Axol Bioscience Science Scholarship recipient, Nataly Martynyuk, is a PhD student at the Brain Repair Centre, Department of Clinical Neurosciences, University of Cambridge, UK. Her research focuses on the actions of alpha-chimerins in mechanisms relevant to dendritic spine formation and neurodegeneration. Nataly reviews the development and degeneration of dopaminergic neurons and discusses the role of dopamine and alpha-synuclein in Parkinson's disease.

You wake up to a bright-lit room, stretch and make your first unstable steps. A coffee tin comes into sight. Nothing smells better than that first cup of morning coffee. The fifth one in the afternoon is not nearly as satisfying, but you keep reaching for it nervously. What is common between movement, vision, olfaction, reward, addiction, stress, and digestion?

Dopaminergic neurons

Dopamine (DA) is a catecholamine neurotransmitter used in a variety of neuronal systems including midbrain, hypothalamus, retinal and olfactory systems, some sympathetic ganglia, and enteric nervous system (ENS) (1–3). Being an intermediate product in the biosynthesis of other catecholamines, DA used to be confused for a by-product of those reactions, but was later recognized as an independent neurotransmitter.

The most prominent source of DA in humans is dopaminergic neurons of the ventral midbrain (Fig. 1). The voluntary movement-controlling nigrostriatal pathway extends from the substantia nigra pars compacta (SNpc) to the dorsal striatum that includes putamen and caudate nuclei. Motivation and reward are governed by the overlapping mesolimbic and mesocortical pathways that spread from the ventral tegmental area (VTA) to the septum, amygdala, hippocampus, nucleus accumbens, and olfactory tubercle (mesolimbic) or the cingulate, prefrontal, and perirhinal cortices (mesocortical). There are almost 600,000 DA-producing cells in the human mesencephalon, which represent around 90% of all dopaminergic neurons in the brain while still comprising only 1% of the total brain neuronal number (1,4).

Midbrain dopaminergic neuron development 

Neural progenitors that are destined to become midbrain dopaminergic neurons start acquiring their fate early in the embryonic development from a single group of embryonic cells, around embryonic days E7.5-E9.5 in mice, corresponding approximately to an embryonic day 17 in humans according to the Carnegie staging (stage 8-10), when the floor plate and the midbrain-hindbrain junction develop. These precursors exit the cell cycle at around E10.5-11.5 in mice (Carnegie stage 12-14, embryonic days 28-33 in humans) to become maturing postmitotic dopaminergic neurons (1,5). Many maturing dopaminergic neurons reach their targets by murine E14.5 (6) (Carnegie stage 20, human embryonic day 52) (Fig. 2). 

Midbrain dopaminergic neuron development.
Figure 2. Midbrain dopaminergic neuron differentiation. Author’s illustration, watercolour on paper.

Neurodegeneration in Parkinson’s disease 

Long before DA was discovered in 1958, people were aware of the drastic motor impairments of the ‘shaking palsy’ of the older age described by James Parkinson in 1817 (7). This debilitating disease progresses from affecting around 1% of the population at 65 to 5% at 85 years (8) with symptoms worsening over time. Its tremor, bradykinesia, rigidity and ‘freezing’ result specifically from neuronal loss in the SNpc.

Substantia nigra (SN) derives its name from the accumulation of neuromelanin that gives this area a characteristic dark color. However, in Parkinson’s disease (PD) patients neuromelanin-containing cells are mostly lost, resulting in the SN pallor. Out of all cells in the brain, why such selectivity?

Lewy bodies and α-synuclein 

In 95% of PD cases, there is no apparent genetic cause. A striking line of evidence shows that neurodegeneration might start in the areas exposed to the toxic insults from the outside: the ENS and olfactory bulb, from where it spreads through synaptic contacts retrogradely to the brain (3,9). The cellular pathology includes, in most cases, formation of intraneuronal aggregates termed Lewy bodies that contain primarily a highly ubiquitinated filamentous α-synuclein. It is suggested that abnormal concentration and/or handling of this protein is the main reason for cellular demise. Alpha-synuclein oligomers affect cellular metabolism at the levels of protein degradation and recycling as well as at the level of mitochondrial and endoplasmic reticulum functions (10,11). Abnormal α-synuclein may be either the primary cause or the consequence of the previously affected cellular metabolism. This view is supported by the functions of genes known to give rise to familial PD forms (Table 1). 

