Pantothenate kinase-associated neurodegeneration (formerly Hallervorden-Spatz syndrome)

Pantothenate Kinase Activation Restores Brain Coenzyme A in a Mouse Model of Pantothenate Kinase-Associated Neurodegeneration

Pantothenate kinase-associated neurodegeneration (PKAN) is characterized by a motor disorder with combinations of dystonia, parkinsonism, and spasticity, leading to premature death. PKAN is caused by mutations in the PANK2 gene that result in loss or reduction of PANK2 protein function. PANK2 is one of three kinases that initiate and regulate coenzyme A biosynthesis from vitamin B5, and the ability of BBP-671, an allosteric activator of pantothenate kinases, to enter the brain and elevate coenzyme A was investigated. The metabolic stability, protein binding, and membrane permeability of BBP-671 all suggest that it has the physical properties required to cross the blood-brain barrier. BBP-671 was detected in plasma, liver, cerebrospinal fluid, and brain following oral administration in rodents, demonstrating the ability of BBP-671 to penetrate the brain. The pharmacokinetic and pharmacodynamic properties of orally administered BBP-671 evaluated in cannulated rats showed that coenzyme A (CoA) concentrations were elevated in blood, liver, and brain. BBP-671 elevation of whole-blood acetyl-CoA served as a peripheral pharmacodynamic marker and provided a suitable method to assess target engagement. BBP-671 treatment elevated brain coenzyme A concentrations and improved movement and body weight in a PKAN mouse model. Thus, BBP-671 crosses the blood-brain barrier to correct the brain CoA deficiency in a PKAN mouse model, resulting in improved locomotion and survival and providing a preclinical foundation for the development of BBP-671 as a potential treatment of PKAN. SIGNIFICANCE STATEMENT: The blood-brain barrier represents a major hurdle for drugs targeting brain metabolism. This work describes the pharmacokinetic/pharmacodynamic properties of BBP-671, a pantothenate kinase activator. BBP-671 crosses the blood-brain barrier to correct the neuron-specific coenzyme A (CoA) deficiency and improve motor function in a mouse model of pantothenate kinase-associated neurodegeneration. The central role of CoA and acetyl-CoA in intermediary metabolism suggests that pantothenate kinase activators may be useful in modifying neurological metabolic disorders.

Advances in Mitochondria-Targeted Drug Delivery

Mitochondria are dynamic organelles that play a crucial role in numerous cellular activities [...].

Natural Molecules and Neuroprotection: Kynurenic Acid, Pantethine and α-Lipoic Acid

The incidence of neurodegenerative diseases has increased greatly worldwide due to the rise in life expectancy. In spite of notable development in the understanding of these disorders, there has been limited success in the development of neuroprotective agents that can slow the progression of the disease and prevent neuronal death. Some natural products and molecules are very promising neuroprotective agents because of their structural diversity and wide variety of biological activities. In addition to their neuroprotective effect, they are known for their antioxidant, anti-inflammatory and antiapoptotic effects and often serve as a starting point for drug discovery. In this review, the following natural molecules are discussed: firstly, kynurenic acid, the main neuroprotective agent formed via the kynurenine pathway of tryptophan metabolism, as it is known mainly for its role in glutamate excitotoxicity, secondly, the dietary supplement pantethine, that is many sided, well tolerated and safe, and the third molecule, α-lipoic acid is a universal antioxidant. As a conclusion, because of their beneficial properties, these molecules are potential candidates for neuroprotective therapies suitable in managing neurodegenerative diseases.

Novel mutations in PANK2 and PLA2G6 genes in patients with neurodegenerative disorders: two case reports

BACKGROUND

Neurodegeneration with brain iron accumulation (NBIA) is a genetically heterogeneous group of disorders associated with progressive impairment of movement, vision, and cognition. The disease is initially diagnosed on the basis of changes in brain magnetic resonance imaging which indicate an abnormal brain iron accumulation in the basal ganglia. However, the diagnosis of specific types should be based on both clinical findings and molecular genetic testing for genes associated with different types of NBIA, including PANK2, PLA2G6, C19orf12, FA2H, ATP13A2, WDR45, COASY, FTL, CP, and DCAF17. The purpose of this study was to investigate disease-causing mutations in two patients with distinct NBIA disorders.

