Elucidating how inner mitochondrial membrane remodeling regulates mtDNA quality control

线粒体是细胞膜结合的细胞器，不仅产生细胞功能所需的能量，还参与细胞死亡、钙稳态、炎症和免疫等多种细胞途径。线粒体是动态的细胞器，根据两个相反的事件：融合和裂变的细胞要求，不断调整自己的形状。例如，线粒体分裂(分裂)代表一个实体形成两个线粒体，不仅对线粒体DNA(MtDNA)传递至关重要，而且对细胞器在细胞内的分布和运动也是至关重要的。另一方面，线粒体融合允许两个线粒体内容的有效混合，因此被认为是促进受损线粒体互补的一种保障机制。适当的线粒体动力学对细胞的存活至关重要，线粒体形态的改变有助于多种疾病的发病。因此，破译线粒体适应其形状的分子机制，并在病理条件下调节线粒体的形态，目前处于线粒体研究的前沿。线粒体含有自己的环状DNA(MtDNA)，其复制独立于核DNA复制进行，只编码一小部分线粒体蛋白。线粒体DNA是以紧凑的结构组织起来的，称为类核(平均每个类核约有1.4个线粒体DNA分子)，这些分子与线粒体内膜(IMM)物理上相关。因此，了解IMM动力学事件对于揭示线粒体基因组自我自主调节的秘密以及揭开IMM重塑和mtDNA水平之间的相互作用是至关重要的，这是确保适当的mtDNA质量控制的机制。异质性是指在单个细胞或组织中可以发现两种不同的线粒体遗传背景的情况。突变mtDNA的基础异质性水平在所有人类中都存在，约为1-2%。然而，当突变mtDNA分子的拷贝数达到50%~80%左右(称为生化阈值)时，线粒体功能障碍就会发生，导致能量产生减少，并出现肌肉无力、运动障碍和听力和视觉缺陷等临床表现。生化阈值不仅取决于突变的类型，还取决于受影响的组织和细胞类型。因此，保持较低比例的突变mtDNA拷贝可以充分改善受异质性mtDNA突变影响的患者的临床症状。在此背景下，该项目的总体目标是阐明IMM构象变化的分子机制，并了解这些动态形状变化如何直接调节mtDNA在健康和疾病中的含量和分布。为了解决这些基本问题，我们将(1)通过研究一种控制IMM区划的新机制来定义支配IMM动力学的事件，这是一个允许在单个线粒体内形成不同IMM结构的过程。然后我们将(2)研究如何需要这些IMM动力学来分离随后将被作为降解目标的含有mtDNA的类核。最后，使用不同的异质性细胞模型，我们将(3)探索这些IMM动力学如何调节突变的mtDNA池并挽救与生化相关的缺陷。总而言之，该项目将阐明一种专门用于规范线粒体DNA质量控制的新机制。这不仅标志着对线粒体生理学的基本理解的重大进步，而且还将提出一种新的范式，可能为调节病理条件下线粒体DNA的分布和水平开辟新的治疗策略。

Mitochondria are membrane bound organelle which not only produce the energy required for cellular functions but are also involved in numerous cellular pathways including cell death, calcium homeostasis, inflammation and immunity. Mitochondria are dynamic organelles that constantly adapt their shape depending on cellular requirements by two opposing events: fusion and fission. For example, mitochondrial division (fission), which represents the formation of two mitochondria from one entity, is crucial not only for mitochondrial DNA (mtDNA) transmission but also for organelle distribution and movement within the cell. On the other hand, mitochondrial fusion allows the efficient mix of contents of two mitochondria and therefore is considered as a safeguard mechanism that facilitates the complementation of damaged mitochondria. Proper mitochondrial dynamics are essential for cell viability and altered mitochondrial morphology contributes to the pathogenesis of multiple diseases. Thus, deciphering the molecular mechanisms by which mitochondria adapt their shape, and modulating mitochondrial morphology in pathological conditions are currently at the forefront of mitochondrial research. Mitochondria contain their own circular DNA (mtDNA), whose replication occurs independently of nuclear DNA replication and only encodes a small set of mitochondrial proteins. mtDNA is organized in compacted structures named nucleoids (around 1.4 mtDNA molecules per nucleoid on average), which are physically associated with the inner mitochondrial membrane (IMM). Therefore, understanding IMM dynamics events are essential to unveil the secrets underlying the self-autonomous regulation of the mitochondrial genome and to unravel the interplay between IMM remodeling and mtDNA levels, a mechanism ensuring a proper mtDNA quality control. Heteroplasmy is a condition where two different mitochondrial genetic backgrounds can be found in a single cell or tissue. Basal heteroplasmy levels, ~1-2% of mutant mtDNA, are found in all humans. However, when the number of copies of mutant mtDNA molecules reach a percentage around 50 to 80% (knows as biochemical threshold), mitochondrial dysfunction will take place leading to decrease energy production and the appearance of clinical manifestations including muscle weakness, movements disorders and hearing and vision defects. The biochemical threshold depends not only on the type of mutation but also on the tissues and cell type affected. Therefore, maintaining a low percentage of mutant mtDNA copies can be an adequate strategy to ameliorate the clinical symptomatology of patients affected by heteroplasmic mtDNA mutations. In this context, the overall aim of the project is to elucidate the molecular mechanisms underlying IMM conformational changes and to understand how these dynamic shape transitions directly regulate mtDNA content and distribution in both health and disease. To address these fundamental questions, we will (1) define the events governing IMM dynamics by studying a new mechanism controlling IMM compartmentalization, a process allowing the formation of different IMM structures inside one single mitochondrion. We will then (2) investigate how these IMM dynamics are required to isolate mtDNA-containing nucleoids that will be subsequently targeted for degradation. Finally, using different cellular models of heteroplasmy, we will (3) explore how these IMM dynamics could modulate the pools of mutant mtDNA and rescue the biochemical-associated defects. Together, this project will shed light on a new mechanism specifically dedicated to regulate mtDNA quality control. This will not only mark a significant advance in the fundamental understanding of mitochondrial physiology, but will also propose a new paradigm that could open new therapeutic strategies for the modulation of mtDNA distribution and levels in pathological conditions.