Metallothioneins in Neurodegenerative Diseases: Metal Homeostasis, Autoimmunity, and Therapeutic Potential

Neurodegenerative diseases, including multiple sclerosis, Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis, are characterized by progressive neuronal loss and are frequently linked to metal dysregulation, oxidative stress, and immune dysfunction. Metallothioneins (MTs), a family of cysteine-rich, metal-binding proteins, are critical in maintaining metal homeostasis, mitigating oxidative damage, and modulating immune responses, functions highly relevant in these pathologies. MTs regulate essential metals like copper and iron by preventing their participation in harmful redox reactions and control zinc availability for enzymatic and signaling processes. They also detoxify neurotoxic metal(oid)s such as cadmium, mercury, lead, and arsenic, thereby reducing their adverse neurological and immunological effects. In autoimmune neurodegeneration, MTs modulate pro- and anti-inflammatory cytokines (e.g., IL-6, TNF-α, IL-10) and influence immune cell activity, particularly microglia and T cells, which are central to neuroinflammation and autoimmunity. Through these mechanisms, MTs play a dual role in sustaining immune homeostasis and counteracting oxidative stress. Their capacity to integrate metal regulation with immune modulation positions them as promising therapeutic targets, with preclinical and some clinical evidence supporting strategies to enhance MT expression or develop MT-mimetic agents to address both metal dysregulation and immune imbalance. Additionally, MTs show emerging utility as biomarkers, as alterations in MT isoform expression and metal-bound complexes in biofluids have been associated with disease onset, progression, and therapeutic response in specific neurodegenerative conditions. This article reviews the multifaceted roles of MTs in neurodegenerative diseases, emphasizing their function in metal and immune regulation and their emerging potential as therapeutic targets and clinical biomarkers.