The potential for transition metal-mediated neurodegeneration in amyotrophic lateral sclerosis

Abstract

Modulations of the potentially toxic transition metals iron (Fe) and copper (Cu) are implicated in the neurodegenerative process in a variety of human disease states including amyotrophic lateral sclerosis (ALS). However, the precise role played by these metals is still very much unclear, despite considerable clinical and experimental data suggestive of a role for these elements in the neurodegenerative process. The discovery of mutations in the antioxidant enzyme Cu/Zn superoxide dismutase 1 (SOD-1) in ALS patients established the first known cause of ALS. Recent data suggest that various mutations in SOD-1 affect metal-binding of Cu and Zn, in turn promoting toxic protein aggregation. Copper homeostasis is also disturbed in ALS, and may be relevant to ALS pathogenesis. Another set of interesting observations in ALS patients involves the key nutrient Fe. In ALS patients, Fe loading can be inferred by studies showing increased expression of serum ferritin, an Fe-storage protein, with high serum ferritin levels correlating to poor prognosis. Magnetic resonance imaging of ALS patients shows a characteristic T2 shortening that is attributed to the presence of Fe in the motor cortex. In mutant SOD-1 mouse models, increased Fe is also detected in the spinal cord and treatment with Fe-chelating drugs lowers spinal cord Fe, preserves motor neurons, and extends lifespan. Inflammation may play a key causative role in Fe accumulation, but this is not yet conclusive. Excess transition metals may enhance induction of endoplasmic reticulum (ER) stress, a system that is already under strain in ALS. Taken together, the evidence suggests a role for transition metals in ALS progression and the potential use of metal-chelating drugs as a component of future ALS therapy.

Keywords: ER stress; amyotrophic lateral sclerosis; copper; iron; neurodegeneration; transition metals.

Figure 1

Key features of Fe and Cu uptake and release by the neuron. Iron (Fe) is either acquired by non-transferrin bound Fe (NTBI) from low-molecular weight complexes with citrate or ascorbate or by endocytosis of the transferrin (Tf)-transferrin receptor complex (TfR). Once endocytosed, a decrease in pH and the action of a ferrioxidase enables Fe²⁺ release to the labile Fe pool (LIP) by divalent metal transporter 1 (DMT1) where it may be stored in the Fe-storage protein ferritin, or directed to organelles such as lysosomes. Copper (Cu) uptake occurs at neuron surface by copper transporter-1 (CTR1). Following endocytosis, Cu²⁺ ions are incorporated into Cu-metallothionein (Cu-MT), cytochrome c, or superoxide dismutase 1 (SOD-1). Ferroportin (Fpn), the only known Fe exporter from neurons, relies on the Cu-containing metalloenzymes hephaestin (Hep) or ceruloplasmin (Cp) for activity. Hepcidin (Hp) activity can internalize Fpn.

Figure 2

(A) The caudal-to-rostral (tail-to-head) pattern of mRNA expression of iron metabolism proteins in young and old SOD-1(G37R) mice provides circumstantial evidence that Fe is involved in the neurodegenerative process. Advanced neurodegeneration and Fe loading is evident in the lumbar region 12-month-old mice, where 8 months earlier, the expression pattern of proteins of Fe metabolism favored net Fe accumulation. (B) Treatment of SOD-1(G37R) mice with the Fe chelator SIH extends lifespan in SOD-1(G37R) mice. SOD-1(G37R) mice were given SIH either once or twice a week (n = 12) from 8 months of age, a Kaplan–Meier graph shows the percentage of animals surviving with age. (C,D) Vehicle control treated 50 week-old SOD-1(G37R) mice showed loss of neurons whereas SIH treatment results in neuronal preservation (arrows) as indicated by cresyl violet-stained lumbar spinal cord tissue sections. Scale bar, 50 μM. [(B–D) reproduced with permission and minor modification from Jeong et al., 2009].

Figure 3

Iron (Fe) accumulation in the motor cortex of an ALS patient. The 7T magnetic resonance imaging (MRI) scans reveal Fe accumulation in the motor cortex hand-knob region in a 51-year-old ALS patient (A) (arrows), compared to a healthy control. (B) Postmortem Fe accumulation in the middle and deeper layers of cortical gray matter and at the gray–white junction by Pearl’s staining (arrowheads indicate the pial surface while arrows indicate the gray–white junction). (C) At higher magnification, Pearl’s staining detects Fe in cells with irregular processes suggestive of microglia in the ALS motor cortex. (D) Scale bars: C, 1 mm; D, 10 μM. Reproduced with permission and minor modification from Kwan et al. (2012).

Figure 4

Impact of excess iron (Fe) on indicators of ER stress and calcium (Ca²⁺) signaling. ER stress results in the activation of the IRE1, PERK, and ATF6 apoptotic pathways. Loss of BiP induces oligomerization of IRE1 or PERK or activates ATF6 which downstream results in JNK or CHOP mediated apoptosis. High Fe in cells and animal models has been shown to induce BiP, CHOP, and calrectulin (CRT) expression. Excess Fe and resultant redox processes may inhibit function of the SERCA Ca²⁺ pump, decreasing ER Ca²⁺ levels, and leading to CHOP induction, on the other hand, Fe has been shown to inhibit the ryanodine receptor (RyR) ER Ca²⁺ exporter. Induction of CRT by Fe could suggest that Fe participates in causing protein mis-folds or results in perturbed Ca²⁺ balance in the ER.