Background: Regional heterogeneity of the macula and peripheral retinas contributes to the differences in retinal physiology, metabolic demand and susceptibility to macular diseases such as age-related macular degeneration (AMD), diabetic macular oedema (DME) and Macular telangiectasia type 2 (MacTel). Extracellular vesicles (EVs) released from the retinal pigment epithelium (RPE)-choroid are increasingly recognised as mediators of extracellular communication and correlate with the molecular biological states of their tissue of origin. However, regional variation in EV composition in human RPE-Choroid tissues remains poorly characterised. Methods: Human macular and peripheral RPE-choroid explants from non-diseased donor eyes (n = 4) were cultured ex vivo using a transwell system. EV and EV-depleted conditioned media were collected by differential ultracentrifugation, respectively. EVs were characterised by transmission electron microscopy (TEM) and nanoparticle tracking analysis (NTA). Proteomic profiling was performed using LC-MS/MS followed by multivariate, pathway, and differential expression analyses. Results: TEM and NTA confirmed the presence of vesicle-like particles in a 119-140 nm size range, indicating small EVs in both regions. Proteomic analysis demonstrated a region-associated separation between the macular and peripheral samples in both EV proteome and EV-depleted soluble secretome datasets. GO enrichment analysis revealed that macular EV proteomes were enriched for wound healing, cell-substrate adhesion, and focal adhesion-related terms with a high abundance of integrin and annexin family members, whereas peripheral EV proteomes were enriched for retinoid- and vitamin-binding terms. In the EV-depleted soluble secretome, macular-enriched proteins were associated with actin binding and extracellular matrix-related terms, while peripheral-enriched proteins were correlated with RNA localisation and nuclear compartment terms. Comparative analysis identified EV-specific, secretome-specific, and shared extracellular protein pools. Several EV-specific markers and membrane proteins such as annexins and integrins, showed a relatively high enrichment in macular EV proteomes, whereas CD63 is more abundant in peripheral EV proteomes. In contrast, EV-depleted soluble secretome contains cytokine- and ligand-associated proteins, including MIF, SPP1, and IL6, which may suggest that EVs and soluble secreted proteins represent partially distinct extracellular signalling compartments. Conclusions: Human macular and peripheral RPE-choroid explants released secretory proteins in a regional differentiated manner, supported by PLS-DA and GO enrichment analyses of both EV proteomes and EV-depleted soluble secretome. Further analysis of the EV proteomes may show that such differences were also able to be reflected in protein categories such as EV-specific markers, mitochondrial and membrane proteins. These findings provide a foundation for future investigations into the role of EV-mediated communication in macular diseases and may support the development of region-specific extracellular biomarkers and therapeutic targets.

