Caspase Dependent and Independent Pathways for Cell Death Mechanisms

Jun Chen, M.D.

Department of Neurology
University of Pittsburgh, Pittsburgh, Pennsylvania

Abstract

Recent studies based on the examination of postmortem brains from Parkinson’s disease (PD) patients and animal models of PD have suggested that at least part of the neuronal loss in PD pathogenesis is due to a genetically controlled process called apoptosis. Apoptosis is a cell suicide program, also termed programmed cell death, in which a range of cellular molecules are inappropriately activated and subsequently destroy the cell. The central execution molecules for apoptosis in mammalian cells are a group of enzymes called caspases.

Among the 14 caspases that have been identified so far, caspase-3 appears to play a central ole in the final execution of cell death. Caspase-3, upon activation, cleaves a variety of essential cytoplasmic and nuclear proteins, ensuring the inevitability of cell death. Caspase inhibitors can block apoptosis completely under many circumstances.

Dr. Chen and his associates have begun to study the casepase-related mechanism of neuronal apoptosis in experimental models of PD. They found that 6-OHDA, a neurotoxin that can reproduce many characteristic features of PD in animals, induced caspase-3 dependent apoptosis in cultured dopaminergic neurons. This exciting observation warrants them to further elucidate the mechanism by which caspase-3 mediates 6-OHDA’s cell killing effect in dopaminergic neurons. Dr. Chen believes that this research has great potential from the perspective of the development of novel and effective therapeutic interventions for PD patients.

Progress Report (as of 8/2002)

The scientific objective of the proposal was to understand the role of specific cystine proteases such as caspases in the degeneration of dopaminergic neurons in models of Parkinson's disease. Since receiving the seed funds, we have made significant progress. First, we have studied in the MN9D cellular model of PD the role of mitochondria-dependent cell death pathway in 6-OHDA-induced dopaminergic neurodegeneration. We found that 6-OHDA, a PD-relevant insult, induced the release of cytochrome c, an apoptogenic factor, from the mitochondria, and subsequent activation of caspase-3 in MN9D cells. We also found that when a gene called caspase-9 dominant negative inhibitor (caspase-9dn) was overexpressed in cells using viral vector technology this 6-OHDA-induced neurodegeneration was prevented.

The significance of these findings is two-fold. These results defined the cytochrome c/caspase-9, so called intrinsic apoptosis signaling pathway, as the main mechanism responsible for PD-relevant neurodegeneration. Furthermore, the results suggest that augmentation of the expression of caspase-9dn or related gene products in dopaminergic neurons may be a novel strategy to prevent neuronal loss in PD.

The second important finding that we made was from studies in the model of rotenone neurotoxicity in primary neurons. Rotenone is a mitochondrial complex I inhibitor, which mimics some important pathologic components of PD. We found that rotenone killed neurons via programmed cell death (or apoptosis), an active cell death process that requires the synthesis of new gene products and the activation of several different caspases. We also found that rotenone's cell-killing effect could be attenuated by treating neurons with specific caspase inhibitors against caspase-3 or caspase-1. These results thus emphasize the importance of the caspase-dependent cell death-executing mechanisms in underlying neurodegenration in PD pathogenesis. Thus, antagonizing the effect of specific caspases is a promising strategy to prevent or decrease dopaminergic neurodegeneration in PD.

Publications
Two manuscripts have been submitted to report the above findings:

Two caspase mediated apoptotic pathways induced by rotenone toxicity in cortical neuronal cells.

Funding resulting from seed grant research
An NIH grant "Apoptosis Execution Pathways in Dopaminergic Cell Death" has been funded by the NINDS for the budget period 10/1/02-9/30/06 (PI: Jun Chen)