Updates on sphingolipids: Spotlight on retinopathy

Sphingosine-1-phosphate in the regulation of diabetes mellitus: a scientometric study to an in-depth review

Diabetes is a significant global health issue, causing extensive morbidity and mortality, and represents a serious threat to human health. Recently, the bioactive lipid molecule Sphingosine-1-Phosphate has garnered considerable attention in the field of diabetes research. The aim of this study is to comprehensively understand the mechanisms by which Sphingosine-1-Phosphate regulates diabetes. Through comprehensive bibliometric analysis and an in-depth review of relevant studies, we investigated and summarized various mechanisms through which Sphingosine-1-Phosphate acts in prediabetes, type 1 diabetes, type 2 diabetes, and their complications (such as diabetic nephropathy, retinopathy, cardiovascular disease, neuropathy, etc.), including but not limited to regulating lipid metabolism, insulin sensitivity, and inflammatory responses. This scholarly work not only unveils new possibilities for using Sphingosine-1-Phosphate in diabetes treatment but also offers fresh insights and recommendations for future research directions to researchers.

Sphingosine-1-phosphate signaling regulates the ability of Müller glia to become neurogenic, proliferating progenitor-like cells

The purpose of these studies is to investigate how Sphingosine-1-phosphate (S1P) signaling regulates glial phenotype, dedifferentiation of Müller glia (MG), reprogramming into proliferating MG-derived progenitor cells (MGPCs), and neuronal differentiation of the progeny of MGPCs in the chick retina. We found that S1P-related genes are highly expressed by retinal neurons and glia, and levels of expression were dynamically regulated following retinal damage. Drug treatments that activate S1P receptor 1 (S1PR1) or increase levels of S1P suppressed the formation of MGPCs. Conversely, treatments that inhibit S1PR1 or decrease levels of S1P stimulated the formation of MGPCs. Inhibition of S1P receptors or S1P synthesis significantly enhanced the neuronal differentiation of the progeny of MGPCs. We report that S1P-related gene expression in MG is modulated by microglia and inhibition of S1P receptors or S1P synthesis partially rescues the loss of MGPC formation in damaged retinas missing microglia. Finally, we show that TGFβ/Smad3 signaling in the resting retina maintains S1PR1 expression in MG. We conclude that the S1P signaling is dynamically regulated in MG and MGPCs in the chick retina, and activation of S1P signaling depends, in part, on signals produced by reactive microglia.

Hyaluronan-Sphingosine Polymersomes for Treatment of Ocular Neovascularization: Synthesis and Evaluation

Ocular neovascularization is a hallmark of several sight-threatening diseases, including diabetic retinopathy and age-related macular degeneration. Currently, available treatments are limited and often associated with side effects. Therefore, a novel approach to ocular neovascularization treatment through utilization of polymersomes from self-assembled sphingosine-grafted hyaluronic acid (HA-Sph) amphiphilic polymers is presented. The polymersomes are generated in spherical morphologies and sizes between 97.95 - 161.9 nm with homogenous size distributions. Experiments reveal that HA-Sph polymersomes, with concentrations ≥150 µg mL-1, significantly inhibit the proliferation of human umbilical vein endothelial cells (HUVECs), while concurrently promoting the proliferation of retinal pigment epithelial cells. The polymersomes demonstrate gradual disintegration in vitro, leading to sustained release of sphingosine, which prolongs the inhibition of HUVEC proliferation (from 87.5% at 24 h to 35.2% viability at 96 h). The efficacy of polymersomes in inhibiting angiogenesis is confirmed through tube formation assay, revealing a substantial reduction in tube length compared to the control group. The findings also validate the ocular penetration capability of polymersomes through ex vivo whole porcine eye ocular penetration study, indicating their suitability for topical administration. Potentially, HA-Sph polymersomes can be harnessed to develop intricate drug delivery systems that protect the retina and effectively treat ocular diseases.

