Heme oxygenase-1 modulates microRNA expression in cultured astroglia: implications for chronic brain disorders.

BACKGROUND: Over-expression of the heme-degrading enzyme, heme oxygenase-1 (HO-1) promotes iron deposition, mitochondrial damage, and autophagy in astrocytes and enhances the vulnerability of nearby neuronal constituents to oxidative injury. These neuropathological features and aberrant brain microRNA (miRNA) expression patterns have been implicated in the etiopathogeneses of various neurodevelopmental and aging-related neurodegenerative disorders. OBJECTIVE: To correlate glial HO-1 overexpression with altered miRNA patterns, which have been linked to the aforementioned "core" neuropathological features. METHODS: miRNA microchip assays were performed on HMOX1- and sham-transfected primary rat astroglia and affected miRNAs were further validated by qPCR. The roles of the heme degradation products, carbon monoxide (CO), iron (Fe) and bilirubin on miRNA expression were assessed and salient mRNA targets of the impacted miRNAs were ascertained. RESULTS: In HMOX1-transfected astrocytes, rno-miR-140*, rno-miR-17, and rno-miR-16 were significantly up-regulated, and rno-miR-297, rno-miR-206, rno-miR-187, rno-miR-181a, rno-miR-138 and rno-miR-29c were down-regulated, compared to sham-transfected controls. CO and Fe were implicated in the HMOX1 effects, whereas bilirubin was inert or counteracted the HMOX1-related changes. mRNA levels of Ngfr, Vglut1, Mapk3, Tnf-α, and Sirt1, known targets of the down-regulated miRNAs and abnormal in various human brain disorders, were significantly increased in the HMOX-1-transfected astrocytes. CONCLUSIONS: In chronic CNS disorders, altered expression of salient miRNAs and their mRNA targets may contribute to the neural damage accruing from the over-expression of glial HO-1.

Schizophrenia-like features in transgenic mice overexpressing human HO-1 in the astrocytic compartment.

Delineation of key molecules that act epigenetically to transduce diverse stressors into established patterns of disease would facilitate the advent of preventive and disease-modifying therapeutics for a host of neurological disorders. Herein, we demonstrate that selective overexpression of the stress protein heme oxygenase-1 (HO-1) in astrocytes of novel GFAP.HMOX1 transgenic mice results in subcortical oxidative stress and mitochondrial damage/autophagy; diminished neuronal reelin content (males); induction of Nurr1 and Pitx3 with attendant suppression of their targeting miRNAs, 145 and 133b; increased tyrosine hydroxylase and α-synuclein expression with downregulation of the targeting miR-7b of the latter; augmented dopamine and serotonin levels in basal ganglia; reduced D1 receptor binding in nucleus accumbens; axodendritic pathology and altered hippocampal cytoarchitectonics; impaired neurovascular coupling; attenuated prepulse inhibition (males); and hyperkinetic behavior. The GFAP.HMOX1 neurophenotype bears resemblances to human schizophrenia and other neurodevelopmental conditions and implicates glial HO-1 as a prime transducer of inimical (endogenous and environmental) influences on the development of monoaminergic circuitry. Containment of the glial HO-1 response to noxious stimuli at strategic points of the life cycle may afford novel opportunities for the effective management of human neurodevelopmental and neurodegenerative conditions.

Unregulated brain iron deposition in transgenic mice over-expressing HMOX1 in the astrocytic compartment.

The mechanisms responsible for pathological iron deposition in the aging and degenerating mammalian CNS remain poorly understood. The stress protein, HO-1 mediates the degradation of cellular heme to biliverdin/bilirubin, free iron, and CO and is up-regulated in the brains of persons with Alzheimer's disease and Parkinson's disease. HO-1 induction in primary astroglial cultures promotes deposition of non-transferrin iron, mitochondrial damage and macroautophagy, and predisposes cocultured neuronal elements to oxidative injury. To gain a better appreciation of the role of glial HO-1 in vivo, we probed for aberrant brain iron deposition using Perls' method and dynamic secondary ion mass spectrometry in novel, conditional GFAP.HMOX1 transgenic mice that selectively over-express human HO-1 in the astrocytic compartment. At 48 weeks, the GFAP.HMOX1 mice exhibited increased deposits of glial iron in hippocampus and other subcortical regions without overt changes in iron-regulatory and iron-binding proteins relative to age-matched wild-type animals. Dynamic secondary ion mass spectrometry revealed abundant FeO⁻ signals in the transgenic, but not wild-type, mouse brain that colocalized to degenerate mitochondria and osmiophilic cytoplasmic inclusions (macroautophagy) documented by TEM. Sustained up-regulation of HO-1 in astrocytes promotes pathological brain iron deposition and oxidative mitochondrial damage characteristic of Alzheimer's disease-affected neural tissues. Curtailment of glial HO-1 hyperactivity may limit iron-mediated cytotoxicity in aging and degenerating neural tissues.

