Retinal insulin receptors. 1. Structural heterogeneity and functional characterization

Purification of a Src family tyrosine protein kinase from bovine retinas

Since tyrosine phosphorylation appears to play important functions in photoreceptor cells, we searched here for retinal nonreceptor tyrosine kinases of the Src family. We demonstrated that Src family tyrosine kinases were present in the cytosolic fraction of extracted bovine retinas. A Src family tyrosine kinase with an apparent molecular mass of about 62 kDa was purified to homogeneity from the soluble fraction of dark-adapted bovine retinas after three consecutive purification steps: ω-aminooctyl-agarose hydrophobic chromatography, Cibacron blue 3GA-agarose pseudo-affinity chromatography, and α-casein-agarose affinity chromatography. The purified protein was subjected to N-terminal amino acid sequencing and the sequence Gly-Ile-Ile-Lys-Ser-Glu-Glu was obtained, which displayed homology with the first seven residues of the Src family tyrosine kinase c-Yes from Bos taurus (Gly-Cys-Ile-Lys-Ser-Lys-Glu). Although the cytosolic fraction from dark-adapted retinas contained tyrosine kinases of the Src family capable of phosphorylating the α-subunit of transducin, which is the heterotrimeric G protein involved in phototransduction, the purified tyrosine kinase was not capable of using transducin as a substrate. The cellular role of this retinal Src family member remains to be found.

Diabetes reduces basal retinal insulin receptor signaling: reversal with systemic and local insulin

Diabetic retinopathy is characterized by early onset of neuronal cell death. We previously showed that insulin mediates a prosurvival pathway in retinal neurons and that normal retina expresses a highly active basal insulin receptor/Akt signaling pathway that is stable throughout feeding and fasting. Using the streptozotocin-induced diabetic rat model, we tested the hypothesis that diabetes diminishes basal retinal insulin receptor signaling concomitantly with increased diabetes-induced retinal apoptosis. The expression, phosphorylation status, and/or kinase activity of the insulin receptor and downstream signaling proteins were investigated in retinas of age-matched control, diabetic, and insulin-treated diabetic rats. Four weeks of diabetes reduced basal insulin receptor kinase, insulin receptor substrate (IRS)-1/2-associated phosphatidylinositol 3-kinase, and Akt kinase activity without altering insulin receptor or IRS-1/2 expression or tyrosine phosphorylation. After 12 weeks of diabetes, constitutive insulin receptor autophosphorylation and IRS-2 expression were reduced, without changes in p42/p44 mitogen-activated protein kinase or IRS-1. Sustained systemic insulin treatment of diabetic rats prevented loss of insulin receptor and Akt kinase activity, and acute intravitreal insulin administration restored insulin receptor kinase activity. Insulin treatment restored insulin receptor-beta autophosphorylation in rat retinas maintained ex vivo, demonstrating functional receptors and suggesting loss of ligand as a cause for reduced retinal insulin receptor/Akt pathway activity. These results demonstrate that diabetes progressively impairs the constitutive retinal insulin receptor signaling pathway through Akt and suggests that loss of this survival pathway may contribute to the initial stages of diabetic retinopathy.

Effects of insulin-induced acute hypoglycemia and normoglycemic hyperinsulinemia on the retinal uptake and ocular metabolism of glucose in rabbits

Glucose is the principal metabolic substrate for the retina in mammals, being essential for maintaining the functional activity of the retina; it can be supplied to the tissue by both vitreous humor and blood. Yet, the impact of hypoglycemia on retinal glucose metabolism has been poorly investigated. We have therefore studied the effects of acute insulin-induced hypoglycemia on the glucose uptake and metabolism in the retina, by analyzing the hypoglycemia-induced changes in the ocular distribution and metabolic fate of [3H]-2-deoxy-D-glucose (2-DG) and [14C]-D-glucose, both injected in the vitreous body. Rabbits were rendered hypoglycemic by subcutaneous injection of insulin (0.8 and 1.2 IU/kg). Insulin-induced hypoglycemia increased both retinal [3H]-radioactivity levels and retina to vitreous humor ratio of [3H]-radioactivity levels ([3H]-[R/VH]). Radio-chromatography showed that hypoglycemia did not induce any change in the retinal conversion of 2-DG to 2-DG-6-phosphate, but increased the conversion of [14C]-D-glucose to [14C]-lactate. Normoglycemic hyperinsulinemia caused no change in either retinal [3H]-radioactivity levels or [3H]-[R/VH] while decreasing retinal [14C]-radioactivity levels and retina to vitreous ratios of 14C-radioactivity levels. These results indicate that acute hypoglycemia increases the uptake rate of glucose by the retina and suggest that normoglycemic hyperinsulinemia may decrease retinal lactate, possibly stimulating its removal from the retina.

