Investigation of the Effects of the Short QT Syndrome D172N Kir2.1 Mutation on Ventricular Action Potential Profile Using Dynamic Clamp

The congenital short QT syndrome (SQTS) is a cardiac condition that leads to abbreviated ventricular repolarization and an increased susceptibility to arrhythmia and sudden death. The SQT3 form of the syndrome is due to mutations to the KCNJ2 gene that encodes Kir2.1, a critical component of channels underlying cardiac inwardly rectifying K⁺ current, I_K1. The first reported SQT3 KCNJ2 mutation gives rise to the D172N Kir2.1 mutation, the consequences of which have been studied on recombinant channels in vitro and in ventricular cell and tissue simulations. The aim of this study was to establish the effects of the D172N mutation on ventricular repolarization through real-time replacement of I_K1 using the dynamic clamp technique. Whole-cell patch-clamp recordings were made from adult guinea-pig left ventricular myocytes at physiological temperature. Action potentials (APs) were elicited at 1 Hz. Intrinsic I_K1 was inhibited with a low concentration (50 µM) of Ba²⁺ ions, which led to AP prolongation and triangulation, accompanied by a ∼6 mV depolarization of resting membrane potential. Application of synthetic I_K1 through dynamic clamp restored AP duration, shape and resting potential. Replacement of wild-type (WT) I_K1 with heterozygotic (WT-D172N) or homozygotic (D172N) mutant formulations under dynamic clamp significantly abbreviated AP duration (APD₉₀) and accelerated maximal AP repolarization velocity, with no significant hyperpolarization of resting potential. Across stimulation frequencies from 0.5 to 3 Hz, the relationship between APD₉₀ and cycle length was downward shifted, reflecting AP abbreviation at all stimulation frequencies tested. In further AP measurements at 1 Hz from hiPSC cardiomyocytes, the D172N mutation produced similar effects on APD and repolarization velocity; however, resting potential was moderately hyperpolarized by application of mutant I_K1 to these cells. Overall, the results of this study support the major changes in ventricular cell AP repolarization with the D172N predicted from prior AP modelling and highlight the potential utility of using adult ventricular cardiomyocytes for dynamic clamp exploration of functional consequences of Kir2.1 mutations.

Impaired chromaffin cell excitability and exocytosis in autistic Timothy syndrome TS2-neo mouse rescued by L-type calcium channel blockers

KEY POINTS

Tymothy syndrome (TS) is a multisystem disorder featuring cardiac arrhythmias, autism and adrenal gland dysfunction that originates from a de novo point mutation in the gene encoding the Cav1.2 (CACNA1C) L-type channel. To study the role of Cav1.2 channel signals in autism, the autistic TS2-neo mouse has been generated bearing the G406R point-mutation associated with TS type-2. Using heterozygous TS2-neo mice, we report that the G406R mutation reduces the rate of inactivation and shifts leftward the activation and inactivation of L-type channels, causing marked increase of resting Ca²⁺ influx ('window' Ca²⁺ current). The increased 'window current' causes marked reduction of Na_V channel density, switches normal tonic firing to abnormal burst firing, reduces mitochondrial metabolism, induces cell swelling and decreases catecholamine release. Overnight incubations with nifedipine rescue Na_V channel density, normal firing and the quantity of catecholamine released. We provide evidence that chromaffin cell malfunction derives from altered Cav1.2 channel gating.

ABSTRACT

L-type voltage-gated calcium (Cav1) channels have a key role in long-term synaptic plasticity, sensory transduction, muscle contraction and hormone release. A point mutation in the gene encoding Cav1.2 (CACNA1C) causes Tymothy syndrome (TS), a multisystem disorder featuring cardiac arrhythmias, autism spectrum disorder (ASD) and adrenal gland dysfunction. In the more severe type-2 form (TS2), the missense mutation G406R is on exon 8 coding for the IS6-helix of the Cav1.2 channel. The mutation causes reduced inactivation and induces autism. How this occurs and how Cav1.2 gating-changes alter cell excitability, neuronal firing and hormone release on a molecular basis is still largely unknown. Here, using the TS2-neo mouse model of TS we show that the G406R mutation altered excitability and reduced secretory activity in adrenal chromaffin cells (CCs). Specifically, the TS2 mutation reduced the rate of voltage-dependent inactivation and shifted leftward the activation and steady-state inactivation of L-type channels. This markedly increased the resting 'window' Ca²⁺ current that caused an increased percentage of CCs undergoing abnormal action potential (AP) burst firing, cell swelling, reduced mitochondrial metabolism and decreased catecholamine release. The increased 'window' Ca²⁺ current caused also decreased Na_V channel density and increased steady-state inactivation, which contributed to the increased abnormal burst firing. Overnight incubation with the L-type channel blocker nifedipine rescued the normal AP firing of CCs, the density of functioning Na_V channels and their steady-state inactivation. We provide evidence that CC malfunction derives from the altered Cav1.2 channel gating and that dihydropyridines are potential therapeutics for ASD.

