A sensitive post-DAB enhancement technique for demonstration of iron in the central nervous system

Local Administrations of Iron Oxide Nanoparticles in the Prefrontal Cortex and Caudate Putamen of Rats Do Not Compromise Working Memory and Motor Activity

Iron oxide nanoparticles (IONPs) have come into focus for their use in medical applications although possible health risks for humans, especially in terms of brain functions, have not yet been fully clarified. The present study investigates the effects of IONPs on neurobehavioural functions in rats. For this purpose, we infused dimercaptosuccinic acid-coated IONPs into the medial prefrontal cortex (mPFC) and caudate putamen (CPu). Saline (VEH) and ferric ammonium citrate (FAC) were administered as controls. One- and 4-week post-surgery mPFC-infused animals were tested for their working memory performance in the delayed alternation T-maze task and in the open field (OF) for motor activity, and CPu-infused rats were tested for their motor activity in the OF. After completion of the experiments, the brains were examined histologically and immunohistochemically. We did not observe any behavioural or structural abnormalities in the rats after administration of IONPs in the mPFC and the CPu. In contrast, administration of FAC into the CPu resulted in decreased motor activity and increased the number of microglia in the mPFC. Perls' Prussian blue staining revealed that FAC- and IONP-treated rats had more iron-containing ramified cells than VEH-treated rats, indicating iron uptake by microglia. Our results demonstrate that local infusions of IONPs into selected brain regions have no adverse impact on locomotor behaviour and working memory.

Uncovering the dynamics of cellular responses induced by iron-carbohydrate complexes in human macrophages using quantitative proteomics and phosphoproteomics

Iron-carbohydrate complexes are widely used to treat iron deficiencies. Macrophages play a crucial role in the uptake and fate of these nanomedicines, however, how complexed iron carbohydrates are taken up and metabolized by macrophages is still not fully understood. Using a (phospho-)proteomics approach, we assessed differences in protein expression and phosphorylation in M2 macrophages triggered by iron sucrose (IS). Our results show that IS alters the expression of multiple receptors, indicative of a complex entry mechanism. Besides, IS induced an increase in intracellular ferritin, the loss of M2 polarization, protective mechanisms against ferroptosis, and an autophagic response. These data indicate that macrophages can use IS as a source of iron for its storage and later release, however, the excess of iron can cause oxidative stress, which can be successfully regulated by the cells. When comparing IS with ferric carboxymaltose (FCM) and iron isomaltoside-1000 (IIM), complexes with a higher carbohydrate ligand stability, we observed that FCM and IIM are metabolized at a slower rate, and trigger M2 polarization loss to a lower extent. These results indicate that the surface characteristics of the iron-carbohydrate complexes may influence the cell responses. Our data show that the application of (phospho-)proteomics can lead to a better understanding of metabolic processes, including the uptake, biodegradation and bioavailability of nanomedicines.

Prussian Blue Staining to Visualize Iron Oxide Nanoparticles

Iron deposits in cells and tissues can be detected by ex vivo histological examination through the Prussian blue (PB) staining. This practical, inexpensive, and highly sensitive technique involves the treatment of fixed tissue sections and cells with acid solutions of ferrocyanides that combine with ferric ion forming a bright blue pigment (i.e., ferric ferrocyanide). The staining can be applied to visualize iron oxide nanoparticles (IONPs), versatile magnetic nanosystems that are used in various biomedical applications and whose localization is usually required at a higher resolution than that enabled by in vivo tracking techniques.

Prussian blue technique is prone to yield false negative results in magnetoreception research

Perls’s Prussian blue staining technique has been used in magnetoreception research to screen tissues for iron-rich structures as proxies for putative magnetoreceptor structures based on magnetic particles. However, seemingly promising structural candidates in the upper beak of birds detected with Prussian blue turned out to be either irreproducible or located in non-neuronal cells, which has spurred a controversy that has not been settled yet. Here we identify possible pitfalls in the previous works and apply the Prussian blue technique to tissues implicated in magnetic-particle-based magnetoreception, in an effort to reassess its suitability for staining single-domain magnetite, i.e., the proposed magnetic substrate for the interaction with the external magnetic field. In the upper beak of night-migratory songbirds, we found staining products in great numbers, but not remotely associated with fiber terminals of the traced ophthalmic branch of the trigeminal nerve. Surprisingly, staining products were absent from the lamina propria in the olfactory rosette of rainbow trout where candidate magnetoreceptor structures were identified with different techniques earlier. Critically, magnetosome chains in whole cells of magnetotactic bacteria remained unstained. The failure to label single-domain magnetite in positive control samples is a serious limitation of the technique and suggests that two most influential but antipodal studies conducted previously stood little chances of obtaining correct positive results under the assumption that magnetosome-like particles were present in the tissues. Nonetheless, the staining technique appears suitable to identify tissue contamination with iron-rich fine dust trapped in epithelia already in vivo.

Ferrostatin-1 Ameliorates Liver Dysfunction via Reducing Iron in Thioacetamide-induced Acute Liver Injury in Mice

Background and Aims: Hepatic iron overload always leads to oxidative stress, which has been found to be involved in the progression of liver disease. However, whether iron disorder is involved in acute liver disease and the further molecular mechanisms remain unclear.

Methods: A mice model of acute liver injury (ALI) was established via intraperitoneal injection of thioacetamide (TAA) (250 mg/kg/day) for 3 consecutive days. Ferrostatin-1 (Fer-1) was administered intraperitoneally (2.5 μM/kg/day) starting 3 days before TAA treatment. Deferoxamine (DFO) was intraperitoneally injected (200 mg/kg/day) with TAA treatment for 3 days. We further observed the effect of Fer-1 on TAA model with high-iron diet feeding. ALI was confirmed using histological examination and liver function activity. Moreover, expressions of iron metabolism and ferroptosis proteins were measured by Western blot analysis.

Results: The study revealed that the iron accumulation and ferroptosis contributed to TAA-induced ALI pathogenesis. TAA induced prominent inflammation and vacuolar degeneration in the liver as well as liver dysfunction. In addition, protein expression of the cystine/glutamate antiporter SLC7A11 (xCT) and glutathione peroxidase 4 (GPX4) was significantly decreased in the liver, while transferrin receptor 1 (TfR1), ferroportin (Fpn) and light chain of ferritin (Ft-L) expression levels were increased after TAA exposure. As the same efficiency as DFO, pre-administration of Fer-1 significantly decreased TAA-induced alterations in the plasma ALT, AST and LDH levels compared with the TAA group. Moreover, both Fer-1 and DFO suppressed TfR1, Fpn and Ft-L protein expression and decreased iron accumulation, but did not affect xCT or GPX4 expression in the liver. Both Fer-1and DFO prevented hepatic ferroptosis by reducing the iron content in the liver. Furthermore, Fer-1 also reduced iron and reversed liver dysfunction under iron overload conditions.

