Expression of lactoferrin receptors is increased in the mesencephalon of patients with Parkinson disease

Pharmacokinetics and brain uptake of lactoferrin in rats

Lactoferrin (Lf) is an iron-binding glycoprotein belonging to the transferrin (Tf) family. Lf was reported to cross the blood brain barrier (BBB) via receptor-mediated transcytosis in an in vitro model of the BBB. In the present study, we compared the in vivo brain uptake of Lf with that of OX26, an anti-Tf receptor antibody, and Tf. These three proteins were radiolabeled with 125I and administered to rats by i.v. injection. We found that Lf was more rapidly eliminated from the blood compared with OX26 and Tf (The half-life of Lf was approximately 8 and 6 times shorter than that of OX26 and Tf, respectively; the area under the blood concentration-time curve of Lf was approximately 15 and 17 times smaller than that of OX26 and Tf, respectively), and mainly accumulated in the liver, spleen, and kidney. Markedly high brain uptake was observed for Lf relative to Tf and OX26. Lf might be useful as a ligand for facilitating drug delivery into the brain.

Altered regulation of iron transport and storage in Parkinson's disease

Parkinson's disease (PD) is characterized by the death of dopaminergic neurons in the substantia nigra. This neuronal degeneration is associated with a strong microglial activation and iron accumulation in the affected brain structures. The increased iron content may result from an increased iron penetration into the brain parenchyma due to a higher expression of lactoferrin and lactoferrin receptors at the level of the blood vessels and dopaminergic neurons in the substantia nigra in PD. Iron may also accumulate in microglial cells after phagocytosis of dopaminergic neurons. These effects may be reinforced by a lack of up-regulation of the iron storage protein ferritin, as suggested by an absence of change in iron regulatory protein 1 (IRP-1) control of ferritin mRNA translation in PD. Thus, a dysregulation of the labile iron pool may participate in the degenerative process affecting dopaminergic neurons in PD.

Potential sources of increased iron in the substantia nigra of parkinsonian patients

Histopathological, biochemical and in vivo brain imaging techniques, such as magnetic resonance imaging and transcranial sonography, revealed a consistent increase of substantia nigra (SN) iron in Parkinson's disease (PD). Increased iron deposits in the SN may have genetic and non-genetic causes. There are several rare movement disorders associated with neurodegeneration, and genetic abnormalities in iron regulation resulting in iron deposition in the brain. Non-genetic causes of increased SN iron may be the result of a disturbed or open blood-brain-barrier, local changes in the normal iron-regulatory systems, intraneuronal transportation of iron from iron-rich area into the SN and release of iron from intracellular iron storage molecules. Major iron stores are ferritin and haemosiderin in glial cells as well as neuromelanin in neurons. Age- and disease dependent overload of iron storage proteins may result in iron release upon reduction. Consequently, the low molecular weight chelatable iron complexes may trigger redox reactions leading to damage of biomolecules. Additionally, upon neurodegeneration there is strong microglial activation which can be another source of high iron concentrations in the brain.

Neuroinflammatory processes in Parkinson's disease

In Parkinson's disease (PD), post-mortem examination reveals a loss of dopaminergic (DA) neurons in the substantia nigra (SN) associated with a massive astrogliosis and the presence of activated microglial cells. Similarly, microglial activation has also been reported to be associated with the loss of DA neurons in animal models of PD induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), rotenone, annonacine and lipopolysaccharide (LPS). Recent evidence suggests that the disease may progress even when the initial cause of neuronal degeneration has disappeared, raising the possibility that toxic substances released by glial cells could be involved in the propagation of neuronal degeneration. Inhibition of the glial reaction and the inflammatory processes may thus represent a therapeutic target to reduce neuronal degeneration in PD.

Changes in vascularization in substantia nigra pars compacta of monkeys rendered parkinsonian

The degeneration of nigral dopaminergic neurons in Parkinson's disease is believed to be associated with a glial reaction and inflammatory changes. In turn, local factors may induce changes in vascularization and contribute to neuronal vulnerability. Among these factors, Vascular Endothelial Growth Factor (VEGF) is released in adults under pathological conditions and is thought to induce angiogenesis. In order to determine whether changes in brain vasculature are observed in the affected brain regions in parkinsonism, we quantitatively analysed the VEGF-expressing cells and blood vessels in the substantia nigra of monkeys rendered parkinsonian by MPTP injection and compared the results with those obtained in control monkeys. Using stereological methods, we observed an increase in the number of VEGF-expressing neurons and an increase of the number of blood vessels and their volume occupying the substantia nigra pars compacta of monkeys rendered parkinsonian by chronic MPTP intoxication. These changes in vascularization may therefore modify the neuronal availability of blood nutrients, blood cells or toxic substances and neuronal susceptibility to parkinsonism.

Heme oxygenase expression in human central nervous system disorders

In the normal mammalian CNS, heme oxygenase-2 (HO-2) is constitutively, abundantly, and fairly ubiquitously expressed, whereas heme oxygenase-1 (HO-1) mRNA and protein are confined to small populations of scattered neurons and neuroglia. Unlike ho-2, the ho-1 gene in neural (and many systemic) tissues is exquisitely sensitive to upregulation by a host of pro-oxidant and other noxious stimuli. In Alzheimer disease, HO-1 immunoreactivity is significantly augmented in neurons and astrocytes of the hippocampus and cerebral cortex relative to age-matched, nondemented controls and colocalizes to senile plaques, neurofibrillary tangles, and corpora amylacea. In Parkinson disease, HO-1 decorates Lewy bodies of affected dopaminergic neurons and is highly overexpressed in astrocytes residing within the substantia nigra. The ho-1 gene is also upregulated in glial cells within multiple sclerosis plaques; in the vicinity of human cerebral infarcts, hemorrhages, and contusions; and in various other degenerative and nondegenerative human CNS disorders. The products of the heme oxygenase reaction, free ferrous iron, carbon monoxide, and biliverdin/bilirubin, are all biologically active molecules that may profoundly influence tissue redox homeostasis under a wide range of pathophysiological conditions. Evidence adduced from whole animal and in vitro studies indicates that enhanced HO-1 activity may either ameliorate or exacerbate neural injury, effects likely contingent upon the specific model employed, the duration and intensity of HO-1 induction, and the chemistry of the local redox microenvironment. HO-1 hyperactivity also promotes mitochondrial sequestration of nontransferrin iron in oxidatively challenged astroglia and may thereby contribute to the pathological iron deposition and bioenergetic failure amply documented in aging and degenerating human neural tissues.

