Metabolic Deregulation of the Blood-Outer Retinal Barrier in Retinitis Pigmentosa

Tissue Inhibitor of Metalloproteinase 3 (TIMP3) mutations increase glycolytic activity and dysregulate glutamine metabolism in RPE cells

OBJECTIVES

Mutations in Tissue Inhibitor of Metalloproteinases 3 (TIMP3) cause Sorsby's Fundus Dystrophy (SFD), a dominantly inherited, rare form of macular degeneration that results in vision loss. TIMP3 is synthesized primarily by retinal pigment epithelial (RPE) cells, which constitute the outer blood-retinal barrier. One major function of RPE is the synthesis and transport of vital nutrients, such as glucose, to the retina. Recently, metabolic dysfunction in RPE cells has emerged as an important contributing factor in retinal degenerations. We set out to determine if RPE metabolic dysfunction was contributing to SFD pathogenesis.

METHODS

Quantitative proteomics was conducted on RPE of mice expressing the S179C variant of TIMP3, known to be causative of SFD in humans. Proteins found to be differentially expressed (P < 0.05) were analyzed using statistical overrepresentation analysis to determine enriched pathways, processes, and protein classes using g:profiler and PANTHER Gene Ontology. We examined the effects of mutant TIMP3 on RPE metabolism using human ARPE-19 cells expressing mutant S179C TIMP3 and patient-derived induced pluripotent stem cell-derived RPE (iRPE) carrying the S204C TIMP3 mutation. RPE metabolism was directly probed using isotopic tracing coupled with GC/MS analysis. Steady state [U-13C6] glucose isotopic tracing was preliminarily conducted on S179C ARPE-19 followed by [U-13C6] glucose and [U-13C5] glutamine isotopic tracing in SFD iRPE cells.

RESULTS

Quantitative proteomics and enrichment analysis conducted on RPE of mice expressing mutant S179C TIMP3 identified differentially expressed proteins that were enriched for metabolism-related pathways and processes. Notably these results highlighted dysregulated glycolysis and glucose metabolism. Stable isotope tracing experiments with [U-13C6] glucose demonstrated enhanced glucose utilization and glycolytic activity in S179C TIMP3 APRE-19 cells. Similarly, [U-13C6] glucose tracing in SFD iRPE revealed increased glucose contribution to glycolysis and the TCA cycle. Additionally, [U-13C5] glutamine tracing found evidence of altered malic enzyme activity.

CONCLUSIONS

This study provides important information on the dysregulation of RPE glucose metabolism in SFD and implicates a potential commonality with other retinal degenerative diseases, emphasizing RPE cellular metabolism as a therapeutic target.

Metabolic plasticity in a Pde6bSTOP/STOP retinitis pigmentosa mouse model following rescue

OBJECTIVE

Retinitis pigmentosa (RP) is a hereditary retinal disease characterized by progressive photoreceptor degeneration, leading to vision loss. The best hope for a cure for RP lies in gene therapy. However, given that RP patients are most often diagnosed in the midst of ongoing photoreceptor degeneration, it is unknown how the retinal proteome changes as RP disease progresses, and which changes can be prevented, halted, or reversed by gene therapy.

METHODS

Here, we used a Pde6b-deficient RP gene therapy mouse model and performed untargeted proteomic analysis to identify changes in protein expression during degeneration and after treatment.

RESULTS

We demonstrated that Pde6b gene restoration led to a novel form of homeostatic plasticity in rod phototransduction which functionally compensates for the decreased number of rods. By profiling protein levels of metabolic genes and measuring metabolites, we observed an upregulation of proteins associated with oxidative phosphorylation in mutant and treated photoreceptors.

CONCLUSION

In conclusion, the metabolic demands of the retina differ in our Pde6b-deficient RP mouse model and are not rescued by gene therapy treatment. These findings provide novel insights into features of both RP disease progression and long-term rescue with gene therapy.

The LCHADD Mouse Model Recapitulates Early-Stage Chorioretinopathy in LCHADD Patients

Purpose

Recent studies have shown that the retinal pigment epithelium (RPE) relies on fatty acid oxidation (FAO) for energy, however, its role in overall retinal health is unknown. The only FAO disorder that presents with chorioretinopathy is long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD). Studying the molecular mechanisms can lead to new treatments for patients and elucidate the role of FAO in the RPE. This paper characterizes the chorioretinopathy progression in a recently reported LCHADD mouse model.

Methods

Visual assessments, such as optokinetic tracking and fundus imaging, were performed in wildtype (WT) and LCHADD mice at 3, 6, 10, and 12 months of age. Retinal morphology was analyzed in 12-month retinal cross-sections using hematoxylin and eosin (H&E), RPE65, CD68, and TUNEL staining, whereas RPE structure was assessed using transmission electron microscopy (TEM). Acylcarnitine profiles were measured in isolated RPE/sclera samples to determine if FAO was blocked. Bulk RNA-sequencing of 12 month old male WT mice and LCHADD RPE/sclera samples assessed gene expression changes.

Results

LCHADD RPE/sclera samples had a 5- to 7-fold increase in long-chain hydroxyacylcarnitines compared to WT, suggesting an impaired LCHAD step in long-chain FAO. LCHADD mice have progressively decreased visual performance and increased RPE degeneration starting at 6 months. LCHADD RPE have an altered structure and a two-fold increase in macrophages in the subretinal space. Finally, LCHADD RPE/sclera have differentially expressed genes compared to WT, including downregulation of genes important for RPE function and angiogenesis.

Conclusions

Overall, this LCHADD mouse model recapitulates early-stage chorioretinopathy seen in patients with LCHADD and is a useful model for studying LCHADD chorioretinopathy.

Targeting miR-181a/b in retinitis pigmentosa: implications for disease progression and therapy

BACKGROUND

Retinitis pigmentosa (RP) is a genetically heterogeneous group of degenerative disorders causing progressive vision loss due to photoreceptor death. RP affects other retinal cells, including the retinal pigment epithelium (RPE). MicroRNAs (miRs) are implicated in RP pathogenesis, and downregulating miR-181a/b has shown therapeutic benefit in RP mouse models by improving mitochondrial function. This study investigates the expression profile of miR-181a/b in RPE cells and the neural retina during RP disease progression. We also evaluate how miR-181a/b downregulation, by knocking out miR-181a/b-1 cluster in RPE cells, confers therapeutic efficacy in an RP mouse model and explore the mechanisms underlying this process.

RESULTS

Our findings reveal distinct expression profiles, with downregulated miR-181a/b in RPE cells suggesting a protective response and upregulated miR-181a/b in the neural retina indicating a role in disease progression. We found that miR-181a/b-2, encoded in a separate genomic cluster, compensates for miR-181a/b-1 ablation in RPE cells at late time points. The transient downregulation of miR-181a/b in RPE cells at post-natal week 6 (PW6) led to improved RPE morphology, retarded photoreceptor degeneration and decreased RPE aerobic glycolysis.

CONCLUSIONS

Our study elucidates the underlying mechanisms associated with the therapeutic modulation of miR-181a/b, providing insights into the metabolic processes linked to its RPE-specific downregulation. Our data further highlights the impact of compensatory regulation between miR clusters with implications for the development of miR-based therapeutics.

