A cutting-edge MRI technique to detect iron deposits in different brain regions can track declines in thinking, memory and movement in people with Parkinson’s disease, finds a new UCL-led study.
The findings, published in the Journal of Neurology, Neurosurgery, and Psychiatry, suggest that measures of brain iron might eventually help predict which people with Parkinson’s will develop dementia.
“Iron in the brain is of growing interest to people researching neurodegenerative diseases such as Parkinson’s and dementias.
As you get older, iron accumulates in the brain, but it’s also linked to the build-up of harmful brain proteins, so we’re starting to find evidence that it could be useful in monitoring disease progression, and potentially even in diagnostics,” said the study’s lead author, Dr Rimona Weil (UCL Queen Square Institute of Neurology).
The study involved 97 people with Parkinson’s disease, who had been diagnosed within the last 10 years, along with 37 people without the condition, as a control (comparison) group.
They were tested for their thinking and memory as well as for their motor function.
Parkinson’s disease is a progressive condition of brain degeneration resulting in tremors, stiffness and slowness of movement. Close to 50% of people with the condition end up developing dementia, but the timing and severity vary substantially.
Currently there are no reliable measures to track Parkinson’s progression in the brain, so clinicians rely on monitoring symptoms.
Conventional brain imaging fails to track progression until quite a late stage, when large-scale brain volume loss can be detected.
Iron accumulates in people’s brains as part of the normal ageing process, partly due to increased permeability in the blood-brain barrier.
Excess iron can have toxic effects leading to proteins being irreversibly modified.
Recent studies have found that when proteins linked to Alzheimer’s disease (amyloid and tau, which are also linked to Parkinson’s dementia) build up, iron also accumulates in the affected brain areas.
In the current study, researchers used a new technique, called quantitative susceptibility mapping,* to map iron levels in the brain based on MRI (magnetic resonance imaging) scans.
They found that iron accumulation in the hippocampus and thalamus brain regions was associated with poor memory and thinking scores.
Iron in the putamen brain region was associated with poor movement scores, suggesting a more advanced stage of the disease.
In Parkinson’s disease, the hippocampus and thalamus are known to be associated with thinking and memory, and the putamen with movement scores, so the researchers say it’s very promising that iron deposition was specifically detected in those areas.
Areas with iron accumulation in the brains of people with Parkinson’s, correlated with risk of cognitive decline. The image is credited to George Thomas et al.
The findings suggest that iron deposition could be valuable to track if a treatment is working in a clinical trial, and might eventually be helpful for early diagnosis of Parkinson’s or other neurodegenerative diseases.
Dr Weil has previously found in a 2019 study** that a suite of vision tests may be helpful to predict cognitive decline in Parkinson’s.
She and her colleagues hope that further research will determine if the vision tests and iron measures could be helpful to predict which people with dementia are likely to develop dementia.
First author, PhD student George Thomas (UCL Queen Square Institute of Neurology), said: “It’s really promising to see measures like this which can potentially track the varying progression of Parkinson’s disease, as it could help clinicians devise better treatment plans for people based on how their condition manifests.”
Co-author Dr Julio Acosta-Cabronero (Tenoke Ltd. and Wellcome Centre for Human Neuroimaging, UCL) added:
“We were surprised at how well the iron levels measured in different regions of the brain with MRI were correlated with cognitive and motor skills.
We hope that brain iron measurement could be useful for a wide range of conditions, such as to gauge dementia severity or to see which brain regions are affected by other movement, neuromuscular and neuroinflammatory disorders, stroke, traumatic brain injury and drug abuse.”
The researchers are now following up the same study participants to see how their disease is progressing, whether they develop dementia, and how such measures correlate with changes in iron levels over time.
Funding: The study was supported by Wellcome, the National Institute for Health Research, the Medical Research Council, Parkinson’s UK, Movement Disorders Society, ESRC, and the Cure Parkinson’s Trust.
Iron is involved in an abundant number of cellular processes in the brain including mitochondrial respiration, myelin synthesis, DNA synthesis, oxygen transportation, neurotransmitter synthesis and cellular metabolism (Stankiewicz and Brass, 2009; Ward et al., 2014). In the CNS, iron is present in neurons, oligodendrocytes, astroglia and microglia cells.
Within the cell, iron mediates essential functions due to its capability to participate in reaction of electron transfer thereby switching between two states: ferrous (II) and ferric (III) iron. This mechanism represents a double-edge sword because distinct levels of reactive oxygen species (ROS) are produced during these reactions enabling calcium-mediated basal synaptic transmission and long-term potentiation (Muñoz et al., 2011), but on the other hand may catalyze the so-called Fenton reaction, which generates ROS in high amounts detrimental for cellular function and eventually survival (Halliwell, 2006).
Thus, tight regulation of intracellular iron homeostasis is required which is facilitated by sequestration of iron into iron-binding proteins like heme, ferritin, neuromelanin, and iron-sulfur clusters among others (Kruszewski, 2003).
