Symmetrical Bispyridinium Compounds Act as Open Channel Blockers of Cation-Selective Ion Channels

Current treatments against organophosphate poisoning (OPP) do not directly address effects mediated by the overstimulation of nicotinic acetylcholine receptors (nAChR). Non-oxime bispyridinium compounds (BPC) promote acetylcholine esterase-independent recovery of organophosphate-induced paralysis. Here, we test the hypothesis that they act by positive modulatory action on nAChRs. Using two-electrode voltage clamp analysis in combination with mutagenesis and molecular docking analysis, the potency and molecular mode of action of a series of nine BPCs was investigated on human α7 and muscle-type nAChRs expressed in Xenopus laevis oocytes. The investigated BPCs inhibited α7 and/or muscle-type nAChRs with IC50 values in the high nanomolar to high micromolar range. Further analysis of the most potent analogues revealed a noncompetitive, voltage-dependent inhibition. Co-application with the α7-selective positive allosteric modulator PNU120596 and generation of α7/5HT3 receptor chimeras excluded direct interaction with the PNU120596 binding site and binding to the extracellular domain of the α7 nAChR, suggesting that they act as open channel blockers (OCBs). Molecular docking supported by mutagenesis localized the BPC binding area in the outer channel vestibule between the extracellular and transmembrane domains. Analysis of BPC action on other cation-selective channels suggests a rather nonspecific inhibition of pentameric cation channels. BPCs have been shown to ameliorate organophosphate-induced paralysis in vitro and in vivo. Our data support molecular action as OCBs at α7 and muscle-type nAChRs and suggest that their positive physiological effects are more complex than anticipated and require further investigation.

Functional evaluation of epilepsy-associated KCNT1 variants in multiple cellular systems reveals a predominant gain of function impact on channel properties

OBJECTIVE

Gain of function variants in the sodium-activated potassium channel KCNT1 have been associated with pediatric epilepsy disorders. Here, we systematically examine a spectrum of KCNT1 variants and establish their impact on channel function in multiple cellular systems.

METHODS

KCNT1 variants identified from published reports and genetic screening of pediatric epilepsy patients were expressed in Xenopus oocytes and HEK cell lines. Variant impact on current magnitude, current-voltage relationships, and sodium ion modulation were examined.

RESULTS

We determined basic properties of KCNT1 in Xenopus oocyte and HEK systems, including the role of extra- and intracellular sodium in regulating KCNT1 activity. The most common six KCNT1 variants demonstrated strong gain of function (GOF) effects on one or more channel properties. Analysis of 36 total variants identified phenotypic heterogeneity but a strong tendency for pathogenic variants to exert GOF effects on channel properties. By controlling intracellular sodium, we demonstrate that multiple pathogenic KCNT1 variants modulate channel voltage dependence by altering the sensitivity to sodium ions.

SIGNIFICANCE

This study represents the largest systematic functional examination of KCNT1 variants to date. We both confirm previously reported GOF channel phenotypes and expand the number of variants with in vitro GOF effects. Our data provide further evidence that novel KCNT1 variants identified in epilepsy patients lead to disease through generalizable GOF mechanisms including increases in current magnitude and/or current-voltage relationships.

Functional alterations by a subgroup of neonicotinoid pesticides in human dopaminergic neurons

Neonicotinoid pesticides, originally developed to target the insect nervous system, have been reported to interact with human receptors and to activate rodent neurons. Therefore, we evaluated in how far these compounds may trigger signaling in human neurons, and thus, affect the human adult or developing nervous system. We used SH-SY5Y neuroblastoma cells as established model of nicotinic acetylcholine receptor (nAChR) signaling. In parallel, we profiled dopaminergic neurons, generated from LUHMES neuronal precursor cells, as novel system to study nAChR activation in human post-mitotic neurons. Changes of the free intracellular Ca²⁺ concentration ([Ca²⁺]_i) were used as readout, and key findings were confirmed by patch clamp recordings. Nicotine triggered typical neuronal signaling responses that were blocked by antagonists, such as tubocurarine and mecamylamine. Pharmacological approaches suggested a functional expression of α7 and non-α7 nAChRs on LUHMES cells. In this novel test system, the neonicotinoids acetamiprid, imidacloprid, clothianidin and thiacloprid, but not thiamethoxam and dinotefuran, triggered [Ca²⁺]_i signaling at 10-100 µM. Strong synergy of the active neonicotinoids (at low micromolar concentrations) with the α7 nAChR-positive allosteric modulator PNU-120596 was observed in LUHMES and SH-SY5Y cells, and specific antagonists fully inhibited such signaling. To provide a third line of evidence for neonicotinoid signaling via nAChR, we studied cross-desensitization: pretreatment of LUHMES and SH-SY5Y cells with active neonicotinoids (at 1-10 µM) blunted the signaling response of nicotine. The pesticides (at 3-30 µM) also blunted the response to the non-α7 agonist ABT 594 in LUHMES cells. These data show that human neuronal cells are functionally affected by low micromolar concentrations of several neonicotinoids. An effect of such signals on nervous system development is a toxicological concern.