Light regulation of the insulin receptor in the retina

The peptide hormone insulin binds its cognate cell-surface receptors to activate a coordinated biochemical-signaling network and to induce intracellular events. The retina is an integral part of the central nervous system and is known to contain insulin receptors, although their function is unknown. This article, describes recent studies that link the photobleaching of rhodopsin to tyrosine phosphorylation of the insulin receptor and subsequent activation of phosphoinositide 3- kinase (PI3K). We recently found a light-dependent increase in tyrosine phosphorylation of the insulin receptor-beta-subunit (IR beta) and an increase in PI3K enzyme activity in isolated rod outer segments (ROS) and in anti-phosphotyrosine (PY) and anti-IR beta immunoprecipitates of retinal homogenates. The light effect, which was localized to photoreceptor neurons, is independent of insulin secretion. Our results suggest that light induces tyrosine phosphorylation of IR beta in outer-segment membranes, which leads to the binding of p85 through its N-terminal SH2 domain and the generation of PI-3,4,5-P3. We suggest that the physiological role of this process may be to provide neuroprotection of the retina against light damage by activating proteins that protect against stress-induced apoptosis. The studies linking PI3K activation through tyrosine phosphorylation of IR beta now provide physiological relevance for the presence of these receptors in the retina.

Characterization of insulin signaling in rat retina in vivo and ex vivo

Insulin receptor (IR) signaling cascades have been studied in many tissues, but retinal insulin action has received little attention. Retinal IR signaling and activity were investigated in vivo in rats that were freely fed, fasted, or injected with insulin by phosphotyrosine immunoblotting and by measuring kinase activity. A retina explant system was utilized to investigate the IR signaling cascade, and immunohistochemistry was used to determine which retinal cell layers respond to insulin. Basal IR activity in the retina was equivalent to that in brain and significantly greater than that of liver, and it remained constant between freely fed and fasted rats. Furthermore, IR signaling increased in the retina after portal vein administration of supraphysiological doses of insulin. Ex vivo retinas responded to 10 nM insulin with IR beta-subunit (IRbeta) and IR substrate-2 (IRS-2) tyrosine phosphorylation and AktSer473 phosphorylation. The retina expresses mRNA for all three Akt isoforms as determined by in situ hybridization, and insulin specifically increases Akt-1 kinase activity. Phospho-AktSer473 immunoreactivity increases in retinal nuclear cell layers with insulin treatment. These results demonstrate that the retinal IR signaling cascade to Akt-1 possesses constitutive activity, and that exogenous insulin further stimulates this prosurvival pathway. These findings may have implications in understanding normal and dysfunctional retinal physiology.

In vivo regulation of phosphoinositide 3-kinase in retina through light-induced tyrosine phosphorylation of the insulin receptor beta-subunit

Recently, we have shown that phosphoinositide 3-kinase (PI3K) in bovine rod outer segment (ROS) is activated in vitro by tyrosine phosphorylation of the C-terminal tail of the insulin receptor (Rajala, R. V. S., and Anderson, R. E. (2001) Invest. Ophthal. Vis. Sci. 42, 3110-3117). In this study, we have investigated the in vivo mechanism of PI3K activation in the rodent retina and report the novel finding that light stimulates tyrosine phosphorylation of the beta-subunit of the insulin receptor (IRbeta) in ROS membranes, which leads to the association of PI3K enzyme activity with IRbeta. Retinas from light- or dark-adapted mice and rats were homogenized and immunoprecipitated with antibodies against phosphotyrosine, IRbeta, or the p85 regulatory subunit of PI3K, and PI3K activity was measured using PI-4,5-P(2) as substrate. We observed a light-dependent increase in tyrosine phosphorylation of IRbeta and an increase in PI3K enzyme activity in isolated ROS and in anti-phosphotyrosine and anti-IRbeta immunoprecipitates of retinal homogenates. The light effect was localized to photoreceptor neurons and is independent of insulin secretion. Our results suggest that light induces tyrosine phosphorylation of IRbeta in outer segment membranes, which leads to the binding of p85 through its N-terminal Src homology 2 domain and the generation of PI-3,4,5-P(3). We suggest that the physiological role of this process may be to provide neuroprotection of the retina against light damage by activating proteins that protect against stress-induced apoptosis.

