Heterogeneous ventricular sympathetic innervation, altered beta-adrenergic receptor expression, and rhythm instability in mice lacking the p75 neurotrophin receptor

Molecular and cellular neurocardiology in heart disease

This paper updates and builds on a previous White Paper in this journal that some of us contributed to concerning the molecular and cellular basis of cardiac neurobiology of heart disease. Here we focus on recent findings that underpin cardiac autonomic development, novel intracellular pathways and neuroplasticity. Throughout we highlight unanswered questions and areas of controversy. Whilst some neurochemical pathways are already demonstrating prognostic viability in patients with heart failure, we also discuss the opportunity to better understand sympathetic impairment by using patient specific stem cells that provides pathophysiological contextualization to study 'disease in a dish'. Novel imaging techniques and spatial transcriptomics are also facilitating a road map for target discovery of molecular pathways that may form a therapeutic opportunity to treat cardiac dysautonomia.

Semaphorins and the bone marrow microenvironment: New candidates that influence the hematopoietic system

The bone marrow is a haven for hematopoietic and non-hematopoietic cells, creating complex micro-anatomical regions called niches. These distinct niches all participate in an intricate orchestra of cellular interactions that regulates the hematopoietic stem cell and its progenies. In this review, we provide a detailed description of the three most well-known bone marrow niches and their participation in hematopoiesis. We use pre-clinical data, including different in vitro and in vivo studies to discuss how a group of proteins called Semaphorins could potentially modulate both hematopoietic and non-hematopoietic cells, establishing links between the niches, semaphorins, and hematopoietic regulation. Thus, here we provide a deep dive into the inner functioning of the bone marrow and discuss the overarching implications that semaphorins might have on blood formation.

Heterogeneous cardiac sympathetic innervation gradients promote arrhythmogenesis in murine dilated cardiomyopathy

Ventricular arrhythmias (VAs) in heart failure are enhanced by sympathoexcitation. However, radiotracer studies of catecholamine uptake in failing human hearts demonstrate a proclivity for VAs in patients with reduced cardiac sympathetic innervation. We hypothesized that this counterintuitive finding is explained by heterogeneous loss of sympathetic nerves in the failing heart. In a murine model of dilated cardiomyopathy (DCM), delayed PET imaging of sympathetic nerve density using the catecholamine analog [11C]meta-Hydroxyephedrine demonstrated global hypoinnervation in ventricular myocardium. Although reduced, sympathetic innervation in 2 distinct DCM models invariably exhibited transmural (epicardial to endocardial) gradients, with the endocardium being devoid of sympathetic nerve fibers versus controls. Further, the severity of transmural innervation gradients was correlated with VAs. Transmural innervation gradients were also identified in human left ventricular free wall samples from DCM versus controls. We investigated mechanisms underlying this relationship by in silico studies in 1D, 2D, and 3D models of failing and normal human hearts, finding that arrhythmogenesis increased as heterogeneity in sympathetic innervation worsened. Specifically, both DCM-induced myocyte electrical remodeling and spatially inhomogeneous innervation gradients synergistically worsened arrhythmogenesis. Thus, heterogeneous innervation gradients in DCM promoted arrhythmogenesis. Restoration of homogeneous sympathetic innervation in the failing heart may reduce VAs.

From the sideline: Tissue-specific nucleoporin function in health and disease, an update

The subcellular compartmentalisation of eukaryotic cells requires selective exchange between the cytoplasm and the nucleus. Intact nucleocytoplasmic transport is vital for normal cell function and mutations in the executing machinery have been causally linked to human disease. Central players in nucleocytoplasmic exchange are nuclear pore complexes (NPCs), which are built from ~30 distinct proteins collectively termed nucleoporins. Aberrant nucleoporin expression was detected in human cancers and autoimmune diseases since quite some time, while it was through the increasing use of next generation sequencing that mutations in nucleoporin genes associated with mainly rare hereditary diseases were revealed. The number of newly identified mutations is steadily increasing, as is the number of diseases. Mutational hotspots have emerged: mutations in the scaffold nucleoporins seemingly affect primarily inner organs, such as heart, kidney, and ovaries, whereas genetic alterations in peripheral, cytoplasmic nucleoporins affect primarily the central nervous system and development. In this review, we summarise latest insights on altered nucleoporin function in the context of human hereditary disorders, with a focus on those where mechanistic insights are beginning to emerge.

A review of the historical use of sex as a biological variable in the American Journal of Physiology-Heart and Circulatory Physiology

Despite National Institute of Health (NIH) mandates requiring sex as a biological variable (SABV), female underrepresentation persists in research, driving the American Journal of Physiology-Heart and Circulatory Physiology (Am J Physiol-Heart Circ) to publish SABV expectations in 2021. To determine progress within the Am J Physiol-Heart Circ, this mini-review evaluated SABV during the first 6 mo of each decade from 1980 to 2020, and 2019, to mitigate pandemic influence. Of the 1,205 articles published, 1,087 articles were included in this review (articles without original research subjects were excluded), of which 72.9% identified subjects. There were consistently fewer female human participants than males, except within 2019 (1980: females n = 3, males n = 5; 1990: females n = 70, males n = 199; 2000: females n = 305, males n = 355; 2010: females n = 186, males n = 472; 2019: females n = 1,695, males n = 1,550; 2020: females n = 1,157, males n = 1,222) and fewer female animals than males (1980: females n = 58, males n = 1,291; 1990: females n = 447, males n = 2,628; 2000: females n = 590, males n = 3,083; 2010: females n = 663, males n = 4,517; 2019: females n = 338, males n = 1,340; 2020: females n = 1,372, males n = 1,973). Only 16 (12.3%) articles including humans discussed SABV from 1980 to 2020. There are persistent SABV disparities within Am J Physiol-Heart Circ with some improvements in recent years. It is imperative that organizations such as the American Physiological Society and NIH foster an expectation of SABV as the norm, not the exception.

Pathophysiological functions of semaphorins in the sympathetic nervous system

Upon exposure to external stressors, the body senses them and activates the sympathetic nervous system (SNS) to maintain the homeostasis, which is known as the "fight-or-flight" response. Recent studies have revealed that the SNS also plays pivotal roles in regulating immune responses, such as hematopoiesis, leukocyte mobilization, and inflammation. Indeed, overactivation of the SNS causes many inflammatory diseases, including cardiovascular diseases, metabolic disorders, and autoimmune diseases. However, the molecular basis essential for SNS-mediated immune regulation is not completely understood. In this review, we focus on axon guidance cues, semaphorins, which play multifaceted roles in neural and immune systems. We summarize the functions of semaphorins in the crosstalk between the SNS and the immune system, exploring its pathophysiological roles.

