The autonomic nervous system tightly regulates cardiac function in health and disease. Glia cells are indispensable for normal neuronal function in the central and peripheral nervous system, but their role in the heart is vastly understudied. Therefore, we aimed to characterize glia cells of the heart and its autonomic structures in mice and men.  
 

We studied three transgenic mouse lines expressing tdTomato under the control of different glia-specific promoters (Sox10-Cre:tdTomato, P0/MPZ Cre:tdTomato and inducible PLP1-CreERT:tdTomato mice 5 days after one injection of tamoxifen, n=18 animals, aged 5-26 weeks). Hearts, cardiac sympathetic and parasympathetic ganglia were isolated and native transgene expression systematically studied. Immunohistochemical co-staining was performed in whole mounts and vibratome sections using cardiomyocyte (Troponin T, Trop), sympathetic (Tyrosine Hydroxylase, TH) and parasympathetic (Choline Acetyltransferase, ChAT) nerve markers. Glial gene expression was studied via single cell RNA sequencing in left ventricular tissue of healthy wildtype mice. The presence of glia cells was confirmed by immunohistochemistry in human left ventricular tissue. As we have shown previously that glia cells react to nerve damage in the atria with release of the neurotrophic factor S100B, we characterized ventricular myocardial nerve damage induced by epicardial cryoablation in Langendorff-perfused wildtype mouse hearts (n=14) and in plasma of patients receiving catheter ablation in the left ventricle (n=19). 

TdTomato-expression was detected in all three transgenic mouse lines accompanying cardiac autonomic structures: (1) in satellite cells in parasympathetic and sympathetic ganglia (2) accompanying large endo- and epicardial nerve fibers, (3) in form of a very delicate network of cells on the atrial and ventricular epicardium and (4) distributed throughout the left and right ventricular myocardium.  Co-staining revealed terminal Schwann cells at the sympathetic neuro-cardiac junction where nerve endings converge onto cardiomyocytes. RNAsequencing confirmed the expression of glial genes in the healthy heart (e.g. Cnp, Cspg4, Gdnf, Mbp, S100b, Sox10, Plp1). Immunohistochemistry in human left ventricular heart tissue confirmed the presence of glial cells. In line with this, damage of the innervated left ventricular myocardium results in release of S100B in murine wildtype hearts (5.2±2.2 to 23.0±5.9, P=0.0030). S100B release was also detectable in patients receiving catheter ablation of the left ventricle (45.5±7.4 vs 85.7±14.0 pg/ml; P=0.0011). 
 

Glia cells accompany cardiac autonomic structures up to the smallest unit and release neurotrophic factors upon myocardial nerve damage in mice and men. Still, they are a completely overlooked cell population. Whether and how cardiac glia contribute to autonomic regulation or even regeneration upon cardiac nerve damage needs to be determined.