to improve the drug development process. This study presents a replicable model of neurodegenerative pathology through the use of engineered neural tissue co-cultures that can incorporate cells from various sources and allow degeneration and protection of neurons to be observed easily and measured, following exposure to neurotoxic compounds – okadaic acid and 1-methyl-4-phenylpyridinium. Furthermore, the technology has been miniaturised through development of a mould with 6 mm length that recreates the advantageous features of engineered neural tissue co-cultures at a scale suitable for commercial research and development. Integration of human-derived induced pluripotent stem cells aids more accurate modelling of human diseases, creating new possibilities for engineered neural tissue co-cultures and their use in drug screening.

Quantitative characterization of capsaicin-induced TRPV1 ion channel activation in HEK293 cells by impedance spectroscopy

The analysis of receptor activity, especially in its native cellular environment, has always been of great interest to evaluate its intrinsic but also downstream biological activity. An important group of cellular receptors are ion channels. Since they are involved in a broad range of crucial cell functions, they represent important therapeutic targets. Thus, novel analytical techniques for the quantitative monitoring and screening of biological receptor activity are of great interest. In this context, we developed an impedance spectroscopy-based label-free and non-invasive monitoring system that enabled us to analyze the activation of the transient receptor potential channel Vanilloid 1 (TRPV1) in detail. TRPV1 channel activation by capsaicin resulted in a reproducible impedance decrease. Moreover, concentration response curves with an EC₅₀ value of 0.9 μM could be determined. Control experiments with non TRPV1 channel expressing HEK cells as well as experiments with the TRPV1 channel blocker ruthenium red validated the specificity of the observed impedance decrease. More strikingly, through correlative studies with a cytoskeleton restructuring inhibitor mixture and equivalent circuit analysis of the acquired impedance spectra, we could quantitatively discriminate between the direct TRPV1 channel activation and downstream-induced biological effects. In summary, we developed a quantitative impedimetric monitoring system for the analysis of TRPV1 channel activity as well as downstream-induced biological activity in living cells. It has the capabilities to identify novel ion channel activators as well as inhibitors for the TRPV1 channel but could also easily be applied to other ion channel-based receptors.

A novel 384-multiwell microelectrode array for the impedimetric monitoring of Tau protein induced neurodegenerative processes

Over the last decades, countless bioelectronic monitoring systems were developed for the analysis of cells as well as complex tissues. Most studies addressed the sensitivity and specificity of the bioelectronic detection method in comparison to classical molecular biological assays. In contrast, the up scaling as a prerequisite for the practical application of these novel bioelectronic monitoring systems is mostly only discussed theoretically. In this context, we developed a novel 384-multiwell microelectrode array (MMEA) based measurement system for the sensitive label-free real-time monitoring of neurodegenerative processes by impedance spectroscopy. With respect to the needs of productive screening systems for robust and reproducible measurements on high numbers of plates, we focused on reducing the critical contacting of more than 400 electrodes for a 384-MMEA. Therefore, we introduced an on top array of immersive counter electrodes that are individually addressed by a multiplexer and connected all measurement electrodes on the 384-MMEA to a single contact point. More strikingly, our novel approach provided a comparable signal stability and sensitivity similar to an array with integrated counter electrodes. Next, we optimized a SH-SY5Y cell based tauopathy model by introducing a novel 5-fold Tau mutation eliminating the need of artificial tauopathy induction. In combination with our novel 384-MMEA based measurement system, the concentration and time dependent neuroregenerative effect of the kinase inhibitor SRN-003-556 could be quantitatively monitored. Thus, our novel screening system could be a useful tool to identify and develop potential novel therapeutics in the field of Tau-related neurodegenerative diseases.