Conclusion: These findings indicate a role of TAA-induced iron accumulation and ferroptosis in the pathogenesis of ALI model. The effect of Fer-1 was consistent with that of DFO, which prevented hepatic ferroptosis by reducing the iron content in the liver. Thus, Fer-1 might be a useful reagent to reverse liver dysfunction and decreasing the iron content of the liver may be a potential therapeutic strategy for ALI.

Iron Deposition in the Brain After Aneurysmal Subarachnoid Hemorrhage

BACKGROUND

After aneurysmal subarachnoid hemorrhage (SAH), thrombus forms over the cerebral cortex and releases hemoglobin. When extracellular, hemoglobin is toxic to neurones. High local hemoglobin concentration overwhelms the clearance capacity of macrophages expressing the hemoglobin-haptoglobin scavenger receptor CD163. We hypothesized that iron is deposited in the cortex after SAH and would associate with outcome.

METHODS

Two complementary cross-sectional studies were conducted. Postmortem brain tissue from 39 SAH (mean postictal interval of 9 days) and 22 control cases was studied with Perls' staining for iron and immunolabeling for CD163, ADAM17 (a disintegrin and metallopeptidase domain 17), CD68, and Iba1 (ionized calcium binding adaptor molecule 1). In parallel, to study the persistence of cortical iron and its relationship to clinical outcome, we conducted a susceptibility-weighted imaging study of 21 SAH patients 6 months postictus and 10 control individuals.

RESULTS

In brain tissue from patients dying soon after SAH, the distribution of iron deposition followed a gradient that diminished with distance from the brain surface. Iron was located intracellularly (mainly in macrophages, and occasionally in microglia, neurones, and glial cells) and extracellularly. Microglial activation and motility markers were increased after SAH, with a similar inward diminishing gradient. In controls, there was a positive correlation between CD163 and iron, which was lost after SAH. In SAH survivors, iron-sensitive imaging 6 months post-SAH confirmed persistence of cortical iron, related to the size and location of the blood clot immediately after SAH, and associated with cognitive outcome.

CONCLUSIONS

After SAH, iron deposits in the cortical gray matter in a pattern that reflects proximity to the brain surface and thrombus and is related to cognitive outcome. These observations support therapeutic manoeuvres which prevent the permeation of hemoglobin into the cortex after SAH.

Effects of Local Administration of Iron Oxide Nanoparticles in the Prefrontal Cortex, Striatum, and Hippocampus of Rats

Iron oxide nanoparticles (IONPs) are used for diverse medical approaches, although the potential health risks, for example adverse effects on brain functions, are not fully clarified. Several in vitro studies demonstrated that the different types of brain cells are able to accumulate IONPs and reported a toxic potential for IONPs, at least for microglia. However, little information is available for the in vivo effects of direct application of IONPs into the brain over time. Therefore, we examined the cellular responses and the distribution of iron in the rat brain at different time points after local infusion of IONPs into selected brain areas. Dispersed IONPs or an equivalent amount of low molecular weight iron complex ferric ammonium citrate or vehicle were infused into the medial prefrontal cortex (mPFC), the caudate putamen (CPu), or the dorsal hippocampus (dHip). Rats were sacrificed 1 day, 1 week, or 4 weeks post-infusion and brain sections were histologically examined for treatment effects on astrocytes, microglia, and neurons. Glial scar formation was observed in the mPFC and CPu 1 week post-infusion independent of the substance and probably resulted from the infusion procedure. Compared to vehicle, IONPs did not cause any obvious additional adverse effects and no additional tissue damage, while the infusion of ferric ammonium citrate enhanced neurodegeneration in the mPFC. Results of iron staining indicate that IONPs were mainly accumulated in microglia. Our results demonstrate that local infusions of IONPs in selected brain areas do not cause any additional adverse effects or neurodegeneration compared to vehicle.

Iron accumulation in the choroid plexus, ependymal cells and CNS parenchyma in a rat strain with low-grade haemolysis of fragile macrocytic red blood cells

Iron accumulation in the CNS is associated with many neurological diseases via amplification of inflammation and neurodegeneration. However, experimental studies on iron overload are challenging, since rodents hardly accumulate brain iron in contrast to humans. Here, we studied LEWzizi rats, which present with elevated CNS iron loads, aiming to characterise choroid plexus, ependymal, CSF and CNS parenchymal iron loads in conjunction with altered blood iron parameters and, thus, signifying non‐classical entry sites for iron into the CNS. Non‐haem iron in formalin‐fixed paraffin‐embedded tissue was detected via DAB‐enhanced Turnbull Blue stainings. CSF iron levels were determined via atomic absorption spectroscopy. Ferroportin and aquaporin‐1 expression was visualised using immunohistochemistry. The analysis of red blood cell indices and serum/plasma parameters was based on automated measurements; the fragility of red blood cells was manually determined by the osmotic challenge. Compared with wild‐type animals, LEWzizi rats showed strongly increased iron accumulation in choroid plexus epithelial cells as well as in ependymal cells of the ventricle lining. Concurrently, red blood cell macrocytosis, low‐grade haemolysis and significant haemoglobin liberation from red blood cells were apparent in the peripheral blood of LEWzizi rats. Interestingly, elevated iron accumulation was also evident in kidney proximal tubules, which share similarities with the blood–CSF barrier. Our data underscore the importance of iron gateways into the CNS other than the classical route across microvessels in the CNS parenchyma. Our findings of pronounced choroid plexus iron overload in conjunction with peripheral iron overload and increased RBC fragility in LEWzizi rats may be seminal for future studies of human diseases, in which similar constellations are found.

