Loss of synaptic connectivity, particularly in second order neurons is a key feature of diabetic retinal neuropathy in the Ins2Akita mouse

Electroretinographic evidence suggesting that the type 2 diabetic retinopathy of the sand rat Psammomys obesus is comparable to that of humans

Purpose

Type 2 diabetic retinopathy is the main cause of acquired blindness in adults. The aim of this work was to examine the retinal function of the sand rat Psammomys obesus as an animal model of diet-induced type 2 diabetes when subjected to a hypercaloric regimen.

Materials and methods

Hyperglycemia was induced in Psammomys obesus by high caloric diet (4 kcal/g). The visual function of control (n = 7) and diabetic (n = 7) adult rodents were followed up during 28 consecutive weeks with full-field electroretinogram(ERG) recordings evoked to flashes of white light according to the standard protocol of the International Society for Clinical Electrophysiology of Vision protocol (ISCEV).

Results

Twenty-eight weeks following the induction of diabetes, results revealed significantly reduced and delayed photopic and scotopic ERG responses in diabetic rats compared to control rats. More specifically, we noted a significant decrease in the amplitude of the dark-adapted 0.01ERG (62%), a- and b-wave amplitudes of the dark-adapted 3.0 ERG (33.6%, 55.1%) and the four major oscillatory potentials components (OP1-OP4) (39.0%, 75.2%, 54.8% and 53.7% respectively). In photopic conditions, diabetic rats showed a significant decrease in a- and b-wave (30.4%, 43.4%), photopic negative response (55.3%), 30 Hz flicker (63.7%), OP1-OP4(51.6%, 61.8%, 68.3% and 47.5% respectively) and S-cone (34.7%). Significantly delayed implicit times were observed for all ERG components in the diabetic animals. Results obtained are comparable to those characterizing the retinal function of patients affected with advanced stage of diabetic retinopathy.

Conclusion

Psammomys obesus is a useful translational model to study the pathophysiology of diabetic retinopathy in order to explore new therapeutic avenues in human patients.

Neurodegeneration in diabetic retinopathy: Potential for novel therapies

The complex pathology of diabetic retinopathy (DR) affects both vascular and neural tissue. The characteristics of neurodegeneration are well-described in animal models but have more recently been confirmed in the clinical setting, mostly by using non-invasive imaging approaches such as spectral domain optical coherence tomography (SD-OCT). The most frequent observations report loss of tissue in the nerve fiber layer and inner plexiform layer, confirming earlier findings from animal models. In several cases the reduction in inner retinal layers is reported in patients with little evidence of vascular lesions or macular edema, suggesting that degenerative loss of neural tissue in the inner retina can occur after relatively short durations of diabetes. Animal studies also suggest that neurodegeneration leading to retinal thinning is not limited to cell death and tissue loss but also includes changes in neuronal morphology, reduced synaptic protein expression and alterations in neurotransmission, including changes in expression of neurotransmitter receptors as well as neurotransmitter release, reuptake and metabolism. The concept of neurodegeneration as an early component of DR introduces the possibility to explore alternative therapies to prevent the onset of vision loss, including neuroprotective therapies and drugs targeting individual neurotransmitter systems, as well as more general neuroprotective approaches to preserve the integrity of the neural retina. In this review we consider some of the evidence for progressive retinal neurodegeneration in diabetes, and explore potential neuroprotective therapies.

Early Functional and Morphologic Abnormalities in the Diabetic Nyxnob Mouse Retina

Purpose

The electroretinogram c-wave is generated by the summation of the positive polarity hyperpolarization of the apical RPE membrane and a negative polarity slow PIII response of Müller glia cells. Therefore, the c-wave reduction noted in prior studies of mouse models of diabetes could reflect a reduction in the RPE component or an increase in slow PIII. The present study used a genetic approach to distinguish between these two alternatives.

Methods

Nyx^nob mice lack the ERG b-wave, revealing the early phase of slow PIII. To visualize changes in slow PIII due to diabetes, Nyx^nob mice were given streptozotocin (STZ) injections to induce diabetes or received vehicle as a control. After 1, 2, and 4 weeks of sustained hyperglycemia (>250 mg/dL), standard strobe flash ERG and dc-ERG testing were conducted. Histological analysis of the retina was performed.

