Elucidating how inner mitochondrial membrane remodeling regulates mtDNA quality control

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.

In the past two decades, multiple studies have shown how mitochondria adapt their shape depending on the metabolic context. These investigations have been mainly focused on understanding the molecular contribution of other organelles and of the proteins involved in outer mitochondrial membrane dynamics. However, during the mitochondrial division process, how the inner mitochondrial membrane (IMM) is remodeled and which proteins are responsible for regulating these dynamics are currently unknown. In addition, while mtDNA is physically associated to the IMM, how IMM dynamics regulates mtDNA distribution, replication or quality control are still open questions. Thus, 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) combine molecular biology, proteomics and state-of-the-art microscopy to precisely define the processes regulating IMM dynamics by studying a new mechanism controlling IMM compartmentalization. We will then (2) investigate, by coupling cell biology and super-resolution live cell imaging to single cell analysis, 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 the manipulation of IMM dynamics could modulate the pools of mutant mtDNA and rescue the biochemical-associated defects. Together, this project will shed the light on a new mechanism specifically dedicated to regulate mtDNA quality control. This would not only mark a significant advance in the fundamental understanding of mitochondrial physiology, but would also propose a new paradigm that will open new therapeutic strategies for the modulation of mtDNA distribution and levels in pathological conditions.