Endoplasmic reticulum in the axon: Insights into structural dynamics and implications in neurodegeneration

The endoplasmic reticulum (ER) is an interconnected and highly dynamic organelle essential for multiple cellular functions. In neurons, the ER extends into axons, where it plays a pivotal role in maintaining neuronal polarity. The unique structural and dynamic adaptations of the axonal ER enable it to meet the specialized demands of neurons, ranging from compartmentalized physiological regulation to long-distance intracellular communication. Recent studies have shown that axonal ER supports the regulation of organelle remodeling and trafficking in a spatiotemporal manner, processes that become compromised in aged neurons. Moreover, disruptions in the structure and dynamics of the axonal ER have increasingly become associated with neurodegenerative diseases, including hereditary spastic paraplegia, amyotrophic lateral sclerosis, and peripheral neuropathies. This review synthesizes current knowledge of axonal ER biology, highlighting its structural and dynamic characteristics, its impact on organelle arrangement and distribution, and its pathological implications in neurodegeneration. By consolidating recent advances, this review outlines emerging questions and future directions in axonal ER research, a field gaining recognition for its contribution to neuronal dysfunction and neurodegenerative pathomechanisms.