Microarray analysis reveals complex remodeling of cardiac ion channel expression with altered thyroid status: relation to cellular and integrated electrophysiology

[Arrhythmias in thyroid dysfunction]

Thyroid dysfunction is associated with characteristic changes in heart rate and arrhythmias. Thyroid hormones act through genomic and non-genomic effects on myocytes and influence contractility, relaxation and action potential duration through a variety of mechanisms. Atrial fibrillation is the most common arrhythmia associated with thyroid dysfunction, it occurs in both euthyroidism and hyperthyroidism in clear association with T4 levels. Mechanistically, in the hyperthyroid state, increased automaticity and triggered activity, together with a shortened refractory period and slowing of the conduction speed, lead to the initiation and maintenance of multiple intraatrial reentry circuits. Influences from the autonomic nervous system and hemodynamics controlled by thyroid hormones act as modulators for arrhythmias, which are promoted by a corresponding substrate (significant impact of comorbidities). Concerning therapy, in addition to treating hyperthyroidism, the initial therapeutic focus is on adequate rate control and anticoagulation in patients with a high risk of thromboembolism. Ablation of atrial fibrillation can be considered later on, although there is an increased likelihood of recurrence compared to patients without hyperthyroidism.Prolongation of the QT interval and increase in QT dispersion are involved in the formation of ventricular arrhythmias. Epidemiological data suggest an association of elevated T4 levels with ventricular arrhythmias and sudden cardiac death. However, this seems to be mainly relevant for patients with underlying cardiac disease (e.g. ICD users).

Severe hypothyroidism as a trigger for Brugada-type ECG abnormalities: a case report and literature review

Brugada syndrome (BrS) is an inherited disorder that can cause ventricular fibrillation and sudden cardiac death in individuals with otherwise structurally normal hearts. Several provoking factors are known to potentially unmask or exacerbate a typical Brugada ECG pattern in predisposed subjects. Hypothyroidism has been suggested as one of these triggers, but the exact mechanisms underlying this relationship remain poorly understood. Moreover, the severity of thyroid dysfunction beyond which a Brugada-type ECG alteration might be triggered is still unclear. We report the case of a 33-year-old male who displayed a Brugada type 1 ECG pattern and was diagnosed with severe hypothyroidism (TSH > 100 mU/L with undetectable levels of fT4 and fT3). Hormonal replacement therapy with levothyroxine was initiated at increasing doses; serial biochemical and ECG controls were performed, initially every 3 weeks up to 15 weeks and afterward every 3 months. The regression of typical Brugada ECG waveforms could be seen at an early stage, when the patient was still taking a low dose of levothyroxine (37.5 µg/day, i.e., one-fourth of his final requirements of 150 µg/day), and laboratory tests still showed a marked alteration of thyroid hormonal parameters. Hypothyroidism may act as a trigger for Brugada-type ECG abnormalities, but a very severe alteration of the hormonal parameters is necessary to prompt these alterations. In our case, the initiation of replacement therapy with levothyroxine rapidly reversed the ECG modifications, even at a low subtherapeutic dose.

Deciphering the roles of triiodothyronine (T3) and thyroid-stimulating hormone (TSH) on cardiac electrical remodeling in clinical and experimental hypothyroidism

Hypothyroidism is the most frequent endocrine pathology. Although clinical or overt hypothyroidism has been traditionally associated to low T3 / T4 and high thyrotropin (TSH) circulating levels, other forms exist such as subclinical hypothyroidism, characterized by normal blood T3 / T4 and high TSH. In its different forms is estimated to affect approximately 10% of the population, especially women, in a 5:1 ratio with respect to men. Among its consequences are alterations in cardiac electrical activity, especially in the repolarization phase, which is accompanied by an increased susceptibility to cardiac arrhythmias. Although these alterations have traditionally been attributed to thyroid hormone deficiency, recent studies, both clinical trials and experimental models, demonstrate a fundamental role of TSH in cardiac electrical remodeling. Thus, both metabolic thyroid hormones and TSH regulate cardiac ion channel expression in many and varied ways. This means that the different combinations of hormones that predominate in different types of hypothyroidism (overt, subclinic, primary, central) can generate different forms of cardiac electrical remodeling. These new findings are raising the relevant question of whether serum TSH reference ranges should be redefined.

Circadian regulation of Ca V 1.2 expression by RORα in the mouse heart

Background

In addition to show autonomous beating rhythmicity, the physiological functions of the heart present daily periodic oscillations. Notably the ventricular repolarization itself varies throughout the circadian cycle which was mainly related to the periodic expression of K + channels. However, the involvement of the L-type Ca 2+ channel (Ca V 1.2 encoded by Cacna1c gene) in these circadian variations remains elusive.

Methods

We used a transgenic mouse model (PCa-luc) that expresses the luciferase reporter under the control of the cardiac Cacna1c promoter and analyzed promoter activity by bioluminescent imaging, qPCR, immunoblot, Chromatin immunoprecipitation assay (ChIP) and Ca V 1.2 activity.

Results

Under normal 12:12h light-dark cycle, we observed in vivo a biphasic diurnal variation of promoter activities peaking at 9 and 19.5 Zeitgeber time (ZT). This was associated with a periodicity of Cacna1c mRNA levels preceding 24-h oscillations of Ca V 1.2 protein levels in ventricle (with a 1.5 h phase shift) but not in atrial heart tissues. The periodicity of promoter activities and Ca V 1.2 proteins, which correlated with biphasic oscillations of L-type Ca 2+ current conductance, persisted in isolated ventricular cardiomyocytes from PCa-Luc mice over the course of the 24-h cycle, suggesting an endogenous cardiac circadian regulation. Comparison of 24-h temporal patterns of clock gene expressions in ventricles and atrial tissues of the same mice revealed conserved circadian oscillations of the core clock genes except for the retinoid-related orphan receptor α gene (RORα), which remained constant throughout the course of a day in atrial tissues. In vitro we found that RORα is recruited to two specific regions on the Cacna1c promoter and that incubation with specific RORα inhibitor disrupted 24-h oscillations of ventricular promoter activities and Ca V 1.2 protein levels. Similar results were observed for pore forming subunits of the K + transient outward currents, K V 4.2 and K V 4.3.

Conclusions

These findings raise the possibility that the RORα-dependent rhythmic regulation of cardiac Ca V 1.2 and K V 4.2/4.3 throughout the daily cycle may play an important role in physiopathology of heart function.

Minor perturbations of thyroid homeostasis and major cardiovascular endpoints—Physiological mechanisms and clinical evidence

It is well established that thyroid dysfunction is linked to an increased risk of cardiovascular morbidity and mortality. The pleiotropic action of thyroid hormones strongly impacts the cardiovascular system and affects both the generation of the normal heart rhythm and arrhythmia. A meta-analysis of published evidence suggests a positive association of FT4 concentration with major adverse cardiovascular end points (MACE), but this association only partially extends to TSH. The risk for cardiovascular death is increased in both subclinical hypothyroidism and subclinical thyrotoxicosis. Several published studies found associations of TSH and FT4 concentrations, respectively, with major cardiovascular endpoints. Both reduced and elevated TSH concentrations predict the cardiovascular risk, and this association extends to TSH gradients within the reference range. Likewise, increased FT4 concentrations, but high-normal FT4 within its reference range as well, herald a poor outcome. These observations translate to a monotonic and sensitive effect of FT4 and a U-shaped relationship between TSH and cardiovascular risk. Up to now, the pathophysiological mechanism of this complex pattern of association is poorly understood. Integrating the available evidence suggests a dual etiology of elevated FT4 concentration, comprising both ensuing primary hypothyroidism and a raised set point of thyroid function, e. g. in the context of psychiatric disease, chronic stress and type 2 allostatic load. Addressing the association between thyroid homeostasis and cardiovascular diseases from a systems perspective could pave the way to new directions of research and a more personalized approach to the treatment of patients with cardiovascular risk.