CASE PRESENTATION

Whole Exome sequencing using Next Generation Illumina Sequencing was used to enrich all exons of protein-coding genes as well as some other important genomic regions in these two affected patients. A deleterious homozygous four-nucleotide deletion causing frameshift deletion in PANK2 gene (c.1426_1429delATGA, p.M476 fs) was identified in an 8 years old girl with dystonia, bone fracture, muscle rigidity, abnormal movement, lack of coordination and chorea. In addition, our study revealed a novel missense mutation in PLA2G6 gene (c.3G > T:p.M1I) in one and half-year-old boy with muscle weakness and neurodevelopmental regression (speech, motor and cognition). The novel mutations were also confirmed by Sanger sequencing in the proband and their parents.

CONCLUSIONS

Current study uncovered two rare novel mutations in PANK2 and PLA2G6 genes in patients with NBIA disorder and such studies may help to conduct genetic counseling and prenatal diagnosis more accurately for individuals at the high risk of these types of disorders.

A variation in PANK2 gene is causing Pantothenate kinase-associated Neurodegeneration in a family from Jammu and Kashmir - India

Pantothenate kinase-associated neurodegeneration is a rare hereditary neurodegenerative disorder associated with nucleotide variation(s) in mitochondrial human Pantothenate kinase 2 (hPanK2) protein encoding PANK2 gene, and is characterized by symptoms of extra-pyramidal dysfunction and accumulation of non-heme iron predominantly in the basal ganglia of the brain. In this study, we describe a familial case of PKAN from the State of Jammu and Kashmir (J&K), India based on the clinical findings and genetic screening of two affected siblings born to consanguineous normal parents. The patients present with early-onset, progressive extrapyramidal dysfunction, and brain Magnetic Resonance imaging (MRI) suggestive of symmetrical iron deposition in the globus pallidi. Screening the PANK2 gene in the patients as well as their unaffected family members revealed a functional single nucleotide variation, perfectly segregating in the patient’s family in an autosomal recessive mode of inheritance. We also provide the results of in-silico analyses, predicting the functional consequence of the identified PANK2 variant.

First successful trial of preimplantation genetic diagnosis for pantothenate kinase-associated neurodegeneration

PURPOSE

We aim to present a case of a healthy infant born after intracytoplasmic sperm injection-in vitro fertilization (ICSI-IVF) with a preimplantation genetic diagnosis (PGD) for pantothenate kinase-associated neurodegeneration (PKAN) due to PANK2 mutation.

METHODS

ICSI-IVF was performed on a Thai couple, 34-year-old female and 33-year-old male, with a family history of PKAN in their first child. Following fertilization, each of the embryos were biopsied in the cleavage stage and subsequently processed for whole-genome amplification. Genetic status of the embryos was diagnosed by linkage analysis and direct mutation testing using primer extension-based mini-sequencing. Comprehensive chromosomal aneuploidy screening was performed using a next-generation sequencing-based strategy.

RESULTS

Only a single cycle of ICSI-IVF was processed. There were seven embryos from this couple-two were likely affected, three were likely carriers, one was likely unaffected, and one failed in target genome amplification. Aneuploidy screening was performed before making a decision on embryo transfer, and only one unaffected embryo passed the screening. That embryo was transferred in a frozen thawed cycle, and the pregnancy was successful. The diagnosis was confirmed by amniocentesis, which presented with a result consistent with PGD. At 38 weeks of gestational age, a healthy male baby was born. Postnatal genetic confirmation was also consistent with PGD and the prenatal results. At the age of 24 months, the baby presented with normal growth and development lacking any neurological symptoms.

CONCLUSIONS

We report the first successful trial of PGD for PKAN in a developing country using linkage analysis and mini-sequencing in cleavage stage embryos.

