The relationship between α-synuclein and the proteasome and its role in Parkinson’s pathology

Parkinson’s disease (PD) is a movement disorder characterized by the appearance of inclusions known as Lewy bodies, most prominently in the substantia nigra pars compacta. The synaptic protein α-synuclein (α-syn) and polyubiquitin are major components of Lewy bodies. The objective of this study was, firstly, to evaluate whether α-syn inhibits proteolytic function of the proteasome and, secondly, to determine the effects of proteasome inhibition on α-syn solubility and localization. α-Syn inhibited 20S proteasome activity reversibly in vitro, while α-syn overexpression did not affect activity in neuroblastoma (SH-SY5Y and SK-N-BE) or HEK293 cells nor in α-syn transgenic mice in vivo. A reciprocal approach was used to analyze the effects of pharmacological proteasome inhibition (with MG132 and epoxomicin) on α-syn solubility in SHSY5Y cells. This treatment did not lead to α-syn aggregation, as seen in PD. However, it induced a shift from more soluble α-syn toward more membrane bound α-syn in endogenous (mock) and WT α-syn transfected cells. This treatment also led to the clearing of nuclei of αsyn and ubiquitin, as well as to cytoplasmic α-syn inclusions devoid of polyubiquitin in a small percentage of the cells. The combination of proteasome inhibition with serum deprivation, which is known to produce oxidative dysfunction, caused the appearance of high molecular weight α-syn species, such as those found in Lewy bodies. So far these data suggest that, although not observed in our in vivo models, high concentrations of α-syn can interfere with proteasome function under certain conditions, while proteasome inhibition leads to structural changes in α-syn that may precede neurodegeneration. However, more than one condition (e.g. oxidative stress and proteasome inhibition) needs to be met to induce pathological changes.