Unraveling the evolutionary origin of the complex Nuclear Receptor Element (cNRE), a cis-regulatory module required for preferential expression in the atrial chamber.

Cardiac function requires appropriate proteins in each chamber. Atria requires slow myosin to act as reservoirs, while ventricles demand fast myosin for swift pumping. Myosins are thus under chamber-biased cis-regulation, with myosin gene expression imbalances leading to congenital heart dysfunction. To identify regulatory inputs leading to cardiac chamber-biased expression, we computationally and molecularly dissected the quail Slow Myosin Heavy Chain III (SMyHC III) promoter that drives preferential expression to the atria. We show that SMyHC III gene states are orchestrated by a complex Nuclear Receptor Element (cNRE) of 32 base pairs. Using transgenesis in zebrafish and mice, we demonstrate that preferential atrial expression is achieved by a combinatorial regulatory input composed of atrial activation motifs and ventricular repression motifs. Using comparative genomics, we show that the cNRE might have emerged from an endogenous viral element through infection of an ancestral host germline, revealing an evolutionary pathway to cardiac chamber-specific expression.

The Differences in the Developmental Stages of the Cardiomyocytes and Endothelial Cells in Human and Mouse Embryos at the Single-Cell Level.

Our research focuses on expression patterns in human and mouse embryonic cardiomyocytes and endothelial cells at the single-cell level. We analyzed single-cell datasets containing different species, cardiac chambers, and cell types. We identified developmentally dynamic genes associated with different cellular lineages in the heart and explored their expression and possible roles during cardiac development. We used dynamic time warping, a method that aligns temporal sequences, to compare these developmental stages across two species. Our results indicated that atrial cardiomyocytes from E9.5 to E13.5 in mice corresponded to a human embryo age of approximately 5-6 weeks, whereas in ventricular cardiomyocytes, they corresponded to a human embryo age of 13-15 weeks. The endothelial cells in mouse hearts corresponded to 6-7-week-old human embryos. Next, we focused on expression changes in cardiac transcription factors over time in different species and chambers, and found that Prdm16 might be related to interspecies cardiomyocyte differences. Moreover, we compared the developmental trajectories of cardiomyocytes differentiated from human pluripotent stem cells and embryonic cells. This analysis explored the relationship between their respective developments and provided compelling evidence supporting the relevance of our dynamic time-warping results. These significant findings contribute to a deeper understanding of cardiac development across different species.

25-hydroxycholesterol triggers antioxidant signaling in mouse atria.

Oxysterol, 25-hydroxycholesterol (25HC), is a potent regulator of immune reactions, its synthesis greatly increases by macrophages during inflammation. We hypothesize that 25HC can have cardioprotective effects by limiting consequences of excessive ß-adrenoceptor (ßAR) stimulation, particularly reactive oxygen species (ROS) production, in mouse atria. Isoproterenol, a ßAR agonist, increased extra- and intracellular levels of ROS. This enhancement of ROS production was suppressed by NADPH oxidase antagonists as well as 25HC. Inhibition of ß3ARs, Gi protein and protein kinase Cε prevented the effect of 25HC on isoproterenol-dependent ROS synthesis. Furthermore, 25HC suppressed isoproterenol-induced lipid peroxidation and mitochondrial ROS generation as well as ROS-dependent component of positive inotropic response to isoproterenol. Additionally, 25HC decreased mitochondrial ROS production and lipid peroxidation induced by antimycin A, a mitochondrial poison. Thus, 25HC exerts antioxidant properties alleviating mitochondrial dysfunction-induced and ßAR-dependent cardiac oxidative damage. In the latter case, 25HC can act via signaling mechanism engaging ß3ARs, Gi protein and protein kinase Cε.

Naringin activates semaphorin 3A to ameliorate TGF-ß-induced endothelial-to-mesenchymal transition related to atrial fibrillation.