Results

A reduced c-wave was noted at the 1 week time point, and persisted at later time points. In comparison, slow PIII amplitudes were unaffected after 1 week of hyperglycemia, but were significantly reduced in STZ mice at the 2-week time point. The decrease in amplitude occurred before any identifiable decrease to the a-wave. At the later time point, the a-wave became involved, although the slow PIII reductions were more pronounced. Morphological abnormalities in the RPE, including increased thickness and altered melanosome distribution, were identified in diabetic animals.

Conclusions

Because the c-wave and slow PIII were both reduced, these results demonstrated that diabetes-induced reductions to the c-wave cannot be attributed to an early increase in the Müller glia-derived potassium conductance. Furthermore, because the a-wave, slow PIII and c-wave reductions were not equivalent, and varied in their onset, the reductions cannot reflect the same mechanism, such as a change in membrane resistance. The presence of small changes to RPE architecture indicate that the c-wave reductions present in diabetic mice likely represents a primary change in the RPE induced by hyperglycemia.

Study of retinal neurodegeneration and maculopathy in diabetic Meriones shawi: A particular animal model with human-like macula

The purpose of this work was to evaluate a potentially useful animal model, Meriones shawi (M.sh)-developing metabolic X syndrome, diabetes and possessing a visual streak similar to human macula-in the study of diabetic retinopathy and diabetic macular edema (DME). Type 2 diabetes (T2D) was induced by high fat diet administration in M.sh. Body weights, blood glucose levels were monitored throughout the study. Diabetic retinal histopathology was evaluated 3 and 7 months after diabetes induction. Retinal thickness was measured, retinal cell types were labeled by immunohistochemistry and the number of stained elements were quantified. Apoptosis was determined with TUNEL assay. T2D induced progressive changes in retinal histology. A significant decrease of retinal thickness and glial reactivity was observed without an increase in apoptosis rate. Photoreceptor outer segment degeneration was evident, with a significant decrease in the number of all cones and M-cone subtype, but-surprisingly-an increase in S-cones. Damage of the pigment epithelium was also confirmed. A decrease in the number and labeling intensity of parvalbumin- and calretinin-positive amacrine cells and a loss of ganglion cells was detected. Other cell types showed no evident alterations. No DME-like condition was noticed even after 7 months. M.sh could be a useful model to study the evolution of diabetic retinal pathology and to identify the role of hypertension and dyslipidemia in the development of the reported alterations. Longer follow up would be needed to evaluate the potential use of the visual streak in modeling human macular diseases.

Diabetic retinopathy and dysregulated innate immunity

Diabetic retinopathy (DR) is the progressive degeneration of retinal blood vessels and neurons. Inflammation is known to play an important role in the pathogenesis of DR. During diabetes, metabolic disorder leads to the release of damage-associated molecular patterns (DAMPs) both in the retina and elsewhere in the body. The innate immune system provides the first line of defense against the DAMPs. In the early stages of DR when the blood retinal barrier (BRB) is intact, retinal microglia and the complement system are activated at low levels. This low-level of inflammation (para-inflammation) is believed to be essential to maintain homeostasis and restore functionality. However, prolonged stimulation by DAMPs in the diabetic eye leads to maladaptation of the innate immune system and dysregulated para-inflammation may contribute to DR development. In the advanced stages of DR where immune privilege is comprised, circulating immune cells and serum proteins may infiltrate the retina and participate in retinal chronic inflammation and retinal vascular and neuronal damage. This review discusses how the innate immune system is activated in diabetes and DR. The view also discusses why the protective immune response becomes detrimental in DR.

Serum retinol-binding protein-induced endothelial inflammation is mediated through the activation of toll-like receptor 4

PURPOSE

Elevation of serum retinol-binding protein 4 (RBP4) induces inflammation in primary human retinal microvascular endothelial cells (HRECs) via a retinol-independent mechanism; thus, it may play a causative role in the development and progression of vascular lesions in diabetic retinopathy (DR). Since HRECs do not express the classical RBP4 receptor, stimulated by retinoic acid gene 6 (STRA6), this study focuses on identifying the endothelial cell receptor and signaling that mediate RBP4-induced inflammation.

