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Programmed cell death of embryonic motoneurons triggered through the Fas death receptor
C Raoul1, C E Henderson, B Pettmann
1Institut National de la Santé et de la Recherche Médicale U.382, Developmental Biology Institute of Marseille (CNRS), Institut National de la Santé et de la Recherche Médicale, Université de la Mediterranee, AP Marseille, France.
Abstract:
About 50% of spinal motoneurons undergo programmed cell death (PCD) after target contact, but little is known about how this process is initiated. Embryonic motoneurons coexpress the death receptor Fas and its ligand FasL at the stage at which PCD is about to begin. In the absence of trophic factors, many motoneurons die in culture within 2 d. Most (75%) of these were saved by Fas-Fc receptor body, which blocks interactions between Fas and FasL, or by the caspase-8 inhibitor tetrapeptide IETD. Therefore, activation of Fas by endogenous FasL underlies cell death induced by trophic deprivation. In the presence of neurotrophic factors, exogenous Fas activators such as soluble FasL or anti-Fas antibodies triggered PCD of 40-50% of purified motoneurons over the following 3-5 d; this treatment led to activation of caspase-3, and was blocked by IETD. Sensitivity to Fas activation is regulated: motoneurons cultured for 3 d with neurotrophic factors became completely resistant. Levels of Fas expressed by motoneurons varied little, but FasL was upregulated in the absence of neurotrophic factors. Motoneurons resistant to Fas activation expressed high levels of FLICE-inhibitory protein (FLIP), an endogenous inhibitor of caspase-8 activation. Our results suggest that Fas can act as a driving force for motoneuron PCD, and raise the possibility that active triggering of PCD may contribute to motoneuron loss during normal development and/or in pathological situations.
Insights
Spinal motoneurons use the Fas receptor pathway to initiate programmed cell death (PCD) when deprived of trophic factors. Neurotrophic factors regulate sensitivity to Fas-mediated PCD, involving caspase-8 and FLIP.
Area of Science:
- Neuroscience
- Developmental Biology
- Cell Biology
Background:
- Approximately 50% of spinal motoneurons undergo programmed cell death (PCD) post-target contact.
- The precise mechanisms initiating this PCD are not fully understood.
Purpose of the Study:
- To investigate the role of the Fas receptor pathway in motoneuron PCD.
- To determine how neurotrophic factors regulate motoneuron sensitivity to PCD.
Main Methods:
- Cultured embryonic motoneurons were treated with trophic factors, FasL, anti-Fas antibodies, and caspase inhibitors.
- Expression levels of Fas, FasL, and FLIP were analyzed.
- Caspase-3 and caspase-8 activation were assessed.
Main Results:
- Trophic factor deprivation induced motoneuron death mediated by Fas/FasL interactions and caspase-8 activation.
- Exogenous Fas activation triggered PCD, which was blocked by caspase-8 inhibition.
- Motoneurons became resistant to Fas activation upon prolonged culture with neurotrophic factors, correlating with increased FLIP expression.
Conclusions:
- Fas receptor signaling is a key driver of motoneuron PCD, particularly under conditions of trophic deprivation.
- Neurotrophic factors modulate motoneuron susceptibility to Fas-mediated apoptosis via regulation of FLIP.
- Active triggering of PCD through Fas may contribute to normal development and pathological conditions involving motoneuron loss.