Abstract Purpose Alanine transaminases (ALT), encoded by the GPT gene, catalyzes the reversible conversion of pyruvate and glutamate to alanine and alpha-ketoglutarate, thereby correlating carbohydrate and amino acid metabolism. However, its role in the human neural retina remains unclear. This study aimed to explore the expression, localization, and metabolic function of ALT in the human neural retina and its potential involvement in retinal diseases. Methods ALT1 and ALT2 expression and localization were examined in the retinas of healthy and diabetic retinopathy (DR) donors via immunoblotting and immunofluorescence. ALT function was assessed in ex vivo human retinal explants using pharmacological inhibition with beta-chloro-L-alanine (BCLA), followed by the analyses of enzyme activity, tissue injury, and transcriptomic responses. Stable-isotope tracing with 13 C-and 15 N-labelled substrates combined with GC-MS was used to define ALT-dependent carbon and nitrogen fluxes in macular and peripheral retinas. Redox level (NADPH/NADP + ) was also evaluated under tert-butyl hydroperoxide-induced oxidative stress. Results ALT1 and ALT2 were both expressed in the human neural retina, with prominent localization in Müller glia and photoreceptor inner segments. ALT1 displayed a diffuse cytoplasmic distribution, whereas ALT2 demonstrated a punctate pattern consistent with mitochondrial localization. In DR retinas, ALT1 expression was spatially disorganized and heterogeneous, while ALT2 remained comparatively preserved. Inhibition of ALT with BCLA markedly reduced ALT activity without causing overt cytotoxicity or major transcriptional changes. Isotope tracing demonstrated that retinal ALT predominantly channels pyruvate-derived carbon into alanine, whereas alanine was minimally contributed to pyruvate production under basal conditions. ALT inhibition suppressed alanine synthesis and release, redirected nitrogen flux towards glutamate, glutamine, and aspartate, and uncovered distinct metabolic adaptations in macular but not peripheral retinas. Under oxidative stress, ALT inhibition induced the decrease of NADP + /NADPH ratio and LDH release, indicating improved redox balance and reduced tissue injury. Conclusions ALT is previously unrecognized as a regulator of carbon and nitrogen partitioner in the human neural retina, contributing to redox homeostasis under stress. The altered distribution of ALT1 in DR retina and the protective metabolic effects of ALT inhibition suggest ALT as a potential contributor to retinal metabolic vulnerability and a candidate therapeutic target in retinal diseases.

The macula is preferentially affected in some common retinal diseases (such as age-related macular degeneration, diabetic retinopathy and macular telangiectasia type 2), whereas most inherited retinal degenerations (e.g., retinitis pigmentosa) tend to initially affect the peripheral retina. This pattern suggests the macula may have intrinsic vulnerabilities in its oxidative stress defences, compared to the periphery. Profiling of single-cell level transcriptional changes found that the peripheral retina exhibited greater transcriptional alterations than the macula in response to stress. One pronounced change was in a subgroup of Müller glia (MG) that was dominant in the peripheral retina. Genes more abundantly expressed in peripheral MG were mainly associated with redox regulation, oxidative stress responses and cellular detoxification and were more influenced by oxidative insults, such as light-induced stress. In contrast, genes highly expressed in macular MG were primarily involved in cellular homeostasis and neuroprotection, showing less responsiveness to oxidative challenges. Notably, Metallothionein 1 (MT1), A-Kinase Anchor Protein 12 (AKAP12) and MAF BZIP Transcription Factor F (MAFF) were significantly more expressed in peripheral MG than in macular MG, indicating a region-specific redox regulatory mechanism. Knockdown of these genes in primary MG led to decreased viability under oxidative stress, suggesting their role in antioxidant defence. Our findings indicate that macular MG prioritise retinal function over redox adaptation, which may contribute to their vulnerability to degenerative diseases associated with oxidative damage. These insights underscore the importance of region-specific redox homeostasis in retinal health and disease.

The predilection of many conditions, such as macular telangiectasia type 2, for the human macula suggests it may be more susceptible to stress than the peripheral retina. In this study, we have comprehensively investigated the transcriptomic profiling of the macula and peripheral retina in response to stress. We conducted single-cell RNA sequencing analysis on the macula and peripheral retina of four donors cultured ex vivo with or without exposure to light stress. We found that the peripheral retina generally exhibited more transcriptional changes than the macula in response to stress. Interestingly, one of the most pronounced changes was observed in a subgroup of Müller cells that are dominant in the peripheral retina. Genes more abundantly expressed in peripheral retinal Müller cells were mainly associated with stress responses and were more influenced by light stress. In contrast, genes that were highly expressed in Müller cells that predominate in the macula played roles in cellular function and were less influenced by light stress. We identified that Metallothionein 1 (MT1), A Kinase Anchor Protein 12 (AKAP12) and MAF BZIP Transcription Factor F (MAFF) were more abundantly expressed in peripheral Müller cells than in macular Müller cells. We found that these genes were also activated in the mouse retina in the early stages of development of subretinal neovascularisation. Knockdown of the MT1, AKAP12 and MAFF genes in human primary Müller cells reduced cell viability in response to light stress and disrupted several stress response pathways. Taken together, our findings indicate that macular Müller cells are more directed toward maintaining retinal cell function rather than mounting a stress response when they are exposed to acute stress, which may contribute to the macula’s vulnerability to degenerative disease.### Competing Interest StatementThe authors have declared no competing interest.

