Immunomodulatory role of non-neuronal cholinergic signaling in myocardial injury

Research progress in myocardial function and diseases related to muscarinic acetylcholine receptor (Review)

Muscarinic acetylcholine (ACh) receptors (also known as M receptors) are widely distributed in all organs and tissues of the body, mainly playing a role in cholinergic nerve conduction. There are five known subtypes of muscarinic ACh receptors, but their pharmacological mechanisms of action on myocardial function have remained to be clearly defined. Functional myocardial diseases and myocardial injuries, such as arrhythmia, myocardial ischemia, myocarditis and myocardial fibrosis, may be affected by muscarinic ACh receptors. This article reviews the research progress of the regulation of myocardial function by muscarinic ACh receptors and related diseases, with the aim of developing better strategies and providing references for further revealing and clarifying the signal transduction and mechanisms of muscarinic ACh receptors in cardiomyocytes, and finding potential myocardial protective drugs that act on muscarinic ACh receptors.

α7nAChR on B cells directs T cell differentiation to prevent viral myocarditis

Patients with viral myocarditis (VMC) exhibit evident autonomic nervous system imbalance, and adverse cardiac remodeling is involved in impaired cholinergic function. The α7 nicotinic acetylcholine receptor (α7nAChR), which is a neurotransmitter receptor, exerts immunoregulatory effects. Recent advances have illuminated the evolution and functions of peripheral and cardiac B cells in heart disease. However, the role of α7nAChR expressed by B cells in the progression of VMC has not been established. We revealed the neuroimmune communication landscape in the heart and found that the phenotypes of cardiac and splenic B cells and their α7nAChR expression changed dynamically during the progression of VMC to dilated cardiomyopathy. α7nAChR on B cells serves as a negative regulator by inhibiting their proinflammatory functions and signaling pathways. B cell-specific α7nAChR deficiency exacerbated myocardial inflammation, fibrosis, and cardiac dysfunction. However, these effects were abrogated in non-B cells from mice with IL-17A knockdown. Enhanced degradation of acetylcholine leads to an imbalance in cholinergic signaling, resulting in impaired neurotransmission. The acetylcholinesterase inhibitor pyridostigmine bromide could improve cardiac remodeling and prevent the progression of VMC to the chronic phase, which was partly dependent on the α7nAChR on B cells. Our findings provide notable insights into cardiac-neural-immune communication during myocardial injury.

Protective role of acetylcholine and the cholinergic system in the injured heart

This review explores the roles of the cholinergic system in the heart, comprising the neuronal and non-neuronal cholinergic systems. Both systems are essential for maintaining cardiac homeostasis by regulating the release of acetylcholine (ACh). A reduction in ACh release is associated with the early onset of cardiovascular diseases (CVDs), and increasing evidence supports the protective roles of ACh against CVD. We address the challenges and limitations of current strategies to elevate ACh levels, including vagus nerve stimulation and pharmacological interventions such as cholinesterase inhibitors. Additionally, we introduce alternative strategies to increase ACh in the heart, such as stem cell therapy, gene therapy, microRNAs, and nanoparticle drug delivery methods. These findings offer new insights into advanced treatments for regenerating the injured human heart.

Mini Review: the non-neuronal cardiac cholinergic system in type-2 diabetes mellitus

Diabetic heart disease remains the leading cause of death in individuals with type-2 diabetes mellitus (T2DM). Both insulin resistance and metabolic derangement, hallmark features of T2DM, develop early and progressively impair cardiovascular function. These factors result in altered cardiac metabolism and energetics, as well as coronary vascular dysfunction, among other consequences. Therefore, gaining a deeper understanding of the mechanisms underlying the pathophysiology of diabetic heart disease is crucial for developing novel therapies for T2DM-associated cardiovascular disease. Cardiomyocytes are equipped with the cholinergic machinery, known as the non-neuronal cardiac cholinergic system (NNCCS), for synthesizing and secreting acetylcholine (ACh) as well as possessing muscarinic ACh receptor for ACh binding and initiating signaling cascade. ACh from cardiomyocytes regulates glucose metabolism and energetics, endothelial function, and among others, in an auto/paracrine manner. Presently, there is only one preclinical animal model - diabetic db/db mice with cardiac-specific overexpression of choline transferase (Chat) gene - to study the effect of activated NNCCS in the diabetic heart. In this mini-review, we discuss the physiological role of NNCCS, the connection between NNCCS activation and cardiovascular function in T2DM and summarize the current knowledge of S-Nitroso-NPivaloyl-D-Penicillamine (SNPiP), a novel inducer of NNCCS, as a potential therapeutic strategy to modulate NNCCS activity for diabetic heart disease.

