Stimulating ideas for heart regeneration: the future of nerve-directed heart therapy

Optogenetic Stimulation of the Cardiac Vagus Nerve to Promote Heart Regenerative Repair after Myocardial Infarction

Background: It had been shown that selective cardiac vagal activation holds great potential for heart regeneration. Optogenetics has clinical translation potential as a novel means of modulating targeted neurons. This study aimed to investigate whether cardiac vagal activation via optogenetics could improve heart regenerative repair after myocardial infarction (MI) and to identify the underlying mechanism. Methods: We used an adeno-associated virus (AAV) as the vector to deliver ChR2, a light-sensitive protein, to the left nodose ganglion (LNG). To assess the effects of the cardiac vagus nerve on cardiomyocyte (CM) proliferation and myocardial regeneration in vivo, the light-emitting diode illumination (470 nm) was applied for optogenetic stimulation to perform the gain-of-function experiment and the vagotomy was used as a loss-of-function assay. Finally, sequencing data and molecular biology experiments were analyzed to determine the possible mechanisms by which the cardiac vagus nerve affects myocardial regenerative repair after MI. Results: Absence of cardiac surface vagus nerve after MI was more common in adult hearts with low proliferative capacity, causing a poor prognosis. Gain- and loss-of-function experiments further demonstrated that optogenetic stimulation of the cardiac vagus nerve positively regulated cardiomyocyte (CM) proliferation and myocardial regeneration in vivo. More importantly, optogenetic stimulation attenuated ventricular remodeling and improved cardiac function after MI. Further analysis of sequencing results and flow cytometry revealed that cardiac vagal stimulation activated the IL-10/STAT3 pathway and promoted the polarization of cardiac macrophages to the M2 type, resulting in beneficial cardiac regenerative repair after MI. Conclusions: Targeting the cardiac vagus nerve by optogenetic stimulation induced macrophage M2 polarization by activating the IL-10/STAT3 signaling pathway, which obviously optimized the regenerative microenvironment and then improved cardiac function after MI.

Parasympathetic and sympathetic axons are bundled in the cardiac ventricles and undergo physiological reinnervation during heart regeneration

Sympathetic innervation influences homeostasis, repair, and pathology in the cardiac ventricles; in contrast, parasympathetic innervation is considered to have minimal contribution and influence in the ventricles. Here, we use genetic models, whole-mount imaging, and three-dimensional modeling to define cardiac nerve architecture during development, disease, and regeneration. Our approach reveals that parasympathetic nerves extensively innervate the cardiac ventricles. Furthermore, we identify that parasympathetic and sympathetic axons develop synchronously and are bundled throughout the ventricles. We further investigate cardiac nerve remodeling in the regenerative neonatal and the non-regenerative postnatal mouse heart. Our results show that the regenerating myocardium undergoes a unique process of physiological reinnervation, where proper nerve distribution and architecture is reestablished, in stark contrast to the non-regenerating heart. Mechanistically, we demonstrate that physiological reinnervation during regeneration is dependent on collateral artery formation. Our results reveal clinically significant insights into cardiac nerve plasticity which can identify new therapies for cardiac disease.

Research progress in the cardiac lymphatic system and myocardial repair after myocardial infarction

The lymphatic system of the heart plays an important role in the repair process after myocardial injury and may regulate normal tissue homeostasis and natural regeneration via maintaining fluid homeostasis and controlling the inflammatory response. The lymphatic system in the heart is activated after myocardial injury and is involved in the scarring process of the heart. Recent studies on the lymphatic system and myocardial repair of the heart have developed rapidly, and the mechanisms for lymphangiogenesis and lymphatic endothelial cell secretion have been elucidated by different animal models. A deep understanding of the structural, molecular, and functional characteristics of the lymphatic system of the heart can help develop therapies that target the lymphatic system in the heart. Summarizing the progress in studies on targets related to myocardial repair and the cardiac lymphatic system is helpful to provide potential new targets and strategies for myocardial repair therapy after myocardial infarction.

Heart development and regeneration-a multi-organ effort

Development of the heart, from early morphogenesis to functional maturation, as well as maintenance of its homeostasis are tasks requiring collaborative efforts of cardiac tissue and different extra-cardiac organ systems. The brain, lymphoid organs, and gut are among the interaction partners that can communicate with the heart through a wide array of paracrine signals acting at local or systemic level. Disturbance of cardiac homeostasis following ischemic injury also needs immediate response from these distant organs. Our hearts replace dead muscles with non-contractile fibrotic scars. We have learned from animal models capable of scarless repair that regenerative capability of the heart does not depend only on competency of the myocardium and cardiac-intrinsic factors but also on long-range molecular signals originating in other parts of the body. Here, we provide an overview of inter-organ signals that take part in development and regeneration of the heart. We highlight recent findings and remaining questions. Finally, we discuss the potential of inter-organ modulatory approaches for possible therapeutic use.

