The Heart’s Pacemaker Mimics Brain Cytoarchitecture and Function: Autonomic innervation, a peripheral glial cell web, and a novel S100B expressing interstitial cell type impart structural and functional complexity to the sinoatrial node

Abstract

Objectives: This study sought to describe the 3D cytoarchitecture of sinoatrial node tissue, in-cluding autonomic innervation, peripheral glial cells, and pacemaker cells. Background: The sinoatrial node of the heart produces rhythmic action potentials (AP), gener-ated via calcium signaling within and among pacemaker cells. Our previous work has described the SAN as composed of an HCN4-expressing pacemaker cell meshwork, which merges with a network of CNX43+/F-actin+ cells. It is also known that sympathetic and parasympathetic in-nervation from epicardial ganglia create an autonomic plexus in the sinoatrial node, which modulates heart rate and rhythm. However, the anatomical details of the interaction of this plexus with the pacemaker cell meshwork have yet to be described. Methods: 3D confocal laser-scanning microscopy of triple immunolabeled SAN whole mount preparations with combinations of antibodies for HCN4, S100B, GFAP, ChAT+ or VAChT+, and TH, and transfer electron microscopy (TEM). Results: The SAN exhibited heterogeneous autonomic innervation, which was accompanied by a web of peripheral glial cells (PGCs). Further, we identified a novel S100B+/GFAP- interstitial cell population, with unique morphology and distinct distribution pattern, creating complex in-teractions with other cell types in the node. TEM images showed a similar population of cells, here identified as telocytes, which appeared to secrete vesicles towards pacemaker cells. Appli-cation of S100B protein to SAN preparations induced distinct changes in rhythmogenic calcium signaling. Conclusions: The autonomic plexus and its associated peripheral glial cell web, a novel network of S100B expressing interstitial cells resembling telocytes, and a meshwork of HCN4+ cells interact to impart structural complexity to the sinoatrial node.