Sympathetic hyperinnervation and inflammatory cell NGF synthesis following myocardial infarction in rats

Putting together the clues of the everlasting neuro-cardiac liaison

Starting from the late embryonic development, the sympathetic nervous system extensively innervates the heart and modulates its activity during the entire lifespan. The distribution of myocardial sympathetic processes is finely regulated by the secretion of limiting amounts of pro-survival neurotrophic factors by cardiac cells. Norepinephrine release by the neurons rapidly modulates myocardial electrophysiology, and increases the rate and force of cardiomyocyte contractions. Sympathetic processes establish direct interaction with cardiomyocytes, characterized by the presence of neurotransmitter vesicles and reduced cell-cell distance. Whether such contacts have a functional role in both neurotrophin- and catecholamine-dependent communication between the two cell types, is poorly understood. In this review we will address the effects of the sympathetic neuron activity on the myocardium and the hypothesis that the direct neuro-cardiac contact might have a key role both in norepinephrine and neurotrophin mediated signaling. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

The nervous heart

Many cardiac electrophysiological abnormalities are accompanied by autonomic nervous system dysfunction. Here, we review mechanisms by which the cardiac nervous system controls normal and abnormal excitability and may contribute to atrial and ventricular tachyarrhythmias. Moreover, we explore the potential antiarrhythmic and/or arrhythmogenic effects of modulating the autonomic nervous system by several strategies, including ganglionated plexi ablation, vagal and spinal cord stimulations, and renal sympathetic denervation as therapies for atrial and ventricular arrhythmias.

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.

Endocardial Endothelial Dysfunction Progressively Disrupts Initially Anti then Pro-Thrombotic Pathways in Heart Failure Mice

Objective

An experimental model of endocardial thrombosis has not been developed and endocardial endothelial dysfunction in heart failure (HF) is understudied. We sought to determine whether disruption of the endothelial anti-coagulant activated protein C (APC) pathway in CREB_A133 HF mice promotes endocardial thrombosis in the acute decompensated phase of the disease, and whether alterations in von Willebrand factor (vWF) secretion from HF endocardium reduces thrombus formation as HF stabilizes.

Approach and results

Echocardiography was used to follow HF development and to detect endocardial thrombi in CREB_A133 mice. Endocardial thrombi incidence was confirmed with immunohistochemistry and histology. In early and acute decompensated phases of HF, CREB_A133 mice had the highest incidence of endocardial thrombi and these mice also had a shorter tail-bleeding index consistent with a pro-thrombotic milieu. Both APC generation, and expression of receptors that promote APC function (thrombomodulin, endothelial protein C receptor, protein S), were suppressed in the endocardium of acute decompensated HF mice. However, in stable compensated HF mice, an attenuation occurred for vWF protein content and secretion from endocardial endothelial cells, vWF-dependent platelet agglutination (by ristocetin), and thrombin generation on the endocardial surface.

Conclusions

CREB_A133 mice develop HF and endocardial endothelial dysfunction. Attenuation of the anti-coagulant APC pathway promotes endocardial thrombosis in early and acute decompensated phases of HF. However, in stable compensated HF mice, disruptions in endothelial vWF expression and extrusion may actually reduce the incidence of endocardial thrombosis.

Inhibition of Notch signaling pathway attenuates sympathetic hyperinnervation together with the augmentation of M2 macrophages in rats post-myocardial infarction

Inflammation-dominated sympathetic sprouting adjacent to the necrotic region following myocardial infarction (MI) has been implicated in the etiology of arrhythmias resulting in sudden cardiac death; however, the mechanisms responsible remain to be elucidated. Although being a key immune mediator, the role of Notch has yet to be explored. We investigated whether Notch regulates macrophage responses to inflammation and affects cardiac sympathetic reinnervation in rats undergoing MI. MI was induced by coronary artery ligation. A high level of Notch intracellular domain was observed in the macrophages that infiltrated the infarct area at 3 days post-MI. The administration of the Notch inhibitor N-N-(3,5-difluorophenacetyl-L-alanyl)-S-phenylglycine-t-butyl ester (DAPT) (intravenously 30 min before MI and then daily until death) decreased the number of macrophages and significantly increased the M2 macrophage activation profile in the early stages and attenuated the expression of nerve growth factor (NGF). Eventually, NGF-induced sympathetic hyperinnervation was blunted, as assessed by the immunofluorescence of tyrosine hydroxylase. At 7 days post-MI, the arrhythmia score of programmed electric stimulation in the vehicle-treated infarcted rats was higher than that in rats treated with DAPT. Further deterioration in cardiac function and decreases in the plasma levels of TNF-α and IL-1β were also detected. In vitro studies revealed that LPS/IFN-γ upregulated the surface expression of NGF in M1 macrophages in a Notch-dependent manner. We concluded that Notch inhibition during the acute inflammatory response phase is associated with the downregulation of NGF, probably through a macrophage-dependent pathway, thus preventing the process of sympathetic hyperinnervation.

Splicing Regulation of Pro-Inflammatory Cytokines and Chemokines: At the Interface of the Neuroendocrine and Immune Systems

Alternative splicing plays a key role in posttranscriptional regulation of gene expression, allowing a single gene to encode multiple protein isoforms. As such, alternative splicing amplifies the coding capacity of the genome enormously, generates protein diversity, and alters protein function. More than 90% of human genes undergo alternative splicing, and alternative splicing is especially prevalent in the nervous and immune systems, tissues where cells need to react swiftly and adapt to changes in the environment through carefully regulated mechanisms of cell differentiation, migration, targeting, and activation. Given its prevalence and complexity, this highly regulated mode of gene expression is prone to be affected by disease. In the following review, we look at how alternative splicing of signaling molecules—cytokines and their receptors—changes in different pathological conditions, from chronic inflammation to neurologic disorders, providing means of functional interaction between the immune and neuroendocrine systems. Switches in alternative splicing patterns can be very dynamic and can produce signaling molecules with distinct or antagonistic functions and localization to different subcellular compartments. This newly discovered link expands our understanding of the biology of immune and neuroendocrine cells, and has the potential to open new windows of opportunity for treatment of neurodegenerative disorders.

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.

