The Experts below are selected from a list of 168 Experts worldwide ranked by ideXlab platform
Werner Baumgartner - One of the best experts on this subject based on the ideXlab platform.
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Model of water and electrolyte transport through simple epithelia and IC width regulation via LI-cadherins.
2018Co-Authors: Yana Vereshchaga, Nikita Arnold, Werner BaumgartnerAbstract:a) The model comprises four compartments, which are (1) the lumen of the organ (e.g. the gut), (2) the lateral Intercellular Cleft (IC), where the values of c(y), v(y), p(y) are calculated, (3) the cytoplasm of the cell, and (4) the interstitial tissue. In the lumen a given concentration of electrolytes is assumed. The tight junctions (TJ) separate the lumen (1) and the IC (2), and are assumed to be impermeable to the electrolyte and permeable to water with a permeability coefficient kTJ. The concentration of electrolytes in the cytoplasm is assumed to be constant c3 (except in the case in which we explored its influence on the direction of water flow). ATPases are assumed to pump the electrolyte through the lateral membrane into the lateral Intercellular Cleft. The interstitial tissue is assumed to exhibit a constant electrolyte concentration c4, which is maintained by the blood vessels located there. The important compartment is the IC. Water enters this compartment through the TJ, aquaporins or from the interstitial tissue. Ions enter through the lateral membrane due to the ATPases and leave the IC due to diffusion and due to the water flux, flushing the lateral Intercellular Cleft. b) The width of the lateral Intercellular Cleft b depends on the binding activity of the 7D-cadherins, which in turn depends on the extracellular Ca2+ level. High Ca2+ concentration triggers LI-cadherin binding and low one courses protein disruption. In the publication [12] we reported the results with a designed peptide that disrupts the LI-cadherins in the epithelium CACO2 and the Cleft becomes broader as a result.
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Physiological relevance of epithelial geometry: New insights into the standing gradient model and the role of LI cadherin
2018Co-Authors: Yana Vereshchaga, Nikita Arnold, Werner BaumgartnerAbstract:We introduce a mathematical model of an absorbing leaky epithelium to reconsider the problem formulated by Diamond and Bossert in 1967: whether “… some distinctive physiological properties of epithelia might arise as geometrical consequences of epithelial ultrastructure”. A standing gradient model of the Intercellular Cleft (IC) is presented that includes tight junctions (TJ) and ion channels uniformly distributed along the whole Cleft. This nonlinear system has an intrinsic homogeneous concentration and the spatial scale necessary to establish it along the Cleft. These parameters have not been elucidated so far. We further provide non-perturbative analytical approximations for a broad range of parameters. We found that narrowing of the IC increases ion concentration dramatically and can therefore prevent outflow through tight junctions (TJs) and the lateral membrane, as long as extremely high luminal osmolarities are not reached. Our model predicts that the system is to some extent self-regulating and thereby prevents fluxes into the lumen. Recent experimental evidence has shown that liver-intestine (LI) cadherin can control the up/down flux in intestines via regulation of the Cleft width. This finding is in full agreement with predictions of our model. We suggest that LI-cadherin may increase water transport through epithelia via sequential narrowing of the Cleft, starting from the highest concentration area at the beginning of the Cleft and triggering a propagating squeezing motion.
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Water transport through the intestinal epithelial barrier under different osmotic conditions is dependent on LI-cadherin trans-interaction.
Tissue barriers, 2017Co-Authors: Agnes Weth, Yana Vereshchaga, Carsten Dippl, Yasmin Striedner, Irene Tiemann-boege, Nikola Golenhofen, Britta Bartelt-kirbach, Werner BaumgartnerAbstract:In the intestine water has to be reabsorbed from the chymus across the intestinal epithelium. The osmolarity within the lumen is subjected to high variations meaning that water transport often has to take place against osmotic gradients. It has been hypothesized that LI-cadherin is important in this process by keeping the Intercellular Cleft narrow facilitating the buildup of an osmotic gradient allowing water reabsorption. LI-cadherin is exceptional among the cadherin superfamily with respect to its localization along the lateral plasma membrane of epithelial cells being excluded from adherens junction. Furthermore it has 7 but not 5 extracellular cadherin repeats (EC1-EC7) and a small cytosolic domain. In this study we identified the peptide VAALD as an inhibitor of LI-cadherin trans-interaction by modeling the structure of LI-cadherin and comparison with the known adhesive interfaces of E-cadherin. This inhibitory peptide was used to measure LI-cadherin dependency of water transport through a monolayer of epithelial CACO2 cells under various osmotic conditions. If LI-cadherin trans-interaction was inhibited by use of the peptide, water transport from the luminal to the basolateral side was impaired and even reversed in the case of hypertonic conditions whereas no effect could be observed at isotonic conditions. These data are in line with a recently published model predicting LI-cadherin to keep the width of the lateral Intercellular Cleft small. In this narrow Cleft a high osmolarity can be achieved due to ion pumps yielding a standing osmotic gradient allowing water absorption from the gut even if the faeces is highly hypertonic.
