Tetrahydrobiopterin Scavenges Superoxide in Dopaminergic Neurons

Abstract

Increased oxidative stresses are implicated in the pathogenesis of Parkinson's disease, and dopaminergic neurons may be intrinsically susceptible to oxidative damage. However, the selective presence of tetrahydrobiopterin (BH 4 ) makes dopaminergic neurons more resistant to oxidative stress caused by glutathione depletion. To further investigate the mechanisms of BH 4 protection, we examined the effects of BH 4 on superoxide levels in individual living mesencephalic neurons. Dopaminergic neurons have intrinsically lower levels of superoxide than nondopaminergic neurons. In addition, inhibiting BH 4 synthesis increased superoxide in dopaminergic neurons, while BH 4 supplementation decreased superoxide in nondopaminergic cells. BH 4 is also a cofactor in catecholamine and NO production. In order to exclude the possibility that the antioxidant effects of BH 4 are mediated by dopamine and NO, we used fibroblasts in which neither catecholamine nor NO production occurs. In fibroblasts, BH 4 decreased baseline reactive oxygen species, and attenuated reactive oxygen species increase by rotenone and antimycin A. Physiologic concentrations of BH 4 directly scavenged superoxide generated by potassium superoxide in vitro. We hypothesize that BH 4 protects dopaminergic neurons from ordinary oxidative stresses generated by dopamine and its metabolites and that environmental insults or genetic defects may disrupt this intrinsic capacity of dopaminergic neurons and contribute to their degeneration in Parkinson's disease. Parkinson's disease (PD)1 is characterized by the selective degeneration of dopaminergic neurons in the substantia nigra (SN) pars compacta. While the etiology and pathogenesis of this cell death in most cases of PD remain unknown, it has been widely hypothesized that increased oxidative stress and mitochondrial dysfunction contribute to dopaminergic neuronal degeneration. Autopsy studies have noted numerous abnormalities indicative of increased oxidative damage in the SN (1-5).In particular, decreased glutathione in the SN has been found in autopsy brains from individuals with incidental Lewy bodies, presumed to represent presymptomatic PD (6). In addition, mitochondrial complex I activity is also decreased in the SN of PD patients (7, 8), and complex I inhibition can increase the formation of reactive oxygen species (9, 10). Although these findings suggest that dopaminergic neurons are susceptible to oxidative stress, studies in vivo have not observed dopaminergic neuronal death following the oxidative stress generated by glutathione depletion (11-13). Furthermore, mesencephalic dopaminergic neurons in primary monolayer (14) and reaggregate (15) cultures are less susceptible to the toxicity of glutathione depletion than nondopaminergic neurons.To explain this unanticipated resistance of dopaminergic neurons to oxidative stress, we hypothesized that dopaminergic neurons may contain additional antioxidants for the metabolism of ROS. One candidate is tetrahydrobiopterin (BH 4 ), which is present abundantly in do...

1-Methyl-4-phenylpyridinium-induced Apoptosis in Cerebellar Granule Neurons Is Mediated by Transferrin Receptor Iron-dependent Depletion of Tetrahydrobiopterin and Neuronal Nitric-oxide Synthase-derived Superoxide

“…Intracellular BH 4 content was reduced by treating cells with inhibitors of BH 4 de novo synthesis. In agreement with a previous report (53) , Fig. 3 (A and B) shows that inhibiting BH 4 synthesis by incubating cells with DAHP (1 mM) and NAS (100 M) for 24 h caused an increase in superoxide generation in CGNs.…”

Distinct Mechanisms of Neurodegeneration Induced by Chronic Complex I Inhibition in Dopaminergic and Non-dopaminergic Cells

“…We observed no significant activity differences in any of these enzymes between cell types (Table I). We have previously found tetrahydrobiopterin (BH 4 ), a cofactor for tyrosine hydroxylase, to have antioxidant activity, scavenge superoxide, and protect cells from oxidative stress (28,29). As expected from having a dopaminergic phenotype, MN9D cells had much higher BH 4 levels compared with MN9X (Table I).…”

“…Although it is difficult to separate differences in oxidant production and antioxidant capacities in whole cells, the higher glutathione levels in dopaminergic midbrain neurons compared with non-dopaminergic cells we have previously reported, as well as the presence of tetrahydrobiopterin, a scavenger of superoxide, only in dopaminergic neurons (28,29) likely are significant contributors to this increased antioxidant capacity in dopaminergic neurons. In the present study, we found no differences in activity levels of major antioxidant enzymes between MN9D and MN9X cells.…”

“…We have previously shown that primary midbrain dopaminergic neurons produce less reactive oxygen species than nondopaminergic midbrain neurons (13), and are more resistant to the oxidant stresses of glutathione depletion or H 2 O 2 treatment (29). Although it is difficult to separate differences in oxidant production and antioxidant capacities in whole cells, the higher glutathione levels in dopaminergic midbrain neurons compared with non-dopaminergic cells we have previously reported, as well as the presence of tetrahydrobiopterin, a scavenger of superoxide, only in dopaminergic neurons (28,29) likely are significant contributors to this increased antioxidant capacity in dopaminergic neurons.…”

Sepiapterin reductase expression is increased in Parkinson's disease brain tissue

“…There is conflicting evidence whether BH 4 is protective or toxic to dopaminergic neurons. Extracellular BH 4 has been shown to increase the rate of apoptotic cell death in DA neurons (Choi et al, 2000;Choi et al, 2003;Kim et al, 2004), while intracellular BH 4 appears to protect DA neurons against oxidative damage (Choi et al, 2003;Nakamura et al, 2001). Treatment of cerebellar granule neurons with sepiapterin attenuated 1-methyl-4-phenylpyridinium-induced (MPP+) toxicity (Shang et al, 2004).…”