Iron-enriched oligodendrocytes: a reexamination of their spatial distribution

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.

Delayed oligodendrocyte degeneration induced by brief exposure to hydrogen peroxide

An in vitro model system of cultured oligodendrocytes was used to determine the susceptibility of these cells to oxidative stress induced by 15 min exposure to millimolar concentrations of hydrogen peroxide (H2O2). Following the exposure, the cells were incubated in normal growth medium, and analyzed at different time points. Although no cell loss was observed during the exposure period, there was a progressive depletion of adherent cells during the postexposure period as seen from either the number of recoverable nuclei, or from total RNA content of the cultures. Both the rate and the extent of cell deletion was directly dependent on H2O2 concentration. Cell death was preceded by structural alterations in the nuclear envelope resulting in "fragile" nuclei which disintegrated during isolation. Northern blot analysis showed that the expression of myelin-specific genes was rapidly downregulated in H2O2-treated cells. On the other hand, the expression of antiapoptotic gene, bcl-2 featured massive but transient upregulation. Oligodendrocyte degeneration also featured genomic DNA degradation into high molecular weight fragments, which are likely to represent cleaved chromosomal loops. The results demonstrate vulnerability of oligodendrocytes to oxidative stress that induces rapid degeneration and ultimately leads to delayed cell death. This feature is highly relevant to oligodendrocyte damage and depletion following ischemic, traumatic, or inflammatory insults to the central nervous system (CNS).

Iron deposits in the central nervous system of SJL mice with experimental allergic encephalomyelitis

Iron has been proposed to promote oxidative tissue damage in multiple sclerosis (MS). In order to gain insights about how iron gets processed during MS, the deposition of iron was investigated in the CNS of mice with experimental allergic encephalomyelitis (EAE), which is a commonly used animal model of MS. Control mice (adjuvant only) and EAE mice (myelin basic protein plus adjuvant), were sacrificed at 4-8 days (preclinical phase), 10-13 days (clinical phase), or 18 days (recovery phase) post injection. Sections from the cerebrum, hindbrain, and cervical, thoracic and lumbar spinal cord were stained as previously described (J. Neurosci. Res. 29:413, 1991), and scored blindly for histopathological staining. There was minimal histopathological staining at any age in control animals or during the preclinical stage in EAE animals. At the clinical stage of EAE, stained pathological features (macrophages, extravasated RBC and granular staining) were significantly increased compared to the preclinical stage. In the recovery phase, macrophage and granular staining persisted but there was loss of extravasated RBC. Dual labeling studies revealed that granular deposits were present in astrocytes and in locations that appeared to be extracellular. In order to gain insights about the origin of iron deposits in EAE mice, additional studies were performed on brains of mice with extravasated blood lesions. These brains had granular, macrophage and RBC staining. Thus, each of the stained features in EAE animals could be due to the extravasation of blood which occurs in the SJL model of EAE, although some of the iron could have originated from myelin and oligodendrocytes damaged during EAE.

Desferrioxamine suppresses experimental allergic encephalomyelitis induced by MBP in SJL mice

Data from several studies indicate that free radicals have a pathogenic role in experimental allergic encephalomyelitis (EAE). Iron can contribute to free radical damage by catalyzing the formation of hydroxyl radical, inducing secondary initiation of lipid peroxidation and by promoting the oxidation of proteins. The iron chelator, desferrioxamine, can limit these oxidative reactions and it can scavenge peroxynitrite independent of iron chelation. Two previous studies have examined the therapeutic value of desferrioxamine in EAE. One study observed an effect when disease was induced by spinal cord homogenates (J. Exp. Med. 160, p. 1532, 1984), but a second study found no therapeutic value of desferrioxamine for myelin basic protein (MBP)-induced EAE (J. Neuroimmunol. 17, p. 127, 1988). In the second study, the drug was only administered during the preclinical stages of disease. Since desferrioxamine scavenges free radicals and prevents their formation, we hypothesized that the drug should be given during the active stage of disease to have therapeutic value. We first demonstrated that the drug enters the CNS around inflammatory cells in EAE animals. In animals treated during the active stage of MBP-induced EAE, the clinical signs were significantly reduced compared to vehicle-treated animals. The iron-bound form of this drug, ferrioxamine, was without therapeutic value. A derivative of desferrioxamine, hydroxylethyl starch (HES)-desferrioxamine, has a greater plasma half-life than desferrioxamine and it was also tested. Although there was a suggestion of improvement in these animals, the effects were less than that observed for desferrioxamine which may be related to the greater molecular size of HES-desferrioxamine. In summary, these data suggest that chelation of iron is an effective therapeutic target for EAE.

