Proteasome inhibition leads to microglial activation and progressive nigral dopaminergic cell death in rats

Atsuko Hikima, Sarah Rose, Peter Jenner. King’s College London, London, United Kingdom.

Background: Parkinson’s disease (PD) is the second most common neurodegenerative disorder pathologically characterized by the degeneration of dopaminergic neurones in the substantia nigra pars compacta (SNpc) and the loss of nigro-striatal dopamine system, resulting in motor dysfunction. Evidence of disruption of the ubiquitin-proteasome system and inclusion body formation suggests a role for proteasomal dysfunction in the process of cell death. Recent studies have shown that systemic administration of a proteasomal inhibitor, PSI causes a reduction in the number of dopaminergic neurones in the SN with an accompanying inflammatory response, inclusion body formation and apoptotic cell death. However, how inflammation is involved in the progression of PSI induced cell death is unknown and this is the main aim of this study.

Methods: Female Wistar rats (200 – 250g) were treated with either PSI (5.0mg/kg, s.c.) or vehicle (100% DMSO, 1.0ml/kg, s.c.) on days 1, 3, 5, 8, 10 and 12. Spontaneous locomotor activity was assessed for up to day 52 following PSI administration, using automated locomotor activity equipment. On days, 1, 12, 19, 26 and 52, the number of tyrosine hydroxylase (TH), glial fibrillary acidic protein (GFAP), OX-6 and ED-1 positive cells was determined in the SNpc by immunohistochemistry and levels of dopamine and DOPAC were determined in the striatum by HPLC with electrochemical detection.

Results: Spontaneous locomotor activity showed no significant change up to day 52 following PSI administration. Similarly, there was no significant effect of PSI administration on dopamine and DOPAC concentrations and dopamine turnover (DOPAC/dopamine) in the striatum. However, PSI administration induced a time-related nigral cell loss in the SNpc over 52 days. The number of TH-positive neurones started decreasing at day 19 and reached a significant reduction of 46% compared to vehicle-treated controls by day 52. Administration of PSI had no effect on the number of GFAP-positive astrocytes in the SNpc. There was a small increase in the number of OX-6-positive microglia up to day 52 although PSI treatment induced ED-1-positive macrophages on day 19. After these time points, ED-1-positive macrophages disappeared from the SNpc.

Conclusion: PSI treatment induced progressive nigral dopaminergic cell loss as previously described. This was accompanied by microglial activation but no astrocytosis in the SNpc at early time points. This microglial activation may contribute to the progression of cell death in this model although further studies are required to establish their role.