Stimulation of DNA synthesis in Balbc 3T3 cells by peripheral nerve degenerating in vitro

The release of mitogenic substances from degenerating peripheral nerves was detected and characterized in vitro. Cultures of serum-starved, subconfluent $\text{Balb}\text{c}$ 3T3 cells were exposed 24 h to myelinated peripheral nerve fascicles, with ³H]thymidine added during the last 3 h. Cells exposed to peripheral nerves incorporated twice as much ³H]thymidine as control cultures without nerves (P < 0.005). Autoradiography showed a graded decrease in labeling index with increasing distance from nerves. The mitogenic response varied in a dose-dependent manner with increasing nerve length. Also, the response varied according to the degree of myelination. Myelinated sciatic nerve fascicles caused greater incorporation of ³H]thymidine (P < 0.005) than unmyelinated abdominal vagus nerves of similar size, suggesting myelin-derived growth factor activity. Evidence from other laboratories has led to the hypothesis that during peripheral nerve injury, myelin proteins are degraded by lysosome-derived acid proteinases yielding mitogenic polypeptide fragments. We report that the addition of the acid proteinase inhibitor, pepstatin, to the culture media caused a small but significant decrease (P < 0.05) in the mitogenic effect of peripheral nerves. The work supports the concept that the cell proliferation accompanying Wallerian degeneration is stimulated by mitogens released by the injured nerve.