Molecular mechanisms of retinal ischemia

Abstract

Each day, the retina converts an immense number of photons into chemical signals that are then transported to higher order neural centers for interpretation. This process of photo transduction requires large quantities of cellular energy and anabolic precursors, making the retina one of the most metabolically active tissues in the body. With such a large metabolic demand, the retina is understandably sensitive to perturbations in perfusion and hypoxia. Indeed, retinal ischemia underlies many prevalent retinal disorders including diabetic retinopathy (DR), retinal vein occlusion (RVO), and retinopathy of prematurity (ROP). Retinal ischemia leads to the expression of growth factors, cytokines, and other cellular mediators which promote inflammation, vascular dysfunction, and ultimately, vision loss. This review aims to highlight the most recent and compelling findings that have advanced our understanding of the molecular mechanisms underlying retinal ischemias.

Figure 1

Molecular mechanisms regulating HIF activation. All metazoans share an evolutionarily conserved heterodimer system known as hypoxia-inducible factor 1 (HIF-1) which indirectly senses intracellular oxygen concentrations and acts as a transcription factor to initiate a cellular response to hypoxia. HIF-1 consists of two subunits, HIF-1α and HIF-1β, both of which are constitutively expressed at the transcriptional and translational levels. Regulation of the HIF-1 heterodimer system primarily occurs through post-translational modifications of HIF-1α which is hydroxylated by the prolyl hydroxylase PHD2 and the asparaginyl hydroxylase FIH-1. Hydroxylated HIF-1α binds von Hippel-Lindau (VHL) which recruits ubiquitin ligases and targets HIF-1α for proteasomal degradation. PHD2 and FIH-1-mediated hydroxylation of HIF-1α utilizes O₂ as a substrate and is thus limited by the availability of oxygen, providing a real-time sensor of intracellular O₂ concentrations. During hypoxia, HIF-1α hydroxylation is inhibited, preventing its degradation and allowing heterodimerization with HIF-1β. This unit directly interacts with DNA at hypoxia response elements (HREs) and induces the transcription of a diverse set of genes including those involved in angiogenesis, metabolism, and inflammation [7]. In PHD2 and FIH-1-mediated hydroxylation of HIF-1α, one oxygen atom is inserted into HIF-1α while the remaining oxygen atom is inserted into α-ketoglutarate, splitting it into CO₂ and succinate. Thus, independent of hypoxia, reduced α-ketoglutarate levels can act as a limiting reagent in the hydroxylation of HIF-1α, leading to its activation. In addition, increased mitochondrial-derived ROS can inhibit hydroxylation of HIF-1α through an unknown mechanism, leading to its activation.

Figure 2

Novel insights into the metabolic alterations in retinal ischemias. (a) In the normal state, photoreceptors (depicted here in grey) rely on aerobic glycolysis for synthesis of anabolic precursors and β-oxidation for production of ATP. When both β-oxidation and glycolysis are impaired, as in Vldlr^−/− mice, α-ketogluterate levels are reduced leading to activation of HIF-1α and subsequent neovascularization. In pseudohypoxic RPE cells (depicted here in blue), β-oxidation regulatory proteins are reduced and long-chain acylcarnitines aggregate, suggesting impaired lipid handling. In addition, TCA cycle enzymes are downregulated and glycolytic enzymes are increased, suggesting a shift to anaerobic glycolysis. (b) Metabolomic studies on vitreous humor from PDR patients (depicted here as upper eye) revealed several metabolic alterations including increased proline, citrulline, arginine, acylcarnitines, pentose phosphates, and purine intermediates, and decreased glycolytic intermediates and xanthine. Metabolomic analysis of retinas from diabetic db/db mice (depicted here as lower eye) revealed increased acylcarnitines and glycolytic metabolites.

Figure 3

Novel insights into the inflammatory responses to retinal ischemias. (a) CD45^hi CD11b⁺ macrophages are increased in mouse retinas with pericyte free vessels and promote vascular dysfunction through paracrine growth factor release. (b) Neovascular endothelial cells and microglia enter a senescent-like state in hypoxic mouse retinas and develop a pro-inflammatory secretory phenotype. (c) Leukostasis-mediated vascular plugging occurs in response to increased retinal VEGF and results in secondary regions of retinal hypoxia. (d) FOXP3⁺ Tregs improve vascular dysfunction in hypoxic mouse retinas by decreasing microglial activation via direct cell contact.

Figure 4

Novel insights into vascular dysfunction in retinal ischemias. (a) Normal retinal vasculature is composed of a heterogeneous assortment of ECs while neovascularization is homogenous and composed of clonally expanded ECs originating from a previously quiescent EC. (b) Changes in gut flora, induced by intermittent fasting, alters BA metabolism, leading to increased TUDCA. TUDCA travels via the blood to the neural retina where it signals through its cognate receptor and protects the retinal vasculature.