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

Mechanism of Cone Degeneration in Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a group of genetic disorders resulting in inherited blindness due to the degeneration of rod and cone photoreceptors. The various mechanisms underlying rod degeneration primarily rely on genetic mutations, leading to night blindness initially. Cones gradually degenerate after rods are almost eliminated, resulting in varying degrees of visual disability and blindness. The mechanism of cone degeneration remains unclear. An understanding of the mechanisms underlying cone degeneration in RP, a highly heterogeneous disease, is essential to develop novel treatments of RP. Herein, we review recent advancements in the five hypotheses of cone degeneration, including oxidative stress, trophic factors, metabolic stress, light damage, and inflammation activation. We also discuss the connection among these theories to provide a better understanding of secondary cone degeneration in RP. Five current mechanisms of cone degenerations in RP Interactions among different pathways are involved in RP.

Ribosomal targeting strategy and nuclear labeling to analyze photoreceptor phosphoinositide signatures

Reversible phosphorylation of phosphatidylinositol by phosphoinositide (PI) kinases and phosphatases generates seven distinct phosphoinositide phosphates, called phosphoinositides or PIPs. All seven PIPs are formed in the retina and photoreceptor cells. Around 50 genes in the mammalian genome encode PI kinases and PI phosphatases. There are no studies available on the distribution of these enzymes in the retina and photoreceptors.

AIM

To employ Ribosomal Targeting Strategy and Nuclear Labeling to Analyze Phosphoinositide Signatures in rod-photoreceptor cells.

METHODS

HA-tagging of ribosomal protein Rpl22 was induced with Cre-recombinase under the control of the rhodopsin promoter. Actively translating mRNAs associated with polyribosomes were isolated by immunoprecipitation with HA antibody, followed by RNA isolation and gene identification. We also isolated biotinylated-rod nuclei from NuTRAP mice under the control of the rhodopsin-Cre promoter and analyzed nuclear phosphoinositides.

RESULTS

Our results indicate that the expression of class I and class III PI 3-kinase, PI4K IIIβ, PI 5-kinase, PIKfyve, PI3-phosphatases, MTMR2, 4, 6, 7, 14, PI4-phosphatase, TMEM55A, PI 5-phosphatases, SYNJI, INPP5B, INPP5E, INPP5F, SKIP and other phosphatases with dual substrate specificity, PTPMT1, SCAM1, and FIG4 are highly enriched in rod photoreceptor cells compared with the retina and cone-like retina. Our analysis identified the presence of PI(4)P, PI(3,4)P2, PI(3,5)P2, and PI(4,5)P2 in the rod nuclei.

CONCLUSIONS

Our studies for the first time demonstrate the expression of PI kinases, PI phosphatases, and nuclear PIPs in rod photoreceptor cells. The NuTRAP mice may be useful not only for epigenetic and transcriptomic studies but also for in vivo cell-specific lipidomics research.

Insulin-like growth factor 1 receptor mediates photoreceptor neuroprotection

Insulin-like growth factor I (IGF-1) is a neurotrophic factor and is the ligand for insulin-like growth factor 1 receptor (IGF-1R). Reduced expression of IGF-1 has been reported to cause deafness, mental retardation, postnatal growth failure, and microcephaly. IGF-1R is expressed in the retina and photoreceptor neurons; however, its functional role is not known. Global IGF-1 KO mice have age-related vision loss. We determined that conditional deletion of IGF-1R in photoreceptors and pan-retinal cells produces age-related visual function loss and retinal degeneration. Retinal pigment epithelial cell-secreted IGF-1 may be a source for IGF-1R activation in the retina. Altered retinal, fatty acid, and phosphoinositide metabolism are observed in photoreceptor and retinal cells lacking IGF-1R. Our results suggest that the IGF-1R pathway is indispensable for photoreceptor survival, and activation of IGF-1R may be an essential element of photoreceptor and retinal neuroprotection.

Beyond Genetics: The Role of Metabolism in Photoreceptor Survival, Development and Repair

Vision commences in the retina with rod and cone photoreceptors that detect and convert light to electrical signals. The irreversible loss of photoreceptors due to neurodegenerative disease leads to visual impairment and blindness. Interventions now in development include transplanting photoreceptors, committed photoreceptor precursors, or retinal pigment epithelial (RPE) cells, with the latter protecting photoreceptors from dying. However, introducing exogenous human cells in a clinical setting faces both regulatory and supply chain hurdles. Recent work has shown that abnormalities in central cell metabolism pathways are an underlying feature of most neurodegenerative disorders, including those in the retina. Reversal of key metabolic alterations to drive retinal repair thus represents a novel strategy to treat vision loss based on cell regeneration. Here, we review the connection between photoreceptor degeneration and alterations in cell metabolism, along with new insights into how metabolic reprogramming drives both retinal development and repair following damage. The potential impact of metabolic reprogramming on retinal regeneration is also discussed, specifically in the context of how metabolic switches drive both retinal development and the activation of retinal glial cells known as Müller glia. Müller glia display latent regenerative properties in teleost fish, however, their capacity to regenerate new photoreceptors has been lost in mammals. Thus, re-activating the regenerative properties of Müller glia in mammals represents an exciting new area that integrates research into developmental cues, central metabolism, disease mechanisms, and glial cell biology. In addition, we discuss this work in relation to the latest insights gleaned from other tissues (brain, muscle) and regenerative species (zebrafish).

Diabetic Retinopathy and Insulin Insufficiency: Beta Cell Replacement as a Strategy to Prevent Blindness

Diabetic retinopathy (DR) is a potentially devastating complication of diabetes because it puts patients at risk of blindness. Diabetes is a common cause of blindness in the U.S. and worldwide and is dramatically increasing in global prevalence. Thus new approaches are needed to prevent this dreaded complication. There is extensive data that indicates beta cell secretory failure is a risk factor for DR, independent of its influence on glycemic control. This perspective article will provide evidence for insufficient endogenous insulin secretion as an important factor in the development of DR. The areas of evidence discussed are: (a) Presence of insulin receptors in the retina, (b) Clinical studies that show an association of beta cell insufficiency with DR, (c) Treatment with insulin in type 2 diabetes, a marker for endogenous insulin deficiency, is an independent risk factor for DR, (d) Recent clinical studies that link DR with an insulin deficient form of type 2 diabetes, and (e) Beta cell replacement studies that demonstrate endogenous insulin prevents progression of DR. The cumulative data drive our conclusion that beta cell replacement will have an important role in preventing DR and/or mitigating its severity in both type 1 diabetes and insulinopenic type 2 diabetes.