The location of insulin receptors in bovine retina and isolated retinal cells

PURPOSE

The binding of insulin to its cell-surface receptor is the sole means by which the hormone influences cellular activity. The location of insulin receptors in bovine retina and on isolated retinal cells was investigated to determine the specific cells sensitive to insulin.

METHODS

Insulin receptors were located in frozen retinal sections prepared from enucleated bovine eyes, with polyclonal anti-insulin receptor antibodies using an immuno-peroxidase method. Isolated cells were obtained by enzymatic and physical dispersion of bovine retinal tissue. Insulin receptors on isolated cells were located by a monoclonal anti-insulin receptor antibody using an immunogold silver staining technique.

RESULTS

Insulin receptors demonstrated a widespread distribution throughout the bovine retina, being present in all retinal layers. A particular association with the plexiform layers and Müller cells was identified in the frozen sections. Consistent with these findings, insulin receptors were predominantly located on dendritic processes of isolated retinal neurones and on Müller cells.

CONCLUSIONS

The widespread distribution of retinal insulin receptors in the bovine retina supports the hypothesis that insulin has a role in regulating retinal activity. Insulin receptors associated with plexiform regions suggests that insulin may influence neural activity, while receptors on Müller cells indicate that insulin may have a role in metabolic or functional mechanisms in bovine retina.

Insulin rescues retinal neurons from apoptosis by a phosphatidylinositol 3-kinase/Akt-mediated mechanism that reduces the activation of caspase-3

The ability of insulin to protect neurons from apoptosis was examined in differentiated R28 cells, a neural cell line derived from the neonatal rat retina. Apoptosis was induced by serum deprivation, and the number of pyknotic cells was counted. p53 and Akt were examined by immunoblotting after serum deprivation and insulin treatment, and caspase-3 activation was examined by immunocytochemistry. Serum deprivation for 24 h caused approximately 20% of R28 cells to undergo apoptosis, detected by both pyknosis and activation of caspase-3. 10 nm insulin maximally reduced the amount of apoptosis with a similar potency as 1.3 nm (10 ng/ml) insulin-like growth factor 1, which acted as a positive control. Insulin induced serine phosphorylation of Akt, through the phosphatidylinositol (PI) 3-kinase pathway. Inhibition of PI 3-kinase with wortmannin or LY294002 blocked the ability of insulin to rescue the cells from apoptosis. SN50, a peptide inhibitor of NF-kappaB nuclear translocation, blocked the rescue effect of insulin, but neither insulin or serum deprivation induced phosphorylation of IkappaB. These results suggest that insulin is a survival factor for retinal neurons by activating the PI 3-kinase/Akt pathway and by reducing caspase-3 activation. The rescue effect of insulin does not appear to be mediated by NF-kappaB or p53. These data suggest that insulin provides trophic support for retinal neurons through a PI 3-kinase/Akt-dependent pathway.

