alpha-Synuclein biology and pathphysiology in Parkinson's disease

Parkinson's disease (PD) is the most common movement disorder affecting the elderly. Pathologically, PD is characterized by the formation of Lewy bodies and death of dopaminergic neurons in the substantia nigra region of the brain. Mechanisms underlying PD are unknown, but discoveries that mutations in alpha-synuclein can cause familial PD and alpha-synuclein accumulation in Lewy bodies suggest that alpha-synuclein participates in the pathophysiology of PD.;In the first part of our research, we investigated biological functions of alpha-synuclein. We showed that alpha-synuclein shares physical and functional homology with 14-3-3 proteins, which comprise a family of ubiquitous cytoplasmic chaperones. Regions of alpha-synuclein and 14-3-3 share over 40% protein sequence homology. In addition, alpha-synuclein binds to proteins known to associate with 14-3-3, including protein kinase C, BAD, extracellular regulated kinase and also Parkin. The binding profile of alpha-synuclein suggests that it might act as a protein chaperone, and alterations of alpha-synuclein binding characteristics can alter cell homeostasis and contribute to neurodegeneration.;The second part of our research focused on the pathophysiology of PD. Based on the knowledge that metals stimulate protein aggregation, we developed a cell culture model of alpha-synuclein aggregation. Then we investigated the modulation of aggregation by iron and magnesium. Iron accumulates with age in the substantia nigra and patients with PD show increased levels of iron. We have found that iron binds to alpha-synuclein and induces aggregation. Iron and free radical generators, such as dopamine or hydrogen peroxide, stimulate the production of intracellular aggregates containing alpha-synuclein and ubiquitin in neuronal cell lines over-expressing alpha-synuclein. The aggregates can be identified by immunocytochemistry, electron microscopy, or by thioflavine S staining. The amount of aggregation that occurs in the cells is dependent on the amount and type of alpha-synuclein expressed. The amount of alpha-synuclein aggregation in cells in culture follows a rank order potency of mutants > wild type > untransfected.;We were also able to show that magnesium binds wild type alpha-synuclein and inhibits its aggregation. In contrast, magnesium neither binds to the A53T mutant of alpha-synuclein nor inhibits its aggregation. In addition to decreasing aggregation of wild type alpha-synuclein, magnesium also decreases toxicity developed under conditions associated with alpha-synuclein aggregation. These data suggest that magnesium might inhibit alpha-synuclein aggregation in vivo and thereby slow the progression of PD.;In conclusion, protein homology and binding profiles of alpha-synuclein suggest that alpha-synuclein acts as a protein chaperone. Based on our cell culture model, we propose that changes in iron and magnesium metabolism can lead to alpha-synuclein aggregation in the substantia nigra, thereby leading to PD pathogenesis.