The identity of what triggers the loss of dopaminergic neurons containing neuromelanin
in Parkinson's disease (PD) is still unknown. Fifty years since its introduction in
PD therapy, L-dopa is still the gold-standard drug despite severe side effects observed
after 4 to 6 years of being treated with it. There are no new therapies that can halt
or slow down the progression of the disease and much of the research efforts in this
context have been destined to treat L-dopa-induced dyskinesia. There is huge concern
about the difficulties that have been observed in the translation of successful preclinical
results into clinical studies and new therapies in PD. The discovery of genes associated
with familiar forms of PD has made an enormous input into basic research, which seeks
to understand the degenerative process resulting in the loss of dopaminergic neurons
in the nigrostriatal system. Several mechanisms have been suggested to be involved
in the degeneration of nigrostriatal neurons in PD, including mitochondrial dysfunction,
endoplasmic reticulum stress, lysosomal and proteasomal protein degradation dysfunction,
the formation of neurotoxic alpha-synuclein (SNCA) oligomers, neuroinflammation, and
oxidative stress.
Mitochondrial Dysfunction
The brain is completely dependent on chemical energy (ATP) in order to perform the
release of neurotransmitters such as dopamine. Therefore, the existence of functional
mitochondria is essential to the performed role of a dopaminergic neuron, i.e., to
release dopamine. Postmortem brains with PD presented a deficiency in Complex I activity
(Shapira et al., 1990; Esteves et al., 2011). Reduced Complex I activity in platelet
mitochondria, purified from patients with idiopathic PD, has been observed (Esteves
et al., 2011). CHCHD2 mutation in PD patient fibroblasts reduces oxidative phosphorylation
in Complexes I and IV and induces fragmentation of the mitochondrial reticular morphology
(Lee et al., 2018). A meta-analysis supports the deficit in Complexes I and IV in
the case of peripheral blood, the frontal cortex, the cerebellum and the substantia
nigra in PD (Holper et al., 2018). Analysis of mitochondria morphology in PD samples
compared to controls revealed a significant decrease in the number of healthy mitochondria
per cell. Several genes associated with familial forms of PD (PINK-1, DJ-1, Parkin,
HTRA2) are linked to mitochondrial impairment (Larsen et al., 2018). Parkinson's disease,
associated with vacuolar protein sorting 35 mutation, affects Complex I activity (Zhou
et al., 2017). PINK1 and DJ-1 mutation induce energetic inefficiency (Lopez-Fabuel
et al., 2017). SNCA induces mitochondrial dysfunction (Devi et al., 2008; Chinta et
al., 2010; Nakamura et al., 2011; Martínez et al., 2018).
Endoplasmic Reticulum Stress
Endoplasmic reticulum is involved in secretory protein translocation and the quality
control of secretory protein folding. Misfolded or unfolded proteins in the lumen
accumulate under endoplasmic reticulum stress, which causing an integrated adaptive
response identified as the unfolded protein response (UPR), which seeks to restore
proteostasis within the secretory pathway (Cabral-Miranda and Hetz, 2018).
The UPR activation markers, phosphorylated eukaryotic initiation factor 2alpha and
phosphorylated pancreatic endoplasmic reticulum kinase, were detected in dopaminergic
neurons containing neuromelanin in the substantia nigra of PD patients. Interestingly,
phosphorylated pancreatic endoplasmic reticulum kinase was colocalized with an increased
level of SNCA (Hoozemans et al., 2007). Neuropathological analysis of PD postmortem
brain tissue revealed that pIRE1α is expressed within neurons containing elevated
levels of α-synuclein or Lewy bodies (Heman-Ackah et al., 2017). SNCA triggers endoplasmic
reticulum stress via the protein kinase RNA-like endoplasmic reticulum kinase/eukaryotic
translation initiation factor 2α signaling pathway (Liu et al., 2018). N370S mutation
and β-glucocerebrosidase-1 retention within the endoplasmic reticulum induce endoplasmic
reticulum stress activation, triggering UPR and Golgi apparatus fragmentation (García-Sanz
et al., 2017). It has been reported that endoplasmic reticulum stress activates the
chaperone-mediated autophagy pathway via an EIF2AK3/PERK-MAP2K4/MKK4-MAPK14/p38-dependent
manner (Li et al., 2018).
