Changes in axonally transported proteins during axon regeneration in toad retinal ganglion cells.

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Abstract
In an effort to understand the regulation of the transition of a mature neuron to the growth, or regenerating, state, the composition of the axonally transported proteins in the retinal ganglion cells of the toad B. marinus was analyzed after inducing axon regeneration by crushing the optic nerve. At increasing intervals after axotomy, the retinal ganglion cells were labeled with [35S]methionine and the labeled transported polypeptides in the crushed optic nerve subsequently analyzed by means of 1- and 2-dimensional electrophoretic techniques. While the transition from the mature to the regenerating state does not require a gross qualitative alteration in the composition of axonally transported proteins, the relative labeling of a small subset of rapidly transported proteins is altered dramatically (changes of > 20-fold) and reproducibly (> 30 animals) by axotomy. One of these growth-associated proteins (GAP) was soluble in an aqueous buffer, while 3 were associated with a crude membrane fraction. The labeling of all 3 of the membrane-associated GAP increased during the first 8 days after axotomy and they continued to be labeled for at least 4 wk. The modulation of these proteins after axotomy is consistent with the possibility that they are involved in growth-specific functions and that the altered expression of a small number of genes is a crucial regulatory event in the transition of a mature neuron to a growth state. In addition to these selective changes in rapidly transported proteins, the following more general metabolic correlates of the regeneration process were observed. The total radioactive label associated with the most rapidly transported proteins (groups I and II) increased 3- to 4-fold during the first 8 days after the nerve was crushed, while the total label associated with more slowly moving proteins (group IV) increased about 10-fold during this same period. Among these more slowly transported polypeptides, 5 were observed whose labeling increased much more than the average. Three of these 5 polypeptides resemble actin and .alpha.- and .beta.-tubulin in their electrophoretic properties.