Division of Cardiology, Department of Medicine, Vagelos College of Physicians and Surgeons, Columbia University, New York, NY, United States.
Division of Cardiology, Department of Medicine, Vagelos College of Physicians and Surgeons, Columbia University, New York, NY, United States; Department of Medicine, University of Mississippi, Jackson, MS, United States.
J Struct Biol. 2024 Sep;216(3):108110. doi: 10.1016/j.jsb.2024.108110. Epub 2024 Jul 14.
Atrial fibrillation (AF) is the most common clinical arrhythmia, however there is limited understanding of its pathophysiology including the cellular and ultrastructural changes rendered by the irregular rhythm, which limits pharmacological therapy development. Prior work has demonstrated the importance of reactive oxygen species (ROS) and mitochondrial dysfunction in the development of AF. Mitochondrial structure, interactions with other organelles such as sarcoplasmic reticulum (SR) and T-tubules (TT), and degradation of dysfunctional mitochondria via mitophagy are important processes to understand ultrastructural changes due to AF. However, most analysis of mitochondrial structure and interactome in AF has been limited to two-dimensional (2D) modalities such as transmission electron microscopy (EM), which does not fully visualize the morphological evolution of the mitochondria during mitophagy. Herein, we utilize focused ion beam-scanning electron microscopy (FIB-SEM) and perform reconstruction of three-dimensional (3D) EM from murine left atrial samples and measure the interactions of mitochondria with SR and TT. We developed a novel 3D quantitative analysis of FIB-SEM in a murine model of AF to quantify mitophagy stage, mitophagosome size in cardiomyocytes, and mitochondrial structural remodeling when compared with control mice. We show that in our murine model of spontaneous and continuous AF due to persistent late sodium current, left atrial cardiomyocytes have heterogenous mitochondria, with a significant number which are enlarged with increased elongation and structural complexity. Mitophagosomes in AF cardiomyocytes are located at Z-lines where they neighbor large, elongated mitochondria. Mitochondria in AF cardiomyocytes show increased organelle interaction, with 5X greater contact area with SR and are 4X as likely to interact with TT when compared to control. We show that mitophagy in AF cardiomyocytes involves 2.5X larger mitophagosomes that carry increased organelle contents. In conclusion, when oxidative stress overcomes compensatory mechanisms, mitophagy in AF faces a challenge of degrading bulky complex mitochondria, which may result in increased SR and TT contacts, perhaps allowing for mitochondrial Ca maintenance and antioxidant production.
心房颤动（AF）是最常见的临床心律失常，但对其病理生理学的了解有限，包括不规则节律引起的细胞和超微结构变化，这限制了药物治疗的发展。先前的工作已经证明了活性氧（ROS）和线粒体功能障碍在 AF 发展中的重要性。线粒体结构、与肌浆网（SR）和 T 小管（TT）等细胞器的相互作用以及通过线粒体自噬降解功能失调的线粒体是理解 AF 引起的超微结构变化的重要过程。然而，大多数关于 AF 中线粒体结构和相互作用组的分析仅限于二维（2D）模式，如透射电子显微镜（EM），这不能完全可视化线粒体在自噬过程中的形态演变。在此，我们利用聚焦离子束扫描电子显微镜（FIB-SEM）并对来自鼠左心房样本的三维（3D）EM 进行重建，并测量线粒体与 SR 和 TT 的相互作用。我们开发了一种新型的 FIB-SEM 在 AF 小鼠模型中的 3D 定量分析方法，用于量化心肌细胞中线粒体自噬阶段、自噬体大小以及与对照小鼠相比的线粒体结构重塑。我们表明，在我们由于持续的晚期钠电流而导致自发性和持续性 AF 的小鼠模型中，左心房心肌细胞具有异质性的线粒体，其中大量线粒体增大，伸长和结构复杂性增加。AF 心肌细胞中的自噬体位于 Z 线，在那里它们与大的、伸长的线粒体相邻。AF 心肌细胞中的线粒体显示出增加的细胞器相互作用，与 SR 的接触面积增加 5 倍，与 TT 的相互作用增加 4 倍。我们表明，AF 心肌细胞中的自噬涉及携带增加的细胞器内容的 2.5 倍更大的自噬体。总之，当氧化应激超过代偿机制时，AF 中的自噬面临降解体积庞大的复杂线粒体的挑战，这可能导致 SR 和 TT 接触增加，也许允许线粒体 Ca 维持和抗氧化剂产生。