Nerve Growth Factor transfer from cardiomyocytes to innervating sympathetic neurons activates TrkA receptors at the neuro-cardiac junction

The integration of ex vivo and in vitro data, described in this manuscript, together with our previous demonstration that sympathetic neurons (SNs) contact target cardiomyocytes (CMs) at the neuro-cardiac junction (NCJ), which underlies intercellular synaptic communication (Prando et al., 47), demonstrate that: CMs are the cell source of Nerve Growth Factor (NGF), required to sustain innervating cardiac SNs; NCJ is the place of the intimate liaison, between SNs and CMs, allowing on the one hand neurons to peremptorily control CM activity, and on the other, CMs to adequately sustain the contacting, everchanging, neuronal actuators; alterations in NCJ integrity may compromise the efficiency of 'CM-to-SN' signaling, thus representing a potentially novel mechanism of sympathetic denervation in cardiac diseases. ABSTRACT: Background Sympathetic neurons densely innervate the myocardium with non-random topology and establish structured contacts (i.e. neuro-cardiac junctions, NCJ) with cardiomyocytes, allowing synaptic intercellular communication. Establishment of heart innervation is regulated by molecular mediators released by myocardial cells. The mechanisms underlying maintenance of cardiac innervation in the fully developed heart, are, however, less clear. Notably, several cardiac diseases, primarily affecting cardiomyocytes, are associated to sympathetic denervation, supporting that retrograde 'cardiomyocyte-to-sympathetic neuron' communication is essential for heart cellular homeostasis. Objective We aimed to determine whether cardiomyocytes provide Nerve Growth Factor (NGF) to sympathetic neurons, and the role of the NCJ in supporting such retrograde neurotrophic signaling. Methods and Results Immunofluorescence on murine and human heart slices shows that NGF and its receptor, Tropomyosin-receptor-kinase-A, accumulate respectively in the pre- and post-junctional sides of the NCJ. Confocal immunofluorescence, scanning ion conductance microscopy and molecular analyses, in co-cultures, demonstrate that cardiomyocytes feed NGF to sympathetic neurons, and that such mechanism requires a stable intercellular contact at the NCJ. Consistently, cardiac fibroblasts, devoid of NCJ, are unable to sustain SN viability. ELISA assay and competition binding experiments suggest that this depends on the NCJ being an insulated microenvironment, characterized by high [NGF]. In further support, real-time imaging of Tropomyosin-receptor-kinase-A-vesicle movements demonstrate that efficiency of neurotrophic signaling parallels the maturation of such structured intercellular contacts. Conclusions Altogether, our results demonstrate the mechanisms which link sympathetic neuron survival to neurotrophin release by directly innervated cardiomyocytes, conceptualizing sympathetic neurons as cardiomyocyte-driven heart drivers. Abstract figure legend Sympathetic neuron (SN, green) varicosities establish synaptic contacts with target cardiomyocytes (CMs, pink), which we previously called Neuro-Cardiac Junction (NCJ, Prando et al. J Physiol 47). At NCJs, CMs release selectively NGF, which by activating TrkA signaling, is key to sustain neuronal survival. This article is protected by copyright. All rights reserved.

A Primer on Incorporating Sex as a Biological Variable into the Conduct and Reporting of Basic and Clinical Research Studies

The recent move to require sex as a biological variable (SABV), which includes gender, into the reporting of research published by the American Journal of Physiology - Heart and Circulatory Physiology follows a growing, and much-needed, trend by journals. Understandably, there is concern over how to do this without adding considerable work, especially if one's primary research focus is not on elucidating sex/gender differences. The purpose of this article is to provide additional guidance and examples on how to incorporate SABV into the conduct and reporting of basic and clinical research. Using examples from our research, which includes both studies focused and not focused on sex/gender differences, we offer suggestions for how to incorporate SABV into basic and clinical research studies.

Xiao-Qing-Long-Tang Maintains Cardiac Function during Heart Failure with Reduced Ejection Fraction in Salt-Sensitive Rats by Regulating the Imbalance of Cardiac Sympathetic Innervation

Objective

The anatomical and functional imbalances of sympathetic nerves are associated with cardiovascular disease progression. Xiao-Qing-Long-Tang (XQLT), an ancient Chinese herbal formula, has been used to treat cardiovascular diseases in eastern Asia for thousands of years. We determined the effect of XQLT in maintaining cardiac function during heart failure with reduced ejection fraction (HFrEF) with respect to its neurobiological effects in salt-sensitive rats.

Methods

Dahl salt-sensitive (DS) rats were fed a high-salt diet to establish an HFrEF model and were divided into model (DS, administered normal saline) and XQL groups (administrated XQLT) randomly, with SS-13BN rats being used as the control. The bodyweight and blood pressure of rats were observed regularly. Electrocardiogram, echocardiography, and plasma N-terminal pro-B-type natriuretic peptide (NT-proBNP) were determined to assess cardiac function. The sympathetic tune and myocardial morphological changes were evaluated. Western blot and qRT-PCR were used to assay the expression of the nerve growth factor (NGF) and leukemia inhibitory factor (LIF). Tyrosine hydroxylase (TH), choline acetyltransferase (CHAT), and growth-associated protein 43 (GAP43) were assayed to confirm sympathetic remodeling. The micromorphological changes in cardiac sympathetic nerve endings were observed by transmission electron microscopy.

Results

Four weeks after XQLT treatment, cardiac function and bodyweight were higher and blood pressure was lower than that of the DS group. Myocardial noradrenaline (NA) increased, while the plasma NA level decreased significantly. The morphology demonstrated that XQLT significantly alleviated myocardial damage. XQLT decreased the expression of LIF, increased the expression of NGF, enhanced the TH+/GAP43+ and TH+/CHAT + positive nerve fiber density, and improved the TH and GAP43 protein expression, but had no effect on CHAT. Moreover, XQLT improved the micromorphology of sympathetic nerve endings in the myocardium.

Conclusion

XQLT maintains cardiac function during HFrEF in salt-sensitive rats, in part, by regulating the imbalance of cardiac sympathetic innervation.

Adrenergic supersensitivity and impaired neural control of cardiac electrophysiology following regional cardiac sympathetic nerve loss

Myocardial infarction (MI) can result in sympathetic nerve loss in the infarct region. However, the contribution of hypo-innervation to electrophysiological remodeling, independent from MI-induced ischemia and fibrosis, has not been comprehensively investigated. We present a novel mouse model of regional cardiac sympathetic hypo-innervation utilizing a targeted-toxin (dopamine beta-hydroxylase antibody conjugated to saporin, DBH-Sap), and measure resulting electrophysiological and Ca²⁺ handling dynamics. Five days post-surgery, sympathetic nerve density was reduced in the anterior left ventricular epicardium of DBH-Sap hearts compared to control. In Langendorff-perfused hearts, there were no differences in mean action potential duration (APD₈₀) between groups; however, isoproterenol (ISO) significantly shortened APD₈₀ in DBH-Sap but not control hearts, resulting in a significant increase in APD₈₀ dispersion in the DBH-Sap group. ISO also produced spontaneous diastolic Ca²⁺ elevation in DBH-Sap but not control hearts. In innervated hearts, sympathetic nerve stimulation (SNS) increased heart rate to a lesser degree in DBH-Sap hearts compared to control. Additionally, SNS produced APD₈₀ prolongation in the apex of control but not DBH-Sap hearts. These results suggest that hypo-innervated hearts have regional super-sensitivity to circulating adrenergic stimulation (ISO), while having blunted responses to SNS, providing important insight into the mechanisms of arrhythmogenesis following sympathetic nerve loss.

