alpha-synuclein

alpha-synuclein is a presynaptic neuronal protein encoded by the SNCA gene. It is intrinsically disordered and abundant in the brain, particularly at synaptic vesicle terminals. Its precise physiological function remains under investigation but is implicated in neurotransmitter release and synaptic plasticity. The protein is most famously known for its central role in the pathogenesis of several neurodegenerative disorders collectively termed synucleinopathies.

Structure and function

The primary structure of this protein consists of 140 amino acids, which can be divided into three distinct domains. The N-terminal region contains amphipathic repeats that facilitate membrane binding, particularly to lipid bilayers of synaptic vesicles. The central non-amyloid-β component (NAC) domain is hydrophobic and crucial for its propensity to aggregate. The acidic C-terminal tail is highly flexible and interacts with various biomolecules. In its native state, it is considered an intrinsically disordered protein, lacking a fixed tertiary structure. Its physiological functions are believed to involve the regulation of synaptic vesicle trafficking, the assembly of the SNARE complex, and modulation of dopamine release. Research at institutions like the National Institutes of Health and Max Planck Society has shown it may act as a molecular chaperone for the soluble NSF attachment protein receptor.

Role in disease

The pathological aggregation of this protein is the defining hallmark of several major neurodegenerative diseases. Its abnormal accumulation into Lewy bodies and Lewy neurites is the primary pathological feature of Parkinson's disease and dementia with Lewy bodies. Furthermore, its aggregation is the main component of glial cytoplasmic inclusions found in multiple system atrophy. These disorders are clinically distinct but share this common pathological proteinopathy. The presence of these inclusions is associated with progressive neuronal loss in regions like the substantia nigra and cerebral cortex, leading to characteristic motor and cognitive symptoms. The Braak staging hypothesis describes the progressive spread of this pathology through the brainstem and forebrain.

Genetics and regulation

The gene encoding this protein, SNCA, is located on chromosome 4 (4q22.1) in humans. Both point mutations and multiplications (duplications and triplications) of this gene are causally linked to rare familial forms of Parkinson's disease. Notable pathogenic mutations include A53T, first identified in families from Contursi Terme, Italy, as well as A30P and E46K. Gene dosage effects are evident, with triplication carriers often presenting with a more severe phenotype akin to dementia with Lewy bodies. Regulation of its expression is complex, involving elements in the promoter region and enhancers, and is influenced by various transcription factors. Epigenetic modifications studied at the Salk Institute for Biological Studies also play a significant role in its regulation.

Aggregation and pathology

The process of aggregation follows a nucleation-dependent polymerization model, leading to the formation of toxic oligomers, protofibrils, and ultimately mature amyloid fibrils. The hydrophobic NAC region is essential for this self-assembly. Post-translational modifications such as phosphorylation at serine 129, ubiquitination, and nitration can accelerate aggregation and are highly enriched in pathological inclusions. The spread of pathology throughout the brain is thought to occur via a prion-like mechanism, where misfolded templates seed aggregation in recipient cells. This process may involve trans-synaptic transmission, a concept supported by research from University College London and the Karolinska Institutet.

Research and therapeutic approaches

Current research strategies aim to modulate levels, prevent aggregation, or enhance clearance of the pathogenic protein. Immunotherapeutic approaches, including active vaccination and passive administration of monoclonal antibodies targeting various epitopes, are in clinical trials sponsored by companies like Prothena and Biogen. Small molecule inhibitors, such as anle138b developed at the Max Planck Society, aim to block oligomer formation. Gene-silencing techniques utilizing antisense oligonucleotides and RNA interference are being pursued by Wave Life Sciences and others. Furthermore, strategies to boost cellular clearance mechanisms like the ubiquitin-proteasome system and autophagy-lysosomal pathway are under investigation at centers like the Whitehead Institute for Biomedical Research.

Category:Proteins Category:Neurodegenerative disorders