Selective dopaminergic vulnerability: 3,4-dihydroxyphenylacetaldehyde targets mitochondria

Abstract

Parkinson’s disease (PD) is a major cause of age-related morbidity and mortality, present in nearly 1% of individuals at ages 70–79 and ∼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 ≥ 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.

Introduction

Selective vulnerability of dopaminergic neurons is manifest in the progressive loss of these neurons in both aging and PD. The mechanism(s) underlying selective vulnerability remain(s) unclear, but evidence from a variety of experimental approaches connects dopamine toxicity, MAO activity, and mitochondrial dysfunction. For example, inhibition of mitochondrial respiration by monoamine-derived H₂O₂ [1], L-dopa mediated inhibition of respiration [2], and dopamine auto-oxidation [3] have all been proposed to contribute to the oxidative stress [4] and mitochondrial dysfunction [5], [6], [7], [8] associated with the dopaminergic deficits in PD. The dopamine auto-oxidation product, 6-OH dopamine, can also stimulate induction of the Ca²⁺-mediated mitochondrial permeability transition (PT) [3], an event implicated in cell death under some circumstances [9], [10], [11], [12], [13]. The concentrations of both 6-OH-dopamine and Ca²⁺ required, however, are well beyond those known to occur in vivo [3].

DOPAL is the initial product of MAO activity on dopamine, and it is the precursor of 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylethanol (DOPET). Recent work shows that severe metabolic inhibition (rotenone coupled with glucose deprivation) increases the concentration of DOPAL in neuronally differentiated PC12 cells [14]. The potential toxicity of monoamine-derived aldehydes [15] suggests that the reactive aldehyde DOPAL may be a physiological mediator of neurotoxicity. This hypothesis is supported by recent evidence that DOPAL can exacerbate toxicity induced by severe metabolic inhibition and by evidence that doses of 100 μM and 500 μM DOPAL can increase lactate dehydrogenase release from neuronally differentiated pheochromocytoma (PC12) cells and the neuroblastoma line SK-N-SH [14]. Involvement of a toxic monoamine metabolite in neurotoxicity is consistent with evidence that the aldehyde metabolites of catecholamines generate free radicals [16] and induce apoptotic neuronal death in catecholaminergic neurons [17], [18]. The catecholamines themselves or their other metabolites do not. Other reactive aldehydes, including the lipid peroxidation byproduct 4-hydroxyhexenal and DOPEGAL [3,4-dihydroxyphenylglycolaldehyde], the monoamine metabolite of epinephrine and norepinephrine, are also potent co-stimulators of the mitochondrial PT [16], [19].

A potential role for dopamine metabolites in neuronal toxicity is supported by evidence that millimolar dopamine can kill undifferentiated PC12 cells in a manner that may, based on cyclosporin A studies, involve the PT [20]. Involvement of MAO is equivocal because the nonspecific MAO inhibitor pargyline protects, but so does deprenyl, which selectively inhibits the MAO-B isoform not present in PC12 cells. As noted by the authors of ref. [20], however, deprenyl’s protection may be due to its antioxidant capacity. Deprenyl may also have other non-MAO-dependent effects on the PT (B.S.K./W.J.B., manuscript in preparation). Dopamine-induced cytotoxicity is inhibited by bcl-2 [21]. The protective effects of bcl-2 are consistent with possible mitochondrial involvement in dopamine toxicity, but give no insight into the active metabolite. We now report that DOPAL, at physiological levels, is a highly potent mediator of cell death that is active at the mitochondrial level.