Enteric glia mediate neuronal outgrowth through release of neurotrophic factors

Previous studies have shown that transplanted enteric glia enhance axonal regeneration, reduce tissue damage, and promote functional recovery following spinal cord injury. However, the mechanisms by which enteric glia mediate these beneficial effects are unknown. Neurotrophic factors can promote neuronal differentiation, survival and neurite extension. We hypothesized that enteric glia may exert their protective effects against spinal cord injury partially through the secretion of neurotrophic factors. In the present study, we demonstrated that primary enteric glia cells release nerve growth factor, brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor over time with their concentrations reaching approximately 250, 100 and 50 pg/mL of culture medium respectively after 48 hours. The biological relevance of this secretion was assessed by incubating dissociated dorsal root ganglion neuronal cultures in enteric glia-conditioned medium with and/or without neutralizing antibodies to each of these proteins and evaluating the differences in neurite growth. We discovered that conditioned medium enhances neurite outgrowth in dorsal root ganglion neurons. Even though there was no detectable amount of neurotrophin-3 secretion using ELISA analysis, the neurite outgrowth effect can be attenuated by the antibody-mediated neutralization of each of the aforementioned neurotrophic factors. Therefore, enteric glia secrete nerve growth factor, brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor and neurotrophin-3 into their surrounding environment in concentrations that can cause a biological effect.

Transplantation of iPSc ameliorates neural remodeling and reduces ventricular arrhythmias in a post-infarcted swine model

Neural remodeling after myocardial infarction (MI) may cause malignant ventricular arrhythmia, which is the main cause of sudden cardiac death following MI. Herein, we aimed to examine whether induced pluripotent stem cells (iPSc) transplantation can ameliorate neural remodeling and reduce ventricular arrhythmias (VA) in a post-infarcted swine model. Left anterior descending coronary arteries were balloon-occluded to generate MI. Animals were then divided into Sham, PBS control, and iPS groups. Dynamic electrocardiography programmed electric stimulation were performed to evaluate VA. The spatial distribution of vascularization, Cx43 and autonomic nerve regeneration were evaluated by immunofluorescence staining. Associated protein expression was detected by Western blotting. Likewise, we measured the enzymatic activities of superoxide dismutase and content of malondialdehyde. Six weeks later, the number of blood vessels increased significantly in the iPSc group. The expression of vascular endothelial growth factor and connexin 43 in the iPS group was significantly higher than the PBS group; however, the levels of nerve growth factor and tyrosine hydroxylase were lower. The oxidative stress was ameliorated by iPSc transplantation. Moreover, the number of sympathetic nerves in the iPSc group was reduced, while the parasympathetic nerve fibers had no obvious change. The transplantation of iPSc also significantly decreased the low-/high-frequency ratio and arrhythmia score of programmed electric stimulation-induced VA. In conclusion, iPSc intramyocardial transplantation reduces vulnerability to VAs, and the mechanism was related to the remodeling amelioration of autonomic nerves and gap junctions. Moreover, possible mechanisms of iPSc transplantation in improving neural remodeling may be related to attenuated oxidative stress and inflammatory response.

Participation of interleukin 17A in neuroimmune interactions

Inflammation involving the helper T cell 17 (Th17) subset of lymphocytes has been implicated in a number of diseases that affect the nervous system. As the canonical cytokine of Th17 cells, interleukin 17A (IL-17A) is thought to contribute to these neuroimmune interactions. The main receptor for IL-17A is expressed in many neural tissues. IL-17A has direct effects on neurons but can also impact neural function via signaling to satellite cells and immune cells. In the central nervous system, IL-17A has been associated with neuropathology in multiple sclerosis, epilepsy syndromes and ischemic brain injury. Effects of IL-17A at the level of dorsal root ganglia and the spinal cord may contribute to enhanced nociception during neuropathic and inflammatory pain. Finally, IL-17A plays a role in sympathetic axon growth and regeneration of damaged axons that innervate the cornea. Given the widespread effects of IL-17A on neural tissues, it will be important to determine whether selectively mitigating the damaging effects of this cytokine while augmenting its beneficial effects is a possible strategy to treat inflammatory damage to the nervous system.

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.

Triactome: neuro-immune-adipose interactions. Implication in vascular biology

Understanding how the precise interactions of nerves, immune cells, and adipose tissue account for cardiovascular and metabolic biology is a central aim of biomedical research at present. A long standing paradigm holds that the vascular wall is composed of three concentric tissue coats (tunicae): intima, media, and adventitia. However, large- and medium-sized arteries, where usually atherosclerotic lesions develop, are consistently surrounded by periadventitial adipose tissue (PAAT), we recently designated tunica adiposa (in brief, adiposa like intima, media, and adventitia). Today, atherosclerosis is considered an immune-mediated inflammatory disease featured by endothelial dysfunction/intimal thickening, medial atrophy, and adventitial lesions associated with adipose dysfunction, whereas hypertension is characterized by hyperinnervation-associated medial thickening due to smooth muscle cell hypertrophy/hyperplasia. PAAT expansion is associated with increased infiltration of immune cells, both adipocytes and immunocytes secreting pro-inflammatory and anti-inflammatory (metabotrophic) signaling proteins collectively dubbed adipokines. However, the role of vascular nerves and their interactions with immune cells and paracrine adipose tissue is not yet evaluated in such an integrated way. The present review attempts to briefly highlight the findings in basic and translational sciences in this area focusing on neuro-immune-adipose interactions, herein referred to as triactome. Triactome-targeted pharmacology may provide a novel therapeutic approach in cardiovascular disease.

Cardiac sympathetic innervation and PGP9.5 expression by cardiomyocytes after myocardial infarction: effects of central MR blockade

Central mechanisms involving mineralocorticoid receptor (MR) activation contribute to an increase in sympathetic tone after myocardial infarction (MI). We hypothesized that this central mechanism also contributes to cardiac sympathetic axonal sprouting and that central MR blockade reduces cardiac sympathetic hyperinnervation post-MI. Post-MI, tyrosine hydroxylase (TH) and norepinephrine transporter protein content in the noninfarcted base of the heart remained unaltered. In contrast, protein gene product (PGP)9.5 protein was increased twofold in the base of the heart and sixfold in the peri-infarct area at 1 wk post-MI and was associated with increased ubiquitin expression. These changes persisted to a lesser extent at 4 wk post-MI and were no longer present at 12 wk. Cardiac myocytes rather than sympathetic axons were the main source of this elevated PGP9.5 expression. At 7–10 days post-MI, in the peri-infarct area, sympathetic hyperinnervation was observed with a fourfold increase in growth-associated protein 43, a twofold increase in TH, and a 50% increase in PGP9.5-positive fibers compared with the epicardial side of the left ventricle in sham rats. Central infusion of the MR blocker eplerenone markedly attenuated these increases in nerve densities but did not affect overall cardiac PGP9.5 and ubiquitin protein overexpression. We conclude that central MR activation contributes to sympathetic hyperinnervation, possibly by decreasing cardiac sympathetic activity post-MI, or by affecting other mechanisms, such as the expression of nerve growth factor. Marked PGP9.5 expression occurs in cardiomyocytes early post-MI, which may contribute to the increase in ubiquitin.

