Divergent regulation of KCNQ1/E1 by targeted recruitment of protein kinase A to distinct sites on the channel complex

The slow delayed rectifier potassium current, IKs, conducted through pore-forming Q1 and auxiliary E1 ion channel complexes is important for human cardiac action potential repolarization. During exercise or fright, IKs is up-regulated by protein kinase A (PKA)-mediated Q1 phosphorylation to maintain heart rhythm and optimum cardiac performance. Sympathetic up-regulation of IKs requires recruitment of PKA holoenzyme (two regulatory - RI or RII - and two catalytic Cα subunits) to Q1 C-terminus by an A kinase anchoring protein (AKAP9). Mutations in Q1 or AKAP9 that abolish their functional interaction result in long QT syndrome type 1 and 11, respectively, which increases the risk of sudden cardiac death during exercise. Here, we investigated the utility of a targeted protein phosphorylation (TPP) approach to reconstitute PKA regulation of IKs in the absence of AKAP9. Targeted recruitment of endogenous Cα to E1-YFP using a GFP/YFP nanobody (nano) fused to RIIα enabled acute cAMP-mediated enhancement of IKs, reconstituting physiological regulation of the channel complex. By contrast, nano-mediated tethering of RIIα or Cα to Q1-YFP constitutively inhibited IKs by retaining the channel intracellularly in the endoplasmic reticulum and Golgi. Proteomic analysis revealed that distinct phosphorylation sites are modified by Cα targeted to Q1-YFP compared to free Cα. Thus, functional outcomes of synthetically recruited PKA on IKs regulation is critically dependent on the site of recruitment within the channel complex. The results reveal insights into divergent regulation of IKs by phosphorylation across different spatial and time scales, and suggest a TPP approach to develop new drugs to prevent exercise-induced sudden cardiac death.

Acute phosphatidylinositol 4,5 bisphosphate depletion destabilizes sarcolemmal expression of cardiac L-type Ca2+ channel CaV1.2

Ca_V1.2 channels are critical players in cardiac excitation-contraction coupling, yet we do not understand how they are affected by an important therapeutic target of heart failure drugs and regulator of blood pressure, angiotensin II. Signaling through G_q-coupled AT1 receptors, angiotensin II triggers a decrease in PIP₂, a phosphoinositide component of the plasma membrane (PM) and known regulator of many ion channels. PIP₂ depletion suppresses Ca_V1.2 currents in heterologous expression systems but the mechanism of this regulation and whether a similar phenomenon occurs in cardiomyocytes is unknown. Previous studies have shown that Ca_V1.2 currents are also suppressed by angiotensin II. We hypothesized that these two observations are linked and that PIP₂ stabilizes Ca_V1.2 expression at the PM and angiotensin II depresses cardiac excitability by stimulating PIP₂ depletion and destabilization of Ca_V1.2 expression. We tested this hypothesis and report that Ca_V1.2 channels in tsA201 cells are destabilized after AT1 receptor-triggered PIP₂ depletion, leading to their dynamin-dependent endocytosis. Likewise, in cardiomyocytes, angiotensin II decreased t-tubular Ca_V1.2 expression and cluster size by inducing their dynamic removal from the sarcolemma. These effects were abrogated by PIP₂ supplementation. Functional data revealed acute angiotensin II reduced Ca_V1.2 currents and Ca²⁺ transient amplitudes thus diminishing excitation-contraction coupling. Finally, mass spectrometry results indicated whole-heart levels of PIP₂ are decreased by acute angiotensin II treatment. Based on these observations, we propose a model wherein PIP₂ stabilizes Ca_V1.2 membrane lifetimes, and angiotensin II-induced PIP₂ depletion destabilizes sarcolemmal Ca_V1.2, triggering their removal, and the acute reduction of Ca_V1.2 currents and contractility.

Mechanisms and Regulation of Cardiac CaV1.2 Trafficking

During cardiac excitation contraction coupling, the arrival of an action potential at the ventricular myocardium triggers voltage-dependent L-type Ca²⁺ (Ca_V1.2) channels in individual myocytes to open briefly. The level of this Ca²⁺ influx tunes the amplitude of Ca²⁺-induced Ca²⁺ release from ryanodine receptors (RyR2) on the junctional sarcoplasmic reticulum and thus the magnitude of the elevation in intracellular Ca²⁺ concentration and ultimately the downstream contraction. The number and activity of functional Ca_V1.2 channels at the t-tubule dyads dictates the amplitude of the Ca²⁺ influx. Trafficking of these channels and their auxiliary subunits to the cell surface is thus tightly controlled and regulated to ensure adequate sarcolemmal expression to sustain this critical process. To that end, recent discoveries have revealed the existence of internal reservoirs of preformed Ca_V1.2 channels that can be rapidly mobilized to enhance sarcolemmal expression in times of acute stress when hemodynamic and metabolic demand increases. In this review, we provide an overview of the current thinking on Ca_V1.2 channel trafficking dynamics in the heart. We highlight the numerous points of control including the biosynthetic pathway, the endosomal recycling pathway, ubiquitination, and lysosomal and proteasomal degradation pathways, and discuss the effects of β-adrenergic and angiotensin receptor signaling cascades on this process.

