Acidic dopamine metabolites are actively extruded from PC12 cells by a novel sulfonylurea-sensitive transporter

Enhanced tyrosine hydroxylase activity induces oxidative stress, causes accumulation of autotoxic catecholamine metabolites, and augments amphetamine effects in vivo

In Parkinson's disease, dopamine‐containing nigrostriatal neurons undergo profound degeneration. Tyrosine hydroxylase (TH) is the rate‐limiting enzyme in dopamine biosynthesis. TH increases in vitro formation of reactive oxygen species, and previous animal studies have reported links between cytosolic dopamine build‐up and oxidative stress. To examine effects of increased TH activity in catecholaminergic neurons in vivo, we generated TH‐over‐expressing mice (TH‐HI) using a BAC‐transgenic approach that results in over‐expression of TH with endogenous patterns of expression. The transgenic mice were characterized by western blot, qPCR, and immunohistochemistry. Tissue contents of dopamine, its metabolites, and markers of oxidative stress were evaluated. TH‐HI mice had a 3‐fold increase in total and phosphorylated TH levels and an increased rate of dopamine synthesis. Coincident with elevated dopamine turnover, TH‐HI mice showed increased striatal production of H₂O₂ and reduced glutathione levels. In addition, TH‐HI mice had elevated striatal levels of the neurotoxic dopamine metabolites 3,4‐dihydroxyphenylacetaldehyde and 5‐S‐cysteinyl‐dopamine and were more susceptible than wild‐type mice to the effects of amphetamine and methamphetamine. These results demonstrate that increased TH alone is sufficient to produce oxidative stress in vivo, build up autotoxic dopamine metabolites, and augment toxicity.

The Catecholaldehyde Hypothesis for the Pathogenesis of Catecholaminergic Neurodegeneration: What We Know and What We Do Not Know

3,4-Dihydroxyphenylacetaldehyde (DOPAL) is the focus of the catecholaldehyde hypothesis for the pathogenesis of Parkinson’s disease and other Lewy body diseases. The catecholaldehyde is produced via oxidative deamination catalyzed by monoamine oxidase (MAO) acting on cytoplasmic dopamine. DOPAL is autotoxic, in that it can harm the same cells in which it is produced. Normally, DOPAL is detoxified by aldehyde dehydrogenase (ALDH)-mediated conversion to 3,4-dihydroxyphenylacetic acid (DOPAC), which rapidly exits the neurons. Genetic, environmental, or drug-induced manipulations of ALDH that build up DOPAL promote catecholaminergic neurodegeneration. A concept derived from the catecholaldehyde hypothesis imputes deleterious interactions between DOPAL and the protein alpha-synuclein (αS), a major component of Lewy bodies. DOPAL potently oligomerizes αS, and αS oligomers impede vesicular and mitochondrial functions, shifting the fate of cytoplasmic dopamine toward the MAO-catalyzed formation of DOPAL—destabilizing vicious cycles. Direct and indirect effects of DOPAL and of DOPAL-induced misfolded proteins could “freeze” intraneuronal reactions, plasticity of which is required for neuronal homeostasis. The extent to which DOPAL toxicity is mediated by interactions with αS, and vice versa, is poorly understood. Because of numerous secondary effects such as augmented spontaneous oxidation of dopamine by MAO inhibition, there has been insufficient testing of the catecholaldehyde hypothesis in animal models. The clinical pathophysiological significance of genetics, emotional stress, environmental agents, and interactions with numerous proteins relevant to the catecholaldehyde hypothesis are matters for future research. The imposing complexity of intraneuronal catecholamine metabolism seems to require a computational modeling approach to elucidate clinical pathogenetic mechanisms and devise pathophysiology-based, individualized treatments.

