**3. Ferroptosis in Neurodegenerative Diseases**

Iron is vital to the physiology of all human tissues. However, under certain conditions it can be harmful, especially for the brain. Although the cellular metabolism of the CNS requires iron as a redox metal for energy generation, mainly the production of ATP, nervous tissue is vulnerable to oxidative damage generated by excess iron and decreased antioxidant systems [110–113]. The explicit identification of ferroptosis in vivo is hampered by the lack of specific biomarkers, due to several factors that may be associated with the ferroptotic process. However, there is considerable evidence that implicates ferroptosis in the pathophysiology of neurodegeneration. Ferroptosis involves the accumulation of brain iron, glutathione depletion and lipid peroxidation simultaneously, which triggers a cascade of events including activation of inflammation, neurotransmitter oxidation, neuronal communication failure, myelin sheath degeneration, astrocyte dysregulation, dementia and cell death. Iron or free iron overload can initiate lipid peroxidation in neurons, astrocytes, oligodendrocytes, microglia and Schwann cells. In addition, the low activity of GPx4 and the glutathione system have been shown to be associated with ferroptosis in motor neurodegeneration [114–117].

Recently, it has been proposed that the modulation of ferroptosis may be beneficial for neurodegenerative diseases and that inhibition of ferroptosis by GPx4 could provide protective mechanisms against neurodegeneration [118]. First, it was demonstrated that the non-oxidative form of dopamine is a strong inhibitor of ferroptotic cell death. Dopamine reduced erastin-induced ferrous iron accumulation, glutathione depletion, and malondialdehyde production. Moreover, dopamine increased the stability of GPx4 [119]. The GPx4 enzyme is essential for the survival of parvalbumin-positive interneurons and prevention of seizures, as well as protection against ferroptosis in animal models [42,120]. Next, in a study with PC12 cell line (a model system for neurobiological and neurochemical studies), cell death was induced by *tert*-butylhydroperoxide (t-BHP), a widespread inducer of oxidative stress; it was observed that t-BHP increased the generation of lipid ROS, decreased the expression of GPx4 and the ratio of GSH/GSSG. All these effects could be reversed by the ferroptosis inhibitor, ferrostatin-1 and deferoxamine, iron chelator. In addition, JNK1/2 and ERK1/2 were activated upstream from the ferroptosis and mitochondrial dysfunction [121].

In addition, in C57BL/6 J male mice treated with arsenite for 6 months, it was observed that arsenite induced ferroptotic cell death in neurons by the accumulation of reactive oxygen species and lipid peroxidation products, disruption of Fe2<sup>+</sup> homeostasis, depletion of glutathione and adenosine triphosphate, inhibition of system xc−, activation of mitogen-activated protein kinases and mitochondrial voltage-dependent anion channel pathways, and upregulation of endoplasmic reticulum stress [122]. This is an important issue because arsenite (inorganic arsenic) has been associated with neural loss and Alzheimer's and Parkinson's diseases as well as amyotrophic lateral sclerosis (ALS) [123–130]. An in vitro study showed that exposure to paraquat and maneb induced ferroptosis in dopaminergic SHSY5Y cells, associated with the activation of NADPH oxidase. The activation of NADPH oxidase contributed to the dopaminergic neurodegeneration associated with lipid peroxidation and neuroinflammation [131].

In a multiple sclerosis model and in an experimental autoimmune encephalomyelitis (EAE) animal model [124], it has been observed that mRNA expression of the cytoplasmic, mitochondrial and nuclear GPx4 enzyme decreased in multiple sclerosis gray matter and in the spinal cord of EAE. Neuronal GPx4 was lower in EAE spinal cords than in controls. Moreover, γ-glutamylcysteine ligase and cysteine/glutamate antiporter were diminished in EAE, which is associated with high accumulation of lipid peroxidation products and the reduction in the proportion of the docosahexaenoic acid in non-myelin lipids. These results, together with the presence of abnormal neuronal mitochondrial morphology, which includes an irregular matrix, ruptured external membrane and reduced/absent ridges, are consistent with the occurrence of ferroptotic damage in inflammatory demyelinating disorders [132].

In fact, iron can bind to IRPs, leading to the dissociation of IRPs from the IRE and altered translation of the target transcripts. Recently, an IRE was found in the 5- -UTR of the amyloid precursor protein (APP) and α-synuclein transcripts (α-Syn). The levels of α-Syn, APP and amyloid β (Aβ) peptide, as well as protein aggregation, can be negatively regulated by IRPs, but are regulated positively in the presence of iron accumulation. Therefore, it has been suggested that the inhibition of the IRE-modulated expression of APP and α-Syn or iron chelation in patient brains has therapeutic significance for human neurodegenerative diseases [133].