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.

Neurodegenerative disease and iron storage in the brain

PURPOSE OF REVIEW

Iron is very important for normal regulation of various metabolic pathways. Neurons store iron in the form of ferrous ion or neuromelanin. In specific disorders the axonal transport of iron is impaired, leading to iron deposition which in the presence of reactive oxygen species results in neurodegeneration.

RECENT FINDINGS

Recent developments in genetics, including the finding of mutations in the pantothenate kinase gene and ferritin light chain gene, have demonstrated a direct relationship between the presence of a mutation in the iron-regulatory pathways and iron deposition in the brain resulting in neurodegeneration. These two disorders now add to our understanding of the mechanism of disease due to dysfunction of iron-regulatory pathways. In addition to these disorders there may be several other mutations of iron-regulatory genes or related genes that are yet to be found. The animal models of disease have also added value to this area.

SUMMARY

In this review we provide a summary of recent developments in the field of movement disorders with abnormalities in iron transport, and the current evidence in neurodegenerative disorders such as Parkinson's disease.

Lactoferrin enhances peripheral opioid-mediated antinociception via nitric oxide in rats

Lactoferrin (LF) is a multifunctional protein found in various biological fluids. However, the peripheral action of lactoferrin remains unknown. In this study, peripherally applied bovine lactoferrin showed antinociceptive effect that was reversed by a mu-opioid receptor antagonist, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-NH(2) (CTOP), or by a nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), but not by an inactive enantiomer of L-NAME, N(G)-nitro-D-arginine methyl ester (D-NAME), during phase 1 and phase 2 in the rat formalin test. Peripheral coadministration of a micro-opioid receptor agonist, morphine, with subeffective dose of bovine lactoferrin produced a potentiated antinociceptive effect compared to that of morphine alone during both phases in the formalin test. This potentiated antinociception by morphine with bovine lactoferrin was reversed by CTOP or by L-NAME. These results suggest that bovine lactoferrin exerts an antinociceptive activity via potentiation of the peripheral micro-opioidergic system, and that nitric oxide (NO) is involved in this potentiation.

A nuclear microscopic and histochemical study of iron concentrations and distribution in the midbrain of two age groups of monkeys unilaterally injected with MPTP

The present study was carried out to elucidate the concentration and distribution of iron in the substantia nigra of two age groups of monkeys after experimental hemi-Parkinsonism induced by unilateral internal carotid injections of MPTP. Iron levels and distribution were detected using the nuclear microscope, which is able to provide structural and quantitative elemental analysis of biological tissue down to the parts per million (ppm) level of analytical sensitivity. Five weeks after unilateral lesioning with MPTP, we observed a 30-65% loss of neurons in the injected substantia nigra of each monkey, compared with the contralateral control 'non-lesioned' side. In monkeys less than 7 years of age, the iron was distributed fairly uniformly and showed little evidence of focal deposits. In monkeys greater than 7 years of age, we observed many dense focal deposits of iron in the substantia nigra. A comparison between iron distributions in nuclear microscopic scans and cell distributions in the same sections stained by the Nissl technique showed that areas containing high iron concentrations were present not where large-diameter neurons with abundant Nissl substance (presumed dopaminergic neurons) were located but in a region ventral to these cell bodies, i.e., in the substantia nigra pars reticulata. These distributions were present on the control side as well as the MPTP-injected side. Since a previous study has shown that unilateral MPTP injection results in lesions of the substantia nigra of the same side but negligible injury to the opposite side, this implies that the iron deposits existed in the older monkeys before MPTP injections (i.e. they occurred normally). The accumulation of iron in the substantia nigra with age suggests the possibility of localised damage to neurons through the catalysis of free radicals.

Bovine lactoferrin has a nitric oxide-dependent hypotensive effect in rats

Lactoferrin (LF) is a multifunctional protein that is found in milk, neutrophils, and other biological fluids. Under inflammatory conditions, LF production is increased in the periphery by neutrophils. However, the cardiovascular function of LF is still unknown. In the present study, we investigated the effect of bovine LF (BLF) on the mean blood pressure (MBP) and heart rate (HR) in urethane-anesthetized rats and the vascular function of BLF in the rat thoracic aorta. Intravenous injection of BLF produced dose-dependent decreases in MBP but did not affect HR, while the opioid agonist morphine decreased both MBP and HR. The hypotensive effect of BLF was not altered by naloxone methiodide, which cannot pass through the blood-brain barrier, but was significantly reduced by naloxone hydrochloride, which does pass through the blood-brain barrier. BLF-induced hypotension was completely blocked by the NO synthase inhibitor NG-nitro-l-arginine methyl ester (l-NAME) but not by the inactive enantiomer of l-NAME, NG-nitro-d-arginine methyl ester (d-NAME). BLF-induced hypotension was not altered by the muscarinic ACh receptor antagonist atropine or the cyclooxygenase inhibitor diclofenac. BLF produced relaxation in endothelium-intact but not endothelium-denuded aortic rings precontracted with phenylephrine. The relaxation evoked by BLF was completely blocked by l-NAME but not by d-NAME or the ATP-sensitive potassium channel blocker glibenclamide. These results suggest that BLF causes hypotension via an endothelium-dependent vasodilation that is strongly mediated by NO production and that BLF-induced hypotension also may be mediated by the central opioidergic system.

Dopaminergic cell death precedes iron elevation in MPTP-injected monkeys

Though increasing lines of evidence suggest that iron accumulation and iron-induced oxidative stress might be important pathological factors responsible for substantia nigra (SN) cell death in Parkinson's disease (PD), it is still unknown whether iron accumulation is a primary cause or consequence of nigral cell death. Using nuclear microscopy, iron histochemistry, TUNEL method for apoptosis detection, and tyrosine hydroxylase (TH) immunohistochemistry, the present study investigated possible changes in iron contents in the SN and correlations of dopaminergic cell death progression with the process of iron accumulation in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine(MPTP)-induced parkinsonian monkey from 1 d to 18 months after MPTP administration. Our study demonstrated that apoptosis occurred in the ipsilateral SN at 1 d after MPTP injection and the number of TH-positive cells decreased significantly from 1 week onward. However, iron content was significantly increased in the ipsilateral SN from 4.5 months to 18 months after MPTP injection, and the iron increase was significantly correlated to the extent of dopaminergic cell death. These results suggest that dopaminergic cell death induced by MPTP administration might lead to iron accumulation in the monkey SN, and increased iron might contribute to the progression of nigral degeneration.