Nardilysin controls cardiac sympathetic innervation patterning through regulation of p75 neurotrophin receptor

Cardiac sympathetic innervation is critically involved in the regulation of circulatory dynamics. However, the molecular mechanism for the innervation patterning has remained elusive. Here, we demonstrate that nardilysin (NRDC, Nrdc), an enhancer of ectodomain shedding, regulates cardiac sympathetic innervation. Nardilysin-deficient (Nrdc^-/- ) mice show hypoplastic hearts, hypotension, bradycardia, and abnormal sympathetic innervation patterning. While the innervation of left ventricle (LV) of wild-type mice is denser in the subepicardium than in the subendocardium, Nrdc^-/- LV lacks such a polarity and is uniformly and more abundantly innervated. At the molecular level, the full-length form of p75 neurotrophin receptor (p75^NTR , Ngfr) is increased in Nrdc^-/- LV due to the reduced ectodomain shedding of p75^NTR . Importantly, the reduction of p75^NTR rescued the abnormal innervation phenotype of Nrdc^-/- mice. Moreover, sympathetic neuron-specific, but not cardiomyocyte-specific deletion of Nrdc recapitulated the abnormal innervation patterning of Nrdc^-/- mice. In conclusion, neuronal nardilysin critically regulates cardiac sympathetic innervation and circulatory dynamics via modulation of p75^NTR .

Targeting the β-adrenergic receptor in the clinical management of congenital long QT syndrome

The long QT syndrome (LQTS) is largely treated pharmacologically with β-blockers, despite the role of sympathetic activity in LQTS being poorly understood. Using the trigger-substrate model of cardiac arrhythmias in this review, we amalgamate current experimental and clinical data from both animal and human studies to explain the mechanism of adrenergic stimulation and blockade on LQT arrhythmic risk and hence assess the efficacy of β-adrenoceptor blockade in the management of LQTS. In LQTS1 and LQTS2, sympathetic stimulation increases arrhythmic risk by enhancing early afterdepolarizations and transmural dispersion of repolarization. β-Blockers successfully reduce cardiac events by reducing these triggers and substrates; however, these effects are less marked in LQTS2 compared with LQTS1. In LQTS3, clinical and experimental investigations of the effects of sympathetic stimulation and β-blocker use have produced contradictory findings, resulting in significant clinical uncertainty. We offer explanations for these contradicting results relating to study sample size, the dose of the β-blocker administered associated with its off-target Na⁺ channel effects, as well as the type of β-blocker used. We conclude that the antiarrhythmic efficacy of β-blockers is a genotype-specific phenomenon, and hence the use of β-blockers in clinical practice should be genotype dependent.

Low-Frequency Oscillations in Cardiac Sympathetic Neuronal Activity

Sudden cardiac death caused by ventricular arrhythmias is among the leading causes of mortality, with approximately half of all deaths attributed to heart disease worldwide. Periodic repolarization dynamics (PRD) is a novel marker of repolarization instability and strong predictor of death in patients post-myocardial infarction that is believed to occur in association with low-frequency oscillations in sympathetic nerve activity. However, this hypothesis is based on associations of PRD with indices of sympathetic activity that are not directly linked to cardiac function, such as muscle vasoconstrictor activity and the variability of cardiovascular autospectra. In this review article, we critically evaluate existing scientific evidence obtained primarily in experimental animal models, with the aim of identifying the neuronal networks responsible for the generation of low-frequency sympathetic rhythms along the neurocardiac axis. We discuss the functional significance of rhythmic sympathetic activity on neurotransmission efficacy and explore its role in the pathogenesis of ventricular repolarization instability. Most importantly, we discuss important gaps in our knowledge that require further investigation in order to confirm the hypothesis that low frequency cardiac sympathetic oscillations play a causative role in the generation of PRD.

Sex differences in sympathetic gene expression and cardiac neurochemistry in Wistar Kyoto rats

The stellate ganglia are the predominant source of sympathetic innervation to the heart. Remodeling of sympathetic nerves projecting to the heart has been observed in several cardiovascular diseases, and sympathetic dysfunction contributes to cardiac pathology. Wistar Kyoto rats are a common model for the study of cardiovascular diseases, but we lack a profile of the baseline transcriptomic and neurochemical characteristics of their cardiac sympathetic neurons. Most studies of cardiovascular disease have used male animals only, but in the future both male and female animals will be used for these types of studies; therefore, we sought to characterize the transcriptome of male and female stellate ganglia and to correlate that with catecholamine and acetylcholine content in the heart. We have generated a dataset of baseline RNA expression in male and female Wistar Kyoto rat stellate ganglia using RNA-seq, and have measured neurotransmitter levels in heart and stellate ganglia using HPLC and mass spectrometry. We identified numerous gene expression differences between male and female stellates, including genes encoding important developmental factors, receptors and neuropeptides. Female hearts had significantly higher neurotransmitter content than male hearts; however, no significant differences were detected in expression of the genes encoding neurotransmitter synthetic enzymes. Similarly, no statistically significant differences were identified between the sexes in cardiac tyrosine hydroxylase levels.

Mechanisms of organophosphorus pesticide toxicity in the context of airway hyperreactivity and asthma

Numerous epidemiologic studies have identified an association between occupational exposures to organophosphorus pesticides (OPs) and asthma or asthmatic symptoms in adults. Emerging epidemiologic data suggest that environmentally relevant levels of OPs may also be linked to respiratory dysfunction in the general population and that in utero and/or early life exposures to environmental OPs may increase risk for childhood asthma. In support of a causal link between OPs and asthma, experimental evidence demonstrates that occupationally and environmentally relevant OP exposures induce bronchospasm and airway hyperreactivity in preclinical models. Mechanistic studies have identified blockade of autoinhibitory M2 muscarinic receptors on parasympathetic nerves that innervate airway smooth muscle as one mechanism by which OPs induce airway hyperreactivity, but significant questions remain regarding the mechanism(s) by which OPs cause neuronal M2 receptor dysfunction and, more generally, how OPs cause persistent asthma, especially after developmental exposures. The goals of this review are to 1) summarize current understanding of OPs in asthma; 2) discuss mechanisms of OP neurotoxicity and immunotoxicity that warrant consideration in the context of OP-induced airway hyperreactivity and asthma, specifically, inflammatory responses, oxidative stress, neural plasticity, and neurogenic inflammation; and 3) identify critical data gaps that need to be addressed in order to better protect adults and children against the harmful respiratory effects of low-level OP exposures.

