Axonal transport and release of transferrin in nerves of regenerating amphibian limbs.

Transferrin, a plasma protein required for proliferation of normal and malignant cells, is abundant in peripheral nerves of birds and mammals and becomes more concentrated in this tissue during nerve regeneration. We are testing the hypothesis that this factor is involved in the growth-promoting effect of nerves during the early, avascular phase of amphibian limb regeneration. A sensitive enzyme-linked immunosorbent assay for axolotl transferrin was developed and used to determine whether this protein meets certain criteria expected of the trophic factor(s) from nerves. During limb regeneration adult sciatic nerves greatly increased their content of transferrin, which immunohistochemistry revealed was distributed in both axons and Schwann cells. Using the double ligature method with sciatic nerves in vivo, it was determined that transferrin is carried by fast anterograde axonal transport at all stages of limb regeneration. An approach based on multicompartment organ culture demonstrated that fast-transported transferrin was secreted in physiologically significant amounts at distal ends of regenerating axons. Finally, the concentration of transferrin in the distal region of larval axolotl limb stumps was found to decrease directly and rapidly in response to axotomy. Since transferrin is important for both axonal regeneration and cell cycling, the present data have significance for various aspects of nerve's trophic activity during limb regeneration.