Oxidative stress and cellular pathologies in Parkinson’s disease
Puspita et al. Molecular Brain
Oxidative stress and cellular pathologies in Parkinson's disease
Lesly Puspita 0
Sun Young Chung 1
Jae-won Shim 0
0 Soonchunhyang Institute of Medi-bio Science (SIMS), Soonchunhyang University , 25, Bongjeong-ro, Dongnam-gu, Cheonan-si 31151 , South Korea
1 Center for Stem Cell Biology, Sloan-Kettering Institute , New York, NY 10065 , USA
Parkinson's disease (PD) is a chronic and progressive neurodegeneration of dopamine neurons in the substantia nigra. The reason for the death of these neurons is unclear; however, studies have demonstrated the potential involvement of mitochondria, endoplasmic reticulum, α-synuclein or dopamine levels in contributing to cellular oxidative stress as well as PD symptoms. Even though those papers had separately described the individual roles of each element leading to neurodegeneration, recent publications suggest that neurodegeneration is the product of various cellular interactions. This review discusses the role of oxidative stress in mediating separate pathological events that together, ultimately result in cell death in PD. Understanding the multi-faceted relationships between these events, with oxidative stress as a common denominator underlying these processes, is needed for developing better therapeutic strategies.
Alpha-synuclein; Dopamine neurons; Mitochondria; Oxidative stress; Parkinson's disease; Reactive oxygen species; Unfolded protein response
Parkinson’s disease (PD) is the second most common
neurodegenerative disorder, characterized by serious
movement disturbances such as tremor, rigidity, and
bradykinesia. It is a chronic condition attributed by the
selective degeneration of dopamine (DA) neurons in the
midbrain substantia nigra (SN). By the time patients
experience these symptoms, a portion of DA neurons
that project from the SN to the striatum have already
]. Appearance of insoluble inclusions in
neurons called Lewy bodies, which mainly consist of
αsynuclein, is a major hallmark of this disease . Based
on its progressive nature, it is unlikely that the disease
pathogenesis is triggered by an acute toxicity that kills
cells directly. Instead, it is possible that an ongoing
process such as oxidation is responsible for the gradual
dysfunction that manifests across myriad cellular
pathways throughout the disease trajectory.
Most PD cases are sporadic, with only about 10%
associated with an inherited genetic component. Even
though familial cases comprise only a minor subset of
the overall PD pool, examining PD-related monogenic
mutations is a valuable method of understanding disease
pathogenesis and cell death which may have implications
for studying the disease at large. PTEN induced putative
kinase 1 (PINK1), Parkin, DJ-1, leucine-rich repeat
kinase 2 (LRRK2) and α-synuclein are among the proteins
which have been strongly linked to the familial forms of
]. Of these, α-synuclein is most commonly
associated with PD pathogenesis for its predominance in
Lewy bodies, which develop and aggregate throughout
disease progression [
]. PINK1 and Parkin are
involved in mitochondria-related autophagy, whereas the
loss of function of these proteins leads to the
accumulation of damaged mitochondria [
]. DJ-1 is involved
in a wide range of cellular functions, two of which
include its roles as a sensor for oxidative stress and as a
redox-chaperone protein [
]. The physiological
function of LRRK2 is less understood but neurons with
mutations in this protein exhibit greater vulnerability to
mitochondrial toxins .
Reactive oxygen species (ROS) are normally produced
in the cell during mitochondrial electron transfer chain
(ETC) or redox reactions and are in fact a necessary
component of cellular homeostasis. As an example,
several enzymes in mitogen-activated protein kinase and
phosphoinositide 3-kinase pathways, which are pivotal in
mediating cellular responses to growth hormones and
cytokines, are regulated directly by ROS [
despite the importance of ROS in normal physiology,
antioxidant proteins like superoxide dismutase (SOD)
and glutathione (GSH) also prevent ROS levels from
getting too high [
]. Failure of these antioxidants in
regulating ROS levels leads to oxidative stress, which can
have a variety of detrimental effects. Random oxidation
of macromolecules inside the cell can damage cellular
structures and even lead to cell death [
publications have reported evidence of oxidative stress
through the detection of oxidized DNA, lipids, and
proteins in the brain tissues of both familial and sporadic
PD patients [
]. Since oxidative stress increases the
chance of spontaneous mutations, it is possible that this
can trigger mutations that make cells more vulnerable
to dysfunction. Interestingly, in the SN of healthy
individuals, the concentration of oxidized proteins was
found to be twice that of the caudate, putamen, and
the frontal cortex, indicating that susceptibility of SN
to oxidative stress may contribute to the selective
neuronal degeneration .
While many studies in the past have examined
dysfunctional cellular processes in PD independently of
each other, in our recent study, we sought to develop a
more comprehensive understanding of the disease by
examining how those processes might be interconnected
]. Using induced pluripotent stem cell (iPSC)-derived
midbrain DA neurons from patients with PINK1 or
Parkin mutations, we first noted the presence of
abnormal mitochondria. We also observed cytosolic
αsynuclein and DA accumulation, with increased
sensitivity toward oxidative stress-inducing agents, in the
mutant lines [
]. Similarly, another group which had
utilized LRRK2 mutant iPSC-derived neurons noted
elevated expression of genes involved in oxidative stress
regulation, and α-synuclein levels. Moreover, they
observed cells’ increased vulnerability towards H2O2,
6hydroxydopamine (6-OHDA), and MG132, a
proteasome inhibitor [
]. Together, these results reflect the
idea that a single mutation can profoundly disrupt
cellular homeostasis, implying that PD progression may be
the result of a multitude of interactions between the
various pathogenic phenotypes linked to cellular stress.
While oxidative stress has been thoroughly researched,
advances in stem cell technology have engendered a
wide range of tools with which to study and model
diseases in vitro. As demonstrated by our study as well as
many others, iPSCs have made it possible to study
specific disease mutations using patient derived cells, which
has been especially valuable in modeling
neurodegenerative diseases which lack authentic animal models.
Moreover, because PD is diagnosed only when the
degeneration of midbrain DA neurons has already
progressed considerably, neurons from PD patient-derived
iPSCs enable researchers to carefully track even minor
disturbances that precede major pathogenic processes.
Based on our iPSC-based findings demonstrating the
contribution of oxidative stress toward triggering
dysfunctional processes, this review explores how
oxidative stress may play a central role in mediating disease
progression (Fig. 1).
The mitochondrion is a key site of ROS production and a target of ROS-induced damage
In the inner membrane of mitochondria, electrons are
transferred through a series of protein complexes via
redox reactions to oxygen, the last electron acceptor. As
the electrons pass, some protons are translocated by the
electron carriers from the matrix to the mitochondrial
intermembrane space, thereby creating a proton
gradient. Protons flow back into the mitochondrial matrix
following its gradient, concurrently providing energy for
the ATP synthase to phosphorylate ADP into ATP. This
entire process, which is a critical means of energy
production, produces ROS as a major byproduct [
Premature electron leakage in ETC Complex I
(Nicotinamide adenine dinucleotide [NADH] dehydrogenase)
and Complex III (cytochrome bc1) to oxygen, is the
main source of mitochondrial superoxide anions (O2−)
]. Production of ROS from the mitochondrial
action is physiologic, but dysfunction of ETC in damaged
mitochondrial causes excessive ROS production, which
is quite detrimental to cells.
The involvement of mitochondria in PD pathogenesis
was first brought to light after individuals consumed illicit
drugs contaminated with
1-methyl-4-phenyl-1,2,3,6-tetrabydropyridine (MPTP). Symptoms resembling those
present in PD were observed soon after drug intake with
postmortem analyses revealing destruction of the SN [
Subsequent studies explained that
1-methyl-4-phenylpyridinium (MPP+), the toxic bioactive form of MPTP,
undergoes oxidation by monoamine oxidase B (MAO-B) and
enters the DA-producing neurons in the SN via the DA
reuptake system [
]. Upon entering the cell, MPP+
inhibits the mitochondrial ETC Complex I enzyme, and
NADH-ubiquinone oxidoreductase (EC 184.108.40.206), resulting
in electron leakage and ROS generation in mitochondria
]. Similarly, rotenone, a pesticide, also induces
parkinsonism by inhibiting ETC Complex I. Due to its
hydrophobicity, rotenone easily crosses biological membranes
independently of the DA transporter and its delivery
causes systemic inhibition of the mitochondrial ETC.
