Iron modulates the differentiation of a distinct population of glial precursor cells into oligodendrocytes

Iron and Ferroptosis More than a Suspect: Beyond the Most Common Mechanisms of Neurodegeneration for New Therapeutic Approaches to Cognitive Decline and Dementia

Neurodegeneration is a multifactorial process that involves multiple mechanisms. Examples of neurodegenerative diseases are Parkinson's disease, multiple sclerosis, Alzheimer's disease, prion diseases such as Creutzfeldt-Jakob's disease, and amyotrophic lateral sclerosis. These are progressive and irreversible pathologies, characterized by neuron vulnerability, loss of structure or function of neurons, and even neuron demise in the brain, leading to clinical, functional, and cognitive dysfunction and movement disorders. However, iron overload can cause neurodegeneration. Dysregulation of iron metabolism associated with cellular damage and oxidative stress is reported as a common event in several neurodegenerative diseases. Uncontrolled oxidation of membrane fatty acids triggers a programmed cell death involving iron, ROS, and ferroptosis, promoting cell death. In Alzheimer's disease, the iron content in the brain is significantly increased in vulnerable regions, resulting in a lack of antioxidant defenses and mitochondrial alterations. Iron interacts with glucose metabolism reciprocally. Overall, iron metabolism and accumulation and ferroptosis play a significant role, particularly in the context of diabetes-induced cognitive decline. Iron chelators improve cognitive performance, meaning that brain iron metabolism control reduces neuronal ferroptosis, promising a novel therapeutic approach to cognitive impairment.

Glial Cell Metabolic Profile Upon Iron Deficiency: Oligodendroglial and Astroglial Casualties of Bioenergetic Adjustments

Iron deficiency (ID) represents one of the most prevalent nutritional deficits, affecting almost two billion people worldwide. Gestational iron deprivation induces hypomyelination due to oligodendroglial maturation deficiencies and is thus a useful experimental model to analyze oligodendrocyte (OLG) requirements to progress to a mature myelinating state. A previous proteomic study in the adult ID brain by our group demonstrated a pattern of dysregulated proteins involved in the tricarboxylic acid cycle and mitochondrial dysfunction. The aim of the present report was to assess bioenergetics metabolism in primary cultures of OLGs and astrocytes (ASTs) from control and ID newborns, on the hypothesis that the regulation of cell metabolism correlates with cell maturation. Oxygen consumption and extracellular acidification rates were measured using a Seahorse extracellular flux analyzer. ID OLGs and ASTs both exhibited decreased spare respiratory capacity, which indicates that ID effectively induces mitochondrial dysfunction. A decrease in glycogen granules was observed in ID ASTs, and an increase in ROS production was detected in ID OLGs. Immunolabeling of structural proteins showed that mitochondrial number and size were increased in ID OLGs, while an increased number of smaller mitochondria was observed in ID ASTs. These results reflect an unfavorable bioenergetic scenario in which ID OLGs fail to progress to a myelinating state, and indicate that the regulation of cell metabolism may impact cell fate decisions and maturation.

Human Glial Progenitor Cells Effectively Remyelinate the Demyelinated Adult Brain

Neonatally transplanted human glial progenitor cells (hGPCs) can myelinate the brains of myelin-deficient shiverer mice, rescuing their phenotype and survival. Yet, it has been unclear whether implanted hGPCs are similarly able to remyelinate the diffusely demyelinated adult CNS. We, therefore, ask if hGPCs could remyelinate both congenitally hypomyelinated adult shiverers and normal adult mice after cuprizone demyelination. In adult shiverers, hGPCs broadly disperse and differentiate as myelinating oligodendrocytes after subcortical injection, improving both host callosal conduction and ambulation. Implanted hGPCs similarly remyelinate denuded axons after cuprizone demyelination, whether delivered before or after demyelination. RNA sequencing (RNA-seq) of hGPCs back from cuprizone-demyelinated brains reveals their transcriptional activation of oligodendrocyte differentiation programs, while distinguishing them from hGPCs not previously exposed to demyelination. These data indicate the ability of transplanted hGPCs to disperse throughout the adult CNS, to broadly myelinate regions of dysmyelination, and also to be recruited as myelinogenic oligodendrocytes later in life, upon demyelination-associated demand.

Glial restricted precursor cells in central nervous system disorders: Current applications and future perspectives

The crosstalk between glial cells and neurons represents an exceptional feature for maintaining the normal function of the central nervous system (CNS). Increasing evidence has revealed the importance of glial progenitor cells in adult neurogenesis, reestablishment of cellular pools, neuroregeneration, and axonal (re)myelination. Several types of glial progenitors have been described, as well as their potentialities for recovering the CNS from certain traumas or pathologies. Among these precursors, glial-restricted precursor cells (GRPs) are considered the earliest glial progenitors and exhibit tripotency for both Type I/II astrocytes and oligodendrocytes. GRPs have been derived from embryos and embryonic stem cells in animal models and have maintained their capacity for self-renewal. Despite the relatively limited knowledge regarding the isolation, characterization, and function of these progenitors, GRPs are promising candidates for transplantation therapy and reestablishment/repair of CNS functions in neurodegenerative and neuropsychiatric disorders, as well as in traumatic injuries. Herein, we review the definition, isolation, characterization and potentialities of GRPs as cell-based therapies in different neurological conditions. We briefly discuss the implications of using GRPs in CNS regenerative medicine and their possible application in a clinical setting. MAIN POINTS: GRPs are progenitors present in the CNS with differentiation potential restricted to the glial lineage. These cells have been employed in the treatment of a myriad of neurodegenerative and traumatic pathologies, accompanied by promising results, herein reviewed.

Age-Related Changes and Sex-Related Differences in Brain Iron Metabolism

Iron is an essential element that participates in numerous cellular processes. Any disruption of iron homeostasis leads to either iron deficiency or iron overload, which can be detrimental for humans' health, especially in elderly. Each of these changes contributes to the faster development of many neurological disorders or stimulates progression of already present diseases. Age-related cellular and molecular alterations in iron metabolism can also lead to iron dyshomeostasis and deposition. Iron deposits can contribute to the development of inflammation, abnormal protein aggregation, and degeneration in the central nervous system (CNS), leading to the progressive decline in cognitive processes, contributing to pathophysiology of stroke and dysfunctions of body metabolism. Besides, since iron plays an important role in both neuroprotection and neurodegeneration, dietary iron homeostasis should be considered with caution. Recently, there has been increased interest in sex-related differences in iron metabolism and iron homeostasis. These differences have not yet been fully elucidated. In this review we will discuss the latest discoveries in iron metabolism, age-related changes, along with the sex differences in iron content in serum and brain, within the healthy aging population and in neurological disorders such as multiple sclerosis, Parkinson's disease, Alzheimer's disease, and stroke.

