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

Protein tyrosine phosphatase 1B: a novel molecular target for retinal degenerative diseases

Protein tyrosine phosphatase 1B (PTP1B) is considered as a major negative regulator of insulin receptor (IR) signaling. IR signaling in retina has been demonstrated to be neuroprotective. Photoreceptor specific deletion of PTP1B results in enhanced retinal IR-mediated neuroprotection indicating the importance of PTP1B as a negative regulator in the retina. Elevated levels of retinal PTP1B activity has been observed in mice lacking retinal pigment epithelium (Rpe65^-/-), a mouse model of leber congenital amaurosis (LCA-type 2), retinitis pigmentosa and diabetic retinopathy animal models. This enhanced PTP1B activity could down regulate the IR signaling which may contribute to the death of photoreceptor neurons and ultimately lead to retinal degenerations. The potential therapeutic agents that specifically reduce or inhibit the PTP1B activity could be beneficial in protecting or delaying the photoreceptor cell death in the retinal degenerative diseases.

Impact of diabetes on alpha-crystallins and other heat shock proteins in the eye

Diabetes and its related complications represent a major growing health concern and economic burden worldwide. Ocular manifestations of diabetes include cataractogenesis and retinopathy, the latter being the leading cause of blindness in the working-age population. Despite numerous studies and recent progress, the exact pathophysiology of the disease remains to be fully elucidated and development of new and improved therapeutic strategies for this chronic condition are greatly needed. Heat shock proteins (Hsps) are highly conserved families of proteins, which are generally regarded as protective molecules that play a wide variety of roles and can be expressed in response to different types of cellular stresses. In recent years, numerous studies have reported their implication in various ocular diseases including diabetic retinopathy. The present review focuses on the potential implication of Hsps in ocular diabetic complications and discusses their specific mechanisms of regulation with respect to their expression, functions and alteration during diabetes. The review will conclude by examining the potential of Hsps as therapeutic agents or targets for the treatment of diabetic retinopathy.

Role of β-adrenergic receptor regulation of TNF-α and insulin signaling in retinal Muller cells

PURPOSE

The goal of this study was to determine the relationship of TNF-α and the downregulation of insulin receptor signaling in retinal Müller cells cultured under hyperglycemic conditions and the role of β-adrenergic receptors in regulating these responses.

METHODS

Retinal Müller cells were cultured in normal (5 mM) or high (25 mM) glucose until 80% confluent and then were reduced to 2% serum for 18 to 24 hours. The cells were then treated with 10 μM salmeterol followed by Western blot analysis or ELISA. For TNF-α inhibitory studies, the cells were treated with 5 ng/mL of TNF-α for 30 minutes or by a 30-minute pretreatment with TNF-α followed by salmeterol for 6 hours. In the TNF-α short hairpin (sh)RNA experiments, the cells were cultured until 90% confluent, followed by transfection with TNF-α shRNA for 18 hours.

RESULTS

TNF-α-only treatments of Müller cells resulted in significant decreases of tyrosine phosphorylation of the insulin receptor and Akt in high-glucose conditions. Salmeterol (10 μM), a β-2-adrenergic receptor agonist, significantly increased phosphorylation of both insulin receptor and Akt. TNF-α shRNA significantly decreased phosphorylation of IRS-1(Ser307), which was further decreased after salmeterol+TNF-α shRNA. Both TNF-α shRNA and salmeterol significantly reduced death of the retinal Müller cells.

CONCLUSIONS

These studies demonstrate that β-adrenergic receptor agonists in vitro can restore the loss of insulin receptor activity noted in diabetes. By decreasing the levels of TNF-α and decreasing the phosphorylation of IRS-1(Ser307) while increasing tyrosine phosphorylation of insulin receptor, these results suggest a possible mechanism by which restoration of β-adrenergic receptor signaling may protect the retina against diabetes-induced damage.

Differential roles of hyperglycemia and hypoinsulinemia in diabetes induced retinal cell death: evidence for retinal insulin resistance

Diabetes pathology derives from the combination of hyperglycemia and hypoinsulinemia or insulin resistance leading to diabetic complications including diabetic neuropathy, nephropathy and retinopathy. Diabetic retinopathy is characterized by numerous retinal defects affecting the vasculature and the neuro-retina, but the relative contributions of the loss of retinal insulin signaling and hyperglycemia have never been directly compared. In this study we tested the hypothesis that increased retinal insulin signaling and glycemic normalization would exert differential effects on retinal cell survival and retinal physiology during diabetes. We have demonstrated in this study that both subconjunctival insulin administration and systemic glycemic reduction using the sodium-glucose linked transporter inhibitor phloridzin affected the regulation of retinal cell survival in diabetic rats. Both treatments partially restored the retinal insulin signaling without increasing plasma insulin levels. Retinal transcriptomic and histological analysis also clearly demonstrated that local administration of insulin and systemic glycemia normalization use different pathways to counteract the effects of diabetes on the retina. While local insulin primarily affected inflammation-associated pathways, systemic glycemic control affected pathways involved in the regulation of cell signaling and metabolism. These results suggest that hyperglycemia induces resistance to growth factor action in the retina and clearly demonstrate that both restoration of glycemic control and retinal insulin signaling can act through different pathways to both normalize diabetes-induced retinal abnormality and prevent vision loss.

Insulin receptor substrate 2 and FoxO3a signalling are involved in E-cadherin expression and transforming growth factor-β1-induced repression in kidney epithelial cells

Insulin receptor substrate (IRS) proteins comprise a family of adaptor molecules that integrate extracellular signals from insulin and other ligands to intracellular effectors such as phosphoinositide 3-kinase and mitogen-activated protein kinase. The predominant forms of IRS protein in humans, IRS1 and IRS2, are widely expressed. Despite structural similarities, IRS1 and IRS2 display distinct signalling modalities, and mice lacking these proteins present with distinct phenotypes. Transforming growth factor (TGF)-β1 is the primary cytokine shown to induce epithelial-mesenchymal transition. Recent data have demonstrated a role for IRS1 in TGF-β1-induced epithelial-mesenchymal transition in lung epithelial cells. In the present study, we report data showing that TGF-β1 signals via IRS2 in kidney epithelial cells. Small interfering RNA (siRNA)-mediated targeting of IRS2 increased E-cadherin expression, although it did not alter TGF-β1-mediated E-cadherin repression. Phosphorylation of the downstream target of IRS2/Akt signalling, FoxO3a, was induced on Ser253 and, to a lesser extent, on Thr32. Transfection of FoxO3aThr32Ala mutant for 24 h greatly reduced FoxO3a phosphorylation on Ser253 but over-expression of FoxO3a Ser253Ala did not effect Thr32 phosphorylation, suggesting that a distinct order of phosphorylation of FoxO3a is required for physiological function in cells. Transfection of FoxO3a Ser253Ala mutant partially inhibited TGF-β1-mediated E-cadherin repression at 24 h. Taken together, these data highlight novel roles for IRS2 and FoxO3a in the regulation of kidney epithelial cells by E-cadherin.

Phosphoinositide 3-kinase signaling in retinal rod photoreceptors

PURPOSE

Phosphoinositide 3-kinase (PI3K) consists of a p110 catalytic protein and a p85α regulatory protein, required for the stabilization and localization of p110-PI3K activity. The biological significance of PI3K was investigated in vertebrate rod photoreceptors by deleting its regulatory p85α protein and examining its role in photoreceptor structure, function, and protein trafficking.

METHODS

Mice that expressed Cre recombinase in rods were bred to mice with a floxed p85α (pik3r1) regulatory subunit of PI3K to generate a conditional deletion of pik3r1 in rods. Functional and structural changes were determined by ERG and morphometric analysis, respectively. PI3K activity was measured in retinal homogenates immunoprecipitated with an anti-PY antibody. Akt activation was determined by Western blot analysis with a pAkt antibody.

