isolation-culture-functional-characterization-adult-mouse

Isolation, Culture, and Functional Characterization of Adult Mouse Cardiomyoctyes

The use of primary cardiomyocytes (CMs) in culture has provided a powerful complement to murine models of heart disease in advancing our understanding of ...

Beth Israel Deaconess Medical Center, Harvard Medical School

Acute stress provokes lethal cardiac arrhythmias in the hereditary long QT syndrome. Here we provide a novel molecular mechanism linking beta-adrenergic signaling and altered human ether-a-go-go related gene (HERG) channel activity. Stress stimulates beta-adrenergic receptors, leading to cAMP elevations that can regulate HERG K+ channels both directly and via phosphorylation by cAMP-dependent protein kinase (PKA). We show that HERG associates with 14-3-3epsilon to potentiate cAMP/PKA effects upon HERG. The binding of 14-3-3 occurs simultaneously at the N- and C-termini of the HERG channel. 14-3-3 accelerates and enhances HERG activation, an effect that requires PKA phosphorylation of HERG and dimerization of 14-3-3. The interaction also stabilizes the lifetime of the PKA-phosphorylated state of the channel by shielding the phosphates from cellular phosphatases. The net result is a prolongation of the effect of adrenergic stimulation upon HERG activity. Thus, 14-3-3 interactions with HERG may provide a unique mechanism for plasticity in the control of membrane excitability and cardiac rhythm.

14-3-3 amplifies and prolongs adrenergic stimulation of HERG K+ channel activity.

KCNEs are a family of genes encoding small integral membrane proteins whose role in governing voltage-gated potassium channel gating is emerging. Whether each member of this homologous family interacts with channel proteins in the same manner is unknown; however, it is clear that the functional effect of each KCNE on channel gating is different. The specificity of KCNE1 (minK) and KCNE3 control of activation of the potassium channel KvLQT1 maps to a triplet of amino acids within the KCNE transmembrane domain by chimera analysis. We now define the structural determinants of functional specificity within this triplet. The central amino acid of the triplet (Thr-58 of minK and Val-72 of KCNE3) is essential for the specific control of voltage-dependent channel activation characteristics of both minK and KCNE3. Using site-directed mutations that substitute minK and KCNE3 residues, we determined that a hydroxylated central amino acid is necessary for the slow sigmoidal activation produced by minK. The precise spacing of the hydroxyl group was required for minK-like activation. An aliphatic amino acid substituted at position 58 of minK is capable of reproducing KCNE3-like kinetics and voltage-independent constitutive current activation. The bulk of the central residue is another critical parameter, indicating precise positioning of this portion of the KCNE proteins within the channel complex. An intermediate phenotype produced by several smaller aliphatic-substituted mutants yields conditional voltage independence that is distinct from the voltage-dependent gating process, suggesting that KCNE3 traps the channel in a stable open state. From these results, we propose a model of KCNE-potassium channel interaction where the functional consequence depends on the precise contact at a single amino acid.

A single transmembrane site in the KCNE-encoded proteins controls the specificity of KvLQT1 channel gating.

K(+) channels play a central role in determining resting membrane potential and cellular excitability. There is growing recognition that the channels exist not as independent units but as macromolecular complexes able to integrate a plethora of cellular signals to fine-tune channel activities. Interaction of K(+) channels with accessory proteins and subunits is increasingly reported as providing mechanisms for channels to respond to a variety of stimuli beyond just changes in membrane potential. One such association is that between some voltage-gated K(+) channels and the proteins encoded by the KCNE family of genes. The significance of these interactions is manifest in reports of genetic disorders such as the Long QT Syndrome linked to KCNE mutations and proarrhythmic drug susceptibilities from KCNE polymorphisms. The mechanism by which KCNE-encoded proteins control channel behavior is an emerging story. This article reviews some of the recent work addressing the prototypical KCNE-channel interaction between minK and KvLQT1.

KCNE regulation of KvLQT1 channels: structure-function correlates.

Cardiac I(Ks), the slowly activated delayed-rectifier K(+) current, is produced by the protein complex composed of alpha- and beta-subunits: KvLQT1 and minK. Mutations of genes encoding KvLQT1 and minK are responsible for the hereditary long QT syndrome (loci LQT1 and LQT5, respectively). MinK-L51H fails to traffic to the cell surface, thereby failing to produce effective I(Ks). We examined the effects that minK-L51H and an endoplasmic reticulum (ER)-targeted minK (minK-ER) exerted over the electrophysiology and biosynthesis of coexpressed KvLQT1. Both minK-L51H and minK-ER were sequestered primarily in the ER as confirmed by lack of plasma membrane expression. Glycosylation and immunofluorescence patterns of minK-L51H were qualitatively different for minK-ER, suggesting differences in trafficking. Cotransfection with the minK mutants resulted in reduced surface expression of KvLQT1 as assayed by whole cell voltage clamp and immunofluorescence. MinK-L51H reduced current amplitude by 91% compared with wild-type (WT) minK/KvLQT1, and the residual current was identical to KvLQT1 without minK. The phenotype of minK-L51H on I(Ks) was not dominant because coexpressed WT minK rescued the current and surface expression. Collectively, our data suggest that ER quality control prevents minK-L51H/KvLQT1 complexes from trafficking to the plasma membrane, resulting in decreased I(Ks). This is the first demonstration that a minK LQT mutation is capable of conferring trafficking defects onto its associated alpha-subunit.