NUP155 insufficiency recalibrates a pluripotent transcriptome with network remodeling of a cardiogenic signaling module

BACKGROUND

Atrial fibrillation is a cardiac disease driven by numerous idiopathic etiologies. NUP155 is a nuclear pore complex protein that has been identified as a clinical driver of atrial fibrillation, yet the precise mechanism is unknown. The present study employs a systems biology algorithm to identify effects of NUP155 disruption on cardiogenicity in a model of stem cell-derived differentiation.

METHODS

Embryonic stem (ES) cell lines (n = 5) with truncated NUP155 were cultured in parallel with wild type (WT) ES cells (n = 5), and then harvested for RNAseq. Samples were run on an Illumina HiSeq 2000. Reads were analyzed using Strand NGS, Cytoscape, DAVID and Ingenuity Pathways Analysis to deconvolute the NUP155-disrupted transcriptome. Network topological analysis identified key features that controlled framework architecture and functional enrichment.

RESULTS

In NUP155 truncated ES cells, significant expression changes were detected in 326 genes compared to WT. These genes segregated into clusters that enriched for specific gene ontologies. Deconvolution of the collective framework into discrete sub-networks identified a module with the highest score that enriched for Cardiovascular System Development, and revealed NTRK1/TRKA and SRSF2/SC35 as critical hubs within this cardiogenic module.

CONCLUSIONS

The strategy of pluripotent transcriptome deconvolution used in the current study identified a novel association of NUP155 with potential drivers of arrhythmogenic AF. Here, NUP155 regulates cardioplasticity of a sub-network embedded within a larger framework of genome integrity, and exemplifies how transcriptome cardiogenicity in an embryonic stem cell genome is recalibrated by nucleoporin dysfunction.

Transient denervation of viable myocardium after myocardial infarction does not alter arrhythmia susceptibility

Cardiac sympathetic nerves stimulate heart rate and force of contraction. Myocardial infarction (MI) leads to the loss of sympathetic nerves within the heart, and clinical studies have indicated that sympathetic denervation is a risk factor for arrhythmias and cardiac arrest. Two distinct types of denervation have been identified in the mouse heart after MI caused by ischemia-reperfusion: transient denervation of peri-infarct myocardium and sustained denervation of the infarct. Sustained denervation is linked to increased arrhythmia risk, but it is not known whether acute nerve loss in peri-infarct myocardium also contributes to arrhythmia risk. Peri-infarct sympathetic denervation requires the p75 neurotrophin receptor (p75NTR), but removal of p75NTR alters the pattern of sympathetic innervation in the heart and increases spontaneous arrhythmias. Therefore, we targeted the p75NTR coreceptor sortilin and the p75NTR-induced protease tumor necrosis factor-α-converting enzyme/A disintegrin and metalloproteinase domain 17 (TACE/ADAM17) to selectively block peri-infarct denervation. Sympathetic nerve density was quantified using immunohistochemistry for tyrosine hydroxylase. Genetic deletion of sortilin had no effect on the timing or extent of axon degeneration, but inhibition of TACE/ADAM17 with the protease inhibitor marimastat prevented the loss of axons from viable myocardium. We then asked whether retention of nerves in peri-infarct myocardium had an impact on cardiac electrophysiology 3 days after MI using ex vivo optical mapping of transmembrane potential and intracellular Ca2+. Preventing acute denervation of viable myocardium after MI did not significantly alter cardiac electrophysiology or Ca2+ handling, suggesting that transient denervation at this early time point has minimal impact on arrhythmia risk. NEW & NOTEWORTHY Sympathetic denervation after myocardial infarction is a risk factor for arrhythmias. We asked whether transient loss of nerves in viable myocardium contributed to arrhythmia risk. We found that targeting protease activity could prevent acute peri-infarct denervation but that it did not significantly alter cardiac electrophysiology or Ca2+ handling 3 days after myocardial infarction.

Renal denervation in male rats with heart failure improves ventricular sympathetic nerve innervation and function

Heart failure is characterized by the loss of sympathetic innervation to the ventricles, contributing to impaired cardiac function and arrhythmogenesis. We hypothesized that renal denervation (RDx) would reverse this loss. Male Wistar rats underwent myocardial infarction (MI) or sham surgery and progressed into heart failure for 4 wk before receiving bilateral RDx or sham RDx. After additional 3 wk, left ventricular (LV) function was assessed, and ventricular sympathetic nerve fiber density was determined via histology. Post-MI heart failure rats displayed significant reductions in ventricular sympathetic innervation and tissue norepinephrine content (nerve fiber density in the LV of MI+sham RDx hearts was 0.31 ± 0.05% vs. 1.00 ± 0.10% in sham MI+sham RDx group, P < 0.05), and RDx significantly increased ventricular sympathetic innervation (0.76 ± 0.14%, P < 0.05) and tissue norepinephrine content. MI was associated with an increase in fibrosis of the noninfarcted ventricular myocardium, which was attenuated by RDx. RDx improved LV ejection fraction and end-systolic and -diastolic areas when compared with pre-RDx levels. This is the first study to show an interaction between renal nerve activity and cardiac sympathetic nerve innervation in heart failure. Our findings show denervating the renal nerves improves cardiac sympathetic innervation and function in the post-MI failing heart.

Differential roles for cortical versus sub-cortical noradrenaline and modulation of impulsivity in the rat

RATIONALE

Atomoxetine is a noradrenaline re-uptake inhibitor licensed for the treatment of adult and childhood attention deficit hyperactivity disorder. Although atomoxetine has established efficacy, the mechanisms which mediate its effects are not well understood.

OBJECTIVES

In this study, we investigated the role of cortical versus sub-cortical noradrenaline by using focal dopamine beta hydroxylase-saporin-induced lesions, to the prefrontal cortex (n = 16) or nucleus accumbens shell (n = 18).

METHODS

Healthy animals were tested by using the forced-choice serial reaction time task to assess the impact of the lesion on baseline performance and the response to atomoxetine and the psychostimulant amphetamine.

RESULTS

We observed attenuation in the efficacy of atomoxetine in animals with lesions to the nucleus accumbens shell, but not the prefrontal cortex. Amphetamine-induced increases in premature responses were potentiated in animals with lesions to the prefrontal cortex, but not the nucleus accumbens shell.