The loss of semaphorin 3A (Sema3A), which is related to endothelial-to-mesenchymal transition (EndMT) in atrial fibrosis, is implicated in the pathogenesis of atrial fibrillation (AF). To explore the mechanisms by which EndMT affects atrial fibrosis and assess the potential of a Sema3A activator (naringin) to prevent atrial fibrosis by targeting transforming growth factor-beta (TGF-ß)-induced EndMT, we used human atria, isolated human atrial endocardial endothelial cells (AEECs), and used transgenic mice expressing TGF-ß specifically in cardiac tissues (TGF-ß transgenic mice). We evaluated an EndMT marker (Twist), a proliferation marker (proliferating cell nuclear antigen; PCNA), and an endothelial cell (EC) marker (CD31) through triple immunohistochemistry and confirmed that both EndMT and EC proliferation contribute to atrial endocardial fibrosis during AF in TGF-ß transgenic mice and AF patient tissue sections. Additionally, we investigated the impact of naringin on EndMT and EC proliferation in AEECs and atrial fibroblasts. Naringin exhibited an antiproliferative effect, to which AEECs were more responsive. Subsequently, we downregulated Sema3A in AEECs using small interfering RNA to clarify a correlation between the reduction in Sema3A and the elevation of EndMT markers. Naringin treatment induced the expression of Sema3A and a concurrent decrease in EndMT markers. Furthermore, naringin administration ameliorated AF and endocardial fibrosis in TGF-ß transgenic mice by stimulating Sema3A expression, inhibiting EndMT markers, reducing atrial fibrosis, and lowering AF vulnerability. This suggests therapeutic potential for naringin in AF treatment.

Atrial proteomic profiling reveals a switch towards profibrotic gene expression program in CREM-IbΔC-X mice with persistent atrial fibrillation.

BACKGROUND: Overexpression of the CREM (cAMP response element-binding modulator) isoform CREM-IbΔC-X in transgenic mice (CREM-Tg) causes the age-dependent development of spontaneous AF. PURPOSE: To identify key proteome signatures and biological processes accompanying the development of persistent AF through integrated proteomics and bioinformatics analysis. METHODS: Atrial tissue samples from three CREM-Tg mice and three wild-type littermates were subjected to unbiased mass spectrometry-based quantitative proteomics, differential expression and pathway enrichment analysis, and protein-protein interaction (PPI) network analysis. RESULTS: A total of 98 differentially expressed proteins were identified. Gene ontology analysis revealed enrichment for biological processes regulating actin cytoskeleton organization and extracellular matrix (ECM) dynamics. Changes in ITGAV, FBLN5, and LCP1 were identified as being relevant to atrial fibrosis and structural based on expression changes, co-expression patterns, and PPI network analysis. Comparative analysis with previously published datasets revealed a shift in protein expression patterns from ion-channel and metabolic regulators in young CREM-Tg mice to profibrotic remodeling factors in older CREM-Tg mice. Furthermore, older CREM-Tg mice exhibited protein expression patterns reminiscent of those seen in humans with persistent AF. CONCLUSIONS: This study uncovered distinct temporal changes in atrial protein expression patterns with age in CREM-Tg mice consistent with the progressive evolution of AF. Future studies into the role of the key differentially abundant proteins identified in this study in AF progression may open new therapeutic avenues to control atrial fibrosis and substrate development in AF.

ROS in diabetic atria regulate SK2 degradation by Atrogin-1 through the NF-κB signaling pathway.

One of the independent risk factors for atrial fibrillation is diabetes mellitus (DM); however, the underlying mechanisms causing atrial fibrillation in DM are unknown. The underlying mechanism of Atrogin-1-mediated SK2 degradation and associated signaling pathways are unclear. The aim of this study was to elucidate the relationship among reactive oxygen species (ROS), the NF-κB signaling pathway, and Atrogin-1 protein expression in the atrial myocardia of DM mice. We found that SK2 expression was downregulated comitant with increased ROS generation and enhanced NF-κB signaling activation in the atrial cardiomyocytes of DM mice. These observations were mimicked by exogenously applicating H2O2 and by high glucose culture conditions in HL-1 cells. Inhibition of ROS production by diphenyleneiodonium chloride or silencing of NF-κB by siRNA decreased the protein expression of NF-κB and Atrogin-1 and increased that of SK2 in HL-1 cells with high glucose culture. Moreover, chromatin immunoprecipitation assay demonstrated that NF-κB/p65 directly binds to the promoter of the FBXO32 gene (encoding Atrogin-1), regulating the FBXO32 transcription. Finally, we evaluated the therapeutic effects of curcumin, known as a NF-κB inhibitor, on Atrogin-1 and SK2 expression in DM mice and confirmed that oral administration of curcumin for 4 weeks significantly suppressed Atrogin-1 expression and protected SK2 expression against hyperglycemia. In summary, the results from this study indicated that the ROS/NF-κB signaling pathway participates in Atrogin-1-mediated SK2 regulation in the atria of streptozotocin-induced DM mice.

