Cell death mechanisms in Parkinson's disease

While the causes of neuronal death in Parkinson's disease (PD) and other neurodegenerative disorders are still unknown, several mechanisms are under discussion: programmed vs. passive cell death (apoptosis vs. necrosis), mainly based on conflicting results on the rare presence or absence of DNA fragmentation in substantia nigra neurons using the in situ DNA-labeling (TUNEL) method.

In 4 cases of Parkinson's disease (PD), 2 cases of Dementia with Lewy bodies (DLB) and 3 age-matched controls, the TUNEL/ISEL method was used to detect DNA fragmentation in substantia nigra locus coeruleus and cerebral cortex [method by Gold et al. (1994)]. In addition, immunohistochemistry was performed for an array of apoptosis-related proteins, i.e. the recently described apoptosis specific protein cJun/AP1 (ASP), the proto-oncogenes c-Jun, c-Jun AP1, Bcl2, Bax, Bcl-x, p53, CD 95 (Fas/Apo-1), activated caspase 3, several heat shock proteins (alpha-B crystallin, ubiquitin), and alpha-synuclein.

None of the cases of PD, DLB, and controls showed convincing TUNEL-positivity nor morphologic signs of apoptosis in nigral, locus coeruleus or cortical neurons with or without Lewy bodies but variable numbers of TUNEL-positive astrocytes and microglial cells in substantia nigra of PD and DLB. There were no significant differences in the expression of c-Jun, ASP, Bcl-2, Bax, and Bcl-x in substantia nigra neurons between PD, DLB, and controls nor between cortical and subcortical neurons with and without Lewy bodies. No expression of p53, and activated caspase 3, or any of the examined stress proteins was seen in neurons, while reactive astroglia and microglia were decorated by antibodies to Bcl-2, Bax, alpha-B-crystallin and less, to Bcl-x and caspase 3. Lewy bodies, dystrophic neurites and axonal spheroids, all being negative for the applied apoptosis regulating proteins, showed strong expression of the examined stress proteins and of alpha-synuclein.

These findings which are in line with previous results in Alzheimer's disease (Stadelmann et al., 1998) and Parkinson's disease (Banati et al., 1999) suggest that mechanisms distinct from classical apoptosis play a central role in the pathogenesis of PD and related neurodegenerative diseases. Further studies are warranted to elucidate the intracellular cascade of events leading to cell death in these disorders showing slow progression over many years.