Tempol, a superoxide dismutase mimetic, inhibits Wallerian degeneration following spinal cord injury by preventing glutathione depletion and aldose reductase activation.

Abstract

Spinal cord contusion injury results in Wallerian degeneration of spinal cord axonal tracts, which are necessary for locomotor function. Axonal swelling and loss of axonal density at the contusion site, characteristic of Wallerian degeneration, commence within hours of injury. Tempol, a superoxide dismutase mimetic, was previously shown to reduce the loss of spinal cord white matter and improve locomotor function in an experimental model of spinal cord contusion, suggesting that tempol treatment might inhibit Wallerian degeneration of spinal cord axons. Here, we report that tempol partially inhibits Wallerian degeneration, resulting in improved locomotor recovery. We previously reported that Wallerian degeneration is reduced by inhibitors of aldose reductase (AR), which converts glucose to sorbitol in the polyol pathway. We observed that tempol inhibited sorbitol production in the injured spinal cord to the same extent as the AR inhibitor, sorbinil. Tempol also prevented post-contusion upregulation of AR (AKR1B10) protein expression within degenerating axons, as previously observed for AR inhibitors. Additionally, we hypothesized that tempol inhibits axonal degeneration by preventing loss of the glutathione pool due to polyol pathway activity. Consistent with our hypothesis, tempol treatment resulted in greater glutathione content in the injured spinal cord, which was correlated with increased expression and activity of γ-glutamylcysteine ligase (γGCL; EC 6.3.2.2), the rate-limiting enzyme for glutathione synthesis. Administration of the γGCL inhibitor buthionine sulfoximine abolished all observed effects of tempol administration. Together, these results support a pathological role for polyol pathway activation in glutathione depletion, resulting in Wallerian degeneration after spinal cord injury. Interestingly, methylprednisolone, oxandrolone, and clenbuterol, which are known to spare axonal tracts after spinal cord injury, were equally effective in inhibiting polyol pathway activation. These results suggest that prevention of AR activation is a common target of many disparate post-SCI interventions.