Exploring Missense Mutations in Tyrosine Kinases Implicated with Neurodegeneration

Protein kinases are one of the largest families of evolutionarily related proteins and the third most common protein class of human genome. All the protein kinases share the same structural organization. They are made up of an extracellular domain, transmembrane domain and an intra cellular kinase domain. Missense mutations in these kinases have been studied extensively and correlated with various neurological disorders. Individual mutations in the kinase domain affect the functions of protein. The enhanced or reduced expression of protein leads to hyperactivation or inactivation of the signalling pathways, resulting in neurodegeneration. Here, we present extensive analyses of missense mutations in the tyrosine kinase focussing on the neurodegenerative diseases encompassing structure function relationship. This is envisaged to enhance our understanding about the neurodegeneration and possible therapeutic measures.

Pallidal neuronal apolipoprotein E in pantothenate kinase-associated neurodegeneration recapitulates ischemic injury to the globus pallidus

Pantothenate kinase-associated neurodegeneration (PKAN) is a progressive movement disorder that is due to mutations in PANK2. Pathologically, it is a member of a class of diseases known as neurodegeneration with brain iron accumulation (NBIA) and features increased tissue iron and ubiquitinated proteinaceous aggregates in the globus pallidus. We have previously determined that these aggregates represent condensed residue derived from degenerated pallidal neurons. However, the protein content, other than ubiquitin, of these aggregates remains unknown. In the present study, we performed biochemical and immunohistochemical studies to characterize these aggregates and found them to be enriched in apolipoprotein E that is poorly soluble in detergent solutions. However, we did not determine a significant association between APOE genotype and the clinical phenotype of disease in our database of 81 cases. Rather, we frequently identified similar ubiquitin- and apolipoprotein E-enriched lesions in these neurons in non-PKAN patients in the penumbrae of remote infarcts that involve the globus pallidus, and occasionally in other brain sites that contain large γ-aminobutyric acid (GABA)ergic neurons. Our findings, taken together, suggest that tissue or cellular hypoxic/ischemic injury within the globus pallidus may underlie the pathogenesis of PKAN.

Pantethine treatment is effective in recovering the disease phenotype induced by ketogenic diet in a pantothenate kinase-associated neurodegeneration mouse model

Pantothenate kinase-associated neurodegeneration, caused by mutations in the PANK2 gene, is an autosomal recessive disorder characterized by dystonia, dysarthria, rigidity, pigmentary retinal degeneration and brain iron accumulation. PANK2 encodes the mitochondrial enzyme pantothenate kinase type 2, responsible for the phosphorylation of pantothenate or vitamin B5 in the biosynthesis of co-enzyme A. A Pank2 knockout (Pank2^−/−) mouse model did not recapitulate the human disease but showed azoospermia and mitochondrial dysfunctions. We challenged this mouse model with a low glucose and high lipid content diet (ketogenic diet) to stimulate lipid use by mitochondrial beta-oxidation. In the presence of a shortage of co-enzyme A, this diet could evoke a general impairment of bioenergetic metabolism. Only Pank2^−/− mice fed with a ketogenic diet developed a pantothenate kinase-associated neurodegeneration-like syndrome characterized by severe motor dysfunction, neurodegeneration and severely altered mitochondria in the central and peripheral nervous systems. These mice also showed structural alteration of muscle morphology, which was comparable with that observed in a patient with pantothenate kinase-associated neurodegeneration. We here demonstrate that pantethine administration can prevent the onset of the neuromuscular phenotype in mice suggesting the possibility of experimental treatment in patients with pantothenate kinase-associated neurodegeneration.

Eye-of-the-tiger-sign in a 48 year healthy adult

We report a healthy adult male, who underwent, as a control subject, part of a Huntington's disease study, extensive testing during three visits in a two year follow-up, including clinical examination and 3.0 T MRI scans. The T2-weighted MRI sequences revealed the "eye-of-the-tiger-sign". No clinical abnormalities in either motor, cognitive or behavioural domains were observed. PKAN2 and FTL gene mutation analysis were negative. This finding implies that an eye-of-the-tiger sign, which is considered a pathognomonic feature of neurodegeneration with brain iron accumulation (NBIA), can occur without any clinical symptoms.

