Overexpression of SPARC in human trabecular meshwork increases intraocular pressure and alters extracellular matrix

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.

Alternative application of an iTrack microcatheter and canaloplasty: case report and literature review

INTRODUCTION

Glaucoma is the leading cause of irreversible blindness worldwide. Schlemm's canal surgery using an iTrack flexible microcatheter has become popular because of its high quality-of-life issues and the growing demand for less invasive but effective procedures. The unique design of the microcatheter makes it a multimodal tool, which can be used not only in the field of antiglaucoma surgery but also as a drug delivery system to treat various conditions.

AREAS COVERED

This review presents an update on the selected aspects of a drug delivery system using the iTrack microcatheter, including glaucoma gene therapy and posterior-segment diseases, both in animal models and human patients. The authors also report the case of a patient with branch retinal vein occlusion treated with suprachoroidal bevacizumab in the submacular region administered with the iTrack catheter.

EXPERT OPINION

The findings presented in this study may indicate that the application of a microcatheter in open-angle glaucoma gene therapy is reasonable and can be combined with full or partial surgical canaloplasty procedures. Translation of this potential into a treatment modality would require overcoming multiple barriers.

SPARC-YAP/TAZ inhibition prevents the fibroblasts-myofibroblast transformation

BACKGROUND

Fibrotic scar is a severe side effect of trabeculectomy, resulting in unsatisfactory outcomes for glaucoma surgery. Accumulating evidence showed human Tenon's fibroblasts (HTFs) play an important role in fibrosis formation. We previously reported that the aqueous level of secreted protein acidic and rich in cysteine (SPARC) was higher in the patients with primary angle closure glaucoma, which was associated with the failure of trabeculectomy. In this study, the potential effect and mechanism of SPARC in promoting fibrosis were explored by using HTFs.

METHODS

HTFs were employed in this study and examined under a phase-contrast microscope. Cell viability was determined by CCK-8. The expressions of SPARC-YAP/TAZ signaling and the fibrosis-related markers were examined with reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR), Western blot, and immunofluorescence, subcellular fractionation was conducted to further determined the variation of YAP and phosphorylated YAP. The differential gene expressions were analyzed with RNA sequencing (RNAseq), followed by Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses.

RESULTS

Exogenous SPARC induced HTFs-myofibroblast transformation, as evidenced by the increased expression of α-SMA, collagen I and fibronectin in both protein and mRNA levels. SPARC knockdown decreased the expressions of the above genes in TGF-β2-treated HTFs. KEGG analysis showed that the Hippo signaling pathway was mostly enriched. SPARC treatment increased the expressions of YAP, TAZ, CTGF and CYR61 as well as enhanced YAP translocation from cytoplasm to nucleus, and decreased the phosphorylation of YAP and LAST1/2, which was reversed by SPARC knockdown. Knockdown of YAP1 decreased the fibrosis-related markers, such as α-SMA, collagen I and Fibronectin, in SPARC-treated HTFs.

CONCLUSIONS

SPARC induced HTFs-myofibroblast transformation via activating YAP/TAZ signaling. Targeting SPARC-YAP/TAZ axis in HTFs might provide a novel strategy for inhibiting fibrosis formation after trabeculectomy.

The Extracellular Matrix: Its Composition, Function, Remodeling, and Role in Tumorigenesis

The extracellular matrix (ECM) is a ubiquitous member of the body and is key to the maintenance of tissue and organ integrity. Initially thought to be a bystander in many cellular processes, the extracellular matrix has been shown to have diverse components that regulate and activate many cellular processes and ultimately influence cell phenotype. Importantly, the ECM's composition, architecture, and stiffness/elasticity influence cellular phenotypes. Under normal conditions and during development, the synthesized ECM constantly undergoes degradation and remodeling processes via the action of matrix proteases that maintain tissue homeostasis. In many pathological conditions including fibrosis and cancer, ECM synthesis, remodeling, and degradation is dysregulated, causing its integrity to be altered. Both physical and chemical cues from the ECM are sensed via receptors including integrins and play key roles in driving cellular proliferation and differentiation and in the progression of various diseases such as cancers. Advances in 'omics' technologies have seen an increase in studies focusing on bidirectional cell-matrix interactions, and here, we highlight the emerging knowledge on the role played by the ECM during normal development and in pathological conditions. This review summarizes current ECM-targeted therapies that can modify ECM tumors to overcome drug resistance and better cancer treatment.