Subretinal fibrosis is a major untreatable cause of poor outcomes in neovascular age-related macular degeneration. Mouse models of subretinal fibrosis all possess a degree of invasiveness and tissue damage not typical of fibrosis progression. This project characterises JR5558 mice as a model to study subretinal fibrosis. Fundus and optical coherence tomography (OCT) imaging was used to non-invasively track lesions. Lesion number and area were quantified with ImageJ. Retinal sections, wholemounts and Western blots were used to characterise alterations. Subretinal lesions expand between 4 and 8 weeks and become established in size and location around 12 weeks. Subretinal lesions were confirmed to be fibrotic, including various cell populations involved in fibrosis development. Müller cell processes extended from superficial retina into subretinal lesions at 8 weeks. Western blotting revealed increases in fibronectin (4 wk and 8 wk, p < 0.001), CTGF (20 wks, p < 0.001), MMP2 (12 wks and 20 wks p < 0.05), αSMA (12 wks and 20 wks p < 0.05) and GFAP (8 wk and 12 wk, p ≤ 0.01), consistent with our immunofluorescence results. Intravitreal injection of Aflibercept reduced subretinal lesion growth. Our study provides evidence JR5558 mice have subretinal fibrotic lesions that grow between 4 and 8 weeks and confirms this line to be a good model to study subretinal fibrosis development and assess treatment options.

The common final pathway to blindness in many forms of retinal degeneration is the death of the light-sensitive primary retinal neurons. However, the normally light-insensitive second- and third-order neurons persist optogenetic gene therapy aims to restore sight by rendering such neurons light-sensitive. Here, we investigate whether bReaChES, a newly described high sensitivity Type I opsin with peak sensitivity to long-wavelength visible light, can restore vision in a murine model of severe early-onset retinal degeneration. Intravitreal injection of an adeno-associated viral vector carrying the sequence for bReaChES downstream of the calcium calmodulin kinase IIα promoter resulted in sustained retinal expression of bReaChES. Retinal ganglion cells (RGCs) expressing bReaChES generated action potentials at light levels consistent with bright indoor lighting (from 13.6 log photons cm −2 s −1 ). They could also detect flicker at up to 50 Hz, which approaches the upper temporal limit of human photopic vision. Topological response maps of bReaChES-expressing RGCs suggest that optogenetically activated RGCs may demonstrate similar topographical responses to RGCs stimulated by photoreceptor activation. Furthermore, treated dystrophic mice displayed restored cortical neuronal activity in response to light and rescued behavioral responses to a looming stimulus that simulated an aerial predator. Finally, human surgical retinal explants exposed to the bReaChES treatment vector demonstrated transduction. Together, these findings suggest that intravitreal gene therapy to deliver bReaChES to the retina may restore vision in human retinal degeneration in vivo at ecologically relevant light levels with spectral and temporal response characteristics approaching those of normal human photopic vision.

