Protein Synthesis and Release by Normal and Lesioned Axolotl Peripheral Nerves

Previous studies in urodeles (Holder et al., 1982, J. Physiol. 326: 371; Holder et al., 1984, Proc. R. Soc. Lond. B 222: 477; Aaronson et al., 1995, Neuroscience 66: 201) have shown that regenerating axons of peripheral nerves tend to grow toward distal nerve stumps, which is consistent with the hypothesis that axonal growth may be stimulated by factors released from degenerating nerves. In the present study we used two-dimensional gel electrophoresis and autoradiography to compare the incorporation of radiolabeled methionine into proteins which are synthesized and released in vitro by segments of normal and previously cut axolotl sciatic nerves, within the isoelectric point range 2.4-10.6 and molecular weight range 3.6-200 kDa. In the distal portion of nerves cut 7 days previously in vivo , the synthesis of at least six secreted proteins was significantly greater than in undamaged nerves. The possible cellular sources of these proteins was assessed by comparing protein release from normal nerves with nerve segments maintained in culture for 7 days (in which the contribution from recruited macrophages would be expected to be minimal) and segments of nerve which had been frozen and then replaced in situ for 7 days (in which the contribution from sheath cells would be expected to be minimal). This revealed that five of the up-regulated proteins from the lesioned nerves, with apparent molecular weight (kDa)/isoelectric point values of approximately 120/4, 20/5, 19/5.3, 10.5/6.4-6.5, and 7/4.2 are predominantly sheath cell products, while one (19/5.8-6.0) may be secreted mainly by macrophages (or other cells) which center the frozen nerve segments. The identity of these proteins and their possible involvement in nerve repair remain to be determined.