Hippo-Yap Signaling Maintains Sinoatrial Node Homeostasis

Connexin 43 dephosphorylation mediates the Dchs1/YAP/TEAD signaling pathway to induce cardiac fibrosis

BACKGROUND

The gap junction protein connexin 43 (Cx43) has been implicated in the development of cardiac fibrosis. Our previous findings revealed that Cx43 dephosphorylation at serine 282 (S282) is related to cardiomyocyte apoptosis and arrhythmias in hearts damaged by ischemia/reperfusion. In this study, we investigated the role of Cx43 S282 phosphorylation in cardiac fibrosis.

METHODS

We used angiotensin II (Ang II) intervention in mice to establish an in vivo cardiac fibrosis model and transforming growth factor β-1 (TGF-β1) intervention in cardiac myofibroblasts to establish an in vitro fibrosis model. The expression of Cx43 S282 phosphorylation was examined in the in vivo and in vitro models. To further confirm the effect of Cx43 S282 phosphorylation on cardiac fibrosis, we transfected cardiac myofibroblasts with lentiviral bodies in vitro, and injected myocardium with adenovirus in vivo to establish the over-expression of phosphorylation of Cx43 S282 locus and mutant groups. We sequenced the mRNA of the in vitro group using gene set enrichment analysis (GSEA) and normalized enrichment scoring (NES) to investigate the signaling pathway by which p282-Cx43 affects myocardial fibrosis (MF). The role of the Hippo signaling pathway in phosphorylation at the Cx43 282 site was further validated.

RESULTS

In an in vivo and in vitro model of cardiac fibrosis, the level of phosphorylation of Cx43 S282 was reduced. Mutation of Cx43 S282 to a less phosphorylatable form (S282A) resulted in elevated levels of fibrosis markers, suggesting a critical antifibrotic role for phosphorylated Cx43 S282. Increased phosphorylation of Cx43 S282 produced an inhibitory effect on fibrosis. Enrichment analysis of mRNA sequencing in the mutant model group indicated that the Hippo signaling pathway was involved in the fibrosis process. Cx43 S282 phosphorylation increased the expression of Dchs1 gene, which activates the phosphorylation of yes-associated protein (YAP) and inhibits the YAP/TEAD signaling pathway to inhibit fibrosis development.

CONCLUSIONS

This study suggests that the phosphorylation of Cx43 S282 could be an effective antifibrotic target in cardiac fibroblasts. This indicates a novel mechanism and a molecular target that may hold promise for treating cardiac fibrosis.

HCN4 in the atrioventricular node

Hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) drives the funny current in cardiac pacemaker regions. Its involvement in sinoatrial node pacemaker generation is well known, but its function in the atrioventricular (AV) node (AVN) has not intensively been studied. HCN4 is expressed in the AVN, and its expression within the AVN seems similar across mammalian species with HCN4 presence in the inferior nodal extensions, compact node, and AV bundle. The main direct regulators of HCN4 are cAMP and protein kinase A. In addition, indirect regulators may affect HCN4 via trafficking and localization. However, these effects are underexplored in the AVN. AVN-specific effects in knockout and knockin mice include reduced funny current density and increased AV block. HCN4 expression in the AVN could be affected by aging, exercise, heart failure, and diabetes. This could underlie changes in PR interval, atria-His interval, Wenckebach cycle length, and AVN effective refractory period. Clinical reports link the HCN4 variant G1097W to AV block. Other clinical data come from studies assessing ivabradine, an HCN4 inhibitor. In animals, ivabradine resulted in prolonged PR and atrial-his intervals. To date, uncertainty regarding the role of HCN4 in the AVN remains. However, AVN-focused studies suggest HCN4's importance for AVN function. This review summarizes recent findings and highlights the involvement of HCN4 in normal and pathological AVN function.

