Expression of the Igf–1 Receptor in Developing Photoreceptors of the Mouse Retina and the Importance of Igf–1 Signaling During Photoreceptor Differentiation

Abstract

Purpose: : Previous studies have demonstrated the importance of IGF–1 for proliferation, differentiation and survival of retinal neurons. A recent study demonstrated that IGF–1 signaling is required specifically for rod photoreceptor survival. Our previous work also suggests that IGF–1 promotes rod differentiation of in vitro expanded retinal progenitor cells. However, the precise role of IGF–1 in developing photoreceptors (PRs) is not well defined. The aims of this study are first to characterize the expression of the IGF–1 receptor in the developing mouse retina, specifically developing PRs and second to study the effects of inhibition of molecules in the IGF–1 signaling pathway on photoreceptor differentiation.

Methods: : Expression of the IGF–1 receptor was analyzed in embryonic ages E15, E17 and at postnatal days P1, P5, P10 and adult retinal tissue by standard immunohistochemical techniques. The importance of IGF–1 signaling for appropriate rod PR genesis and differentiation was studied in developing retinal explants. Explants from P0 C57Bl/6 mice were cultured in media containing inhibitors of the MAP kinase and Akt signaling pathways for 7 and 14 days. Explants were either homogenized for protein extraction or dissociated. Protein samples were used to assay expression of rhodopsin and caspase 8. Dissociated cells were plated on chamber slides and labeled with anti–rhodopsin antibody.

Results: : Immunoreactivity (IR) for the IGF–1 receptor (IGF–1R) was observed throughout the developing retina, including presumptive PRs in the outer neuroblastic layer. By P5 relatively intense IGF–1R–IR was observed throughout the developing outer nuclear layer. Further, IGF–1R–IR was localized to rhodopsin positive cells during development and in the mature retina. Treatment of P0 retinal explants with the Akt inhibitor Triciribine decreased the expression of rhodopsin compared to explants cultured without inhibitor. The inhibition of MEK by PD 98059 did not significantly affect the expression of rhodopsin compared with the control.

Conclusions: : Our results demonstrate that IGF–1R is expressed by developing as well as mature PRs. Inhibition of IGF–1 signaling during rod PR differentiation suggests that IGF–1 signaling is important for rod PR differentiation and this affect is mediated through Akt.

Keywords: retinal development • photoreceptors • retinal culture