Human neuronal signaling and communication assays to assess functional neurotoxicity

Prediction of drug toxicity on the human nervous system still relies mainly on animal experiments. Here, we developed an alternative system allowing assessment of complex signaling in both individual human neurons and on the network level. The LUHMES cultures used for our approach can be cultured in 384-well plates with high reproducibility. We established here high-throughput quantification of free intracellular Ca2+ concentrations [Ca2+]i as broadly applicable surrogate of neuronal activity and verified the main processes by patch clamp recordings. Initially, we characterized the expression pattern of many neuronal signaling components and selected the purinergic receptors to demonstrate the applicability of the [Ca2+]i signals for quantitative characterization of agonist and antagonist responses on classical ionotropic neurotransmitter receptors. This included receptor sub-typing and the characterization of the anti-parasitic drug suramin as modulator of the cellular response to ATP. To exemplify potential studies on ion channels, we characterized voltage-gated sodium channels and their inhibition by tetrodotoxin, saxitoxin and lidocaine, as well as their opening by the plant alkaloid veratridine and the food-relevant marine biotoxin ciguatoxin. Even broader applicability of [Ca2+]i quantification as an end point was demonstrated by measurements of dopamine transporter activity based on the membrane potential-changing activity of this neurotransmitter carrier. The substrates dopamine or amphetamine triggered [Ca2+]i oscillations that were synchronized over the entire culture dish. We identified compounds that modified these oscillations by interfering with various ion channels. Thus, this new test system allows multiple types of neuronal signaling, within and between cells, to be assessed, quantified and characterized for their potential disturbance.

Cross-site and cross-platform variability of automated patch clamp assessments of drug effects on human cardiac currents in recombinant cells

Automated patch clamp (APC) instruments enable efficient evaluation of electrophysiologic effects of drugs on human cardiac currents in heterologous expression systems. Differences in experimental protocols, instruments, and dissimilar site procedures affect the variability of IC50 values characterizing drug block potency. This impacts the utility of APC platforms for assessing a drug's cardiac safety margin. We determined variability of APC data from multiple sites that measured blocking potency of 12 blinded drugs (with different levels of proarrhythmic risk) against four human cardiac currents (hERG [IKr], hCav1.2 [L-Type ICa], peak hNav1.5, [Peak INa], late hNav1.5 [Late INa]) with recommended protocols (to minimize variance) using five APC platforms across 17 sites. IC50 variability (25/75 percentiles) differed for drugs and currents (e.g., 10.4-fold for dofetilide block of hERG current and 4-fold for mexiletine block of hNav1.5 current). Within-platform variance predominated for 4 of 12 hERG blocking drugs and 4 of 6 hNav1.5 blocking drugs. hERG and hNav1.5 block. Bland-Altman plots depicted varying agreement across APC platforms. A follow-up survey suggested multiple sources of experimental variability that could be further minimized by stricter adherence to standard protocols. Adoption of best practices would ensure less variable APC datasets and improved safety margins and proarrhythmic risk assessments.

A systematic strategy for estimating hERG block potency and its implications in a new cardiac safety paradigm

Introduction

hERG block potency is widely used to calculate a drug's safety margin against its torsadogenic potential. Previous studies are confounded by use of different patch clamp electrophysiology protocols and a lack of statistical quantification of experimental variability. Since the new cardiac safety paradigm being discussed by the International Council for Harmonisation promotes a tighter integration of nonclinical and clinical data for torsadogenic risk assessment, a more systematic approach to estimate the hERG block potency and safety margin is needed.

Methods

A cross-industry study was performed to collect hERG data on 28 drugs with known torsadogenic risk using a standardized experimental protocol. A Bayesian hierarchical modeling (BHM) approach was used to assess the hERG block potency of these drugs by quantifying both the inter-site and intra-site variability. A modeling and simulation study was also done to evaluate protocol-dependent changes in hERG potency estimates.

Results

A systematic approach to estimate hERG block potency is established. The impact of choosing a safety margin threshold on torsadogenic risk evaluation is explored based on the posterior distributions of hERG potency estimated by this method. The modeling and simulation results suggest any potency estimate is specific to the protocol used.

Discussion

This methodology can estimate hERG block potency specific to a given voltage protocol. The relationship between safety margin thresholds and torsadogenic risk predictivity suggests the threshold should be tailored to each specific context of use, and safety margin evaluation may need to be integrated with other information to form a more comprehensive risk assessment.

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hERG potency/safety margin is a widely used nonclinical cardiac safety strategy.

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A new regulatory paradigm promotes the integration of nonclinical and clinical data.