Iron levels in the brain and body increase sharply up to 30 years of age due to a metabolic need during the growth process and remain stable during adulthood (Bolognin et al., 2009; Stankiewicz and Brass, 2009; Apostolakis and Kypraiou, 2017). In the aging brain however, region-specific increase of total iron is observed, probably triggered by inflammation, increased blood-brain barrier (BBB) permeability, redistribution of iron within the brain, and changes in iron homeostasis (Conde and Streit, 2006; Farrall and Wardlaw, 2009; Ward et al., 2014) and shows highest iron levels in the basal ganglia.
In addition, iron accumulation varies among brain cell types, as neurons, micro- and astroglia accumulate iron over their lifespan, whereas oligodendroglial iron levels remain stable (Connor et al., 1990). Changes in regional iron distribution have been demonstrated consistently in neurodegenerative diseases via magnetic resonance imaging (MRI) (Duyn, 2010) and at post-mortem examinations (Hare et al., 2012) and since aging represents the number one risk factor for neurodegeneration, there may be a link between age-related iron accumulation and neurodegeneration (Zecca et al., 2001, 2004; Ward et al., 2014).
Alzheimer’s disease (AD), Parkinson’s disease (PD), multiple system atrophy (MSA), dementia with Lewy bodies, amyotrophic lateral sclerosis, Huntington’s disease (HD), frontotemporal dementia, corticobasal degeneration, and progressive supranuclear palsy (PSP) are primarily characterized by the deposition of insoluble protein aggregates which colocalize with iron (Coffey et al., 1989; Berg and Hochstrasser, 2006; Muller and Leavitt, 2014; Wang et al., 2016; Fernández et al., 2017; Lee et al., 2017; Sheelakumari et al., 2017; Kaindlstorfer et al., 2018; Lane et al., 2018; Moreau et al., 2018), suggesting a link between those clinically and pathologically distinct disease entities. This raises the question whether iron dyshomeostasis represents a critical factor in initiating neurodegeneration, whether it contributes to acceleration of widespread pathology as a result of nerve cell death and the consecutive release of intracellular components or whether neurodegeneration and iron accumulation constitute two completely unrelated events appearing in parallel.
In this review we aim to outline the potential links between pathophysiological mechanisms and the role of iron in neurodegeneration.
Brain Iron Metabolism in Health and Aging
Both excess and deficiency of iron lead to impaired brain function, thus tight regulation of iron metabolism is critical.
Iron enters the brain either bound to transferrin, thereby crossing the BBB or the blood-cerebrospinal fluid (CSF) barrier (Moos and Morgan, 2000), or possibly unbound, especially in conditions that result in iron overload as transferrin becomes saturated with iron. The exact uptake mechanisms in the latter case are unknown, but it is hypothesized that free serum iron might be reduced by cellular reductants such as ascorbate (Lane and Lawen, 2008; Lane et al., 2010; Lane and Richardson, 2014) to then be imported by divalent metal transporter 1 (DMT1) or ZRT/IRT-like proteins (ZIPs) like ZIP14 or ZIP8 (Jenkitkasemwong et al., 2012). Iron uptake into astroglia is thought to be mediated mainly by non-Tf bound iron (Ashraf et al., 2018), as TfR1 has so far only been reported in vitro(Pelizzoni et al., 2013).
DMT1, a transporter located in cellular and endosomal membranes, is found in astrocytes and is believed to facilitate transport of non-transferrin bound iron into the cytoplasm (Xu and Ling, 1994). How iron is exported from the endothelial cells is still elusive (Ward et al., 2014). Within the CSF, iron occurs mainly as holo-transferrin (two ferric iron atoms bound to apo-transferrin) that interacts with TfR1. Neurons internalize the Tf-TfR1 complex into endosomes, where iron is separated from transferrin after acidification, converted into its ferrous form via reductase STEAP3 (Ohgami et al., 2005) and transported into the cytoplasm via DMT1 (Moos and Morgan, 2004).
Iron, prone to contribute to oxidative stress, can be (i) stored within ferritin (Zecca et al., 2004), (ii) imported into mitochondria, probably via so-called mitoferrins and TfR2 (Mastroberardino et al., 2009; Horowitz and Greenamyre, 2010), to enable biosynthesis of heme and iron-sulfur clusters and contribute in the respiratory chain reaction, or (iii) be released from the cell via ferroportin 1 (Ward et al., 2014).
Intracellular iron homeostasis is tightly modulated by the iron regulatory protein (IRP) and iron-responsive element (IRE) signaling pathways (Pantopoulos, 2004; Zhang D.L. et al., 2014). IRP1 and IRP2 are two RNA-binding proteins that interact with IREs, non-coding sequences of messenger RNA (mRNA) transcripts to alter translation of ferritin, ferroportin and TfR mRNA. Ferritin H and L subunits or ferroportin mRNA transcripts carry IREs within the 5′-untranslated region (UTR), whereas mRNA transcripts for TfR and DMT-1 carry IRE motifs at the 3′-UTR.
