脑铁代谢和神经变性性疾病

最近关于脑铁代谢研究的新成果,尤其是与脑铁转运、储存、调节相关的某些突变基因的发现,足以得出以下结论,即异常增高的脑铁至少是部份神经变性疾病的起始原因。研究显示,脑铁过量积聚主要是由于遗传性和非遗传性因素所引起的某些服铁代谢蛋白功能异常或表达失控。正是异常增高的脑铁触发一系列病理反应,最终导致神经为性性疾病病人服神经元死亡。本文简要叙述了目前对服铁分布、功能和脑铁代谢蛋白的认识,讨论了内铁转运机制以及服铁和神经变性性疾病之间的关系研究的新进展。     

Iron and ER stress in neurodegenerative disease

Neurodegenerative disease is a condition in which subpopulations of neuronal cells of the brain and spinal cord are selectively lost. A common event in many neurodegenerative diseases is the increased level of endoplasmic reticulum (ER) stress caused by accumulation and deposits of inclusion bodies that contain abnormal aggregated proteins. However, the basis of how ER stress contributes to the selective neuronal vulnerability and degeneration remain elusive. Iron accumulation in the central nerve system is consistently present in many neurodegenerative diseases. In the past 5?years we have begun to show a relationship between polymorphisms in the HFE (high iron) gene and the risk of neurodegenerative disorders. Recent findings have suggested a connection between ER stress and iron metabolism and neurodegeneration. Here we review how the different levels of chronic ER stress contribute to the different fates of neurons, namely the adaptive response and neuronal death. And, we discuss the roles of iron and HFE genotype in selective neuronal vulnerability and degeneration through modifying the ER stress level.     

Apoptosis as the Mechanism of Neurodegeneration in Batten's Disease

Abstract: Batten's disease is a genetic neurodegenerative disease of childhood. Its hallmarks are retinitis pigmentosa and neuronal degeneration. As some types of photoreceptor death in mice are mediated by apoptosis, we investigated whether apoptosis is responsible for retinal and neuronal degeneration in the late infantile and juvenile forms of Batten's disease. Using the terminal dUDP nick end-labeling (TUNEL) staining method, we detected apoptotic neuronal cells in brain from patients and a canine model and in brain and retina from an ovine model for Batten's disease. We confirmed apoptosis by flow cytometry, electron microscopy, and DNA laddering. This is the first inherited neurodegenerative disease involving brain and retina in which apoptosis has been established as the mechanism of neuronal and photoreceptor cell death in both humans and animal models.     

Parkinson disease: from pathology to molecular disease mechanisms

Parkinson disease (PD) is a complex neurodegenerative disorder with both motor and nonmotor symptoms owing to a spreading process of neuronal loss in the brain. At present, only symptomatic treatment exists and nothing can be done to halt the degenerative process, as its cause remains unclear. Risk factors such as aging, genetic susceptibility, and environmental factors all play a role in the onset of the pathogenic process but how these interlink to cause neuronal loss is not known. There have been major advances in the understanding of mechanisms that contribute to nigral dopaminergic cell death, including mitochondrial dysfunction, oxidative stress, altered protein handling, and inflammation. However, it is not known if the same processes are responsible for neuronal loss in nondopaminergic brain regions. Many of the known mechanisms of cell death are mirrored in toxin-based models of PD, but neuronal loss is rapid and not progressive and limited to dopaminergic cells, and drugs that protect against toxin-induced cell death have not translated into neuroprotective therapies in humans. Gene mutations identified in rare familial forms of PD encode proteins whose functions overlap widely with the known molecular pathways in sporadic disease and these have again expanded our knowledge of the neurodegenerative process but again have so far failed to yield effective models of sporadic disease when translated into animals. We seem to be missing some key parts of the jigsaw, the trigger event starting many years earlier in the disease process, and what we are looking at now is merely part of a downstream process that is the end stage of neuronal death.     