Mice deficient in UXT exhibit retinitis pigmentosa-like features via aberrant autophagy activation

UXT (ubiquitously expressed prefoldin like chaperone), a small chaperone-like protein, is widely expressed in diverse human and mouse tissues and is more abundant in retina and kidney. However, the functional characterization of UXT at tissue level was largely unknown. Here, we reported that mice deficient in UXT exhibited salient features of retinal degenerative disease, similar to retinitis pigmentosa. Conditional knockout (CKO) of Uxt led to retinal degeneration and pigmentation in mice retina along with significant alterations of retinitis pigmentosa-related genes, which indicated UXT might be associated with retinal degenerative disease sharing key features to retinitis pigmentosa. Consistently, the electroretinogram (ERG) responses were dramatically impaired in uxt CKO retinas. Strong degenerative features were observed in uxt CKO retinas, including specific and progressive reduction of photoreceptor cells and increased numbers of apoptotic cells. Intriguingly, macroautophagic/autophagic flux was enhanced in uxt CKO retina. Mechanistically, we found UXT was indispensable to suppress photoreceptor apoptotic cell death by inhibiting autophagy through regulating the activity of MTOR (mechanistic target of rapamycin kinase), a key negative regulator of autophagy. Conversely, knockdown of UXT induced the robust expression of the canonical autophagy-related genes and boosted autophagic flux and apoptosis, finally resulting in severe retina degeneration in uxt CKO mice. Taken together, our study reveals a vital role of UXT in preventing retina from degeneration. The loss of UXT results in a hyper-autophagic state leading to massive retinal degeneration. Therefore, UXT may be a crucial target for retinal degenerative disease.Abbreviations: 3-ma: 3-methyladenine; casp3: caspase 3; cko: conditional knockout; erg: electroretinogram; gapdh: glyceraldehyde-3-phosphate dehydrogenase; map1lc3b/lc3b: microtubule-associated protein 1 light chain 3; mtor: mechanistic target of rapamycin kinase; parp: poly (adp-ribose) polymerase family; rna-seq: rna sequencing; rp: retinitis pigmentosa; rps6kb1/s6k: ribosomal protein s6 kinase b1; sqstm1: sequestosome 1; tunel: terminal deoxynucleotidyl transferase mediated dutp nick-end labeling; uxt: ubiquitously expressed prefoldin like chaperone.

Regulation of Phosphoinositide Levels in the Retina by Protein Tyrosine Phosphatase 1B and Growth Factor Receptor-Bound Protein 14

Protein tyrosine kinases and protein phosphatases play a critical role in cellular regulation. The length of a cellular response depends on the interplay between activating protein kinases and deactivating protein phosphatases. Protein tyrosine phosphatase 1B (PTP1B) and growth factor receptor-bound protein 14 (Grb14) are negative regulators of receptor tyrosine kinases. However, in the retina, we have previously shown that PTP1B inactivates insulin receptor signaling, whereas phosphorylated Grb14 inhibits PTP1B activity. In silico docking of phosphorylated Grb14 and PTP1B indicate critical residues in PTP1B that may mediate the interaction. Phosphoinositides (PIPs) are acidic lipids and minor constituents in the cell that play an important role in cellular processes. Their levels are regulated by growth factor signaling. Using phosphoinositide binding protein probes, we observed increased levels of PI(3)P, PI(4)P, PI(3,4)P₂, PI(4,5)P2, and PI(3,4,5)P₃ in PTP1B knockout mouse retina and decreased levels of these PIPs in Grb14 knockout mouse retina. These observations suggest that the interplay between PTP1B and Grb14 can regulate PIP metabolism.

Effect of selectively knocking down key metabolic genes in Müller glia on photoreceptor health

The importance of Müller glia for retinal homeostasis suggests that they may have vulnerabilities that lead to retinal disease. Here, we studied the effect of selectively knocking down key metabolic genes in Müller glia on photoreceptor health. Immunostaining indicated that murine Müller glia expressed insulin receptor (IR), hexokinase 2 (HK2) and phosphoglycerate dehydrogenase (PHGDH) but very little pyruvate dehydrogenase E1 alpha 1 (PDH-E1α) and lactate dehydrogenase A (LDH-A). We crossed Müller glial cell-CreER (MC-CreER) mice with transgenic mice carrying a floxed IR, HK2, PDH-E1α, LDH-A, or PHGDH gene to study the effect of selectively knocking down key metabolic genes in Müller glia cells on retinal health. Selectively knocking down IR, HK2, or PHGDH led to photoreceptor degeneration and reduced electroretinographic responses. Supplementing exogenous l-serine prevented photoreceptor degeneration and improved retinal function in MC-PHGDH knockdown mice. We unexpectedly found that the levels of retinal serine and glycine were not reduced but, on the contrary, highly increased in MC-PHGDH knockdown mice. Moreover, dietary serine supplementation, while rescuing the retinal phenotypes caused by genetic deletion of PHGDH in Müller glial cells, restored retinal serine and glycine homeostasis probably through regulation of serine transport. No retinal abnormalities were observed in MC-CreER mice crossed with PDH-E1α- or LDH-A-floxed mice despite Cre expression. Our findings suggest that Müller glia do not complete glycolysis but use glucose to produce serine to support photoreceptors. Supplementation with exogenous serine is effective in preventing photoreceptor degeneration caused by PHGDH deficiency in Müller glia.

Photoreceptor metabolic reprogramming: current understanding and therapeutic implications

Acquired and inherited retinal disorders are responsible for vision loss in an increasing proportion of individuals worldwide. Photoreceptor (PR) death is central to the vision loss individuals experience in these various retinal diseases. Unfortunately, there is a lack of treatment options to prevent PR loss, so an urgent unmet need exists for therapies that improve PR survival and ultimately, vision. The retina is one of the most energy demanding tissues in the body, and this is driven in large part by the metabolic needs of PRs. Recent studies suggest that disruption of nutrient availability and regulation of cell metabolism may be a unifying mechanism in PR death. Understanding retinal cell metabolism and how it is altered in disease has been identified as a priority area of research. The focus of this review is on the recent advances in the understanding of PR metabolism and how it is critical to reduction-oxidation (redox) balance, the outer retinal metabolic ecosystem, and retinal disease. The importance of these metabolic processes is just beginning to be realized and unraveling the metabolic and redox pathways integral to PR health may identify novel targets for neuroprotective strategies that prevent blindness in the heterogenous group of retinal disorders.

Signaling roles of phosphoinositides in the retina

The field of phosphoinositide signaling has expanded significantly in recent years. Phosphoinositides (also known as phosphatidylinositol phosphates or PIPs) are universal signaling molecules that directly interact with membrane proteins or with cytosolic proteins containing domains that directly bind phosphoinositides and are recruited to cell membranes. Through the activities of phosphoinositide kinases and phosphoinositide phosphatases, seven distinct phosphoinositide lipid molecules are formed from the parent molecule, phosphatidylinositol. PIP signals regulate a wide range of cellular functions, including cytoskeletal assembly, membrane budding and fusion, ciliogenesis, vesicular transport, and signal transduction. Given the many excellent reviews on phosphoinositide kinases, phosphoinositide phosphatases, and PIPs in general, in this review, we discuss recent studies and advances in PIP lipid signaling in the retina. We specifically focus on PIP lipids from vertebrate (e.g., bovine, rat, mouse, toad, and zebrafish) and invertebrate (e.g., Drosophila, horseshoe crab, and squid) retinas. We also discuss the importance of PIPs revealed from animal models and human diseases, and methods to study PIP levels both in vitro and in vivo. We propose that future studies should investigate the function and mechanism of activation of PIP-modifying enzymes/phosphatases and further unravel PIP regulation and function in the different cell types of the retina.