Triiodothyronine and dexamethasone alter potassium channel expression and promote electrophysiological maturation of human-induced pluripotent stem cell-derived cardiomyocytes

BACKGROUND

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have emerged as a promising tool for disease modeling and drug development. However, hiPSC-CMs remain functionally immature, which hinders their utility as a model of human cardiomyocytes.

OBJECTIVE

To improve the electrophysiological maturation of hiPSC-CMs.

METHODS AND RESULTS

On day 16 of cardiac differentiation, hiPSC-CMs were treated with 100 nmol/L triiodothyronine (T3) and 1 μmol/L Dexamethasone (Dex) or vehicle for 14 days. On day 30, vehicle- and T3 + Dex-treated hiPSC-CMs were dissociated and replated either as cell sheets or single cells. Optical mapping and patch-clamp technique were used to examine the electrophysiological properties of vehicle- and T3 + Dex-treated hiPSC-CMs. Compared to vehicle, T3 + Dex-treated hiPSC-CMs had a slower spontaneous beating rate, more hyperpolarized resting membrane potential, faster maximal upstroke velocity, and shorter action potential duration. Changes in spontaneous activity and action potential were mediated by decreased hyperpolarization-activated current (I_f) and increased inward rectifier potassium currents (I_K1), sodium currents (I_Na), and the rapidly and slowly activating delayed rectifier potassium currents (I_Kr and I_Ks, respectively). Furthermore, T3 + Dex-treated hiPSC-CM cell sheets (hiPSC-CCSs) exhibited a faster conduction velocity and shorter action potential duration than the vehicle. Inhibition of I_K1 by 100 μM BaCl₂ significantly slowed conduction velocity and prolonged action potential duration in T3 + Dex-treated hiPSC-CCSs but had no effect in the vehicle group, demonstrating the importance of I_K1 for conduction velocity and action potential duration.

CONCLUSION

T3 + Dex treatment is an effective approach to rapidly enhance electrophysiological maturation of hiPSC-CMs.

Levothyroxine Treatment and the Risk of Cardiac Arrhythmias - Focus on the Patient Submitted to Thyroid Surgery

Levothyroxine (LT4) is used to treat frequently encountered endocrinopathies such as thyroid diseases. It is regularly used in clinical (overt) hypothyroidism cases and subclinical (latent) hypothyroidism cases in the last decade. Suppressive LT4 therapy is also part of the medical regimen used to manage thyroid malignancies after a thyroidectomy. LT4 treatment possesses dual effects: substituting new-onset thyroid hormone deficiency and suppressing the local and distant malignancy spreading in cancer. It is the practice to administer LT4 in less-than-high suppressive doses for growth control of thyroid nodules and goiter, even in patients with preserved thyroid function. Despite its approved safety for clinical use, LT4 can sometimes induce side-effects, more often recorded with patients under treatment with LT4 suppressive doses than in unintentionally LT4-overdosed patients. Cardiac arrhythmias and the deterioration of osteoporosis are the most frequently documented side-effects of LT4 therapy. It also lowers the threshold for the onset or aggravation of cardiac arrhythmias for patients with pre-existing heart diseases. To improve the quality of life in LT4-substituted patients, clinicians often prescribe higher doses of LT4 to reach low normal TSH levels to achieve cellular euthyroidism. In such circumstances, the risk of cardiac arrhythmias, particularly atrial fibrillation, increases, and the combined use of LT4 and triiodothyronine further complicates such risk. This review summarizes the relevant available data related to LT4 suppressive treatment and the associated risk of cardiac arrhythmia.

Alternative splicing potentiates dysfunction of early-onset epileptic encephalopathy SCN2A variants

Epileptic encephalopathies are severe forms of infantile-onset epilepsy often complicated by severe neurodevelopmental impairments. Some forms of early-onset epileptic encephalopathy (EOEE) have been associated with variants in SCN2A, which encodes the brain voltage-gated sodium channel NaV1.2. Many voltage-gated sodium channel genes, including SCN2A, undergo developmentally regulated mRNA splicing. The early onset of these disorders suggests that developmentally regulated alternative splicing of NaV1.2 may be an important consideration when elucidating the pathophysiological consequences of epilepsy-associated variants. We hypothesized that EOEE-associated NaV1.2 variants would exhibit greater dysfunction in a splice isoform that is prominently expressed during early development. We engineered five EOEE-associated NaV1.2 variants (T236S, E999K, S1336Y, T1623N, and R1882Q) into the adult and neonatal splice isoforms of NaV1.2 and performed whole-cell voltage clamp to elucidate their functional properties. All variants exhibited functional defects that could enhance neuronal excitability. Three of the five variants (T236S, E999K, and S1336Y) exhibited greater dysfunction in the neonatal isoform compared with those observed in the adult isoform. Computational modeling of a developing cortical pyramidal neuron indicated that T236S, E999K, S1336Y, and R1882Q showed hyperexcitability preferentially in immature neurons. These results suggest that both splice isoform and neuronal developmental stage influence how EOEE-associated NaV1.2 variants affect neuronal excitability.

The Assessment of Ventricular Arrhythmia Indicators Based on Electrocardiography in Patients With Differentiated Thyroid Cancer Followed Up With Levothyroxine Suppression

Levothyroxine suppression therapy (LST) can cause some unfavorable effects on the cardiovascular system in patients with differentiated thyroid cancer (DTC). The aim of this study was to evaluate ventricular arrhythmia predictors based on electrocardiography (ECG) in patients with DTC with LST. The ECG parameters including QT, corrected QT (QTc), Tp-e intervals, Tp-e/QT, and Tp-e/QTC ratios of 265 patients with DTC who met the inclusion criteria were compared with 100 controls. No difference was observed in the number of patients with DTC and controls with prolonged and borderline QTc interval (P = .273). Tp-e interval, Tp-e/QT, and Tp-e/QTc ratios were significantly higher in patients (P = .002, P = .02, P = .003; respectively). Linear regression analysis suggested that male gender was a predictor of higher Tp-e interval, Tp-e/QT, and Tp-e/QTc ratios (β = 4.322, R ² = 0.024, P = .042; β = 0.016, R ² = 0.048, P = .005; β = 0.015, R ² = 0.044, P = .006, respectively). A higher serum fT4 level was found to be associated with a higher Tp-e/QT ratio (β = 0.018, R ² = 0.089, P = .007). Ventricular arrhythmia indicators were found to be higher in patients with DTC with LST. Defining ventricular arrhythmia predictors through ECG, an easily accessible cardiac diagnostic tool, can be potentially useful in raising awareness of the possible cardiac harm of LST.

Relative role of T-tubules disruption and decreased SERCA2 on contractile dynamics of isolated rat ventricular myocytes

AIMS

Ventricular myocytes (VM) depolarization activates L-type Ca2+ channels (LCC) allowing Ca2+ influx (ICa) to synchronize sarcoplasmic reticulum (SR) Ca2+ release, via Ca2+-release channels (RyR2). The resulting whole-cell Ca2+ transient triggers contraction, while cytosolic Ca2+ removal by SR Ca2+ pump (SERCA2) and sarcolemmal Na+/Ca2+ exchanger (NCX) allows relaxation. In diseased hearts, extensive VM remodeling causes heterogeneous, blunted and slow Ca2+ transients. Among remodeling changes are: A) T-tubules disorganization. B) Diminished SERCA2 and low SR Ca2+. However, those often overlap, hindering their relative contribution to contractile dysfunction (CD). Furthermore, few studies have assessed their specific impact on the spatiotemporal Ca2+ transient properties and contractile dynamics simultaneously. Therefore, we sought to perform a quantitative comparison of how heterogeneous and slow Ca2+ transients, with different underlying determinants, affect contractile performance.

