Characterization of depolarization-evoked ERG K currents in interstitial cells of Cajal

Gastrointestinal Symptoms and Channelopathy-Associated Epilepsy

OBJECTIVE

To determine the prevalence of and identify factors associated with gastrointestinal (GI) symptoms among children with channelopathy-associated developmental and epileptic encephalopathy (DEE).

STUDY DESIGN

Parents of 168 children with DEEs linked to SCN1A (n = 59), KCNB1 (n = 31), or KCNQ2 (n = 78) completed online CLIRINX surveys about their children's GI symptoms. Our analysis examined the prevalence, frequency, and severity of GI symptoms, as well as DEE type, functional mobility, feeding difficulties, ketogenic diet, antiseizure medication, autism spectrum disorder (ASD), and seizures. Statistical analyses included the χ² test, Wilcoxon rank-sum analysis, and multiple logistic regression.

RESULTS

GI symptoms were reported in 92 of 168 patients (55%), among whom 63 of 86 (73%) reported daily or weekly symptoms, 29 of 92 (32%) had frequent or serious discomfort, and 13 of 91 (14%) had frequent or serious appetite disturbances as a result. The prevalence of GI symptoms varied across DEE cohorts with 44% of SCN1A-DEE patients, 35% of KCNB1-DEE patients, and 71% of KCNQ2-DEE patients reporting GI symptoms in the previous month. After adjustment for DEE type, current use of ketogenic diet (6% reported), and gastrostomy tube (13% reported) were both associated with GI symptoms in a statistically, but not clinically, significant manner (P < .05). Patient age, functional mobility, feeding difficulties, ASD, and seizures were not clearly associated with GI symptoms. Overall, no individual antiseizure medication was significantly associated with GI symptoms across all DEE cohorts.

CONCLUSIONS

GI symptoms are common and frequently severe in patients with DEE.

Current applications of mathematical models of the interstitial cells of Cajal in the gastrointestinal tract

The interstitial cells of Cajal (ICC) form interconnected networks throughout the gastrointestinal (GI) tract. ICC act as the pacemaker cells that initiate the rhythmic bioelectrical slow waves and intermediary between the GI musculature and nerves, both of which are critical to GI motility. Disruptions to the number of ICC and the integrity of ICC networks have been identified as a key pathophysiological mechanism in a number of clinically challenging GI disorders. The current analyses of ICC generally rely on either functional recordings taken directly from excised tissue or morphological analysis based on images of labeled ICC, where the structural-functional relationship is investigated in an associative manner rather than mechanistically. On the other hand, computational physiology has played a significant role in facilitating our understanding of a number of physiological systems in both health and disease, and investigations in the GI field are beginning to incorporate several mathematical models of the ICC. The main aim of this review is to present the major modeling advances in GI electrophysiology, in order to introduce a multi-scale framework for mathematically quantifying the functional consequences of ICC degradation at both cellular and tissue scales. The outcomes will inform future investigators utilizing modeling techniques in their studies. This article is categorized under: Metabolic Diseases > Computational Models.

Mechanisms of Electrical Activation and Conduction in the Gastrointestinal System: Lessons from Cardiac Electrophysiology

The gastrointestinal (GI) tract is an electrically excitable organ system containing multiple cell types, which coordinate electrical activity propagating through this tract. Disruption in its normal electrophysiology is observed in a number of GI motility disorders. However, this is not well characterized and the field of GI electrophysiology is much less developed compared to the cardiac field. The aim of this article is to use the established knowledge of cardiac electrophysiology to shed light on the mechanisms of electrical activation and propagation along the GI tract, and how abnormalities in these processes lead to motility disorders and suggest better treatment options based on this improved understanding. In the first part of the article, the ionic contributions to the generation of GI slow wave and the cardiac action potential (AP) are reviewed. Propagation of these electrical signals can be described by the core conductor theory in both systems. However, specifically for the GI tract, the following unique properties are observed: changes in slow wave frequency along its length, periods of quiescence, synchronization in short distances and desynchronization over long distances. These are best described by a coupled oscillator theory. Other differences include the diminished role of gap junctions in mediating this conduction in the GI tract compared to the heart. The electrophysiology of conditions such as gastroesophageal reflux disease and gastroparesis, and functional problems such as irritable bowel syndrome are discussed in detail, with reference to ion channel abnormalities and potential therapeutic targets. A deeper understanding of the molecular basis and physiological mechanisms underlying GI motility disorders will enable the development of better diagnostic and therapeutic tools and the advancement of this field.