RESULTS

Light-induced stress increased PI3K activity in retinal immunoprecipitates and phosphorylation of Akt. There was no effect of pik3r1 deletion on retinal structure. However, twin flash electroretinography revealed a slight delay in recovery kinetics in pik3r1 knockout (KO) mice compared with wild-type controls. The movement of arrestin in the pik3r1 KO mice was slower than that in the wild-type mouse retinas at 5 minutes of exposure to light. At 10 minutes of exposure, the ROS localization of arrestin was almost identical between the wild-type and pik3r1 KO mice.

CONCLUSIONS

The results provide the first direct evidence that rods use PI3K-generated phosphoinositides for photoreceptor function. The lack of phenotype in pik3r1 KO rod photoreceptors suggests a redundant role in controlling PIP(3) synthesis.

Blue light stress in retinal neuronal (R28) cells is dependent on wavelength range and irradiance

The aim of our study was to elucidate the role of wavelength and irradiance in blue light retinal damage. We investigated the impact of blue light emitted from light-emitting diode (LED) modules with peaks at either 411nm (half bandwidth 17nm) or 470nm (half bandwidth 25nm) at defined irradiances of 0.6, 1.5 and 4.5W/m(2) for 411nm and 4.5W/m(2) for 470nm on retinal neuronal (R28) cells in vitro. We observed a reduction in metabolic activity and transmembrane potential of mitochondria when cells were irradiated at 411nm at higher irradiances. Furthermore, production of mitochondrial superoxide radicals increased significantly when cells were irradiated with 411nm light at 4.5W/m(2) . In addition, such irradiation caused an activation of the antioxidative glutathion system. Using vital staining, flow cytometry and western blotting, we were able to show that apoptosis only took place when cells were exposed to 411nm blue light at higher irradiances; necrosis was not observed. Enhanced caspase-3 cleavage product levels confirmed that this effect was dependent on light irradiance. Significant alterations of the above-mentioned parameters were not observed when cells were irradiated with 471nm light despite a high irradiance of 4.5W/m(2) , indicating that the cytotoxic effect of blue light is highly dependent on wavelength. The observed phenomena in R28 cells at 411nm (4.5W/m(2) ) point to an apoptosis pathway elicited by direct mitochondrial damage and increased oxidative stress. Thus, light of 411nm should act via impairment of mitochondrial function by compromising the metabolic situation of these retinal neuronal cells.

Deletion of the p85alpha regulatory subunit of phosphoinositide 3-kinase in cone photoreceptor cells results in cone photoreceptor degeneration

PURPOSE

Downregulation of the retinal insulin/mTOR pathway in mouse models of retinitis pigmentosa is linked to cone cell death, which can be delayed by systemic administration of insulin. A classic survival kinase linking extracellular trophic/growth factors with intracellular antiapoptotic pathways is phosphoinositide 3-kinase (PI3K), which the authors have shown to protect rod photoreceptors from stress-induced cell death. The role of PI3K in cones was studied by conditional deletion of its p85α regulatory subunit.

METHODS

Mice expressing Cre recombinase in cones were bred to mice with a floxed pi3k gene encoding the p85α regulatory subunit of the PI3K and were back-crossed to ultimately generate offspring with cone-specific p85α knockout (cKO). Cre expression and cone-specific localization were confirmed by Western blot analysis and immunohistochemistry (IHC), respectively. Cone structural integrity was determined by IHC using peanut agglutinin and an M-opsin-specific antibody. Electroretinography (ERG) was used to assess rod and cone photoreceptor function. Retinal structure was examined by light and electron microscopy.

RESULTS

An age-related cone degeneration was found in cKO mice, evidenced by a reduction in photopic ERG amplitudes and loss of cone cells. By 12 months of age, approximately 78% of cones had died, and progressive disorganization of synaptic ultrastructure was noted in surviving cone terminals in cKO retinas. Rod viability was unaffected in p85α cKO mice.

CONCLUSIONS

The present study suggests that PI3K signaling pathway is essential for cone survival in the mouse retina.

The PI3K/Akt/mTOR pathway mediates retinal progenitor cell survival under hypoxic and superoxide stress

Oxygen (O₂) tension has emerged as a major regulator of stem cell (SC) biology. Low O₂ concentrations that are toxic to mature cells can confer advantage to stem and early progenitors, while superoxide stress remains a constant threat in aerobic biology and may be partially avoided through sequestration of SCs in the relatively hypoxic stem or regenerative niche. Using primary retina-derived retinal progenitor cells (RPCs) and the R28 progenitor cell line in vitro, we show that RPCs are sensitive to hydrogen peroxide (H₂O₂) induced damage and resistant to moderate levels of low oxygen stress (1% O₂). Under hypoxic conditions, multipotent RPCs upregulate Epo receptors, and Epo, along with insulin, protects against both superoxide- and severe hypoxia- (0.25% O₂) induced apoptosis through activation of the canonical PI3K/Akt/mTOR pathway. This survival advantage is sensitive to inhibitors of PI3K and mTOR. We further demonstrate phosphorylation of the p70S6 ribosomal kinase, a downstream mediator of PI3K/Akt/mTOR and translational activator. Overall, these data confirm that RPCs are sensitive to superoxide stress and resistant to hypoxia and that this resistance is mediated in part by Epo. They further suggest that manipulation of RPCs ex vivo prior to ocular delivery, or the in vivo delivery of exogenous survival factors at the time of cell implantation, could enhance the success of regenerative therapies aimed to restore sight.

Impaired glucose tolerance plus hyperlipidaemia induced by diet promotes retina microaneurysms in New Zealand rabbits

With the increasing prevalence of diabetes mellitus and metabolic syndrome worldwide, experimental models are required to better understand the pathophysiology and therapeutic approaches to preserve pancreatic beta cells, attenuate atherosclerosis and protect target organs. The aims of this study were to develop an experimental model of impaired glucose tolerance combined with hypercholesterolaemia induced by diet and assess metabolic alterations and target organ lesions. New Zealand male rabbits were fed high-fat/high-sucrose (10/40%) and cholesterol-enriched diet for 24 weeks, when they were sacrificed. Biochemistry, fundus photographs with fluorescein angiography and pathological analyses were performed. Cholesterol-fed and normal animals of same age were compared.

RESULTS

The animals with diet-induced impaired glucose tolerance combined with hypercholesterolaemia gained weight, increased blood glucose, total cholesterol, LDL-C and triglycerides and decreased HDL-C (P < 0.05 vs. baseline). Fructosamine levels and the homeostasis model assessment of insulin resistance (HOMA-IR) index were increased, while there was a reduction in the HOMA-β (P < 0.05 for all vs. baseline). Histomorphologic findings of this model were aortic atherosclerosis, hepatic steatofibrosis and glomerular macrophage infiltration. Early clinical features of diabetic retinopathy with hyperfluorescent dots consistent with presence of retina microaneurysms were seen since week 12, progressing up to the end of the experiment (P < 0.0005 vs. baseline and 12 weeks). Our model reproduced several metabolic characteristics of human diabetes mellitus and promoted early signs of retinopathy. This non-expensive model is suitable for studying mechanistic pathways and allowing novel strategic approaches.

