The Effects of Mechanical Stretch on Integrins and Filopodial-Associated Proteins in Normal and Glaucomatous Trabecular Meshwork Cells

Fibrosis-Related Gene and Protein Expression in Normal and Glaucomatous Trabecular Meshwork Cells

Purpose

Glaucomatous trabecular meshwork (GTM) tissue is characterized by excess fibrotic-like extracellular matrices, which negatively impacts aqueous humor outflow. Endothelial-to-mesenchymal transition (EndMT) is the process by which tissues develop fibrosis. In this study, we investigated fibrotic-related gene and protein profiles of non-glaucomatous trabecular meshwork (NTM) and GTM cells.

Methods

Primary cells were cultured from NTM (n = 6) and GTM (n = 5) age-matched cadaver eyes. RNA was harvested and mRNA profiling of 750 genes was performed using the human fibrosis panel (NanoString). Quantitative PCR (qPCR), Western blotting, and immunofluorescence microscopy were performed. A matrix metalloproteinase (MMP) fluorogenic assay was used to quantitate enzyme activity.

Results

Classic EndMT biomarkers, α-SMA, SNAI2, TWIST1, TWIST2, and VIM, were upregulated in GTM cells, whereas increased phosphorylated SMAD2-3 indicated increased TGFβ signaling. GTM cells had increased deposition of FN-EDA fibronectin fibrils, but reduced amounts of FN-EDB fibrils, and altered immunostaining of active α5β1 and αvβ3 integrins. NanoString analysis showed that 2 genes were upregulated and 28 genes were downregulated in GTM cells compared with NTM cells. Western immunoblotting confirmed increased protein levels of N-cadherin and decreased MMP2, CHI3L1, COL6A3, and SERPINF1 proteins in GTM cells. Whereas MMP2 gene and protein levels were reduced, there was increased MMP activity.

Conclusions

Increased expression of α-SMA, FN-EDA, N-cadherin, SNAI2, TWISTs, VIM, TGFβ signaling, and MMP activity are consistent with GTM cells acquiring an EndMT phenotype. In combination with tissue studies, cultured GTM cells are a useful in vitro model for studying the fibrotic process in glaucoma.

TRPV4 overactivation enhances cellular contractility and drives ocular hypertension in TGFβ2 overexpressing eyes

The risk for developing primary open-angle glaucoma (POAG) correlates with the magnitude of ocular hypertension (OHT) and the concentration of transforming growth factor-β2 (TGFβ2) in the aqueous humor. Effective treatment of POAG requires detailed understanding of interaction between pressure sensing mechanisms in the trabecular meshwork (TM) and biochemical risk factors. Here, we employed molecular, optical, electrophysiological and tonometric strategies to establish the role of TGFβ2 in transcription and functional expression of mechanosensitive channel isoforms alongside studies of TM contractility in biomimetic hydrogels, and intraocular pressure (IOP) regulation in a mouse model of TGFβ2 -induced OHT. TGFβ2 upregulated expression of TRPV4 and PIEZO1 transcripts and time-dependently augmented functional TRPV4 activation. TRPV4 activation induced TM contractility whereas pharmacological inhibition suppressed TGFβ2-induced hypercontractility and abrogated OHT in eyes overexpressing TGFβ2. Trpv4-deficient mice resisted TGFβ2-driven increases in IOP. Nocturnal OHT was not additive to TGFβ-evoked OHT. Our study establishes the fundamental role of TGFβ as a modulator of mechanosensing in nonexcitable cells, identifies TRPV4 channel as the final common mechanism for TM contractility and circadian and pathological OHT and offers insights future treatments that can lower IOP in the sizeable cohort of hypertensive glaucoma patients that resist current treatments.

