Regulation of heart rate and the pacemaker current by phosphoinositide 3-kinase signaling

Role of Endothelin-1 in Right Atrial Arrhythmogenesis in Rabbits with Monocrotaline-Induced Pulmonary Arterial Hypertension

Atrial arrhythmias are considered prominent phenomena in pulmonary arterial hypertension (PAH) resulting from atrial electrical and structural remodeling. Endothelin (ET)-1 levels correlate with PAH severity and are associated with atrial remodeling and arrhythmia. In this study, hemodynamic measurement, western blot analysis, and histopathology were performed in the control and monocrotaline (MCT, 60 mg/kg)-induced PAH rabbits. Conventional microelectrodes were used to simultaneously record the electrical activity in the isolated sinoatrial node (SAN) and right atrium (RA) tissue preparations before and after ET-1 (10 nM) or BQ-485 (an ET-A receptor antagonist, 100 nM) perfusion. MCT-treated rabbits showed an increased relative wall thickness in the pulmonary arterioles, mean cell width, cross-sectional area of RV myocytes, and higher right ventricular systolic pressure, which were deemed to have PAH. Compared to the control, the spontaneous beating rate of SAN–RA preparations was faster in the MCT-induced PAH group, which can be slowed down by ET-1. MCT-induced PAH rabbits had a higher incidence of sinoatrial conduction blocks, and ET-1 can induce atrial premature beats or short runs of intra-atrial reentrant tachycardia. BQ 485 administration can mitigate ET-1-induced RA arrhythmogenesis in MCT-induced PAH. The RA specimens from MCT-induced PAH rabbits had a smaller connexin 43 and larger ROCK1 and phosphorylated Akt than the control, and similar PKG and Akt to the control. In conclusion, ET-1 acts as a trigger factor to interact with the arrhythmogenic substrate to initiate and maintain atrial arrhythmias in PAH. ET-1/ET-A receptor/ROCK signaling may be a target for therapeutic interventions to treat PAH-induced atrial arrhythmias.

PI3K/Akt pathway mediates the positive inotropic effects of insulin in Langendorff-perfused rat hearts

Insulin exerts positive inotropic effects on cardiac muscle; however, the relationship between cardiac contractility and phosphoinositol-3-kinase/Akt (PI3K/Akt) activation remains unclear. We hypothesized that the positive inotropic effects of insulin are dose-dependent and mediated via the PI3K/Akt pathway in isolated normal rat hearts. The Institutional Animal Investigation Committee approved the use of hearts excised from rats under pentobarbital anesthesia. The hearts were perfused at a constant pressure using the Langendorff technique. After stabilization (baseline), the hearts were randomly divided into the following four insulin (Ins) groups: 1) Ins0 (0 IU/L), 2) Ins0.5 (0.5 IU/L), 3) Ins5 (5 IU/L), and 4) Ins50 (50 IU/L) (n = 8 in each group). To clarify the role of the PI3K/Akt pathway in insulin-dependent inotropic effects, we also treated the insulin groups with the PI3K inhibitor wortmannin (InsW): 5) InsW0 (0 IU/L), 6) InsW0.5 (0.5 IU/L), 7) InsW5 (5 IU/L), and 8) InsW50 (50 IU/L). Hearts were perfused with Krebs–Henseleit buffer solution with or without wortmannin for 10 min, followed by 20 min perfusion with the solution containing each concentration of insulin. The data were recorded as the maximum left ventricular derivative of pressure development (LV dP/dt max). Myocardial p-Akt levels were measured at 3 min, 5 min, and at the end of the perfusion. In the Ins groups, LV dP/dt max in Ins5 and Ins50 increased by 14% and 48%, respectively, 3 min after insulin perfusion compared with the baseline. Tachyphylaxis was observed after 10 min in the Ins5 and Ins50 treatment groups. Wortmannin partially inhibited the positive inotropic effect of insulin; although insulin enhanced p-Akt levels at all time points compared with the control group, this increase was suppressed in the presence of wortmannin. The positive inotropic effect of insulin is dose-dependent and consistent with Akt activation. This effect mediated by high doses of insulin on cardiac tissue was temporary and caused tachyphylaxis, potentially triggered by Akt overactivation, which leads beta 1 deactivation.