HRAS-Mutant Cardiomyocyte Model of Multifocal Atrial Tachycardia.

BACKGROUND: Germline HRAS gain-of-function pathogenic variants cause Costello syndrome (CS). During early childhood, 50% of patients develop multifocal atrial tachycardia, a treatment-resistant tachyarrhythmia of unknown pathogenesis. This study investigated how overactive HRAS activity triggers arrhythmogenesis in atrial-like cardiomyocytes (ACMs) derived from human-induced pluripotent stem cells bearing CS-associated HRAS variants. METHODS: HRAS Gly12 mutations were introduced into a human-induced pluripotent stem cells-ACM reporter line. Human-induced pluripotent stem cells were generated from patients with CS exhibiting tachyarrhythmia. Calcium transients and action potentials were assessed in induced pluripotent stem cell-derived ACMs. Automated patch clamping assessed funny currents. HCN inhibitors targeted pacemaker-like activity in mutant ACMs. Transcriptomic data were analyzed via differential gene expression and gene ontology. Immunoblotting evaluated protein expression associated with calcium handling and pacemaker-nodal expression. RESULTS: ACMs harboring HRAS variants displayed higher beating rates compared with healthy controls. The hyperpolarization activated cyclic nucleotide gated potassium channel inhibitor ivabradine and the Nav1.5 blocker flecainide significantly decreased beating rates in mutant ACMs, whereas voltage-gated calcium channel 1.2 blocker verapamil attenuated their irregularity. Electrophysiological assessment revealed an increased number of pacemaker-like cells with elevated funny current densities among mutant ACMs. Mutant ACMs demonstrated elevated gene expression (ie, ISL1, TBX3, TBX18) related to intracellular calcium homeostasis, heart rate, RAS signaling, and induction of pacemaker-nodal-like transcriptional programming. Immunoblotting confirmed increased protein levels for genes of interest and suppressed MAPK (mitogen-activated protein kinase) activity in mutant ACMs. CONCLUSIONS: CS-associated gain-of-function HRASG12 mutations in induced pluripotent stem cells-derived ACMs trigger transcriptional changes associated with enhanced automaticity and arrhythmic activity consistent with multifocal atrial tachycardia. This is the first human-induced pluripotent stem cell model establishing the mechanistic basis for multifocal atrial tachycardia in CS.

Administration of USP7 inhibitor p22077 alleviates Angiotensin II (Ang II)-induced atrial fibrillation in Mice.

Atrial fibrillation (AF), the most common cardiac arrhythmia, is an important contributor to mortality and morbidity. Ubquitin-specific protease 7 (USP7), one of the most abundant ubiquitin-specific proteases (USP), participated in many cellular events, such as cell proliferation, apoptosis, and tumourigenesis. However, its role in AF remains unknown. Here, the mice were treated with Ang II infusion to induce the AF model. Echocardiography was used to measure the atrial diameter. Electrical stimulation was programmed to measure the induction and duration of AF. The changes in atrial remodeling were measured using routine histologic analysis. Here, a significant increase in USP7 expression was observed in Ang II-stimulated atrial cardiomyocytes and atrial tissues, as well as in atrial tissues from patients with AF. The administration of p22077, the inhibitor of USP7, attenuated Ang II-induced inducibility and duration of AF, atrial dilatation, connexin dysfunction, atrial fibrosis, atrial inflammation, and atrial oxidase stress, and then inhibited the progression of AF. Mechanistically, the administration of p22077 alleviated Ang II-induced activation of TGF-ß/Smad2, NF-κB/NLRP3, NADPH oxidases (NOX2 and NOX4) signals, the up-regulation of CX43, ox-CaMKII, CaMKII, Kir2.1, and down-regulation of SERCA2a. Together, this study, for the first time, suggests that USP7 is a critical driver of AF and revealing USP7 may present a new target for atrial fibrillation therapeutic strategies.