Insulin-like growth factor-I is a potential trophic factor for amacrine cells

In this study we show that insulin-like growth factor (IGF)-I selectively promotes survival and differentiation of amacrine neurons. In cultures lacking this factor, an initial degeneration pathway, selectively affecting amacrine neurons, led to no lamellipodia development and little axon outgrowth. Cell lysis initially affected 50% of amacrine neurons; those remaining underwent apoptosis leading to the death of approximately 95% of them by day 10. Apoptosis was preceded by a marked increase in c-Jun expression. Addition of IGF-I or high concentrations (over 1 microM) of either insulin or IGF-II to the cultures prevented the degeneration of amacrine neurons, stimulated their neurite outgrowth, increased phospho-Akt expression and decreased c-Jun expression. The high insulin and IGF-II concentrations required to protect amacrine cells suggest that these neurons depend on IGF-I for their survival, IGF-II and insulin probably acting through IGF-I receptors to mimic IGF-I effects. Inhibition of phosphatidylinositol-3 kinase (PI 3-kinase) with wortmannin blocked insulin-mediated survival. Wortmannin addition had similar effects to IGF-I deprivation: it prevented neurite outgrowth, increased c-Jun expression and induced apoptosis. These results suggest that IGF-I is essential for the survival and differentiation of amacrine neurons, and activation of PI 3-kinase is involved in the intracellular signaling pathways mediating these effects.

Insulin receptor and insulin receptor substrate-I in rat retinae

Insulin receptor substrate-I (IRS-I) is a major cytosolic substrate of the insulin receptor Expression of insulin receptor and IRS-I, and the distribution of these components of the insulin-signalling pathway, were investigated in rat retinae. Insulin receptor and IRS-I were located in retinal sections with anti-insulin receptor and anti-IRS-I antibodies. Reverse transcription-polymerase chain reaction (RT-PCR) of retinal mRNA was performed with primers specific for insulin receptor and IRS-I gene sequences. Immunohistochemistry demonstrated distinct but closely associated staining patterns for insulin receptor and IRS-I throughout rat retinae. The RT-PCR product from rat retinal insulin receptor mRNA corresponded to the high affinity insulin receptor isoform. The RT-PCR product for retinal IRS-I mRNA agreed with that predicted from the gene sequence. The expression of IRS- I and insulin receptors indicates a signalling mechanism by which insulin can influence retinal metabolism or function.

Characterization of [2-3H]deoxy-D-glucose uptake in retina and retinal pigment epithelium of normal and diabetic rats

The outer blood-retinal barrier which results from the tight junctions between retinal pigment epithelial cells (RPE) restricts the flow of nutrients reaching the retina. We characterize the transport of [2-3H]deoxy-D-glucose (2-DG) across isolated mammalian neural retina and RPE in terms of their kinetics constants. In addition, the effect of insulin on glucose transport was studied by using streptozotocin-induced diabetic rats. RPE accumulates 2-DG by a temperature-sensitive and energy-dependent complex kinetics mechanism. The retina takes up 2-DG by an energy and Na(+)-dependent saturable system with an apparent Km of 2 mM. Insulin induced an increase of 2-DG uptake by normal retina. The retina of diabetic rats shows lower levels of 2-DG accumulation. These levels can be returned to the normal ones by exposure to insulin. Although insulin does not affect, significantly, 2-DG accumulation by RPE, 2-DG uptake of RPE from diabetic rats shows a normal saturable kinetics with an apparent Km of 20 mM. Those findings suggest the presence of different types of glucose transporters in retina and RPE. Insulin-sensitive glucose transport in retina might be involved in the manifestation of diabetic retinopathy.

Insulin in the vitreous of the normal and streptozotocin-induced diabetic rat

Insulin has been detected by ELISA in the vitreous of the normal and streptozotocin-diabetic rat at levels for both about 1% of those in serum. 131I-labeled insulin, administered to conscious rats via an indwelling cannula in the right atrium, was found to cross the blood-ocular barrier into the vitreous. Autoradiographic gel analysis showed the peptide was transferred as an intact molecule. Vitreous insulin levels reflected serum levels as seen in relatively constant vitreous-to-serum insulin ratios over a wide range of serum insulin concentrations. The rate of blood-to-vitreous passage of insulin was about the same in normal as in diabetic rats (fasting serum glucose greater than or equal to 21 mM). At least a portion of vitreous insulin is therefore of pancreatic origin, and retinal tissue in the normal and diabetic animal is thus accessible to circulating hormone. The blood-ocular barrier is unaltered in streptozotocin diabetes with regard to insulin passage.