Neurotrophic and metabotrophic potential of nerve growth factor and brain-derived neurotrophic factor: Linking cardiometabolic and neuropsychiatric diseases

One of biggest recent achievements of neurobiology is the study on neurotrophic factors. The neurotrophins are exciting examples of these factors. They belong to a family of proteins consisting of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), NT-4/5, NT-6, and NT-7. Today, NGF and BDNF are well recognized to mediate a dizzying number of trophobiological effects, ranging from neurotrophic through immunotrophic and epitheliotrophic to metabotrophic effects. These are implicated in the pathogenesis of various diseases. In the same vein, recent studies in adipobiology reveal that this tissue is the body’s largest endocrine and paracrine organ producing multiple signaling proteins collectively termed adipokines, with NGF and BDNF being also produced from adipose tissue. Altogether, neurobiology and adipobiology contribute to the improvement of our knowledge on diseases beyond obesity such as cardiometabolic (atherosclerosis, type 2 diabetes, and metabolic syndrome) and neuropsychiatric (e.g., Alzheimer’s disease and depression) diseases. The present review updates evidence for (1) neurotrophic and metabotrophic potentials of NGF and BDNF linking the pathogenesis of these diseases, and (2) NGF- and BDNF-mediated effects in ampakines, NMDA receptor antagonists, antidepressants, selective deacetylase inhibitors, statins, peroxisome proliferator-activated receptor gamma agonists, and purinergic P2X₃ receptor up-regulation. This may help to construct a novel paradigm in the field of translational pharmacology of neuro-metabotrophins, particularly NGF and BDNF.

Decreased adrenoceptor stimulation in heart failure rats reduces NGF expression by cardiac parasympathetic neurons

Postganglionic cardiac parasympathetic and sympathetic nerves are physically proximate in atrial cardiac tissue allowing reciprocal inhibition of neurotransmitter release, depending on demands from central cardiovascular centers or reflex pathways. Parasympathetic cardiac ganglion (CG) neurons synthesize and release the sympathetic neurotrophin nerve growth factor (NGF), which may serve to maintain these close connections. In this study we investigated whether NGF synthesis by CG neurons is altered in heart failure, and whether norepinephrine from sympathetic neurons promotes NGF synthesis. NGF and proNGF immunoreactivity in CG neurons in heart failure rats following chronic coronary artery ligation was investigated. NGF immunoreactivity was decreased significantly in heart failure rats compared to sham-operated animals, whereas proNGF expression was unchanged. Changes in neurochemistry of CG neurons included attenuated expression of the cholinergic marker vesicular acetylcholine transporter, and increased expression of the neuropeptide vasoactive intestinal polypeptide. To further investigate norepinephrine's role in promoting NGF synthesis, we cultured CG neurons treated with adrenergic receptor (AR) agonists. An 82% increase in NGF mRNA levels was detected after 1h of isoproterenol (β-AR agonist) treatment, which increased an additional 22% at 24h. Antagonist treatment blocked isoproterenol-induced increases in NGF transcripts. In contrast, the α-AR agonist phenylephrine did not alter NGF mRNA expression. These results are consistent with β-AR mediated maintenance of NGF synthesis in CG neurons. In heart failure, a decrease in NGF synthesis by CG neurons may potentially contribute to reduced connections with adjacent sympathetic nerves.

Cardiac fibroblasts regulate sympathetic nerve sprouting and neurocardiac synapse stability

Sympathetic nervous system (SNS) plays a key role in cardiac homeostasis and its deregulations always associate with bad clinical outcomes. To date, little is known about molecular mechanisms regulating cardiac sympathetic innervation. The aim of the study was to determine the role of fibroblasts in heart sympathetic innervation. RT-qPCR and western-blots analysis performed in cardiomyocytes and fibroblasts isolated from healthy adult rat hearts revealed that Pro-Nerve growth factor (NGF) and pro-differentiating mature NGF were the most abundant neurotrophins expressed in cardiac fibroblasts while barely detectable in cardiomyocytes. When cultured with cardiac fibroblasts or fibroblast-conditioned medium, PC12 cells differentiated into/sympathetic-like neurons expressing axonal marker Tau-1 at neurites in contact with cardiomyocytes. This was prevented by anti-NGF blocking antibodies suggesting a paracrine action of NGF secreted by fibroblasts. When co-cultured with cardiomyocytes to mimic neurocardiac synapse, differentiated PC12 cells exhibited enhanced norepinephrine secretion as quantified by HPLC compared to PC12 cultured alone while co-culture with fibroblasts had no effect. However, when supplemented to PC12-cardiomyocytes co-culture, fibroblasts allowed long-term survival of the neurocardiac synapse. Activated fibroblasts (myofibroblasts) isolated from myocardial infarction rat hearts exhibited significantly higher mature NGF expression than normal fibroblasts and also promoted PC12 cells differentiation. Within the ischemic area lacking cardiomyocytes and neurocardiac synapses, tyrosine hydroxylase immunoreactivity was increased and associated with local anarchical and immature sympathetic hyperinnervation but tissue norepinephrine content was similar to that of normal cardiac tissue, suggesting depressed sympathetic function. Collectively, these findings demonstrate for the first time that fibroblasts are essential for the setting of cardiac sympathetic innervation and neurocardiac synapse stability. They also suggest that neurocardiac synapse functionality relies on a triptych with tight interaction between sympathetic nerve endings, cardiomyocytes and fibroblasts. Deregulations of this triptych may be involved in pathophysiology of cardiac diseases.

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).

