Postnatal development of rat retina: a continuous observation and comparison between the organotypic retinal explant model and in vivo development

JOURNAL/nrgr/04.03/01300535-202503000-00033/figure1/v/2024-06-17T092413Z/r/image-tiff The organotypic retinal explant culture has been established for more than a decade and offers a range of unique advantages compared with in vivo experiments and cell cultures. However, the lack of systematic and continuous comparison between in vivo retinal development and the organotypic retinal explant culture makes this model controversial in postnatal retinal development studies. Thus, we aimed to verify the feasibility of using this model for postnatal retinal development studies by comparing it with the in vivo retina. In this study, we showed that postnatal retinal explants undergo normal development, and exhibit a consistent structure and timeline with retinas in vivo. Initially, we used SOX2 and PAX6 immunostaining to identify retinal progenitor cells. We then examined cell proliferation and migration by immunostaining with Ki-67 and doublecortin, respectively. Ki-67- and doublecortin-positive cells decreased in both in vivo and explants during postnatal retinogenesis, and exhibited a high degree of similarity in abundance and distribution between groups. Additionally, we used Ceh-10 homeodomain-containing homolog, glutamate-ammonia ligase (glutamine synthetase), neuronal nuclei, and ionized calcium-binding adapter molecule 1 immunostaining to examine the emergence of bipolar cells, Müller glia, mature neurons, and microglia, respectively. The timing and spatial patterns of the emergence of these cell types were remarkably consistent between in vivo and explant retinas. Our study showed that the organotypic retinal explant culture model had a high degree of consistency with the progression of in vivo early postnatal retina development. The findings confirm the accuracy and credibility of this model and support its use for long-term, systematic, and continuous observation.

Unveiling enigmatic essence of Sphingolipids: A promising avenue for glaucoma treatment

Treatment of glaucoma, the leading cause of irreversible blindness, remains challenging. The apoptotic loss of retinal ganglion cells (RGCs) in glaucoma is the pathological hallmark. Current treatments often remain suboptimal as they aim to halt RGC loss secondary to reduction of intraocular pressure. The pathophysiological targets for exploring direct neuroprotective approaches, therefore are highly relevant. Sphingolipids have emerged as significant target molecules as they are not only the structural components of various cell constituents, but they also serve as signaling molecules that regulate molecular pathways involved in cell survival and death. Investigations have shown that a critical balance among various sphingolipid species, particularly the ceramide and sphingosine-1-phosphate play a role in deciding the fate of the cell. In this review we briefly discuss the metabolic interconversion of sphingolipid species to get an insight into "sphingolipid rheostat", the dynamic balance among metabolites. Further we highlight the role of sphingolipids in the key pathophysiological mechanisms that lead to glaucomatous loss of RGCs. Lastly, we summarize the potential drug candidates that have been investigated for their neuroprotective effects in glaucoma via their effects on sphingolipid axis.

Spinster homolog 2 reduces malignancies of glioblastoma via PTEN/PI3K/AKT pathway