Modeling of stochastic behavior of pacemaker potential in interstitial cells of Cajal

It is widely accepted that interstitial cells of Cajal (ICCs) generate pacemaker potentials to propagate slow waves along the whole gastrointestinal tract. Previously, we constructed a biophysically based model of ICCs in mouse small intestine to explain the pacemaker mechanism. Our previous model, however, could not explain non-uniformity of pacemaker potentials and random occurrence of unitary potentials, thus we updated our model. The inositol 1,4,5-trisphosphate (IP3)-mediated Ca(2+) mobilization is a key event to drive the cycle of pacemaker activity and was updated to reproduce its stochastic behavior. The stochasticity was embodied by simulating random opening and closing of individual IP3-mediated Ca(2+) channel. The updated model reproduces the stochastic features of pacemaker potentials in ICCs. Reproduced pacemaker potentials are not uniform in duration and interval. The resting and peak potentials are -75.5 ± 1.1 mV and -0.8 ± 0.5 mV, respectively (n = 55). Frequency of pacemaker potential is 14.3 ± 0.4 min(-1) (n = 10). Width at half-maximal amplitude of pacemaker potential is 902 ± 6 ms (n = 55). There are random events of unitary potential-like depolarization. Finally, we compared our updated model with a recently published model to speculate which ion channel is the best candidate to drive pacemaker depolarization. In conclusion, our updated mathematical model could now reproduce stochastic features of pacemaker activity in ICCs.

Targeting ion channels for the treatment of gastrointestinal motility disorders

Gastrointestinal (GI) functional and motility disorders are highly prevalent and responsible for long-term morbidity and sometimes mortality in the affected patients. It is estimated that one in three persons has a GI functional or motility disorder. However, diagnosis and treatment of these widespread conditions remains challenging. This partly stems from the multisystem pathophysiology, including processing abnormalities in the central and peripheral (enteric) nervous systems and motor dysfunction in the GI wall. Interstitial cells of Cajal (ICCs) are central to the generation and propagation of the cyclical electrical activity and smooth muscle cells (SMCs) are responsible for electromechanical coupling. In these and other excitable cells voltage-sensitive ion channels (VSICs) are the main molecular units that generate and regulate electrical activity. Thus, VSICs are potential targets for intervention in GI motility disorders. Research in this area has flourished with advances in the experimental methods in molecular and structural biology and electrophysiology. However, our understanding of the molecular mechanisms responsible for the complex and variable electrical behavior of ICCs and SMCs remains incomplete. In this review, we focus on the slow waves and action potentials in ICCs and SMCs. We describe the constituent VSICs, which include voltage-gated sodium (Na(V)), calcium (Ca(V)), potassium (K(V), K(Ca)), chloride (Cl(-)) and nonselective ion channels (transient receptor potentials [TRPs]). VSICs have significant structural homology and common functional mechanisms. We outline the approaches and limitations and provide examples of targeting VSICs at the pores, voltage sensors and alternatively spliced sites. Rational drug design can come from an integrated view of the structure and mechanisms of gating and activation by voltage or mechanical stress.

Biophysically based modeling of the interstitial cells of cajal: current status and future perspectives

Gastrointestinal motility research is progressing rapidly, leading to significant advances in the last 15 years in understanding the cellular mechanisms underlying motility, following the discovery of the central role played by the interstitial cells of Cajal (ICC). As experimental knowledge of ICC physiology has expanded, biophysically based modeling has become a valuable tool for integrating experimental data, for testing hypotheses on ICC pacemaker mechanisms, and for applications in in silico studies including in multiscale models. This review is focused on the cellular electrophysiology of ICC. Recent evidence from both experimental and modeling domains have called aspects of the existing pacemaker theories into question. Therefore, current experimental knowledge of ICC pacemaker mechanisms is examined in depth, and current theories of ICC pacemaking are evaluated and further developed. Existing biophysically based ICC models and their physiological foundations are then critiqued in light of the recent advances in experimental knowledge, and opportunities to improve these models are identified. The review concludes by examining several potential clinical applications of biophysically based ICC modeling from the subcellular through to the organ level, including ion channelopathies and ICC network degradation.