Metal bashing: iron deficiency and manganese overexposure impact on peripheral nerves

Iron (Fe) deficiency (FeD) and manganese (Mn) overexposure (MnOE) may result in several neurological alterations in the nervous system. Iron deficiency produces unique neurological deficits due to its elemental role in central nervous system (CNS) development and myelination, which might persist after normalization of Fe in the diet. Conversely, MnOE is associated with diverse neurocognitive deficits. Despite these well-known neurotoxic effects on the CNS, the influence of FeD and MnOE on the peripheral nervous system (PNS) remains poorly understood. The aim of the present investigation was to examine the effects of developmental FeD and MnOE or their combination on the sciatic nerve of young and adult rats. The parameters measured included divalent metal transporter 1 (DMT1), transferrin receptor (TfR), myelin basic protein (MBP) and peripheral myelin protein 22 (PMP22) expression, as well as Fe levels in the nerve. Our results showed that FeD produced a significant reduction in MBP and PMP22 content at P29, which persisted at P60 after Fe-sufficient diet replenishment regardless of Mn exposure levels. At P60 MnOE significantly increased sciatic nerve Fe content and DMT1 expression. However, the combination of FeD and MnOE produced no marked motor skill impairment. Evidence indicates that FeD appears to hinder developmental peripheral myelination, while MnOE may directly alter Fe homeostasis. Further studies are required to elucidate the interplay between these pathological conditions.

Monitoring of the Cytoskeleton-Dependent Intracellular Trafficking of Fluorescent Iron Oxide Nanoparticles by Nanoparticle Pulse-Chase Experiments in C6 Glioma Cells

Iron oxide nanoparticles (IONPs) are used for various biomedical and therapeutic approaches. To investigate the uptake and the intracellular trafficking of IONPs in neural cells we have performed nanoparticle pulse-chase experiments to visualize the internalization and the fate of fluorescent IONPs in C6 glioma cells and astrocyte cultures. Already a short exposure to IONPs for 10 min at 4 °C (nanoparticle pulse) allowed binding of substantial amounts of nanoparticles to the cells, while internalization of IONPs into the cell was prevented. The uptake of bound IONPs and the intracellular trafficking was started by increasing the temperature to 37 °C (chase period). While hardly any cellular fluorescence nor any iron staining was detectable directly after the nanoparticle pulse, dotted cellular fluorescence and iron patterns appeared already within a few minutes after start of the chase incubation and became intensified in the perinuclear region during further incubation for up to 90 min. Longer chase incubations resulted in separation of the fluorescent coat from the core of the internalized IONPs. Disruption of actin filaments in C6 cells strongly impaired the internalization of IONPs, whereas destabilization of microtubules traped IONP-containing vesicles to the plasma membrane. In conclusion, nanoparticle pulse-chase experiments allowed to synchronize the cellular uptake of fluorescent IONPs and to identify for C6 cells an actin-dependent early and a microtubule-dependent later process in the intracellular trafficking of fluorescent IONPs.

S-Nitrosylation of Divalent Metal Transporter 1 Enhances Iron Uptake to Mediate Loss of Dopaminergic Neurons and Motoric Deficit

Elevated iron deposition has been reported in Parkinson's disease (PD). However, the route of iron uptake leading to high deposition in the substantia nigra is unresolved. Here, we show a mechanism in enhanced Fe²⁺ uptake via S-nitrosylation of divalent metal transporter 1 (DMT1). While DMT1 could be S-nitrosylated by exogenous nitric oxide donors, in human PD brains, endogenously S-nitrosylated DMT1 was detected in postmortem substantia nigra. Patch-clamp electrophysiological recordings and iron uptake assays confirmed increased Mn²⁺ or Fe²⁺ uptake through S-nitrosylated DMT1. We identified two major S-nitrosylation sites, C23 and C540, by mass spectrometry, and DMT1 C23A or C540A substitutions abolished nitric oxide (NO)-mediated DMT1 current increase. To evaluate in vivo significance, lipopolysaccharide (LPS) was stereotaxically injected into the substantia nigra of female and male mice to induce inflammation and production of NO. The intranigral LPS injection resulted in corresponding increase in Fe²⁺ deposition, JNK activation, dopaminergic neuronal loss and deficit in motoric activity, and these were rescued by the NO synthase inhibitor l-NAME or by the DMT1-selective blocker ebselen. Lentiviral knockdown of DMT1 abolished LPS-induced dopaminergic neuron loss.SIGNIFICANCE STATEMENT Neuroinflammation and high cytoplasmic Fe²⁺ levels have been implicated in the initiation and progression of neurodegenerative diseases. Here, we report the unexpected enhancement of the functional activity of transmembrane divalent metal transporter 1 (DMT1) by S-nitrosylation. We demonstrated that S-nitrosylation increased DMT1-mediated Fe²⁺ uptake, and two cysteines were identified by mass spectrometry to be the sites for S-nitrosylation and for enhanced iron uptake. One conceptual advance is that while DMT1 activity could be increased by external acidification because the gating of the DMT1 transporter is proton motive, we discovered that DMT1 activity could also be enhanced by S-nitrosylation. Significantly, lipopolysaccharide-induced nitric oxide (NO)-mediated neuronal death in the substantia nigra could be ameliorated by using l-NAME, a NO synthase inhibitor, or by ebselen, a DMT1-selective blocker.

Developmental SMAD6 loss leads to blood vessel hemorrhage and disrupted endothelial cell junctions

The BMP pathway regulates developmental processes including angiogenesis, yet its signaling outputs are complex and context-dependent. Recently, we showed that SMAD6, an intracellular BMP inhibitor expressed in endothelial cells, decreases vessel sprouting and branching both in vitro and in zebrafish. Genetic deletion of SMAD6 in mice results in poorly characterized cardiovascular defects and lethality. Here, we analyzed the effects of SMAD6 loss on vascular function during murine development. SMAD6 was expressed in a subset of blood vessels throughout development, primarily in arteries, while expression outside of the vasculature was largely confined to developing cardiac valves with no obvious embryonic phenotype. Mice deficient in SMAD6 died during late gestation and early stages of postnatal development, and this lethality was associated with vessel hemorrhage. Mice that survived past birth had increased branching and sprouting of developing postnatal retinal vessels and disorganized tight and adherens junctions. In vitro, knockdown of SMAD6 led to abnormal endothelial cell adherens junctions and increased VE-cadherin endocytosis, indicative of activated endothelium. Thus, SMAD6 is essential for proper blood vessel function during murine development, where it appears to stabilize endothelial junctions to prevent hemorrhage and aberrant angiogenesis.

