Inhibition of defective adenylosuccinate lyase by HNE: a neurological disease that may be affected by oxidative stress

Intraneuronal accumulation of amyloid-β peptides as the pathomechanism linking autism and its co-morbidities: epilepsy and self-injurious behavior - the hypothesis

Autism spectrum disorder (ASD) is associated with enhanced processing of amyloid-β precursor protein (APP) by secretase-α, higher blood levels of sAPPα and intraneuronal accumulation of N-terminally truncated Aβ peptides in the brain cortex - mainly in the GABAergic neurons expressing parvalbumin - and subcortical structures. Brain Aβ accumulation has been also described in epilepsy-the frequent ASD co-morbidity. Furthermore, Aβ peptides have been shown to induce electroconvulsive episodes. Enhanced production and altered processing of APP, as well as accumulation of Aβ in the brain are also frequent consequences of traumatic brain injuries which result from self-injurious behaviors, another ASD co-morbidity. We discuss distinct consequences of accumulation of Aβ in the neurons and synapses depending on the Aβ species, their posttranslational modifications, concentration, level of aggregation and oligomerization, as well as brain structures, cell types and subcellular structures where it occurs. The biological effects of Aβ species which are discussed in the context of the pathomechanisms of ASD, epilepsy, and self-injurious behavior include modulation of transcription-both activation and repression; induction of oxidative stress; activation and alteration of membrane receptors' signaling; formation of calcium channels causing hyper-activation of neurons; reduction of GABAergic signaling - all of which lead to disruption of functions of synapses and neuronal networks. We conclude that ASD, epilepsy, and self-injurious behaviors all contribute to the enhanced production and accumulation of Aβ peptides which in turn cause and enhance dysfunctions of the neuronal networks that manifest as autism clinical symptoms, epilepsy, and self-injurious behaviors.

The Role of Autophagy, Mitophagy and Lysosomal Functions in Modulating Bioenergetics and Survival in the Context of Redox and Proteotoxic Damage: Implications for Neurodegenerative Diseases

Redox and proteotoxic stress contributes to age-dependent accumulation of dysfunctional mitochondria and protein aggregates, and is associated with neurodegeneration. The free radical theory of aging inspired many studies using reactive species scavengers such as alpha-tocopherol, ascorbate and coenzyme Q to suppress the initiation of oxidative stress. However, clinical trials have had limited success in the treatment of neurodegenerative diseases. We ascribe this to the emerging literature which suggests that the oxidative stress hypothesis does not encompass the role of reactive species in cell signaling and therefore the interception with reactive species with antioxidant supplementation may result in disruption of redox signaling. In addition, the accumulation of redox modified proteins or organelles cannot be reversed by oxidant intercepting antioxidants and must then be removed by alternative mechanisms. We have proposed that autophagy serves this essential function in removing damaged or dysfunctional proteins and organelles thus preserving neuronal function and survival. In this review, we will highlight observations regarding the impact of autophagy regulation on cellular bioenergetics and survival in response to reactive species or reactive species generating compounds, and in response to proteotoxic stress.

The link between intraneuronal N-truncated amyloid-β peptide and oxidatively modified lipids in idiopathic autism and dup(15q11.2-q13)/autism

BACKGROUND

Autism is a neurodevelopmental disorder of unknown etiopathogenesis associated with structural and functional abnormalities of neurons and increased formation of reactive oxygen species. Our previous study revealed enhanced accumulation of amino-terminally truncated amyloid-β (Aβ) in brain neurons and glia in children and adults with autism. Verification of the hypothesis that intraneuronal Aβ may cause oxidative stress was the aim of this study.

RESULTS

The relationships between neuronal Aβ and oxidative stress markers-4-hydroxy-2-nonenal (HNE) and malondialdehyde (MDA)-were examined in the frontal cortex from individuals aged 7-32 years with idiopathic autism or with chromosome 15q11.2-q13 duplications (dup(15)) with autism, and age-matched controls. Quantification of confocal microscopy images revealed significantly higher levels of neuronal N-truncated Aβ and HNE and MDA in idiopathic autism and dup(15)/autism than in controls. Lipid peroxidation products were detected in all mitochondria and lipofuscin deposits, in numerous autophagic vacuoles and lysosomes, and in less than 5% of synapses. Neuronal Aβ was co-localized with HNE and MDA, and increased Aβ levels correlated with higher levels of HNE and MDA.

CONCLUSIONS

The results suggest a self-enhancing pathological process in autism that is initiated by intraneuronal deposition of N-truncated Aβ in childhood. The cascade of events includes altered APP metabolism and abnormal intracellular accumulation of N-terminally truncated Aβ which is a source of reactive oxygen species, which in turn increase the formation of lipid peroxidation products. The latter enhance Aβ deposition and sustain the cascade of changes contributing to metabolic and functional impairments of neurons in autism of an unknown etiology and caused by chromosome 15q11.2-q13 duplication.