Sympathetic axonopathies and hyperinnervation in the small intestine smooth muscle of aged Fischer 344 rats

It is well documented that the intrinsic enteric nervous system of the gastrointestinal (GI) tract sustains neuronal losses and reorganizes as it ages. In contrast, age-related remodeling of the extrinsic sympathetic projections to the wall of the gut is poorly characterized. The present experiment, therefore, surveyed the sympathetic projections to the aged small intestine for axonopathies. Furthermore, the experiment evaluated the specific prediction that catecholaminergic inputs undergo hyperplastic changes. Jejunal tissue was collected from 3-, 8-, 16-, and 24-month-old male Fischer 344 rats, prepared as whole mounts consisting of the muscularis, and processed immunohistochemically for tyrosine hydroxylase, the enzymatic marker for norepinephrine, and either the protein CD163 or the protein MHCII, both phenotypical markers for macrophages. Four distinctive sympathetic axonopathy profiles occurred in the small intestine of the aged rat: (1) swollen and dystrophic terminals, (2) tangled axons, (3) discrete hyperinnervated loci in the smooth muscle wall, including at the bases of Peyer's patches, and (4) ectopic hyperplastic or hyperinnervating axons in the serosa/subserosal layers. In many cases, the axonopathies occurred at localized and limited foci, involving only a few axon terminals, in a pattern consistent with incidences of focal ischemic, vascular, or traumatic insult. The present observations underscore the complexity of the processes of aging on the neural circuitry of the gut, with age-related GI functional impairments likely reflecting a constellation of adjustments that range from selective neuronal losses, through accumulation of cellular debris, to hyperplasias and hyperinnervation of sympathetic inputs.

Sympathetic cardiac hyperinnervation and atrial autonomic imbalance in diet-induced obesity promote cardiac arrhythmias

Obesity increases the risk of arrhythmias and sudden cardiac death, but the mechanisms are unknown. This study tested the hypothesis that obesity-induced cardiac sympathetic outgrowth and hyperinnervation promotes the development of arrhythmic events. Male Sprague-Dawley rats (250-275 g), fed a high-fat diet (33% kcal/fat), diverged into obesity-resistant (OR) and obesity-prone (OP) groups and were compared with rats fed normal chow (13% kcal/fat; CON). In vitro experiments showed that both OR and OP rats exhibited hyperinnervation of the heart and high sympathetic outgrowth compared with CON rats, even though OR rats are not obese. Despite the hyperinnervation and outgrowth, we showed that, in vivo, OR rats were less susceptible to arrhythmic events after an intravenous epinephrine challenge compared with OP rats. On examining total and stimulus-evoked neurotransmitter levels in an ex vivo system, we demonstrate that atrial acetylcholine content and release were attenuated in OP compared with OR and CON groups. OP rats also expressed elevated atrial norepinephrine content, while norepinephrine release was suppressed. These findings suggest that the consumption of a high-fat diet, even in the absence of overt obesity, stimulates sympathetic outgrowth and hyperinnervation of the heart. However, normalized cardiac parasympathetic nervous system control may protect the heart from arrhythmic events.

The neurogenic response of cardiac resident nestin(+) cells was associated with GAP43 upregulation and abrogated in a setting of type I diabetes

Background

Cardiac nestin⁽⁺⁾ cells exhibit properties of a neural progenitor/stem cell population characterized by the de novo synthesis of neurofilament-M in response to ischemic injury and 6-hydroxydopamine administration. The induction of growth associated protein 43 (GAP43) was identified as an early event of neurogenesis. The present study tested the hypothesis that the de novo synthesis of neurofilament-M by nestin⁽⁺⁾ cells was preceded by the transient upregulation of GAP43 during the acute phase of reparative fibrosis in the infarcted male rat heart. Secondly, a seminal feature of diabetes is impaired wound healing secondary to an inadequate neurogenic response. In this regard, an additional series of experiments tested the hypothesis that the neurogenic response of cardiac nestin⁽⁺⁾ cells was attenuated in a setting of type I diabetes.

Methods

The neurogenic response of cardiac nestin⁽⁺⁾ cells was examined during the early phase of reparative fibrosis following permanent ligation of the left anterior descending coronary artery in the adult male rat heart. The experimental model of type I diabetes was created following a single injection of streptozotocin in adult male rats. The impact of a type I diabetic environment on the neurogenic response of cardiac nestin⁽⁺⁾ cells was examined during myocardial infarction and following the administration of 6-hydroxydopamine.

Results

During the early phase of scar formation/healing, the density of GAP43/nestin⁽⁺⁾ fibres innervating the peri-infarct/infarct region was significantly increased, whereas neurofilament-M/nestin⁽⁺⁾ fibres were absent. With ongoing scar formation/healing, a temporal decrease of GAP43/nestin⁽⁺⁾ fibre density and a concomitant increase in the density of innervating neurofilament-M/nestin⁽⁺⁾ fibres were observed. The neurogenic response of cardiac nestin⁽⁺⁾ cells during scar formation/healing was inhibited following the superimposition of type I diabetes. The de novo synthesis of neurofilament-M by nestin⁽⁺⁾ cells after 6-hydroxydopamine administration was likewise attenuated in the heart of type I diabetic rats whereas the density of GAP43/nestin⁽⁺⁾ fibres remained elevated.

Conclusion

The transient upregulation of GAP43 apparently represents a transition event during the acquisition of a neuronal-like phenotype and a type I diabetic environment attenuated the neurogenic response of cardiac nestin⁽⁺⁾ cells to ischemia and 6-hydroxydopamine.

