Identification of the novel role of sterol regulatory element binding proteins (SREBPs) in mechanotransduction and intraocular pressure regulation

High Glucose-induced transcriptomic changes in human trabecular meshwork cells

Glaucoma is a leading cause of irreversible blindness, often associated with elevated intraocular pressure (IOP) due to trabecular meshwork (TM) dysfunction. Diabetes mellitus (DM) is recognized as a significant risk factor for glaucoma; however, the molecular mechanisms through which hyperglycemia affects TM function remain unclear. This study investigated the impact of high glucose on gene expression in human TM (HTM) cells to uncover pathways that contribute to TM dysfunction and glaucoma pathogenesis under diabetic conditions. Primary HTM cells were cultured under normoglycemic (5.5 mM) and hyperglycemic (30 mM) conditions for seven days, followed by mRNA sequencing (mRNA-seq) to identify differentially expressed genes, with quantitative PCR (qPCR) used for confirmatory analysis. STRING network analysis was performed to predict potential interactions among upregulated and downregulated genes. mRNA-seq analysis revealed 25 significantly differentially expressed genes in high glucose conditions, including upregulated genes associated with oxidative stress, apoptosis, autophagy, immune response, and fibrosis. Notably, TXNIP gene was significantly upregulated, indicating increased oxidative stress and apoptosis in TM cells, while downregulation of autophagy-related genes, such as HSPA6 and LAMP3, suggests compromised protein quality control. Immune response genes, including CCL7 and CHI3L1, were upregulated, suggesting an inflammatory response to oxidative stress. Increased expression of fibrosis-related genes, such as SNAI1, FGF7, and KRT19, and an increase in ECM proteins like Collagen 1 and FN accumulation and fibril formation suggest increased fibrosis of TM in diabetic conditions, potentially elevating IOP. Metabolic changes in diabetic patients could therefore lead to TM dysfunction, impair aqueous humor outflow, and elevate IOP, thereby increasing glaucoma risk. Targeting oxidative stress and fibrosis pathways offers therapeutic strategies to mitigate glaucoma progression in diabetic populations.

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.

Ocular manifestations of SREBF1-associated hereditary mucoepithelial dysplasia

Hereditary mucoepithelial dysplasia (HMD) is a rare autosomal dominant dysplastic dyskeratotic epithelial syndrome caused by pathogenic variants in the SREBF1 gene. This syndrome is associated with a variety of ocular conditions, including cataracts, nystagmus, keratitis, meibomian gland dysfunction (MGD), and decreased visual acuity. We report the case of a boy followed from 1 to 7 years of age who had a confirmed HMD-associated variant in the SREBF1 gene. The patient has severe MGD, with resulting keratitis and photosensitivity, and bilateral glaucoma, which has not previously been reported in association with HMD. The gene affected in HMD negatively affects gap junctions and lipid biosynthesis, which are important in the stability of the trabecular meshwork.

High Glucose-Induced Transcriptomic Changes in Human Trabecular Meshwork Cells

Glaucoma is a leading cause of irreversible blindness, often associated with elevated intraocular pressure (IOP) due to trabecular meshwork (TM) dysfunction. Diabetes mellitus (DM) is recognized as a significant risk factor for glaucoma; however, the molecular mechanisms through which hyperglycemia affects TM function remain unclear. This study investigated the impact of high glucose on gene expression in human TM (HTM) cells to uncover pathways that contribute to TM dysfunction and glaucoma pathogenesis under diabetic conditions. Primary HTM cells were cultured under normoglycemic (5.5 mM) and hyperglycemic (30 mM) conditions for seven days, followed by mRNA sequencing (mRNA-seq) to identify differentially expressed genes, with quantitative PCR (qPCR) used for confirmatory analysis. STRING network analysis was performed to predict interactions among upregulated and downregulated proteins. mRNA-seq analysis revealed 25 significantly differentially expressed genes in high glucose conditions, including upregulated genes associated with oxidative stress, apoptosis, autophagy, immune response, and fibrosis. Notably, TXNIP was significantly upregulated, indicating increased oxidative stress and apoptosis in TM cells, while downregulation of autophagy-related genes, such as HSPA6 and LAMP3, suggests compromised protein quality control. Immune response genes, including CCL7 and CHI3L1, were upregulated, suggesting an inflammatory response to oxidative stress. Increased expression of fibrosis-related genes, such as SNAI1, FGF7, and KRT19, supports the hypothesis of ECM accumulation in diabetic conditions, potentially elevating IOP. Chronic hyperglycemia in diabetic patients could therefore lead to TM dysfunction, impair aqueous humor outflow, and elevate IOP, thereby increasing glaucoma risk. Targeting oxidative stress and fibrosis pathways offers therapeutic strategies to mitigate glaucoma progression in diabetic populations.

