The ubiquitin-proteasome pathway is necessary for maintenance of the postmitotic status of neurons.

The ubiquitin-proteasome pathway (UPP) has been implicated in the regulation of a number of key cellular processes in mammalian cells, including the cell cycle and apoptosis. Furthermore, defects in the UPP have been implicated in neurodegenerative disorders such as Parkinson's disease (PD), as mutations in the ubiquitin ligase Parkin underlie a familial form of parkinsonism and ubiquitinated inclusions are a defining hallmark of PD pathology. To functionally dissect molecular components of the UPP in postmitotic neurons, we used RNA interference to knock down genes that encode genetically characterized components of the UPP. Here, we show that knockdown of two such components, the ubiquitin ligase scaffolding protein Cullin-1 (Cul-1) and the proteasome-associated deubiquitinating protein Pad-1, lead to cell cycle reactivation and apoptosis in subsets of postmitotic neurons. Furthermore, knockdown of Cul-1 appears to specifically affect the dopaminergic population. These data support the hypothesis that the UPP normally functions to regulate cell-cycle reentry in postmitotic neurons and further implicate this pathway in dopamine neuron degeneration.