Cystatin B deficiency sensitizes neurons to oxidative stress in progressive myoclonus epilepsy, EPM1.

The progressive myoclonus epilepsies, featuring the triad of myoclonus, seizures, and ataxia, comprise a large group of inherited neurodegenerative diseases that remain poorly understood and refractory to treatment. The Cystatin B gene is mutated in one of the most common forms of progressive myoclonus epilepsy, Unverricht-Lundborg disease (EPM1). Cystatin B knockout in a mouse model of EPM1 triggers progressive degeneration of cerebellar granule neurons. Here, we report impaired redox homeostasis as a key mechanism by which Cystatin B deficiency triggers neurodegeneration. Oxidative stress induces the expression of Cystatin B in cerebellar granule neurons, and EPM1 patient-linked mutation of the Cystatin B gene promoter impairs oxidative stress induction of Cystatin B transcription. Importantly, Cystatin B knockout or knockdown sensitizes cerebellar granule neurons to oxidative stress-induced cell death. The Cystatin B deficiency-induced predisposition to oxidative stress in neurons is mediated by the lysosomal protease Cathepsin B. We uncover evidence of oxidative damage, reflected by depletion of antioxidants and increased lipid peroxidation, in the cerebellum of Cystatin B knock-out mice in vivo. Collectively, our findings define a pathophysiological mechanism in EPM1, whereby Cystatin B deficiency couples oxidative stress to neuronal death and degeneration, and may thus provide the basis for novel treatment approaches for the progressive myoclonus epilepsies.

Heme oxygenase-1 and neurodegeneration: expanding frontiers of engagement.

The heme oxygenases (HOs), responsible for the degradation of heme to biliverdin/bilirubin, free iron and CO, have been heavily implicated in mammalian CNS aging and disease. In normal brain, the expression of HO-2 is constitutive, abundant and fairly ubiquitous, whereas HO-1 mRNA and protein are confined to small populations of scattered neurons and neuroglia. In contradistinction to HO-2, the ho-1 gene (Hmox1) is exquisitely sensitive to induction by a wide range of pro-oxidant and other stressors. In Alzheimer disease and mild cognitive impairment, immunoreactive HO-1 protein is over-expressed in neurons and astrocytes of the cerebral cortex and hippocampus relative to age-matched, cognitively intact controls and co-localizes to senile plaques, neurofibrillary tangles, and corpora amylacea. In Parkinson disease, HO-1 is markedly over-expressed in astrocytes of the substantia nigra and decorates Lewy bodies in affected dopaminergic neurons. HMOX1 is also up-regulated in glial cells surrounding human cerebral infarcts, hemorrhages and contusions, within multiple sclerosis plaques, and in other degenerative and inflammatory human CNS disorders. Heme-derived free ferrous iron, CO, and biliverdin/bilirubin are biologically active substances that have been shown to either ameliorate or exacerbate neural injury contingent upon specific disease models employed, the intensity and duration of HO-1 expression and the nature of the prevailing redox microenvironment. In 'stressed' astroglia, HO-1 hyperactivity promotes mitochondrial sequestration of non-transferrin iron and macroautophagy and may thereby contribute to the pathological iron deposition and bioenergetic failure amply documented in Alzheimer disease, Parkinson disease and other aging-related neurodegenerative disorders. Glial HO-1 expression may also impact cell survival and neuroplasticity in these conditions by modulating brain sterol metabolism and proteosomal degradation of neurotoxic protein aggregates.

HO-1-mediated macroautophagy: a mechanism for unregulated iron deposition in aging and degenerating neural tissues.

