Iron homeostasis in the central nervous system.

摘要

Iron is an essential, yet highly reactive, metal that needs to be tightly regulated. Excess iron in the central nervous system (CNS) can lead to free radical formation and neurodegeneration. My Ph.D. thesis work is focused on two animal models of neurodegeneration: one uses a null mutation of ceruloplasmin (CP), which in humans causes iron accumulation and neurodegeneration in the CNS; and the other is a gain of function mutation in the superoxide dismutase 1 (SOD1), which causes amyotrophic lateral sclerosis (ALS) in mice/humans. CP is a ferroxidase that converts highly toxic ferrous iron to its non-toxic ferric form. A GPI-anchored form of CP (GPI-CP), expressed by astrocytes, is the major form of CP in the CNS. In my Master's work, the role of CP in iron influx and efflux in vitro using astrocyte cultures from CP-/- and CP +/+ mice was characterized and revealed that iron efflux is completely absent while iron influx into astrocytes is unaffected in the absence of CP.;To understand how the lack of CP causes iron accumulation and neurodegeneration, I carried out a detailed analysis of iron accumulation, dysregulation of iron homeostasis proteins, and loss of astrocytes and neurons in the cerebellum of CP-/- mice during aging. Abnormal iron accumulation is first detected in CP-/- mice by 12 months and peaks at 24 months. Iron accumulation occurs in astrocytes, but not in Purkinje neurons and large neurons in the deep nuclei. The iron importer DMT1 is abnormally increased in these large neurons but not in astrocytes, while ferritin expression is increased in astrocytes. There is also a marked loss of astrocytes and Purkinje neurons in CP-/- mice with age. The loss of astrocytes is likely to be related to iron accumulation while the loss of the Purkinje neurons is likely due to the loss of astrocytic support and lack of sufficient supply of iron from astrocytes.;To assess the contribution of iron to the pathogenesis of ALS, I studied the involvement of iron accumulation and the dysregulation of iron homeostasis in a mouse model of ALS (SOD1G37R). I found excessive iron accumulation in the large motor neurons and glia in the spinals cords of 12-month old SOD1 mice. There was dysregulation in the expression of iron homeostasis proteins, which correlate with the progression of the disease. This iron accumulation in the motor neurons may be due to impaired anterograde axonal transport, as evidenced by a nerve ligation study I carried out. Furthermore, purified mitochondria from SOD1 mice show increased iron accumulation, suggesting a role for iron in mitochondrial dysfunction. Importantly, treatment with a lipophilic iron chelator before the onset of clinical symptoms (8 months of age) extended lifespan by an extra 4.8 weeks, thus pointing to an important role for iron in the progression of disease.;Iron in the CNS is thought to play an important role in many neuroinflammatory conditions due to its redox activity. My work will help further the understanding of how disruption of iron homeostasis can contribute to cell death under various neurodegenerative conditions.;For my Doctoral thesis, I continued to study the role of GPI-CP in astrocytes and showed that GPI-CP is physically associated with the iron efflux transporter ferroportin (FPN). I also assessed whether FPN, a transmembrane protein, is mobilized from non-lipid raft to lipid raft regions of the membrane in response to cellular iron status. Using astrocyte cultures incubated in different iron concentrations, I found that under high iron conditions, there is a rapid relocation of FPN into lipid rafts containing GPI-CP. On the other hand, cells treated with an iron chelator showed decreased FPN expression on the membrane. Therefore, formation of the GPI-CP/FPN complex is essential for cellular iron efflux and the generation of this complex is regulated by cellular iron levels.