G-protein-coupled Receptor Rhodopsin Regulates the Phosphorylation of Retinal Insulin Receptor

Abstract

We have shown previously that phosphoinositide 3-kinase in the retina is activated in vivo through light-induced tyrosine phosphorylation of the insulin receptor (IR). The light effect is localized to photoreceptor neurons and is independent of insulin secretion (Rajala, R. V., McClellan, M. E., Ash, J. D., and Anderson, R. E. (2002) J. Biol. Chem. 277, 43319 -43326). These results suggest that there exists a cross-talk between phototransduction and other signal transduction pathways. In this study, we examined the stage of phototransduction that is coupled to the activation of the IR. We studied IR phosphorylation in mice lacking the rod-specific ␣-subunit of transducin to determine if phototransduction events are required for IR activation. To confirm that light-induced tyrosine phosphorylation of the IR is signaled through bleachable rhodopsin, we examined IR activation in retinas from RPE65 ؊/؊ mice that are deficient in opsin chromophore. We observed that IR phosphorylation requires the photobleaching of rhodopsin but not transducin signaling. To determine whether the light-dependent activation of IR is mediated through the rod or cone transduction pathway, we studied the IR activation in mice lacking opsin, a mouse model of pure cone function. No light-dependent activation of the IR was found in the retinas of these mice. We provide evidence for the existence of a light-mediated IR pathway in the retina that is different from the known insulin-mediated pathway in nonneuronal tissues. These results suggest that IR phosphorylation in rod photoreceptors is signaled through the G-protein-coupled receptor rhodopsin. This is the first study demonstrating that rhodopsin can initiate signaling pathway(s) in addition to its classical phototransduction. Insulin receptor (IR)2 activation has been shown to rescue retinal neurons from apoptosis through a phosphoinositide 3-kinase (PI3K) cascade (1). Particular insights have come from observations that IR substrate-2 (principle substrate of the IR) knock-out mice lose up to 50% of their photoreceptors by 2 weeks of age, due to increased apoptosis (2). Very recently, we have shown that Ak2, another downstream target of the IR, is essential for photoreceptor survival and maintenance, since ablation of this protein in the retina resulted in stress-induced retinal degeneration (3). Specific deletion of bcl-xl (downstream effector of Akt) from rod photoreceptor cells also results in stress-induced retinal degeneration (4). These studies clearly suggest that the IR pathway is important for photoreceptor survival and maintenance.Many retinal degenerative diseases show an early loss of rod cells that is followed by loss of cone cells, and the pathological phenotype for this loss is apoptosis (5-7). The induction of cell death is a highly regulated process and can be suppressed by a variety of extracellular signals (8, 9). The ability of trophic factors to promote survival is mediated, at least in part, by PI3K (1, 10 -12). Activation of PI3K is a critical step in signal transdu...

Loss of Neuroprotective Survival Signal in Mice Lacking Insulin Receptor Gene in Rod Photoreceptor Cells

“…Under the stress conditions, homozygous PTP-1B knock-out mice did not undergo stress-induced photoreceptor degeneration. 3 These studies further provide assurance that Cre has no deleterious effects on photoreceptor structure and function.…”

“…We previously reported that light induces tyrosine phosphorylation of the retinal IR and that this activation leads to the binding of PI3K to rod outer segment (ROS) membranes (2). More recently, we demonstrated that IR activation is mediated through the G-protein-coupled receptor rhodopsin (3). IR signaling is also involved in 17␤-estradiolmediated neuroprotection in the retina (4).…”

Phosphorylation of G Protein-coupled Receptor Kinase 1 (GRK1) Is Regulated by Light but Independent of Phototransduction in Rod Photoreceptors

“…Surprisingly, the Tr␣ Ϫ/Ϫ mice displayed normal, light-dependent dephosphorylation of GRK1, despite maintaining elevated levels of Ca 2ϩ . Previously, Rajala et al (43) reported that phosphorylation of the insulin receptor on tyrosine is high in the light and low in the dark, even in Tr␣ Ϫ/Ϫ mice and speculated that arrestin may activate a signaling pathway that is independent of transducin. However, we found that the light-dependence of GRK1 dephosphorylation in Arr1 Ϫ/Ϫ mice was also normal.…”

Phosphoinositide 3-kinase signaling in the vertebrate retina

“…Class 1A PI3K ␣ has been extensively studied in the retina and photoreceptors ( 14,15,(48)(49)(50)(51)(52). The retinal transducin ␤ ␥ subunits can stimulate G-protein-regulated PI3K, presumably PI3K ␥ or class 1A PI3K ␤ ( 53 ).…”

“…This novel pathway is independent of insulin secretion but dependent on photoreceptor neurons ( 50 ). We subsequently found that this pathway is mediated through the photobleaching of GPCR rhodopsin but is independent of transducin activation ( 49 ). Compared with the liver IR, retinal IR has a higher basal level of autophosphorylation ( 52 ), which is lightdependent ( 50 ).…”