Insulin signaling in retinal neurons is regulated within cholesterol-enriched membrane microdomains

Neuronal cell death is an early pathological feature of diabetic retinopathy. We showed previously that insulin receptor signaling is diminished in retinas of animal models of diabetes and that downstream Akt signaling is involved in insulin-mediated retinal neuronal survival. Therefore, further understanding of the mechanisms by which retinal insulin receptor signaling is regulated could have therapeutic implications for neuronal cell death in diabetes. Here, we investigate the role of cholesterol-enriched membrane microdomains to regulate PKC-mediated inhibition of Akt-dependent insulin signaling in R28 retinal neurons. We demonstrate that PKC activation with either a phorbol ester or exogenous application of diacylglycerides impairs insulin-induced Akt activation, whereas PKC inhibition augments insulin-induced Akt activation. To investigate the mechanism by which PKC impairs insulin-stimulated Akt activity, we assessed various upstream mediators of Akt signaling. PKC activation did not alter the tyrosine phosphorylation of the insulin receptor or IRS-2. Additionally, PKC activation did not impair phosphatidylinositol 3-kinase activity, phosphoinositide-dependent kinase phosphorylation, lipid phosphatase (PTEN), or protein phosphatase 2A activities. Thus, we next investigated a biophysical mechanism by which insulin signaling could be disrupted and found that disruption of lipid microdomains via cholesterol depletion blocks insulin-induced Akt activation and reduces insulin receptor tyrosine phosphorylation. We also demonstrated that insulin localizes phosphorylated Akt to lipid microdomains and that PMA reduces phosphorylated Akt. In addition, PMA localizes and recruits PKC isotypes to these cholesterol-enriched microdomains. Taken together, these results demonstrate that both insulin-stimulated Akt signaling and PKC-induced inhibition of Akt signaling depend on cholesterol-enriched membrane microdomains, thus suggesting a putative biophysical mechanism underlying insulin resistance in diabetic retinopathy.

Investigating the establishment of primary cell culture from different abalone (Haliotis midae) tissues

The abalone, Haliotis midae, is the most valuable commodity in South African aquaculture. The increasing demand for marine shellfish has stimulated research on the biology and physiology of target species in order to improve knowledge on growth, nutritional requirements and pathogen identification. The slow growth rate and long generation time of abalone restrict efficient design of in vivo experiments. Therefore, in vitro systems present an attractive alternative for short term experimentation. The use of marine invertebrate cell cultures as a standardised and controlled system to study growth, endocrinology and disease contributes to the understanding of the biology of economically important molluscs. This paper investigates the suitability of two different H. midae tissues, larval and haemocyte, for establishing primary cell cultures. Cell cultures are assessed in terms of culture initiation, cell yield, longevity and susceptibility to contamination. Haliotis midae haemocytes are shown to be a more feasible tissue for primary cell culture as it could be maintained without contamination more readily than larval cell cultures. The usefulness of short term primary haemocyte cultures is demonstrated here with a growth factor trial. Haemocyte cultures can furthermore be used to relate phenotypic changes at the cellular level to changes in gene expression at the molecular level.

Application of isoproterenol inhibits diabetic-like changes in the rat retina

Diabetic retinopathy is the leading cause of blindness to working-age adults. We have recently shown that surgical removal or genetic manipulations to eliminate sympathetic neurotransmission produces many of the retinal changes similar to rodent diabetic retinopathy with normal glucose levels. We hypothesized that application of a beta-adrenergic receptor agonist, isoproterenol, could reach the retina to elicit normal cellular signaling and inhibit the functional and morphological markers of early stage diabetic retinopathy in the rat. Rats were made diabetic by injection of 60 mg/kg streptozotocin. Within 3 days of diabetes-induction, rats were placed into 1 of 3 groups (control, diabetes, or diabetic + isoproterenol). Dose and time course studies were done for isoproterenol using a PKA ELISA and CREB analyses. Once the optimal dose and time course were established, electrical activity of the retina was analyzed by electroretinogram each month for the 8-month study. Western blotting was done for insulin receptor signaling and Akt and ELISA analyses for TNFalpha concentration and cleavage of caspase 3 at 2- and 8-months of diabetes. Diabetes-induced degeneration of neural cells and retinal thickness were assessed at 2 months, while degenerate capillaries were quantitated at 8 months of treatment. Daily application of 50 mM isoproterenol was effective in inhibiting the diabetes-induced loss of a- and b-wave and oscillatory potential amplitudes in the electroretinogram. Isoproterenol blocked the increase in TNFalpha and apoptosis in the diabetic retina. The numbers of degenerate capillaries were also reduced in the treated + diabetes retina. These data strongly suggest that loss of beta-adrenergic receptor signaling may be a key factors in early stage diabetic retinopathy. Resolution of this loss of adrenergic receptor signaling can inhibit some of the hallmarks of diabetic retinopathy in the retina.

Rhodopsin-regulated insulin receptor signaling pathway in rod photoreceptor neurons

The retina is an integral part of the central nervous system and retinal cells are known to express insulin receptors (IR), although their function is not known. This article describes recent studies that link the photoactivation of rhodopsin to tyrosine phosphorylation of the IR and subsequent activation of phosphoinositide 3-kinase, a neuron survival factor. Our studies suggest that the physiological role of this process is to provide neuroprotection of the retina against light damage by activating proteins that protect against stress-induced apoptosis. We focus mainly on our recently identified regulation of the IR pathway through the G-protein-coupled receptor rhodopsin. Various mutant and knockout proteins of phototransduction cascade have been used to study the light-induced activation of the retinal IR. Our studies suggest that rhodopsin may have additional previously uncharacterized signaling functions in photoreceptors.

Phosphoinositide 3-kinase signaling in the vertebrate retina

The phosphoinositide (PI) cycle, discovered over 50 years ago by Mabel and Lowell Hokin, describes a series of biochemical reactions that occur on the inner leaflet of the plasma membrane of cells in response to receptor activation by extracellular stimuli. Studies from our laboratory have shown that the retina and rod outer segments (ROSs) have active PI metabolism. Biochemical studies revealed that the ROSs contain the enzymes necessary for phosphorylation of phosphoinositides. We showed that light stimulates various components of the PI cycle in the vertebrate ROS, including diacylglycerol kinase, PI synthetase, phosphatidylinositol phosphate kinase, phospholipase C, and phosphoinositide 3-kinase (PI3K). This article describes recent studies on the PI3K-generated PI lipid second messengers in the control and regulation of PI-binding proteins in the vertebrate retina.

Deletion of tau attenuates heat shock-induced injury in cultured cortical neurons

The microtubule-associated protein tau has been implicated in beta-amyloid- and glutamate-induced neurotoxicity. However, the potential role of tau in response to other insults to neurons remains unclear. In this study, we examined whether deletion of tau would change cell injury induced by heat shock in primary cultures of cortical neurons. After 30 min of a 45 degrees C heat shock, lactate dehydrogenase (LDH) release increased, reaching a peak at 6 hr in wild-type (WT) neurons. A significantly lower LDH release, with a peak delayed by 24 hr, was detected in tau knockout (TKO) neurons. After heat shock treatment, MAP-2 and tubulin staining of the processes of WT neurons revealed more dramatic abnormalities than in TKO neurons. Both WT and TKO neurons exhibited a similar elevation of HSP70 level but different time courses of Akt phosphorylation. In contrast to an early, brief response in WT neurons, TKO neurons displayed a late, but long-lasting increase in phosphorylation of Akt and its downstream target, glycogen synthase kinase 3beta. Additionally, inhibition of Akt activity aggravated the cell morbidity caused by heat shock exposure in both WT and TKO neurons, indicating a protective role of Akt against cell injury. In conclusion, our results demonstrate that deletion of tau attenuated heat shock-induced neuronal injury. Enhanced Akt response in the absence of endogenous tau is suggested to represent a compensatory mechanism for regulating cell reactions to stress stimuli.