Gene Therapies for Retinitis Pigmentosa that Target Glucose Metabolism

Retinitis pigmentosa is a blinding disease wherein rod photoreceptors are affected first, due to the expression of a disease gene, leading to the loss of dim light vision. In many cases, cones do not express the disease gene, yet they are also affected and eventually die, typically after most of the rods in their neighborhood have died. The cause of secondary cone death is unclear. Photoreceptors are one of the most energy-demanding cell types in the body and consume a high amount of glucose. At an early stage of degeneration, the cones appear to have a shortage of glucose to fuel their metabolism. This review focuses on gene therapy approaches that address this potential metabolic shortcoming.

Targeting posterior eye infections with colloidal carriers: The case of Ganciclovir

The ocular system, unlike any other human body organ, is a system in which foreign bodies appear quite defenceless in front of the eye. Several infections of the ocular system occur due to various opportunistic conditions. Cytomegalovirus (CMV) is one of the opportunivores that causes several posterior eye infections. Ganciclovir (GCV),9-(2-hydroxy-1-(hydroxymethyl) ethoxymethyl), is aguanine-antiviral agent primarily used to treat CMV diseases. However, the major challenge is of lower bioavailability. Hence, GCV must be dosed repeatedly to enhance drug absorption. but this causes side effects like neutropenia and bone marrow suppression. So, formulators have used alternative formulation strategies such as prodrug formulation and colloidal drug delivery systems. In the prodrug strategy, they attempted to bind various compounds into the parent drug to increase the permeability and bioavailability of GCV. In colloidal drug delivery systems, mucoadhesive microspheres, nanoparticles, Niosome and liposome were employed to extend the drug residence time at the application site. This paper discusses several colloidal carriers combined with GCV to treat opportunistic CMV infection in the posterior ocular system. It reviews the limitations of conventional ocular therapy and explores various novel formulation approaches to improve the ocular bioavailability of GCV in the posterior chamber of the eye.

Metabolic transcriptomics dictate responses of cone photoreceptors to retinitis pigmentosa

Most mutations in retinitis pigmentosa (RP) arise in rod photoreceptors, but cone photoreceptors, responsible for high-resolution daylight and color vision, are subsequently affected, causing the most debilitating features of the disease. We used mass spectroscopy to follow 13C metabolites delivered to the outer retina and single-cell RNA sequencing to assess photoreceptor transcriptomes. The S cone metabolic transcriptome suggests engagement of the TCA cycle and ongoing response to ROS characteristic of oxidative phosphorylation, which we link to their histone modification transcriptome. Tumor necrosis factor (TNF) and its downstream effector RIP3, which drive ROS generation via mitochondrial dysfunction, are induced and activated as S cones undergo early apoptosis in RP. The long/medium-wavelength (L/M) cone transcriptome shows enhanced glycolytic capacity, which maintains their function as RP progresses. Then, as extracellular glucose eventually diminishes, L/M cones are sustained in long-term dormancy by lactate metabolism.

Metabolism: How removal of damaged cells impacts energy availability in the retina

Once thought to be a quiescent process, elimination of damaged cells by professional phagocytes is now understood to modulate metabolite availability within tissues. A new study reveals that the retinal pigment epithelium serves as a local source of insulin after engulfment of damaged photoreceptors.

Separate lifetime signatures of macaque S cones, M/L cones, and rods observed with adaptive optics fluorescence lifetime ophthalmoscopy

In the retina, several molecules involved in metabolism, the visual cycle, and other roles exhibit intrinsic fluorescence. The overall properties of retinal fluorescence depend on changes to the composition of these molecules and their environmental interactions due to transient functional shifts, especially in disease. This behooves the understanding of the origins and deviations of these properties within the multilayered retina at high lateral and axial resolution. Of particular interest is the fluorescence lifetime, a potential biomarker of function and disease independent of fluorescence intensity that can be measured in the retina with adaptive optics fluorescence lifetime ophthalmoscopy (AOFLIO). This work demonstrates the utility of the phasor method of analysis, an alternate approach to traditional multiexponential fitting, to evaluate photoreceptor two-photon excited AOFLIO data and separate them based on functional differences. Phasor analysis on fluorescence lifetime decay data allowed the repeatable segregation of S from M/L cones, likely from differences in functional or metabolic demands. Furthermore, it is possible to track the lifetime changes in S cones after photodamage. Phasor analysis increases the sensitivity of AOFLIO to functional differences between cells and has the potential to improve our understanding of pathways involved in normal and diseased conditions at the cellular scale throughout the retina.

Phagocytosis in the retina promotes local insulin production in the eye

The retina is highly metabolically active, relying on glucose uptake and aerobic glycolysis. Situated in close contact to photoreceptors, a key function of cells in the retinal pigment epithelium (RPE) is phagocytosis of damaged photoreceptor outer segments (POS). Here we identify RPE as a local source of insulin in the eye that is stimulated by POS phagocytosis. We show that Ins2 messenger RNA and insulin protein are produced by RPE cells and that this production correlates with RPE phagocytosis of POS. Genetic deletion of phagocytic receptors ('loss of function') reduces Ins2, whereas increasing the levels of the phagocytic receptor MerTK ('gain of function') increases Ins2 production in male mice. Contrary to pancreas-derived systemic insulin, RPE-derived local insulin is stimulated during starvation, which also increases RPE phagocytosis. Global or RPE-specific Ins2 gene deletion decreases retinal glucose uptake in starved male mice, dysregulates retinal physiology, causes defects in phototransduction and exacerbates photoreceptor loss in a mouse model of retinitis pigmentosa. Collectively, these data identify RPE cells as a phagocytosis-induced local source of insulin in the retina, with the potential to influence retinal physiology and disease.

Gene-agnostic therapeutic approaches for inherited retinal degenerations

Inherited retinal diseases (IRDs) are associated with mutations in over 250 genes and represent a major cause of irreversible blindness worldwide. While gene augmentation or gene editing therapies could address the underlying genetic mutations in a small subset of patients, their utility remains limited by the great genetic heterogeneity of IRDs and the costs of developing individualised therapies. Gene-agnostic therapeutic approaches target common pathogenic pathways that drive retinal degeneration or provide functional rescue of vision independent of the genetic cause, thus offering potential clinical benefits to all IRD patients. Here, we review the key gene-agnostic approaches, including retinal cell reprogramming and replacement, neurotrophic support, immune modulation and optogenetics. The relative benefits and limitations of these strategies and the timing of clinical interventions are discussed.

Overexpression of Rhodopsin or Its Mutants Leads to Energy Metabolism Dysfunction in 661w Cells

Purpose

Retinitis pigmentosa (RP) is a heterogeneous group of inherited disorders characterized by photoreceptor degeneration. The rhodopsin gene (RHO) is the most frequent cause of autosomal dominant RP (ADRP), yet it remains unclear how RHO mutations cause heterogeneous phenotypes. Energy failure is a main cause of the secondary cone death during RP progression; however, its role in primary rod death induced by ADRP RHO mutants is unknown.

Methods

Three RHO missense mutations were chosen from different clinical classes. Wild-type (WT) RHO and its mutants, P23H (class B1), R135L (class A), and G188R (class B2), were overexpressed in 661w cells, a mouse photoreceptor cell line, and their effects on oxidative phosphorylation (OXPHOS) and aerobic glycolysis were compared separately.

