Insulin-like growth factor-I influences the initiation of myelination: studies of the anterior commissure of transgenic mice.

The effects of insulin-like growth factor-I (IGF-I) on the initiation of myelination have been examined in the posterior portion of anterior commissure of the transgenic (Tg) mice that overexpress IGF-I in brain, and those which exhibit ectopic brain expression of IGF binding protein-1 (IGFBP-1), an inhibitor of IGF actions. In IGF-I Tg mice the number of ensheathed axons was increased compared to their normal littermates. The distribution of the axon size further showed that both small (< 300 nm) and large (> 700 nm) axons were more often ensheathed and myelinated. The opposite was true in IGFBP-1 Tg mice. These data indicate that IGF-I increases myelin ensheathment independent of axon diameter, and support the hypothesis that the absolute axon size is not the sole determinant for the initiation of myelination in CNS.