The role of NADPH oxidase (NOX) enzymes in neurodegenerative disease

Recently, mounting evidence implicating reactive oxygen species (ROS) generated by NADPH oxidase (NOX) enzymes in the pathogenesis of several neurodegenerative diseases including Amyotrophic lateral sclerosis (ALS), Alzheimer’s (AD), Parkinson’s (PD) and polyglutamine disease, have arisen. NOX enzymes are transmembrane proteins and generate reactive oxygen species by transporting electrons across lipid membranes. Under normal healthy conditions, low levels of ROS produced by NOX enzymes have been shown to play a role in neuronal differentiation and synaptic plasticity. However, in chronic neurodegenerative diseases over-activation of NOX in neurons, as well as in astrocytes and microglia, has been linked to pathogenic processes such as oxidative stress, exitotoxicity and neuroinflammation. In this review, we summarize the current knowledge about NOX functions in the healthy central nervous system and especially the role of NOX enzymes in neurodegenerative disease processes.     

Viruses in neurodegenerative diseases: More than just suspects in crimes

Neurodegenerative diseases (NDs) such as Alzheimer’s and Parkinson’s disease are fatal neurological diseases that can be of idiopathic, genetic, or even infectious origin, as in the case of transmissible spongiform encephalopathies. The etiological factors that lead to neurodegeneration remain unknown but likely involve a combination of aging, genetic risk factors, and environmental stressors. Accumulating evidence hints at an association of viruses with neurodegenerative disorders and suggests that virus-induced neuroinflammation and perturbation of neuronal protein quality control can be involved in the early steps of disease development. In this review, we focus on emerging evidence for a correlation between NDs and viral infection and discuss how viral manipulations of cellular processes can affect the formation and dissemination of disease-associated protein aggregates.     

Environmental-induced oxidative stress in neurodegenerative disorders and aging

The aetiology of most neurodegenerative disorders is multifactorial and consists of an interaction between environmental factors and genetic predisposition. Free radicals derived primarily from molecular oxygen have been implicated and considered as associated risk factors for a variety of human disorders including neurodegenerative diseases and aging. Damage to tissue biomolecules, including lipids, proteins and DNA, by free radicals is postulated to contribute importantly to the pathophysiology of oxidative stress. The potential of environmental exposure to metals, air pollution and pesticides as well as diet as risk factors via the induction of oxidative stress for neurodegenerative diseases and aging is discussed. The role of genetic background is discussed on the light of the oxidative stress implication, focusing on both complex neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis) and monogenic neurological disorders (Huntington's disease, Ataxia telangiectasia, Friedreich Ataxia and others). Emphasis is given to role of the repair mechanisms of oxidative DNA damage in delaying aging and protecting against neurodegeneration. The emerging interplay between environmental-induced oxidative stress and epigenetic modifications of critical genes for neurodegeneration is also discussed.     

Mitochondrial dysfunction: A potential link between neuroinflammation and neurodegeneration?

Dysfunctional mitochondria are thought to play a cardinal role in the pathogenesis of various neurological disorders, such as multiple sclerosis, Alzheimer’s disease, Parkinson’s disease and stroke. In addition, neuroinflammation is a common denominator of these diseases. Both mitochondrial dysfunction and neuroinflammatory processes lead to increased production of reactive oxygen species (ROS) which are detrimental to neurons. Therefore, neuroinflammation is increasingly recognized to contribute to processes underlying neurodegeneration. Here we describe the involvement of mitochondrial (dys)function in various neurological disorders and discuss the putative link between mitochondrial function and neuroinflammation.     

Neurodegenerative diseases and therapeutic strategies using iron chelators

This review will summarise the current state of our knowledge concerning the involvement of iron in various neurological diseases and the potential of therapy with iron chelators to retard the progression of the disease. We first discuss briefly the role of metal ions in brain function before outlining the way by which transition metal ions, such as iron and copper, can initiate neurodegeneration through the generation of reactive oxygen and nitrogen species. This results in protein misfolding, amyloid production and formation of insoluble protein aggregates which are contained within inclusion bodies. This will activate microglia leading to neuroinflammation. Neuroinflammation plays an important role in the progression of the neurodegenerative diseases, with activated microglia releasing pro-inflammatory cytokines leading to cellular cell loss. The evidence for metal involvement in Parkinson's and Alzheimer's disease as well as Friedreich's ataxia and multiple sclerosis will be presented. Preliminary results from trials of iron chelation therapy in these neurodegenerative diseases will be reviewed.     

