Enhancing mitochondrial proteostasis reduces amyloid-beta proteotoxicity

Alzheimer's disease is a common and devastating disease characterized by aggregation of the amyloid-beta peptide. However, we know relatively little about the underlying molecular mechanisms or how to treat patients with Alzheimer's disease. Here we provide bioinformatic and experimental evidence of a conserved mitochondrial stress response signature present in diseases involving amyloid-beta proteotoxicity in human, mouse and Caenorhabditis elegans that involves the mitochondrial unfolded protein response and mitophagy pathways. Using a worm model of amyloid-beta proteotoxicity, GMC101, we recapitulated mitochondrial features and confirmed that the induction of this mitochondrial stress response was essential for the maintenance of mitochondrial proteostasis and health. Notably, increasing mitochondrial proteostasis by pharmacologically and genetically targeting mitochondrial translation and mitophagy increases the fitness and lifespan of GMC101 worms and reduces amyloid aggregation in cells, worms and in transgenic mouse models of Alzheimer's disease. Our data support the relevance of enhancing mitochondrial proteostasis to delay amyloid-beta proteotoxic diseases, such as Alzheimer's disease.

Enhancing mitochondrial quality control to fight neuromuscular degeneration in aging and disease

Mario Romani

Loss of mitochondrial function and proteostasis typify aging and age-associated degenerative disorders such as Alzheimer's disease and muscle aging. To date, no cure or preventive measure is available to manage these conditions. Alterations of cellular proteostasis, such as accumulation of misfolded or aggregated proteins, can directly affect mitochondrial homeostasis leading to activation of specific mitochondrial stress pathways capable of maintaining mitochondrial function. However, the contribution of these pathways in age-associated diseases characterized by proteotoxic aggregates is largely unknown. My thesis aims to understand the connection between cellular and mitochondrial proteostasis with a particular focus on mitochondrial stress pathways involved in the response to proteotoxic stress. To this end, I performed 2 studies: Reduction of Amyloid-beta proteotoxicity through activation of mitochondrial quality control: Accumulation of Amyloid-beta (Aß) peptidic aggregates, generated by the miscleavage of amyloid precursor protein (APP), is often associated with mitochondrial dysfunction. However, it is largely unknown how mitochondria react to these proteotoxic insults. We identified a cross-species conserved mitochondrial stress response, from nematodes to humans, activated during Aß proteotoxic stress. This response entails key mitochondrial quality control pathways such as the mitochondrial unfolded protein response (UPRmt) and mitophagy. Importantly, activation of these pathways through administration of mitochondrial regulators Doxycycline and the NAD+ booster nicotinamide riboside (NR), was sufficient to reduce Aß accumulation and proteotoxicity in nematodes and human cell lines. Moreover, boosting mitochondrial quality control in vivo in a mouse model of Alzheimer's disease with the compound NR led to a reduction of brain Aß plaques and preservation of cognitive function. Skeletal muscle aging is characterized by amyloid-like protein aggregates: Skeletal muscle aging is characterized by accumulation of dysfunctional mitochondria and misfolded proteins. However, the connection between these two hallmarks of muscle aging remains elusive. We found that aged skeletal muscles present a similar perturbation of mitochondrial and APP processing pathways when compared to muscles from inclusion body myositis patients (IBM), a disease characterized by intracellular protein deposits containing also Aß aggregates. In line with this, we identified amyloid-like aggregates in aged muscle closely resembling the ones observed during IBM with concomitant mitochondrial dysfunction. Strategies aiming at restoring mitochondrial homeostasis led to activation of the mitochondrial quality control accompanied by a reduction of amyloid-like aggregates in vitro and in vivo in mammalian and nematode systems. In summary, our work demonstrated that amyloidosis characterizes muscle, and possibly brain, aging in a similar fashion to what is observed in muscle and brain tissues from IBM and Alzheimer's patients, respectively. Importantly, restoring mitochondrial homeostasis through activation of mitochondrial quality control is sufficient to alleviate protein aggregation and improve tissue function in aging, IBM and Alzheimer's disease models.