METHODS

We used two experimental models: A) formamide-induced acute "detubulation", where VM retain functional RyR2 and SERCA2, but lack T-tubules-associated LCC and NCX. B) Intact VM from hypothyroid rats, presenting decreased SERCA2 and SR Ca2+, but maintained T-tubules. By confocal imaging of Fluo-4-loaded VM, under field-stimulation, simultaneously acquired Ca2+ transients and shortening, allowing direct correlations.

KEY FINDINGS

We found near-linear correlations among key parameters of altered Ca2+ transients, caused independently by T-tubules disruption or decreased SR Ca2+, and shortening and relaxation, SIGNIFICANCE: Unrelated structural and molecular alterations converge in similarly abnormal Ca2+ transients and CD, highlighting the importance of independently reproduce disease-specific alterations, to quantitatively assess their impact on Ca2+ signaling and contractility, which would be valuable to determine potential disease-specific therapeutic targets.

The 10-Year Prognosis and Prevalence of Brugada-Type Electrocardiograms in Elderly Women: A Longitudinal Nationwide Community-Based Prospective Study

BACKGROUND

Brugada syndrome is a disorder associated with sudden cardiac death and characterized by an abnormal electrocardiogram (ECG). Previous studies were predominantly conducted in men, and the data on long-term prognosis are limited. Information about women, especially elderly women, is lacking.

OBJECTIVE

The aim of this study was to investigate the long-term prognosis of the Brugada ECG pattern in elderly women.

METHOD

We investigated the 10-year prognosis of the Brugada ECG pattern in elderly women in a nationwide community-based population in Taiwan. Community-dwelling women older than 55 years were prospectively recruited from December 2008 to March 2013 by a stratified random sampling method. All enrolled individuals were followed up annually until April 2019, and the cause of death was documented by citizen death records.

RESULTS

Among 2597 women, 60 (2.31%) had a Brugada-type ECG, and this prevalence was higher than the mean global prevalence of 0.23%. One woman had a type 1 ECG (0.04%), whereas 15 (0.58%) and 44 (1.70%) women had type 2 and type 3 ECG patterns, respectively. Cox survival analysis revealed that all-cause mortality and cardiac mortality were similar in the individuals with and without a Brugada-type ECG during a mean follow-up of 96.1 ± 20.5 months.

CONCLUSIONS

Our findings suggest that Brugada ECG patterns are not infrequent in elderly women but are not associated with increased risk of mortality in long-term follow-up; these findings may help reduce unnecessary anxiety for physicians, nurses, allied health caregivers, and patients.

Pro-Arrhythmic Signaling of Thyroid Hormones and Its Relevance in Subclinical Hyperthyroidism

A perennial task is to prevent the occurrence and/or recurrence of most frequent or life-threatening cardiac arrhythmias such as atrial fibrillation (AF) and ventricular fibrillation (VF). VF may be lethal in cases without an implantable cardioverter defibrillator or with failure of this device. Incidences of AF, even the asymptomatic ones, jeopardize the patient's life due to its complication, notably the high risk of embolic stroke. Therefore, there has been a growing interest in subclinical AF screening and searching for novel electrophysiological and molecular markers. Considering the worldwide increase in cases of thyroid dysfunction and diseases, including thyroid carcinoma, we aimed to explore the implication of thyroid hormones in pro-arrhythmic signaling in the pathophysiological setting. The present review provides updated information about the impact of altered thyroid status on both the occurrence and recurrence of cardiac arrhythmias, predominantly AF. Moreover, it emphasizes the importance of both thyroid status monitoring and AF screening in the general population, as well as in patients with thyroid dysfunction and malignancies. Real-world data on early AF identification in relation to thyroid function are scarce. Even though symptomatic AF is rare in patients with thyroid malignancies, who are under thyroid suppressive therapy, clinicians should be aware of potential interaction with asymptomatic AF. It may prevent adverse consequences and improve the quality of life. This issue may be challenging for an updated registry of AF in clinical practice. Thyroid hormones should be considered a biomarker for cardiac arrhythmias screening and their tailored management because of their multifaceted cellular actions.

High Thyrotropin Is Critical for Cardiac Electrical Remodeling and Arrhythmia Vulnerability in Hypothyroidism

Background: Hypothyroidism, the most common endocrine disease, induces cardiac electrical remodeling that creates a substrate for ventricular arrhythmias. Recent studies report that high thyrotropin (TSH) levels are related to cardiac electrical abnormalities and increased mortality rates. The aim of the present work was to investigate the direct effects of TSH on the heart and its possible causative role in the increased incidence of arrhythmia in hypothyroidism. Methods: A new rat model of central hypothyroidism (low TSH levels) was created and characterized together with the classical propylthiouracil-induced primary hypothyroidism model (high TSH levels). Electrocardiograms were recorded in vivo, and ionic currents were recorded from isolated ventricular myocytes in vitro by the patch-clamp technique. Protein and mRNA were measured by Western blot and quantitative reverse transcription polymerase chain reaction in rat and human cardiac myocytes. Adult human action potentials were simulated in silico to incorporate the experimentally observed changes. Results: Both primary and central hypothyroidism models increased the L-type Ca2+ current (ICa-L) and decreased the ultra-rapid delayed rectifier K+ current (IKur) densities. However, only primary but not central hypothyroidism showed electrocardiographic repolarization abnormalities and increased ventricular arrhythmia incidence during caffeine/dobutamine challenge. These changes were paralleled by a decrease in the density of the transient outward K+ current (Ito) in cardiomyocytes from animals with primary but not central hypothyroidism. In vitro treatment with TSH for 24 hours enhanced isoproterenol-induced spontaneous activity in control ventricular cells and diminished Ito density in cardiomyocytes from control and central but not primary hypothyroidism animals. In human myocytes, TSH decreased the expression of KCND3 and KCNQ1, Ito, and the delayed rectifier K+ current (IKs) encoding proteins in a protein kinase A-dependent way. Transposing the changes produced by hypothyroidism and TSH to a computer model of human ventricular action potential resulted in enhanced occurrence of early afterdepolarizations and arrhythmia mostly in primary hypothyroidism, especially under β-adrenergic stimulation. Conclusions: The results suggest that suppression of repolarizing K+ currents by TSH underlies most of the electrical remodeling observed in hypothyroidism. This work demonstrates that the activation of the TSH-receptor/protein kinase A pathway in the heart is responsible for the cardiac electrical remodeling and arrhythmia generation seen in hypothyroidism.

Thyroid and Cardiovascular Disease: Research Agenda for Enhancing Knowledge, Prevention, and Treatment

Thyroid hormones have long been known to have a range of effects on the cardiovascular system. However, significant knowledge gaps exist concerning the precise molecular and biochemical mechanisms governing these effects and the optimal strategies for management of abnormalities in thyroid function in patients with and without preexisting cardiovascular disease. In September 2017, the National Heart, Lung, and Blood Institute convened a Working Group with the goal of developing priorities for future scientific research relating thyroid dysfunction to the progression of cardiovascular disease. The Working Group reviewed and discussed the roles of normal thyroid physiology, the consequences of thyroid dysfunction, and the effects of therapy in 3 cardiovascular areas: cardiac electrophysiology and arrhythmias, the vasculature and atherosclerosis, and the myocardium and heart failure. This report describes the current state of the field, outlines barriers and challenges to progress, and proposes research opportunities to advance the field, including strategies for leveraging novel approaches using omics and big data. The Working Group recommended research in 3 broad areas: (1) investigation into the fundamental biology relating thyroid dysfunction to the development of cardiovascular disease and into the identification of novel biomarkers of thyroid hormone action in cardiovascular tissues; (2) studies that define subgroups of patients with thyroid dysfunction amenable to specific preventive strategies and interventional therapies related to cardiovascular disease; and (3) clinical trials focused on improvement in cardiovascular performance and cardiovascular outcomes through treatment with thyroid hormone or thyromimetic drugs.

