Hyman M. Schipper

Oxidative stress and aberrant signaling in aging and cognitive decline.

Brain aging is associated with a progressive imbalance between antioxidant defenses and intracellular concentrations of reactive oxygen species (ROS) as exemplified by increases in products of lipid peroxidation, protein oxidation, and DNA oxidation. Oxidative conditions cause not only structural damage but also changes in the set points of redox‐sensitive signaling processes including the insulin receptor signaling pathway. In the absence of insulin, the otherwise low insulin receptor signaling is strongly enhanced by oxidative conditions. Autophagic proteolysis and sirtuin activity, in turn, are downregulated by the insulin signaling pathway, and impaired autophagic activity has been associated with neurodegeneration. In genetic studies, impairment of insulin receptor signaling causes spectacular lifespan extension in nematodes, fruit flies, and mice. The predicted effects of age‐related oxidative stress on sirtuins and autophagic activity and the corresponding effects of antioxidants remain to be tested experimentally. However, several correlates of aging have been shown to be ameliorated by antioxidants. Oxidative damage to mitochondrial DNA and the electron transport chain, perturbations in brain iron and calcium homeostasis, and changes in plasma cysteine homeostasis may altogether represent causes and consequences of increased oxidative stress. Aging and cognitive decline thus appear to involve changes at multiple nodes within a complex regulatory network.

Diagnosis and treatment of dementia: 2. Diagnosis

Background: Dementia can now be accurately diagnosed through clinical evaluation, cognitive screening, basic laboratory evaluation and structural imaging. A large number of ancillary techniques are also available to aid in diagnosis, but their role in the armamentarium of family physicians remains controversial. In this article, we provide physicians with practical guidance on the diagnosis of dementia based on recommendations from the Third Canadian Consensus Conference on the Diagnosis and Treatment of Dementia, held in March 2006. Methods: We developed evidence-based guidelines using systematic literature searches, with specific criteria for study selection and quality assessment, and a clear and transparent decision-making process. We selected studies published from January 1996 to December 2005 that pertained to key diagnostic issues in dementia. We graded the strength of evidence using the criteria of the Canadian Task Force on Preventive Health Care. Results: Of the 1591 articles we identified on all aspects of dementia diagnosis, 1095 met our inclusion criteria; 620 were deemed to be of good or fair quality. From a synthesis of the evidence in these studies, we made 32 recommendations related to the diagnosis of dementia. There are clinical criteria for diagnosing most forms of dementia. A standard diagnostic evaluation can be performd by family physicians over multiple visits. It involves a clinical history (from patient and caregiver), a physical examination and brief cognitive testing. A list of core laboratory tests is recommended. Structural imaging with computed tomography or magnetic resonance imaging is recommended in selected cases to rule out treatable causes of dementia or to rule in cerebrovascular disease. There is insufficient evidence to recommend routine functional imaging, measurement of biomarkers or neuropsychologic testing. Interpretation: The diagnosis of dementia remains clinically integrative based on history, physical examination and brief cognitive testing. A number of core laboratory tests are also recommended. Structural neuroimaging is advised in selected cases. Other diagnostic approaches, including functional neuroimaging, neuropsychological testing and measurement of biomarkers, have shown promise but are not yet recommended for routine use by family physicians.

Neural Heme Oxygenase-1 Expression in Idiopathic Parkinson's Disease☆

Heme oxygenase-1 is a cellular stress protein expressed in brain and other tissues in response to oxidative challenge and other noxious stimuli. In the present study, immunohistochemistry was used to assess HO-1 expression in various postmortem human brain specimens derived from PD and control subjects. In the substantia nigra of both PD and control specimens, moderate HO-1 immunoreactivity was consistently observed in neuromelanin-containing (dopaminergic) neurons. Lewy bodies in PD nigra neurons exhibited intense HO-1 immunostaining in their peripheries. In both PD and control specimens, neuronal HO-1 staining was faint or nondetectable in the other brain regions surveyed. The fraction of GFAP-positive astroglia expressing HO-1 in PD substantia nigra (77.1 +/- 12.3) was significantly greater than that observed in the substantia nigra of control subjects (18.7 +/- 7.1; P = 0.0015). In the other regions examined, percentages of GFAP-positive astroglia coexpressing HO-1 were relatively low and did not differ significantly (P > 0.05) between control and PD specimens. Upregulation of HO-1 in the substantia nigra of PD subjects supports the view that the affected tissue is experiencing chronic oxidative stress. In addition, excessive cellular levels of heme-derived free iron and carbon monoxide resulting from HO-1 overactivity may contribute to the pathogenesis of PD.