The S341P mutant MYOC renders the trabecular meshwork sensitive to cyclic mechanical stretch

The trabecular meshwork (TM) plays an essential role in the circulation of aqueous humor by sensing mechanical stretch. The balance between the outflow and inflow of aqueous humor is critical in regulating intraocular pressure (IOP). A dysfunctional TM leads to resistance to the outflow of aqueous humor, resulting in an elevated IOP, a major risk factor for glaucoma. It is widely accepted that mutant myocilin (MYOC) can cause damage to the TM. However, few studies have investigated how TM cells carrying mutant MYOC respond to cyclic mechanical stretch (CMS) and whether these cells are more sensitive to CMS under this genetic background. In this study, we applied mechanical stretch to TM cells using the Flexcell system to mimic physiological stress. In addition, we performed genome-wide transcriptome analysis and oxidized lipidomics to systematically compare the gene expression and oxylipin profiles of non-stretched control human primary TM cells, human primary TM cells under CMS (TM-CMS), and human primary TM cells overexpressing MYOCS341P under CMS (S341P-CMS). We found that TM cells that overexpressed MYOCS341P were more sensitive to mechanical stress. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that downregulated genes were most enriched in oxidative phosphorylation, indicating mitochondria dysfunction and the likelihood of oxidative stress. Oxidized lipidomics analysis revealed significant changes in oxylipin profiles between the S341P-CMS and TM-CMS groups. Through further genome-wide transcriptomic analysis, we identified several genes that may be involved in the sensitivity of TM cells that overexpressed MYOCS341P to mechanical stress, including SARM1, AHNAK2, NT5C, and SOX8. The importance of these genes was validated by quantitative real-time PCR. Collectively, our findings indicate that mitochondrial dysfunction may contribute to the damage that occurs to TM cells with a MYOCS341P background under mechanical stretch.

The Role and Mechanism of Nicotinamide Riboside in Oxidative Damage and a Fibrosis Model of Trabecular Meshwork Cells

Purpose

To investigate the potential effects and mechanism of nicotinamide riboside (NR) on the oxidative stress and fibrosis model of human trabecular meshwork (HTM) cell line cells.

Methods

HTM cells were pretreated with NR, followed by the induction of oxidative injury and fibrosis by hydrogen peroxide (H2O2) and TGF-β2, respectively. Cell viability was tested using Hoechst staining and MTT assays, cell proliferation was assessed by EdU assay, and cell apoptosis was detected by flow cytometry and western blotting. DCFH-DA and DHE probes were used to measure the level of reactive oxygen species (ROS), and MitoTracker staining was used to measure the mitochondrial membrane potential (MMP). Fibrotic responses, including cell migration and deposition of extracellular matrix (ECM) proteins, were detected via Transwell assays, qRT-PCR, and immunoblotting.

Results

NR pretreatment improved the viability, proliferation, and MMP of H2O2-treated HTM cells. Compared to cells treated solely with H2O2, HTM cells treated with both NR and H2O2, exhibited a reduced rate of apoptosis and generation of ROS. Compared with H2O2 pretreatment, NR pretreatment upregulated expression of the JAK2/Stat3 pathway but inhibited mitogen-activated protein kinase (MAPK) pathway expression. Moreover, 10-ng/mL TGF-β2 promoted cell proliferation and migration, which were inhibited by NR pretreatment. Both qRT-PCR and immunoblotting showed that NR inhibited the expression of fibronectin in a TGF-β2-induced fibrosis model.

Conclusions

NR has a protective effect on oxidative stress and fibrosis in HTM cells, which may be related to the JAK2/Stat3 pathway and MAPK pathway.

Translational Relevance

Our research provides the ongoing data for potential therapy of NAD+ precursors in glaucoma.

Digital spatial profiling of segmental outflow regions in trabecular meshwork reveals a role for ADAM15