Metabolomic-derived endotypes of age-related macular degeneration (AMD): a step towards identification of disease subgroups

Age-related macular degeneration (AMD) is a leading cause of blindness worldwide, with a complex pathophysiology and phenotypic diversity. Here, we apply Similarity Network Fusion (SNF) to cluster AMD patients into putative metabolomics-derived endotypes. Using a discovery cohort of 163 AMD patients from Boston, US, and a validation cohort of 214 patients from Coimbra, Portugal, we identified four distinct metabolomics-derived endotypes with varying retinal structural and functional characteristics, confirmed across both cohorts. Patients clustered into Endotype 1 exhibited a milder form of AMD and were characterized by low levels of amino acids in specific metabolic pathways. Meanwhile, patients clustered into both Endotype 3 and 4 were associated with more severe AMD and exhibited low levels of fatty acid metabolites and elevated levels of sphingomyelins and fatty acid metabolites, respectively. These preliminary findings indicate that metabolomics-derived endotyping may offer a refined strategy for categorizing AMD patients based on their specific pathophysiological underpinnings, rather than relying solely on traditional observational clinical indicators.

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.

Untargeted and temporal analysis of retinal lipidome in bacterial endophthalmitis

Bacterial endophthalmitis is a blinding infectious disease typically acquired during ocular surgery. We previously reported significant alterations in retinal metabolism during Staphylococcus (S) aureus endophthalmitis. However, the changes in retinal lipid composition during endophthalmitis are unknown. Here, using a mouse model of S. aureus endophthalmitis and an untargeted lipidomic approach, we comprehensively analyzed temporal alterations in total lipids and oxylipin in retina. Our data showed a time-dependent increase in the levels of lipid classes, sphingolipids, glycerolipids, sterols, and non-esterified fatty acids, whereas levels of phospholipids decreased. Among lipid subclasses, phosphatidylcholine decreased over time. The oxylipin analysis revealed increased prostaglandin-E2, hydroxyeicosatetraenoic acids, docosahexaenoic acid, eicosapentaenoic acid, and α-linolenic acid. In-vitro studies using mouse bone marrow-derived macrophages showed increased lipid droplets and lipid-peroxide formation in response to S. aureus infection. Collectively, these findings suggest that S. aureus-infection alters the retinal lipid profile, which may contribute to the pathogenesis of bacterial endophthalmitis.

Plasma Lipidomics Profiles Highlight the Associations of the Dual Antioxidant/Pro-oxidant Molecules Sphingomyelin and Phosphatidylcholine with Subclinical Atherosclerosis in Patients with Type 1 Diabetes

Here, we report on our study of plasma lipidomics profiles of patients with type 1 diabetes (T1DM) and explore potential associations. One hundred and seven patients with T1DM were consecutively recruited. Ultrasound imaging of peripheral arteries was performed using a high image resolution B-mode ultrasound system. Untargeted lipidomics analysis was performed using UHPLC coupled to qTOF/MS. The associations were evaluated using machine learning algorithms. SM(32:2) and ether lipid species (PC(O-30:1)/PC(P-30:0)) were significantly and positively associated with subclinical atherosclerosis (SA). This association was further confirmed in patients with overweight/obesity (specifically with SM(40:2)). A negative association between SA and lysophosphatidylcholine species was found among lean subjects. Phosphatidylcholines (PC(40:6) and PC(36:6)) and cholesterol esters (ChoE(20:5)) were associated positively with intima-media thickness both in subjects with and without overweight/obesity. In summary, the plasma antioxidant molecules SM and PC differed according to the presence of SA and/or overweight status in patients with T1DM. This is the first study showing the associations in T1DM, and the findings may be useful in the targeting of a personalized approach aimed at preventing cardiovascular disease in these patients.