Oxidative stress, deposition of non-transferrin iron, and mitochondrial insufficiency occur in the brains of patients with Alzheimer disease (AD) and Parkinson disease (PD). We previously demonstrated that heme oxygenase-1 (HO-1) is up-regulated in AD and PD brain and promotes the accumulation of non-transferrin iron in astroglial mitochondria. Herein, dynamic secondary ion mass spectrometry (SIMS) and other techniques were employed to ascertain (i) the impact of HO-1 over-expression on astroglial mitochondrial morphology in vitro, (ii) the topography of aberrant iron sequestration in astrocytes over-expressing HO-1, and (iii) the role of iron regulatory proteins (IRP) in HO-1-mediated iron deposition. Astroglial hHO-1 over-expression induced cytoplasmic vacuolation, mitochondrial membrane damage, and macroautophagy. HO-1 promoted trapping of redox-active iron and sulfur within many cytopathological profiles without impacting ferroportin, transferrin receptor, ferritin, and IRP2 protein levels or IRP1 activity. Thus, HO-1 activity promotes mitochondrial macroautophagy and sequestration of redox-active iron in astroglia independently of classical iron mobilization pathways. Glial HO-1 may be a rational therapeutic target in AD, PD, and other human CNS conditions characterized by the unregulated deposition of brain iron.

Brain sterol dysregulation in sporadic AD and MCI: relationship to heme oxygenase-1.

The objective of this study was to ascertain the impact of aging and Alzheimer's disease (AD) on brain cholesterol (CH), CH precursors, and oxysterol homeostasis. Altered CH metabolism and up-regulation of heme oxygenase-1 (HO-1) are characteristic of AD-affected neural tissues. We recently determined that HO-1 over-expression suppresses total CH levels by augmenting liver X receptor-mediated CH efflux and enhances oxysterol formation in cultured astroglia. Lipids and proteins were extracted from postmortem human frontal cortex derived from subjects with sporadic AD, mild cognitive impairment (MCI), and no cognitive impairment (n = 17 per group) enrolled in the Religious Orders Study, an ongoing clinical-pathologic study of aging and AD. ELISA was used to quantify human HO-1 protein expression from brain tissue and gas chromatography-mass spectrometry to quantify total CH, CH precursors, and relevant oxysterols. The relationships of sterol/oxysterol levels to HO-1 protein expression and clinical/demographic variables were determined by multivariable regression and non-parametric statistical analyses. Decreased CH, increased oxysterol and increased CH precursors concentrations in the cortex correlated significantly with HO-1 levels in MCI and AD, but not no cognitive impairment. Specific oxysterols correlated with disease state, increasing neuropathological burden, neuropsychological impairment, and age. A model featuring compensated and de-compensated states of altered sterol homeostasis in MCI and AD is presented based on the current data set and our earlier in vitro work.

Parkinsonian features in aging GFAP.HMOX1 transgenic mice overexpressing human HO-1 in the astroglial compartment.

Epigenetic influences mediating brain iron deposition, oxidative mitochondrial injury, and macroautophagy in Parkinson disease and related conditions remain enigmatic. Here, we show that selective overexpression of the stress protein, heme oxygenase-1 (HO-1) in astrocytes of GFAP.HMOX1 transgenic mice between 8.5 and 19 months of age results in nigrostriatal hypodopaminergia associated with locomotor incoordination and stereotypy; downregulation of tyrosine hydroxylase, DAT, LMX1B, Nurr1, Pitx3 and DJ-1 mRNA and/or protein; overproduction of α-synuclein and ubiquitin; oxidative stress; basal ganglia siderosis; mitochondrial damage/mitophagy; and augmented GABAergic systems (increased GABA, GAD67 and reelin). The neurophenotype of these GFAP.HMOX18.5-19m mice is highly consistent with parkinsonism and differs dramatically from the schizophrenia-like features previously documented in younger GFAP.HMOX10-12m mice. Common stressors may elicit either early-onset developmental (schizophrenia) or later-life degenerative (PD) brain disorders depending on whether the glial HO-1 response is engaged prior to or following the maturation of dopaminergic circuitry. Curtailment of glial HO-1 transduction at strategic points of the life course may confer neuroprotection in human degenerative and developmental central nervous system disorders.

Astroglial heme oxygenase-1 and the origin of corpora amylacea in aging and degenerating neural tissues.

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 transfected with flag-tagged human HO-1 cDNA, (ii) brain sections derived from 19month-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), and α-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 the MCI 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.