MAPK Pathways in Ocular Pathophysiology: Potential Therapeutic Drugs and Challenges

Mitogen-activated protein kinase (MAPK) pathways represent ubiquitous cellular signal transduction pathways that regulate all aspects of life and are frequently altered in disease. Once activated through phosphorylation, these MAPKs in turn phosphorylate and activate transcription factors present either in the cytoplasm or in the nucleus, leading to the expression of target genes and, as a consequence, they elicit various biological responses. The aim of this work is to provide a comprehensive review focusing on the roles of MAPK signaling pathways in ocular pathophysiology and the potential to influence these for the treatment of eye diseases. We summarize the current knowledge of identified MAPK-targeting compounds in the context of ocular diseases such as macular degeneration, cataract, glaucoma and keratopathy, but also in rare ocular diseases where the cell differentiation, proliferation or migration are defective. Potential therapeutic interventions are also discussed. Additionally, we discuss challenges in overcoming the reported eye toxicity of some MAPK inhibitors.

Genome-Wide RNA Sequencing of Human Trabecular Meshwork Cells Treated with TGF-β1: Relevance to Pseudoexfoliation Glaucoma

Pseudoexfoliation glaucoma (XFG) is an aggressive form of secondary open angle glaucoma, characterised by the production of exfoliation material and is estimated to affect 30 million people worldwide. Activation of the TGF-β pathway by TGF-β1 has been implicated in the pathogenesis of pseudoexfoliation glaucoma. To further investigate the role of TGF-β1 in glaucomatous changes in the trabecular meshwork (TM), we used RNA-Seq to determine TGF-β1 induced changes in the transcriptome of normal human trabecular meshwork (HTM) cells. The main purpose of this study was to perform a hypothesis-independent RNA sequencing analysis to investigate genome-wide alterations in the transcriptome of normal HTMs stimulated with TGF-β1 and investigate possible pathophysiological mechanisms driving XFG. Our results identified multiple differentially expressed genes including several genes known to be present in exfoliation material. Significantly altered pathways, biological processes and molecular functions included extracellular matrix remodelling, Hippo and Wnt pathways, the unfolded protein response, oxidative stress, and the antioxidant system. This cellular model of pseudoexfoliation glaucoma can provide insight into disease pathogenesis and support the development of novel therapeutic interventions.

Effect of SPARC Suppression in Mice, Perfused Human Anterior Segments, and Trabecular Meshwork Cells

Purpose

Secreted protein, acidic and rich in cysteine (SPARC) elevates intraocular pressure (IOP), increases certain structural extracellular matrix (ECM) proteins in the juxtacanalicular trabecular meshwork (JCT), and decreases matrix metalloproteinase (MMP) protein levels in trabecular meshwork (TM) endothelial cells. We investigated SPARC as a potential target for lowering IOP. We hypothesized that suppressing SPARC will decrease IOP, decrease structural JCT ECM proteins, and alter the levels of MMPs and/or their inhibitors.

Methods

A lentivirus containing short hairpin RNA of human SPARC suppressed SPARC in mouse eyes and perfused cadaveric human anterior segments with subsequent IOP measurements. Immunohistochemistry determined structural correlates. Human TM cell cultures were treated with SPARC suppressing lentivirus. Quantitative reverse transcriptase polymerase chain reaction (PCR), immunoblotting, and zymography determined total RNA, relative protein levels, and MMP enzymatic activity, respectively.

Results

Suppressing SPARC decreased IOP in mouse eyes and perfused human anterior segments by approximately 20%. Histologically, this correlated to a decrease in collagen I, IV, and VI in both the mouse TM and human JCT regions; in the mouse, fibronectin was also decreased but not in the human. In TM cells, collagen I and IV, fibronectin, MMP-2, and tissue inhibitor of MMP-1 were decreased. Messenger RNA of the aforementioned genes was not changed. Plasminogen activator inhibitor 1 (PAI-1) was upregulated in vitro by quantitative PCR and immunoblotting. MMP-1 activity was reduced in vitro by zymography.

Conclusions

Suppressing SPARC decreased IOP in mice and perfused cadaveric human anterior segments corresponding to qualitative structural changes in the JCT ECM, which do not appear to be the result of transcription regulation.

