Autonomic neural remodeling of the pulmonary vein-left atrium junction in a prolonged right atrial pacing canine model

Sinus node dysfunction and atrial fibrillation-Relationships, clinical phenotypes, new mechanisms, and treatment approaches

Although the anatomical basis of the pathogenesis of sinus node dysfunction (SND) and atrial fibrillation (AF) is located primarily in the left and right atria, increasing evidence suggests a strong correlation between SND and AF, in terms of both clinical presentation and formation mechanisms. However, the exact mechanisms underlying this association are unclear. The relationship between SND and AF may not be causal, but is likely to involve common factors and mechanisms, including ion channel remodeling, gap junction abnormalities, structural remodeling, genetic mutations, neuromodulation abnormalities, the effects of adenosine on cardiomyocytes, oxidative stress, and viral infections. Ion channel remodeling manifests primarily as alterations in the "funny" current (I_f) and Ca²⁺ clock associated with cardiomyocyte autoregulation, and gap junction abnormalities are manifested primarily as decreased expression of connexins (Cxs) mediating electrical impulse propagation in cardiomyocytes. Structural remodeling refers primarily to fibrosis and cardiac amyloidosis (CA). Some genetic mutations can also cause arrhythmias, such as SCN5A, HCN4, EMD, and PITX2. The intrinsic cardiac autonomic nervous system (ICANS), a regulator of the heart's physiological functions, triggers arrhythmias.In addition, we discuss arrhythmias caused by viral infections, notably Coronavirus Disease 2019 (COVID-19). Similarly to upstream treatments for atrial cardiomyopathy such as alleviating CA, ganglionated plexus (GP) ablation acts on the common mechanisms between SND and AF, thus achieving a dual therapeutic effect.

Heart rate variability in atrial fibrillation: The balance between sympathetic and parasympathetic nervous system

BACKGROUND

Atrial fibrillation (AF) is the commonest abnormal heart rhythm with significant related morbidity and mortality. Several pathophysiologic mechanisms have been advocated to explain the onset of AF. There has been increasing evidence that abnormalities of the autonomic nervous system (ANS) that includes sympathetic, parasympathetic and intrinsic neural network are involved in the pathogenesis of AF. This review will consider the anatomical and pathophysiological concepts of the cardiac neuronal network and discuss how it can be investigated.

DESIGN

Relevant articles for this review were selected primarily from Ovid Medline and Embase databases (see appendix). We searched for key terms "atrial fibrillation," "AF," "autonomic dysfunction," "autonomic nervous system," "heart rate variability" and "HRV" to gather relevant studies. Duplicate papers were excluded.

RESULTS

Heart is richly innervated by autonomic nerves. Both sympathetic and parasympathetic systems interact in developing AF along with cardiac ganglionated plexi (GP). Thus autonomic dysfunction is present in AF. There are methods including selective ablation that reduce autonomic innervation and show to reduce the incidence of spontaneous or induced atrial arrhythmias. Heart rate variability (HRV) is a useful tool to assess sympathetic and parasympathetic influences on disease states. HRV can be improved following intervention and is thus a useful application in assessing autonomic dysfunction in patients with AF.

CONCLUSION

ANS plays a crucial role in the development, propagation and complexity of AF. Assessment of the autonomic involvement in the propagation of AF may help in explaining why certain patients with AF do not benefit from cardioversion or ablation.

Chronic stimulation of the sigma-1 receptor ameliorates autonomic nerve dysfunction and atrial fibrillation susceptibility in a rat model of depression

The present study aimed to assess the effect of sigma-1 receptor (S1R) stimulation on autonomic nerve dysfunction and susceptibility to atrial fibrillation (AF) in a rat depression model. Male rats were randomly divided into one of the following four treatment groups: saline [control (CTL)]; saline + intragastric administration of SA4503, an agonist of S1R (CTS); chronic unpredictable mild stress (CUMS) to produce depression (MDD); and CUMS + intragastric administration of SA4503 (MDS). Depression-like behaviors, such as reduced sucrose preference, decreased body weight gain, and increased immobility time during forced swimming, improved in the MDS group after 4 wk of SA4503 treatment. Compared with rats in the CTL group, rats in the MDD group showed significantly augmented sympathetic activity, reduced parasympathetic activity, decreased heart rate variability, and lowered S1R expression in the atrium and hippocampus (all P < 0.01). However, rats in the MDS group showed mitigated aforementioned alterations and improved electrical remodeling compared with rats in the MDD group (all P < 0.01). Furthermore, rats in the MDS group showed shortened activation latencies, increased effective refractory periods, and lowered frequency of AF incidence duration and fibrosis compared with rats in the MDD group (all P < 0.01). The results indicate that S1R stimulation reduces sympathetic activity and susceptibility to AF by improving depressive behaviors, modulating cardiac autonomic nerve balance, lightening nerve remodeling, and upregulating S1R and ion channel protein expression. NEW & NOTEWORTHY Chronic stimulation of the sigma-1 receptor (S1R) ameliorates depression-induced autonomic nerve dysfunction by modulating the imbalance between overactivated sympathetic activity and decreased vagal activity. Chronic S1R stimulation alleviates atrial electrical remodeling, fibrosis, and susceptibility to atrial fibrillation (AF). The S1R agonist may target the underlying mechanisms related to AF occurrence. The results indicate that the S1R could be a potential clinical target for atrial arrhythmia, especially when it is combined with major depressive disorders.

