Alpha-synuclein is a protein that is abundant in the human brain. Smaller amounts are found in the heart, muscles, and other tissues. In the brain, alpha-synuclein is found mainly at the tips of nerve cells (neurons) in specialized structures called presynaptic terminals. Within these structures, alpha-synuclein interacts with phospholipids and proteins. Presynaptic terminals release chemical messengers, called neurotransmitters, from compartments known as synaptic vesicles. The release of neurotransmitters relays signals between neurons and is critical for normal brain function.
Although the function of alpha-synuclein is not well understood, studies suggest that it plays an important role in maintaining a supply of synaptic vesicles in presynaptic terminals. It may also help regulate the release of dopamine, a type of neurotransmitter that is critical for controlling the start and stop of voluntary and involuntary movements.
The human Alpha-synuclein protein is made of 140 amino acids, and is encoded by the SNCAgene. An alpha-synuclein fragment, known as the non-Abeta component (NAC) of Alzheimer's diseaseamyloid, originally found in an amyloid-enriched fraction, was shown to be a fragment of its precursor protein, NACP. It was later determined that NACP was the human homologue of Torpedo synuclein. Therefore, NACP is now referred to as human alpha-synuclein.
It has been established that alpha-synuclein is extensively localized in the nucleus of mammalian brain neurons, suggesting a role of alpha-synuclein in the nucleus. Synuclein is however found predominantly in the presynaptic termini, in both free or membrane-bound forms, with roughly 15% of synuclein being membrane-bound in any moment in neurons.
Recently, it has been shown that alpha-synuclein is localized in neuronal mitochondria. Alpha-synuclein is highly expressed in the mitochondria in olfactory bulb, hippocampus, striatum,and thalamus, where the cytosolic alpha-synuclein is also rich. However, the cerebral cortex and cerebellum are two exceptions, which contain rich cytosolic alpha-synuclein but very low levels of mitochondrial alpha-synuclein. It has been shown that alpha-synuclein is localized in the inner membrane of mitochondria, and that the inhibitory effect of alpha-synuclein on complex I activity of mitochondrial respiratory chain is dose-dependent. Thus, it is suggested that alpha-synuclein in mitochondria is differentially expressed in different brain regions and the background levels of mitochondrial alpha-synuclein may be a potential factor affecting mitochondrial function and predisposing some neurons to degeneration.
At least three isoforms of synuclein are produced through alternative splicing. The majority form of the protein, and the one most investigated, is the full 140 aminoacids-long transcript. Other isoforms are alpha-synuclein-126, where exon 3 is lost and lacks residues 41-54; and alpha-synuclein-112, which lacks residue 103-130 due to loss of exon 5.
Alpha-synuclein in solution is considered to be an intrinsically disordered protein, i.e. it lacks a single stable 3D structure. As of 2014, an increasing number of reports suggest, however, the presence of partial structures or mostly structured oligomeric states in the solution structure of alpha-synuclein even in the absence of lipids. This trend is also supported by a large number of single molecule (optical tweezers) measurements on single copies of monomeric alpha-synuclein as well as covalently enforced dimers or tetramers of alpha-synuclein.
Alpha-synuclein is specifically upregulated in a discrete population of presynaptic terminals of the brain during a period of acquisition-related synaptic rearrangement. It has been shown that alpha-synuclein significantly interacts with tubulin, and that alpha-synuclein may have activity as a potential microtubule-associated protein, like tau.
Recent evidence suggests that alpha-synuclein functions as a molecular chaperone in the formation of SNARE complexes. In particular, it simultaneously binds to phospholipids of the plasma membrane via its N-terminus domain and to synaptobrevin-2 via its C-terminus domain, with increased importance during synaptic activity. Indeed, there is growing evidence that alpha-synuclein is involved in the functioning of the neuronal Golgi apparatus and vesicle trafficking.
Apparently, alpha-synuclein is essential for normal development of the cognitive functions. Knock-out mice with the targeted inactivation of the expression of alpha-synuclein show impaired spatial learning and working memory.
Interaction with lipid membranes
Experimental evidence has been collected on the interaction of alpha-synuclein with membrane and its involvement with membrane composition and turnover. Yeast genome screening has found that several genes that deal with lipid metabolism play a role in alpha-synuclein toxicity. Conversely, alpha-synuclein expression levels can affect the viscosity and the relative amount of fatty acids in the lipid bilayer.
Alpha-synuclein is known to directly bind to lipid membranes, associating with the negatively charged surfaces of phospholipids. Alpha-synuclein forms an extended helical structure on small unilamellar vesicles. A preferential binding to small vesicles has been found. The binding of alpha-synuclein to lipid membranes has complex effects on the latter, altering the bilayer structure and leading to the formation of small vesicles. Alpha-synuclein has been shown to bend membranes of negatively charged phospholipid vesicles and form tubules from large lipid vesicles. Using cryo-EM it was shown that these are micellar tubes of ~5-6 nm diameter. Alpha-synuclein has also been shown to form lipid disc-like particles similar to apolipoproteins. Studies have also suggested a possible antioxidant activity of alpha-synuclein in the membrane.
