Programmed cell death of embryonic motoneurons triggered through the Fas death receptor

About 50% of spinal motoneurons undergo programmed cell death (PCD) after t arget contact, but little is known about how this process is initiated. Emb ryonic motoneurons coexpress the death receptor Fas and its ligand Fast 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 save d by Fas-Fc receptor body, which blocks interactions between Fas and Fast, or by the caspase-8 inhibitor tetrapeptide IETD, Therefore, activation of F as by endogenous Fast underlies cell death induced by trophic deprivation. In the presence of neurotrophic factors, exogenous Fas activators such as s oluble Fast or anti-Fas antibodies triggered PCD of 40-50% of purified moto neurons over the following 3-5 d; this treatment led to activation of caspa se-3, and was blocked by IETD. Sensitivity to Fas activation is regulated: motoneurons cultured for 3 d with neurotrophic factors became completely re sistant. Levels of Fas expressed by motoneurons varied little, but Fast 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 possibilit y that active triggering of PCD may contribute to motoneuron loss during no rmal development and/or in pathological situations.