Metabolic syndrome--from the neurotrophic hypothesis to a theory

Metabolic syndrome (MetS) is a complex and heterogeneous disease characterized by central obesity, impaired glucose metabolism, dyslipidemia, arterial hypertension, insulin resistance and high-sensitivity C-reactive protein. In 2006, a neurotrophic hypothesis of the etiopathogenesis of MetS was launched. This hypothesis considered the neurotrophins a key factor in MetS development. Chronic inflammatory and/or psychoemotional distress provoke a series of neuroimmunoendocrine interactions such as increased tissue and plasma levels of proinflammatory cytokines and neurotrophins, vegetodystonia, disbalance of neurotransmitters, hormones and immunity markers, activation of the hypothalamo-pituitary-adrenal axis, insulin resistance, and atherosclerosis. An early and a late clinical stage in the course of MetS are defined. Meanwhile, evidence of supporting results from the world literature accumulates. This enables the transformation of the definition of the neurotrophic hypothesis into a neurotrophic theory of MetS. The important role of two neurotrophic factors, i.e. the nerve growth factor and brain-derived neurotrophic factor as well as of the proinflammatory cytokines, neurotransmitters, adipokines and, especially, of leptin for the development of MetS, obesity and type 2 diabetes mellitus is illustrated. There are reliable scientific arguments that the metabotrophic deficit due to reduced neurotrophins could be implicated in the pathogenesis of MetS, type 2 diabetes mellitus, and atherosclerosis as well. A special attention is paid to the activity of the hypothalamo-pituitary-adrenal axis after stress. The application of the neurotrophic theory of MetS could contribute to the etiological diagnosis and individualized management of MetS by eliminating the chronic distress, hyponeurotrophinemia and consequent pathology. It helps estimating the risk, defining the prognosis and implementing the effective prevention of this socially significant disease as evidenced by the dramatic recent growth of the world publication output on this interdisciplinary topic.

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.

Bone morphogenetic protein 4 mediates estrogen-regulated sensory axon plasticity in the adult female reproductive tract

Peripheral axons are structurally plastic even in the adult, and altered axon density is implicated in many disorders and pain syndromes. However, mechanisms responsible for peripheral axon remodeling are poorly understood. Physiological plasticity is characteristic of the female reproductive tract: vaginal sensory innervation density is low under high estrogen conditions, such as term pregnancy, whereas density is high in low-estrogen conditions, such as menopause. We exploited this system in rats to identify factors responsible for adult peripheral neuroplasticity. Calcitonin gene-related peptide-immunoreactive sensory innervation is distributed primarily within the vaginal submucosa. Submucosal smooth muscle cells express bone morphogenetic protein 4 (BMP4). With low estrogen, BMP4 expression was elevated, indicating negative regulation by this hormone. Vaginal smooth muscle cells induced robust neurite outgrowth by cocultured dorsal root ganglion neurons, which was prevented by neutralizing BMP4 with noggin or anti-BMP4. Estrogen also prevented axon outgrowth, and this was reversed by exogenous BMP4. Nuclear accumulation of phosphorylated Smad1, a primary transcription factor for BMP4 signaling, was high in vagina-projecting sensory neurons after ovariectomy and reduced by estrogen. BMP4 regulation of innervation was confirmed in vivo using lentiviral transduction to overexpress BMP4 in an estrogen-independent manner. Submucosal regions with high virally induced BMP4 expression had high innervation density despite elevated estrogen. These findings show that BMP4, an important factor in early nervous system development and regeneration after injury, is a critical mediator of adult physiological plasticity as well. Altered BMP4 expression may therefore contribute to sensory hyperinnervation, a hallmark of several pain disorders, including vulvodynia.

Simvastatin attenuates sympathetic hyperinnervation to prevent atrial fibrillation during the postmyocardial infarction remodeling process

Statin, as a 3-hydroxy-3-methyl-glutaryl-CoA reductase inhibitor, has been shown to prevent atrial fibrillation (AF) due to its anti-inflammatory and antioxidant effects. However, it is still not known whether statin can improve autonomic remodeling to prevent AF. In the present study, using an in vivo rat myocardial infarction (MI) model, we aimed to test whether simvastatin can attenuate nerve sprouting and sympathetic hyperinnervation to prevent AF during the post-MI remodeling process. Our data demonstrate that simvastatin, delivered 3 days after MI for 4 wk, can result in significant decreases in plasma levels of both TNF-α (239 ± 23 pg/ml) and IL-1β (123 ± 11 pg/ml) compared with MI rats without therapy (TNF-α, 728 ± 57 pg/ml; IL-1β, 213 ± 21 pg/ml; P < 0.05), which, however, were still higher than sham-operated rats (TNF-α, 194 ± 20 pg/ml; IL-1β, 75 ± 8 pg/ml; P < 0.05). The similar pattern of changes in inflammation responses was also observed in TNF-α and IL-1β protein expression in the left atrium free wall. The suppressed inflammation responses were associated with reduced superoxide and malondialdehyde generation in the atrium. These changes account for decreases in neural growth factor expression at levels of both mRNA (1.2 ± 0.09 AU vs. MI group, 1.78 ± 0.16 AU) and protein (1.57 ± 0.17 AU vs. MI group, 2.24 ± 0.19 AU; P < 0.05), thus resulting in reduced nerve sprouting and sympathetic hyperinnervation. Accordingly, the rate adaptation of the atrial effective refractory period also recovered, leading to the decreased inducibility of AF. These data suggest that simvastatin administration after MI can prevent AF through reduced sympathetic hyperinnervation.

Neurotrophins regulate cholinergic synaptic transmission in cultured rat sympathetic neurons through a p75-dependent mechanism

The sympathetic nervous system regulates many essential physiological systems, and its dysfunction is implicated in cardiovascular diseases. Mechanisms that control the strength of sympathetic output are therefore potential targets for the management of these disorders. Here we show that neurotrophins rapidly potentiate cholinergic transmission between cultured rat sympathetic neurons. We found that brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), acting at the p75 receptor, increased the amplitude of excitatory postsynaptic currents (EPSCs). We observed increased amplitude but not frequency of miniature synaptic currents after p75 activation, suggesting that p75 acts postsynaptically to modulate transmission at these synapses. This neurotrophic modulation enhances cholinergic EPSCs via sphingolipid signaling. Application of sphingolactone-24, an inhibitor of neutral sphingomyelinase, blocked the effect of BDNF, implicating a sphingolipid pathway. Furthermore, application of the p75-associated sphingolipid second messengers C(2)-ceramide and d-erythro-sphingosine restricted to the postsynaptic cell mimicked BDNF application. Postsynaptic blockade of ceramide production with fumonisin, a ceramide synthase inhibitor, blocked the effects of BDNF and d-erythro-sphingosine, implicating ceramide or ceramide phosphate as the active signal. Together these data suggest that neurotrophin signaling, which occurs in vivo via release from sympathetic neurons and target tissues such as the heart, acutely regulates the strength of the sympathetic postganglionic response to central cholinergic inputs. This pathway provides a potential mechanism for modulating the strength of sympathetic drive to target organs such as the heart and could play a role in the development of cardiovascular diseases.