Transient outward potassium current in ICC

Interstitial cells of Cajal (ICC) are the pacemakers of the gut, initiating slow-wave activity. Several ion channels have been identified that contribute to the depolarization phase of the slow wave. Our aim was to contribute to knowledge about the identity and role of ICC potassium channels in pacemaking. Here we describe a transient outward potassium current in cell-attached patches of ICC. This current was activated almost instantaneously at potentials positive of the resting membrane potential and inactivated as a single exponential or biexponential with time constants that varied widely from patch to patch. Averaged traces gave a biexponential inactivation with time constants of approximately 40 and approximately 500 ms, with no clear voltage dependence. Analysis of single-channel openings and closings indicated a channel conductance of 5 pS and permeability sequence of K(+) (111) > Na(+) (1) > N-methyl-d-glucamine(+) (0.11). The current was completely blocked by 20 microM clotrimazole but was unaffected by 20 microM ketoconazole, 10 microM E4031, or 20 microM clofilium; 5 mM 4-aminopyridine slowed the activation of the current. The transient outward current may be important in moderating the upstroke of the pacemaker potential.

Biophysically based mathematical modeling of interstitial cells of Cajal slow wave activity generated from a discrete unitary potential basis

Spontaneously rhythmic pacemaker activity produced by interstitial cells of Cajal (ICC) is the result of the entrainment of unitary potential depolarizations generated at intracellular sites termed pacemaker units. In this study, we present a mathematical modeling framework that quantitatively represents the transmembrane ion flows and intracellular Ca2+ dynamics from a single ICC operating over the physiological membrane potential range. The mathematical model presented here extends our recently developed biophysically based pacemaker unit modeling framework by including mechanisms necessary for coordinating unitary potential events, such as a T-Type Ca2+ current, Vm-dependent K+ currents, and global Ca2+ diffusion. Model simulations produce spontaneously rhythmic slow wave depolarizations with an amplitude of 65 mV at a frequency of 17.4 cpm. Our model predicts that activity at the spatial scale of the pacemaker unit is fundamental for ICC slow wave generation, and Ca2+ influx from activation of the T-Type Ca2+ current is required for unitary potential entrainment. These results suggest that intracellular Ca2+ levels, particularly in the region local to the mitochondria and endoplasmic reticulum, significantly influence pacing frequency and synchronization of pacemaker unit discharge. Moreover, numerical investigations show that our ICC model is capable of qualitatively replicating a wide range of experimental observations.

A Ca(2+)-activated Cl(-) conductance in interstitial cells of Cajal linked to slow wave currents and pacemaker activity

Interstitial cells of Cajal (ICC) are unique cells that generate electrical pacemaker activity in gastrointestinal (GI) muscles. Many previous studies have attempted to characterize the conductances responsible for pacemaker current and slow waves in the GI tract, but the precise mechanism of electrical rhythmicity is still debated. We used a new transgenic mouse with a bright green fluorescent protein (copGFP) constitutively expressed in ICC to facilitate study of these cells in mixed cell dispersions. We found that ICC express a specialized 'slow wave' current. Reversal of tail current analysis showed this current was due to a Cl(-) selective conductance. ICC express ANO1, a Ca(2+)-activated Cl(-) channel. Slow wave currents are not voltage dependent, but a secondary voltage-dependent process underlies activation of these currents. Removal of extracellular Ca(2+), replacement of Ca(2+) with Ba(2+), or extracellular Ni(2+) (30 microm) blocked the slow wave current. Single Ca(2+)-activated Cl() channels with a unitary conductance of 7.8 pS were resolved in excised patches of ICC. These are similar in conductance to ANO1 channels (8 pS) expressed in HEK293 cells. Slow wave current was blocked in a concentration-dependent manner by niflumic acid (IC(50) = 4.8 microm). Slow wave currents are associated with transient depolarizations of ICC in current clamp, and these events were blocked by niflumic acid. These findings demonstrate a role for a Ca(2+)-activated Cl(-) conductance in slow wave current in ICC and are consistent with the idea that ANO1 participates in pacemaker activity.