Iron quantification with susceptibility

Iron is an essential trace element involved in a variety of biological mechanisms in the human body. Disturbances of iron homeostasis have been observed in several inflammatory and degenerative diseases, which have raised strong interest in non-invasive iron mapping techniques. Numerous MRI techniques have been proposed so far, mostly based on the field changes induced by the magnetic properties of iron. Each of these approaches has a specific sensitivity for iron and its microstructural environment. Quantitative susceptibility mapping is the latest development and provides a direct measure of bulk susceptibility. However, field changes induced by iron are not always directly related to the concentration of iron, but rather reflect the structure of iron compounds and its cellular distribution. This review provides an overview of the most relevant iron compounds in the human body, their magnetic properties and their cellular distribution. In addition, MRI methods based on direct or indirect susceptibility changes are presented and discussed with respect to technical aspects and clinical applicability. Copyright © 2016 John Wiley & Sons, Ltd.

Loss of NCB5OR in the cerebellum disturbs iron pathways, potentiates behavioral abnormalities, and exacerbates harmaline-induced tremor in mice

Iron dyshomeostasis has been implicated in many diseases, including a number of neurological conditions. Cytosolic NADH cytochrome b5 oxidoreductase (NCB5OR) is ubiquitously expressed in animal tissues and is capable of reducing ferric iron in vitro. We previously reported that global gene ablation of NCB5OR resulted in early-onset diabetes and altered iron homeostasis in mice. To further investigate the specific effects of NCB5OR deficiency on neural tissue without contributions from known phenotypes, we generated a conditional knockout (CKO) mouse that lacks NCB5OR only in the cerebellum and midbrain. Assessment of molecular markers in the cerebellum of CKO mice revealed changes in pathways associated with cellular and mitochondrial iron homeostasis. (59)Fe pulse-feeding experiments revealed cerebellum-specific increased or decreased uptake of iron by 7 and 16 weeks of age, respectively. Additionally, we characterized behavioral changes associated with loss of NCB5OR in the cerebellum and midbrain in the context of dietary iron deprivation-evoked generalized iron deficiency. Locomotor activity was reduced and complex motor task execution was altered in CKO mice treated with an iron deficient diet. A sucrose preference test revealed that the reward response was intact in CKO mice, but that iron deficient diet consumption altered sucrose preference in all mice. Detailed gait analysis revealed locomotor changes in CKO mice associated with dysfunctional proprioception and locomotor activation independent of dietary iron deficiency. Finally, we demonstrate that loss of NCB5OR in the cerebellum and midbrain exacerbated harmaline-induced tremor activity. Our findings suggest an essential role for NCB5OR in maintaining both iron homeostasis and the proper functioning of various locomotor pathways in the mouse cerebellum and midbrain.

MBD5 regulates iron metabolism via methylation-independent genomic targeting of Fth1 through KAT2A in mice

Ferritin plays important roles in iron metabolism and controls iron absorption in the intestine. The ferritin subunits ferritin heavy chain (Fth1) and ferritin light chain (Ftl1) are tightly regulated at both the transcriptional and post-transcriptional levels. However, mechanisms of maintaining stable, basal expression of Fth1 are poorly understood. Here, we show that global deletion of Mbd5 in mice induces an iron overload phenotype. Liver and serum iron levels in Mbd5(-/-) mice were 3·2-fold and 1·5-fold higher respectively, than wild-type littermates; moreover, serum ferritin was increased >5-fold in the Mbd5(-/-) mice. Mbd5 encodes a member of the methyl-CpG binding domain family; however, the precise function of this gene is poorly understood. Here, we found that intestinal Fth1 mRNA levels were decreased in Mbd5(-/-) mice. Loss of Fth1 expression in the intestine could lead to iron over-absorption. Furthermore, deleting Mbd5 specifically in the intestine resulted in a phenotype similar to that of conditional deletion of Fth1 mice. An Fth1 promoter-report luciferase assay indicated that overexpression of Mbd5 enhanced Fth1 transcription in a dose-dependent manner. Histone H4 acetylation of the Fth1 promoter was reduced in the intestine of Mbd5(-/-) mice and further analysis showed that histone acetyltransferase KAT2A was essential for MBD5-induced Fth1 transcription.

Biochemical and biophysical methods for studying mitochondrial iron metabolism

Iron is a heavily utilized element in organisms and numerous mechanisms accordingly regulate the trafficking, metabolism, and storage of iron. Despite the high regulation of iron homeostasis, several diseases and mutations can lead to the misregulation and often accumulation of iron in the cytosol or mitochondria of tissues. To understand the genesis of iron overload, it is necessary to employ various techniques to quantify iron in organisms and mitochondria. This chapter discusses techniques for determining the total iron content of tissue samples, ranging from colorimetric determination of iron concentrations, atomic absorption spectroscopy, inductively coupled plasma-optical emission spectroscopy, and inductively coupled plasma-mass spectrometry. In addition, we discuss in situ techniques for analyzing iron including electron microscopic nonheme iron histochemistry, electron energy loss spectroscopy, synchrotron X-ray fluorescence imaging, and confocal Raman microscopy. Finally, we discuss biophysical methods for studying iron in isolated mitochondria, including ultraviolet-visible, electron paramagnetic resonance, X-ray absorbance, and Mössbauer spectroscopies. This chapter should aid researchers to select and interpret mitochondrial iron quantifications.

The metabolism and toxicity of hemin in astrocytes

Hemin is cytotoxic, and contributes to the brain damage that accompanies hemorrhagic stroke. In order to better understand the basis of hemin toxicity in astrocytes, the present study quantified hemin metabolism and compared it to the pattern of cell death. Heme oxygenase-1 (HO-1) expression was first evident after 2 h incubation with hemin, with maximal expression being observed by 24 h. Despite the induction of HO-1, it was found that the proportion of hemin metabolized by astrocytes remained fairly constant throughout the 24 h period, with 70-80% of intracellular hemin remaining intact. A period of cell loss began after 2 h exposure to hemin, which gradually increased in severity to reach a maximum by 24 h. This cell loss could not be attenuated by the iron chelator, 1,10-phenanthroline, or by several antioxidant compounds (Trolox, N-acetyl-L-cysteine and N-tert-butyl-α-phenylnitrone), indicating that the mechanism of hemin toxicity does not involve iron. While these results make it unlikely that hemin toxicity is due to interactions with endogenous H(2)O(2), hemin toxicity was increased in the presence of supraphysiological levels of H(2)O(2) and this increase was ameliorated by PHEN, indicating that the iron released from hemin can be toxic under some pathological conditions. However, when H(2)O(2) is present at physiological levels, the toxicity of hemin appears to be caused by other mechanisms that may involve bilirubin and carbon monoxide in this model system.