Discovery of 1,3,4-oxidiazole scaffold compounds as inhibitors of superoxide dismutase expression

The treatment of neurodegenerative diseases is difficult because of multiple etiologies and the interplay of genetics and environment as precipitating factors. In the case of amyotrophic lateral sclerosis (ALS), we have knowledge of a handful of genes that cause disease when mutated. However, drugs to counteract the effect of genetic mutations have not yet been found. One of the causative genes, Cu, Zn-superoxide dismutase (SOD1) is responsible for about 10–15% of the genetically linked autosomal dominant disease. Our rationale was that compounds that reduce expression of the mutant protein would be beneficial to slow onset and/or disease progression. We screened candidate compounds using a cell-based in vitro assay for those that reduce mutant SOD1 (G93A) protein expression. This led to the discovery of 2-[3-iodophenyl)methylsulfanyl]-5pyridin-4-yl-1,3,4-oxadiazole, a known protein kinase inhibitor that decreases G93A-SOD1 expression in vitro and in the brain and spinal cord in vivo. However, this compound has a biphasic dose response curve and a likely toxophore which limit its therapeutic window for chronic disease such as ALS. Therefore, we designed and tested a focused library of analogs for their ability to decrease SOD1 expression in vitro. This exercise resulted in the identification of a lead compound with improved drug-like characteristics and activity. Development of small molecules that reduce the expression of etiologically relevant toxic proteins is a strategy that may also be extended to familial ALS linked to gain of function mutations in other genes.     

Molecular mediators, environmental modulators and experience-dependent synaptic dysfunction in Huntington's disease

Huntington's disease (HD) is an autosomal dominant disorder in which there is progressive neurodegeneration producing motor, cognitive and psychiatric symptoms. HD is caused by a trinucleotide (CAG) repeat mutation, encoding an expanded polyglutamine tract in the huntingtin protein. At least eight other neurodegenerative diseases are caused by CAG/glutamine repeat expansions in different genes. Recent evidence suggests that environmental factors can modify the onset and progression of Huntington's disease and possibly other neurodegenerative disorders. This review outlines possible molecular and cellular mechanisms mediating the polyglutamine-induced toxic 'gain of function' and associated gene-environment interactions in HD. Key aspects of pathogenesis shared with other neurodegenerative diseases may include abnormal protein-protein interactions, selective disruption of gene expression and 'pathological plasticity' of synapses in specific brain regions. Recent discoveries regarding molecular mechanisms of pathogenesis are guiding the development of new therapeutic approaches. Knowledge of gene-environment interactions, for example, could lead to development of 'enviromimetics' which mimic the beneficial effects of specific environmental stimuli. The effects of environmental enrichment on brain and behaviour will also be discussed, together with the general implications for neuroscience research involving animal models.     

Risk conferring genes in multiple sclerosis

Multiple sclerosis (MS) is characterized by inflammation, axonal and oligodendrocyte pathology and progressive neurological disability. Epidemiologic data indicate that MS may be caused by interplay of genetic and environmental factors. Large samples collected in cooperative efforts and new technologies such as high throughput single nucleotide polymorphism (SNP) genotyping allowed recently to discover non-HLA genes associated with MS susceptibility that are mostly involved in the immune response. In addition, several studies indicate an effect of genetic variations on disease onset, progression and response to therapy. However, the polymorphisms discovered so far explain the genetic variation in MS only in part and are mostly common variants that have only low impact on MS susceptibility. Functional studies are required to validate the importance of the newly identified SNPs. Taking into account the interplay of genetic and environmental factors a combination of genome wide genotyping including HLA-typing and genome wide expression profiling as well as a collection on relevant or putatively relevant environmental factors in patients well characterized clinically and by MRI is a promising way to identify new disease relevant biomarkers.     

Shared mechanisms among neurodegenerative diseases: from genetic factors to gene networks

Neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis are pressing health concerns in modern societies for which effective therapies are still lacking. Recent high-throughput genomic technologies have enabled genome-scale, multidimensional investigations to facilitate a better understanding of the underlying mechanisms and the identification of novel targets. Here we review the molecular insights gained through such studies, and compare the similarities and differences between neurodegenerative diseases revealed by systems genomics and gene network modelling approaches. We focus specifically on the shared mechanisms at multiple molecular scales ranging from genetic factors to gene expression to network-level features of gene regulation, and whenever possible also point out mechanisms that distinguish one disease from another. Our review sets the stage for similar genomewide inspection in the future on shared/distinct features of neurodegenerative diseases at the levels of cellular, proteomic or epigenomic signatures, and how these features may interact to determine the progression and treatment response of different diseases afflicting the same individual.     

