Abstract 3656: Estradiol Improves Neural Functional Recovery After Nerve Injury by Reducing Inhibition of the Hedgehog-signaling Pathway

Background: Prior data indicate that estradiol (E2) favorably affects neovascularization in injured tissue, and that Sonic Hedgehog (Shh)-induced neovascularization is critical for nerve regeneration; however, little is known about the relationship between E2 and Shh signaling. We tested the hypothesis that E2 potentiates nerve regeneration via Shh-activated neoangiogen-esis. Methods and Results: Ovariectomized mice were subjected to unilateral sciatic nerve crush injury and treated with either E2 (via locally administered biodegradable polymer) or placebo (polymer alone). E2 treatment resulted in greater functional recovery of the nerve as evaluated by both exercise duration (P<0.05) and motor nerve conduction velocity (P<0.05), and intraneural vascularity was significantly higher in E2-treated animals than in the placebo-treatment group (P<0.05). Analyses of mRNA expression in the injured nerves of placebo-treated mice showed that the Shh pathway was activated immediately after injury, but Shh expression declined substantially within 7 days. Shh activation persisted in the E2-treatment group; Shh expression on day 7 was greater in E2-treated animals than in the placebo-treatment group (P<0.05). The expression of Hedgehog-interacting protein (HIP), an endogenous Shh inactivator, was significantly downregulated 24 hours after E2 treatment, and HIP downregulation was attenuated by the E2-receptor blocker ICI, suggesting that E2 augments Shh signaling both my enhancing Shh expression and by inhibiting HIP. Inhibition of HIP expression by E2 was also observed in vitro in Schwann cells and endothelial cells. The expression of Ptc1 (Shh receptor) and Gli1 (a downstream target of Shh) was evaluated in Ptc1-LacZ and Gli1-LacZ mice after nerve-crush injury. X-gal staining demonstrated that E2 treatment upregulated both Ptc1 and Gli1 expression within 3 days of surgery. Conclusions: E2 accelerates the functional recovery of injured nerves by enhancing angiogenesis. We propose that these effects occur in part through the inhibition of the Shh inactivator HIP, thereby preserving Shh activity after nerve injury. These findings may have identified a novel regulatory mechanism for nerve repair via E2-enhaced, Shh-mediated angiogenesis.