Early evidence of delayed oligodendrocyte maturation in the mouse model of mucolipidosis type IV

Mucolipidosis type IV (MLIV) is a lysosomal disease caused by mutations in the MCOLN1 gene that encodes the endolysosomal transient receptor potential channel mucolipin-1, or TRPML1. MLIV results in developmental delay, motor and cognitive impairments, and vision loss. Brain abnormalities include thinning and malformation of the corpus callosum, white-matter abnormalities, accumulation of undegraded intracellular 'storage' material and cerebellar atrophy in older patients. Identification of the early events in the MLIV course is key to understanding the disease and deploying therapies. The Mcoln1-/- mouse model reproduces all major aspects of the human disease. We have previously reported hypomyelination in the MLIV mouse brain. Here, we investigated the onset of hypomyelination and compared oligodendrocyte maturation between the cortex/forebrain and cerebellum. We found significant delays in expression of mature oligodendrocyte markers Mag, Mbp and Mobp in the Mcoln1-/- cortex, manifesting as early as 10 days after birth and persisting later in life. Such delays were less pronounced in the cerebellum. Despite our previous finding of diminished accumulation of the ferritin-bound iron in the Mcoln1-/- brain, we report no significant changes in expression of the cytosolic iron reporters, suggesting that iron-handling deficits in MLIV occur in the lysosomes and do not involve broad iron deficiency. These data demonstrate very early deficits of oligodendrocyte maturation and critical regional differences in myelination between the forebrain and cerebellum in the mouse model of MLIV. Furthermore, they establish quantitative readouts of the MLIV impact on early brain development, useful to gauge efficacy in pre-clinical trials.

Potential role of iron in repair of inflammatory demyelinating lesions

Inflammatory destruction of iron-rich myelin is characteristic of multiple sclerosis (MS). Although iron is needed for oligodendrocytes to produce myelin during development, its deposition has also been linked to neurodegeneration and inflammation, including in MS. We report perivascular iron deposition in multiple sclerosis lesions that was mirrored in 72 lesions from 13 marmosets with experimental autoimmune encephalomyelitis. Iron accumulated mainly inside microglia/macrophages from 6 weeks after demyelination. Consistently, expression of transferrin receptor, the brain's main iron-influx protein, increased as lesions aged. Iron was uncorrelated with inflammation and postdated initial demyelination, suggesting that iron is not directly pathogenic. Iron homeostasis was at least partially restored in remyelinated, but not persistently demyelinated, lesions. Taken together, our results suggest that iron accumulation in the weeks after inflammatory demyelination may contribute to lesion repair rather than inflammatory demyelination per se.

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.

Iron as a model nutrient for understanding the nutritional origins of neuropsychiatric disease

Adequate nutrition during the pre- and early-postnatal periods plays a critical role in programming early neurodevelopment. Disruption of neurodevelopment by nutritional deficiencies can result not only in lasting functional deficits, but increased risk of neuropsychiatric disease in adulthood. Historical periods of famine such as the Dutch Hunger Winter and the Chinese Famine have provided foundational evidence for the long-term effects of developmental malnutrition on neuropsychiatric outcomes. Because neurodevelopment is a complex process that consists of many nutrient- and brain-region-specific critical periods, subsequent clinical and pre-clinical studies have aimed to elucidate the specific roles of individual macro- and micronutrient deficiencies in neurodevelopment and neuropsychiatric pathologies. This review will discuss developmental iron deficiency (ID), the most common micronutrient deficiency worldwide, as a paradigm for understanding the role of early-life nutrition in neurodevelopment and risk of neuropsychiatric disease. We will review the epidemiologic data linking ID to neuropsychiatric dysfunction, as well as the underlying structural, cellular, and molecular mechanisms that are thought to underlie these lasting effects. Understanding the mechanisms driving lasting dysfunction and disease risk is critical for development and implementation of nutritional policies aimed at preventing nutritional deficiencies and their long-term sequelae.

The Divalent Metal Transporter 1 (DMT1) Is Required for Iron Uptake and Normal Development of Oligodendrocyte Progenitor Cells

The divalent metal transporter 1 (DMT1) is a multimetal transporter with a primary role in iron transport. Although DMT1 has been described previously in the CNS, nothing was known about the role of this metal transporter in oligodendrocyte maturation and myelination. To determine whether DMT1 is required for oligodendrocyte progenitor cell (OPC) maturation, we used siRNAs and the Cre-lox system to knock down/knock out DMT1 expression in vitro as well as in vivo Blocking DMT1 synthesis in primary cultures of OPCs reduced oligodendrocyte iron uptake and significantly delayed OPC development. In vivo, a significant hypomyelination was found in DMT1 conditional knock-out mice in which DMT1 was postnatally deleted in NG2- or Sox10-positive OPCs. The brain of DMT1 knock-out animals presented a decrease in the expression levels of myelin proteins and a substantial reduction in the percentage of myelinated axons. This reduced postnatal myelination was accompanied by a decrease in the number of myelinating oligodendrocytes and a rise in proliferating OPCs. Furthermore, using the cuprizone model of demyelination, we established that DMT1 deletion in NG2-positive OPCs lead to less efficient remyelination of the adult brain. These results indicate that DMT1 is vital for OPC maturation and for the normal myelination of the mouse brain.SIGNIFICANCE STATEMENT To determine whether divalent metal transporter 1 (DMT1), a multimetal transporter with a primary role in iron transport, is essential for oligodendrocyte development, we created two conditional knock-out mice in which DMT1 was postnatally deleted in NG2- or Sox10-positive oligodendrocyte progenitor cells (OPCs). We have established that DMT1 is necessary for normal OPC maturation and is required for an efficient remyelination of the adult brain. Since iron accumulation by OPCs is indispensable for myelination, understanding the iron incorporation mechanism as well as the molecules involved is critical to design new therapeutic approaches to intervene in diseases in which the myelin sheath is damaged or lost.