The structure and function of TRIP8b, an auxiliary subunit of hyperpolarization-activated cyclic-nucleotide gated channels

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed throughout the mammalian central nervous system (CNS). These channels have been implicated in a wide range of diseases, including Major Depressive Disorder and multiple subtypes of epilepsy. The diversity of functions that HCN channels perform is in part attributable to differences in their subcellular localization. To facilitate a broad range of subcellular distributions, HCN channels are bound by auxiliary subunits that regulate surface trafficking and channel function. One of the best studied auxiliary subunits is tetratricopeptide-repeat containing, Rab8b-interacting protein (TRIP8b). TRIP8b is an extensively alternatively spliced protein whose only known function is to regulate HCN channels. TRIP8b binds to HCN pore-forming subunits at multiple interaction sites that differentially regulate HCN channel function and subcellular distribution. In this review, we summarize what is currently known about the structure and function of TRIP8b isoforms with an emphasis on the role of this auxiliary subunit in health and disease.

Trafficking of Stretch-Regulated TRPV2 and TRPV4 Channels Inferred Through Interactomics

Transient receptor potential cation channels are emerging as important physiological and therapeutic targets. Within the vanilloid subfamily, transient receptor potential vanilloid 2 (TRPV2) and 4 (TRPV4) are osmo- and mechanosensors becoming critical determinants in cell structure and activity. However, knowledge is scarce regarding how TRPV2 and TRPV4 are trafficked to the plasma membrane or specific organelles to undergo quality controls through processes such as biosynthesis, anterograde/retrograde trafficking, and recycling. This review lists and reviews a subset of protein–protein interactions from the TRPV2 and TRPV4 interactomes, which is related to trafficking processes such as lipid metabolism, phosphoinositide signaling, vesicle-mediated transport, and synaptic-related exocytosis. Identifying the protein and lipid players involved in trafficking will improve the knowledge on how these stretch-related channels reach specific cellular compartments.

Development of a High-Throughput Flow Cytometry Assay to Monitor Defective Trafficking and Rescue of Long QT2 Mutant hERG Channels

Long QT Syndrome (LQTS) is an acquired or inherited disorder characterized by prolonged QT interval, exertion-triggered arrhythmias, and sudden cardiac death. One of the most prevalent hereditary LQTS subtypes, LQT2, results from loss-of-function mutations in the hERG channel, which conducts I_Kr, the rapid component of the delayed rectifier K⁺ current, critical for cardiac repolarization. The majority of LQT2 mutations result in Class 2 deficits characterized by impaired maturation and trafficking of hERG channels. Here, we have developed a high-throughput flow cytometric assay to analyze the surface and total expression of wild-type (WT) and mutant hERG channels with single-cell resolution. To test our method, we focused on 16 LQT2 mutations in the hERG Per-Arnt-Sim (PAS) domain that were previously studied via a widely used biochemical approach that compares levels of 135-kDa immature and 155-kDa fully glycosylated hERG protein to infer surface expression. We confirmed that LQT2 mutants expressed in HEK293 cells displayed a decreased surface density compared to WT hERG, and were differentially rescued by low temperature. However, we also uncovered some notable differences from the findings obtained via the biochemical approach. In particular, three mutations (N33T, R56Q, and A57P) with apparent WT-like hERG glycosylation patterns displayed up to 50% decreased surface expression. Furthermore, despite WT-like levels of complex glycosylation, these mutants have impaired forward trafficking, and exhibit varying half-lives at the cell surface. The results highlight utility of the surface labeling/flow cytometry approach to quantitatively assess trafficking deficiencies associated with LQT2 mutations, to discern underlying mechanisms, and to report on interventions that rescue deficits in hERG surface expression.

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.

Functional interaction with filamin A and intracellular Ca2+ enhance the surface membrane expression of a small-conductance Ca2+-activated K+ (SK2) channel

For an excitable cell to function properly, a precise number of ion channel proteins need to be trafficked to distinct locations on the cell surface membrane, through a network and anchoring activity of cytoskeletal proteins. Not surprisingly, mutations in anchoring proteins have profound effects on membrane excitability. Ca(2+)-activated K(+) channels (KCa2 or SK) have been shown to play critical roles in shaping the cardiac atrial action potential profile. Here, we demonstrate that filamin A, a cytoskeletal protein, augments the trafficking of SK2 channels in cardiac myocytes. The trafficking of SK2 channel is Ca(2+)-dependent. Further, the Ca(2+) dependence relies on another channel-interacting protein, α-actinin2, revealing a tight, yet intriguing, assembly of cytoskeletal proteins that orchestrate membrane expression of SK2 channels in cardiac myocytes. We assert that changes in SK channel trafficking would significantly alter atrial action potential and consequently atrial excitability. Identification of therapeutic targets to manipulate the subcellular localization of SK channels is likely to be clinically efficacious. The findings here may transcend the area of SK2 channel studies and may have implications not only in cardiac myocytes but in other types of excitable cells.