Notably, the degeneration is specific to midbrain DA
neurons, while other DA-producing neurons in the ventral
tegmental area (VTA) may be relatively spared [
decline of Complex I activity and elevated intracellular
ROS have been verified in the SN of the post-mortem
brain of PD patients [
Further supporting the importance of the
mitochondria and its relevance in PD is the fact that PD-related
genes such as PINK1, PARK2 (Parkin), DJ-1 and LRRK2
encode proteins that regulate mitochondrial and ROS
3, 4, 6, 7, 36
]. PINK1 is a mitochondrial
protein that is degraded rapidly in healthy mitochondria.
In defective mitochondria, which may exhibit high levels
of oxidative stress, decreased membrane potential, or the
presence of misfolded proteins, the degradation of PINK1
is impeded, leading to the accumulation of PINK1 on the
mitochondrial outer membrane. PINK1, which
phosphorylates Parkin at Ser65, induces E3 ubiquitin ligase activity
of the enzyme and its recruitment to the mitochondria.
Parkin modifies proteins on the mitochondrial membrane
by adding ubiquitin chains that function as signals for
autophagy. The mitochondria-specific autophagy process,
also known as mitophagy, ultimately results in
mitochondria engulfment and degradation [
] which has been
demonstrated experimentally where the systemic
knockout of Parkin in mice resulted in elevated intracellular
ROS levels in the VTA and a reduction in proteins
involved in ETC and oxidative stress regulation . In
drosophila, PINK1 null mutants exhibited reduced
mitochondrial membrane potential, suboptimal ETC activity,
as well as a reduction in synaptic neurotransmitter release
in neural cells [
]. Moreover, accumulation of damaged
mitochondria in neuronal axons, as observed in PINK1
knock-out mutants, could be a source of ROS and
oxidative damage [
Defects in mitophagy and increased oxidative stress
might also partially explain the specificity of the
phenotypes in DA neurons. As shown in PINK1 null mutants
that display reduced neurotransmitter release,
mitochondria are critical in cells with actively firing axons [
an unusual case of a woman with a homozygote recessive
Parkin mutation, she remained free of PD symptoms even
through her eighth decade, while her relative carrying the
same mutation exhibited early onset of PD. When
comparing their fibroblasts, it was found that Nip3-like
protein X, which may mediate a Parkin/PINK1-independent
pathway in eliminating damaged mitochondria, was highly
upregulated in the asymptomatic carrier. Levels of
mitochondrial membrane potential, oxygen consumption rate,
and resistance capacity toward protonophore carbonyl
cyanide-m-chlorophenylhydrazone (CCCP) were higher in
fibroblasts derived from the asymptomatic carrier than
those of the individual with PD symptoms. Although the
study was conducted in fibroblasts and not midbrain DA
neurons, the data strongly suggested that failure in
mitochondria clearance or in other words, accumulation of
defective mitochondria, was an important factor in
mediating PD pathology [
]. Another protein related to a
recessive form of PD, DJ-1, contains a cysteine residue
(C106) that is vulnerable to oxidation during oxidative
stress. Oxidation of C106 leading to the formation of
cysteine-sulfinic acid has been verified using mass
] and crystal analysis of DJ-1. During
oxidative stress, the oxidized DJ-1 may translocate to the
outer membrane of mitochondria and it has been
shown to prevent MPP + −induced cell death, the
mechanism of which remains unclear [
]. In line
with this finding, homozygous mutation of DJ-1 has
been linked to increased mitochondrial oxidative
stress in human iPSC-derived DA neurons, a feature
that was accompanied by accumulation of α-synuclein
and oxidized form of DA [
As one of the main sites of ROS production,
mitochondria are particularly susceptible to oxidative
stressinduced damage. Unlike nuclear DNA, mitochondrial
DNA (mtDNA) are unprotected by histone proteins and
therefore are easy targets of oxidation [
production and mtDNA damage has been shown to increase
with age, up to 10–20 folds higher than in nuclear DNA
]. Since most of the proteins coded by mtDNA
are involved in ETC, mutations and deletions in mtDNA
would likely disturb ETC and increase ROS formation,
creating a vicious cycle further inflicting mitochondrial
damage . Another mechanism of how this cycle
might work involves nitrosative stress induced by either
mitochondrial toxins or mutated α-synuclein proteins.
In Ryan et al., these were shown to cause sulfonation on
myocyte-specific enhancer factor 2C (MEF2C). This
modification inhibits MEF2C transcriptional activity and
consequently, decreases expression of the target genes.
One of the important genes regulated by MEF2C
encodes peroxisome proliferator-activated receptor gamma
coactivator 1-alpha (PGC-1a), a master regulator of
mitochondria biogenesis. Therefore, failure to express
PGC-1a implies dysfunction among mitochondria [
ER protein folding and calcium storage functions are prominent sources of ROS
The endoplasmic reticulum (ER) is the site of secretory
protein production and post-translational modifications
such as protein folding and glycosylation. Protein folding
is a process that is greatly affected by the redox status of
the ER lumen as the formation of disulfide bonds requires
a highly oxidizing environment. During disulfide bond
formation, electrons are transferred from the target
protein to oxygen by protein disulfide isomerase and ER
oxidoreductin-1, which forms ROS as byproduct [
Quantitative analyses of protein synthesis and processing
suggest that disulfide bond formation produces
approximately 25% of total ROS in the ER lumen .
Another vital role of the ER entails regulation of
intracellular calcium, involving the release or absorption of
calcium to regulate its cytoplasmic concentration levels.
Failure to maintain the calcium concentration in
homeostasis and accumulation of misfolded proteins may lead to
the activation of the unfolded protein response (UPR)
]. UPR is a protective mechanism which is initiated by
three proteins in the ER membrane: inositol-requiring
enzyme 1 (IRE1), activating transcription factor (ATF) 6, and
pancreatic ER kinase (PKR)-like ER kinase (PERK) which
each bind to GRP78/BIP (binding immunoglobulin
protein), a chaperone in the ER lumen. In the presence of
misfolded proteins, BIP dissociates from the membrane
proteins to bind the misfolded proteins. Dissociation from
BIP activates the three membrane proteins and the
following pathways. IRE1 splices the intron in X-box binding
protein 1 (XBP1) RNA to produce its translationally active
form. After dissociation from BIP, ATF6 is translocated
from the ER to the Golgi apparatus, where it is cleaved
and activated. Both XBP1 and ATF6 act as transcriptional
regulators of ER chaperones and ER-associated
degradation pathways, which are essential for reducing ER stress
and promoting cell survival. Lastly, BIP dissociation
triggers autophosphorylation of PERK into phosphorylated
PERK (pPERK). Phosphorylation of initiation factor 1
subunit α (eIF2α) by p-PERK results in global attenuation of
protein translation. However, the attenuation does not
apply to certain PERK-downstream proteins like ATF4.
Prolonged expression of ATF4 can trigger the expression
of another transcription factor, C/EBP homologous
protein (CHOP), and the downstream apoptotic pathway
]. Activation of the ATF4/CHOP pathway could
lead to apoptosis and has thus far been suggested as a part
of the neuronal apoptotic signaling pathway .
The ER’s function in regulating calcium and the
downstream events can greatly affect mitochondria. Calcium
leakage from the ER into the cytosol due to ER stress
can trigger excessive calcium intake by the mitochondria
via mitochondrial calcium uniporter (MCU) [
ERto-mitochondria transfer of calcium is facilitated by
mitochondrial-associated membrane (MAM) which brings
both organelles in close proximity [
]. As visualized
in human fibroblasts using green fluorescence
proteintagged ER membrane protein and the mitochondrial dye
tetramethylrhodamine methyl ester, enhanced proximity
of both organelles was observed in fibroblasts containing
Parkin mutation compared to those of control fibroblasts
]. As expected, calcium transfer to mitochondria was
equally increased. Downregulation of Parkin substrate
mitofusin 2, another MAM tethering protein [
exogenous expression of Parkin was shown to rescue the
disturbance of calcium homeostasis in the mutant
]. Moreover, it has been found that increased
calcium in the mitochondria leads to the stimulation of ETC
and can exacerbate ROS formation, or induce the
activation of mitochondria-related apoptotic pathways [
In PD cases, immunoreactivity of pPerk and peIF2a
were observed by Hoozemans and colleagues in the SN
of patients compared to those of healthy individuals
]. Another study revealed that treatment with either
MPP+ or 6-OHDA induced the changing of UPR
proteins such as expression of BIP and CHOP, and
phosphorylation of PERK and eIF2α, while relieving ER stress
with salubrinal, a selective inhibitor of eIF2α
dephosphorylation, attenuated mitochondrial toxin-induced cell
]. ER stress might also impact cellular oxidative
stress through regulation of mitochondrial clearance.