Neuropathological Consequences of Gestational Exposure to Concentrated Ambient Fine and Ultrafine Particles in the Mouse

Increasing evidence indicates that the central nervous system (CNS) is a target of air pollution. We previously reported that postnatal exposure of mice to concentrated ambient ultrafine particles (UFP; ≤100 nm) via the University of Rochester HUCAPS system during a critical developmental window of CNS development, equivalent to human 3rd trimester, produced male-predominant neuropathological and behavioral characteristics common to multiple neurodevelopmental disorders, including autism spectrum disorder (ASD), in humans. The current study sought to determine whether vulnerability to fine (≤2.5 μm) and UFP air pollution exposure extends to embryonic periods of brain development in mice, equivalent to human 1st and 2nd trimesters. Pregnant mice were exposed 6 h/day from gestational days (GDs) 0.5-16.5 using the New York University VACES system to concentrated ambient fine/ultrafine particles at an average concentration of 92.69 μg/m3 over the course of the exposure period. At postnatal days (PNDs) 11-15, neuropathological consequences were characterized. Gestational air pollution exposures produced ventriculomegaly, increased corpus callosum (CC) area and reduced hippocampal area in both sexes. Both sexes demonstrated CC hypermyelination and increased microglial activation and reduced total CC microglia number. Analyses of iron deposition as a critical component of myelination revealed increased iron deposition in the CC of exposed female offspring, but not in males. These findings demonstrate that vulnerability of the brain to air pollution extends to gestation and produces features of several neurodevelopmental disorders in both sexes. Further, they highlight the importance of the commonalities of components of particulate matter exposures as a source of neurotoxicity and common CNS alterations.

Intraspinal TLR4 activation promotes iron storage but does not protect neurons or oligodendrocytes from progressive iron-mediated damage

Iron is essential for basic cellular functions but in excess is highly toxic. For this reason, free iron and iron storage are controlled in the periphery by elaborate regulatory mechanisms. In contrast, iron regulation in the central nervous system (CNS) is not well defined. Given that excess iron is present after trauma, hemorrhagic stroke and neurodegeneration, understanding normal iron regulation and promoting iron uptake in CNS pathology is crucial. Peripherally, toll-like receptor 4 (TLR4) activation promotes iron sequestration by macrophages. Notably, iron-rich sites of CNS pathology typically contain TLR4 agonists, which may promote iron uptake. Indeed, our recent work showed impaired iron storage after acute spinal cord injury in mice with TLR4 deficiency. Here we used a reductionist model to ask if TLR4 activation in the CNS stimulates iron uptake and promotes neuroprotection from iron-induced toxicity. For this, we measured the ability of microglia/macrophages to sequester exogenous iron and prevent pathology with and without concomitant intraspinal TLR4 activation. Results show that, similar to the periphery, activating intraspinal TLR4 via focal LPS injection increased mRNA encoding iron uptake and storage proteins and promoted iron sequestration into ferritin-expressing macrophages. However, this did not prevent oligodendrocyte and neuron loss. Moreover, replacement of oligodendrocytes by progenitor cells - a normally robust response to in vivo macrophage TLR4 activation - was significantly reduced if iron was present concomitant with TLR4 activation. Thus, while TLR4 signaling promotes CNS iron uptake, future work needs to determine ways to enhance iron removal without blocking the reparative effects of innate immune receptor signaling.

EGF Enhances Oligodendrogenesis from Glial Progenitor Cells

Emerging evidence indicates that epidermal growth factor (EGF) signaling plays a positive role in myelin development and repair, but little is known about its biological effects on the early generation and differentiation of oligodendrocyte (OL) lineage cells. In this study, we investigated the role of EGF in early OL development with isolated glial restricted precursor (GRP) cells. It was found that EGF collaborated with Platelet Derived Growth Factor-AA (PDGFaa) to promote the survival and self-renewal of GRP cells, but predisposed GRP cells to develop into O4^- early-stage oligodendrocyte precursor cells (OPCs) in the absence of or PDGFaa. In OPCs, EGF synergized with PDGFaa to maintain their O4 negative antigenic phenotype. Upon PDGFaa withdrawal, EGF promoted the terminal differentiation of OPCs by reducing apoptosis and increasing the number of mature OLs. Together, these data revealed that EGF is an important mitogen to enhance oligodendroglial development.

Astrocytes in Oligodendrocyte Lineage Development and White Matter Pathology

White matter is primarily composed of myelin and myelinated axons. Structural and functional completeness of myelin is critical for the reliable and efficient transmission of information. White matter injury has been associated with the development of many demyelinating diseases. Despite a variety of scientific advances aimed at promoting re-myelination, their benefit has proven at best to be marginal. Research suggests that the failure of the re-myelination process may be the result of an unfavorable microenvironment. Astrocytes, are the most ample and diverse type of glial cells in central nervous system (CNS) which display multiple functions for the cells of the oligodendrocytes lineage. As such, much attention has recently been drawn to astrocyte function in terms of white matter myelin repair. They are different in white matter from those in gray matter in specific regards to development, morphology, location, protein expression and other supportive functions. During the process of demyelination and re-myelination, the functions of astrocytes are dynamic in that they are able to change functions in accordance to different time points, triggers or reactive pathways resulting in vastly different biologic effects. They have pivotal effects on oligodendrocytes and other cell types in the oligodendrocyte lineage by serving as an energy supplier, a participant of immunological and inflammatory functions, a source of trophic factors and iron and a sustainer of homeostasis. Astrocytic impairment has been shown to be directly linked to the development of neuromyelities optica (NMO). In addition, astroctyes have also been implicated in other white matter conditions such as psychiatric disorders and neurodegenerative diseases such as Alzheimer’s disease (AD), multiple sclerosis (MS) and amyotrophic lateral sclerosis (ALS). Inhibiting specifically detrimental signaling pathways in astrocytes while preserving their beneficial functions may be a promising approach for remyelination strategies. As such, the ability to manipulate astrocyte function represents a novel therapeutic approach that can repair the damaged myelin that is known to occur in a variety of white matter-related disorders.

Dietary LC-PUFA in iron-deficient anaemic pregnant and lactating guinea pigs induce minor defects in the offsprings' auditory brainstem responses

OBJECTIVES

We previously demonstrated that a mild pre-natal/early post-natal iron-deficient anaemic (IDA) diet devoid of long-chain polyunsaturated fatty acids (LC-PUFA) affected development, neurophysiology, and cerebral lipid biochemistry of the guinea pigs' progeny. Impacts of dietary LC-PUFA on altered cerebral development resulting from pre-natal IDA are unknown. To address this health issue, impacts of mild gestational IDA in the presence of dietary LC-PUFA on the offsprings' neural maturation were studied in guinea pigs using auditory brainstem responses (ABRs) and assessments of brain fatty acids (FAs).

METHODS

Female guinea pigs (n = 10/group) were fed an iron sufficient (IS) or IDA diet (146 and 12.7 mg iron/kg, respectively) with physiological amounts of LC-PUFA, during the gestation and lactation periods. From post-natal day (PNd) 9 onwards, the IS + PUFA diet was given to both groups of weaned offspring. Cerebral tissue and offsprings' ABR were collected on PNd24.