Enhanced oligodendrocyte maturation and myelination in a mouse model of Timothy syndrome

To study the role of L-type voltage-gated Ca++ channels in oligodendrocyte development, we used a mouse model of Timothy syndrome (TS) in which a gain-of-function mutation in the α1 subunit of the L-type Ca++ channel Cav1.2 gives rise to an autism spectrum disorder (ASD). Oligodendrocyte progenitor cells (OPCs) isolated from the cortex of TS mice showed greater L-type Ca++ influx and displayed characteristics suggestive of advanced maturation compared to control OPCs, including a more complex morphology and higher levels of myelin protein expression. Consistent with this, expression of Cav1.2 channels bearing the TS mutation in wild-type OPCs triggered process formation and promoted oligodendrocyte-neuron interaction via the activation of Ca++ /calmodulin-dependent protein kinase II. To ascertain whether accelerated OPC maturation correlated with functional enhancements, we examined myelination in the TS brain at different postnatal time points. The expression of myelin proteins was significantly higher in the corpus callosum, cortex and striatum of TS animals, and immunohistochemical analysis for oligodendrocyte stage-specific markers revealed an increase in the density of myelinating oligodendrocytes in several areas of the TS brain. Along the same line, electron microscopy studies in the corpus callosum of TS animals showed significant increases both in the percentage of myelinated axons and in the thickness of myelin sheaths. In summary, these data indicate that OPC development and oligodendrocyte myelination is enhanced in the brain of TS mice, and suggest that this mouse model of a syndromic ASD is a useful tool to explore the role of L-type Ca++ channels in myelination.

Regional variation of the inwardly rectifying potassium current in the canine heart and the contributions to differences in action potential repolarization

The inward rectifier potassium current, IK1, contributes to the terminal phase of repolarization of the action potential (AP), as well as the value and stability of the resting membrane potential. Regional variation in IK1 has been noted in the canine heart, but the biophysical properties have not been directly compared. We examined the properties and functional contribution of IK1 in isolated myocytes from ventricular, atrial and Purkinje tissue. APs were recorded from canine left ventricular midmyocardium, left atrial and Purkinje tissue. The terminal rate of repolarization of the AP in ventricle, but not in Purkinje, depended on changes in external K(+) ([K(+)]o). Isolated ventricular myocytes had the greatest density of IK1 while atrial myocytes had the lowest. Furthermore, the outward component of IK1 in ventricular cells exhibited a prominent outward component and steep negative slope conductance, which was also enhanced in 10 mM [K(+)]o. In contrast, both Purkinje and atrial cells exhibited little outward IK1, even in the presence of 10 mM [K(+)]o, and both cell types showed more persistent current at positive potentials. Expression of Kir2.1 in the ventricle was 76.9-fold higher than that of atria and 5.8-fold higher than that of Purkinje, whereas the expression of Kir2.2 and Kir2.3 subunits was more evenly distributed in Purkinje and atria. Finally, AP clamp data showed distinct contributions of IK1 for each cell type. IK1 and Kir2 subunit expression varies dramatically in regions of the canine heart and these regional differences in Kir2 expression likely underlie regional distinctions in IK1 characteristics, contributing to variations in repolarization in response to in [K(+)]o changes.

Cardiac mechanosensitivity and stretch-activated ion channels

Mechanosensitivity is a ubiquitous property of cells, and mechanosensitive ion channels (MSCs) are hypothesized to be the transducers. In the heart, MSCs are likely to account for changes in beating rate as a function of filling and for initiating stretch-induced arrhythmias (for example, following a myocardial infarction). Pharmacological agents that affect MSCs may provide a new class of antiarrhythmic drugs. © 1997, Elsevier Science Inc. (Trends Cardiovasc Med 1997;7:4-8).

