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.