Implications of High Neuronal Cyclophilin D Levels for Spinal Cord Injury

The immunosuppressant drug Cyclosporin A is currently in clinical trials as a potential treatment for traumatic brain injury. This follows up on promising experimental results demonstrating that CsA improves tissue sparing following rodent traumatic brain injury, although it is less clear if CsA improves functional outcomes. CsA has also been administered following experimental spinal cord injury, with contrasting results. CsA is thought to protect brain and spinal cord tissue by binding to cyclophilin D (CypD), a mitochondrial protein that gates the opening of a mitochondrial permeability transition pore (mPTP). Prolonged mPTP opening results in mitochondrial dysfunction and cell death. We recently found that neuronal mitochondria contain high levels of CypD, and therefore require high CsA levels to inhibit mPTP opening. This may explain the apparent resistance of brain and neuronal mitochondria to CsA-mediated inhibition ofmPT in some previous studies. Unfortunately, the toxicity of CsA limits the dose that can be administered in vivo. To examine the hypothesis that CypD inhibition is protective following spinal cord injury, we will use genetic and newer pharmacologic approaches. Using primary neuronal cultures prepared from CypD knockout mice (ppifl-), wild-type mice, and heterozygotes (ppi/I-) we will evaluate the hypothesis that the high CypD content of neuronal mitochondria enhances vulnerability to excitotoxic insult and requires high levels of CypD inhibitors for neuroprotection. These mice will also be used for in vivo studies examining the influence of mitochondrial CypD content on the extent of secondary degeneration and locomotor impairment following contusion injury to the spinal cord. Using CsA analogs with greater specificity for CypD and reduced toxicity, namely NIM811 and Debio025, we will compare effects of moderate (lO/mg/kg) and high (50 mg/kg) levels of the inhibitors on tissue sparing and locomotor function following spinal cord injury in the rat. Together, these results will provide a clear indication regarding the potential use of CypD inhibitors as a treatment to minimize the secondary damage and improve functional outcome following spinal cord injury.