Lactoferrin enhances opioid-mediated analgesia via nitric oxide in the rat spinal cord

Lactoferrin (LF) is a multifunctional protein that is found in milk, neutrophils, and other biological fluids, and its receptors have also been identified in the central nervous system. Recently, we found that bovine milk-derived LF (BLF) produced analgesia via a mu-opioid receptor-mediated response in the spinal cord. However, the precise mechanism of this analgesic effect remains unclear. In this study, spinally applied BLF produced analgesia that was reversed by coadministration with a nitric oxide (NO) synthase inhibitor, NG-nitro-l-arginine methyl ester, during phases 1 and 2 in the formalin test. Spinal coadministration of a mu-opioid receptor agonist, morphine, with a subeffective dose of BLF produced a much more highly potentiated analgesia than that produced by morphine alone during phases 1 and 2 in the formalin test. This potentiated analgesia by morphine with BLF was reversed by a mu-opioid receptor antagonist, d-Phe-Cys-Tyr-d-Trp-Orn-Thr-NH2, or by NG-nitro-l-arginine methyl ester. In the tail-flick test, continuous spinal infusion of morphine via an osmotic minipump over 6 days resulted in development of tolerance by day 4, but no tolerance of BLF was observed throughout the experiment. These results suggest that BLF acts as an enhancer of the spinal opioidergic system via an NO-mediated mechanism.

Iron misregulation in the brain: a primary cause of neurodegenerative disorders

High iron concentrations in the brains of patients and the discovery of mutations in the genes associated with iron metabolism in the brain suggest that iron misregulation in the brain plays a part in neuronal death in some neurodegenerative disorders, such as Alzheimer's, Parkinson's, and Huntington's diseases and Hallervorden-Spatz syndrome. Iron misregulation in the brain may have genetic and non-genetic causes. The disrupted expression or function of proteins involved in iron metabolism increases the concentration of iron in the brain. Disturbances can happen at any of several stages in iron metabolism (including uptake and release, storage, intracellular metabolism, and regulation). Increased brain iron triggers a cascade of deleterious events that lead to neurodegeneration. An understanding of the process of iron regulation in the brain, the proteins important in this process, and the effects of iron misregulation could help to treat or prevent neurodegenerative disorders.

Novel function of bovine milk-derived lactoferrin on antinociception mediated by mu-opioid receptor in the rat spinal cord

Lactoferrin (LF) is a multifunctional protein that is found in milk, neutrophils, and other biological fluids. Although LF and the LF receptor have been identified in the central nervous system (CNS), the physiological role of LF remains unknown. We found that bovine milk-derived LF (BLF) reduces nociception in various pain models, as shown by the formalin test, hot plate test, and acetic acid writhing test in rats. Intraperitoneal (i.p.) administration of BLF significantly inhibited nociception in these pain models. These antinociceptive effects were also confirmed in BLF-fed rats. The antinociceptive effects of BLF were blocked by naloxone treatment, even though prostaglandin E(2) (PGE(2)) production in the ascites fluid that accumulated during the writhing test was not affected by BLF. Intrathecal (i.t.) application of BLF caused marked antinociceptive effects that were reversed by co-administration of a specific mu-opioid receptor antagonist, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-NH(2) (CTOP), or by naloxone during the formalin test. We conclude that LF possesses mu-opioid receptor-mediated antinociceptive activity in the spinal cord.

Iron in neurodegenerative disorders

Increasing evidence implicates a role of iron in the pathogenesis of numerous neurodegenerative diseases due to its capacity to enhance production of toxic reactive radicals and to induce protein aggregation. The underlying mechanism of iron accumulation in areas of the brain specific for the respective disease, however, is still unknown. Recent molecular and biochemical studies provide new insights into the consequences of impairment of brain iron metabolism. This review summarizes our understanding of the regulation of iron in the brain and defines the current knowledge on the involvement of iron metabolism in neurodegenerative diseases with genetically determined iron accumulation in the brain.

Role of free radicals in the neurodegenerative diseases: therapeutic implications for antioxidant treatment

Free radicals and other so-called 'reactive species' are constantly produced in the brain in vivo. Some arise by 'accidents of chemistry', an example of which may be the leakage of electrons from the mitochondrial electron transport chain to generate superoxide radical (O2*-). Others are generated for useful purposes, such as the role of nitric oxide in neurotransmission and the production of O2*- by activated microglia. Because of its high ATP demand, the brain consumes O2 rapidly, and is thus susceptible to interference with mitochondrial function, which can in turn lead to increased O2*- formation. The brain contains multiple antioxidant defences, of which the mitochondrial manganese-containing superoxide dismutase and reduced glutathione seem especially important. Iron is a powerful promoter of free radical damage, able to catalyse generation of highly reactive hydroxyl, alkoxyl and peroxyl radicals from hydrogen peroxide and lipid peroxides, respectively. Although most iron in the brain is stored in ferritin, 'catalytic' iron is readily mobilised from injured brain tissue. Increased levels of oxidative damage to DNA, lipids and proteins have been detected by a range of assays in post-mortem tissues from patients with Parkinson's disease, Alzheimer's disease and amyotrophic lateral sclerosis, and at least some of these changes may occur early in disease progression. The accumulation and precipitation of proteins that occur in these diseases may be aggravated by oxidative damage, and may in turn cause more oxidative damage by interfering with the function of the proteasome. Indeed, it has been shown that proteasomal inhibition increases levels of oxidative damage not only to proteins but also to other biomolecules. Hence, there are many attempts to develop antioxidants that can cross the blood-brain barrier and decrease oxidative damage. Natural antioxidants such as vitamin E (tocopherol), carotenoids and flavonoids do not readily enter the brain in the adult, and the lazaroid antioxidant tirilazad (U-74006F) appears to localise in the blood-brain barrier. Other antioxidants under development include modified spin traps and low molecular mass scavengers of O2*-. One possible source of lead compounds is the use of traditional remedies claimed to improve brain function. Little is known about the impact of dietary antioxidants upon the development and progression of neurodegenerative diseases, especially Alzheimer's disease. Several agents already in therapeutic use might exert some of their effects by antioxidant action, including selegiline (deprenyl), apomorphine and nitecapone.