RESULTS

There was no difference in peripheral and brainstem transmission times (BTTs) between IS + PUFA and IDA + PUFA siblings (n = 10/group); the neural synchrony was also similar in both groups. Despite the absence of differences in auditory thresholds, IDA + PUFA siblings demonstrated a sensorineural hearing loss in the extreme range of frequencies (32, 4, and 2 kHz), as well as modified brain FA profiles compared to the IS + PUFA siblings.

DISCUSSION

The present study reveals that siblings born from dams exposed to a moderate IDA diet including balanced physiological LC-PUFA levels during pregnancy and lactation demonstrate minor impairments of ABR compared to the control siblings, particularly on the auditory acuity, but not on neural synchrony, auditory nerve velocity and BTT.

Impaired myelination and reduced brain ferric iron in the mouse model of mucolipidosis IV

Mucolipidosis type IV (MLIV) is a lysosomal storage disease caused by mutations in the MCOLN1 gene, which encodes the lysosomal transient receptor potential ion channel mucolipin-1 (TRPML1). MLIV causes impaired motor and cognitive development, progressive loss of vision and gastric achlorhydria. How loss of TRPML1 leads to severe psychomotor retardation is currently unknown, and there is no therapy for MLIV. White matter abnormalities and a hypoplastic corpus callosum are the major hallmarks of MLIV brain pathology. Here, we report that loss of TRPML1 in mice results in developmental aberrations of brain myelination as a result of deficient maturation and loss of oligodendrocytes. Defective myelination is evident in Mcoln1(-/-) mice at postnatal day 10, an active stage of postnatal myelination in the mouse brain. Expression of mature oligodendrocyte markers is reduced in Mcoln1(-/-) mice at postnatal day 10 and remains lower throughout the course of the disease. We observed reduced Perls' staining in Mcoln1(-/-) brain, indicating lower levels of ferric iron. Total iron content in unperfused brain is not significantly different between Mcoln1(-/-) and wild-type littermate mice, suggesting that the observed maturation delay or loss of oligodendrocytes might be caused by impaired iron handling, rather than by global iron deficiency. Overall, these data emphasize a developmental rather than a degenerative disease course in MLIV, and suggest that there should be a stronger focus on oligodendrocyte maturation and survival to better understand MLIV pathogenesis and aid treatment development.

Iron in multiple sclerosis: roles in neurodegeneration and repair

MRI and histological studies have shown global alterations in iron levels in the brains of patients with multiple sclerosis (MS), including increases in the iron stored by macrophages and microglia. Excessive free iron can be toxic, and accumulation of iron in MS has generally been thought to be detrimental. However, iron maintains the integrity of oligodendrocytes and myelin, and facilitates their regeneration following injury. The extracellular matrix, a key regulator of remyelination, might also modulate iron levels. This Review highlights key histological and MRI studies that have investigated changes in iron distribution associated with MS. Potential sources of iron, as well as iron regulatory proteins and the detrimental roles of excessive iron within the CNS, are also discussed, with emphasis on the importance of iron within cells for oxidative metabolism, proliferation and differentiation of oligodendrocytes, and myelination. In light of the beneficial and detrimental properties of iron within the CNS, we present considerations for treatments that target iron in MS. Such treatments must balance trophic and toxic properties of iron, by providing sufficient iron levels for remyelination and repair while avoiding excesses that might overwhelm homeostatic mechanisms and contribute to damage.

A delicate balance: Iron metabolism and diseases of the brain

Iron is the most abundant transition metal within the brain, and is vital for a number of cellular processes including neurotransmitter synthesis, myelination of neurons, and mitochondrial function. Redox cycling between ferrous and ferric iron is utilized in biology for various electron transfer reactions essential to life, yet this same chemistry mediates deleterious reactions with oxygen that induce oxidative stress. Consequently, there is a precise and tightly controlled mechanism to regulate iron in the brain. When iron is dysregulated, both conditions of iron overload and iron deficiencies are harmful to the brain. This review focuses on how iron metabolism is maintained in the brain, and how an alteration to iron and iron metabolism adversely affects neurological function.

DMT1 as a candidate for non-transferrin-bound iron uptake in the peripheral nervous system

Iron, either in its chelated form or as holotransferrin (hTf), prevents the dedifferentiation of Schwann cells (SC), cells responsible for the myelination of the peripheral nervous system (PNS). This dedifferentiation is promoted by serum deprivation through cAMP release, PKA activation, and CREB phosphorylation. Since iron elicits its effect in a transferrin (Tf)-free environment, in this work we postulate that non-transferrin-bound iron (NTBI) uptake must be involved. Divalent metal transporter 1(DMT1) has been widely described in literature as a key player in iron metabolism, but never before in the PNS context. The presence of DMT1 was demonstrated in nerve homogenate, isolated adult-rat myelin, and cultured SC by Western Blot (WB) analysis and confirmed through its colocalization with S-100β (SC marker) by immunocytochemical and immunohistochemical analyses. Furthermore, the existence of its mRNA was verified in sciatic nerve homogenate by RT-PCR and throughout SC maturational stages. Finally, we describe DMT1's subcellular location in the plasma membrane by confocal microscopy of SC and WB of different subcellular fractions. These data allow us to suggest the participation of DMT1 as part of a Tf independent iron uptake mechanism in SC and lead us to postulate a crucial role for iron in SC maturation and, as a consequence, in PNS myelination.

The influence of micronutrients in cell culture: a reflection on viability and genomic stability

Micronutrients, including minerals and vitamins, are indispensable to DNA metabolic pathways and thus are as important for life as macronutrients. Without the proper nutrients, genomic instability compromises homeostasis, leading to chronic diseases and certain types of cancer. Cell-culture media try to mimic the in vivo environment, providing in vitro models used to infer cells' responses to different stimuli. This review summarizes and discusses studies of cell-culture supplementation with micronutrients that can increase cell viability and genomic stability, with a particular focus on previous in vitro experiments. In these studies, the cell-culture media include certain vitamins and minerals at concentrations not equal to the physiological levels. In many common culture media, the sole source of micronutrients is fetal bovine serum (FBS), which contributes to only 5-10% of the media composition. Minimal attention has been dedicated to FBS composition, micronutrients in cell cultures as a whole, or the influence of micronutrients on the viability and genetics of cultured cells. Further studies better evaluating micronutrients' roles at a molecular level and influence on the genomic stability of cells are still needed.