Thyroid and Cardiovascular Disease: Research Agenda for Enhancing Knowledge, Prevention, and Treatment

Thyroid hormones have long been known to have a range of effects on the cardiovascular system. However, significant knowledge gaps exist concerning the precise molecular and biochemical mechanisms governing these effects and the optimal strategies for management of abnormalities in thyroid function in patients with and without preexisting cardiovascular disease. In September 2017, The National Heart, Lung, and Blood Institute convened a Working Group with the goal of developing priorities for future scientific research relating thyroid dysfunction to the progression of cardiovascular disease. The Working Group reviewed and discussed the roles of normal thyroid physiology, the consequences of thyroid dysfunction, and the effects of therapy in three cardiovascular areas: cardiac electrophysiology and arrhythmias, the vasculature and atherosclerosis, and the myocardium and heart failure. This report describes the current state of the field, outlines barriers and challenges to progress, and proposes research opportunities to advance the field, including strategies for leveraging novel approaches using omics and big data. The Working Group recommended research in three broad areas: 1) investigation into the fundamental biology relating thyroid dysfunction to the development of cardiovascular disease and into the identification of novel biomarkers of thyroid hormone action in cardiovascular tissues; 2) studies that define subgroups of patients with thyroid dysfunction amenable to specific preventive strategies and interventional therapies related to cardiovascular disease; and 3) clinical trials focused on improvement in cardiovascular performance and cardiovascular outcomes through treatment with thyroid hormone or thyromimetic drugs.

Adult-Onset Hypothyroidism Alters the Metaplastic Properties of Dentate Granule Cells by Decreasing Akt Phosphorylation

The expression of homosynaptic long-term depression (LTD) governs the subsequent induction of long-term potentiation (LTP) at hippocampal synapses. This process, called metaplasticity, is associated with a transient increase in the levels of several kinases, such as extracellular signal-regulated protein kinases 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK), and Akt kinase. It has been increasingly realized that the chemical changes in the hippocampus caused by hypothyroidism may be the key underlying causes of the learning deficits, memory loss, and impaired LTP associated with this disease. However, the functional role of thyroid hormones in the "plasticity of synaptic plasticity" has only begun to be elucidated. To address this issue, we sought to determine whether the administration of 6-n-propyl-2-thiouracil (PTU) alters the relationship between priming and the induction of subsequent LTP and related signaling molecules. The activation of ERK1/2, JNK, and Akt was measured in the hippocampus at least 95 min after priming onset. We found that priming stimulation at 5 Hz for 3 s negatively impacted the induction of LTP by subsequent tetanic stimulation in hypothyroid animals, as manifested by a more rapid decrease in the fEPSP slope and population spike amplitude. This phenomenon was accompanied by lower levels of phosphorylated Akt in the surgically removed hippocampus of the hypothyroid rats compared to the euthyroid rats. The metaplastic response and the expression of these proteins in the 1-Hz-primed hippocampus were not different between the two groups. These observations suggest that decreased PI3K/Akt signaling may be involved in the compromised metaplastic regulation of LTP observed in hypothyroidism, which may account for the learning difficulties/cognitive impairments associated with this condition.

Molecular Network-Based Identification of Competing Endogenous RNAs in Thyroid Carcinoma

RNAs may act as competing endogenous RNAs (ceRNAs), a critical mechanism in determining gene expression regulations in many cancers. However, the roles of ceRNAs in thyroid carcinoma remains elusive. In this study, we have developed a novel pipeline called Molecular Network-based Identification of ceRNA (MNIceRNA) to identify ceRNAs in thyroid carcinoma. MNIceRNA first constructs micro RNA (miRNA)-messenger RNA (mRNA)long non-coding RNA (lncRNA) networks from miRcode database and weighted correlation network analysis (WGCNA), based on which to identify key drivers of differentially expressed RNAs between normal and tumor samples. It then infers ceRNAs of the identified key drivers using the long non-coding competing endogenous database (lnCeDB). We applied the pipeline into The Cancer Genome Atlas (TCGA) thyroid carcinoma data. As a result, 598 lncRNAs, 1025 mRNAs, and 90 microRNA (miRNAs) were inferred to be differentially expressed between normal and thyroid cancer samples. We then obtained eight key driver miRNAs, among which hsa-mir-221 and hsa-mir-222 were key driver RNAs identified by both miRNA-mRNA-lncRNA and WGCNA network. In addition, hsa-mir-375 was inferred to be significant for patients' survival with 34 associated ceRNAs, among which RUNX2, DUSP6 and SEMA3D are known oncogenes regulating cellular proliferation and differentiation in thyroid cancer. These ceRNAs are critical in revealing the secrets behind thyroid cancer progression and may serve as future therapeutic biomarkers.

Global gene expression profile of peripheral blood mononuclear cells challenged with Theileria annulata in crossbred and indigenous cattle

Bovine tropical theileriosis is an important haemoprotozoan disease associated with high rates of morbidity and mortality particularly in exotic and crossbred cattle. It is one of the major constraints of the livestock development programmes in India and Southeast Asia. Indigenous cattle (Bos indicus) are reported to be comparatively less affected than exotic and crossbred cattle. However, genetic basis of resistance to tropical theileriosis in indigenous cattle is not well documented. Recent studies incited an idea that differentially expressed genes in exotic and indigenous cattle play significant role in breed specific resistance to tropical theileriosis. The present study was designed to determine the global gene expression profile in peripheral blood mononuclear cells derived from indigenous (Tharparkar) and cross-bred cattle following in vitro infection of T. annulata (Parbhani strain). Two separate microarray experiments were carried out each for cross-bred and Tharparkar cattle. The cross-bred cattle showed 1082 differentially expressed genes (DEGs). Out of total DEGs, 597 genes were down-regulated and 485 were up-regulated. Their fold change varied from 2283.93 to -4816.02. Tharparkar cattle showed 875 differentially expressed genes including 451 down-regulated and 424 up-regulated. The fold change varied from 94.93 to -19.20. A subset of genes was validated by qRT-PCR and results were correlated well with microarray data indicating that microarray results provided an accurate report of transcript level. Functional annotation study of DEGs confirmed their involvement in various pathways including response to oxidative stress, immune system regulation, cell proliferation, cytoskeletal changes, kinases activity and apoptosis. Gene network analysis of these DEGs plays an important role to understand the interaction among genes. It is therefore, hypothesized that the different susceptibility to tropical theileriosis exhibited by indigenous and crossbred cattle is due to breed-specific differences in the dealing of infected cells with other immune cells, which ultimately influence the immune response responded against T. annulata infection.

Thyroid stimulating hormone directly modulates cardiac electrical activity

BACKGROUND

The electrocardiogram of hypothyroid patients shows a series of abnormalities of cardiac repolarization due to a reduction of some repolarizing K(+) currents and an increase of the L-type calcium current. Experimental and clinical works call into question the unique role of T3 and T4 in these mechanisms and correlate increased serum TSH levels with the repolarization abnormalities in patients with both subclinical and overt hypothyroidism. In this context, the aim of the present study was to investigate the direct effects of TSH upon cardiac electrical properties.

METHODS

The action potential recording and the ion channel subunits mRNA expression were obtained from left ventricle of adult rats. Additionally, the repolarizing K(+) currents and the L-type Ca(2+) current (ICa-L) were recorded in isolated rat adult ventricular myocytes by the patch-clamp technique.

RESULTS

24h exposure to TSH lengthened the action potential and slightly depolarized the resting membrane potential. TSH- receptor activation causes a reduction of the amplitude of Ito and IK1 currents caused by a reduction in channels expression. However, TSH had no effect on ICa-L, IK or IKur.

