Exploring the skin as an open window onto neurodegenerative diseases.

Neurodegenerative diseases (NDDs), including Parkinson's disease and Alzheimer's disease, are major age-related disorders characterized by progressive neuronal degeneration and a decline in cognitive and motor functions. Managing NDDs poses an increasing healthcare challenge as the global population ages. The onset of NDDs is linked to protein misfolding, oxidative stress, dysfunction of mitochondria and lysosomes, and neuroinflammation. Clinical manifestations of NDDs only appear after substantial neuronal damage has already occurred. This underscores the urgent need for accessible tissue biomarkers to enable early diagnosis, disease monitoring and assessment of therapeutic efficacy. The skin has emerged as a valuable peripheral indicator of neurodegeneration, sharing embryological origin, gene expression profiles, protein alterations and cellular dysfunctions with the brain. Notably, pathological protein deposits, which are hallmarks of NDDs, such as beta-amyloid, tau proteins, and oligomeric alpha-synuclein, have been observed in the skin. Increasing evidence links NDDs with various pathological skin conditions, including melanoma and inflammatory diseases. This review aims to explore the potential of the skin as a window into neurodegenerative processes at an early stage, before clinical signs arise. The main advantages of using skin as a source of NDD biomarkers are its accessibility and the minimally invasive sampling methods such as stratum corneum collection, sebum and volatile compounds analysis, and biopsies. Immunohistochemistry and omics approaches applied to skin samples provide valuable insights into NDD pathophysiology and facilitate biomarker discovery for early diagnosis and disease monitoring. NDDs are multisystemic disorders and new findings in skin research highlight the value of peripheral tissues for investigating central nervous system alterations enabling earlier neuroprotective interventions.