METHODS

HRECs were treated with a toll-like receptor 4 (TLR4) small molecule inhibitor (Cli95, also known as TAK-242), TLR4 neutralizing antibody, or mitogen-activated protein kinase (MAPK) inhibitors before treatment with purified recombinant RBP4. The HREC inflammatory response was quantified by in vitro leukostasis assays, western blotting, and enzyme-linked immunosorbent assay (ELISA). To understand how the serum binding partner for RBP4, transthyretin (TTR), may affect RBP4 activity, we also measured RBP4 and TTR levels in serum and retinal lysates from RBP4-Tg and wild-type mice.

RESULTS

TLR4 inhibition significantly reduced RBP4-induced expression of pro-inflammatory proteins and in vitro leukostasis. RBP4 treatment significantly increased phosphoactivation of p38 and c-Jun N-terminal protein kinase (JNK). The p38 inhibitor (SB203580) attenuated RBP4-stimulated vascular cell adhesion molecule 1 (VCAM-1), intracellular adhesion molecule 1 (ICAM-1), monocyte chemoattractant protein (MCP-1), and interleukin 6 (IL-6) production, while the JNK inhibitor (SP600125) reduced RBP4-stimulated sICAM-1, endothelial cell selectin (E-selectin), and MCP-1 production. The MAPK inhibitors only showed partial (50-70%) suppression of the RBP4-stimulated proinflammatory response. Moreover, TLR4 inhibition did not decrease RBP4-induced MAPK phosphoactivation, suggesting that RBP4-mediated MAPK activation is TLR4 independent and occurs through a secondary unknown receptor. We also found that the RBP4/TTR molar ratio was exceptionally high in the retina of RBP4-Tg mice, indicating an abundance of TTR-free RBP4.

CONCLUSIONS

RBP4-induced inflammation is largely mediated by TLR4, and in part, through JNK and p38 MAPK signaling. The high TTR/RBP4 molar ratio in serum likely protects the endothelium from the proinflammatory effects of RBP4 in vivo, whereas elevation of serum RBP4 causes a significant increase in TTR-free RBP4 in retinal tissue. This offers insight into how RBP4-Tg mice can develop retinal neurodegeneration without coincident retinal microvascular pathology.

Adrenomedullin Suppresses Vascular Endothelial Growth Factor-Induced Vascular Hyperpermeability and Inflammation in Retinopathy

Diabetic macular edema (DME) is caused by blood-retinal barrier breakdown associated with retinal vascular hyperpermeability and inflammation, and it is the major cause of visual dysfunction in diabetic retinopathy. Adrenomedullin (ADM) is an endogenous peptide first identified as a strong vasodilator. ADM is expressed in the eyes and is up-regulated in various eye diseases, although the pathophysiological significance is largely unknown. We investigated the effect of ADM on DME. In Kimba mice, which overexpress human vascular endothelial growth factor in their retinas, the capillary dropout, vascular leakage, and vascular fragility characteristic of diabetic retinopathy were observed. Intravitreal or systemic administration of ADM to Kimba mice ameliorated both the capillary dropout and vascular leakage. Evaluation of the transendothelial electrical resistance and fluorescein isothiocyanate-dextran permeability of an endothelial cell monolayer using TR-iBRB retinal capillary endothelial cells revealed that vascular endothelial growth factor enhanced vascular permeability but that co-administration of ADM suppressed the effect, in part by enhancing tight junction formation between endothelial cells. In addition, a comprehensive PCR array analysis showed that ADM administration suppressed various molecules related to inflammation and NF-κB signaling within retinas. From these results, we suggest that by exerting inhibitory effects on retinal inflammation, vascular permeability, and blood-retinal barrier breakdown, ADM could serve as a novel therapeutic agent for the treatment of DME.