CONCLUSION

These results support the idea that some of the electrical disturbances seen in hypothyroid hearts, such as the Ito and IK1 current reduction, could be caused not by low T3 but by the elevation of circulating TSH.

Second degree AV block and severely impaired contractility in cardiac myxedema: a case report

The heart is a major target organ for thyroid hormone action. Severe overt hypothyroidism can result in diastolic hypertension, lowered cardiac output, impaired left ventricular contractility and diastolic relaxation, pericardial effusion and bradycardia. However, the function of the atrial pacemaker is usually normal and the degree by which the heart rate slows down is often modest. Here we report the case of a 20 year old male Caucasian with severe overt hypothyroidism. He presented with syncopation due to second degree atrioventricular block type Mobitz 2 and heart failure with reduced ejection fraction (38 %). Laboratory testing revealed a severe overt hypothyroidism with markedly elevated TSH (>100 mIU/L) and reduced fT3 and fT4 levels. The condition was caused by hypothyroid Graves’ disease (Graves’ disease with Hashimoto component). Although magnetic resonance imaging of the heart demonstrated decreased cardiac contractility and pericardial effusion, suggesting peri-myocarditis, plasma levels for BNP and troponin I were low. A possible infectious cause was unlikely, since testing for cardiotropic viruses was negative. The patient was treated with intravenous levothyroxine and after peripheral euthyroidism had been achieved, left ventricular ejection fraction returned to normal and pericardial effusion dissolved. Additionally, bradycardiac episodes abated, although intermittent second degree AV block was still occasionally present during the night. In conclusion, overt hypothyroidism may be associated by cardiac myxedema affecting both electrophysiology and contractility, observations that underscore the necessity of thyroid testing in different phenotypes of heart failure.

Measurement and interpretation of electrocardiographic QT intervals in murine hearts

Alterations in ECG QT intervals correlate with the risk of potentially fatal arrhythmias, for which transgenic murine hearts are becoming increasingly useful experimental models. However, QT intervals are poorly defined in murine ECGs. As a consequence, several different techniques have been used to measure murine QT intervals. The present work develops a consistent measure of the murine QT interval that correlates with changes in the duration of ventricular myocyte action potentials (APs). Volume-conducted ECGs were compared with simultaneously recorded APs, obtained using floating intracellular microelectrodes in Langendorff-perfused mouse hearts. QT intervals were measured from the onset of the QRS complex. The interval, Q-APR₉₀, measured to the time at 90% AP recovery, was compared with two measures of the QT interval. QT1 was measured to the recovery of the ECG trace to the isoelectric baseline for entirely positive T-waves or to the trough of any negative T-wave undershoot. QT2—used extensively in previous studies—was measured to the return of any ECG trough to the isoelectric baseline. QT1, but not QT2, closely correlated with changes in Q-APR₉₀. These findings were confirmed over a range of pacing rates, in low K⁺ concentration solutions, and in Scn5a+/ΔKPQ hearts used to model human long QT syndrome. Application of this method in whole anesthetized mice similarly demonstrated a prolonged corrected QT (QTc) in Scn5a+/ΔKPQ hearts. We therefore describe a robust method for the determination of QT and QTc intervals that correlate with the duration of ventricular myocyte APs in murine hearts.

Short-term thyroid hormone excess affects the heart but does not affect adrenal activity in rats

Background

Hyperthyroidism (Hy) exerts a broad range of influences on a variety of physiological parameters. Its disruptive effect on cardiovascular system is one of its most remarkable impacts. Moreover, Hy has been clinically associated with stress - induced hyperactivation of the hypothalamic-pituitary-adrenal axis.

Objective

Evaluate the impact of short-term Hy on cardiac performance and adrenal activity of rats.

Methods

Induction of Hy in Wistar rats through injections of T3 (150 µg/kg) for 10 days (hyperthyroid group - HG) or vehicle (control group). The cardiovascular performance was evaluated by: echocardiography (ECHO); heart weight/body weight (mg/gr) ratio; contractility of isolated papillary muscles (IPM) and direct measurement of blood pressures. Adrenal activity was evaluated by adrenal weight/body weight (mg/gr) ratio and 24-hour fecal corticosterone (FC) levels on the, 5^th and 10^th days of T3 treatment.

Results

In HG, the ECHO showed reduction of the End Systolic and End Diastolic Volumes, Ejection, Total Diastolic and Isovolumic Relaxation Times, Diastolic and Systolic Areas and E/A ratio. Heart Rate, Ejection Fraction and Cardiac Output increased. The heart weight/body weight ratio was higher. Similarly, in IPM, the maximum rate of force decay during relaxation was higher in all extracellular calcium concentrations. Systolic blood pressure (SBP) levels were higher. (p ≤ 0.05). On the other hand, there was no difference in the adrenal weight/body weight ratio or in the 24-hour FC levels.

Conclusions

Hy induces positive inotropic, chronotropic and lusitropic effects on the heart by direct effects of T3 and increases SBP. Those alterations are not correlated with changes in the adrenal activity.

Microarray analysis in cardiac arrhythmias: a new perspective?

The opportunity to distinguish an accurate set of genes associated with multigenic diseases such as cardiomyopathies or cardiac arrhythmias was very limited before the genomic era. Numerous methods of measuring RNA abundance exist, including northern blotting, multiplex polymerase chain reaction (PCR), and quantitative real-time reverse transcriptase-PCR. However, these techniques might be used to assess the expression levels of only 10-50 genes at time. Today, DNA microarrays provide us with opportunity to simultaneously analyze tens of thousands of genes, giving a remarkable possibility to investigate the genomic contribution to cardiovascular diseases. A particular tissue at any stage of health or disease may be used to generate a genomic profile. Microarray techniques are already used in infectious diseases, oncology, and pharmacology to facilitate clinicians, risk-stratify patients, as well as to predict and assess therapeutic responses to drugs. In this paper, we describe recent advances in the use of various types of microarray technique in the diagnosis of arrhythmogenic heart disease. We also highlight other strategies and methods of differential gene typing comparing with pros and cons of microarray analysis.

Diet-induced obesity causes long QT and reduces transcription of voltage-gated potassium channels

In humans, obesity is associated with long QT, increased frequency of premature ventricular complexes, and sudden cardiac death. The mechanisms of the pro-arrhythmic electrophysiologic remodeling of obesity are poorly understood. We tested the hypothesis that there is decreased expression of voltage-gated potassium channels in the obese heart, leading to long QT. Using implanted telemeters, we found that diet-induced obese (DIO) wild-type mice have impaired cardiac repolarization, demonstrated by long QT, as well as more frequent ventricular ectopy, similar to obese humans. DIO mice have reduced protein and mRNA levels of the potassium channel Kv1.5 caused by a reduction of the transcription factor cyclic AMP response element binding protein (CREB) in DIO hearts. We found that CREB knock-down by siRNA reduces Kv1.5, CREB binds to the Kv1.5 promoter in the heart, and CREB increases transcription of mouse and human Kv1.5 promoters. The reduction in CREB protein during lipotoxicity can be rescued by inhibiting protein kinase D (PKD). Our results identify a mechanism for obesity-induced electrophysiologic remodeling in the heart, namely PKD-induced reduction of CREB, which in turn decreases expression of the potassium channel Kv1.5.

Mechanisms responsible for the altered cardiac repolarization dispersion in experimental hypothyroidism

AIMS

To identify the causes for the inhomogeneity of ventricular repolarization and increased QT dispersion in hypothyroid mice.