Iron deposits in multiple sclerosis and Alzheimer's disease brains

Iron may contribute to the pathogenesis of neurological diseases by promoting oxidative damage. The localization of iron in multiple sclerosis (MS) and Alzheimer's disease (AD) brains was investigated to further the understanding of its pathogenic role in these disease states. Earlier studies, utilizing a standard Perls' stain, yielded conflicting reports regarding the distribution of iron deposits in MS brains, and a previous study on AD brains utilized a diaminobenzidine (DAB) enhanced version of this stain. In the present study, a modified version of the DAB-enhanced stain was used; it utilizes sodium borohydride, proteinase K, Triton X-100 and xylenes to increase the accessibility of tissue iron to histochemical reagents. This modified method can reveal iron deposits that are missed by the Perls' or DAB-enhanced Perls' stains. In addition to its normal deposition in oligodendrocytes and myelin, iron was detected in reactive microglia, ameboid microglia and macrophages in MS brains. In AD brains, three types of plaques were stained: dense core, clear core and amorphous plaques. Punctate staining was also observed in neurons in the corticies of AD brains. The structure accounting for punctate labeling may be damaged mitochondria, lipofuscin or amyloid deposits. Dense core plaques, clear plaques and punctate labeling were not detected in the previous AD study which utilized only the DAB-enhanced Perls' stain. The labeling of these additional structures illustrates the benefit of the modified method. In summary, the localization of iron deposition in MS and AD brains indicates potential sites where iron could promote oxidative damage in these disease states.

Desferrioxamine in chronic progressive multiple sclerosis: a pilot study

Chronic progressive Multiple Sclerosis is refractory to many conventional treatments. We performed a pilot study testing desferroxamine (DFO) as a candidate in the treatment of chronic progressive Multiple Sclerosis. DFO was given daily by 8 h subcutaneous infusions at a dose of 2 grams daily for 7 days, followed by 1 gram daily for 7 days. Eighteen of 19 individuals completed the full dose of 21 grams. One patient was unable to complete the course due to nausea. No acute deterioration of neurological status was seen during the administration of DFO. No worsening of vision or hearing was noted except that the one patient who was unable to tolerate the medication had a transient reduction in hearing. All patients had a local redness at the injection site. None of the patients had any sudden worsening during or shortly after the treatment. This pilot study suggests that DFO is relatively well tolerated by Multiple Sclerosis patients when given in a short course of therapy.

Complexity analysis of oligodendroglial processes expressing myelin-associated glycoprotein

Oligodendroglia synthesize myelin in the mammalian central nervous system. Mature oligodendroglia have been identified in culture by two criteria; the expression of molecules characteristic of myelin, such as galactocerebroside (galC) and myelin-associated glycoprotein (MAG), and the elaboration of complex processes. Myelin gene expression can be documented by the binding of specific antibodies and antisera to the myelin-specific molecules; process complexity can be described by the fractal dimension, D. In this study, anti-MAG antisera was used to document MAG expression in the processes of oligodendroglia. Eighty percent of the galC+ oligodendroglia bound anti-MAG antiserum. With time in culture, MAG immunoreactivity seemed to extend from the cell soma into the oligodendroglial processes. To quantify this observation, fractal dimensions were calculated using either galC or MAG immunoreactivity to visualize oligodendroglial processes. A fractal dimension of 1.5 was calculated for O1+ processes by day 4 of culture; this value for D remained constant over the course of 1 month in culture. The fractal dimension calculated for MAG+ processes increased from 1.2 to 1.5 over the course of 28 days in culture. This change in fractal dimension confirms our visual impression that galC-containing processes acquire MAG slowly over the course of several weeks in culture.