Iron and Parkinson's disease

Multiple studies implicate iron in the pathophysiology of Parkinson's disease (PD). In the brains of patients with PD, iron levels are elevated and the levels of iron-binding proteins are abnormal. Iron has been suspected to contribute to PD because Fe(II) is known to promote oxidative damage. Recent studies suggest that an additional mechanism by which iron might contribute to PD is by inducing aggregation of the alpha-synuclein, which is a protein that accumulates in Lewy bodies in PD.

Lactoferrin is synthesized by activated microglia in the human substantia nigra and its synthesis by the human microglial CHME cell line is upregulated by tumor necrosis factor alpha or 1-methyl-4-phenylpyridinium treatment

The presence of the iron-binding protein lactoferrin (Lf) in some specific areas of the central nervous system and particularly in the normal human substantia nigra, where it is found in dopaminergic (DA) neurons and some glial cells, led us to investigate Lf synthesis in this area. Lf mRNA were identified using in situ hybridization and found in small ameboid cells. These cells were identified using immunocytochemistry as activated microglia since they exhibited macrophage markers such as the CD68 and the CR1 antigens. Double immunofluorescent labeling confirmed that the two Lf immunostained cell populations were activated microglia and DA neurons. Since activated microglia contained both Lf and its messenger, these cells are the Lf producing cells. The presence of Lf in DA neurons in which no Lf messengers were visible, might be due to an endocytosis mechanism, DA neurons probably internalizing Lf produced in microglial cells located in their neighborhood. In neuropathological disorders, such as Alzheimer's and Parkinson's diseases, inflammatory process and oxidative stress are events that contribute to neuronal death. Since Lf concentration increases during these pathologies, we studied the level of Lf expression under these different stresses and showed, using RT-PCR, that the immortalized human embryonic microglial CHME cell line produced Lf transcripts under tumor necrosis factor alpha or 1-methyl-4-phenylpyridinium treatment whereas untreated cells did not. These data confirm that Lf is produced only when microglia are activated.

Human neutrophil lactoferrin trans-activates the matrix metalloproteinase 1 gene through stress-activated MAPK signaling modules

It has been proposed that human neutrophil lactoferrin (Lf) could be involved in gene expression as a DNA-binding protein after its translocation into the nucleus. However, the molecular basis of Lf action has not been defined, and Lf-regulated target genes have not been identified. We report here that overexpressed Lf functions as a specific trans-activator of matrix metalloproteinase 1 (MMP1) gene, and that induction of this AP-1-responsive gene is mediated via the stress-activated MAPK signaling modules. Transactivation of the MMP1 promoter by overexpressed Lf requires the presence of an AP-1 binding site. In gel shift experiments, Lf did not interact directly with AP-1-containing fragments of the MMP1 promoter. However, nuclear extracts from Lf-expressing cells contained increased levels of proteins that bound to AP-1 elements. This Lf-induced AP-1 DNA binding activity was reduced by a p38 MAPK inhibitor. Inhibitors of the MEK kinases had little effect on Lf-induced AP-1. However, expression of dominant-negative MKK4 or JNK1 inhibited Lf-induced gene expression. The JNK activity stimulated by Lf correlates with the enhanced AP-1 binding ability. These findings demonstrate that the Lf-induced activation of AP-1 is mediated via JNK and p38 MAPK pathways.

Brain iron pathways and their relevance to Parkinson's disease

A central role of iron in the pathogenesis of Parkinson's disease (PD), due to its increase in substantia nigra pars compacta dopaminergic neurons and reactive microglia and its capacity to enhance production of toxic reactive oxygen radicals, has been discussed for many years. Recent transcranial ultrasound findings and the observation of the ability of iron to induce aggregation and toxicity of alpha-synuclein have reinforced the critical role of iron in the pathogenesis of nigrostriatal injury. Presently the mechanisms involved in the disturbances of iron metabolism in PD remain obscure. In this review we summarize evidence from recent studies suggesting disturbances of iron metabolism in PD at possibly different levels including iron uptake, storage, intracellular metabolism, release and post-transcriptional control. Moreover we outline that the interaction of iron with other molecules, especially alpha-synuclein, may contribute to the process of neurodegeneration. Because many neurodegenerative diseases show increased accumulation of iron at the site of neurodegeneration, it is believed that maintenance of cellular iron homeostasis is crucial for the viability of neurons.

Iron and iron management proteins in neurobiology

The ability of the brain to store a readily bioavailable source of iron is essential for normal neurologic function because both iron deficiency and iron excess in the brain have serious neurologic consequences. The blood-brain barrier presents unique challenges to timely and adequate delivery of iron to the brain. The regional compartmentalization of neurologic function and a myriad of cell types provide additional challenges. Furthermore, iron-dependent events within the central nervous system (CNS) are age dependent (e.g., myelination) or region specific (e.g., dopamine synthesis). Thus the mechanisms for maintaining the delicate balance of CNS iron concentration must be considered on a region-specific and age-specific basis. Confounding factors that influence brain iron acquisition in addition to age-specific and region-specific requirements are dietary factors and disease. This article raises and addresses the novel concept of regional regulation of brain iron uptake by reviewing the developmental patterns of iron accumulation and expression of proteins responsible for maintaining iron homeostasis in a region-specific and cell-specific manner. Understanding these mechanisms is essential for generating insights into diseases such as Hallervorden-Spatz syndrome, in which excess iron accumulation in the brain plays a significant role in the disease process, and should also unveil windows of opportunity for replenishing the brain in a state of iron deficiency.

Iron and neurodegenerative disorders

The brain shares with other organs the need for a constant and readily available supply of iron and has a similar array of proteins available to it for iron transport, storage, and regulation. However, unlike other organs, the brain places demands on iron availability that are regional, cellular, and age sensitive. Failure to meet these demands for iron with an adequate supply in a timely manner can result in persistent neurological and cognitive dysfunction. Consequently, the brain has developed mechanisms to maintain a continuous supply of iron. However, in a number of common neurodegenerative disorders, there appears to be an excess accumulation of iron in the brain that suggests a loss of the homeostatic mechanisms responsible for regulating iron in the brain. These systems are reviewed in this article. As a result of a loss in iron homeostasis, the brain becomes vulnerable to iron-induced oxidative stress. Oxidative stress is a confounding variable in understanding the cell death that may result directly from a specific disease and is a contributing factor to the disease process. The underlying pathogenic event in oxidative stress is cellular iron mismanagement.

