Iron accumulation and dysregulation in the putamen in fragile X-associated tremor/ataxia syndrome

Abstract

Background: Fragile X-associated tremor/ataxia syndrome is an adult-onset disorder associated with premutation alleles of the FMR1 gene. This disorder is characterized by progressive action tremor, gait ataxia, and cognitive decline. Fragile X-associated tremor/ataxia syndrome pathology includes dystrophic white matter and intranuclear inclusions in neurons and astrocytes. We previously demonstrated that the transport of iron into the brain is altered in fragile X-associated tremor/ataxia syndrome; therefore, we also expect an alteration of iron metabolism in brain areas related to motor control. Iron is essential for cell metabolism, but uncomplexed iron leads to oxidative stress and contributes to the development of neurodegenerative diseases. We investigated a potential iron modification in the putamen - a structure that participates in motor learning and performance - in fragile X-associated tremor/ataxia syndrome.

Methods: We used samples of putamen obtained from 9 fragile X-associated tremor/ataxia syndrome and 9 control cases to study iron localization using Perl's method, and iron-binding proteins using immunostaining.

Results: We found increased iron deposition in neuronal and glial cells in the putamen in fragile X-associated tremor/ataxia syndrome. We also found a generalized decrease in the amount of the iron-binding proteins transferrin and ceruloplasmin, and decreased number of neurons and glial cells that contained ceruloplasmin. However, we found increased levels of iron, transferrin, and ceruloplasmin in microglial cells, indicating an attempt by the immune system to remove the excess iron.

Conclusions: Overall, found a deficit in proteins that eliminate extra iron from the cells with a concomitant increase in the deposit of cellular iron in the putamen in Fragile X-associated tremor/ataxia syndrome. © 2017 International Parkinson and Movement Disorder Society.

Keywords: CGG; FXTAS; dementia; fragile X; motor disorder; neurodegeneration; repeat-expansion disorder.

Figure 1

Iron bound to hemosiderin (blue) accumulated in the putamen in FXTAS. Iron (blue) in sagittal (A) and coronal (B) sections of capillaries showed a great amount of iron in the walls and within the capillaries. Iron was present in the putamen parenchyma in FXTAS (C, E-G) but not in control tissue (D, H). Arrows point to iron deposits. Tissue is co-stained with nuclear fast red. I-K: The amount of iron was increased while the amount of Tf and Cp was decreased in the putamen in FXTAS. Iron: blue, Perl’s method; Tf and Cp: brown, immunostaining. Tf: Transferrin; Cp: ceruloplasmin. Scale bar: A-D: 200 μm; E-H: 100 μm.

Figure 2

The number of oligodendrocytes containing iron drastically increased, while the number of neurons containing iron marginally increased in FXTAS. However, the number of astrocytes containing iron was similar in FXTAS and control cases. A, B, C: Representative cell-types (A: neurons; B: astrocytes; C: oligodendrocytes) containing iron in FXTAS. D: Number of cells containing iron for each control and FXTAS case included in this study. Iron: blue, Perl’s method. CT: control. Calibration bar: 5 μm.

Figure 3

The number of neurons, astrocytes and oligodendrocytes containing Tf was similar in FXTAS and control cases. A, B, C: Representative cell-types (A: neurons; B: astrocytes; C: oligodendrocytes) containing Tf in FXTAS. D: Number of cells containing Tf for each control and FXTAS case included in this study. Tf: blue. Tf: Transferrin. CT: control. Calibration bar: 5 μm.

Figure 4

The number of neurons, astrocytes and oligodendrocytes containing Cp drastically decreased in FXTAS when compared to control cases. A, B, C: Representative cell-types (A: neurons; B: astrocytes; C: oligodendrocytes) containing Cp in FXTAS. D: Number of cells containing Cp for each control and FXTAS case included in this study. Cp: blue. Cp: ceruloplasmin. CT: control. Calibration bar: 5 μm.

Figure 5

The number of microglial cells containing iron, Tf, and and Cp was increased in the putamen in FXTAS. A-C: Representative microglial cells containing iron, Tf and Cp. D-F: Number of microglial cells containing iron, Tf and Cp for each control and FXTAS case included in this study. Iron, Tf and Cp: blue. Tf: Transferrin. Cp: ceruloplasmin; CT: control. Calibration bar: 5 μm.