Oxidative stress creates a unique, CaMKII-mediated substrate for atrial fibrillation in heart failure

The precise mechanisms by which oxidative stress (OS) causes atrial fibrillation (AF) are not known. Since AF frequently originates in the posterior left atrium (PLA), we hypothesized that OS, via calmodulin-dependent protein kinase II (CaMKII) signaling, creates a fertile substrate in the PLA for triggered activity and reentry. In a canine heart failure (HF) model, OS generation and oxidized-CaMKII-induced (Ox-CaMKII-induced) RyR2 and Nav1.5 signaling were increased preferentially in the PLA (compared with left atrial appendage). Triggered Ca2+ waves (TCWs) in HF PLA myocytes were particularly sensitive to acute ROS inhibition. Computational modeling confirmed a direct relationship between OS/CaMKII signaling and TCW generation. CaMKII phosphorylated Nav1.5 (CaMKII-p-Nav1.5 [S571]) was located preferentially at the intercalated disc (ID), being nearly absent at the lateral membrane. Furthermore, a decrease in ankyrin-G (AnkG) in HF led to patchy dropout of CaMKII-p-Nav1.5 at the ID, causing its distribution to become spatially heterogeneous; this corresponded to preferential slowing and inhomogeneity of conduction noted in the HF PLA. Computational modeling illustrated how conduction slowing (e.g., due to increase in CaMKII-p-Nav1.5) interacts with fibrosis to cause reentry in the PLA. We conclude that OS via CaMKII leads to substrate for triggered activity and reentry in HF PLA by mechanisms independent of but complementary to fibrosis.

GCH1 attenuates cardiac autonomic nervous remodeling in canines with atrial-tachypacing via tetrahydrobiopterin pathway regulated by microRNA-206

BACKGROUND/AIMS

Cardiac autonomic nerve remodeling (ANR) is an important mechanism of atrial fibrillation (AF). GTP cyclohydrolase I, encoded by GCH1, is the rate-limiting enzyme in de novo synthesis of tetrahydrobiopterin (BH4), an essential cofactor for nitric oxide (NO) synthesis. Previous studies reported that increased BH4 and NO content negatively regulated nerve regeneration. This study investigated the effects of GCH1 on ANR via BH4 pathway, regulated by microRNA-206 (miR-206).

METHODS AND RESULTS

In canines, atrial tachypacing (A-TP), together with miR-206 overexpression, increased PGP9.5 level and inhibited GCH1 expression by quantitative real-time polymerase chain reaction and western blot analysis. GCH1 was validated to be a direct target of miR-206 by luciferase assays. Meanwhile, miR-206 overexpression by lentiviruses infection into right superior pulmonary vein fat pad decreased GCH1 expression to ∼40% and further reduced BH4 and NO content compared with the control canines. After infection of GCH1 overexpression lentiviruses for two weeks, atrial effective refractory period was increased compared with the control group (105.8 ± 1.537 ms vs 99.17 ± 2.007 ms, P < 0.05). Moreover, GCH1 overexpression attenuated canines' atrial PGP9.5 level to ∼56% of the controls. In myocardial cells, transfection of GCH1 overexpression lentiviruses also decreased PGP9.5 expression to 26% of the control group. In patients, plasma was collected and miR-206 expression was upregulated in AF patients (n = 18) than the controls (n = 12).

CONCLUSIONS

Our findings suggested that GCH1 downregulation exacerbated ANR by decreasing atrial BH4 and NO content modulated by miR-206 in A-TP canines. This indicates that GCH1 may prevent the initiation of AF through inhibiting ANR.

Long-Term Effects of Ganglionated Plexi Ablation on Electrophysiological Characteristics and Neuron Remodeling in Target Atrial Tissues in a Canine Model

BACKGROUND

The long-term effects of ganglionated plexi ablation on electrophysiological characteristics and neuron remodeling in target atrial tissues remain unclear.

