Mitochondrial oxidative capacity and NAD(+) biosynthesis are reduced in human sarcopenia across ethnicities

The causes of impaired skeletal muscle mass and strength during aging are well-studied in healthy populations. Less is known on pathological age-related muscle wasting and weakness termed sarcopenia, which directly impacts physical autonomy and survival. Here, we compare genome-wide transcriptional changes of sarcopenia versus age-matched controls in muscle biopsies from 119 older men from Singapore, Hertfordshire UK and Jamaica. Individuals with sarcopenia reproducibly demonstrate a prominent transcriptional signature of mitochondrial bioenergetic dysfunction in skeletal muscle, with low PGC-1 alpha/ERR alpha signalling, and downregulation of oxidative phosphorylation and mitochondrial proteostasis genes. These changes translate functionally into fewer mitochondria, reduced mitochondrial respiratory complex expression and activity, and low NAD(+) levels through perturbed NAD(+) biosynthesis and salvage in sarcopenic muscle. We provide an integrated molecular profile of human sarcopenia across ethnicities, demonstrating a fundamental role of altered mitochondrial metabolism in the pathological loss of skeletal muscle mass and function in older people.

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

NAD+ boosting reduces age-associated amyloidosis and restores mitochondrial homeostasis in muscle

Johan Auwerx, Hao Li, Mario Romani, Vincenzo Sorrentino, Hongbo Zhang

Aging is characterized by loss of proteostasis and mitochondrial homeostasis. Here, we provide bioinformatic evidence of dysregulation of mitochondrial and proteostasis pathways in muscle aging and diseases. Moreover, we show accumulation of amyloid-like deposits and mitochondrial dysfunction during natural aging in the body wall muscle of C. elegans, in human primary myotubes, and in mouse skeletal muscle, partially phenocopying inclusion body myositis (IBM). Importantly, NAD+ homeostasis is critical to control age-associated muscle amyloidosis. Treatment of either aged N2 worms, a nematode model of amyloid-beta muscle proteotoxicity, human aged myotubes, or old mice with the NAD+ boosters nicotinamide riboside (NR) and olaparib (AZD) increases mitochondrial function and muscle homeostasis while attenuating amyloid accumulation. Hence, our data reveal that age-related amyloidosis is a contributing factor to mitochondrial dysfunction and that both are features of the aging muscle that can be ameliorated by NAD+ metabolism-enhancing approaches, warranting further clinical studies.

2021

Mitochondrial unfolded protein response, its molecular mechanism and physiological impact

Virginija Jovaisaite

Mitochondria are the main producers of ATP, the energy currency of the cell, and they perform a wide array of other fundamental functions. These organelles are essential not only for cellular metabolism, but also for organismal physiology and lifespan. A recently discovered mitochondrial unfolded protein response (UPRmt) maintains the mitochondrial proteostasis by managing protein quality control (PQC) network during proteotoxic stress. Recent findings show that activation of this repair pathway improves mitochondrial function and is associated with increased longevity. My thesis aims to expand the knowledge about the UPRmt by exploring its molecular mechanism and impact on mitochondrial stress induced longevity. We performed three studies: Identification of two epigenetic UPRmt regulators (chapter 2). In collaboration with Prof. Dillinâs laboratory (UCB, USA), we identified the conserved histone demethylases jmjd-1.2/PHF8 and jmjd-3.1/JMJD3 as positive regulators of UPRmt and longevity in response to mitochondrial electron transport chain (ETC) dysfunction across species. A systems genetics analysis of data from the BXD mouse genetic reference population (GRP) further indicates conserved roles of the mammalian orthologs of these demethylases in longevity and UPRmt signaling. These findings illustrate an evolutionary conserved epigenetic mechanism that determines the rate of aging, downstream of mitochondrial perturbations. Relation between UPRmt and TCA cycle (chapter 3). We investigated the effects of genetic perturbations of the TCA cycle on the UPRmt and lifespan. We found that downregulation of several TCA cycle enzymes induces UPRmt, but they have divergent effects on lifespan. Restricting the knockdown of these genes to early larval development stages had positive effects on longevity, which was dependent on UPRmt. We analysed the transcriptomes of C. elegans with knockdown of these TCA cycle genes and compared them with those obtained after the knockdown of other mitochondrial genes, which are known to induce the UPRmt. We identified a core set of transcripts, which change in common and which likely represent a signature of mitochondrial dysfunction associated with UPRmt induction. Bioinformatic analysis of UPRmt (chapter 4). In two collaborative studies, we investigated the UPRmt using newly collected data from the BXD mouse GRP. In the first study, we quantified the transcriptome and proteome from 40 strains of the BXD mouse GRP on two different diets. Integrated molecular profiles were used to characterize the UPRmt, which shows strikingly variant responses at the transcript and protein level that are conserved between C.elegans, mice and humans. In the second study, whole genomes of BXD GRP strains were sequenced to detect sequence variants, which were then used to find novel associations within a high coverage phenome of various traits. One of the novel associations included a missense mutation in fumarate hydratase that controls variation in the UPRmt in both mouse and C. elegans. In summary, our work characterized novel epigenetic regulators of UPRmt and associated mitochondrial stress induced longevity, described the connection between the UPRmt and the TCA cycle, and illustrated the conservation of UPRmt in a mouse GRP. These results expand the knowledge about the UPRmt and promote further investigation of its role in longevity and mitochondrial diseases.