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Lack of uncertainty quantification hindered using hERG potency in the new paradigm.

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A systematic method was established to address this limitation.

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Analysis supports using different safety margin thresholds in different context.

Catch and Patch: A Pipette-Based Approach for Automating Patch Clamp That Enables Cell Selection and Fast Compound Application

Manual patch clamp, the gold standard of electrophysiology, represents a powerful and versatile toolbox to stimulate, modulate, and record ion channel activity from membrane fragments and whole cells. The electrophysiological readout can be combined with fluorescent or optogenetic methods and allows for ultrafast solution exchanges using specialized microfluidic tools. A hallmark of manual patch clamp is the intentional selection of individual cells for recording, often an essential prerequisite to generate meaningful data. So far, available automation solutions rely on random cell usage in the closed environment of a chip and thus sacrifice much of this versatility by design. To parallelize and automate the traditional patch clamp technique while perpetuating the full versatility of the method, we developed an approach to automation, which is based on active cell handling and targeted electrode placement rather than on random processes. This is achieved through an automated pipette positioning system, which guides the tips of recording pipettes with micrometer precision to a microfluidic cell handling device. Using a patch pipette array mounted on a conventional micromanipulator, our automated patch clamp process mimics the original manual patch clamp as closely as possible, yet achieving a configuration where recordings are obtained from many patch electrodes in parallel. In addition, our implementation is extensible by design to allow the easy integration of specialized equipment such as ultrafast compound application tools. The resulting system offers fully automated patch clamp on purposely selected cells and combines high-quality gigaseal recordings with solution switching in the millisecond timescale.

Early identification of hERG liability in drug discovery programs by automated patch clamp

Blockade of the cardiac ion channel coded by human ether-à-gogo-related gene (hERG) can lead to cardiac arrhythmia, which has become a major concern in drug discovery and development. Automated electrophysiological patch clamp allows assessment of hERG channel effects early in drug development to aid medicinal chemistry programs and has become routine in pharmaceutical companies. However, a number of potential sources of errors in setting up hERG channel assays by automated patch clamp can lead to misinterpretation of data or false effects being reported. This article describes protocols for automated electrophysiology screening of compound effects on the hERG channel current. Protocol details and the translation of criteria known from manual patch clamp experiments to automated patch clamp experiments to achieve good quality data are emphasized. Typical pitfalls and artifacts that may lead to misinterpretation of data are discussed. While this article focuses on hERG channel recordings using the QPatch (Sophion A/S, Copenhagen, Denmark) technology, many of the assay and protocol details given in this article can be transferred for setting up different ion channel assays by automated patch clamp and are similar on other planar patch clamp platforms.

Dominant-negative effects of KCNQ2 mutations are associated with epileptic encephalopathy

OBJECTIVE

Mutations in KCNQ2 and KCNQ3, encoding the voltage-gated potassium channels KV 7.2 and KV 7.3, are known to cause benign familial neonatal seizures mainly by haploinsufficiency. Here, we set out to determine the disease mechanism of 7 de novo missense KCNQ2 mutations that were recently described in patients with a severe epileptic encephalopathy including pharmacoresistant seizures and pronounced intellectual disability.

METHODS

Mutations were inserted into the KCNQ2 cDNA. Potassium currents were recorded using 2-microelectrode voltage clamping, and surface expression was analyzed by a biotinylation assay in cRNA-injected Xenopus laevis oocytes.

RESULTS

We observed a clear loss of function for all mutations. Strikingly, 5 of 7 mutations exhibited a drastic dominant-negative effect on wild-type KV 7.2 or KV 7.3 subunits, either by globally reducing current amplitudes (3 pore mutations) or by a depolarizing shift of the activation curve (2 voltage sensor mutations) decreasing potassium currents at the subthreshold level at which these channels are known to critically influence neuronal firing. One mutation significantly reduced surface expression. Application of retigabine, a recently marketed KV 7 channel opener, partially reversed these effects for the majority of analyzed mutations.

INTERPRETATION

The development of severe epilepsy and cognitive decline in children carrying 5 of the 7 studied KCNQ2 mutations can be related to a dominant-negative reduction of the resulting potassium current at subthreshold membrane potentials. Other factors such as genetic modifiers have to be postulated for the remaining 2 mutations. Retigabine or similar drugs may be used as a personalized therapy for this severe disease.

