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

Abstract

Insulin receptor (IR) signaling provides a trophic signal for transformed retinal neurons in culture, but the role of IR activity in vivo is unknown. We previously reported that light causes increased tyrosine phosphorylation of the IR in vivo, which leads to the downstream activation of the phosphoinositide 3-kinase and Akt pathway in rod photoreceptor cells. The functional role of IR in rod photoreceptor cells is not known. We observed that light stress induced tyrosine phosphorylation of the IR in rod photoreceptor cells, and we hypothesized that IR activation is neuroprotective. To determine whether IR has a neuroprotective role on rod photoreceptor cells, we used the Cre/lox system to specifically inactivate the IR gene in rod photoreceptors. Rodspecific IR knock-out mice have reduced the phosphoinositide 3-kinase and Akt survival signal in rod photoreceptors. The resultant mice exhibited no detectable phenotype when they were raised in dim cyclic light. However, reduced IR expression in rod photoreceptors significantly decreased retinal function and caused the loss of photoreceptors in mice exposed to bright light stress. These results indicate that reduced expression of IR in rod photoreceptor cells increases their susceptibility to lightinduced photoreceptor degeneration. These data suggest that the IR pathway is important for photoreceptor survival and that activation of the IR may be an essential element of photoreceptor neuroprotection. Insulin receptor (IR)2 signaling provides a trophic signal for transformed retinal neurons in culture (1), but the role of the IR in vivo is unknown. IR activation has been shown to rescue retinal neurons from apoptosis through a phosphoinositide 3-kinase (PI3K) cascade (1). 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). Recent evidence suggests a down-regulation of IR kinase activity in diabetic retinopathy that is associated with the deregulation of downstream signaling molecules (5). Deletion of several downstream effector molecules of the IR signaling pathway, such as IRS-2 (6), Akt2 (7), and Bcl-xl (8), in the retina resulted in a photoreceptor degeneration phenotype. These studies clearly indicate the importance of the IR signaling pathway in the retina.The IR is highly conserved, and the high degree of IR signaling homology between Caenorhabditis elegans, Drosophila, and humans suggests functional conservation in mammalian retina. The IR regulates neuronal survival in C. elegans (9). In Drosophila, the IR serves an important function to guide retinal photoreceptor axons from the retina to the brain during development (10), and the IR influences the size and number of photoreceptors (11). The lack of IR activation leads to neurod...

Phosphoinositide 3-kinase signaling in the vertebrate retina

“…Supporting this hypothesis are the data that show deletion of Akt2, one of the isoforms of Akt, results in stress-induced photoreceptor degeneration ( 135 ). Furthermore, rod-specifi c IR knockout mice have reduced PI3K and Akt recruitment to the membranes, suggesting that the IR regulates the downstream PI3K and Akt survival signal in rod photoreceptors ( 148 ). The IR knockout mice exhibited no detectable phenotype when they were raised in dim cyclic light.…”

“…There are no direct studies available on the disruption of the p110 subunit of PI3K in retinal cells. However, our previous work demonstrated that PI3K activity in photoreceptors is regulated through the IR ( 148 ). Rod-specifi c IR knockout mice have reduced PI3K and Akt association with the membranes, suggesting that the IR regulates the downstream a new mechanism in which the PI3K/Akt cassette functions as a master regulator of cellular metabolism.…”

“…The IR knockout mice exhibited no detectable phenotype when they were raised in dim cyclic light. However, reduced IR expression in rod photoreceptors signifi cantly decreased retinal function and caused the loss of photoreceptors in mice exposed to bright light stress ( 148 ).…”

“…Phosphoinositides are the key molecules for regulation of actin cytoskeletal organization and membrane traffi c from the plasma membrane ( 147 ). In photoreceptors, we previously reported that the IR regulates PI3K and Akt activation ( 49,50,148 ). Further, we recently reported that 3 ′ -phosphoinostides generated through light-induced tyrosine phosphorylation of IR regulates the reorganization of actin cytoskeleton ( 61 ).…”

Insulin Receptor Signaling in Cones

“…The lack of IR activation leads to neurodegeneration in brain/neuron-specific IR knock-out mice (14). We have previously reported that conditional deletion of IR in rod photoreceptor neurons resulted in a stressinduced photoreceptor degeneration (15).…”