Dopamine Oxidation and Parkinson's Disease
One of the most characteristic features of the pathology of PD, which results in the
onset of motor symptoms, is the massive loss of dopaminergic neurons containing neuromelanin
in the nigrostriatal system. As mentioned before, several mechanisms, including mitochondrial
dysfunction and endoplasmic reticulum stress, have been proposed as being involved
in the degeneration of the nigrostriatal neurons in PD, but the question concerns
what triggers these mechanisms in dopaminergic neurons containing neuromelanin. Many
times, it has been suggested that the involvement of exogenous neurotoxins triggers
these mechanisms, but the severe Parkinsonism induced by MPTP in just 3 days in drug
addicts who used synthetic drugs contaminated with this compound undermines this idea
(Williams, 1986). The rate of the degenerative process in PD takes years (Braak et
al., 2004). The extremely slow degeneration of the nigrostriatal neurons and slow
progression of the disease challenge the possible role of exogenous neurotoxins in
the loss of dopaminergic neurons containing neuromelanin, suggesting that some endogenous
neurotoxin must trigger these mechanisms. A neurotoxic event, triggered by an endogenous
neurotoxin, will affect a single neuron without propagative effects, which explains
the extremely slow rate of this degenerative process in PD. Among possible endogenous
neurotoxins are the neurotoxic SNCA oligomers. However, the prion-like hypothesis
of SNCA in PD pathogenesis is based on the propagation (neuron-to-neuron transfer)
of neurotoxic SNCA oligomers (Brundin and Melki, 2017). According to this prion-like
hypothesis, a relatively rapid process is expected, in contrasting with what happens
in PD, which takes years. In addition, what triggers the formation of neurotoxic SNCA
oligomers inside the dopaminergic neurons containing neuromelanin? Braak stage hypothesis
use the intraneuronal inclusion bodies to follow the development of Parkinson's disease
where SNCA is one of the aggregated proteins (Braak et al., 2004). What induces SNCA
aggregation in other brain region involved in non-motor symptoms remains unclear.
A possible explanation is that an endogenous neurotoxin is formed inside dopaminergic
neurons containing neuromelanin during dopamine oxidation. The formation of the pigment
called neuromelanin in these neurons is the result of dopamine oxidation into ortho(o)-quinones,
which is a pathway that involves the formation of three o-quinones in a sequential
manner (dopamine → dopamine o-quinone → aminochrome → 5,6- indolequinone → neuromelanin).
Dopamine o-quinone is able to form adducts with proteins, such as ubiquitin carboxy-terminal
hydrolase L1 (UCHL-1) and Parkinsonism-associated deglycase (DJ-1, PARK7), as well
as ubiquinol-cytochrome c reductase core protein 1, glucose-regulated protein 75/mitochondrial
HSP70/mortalin, mitofilin, mitochondrial creatine kinase and glutathione peroxidase-4,
and a human dopamine transporter (Whitehead et al., 2001; Van Laar et al., 2009; Hauser
et al., 2013). Incubation of purified tyrosine hydroxylase with dopamine and tyrosinase
also forms adducts with dopamine (Xu et al., 1998). Dopamine o-quinone induces mitochondrial
dysfunction (Berman and Hastings, 1999). Exposure of cells to dopamine induced the
formation of dopamine adducts with parkin (LaVoie et al., 2005), but the identity
of the o-quinone involved in this reaction (dopamine o-quinone or aminochrome) is
not clear. Dopamine o-quinone is completely unstable at physiological pH and cyclizes
immediately into aminochrome; thus, the question concerns whether dopamine o-quinone
has the opportunity to form adducts with parkin in the cell cytosol overcrowded with
other proteins, molecules and organelles.
