Science & Progress: Why does Parkinson's disease progress so much differently in different patients?

*For medical professionals only

Parkinson's disease (PD) is the second largest neurological degenerative disease in the world, with the degeneration and death of neurons caused by the degeneration and death of neurons α-synuclein (α-Syn) in neurons
.
This pathological protein has an important feature, the "prion-like" property, that is, the misfolded α-Syn can travel along neurons in the brain, but the mechanism of transmission is not clear
.

In recent years, the concept of "intestinal-brain axis" has gradually become popular in academia, believing that an imbalance in the intestinal flora can also lead to intestinal nerve α-Syn misfolding, and these pathological α-Syn can travel along the vagus nerve and pass through the dorsal nucleus of the medulla vagus nerve to the midbrain, resulting in the death of dopaminergic neurons in the midbrain and the appearance of PD motor symptoms[1].

There is already evidence that neuronal activity may have an effect on the spread and aggregation of pathological proteins, but the specific mechanism of influence has not been studied [2].

Recently, scientists from the German Neurodegenerative Diseases Research Center and the Parkinson's Disease Research Association of the United States have jointly found that the speed of α-Syn from the medulla oblongata to the midbrain is closely related to neuronal activity, and oxidative stress caused by neuronal overactivation can accelerate the spread
of α-Syn.
The results were published in the journal Science advances
.

Since in the theory of "intestinal-brain axis", the dorsal nucleus of the medulla oblongata is the starting point for the transmission of pathological proteins into the brain, the researchers used the LoxP-Cre technique to specifically express human-derived α-Syn
in the dorsal nucleus of the vagus nerve in mice.

Transfer into a human-derived α-Syn expression sequence in mice that can only be expressed under Cre induction, and by injecting Cre-AAV in the vagus nerve, an increase
in the level of human-derived α-Syn RNA and protein in the dorsal nucleus of the vagus nerve can be observed after 4-6 weeks of injection.
In the upstream brain region (pons, midbrain, telencephalon), transcription of human-derived α-Syn mRNA cannot be observed, but deposition of human-α-Syn is observed
.
This suggests that human-α-Syn can spontaneously travel from medulla oblonga to the midbrain and telencephalon in mouse brains
.

Shows spontaneous transmission in the brain 4-6 weeks after injection of human-derived α-Syn

Next, to see the effect of neuronal activity on transmission, the researchers used another tool: only by specific drug-activated receptors (DREADDs
).
Also by AAV virus injection, two receptors are expressed, hM3D (excitation receptor) and hM4D (inhibitory receptor), respectively
, in the dorsal nucleus of the vagus nerve.
In the case of these two receptors expressing in neurons, the drug CNO is injected into the abdominal cavity of mice, and neuronal firing is regulated by the
drug.
Neurons expressing hM3D are significantly activated and neurons expressing hM4D are significantly inhibited
.
Through c-fos staining and electrophysiological experiments, we can determine the activation of neurons
.

The researchers first injected Cre and DREADD mixed AAV into the vagus nerve on one side of the mouse, and injected CNO or normal saline
into the intraperitoneal cavity of the mice 3 weeks later.
Brain observation
after 2 weeks of CNO/normal saline injection.
The results showed that in the case of neuronal overactivation, the transmission density (number of positive neurons) and transmission distance (distance of positive neurons from the dorsal nucleus of the vagus nerve) of the human-derived α-Syn from the upper end of the medulla oblongata were significantly improved
.
Under the condition that the electrical activity of neurons is inhibited, the transmission of α-Syn is also significantly shortened
.

The left figure shows that the α-Syn transmission ability is increased after neuronal overactivation, and the right figure shows that the α-Syn transmission ability is weakened after neuronal overinhibition

Overactivation of neurons is often accompanied by oxidative stress responses, and nitrolation modifications are a key mechanism
mediating α-Syn deposition.
Previous studies have shown that the production of superoxides can mediate the nitrolation of α-Syn[4].

Therefore, in the next experiments, the researchers injected a superoxide fluorescent probe (DHE) into mice abdominally based on the above model to explore the transmission pathway
of neuron overactivation → peroxide production →α-Syn.