Recurrent Adrenergic Stress Provokes Persistent Myocarditis in PD-1-Deficient Mice

It is unclear how the immune system initiates effective tissue repair responses without also simultaneously activating adaptive immune responses to self-antigens released by damaged or necrotic cells. We studied the role of repetitive adrenergic mediated stress on cardiac injury wild-type and programmed death-1-deficient (PD-1-/-) mice treated with 3 intraperitoneal low doses of isoproterenol followed by an intraperitoneal injection of high-dose ISO 7 days later (ISOprimed/ISOinjury). Repetitive adrenergic stress in ISOprimed/ISOinjury PD-1-/- mice resulted in a persistent dysregulated myocardial inflammatory response characterized by the expansion of autoreactive effector CD8+ T cells, increased cardiac hypertrophy, mild left ventricular dysfunction, and increased lethality when compared with ISOprimed/ISOinjury wild-type mice.

Recurrent Myocardial Injury Leads to Disease Tolerance in a Murine Model of Stress-Induced Cardiomyopathy

Whereas the innate immune response to an initial episode of cardiac injury has been studied extensively, the response of the immune system to recurrent cardiac tissue injury is not well understood. Specifically, it is not known whether the immune system adapts to the initial episode of cardiac injury and whether any adaptations that occur lead to immune cell hypo-responsiveness or, alternatively, immune cell hyper-responsiveness. Here, we studied the role of adrenergic-mediated stress using a simple model of reversible stress-induced cardiomyopathy, and show that isoproterenol-induced tissue injury and inflammation are sufficient to protect the heart from the myopathic effects of a subsequent exposure to isoproterenol. Remarkably, pharmacological depletion of macrophages partially attenuated the isoproterenol-induced cytoprotective response, suggesting that immune-mediated tissue repair mechanisms confer tolerance to subsequent tissue damage.

Refining the reproducibility of a murine model of stress-induced reversible cardiomyopathy

Despite the many advantages of isoproterenol (Iso)-induced models of cardiomyopathy, the extant literature suggests that the reproducibility of the Iso-induced stress cardiomyopathy phenotype varies considerably depending on the dose of Iso used, the mode of administration of Iso (subcutaneous vs. intraperitoneal), and the species of the animal that is being studied. Recently, we have shown that a single injection of Iso into female C57BL/6J mice provokes transient myocardial injury that is characterized by a brisk release of troponin I within 1 h, as well as a self-limited myocardial inflammatory response that is associated with increased myocardial tissue edema, inferoapical regional left ventricular (LV) wall motion abnormalities, and a transient decrease in global LV function, which were completely recovered by day 7 after the Iso injection (i.e., stress-induced reversible cardiomyopathy). Here we expand upon this initial report in this model by demonstrating important sexually dimorphic differences in the response to Iso-induced tissue injury, the ensuing myocardial inflammatory response, and changes in LV structure and function. We also provide information with respect to enhancing the reproducibility in this model by optimizing animal welfare during the procedure. The acute Iso-induced myocardial injury model provides a low-cost, relatively high-throughput small-animal model that mimics human disease (e.g., Takotsubo cardiomyopathy). Given that the model can be performed in different genetic backgrounds, as well as different experimental conditions, the acute Iso injury model should provide the cardiovascular community with a valuable nonsurgical animal model for understanding the myocardial response to tissue injury.NEW & NOTEWORTHY The present study highlights the importance of sexual dimorphism with respect to isoproterenol injury, as well as the importance of animal handling and welfare to obtain reproducible results from investigator to investigator. Based on serial observations of animal recovery (locomotor activity and grooming behavior), troponin I release, and inflammation, we identified that the method used to restrain the mice for the intraperitoneal injection was the single greatest source of variability in this model.