Sodium 4-Phenylbutyrate Reduces Ocular Hypertension by Degrading Extracellular Matrix Deposition via Activation of MMP9

Ocular hypertension (OHT) is a serious adverse effect of the widely prescribed glucocorticoid (GC) therapy and, if left undiagnosed, it can lead to glaucoma and complete blindness. Previously, we have shown that the small chemical chaperone, sodium-4-phenylbutyrate (PBA), rescues GC-induced OHT by reducing ocular endoplasmic reticulum (ER) stress. However, the exact mechanism of how PBA rescues GC-induced OHT is not completely understood. The trabecular meshwork (TM) is a filter-like specialized contractile tissue consisting of TM cells embedded within extracellular matrix (ECM) that controls intraocular pressure (IOP) by constantly regulating aqueous humor (AH) outflow. Induction of abnormal ECM deposition in TM is a hallmark of GC-induced OHT. Here, we investigated whether PBA reduces GC-induced OHT by degrading abnormal ECM deposition in TM using mouse model of GC-induced OHT, ex vivo cultured human TM tissues and primary human TM cells. We show that topical ocular eye drops of PBA (1%) significantly lowers elevated IOP in mouse model of GC-induced OHT. Importantly, PBA prevents synthesis and deposition of GC-induced ECM in TM. We report for the first time that PBA can degrade existing abnormal ECM in normal human TM cells/tissues by inducing matrix metalloproteinase (MMP)9 expression and activity. Furthermore, inhibition of MMPs activity by chemical-inhibitor (minocycline) abrogated PBA's effect on ECM reduction and its associated ER stress. Our study indicates a non-chaperone activity of PBA via activation of MMP9 that degrades abnormal ECM accumulation in TM.

Diverse effects of pan-ROCK and ROCK2 inhibitors on 2 D and 3D cultured human trabecular meshwork (HTM) cells treated with TGFβ2

A pan-ROCK-inhibitor, ripasudil (Rip), and a ROCK2 inhibitor, KD025, were used To study the effects of Rho-associated coiled-coil containing protein kinase (ROCK)1 and 2 on two-dimensional (2D) and three-dimensional (3D) cultures of a TGFβ2-treated human trabecular meshwork (HTM) cells. In the presence of 5 ng/mL TGFβ2, the effects of these inhibitors were characterized by transendothelial electrical resistance (TEER), FITC-dextran permeability, and the size and stiffness of 3D sphenoids, the expression of extracellular matrix (ECM) including collagen1, 4 and 6, and fibronectin, α-smooth muscle actin, a tissue inhibitor of metalloproteinase (TIMP)1–4, and matrix metalloproteinase (MMP)2, 9 and 14. TGFβ2 caused a significant increase in the TEER values, and decrease in FITC-dextran permeability, as well as a decrease in the sizes and stiffness of the 3D sphenoids. In the presence of ROCK inhibitors, the TGFβ2-induced effects of the TEER and FITC-dextran permeability were inhibited, especially by KD025. Rip induced a significant increase in sizes and a decrease in the stiffness of the TGFβ2-treated 3D sphenoids, although the effects of KD025 were weaker. Gene expressions of most of the ECMs, TIMP2 and MMP9 of 2D and 3D HTM cells were significantly up-regulated by TGFβ2. Those were significantly and differently modulated by Rip or KD025.

Secreted protein acidic and rich in cysteine (SPARC) knockout mice have greater outflow facility

Purpose

Secreted protein acidic and rich in cysteine (SPARC) is a matricellular protein that regulates intraocular pressure (IOP) by altering extracellular matrix (ECM) homeostasis within the trabecular meshwork (TM). We hypothesized that the lower IOP previously observed in SPARC -/- mice is due to a greater outflow facility.

Methods

Mouse outflow facility (C_live) was determined by multiple flow rate infusion, and episcleral venous pressure (P_e) was estimated by manometry. The animals were then euthanized, eliminating aqueous formation rate (F_in) and P_e. The C value was determined again (C_dead) while F_in was reduced to zero. Additional mice were euthanized for immunohistochemistry to analyze ECM components of the TM.