Aminochrome has been reported to be neurotoxic on account of inducing mitochondrial
dysfunction, endoplasmic reticulum stress, autophagy dysfunction, proteasomal dysfunction,
oxidative stress, neuroinflammation, the disruption of the cytoskeleton architecture
and the formation of neurotoxic SNCA oligomers (Arriagada et al., 2004; Zafar et al.,
2006; Fuentes et al., 2007; Zhou and Lim, 2009; Paris et al., 2010, 2011; Aguirre
et al., 2012; Muñoz et al., 2012, 2015; Huenchuguala et al., 2014, 2017; Xiong et
al., 2014; Briceño et al., 2016; Santos et al., 2017; de Araújo et al., 2018; Segura-Aguilar
and Huenchuguala, 2018) (Figure 1).
Figure 1
Neuroprotection against aminochrome-induced neurotoxicity. In dopaminergic neurons,
DT-diaphorase catalyzes the two-electron reduction of aminochrome into leukoaminochrome,
preventing aminochrome-induced endoplasmic reticulum stress and mitochondrial dysfunction.
Leukoaminochrome is rearranged into 5,6-dihydroxyindole, which oxidizes into 5,6-indolequinone
and polymerizes into neuromelanin. In astrocytes, GSTM2 is able to conjugate both
dopamine o-quinone and aminochrome with GSH and DT-diaphorase can reduce aminochrome
with two-electron to leukoaminochrome. However, astrocytes secrete the enzyme GSTM2,
whose dopaminergic neurons internalize in the cytosol. GSTM2 inside the dopaminergic
neurons conjugates both dopamine o-quinone and aminochrome with GSH, whose stable
products are eliminated from dopaminergic neurons.
5,6-Indolequinone, the precursor of neuromelanin, is able to form adducts with SNCA
(Bisaglia et al., 2010). Dopaminochrome has also been reported to form adducts with
SNCA (Norris et al., 2005) and to be neurotoxic in cell cultures (Linsenbardt et al.,
2009, 2012). The unilateral injection of dopaminochrome induced degeneration of the
dopaminergic neurons within the substantia nigra (Touchette et al., 2015). However,
the structure of dopaminochrome has not been determined by NMR; nor do we know the
nature of this structure. The dopaminochrome structure is different to the aminochrome
structure because dopaminochrome has an absorption maximum of 303 and 479 nm (Ochs
et al., 2005), while aminochrome has an absorption maximum of 280 and 475 nm and its
structure has been confirmed by NMR (Paris et al., 2010).
Aminochrome and Parkinson's Disease
Dopamine oxidation into neuromelanin is a normal and harmless pathway because neuromelanin
accumulates with age, with dopaminergic neurons containing neuromelanin remaining
intact in the substantia nigra of healthy seniors (Zecca et al., 2002). Aminochrome
is the most stable and studied o-quinone formed during dopamine oxidation into neuromelanin.