Proteomic analysis uncovers clusterin-mediated disruption of actin-based contractile machinery in the trabecular meshwork to lower intraocular pressure

Glaucoma, a major cause of blindness, is characterized by elevated intraocular pressure (IOP) due to improper drainage of aqueous humor via the trabecular meshwork (TM) outflow pathway. Our recent work identified that loss of clusterin resulted in elevated IOP. This study delves deeper to elucidate the role of clusterin in IOP regulation. Employing an ex vivo human anterior segment perfusion model, we established that constitutive expression and secretion as well as exogenous addition of clusterin can significantly lower IOP. Interestingly, clusterin significantly lowered transforming growth factor β2 (TGFβ2)-induced IOP elevation. This effect was linked to the suppression of extracellular matrix (ECM) deposition and, highlighting the crucial role of clusterin in maintaining ECM equilibrium. A comprehensive global proteomic approach revealed the broad impact of clusterin on TM cell structure and function by identifying alterations in protein expression related to cytoskeletal organization, protein processing, and cellular mechanics, following clusterin induction. These findings underscore the beneficial modulation of TM cell structure and functionality by clusterin. Specifically, clusterin influences the actin-cytoskeleton and focal adhesion dynamics, which are instrumental in cell contractility and adhesion processes. Additionally, it suppresses the activity of proteins critical in TGFβ2, G-protein, and JAK-STAT signaling pathways, which are vital for the regulation of ocular pressure. By delineating these targeted effects of clusterin within the TM outflow pathway, our findings pave the way for novel treatment strategies aimed at mitigating the progression of ocular hypertension and glaucoma through targeted molecular interventions.

Regulatory role of cholesterol in modulating actin dynamics and cell adhesive interactions in the trabecular meshwork

The trabecular meshwork (TM) tissue plays a crucial role in maintaining intraocular pressure (IOP) homeostasis. Increased TM contractility and stiffness are directly correlated with elevated IOP. Although cholesterol is known to be a determinant of glaucoma occurrence and elevated IOP, the underlying mechanisms remain elusive. In this study, we used human TM (HTM) cells to unravel the effects of cholesterol on TM stiffness. We achieved this by performing acute cholesterol depletion with Methyl-β-cyclodextrin (MβCD) and cholesterol enrichment/replenishment with MβCD cholesterol complex (CHOL). Interestingly, cholesterol depletion triggered notable actin depolymerization and decreased focal adhesion formation, while enrichment/replenishment promoted actin polymerization, requiring the presence of actin monomers. Using a specific reporter of phosphatidylinositol 4,5-bisphosphate (PIP2), we demonstrated that cholesterol depletion decreases PIP2 levels on the cell membrane, whereas enrichment increases them. Given the critical role of PIP2 in actin remodeling and focal adhesion formation, we postulate that cholesterol regulates actin dynamics by modulating PIP2 levels on the membrane. Furthermore, we showed that cholesterol levels regulate integrin α5β1 and αVβ3 distribution and activation, subsequently altering cell-extracellular matrix (ECM) interactions. Notably, the depletion of cholesterol, as a major lipid constituent of the cell membrane, led to a decrease in HTM cell membrane tension, which was reversed upon cholesterol replenishment. Overall, our systematic exploration of cholesterol modulation on TM stiffness highlights the critical importance of maintaining appropriate membrane and cellular cholesterol levels for achieving IOP homeostasis.