Neuron-Derived Extracellular Vesicles: Emerging Biomarkers and Functional Mediators in Alzheimer's Disease, With Comparative Insights Into Neurodevelopment and Aging.

Alzheimer's disease (AD) is one of the most common neurodegenerative disorders characterized by the accumulation of amyloid-β (Aβ) peptide and phosphorylated tau protein in the brain. Despite intensive efforts, early diagnosis and monitoring of AD remain challenging due to the lack of reliable biomarkers that can detect the disease in its preclinical stages. As a result, there exists a requirement for novel approaches to the diagnosis and treatment of the disease. Extracellular vesicles provide the transfer of Aβ peptides and tau proteins between the cells and participates in the spreading/propagation of disease pathology. Neuron-derived extracellular vesicles (NDEVs) that are found in plasma have emerged as promising candidates, especially for biomarker studies on neurodegenerative diseases because they are reachable and comparable with cerebrospinal fluid (CSF) studies. In addition to known proteins, synaptic proteins, transcription factors, or microRNAs have been suggested as new biomarkers, aiming to help differential or early diagnosis. Beyond their involvement in AD pathology, NDEVs also play essential roles in neurodevelopment and aging by mediating cell-to-cell communication and regulating processes such as synaptic formation, neuronal differentiation, and neuroinflammation. Age-related alterations in EV composition and secretion may contribute to the decline in neuroplasticity, thereby increasing susceptibility to neurodegenerative diseases like AD. Several challenges such as heterogeneous isolation of NDEVs limit the widespread clinical application of them as biomarkers for AD. Furthermore, the lack of standardized protocols for vesicle isolation and molecular analysis poses a barrier to reproducibility and clinical validation. The aim of this review is to elucidate the role of NDEVs in AD pathogenesis in comparison with their functions in neurodevelopment and aging, evaluate their potential as biomarkers for early diagnosis, while addressing the challenges in their isolation, characterization, and clinical application.