Putative ATF4 binding sites were found upstream of the
transcriptional start site and in the first intron of the
human PARK2 gene. To verify this, ER stress was triggered
in SH-SY5Y, human embryonic kidney T293 cells, and
mouse embryonic fibroblasts by using the ER Ca2 +
−ATPase inhibitor thapsigargin, the N-glycosylation
inhibitor tunicamycin, and inducing amino acid starvation.
In all experiments, upregulation of Parkin mRNA was
observed. Data from a luciferase reporter assay and
chromatin immunoprecipitation provided further
evidence that ATF4 functions as a transcriptional regulator
of Parkin [
]. A separate study also supported this idea
by showing that ATF4 protects from neuronal apoptosis
by regulating the level of Parkin [
]. Altogether the
studies suggest the failure in degradation of Parkin’s
substrates that is required during UPR activation may
contribute to the neurodegeneration in PD with Parkin
mutation. Nonetheless, further research is needed to
determine whether ER stress is directly involved in the
pathogenic mechanism of Parkin-associated PD.
Increased risk of developing sporadic PD after
overexposure to manganese, copper, iron and mercury has been
studied for decades [
] where it has been suggested
that α-synuclein, ER stress, and oxidative stress are
involved in manganese toxicity in neurons. After 24 h of
manganese treatment, α-synuclein oligomerization,
elevated ROS, and oxidative damage of macromolecules have
been observed in primary neuronal cultures. As an
example, pre-treatment with GSH partially rescued the
αsynuclein oligomerization and neuronal damage while
H2O2 accelerated the process . Xu et al. demonstrated
that manganese treatment on rat brain slices induced
expression of UPR proteins and apoptotic cell death [
the rat models where α-synuclein expression was silenced
by siRNA, apoptosis by manganese treatment was less
pronounced. Furthermore, pPERK, pEIF2a, and ATF4
protein levels were also lower than those of wild type
(WT) rats, despite the absence of changes in pIRE1 and
sXBP1, suggesting α-synuclein involvement in the UPR
PERK pathway [
]. In mice with mutant A53T,
interaction between α-synuclein with ER chaperones in ER
lumen was observed, indicating that abnormalities in
αsynuclein alone could trigger ER stress and the
downstream response [
]. Activation of IRE1α/XBP1 axis of
UPR was also found in iPSC-derived DA neurons obtained
from patients with α-synuclein triplication. In support of
this, postmortem analyses conducted in the same study
verified the presence of pIRE1 together with elevated level
of α-synuclein in the brain [
]. Additionally, a recent
study with an animal model reported that tunicamycin, an
ER stress inducer also affected the aggregation process of
Alpha-synuclein is affected by and contributes to oxidative stress, by binding with iron and mitochondrial membrane proteins
Alpha-synuclein is a 140 kDA protein encoded by the
SNCA gene. As the main component of Lewy bodies,
αsynuclein is a well-known player in PD pathogenesis as
duplication, triplication, and point mutations in its
Nterminal region (A30P, A53T and E46K) are connected
to familial PD [
]. A growing body of work suggests
that the monomer and tetramer types are the
physiological forms of α-synuclein, while oligomers and fibrils
are the pathogenic forms [
]. Abundance of fibril
αsynuclein was detected in Lewy bodies in several studies;
however, abnormal accumulation of the soluble
monomer form that leads to formation of oligomers and fibrils
has also been proposed as a key pathogenic event in the
early stages of PD [
Spontaneous oligomerization and fibrilization of
αsynuclein have also been observed in vitro, with mutated
α-synuclein oligomerizing faster than the WT form of
the protein [
]. In the same study, DA treatment
increased the rate of polymerization in both mutated and
WT forms of α-synuclein. Evidence of α-synuclein
accumulation as a signature of disease initiation was shown
in Nurr1+/tyrosine hydroxylase (TH) + neurons derived
from a patient’s iPSCs. Additionally, α-synuclein
accumulation was observed in neurons derived from patients
with PINK1 or Parkin mutations, along with abnormal
mitochondrial morphology and increased sensitivity
towards oxidative stress [
]. Deas et al. suggested that
interactions between α-synuclein oligomers and metal ions
may induce oxidative stress in human iPSC-derived
neurons. Neurons with α-synuclein triplication were
reported to have a higher basal level of oxidative stress.
When monomer, oligomer, or fibril forms of exogenous
α-synuclein were added, α-synuclein oligomers triggered
oxidative stress more potently than monomers and
fibrils. Neurons treated only with oligomer α-synuclein
demonstrated a reduction in the level of GSH and an
increase in lipid peroxidation [
]. The propensity of
oligomers to induce ROS production was significantly
reduced in the presence of metal chelators such as
deferoxamine, indicating that α-synuclein oligomers produce
superoxide radicals by binding to transition metal ions
such as copper and iron [
]. In vitro incubation of
αsynuclein with iron resulted in the formation of H2O2
and hydroxyl radicals, a finding that supported iron-rich
SN neurons’ selective vulnerability toward oxidative
]. Increased iron has also been detected in
the SN of postmortem PD brains as well as living PD
patients using magnetic resonance imaging [
Greater colocalization of iron and DA was found in the
SN compared to those in the VTA , and given the
ability of both substances to modify α-synuclein [
this can explain the selective vulnerability of this region.
Oxidative stress upon accumulation of α-synuclein can
also be mediated by direct interaction between
αsynuclein and mitochondrial membrane protein. In fact,
α-synuclein has been demonstrated to disturb the
translocation of nuclear-encoded mitochondrial proteins into
the mitochondria by binding to translocase of the outer
membrane (TOM)20 in both rotenone-treated and
human α-synuclein-overexpressing animal models. The
binding prevents the interaction of TOM20 with the
coreceptor TOM22 and the subsequent translocation of
mitochondrial-targeted proteins, which include some
subunits of Complex I. As a result, mitochondrial ETC
is rendered defective and intracellular oxidative stress
While α-synuclein toxicity can contribute in elevating
cellular oxidative stress, it has also been suggested that
oxidative stress can trigger the toxicity of α-synuclein.
One consequence of chronic oxidative stress is lipid
peroxidation of polyunsaturated fatty acids in the cell
membrane, as observed in post-mortem SN [
]. A product
of lipid peroxidation, 4-hyroxy-2-nonenal, prevents
fibrillation of α-synuclein and supports the formation of
secondary beta sheets and toxic soluble oligomers in a
dose-dependent manner [
]. Thus, oxidative stress
can also influence α-synuclein toxicity and mediate PD
pathogenesis. Incubation of α-synuclein monomers with
cytochrome c/H2O2 led to α-synuclein aggregation
in vitro by catalyzing the crosslinking of α-synuclein
tyrosine residues through 3,3′-dityrosine bond
]. Colocalization of cytochrome c and
αsynuclein has also been detected in the Lewy body of PD
Since accumulation of α-synuclein monomers initiates
the formation of aggregates, some studies have shifted
their focus toward the α-synuclein degradation process.
Both ubiquitin-proteasome system (UPS) and
autophagylysosomal pathway have been linked with α-synuclein
], with more recent publications suggesting
that UPS is the main α-synuclein degradation pathway
under normal physiological conditions, while the
lysosomal pathway is more responsive to stress or when
αsynuclein is overly abundant [
autophagy, a subtype of lysosomal pathway, works with
the help of cytosolic chaperone Hsc70 that recognizes the
KFERQ peptide motif in α-synuclein [
αsynuclein is delivered to a receptor in the lysosomal
membrane and translocated into the lysosome, where
enzymatic degradation takes place. However, in a highly
oxidizing environment, the oxidized form of α-synuclein
cannot be efficiently degraded, resulting in its
accumulation and aggregation [
]. Its accumulation also affects
vesicle trafficking between the ER and the Golgi, and
reduces lysosomal degradation capacity [
this, α-synuclein had been shown to inhibit ER-Golgi
transit of COPII vesicles which carries ATF6 to its activation
site at the Golgi apparatus. Consequently, the protective
ATF6 pathway of UPR signaling was blocked [
Oxidative stress and α-synuclein accumulation
preceding diminished lysosomal proteolysis has also been
observed in patient-derived DJ-1 mutant DA neurons [
Attenuation of this process results in the buildup of
cargo proteins, which can trigger ER stress and activate
the UPR response system, and prolonged ER stress
caused by misfolded proteins has been linked to ROS
production, possibly due to higher levels of protein
folding activity in the ER lumen. Degradation of these
misfolded proteins mitigates oxidative stress and
associated cell death [
Elevated intracellular DA promotes oxidation and
increases SN DA neurons’ vulnerability towards oxidative
It is important to highlight that despite their relevance
in PD phenotypes, α-synuclein, PINK1, Parkin, and DJ-1,
are not exclusively expressed in midbrain DA neurons
3, 4, 6, 7, 36
]. The specific neurodegeneration in loss of
function mutants may be caused by the nature of the
neurons themselves in their ability to produce and
release DA, which can be highly reactive. Its metabolism
can lead to the production of ROS byproducts such as
hydrogen peroxide. DA synthesis involves several
enzymatic reactions beginning with tyrosine hydroxylase
catalyzing the hydroxylation of L-tyrosine at the phenol ring to
produce L-3,4-dihydroxyphenylalanine (L-DOPA). Next,
aromatic acid decarboxylase converts L-DOPA to DA.