Study of retinal alterations in a high fat diet-induced type ii diabetes rodent: Meriones shawi

Diabetic retinopathy is a common complication of type 2 diabetes and the leading cause of blindness in adults of working age. The aim of this work was to study the repercussions of high fat diet (HFD) induced diabetes on the retina of Meriones shawi (M.sh). Two groups of six M.sh each was studied. Group I was a normal control, fed with standard laboratory granules. In Group II, rodents received a HFD of enriched laboratory granules, for a period of 3 months. Body weight and plasma glucose were determined in the two groups. Retinal sections of the two groups were stained with the Hematoxylin-Eosin. Photoreceptors were identified by immunolabeling for rhodopsin (rods) and PNA (cones). Gliosis and microglial activation were identified by immunolabeling for GFAP and Iba-1. Labeling of calretinin and parvalbumin were also carried out to study the AII amacrine cells. Retinal layers thicknesses, gliosis, and specific neural cell populations were quantified by microscopy. The body weight (+77%) and plasma glucose (+108%) were significantly greater in the HFD rodents. Three months of HFD induced a significant loss of 38.77% of cone photoreceptors, as well as gliosis and an increase of 70.67% of microglial cells. Calcium homeostatic enzymes were depleted. This work shows that HFD in Meriones shawi induces a type II diabetes-like condition that causes loss of retinal neurons and photoreceptors, as well as gliosis. Meriones shawi could be a useful experimental animal model for this physiopathology particularly in the study of retinal neuro-glial alterations in Type II diabetes.

Insights from Genetic Model Systems of Retinal Degeneration: Role of Epsins in Retinal Angiogenesis and VEGFR2 Signaling

The retina is a light sensitive tissue that contains specialized photoreceptor cells called rods and cones which process visual signals. These signals are relayed to the brain through interneurons and the fibers of the optic nerve. The retina is susceptible to a variety of degenerative diseases, including age-related macular degeneration (AMD), diabetic retinopathy (DR), retinitis pigmentosa (RP) and other inherited retinal degenerations. In order to reveal the mechanism underlying these diseases and to find methods for the prevention/treatment of retinal degeneration, animal models have been generated to mimic human eye diseases. In this paper, several well-characterized and commonly used animal models are reviewed. Of particular interest are the contributions of these models to our understanding of the mechanisms of retinal degeneration and thereby providing novel treatment options including gene therapy, stem cell therapy, nanomedicine, and CRISPR/Cas9 genome editing. Role of newly-identified adaptor protein epsins from our laboratory is discussed in retinal angiogenesis and VEGFR2 signaling.

Insulin signaling controls neurotransmission via the 4eBP-dependent modification of the exocytotic machinery

Altered insulin signaling has been linked to widespread nervous system dysfunction including cognitive dysfunction, neuropathy and susceptibility to neurodegenerative disease. However, knowledge of the cellular mechanisms underlying the effects of insulin on neuronal function is incomplete. Here, we show that cell autonomous insulin signaling within the Drosophila CM9 motor neuron regulates the release of neurotransmitter via alteration of the synaptic vesicle fusion machinery. This effect of insulin utilizes the FOXO-dependent regulation of the thor gene, which encodes the Drosophila homologue of the eif-4e binding protein (4eBP). A critical target of this regulatory mechanism is Complexin, a synaptic protein known to regulate synaptic vesicle exocytosis. We find that the amounts of Complexin protein observed at the synapse is regulated by insulin and genetic manipulations of Complexin levels support the model that increased synaptic Complexin reduces neurotransmission in response to insulin signaling.

The Effects of Diabetic Retinopathy and Pan-Retinal Photocoagulation on Photoreceptor Cell Function as Assessed by Dark Adaptometry

Purpose

The pathophysiology of vision loss in persons with diabetic retinopathy (DR) is complex and incompletely defined. We hypothesized that retinal pigment epithelium (RPE) and rod and cone photoreceptor dysfunction, as measured by dark adaptometry, would increase with severity of DR, and that pan-retinal photocoagulation (PRP) would exacerbate this dysfunction.

Methods

Dark adaptation (DA) was measured in subjects with diabetes mellitus and healthy controls. Dark adaptation was measured at 5° superior to the fovea following a flash bleach, and the data were analyzed to yield cone and rod sensitivity curves. Retinal layer thicknesses were quantified using spectral-domain optical coherence tomography (OCT).