The conundrum of iron in multiple sclerosis--time for an individualised approach

Although the involvement of immune mechanisms in multiple sclerosis (MS) is undisputed, some argue that there is insufficient evidence to support the hypothesis that MS is an autoimmune disease, and that the difference between immune- and autoimmune disease mechanisms has yet to be clearly delineated. Uncertainties surrounding MS disease pathogenesis and the modest efficacy of currently used disease modifying treatments (DMTs) in the prevention of disability, warrant the need to explore other possibilities. It is evident from the literature that people diagnosed with MS differ widely in symptoms and clinical outcome--some patients have a benign disease course over many years without requiring any DMTs. Attempting to include all patients into a single entity is an oversimplification and may obscure important observations with therapeutic consequences. In this review we advocate an individualised approach named Pathology Supported Genetic Testing (PSGT), in which genetic tests are combined with biochemical measurements in order to identify subgroups of patients requiring different treatments. Iron dysregulation in MS is used as an example of how this approach may benefit patients. The theory that iron deposition in the brain contributes to MS pathogenesis has caused uncertainty among patients as to whether they should avoid iron. However, the fact that a subgroup of people diagnosed with MS show clinical improvement when they are on iron supplementation emphasises the importance of individualised therapy, based on genetic and biochemical determinations.

Iron and holotransferrin induce cAMP-dependent differentiation of Schwann cells

The differentiation of myelin-forming Schwann cells (SC) is completed with the appearance of myelin proteins MBP and P(0) and a concomitant downregulation of markers GFAP and p75NTR, which are expressed by immature and adult non-myelin-forming SC. We have previously demonstrated that holotransferrin (hTf) can prevent SC dedifferentiation in culture (Salis et al., 2002), while apotransferrin (aTf) cannot. As a consequence, we used pure cultured SC and submitted them to serum deprivation in order to promote dedifferentiation and evaluate the prodifferentiating ability of ferric ammonium citrate (FAC) through the expression of MBP, P(0), p75NTR and c-myc. The levels of cAMP, CREB and p-CREB were also measured. Results show that Fe(3+), either in its free form or as hTf, can prevent the dedifferentiation promoted by serum withdrawal. Both FAC and hTf were proven to promote differentiation, probably through the increase in cAMP levels and CREB phosphorylation, as well as levels of reactive oxygen species. This effect was inhibited by deferroxamine (Dfx, an iron chelator), H9 (a cAMP-PKA antagonist) and N-acetylcysteine (NAC, a powerful antioxidant).

Ferritin stimulates oligodendrocyte genesis in the adult spinal cord and can be transferred from macrophages to NG2 cells in vivo

Injured CNS tissue often contains elevated iron and its storage protein ferritin, which may exacerbate tissue damage through pro-oxidative mechanisms. Therefore, therapeutic studies often target iron reduction as a neuroprotective strategy. However, iron may be crucial for oligodendrocyte replacement and remyelination. For instance, we previously showed that intraspinal toll-like receptor 4 macrophage activation induced the generation of new ferritin-positive oligodendrocytes, and that iron chelation significantly reduced this oligodendrogenic response. Since macrophages can secrete ferritin, we hypothesize that ferritin is a macrophage-derived signal that promotes oligodendrogenesis. To test this, we microinjected ferritin into intact adult rat spinal cords. Within 6 h, NG2+ progenitor cells proliferated and accumulated ferritin. By 3 d, many of these cells had differentiated into new oligodendrocytes. However, acute neuron and oligodendrocyte toxicity occurred in gray matter. Interestingly, ferritin-positive NG2 cells and macrophages accumulated in the area of cell loss, revealing that NG2 cells thrive in an environment that is toxic to other CNS cells. To test whether ferritin can be transferred from macrophages to NG2 cells in vivo, we loaded macrophages with fluorescent ferritin then transplanted them into intact spinal white matter. Within 3-6 d, proliferating NG2 cells migrated into the macrophage transplants and accumulated fluorescently labeled ferritin. These results show that activated macrophages can be an in vivo source of ferritin for NG2 cells, which induces their proliferation and differentiation into new oligodendrocytes. This work has relevance for conditions in which iron-mediated injury and/or repair likely occur, such as hemorrhage, stroke, spinal cord injury, aging, Parkinson's disease, and Alzheimer's disease.

Iron efflux from astrocytes plays a role in remyelination

How iron is delivered to the CNS for myelination is not well understood. We assessed whether astrocytes can provide iron to cells in the CNS for remyelination. To study this we generated a conditional deletion of the iron efflux transporter ferroportin (Fpn) in astrocytes, and induced focal demyelination in the mouse spinal cord dorsal column by microinjection of lysophosphatidylcholine (LPC). Remyelination assessed by electron microscopy was reduced in astrocyte-specific Fpn knock-out mice compared with wild-type controls, as was proliferation of oligodendrocyte precursor cells (OPCs). Cell culture work showed that lack of iron reduces the ability of microglia to express cytokines (TNF-α and IL-1β) involved in remyelination. Furthermore, astrocytes in culture express high levels of FGF-2 in response to IL-1β, and IGF-1 in response to TNF-α stimulation. FGF-2 and IGF-1 are known to be important for myelination. Reduction in IL-1β and IGF-1 were also seen in astrocyte-specific Fpn knock-out mice after LPC-induced demyelination. These data suggest that iron efflux from astrocytes plays a role in remyelination by either direct effects on OPCs or indirectly by affecting glial activation.

Iron efflux from oligodendrocytes is differentially regulated in gray and white matter

Accumulation of iron occurs in the CNS in several neurodegenerative diseases. Iron is essential for life but also has the ability to generate toxic free radicals if not properly handled. Iron homeostasis at the cellular level is therefore important to maintain proper cellular function, and its dysregulation can contribute to neurodegenerative diseases. Iron export, a key mechanism to maintain proper levels in cells, occurs via ferroportin, a ubiquitously expressed transmembrane protein that partners with a ferroxidase. A membrane-bound form of the ferroxidase ceruloplasmin is expressed by astrocytes in the CNS and regulates iron efflux. We now show that oligodendrocytes use another ferroxidase, called hephaestin, which was first identified in enterocytes in the gut. Mice with mutations in the hephaestin gene (sex-linked anemia mice) show iron accumulation in oligodendrocytes in the gray matter, but not in the white matter, and exhibit motor deficits. This was accompanied by a marked reduction in the levels of the paranodal proteins contactin-associated protein 1 (Caspr) and reticulon-4 (Nogo A). We show that the sparing of iron accumulation in white matter oligodendrocytes in sex-linked anemia mice is due to compensatory upregulation of ceruloplasmin in these cells. This was further confirmed in ceruloplasmin/hephaestin double-mutant mice, which show iron accumulation in both gray and white matter oligodendrocytes. These data indicate that gray and white matter oligodendrocytes can use different iron efflux mechanisms to maintain iron homeostasis. Dysregulation of such efflux mechanisms leads to iron accumulation in the CNS.