Cancer Chemotherapy-Induced Sinus Bradycardia: A Narrative Review of a Forgotten Adverse Effect of Cardiotoxicity

Cardiotoxicity is a common adverse effect of anticancer drugs (ACDs), including the so-called targeted drugs, and increases morbidity and mortality in patients with cancer. Attention has focused mainly on ACD-induced heart failure, myocardial ischemia, hypertension, thromboembolism, QT prolongation, and tachyarrhythmias. Yet, although an increasing number of ACDs can produce sinus bradycardia (SB), this proarrhythmic effect remains an underappreciated complication, probably because of its low incidence and severity since most patients are asymptomatic. However, SB merits our interest because its incidence increases with the aging of the population and cancer is an age-related disease and because SB represents a risk factor for QT prolongation. Indeed, several ACDs that produce SB also prolong the QT interval. We reviewed published reports on ACD-induced SB from January 1971 to November 2020 using the PubMed and EMBASE databases. Published reports from clinical trials, case reports, and recent reviews were considered. This review describes the associations between ACDs and SB, their clinical relevance, risk factors, and possible mechanisms of onset and treatment.

Regulation of sinus node pacemaking and atrioventricular node conduction by HCN channels in health and disease

The funny current, I_f, was first recorded in the heart 40 or more years ago by Dario DiFrancesco and others. Since then, we have learnt that I_f plays an important role in pacemaking in the sinus node, the innate pacemaker of the heart, and more recently evidence has accumulated to show that I_f may play an important role in action potential conduction through the atrioventricular (AV) node. Evidence has also accumulated to show that regulation of the transcription and translation of the underlying Hcn genes plays an important role in the regulation of sinus node pacemaking and AV node conduction under normal physiological conditions - in athletes, during the circadian rhythm, in pregnancy, and during postnatal development - as well as pathological states - ageing, heart failure, pulmonary hypertension, diabetes and atrial fibrillation. There may be yet more pathological conditions involving changes in the expression of the Hcn genes. Here, we review the role of I_f and the underlying HCN channels in physiological and pathological changes of the sinus and AV nodes and we begin to explore the signalling pathways (microRNAs, transcription factors, GIRK4, the autonomic nervous system and inflammation) involved in this regulation. This review is dedicated to Dario DiFrancesco on his retirement.

Cohesin-protein Shugoshin-1 controls cardiac automaticity via HCN4 pacemaker channel

Endogenous cardiac pacemaker function regulates the rate and rhythm of cardiac contraction. The mutation p.Lys23Glu in the cohesin protein Shugoshin-1 causes severe heart arrhythmias due to sinoatrial node dysfunction and a debilitating gastrointestinal motility disorder, collectively termed the Chronic Atrial and Intestinal Dysrhythmia Syndrome, linking Shugoshin-1 and pacemaker activity. Hyperpolarization-activated, cyclic nucleotide-gated cation channel 4 (HCN4) is the predominant pacemaker ion-channel in the adult heart and carries the majority of the "funny" current, which strongly contributes to diastolic depolarization in pacemaker cells. Here, we study the mechanism by which Shugoshin-1 affects cardiac pacing activity with two cell models: neonatal rat ventricular myocytes and Chronic Atrial and Intestinal Dysrhythmia Syndrome patient-specific human induced pluripotent stem cell derived cardiomyocytes. We find that Shugoshin-1 interacts directly with HCN4 to promote and stabilize cardiac pacing. This interaction enhances funny-current by optimizing HCN4 cell-surface expression and function. The clinical p.Lys23Glu mutation leads to an impairment in the interaction between Shugoshin-1 and HCN4, along with depressed funny-current and dysrhythmic activity in induced pluripotent stem cell derived cardiomyocytes derived from Chronic Atrial and Intestinal Dysrhythmia Syndrome patients. Our work reveals a critical non-canonical, cohesin-independent role for Shugoshin-1 in maintaining cardiac automaticity and identifies potential therapeutic avenues for cardiac pacemaking disorders, in particular Chronic Atrial and Intestinal Dysrhythmia Syndrome.

