The Heart's Pacemaker Mimics Brain Cytoarchitecture and Function: Novel Interstitial Cells Expose Complexity of the SAN.

Background: The sinoatrial node (SAN) of the heart produces rhythmic action potentials, generated via calcium signaling within and among pacemaker cells. Our previous work has described the SAN as composed of a hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 (HCN4)-expressing pacemaker cell meshwork, which merges with a network of connexin 43 ⁺ /F-actin ⁺ cells. It is also known that sympathetic and parasympathetic innervation create an autonomic plexus in the SAN that 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.
Objectives: This study sought to describe the 3-dimensional cytoarchitecture of the mouse SAN, including autonomic innervation, peripheral glial cells, and pacemaker cells.
Methods: The cytoarchitecture of SAN whole-mount preparations was examined by three-dimensional confocal laser-scanning microscopy of triple immunolabeled with combinations of antibodies for HCN4, S100 calcium-binding protein B (S100B), glial fibrillary acidic protein (GFAP), choline acetyltransferase, or vesicular acetylcholine transporter, and tyrosine hydroxylase, and transmission electron microscopy.
Results: The SAN exhibited heterogeneous autonomic innervation, which was accompanied by a web of peripheral glial cells and a novel S100B ⁺ /GFAP ^- interstitial cell population, with a unique morphology and a distinct distribution pattern, creating complex interactions with other cell types in the node, particularly with HCN4-expressing cells. Transmission electron microscopy identified a similar population of interstitial cells as telocytes, which appeared to secrete vesicles toward pacemaker cells. Application of S100B to SAN preparations desynchronized Ca ²⁺ signaling in HCN4-expressing cells and increased variability in SAN impulse rate and rhythm.
Conclusions: The autonomic plexus, peripheral glial cell web, and a novel S100B ⁺ /GFAP ^- interstitial cell type embedded within the HCN4 ⁺ cell meshwork increase the structural and functional complexity of the SAN and provide a new regulatory pathway of rhythmogenesis.
Competing Interests: Funding Support and Author Disclosures This work was supported by the Intramural Research Program of the National Institutes of Health, National Institute on Aging. The authors have reported that they have no relationships or conflicts of interest relevant to the contents of this paper to disclose.
(Published by Elsevier Inc.)