Impact of mitochondrial DYNAMIcs and genome instability on the development of age-related CArdiac disorders – DYNAMICA

Our project is structured around three work packages (WP):

WP1: Exploring how the heart adapts to the presence of cells that have lost mitochondrial activity during aging.

Given the low proportion of these defective cells in aging hearts and their random distribution, it is highly likely that their pathological role results from adverse effects on surrounding "normal" cells. We propose to analyze these adaptations using a multi-omics approach (DNA sequencing, metabolite analysis) as well as tissue analysis to detect potential remodeling at the cellular and tissue levels in the heart, using a mouse model reproducing this phenomenon of cardiac mitochondrial aging.

WP2: Determining whether modulation of mitochondrial dynamics in the heart can interfere with the accumulation of mtDNA mutations and deficient cells during aging.

We have shown that cells deficient in mitochondrial activity affect heart function once a critical threshold is exceeded. We therefore propose that any intervention aimed at slowing the accumulation of mtDNA deletions, and consequently the development of these deficient cells, should delay the onset of cardiac dysfunction. As experimental and clinical data suggest that mitochondrial dynamics play a role in mtDNA quality control, we will use a genetic approach to alter mitochondrial fusion and fission in our mouse model of cardiac mitochondrial aging. Hearts will be analyzed to identify mtDNA deletions, the presence of cells deficient in mitochondrial activity, as well as biomarkers and molecular pathways identified in WP1. Cardiac function will be assessed through electrocardiogram analysis using implanted telemetry chips.

WP3: Determining whether pharmacological approaches targeting mitochondrial dynamics and mitophagy can reproduce the effects observed in WP2.

The identification of mitochondrial dysfunction as a major contributor to cardiovascular diseases has opened a new field of research for the development of therapies targeting mitochondria and their quality control systems, including mitochondrial dynamics and mitophagy. The availability of compounds known to modulate these processes now allows us to test their effects in vivo in the context of accelerated mtDNA deletions in the heart of our mouse model, to determine whether they have a beneficial effect on cardiac function.

The main results obtained during the DYNAMICA project are :

Metabolite Analysis (WP1):

A specific class of lipids was found in increased quantities in the plasma of mice with accelerated mitochondrial aging in the heart, starting at 12 months of age, before the onset of cardiac arrhythmias. This suggests that the increased concentration of these particular lipids in the plasma is an early marker of cardiac dysfunction.

Influence of Mitochondrial Fusion on the Accumulation of mtDNA Mutations and the Presence of Cells with impaired mitochondrial function (WP2):

After developing models with impaired mitochondrial fusion or fission in conjunction with accelerated mitochondrial aging, we validated these models (showing decreased expression of proteins involved in these processes). While fission impairment showed no significant impact, our results highlighted a protective role of mitochondrial fusion against the accumulation of deficient cardiac cells, without influencing the overall rate of mtDNA mutations. Specifically, the number of cells deficient in mitochondrial activity doubled when fusion was impaired in our mouse model.

Treatment of Mice with Molecules Modulating Mitochondrial Fusion and Mitophagy (WP3):

We treated our mouse model of accelerated mitochondrial aging in the heart for two months, at two different ages (18 and 24 months), using a molecule promoting fusion and another promoting mitochondrial turnover via mitophagy. Our results showed limited effects of the pro-fusion molecule at the tested dose. However, the mitophagy-stimulating molecule had a generally beneficial impact on the mtDNA mutation rate and the proportion of cells deficient in mitochondrial activity.

Overall, the DYNAMICA project has provided new insights into the development of heart diseases and should stimulate further research to develop therapeutic strategies targeting mitochondrial dynamics to improve cardiac health during aging.

In particular, we have highlighted a crucial role of mitochondrial fusion in preventing the accumulation of cells with impaired mitochondrial function during cardiac aging. This may occur through the control of the distribution of mutated mtDNA molecules within the cellular mitochondrial network, and we will conduct further experiments to investigate this phenomenon.

The electrocardiogram analysis, which will be carried out in the coming weeks, should help determine if the pharmacological treatment approach we used had a beneficial impact on cardiac arrhythmias in our study model, thereby opening promising perspectives for treating these cardiac disorders in humans.

Cardiovascular diseases are a major issue in public health. They are among the leading causes of death worldwide, with age being one of the main risk factors. Indeed, aging is accompanied by increased incidence of cardiac dysfunction and worsens the outcome of ischemic events. Although the origins of cardiac diseases are multifactorial, many studies point to a critical role of mitochondria, which are crucial organelles involved in many metabolic and cellular processes. In particular, the instability of the mitochondrial genome (mtDNA) during aging, reflected by the accumulation of mtDNA point mutations and large deletions in many tissues, appears as a critical parameter. Once they exceed a deleterious threshold, mtDNA mutations leads to the development of tissue mosaics of mitochondrial deficiency, represented by few cells with severe mitochondrial dysfunction embedded among numerous cells with normal mitochondrial function, found in many tissues including brain, skeletal muscle, colon and the heart. Until recently, the physiological relevance of such mosaics for the pathogenesis of aging was unknown. To address this question, we have generated a novel mouse model, which can accumulate mtDNA deletions in a tissue specific (cre-lox system) and accelerated way, due to impaired activity of the mitochondrial helicase during mtDNA replication. We have shown that in the heart, this progressively induces the accumulation of a low proportion of cardiomyocytes with severe mitochondrial dysfunction, which indeed are sufficient to promote cardiac arrhythmias and facilitate the occurrence of serious arrhythmias after myocardial cryoinfarction. Although those observations highlight the importance of mtDNA integrity for cardiac physiology, little is known about the mechanisms involved. Interestingly, the high plasticity of mitochondria, which are organized into a dynamic network resulting from a fine balance between fusion and fission events, appears to play a role in the quality control of mtDNA integrity. Thus, we hypothesize that mtDNA instability and the ensuing mosaic pattern of mitochondrial deficiency leads to maladaptation processes in the heart that ultimately contributes to the development and severity of age-related cardiovascular diseases, and that therapies aimed at modulating mitochondrial dynamics in cardiomyocytes might delay their onset or ameliorate their outcomes. To address these hypotheses, we propose a research program divided into three work packages: WP1) We will explore how the heart respond to mtDNA instability and increasing loads of cardiomyocytes with mitochondrial dysfunction during aging. We propose to unravel these adaptations using a multi-omics approach (genomics, transcriptomics, metabolomics) to detect potential biomarkers, as well as histological analysis to detect potential remodelling at the cellular and tissue level. WP2) We will determine whether the modulation of mitochondrial dynamics in the heart can interfere with the accumulation of mtDNA deletions and defective cardiomyocytes during aging. We will genetically induce a systemic impairment of mitochondrial fusion or fission in our mouse model of mtDNA instability and analyse the phenotypic changes (mtDNA deletions, proportion of defective cardiomyocytes biomarkers and pathways identified in WP1). Moreover, cardiac function will be explored (electrocardiograms, echocardiography), and the impact on age-related cardiomyopathies will be investigated in the context of ischemia/reperfusion injuries. WP3) We will determine if pharmacological approaches using drugs known to modulate mitochondrial dynamics can recapitulate the effects observed in WP2 in our mice with mtDNA instability in the heart, and determine whether they have a beneficial effect on their cardiac function. Our project should thus stimulate new research to unravel therapeutic strategies for cardiovascular diseases, in order to promote pre-clinical data to clinical trials.