Is fat the future for saving sight? Bioactive lipids and their impact on glaucoma

High-fat diet alters retinal lipid composition and gene expression networks in mice

BACKGROUND

High-fat diet (HFD) was suggested to be associated with several retinal diseases, including age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy (DR). Nevertheless, our understanding of the mechanisms governing retinal lipid metabolic homeostasis remains limited, with little attention focused on the influence of HFD on different retinal cell types. To address this gap, we established a high-fat model using mice fed with HFD for a duration of 6 months. Then, we conducted a comparative analysis of the retinal lipidome and proteome between normal diet (ND) and HFD-fed mice to explore the impacts of HFD on retinal lipid metabolism and gene expression network. Furthermore, we also investigated the impacts of HFD on retina in single-cell resolution by single-cell transcriptome sequencing.

RESULTS

We found that a long-term HFD significantly altered the lipid composition of the retina, with a dramatically elevated cholesterylesters (CE), phosphatidylcholine (PC), and phosphatidylglycerol (PG) level and a decreased eicosanoid level. Proteomic analysis revealed that the primary bile acid biosynthesis pathway was over-activated in HFD retinas. By using single-cell transcriptome analysis, we identified different regulation of gene expression in MG and rod cells in a high-fat environment, whereas the previously identified activation of the bile acid synthesis pathway was predominantly found in MG cells, and may be regulated by alternative pathways of bile acid synthesis, suggesting the critical roles of MG cells in retinal lipid metabolism.

CONCLUSIONS

Taken together, by multi-omics studies, we unveiled that HFD leading to the development of retinal diseases may be regulated by alternative pathways of bile acid synthesis, and our study will shed light on the treatment of these diseases.

Lipids and lipid regulators in intraocular pressure homeostasis

Increased intraocular pressure is the strongest correlated modifiable risk factor for developing primary open-angle glaucoma (POAG). Lipids have long been known to be a major constituent of aqueous humor. Lipid mediators, prostaglandins for example, are the first-line treatment for glaucoma. Innovative technologies have made the investigation of lipids in small quantities possible, and interest in identifying lipids as new pharmacological targets has grown in ophthalmology. There is expanding evidence to suggest that lipids and their active metabolites play a role in POAG pathophysiology, as differences between control and diseased eyes have now been demonstrated. The role of these differences is yet to be determined and is the subject of this review.

Development and characterization of a chronic high intraocular pressure model in New Zealand white rabbits for glaucoma research

Glaucoma is a neurodegenerative disease characterized by visual field loss associated with optic nerve damage and ocular hypertension. The biological basis for the elevated intraocular pressure (IOP) is largely unknown, such that lowering the IOP is currently the only established treatment. Several animal models have been developed to elucidate the mechanism underlying the increased IOP and for use in drug discovery research, but their utility is often limited by the occurrence of severe intraocular inflammation and by technical challenges. In this study, we developed a rabbit glaucoma model that does not require experimental disease induction. Rabbits were chosen as the model because their eyeballs are similar in size to those of humans, and they are easy to breed. By crossing rabbit strains with inherited glaucoma, as indicated by obvious buphthalmos, we produced a strain that exhibits ocular hypertension. The IOP of the Ocular Hypertension (OH) rabbits was significantly higher than that of the wild type (WT; normal New Zealand white rabbits) from the age of 3 weeks to at least 22 weeks. The significantly larger corneal diameter of the OH rabbits indicated ocular enlargement, whereas there was no significant difference in corneal thickness compared with WT rabbits. Anterior segment ocular coherence tomography and gonioscopic observations revealed an open angle in the OH rabbits. Hematoxylin and eosin (HE) staining together with Masson's trichrome staining showed abnormal collagen accumulation in the angle of the OH rabbit's eyes. Furthermore, aqueous humor (AH) outflow imaging following an intravitreal injection of a fluorescent probe into the anterior chamber for tissue-section analysis revealed retention of the probe in the area of collagen deposition in the OH eyes. The OH rabbits also had a time-dependent increase in the cup/disc ratio. In conclusion, investigations using our newly developed rabbit model of open-angle ocular hypertension showed that abnormal accumulation of extracellular matrix at the angle increased AH outflow resistance in the conventional outflow pathway, leading to a high IOP. Furthermore, the OH rabbits exhibited glaucomatous optic disc cupping over time. These findings suggest the utility of the OH rabbits as a model for open-angle glaucoma (OAG).