Direct chemosensitivity monitoring ex vivo on undissociated melanoma tumor tissue by impedance spectroscopy

Stage III/IV melanoma remains incurable in most cases due to chemotherapeutic resistance. Thus, predicting and monitoring chemotherapeutic responses in this setting offer great interest. To overcome limitations of existing assays in evaluating the chemosensitivity of dissociated tumor cells, we developed a label-free monitoring system to directly analyze the chemosensitivity of undissociated tumor tissue. Using a preparation of tumor micro-fragments (TMF) established from melanoma biopsies, we characterized the tissue organization and biomarker expression by immunocytochemistry. Robust generation of TMF was established successfully and demonstrated on a broad range of primary melanoma tumors and tumor metastases. Organization and biomarker expression within the TMF were highly comparable with tumor tissue, in contrast to dissociated, cultivated tumor cells. Using isolated TMF, sensitivity to six clinically relevant chemotherapeutic drugs (dacarbazine, doxorubicin, paclitaxel, cisplatin, gemcitabine, and treosulfan) was determined by impedance spectroscopy in combination with a unique microcavity array technology we developed. In parallel, comparative analyses were performed on monolayer tumor cell cultures. Lastly, we determined the efficacy of chemotherapeutic agents on TMF by impedance spectroscopy to obtain individual chemosensitivity patterns. Our results demonstrated nonpredictable differences in the reaction of tumor cells to chemotherapy in TMF by comparison with dissociated, cultivated tumor cells. Our direct impedimetric analysis of melanoma biopsies offers a direct ex vivo system to more reliably predict patient-specific chemosensitivity patterns and to monitor antitumor efficacy.

A novel 96-well multielectrode array based impedimetric monitoring platform for comparative drug efficacy analysis on 2D and 3D brain tumor cultures

Aggressive cancer entities like neuroblastoma and glioblastoma multiforme are still difficult to treat and have discouraging prognosis in malignant stage. Since each tumor has its own characteristics concerning the sensitivity towards different chemotherapeutics and moreover, can obtain resistance, the development of novel chemotherapeutics with a broad activity spectrum, high efficacy and minimum side effects is a continuous process. Sophisticated in vitro assays for comprehensive prediction of in vivo drug efficacy and side effects represent an actual bottleneck in the drug development process. In this context, we developed a novel in vitro 2D and 3D multiwell-multielectrode device for drug efficacy monitoring based on direct real-time impedance spectroscopy measurement in combination with our unique 96-well multielectrode arrays and microcavity arrays. For demonstration, we used three neuro- and glioblastoma cell lines that were cultured as monolayer and multicellular tumor spheroids for recapitulating in vivo conditions. Using our novel 96-well multielectrode array based system it was possible to detect time and concentration dependent responses concerning treatment with doxorubicin, etoposide and vincristine. While all tested chemotherapeutics revealed high potency for apoptosis induction in neuroblastoma cells, etoposide was ineffective for glioblastoma cell lines. Determination of IC50 values allowed us to compare drug efficacy in 2D and 3D culture models and moreover, revealed chemotherapeutic and tumor cell line specific activity patterns. These pharmacokinetic patterns are of great interest in the context of preclinical drug development. Thus, impedance spectroscopy based monitoring systems could be used for the fast in vitro based in vivo prediction of novel anti-tumor drugs.

Quantitative impedimetric NPY-receptor activation monitoring and signal pathway profiling in living cells

Label-free and non-invasive monitoring of receptor activation and identification of the involved signal pathways in living cells is an ongoing analytic challenge and a great opportunity for biosensoric systems. In this context, we developed an impedance spectroscopy-based system for the activation monitoring of NPY-receptors in living cells. Using an optimized interdigital electrode array for sensitive detection of cellular alterations, we were able for the first time to quantitatively detect the NPY-receptor activation directly without a secondary or enhancer reaction like cAMP-stimulation by forskolin. More strikingly, we could show that the impedimetric based NPY-receptor activation monitoring is not restricted to the Y1-receptor but also possible for the Y2- and Y5-receptor. Furthermore, we could monitor the NPY-receptor activation in different cell lines that natively express NPY-receptors and proof the specificity of the observed impedimetric effect by agonist/antagonist studies in recombinant NPY-receptor expressing cell lines. To clarify the nature of the observed impedimetric effect we performed an equivalent circuit analysis as well as analyzed the role of cell morphology and receptor internalization. Finally, an antagonist based extensive molecular signal pathway analysis revealed small alterations of the actin cytoskeleton as well as the inhibition of at least L-type calcium channels as major reasons for the observed NPY-induced impedance increase. Taken together, our novel impedance spectroscopy based NPY-receptor activation monitoring system offers the opportunity to identify signal pathways as well as for novel versatile agonist/antagonist screening systems for identification of novel therapeutics in the field of obesity and cancer.

