Nitrosative stress in Parkinson's disease

Abstract

Parkinson's Disease (PD) is a neurodegenerative disorder characterized, in part, by the loss of dopaminergic neurons within the nigral-striatal pathway. Multiple lines of evidence support a role for reactive nitrogen species (RNS) in degeneration of this pathway, specifically nitric oxide (NO). This review will focus on how RNS leads to loss of dopaminergic neurons in PD and whether RNS accumulation represents a central signal in the degenerative cascade. Herein, we provide an overview of how RNS accumulates in PD by considering the various cellular sources of RNS including nNOS, iNOS, nitrate, and nitrite reduction and describe evidence that these sources are upregulating RNS in PD. We document that over 1/3 of the proteins that deposit in Lewy Bodies, are post-translationally modified (S-nitrosylated) by RNS and provide a broad description of how this elicits deleterious effects in neurons. In doing so, we identify specific proteins that are modified by RNS in neurons which are implicated in PD pathogenesis, with an emphasis on exacerbation of synucleinopathy. How nitration of alpha-synuclein (aSyn) leads to aSyn misfolding and toxicity in PD models is outlined. Furthermore, we delineate how RNS modulates known PD-related phenotypes including axo-dendritic-, mitochondrial-, and dopamine-dysfunctions. Finally, we discuss successful outcomes of therapeutics that target S-nitrosylation of proteins in Parkinson's Disease related clinical trials. In conclusion, we argue that targeting RNS may be of therapeutic benefit for people in early clinical stages of PD.

Fig. 1. Sources of reactive nitrogen species.

Various formulas showing the production of reactive nitrogen species are displayed (a). The known reactions between RNS and thiols are also shown (b). A cartoon depicting the primary effects of various RNS and their intermediates (c). This figure was created using Biorender.

Fig. 2. Primary production of NO occurs at the nitric oxide synthase (NOS).

NOS may be activated by various ions, proteins, or enzymes. To produce nitric oxide, NOS requires oxygen, NADPH and L-arginine. The reductase (containing FMN) and oxygenase (heme-containing) domain are bound by calmodulin (CaM). To produce nitric oxide, oxygen and NADPH enter the reductase domain facilitating the transfer of the electron from the FAD, to the FMN, to the heme. This electron, along with oxygen, is needed to convert L-arginine into N-hydroxyl-L-arginine (monooxygenase reaction I). Next, a second monooxygenase reaction involving an electron, oxygen molecule and co-factor BH4 occurs to convert N-hydroxyl-L-arginine to L-citrulline and nitric oxide (NO). BH4 is oxidized to BH2, which can then be reduced back to BH4, balancing NOS dimers in an on (coupled)/off (uncoupled) state. NOS subunits are held together by zinc (Zn) ions that bind the oxygenase domains. Output from NOS includes L-citrulline, water, NADP and nitric oxide. This figure was created using Biorender.

Fig. 3. RNS-induced dysfunctions in Parkinson’s disease.

This schematic highlights the major phenotypes that arise as a consequence of RNS in PD. This includes axo-dendritic defects (a), mitochondrial defects (b) and dopamine-induced defects (c) discussed in this review. This figure was created using Biorender.

Fig. 4. Major pathways of protein de-nitrosylation.

Two major de-nitrosylation pathways reduce s-nitrosocysteine in cells: the thioredoxin reductase (Trx) family of redox sensors that includes Glutaredoxin (Grx), and the indirect S-nitrosoglutathione reductase (GSNOR) system. a Trx removes the s-nitrosyl-group from the protein-SNO (either through Trx-mediated trans-nitrosylation or Trx-disulfide-bond formation within the protein-SNO) and then b generates a disulfide bond, freeing HON or NO + H⁺. Alternatively, the GSNOR system c first requires that glutathione (GSH) react with the s-nitrosocysteine of an S-nitrosylated protein, transferring the nitrosyl-group to the thiol of the cysteinyl-group of GSH generating GSNO. d Next, the enzyme S-nitrosoglutathione reductase (GSNOR) and co-enzyme NADH catalyzes the conversion of GSNO to an N-hydrosulfinamide (GSNOH) intermediate and subsequently to glutathione-disulfide (protein-GSSG) and hydroxylamine (not shown). e Protein-GSSG is then reduced to GSH through the activity of glutathione reductase (GR) and co-enzyme NADPH. f The Grx member of the Trx family has two modes of SNO reduction. Grx can form an oxidized Grx intermediate (Grx-SNO) on one cysteine residue following transfer of the nitrosyl-group from protein-SNO. This can either be reduced though the GSH pathway described above (mono-thiol reaction) or through the traditional Trx system (diol-thiol reaction) by producing a disulfide bond. g The now oxidized Grx is reduced by two GSH molecules to restore the cycle. This figure was created using Biorender.