CONCLUSIONS

These data suggest that noradrenaline in the nucleus accumbens shell plays an important role in the effects of atomoxetine. Under these conditions, prefrontal cortex noradrenaline did not appear to contribute to atomoxetine's effects suggesting a lack of cortical-mediated "top-down" modulation. Noradrenaline in the prefrontal cortex appears to contribute to the modulation of impulsive responding in amphetamine-treated animals, with a loss of noradrenaline associated with potentiation of its effects. These data demonstrate a potential dissociation between cortical and sub-cortical noradrenergic mechanisms and impulse control in terms of the actions of atomoxetine and amphetamine.

Molecular and cellular neurocardiology: development, and cellular and molecular adaptations to heart disease

The nervous system and cardiovascular system develop in concert and are functionally interconnected in both health and disease. This white paper focuses on the cellular and molecular mechanisms that underlie neural-cardiac interactions during development, during normal physiological function in the mature system, and during pathological remodelling in cardiovascular disease. The content on each subject was contributed by experts, and we hope that this will provide a useful resource for newcomers to neurocardiology as well as aficionados.

Myocardial Infarction Causes Transient Cholinergic Transdifferentiation of Cardiac Sympathetic Nerves via gp130

Sympathetic and parasympathetic control of the heart is a classic example of norepinephrine (NE) and acetylcholine (ACh) triggering opposing actions. Sympathetic NE increases heart rate and contractility through activation of β receptors, whereas parasympathetic ACh slows the heart through muscarinic receptors. Sympathetic neurons can undergo a developmental transition from production of NE to ACh and we provide evidence that mouse cardiac sympathetic nerves transiently produce ACh after myocardial infarction (MI). ACh levels increased in viable heart tissue 10-14 d after MI, returning to control levels at 21 d, whereas NE levels were stable. At the same time, the genes required for ACh synthesis increased in stellate ganglia, which contain most of the sympathetic neurons projecting to the heart. Immunohistochemistry 14 d after MI revealed choline acetyltransferase (ChAT) in stellate sympathetic neurons and vesicular ACh transporter immunoreactivity in tyrosine hydroxylase-positive cardiac sympathetic fibers. Finally, selective deletion of the ChAT gene from adult sympathetic neurons prevented the infarction-induced increase in cardiac ACh. Deletion of the gp130 cytokine receptor from sympathetic neurons prevented the induction of cholinergic genes after MI, suggesting that inflammatory cytokines induce the transient acquisition of a cholinergic phenotype in cardiac sympathetic neurons. Ex vivo experiments examining the effect of NE and ACh on rabbit cardiac action potential duration revealed that ACh blunted both the NE-stimulated decrease in cardiac action potential duration and increase in myocyte calcium transients. This raises the possibility that sympathetic co-release of ACh and NE may impair adaptation to high heart rates and increase arrhythmia susceptibility.

SIGNIFICANCE STATEMENT

Sympathetic neurons normally make norepinephrine (NE), which increases heart rate and the contractility of cardiac myocytes. We found that, after myocardial infarction, the sympathetic neurons innervating the heart begin to make acetylcholine (ACh), which slows heart rate and decreases contractility. Several lines of evidence confirmed that the source of ACh was sympathetic nerves rather than parasympathetic nerves that are the normal source of ACh in the heart. Global application of NE with or without ACh to ex vivo hearts showed that ACh partially reversed the NE-stimulated decrease in cardiac action potential duration and increase in myocyte calcium transients. That suggests that sympathetic co-release of ACh and NE may impair adaptation to high heart rates and increase arrhythmia susceptibility.

ERK5 induces ankrd1 for catecholamine biosynthesis and homeostasis in adrenal medullary cells

Extracellular signal-regulated kinases (ERKs) play important roles in proliferation, differentiation and gene expression. In our previous study, we demonstrated that both ERK5 and ERK1/2 were responsible for neurite outgrowth and tyrosine hydroxylase (TH) expression in rat pheochromocytoma cells (PC12) (J Biol Chem 284, 23,564-23,573, 2009). However, the functional differences between ERK5 and ERK1/2 signaling in neural differentiation remain unclear. In the present study, we show that ERK5, but not ERK1/2 regulates TH levels in rat sympathetic neurons. Furthermore, microarray analysis performed in PC12 cells using ERK5 and ERK1/2-specific inhibitors, identified ankyrin repeat domain 1 (ankrd1) as an ERK5-dependent and ERK1/2-independent gene. Here, we report a novel role of the ERK5/ankrd1 signaling in regulating TH levels and catecholamine biosynthesis. Ankrd1 mRNA was induced by nerve growth factor in time- and concentration-dependent manners. TH levels were reduced by ankrd1 knockdown with no changes in the mRNA levels, suggesting that ankrd1 was involved in stabilization of TH protein. Interestingly, ubiquitination of TH was enhanced and catecholamine biosynthesis was reduced by ankrd1 knockdown. Finally, we examined the relationship of ERK5 to TH levels in human adrenal pheochromocytomas. Whereas TH levels were correlated with ERK5 levels in normal adrenal medullas, ERK5 was down-regulated and TH was up-regulated in pheochromocytomas, indicating that TH levels are regulated by alternative mechanisms in tumors. Taken together, ERK5 signaling is required for catecholamine biosynthesis during neural differentiation, in part to induce ankrd1, and to maintain appropriate TH levels. This pathway is disrupted in pathological conditions.

Cardiac Sympathetic Nerve Sprouting and Susceptibility to Ventricular Arrhythmias after Myocardial Infarction

Ventricular arrhythmogenesis is thought to be a common cause of sudden cardiac death following myocardial infarction (MI). Nerve remodeling as a result of MI is known to be an important genesis of life-threatening arrhythmias. It is hypothesized that neural modulation might serve as a therapeutic option of malignant arrhythmias. In fact, left stellectomy or β-blocker therapy is shown to be effective in the prevention of ventricular tachyarrhythmias (VT), ventricular fibrillation (VF), and sudden cardiac death (SCD) after MI both in patients and in animal models. Results from decades of research already evidenced a positive relationship between abnormal nerve density and ventricular arrhythmias after MI. In this review, we summarized the molecular mechanisms involved in cardiac sympathetic rejuvenation and mechanisms related to sympathetic hyperinnervation and arrhythmogenesis after MI and analyzed the potential therapeutic implications of nerve sprouting modification for ventricular arrhythmias and SCD control.

Targeting protein tyrosine phosphatase σ after myocardial infarction restores cardiac sympathetic innervation and prevents arrhythmias

Millions of people suffer a myocardial infarction (MI) every year, and those who survive have increased risk of arrhythmias and sudden cardiac death. Recent clinical studies have identified sympathetic denervation as a predictor of increased arrhythmia susceptibility. Chondroitin sulfate proteoglycans present in the cardiac scar after MI prevent sympathetic reinnervation by binding the neuronal protein tyrosine phosphatase receptor σ (PTPσ). Here we show that the absence of PTPσ, or pharmacologic modulation of PTPσ by the novel intracellular sigma peptide (ISP) beginning 3 days after injury, restores sympathetic innervation to the scar and markedly reduces arrhythmia susceptibility. Using optical mapping we observe increased dispersion of action potential duration, supersensitivity to β-adrenergic receptor stimulation and Ca(2+) mishandling following MI. Sympathetic reinnervation prevents these changes and renders hearts remarkably resistant to induced arrhythmias.

