title: Modulation of the unfolded protein response in development, ageing and exercise, regulates mitochondrial ER contact sites and mitochondrial remodelling
Creators: name: Casas Martínez, José C.
Publisher: name: University of Galway
Year: 2024
Subjects: subject: Medicine
subject: Physiology
subject: FOS: Biological sciences
subject: Unfolded protein
subject: Ageing
subject: Exercise
subject: Mitochondrial remodelling

description: Mitochondria and the endoplasmic reticulum (ER) have a synergistic relationship and are key regulatory hubs in maintaining cell homeostasis. Communication between these organelles is mediated by mitochondria ER contact sites (MERCS), allowing the exchange of material and information, modulating calcium homeostasis, redox signalling, lipid transfer and the regulation of mitochondrial dynamics. MERCS are dynamic structures that allow cells to respond to changes in the intracellular environment under normal homeostatic conditions, while their assembly/disassembly are affected by pathophysiological conditions such as ageing and disease. Disruption of protein folding in the ER lumen can activate the Unfolded Protein Response (UPR), promoting the remodelling of ER membranes and MERCS formation. The UPR stress receptor kinases PERK and IRE1 are located at or close to MERCS. UPR signalling can be adaptive or maladaptive, depending on whether the disruption in protein folding or ER stress is transient or sustained. Adaptive UPR signalling via MERCS can increase mitochondrial calcium import, mitochondrial metabolism and dynamics, while maladaptive UPR signalling can result in excessive calcium import and activation of apoptotic pathways. Targeting UPR signalling and the assembly of MERCS is an attractive therapeutic approach for a range of age-related conditions such as neurodegeneration and sarcopenia. This project identified a key adaptive response of the UPR during development that regulates mitochondrial capacity. Bolus addition of a low concentration of Tunicamycin (TM) to myoblasts resulted in increased mitochondrial content, respiration, mitochondrial turnover and MERCS formation. The adaptive response was dependent on the PERK arm of the UPR and resulted in improved myogenesis as assessed by myotube formation. The resulting myotubes maintained the increase in MERCS and mitochondrial morphology. However, when myotubes were treated with TM, the adaptive response was not evident, suggesting that the activation of the adaptive UPR was dependent on the developmental stage of the cells. This study also identified a key role of the UPR in the determination of lifespan and health span in C. elegans. The pharmacological activation of the adaptive UPR, by induction of physiological levels of ER stress during hatching of C. elegans embryos promoted an extension in lifespan. This adaptative response was specific to the early stages of development as activation in adult nematodes induced a deleterious reduction in lifespan. The adaptive response increased the mitochondrial content and turnover of C. elegans of treated embryos that was maintained in adulthood. A TEM analysis of the primed adult worms identified increased formation of mitochondria-ER contact sites (MERCS). The physiological fitness of the nematode during ageing was examined and improved fitness was conserved throughout adulthood following TM treatment. C. elegans subjected to a TM treatment during early-development, activated the UPR and resulted in a series of cellular adaptative responses. To investigate the mechanisms of the adaptive response in C. elegans, strains with null mutations for the different arms of the C. elegans UPRER and UPRmt were employed. An epistasis analysis of the pek-1 (-);atfs-1 (-) double mutant strain demonstrated that the adaptations to the UPR activation are controlled by a signalling pathway that involves PEK-1 and ATFS-1. In response to exercise, C. elegans experienced an increase in the levels of oxidative stress and ER stress. This project demonstrated that PEK-1 and PRDX-2 are required for the activation of the adaptive response, specifically PRDX-2 was essential for the formation of MERCS following TM treatment. PEK-1/ATFS-1, PRDX-2/ATFS-1 and PRDX 2/ATF-6 axis were identified as key adaptive signalling pathways induced after a chronic exercise intervention in the worms. This project provides novel insights into the interplay between ER stress, UPR signalling, and mitochondrial dynamics, utilising C. elegans and mammalian cell lines to investigate the UPRER-UPRmt axis regulation of mitochondria and ER dynamics. Key discoveries include PEK-1/PERK as a pivotal regulator of mitochondrial morphology through MERCS assembly and PRDX2 as a key regulator of mitochondrial adaptations post-exercise, independent of PERK, opening new avenues for exploring redox regulation and oxidative stress management.