by Trends Biochem. used a targeted deletion strategy to specifically inactivate the IR gene in rod photoreceptors (10). Reduced IR expression in rod photoreceptors significantly decreased retinal function and caused the loss of photoreceptors in mice exposed to bright light stress (10). IR activation has been shown to rescue retinal neurons from apoptosis through the phosphoinositide 3-kinase (PI3K) cascade (9). We previously reported that light induces tyrosine Benznidazole phosphorylation of the retinal IR and that this activation leads to the binding of PI3K to rod outer segment (ROS) membranes (11). More recently, we exhibited that light-dependent IR activation is usually mediated through the G-protein-coupled receptor rhodopsin (12), the major protein in ROS. IR signaling is also involved in 17?-estradiol-mediated neuroprotection in the retina (13). Recent evidence suggests a down-regulation of IR kinase activity in diabetic retinopathy that is associated with the deregulation of downstream signaling molecules (14). Our laboratory has shown that light-induced activation of the IR leads to the activation of downstream effectors, PI3K and Akt (12,15). Activated Akt phosphorylates and inactivates components Benznidazole of the apoptotic machinery (16-19). There are three isoforms of Akt (20-25) and we found that all three isoforms are expressed in rod photoreceptor cells (26). We also found that physiological light-activated IR results in the activation of Akt1 and Akt3 but not in the activation of the Akt2 isoform (15). Deletion of several downstream effector molecules of the IR signaling pathway, such as IRS-2 (27), Akt2 (26), and bcl-xl (28), in the retina resulted in photoreceptor degeneration. These studies clearly indicate the importance of the IR signaling pathway in the retina. The IR is usually highly conserved with a high degree of IR signaling homology between and humans suggests functional conservation in the mammalian retina. The IR regulates neuronal survival in (29). In the IR serves an important function to guide retinal photoreceptor axons from the retina to the brain during development (30) and the IR influences the size and number of photoreceptors (31). Mutation in either IR autophosphorylation sites (30) or its binding partner Dock (32) in results in a severe photoreceptor axonal misguidance phenotype. In humans, defects in IR signaling in the central nervous system are associated with Alzheimer’s disease (33-35). The lack of IR activation leads to neurodegeneration in brain/neuron-specific IR knock-out mice (36). These studies clearly suggest that the IR pathway is usually important for neuronal survival and maintenance. This article focuses on our recently identified regulation of the IR pathway through the G-protein-coupled receptor rhodopsin. Our studies suggest that rhodopsin photoexcitation may trigger signaling events alternative to the classical transducin activation. Increased IR Phosphorylation and PI3K Enzyme Activity Associated with IRs of Light-adapted Rat Retinas Ghalayini et al (37) previously reported that light stimulates tyrosine phosphorylation of multiple proteins in ROS in vivo. To determine whether light has an effect on PI3K activity and phosphorylation of the IR, rats were dark-adapted overnight, and one-half were subjected to normal room light for 30 min (11). Retinal lysates were immunoprecipitated with anti-PY-99 and anti-IR antibodies. The PI3K activity was higher in retinas from light-adapted rats, compared to those from dark-adapted animals (Fig. 1). These experiments suggested the light-induced activation of PI3K through light-induced tyrosine phosphorylation of IR (11). Open in a separate window Physique 1 PI3K enzyme activity in anti-PY-99 and anti-IR immunoprecipitates from dark- and light-adapted rat retina homogenates. PI3K activity was measured from anti-IR immunoprecipitates of lysates from Benznidazole light- and dark-adapted retinas. PI3K activity was measured using PI-4,5-P2 and [32P]ATP as substrate. The radioactive spots of PI-3,4,5-P3 (A, C) were scraped from Ptgfr TLC plates and counted (B,D). Data are mean SD, n=6, *p 0.05. Reprinted with permission from (11). The catalytic loops within the tyrosine kinase domain name of the IR contain three (Y1158, Y1162 and Y1163) tyrosine motifs (38,39). It is generally believed that autophosphorylation within the activation loop proceeds in a processive manner initiated at the second tyrosine (1162), followed by phosphorylation at the first tyrosine (1158).