METHODS

We studied the effects of 5-propyl-2-thiouracil-induced hypothyroidism on the ECG, action potential (AP) and current density of the repolarizing potassium currents I(to,fast), I(to,slow), I(K,slow) and I(ss) in enzymatically isolated myocytes from three different regions of mouse heart: right ventricle (RV), epicardium of the left ventricle (Epi-LV) and interventricular septum. K(+) currents were recorded with the patch-clamp technique. Membranes from isolated ventricular myocytes were extracted by centrifugation. Kv4.2, Kv4.3, KChIP and Na/Ca exchanger proteins were visualized by Western blot.

RESULTS

The frequency or conduction velocity was not changed by hypothyroidism, but QTc was prolonged. Neither resting membrane potential nor AP amplitude was modified. The action potential duration (APD)(90) increased in the RV and Epi-LV, but not in the septum. Hypothyroid status has no effect either on I(to,slow), I(k,slow) or I(ss) in any of the regions analysed. However, I(to,fast) was significantly reduced in the Epi-LV and in the RV, whereas it was not altered in cells from the septum. Western blot analysis reveals a reduction in Kv4.2 and Kv4.3 protein levels in both the Epi-LV and the RV and an increase in Na/Ca exchanger.

CONCLUSION

From these results we suggest that the regional differences in APD lengthening, and thus in repolarization inhomogeneity, induced by experimental hypothyroidism are at least partially explained by the uneven decrease in I(to,fast) and the differences in the relative contribution of the depolarization-activated outward currents to the repolarization process.

The redox imbalance and the reduction of contractile protein content in rat hearts administered with L-thyroxine and Doxorubicin

Oxidative stress and disorders in calcium balance play a crucial role in the doxorubicin-induced cardiotoxicity. Moreover, many cardiotoxic targets of doxorubicin are regulated by iodothyronine hormones. The aim of the study was to evaluate effects of tetraiodothyronine (0.2, 2 mg/L) on oxidative stress in the cardiac muscle as well as contractility and cardiomyocyte damage markers in rats receiving doxorubicin (1.5 mg/kg) once a week for ten weeks. Doxorubicin was administered alone (DOX) or together with a lower (0.2T₄ + DOX) and higher dose of tetraiodothyronine (2T₄ + DOX). Two groups received only tetraiodothyronine (0.2T₄, 2T₄). Coadministration of tetraiodothyronine and doxorubicin increased the level of lipid peroxidation products and reduced RyR2 level when compared to untreated control and group exposed exclusively to doxorubicin. Insignificant differences in SERCA2 and occasional histological changes were observed. In conclusion, an increase of tetraiodothyronine level may be an additional risk factor of redox imbalance and RyR2 reduction in anthracycline cardiotoxicity.

Distribution of histone3 lysine 4 trimethylation at T3-responsive loci in the heart during reversible changes in gene expression

Expression in the adult heart of a number of cardiac genes, including the two genes comprising the cardiac myosin heavy chain locus (Myh), is controlled by thyroid hormone (T3) levels, but there is minimal information concerning the epigenetic status of the genes when their expressions change. We fed mice normal chow or a propyl thio uracil (PTU, an inhibitor of T3 production) diet for 6 weeks, or the PTU diet for 6 weeks followed by normal chow for a further 2 weeks. Heart ventricles from these groups were then used for ChIP-seq analyses with an antibody to H3K4me3, a well-documented epigenetic marker of gene activation. The resulting data show that, at the Myh7 locus, H3K4me3 modifications are induced primarily at 5' transcribed region in parallel with increased expression of beta myosin heavy chain (MHC). At the Myh6 locus, decreases in H3K4me3 modifications occurred at the promoter and 5' transcribed region. Extensive H3K4me3 modifications also occurred at the intergenic region between the two Myh genes, which extended into the 3' transcribed region of Myh7. The PTU-induced changes in H3K4me3 levels are, for the most part, reversible but are not invariably complete. We found full restoration of Myh6 gene expression upon PTU withdrawal; however, the H3K4me3 pattern was only partially restored at Myh6, suggesting that full reexpression of Myh6 does not require that the H3K4me3 modifications return fully to the untreated conditions. Together, our data show that the H3K4me3 modification is an epigenetic marker closely associated with changes in Myh gene expression.

Cardiac ventricular repolarization reserve: a principle for understanding drug-related proarrhythmic risk

Cardiac repolarization abnormalities can be caused by a wide range of cardiac and non-cardiac compounds and may lead to the development of life-threatening Torsades de Pointes (TdP) ventricular arrhythmias. Drug-induced torsades de pointes is associated with unexpected and unexplained sudden cardiac deaths resulting in the withdrawal of several compounds in the past. To better understand the mechanism of such unexpected sudden cardiac deaths, the concept of repolarization reserve has recently emerged. According to this concept, pharmacological, congenital or acquired impairment of one type of transmembrane ion channel does not necessarily result in excessive repolarization changes because other repolarizing currents can take over and compensate. In this review, the major factors contributing to repolarization reserve are discussed in the context of their clinical significance in physiological and pathophysiological conditions including drug administration, genetic defects, heart failure, diabetes mellitus, gender, renal failure, hypokalaemia, hypothyroidism and athletes' sudden deaths. In addition, pharmacological support of repolarization reserve as a possible therapeutic option is discussed. Some methods for the quantitative estimation of repolarization reserve are also recommended. It is concluded that repolarization reserve should be considered by safety pharmacologists to better understand, predict and prevent previously unexplained drug-induced sudden cardiac deaths.

Comparative phenotypic assessment of cardiac pathology, physiology, and gene expression in C3H/HeJ, C57BL/6J, and B6C3F1/J mice

Human cardiomyopathies often lead to heart failure, a major cause of morbidity and mortality in industrialized nations. Described here is a phenotypic characterization of cardiac function and genome-wide expression from C3H/HeJ, C57BL/6J, and B6C3F1/J male mice. Histopathologic analysis identified a low-grade background cardiomyopathy (murine progressive cardiomyopathy) in eight of nine male C3H/HeJ mice (age nine to ten weeks), but not in male C57BL/6J and in only of ten male B6C3F1/J mice. The C3H/HeJ mouse had an increased heart rate and a shorter RR interval compared to the B6C3F1/J and C57BL/6J mice. Cardiac genomic studies indicated the B6C3F1/J mice exhibited an intermediate gene expression phenotype relative to the 2 parental strains. Disease-centric enrichment analysis indicated a number of cardiomyopathy-associated genes were induced in B6C3F1/J and C3H/HeJ mice, including Myh7, My14, and Lmna and also indicated differential expression of genes associated with metabolic (e.g., Pdk2) and hypoxic stress (e.g. Hif1a). A novel coexpression and integrated pathway network analysis indicated Prkaa2, Pdk2, Rhoj, and Sgcb are likely to play a central role in the pathophysiology of murine progressive cardiomyopathy in C3H/HeJ mice. Our studies indicate that genetically determined baseline differences in cardiac phenotype have the potential to influence the results of cardiotoxicity studies.

Altered in vivo left ventricular torsion and principal strains in hypothyroid rats

The twisting and untwisting motions of the left ventricle (LV) lead to efficient ejection of blood during systole and filling of the ventricle during diastole. Global LV mechanical performance is dependent on the contractile properties of cardiac myocytes; however, it is not known how changes in contractile protein expression affect the pattern and timing of LV rotation. At the myofilament level, contractile performance is largely dependent on the isoforms of myosin heavy chain (MHC) that are expressed. Therefore, in this study, we used MRI to examine the in vivo mechanical consequences of altered MHC isoform expression by comparing the contractile properties of hypothyroid rats, which expressed only the slow β-MHC isoform, and euthyroid rats, which predominantly expressed the fast α-MHC isoform. Unloaded shortening velocity (V(o)) and apparent rate constants of force development (k(tr)) were measured in the skinned ventricular myocardium isolated from euthyroid and hypothyroid hearts. Increased expression of β-MHC reduced LV torsion and fiber strain and delayed the development of peak torsion and strain during systole. Depressed in vivo mechanical performance in hypothyroid rats was related to slowed cross-bridge performance, as indicated by significantly slower V(o) and k(tr), compared with euthyroid rats. Dobutamine infusion in hypothyroid hearts produced smaller increases in torsion and strain and aberrant transmural torsion patterns, suggesting that the myocardial response to β-adrenergic stress is compromised. Thus, increased expression of β-MHC alters the pattern and decreases the magnitude of LV rotation, contributing to reduced mechanical performance during systole, especially in conditions of increased workload.