Comparative study of cerebellar degeneration in canine neuroaxonal dystrophy, cerebellar cortical abiotrophy, and neuronal ceroid-lipofuscinosis

The cerebellar lesions of three dogs with canine neuroaxonal dystrophy (NAD), one dog with cerebellar cortical abiotrophy (CCA), and 4 dogs with neuronal ceroid-lipofuscinosis (NCL) were examined to understand their pathogeneses. Purkinje cell loss was most severe in the vermis of a dog with CCA, and granule cell loss was most prominent in the cerebellar hemisphere of dogs with NCL. Immunohistochemically, CD3-and HLA-DR-positive cells were most frequent in the dogs with NCL, and moderate in dogs with NAD, but not in a dog with CCA. The number of cleaved caspase 3-positive cells was prominent in a dog with CCA, but no significant in the dogs with NAD. The results indicate different pathway of neuronal loss of these canine neuronal disorders.

Immunohistochemical Features of Dystrophic Axons in Papillon Dogs with Neuroaxonal Dystrophy

The immunohistochemical features of dystrophic axons in brain tissues of Papillon dogs with neuroaxonal dystrophy (NAD) were examined in comparison with 1 dog with cerebellar cortical abiotrophy (CCA) and a dog without neurologic signs. Histologically, many dystrophic axons were observed throughout the central nervous system of all dogs with NAD. These axonal changes were absent in the dog with CCA and in the control dog. Severe Purkinje cell loss was found in the dog with CCA, whereas the lesions were milder in all dogs with NAD. Immunohistochemically, the many dystrophic axons were positive for neurofilaments, tau, α/β-synuclein, HSP70, ubiquitin, synaptophysin, syntaxin-1, and synaptosomal-associated protein-25 (SNAP-25). A few dystrophic axons were positive for α-synuclein. In addition, these dystrophic axons, especially in the nucleus gracilis, cuneatus, olivaris, and spinal tract of the trigeminal nerve, were intensely immunopositive for the 3 calcium-binding proteins calretinin, calbindin, and parvalbumin. The accumulation of synapse-associated proteins in the dystrophic axons may indicate dysfunction of the synapse at the presynaptic portion. The accumulation of α-synuclein in the dystrophic axon and region-specific appearance of calcium-binding protein-positive spheroids are considered as unique features in NAD of Papillon dogs, providing the key to elucidate the pathogenesis.

Good response to IL-1beta blockade by anakinra in a 23-year-old CINCA/NOMID patient without mutations in the CIAS1 gene. Cytokine profiles and functional studies

Chronic infantile neurological cutaneous and articular (CINCA) syndrome is an autoinflammatory disease, defined by the triad of urticarial rash, neurological manifestations, and arthropathy, accompanied by recurrent fevers and systemic inflammation. Increasing neurological deficits result from aseptic meningitis. Sensorineural hearing loss and progressive loss of vision caused by keratoconjunctivitis or papilloedema may emerge. An autosomal-dominant inheritance is suspected although sporadic cases are reported frequently. Sixty per cent of CINCA patients carry mutations in the cold-induced autoinflammatory syndrome (CIAS1) gene. We report the favourable response of a 23-year-old CINCA patient without CIAS1 mutations to treatment with the recombinant interleukin-1 (IL-1) receptor antagonist anakinra.

Neurodegeneration associated with genetic defects in phospholipase A(2)

OBJECTIVE

Mutations in the gene encoding phospholipase A(2) group VI (PLA2G6) are associated with two childhood neurologic disorders: infantile neuroaxonal dystrophy (INAD) and idiopathic neurodegeneration with brain iron accumulation (NBIA). INAD is a severe progressive psychomotor disorder in which axonal spheroids are found in brain, spinal cord, and peripheral nerves. High globus pallidus iron is an inconsistent feature of INAD; however, it is a diagnostic criterion of NBIA, which describes a clinically and genetically heterogeneous group of disorders that share this hallmark feature. We sought to delineate the clinical, radiographic, pathologic, and genetic features of disease resulting from defective phospholipase A(2).