In this study we used a spatial transcriptomics approach to identify genes specifically associated with either high or low outflow regions in the trabecular meshwork (TM) that could potentially affect aqueous humor outflow in vivo. High and low outflow regions were identified and isolated from organ cultured human anterior segments perfused with fluorescently-labeled 200 nm FluoSpheres. The NanoString GeoMx Digital Spatial Profiler (DSP) platform was then used to identified genes in the paraffin embedded tissue sections from within those regions. These transcriptome analyses revealed that 16 genes were statistically upregulated in high outflow regions and 57 genes were statistically downregulated in high outflow regions when compared to low outflow regions. Gene ontology enrichment analysis indicated that the top three biological categories of these differentially expressed genes were ECM/cell adhesion, signal transduction, and transcription. The ECM/cell adhesion genes that showed the largest differential expression (Log2FC ±1.5) were ADAM15, BGN, LDB3, and CRKL. ADAM15, which is a metalloproteinase that can bind integrins, was upregulated in high outflow regions, while the proteoglycan BGN and two genes associated with integrin signaling (LDB3, and CRKL) were downregulated. Immunolabeling studies supported the differential expression of ADAM15 and showed that it was specifically upregulated in high outflow regions along the inner wall of Schlemm's canal and in the juxtacanalicular (JCT) region of the TM. In addition to these genes, the studies showed that genes for decorin, a small leucine-rich proteoglycan, and the α8 integrin subunit were enriched in high outflow regions. These studies identify several novel genes that could be involved in segmental outflow, thus demonstrating that digital spatial profiling could be a useful approach for understanding segmental flow through the TM. Furthermore, this study suggests that changes in the expression of genes involved in regulating the activity and/or organization of the ECM and integrins in the TM are likely to be key players in segmental outflow.

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.

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

Trabecular meshwork (TM) cells are contractile and mechanosensitive, and they aid in maintaining intraocular pressure (IOP) homeostasis. Lipids are attributed to modulating TM contractility, with poor mechanistic understanding. In this study using human TM cells, we identify the mechanosensing role of the transcription factors sterol regulatory element binding proteins (SREBPs) involved in lipogenesis. By constitutively activating SREBPs and pharmacologically inactivating SREBPs, we have mechanistically deciphered the attributes of SREBPs in regulating the contractile properties of TM. The pharmacological inhibition of SREBPs by fatostatin and molecular inactivation of SREBPs ex vivo and in vivo, respectively, results in significant IOP lowering. As a proof of concept, fatostatin significantly decreased the SREBPs responsive genes and enzymes involved in lipogenic pathways as well as the levels of the phospholipid, cholesterol, and triglyceride. Further, we show that fatostatin mitigated actin polymerization machinery and stabilization, and decreased ECM synthesis and secretion. We thus postulate that lowering lipogenesis in the TM outflow pathway can hold the key to lowering IOP by modifying the TM biomechanics.

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

Trabecular meshwork (TM) cells are highly contractile and mechanosensitive to aid in maintaining intraocular pressure (IOP) homeostasis. Lipids are attributed to modulating TM contractility with poor mechanistic understanding. In this study using human TM cells, we identify the mechanosensing role of the transcription factors sterol regulatory element binding proteins (SREBPs) involved in lipogenesis. By constitutively activating SREBPs and pharmacologically inactivating SREBPs, we have mechanistically deciphered the attributes of SREBPs in regulating the contractile properties of TM. The pharmacological inhibition of SREBPs by fatostatin and molecular inactivation of SREBPs ex vivo and in vivo respectively results in significant IOP lowering. As a proof of concept, fatostatin significantly decreased the SREBPs responsive genes and enzymes involved in lipogenic pathways as well as the levels of the phospholipid, cholesterol, and triglyceride. Further, we show that fatostatin mitigated actin polymerization machinery and stabilization, and decreased ECM synthesis and secretion. We thus postulate that lowering lipogenesis in the TM outflow pathway can hold the key to lowering IOP by modifying the TM biomechanics.

Synopsis

In this study, we show the role of lipogenic transcription factors sterol regulatory element binding proteins (SREBPs) in the regulation of intraocular pressure (IOP). ( Synopsis Figure - Created using Biorender.com ) SREBPs are involved in the sensing of changes in mechanical stress on the trabecular meshwork (TM). SREBPs aid in transducing the mechanical signals to induce actin polymerization and filopodia/lamellipodia formation.SREBPs inactivation lowered genes and enzymes involved in lipogenesis and modified lipid levels in TM.SREBPs activity is a critical regulator of ECM engagement to the matrix sites.Inactivation of SCAP-SREBP pathway lowered IOP via actin relaxation and decreasing ECM production and deposition in TM outflow pathway signifying a novel relationship between SREBP activation status and achieving IOP homeostasis.