Mitochondrial dysfunction is a key link involved in the pathogenesis of sick sinus syndrome: a review

Sick sinus syndrome (SSS) is a grave medical condition that can precipitate sudden death. The pathogenesis of SSS remains incompletely understood. Existing research postulates that the fundamental mechanism involves increased fibrosis of the sinoatrial node and its surrounding tissues, as well as disturbances in the coupled-clock system, comprising the membrane clock and the Ca2+ clock. Mitochondrial dysfunction exacerbates regional tissue fibrosis and disrupts the functioning of both the membrane and calcium clocks. This plays a crucial role in the underlying pathophysiology of SSS, including mitochondrial energy metabolism disorders, mitochondrial oxidative stress damage, calcium overload, and mitochondrial quality control disorders. Elucidating the mitochondrial mechanisms involved in the pathophysiology of SSS and further investigating the disease's mechanisms is of great significance.

CMTM5 influences Hippo/YAP axis to promote ferroptosis in glioma through regulating WWP2-mediated LATS2 ubiquitination

Glioma, a common malignancy, is characterized by high morbidity and mortality. Promoting ferroptosis can delay tumor progression. Here, we aimed to explore the underlying mechanism of ferroptosis in glioma. In vitro and in vivo experiments were conducted using glioma cells and nude mice. The expression of genes and proteins was evaluated by RT-qPCR, Western blot assay, and immunohistochemical staining. Malignant activities of glioma cells were evaluated using MTT, EdU, and Transwell assays. The levels of Fe2+, lipid reactive oxygen species, and malondialdehyde were determined using commercial kits. The interplays among CMTM5, WWP2, and LATS2 were validated using Co-immunoprecipitation assay. The UALCAN database predicted downregulation of CMTM5 expression in glioma, and low expression of CMTM5 was associated with poor survival outcomes. CMTM5 overexpression inhibited cell growth and invasion and promoted ferroptosis of glioma cells. Besides, CMTM5 protein interacted with WWP2 protein and decreased WWP2 expression. WWP2 silencing attenuated LATS2 ubiquitination to enhance LATS2 expression and phosphorylation of YAP1. CMTM5 exerted a suppressive effect on cell growth and invasion and promoted ferroptosis of glioma cells by regulating the WWP2/LATS2 pathway. In the in vivo experiments, CMTM5 overexpression suppressed tumor growth and enhanced ferroptosis. CMTM5 regulated Hippo/YAP signaling to inhibit cell growth and invasion and to promote ferroptosis in glioma by regulating WWP2-mediated LATS2 ubiquitination, thereby attenuating glioma progression.

Common SYNE2 Genetic Variant Associated With Atrial Fibrillation Lowers Expression of Nesprin-2α1 With Downstream Effects on Nuclear and Electrophysiological Traits

BACKGROUND

Atrial fibrillation GWAS (genome-wide association studies) identified significant associations for rs1152591 and linked variants in the SYNE2 gene encoding Nesprin-2, which connects the nuclear membrane with the cytoskeleton.

METHODS

Reporter gene vector transfection and CRISPR-Cas9 editing were used to identify the causal variant regulating the expression of SYNE2α1. After SYNE2 knockdown or SYNE2α1 overexpression in human stem cell-derived cardiomyocytes, nuclear phenotypes were assessed by imaging and atomic force microscopy. Gene expression was assessed by RNAseq and gene set enrichment analysis. Fura-2 AM staining assessed calcium transients. Optical mapping assessed action potential duration and conduction velocity.

RESULTS

The risk allele of rs1152591 had lower promoter and enhancer activity and was significantly associated with lower expression of the short SYNE2α1 isoform in human stem cell-derived cardiomyocytes, without an effect on the expression of the full-length SYNE2 mRNA. SYNE2α1 overexpression had dominant negative effects on the nucleus with its overexpression or SYNE2 knockdown leading to increased nuclear area and decreased nuclear stiffness. Gene expression results from SYNE2α1 overexpression demonstrated both concordant and nonconcordant effects with SYNE2 knockdown. SYNE2α1 overexpression had a gain of function on electrophysiology, leading to significantly faster calcium reuptake and decreased assessed action potential duration, while SYNE2 knockdown showed both shortened assessed action potential duration and decreased conduction velocity.

CONCLUSIONS

rs1152591 was identified as a causal atrial fibrillation variant, with the risk allele decreasing SYNE2α1 expression. Downstream effects of SYNE2α1 overexpression include changes in nuclear stiffness and electrophysiology, which may contribute to the mechanism for the risk allele's association with AF.

