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.

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.

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.