Evidence for an insulin-like growth factor autocrine-paracrine system in the retinal photoreceptor-pigment epithelial cell complex

The interphotoreceptor matrix (IPM), lying between retinal photoreceptor and pigment epithelial (RPE) cells, contains insulin-like growth factor I (IGF-I) immunoreactivity that co-elutes with authentic human IGF-I in HPLC analyses. Cultured human RPE cells synthesize and release IGF-I, raising the possibility that the RPE serves as a source of IPM IGF-I in vivo. Photoreceptor rod outer segments and cultured monkey RPE cells express specific IGF-I receptors with alpha-subunits of 120 and 138 kDa, respectively. They thus appear to be of the "brain" (in photoreceptors) and "peripheral" (in RPE cells) receptor subtypes. Additionally, the IPM contains high levels of an IGF binding protein (IGF-BP) that specifically binds IGF-I and IGF-II. The IPM-BP is visualized as a single radiographic band by both ligand blot and affinity cross-linking procedures. With enzymes specific for removing N- and O-linked oligosaccharides, the IPM-BP was found to contain O- but not N-linked glycosylated side chains. The distinctive size and glycosylation pattern of the IPM-BP indicate that it is not derived from the vitreous or serum but instead is synthesized locally. The presence of IGF-I and IGF-BP in the IPM, together with the presence of IGF-I receptors on both photoreceptor and RPE cells, suggests the presence of an outer retina autocrine-paracrine system.

Insulin and IGF-I binding in developing chick neural retina and pigment epithelium: a characterization of binding and structural differences

We have characterized insulin and insulin-like growth factor I (IGF-I) binding sites in developing chick retina and pigment epithelium (10- and 14-day embryonic, and 2-week post-hatched). For comparison, binding sites in brain and liver were also examined. Both the retina and pigment epithelium (PE) contain separate, specific, high affinity binding sites for insulin and IGF-I. In both tissues, IGF-I binding exceeds insulin binding by two to threefold. Insulin and IGF-I binding in the retina is four to six times greater than in PE. Insulin and IGF-I binding in the retina and PE exhibit independent developmental regulation. In the retina, the number of binding sites decreases by approximately 50% between embryonic day 10 and 2 weeks post-hatching. In the PE, binding decreases slightly between embryonic day 10 and 14 and then, in the 2-week post-hatched chick, increases threefold. Insulin receptor binding subunits in the retina and brain are similar in that both are neuraminidase insensitive and have apparent molecular weights of 116 kD. In contrast, binding subunits in the PE and liver have higher molecular weights (about 126 kD), and are sensitive to neuraminidase. At the embryonic stages examined, the levels of retinal insulin and IGF-I binding exceed those of the brain by five to 13-fold. Taken together, these data suggest that the retina is a major target of insulin and IGF-I and that the binding of these growth factors is developmentally regulated.

Insulin and insulin-like growth factor receptors in the nervous system

Insulin and the insulin-like growth factors (I and II) are homologous peptides essential to normal metabolism as well as growth. These peptide hormones are present in the brain, and, based on biosynthetic labeling studies as well as evidence for local gene expression, they are synthesized by nervous tissue as well as being taken up by the brain from the peripheral circulation. Furthermore, the presence of insulin and IGF receptors in the brain, on both neuronal and glial cells, also suggests a role for these peptides in the nervous system. Thus, these ligands affect brain electrical activity, either as neurotransmitters or as neuromodulators, altering the release and re-uptake of other neurotransmitters. The insulin and IGF-I and -II receptors found in the brain exhibit a lower molecular weight than corresponding receptors on peripheral tissues, primarily caused by alterations in glycosylation. Despite these alterations, both brain insulin and IGF-I receptors exhibit tyrosine kinase activity in cell-free systems, as do their peripheral counterparts. Brain insulin and IGF-I receptors are developmentally regulated, with the highest levels appearing in fetal or perinatal life. However, the altered glycosylation of brain receptors does not appear until late in fetal development. The receptors are widely distributed in the brain, but especially enriched in the circumventricular organs, choroid plexus, hypothalamus, cerebellum, and olfactory bulb. These studies on the insulin and IGF receptor in brain, add strong support to the suggestion that insulin and IGFs are important neuroactive substances, regulating growth, development, and metabolism in the brain.

