A study in mice mapped two specialized populations of nerve cells in the heart, revealing how one supports daily heart function and the other helps prevent electrical collapse during severe stress.
Research: The intrinsic cardiac nervous system is essential to heart function and survival
Recent Mouse Research Published in the Journal cell suggest that the intrinsic cardiac nervous system (ICNS) is essential for the proper functioning of the mouse heart. Researchers integrated genetic, imaging, and cell-specific neuromodulatory studies in adult mice to identify two distinct types of intrinsic cardiac neurons (ICNs). One is important for regulating heart rate and supporting coronary perfusion, and the other is important for maintaining electrical stability under extreme stress. Further mechanistic studies of ICNS structure and function may inform future cell type-targeted neuromodulatory therapies, but the relevance of these findings to humans remains unclear.
The ICNS is an important node in the communication between the heart and the brain and is involved in several cardiac diseases. However, its functional organization remains poorly understood as most studies mainly focus on the extrinsic autonomic circuit. Currently used therapies, such as radiofrequency ablation, electrical stimulation, and cryoablation, broadly target components of the cardiac nervous system without cell type specificity and often have mixed results. A better understanding of ICNS could help researchers design more targeted treatments in the future, but its clinical benefit in humans has yet to be tested.
About research
In this study, the researchers integrated genetic studies and imaging in adult mice to investigate the role of ICNS in cardiac function. They conducted single-cell RNA sequencing (scRNA-seq), gene labeling studies, high-resolution volumetric imaging, and cell-specific neuromodulatory studies to dissect the complex ICNS framework. To characterize ICN subtypes, the research team performed adeno-associated virus (AAV)-based genetic labeling to map neuronal morphology and cell type-specific innervation patterns.
Researchers have developed a genetically engineered mouse model to selectively label, target, and manipulate specific cell types within the ICNS. They performed fluorescence-activated cell sorting (FACS) to purify green fluorescent protein-labeled (GFP) ICNs from anterior, posterior, and mixed cardiac regions. RNAscope analysis validated the single-cell findings and confirmed the in situ distribution.
The research team identified molecularly distinct ICN subtypes. These populations received different inputs from extrinsic circuits and innervated different regions of the cardiac system to perform different functions. They also conducted diphtheria toxin (DT)-mediated ablation experiments and evaluated its efficacy. Single-cell RNA sequencing of 2,870 ICNs identified four transcriptional clusters visualized using uniform manifold approximation and projection (UMAP), with Npy-positive cells and Ddah1-positive cells comprising the two major molecular classes. They also exposed the animals to physical restraint, sustained heat exposure, and sympathetic hyperactivation with epinephrine and caffeine to study ICNS function under stressful conditions.
result
The ICNS is located at the interface between external autonomic inputs and cardiac cells, and is positioned to integrate local and central signals. Through genetic labeling, the researchers identified GFP-positive ICNs located across several anterior and posterior ganglia in the atrium. Most ICNs were cholinergic. The research team discovered that neurons expressing neuropeptide Y (Npy) and dimethylarginine dimethylaminohydrolase 1 (Ddah1) are the two main molecular subtypes of ICNS. These ICNs differed in their neural inputs and cardiac projections. Npy-positive ICNs preferentially received vagal input and supported coronary perfusion by regulating heart rate through the parasympathetic nervous system and modulating aortic root mechanics. Removal of Npy-positive ICN caused lethal heart failure in mice.
Ddah1-positive ICNs, which are abundant in the posterior ganglia, receive sympathetic input and were found to be important for maintaining electrical stability of the heart under extreme physiological or psychological stress. Ablation increased susceptibility to malignant arrhythmias and sudden cardiac arrest. Activation of these ICNs exerted a cardioprotective function by improving survival during epinephrine-caffeine-induced sympathetic hyperactivation in mice.
Although Ddah1-positive neurons frequently coexpressed dopamine beta hydroxylase (Dbh), this molecular signature did not establish classical sympathetic identity. Separate monosynaptic tracings showed that Ddah1-positive ICNs receive direct sympathetic input. Npy-positive neurons frequently express prostaglandin E receptor 3 (PTGER3), representing distinct molecular subtypes whose functional relevance has not been tested. Npy-positive ICNs most closely resemble intestinal inhibitory motor neurons at the transcriptomic level, but their precise downstream effects have not been tested. Ddah1-positive ICNs shared transcriptomic features with specific sensory and interneuron-like populations in the intestine, but their sensory modalities and circuit functions remain unknown.
Npy-positive ICN projections extensively innervated the heart, reaching the atrial appendages, ventricular tissue, apex of the heart, and pulmonary vein regions. Npy-positive fibers were targeted to the atrioventricular (AV) and sinoatrial (SA) nodes, suggesting that these ICNs regulate cardiac rhythm. They also closely surround the root of the aorta, suggesting a role in regulating cardiac outflow. In contrast, Ddah1-positive ICNs showed limited innervation confined to left atrial tissue, pulmonary veins, and pulmonary arteries.
Specifically, these ICNs targeted the opening where the pulmonary veins enter the left atrium, an area associated with atrial fibrillation and arrhythmias. ICNs expressing Ddah1 also showed relatively sparse local projections, but a more extensive network of nerve fibers connecting ICN ganglia. This finding is consistent with spatially limited communication between these ICNs and cardiac targets, but also suggests a potential role in local circuit integration.
conclusion
The findings indicate that the ICNS neural network is important in regulating cardiac function in mice and may ultimately help guide neuromodulation targeting more precise cell types, but therapeutic efficacy in humans has not been established. Future studies in mice should use single neuron tracking and cell-specific functional mapping to determine whether individual Npy-positive neurons are multifunctional or whether the Npy-positive population constitutes a specialized subgroup. Future studies should also reveal the precise mode of action, sensory modality, and downstream targets of Ddah1-positive ICNs, and whether the molecular and functional organization of these ICNs is conserved in the human heart.
