Astroglial heme oxygenase-1 and the origin of corpora amylacea in aging and degenerating neural tissues.
Exp Neurol. 2014 Apr;254:78-89.
- 1Lady Davis Institute, Jewish General Hospital, 3755 Cote Ste. Catherine Road, Montreal, Quebec H3T 1E2, Canada.
- 2Lady Davis Institute, Jewish General Hospital, 3755 Cote Ste. Catherine Road, Montreal, Quebec H3T 1E2, Canada; Dept. of Neurology and Neurosurgery, McGill University, Montreal, Quebec H3A 2B4, Canada.
- 3Rush Alzheimer’s Disease Center, Rush University Medical Center, Chicago, IL 60612, USA; Department of Neurological Sciences, Rush University Medical Center, Chicago, IL 60612, USA.
- 4Lady Davis Institute, Jewish General Hospital, 3755 Cote Ste. Catherine Road, Montreal, Quebec H3T 1E2, Canada; Dept. of Neurology and Neurosurgery, McGill University, Montreal, Quebec H3A 2B4, Canada. Electronic address: firstname.lastname@example.org.
Background: Corpora amylacea (CA) are glycoproteinaceous (predominantly glial and extracellular) inclusions that accumulate in normal aging brain and, to a greater extent, in Alzheimer disease (AD). Previous pharmacological evidence suggested that up-regulation of endogenous heme oxygenase-1 (HO-1) in astrocytes promotes transformation of normal mitochondria to CA-like inclusions. Here, we determined whether 1) HMOX1 transfection fosters the accumulation of CA-like inclusions in cultured rat astroglia; 2) the HMOX1 transgene promotes CA formation in the brains of aging GFAP.HMOX1 mice; and 3) brain mitochondrial damage and CA biogenesis are augmented in persons with mild cognitive impairment (MCI), a harbinger of AD.
Methods: CA were ascertained in (i) neonatal rat astroglia transfectedwith flag-tagged human HO-1 cDNA, (ii) brain sections derived from 19 month-old GFAP.HMOX1 and wild-type (WT) mice, and (iii) post-mortem hippocampal sections from individuals with mild (MCI) and no cognitive impairment (NCI) after staining with PAS or antisera against HO-1, ubiquitin (Ub), manganese superoxide dismutase (MnSOD), andα-synuclein or tyrosine hydroxylase (TH).
Results: HMOX1 transfection induced cytoplasmic vacuolation and the accumulation of PAS+ inclusions in cultured astroglia. Numerous CA-like inclusions stained with PAS and immunoreactive for HO-1, Ub and MnSOD were observed in the brains of GFAP.HMOX1 mice, but were rarely encountered in age-matched, WT controls. Numbers of HO-1-positive CA were significantly increased in certain hippocampal strata of MCI subjects relative to NCI preparations. MnSOD and Ub proteins co-localized to CA in both the control and MCI specimens.
Conclusions: HO-1 promotes mitochondrial damage and CA biogenesis in astrocyte cultures and in the intact aging brain. CA formation is enhanced in theMCI hippocampus and thus occurs relatively early in the pathogenesis of AD. Glial HO-1 suppression may attenuate bioenergetic failure and slow disease progression in AD and other neurodegenerative conditions featuring accelerated accumulation of CA. Copyright © 2014 Elsevier Inc.
KEYWORDS: Alzheimer disease; Astrocyte; Corpora amylacea; Heme oxygenase-1; Mild cognitive impairment; Mitochondria; Transgenic mice
Corpora amylacea (CA) are glycoproteinaceous, ubiquitinated cytoplasmic inclusions that accumulate in periventricular and subpial regions of the human brain in the course of normal aging, and to a greater extent in patients with mild cognitive impairment (MCI), Alzheimer disease (AD), mesial temporal (hippocampal) sclerosis and other neurodegenerative conditions (Schipper and Cissé 1995, Cavanagh 1999, Song, Zukor et al. 2014). CA are thus not unlike neurofibrillary tangles and senile plaques, which are components of normal brain aging when they occur at low densities but are considered hallmark neuropathological lesions of AD when present in abundance. In the CNS, CA are most frequently encountered in astrocytes and as extracellular concretions. CA may occasionally arise within neuritic processes and have been documented in various non-neural tissues. The subcellular origins and mechanisms responsible for formation of CA have remained enigmatic despite the fact that many of their structural, tinctorial and histochemical properties have been elucidated during the 150 years or so since their discovery. Human CA share many topographical, histochemical and antigenic features with iron-laden (Gomori-positive) astrocytic granules which accumulate in the mammalian subcortex with advancing aging and in response to x-irradiation, cysteamine (CSH) exposure and other stressors (Schipper 2004). In 1995, we demonstrated consistent co-localization of two mitochondrial proteins, HSP60 and sulfite oxidase, to Gomori-positive astrocyte granules and CA in subependymal areas of senescent and AD human brain and in smears of isolated human CA. Nucleic acids