MiR-196a-5p hinders vascular smooth muscle cell proliferation and vascular remodeling via repressing BACH1 expression

Hyperproliferation of vascular smooth muscle cells (VSMCs) is a driver of hypertensive vascular remodeling. This study aimed to uncover the mechanism of BTB and CNC homology 1 (BACH1) and microRNAs (miRNAs) in VSMC growth and hypertensive vascular remodeling. With the help of TargetScan, miRWalk, miRDB, and miRTarBase online database, we identified that BACH1 might be targeted by miR-196a-5p, and overexpressed in VSMCs and aortic tissues from spontaneously hypertensive rats (SHRs). Gain- and loss-of-function experiments demonstrated that miR-196a-5p suppressed VSMC proliferation, oxidative stress and hypertensive vascular remodeling. Double luciferase reporter gene assay and functional verification showed that miR-196a-5p cracked down the transcription and translation of BACH1 in both Wistar Kyoto rats (WKYs) and SHRs. Silencing BACH1 mimicked the actions of miR-196a-5p overexpression on attenuating the proliferation and oxidative damage of VSMCs derived from SHRs. Importantly, miR-196a-5p overexpression and BACH1 knockdown cooperatively inhibited VSMC proliferation and oxidative stress in SHRs. Furthermore, miR-196a-5p, if knocked down in SHRs, aggravated hypertension, upregulated BACH1 and promoted VSMC proliferation, all contributing to vascular remodeling. Taken together, targeting miR-196a-5p to downregulate BACH1 may be a promising strategy for retarding VSMC proliferation and hypertensive vascular remodeling.

Segregation of morphogenetic regulatory function of Shox2 from its cell fate guardian role in sinoatrial node development

Shox2 plays a vital role in the morphogenesis and physiological function of the sinoatrial node (SAN), the primary cardiac pacemaker, manifested by the formation of a hypoplastic SAN and failed differentiation of pacemaker cells in Shox2 mutants. Shox2 and Nkx2-5 are co-expressed in the developing SAN and regulate the fate of the pacemaker cells through a Shox2-Nkx2-5 antagonistic mechanism. Here we show that simultaneous inactivation of Nkx2-5 in the SAN of Shox2 mutants (dKO) rescued the pacemaking cell fate but not the hypoplastic defects, indicating uncoupling of SAN cell fate determination and morphogenesis. Single-cell RNA-seq revealed that the presumptive SAN cells of Shox2-/- mutants failed to activate pacemaking program but remained in a progenitor state preceding working myocardium, while both wildtype and dKO SAN cells displayed normal pacemaking cell fate with similar cellular state. Shox2 thus acts as a safeguard but not a determinant to ensure the pacemaking cell fate through the Shox2-Nkx2-5 antagonistic mechanism, which is segregated from its morphogenetic regulatory function in SAN development.

The important role of miR-1-3p in cancers

Cancer is a malignant tumor that seriously threatens human life and health. At present, the main treatment methods include surgical resection, chemotherapy, radiotherapy, and immunotherapy. However, the mechanism of tumor occurrence and development is complex, and it produces resistance to some traditional treatment methods, leading to treatment failure and a high mortality rate for patients. Therefore, exploring the molecular mechanisms of tumor occurrence, development, and drug resistance is a very important task. MiRNAs are a type of non-coding small RNA that regulate a series of biological effects by binding to the 3'-UTR of the target mRNA, degrading the mRNA, or inhibiting its translation. MiR-1-3p is an important member of them, which is abnormally expressed in various tumors and closely related to the occurrence and development of tumors. This article introduces miR-1-3p from multiple aspects, including its production and regulation, role in tumor occurrence and development, clinical significance, role in drug resistance, and approaches for targeting miR-1-3p. Intended to provide readers with a comprehensive understanding of the important role of miR-1-3p in tumors.