Results

The C_live and C_dead of SPARC -/- mice were 0.014 ± 0.002 μL/min/mmHg and 0.015 ± 0.002 μL/min/mmHg, respectively (p = 0.376, N/S). Compared to the C_live = 0.010 ± 0.002 μL/min/mmHg and C_dead = 0.011 ± 0.002 μL/min/mmHg in the WT mice (p = 0.548, N/S), the C_live and C_dead values for the SPARC -/- mice were higher. P_e values were estimated to be 8.0 ± 0.2 mmHg and 8.3 ± 0.7 mmHg in SPARC -/- and WT mice, respectively (p = 0.304, N/S). Uveoscleral outflow (F_u) was 0.019 ± 0.007 μL/min and 0.022 ± 0.006 μL/min for SPARC -/- and WT mice, respectively (p = 0.561, N/S). F_in was 0.114 ± 0.002 μL/min and 0.120 ± 0.016 μL/min for SPARC -/- and WT mice (p = 0.591, N/S). Immunohistochemistry demonstrated decreases of collagen types IV and VI, fibronectin, laminin, PAI-1, and tenascin-C within the TM of SPARC -/- mice (p < 0.05).

Conclusions

The lower IOP of SPARC -/- mice is due to greater aqueous humor outflow facility through the conventional pathway. Corresponding changes in several matricellular proteins and ECM structural components were noted in the TM of SPARC -/- mice.

TGFβ2-Hepcidin Feed-Forward Loop in the Trabecular Meshwork Implicates Iron in Glaucomatous Pathology

Purpose

Elevated levels of transforming-growth-factor (TGF)-β2 in the trabecular meshwork (TM) and aqueous humor are associated with primary open-angle glaucoma (POAG). The underlying mechanism includes alteration of extracellular matrix homeostasis through Smad-dependent and independent signaling. Smad4, an essential co-Smad, upregulates hepcidin, the master regulator of iron homeostasis. Here, we explored whether TGF-β2 upregulates hepcidin, implicating iron in the pathogenesis of POAG.

Methods

Primary human TM cells and human and bovine ex vivo anterior segment organ cultures were exposed to bioactive TGF-β2, hepcidin, heparin (a hepcidin antagonist), or N-acetyl carnosine (an antioxidant), and the change in the expression of hepcidin, ferroportin, ferritin, and TGF-β2 was evaluated by semiquantitative RT-PCR, Western blotting, and immunohistochemistry. Increase in reactive oxygen species (ROS) was quantified with dihydroethidium, an ROS-sensitive dye.

Results

Primary human TM cells and bovine TM tissue synthesize hepcidin locally, which is upregulated by bioactive TGF-β2. Hepcidin downregulates ferroportin, its downstream target, increasing ferritin and iron-catalyzed ROS. This causes reciprocal upregulation of TGF-β2 at the transcriptional and translational levels. Heparin downregulates hepcidin, and reduces TGF-β2-mediated increase in ferritin and ROS. Notably, both heparin and N-acetyl carnosine reduce TGF-β2-mediated reciprocal upregulation of TGF-β2.

Conclusions

The above observations suggest that TGF-β2 and hepcidin form a self-sustained feed-forward loop through iron-catalyzed ROS. This loop is partially disrupted by a hepcidin antagonist and an anti-oxidant, implicating iron and ROS in TGF-β2-mediated POAG. We propose that modification of currently available hepcidin antagonists for ocular use may prove beneficial for the therapeutic management of TGF-β2-associated POAG.

Wnt Activation After Inhibition Restores Trabecular Meshwork Cells Toward a Normal Phenotype

Purpose

Wnt is a spatiotemporally regulated signaling pathway whose inhibition is associated with glaucoma, elevated intraocular pressure (IOP), and cell stiffening. Whether such changes are permanent or may be reversed is unclear. Here, we determine if activation of Wnt pathway after inhibition reverses the pathologic phenotype.

Methods

Primary human trabecular meshwork (hTM) cells from nonglaucomatous donors were cultured for 12 days in the absence or presence of Wnt modulators: (i) LGK974 (Porcn inhibitor, 10 µM); (ii) LY2090314 (pGSK3β inhibitor, 250 nM); or (iii) 9 days of LGK974 followed by 3 days of LY2090314. Wnt modulation were determined by Western blotting and extracellular matrix (ECM) related genes were evaluated by quantitative PCR. Cytoskeletal morphology was determined by immunofluorescence and cell stiffness by atomic force microscopy.