Serine/threonine kinase akt activation regulates the activity of retinal serine/threonine phosphatases, PHLPP and PHLPPL

In our previous studies, we have shown that insulin receptor (IR) activation leads to the activation of phosphoinositide 3-kinase (PI3K) and Akt activation in rod photoreceptors. This pathway is functionally important for photoreceptor survival as deletion of IR and one of the isoforms of Akt (Akt2) resulted in stress-induced photoreceptor degeneration. However, the molecular mechanism of this degeneration is not known. Akt signaling is known to be regulated by the serine/threonine phosphatases, PH domain and leucine-rich repeat protein phosphatases (PHLPP) and PHLPP-like (PHLPPL). In this study, we characterized these two phosphatases in the retina and examined the role of IR, PI3K, and Akt signaling on the activity of PHLPP and PHLPPL. Most of the studies published on PHLPP and PHLPPL are directed toward Akt dephosphorylation; however, there are no studies available to date on how the enzyme activities of these phosphatases are regulated. We made a novel finding in this study that both PHLPP and PHLPPL activities were significantly decreased in the presence of insulin ex vivo. The insulin-induced decrease of phosphatase activities were PI3K-dependent as pre-treatment of ex vivo retinal cultures with LY294002 significantly reversed the insulin-induced inhibition. It has been shown previously that PHLPP and PHLPPL regulate the dephosphorylation of Akt isoforms, and our results demonstrate for the first time that retinal PHLPP and PHLPPL activities are under the control of the IR-activated PI3K/Akt pathway.

Enhanced retinal insulin receptor-activated neuroprotective survival signal in mice lacking the protein-tyrosine phosphatase-1B gene

Protein-tyrosine phosphatase 1B (PTP1B) has been implicated in the negative regulation of insulin signaling. We previously demonstrated that light-induced tyrosine phosphorylation of the retinal insulin receptor (IR) results in the activation of phosphoinositide 3-kinase/Akt survival pathway in rod photoreceptor cells. The molecular mechanism behind light-induced activation of IR is not known. We investigated the in vivo mechanism of IR activation and found that PTP1B activity in dark-adapted retinas was significantly higher than in light-adapted retinas. We made a novel finding in this study that the light-dependent regulation of PTP1B activity is signaled through photobleaching of rhodopsin. Conditional deletion of PTP1B in rod photoreceptors by the Cre-loxP system resulted in enhanced IR signaling. Further PTP1B activity negatively regulated the neuroprotective survival signaling in the retina. One of the challenging questions in the retina research is how mutations in human rhodopsin gene slowly disable and eventually disrupt photoreceptor functions. Our studies suggest that a defect in the photobleaching of rhodopsin and mutation in rhodopsin gene enhances the activity of PTP1B, and this activated activity could down-regulate the IR survival signaling. Our studies suggest that PTP1B antagonists could be potential therapeutic agents to treat stress-induced photoreceptor degenerations and provide further evidence that rhodopsin photoexcitation may trigger signaling events alternative to the classic phototransduction.

Protective role of pigment epithelium-derived factor (PEDF) in early phase of experimental diabetic retinopathy

BACKGROUND

Pigment epithelium-derived factor (PEDF) is the most potent inhibitor of angiogenesis in the mammalian eye, thus suggesting that PEDF may protect against proliferative diabetic retinopathy. However, a role for PEDF in early diabetic retinopathy remains to be elucidated. We investigated here whether and how PEDF could prevent the development of diabetic retinopathy.

METHODS

Streptozotocin-induced diabetic rats were treated with or without intravenous injection of PEDF for 4 weeks. Early neuronal derangements were evaluated by electroretinogram (ERG) and immunofluorescent staining of glial fibrillary acidic protein (GFAP). Expression of PEDF and 8-hydroxydeoxyguanosine (8-OHdG), a marker of oxidative stress, was localized by immunofluorescence. Vascular endothelial growth factor (VEGF) and p22phox expression were evaluated with western blots. Breakdown of blood retinal barrier (BRB) was quantified with fluorescein isothiocynate (FITC)-conjugated dextran. NADPH oxidase activity was measured with lucigenin luminescence.

RESULTS

Retinal PEDF levels were reduced, and amplitudes of a- and b-wave in the ERG were decreased in diabetic rats, which were in parallel with GFAP overexpression in the Müller cells. Further, retinal 8-OHdG, p22phox and VEGF levels and NADPH oxidase activity were increased, and BRB was broken in diabetic rats. Administration of PEDF ameliorated all of the characteristic changes in early diabetic retinopathy.

CONCLUSIONS

Results suggest that PEDF could prevent neuronal derangements and vascular hyperpermeability in early diabetic retinopathy via inhibition of NADPH oxidase-driven oxidative stress generation. Substitution of PEDF may offer a promising strategy for halting the development of diabetic retinopathy.

Growth factor receptor-bound protein 14 undergoes light-dependent intracellular translocation in rod photoreceptors: functional role in retinal insulin receptor activation

Growth factor receptor-bound protein 14 (Grb14) is involved in growth factor receptor tyrosine kinase signaling. Here we report that light causes a major redistribution of Grb14 among the individual subcellular compartments of the retinal rod photoreceptor. Grb14 is localized predominantly to the inner segment, nuclear layer, and synapse in dark-adapted rods, whereas in the light-adapted rods, Grb14 redistributed throughout the entire cell, including the outer segment. The translocation of Grb14 requires photoactivation of rhodopsin, but not signaling through the phototransduction cascade, and is not based on direct Grb14-rhodopsin interactions. We previously hypothesized that Grb14 protects light-dependent insulin receptor (IR) activation in rod photoreceptors against dephosphorylation by protein tyrosine phosphatase 1B. Consistent with this hypothesis, we failed to observe light-dependent IR activation in Grb14(-/-) mouse retinas. Our studies suggest that Grb14 translocates to photoreceptor outer segments after photobleaching of rhodopsin and protects IR phosphorylation in rod photoreceptor cells. These results demonstrate that Grb14 can undergo subcellular redistribution upon illumination and suggest that rhodopsin photoexcitation may trigger signaling events alternative to the classical transducin activation.

Cytoskeletal components enhance the autophosphorylation of retinal insulin receptor

Insulin receptor (IR) signaling provides a trophic signal for transformed retinal neurons in culture, and we recently reported that deletion of IR from rod photoreceptors resulted in stress-induced photoreceptor degeneration. Retinal insulin receptor has a high basal level autophosphorylation compared to liver and the reasons for higher autophosphorylation are not known. In the current study we report a novel finding that cytoplasmic actin associates with and activates the retinal IR in vivo. Similar to insulin, actin also induced autophosphorylation at tyrosines 1158, 1162 and 1163 in the catalytic loop of IR. Our studies also suggest that globular actin activates the retinal IR more effectively than does filamentous actin. Retinal IR kinase activity has been shown to decrease in hyperglycemia and we found a decreased binding of actin to the IR under hyperglycemia. This is the first study which demonstrates that cytoplasmic actin regulates autophosphorylation of the retinal IR.

Unoprostone isopropyl rescues retinal progenitor cells from apoptosisin vitro

PURPOSE

Unoprostone isopropyl is an ocular hypotensive that was originally produced as a prostaglandin F2alpha analogue and is eventually recognized as a synthetic docosanoid. The compound is recently suggested to have potent neuroprotective ability in the retina. The purpose of this study is to test whether and how the biologically active metabolites of unoprostone isopropyl rescue retinal neuro-glial progenitor cells from apoptosis.

METHODS

R28 cells were deprived of serum for 24 hr with or without varying concentrations of unoprostone metabolite M1 or M2 or vehicle in the presence or absence of specific inhibitors against several types of signal transduction proteins. Immunocytochemistry against activated caspase-3 with Hoechst nuclear staining was performed.