METHODS AND RESULTS

Dogs in group 1 (control, n=8) were not subjected to ganglionated plexi ablation and observed for 1 month, and dogs in groups 2 to 4 (ablation groups, n=8 each) underwent ablation of the right-sided ganglionated plexi and observed for 1, 6, and 12 months, respectively. Atrial electrophysiological characteristics were examined before ablation, immediately and continuously after ablation. Target atrial tissues were subjected to immunohistochemical staining and Western blot analysis. Atrial effective refractory period was significantly prolonged immediately after ablation (P<0.001), and persisted for 1 month (P<0.05). Nerve densities decreased 1 month after ablation (P<0.001). These parameters reverted to preablation levels after 6 and 12 months. In the ablation groups, atrial fibrillation was induced in 5 of 8 dogs after 1 month and in all animals after 6 and 12 months. Atrial fibrillation was not observed in the control group and in the experimental groups immediately after ablation. Moreover, the expression of the growth-associated protein 43 was upregulated after ablation.

CONCLUSIONS

Ganglionated plexi ablation effectively prolonged atrial effective refractory period for a short period, but the long-term effects on atrial effective refractory period and the suppression of atrial fibrillation induction were not persistent. Targeted atrial neuron remodeling may be an important mechanism underlying the observed electrophysiological changes.

MicroRNA profiling of atrial fibrillation in canines: miR-206 modulates intrinsic cardiac autonomic nerve remodeling by regulating SOD1

Background

A critical mechanism in atrial fibrillation (AF) is cardiac autonomic nerve remodeling (ANR). MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the post-transcriptional level. Numerous miRNAs are involved in diseases of the nervous and cardiovascular systems.

Objective

We aimed to assess the underlying role of miRNAs in regulating cardiac ANR in AF by right atrial tachypacing (A-TP) in canines.

Methods and Results

Following 4-week A-TP, the superior left ganglionated plexuses (SLGPs), which are embedded in the fat pads of the left atrium, were subjected to miRNA expression profiling to screen preferentially expressed miRNAs. Sixteen miRNAs showed significantly differential expression between the control and A-TP groups, including miR-206, miR-203, miR-224 and miR-137. In particular, we focused on miR-206, which was elevated ~10-fold in A-TP dogs. Forced expression of miR-206 through lentiviral infection based on A-TP in vivo significantly shortened the atrial effective refractory period (AERP) (81 ± 7 vs. 98 ± 7 ms, P < 0.05). Immunohistochemical analysis showed that the regeneration of nerves increased more than 2-fold by miR-206 overexpression (P < 0.01). The expression of superoxide dismutase 1 (SOD1) was repressed by miR-206 overexpression by Western blot and luciferase assay, indicative of SOD1 as a direct target of miR-206. Overexpression of miR-206 increased reactive oxygen species (ROS) levels in vitro and in vivo, whereas miR-206 silencing attenuated irradiation- or A-TP-induced ROS. Knockdown of SOD1 effectively abolished ROS reduction caused by miR-206 silencing.

Conclusions

Our results found the differential expression of miRNAs in response to ANR in AF and elucidated the important role of miR-206 by targeting SOD1. The study illustrated the novel molecular mechanism of ANR and indicated a potential therapeutic target for AF.

Biatrial neuroablation attenuates atrial remodeling and vulnerability to atrial fibrillation in canine chronic rapid atrial pacing

AIMS

We investigated the proposition that an intact cardiac nervous system may contribute to electrophysiological remodeling and increased vulnerability to atrial fibrillation (AF) following chronic rapid atrial pacing (RAP).

METHODS AND RESULTS

Baseline study was conducted prior to ablating right and left ganglionated plexuses (RAGP, LAGP) in 11 anesthetized canines (Neuroablation group) and in 11 canines without neuroablation (Intact GP). After being subjected to RAP (400 beats/min) for 6 weeks, animals were reanesthetized for terminal study. The ERP shortening typical of chronic RAP was significantly more pronounced in the Intact GP (baseline: 112 ± 12 to terminal: 80 ± 11 ms) than in the Neuroablation group (113 ± 18 to 102 ± 21 ms, p < .001), and AF inducibility (extrastimulus protocol) showed significantly greater increment in the Intact GP (baseline: 23 ± 19% to terminal: 60 ± 17% of trials) than in the Neuroablation group (18 ± 15% to 27 ± 17%, p = 0.029). Negative chronotropic responses to right vagus nerve stimulation were markedly reduced immediately after the neuroablation procedure but had recovered at terminal study. Vagally-evoked repolarization changes (from 191 unipolar electrograms) occurred in a majority of Intact GP animals in the superior, middle and inferior RA free wall, and in the LA appendage. In the Neuroablation group, repolarization changes were restricted to the superior RA free wall but none occurred in the inferior RA and only infrequently in the LA appendage, yielding significantly smaller affected areas in Neuroablation than in Intact GP animals.

CONCLUSION

Persistent functional denervation in LA and RA regions other than RA pacemaker areas may contribute to prevent the development of a tachycardia-dependent AF substrate.