What is the role of alpha-synuclein in the pathogenesis of Parkinson disease (PD)?

Updated: Jan 24, 2019
  • Author: Robert A Hauser, MD, MBA; Chief Editor: Selim R Benbadis, MD  more...
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Answer

Abnormally aggregated alpha-synuclein is the major component of Lewy bodies and Lewy neurites, which are characteristic pathologic findings in Parkinson disease. Missense mutations and multiplications in the SNCA gene that encodes alpha-synuclein, although rare, cause autosomal dominant Parkinson disease. However, genome-wide association studies have also demonstrated a link between SNCA and sporadic Parkinson disease.

Dysfunction of alpha-synuclein appears to play a central role in the pathogenesis of Parkinson disease, and understanding its relationship to the disease process holds major promise for the development of a cure.

Alpha-synuclein is a 140-amino-acid protein that is unfolded at neutral pH. However, when bound to membranes or vesicles containing acidic phospholipids, it takes on an alpha-helical structure. Normally, alpha-synuclein is found mainly in neuronal presynaptic terminals and may play a role in assembly and function of SNARE (soluble N-ethylmaleimide-sensitive factor activating protein receptor) proteins that are involved in neurotransmitter release.

Under certain conditions, alpha-synuclein aggregates into oligomers that are gradually converted to the beta–sheet-rich fibrillary structures that form Lewy bodies and neurites in Parkinson disease. Most evidence currently suggests that it is the intermediate soluble oligomers that are toxic to neurons.

Multiple mechanisms have been suggested as to how abnormally aggregated alpha-synuclein could exert neurotoxicity. [12] One hypothesis suggests that oligomeric alpha-synuclein can promote formation of ion-permeable pores on neuronal membranes, leading to increased calcium influx. Aberrant pore formation could also lead to neurotransmitter leaks from synaptic vesicles into the cytosol. In addition, overexpression of alpha-synuclein has been demonstrated to impair mitochondrial complex I activity, and oligomeric alpha-synuclein may have a direct effect on mitochondrial membranes. Other lines of evidence suggest that oligomerization of alpha-synuclein could cause cytoskeletal disruption, possibly by an effect on the microtubule-stabilizing protein, tau. [13]

Elevated levels of alpha-synuclein promote abnormal aggregation. levels are normally regulated by a balance between synthesis and degradation. SNCA multiplications lead to increased synthesis of alpha-synuclein and can cause Parkinson disease. Alpha-synuclein appears to be degraded by the ubiquitin proteasome system and the autophagy-lysosome pathway. Several genetic mutations associated with Parkinson disease may lead to decreased alpha-synuclein degradation. For example, increased risk of Parkinson disease in carriers of GBA (beta-glucocerebrosidase gene) mutations, which encode for the lysosomal enzyme glucocerebrosidase, may be due to lysosomal dysfunction and consequent alpha-synuclein accumulation and oligomerization.

How the Parkinson disease process begins is not known. Once it is initiated, however, it may propagate by a prionlike process in which misconformed proteins induce the templated misfolding of other protein molecules. In Parkinson disease, synuclein pathology begins in the lower brainstem and olfactory bulb, ascends up the midbrain, and eventually affects the neocortex. One set of observations in support of a prionlike process comes from experience with fetal dopaminergic grafts transplanted into the striata of patients with Parkinson disease, because these grafts develop Lewy bodies, suggesting host-graft transmission of disease. [14]

Preventing the propagation of abnormal alpha-synuclein aggregation may be the key to slowing or stopping Parkinson disease progression.


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