The molecular mechanisms of glioblastoma (GBM) are unclear, and the prognosis is poor. Spinster homolog 2 (SPNS2) is reportedly involved in pathological processes such as immune response, vascular development, and cancer. However, the biological function and molecular role of SPNS2 in GBM are unclear. SPNS2 is aberrantly low expressed in glioma. Survival curves, risk scores, prognostic nomograms, and univariate and multifactorial Cox regression analyses showed that SPNS2 is an independent prognostic indicator significantly associated with glioma progression and prognosis. Cell function assays and in vivo xenograft transplantation were performed that downregulation of SPNS2 promoted GBM cell growth, migration, invasion, epithelial-mesenchymal transition (EMT), anti-apoptosis, drug resistance, and stemness, while overexpression of SPNS2 had the opposite effect. Meanwhile, the functional enrichment and signaling pathways of SPNS2 in the Cancer Genome Atlas (TCGA), Chinese Glioma Genome Atlas (CGGA), and RNA sequencing were analyzed by Gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and Gene set enrichment analysis (GSEA). The above results were related to the inhibition of the PTEN/PI3K/AKT pathway by SPNS2. In addition, we predicted that SPNS2 is closely associated with immune infiltration in the tumor microenvironment by four immune algorithms, ESTIMATE, TIMER, CIBERSORT, and QUANTISEQ. In particular, SPNS2 was negatively correlated with the infiltration of most immune cells, immunomodulators, and chemokines. Finally, single-cell sequencing analysis also revealed that SPNS2 was remarkably correlated with macrophages, and downregulation of SPNS2 promotes the expression of M2-like macrophages. This study provides new evidence that SPNS2 inhibits malignant progression, stemness, and immune infiltration of GBM cells through PTEN/PI3K/AKT pathway. SPNS2 may become a new diagnostic indicator and potential immunotherapeutic target for glioma.

Enhancing Spns2/S1P in macrophages alleviates hyperinflammation and prevents immunosuppression in sepsis

Sepsis is a leading cause of in-hospital mortality resulting from a dysregulated response to infection. Novel immunomodulatory therapies targeting macrophage metabolism have emerged as an important focus for current sepsis research. However, understanding the mechanisms underlying macrophage metabolic reprogramming and how they impact immune response requires further investigation. Here, we identify macrophage-expressed Spinster homolog 2 (Spns2), a major transporter of sphingosine-1-phosphate (S1P), as a crucial metabolic mediator that regulates inflammation through the lactate-reactive oxygen species (ROS) axis. Spns2 deficiency in macrophages significantly enhances glycolysis, thereby increasing intracellular lactate production. As a key effector, intracellular lactate promotes pro-inflammatory response by increasing ROS generation. The overactivity of the lactate-ROS axis drives lethal hyperinflammation during the early phase of sepsis. Furthermore, diminished Spns2/S1P signaling impairs the ability of macrophages to sustain an antibacterial response, leading to significant innate immunosuppression in the late stage of infection. Notably, reinforcing Spns2/S1P signaling contributes to balancing the immune response during sepsis, preventing both early hyperinflammation and later immunosuppression, making it a promising therapeutic target for sepsis.

Structural and functional insights into Spns2-mediated transport of sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is an important signaling sphingolipid that regulates the immune system, angiogenesis, auditory function, and epithelial and endothelial barrier integrity. Spinster homolog 2 (Spns2) is an S1P transporter that exports S1P to initiate lipid signaling cascades. Modulating Spns2 activity can be beneficial in treatments of cancer, inflammation, and immune diseases. However, the transport mechanism of Spns2 and its inhibition remain unclear. Here, we present six cryo-EM structures of human Spns2 in lipid nanodiscs, including two functionally relevant intermediate conformations that link the inward- and outward-facing states, to reveal the structural basis of the S1P transport cycle. Functional analyses suggest that Spns2 exports S1P via facilitated diffusion, a mechanism distinct from other MFS lipid transporters. Finally, we show that the Spns2 inhibitor 16d attenuates the transport activity by locking Spns2 in the inward-facing state. Our work sheds light on Spns2-mediated S1P transport and aids the development of advanced Spns2 inhibitors.

S1P promotes corneal trigeminal neuron differentiation and corneal nerve repair via upregulating nerve growth factor expression in a mouse model