A Ca(2+)-activated Cl(-) conductance in interstitial cells of Cajal linked to slow wave currents and pacemaker activity

Interstitial cells of Cajal (ICC) are unique cells that generate electrical pacemaker activity in gastrointestinal (GI) muscles. Many previous studies have attempted to characterize the conductances responsible for pacemaker current and slow waves in the GI tract, but the precise mechanism of electrical rhythmicity is still debated. We used a new transgenic mouse with a bright green fluorescent protein (copGFP) constitutively expressed in ICC to facilitate study of these cells in mixed cell dispersions. We found that ICC express a specialized 'slow wave' current. Reversal of tail current analysis showed this current was due to a Cl(-) selective conductance. ICC express ANO1, a Ca(2+)-activated Cl(-) channel. Slow wave currents are not voltage dependent, but a secondary voltage-dependent process underlies activation of these currents. Removal of extracellular Ca(2+), replacement of Ca(2+) with Ba(2+), or extracellular Ni(2+) (30 microm) blocked the slow wave current. Single Ca(2+)-activated Cl() channels with a unitary conductance of 7.8 pS were resolved in excised patches of ICC. These are similar in conductance to ANO1 channels (8 pS) expressed in HEK293 cells. Slow wave current was blocked in a concentration-dependent manner by niflumic acid (IC(50) = 4.8 microm). Slow wave currents are associated with transient depolarizations of ICC in current clamp, and these events were blocked by niflumic acid. These findings demonstrate a role for a Ca(2+)-activated Cl(-) conductance in slow wave current in ICC and are consistent with the idea that ANO1 participates in pacemaker activity.

Ether-a-go-go-related gene 3 is the main candidate for the E-4031-sensitive potassium current in the pacemaker interstitial cells of Cajal

The interstitial cells of Cajal (ICC), as pacemaker cells of the gut, contribute to rhythmic peristalsis and muscle excitability through initiation of slow-wave activity, which subsequently actively propagates into the musculature. An E-4031-sensitive K(+) current makes a critical contribution to membrane potential in ICC. This study provides novel features of this current in ICC in physiological solutions and seeks to identify the channel isoform. In situ hybridization and Kit immunohistochemistry were combined to assess ether-a-go-go-related gene (ERG) mRNA expression in ICC in mouse jejunum, while the translated message was assessed by immunofluorescence colocalization of ERG and Kit proteins. E-4031-sensitive currents in cultured ICC were studied by the whole cell patch-clamp method, with physiological K(+) concentration in the bath and the pipette. In situ hybridization combined with Kit immunohistochemistry detected m-erg1 and m-erg3, but not m-erg2, mRNA in ICC. ERG3 protein was colocalized with Kit-immunoreactive ICC in jejunum sections, but ERG1 protein was visualized only in the smooth muscle cells. At physiological K(+) concentration, the E-4031-sensitive outward current in ICC was different from its counterpart in cardiac and gut smooth muscle cells. In particular, inactivation upon depolarization and recovery from inactivation by hyperpolarization were modest in ICC. In summary, the E-4031-sensitive currents influence the kinetics of the pacemaker activity in ICC and contribute to maintenance of the resting membrane potential in smooth muscle cells, which together constitute a marked effect on tissue excitability. Whereas this current is mediated by ERG1 in smooth muscle, it is primarily mediated by ERG3 in ICC.

Quantitative cellular description of gastric slow wave activity

Interstitial cells of Cajal (ICC) are responsible for the spontaneous and omnipresent electrical activity in the stomach. A quantitative description of the intracellular processes whose coordinated activity is believed to generate electrical slow waves has been developed and is presented here. In line with recent experimental evidence, the model describes how the interplay between the mitochondria and the endoplasmic reticulum in cycling intracellular Ca(2+) provides the primary regulatory signal for the initiation of the slow wave. The major ion channels that have been identified as influencing slow wave activity have been modeled according to data obtained from isolated ICC. The model has been validated by comparing the simulated profile of the slow waves with experimental recordings and shows good correspondence in terms of frequency, amplitude, and shape in both control and pharmacologically altered conditions.