Results

Wnt activation was confirmed by downregulation of pGSK3β (0.3-fold; P < 0.01), overexpression of AXIN2 (6.7-fold; P < 0.001), and LEF1 (3.8-fold; P < 0.001). Wnt inhibition resulted in dramatic changes in F-actin, which were resolved with subsequent Wnt activation. Concurrently, cell stiffness that was elevated with Wnt inhibition (11.86 kPa; P < 0.01) decreased with subsequent Wnt activation (4.195 kPa; P < 0.01) accompanied by significant overexpression of phosphorylated YAP (1.8-fold; P < 0.001) and TAZ (1.4-fold; P < 0.001). Additionally, Wnt activation after inhibition significantly repressed ECM genes (SPARC and CTGF, P < 0.01), cross-linking genes (LOX and TGM2, P < 0.05), inhibitors of matrix metalloproteinases (TIMP1 and PAI1, P < 0.001), and overexpressed MMP 1/9/14 (P < 0.01).

Conclusions

These data strongly demonstrate that, in normal hTM cells, activation of the Wnt pathway reverses the pathological phenotype caused by Wnt inhibition and may thus be a viable therapeutic for lowering IOP.

The extracellular matrix in development

As the crucial non-cellular component of tissues, the extracellular matrix (ECM) provides both physical support and signaling regulation to cells. Some ECM molecules provide a fibrillar environment around cells, while others provide a sheet-like basement membrane scaffold beneath epithelial cells. In this Review, we focus on recent studies investigating the mechanical, biophysical and signaling cues provided to developing tissues by different types of ECM in a variety of developing organisms. In addition, we discuss how the ECM helps to regulate tissue morphology during embryonic development by governing key elements of cell shape, adhesion, migration and differentiation.

Summary: This Review discusses our current understanding of how the extracellular matrix helps guide developing tissues by influencing cell adhesion, migration, shape and differentiation, emphasizing the biophysical cues it provides.

C3 Transferase-Expressing scAAV2 Transduces Ocular Anterior Segment Tissues and Lowers Intraocular Pressure in Mouse and Monkey

Glaucoma is a lifelong disease with elevated intraocular pressure (IOP) as the main risk factor, and reduction of IOP remains the major treatment for this disease. However, current IOP-lowering therapies are far from being satisfactory. We have demonstrated that the lentivirus-mediated exoenzyme C3 transferase (C3) expression in rat and monkey eyes induced relatively long-term IOP reduction. We now show that intracameral injection of self-complementary AAV2 containing a C3 gene into mouse and monkey eyes resulted in morphological changes in trabecular meshwork and IOP reduction. The vector-transduced corneal endothelium and the C3 transgene expression, not vector itself, induced corneal edema as a result of actin-associated endothelial barrier disruption. There was a positive (quadratic) correlation between measured IOP and grade of corneal edema. This is the first report of using an AAV to transduce the trabecular meshwork of monkeys with a gene capable of altering cellular structure and physiology, indicating a potential gene therapy for glaucoma.

Fidelity of long-term cryopreserved adipose-derived stem cells for differentiation into cells of ocular and other lineages

Adipose-Derived Stem Cells (ADSCs) have an important contribution in regenerative medicine ranging from testing stem cell therapy for disease treatment in pre-clinical models to clinical trials. For immediate use of stem cells for therapy, there is a requirement of the high dose of stem cells at different time points which can be met by cryopreservation. In this study, we evaluated the characteristics of long-term cryopreserved ADSCs and their regenerative potential after an average of twelve-year cryopreservation. Revived ADSCs were examined for cell viability and proliferation by trypan blue, Calcein/Hoechst and MTT assay. Expression of stem cell markers was examined by flow cytometry, immunostaining and qPCR. Colony forming efficiency and spheroid formation ability were also assessed. Multilineage differentiation potential was evaluated by induction into osteocytes, adipocytes, neural cells, corneal keratocytes and trabecular meshwork (TM) cells. Post-thaw, ADSCs maintained expression of stem cell markers CD90, CD73, CD105, CD166, NOTCH1, STRO-1, ABCG2, OCT4, KLF4. ADSCs retained colony and spheroid forming potential. These cells were able to differentiate into osteocytes, confirmed by Alizarin Red S staining and elevated expression of osteocalcin and osteopontin; into adipocytes by Oil Red O staining and elevated expression of PPARγ2. ADSCs could differentiate into neural cells, stained positive to β-III tubulin, neurofilament, GFAP as well as elevated expression of nestin and neurofilament mRNAs. ADSCs could also give rise to corneal keratocytes expressing keratocan, keratan sulfate, ALDH and collagen V, and to TM cells expressing CHI3L1 and AQP1. Differentiated TM cells responded to dexamethasone treatment with increased Myocilin expression, which could be used as in vitro glaucoma model for further studies. Conditioned medium from ADSCs was found to impart a regenerative effect on primary TM cells. In conclusion, ADSCs maintained their stemness and multipotency after long-term cryopreservation with variability between different donors. This study can have great repercussions in regenerative medicine and pave the way for future clinical trials using cryopreserved ADSCs.

