內質網蛋白TXNDC5在心房纖維化及心房顫動的角色

Abstract

心房顫動(atrial fibrillation, AF)是成人最常見的心律不整之一，會造成心輸出量下降及在心房或心耳形成血栓，繼而引發心臟衰竭、中風或器官栓塞，使得心血管功能惡化或甚死亡的風險大增。AF的風險會隨著年紀增長而逐漸上升，由於全球人口老化加速，AF的發生率與盛行率都明顯增加。目前針對AF的有效治療十分有限，多半以控制心跳及減少栓塞風險為主，仍需要針對AF本身的致病機轉深入研究，才能找出新型預防及治療AF的策略。目前對於心房顫動形成的機轉的認知，是一連串複雜的心房結構與電生理功能異常所組成，但這些異常變化的分子機轉，仍存在許多未知，亟需深入探討研究。許多研究已經指出，心房纖維化在心房結構與電生理功能異常上具有重要角色，是造成心房顫動發生的重要關鍵之一。相較於心室，心房更容易在受到病理性刺激時，產生纖維化，進而誘發心律不整。因此，找出引起心房纖維化的致病分子機轉，是發展新型AF治療方向的重要關鍵。我們的研究團隊，最近發現一個內質網蛋白—硫氧還原蛋白5(thioredoxin domain containing 5，TXNDC5)在心臟衰竭與心室纖維化上有重要角色，因此本實驗想探討TXNDC5在心房纖維化及AF所扮演的角色。首先，我們發現在相較於正常心律(normal sinus rhythm)的病人心房組織，AF病人心房組織有較高表現的TXNDC5。進一步的實驗也證實，TXNDC5上升會促進心房纖維母細胞的活化增生及ECM的製造。研究結果也顯示，TXNDC5 會透過自身的蛋白雙硫鍵異構酶 (protein disulfide isomerase) 活性，造成活性氧類 (reactive oxygen species，ROS)上升，進而活化c-Jun N末端激酶(JNK)從而促進心房纖維母細胞的活化與增生及ECM蛋白的產生。而乙型轉化生長因子（transforming growth factor beta, TGF-β）會透過內質網壓力(endoplasmic reticulum stress，ER stress)活化下游的轉錄因子並促進TXNDC5的上升。最後，將小鼠纖維母細胞中的TXNDC5基因剔除後，能減輕小鼠心房纖維化及電刺激誘發AF的頻率。這些實驗結果顯示TXNDC5在心房纖維化與心房顫動的發生確實扮演十分重要的角色，並且有機會能作為心房纖維化與心房顫動的有效治療策略。

Introduction: Atrial fibrosis, a hallmark of atrial fibrillation (AF), provides substrates to initiate/propagate fibrillation waves in the atria. We have recently identified an endoplasmic reticulum (ER) protein thioredoxin domain containing 5 (TXNDC5) as a critical mediator of cardiac ventricular fibrosis. We hypothesized that TXNDC5 could also play an important role in the pathogenesis of atrial fibrosis and fibrillation. Methods: RNA sequencing was performed on the atrial samples from patients undergoing open-heart surgery with normal sinus rhythm or AF. TXNDC5 and extracellular matrix (ECM) expression were quantified by quantitative PCR and western blotting in human atrial fibroblasts (hAF). Additionally, TXNDC5 was globally (Txndc5-/-) or conditionally (Txndc5fl/fl; Col1a2-Cre/ERT2) knocked out in a transgenic mouse model (a-MHC-TGFβ cys33ser), which develop extensive atrial fibrosis and inducible AF. Masson's trichrome and picrosirius red staining were used to analyze fibrotic tissue area. AF inducibility was assessed by transesophageal atrial stimulation. Results: TXNDC5 expression levels were significantly upregulated in the atrial tissue from patients with AF, and were positively correlated with those of transcripts encoding transforming growth factor β1 (TGFβ1) and ECM proteins in human atrial tissue. TXNDC5 is required for TGFβ1-induced fibroblast activation, proliferation, and ECM production. Also, elevated TXNDC5 expression level is sufficient to trigger these fibrogenic effects through c-Jun N-terminal kinase (JNK) signaling mediated by PDI activity of TXNDC5. In vivo study showed that global or conditional deletion of TXNDC5 in a-MHC-TGFβcys33ser mice considerably mitigated the extent of atrial fibrosis. Lastly, transesophageal atrial burst pacing induced AF 83% (5 out of 6) in a-MHC-TGFcys33ser mice, whereas conditional TXNDC5 knockout markedly reduced the inducibility of AF (22%, 2 out of 9) in a-MHC-TGFcys33ser mice. Conclusion: TXNDC5 augments atrial fibrosis by promoting cardiac fibroblast proliferation and ECM protein production via JNK signaling activation. Targeted deletion of Txndc5 protects against TGFβ induced atrial fibrosis and AF. Thus, TXNDC5 could be a new therapeutic target for atrial fibrosis and fibrillation.