Interleukin-33/ST2 axis involvement in atrial remodeling and arrhythmogenesis.

Interleukin (IL)-33, a cytokine involved in immune responses, can activate its receptor, suppression of tumorigenicity 2 (ST2), is elevated during atrial fibrillation (AF). However, the role of IL-33/ST2 signaling in atrial arrhythmia is unclear. This study explored the pathological effects of the IL-33/ST2 axis on atrial remodeling and arrhythmogenesis. Patch clamping, confocal microscopy, and Western blotting were used to analyze the electrical characteristics of and protein activity in atrial myocytes (HL-1) treated with recombinant IL-33 protein and/or ST2-neutralizing antibodies for 48 hrs. Telemetric electrocardiographic recordings, Masson's trichrome staining, and immunohistochemistry staining of the atrium were performed in mice receiving tail vein injections with nonspecific immunoglobulin (control), IL-33, and IL-33 combined with anti-ST2 antibody for 2 weeks. IL-33-treated HL-1 cells had a reduced action potential duration, lower L-type Ca2+ current, greater sarcoplasmic reticulum (SR) Ca2+ content, increased Na+/Ca2+ exchanger (NCX) current, elevation of K+ currents, and increased intracellular calcium transient. IL-33-treated HL-1 myocytes had greater activation of the calcium-calmodulin-dependent protein kinase II (CaMKII)/ryanodine receptor 2 (RyR2) axis and nuclear factor kappa B (NF-κB) / NLR family pyrin domain containing 3 (NLRP3) signaling than did control cells. IL-33 treated cells also had greater expression of Nav1.5, Kv1.5, NCX, and NLRP3 than did control cells. Pretreatment with neutralizing anti-ST2 antibody attenuated IL-33-mediated activation of CaMKII/RyR2 and NF-κB/NLRP3 signaling. IL-33-injected mice had more atrial ectopic beats and increased AF episodes, greater atrial fibrosis, and elevation of NF-κB/NLRP3 signaling than did controls or mice treated with IL-33 combined with anti-ST2 antibody. Thus, IL-33 recombinant protein treatment promotes atrial remodeling through ST2 signaling. Blocking the IL-33/ST2 axis might be an innovative therapeutic approach for patients with atrial arrhythmia and elevated serum IL-33.

The role of cellular senescence in profibrillatory atrial remodelling associated with cardiac pathology.

AIMS: Cellular senescence is a stress-related or aging response believed to contribute to many cardiac conditions; however, its role in atrial fibrillation (AF) is unknown. Age is the single most important determinant of the risk of AF. The present study was designed to (i) evaluate AF susceptibility and senescence marker expression in rat models of aging and myocardial infarction (MI), (ii) study the effect of reducing senescent-cell burden with senolytic therapy on the atrial substrate in MI rats, and (iii) assess senescence markers in human atrial tissue as a function of age and the presence of AF. METHODS AND RESULTS: AF susceptibility was studied with programmed electrical stimulation. Gene and protein expression was evaluated by immunoblot or immunofluorescence (protein) and digital polymerase chain reaction (PCR) or reverse transcriptase quantitative PCR (messenger RNA). A previously validated senolytic combination, dasatinib and quercetin, (D+Q; or corresponding vehicle) was administered from the time of sham or MI surgery through 28 days later. Experiments were performed blinded to treatment assignment. Burst pacing-induced AF was seen in 100% of aged (18-month old) rats, 87.5% of young MI rats, and 10% of young control (3-month old) rats (P ≤ 0.001 vs. each). Conduction velocity was slower in aged [both left atrium (LA) and right atrium (RA)] and young MI (LA) rats vs. young control rats (P ≤ 0.001 vs. each). Atrial fibrosis was greater in aged (LA and RA) and young MI (LA) vs. young control rats (P < 0.05 for each). Senolytic therapy reduced AF inducibility in MI rats (from 8/9 rats, 89% in MI vehicle, to 0/9 rats, 0% in MI D + Q, P < 0.001) and attenuated LA fibrosis. Double staining suggested that D + Q acts by clearing senescent myofibroblasts and endothelial cells. In human atria, senescence markers were upregulated in older (≥70 years) and long-standing AF patients vs. individuals ≤60 and sinus rhythm controls, respectively. CONCLUSION: Our results point to a potentially significant role of cellular senescence in AF pathophysiology. Modulating cell senescence might provide a basis for novel therapeutic approaches to AF.

