Dissociation between iron accumulation and ferritin upregulation in the aged substantia nigra: attenuation by dietary restriction

Abstract

Despite regulation, brain iron increases with aging and may enhance aging processes including neuroinflammation. Increases in magnetic resonance imaging transverse relaxation rates, R2 and R2*, in the brain have been observed during aging. We show R2 and R2* correlate well with iron content via direct correlation to semi-quantitative synchrotron-based X-ray fluorescence iron mapping, with age-associated R2 and R2* increases reflecting iron accumulation. Iron accumulation was concomitant with increased ferritin immunoreactivity in basal ganglia regions except in the substantia nigra (SN). The unexpected dissociation of iron accumulation from ferritin-upregulation in the SN suggests iron dyshomeostasis in the SN. Occurring alongside microgliosis and astrogliosis, iron dyshomeotasis may contribute to the particular vulnerability of the SN. Dietary restriction (DR) has long been touted to ameliorate brain aging and we show DR attenuated agerelated in vivo R2 increases in the SN over ages 7 – 19 months, concomitant with normal iron-induction of ferritin expression and decreased microgliosis. Iron is known to induce microgliosis and conversely, microgliosis can induce iron accumulation, which of these may be the initial pathological aging event warrants further investigation. We suggest iron chelation therapies and anti-inflammatory treatments may be putative ‘antibrain aging’ therapies and combining these strategies may be synergistic.

Regional Distributions of Iron, Copper and Zinc and Their Relationships With Glia in a Normal Aging Mouse Model

“…Consistent with previous reports including our own (Walker et al, 2016), iron was present at higher concentrations in the substantia nigra and globus pallidus compared to that in the striatum, cortex, and hippocampus at the older ages (Hallgren and Sourander, 1958). The iron levels increased with age predominantly in the basal ganglia, suggesting iron to be an age-dependent enriched metal in the brain (Connor et al, 1990).…”

“…Indeed, increased ferritin mirrors the iron deposition in the globus pallidus and striatum we observed. However, a dissociation between iron accumulation and ferritin upregulation was observed in the aged substantia nigra, where higher levels of iron were not associated with increased levels of ferritin as we and others have reported previously (Benkovic and Connor, 1993;Walker et al, 2016). Ferritin is composed of 24 subunits of two types, heavy and light chains, forming a soluble hollow shell capable of storing up to 4,500 ferric iron atoms (Harrison and Arosio, 1996).…”

“…We and others have previously reported increased microglial and astroglial cell numbers with aging in the basal ganglia, providing evidence for a more primed or inflammatory profile (Codazzi et al, 2015;Walker et al, 2016;Boisvert et al, 2018). Aging is associated with chronically elevated levels of circulating cytokines including TNFα, IL1β and TGFβ, with glial cells being major driving factors for brain aging (von Bernhardi et al, 2010). Interestingly, microglial and astroglial iron homeostasis has been shown to be differentially regulated by TNFα and TGFβ.…”

“…Assessment of microglia and astrocytes by ionized calcium binding adaptor molecule 1 (Iba1) and glial fibrillary acidic protein (GFAP) immunohistochemistry (IHC), respectively, was also performed. We have focussed on the basal ganglia which we and others have demonstrated to accumulate iron with aging (Aquino et al, 2009;Walker et al, 2016). We have previously reported the quantitative assessments of iron; and ferritin-, Iba1-and GFAP-immunopositive cells in the basal ganglia at these ages (Walker et al, 2016).…”

“…We have focussed on the basal ganglia which we and others have demonstrated to accumulate iron with aging (Aquino et al, 2009;Walker et al, 2016). We have previously reported the quantitative assessments of iron; and ferritin-, Iba1-and GFAP-immunopositive cells in the basal ganglia at these ages (Walker et al, 2016). Here, we have extended the study/reanalyzed the data to include both qualitative and quantitative assessments of other metals, and particularly focussed on investigating the relationships between metals and ferritin-, Iba1-and GFAP-immunopositive cells in the basal ganglia.…”