A novel 3D label-free monitoring system of hES-derived cardiomyocyte clusters: a step forward to in vitro cardiotoxicity testing

Unexpected adverse effects on the cardiovascular system remain a major challenge in the development of novel active pharmaceutical ingredients (API). To overcome the current limitations of animal-based in vitro and in vivo test systems, stem cell derived human cardiomyocyte clusters (hCMC) offer the opportunity for highly predictable pre-clinical testing. The three-dimensional structure of hCMC appears more representative of tissue milieu than traditional monolayer cell culture. However, there is a lack of long-term, real time monitoring systems for tissue-like cardiac material. To address this issue, we have developed a microcavity array (MCA)-based label-free monitoring system that eliminates the need for critical hCMC adhesion and outgrowth steps. In contrast, feasible field potential derived action potential recording is possible immediately after positioning within the microcavity. Moreover, this approach allows extended observation of adverse effects on hCMC. For the first time, we describe herein the monitoring of hCMC over 35 days while preserving the hCMC structure and electrophysiological characteristics. Furthermore, we demonstrated the sensitive detection and quantification of adverse API effects using E4031, doxorubicin, and noradrenaline directly on unaltered 3D cultures. The MCA system provides multi-parameter analysis capabilities incorporating field potential recording, impedance spectroscopy, and optical read-outs on individual clusters giving a comprehensive insight into induced cellular alterations within a complex cardiac culture over days or even weeks.

Impedance spectroscopy--an outstanding method for label-free and real-time discrimination between brain and tumor tissue in vivo

Until today, brain tumors especially glioblastoma are difficult to treat and therefore, results in a poor survival rate of 0-14% over five years. To overcome this problem, the development of novel therapeutics as well as optimization of neurosurgical procedures to remove the tumor tissue are subject of intensive research. The main problem of the tumor excision, as the primary clinical intervention is the diffuse infiltration of the tumor cells in unaltered brain tissue that complicates the complete removal of residual tumor cells. In this context, we are developing novel approaches for the label-free discrimination between tumor tissue and unaltered brain tissue in real-time during the surgical process. Using our impedance spectroscopy-based measurement system in combination with flexible microelectrode arrays we could successfully demonstrate the discrimination between a C6-glioma and unaltered brain tissue in an in vivo rat model. The analysis of the impedance spectra revealed specific impedance spectrum shape characteristics of physiologic neuronal tissue in the frequency range of 10-500 kHz that were significantly different from the tumor tissue. Moreover, we used an adapted equivalent circuit model to get a deeper understanding for the nature of the observed effects. The impedimetric label-free and real-time discrimination of tumor from unaltered brain tissue offers the possibility for the implementation in surgical instruments to support surgeons to decide, which tissue areas should be removed and which should be remained.

Induced tauopathy in a novel 3D-culture model mediates neurodegenerative processes: a real-time study on biochips