An LQT mutant minK alters KvLQT1 trafficking.

Potassium channels control the resting membrane potential and excitability of biological tissues. Many voltage-gated potassium channels are controlled through interactions with accessory subunits of the KCNE family through mechanisms still not known. Gating of mammalian channel KCNQ1 is dramatically regulated by KCNE subunits. We have found that multiple segments of the channel pore structure bind to the accessory protein KCNE1. The sites that confer KCNE1 binding are necessary for the functional interaction, and all sites must be present in the channel together for proper regulation by the accessory subunit. Specific gating control is localized to a single site of interaction between the ion channel and accessory subunit. Thus, direct physical interaction with the ion channel pore is the basis of KCNE1 regulation of K+ channels.

KCNE1 binds to the KCNQ1 pore to regulate potassium channel activity.

The kinetics of voltage-dependent inactivation of the rapidly activating delayed rectifier, IKr, are unique among K+ channels. The human ether-a-gogo-related gene (HERG) encodes the pore-forming subunit of IKr and shares a high degree of homology with ether-a-gogo (EAG) channels that do not inactivate. Within those segments thought to contribute to the channel pore, HERG possesses several serine residues that are not present in EAG channels. Two of these serines, S620 and S631, are known to be required for inactivation. We now show that a third serine, S641, which resides in the outer portion of the sixth transmembrane segment, is also critical for normal inactivation. As with the other serines, S641 is also involved in maintaining ion selectivity of the HERG channel and alters sensitivity to block by E4031. Larger charged or polar substitutions (S641D and S641T) disrupted C-type inactivation in HERG. Smaller aliphatic and more conservative substitutions (S641A and S641C) facilitated C-type inactivation. Our data show that, like S620 and S631, S641 is another key residue for the rapid inactivation. The altered inactivation of mutations at S620, S631, and S641 were dominant, suggesting that a network of hydroxyl side chains is required for the unique inactivation, permeation, and rectification of HERG channels.

S641 contributes HERG K+ channel inactivation.

We report an infant who, after prolonged intravenous propofol infusion for treatment of status epilepticus, developed dramatic cardiac conduction disturbances and tachyarrhythmias in the setting of only mild metabolic acidosis and good ventricular function. Certain electrocardiographic findings were similar to those observed in the Brugada syndrome as well as in adult patients with propofol toxicity who suffered fatal ventricular arrhythmias. This case illustrates that serious arrhythmias can occur during prolonged high-dose propofol infusion in young patients, probably through a direct electrophysiologic membrane effect.

Cardiac conduction disturbances and ventricular tachycardia after prolonged propofol infusion in an infant.

The KCNE1 gene product (minK protein) associates with the cardiac KvLQT1 potassium channel (encoded by KCNQ1) to create the cardiac slowly activating delayed rectifier, I(Ks). Mutations throughout both genes are linked to the hereditary cardiac arrhythmias in the Long QT Syndrome (LQTS). KCNE1 exerts its specific regulation of KCNQ1 activation via interactions between membrane-spanning segments of the two proteins. Less detailed attention has been focused on the role of the KCNE1 C-terminus in regulating channel behavior. We analyzed the effects of an LQT5 point mutation (D76N) and the truncation of the entire C-terminus (Delta70) on channel regulation, assembly and interaction. Both mutations significantly shifted voltage dependence of activation in the depolarizing direction and decreased I(Ks) current density. They also accelerated rates of channel deactivation but notably, did not affect activation kinetics. Truncation of the C-terminus reduced the apparent affinity of KCNE1 for KCNQ1, resulting in impaired channel formation and presentation of KCNQ1/KCNE1 complexes to the surface. Complete saturation of KCNQ1 channels with KCNE1-Delta70 could be achieved by relative over-expression of the KCNE subunit. Rate-dependent facilitation of K(+) conductance, a key property of I(Ks) that enables action potential shortening at higher heart rates, was defective for both KCNE1 C-terminal mutations, and may contribute to the clinical phenotype of arrhythmias triggered by heart rate elevations during exercise in LQTS mutations. These results support several roles for KCNE1 C-terminus interaction with KCNQ1: regulation of channel assembly, open-state destabilization, and kinetics of channel deactivation.

Functional interactions between KCNE1 C-terminus and the KCNQ1 channel.

Mutations in several ion channel genes have been reported to cause rare cases of familial atrial fibrillation (AF).

Mutation in the S3 segment of KCNQ1 results in familial lone atrial fibrillation.

Rare ion channel polymorphisms: separating signal from noise.

Left ventricular (LV) pseudoaneurysm is a rare complication after myocardial infarction and cardiac surgery. Standard treatment remains surgical correction; however, percutaneous closure has been attempted in high risk surgical patients. We report a case of three dimensional echocardiography and cardiac CT defined LV pseudoaneurysm which was closed percutaneously using intracardiac echocardiography (ICE) and fluoroscopy guidance. Appropriate planning and guidance proved essential to the procedure with an excellent outcome. Percutaneous closure of LV pseudoaneurysms is safe and feasible in high risk surgical patients and with appropriate imaging modalities may be an alternative to surgical correction. Â© 2013 Wiley Periodicals, Inc.

Yonathan Melman

Cardiovascular Institute

Beth Israel Deaconess Medical Center, Harvard Medical School

Isolation, Culture, and Functional Characterization of Adult Mouse Cardiomyoctyes