Defining Cardiac Nerve Architecture During Development, Disease, and Regeneration

Cardiac nerves regulate neonatal mouse heart regeneration and are susceptible to pathological remodeling following adult injury. Understanding cardiac nerve remodeling can lead to new strategies to promote cardiac repair. Our current understanding of cardiac nerve architecture has been limited to two-dimensional analysis. Here, we use genetic models, whole-mount imaging, and three-dimensional modeling tools to define cardiac nerve architecture and neurovascular association during development, disease, and regeneration. Our results demonstrate that cardiac nerves sequentially associate with coronary veins and arteries during development. Remarkably, our results reveal that parasympathetic nerves densely innervate the ventricles. Furthermore, parasympathetic and sympathetic nerves develop synchronously and are intertwined throughout the ventricles. Importantly, the regenerating myocardium reestablishes physiological innervation, in stark contrast to the non-regenerating heart. Mechanistically, reinnervation during regeneration is dependent on collateral artery formation. Our results reveal how defining cardiac nerve remodeling during homeostasis, disease, and regeneration can identify new therapies for cardiac disease.

Validation and Reliability of a Novel Vagus Nerve Neurodynamic Test and Its Effects on Heart Rate in Healthy Subjects: Little Differences Between Sexes

Background

The vagus nerve (VN), also called the pneumogastric nerve, connects the brainstem to organs contained in the chest and abdomen. Physiologically, VN stimulation can rapidly affect cardiac activity and heart rate (HR). VN neuropathy can increase the risk of arrhythmias and sudden death. Therefore, a selective test of VN function may be very useful. Since peripheral neurodynamic tests (NDT) are reliable for the assessment of neuropathies in somatic nerves, we aimed to validate a novel NDT to assess VN activity, namely, the VN-NTD.

Methods

In this cross-sectional double-blind, sex-balanced study, 30 participants (15 females) completed a checklist of autonomic dysfunction symptoms. During the VN-NDT administration, HR and symptoms (i.e., mechanical allodynia) were monitored in parallel to a real-time ultrasonography imaging (USI) and motion capture analysis of the neck. The VN-NDT impact on HR and its accuracy for autonomic symptoms reported in the last 7 days were tested.

Results

The VN-NDT induced a significant HR reduction of about 12 and 8 bpm in males and females [t(1, 119) = 2.425; p < 0.017; η_p² = 0.047, 95% confidence interval (CI): 0.93–9.18], respectively. No adverse events were observed during VN-NDT. A substantial interexaminer agreement between the evaluators in symptoms induction by VN-NDT was detected [F(1, 119) = 0.540; p = 0.464; η_p² = 0.005, low effect]. Notably, mechanical allodynia accuracy for gastrointestinal dysfunctions was excellent (p < 0.05; 95% CI: 0.52–0.73; p < 0.001; 95% CI: 0.81–0.96).

Conclusions

The novel VN-NDT is a valid and accurate test capable of detecting VN activation with high sensitivity. Data provided are suitable for both sexes as a hallmark of HR variation due to VN normal response. The proposed VN-NDT may be reliable as daily routine neurological examination tests for the evaluation of neuropathic signs related to neuroinflammation of the VN.

Clinical Trial Registration

www.ClinicalTrials.gov, identifier NCT04192877.

Heart regeneration: beyond new muscle and vessels

The most striking consequence of a heart attack is the loss of billions of heart muscle cells, alongside damage to the associated vasculature. The lost cardiovascular tissue is replaced by scar formation, which is non-functional and results in pathological remodelling of the heart and ultimately heart failure. It is, therefore, unsurprising that the heart regeneration field has centred efforts to generate new muscle and blood vessels through targeting cardiomyocyte proliferation and angiogenesis following injury. However, combined insights from embryological studies and regenerative models, alongside the adoption of -omics technology, highlight the extensive heterogeneity of cell types within the forming or re-forming heart and the significant crosstalk arising from non-muscle and non-vessel cells. In this review, we focus on the roles of fibroblasts, immune, conduction system, and nervous system cell populations during heart development and we consider the latest evidence supporting a function for these diverse lineages in contributing to regeneration following heart injury. We suggest that the emerging picture of neurologically, immunologically, and electrically coupled cell function calls for a wider-ranging combinatorial approach to heart regeneration.

Large Animal Models of Cell-Free Cardiac Regeneration

The adult mammalian heart lacks the ability to sufficiently regenerate itself, leading to the progressive deterioration of function and heart failure after ischemic injuries such as myocardial infarction. Thus far, cell-based therapies have delivered unsatisfactory results, prompting the search for cell-free alternatives that can induce the heart to repair itself through cardiomyocyte proliferation, angiogenesis, and advantageous remodeling. Large animal models are an invaluable step toward translating basic research into clinical applications. In this review, we give an overview of the state-of-the-art in cell-free cardiac regeneration therapies that have been tested in large animal models, mainly pigs. Cell-free cardiac regeneration therapies involve stem cell secretome- and extracellular vesicles (including exosomes)-induced cardiac repair, RNA-based therapies, mainly regarding microRNAs, but also modified mRNA (modRNA) as well as other molecules including growth factors and extracellular matrix components. Various methods for the delivery of regenerative substances are used, including adenoviral vectors (AAVs), microencapsulation, and microparticles. Physical stimulation methods and direct cardiac reprogramming approaches are also discussed.