NO Synthase and NO-Dependent Signal Pathways in Brain Aging and Neurodegenerative Disorders: The Role of Oxidant/Antioxidant Balance

Nitric oxide and other reactive nitrogen species appear to play several crucial roles in the brain. These include physiological processes such as neuromodulation, neurotransmission and synaptic plasticity, and pathological processes such as neurodegeneration and neuroinflammation. There is increasing evidence that glial cells in the central nervous system can produce nitric oxide in vivo in response to stimulation by cytokines and that this production is mediated by the inducible isoform of nitric oxide synthase. Although the etiology and pathogenesis of the major neurodegenerative and neuroinflammatory disorders (Alzheimer's disease, amyothrophic lateral sclerosis, Parkinson's disease, Huntington's disease and multiple sclerosis) are unknown, numerous recent studies strongly suggest that reactive nitrogen species play an important role. Furthermore, these species are probably involved in brain damage following ischemia and reperfusion, Down's syndrome and mitochondrial encephalopathies. Recent evidence also indicates the importance of cytoprotective proteins such as heat shock proteins (HSPs) which appear to be critically involved in protection from nitrosative and oxidative stress. In this review, evidence for the involvement of nitrosative stress in the pathogenesis of the major neurodegenerative/ neuroinflammatory diseases and the mechanisms operating in brain as a response to imbalance in the oxidant/antioxidant status are discussed.     

Extracellular αB-crystallin modulates the inflammatory responses

Neuroinflammation is considered a challenging clinical problem. Chronic inflammatory responses play important roles in the onset and progression of various neurodegenerative diseases, including multiple sclerosis (MS). Previous studies have shown that astrocytes express small heat shock protein αB-crystallin (CRYAB) which is capable of inhibiting inflammatory responses in astrocytes per se. However, the underlying mechanisms of CRYAB-induced modulation of neuroinflammation are still not fully understood. In the present study, we investigated the role of extracellular CRYAB in the interaction between microglia and astrocytes in the context of MS-associated neuroinflammation. We found that the expression of CRYAB was profoundly increased in EAE mice. CRYAB was preferentially expressed in astrocytes and could be secreted via exosomes. Levels of exosomal CRYAB secreted from astrocytes were markedly increased under stress conditions. Furthermore, incubation of immortalized astrocytes or microglia cell lines with CRYAB remarkably suppressed astrocytes and microglia-mediated inflammatory responses in both autocrine and paracrine manners. Our results reveal a novel function for extracellular CRYAB in the regulation of neuroinflammation. Targeting extracellular CRYAB-modulated neuroinflammation is a potential therapeutic intervention for MS.     

HMGB1 in Development and Diseases of the Central Nervous System

High mobility group box 1 (HMGB1) is widely expressed in cells of vertebrates in two forms: a nuclear "architectural" factor and a secreted inflammatory factor. During early brain development, HMGB1 displays a complex temporal and spatial distribution pattern in the central nervous system. It facilitates neurite outgrowth and cell migration critical for processes, such as forebrain development. During adulthood, HMGB1 serves to induce neuroinflammation after injury, such as lesions in the spinal cord and brain. Receptor for advanced glycation end products and Toll-like receptors signal transduction pathways mediate HMGB1-induced neuroinflammation and necrosis. Increased levels of endogenous HMGB1 have also been detected in neurodegenerative diseases. However, in Huntington's disease, HMGB1 has been reported to protect neurons through activation of apurinic/apyrimidinic endonuclease and 5'-flap endonuclease-1, whereas in other neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and multiple sclerosis, HMGB1 serves as a risk factor for memory impairment, chronic neurodegeneration, and progression of neuroinflammation. Thus, HMGB1 plays important and double-edged roles during neural development and neurodegeneration. The HMGB1-mediated pathological mechanisms have remained largely elusive. Knowledge of these mechanisms is likely to lead to therapeutic targets for neurological diseases.     

