from a patient who had Parkinson's disease. Alpha synuclein, having no single, well-defined tertiary structure, is an
intrinsically disordered protein, with a pI value of 4.7, which, under certain pathological conditions, can
misfold in a way that exposes its core
hydrophobic residues to the intracellular milieu, thus providing the opportunity for
hydrophobic interactions to occur with a similar, equally exposed protein. In 2011, two groups published their findings that unmutated α-synuclein forms a stably folded tetramer that resists
aggregation, asserting that this folded tetramer represented the relevant in vivo structure in cells, thereby relieving alpha synuclein of its disordered status. Proponents of the
tetramer hypothesis argued that
in vivo cross-linking in bacteria, primary neurons and human erythroleukemia cells confirmed the presence of labile, tetrameric species. However, despite numerous in-cell NMR reports demonstrating that alpha synuclein is indeed monomeric and disordered in intact
E. coli cells, it is still a matter of debate in the field despite an ever growing mountain of conflicting reports. 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. In vitro models of synucleinopathies revealed that aggregation of alpha-synuclein may lead to various cellular disorders including microtubule impairment, synaptic and mitochondrial dysfunctions,
oxidative stress as well as dysregulation of Calcium signaling, proteasomal and lysosomal pathway. 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 alpha-synuclein seed amplification assay (SAA) using cerebrospinal fluid demonstrated high diagnostic performance in distinguishing synucleinopathies with Lewy bodies from control subjects. Additionally, SAA kinetics are associated with cognitive impairment and predict the development of dementia. 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. One theory is that the majority of alpha-synuclein aggregates are located in the presynapse as smaller deposits which causes synaptic dysfunction. 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. Five
point mutations have been identified thus far:
A53T, A30P, E46K, H50Q, and G51D; however, in total, nineteen mutations in the SNCA gene have been associated with parkinsonism: A18T, A29S, A53E, A53V, E57A, V15A, T72M, L8I, V15D, M127I, P117S, M5T, G93A, E83Q, and A30G. It has been reported that some mutations influence the initiation and amplification steps of the aggregation process. 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. Over-expression of human wild-type or A53T-mutant alpha-synuclein in primates drives deposition of alpha-synuclein in the ventral midbrain, degeneration of the dopaminergic system and impaired motor performance. Although the accumulation and aggregation of alpha-synuclein in most Parkinson's disease patients primarily result from posttranscriptional mechanisms, targeting its production remains a potential therapeutic approach. Research indicates that
microRNA-7 and the naturally occurring small molecule
quercetin can reduce alpha-synuclein levels under experimental conditions. Certain sections of the alpha-synuclein protein may play a role in the
tauopathies. In children with autism spectrum disorders, the serum levels of alpha-synuclein have been reported to be significantly higher. A correlation to higher levels of e.coli and pro-inflammatory gut microbiome in those patients and an approach through disease modifying polysaccharides has been reported in clinical pilot studies. A
prion form of the protein alpha-synuclein may be a causal agent for the disease
multiple system atrophy. Self-replicating "prion-like" amyloid assemblies of alpha-synuclein have been described that are invisible to the amyloid dye Thioflavin T and that can acutely spread in neurons in vitro and in vivo.
Antibodies against alpha-synuclein have replaced antibodies against
ubiquitin as the gold standard for
immunostaining of Lewy bodies. The central panel in the figure to the right 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, allowing small molecules to be translocated from one side of the membrane to the other through the hollow oligomer. 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. == Protein-protein interactions ==