Identifying a window of vulnerability during fetal development in a maternal iron restriction model

It is well acknowledged from observations in humans that iron deficiency during pregnancy can be associated with a number of developmental problems in the newborn and developing child. Due to the obvious limitations of human studies, the stage during gestation at which maternal iron deficiency causes an apparent impairment in the offspring remains elusive. In order to begin to understand the time window(s) during pregnancy that is/are especially susceptible to suboptimal iron levels, which may result in negative effects on the development of the fetus, we developed a rat model in which we were able to manipulate and monitor the dietary iron intake during specific stages of pregnancy and analyzed the developing fetuses. We established four different dietary-feeding protocols that were designed to render the fetuses iron deficient at different gestational stages. Based on a functional analysis that employed Auditory Brainstem Response measurements, we found that maternal iron restriction initiated prior to conception and during the first trimester were associated with profound changes in the developing fetus compared to iron restriction initiated later in pregnancy. We also showed that the presence of iron deficiency anemia, low body weight, and changes in core body temperature were not defining factors in the establishment of neural impairment in the rodent offspring.Our data may have significant relevance for understanding the impact of suboptimal iron levels during pregnancy not only on the mother but also on the developing fetus and hence might lead to a more informed timing of iron supplementation during pregnancy.

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.

Iron chelator deferoxamine alters iron-regulatory genes and proteins and suppresses osteoblast phenotype in fetal rat calvaria cells

There are few studies describing the extent to which low iron status affects osteoblastogenesis, despite evidence that iron deficiency produces adverse effects on bone density. The purpose of this study was to evaluate alterations in intracellular iron status by measuring iron-regulated gene and protein expression and to describe development of osteoblast phenotype in primary cells treated with iron chelator deferoxamine (DFOM) during differentiation. Using the well-described fetal rat calvaria model, cells were incubated with 0-8 microM DFOM throughout differentiation (confluence to day (D) 21), or only during early differentiation (confluence to D13-15) or late differentiation (D13-15 to D21). Changes in intracellular iron status were determined by measuring alterations in gene and protein expression of transferrin receptor and ferritin light chain and heavy chain. Development of osteoblast phenotype was monitored by measuring expression of genes that are known to be up-regulated during differentiation, analyzing the percentage of mineralized surface area, and counting the number of multi-layered bone nodules at the end of culture. Results indicate that treatment throughout differentiation with 8 microM DFOM alters iron-regulated genes and proteins by mid-differentiation (D13-15) in a pattern consistent with iron deficiency with concomitant down-regulation of osteoblast phenotype genes, especially osteocalcin. Additionally, alkaline phosphatase staining was lower and there was about 70% less mineralized surface area (p<0.05) by D21 in wells treated throughout differentiation with 8 microM DFOM compared to control. Down-regulation of osteocalcin and alkaline phosphatase mRNA (p<0.05) and suppressed mineralization (p<0.05) was also evident at D21 in cells treated only during early differentiation. In contrast, treatment during late differentiation did not alter osteoblastic outcomes by D21. In conclusion, it appears that iron is required for normal osteoblast phenotype development, and that early rather than late differentiation events may be more sensitive to iron availability.

Oligodendrogenesis: the role of iron

Iron seems to be an essential factor in myelination and oligodendrocyte (OLGc) biology. However, the specific role of iron in these processes remains to be elucidated. Iron deficiency (ID) imposed to developing rats has been a relevant model to understand the role of iron in oligodendrogenesis and myelination. During early development ID causes specific changes in myelin composition, including a lower relative content of cholesterol, proteolipid protein (PLP), and myelin basic protein 21 (MBP21). These changes could be a consequence of the adverse effects of ID on OLGc development and function. We subsenquently studied the possible corrective effect of a single intracranial injection (ICI) of apotransferrin (aTf) on myelin formation in ID rats OLGc migration and differentiation after an ICI of aTf was evaluated at 3 days of age. ID increased the number of proliferating and undifferentiated cells in the corpus callosum (CC), while a single aTf injection reverts these effects, increasing the number of mature cells and myelin formation. Overall, results of a series of studies supports the concept that iron may affect OLGc development at early stages of embryogenesis rather than during late development. Myelin composition is altered by a limited iron supply, changes that can be reverted by a single injection of aTf.

A history of iron deficiency anemia during infancy alters brain monoamine activity later in juvenile monkeys

Both during and after a period of iron deficiency (ID), iron-dependent neural processes are affected, which raises the potential concern that the anemia commonly experienced by many growing infants could have a protracted effect on the developing brain. To further investigate the effects of ID on the immature brain, 49 infant rhesus monkeys were evaluated across the first year of life. The mothers, and subsequently the infants after weaning, were maintained on a standardized diet containing 180 mg/kg of iron and were not provided other iron-rich foods as treats or supplements. As the infants grew, they were all screened with hematological tests, which documented that 16 (33.3%) became markedly ID between 4 and 8 months of age. During this anemic period and subsequently at 1 year of age, cerebrospinal fluid (CSF) specimens were collected to compare monoamine activity in the ID and iron-sufficient infants. Monoamine neurotransmitters and metabolite levels were normal at 4 and 8 months of age, but by 1 year the formerly anemic monkeys had significantly lower dopamine and significantly higher norepinephrine levels. These findings indicate that ID can affect the developmental trajectory of these two important neurotransmitter systems, which are associated with emotionality and behavioral performance, and further that the impact in the young monkey was most evident during the period of recovery.

Vitamin and mineral deficiencies in the developed world and their effect on the eye and vision

Vitamin and mineral deficiencies are common in developing countries, but also occur in developed countries. We review micronutrient deficiencies for the major vitamins A, cobalamin (B(12)), biotin (vitamin H), vitamins C and E, as well as the minerals iron, and zinc, in the developed world, in terms of their relationship to systemic health and any resulting ocular disease and/or visual dysfunction. A knowledge of these effects is important as individuals with consequent poor ocular health and reduced visual function may present for ophthalmic care.

Iron metabolism in Parkinsonian syndromes

Growing evidence suggests an involvement of iron in the pathophysiology of neurodegenerative diseases. Several of the diseases are associated with parkinsonian syndromes, induced by degeneration of basal ganglia regions that contain the highest amount of iron within the brain. The group of neurodegenerative disorders associated with parkinsonian syndromes with increased brain iron content can be devided into two groups: (1) parkinsonian syndromes associated with brain iron accumulation, including Parkinson's disease, diffuse Lewy body disease, parkinsonian type of multiple system atrophy, progressive supranuclear palsy, corticobasal ganglionic degeneration, and Westphal variant of Huntington's disease; and (2) monogenetically caused disturbances of brain iron metabolism associated with parkinsonian syndromes, including aceruloplasminemia, hereditary ferritinopathies affecting the basal ganglia, and panthotenate kinase associated neurodegeneration type 2. Although it is still a matter of debate whether iron accumulation is a primary cause or secondary event in the first group, there is no doubt that iron-induced oxidative stress contributes to neurodegeneration. Parallels concerning pathophysiological as well as clinical aspects can be drawn between disorders of both groups. Results from animal models and reduction of iron overload combined with at least partial relief of symptoms by application of iron chelators in patients of the second group give hope that targeting the iron overload might be one possibility to slow down the neurodegenerative cascade also in the first group of inevitably progressive neurodegenerative disorders.

