Programmed cell death (PCD) is a highly regulated process that results in the orderly destruction of a cell. Many different forms of PCD may be distinguished, including apoptosis, PARthanatos, and cGMP-dependent cell death. Misregulation of PCD mechanisms may be the underlying cause of neurodegenerative diseases of the retina, including hereditary retinal degeneration (RD). RD relates to a group of diseases that affect photoreceptors and that are triggered by gene mutations that are often well known nowadays. Nevertheless, the cellular mechanisms of PCD triggered by disease-causing mutations are still poorly understood, and RD is mostly still untreatable. While investigations into the neurodegenerative mechanisms of RD have focused on apoptosis in the past two decades, recent evidence suggests a predominance of non-apoptotic processes as causative mechanisms. Research into these mechanisms carries the hope that the knowledge created can eventually be used to design targeted treatments to prevent photoreceptor loss. Hence, in this review, we summarize studies on PCD in RD, including on apoptosis, PARthanatos, and cGMP-dependent cell death. Then, we focus on a possible interplay between these mechanisms, covering cGMP-signaling targets, overactivation of poly(ADP-ribose)polymerase (PARP), energy depletion, Ca²⁺-permeable channels, and Ca²⁺-dependent proteases. Finally, an outlook is given into how specific features of cGMP-signaling and PARthanatos may be targeted by therapeutic interventions.

The retina has the highest relative energy consumption of any tissue, depending on a steady supply of glucose from the bloodstream. Glucose uptake is mediated by specific transporters whose regulation and expression are critical for the pathogenesis of many diseases, including diabetes and diabetic retinopathy. Here, we used immunofluorescence to show that glucose transporter-2 (GLUT2) is expressed in horizontal cells of the mouse neuroretina in proximity to inner retinal capillaries. To study the function of GLUT2 in the murine retina, we used organotypic retinal explants, cultivated under entirely controlled, serum free conditions, and exposed them to streptozotocin, a cytotoxic drug transported exclusively by GLUT2. Contrary to our expectations, streptozotocin did not measurably affect horizontal cell viability, while it ablated rod and cone photoreceptors in a concentration-dependent manner. Staining for poly-ADP-ribose (PAR) indicated that the detrimental effect of streptozotocin on photoreceptors may be associated with DNA-damage. The negative effect of streptozotocin on the viability of rod photoreceptors was counteracted by co-administration of either the inhibitor of connexin-formed hemi-channels meclofenamic acid or the blocker of clathrin-mediated endocytosis dynasore. Remarkably, cone photoreceptors were not protected from streptozotocin induced degeneration by neither of the two drugs. Overall, these data suggests the existence of a GLUT2-dependent glucose transport shuttle, from horizontal cells into photoreceptor synapses. Moreover, our study points at different glucose uptake mechanisms in rod and cone photoreceptors.

Iridoschisis is a rare condition that is characterized by the separation of the iris stroma into layers, with portions of the anterior layer floating freely in the aqueous humour. Here, we report three cases of iridoschisis that were complicated by either a cataract or glaucoma. Based on these cases, we speculate that the scope of iridoschisis has a rare association with intraocular pressure and the loss of corneal endothelial cells after surgery, which is mainly due to the surgery and not iridocorneal mechanical contact. We report three cases of iridoschisis, two of which were complicated by cataracts and the third by glaucoma. Patient 1 was a 69-year-old man with bilateral iridoschisis complicated by a cataract but not glaucoma, even though the entire anterior layer of the iris stroma in the right eye presented as white atrophic strands. To prevent the detached iris fibrils from invading the phacoemulsification tip and the irrigation/aspiration hand piece port, the separated iris stroma that was floating freely in the aqueous humour was cut with scissors immediately before the cataract extraction. Patient 2 was an 87-year-old woman with iridoschisis complicated by a cataract in the right eye. We successfully performed cataract surgery on the right eye without a pupillary device. Patient 3 was a 66-year-old man who presented with increased intraocular pressure with bilateral iridoschisis. He was discharged and prescribed with a combination of four glaucoma drugs. Patients with iridoschisis should have continuous follow-up because complications may occur, and extra care from ophthalmologists is required.

