28 Jul 2000

Reported by Benjamin Wolozin, Loyola University Medical Center www.meddean.luc.edu/lumen/DeptWebs/pharm/wolozin.htm

Increasing attention has focused on the role of synucleins in neurodegeneration. α-synuclein was originally cloned by Richard Scheller from rat brain (1). Subsequently, it was cloned by Julia George and David Clayton from the finch brain (2), and by Ueda, Masliah, Saitoh and colleagues from human brain (3). At the oral session titled, "The Synucleins and Lewy Body-Related Diseases," Julia George reviewed some of the fundamental biology of α-synuclein (1291) (4). She pointed out that it exists as a natively unfolded protein, but develops helical structure upon exposure to acidic phospholipids (5). In most cases, α-synuclein exists as a monomeric protein in the α-synuclein/lipid complexes. However, certain phospholipids are able to induce oligomerization of α-synuclein into SDS-stable complexes containing two to four α-synuclein molecules (1291). This raises the possibility that the interaction of α-synuclein with particular cellular lipids could influence its pathological aggregation. Earlier in the meeting, Sharon, Goldberg and Selkoe presented evidence indicating that some membrane-bound α-synuclein (from brains of transgenic mice and MES23.5 cells) can be detected as higher molecular weight oligomers, although most of the α-synuclein exists as a cytoplasmic, monomeric protein (52). John Trojanowski's presentation reviewed research on the pathology of the synucleins (1290, also see www.med.upenn.edu/cndr) (6). α-synuclein has been shown to accumulate in a variety of forms in multiple different diseases. α-synuclein accumulates in Lewy bodies in a variety of diseases such Parkinson's disease, diffuse Lewy body disease and familial Alzheimer's disease. Lippa and colleagues showed evidence suggesting that the presence of Lewy bodies in familial Alzheimer's disease is associated with unusual clinical presentations that include visual hallucinations, fluctuating mentation and other psychotic features (173). Trojanowski also showed that α-synuclein is associated with other types of pathologies (1290). Neurites in multiples diseases including Parkinson's disease, diffuse Lewy body disease, familial Alzheimer's disease and neurodegeneration with brain iron accumulation type I (1290, 240). Glia can also develop accumulation of α-synuclein in multiple systems atrophy and in amyotrophic lateral sclerosis. Thus, α-synuclein aggregates are not restricted to neurons. Trojanowski's group has also shown that β- and c-synuclein also accumulate along with α-synuclein, but the amount of β- and c-synuclein that aggregate are much less than that for α-synuclein. The amount of aggregation appears to correlate with the inherent tendency of the proteins to aggregate, because studies show that β- and c-synuclein do not aggregate as readily as α-synuclein in vitro (1290).

The mechanisms underlying synuclein aggregation in neurodegeneration remain controversial, but it is becoming increasingly clear that protein oxidation plays an important role. Harry Ischiropoulos presented his research investigating the putative role of tyrosine nitration in α-synuclein aggregation (1292) (7). α-synuclein has four tyrosines that could be substrates for nitration induced cross-linking. Ischiropoulos showed that α-synuclein could be nitrated and that peroxynitrate could induce cross-linking of α-synuclein in vitro. Although all four tyrosines in α-synuclein can be nitrated, Ischiropoulos showed that nitration of the three tyrosines at the C-terminus of α-synuclein is required for oligomerization of α-synuclein (1292). Peroxynitrate can also induce oligomerization of α-synuclein in cell culture; however, only a small number (~25%) of cells actually show α-synuclein aggregates under these conditions. Other studies from the group of Ischiropoulos, Lee and Trojanowski show that Lewy bodies contain 3-nitrotyrosine and nitrated α-synuclein, which indicates that α-synuclein is nitrated in brains of patients with Parkinson's disease (240). Whether nitration of α-synuclein is a primary or secondary event in its aggregation in neurodegeneration remains to be determined.