Results

Here, we report that energy failure is an early event in the cell death caused by overexpression of WT RHO and its mutants. RHO overexpression leads to OXPHOS deficiency, which might be a result of mitochondrial loss. Nonetheless, only in WT RHO and P23H groups, energy stress triggers AMP-activated protein kinase activation and metabolic reprogramming to increase glycolysis. Metabolic reprogramming impairment in R135L and G188R groups might be the reason why energy failure and cell injury are much more severe in those groups.

Conclusions

Our results imply that overexpression of RHO missense mutants have distinct impacts on the two energy metabolic pathways, which might be related to their heterogeneous phenotypes.

RIP140-Mediated NF-κB Inflammatory Pathway Promotes Metabolic Dysregulation in Retinal Pigment Epithelium Cells

Metabolic dysregulation of the retinal pigment epithelium (RPE) has been implicated in age-related macular degeneration (AMD). However, the molecular regulation of RPE metabolism remains unclear. RIP140 is known to affect oxidative metabolism and mitochondrial biogenesis by negatively controlling mitochondrial pathways regulated by PPAR-γ co-activator-1 α(PGC-1α). This study aims to disclose the effect of RIP140 on the RPE metabolic program in vitro and in vivo. RIP140 protein levels were assayed by Western blotting. Gene expression was tested using quantitative real-time PCR (qRT-PCR), ATP production, and glycogen concentration assays, and the release of inflammatory factors was analyzed by commercial kits. Mice photoreceptor function was measured by electroretinography (ERG). In ARPE-19 cells, RIP140 overexpression changed the expression of the key metabolic genes and lipid processing genes, inhibited mitochondrial ATP production, and enhanced glycogenesis. Moreover, RIP140 overexpression promoted the translocation of NF-κB and increased the expression and production of IL-1β, IL-6, and TNF-α in ARPE-19 cells. Importantly, we also observed the overexpression of RIP140 through adenovirus delivery in rat retinal cells, which significantly decreased the amplitude of the a-wave and b-wave measured by ERG assay. Therapeutic strategies that modulate the activity of RIP140 could have clinical utility for the treatment of AMD in terms of preventing RPE degeneration.

Dissecting Regulators of Aging and Age-Related Macular Degeneration in the Retinal Pigment Epithelium

Age-related macular degeneration (AMD), the leading cause of blindness in elderly populations, involves the loss of central vision due to progressive dysfunction of the retinal pigment epithelium (RPE) and subsequent loss of light-sensing photoreceptors. While age is a key risk factor, not every aged individual develops AMD. Thus, the critical question is what specific cellular changes tip the balance from healthy aging to disease. To distinguish between changes associated with aging and AMD, we compared the RPE proteome in human eye bank tissue from nondiseased donors during aging (n = 50, 29-91 years) and in donors with AMD (n = 36) compared to age-matched donors without disease (n = 28). Proteins from RPE cells were separated on two-dimensional gels, analyzed for content, and identified using mass spectrometry. A total of 58 proteins displayed significantly altered content with either aging or AMD. Proteins involved in metabolism, protein turnover, stress response, and cell death were altered with both aging and AMD. However, the direction of change was predominantly opposite. With aging, we detected an overall decrease in metabolism and reductions in stress-associated proteins, proteases, and chaperones. With AMD, we observed upregulation of metabolic proteins involved in glycolysis, TCA, and fatty acid metabolism, with a concurrent decline in oxidative phosphorylation, suggesting a reprogramming of energy utilization. Additionally, we detected upregulation of proteins involved in the stress response and protein turnover. Predicted upstream regulators also showed divergent results, with inhibition of inflammation and immune response with aging and activation of these processes with AMD. Our results support the idea that AMD is not simply advanced aging but rather the culmination of perturbed protein homeostasis, defective bioenergetics, and increased oxidative stress within the aging RPE, exacerbated by environmental factors and the genetic background of an individual.

Neuroinflammation in retinitis pigmentosa: Therapies targeting the innate immune system

Retinitis pigmentosa (RP) is an important cause of irreversible blindness worldwide and lacks effective treatment strategies. Although mutations are the primary cause of RP, research over the past decades has shown that neuroinflammation is an important cause of RP progression. Due to the abnormal activation of immunity, continuous sterile inflammation results in neuron loss and structural destruction. Therapies targeting inflammation have shown their potential to attenuate photoreceptor degeneration in preclinical models. Regardless of variations in genetic background, inflammatory modulation is emerging as an important role in the treatment of RP. We summarize the evidence for the role of inflammation in RP and mention therapeutic strategies where available, focusing on the modulation of innate immune signals, including TNFα signaling, TLR signaling, NLRP3 inflammasome activation, chemokine signaling and JAK/STAT signaling. In addition, we describe epigenetic regulation, the gut microbiome and herbal agents as prospective treatment strategies for RP in recent advances.

Cellular and molecular alterations in neurons and glial cells in inherited retinal degeneration

Multiple gene mutations have been associated with inherited retinal dystrophies (IRDs). Despite the spectrum of phenotypes caused by the distinct mutations, IRDs display common physiopathology features. Cell death is accompanied by inflammation and oxidative stress. The vertebrate retina has several attributes that make this tissue vulnerable to oxidative and nitrosative imbalance. The high energy demands and active metabolism in retinal cells, as well as their continuous exposure to high oxygen levels and light-induced stress, reveal the importance of tightly regulated homeostatic processes to maintain retinal function, which are compromised in pathological conditions. In addition, the subsequent microglial activation and gliosis, which triggers the secretion of pro-inflammatory cytokines, chemokines, trophic factors, and other molecules, further worsen the degenerative process. As the disease evolves, retinal cells change their morphology and function. In disease stages where photoreceptors are lost, the remaining neurons of the retina to preserve their function seek out for new synaptic partners, which leads to a cascade of morphological alterations in retinal cells that results in a complete remodeling of the tissue. In this review, we describe important molecular and morphological changes in retinal cells that occur in response to oxidative stress and the inflammatory processes underlying IRDs.

Drug Discovery Strategies for Inherited Retinal Degenerations

Inherited retinal degeneration is a group of blinding disorders afflicting more than 1 in 4000 worldwide. These disorders frequently cause the death of photoreceptor cells or retinal ganglion cells. In a subset of these disorders, photoreceptor cell death is a secondary consequence of retinal pigment epithelial cell dysfunction or degeneration. This manuscript reviews current efforts in identifying targets and developing small molecule-based therapies for these devastating neuronal degenerations, for which no cures exist. Photoreceptors and retinal ganglion cells are metabolically demanding owing to their unique structures and functional properties. Modulations of metabolic pathways, which are disrupted in most inherited retinal degenerations, serve as promising therapeutic strategies. In monogenic disorders, great insights were previously obtained regarding targets associated with the defective pathways, including phototransduction, visual cycle, and mitophagy. In addition to these target-based drug discoveries, we will discuss how phenotypic screening can be harnessed to discover beneficial molecules without prior knowledge of their mechanisms of action. Because of major anatomical and biological differences, it has frequently been challenging to model human inherited retinal degeneration conditions using small animals such as rodents. Recent advances in stem cell-based techniques are opening new avenues to obtain pure populations of human retinal ganglion cells and retinal organoids with photoreceptor cells. We will discuss concurrent ideas of utilizing stem-cell-based disease models for drug discovery and preclinical development.