The Programmed Death-1 Signaling Axis Modulates Inflammation and LV Structure/Function in a Stress-Induced Cardiomyopathy Model

The role of immune checkpoints in the setting of tissue injury remains unknown. Using an experimental model of isoproterenol (ISO)-induced stress cardiomyopathy, we show that ISO-induced myocardial injury provokes tissue-autonomous up-regulation of the programmed death-1 (PD-1):programmed death ligand (PD-L) axis in cardiac resident innate immune cells and T cells. PD-1 signaling was responsible for modulating the acute inflammatory response, as well as normalization of impaired left ventricular structure and function after ISO injection. Necrotic cardiac extracts were sufficient to increase the expression of PD-1 in macrophages and T cells in vitro. Viewed together these studies suggest that the PD-1:PD-L signaling axis regulates immune responses to cardiac tissue injury and is important for restoring myocardial homeostasis.

Muscarinic receptor regulation of chronic pain-induced atrial fibrillation

Atrial fibrillation (AF), one of the most common arrhythmias, is associated with chronic emotional disorder. Chronic pain represents a psychological instability condition related to cardiovascular diseases, but the mechanistic linkage connecting chronic pain to AF occurrence remains unknown. Wild-type C57BL/6J male mice were randomly divided into sham and chronic pain groups. Autonomic nerve remodeling was reflected by the increased atrial parasympathetic tension and muscarinic acetylcholine receptor M2 expression. AF susceptibility was assessed through transesophageal burst stimulation in combination with electrocardiogram recording and investigating AERP in Langendorff perfused hearts. Our results demonstrated the elevated protein expression of muscarinic acetylcholine receptor M2 in the atria of mice subjected to chronic pain stress. Moreover, chronic pain induced the increase of atrial PR interval, and atrial effective refractory periods as compared to the sham group, underlying the enhanced susceptibility of AF. Thus, autonomic cholinergic nerve may mediate mice AF in the setting of chronic pain.

Transcutaneous Vagus Nerve Stimulation Ameliorates the Phenotype of Heart Failure With Preserved Ejection Fraction Through Its Anti-Inflammatory Effects

BACKGROUND

A systemic proinflammatory state plays a central role in the development of heart failure with preserved ejection fraction (HFpEF). Low-level transcutaneous vagus nerve stimulation (LLTS) suppresses inflammation in animals and humans, mediated by an α7nAchR (alpha7 nicotinic acetylcholine receptor)-dependent pathway. We examined the effects of LLTS on cardiac function, inflammation, and fibrosis in the presence of α7nAchR pharmacological blockade in a rat model of HFpEF.

METHODS

Dahl salt-sensitive rats at 7 weeks of age were treated with high-salt diet for 6 weeks to induce HFpEF, followed by 4 weeks of (1) LLTS, (2) LLTS with the α7nAchR blocker methyllycaconitine, (3) sham, and (4) olmesartan. Blood pressure, cardiac function by echocardiography, heart rate variability, and serum cytokines were measured at 13 and 17 weeks of age. Cardiac fibrosis, inflammatory cell infiltration, and gene expression were determined at 17 weeks.

RESULTS

LLTS attenuated the increase in blood pressure; improved cardiac function; decreased inflammatory cytokines, macrophage infiltration, and fibrosis; and improved survival compared with other groups. Methyllycaconitine attenuated these effects, whereas olmesartan did not improve cardiac function or fibrosis despite maintaining similar blood pressure as LLTS. Heart rate variability was similarly improved in the LLTS and LLTS plus methyllycaconitine groups but remained low in the other groups. LLTS reversed the dysregulated inflammatory signaling pathways in HFpEF hearts.