Impaired iron homeostasis in Parkinson's disease

Despite physiological systems designed to achieve iron homeostasis, increased concentrations of brain iron have been demonstrated in a range of neurodegenerative diseases. These including the parkinsonian syndromes, the trinucleotide repeat disorders and the dementia syndromes. The increased brain iron is confined to those brain regions most affected by the degeneration characteristic of the particular disorder and is suggested to stimulate cell damage via oxidative mechanisms. Changes in central iron homeostasis have been most closely investigated in PD, as this disorder is well characterised both clinically and pathologically. PD is associated with a significant increase in iron in the degenerating substantia nigra (SN) and is measureable in living PD patients and in post-mortem brain. This increase, however, occurs only in the advanced stages of the disease, suggesting that this phenonoma may be a secondary, rather than a primary initiating event, a hypothesis also supported by evidence from animal experiments. The source of the increased iron is unknown but a variety of changes in iron homeostasis have been identified in PD, both in the brain and in the periphery. The possibility that an increased amount of iron may be transported into the SN is supported by data demonstrating that one form of the iron-binding glycoprotein transferrin family, lactotransferrin, is increased in surviving neurons in the SN in the PD brain and that this change is associated with increased numbers of lactotransferrin receptors on neurons and microvessels in the parkinsonian SN. These changes could represent one mechanism by which iron might concentrate within the PD SN. Alternatively, the measured increased in iron might result from a redistribution of ferritin iron stores. Ferritin is located in glial cells while the degenerating neurons do not stain positive for ferritin. As free radicals are highly reactive, it is unlikely that glial-derived free radicals diffuse across the intracellular space in sufficent quantities to damage neuronal constituents. If intracellular iron release contributes to neuronal damage it seems more probable that an intraneuronal iron source is responsible for oxidant-mediated damage. Such a iron source is neuromelanin (NM), a dark-coloured pigment found in the dopaminergic neurons of the human SN. In the normal brain, NM has the ability to bind a variety of metals, including iron, and increased NM-bound iron is reported in the parkinsonian SN. The consequences of these phenomena for the cell have not yet been clarified. In the absence of significant quantities of iron NM can act as an antioxidant, in that it can interact with and inactivate free radicals. On the other hand, in the presence of iron NM appears to act as a proxidant, increasing the rate of free radical production and thus the oxidative load within the vulnerable neurons. Given that increased iron is only apparent in the advanced stages of the disease it is unlikely that NM is of importance for the primary aetiology of PD. A localised increase in tissue iron and its interaction with NM may be, however, important as a secondary mechanism by increasing the oxidative load on the cell, thereby driving neurodegeneration.

Brain iron transport and neurodegeneration

Despite years of investigation, it is still not known why iron levels are abnormally high in some regions of the brain in neurodegenerative disorders. Also, it is not clear whether iron accumulation in the brain is an initial event that causes neuronal death or is a consequence of the disease process. Here, we propose that iron and iron-induced oxidative stress constitute a common mechanism that is involved in the development of neurodegeneration. Also, we suggest that, at least in some neurodegenerative disorders, brain iron misregulation is an initial cause of neuronal death and that this misregulation might be the result of either genetic or non-genetic factors.

Assessment of plasma lactoferrin in Parkinson's disease

Iron may play an important role in the pathogenesis of Parkinson's disease (PD). Recent studies have shown that the iron-transporting glycoprotein lactoferrin (LF) and its receptor are increased in the substantia nigra (SN) in PD. We investigated whether plasma levels of LF are altered in dopa-responsive PD. Plasma LF was not different between patients with PD (n = 23; 306 +/- 116 [mean +/- standard deviation] ng/ml) and age- and sex-matched healthy control subjects (n = 15; 359 +/- 126 ng/ml ). However, LF was inversely correlated with PD severity (r = -0.68, P = 0.002), an association that remained significant after adjustment for treatment with levodopa, monoaminooxidase inhibitors, and dopa agonists (r = -0.53, P = 0.017). Plasma transferrin and ferritin levels were not different between groups and neither correlated with disease severity nor with LF levels. Together with the result of increased nigral lactoferrin, this finding is compatible with the hypothesis of an imbalance between LF levels in blood and SN in progressing PD. Larger and particularly longitudinal studies and measurements of LF in cerebrospinal fluid are warranted to further examine the role of LF in PD.

Tumor necrosis factor-alpha increases lactoferrin transcytosis through the blood-brain barrier

Lactoferrin (Lf) is an iron-binding protein involved in host defense against infection and severe inflammation, which accumulates in the brain during neurodegenerative disorders. Prior to determining Lf function in pathological brain tissues, we investigated its transport through the blood-brain barrier (BBB) in inflammatory conditions. For this purpose, we used a reconstituted BBB model consisting of the coculture of bovine brain capillary endothelial cells (BBCECs) and astrocytes in the presence of tumor necrosis factor-alpha (TNF-alpha). As TNF-alpha can be either synthesized by brain glial cells or present in circulating blood, BBCECs were exposed to this cytokine at their luminal or abluminal side. We have been able to demonstrate that in the presence of TNF-alpha, whatever the type of exposure, BBCECs were activated and Lf transport through the activated BBCECs was markedly increased. Lf was recovered intact at the abluminal side of the cells, suggesting that increased Lf accumulation may occur in immune-mediated pathophysiology. This process was transient as 20 h later, cells were in a resting state and Lf transendothelial traffic was back to normal. The enhancement of Lf transcytosis seems not to involve the up-regulation of the Lf receptor but rather an increase in the rate of transendothelial transport.

Time course of dopaminergic cell death and changes in iron, ferritin and transferrin levels in the rat substantia nigra after 6-hydroxydopamine (6-OHDA) lesioning

Parkinson's disease is characterized by dopaminergic cell death in the substantia nigra. The underlying mechanism is, however, unknown. Though there are increasing lines of evidence showing iron accumulation in the Parkinsonian substantia nigra, it still remains obscure whether increased iron is the primary cause of dopaminergic cell death, or just a consequence of the pathological process. It is also unclear how iron gains access to the Parkinsonian SN. To gain more understanding in these areas, the present study investigated the time course of dopaminergic cell death and of changes in the level of iron, ferritin and transferrin. The results showed that iron was increased after the significant nigral dopaminergic cell death induced by 6-hydroxydopamine injection into the rat substantia nigra. On the other hand, the expression of transferrin was decreased. However, there was a temporal increase in the number of ferritin positive microglia. The results indicated that iron increase was not the primary cause of dopaminergic cell death in the Parkinsonian rat. It was most likely the result of an accumulation of iron-laden microglia.