RESULTS

Up to 15%of R28 cells became pyknotic and activated caspase-3 immunoreactive after 24-hr serum withdrawal. M1, but not M2, significantly reduced apoptotic cells in a dose-dependent fashion with a maximal effect at 100 microM (p < .0001). LY294002, the phosphatidylinositol 3-OH kinase (PI3K) inhibitor, and KT5823, the protein kinase G (PKG) inhibitor, reversed the antiapoptotic effect of M1.

CONCLUSIONS

The unoprostone metabolite M1 protects retinal neuro-glial progenitor R28 cells from apoptosis induced by serum deprivation via the PI3K and PKG pathways.

Insulin receptor signaling regulates actin cytoskeletal organization in developing photoreceptors

The insulin receptor (IR) and IR signaling proteins are widely distributed throughout the CNS. IR signaling provides a trophic signal for transformed retinal neurons in culture and we recently reported that deletion of IR in rod photoreceptors by Cre/lox system resulted in stress-induced photoreceptor degeneration. These studies suggest a neuroprotective role of IR in rod photoreceptor cell function. However, there are no studies available on the role of insulin-induced IR signaling in the development of normal photoreceptors. To examine the role of insulin-induced IR signaling, we analyzed cultured neuronal cells isolated from newborn rodent retinas. In insulin-lacking cultures, photoreceptors from wild-type rat retinas exhibited an abnormal morphology with a wide axon cone and disorganization of the actin and tubulin cytoskeleton. Photoreceptors from IR knockout mouse retinas also exhibited a similar abnormal morphology. A novel finding in this study was that addition of docosahexaenoic acid, a photoreceptor trophic factor, restored normal axonal outgrowth in insulin-lacking cultures. These data suggest that IR signaling pathways regulate actin and tubulin cytoskeletal organization in photoreceptors; they also imply that insulin and docosahexaenoic acid activate at least partially overlapping signaling pathways that are essential for the development of normal photoreceptors.

Wnt14 inhibits death of retinal precursor cells

An earlier proteomics study from our laboratory showed that Wnt14, a member of the Wnt family that regulates the development of vertebrates, was one of the proteins expressed transiently during the development of the chick retina. The purpose of this study was to determine in more detail the changes in the expression of Wnt14 during the development of the chick retina, and to investigate the biological function of Wnt14. Endogenous Wnt14 is located in the retinal ganglion cell layer, and is expressed in the chick retina on embryonic days (ED) 7, ED11, and ED15. The level of Wnt14 is transiently decreased on ED11. In vitro analysis showed that an over-expression of Wnt14 reduced the activation of caspase-3 and inhibited the death of R28 cells induced by serum deprivation or exposure to glutamate. An interferon-induced protein was identified as the protein that was bound to Wnt14. These results suggest that a stable expression of Wnt14 inhibits cell death by inactivating caspase-3 in the developing retina.

Selective loss of inner retinal layer thickness in type 1 diabetic patients with minimal diabetic retinopathy

PURPOSE

To determine whether type 1 diabetes preferentially affects the inner retinal layers by comparing the thickness of six retinal layers in type 1 diabetic patients who have no or minimal diabetic retinopathy (DR) with those of age- and sex-matched healthy controls.

METHODS

Fifty-seven patients with type 1 diabetes with no (n = 32) or minimal (n = 25) DR underwent full ophthalmic examination, stereoscopic fundus photography, and optical coherence tomography (OCT). After automated segmentation of intraretinal layers of the OCT images, mean thickness was calculated for six layers of the retina in the fovea, the pericentral area, and the peripheral area of the central macula and were compared with those of an age- and sex-matched control group.

RESULTS

In patients with minimal DR, the mean ganglion cell/inner plexiform layer was 2.7 microm thinner (95% confidence interval [CI], 2.1-4.3 microm) and the mean inner nuclear layer was 1.1 microm thinner (95% CI, 0.1-2.1 microm) in the pericentral area of the central macula compared to those of age-matched controls. In the peripheral area, the mean ganglion cell/inner plexiform layer remained significantly thinner. No other layers showed a significant difference.

CONCLUSIONS

Thinning of the total retina in type 1 diabetic patients with minimal retinopathy compared with healthy controls is attributed to a selective thinning of inner retinal layers and supports the concept that early DR includes a neurodegenerative component.

Regulation of forkhead transcription factor FoxO3a contributes to calorie restriction-induced prevention of Alzheimer's disease-type amyloid neuropathology and spatial memory deterioration

Forkhead transcription factor FoxO3a, also known as DAF-16 in Caenorhabditis elegans, is a key regulator of the insulin receptor (IR)/insulin-like growth factor-I signaling pathway mediated extension of life span in worms and yeast. In this study, we report that calorie restriction (CR)-mediated activation of the IR signaling pathway leads to hyperphosphorylation of FoxO3a transcription factor and, consequently, its exclusion from the nucleus. This inactivation of FoxO3a activity is correlated with attenuation of Alzheimer's disease (AD)-type amyloid neuropathology and with preservation of spatial reference memory in the Tg2576 mouse model of AD. Further, in vitro studies reveal that exogenous expression of viral, triple-mutant, constitutively active FoxO3a resulting in increased nuclear FoxO3a activity in primary neuron cultures derived from Tg2576 mouse embryos, causally promotes AD amyloid-beta peptide (Abeta) levels by inhibiting nonamyloidogenic alpha-secretase activity, indicating the existence of an inverse correlation between FoxO3a activity and cerebral Abeta amyloidosis. Moreover, we report for the first time that the exclusion of the FoxO3a transcription factor from the nucleus in combination with inhibition of nuclear FoxO3a activity by SIRT1-mediated deacetylation in response to CR is a mechanism resulting in the repression of Rho-associated protein kinase-1 gene expression, thereby activating nonamyloidogenic alpha-secretase processing of the amyloid precursor protein and lowering Abeta generation. This study provides a novel metabolic pathway for prevention and/or treatment of AD.

Retinal insulin receptor signaling in hyperosmotic stress

In the diabetic eye, the increased accumulation of sorbitol in the retina has been implicated in the pathogenesis of diabetic retinopathy (DR). Neurodegeneration is an important component of DR as demonstrated by increased neural apoptosis in the retina during experimental and human diabetes. Insulin receptor (IR) activation has been shown to rescue retinal neurons from apoptosis through a phosphoinositide 3-kinase and protein kinase B (Akt) survival cascade. In this study, we examined the IR signaling in sorbitol-induced hyperosmotic stressed retinas.

Diabetes reduces autophosphorylation of retinal insulin receptor and increases protein-tyrosine phosphatase-1B activity

PURPOSE

Protein-tyrosine phosphatase-1B (PTP1B) has been implicated in the negative regulation of insulin signaling. The expression, activity, and functional role of PTP1B in the retina are unknown. In this study, the authors examined the relationship between the retinal insulin receptor (IR) and PTP1B in normal and diabetic mouse retinas.

METHODS

IR and PTP1B localization was examined by immunohistochemistry. The activation of IR was analyzed using specific antibodies against phosphotyrosine. PTP1B activity was determined in anti-PTP1B immunoprecipitates. Glutathione-S-transferase fusion proteins containing wild-type and catalytically inactive mutant PTP1B was used to study the interaction between IR and PTP1B. Anti-IR immunoprecipitates and the cytoplasmic domain of purified IR were incubated in the presence of ATP, and the autophosphorylation of IR with antiphosphotyrosine antibody was analyzed.