New aspects of endocrine control of atrial fibrillation and possibilities for clinical translation

Hormones are potent endo-, para-, and autocrine endogenous regulators of the function of multiple organs, including the heart. Endocrine dysfunction promotes a number of cardiovascular diseases, including atrial fibrillation (AF). While the heart is a target for endocrine regulation, it is also an active endocrine organ itself, secreting a number of important bioactive hormones that convey significant endocrine effects, but also through para-/autocrine actions, actively participate in cardiac self-regulation. The hormones regulating heart-function work in concert to support myocardial performance. AF is a serious clinical problem associated with increased morbidity and mortality, mainly due to stroke and heart failure. Current therapies for AF remain inadequate. AF is characterized by altered atrial function and structure, including electrical and profibrotic remodelling in the atria and ventricles, which facilitates AF progression and hampers its treatment. Although features of this remodelling are well-established and its mechanisms are partly understood, important pathways pertinent to AF arrhythmogenesis are still unidentified. The discovery of these missing pathways has the potential to lead to therapeutic breakthroughs. Endocrine dysfunction is well-recognized to lead to AF. In this review, we discuss endocrine and cardiocrine signalling systems that directly, or as a consequence of an underlying cardiac pathology, contribute to AF pathogenesis. More specifically, we consider the roles of products from the hypothalamic-pituitary axis, the adrenal glands, adipose tissue, the renin–angiotensin system, atrial cardiomyocytes, and the thyroid gland in controlling atrial electrical and structural properties. The influence of endocrine/paracrine dysfunction on AF risk and mechanisms is evaluated and discussed. We focus on the most recent findings and reflect on the potential of translating them into clinical application.

RNAseq shows an all-pervasive day-night rhythm in the transcriptome of the pacemaker of the heart

Physiological systems vary in a day-night manner anticipating increased demand at a particular time. Heart is no exception. Cardiac output is primarily determined by heart rate and unsurprisingly this varies in a day-night manner and is higher during the day in the human (anticipating increased day-time demand). Although this is attributed to a day-night rhythm in post-translational ion channel regulation in the heart's pacemaker, the sinus node, by the autonomic nervous system, we investigated whether there is a day-night rhythm in transcription. RNAseq revealed that ~ 44% of the sinus node transcriptome (7134 of 16,387 transcripts) has a significant day-night rhythm. The data revealed the oscillating components of an intrinsic circadian clock. Presumably this clock (or perhaps the master circadian clock in the suprachiasmatic nucleus) is responsible for the rhythm observed in the transcriptional machinery, which in turn is responsible for the rhythm observed in the transcriptome. For example, there is a rhythm in transcripts responsible for the two principal pacemaker mechanisms (membrane and Ca²⁺ clocks), transcripts responsible for receptors and signalling pathways known to control pacemaking, transcripts from genes identified by GWAS as determinants of resting heart rate, and transcripts from genes responsible for familial and acquired sick sinus syndrome.

Regulation of HCN2 Current by PI3K/Akt Signaling

It has long been known that heart rate is regulated by the autonomic nervous system. Recently, we demonstrated that the pacemaker current, I_f, is regulated by phosphoinositide 3-kinase (PI3K) signaling independently of the autonomic nervous system. Inhibition of PI3K in sinus node (SN) myocytes shifts the activation of I_f by almost 16 mV in the negative direction. I_f in the SN is predominantly mediated by two members of the HCN gene family, HCN4 and HCN1. Purkinje fibers also possess I_f and are an important secondary pacemaker in the heart. In contrast to the SN, they express HCN2 and HCN4, while ventricular myocytes, which do not normally pace, express HCN2 alone. In the current work, we investigated PI3K regulation of HCN2 expressed in HEK293 cells. Treatment with the PI3K inhibitor PI-103 caused a negative shift in the activation voltage and a dramatic reduction in the magnitude of the HCN2 current. Similar changes were also seen in cells treated with an inhibitor of the protein kinase Akt, a downstream effector of PI3K. The effects of PI-103 were reversed by perfusion of cells with phosphatidylinositol 3,4,5-trisphosphate (the second messenger produced by PI3K) or active Akt protein. We identified serine 861 in mouse HCN2 as a putative Akt phosphorylation site. Mutation of S861 to alanine mimicked the effects of Akt inhibition on voltage dependence and current magnitude. In addition, the Akt inhibitor had no effect on the mutant channel. These results suggest that Akt phosphorylation of mHCN2 S861 accounts for virtually all of the observed actions of PI3K signaling on the HCN2 current. Unexpectedly, Akt inhibition had no effect on I_f in SN myocytes. This result raises the possibility that diverse PI3K signaling pathways differentially regulate HCN-induced currents in different tissues, depending on the isoforms expressed.