Paradoxically, aminochrome under certain conditions can be neurotoxic as a result
of inducing mitochondrial dysfunction (Arriagada et al., 2004; Paris et al., 2011;
Aguirre et al., 2012; Huenchuguala et al., 2017; Segura-Aguilar and Huenchuguala,
2018), endoplasmic reticulum stress (Xiong et al., 2014), the formation of neurotoxic
SNCA oligomers (Muñoz et al., 2015; Muñoz and Segura-Aguilar, 2017), proteasome dysfunction
(Zafar et al., 2006; Zhou and Lim, 2009), autophagy dysfunction (Muñoz et al., 2012;
Huenchuguala et al., 2014), lysosome dysfunction (Meléndez et al., 2018), neuroinflammation
(Santos et al., 2017; de Araújo et al., 2018), cytoskeleton architecture disruption
(Paris et al., 2010; Briceño et al., 2016) and oxidative stress (Arriagada et al.,
2004). Aminochrome in vivo induces neuronal dysfunction as a consequence of mitochondrial
dysfunction, decreased axonal transport resulting in a significant decrease in the
number of synaptic monoaminergic vesicles, reduced dopamine release accompanied by
an increase in GABA levels, and a dramatic change in the neurons' morphology characterized
as cell shrinkage (Herrera et al., 2016). The explanation as to why dopamine oxidation
into neuromelanin is not a harmful pathway, despite the formation of potential neurotoxic
o-quinones, is because the existence of two enzymes [DT-diaphorase and glutathione
transferase M2-2 (GSTM2)], which are able to prevent aminochrome neurotoxicity. DT-diaphorase
is expressed in dopaminergic neurons and astrocytes and catalyzes the two-electron
reduction of aminochrome into leukoaminochrome, preventing aminochrome one-electron
reduction into the leukoaminochrome o-semiquinone radical, catalyzed by flavoenzymes
that transfer one electron and use NADH or NADPH. DT-diaphorase prevents aminochrome-induced
cell death (Lozano et al., 2010), mitochondrial dysfunction (Arriagada et al., 2004;
Paris et al., 2011; Muñoz et al., 2012), cytoskeleton architecture disruption (Paris
et al., 2010), lysosomal dysfunction (Meléndez et al., 2018), the formation of neurotoxic
SNCA oligomers (Muñoz et al., 2015; Muñoz and Segura-Aguilar, 2017), oxidative stress
(Arriagada et al., 2004); dopaminergic neurons' degeneration in vivo (Herrera-Soto
et al., 2017) and astrocytes dell death (Huenchuguala et al., 2016). GSTM2 catalyzes
the GSH conjugation of aminochrome into 4-S-glutathionyl-5,6-dihydroxyindoline, which
is resistant to biological oxidizing agents such as oxygen, hydrogen peroxide, and
superoxide (Segura-Aguilar et al., 1997). GSTM2 also catalyzes the GSH conjugation
of dopamine o-quinone into 5-glutathionyl-dopamine (Dagnino-Subiabre et al., 2000),
which degrades into 5-cysteinyl-dopamine. Interestingly, 5-cysteinyl-dopamine is a
stable metabolite that can be eliminated from the cells. 5-Cysteinyl-dopamine has
been found in substantia nigra, caudate nucleus, putamen, globus pallidus, neuromelanin,
and the cerebrospinal fluid of PD patients (Rosengren et al., 1985; Carstam et al.,
1991; Cheng et al., 1996). GSTM2 prevents aminochrome-induced cell death, mitochondrial
dysfunction, autophagy, and lysosome dysfunction (Huenchuguala et al., 2014; Segura-Aguilar,
2017a; Segura-Aguilar and Huenchuguala, 2018). The GSH conjugation of aminochrome
prevents the formation of neurotoxic SNCA oligomers by generating nontoxic SNCA oligomers
(Huenchuguala et al., 2018). GSTM2 is expressed in human astrocytes and it has been
reported that astrocytes secrete GSTM2, while dopaminergic neurons are able to internalize
this enzyme into the cytosol, protecting these neurons against aminochrome-induced
neurotoxicity (Cuevas et al., 2015; Segura-Aguilar, 2015, 2017b).
Mitochondrial dysfunction and endoplasmic reticulum stress are two very important
mechanisms involved in the loss of dopaminergic neurons containing neuromelanin in
the nigrostriatal neurons in idiopathic PD. However, the question concerns the common
denominator in these mechanisms: i.e., what triggers these mechanisms in dopaminergic
neurons containing neuromelanin in the nigrostriatal system? We propose that aminochrome
is the endogenous neurotoxin that triggers mitochondrial dysfunction and endoplasmic
reticulum stress because aminochrome is formed inside dopaminergic neurons of the
nigrostriatal system. In addition, aminochrome also triggers other mechanisms involved
in the loss of dopaminergic neurons in the nigrostriatal system, such as the formation
of neurotoxic SNCA oligomers, oxidative stress, neuroinflammation, and proteasomal
and lysosomal protein degradation dysfunction.
Author Contributions
The author confirms being the sole contributor of this work and has approved it for
publication.
Conflict of Interest Statement
The author declares that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.