RESULTS

Immunohistochemical analysis of PTP1B shows that it is predominantly expressed in nonphotoreceptor layers of the retina, though it is clearly expressed in the inner segments of the rod photoreceptors. The IR is predominately expressed in rod inner segments. Biochemical analysis of rod outer segments indicates the presence of IR and PTP1B. Retinal IR exhibits a high level of basal autophosphorylation, and this autophosphorylation is reduced in diabetic mouse retinas. In vitro, PTP1B is able to dephosphorylate the autophosphorylated IR. Substrate mutant-trap results indicate a stable interaction between IR and PTP1B. Further, PTP1B activity was increased in diabetic mouse retinas.

CONCLUSIONS

These studies indicate that diabetes reduces the autophosphorylation of retinal IR and increased PTP1B activity. Further, PTP1B regulates the state of IR phosphorylation in the retina.

Activation and membrane binding of retinal protein kinase Balpha/Akt1 is regulated through light-dependent generation of phosphoinositides

Akt is a phospholipid-binding protein and the downstream effector of the phosphoinositide 3-kinase (PI3K) pathway. Akt has three isoforms: Akt1, Akt2, and Akt3. All of these isoforms are expressed in rod photoreceptor cells, but the individual functions of each isoform are not known. In this study, we found that light induces the activation of Akt1. The membrane binding of Akt1 to rod outer segments (ROS) is insulin receptor (IR)/PI3K-dependent as demonstrated by reduced binding of Akt1 to ROS membranes of photoreceptor-specific IR knockout mice. Membrane binding of Akt1 is mediated through its Pleckstrin homology (PH) domain. To determine whether binding of the PH domain of Akt1 to photoreceptor membranes is regulated by light, various green fluorescent protein (GFP)/Akt1-PH domain fusion proteins were expressed in rod photoreceptors of transgenic Xenopus laevis under the control of the Xenopus opsin promoter. The R25C mutant PH domain of Akt1, which does not bind phosphoinositides, failed to associate with plasma membranes in a light-dependent manner. This study suggests that light-dependent generation of phosphoinositides regulates the activation and membrane binding of Akt1 in vivo. Our results also suggest that actin cytoskeletal organization may be regulated through light-dependent generation of phosphoinositides.

Nipradilol protects rat retinal ganglion cells from apoptosis induced by serum deprivation in vitro and by diabetes in vivo

PURPOSE

To investigate if nipradilol has an anti-apoptotic effect in serum-deprived RGC-5 cells and in the streptozotocin-induced diabetic rat retina.

METHODS

Apoptosis was quantified by activated caspase-3 immunohistochemistry or terminal dUTP nick end-labeling assay.

RESULTS

Nipradilol dose-dependently suppressed apoptosis in a protein kinase A- and G-dependent manner and counteracted glutamate-induced calcium entry in the RGC-5 cells and reduced apoptotic cells in the retinal ganglion cell layer of 4- and 12-week diabetic retinas compared to controls when instilled for 5 days. Removal of the nitric oxide moiety from nipradilol blocked these effects.

CONCLUSIONS

Nipradilol protects RGCs from apoptosis induced by serum-deprivation in vitro and by diabetes in vivo. The NO-related signaling pathway mediates the anti-apoptotic ability of nipradilol.

Phospho-Site-Specific Antibody Microarray to Study the State of Protein Phosphorylation in the Retina

Neurodegeneration is an important component of diabetic retinopathy as demonstrated by increased neural apoptosis in the retina during experimental and human diabetes. Accumulation of sorbitol and fructose and the generation or enhancement of oxidative stress has been reported in the whole retina of diabetic animals. Aldose reductase (AR), the first and the rate limiting enzyme in the pathway reduces glucose to sorbitol and the diabetic complications are prevented by drugs that inhibit AR. In this study we examined the phosphorylation state of various retinal proteins in response to sorbitol-treatment by phosphor-site-specific antibody microarray. Our results suggest that various retinal protein kinases and cytoskeletal proteins either activated or down regulated in response to sorbitol treatment. Further, our study also indicates the activation of retinal insulin- and insulin growth factor 1 receptor and their downstream signaling proteins such as phosphoinositide 3-kinanse and protein kinase B (Akt). Understanding the regulation of retinal proteins involved in polyol (sorbitol) pathway would help to design therapeutic agents for the treatment of diabetic retinopathy.

Autophagy induction and autophagosome clearance in neurons: relationship to autophagic pathology in Alzheimer's disease

Macroautophagy, a major pathway for organelle and protein turnover, has been implicated in the neurodegeneration of Alzheimer's disease (AD). The basis for the profuse accumulation of autophagic vacuoles (AVs) in affected neurons of the AD brain, however, is unknown. In this study, we show that constitutive macroautophagy in primary cortical neurons is highly efficient, because newly formed autophagosomes are rapidly cleared by fusion with lysosomes, accounting for their scarcity in the healthy brain. Even after macroautophagy is strongly induced by suppressing mTOR (mammalian target of rapamycin) kinase activity with rapamycin or nutrient deprivation, active cathepsin-positive autolysosomes rather than LC3-II-positive autophagosomes predominate, implying efficient autophagosome clearance in healthy neurons. In contrast, selectively impeding late steps in macroautophagy by inhibiting cathepsin-mediated proteolysis within autolysosomes with cysteine- and aspartyl-protease inhibitors caused a marked accumulation of electron-dense double-membrane-limited AVs, containing cathepsin D and incompletely degraded LC3-II in perikarya and neurites. Similar structures accumulated in large numbers when fusion of autophagosomes with lysosomes was slowed by disrupting their transport on microtubules with vinblastine. Finally, we find that the autophagic vacuoles accumulating after protease inhibition or prolonged vinblastine treatment strongly resembled AVs that collect in dystrophic neurites in the AD brain and in an AD mouse model. We conclude that macroautophagy is constitutively active and highly efficient in healthy neurons and that the autophagic pathology observed in AD most likely arises from impaired clearance of AVs rather than strong autophagy induction alone. Therapeutic modulation of autophagy in AD may, therefore, require targeting late steps in the autophagic pathway.

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

PURPOSE

The authors previously reported that physiological light induces the tyrosine phosphorylation of insulin receptors (IRs), which leads to the activation of the phosphoinositide 3-kinase (PI3K) and Akt (serine/threonine protein kinase B) survival pathway in rod photoreceptor cells. Tissue-specific deletion of IRs from photoreceptors resulted in stress-induced photoreceptor degeneration. Insulin growth factor 1 receptor (IGF-1R) is highly related in sequence and structure to the IR and shares 70% sequence identity overall and 84% identity within the tyrosine kinase domain. The role of IGF-1R in photoreceptor function is unknown. In this study the authors examined IGF-1R signaling in rod outer segment (ROS) membranes.

METHODS

IGF-1R localization was examined in the plasma and disc membranes of ROS. Activation of the IGF-1R/PI3K/Akt pathway was analyzed using specific antibodies against phospho-tyrosine, IGF-1R, and phospho-Akt. PI3K activity was determined in the anti-phospho-tyrosine and anti-IGF-1R immunoprecipitates. Glutathione-S-transferase fusion proteins containing two Src homology 2 (SH2) domains of the p85 subunit of PI3K and their mutants were used to study the molecular interaction between IGF-1R and p85. In vivo IGF-1R signaling was studied in rats exposed to physiological light or to constant light.

RESULTS

IGF-1R is predominately localized to plasma membranes of ROS. These studies indicate that light stress results in an increase in tyrosine phosphorylation of IGF-1R and an increase in PI3K enzyme activity in anti-phosphotyrosine and anti-IGF-1R immunoprecipitates of ROS and retinal homogenates. The authors observed that light stress induces tyrosine phosphorylation of IGF-1R in ROS membranes, which leads to the binding of p85 through N-SH2 and C-SH2 domains. Finally, the authors observed a significant activation of Akt in light-stressed retinas, indicating activation of the Akt survival pathway downstream of IGF-1R activation.

