Nitric Oxide and Superoxide Anion Differentially Activate Poly(ADP-ribose) Polymerase-1 and Bax to Induce Nuclear Translocation of Apoptosis-Inducing Factor and Mitochondrial Release of Cytochrome c after Spinal Cord Injury

We reported previously that complete spinal cord transection (SCT) results in depression of mitochondrial respiratory chain enzyme activity that triggers apoptosis via sequential activations of apoptosis-inducing factor (AIF)- and caspase- dependent cascades in the injured spinal cord. This study tested the hypothesis that nitric oxide (NO) and superoxide anion (O-2(center dot-)) serve as the interposing signals between SCT and impaired mitochondrial respiratory functions. Adult Sprague-Dawley rats manifested a significant increase in NO or O-2(center dot-) level in the injured spinal cord during the first 3 days after SCT. The augmented O-2(center dot-) production, along with concomitant reduction in mitochondrial respiratory chain enzyme activity or ATP level, nuclear translocation of AIF, cytosolic release of cytochrome c, and DNA fragmentation were reversed by osmotic minipump infusion of a NO trapping agent, carboxy-PTIO, or a superoxide dismutase mimetic, tempol, into the epicenter of the transected spinal cord. Intriguingly, carboxy-PTIO significantly suppressed upregulation of poly(ADP-ribose) polymerase-1 (PARP-1) in the nucleus, attenuated nuclear translocation of AIF, inhibited mitochondrial translocation of Bax and antagonized mitochondrial release of cytochrome c; whereas tempol only inhibited the later two cellular events after SCT. We conclude that overproduction of NO and O-2(center dot-) in the injured spinal cord promulgates mitochondrial dysfunction and triggers AIF- and caspase- dependent apoptotic signaling cascades via differential upregulation of nuclear PARP-1 and mitochondrial translocation of Bax.