Relationship of iron to oligodendrocytes and myelination

Oligodendrocytes are the predominant iron-containing cells in the brain. Iron-containing oligodendrocytes are found near neuronal cell bodies, along blood vessels, and are particularly abundant within white matter tracts. Iron-positive cells in white matter are present from birth and eventually reside in defined patches of cells in the adult. These patches of iron-containing cells typically have a blood vessel in their center. Ferritin, the iron storage protein, is also expressed early in development in oligodendrocytes in a regional and cellular pattern similar to that seen for iron. Recently, the functionally distinct subunits of ferritin have been analyzed; only heavy (H)-chain ferritin is found in oligodendrocytes early in development. H-ferritin is associated with high iron utilization and low iron storage. Consistent with the expression of H-ferritin is the expression of transferrin receptors (for iron acquisition) on immature oligodendrocytes. Transferrin protein accumulation and mRNA expression in the brain are both dependent on a viable population of oligodendrocytes and may have an autocrine function to assist oligodendrocytes in iron acquisition. Although apparently the majority of oligodendrocytes in white matter tracts contain ferritin, transferrin, and iron, not all of them do, indicating that there is a subset of oligodendrocytes in white matter tracts. The only known function of oligodendrocytes is myelin production, and both a direct and indirect relationship exists between iron acquisition and myelin production. Iron is directly involved in myelin production as a required co-factor for cholesterol and lipid biosynthesis and indirectly because of its requirement for oxidative metabolism (which occurs in oligodendrocytes at a higher rate than other brain cells). Factors (such as cytokines) and conditions such as iron deficiency may reduce iron acquisition by oligodendrocytes and the susceptibility of oligodendrocytes to oxidative injury may be a result of their iron-rich cytoplasm. Thus, the many known phenomena that decrease oligodendrocyte survival and/or myelin production may mediate their effect through a final common pathway that involves disruptions in iron availability or intracellular management of iron.

The distribution of iron in the brain: a phylogenetic analysis using iron histochemistry

Histochemical procedures can be used to detect the cellular distribution of iron in the brain. The objective of the present study was to determine if the cellular distribution of iron enrichment is conserved between animals on different branches of the phylogenetic tree. This information can facilitate our understanding of the role of iron enrichment in cells of the brain. The animals studied were the mouse, rat, chicken, frog, fish and fly. In order to optimize the detection of iron, two histochemical staining methods and three fixatives per staining method were examined for each species. The results indicated that there was no single cell type that displayed iron enrichment in each of the species examined. In three out of five species in the phylum chordata, iron was enriched in oligodendrocytes; the exceptions to this were the fish and frog, which had iron enrichment in neurons but not oligodendrocytes. Iron was enriched in ependymal cells and endothelial cells in four out of the five species in the phylum chordata with the fish and the mouse being the respective exceptions. Myelin was stained in the mouse and rat, and microglia were occasionally observed in the rat and chicken. Astrocyte staining was not observed in any of the species examined. In the fly third instar larvae, iron enrichment was found in border glia and in neuropil. The absence of a conserved staining pattern between species suggests that iron enrichment probably does not play a role in the main functions that have been attributed to those cells that were stained. These findings, taken together with previously published data on the distribution of ferritin and transferrin, suggests that iron-enriched cells serve as stores of iron for the brain.

Cellular distribution of iron, transferrin, and ferritin in the hypotransferrinemic (Hp) mouse brain

Hypotransferrinemic (Hp) mice have a point mutation or small deletion in the transferrin (Tf) gene, resulting in defective splicing of precursor Tf mRNA. Hp animals produce < 1% of normal Tf levels and require supplemental serum or purified Tf for survival. Because of the lack of endogenous brain Tf, we examined regional and cellular distributions of iron and iron regulatory proteins (Tf and ferritin) in selected brain regions of Hp mice. The regional distribution of iron, Tf, and ferritin in Hp brain was similar to normal except for the pattern of iron staining in hippocampus. The cellular distribution of iron, ferritin, and Tf was similar between Hp and normal animals. The predominant cell type staining for Tf and iron was oligodendrocytes. Qualitative observations suggest that the number of cells staining for iron was similar between Hp and normal mice, whereas the number of Hp Tf-positive cells was reduced. Ferritin immunostaining was similar in both cases. However, ferritin-positive cells were predominantly astrocytes, an observation unique to mice among species studied previously. Western blot analysis revealed that Tf present in Hp brain was of exogenous origin (from supplemental injections). Presumably, Tf transports the iron found in Hp oligodendrocytes. These data demonstrate that, despite reduced endogenous Hp brain Tf, iron and plasma Tf migrate or are transported to the appropriate cells (oligodendrocytes), bringing into question the role of endogenous brain Tf in extracellular iron transport.