Tauopathies including Alzheimer's disease represent one of the major health problems of aging population worldwide. Therefore, a better understanding of tau-dependent pathologies and consequently, tau-related intervention strategies is highly demanded. In recent years, several tau-focused therapies have been proposed with the aim to stop disease progression. However, to develop efficient active pharmaceutical ingredients for the broad treatment of Alzheimer's disease patients, further improvements are necessary for understanding the detailed neurodegenerative processes as well as the mechanism and side effects of potential active pharmaceutical ingredients (API) in the neuronal system. In this context, there is a lack of suitable complex in vitro cell culture models recapitulating major aspects of taupathological degenerative processes in sufficient time and reproducible manner.Herewith, we describe a novel 3D SH-SY5Y cell-based, tauopathy model that shows advanced characteristics of matured neurons in comparison to monolayer cultures without the need of artificial differentiation promoting agents. Moreover, the recombinant expression of a novel highly pathologic fourfold mutated human tau variant lead to a fast and emphasized degeneration of neuritic processes. The neurodegenerative effects could be analyzed in real time and with high sensitivity using our unique microcavity array-based impedance spectroscopy measurement system. We were able to quantify a time- and concentration-dependent relative impedance decrease when Alzheimer's disease-like tau pathology was induced in the neuronal 3D cell culture model. In combination with the collected optical information, the degenerative processes within each 3D-culture could be monitored and analyzed. More strikingly, tau-specific regenerative effects caused by tau-focused active pharmaceutical ingredients could be quantitatively monitored by impedance spectroscopy.Bringing together our novel complex 3D cell culture taupathology model and our microcavity array-based impedimetric measurement system, we provide a powerful tool for the label-free investigation of tau-related pathology processes as well as the high content analysis of potential active pharmaceutical ingredient candidates.

Electrochemical investigations into Tau protein phosphorylations

Hyperphosphorylation of Tau, a protein that stabilizes microtubules, leads to the breakdown of the microtubular structure and ultimately to the formation of neurofibrillar tangles within neurons. Here, we report monitoring of Tau phosphorylations electrochemically, using Tau protein films chemically linked to gold surfaces and 5'-γ-ferrocenyl (Fc) adenosine triphosphate (Fc-ATP) as a co-substrate. Fc-phosphorylation reactions of Tau are explored using the three protein kinases, glycogen synthase kinase (GSK-3β), sarcoma (Src)-related kinase, and protein kinase A (PKA), which catalyze Fc-phosphorylation of different residues and regions within Tau. The kinetic parameters of the biochemical process (K(M) and V(max)) were determined.

An electrode array for electrochemical immuno-sensing using the example of impedimetric tenascin C detection

Electrochemical biosensors allow simple, fast and sensitive analyte detection for various analytical problems. Especially immunosensors are favourable due to specificity and affinity of antigen recognition by the associated antibody. We present a novel electrode array qualified for parallel analysis and increased sample throughput. The chip has nine independent sample chambers. Each chamber contains a circular gold working electrode with a diameter of 1.9 mm that is surrounded by a ring-shaped auxiliary electrode with a platinum surface. The corresponding silver/silver chloride reference electrodes are embedded in a sealing lid. The chip is open to the full range of electrochemical real-time detection methods. Among these techniques, impedance spectroscopy is an attractive tool to detect fast and label-free interfacial changes originating from the biorecognition event at the electrode surface. The capabilities of the novel electrode array are demonstrated using the example of tumour marker tenascin C detection. This glycoprotein of the extracellular matrix is expressed in cancerous tissues, especially in solid tumours such as glioma or breast carcinoma. Electrodes covered with specific antibodies were exposed to tenascin C containing samples. Non-occupied binding sites were identified using a secondary peroxidase-conjugated antibody that generated an insoluble precipitate on the electrode in a subsequent amplification procedure. The charge transfer resistance obtained from impedimetric analysis of ferri-/ferrocyanide conversion at the electrode served as analytic parameter. This assay detected 14 ng (48 fmol) tenascin C that is sufficient for clinical diagnostics. The electrode surface could be regenerated at least 20-fold without loss of its analytical performance.

