Molecular and Functional Remodeling of the Superior and Inferior Sinoatrial Nodes in a Rat Model of Hypertrophic Cardiomyopathy

Objectives: This study aimed to determine the transcriptional basis of functional silencing of one of the two dominant pacemakers in failing rat hearts. Background: Recently, our laboratory presented functional and molecular evidence for the presence of two competing sinoatrial node (SAN) pacemakers in the healthy human and rat hearts. Anatomically localized near the superior vena cava and inferior vena cava, the superior and inferior SANs (sSAN and iSAN), respectively, preferentially control fast and slow normal heart rates. However, only one dominant pacemaker, primarily sSAN, was functional in the failing rat heart with hypertrophic cardiomyopathy. Methods: Ascending aortic constriction was performed on 1-week-old male Sprague-Dawley rat pups to induce left ventricular hypertrophy and heart failure. The dominant pacemaker was anatomically mapped in adult (8-12 weeks old) healthy and failing rat hearts using optical mapping in isolated right atrial tissue preparations. RNA sequencing was used to identify regional sSAN/iSAN gene expression differences between healthy and failing rat hearts. Results: In all failing rat hearts optically mapped in this study (n=4), only the sSAN pacemaker was functional, while the iSAN was silent. Compared to healthy rat hearts, a total of 3,640 genes were downregulated and 4,518 genes were upregulated in failing rat hearts. The functional quiescence of the iSAN in these failing rat hearts may be explained by their downregulation of sodium, potassium, and calcium ion channels as well as their downregulation of certain structural genes, including ankyrin, titin, and myosin heavy chain. Moreover, the iSAN showed predominant downregulation of several key transcription factors such as TBX5, TBX3, SHOX2, and SMAD9. Conclusions: Pressure-overload-induced heart failure resulted in significant downregulation of critical transcription factors, ion channels, and structural transcripts of the iSAN, which could explain the functional silencing of the iSAN in failing rat hearts. Overall design: RNA-Seq of healthy and failing rat heart samples from inferior and superior sino-atrial nodes, left atria, right atria, and left ventricles.

研究目的：本研究旨在明确衰竭大鼠心脏中两个主要起搏点之一发生功能沉默的转录基础。
研究背景：本实验室近期已通过功能与分子实验证据证实，健康人类与大鼠心脏中存在两个相互竞争的窦房结（sinoatrial node, SAN）起搏点。上窦房结（sSAN）与下窦房结（iSAN）分别解剖定位于上腔静脉与下腔静脉附近，分别优先调控正常心率的快、慢节律。然而在患有肥厚型心肌病的衰竭大鼠心脏中，仅存在一个主导起搏点（主要为上窦房结）具有功能活性。
研究方法：对1周龄雄性斯普拉-道来（Sprague-Dawley）大鼠幼崽实施升主动脉缩窄术，以诱导左心室肥厚与心力衰竭。采用离体右心房组织标本的光学标测技术，对成年（8~12周龄）健康与衰竭大鼠心脏的主导起搏点进行解剖定位。通过RNA测序（RNA sequencing, RNA-seq）鉴定健康与衰竭大鼠心脏中不同区域的上、下窦房结的基因表达差异。
研究结果：本研究中完成光学标测的所有衰竭大鼠心脏样本（n=4）均仅上窦房结起搏点具有功能活性，而下窦房结处于沉默状态。与健康大鼠心脏相比，衰竭大鼠心脏中共计3640个基因表达下调，4518个基因表达上调。衰竭大鼠心脏下窦房结的功能沉默可通过其钠、钾、钙离子通道相关基因以及锚蛋白（ankyrin）、肌联蛋白（titin）、肌球蛋白重链（myosin heavy chain）等结构基因的表达下调得到解释。此外，下窦房结还表现出TBX5、TBX3、SHOX2、SMAD9等多个关键转录因子的显著表达下调。
研究结论：压力负荷诱导的心力衰竭可导致下窦房结的关键转录因子、离子通道及结构转录本显著表达下调，这可解释衰竭大鼠心脏中下窦房结的功能沉默现象。
整体实验设计：对健康与衰竭大鼠心脏的上、下窦房结、左心房、右心房及左心室样本进行RNA测序。