Active transport by vesicular monoamine transporter 2
(VMAT2) mediates DA storage in vesicles, an important
step in protecting DA which is easily oxidized in the
cytoplasm. In action potentials, DA vesicles fuse with the
presynaptic membrane at the terminal button and are
released into the synapse where they engage with receptors
in the post-synaptic membranes. After binding, DA
molecules can be reabsorbed into the cytosol via the DA
transporter where it may undergo several different fates. It can
be transported back into storage vesicles by VMAT2 and
reused in the next axon firing, or be degraded by MAO,
which leads to the production of
3,4-dihydroxyphenylacetaldehyde (DOPAL) and H2O2, two potent oxidizing
]. The enzyme aldehyde dehydrogenase
turns DOPAL into the less reactive
3,4-dihydroxyphenylacetic acid (DOPAC) .
In the presence of iron in the cytosol, DA can be
oxidized into DA-quinone (DAQ)—a highly reactive and
toxic compound [
]. In fact, the presence of the
oxidized form of DA has been verified in iPSC-derived
DA neurons containing parkin, PINK1, LRRK2
mutations or SNCA triplication . DAQ binds covalently
with free cysteine and cysteine residues on proteins,
which can drastically alter their function [
may also be involved in mitochondrial depolarization
after DA exposure  and its binding to
mitochondrial proteins can inhibit Complex I and IV. This was
supported by the fact that treatment with quinone
scavenger, GSH, reversed this effect. Proteomic analysis of
isolated rat mitochondria in the same experiment
revealed that mitochondrial proteins had been modified
covalently by DAQ, including ubiquinol-cytochrome c
reductase core protein 1 [
]. A recent study by Bondi
et al. using SH-S5Y5 cells interestingly showed that DA
treatment did not induce PINK and Parkin localization
in the mitochondria, as CCCP treatment did. This
means that despite inducing depolarization, DA did not
activate the PINK1-Parkin autophagy pathway that was
necessary to get rid of defective mitochondria [
DJ-1 mutant human DA neurons, increased mitochondrial
oxidative stress and accumulation of α-synuclein could be
reversed with α-methyl-p-tyrosine, a competitive inhibitor
of TH, preventing DA synthesis [
]. Accumulation of
defective mitochondria by DA modification resembling
mitochondria-related abnormalities were observed in
several in vitro PD modeling studies involving PINK1 or
Parkin mutations thus providing a basis for what may
occur in sporadic cases and how the phenotypes mimic
those of familial PD cases [
25, 39, 40
]. Moreover, a more
oxidative environment due to defective mitochondria can
further stimulate DA oxidation to DAQ, as indicated by
increased binding of DAQ to cysteine-containing proteins
in the striatum of animal models upon MPTP treatment
]. The data altogether propose an alternate
mechanism involving a positive feedback loop among PD
elements that conditions the neurons into a state of chronic
DA has also been shown to react and alter the
function of PD related proteins, such as DJ-1, Parkin
and α-synuclein [
106, 109, 110
]. In DA neurons, DA
modification on Parkin leads to decreased solubility,
functional inactivation, and subsequent accumulation of
its ubiquitin ligase E3 substrates, including Synphilin-1
and Parkin, itself [
]. Interestingly, catechol-modified
Parkin was only found in the SN of normal human brain
tissue, but not in other areas such as caudate-putamen,
cerebellum, and adjacent red nucleus [
Overexpression of α-synuclein in human primary DA neurons
resulted in degeneration, a phenotype that was not observed
in non-DA cells nor β-synuclein-overexpressing cells.
Inhibition of DA production by α-methyl-p-tyrosine, a TH
inhibitor, or antioxidant vitamin E reversed the
αsynuclein overexpression-induced damage, supporting the
hypothesis that DA fueled-oxidative stress plays a key role
in mediating α-synuclein toxicity [
another study revealed that DOPAL-induced α-synuclein
oligomers damaged cellular vesicles by permeabilizing
cholesterol-containing vesicular membranes and inducing
leakage of DA from vesicles into the cytosol [
non-covalent modification on α-synuclein was also
observed to stabilize the protofibril form [
Changes in the level of cytosolic DA during PD
progression remains a controversial subject, with studies
arguing for elevation or [
], reduction  as a
cause of PD-related phenotypes. This contrast may be
due to the decline in cytosolic DA, a feature of PD in its
later stages when neurons are no longer able to produce
DA in contrast to the earlier stages when
overproduction of DA may be triggered by cellular dysfunction. The
idea of DA overproduction is also supported by studies
reporting the role of α-synuclein in negatively regulating
DA vesicle release. It was found that a 2–3-fold
αsynuclein overexpression in hippocampal and ventral
midbrain neurons impeded synaptic vesicle release [
while α-synuclein knock-out mice displayed stronger
release of DA upon stimulus [
]. Two potential reasons
behind this could be a reduction in the number of
vesicles available for release [
] or inhibition of
vesicle docking by α-synuclein oligomers . A
follow-up study demonstrated that α-synuclein
oligomers prevented soluble N-ethylmaleimide-sensitive
factor attachment protein receptor (SNARE) complex
formation which is necessary for vesicle docking, by
binding to synaptobrevin-2, a vesicle-associated
membrane protein [
]. These studies collectively
demonstrate synergism between α-synuclein and DA in
promoting oxidative stress in DA neurons (Fig. 2).
The mechanism of neurodegeneration in PD still
remains a controversial subject. Although PD entails a
wide variety of cellular phenotypes, it is possible to
decipher the events involved in the disease trajectory by
studying the genetic forms of PD with the hopes of
extrapolating gained insights toward the sporadic,
nonfamilial forms. Mutations in PINK1, Parkin, DJ-1, and
LRRK2 result in mitochondrial perturbations and
elevations in oxidative stress. Utilization of DA as a
neurotransmitter renders midbrain DA neurons more prone
to damage given the potential for production of
oxidative and reactive byproducts in DA metabolism. Elevated
ROS levels also seem to be involved in metal-exposure
related toxicity suspected to cause sporadic PD. These
studies hint at the idea that oxidative stress plays a
central role across a variety of PD linked phenotypes.
Additionally, chronic oxidative stress, beyond the load
that can be adequately regulated by homeostasis, can
impact macromolecules inside the cell and result in
cell death [
Considering the complexity and singularity of each PD
case, a great deal of effort is required to understand
general PD pathology. Collecting PD case studies is an
essential step in gaining a better idea of the underlying
biology and overall landscape of the disease progression.
