Involvement of IGF-1 and IGFBP-1 and -3 in Axon-Schwann Cell Interactions

Insulin-like growth factor-I (IGF-I) promotes proliferation, differentiation and survival of Schwann cells (SC). Moreover, an effect of IGF-I on axon regeneration has been proposed. However, although IGF-I expression has been shown in postnatal SC and in SC precursors thus suggesting an autocrine regulation of SC survival, release of the trophic factor by SC has not been clearly demonstrated. To investigate the role of the IGF system, we studied the supernatant of primary SC cultures, purified sensory neuron cultures, and co-cultures of SC and sensory neurons. Moreover, since in its free form IGF-I interacts with specific binding proteins (IGFBPs) whose active role in modulating the effects of IGF-I is increasingly recognized, in the same culture systems we also evaluated levels of IGFBP-1 and -3. We established SC, sensory neuron and SC/sensory neuron cultures according to classical procedures. Purified SC (87,500 cells/dish) were grown in presence of serum, forskolin, cholera toxin and bovine pituitary extract. Purified sensory neurons were grown in the presence of serum and NGF; part of it was maintained alone and the rest was added with SC (87,500 cells/dish) and grown in a co-culture system. After 1 month all cultures were washed with PBS and grown for at least 4 days in a serum-free medium before collecting their conditioned media (CM) for chromatography assays. Similar IGF-I levels were found into the CM of SC and of sensory neurons when grown as purified cultures (94.40 vs 73.80 ng/5×10⁶ cell). Instead, higher amounts of IGF-I were detected into the medium of SC/sensory neurons co-cultures (277.70 ng/5×10⁶ cell). IGFBP-1 and -3 production was also found in both SC and sensory neuron purified cultures, but their expression was clearly higher in neurons compared to SC (0.74 vs 0.28 ng/5×10⁶ cell for IGFBP-1 and 4.48 vs 1.77 ng/5×10⁶ cell for IGFBP-3). Furthermore, SC/sensory neuron co-cultures maintained high levels of IGFBP-3 (3.93 ng/5×10⁶) but not of IGFBP-1 (0.28 ng/5×10⁶). In conclusion, IGF-1 is produced by both SC and pure sensory neurons, but co-culturing the two cell populations results in a possible reciprocal stimulation to synthesize this growth factor. Moreover, the decrease of IGFBP-1 in the medium of co-cultures could be due to the production of IGFBP proteases by SC. This action could be a mechanism to increase IGF-1 availability to the specific receptor.