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The function of 7D-cadherins: a mathematical model predicts physiological importance for water transport through simple epithelia.
Theoretical biology & medical modelling, 2011Co-Authors: Mareike Ahl, Agnes Weth, Sebastian Walcher, Werner BaumgartnerAbstract:Background: 7D-cadherins like LI-cadherin are cell adhesion molecules and represent exceptional members of the cadherin superfamily. Although LI-cadherin was shown to act as a functional Ca 2+ -dependent adhesion molecule, linking neighboring cells together, and to be dysregulated in a variety of diseases, the physiological role is still enigmatic. Interestingly 7D-cadherins occur only in the lateral plasma membranes of cells from epithelia of water transporting tissues like the gut, the liver or the kidney. Furthermore LI-cadherin was shown to exhibit a highly cooperative Ca 2+ -dependency of the binding activity. Thus it is tempting to assume that LI-cadherin regulates the water transport through the epithelium in a passive fashion by changing its binding activity in dependence on the extracellular Ca 2+ . Results: We developed a simple mathematical model describing the epithelial lining of a lumen with a content of variable osmolarity covering an interstitium of constant osmolarity. The width of the lateral Intercellular Cleft was found to influence the water transport significantly. In the case of hypertonic luminal content a narrow Cleft is necessary to further increase concentration of the luminal content. If the Cleft is too wide, the water flux will change direction and water is transported into the lumen. Electron microscopic images show that in fact areas of the gut can be found where the lateral Intercellular Cleft is narrow throughout the lateral cell border whereas in other areas the lateral Intercellular Cleft is widened. Conclusions: Our simple model clearly predicts that changes of the width of the lateral Intercellular Cleft can regulate the direction and efficiency of water transport through a simple epithelium. In a narrow Cleft the cells can increase the concentration of osmotic active substances easily by active transport whereas if the Cleft is wide, friction is reduced but the cells can hardly build up high osmotic gradients. It is now tempting to speculate that 7D-cadherins, owing to their location and their Ca 2+ -dependence, will adapt their binding activity and thereby the width of the lateral Intercellular Cleft automatically as the Ca 2+ -concentration is coupled to the overall electrolyte concentration in the lateral Intercellular Cleft. This could provide a way to regulate the water resorption in a passive manner adapting to different osmotic conditions.
Steven Poelzing - One of the best experts on this subject based on the ideXlab platform.
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Intercellular Sodium Regulates Repolarization in Cardiac Tissue with Sodium Channel Gain of Function.
Biophysical journal, 2020Co-Authors: Madison B. Nowak, Amara Greer-short, Xiaoping Wan, Isabelle Deschenes, Seth H. Weinberg, Steven PoelzingAbstract:In cardiac myocytes, action potentials are initiated by an influx of sodium (Na+) ions via voltage-gated Na+ channels. Na+ channel gain of function (GOF), arising in both inherited conditions associated with mutation in the gene encoding the Na+ channel and acquired conditions associated with heart failure, ischemia, and atrial fibrillation, enhance Na+ influx, generating a late Na+ current that prolongs action potential duration (APD) and triggering proarrhythmic early afterdepolarizations (EADs). Recent studies have shown that Na+ channels are highly clustered at the myocyte intercalated disk, facilitating formation of Na+ nanodomains in the Intercellular Cleft between cells. Simulations from our group have recently predicted that narrowing the width of the Intercellular Cleft can suppress APD prolongation and EADs in the presence of Na+ channel mutations because of increased Intercellular Cleft Na+ ion depletion. In this study, we investigate the effects of modulating multiple extracellular spaces, specifically the Intercellular Cleft and bulk interstitial space, in a novel computational model and experimentally via osmotic agents albumin, dextran 70, and mannitol. We perform optical mapping and transmission electron microscopy in a drug-induced (sea anemone toxin, ATXII) Na+ channel GOF isolated heart model and modulate extracellular spaces via osmotic agents. Single-cell patch-clamp experiments confirmed that the osmotic agents individually do not enhance late Na+ current. Both experiments and simulations are consistent with the conclusion that Intercellular Cleft narrowing or expansion regulates APD prolongation; in contrast, modulating the bulk interstitial space has negligible effects on repolarization. Thus, we predict that Intercellular Cleft Na+ nanodomain formation and collapse critically regulates cardiac repolarization in the setting of Na+ channel GOF.