Rationale: Müller cells play an essential role in maintaining the health of retinal photoreceptors. Dysfunction of stressed Müller cells often results in photoreceptor degeneration. However, how these cells communicate under stress and the signalling pathways involved remain unclear. In this study, we inhibited the MAPK (ERK1/2) signalling, mainly activated in Müller cells, evaluated the protective effects on the photoreceptors and further explored the signalling communication between stressed Müller cells and degenerating photoreceptors. Methods: We evaluated the changes of MAPK (ERK1/2) signalling and its downstream targets in human retinal explants treated with PD98059, a specific phosphorylated ERK1/2 inhibitor, by western blot and immunostaining. We further assessed photoreceptor degeneration by TUNEL staining and outer nuclear layer thickness. We also injected PD98059 into the eyes of mice exposed to photo-oxidative stress. We evaluated the protective effects on photoreceptor degeneration by optical coherence tomography (OCT) and electroretinography (ERG). The crosstalk between Müller cells and photoreceptors was further dissected based on the changes of transcription factors by RNA sequencing and protein profiles of multiple signalling pathways. Results: We found that MAPK (ERK1/2) signalling was mainly activated in Müller cells under stress, both ex vivo and in vivo. PD98059 inhibited the phosphorylation of ERK1/2, reduced expression of the gliotic marker glial fibrillary acidic protein (GFAP) in Müller cells and increased levels of the neuroprotective factor, interphotoreceptor retinoid-binding protein (IRBP) in photoreceptors. Inhibition of pERK1/2 also reduced retinal photo-oxidative damage in mice retinas assessed by OCT and ERG. We also identified that the JAK/STAT3 signalling pathway might mediate signalling transduction from Müller cells to photoreceptors. Conclusion: MAPK (ERK1/2) deactivation through chemical inhibition, mainly in stressed Müller cells, can alleviate gliosis in Müller cells and restore the expression of IRBP in photoreceptors, which appears to prevent retinal degeneration. Our findings suggested a new way to prevent photoreceptor degeneration by manipulating the stress response in Müller cells.

The Pentose Phosphate Pathway (PPP), a metabolic offshoot of the glycolytic pathway, provides protective metabolites and molecules essential for cell redox balance and survival. Transketolase (TKT) is the critical enzyme that controls the extent of “traffic flow” through the PPP. Here, we explored the role of TKT in maintaining the health of the human retina. We found that Müller cells were the primary retinal cell type expressing TKT in the human retina. We further explored the role of TKT in human Müller cells by knocking down its expression in primary cultured Müller cells (huPMCs), isolated from the human retina (11 human donors in total), under light-induced oxidative stress. TKT knockdown and light stress reduced TKT enzymatic activities and the overall metabolic activities of huPMCs with no detectable cell death. TKT knockdown restrained the PPP traffic flow, reduced the expression of NAD(P)H Quinone Dehydrogenase 1 (NQO1), impaired the antioxidative response of NRF2 to light stress and aggravated the endoplasmic reticulum (ER) stress. TKT knockdown also inhibited overall glucose intake, reduced expression of Dihydrolipoamide dehydrogenase (DLD) and impaired the energy supply of the huPMCs. In summary, Müller cell-mediated TKT activity plays a critical protective role in the stressed retina. Knockdown of TKT disrupted the PPP and impaired overall glucose utilisation by huPMCs and rendered huPMCs more vulnerable to light stress by impairing energy supply and antioxidative NRF2 responses.

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.

