Insulin-like growth factor-I protects axotomized rat retinal ganglion cells from secondary death via PI3-K-dependent akt phosphorylation and inhibition of caspase-3 in vivo

Recently we have shown that the majority of retinal ganglion cells (RGCs) d ies via activation of caspase-3 after transection of the optic nerve (ON) i n the adult rat. In the present study we investigated whether insulin-like growth factor-I (IGF-I), an important factor in retinal development, preven ts secondary death of RGCs after axotomy. Moreover, we studied potential in tracellular mechanisms of IGF-mediated neuroprotection in more detail. Our results indicate that intraocular application of IGF-I protects RGCs from d eath after ON transection in a dose-dependent manner. We show reduced caspa se-3 activity as one possible neuroprotective mechanism of IGF-I treatment in vivo. Caspase-3 mRNA expression remained unchanged. Because caspase inhi bition can be mediated by Akt in vitro, we examined phosphorylation of Akt after axotomy and under IGF treatment. Western blot analysis revealed decre ased Akt phosphorylation after axotomy without treatment and an increased p hosphorylation of Akt under treatment with IGF-I. This strong increase coul d be reduced by simultaneous injection of wortmannin (WM), a potent inhibit or of phosphatidylinositol 3-kinase (PI3-K). To prove the pathway suggested by these experiments as relevant for the in vivo situation, we assessed th e number of RGCs 14 d after ON transection under a combined treatment strat egy of IGF-I and WM. As expected, WM significantly reduced the neuroprotect ive effects of IGF-I. In summary, we show for the first time in vivo that I GF is neuroprotective via PI3-K-dependent Akt phosphorylation and by inhibi tion of caspase-3.