Cryogenic spinal cord injury induces astrocytic gene expression of insulin-like growth factor I and insulin-like growth factor binding protein 2 during myelin regeneration.
To study injury-induced astrocytic responses associated with regrowth of axons and regeneration of myelin, the method of Collins and colleagues was used to make focal cryogenic lesions in spinal cords of adult rats (Collins et al.: J Neuropathol Exp Neurol 45: 742-757, 1986). The duration of cryogenic injury (CI), the size of the cryode, and its temperature were chosen to destroy all myelin sheaths and axons without producing cavities or hemorrhages. Messenger RNA and peptide distributions of insulin-like growth factor I (IGF-I), IGF-I receptor (IGFR-I), IGF binding protein 2 (IGFBP-2), glial fibrillary acidic protein (GFAP), and myelin basic protein (MBP) were studied 3-56 days after CI by in situ hybridization and immunocytochemistry. At 3 days, vimentin-positive, GFAP-negative astrocyte-like cells in the lesion expressed IGF-I mRNA and peptide and 7 days after CI, both were expressed by typical GFAP-positive, hypertrophic astrocytes, many of which also were vimentin-positive. Levels of IGF-I, IGFBP-2, and GFAP mRNA and peptide were higher in lesion astrocytes after 14 days. They attained maximum levels at 21-28 days before declining to near control levels at 56 days. Decreasing relative levels of oligodendroglial MBP mRNA were found in and around lesions 7-14 days after CI; subsequently, rising levels accompanied remyelination. At 28 and 56 days after CI, some transferrin-positive, oligodendroglia-like cells also were immunostained by anti-IGFR-I. Our findings suggest that early astrocytic production of IGF-I and IGFBP-2 may be involved in the myelin regeneration which occurs in this model of spinal cord injury.