Results

The sample consisted of 23 controls and 73 diabetic subjects. Subjects with moderate nonproliferative diabetic retinopathy (NPDR) exhibited significant impairment of rod recovery rate compared with control subjects (P = 0.04). Cone sensitivity was impaired in subjects with proliferative diabetic retinopathy (PDR) (type 1 diabetes mellitus [T1DM]: P = 0.0047; type 2 diabetes mellitus [T2DM]: P < 0.001). Subjects with untreated PDR compared with subjects treated with PRP exhibited similar rod recovery rates and cone sensitivities. Thinner RPE as assessed by OCT was associated with slower rod recovery and lower cone sensitivity, and thinner photoreceptor inner segment/outer segment layer was associated with lower cone sensitivity.

Conclusions

The results suggest that RPE and photoreceptor cell dysfunction, as assessed by cone sensitivity level and rod- and RPE-mediated dark adaptation, progresses with worsening DR, and rod recovery dysfunction occurs earlier than cone dysfunction. Function was preserved following PRP. The findings suggest multiple defects in retinoid function and provide potential points to improve visual function in persons with PDR.

Retinal neurodegeneration may precede microvascular changes characteristic of diabetic retinopathy in diabetes mellitus

Diabetic retinopathy (DR) has long been recognized as a microvasculopathy, but retinal diabetic neuropathy (RDN), characterized by inner retinal neurodegeneration, also occurs in people with diabetes mellitus (DM). We report that in 45 people with DM and no to minimal DR there was significant, progressive loss of the nerve fiber layer (NFL) (0.25 μm/y) and the ganglion cell (GC)/inner plexiform layer (0.29 μm/y) on optical coherence tomography analysis (OCT) over a 4-y period, independent of glycated hemoglobin, age, and sex. The NFL was significantly thinner (17.3 μm) in the eyes of six donors with DM than in the eyes of six similarly aged control donors (30.4 μm), although retinal capillary density did not differ in the two groups. We confirmed significant, progressive inner retinal thinning in streptozotocin-induced "type 1" and B6.BKS(D)-Lepr(db)/J "type 2" diabetic mouse models on OCT; immunohistochemistry in type 1 mice showed GC loss but no difference in pericyte density or acellular capillaries. The results suggest that RDN may precede the established clinical and morphometric vascular changes caused by DM and represent a paradigm shift in our understanding of ocular diabetic complications.

Involvement of Autophagic Pathway in the Progression of Retinal Degeneration in a Mouse Model of Diabetes

The notion that diabetic retinopathy (DR) is essentially a micro-vascular disease has been recently challenged by studies reporting that vascular changes are preceded by signs of damage and loss of retinal neurons. As to the mode by which neuronal death occurs, the evidence that apoptosis is the main cause of neuronal loss is far from compelling. The objective of this study was to investigate these controversies in a mouse model of streptozotocin (STZ) induced diabetes. Starting from 8 weeks after diabetes induction there was loss of rod but not of cone photoreceptors, together with reduced thickness of the outer and inner synaptic layers. Correspondingly, rhodopsin expression was downregulated and the scotopic electroretinogram (ERG) is suppressed. In contrast, cone opsin expression and photopic ERG response were not affected. Suppression of the scotopic ERG preceded morphological changes as well as any detectable sign of vascular alteration. Only sparse apoptotic figures were detected by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay and glia was not activated. The physiological autophagy flow was altered instead, as seen by increased LC3 immunostaining at the level of outer plexiform layer (OPL) and upregulation of the autophagic proteins Beclin-1 and Atg5. Collectively, our results show that the streptozotocin induced DR in mouse initiates with a functional loss of the rod visual pathway. The pathogenic pathways leading to cell death develop with the initial dysregulation of autophagy well before the appearance of signs of vascular damage and without strong involvement of apoptosis.

Diabetic Retinopathy: Animal Models, Therapies, and Perspectives

Diabetic retinopathy (DR) is one of the major complications of diabetes. Although great efforts have been made to uncover the mechanisms underlying the pathology of DR, the exact causes of DR remain largely unknown. Because of multifactor involvement in DR etiology, currently no effective therapeutic treatments for DR are available. In this paper, we review the pathology of DR, commonly used animal models, and novel therapeutic approaches. Perspectives and future directions for DR treatment are discussed.