Biological events considered in this review cannot
explain the full spectrum of phenotypes present in PD like
pathological events that occur on an intercellular level
such as neuroinflammation [
], gut microbiota
], or the prion-hypothesis of
]; however, mounting evidence pointing to
oxidative stress as a common denominator provides
hope for developing a more thorough understanding
that could explain the complex cellular pathologies. This
review proposes intracellular oxidative stress mitigation
as major path toward regenerative treatment. Treatment
with antioxidants, identification of appropriate
antioxidant therapeutic candidates as well as efficient delivery
methods across the blood brain barrier are major
hurdles that would need to be resolved for building the
groundwork for PD treatment in the context of our
proposed paradigm. An additional implication is the
identification of elements linked to oxidative stress as
potential diagnostic targets for PD, including
upregulation of lipid hydroperoxide and SOD activity, and
downregulation of antioxidant factors like sulfhydryl groups
and catalase activity in the blood [
]. Advances in the
area of in vitro disease modeling have illuminated novel
insights regarding PD and yielded new ways of studying
complex cellular phenotypes. Despite these advantages,
current in vitro or even animal disease models are
limited by their inability to recapitulate the disease in an
aged condition, which is especially relevant to studying
neurodegenerative diseases which are chronic conditions
that occur late in life. This issue remains a critical
challenge given that there are no definitive methods of
closely mimicking the naturally aged state of cells;
nevertheless, given the rapid and continued progress in the
field of disease modeling particularly in the context of
neurological disorders, there is hope that these strides
can soon lead to the development of effective
6-OHDA: 6-hydroxydopamine; ATF: Activation transcription factor;
CCCP: Carbonyl cyanide-m-chlorophenylhydrazone; CHOP: C/EBP homologous
protein; DA: Dopamine; DAQ: DA-quinone; DOPAC: 3,4-dihydroxyphenylacetic
acid; DOPAL: 3,4-dihydroxyphenylacetaldehyde; ER: Endoplasmic reticulum;
GSH: Glutathione; iPSC: Induced pluripotent stem cell; IRE1: Inositol-requiring
enzyme 1; L-DOPA: L-3,4-dihydroxyphenylalanine; LRRK2: Leucine-rich repeat
kinase 2; MAM: Mitochondrial-associated membrane; MAO-B: Monoamine
oxidase B; MCU: Mitochondrial calcium uniporter; MEF2C: Myocyte-specific
enhancer factor 2C; MPP + : 1-methyl-4-phenylpyridinium; MPTP:
1-methyl-4phenyl-1,2,3,6-tetrabydropyridine; mtDNA: Mitochondrial DNA;
NADH: Nicotinamide adenine dinucleotide; PERK: Pancreatic ER kinase-like
ER kinase; PD: Parkinson’s disease; PGC-1α: Peroxisome
proliferatoractivated receptor gamma coactivator 1-alpha; PINK1: PTEN induced
putative kinase 1; ROS: Reactive oxygen species; SN: Substantia nigra;
SNARE: Soluble N-ethylmaleimide-sensitive factor attachment protein
receptor; SOD: Superoxide dismutase; TH: Tyrosine hydroxylase;
TOM: Translocase of the outer membrane; UPR: Unfolded protein
response; UPS: Ubiquitin-proteasome system; VMAT2: Vesicular
monoamine transporter 2; VTA: Ventral tegmental area; WT: Wild type;
XBP1: X-box binding protein 1
This work was supported by the National Research Foundation of Korea (NRF)
grant (NRF-2017M3A9B4062415; NRF-2017R1A2B4003018; NRF-2016K1A4A3914725)
and by the International Science and Business Belt Program (2015 K000278) funded
by the Korea government (MSIP) for writing the manuscript.
Availability of data and materials
LP and SJ contributed to manuscript composition as well as construction of
figs. SC provided constructive suggestions and comments to improve the
manuscript. All authors read and approved the final manuscript.
Ethics approval and consent to participate
Consent for publication
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
1. Bellucci A , Mercuri NB , Venneri A , Faustini G , Longhena F , Pizzi M , et al. Review: Parkinson's disease: from synaptic loss to connectome dysfunction . Neuropathol Appl Neurobiol . 2016 ; 42 : 77 - 94 .
2. Tabbal SD , Tian LL , Karimi M , Brown CA , Loftin SK , Perlmutter JS . Low nigrostriatal reserve for motor parkinsonism in nonhuman primates . Exp Neurol . 2012 ; 237 : 355 - 62 .
3. Valente EM , Abou-Sleiman PM , Caputo V , Muqit MM , Harvey K , Gispert S , et al. Hereditary early-onset Parkinson's disease caused by mutations in PINK1 . Science . 2004 ; 304 : 1158 - 60 .
4. Polymeropoulos MH , Lavedan C , Leroy E , Ide SE , Dehejia A , Dutra A , et al. Mutation in the alpha-synuclein gene identified in families with Parkinson's disease . Science . 1997 ; 276 : 2045 - 7 .
5. Di Fonzo A , Rohe CF , Ferreira J , Chien HF , Vacca L , Stocchi F , et al. A frequent LRRK2 gene mutation associated with autosomal dominant Parkinson's disease . Lancet . 2005 ; 365 : 412 - 5 .
6. Nichols WC , Pankratz N , Hernandez D , Paisan-Ruiz C , Jain S , Halter CA , et al. Genetic screening for a single common LRRK2 mutation in familial Parkinson's disease . Lancet . 2005 ; 365 : 410 - 2 .
7. Bonifati V , Rizzu P , van Baren MJ , Schaap O , Breedveld GJ , Krieger E , et al. Mutations in the DJ-1 gene associated with autosomal recessive early-onset parkinsonism . Science . 2003 ; 299 : 256 - 9 .
8. Spillantini MG , Schmidt ML , Lee VM , Trojanowski JQ , Jakes R , Goedert M . Alpha-synuclein in Lewy bodies . Nature . 1997 ; 388 : 839 - 40 .
9. Bellucci Zaltieri M , Navarria L , Grigoletto J , Missale C , Spano PA. From α- synuclein to synaptic dysfunctions: new insights into the pathophysiology of Parkinson's disease . Brain Res . 2012 ; 1476 : 183 - 202 .
10. Lazarou M , Sliter DA , Kane LA , Sarraf SA , Wang C , Burman JL , et al. The ubiquitin kinase PINK1 recruits autophagy receptors to induce mitophagy . Nature . 2015 ; 524 : 309 - 14 .
11. Pickrell AM , Youle RJ . The roles of PINK1, Parkin, and mitochondrial Fidelity in Parkinson's disease . Neuron . 2015 ; 85 : 257 - 73 .
12. Canet-Avilés Wilson MA , Miller DW , Ahmad R , McLendon C , Bandyopadhyay S , Baptista MJ , Ringe D , Petsko GA , Cookson MRRM . The Parkinson's disease protein DJ-1 is neuroprotective due to cysteine-sulfinic acid-driven mitochondrial localization . Proc Natl Acad Sci U S A . 2004 ; 101 : 9103 - 8 .
13. Zondler L , Miller-Fleming L , Repici M , Gonçalves S , Tenreiro S , RosadoRamos R , et al. DJ-1 interactions with α-synuclein attenuate aggregation and cellular toxicity in models of Parkinson's disease . Cell Death Dis . 2014 ; 5 : e1350 .
14. Cooper O , Seo H , Andrabi S , Guardia-Laguarta C , Graziotto J , Sundberg M , et al. Pharmacological rescue of mitochondrial deficits in iPSC-derived neural cells from patients with familial Parkinson's disease . Sci Transl Med . 2012 ; 4 : 141ra90 .
15. Ray Huang BW , Tsuji YPD . Reactive oxygen species (ROS) homeostasis and redox regulation in cellular signaling . Cell Signal . 2012 ; 24 : 981 - 90 .
16. Seo Ahn Y , Lee SR , Yeo CY , Hur KCJH . The major target of the endogenously generated reactive oxygen species in response to insulin stimulation is phosphatase and tensin homolog and not phosphoinositide-3 kinase (PI-3 kinase) in the PI-3 kinase/Akt pathway . Mol Biol Cell . 2005 ; 16 : 348 - 57 .
17. Fujino Noguchi T , Matsuzawa A , Yamauchi S , Saitoh M , Takeda K , Ichijo HG . Thioredoxin and TRAF family proteins regulate reactive oxygen speciesdependent activation of ASK1 through reciprocal modulation of the Nterminal homophilic interaction of ASK1 . Mol Cell Biol . 2007 ; 27 : 8152 - 63 .
18. Indo HP , Yen HC , Nakanishi I , Matsumoto K , Tamura M , Nagano Y , et al. A mitochondrial superoxide theory for oxidative stress diseases and aging . J Clin Biochem Nutr . 2015 ; 56 : 1 - 7 .
19. Rego AC , Oliveira CR . Mitochondrial dysfunction and reactive oxygen species in excitotoxicity and apoptosis: implications for the pathogenesis of neurodegenerative diseases . Neurochem Res . 2003 ; 28 : 1563 - 74 .
20. Sies Berndt C , Jones DPH . Oxidative stress . Annu Rev Biochem . 2017 ; 86
21. Wiseman H , Halliwell B . Damage to DNA by reactive oxygen and nitrogen species: role in inflammatory disease and progression to cancer . Biochem J . 1996 ; 313 (Pt 1): 17 - 29 .
22. Bosco DA , Fowler DM , Zhang Q , Nieva J , Powers ET , Wentworth P Jr, et al. Elevated levels of oxidized cholesterol metabolites in Lewy body disease brains accelerate alpha-synuclein fibrilization . Nat Chem Biol . 2006 ; 2 : 249 - 53 .
23. Nakabeppu Y , Tsuchimoto D , Yamaguchi H , Sakumi K. Oxidative damage in nucleic acids and Parkinson's disease . J Neurosci Res . 2007 ; 85 : 919 - 34 .
