Involvement of mitochondrial signaling pathways in the mechanism of Fas-mediated apoptosis after spinal cord injury

Activation of the Fas receptor has been recently linked to apoptotic cell death after spinal cord injury (SCI). Although it is generally considered that Fas activation mediates apoptosis predominantly through the extrinsic pathway, we hypothesized that intrinsic mitochondrial signaling could be involved in the underlying mechanism of Fas-induced apoptosis after SCI. In the present study, we utilized the Fejota(TM) clip compression model of SCI at T5-6 in C57BL/6 Fas-deficient (lpr) and wild-type mice. Complementary studies were conducted using an in vitro model of trauma or a Fas-activating antibody to induce apoptosis in primary neuronal-glial mixed spinal cord cultures. After in vivo SCI, lpr mice, in comparison with wild-type mice, exhibited reduced numbers of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells at the lesion, reduced expression of truncation of Bid (tBid), apoptosis-inducing factor, activated caspase-9 and activated caspase-3, and increased expression of the antiapoptotic proteins Bcl-2 and Bcl-xL. After in vitro neurotrauma or the induction of Fas signaling by the Jo2 activating antibody, lpr spinal cord cultures showed an increased proportion of cells retaining mitochondrial membrane integrity and a reduction of tBid expression, caspase-9 and caspase-3 activation, and TUNEL-positive cells as compared to wild-type spinal cord cultures. The neutralization of Fas ligand (FasL) protected against traumatically induced or Fas-mediated caspase-3 activation and the loss of mitochondrial membrane potential and tBid expression in wild-type spinal cord cultures. However, in lpr spinal cord cultures, FasL neutralization had no protective effects. In summary, these data provide direct evidence for the induction of intrinsic mitochondrial signaling pathways following Fas activation after SCI.