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Revealing the Concealed Nature of Long-QT Type 3 Syndrome.
Circulation. Arrhythmia and electrophysiology, 2017Co-Authors: Amara Greer-short, Steven Poelzing, Sharon A. George, Seth H. WeinbergAbstract:Gain-of-function mutations in the voltage-gated sodium channel (Nav1.5) are associated with the long-QT-3 (LQT3) syndrome. Nav1.5 is densely expressed at the intercalated disk, and narrow Intercellular separation can modulate cell-to-cell coupling via extracellular electric fields and depletion of local sodium ion nanodomains. Models predict that significantly decreasing Intercellular Cleft widths slows conduction because of reduced sodium current driving force, termed "self-attenuation." We tested the novel hypothesis that self-attenuation can "mask" the LQT3 phenotype by reducing the driving force and late sodium current that produces early afterdepolarizations (EADs). Acute interstitial edema was used to increase Intercellular Cleft width in isolated guinea pig heart experiments. In a drug-induced LQT3 model, acute interstitial edema exacerbated action potential duration prolongation and produced EADs, in particular, at slow pacing rates. In a computational cardiac tissue model incorporating extracellular electric field coupling, Intercellular Cleft sodium nanodomains, and LQT3-associated mutant channels, myocytes produced EADs for wide Intercellular Clefts, whereas for narrow Clefts, EADs were suppressed. For both wide and narrow Clefts, mutant channels were incompletely inactivated. However, for narrow Clefts, late sodium current was reduced via self-attenuation, a protective negative feedback mechanism, masking EADs. We demonstrated a novel mechanism leading to the concealing and revealing of EADs in LQT3 models. Simulations predict that this mechanism may operate independent of the specific mutation, suggesting that future therapies could target Intercellular Cleft separation as a compliment or alternative to sodium channels. © 2017 American Heart Association, Inc.
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Ephaptic Self-Attenuation Conceals Early Afterdepolarizations Associated with Long QT-3 Syndrome
Biophysical Journal, 2016Co-Authors: Steven Poelzing, Amara Greer-short, Donald K. Jessup, Seth H. WeinbergAbstract:Gain-of-function mutations in the cardiac voltage-gated sodium channel (Nav1.5) are associated with the long QT-3 (LQT3) syndrome. Early afterdepolarizations (EADs) are common in isolated myocyte models of LQT3, but rare in tissue and patients with LQT3 mutations. We have shown that Nav1.5 is densely expressed at the intercalated disk and narrowing Intercellular separation can support an alternative form of cell-to-cell coupling known as ephaptic coupling (EpC). EpC occurs when sodium channels in the depolarizing cell decrease the Intercellular Cleft potential, depolarizing the apposing membrane from the extracellular rather than the intracellular domain. Critically, EpC models predict that decreasing Intercellular separation slows conduction due to reduced sodium current driving force. Here, we test the novel hypothesis that ephaptic self-attenuation can “mask” the LQT3 phenotype by reducing the driving force and late sodium current that produces EADs.We tested our hypothesis both in a pharmacological LQT3 model in isolated guinea pig hearts experiments (n=3), paced at 500 ms basic cycle length using late sodium current agonist ATXII (7nM), and a computational model. Acute interstitial edema, induced by 20 g/l mannitol, increased Intercellular Cleft width but did not alter action potential duration (APD). ATXII prolonged APD by 80±11 ms, and ATXII plus mannitol prolonged APD by 165±7 ms and produced EADs. In a computational model incorporating EpC, a recent LRd formulation, and a sodium channel LQT3-associated mutant model, we show that for wide Clefts, mutant myocytes consistently produce EADs, while for small Clefts and wild-type myocytes, EADs are suppressed. For both large and small widths, the mutant sodium channel incompletely inactivates, but only for small Cleft widths is late sodium current sufficiently reduced, suppressing EADs. These data demonstrate that Intercellular edema in LQT3 underlies the formation of EADs presumably by an ephaptically-mediated mechanism.
