Shu Wen Li

Neurotoxicity of MAO metabolites of catecholamine neurotransmitters: role in neurodegenerative diseases.

The monoamine oxidase (MAO) metabolites of norepinephrine (NE) or epinephrine (EPI) and of dopamine (DA) are 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL) and 3,4-dihydroxyphenylacetaldehyde (DOPAL), respectively. The toxicity of these catecholamine (CA) MAO metabolites was predicted over 50 years ago. However, until our recent chemical synthesis of these CA aldehyde metabolites, the hypothesis about their toxicity could not be tested. The present paper reviews recent knowledge gained about these compounds. Topics to be reviewed include: chemical synthesis and properties of DOPEGAL and DOPAL; in vitro and in vivo toxicity of CA aldehydes; subcellular mechanisms of toxicity; free radical formation by DOPEGAL versus DOPAL; mechanisms of accumulation of CA aldehydes in Alzheimers disease (AD) and Parkinsons disease (PD) and potential therapeutic targets in Alzheimers disease and Parkinsons disease.

Aggregation of α-synuclein by DOPAL, the monoamine oxidase metabolite of dopamine

Parkinson’s disease (PD) is a neurodegenerative disease characterized by the selective loss of dopamine (DA) neurons and the presence of α-synuclein (AS) aggregates as Lewy bodies (LBs) in the remaining substantia nigra (SN) neurons. A continuing puzzle in studying PD pathogenesis is that although AS is expressed throughout the brain, LBs and selective dopaminergic cell loss lead to characteristic clinical signs of PD, suggesting that there is a link between AS aggregation and DA metabolism. One potential candidate for this link is the monoamine oxidase (MAO) metabolite of DA, 3,4-dihydroxyphenylacetaldehyde (DOPAL), as neither DA nor DA metabolites other than DOPAL are toxic to SN neurons at physiological concentrations. We tested DOPAL-induced AS aggregation in a cell-free system, in vitro in DA neuron cultures and in vivo with stereotactic injections into the SN of Sprague–Dawley rats by Western blots, fluorescent confocal microscopy and immunohistochemistry. We demonstrate that DOPAL in physiologically relevant concentrations, triggers AS aggregation in the cell-free system, and in cell cultures resulting in the formation of potentially toxic AS oligomers and aggregates. Furthermore, DOPAL injection into the SN of Sprague–Dawley rats resulted in DA neuron loss and the accumulation of high molecular weight oligomers of AS detected by Western blot. Our findings support the hypothesis that DA metabolism via DOPAL can cause both DA neuron loss and AS aggregation observed in PD.

3,4-Dihydroxyphenylacetaldehyde is the toxic dopamine metabolite in vivo: implications for Parkinson's disease pathogenesis.

In Parkinsons disease (PD), there is a highly selective loss of dopamine (DA) neurons in the substantia nigra (SN) greater than in the ventral tegmental area (VTA). The simplest explanation for selective DA neuron loss in PD is that DA is toxic and, because only DA neurons contain significant amounts of DA, this highly localized synthesis of DOPAL accounts for selective vulnerability of DA neurons. However, the large concentrations of DA required to produce in vivo toxicity cast doubt on its role in PD pathogenesis. Alpha-synuclein (alpha-syn) is the major component of the Lewy body, the pathological marker of PD, and is genetically linked to the disease. Recent studies indicate that alpha-syn neurotoxicity is mediated by a free radical generating metabolite of DA. Here we test the hypothesis that 3,4-dihydroxyphenylacetaldehyde (DOPAL), the monamine oxidase metabolite of DA, mediates DA toxicity in vivo. We injected DOPAL, DA and its oxidative, reduced and methylated metabolites into rat SN and VTA. Five days post-surgery, the injection sites were evaluated in Nissl preparations and with tyrosine hydroxylase (for DA neurons), neuronal nuclear antigen (for neurons) and glial fibrillary acidic protein (for astrocytes) immunoreactivities. Lesion size in SN vs. VTA was compared using morphometry. DOPAL at concentrations as low as 100 ng was toxic to DA SN neurons>DA VTA neurons>glia. Neither DA nor its other metabolites showed evidence of neurotoxicity at fivefold higher doses. However, 20 microg of DA produced lesions in the SN and VTA. We conclude that DOPAL is the toxic DA metabolite in vivo. Implications for a unified hypothesis for PD pathogenesis are discussed.