A histochemical study of iron-positive cells in the developing rat brain

The establishment of normal iron levels in the neonatal brain is critical for normal neurological development. Studies have shown that both iron uptake and iron concentration in the brain are relatively high during neonatal development. This histochemical study was undertaken to determine the pattern of iron development at the cellular level in the rat forebrain. Iron-stained cells were observed as early as postnatal day (PND) 3, which was the earliest time point examined. At PND 3, there were four major foci of iron-containing cells: the subventricular zone and three areas within the subcortical white matter. These latter foci are associated with myelinogenic regions. The blood vessels were prominently stained for iron throughout the brain. At PND 7, as in PND 3, the majority of the iron-containing cells were in white matter. However, there were also patches of iron staining located specifically in the layer IV of the somatosensory cortex. These cortical patches were no longer visible by PND 14. At PND 14, numerous iron-stained cells were dispersed throughout white matter regions and the tanycytes aligning the third ventricle were prominently stained. The blood vessel staining was less prominent than at earlier time periods. By PND 28, the adult pattern of iron staining was emerging. Iron-stained cells were aligned in rows in white matter and had an apparent preference for a location near blood vessels. This clustering of iron-positive cells around blood vessels gave the white matter a "patchy" appearance. The pattern of development, cell distribution, and morphological appearance of the iron-stained cells are consistent with that reported for oligodendrocytes. That iron-positive cells in the neonate may be oligodendrocytes is consistent with the reports for iron staining in adult brains. The recent reports that oligodendrocytes are highly susceptible to oxidative damage would be consistent with the high iron levels found in these cells. These results indicate that oligodendrocytes play a major role in the development of iron homeostasis in the brain. The role of iron in oligodendrocytes may be associated with metabolic demands of myelinogenesis, including cholesterol and fatty acid synthesis. However, these cells may be a morphologically similar but functionally distinct subset of oligodendrocytes whose function is to regulate the availability of iron in the brain.

Ferritin, transferrin, and iron in selected regions of the adult and aged rat brain

Iron is necessary for normal neural function but it must be stringently regulated to avoid iron-induced oxidative injury. The regulation of systemic iron is through the proteins transferrin (iron mobilization) and ferritin (iron sequestration). This study examines the cellular and regional distribution of iron and the iron-related proteins ferritin and transferrin in selected regions of the adult and aged rat brain. This information is a necessary prerequisite to understanding the mechanism by which iron homeostasis is maintained in the brain. The predominant cell type containing ferritin, transferrin, and iron throughout the brain at all ages is the oligodendrocyte. Neurons in most brain regions contain granular iron deposits which become more apparent with age. Ferritin and iron are also present in microglial cells in all brain regions, but are particularly abundant in the hippocampus. These latter cells visibly increase in number in all brain regions as the animal approaches senescence. Another area in which immunostaining is notable is surrounding the III ventricle, where transferrin is found in the choroid plexus and ependyma and ferritin and iron are present in tanycytes. The results of this study indicate an important role for neuroglia in the regulation of iron in the brain and also implies that a transport system may exist for the transfer of iron between the brain and cerebrospinal fluid. In the normal rodent brain, the principal cell of iron regulation is the oligodendrocyte; however, the role of microglial cells in the sequestration and detoxification of iron may be significant, particularly as the animal ages. With age there is an increase in stainable iron in neurons without a concomitant increase in neuronal ferritin immunostaining, suggesting a ferritin independent accumulation of neuronal iron with age.

Satellite oligodendrocytes and myelin are displaced in the cortex of the reeler mouse

The spatial distribution of satellite (perineuronal) oligodendrocytes and myelin was studied in the cortices of normal and reeler mice. In normal mice, satellite oligodendrocytes were concentrated in the inner third of the cortex. In reeler mice, satellite oligodendrocytes were present beneath the pial surface and distributed throughout the width of the cortex. In reeler mice, but not normal mice, myelin was present in patches beneath the pia and distributed throughout the width of the cortex. The abnormal position of satellite oligodendrocytes and myelin coincides with the displacement of neurons and axons in the cortex of reeler mice. These studies indicate that the distribution of cortical oligodendrocytes is influenced by neuronal/axonal position.

