Abstract |
Alpha-synuclein is a 140 amino acid neuronal protein that has been associated
with several neurodegenerative diseases. A point mutation
in the gene coding for the a-synuclein protein was the first discovery linking this
protein to a rare familial form of Parkinson’s disease. Subsequently, other
mutations in the a-synuclein gene have been identified in familial Parkinson’s
disease. The aggregated proteinaceous inclusions called Lewy bodies found in
Parkinson’s disease and cortical Lewy body dementia were discovered to be
predominantly a-synuclein. Aberrant aggregation of a-synuclein has been
detected in an increasing number of neurodegenerative diseases, collectively
known as synucleopathies.
a-synuclein exists physiologically in both soluble and membrane-bound
states, in unstructured and a-helical conformations, respectively. The
physiological function of a-synuclein appears to require its translocation between
these subcellular compartments and interconversion between the 2
conformations. a-synuclein’s propensity to oligomerize and form fibrils may be
central to its toxicity. A precursor of those filamental structures, called protofibrils
appears to be the most toxic formation.
Thus, agents that block or prevent the formation of oligomers and/or
fibrils might be neuroprotective. Already, several compounds have been tested for
their ability to inhibit the assembly of a-synuclein into filaments in vitro.
Biochemical analysis revealed the formation of soluble oligomeric a-synuclein in
the presence of inhibitory compounds, suggesting that this may be the
mechanism by which filament formation is inhibited. Unlike a-synuclein filaments
and protofibrils, these soluble oligomeric species did not reduce the viability of
cells.
Until recently, a-synuclein was considered to exist as a conventional
cytosolic protein. However, a series of new observations demonstrate the
presence of a-synuclein in extracellular biological fluids, such as cerebrospinal
fluid and plasma. The exact mechanism of this secretion has not been
characterized. Thus, exocytosis of a-synuclein may be an important mechanism of
amplifying and spreading degenerative changes from a small group of cells to its
surrounding tissues, and it potentially provides therapeutic targets for halting the
progression of the disease.
In the present study we examine the possible cytotoxic effect of secreted
a-synuclein species and the role of three inhibitory compounds: Congo red,
Chrysamine G, Scyllo-inositol. These compounds have been used so far in studies
with oligomeric aggregates of amyloid W peptide, which is responsible for
cytotoxicity in Alzheimer’s disease and it has been found that they reduce or even
inhibit Ab-induced toxicity.
However, these compounds are themselves cytotoxic. Due to this, we
performed a number of experiments in order to determine the ‘‘ideal’’ dose, and
with this we mean the dose that has the most reducing result in aggregation and
simultaneously the less toxic effect in cells’ survival.
Our results showed that secreted a-synuclein species are cytotoxic. On the
other hand, when the three compounds were added to conditioned medium,
containing naturally secreted a-synuclein species, they produced a reduction in
the cytotoxic effects caused by the application of this medium on differentiated
neuroblastoma cells.
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