Selective dopaminergic vulnerability: 3,4-dihydroxyphenylacetaldehyde targets mitochondria

Parkinsons disease (PD) is a major cause of age-related morbidity and mortality, present in nearly 1% of individuals at ages 70-79 and approximately 2.5% of individuals at age 85. L-DOPA (L-dihydroxyphenylalanine), which is metabolized to dopamine by dopa decarboxylase, is the primary therapy for PD, but may also contribute to disease progression. Association between mitochondrial dysfunction, monoamine oxidase (MAO) activity, and dopaminergic neurotoxicity has been repeatedly observed, but the mechanisms underlying selective dopaminergic neuron depletion in aging and neurodegenerative disorders remain unclear. We now report that 3,4-dihydroxyphenylacetaldehyde (DOPAL), the MAO metabolite of dopamine, is more cytotoxic in neuronally differentiated PC12 cells than dopamine and several of its metabolites. In isolated, energetically compromised mitochondria, physiological concentrations of DOPAL induced the permeability transition (PT), a trigger for cell death. Dopamine was > 1000-fold less potent. PT inhibitors protected both mitochondria and cells against DOPAL. Sensitivity to DOPAL was reduced > or = 30-fold in fully energized mitochondria, suggesting that mitochondrial respiration may increase resistance to PT induction by the endogenous DOPAL in the substantia nigra. These data provide a potential mechanism of action for L-DOPA-mediated neurotoxicity and suggest two potentially interactive mechanisms for the selective vulnerability of neurons exposed to dopamine.

3,4-Dihydroxyphenylacetaldehyde and hydrogen peroxide generate a hydroxyl radical: Possible role in Parkinson's disease pathogenesis

3,4-Dihydroxyphenylacetaldehyde (DOPAL) and 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL), the monoamine oxidase (MAO) metabolites of dopamine (DA) and norepinephrine (NE), respectively, are toxic to catecholamine (CA) neurons in vitro and in vivo. DOPEGAL generates a free radical and activates mitochondrial permeability transition, a mechanism implicated in neuron death. To determine if DOPAL and other DA metabolites generate the hydroxyl radical in the presence of H(2)O(2), we used HPLC-EC to detect salicylate hydroxylation products. To determine the relative reducing capacity of DOPAL and DOPEGAL we used cyclic voltammetry to measure their reduction potentials. Results indicate that DOPAL, but not DOPEGAL, DA or other DA metabolites, generates hydroxyl radicals. Atomic absorption spectroscopy and heavy metal screening indicate that this result is not due to contamination of DOPAL with iron or other heavy metals. DOPAL reduction potential (161 mV) is lower than that of DOPEGAL (235 mV). DOPAL is present in human substantia nigra. The implications of these findings to CA neuronal death in degenerative brain diseases are discussed.

Accumulation of 3,4-dihydroxyphenylglycolaldehyde, the neurotoxic monoamine oxidase A metabolite of norepinephrine, in locus ceruleus cell bodies in Alzheimer's disease : Mechanism of neuron death

3,4-Dihydroxyphenylglycolaldehyde (DOPEGAL) is the neurotoxic monoamine oxidase A (MAO-A) metabolite of norepinephrine (NE). NE neurons in the locus ceruleus (LC) die in Alzheimers disease (AD). To determine if DOPEGAL could contribute to NE neuron death in AD we measured levels of DOPEGAL, NE and their synthesizing enzymes in LC from AD and matched controls. We found 2.8- and 3.6-fold increases in DOPEGAL and MAO-A in AD LC neuronal cell bodies compared to controls. NE and dopamine beta-hydroxylase were increased by 3.8- and 10.7-fold, respectively. Implications for the mechanism of neuron death in AD are discussed.