The plating of rat scar myofibroblasts on matrigel unmasks a novel phenotype; the self assembly of lumen-like structures

During tissue healing, the primary role of myofibroblasts involves the synthesis and deposition of collagen. However, it has also been reported that selective populations of myofibroblasts can acquire the phenotype and/or differentiate to other cells types. The present study tested the hypothesis that myofibroblasts isolated from the scar of the ischemically damaged rat heart can recapitulate an endothelial cell-like response when plated in a permissive in vitro environment. Scar myofibroblasts, neonatal and adult ventricular fibroblasts express smooth muscle α-actin, collagen α(1) type 1 and a panel of pro-fibrotic and pro-angiogenic peptide growth factor mRNAs. Myofibroblasts plated alone on matrigel led to the self assembly of lumen-like structures whereas neonatal and adult rat ventricular fibroblasts were unresponsive. Myofibroblasts labeled with the fluorescent cell tracker CM-DiI were injected in the viable myocardium of 3-day post-myocardial infarcted Sprague-Dawley rats and sacrificed 7 days later. Injected CM-DiI-labeled myofibroblasts were detected predominantly in the peri-infarct/infarct region, highlighting their migration to the damaged region. However, engrafted myofibroblasts in the peri-infarct/infarct region were unable to adopt an endothelial cell-like phenotype or lead to the de novo formation of CM-DiI-labeled blood vessels. The non-permissive nature of the infarct region may be attributed at least in part to the presence of growth-promoting stimuli as TGF-β and the β-adrenergic agonist isoproterenol inhibited the self assembly of lumen-like structures by myofibroblasts. Thus, when plated in a permissive in vitro environment, scar myofibroblasts can self assemble and form lumen-like structures providing an additional novel phenotype distinguishing this population from normal ventricular fibroblasts.

ProNGF, a cytokine induced after myocardial infarction in humans, targets pericytes to promote microvascular damage and activation

proNGF and p75NTR are induced following fatal myocardial infraction and are required for the development of microvascular injury.

Treatment of acute cardiac ischemia focuses on reestablishment of blood flow in coronary arteries. However, impaired microvascular perfusion damages peri-infarct tissue, despite arterial patency. Identification of cytokines that induce microvascular dysfunction would provide new targets to limit microvascular damage. Pro–nerve growth factor (NGF), the precursor of NGF, is a well characterized cytokine in the brain induced by injury. ProNGF activates p75 neurotrophin receptor (p75^NTR) and sortilin receptors to mediate proapoptotic responses. We describe induction of proNGF by cardiomyocytes, and p75^NTR in human arterioles after fatal myocardial infarction, but not with unrelated pathologies. After mouse cardiac ischemia-reperfusion (I-R) injury, rapid up-regulation of proNGF by cardiomyocytes and p75^NTR by microvascular pericytes is observed. To identify proNGF actions, we generated a mouse expressing a mutant Ngf allele with impaired processing of proNGF to mature NGF. The proNGF-expressing mouse exhibits cardiac microvascular endothelial activation, a decrease in pericyte process length, and increased vascular permeability, leading to lethal cardiomyopathy in adulthood. Deletion of p75^NTR in proNGF-expressing mice rescues the phenotype, confirming the importance of p75^NTR-expressing pericytes in the development of microvascular injury. Furthermore, deficiency in p75^NTR limits infarct size after I-R. These studies identify novel, nonneuronal actions for proNGF and suggest that proNGF represents a new target to limit microvascular dysfunction.

Extracardiac neural remodeling in humans with cardiomyopathy

BACKGROUND

Intramyocardial nerve sprouting after myocardial infarction is associated with ventricular arrhythmias. Whether human stellate ganglia remodel in association with cardiac pathology is unknown. The purpose of this study was to determine whether cardiac pathology is associated with remodeling of the stellate ganglia in humans.

METHODS AND RESULTS

Left stellate ganglia were collected from patients undergoing sympathetic denervation for intractable ventricular arrhythmias and from cadavers, along with intact hearts. Clinical data on patients and cadaveric subjects were reviewed. We classified ganglia from normal, scarred, and nonischemic cardiomyopathic hearts without scar as NL (n=3), SCAR (n=24), and NICM (n=7), respectively. Within left stellate ganglia, neuronal size, density, fibrosis, synaptic density, and nerve sprouting were determined. Nerve density and sprouting were also quantified in cadaveric hearts. Mean neuronal size in normal, scarred, and nonischemic cardiomyopathic hearts without scar groups were 320 ± 4 μm(2), 372 ± 10 μm(2), and 435 ± 10 μm(2) (P=0.002), respectively. No significant differences in neuronal density and fibrosis were present between the groups. Synaptic density in ganglia from SCAR and NICM groups were 57.8 ± 11.2 μm(2)/mm(2) (P=0.084) and 44.5 ± 7.9 μm(2)/mm(2) (P=0.039), respectively, compared with the normal group, 17.8 ± 7 μm(2)/mm(2) (overall P=0.162). There were no significant differences in left stellate ganglia nerve sprouting or myocardial nerve density between the groups.

CONCLUSIONS

Neuronal hypertrophy within left stellate ganglia is associated with chronic cardiomyopathy in humans. Ganglionic and myocardial nerve sprouting and nerve density were not significantly different. These changes may be related to increased cardiac sympathetic signaling and ventricular arrhythmias. Further studies are needed to determine the electrophysiological consequences of extracardiac neuronal remodeling in humans.

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.

