Project Summary/Abstract The sinoatrial node (SA node or SAN) is a minuscule, heterogeneous structure which initiates and sets the rhythm of the heartbeat. Recent insights into embryonic development have pinpointed T-box (TBX) transcription factors as key determinants of the SAN differentiation. Tbx18, in particular, has been shown to be indispensable for the specification of the SA node during development. In the past decade, we have learnt much about the electrophysiological bases of pacemaker cell automaticity. The membrane and Ca2+ clock mechanisms can explain the automaticity at the single, pacemaker cell level. However, heart rhythm cannot arise from mere summation of the spontaneous electrical activities from single pacemaker cells. Rather, the intricate architecture of the sinoatrial node allows the <10,000 nodal pacemaker cells to pace- and-drive the atrial myocardium. However, little is known about how the SAN can self-assemble its structure and the molecular determinants of synchronous sinus rhythm; i.e., how the pacemaker cells and the rich network of non-myocytes become confined into the SA node area to achieve its sole function of pace-and-drive. We hypothesize that Tbx18 induces acute, inflammatory signals, and the acute inflammatory cytokines facilitate homing and/or proliferation of non-myocytes such as fibroblasts and macrophages, and thus self-assembly of the developing SAN. Somatic gene transfer technology, somatic cell reprogramming, and in vitro & in vivo models of electrophysiology are the main tools of this study. This proposal is deliberately mechanistic, with the singular goal of understanding the molecular determinants of synchronous sinus rhythm generation.