Efficient binding of 4/7 α-conotoxins to nicotinic α4β2 receptors is prevented by Arg185 and Pro195 in the α4 subunit

α-Conotoxins are subtype-selective nicotinic acetylcholine receptor (nAChR) antagonists. Although potent α3β2 nAChR-selective α-conotoxins have been identified, currently characterized α-conotoxins show no or only weak affinity for α4β2 nAChRs, which are, besides α7 receptors, the most abundant nAChRs in the mammalian brain. To identify the determinants responsible for this difference, we substituted selected amino acid residues in the ligand-binding domain of the α4 subunit by the corresponding residues in the α3 subunit. Two-electrode voltage clamp analysis of these mutants revealed increased affinity of α-conotoxins MII, TxIA, and [A10L]TxIA at the α4(R185I)β2 receptor. Conversely, α-conotoxin potency was reduced at the reverse α3(I186R)β2 mutant. Replacement of α4Arg185 by alanine, glutamate, and lysine demonstrated that a positive charge in this position prevents α-conotoxin binding. Combination of the R185I mutation with a P195Q mutation outside the binding site but in loop C completely transferred high α-conotoxin potency to the α4β2 receptor. Molecular dynamics simulations of homology models with docked α-conotoxin indicate that these residues control access to the α-conotoxin binding site.

Local administration of DFO-loaded lipid particles improves recovery after end-to-end reconstruction of rat median nerve

PURPOSE

The improvement of regeneration and functional recovery after peripheral nerve injury is a major challenge in neurosurgery. Although microsurgical techniques for nerve reconstruction have seen great advancements over the last years, the clinical outcome with patients is often unsatisfactory. The aim of the present study was to investigate if administration of the iron chelator Deferroxamine (DFO), can improve postoperative outcome in the rat median nerve reconstruction model.

METHODS

After complete transection, the right median nerve was repaired by end-to-end neurorrhaphy. The suture site was wrapped by a 1-cm-long external jugular vein segment, either empty or filled with DFO-loaded lipid particles (Perineurin or with a vehicle (unloaded lipid particles) alone. Functional testing was carried out weekly by means of the grasping test. At the time of withdrawal, 12 weeks post-operatively, muscle tropism recovery was assessed by weighing flexor digitorum sublimis muscle that is innervated by the median nerve only. Before harvesting of the nerve specimens electrophysiological analyses were performed with measuring the latency, the threshold and the conduction velocity. Finally, the repaired nerves were withdrawn for immunocytochemistry with a neurofilament antibody and axon quantitative morphology.

RESULTS

The comparison between the groups showed that intraoperative application of the DFO-loaded lipid particles at the neurorrhaphy site led to a significant increase in the density of regenerating axons as well as to an accelerated recovery of both muscle tropism and motor function. The electrophysiological results demonstrated a decrease of the threshold, a lower latency, and a higher conduction velocity in the Perineurin-treated animals.

CONCLUSIONS

The results of the present study suggest that local administration of Perineurin might have a therapeutic potential for improving the postoperative outcome after microsurgical nerve reconstruction in patients.

Expansion and differentiation of neural progenitors derived from the human adult enteric nervous system

BACKGROUND & AIMS

Neural stem and progenitor cells from the enteric nervous system have been proposed for use in cell-based therapies against specific neurogastrointestinal disorders. Recently, enteric neural progenitors were generated from human neonatal and early postnatal (until 5 years after birth) gastrointestinal tract tissues. We investigated the proliferation and differentiation of enteric nervous system progenitors isolated from human adult gastrointestinal tract.

METHODS

Human enteric spheroids were generated from adult small and large intestine tissues and then expanded and differentiated, depending on the applied cell culture conditions. For implantation studies, spheres were grafted into fetal slice cultures and embryonic aganglionic hindgut explants from mice. Differentiating enteric neural progenitors were characterized by 5-bromo-2-deoxyuridine labeling, in situ hybridization, immunocytochemistry, quantitative real-time polymerase chain reaction, and electrophysiological studies.

RESULTS

The yield of human neurosphere-like bodies was increased by culture in conditional medium derived from fetal mouse enteric progenitors. We were able to generate proliferating enterospheres from adult human small or large intestine tissues; these enterospheres could be subcultured and maintained for several weeks in vitro. Spheroid-derived cells could be differentiated into a variety of neuronal subtypes and glial cells with characteristics of the enteric nervous system. Experiments involving implantation into organotypic intestinal cultures showed the differentiation capacity of neural progenitors in a 3-dimensional environment.

CONCLUSIONS

It is feasible to isolate and expand enteric progenitor cells from human adult tissue. These findings offer new strategies for enteric stem cell research and future cell-based therapies.

Passive transport of macromolecules through Xenopus laevis nuclear envelope

Although nuclear pore complexes (NPC) are considered to be key structures in gene expression, little is known about their regulatory control. In order to explore the regulatory mechanism of passive transport of small macromolecules we examined the influence of different factors on the diffusional pathway of NPCs in isolated Xenopus laevis oocyte nuclei. Diffusion of fluorescence-labeled 10-kD dextran was measured across the nuclear envelope with confocal fluorescence microscopy. Surprisingly, the filling state of the perinuclear Ca(2+) store had no influence on passive transport of 10-kD dextran. Furthermore, nuclear envelope permeability was independent of cytoplasmic pH (pH range 8.3-6.3). In contrast, nuclear swelling, induced by omission of the endogenous cytosolic macromolecules, clearly increased nuclear permeability. An antibody against the glycoprotein gp62, located at the central channel entrance, reduced macromolecule diffusion. In addition, nuclei from transcriptionally active, early developmental stages (stage II) were less permeable compared to transcriptionally inactive, late-developmental-stage (stage VI) nuclei. In stage II nuclei, atomic force microscopy disclosed NPC central channels with plugs that most likely were ribonucleoproteins exiting the nucleus. In conclusion, the difference between macromolecule permeability and previous measurements of electrical resistance strongly indicates separate routes for macromolecules and ions across the nuclear envelope.

