Declining autophagy drives age-related axonal decay through microtubule alterations

The microtubule cytoskeleton plays a pivotal role in maintaining axonal integrity, and its deterioration has been closely associated with neuronal decay during ageing. Autophagy, a crucial cellular degradation process, is essential for sustaining neuronal homeostasis and its dysregulation has been implicated in various neurodegenerative diseases. Changes in autophagy are also associated with ageing. However, the mechanisms by which autophagy dysfunction contributes to neuronal atrophy in the ageing brain remain poorly understood. In this study, we unveil the significant involvement of autophagy in preserving axonal architectural integrity during ageing through regulation of the microtubule cytoskeleton. Our findings indicate that autophagic activity declines in neurons of the aged brain, and the inhibition of autophagy exacerbates age-related alterations in axonal and synaptic microtubules. This dysfunction correlates with an increase in established hallmarks of ageing, including swellings and thinning of axons plus synaptic terminal breakdown. Conversely, pharmacological and genetic strategies to enhance autophagic activity not only rescued age-related microtubule changes, but also inhibited the formation of axonal swellings, the thinning of axons and the deterioration of synaptic terminals. Mechanistically, we find that reactive oxygen species (ROS), a consequence of impaired autophagy, lead to alterations in microtubule networks. In addition, enhancing Eb1 expression can counteract the detrimental effects of deficient autophagy, effectively blocking axonal and synaptic microtubule deterioration. These findings underscore that alterations in autophagy are a critical driver of axonal deterioration during ageing, operating through modifications to the microtubule cytoskeleton as a downstream target. Our findings deliver a cascade of events that underlie neuronal ageing.