METHODS

We identified 56 patients clinically diagnosed with INAD and 23 with idiopathic NBIA and screened their DNA for PLA2G6 mutations.

RESULTS

Eighty percent of patients with INAD had mutations in PLA2G6, whereas mutations were found in only 20% of those with idiopathic NBIA. All patients with two null mutations had a more severe phenotype. On MRI, nearly all mutation-positive patients had cerebellar atrophy, and half showed brain iron accumulation. We observed Lewy bodies and neurofibrillary tangles in association with PLA2G6 mutations.

CONCLUSION

Defects in phospholipase A(2) lead to a range of phenotypes. PLA2G6 mutations are associated with nearly all cases of classic infantile neuroaxonal dystrophy but a minority of cases of idiopathic neurodegeneration with brain iron accumulation, and genotype correlates with phenotype. Cerebellar atrophy predicts which patients are likely to be mutation-positive. The neuropathologic changes that are caused by defective phospholipase A(2) suggest a shared pathogenesis with both Parkinson and Alzheimer diseases.

A novel PANK2 gene mutation: clinical and molecular characteristics of patients short communication

Pantothenate kinase-associated neurodegeneration (PKAN) is a progressive neurodegenerative disorder with autosomal recessive inheritance. The major symptoms of PKAN include the onset before the age of 20 years, progressive pyramidal and extrapyramidal signs, retinitis pigmentosa, optic atrophy, dementia, and iron depositions in the globus pallidus. The authors present 3 patients with proven molecular diagnosis of PKAN, in whom 2 novel mutations of PANK2 gene have been identified.

Vitamin and cofactor biosynthesis pathways in Plasmodium and other apicomplexan parasites

Vitamins are essential components of the human diet. By contrast, the malaria parasite Plasmodium falciparum and related apicomplexan parasites synthesize certain vitamins de novo, either completely or in parts. The various biosynthesis pathways are specific to different apicomplexan parasites and emphasize the distinct requirements of these parasites for nutrients and growth factors. The absence of vitamin biosynthesis in humans implies that inhibition of the parasite pathways might be a way to interfere specifically with parasite development. However, the roles of biosynthesis and uptake of vitamins in the regulation of vitamin homeostasis in parasites needs to be established first. In this article, the procurement of vitamins B(1), B(5) and B(6) by Plasmodium and other apicomplexan parasites is discussed.

Activation of human mitochondrial pantothenate kinase 2 by palmitoylcarnitine

The human isoform 2 of pantothenate kinase (PanK2) is localized to the mitochondria, and mutations in this protein are associated with a progressive neurodegenerative disorder. PanK2 inhibition by acetyl-CoA is so stringent (IC50 < 1 microM) that it is unclear how the enzyme functions in the presence of intracellular CoA concentrations. Palmitoylcarnitine was discovered to be a potent activator of PanK2 that functions to competitively antagonize acetyl-CoA inhibition. Acetyl-CoA was a competitive inhibitor of purified PanK2 with respect to ATP. The interaction between PanK2 and acetyl-CoA was stable enough that a significant proportion of the purified protein was isolated as the PanK2.acetyl-CoA complex. The long-chain acylcarnitine activation of PanK2 explains how PanK2 functions in vivo, by providing a positive regulatory mechanism to counteract the negative regulation of PanK2 activity by acetyl-CoA. Our results suggest that PanK2 is located in the mitochondria to sense the levels of palmitoylcarnitine and up-regulate CoA biosynthesis in response to an increased mitochondrial demand for the cofactor to support beta-oxidation.

