Insulin growth factor 1 receptor/PI3K/AKT survival pathway in outer segment membranes of rod photoreceptors.

Insulin growth factor I receptor (IGF-1R) is involved in the proliferation of many retinal cell types: retinal pigment epithelial cells,1,2 rod3 and cone4 photoreceptor cells, and Muller cells.5 IGF-1R activation protects cultured retinal neurons from apoptosis through a phosphoinositide-3-kinase (PI3K) cascade.6 Mice lacking insulin receptor substrate-2, the principal substrate of IGF-1R, experience photoreceptor degeneration.7 IGF-1R activation has been shown to protect the retinal ganglion cells from death through PI3K-dependent Akt phosphorylation and inhibition of caspase-3 in vivo.8 These studies suggest the importance of the IGF-1R/PI3K/Akt signaling pathway in the retina. PI3K is an important component in the signal transduction cascade initiated by various receptor tyrosine kinase-specific growth factors. This kinase is a heterodimeric protein with a 110-kDa catalytic subunit (p110) and an 85-kDa regulatory adaptor subunit (p85) with two Src homology (SH2) domains9 that interact with the cytosolic portion of the tyrosine kinase domain of the receptor. PI3K catalyzes the phosphorylation of the 3-OH position of inositol head groups of phosphoinositide lipids, namely phosphatidylinositol (PI), phosphatidylinositol-4-phosphate (PI-4-P), and phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2). This results in the formation of PI-3-P, PI-3,4-P2 and PI-3,4,5-P3, respectively. These are collectively termed 3′-phosphoinositide lipids.9 Several studies have highlighted the importance of PI3K-generated phosphoinositides in the regulation of cell proliferation, cellular differentiation, cytoskeletal re-organization, membrane trafficking, glucose transport, survival and metabolism, and the prevention of apoptosis.10 Our previous study shows that bovine rod outer segments (ROS) contain the class 1 p85-p110 enzyme complex, which is significantly active in light-adapted retinas in vitro11 and which can be activated in vivo through light-induced tyrosine phosphorylation of the insulin receptor (IR) in ROS.12 We also demonstrated that photobleaching of rhodopsin regulates the phosphorylation state of IR.13 IR, IGF-1R, and insulin-related receptor (IRR) belong to the same receptor-tyrosine kinase (RTK) family.14,15 These receptors are composed of two extracellular α-subunits containing the ligand-binding domain and two transmembrane β-subunits containing tyrosine kinase activity. IGF-1R and IR share 70% sequence identity overall and 84% identity within the tyrosine kinase catalytic domains. The mechanism for receptor activation is thought to be similar for IR and IGF-1R.16-19 We recently reported that IR activation is neuroprotective because its deletion in rods resulted in stress-induced photoreceptor degeneration.20 This functional phenotype in IR knockout mice is attributed to the reduced PI3K/Akt survival signal in rods.20 Because of the structural similarity between IR and IGF-1R, we hypothesized that IGF-1R activation might also result in the activation of PI3K/Akt survival pathway in rod photoreceptors. To test this hypothesis, we studied the IGF-1R/PI3K/Akt neuron survival pathway in physiological light and light stressed retinas in vivo.