Animals pretreated with plasmalogen supplements are protected against the neurotoxin MPTP. MPTP selectively targets dopamine neurons and creates symptoms and neuropathology similar to Parkinson’s disease.
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Parkinson’s disease is a neurodegenerative disease that affects a specific type of neuron, called a dopamine neuron, in a particular region of the brain, called the substantia nigra. MPTP is a neurotoxin that selectively targets and kills these dopamine neurons and creates Parkinson-like symptoms. This seminar describes the biochemical mechanism of MPTP and how plasmalogen precursor treatment protects dopamine neurons from neurodegeneration.
MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) was accidentally discovered as a selective dopamine neuron neurotoxin by a group of amateur drug makers in California in the 1980s. They were trying to make designer heroin that was not on the U.S. Drug Enforcement Agency (DEA) controlled substance list. However, the final drug material that they made was contaminated with MPTP. When they injected themselves with this new “synthetic” heroin, they ended up having a toxic reaction that resulted in them exhibiting symptoms of Parkinson’s disease. After a bit of scientific detective work, MPTP was discovered as the cause of neurodegeneration. From this discovery, a whole bunch of research was done to understand all the nitty-gritty details of how MPTP kills dopamine neurons. MPTP is a prodrug or precursor to MPP+. MPTP enters the brain and then gets metabolized to MPP+, the toxic agent.
Parkinson’s disease is a movement disorder caused by the degeneration of a specific type of neuron in the brain called a dopamine neuron. Dopamine neurons are called dopamine neurons because they use dopamine as their neurotransmitter. The principle of dopamine neurotransmission is the same as I described earlier for acetylcholine neurons. After the released dopamine interacts with the postsynaptic neuron, it is re-absorbed back into the presynaptic neuron. There is a special dopamine-specific transporter that performs this task. It turns out that this transporter recognizes MPP+ as if it is dopamine. Since only dopamine neurons have the dopamine transporter, MPP+ is selectively absorbed and concentrated into dopamine neurons. MPP+ is toxic to all cells and neurons. It is selectively toxic to dopamine neurons because of the dopamine transporter. Since MPP+ selectively targets only one neuron cell type, it is an invaluable scientific model to study the mechanisms of neurodegeneration applicable to other neurons without having the whole brain affected. Once MPP+ is in the presynaptic dopamine neuron, it has the following negative consequences.
- MPP+ is also recognized by the protein that transports dopamine into the presynaptic vesicles (called the vesicular monoamine transporter or VMAT). These presynaptic vesicles store the dopamine neurotransmitters before their release during a neuron transmission event. MPP+ pushes dopamine out of the vesicles resulting in increased cytosolic dopamine and dopamine leaking out of the cell when at rest. This results in less dopamine being released by the neuron during a neuron transmission pulse. Sequestering the MPP+ in the vesicles reduces dopamine transmission but also reduces the toxicity of MPP+ to the neuron.
- MPP+ induces reactive oxygen species (ROS), commonly referred to as “oxidative stress”. This is what ultimately causes the death of the neuron.
- MPP+ also inhibits the transport of mitochondria from the cell body through the axon to the synapse region. This effect of MPP+ is completely neutralized by restoring glutathione with N-acetylcysteine.
The terms “oxidative stress” and “reactive oxygen species or ROS” are frequently used when describing a negative or toxic effect on a cell system. ROS are natural and essential products of normal and healthy human biochemistry. They are also vital signaling molecules. Like all things, it is when they are found in excess and in the wrong places that they cause problems. MPP+ toxicity is as good a model as any to explain ROS biochemistry and how it can kill if it is not controlled.