The study found that in the over-activated side of the brain of neurons, there was also an increase in superoxide signaling and an increase in the spread of human-derived α-Syn, and the nitrolification level of human-α-Syn was also significantly improved
.
This oxidative stress response also affects mitochondrial function in neurons, and mitochondrial nitrocellation damage
was also found in over-activated neurons by staining nitrocellulated NDUFB8.

Oxidative stress (upper left), mitochondrial injury (lower left), and α-Syn nitrolation modification (right) levels are elevated after neuronal overactivation

To summarize the above research results: human-derived α-Syn can be transmitted
between mouse neurons.
In the case of neuronal hyperactivation, increased α-Syn transmission capacity, increased nitrocellation levels, increased oxidative stress of neurons, and increased
mitochondrial damage can be observed simultaneously.
So, is there a direct relationship between the increased ability of α-Syn to transmit and oxidative stress?

SOD is very familiar to everyone, is a superoxide clearing protein, with a strong antioxidant capacity
.
The researchers also stained nitroated SOD2 and found that nitrogenated SOD2 also increased in over-activated neurons
.
So the researchers overexpressed SOD2 on the basis of the previous mouse model, which was also expressed in the dorsal nucleus of the vagus nerve through AAV injection (this group of mice suffered too much
).

After SOD2 overexpression, the expression level of human-derived α-Syn did not decrease at the mRNA level, but by calculating the number of α-Syn-positive neurons, the distance of the positive range from the dorsal nucleus of the vagus nerve, and the staining intensity of the α-Syn, the researchers observed a significant decrease in the transmission range and transmission efficiency of α-Syn, and the nitrocellation level of α-Syn was also significantly reduced
.

Correspondingly, oxidative stress and mitochondrial damage within neurons are significantly reduced
.
Staining using oligomers for human-derived α-Syn found that the number of oligomers produced after SOD2 overexpression also decreased significantly, indicating that the inhibition of oxidative stress inhibited not only the spread of α-Syn, but also the aggregation
of α-Syn.

The number of cells, propagation distance, and propagation density of α-Syn after SOD2 overexpression decreased, and the nitrocellation level of α-Syn decreased

In the clinic, the same is Parkinson's disease, the clinical progress of different patients is very different: some patients have symptoms that are still very mild for more than ten years, and they are no different from normal people after taking dopamine replacement drugs; Some patients progress rapidly to advanced pathology within 5 years and are unable to take care of themselves, which may be related
to the speed of α-Syn transmission and the degree of neuronal death.

The findings of this study link neuronal activity, oxidative stress to evidence related to mitochondrial damage and α-Syn transmission and deposition into a clue that shows us their causal relationship
.
Maybe we can't regulate neuronal activity, but we can regulate oxidative stress, and maybe one day in the future, we can usher in new disease-modifying treatments for Parkinson's
disease.

How pathological proteins are produced and how they are transmitted has always been the focus of
continuous exploration in the field of neurodegenerative diseases.
Despite many setbacks and failures in the clinical study of monoclonal antibodies against α-Syn, with each step forward, we are closer to the truth
.
I also hope that one day, the diagnosis and treatment of Parkinson's disease can really make a breakthrough
.

Resources:

[1] Ulusoy A, Phillips RJ, Helwig M, Klinkenberg M, Powley TL, Di Monte DA.
Brain-to-stomach transfer of α-synuclein via vagal preganglionic projections.
Acta Neuropathol.
2017; 133(3):381-393.
doi:10.
1007/s00401-016-1661-y

[2] Wu Q, Shaikh MA, Meymand ES, et al.
Neuronal activity modulates alpha-synuclein aggregation and spreading in organotypic brain slice cultures and in vivo.
Acta Neuropathol.
2020; 140(6):831-849.
doi:10.
1007/s00401-020-02227-6

[3] Helwig M, Ulusoy A, Rollar A, et al.
Neuronal hyperactivity-induced oxidant stress promotes in vivo α-synuclein brain spreading.
Sci Adv.
2022; 8(35):eabn0356.
doi:10.
1126/sciadv.
abn0356

[4] Schildknecht S, Gerding HR, Karreman C, et al.
Oxidative and nitrative alpha-synuclein modifications and proteostatic stress: implications for disease mechanisms and interventions in synucleinopathies.
J Neurochem.
2013; 125(4):491-511.
doi:10.
1111/jnc.
12226

The author of this article Yan Yiqun

Editor-in-charge Daisi Rain