The catecholaldehyde hypothesis: where MAO fits in

Monoamine oxidase (MAO) plays a central role in the metabolism of the neurotransmitters dopamine, norepinephrine, and serotonin. This brief review focuses on 3,4-dihydroxyphenylacetaldehyde (DOPAL), which is the immediate product of MAO acting on cytoplasmic dopamine. DOPAL is toxic; however, normally DOPAL is converted via aldehyde dehydrogenase (ALDH) to 3,4-dihydroxyphenylacetic acid (DOPAC), which rapidly exits the neurons. In addition to vesicular uptake of dopamine via the vesicular monoamine transporter (VMAT), the two-enzyme sequence of MAO and ALDH keeps cytoplasmic dopamine levels low. Dopamine oxidizes readily to form toxic products that could threaten neuronal homeostasis. The catecholaldehyde hypothesis posits that diseases featuring catecholaminergic neurodegeneration result from harmful interactions between DOPAL and the protein alpha-synuclein, a major component of Lewy bodies in diseases such as Parkinson disease, dementia with Lewy bodies, and pure autonomic failure. DOPAL potently oligomerizes alpha-synuclein, and alpha-synuclein oligomers impede vesicular functions, shifting the fate of cytoplasmic dopamine toward MAO-catalyzed formation of DOPAL-a vicious cycle. When MAO deaminates dopamine to form DOPAL, hydrogen peroxide is generated; and DOPAL, hydrogen peroxide, and divalent metal cations react to form hydroxyl radicals, which peroxidate lipid membranes. Lipid peroxidation products in turn inhibit ALDH, causing DOPAL to accumulate-another vicious cycle. MAO inhibition decreases DOPAL formation but concurrently increases the spontaneous oxidation of dopamine, potentially trading off one form of toxicity for another. These considerations rationalize a neuroprotection strategy based on concurrent treatment with an MAO inhibitor and an anti-oxidant.

Roles of catechol neurochemistry in autonomic function testing

Catechols are a class of compounds that contain adjacent hydroxyl groups on a benzene ring. Endogenous catechols in human plasma include the catecholamines norepinephrine, epinephrine (adrenaline), and dopamine; the catecholamine precursor DOPA, 3,4-dihydroxyphenylglycol (DHPG), which is the main neuronal metabolite of norepinephrine; and 3,4-dihydroxyphenylacetic acid (DOPAC), which is the main neuronal metabolite of dopamine. In the diagnostic evaluation of patients with known or suspected dysautonomias, measurement of plasma catechols is rarely diagnostic but often is informative. This review summarizes the roles of clinical catechol neurochemistry in autonomic function testing.

L-Tyrosine availability affects basal and stimulated catecholamine indices in prefrontal cortex and striatum of the rat

We previously found that L-tyrosine (L-TYR) but not D-TYR administered by reverse dialysis elevated catecholamine synthesis in vivo in medial prefrontal cortex (MPFC) and striatum of the rat (Brodnik et al., 2012). We now report L-TYR effects on extracellular levels of catecholamines and their metabolites. In MPFC, reverse dialysis of L-TYR elevated in vivo levels of dihydroxyphenylacetic acid (DOPAC) (L-TYR 250-1000 μM), homovanillic acid (HVA) (L-TYR 1000 μM) and 3-methoxy-4-hydroxyphenylglycol (MHPG) (L-TYR 500-1000 μM). In striatum L-TYR 250 μM elevated DOPAC. We also examined L-TYR effects on extracellular dopamine (DA) and norepinephrine (NE) levels during two 30 min pulses (P2 and P1) of K+ (37.5 mM) separated by t = 2.0 h. L-TYR significantly elevated the ratio P2/P1 for DA (L-TYR 125 μM) and NE (L-TYR 125-250 μM) in MPFC but lowered P2/P1 for DA (L-TYR 250 μM) in striatum. Finally, we measured DA levels in brain slices using ex-vivo voltammetry. Perfusion with L-TYR (12.5-50 μM) dose-dependently elevated stimulated DA levels in striatum. In all the above studies, D-TYR had no effect. We conclude that acute increases within the physiological range of L-TYR levels can increase catecholamine metabolism and efflux in MPFC and striatum. Chronically, such repeated increases in L-TYR availability could induce adaptive changes in catecholamine transmission while amplifying the metabolic cost of catecholamine synthesis and degradation. This has implications for neuropsychiatric conditions in which neurotoxicity and/or disordered L-TYR transport have been implicated.