The hERG K+ channel: target and antitarget strategies in drug development

The human ether-à-go-go related gene (hERG) K+ channel is of great interest for both basic researchers and clinicians because its blockade by drugs can lead to QT prolongation, which is a risk factor for torsades de pointes, a potentially life-threatening arrhythmia. A growing list of agents with "QT liability" have been withdrawn from the market or restricted in their use, whereas others did not even receive regulatory approval for this reason. Thus, hERG K+ channels have become a primary antitarget (i.e. an unwanted target) in drug development because their blockade causes potentially serious side effects. On the other hand, the recent identification and functional characterization of hERG K+ channels not only in the heart, but also in several other tissues (e.g. neurons, smooth muscle and cancer cells) may have far reaching implications for drug development for a possible exploitation of hERG as a target, especially in oncology and cardiology.

Clotrimazole-sensitive K+ currents regulate pacemaker activity in interstitial cells of Cajal

Interstitial cells of Cajal (ICC) are pacemaker cells for gut peristaltic motor activity. Compared with cardiac pacemaker cells, little is known about mechanisms that regulate ICC excitability. The objective of the present study was to investigate a potential role for clotrimazole (CTL)-sensitive K currents (I(CTL)) in the regulation of ICC excitability and pacemaker activity. ICC were studied in situ and in short-term culture by using the whole cell patch-clamp configuration. In situ, ICC exhibited spontaneous transient inward currents followed by transient outward currents. CTL blocked outward currents, thereby increasing the net inward currents, and depolarized ICC, thereby establishing CTL-sensitive channels as regulators of ICC pacemaker activity. In short-term culture, a I(CTL) was identified that showed increased conductance when depolarized from the resting membrane potential to 0 mV and subsequent inward rectification at further depolarized potentials. The I(CTL) markedly increased with increasing intracellular calcium and was insensitive to the ether-à-go-go-related K channel blocker E-4031 and the large-conductance calcium-activated K channel blocker iberiotoxin. I(CTL) contributed 3-9 nS to the whole cell conductance at 0 mV membrane potential under physiological conditions; it was fast activating (tau = 88 ms), showed little time-dependent inactivation, and exhibited a deactivation time constant of 38 ms. The nitric oxide donor sodium nitroprusside (SNP) increased I(CTL). Single-channel activity, activated by calcium and SNP, was inhibited by CTL, with a single-channel conductance of approximately 38 pS. In summary, ICC generate a I(CTL) on depolarization through an intermediate-conductance calcium-activated K channel that regulates pacemaker activity and ICC excitability.

Muscarinic regulation of ether-a-go-go-related gene K+ currents in interstitial cells of Cajal

The interstitial cells of Cajal (ICC) of the myenteric plexus generate a set of currents that evoke a pacemaker potential that sets the initial conditions for the contraction frequency and duration of the electrically coupled intestinal musculature. The synapse-like contacts between ICC and myenteric motor nerves highlight the potential role of the enteric nervous system in regulating the pacemaking currents in ICC. The objective of the present study was to investigate muscarinic regulation of the ether-a-go-go-related gene (ERG) K(+) current. Immunoreactivity of the M(3) receptor (M(3)R) but not the M(2) receptor was detected on murine jejunal ICC-Auerbach's plexus (ICC-AP). The muscarinic agonist bethanechol reduced hyperpolarization-evoked peak ERG currents at -100 mV by 23 +/- 1% and increased both fast and slow time constants of deactivation, resulting in increased steady-state currents between -55 and -35 mV. Bethanechol also increased depolarization-evoked steady-state currents by 59 +/- 10% at -40 mV, whereas currents were decreased at potentials positive to 0 mV. The half-maximal voltage of activation was shifted 11.9 mV leftward. Interestingly, the time constant of activation increased only at -40 mV. Atropine prevented and 2 muM E4031 [1-[2-(6-methyl-2-pyridyl)-ethyl-4-(methylsulfonylaminobenzoyl)piperidine] inhibited bethanechol-affected currents. The effect of bethanechol was mimicked by protein kinase C (PKC) activation and diminished by PKC inhibition. Our results indicate that the ERG K(+) channel in ICC is affected by stimulation of muscarinic receptors, probably the M(3)R, via a PKC-dependent mechanism. Modulation of the ERG K(+) current in ICC-AP will affect the kinetics of pacemaking in the intestinal musculature.