Role of calcineurin in calcium-mediated hypoxic injury to white matter

Background context: Calcium influx into cells is responsible for initiating the "final pathway" to cell death in neuronal tissue after traumatic or hypoxic injury. The specific pathways in this cascade are myriad and the importance each one plays is controversial. It is clear, though, that blocking individual pathways confers protection to these tissues. Purpose: In the present study we examined the role of Cyclosporin A (CsA), FK-506 and rapamycin in modulating the effects of Ca²⁺ influx through their interactions with immunophilins and specifically the end result of calcineurin modulation. Methods: Dorsal columns were isolated from the spinal cord of adult rats and injured by exposure to hypoxic conditions for 60 minutes. The samples were monitored electrophysiologically in an in vitro recording chamber (maintained at 37 C°) during injury, and the compound action potential (CAP) was monitored with glass microelectrodes. The dorsal column was exposed to hypoxic Ringers solution alone or with the different immunosuppressants and compared with baseline readings. Functional recovery of the dorsal column was then assessed by recovery of the CAP. Results: The mean CAP decreased to about 20% of baseline control levels during hypoxia and returned 53.8±7.6% of baseline (p<.05) after reoxygenation. CsA, an immunosuppressant known to inhibit calcineurin, promoted a significantly greater recovery of CAP amplitude to 76.8±5.2% and 72.1±13.2% of control (p<.05) after hypoxic injury and reoxygenation of dorsal column white matter when applied at concentrations of 1 μM and 10 μM, respectively. FK-506, which also inhibits calcineurin, was applied at a concentration of 0.1 μM, and promoted CAP amplitude recovery to 82.6±5.0% of control after hypoxic injury and reoxygenation of dorsal column white matter. The addition of rapamycin (1 μM), which binds to the same immunophilin as FK-506, to the FK-506 (0.1 μM) solution during hypoxic injury showed recovery of CAP amplitudes to only 56.9±6.7% of control. Electron microscopy revealed remarkable protection of axons and prevention of organelle disruption in segments treated with CsA and FK-506 during hypoxia when compared with hypoxic controls. Conclusion: In conclusion, both CsA and FK-506 confer in vitro protection to dorsal columns during hypoxic injury at physiological temperatures, and rapamycin blocks the protective effect of FK-506. Thus, calcineurin may play an important role in the physiology of neuronal injury.