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 ensheated axons was increased compared to their normal littermates. The distribution of the axon size further showed that both small (700 nm) axons were more often ensheated 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 is not sole determinant for the initiation of myelination in CNS.