The biology of neurotrophins: cardiovascular function

This chapter addresses the role of neurotrophins in the development of the heart, blood vessels, and neural circuits that control cardiovascular function, as well as the role of neurotrophins in the mature cardiovascular system. The cardiovascular system includes the heart and vasculature whose functions are tightly controlled by the nervous system. Neurons, cardiomyocytes, endothelial cells, vascular smooth muscle cells, and pericytes are all targets for neurotrophin action during development. Neurotrophin expression continues throughout life, and several common pathologies that impact cardiovascular function involve changes in the expression or activity of neurotrophins. These include atherosclerosis, hypertension, diabetes, acute myocardial infarction, and heart failure. In many of these conditions, altered expression of neurotrophins and/or neurotrophin receptors has direct effects on vascular endothelial and smooth muscle cells in addition to effects on nerves that modulate vascular resistance and cardiac function. This chapter summarizes the effects of neurotrophins in cardiovascular physiology and pathophysiology.

Sympathetic denervation of peri-infarct myocardium requires the p75 neurotrophin receptor

Development of cardiac sympathetic heterogeneity after myocardial infarction contributes to ventricular arrhythmias and sudden cardiac death. Regions of sympathetic hyperinnervation and denervation appear in the viable myocardium beyond the infarcted area. While elevated nerve growth factor (NGF) is implicated in sympathetic hyperinnervation, the mechanisms underlying denervation are unknown. Recent studies show that selective activation of the p75 neurotrophin receptor (p75(NTR)) in sympathetic neurons causes axon degeneration. We used mice that lack p75(NTR) to test the hypothesis that activation of p75(NTR) causes peri-infarct sympathetic denervation after cardiac ischemia-reperfusion. Wild type hearts exhibited sympathetic denervation adjacent to the infarct 24h and 3 days after ischemia-reperfusion, but no peri-infarct sympathetic denervation occurred in p75(NTR)-/- mice. Sympathetic hyperinnervation was found in the distal peri-infarct myocardium in both genotypes 3 days after MI, and hyperinnervation was increased in the p75(NTR)-/- mice. By 7 days after ischemia-reperfusion, cardiac sympathetic innervation density returned back to sham-operated levels in both genotypes, indicating that axonal pruning did not require p75(NTR). Prior studies revealed that proNGF is elevated in the damaged left ventricle after ischemia-reperfusion, as is mRNA encoding brain-derived neurotrophic factor (BDNF). ProNGF and BDNF preferentially bind p75(NTR) rather than TrkA on sympathetic neurons. Immunohistochemistry using Bdnf-HA mice confirmed the presence of BDNF or proBDNF in the infarct after ischemia-reperfusion. Thus, at least two p75(NTR) ligands are elevated in the left ventricle after ischemia-reperfusion where they may stimulate p75(NTR)-dependent denervation of peri-infarct myocardium. In contrast, NGF-induced sympathetic hyperinnervation in the distal peri-infarct ventricle is attenuated by p75(NTR).

STAT3 integrates cytokine and neurotrophin signals to promote sympathetic axon regeneration

The transcription factor STAT3 has been implicated in axon regeneration. Here we investigate a role for STAT3 in sympathetic nerve sprouting after myocardial infarction (MI) - a common injury in humans. We show that NGF stimulates serine phosphorylation (S727) of STAT3 in sympathetic neurons via ERK1/2, in contrast to cytokine phosphorylation of Y705. Maximal sympathetic axon regeneration in vitro requires phosphorylation of both S727 and Y705. Furthermore, cytokine signaling is necessary for NGF-induced sympathetic nerve sprouting in the heart after MI. Transfection studies in neurons lacking STAT3 suggest two independent pools of STAT3, phosphorylated on either S727 or Y705, that regulate sympathetic regeneration via both transcriptional and non-transcriptional means. Additional data identify STAT3-microtubule interactions that may complement the well-characterized role of STAT3 stimulating regeneration associated genes. These data show that STAT3 is critical for sympathetic axon regeneration in vitro and in vivo, and identify a novel non-transcriptional mode of action.

Infarct-derived chondroitin sulfate proteoglycans prevent sympathetic reinnervation after cardiac ischemia-reperfusion injury

Sympathetic nerves can regenerate after injury to reinnervate target tissues. Sympathetic regeneration is well documented after chronic cardiac ischemia, so we were surprised that the cardiac infarct remained denervated following ischemia-reperfusion (I-R). We used mice to ask if the lack of sympathetic regeneration into the scar was due to blockade by inhibitory extracellular matrix within the infarct. We found that chondroitin sulfate proteoglycans (CSPGs) were present in the infarct after I-R, but not after chronic ischemia, and that CSPGs caused inhibition of sympathetic axon outgrowth in vitro. Ventricle explants after I-R and chronic ischemia stimulated sympathetic axon outgrowth that was blocked by nerve growth factor antibodies. However, growth in I-R cocultures was asymmetrical, with axons growing toward the heart tissue consistently shorter than axons growing in other directions. Growth toward I-R explants was rescued by adding chondroitinase ABC to the cocultures, suggesting that I-R infarct-derived CSPGs prevented axon extension. Sympathetic ganglia lacking protein tyrosine phosphatase sigma (PTPRS) were not inhibited by CSPGs or I-R explants in vitro, suggesting PTPRS is the major CSPG receptor in sympathetic neurons. To test directly if infarct-derived CSPGs prevented cardiac reinnervation, we performed I-R in ptprs-/- and ptprs+/- mice. Cardiac infarcts in ptprs-/- mice were hyperinnervated, while infarcts in ptprs+/- littermates were denervated, confirming that CSPGs prevent sympathetic reinnervation of the cardiac scar after I-R. This is the first example of CSPGs preventing sympathetic reinnervation of an autonomic target following injury, and may have important consequences for cardiac function and arrhythmia susceptibility after myocardial infarction.

Autonomic cardiac innervation: development and adult plasticity

Autonomic cardiac neurons have a common origin in the neural crest but undergo distinct developmental differentiation as they mature toward their adult phenotype. Progenitor cells respond to repulsive cues during migration, followed by differentiation cues from paracrine sources that promote neurochemistry and differentiation. When autonomic axons start to innervate cardiac tissue, neurotrophic factors from vascular tissue are essential for maintenance of neurons before they reach their targets, upon which target-derived trophic factors take over final maturation, synaptic strength and postnatal survival. Although target-derived neurotrophins have a central role to play in development, alternative sources of neurotrophins may also modulate innervation. Both developing and adult sympathetic neurons express proNGF, and adult parasympathetic cardiac ganglion neurons also synthesize and release NGF. The physiological function of these “non-classical” cardiac sources of neurotrophins remains to be determined, especially in relation to autocrine/paracrine sustenance during development.

