ABSTRACT
Chronic or intermittent extravasations of blood into the subarachnoid space, and dissemination of heme by circulating cerebrospinal fluid, are the only established causes of superficial siderosis of the central nervous system (CNS). We studied the autopsy tissues of nine patients by iron histochemistry, immunocytochemistry, single- and double-label immunofluorescence, electron microscopy of ferritin, and high-definition X-ray fluorescence. In one case, frozen brain tissue was available for quantitative assay of total iron and ferritin. Siderotic tissues showed extensive deposits of iron and ferritin, and infiltration of the cerebellar cortex was especially severe. In addition to perivascular collections of hemosiderin-laden macrophages, affected tissues displayed iron-positive anuclear foamy structures in the neuropil that resembled axonal spheroids. They were especially abundant in eighth cranial nerves and spinal cord. Double-label immunofluorescence of the foamy structures showed co-localization of neurofilament protein and ferritin but comparable merged images of myelin-basic protein and ferritin, and ultrastructural visualization of ferritin, did not allow the conclusion that axonopathy was simply due to dilatation and rupture of fibers. Heme-oxygenase-1 (HO-1) immunoreactivity persisted in macrophages of siderotic cerebellar folia. Siderosis caused a large increase in total CNS iron but high-definition X-ray fluorescence of embedded tissue blocks excluded the accumulation of other metals. Holoferritin levels greatly exceeded the degree of iron accumulation. The susceptibility of the cerebellar cortex is likely due to Bergmann glia that serve as conduits for heme; and the abundance of microglia. Both cell types biosynthesize HO-1 and ferritin in response to heme. The eighth cranial nerves are susceptible because they consist of CNS axons, myelin, and neuroglial tissue along their subarachnoid course. The persistence of HO-1 protein implies continuous exposure of CNS to free heme or an excessively sensitive transcriptional response of the HO-1 gene. The conversion of heme iron to hemosiderin probably involves both translational and transcriptional activation of ferritin biosynthesis.
Subject(s)
Central Nervous System Diseases/pathology , Central Nervous System/pathology , Siderosis/pathology , Adult , Aged , Central Nervous System/metabolism , Central Nervous System Diseases/etiology , Central Nervous System Diseases/metabolism , Cerebral Cortex/metabolism , Cerebral Cortex/pathology , Cranial Nerves/metabolism , Cranial Nerves/pathology , Female , Ferritins/metabolism , Heme/cerebrospinal fluid , Heme Oxygenase-1/metabolism , Hemosiderin/metabolism , Humans , Iron/metabolism , Male , Microglia/metabolism , Microglia/pathology , Middle Aged , Retrospective Studies , Siderosis/etiology , Siderosis/metabolismABSTRACT
Frataxin deficiency in Friedreich's ataxia (FRDA) causes cardiac, endocrine, and nervous system manifestations. Frataxin is a mitochondrial protein, and adequate amounts are essential for cellular iron homeostasis. The main histological lesion in the brain of FRDA patients is neuronal atrophy and a peculiar proliferation of synaptic terminals in the dentate nucleus termed grumose degeneration. This cerebellar nucleus may be especially susceptible to FRDA because it contains abundant iron. We examined total iron and selected iron-responsive proteins in the dentate nucleus of nine patients with FRDA and nine normal controls by biochemical and microscopic techniques. Total iron (1.53 +/- 0.53 mumol/g wet weight) and ferritin (206.9 +/- 46.6 mug/g wet weight) in FRDA did not significantly differ from normal controls (iron: 1.78 +/- 0.88 mumol/g; ferritin: 210.9 +/- 9.0 mug/g) but Western blots exhibited a shift to light ferritin subunits. Immunocytochemistry of the dentate nucleus revealed loss of juxtaneuronal ferritin-containing oligodendroglia and prominent ferritin immunoreactivity in microglia and astrocytes. Mitochondrial ferritin was not detectable by immunocytochemistry. Stains for the divalent metal transporter 1 confirmed neuronal loss while endothelial cells reacting with antibodies to transferrin receptor 1 protein showed crowding of blood vessels due to collapse of the normal neuropil. Regions of grumose degeneration were strongly reactive for ferroportin. Purkinje cell bodies, their dendrites and axons, were also ferroportin-positive, and it is likely that grumose degeneration is the morphological manifestation of mitochondrial iron dysmetabolism in the terminals of corticonuclear fibers. Neuronal loss in the dentate nucleus is the likely result of trans-synaptic degeneration.
