A commentary on studies of brain iron accumulation during ageing

Abstract

Brain iron content is widely reported to increase during "ageing", across multiple species from nematodes, rodents (mice and rats) and humans. Given the redox-active properties of iron, there has been a large research focus on iron-mediated oxidative stress as a contributor to tissue damage during natural ageing, and also as a risk factor for neurodegenerative disease. Surprisingly, however, the majority of published studies have not investigated brain iron homeostasis during the biological time period of senescence, and thus knowledge of how brain homeostasis changes during this critical stage of life largely remains unknown. This commentary examines the literature published on the topic of brain iron homeostasis during ageing, providing a critique on limitations of currently used experimental designs. The commentary also aims to highlight that although much research attention has been given to iron accumulation or iron overload as a pathological feature of ageing, there is evidence to support functional iron deficiency may exist, and this should not be overlooked in studies of ageing or neurodegenerative disease.

Keywords: Brain rust; Metallomics; Senescence; X-ray fluorescence; XFM.

Fig. 1

Schematic of Fe transport into and within the brain: A large fraction of Fe destined for the brain is transported in the blood stream bound to transferrin (the main Fe-transport protein), however substantial pool of non-transferrin bound Fe is also present, such as Fe bound to albumin or in low-molecular-weight complexes with molecules such as ATP or citrate. At the blood–brain barrier (e.g. endothelial cell apical surface), transferrin bound Fe enters endothelial cells via transferrin receptor-mediated internalisation on the luminal surface. Alternatively, labile of Fe³⁺ may be reduced by reductases on endothelial cell surface, which could enable Fe²⁺ via divalent metal transporters (DMT). Direct entry of low-molecular-weight complexes of Fe³⁺ into endothelial cells, such as Fe³⁺–citrate complexes could also be possible. Within cell cytoplasm, Fe²⁺ may be oxidised via enzymes with ferroxidase activity, enabling subsequent export of Fe³⁺ through ferroportin. Release of Fe³⁺ from abluminal membrane of endothelial cells, via ferroportin, is one pathway through which Fe³⁺ could enter the brain interstitial fluid. Once in interstitial fluid, Fe³⁺ may exist as low-molecular-weight or labile Fe³⁺ complexes, it may be taken up via apo-transferrin, or it may be reduced to Fe²⁺ where it can enter other brain cells (astrocytes, oligodendrocytes, neurons, etc.) through membrane-bound DMT. Transferrin receptors on cell membranes of neurons present a major pathway for Fe entry, from the interstitial Fe pool, into brain cells. It is not clear if transferrin receptors enable Fe entry into glia in vivo. Similarly, there is conflicting literature regarding the in vivo role of DMT between different brain cells. Fe³⁺ bound to low-molecular-weight complexes, such as Fe³⁺–citrate represents another pathway through which Fe enters brain cells from the interstitial fluid. Excess Fe within brain cells is sequestered/stored as ferritin, with glial cells displaying far greater capacity for Fe storage than neurons. Note: This schematic is for general illustration purposes, and is not meant to convey an exhaustive summary of Fe import and transport mechanisms. White arrows indicate pathways where diverging opinions exist in the literature. Schematic adapted from literature references [, –37]

Fig. 2

Direct elemental mapping techniques such as X-ray fluorescence microscopy (XFM) have emerged as valuable tools to characterise brain Fe homeostasis during ageing. A Shows a multi-colour XFM overlap image of K (green), Fe (red) and Zn (blue) in a mouse hippocampus. White box in A shows approximate anatomical location of the elemental maps shown in panels B and C. B, C XFM elemental maps of Fe in a 5-month-old and 24-month-old C57Bl6 mouse. White arrows indicate location of hippocampal CA1 pyramidal layer where age-related Fe increase was not observed, while asterisks highlight the corpus callosum white matter where age-related Fe increase is observed, as described in D statistical analysis. Scale bar in A = 500 µm, B, C = 200 µm. Panel A reproduced with permission from reference [57], and Panels B–D reproduced with permission from reference [45]

Fig. 3

Schematic showing the approximate association between age and biological timeline of life (development, adulthood, senescence) in mice, rats and humans. The majority of pre-clinical studies in rodents have characterised changes in brain Fe levels during adulthood (blue shaded region), with very few studies characterising brain Fe homeostasis across the period of senescence (red shaded region). Schematic developed from data contained in references [69] and [70]