Cellular iron status influences the functional relationship between microglia and oligodendrocytes

Previously, we have reported that there is a spatiotemporal relationship between iron accumulation in microglia and oligodendrocytes during normal development and in remyelination following injury. This in vivo observation has prompted us to develop a cell culture model to test the relationship between iron status of microglia and survival of oligodendrocytes. We found that conditioned media from iron-loaded microglia increases the survival of oligodendrocytes; but conditioned media from iron loaded activated microglia is toxic to oligodendrocytes. In the trophic condition, one of the proteins released by iron-loaded microglia is H-ferritin, and transfecting the microglia with siRNA for H-ferritin blocks the trophic response on oligodendrocytes. Lipopolysaccharide (LPS) activation decreases the amount of H-ferritin that is released from microglia and increases the release of the proinflammatory cytokines tumor necrosis factor-alpha and interleukin-1. LPS activation of iron-enriched microglia results in the activation of NF-kB and greater release of cytokines when compared with that of control microglia; whereas treating microglia with an iron chelator is associated with less NF-kB activation and less release of cytokines. These results indicate that microglia play an important role in iron homoeostasis and that their iron status can influence how microglia influence growth and survival of oligodendrocytes. The results further indicate that ferritin, released by microglia, is a significant source of iron for oligodendrocytes.

Age-related myelin breakdown: a developmental model of cognitive decline and Alzheimer's disease

A hypothetical model of Alzheimer's disease (AD) as a uniquely human brain disorder rooted in its exceptional process of myelination is presented. Cortical regions with the most protracted development are most vulnerable to AD pathology, and this protracted development is driven by oligodendrocytes, which continue to differentiate into myelin producing cells late into the fifth decade of life. The unique metabolic demands of producing and maintaining their vast myelin sheaths and synthesizing the brain's cholesterol supply make oligodendrocytes especially susceptible to a variety of insults. Their vulnerability increases with increasing age at differentiation as later-differentiating cells myelinate increasing numbers of axonal segments. These vulnerable late-differentiating cells drive the protracted process of intracortical myelination and by increasing local cholesterol and iron levels, progressively increase the toxicity of the intracortical environment forming the basis for the age risk factor for AD. At older ages, the roughly bilaterally symmetrical continuum of oligodendrocyte vulnerability manifests as a progressive pattern of myelin breakdown that recapitulates the developmental process of myelination in reverse. The ensuing homeostatic responses to myelin breakdown further increase intracortical toxicity and results in the relentless progression and non-random anatomical distribution of AD lesions that eventually cause neuronal dysfunction and degeneration. This process causes a slowly progressive disruption of neural impulse transmission that degrades the temporal synchrony of widely distributed neural networks underlying normal brain function. The resulting network "disconnections" first impact functions that are most dependent on large-scale synchronization including higher cognitive functions and formation of new memories. Multiple genetic and environmental risk factors (e.g. amyloid beta-peptide and free radical toxicity, head trauma, anoxia, cholesterol levels, etc.) can contribute to the cognitive deficits observed in aging and AD through their impact on the life-long trajectory of myelin development and breakdown. This development-to-degeneration model is testable through imaging and post mortem methods and highlights the vital role of myelin in impulse transmission and synchronous brain function. The model offers a framework that explains the anatomical distribution and progressive course of AD pathology, some of the failures of promising therapeutic interventions, and suggests further testable hypotheses as well as novel approaches for intervention efforts.

Brain Ferritin Iron as a Risk Factor for Age at Onset in Neurodegenerative Diseases

Tissue iron can promote oxidative damage. Brain iron increases with age and is abnormally elevated early in the disease process in several neurodegenerative disorders, including Alzheimer's disease (AD) and Parkinson's disease (PD). Higher iron levels in males may contribute to higher risk for younger-onset PD and recent studies have linked the presence of the hemochromatosis gene with a younger age at onset of AD. We examined whether age at onset of PD and AD was associated with increased brain ferritin iron. Ferritin iron can be measured with specificity in vivo with MRI utilizing the field-dependent relaxation rate increase (FDRI) method. FDRI was assessed in three basal ganglia regions (caudate, putamen, and globus pallidus) and frontal lobe white matter for younger- and older-onset male PD and AD patients and healthy controls. Significant increases in basal ganglia FDRI levels were observed in the younger-onset groups of both diseases compared to their respective control groups, but were absent in the older-onset patients. The results support the suggestion that elevated ferritin iron and its associated toxicity is a risk factor for age at onset of neurodegenerative diseases such as AD and PD. Clinical phenomena such as gender-associated risk of developing neurodegenerative diseases and the age at onset of such diseases may be associated with brain iron levels. In vivo MRI can measure and track brain ferritin iron levels and provides an opportunity to design therapeutic interventions that target high-risk populations early in the course of illness, possibly even before symptoms appear.

Getting a GR(i)P on oligodendrocyte development

One of the most extensively studied of mammalian cells is the oligodendrocyte, the myelin-forming cell of the central nervous system. The ancestry and development of this cell have been studied with every approach utilized by developmental biologists. Such detailed efforts have the potential of providing paradigms of relevance to those interested in analyzing the ancestry and development of any cell type. One of the striking features of studies on the development of oligodendrocytes is that different analytical approaches have led to strikingly different theoretical views regarding the ancestry of these cells. On one extreme is the hypothesis that the steps leading to the generation of oligodendrocytes begin with the generation of a glial-restricted precursor (GRP) cell from neuroepithelial stem cells. GRP cells are thought to be capable of giving rise to all glial cells (including oligodendrocytes and multiple astrocyte populations), but not to neurons, a process that appears to require progression through further stages of greater lineage restriction. On the other extreme is the hypothesis that oligodendrocytes are derived from a precursor cell that generates only motor neurons and oligodendrocytes, with astrocytes being generated through a separate lineage. In this review, we critically consider the various contributions to understanding the ancestry of oligodendrocytes, with particular attention to the respective merits of the GRP cell vs. the motor neuron-oligodendrocyte precursor (MNOP) cell hypothesis. We draw the conclusion that, at present, the strengths of the GRP cell hypothesis outweigh those of the MNOP hypothesis and other hypotheses suggesting oligodendrocytes are developmentally more related to motor neurons than to astrocytes. Moreover, it is clear from existing data that, following the period of motor neuron generation, the major glial precursor cell in the embryonic spinal cord is the GRP cell, and that multiple previous studies on the earliest stages of oligodendrocyte generation in the developing spinal cord have been focused on a differentiation stage of GRP cells.