Sustained intraocular VEGF neutralization results in retinal neurodegeneration in the Ins2(Akita) diabetic mouse

Current therapies that target vascular endothelial growth factor (VEGF) have become a mainstream therapy for the management of diabetic macular oedema. The treatment involves monthly repeated intravitreal injections of VEGF inhibitors. VEGF is an important growth factor for many retinal cells, including different types of neurons. In this study, we investigated the adverse effect of multiple intravitreal anti-VEGF injections (200 ng/μl/eye anti-mouse VEGF164, once every 2 weeks totalling 5-6 injections) to retinal neurons in Ins2(Akita) diabetic mice. Funduscopic examination revealed the development of cotton wool spot-like lesions in anti-VEGF treated Ins2(Akita) mice after 5 injections. Histological investigation showed focal swellings of retinal nerve fibres with neurofilament disruption. Furthermore, anti-VEGF-treated Ins2(Akita) mice exhibited impaired electroretinographic responses, characterized by reduced scotopic a- and b-wave and oscillatory potentials. Immunofluorescent staining revealed impairment of photoreceptors, disruptions of synaptic structures and loss of amacrine and retinal ganglion cells in anti-VEGF treated Ins2(Akita) mice. Anti-VEGF-treated WT mice also presented mild amacrine and ganglion cell death, but no overt abnormalities in photoreceptors and synaptic structures. At the vascular level, exacerbated albumin leakage was observed in anti-VEGF injected diabetic mice. Our results suggest that sustained intraocular VEGF neutralization induces retinal neurodegeneration and vascular damage in the diabetic eye.

Intravitreal AAV2.COMP-Ang1 Prevents Neurovascular Degeneration in a Murine Model of Diabetic Retinopathy

Diabetic retinopathy (DR) is the leading cause of blindness in the working-age population in the U.S. The vision-threatening processes of neuroglial and vascular dysfunction in DR occur in concert, driven by hyperglycemia and propelled by a pathway of inflammation, ischemia, vasodegeneration, and breakdown of the blood retinal barrier. Currently, no therapies exist for normalizing the vasculature in DR. Here, we show that a single intravitreal dose of adeno-associated virus serotype 2 encoding a more stable, soluble, and potent form of angiopoietin 1 (AAV2.COMP-Ang1) can ameliorate the structural and functional hallmarks of DR in Ins2Akita mice, with sustained effects observed through six months. In early DR, AAV2.COMP-Ang1 restored leukocyte-endothelial interaction, retinal oxygenation, vascular density, vascular marker expression, vessel permeability, retinal thickness, inner retinal cellularity, and retinal neurophysiological response to levels comparable with nondiabetic controls. In late DR, AAV2.COMP-Ang1 enhanced the therapeutic benefit of intravitreally delivered endothelial colony-forming cells by promoting their integration into the vasculature and thereby stemming further visual decline. AAV2.COMP-Ang1 single-dose gene therapy can prevent neurovascular pathology, support vascular regeneration, and stabilize vision in DR.

Wnt signaling in age-related macular degeneration: human macular tissue and mouse model

Background

The wingless-type MMTV integration site (Wnt) signaling is a group of signal transduction pathways. In canonical Wnt pathway, Wnt ligands bind to low-density lipoprotein receptor-related protein 5 or 6 (LRP5 or LRP6), resulting in phosphorylation and activation of the receptor. We hypothesize that canonical Wnt pathway plays a role in the retinal lesion of age-related macular degeneration (AMD), a leading cause of irreversible central visual loss in elderly.

Methods

We examined LRP6 phosphorylation and Wnt signaling cascade in human retinal sections and plasma kallistatin, an endogenous inhibitor of the Wnt pathway in AMD patients and non-AMD subjects. We also used the Ccl2^−/−/Cx3cr1^−/−/rd8 and Ccl2^−/−/Cx3cr1^gfp/gfp mouse models with AMD-like retinal degeneration to further explore the involvement of Wnt signaling activation in the retinal lesions in those models and to preclinically evaluate the role of Wnt signaling suppression as a potential therapeutic option for AMD.