The mechanism of 25-hydroxycholesterol-mediated suppression of atrial ß1-adrenergic responses.

25-Hydroxycholesterol (25HC) is a biologically active oxysterol, whose production greatly increases during inflammation by macrophages and dendritic cells. The inflammatory reactions are frequently accompanied by changes in heart regulation, such as blunting of the cardiac ß-adrenergic receptor (AR) signaling. Here, the mechanism of 25HC-dependent modulation of responses to ß-AR activation was studied in the atria of mice. 25HC at the submicromolar levels decreased the ß-AR-mediated positive inotropic effect and enhancement of the Ca2+ transient amplitude, without changing NO production. Positive inotropic responses to ß1-AR (but not ß2-AR) activation were markedly attenuated by 25HC. The depressant action of 25HC on the ß1-AR-mediated responses was prevented by selective ß3-AR antagonists as well as inhibitors of Gi protein, Gßγ, G protein-coupled receptor kinase 2/3, or ß-arrestin. Simultaneously, blockers of protein kinase D and C as well as a phosphodiesterase inhibitor did not preclude the negative action of 25HC on the inotropic response to ß-AR activation. Thus, 25HC can suppress the ß1-AR-dependent effects via engaging ß3-AR, Gi protein, Gßγ, G protein-coupled receptor kinase, and ß-arrestin. This 25HC-dependent mechanism can contribute to the inflammatory-related alterations in the atrial ß-adrenergic signaling.

Why is early-onset atrial fibrillation uncommon in patients with Duchenne muscular dystrophy? Insights from the mdx mouse.

AIMS: A reduction in both dystrophin and neuronal nitric oxide synthase (NOS1) secondary to microRNA-31 (miR-31) up-regulation contributes to the atrial electrical remodelling that underpins human and experimental atrial fibrillation (AF). In contrast, patients with Duchenne muscular dystrophy (DMD), who lack dystrophin and NOS1 and, at least in the skeletal muscle, have raised miR-31 expression, do not have increase susceptibility to AF in the absence of left ventricular (LV) dysfunction. Here, we investigated whether dystrophin deficiency is also associated with atrial up-regulation of miR-31, loss of NOS1 protein, and increased AF susceptibility in young mdx mice. METHODS AND RESULTS: Echocardiography showed normal cardiac structure and function in 12-13 weeks mdx mice, with no indication by assay of hydroxyproline that atrial fibrosis had developed. The absence of dystrophin in mdx mice was accompanied by an overall reduction in syntrophin and a lower NOS1 protein content in the skeletal muscle and in the left atrial and ventricular myocardium, with the latter occurring alongside reduced Nos1 transcript levels (exons 1-2 by quantitative polymerase chain reaction) and an increase in NOS1 polyubiquitination [assessed using tandem polyubiquitination pulldowns; P < 0.05 vs. wild type (WT)]. Neither the up-regulation of miR-31 nor the substantial reduction in NOS activity observed in the skeletal muscle was present in the atrial tissue of mdx mice. At difference with the skeletal muscle, the mdx atrial myocardium showed a reduction in the constitutive NOS inhibitor, caveolin-1, coupled with an increase in NOS3 serine1177 phosphorylation, in the absence of differences in the protein content of other NOS isoforms or in the relative expression NOS1 splice variants. In line with these findings, transoesophageal atrial burst pacing revealed no difference in AF susceptibility between mdx mice and their WT littermates. CONCLUSION: Dystrophin depletion is not associated with atrial miR-31 up-regulation, reduced NOS activity, or increased AF susceptibility in the mdx mouse. Compared with the skeletal muscle, the milder atrial biochemical phenotype may explain why patients with DMD do not exhibit a higher prevalence of atrial arrhythmias despite a reduction in NOS1 content.

Inhibition of the acetylcholine-regulated potassium current prevents transient apnea-related atrial arrhythmogenic changes in a porcine model.

