Dopamine Stabilization and Neurodegeneration with Alpha Synuclein Oligomers

As opposed to a monomer, which has only one unit, or a polymer, which has many units, an oligomeric protein is made up of a few repeating units. Oligo – is derived from the Ancient Greek word meaning “few” and mer comes from the Ancient Greek word for “part.”

Alpha synuclein is a protein involved in Parkinson’s Disease (PD), a common neurodegenerative disorder. While alpha synuclein is usually found in the presynaptic terminals of neurons, it aggregates to form larger fibrils and aggregates called Lewy bodies in certain diseases, known as synucleinopathies.

It forms intermediate species called oligomers during this aggregation process. These oligomers are of interest because rather than Lewy bodies, they are increasingly thought to be the toxic species that result in neuron death in synucleinopathic diseases.^1,2

Alpha synuclein monomers can aggregate to form a variety of oligomeric species. If aggregation continues, they form protofibrils and fibrils as found in Lewy bodies. Image Credit: StressMarq Biosciences

How Dopamine Induces Alpha Synuclein Oligomerization

Dopamine is a neurotransmitter which can prevent the aggregation of alpha synuclein into fibrils, stabilizing it in its oligomeric form. Dopamine oxidation is thought to be part of this process.

Dopamine Oxidation Mechanism

The oxidation of the catechol ring to form dopamine-quinone is the first step in dopamine oxidation. This species is then able to react to form dopaminochrome, which then can be converted into 5, 6-indoloquinone and ultimately, neuromelanin.³

Dopamine oxidation mechanism. Both dopaminochrome and DOPAL are thought to interact with alpha synuclein in a way that promotes oligomer formation. Image Credit: StressMarq Biosciences

Interactions with Alpha Synuclein: Oligomer Stabilization

Through interactions in different regions, dopamine and its derivatives are thought to inhibit alpha synuclein aggregation and kinetically stabilize oligomers.

C-terminus: Y125EMPS129

Computational modeling suggests that the aromatic ring of dopamine interacts with this 5-amino acid sequence of alpha synuclein in a non-covalent, non-specific, hydrophobic way.⁴ Yet, there is also evidence that the oxidation of the methionine residue in this region, and not binding with dopamine, prevents fibrillization.⁵

This motif is needed for oligomer stabilization in any case,^6-9 and the mutation of Y125EMPS129 to F125AAFA129 stops this by inhibiting interaction with dopamine.^7,11

NAC Region: E83

It is thought that dopamine derivatives also have long-range electrostatic interactions with E83 in the NAC region.⁴ The mutation E83A hinders dopamine’s ability to prevent fibril formation.⁴

Image Credit: StressMarq Biosciences

Oligomerization Mechanism

Dopamine and its derivatives not only kinetically stabilize oligomers, but in addition, they can induce oligomerization of monomers. Dopamine can prompt dimerization, with residues 43 through 60 in the N-terminal forming the dimer interface.¹⁰

Dopamine also induces trimerization, where monomers overlap but do not have associate end-to-end.¹¹ By cross-linking with dopamine-quinone and dopamine-melanin, the oxidation of four methionine residues at the C and N-terminals inhibits end-to-end associations and beta-sheet formation.¹¹

Dopaminochrome can prompt reversible conformational alterations which promote oligomer formation¹² and DOPAL, which is a toxic dopamine metabolite, is thought to induce monomer aggregation by cross-linking alpha synuclein lysine residues.¹³

Alpha Synuclein Oligomer Structure

There are a number of types of alpha synuclein oligomers, both synthesized in labs, and found in living systems. Alpha synuclein oligomers which are derived from post-mortem brains can range from dimers (two units) to 150 units.^1,14

Evidence for Oligomer Toxicity

Increasingly, alpha synuclein oligomers are recognized as a toxic species in synucleinopathies. In vitro experiments and animal models have shown their toxicity, as cell death and degeneration happens when there are oligomers are present, even in the absence of large alpha synuclein aggregates.^15-18