 

Cardiac autonomic nerves are closely spatially associated in cardiac plexuses, ganglia and pacemaker regions and so are sensitive to release of neurotransmitter, neuropeptides and trophic factors from adjacent nerves. As such, in many cardiac pathologies, it is an imbalance within the two arms of the autonomic system that is critical for disease progression. Although this crosstalk between sympathetic and parasympathetic nerves has been well established for adult nerves, it is unclear whether a degree of paracrine regulation occurs across the autonomic limbs during development. Aberrant nerve remodeling is a common occurrence in many adult cardiovascular pathologies, and the mechanisms regulating outgrowth or denervation are disparate. However, autonomic neurons display considerable plasticity in this regard with neurotrophins and inflammatory cytokines having a central regulatory function, including in possible neurotransmitter changes. Certainly, neurotrophins and cytokines regulate transcriptional factors in adult autonomic neurons that have vital differentiation roles in development. Particularly for parasympathetic cardiac ganglion neurons, additional examinations of developmental regulatory mechanisms will potentially aid in understanding attenuated parasympathetic function in a number of conditions, including heart failure.

A nonsynonymous polymorphism in semaphorin 3A as a risk factor for human unexplained cardiac arrest with documented ventricular fibrillation

Unexplained cardiac arrest (UCA) with documented ventricular fibrillation (VF) is a major cause of sudden cardiac death. Abnormal sympathetic innervations have been shown to be a trigger of ventricular fibrillation. Further, adequate expression of SEMA3A was reported to be critical for normal patterning of cardiac sympathetic innervation. We investigated the relevance of the semaphorin 3A (SEMA3A) gene located at chromosome 5 in the etiology of UCA. Eighty-three Japanese patients diagnosed with UCA and 2,958 healthy controls from two different geographic regions in Japan were enrolled. A nonsynonymous polymorphism (I334V, rs138694505A>G) in exon 10 of the SEMA3A gene identified through resequencing was significantly associated with UCA (combined P = 0.0004, OR 3.08, 95%CI 1.67-5.7). Overall, 15.7% of UCA patients carried the risk genotype G, whereas only 5.6% did in controls. In patients with SEMA3A(I334V), VF predominantly occurred at rest during the night. They showed sinus bradycardia, and their RR intervals on the 12-lead electrocardiography tended to be longer than those in patients without SEMA3A(I334V) (1031±111 ms versus 932±182 ms, P = 0.039). Immunofluorescence staining of cardiac biopsy specimens revealed that sympathetic nerves, which are absent in the subendocardial layer in normal hearts, extended to the subendocardial layer only in patients with SEMA3A(I334V). Functional analyses revealed that the axon-repelling and axon-collapsing activities of mutant SEMA3A(I334V) genes were significantly weaker than those of wild-type SEMA3A genes. A high incidence of SEMA3A(I334V) in UCA patients and inappropriate innervation patterning in their hearts implicate involvement of the SEMA3A gene in the pathogenesis of UCA.

Altered atrial neurotransmitter release in transgenic p75(-/-) and gp130 KO mice

Heart rate is controlled by stimulatory sympathetic and inhibitory parasympathetic nerves innervating the sino-atrial node and cardiac conduction system. Sympathetic release of norepinephrine (NE) and parasympathetic release of acetylcholine (ACh) are controlled by the central nervous system, and by pre-synaptic inhibition of transmitter release within the atria. An increase in cardiac sympathetic transmission relative to parasympathetic transmission is pathological as it can lead to disturbances in heart rhythm, catecholaminergic toxicity and development of arrhythmias or fibrillation. Mice lacking the p75 neurotrophin receptor (p75(-/-)) have elevated atrial NE but a low heart rate suggesting autonomic dysregulation. Similarly, mice whose sympathetic neurons lack the gp130 cytokine receptor (gp130 KO) have a normal heart rate but enhanced bradycardia after vagal nerve stimulation. What is unclear is whether cardiac autonomic disturbances in these animals reflect systemic alterations in nerve activity or whether localized defects in neurotransmitter stores or release are involved. To examine local stimulus-evoked release of neurotransmitters, we have developed a novel method for simultaneous quantification of both NE and ACh after ex vivo atrial field stimulation. Using HPLC with electrochemical detection for NE, and HPLC with mass spectrometry for ACh, we found that following field stimulation NE release was impaired in p75(-/-) atria while ACh content and release was elevated in gp130 KO atria. Thus, alterations in localized transmitter release from atrial explants are consistent with in vivo deficits in heart rate control, suggesting peripheral alterations in autonomic transmission in these mice.

P75 neurotrophin receptor is a regulatory factor in sudden cardiac death with myocardial infarction

Sudden cardiac death (SCD) is a major cause of morbidity and mortality in patients with coronary artery diseases and myocardial infarction (MI). Sympathetic stimulation and sympathetic neural remodeling are important in the generation of SCD in diseased heart. The balance of nerve growth factor (NGF) and semaphoring 3A determines the sympathetic innervation patterning. Recently studies showed that P75 neurotrophin receptor (P75 NTR) is the main receptor for NGF mediates sympathetic hyperinnervation in the heart, and also interacts with semaphoring 3A. Sympathetic axons lacking P75 NTR are more sensitive to semaphoring 3A in vitro than control neurons, resulting in decreased sympathetic innervation in the left ventricular subendocardium. P75 NTR(-/-) mice had increased sympathetic heterogeneity and more spontaneous ventricular arrhythmias. Based on current studies, we present a hypothesis that P75 NTR plays an important regulatory role in sudden cardiac after myocardial infarction and hope to find new therapeutic target for SCD.

Development, maturation, and transdifferentiation of cardiac sympathetic nerves

The heart is electrically and mechanically controlled as a syncytium by the autonomic nervous system. The cardiac nervous system comprises the sympathetic, parasympathetic, and sensory nervous systems that together regulate heart function on demand. Sympathetic electric activation was initially considered the main regulator of cardiac function; however, modern molecular biotechnological approaches have provided a new dimension to our understanding of the mechanisms controlling the cardiac nervous system. The heart is extensively innervated, although the innervation density is not uniform within the heart, being high in the subepicardium and the special conduction system. We and others showed previously that the balance between neural chemoattractants and chemorepellents determine cardiac nervous development, with both factors expressed in heart. Nerve growth factor is a potent chemoattractant synthesized by cardiomyocytes, whereas Sema3a is a neural chemorepellent expressed specifically in the subendocardium. Disruption of this well-organized molecular balance and innervation density can induce sudden cardiac death due to lethal arrhythmias. In diseased hearts, various causes and mechanisms underlie cardiac sympathetic abnormalities, although their detailed pathology and significance remain contentious. We reported that cardiac sympathetic rejuvenation occurs in cardiac hypertrophy and, moreover, interleukin-6 cytokines secreted from the failing myocardium induce cholinergic transdifferentiation of the cardiac sympathetic system via a gp130 signaling pathway, affecting cardiac performance and prognosis. In this review, we summarize the molecular mechanisms involved in sympathetic development, maturation, and transdifferentiation, and propose their investigation as new therapeutic targets for heart disease.

