Emerging concepts in the pharmacogenomics of arrhythmias: ion channel trafficking

Kv Channel Ancillary Subunits: Where Do We Go from Here?

Voltage-gated potassium (Kv) channels each comprise four pore-forming α-subunits that orchestrate essential duties such as voltage sensing and K+ selectivity and conductance. In vivo, however, Kv channels also incorporate regulatory subunits-some Kv channel specific, others more general modifiers of protein folding, trafficking, and function. Understanding all the above is essential for a complete picture of the role of Kv channels in physiology and disease.

Identification of small-molecule ion channel modulators in C. elegans channelopathy models

Ion channels are important therapeutic targets, but the discovery of ion channel drugs remains challenging due to a lack of assays that allow high-throughput screening in the physiological context. Here we report C. elegans phenotype-based methods for screening ion channel drugs. Expression of modified human ether-a-go-go-related gene (hERG) potassium channels in C. elegans results in egg-laying and locomotive defects, which offer indicators for screening small-molecule channel modulators. Screening in worms expressing hERG^A561V, which carries a trafficking-defective mutation A561V known to associate with long-QT syndrome, identifies two functional correctors Prostratin and ingenol-3,20-dibenzoate. These compounds activate PKCε signaling and consequently phosphorylate S606 at the pore region of the channel to promote hERG^A561V trafficking to the plasma membrane. Importantly, the compounds correct electrophysiological abnormalities in hiPSC-derived cardiomyocytes bearing a heterozygous CRISPR/Cas9-edited hERG^A561V. Thus, we have developed an in vivo high-throughput method for screening compounds that have therapeutic potential in treating channelopathies.

Mutations in the voltage-gated K⁺ channel human ether-a-go-go-related gene (hERG) lead to Long-QT syndrome, causing life-threatening cardiac arrhythmia. Here the authors use C. elegans as a platform to run a channelopathy drug screen, identifying drugs to target hERG mutants.

A forward genetic screen identifies chaperone CNX-1 as a conserved biogenesis regulator of ERG K+ channels

The human ether-a-go-go-related gene (hERG) encodes a voltage-gated potassium channel that controls repolarization of cardiac action potentials. Accumulating evidence suggests that most disease-related hERG mutations reduce the function of the channel by disrupting protein biogenesis of the channel in the endoplasmic reticulum (ER). However, the molecular mechanism underlying the biogenesis of ERG K⁺ channels is largely unknown. By forward genetic screening, we identified an ER-located chaperone CNX-1, the worm homologue of mammalian chaperone Calnexin, as a critical regulator for the protein biogenesis of UNC-103, the ERG-type K⁺ channel in Caenorhabditis elegans Loss-of-function mutations of cnx-1 decreased the protein level and current density of the UNC-103 K⁺ channel and suppressed the behavioral defects caused by a gain-of-function mutation in unc-103 Moreover, CNX-1 facilitated tetrameric assembly of UNC-103 channel subunits in a liposome-assisted cell-free translation system. Further studies showed that CNX-1 act in parallel to DNJ-1, another ER-located chaperone known to regulate maturation of UNC-103 channels, on controlling the protein biogenesis of UNC-103. Importantly, Calnexin interacted with hERG proteins in the ER in HEK293T cells. Deletion of calnexin reduced the expression and current densities of endogenous hERG K⁺ channels in SH-SY5Y cells. Collectively, we reveal an evolutionarily conserved chaperone CNX-1/Calnexin controlling the biogenesis of ERG-type K⁺ channels.

Chansporter complexes in cell signaling

Ion channels facilitate diffusion of ions across cell membranes for such diverse purposes as neuronal signaling, muscular contraction, and fluid homeostasis. Solute transporters often utilize ionic gradients to move aqueous solutes up their concentration gradient, also fulfilling a wide variety of tasks. Recently, an increasing number of ion channel-transporter ('chansporter') complexes have been discovered. Chansporter complex formation may overcome what could otherwise be considerable spatial barriers to rapid signal integration and feedback between channels and transporters, the ions and other substrates they transport, and environmental factors to which they must respond. Here, current knowledge in this field is summarized, covering both heterologous expression structure/function findings and potential mechanisms by which chansporter complexes fulfill contrasting roles in cell signaling in vivo.

Over-expression of microRNA-1 causes arrhythmia by disturbing intracellular trafficking system

Dysregulation of intracellular trafficking system plays a fundamental role in the progression of cardiovascular disease. Up-regulation of miR-1 contributes to arrhythmia, we sought to elucidate whether intracellular trafficking contributes to miR-1-driven arrhythmia. By performing microarray analyses of the transcriptome in the cardiomyocytes-specific over-expression of microRNA-1 (miR-1 Tg) mice and the WT mice, we found that these differentially expressed genes in miR-1 Tg mice were significantly enrichment with the trafficking-related biological processes, such as regulation of calcium ion transport. Also, the qRT-PCR and western blot results validated that Stx6, Braf, Ube3a, Mapk8ip3, Ap1s1, Ccz1 and Gja1, which are the trafficking-related genes, were significantly down-regulated in the miR-1 Tg mice. Moreover, we found that Stx6 was decreased in the heart of mice after myocardial infarction and in the hypoxic cardiomyocytes, and further confirmed that Stx6 is a target of miR-1. Meanwhile, knockdown of Stx6 in cardiomyocytes resulted in the impairments of PLM and L-type calcium channel, which leads to the increased resting ([Ca²⁺]_i). On the contrary, overexpression of Stx6 attenuated the impairments of miR-1 or hypoxia on PLM and L-type calcium channel. Thus, our studies reveals that trafficking-related gene Stx6 may regulate intracellular calcium and is involved in the occurrence of cardiac arrhythmia, which provides new insights in that miR-1 participates in arrhythmia by regulating the trafficking-related genes and pathway.

