Impatto dell’invecchiamento e dell’inattività sull’integrità della giunzione neuromuscolare e sulla funzione mitocondriale nell’uomo

Skeletal muscle constitutes approximately 30–40% of total body mass and is essential for voluntary movement, posture, and metabolic homeostasis. Proper neuromuscular function depends on the integrity of the neuromuscular junction (NMJ) and the energetic support provided by mitochondria. This study aimed to investigate the effects of ageing inactivity on NMJ structure and function, mitochondrial adaptations, and their potential interplay in human skeletal muscle. To achieve this aim, 10 older (65+ years) and 9 young (18-29 years) participants underwent a 10- and 21-day bed rest period, respectively, with vastus lateralis muscle biopsies collected before and after the inactivity period. Specifically, we analysed the morphology and structural integrity of the NMJ, as well as several aspects of mitochondrial function and structure, including respiratory capacity, ROS production, the organisation of mitochondrial complexes and supercomplexes, mitochondrial dynamics, volume density, and morphology. We also assessed relevant gene expression profiles. Older adults were particularly susceptible, displaying early signs of NMJ remodelling, including reduced overlap between pre- and postsynaptic terminals and impaired neuromuscular transmission after only 10 days of bed rest. These structural changes were mirrored by elevated circulating C-terminal agrin fragment (CAF), an indirect biomarker of NMJ integrity, confirming early remodelling. In contrast, younger adults maintained NMJ structure and function during the first 10 days, with clear morphological and functional impairment emerging only after 21 days. Mitochondrial adaptations paralleled these observations. Older adults exhibited increased ROS production, reduced mitochondrial content, and shifts in fission–fusion balance, whereas young adults largely preserved mitochondrial function and morphology, showing only changes in phosphorylated DRP1. Electron microscopy suggested that presynaptic mitochondria at the NMJ may represent an early vulnerable site, potentially initiating subsequent NMJ remodelling. Transcriptomic profiling confirmed inactivity as the dominant driver of gene expression changes. In older adults, bed rest induced rapid suppression of respiratory chain and mitochondrial ribosomal genes together with enrichment of mitophagy and antioxidant pathways, consistent with elevated ROS at 10 days of bed rest. In contrast, younger adults displayed a delayed transcriptional response, with early activation of stress and lysosomal programmes at 10 days and later engagement of metabolic and structural pathways at 21 days of bed rest. Together, these findings highlight the critical role of mitochondria in maintaining neuromuscular integrity, emphasising that ageing accelerates the adverse consequences of inactivity and that mitochondria may act as key early mediators of NMJ vulnerability.