Norepinephrine transmitter metabolite generates free radicals and activates mitochondrial permeability transition: a mechanism for DOPEGAL-induced apoptosis

3,4-Dihydroxyphenylglycolaldehyde (DOPEGAL) is the monoamine oxidase A metabolite of norepinephrine (NE) and epinephrine. DOPEGAL, but neither NE nor its other metabolites induces apoptosis in differentiated PC-12 cells by an unknown mechanism. To study the mechanism of DOPEGAL-induced apoptosis, we tested DOPEGAL and NE for their capacity to generate free radicals and to induce mitochondrial permeability transition (PT). Results show that DOPEGAL but not NE forms reactive free radical intermediates under oxidative stress and enhances Ca2+-mediated induction of the mitochondrial PT. Linkage of these events to apoptosis is described. Implications for degenerative diseases are discussed.

Catecholamine monoamine oxidase a metabolite in adrenergic neurons is cytotoxic in vivo

3,4-Dihydroxyphenylglycolaldehyde is the monoamine oxidase-A metabolite of two catecholamine neurotransmitters, epinephrine and norepinephrine. This aldehyde metabolite and its synthesizing enzymes increase in cell bodies of catecholamine neurons in Alzheimers disease. To test the hypothesis that 3,4-dihydroxyphenylglycolaldehyde, but not epinephrine or its major metabolite 4-hydroxy-3-methoxyphenylglycol, is a neurotoxin, we injected 3,4-dihydroxyphenylglycolaldehyde onto adrenergic neurons in the rostral ventrolateral medulla. Injections of epinephrine or 4-hydroxy-3-methoxyphenylglycol were made into the same area of controls. A dose response and time study were performed. Adrenergic neurons were identified by their content of the epinephrine synthesizing enzyme, phenylethanolamine N-methyltransferase, immunohistochemically. Apoptosis was evaluated by in situ terminal deoxynucleotidyl-transferase mediated dUTP nick end label staining. Injection of 3,4-dihydroxyphenylglycolaldehyde in amounts as low as 50 ng results in loss of adrenergic neurons and apoptosis after 18 h. The degree of neurotoxicity is dose and time dependent. Doses of 3,4-dihydroxyphenylglycolaldehyde 10-fold higher produce necrosis. Neither epinephrine nor 4-hydroxy-3-methoxyphenylglycol are toxic. A 2.5 microg injection of 3,4-dihydroxyphenylglycolaldehyde is toxic to cortical neurons but not glia. Active uptake of the catecholamine-derived aldehyde into differentiated PC-12 cells is demonstrated. Implications of these findings for catecholamine neuron death in neurodegenerative diseases are discussed.

Norepinephrine transmitter metabolite induces apoptosis in differentiated rat pheochromocytoma cells

3,4-Dihydroxyphenylglycolaldehyde (DOPEGAL) is the monoamine oxidase A metabolite of norepinephrine and epinephrine. DOPEGAL, but not other metabolites, kills differentiated PC-12 cells. However, the type of DOPEGAL induced cell death, whether necrosis or apoptosis, is not known. To determine the type of cell death triggered by DOPEGAL, PC-12 cells cultured in the presence or absence of 30 microM DOPEGAL were examined by electron microscopy and DNA agarose gel electrophoresis for characteristic features of apoptosis. Results indicate that DOPEGAL induces apoptosis in these cells. Implications for degenerative diseases are discussed.

Catecholamine-Derived Aldehyde Neurotoxins

Catecholamine derived aldehydes are the products of monoamine oxidase (MAO) action on catecholamines (CA). There are three major CA in human tissues: norepinephrine (NE), epinephrine (Epi), and dopamine (DA). All three are central neurotransmitters and, in addition, NE and Epi are hormones secreted by the adrenal medulla. MAO has two isoforms: A and B. NE and Epi are the preferred substrates for MAO A (Rivett et al., 1982). DA is metabolized by both MAO A and B (Rivett et al., 1982). The MAO A product of either NE or Epi is 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL). The MAO product of DA is 3,4-dihydroxyphenylacetaldehyde (DOP-AL). Researchers initially considered these aldehydes, synthesized on the outer mitochondrial membrane, merely ephemeral intermediates in CA metabolism. However, the synthesis of sufficient quantities of chemically pure CA aldehydes has led to an understanding of their role in mitochondrially mediated apoptotic neuron death.