Neuroprotection by 7-Nitroindazole Against Iron-Induced Hippocampal Neurotoxicity

(1) Iron plays an important role in maintaining normal brain function. In some neurodegenerative disorders including Parkinson's and Alzheimer's disease, iron levels rise in the brain. It is known that higher iron levels induce neuronal hyperactivity and oxidative stress. A body of evidence indicates a relationship between neuronal death and nitric oxide (NO). The aim of present study was to evaluate the effects of NO produced by neuronal nitric oxide synthase (nNOS) on iron-induced neuronal death. (2) Animals were classified into four groups: control, iron, iron+7-nitroindazole, and iron+vehicle. Rats in iron, iron+7-nitroindazole, and iron+vehicle groups received intracerebroventricular (i.c.v.) FeCl3 injection (200 mM, in 2.5 microl). Rats belonging to control groups received the same amount of saline into the cerebral ventricles. All animals were kept alive for 10 days following the operation. Animals in iron+7-nitroindazole group received intraperitoneal 7-nitroindazole (30 mg/kg/day) injections once a day during this period, while the rats belonging to vehicle group received daily intraperitoneal injection of peanut oil. After 10 days, rats were perfused intracardially under deep urethane anesthesia. Removed brains were processed using the standard histological techniques. (3) The total number of neurons in hippocampus of all rats was estimated with the unbiased stereological techniques. Results of present study show that 7-nitroindazole decreased mean neuron loss from 43% to 11%. Treatment of peanut oil alone did not affect iron-induced hippocampal cell loss with respect to iron group values. (4) Findings of our study suggest that 7-nitroindazole may have neuroprotective effects against iron-induced hippocampal neurotoxicity by inhibiting nNOS.     

Mitochondrial mechanisms of estrogen neuroprotection

Mitochondria have become a primary focus in our search not only for the mechanism(s) of neuronal death but also for neuroprotective drugs and therapies that can delay or prevent Alzheimer's disease and other chronic neurodegenerative conditions. This is because mitochrondria play a central role in regulating viability and death of neurons, and mitochondrial dysfunction has been shown to contribute to neuronal death seen in neurodegenerative diseases. In this article, we review the evidence for the role of mitochondria in cell death and neurodegeneration and provide evidence that estrogens have multiple effects on mitochondria that enhance or preserve mitochondrial function during pathologic circumstances such as excitotoxicity, oxidative stress, and others. As such, estrogens and novel non-hormonal analogs have come to figure prominently in our efforts to protect neurons against both acute brain injury and chronic neurodegeneration.     

Altered Brain Metabolism of Iron as a Cause of Neurodegenerative Diseases?

Abstract: Iron is the most abundant metal in the human body (Pollitt and Leibel, 1982; Youdim, 1988), and the brain, like the liver, contains a substantially higher concentration of iron than of any other metal (Yehuda and Youdim, 1988). Within the brain, iron shows an uneven distribution, with high levels in the basal ganglia (substantia nigra, putamen, caudate nucleus, and globus pallidus), red nucleus, and dentate nucleus (Spatz, 1922; Hallgren and Sourander, 1958; Hill and Switzer, 1984; Riederer et al., 1989). Iron deposition in the brain is mainly in organic storage forms such as ferritin but not hemosiderin (Hallgren and Sourander, 1958; Octave et al., 1983), with relatively little in a free and reactive form. Although the function of a regionally high brain iron content is unknown, the homeostasis of brain iron is thought to be necessary for normal brain function, especially in learning and memory (Youdim et al., 1989; Yehuda and Youdim, 1989; Pollit and Metallinos-Katsaras, 1990; Youdim, 1990). Thus, a high content of brain iron may be essential, particularly during development, but its presence means that injury to brain cells may release iron ions that can lead to oxidative stress via formation of oxygen free radicals. Such radicals are thought to be involved in lipid peroxidation of the cell membrane, leading to increased membrane fluidity, disturbance of calcium homeostasis, and finally cell death (Youdim et al., 1989; Halliwell, 1992). Iron is an essential participant in many metabolic processes, including (a) DNA, RNA, and protein synthesis, (b) as a cofactor of many heme and nonheme enzymes, (c) the formation of myelin, and (d) the development of the neuronal dendritric tree (Ben-Shachar et al., 1986; Youdim et al., 1991b). A deficiency of iron metabolism would therefore be expected to alter some or all of these processes (Jacobs and Worwood, 1980; Youdim, 1985, 1988). Studies of iron distribution in the human brain have demonstrated that the degree of iron deposition, primarily in the basal ganglia (a predominantly dopamine structure), increases with age (Hallgren and Sourander, 1958) and in certain disorders, most notably the basal ganglia disorders (Seitelberger, 1964). This review will present some of the experimental evidence indicating a role of disturbed iron metabolism as a cause of the neurodegenerative disorder Parkinson's disease and possibly other neurodegenerative disorders such as Alzheimer's disease. In addition, some of the neurochemical and histochemical findings obtained at autopsy from analyses of the brain from patients with neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, Huntington's disease, and progressive supranuclear palsy (Steele-Richardson-Olszewski's disease) will be discussed. Special attention will be paid to clarifying the possible implication of the observed changes in the etiology of neurodegenerative disorders.     

