Introduction

Synucleins are soluble proteins found primarily in nervous system tissue and in specific tumors. There are three families of synuclein: alpha-synuclein, beta-synuclein, and gamma-synuclein. The alpha and beta variants are seen in presynaptic terminals, whereas the gamma variant appears in the peripheral nervous system and retina. The gamma variant is also expressed in breast tumors and serves as a marker for disease progression. Alpha-synuclein is the more studied variant due to its link to Parkinson’s disease, and hence will be the focus of this article.

Alpha-synuclein is the product of the SNCA gene. It is a small protein that occurs in neurons near synaptic vesicles within presynaptic terminals. Recent studies have shown that alpha-synuclein functions in synaptic vesicle recycling, but the exact mechanism is unclear. The protein becomes pathologic when it aggregates and leads to neuron dysfunction, then subsequent cell death.[1][2][3][4]

Issues of Concern

The synucleins are proteins of concern due to their implication in synucleinopathies. Synucleinopathies include Parkinson disease, dementia with Lewy Bodies, and multiple system atrophy.

Alpha-synuclein has strong links to Parkinson disease. Research shows that alpha-synuclein accumulation results in impaired lysosomal autophagy. Additionally, mutations in alpha-synuclein lead to the generation of aberrant proteins, which causes further impairment in lysosomal proteolytic pathways and subsequent neuronal cell death.[5]

Beta-synuclein is generally neuroprotective as it can inhibit alpha-synuclein aggregation, but in specific cases, it can be pathologic. Rare cases of dementia with Lewy bodies have been found with point mutations in beta-synuclein, leading to a neurotoxic effect.[6]

Multiple system atrophy characteristically demonstrates autonomic failure, parkinsonism, and ataxia. It is a sporadic, adult-onset, progressive disorder. The pathologic hallmark presentation of multiple system atrophy is glial cytoplasmic inclusions of misfolded alpha-synuclein, found in oligodendrocytes. Multiple system atrophy predominantly affects the striatonigral and olivopontocerebellar structures. Multiple system atrophy has two main motor phenotypes, parkinsonian and cerebellar. Clinical differentiation from Parkinson disease is not possible and requires neuropathology for definitive diagnosis.[7][8] 

In the animal model, research has shown that the lack of gamma-synuclein in the central nervous system can decrease the ability to use information (memory) to manage and solve tasks and problems. Still, with the animal model, gamma-synuclein has been shown to recruit macrophages in the area of damage to the optic nerve and stimulate scar tissue production.

In both animal studies just mentioned, the elderly mice showed a reduction in the presence of this protein.

Cellular Level

The SNCA gene encodes synuclein, which is made up of 140 amino acids. The resulting amino acid sequence can code for alpha helixes via binding to negatively charged lipids or beta-sheets with prolonged incubation. The protein is made up of three regions: an amino terminus, central hydrophobic, and a carboxyl terminus. The amino terminus regions lead to the alpha-helix shape with apolipoprotein lipid-binding motifs. The central hydrophobic regions can lead to beta-sheet configurations. Lastly, the carboxyl terminus is generally unstructured.[6]

Alpha-synuclein is present in living structures in a perennial balance between the soluble state and a state in which it links to the membrane. Membrane bonding occurs thanks to the presence of phosphatidylinositol or phosphatidylserine; at the brain level, the protein binds to small vesicles (about 40-nm in diameter). Alpha-synuclein can create different polymorphic aggregates, which have been called strains. This aggregation capacity, most likely, is one reason for the different types of synucleinopathies found; each strain recruits other proteins differently, always creating different dysfunctional patterns.

Beta-synuclein is made up of 134 amino acids, while gamma-synuclein is made up of 127 amino acids, which bind membrane phospholipids.

Development

Alpha-synuclein expression inducement occurs during neuronal development. It occurs after neuronal phenotype and the creation of synaptic connections. Additionally, alpha-synuclein levels fluctuate in conditions that alter neuroplasticity or result in neural injury.[6]

In the human model, alpha-synuclein is present in the fetus since the fifteenth week of gestation; during the fetal period, the protein is present in all organs.

Organ Systems Involved

Alpha-synuclein is predominantly found in the nervous system, composing 1% of total cytosolic protein. Alpha-synuclein is also abundant in erythrocytes and platelets, although for reasons unknown.[6] Functional alteration of this protein causes anemia and incomplete maturation of the red cells.

Alpha-synuclein is also present in the CSF of patients with Parkinson disease and healthy subjects.

This protein resides mainly in the brain and is found in specific brain areas with a higher percentage (such as the beta form): in the cerebellum, in the hippocampus, in the neocortex area, in the thalamus, and striatum. Alpha-synuclein occurs in small percentages in other anatomical areas, such as muscles, kidneys, motor neurons, liver, heart, and lungs, as well as in the olfactory epithelium, lymphocytes, and testes. Gamma-synuclein is found mainly in the peripheral nervous system and the sympathetic system, and the brain area.