Gene
Inheritance

Normal function

PD-causing mutation

Symptoms

SNCA (α-synuclein)
AD
Regulates vesicle traffic through phospholipase C&D modulation;
Antimicrobial role

Duplication;
Triplication;
Mutations promoting aggregation:
A53T, A30P, E46K

Lewy bodies present, onset 25-60 yo
Parkin
AR
E3 ubiquitin ligase
Loss of function
Parkinsonism with no Lewy bodies, onset juvenile to 40 yo
Ubiquitin carboxy-terminal hydrolase L1/
UCHL1

AR?
Ubiquitin hydrolase and ligase
Loss of function?
Lewy bodies present, possible early onset, classical PD
PTEN-induced putative kinase/
PINK1

AR
Mitochondrial kinase
Loss of function
Early onset parkinsonism
DJ1
AR
Molecular chaperone;
Antioxidant

Loss of function
Early onset parkinsonism
Leucine rich repeat kinase 2/
LRRK2 (dardarin)

AD
Kinase?
G2019S Lewy bodies present, classical PD, onset 40-60 yo

Table 1. Genes involved in familial Parkinson’s disease. Adapted from Wood-Kaczmar A, Gandhi S, Wood NW, Understanding the molecular causes of Parkinson’s disease, 2006 and Kim SW, Ko HS, Dawson VL, et al., Recent advances in our understanding of Parkinson’s disease, 2005. (AD, autosomal dominant; AR, autosomal recessive.)

It is curious that Lewy bodies, Lewy neurites and neurodegeneration are also observed in other catecholaminergic nuclei such as noradrenergic locus coeruleus, serotonergic and cholinergic systems, olfactory bulb, cerebral cortex, and autonomic nervous system including ENS (3,12). Nevertheless, the PD clinical symptoms correlate most reliably to the SNpc neurodegeneration. Dopaminergic neurons seem to harbor some internal weakness that gives the disease away while most other systems keep holding on. This weakness might be, at the same time, at the very core of their function.

A vicious cycle

Dopamine oxidation and radical generation 

DA readily oxidizes at normal pH to produce toxic semiquinones (SQ), superoxide anions, hydroxyl radicals, peroxinitrates, and nitrate radicals. Enzymatic deamination by monoamine oxidase-B (MAO-B) results in the non-toxic 3,4-dihyroxyphenyl-acetaldehyde (DOPAA) and 3,4-dihydroxyphenylacetic acid (DOPAC) production; however, hydrogen peroxide is still generated. Peroxide is neutralized by catalase or glutathione peroxidase (GPx) in healthy cells. This outcome can be influenced by the presence of iron, which allows for the toxic hydroxyl radical production (13,14) (Table 2).  

Pathways of dopamine oxidation.
Table 2. Pathways of dopamine oxidation. Toxic species produced are highlighted in bold.

Neuromelanin, that is abundant in SNpc and other catecholaminergic brain stem nuclei, has an intrinsic iron binding affinity (14,15). This metal is an important cofactor in many enzymatic reactions, but when its concentration becomes too high, it is more likely to take a side road. In accordance with this, SN of PD patients show an increased iron content (16–18). Coupled with the radical-producing DA metabolism, this can put an additional strain on mitochondria and the general antioxidant systems (19–22).

Alpha-synuclein and oxidative stress 

Alpha-synuclein localizes to the mitochondria and can induce oxidative stress (23). Once again, dopaminergic neurons take the brunt of the α-synuclein pathological actions. DA-quinones form covalent adducts with this protein, inhibiting aggregation of oligomers into fibrils (24). Accumulating oligomers, or protofibrils, are capable of forming pores in DA-containing synaptic vesicles, and this property is enhanced by the PD-causing mutations (25,26). This odd function could be a reminiscence of the antimicrobial function of α-synuclein (27) applied in an unfortunate mistake. Combined with a potential loss of normal function in synaptic vesicle cycling (14,28,29), PD-inducing α-synuclein mutations lead to the enhanced DA concentration in the cytoplasm, where oxidation takes place. Toxic concentrations of wild-type α-synuclein are achieved through toxin exposure (9) or gene multiplications. DA packaging into vesicles protects the neurotransmitter from unwanted oxidative reactions, so an increased ratio of DA transporter (DAT) on the cell membrane that transfers DA from the synaptic cleft into the cytoplasm to vesicular DA transporter that aids DA packaging (VMAT2) confers a genetic predisposition to PD (30). Therefore, cells get caught up in a vicious DA cycle: oxidation causes DA-quinone formation, which then promotes toxic α-synuclein accumulation, that further increases cytoplasmic DA concentration and, therefore, oxidative stress. 