Gene Therapy for Rhodopsin Mutations

Mutations in RHO, the gene for rhodopsin, account for a large fraction of autosomal-dominant retinitis pigmentosa (adRP). Patients fall into two clinical classes, those with early onset, pan retinal photoreceptor degeneration, and those who experience slowly progressive disease. The latter class of patients are candidates for photoreceptor-directed gene therapy, while former may be candidates for delivery of light-responsive proteins to interneurons or retinal ganglion cells. Gene therapy for RHO adRP may be targeted to the mutant gene at the DNA or RNA level, while other therapies preserve the viability of photoreceptors without addressing the underlying mutation. Correcting the RHO gene and replacing the mutant RNA show promise in animal models, while sustaining viable photoreceptors has the potential to delay the loss of central vision and may preserve photoreceptors for gene-directed treatments.

Glucose uptake by GLUT1 in photoreceptors is essential for outer segment renewal and rod photoreceptor survival

Photoreceptors consume glucose supplied by the choriocapillaris to support phototransduction and outer segment (OS) renewal. Reduced glucose supply underlies photoreceptor cell death in inherited retinal degeneration and age-related retinal disease. We have previously shown that restricting glucose transport into the outer retina by conditional deletion of Slc2a1 encoding GLUT1 resulted in photoreceptor loss and impaired OS renewal. However, retinal neurons, glia, and the retinal pigment epithelium play specialized, synergistic roles in metabolite supply and exchange, and the cell-specific map of glucose uptake and utilization in the retina is incomplete. In these studies, we conditionally deleted Slc2a1 in a pan-retinal or rod-specific manner to better understand how glucose is utilized in the retina. Using non-invasive ocular imaging, electroretinography, and histochemical and biochemical analyses we show that genetic deletion of Slc2a1 from retinal neurons and Müller glia results in reduced OS growth and progressive rod but not cone photoreceptor cell death. Rhodopsin levels were severely decreased even at postnatal day 20 when OS length was relatively normal. Arrestin levels were not changed suggesting that glucose uptake is required to synthesize membrane glycoproteins. Rod-specific deletion of Slc2a1 resulted in similar changes in OS length and rod photoreceptor cell death. These studies demonstrate that glucose is an essential carbon source for rod photoreceptor cell OS maintenance and viability.

Mesenchymal stem cells-derived small extracellular vesicles alleviate diabetic retinopathy by delivering NEDD4

Background

As a leading cause of vision decline and severe blindness in adults, diabetic retinopathy (DR) is characterized by the aggravation of retinal oxidative stress and apoptosis in the early stage. Emerging studies reveal that mesenchymal stem cells-derived small extracellular vesicles (MSC-sEV) treatment represents a promising cell-free approach to alleviate ocular disorders. However, the repairing effects of MSC-sEV in DR remain largely unclear. This study aimed at exploring the role and the underlying mechanism of MSC-sEV in hyperglycemia-induced retinal degeneration.

Methods

In vivo, we used streptozotocin (STZ) to establish diabetic rat model, followed by the intravitreal injection of MSC-sEV to determine the curative effect. The cell viability and antioxidant capacity of retinal pigment epithelium (RPE) cells stimulated with high-glucose (HG) medium after MSC-sEV treatment were analyzed in vitro. By detecting the response of cell signaling pathways in MSC-sEV-treated RPE cells, we explored the functional mechanism of MSC-sEV. Mass spectrometry was performed to reveal the bioactive protein which mediated the role of MSC-sEV.

Results

The intravitreal injection of MSC-sEV elicited antioxidant effects and counteracted retinal apoptosis in STZ-induced DR rat model. MSC-sEV treatment also reduced the oxidative level and enhanced the proliferation ability of RPE cells cultured in HG conditions in vitro. Further studies showed that the increased level of phosphatase and tensin homolog (PTEN) inhibited AKT phosphorylation and nuclear factor erythroid 2-related factor 2 (NRF2) expression in RPE cells stimulated with HG medium, which could be reversed by MSC-sEV intervention. Through mass spectrometry, we illustrated that MSC-sEV-delivered neuronal precursor cell-expressed developmentally downregulated 4 (NEDD4) could cause PTEN ubiquitination and degradation, activate AKT signaling and upregulate NRF2 level to prevent DR progress. Moreover, NEDD4 knockdown impaired MSC-sEV-mediated retinal therapeutic effects.

Conclusions

Our findings indicated that MSC-sEV ameliorated DR through NEDD4-induced regulation on PTEN/AKT/NRF2 signaling pathway, thus revealing the efficiency and mechanism of MSC-sEV-based retinal protection and providing new insights into the treatment of DR.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02983-0.

A mathematical model of GLUT1 modulation in rods and RPE and its differential impact in cell metabolism

We present a mathematical model of key glucose metabolic pathways in two cells of the human retina: the rods and the retinal pigmented epithelium (RPE). Computational simulations of glucose transporter 1 (GLUT1) inhibition in the model accurately reproduce experimental data from conditional knockout mice and reveal that modification of GLUT1 expression levels of both cells differentially impacts their metabolism. We hypothesize that, under glucose scarcity, the RPE's energy producing pathways are altered in order to preserve its functionality, impacting the photoreceptors' outer segment renewal. On the other hand, when glucose is limited in the rods, aerobic glycolysis is preserved, which maintains the lactate contribution to the RPE.

Challenges to Gene Editing Approaches in the Retina

Retinal gene therapy has recently been at the cutting edge of clinical development in the diverse field of genetic therapies. The retina is an attractive target for genetic therapies such as gene editing due to the distinctive anatomical and immunological features of the eye, known as immune privilege, so that inherited retinal diseases (IRDs) have been studied in several clinical studies. Thus, rapid strides are being made toward developing targeted treatments for IRDs. Gene editing in the retina faces a group of heterogenous challenges, including editing efficiencies, off-target effects, the anatomy of the target organ, immune responses, inactivation, and identifying optimal application methods. As clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated nuclease (Cas) based technologies are at the forefront of current gene editing advances, their specific editing efficiency challenges and potential off-target effects were assessed. The immune privilege of the eye reduces the likelihood of systemic immune responses following retinal gene therapy, but possible immune responses must not be discounted. Immune responses to gene editing in the retina may be humoral or cell mediated, with immunologically active cells, including microglia, implicated in facilitating possible immune responses to gene editing. Immunogenicity of gene therapeutics may also lead to the inactivation of edited cells, reducing potential therapeutic benefits. This review outlines the broad spectrum of potential challenges currently facing retinal gene editing, with the goal of facilitating further advances in the safety and efficacy of gene editing therapies.

Insights into pathological mechanisms and interventions revealed by analyzing a mathematical model for cone metabolism

This work analyzes a mathematical model for the metabolic dynamics of a cone photoreceptor, which is the first model to account for energy generation from fatty acids oxidation of shed photoreceptor outer segments (POS). Multiple parameter bifurcation analysis shows that joint variations in external glucose, the efficiency of glucose transporter 1 (GLUT1), lipid utilization for POS renewal, and oxidation of fatty acids affect the cone’s metabolic vitality and its capability to adapt under glucose-deficient conditions. The analysis further reveals that when glucose is scarce, cone viability cannot be sustained by only fueling energy production in the mitochondria, but it also requires supporting anabolic processes to create lipids necessary for cell maintenance and repair. In silico experiments are used to investigate how the duration of glucose deprivation impacts the cell without and with a potential GLUT1 or oxidation of fatty acids intervention as well as a dual intervention. The results show that for prolonged duration of glucose deprivation, the cone metabolic system does not recover with higher oxidation of fatty acids and requires greater effectiveness of GLUT1 to recover. Finally, time-varying global sensitivity analysis (GSA) is applied to assess the sensitivity of the model outputs of interest to changes and uncertainty in the parameters at specific times. The results reveal a critical temporal window where there would be more flexibility for interventions to rescue a cone cell from the detrimental consequences of glucose shortage.