PDK1-AKT signaling pathway regulates the expression and function of cardiac hyperpolarization-activated cyclic nucleotide-modulated channels

AIM

The enzyme 3-phosphoinositide-dependent protein kinase-1 (PDK1) is associated with cardiac and pathological remodeling and ion channel function regulation. However, whether it regulates hyperpolarization-activated cyclic nucleotide-modulated channels (HCNs) remains unclear.

MAIN METHODS

In the atrial myocytes of heart-specific PDK1 "knockout" mouse model and neonatal mice, protein kinase B (AKT)-related inhibitors or agonists as well as knockdown or overexpression plasmids were used to study the relationship between PDK1 and HCNs.

KEY FINDINGS

HCN1 expression and AKT phosphorylation at the Thr308 site were significantly decreased in atrial myocytes after PDK1 knockout or inhibition; in contrast, HCN2 and HCN4 levels were significantly increased. Also, a similar trend of HCNs expression has been observed in cultured atrial myocytes after PDK1 inhibition, as further demonstrated via immunofluorescence and patch-clamp experiments. Moreover, these results of PDK1 overexpression indicate an opposite trend compared with the previous experimental results. However, the results of PDK1 inhibition or overexpression could be reversed by activating or inhibiting AKT, respectively.

SIGNIFICANCE

These results indicate that the PDK1-AKT signaling pathway is involved in the regulation of HCN mRNA transcription, protein expression, HCN current density, and cell membrane location.

Atrial remodeling and atrial fibrillation in acquired forms of cardiovascular disease

Atrial fibrillation (AF) is prevalent in common conditions and acquired forms of heart disease, including diabetes mellitus (DM), hypertension, cardiac hypertrophy, and heart failure. AF is also prevalent in aging. Although acquired heart disease is common in aging individuals, age is also an independent risk factor for AF. Importantly, not all individuals age at the same rate. Rather, individuals of the same chronological age can vary in health status from fit to frail. Frailty can be quantified using a frailty index, which can be used to assess heterogeneity in individuals of the same chronological age. AF is thought to occur in association with electrical remodeling due to changes in ion channel expression or function as well as structural remodeling due to fibrosis, myocyte hypertrophy, or adiposity. These forms of remodeling can lead to triggered activity and electrical re-entry, which are fundamental mechanisms of AF initiation and maintenance. Nevertheless, the underlying determinants of electrical and structural remodeling are distinct in different conditions and disease states. In this focused review, we consider the factors leading to atrial electrical and structural remodeling in human patients and animal models of acquired cardiovascular disease or associated risk factors. Our goal is to identify similarities and differences in the cellular and molecular bases for atrial electrical and structural remodeling in conditions including DM, hypertension, hypertrophy, heart failure, aging, and frailty.

A Brief History of Pacemaking

Cardiac pacemaking is a most fundamental cardiac function, thoroughly investigated for decades with a multiscale approach at organ, tissue, cell and molecular levels, to clarify the basic mechanisms underlying generation and control of cardiac rhythm. Understanding the processes involved in pacemaker activity is of paramount importance for a basic physiological knowledge, but also as a way to reveal details of pathological dysfunctions useful in the perspective of a therapeutic approach. Among the mechanisms involved in pacemaking, the "funny" (I_f) current has properties most specifically fitting the requirements for generation and control of repetitive activity, and has consequently received the most attention in studies of the pacemaker function. Present knowledge of the basic mechanisms of pacemaking and the properties of funny channels has led to important developments of clinical relevance. These include: (1) the successful development of heart rate-reducing agents, such as ivabradine, able to control cardiac rhythm and useful in the treatment of diseases such as coronary artery disease, heart failure and tachyarrhythmias; (2) the understanding of the genetic basis of disorders of cardiac rhythm caused by HCN channelopathies; (3) the design of strategies to implement biological pacemakers based on transfer of HCN channels or of stem cell-derived pacemaker cells expressing I_f, with the ultimate goal to replace electronic devices. In this review, I will give a brief historical account of the discovery of the funny current and the development of the concept of I_f-based pacemaking, in the context of a wider, more complex model of cardiac rhythmic function.

Comparing pathways for long-term heart rate modulation by the funny current

Heart rate control by the funny current (I_f) involves both fast, cAMP-dependent, and slow, membrane expression–based mechanisms to adapt to different needs.