Activation and inactivation of mechanosensitive currents in the chick heart

The behavior of MS channels in embryonic chick ventricular myocytes activated by direct mechanical stimulation is strongly affected by inactivation. The amplitude of the current is dependent not only on the amplitude of the stimulus, but also the history of stimulation. The MS current inactivation appears to be composed of at least two contributions: (i) rearrangement of the cortical tension transducing elements and (ii) blocking action of an autocrine agent released from the cell. With discrete mechanical stimuli, the MS current amplitude in the second press of a double press protocol was always smaller than the amplitude of the first MS current. Occasionally, a large MS current occurred when the cell was first stimulated, but subsequently the cell became unresponsive. For a series of stimuli of varying amplitudes, the order in which they were applied to the cell affected the size of the observed MS current for a given stimulus magnitude. When continuous sinusoidal stimulation was applied to the cells, the MS current envelope either reached a steady state, or inactivated. With sinusoidal stimulation, the MS response could be enhanced or restored by simple perfusion of fluid across the cell. This suggests that mechanical stimulation of the cells produces an autocrine inhibitor of MS channels as well as resulting in cortical rearrangement.

Stretch-activated whole cell currents in adult rat cardiac myocytes

Mechanoelectric transduction can initiate cardiac arrhythmias. To examine the origins of this effect at the cellular level, we made whole cell voltage-clamp recordings from acutely isolated rat ventricular myocytes under controlled strain. Longitudinal stretch elicited noninactivating inward cationic currents that increased the action potential duration. These stretch-activated currents could be blocked by 100 microM Gd(3+) but not by octanol. The current-voltage relationship was nearly linear, with a reversal potential of approximately -6 mV in normal Tyrode solution. Current density varied with sarcomere length (SL) according to I (pA/pF) = 8.3 - 5.0 SL (microm). Repeated attempts to record single channel currents from stretch-activated ion channels failed, in accord with the absence of such data from the literature. The inability to record single channel currents may be a result of channels being located on internal membranes such as the T tubules or, possibly, inactivation of the channels by the mechanics of patch formation.

Whole-cell mechanosensitive currents in rat ventricular myocytes activated by direct stimulation

Mechanosensitive channels may have a significant role in the development of cardiac arrhythmia following infarction, but the data on mechanical responses at the cellular level are limited. Mechanosensitivity is a ubiquitous property of cells, and although the structure of bacteriological mechanosensitive ion channels is becoming known by cloning, the structure and force transduction pathway in eukaryotes remains elusive. Isolated adult rat ventricular myocytes were voltage clamped and stimulated with a mechanical probe. The probe was set in sinusoidal motion (either in, or normal to, the plane of the cell membrane), and then slowly lowered onto the cell. The sinusoidal frequency was held constant at 1 Hz but the stimulation amplitude was increased and the probe gradually lowered until a mechanically sensitive whole cell current was seen, which usually followed several minutes of stimulation. The whole cell mechanosensitive current in rat cells had two components: (i) a brief large inward current spike current; (ii) a more sustained smaller inward current. The presence of the initial sharp inward current suggests that some structure within the cell either relaxes or is broken, exposing the mechanosensitive element(s) to stress. Metabolic changes induced by continued stress prior to the mechanosensitive response may weaken the elements that break producing the spike, or simple stress-induced fracture of the cytoskeleton itself may occur.

Contribution of a swelling-activated chloride current to changes in the cardiac action potential

The purpose of this investigation was to determine to what extent the swelling-activated Cl- current (ICl,swell) contributes to swelling-induced changes in the resting membrane potential and action potential duration (APD) in ventricular myocytes. Action potentials were recorded from guinea pig ventricular myocytes using conventional whole cell recording techniques. Cell swelling caused initial lengthening followed by a variable shortening of APD. In 59% of cells this secondary APD shortening had a 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive component, consistent with a contribution from ICl,swell. Furthermore, DIDS partially antagonized the depolarization of the resting membrane potential that occurred during cell swelling. We have modeled the ICl,swell using the Oxsoft Heart computer program. Action potential changes predicted by the model agree well with the observed DIDS-sensitive component of the change in the action potential during cell swelling. We conclude that activation of ICl,swell contributes to shortening of APD and depolarization of the resting membrane potential during cell swelling in cardiac myocytes.