Design of mutant beta2 subunits as decoy molecules to reduce the expression of functional Ca2+ channels in cardiac cells

β subunits of voltage-gated calcium channels in cardiovascular diseases

Calcium signaling is required in bodily functions essential for survival, such as muscle contractions and neuronal communications. Of note, the voltage-gated calcium channels (VGCCs) expressed on muscle and neuronal cells, as well as some endocrine cells, are transmembrane protein complexes that allow for the selective entry of calcium ions into the cells. The α1 subunit constitutes the main pore-forming subunit that opens in response to membrane depolarization, and its biophysical functions are regulated by various auxiliary subunits-β, α2δ, and γ subunits. Within the cardiovascular system, the γ-subunit is not expressed and is therefore not discussed in this review. Because the α1 subunit is the pore-forming subunit, it is a prominent druggable target and the focus of many studies investigating potential therapeutic interventions for cardiovascular diseases. While this may be true, it should be noted that the direct inhibition of the α1 subunit may result in limited long-term cardiovascular benefits coupled with undesirable side effects, and that its expression and biophysical properties may depend largely on its auxiliary subunits. Indeed, the α2δ subunit has been reported to be essential for the membrane trafficking and expression of the α1 subunit. Furthermore, the β subunit not only prevents proteasomal degradation of the α1 subunit, but also directly modulates the biophysical properties of the α1 subunit, such as its voltage-dependent activities and open probabilities. More importantly, various isoforms of the β subunit have been found to differentially modulate the α1 subunit, and post-translational modifications of the β subunits further add to this complexity. These data suggest the possibility of the β subunit as a therapeutic target in cardiovascular diseases. However, emerging studies have reported the presence of cardiomyocyte membrane α1 subunit trafficking and expression in a β subunit-independent manner, which would undermine the efficacy of β subunit-targeting drugs. Nevertheless, a better understanding of the auxiliary β subunit would provide a more holistic approach when targeting the calcium channel complexes in treating cardiovascular diseases. Therefore, this review focuses on the post-translational modifications of the β subunit, as well as its role as an auxiliary subunit in modulating the calcium channel complexes.

Prenatal Hypoxia Induced Dysfunction in Cerebral Arteries of Offspring Rats

Background

Hypoxia during pregnancy could cause abnormal development and lead to increased risks of vascular diseases in adults. This study determined angiotensin II (AII)‐mediated vascular dysfunction in offspring middle cerebral arteries (MCA).

Methods and Results

Pregnant rats were subjected to hypoxia. Vascular tension in offspring MCA by AII with or without inhibitors, calcium channel activities, and endoplasmic reticulum calcium stores were tested. Whole‐cell patch clamping was used to investigate voltage‐dependent calcium channel currents. mRNA expression was tested using quantitative real‐time polymerase chain reaction. AII‐mediated MCA constriction was greater in male offspring exposed to prenatal hypoxia. AT1 and AT2 receptors were involved in the altered AII‐mediated vasoconstriction. Prenatal hypoxia increased baseline activities of L‐type calcium channel currents in MCA smooth muscle cells. However, calcium currents stimulated by AII were not significantly changed, whereas nifedipine inhibited AII‐mediated vasoconstrictions in the MCA. Activities of IP₃/ryanodine receptor–operated calcium channels, endoplasmic reticulum calcium stores, and sarcoendoplasmic reticulum membrane Ca²⁺‐ATPase were increased. Prenatal hypoxia also caused dysfunction of vasodilatation via the endothelium NO synthase. The mRNA expressions of AT1A, AT1B, AT2R, Cav1.2α1C, Cav3.2α1H, and ryanodine receptor RyR2 were increased in the prenatal‐hypoxia group.

Conclusions

Hypoxia in pregnancy could induce dysfunction in both contraction and dilation in the offspring MCA. AII‐increased constriction in the prenatal‐hypoxia group was not mainly dependent on the L‐type and T‐type calcium channels; it might predominantly rely on the AII receptors, IP₃/ryanodine receptors, and the endoplasmic reticulum calcium store as well as calcium ATPase.

