Dopaminergic neurons in rat ventral midbrain cultures undergo selective apoptosis and form inclusions, but do not up-regulate iHSP70, following proteasomal inhibition.

Dysfunction of the ubiquitin-dependent protein degradation system, either at the level of the proteasome itself, or at the level of ubiquitination, may play a role in the pathogenesis of Parkinson's disease (PD) and other related neurodegenerative disorders. We have employed a cellular model of this dysfunction in which lactacystin or epoxomicin, selective pharmacological inhibitors of the proteasome, are applied to primary cultures of embryonic rat ventral midbrain. Proteasomal inhibition with either agent led to apoptotic death specifically within phenotypically defined tyrosine hydroxylase (TH)-positive dopaminergic neurons, with little or no apoptotic death induced in GABAergic neurons. Inhibition of the proteasome also led to the formation of ubiquitin and alpha-synuclein-positive cytoplasmic inclusions in TH-positive and TH-negative neurons. Inclusions were observed in viable as well as apoptotic neurons, and required new or ongoing transcription. Tyrosine hydroxylase immunolabeling was often present within the inclusions. Such mislocalization may lead to dysfunction of dopamine biosynthesis. Interestingly, dopaminergic neurons, unlike other neurons within these cultures or cultured cortical neurons, failed to induce the chaperone Hsp70 in response to proteasomal inhibition. This failure may explain in part the increased sensitivity of these neurons to proteasomal inhibitors.