Results

We found higher levels of LRP6 (a key Wnt signaling receptor) protein phosphorylation and transcripts of the Wnt pathway-targeted genes, as well as higher beta-catenin protein in AMD macula compared to controls. Kallistatin was decreased in the plasma of AMD patients. Retinal non-phosphorylated-β-catenin and phosphorylated-LRP6 were higher in Ccl2^−/−/Cx3cr1^−/−/rd8 mice than that in wild type. Intravitreal administration of an anti-LRP6 antibody slowed the progression of retinal lesions in Ccl2^−/−/Cx3cr1^−/−/rd8 and Ccl2^−/−/Cx3cr1^gfp/gfp mice. Electroretinography of treated eyes exhibited larger amplitudes compared to controls in both mouse models. A2E, a retinoid byproduct associated with AMD was lower in the treated eyes of Ccl2^−/−/Cx3cr1^−/−/rd8 mice. Anti-LRP6 also suppressed the expression of Tnf-α and Icam-1 in Ccl2^−/−/Cx3cr1^−/−/rd8 retinas.

Conclusions

Wnt signaling may be disturbed in AMD patients, which could contribute to the retinal inflammation and increased A2E levels found in AMD. Aberrant activation of canonical Wnt signaling might also contribute to the focal retinal degenerative lesions of mouse models with Ccl2 and Cx3cr1 deficiency, and intravitreal administration of anti-LRP6 antibody could be beneficial by deactivating the canonical Wnt pathway.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0683-x) contains supplementary material, which is available to authorized users.

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.

Primary retinal cultures as a tool for modeling diabetic retinopathy: an overview

Experimental models of diabetic retinopathy (DR) have had a crucial role in the comprehension of the pathophysiology of the disease and the identification of new therapeutic strategies. Most of these studies have been conducted in vivo, in animal models. However, a significant contribution has also been provided by studies on retinal cultures, especially regarding the effects of the potentially toxic components of the diabetic milieu on retinal cell homeostasis, the characterization of the mechanisms on the basis of retinal damage, and the identification of potentially protective molecules. In this review, we highlight the contribution given by primary retinal cultures to the study of DR, focusing on early neuroglial impairment. We also speculate on possible themes into which studies based on retinal cell cultures could provide deeper insight.

Photoreceptors in diabetic retinopathy

Although photoreceptors account for most of the mass and metabolic activity of the retina, their role in the pathogenesis of diabetic retinopathy has been largely overlooked. Recent studies suggest that photoreceptors might play a critical role in the diabetes-induced degeneration of retinal capillaries, and thus can no longer be ignored. The present review summarizes diabetes-induced alterations in photoreceptor structure and function, and provides a rationale for further study of a role of photoreceptors in the pathogenesis of the retinopathy.

DBA/2J mice are susceptible to diabetic nephropathy and diabetic exacerbation of IOP elevation

Some pathological manifestations of diabetes in the eye include retinopathy, cataracts and elevated intraocular pressure (IOP). Loss of retinal ganglion cells (RGCs) in non-proliferative stages of diabetic retinopathy and small increases in IOP in diabetic patients has raised the possibility that diabetes affects the development and progression of ocular hypertension and glaucoma. The Ins2Akita mutation is known to cause diabetes and retinopathy on a C57BL/6J (B6) background by as early as 3 months of age. Here, the impact of the Akita mutation on glaucoma was assessed using DBA/2J (D2) mice, a widely used mouse model of ocular hypertension induced glaucoma. In D2.Ins2Akita/+ mice, the contribution of diabetes to vascular permeability, IOP elevation, RGC loss, and glaucoma development was assessed. D2.Ins2Akita/+ mice developed a severe diabetic nephropathy and early mortality between 6-8 months of age. This agrees with previous reports showing that the D2 background is more susceptible to diabetes than the B6 background. In addition, D2.Ins2Akita/+ mice had vascular leakage, astrocyte reactivity and a significant increase in IOP. However no RGC loss and no anterograde axonal transport dysfunction were found at 8.5 months of age. Therefore, our data show that despite severe diabetes and an increased IOP compared to controls, RGCs do not lose axon transport or degenerate. This may be due to a DBA/2J-specific genetic modifier(s) that could provide novel and important avenues for developing new therapies for diabetic retinopathy and possibly glaucoma.