Age and the decline of natural defense systems 

The reason the consequences of such reactions lie dormant, even in case of familial PD, for decades is likely to be due to the natural cell defense systems. With age, ubiquitin-proteasome system (UPS) is more likely to accumulate impairments (31) that increase a chance of protein misfolding abnormalities. Additionally, age-dependent cellular shrinkage promotes protein crowding in the cytoplasm that could enhance apparent α-synuclein and dopamine concentrations (32) at the same time when antioxidant systems become depleted (21). As mentioned above, α-synuclein pathology is observed in multiple other areas of the nervous system. In fact, olfactory and enteric systems are at the forefront of the pathology development, and those areas manifest characteristic dysfunctions such as anosmia and increased colonic transit times before other symptoms develop (3). Interestingly, both of those systems utilize DA as a neurotransmitter (34,35). Damage to the other brain areas eventually contributes to the cognitive decline and dementia. Nevertheless, when protein misfolding and oxidative stress strike, the chain that separates a healthy adult from disability is only as strong as its weakest link.  

Mending the weakest link

Dopamine replacement therapy  

Various DA-replacement therapies are employed to control PD, such as a DA precursor L-DOPA (L-3,4-dihydroxyphenylalanine) and DA agonists. While L-DOPA can efficiently ameliorate PD symptoms in many patients, it also increases DA concentration and encourages the vicious circle of progressing degeneration in remaining neurons (36). DA agonists are a more preferable course of treatment as they decrease endogenous DA progression while restoring the subthalamic nuclei inhibition that ameliorates ecitotoxicity (37). On the other hand, these drugs often have serious side effects including hallucinations due to their non-specific effects. Considering the role of oxidative stress in PD progression, it should be beneficial to use an antioxidant therapy, but up to date it has proven to have only a limited success (38,39). It is noteworthy that coffee and tobacco appear protective against PD (40), although the exact neuroprotective compounds might not be caffeine and nicotine themselves (41). 

Dopaminergic neuron grafts 

An obvious solution to the SN neurodegeneration is cell replacement. Embryonic post-mitotic ventral mesencephalon neurons, that constitute mostly of developing, but differentiated past the neural stem cell stage, dopaminergic neurons, can be extracted from 6- to 9-week-old embryos and successfully grafted into the SN of PD patients (42,43). Even though graft-induced dyskinesias (GID) were observed as a side effect of initial transplants (44,45), nowadays better understanding of the mechanisms underlying GID and perfection of the technique made this procedure much safer and more effective. A “TRANSEURO” trial is currently using fetal tissue for the grafts, stating however that using embryonic tissue presents immunological, ethical, and logistical issues (46).

Human iPSC-derived dopaminergic neurons 

A safer and more reliable way of providing a dopaminergic neuron graft would be the re-programming of patient’s own cells. If cells come from the patient’s body, there is a much lower risk of rejection, and since these cells can be grown in the laboratory, there are no ethical and transportation issues that are associated with the aborted embryonic material. It also means that the cells will always be readily available, and the grafts will be more well-defined creating more predictable outcomes. As our understanding of the natural midbrain DA neuron development progressed, it became possible to recapitulate those events in vitro. The technology allowing for the derivation of DA-producing neurons from the human induced pluripotent stem cells (iPSCs) already exists. Human iPSC-derived dopaminergic neurons (Fig. 3) have already shown promising results in rats (47). Combined with the targeted gene editing to remove PD-predisposing mutations in DA neurons and co-transplantation of other cells such as astrocytes (48), SN grafts offer a promising treatment for the motor symptoms of the PD. Although they do not cure neurodegeneration, they can bring patients their life quality, life expectancy, and the solid ground beneath their feet back.

human iPSC-derived dopaminergic neuron progenitor kit


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