CONCLUSIONS

Light stress induced the activation of PI3K through activation and binding of IGF-1R, which leads to activation of the Akt survival pathway in photoreceptors.

Overexpression of Pax6 results in microphthalmia, retinal dysplasia and defective retinal ganglion cell axon guidance

Background

The transcription factor Pax6 is expressed by many cell types in the developing eye. Eyes do not form in homozygous loss-of-function mouse mutants (Pax6^Sey/Sey) and are abnormally small in Pax6^Sey/+ mutants. Eyes are also abnormally small in PAX77 mice expressing multiple copies of human PAX6 in addition to endogenous Pax6; protein sequences are identical in the two species. The developmental events that lead to microphthalmia in PAX77 mice are not well-characterised, so it is not clear whether over- and under-expression of Pax6/PAX6 cause microphthalmia through similar mechanisms. Here, we examined the consequences of over-expression for the eye and its axonal connections.

Results

Eyes form in PAX77^+/+ embryos but subsequently degenerate. At E12.5, we found no abnormalities in ocular morphology, retinal cell cycle parameters and the incidence of retinal cell death. From E14.5 on, we observed malformations of the optic disc. From E16.5 into postnatal life there is progressively more severe retinal dysplasia and microphthalmia. Analyses of patterns of gene expression indicated that PAX77^+/+ retinae produce a normal range of cell types, including retinal ganglion cells (RGCs). At E14.5 and E16.5, quantitative RT-PCR with probes for a range of molecules associated with retinal development showed only one significant change: a slight reduction in levels of mRNA encoding the secreted morphogen Shh at E16.5. At E16.5, tract-tracing with carbocyanine dyes in PAX77^+/+ embryos revealed errors in intraretinal navigation by RGC axons, a decrease in the number of RGC axons reaching the thalamus and an increase in the proportion of ipsilateral projections among those RGC axons that do reach the thalamus. A survey of embryos with different Pax6/PAX6 gene dosage (Pax6^Sey/+, Pax6^+/+, PAX77⁺ and PAX77^+/+) showed that (1) the total number of RGC axons projected by the retina and (2) the proportions that are sorted into the ipsilateral and contralateral optic tracts at the optic chiasm vary differently with gene dosage. Increasing dosage increases the proportion projecting ipsilaterally regardless of the size of the total projection.

Conclusion

Pax6 overexpression does not obviously impair the initial formation of the eye and its major cell-types but prevents normal development of the retina from about E14.5, leading eventually to severe retinal degeneration in postnatal life. This sequence is different to that underlying microphthalmia in Pax6^+/- heterozygotes, which is due primarily to defects in the initial stages of lens formation. Before the onset of severe retinal dysplasia, Pax6 overexpression causes defects of retinal axons, preventing their normal growth and navigation through the optic chiasm.

Loss of neuroprotective survival signal in mice lacking insulin receptor gene in rod photoreceptor cells

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. Rod-specific 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 light-induced 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.

PI3 kinase/Akt activation mediates estrogen and IGF-1 nigral DA neuronal neuroprotection against a unilateral rat model of Parkinson's disease

Recently, using the medial forebrain bundle (MFB) 6-hydroxydopmaine (6-OHDA) lesion rat model of Parkinson's disease (PD), we have demonstrated that blockade of central IGF-1 receptors (IGF-1R) attenuated estrogen neuroprotection of substantia nigra pars compacta (SNpc) DA neurons, but exacerbated 6-OHDA lesions in IGF-1 only treated rats (Quesada and Micevych [2004]: J Neurosci Res 75:107-116). This suggested that the IGF-1 system is a central mechanism through which estrogen acts to protect the nigrostriatal DA system. Moreover, these results also suggest that IGF-1R-induced intracellular signaling pathways are involved in the estrogen mechanism that promotes neuronal survival. In vitro, two convergent intracellular signaling pathways used by estrogen and IGF-1, the mitogen-activated protein kinase (MAPK/ERK), and phosphatidyl-inositol-3-kinase/Akt (PI3K/Akt), have been demonstrated to be neuroprotective. Continuous central infusions of MAPK/ERK and PI3K/Akt inhibitors were used to test the hypothesis that one or both of these signal transduction pathways mediates estrogen and/or IGF-1 neuroprotection of SNpc DA neurons after a unilateral administration of 6-OHDA into the MFB of rats. Motor behavior tests and tyrosine hydroxylase immunoreactivity revealed that the inhibitor of the PI3K/Akt pathway (LY294002) blocked the survival effects of both estrogen and IGF-1, while an inhibitor of the MAPK/ERK signaling (PD98059) was ineffective. Western blot analyses showed that estrogen and IGF-1 treatments increased PI3K/Akt activation in the SN; however, MAPK/ERK activation was decreased in the SN. Indeed, continuous infusions of inhibitors blocked phosphorylation of PI3K/Akt and MAPK/ERK. These findings indicate that estrogen and IGF-1-mediated SNpc DA neuronal protection is dependent on PI3K/Akt signaling, but not on the MAPK/ERK pathway.

Progesterone potentiates IP(3)-mediated calcium signaling through Akt/PKB

The activity of cells critically depends on the control of their cytosolic free calcium ion (Ca(2+)) concentration. The objective of the present study was to identify mechanisms of action underlying the control of the gain of intracellular Ca(2+) release by circulating gonadal steroid hormones. Acute stimulation of isolated neurons with progesterone led to IP(3)R-mediated Ca(2+) transients that depend on the activation of the PI3 kinase/Akt/PKB signaling pathway. These results were confirmed at the molecular level and phosphorylation of IP(3)R type 1 by Akt/PKB was identified as the mechanism of action. Hence, it is likely that circulating gonadal steroid hormones control neuronal activity including phosporylation status through receptor- and kinase-mediated signaling. With a direct control of the gain of the Ca(2+) second messenger system as a signaling gatekeeper for neuronal activity the present study identifies a novel pathway for interaction of the endocrine and central nervous system.

Differential, age-dependent MEK-ERK and PI3K-Akt activation by insulin acting as a survival factor during embryonic retinal development

Programmed cell death is a genuine developmental process of the nervous system, affecting not only projecting neurons but also proliferative neuroepithelial cells and young neuroblasts. The embryonic chick retina has been employed to correlate in vivo and in vitro studies on cell death regulation. We characterize here the role of two major signaling pathways, PI3K-Akt and MEK-ERK, in controlled retinal organotypic cultures from embryonic day 5 (E5) and E9, when cell death preferentially affects proliferating neuroepithelial cells and ganglion cell neurons, respectively. The relative density of programmed cell death in vivo was much higher in the proliferative and early neurogenic stages of retinal development (E3-E5) than during neuronal maturation and synaptogenesis (E8-E19). In organotypic cultures from E5 and E9 retinas, insulin, as the only growth factor added, was able to completely prevent cell death induced by growth factor deprivation. Insulin activated both the PI3K-Akt and the MEK-ERK pathways. Insulin survival effect, however, was differentially blocked at the two stages. At E5, the effect was blocked by MEK inhibitors, whereas at E9 it was blocked by PI3K inhibitors. The cells which were found to be dependent on insulin activation of the MEK-ERK pathway at E5 were mostly proliferative neuroepithelial cells. These observations support a remarkable specificity in the regulation of early neural cell death.

Aging-related changes in the expression of apoptosis-associated molecules in neurosecretory cells of the mouse hypothalamus

The aim of the present work was to identify the characteristics of the control of apoptosis in neurosecretory centers of aged (18 months) mice on exposure to the immunomodulator interferon-alpha (IA) in comparison with young (two months) mice. Age-related activation of apoptosis in the supraoptic (SON) and paraventricular (PVN) nuclei was found to be mediated by different pathways and could result from changes in the ratio of synthesis of pro-(Bax) and antiapoptotic (Mcl-1, Bcl-2) molecules. In addition, the apoptosis signal cascade in young mice treated with IA was identical in both nuclei, while in aged mice there were differences between the SON and the PVN.