A different model for α-synuclein aggregation was presented by myself, Benjamin Wolozin (1293) (8). Previous studies by Hashimoto, Masliah and colleagues and by Paik, Kim and colleagues indicated that metals could induce oligomerization of α-synuclein in vitro (9, 10). Work in my laboratory shows that FeCl2 might also be an important inducer of α-synuclein aggregation. Neuroblastoma cells overexpressing A53T α-synuclein developed aggregates upon exposure to 0.3-10 mM FeCl2. Wild-type and A30P α-synuclein also developed aggregates but only when neuroblastoma cells or primary cortical neurons were treated with both iron and an oxidative generator, such as dopamine or hydrogen peroxide. The aggregates could be detected by immunoblot, immunohistochemistry, electron microscopy or thioflavine-S histochemistry. The amount of aggregate was dose dependent, being present in 100% of the cells with high-dose treatment and 25% of the cells at the milder treatments. As with the nitro-tyrosine story, the role of iron in α-synucleinopathies appears to have strong clinical relevance. Most, if not all, of the synucleinopathies appear to be associated with iron accumulation, and George Perry presented evidence indicative that reactive iron (FeII) can be detected in Lewy bodies (969) (11).

One of the most pressing needs in the field of synuclein research is identification and distribution of good animal models mimicking Lewy body diseases. Lee presented recent results obtained with a strain of transgenic mice overexpressing α-synuclein using a prion promoter (1294). Mice expressing A53T α-synuclein (but not wild-type or A30P α-synuclein) develop severe motor pathology at about 11 months of age. The behavioral characteristics of the mice have not been fully characterized, but comments from neurologists in the audience suggest that the behavior appeared to be dystonic. Lee had performed some preliminary analyses of pathology and observed the presence of α-synuclein accumulations in the deep nuclei of the cerebellum and in the brain stem, but no obvious pathology in the substantia nigra.

The transgenic α-synuclein mouse model that best resembles Lewy body diseases is that developed by Masliah, Mucke and colleagues (12). This mouse develops motor deficits at 11 months of age, shows loss of dopaminergic markers and the presence of α-synuclein accumulations in the substiantia nigra. Having developed this model, the group has begun to explore factors that modulate α-synuclein pathology. Masliah presented a new study in which they crossed the transgenic mice overexpressing α-synuclein with trangenic mice overexpressing β-synuclein (1295). They observed that overexpressing β-synuclein can reduce α-synuclein aggregation, pathology and motor deficits-the performance of the α/β-synuclein overexpressors was equal to that of control mice, although slightly below that of mice overexpressing β-synuclein alone. The aggregation-inhibiting ability of β-synuclein might result from a direct interaction between α- and β-synuclein, because Masliah showed that β-synuclein could also inhibit aggregation of α-synuclein in vitro. Although they have yet to show coassociation of α- and β-synuclein, it appears likely that there is some interaction between the two types of proteins.

In summary, α-synuclein has a strong tendency to aggregate. A consensus is emerging indicating that oxidative factors, such as tyrosine nitration or iron-induced oxidation, increases the tendency of α-synuclein to aggregate. The aggregation can be modified by interaction with other factors, such as homologous proteins (β-synuclein), which inhibits aggregation, or long chained lipids, which induce oligomerization. Aggregation of α-synuclein is likely to be dose-dependent because mice that overexpress α-synuclein develop accumulations of α-synuclein aggregates and associated motor deficits. A dose dependence of aggregation is also seen in vitro and in cell lines. Simply overexpressing α-synuclein, though, does not appear to be sufficient to mimic Lewy body disease because the resulting pathology that occurs in transgenic mice does not often mimic that seen in human neurodegenerative disease. Understanding the factors that increase α-synuclein aggregation, and the mechanism of toxicity of aggregated α-synuclein have emerged as pressing questions.

References:
52. Sharon R, Goldberg M, Selkoe DJ. Human α-synuclein exists in both monomeric and polymeric membrane-bound forms in transgenic mice and Mes23.5 dopaminergic cells overexpressing WT and A53T mutant α-synuclein.

173. Lippa CF, Nee L, Pollen D, Lee VMY, Trojanowski JQ. Lewy bodies may influence symptomatology in some cases of presenilin-related Alzheimer's disease.

240. Duda JE, Giasson BI, Chen Q, Souza JM, Murray IVJ, Ischiropoulos Lee VMY, Trojanowki JQ. α-synuclein nitration in neuropathological inclusion.

969. Perry G et al. Neurodegenerative disease: role of free radical damage.

1290. Trojanowski JQ. Overview of the synucleinopathies.

1291. George J. Synuclein and synelfin: the songbird story.

1292. Ischiropoulos H. Mechanisms of oxidative injury in neurodegenerative synucleinopathies.

1293. Wolozin B. α-synuclein in Lewy body diseases.

1294. Lee M et al. Synucleins: in vitro and in vivo studies.

1295. Masliah E. Mechanisms of synuclein and NAC fibrilogenesis.

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Papers

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