CONCLUSIONS

Neuromodulation with LLTS improved cardiac function in a rat model of HFpEF through its anti-inflammatory and antifibrotic effects. These results provide the basis for further clinical trials in humans.

Brain-heart communication in health and diseases

Tight connections between the brain and heart have attracted a considerable amount of attention. This review focuses on the anatomical (extrinsic cardiac autonomic nervous system and intrinsic cardiac autonomic nervous system) and functional (neuroendocrine-heart axis and neuroimmune-heart axis) connections between the brain and heart, the linkage between central nervous system diseases and cardiovascular diseases, the harm of sympathetic hyperactivity to the heart, and current neuromodulation therapies. Depression is a comorbidity of cardiovascular diseases, and the two are causally related. This review summarizes the mechanisms and treatment of depression and cardiovascular diseases, providing theoretical evidence for basic research and clinical studies to improve treatment options.

Nerve-macrophage interactions in cardiovascular disease

The heart is highly innervated by autonomic neurons, and dynamic autonomic regulation of the heart and blood vessels is essential for animals to carry out the normal activities of life. Cardiovascular diseases, including heart failure and myocardial infarction, are often characterized in part by an imbalance in autonomic nervous system activation, with excess sympathetic and diminished parasympathetic activation. Notably, however, this is often accompanied by chronic inflammation within the cardiovascular tissues, which suggests there are interactions between autonomic dysregulation and inflammation. Recent studies have been unraveling the mechanistic links between autonomic nerves and immune cells within cardiovascular disease. The autonomic nervous system and immune system also act in concert to coordinate the actions of multiple organs that not only maintain homeostasis but also likely play key roles in disease-disease interactions, such as cardiorenal syndrome and multimorbidity. In this review, we summarize the physiological and pathological interactions between autonomic nerves and macrophages in the context of cardiovascular disease.

B cells modulate the expression of MHC-II on cardiac CCR2- macrophages

The uninjured murine heart contains a heterogeneous population of macrophages with disparate ontogenies and functions. These macrophages are often associated with blood vessels and can be subclassified based on the expression of CC chemokine receptor 2 (CCR2) and major histocompatibility complex class II (MHC-II). The biological cues that modulate these macrophage pool subpopulations have not been completely identified. It has been recently shown that a sub-population of circulating naïve B cells adheres to the myocardial microvasculature. We hypothesized that B cells might modulate the phenotype of myocardial macrophages. To test this hypothesis, we analyzed both the relative location of B cells and macrophages in myocardial histological section and the prevalence of myocardial macrophage subsets in hearts from B cell-deficient mice (μMT) and mice depleted of B cells through administration of an anti-CD20 antibody. We found that B cells pause in the microvasculature in proximity of macrophages and modulate the number of myocardial CCR2^-MHC-II^high cells. Through in vitro studies we found that this is likely the result of a paracrine effect of B cells on the expression of MHC-II in CCR2^- cells. These results reveal an unexpected relationship between B cells and resident macrophages and, highlighting a direct intramyocardial effect of circulating B cells, challenge the currently held belief that naïve recirculating B lymphocytes merely shuttle between lymphoid stations.

Increased cholinergic activity under conditions of low estrogen leads to adverse cardiac remodeling