24. Floor E , Wetzel MG . Increased protein oxidation in human substantia nigra pars compacta in comparison with basal ganglia and prefrontal cortex measured with an improved dinitrophenylhydrazine assay . J Neurochem . 1998 ; 70 : 268 - 75 .
25. Chung SY , Kishinevsky S , Mazzulli JR , Graziotto J , Mrejeru A , Mosharov EV , et al. Parkin and PINK1 patient iPSC-derived midbrain dopamine neurons exhibit mitochondrial dysfunction and alpha-Synuclein accumulation . Stem Cell Reports . 2016 ; 7 : 664 - 77 .
26. Nguyen HN , Byers B , Cord B , Shcheglovitov A , Byrne J , Gujar P , et al. LRRK2 mutant iPSC-derived DA neurons demonstrate increased susceptibility to oxidative stress . Cell Stem Cell . 2011 ; 8 : 267 - 80 .
27. Loschen Azzi A , Richter C , Flohé LG . Superoxide radicals as precursors of mitochondrial hydrogen peroxide . FEBS Lett . 1974 ; 42 : 68 - 72 .
28. Drose S , Brandt U. The mechanism of mitochondrial superoxide production by the cytochrome bc1 complex . J Biol Chem . 2008 ; 283 : 21649 - 54 .
29. Kussmaul L , Hirst J. The mechanism of superoxide production by NADH: ubiquinone oxidoreductase (complex I) from bovine heart mitochondria . Proc Natl Acad Sci U S A . 2006 ; 103 : 7607 - 12 .
30. Hala Vilhelmova M , Hartmanova I , Pink WK . Chronic parkinsonism in humans due to product of meperidine-analog synthesis . Science . 1983 ; 219 : 979 - 80 .
31. Javitch JA , D'Amato RJ , Strittmatter SM , Snyder SH . Parkinsonism-inducing neurotoxin , N-methyl-4-phenyl-1 , 2 , 3 ,6 -tetrahydropyridine: uptake of the metabolite N-methyl-4-phenylpyridine by dopamine neurons explains selective toxicity . Proc Natl Acad Sci U S A . 1985 ; 82 : 2173 - 7 .
32. Mizuno Y , Sone N , Saitoh T . Effects of 1-methyl-4-phenyl-1 , 2 , 3 ,6 - tetrahydropyridine and 1-methyl-4-phenylpyridinium ion on activities of the enzymes in the electron transport system in mouse brain . J Neurochem . 1987 ; 48 : 1787 - 93 .
33. Betarbet R , Sherer TB , MacKenzie G , Garcia-Osuna M , Panov AV , Greenamyre JT . Chronic systemic pesticide exposure reproduces features of Parkinson's disease . Nat Neurosci . 2000 ; 3 : 1301 - 6 .
34. Schapira AH , Cooper JM , Dexter D , Clark JB , Jenner P , Marsden CD . Mitochondrial complex I deficiency in Parkinson's disease . J Neurochem . 1990 ; 54 : 823 - 7 .
35. Parker Jr WD , Parks JK , Swerdlow RH , Complex I. Deficiency in Parkinson's disease frontal cortex . Brain Res . 2008 ; 1189 : 215 - 8 .
36. Gilks Abou-Sleiman PM , Gandhi S , Jain S , Singleton A , Lees AJ , Shaw K , Bhatia KP , Bonifati V , Quinn NP , Lynch JWP . A common LRRK2 mutation in idiopathic Parkinson's disease . Lancet . 2005 ; 365 : 415 - 6 .
37. Palacino JJ , Sagi D , Goldberg MS , Krauss S , Motz C , Wacker M , et al. Mitochondrial dysfunction and oxidative damage in parkin-deficient mice . J Biol Chem . 2004 ; 279 : 18614 - 22 .
38. Morais VA , Verstreken P , Roethig A , Smet J , Snellinx A , Vanbrabant M , et al. Parkinson's disease mutations in PINK1 result in decreased complex I activity and deficient synaptic function . EMBO Mol Med . 2009 ; 1 : 99 - 111 .
39. Wang X , Winter D , Ashrafi G , Schlehe J , Wong YL , Selkoe D , et al. PINK1 and Parkin target Miro for phosphorylation and degradation to arrest mitochondrial motility . Cell . 2011 ; 147 : 893 - 906 .
40. Koentjoro B , Park JS , Sue CM . Nix restores mitophagy and mitochondrial function to protect against PINK1/Parkin-related Parkinson's disease . Sci Rep . 2017 ; 7 : 44373 .
41. Kinumi Kimata J , Taira T , Ariga H , Niki ET . Cysteine-106 of DJ-1 is the most sensitive cysteine residue to hydrogen peroxide-mediated oxidation in vivo in human umbilical vein endothelial cells . Biochem Biophys Res Commun . 2004 ; 317 : 722 - 8 .
42. Burbulla LF , Song P , Mazzulli JR , Zampese E , Wong YC , Jeon S , et al. Dopamine oxidation mediates mitochondrial and lysosomal dysfunction in Parkinson's disease . Science . 2017 ; 357 : 1255 - 61 .
43. Richter Park JW , Ames BNC . Normal oxidative damage to mitochondrial and nuclear DNA is extensive . Proc Natl Acad Sci . 1988 ; 85 : 6465 - 7 .
44. Cadenas E , Davies KJ . Mitochondrial free radical generation, oxidative stress, and aging . Free Radic Biol Med . 2000 ; 29 : 222 - 30 .
45. Lee HC , Chang CM , Chi CW . Somatic mutations of mitochondrial DNA in aging and cancer progression . Ageing Res Rev . 2010 ; 9 ( Suppl 1 ): S47 - 58 .
46. Madamanchi NR , Runge MS . Mitochondrial dysfunction in atherosclerosis . Circ Res . 2007 ; 100 : 460 - 73 .
47. Ryan SD , Dolatabadi N , Chan SF , Zhang X , Akhtar MW , Parker J , et al. Erratum: isogenic human iPSC parkinson's model shows nitrosative stress-induced dysfunction in MEF2-PGC1α transcription . Cell . 2013 ; 155 : 1652 - 3 .
48. Pollard MG , Travers KJ , Weissman JS . Ero1p: a novel and ubiquitous protein with an essential role in oxidative protein folding in the endoplasmic reticulum . Mol Cell . 1998 ; 1 : 171 - 82 .
49. Tu BP , Weissman JS . The FAD-and O 2-dependent reaction cycle of Ero1- mediated oxidative protein folding in the endoplasmic reticulum . Mol Cell . 2002 ; 10 : 983 - 94 .
50. Princiotta MF , Finzi D , Qian SB , Gibbs J , Schuchmann S , Buttgereit F , et al. Quantitating protein synthesis, degradation, and endogenous antigen processing . Immunity . 2003 ; 18 : 343 - 54 .
51. Rao Ellerby HM , Bredesen DERV . Coupling endoplasmic reticulum stress to the cell death program . Cell Death Differ . 2004 ; 11 : 372 - 80 .
52. Han J , Kaufman RJ . The role of ER stress in lipid metabolism and lipotoxicity . J Lipid Res . 2016 ; 57 : 1329 - 38 .
53. Krebs J , Agellon LB , Michalak M. Ca(2+) homeostasis and endoplasmic reticulum (ER) stress: an integrated view of calcium signaling . Biochem Biophys Res Commun . 2015 ; 460 : 114 - 21 .
54. Galehdar Swan P , Fuerth B , Callaghan SM , Park DS , Cregan SPZ . Neuronal apoptosis induced by endoplasmic reticulum stress is regulated by ATF4- CHOP-mediated induction of the Bcl-2 homology 3-only member PUMA . J Neurosci . 2010 ; 30 : 16938 - 48 .
55. Deniaud A , Sharaf el dein O , Maillier E , Poncet D , Kroemer G , Lemaire C , et al. Endoplasmic reticulum stress induces calcium-dependent permeability transition, mitochondrial outer membrane permeabilization and apoptosis . Oncogene . 2008 ; 27 : 285 - 99 .
56. Van Coppenolle F , Vanden Abeele F , Slomianny C , Flourakis M , Hesketh J , Dewailly E , et al. Ribosome-translocon complex mediates calcium leakage from endoplasmic reticulum stores . J Cell Sci . 2004 ; 117 (Pt 18): 4135 - 42 .
57. Hammadi M , Oulidi A , Gackiere F , Katsogiannou M , Slomianny C , Roudbaraki M , et al. Modulation of ER stress and apoptosis by endoplasmic reticulum calcium leak via translocon during unfolded protein response: involvement of GRP78 . FASEB J . 2013 ; 27 : 1600 - 9 .
