The effect of copy number on α-synuclein's toxicity and its protective role in Bax-induced apoptosis, in yeast

Apoptosis is a form of programmed cell death which is essential for the growth of dividing human cells whereas, in contrast, it is deleterious for post-mitotic cells such as neurons. Bax and alpha-synuclein are two human proteins which play a role in the induction of neuronal apoptosis in neurodegenerative diseases like Alzheimer's and Parkinson's. Human Bax and alpha-synuclein also induce cell death when expressed in baker's yeast, Saccharomyces cerevisiae. Quite unexpectedly, the human alpha-synuclein gene had been identified as an inhibitor of pro-apoptotic Bax using a yeast-based screen of a human hippocampal cDNA library. Plasmids were constructed with different promoters, which allow expression of wildtype and Parkinson's disease (PD)-related mutant alpha-synuclein genes, from (i) multi-copy 2 mu (episomal) plasmids and (ii) integrative plasmids that compel expression of genes from chromosomal sites in varying copy numbers (1-3). All alpha-synuclein-containing plasmids were introduced, through transformation, into a yeast strain which already contained a chromosomally integrated copy of Bax. It is for the first time that it was observed that, depending on gene dosage, only wildtype alpha-synuclein is anti-apoptotic while mutant alpha-synuclein is not. The results also indicate that wildtype alpha-synuclein has a remarkable ability to manifest two contrasting effects depending on its level of expression: (i) normally, it would negate apoptosis but (ii) when overexpressed, it tends to induce apoptosis which is probably what happens in PD.