Insulin inhibits inflammation-induced cone death in retinal detachment

BACKGROUND

Rhegmatogenous retinal detachment (RD) involving the macula is a major cause of visual impairment despite high surgical success rate, mainly because of cone death. RD causes the infiltration of activated immune cells, but it is not clear whether and how infiltrating inflammatory cells contribute to cone cell loss.

METHODS

Vitreous samples from patients with RD and from control patients with macular hole were analyzed to characterize the inflammatory response to RD. A mouse model of RD and retinal explants culture were then used to explore the mechanisms leading to cone death.

RESULTS

Analysis of vitreous samples confirms that RD induces a marked inflammatory response with increased cytokine and chemokine expression in humans, which is closely mimicked by experimental murine RD. In this model, we corroborate that myeloid cells and T-lymphocytes contribute to cone loss, as the inhibition of their accumulation by Thrombospondin 1 (TSP1) increased cone survival. Using monocyte/retinal co-cultures and TSP1 treatment in RD, we demonstrate that immune cell infiltration downregulates rod-derived cone viability factor (RdCVF), which physiologically regulates glucose uptake in cones. Insulin and the insulin sensitizers rosiglitazone and metformin prevent in part the RD-induced cone loss in vivo, despite the persistence of inflammation CONCLUSION: Our results describe a new mechanism by which inflammation induces cone death in RD, likely through cone starvation due to the downregulation of RdCVF that could be reversed by insulin. Therapeutic inhibition of inflammation and stimulation of glucose availability in cones by insulin signaling might prevent RD-associated cone death until the RD can be surgically repaired and improve visual outcome after RD.

TRIAL REGISTRATION

ClinicalTrials.gov NCT03318588.

Loss of Class III Phosphoinositide 3-Kinase Vps34 Results in Cone Degeneration

The major pathway for the production of the low-abundance membrane lipid phosphatidylinositol 3-phosphate (PI(3)P) synthesis is catalyzed by class III phosphoinositide 3-kinase (PI3K) Vps34. The absence of Vps34 was previously found to disrupt autophagy and other membrane-trafficking pathways in some sensory neurons, but the roles of phosphatidylinositol 3-phosphate and Vps34 in cone photoreceptor cells have not previously been explored. We found that the deletion of Vps34 in neighboring rods in mouse retina did not disrupt cone function up to 8 weeks after birth, despite diminished rod function. Immunoblotting and lipid analysis of cones isolated from the cone-dominant retinas of the neural retina leucine zipper gene knockout mice revealed that both PI(3)P and Vps34 protein are present in mouse cones. To determine whether Vps34 and PI(3)P are important for cone function, we conditionally deleted Vps34 in cone photoreceptor cells of the mouse retina. Overall retinal morphology and rod function appeared to be unaffected. However, the loss of Vps34 in cones resulted in the loss of structure and function. There was a substantial reduction throughout the retina in the number of cones staining for M-opsin, S-opsin, cone arrestin, and peanut agglutinin, revealing degeneration of cones. These studies indicate that class III PI3K, and presumably PI(3)P, play essential roles in cone photoreceptor cell function and survival.

Power to see-Drivers of aerobic glycolysis in the mammalian retina: A review

The mammalian retina converts most glucose to lactate rather than catabolizing it completely to carbon dioxide via oxidative phosphorylation, despite the availability of oxygen. This unusual metabolism is known as aerobic glycolysis or the Warburg effect. Molecules and pathways that drive aerobic glycolysis have been identified and thoroughly studied in the context of cancer but remain relatively poorly understood in the retina. Here, we review recent research on the molecular mechanisms that underly aerobic glycolysis in the retina, focusing on key glycolytic enzymes including hexokinase 2 (HK2), pyruvate kinase M2 (PKM2) and lactate dehydrogenase A (LDHA). We also discuss the potential involvement of cell signalling and transcriptional pathways including phosphoinositide 3-kinase (PI3K) signalling, fibroblast growth factor receptor (FGFR) signalling, and hypoxia-inducible factor 1 (HIF-1), which have been implicated in driving aerobic glycolysis in the context of cancer.

Mouse Models of Inherited Retinal Degeneration with Photoreceptor Cell Loss

Inherited retinal degeneration (RD) leads to the impairment or loss of vision in millions of individuals worldwide, most frequently due to the loss of photoreceptor (PR) cells. Animal models, particularly the laboratory mouse, have been used to understand the pathogenic mechanisms that underlie PR cell loss and to explore therapies that may prevent, delay, or reverse RD. Here, we reviewed entries in the Mouse Genome Informatics and PubMed databases to compile a comprehensive list of monogenic mouse models in which PR cell loss is demonstrated. The progression of PR cell loss with postnatal age was documented in mutant alleles of genes grouped by biological function. As anticipated, a wide range in the onset and rate of cell loss was observed among the reported models. The analysis underscored relationships between RD genes and ciliary function, transcription-coupled DNA damage repair, and cellular chloride homeostasis. Comparing the mouse gene list to human RD genes identified in the RetNet database revealed that mouse models are available for 40% of the known human diseases, suggesting opportunities for future research. This work may provide insight into the molecular players and pathways through which PR degenerative disease occurs and may be useful for planning translational studies.

Phosphoinositides in Retinal Function and Disease

Phosphatidylinositol and its phosphorylated derivatives, the phosphoinositides, play many important roles in all eukaryotic cells. These include modulation of physical properties of membranes, activation or inhibition of membrane-associated proteins, recruitment of peripheral membrane proteins that act as effectors, and control of membrane trafficking. They also serve as precursors for important second messengers, inositol (1,4,5) trisphosphate and diacylglycerol. Animal models and human diseases involving defects in phosphoinositide regulatory pathways have revealed their importance for function in the mammalian retina and retinal pigmented epithelium. New technologies for localizing, measuring and genetically manipulating them are revealing new information about their importance for the function and health of the vertebrate retina.