Histone Deacetylase Inhibitors Prolong Cardiac Repolarization through Transcriptional Mechanisms

Histone deacetylase (HDAC) inhibitors are an emerging class of anticancer agents that modify gene expression by altering the acetylation status of lysine residues of histone proteins, thereby inducing transcription, cell cycle arrest, differentiation, and cell death or apoptosis of cancer cells. In the clinical setting, treatment with HDAC inhibitors has been associated with delayed cardiac repolarization and in rare instances a lethal ventricular tachyarrhythmia known as torsades de pointes. The mechanism(s) of HDAC inhibitor-induced effects on cardiac repolarization is unknown. We demonstrate that administration of structurally diverse HDAC inhibitors to dogs causes delayed but persistent increases in the heart rate corrected QT interval (QTc), an in vivo measure of cardiac repolarization, at timepoints far removed from the Tmax for parent drug and metabolites. Transcriptional profiling of ventricular myocardium from dogs treated with various HDAC inhibitors demonstrated effects on genes involved in protein trafficking, scaffolding and insertion of various ion channels into the cell membrane as well as genes for specific ion channel subunits involved in cardiac repolarization. Extensive in vitro ion channel profiling of various structural classes of HDAC inhibitors (and their major metabolites) by binding and acute patch clamp assays failed to show any consistent correlations with direct ion channel blockade. Drug-induced rescue of an intracellular trafficking-deficient mutant potassium ion channel, hERG (G601S), and decreased maturation (glycosylation) of wild-type hERG expressed by CHO cells in vitro correlated with prolongation of QTc intervals observed in vivo The results suggest that HDAC inhibitor-induced prolongation of cardiac repolarization may be mediated in part by transcriptional changes of genes required for ion channel trafficking and localization to the sarcolemma. These data have broad implications for the development of these drug classes and suggest that the optimal time to assess potentially transcriptionally mediated physiologic effects will be delayed relative to an epigenetic drug's Tmax/Cmax.

Identifying potential functional impact of mutations and polymorphisms: linking heart failure, increased risk of arrhythmias and sudden cardiac death

Researchers and clinicians have discovered several important concepts regarding the mechanisms responsible for increased risk of arrhythmias, heart failure, and sudden cardiac death. One major step in defining the molecular basis of normal and abnormal cardiac electrical behavior has been the identification of single mutations that greatly increase the risk for arrhythmias and sudden cardiac death by changing channel-gating characteristics. Indeed, mutations in several genes encoding ion channels, such as SCN5A, which encodes the major cardiac Na⁺ channel, have emerged as the basis for a variety of inherited cardiac arrhythmias such as long QT syndrome, Brugada syndrome, progressive cardiac conduction disorder, sinus node dysfunction, or sudden infant death syndrome. In addition, genes encoding ion channel accessory proteins, like anchoring or chaperone proteins, which modify the expression, the regulation of endocytosis, and the degradation of ion channel a-subunits have also been reported as susceptibility genes for arrhythmic syndromes. The regulation of ion channel protein expression also depends on a fine-tuned balance among different other mechanisms, such as gene transcription, RNA processing, post-transcriptional control of gene expression by miRNA, protein synthesis, assembly and post-translational modification and trafficking. The aim of this review is to inventory, through the description of few representative examples, the role of these different biogenic mechanisms in arrhythmogenesis, HF and SCD in order to help the researcher to identify all the processes that could lead to arrhythmias. Identification of novel targets for drug intervention should result from further understanding of these fundamental mechanisms.

A case for pharmacogenomics in management of cardiac arrhythmias

Disorders of the cardiac rhythm are quite prevalent in clinical practice. Though the variability in drug response between individuals has been extensively studied, this information has not been widely used in clinical practice. Rapid advances in the field of pharmacogenomics have provided us with crucial insights on inter-individual genetic variability and its impact on drug metabolism and action. Technologies for faster and cheaper genetic testing and even personal genome sequencing would enable clinicians to optimize prescription based on the genetic makeup of the individual, which would open up new avenues in the area of personalized medicine. We have systematically looked at literature evidence on pharmacogenomics markers for anti-arrhythmic agents from the OpenPGx consortium collection and reason the applicability of genetics in the management of arrhythmia. We also discuss potential issues that need to be resolved before personalized pharmacogenomics becomes a reality in regular clinical practice.

Visualizing cardiac ion channel trafficking pathways

Understanding cardiac electrical and mechanical function requires knowledge of cardiac muscle at the subcellular level. Traditional biochemical and electrophysiological techniques have provided invaluable information in the description of ion channels and their occurrence in various tissues. This knowledge is the basis for our current ability to understand how subcellular ion channel localization occurs and is regulated. We are now in an era whereby individual ion channels can be followed from the moment of their synthesis to placement on the plasma membrane, movements within the membrane, internalization back into the cytoplasm, and degradation. Such insight opens many possibilities for the dissection of regulatory elements governing ion channel expression and function, which will in turn be translated to future therapies for cardiac disease. In this chapter, we discuss the structure of cardiomyocytes and their submembrane domains, the ion channels that we study, and the techniques that can be employed to visualize cardiac ion channel trafficking in real time.