Characterization of Insulin-Like Growth Factor Binding Proteins Produced in the Rat Central Nervous System*

Abstract

Insulin-like growth factor binding proteins (IGFBP) are thought to modulate the biological actions of the insulin-like growth factors (IGF), including possible regulatory roles in the growth and differentiation of the central nervous system. Extracellular fluids usually contain a mixture of IGFBPs, three of which have been cloned, sequenced, and designated IGFBP-1, -2, and -3. We used Western ligand blotting, immunoprecipitation, and competitive binding analysis to characterize IGFBPs found in fetal and adult rat cerebrospinal fluid (CSF) and IGFBPs produced by cultures of neonatal rat choroid plexus, astrocytes, and C6 glial cells. Pooled rat CSF contains primarily IGFBP-2 (a narrow band at Mr = 29,000), lesser quantities of IGFBP-3 (a multicomponent broad band at Mr = 37,500-43,000), and trace amounts of low mol wt IGFBPs. Conditioned medium from cultures of choroid plexus cells contained a single binding protein corresponding to IGFBP-2, whereas C6 cells made predominately an IGFBP corresponding to IGFBP-3. Astrocytes secreted two IGFBPs corresponding to IGFBP-2 and -3, primarily IGFBP-3. Neonatal CSF contained substantially more binding activity corresponding to IGFBP-2 than did adult CSF. In all samples showing Western ligand binding profiles corresponding to IGFBP-2, identification was established by immunoprecipitation. Competitive binding analysis performed on choroid plexus IGFBP showed preferential high affinity binding for IGF-II compared with that for IGF-I. In conclusion, CSF contains a mixture of distinct IGFBPs, primarily IGFBP-2. The other IGFBPs found in CSF are capable of being synthesized locally within the central nervous system by glial cells and neurons, suggesting that they are not derived from plasma by transport across the blood-brain barrier.