Ilaria Pelizzoni

Iron handling in hippocampal neurons: activity-dependent iron entry and mitochondria-mediated neurotoxicity

The characterization of iron handling in neurons is still lacking, with contradictory and incomplete results. In particular, the relevance of non‐transferrin‐bound iron (NTBI), under physiologic conditions, during aging and in neurodegenerative disorders, is undetermined. This study investigates the mechanisms underlying NTBI entry into primary hippocampal neurons and evaluates the consequence of iron elevation on neuronal viability. Fluorescence‐based single cell analysis revealed that an increase in extracellular free Fe2+ (the main component of NTBI pool) is sufficient to promote Fe2+ entry and that activation of either N‐methyl‐d‐aspartate receptors (NMDARs) or voltage operated calcium channels (VOCCs) significantly potentiates this pathway, independently of changes in intracellular Ca2+ concentration ([Ca2+]i). The enhancement of Fe2+ influx was accompanied by a corresponding elevation of reactive oxygen species (ROS) production and higher susceptibility of neurons to death. Interestingly, iron vulnerability increased in aged cultures. Scavenging of mitochondrial ROS was the most powerful protective treatment against iron overload, being able to preserve the mitochondrial membrane potential and to safeguard the morphologic integrity of these organelles. Overall, we demonstrate for the first time that Fe2+ and Ca2+ compete for common routes (i.e. NMDARs and different types of VOCCs) to enter primary neurons. These iron entry pathways are not controlled by the intracellular iron level and can be harmful for neurons during aging and in conditions of elevated NTBI levels. Finally, our data draw the attention to mitochondria as a potential target for the treatment of the neurodegenerative processes induced by iron dysmetabolism.

Iron uptake in quiescent and inflammation-activated astrocytes: A potentially neuroprotective control of iron burden

Astrocytes play a crucial role in proper iron handling within the central nervous system. This competence can be fundamental, particularly during neuroinflammation, and neurodegenerative processes, where an increase in iron content can favor oxidative stress, thereby worsening disease progression. Under these pathological conditions, astrocytes undergo a process of activation that confers them either a beneficial or a detrimental role on neuronal survival. Our work investigates the mechanisms of iron entry in cultures of quiescent and activated hippocampal astrocytes. Our data confirm that the main source of iron is the non-transferrin-bound iron (NTBI) and show the involvement of two different routes for its entry: the resident transient receptor potential (TRP) channels in quiescent astrocytes and the de novo expressed divalent metal transporter 1 (DMT1) in activated astrocytes, which accounts for a potentiation of iron entry. Overall, our data suggest that at rest, but even more after activation, astrocytes have the potential to buffer the excess of iron, thereby protecting neurons from iron overload. These findings further extend our understanding of the protective role of astrocytes under the conditions of iron-mediated oxidative stress observed in several neurodegenerative conditions.

Expression of divalent metal transporter 1 in primary hippocampal neurons: reconsidering its role in non‐transferrin‐bound iron influx

J. Neurochem. (2012) 120, 269–278.

β-Secretase activity in rat astrocytes: Translational block of BACE1 and modulation of BACE2 expression

BACE1 and BACE2 are two closely related membrane‐bound aspartic proteases. BACE1 is widely recognized as the neuronal β‐secretase that cleaves the amyloid‐β precursor protein, thus allowing the production of amyloid‐β, i.e. the peptide that has been proposed to trigger the neurodegenerative process in Alzheimer’s disease. BACE2 has ubiquitous expression and its physiological and pathological role is still unclear. In light of a possible role of glial cells in the accumulation of amyloid‐β in brain, we have investigated the expression of these two enzymes in primary cultures of astrocytes. We show that astrocytes possess β‐secretase activity and produce amyloid‐β because of the activity of BACE2, but not BACE1, the expression of which is blocked at the translational level. Finally, our data demonstrate that changes in the astrocytic phenotype during neuroinflammation can produce both a negative as well as a positive modulation of β‐secretase activity, also depending on the differential responsivity of the brain regions.

Astrocytes acquire resistance to iron-dependent oxidative stress upon proinflammatory activation

BackgroundAstrocytes respond to local insults within the brain and the spinal cord with important changes in their phenotype. This process, overall known as “activation”, is observed upon proinflammatory stimulation and leads astrocytes to acquire either a detrimental phenotype, thereby contributing to the neurodegenerative process, or a protective phenotype, thus supporting neuronal survival. Within the mechanisms responsible for inflammatory neurodegeneration, oxidative stress plays a major role and has recently been recognized to be heavily influenced by changes in cytosolic iron levels. In this work, we investigated how activation affects the competence of astrocytes to handle iron overload and the ensuing oxidative stress.MethodsCultures of pure cortical astrocytes were preincubated with proinflammatory cytokines (interleukin-1β and tumor necrosis factor α) or conditioned medium from lipopolysaccharide-activated microglia to promote activation and then exposed to a protocol of iron overload.ResultsWe demonstrate that activated astrocytes display an efficient protection against iron-mediated oxidative stress and cell death. Based on this evidence, we performed a comprehensive biochemical and molecular analysis, including a transcriptomic approach, to identify the molecular basis of this resistance.ConclusionsWe propose the protective phenotype acquired after activation not to involve the most common astrocytic antioxidant pathway, based on the Nrf2 transcription factor, but to result from a complex change in the expression and activity of several genes involved in the control of cellular redox state.