Inflammation alters the expression of DMT1, FPN1 and hepcidin,and it causes iron accumulation in central nervous system cells

Inflammation and iron accumulation are present in a variety of neurodegenerative diseases that include Alzheimer's disease and Parkinson's disease. The study of the putative association between inflammation and iron accumulation in central nervous system cells is relevant to understand the contribution of these processes to the progression of neuronal death. In this study, we analyzed the effects of the inflammatory cytokines tumor necrosis factor alpha (TNF‐α) and interleukin 6 (IL‐6) and of lipopolysaccharide on total cell iron content and on the expression and abundance of the iron transporters divalent metal transporter 1 (DMT1) and Ferroportin 1 (FPN1) in neurons, astrocytes and microglia obtained from rat brain. Considering previous reports indicating that inflammatory stimuli induce the systemic synthesis of the master iron regulator hepcidin, we identified brain cells that produce hepcidin in response to inflammatory stimuli, as well as hepcidin‐target cells. We found that inflammatory stimuli increased the expression of DMT1 in neurons, astrocytes, and microglia. Inflammatory stimuli also induced the expression of hepcidin in astrocytes and microglia, but not in neurons. Incubation with hepcidin decreased the expression of FPN1 in the three cell types. The net result of these changes was increased iron accumulation in neurons and microglia but not in astrocytes. The data presented here establish for the first time a causal association between inflammation and iron accumulation in brain cells, probably promoted by changes in DMT1 and FPN1 expression and mediated in part by hepcidin. This connection may potentially contribute to the progression of neurodegenerative diseases by enhancing iron‐induced oxidative damage.     

ApoE receptor 2 controls neuronal survival in the adult brain

A central pathogenic feature of neurodegenerative diseases and neurotrauma is the death of neurons. A mechanistic understanding of the factors and conditions that induce the dysfunction and death of neurons is essential for devising effective treatment strategies against neuronal loss after trauma or during aging. Because Apolipoprotein E (ApoE) is a major risk factor for several neurodegenerative diseases, including Alzheimer's disease , a direct or indirect role of ApoE receptors in the disease process is likely. Here we have used gene targeting in mice to investigate possible roles of ApoE receptors in the regulation of neuronal survival. We demonstrate that a differentially spliced isoform of an ApoE receptor, ApoE receptor 2 (Apoer2), is essential for protection against neuronal cell loss during normal aging. Furthermore, the same splice form selectively promotes neuronal cell death after injury through mechanisms that may involve serine/threonine kinases of the Jun N-terminal kinase (JNK) family. These findings raise the possibility that ApoE and its receptors cooperatively regulate common mechanisms that are essential to neuronal survival in the adult brain.     

Iron, brain ageing and neurodegenerative disorders

There is increasing evidence that iron is involved in the mechanisms that underlie many neurodegenerative diseases. Conditions such as neuroferritinopathy and Friedreich ataxia are associated with mutations in genes that encode proteins that are involved in iron metabolism, and as the brain ages, iron accumulates in regions that are affected by Alzheimer's disease and Parkinson's disease. High concentrations of reactive iron can increase oxidative-stress induced neuronal vulnerability, and iron accumulation might increase the toxicity of environmental or endogenous toxins. By studying the accumulation and cellular distribution of iron during ageing, we should be able to increase our understanding of these neurodegenerative disorders and develop new therapeutic strategies.     

The relevance of iron in the pathogenesis of Parkinson's disease

Alterations of iron levels in the brain has been observed and documented in a number of neurodegenerative disorders including Parkinson's disease (PD). The elevated nigral iron levels observed in PD may reflect a dysfunction of brain iron homeostasis. Under normal physiological conditions excess iron can be sequestrated in ferritin and neuromelanin. Alternatively, the excess iron may represent a component of brain iron deposition associated with ageing. The aetiology of idiopathic PD largely remains an enigma. However, intensive investigations have provided a host of putative mechanisms that might contribute to the pathogenesis underlying the characteristic degeneration of the dopaminergic neurons in the substantia nigra (SN). The mechanisms proposed include oxidative (and nitrative) stress, inflammation, excitotoxicity, mitochondrial dysfunction, altered proteolysis and finally apoptotic induced cell death. Iron-mediated cellular destruction is mediated primarily via reactive oxygen or/and nitrogen species induced oxidative stress. Furthermore, these pathogenic mechanisms appear to be closely interlinked to the cascade of events leading to cellular death. There are conflicting reports about the stage during disease progression at which nigral iron change occurs in PD. Some have found that there are no changes in iron content SN in asymptomatic incidental Lewy body disease, suggesting it may represent a secondary event in the cascade of neuronal degeneration. In contrast, others have found an elevation of iron in SN in pre-clinical stages. These discrepancies may be attributed to the occurrence of different sub-groups of the disease. This concurs with the notion that PD represents a group of related diseases with a number of potential pathogenic pathways.     

