Initial cholinergic differentiation in embryonic chick retina is responsive to insulin and cell-cell interactions
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Published on: November 2, 2015
R E Hausman1, G D Sagar, B H Shah
1Department of Biology, Boston University, MA 02215.
Insights
Insulin stimulates early cholinergic differentiation in chick retinal neurons by increasing choline acetyltransferase (ChAT) activity. However, it inhibits GABAergic differentiation, indicating a selective role in neuronal development.
Area of Science:
- Neuroscience
- Developmental Biology
- Cell Biology
Background:
- Insulin influences neuronal development, with prior studies showing its role in cholinergic differentiation between embryonic days 9-11.
- The precise role of insulin in the initial stages of cholinergic and GABAergic differentiation in embryonic chick retinal neurons remains to be fully elucidated.
Purpose of the Study:
- To investigate the effects of insulin and Insulin-like Growth Factor 1 (IGF-1) on the early differentiation of cholinergic and GABAergic neurons in embryonic chick retinal cells.
- To determine if insulin influences the expression of choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) activities at the onset of their developmental increase.
Main Methods:
- Cultured embryonic chick retinal cells and developing retinal tissue were treated with insulin and IGF-1.
- Choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) activities were measured as markers for cholinergic and GABAergic differentiation, respectively.
- Choline uptake was also assessed to evaluate cholinergic neuron function.
Main Results:
- Insulin significantly stimulated the initial increase in ChAT activity, indicating promotion of cholinergic differentiation.
- Insulin inhibited GAD activity, suggesting a suppressive effect on GABAergic differentiation.
- Insulin also stimulated choline uptake after a delay, and IGF-1 had no significant effect on either differentiation pathway.
Conclusions:
- Cholinergic differentiation in embryonic chick retinal neurons is initiated by insulin signaling.
- Insulin plays a selective role, promoting cholinergic and inhibiting GABAergic differentiation.
- Both insulin-mediated signals and cell contact are crucial for normal cholinergic differentiation in the developing chick retina.
Abstract:
Previous work [Kyriakis et al., Proc. Natl. Acad. Sci. U.S.A., 84 (1987) 7463-7467] had shown that insulin, when added during a window of binding from embryonic days 9-11, stimulates the normal developmental increase in choline acetyltransferase (ChAT) activity (a marker for cholinergic differentiation) in cultured embryonic chick retinal neurons. Here, we investigated the effect of insulin and IGF 1 on embryonic chick retinal neurons at the stage of development (embryonic day 6) when ChAT activity is first expressed. We investigated insulin peptide effects in retinal tissue developing in vitro as well as in cultures of retinal cells. We show that insulin also stimulated the initial embryonic increase in ChAT activity but had no stimulatory effect on glutamic acid decarboxylase activity (a marker for GABAergic differentiation), an enzyme whose activity also increases developmentally in the same retinal neurons. In fact, insulin inhibited the expression of GAD activity in the retina. The insulin-mediated increase in ChAT activity was independent of normal cell-cell interactions but could not replace them. Insulin also stimulated choline uptake but only after a two day delay, suggesting that the normal program for cholinergic differentiation in the chick retina was induced by insulin. IGF 1 did not have any effect on either cholinergic or GABAergic differentiation. We conclude that cholinergic differentiation in chick embryo retinal neurons is dependent on both insulin- and cell contact-mediated signals.
