Electron tomography of degenerating neurons in mice with abnormal regulation of iron metabolism

Abstract

Previous studies have shown that IRP1 +/− IRP2 −/− knockout mice develop progressive neurodegenerative symptoms similar to those observed in human movement disorders such as Parkinson's disease. Histological investigations using optical microscopy show that these IRP knockout mice display accumulation of ferritin in axonal tracts in the brain, suggesting a possible role for excess ferritin in mediating axonal degeneration. Direct observation of the 3D distribution of ferritin by electron tomography indicates that ferritin amounts are increased by 3-to 4-fold in selected regions of the brain, and structural damage is observed within the axon as evidenced by the loss of the internal network of filaments, and the invaginations of neighboring oligodendrocyte membranes into the axonal medium. While optical microscopic investigations suggest that there is a large increase in ferritin in the presumptive axonal regions of the IRP knockout mice, electron tomographic studies reveal that most of the excess ferritin is localized to double-walled vesicular compartments which are present in the interior of the axon and appear to represent invaginations of the oligodendrocyte cells into the axon. The amount of ferritin observed in the axonal space of the knockout mice is at least 10-fold less than the amount of ferritin observed in wild-type mouse axons. The surprising conclusion from our analysis, therefore, is that despite the overall increase in ferritin levels in the knockout mouse brain, ferritin is absent from axons of degenerating neurons, suggesting that trafficking is compromised in early stages of this type of neuronal degeneration.

Dysregulation of Iron Homeostasis in the CNS Contributes to Disease Progression in a Mouse Model of Amyotrophic Lateral Sclerosis

“…A variety of structural, metabolic, and synaptic elements are transported down the axon by anterograde transport. There is some indirect evidence to suggest that iron may also be transported down the axon by anterograde transport (Wu et al, 2004;Zhang et al, 2005). We now provide evidence that ligation of the sciatic nerve in wild-type mice results in iron accumulation in the cell bodies of the ventral horn motor neurons.…”

Essential role for autophagy protein Atg7 in the maintenance of axonal homeostasis and the prevention of axonal degeneration

“…6A b and c and SI Fig. 12A) (25,26), some of them appeared to originate from axonal subsurface cisternae (27) or smooth endoplasmic reticulum (29) (Fig. 6A d and e).…”

“…6A b and c, SI Fig. 12 A), reminiscent of invasion by oligodendrocytic processes (25,26). Some of them appear to be continuous with the axonal plasma membrane (Fig.…”

Site-specific 3D imaging of cells and tissues with a dual beam microscope

“…It was then loaded onto a Model 916 high-tilt holder (Gatan, Pleasanton, CA) and imaged at 12 000× magnification using a Tecnai F30 transmission electron microscope (FEI, Hillsboro, OR) operating at 300 kV. Images were recorded using a Gatan Ultrascan 2k × 2k CCD camera in conjunction with the FEI tomography software suite, and filtered images were generated and processed as previously described (Zhang et al, 2005) using the IMOD software suite (Kremer et al, 1996).…”

“…A number of powerful approaches have been developed and applied to explore structure-function relationships in macromolecular assem-blies spanning a wide range of sizes (Subramaniam and Milne, 2004). In particular, methods such as electron tomography are now at a stage where they can bridge a critical imaging gap between cellular structure and protein structure (Forster et al, 2005;Marsh et al, 2004;McIntosh et al, 2005;Subramaniam, 2005;Zhang et al, 2005).…”