BACKGROUND: More than 50% of patients with atrial fibrillation (AF) suffer from sleep disordered breathing (SDB). Obstructive respiratory events contribute to a transient, vagally mediated atrial arrhythmogenic substrate, which is resistant to most available antiarrhythmic drugs. OBJECTIVE: The purpose of this study was to investigate the effect of pharmacologic inhibition of the G-protein-gated acetylcholine-regulated potassium current (IK,ACh) with and without acute autonomic nervous system activation by nicotine in a pig model for obstructive respiratory events. METHODS: In 21 pigs, SDB was simulated by applying an intermittent negative upper airway pressure (INAP). AF inducibility and atrial effective refractory periods (aERPs) were determined before and during INAP by an S1S2 atrial pacing-protocol. Pigs were randomized into 3 groups-group 1: vehicle (n = 4); group 2: XAF-1407 (IK,ACh inhibitor) (n = 7); and group 3: nicotine followed by XAF-1407 (n = 10). RESULTS: In group 1, INAP shortened aERP (ΔaERP -42.6 ms; P = .004) and transiently increased AF inducibility from 0% to 31%. In group 2, XAF-1407 prolonged aERP by 25.2 ms (P = .005) during normal breathing and prevented INAP-induced aERP shortening (ΔaERP -3.6 ms; P = .3) and AF inducibility. In group 3, INAP transiently shortened aERP during nicotine perfusion (ΔaERP -33.6 ms; P = .004) and increased AF inducibility up to 61%, which both were prevented by XAF-1407. CONCLUSION: Simulated obstructive respiratory events transiently shorten aERP and increase AF inducibility, which can be prevented by the IK,ACh-inhibitor XAF-1407. XAF-1407 also prevents these arrhythmogenic changes induced by obstructive respiratory events during nicotine perfusion. Whether IK,ACh channels represent a target for SDB-related AF in humans warrants further study.

Sphingolipids: drivers of cardiac fibrosis and atrial fibrillation.

Sphingolipids (SLs) are vital constituents of the plasma membrane of animal cells and concurrently regulate numerous cellular processes. An escalating number of research have evinced that SLs assume a crucial part in the progression of tissue fibrosis, a condition for which no efficacious cure exists as of now. Cardiac fibrosis, and in particular, atrial fibrosis, is a key factor in the emergence of atrial fibrillation (AF). AF has become one of the most widespread cardiac arrhythmias globally, with its incidence continuing to mount, thereby propelling it to the status of a major public health concern. This review expounds on the structure and biosynthesis pathways of several pivotal SLs, the pathophysiological mechanisms of AF, and the function of SLs in cardiac fibrosis. Delving into the influence of sphingolipid levels in the alleviation of cardiac fibrosis offers innovative therapeutic strategies to address cardiac fibrosis and AF.

Cantharidin and sodium fluoride attenuate the negative inotropic effects of carbachol in the isolated human atrium.

Carbachol, an agonist at muscarinic receptors, exerts a negative inotropic effect in human atrium. Carbachol can activate protein phosphatases (PP1 or PP2A). We hypothesized that cantharidin or sodium fluoride, inhibitors of PP1 and PP2A, may attenuate a negative inotropic effect of carbachol. During bypass-surgery trabeculae carneae of human atrial preparations (HAP) were obtained. These trabeculae were mounted in organ baths and electrically stimulated (1 Hz). Force of contraction was measured under isometric conditions. For comparison, we studied isolated electrically stimulated left atrial preparations (LA) from mice. Cantharidin (100 µM) and sodium fluoride (3 mM) increased force of contraction in LA (n = 5-8, p < 0.05) by 113% ± 24.5% and by 100% ± 38.2% and in HAP (n = 13-15, p < 0.05) by 625% ± 169% and by 196% ± 23.5%, respectively. Carbachol (1 µM) alone exerted a rapid transient maximum negative inotropic effect in LA (n = 6) and HAP (n = 14) to 46.9% ± 3.63% and 19.4% ± 3.74%, respectively (p < 0.05). These negative inotropic effects were smaller in LA (n = 4-6) and HAP (n = 9-12) pretreated with 100 µM cantharidin and amounted to 58.0% ± 2.27% and 59.2% ± 6.19% or 3 mM sodium fluoride to 63.7% ± 9.84% and 46.3% ± 5.69%, (p < 0.05). We suggest that carbachol, at least in part, exerts a negative inotropic effect in the human atrium by stimulating the enzymatic activity of PP1 and/or PP2A.