Elevated cerebrospinal fluid ratios of cysteinyl-dopamine/3,4-dihydroxyphenylacetic acid in parkinsonian synucleinopathies

INTRODUCTION

There is intense interest in identifying cerebrospinal fluid (CSF) biomarkers of Parkinson's disease (PD), both for early diagnosis and to track effects of putative treatments. Nigrostriatal dopamine depletion characterizes PD. Predictably, CSF levels of 3,4-dihydroxyphenylacetic acid (DOPAC), the main neuronal metabolite of dopamine, are decreased in PD, even in patients with recent onset of the movement disorder. Whether low CSF DOPAC is associated specifically with parkinsonism has been unclear. In the neuronal cytoplasm dopamine undergoes not only enzymatic oxidation to form DOPAC but also spontaneous oxidation to form 5-S-cysteinyl-dopamine (Cys-DA). Theoretically, oxidative stress or decreased activity of aldehyde dehydrogenase (ALDH) in the residual nigrostriatal dopaminergic neurons would increase CSF Cys-DA levels with respect to DOPAC levels. PD, parkinsonian multiple system atrophy (MSA-P), and pure autonomic failure (PAF) are synucleinopathies; however, PAF does not entail parkinsonism. We examined whether an elevated Cys-DA/DOPAC ratio provides a specific biomarker of parkinsonism in synucleinopathy patients.

METHODS

CSF catechols were assayed in PD (n = 24), MSA-P (n = 32), PAF (n = 18), and control subjects (n = 32).

RESULTS

Compared to controls, CSF DOPAC was decreased in PD and MSA-P (p < 0.0001 each). In both diseases Cys-DA/DOPAC ratios averaged more than twice control (0.14 ± 0.02 and 0.13 ± 0.02 vs. 0.05 ± 0.01, p < 0.0001 each), whereas in PAF the mean Cys-DA/DOPAC ratio was normal (0.05 ± 0.01).

CONCLUSIONS

CSF Cys-DA/DOPAC ratios are substantially increased in PD and MSA-P and are normal in PAF. Thus, in synucleinopathies an elevated CSF Cys-DA/DOPAC ratio seems to provide a specific biomarker of parkinsonism.

Tyrosine depletion lowers in vivo DOPA synthesis in ventral hippocampus

In vivo dopamine synthesis in the medial prefrontal cortex of the rat is sensitive to the availability of tyrosine. Whether other limbic cortical dopamine terminal regions are similarly tyrosine-dependent is not known. In this study we examined the effects of tyrosine depletion on dopamine synthesis and catecholamine levels in the ventral hippocampus. A tyrosine- and phenylalanine-free neutral amino acid mixture was used to lower brain tyrosine levels in rats undergoing in vivo microdialysis. In one group, NSD-1015 was included in perfusate to permit measurement of DOPA levels. In a second group, NSD-1015 was not included in perfusate so that catecholamine levels could be assayed. Tyrosine depletion significantly lowered DOPA levels in the NSD-1015 treated group and lowered DOPAC but not dopamine or noradrenaline levels in the group not exposed to NSD-1015. We conclude that while catecholamine synthesis in the ventral hippocampus declines when tyrosine availability is lowered, under basal conditions, compensatory mechanisms are able to maintain stable extracellular catecholamine levels.

Catechol and aldehyde moieties of 3,4-dihydroxyphenylacetaldehyde contribute to tyrosine hydroxylase inhibition and neurotoxicity

