An immunohistochemical and in situ hybridization study of insulin-like growth factor I within fetal neuron cell cultures

The insulin-like growth factor (IGF) receptor type 1 (IGF1R) as an essential component of the signalling network regulating neurogenesis

The insulin-like growth factor receptor type 1 (IGF1R) signalling pathway is activated in the mammalian nervous system from early developmental stages. Its major effect on developing neural cells is to promote their growth and survival. This pathway can integrate its action with signalling pathways of growth and morphogenetic factors that induce cell fate specification and selective expansion of specified neural cell subsets. This suggests that during developmental and adult neurogenesis cellular responses to many signalling factors, including ligands of Notch, sonic hedgehog, fibroblast growth factor family members, ligands of the epidermal growth factor receptor, bone morphogenetic proteins and Wingless and Int-1, may be modified by co-activation of the IGF1R. Modulation of cell migration is another possible role that IGF1R activation may play in neurogenesis. Here, I briefly overview neurogenesis and discuss a role for IGF1R-mediated signalling in the developing and mature nervous system with emphasis on crosstalk between the signalling pathways of the IGF1R and other factors regulating neural cell development and migration. Studies on neural as well as on non-neural cells are highlighted because it may be interesting to test in neurogenic paradigms some of the models based on the information obtained in studies on non-neural cell types.

Neuronal synthesized insulin roles on neural differentiation within fetal rat neuron cell cultures

We previously, described the production and secretion of insulin by fetal neurons in culture and demonstrated that neuronal synthesized insulin [I(n)] promoted neurofilament distribution and axonal growth. In this study we investigated the role of I(n) in promoting neural differentiation. Stem cells from 16 day gestational age rat brains were cultured in an insulin-free defined medium (IFDM) and treated with: 5, 20 or 100 ng/ml of exogenous insulin, 100 ng/ml insulin-like growth factor I (IGF-I) or an anti-insulin antibody. The neurons were studied at 1 and 3 days of incubation. The total number of cells showed no significant difference (P>0.05) in any of the media used, except the IFDM at day 3 of incubation treated with the anti-insulin antibody (P<0.05) and IGF-I to 20 ng/ml of insulin (P<0.05). No significant difference (P>0.05) was found in the number of differentiated neurons incubated in the IFMD, in which the neurons produce and secrete I(n), between days 1 and 3 of incubation, but neural differentiation decreased significantly (P<0.05) when treated with the anti-insulin antibody. Exogenous insulin significantly increased (P<0.05) the number of differentiated neurons compared to the IFDM. A significant reduction (P<0.05) of differentiated neurons was observed at day 3 of incubation with IGF-I compared to all the different media. Thus, I(n) has a role in promoting neural differentiation and growth, but exogenous insulin promoted neural differentiation and growth beyond I(n).

Brain endogenous insulin effects on neurite growth within fetal rat neuron cell cultures

We have previously described insulin to be synthesized "de novo" within the fetal rat brain and that brain endogenous insulin [I(n)] promoted neurofilament distribution within fetal neurons. In this study, we investigated the role of I(n) in neuron axonal growth. Rat fetal brain stem cells from 16-day gestational age were cultured in an IFDM and treated with an insulin antibody. In addition, the cell cultures were also treated in defined medium with the addition of: 5 ng, 20 ng or 100 ng/ml of insulin or 100 ng/ml insulin-like growth factor 1 (IGF-1). The neuron cell cultures were studied at 1 and 3 days of incubation. The presence of preproinsulin mRNA and insulin immunoreaction confirmed the "de novo" synthesis of insulin by the fetal neuron cell cultures. Axonal growth was similar by day 1 of the study in all the media, but in insulin medium containing 100 ng/ml of insulin the axonal length was significantly longer. By day 3 of incubation I(n) promoted axonal growth. Treating the neurons with an insulin antibody confirmed these findings, with a significant decrease in axonal length (p<0.05). The treatment with different concentrations of exogenous insulin did not promote axonal growth beyond I(n) by day 3 of incubation. IGF-1 did not promote axonal growth by day 3 of incubation. In summary, I(n) may promote axonal growth during brain development.

The expression of insulin-like growth factor-1 during adipogenesis in vivo: effect of thyroxine

The expression of insulin-like growth factor-1 (IGF-1) was examined in subcutaneous (SQ) adipose tissue from 105-day pig fetuses by ribonuclease protection assays and in situ hybridization using a porcine IGF-1 riboprobe. Fetuses were hypophysectomized (hypox) at 70 days of gestation and at 90 days thyroxine (T4) pellets were implanted into some of the hypox fetuses. Fetuses were removed and SQ tissues collected on day 105 of gestation. RNase protection assays followed by scanning laser densitometry revealed that IGF-1 mRNA in SQ adipose tissue in hypox fetuses was significantly decreased to 19.8 +/- 1.2% of control values. T4 treatment increased the expression of IGF-1 by 174 +/- 26.6% of values for hypox fetuses. Using in situ hybridization we showed that fetal hypophysectomy reduced and T4 treatment increased expression of IGF-1 mRNA in the outer SQ adipose tissue layer (P < 0.05). However, T4 treatment after hypox did not influence IGF-1 expression in the inner SQ layer (P > 0.05). IGF-1 was expressed around capillaries, in small fat cells, and in fibroblasts in loose and dense connective tissue. Large fat cells, however, did not express IGF-1. Collectively, we concluded that (1) IGF-1 mRNA was decreased after hypox and increased by T4 treatment in SQ tissue of 105-day fetuses; (2) The expression of the IGF-1 gene was more sensitive to T4 treatment after hypox in outer SQ tissue than in inner SQ tissue; (3) Most stromal cells produced IGF-1 mRNA, and as a result they may influence adipogenesis in the outer layer of SQ adipose tissue; and (4) Once fat cells enlarged, expression of IGF-1 was not detected. Therefore, these studies provide evidence that IGF-1 may mediate the influence of T4 on fetal adipogenesis.