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

Glaucoma is characterized by a continuous loss of retinal ganglion cells. The cause of glaucoma is associated with an increase in intraocular pressure (IOP), but the underlying pathophysiology is diverse and, in most cases, unknown. There is an indisputable unmet need to identify new pathways involved in glaucoma pathogenesis. Increasing evidence suggests that bioactive lipids may be critical in the development and progression of glaucoma. Preclinical and clinical bioactive lipid targets exist and are being developed. In this review, we aim to shed light on the potential of bioactive lipids for the prevention, diagnosis, prognosis, and treatment of glaucoma by asking the question "is fat the future for saving sight".

Single-cell transcriptomics-based multidisease analysis revealing the molecular dynamics of retinal neurovascular units under inflammatory and hypoxic conditions

The retinal neurovascular unit (NVU) is paramount to maintaining the homeostasis of the retina and determines the progression of various diseases, including diabetic retinopathy (DR), glaucoma, and retinopathy of prematurity (ROP). Although some studies have investigated these diseases, a combined analysis of disease-wide etiology in the NUV at the single-cell level is lacking. Herein, we constructed an atlas of the NVU under inflammatory and hypoxic conditions by integrating single-cell transcriptome data from retinas from wild-type, AireKO, and NdpKO mice. Based on the heterogeneity of the NVU structure and transcriptome diversity under normal and pathological conditions, we discovered two subpopulations of Müller cells: Aqp4^hi and Aqp4^lo cells. Specifically, Aqp4^lo cells expresses phototransduction genes and represent a special type of Müller cell distinct from Aqp4^hi cells, classical Müller cells. AireKO mice exhibit experimental autoimmune uveitis (EAU) with severe damage to the NVU structure, mainly degeneration of Aqp4^hi cells. NdpKO mice exhibited familial exudative vitreoretinopathy (FEVR), with damage to the endothelial barrier, endothelial cell tight junction destruction and basement membrane thickening, accompanied by the reactive secretion of proangiogenic factors by Aqp4^hi cells. In both EAU and FEVR, Aqp4^hi cells are a key factor leading to NVU damage, and the mechanism by which they are generated is regulated by different transcription factors. By studying the pattern of immune cell infiltration in AireKO mice, we constructed a regulatory loop of "inflammatory cells/NVU - monocytes - APCs - Ifng⁺ T cells", providing a new target for blocking the inflammatory cascade. Our elucidation of the cell-specific molecular changes, cell-cell interactions and transcriptional mechanisms of the retinal NVU provides new insights to support the development of multipurpose drugs to block or even reverse NVU damage.

Reduced sphingolipid biosynthesis modulates proteostasis networks to enhance longevity

As the elderly population increases, chronic, age-associated diseases are challenging healthcare systems around the world. Nutrient limitation is well known to slow the aging process and improve health. Regrettably, practicing nutrient restriction to improve health is unachievable for most people. Alternatively, pharmacological strategies are being pursued including myriocin which increases lifespan in budding yeast. Myriocin impairs sphingolipid synthesis, resulting in lowered amino acid pools which promote entry into a quiescent, long-lived state. Here we present transcriptomic data during the first 6 hours of drug treatment that improves our mechanistic understanding of the cellular response to myriocin and reveals a new role for ubiquitin in longevity. Previously we found that the methionine transporter Mup1 traffics to the plasma membrane normally in myriocin-treated cells but is not active and undergoes endocytic clearance. We now show that UBI4, a gene encoding stressed-induced ubiquitin, is vital for myriocin-enhanced lifespan. Furthermore, we show that Mup1 fused to a deubiquitinase domain impairs myriocin-enhanced longevity. Broader effects of myriocin treatment on ubiquitination are indicated by our finding of a significant increase in K63-linked ubiquitin polymers following myriocin treatment. Although proteostasis is broadly accepted as a pillar of aging, our finding that ubiquitination of an amino acid transporter promotes longevity in myriocin-treated cells is novel. Addressing the role of ubiquitination/deubiquitination in longevity has the potential to reveal new strategies and targets for promoting healthy aging.