Seth H. Weinberg - One of the best experts on this subject based on the ideXlab platform.
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Intercellular Sodium Regulates Repolarization in Cardiac Tissue with Sodium Channel Gain of Function.
Biophysical journal, 2020Co-Authors: Madison B. Nowak, Amara Greer-short, Xiaoping Wan, Isabelle Deschenes, Seth H. Weinberg, Steven PoelzingAbstract:In cardiac myocytes, action potentials are initiated by an influx of sodium (Na+) ions via voltage-gated Na+ channels. Na+ channel gain of function (GOF), arising in both inherited conditions associated with mutation in the gene encoding the Na+ channel and acquired conditions associated with heart failure, ischemia, and atrial fibrillation, enhance Na+ influx, generating a late Na+ current that prolongs action potential duration (APD) and triggering proarrhythmic early afterdepolarizations (EADs). Recent studies have shown that Na+ channels are highly clustered at the myocyte intercalated disk, facilitating formation of Na+ nanodomains in the Intercellular Cleft between cells. Simulations from our group have recently predicted that narrowing the width of the Intercellular Cleft can suppress APD prolongation and EADs in the presence of Na+ channel mutations because of increased Intercellular Cleft Na+ ion depletion. In this study, we investigate the effects of modulating multiple extracellular spaces, specifically the Intercellular Cleft and bulk interstitial space, in a novel computational model and experimentally via osmotic agents albumin, dextran 70, and mannitol. We perform optical mapping and transmission electron microscopy in a drug-induced (sea anemone toxin, ATXII) Na+ channel GOF isolated heart model and modulate extracellular spaces via osmotic agents. Single-cell patch-clamp experiments confirmed that the osmotic agents individually do not enhance late Na+ current. Both experiments and simulations are consistent with the conclusion that Intercellular Cleft narrowing or expansion regulates APD prolongation; in contrast, modulating the bulk interstitial space has negligible effects on repolarization. Thus, we predict that Intercellular Cleft Na+ nanodomain formation and collapse critically regulates cardiac repolarization in the setting of Na+ channel GOF.
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Revealing the Concealed Nature of Long-QT Type 3 Syndrome.
Circulation. Arrhythmia and electrophysiology, 2017Co-Authors: Amara Greer-short, Steven Poelzing, Sharon A. George, Seth H. WeinbergAbstract:Gain-of-function mutations in the voltage-gated sodium channel (Nav1.5) are associated with the long-QT-3 (LQT3) syndrome. Nav1.5 is densely expressed at the intercalated disk, and narrow Intercellular separation can modulate cell-to-cell coupling via extracellular electric fields and depletion of local sodium ion nanodomains. Models predict that significantly decreasing Intercellular Cleft widths slows conduction because of reduced sodium current driving force, termed "self-attenuation." We tested the novel hypothesis that self-attenuation can "mask" the LQT3 phenotype by reducing the driving force and late sodium current that produces early afterdepolarizations (EADs). Acute interstitial edema was used to increase Intercellular Cleft width in isolated guinea pig heart experiments. In a drug-induced LQT3 model, acute interstitial edema exacerbated action potential duration prolongation and produced EADs, in particular, at slow pacing rates. In a computational cardiac tissue model incorporating extracellular electric field coupling, Intercellular Cleft sodium nanodomains, and LQT3-associated mutant channels, myocytes produced EADs for wide Intercellular Clefts, whereas for narrow Clefts, EADs were suppressed. For both wide and narrow Clefts, mutant channels were incompletely inactivated. However, for narrow Clefts, late sodium current was reduced via self-attenuation, a protective negative feedback mechanism, masking EADs. We demonstrated a novel mechanism leading to the concealing and revealing of EADs in LQT3 models. Simulations predict that this mechanism may operate independent of the specific mutation, suggesting that future therapies could target Intercellular Cleft separation as a compliment or alternative to sodium channels. © 2017 American Heart Association, Inc.
