Analysis of caspase-independent neuronal death

Apoptosis is a form of cell death that occurs throughout development to shape and refine many organ systems, including the nervous system. Apoptosis also occurs in some pathological conditions of the nervous system. Recent studies have identified a critical role for a family of proteases, known as caspases, in this process. Pharmacologic or genetic inhibition of caspases prevents many of the events associated with apoptosis, such as chromatin condensation, DNA fragmentation, and degradation of certain cellular proteins. However, cells that are protected against apoptosis by such means eventually die by a distinct mechanism that is caspase-independent.; In these experiments, we focused on nerve growth factor (NGF)-deprivation-induced death of sympathetic neurons, a well-characterized in vitro model of naturally occurring cell death. Inhibition of caspase activity prevents NGF-deprived sympathetic neurons from undergoing apoptosis and prolongs their survival. However, because caspase inhibitors do not prevent the drop in metabolic activity caused be NGF deprivation, cells saved with a caspase inhibitor, such as boc-aspartyl-fluoromethylketone (BAF), have low levels of metabolic activity. To investigate how this may contribute to the eventual death of these cells, we examined the bioenergetic status of NGF-deprived, BAF-saved sympathetic neurons. Oxidative phosphorylation, which contributes to ATP generation in NGF-maintained neurons, does not contribute significantly to ATP production in NGF-deprived, BAF-saved neurons. Instead, these cells rely more heavily on glycolysis for ATP production.; While mitochondria in NGF-deprived, BAF-saved cells do not produce ATP, we found that they still maintain mitochondrial membrane potential (ΔΨm) by electron transport. The maintenance of ΔΨm may be important to the viability of BAF-saved cells, since death of sympathetic neurons in the presence of a caspase inhibitor is associated with loss of ΔΨm. Inhibition of the mitochondrial permeability transition pore, a transient channel which connects the mitochondrial matrix to the cytosol, greatly attenuates both the loss of ΔΨm and the death of NGF-deprived, BAF-saved neurons. Taken together, the findings in these studies may promote an understanding a better understanding of the utility and limitations of the therapeutic use of caspase inhibitors and suggest adjunctive approaches to improving their efficacy.