First we eat, then we do everything else: The dynamic metabolic regulation of efferocytosis

Clearance of apoptotic cells, or "efferocytosis," is essential for diverse processes including embryonic development, tissue turnover, organ regeneration, and immune cell development. The human body is estimated to remove approximately 1% of its body mass via apoptotic cell clearance daily. This poses several intriguing cell metabolism problems. For instance, phagocytes such as macrophages must induce or suppress metabolic pathways to find, engulf, and digest apoptotic cells. Then, phagocytes must manage the potentially burdensome biomass of the engulfed apoptotic cell. Finally, phagocytes reside in complex tissue architectures that vary in nutrient availability, the types of dying cells or debris that require clearance, and the neighboring cells they interact with. Here, we review advances in our understanding of these three key areas of phagocyte metabolism. We end by proposing a model of efferocytosis that integrates recent findings and establishes a new paradigm for testing how efferocytosis prevents chronic inflammatory disease and autoimmunity.

Metabolic rescue of cone photoreceptors in retinitis pigmentosa

Retinitis pigmentosa (RP) encompasses a group of inherited retinal dystrophies characterized by the primary degeneration of rod and cone photoreceptors. It is a leading cause of visual disability, with an incidence of ~1 in 7000 persons. Although most RP is nonsyndromic, 20%-30% of patients with RP also have an associated nonocular condition. The gene mutations responsible for RP occur overwhelmingly in rod photoreceptors. Visual loss frequently begins with night blindness in adolescence, followed by concentric visual field loss, reflecting the principal dysfunction of rod photoreceptors. Although the visual disability from rod dysfunction is significant, it is the subsequent loss of central vision later in life due to cone degeneration that is catastrophic. Until recently, the reason for cone dysfunction in RP was unknown. However, it is now recognized that cones degenerate, losing outer segment (OS) synthesis and inner segment (IS) disassembly because of glucose starvation following rod demise. Rod OS phagocytosis by the apical microvilli of retinal pigment epithelium is necessary to transport glucose from the choriocapillaris to the subretinal space. Although cones lose OS with the onset of rod degeneration in RP, regardless of the gene mutation in rods, cone nuclei remain viable for years (i.e. enter cone dormancy) so that therapies aimed at reversing glucose starvation can prevent and/or recover cone function and central vision.

cGMP-PKG dependent transcriptome in normal and degenerating retinas: Novel insights into the retinitis pigmentosa pathology

Retinitis Pigmentosa represents a group of genetic disorders that cause progressive vision loss via degeneration of photoreceptors, but there is in principle no treatment available. For any therapy development, a deeper comprehension of the disease-leading mechanism(s) at the molecular level is needed. Here we focused on the cGMP-PKG system, which has been suggested to be a driver in several models of the disease. To gain insights in its downstream signaling we manipulated the cGMP-PKG system with the aid of organotypic retinal explant cultures from either a mouse-based disease model, i.e. the rd1 mouse, or its healthy wild-type counterpart (wt), by adding different types of cGMP analogues to either inhibit or activate PKG in retinal explants from rd1 and wt, respectively. An RNA sequencing was then performed to study the cGMP-PKG dependent transcriptome. Expression changes of gene sets related to specific pathways or functions, that fulfilled criteria involving that the changes should match PKG activation and inhibition, were determined via bioinformatics. The analyses highlighted that several gene sets linked to oxidative phosphorylation and mitochondrial pathways were regulated by this enzyme system. Specifically, the expression of such pathway components was upregulated in the rd1 treated with PKG inhibitor and downregulated in the wt with PKG activator treatment, suggesting that cGMP-PKG act as a negative regulator in this context. Downregulation of energy production pathways may thus play an integral part in the mechanism behind the degeneration for at least several RP mutations.

Absence of retbindin blocks glycolytic flux, disrupts metabolic homeostasis, and leads to photoreceptor degeneration

We previously reported a model of progressive retinal degeneration resulting from the knockout of the retina-specific riboflavin binding protein, retbindin (Rtbdn ^-/- ). We also demonstrated a reduction in neural retinal flavins as a result of the elimination of RTBDN. Given the role of flavins in metabolism, herein we investigated the underlying mechanism of this retinal degeneration by performing metabolomic analyses on predegeneration at postnatal day (P) 45 and at the onset of functional degeneration in the P120 retinas. Metabolomics of hydrophilic metabolites revealed that individual glycolytic products accumulated in the P45 Rtbdn ^-/- neural retinas along with the elevation of pentose phosphate pathway, while TCA cycle intermediates remained unchanged. This was confirmed by using ¹³C-labeled flux measurements and immunoblotting, revealing that the key regulatory step of phosphoenolpyruvate to pyruvate was inhibited via down-regulation of the tetrameric pyruvate kinase M2 (PKM2). Separate metabolite assessments revealed that almost all intermediates of acylcarnitine fatty acid oxidation, ceramides, sphingomyelins, and multiple toxic metabolites were significantly elevated in the predegeneration Rtbdn ^-/- neural retina. Our data show that lack of RTBDN, and hence reduction in flavins, forced the neural retina into repurposing glucose for free-radical mitigation over ATP production. However, such sustained metabolic reprogramming resulted in an eventual metabolic collapse leading to neurodegeneration.

Retina Metabolism and Metabolism in the Pigmented Epithelium: A Busy Intersection

The outer retina is nourished from the choroid, a capillary bed just inside the sclera. O₂, glucose, and other nutrients diffuse out of the choroid and then filter through a monolayer of retinal pigment epithelium (RPE) cells to fuel the retina. Recent studies of energy metabolism have revealed striking differences between retinas and RPE cells in the ways that they extract energy from fuels. The purpose of this review is to suggest and evaluate the hypothesis that the retina and RPE have complementary metabolic roles that make them depend on each other for survival and for their abilities to perform essential and specialized functions. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Retinal Pigment Epithelium Remodeling in Mouse Models of Retinitis Pigmentosa

In retinitis pigmentosa (RP), one of many possible genetic mutations causes rod degeneration, followed by cone secondary death leading to blindness. Accumulating evidence indicates that rod death triggers multiple, non-cell-autonomous processes, which include oxidative stress and inflammation/immune responses, all contributing to cone demise. Inflammation relies on local microglia and recruitment of immune cells, reaching the retina through breakdowns of the inner blood retinal barrier (iBRB). Leakage in the inner retina vasculature suggests similarly altered outer BRB, formed by junctions between retinal pigment epithelium (RPE) cells, which are crucial for retinal homeostasis, immune response, and privilege. We investigated the RPE structural integrity in three models of RP (rd9, rd10, and Tvrm4 mice) by immunostaining for zonula occludens-1 (ZO-1), an essential regulatory component of tight junctions. Quantitative image analysis demonstrated discontinuities in ZO-1 profiles in all mutants, despite different degrees of photoreceptor loss. ZO-1 interruption zones corresponded to leakage of in vivo administered, fluorescent dextran through the choroid-RPE interface, demonstrating barrier dysfunction. Dexamethasone, administered to rd10 mice for rescuing cones, also rescued RPE structure. Thus, previously undetected, stereotyped abnormalities occur in the RPE of RP mice; pharmacological targeting of inflammation supports a feedback loop leading to simultaneous protection of cones and the RPE.