The structure and function of central nervous system myelin

Multiple sclerosis (MS) is characterized by the active degradation of central nervous system myelin, a multilamellar membrane system that insulates nerve axons. MS arises from complex interactions between genetic, immunological, infective, and biochemical mechanisms. Although the circumstances of MS etiology remain hypothetical, one persistent theme involves immune system recognition of myelin-specific antigens derived from myelin basic protein, the most abundant extrinsic myelin membrane protein, and/or another equally suitable myelin protein or lipid. Knowledge of the biochemical and physical-chemical properties of myelin proteins, and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to understanding how and why these antigens become selected during the development of MS. This article focuses on the current understanding of the molecular basis of MS as it may relate to the protein and lipid components of myelin, which dictate myelin morphology on the basis of protein-lipid and lipid-lipid interactions, and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.

The role of reactive oxygen species in the pathogenesis of multiple sclerosis

Although reactive oxygen species are thought to mediate cellular damage in many disease states the role of reactive oxygen species in the pathogenesis of multiple sclerosis is unknown. Data from biochemical, histochemical and pharmacological studies have been evaluated to determine if the necessary conditions exist for the formation of reactive oxygen species during a demyelination episode of multiple sclerosis. This evaluation found that not only do the necessary conditions exist for the formation of reactive oxygen species but that these species may play a significant pathogenic role in this disease. A hypothesis describing a detailed role of reactive oxygen species in the pathogenesis of multiple sclerosis is put forth.

Iron accumulation in the rat basal ganglia after excitatory amino acid injections--dissociation from neuronal loss

The current study examines in an animal model the relation of excessive iron accumulation in the basal ganglia to the pathology of Parkinsonism and Hallervoden-Spatz disease. Following a unilateral microinjection of excitatory amino acids, kainate, or quinolinate to the anterior olfactory nucleus/ventral striatal region, an increase in histochemical iron concentration was observed in the ipsilateral ventral pallidum, the islands of calleja, the globus pallidus, the entopeduncular nucleus, the ventral thalamus, and the substantia nigra pars reticulata. The iron was observed both in glia and as intensification of patches in the neuropil. In a second group of rats, after microinjection of ibotenate or quisqualate to the nucleus basalis of Meynert, iron accumulated in the ipsilateral entopeduncular nucleus and pars reticulata of substantia nigra. Increased iron accumulation, compared to that in the contralateral side, was stable for months after a single microinjection. In the basal ganglia distal from the site of EAA injection, no gross morphological changes were associated with the increased iron accumulation. The implications of these findings to the pathology of Parkinson's and Hallervorden-Spatz diseases are discussed.

Oligodendrocytes and myelin sheaths in normal, quaking and shiverer brains are enriched in iron

Early studies employing iron histochemical techniques found a spatially restricted distribution of oligodendrocytes and myelin enriched in iron. In particular, oligodendrocytes and myelin positively stained for iron were found in sparsely myelinated brain regions but not in densely myelinated tracts. Subsequent studies modified the iron histochemical technique and demonstrated that oligodendrocytes and myelin were stained in densely myelinated brain regions but the staining occurred in patches rather than uniformly throughout densely myelinated tracts. This study further modified the iron histochemistry technique to establish that oligodendrocytes and myelin are enriched in iron throughout densely myelinated tracts. This finding supports biochemical studies that detected high levels of iron in white matter (Hallgren and Sourander, J Neurochem 3:41-51, 1958) and myelin fractions of brain homogenates (Rajan et al., Life Sci 18:423-432, 1976). Thus, this histochemical study and earlier biochemical studies indicate that white matter is a major site of iron concentration within the brain. The present study also examined the distribution of iron in oligodendrocytes and myelin from the dysmyelinating mutant mice quaking and shiverer. Results from these studies demonstrate that oligodendrocytes and myelin are enriched in iron in both quaking and shiverer brains. An unexpected finding was an intense staining of oval structures within the oligodendrocyte cytoplasm. This result indicates a concentration of iron in these structures and may be important for understanding how high concentrations of iron are processed by oligodendrocytes.