Des (1-3) IGF-I potently enhances differentiated cell growth in olfactory bulb organ culture

We recently provided evidence that newborn rat olfactory bulb (OB) could be maintained in serum-free organ culture with combinations of insulin-like growth factor-I (IGF-I) and basic fibroblast growth factor (bFGF), both of which are locally synthesized. Des (1-3), or truncated, IGF-I is a potent analog of IGF-I isolated from rat and human brain. We proposed in this study to examine the effects of des (1-3) IGF-I on cell function, morphology and on neuronal and glial cell differentiation in our cultured OB model, using cell-specific immunostains for neurons (150 kDa neurofilament) and glial cells (glial fibrillary associated protein--GFAP). OB were cultured in Iscove's serum-free medium containing IGF-I or des (1-3) IGF-I both alone or in combination with bFGF. Dose dependent responses of 14C amino acid uptake showed des (1-3) IGF-I to be 3-5 fold more potent than IGF-I with a half maximal response at about 20 ng/ml in comparison to 100 ng/ml of IGF-I. The maximum response to IGF-I +/- bFGF was seen at 150 ng/ml; a ten-fold higher dose of insulin +/- bFGF was required to achieve the same response. While morphology was close to fresh 6 day OB following culture with IGF-I (150 ng/ml) and bFGF (25 ng/ml), the substitution of des (1-3) IGF-I at 50 ng/ml markedly improved morphology. Neurons were identified following culture in IGF-I or bFGF alone, but showed greater organisation in the mitral layer following combined IGF-I/bFGF culture. However, in contrast to IGF-I (150 ng/ml), des (1-3) IGF-I (50 ng/ml) supported marked neuronal expression. Furthermore, when des (1-3) IGF-I (50 ng/ml) was substituted for IGF-I, in combination with bFGF, the pattern of enhanced neuronal expression in the mitral layer was very close to that seen in the fresh 6 day bulb, with dendrites projecting to the glomerular layer. In OBs treated with no growth factors, or either IGF-I, des (1-3) IGF-I or bFGF alone, glial expression was widespread and poorly organised, suggesting an injury response. In contrast, following treatment with combinations of bFGF with IGF-I or des (1-3) IGF-I, a more ordered, though enhanced glial response was seen in glomerular and granule cell layers.(ABSTRACT TRUNCATED AT 400 WORDS)