Hippo-deficient cardiac fibroblasts differentiate into osteochondroprogenitors

Cardiac fibrosis, a common pathophysiology associated with various heart diseases, occurs from the excess deposition of extracellular matrix (ECM) 1 . Cardiac fibroblasts (CFs) are the primary cells that produce, degrade, and remodel ECM during homeostasis and tissue repair 2 . Upon injury, CFs gain plasticity to differentiate into myofibroblasts 3 and adipocyte-like 4,5 and osteoblast-like 6 cells, promoting fibrosis and impairing heart function 7 . How CFs maintain their cell state during homeostasis and adapt plasticity upon injury are not well defined. Recent studies have shown that Hippo signalling in CFs regulates cardiac fibrosis and inflammation 8-11 . Here, we used single-nucleus RNA sequencing (snRNA-seq) and spatially resolved transcriptomic profiling (ST) to investigate how the cell state was altered in the absence of Hippo signaling and how Hippo-deficient CFs interact with macrophages during cardiac fibrosis. We found that Hippo-deficient CFs differentiate into osteochondroprogenitors (OCPs), suggesting that Hippo restricts CF plasticity. Furthermore, Hippo-deficient CFs colocalized with macrophages, suggesting their intercellular communications. Indeed, we identified several ligand-receptor pairs between the Hippo-deficient CFs and macrophages. Blocking the Hippo-deficient CF-induced CSF1 signaling abolished macrophage expansion. Interestingly, blocking macrophage expansion also reduced OCP differentiation of Hippo-deficient CFs, indicating that macrophages promote CF plasticity.

Transforming Growth Factor Beta Promotes Inflammation and Tumorigenesis in Smad4-Deficient Intestinal Epithelium in a YAP-Dependent Manner

Transforming growth factor beta (TGF-β), a multifunctional cytokine, plays critical roles in immune responses. However, the precise role of TGF-β in colitis and colitis-associated cancer remains poorly defined. Here, it is demonstrated that TGF-β promotes the colonic inflammation and related tumorigenesis in the absence of Smad family member 4 (Smad4). Smad4 loss in intestinal epithelium aggravates colitis and colitis-associated neoplasia induced by dextran sulfate sodium (DSS) and azoxymethane/dextran sulfate sodium (AOM/DSS), leading to over-activated immune responses and increased TGF-β1 levels. In Smad4-deficient organoids, TGF-β1 stimulates spheroid formation and impairs intestinal stem cell proliferation and lineage specification. YAP, whose expression is directly upregulated by TGF-β1 after Smad4 deletion, mediates the effect of TGF-β1 by interacting with Smad2/3. Attenuation of YAP/TAZ prevents TGF-β1-induced spheroid formation in Smad4-/- organoids and alleviates colitis and colitis-associated cancer in Smad4-deficient mice. Collectively, these results highlight an integral role of the TGF-β/Smad4 axis in restraining intestinal inflammation and tumorigenesis and suggest TGF-β or YAP signaling as therapeutic targets for these gastrointestinal diseases intervention.

Emerging Signaling Regulation of Sinoatrial Node Dysfunction

PURPOSE OF REVIEW

The sinoatrial node (SAN), the natural pacemaker of the heart, is responsible for generating electrical impulses and initiating each heartbeat. Sinoatrial node dysfunction (SND) causes various arrhythmias such as sinus arrest, SAN block, and tachycardia/bradycardia syndrome. Unraveling the underlying mechanisms of SND is of paramount importance in the pursuit of developing effective therapeutic strategies for patients with SND. This review provides a concise summary of the most recent progress in the signaling regulation of SND.

RECENT FINDINGS

Recent studies indicate that SND can be caused by abnormal intercellular and intracellular signaling, various forms of heart failure (HF), and diabetes. These discoveries provide novel insights into the underlying mechanisms SND, advancing our understanding of its pathogenesis. SND can cause severe cardiac arrhythmias associated with syncope and an increased risk of sudden death. In addition to ion channels, the SAN is susceptible to the influence of various signalings including Hippo, AMP-activated protein kinase (AMPK), mechanical force, and natriuretic peptide receptors. New cellular and molecular mechanisms related to SND are also deciphered in systemic diseases such as HF and diabetes. Progress in these studies contributes to the development of potential therapeutics for SND.

