Approaches to prevent dopamine quinone-induced neurotoxicity

Vulnerability of mesostriatal dopaminergic neurons in Parkinson's disease

The term vulnerability was first associated with the midbrain dopaminergic neurons 85 years ago, before they were identified as monoaminergic neurons, when Foix and Nicolesco (1925) reported the loss of neuromelanin containing neurons in the midbrain of patients with post-encephalitic Parkinson's disease (PD). A few years later, Hassler (1938) showed that degeneration is more intense in the ventral tier of the substantia nigra compacta than in its dorsal tier and the ventral tegmental area (VTA), outlining the concept of differential vulnerability of midbrain dopaminergic (DA-) neurons. Nowadays, we know that other neuronal groups degenerate in PD, but the massive loss of nigral DA-cells is its pathological hallmark, having a pivotal position in the pathophysiology of the disease as it is responsible for the motor symptoms. Data from humans as well as cellular and animal models indicate that DA-cell degeneration is a complex process, probably precipitated by the convergence of different risk factors, mediated by oxidative stress, and involving pathogenic factors arising within the DA-neuron (intrinsic factors), and from its environment and distant interconnected brain regions (extrinsic factors). In light of current data, intrinsic factors seem to be preferentially involved in the first steps of the degenerative process, and extrinsic factors in its progression. A controversial issue is the relative weight of the impairment of common cell functions, such as energy metabolism and proteostasis, and specific dopaminergic functions, such as pacemaking activity and DA handling, in the pathogenesis of DA-cell degeneration. Here we will review the current knowledge about the relevance of these factors at the beginning and during the progression of PD, and in the differential vulnerability of midbrain DA-cells.

Paraquat induces cyclooxygenase-2 (COX-2) implicated toxicity in human neuroblastoma SH-SY5Y cells

Paraquat produces dopaminergic pathologies of Parkinson's disease, in which cyclooxygenase-2 (COX-2) is implicated. However, it is unclear whether paraquat induces toxicity within dopaminergic neurons through COX-2. To address this, human neuroblastoma SH-SY5Y cells were treated with paraquat and then the involving mechanism of COX-2 was investigated. We initially examined the involvement of COX-2 in paraquat-induced toxicity. Data suggest that COX-2 is implicated in paraquat-induced reduction of viability in SY5Y cells. Then, to confirm the presence of COX-2 in SY5Y cells, we examined COX-2 mRNA and protein levels, which are regulated by NF-κB. Data indicate that paraquat activates NF-κB and up-regulates COX-2. We then checked quinone-bound proteins as quinones produced by COX-2 bind to intracellular proteins. Paraquat obviously forms quinone-bound proteins, in particular, quinone-bound DJ-1 and this formation is attenuated by meloxicam. Finally, we investigated antioxidant system including nuclear factor erythroid-related factor 2 (Nrf2), gamma glutamylcysteine synthetase (γGCS), and glutathione (GSH) as DJ-1 is linked to Nrf2 and Nrf2 regulates γGCS expression and γGCS is a GSH synthesis enzyme. Paraquat decreases protein levels of Nrf2 and γGCS and intracellular GSH level and these decreases are alleviated by meloxicam. Therefore, collectively, our data indicate that paraquat induces COX-2 implicated toxicity in SY5Y cells. In conclusion, current findings support the idea that paraquat might produce toxicity in dopaminergic neurons through COX-2.

Induction of parkinsonism-related proteins in the spinal motor neurons of transgenic mouse carrying a mutant SOD1 gene

Amyotrophic lateral sclerosis is a progressive and fatal disease caused by selective death of motor neurons, and a number of these patients carry mutations in the superoxide dismutase 1 (SOD1) gene involved in ameliorating oxidative stress. Recent studies indicate that oxidative stress and disruption of mitochondrial homeostasis is a common mechanism for motor neuron degeneration in amyotrophic lateral sclerosis and the loss of midbrain dopamine neurons in Parkinson's disease. Therefore, the present study investigated the presence and alterations of familial Parkinson's disease-related proteins, PINK1 and DJ-1, in spinal motor neurons of G93ASOD1 transgenic mouse model of amyotrophic lateral sclerosis. Following onset of disease, PINK1 and DJ-1 protein expression increased in the spinal motor neurons. The activated form of p53 also increased and translocated to the nuclei of spinal motor neurons, followed by increased expression of p53-activated gene 608 (PAG608). This is the first report demonstrating that increased expression of PAG608 correlates with activation of phosphorylated p53 in spinal motor neurons of an amyotrophic lateral sclerosis model. These results provide further evidence of the profound correlations between spinal motor neurons of amyotrophic lateral sclerosis and parkinsonism-related proteins.