Autophagy, lysosome dysfunction and mTOR inhibition in MNU-induced photoreceptor cell damage

Progressive photoreceptor death is the main cause of retinal degeneration diseases. Determining the underlying mechanism of this process is essential for therapy improvement. Autophagy has long been considered to be involved in neuronal degeneration diseases, and the regulation of autophagy is thought to have potential implications for neurodegenerative disease therapies. However, whether autophagy is protective or destructive varies among diseases and is controversial. In the present study, we established an N-methyl-N-nitrosourea (MNU)-induced photoreceptor cell damage model in vitro that faithfully replicated photoreceptor cell death in retinal degeneration diseases. Cell viability was tested by 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxy-methoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assays. Reactive oxygen species (ROS) levels were assessed through 2,7-dichlorodihydrofluorescein diacetate (DCFH-DA) fluorescence. Autophagy was confirmed by observing autophagosomes using transmission electron microscopy (TEM). A lysosome tracker was used to identify acidic lysosomes in cells. We also measured the expression of some proteins related to autophagy, apoptosis and lysosomal degradation by western blot and immunofluorescence assays. We found that MNU could decrease photoreceptor cell viability in a time- and dose-dependent manner, and this change was accompanied by concomitant increases in ROS and the expression of the apoptosis-inducing protein cleaved caspase-3. Moreover, autophagy was activated by MNU treatment during this process. Inhibition of autophagy with 3-methyladenine accelerated cell damage. Lysosome dysfunction was confirmed by autophagosome enlargement and increased cathepsin expression, which was accompanied by mTOR dephosphorylation. In conclusion, autophagy was activated through inhibition of the PI3K/mTOR pathway in the context of MNU-induced photoreceptor cell death. Prolonged mTOR dephosphorylation and autophagy activation resulted in autophagic vacuole accumulation, as indicated by inefficient degradation in lysosomes, and further led to apoptosis.

Therapeutic Benefits from Nanoparticles: The Potential Significance of Nanoscience in Retinal Degenerative Diseases

Several nanotechnology podiums have gained remarkable attention in the area of medical sciences, including diagnostics and treatment. In the past decade, engineered multifunctional nanoparticles have served as drug and gene carriers. The most important aspect of translating nanoparticles from the bench to bedside is safety. These nanoparticles should not elicit any immune response and should not be toxic to humans or the environment. Lipid-based nanoparticles have been shown to be the least toxic for in vivo applications, and significant progress has been made in gene and drug delivery employing lipid-based nanoassemblies. Several excellent reviews and reports discuss the general use and application of lipid-based nanoparticles; our review focuses on the application of lipid-based nanoparticles for the treatment of ocular diseases, and recent advances in and updates on their use.

Endogenous insulin signaling in the RPE contributes to the maintenance of rod photoreceptor function in diabetes

In diabetes, there are two major physiological aberrations: (i) Loss of insulin signaling due to absence of insulin (type 1 diabetes) or insulin resistance (type 2 diabetes) and (ii) increased blood glucose levels. The retina has a high proclivity to damage following diabetes, and much of the pathology seen in diabetic retinopathy has been ascribed to hyperglycemia and downstream cascades activated by increased blood glucose. However, less attention has been focused on the direct role of insulin on retinal physiology, likely due to the fact that uptake of glucose in retinal cells is not insulin-dependent. The retinal pigment epithelium (RPE) is instrumental in maintaining the structural and functional integrity of the retina. Recent studies have suggested that RPE dysfunction is a precursor of, and contributes to, the development of diabetic retinopathy. To evaluate the role of insulin on RPE cell function directly, we generated a RPE specific insulin receptor (IR) knockout (RPEIRKO) mouse using the Cre-loxP system. Using this mouse, we sought to determine the impact of insulin-mediated signaling in the RPE on retinal function under physiological control conditions as well as in streptozotocin (STZ)-induced diabetes. We demonstrate that loss of RPE-specific IR expression resulted in lower a- and b-wave electroretinogram amplitudes in diabetic mice as compared to diabetic mice that expressed IR on the RPE. Interestingly, RPEIRKO mice did not exhibit significant differences in the amplitude of the RPE-dependent electroretinogram c-wave as compared to diabetic controls. However, loss of IR-mediated signaling in the RPE reduced levels of reactive oxygen species and the expression of pro-inflammatory cytokines in the retina of diabetic mice. These results imply that IR-mediated signaling in the RPE regulates photoreceptor function and may play a role in the generation of oxidative stress and inflammation in the retina in diabetes.

Ribosomal protein S6 kinase 1 promotes the survival of photoreceptors in retinitis pigmentosa

Retinitis pigmentosa (RP) is a heterogeneous group of inherited disorders caused by mutations in genes that are mostly expressed by rod photoreceptors, which results in initial death of rods followed by cone photoreceptors. The molecular mechanisms that lead to both rod and cone degeneration are not yet fully understood. The mTOR pathway is implicated in RP. However, it remains unclear whether S6K1 plays an essential role downstream of the mTOR pathway in mediating photoreceptor survival in RP. Our in vitro studies demonstrated that PTEN (phosphatase and tensin homolog) overexpression deactivated mTOR activity and induced 661W cone cell apoptosis. In addition, we identified that S6K1 but not 4EBP1 was the downstream effector of PTEN neurotoxicity using gain- and loss-of-function approaches. Moreover, our in vivo data corroborated the results of our in vitro studies. S6K1 overexpression either in rods or cones promoted these cell survival and function and improved visual performance in the rd10 mouse model of RP. Our data demonstrated that S6K1 was the downstream effector of mTOR and that S6K1 was critical for both rod and cone survival in RP. Our findings make a strong case for targeting S6K1 as a promising therapeutic strategy for promoting the survival of photoreceptors in RP.

Pyruvate kinase M2 isoform deletion in cone photoreceptors results in age-related cone degeneration

The tumor form of pyruvate kinase M2 has been suggested to promote cellular anabolism by redirecting the metabolism to cause accumulation of glycolytic intermediates and increasing flux through the pentose phosphate pathway, which is a metabolic pathway parallel to glycolysis. Both rod and cone photoreceptors express the tumor form of pyruvate kinase M2. Recent studies from our laboratory show that PKM2 is functionally important for rod photoreceptor structure, function, and viability. However, the functional role of PKM2 in cones is not known. In this study, we conditionally deleted PKM2 in cones (cone-cre PKM2-KO) and found that loss of PKM2 results in the upregulation of PKM1 and a significant loss of cone function and cone degeneration in an age-dependent manner. Gene expression studies on cone-cre PKM2-KO show decreased expression of genes regulating glycolysis, PPP shunt, and fatty acid biosynthesis. Consistent with these observations, cones lacking PKM2 have significantly shorter cone outer segments than cones with PKM2. Our studies clearly suggest that PKM2 is essential for the anabolic process in cones to keep them alive for normal functioning and to support cone structure.

Activation of oncogenic tyrosine kinase signaling promotes insulin receptor-mediated cone photoreceptor survival

In humans, daylight vision is primarily mediated by cone photoreceptors. These cells die in age-related retinal degenerations. Prolonging the life of cones for even one decade would have an enormous beneficial effect on usable vision in an aging population. Photoreceptors are postmitotic, but shed 10% of their outer segments daily, and must synthesize the membrane and protein equivalent of a proliferating cell each day. Although activation of oncogenic tyrosine kinase and inhibition of tyrosine phosphatase signaling is known to be essential for tumor progression, the cellular regulation of this signaling in postmitotic photoreceptor cells has not been studied. In the present study, we report that a novel G-protein coupled receptor–mediated insulin receptor (IR) signaling pathway is regulated by non-receptor tyrosine kinase Src through the inhibition of protein tyrosine phosphatase IB (PTP1B). We demonstrated the functional significance of this pathway through conditional deletion of IR and PTP1B in cones, in addition to delaying the death of cones in a mouse model of cone degeneration by activating the Src. This is the first study demonstrating the molecular mechanism of a novel signaling pathway in photoreceptor cells, which provides a window of opportunity to save the dying cones in retinal degenerative diseases.