Fine tuning contractility: atrial sarcomere function in health and disease.

The molecular mechanisms of sarcomere proteins underlie the contractile function of the heart. Although our understanding of the sarcomere has grown tremendously, the focus has been on ventricular sarcomere isoforms due to the critical role of the ventricle in health and disease. However, atrial-specific or -enriched myofilament protein isoforms, as well as isoforms that become expressed in disease, provide insight into ways this complex molecular machine is fine-tuned. Here, we explore how atrial-enriched sarcomere protein composition modulates contractile function to fulfill the physiological requirements of atrial function. We review how atrial dysfunction negatively affects the ventricle and the many cardiovascular diseases that have atrial dysfunction as a comorbidity. We also cover the pathophysiology of mutations in atrial-enriched contractile proteins and how they can cause primary atrial myopathies. Finally, we explore what is known about contractile function in various forms of atrial fibrillation. The differences in atrial function in health and disease underscore the importance of better studying atrial contractility, especially as therapeutics currently in development to modulate cardiac contractility may have different effects on atrial sarcomere function.

TRPV2 inhibitor tranilast prevents atrial fibrillation in rat models of pulmonary hypertension.

Atrial fibrillation (AF) is common in pulmonary hypertension (PH), whereas the mechanisms and treatments remain to be explored. TRPV2 regulates the structure and function of the cardiovascular system; however, little attention has been given to its role in AF. This study was to determine whether TRPV2 was involved in PH-induced AF and the effects of TRPV2 inhibitor tranilast on AF in rat models of PH. Monocrotaline (MCT) and SU5416/hypoxia (SuHx)-induced PH models were performed to detect atrial electrophysiological parameters. Daily tranilast (a TRPV2 inhibitor) or saline was given starting 1 day before PH establishment. PH increased the susceptibility to AF, with TRPV2 up-regulated in the right atria. Compared to PH rats, tranilast reduced AF inducibility and the prolongations of ERP and APD; mitigated cardiopulmonary remodeling and the increases in P-wave duration and P-R interval; partially reversed the down-regulation of ion channels such as Cav1.2, Nav1.5, Kv4.3, Kv4.2, Kv1.5, Kir2.1, Kir3.1, Kir3.4 as well as connexin (Cx) 40 and Cx43; improved right atrial (RA) fibrosis, enlargement, and myocardial hypertrophy; decreased the accumulation of inflammatory cells; down-regulated inflammatory indicators such as TNF-α, IL-1ß, CXCL1, and CXCL2; and inhibited the activation of the PI3K-AKT-NF-κB signaling pathway. Our results reveal that TRPV2 participates in PH-induced AF, and TRPV2 inhibitor tranilast prevents PH-induced RA remodeling. TRPV2 might be a promising target for PH-induced AF.

Chronic intermittent hypoxia remodels catecholaminergic nerve innervation in mouse atria.

Chronic intermittent hypoxia (CIH, a model for sleep apnoea) is a major risk factor for several cardiovascular diseases. Autonomic imbalance (sympathetic overactivity and parasympathetic withdrawal) has emerged as a causal contributor of CIH-induced cardiovascular disease. Previously, we showed that CIH remodels the parasympathetic pathway. However, whether CIH induces remodelling of the cardiac sympathetic innervation remains unknown. Mice (male, C57BL/6J, 2-3 months) were exposed to either room air (RA, 21% O2 ) or CIH (alternating 21% and 5.7% O2 , every 6 min, 10 h day-1 ) for 8-10 weeks. Flat-mounts of their left and right atria were immunohistochemically labelled for tyrosine hydroxylase (TH, a sympathetic marker). Using a confocal microscope (or fluorescence microscope) and Neurlocudia 360 digitization and tracing system, we scanned both the left and right atria and quantitatively analysed the sympathetic axon density in both groups. The segmentation data was mapped onto a 3D mouse heart scaffold. Our findings indicated that CIH significantly remodelled the TH immunoreactive (-IR) innervation of the atria by increasing its density at the sinoatrial node, the auricles and the major veins attached to the atria (P < 0.05, n = 7). Additionally, CIH increased the branching points of TH-IR axons and decreased the distance between varicosities. Abnormal patterns of TH-IR axons around intrinsic cardiac ganglia were also found following CIH. We postulate that the increased sympathetic innervation may further amplify the effects of enhanced CIH-induced central sympathetic drive to the heart. Our work provides an anatomical foundation for the understanding of CIH-induced autonomic imbalance. KEY POINTS: Chronic intermittent hypoxia (CIH, a model for sleep apnoea) causes sympathetic overactivity, cardiovascular remodelling and hypertension. We determined the effect of CIH on sympathetic innervation of the mouse atria. In vivo CIH for 8-10 weeks resulted in an aberrant axonal pattern around the principal neurons within intrinsic cardiac ganglia and an increase in the density, branching point, tortuosity of catecholaminergic axons and atrial wall thickness. Utilizing mapping tool available from NIH (SPARC) Program, the topographical distribution of the catecholaminergic innervation of the atria were integrated into a novel 3D heart scaffold for precise anatomical distribution and holistic quantitative comparison between normal and CIH mice. This work provides a unique neuroanatomical understanding of the pathophysiology of CIH-induced autonomic remodelling.

