Induction of tumor necrosis factor-α in Schwann cells after gradual elongation of rat sciatic nerve

Background. Although limb lengthening has become a common treatment, the biochemical responses underlying the adaptation of elongated nerves are unclear. The purpose of this study was to clarify whether expression of cytokines and neurotrophins is altered in gradually elongated peripheral nerves. Methods. Left sciatic nerves of adult rats were elongated by lengthening the femur up to 20mm at a rate of 1, 2, or 20mm/day. The ipsilateral and contralateral sciatic nerves of each group were resected 1, 4, 8, and 16 days after 20mm of lengthening. mRNAs for interleukin-1, interleukin-6, tumor necrosis factor-alpha (TNF alpha), nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 were semiquantified by reverse transcription-polymerase chain reaction. Histological changes were assessed by immunoperoxidase and immunofluorescence staining. Results. Expression of TNFa mRNA was markedly induced in the ipsilateral sciatic nerves of the gradually elongated, 1 mm/day and 2 mm/day groups, although to a lesser extent than in the acutely elongated, 20 mm/day group. In contrast, mRNAs for other factors remained undetectable. The mRNA level for TNF alpha in each group was highest 1 day after 20 mm of lengthening. The highly up-regulated level in the acute group declined rapidly within 4 days and slowly thereafter; in contrast, the decrease in the gradual groups was always slow. Even 16 days later, the levels in all groups remained significantly elevated. Unexpectedly, TNFa mRNA expression was also induced in the contralateral side of all groups. Immunohistochemical staining showed that TNF alpha-immunoreactive cells in gradually elongated nerves were also positive for S-100 protein but negative for proliferating nuclear cell antigen, indicating that TNFa was produced by nonproliferating Schwarm cells. Conclusions. Gradual nerve elongation by limb lengthening induces production of TNFa in Schwarm cells. Presumably, TNFa plays a critical role in the adaptation of peripheral nerves to elongation.