Genetic and Physiologic Dissection of the Vertebrate Cardiac Conduction System

Abstract

Vertebrate hearts depend on highly specialized cardiomyocytes that form the cardiac conduction system (CCS) to coordinate chamber contraction and drive blood efficiently and unidirectionally throughout the organism. Defects in this specialized wiring system can lead to syncope and sudden cardiac death. Thus, a greater understanding of cardiac conduction development may help to prevent these devastating clinical outcomes. Utilizing a cardiac-specific fluorescent calcium indicator zebrafish transgenic line, Tg(cmlc2:gCaMP)^s878, that allows for in vivo optical mapping analysis in intact animals, we identified and analyzed four distinct stages of cardiac conduction development that correspond to cellular and anatomical changes of the developing heart. Additionally, we observed that epigenetic factors, such as hemodynamic flow and contraction, regulate the fast conduction network of this specialized electrical system. To identify novel regulators of the CCS, we designed and performed a new, physiology-based, forward genetic screen and identified for the first time, to our knowledge, 17 conduction-specific mutations. Positional cloning of hobgoblin^s634 revealed that tcf2, a homeobox transcription factor gene involved in mature onset diabetes of the young and familial glomerulocystic kidney disease, also regulates conduction between the atrium and the ventricle. The combination of the Tg(cmlc2:gCaMP)^s878 line/in vivo optical mapping technique and characterization of cardiac conduction mutants provides a novel multidisciplinary approach to further understand the molecular determinants of the vertebrate CCS. Aberrant electrical activity of the heart, otherwise known as cardiac arrhythmia, may disrupt heart contractions, leading to loss of consciousness and sudden death. Every year, approximately 450,000 individuals in the United States die suddenly from this event. Currently, the only proven preventive therapy for sudden cardiac death is the automatic implantable cardioverter defibrillator, which carries a significant burden and cost to the patient. Greater understanding of the cardiac conduction system, which coordinates rhythmic beating of the heart, may lead to novel and safer therapeutic options for these patients. Working with zebrafish, a productive model system for understanding human disease, we have developed a cardiac-specific fluorescent calcium indicator zebrafish transgenic line to analyze the formation of the cardiac conduction system. Using this fluorescent transgenic line, we have observed four distinct physiologic cardiac conduction stages that correspond to cellular and anatomic changes of the developing heart. Furthermore, we have designed and performed a new, physiology-based, forward genetic screen to identify cardiac conduction mutants that would have escaped discovery in previous screens. Overall, these studies may prove rewarding toward developing therapeutic options aimed at maintaining and/or improving overall cardiac health.