Insulin rescues ES cell-derived neural progenitor cells from apoptosis by differential regulation of Akt and ERK pathways

Transplantation of embryonic stem (ES) cell-derived neural progenitor cells (ES-NPCs) is one promising technology for the treatment of spinal cord injury. Promoting ES-NPC survival at the lesion site is critical for the successful treatment. We tested the role of insulin in promoting mouse ES-NPC survival. Cultured ES-NPCs survived when maintained in normoxia but underwent apoptosis when exposed to hypoxia. Insulin rescued ES-NPCs from hypoxia-induced cell death. This effect could be blocked by the phosphatidylinositol 3-kinase (PI3K)/Akt pathway inhibitor LY294002. In contrast, mitogen-activated protein kinase (MAP)/extracellular-signal-regulated kinase (ERK) pathway inhibitor U0126 potentiated insulin-mediated survival. Immunoblots revealed that insulin upregulated activation of Akt and inhibited ERK activation through the PI3K pathway. In addition, we showed that insulin reduced the activation of caspase-3, the key executor of apoptosis. In summary, our data suggest that insulin prevent apoptosis in ES-NPCs by activating Akt and inhibiting ERK through the PI3K pathway.

Interleukin-1β enhances the effect of serum deprivation on rat annular cell apoptosis

Excessive apoptosis of disc cells is believed to play an important role in intervertebral disc (IVD) degeneration. It has been shown that interleukin-1beta (IL-1beta) is involved in the failure of disc matrix by suppressing the synthesis of matrix components and stimulating the expression of matrix metalloproteinases. However, whether IL-1beta induces disc cell apoptosis is still unclear. The objective of this study was to investigate the effect of IL-1beta on the apoptosis of rat annular cells cultured with or without serum supplement. First-passage rat annular cells were cultured with 0% or 10% fetal bovine serum (FBS) supplement and stimulated with 0, 10, 20 or 50 ng/ml IL-1beta for 12, 24 or 48 h. Apoptotic incidences were quantified by flow cytometry, morphologic changes in apoptotic cells were visualized by Hoechst 33258 staining and phase-contrast microscopy, and caspase-3 activity was also determined. When rat annular cells were cultured with 10% FBS supplement, no significant changes in apoptotic incidences, apoptotic morphology and caspase-3 activity were observed even when cells were stimulated with 50 ng/ml IL-1beta for 48 h. In contrast, serum deprivation for 24 h led to an increase in apoptotic incidences, the number of apoptotic nuclei and caspase-3 activity, and IL-1beta significantly increased the effects of serum deprivation in a dose-dependent manner. Our results indicate that IL-1beta alone is not a sufficient stimulus to induce disc cell apoptosis and that in order to suppress disc cell apoptosis, improving the nutrient supply to the disc may be more effective than antagonizing the adverse effects of IL-1beta.

Multifocal Electroretinogram Responses of the Clinically Normal Retinal Areas in Diabetes

AIMS

To investigate the degree of implicit time delay in aretinopathic hexagons according to the grading of diabetic retinopathy and factors that affect the mfERG responses of aretinopathic areas.

METHODS

MfERG was recorded using the RETIscan system (Roland consult, Brandenbrug, Germany) in 40 eyes of 20 nondiabetic controls, 32 eyes of 16 diabetic patients without retinopathy, and 96 eyes of 48 diabetic patients with nonproliferative diabetic retinopathy (NPDR). Hexagons are divided into retinopathic and aretinopathic hexagons, which were selected using fundus photographs and fluorescence angiography. Relative amplitude and implicit time delay were compared between patient groups and controls.

RESULTS

The mean implicit time delay in aretinopathic hexagons was significantly different between each subject group and correlated with the retinopathy severity of the whole retina, as well as in retinopathic hexagons. In all three NPDR groups, implicit time was significantly more delayed in retinopathic hexagons compared to aretinopathic hexagons. Relative amplitude was significantly decreased only in the severe NPDR group in both retinopathic and aretinopathic hexagons. The duration of diabetes and glycemic control status did not correlate with the local mfERG responses.

CONCLUSIONS

Local mfERG showed considerable implicit time delays in clinically normal retinal areas that correlated significantly with the severity of diabetic retinopathy of the whole retina.

A neuroprotective herbal mixture inhibits caspase-3-independent apoptosis in retinal ganglion cells

It was previously demonstrated that Menta-FX, a mixture of Panax quinquefolius L. (PQE), Ginkgo biloba (GBE), and Hypericum perforatum extracts (HPE), enhances retinal ganglion cell survival after axotomy. However, the mechanisms of neuroprotection remain unknown. The aim of this study is to elucidate the neuroprotective mechanisms of Menta-FX. Since PQE, GBE and HPE have all been observed to display anti-oxidative property, the involvement of anti-oxidation in Menta-FX's neuroprotective effect was investigated. Menta-FX lowered nitric oxide (NO) content in axotomized retinas without affecting nitric oxide synthase activity, suggesting that Menta-FX possibly exhibited a NO scavenging property. In addition, the effect of Menta-FX on the frequency of axotomy-induced nuclear fragmentation and caspase-3 activation was investigated. Menta-FX treatment significantly reduced nuclear fragmentation in axotomized retinas. Surprisingly, Menta-FX had no effect on caspase-3 activation, but selectively lowered caspase-3-independent nuclear fragmentation in axotomized retinal ganglion cells. In addition, inhibition of PI3K activity by intravitreal injection of wortmannin, a phosphoinositide-3 kinase (PI3K) inhibitor, completely abolished the neuroprotective effect of Menta-FX, indicating that Menta-FX's neuroprotective effect was PI3K-dependent. Data here suggest that Menta-FX displayed a PI3K-dependent, selective inhibition on a caspase-3-independent apoptotic pathway in axotomized RGCs, thus, highlighting the potential use of herbal remedies as neuroprotective agents for other neurodegenerative diseases.

Changes in select redox proteins of the retinal pigment epithelium in age-related macular degeneration

PURPOSE

To examine changes of select reduction-oxidation (redox) sensitive proteins from human donor retinal pigment epithelium (RPE) at four stages of age-related macular degeneration (AMD).

DESIGN

Experimental study.

METHODS

Human donor eyes were obtained from the Minnesota Lions Eye Bank and graded using the Minnesota Grading System (MGS) into four stages that correspond to stages defined by the age-related eye disease study (AREDS). Protein content in RPE homogenates was measured using Western immunoblotting with protein-specific antibodies.

RESULTS

The content of several antioxidant enzymes and specific proteins that facilitate refolding or degradation of oxidatively damaged proteins increased significantly in MGS stage 3. These proteins are involved in the primary (copper-zinc superoxide dismutase [CuZnSOD], manganese superoxide dismutase [MnSOD], and catalase) and secondary (heat shock protein [HSP] 27, HSP 90, and proteasome) defense against oxidative damage. Additionally, the insulin pro-survival receptor exhibited disease-related upregulation.

CONCLUSIONS

The pattern of protein changes identified in human donor tissue graded using the MGS support the role of oxidative mechanisms in the pathogenesis and progression of AMD. The MGS uses nearly identical clinical definitions and grading criteria of AMD that are used in the AREDS, so our results apply to clinical and epidemiologic studies using similar definitions. Results from our protein analysis of human donor tissue helps to explain altered oxidative stress regulation and cell-survival pathways that occur in progressive stages of AMD.