Uptake, metabolism and toxicity of hemin in cultured neurons

Following hemorrhagic stroke, red blood cells lyse and release neurotoxic hemin into the interstitial space. The present study investigates whether neurons can accumulate and metabolize hemin. We demonstrate that cultured neurons express the heme carrier protein 1 (HCP1), and that this transporter appears to contribute to the time- and concentration-dependent accumulation of hemin by neurons. Although exposure of neurons to hemin stimulates the synthesis of the iron storage protein ferritin, approximately 80% of the hemin accumulated by neurons remains intact. Within 24h of incubation, substantial neurotoxicity was observed that was not attenuated by the cell permeable, selective ferrous iron chelator, 1,10-phenanthroline. These results demonstrate that while neurons efficiently accumulate hemin they slowly degrade it, and they support the conclusion that intact hemin is more neurotoxic than the iron released from the breakdown of hemin. Further investigations are required to determine the basis of this neurotoxicity.

Apotransferrin-induced recovery after hypoxic/ischaemic injury on myelination

We have previously demonstrated that aTf (apotransferrin) accelerates maturation of OLs (oligodendrocytes) in vitro as well as in vivo. The purpose of this study is to determine whether aTf plays a functional role in a model of H/I (hypoxia/ischaemia) in the neonatal brain. Twenty-four hours after H/I insult, neonatal rats were intracranially injected with aTf and the effects of this treatment were evaluated in the CC (corpus callosum) as well as the SVZ (subventricular zone) at different time points. Similar to previous studies, the H/I event produced severe demyelination in the CC. Demyelination was accompanied by microglial activation, astrogliosis and iron deposition. Ferritin levels increased together with lipid peroxidation and apoptotic cell death. Histological examination after the H/I event in brain tissue of aTf-treated animals (H/I aTF) revealed a great number of mature OLs repopulating the CC compared with saline-treated animals (H/I S). ApoTf treatment induced a gradual increase in MBP (myelin basic protein) and myelin lipid staining in the CC reaching normal levels after 15 days. Furthermore, significant increase in the number of OPCs (oligodendroglial progenitor cells) was found in the SVZ of aTf-treated brains compared with H/I S. Specifically, there was a rise in cells positive for OPC markers, i.e. PDGFRα and SHH⁺ cells, with a decrease in cleaved-caspase-3⁺ cells compared with H/I S. Additionally, neurospheres from aTf-treated rats were bigger in size and produced more O4/MBP⁺ cells. Our findings indicate a role for aTf as a potential inducer of OLs in neonatal rat brain in acute demyelination caused by H/I and a contribution to the differentiation/maturation of OLs and survival/migration of SVZ progenitors after demyelination in vivo.

Oligodendrogenesis in iron-deficient rats: effect of apotransferrin

In rats, iron deficiency produces an alteration in myelin formation. However, there is limited information on the effects of this condition on oligodendroglial cell (OLGc) proliferation and maturation. In the present study, we further analyzed the hypomyelination associated with iron deficiency by studying the dynamics of oligodendrogenesis. Rats were fed control (40 mg Fe/kg) or iron-deficient (4 mg Fe/kg) diets from gestation day 5 until postnatal day 3 (P3) or 11 (P11). OLGc proliferation, migration and differentiation were investigated before and after an intracranial injection of apotransferrin at 3 days of age (P3). The proliferating cell population was evaluated at P3. Iron-deficient (ID) animals showed an increase in the oligodendrocyte precursors cell (OPC) population in comparison with controls. The overall pattern of migration of cells labeled with BrdU was investigated at P11. Iron deficiency increased the amount of BrdU(+) cells in the corpus callosum (CC) and decreased OLGc maturation and myelin formation. Changes in nerve conduction were analyzed by measuring visual evoked potentials. Latency and amplitude were significantly disturbed in ID rats compared with controls. Both parameters were substantially normalized when animals were treated with a single intracranial injection of 350 ng apotransferrin (aTf). The current results give support to the idea that iron deficiency increases the number of proliferating and undifferentiated cells in the CC compared with the control. Treatment with aTf almost completely reverted the effects of iron deficiency, both changing the migration pattern and increasing the number of mature cells in the CC and myelin formation.

The putative heme transporter HCP1 is expressed in cultured astrocytes and contributes to the uptake of hemin

Hemin, which is toxic to brain cells, has been reported to be taken up by cultured astrocytes; however, the mechanism of uptake is currently unknown. The present study investigated the mechanism of hemin uptake by rat primary astrocyte cultures. In medium containing 10% fetal calf serum, cultured astrocytes failed to accumulate significant amounts of heme-iron, while in serum-free medium the accumulation of heme-iron was found to be time- and concentration-dependent. After 6 h of incubation with 24 muM hemin, cells contained 36.2 +/- 2.4 nmol heme-iron/mg protein, which was 21% of the applied hemin. These results suggest that the accumulation of hemin in astrocytes does not require serum proteins such as hemopexin. A potential mechanism of hemin uptake in astrocytes involves the heme carrier protein 1 (HCP1), which is reported to mediate hemin uptake into intestinal cells. RT-PCR analysis revealed that astrocyte cultures contained HCP1 mRNA, and immunocytochemical staining and Western blot analysis confirmed the expression of HCP1 protein in cultured astrocytes. The functionality of HCP1 in astrocytes was demonstrated by incubating cells with zinc protoporphyrin IX (ZnPPIX), which is known to be transported into cells via HCP1, and ZnPPIX autofluorescence was detected in HCP1-positive astrocytes. In addition, ZnPPIX was found to attenuate the accumulation of heme-iron by astrocytes. These results are the first to demonstrate that cultured astrocytes contain functional HCP1 and that this transporter contributes to hemin uptake by astrocytes. HCP1 may therefore provide a new target for reducing hemin-related toxicity in brain cells.