Cholinesterase inhibitors are used in postmenopausal women for the treatment of neurodegenerative diseases. Despite their widespread use in the clinical practice, little is known about the impact of augmented cholinergic signaling on cardiac function under reduced estrogen conditions. To address this gap, we subjected a genetically engineered murine model of systemic vesicular acetylcholine transporter overexpression (Chat-ChR2) to ovariectomy and evaluated cardiac parameters. Left-ventricular function was similar between Chat-ChR2 and wild-type (WT) mice. Following ovariectomy, WT mice showed signs of cardiac hypertrophy. Conversely, ovariectomized (OVX) Chat-ChR2 mice evolved to cardiac dilation and failure. Transcript levels for cardiac stress markers atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) were similarly upregulated in WT/OVX and Chat-ChR2/OVX mice. 17β-Estradiol (E2) treatment normalized cardiac parameters in Chat-ChR2/OVX to the Chat-ChR2/SHAM levels, providing a link between E2 status and the aggravated cardiac response in this model. To investigate the cellular basis underlying the cardiac alterations, ventricular myocytes were isolated and their cellular area and contractility were assessed. Myocytes from WT/OVX mice were wider than WT/SHAM, an indicative of concentric hypertrophy, but their fractional shortening was similar. Conversely, Chat-ChR2/OVX myocytes were elongated and presented contractile dysfunction. E2 treatment again prevented the structural and functional changes in Chat-ChR2/OVX myocytes. We conclude that hypercholinergic mice under reduced estrogen conditions do not develop concentric hypertrophy, a critical compensatory adaptation, evolving toward cardiac dilation and failure. This study emphasizes the importance of understanding the consequences of cholinesterase inhibition, used clinically to treat dementia, for cardiac function in postmenopausal women.

Cholinergic modulation of the immune system - A novel therapeutic target for myocardial inflammation

The immune system and the nervous system depend on each other for their fine tuning and working, thus cooperating to maintain physiological homeostasis and prevent infections. The cholinergic system regulates the mobilization, differentiation, secretion, and antigen presentation of adaptive and innate immune cells mainly through α7 nicotinic acetylcholine receptors (α7nAChRs). The neuro-immune interactions are established and maintained by the following mechanisms: colocalization of immune and neuronal cells at defined anatomical sites, expression of the non-neuronal cholinergic system by immune cells, and the acetylcholine receptor-mediated activation of intracellular signaling pathways. Based on these immunological mechanisms, the protective effects of cholinergic system in animal models of diseases were summarized in this paper, such as myocardial infarction/ischemia-reperfusion, viral myocarditis, and endotoxin-induced myocardial damage. In addition to maintaining hemodynamic stability and improving the energy metabolism of the heart, both non-neuronal acetylcholine and neuronal acetylcholine in the heart can alleviate myocardial inflammation and remodeling to exert a significant cardioprotective effect. The new findings on the role of cholinergic agonists and vagus nerve stimulation in immune regulation are updated, so as to develop improved approaches to treat inflammatory heart disease.

Characteristic Effects of the Cardiac Non-Neuronal Acetylcholine System Augmentation on Brain Functions

Since the discovery of non-neuronal acetylcholine in the heart, this specific system has drawn scientific interest from many research fields, including cardiology, immunology, and pharmacology. This system, acquired by cardiomyocytes independent of the parasympathetic nervous system of the autonomic nervous system, helps us to understand unsolved issues in cardiac physiology and to realize that the system may be more pivotal for cardiac homeostasis than expected. However, it has been shown that the effects of this system may not be restricted to the heart, but rather extended to cover extra-cardiac organs. To this end, this system intriguingly influences brain function, specifically potentiating blood brain barrier function. Although the results reported appear to be unusual, this novel characteristic can provide us with another research interest and therapeutic application mode for central nervous system diseases. In this review, we discuss our recent studies and raise the possibility of application of this system as an adjunctive therapeutic modality.

Protective and anti-inflammatory effects of acetylcholine in the heart

The innate and adaptive immune systems play an important role in the development of cardiac diseases. Therefore, it has become critical to identify molecules that can modulate inflammation in the injured heart. In this regard, activation of the cholinergic system in animal models of heart disease has been shown to exert protective actions that include immunomodulation of cardiac inflammation. In this mini-review, we briefly present our current understanding on the cardiac cellular sources of acetylcholine (ACh) (neuronal vs. nonneuronal), followed by a discussion on its contribution to the regulation of inflammatory cells. Although the mechanism behind ACh-mediated protection still remains to be fully elucidated, the beneficial immunomodulatory role of the cholinergic signaling emerges as a potential key regulator of cardiac inflammation.