of mitochondrial origin were also identified in these inclusions by co-immunolocalization of sulfite oxidase and DNA (Schipper and Cissé 1995). These observations indicated the presence of mitochondrial constituents within CA, corroborating earlier ultrastructural evidence of mitochondrial remnants in CA of the optic nerve (Woodford and Tso 1980) and senescent human astrocytes (Gertz, Cervos-Navarro et al. 1985). We determined that extended (90 day) treatment of cultured rat astroglia with CSH (Schipper and Cissé 1995) and subcutaneous administration of CSH to young adult rats (Schipper 1998) engender the formation of large, spherical inclusions that stain for iron-mediated pseudoperoxidase activity, periodic acid-Schiff (PAS), ubiquitin and mitochondrial epitopes. We also demonstrated that CA in human brain ((Schipper, Cissé et al. 1995) and in CSH-exposed astroglia are immunoreactive for the heme-degrading enzyme, heme oxygenase-1 (HO-1), and that dexamethasone suppression of the Hmox1 gene interferes with the formation of CA in CSH-treated glial cultures (Sahlas, Liberman et al. 2002). These results suggested that HO-1, a stress protein that is highly inducible by a broad array of oxidative and pro-inflammatory stimuli (Schipper, Song et al. 2009), may play an important role in the biogenesis of CA. In support of this formulation, we demonstrated that (i) transient transfection of human HMOX1 cDNA elicits the production of CA-like inclusions in cultured rat astroglia (Zukor et al. 2006) and (ii) CA accumulate in subcortical tissues of GFAP.HMOX1 transgenic mice which conditionally and selectively express human HO-1 in the astrocytic compartment (Song, Zukor et al. 2014). On the basis of these findings we concluded that (i) iron-rich (Gomori-positive) astrocyte granules are structural precursors of CA in senescent mammalian brain, (ii) degenerate, metal-laden mitochondria are the subcellular precursors of Gomori-positive inclusions and CA in senescent astroglia (and possibly other tissues) and (iii) the biogenesis of CA (and Gomori-positive glial granules) in the aging and diseased CNS is contingent upon the antecedent induction of Hmox1 (ibid.). A model for the biogenesis of CA is presented in Fig. 1. The model predicts that glial HO-1 suppression (Gupta, Lacoste et al. 2014) may attenuate pathological iron deposition and mitochondrial insufficiency (bioenergetic failure) and thereby slow disease progression in AD and other neurodegenerative conditions featuring the accelerated accumulation of CA.
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Fig 1. In the aging and degenerating CNS, oxidative stress generated by effete mitochondria, dopamine metabolism, amyloid deposition and the secretion of pro-inflammatory cytokines induces the HMOX1 gene in resident astroglia. 2) Free ferrous iron and carbon monoxide (CO), liberated intracellularly as products of HO-1-mediated heme degradation, subject the mitochondrial compartment to (further) oxidative stress. 3) The redox challenge stimulates opening of the mitochondrial permeability transition pore, which facilitates swelling of the organelle, disruption of cristae and sequestration of non-transferrin-derived iron (and occasionally other metals) within the mitochondrial matrix. 4) The distended mitochondria become autofluorescent (oxidized flavoproteins?) and engage lysosomes enriched for cathepsin D in a complex autophagic process, resulting in the formation of Gomori-positive cytoplasmic granules. Ferrous iron trapped within these inclusions behaves as a non-enzymatic (or pseudo-) peroxidase activity capable of converting inert catechols and MPTP into potential neurotoxins. 5) MnSOD and several redox-sensitive heat shock proteins are induced in these cells as a defence against accruing oxidative damage. Certain stress proteins, including HO-1, HSP27 and ubiquitin, become incorporated within the nascent inclusions. ATP depletion secondary to mitochondrial insufficiency may interfere with the disposal of ubiquitin-tagged proteins within the gliosomes by the ubiquitin-proteasome system. 6) A proportion of the iron-rich (Gomori) granules undergoes progressive glycation resulting in the quenching of autofluorescence and culminating in the formation of mature CA. 7) In some cases (not illustrated), degeneration of the host cell results in deposition of mature, insoluble CA within the extracellular space. Aβ, amyloid beta; CB, cathepsin B; CD, cathepsin D; CO, carbon monoxide; Fe2, ferrous iron; G, glycation; HO-1, heme oxygenase-1; HSP, heat shock protein; IL-1β, interleukin-1β; M-DNA, mitochondrial DNA; MnSOD, manganese superoxide dismutase; M-PROT, mitochondrial protein; MTP, mitochondrial permeability transition pore; TNFα, tumor necrosis factor-α; Ub, ubiquitin. [Modified from Schipper HM,