Severity of Neurodegeneration Correlates with Compromise of Iron Metabolism in Mice with Iron Regulatory Protein Deficiencies

Abstract

In mammals, iron regulatory proteins 1 and 2 (IRP1 and IRP2) posttranscriptionally regulate expression of several iron metabolism proteins including ferritin and transferrin receptor. Genetically engineered mice that lack IRP2, but have the normal complement of IRP1, develop adult-onset neurodegenerative disease associated with inappropriately high expression of ferritin in degenerating neurons. Here, we report that mice that are homozygous for a targeted deletion of IRP2 and heterozygous for a targeted deletion of IRP1 (IRP1+/- IRP2-/-) develop a much more severe form of neurodegeneration, characterized by widespread axonopathy and eventually by subtle vacuolization in several areas, particularly in the substantia nigra. Axonopathy develops in white matter tracts in which marked increases in ferric iron and ferritin expression are detected. Axonal degeneration is significant and widespread before evidence for abnormalities or loss of neuronal cell bodies can be detected. Ultimately, neuronal cell bodies degenerate in the substantia nigra and some other vulnerable areas, microglia are activated, and vacuoles appear. Mice manifest gait and motor impairment at stages when axonopathy is pronounced, but neuronal cell body loss is minimal. These observations suggest that therapeutic strategies that aim to revitalize neurons by treatment with neurotrophic factors may be of value in IRP2-/- and IRP1+/- IRP2-/- mouse models of neurodegeneration.

Down-regulation of Iron Regulatory Protein 1 Activities and Expression in Superoxide Dismutase 1 Knock-out Mice Is Not Associated with Alterations in Iron Metabolism

“…It is noteworthy that misregulation of iron metabolism, e.g. that associated with IRP2 deficiency, is in general associated with severe neurodegenerative pathologies (57). On the other hand, there is a discrepancy between the intact iron metabolism in SOD1 Ϫ/Ϫ mice as shown here and the altered iron homeostasis reported in Cu,Zn-SOD-deficient bacteria (7) and yeast (8,9).…”

Excess Capacity of the Iron Regulatory Protein System

“…Instead, we observed that further knockdown of IRP1 and IRP2 was sufficient to completely abolish the response of ferritin H and TfR1 to iron challenge, suggesting that although other IRE-binding proteins may exist, they cannot substitute to maintain iron homeostasis when IRP1 and IRP2 are greatly diminished. The essential requirement for IRP1 or IRP2 is supported by the finding that double knockouts of IRP1 and IRP2 are not viable (35,37), although results in mice may be complicated by requirements for IRP1 or IRP2 at particular phases of development or embryogenesis. Here, we demonstrate that in the context of a single cell type, an appropriate response to iron or iron deprivation cannot be mounted when levels of IRP1 and IRP2 fall below a certain threshold.…”

“…This has not been possible in mouse models, because constitutive double knockouts are not viable (35,37). Knockdown of both IRPs led to a greater reduction in translational repression of a luciferase reporter than knockdown of either IRP alone (Fig.…”

“…Because combined inactivation of both IRP1 and IRP2 results in embryonic lethality in the mouse (35,37), IRP function that would have been elucidated with such models remain unexplored. We undertook to understand and measure the cellular effects of deficiency of IRP1 and IRP2, both singly and in combination, in epithelial cells using siRNA technology to efficiently knock down IRP1 and IRP2.…”

Advantages and disadvantages of the animal models v. in vitro studies in iron metabolism: a review

“…Transgenic knockout mouse models of IRP1, IRP2 indicate that the double knockout is embryologically lethal (Smith et al, 2004). Observation of no overt phenotype for IRP1-/-knockout mouse is rather surprising and suggests that IRP2 can compensate for the loss of IRP1.…”