Iron is an essential element for the development and functionality of the brain, and anomalies in its distribution and concentration in brain tissue have been found to be associated with the most frequent neurodegenerative diseases. When magnetic resonance techniques allowed iron quantification in vivo, it was confirmed that the alteration of brain iron homeostasis is a common feature of many neurodegenerative diseases. However, whether iron is the main actor in the neurodegenerative process, or its alteration is a consequence of the degenerative process is still an open question. Because the different iron-related pathogenic mechanisms are specific for distinctive diseases, identifying the molecular mechanisms common to the various pathologies could represent a way to clarify this complex topic. Indeed, both iron overload and iron deficiency have profound consequences on cellular functioning, and both contribute to neuronal death processes in different manners, such as promoting oxidative damage, a loss of membrane integrity, a loss of proteostasis, and mitochondrial dysfunction. In this review, with the attempt to elucidate the consequences of iron dyshomeostasis for brain health, we summarize the main pathological molecular mechanisms that couple iron and neuronal death.

Aims: The endoplasmic reticulum (ER) is involved in many functions, including protein folding, redox homeostasis, and Ca ²⁺ storage and signaling. To perform these multiple tasks, the ER is composed of distinct, specialized subregions, amongst which mitochondrial-associated ER membranes (MAM) emerge as key signaling hubs. How these multiple functions are integrated with one another in living cells remains unclear. Results: Here we show that Ero1α, a key controller of oxidative folding and ER redox homeostasis, is enriched in MAM and regulates Ca ²⁺ fluxes. Downregulation of Ero1α by RNA interference inhibits mitochondrial Ca ²⁺ fluxes and modifies the activity of mitochondrial Ca ²⁺ uniporters. The overexpression of redox active Ero1α increases passive Ca ²⁺ efflux from the ER, lowering [Ca ²⁺ ] _ER and mitochondrial Ca ²⁺ fluxes in response to IP3 agonists. Innovation: The unexpected observation that Ca ²⁺ fluxes are affected by either increasing or decreasing the levels of Ero1α reveals a pivotal role for this oxidase in the early secretory compartment and implies a strict control of its amounts. Conclusions: Taken together, our results indicate that the levels, subcellular localization, and activity of Ero1α coordinately regulate Ca ²⁺ and redox homeostasis and signaling in the early secretory compartment. Antioxid. Redox Signal. 16, 1077–1087.

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Neuroferritinopathy (NF) is a movement disorder caused by alterations in the L-ferritin gene that generate cytosolic free iron. NF is a unique pathophysiological model for determining the direct consequences of cell iron dysregulation. We established lines of induced pluripotent stem cells from fibroblasts from two NF patients and one isogenic control obtained by CRISPR/Cas9 technology. NF fibroblasts, neural progenitors, and neurons exhibited the presence of increased cytosolic iron, which was also detectable as: ferritin aggregates, alterations in the iron parameters, oxidative damage, and the onset of a senescence phenotype, particularly severe in the neurons. In this spontaneous senescence model, NF cells had impaired survival and died by ferroptosis. Thus, non-ferritin-bound iron is sufficient per se to cause both cell senescence and ferroptotic cell death in human fibroblasts and neurons. These results provide strong evidence supporting the primary role of iron in neuronal aging and degeneration.

In 2001, a new type of human ferritin was identified by searching for homologous sequences to H-ferritin in the human genome. After the demonstration that this ferritin is located specifically in the mitochondrion, it was called mitochondrial ferritin. Studies on the properties of this new type of ferritin have been limited by its very high homology with the cytosolic H-ferritin, which is expressed at higher levels in cells. This great similarity made it difficult to obtain specific antibodies against the mitochondrial ferritin devoid of cross-reactivity with cytosolic ferritin. Thus, the knowledge of the physiological role of mitochondrial ferritin is still incomplete despite 20 years of research. In this review, we summarize the literature on mitochondrial ferritin expression regulation and its physical and biochemical properties, with particular attention paid to the differences with cytosolic ferritin and its role in physiological condition. Until now, there has been no evidence that the alteration of the mitochondrial ferritin gene is causative of any disorder; however, the identified association of the mitochondrial ferritin with some disorders is discussed.

Neurodegeneration associated with defective pantothenate kinase-2 (PKAN) is an early-onset monogenic autosomal-recessive disorder. The hallmark of the disease is the massive accumulation of iron in the globus pallidus brain region of patients. PKAN is caused by mutations in the PANK2 gene encoding the mitochondrial enzyme pantothenate kinase-2, whose function is to catalyze the first reaction of the CoA biosynthetic pathway. To date, the way in which this alteration leads to brain iron accumulation has not been elucidated. Starting from previously obtained hiPS clones, we set up a differentiation protocol able to generate inhibitory neurons. We obtained striatal-like medium spiny neurons composed of approximately 70–80% GABAergic neurons and 10–20% glial cells. Within this mixed population, we detected iron deposition in both PKAN cell types, however, the viability of PKAN GABAergic neurons was strongly affected. CoA treatment was able to reduce cell death and, notably, iron overload. Further differentiation of hiPS clones in a pure population of astrocytes showed particularly evident iron accumulation, with approximately 50% of cells positive for Perls staining. The analysis of these PKAN astrocytes indicated alterations in iron metabolism, mitochondrial morphology, respiratory activity, and oxidative status. Moreover, PKAN astrocytes showed signs of ferroptosis and were prone to developing a stellate phenotype, thus gaining neurotoxic features. This characteristic was confirmed in iPS-derived astrocyte and glutamatergic neuron cocultures, in which PKAN glutamatergic neurons were less viable in the presence of PKAN astrocytes. This newly generated astrocyte model is the first in vitro disease model recapitulating the human phenotype and can be exploited to deeply clarify the pathogenetic mechanisms underlying the disease.