The Warburg Effect Mediator Pyruvate Kinase M2 Expression and Regulation in the Retina

The tumor form of pyruvate kinase M2 (PKM2) undergoes tyrosine phosphorylation and gives rise to the Warburg effect. The Warburg effect defines a pro-oncogenic metabolism switch such that cancer cells take up more glucose than normal tissue and favor incomplete oxidation of glucose, even in the presence of oxygen. Retinal photoreceptors are highly metabolic and their energy consumption is equivalent to that of a multiplying tumor cell. In the present study, we found that PKM2 is the predominant isoform in both rod- and cone-dominant retina, and that it undergoes a light-dependent tyrosine phosphorylation. We also discovered that PKM2 phosphorylation is signaled through photobleaching of rhodopsin. Our findings suggest that phosphoinositide 3-kinase activation promotes PKM2 phosphorylation. Light and tyrosine phosphorylation appear to regulate PKM2 to provide a metabolic advantage to photoreceptor cells, thereby promoting cell survival.

Autophagy supports color vision

Cones comprise only a small portion of the photoreceptors in mammalian retinas. However, cones are vital for color vision and visual perception, and their loss severely diminishes the quality of life for patients with retinal degenerative diseases. Cones function in bright light and have higher demand for energy than rods; yet, the mechanisms that support the energy requirements of cones are poorly understood. One such pathway that potentially could sustain cones under basal and stress conditions is macroautophagy. We addressed the role of macroautophagy in cones by examining how the genetic block of this pathway affects the structural integrity, survival, and function of these neurons. We found that macroautophagy was not detectable in cones under normal conditions but was readily observed following 24 h of fasting. Consistent with this, starvation induced phosphorylation of AMPK specifically in cones indicating cellular starvation. Inhibiting macroautophagy in cones by deleting the essential macroautophagy gene Atg5 led to reduced cone function following starvation suggesting that cones are sensitive to systemic changes in nutrients and activate macroautophagy to maintain their function. ATG5-deficiency rendered cones susceptible to light-induced damage and caused accumulation of damaged mitochondria in the inner segments, shortening of the outer segments, and degeneration of all cone types, revealing the importance of mitophagy in supporting cone metabolic needs. Our results demonstrate that macroautophagy supports the function and long-term survival of cones providing for their unique metabolic requirements and resistance to stress. Targeting macroautophagy has the potential to preserve cone-mediated vision during retinal degenerative diseases.

Metabolic and redox signaling in the retina

Visual perception by photoreceptors relies on the interaction of incident photons from light with a derivative of vitamin A that is covalently linked to an opsin molecule located in a special subcellular structure, the photoreceptor outer segment. The photochemical reaction produced by the photon is optimal when the opsin molecule, a seven-transmembrane protein, is embedded in a lipid bilayer of optimal fluidity. This is achieved in vertebrate photoreceptors by a high proportion of lipids made with polyunsaturated fatty acids, which have the detrimental property of being oxidized and damaged by light. Photoreceptors cannot divide, but regenerate their outer segments. This is an enormous energetic challenge that explains why photoreceptors metabolize glucose through aerobic glycolysis, as cancer cells do. Uptaken glucose produces metabolites to renew that outer segment as well as reducing power through the pentose phosphate pathway to protect photoreceptors against oxidative damage.

Phosphatidylinositol-3-phosphate is light-regulated and essential for survival in retinal rods

Phosphoinositides play important roles in numerous intracellular membrane pathways. Little is known about the regulation or function of these lipids in rod photoreceptor cells, which have highly active membrane dynamics. Using new assays with femtomole sensitivity, we determined that whereas levels of phosphatidylinositol-3,4-bisphosphate and phosphatidylinositol-3,4,5-trisphosphate were below detection limits, phosphatidylinositol-3-phosphate (PI(3)P) levels in rod inner/outer segments increased more than 30-fold after light exposure. This increase was blocked in a rod-specific knockout of the PI-3 kinase Vps34, resulting in failure of endosomal and autophagy-related membranes to fuse with lysosomes, and accumulation of abnormal membrane structures. At early ages, rods displayed normal morphology, rhodopsin trafficking, and light responses, but underwent progressive neurodegeneration with eventual loss of both rods and cones by twelve weeks. The degeneration is considerably faster than in rod knockouts of autophagy genes, indicating defects in endosome recycling or other PI(3)P-dependent membrane trafficking pathways are also essential for rod survival.

Class I Phosphoinositide 3-Kinase Exerts a Differential Role on Cell Survival and Cell Trafficking in Retina

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that phosphorylates the 3'OH of the inositol ring of phosphoinositides. They are responsible for coordinating a diverse range of cell functions including proliferation, cell survival, degranulation, vesicular trafficking, and cell migration. The PI 3-kinases are grouped into three distinct classes: I, II, and III. Class III PI3K has been shown to be involved in intracellular protein trafficking, whereas class I PI3K is known to regulate cell survival following activation of cell surface receptors. However, studies from our laboratory and others have shown that class I PI3K may also be involved in photoreceptor protein trafficking. Therefore, to learn more about the role of class I and class III P13K in trafficking and to understand the impact of the lipid content of trafficking cargo vesicles, we developed a methodology to isolate trafficking vesicles from retinal tissue. PI3K class I and III proteins were enriched in our extracted trafficking vesicle fraction. Moreover, levels of ether phosphatidylethanolamine (PE) and ether phosphatidylcholine (PC) were significantly higher in the trafficking vesicle fraction than in total retina. These two lipid classes have been suggested to be involved with fusion/targeting of trafficking vesicles.

Transgenic Mice Overexpressing Serum Retinol-Binding Protein Develop Progressive Retinal Degeneration through a Retinoid-Independent Mechanism

Serum retinol-binding protein 4 (RBP4) is the sole specific transport protein for retinol in the blood, but it is also an adipokine with retinol-independent, proinflammatory activity associated with obesity, insulin resistance, type 2 diabetes, and cardiovascular disease. Moreover, two separate studies reported that patients with proliferative diabetic retinopathy have increased serum RBP4 levels compared to patients with mild or no retinopathy, yet the effect of increased levels of RBP4 on the retina has not been studied. Here we show that transgenic mice overexpressing RBP4 (RBP4-Tg mice) develop progressive retinal degeneration, characterized by photoreceptor ribbon synapse deficiency and subsequent bipolar cell loss. Ocular retinoid and bisretinoid levels are normal in RBP4-Tg mice, demonstrating that a retinoid-independent mechanism underlies retinal degeneration. Increased expression of pro-interleukin-18 (pro-IL-18) mRNA and activated IL-18 protein and early-onset microglia activation in the retina suggest that retinal degeneration is driven by a proinflammatory mechanism. Neither chronic systemic metabolic disease nor other retinal insults are required for RBP4 elevation to promote retinal neurodegeneration, since RBP4-Tg mice do not have coincident retinal vascular pathology, obesity, dyslipidemia, or hyperglycemia. These findings suggest that elevation of serum RBP4 levels could be a risk factor for retinal damage and vision loss in nondiabetic as well as diabetic patients.