Parkinson's disease (PD) is a progressive neurodegenerative disorder which leads to the selective loss of dopaminergic neurons. This causes a decrease in the important neurotransmitter dopamine (DA), which is essential for coordinated movement. Previous studies have implicated the monoamine oxidase metabolite of DA, 3,4-dihydroxphenylacetaldehyde (DOPAL), in the pathogenesis of PD and have shown it to be a reactive intermediate capable of protein modification. DOPAL also has demonstrated the ability to cause mitochondrial dysfunction and lead to significant inhibition of the rate-limiting enzyme in DA synthesis, tyrosine hydroxylase (TH). The current study was undertaken to investigate four analogs of DOPAL, including a novel nitrile analog, to determine how the structure of DOPAL is related to its toxicity and inhibition of TH. Both mitochondrial function and inhibition of TH in cell lysate were investigated. Furthermore, a novel whole cell assay was designed to determine the consequence to enzyme action when DOPAL levels were elevated. The results presented here demonstrate that changes to DOPAL structure lead to a decrease in toxicity and inhibition of enzyme activity as compared to the parent compound. Furthermore, the production of superoxide anion but not hydrogen peroxide increased in the presence of elevated DOPAL. These results reveal the toxicity of DOPAL and demonstrate that both the catechol and aldehyde are required to potently inhibit TH activity.

Dopac distribution and regulation in striatal dopaminergic varicosities and extracellular space

DOPAC, the major intermediate metabolite of dopamine, is found in the cytosolic compartment of dopaminergic terminals/varicosities and in the extracellular space. It has been proposed that extracellular DOPAC is derived from newly synthesized dopamine rather than from dopamine in the signaling pool. On the basis of literature data supporting such a concept, we hypothesize a DOPAC synthesis/secretory complex producing extracellular DOPAC and use a computational simulation model of dopaminergic varicosities to estimate the distribution of DOPAC between cytosolic and extracellular compartments, amount of newly synthesized dopamine entering the DOPAC synthesis/secretory complex, and potential regulatory processes in the complex. Results suggest that about two-thirds of DOPAC is in the extracellular space. Approximately one-third of newly synthesized dopamine is immediately processed to DOPAC, which is then secreted into extracellular space. Extracellular DOPAC concentration is approximately 300 times higher than extracellular dopamine, and cytosolic DOPAC is ∼18-fold higher than cytosolic dopamine. We suggest that the high levels of extracellular DOPAC coupled with evidence for its production from newly synthesized dopamine imply the existence of an as yet undiscovered regulatory/signaling role for DOPAC.

Leishmania amazonensis: Metabolic adaptations induced by resistance to an ABC transporter blocker

We compared growth rate, cell glucose turnover and expression of ATP-binding-cassette (ABC) transporters in Leishmania amazonensis (LTB0016; LTB) versus LTB(160) selected for resistance against the ABC transporter blocker glibenclamide. Additionally, we evaluated the influence of drug-resistance on Leishmania sensitivity against 2-mercaptoacetate and 2-deoxyglucose. Our data demonstrate that (1) LTB(160) and LTB constitutively express ABC transporters for neutral substrates, (2) glibenclamide resistance induces the expression of organic anion ABC transporters, members of the drug resistance associated transporters subfamily, (3) LTB(160) parasites use less glucose as energy substrate and exhibit a slower glucose uptake than LTB cells, and (4) LTB(160) parasites are less sensitive to 2-mercaptoacetate and 2-deoxyglucose than the glibenclamide-sensitive Leishmania LTB. Together these and previous results indicate that the metabolic adaptations expressed in drug-resistant LTB(160) differ from those described for mammalian drug resistant cells and constitute general mechanisms that underlie drug resistance in Leishmania and may be helpful for identifying alternative strategies to circumvent drug resistance in leishmaniasis.

Polymeric gene carrier for insulin secreting cells: poly(L-lysine)-g-sulfonylurea for receptor mediated transfection

