Time‐dependent effects of insulin on Schwann cell proliferation in the in vitro regenerating adult frog sciatic nerve

The present study showed that insulin (0.01 μg/ml, ≈︂ 2 nM) inhibited [3H]‐thymidine incorporation in support cells, most likely Schwann cells, of the cultured frog sciatic nerve. A 25–35% inhibition took place in regenerating nerve preparations as well as in preparations devoid of neuronal protein synthesis, i.e., in isolated 5 mm nerve segments and in gangliectomized nerves, suggesting that the effect was direct and not mediated via the neuronal cells. The inhibition by insulin was time‐dependent in that an effect was seen after 4 days but not at shorter or at longer periods of culturing. In separate experiments biotinylated insulin was shown to be taken up by Schwann cells in the regenerating nerve. Addition of serum increased the [3H]‐thymidine incorporation severalfold and abolished the inhibitory action of insulin. Our results suggest that insulin, at a certain stage of the regeneration programme, exerts a direct, inhibitory effect on the proliferation of the Schwann cells in the cultured frog sciatic nerve. © 1993 Wiley‐Liss, Inc

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