Loss of mTOR signaling affects cone function, cone structure and expression of cone specific proteins without affecting cone survival

Cones are the primary photoreceptor (PR) cells responsible for vision in humans. They are metabolically highly active requiring phosphoinositide 3-kinase (PI3K) activity for long-term survival. One of the downstream targets of PI3K is the kinase mammalian target of rapamycin (mTOR), which is a key regulator of cell metabolism and growth, integrating nutrient availability and growth factor signals. Both PI3K and mTOR are part of the insulin/mTOR signaling pathway, however if mTOR is required for long-term PR survival remains unknown. This is of particular interest since deregulation of this pathway in diabetes results in reduced PR function before the onset of any clinical signs of diabetic retinopathy. mTOR is found in two distinct complexes (mTORC1 & mTORC2) that are characterized by their unique accessory proteins RAPTOR and RICTOR respectively. mTORC1 regulates mainly cell metabolism in response to nutrient availability and growth factor signals, while mTORC2 regulates pro-survival mechanisms in response to growth factors. Here we analyze the effect on cones of loss of mTORC1, mTORC2 and simultaneous loss of mTORC1 & mTORC2. Interestingly, neither loss of mTORC1 nor mTORC2 affects cone function or survival at one year of age. However, outer and inner segment morphology is affected upon loss of either complex. In contrast, concurrent loss of mTORC1 and mTORC2 leads to a reduction in cone function without affecting cone viability. The data indicates that PI3K mediated pro-survival signals diverge upstream of both mTOR complexes in cones, suggesting that they are independent of mTOR activity. Furthermore, the data may help explain why PR function is reduced in diabetes, which can lead to deregulation of both mTOR complexes simultaneously. Finally, although mTOR is a key regulator of cell metabolism, and PRs are metabolically highly active, the data suggests that the role of mTOR in regulating the metabolic transcriptome in healthy cones is minimal.

The p110α isoform of phosphoinositide 3-kinase is essential for cone photoreceptor survival

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that phosphorylates the 3'OH of the inositol ring of phosphoinositides (PIs). They are responsible for coordinating a diverse range of cellular functions. Class IA PI3K is a heterodimeric protein composed of a regulatory p85 and a catalytic p110 subunit. In this study, we conditionally deleted the p110α-subunit of PI3K in cone photoreceptor cells using the Cre-loxP system. Cone photoreceptors allow for color vision in bright light (daylight vision). Cone-specific deletion of p110α resulted in cone degeneration. Our studies suggest that PI3K signaling is essential for cone photoreceptor functions.

Integrative analysis of ocular complications in atherosclerosis unveils pathway convergence and crosstalk

Atherosclerosis is a life-threatening disease and a major cause of mortalities worldwide. While many of the atherosclerotic sequelae are reflected as microvascular effects in the eye, the molecular mechanisms of their development is not yet known. In this study, we employed a systems biology approach to unveil the most significant events and key molecular mediators of ophthalmic sequelae caused by atherosclerosis. Literature mining was used to identify the proteins involved in both atherosclerosis and ophthalmic diseases. A protein-protein interaction (PPI) network was prepared using the literature-mined seed nodes. Network topological analysis was carried out using Cytoscape, while network nodes were annotated using database for annotation, visualization and integrated discovery in order to identify the most enriched pathways and processes. Network analysis revealed that mitogen-activated protein kinase 1 (MAPK1) and protein kinase C occur with highest betweenness centrality, degree and closeness centrality, thus reflecting their functional importance to the network. Our analysis shows that atherosclerosis-associated ophthalmic complications are caused by the convergence of neurotrophin signaling pathways, multiple immune response pathways and focal adhesion pathway on the MAPK signaling pathway. The PPI network shares features with vasoregression, a process underlying multiple vascular eye diseases. Our study presents a first clear and composite picture of the components and crosstalk of the main pathways of atherosclerosis-induced ocular diseases. The hub bottleneck nodes highlight the presence of molecules important for mediating the ophthalmic complications of atherosclerosis and contain five established drug targets for future therapeutic modulation efforts.

R9AP overexpression alters phototransduction kinetics in iCre75 mice

PURPOSE

Determine the impact of rod photoreceptor-specific expression of Cre recombinase on the kinetics of phototransduction in the mouse eye and identify changes in gene expression that underlie any observed phenotypic differences.

METHODS

Transretinal ERG and single-cell suction electrode recordings were used to measure the kinetics of phototransduction in a mouse line exhibiting rod photoreceptor-specific Cre recombinase expression, and the results were compared with those from control non-Cre-expressing littermates. Gene expression changes were evaluated using RNA sequencing transcriptome analysis. The pattern of expression of Rgs9bp was determined by mapping sequencing reads to the mouse genome and performing 3'-rapid amplification of cDNA ends (3'-RACE).

RESULTS

Expression of the rod-specific iCre75 transgene was accompanied by accelerated phototransduction inactivation, likely due to overexpression of the Rgs9bp gene, which encodes the Rgs9 anchor protein (R9AP). R9AP upregulation stabilized the RGS9 GAP complex, altering phototransduction kinetics. 3'-Race identified an abundant, unexpected Rgs9bp-Prm1 fusion mRNA in Cre-expressing mouse retinas, which was determined to be derived from a second transgene present in the iCre75 line.

CONCLUSIONS

Here we report the presence of a second, R9AP-expressing transgene in the iCre75 mouse line, leading to altered kinetics of phototransduction. These results highlight an important caveat that must be considered when utilizing this mouse line for rod photoreceptor-specific gene loss of function studies.

The CRB1 and adherens junction complex proteins in retinal development and maintenance

The early developing retinal neuroepithelium is composed of multipotent retinal progenitor cells that differentiate in a time specific manner, giving rise to six major types of neuronal and one type of glial cells. These cells migrate and organize in three distinct nuclear layers divided by two plexiform layers. Apical and adherens junction complexes have a crucial role in this process by the establishment of polarity and adhesion. Changes in these complexes disturb the spatiotemporal aspects of retinogenesis, leading to retinal degeneration resulting in mild or severe impairment of retinal function and vision. In this review, we summarize the mouse models for the different members of the apical and adherens junction protein complexes and describe the main features of their retinal phenotypes. The knowledge acquired from the different mutant animals for these proteins corroborate their importance in retina development and maintenance of normal retinal structure and function. More recently, several studies have tried to unravel the connection between the apical proteins, important cellular signaling pathways and their relation in retina development. Still, the mechanisms by which these proteins function remain largely unknown. Here, we hypothesize how the mammalian apical CRB1 complex might control retinogenesis and prevents onset of Leber congenital amaurosis or retinitis pigmentosa.