The retina consumes massive amounts of energy, yet its metabolism and substrate exploitation remain poorly understood. Here, we used a murine explant model to manipulate retinal energy metabolism under entirely controlled conditions and utilized 1H-NMR spectroscopy-based metabolomics, in situenzyme detection, and cell viability readouts to uncover the pathways of retinal energy production. Our experimental manipulations resulted in varying degrees of photoreceptor degeneration, while the inner retina and retinal pigment epithelium were essentially unaffected. This selective vulnerability of photoreceptors suggested very specific adaptations in their energy metabolism. Rod photoreceptors were found to rely strongly on oxidative phosphorylation, but only mildly on glycolysis. Conversely, cone photoreceptors were dependent on glycolysis but insensitive to electron transport chain decoupling. Importantly, photoreceptors appeared to uncouple glycolytic and Krebs-cycle metabolism via three different pathways: 1) the mini-Krebs-cycle, fueled by glutamine and branched-chain amino acids, generating N-acetylaspartate; 2) the alanine-generating Cahill-cycle; 3) the lactate-releasing Cori-cycle. Moreover, the metabolomic data indicated a shuttling of taurine and hypotaurine between the retinal pigment epithelium and photoreceptors, likely resulting in an additional net transfer of reducing power to photoreceptors. These findings expand our understanding of retinal physiology and pathology and shed new light on neuronal energy homeostasis and the pathogenesis of neurodegenerative diseases.

The retina consumes massive amounts of energy, yet its metabolism and substrate exploitation remain poorly understood. Here, we used a murine explant model to manipulate retinal energy metabolism under entirely controlled conditions and utilized ¹ H-NMR spectroscopy-based metabolomics, in situenzyme detection, and cell viability readouts to uncover the pathways of retinal energy production. Our experimental manipulations resulted in varying degrees of photoreceptor degeneration, while the inner retina and retinal pigment epithelium were essentially unaffected. This selective vulnerability of photoreceptors suggested very specific adaptations in their energy metabolism. Rod photoreceptors were found to rely strongly on oxidative phosphorylation, but only mildly on glycolysis. Conversely, cone photoreceptors were dependent on glycolysis but insensitive to electron transport chain decoupling. Importantly, photoreceptors appeared to uncouple glycolytic and Krebs-cycle metabolism via three different pathways: 1) the mini-Krebs-cycle, fueled by glutamine and branched-chain amino acids, generating N-acetylaspartate; 2) the alanine-generating Cahill-cycle; 3) the lactate-releasing Cori-cycle. Moreover, the metabolomic data indicated a shuttling of taurine and hypotaurine between the retinal pigment epithelium and photoreceptors, likely resulting in an additional net transfer of reducing power to photoreceptors. These findings expand our understanding of retinal physiology and pathology and shed new light on neuronal energy homeostasis and the pathogenesis of neurodegenerative diseases. Graphical abstract Retinal photoreceptors employ both glucose and glutamate as fuels. While rod photoreceptors rely strongly on oxidative phosphorylation and the N-acetylaspartate producing mini-Krebs-cycle, cone photoreceptors rely on the lactate-producing Cori cycle and the oxidative, alanine producing Cahill cycle. Highlights: The retina utilizes a complex energy switchboard consisting of the Krebs cycle, mini-Krebs cycle, Cahill cycle, and Cori cycle. Mini-Krebs cycle runs more efficiently than ‘full’ Krebs cycle. Alanine transaminase decouples glycolysis from the Krebs cycle. Lactate, alanine, and N-acetylaspartate are distinctive energetic pathway signatures.

In his seminal 1920s studies, Otto Warburg found the retina to generate large amounts of lactate. However, it was unclear what retinal cells produced lactate and whether it was a metabolic waste product or used further. Here, we show that lactate produced by rod photoreceptors fuels the energy-intensive function and viability of cone photoreceptors. In an initial expression analysis, we found monocarboxylate transporter-1 (MCT1), lactate-producing lactate-dehydrogenase-A (LDHA), and pyruvate carboxykinase-1 (PCK1) localized to rod photoreceptors, while high-affinity MCT2, pyruvate-producing LDHB, and PCK2 were expressed in cones. We then exposed retina to defined media containing either glucose or lactate as caloric component, and applied specific MCT inhibitors. In glucose-containing medium, 1H-NMR metabolomics showed rod MCT1 inhibition to increase retinal lactate, suggesting rods as a major source of lactate. In lactate-only medium, functional recordings using micro-electroretinography showed decreased rod function, while cone function was maintained. In glucose-containing medium, blocking rod MCT1 abolished cone function. Long-term treatment with MCT inhibitors selectively decreased photoreceptor viability. Conversely, supplementing the defined medium with lactate preserved cone viability in the rd1 mouse model for Retinitis Pigmentosa. Together, our data suggest that lactate shuttling from rods is crucial for cone function and viability. This may explain cone degeneration seen in various retinal diseases and provides an entirely new avenue for metabolism-based treatment development. The discovery of a lactate-shuttle between two functionally similar, yet distinct types of neurons may have far-reaching implications for our understanding of the central nervous system in general.