TRANSFERRIN IS NECESSARY AND SUFFICIENT FOR THE NEURAL EFFECT ON GROWTH IN AMPHIBIAN LIMB REGENERATION BLASTEMAS

Cell proliferation during the early phase of growth in regenerating am phibian limbs requires a permissive influence of nerves. Based on anal yses of proliferative activity in denervated blastemas, it was propose d that nerves provide factors important for cells to complete the prol iferative cycle rather than for mitogenesis itself. One such factor, t he iron-transport protein transferrin (Tf), is abundant in regeneratin g peripheral nerves where it is axonally transported and released at g rowth cones. Using blastemas in organ culture, which have been widely used in previous investigations of the neural effect on growth, it was shown here that the growth-promoting activity of neural extract was c ompletely removed by immune-absorption with antiserum against Tf and r estored by addition of Tf. Purified Ti or a low molecular weight ferri c ionophore were as active as the neural extract in this assay, indica ting that the trophic effect of Tf involves its capacity for iron deli very. Both Tf and ferric ionophore also maintained DNA synthesis in de nervated blastemas in vivo. A dose-response assay indicated that purif ied axolotl Tf stimulates growth of cultured blastemal cells at concen trations as low as 100 ng/mL. The Ti mRNA in axolotl nervous tissue wa s shown by northern analysis to be similar in size to that of liver. T hese results are discussed together with those from previous in vitro studies of blastemal growth and support the hypothesis that cell divis ion in the blastema depends on axonally released Tf during the early, nerve-dependent phase of limb regeneration.