Inhibition of Drp1 provides neuroprotection in vitro and in vivo

Impaired regulation of mitochondrial dynamics, which shifts the balance towards fission, is associated with neuronal death in age-related neurodegenerative diseases, such as Alzheimer's disease or Parkinson's disease. A role for mitochondrial dynamics in acute brain injury, however, has not been elucidated to date. Here, we investigated the role of dynamin-related protein 1 (Drp1), one of the key regulators of mitochondrial fission, in neuronal cell death induced by glutamate toxicity or oxygen-glucose deprivation (OGD) in vitro, and after ischemic brain damage in vivo. Drp1 siRNA and small molecule inhibitors of Drp1 prevented mitochondrial fission, loss of mitochondrial membrane potential (MMP), and cell death induced by glutamate or tBid overexpression in immortalized hippocampal HT-22 neuronal cells. Further, Drp1 inhibitors protected primary neurons against glutamate excitotoxicity and OGD, and reduced the infarct volume in a mouse model of transient focal ischemia. Our data indicate that Drp1 translocation and associated mitochondrial fission are key features preceding the loss of MMP and neuronal cell death. Thus, inhibition of Drp1 is proposed as an efficient strategy of neuroprotection against glutamate toxicity and OGD in vitro and ischemic brain damage in vivo.     

4-hydroxynonenal and neurodegenerative diseases

The development of oxidative stress, in which production of highly reactive oxygen species (ROS) overwhelms antioxidant defenses, is a feature of many neurological diseases: ischemic, inflammatory, metabolic and degenerative. Oxidative stress is increasingly implicated in a number of neurodegenerative disorders characterized by abnormal filament accumulation or deposition of abnormal forms of specific proteins in affected neurons, like Alzheimer's disease (AD), Pick's disease, Lewy bodies related diseases, amyotrophic lateral sclerosis (ALS), and Huntington disease. Causes of neuronal death in neurodegenerative diseases are multifactorial. In some familiar cases of ALS mutation in the gene for Cu/Zn superoxide dismutase (SOD1) can be identified. In other neurodegenerative diseases ROS have some, usually not clear, role in early pathogenesis or implications on neuronal death in advanced stages of illness. The effects of oxidative stress on "post-mitotic cells", such as neurons may be cumulative, hence, it is often unclear whether oxidative damage is a cause or consequence of neurodegeneration. Peroxidation of cellular membrane lipids, or circulating lipoprotein molecules generates highly reactive aldehydes among which one of most important is 4-hydroxynonenal (HNE). The presence of HNE is increased in brain tissue and cerebrospinal fluid of AD patients, and in spinal cord of ALS patients. Immunohistochemical studies show presence of HNE in neurofibrilary tangles and in senile plaques in AD, in the cytoplasm of the residual motor neurons in sporadic ALS, in Lewy bodies in neocortical and brain stem neurons in Parkinson's disease (PD) and in diffuse Lewy bodies disease (DLBD). Thus, increased levels of HNE in neurodegenerative disorders and immunohistochemical distribution of HNE in brain tissue indicate pathophysiological role of oxidative stress in these diseases, and especially HNE in formation of abnormal filament deposites.     

Ceruloplasmin immunoreactivity in neurodegenerative disorders

Ceruloplasmin (CP) is a 132kd cuproprotein which, together with transferrin, provides the majority of anti-oxidant capacity in serum. Increased iron deposition and lipid peroxidation in the basal ganglia of subjects with hereditary CP deficiency suggest that CP may serve as an anti-oxidant in the brain as well. The present study compared CP immunoreactivity in brain specimens from normal controls and subjects with neurodegenerative disorders (Alzheimer's disease [AD], Parkinson's disease [PD], progressive supranuclear palsy [PSP], and Huntington's disease [HD]) (n = 5 per group). The relative intensity of neuronal CP staining and the numbers of CP-stained neurons per 25x microscope field were determined in hippocampus (CA1, subiculum, and parahippocampal gyrus), parietal cortex, frontal cortex, substantia nigra, and caudate. CP was detected in both neurons and astrocytes in all specimens, and in senile plaques and occasional neurofibrillary tangles in AD brain. Neuronal CP staining intensity tended to increase in most AD brain regions, but was statistically significant vs controls only in the CA1 region of hippocampus (p = .016). Neuronal CP staining in brain specimens from other neurodegenerative disorders showed a slight but nonsignificant increase vs controls. The numbers of CP-stained neurons per field did not differ between the various neurodegenerative disorders and controls. These results suggest that a modest increase in neuronal CP content is present in the AD brain, and lesser elevations in neuronal CP occur in the other neurodegenerative disorders in this study. Though CP functions as both an acute phase protein and an anti-oxidant in peripheral tissues, whether it does so in the brain remains to be determined.     