Iron entry in neurons and astrocytes: a link with synaptic activity

Iron plays a fundamental role in the development of the central nervous system (CNS) as well as in several neuronal functions including synaptic plasticity. Accordingly, neuronal iron supply is tightly controlled: it depends not only on transferrin-bound iron but also on non-transferrin-bound iron (NTBI), which represents a relevant quote of the iron physiologically present in the cerebrospinal fluid (CSF). Different calcium permeable channels as well as the divalent metal transporter 1 (DMT1) have been proposed to sustain NTBI entry in neurons and astrocytes even though it remains an open issue. In both cases, it emerges that the control of iron entry is tightly linked to synaptic activity. The iron-induced oxidative tone can, in physiological conditions, positively influence the calcium levels and thus the synaptic plasticity. On the other hand, an excess of iron, with the ensuing uncontrolled production of reactive oxygen species (ROS), is detrimental for neuronal survival. A protective mechanism can be played by astrocytes that, more resistant to oxidative stress, can uptake iron, thereby buffering its concentration in the synaptic environment. This competence is potentiated when astrocytes undergo activation during neuroinflammation and neurodegenerative processes. In this minireview we focus on the mechanisms responsible for NTBI entry in neurons and astrocytes and on how they can be modulated during synaptic activity. Finally, we speculate on the relevance they may have in both physiological and pathological conditions.

Iron and calcium in the central nervous system: a close relationship in health and sickness.

Iron and calcium are required for general cellular functions, as well as for specific neuronal-related activities. However, a pathological increase in their levels favours oxidative stress and mitochondrial damage, leading to neuronal death. Neurodegeneration can thus be determined by alterations in ionic homoeostasis and/or pro-oxidative-antioxidative equilibrium, two conditions that vary significantly in different kinds of brain cell and also with aging. In the present review, we re-evaluate recent data on NTBI (non-transferrin bound iron) uptake that suggest a strict interplay with the mechanisms of calcium control. In particular, we focus on the use of common entry pathways and on the way cytosolic calcium can modulate iron entry and determine its intracellular accumulation.

Friedreich ataxia-induced pluripotent stem cell-derived neurons show a cellular phenotype that is corrected by a benzamide HDAC inhibitor

We employed induced pluripotent stem cell (iPSC)-derived neurons obtained from Friedreich ataxia (FRDA) patients and healthy subjects, FRDA neurons and CT neurons, respectively, to unveil phenotypic alterations related to frataxin (FXN) deficiency and investigate if they can be reversed by treatments that upregulate FXN. FRDA and control iPSCs were equally capable of differentiating into a neuronal or astrocytic phenotype. FRDA neurons showed lower levels of iron–sulfur (Fe–S) and lipoic acid-containing proteins, higher labile iron pool (LIP), higher expression of mitochondrial superoxide dismutase (SOD2), increased reactive oxygen species (ROS) and lower reduced glutathione (GSH) levels, and enhanced sensitivity to oxidants compared with CT neurons, indicating deficient Fe–S cluster biogenesis, altered iron metabolism, and oxidative stress. Treatment with the benzamide HDAC inhibitor 109 significantly upregulated FXN expression and increased Fe–S and lipoic acid-containing protein levels, downregulated SOD2 levels, normalized LIP and ROS levels, and almost fully protected FRDA neurons from oxidative stress-mediated cell death. Our findings suggest that correction of FXN deficiency may not only stop disease progression, but also lead to clinical improvement by rescuing still surviving, but dysfunctional neurons.

P3-352: Beta-secretase modulation and amyloid-beta production in rat brain primary cultures

Background: The processing of the amyloid-beta precursor protein by a beta-secretase is the rate-limiting step in the production of amyloid-beta within the central nervous system. Two beta-secretases have been identified, BACE1, mainly expressed in neurons, and BACE2, with a more ubiquitous expression pattern. Their expression is tightly regulated at both the transcriptional and translational level. In primary cultures from rat brain, we investigated: i) the regulation of BACE1 expression in neurons; ii) the regulation of beta-secretase activity, in resting and activated astrocytes, and its contribution to amyloid-beta production. Methods: Primary cultures of neurons and astrocytes from rat brain were employed. The translational control of BACE1 expression was evaluated by transfection of reporter genes preceded by the BACE1 transcript leader. For the evaluation of endogenous beta-secretases, we tested their activity by an in vitro assay, and BACE1/2 expression by RNA analysis and Western blotting. Results: In a previous work we demonstrated that the translation driven by the BACE1 transcript leader (as evaluated by reporter genes) is higher in neurons than in astrocytes or other commonly used cell lines. In the present study, we investigated the molecular determinants of this control by analyzing the features of the BACE1 mRNA and monitoring changes in endogenous BACE1 upon various treatments. This analysis revealed the presence of both inhibitory and stimulatory sequences in the transcript leader. Furthermore, we demonstrated the presence of a translational block that is specifically bypassed in neurons. In parallel, we studied how betasecretase activity is regulated in astrocytes. We did not detect BACE1 protein, even under conditions of astrocyte activation induced by various stimuli (including pro-inflammatory cytokines). Our data show that betasecretase activity in astrocytes was due to the expression of BACE2. On the other hand, activated astrocytes displayed a reduced BACE2 expression as well as a decrease in beta-secretase activity. We are currently investigating the role of astrocyte activation on the production of amyloid-beta. Conclusions: BACE1 expression is regulated at the translational level by a neuronal-specific mechanism. Astrocytes do not express BACE1 and, upon activation with pro-inflammatory cytokines, they down-regulate BACE2 expression, thus decreasing beta-secretase activity and, likely, amyloidbeta production.