MPP+ is absorbed into mitochondria where it binds to and inhibits a key protein of the electron transport chain (ETC). This is what triggers a chain of reactions that a normal neuron cannot survive, but a plasmalogen supplemented one can. The human body uses oxygen in many different processes. It uses oxygen to detoxify and remove xenobiotics and to make metabolic intermediates. But oxygen is mostly used to burn hydrocarbons and make energy. The energy released from this reaction is used to drive all the processes of the body. Each of the trillions of cells in your body is on fire, burning hydrocarbons and creating carbon dioxide and water. This is a tightly controlled process. The main purpose of the mitochondria is to convert hydrocarbon energy (an acetyl-CoA metabolite) into biochemical energy (ATP). The mitochondria do this by burning the hydrocarbon into carbon dioxide and water, just like your car engine, and capturing the energy released in ATP. It is supposed to work in two steps. The first step is the citric acid cycle, where the creation of carbon dioxide and the high-energy protonated metabolites NADH and FADH2 occurs. These high-energy metabolites are then handed over to the ETC, which will ultimately use oxygen to convert these protons into water and capture their energy in ATP. The first step of the ETC is to recycle NADH back to NAD+ so that it can go back and convert more acetyl-CoA into carbon dioxide. MPP+ blocks this process and results in both a depletion of NAD+ (which shuts down the citric acid cycle) and an accumulation of NADH (which must go somewhere). If NADH cannot be processed by the ETC, it ends up being processed by NADH oxidase, which is in the outer plasma membrane of the cell, especially at the synapse region of a neuron. Here, it will combine with molecular oxygen and an electron donor to recycle the NAD+ and create a superoxide radical (O2-). This superoxide radical is then neutralized by superoxide dismutase (SOD) into peroxide and then the peroxide is neutralized by catalase or glutathione peroxidase. Like all systems, there are limits. If too much superoxide and peroxide is being formed outside the cell, then these ROS begin to oxidize the lipids in the membrane. A cell is not supposed to have oxidized membrane lipids. This only happens to cells that have serious internal metabolism issues. When this happens, it acts as a signal to your body’s immune system that this cell is about to die. This attracts the microglia to come to the cell and finish the death sentence and clean up the mess. This is the biochemical process of inflammation.
These features of MPP+ toxicity make this an ideal model to investigate the effects of ADG plasmalogen precursors as a neuroprotectant. The process to create a modest but reproducible degeneration of dopamine neurons is to inject the MPTP toxin in mice four times, every two hours on Day 0, and then wait five days for the degeneration to stabilize. We performed three experiments with the DHA-ADG:
- Treatment with three different dose levels of DHA-ADG for five days before injecting the MPTP toxin.
- Treatment with three different dose levels of DHA-ADG starting one hour after injecting the MPTP toxin and continuing for the next five days.
- Treatment with three different dose levels of DHA-ADG on normal mice for ten days.
The results were dramatic:
- MPTP treatment resulted in a persistent and residual 15-20% reduction in blood plasmalogen levels five days after treatment with the neurotoxin. The pre-treatment of 10 mg/kg and 50 mg/kg of the plasmalogen precursor completely prevented MPTP depletion of plasmalogens.
- MPTP treatment resulted in the degeneration of approximately 50% of the dopamine neurons and the depletion of both dopamine and serotonin levels. Both the 10 mg/kg and the 50 mg/kg doses resulted in complete protection from neurodegeneration.
- In the post-treatment study, no effect was observed at 10 mg/kg, but a 50% improvement was seen at the 50 mg/kg dose.
These data contain several important observations.
- They show that plasmalogen reserves are affected by oxidative neurotoxins.
- That neurodegeneration takes a significant amount of time.
- That a pre-existing reserve capacity of plasmalogens can absorb insults and prevent this depletion.
- The mechanism of neurodegeneration by MPTP has been extensively studied. We know that MPTP only becomes toxic to neurons after it crosses the blood-brain barrier and after it is activated by the glia into MPP+ and after it is taken up into the dopamine neuron.
- To protect the dopamine neuron, the DHA-plasmalogen precursor must be acting at or in the dopamine neuron. Therefore, the DHA-plasmalogen precursor enters and is active in the brain.
- We know that the neuronal death caused by MPTP is caused by ROS at the membrane. Therefore, the DHA-plasmalogen precursor must be deactivating it, which is what the vinyl ether bond in plasmalogens is designed to do.
- The pre-treatment with a DHA-plasmalogen precursor to create a DHA-plasmalogen reserve and the maintenance of DHA-plasmalogen supply via continued supplementation performed exactly as predicted.
Since the mouse studies were purely focused on neuropathology endpoints and did not evaluate neurological function, we also investigated DHA-plasmalogen precursor therapy on monkeys with confirmed Parkinson’s disease caused by past MPTP toxicity. These monkeys respond to L-DOPA therapy in the same way that humans do. After being chronically treated with L-DOPA, these monkeys get side-effects from L-DOPA called L-DOPA induced dyskinesias or LID, which are abnormal limb movements. When we treated these monkeys with 50 mg/kg of the DHA-plasmalogen precursor, there was a noticeable and significant reduction in LID after only two days, and this effect was observed for the entire 12 days of the study. These results indicate that DHA-plasmalogen supplementation has a positive neurological effect in pre-existing neurologically damaged animals.
Overall, these two MPTP neurodegeneration studies show that DHA-plasmalogen supplementation is neuroprotective against oxidative neurotoxins and neurologically active in neurologically damaged animals.
Dr. Goodenowe explains the relevant research and literature regarding plasmalogen precursors and the prevention neurodegeneration using Parkinson’s disease as an example in seminar C103 – Plasmalogen Supplementation Prevents Neurodegeneration.