Interleukin-17A increases neurite outgrowth from adult postganglionic sympathetic neurons

Inflammation can profoundly alter the structure and function of the nervous system. Interleukin (IL)-17 has been implicated in the pathogenesis of several inflammatory diseases associated with nervous system plasticity. However, the effects of IL-17 on the nervous system remain unexplored. Cell and explant culture techniques, immunohistochemistry, electrophysiology, and Ca2+ imaging were used to examine the impact of IL-17 on adult mouse sympathetic neurons. Receptors for IL-17 were present on postganglionic neurons from superior mesenteric ganglia (SMG). Supernatant from activated splenic T lymphocytes, which was abundant in IL-17, dramatically enhanced axonal length of SMG neurons. Importantly, IL-17-neutralizing antiserum abrogated the neurotrophic effect of splenocyte supernatant, and incubation of SMG neurons in IL-17 (1 ng/ml) significantly potentiated neurite outgrowth. The neurotrophic effect of IL-17 was accompanied by inhibition of voltage-dependent Ca2+ influx and was recapitulated by incubation of neurons in a blocker of N-type Ca2+ channels (ω-conotoxin GVIA; 30 nM). IL-17-induced neurite outgrowth in vitro appeared to be independent of glia, as treatment with a glial toxin (AraC; 5 μM) did not affect the outgrowth response to IL-17. Moreover, application of the cytokine to distal axons devoid of glial processes enhanced neurite extension. An inhibitor of the NF-κB pathway (SC-514; 20 μM) blocked the effects of IL-17. These data represent the first evidence that IL-17 can act on sympathetic somata and distal neurites to enhance neurite outgrowth, and identify a novel potential role for IL-17 in the neuroanatomical plasticity that accompanies inflammation.

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 expression level of ecto-NTP diphosphohydrolase1/CD39 modulates exocytotic and ischemic release of neurotransmitters in a cellular model of sympathetic neurons

Once released, norepinephrine is removed from cardiac synapses via reuptake into sympathetic nerves, whereas transmitter ATP is catabolized by ecto-NTP diphosphohydrolase 1 (E-NTPDase1)/CD39, an ecto-ATPase. Because ATP is known to modulate neurotransmitter release at prejunctional sites, we questioned whether this action may be ultimately controlled by the expression of E-NTPDase1/CD39 at sympathetic nerve terminals. Accordingly, we silenced E-NTPDase1/CD39 expression in nerve growth factor-differentiated PC12 cells, a cellular model of sympathetic neuron, in which dopamine is the predominant catecholamine. We report that E-NTPDase1/CD39 deletion markedly increases depolarization-induced exocytosis of ATP and dopamine and increases ATP-induced dopamine release. Moreover, overexpression of E-NTPDase1/CD39 resulted in enhanced removal of exogenous ATP, a marked decrease in exocytosis of ATP and dopamine, and a large decrease in ATP-induced dopamine release. Administration of a recombinant form of E-NTPDase1/CD39 reproduced the effects of E-NTPDase1/CD39 overexpression. Exposure of PC12 cells to simulated ischemia elicited a release of ATP and dopamine that was markedly increased in E-NTPDase1/CD39-silenced cells and decreased in E-NTPDase1/CD39-overexpressing cells. Therefore, transmitter ATP acts in an autocrine manner to promote its own release and that of dopamine, an action that is controlled by the level of E-NTPDase1/CD39 expression. Because ATP availability greatly increases in myocardial ischemia, recombinant E-NTPDase1/CD39 therapeutically used may offer a novel approach to reduce cardiac dysfunctions caused by excessive catecholamine release.

ß-adrenoceptor blockers increase cardiac sympathetic innervation by inhibiting autoreceptor suppression of axon growth

β-Adrenoceptor antagonists are used widely to reduce cardiovascular sympathetic tone, but withdrawal is accompanied by sympathetic hyperactivity. Receptor supersensitivity accounts for some but not all aspects of this withdrawal syndrome. Therefore, we investigated effects of β-blockers on sympathetic innervation. Rats received infusions of adrenergic receptor blockers or saline for 1 week. The nonselective β-blocker propranolol and the β(1)-antagonist metoprolol both increased myocardial sympathetic axon density. At 2 d after propranolol discontinuation, β-receptor sensitivity and responsiveness to isoproterenol were similar to controls. However, tyramine-induced mobilization of norepinephrine stores produced elevated ventricular contractility consistent with enhanced sympathetic neuroeffector properties. In addition, rats undergoing discontinuation showed exaggerated increases in mean arterial pressure in response to air puff or noise startle. In sympathetic neuronal cell cultures, both propranolol and metoprolol increased axon outgrowth but the β(2)-blocker ICI 118551 did not. Norepinephrine synthesis suppression by α-methyl-p-tyrosine also increased sprouting and concurrent dobutamine administration reduced it, confirming that locally synthesized norepinephrine inhibits outgrowth via β(1)-adrenoceptors. Immunohistochemistry revealed β(1)-adrenoceptor protein on sympathetic axon terminations. In rats with coronary artery ligation, propranolol reversed heart failure-induced ventricular myocardial sympathetic axon depletion, but did not affect infarct-associated sympathetic hyperinnervation. We conclude that sympathetic neurons possess β(1)-autoreceptors that negatively regulate axon outgrowth. Chronic β-adrenoceptor blockade disrupts this feedback system, leading to ventricular sympathetic axon proliferation and increased neuroeffector gain, which are likely to contribute to β-blocker withdrawal syndrome.

Sodium depletion increases sympathetic neurite outgrowth and expression of a novel TMEM35 gene-derived protein (TUF1) in the rat adrenal zona glomerulosa

The adrenal zona glomerulosa (ZG) secretes aldosterone to regulate sodium balance. Chronic sodium restriction increases aldosterone accompanied by ZG expansion. The ZG is innervated by sympathetic, vasoactive intestinal polypeptide (VIP) and neuropeptide tyrosine (NPY), and sensory, calcitonin gene-related peptide, nerves. It is unclear whether innervation is affected by ZG growth. Therefore, we measured neurite outgrowth in the ZG of adult male rats after dietary sodium manipulation. In response to 1 wk sodium restriction, VIP and NPY fibers elongated in parallel with expansion of the ZG, shown by aldosterone synthase (AS) expression, but calcitonin gene-related peptide fibers were not affected. Sodium repletion resulted in parallel regression in VIP and NPY fiber length and AS expression. These results show that sympathetic, but not sensory, innervation is coordinated with ZG growth. Mediators underlying changes in innervation are unknown; therefore, we characterized a novel gene TMEM35 [termed the unknown factor-1 (TUF1) due to its unknown function] that shows extensive overlap with AS in ZG. After sodium restriction, TUF1 expanded in parallel with the ZG. TUF1 bound the low-affinity neurotrophin receptor, p75NTR, which was expressed in NPY fibers and showed a response similar to TUF1 after sodium manipulation. TUF1- p75NTR binding was competitively displaced by nerve growth factor but not by TUF1 lacking the p75NTR binding motif. Moreover, TUF1 mRNA in rat ZG cells increased after angiotensin II exposure in vitro. Collectively, these findings suggest that TMEM35/TUF1 is a candidate for modulating neurite outgrowth in the ZG after sodium depletion.