Defective autophagy in Parkinson's disease: role of oxidative stress

Parkinson's disease (PD) is a paradigmatic example of neurodegenerative disorder with a critical role of oxidative stress in its etiopathogenesis. Genetic susceptibility factors of PD, such as mutations in Parkin, PTEN-induced kinase 1, and DJ-1 as well as the exposure to pesticides and heavy metals, both contribute to altered redox balance and degeneration of dopaminergic neurons in the substantia nigra. Dysregulation of autophagy, a lysosomal-driven process of self degradation of cellular organelles and protein aggregates, is also implicated in PD and PD-related mutations, and environmental toxins deregulate autophagy. However, experimental evidence suggests a complex and ambiguous role of autophagy in PD since either impaired or abnormally upregulated autophagic flux has been shown to cause neuronal loss. Finally, it is generally believed that oxidative stress is a strong proautophagic stimulus. However, some evidence coming from neurobiology as well as from other fields indicate an inhibitory role of reactive oxygen species and reactive nitrogen species on the autophagic machinery. This review examines the scientific evidence supporting different concepts on how autophagy is dysregulated in PD and attempts to reconcile apparently contradictory views on the role of oxidative stress in autophagy regulation. The complex relationship between autophagy and oxidative stress is also considered in the context of the ongoing search for a novel PD therapy.

Autophagy, mitochondria and oxidative stress: cross-talk and redox signalling

Reactive oxygen and nitrogen species change cellular responses through diverse mechanisms that are now being defined. At low levels, they are signalling molecules, and at high levels, they damage organelles, particularly the mitochondria. Oxidative damage and the associated mitochondrial dysfunction may result in energy depletion, accumulation of cytotoxic mediators and cell death. Understanding the interface between stress adaptation and cell death then is important for understanding redox biology and disease pathogenesis. Recent studies have found that one major sensor of redox signalling at this switch in cellular responses is autophagy. Autophagic activities are mediated by a complex molecular machinery including more than 30 Atg (AuTophaGy-related) proteins and 50 lysosomal hydrolases. Autophagosomes form membrane structures, sequester damaged, oxidized or dysfunctional intracellular components and organelles, and direct them to the lysosomes for degradation. This autophagic process is the sole known mechanism for mitochondrial turnover. It has been speculated that dysfunction of autophagy may result in abnormal mitochondrial function and oxidative or nitrative stress. Emerging investigations have provided new understanding of how autophagy of mitochondria (also known as mitophagy) is controlled, and the impact of autophagic dysfunction on cellular oxidative stress. The present review highlights recent studies on redox signalling in the regulation of autophagy, in the context of the basic mechanisms of mitophagy. Furthermore, we discuss the impact of autophagy on mitochondrial function and accumulation of reactive species. This is particularly relevant to degenerative diseases in which oxidative stress occurs over time, and dysfunction in both the mitochondrial and autophagic pathways play a role.

Pediatric neurological syndromes and inborn errors of purine metabolism

This review is devised to gather the presently known inborn errors of purine metabolism that manifest neurological pediatric syndromes. The aim is to draw a comprehensive picture of these rare diseases, characterized by unexpected and often devastating neurological symptoms. Although investigated for many years, most purine metabolism disorders associated to psychomotor dysfunctions still hide the molecular link between the metabolic derangement and the neurological manifestations. This basically indicates that many of the actual functions of nucleosides and nucleotides in the development and function of several organs, in particular central nervous system, are still unknown. Both superactivity and deficiency of phosphoribosylpyrophosphate synthetase cause hereditary disorders characterized, in most cases, by neurological impairments. The deficiency of adenylosuccinate lyase and 5-amino-4-imidazolecarboxamide ribotide transformylase/IMP cyclohydrolase, both belonging to the de novo purine synthesis pathway, is also associated to severe neurological manifestations. Among catabolic enzymes, hyperactivity of ectosolic 5'-nucleotidase, as well as deficiency of purine nucleoside phosphorylase and adenosine deaminase also lead to syndromes affecting the central nervous system. The most severe pathologies are associated to the deficiency of the salvage pathway enzymes hypoxanthine-guanine phosphoribosyltransferase and deoxyguanosine kinase: the former due to an unexplained adverse effect exerted on the development and/or differentiation of dopaminergic neurons, the latter due to a clear impairment of mitochondrial functions. The assessment of hypo- or hyperuricemic conditions is suggestive of purine enzyme dysfunctions, but most disorders of purine metabolism may escape the clinical investigation because they are not associated to these metabolic derangements. This review may represent a starting point stimulating both scientists and physicians involved in the study of neurological dysfunctions caused by inborn errors of purine metabolism with the aim to find novel therapeutical approaches.