Time-course and localization of transferrin receptor expression in the substantia nigra of 6-hydroxydopamine-induced parkinsonian rats

Parkinson's disease is a neurodegenerative disease characterized by dopaminergic cell death in the substantia nigra. The cause of the cell death is, however, obscure. Recently, accumulation of iron in the parkinsonian substantia nigra and iron-catalysed free radical generation have been proposed as possible causes of nigral cell death. The transferrin receptor has been implicated as a possible mediator of this iron accumulation in the parkinsonian substantia nigra. The present study investigated the distribution of transferrin receptor-immunoreactive proteins and its co-localization with tyrosine hydroxylase in the normal rat substantia nigra and their expressions in the parkinsonian substantia nigra from three days to three months after 6-hydroxydopamine lesioning. Computer image analysis of the grey mean of transferrin receptor staining in the microvessels was also employed. The results showed that the transferrin receptor immunolabelling was localized in some neurons and glial cells in the normal substantia nigra pars compacta and pars reticulata, and that about 54% of tyrosine hydroxylase-positive cells were also stained with transferrin receptor. There was a decrease of tyrosine hydroxylase- and transferrin receptor-positive cells in the 6-hydroxydopamine-lesioned substantia nigra. The grey mean of transferrin receptor staining in microvessels in the lesioned substantia nigra was, however, not different from that in the control. It was concluded that transferrin receptors in neurons, glial cells and microvessels might not be responsible for iron accumulation in the parkinsonian substantia nigra. The loss of transferrin receptor-immunopositive cells might, however, partly be accounted for by the death of transferrin receptor-positive dopaminergic cells induced by 6-hydroxydopamine lesioning.

Mitochondrial iron sequestration in dopamine-challenged astroglia: role of heme oxygenase-1 and the permeability transition pore

Little is currently known concerning the mechanisms responsible for the excessive deposition of redox-active iron in the substantia nigra of subjects with Parkinson's disease (PD). In the present study, we demonstrate that dopamine promotes the selective sequestration of non-transferrin-derived iron by the mitochondrial compartment of cultured rat astroglia and that the mechanism underlying this novel dopamine effect is oxidative in nature. We also provide evidence that up-regulation of the stress protein heme oxygenase-1 (HO-1) is both necessary and sufficient for mitochondrial iron trapping in dopamine-challenged astroglia. Finally, we show that opening of the mitochondrial transition pore (MTP) mediates the influx of non-transferrin-derived iron into mitochondria of dopamine-stimulated and HO-1-transfected astroglia. Our findings provide an explanation for the pathological iron sequestration, mitochondrial insufficiency, and amplification of oxidative injury reported in the brains of PD subjects. Pharmacological blockade of transition metal trapping by "stressed" astroglial mitochondria (e.g., using HO-1 inhibitors or modulators of the MTP) may afford effective neuroprotection in patients with PD and other neurological afflictions.

Existing and emerging mechanisms for transport of iron and manganese to the brain

The metals iron (Fe) and manganese (Mn) are essential for normal functioning of the brain. This review focuses on recent developments in the literature pertaining to Fe and Mn transport. These metals are treated together because they appear to share several transport mechanisms. In addition, several neurological diseases such as Alzheimer's Disease, Parkinson's Disease, and Huntington's Disease are all associated with Fe mismanagement in the brain, particularly in the striatum and basal ganglia. Similarly, Mn accumulation in brain also appears to target the same brain regions. Therefore, stringent regulation of the concentration of these metals in the brain is essential. The homeostatic mechanisms for these metals must be understood in order to design neurotoxicity prevention strategies.

Etiology and pathogenesis of Parkinson's disease

Parkinson's disease (PD) is an age-related neurodegenerative disorder that affects approximately 1 million persons in the United States. It is characterized by resting tremor, rigidity, bradykinesia or slowness, gait disturbance, and postural instability. Pathological features include degeneration of dopaminergic neurons in the substantia nigra pars compacta coupled with intracytoplasmic inclusions known as Lewy bodies. Neurodegeneration and Lewy bodies can also be found in the locus ceruleus, nucleus basalis, hypothalamus, cerebral cortex, cranial nerve motor nuclei, and central and peripheral components of the autonomic nervous system. Current treatment consists of a dopamine replacement strategy using primarily the dopamine precursor levodopa. While levodopa provides benefit to virtually all PD patients, after 5-10 years of treatment the majority of patients develop adverse events in the form of dyskinesia (involuntary movements) and fluctuations in motor response. Further, disease progression is associated with the development of dementia, autonomic dysfunction, and postural instability, which do not respond to levodopa therapy. Accordingly, research efforts have been directed toward understanding the etiology and pathogenesis of PD in the hope of developing a more effective therapy that will slow or halt the natural progression of PD. This paper reviews recent advances.

Receptor-mediated transcytosis of lactoferrin through the blood-brain barrier

Lactoferrin (Lf) is an iron-binding protein involved in host defense against infection and severe inflammation; it accumulates in the brain during neurodegenerative disorders. Before determining Lf function in brain tissue, we investigated its origin and demonstrate here that it crosses the blood-brain barrier. An in vitro model of the blood-brain barrier was used to examine the mechanism of Lf transport to the brain. We report that differentiated bovine brain capillary endothelial cells exhibited specific high (Kd = 37.5 nM; n = 90,000/cell) and low (Kd = 2 microM; n = 900,000 sites/cell) affinity binding sites. Only the latter were present on nondifferentiated cells. The surface-bound Lf was internalized only by the differentiated cell population leading to the conclusion that Lf receptors were acquired during cell differentiation. A specific unidirectional transport then occurred via a receptor-mediated process with no apparent intraendothelial degradation. We further report that iron may cross the bovine brain capillary endothelial cells as a complex with Lf. Finally, we show that the low density lipoprotein receptor-related protein might be involved in this process because its specific antagonist, the receptor-associated protein, inhibits 70% of Lf transport.