Ischemia reperfusion injury provokes adverse left ventricular remodeling in dysferlin-deficient hearts through a pathway that involves TIRAP dependent signaling

Cardiac myocytes have multiple cell autonomous mechanisms that facilitate stabilization and repair of damaged sarcolemmal membranes following myocardial injury. Dysferlin is a protein which facilitates membrane repair by promoting membrane resealing. Although prior studies have shown that dysferlin-deficient (Dysf-/-) mouse hearts have an impaired recovery from acute ischemia/reperfusion (I/R) injury ex vivo, the role of dysferlin in mediating the recovery from myocardial injury in vivo is unknown. Here we show that Dysf-/- mice develop adverse LV remodeling following I/R injury secondary to the collateral damage from sustained myocardial inflammation within the infarct zone. Backcrossing Dysf-/- mice with mice lacking signaling through the Toll-Interleukin 1 Receptor Domain-Containing Adaptor Protein (Tirap-/-), attenuated inflammation and abrogated adverse LV remodeling following I/R injury. Subsequent studies using Poloxamer 188 (P188), a membrane resealing reagent, demonstrated that P188 did not attenuate inflammation nor prevent adverse LV remodeling in Dysf-/- mice following I/R injury. Viewed together these studies reveal a previously unappreciated role for the importance of membrane sealing and the resolution of inflammation following myocardial injury.

Obesity, Hypertension, and Cardiac Dysfunction: Novel Roles of Immunometabolism in Macrophage Activation and Inflammation

Obesity and hypertension, which often coexist, are major risk factors for heart failure and are characterized by chronic, low-grade inflammation, which promotes adverse cardiac remodeling. While macrophages play a key role in cardiac remodeling, dysregulation of macrophage polarization between the proinflammatory M1 and anti-inflammatory M2 phenotypes promotes excessive inflammation and cardiac injury. Metabolic shifting between glycolysis and mitochondrial oxidative phosphorylation has been implicated in macrophage polarization. M1 macrophages primarily rely on glycolysis, whereas M2 macrophages rely on the tricarboxylic acid cycle and oxidative phosphorylation; thus, factors that affect macrophage metabolism may disrupt M1/M2 homeostasis and exacerbate inflammation. The mechanisms by which obesity and hypertension may synergistically induce macrophage metabolic dysfunction, particularly during cardiac remodeling, are not fully understood. We propose that obesity and hypertension induce M1 macrophage polarization via mechanisms that directly target macrophage metabolism, including changes in circulating glucose and fatty acid substrates, lipotoxicity, and tissue hypoxia. We discuss canonical and novel proinflammatory roles of macrophages during obesity-hypertension-induced cardiac injury, including diastolic dysfunction and impaired calcium handling. Finally, we discuss the current status of potential therapies to target macrophage metabolism during heart failure, including antidiabetic therapies, anti-inflammatory therapies, and novel immunometabolic agents.

Myocardial B cells are a subset of circulating lymphocytes with delayed transit through the heart

Current models of B lymphocyte biology posit that B cells continuously recirculate between lymphoid organs, without accumulating in peripheral healthy tissues. Nevertheless, B lymphocytes are one of the most prevalent leukocyte populations in the naive murine heart. To investigate this apparent inconsistency in the literature, we conducted a systematic analysis of myocardial B cell ontogeny, trafficking dynamics, histology, and gene expression patterns. We found that myocardial B cells represent a subpopulation of circulating B cells that make close contact with the microvascular endothelium of the heart and arrest their transit as they pass through the heart. The vast majority (>95%) of myocardial B cells remain intravascular, whereas few (<5%) myocardial B cells cross the endothelium into myocardial tissue. Analyses of mice with B cell deficiency or depletion indicated that B cells modulate the myocardial leukocyte pool composition. Analysis of B cell-deficient animals suggested that B cells modulate myocardial growth and contractility. These results transform our current understanding of B cell recirculation in the naive state and reveal a previously unknown relationship between B cells and myocardial physiology. Further work will be needed to assess the relevance of these findings to other organs.