Overexpression of the Large-Conductance, Ca2+-Activated K+ (BK) Channel Shortens Action Potential Duration in HL-1 Cardiomyocytes

Long QT syndrome is characterized by a prolongation of the interval between the Q wave and the T wave on the electrocardiogram. This abnormality reflects a prolongation of the ventricular action potential caused by a number of genetic mutations or a variety of drugs. Since effective treatments are unavailable, we explored the possibility of using cardiac expression of the large-conductance, Ca²⁺-activated K⁺ (BK) channel to shorten action potential duration (APD). We hypothesized that expression of the pore-forming α subunit of human BK channels (hBKα) in HL-1 cells would shorten action potential duration in this mouse atrial cell line. Expression of hBKα had minimal effects on expression levels of other ion channels with the exception of a small but significant reduction in Kv11.1. Patch-clamped hBKα expressing HL-1 cells exhibited an outward voltage- and Ca²⁺-sensitive K⁺ current, which was inhibited by the BK channel blocker iberiotoxin (100 nM). This BK current phenotype was not detected in untransfected HL-1 cells or in HL-1 null cells sham-transfected with an empty vector. Importantly, APD in hBKα-expressing HL-1 cells averaged 14.3 ± 2.8 ms (n = 10), which represented a 53% reduction in APD compared to HL-1 null cells lacking BKα expression. APD in the latter cells averaged 31.0 ± 5.1 ms (n = 13). The shortened APD in hBKα-expressing cells was restored to normal duration by 100 nM iberiotoxin, suggesting that a repolarizing K⁺ current attributed to BK channels accounted for action potential shortening. These findings provide initial proof-of-concept that the introduction of hBKα channels into a cardiac cell line can shorten APD, and raise the possibility that gene-based interventions to increase hBKα channels in cardiac cells may hold promise as a therapeutic strategy for long QT syndrome.

Ion channel remodeling in vascular smooth muscle during hypertension: Implications for novel therapeutic approaches

Ion channels are multimeric, transmembrane proteins that selectively mediate ion flux across the plasma membrane in a variety of cells including vascular smooth muscle cells (VSMCs). The dynamic interplay of Ca(2+) and K(+) channels on the plasma membrane of VSMCs plays a pivotal role in modulating the vascular tone of small arteries and arterioles. The abnormally-elevated arterial tone observed in hypertension thus points to an aberrant expression and function of Ca(2+) and K(+) channels in the VSMCs. In this short review, we focus on the three well-studied ion channels in VSMCs, namely the L-type Ca(2+) (CaV1.2) channels, the voltage-gated K(+) (KV) channels, and the large-conductance Ca(2+)-activated K(+) (BK) channels. First, we provide a brief overview on the physiological role of vascular CaV1.2, KV and BK channels in regulating arterial tone. Second, we discuss the current understanding of the expression changes and regulation of CaV1.2, KV and BK channels in the vasculature during hypertension. Third, based on available proof-of-concept studies, we describe the potential therapeutic approaches targeting these vascular ion channels in order to restore blood pressure to normotensive levels.

Cav1.2, cell proliferation, and new target in atherosclerosis

Ca_v1.2 calcium channels are the principal proteins involved in electrical, mechanical, and/or signaling functions of the cell. Ca_v1.2 couples membrane depolarization to the transient increase in intracellular Ca²⁺ concentration that is a trigger for muscle contraction and CREB-dependent transcriptional activation. The CACNA1C gene coding for the Ca_v1.2 pore-forming α_1C subunit is subject to extensive alternative splicing. This review is the first attempt to follow the association between cell proliferation, Ca_v1.2 expression and splice variation, and atherosclerosis. Based on insights into the association between the atherosclerosis-induced molecular remodeling of Ca_v1.2, proliferation of vascular smooth muscle cells, and CREB-dependent transcriptional signaling, this review will give a perspective outlook for the use of the CACNA1C exon skipping as a new potential gene therapy approach to atherosclerosis.