Characterization and distribution of ferritin binding sites in the adult mouse brain

Studies on iron uptake into the brain have traditionally focused on transport by transferrin. However, transferrin receptors are not found in all brain regions and are especially low in white matter tracts where high iron concentrations have been reported. Several lines of research suggest that a receptor for ferritin, the intracellular storage protein for iron, may exist. We present, herein, evidence for ferritin binding sites in the brains of adult mice. Autoradiographic studies using 125I-recombinant human ferritin demonstrate that ferritin binding sites in brain are predominantly in white matter. Saturation binding analyses revealed a single class of binding sites with a dissociation constant (K(D)) of 4.65 x 10(-9) M and a binding site density (Bmax of 17.9 fmol bound/microg of protein. Binding of radiolabeled ferritin can be competitively displaced by an excess of ferritin but not transferrin. Ferritin has previously been shown to affect cellular proliferation, protect cells from oxidative damage, and deliver iron. The significance of a cellular ferritin receptor is that ferritin is capable of delivering 2,000 times more iron per mole of protein than transferrin. The distribution of ferritin binding sites in brain vis-à-vis transferrin receptor distribution suggests distinct methods for iron delivery between gray and white matter.

The role of iron in neurodegeneration: prospects for pharmacotherapy of Parkinson's disease

Although the aetiology of Parkinson's disease (PD) and related neurodegenerative disorders is still unknown, recent evidence from human and experimental animal models suggests that a misregulation of iron metabolism, iron-induced oxidative stress and free radical formation are major pathogenic factors. These factors trigger a cascade of deleterious events leading to neuronal death and the ensuing biochemical disturbances of clinical relevance. A review of the available data in PD provides the following evidence in support of this hypothesis: (i) an increase of iron in the brain, which in PD selectively involves neuromelanin in substantia nigra (SN) neurons; (ii) decreased availability of glutathione (GSH) and other antioxidant substances; (iii) increase of lipid peroxidation products and reactive oxygen (O2)species (ROS); and (iv) impaired mitochondrial electron transport mechanisms. Most of these changes appear to be closely related to interactions between iron and neuromelanin, which result in accumulation of iron and a continuous production of cytotoxic species leading to neuronal death. Some of these findings have been reproduced in animal models using 6-hydroxydopamine, N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), iron loading and beta-carbolines, although none of them is an accurate model for PD in humans. Although it is not clear whether iron accumulation and oxidative stress are the initial events causing cell death or consequences of the disease process, therapeutic efforts aimed at preventing or at least delaying disease progression by reducing the overload of iron and generation of ROS may be beneficial in PD and related neurodegenerative disorders. Current pharmacotherapy of PD, in addition to symptomatic levodopa treatment, includes 'neuroprotective' strategies with dopamine agonists, monoamine oxidase-B inhibitors (MAO-B), glutamate antagonists, catechol O-methyltransferase inhibitors and other antioxidants or free radical scavengers. In the future, these agents could be used in combination with, or partly replaced by, iron chelators and lazaroids that prevent iron-induced generation of deleterious substances. Although experimental and preclinical data suggest the therapeutic potential of these drugs, their clinical applicability will be a major challenge for future research.

Effects of cytisine on hydroxyl radicals in vitro and MPTP-induced dopamine depletion in vivo

The potential new iron-chelator cytisine and the radical scavenger N-tert-butyl-alpha-(2-sulfophenyl) nitrone (S-PBN) were incubated in a Fenton system and hydroxyl radical formation was measured with the salicylate trapping assay. Both cytisine and S-PBN reduced hydroxyl radical formation in a concentration-dependent manner. For in vivo studies, C57BL/6 mice were injected repeatedly with cytisine (0.5 mg/kg or 2.0 mg/kg s.c.) or saline seven days before and after a single 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) injection (30 mg/kg s.c.). Seven days after MPTP treatment alone dopamine levels were significantly reduced to 12% of the control values (p < 0.001), whereas MPTP + cytisine treatment (2 mg/kg) led to more than twofold higher dopamine levels (p < 0.01) compared with MPTP alone. We have shown for the first time that cytisine attenuates hydroxyl radical formation in vitro and reduces MPTP-induced dopamine depletion. Thus, cytisine may be useful for the treatment of Parkinson's Disease where the chelation of iron ions could prevent neuronal cell death.

Lactoferrin is synthesized by mouse brain tissue and its expression is enhanced after MPTP treatment

The biological role and origin of human lactoferrin (Lf) within the brain in normal and disease processes are as yet uncharted. In this context the origin and expression of brain Lf in normal and MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-treated mice were investigated using immunohisto chemistry, PCR amplification and in situ hybridization. Lf immunostaining was observed both on sections of mouse lactating mammary gland, which was used as a positive control, and brains from young, adult and aged mice. Lf immunoreactivity was present in the pituitary gland, the hippocampus and the cortex of mouse brains and to a greater extent in older mice. After reverse transcription, Lf transcripts were also found in these brain sections. Lf distribution and expression in the MPTP-induced parkinsonian mouse model were next investigated. A marked depletion of dopamine and its metabolites: dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxy indole acetic acid (5-HIAA) occurs in the high dose MPTP-treated mice. The level of Lf expression was found to be greatly increased in the same animals but Lf immunoreactivity detected in the same brain region was not found increased in the affected areas.

Effect of lactoferrin on the phagocytic activity of polymorphonuclear leucocytes isolated from blood of patients with autoimmune diseases and Staphylococcus aureus allergy

Phagocytic number (PN) and phagocytic index (PI) of neutrophils isolated from blood of patients with autoimmune diseases, allergy to Staphylococcus aureus and from blood of healthy individuals were examined. Our results concerning the influence of lactoferrin (Lf); (6.7 mg/l) on the PI of PMN showed that: 1) Lf enhances reliable PI of PMN at the 30-th minute starting the phagocytic reaction in patients with autoimmune disease in an active stage, in blood donors treated as healthy with the presence of autoantibodies, in patients with autoimmune diseases and proved autoantibodies against tissue, cell antigens and collagen, 2) Lf influences non-significantly PI of PMN in patients with autoimmune collagen diseases in remission, 3) Lf increases PI of PMN with 19% only in 58% from the assessed patients with Staphylococcus aureus, and 4) Lf decreases non-significantly PI of PMN in the healthy controls. Our studies on the effect of Lf on the phagocytic activity of PMN suggest that Lf has stronger effect on the PN compared to the PI: 1) Lf enhances with 86% the PN in patients with Staphylococcus aureus, 2) Lf increases PN of PMN in all of the assessed patients with autoimmune collagen diseases in active stage (mean with 72%), and 3) Lf increases PN of PMN in 4 from the 5 investigated healthy controls (mean with 22%). Our results show a "corrective" effect of Lf on the phagocytic functions in the investigated groups of patients. The possible mechanisms, by which Lf increases PN and PI of neutrophils, is discussed: 1) they may concern the antioxidative properties of Lf to block the iron ions in their catalytic inactive form or to take part as ferric-Lf in an oxidative-reduction processes on the plasma membrane and controlling transmembrane transport systems, 2) Lf decreases the negative surface charge and thus enhances the adherent ability of the PMN. Probably to this stimulated adherent ability dues the increased ingestion of bacteria in the presence of Lf, and 3) The "changed" membrane of PMN may have higher number receptors for Lf to bind more molecules of exogenous Lf. The increase of Lf binding which enhances the adherence and aggregation of neutrophils, facilitates the phagocytosis.

