Heterogeneous cardiac sympathetic innervation gradients promote arrhythmogenesis in murine dilated cardiomyopathy

The role of the axonal guidance cue Semaphorin3A in innervation of the postnatal heart in health and disease

During cardiac development the heart is innervated by the autonomous nervous system. After development, neurons of the autonomic nervous system have limited capacity for growth and regeneration. However, in the past decades, it has become clear that cardiac nerves can regenerate after cardiac damage. An excessive amount of re-innervation, so-called sympathetic hyperinnervation, may render patients vulnerable to ventricular arrhythmias and heart failure. Several studies have investigated axonal guidance cues as mediators of cardiac innervation. Axonal guidance cues direct neuronal growth of the axon and play a significant role in the regeneration and remodelling of cardiac autonomic innervation after cardiac damage. This review focusses on current literature regarding the axonal guidance cue group of semaphorins and their function in the healthy and diseased postnatal heart. In light of cardiac innervation, most studies focus on semaphorin 3A (SEMA3A), whereas less is known about the function of the other semaphorin classes. SEMA3A is a neuronal repellent and is associated with a decrease in the density of sympathetic neurons in the heart. Its decline in expression after myocardial infarction plays a role in the development of sympathetic hyperinnervation and the subsequent increased risk of ventricular arrhythmias. In congestive heart failure the opposite occurs: an increase in SEMA3A expression underlies decreased nerve density that may also serve as a substrate for ventricular arrhythmias. Although literature on their role in cardiac innervation is still relatively scarce, semaphorins, in particular SEMA3A, seem relevant candidates to consider when exploring options to modulate pathological innervation patterns in cardiovascular disease.

Hypertension-induced heart failure disrupts cardiac sympathetic innervation

About 26 million people worldwide live with heart failure (HF), and hypertension is the primary cause in 25% of these cases. Autonomic dysfunction and sympathetic hyperactivity accompany cardiovascular diseases, including HF. However, changes in cardiac sympathetic innervation in HF are not well understood. We hypothesized that cardiac sympathetic innervation is disrupted in hypertension-induced HF. Male and female C57BL6/J mice were infused with angiotensin II (ANG II) for 4 wk to generate hypertension leading to HF; controls were infused with saline. ANG II-treated mice displayed HF phenotype, including reduced cardiac function, hypertrophy, and fibrosis. ANG II-treated mice also had significantly reduced sympathetic nerve density in the left ventricle, intraventricular septum, and right ventricle. In the left ventricle, the subepicardium remained normally innervated, whereas the subendocardium was almost devoid of sympathetic nerves. Loss of sympathetic fibers led to loss of norepinephrine content in the left ventricle. Several potential triggers for axon degeneration were tested and ruled out. ANG II-treated mice had increased premature ventricular contractions after isoproterenol and caffeine injection. Although HF can induce a cholinergic phenotype and neuronal hypertrophy in stellate ganglia, ANG II treatment did not induce a cholinergic phenotype or activation of trophic factors in this study. Cardiac neurons in the left stellate ganglion were significantly smaller in ANG II-treated mice, whereas neurons in the right stellate were unchanged. Our findings show that ANG II-induced HF disrupts sympathetic innervation, particularly in the left ventricle. Further investigations are imperative to unveil the mechanisms of denervation in HF and to develop neuromodulatory therapies for patients with autonomic imbalance.NEW & NOTEWORTHY Angiotensin II (ANG II)-induced hypertension leads to a heart failure phenotype and cardiac sympathetic denervation with the endocardial region of the left ventricle being the most affected. Denervation is accompanied by loss of norepinephrine content in the left ventricle and increased premature ventricular contractions (PVCs) after isoproterenol and caffeine injection. ANG II treatment also causes morphological changes in cardiac-projecting left stellate ganglion neurons.

Investigating the Interplay between Cardiovascular and Neurodegenerative Disease

Neurological diseases, including neurodegenerative diseases (NDDs), are the primary cause of disability worldwide and the second leading cause of death. The chronic nature of these conditions and the lack of disease-modifying therapies highlight the urgent need for developing effective therapies. To accomplish this, effective models of NDDs are required to increase our understanding of underlying pathophysiology and for evaluating treatment efficacy. Traditionally, models of NDDs have focused on the central nervous system (CNS). However, evidence points to a relationship between systemic factors and the development of NDDs. Cardiovascular disease and related risk factors have been shown to modify the cerebral vasculature and the risk of developing Alzheimer's disease. These findings, combined with reports of changes to vascular density and blood-brain barrier integrity in other NDDs, such as Huntington's disease and Parkinson's disease, suggest that cardiovascular health may be predictive of brain function. To evaluate this, we explore evidence for disruptions to the circulatory system in murine models of NDDs, evidence of disruptions to the CNS in cardiovascular disease models and summarize models combining cardiovascular disruption with models of NDDs. In this study, we aim to increase our understanding of cardiovascular disease and neurodegeneration interactions across multiple disease states and evaluate the utility of combining model systems.

Sympathetic remodeling and altered angiotensin-converting enzyme 2 localization occur in patients with cardiac disease but are not exacerbated by severe COVID-19

PURPOSE

Remodeling of sympathetic nerves and ACE2 has been implicated in cardiac pathology, and ACE2 also serves as a receptor for SARS-CoV-2. However, there is limited histological knowledge about the transmural distribution of sympathetic nerves and the cellular localization and distribution of ACE2 in human left ventricles from normal or diseased hearts. Goals of this study were to establish the normal pattern for these parameters and determine changes that occurred in decedents with cardiovascular disease alone compared to those with cardiac pathology and severe COVID-19.

METHODS

We performed immunohistochemical analysis on sections of left ventricular wall from twenty autopsied human hearts consisting of a control group, a cardiovascular disease group, and COVID-19 ARDS, and COVID-19 non-ARDS groups.

RESULTS

Using tyrosine hydroxylase as a noradrenergic marker, we found substantial sympathetic nerve loss in cardiovascular disease samples compared to controls. Additionally, we found heterogeneous nerve loss in both COVID-19 groups. Using an ACE2 antibody, we observed robust transmural staining localized to pericytes in the control group. The cardiovascular disease hearts displayed regional loss of ACE2 in pericytes and regional increases in staining of cardiomyocytes for ACE2. Similar changes were observed in both COVID-19 groups.

CONCLUSIONS

Heterogeneity of sympathetic innervation, which occurs in cardiac disease and is not increased by severe COVID-19, could contribute to arrhythmogenesis. The dominant localization of ACE2 to pericytes suggests that these cells would be the primary target for potential cardiac infection by SARS-CoV-2. Regional changes in ACE2 staining by myocytes and pericytes could have complex effects on cardiac pathophysiology.