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Ephaptic Self-Attenuation Conceals Early Afterdepolarizations Associated with Long QT-3 Syndrome
Biophysical Journal, 2016Co-Authors: Steven Poelzing, Amara Greer-short, Donald K. Jessup, Seth H. WeinbergAbstract:Gain-of-function mutations in the cardiac voltage-gated sodium channel (Nav1.5) are associated with the long QT-3 (LQT3) syndrome. Early afterdepolarizations (EADs) are common in isolated myocyte models of LQT3, but rare in tissue and patients with LQT3 mutations. We have shown that Nav1.5 is densely expressed at the intercalated disk and narrowing Intercellular separation can support an alternative form of cell-to-cell coupling known as ephaptic coupling (EpC). EpC occurs when sodium channels in the depolarizing cell decrease the Intercellular Cleft potential, depolarizing the apposing membrane from the extracellular rather than the intracellular domain. Critically, EpC models predict that decreasing Intercellular separation slows conduction due to reduced sodium current driving force. Here, we test the novel hypothesis that ephaptic self-attenuation can “mask” the LQT3 phenotype by reducing the driving force and late sodium current that produces EADs.We tested our hypothesis both in a pharmacological LQT3 model in isolated guinea pig hearts experiments (n=3), paced at 500 ms basic cycle length using late sodium current agonist ATXII (7nM), and a computational model. Acute interstitial edema, induced by 20 g/l mannitol, increased Intercellular Cleft width but did not alter action potential duration (APD). ATXII prolonged APD by 80±11 ms, and ATXII plus mannitol prolonged APD by 165±7 ms and produced EADs. In a computational model incorporating EpC, a recent LRd formulation, and a sodium channel LQT3-associated mutant model, we show that for wide Clefts, mutant myocytes consistently produce EADs, while for small Clefts and wild-type myocytes, EADs are suppressed. For both large and small widths, the mutant sodium channel incompletely inactivates, but only for small Cleft widths is late sodium current sufficiently reduced, suppressing EADs. These data demonstrate that Intercellular edema in LQT3 underlies the formation of EADs presumably by an ephaptically-mediated mechanism.
Jan P Kucera - One of the best experts on this subject based on the ideXlab platform.
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distribution of cardiac sodium channels in clusters potentiates ephaptic interactions in the intercalated disc
The Journal of Physiology, 2018Co-Authors: Echrak Hichri, Hugues Abriel, Jan P KuceraAbstract:Key points It has been proposed that ephaptic conduction, relying on interactions between the sodium (Na+) current and the extracellular potential in intercalated discs, might contribute to cardiac conduction when gap junctional coupling is reduced, but this mechanism is still controversial. In intercalated discs, Na+ channels form clusters near gap junction plaques, but the functional significance of these clusters has never been evaluated. In HEK cells expressing cardiac Na+ channels, we show that restricting the extracellular space modulates the Na+ current, as predicted by corresponding simulations accounting for ephaptic effects. In a high-resolution model of the intercalated disc, clusters of Na+ channels that face each other across the Intercellular Cleft facilitate ephaptic impulse transmission when gap junctional coupling is reduced. Thus, our simulations reveal a functional role for the clustering of Na+ channels in intercalated discs, and suggest that rearrangement of these clusters in disease may influence cardiac conduction. Abstract It has been proposed that ephaptic interactions in intercalated discs, mediated by extracellular potentials, contribute to cardiac impulse propagation when gap junctional coupling is reduced. However, experiments demonstrating ephaptic effects on the cardiac Na+ current (INa) are scarce. Furthermore, Na+ channels form clusters around gap junction plaques, but the electrophysiological significance of these clusters has never been investigated. In patch clamp experiments with HEK cells stably expressing human Nav1.5 channels, we examined how restricting the extracellular space modulates INa elicited by an activation protocol. In parallel, we developed a high-resolution computer model of the intercalated disc to investigate how the distribution of Na+ channels influences ephaptic interactions. Approaching the HEK cells to a non-conducting obstacle always increased peak INa at step potentials near the threshold of INa activation and decreased peak INa at step potentials far above threshold (7 cells, P = 0.0156, Wilcoxon signed rank test). These effects were consistent with corresponding control simulations with a uniform Na+ channel distribution. In the intercalated disc computer model, redistributing the Na+ channels into a central cluster of the disc potentiated ephaptic effects. Moreover, ephaptic impulse transmission from one cell to another was facilitated by clusters of Na+ channels facing each other across the Intercellular Cleft when gap junctional coupling was reduced. In conclusion, our proof-of-principle experiments demonstrate that confining the extracellular space modulates cardiac INa, and our simulations reveal the functional role of the aggregation of Na+ channels in the perinexus. These findings highlight novel concepts in the physiology of cardiac excitation.