Iron homeostasis is maintained in the brain, but not the liver, following mild hypoxia

Alterations in iron metabolism or oxidative damage in response to hypoxic incidents have been examined following re-oxygenation of the hypoxic tissue. To understand the consequences of decreased tissue oxygen on iron load, metal-catalyzed redox activity and oxidative modifications in isolation from re-oxygenation, the present study exposed mice to either normoxia, or mild hypoxia (380 Torr; approximately 10% normobaric oxygen) where the tissue was not allowed to re-oxygenate prior to examination. Brain, liver and skeletal muscle were examined for Fe3+ load, metal-catalyzed redox activity and oxidative modifications to proteins (N(epsilon)-(carboxymethyl)lysine), lipids (4-hydroxynonenal pyrrole) and nucleic acids (8-hydroxyguanosine). Hypoxia induced a 43% increase in the iron content of the liver (P < 0.001) as determined by ICP-MS and a 3.8-fold increase in Fe3+ load (P < 0.001) as determined by Perl's stain. There was a corresponding 2-fold increase in metal-catalyzed redox activity (P < 0.01) in the liver, but no change in the expression of oxidative markers. In contrast, non-significant increases in Fe3+ and metal-catalyzed redox activity were observed in the cerebral cortex, and molecular and granular layers of the hippocampus and cerebellum. Interestingly, hypoxia significantly decreased oxidative modifications to proteins and lipids, but not nucleic acids in most brain regions examined. In addition, hypoxia did not alter the Fe content of skeletal muscle, or the contents of Zn, Cu, Ni or Mn in liver, skeletal muscle, cerebral cortex or hippocampus. Together, these results indicate that there is a tighter regulation of iron metabolism in the brain than the liver, which limits the redistribution of Fe3+ following hypoxia.

Stem cell implantation in ischemic mouse heart: a high-resolution magnetic resonance imaging investigation

Advances in the biology of stem cells have evoked great interest in cell replacement therapies for the regeneration of heart tissue after myocardial infarction. However, results from human trials are controversial, since the destination of the injected cells, their engraftment and their long-term fate have remained unclear. Here we investigate whether transplanted cells can be identified in the intact and lesioned murine myocardium employing high-resolution MRI. Cardiac progenitor cells, expressing the enhanced green fluorescent protein (EGFP), were labeled with ultra-small paramagnetic iron-oxide (USPIO) nanoparticles and transplanted into the intact or injured myocardium of mice. Their precise location was determined with high-resolution MRI and compared with histological tissue sections, stained with Prussian blue for iron content. These experiments showed that iron nanoparticle-loaded cells could be identified at high resolution in the mouse heart. However, ischemic myocardium (after cryoinjury or left coronary artery ligation) was characterized by a signal attenuation similar to that induced by USPIO-labeled cells in T2*-weighted MR images, making detection of labeled stem cells in this area by T2*-sensitive contrast rather difficult. In animals with myocardial injury only, the signal attenuated areas were of the same size in proton density- and T2*-weighted MR images. In injured animals also receiving labeled cells the lesioned area appeared larger in T2*--than in proton density-weighted MR images. This sequence-dependent lesion size change is due to the increased signal loss caused by the iron oxide nanoparticles, most sensitively detectable in the T2*-sensitive images. Thus, using the novel combination of these two parameter weightings, USPIO-labeled cells can be detected at high resolution in ischemic myocardium.

Immunohistochemical and Ultrastructural Findings Related to the Blood–Brain Barrier in the Blood Vessels of the Cerebral White Matter in Aged Dogs

Immunohistochemical and ultrastructural analysis of canine brain tissue was performed to determine whether cerebral capillaries, which form the blood--brain barrier (BBB), display age-related morphological changes in the white matter (WM). A slight decrease in laminin immunolabelling was detected in the basement membranes (BMs) of capillaries in the WM of old dogs, as compared with that in the brains of young dogs. The Prussian blue DAB post-DAB enhancement method detected iron present in macrophages and astrocytes in the WM. Copper/zinc superoxide dismutase, MT-I and -II and MT-III immunoreactivity was detected mainly in reactive astrocytes in the WM of aged dogs. Ultrastructurally, collagen-like fibrils were detected to a variable degree in the spaces between the BMs of capillary endothelial cells and astrocytes in the WM of some aged dogs. These results suggest that age-related morphological changes in capillaries of the WM are associated with BBB dysfunction, leading to the exudation of serum constituents, including harmful substances (e.g., iron), thereby causing tissue damage by oxidative injury. These factors may play a role in the pathogenesis of severe degenerative changes in the WM of aged dogs.

Histochemical detection of ultrasmall superparamagnetic iron oxide (USPIO) contrast medium uptake in experimental brain ischemia

Recently, macrophage infiltration in different central nervous system (CNS) pathologies has been visualized with ultrasmall particles of iron oxide (USPIO) as a new cell-specific contrast medium for MRI. However, validation of these findings at the histological level has been hampered by the fact that the in situ detection of iron uptake by conventional Prussian blue staining is not sensitive enough to detect low amounts of iron in the brain. Here, an improved method for the histochemical detection of USPIO uptake in ischemic brain lesions is reported. The procedure relies on the sequential enhancement of Prussian blue staining by diaminobenzidine and silver/gold impregnation. After photothrombotic cortical brain infarction, this method allowed sensitive in situ detection of iron-laden macrophages which matched both macrophage immunostaining and USPIO-induced signal alterations in high-resolution 7 T MRI. This staining method provides a basis for correlative histological assessment of USPIO-enhanced MRI in a broad spectrum of CNS pathologies.

Brain barrier systems: a new frontier in metal neurotoxicological research

The concept of brain barriers or a brain barrier system embraces the blood-brain interface, referred to as the blood-brain barrier, and the blood-cerebrospinal fluid (CSF) interface, referred to as the blood-CSF barrier. These brain barriers protect the CNS against chemical insults, by different complementary mechanisms. Toxic metal molecules can either bypass these mechanisms or be sequestered in and therefore potentially deleterious to brain barriers. Supportive evidence suggests that damage to blood-brain interfaces can lead to chemical-induced neurotoxicities. This review article examines the unique structure, specialization, and function of the brain barrier system, with particular emphasis on its toxicological implications. Typical examples of metal transport and toxicity at the barriers, such as lead (Pb), mercury (Hg), iron (Fe), and manganese (Mn), are discussed in detail with a special focus on the relevance to their toxic neurological consequences. Based on these discussions, the emerging research needs, such as construction of the new concept of blood-brain regional barriers, understanding of chemical effect on aged or immature barriers, and elucidation of the susceptibility of tight junctions to toxicants, are identified and addressed in this newly evolving field of neurotoxicology. They represent both clear challenges and fruitful research domains not only in neurotoxicology, but also in neurophysiology and pharmacology.