IGF-I receptors in the bovine neural retina: structure, kinase activity and comparison with retinal insulin receptors

The retina contains specific high-affinity receptors for insulin-like growth factor-I (IGF-I). Although IGF-I binding was observed in photoreceptor outer segments, the level of this binding was only 10% of that found in whole retina or mixed preparations of rod outer (ROS) and inner (RIS) segments. The higher IGF-I binding activity in RIS and non-photoreceptor regions of the retina suggests these sites as candidates for putative IGF-I action. Data from crosslinking experiments with and without neuraminidase treatment indicate that the binding subunits of the retinal IGF-I receptor exist in two subpopulations (Mr = 121- and 131 kDa), and that the larger of the two subunits has either a greater number or more exposed sialic acid residues. In these characteristics, the retinal IGF-I receptor is similar to the retinal insulin receptor. Retinal IGF-I and insulin receptors possess kinase activity towards their own beta-subunits, a tyrosine containing copolymer, and various molecular forms and subunits of transducin (T alpha-GDP, T alpha-GTP, T beta). The transducin forms are phosphorylated with different efficiencies (e.g. T alpha-GDP is 10-15 times more effective than T alpha-GTP as substrate). These differences are also observed in basal conditions and may reflect differences in transducin subunit affinity for the IGF-I and insulin receptor. In all retinal areas examined, tracer IGF-I binding is 10 to 20-fold higher than insulin binding; however, autophosphorylation levels are approximately equal.

Anomalous insulin-binding activity in the bovine neural retina: a possible mechanism for regulation of receptor binding specificity

Crude membrane from the bovine neural retina contains one IGF-I and two insulin binding sites. Although both insulin binding sites have a high affinity for insulin (IC50 = 0.1 and 7.0 nM), only one exhibits "classical" specificity and binds insulin with higher affinity than IGF-I. The second insulin binding site is "non-classical" in that it has an equal affinity for IGF-I and insulin. Retinal IGF-I binding exceeds insulin binding by a factor of 10-20. Despite this high level of IGF-I binding it is unlikely that non-classical insulin binding represents insulin binding to an IGF-I receptor because 1) anomalous binding is 30 times greater than that predicted from cross-specificity, 2) low concentrations of unlabeled IGF-I increase IGF-I binding to the IGF-I binding site but do not increase IGF-I binding to the non-classical insulin binding site and 3) the IGF-I receptor's affinity for insulin (and IGF-I) increases greatly during receptor purification. In contrast, the insulin affinity of the non-classical insulin binding site is largely unaffected by this process. Although receptor solubilization and purification had no effect on the insulin receptor's affinity for insulin, it did markedly increase this site's affinity for IGF-I. Thus, the major proportion of purified retinal "insulin receptors" have a higher affinity for IGF-I than insulin. The evidence presented here is consistent with the view that the bovine retina contains one IGF-I and two insulin binding sites and that a detergent-sensitive factor regulates IGF-I affinity of both classes of binding sites.

Retinal insulin receptors: localization using a polyclonal anti-insulin receptor antibody

Although retinal insulin receptors have recently been described biochemically, the location of these receptors within the retina is unknown. The study presented here used a polyclonal anti-insulin receptor antibody (B10), immunofluorescence and immunoelectron microscopy to determine the location of insulin receptors in bovine, monkey and human retina. It was found that antibody immunofluorescence formed discrete bands localized predominantly to photoreceptor and neuronal cell bodies. In addition to the strong association with neuronal perikarya, a lower level of antibody binding was observed in photoreceptor outer segments. In human retina, some of the antibody immunofluorescence also had a pattern that suggested B10 binding to glial-like cells.

Retinal insulin receptors. 2. Characterization and insulin-induced tyrosine kinase activity in bovine retinal rod outer segments

Bovine retinal rod outer segments (ROS) possess specific, high-affinity receptors for insulin. These receptors exhibit an insulin-stimulatable tyrosine-specific activity that is capable of phosphorylating the receptor's own beta-subunit and exogenous substrate. ROS insulin receptors exhibit heterogeneity in the apparent molecular weight of the receptor's alpha-subunit. In this regard, insulin receptors from this single cell type resemble insulin receptors obtained from whole retina, but are unlike receptors from brain and liver.