Trace Elements Dyshomeostasis and Toxic Metals Neurotoxicity in Neurodegenerative Diseases.

Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis, are defined by the progressive loss of neurons through interconnected pathological mechanisms, including oxidative stress, mitochondrial dysfunction, protein aggregation, and neuroinflammation. Accumulating evidence implicates metal dyshomeostasis as a central and multifaceted contributor to these mechanisms, with roles ranging from a primary pathogenic driver in AD and PD, to a secondary amplifier of genetic pathology in HD and ALS, and as a contextual risk modifier in the presence of toxic metals. Essential trace metals such as iron, zinc, copper, manganese, selenium, iodine, and molybdenum are vital for neurotransmission, antioxidant defense, and cellular metabolism. Dysregulation of these metals disrupts redox balance, impairs proteostasis, and activates regulated cell death pathways, including ferroptosis and cuproptosis. Toxic metals, such as lead, cadmium, and mercury, exacerbate neurodegeneration by displacing essential metals, inducing oxidative injury, and promoting protein misfolding and neuroinflammation. This narrative review synthesizes mechanistic, experimental, genetic epidemiological, and clinical evidence to critically evaluate the contributions of both essential and toxic metals to neurodegeneration in AD, PD, HD, and ALS. We examine the genetic, environmental, and physiological determinants of metal homeostasis; the analytical techniques for quantifying metals in clinical samples; and clinical trial data on metal-targeted therapeutic strategies. Notably, iron chelation with deferiprone consistently reduces brain iron on neuroimaging but worsens clinical outcomes in both PD and AD, presenting a translational paradox that requires mechanistic re-evaluation. We also provide methodological recommendations for interpreting Mendelian randomization studies of metal exposures and propose translational priorities to advance metal-targeted diagnostics and therapeutics for neurodegenerative diseases.