Corneal disease was the most critical cause of vision loss. This study aimed to research a new method and provide a theoretical basis for treating corneal injury. A mice corneal epithelial injury model was constructed by the method of mechanical curettage. Models were treated with sphingosine 1-phosphate (S1P) and si-Spns2. An immunofluorescence assay was used to detect βIII-tubulin. The expressions of neurotrophic factor, S1P transporter, and extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathway-related proteins were detected by western blot. Hematoxylin-eosin staining was processed to detect the effect of SIP on corneal repair in mice. si-Spns2 inhibited the effect of S1P. S1P significantly repaired the corneal injury, while si-Spns2 treatment made it more severe. Moreover, S1P could significantly increase the levels of NGF, BDNF, GDNF, Spns2, and p-ERK1/2. si-Spns2 inhibits the effect of S1P in the expression of these proteins. S1P significantly increased axonal differentiation of trigeminal ganglion neurons, which was inhibited after si-Spns2 treatment. S1P promoted corneal trigeminal neuron differentiation and corneal nerve repair via upregulating nerve growth factor expression in a mouse model. Treatment of corneal injury by S1P may be an effective approach.

Postnatal deletion of Spns2 prevents neuroinflammation without compromising blood vascular functions

Protein Spinster homolog 2 (Spns2) is a sphingosine-1-phosphate (S1P) transporter that releases S1P to regulate lymphocyte egress and trafficking. Global deletion of Spns2 (Spns2^-/-) has been shown to reduce disease severity in several autoimmune disease models. To examine whether Spns2 could be exploited as a drug target, we generated and characterized the mice with postnatal knockout of Spns2 (Spns2-Mx1Cre). Our results showed that Spns2-Mx1Cre mice had significantly low number of lymphocytes in blood and lymphoid organs similar to Spns2^-/- mice. Lymph but not plasma S1P levels were significantly reduced in both groups of knockout mice. Our lipidomic results also showed that Spns2 releases different S1P species into lymph. Interestingly, lymphatic vessels in the lymph nodes (LNs) of Spns2^-/- and Spns2-Mx1Cre mice exhibited morphological defects. The structures of high endothelial venules (HEV) in the LNs of Spns2-Mx1Cre mice were disorganized. These results indicate that lack of Spns2 affects both S1P secretion and LN vasculatures. Nevertheless, blood vasculature of these Spns2 deficient mice was not different to controls under homeostasis and vascular insults. Importantly, Spns2-Mx1Cre mice were resistant to multiple sclerosis in experimental autoimmune encephalomyelitis (EAE) models with significant reduction of pathogenic Th17 cells in the central nervous system (CNS). This study suggests that pharmacological inhibition of Spns2 may be exploited for therapeutic applications in treatment of neuroinflammation.

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.

A novel E2F1-regulated lncRNA, LAPAS1, is required for S phase progression and cell proliferation

The transcription factor E2F1 induces both proliferation and apoptosis and is a critical downstream target of the tumor suppressor RB.

Long non-coding RNAs (lncRNAs) are major regulators of many cellular processes, including cell cycle progression and cell proliferation. However, the mode of action as well as the transcriptional regulation of most lncRNAs are only beginning to be understood.

Here, we report that a novel human lncRNA, LAPAS1, is an E2F1- regulated lncRNA that affects S phase progression. Inhibition of LAPAS1 expression increases percentage of S phase cells, and its silencing in synchronized cells delays their progression through S phase. In agreement with its suggested role in cell cycle progression, prolonged inhibition of LAPAS1 attenuates proliferation of human cancer cells.

Our data demonstrate that LAPAS1 predominantly functions in trans to repress expression of Sphingolipid Transporter 2 (SPNS2). Importantly, knockdown of SPNS2 rescues the effect of LAPAS1 silencing on cell cycle and cell proliferation.

Notably, low levels of LAPAS1 are associated with increased survival of kidney cancer patients.

Summarily, we identify LAPAS1 as a novel E2F-regulated lncRNA that has a potential role in human cancer and regulates cell-cycle progression and cell proliferation, at least in part, via regulation of SPNS2.