Perfusion-Perls and -Turnbull methods supplemented by DAB intensification for nonheme iron histochemistry: demonstration of the superior sensitivity of the methods in the liver, spleen, and stomach of the rat

Perfusion-Perls and -Turnbull methods supplemented by the intensification with 3,3'-diaminobenzidine (+ DAB) enabled stronger and more extensive staining of nonheme iron than the Perls + and Turnbull + DAB methods carried out on tissue sections fixed with 10% formalin in 0.9% saline or PBS. The section- and perfusion-Perls + DAB methods are not specific for the demonstration of nonheme ferric iron but also stain nonheme ferrous iron. However, owing to its high sensitivity, the perfusion-Perls + DAB method would provide useful information about nonheme iron deposition regardless of oxidation states in normal and pathological conditions. The perfusion-Turnbull + DAB method is specifically demonstrable of nonheme ferrous iron and the results from this method showed significant stores of nonheme ferrous iron in the hepatocytes, Kupffer cells, splenic macrophages, and gastric parietal cells of the rat. Since nonheme ferrous iron is considered to be critically involved in free radical generation, the perfusion-Turnbull + DAB method would visualize such populations of cells that are at risk from free radical damage.

Quantitative analysis of cell death and ferritin expression in response to cortical iron: implications for hypoxia-ischemia and stroke

Iron that is not bound to storage proteins can catalyse the generation of toxic hydroxyl radicals. Iron can be released from brain storage proteins by hypoxic conditions, such as those that accompany stroke, and the situation can be compounded by iron released from hemoglobin in extravasated blood cells. Despite the neurotoxicity of iron, there is little quantitative data concerning the spatio-temporal extent of its toxicity in vivo. The present study measures the effects of a pathologically relevant concentration of iron (1.0 mM) on neuronal death and on ferritin expression in vivo. Injection of iron (1 microl ferric ammonium citrate) into rat parietal cortex resulted in 7.9-fold more ferritin-labeled cells than did control injections of ammonium citrate at 1 day post-injection. This elevated expression continued for at least 1 week. One day after injection, the mean number of Fluoro-Jade-labeled degenerating neurons in 100 microm sections passing through the center of ferric ammonium citrate injection sites was 664+/-64. This value was 4.5-fold higher than at ammonium citrate injection sites, and this difference increased to 56-fold by day three. By 5 days post-injection, few dying neurons were observed at the control sites, but neurodegeneration continued beyond a week at the iron-injected sites. Thus, iron released during a brief episode of hypoxia-ischemia or during a stroke may be neurotoxic for a protracted period. Therefore, our findings indicate that it may be beneficial to target iron-induced peroxidation throughout the first few weeks following an intracerebral hemorrhage or an hypoxic-ischemic episode.

Toxicology of choroid plexus: special reference to metal-induced neurotoxicities

The chemical stability in the brain underlies normal human thinking, learning, and behavior. Compelling evidence demonstrates a definite capacity of the choroid plexus in sequestering toxic heavy metal and metalloid ions. As the integrity of blood-brain and blood-CSF barriers, both structurally and functionally, is essential to brain chemical stability, the role of the choroid plexus in metal-induced neurotoxicities has become an important, yet under-investigated research area in neurotoxicology. Metals acting on the choroid plexus can be categorized into three major groups. A general choroid plexus toxicant can directly damage the choroid plexus structure such as mercury and cadmium. A selective choroid plexus toxicant may impair specific plexus regulatory pathways that are critical to brain development and function, rather than induce massive pathological alteration. The typical examples in this category include lead-induced alteration in transthyretin production and secretion as well as manganese interaction with iron in the choroid plexus. Furthermore, a sequestered choroid plexus toxicant, such as iron, silver, or gold, may be sequestered by the choroid plexus as an essential CNS defense mechanism. Our current knowledge on the toxicological aspect of choroid plexus research is still incomplete. Thus, the future research needs have been suggested to focus on the role of choroid plexus in early CNS development as affected by metal sequestration in this tissue, to explore how metal accumulation alters the capacity of the choroid plexus in regulation of certain essential elements involved in the etiology of neurodegenerative diseases, and to better understand the blood-CSF barrier as a defense mechanism in overall CNS function.

Concentration of iron and distribution of iron and transferrin after experimental iron overload in rat tissues in vivo: study of the liver, the spleen, the central nervous system and other organs

The purpose of this study was to estimate the iron concentration in the liver, spleen and brain of control rats and rats overloaded with iron and to determine the distribution of iron and of transferrin (TF). Iron was administered to Wistar rats by food supplemented with 3% carbonyl iron for 3 months, or intraperitoneally, or intraveneously as iron polymaltose for 4 months (total administered dose: 300 or 350 mg/rat, respectively). Iron concentration was estimated by atomic absorption spectrophotometry and iron- and TF-distribution histochemically and immunohistochemically, respectively. In control rats the organ with the highest iron content was the spleen, followed by the liver and brain. After iron loading the increase of iron in the liver was greater than that of the spleen; iron concentration in the brain did not change significantly. Distribution of iron in the liver was in Kupffer cells throughout the lobule and in hepatocytes at its periphery. No difference in the number of positive cells or staining intensity for TF was observed between control rats and iron overloaded animals in the liver or central nervous system (CNS); the spleen was negative for TF. Distribution of TF in the liver showed a centrilobular localisation in hepatocytes. TF reaction in the brain occurred in oligodendrocytes, vessel walls, choroid plexus epithelial cells and some neurons. In conclusion, experimental iron overload in rats leads to iron uptake mainly by reticuloendothelial (RE) cells and hepatocytes, indicating that hepatocytes are of particular importance for iron metabolism. Iron uptake by the brain was not significant, probably because the brain is protected against iron overload. Iron overload did not influence location and quantity of TF in the liver and CNS, whereas the visualisation of iron and TF did not coincide. This indicates that TF may have other functions beyond iron transport.

Rapid protein-folding assay using green fluorescent protein

Formation of the chromophore of green fluorescent protein (GFP) depends on the correct folding of the protein. We constructed a "folding reporter" vector, in which a test protein is expressed as an N-terminal fusion with GFP. Using a test panel of 20 proteins, we demonstrated that the fluorescence of Escherichia coli cells expressing such GFP fusions is related to the productive folding of the upstream protein domains expressed alone. We used this fluorescent indicator of protein folding to evolve proteins that are normally prone to aggregation during expression in E. coli into closely related proteins that fold robustly and are fully soluble and functional. This approach to improving protein folding does not require functional assays for the protein of interest and provides a simple route to improving protein folding and expression by directed evolution.