Adaptations of the autonomous nervous system controlling heart rate are impaired by a mutant thyroid hormone receptor-alpha1

Thyroid hormone has profound direct effects on cardiac function, but the hormonal interactions with the autonomic control of heart rate are unclear. Because thyroid hormone receptor (TR)-alpha1 has been implicated in the autonomic control of brown adipose energy metabolism, it might also play an important role in the central autonomic control of heart rate. Thus, we aimed to analyze the role of TRalpha1 signaling in the autonomic control of heart rate using an implantable radio telemetry system. We identified that mice expressing the mutant TRalpha1R384C (TRalpha1+m mice) displayed a mild bradycardia, which becomes more pronounced during night activity or on stress and is accompanied by a reduced expression of nucleotide-gated potassium channel 2 mRNA in the heart. Pharmacological blockage with scopolamine and the beta-adrenergic receptor antagonist timolol revealed that the autonomic control of cardiac activity was similar to that in wild-type mice at room temperature. However, at thermoneutrality, in which the regulation of heart rate switches from sympathetic to parasympathetic in wild-type mice, TRalpha1+m mice maintained sympathetic stimulation and failed to activate parasympathetic signaling. Our findings demonstrate a novel role for TRalpha1 in the adaptation of cardiac activity by the autonomic nervous system and suggest that human patients with a similar mutation in TRalpha1 might exhibit a deficit in cardiac adaptation to stress or physical activity and an increased sensitivity to beta-blockers.

Mutations in potassium channel Kir2.6 cause susceptibility to thyrotoxic hypokalemic periodic paralysis

Thyrotoxic hypokalemic periodic paralysis (TPP) is characterized by acute attacks of weakness, hypokalemia, and thyrotoxicosis of various etiologies. These transient attacks resemble those of patients with familial hypokalemic periodic paralysis (hypoKPP) and resolve with treatment of the underlying hyperthyroidism. Because of the phenotypic similarity of these conditions, we hypothesized that TPP might also be a channelopathy. While sequencing candidate genes, we identified a previously unreported gene (not present in human sequence databases) that encodes an inwardly rectifying potassium (Kir) channel, Kir2.6. This channel, nearly identical to Kir2.2, is expressed in skeletal muscle and is transcriptionally regulated by thyroid hormone. Expression of Kir2.6 in mammalian cells revealed normal Kir currents in whole-cell and single-channel recordings. Kir2.6 mutations were present in up to 33% of the unrelated TPP patients in our collection. Some of these mutations clearly alter a variety of Kir2.6 properties, all altering muscle membrane excitability leading to paralysis.

Thyroid hormone receptor alpha can control action potential duration in mouse ventricular myocytes through the KCNE1 ion channel subunit

AIMS

The reduced heart rate and prolonged QT(end) duration in mice deficient in thyroid hormone receptor (TR) alpha1 may involve aberrant expression of the K(+) channel alpha-subunit KCNQ1 and its regulatory beta-subunit KCNE1. Here we focus on KCNE1 and study whether increased KCNE1 expression can explain changes in cardiac function observed in TRalpha1-deficient mice.

METHODS

TR-deficient, KCNE1-overexpressing and their respective wildtype (wt) mice were used. mRNA and protein expression were assessed with Northern and Western blot respectively. Telemetry was used to record electrocardiogram and temperature in freely moving mice. Patch-clamp was used to measure action potentials (APs) in isolated cardiomyocytes and ion currents in Chinese hamster ovary (CHO) cells.

RESULTS

KCNE1 was four to 10-fold overexpressed in mice deficient in TRalpha1. Overexpression of KCNE1 with a heart-specific promoter in transgenic mice resulted in a cardiac phenotype similar to that in TRalpha1-deficient mice, including a lower heart rate and prolonged QT(end) time. Cardiomyocytes from KCNE1-overexpressing mice displayed increased AP duration. CHO cells transfected with expression plasmids for KCNQ1 and KCNE1 showed an outward rectifying current that was maximal at equimolar plasmids for KCNQ1-KCNE1 and decreased at higher KCNE1 levels.

CONCLUSION

The bradycardia and prolonged QT(end) time in hypothyroid states can be explained by altered K(+) channel function due to decreased TRalpha1-dependent repression of KCNE1 expression.

Bayesian quantitative electrophysiology and its multiple applications in bioengineering

Bayesian interpretation of observations began in the early 1700s, and scientific electrophysiology began in the late 1700s. For two centuries these two fields developed mostly separately. In part that was because quantitative Bayesian interpretation, in principle a powerful method of relating measurements to their underlying sources, often required too many steps to be feasible with hand calculation in real applications. As computer power became widespread in the later 1900s, Bayesian models and interpretation moved rapidly but unevenly from the domain of mathematical statistics into applications. Use of Bayesian models now is growing rapidly in electrophysiology. Bayesian models are well suited to the electrophysiological environment, allowing a direct and natural way to express what is known (and unknown) and to evaluate which one of many alternatives is most likely the source of the observations, and the closely related receiver operating characteristic (ROC) curve is a powerful tool in making decisions. Yet, in general, many people would ask what such models are for, in electrophysiology, and what particular advantages such models provide. So to examine this question in particular, this review identifies a number of electrophysiological papers in bioengineering arising from questions in several organ systems to see where Bayesian electrophysiological models or ROC curves were important to the results that were achieved.

ERRgamma regulates cardiac, gastric, and renal potassium homeostasis

Energy production by oxidative metabolism in kidney, stomach, and heart, is primarily expended in establishing ion gradients to drive renal electrolyte homeostasis, gastric acid secretion, and cardiac muscle contraction, respectively. In addition to orchestrating transcriptional control of oxidative metabolism, the orphan nuclear receptor, estrogen-related receptor gamma (ERRgamma), coordinates expression of genes central to ion homeostasis in oxidative tissues. Renal, gastric, and cardiac tissues subjected to genomic analysis of expression in perinatal ERRgamma null mice revealed a characteristic dysregulation of genes involved in transport processes, exemplified by the voltage-gated potassium channel, Kcne2. Consistently, ERRgamma null animals die during the first 72 h of life with elevated serum potassium, reductions in key gastric acid production markers, and cardiac arrhythmia with prolonged QT intervals. In addition, we find altered expression of several genes associated with hypertension in ERRgamma null mice. These findings suggest a potential role for genetic polymorphisms at the ERRgamma locus and ERRgamma modulators in the etiology and treatment of renal, gastric, and cardiac dysfunction.

Thyroid hormones and cardiac arrhythmias

Thyroid hormone plays an important role in cardiac electrophysiology and Ca2+ handling through both genomic and nongenomic mechanisms of action, while both actions can interfere. Chronic changes in the amount of circulating thyroid hormone due to thyroid dysfunction or systemic disease result in structural, electrophysiological and Ca2+ handling remodeling, while acute changes may affect basal activity of cardiac cells membrane systems. Consequently, long-term or rapid modulation of sarcolemmal ion channels, Ca2+ cycling proteins and intercellular communicating channels by thyroid hormone may affect heart function as well as susceptibility of the heart to arrhythmias. This aspect including pro- and anti-arrhythmic potential of thyroid hormone is highlighted in this review.