Cytosolic iron binds to IRPs and induces a conformational change within the molecule that does not allow attachment to IREs. Decreased iron levels on the other hand facilitate IRP–IRE interaction: IRP binding at the 5′-UTR inhibits further mRNA translation of ferritin subunits and ferroportin; at the 3′-UTR, binding protects against endonuclease cleavage (Pantopoulos, 2004; Zhou and Tan, 2017). Ferritin represents the dominant iron storage protein in the CNS, mostly found in glia and also within neurons, whereas neuromelanin (NM) captures large amounts of iron in certain neuronal populations for longer-term storage (Zucca et al., 2017). Recent studies have demonstrated that human poly-(rC)-binding proteins 1–4 (PCBPs 1–4) are implicated in iron transfer to ferritin (Philpott, 2012; Leidgens et al., 2013; Frey et al., 2014; Yanatori et al., 2014), which is a 24 subunit heteropolymer with heavy chains (H-type ferritin) with ferroxidase activity and light chains (L-type ferritin) crucial for subsequent iron storage. H-type ferritin occurs more abundantly in neurons for rapid mobilization and use, whereas in astro- and microglia L-type ferritin predominates for iron storage.
In oligodendrocytes, both forms of ferritin are expressed (Ashraf et al., 2018). Neuromelanin (NM), a dark brown pigment, is present in dopaminergic neurons of the substantia nigra, the noradrenergic neurons of locus coeruleus, the ventral tegmental area, the ventral reticular formation and the nucleus of the solitary tract in the medulla oblongata (Zecca et al., 2004; Fedorow et al., 2005), but it has also been detected in the putamen, premotor cortex and cerebellum in lower amounts (Zecca et al., 2008; Engelen et al., 2012). Ferritin degradation by the autophagy-lysosome system (Asano et al., 2011) initiates iron release which can then be reutilized or exported, mainly through ferroportin 1 (Biasiotto et al., 2016). This requires ferroxidases ceruloplasmin and hephaestin to oxidize iron for export (Hentze et al., 2004). In addition, heme-oxygenase 1 represents a stress protein which degrades heme to ferrous iron in order to maintain iron homeostasis (Nitti et al., 2018). Systemic ferroportin levels are regulated by circulating hepcidin, the main iron regulatory hormone in the body – during iron overload and inflammation, hepcidin induces ferroportin internalization and degradation (Wang and Pantopoulos, 2011). The origin of hepcidin within the brain is unknown, It may be locally produced or systemically derived by passing the BBB (Vela, 2018). Conditional ferroportin knock-out mice for example do not show any significant intracellular iron accumulation in the brain, nor do they show behavioral or histological deficits compared to wildtype mice (Matak et al., 2016), suggesting that other cellular iron export mechanisms exist.
Iron accumulates as a function of the aging brain and thereby the levels of labile, potentially harmful iron increase (Ward et al., 2014). Iron accumulating at toxic levels within neurons, as seen in neurodegeneration, may lead to cell death via apoptosis, autophagy, necrosis or ferroptosis, a recently discovered mechanism of iron-mediated cell death distinct from apoptosis (Dixon et al., 2012). In glial cells however, iron accumulation triggers the release of pro-inflammatory cytokines, thereby creating a pro-inflammatory environment (Williams et al., 2012) which promotes neurodegeneration (Xu et al., 2012).
The underlying mechanisms of age and disease related region and cell specific differences in iron levels have not been fully elucidated yet, but the distinct iron richness of the basal ganglia may explain its increased vulnerability to neurodegeneration (Hallgren and Sourander, 1958; Zecca et al., 2004; Xu et al., 2012). In neurons of substantia nigra and locus coeruleus, premotor cortex, putamen, and cerebellum, neuromelanin levels are increased for sequestration of iron, however neuromelanin may itself exhibit toxic functions which are further explained in later sections (Zecca et al., 2004, 2008).
Astro- and microglia exhibit increased iron deposition and ferritin concentrations throughout their lifespan. In contrast, oligodendrocytes that constitute the main iron reservoir in the CNS (Connor et al., 1990; Kaindlstorfer et al., 2018) do not accumulate iron by aging. Iron is essential in the development of oligodendrocytes to allow proper axon myelination, as iron deficiency induces hypomyelination that persists even after the iron imbalance has been corrected (Todorich et al., 2009).
The role of iron in oligodendrocytes is further exemplified by Tim-2, a receptor selective for H-ferritin uptake (Todorich et al., 2008). Neuroinflammation in the elderly brain is induced via activated microglia, a lot of which are ferritin-positive (Kaneko et al., 1989). Of those, most exhibit a dystrophic-type morphology, and it has been suggested that iron uptake into the activated microglia cells may represent another source of toxic iron, as seen in neurodegenerative diseases (Rathnasamy et al., 2013; McCarthy et al., 2018; Refolo et al., 2018).