Two structural components in CNGA3 support regulation of cone CNG channels by phosphoinositides

Cyclic nucleotide-gated (CNG) channels in retinal photoreceptors play a crucial role in vertebrate phototransduction. The ligand sensitivity of photoreceptor CNG channels is adjusted during adaptation and in response to paracrine signals, but the mechanisms involved in channel regulation are only partly understood. Heteromeric cone CNGA3 (A3) + CNGB3 (B3) channels are inhibited by membrane phosphoinositides (PIP(n)), including phosphatidylinositol 3,4,5-triphosphate (PIP(3)) and phosphatidylinositol 4,5-bisphosphate (PIP(2)), demonstrating a decrease in apparent affinity for cyclic guanosine monophosphate (cGMP). Unlike homomeric A1 or A2 channels, A3-only channels paradoxically did not show a decrease in apparent affinity for cGMP after PIP(n) application. However, PIP(n) induced an ∼2.5-fold increase in cAMP efficacy for A3 channels. The PIP(n)-dependent change in cAMP efficacy was abolished by mutations in the C-terminal region (R643Q/R646Q) or by truncation distal to the cyclic nucleotide-binding domain (613X). In addition, A3-613X unmasked a threefold decrease in apparent cGMP affinity with PIP(n) application to homomeric channels, and this effect was dependent on conserved arginines within the N-terminal region of A3. Together, these results indicate that regulation of A3 subunits by phosphoinositides exhibits two separable components, which depend on structural elements within the N- and C-terminal regions, respectively. Furthermore, both N and C regulatory modules in A3 supported PIP(n) regulation of heteromeric A3+B3 channels. B3 subunits were not sufficient to confer PIP(n) sensitivity to heteromeric channels formed with PIP(n)-insensitive A subunits. Finally, channels formed by mixtures of PIP(n)-insensitive A3 subunits, having complementary mutations in N- and/or C-terminal regions, restored PIP(n) regulation, implying that intersubunit N-C interactions help control the phosphoinositide sensitivity of cone CNG channels.

Light activation of the insulin receptor regulates mitochondrial hexokinase. A possible mechanism of retinal neuroprotection

The serine/threonine kinase Akt has been shown to mediate the anti-apoptotic activity through hexokinase (HK)-mitochondria interaction. We previously reported that Akt activation in retinal rod photoreceptor cells is mediated through the light-dependent insulin receptor (IR)/PI3K pathway. Our data indicate that light-induced activation of IR/PI3K/Akt results in the translocation of HK-II to mitochondria. We also found that PHLPPL, a serine/threonine phosphatase, enhanced the binding of HK-II to mitochondria. We found a mitochondrial targeting signal in PHLPPL and our study suggests that Akt translocation to mitochondria could be mediated through PHLPPL. Our results suggest that the light-dependent IR/PI3K/Akt pathway regulates hexokinase-mitochondria interaction in photoreceptors. Down-regulation of IR signaling has been associated with ocular diseases of retinitis pigmentosa, diabetic retinopathy, and Leber Congenital Amaurosis-type 2, and agents that enhance the binding interaction between hexokinase and mitochondria may have therapeutic potential against these ocular diseases.

P-Rex2, a Rac-guanine nucleotide exchange factor, is expressed selectively in ribbon synaptic terminals of the mouse retina

Background

Phosphatidylinositol (3,4,5)-trisphosphate-dependent Rac Exchanger 2 (P-Rex2) is a guanine nucleotide exchange factor (GEF) that specifically activates Rac GTPases, important regulators of actin cytoskeleton remodeling. P-Rex2 is known to modulate cerebellar Purkinje cell architecture and function, but P-Rex2 expression and function elsewhere in the central nervous system is unclear. To better understand potential roles for P-Rex2 in neuronal cytoskeletal remodeling and function, we performed widefield and confocal microscopy of specimens double immunolabeled for P-Rex2 and cell- and synapse-specific markers in the mouse retina.

Results

P-Rex2 was restricted to the plexiform layers of the retina and colocalized extensively with Vesicular Glutamate Transporter 1 (VGluT1), a specific marker for photoreceptor and bipolar cell terminals. Double labeling for P-Rex2 and peanut agglutinin, a cone terminal marker, confirmed that P-Rex2 was present in both rod and cone terminals. Double labeling with markers for specific bipolar cell types showed that P-Rex2 was present in the terminals of rod bipolar cells and multiple ON- and OFF-cone bipolar cell types. In contrast, P-Rex2 was not expressed in the processes or conventional synapses of amacrine or horizontal cells.

Conclusions

P-Rex2 is associated specifically with the glutamatergic ribbon synaptic terminals of photoreceptors and bipolar cells that transmit visual signals vertically through the retina. The Rac-GEF function of P-Rex2 implies a specific role for P-Rex2 and Rac-GTPases in regulating the actin cytoskeleton in glutamatergic ribbon synaptic terminals of retinal photoreceptors and bipolar cells and appears to be ideally positioned to modulate the adaptive plasticity of these terminals.

Insulin receptor signaling in cones

In humans, age-related macular degeneration and diabetic retinopathy are the most common disorders affecting cones. In retinitis pigmentosa (RP), cone cell death precedes rod cell death. Systemic administration of insulin delays the death of cones in RP mouse models lacking rods. To date there are no studies on the insulin receptor signaling in cones; however, mRNA levels of IR signaling proteins are significantly higher in cone-dominant neural retina leucine zipper (Nrl) knock-out mouse retinas compared with wild type rod-dominant retinas. We previously reported that conditional deletion of the p85α subunit of phosphoinositide 3-kinase (PI3K) in cones resulted in age-related cone degeneration, and the phenotype was not rescued by healthy rods, raising the question of why cones are not protected by the rod-derived cone survival factors. Interestingly, systemic administration of insulin has been shown to delay the death of cones in mouse models of RP lacking rods. These observations led to the hypothesis that cones may have their own endogenous neuroprotective pathway, or rod-derived cone survival factors may be signaled through cone PI3K. To test this hypothesis we generated p85α(-/-)/Nrl(-/-) double knock-out mice and also rhodopsin mutant mice lacking p85α and examined the effect of the p85α subunit of PI3K on cone survival. We found that the rate of cone degeneration is significantly faster in both of these models compared with respective mice with competent p85α. These studies suggest that cones may have their own endogenous PI3K-mediated neuroprotective pathway in addition to the cone viability survival signals derived from rods.