The β3 subunit contributes to vascular calcium channel upregulation and hypertension in angiotensin II-infused C57BL/6 mice

Voltage-gated L-type Ca(2+) (Ca(v)1.2) channels in vascular smooth muscle cells are a predominant Ca(2+) influx pathway that mediates arterial tone. Channel biogenesis is accomplished when the pore-forming α(1C) subunit coassembles with regulatory Ca(v)β subunits intracellularly, and the multiprotein Ca(v)1.2 channel complex translocates to the plasma membrane to form functional Ca(2+) channels. We hypothesized that the main Ca(v)β isoform in vascular smooth muscle cells, Ca(v)β3, is required for the upregulation of arterial Ca(v)1.2 channels during the development of hypertension, an event associated with abnormal Ca(2+)-dependent tone. Ca(v)1.2 channel expression and function were compared between second-order mesenteric arteries of C57BL/6 wild-type (WT) and Ca(v)β3(-/-) mice infused with saline (control) or angiotensin II (Ang II) for 2 weeks to induce hypertension. The mesenteric arteries of Ang II-infused WT mice showed increased Ca(v)1.2 channel expression and accentuated Ca(2+)-mediated contractions compared with saline-infused WT mice. In contrast, Ca(v)1.2 channels failed to upregulate in mesenteric arteries of Ang II-infused Ca(v)β3(-/-) mice, and Ca(2+)-dependent reactivity was normal in these arteries. Basal systolic blood pressure was not significantly different between WT and Ca(v)β3(-/-) mice (98 ± 2 and 102 ± 3 mm Hg, respectively), but the Ca(v)β3(-/-) mice showed a blunted pressor response to Ang II infusion. Two weeks after the start of Ang II administration, the systolic blood pressure of Ca(v)β3(-/-) mice averaged 149 ± 4 mm Hg compared with 180 ± 5 mm Hg in WT mice. Thus, the Ca(v)β3 subunit is a critical regulatory protein required to upregulate arterial Ca(v)1.2 channels and fully develop Ang II-dependent hypertension in C57BL/6 mice.

Use of escin as a perforating agent on the IonWorks quattro automated electrophysiology platform

The automated electrophysiology platform IonWorks has facilitated the medium-throughput study of ion channel biology and pharmacology. Electrical and chemical access to the cell is by perforated patch, afforded by amphotericin. Permeation of the amphotericin pore is limited to monovalent cations. We describe here the use of the saponin escin as an alternative perforating agent. With respect to the number and robustness of seals formed across a variety of cell and ion channel types, the performance of escin is equal to that of amphotericin. Escin also permits the permeation of larger molecules through its pore. These include nucleotides, important intracellular modulators of ion channel activity that can be used to prevent ion channel rundown of, for instance, Ca(V)1.2. Furthermore, pharmacologic agents such as QX314 can also permeate and be used for mechanistic studies. Escin, in combination with IonWorks, increases the scope of ion channel screening and can facilitate the assay of previously difficult-to-assay targets.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

Modification of cardiovascular ion channels by gene therapy

Delivery of genes to the heart and vasculature for therapeutic purposes is an exciting strategy that is approaching clinical reality. Abnormalities of expression or function of ion channels is central to many cardiovascular diseases and gene delivery to modify ion channels is an appealing alternative to traditional therapy with small-molecule drugs. Potential therapeutic targets include hypertrophy and heart failure, atrioventricular node modification in atrial fibrillation, ventricular tachycardia and hypertension. Numerous approaches for gene delivery are under development, including use of tissue-specific promoters in viral vectors. For other applications, such as development of biological pacemakers, cells can be transduced with pacemaker genes in vitro, and then the cells implanted within the heart. There are short-term hurdles to therapeutic gene delivery to modify cardiovascular ion channels, but in the intermediate and longer term, the outlook is promising.

Delivery of ion channel genes to treat cardiovascular diseases

Modifying ion channel expression and function in the heart and vasculature are potentially useful, novel approaches to managing cardiac hypertrophy, atrial fibrillation and hypertension. Calcium channels play a pivotal role in the heart and vasculature in controlling muscle contraction as well as other aspects of calcium-dependent signaling. The present investigation reports development of mutated L-type calcium channel beta subunits that are delivered by an adenoviral vector to vascular smooth muscle tissue. Wild type subunits serve a chaperone function for the pore-forming alpha(1C) subunit of the calcium channel, localize to the cell membrane and enhance calcium current. Conversely, mutated subunits function as dominant negative, defective chaperone molecules that disrupt targeting to the cell membrane and decrease calcium current. The dominant negative genes can be delivered in vitro and ex vivo, and have the potential to decrease arterial tone and lower blood pressure in vivo.