Yet, there are also non-toxic types of oligomers. The most toxic oligomers are thought to be between 2 and 6 nm in diameter,¹⁴ whereas larger oligomers are thought to be non-toxic but able to seed.¹

Alpha Synuclein Oligomer Toxicity Mechanisms

Through various mechanisms, alpha synuclein oligomers can be toxic to cells:

Interactions with Lipid Membranes and Pore Formation

Alpha synuclein oligomers have exposed hydrophobic surfaces¹⁹ and accessible N-terminal regions which allow them to interact with and destabilize lipid membranes.²⁰ Small, annular oligomers are likely to form pores in vesicle and plasma membranes.^21-25

Since hydrophobic oligomers can bind more tightly to organelles and membranes, oligomer hydrophobicity is thought to be linked with toxicity.²⁶ This would account for the reduced toxicity in larger aggregates with fewer accessible hydrophobic patches.

Mitochondrial Dysfunction

By inducing a slow and progressive increase in NADH, alpha synuclein oligomers can cause mitochondrial dysfunction.³⁰ This is linked with increased production of reactive oxygen species, permeability transition pore (PTP) opening, mitochondrial depolarization, inhibition of complex I, and decreased electron flow through the electron transport chain.³⁰

Endoplasmic Reticulum (ER) Stress

Alpha synuclein oligomers can form within the ER lumen and accumulate within the ER, heightening neuronal sensitivity to ER stress.³¹ This ER stress contributes to neurodegeneration.

Calcium Influx

Via N-type voltage-dependent Ca²⁺, alpha synuclein oligomers can induce Ca²⁺ influx.²⁷ It is thought that this process is mediated by the NAC region.²⁸ While alpha synuclein monomers can also influence calcium signaling, only oligomers cause cell death.²⁹

SNARE Complex Inhibition

Large intracellular dopamine-stabilized alpha synuclein oligomers can prevent the formation of the SNARE complex, which decreases the release of neurotransmitters.³³

Glutamatergic Receptors

Globular oligomers can change the function of glutamatergic receptors, which results in disrupted neuronal function.^31,32

Alpha Synuclein Oligomer Seeding

Different varieties of oligomers exhibit different seeding capacities. Oligomers with higher seeding capacities are linked with lower toxicities.³⁴ Intracellular alpha synuclein oligomers can be released via exosomes, where it is more likely that they are taken up by neighboring cells than free extracellular alpha synuclein oligomers.³⁵

Dopamine-Stabilized Alpha Synuclein Oligomers

Dopamine-induced oligomers have been revealed to auto-aggregate³⁶ and can cross-seed amyloid-beta aggregates.³⁷ Dopamine-induced oligomers have also caused neurodegeneration in mouse and worm in vivo models.³⁸

Yet, it is worth noting that different experimental conditions generate different varieties of oligomers, and the toxicity depends on oligomer synthesis structure and technique. StressMarq has just launched commercially available dopamine-stabilized oligomers which can be used for alpha synuclein research.

As they exhibit potential as a biomarker, oligomers are of interest to researchers as they are present in CSF and plasma, in addition to a treatment target since they are a cause of neurodegeneration.³¹

TEM of Human Recombinant Alpha Synuclein Oligomers (Dopamine HCL Stabilized) (SPR-466). Image Credit: StressMarq Biosciences

References and Further Reading

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2. Wright, J.A., Wang, X., Brown, D.R. Unique copper-induced oligomers mediate alpha-synuclein toxicity. FASEB J. 2009; 23(8):2384-93.
3. Van Diggelen, F., Tepper, A.W.J.W., Apetri, M.M., Otzen, D.E. α-Synuclein oligomers: a study in diversity. Israel J Chem. 2016; 57(7).
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5. Leong, S.L., Pham C.L., Galatis, D., Fodero-Tavoletti, M.T., Perez, K., Hill, A.F., et al. Formation of dopamine-mediated alpha-synuclein-soluble oligomers requires methionine oxidation. Free Rad Biol Med. 2009; 46(10):1328–37.
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Acknowledgments

Produced from materials originally authored by Patricia Thomson from StressMarq Biosciences Inc.

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