Insulin and IGF-II, but not IGF-I, stimulate the in vitro regeneration of adult frog sciatic sensory axons

We used the in vitro regenerating frog sciatic nerve to look for effects of insulin and insulin-like growth factors I and II (IGF-I, IGF-II) on regeneration of sensory axons and on injury induced support cell proliferation in the outgrowth region. In nerves cultured for 11 days, a physiological dose (10 ng/ml, ≈ nM) of insulin or IGF-II increased ganglionic protein synthesis (by 20% and 50%, respectively) as well as the level of newly formed, radiolabelled axonal material distal to a crush injury (both by 80%), compared to untreated, paired controls. In addition, insulin increased the outgrowth distance of the furthest regenerating sensory axons by 10%. The preparation was particularly sensitive to insulin during the first 5 days of culturing. Furthermore, both insulin and IGF-II were found to inhibit proliferation of support cells in the outgrowth region in a manner suggesting effects via their individual receptors. The inhibition, about 30%, was observable after 4 but not 11 days in culture. It is not clear if this reflects a stimulated differentiation of some cells. By contrast, IGF-I lacked effects on both regeneration and proliferation. In conclusion, the results suggest that insulin and IGF-II are involved in the regulation of peripheral nerve regeneration.