Electrophoretic plugging of nuclear pores by using the nuclear hourglass technique

The nuclear hourglass technique (NHT) was recently introduced as a novel technique that measures the electrical nuclear envelope (NE) conductance of isolated Xenopus laevis oocyte nuclei. The main conclusion drawn from NHT work so far is that nuclear pore complexes (NPCs) of oocytes are in an electrically open state under physiological conditions, with a mean conductance of 1.7 nS per NPC. Since nuclear patch-clamp data indicate that usually NPCs are electrically closed, our work has been challenged by the notion that NHT cannot assure a high resistance seal ("gigaseal") between glass wall and NE like that required for patch-clamp experiments. Thus, NHT could have dramatically underestimated NE electrical resistance. Here we demonstrate that NHT does not require a gigaseal for accurate NE conductance measurements. In addition, we present experimental conditions where mean single NPC electrical conductance is reduced 26-fold due to electrophoretic plugging by negatively charged nucleoplasmic macromolecules. In addition, data indicate that under physiological conditions (i.e., when macromolecules are offered in the cytosolic solution) the nuclear surface is heavily folded, underestimating "true" NE surface by a factor of 2.6. When "true" NE surface area is taken into consideration, modified values of mean single NPC conductances of 654 pS for electrically open conditions and 25 pS for electrically plugged conditions can be calculated. We conclude that the large overall NE conductance detected with the nuclear hourglass technique in intact Xenopus laevis oocyte nuclei can be explained by the sum of single NPC conductances in the pS range, as long as open probability is high. This confirms previous patch-clamp work concerning single NPC conductance, but disagrees with the view that mean open probability of NPC channels is usually low.

Evidence for Ca2+- and ATP-sensitive peripheral channels in nuclear pore complexes

In eukaryotic cells the nuclear envelope (NE) serves as a functional barrier between cytosol and nucleoplasm perforated by nuclear pore complexes (NPCs). Both active and passive transport of ions and macromolecules are thought to be mediated by the centrally located large NPC channel. However, 3-dimensional imaging of NPCs based on electron microscopy indicates the existence of additional small channels of unknown function located in the NPC periphery. By means of the recently developed nuclear hourglass technique that measures NE electrical conductance, we evaluated passive electrically driven transport through NPCs. In isolated Xenopus laevis oocyte nuclei, we varied ambient Ca2+ and ATP in the cytosolic solution and/or chelated Ca2+ in the perinuclear stores in order to assess the role of Ca2+ in regulating passive ion transport. We noticed that NE electrical conductance is large under conditions where macromolecule permeability is known to be low. In addition, atomic force microscopy applied to native NPCs detects multiple small pores in the NPC periphery consistent with channel openings. Peripheral pores were detectable only in the presence of ATP. We conclude that NPC transport of ions and macromolecules occurs through different routes. We present a model in which NE ion flux does not occur through the central NPC channel but rather through Ca2+- and ATP-activated peripheral channels of individual NPCs.

Plasma membrane protein clusters appear in CFTR-expressing Xenopus laevis oocytes after cAMP stimulation

Membrane trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) is supposed to be an important mechanism controlled by the intracellular messenger cAMP. This has been shown with fluorescence techniques, electron microscopy and membrane capacitance measurements. In order to visualize protein insertion we applied atomic force microscopy (AFM) to inside-out oriented plasma membrane patches of CFTR-expressing Xenopus laevis oocytes before and after cAMP-stimulation. In a first step, oocytes injected with CFTR-cRNA were voltage-clamped, verifying successful CFTR expression. Water-injected oocytes served as controls. Then, plasma membrane patches were excised, placed (inside out) on glass and scanned by AFM. Before cAMP-stimulation plasma membranes of both water-injected and CFTR-expressing oocytes contained about 200 proteins per micron 2. Molecular protein masses were estimated from molecular volumes measured by AFM. Before cAMP-stimulation, protein distribution showed a peak value of 11 nm protein height corresponding to 475 kDa. During cAMP-stimulation with 1 mM isobutylmethylxanthine (IBMX) plasma membrane protein density increased in water-injected oocytes to 700 proteins per micron 2 while the peak value shifted to 7 nm protein height corresponding to 95 kDa. In contrast, CFTR-expressing oocytes showed after cAMP-stimulation about 400 proteins per micron 2 while protein distribution exhibited two peak values, one peak at 10 nm protein height corresponding to 275 kDa and another one at 14 nm corresponding to 750 kDa. They could represent heteromeric protein clusters associated with CFTR. In conclusion, we visualized plasma membrane protein insertion upon cAMP-stimulation and quantified protein distribution with AFM at molecular level. We propose that CFTR causes clustering of plasma membrane proteins.