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ephaptic effects potentiate the threshold behavior of the cardiac sodium current in a high resolution mathematical model of a narrow Intercellular Cleft
Biophysical Journal, 2017Co-Authors: Echrak Hichri, Hugues Abriel, Jan P KuceraAbstract:Cardiac arrhythmias frequently result from disorders of impulse propagation. Recent studies proposed that electrical potentials occurring in intercalated discs may contribute to cardiac conduction in addition to gap junctional coupling (ephaptic conduction). Furthermore, Na+ channels are found in clusters near gap junctions, suggesting that Na+ channel distribution in intercalated discs may modulate ephaptic Intercellular interactions. Our aim was to investigate computationally how cardiac Na+ current (INa) dynamics are influenced in a narrow extracellular Cleft by the extracellular potential (Ve) and by the distribution of Na+ channels. The model consisted of a disc-shaped Cleft discretized at a high resolution separating either one cell membrane containing Na+ channels and a non-conductive obstacle, or two excitable membranes representing an intercalated disc. INa was modeled using the Hodgkin-Huxley formalism. The intracellular domain of the first cell was subjected to a voltage-clamp activation protocol while the intracellular potential of the second cell was clamped to the resting potential. In the model with one membrane facing a non-conducting obstacle, decreasing Cleft width from 1000 to 20 nm resulted in decreasing INa peak intensity at voltage steps ≥-30 mV. This decrease was caused by the negative Ve in the Cleft, which decreased the driving force for INa. However, at voltage steps just above threshold, INa was activated in a ring-shaped peripheral region. This resulted in larger INa in simulations with Clefts 10-200 nm wide. When total INa was considered, the negative Ve resulted in a Cleft-dependent shift of the steady state activation curve to more negative potentials and a steepening of the curve. Narrowing the Cleft thus lowered and accentuated the threshold of INa. These effects were more prominent in larger discs and when Na+ channels were redistributed in the center of the disc. In the two-cell model, the negative Ve caused by INa in the first cell resulted in activation of INa in the second cell after a ∼1 ms delay for Clefts <70 nm. Therefore, the Ve caused by INa in a narrow restricted extracellular space exerts a major feedback on Na+ channel behavior, modulating the voltage-dependence of activation and the threshold behavior of INa, and thus cellular excitability. These effects are strongly influenced by the spatial distribution of Na+ channels. These findings are relevant for a comprehensive understanding of cardiac excitation.
Amara Greer-short - One of the best experts on this subject based on the ideXlab platform.
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Intercellular Sodium Regulates Repolarization in Cardiac Tissue with Sodium Channel Gain of Function.
Biophysical journal, 2020Co-Authors: Madison B. Nowak, Amara Greer-short, Xiaoping Wan, Isabelle Deschenes, Seth H. Weinberg, Steven PoelzingAbstract:In cardiac myocytes, action potentials are initiated by an influx of sodium (Na+) ions via voltage-gated Na+ channels. Na+ channel gain of function (GOF), arising in both inherited conditions associated with mutation in the gene encoding the Na+ channel and acquired conditions associated with heart failure, ischemia, and atrial fibrillation, enhance Na+ influx, generating a late Na+ current that prolongs action potential duration (APD) and triggering proarrhythmic early afterdepolarizations (EADs). Recent studies have shown that Na+ channels are highly clustered at the myocyte intercalated disk, facilitating formation of Na+ nanodomains in the Intercellular Cleft between cells. Simulations from our group have recently predicted that narrowing the width of the Intercellular Cleft can suppress APD prolongation and EADs in the presence of Na+ channel mutations because of increased Intercellular Cleft Na+ ion depletion. In this study, we investigate the effects of modulating multiple extracellular spaces, specifically the Intercellular Cleft and bulk interstitial space, in a novel computational model and experimentally via osmotic agents albumin, dextran 70, and mannitol. We perform optical mapping and transmission electron microscopy in a drug-induced (sea anemone toxin, ATXII) Na+ channel GOF isolated heart model and modulate extracellular spaces via osmotic agents. Single-cell patch-clamp experiments confirmed that the osmotic agents individually do not enhance late Na+ current. Both experiments and simulations are consistent with the conclusion that Intercellular Cleft narrowing or expansion regulates APD prolongation; in contrast, modulating the bulk interstitial space has negligible effects on repolarization. Thus, we predict that Intercellular Cleft Na+ nanodomain formation and collapse critically regulates cardiac repolarization in the setting of Na+ channel GOF.