The potential of marine resources for retinal diseases: a systematic review of the molecular mechanisms

We rely on vision more than on any other sense to obtain information about our environment. Hence, the loss or even impairment of vision profoundly affects our quality of life. Diet or food components have already demonstrated beneficial effects on the development of retinal diseases. Recently, there has been a growing interest in resources from marine animals and plants for the prevention of retinal diseases through nutrition. Especially fish intake and omega-3 fatty acids have already led to promising results, including associations with a reduced incidence of retinal diseases. However, the underlying molecular mechanisms are insufficiently explained. The aim of this review was to summarize the known mechanistic effects of marine resources on the pathophysiological processes in retinal diseases. We performed a systematic literature review following the PRISMA guidelines and identified 107 studies investigating marine resources in the context of retinal diseases. Of these, 46 studies described the underlying mechanisms including anti-inflammatory, antioxidant, antiangiogenic/vasoprotective, cytoprotective, metabolic, and retinal function effects, which we critically summarize. We further discuss perspectives on the use of marine resources for human nutrition to prevent retinal diseases with a particular focus on regulatory aspects, health claims, safety, and bioavailability.

A Metabolic Landscape for Maintaining Retina Integrity and Function

Neurons have high metabolic demands that are almost exclusively met by glucose supplied from the bloodstream. Glucose is utilized in complex metabolic interactions between neurons and glia cells, described by the astrocyte-neuron lactate shuttle (ANLS) hypothesis. The neural retina faces similar energy demands to the rest of the brain, with additional high anabolic needs to support continuous renewal of photoreceptor outer segments. This demand is met by a fascinating variation of the ANLS in which photoreceptors are the central part of a metabolic landscape, using glucose and supplying surrounding cells with metabolic intermediates. In this review we summarize recent evidence on how neurons, in particular photoreceptors, meet their energy and biosynthetic requirements by comprising a metabolic landscape of interdependent cells.

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.

Retinal glial remodeling by FGF21 preserves retinal function during photoreceptor degeneration

The group of retinal degenerations, retinitis pigmentosa (RP), comprises more than 150 genetic abnormalities affecting photoreceptors. Finding degenerative pathways common to all genetic abnormalities may allow general treatment such as neuroprotection. Neuroprotection may include enhancing the function of cells that directly support photoreceptors, retinal pigment epithelial cells, and Müller glia. Treatment with fibroblast growth factor 21 (FGF21), a neuroprotectant, from postnatal week 4-10, during rod and cone loss in P23H mice (an RP model) with retinal degeneration, preserved photoreceptor function and normalized Müller glial cell morphology. Single-cell transcriptomics of retinal cells showed that FGF21 receptor Fgfr1 was specifically expressed in Müller glia/astrocytes. Of all retinal cells, FGF21 predominantly affected genes in Müller glia/astrocytes with increased expression of axon development and synapse formation pathway genes. Therefore, enhancing retinal glial axon and synapse formation with neurons may preserve retinal function in RP and may suggest a general therapeutic approach for retinal degenerative diseases.

Metabolism in the Zebrafish Retina

Retinal photoreceptors are amongst the most metabolically active cells in the body, consuming more glucose as a metabolic substrate than even the brain. This ensures that there is sufficient energy to establish and maintain photoreceptor functions during and after their differentiation. Such high dependence on glucose metabolism is conserved across vertebrates, including zebrafish from early larval through to adult retinal stages. As the zebrafish retina develops rapidly, reaching an adult-like structure by 72 hours post fertilisation, zebrafish larvae can be used to study metabolism not only during retinogenesis, but also in functionally mature retinae. The interplay between rod and cone photoreceptors and the neighbouring retinal pigment epithelium (RPE) cells establishes a metabolic ecosystem that provides essential control of their individual functions, overall maintaining healthy vision. The RPE facilitates efficient supply of glucose from the choroidal vasculature to the photoreceptors, which produce metabolic products that in turn fuel RPE metabolism. Many inherited retinal diseases (IRDs) result in photoreceptor degeneration, either directly arising from photoreceptor-specific mutations or secondary to RPE loss, leading to sight loss. Evidence from a number of vertebrate studies suggests that the imbalance of the metabolic ecosystem in the outer retina contributes to metabolic failure and disease pathogenesis. The use of larval zebrafish mutants with disease-specific mutations that mirror those seen in human patients allows us to uncover mechanisms of such dysregulation and disease pathology with progression from embryonic to adult stages, as well as providing a means of testing novel therapeutic approaches.

Sodium iodate induces ferroptosis in human retinal pigment epithelium ARPE-19 cells

Sodium iodate (SI) is a widely used oxidant for generating retinal degeneration models by inducing the death of retinal pigment epithelium (RPE) cells. However, the mechanism of RPE cell death induced by SI remains unclear. In this study, we investigated the necrotic features of cultured human retinal pigment epithelium (ARPE-19) cells treated with SI and found that apoptosis or necroptosis was not the major death pathway. Instead, the death process was accompanied by significant elevation of intracellular labile iron level, ROS, and lipid peroxides which recapitulated the key features of ferroptosis. Ferroptosis inhibitors deferoxamine mesylate (DFO) and ferrostatin-1(Fer-1) partially prevented SI-induced cell death. Further studies revealed that SI treatment did not alter GPX4 (glutathione peroxidase 4) expression, but led to the depletion of reduced thiol groups, mainly intracellular GSH (reduced glutathione) and cysteine. The study on iron trafficking demonstrated that iron influx was not altered by SI treatment but iron efflux increased, indicating that the increase in labile iron was likely due to the release of sequestered iron. This hypothesis was verified by showing that SI directly promoted the release of labile iron from a cell-free lysate. We propose that SI depletes GSH, increases ROS, releases labile iron, and boosts lipid damage, which in turn results in ferroptosis in ARPE-19 cells.

Metabolic Analysis of Vitreous/Lens and Retina in Wild Type and Retinal Degeneration Mice

Photoreceptors are the light-sensing cells of the retina and the major cell type affected in most inherited retinal degenerations. Different metabolic pathways sustain their high energetic demand in physiological conditions, particularly aerobic glycolysis. The principal metabolome of the mature retina has been studied, but only limited information is available on metabolic adaptations in response to key developmental events, such as eye opening. Moreover, dynamic metabolic changes due to retinal degeneration are not well understood. Here, we aimed to explore and map the ocular metabolic dynamics induced by eye opening in healthy (wild type) or Pde6b-mutant (retinal degeneration 1, Rd1) mice, in which photoreceptors degenerate shortly after eye opening. To unravel metabolic differences emerging before and after eye opening under physiological and pathophysiological conditions, we performed nuclear magnetic resonance (NMR) spectroscopy-based metabolome analysis of wild type and Rd1 retina and vitreous/lens. We show that eye opening is accompanied by changes in the concentration of selected metabolites in the retina and by alterations in the vitreous/lens composition only in the retinal degeneration context. As such, we identify NAcetylaspartate as a potential novel vitreous/lens marker reflecting progressive retinal degeneration. Thus, our data can help elucidating mechanisms underlying key events in retinal physiology and reveal changes occurring in pathology, while highlighting the importance of the vitreous/lens in the characterization of retinal diseases.

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.