Expression of iron transport proteins and excessive iron accumulation in the brain in neurodegenerative disorders

New findings on the role of LfR (lactotransferrin receptor), MTf (melanotransferrin), CP (ceruloplasmin) and DCT1 (Divalent Cation Transporter) in brain iron transport, obtained during the past 3 years, are important advances in the fields of physiology and pathophysiology of brain iron metabolism. According to these findings, disruption in the expression of these proteins in the brain is probably one of the important causes of the altered brain iron metabolism in age-related neurodegenerative diseases, including Parkinson's Disease, Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. Further studies on the involvement of LfR, MTf and DCT1 in iron uptake by and CP in iron egress from different types of brain cells as well as control mechanisms of expression of these proteins in the brain are critical for elucidating the causes of excessive accumulation of iron in the brain and neuronal death in neurodegenerative diseases.

Transferrin-bound iron uptake by the cultured cerebellar granule cells

Excessive brain iron has been found in several neurodegenerative diseases. However, little information is available about mechanism of iron uptake by different types of brain cells including neurons. In this study, transferrin-bound iron (Tf-Fe) accumulation in the cultured cerebellar granule cell was investigated in vitro. After 5 days of culture, the cells were incubated with 1 microM of double-labelled transferrin (1251-Tf-59Fe) at 37 degrees C for 60 min. The cellular Tf-Fe and transferrin (Tf) uptake was analysed. The result showed (1) Tf uptake by the cells increased rapidly at the first 5 min, reaching its maximum after about 20 min of incubation; (2) Tf-Fe uptake kept increasing in a linear manner during the whole period of incubation; (3) the addition of either NH4Cl or CH3NH2, the blockers of Tf-Fe uptake via inhibiting iron release from Tf within endosomes, decreased the cellular Tf-Fe uptake but had no significant effect on Tf uptake; (4) trypsin and unlabelled Tf-Fe inhibited the uptake rate of Tf-Fe as well as Tf. The results suggested that Tf-Fe transport across the membrane of this type of neuron, much like other mammalian cells, was mediated by Tf-TfR endocytosis. Dysfunction of Tf or TfR would possibly lead to iron irregulation in the brain and consequently cause damage to neuronal functions.

Dendroaxonal transcytosis of transferrin in cultured hippocampal and sympathetic neurons

Previous studies using overexpressed polymeric immunoglobulin receptor in cultured neurons have suggested that these cells may use a dendroaxonal transcytotic pathway (Ikonen et al., 1993; de Hoop et al., 1995). By using a combination of semiquantitative light microscopy, video microscopy, and a biochemical assay, we show that this pathway is used by the endogenous ligand transferrin (Tf) and its receptor. Labeled Tf added to fully mature hippocampal neurons changes the intracellular distribution of its receptor from preferentially dendritic shortly after addition to dendritic and axonal at longer times. Incubation of living neurons with (caged)FITC-Tf followed by uncaging in the dendrites results in the later appearance of fluorescence in the axon of the same cell. In "chambered" sympathetic neurons in culture, 125I-Tf or iron as 55Fe-Tf added to the cell body/dendrite chamber is recovered in the axonal chamber, showing that internalized ligand from the cell body-dendrite area is released at the axonal end. Finally, we show that excitatory neurotransmitters increase Tf receptor transcytosis, whereas inhibitory neurotransmitters reduce it. The dendritic uptake, transcytotic transport, and axonal release of physiologically active Tf demonstrated here could be envisioned for other trophic factors and therefore have important consequences for neuronal anterograde target maturation. Moreover, the changes in transcytosis after neurotransmitter addition may be important in the cellular responses that follow electrical activation.

Dendroaxonal transcytosis of transferrin in cultured hippocampal and sympathetic neurons

Previous studies using overexpressed polymeric immunoglobulin receptor in cultured neurons have suggested that these cells may use a dendroaxonal transcytotic pathway (Ikonen et al., 1993; de Hoop et al., 1995). By using a combination of semiquantitative light microscopy, video microscopy, and a biochemical assay, we show that this pathway is used by the endogenous ligand transferrin (Tf) and its receptor. Labeled Tf added to fully mature hippocampal neurons changes the intracellular distribution of its receptor from preferentially dendritic shortly after addition to dendritic and axonal at longer times. Incubation of living neurons with (caged)FITC-Tf followed by uncaging in the dendrites results in the later appearance of fluorescence in the axon of the same cell. In "chambered" sympathetic neurons in culture, 125I-Tf or iron as 55Fe-Tf added to the cell body/dendrite chamber is recovered in the axonal chamber, showing that internalized ligand from the cell body-dendrite area is released at the axonal end. Finally, we show that excitatory neurotransmitters increase Tf receptor transcytosis, whereas inhibitory neurotransmitters reduce it. The dendritic uptake, transcytotic transport, and axonal release of physiologically active Tf demonstrated here could be envisioned for other trophic factors and therefore have important consequences for neuronal anterograde target maturation. Moreover, the changes in transcytosis after neurotransmitter addition may be important in the cellular responses that follow electrical activation.

The potential role of iron chelators in the treatment of Parkinson's disease and related neurological disorders

Neurodegeneration is characterized by a marked accumulation of iron in the affected brain regions. The reason for this is still unknown. In this article we review the available data on the possible involvement of iron and mediated oxidative stress in the aetiology of Parkinson's disease and related disorders. Iron chelators, if they effectively prevent radical formation, have great therapeutic potential against ischaemia/reperfusion, rheumatoid arthritis, and anthracycline toxicity, which are most likely free radical-mediated. The efficacy of the best established chelating drug desferal in neurodegenerative disease is limited due to its high cerebro- and oculotoxicity. New bioactive chelating agents are currently being developed, among them are oxidative stress activatable iron chelators which are most likely less toxic and can flexibly respond to an increase of free radical formation in the cell.