Monitoring selectivity in kinase-promoted phosphorylation of densely packed peptide monolayers using label-free electrochemical detection

This paper describes remarkably high sensitivities in the label-free detection of kinase-promoted phosphorylation for 14 different peptide substrates on electrode-immobilized monolayers (gold or nitride) using serine/threonine kinases PKA, PKC, and CaMK2. Peptide substrates were preselected using (33)P-labeling in a microarray of 1024 substrates. The three most active peptides (A1-A3, C1-C3, and M1-M3) were investigated using electrochemical impedance spectroscopy (EIS) and ion-sensitive field effect transistors (ISFETs). Some of the peptide substrates, for example, the PKC-specific substrate PPRRSSIRNAH (C1), showed a remarkably high sensitivity in the EIS-based sensor measurements. Our studies revealed that this high sensitivity is primarily due to the monolayer's packing density. Nanoscopic studies demonstrated a distinct disordering of the C1-monolayer upon phosphorylation, while phosphatase-promoted dephosphorylation regenerated the highly ordered peptide monolayer. As a matter of fact, the initial surface packing of the peptide monolayer mainly determined the level of sensitivity, whereas electrostatic repulsion of the redox-active species was found to be much less important.

Distinguishing drug-induced minor morphological changes from major cellular damage via label-free impedimetric toxicity screening

We present a novel perfusion-based microfluidic platform for label-free drug toxicity screening which can single out non-lethal morphological changes from cellular death using electrical impedance spectroscopy. Minor cellular changes such as cell-cell contacts and major cell injury were identified via impedance phase angle analysis and follow-up of impedance magnitude at different frequencies. Having exposed HepG2/C3A cells to acetaminophen (AP), we showed that continuous drug perfusion caused a time and concentration-dependent impedance decrease. Moreover, perfusion of repeated doses revealed altered dielectric properties of the cell culture after recovery from AP exposure. This study highlights the possibility to sense cellular changes long before cellular death takes place, pointing out the remarkable sensitivity advantage of this technique over standard endpoint viability tests and its interest for toxicology.

Real-time monitoring of relaxation and contractility of smooth muscle cells on a novel biohybrid chip

Cardiovascular diseases represent the most common cause of death in industrialized countries. In this context vascular smooth muscle cells (SMCs) are a major key player that is involved in pathological processes like hypertension and atherosclerosis. Therefore the pharmaceutical industry is intensively investigated in developing non-destructive and label-free monitoring techniques for a quantitative detection of SMC characteristics in the field of active pharmaceutical development as well as clinical diagnostics. Hence, we developed a novel multiwell interdigital electrode sensor-array in standardized ANSI 96-well layout. Through optimization of electrode geometry and material as well as passivation/adhesion-layer we obtained a novel biohybrid chip for the sensitive and quantitative detection of SMC contractility as well as relaxation via impedance spectroscopy. For the validation of our multiwell sensor-array we established a SMC culture model derived from primary cells that is switchable from a non-contractile pathological to a functional contractile phenotype. Using the reference compounds acetylcholine (ACh) and amlodipine, we could quantify SMC contraction by an impedance decrease to 40% while SMC relaxation was detectable by an impedance increase to 110%. More strikingly we could monitor aging of the isolated SMC which arose by an attenuated contractility over successive passaging. Demonstrating the performance of our self-developed multiwell sensor-array based impedance measurement setup we provide a suitable sensor-array coupled cell model to study the mechanisms that activated SMCs undergo in response to inflammatory mediators or vessel injury.

Microfluidic impedance-based flow cytometry

Microfabricated flow cytometers can detect, count, and analyze cells or particles using microfluidics and electronics to give impedance-based characterization. Such systems are being developed to provide simple, low-cost, label-free, and portable solutions for cell analysis. Recent work using microfabricated systems has demonstrated the capability to analyze micro-organisms, erythrocytes, leukocytes, and animal and human cell lines. Multifrequency impedance measurements can give multiparametric, high-content data that can be used to distinguish cell types. New combinations of microfluidic sample handling design and microscale flow phenomena have been used to focus and position cells within the channel for improved sensitivity. Robust designs will enable focusing at high flowrates while reducing requirements for control over multiple sample and sheath flows. Although microfluidic impedance-based flow cytometers have not yet or may never reach the extremely high throughput of conventional flow cytometers, the advantages of portability, simplicity, and ability to analyze single cells in small populations are, nevertheless, where chip-based cytometry can make a large impact.