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.

Transcriptional profiling of Alzheimer blood mononuclear cells by microarray.

We evaluated pathomechanisms and systemic manifestations of Alzheimer disease (AD), an aging-related dementing neurodegenerative disorder, by expression profiling. Blood mononuclear cell (BMC) transcriptomes of sporadic AD subjects and aged-matched normal elderly controls (NEC) were compared using the human NIA microarray. Relative to the NEC samples, the Alzheimer BMC exhibited a significant decline in the expression of genes concerned with cytoskeletal maintenance, cellular trafficking, cellular stress response, redox homeostasis, transcription and DNA repair. We observed decreased expression of several genes which may impact amyloid-beta production and the processing of the microtubule-associated protein tau. The microarray results were validated by quantitative real time PCR and revealed gender differences in the levels of altered gene expression. Our findings attest to the systemic nature of gene dys-regulation in sporadic AD, implicate disruption of cytoskeletal integrity, DNA repair mechanisms and cellular defenses in this condition, and suggest novel pathways of beta-amyloid deposition in this disease. BMC are highly accessible and may reflect molecular events germane to the neuropathophysiology of AD.

MicroRNA: Implications for Alzheimer Disease and other Human CNS Disorders

Understanding complex diseases such as sporadic Alzheimer disease (AD) has been a major challenge. Unlike the familial forms of AD, the genetic and environmental risks factors identified for sporadic AD are extensive. MicroRNAs are one of the major noncoding RNAs that function as negative regulators to silence or suppress gene expression via translational inhibition or message degradation. Their discovery has evoked great excitement in biomedical research for their promise as potential disease biomarkers and therapeutic targets. Key microRNAs have been identified as essential for a variety of cellular events including cell lineage determination, proliferation, apoptosis, DNA repair, and cytoskeletal organization; most, if not all, acting to fine-tune gene expression at the post-transcriptional level in a host of cellular signaling networks. Dysfunctional microRNA-mediated regulation has been implicated in the pathogenesis of many disease states. Here, the current understanding of the role of miRNAs in the central nervous system is reviewed with emphasis on their impact on the etiopathogenesis of sporadic AD.

Oxysterols, cholesterol homeostasis, and Alzheimer disease

Aberrant cholesterol metabolism has been implicated in Alzheimer disease (AD) and other neurological disorders. Oxysterols and other cholesterol oxidation products are effective ligands of liver X activated receptor (LXR) nuclear receptors, major regulators of genes subserving cholesterol homeostasis. LXR receptors act as molecular sensors of cellular cholesterol concentrations and effectors of tissue cholesterol reduction. Following their interaction with oxysterols, activation of LXRs induces the expression of ATP‐binding cassette, sub‐family A member 1, a pivotal modulator of cholesterol efflux. The relative solubility of oxysterols facilitates lipid flux among brain compartments and egress across the blood‐brain barrier. Oxysterol‐mediated LXR activation induces local apoE biosynthesis (predominantly in astrocytes) further enhancing cholesterol re‐distribution and removal. Activated LXRs invoke additional neuroprotective mechanisms, including induction of genes governing bile acid synthesis (sterol elimination pathway), apolipoprotein elaboration, and amyloid precursor protein processing. The latter translates into attenuated β‐amyloid production that may ameliorate amyloidogenic neurotoxicity in AD brain. Stress‐induced up‐regulation of the heme‐degrading enzyme, heme oxygenase‐1 in AD‐affected astroglia may impact central lipid homeostasis by promoting the oxidation of cholesterol to a host of oxysterol intermediates. Synthetic oxysterol‐mimetic drugs that activate LXR receptors within the CNS may provide novel therapeutics for management of AD and other neurological afflictions characterized by deranged tissue cholesterol homeostasis.