CNGA3 achromatopsia-associated mutation potentiates the phosphoinositide sensitivity of cone photoreceptor CNG channels by altering intersubunit interactions

Cyclic nucleotide-gated (CNG) channels are critical for sensory transduction in retinal photoreceptors and olfactory receptor cells; their activity is modulated by phosphoinositides (PIPn) such as phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 3,4,5-trisphosphate (PIP3). An achromatopsia-associated mutation in cone photoreceptor CNGA3, L633P, is located in a carboxyl (COOH)-terminal leucine zipper domain shown previously to be important for channel assembly and PIPn regulation. We determined the functional consequences of this mutation using electrophysiological recordings of patches excised from cells expressing wild-type and mutant CNG channel subunits. CNGA3-L633P subunits formed functional channels with or without CNGB3, producing an increase in apparent cGMP affinity. Surprisingly, L633P dramatically potentiated PIPn inhibition of apparent cGMP affinity for these channels. The impact of L633P on PIPn sensitivity depended on an intact amino (NH2) terminal PIPn regulation module. These observations led us to hypothesize that L633P enhances PIPn inhibition by altering the coupling between NH2- and COOH-terminal regions of CNGA3. A recombinant COOH-terminal fragment partially restored normal PIPn sensitivity to channels with COOH-terminal truncation, but L633P prevented this effect. Furthermore, coimmunoprecipitation of channel fragments, and thermodynamic linkage analysis, also provided evidence for NH2-COOH interactions. Finally, tandem dimers of CNGA3 subunits that specify the arrangement of subunits containing L633P and other mutations indicated that the putative interdomain interaction occurs between channel subunits (intersubunit) rather than exclusively within the same subunit (intrasubunit). Collectively, these studies support a model in which intersubunit interactions control the sensitivity of cone CNG channels to regulation by phosphoinositides. Aberrant channel regulation may contribute to disease progression in patients with the L633P mutation.

Preventing retinal apoptosis--is there a common therapeutic theme?

There is an urgent need for therapies for retinal diseases; retinitis pigmentosa sufferers have no treatment options available and those targeted at other retinopathies have shown limited effectiveness. The process of programmed cell death or apoptosis although complex, remains a possible target for the treatment of retinal diseases. Having identified apoptosis in the vertebrate retina in populations of immature neurons as an essential part of development it was proposed that re-activation of these developmental cell death pathways might provide insight into the death mechanisms operating in retinal diseases. However, the discovery that numerous factors initiate and mediate the apoptotic cascade in mature photoreceptors has resulted in a relatively untargeted approach to examining and arresting apoptosis in the retina. In the last 5 years, mouse models have been treated with a diverse range of drugs or factors including anti-oxidants, growth factors, steroid hormones, calcium/calpain inhibitors and tetracycline antibiotics. Therefore to draw a unifying theme from these broad research areas is challenging. However, this review focusses on two targets which are currently under investigation, reactive oxygen species and mammalian target of rapamycin, drawing together the common themes of these research areas.

℮-conome: an automated tissue counting platform of cone photoreceptors for rodent models of retinitis pigmentosa

Background

Retinitis pigmentosa is characterized by the sequential loss of rod and cone photoreceptors. The preservation of cones would prevent blindness due to their essential role in human vision. Rod-derived Cone Viability Factor is a thioredoxin-like protein that is secreted by rods and is involved in cone survival. To validate the activity of Rod-derived Cone Viability Factors (RdCVFs) as therapeutic agents for treating retinitis Pigmentosa, we have developed e-conome, an automated cell counting platform for retinal flat mounts of rodent models of cone degeneration. This automated quantification method allows for faster data analysis thereby accelerating translational research.

Methods

An inverted fluorescent microscope, motorized and coupled to a CCD camera records images of cones labeled with fluorescent peanut agglutinin lectin on flat-mounted retinas. In an average of 300 fields per retina, nine Z-planes at magnification X40 are acquired after two-stage autofocus individually for each field. The projection of the stack of 9 images is subject to a threshold, filtered to exclude aberrant images based on preset variables. The cones are identified by treating the resulting image using 13 variables empirically determined. The cone density is calculated over the 300 fields.

Results

The method was validated by comparison to the conventional stereological counting. The decrease in cone density in rd1 mouse was found to be equivalent to the decrease determined by stereological counting. We also studied the spatiotemporal pattern of the degeneration of cones in the rd1 mouse and show that while the reduction in cone density starts in the central part of the retina, cone degeneration progresses at the same speed over the whole retinal surface. We finally show that for mice with an inactivation of the Nucleoredoxin-like genes Nxnl1 or Nxnl2 encoding RdCVFs, the loss of cones is more pronounced in the ventral retina.

Conclusion

The automated platform ℮-conome used here for retinal disease is a tool that can broadly accelerate translational research for neurodegenerative diseases.

Loss of daylight vision in retinal degeneration: are oxidative stress and metabolic dysregulation to blame?

Retinitis pigmentosa is characterized by loss of night vision, followed by complete blindness. Over 40 genetic loci for retinitis pigmentosa have been identified in humans, primarily affecting photoreceptor structure and function. The availability of excellent animal models allows for a mechanistic characterization of the disease. Metabolic dysregulation and oxidative stress have been found to correlate with the loss of vision, particularly in cones, the type of photoreceptors that mediate daylight and color vision. The evidence that these problems actually cause loss of vision and potential therapeutic approaches targeting them are discussed.

Phosphoinositides and photoreceptors

The importance of phosphoinositides (phosphorylated phosphatidyl inositol derivatives, PIs) for normal cellular function cannot be overstated. Although they represent a small fraction of the total phospholipid within the cell, they are essential regulators of many cellular functions. They direct membrane trafficking by functioning as recruitment factors for vesicular trafficking components, they can modulate ion channel activity through direct binding within cellular membranes, and their hydrolysis generates second messenger signaling molecules. Despite an explosion of information regarding the importance of these lipids in cellular biology, their precise roles in vertebrate retinal photoreceptors has not been established. This review summarizes the literature on potential roles for different phosphoinositides and their regulators in vertebrate rods and cones. A brief description of the importance of PI signaling in other photosensitive cells is also presented. The highly specialized functions of the vertebrate photoreceptor, combined with the established importance of phosphoinositides, promise significant future discoveries in this field.

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.

Phosphorylated Grb14 is an endogenous inhibitor of retinal protein tyrosine phosphatase 1B, and light-dependent activation of Src phosphorylates Grb14

Growth factor receptor-bound protein 14 (Grb14) is an adapter protein implicated in receptor tyrosine kinase signaling. Grb14(-/-) studies highlight both the positive and negative roles of Grb14 in receptor tyrosine kinase signaling in a tissue-specific manner. In this study, we made a novel finding that Grb14 inhibits the activity of PTP1B, the major negative regulator of insulin receptor (IR) signaling, in a phosphorylation-regulated manner. Phosphorylation of Tyr-347 in the BPS domain of Grb14 is critical for interaction with PTP1B, resulting in the competitive inhibition of PTP1B activity. We also found that rhodopsin-regulated Src kinase activation in retina leads to the phosphorylation of Grb14. Further, ablation of Grb14 resulted in significantly elevated retinal PTP1B activity in vivo. PTP1B is known to be regulated by oxidation, glutathionylation, phosphorylation, and SUMOlyation, and our study for the first time demonstrates the inhibition of PTP1B activity in vivo by protein molecule Grb14 in a tissue-specific manner.