Retinal vessel oximetry in children with inherited retinal diseases

BACKGROUND

Retinal vessel oximetry (RO) has been used to show altered metabolic function in patients with inherited retinal diseases (IRDs). The aim of this study was to investigate RO parameters of children with IRDs and presumed IRD carriers (pIRDc) and to compare them to controls.

METHODS

In this cross-sectional cohort study, 142 eyes from 71 Caucasian subjects were included: 40 eyes with IRDs, 26 eyes with pIRDc and 76 control eyes. The oxygen saturation was measured with the Retinal Vessel Analyser (IMEDOS Systems UG, Jena, Germany). Mean oxygen saturations in the peripapillary retinal arterioles (A-SO₂ ; %) and venules (V-SO₂ ; %) were estimated, and their difference (A-V SO₂ ; %) was calculated. In addition, we evaluated the mean diameter in all major retinal arterioles (D-A; μm) and venules (D-V; μm). anova-based linear mixed-effects models were calculated with SPSS^® .

RESULTS

In general, children suffering from IRDs differed from controls when the A-SO₂ and A-V SO₂ were taken into account: both the A-SO₂ and the A-V SO₂ were significantly increased (p = 0.012). In subgroup analyses, children suffering from rod-cone dystrophy (RCD) presented an A-SO₂ increase (99.12 ± 8.24%) when compared to controls (91.33 ± 10.34%, p = 0.014) and pIRDc (92.37 ± 6.57%, p = 0.065). For V-SO₂ significant changes in RCD (67.42 ± 9.19%) were found in comparison with controls (58.24 ± 11.74%, p < 0.041), pIRDc (56.67 ± 7.16%, p = 0.007), cone-rod dystrophies (CRD, 52.17 ± 5.32%, p < 0.001) and inherited macular dystrophies (IMD, 55.74 ± 6.96%, p = 0.004), In addition, A-V SO₂ was decreased in RCD (31.69 ± 3.92%) when measured against CRD (41.9 ± 8.87%, p = 0.017) or IMD (39.52 ± 8.95%, p = 0.059).

CONCLUSION

In general, we found that children with IRDs presented early metabolic changes. Within IRDs, children with RCD showed more affected metabolic changes. Thus, RO may support early screening to rule out IRDs in children, and more precisely may help to differentiate those suffering from RCD.

Rod photoreceptor clearance due to misfolded rhodopsin is linked to a DAMP-immune checkpoint switch

Chronic endoplasmic reticulum stress resulting from misfolding of the visual pigment rhodopsin (RHO) can lead to loss of rod photoreceptors, which initiates retinitis pigmentosa, characterized initially by diminished nighttime and peripheral vision. Cone photoreceptors depend on rods for glucose transport, which the neurons use for assembly of visual pigment-rich structures; as such, loss of rods also leads to a secondary loss of cone function, diminishing high-resolution color vision utilized for tasks including reading, driving, and facial recognition. If dysfunctional rods could be maintained to continue to serve this secondary cone preservation function, it might benefit patients with retinitis pigmentosa, but the mechanisms by which rods are removed are not fully established. Using pigs expressing mutant RHO, we find that induction of a danger-associated molecular pattern (DAMP) "eat me" signal on the surface of mutant rods is correlated with targeting the live cells for (PrCR) by retinal myeloid cells. Glucocorticoid therapy leads to replacement of this DAMP with a "don't eat me" immune checkpoint on the rod surface and inhibition of PrCR. Surviving rods then continue to promote glucose transport to cones, maintaining their viability.

Selective knockdown of hexokinase 2 in rods leads to age-related photoreceptor degeneration and retinal metabolic remodeling

Photoreceptors, the primary site of phototransduction in the retina, require energy and metabolites to constantly renew their outer segments. They preferentially consume most glucose through aerobic glycolysis despite possessing abundant mitochondria and enzymes for oxidative phosphorylation (OXPHOS). Exactly how photoreceptors balance aerobic glycolysis and mitochondrial OXPHOS to regulate their survival is still unclear. We crossed rhodopsin-Cre mice with hexokinase 2 (HK2)-floxed mice to study the effect of knocking down HK2, the first rate-limiting enzyme in glycolysis, on retinal health and metabolic remodeling. Immunohistochemistry and Western blots were performed to study changes in photoreceptor-specific proteins and key enzymes in glycolysis and the tricarboxylic acid (TCA) cycle. Changes in retinal structure and function were studied by optical coherence tomography and electroretinography. Mass spectrometry was performed to profile changes in ¹³C-glucose-derived metabolites in glycolysis and the TCA cycle. We found that knocking down HK2 in rods led to age-related photoreceptor degeneration, evidenced by reduced expression of photoreceptor-specific proteins, age-related reductions of the outer nuclear layer, photoreceptor inner and outer segments and impaired electroretinographic responses. Loss of HK2 in rods led to upregulation of HK1, phosphorylation of pyruvate kinase muscle isozyme 2, mitochondrial stress proteins and enzymes in the TCA cycle. Mass spectrometry found that the deletion of HK2 in rods resulted in accumulation of ¹³C-glucose along with decreased pyruvate and increased metabolites in the TCA cycle. Our data suggest that HK2-mediated aerobic glycolysis is indispensable for the maintenance of photoreceptor structure and function and that long-term inhibition of glycolysis leads to photoreceptor degeneration.

Eucalyptol Inhibits Amyloid-β-Induced Barrier Dysfunction in Glucose-Exposed Retinal Pigment Epithelial Cells and Diabetic Eyes

Hyperglycemia elicits tight junction disruption and blood-retinal barrier breakdown, resulting in diabetes-associated vison loss. Eucalyptol is a natural compound found in eucalyptus oil with diverse bioactivities. This study evaluated that eucalyptol ameliorated tight junctions and retinal barrier function in glucose/amyloid-β (Aβ)-exposed human retinal pigment epithelial (RPE) cells and in db/db mouse eyes. RPE cells were cultured in media containing 33 mM glucose or 5 μM Aβ for 4 days in the presence of 1–20 μM eucalyptol. The in vivo animal study employed db/db mice orally administrated with 10 mg/kg eucalyptol. Nontoxic eucalyptol inhibited the Aβ induction in glucose-loaded RPE cells and diabetic mouse eyes. Eucalyptol reversed the induction of tight junction-associated proteins of ZO-1, occludin-1 and matrix metalloproteinases in glucose- or Aβ-exposed RPE cells and in diabetic eyes, accompanying inhibition of RPE detachment from Bruch’s membrane. Adding eucalyptol to glucose- or Aβ-loaded RPE cells, and diabetic mouse eyes reciprocally reversed induction/activation of apoptosis-related bcl-2, bax, cytochrome C/Apaf-1 and caspases. Eucalyptol attenuated the generation of reactive oxygen species and the induction of receptor for advanced glycation end products in Aβ-exposed RPE cells and diabetic eyes. Eucalyptol may ameliorate RPE barrier dysfunction in diabetic eyes through counteracting Aβ-mediated oxidative stress-induced RPE cell apoptosis.