Glial heme oxygenase-1 expression in Alzheimer disease and mild cognitive impairment.

We determined whether oxidative stress is an early event in the pathogenesis of sporadic Alzheimer disease (AD), and correlated oxidative stress with neuropsychological functions and neurofibrillary pathology in AD and mild cognitive impairment (MCI). Oxidative stress was measured as the percentage of astrocytes expressing heme oxygenase-1 (HO-1) in post mortem temporal cortex and hippocampus after dual HO-1/glial fibrillary acidic protein (GFAP) immunohistochemistry. Glial HO-1 expression in the MCI temporal cortex and hippocampus was significantly greater than in the non-demented group and did not differ from AD values. Astroglial HO-1 expression in the temporal cortex was associated with decreased scores for global cognition, episodic memory, semantic memory and working memory. Hippocampal astroglial HO-1 expression was associated with lower scores for global cognition, semantic memory and perceptual speed. Glial HO-1 immunoreactivity in the temporal cortex, but not hippocampus, correlated with the burden of neurofibrillary pathology. Cortical and hippocampal oxidative stress is a very early event in the pathogenesis of sporadic AD and correlates with the development of specific cognitive deficits in this condition.

Proinflammatory cytokines promote glial heme oxygenase-1 expression and mitochondrial iron deposition: implications for multiple sclerosis.

Proinflammatory cytokines, pathological iron deposition, and oxidative stress have been implicated in the pathogenesis of multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE). HO‐1 mRNA levels and mitochondrial uptake of [55Fe]Cl3‐derived iron were measured in rat astroglial cultures exposed to interleukin‐1β (IL‐1β) or tumor necrosis factor‐α (TNF‐α) alone or in combination with the heme oxygenase‐1 (HO‐1) inhibitors, tin mesoporphyrin (SnMP) or dexamthasone (DEX), or interferon β1b (INF‐β). HO‐1 expression in astrocytes was evaluated by immunohistochemical staining of spinal cord tissue derived from MS and control subjects. IL‐1β or TNF‐α promoted sequestration of non‐transferrin‐derived 55Fe by astroglial mitochondria. HO‐1 inhibitors, mitochondrial permeability transition pore (MTP) blockers and antioxidants significantly attenuated cytokine‐related mitochondrial iron sequestration in these cells. IFN‐β decreased HO‐1 expression and mitochondrial iron sequestration in IL‐1β‐ and TNF‐α‐challenged astroglia. The percentage of astrocytes coexpressing HO‐1 in affected spinal cord from MS patients (57.3% ± 12.8%) was significantly greater (p < 0.05) than in normal spinal cord derived from controls subjects (15.4% ± 8.4%). HO‐1 is over‐expressed in MS spinal cord astroglia and may promote mitochondrial iron deposition in MS plaques. In MS, IFN‐β may attenuate glial HO‐1 gene induction and aberrant mitochondrial iron deposition accruing from exposure to proinflammatory cytokines.

Heme Oxygenase-1: Transducer of Pathological Brain Iron Sequestration under Oxidative Stress

Abstract: Mechanisms responsible for the pathological deposition of redox‐active brain iron in human neurological disorders remain incompletely understood. Heme oxygenase‐1 (HO‐1) is a 32‐kDa stress protein that degrades heme to biliverdin, free iron, and carbon monoxide. In this chapter, we review evidence that (1) HO‐1 is overexpressed in CNS tissues affected by Alzheimers disease (AD), Parkinsons disease (PD), multiple sclerosis (MS), and other degenerative and nondegenerative CNS diseases; (2) the pro‐oxidant effects of dopamine, hydrogen peroxide, b‐amyloid, and proinflammatory cytokines stimulate HO‐1 expression in some of these conditions; and (3) upregulation of HO‐1 in astrocytes exacerbates intracellular oxidative stress and promotes sequestration of nontransferrin‐derived iron by the mitochondrial compartment. A model is presented implicating glial HO‐1 induction as a “final common pathway” leading to pathological iron sequestration and mitochondrial insufficiency in a host of human CNS disorders.