Function

Alpha-synuclein has several roles in synaptic firing, as determined by experimentation over the last decade. It can alter neurotransmitter release, is involved with vesicular trafficking, and works with cysteine string protein-alpha as a chaperone protein in the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes.

In experiments on bovine neural tissue, beta-synuclein has functioned as a constitutive inhibitor of phospholipase D2. Phospholipase D2 is involved in cytoskeleton reorganization and endocytosis at the plasma membrane. Researchers have also suggested beta-synuclein to be a natural negative regulator of alpha-synuclein aggregation – leading to anti-amyloidogenic properties.

Lastly, gamma-synuclein was identified as a marker for breast cancer via cDNA sequencing. The cellular function of gamma-synuclein is unknown.[1][6][9][10][11][12]

The alpha-synuclein protein was discovered within the electric organ of the stingray Torpedo californica in 1988, in the presynaptic area and the nucleus envelope. From these areas derives the name of the protein: synaptic vesicles ("syn") and nuclear envelope ("nuclein"). Although decades have passed since its discovery, its functions remain not fully elucidated, especially in a physiological environment.

Beta-synuclein protein was identified in two animal model studies (1990 and 1992), in the brain area, at the presynaptic level of nerve endings.

Gamma-synuclein was identified in breast cancer in 1997.

Mechanism

Researchers have identified a few pathogenic mechanisms for synucleins, but normal cellular function and mechanism remain unclear. Presynaptic nerve terminals function via the repeated release of neurotransmitters and, as such, require constant cycling of the SNARE complex. Experiments have shown that alpha-synuclein has a nonclassical chaperone role in the maintenance of presynaptic SNARE complex assembly. Alpha-synuclein can be found bound to SNARE protein synaptobrevin-2/vesicle-associated membrane protein 2. This interaction keeps the SNARE complex in an assembled state and prevents the presynaptic nerve terminal from being able to release neurotransmitters.[13]

Alpha-synuclein undergoes several posttranslational changes, particularly in its carboxy-terminal tail portion: ubiquitination; sumoylation (stimulates protein solubility); glycation; glycosylation; nitration (inhibits the binding to lipid vesicles); proteolysis; phosphorylation (inhibiting its aggregation); oxidation; acetylation (increases its shape and helical folding, with aggregation resistance and greater affinity with membrane bonds). These new adaptations will lead to building new structures and different binding affinities.

Related Testing

One of the hallmarks of Parkinson disease is alpha-synuclein containing Lewy body inclusions in the substantia nigra. Parkinson disease is a clinical diagnosis, but eosinophilic inclusions of alpha-synuclein as Lewy bodies are definitive.[14][15]

Additionally, experimentation is underway to find a peripheral biomarker to identify Parkinson disease in patients. Real-time quaking-induced conversion assay functions by vigorous shaking and agitation to induce fibril formation of proteins. Research has revealed that biopsied submandibular gland tissue can be tested via real-time quaking-induced conversion assays for Parkinson disease with high specificity and sensitivity via pathological alpha-synuclein.[16][17]

Pathophysiology

Parkinson’s disease hallmarks include dopaminergic neuron loss in substantia nigra pars compacta and Lewy bodies. The exact mechanism of Parkinson disease is unknown, but it involves alpha-synuclein aggregates, the endo-lysosomal system, and mitochondrial function. Alpha-synuclein is a presynaptic protein that assists in presynaptic vesicle transportation and endocytosis. When it becomes misfolded and aggregates, it becomes toxic to mitochondria and neurons. Increased cellular oxidative stress leads to increased alpha-synuclein aggregation. As alpha-synuclein aggregates, mitochondria and the neuron lysosomal function deteriorate, and the cell eventually dies.[15]

Alpha-synuclein is also involved in other pathologies, such as the Lewy body variant of Alzheimer disease, multiple system atrophy, and brain neurodegeneration with an accumulation of iron (accumulation type I).

Clinical Significance

Synuclein is clinically significant due to its involvement in synucleinopathies – Parkinson disease, dementia with Lewy bodies, and multiple system atrophy. Alpha-synuclein is the target of much research and could serve as a target for therapeutic agents. Suggested therapies include targeting alpha-synuclein aggregates with specific antibodies to encourage clearance, targeting alpha-synuclein monomers in an attempt to increase clearance, decrease synthesis, or stabilize the monomers to prevent aggregation.[18]

Beta-synuclein is an intrinsically disordered protein but has been shown to inhibit alpha-synuclein aggregates, which have implications in the synucleinopathies. The three synuclein domains of beta-synuclein have been demonstrated to inhibit the aggregation of alpha-synuclein. This fact is of clinical significance as the interaction between the synuclein chains can serve as a basis for targeted therapeutics against alpha-synuclein aggregates.[19]

Synuclein-gamma has implications in various types of cancers. For breast cancer, radiation therapy following mastectomy reduces recurrence and improves prognosis. Yet, radiotherapy comes with a litany of side effects and risks. Patients who were positive for synuclein-gamma demonstrated less response to radiation therapy, possibly due to a correlation between gamma-synuclein and expression of radiation repair genes.[20]