Cell death mechanisms in neurodegeneration

Progressive cell loss in specific neuronal populations often associated with typical cytoskeletal protein aggregations is a pathological hallmark of neurodegenerative disorders, but the nature, time course and molecular causes of cell death and their relation to cytoskeletal pathologies are still unresolved. Apoptosis or alternative pathways of cell death have been discussed in Alzheimer's disease and other neurodegenerative disorders. Apoptotic DNA fragmentation in human brain as a sign of neuronal injury is found too frequent as to account for continous neuron loss in these slowly progressive processes. Morphological studies revealed extremely rare apoptotic neuronal death in Alzheimer's disease but yielded mixed results for Parkinson's disease and other neurodegenerative disorders. Based on recent data in human brain, as well as in animal and cell culture models, a picture is beginning to emerge suggesting that, in addition to apoptosis, other forms of programmed cell death may participate in neurodegeneration. Better understanding of the molecular players will further elucidate the mechanisms of cell death in these disorders and their relations to cytoskeletal abnormalities. Susceptible cell populations in a proapoptotic environment show increased vulnerability towards multiple noxious factors discussed in the pathogenesis of neurodegeneration. In conclusion, although many in vivo and in vitro data are in favor of apoptosis involvement in neurodegenerative processes, there is considerable evidence that very complex events may contribute to neuronal death with possible repair mechanisms, the elucidation of which may prove useful for future prevention and therapy of neurodegenerative disorders.     

Impact of endogenous nitric oxide on microglial cell energy metabolism and labile iron pool

Microglial activation is common in several neurodegenerative disorders. In the present study, we used the murine BV-2 microglial cell line stimulated with gamma-interferon and lipopolysaccharide to gain new insights into the effects of endogenously produced NO on mitochondrial respiratory capacity, iron regulatory protein activity, and redox-active iron level. Using polarographic measurement of respiration of both intact and digitonin-permeabilized cells, and spectrophotometric determination of individual respiratory chain complex activity, we showed that in addition to the reversible inhibition of cytochrome-c oxidase, long-term endogenous NO production reduced complex-I and complex-II activities in an irreversible manner. As a consequence, the cellular ATP level was decreased in NO-producing cells, whereas ATPase activity was unaffected. We show that NO up-regulates RNA-binding of iron regulatory protein 1 in microglial cells, and strongly reduces the labile iron pool. Together these results point to a contribution of NO derived from inflammatory microglia to the misregulation of energy-producing reactions and iron metabolism, often associated with the pathogenesis of neurodegenerative disorders.     

Brain iron transport

Brain iron is a crucial participant and regulator of normal physiological activity. However, excess iron is involved in the formation of free radicals, and has been associated with oxidative damage to neuronal and other brain cells. Abnormally high brain iron levels have been observed in various neurodegenerative diseases, including neurodegeneration with brain iron accumulation, Alzheimer's disease, Parkinson's disease and Huntington's disease. However, the key question of why iron levels increase in the relevant regions of the brain remains to be answered. A full understanding of the homeostatic mechanisms involved in brain iron transport and metabolism is therefore critical not only for elucidating the pathophysiological mechanisms responsible for excess iron accumulation in the brain but also for developing pharmacological interventions to disrupt the chain of pathological events occurring in these neurodegenerative diseases. Numerous studies have been conducted, but to date no effort to synthesize these studies and ideas into a systematic and coherent summary has been made, especially concerning iron transport across the luminal (apical) membrane of the capillary endothelium and the membranes of different brain cell types. Herein, we review key findings on brain iron transport, highlighting the mechanisms involved in iron transport across the luminal (apical) as well as the abluminal (basal) membrane of the blood–brain barrier, the blood–cerebrospinal fluid barrier, and iron uptake and release in neurons, oligodendrocytes, astrocytes and microglia within the brain. We offer suggestions for addressing the many important gaps in our understanding of this important topic, and provide new insights into the potential causes of abnormally increased iron levels in regions of the brain in neurodegenerative disorders.