Iron Loading Selectively Increases Hippocampal Levels of Ubiquitinated Proteins and Impairs Hippocampus-Dependent Memory

Abstract

Alterations of brain iron levels have been observed in a number of neurodegenerative disorders. We have previously demonstrated that iron overload in the neonatal period results in severe and persistent memory deficits in the adulthood. Protein degradation mediated by the ubiquitin-proteasome system (UPS) plays a central regulatory role in several cellular processes. Impairment of the UPS has been implicated in the pathogenesis of neurodegenerative disorders. Here, we examined the effects of iron exposure in the neonatal period (12th-14th day of postnatal life) on the expression of proteasome β-1, β-2, and β-5 subunits, and ubiquitinated proteins in brains of 15-day-old rats, to evaluate the immediate effect of the treatment, and in adulthood to assess long-lasting effects. Two different memory types, emotionally motivated conditioning and object recognition were assessed in adult animals. We found that iron administered in the neonatal period impairs both emotionally motivated and recognition memory. Polyubiquitinated protein levels were increased in the hippocampus, but not in the cortex, of adult animals treated with iron. Gene expression of subunits β1 and β5 was affected by age, being higher in the early stages of development in the hippocampus, accompanied by an age-related increase in polyubiquitinated protein levels in adults. In the cortex, gene expression of the three proteasome subunits was significantly higher in adulthood than in the neonatal period. These findings suggest that expression of proteasome subunits and activity are age-dependently regulated. Iron exposure in the neonatal period produces long-lasting harmful effects on the UPS functioning, which may be related with iron-induced memory impairment.

Antiapoptotic effects of cannabidiol in an experimental model of cognitive decline induced by brain iron overload

“…We have evidence that the effects of neonatal iron overload in the brain intensify gradually throughout life. For instance, iron treatment increased the content of ubiquitinated proteins, a marker of UPS-ubiquitin system deterioration, in the hippocampus of adult rats, while no effects were observed when analysis was performed earlier in life 14 . In fact, studies using aged rats and mice suggest that iron accumulates and redistributes in brain regions during life without a coincident increase in ferritin, the main cellular iron storage protein in neurons, suggesting an age-related iron dyshomeostasis 39,40 .…”

“…In cultures of hippocampal slices exposed to iron, ROS formation and lipid peroxidation were increased, in association with Cytochrome c and Caspase 3-dependent apoptotic pathways 37 . Iron overload in the neonatal period induces severe hippocampus-dependent memory deficits, indicating hippocampal dysfunction 5–9,11,14 , while studies performed in humans have correlated iron accumulation in selective brain regions with poor performance in cognitive tests (for a review, see ref. 38 ).…”

“…We have previously described that iron overload induces severe and persistent long-term impairments in spatial, recognition, and emotional memories 5–11 . In molecular analyses, we found lipid peroxidation and oxidative damage associated with iron excess 6 , increased apoptotic markers, Par4 12 and Caspase 3 13 , accumulation of ubiquitinated proteins 14 , and reactive gliosis 15 . Moreover, iron treatment in the neonatal period decreased acetylcholinesterase activity in the striatum 8 and affected the regulation of iron homeostasis proteins in the hippocampus, cortex, and striatum of aged rats 16 .…”

G protein‐coupled oestrogen receptor stimulation ameliorates iron‐ and ovariectomy‐induced memory impairments through the cAMP/PKA/CREB signalling pathway

“…The neonatal iron treatment was performed as described previously. This treatment has been reported to induce memory deficits in male rats . Briefly, 12‐day‐old rat pups received a single oral daily dose (10 mL kg ‐1 solution volume) of vehicle (5% sorbitol in water, control group) or 30 mg kg ‐1 of body weight of Fe 2+ (iron carbonyl; Sigma‐Aldrich, São Paulo, Brazil) via a metallic gastric tube, over 3 consecutive days during days 12‐14 after birth.…”

“…Our laboratory has focused on iron metabolism subsequent to our description of an animal model of iron overload . Iron administration is performed during the neonatal period of rats, the period of maximum absorption of iron through the blood‐brain barrier, and, in adulthood, these animals develop impairments in a variety of memory tasks, including spatial, aversive, and recognition memory . These deficits are associated with increased levels of oxidative stress, apoptotic markers, astrogliosis, mitochondrial damage and impaired ubiquitin‐proteasome functioning .…”