58. De Stefani Raffaello A , Teardo E , Szabò I , Rizzuto RDA . Forty-kilodalton protein of the inner membrane is the mitochondrial calcium uniporter . Nature . 2011 ; 476 : 336 - 40 .
59. Gomez-Suaga P , Paillusson S , Stoica R , Noble W , Hanger DP , Miller CC . The ER-mitochondria tethering complex VAPB-PTPIP51 regulates autophagy . Curr Biol . 2017 ; 27 : 371 - 85 .
60. Paillusson S , Stoica R , Gomez-Suaga P , Lau DH , Mueller S , Miller T , et al. There's something wrong with my MAM; the ER-mitochondria Axis and neurodegenerative diseases . Trends Neurosci . 2016 ; 39 : 146 - 57 .
61. Gautier CA , Erpapazoglou Z , Mouton-Liger F , Muriel MP , Cormier F , Bigou S , et al. The endoplasmic reticulum-mitochondria interface is perturbed in PARK2 knockout mice and patients with PARK2 mutations . Hum Mol Genet . 2016 ; 25 : 2972 - 84 .
62. de Brito OM , Scorrano L . Mitofusin 2 tethers endoplasmic reticulum to mitochondria . Nature . 2008 ; 456 : 605 - 10 .
63. Malhotra JD , Kaufman RJ . Endoplasmic reticulum stress and oxidative stress: a vicious cycle or a double-edged sword? Antioxid Redox Signal . 2007 ; 9 : 2277 - 93 .
64. Joza N , Susin SA , Daugas E , Stanford WL , Cho SK , Li CY , et al. Essential role of the mitochondrial apoptosis-inducing factor in programmed cell death . Nature . 2001 ; 410 : 549 - 54 .
65. Hoozemans JJ , van Haastert ES , Eikelenboom P , de Vos RA , Rozemuller JM , Scheper W. Activation of the unfolded protein response in Parkinson's disease . Biochem Biophys Res Commun . 2007 ; 354 : 707 - 11 .
66. Huang Xu J , Liang M , Hong X , Suo H , Liu J , Yu M , Huang FY . RESP18 is involved in the cytotoxicity of dopaminergic neurotoxins in MN9D cells . Neurotox Res . 2013 ; 24 : 164 - 75 .
67. Bouman Schlierf A , Lutz AK , Shan J , Deinlein A , Kast J , Galehdar Z , Palmisano V , Patenge N , Berg D , Gasser TL . Parkin is transcriptionally regulated by ATF4: evidence for an interconnection between mitochondrial stress and ER stress . Cell Death Differ . 2011 ; 18 : 769 - 82 .
68. Sun X , Liu J , Crary JF , Malagelada C , Sulzer D , Greene LA , et al. ATF4 protects against neuronal death in cellular Parkinson's disease models by maintaining levels of parkin . J Neurosci . 2013 ; 33 : 2398 - 407 .
69. Gorell Johnson CC , Rybicki BA , Peterson EL , Kortsha GX , Brown GG , Richardson RJJM . Occupational exposure to manganese, copper, lead, iron, mercury and zinc and the risk of Parkinson's disease . Neurotoxicology . 1998 ; 20 : 239 - 47 .
70. Dusek P , Roos PM , Litwin T , Schneider SA , Flaten TP , Aaseth J. The neurotoxicity of iron, copper and manganese in Parkinson's and Wilson's diseases . J Trace Elem Med Biol . 2015 ; 31 : 193 - 203 .
71. Xu Wang F , Wu SW , Deng Y , Liu W , Feng S , Yang TY , Xu ZFB . Alpha-synuclein is involved in manganese-induced ER stress via PERK signal pathway in organotypic brain slice cultures . Mol Neurobiol . 2014 ; 49 : 399 - 412 .
72. Colla Coune P , Liu Y , Pletnikova O , Troncoso JC , Iwatsubo T , Schneider BL , Lee MKE . Endoplasmic reticulum stress is important for the manifestations of α-synucleinopathy in vivo . J Neurosci . 2012 ; 32 : 3306 - 20 .
73. Heman-Ackah SM , Manzano R , Hoozemans JJM , Scheper W , Flynn R , Haerty W , et al. Alpha-synuclein induces the unfolded protein response in Parkinson's disease SNCA triplication iPSC-derived neurons . Hum Mol Genet . 2017 ; 0 : 1 - 10 .
74. Cóppola-Segovia V , Cavarsan C , Maia FG , Ferraz AC , Nakao LS , Lima MM , et al. ER stress induced by Tunicamycin triggers α-Synuclein oligomerization, dopaminergic neurons death and locomotor impairment: a new model of Parkinson's disease . Mol Neurobiol . 2017 ; 54 : 5798 - 806 .
75. Bartels T , Choi JG , DJ S. α-Synuclein occurs physiologically as a helically folded tetramer that resists aggregation . Nature . 2011 ; 477 : 107 - 10 .
76. Marques O , Outeiro TF . Alpha-synuclein: from secretion to dysfunction and death . Cell Death Dis . 2012 ; 3 : e350 .
77. Volles MJ , Lansbury Jr PT . Zeroing in on the pathogenic form of alphasynuclein and its mechanism of neurotoxicity in Parkinson's disease . Biochemistry . 2003 ; 42 : 7871 - 8 .
78. Martinez-Vicente M , Talloczy Z , Kaushik S , Massey AC , Mazzulli J , Mosharov EV , et al. Dopamine-modified alpha-synuclein blocks chaperone-mediated autophagy . J Clin Invest . 2008 ; 118 : 777 - 88 .
79. Deas E , Cremades N , Angelova PR , Ludtmann MH , Yao Z , Chen S , et al. Alpha-Synuclein oligomers interact with metal ions to induce oxidative stress and neuronal death in Parkinson's disease . Antioxid Redox Signal . 2016 ; 24 : 376 - 91 .
80. Levin J , Hogen T , Hillmer AS , Bader B , Schmidt F , Kamp F , et al. Generation of ferric iron links oxidative stress to alpha-synuclein oligomer formation . J Park Dis . 2011 ; 1 : 205 - 16 .
81. Tabner BJ , Turnbull S , El-Agnaf O , Allsop D. Production of reactive oxygen species from aggregating proteins implicated in Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases . Curr Top Med Chem . 2001 ; 1 : 507 - 17 .
82. Jellen LC , Lu L , Wang X , Unger EL , Earley CJ , Allen RP , et al. Iron deficiency alters expression of dopamine-related genes in the ventral midbrain in mice . Neuroscience . 2013 ; 252 : 13 - 23 .
83. Dexter DT , Wells FR , Agid F , Agid Y , Lees AJ , Jenner P , et al. Increased nigral iron content in postmortem parkinsonian brain . Lancet . 1987 ; 2 : 1219 - 20 .
84. Michaeli S , Oz G , Sorce DJ , Garwood M , Ugurbil K , Majestic S , et al. Assessment of brain iron and neuronal integrity in patients with Parkinson's disease using novel MRI contrasts . Mov Disord . 2007 ; 22 : 334 - 40 .
85. Pyatigorskaya N , Sharman M , Corvol JC , Valabregue R , Yahia-Cherif L , Poupon F , et al. High nigral iron deposition in LRRK2 and Parkin mutation carriers using R2* relaxometry . Mov Disord . 2015 ; 30 : 1077 - 84 .
86. Hare DJ , Lei P , Ayton S , Roberts BR , Grimm R , George JL , et al. An irondopamine index predicts risk of parkinsonian neurodegeneration in the substantia nigra pars compacta . Chem Sci . 2014 ; 5 : 2160 - 9 .
87. Di Maio R , Barrett PJ , Hoffman EK , Barrett CW , Zharikov A , Borah A , et al. alpha-Synuclein binds to TOM20 and inhibits mitochondrial protein import in Parkinson's disease . Sci Transl Med . 2016 ; 8 : 342ra78 .
88. Dexter Carter CJ , Wells FR , Javoy-Agid F , Agid Y , Lees A , Jenner P , Marsden CDDT . Basal lipid peroxidation in substantia nigra is increased in Parkinson's disease . J Neurochem . 1989 ; 52 : 381 - 9 .
89. Qin Z , Hu D , Han S , Reaney SH , Di Monte DA , Fink AL . Effect of 4-hydroxy-2- nonenal modification on alpha-synuclein aggregation . J Biol Chem . 2007 ; 282 : 5862 - 70 .