Anesthesia for patients with pantothenate-kinase-associated neurodegeneration (Hallervorden-Spatz disease) - a literature review

BACKGROUND

Hallervorden-Spatz disease (HSD) is a rare, progressive neurodegenerative disorder; the new and preferred name for HSD is 'pantothenate-kinase-associated neurodegeneration' (PKAN). Other suggested names are 'neurodegeneration with brain iron accumulation type 1' or 'infantile neuroaxonal dystrophy'. Patients with PKAN have many complications, which lead to numerous anesthetic management challenges. Reports concerning the anesthetic management of patients with PKAN are very limited.

OBJECTIVE

To determine the anesthetic management and techniques as well as relevant complications for patients with PKAN.

METHODS

In this study, we review previously published literature regarding the anesthesia-relevant clinical symptoms, the anesthetic management and techniques, and possible complications for this disorder.

RESULTS

Only four studies describing the anesthetic management and anesthetic techniques in patients with PKAN were found. Anesthesia-relevant symptoms influence the preanesthetic management (eg difficulties in articulation, dementia), the induction of anesthesia (eg oromandibular rigidity, seizures, dysphagia, aspiration) and the postoperative care (eg respiratory disability).

CONCLUSION

Reports concerning the anesthetic management of patients with PKAN are very limited, possibly as a result of the rareness of the disorder. Like many other patients with neurodegenerative diseases, patients with PKAN have many anesthesia-relevant symptoms, leading to numerous anesthetic management challenges. In general, the anesthetic complications associated with PKAN are usually no different from those associated with other neurodegenerative diseases, and the management of these are usually concordant.

Novel compound heterozygous mutations in the PANK2 gene in a Chinese patient with atypical pantothenate kinase-associated neurodegeneration

We investigated the presence of mutations in the pantothenate kinase (PANK2) gene in a 27-year-old male Chinese patient with atypical pantothenate kinase-associated neurodegeneration (PKAN), formerly Hallervorden-Spatz syndrome. Automated DNA sequence analyses revealed compound heterozygous mutations in the exon 3 and 5. This patient had a 10-year history of PKAN characterized by a slight tremor of the right hand when writing at onset and a slow progressive rigidity of the neck and the right arm and resting tremor in upper extremities. Dysarthria, dysphagia, and dystonic-athetoid movements of the face and right fingers were marked. Magnetic resonance showed the typical "eye-of-the-tiger" sign.

Biochemical properties of human pantothenate kinase 2 isoforms and mutations linked to pantothenate kinase-associated neurodegeneration

The PANK2 gene encodes the human pantothenate kinase 2 protein isoforms, and PANK2 mutations are linked to pantothenate kinase-associated neurodegeneration. Two PanK2 protein forms are proteolytically processed to form a mitochondrially localized, mature PanK2. Another isoform arose from a proposed initiation at a leucine codon and was not processed further. The fifth isoform was postulated to arise from an alternative splicing event and was found to encode an inactive protein. Fourteen mutant PanK2 proteins with single amino acid substitutions, associated with either early or late onset disease, were evaluated for activity. The PanK2(G521R), the most frequent mutation in pantothenate kinase-associated neurodegeneration, was devoid of activity and did not fold properly. However, nine of the mutant proteins associated with disease possessed catalytic activities that were indistinguishable from wild type, including the frequently encountered PanK2(T528M) missense mutation. PanK2 was extremely sensitive to feedback inhibition by CoA thioesters (IC50 values between 250 and 500 nM), and the regulation of the active PanK2 mutants was comparable with that of the wild-type protein. Coexpression of the PanK2(G521R) and wild-type PanK2 did not interfere with wild-type enzyme activity, arguing against a dominant negative effect of the PanK2(G521R) mutation in heterozygous patients. These data described the unique biochemical features of the PanK2 isoforms and suggested that catalytic defects may not be the sole cause for the neurodegenerative phenotype.