The atrial and ventricular myocardial proteome of end-stage lamin heart disease.

Lamins A/C (encoded by LMNA gene) can lead to dilated cardiomyopathy (DCM). This pilot study sought to explore the postgenomic phenotype of end-stage lamin heart disease. Consecutive patients with end-stage lamin heart disease (LMNA-group, n = 7) and ischaemic DCM (ICM-group, n = 7) undergoing heart transplantation were prospectively enrolled. Samples were obtained from left atrium (LA), left ventricle (LV), right atrium (RA), right ventricle (RV) and interventricular septum (IVS), avoiding the infarcted myocardial segments in the ICM-group. Samples were analysed using a discovery 'shotgun' proteomics approach. We found that 990 proteins were differentially abundant between LMNA and ICM samples with the LA being most perturbed (16-fold more than the LV). Abundance of lamin A/C protein was reduced, but lamin B increased in LMNA LA/RA tissue compared to ICM, but not in LV/RV. Carbonic anhydrase 3 (CA3) was over-abundant across all LMNA tissue samples (LA, LV, RA, RV, and IVS) when compared to ICM. Transthyretin was more abundant in the LV/RV of LMNA compared to ICM, while sarcomeric proteins such as titin and cardiac alpha-cardiac myosin heavy chain were generally less abundant in RA/LA of LMNA. Protein expression profiling and enrichment analysis pointed towards sarcopenia, extracellular matrix remodeling, deficient myocardial energetics, redox imbalances, and abnormal calcium handling in LMNA samples. Compared to ICM, end-stage lamin heart disease is a biventricular but especially a biatrial disease appearing to have an abundance of lamin B, CA3 and transthyretin, potentially hinting to compensatory responses.

Deletion of the muscle enriched lncRNA Oip5os1 induces atrial dysfunction in male mice with diabetes.

Long ncRNAs (lncRNAs) have been shown to play a biological and physiological role in various tissues including the heart. We and others have previously established that the lncRNA Oip5os1 (1700020I14Rik, OIP5-AS1, Cyrano) is enriched in striated muscles, and its deletion in mice leads to defects in both skeletal and cardiac muscle function. In the present study, we investigated the impact of global Oip5os1 deletion on cardiac function in the setting of streptozotocin (STZ)-induced diabetes. Specifically, we studied male WT and KO mice with or without diabetes for 24 weeks, and phenotyped animals for metabolic and cardiac endpoints. Independent of genotype, diabetes was associated with left ventricular diastolic dysfunction based on a fall in E'/A' ratio. Deletion of Oip5os1 in a setting of diabetes had no significant impact on ventricular function or ventricular weight, but was associated with left atrial dysfunction (reduced fractional shortening) and myopathy which was associated with anesthesia intolerance and premature death in the majority of KO mice tested during cardiac functional assessment. This atrial phenotype was not observed in WT diabetic mice. The most striking molecular difference was a reduction in the metabolic regulator ERRalpha in the atria of KO mice compared with WT mice. There was also a trend for a reduction in Serca2a. These findings highlight Oip5os1 as a gene of interest in aspects of atrial function in the setting of diabetes, highlighting an additional functional role for this lncRNA in cardiac pathological settings.