Favorable effects of resveratrol on sympathetic neural remodeling in rats following myocardial infarction

Oxidative stress and inflammatory response induced by myocardial infarction play important roles in the development of sympathetic neural remodeling. The present study was designed to investigate whether resveratrol can improve sympathetic neural remodeling and hence cause less arrhythmias via its anti-oxidant and anti-inflammatory effects. Male Sprague Dawley rats were randomly assigned to either vehicle or resveratrol (1 mg/kg) treatment for 4 weeks post myocardial infarction. Another group of sham operated rats served as controls. Cardiac electrophysiology examination was performed to evaluate the severity of ventricular arrhythmias. Sympathetic neural remodeling characterized by heterogeneous nerve sprouting and sympathetic hyperinnervation was assessed by immunohistochemistry study. Western blotting and ELISA were used to evaluate inflammatory responses and oxidative stress was also quantified. Resveratrol treatment resulted in less episodes of inducible ventricular arrhythmias which was closely associated with attenuated sympathetic neural remodeling (P<0.001, respectively). Decreased nerve growth factor (NGF) expression was also observed in resveratrol treated rats in the peri-infarct area at 4 weeks after myocardial infarction (P<0.001). Interestingly, beneficial effects of resveratrol were also associated with less inflammatory responses and oxidative stress. Our data indicated that resveratrol can suppress sympathetic neural remodeling process after myocardial infarction via attenuated inflammatory responses and oxidative stress, which in turn leads to less inducibility of ventricular arrhythmias.

Increased sympathoexcitatory reflex induced by myocardial ischemic nociceptive signaling via P2X2/3 receptor in rat superior cervical ganglia

The restructuring of cardiac innervation after myocardial ischemic injury, which is often associated with an increased sympathoexcitatory reflex characterized by an increase in blood pressure and sympathetic nerve activity. This study observed whether P2X(2/3) receptor in rat superior cervical ganglion (SCG) played a role in the increased sympathoexcitatory reflex induced by myocardial ischemic nociceptive signaling. The findings showed that the systolic blood pressure, heart rate and respiration in the myocardial ischemic rats were higher than those in control rats. The content of adenosine 5'-triphosphate (ATP) from myocardial ischemic group was higher than that from control group. After myocardial ischemic rats were treated with selective P2X(2/3) receptor antagonist A-317491, the content of ATP in SCG was reduced. Coexpression value of P2X(2), P2X(3) and tyrosine hydroxylase (TH) in the SCG and myocardial tissues of myocardial ischemic group was increased significantly, compared with that in the SCG and myocardial tissues of control group. The expression value of P2X(2), P2X(3) and TH in the SCG and myocardial tissues of myocardial ischemic rats treated with A-317491 was decreased. The amplitude of the currents was much larger in myocardial ischemic group than that obtained in control group after administration of ATP with the same concentration. After the myocardial ischemic rats were treated with A-317491, ATP-activated currents were reduced. The myocardial ischemic injury induced an increase in the expression of P2X(2/3) receptor colocalization with TH and a hypersensitivity state of SCG neurons to ATP, which led to the increased sympathoexcitatory reflex.

Mechanical stretch induces nerve sprouting in rat sympathetic neurocytes

Sympathetic nerve sprouting (SNS) has been shown to occur after myocardial infarction (MI) and heart failure (HF) and is known to be responsible for the development of lethal arrhythmias. During MI or HF intracardiac cells are exposed to increased mechanical stretch. Molecular mechanisms which trigger sympathetic neural growth are largely unknown. Therefore, this study aimed to investigate the impact of mechanical stretch on rat neonatal sympathetic neurocytes of the superior cervical ganglion (SCG). Mechanical stretch resulted in an increased growth of sympathetic neurocytes. Furthermore, we could demonstrate that SCG neurocytes express nerve growth factor (NGF), ciliary neurotrophic factor (CNTF), neurotrophin-3 (NT-3) and glial derived neurotrophic factor (GDNF) on mRNA and protein level. An increased NGF and CNTF expression, a down-regulated GDNF expression and an unchanged NT-3 expression were identified in the neurocyte cell culture supernatant of neurocytes exposed to mechanical stretch. However, neither brain derived neurotrophic factor (BDNF) mRNA and protein was expressed in SCG neurocytes, nor BDNF could be detected in the cell culture supernatant of SCG neurons. By anti-neurotrophin neutralizing experiments NGF and CNTF were identified as important stretch-induced growth-inducing factors. Losartan, an angiotensin-II type 1 receptor inhibitor, abolished the stretch-induced increase of NGF and CNTF expression and thereby prevented the stretch-induced neural growth. This study provides new molecular mechanisms by which the inhibitory effect of angiotensin-II type 1 receptor blockers on the neural/arrhythmogenic remodeling can be explained. However, further in-vivo studies are required to address this important issue.

Neurotrophins and target interactions in the development and regulation of sympathetic neuron electrical and synaptic properties

The electrical and synaptic properties of neurons are essential for determining the function of the nervous system. Thus, understanding the mechanisms that control the appropriate developmental acquisition and maintenance of these properties is a critical problem in neuroscience. A great deal of our understanding of these developmental mechanisms comes from studies of soluble growth factor signaling between cells in the peripheral nervous system. The sympathetic nervous system has provided a model for studying the role of these factors both in early development and in the establishment of mature properties. In particular, neurotrophins produced by the targets of sympathetic innervation regulate the synaptic and electrophysiological properties of postnatal sympathetic neurons. In this review we examine the role of neurotrophin signaling in the regulation of synaptic strength, neurotransmitter phenotype, voltage-gated currents and repetitive firing properties of sympathetic neurons. Together, these properties determine the level of sympathetic drive to target organs such as the heart. Changes in this sympathetic drive, which may be linked to dysfunctions in neurotrophin signaling, are associated with devastating diseases such as high blood pressure, arrhythmias and heart attack. Neurotrophins appear to play similar roles in modulating the synaptic and electrical properties of other peripheral and central neuronal systems, suggesting that information provided from studies in the sympathetic nervous system will be widely applicable for understanding the neurotrophic regulation of neuronal function in other systems.