Plasma membrane plasticity of Xenopus laevis oocyte imaged with atomic force microscopy

Proteins are known to form functional clusters in plasma membranes. In order to identify individual proteins within clusters we developed a method to visualize by atomic force microscopy (AFM) the cytoplasmic surface of native plasma membrane, excised from Xenopus laevis oocyte and spread on poly-L-lysine coated glass. After removal of the vitelline membrane intact oocytes were brought in contact with coated glass and then rolled off. Inside-out oriented plasma membrane patches left at the glass surface were first identified with the lipid fluorescent marker FM1-43 and then scanned by AFM. Membrane patches exhibiting the typical phospholipid bilayer height of 5 nm showed multiple proteins, protruding from the inner surface of the membrane, with heights of 5 to 20 nm. Modelling plasma membrane proteins as spherical structures embedded in the lipid bilayer and protruding into the cytoplasm allowed an estimation of the respective molecular masses. Proteins ranged from 35 to 2,000 kDa with a peak value of 280 kDa. The most frequently found membrane protein structure (40/microm2) had a total height of 10 nm and an estimated molecular mass of 280 kDa. Membrane proteins were found firmly attached to the poly-L-lysine coated glass surface while the lipid bilayer was found highly mobile. We detected protein structures with distinguishable subunits of still unknown identity. Since X. laevis oocyte is a generally accepted expression system for foreign proteins, this method could turn out to be useful to structurally identify specific proteins in their native environment at the molecular level.

Nuclear pore function viewed with atomic force microscopy

In this review we focus on studies using atomic force microscopy (AFM) to describe the function of nuclear pore complexes (NPC). After a short introduction of AFM we follow the route of cargo molecules from the cytosol into the nucleus. AFM visualizes cargo before translocation into the nucleoplasm, cargo docking at the cytoplasmic NPC surface, cargo passing through the NPC and changes in NPC conformation in response to ATP, Calcium and pH. We discuss AFM experiments on nuclear envelopes on the basis of previous data obtained with more conventional techniques such as electron microscopy, confocal microscopy and other imaging techniques. Finally we draw attention to the recently developed nuclear hourglass technique that serves as a new electrophysiological approach to studying the structure-function relationship of NPC in combination with AFM at a molecular level.

Nuclear pores collapse in response to CO2 imaged with atomic force microscopy

Nuclear pore complexes (NPCs) are the rate-limiting barriers for the exchange of macromolecules (e.g. transcription factors or mRNA) between the nuclear and cytosolic compartments. NPC conformation determines movement of cargo in either direction and thus controls gene expression. ATP and calcium are known to induce an NPC shape change (increase in height and decrease in diameter) indicating pore contraction. Here we report a CO2-induced shape change which is different to the ATP/calcium response. Experiments were performed on the isolated nuclear envelope of Xenopus laevis oocytes. The nuclear envelope was spread on glass and the native cytoplasmic surface was imaged with atomic force microscopy (AFM). The preparation was scanned in a water-saturated 100% O2 atmosphere at room temperature. Exposure to 5% CO2 (95%O2) led over a time course of minutes to a dramatic NPC shape change (decrease in height and decrease in diameter) indicating pore closure. NPCs turned flat and central channel openings virtually disappeared. The CO2 response was only slowly reversible. We conclude that NPCs apparently collapse in response to CO2, a structural change that could lead to the functional isolation of the cell nucleus.

Volume dynamics in migrating epithelial cells measured with atomic force microscopy

Migration of transformed renal epithelial cells (transformed Madin-Darby canine kidney cells, MDCK-F cells) relies on the activity of a Ca(2+)-sensitive K+ channel (IK channel) that is more active at the rear end of these cells. We have postulated that intermittent IK channel activity induces local cell shrinkage at the rear end of migrating MDCK-F cells and thereby supports the cytoskeletal mechanisms of migration. However, due to the complex morphology of MDCK-F cells we have not yet been able to measure volume changes directly. The aim of the present study was to devise a new technique employing atomic force microscopy (AFM) to measure the volume of MDCK-F cells in their physiological environment and to demonstrate its dependence on IK channel activity. The spatial (x, y' and z) co-ordinates of each pixel of the three-dimensional image of MDCK-F cells allow calculation of the volume of the column "underneath" a given pixel. Thus, total cell volume is the sum of all pixel-defined columns. The mean volume of 17 MDCK-F cells was 2500+/-300 fl. Blockade of the IK channel with the specific inhibitor charybdotoxin (CTX) increased cell volume by 17+/-4%; activation of IK by elevating the intracellular [Ca2+] with the Ca2+ ionophore ionomycin decreased cell volume by 19+/-3%. Subtraction images (experimental minus control) reveal that swelling and shrinkage occur predominantly at the rear end of MDCK-F cells. In summary, our experiments show that AFM allows the measurement not only of total cell volume of living cells in their physiological environment but also the tracing of local effects induced by the polarized distribution of K+ channel activity.