Ion-channel mRNA-expression profiling: Insights into cardiac remodeling and arrhythmic substrates

Membrane ion channels and transporters are key determinants of cardiac electrical function. Their expression is affected by cardiac region, hemodynamic properties, heart-rate changes, neurohormones and cardiac disease. One of the important determinants of ion-channel function is the level of ion-channel subunit mRNA expression, which governs the production of ion-channel proteins that traffic to the cell-membrane to form functional ion-channels. Ion-channel mRNA-expression profiling can be performed with cDNA microarrays or high-throughput reverse transcription/polymerase chain reaction (PCR) methods. Expression profiling has been applied to evaluate the dependence of ion-channel expression on cardiac region, revealing the molecular basis of regionally-controlled electrical properties as well as the molecular determinants of specialized electrical functions like pacemaking activity. Ion-channel remodeling occurs with cardiac diseases like heart failure, congenital repolarization abnormalities, and atrial fibrillation, and expression profiling has provided insights into the mechanisms by which these conditions affect cardiac electrical stability. Expression profiling has also shown how hormonal changes, antiarrhythmic drugs, cardiac development and altered heart rate affect ion-channel expression patterns to modify cardiac electrical function and sometimes to produce cardiac rhythm disturbances. This article reviews the information obtained to date with the application of cardiac ion-channel expression profiling. With increasing availability and efficiency of high-throughput PCR methods for ion-channel subunit mRNA-expression characterization, it is likely that the application of ion-channel expression profiling will increase and that it will provide important new insights into the determinants of cardiac electrical function in both physiological and pathological situations.

Molecular determinants of cardiac transient outward potassium current (I(to)) expression and regulation

Rapidly activating and inactivating cardiac transient outward K(+) currents, I(to), are expressed in most mammalian cardiomyocytes, and contribute importantly to the early phase of action potential repolarization and to plateau potentials. The rapidly recovering (I(t)(o,f)) and slowly recovering (I(t)(o,s)) components are differentially expressed in the myocardium, contributing to regional heterogeneities in action potential waveforms. Consistent with the marked differences in biophysical properties, distinct pore-forming (alpha) subunits underlie the two I(t)(o) components: Kv4.3/Kv4.2 subunits encode I(t)(o,f), whereas Kv1.4 encodes I(t)(o,s), channels. It has also become increasingly clear that cardiac I(t)(o) channels function as components of macromolecular protein complexes, comprising (four) Kvalpha subunits and a variety of accessory subunits and regulatory proteins that influence channel expression, biophysical properties and interactions with the actin cytoskeleton, and contribute to the generation of normal cardiac rhythms. Derangements in the expression or the regulation of I(t)(o) channels in inherited or acquired cardiac diseases would be expected to increase the risk of potentially life-threatening cardiac arrhythmias. Indeed, a recently identified Brugada syndrome mutation in KCNE3 (MiRP2) has been suggested to result in increased I(t)(o,f) densities. Continued focus in this area seems certain to provide new and fundamentally important insights into the molecular determinants of functional I(t)(o) channels and into the molecular mechanisms involved in the dynamic regulation of I(t)(o) channel functioning in the normal and diseased myocardium.

Regulation of the Kv2.1 potassium channel by MinK and MiRP1

Kv2.1 is a voltage-gated potassium (Kv) channel alpha-subunit expressed in mammalian heart and brain. MinK-related peptides (MiRPs), encoded by KCNE genes, are single-transmembrane domain ancillary subunits that form complexes with Kv channel alpha-subunits to modify their function. Mutations in human MinK (KCNE1) and MiRP1 (KCNE2) are associated with inherited and acquired forms of long QT syndrome (LQTS). Here, coimmunoprecipitations from rat heart tissue suggested that both MinK and MiRP1 form native cardiac complexes with Kv2.1. In whole-cell voltage-clamp studies of subunits expressed in CHO cells, rat MinK and MiRP1 reduced Kv2.1 current density three- and twofold, respectively; slowed Kv2.1 activation (at +60 mV) two- and threefold, respectively; and slowed Kv2.1 deactivation less than twofold. Human MinK slowed Kv2.1 activation 25%, while human MiRP1 slowed Kv2.1 activation and deactivation twofold. Inherited mutations in human MinK and MiRP1, previously associated with LQTS, were also evaluated. D76N-MinK and S74L-MinK reduced Kv2.1 current density (threefold and 40%, respectively) and slowed deactivation (60% and 80%, respectively). Compared to wild-type human MiRP1-Kv2.1 complexes, channels formed with M54T- or I57T-MiRP1 showed greatly slowed activation (tenfold and fivefold, respectively). The data broaden the potential roles of MinK and MiRP1 in cardiac physiology and support the possibility that inherited mutations in either subunit could contribute to cardiac arrhythmia by multiple mechanisms.

Connexin40 messenger ribonucleic acid is positively regulated by thyroid hormone (TH) acting in cardiac atria via the TH receptor

Thyroid hormone (TH) regulates many cardiac genes via nuclear thyroid receptors, and hyperthyroidism is frequently associated with atrial fibrillation. Electrical activity propagation in myocardium depends on the transfer of current at gap junctions, and connexins (Cxs) 40 and 43 are the predominant junction proteins. In mice, Cx40, the main Cx involved in atrial conduction, is restricted to the atria and fibers of the conduction system, which also express Cx43. We studied cardiac expression of Cx40 and Cx43 in conjunction with electrocardiogram studies in mice overexpressing the dominant negative mutant thyroid hormone receptor-beta Delta337T exclusively in cardiomyocytes [myosin heavy chain (MHC-mutant)]. These mice develop the cardiac hypothyroid phenotype in the presence of normal serum TH. Expression was also examined in wild-type mice rendered hypothyroid or hyperthyroid by pharmacological treatment. Atrial Cx40 mRNA and protein levels were decreased (85 and 55%, respectively; P < 0.001) in MHC-mt mice. Atrial and ventricular Cx43 mRNA levels were not significantly changed. Hypothyroid and hyperthyroid animals showed a 25% decrease and 40% increase, respectively, in Cx40 mRNA abundance. However, MHC-mt mice presented very low Cx40 mRNA expression regardless of whether they were made hypothyroid or hyperthyroid. Atrial depolarization velocity, as represented by P wave duration in electrocardiograms of unanesthetized mice, was extremely reduced in MHC-mt mice, and to a lesser extent also in hypothyroid mice (90 and 30% increase in P wave duration). In contrast, this measure was increased in hyperthyroid mice (19% decrease in P wave duration). Therefore, this study reveals for the first time that Cx40 mRNA is up-regulated by TH acting in cardiac atria via the TH receptor and that this may be one of the mechanisms contributing to atrial conduction alterations in thyroid dysfunctions.

The role of thyroid hormone nuclear receptors in the heart: evidence from pharmacological approaches

This review evaluates the hypothesis that the cardiac effects of amiodarone can be explained—at least partly—by the induction of a local ‘hypothyroid-like condition’ in the heart. Evidence supporting the hypothesis comprises the observation that amiodarone exerts an inhibitory effect on the binding of T3 to thyroid hormone receptors (TR) alpha-1 and beta-1 in vitro, and on the expression of particular T3-dependent genes in vivo. In the heart, amiodarone decreases heart rate and alpha myosin heavy chain expression (mediated via TR alpha-1), and increases sarcoplasmic reticulum calcium-activated ATPase and beta myosin heavy chain expression (mediated via TR beta-1). Recent data show a significant similarity in expression profiles of 8,435 genes in the heart of hypothyroid and amiodarone-treated animals, although similarities do not always exist in transcripts of ion channel genes. Induction of a hypothyroid cardiac phenotype by amiodarone may be advantageous by decreasing energy demands and increasing energy availability.

Genesis and regulation of the heart automaticity

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.