Photomicrographs of regions of substantia nigra in a patient showing Lewy bodies and Lewy neurites in various magnifications
Residues 1-60: An amphipathic N-terminal region dominated by four 11-residue repeats including the consensus sequence KTKEGV. This sequence has a structural alpha helix propensity similar to apolipoproteins-binding domains
Residues 61-95: A central hydrophobic region which includes the non-amyloid-β component (NAC) region, involved in protein aggregation
Residues 96-140: a highly acidic and proline-rich region which has no distinct structural propensity
The use of high-resolution ion-mobility mass spectrometry (IMS-MS) on HPLC-purified alpha-synuclein in vitro has shown alpha-synuclein to be autoproteolytic (self-proteolytic), generating a variety of small molecular weight fragments upon incubation. The 14.46 kDa protein was found to generate numerous smaller fragments, including 12.16 kDa (amino acids 14-133) and 10.44 kDa (40-140) fragments formed through C- and N-terminal truncation and a 7.27 kDa C-terminal fragment (72-140). The 7.27 kDa fragment, which contains the majority of the NAC region, aggregated considerably faster than full-length alpha-synuclein. It is possible that these autoproteolytic products play a role as intermediates or cofactors in the aggregation of alpha-synuclein in vivo.
Positive α-Synuclein staining of a Lewy body in a patient with Parkinson's disease.
Classically considered an unstructured soluble protein, new evidence suggests that unmutated α-synuclein forms a stably folded tetramer that resists aggregation. Nevertheless, alpha-synuclein aggregates to form insoluble fibrils in pathological conditions characterized by Lewy bodies, such as Parkinson's disease, dementia with Lewy bodies and multiple system atrophy. These disorders are known as synucleinopathies. Alpha-synuclein is the primary structural component of Lewy body fibrils. Occasionally, Lewy bodies contain tau protein; however, alpha-synuclein and tau constitute two distinctive subsets of filaments in the same inclusion bodies. Alpha-synuclein pathology is also found in both sporadic and familial cases with Alzheimer's disease.
The aggregation mechanism of alpha-synuclein is uncertain. There is evidence of a structured intermediate rich in beta structure that can be the precursor of aggregation and, ultimately, Lewy bodies. A single molecule study in 2008 suggests alpha-synuclein exists as a mix of unstructured, alpha-helix, and beta-sheet-rich conformers in equilibrium. Mutations or buffer conditions known to improve aggregation strongly increase the population of the beta conformer, thus suggesting this could be a conformation related to pathogenic aggregation. Among the strategies for treating synucleinopathies are compounds that inhibit aggregation of alpha-synuclein. It has been shown that the small molecule cuminaldehyde inhibits fibrillation of alpha-synuclein. The Epstein-Barr virus has been implicated in these disorders.
In rare cases of familial forms of Parkinson's disease, there is a mutation in the gene coding for alpha-synuclein. Three point mutations have been identified thus far: A53T, A30P, and E46K. Genomic duplication and triplication of the gene appear to be a rare cause of Parkinson's disease in other lineages, although more common than point mutations. Hence certain mutations of alpha-synuclein may cause it to form amyloid-like fibrils that contribute to Parkinson's disease.
Events in α-synuclein toxicity. The central panel shows the major pathway for protein aggregation. Monomeric α-synuclein is natively unfolded in solution but can also bind to membranes in an α-helical form. It seems likely that these two species exist in equilibrium within the cell, although this is unproven. From in vitro work, it is clear that unfolded monomer can aggregate first into small oligomeric species that can be stabilized by β-sheet-like interactions and then into higher molecular weight insoluble fibrils. In a cellular context, there is some evidence that the presence of lipids can promote oligomer formation: α-synuclein can also form annular, pore-like structures that interact with membranes. The deposition of α-synuclein into pathological structures such as Lewy bodies is probably a late event that occurs in some neurons. On the left hand side are some of the known modifiers of this process. Electrical activity in neurons changes the association of α-synuclein with vesicles and may also stimulate polo-like kinase 2 (PLK2), which has been shown to phosphorylate α-synuclein at Ser129. Other kinases have also been proposed to be involved. As well as phosphorylation, truncation through proteases such as calpains, and nitration, probably through nitric oxide (NO) or other reactive nitrogen species that are present during inflammation, all modify synuclein such that it has a higher tendency to aggregate. The addition of ubiquitin (shown as a black spot) to Lewy bodies is probably a secondary process to deposition. On the right are some of the proposed cellular targets for α-synuclein mediated toxicity, which include (from top to bottom) ER-golgi transport, synaptic vesicles, mitochondria and lysosomes and other proteolytic machinery. In each of these cases, it is proposed that α-synuclein has detrimental effects, listed below each arrow, although at this time it is not clear if any of these are either necessary or sufficient for toxicity in neurons.
Certain sections of the alpha-synuclein protein may play a role in the tauopathies.
Contursi Terme - the village in Italy where a mutation in the α-synuclein gene led to a family history of Parkinson's disease
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