Ex vivo transfer of therapeutic genes to cells is one of the potential strategies to prolong the life span of cell transplants. However, relatively safe non-viral carriers have not been extensively investigated due to their lower transfection efficiency. In this study, poly(L-lysine)-g-sulfonylurea varying SU content (PLL-SU) was synthesized to promote gene delivery efficacy to an insulin secreting cell line, RINm5F, which is known to express sulfonylurea receptor (SUR). The polymer formed complexes with a model reporter gene of pCMV-Luc (DNA) and the size of resulting particles was around 100 nm. The transfection efficiency of a polymer synthesized with 5 mol% of SU in the reaction feed (PLL-SU5%) to RINm5F cell was at least 5 times higher than that of PLL. The cytotoxicity of PLL-SU5%/DNA complex was equivalent to that of PLL/DNA complex. PLL-SU5% showed less transfection efficiency than PLL to NIH3T3 and HepG2 cells which are SUR negative. In RINm5F cells, the addition of free SU decreased the transfection efficiency of PLL-SU5%/DNA complex, suggesting that the complex shares the same receptors for SU. The PLL-SU5%/DNA complex seems to be internalized via SUR-mediated endocytosis pathway as suggested by vacuolar ATPases inhibition by Bafilomycin A1. It is noted that RINm5F cells treated with PLL-SU5%/DNA complex secreted more insulin than control, untreated cells, suggesting the insulinotropic effect of SU in PLL-SU5%. In conclusion, PLL-SU may be useful for transfer of therapeutic genes into insulin secreting cells.

Iptakalim hydrochloride protects cells against neurotoxin-induced glutamate transporter dysfunction in in vitro and in vivo models

Iptakalim hydrochloride (Ipt), a novel antihypertensive drug, exhibits K(ATP) channel activation. Here, we report that Ipt remarkably protects cells against neurotoxin-induced glutamate transporter dysfunction in in vitro and in vivo models. Chronic exposure of cultured PC12 cells to neurotoxins, such as 6-OHDA, MPP+, or rotenone, decreased overall [3H]-glutamate uptake in a concentration-dependent manner. Pre-treatment using 10 microM Ipt significantly protected cells against neurotoxin-induced glutamate uptake diminishment, and this protection was abolished by the K(ATP) channel blocker glibenclamide (20 microM), suggesting that the protective mechanisms may involve the opening of K(ATP) channels. In 6-OHDA-treated rats (as an in vivo Parkinson's disease model), [3H]-glutamate uptake was significantly lower in synaptosomes isolated from the striatum and cerebral cortex, but not the hippocampus. Pre-conditioning using 10, 50, and 100 microM Ipt significantly restored glutamate uptake impairment and these protections were abolished by blockade of K(ATP) channels. It is concluded that Ipt exhibits substantial protection of cells against neurotoxicity in in vitro and in vivo models. The cellular mechanisms of this protective effect may involve the opening of K(ATP) channels. Collectively, Ipt may serve as a novel and effective drug for PD therapy.

AM-36, a novel neuroprotective agent, profoundly reduces reactive oxygen species formation and dopamine release in the striatum of conscious rats after endothelin-1-induced middle cerebral artery occlusion

Elevated generation of reactive oxygen species (ROS) has been demonstrated during ischemia and reperfusion. Dopamine (DA) autooxidation may contribute to increased ROS generation. The novel neuroprotective agent AM-36 has antioxidant and Na(+) channel blocking activity and reduces neuronal damage in both cortex and striatum after middle cerebral artery (MCA) occlusion. Here we sought in vivo evidence of the ability of AM-36 to inhibit intrastriatal ROS generation and DA release after ischemia. Salicylate hydroxylation coupled with in vivo microdialysis in the striatum of conscious Long Evans rats was performed during MCA occlusion by perivascular microinjection of endothelin-1 (ET-1). AM-36 (6 mg/kg) was administered intraperitoneally 30 min after MCA occlusion. Dialysates were analysed using high performance liquid chromatography with electrochemical detection for the salicylate hydroxylation product, 2,3-dihydroxybenzoic acid (2,3 DHBA) and for DA and metabolites. MCA occlusion resulted in a marked increase in 2,3 DHBA and a secondary increase in all analytes, 180-300 min later. Increased DA release coincided with 2,3 DHBA formation. AM-36 significantly reduced ischemia induced increases in 2,3 DHBA and DA, and infarct volume in the striatum. Significant improvements in a battery of behavioural tests was also found in AM-36 treated rats. This study has demonstrated profound inhibition of ROS generation by a novel compound with antioxidant activity, administered post-ischemia in conscious rats.