Lipid mediators in glaucoma: Unraveling their diverse roles and untapped therapeutic potential

Glaucoma is a complex neurodegenerative disease characterized by optic nerve damage and visual field loss, and remains a leading cause of irreversible blindness. Elevated intraocular pressure (IOP) is a critical risk factor that requires effective management. Emerging research underscores dual roles of bioactive lipid mediators in both IOP regulation, and the modulation of neurodegeneration and neuroinflammation in glaucoma. Bioactive lipids, encompassing eicosanoids, specialized pro-resolving mediators (SPMs), sphingolipids, and endocannabinoids, have emerged as crucial players in these processes, orchestrating inflammation and diverse effects on aqueous humor dynamics and tissue remodeling. Perturbations in these lipid mediators contribute to retinal ganglion cell loss, vascular dysfunction, oxidative stress, and neuroinflammation. Glaucoma management primarily targets IOP reduction via pharmacological agents and surgical interventions, with prostaglandin analogues at the forefront. Intriguingly, additional lipid mediators offer promise in attenuating inflammation and providing neuroprotection. Here we explore these pathways to shed light on their intricate roles, and to unveil novel therapeutic avenues for glaucoma management.

An Autotaxin-Induced Ocular Hypertension Mouse Model Reflecting Physiological Aqueous Biomarker

Purpose

Animal models of ocular hypertension (OH) have been developed to understand the pathogenesis of glaucoma and facilitate drug discovery. However, many of these models are fraught with issues, including severe intraocular inflammation and technical challenges. Lysophosphatidic acid (LPA) is implicated in trabecular meshwork fibrosis and increased resistance of aqueous outflow, factors that contribute to high intraocular pressure (IOP) in human open-angle glaucoma. We aimed to elevate IOP by increasing expression of the LPA-producing enzyme autotaxin (ATX) in mouse eyes.

Methods

Tamoxifen-inducible ATX transgenic mice were developed. Tamoxifen was administered to six- to eight-week-old mice via eye drops to achieve ATX overexpression in the eye. IOP and retinal thickness were measured over time, and retinal flat-mount were evaluated to count retinal ganglion cells (RGCs) loss after three months.

Results

Persistent elevation of ATX expression in mouse eyes was confirmed through immunohistochemistry and LysoPLD activity measurement. ATX Tg mice exhibited significantly increased IOP for nearly two months following tamoxifen treatment, with no anterior segment changes or inflammation. Immunohistochemical analysis revealed enhanced expression of extracellular matrix near the angle after two weeks and three months of ATX induction. This correlated with reduced outflow facility, indicating that sustained ATX overexpression induces angle fibrosis, elevating IOP. Although inner retinal layer thickness remained stable, peripheral retina showed a notable reduction in RGC cell count.

Conclusions

These findings confirm the successful creation of an open-angle OH mouse model, in which ATX expression in the eye prompts fibrosis near the angle and maintains elevated IOP over extended periods.

Understanding the complex genetics and molecular mechanisms underlying glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide. Currently the only effective treatment for glaucoma is to reduce the intraocular pressure, which can halt the progression of the disease. Highlighting the importance of identifying individuals at risk of developing glaucoma and those with early-stage glaucoma will help patients receive treatment before sight loss. However, some cases of glaucoma do not have raised intraocular pressure. In fact, glaucoma is caused by a variety of different mechanisms and has a wide range of different subtypes. Understanding other risk factors, the underlying mechanisms, and the pathology of glaucoma might lead to novel treatments and treatment of underlying diseases. In this review we present the latest research into glaucoma including the genetics and molecular basis of the disease.