90. Bae EJ , Ho DH , Park E , Jung JW , Cho K , Hong JH , et al. Lipid peroxidation product 4-hydroxy-2-nonenal promotes seeding-capable oligomer formation and cell-to-cell transfer of alpha-synuclein . Antioxid Redox Signal . 2013 ; 18 : 770 - 83 .
91. Hashimoto Takeda A , Hsu LJ , Takenouchi T , Masliah EM . Role of cytochrome c as a stimulator of α-synuclein aggregation in Lewy body disease . J Biol Chem . 1999 ; 274 : 28849 - 52 .
92. Ruf RA , Lutz EA , Zigoneanu IG , Pielak GJ . Alpha-Synuclein conformation affects its tyrosine-dependent oxidative aggregation . Biochemistry . 2008 ; 47 : 13604 - 9 .
93. Webb Ravikumar B , Atkins J , Skepper JN , Rubinsztein DCJL . α-Synuclein is degraded by both autophagy and the proteasome . J Biol Chem . 2003 ; 278 : 25009 - 13 .
94. Ebrahimi-Fakhari D , Cantuti-Castelvetri I , Fan Z , Rockenstein E , Masliah E , Hyman BT , et al. Distinct roles in vivo for the ubiquitin-proteasome system and the autophagy-lysosomal pathway in the degradation of alphasynuclein . J Neurosci . 2011 ; 31 : 14508 - 20 .
95. Majeski AE , Dice JF . Mechanisms of chaperone-mediated autophagy . Int J Biochem Cell Biol . 2004 ; 36 : 2435 - 44 .
96. Haynes CM , Titus EA , Cooper AA . Degradation of misfolded proteins prevents ER-derived oxidative stress and cell death . Mol Cell . 2004 ; 15 : 767 - 76 .
97. Cooper AA , Gitler AD , Cashikar A , Haynes CM , Hill KJ , Bhullar B , et al. Alphasynuclein blocks ER-Golgi traffic and Rab1 rescues neuron loss in Parkinson's models . Science . 2006 ; 313 : 324 - 8 .
98. Mazzulli Zunke F , Isacson O , Studer L , Krainc DJR . α-Synuclein-induced lysosomal dysfunction occurs through disruptions in protein trafficking in human midbrain synucleinopathy models . Proc Natl Acad Sci . 2016 ; 113 : 1931 - 6 .
99. Credle Forcelli PA , Delannoy M , Oaks AW , Permaul E , Berry DL , Duka V , Wills J , Sidhu AJJ . α-Synuclein-mediated inhibition of ATF6 processing into COPII vesicles disrupts UPR signaling in Parkinson's disease . Neurobiol Dis . 2015 ; 76 : 112 - 25 .
100. Goldstein DS , Sullivan P , Holmes C , Miller GW , Alter S , Strong R , et al. Determinants of buildup of the toxic dopamine metabolite DOPAL in Parkinson's disease . J Neurochem . 2013 ; 126 : 591 - 603 .
101. Meiser J , Weindl D , Hiller K. Complexity of dopamine metabolism . Cell Commun Signal . 2013 ; 11 : 34 .
102. Graham DG . Oxidative pathways for catecholamines in the genesis of neuromelanin and cytotoxic quinones . Mol Pharmacol . 1978 ; 14 : 633 - 43 .
103. Tse DC , McCreery RL , Adams RN . Potential oxidative pathways of brain catecholamines . J Med Chem . 1976 ; 19 : 37 - 40 .
104. LaVoie MJ , Hastings TG . Dopamine quinone formation and protein modification associated with the striatal neurotoxicity of methamphetamine: evidence against a role for extracellular dopamine . J Neurosci . 1999 ; 19 : 1484 - 91 .
105. Hastings TG , Lewis DA , Zigmond MJ . Role of oxidation in the neurotoxic effects of intrastriatal dopamine injections . Proc Natl Acad Sci U S A . 1996 ; 93 : 1956 - 61 .
106. Van Laar VS , Mishizen AJ , Cascio M , Hastings TG . Proteomic identification of dopamine-conjugated proteins from isolated rat brain mitochondria and SH-SY5Y cells . Neurobiol Dis . 2009 ; 34 : 487 - 500 .
107. Khan FH , Sen T , Maiti AK , Jana S , Chatterjee U , Chakrabarti S . Inhibition of rat brain mitochondrial electron transport chain activity by dopamine oxidation products during extended in vitro incubation: implications for Parkinson's disease . Biochim Biophys Acta . 2005 ; 1741 : 65 - 74 .
108. Bondi Zilocchi M , Mare MG , D'Agostino G , Giovannardi S , Ambrosio S , Fasano M , Alberio TH . Dopamine induces mitochondrial depolarization without activating PINK1-mediated mitophagy . J Neurochem . 2016 ; 136 : 1231 - 91 .
109. Conway KA , Rochet JC , Bieganski RM , Lansbury Jr PT . Kinetic stabilization of the alpha-synuclein protofibril by a dopamine-alpha-synuclein adduct . Science . 2001 ; 294 : 1346 - 9 .
110. LaVoie MJ , Ostaszewski BL , Weihofen A , Schlossmacher MG , Selkoe DJ . Dopamine covalently modifies and functionally inactivates parkin . Nat Med . 2005 ; 11 : 1214 - 21 .
111. Meng F , Yao D , Shi Y , Kabakoff J , Wu W , Reicher J , et al. Oxidation of the cysteine-rich regions of parkin perturbs its E3 ligase activity and contributes to protein aggregation . Mol Neurodegener . 2011 ; 6 : 34 .
112. Xu J , Kao SY , Lee FJ , Song W , Jin LW , Yankner BA . Dopamine-dependent neurotoxicity of alpha-synuclein: a mechanism for selective neurodegeneration in Parkinson disease . Nat Med . 2002 ; 8 : 600 - 6 .
113. Plotegher N , Berti G , Ferrari E , Tessari I , Zanetti M , Lunelli L , et al. DOPAL derived alpha-synuclein oligomers impair synaptic vesicles physiological function . Sci Rep . 2017 ; 7 : 40699 .
114. Bisaglia M , Tosatto L , Munari F , Tessari I , de Laureto PP , Mammi S , et al. Dopamine quinones interact with alpha-synuclein to form unstructured adducts . Biochem Biophys Res Commun . 2010 ; 394 : 424 - 8 .
115. Kitada T , Asakawa S , Hattori N , Matsumine H , Yamamura Y , Minoshima S , et al. Mutations in the parkin gene cause autosomal recessive juvenile parkinsonism . Nature . 1998 ; 392 : 605 - 8 .
116. Nemani VM , Lu W , Berge V , Nakamura K , Onoa B , Lee MK , et al. Increased expression of alpha-synuclein reduces neurotransmitter release by inhibiting synaptic vesicle reclustering after endocytosis . Neuron . 2010 ; 65 : 66 - 79 .
117. Abeliovich A , Schmitz Y , Farinas I , Choi-Lundberg D , Ho WH , Castillo PE , et al. Mice lacking alpha-synuclein display functional deficits in the nigrostriatal dopamine system . Neuron . 2000 ; 25 : 239 - 52 .
118. Choi BK , Choi MG , Kim JY , Yang Y , Lai Y , Kweon DH , et al. Large alphasynuclein oligomers inhibit neuronal SNARE-mediated vesicle docking . Proc Natl Acad Sci U S A . 2013 ; 110 : 4087 - 92 .
119. Zhao J , Yu S , Zheng Y , Yang H , Zhang J . Oxidative modification and its implications for the neurodegeneration of Parkinson's disease . Mol Neurobiol . 2017 ; 54 : 1404 - 18 .
120. Ransohoff RM . How neuroinflammation contributes to neurodegeneration . Science . 2016 ; 353 : 777 - 83 .
121. Sampson TR , Debelius JW , Thron T , Janssen S , Shastri GG , Ilhan ZE , et al. Gut Microbiota Regulate Motor Deficits and Neuroinflammation in a Model of Parkinson's Disease . Cell . 2016 ; 167 : 1469 - 1480 . e12 .
122. Luk KC , Kehm V , Carroll J , Zhang B , O'Brien P , Trojanowski JQ , et al. Pathological -Synuclein transmission initiates Parkinson-like neurodegeneration in nontransgenic mice . Science . 2012 ; 338 : 949 - 53 .
123. de Farias CC , Maes M , Bonifacio KL , Bortolasci CC , de Souza Nogueira A , Brinholi FF , et al. Highly specific changes in antioxidant levels and lipid peroxidation in Parkinson's disease and its progression: disease and staging biomarkers and new drug targets . Neurosci Lett . 2016 ; 617 : 66 - 71 .