Altered norepinephrine content and ventricular function in p75NTR-/- mice after myocardial infarction

Cardiac sympathetic neurons stimulate heart rate and the force of contraction through release of norepinephrine. Nerve growth factor modulates sympathetic transmission through activation of TrkA and p75NTR. Nerve growth factor plays an important role in post-infarct sympathetic remodeling. We used mice lacking p75NTR to examine the effect of altered nerve growth factor signaling on sympathetic neuropeptide expression, cardiac norepinephrine, and ventricular function after myocardial infarction. Infarct size was similar in wildtype and p75NTR-/- mice after ischemia-reperfusion surgery. Likewise, mRNAs encoding vasoactive intestinal peptide, galanin, and pituitary adenylate cyclase activating peptides were identical in wildtype and p75NTR-/- cardiac sympathetic neurons, as was expression of the TrkA neurotrophin receptor. Norepinephrine content was elevated in the base of the p75NTR-/- ventricle compared to wildtype, but levels were identical below the site of occlusion. Left ventricular pressure, dP/dt(MAX), and dP/dt(MIN) were measured under isoflurane anesthesia 3 and 7 days after surgery. Ventricular pressure decreased significantly 3 days after infarction, and deficits in dP/dt(MAX) were revealed by stimulating beta receptors with dobutamine and release of endogenous norepinephrine with tyramine. dP/dt(MIN) was not altered by genotype or surgical group. Few differences were observed between genotypes 3 days after surgery, in contrast to low pressure and dP/dt(MAX) previously reported in control p75NTR-/- animals. Seven days after surgery ventricular pressure and dP/dt(MAX) were significantly lower in p75NTR-/- hearts compared to WT hearts. Thus, the lack of p75NTR did not enhance cardiac function after myocardial infarction.

The role of tumor necrosis factor receptor superfamily members in mammalian brain development, function and homeostasis

Tumor necrosis factor receptor superfamily (TNFRSF) members were initially identified as immunological mediators, and are still commonly perceived as immunological molecules. However, our understanding of the diversity of TNFRSF members' roles in mammalian physiology has grown significantly since the first discovery of TNFRp55 (TNFRSF1) in 1975. In particular, the last decade has provided evidence for important roles in brain development, function and the emergent field of neuronal homeostasis. Recent evidence suggests that TNFRSF members are expressed in an overlapping regulated pattern during neuronal development, participating in the regulation of neuronal expansion, growth, differentiation and regional pattern development. This review examines evidence for non-immunological roles of TNFRSF members in brain development, function and maintenance under normal physiological conditions. In addition, several aspects of brain function during inflammation will also be described, when illuminating and relevant to the non-immunological role of TNFRSF members. Finally, key questions in the field will be outlined.

Cardiac ischemia-reperfusion regulates sympathetic neuropeptide expression through gp130-dependent and independent mechanisms

Cardiac function is regulated by a balance of sympathetic and parasympathetic transmission. Neuropeptide Y (NPY) and galanin (GAL) released from cardiac sympathetic neurons inhibits parasympathetic transmission in the heart. Sympathetic peptides may contribute to autonomic imbalance, which is characterized by increased sympathetic and decreased parasympathetic transmission and contributes to life threatening cardiovascular pathologies. Several gp130 cytokines are increased in the heart after myocardial infarction (MI), and these cytokines stimulate neuropeptide expression in sympathetic neurons. We used mice whose sympathetic neurons lack the gp130 receptor (gp130(DBH-Cre/lox) mice) to ask if cytokine activation of gp130 regulated neuropeptide expression in cardiac sympathetic nerves after MI. Myocardial infarction decreased NPY mRNA through a gp130 independent mechanism and increased VIP and PACAP mRNA via gp130, while GAL mRNA was unchanged. Immunohistochemistry revealed a gp130-dependent increase in PACAP38 in cells of the stellate ganglion after MI, and PACAP was detected in pre-ganglionic fibers of all genotypes and surgical groups. VIP was identified in a few sympathetic nerve fibers in all genotypes and surgical groups. GAL and PACAP38 were not detected in sham hearts, but peptide immunoreactivity was high in the infarct three days after MI. Surprisingly, peptides were abundant in cells that co-labeled with macrophage markers F4/80 and MAC2, but were not detected in sympathetic axons. PACAP protects cardiac myocytes from apoptosis, and GAL stimulates axon regeneration in addition to inhibiting parasympathetic transmission. Thus, these peptides may play an important role in cardiac and neuronal remodeling after ischemia-reperfusion.

Structural neuroplasticity following T5 spinal cord transection: increased cardiac sympathetic innervation density and SPN arborization

When the spinal cord is injured at or below thoracic level 5 (T5), cardiovascular control is markedly unbalanced as the heart and blood vessels innervated by upper thoracic segments remain under brain stem control, whereas the vasculature of the lower body is affected by unregulated spinal reflexes. Importantly, the regulation of heart rate and cardiac function is abnormal after spinal cord injury (SCI) at T5 because sympathetic outflow to the heart is increased. An increase in tonic sympathetic outflow may be attributable to multiple mechanisms, such as increases in cardiac sympathetic innervation density, altered morphology of stellate ganglia neurons, and/or structural neuroplasticity of cardiac sympathetic preganglionic neurons (SPNs). Furthermore, these neuroplastic changes associated with SCI may be mediated by nerve growth factor (NGF). NGF is a neurotrophin that supports the survival and differentiation of sympathetic neurons and enhances target innervation. Therefore, we tested the hypothesis that T5 spinal cord transection (T5X) is associated with an increased left ventricular (LV) NGF content, LV sympathetic innervation density, and cardiac SPN arborization. In intact and paraplegic (9 wk posttransection) rats, LV NGF content (ELISA), LV sympathetic innervation density (tyrosine hydroxylase immunohistochemistry), and cardiac SPN arborization (cholera toxin B immunohistochemistry and Sholl Analysis) were determined. Paraplegia, compared with intact, significantly increased LV NGF content, LV sympathetic innervation density, and cardiac SPN arborization. Thus, altered autonomic behavior following SCI is associated with structural neuroplastic modifications.