Insulin‐like growth factor I stimulates oligodendrocyte development and myelination in rat brain aggregate cultures

Insulin‐like growth factor I (IGF‐I) and high concentrations of insulin have been shown to stimulate an increase in the number of oligodendrocytes that appear in developing monolayer cultures of rat brain cells (McMorris et al., Proc Natl Acad Sci USA 83:822–826, 1986; McMorris et al., Ann NY Acad Sci 605:101–109, 1990; McMorris and Dubois‐Dalcq, J Neurosci Res 21:199–209, 1988). In the present study, we investigated whether IGF‐I or insulin treatment induces a corresponding increase in the synthesis and accumulation of myelin. Aggregate cultures, established from 16‐day‐old fetal rat brains, were treated with either 100 ng/ml IGF‐I or 5,000 ng/ml insulin and analyzed for the number of oligodendrocytes, activity of 2′,3′‐cyclic nucleotide 3′‐phosphohydrolase (CNP), total amount of myelin, and synthesis rate of myelin proteins. Cultures treated with IGF‐I beginning on day 2 after explanation contained 35–80% more oligodendrocytes and had 60–160% higher CNP activity than controls when tested on day 13, 20, or 27. By day 27, treated cultures had 35–90% more myelin than controls. Similar results were observed in response to 5,000 ng/ml insulin, a concentration at which insulin binds to IGF receptors and acts as an analogue of IGF‐I. The synthesis rate of myelin proteins was measured in experiments using 5,000 mg/ml insulin. When treatment was begun at day 20 rather than day 2, cultures did not exhibit an increased number of oligodendrocytes over control during the following 4–6 days. Therefore, this paradigm was used to separate the effects of insulin‐like peptides on the number of oligodendrocytes from the effects on the synthesis rate of myelin proteins and the accumulation of myelin. Cultures treated from day 20 and controls showed no difference in either the myelin yield or the synthesis rate of myelin proteins. These data show that IGF‐I (and insulin at high concentrations) increases the amount of myelin produced in aggregate cultures; and that the increase appears to be due primarily to an increase in the number of oligodendrocytes in the cultures rather than to a direct increase in the rate of myelin protein synthesis or an increase in the amount of myelin synthesized per oligodendrocyte.