Ureteric bud controls multiple steps in the conversion of mesenchyme to epithelia

Conversion of renal mesenchyme into epithelia depends on the ureteric bud, but its specific actions are not established. From conditioned media of ureteric bud cells, we have identified molecules that mimic the growth and epithelialization of mesenchyme in vivo. LIF targets late epithelial progenitors surrounding the ureteric bud, and in combination with survival factors, converts them into nephrons. In contrast, 24p3/Ngal targets early progenitors at the kidney's periphery through an iron-mediated, but a transferrin-independent mechanism. Hence, the ureteric bud controls many steps of cell conversion. A genome wide search for ureteric bud-specific molecules will identify additional pathways that induce morphogenesis.

HoloTransferrin but not ApoTransferrin prevents Schwann cell de-differentiation in culture

Schwann cells (SCs) in culture, without the presence of axons, become de-differentiated, reaching a condition similar to that of their precursor cells. The cytoplasmic accumulation of transferrin (Tf) in the myelinated peripheral nerve has been reported and data in the literature support a role for apoTf in myelination in the CNS. In the present report, we used SC cultures to evaluate the capacity of apoTf and holoTf to prevent cell de-differentiation promoted by fetal calf serum deprivation. SCs incubated in a serum-free medium showed a decrease in the expression of myelin basic protein (MBP) and P(0), markers of mature myelin-forming SCs, together with an increase in the levels of p75NTR and glial fibrillary acidic protein, markers of immature SCs. Treatment with holoTf prevented the decrease in expression of MBP and P(0) and the increase in p75NTR. ApoTf was unable to prevent these changes except when iron was added to the cultures. These results suggest a role for holoTf in the regulation of myelin formation by SCs.

Changes in the expression of cytoskeletal proteins in the CNS of transferrin transgenic mice

Apotransferrin injected intracranially into young rats has been shown in our laboratories to induce an early differentiation of oligodendroglial cells and an increased deposition of myelin. The expression of some myelin-specific proteins such as myelin basic protein (MBP) and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and of their mRNAs were significantly increased in these animals. Also, in the cytoskeleton obtained from isolated myelin, it was found that several microtubule associated proteins (MAPs), particularly the stable tubule only peptide (STOP) and MAP 1B, as well as actin and tubulin were markedly increased. In the present paper, we compare the changes in expression of brain and myelin cytoskeletal proteins in a newly generated transferrin transgenic mouse (Tg), overexpressing the human transferrin gene, with the results obtained in aTf-injected rats. In the myelin cytoskeletal fraction of Tg mice there was a significant increase in the expression of MBP, tubulin, tau and STOP, similarly to what was previously found in the aTf-injected rats. Immunohistochemical studies showed that a variance with what occurs in the aTf-injected model, in which the above mentioned changes were limited to the corpus callosum, in the Tg mice the changes in expression of cytoskeletal proteins were observed in the various anatomical areas studied such as cerebral cortex, brain stem and cerebellum. There was also an increased expression of neurofilaments in the Tg animals, in contrast with results obtained in aTf-injected rats, suggesting that in the Tg mice, the continuous overexpression of Tf might also induce some neuronal changes. Changes in tau, total and acetylated tubulin and MAP 1B were observed in both neurons and OLGc. The increase in STOP was more significant in OLGc while the changes in MAP2 were exclusively found in neurons.

Iron deficiency during embryogenesis and consequences for oligodendrocyte generation in vivo

One of the hallmarks of the pathology of iron deficiency in children is neurological disabilities that are often associated with hypomyelination. It has been hypothesized that this amyelination is mainly due to a disruption of myelin generation during the early postnatal stages when oligodendrocytes mature to generate myelin producing cell. In addition to these suggestions, we have previously provided in vitro data showing that iron affects both the proliferation and differentiation of glial precursor cells leading to a disruption in the generation of oligodendrocytes. We now present evidence demonstrating in vivo that iron deficiency during pregnancy affects the iron levels of various brain tissues in the developing fetus and disrupts not only the proliferation of their glial precursor cells but also disturbs the generation of oligodendrocytes from these precursor cells. In addition, we show that iron deficiency during embryogenesis affects glial lineage cells in a tissue-specific manner. Our studies offer the possibility to begin to comprehend whether any effects that occur during embryogenesis might have an influence on the establishment of the pathological defects that occur as a consequence of iron deficiency.

A morphological study of the developmentally regulated transport of iron into the brain

The distribution of transferrin, transferrin receptor, ferritin and ferric iron was studied in the developing rat brain. Transferrin immunoreactivity (IR) was observed diffusely in the brain from E16 until P10 from where it gradually decreased. The subcellular distribution of transferrin-IR in neurons was compatible with receptor-mediated uptake from P21 and onwards. Transferrin receptor-IR was observed prenatally on cells of neuroectodermal origin in the ventricular zone and in brain capillary endothelial cells (BCECs). In postnatal rats, transferrin receptor-IR in BCECs was most pronounced in rats aged P10-P21 but thereafter decreased in intensity. The neuronal transferrin-receptor IR in postnatal brains was not consistently expressed on neurons until from P21 and onwards. Transferrin receptor-IR was not observed in astrocytes, oligodendrocytes or ramified microglial cells at any age. Ferric iron and ferritin were present in BCECs already from E16, declined from P3-P5, and was absent by P10. There results are discussed with emphasis on the age-dependent transport of transferrin into the developing brain. The upregulated expression of transferrin receptors on BCECs in the second and third postnatal week is compatible with a high need for iron at this age. The neuronal transferrin receptor expression by P21 coincides with a drop in transferrin-IR and iron transport into the brain at this age, suggesting that neuronal transferrin receptor mRNA is posttranscriptionally regulated by the lowered iron availability from this developmental stage onwards.

An iron delivery pathway mediated by a lipocalin

Despite the critical need for iron in many cellular reactions, deletion of the transferrin pathway does not block organogenesis, suggesting the presence of alternative methods to deliver iron. We show that a member of the lipocalin superfamily (24p3/Ngal) delivers iron to the cytoplasm where it activates or represses iron-responsive genes. Iron unloading depends on the cycling of 24p3/Ngal through acidic endosomes, but its pH sensitivity and its subcellular targeting differed from transferrin. Indeed, during the conversion of mesenchyme into epithelia (where we discovered the protein), 24p3/Ngal and transferrin were endocytosed by different cells that characterize different stages of development, and they triggered unique responses. These studies identify an iron delivery pathway active in development and cell physiology.