Neuroproteomics approaches to decipher neuronal regeneration and degeneration

Given the complexity of brain and nerve tissues, systematic approaches are essential to understand normal physiological conditions and functional alterations in neurological diseases. Mass spectrometry-based proteomics is increasingly used in neurosciences to determine both basic and clinical differential protein expression, protein-protein interactions, and post-translational modifications. Proteomics approaches are especially useful to understand the mechanisms of nerve regeneration and degeneration because changes in axons following injury or in disease states often occur without the contribution of transcriptional events in the cell body. Indeed, the current understanding of axonal function in health and disease emphasizes the role of proteolysis, local axonal protein synthesis, and a broad range of post-translational modifications. Deciphering how axons regenerate and degenerate has thus become a postgenomics problem, which depends in part on proteomics approaches. This review focuses on recent proteomics approaches designed to uncover the mechanisms and molecules involved in neuronal regeneration and degeneration. It emerges that the principal degenerative mechanisms converge to oxidative stress, dysfunctions of axonal transport, mitochondria, chaperones, and the ubiquitin-proteasome systems. The mechanisms regulating nerve regeneration also impinge on axonal transport, cytoskeleton, and chaperones in addition to changes in signaling pathways. We also discuss the major challenges to proteomics work in the nervous system given the complex organization of the brain and nerve tissue at the anatomical, cellular, and subcellular levels.

Regulation of the dopamine transporter: aspects relevant to psychostimulant drugs of abuse

Dopaminergic signaling in the brain is primarily modulated by dopamine transporters (DATs), which actively translocate extraneuronal dopamine back into dopaminergic neurons. Transporter proteins are highly dynamic, continuously trafficking between plasmalemmal and endosomal membranes. Changes in DAT membrane trafficking kinetics can rapidly regulate dopaminergic tone by altering the number of transporters present at the cell surface. Various psychostimulant DAT ligands-acting either as amphetamine-like substrates or cocaine-like nontranslocated inhibitors-affect transporter trafficking, triggering rapid insertion or removal of plasmalemmal DATs. In this review, we focus on the effects of psychostimulants of addiction (particularly D-methamphetamine and cocaine) on DAT regulation and membrane trafficking, with an emphasis on how these drugs may influence intracellular signaling cascades and transporter-associated scaffolding proteins to affect DAT regulation. In addition, we consider involvement of presynaptic receptors for dopamine and other ligands in DAT regulation. Finally, we discuss possible implications of transporter regulation to the putative toxicity of several substituted amphetamine derivatives commonly used as recreational drugs, as well as to the design of therapeutics for cocaine addiction.

The structure of dopamine induced alpha-synuclein oligomers

Inclusions of aggregated alpha-synuclein (alpha-syn) in dopaminergic neurons are a characteristic histological marker of Parkinson's disease (PD). In vitro, alpha-syn in the presence of dopamine (DA) at physiological pH forms SDS-resistant non-amyloidogenic oligomers. We used a combination of biophysical techniques, including sedimentation velocity analysis, small angle X-ray scattering (SAXS) and circular dichroism spectroscopy to study the characteristics of alpha-syn oligomers formed in the presence of DA. Our SAXS data show that the trimers formed by the action of DA on alpha-syn consist of overlapping worm-like monomers, with no end-to-end associations. This lack of structure contrasts with the well-established, extensive beta-sheet structure of the amyloid fibril form of the protein and its pre-fibrillar oligomers. We propose on the basis of these and earlier data that oxidation of the four methionine residues at the C- and N-terminal ends of alpha-syn molecules prevents their end-to-end association and stabilises oligomers formed by cross linking with DA-quinone/DA-melanin, which are formed as a result of the redox process, thus inhibiting formation of the beta-sheet structure found in other pre-fibrillar forms of alpha-syn.

Activation of the NRF2 signaling pathway by copper-mediated redox cycling of para- and ortho-hydroquinones

Transcription factor NF-E2 p45-related factor 2 (Nrf2) mediates adaptation to oxidants and electrophiles through up-regulating genes that contain antioxidant response elements (AREs) in their promoters. Using the stably transfected human AREc32 reporter cell line, we found that copper and other transition metals enhanced induction of ARE-driven luciferase by 2-tert-butyl-1,4-hydroquinone (tBHQ) as a result of increased oxidation to 2-tert-butyl-1,4-benzoquinone (tBQ). Following exposure to tBHQ for 30 min, ARE-luciferase activity measured after 24 hr was dependent on the presence of Cu(2+). In contrast, tBQ-induced activity was Cu(2+)-independent. The metal-catalyzed oxidation of tBHQ to tBQ occured rapidly and stoichiometrically. Compounds that share para- or ortho-hydroquinone structures, such as catechol estrogens, dopamine, and l-DOPA, also induced ARE-driven luciferase in a Cu(2+)-dependent manner. Thus, the oxidation of para- and ortho-hydroquinones to quinones represents the rate-limiting step in the activation of Nrf2.