Nuclear hourglass technique: an approach that detects electrically open nuclear pores in Xenopus laevis oocyte

Nuclear pore complexes (NPCs) mediate both active transport and passive diffusion across the nuclear envelope (NE). Determination of NE electrical conductance, however, has been confounded by the lack of an appropriate technical approach. The nuclear patch clamp technique is restricted to preparations with electrically closed NPCs, and microelectrode techniques fail to resolve the extremely low input resistance of large oocyte nuclei. To address the problem, we have developed an approach for measuring the NE electrical conductance of Xenopus laevis oocyte nuclei. The method uses a tapered glass tube, which narrows in its middle part to 2/3 of the diameter of the nucleus. The isolated nucleus is sucked into the narrow part of the capillary by gentle fluid movement, while the resulting change in electrical resistance is monitored. NE electrical conductance was unexpectedly large (7.9 +/- 0.34 S/cm(2)). Evaluation of NPC density by atomic force microscopy showed that this conductance corresponded to 3.7 x 10(6) NPCs. In contrast to earlier conclusions drawn from nuclear patch clamp experiments, NPCs were in an electrically "open" state with a mean single NPC electrical conductance of 1.7 +/- 0.07 nS. Enabling or blocking of active NPC transport (accomplished by the addition of cytosolic extracts or gp62-directed antibodies) revealed this large NPC conductance to be independent of the activation state of the transport machinery located in the center of NPCs. We conclude that peripheral channels, which are presumed to reside in the NPC subunits, establish a high ionic permeability that is virtually independent of the active protein transport mechanism.

ATP-Induced shape change of nuclear pores visualized with the atomic force microscope

Bidirectional transport of molecules between nucleus and cytoplasm through the nuclear pore complexes (NPCs) spanning the nuclear envelope plays a fundamental role in cell function and metabolism. Nuclear import of macromolecules is a two-step process involving initial recognition of targeting signals, docking to the pore and energy-driven translocation. ATP depletion inhibits the translocation step. The mechanism of translocation itself and the conformational changes of the NPC components that occur during macromolecular transport, are still unclear. The present study investigates the effect of ATP on nuclear pore conformation in isolated nuclear envelopes from Xenopus laevis oocytes using the atomic force microscope. All experiments were conducted in a saline solution mimicking the cytosol using unfixed nuclear envelopes. ATP (1 mM) was added during the scanning procedure and the resultant conformational changes of the NPCs were directly monitored. Images of the same nuclear pores recorded before and during ATP exposure revealed dramatic conformational changes of NPCs subsequent to the addition of ATP. The height of the pores protruding from the cytoplasmic surface of the nuclear envelope visibly increased while the diameter of the pore opening decreased. The observed changes occurred within minutes and were transient. The slow-hydrolyzing ATP analogue, ATP-gamma-S, in equimolar concentrations did not exert any effects. The ATP-induced shape change could represent a nuclear pore "contraction."

Using atomic force microscopy to investigate patch-clamped nuclear membrane

Nuclear patch clamp is an emerging research field that aims to disclose the electrical phenomena underlying macromolecular transport across the nuclear envelope (NE), its properties as an ion barrier and its function as an intracellular calcium store. The authors combined the patch clamp technique with atomic force microscopy (AFM) to investigate the structure-function relationship of NE. In principle, patch clamp currents, recorded from the NE can indicate the activity of the nuclear pore complexes (NPCs) and/or of ion channels in the two biomembranes that compose the NE. However, the role of the NPCs is still nuclear because the observed NE current in patch clamp experiments is lower than expected from the known density of the NPCs. Therefore, AFM was applied to link patch clamp currents to structure. The membrane patch was excised from the nuclear envelope and, after electrical evaluation, transferred from the patch pipette to a substrate. We could identify the native nuclear membrane patches with AFM at a lateral and a vertical resolution of 3 nm and 0.1 nm, respectively. It was shown that complete NE together with NPCs can be excised from the nucleus after their functional identification in patch clamp experiments. However, we also show that membranes of the endoplasmic reticulum can contaminate the tip of the patch pipette during nuclear patch clamp experiments. This possibility must be considered carefully in nuclear patch clamp experiments.

