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Exogenous smac induces competence and permits caspase activation in sympathetic neurons
Mohanish Deshmukh1, Chunying Du, Xiaodong Wang
1Department of Cell and Developmental Biology and the Neuroscience Center, University of North Carolina, Chapel Hill, North Carolina 275997090, USA. mohanish@med.unc.edu
Abstract:
Sympathetic neuronal apoptosis after nerve growth factor (NGF) deprivation requires the activation of two events: a protein synthesis-dependent, Bax-dependent release of mitochondrial cytochrome c and a protein synthesis-independent, Bax-independent development of competence. Unlike in most cells, cytosolic cytochrome c is not sufficient to induce cell death in NGF-maintained sympathetic neurons but can do so in neurons that have developed competence. We report that cytosolic cytochrome c-induced apoptosis in competent sympathetic neurons is completely dependent on caspase-9. In addition, the neuroprotective agents KCl and chlorophenylthio-cAMP are potent inhibitors of the development-of-competence pathway in NGF-deprived sympathetic neurons. We also find that the development of competence is reversible. Readdition of NGF reverses competence, and neurons can regain their resistance to cytosolic cytochrome c. Importantly, we examined the mechanism of development of competence and report that the inability of cytochrome c to activate caspases in NGF-maintained sympathetic neurons can be overcome with exogenous Smac that inhibits the inhibitor of apoptosis (IAP) family of proteins. Microinjection of cytochrome c and Smac, but neither alone, induces rapid cell death in NGF-maintained neurons. These data suggest that development of competence may be the result of the loss of the function of one or more members of the IAP family of caspase inhibitors that is needed before cytochrome c can activate caspases and induce cell death in neurons.
Insights
Nerve growth factor (NGF) deprivation triggers sympathetic neuron death through cytochrome c release and competence development. This competence, involving inhibitor of apoptosis proteins (IAPs), is reversible and crucial for apoptosis.
Area of Science:
- Neuroscience
- Cell Biology
- Molecular Biology
Background:
- Sympathetic neuronal apoptosis following nerve growth factor (NGF) deprivation involves distinct pathways.
- Cytosolic cytochrome c induces apoptosis only in competent neurons, not in NGF-maintained neurons.
- The development of competence is a key factor in neuronal susceptibility to apoptosis.
Purpose of the Study:
- To investigate the mechanism of cytochrome c-induced apoptosis in competent sympathetic neurons.
- To identify the role of inhibitor of apoptosis (IAP) proteins in the development of neuronal competence.
- To explore the reversibility of competence and its impact on neuronal survival.
Main Methods:
- Utilized NGF deprivation models in sympathetic neurons.
- Assessed cytochrome c release, caspase activation, and apoptosis induction.
- Employed microinjection techniques with cytochrome c and Smac.
- Investigated the effects of neuroprotective agents like KCl and chlorophenylthio-cAMP.
Main Results:
- Cytosolic cytochrome c induces apoptosis in competent neurons via caspase-9 activation.
- Neuroprotective agents KCl and chlorophenylthio-cAMP inhibit competence development.
- Neuronal competence is reversible upon NGF readdition, restoring resistance to cytochrome c.
- Exogenous Smac, an IAP inhibitor, combined with cytochrome c, induces apoptosis in NGF-maintained neurons.
Conclusions:
- Competence development in sympathetic neurons may involve the downregulation of IAP proteins.
- Loss of IAP function is necessary for cytochrome c to activate caspases and induce apoptosis.
- Understanding competence is crucial for developing therapeutic strategies targeting neuronal survival.