KRAS Affects the Lipid Composition by Regulating Mitochondrial Functions and MAPK Activation in Bovine Mammary Epithelial Cells

Kirsten rat sarcoma viral oncogene homolog (KRAS), or guanosine triphosphatase KRAS, is a proto-oncogene that encodes the small guanosine triphosphatase transductor protein. Previous studies have found that KRAS can promote cytokine secretion, cell chemotaxis, and survival. However, its effects on milk fat synthesis in bovine mammary epithelial cells are unclear. In this study, the effects of KRAS inhibition on cell metabolism, autophagy, oxidative stress, endoplasmic reticulum stress, mitochondrial function, and lipid composition as well as the potential mechanisms were detected in an immortalized dairy cow mammary epithelial cell line (MAC-T). The results showed that inhibition of KRAS changed the lipid composition (especially the triglyceride level), mitochondrial functions, autophagy, and endoplasmic reticulum stress in cells. Moreover, KRAS inhibition regulated the levels of the mammalian target of rapamycin and mitogen-activated protein kinase (extracellular regulated protein kinases, c-Jun N-terminal kinases, p38) activation. These results indicated that regulation of KRAS would affect the synthesis and composition of milk fat. These results are also helpful for exploring the synthesis and secretion of milk fat at the molecular level and provide a theoretical basis for improving the percentage of fat in milk and the yield of milk from cows.

Dynamic lipid turnover in photoreceptors and retinal pigment epithelium throughout life

The retinal pigment epithelium-photoreceptor interphase is renewed each day in a stunning display of cellular interdependence. While photoreceptors use photosensitive pigments to convert light into electrical signals, the RPE supports photoreceptors in their function by phagocytizing shed photoreceptor tips, regulating the blood retina barrier, and modulating inflammatory responses, as well as regenerating the 11-cis-retinal chromophore via the classical visual cycle. These processes involve multiple protein complexes, tightly regulated ligand-receptors interactions, and a plethora of lipids and protein-lipids interactions. The role of lipids in maintaining a healthy interplay between the RPE and photoreceptors has not been fully delineated. In recent years, novel technologies have resulted in major advancements in understanding several facets of this interplay, including the involvement of lipids in phagocytosis and phagolysosome function, nutrient recycling, and the metabolic dependence between the two cell types. In this review, we aim to integrate the complex role of lipids in photoreceptor and RPE function, emphasizing the dynamic exchange between the cells as well as discuss how these processes are affected in aging and retinal diseases.

Urinary Mass Spectrometry Profiles in Age-Related Macular Degeneration

We and others have shown that patients with different severity stages of age-related macular degeneration (AMD) have distinct plasma metabolomic profiles compared to controls. Urine is a biofluid that can be obtained non-invasively and, in other fields, urine metabolomics has been proposed as a feasible alternative to plasma biomarkers. However, no studies have applied urinary mass spectrometry (MS) metabolomics to AMD. This study aimed to assess urinary metabolomic profiles of patients with different stages of AMD and a control group. We included two prospectively designed, multicenter, cross-sectional study cohorts: Boston, US (n = 185) and Coimbra, Portugal (n = 299). We collected fasting urine samples, which were used for metabolomic profiling (Ultrahigh Performance Liquid chromatography—Mass Spectrometry). Multivariable logistic and ordinal logistic regression models were used for analysis, accounting for gender, age, body mass index and use of AREDS supplementation. Results from both cohorts were then meta-analyzed. No significant differences in urine metabolites were seen when comparing patients with AMD and controls. When disease severity was considered as an outcome, six urinary metabolites differed significantly (p < 0.01). In particular, two of the metabolites identified have been previously shown by our group to also differ in the plasma of patients of AMD compared to controls and across severity stages. While there are fewer urinary metabolites associated with AMD than plasma metabolites, this study identified some differences across stages of disease that support previous work performed with plasma, thus highlighting the potential of these metabolites as future biomarkers for AMD.