DeepMiceTL: a deep transfer learning based prediction of mice cardiac conduction diseases using early electrocardiograms

Cardiac conduction disease is a major cause of morbidity and mortality worldwide. There is considerable clinical significance and an emerging need of early detection of these diseases for preventive treatment success before more severe arrhythmias occur. However, developing such early screening tools is challenging due to the lack of early electrocardiograms (ECGs) before symptoms occur in patients. Mouse models are widely used in cardiac arrhythmia research. The goal of this paper is to develop deep learning models to predict cardiac conduction diseases in mice using their early ECGs. We hypothesize that mutant mice present subtle abnormalities in their early ECGs before severe arrhythmias present. These subtle patterns can be detected by deep learning though they are hard to be identified by human eyes. We propose a deep transfer learning model, DeepMiceTL, which leverages knowledge from human ECGs to learn mouse ECG patterns. We further apply the Bayesian optimization and $k$-fold cross validation methods to tune the hyperparameters of the DeepMiceTL. Our results show that DeepMiceTL achieves a promising performance (F1-score: 83.8%, accuracy: 84.8%) in predicting the occurrence of cardiac conduction diseases using early mouse ECGs. This study is among the first efforts that use state-of-the-art deep transfer learning to identify ECG patterns during the early course of cardiac conduction disease in mice. Our approach not only could help in cardiac conduction disease research in mice, but also suggest a feasibility for early clinical diagnosis of human cardiac conduction diseases and other types of cardiac arrythmias using deep transfer learning in the future.

Insulin signaling is critical for sinoatrial node maintenance and function

Insulin and insulin-like growth factor 1 (IGF-1) signaling regulate cellular growth and glucose metabolism in the myocardium. However, their physiological role in the cells of the cardiac conduction system has never been explored. Therefore, we sought to determine the spatiotemporal function of insulin/IGF-1 receptors in the sinoatrial node (SAN). We generated cardiac conduction cell-specific inducible IGF-1 receptor (IGF-1R) knockout (KO) (CSIGF1RKO), insulin receptor (IR) KO (CSIRKO), and IR/IGF-1R double-KO (CSDIRKO) mice and evaluated their phenotypes. Telemetric electrocardiography revealed regular sinus rhythm in CSIGF1RKO mice, indicating that IGF-1R is dispensable for normal pacemaking. In contrast, CSIRKO and CSDIRKO mice exhibited profound sinus bradycardia. CSDIRKO mice showed typical sinus node dysfunction characterized by junctional rhythm and sinus pauses on electrocardiography. Interestingly, the lack of an insulin receptor in the SAN cells of CSIRKO and CSDIRKO mice caused sinus nodal fibrosis. Mechanistically, hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) protein expression significantly decreased in the CSIRKO and CSDIRKO mice relative to the controls. A patch-clamp study of the SAN cells of CSIRKO mice revealed a significant decrease in the funny current, which is responsible for spontaneous diastolic depolarization in the SAN. This result suggested that insulin receptor loss reduces the heart rate via downregulation of the HCN4 channel. Additionally, HCN1 expression was decreased in CSDIRKO mice, explaining their sinus node dysfunction. Our results reveal a previously unrecognized role of insulin/IGF-1 signaling in sinus node structural maintenance and pacemaker function.

Sinus node dysfunction: current understanding and future directions

The sinoatrial node (SAN) is the primary pacemaker of the heart. Normal SAN function is crucial in maintaining proper cardiac rhythm and contraction. Sinus node dysfunction (SND) is due to abnormalities within the SAN, which can affect the heartbeat frequency, regularity, and the propagation of electrical pulses through the cardiac conduction system. As a result, SND often increases the risk of cardiac arrhythmias. SND is most commonly seen as a disease of the elderly given the role of degenerative fibrosis as well as other age-dependent changes in its pathogenesis. Despite the prevalence of SND, current treatment is limited to pacemaker implantation, which is associated with substantial medical costs and complications. Emerging evidence has identified various genetic abnormalities that can cause SND, shedding light on the molecular underpinnings of SND. Identification of these molecular mechanisms and pathways implicated in the pathogenesis of SND is hoped to identify novel therapeutic targets for the development of more effective therapies for this disease. In this review article, we examine the anatomy of the SAN and the pathophysiology and epidemiology of SND. We then discuss in detail the most common genetic mutations correlated with SND and provide our perspectives on future research and therapeutic opportunities in this field.