Metabolic Features of Mouse and Human Retinas: Rods versus Cones, Macula versus Periphery, Retina versus RPE

Photoreceptors, especially cones, which are enriched in the human macula, have high energy demands, making them vulnerable to metabolic stress. Metabolic dysfunction of photoreceptors and their supporting retinal pigment epithelium (RPE) is an important underlying cause of degenerative retinal diseases. However, how cones and the macula support their exorbitant metabolic demand and communicate with RPE is unclear. By profiling metabolite uptake and release and analyzing metabolic genes, we have found cone-rich retinas and human macula share specific metabolic features with upregulated pathways in pyruvate metabolism, mitochondrial TCA cycle, and lipid synthesis. Human neural retina and RPE have distinct but complementary metabolic features. Retinal metabolism centers on NADH production and neurotransmitter biosynthesis. The retina needs aspartate to sustain its aerobic glycolysis and mitochondrial metabolism. RPE metabolism is directed toward NADPH production and biosynthesis of acetyl-rich metabolites, serine, and others. RPE consumes multiple nutrients, including proline, to produce metabolites for the retina.

Inhibition of Mitochondrial Respiration Impairs Nutrient Consumption and Metabolite Transport in Human Retinal Pigment Epithelium

Mitochondrial respiration in mammalian cells not only generates ATP to meet their own energy needs but also couples with biosynthetic pathways to produce metabolites that can be exported to support neighboring cells. However, how defects in mitochondrial respiration influence these biosynthetic and exporting pathways remains poorly understood. Mitochondrial dysfunction in retinal pigment epithelium (RPE) cells is an emerging contributor to the death of their neighboring photoreceptors in degenerative retinal diseases including age-related macular degeneration. In this study, we used targeted-metabolomics and ¹³C tracing to investigate how inhibition of mitochondrial respiration influences the intracellular and extracellular metabolome. We found inhibition of mitochondrial respiration strikingly influenced both the intracellular and extracellular metabolome in primary RPE cells. Intriguingly, the extracellular metabolic changes sensitively reflected the intracellular changes. These changes included substantially enhanced glucose consumption and lactate production; reduced release of pyruvate, citrate, and ketone bodies; and massive accumulation of multiple amino acids and nucleosides. In conclusion, these findings reveal a metabolic signature of nutrient consumption and release in mitochondrial dysfunction in RPE cells. Testing medium metabolites provides a sensitive and noninvasive method to assess mitochondrial function in nutrient utilization and transport.

Aerobic Glycolysis in the Retina: Functional Roles of Pyruvate Kinase Isoforms

One hundred years ago, Otto Heinrich Warburg observed that postmitotic retinal cells are the highest oxygen-consuming cells in the body. He compared these cells to actively growing mitotic tumor cells since both cells reprogram glucose for anabolic processes, which include lipid, protein, and RNA/DNA synthesis, and for antioxidant metabolism. To achieve this metabolic reprogramming, cancer cells preferentially express a less active dimeric form, the M2 isoform of pyruvate kinase (PKM2), which shuttles glucose toward the accumulation of glycolytic intermediates that redirect cell activities into anabolic processes. Similar to cancer cells, retinal photoreceptors predominantly express the M2 isoform of PKM2. This isoform performs both metabolic and non-metabolic functions in photoreceptor cells. This review focuses on the metabolic and non-metabolic roles of pyruvate kinases in photoreceptor cell functions.

The Symbiotic Relationship between the Neural Retina and Retinal Pigment Epithelium Is Supported by Utilizing Differential Metabolic Pathways

The neural retina and retinal pigment epithelium (RPE) maintain a symbiotic metabolic relationship, disruption of which leads to debilitating vision loss. The current study was undertaken to identify the differences in the steady-state metabolite levels and the pathways functioning between bona fide neural retina and RPE. Global metabolomics and cluster analyses identified 650 metabolites differentially modulated between the murine neural retina and RPE. Of these, 387 and 163 were higher in the RPE and the neural retina, respectively. Further analysis coupled with transcript and protein level investigations revealed that under normal physiological conditions, the RPE utilizes the pentose phosphate (>3-fold in RPE), serine (>10-fold in RPE), and sphingomyelin biosynthesis (>5-fold in RPE) pathways. Conversely, the neural retina relied mostly on glycolysis. These results show how the RPE and the neural retina have acquired an efficient, complementary and metabolically diverse symbiotic niche to support each other's distinct functions.

The cell biology of the retinal pigment epithelium

The retinal pigment epithelium (RPE), a monolayer of post-mitotic polarized epithelial cells, strategically situated between the photoreceptors and the choroid, is the primary caretaker of photoreceptor health and function. Dysfunction of the RPE underlies many inherited and acquired diseases that cause permanent blindness. Decades of research have yielded valuable insight into the cell biology of the RPE. In recent years, new technologies such as live-cell imaging have resulted in major advancement in our understanding of areas such as the daily phagocytosis and clearance of photoreceptor outer segment tips, autophagy, endolysosome function, and the metabolic interplay between the RPE and photoreceptors. In this review, we aim to integrate these studies with an emphasis on appropriate models and techniques to investigate RPE cell biology and metabolism, and discuss how RPE cell biology informs our understanding of retinal disease.

Bone marrow mesenchymal stem cells enhance autophagy and help protect cells under hypoxic and retinal detachment conditions

Our study aimed to evaluate the protective role and mechanisms of bone marrow mesenchymal stem cells (BMSCs) in hypoxic photoreceptors and experimental retinal detachment. The cellular morphology, viability, apoptosis and autophagy of hypoxic 661w cells and cells cocultured with BMSCs were analysed. In retinal detachment model, BMSCs were intraocularly transplanted, and then, the retinal morphology, outer nuclear layer (ONL) thickness and rhodopsin expression were studied as well as apoptosis and autophagy of the retinal cells. The hypoxia‐induced apoptosis of 661w cells obviously increased together with autophagy levels increasing and peaking at 8 hours after hypoxia. Upon coculturing with BMSCs, hypoxic 661w cells had a better morphology and fewer apoptosis. After autophagy was inhibited, the apoptotic 661w cells under the hypoxia increased, and the cell viability was reduced, even in the presence of transplanted BMSCs. In retina‐detached eyes transplanted with BMSCs, the retinal ONL thickness was closer to that of the normal retina. After transplantation, apoptosis decreased significantly and retinal autophagy was activated in the BMSC‐treated retinas. Increased autophagy in the early stage could facilitate the survival of 661w cells under hypoxic stress. Coculturing with BMSCs protects 661w cells from hypoxic damage, possibly due to autophagy activation. In retinal detachment models, BMSC transplantation can significantly reduce photoreceptor cell death and preserve retinal structure. The capacity of BMSCs to reduce retinal cell apoptosis and to initiate autophagy shortly after transplantation may facilitate the survival of retinal cells in the low‐oxygen and nutrition‐restricted milieu after retinal detachment.

Tight Junctions of the Outer Blood Retina Barrier

The outer blood retina barrier (oBRB) formed by the retinal pigment epithelium (RPE) is critical for maintaining retinal homeostasis. Critical to this modified neuro-epithelial barrier is the presence of the tight junction structure that is formed at the apical periphery of contacting cells. This tight junction complex mediates size-selective passive diffusion of solutes to and from the outer segments of the retina. Unlike other epithelial cells, the apical surface of the RPE is in direct contact with neural tissue and it is centrally involved in the daily phagocytosis of the effete tips of photoreceptor cells. While much is known about the intracellular trafficking of material within the RPE, less is known about the role of the tight junction complexes in health and diseased states. Here, we provide a succinct overview of the molecular composition of the RPE tight junction complex in addition to highlighting some of the most common retinopathies that involve a dysregulation of RPE integrity