Imaging excised apical plasma membrane patches of MDCK cells in physiological conditions with atomic force microscopy

We combined the patch-clamp technique with atomic force microscopy (AFM) to visualize plasma membrane proteins protruding from the extracellular surface of cultured kidney cells (MDCK cells). To achieve molecular resolution, patches were mechanically isolated from whole MDCK cells by applying the patch-clamp technique. The excised inside-out patches were transferred on freshly cleaved mica and imaged with the AFM in air and under physiological conditions (i. e. in fluid). Thus, the resolution could be increased considerably (lateral and vertical resolutions 5 and 0.1 nm, respectively) as compared to experiments on intact cells, where plasma membrane proteins were hardly detectable. The apical plasma membrane surface of the MDCK cells showed multiple protrusions which could be identified as membrane proteins through the use of pronase. These proteins had a density of about 90 per micron(2), with heights between 1 and 9 nm, and lateral dimensions of 20-60 nm. Their frequency distribution showed a peak value of 3 nm for the protein height. A simplified assumption - modelling plasma membrane proteins as spherical structures protruding from the lipid bilayer - allowed an estimation of the possible molecular weights of these proteins. They range from 50 kDa to 710 kDa with a peak value of 125 kDa. We conclude that AFM can be used to study the molecular structures of membranes which were isolated with the patch-clamp technique. Individual membrane proteins and protein clusters, and their arrangement and distribution in a native plasma membrane can be visualized under physiological conditions, which is a first step for their identification.

Intracellular Ca2+ distribution in migrating transformed epithelial cells

Migration of transformed Madin-Darby canine kidney (MDCK-F) cells depends on the polarized activity of a Ca2+-sensitive K+ channel. We tested whether a gradient of intracellular Ca2+-concentration ([Ca2+]i) underlies the horizontal polarization of K+ channel activity. [Ca2+]i was measured with the fluorescent dye fura-2/AM. Spatial analysis of [Ca2+]i indicated that a horizontal gradient exists, with [Ca2+]i being higher in the cell body than in the lamellipodium. Resting and maximal levels during oscillations of [Ca2+]i in the cell body were found to be 135 +/- 34 and 405 +/- 59 nml/l, respectively, whereas they were 79 +/- 18 and 307 +/- 102 nmol/l in the lamellipodium. This gradient can partially explain the preferential activation of K+ channels in the plasma membrane of the cell body. We applied a local superfusion technique during migration experiments and measurements of [Ca2+]i to test whether its maintenance is due to an uneven distribution of Ca2+ influx into migrating MDCK-F cells. Locally superfusing the cell body of migrating MDCK-F cells with La3+ alone or together with charybdotoxin, a specific blocker of Ca2+-sensitive K+ channels, slowed migration to 47 +/- 10% and 9 +/- 5% of control, respectively. Local blockade of Ca2+ influx into the cell body and the lamellipodium with la3+ was followed by a decrease of [Ca2+]i at both cell poles. This points to Ca2+ influx occurring over the entire cell surface. This conclusion was confirmed by locally superfusing Mn2+ over the cell body and the lamellipodium. Fura-2 fluorescence was quenched in both areas, the decrease of fluorescence being two to three times faster in the cell body than in the lamellipodium. However, this difference is insufficient to account for the observed gradient of [Ca2+]i. We hypothesize that the polarized distribution of intracellular Ca2+ stores contributes significantly to the generation of a gradient of [Ca2+]i.

Extracellular detection of K+ release during migration of transformed Madin-Darby canine kidney cells

Madin Darby canine kidney cells transformed by alkaline stress (MDCK-F cells) constitutively migrate at a rate of about 1 microm.min-1. Migration depends on the intermittent activity of a Ca2+-stimulated, 53-pS K+ channel (KCa channel) that is inhibitable by charybdotoxin. In the present study we examined whether this intermittent KCa channel activity results in a significant K+ loss across the plasma membrane. K+ efflux from MDCK-F cells should result in a transient increase of extracellular K+ ([K+]e) in the close vicinity of a migrating cell. However, due to the rapid diffusion of K+ ions into the virtually infinite extracellular space, such a transient increase in [K+]e was too small to be detected by conventional K+-selective electrodes. Therefore, we developed a "shielded ion-sensitive microelectrode" (SIM) that limited diffusion to a small compartment, formed by a shielding pipette which surrounded the tip of the K+-sensitive microelectrode. The SIM improved the signal to noise ratio by a factor of at least three, thus transient increases of [K+]e in the vicinity of MDCK-F cells became detectable. They occurred at a rate of 1.3 min-1. The cell releases 40 fmol K+ during each burst of intermittent KCa channel activity, which corresponds to about 15% of the total cellular K+ content. Since transmembrane K+ loss must be accompanied by anion loss and therefore leads to a decrease of cell volume, these findings support the hypothesis that intermittent volume changes are a prerequisite for the migration of MDCK-F cells.