EPFL2021

Metabolic control of muscle and muscle stem cell function

Hongbo Zhang

Skeletal muscle composed of myofibers and a small amount of muscle stem cells (MuSCs). It plays important roles in energy metabolism. Some of the key metabolites, such as NAD+ and acetyl-CoA were recently found to regulate muscle and MuSCs function. Most of these functions are related to the cellular mitochondria. In my PhD thesis projects, we aimed at exploring the link between NAD+ levels, mitochondrial function, muscle structural homeostasis and MuSCs senescence. We also investigated the impact of acetyl-CoA on MuSCs activation and proliferation, through the control of protein acetylation. These projects mainly focus on three aspects: Rejuvenation of adult stem cells by NAD+ repletion. Aging is accompanied by SCs senescence. However, the initial signaling events mediating SCs senescence are still not well defined. We demonstrate the importance of mitochondrial activity as a pivotal modulator of MuSCs senescence. MuSCs from aged mice have reduced NAD+ levels, mitochondrial content and capacity for oxidative respiration. Importantly, the induction of the mitochondrial unfolded protein response (UPRmt), subsequent to increasing cellular NAD+ with the precursor nicotinamide riboside (NR), prevents MuSCs senescence and improves muscle function and regeneration in aged mice. NR also prevents MuSCs dysfunction in the mdx mouse, a known mouse model of accelerated MuSCs senescence. Extending these observations to other SC pools and on the organism as a whole, we demonstrate that NR delays neural and melanocyte SCs senescence, while also increasing mouse lifespan. Control of MuSCs function by acetyltransferase KAT2A. MuSCs are essential for skeletal muscle homeostasis and repair. The complete mechanism controlling MuSCs maintenance and differentiation, however, remains elusive. We found that the acetyltransferase KAT2A expression is essential for myogenic differentiation of MuSCs. KAT2A loss-of-function in cells blocks MuSCs differentiation, and in mice impairs muscle regeneration after damage. The regulation of KAT2A in MuSCs function might rely on its ability to enzymatically acetylate other proteins, and in particular the transcription factor PAX7. NAD+ repletion improves muscle function in muscular dystrophy. Duchenne muscular dystrophy (DMD) is caused by dystrophin gene mutations that lead to progressive muscle degeneration. Our bioinformatics analysis in mice and DMD patients suggests a role for NAD+ in protecting the muscle from metabolic and structural degeneration. Validating these findings, we reveal that the mdx mouse is characterized by reductions in muscle NAD+ levels, concurrent to increased PARP activity and reduced expression of nicotinamide phosphoribosyltransferase (NAMPT), the key enzymes for NAD+ consumption and biosynthesis. Replenishing the NAD+ pool by dietary NR supplementation benefits muscle function in mdx and mdx/Utr-/- mice, an effect replicated in C. elegans models of DMD. The beneficial effects of NAD+ repletion in muscular dystrophy are pleiotropic and rely on the improvement in mitochondrial function, structural protein expression and on reductions in general PARylation, inflammation and fibrosis. In summary, our work demonstrates the critical role of NAD+ and Acetyl-coA in maintaining muscle and MuSCs function. Repletion of NAD+ levels, by NR administration, and/or boosting the function of acetyltransferase KAT2A might be attractive strategies to maintain muscle homeostasis and MuSCs function in aging and muscular dystrophy.

EPFL2016

The mitochondrial unfolded protein response in mammalian physiology

Johan Auwerx, Virginija Jovaisaite, Adrienne Joëlle Laurence Mottis

Mitochondria, the main site of cellular energy harvesting, are derived from proteobacteria that evolved within our cells in endosymbiosis. Mitochondria retained vestiges of their proteobacterial genome, the circular mitochondrial DNA, which encodes 13 subunits of the oxidative phosphorylation multiprotein complexes in the electron transport chain (ETC), while the remaining similar to 80 ETC components are encoded in the nuclear DNA (nDNA). A further similar to 1,400 proteins, which are essential for mitochondrial function are also encoded in nDNA. Thus, a majority of mitochondrial proteins are translated in the cytoplasm, then imported, processed, and assembled in the mitochondria. An intricate protein quality control (PQC) network, constituted of chaperones and proteases that refold or degrade defective proteins, maintains mitochondrial proteostasis and ensures the cell and organism health. The mitochondrial unfolded protein response is a relatively recently discovered PQC pathway, which senses the proteostatic disturbances specifically in the mitochondria and resolves the stress by retrograde signaling to the nucleus and consequent transcriptional activation of protective genes. This PQC system does not only transiently resolve the local stress but also can have long-lasting effects on whole body metabolism, fitness, and longevity. A delicate tuning of its activation levels might constitute a treatment of various diseases, such as metabolic diseases, cancer, and neurodegenerative disorders.