MR relaxometry and 1H MR spectroscopy for the determination of iron and metabolite concentrations in PKAN patients

The influence of iron deposits on T2 values and the content of metabolites in the brain of three patients with DNA proved pantothenate kinase-associated neurodegeneration (PKAN, formerly Hallervorden-Spatz syndrome) was studied. An eye-of-the-tiger sign, a typical MR finding for PKAN, was observed in two patients with the same mutation. A hypointensive lesion in a whole globus pallidus was observed in the third patient with the additional mutation. T2 values in the globus pallidus of the patients were about 40% shorter than in controls (71/48 ms in controls vs. patients), which corresponds to the increase of Fe concentration based on the ferritin basis from 17 mg for controls to 48 mg (100 g wet brain weight) in PKAN patients. 1H MR spectroscopy (MRS) has mainly been used to describe neuronal damage represented by decreased NAA (6.4 mmol vs. 9 mmol) and Cr/PCr (7.0 mmol vs. 9.8 mmol) concentrations in the basal ganglia region of the patient group to controls; MRS is much more case-sensitive and describes individual development of the disease as demonstrated in the difference between the spectra of typical PKAN patients (1, 2), and the patient (3) with atypical PKAN development. Any significant changes of metabolite concentration with the exception glutamine, glutamate and GABA were found in the white matter.

Expression of the iron transporter ferroportin in synaptic vesicles and the blood–brain barrier

Iron homeostasis in the mammalian brain is an important and poorly understood subject. Transferrin-bound iron enters the endothelial cells of the blood-brain barrier from the systemic circulation, and iron subsequently dissociates from transferrin to enter brain parenchyma by an unknown mechanism. In recent years, several iron transporters, including the iron importer DMT1 (Ireg1, MTP, DCT1) and the iron exporter ferroportin (SLC11A3, Ireg, MTP1) have been cloned and characterized. To better understand brain iron homeostasis, we have characterized the distribution of ferroportin, the presumed intestinal iron exporter, and have evaluated its potential role in regulation of iron homeostasis in the central nervous system. We discovered using in situ hybridization and immunohistochemistry that ferroportin is expressed in the endothelial cells of the blood-brain barrier, in neurons, oligodendrocytes, astrocytes, and the choroid plexus and ependymal cells. In addition, we discovered using techniques of immunoelectron microscopy and biochemical purification of synaptic vesicles that ferroportin is associated with synaptic vesicles. In the blood-brain barrier, it is likely that ferroportin serves as a molecular transporter of iron on the abluminal membrane of polarized endothelial cells. The role of ferroportin in synaptic vesicles is unknown, but its presence at that site may prove to be of great importance in neuronal iron toxicity. The widespread representation of ferroportin at sites such as the blood-brain barrier and synaptic vesicles raises the possibility that trafficking of elemental iron may be instrumental in the distribution of iron in the central nervous system.

Severity of Neurodegeneration Correlates with Compromise of Iron Metabolism in Mice with Iron Regulatory Protein Deficiencies

In mammals, iron regulatory proteins 1 and 2 (IRP1 and IRP2) posttranscriptionally regulate expression of several iron metabolism proteins including ferritin and transferrin receptor. Genetically engineered mice that lack IRP2, but have the normal complement of IRP1, develop adult-onset neurodegenerative disease associated with inappropriately high expression of ferritin in degenerating neurons. Here, we report that mice that are homozygous for a targeted deletion of IRP2 and heterozygous for a targeted deletion of IRP1 (IRP1+/- IRP2-/-) develop a much more severe form of neurodegeneration, characterized by widespread axonopathy and eventually by subtle vacuolization in several areas, particularly in the substantia nigra. Axonopathy develops in white matter tracts in which marked increases in ferric iron and ferritin expression are detected. Axonal degeneration is significant and widespread before evidence for abnormalities or loss of neuronal cell bodies can be detected. Ultimately, neuronal cell bodies degenerate in the substantia nigra and some other vulnerable areas, microglia are activated, and vacuoles appear. Mice manifest gait and motor impairment at stages when axonopathy is pronounced, but neuronal cell body loss is minimal. These observations suggest that therapeutic strategies that aim to revitalize neurons by treatment with neurotrophic factors may be of value in IRP2-/- and IRP1+/- IRP2-/- mouse models of neurodegeneration.