Rationale: The Notch and transforming growth factor-β (TGFβ) signaling pathways are two intracellular mechanisms that control fibrosis in general but whether they play a major role in retinal fibrosis is less clear. Here we study how these two signaling pathways regulate Müller cell-dominated retinal fibrosis in vitro and in vivo. Methods: Human MIO-M1 Müller cells were treated with Notch ligands and TGFβ1, either alone or in combination. Western blots were performed to study changes in γ-secretase proteases, Notch downstream effectors, endogenous TGFβ1, phosphorylated Smad3 (p-Smad3) and extracellular matrix (ECM) proteins. We also studied the effects of RO4929097, a selective γ-secretase inhibitor, on expression of ECM proteins after ligand stimulation. Müller cell viability was studied by AlamarBlue and cytotoxicity by lactate cytotoxicity assays. Finally, we studied changes in Notch and TGFβ signaling and tested the effect of intravitreal injections of the Notch pathway inhibitor RO4929097 on retinal fibrosis resulted from Sodium iodate (NaIO3)-induced retinal injury in mice. We also studied the safety of intravitreal injections of RO4929097 in normal mice. Results: Treatment of Müller cells with Notch ligands upregulated γ-secretase proteases and Notch downstream effectors, with increased expression of endogenous TGFβ1, TGFβ receptors and p-Smad3. TGFβ1 upregulated the expression of proteins associated with both signaling pathways in a similar manner. Notch ligands and TGFβ1 had additive effects on overexpression of ECM proteins in Müller cells which were inhibited by RO4929097. Notch and TGFβ ligands stimulated Müller cell proliferation which was inhibited by RO4929097 without damaging the cells. NaIO3-induced retinal injury activated both Notch and TGFβ signaling pathways in vivo. Intravitreal injection of RO4929097 prevented Müller cell gliosis and inhibited overexpression of ECM proteins in this murine model. We found no safety concerns for up to 17 days after an intravitreal injection of RO4929097. Conclusions: Inhibiting Notch signaling might be an effective way to prevent retinal fibrosis. This study is of clinical significance in developing a treatment for preventing fibrosis in proliferative vitreoretinopathy, proliferative diabetic retinopathy and wet age-related macular degeneration.

Diabetic macular oedema (DME) is the leading cause of visual impairment in people with diabetes. Intravitreal injections of vascular endothelial growth factor inhibitors or corticosteroids prevent loss of vision by reducing DME, but the injections must be given frequently and usually for years. Here we report laboratory and clinical studies on the safety and efficacy of 670 nm photobiomodulation (PBM) for treatment of centre-involving DME. The therapeutic effect of PBM delivered via a light-emitting diode (LED) device was tested in transgenic mice in which induced Müller cell disruption led to photoreceptor degeneration and retinal vascular leakage. We also developed a purpose-built 670 nm retinal laser for PBM to treat DME in humans. The effect of laser-delivered PBM on improving mitochondrial function and protecting against oxidative stress was studied in cultured rat Müller cells and its safety was studied in pigmented and non-pigmented rat eyes. We then used the retinal laser to perform PBM in an open-label, dose-escalation Phase IIa clinical trial involving 21 patients with centre-involving DME. Patients received 12 sessions of PBM over 5 weeks for 90 s per treatment at a setting of 25, 100 or 200 mW/cm2 for the three sequential cohorts of 6–8 patients each. Patients were recruited from the Sydney Eye Hospital, over the age of 18 and had centre-involving DME with central macular thickness (CMT) of >300 μm with visual acuity of 75–35 Log minimum angle of resolution (logMAR) letters (Snellen visual acuity equivalent of 20/30–20/200). The objective of this trial was to assess the safety and efficacy of laser-delivered PBM at 2 and 6 months. The primary efficacy outcome was change in CMT at 2 and 6 months. LED-delivered PBM enhanced photoreceptor mitochondrial membrane potential, protected Müller cells and photoreceptors from damage and reduced retinal vascular leakage resulting from induced Müller cell disruption in transgenic mice. PBM delivered via the retinal laser enhanced mitochondrial function and protected against oxidative stress in cultured Müller cells. Laser-delivered PBM did not damage the retina in pigmented rat eyes at 100 mW/cm2. The completed clinical trial found a significant reduction in CMT at 2 months by 59 ± 46 μm (p = 0.03 at 200 mW/cm2) and significant reduction at all three settings at 6 months (25 mW/cm2: 53 ± 24 μm, p = 0.04; 100 mW/cm2: 129 ± 51 μm, p < 0.01; 200 mW/cm2: 114 ± 60 μm, p < 0.01). Laser-delivered PBM was well tolerated in humans at settings up to 200 mW/cm2 with no significant side effects. PBM results in anatomical improvement of DME over 6 months and may represent a safe and non-invasive treatment. Further testing is warranted in randomised clinical trials. ClinicalTrials.gov NCT02181400