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Revealing the Concealed Nature of Long-QT Type 3 Syndrome.
Circulation. Arrhythmia and electrophysiology, 2017Co-Authors: Amara Greer-short, Steven Poelzing, Sharon A. George, Seth H. WeinbergAbstract:Gain-of-function mutations in the voltage-gated sodium channel (Nav1.5) are associated with the long-QT-3 (LQT3) syndrome. Nav1.5 is densely expressed at the intercalated disk, and narrow Intercellular separation can modulate cell-to-cell coupling via extracellular electric fields and depletion of local sodium ion nanodomains. Models predict that significantly decreasing Intercellular Cleft widths slows conduction because of reduced sodium current driving force, termed "self-attenuation." We tested the novel hypothesis that self-attenuation can "mask" the LQT3 phenotype by reducing the driving force and late sodium current that produces early afterdepolarizations (EADs). Acute interstitial edema was used to increase Intercellular Cleft width in isolated guinea pig heart experiments. In a drug-induced LQT3 model, acute interstitial edema exacerbated action potential duration prolongation and produced EADs, in particular, at slow pacing rates. In a computational cardiac tissue model incorporating extracellular electric field coupling, Intercellular Cleft sodium nanodomains, and LQT3-associated mutant channels, myocytes produced EADs for wide Intercellular Clefts, whereas for narrow Clefts, EADs were suppressed. For both wide and narrow Clefts, mutant channels were incompletely inactivated. However, for narrow Clefts, late sodium current was reduced via self-attenuation, a protective negative feedback mechanism, masking EADs. We demonstrated a novel mechanism leading to the concealing and revealing of EADs in LQT3 models. Simulations predict that this mechanism may operate independent of the specific mutation, suggesting that future therapies could target Intercellular Cleft separation as a compliment or alternative to sodium channels. © 2017 American Heart Association, Inc.
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Ephaptic Self-Attenuation Conceals Early Afterdepolarizations Associated with Long QT-3 Syndrome
Biophysical Journal, 2016Co-Authors: Steven Poelzing, Amara Greer-short, Donald K. Jessup, Seth H. WeinbergAbstract:Gain-of-function mutations in the cardiac voltage-gated sodium channel (Nav1.5) are associated with the long QT-3 (LQT3) syndrome. Early afterdepolarizations (EADs) are common in isolated myocyte models of LQT3, but rare in tissue and patients with LQT3 mutations. We have shown that Nav1.5 is densely expressed at the intercalated disk and narrowing Intercellular separation can support an alternative form of cell-to-cell coupling known as ephaptic coupling (EpC). EpC occurs when sodium channels in the depolarizing cell decrease the Intercellular Cleft potential, depolarizing the apposing membrane from the extracellular rather than the intracellular domain. Critically, EpC models predict that decreasing Intercellular separation slows conduction due to reduced sodium current driving force. Here, we test the novel hypothesis that ephaptic self-attenuation can “mask” the LQT3 phenotype by reducing the driving force and late sodium current that produces EADs.We tested our hypothesis both in a pharmacological LQT3 model in isolated guinea pig hearts experiments (n=3), paced at 500 ms basic cycle length using late sodium current agonist ATXII (7nM), and a computational model. Acute interstitial edema, induced by 20 g/l mannitol, increased Intercellular Cleft width but did not alter action potential duration (APD). ATXII prolonged APD by 80±11 ms, and ATXII plus mannitol prolonged APD by 165±7 ms and produced EADs. In a computational model incorporating EpC, a recent LRd formulation, and a sodium channel LQT3-associated mutant model, we show that for wide Clefts, mutant myocytes consistently produce EADs, while for small Clefts and wild-type myocytes, EADs are suppressed. For both large and small widths, the mutant sodium channel incompletely inactivates, but only for small Cleft widths is late sodium current sufficiently reduced, suppressing EADs. These data demonstrate that Intercellular edema in LQT3 underlies the formation of EADs presumably by an ephaptically-mediated mechanism.