A critical role for Fas/CD‐95 dependent signaling pathways in the pathogenesis of hyperoxia‐induced brain injury

Objective Prematurely born infants are at risk for development of neurocognitive impairment in later life. Oxygen treatment has been recently identified as a trigger of neuronal and oligodendrocyte apoptosis in the developing rodent brain. We investigated the role of the Fas death receptor pathway in oxygen‐triggered developmental brain injury. Methods Six‐day‐old Wistar rats were exposed to 80% oxygen for various periods (2, 6, 12, 24, 48, and 72 hours), and mice deficient in either Fas (B6.MRL‐ Tnfrsf6 lpr ) or Fas ligand (B6Smn.C3‐ Fasl gld ) and control mice (C57BL/6J) were exposed to 80% oxygen for 24 hours. Polymerase chain reaction, Western blotting, and caspase activity assays of thalamus and cortex tissue were performed. Results Fas and Fas ligand messenger RNA and protein were upregulated. Furthermore, hyperoxia resulted in induction of downstream signaling events of Fas, such as Fas‐associated death domain (FADD), the long and short form of FADD‐like interleukin‐1β–converting enzyme (FLICE) inhibitory protein (FLIP‐L, FLIP‐S), and cleavage of caspase‐8 and caspase‐3. Injection of a selective caspase‐8 inhibitor (TRP801, 1mg/kg) at the beginning of hyperoxia blocked subsequent caspase‐3 cleavage in this model. B6.MRL‐ Tnfrsf6 lpr mice were protected against oxygen‐mediated injury, confirming Fas involvement in hyperoxia‐induced cell death. Mice deficient in Fas ligand did not differ from control animals in the amount of cell death. Interpretation We conclude that neonatal hyperoxia triggers Fas receptor and its downstream signaling events in a Fas ligand–independent fashion. Lack of functional Fas receptors and selective pharmacological inhibition of caspase‐8 prevents activation of caspase‐3 and provides significant neuroprotection. Ann Neurol 2008;64:664–673