Sphingolipids as critical players in retinal physiology and pathology

Sphingolipids have emerged as bioactive lipids involved in the regulation of many physiological and pathological processes. In the retina, they have been established to participate in numerous processes, such as neuronal survival and death, proliferation and migration of neuronal and vascular cells, inflammation, and neovascularization. Dysregulation of sphingolipids is therefore crucial in the onset and progression of retinal diseases. This review examines the involvement of sphingolipids in retinal physiology and diseases. Ceramide (Cer) has emerged as a common mediator of inflammation and death of neuronal and retinal pigment epithelium cells in animal models of retinopathies such as glaucoma, age-related macular degeneration (AMD), and retinitis pigmentosa. Sphingosine-1-phosphate (S1P) has opposite roles, preventing photoreceptor and ganglion cell degeneration but also promoting inflammation, fibrosis, and neovascularization in AMD, glaucoma, and pro-fibrotic disorders. Alterations in Cer, S1P, and ceramide 1-phosphate may also contribute to uveitis. Notably, use of inhibitors that either prevent Cer increase or modulate S1P signaling, such as Myriocin, desipramine, and Fingolimod (FTY720), preserves neuronal viability and retinal function. These findings underscore the relevance of alterations in the sphingolipid metabolic network in the etiology of multiple retinopathies and highlight the potential of modulating their metabolism for the design of novel therapeutic approaches.

Spinster homolog 2 in cancers, its functions and mechanisms

Spinster homolog 2 (SPNS2) is a multi-transmembrane transporter, widely located in the cell membrane and organelle membranes. It transports sphingosine-1-phosphate (S1P) into the extracellular space and the circulatory system, thus alters the concentration and the distribution of S1P, sphingosine-1-phosphate receptor (S1PRs) and S1P related enzymes, meaning that it exerts its functions via S1P signaling pathways. Studies also show that ectopic SPNS2 mediates parts of the physiological process of the cells. As of now, SPNS2 has been reported to participate in physiological processes such as angiogenesis, embryonic development, immune response and metabolisms. It is also associated with the transformation from inflammation to cancer as well as the proliferation and metastasis of cancer cells. In this review, we summarize the functions and the mechanisms of SPNS2 in the pathogenesis of cancer to provide new insights for the diagnosis and the treatments of cancer.

Sphingosine 1-phosphate: Lipid signaling in pathology and therapy

Sphingosine 1-phosphate (S1P), a metabolic product of cell membrane sphingolipids, is bound to extracellular chaperones, is enriched in circulatory fluids, and binds to G protein-coupled S1P receptors (S1PRs) to regulate embryonic development, postnatal organ function, and disease. S1PRs regulate essential processes such as adaptive immune cell trafficking, vascular development, and homeostasis. Moreover, S1PR signaling is a driver of multiple diseases. The past decade has witnessed an exponential growth in this field, in part because of multidisciplinary research focused on this lipid mediator and the application of S1PR-targeted drugs in clinical medicine. This has revealed fundamental principles of lysophospholipid mediator signaling that not only clarify the complex and wide ranging actions of S1P but also guide the development of therapeutics and translational directions in immunological, cardiovascular, neurological, inflammatory, and fibrotic diseases.

Spatiotemporal gene expression patterns reveal molecular relatedness between retinal laminae

In several areas of the central nervous system, neurons are regionally organized into groups or layers that carry out specific activities. In this form of patterning, neurons of distinct types localize their cell bodies to just one or a few of the layers within a structure. However, little is known about whether diverse neuron types within a lamina share molecular features that coordinate their organization. To begin to identify such candidates, we used the laminated murine retina to screen 92 lacZ reporter lines available through the Knockout Mouse Project. Thirty-two of these displayed reporter expression in restricted subsets of inner retina neurons. We then identified the spatiotemporal expression patterns of these genes at key developmental stages. This uncovered several that were heavily enriched in development but reduced in adulthood, including the transcriptional regulator Hmga1. An additional set of genes displayed maturation associated laminar enrichment. Among these, we identified Bbox1 as a novel gene that specifically labels all neurons in the ganglion cell layer but is largely excluded from otherwise molecularly similar neurons in the inner retina. Finally, we established Dbn1 as a new marker enriched in amacrines and Fmnl3 as a marker for subsets of αRGCs. Together, these data provide a spatiotemporal map for laminae-specific molecules and suggest that diverse neuron types within a lamina share coordinating molecular features that may inform their fate or function.