Identifying drug targets for neurological and psychiatric disease via genetics and the brain transcriptome

Discovering drugs that efficiently treat brain diseases has been challenging. Genetic variants that modulate the expression of potential drug targets can be utilized to assess the efficacy of therapeutic interventions. We therefore employed Mendelian Randomization (MR) on gene expression measured in brain tissue to identify drug targets involved in neurological and psychiatric diseases. We conducted a two-sample MR using cis-acting brain-derived expression quantitative trait loci (eQTLs) from the Accelerating Medicines Partnership for Alzheimer’s Disease consortium (AMP-AD) and the CommonMind Consortium (CMC) meta-analysis study (n = 1,286) as genetic instruments to predict the effects of 7,137 genes on 12 neurological and psychiatric disorders. We conducted Bayesian colocalization analysis on the top MR findings (using P<6x10^-7 as evidence threshold, Bonferroni-corrected for 80,557 MR tests) to confirm sharing of the same causal variants between gene expression and trait in each genomic region. We then intersected the colocalized genes with known monogenic disease genes recorded in Online Mendelian Inheritance in Man (OMIM) and with genes annotated as drug targets in the Open Targets platform to identify promising drug targets. 80 eQTLs showed MR evidence of a causal effect, from which we prioritised 47 genes based on colocalization with the trait. We causally linked the expression of 23 genes with schizophrenia and a single gene each with anorexia, bipolar disorder and major depressive disorder within the psychiatric diseases and 9 genes with Alzheimer’s disease, 6 genes with Parkinson’s disease, 4 genes with multiple sclerosis and two genes with amyotrophic lateral sclerosis within the neurological diseases we tested. From these we identified five genes (ACE, GPNMB, KCNQ5, RERE and SUOX) as attractive drug targets that may warrant follow-up in functional studies and clinical trials, demonstrating the value of this study design for discovering drug targets in neuropsychiatric diseases.     

Neuroprotection by Spice-Derived Nutraceuticals: You Are What You Eat!

Numerous lines of evidence indicate that chronic inflammation plays a major role in the development of various neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, brain tumor, and meningitis. Why these diseases are more common among people from some countries than others is not fully understood, but lifestyle factors have been linked to the development of neurodegenerative diseases. For example, the incidence of certain neurodegenerative diseases among people living in the Asian subcontinent, where people regularly consume spices, is much lower than in countries of the western world. Extensive research over the last 10 years has indicated that nutraceuticals derived from such spices as turmeric, red pepper, black pepper, licorice, clove, ginger, garlic, coriander, and cinnamon target inflammatory pathways, thereby may prevent neurodegenerative diseases. How these nutraceuticals modulate various pathways and how they exert neuroprotection are the focus of this review.     

Apolipoprotein E Is a Ligand for Triggering Receptor Expressed on Myeloid Cells 2 (TREM2)

Several heterozygous missense mutations in the triggering receptor expressed on myeloid cells 2 (TREM2) have recently been linked to risk for a number of neurological disorders including Alzheimer disease (AD), Parkinson disease, and frontotemporal dementia. These discoveries have re-ignited interest in the role of neuroinflammation in the pathogenesis of neurodegenerative diseases. TREM2 is highly expressed in microglia, the resident immune cells of the central nervous system. Along with its adaptor protein, DAP12, TREM2 regulates inflammatory cytokine release and phagocytosis of apoptotic neurons. Here, we report apolipoprotein E (apoE) as a novel ligand for TREM2. Using a biochemical assay, we demonstrated high-affinity binding of apoE to human TREM2. The functional significance of this binding was highlighted by increased phagocytosis of apoE-bound apoptotic N2a cells by primary microglia in a manner that depends on TREM2 expression. Moreover, when the AD-associated TREM2-R47H mutant was used in biochemical assays, apoE binding was vastly reduced. Our data demonstrate that apoE-TREM2 interaction in microglia plays critical roles in modulating phagocytosis of apoE-bound apoptotic neurons and establish a critical link between two proteins whose genes are strongly linked to the risk for AD.     

Proteomic analysis of mitochondrial dysfunction in neurodegenerative diseases

Alzheimer’s, Parkinson’s and Huntington’s disease, and amyotrophic lateral sclerosis are the most relevant neurodegenerative syndromes worldwide. The identification of the etiology and additional factors contributing to the onset and progression of these diseases is of great importance in order to develop both preventive and therapeutic intervention. A common feature of these pathologies is the formation of aggregates, containing mutated and/or misfolded proteins, in specific subsets of neurons, which progressively undergo functional impairment and die. The relationship between protein aggregation and the molecular events leading to neurodegeneration has not yet been clarified. In the last decade, several lines of evidence pointed to a major role for mitochondrial dysfunction in the onset of these pathologies. Here, we review how proteomics has been applied to neurodegenerative diseases in order to characterize the relationship existing between protein aggregation and mitochondrial alterations. Moreover, we highlight recent advances in the use of proteomics to identify protein modifications caused by oxidative stress. Future developments in this field are expected to significantly contribute to the full comprehension of the molecular mechanisms at the heart of neurodegeneration.