Expression of ferritin protein and subunit mRNAs in normal and iron deficient rat brain

In non-neuronal tissue, ferritin subunit mRNAs are regulated by post-transcriptional mechanisms leading to decreased ferritin protein synthesis during iron deficiency. Biochemical studies have demonstrated that the cerebral ferritin concentration declines during iron deficiency, suggesting that expression of ferritin subunit mRNAs in the brain may be regulated by mechanisms similar to those of non-neuronal tissue. However, as ferritin expression has been only vaguely studied in brain, this hypothesis remains to be tested. We investigated the influence of dietary iron deficiency on the cellular distribution of ferritin protein using immunohistochemistry and H- and L-ferritin subunit mRNAs by non-radioactive in situ hybridization. Pregnant rats were subjected to an iron depleted diet (6.4 mg/kg) from the day of conception. Litters were kept on the same diet until euthanized at the postnatal age of 10 weeks. This treatment reduced brain iron levels from approximately 57 to 26 microgram/g. Reducing the iron stores reduced histochemical detectable iron and the expression of ferritin immunoreactivity in neurons, oligodendrocyte-like and microglia-like cells. In normal rats, H- and L-ferritin subunit mRNAs were expressed in virtually all neurons and non-neuronal cells. The cerebral expression of the ferritin subunit mRNAs was not affected by iron deficiency. The levels of ferritin subunit mRNAs in the brain were also unaltered from iron deficiency when examined by Northern blotting. In conclusion, brain levels of iron and ferritin protein are highly susceptible to dietary iron deficiency, whereas the cerebral expression of H- and L-ferritin subunit mRNAs remains unchanged.

Silver development in microscopy and bioanalysis: past and present

With the experience accumulated from more than a century of silver applications in biology and medicine, physical development has become a powerful bioanalytical tool for marker amplification in blotting procedures, in situ hybridization, immunocytochemistry, histochemistry, and cytochemistry. Early, empirical techniques of silver impregnation followed by development in a reducing solution (chemical developer), or a solution which contained both silver reducers and silver salts (physical developer) were often capricious and suffered from unwanted silver precipitation caused by light and self-nucleation. To accommodate the modern demand for accurate physical development, various strategies have been devised to counter these problems. One approach has been to introduce organic colloids into the developer to keep the silver ions and reducer molecules apart, whilst excluding light by using a dark-room or by covering the solution. Albumen, gelatin, and complex polysaccharides have all been tested, but gum arabic is preferred. In addition, further control can be achieved by slowing down the rate of development with low pH and by changing from silver nitrate to silver lactate, which dissociates more slowly. Effective colloid protection in a physical developer is also provided by the inclusion of tungsten salts which can delay light-catalysed silver reduction and keep the developer clear for many minutes. The same result has been achieved by complexing the silver salt in the physical developer with very large organic molecules, restricting ionization. 'Light insensitive' commercial designer products have resulted. Probably no single formulation can satisfy all conditions of use, but with increased understanding of the mechanisms of physical developers a more flexible, user-friendly approach is anticipated.

Developmental profile of non-heme iron distribution in the rat brain during ontogenesis

The entry of iron from blood into the developing rat brain was studied by means of non-heme iron-histochemistry. The content of non-heme iron in the endothelial cells was manifest already from E14, declined from P3 to P5, and was almost absent on P10-P15. The choroid plexus epithelial cells of either ventricle was non-heme iron-containing from E14. Non-heme iron-containing macrophages situated in the stroma of the choroid plexus were also observed from E14. From E19, the macrophage-like cells tended to invade into (a) regions with transitory structures like the intermediate zone of the cerebral hemisphere, (b) developing axonal tracts like corpus callosum and internal capsule, and (c) deep layers of the tectum, a region with an extensive degree of naturally occurring cell death. The amoeboid macrophage-like cells observed in the brain parenchyma gradually acquired prolonged extensions and apparently differentiated into ramified microglia-like cells, which later lost their non-heme iron-content. Thus, at P70, non-heme iron-positive microglia-like cells were hardly seen reflecting the transitory event of non-heme iron in microglia-like cells. At P200, non-heme iron-containing microglia cells and oligodendrocytes appeared in manifestly higher number than at P70, a phenomenon probably related to aging. These results delineate for the first time the appearance of iron in the developing brain. The results are of relevance for understanding the potential of iron-deficiency for harming the developing central nervous system, generally by decreased transport of iron through brain capillaries and choroid plexus, and specifically by an impaired modulation of the developing brain parenchyma by iron-containing macrophages.

Increased accumulation of transferrin by motor neurons of the mouse mutant progressive motor neuronopathy (pmn/pmn)

It has been suggested that iron-carrying transferrin exerts growth-factor-like influences on motor neurons. I have evaluated the distribution of proteins related to the intracerebral iron-homeostasis in the mouse mutant progressive motor neuronopathy (pmn/pmn); an autosomal recessive mutant with progressive caudo-cranial motor neuron degeneration. A higher immunoreactivity of transferrin and transferrin receptor in motor neurons of the pmn/pmn mutant compared to that in normal mice was demonstrated. Ferritin was not observed in motor neurons of the pmn/pmn mutant. Transferrin receptors were absent from axons and neuromuscular junctions, indicating that entry of blood-borne, liver-derived transferrin ('liver transferrin') into motor neurons due to uptake and subsequent retrograde axonal transport was unspecific. Due to the selective presence of transferrin receptors on neuronal somata, a more likely mode of entry of transferrin into the motor neurons was by receptor-mediated uptake of brain-derived transferrin ('brain transferrin') at the soma. This study provides data on transferrin accumulation and transferrin receptor expression in diseased motor neurons and adds further insights into influences of proteins related to iron-homeostasis in the diseased PNS.

The ependyma: a protective barrier between brain and cerebrospinal fluid

This review summarizes the current scientific literature concerning the ependymal lining of the cerebral ventricles of the brain with an emphasis on selective barrier function and protective roles for the common ependymal cell. Topics covered include the development, morphology, protein and enzyme expression including reactive changes, and pathology. Some cells lining the neural tube are committed at an early stage to becoming ependymal cells. They serve a secretory function and perhaps act as a cellular/axonal guidance system, particularly during fetal development. In the mature mammalian brain ependymal cells possess the structural and enzymatic characteristics necessary for scavenging and detoxifying a wide variety of substances in the CSF, thus forming a metabolic barrier at the brain-CSF interface.