Sphingolipids as Emerging Mediators in Retina Degeneration

The sphingolipids ceramide (Cer), sphingosine-1-phosphate (S1P), sphingosine (Sph), and ceramide-1-phosphate (C1P) are key signaling molecules that regulate major cellular functions. Their roles in the retina have gained increasing attention during the last decade since they emerge as mediators of proliferation, survival, migration, neovascularization, inflammation and death in retina cells. As exacerbation of these processes is central to retina degenerative diseases, they appear as crucial players in their progression. This review analyzes the functions of these sphingolipids in retina cell types and their possible pathological roles. Cer appears as a key arbitrator in diverse retinal pathologies; it promotes inflammation in endothelial and retina pigment epithelium (RPE) cells and its increase is a common feature in photoreceptor death in vitro and in animal models of retina degeneration; noteworthy, inhibiting Cer synthesis preserves photoreceptor viability and functionality. In turn, S1P acts as a double edge sword in the retina. It is essential for retina development, promoting the survival of photoreceptors and ganglion cells and regulating proliferation and differentiation of photoreceptor progenitors. However, S1P has also deleterious effects, stimulating migration of Müller glial cells, angiogenesis and fibrosis, contributing to the inflammatory scenario of proliferative retinopathies and age related macular degeneration (AMD). C1P, as S1P, promotes photoreceptor survival and differentiation. Collectively, the expanding role for these sphingolipids in the regulation of critical processes in retina cell types and in their dysregulation in retina degenerations makes them attractive targets for treating these diseases.

Stories of spinster with various faces: from courtship rejection to tumor metastasis rejection

The Drosophila spinster (spin) mutant was isolated as a mutant that showed abnormal morphology and function in the nervous system. The spin defect induces neural degeneration similar to human lysosomal storage diseases. Various studies have shown that Spin proteins are localized in lysosomes and participate in the late stages of the autophagic process. Vertebrates have three spinster orthologs, Spns1, Spns2, and Spns3. A defect in Spns1 caused a short lifespan with aberrant lysosomal function in zebrafish. Spns2 was originally isolated as the gene responsible for abnormal heart development and was identified as a sphingosine 1-phosphate transporter in zebrafish. An endothelial cell-specific defect in Spns2 resulted in impaired egress of lymphocytes and the prevention of tumor metastasis in mice. Herein, I reviewed the history of spin/Spns research and discussed the conserved and newly diverged spin/Spns function and possible implications for human diseases.

New insights into functions of the sphingosine-1-phosphate transporter SPNS2

Sphingosine-1-phosphate (S1P) is a potent bioactive signaling molecule that regulates many physiological processes important for development, epithelial and endothelial barrier integrity, and the immune system, as well as for pathologies, such as autoimmune diseases, cancer, and metastasis. Most of the well-known actions of S1P are mediated by five specific G protein-coupled receptors located on the plasma membrane. Because S1P is synthesized intracellularly by two sphingosine kinase isoenzymes, we have proposed the paradigm of inside-out signaling by S1P, suggesting that S1P must be exported out of cells to interact with its receptors. While several transporters of S1P have previously been identified, spinster homologue 2 (SPNS2), a member of the large family of non-ATP-dependent organic ion transporters, has recently attracted much attention as an S1P transporter. Here, we discuss recent advances in understanding the physiological actions of SPNS2 in regulating levels of S1P and the S1P gradient that exists between the high circulating concentrations of S1P and low tissue levels that control lymphocyte trafficking. Special emphasis is on the functions of SPNS2 in inflammatory and autoimmune diseases and its recently discovered unexpected importance in metastasis.