Is "normal tension glaucoma" glaucoma?

Primary open angle glaucoma (POAG) represents a distinct disease entity with elevated intraocular pressure (IOP) as the main risk factor, even though the reasons for why the IOP is elevated remains to be elucidated. It is considered that normal tension glaucoma (NTG) is a subtype of POAG, comprising a special form of glaucomatous neurodegeneration or glaucomatous optic neuropathy (GON) almost exactly the same as that seen in POAG, but the IOP, as named, remains in the statistically normal range. Actually the disease entity of NTG has been a profound confusion and it is difficult to be accurately conceptualized. One of the reasons is that the IOP is closely linked to the occurrence of GON in POAG but not in NTG, and for the latter, it seems that GON is secondary to a number of local or systemic disorders. In recent years, increasing evidences suggest that NTG or IOP independent GON is a non-glaucomatous disease with different disease entities from POAG and with more diverse and complex etiologies. Here we hypothesized that NTG, at least for those with recognizable primary diseases, is not a glaucomatous disease; instead, it represents a group of disorders with GON as a characteristic clinical feature or phenotype.

Prion protein modulates endothelial to mesenchyme-like transition in trabecular meshwork cells: Implications for primary open angle glaucoma

Endothelial-to-mesenchyme-like transition (Endo-MT) of trabecular meshwork (TM) cells is known to be associated with primary open angle glaucoma (POAG). Here, we investigated whether the prion protein (PrP^C), a neuronal protein known to modulate epithelial-to-mesenchymal transition in a variety of cell types, is expressed in the TM, and plays a similar role at this site. Using a combination of primary human TM cells and human, bovine, and PrP-knock-out (PrP^−/−) mouse models, we demonstrate that PrP^C is expressed in the TM of all three species, including endothelial cells lining the Schlemm’s canal. Silencing of PrP^C in primary human TM cells induces aggregation of β1-integrin and upregulation of α-smooth muscle actin, fibronectin, collagen 1A, vimentin, and laminin, suggestive of transition to a mesenchyme-like phenotype. Remarkably, intraocular pressure is significantly elevated in PrP^−/− mice relative to wild-type controls, suggesting reduced pliability of the extracellular matrix and increased resistance to aqueous outflow in the absence of PrP^C. Since PrP^C is cleaved by members of the disintegrin and matrix-metalloprotease family that are increased in the aqueous humor of POAG arising from a variety of conditions, it is likely that concomitant cleavage of PrP^C exaggerates and confounds the pathology by inducing Endo-MT-like changes in the TM.

Matricellular Protein Levels in Aqueous Humor and Surgical Outcomes of Trabeculectomy

Purpose

The purpose of this study was to quantify levels of three matricellular proteins in the aqueous humor in patients with previous acute primary angle closure (APAC) and investigate their correlation with bleb morphology and surgical outcomes of trabeculectomy.

Methods

In this prospective study, aqueous humor samples were collected from 40 previous APAC eyes. Concentrations of three matricellular proteins-secreted protein acidic and rich in cysteine (SPARC), thrombospondin-2, and osteopontin-were measured using multiplexed immunoassays kits. Intraocular pressure was measured using Goldmann application tonometry. Bleb morphology was assessed using anterior segment optical coherence tomography, and bleb score was calculated according to bleb size and reflectivity.

Results

When previous APAC eyes were divided according to surgical outcome 18 months after trabeculectomy, SPARC protein was significantly higher in aqueous humor in the failure group (P = 0.009). When previous APAC eyes were divided according to SPARC level, eyes with low SPARC levels had significantly higher overall success rate compared with eyes with high SPARC levels (P = 0.005 for complete success and P = 0.018 for qualified success). Multiple logistic regression analyses showed that eyes with higher levels of SPARC were more likely to have a failed filtration surgery than were eyes with lower levels of SPARC. For complete success, P = 0.006 and odds ratio (OR) = 6.458; for qualified success, P = 0.033 and OR = 2.608. The level of SPARC was found to have a positive correlation with bleb score (P < 0.001, R2 = 0.471).

Conclusions

In previous APAC patients, the SPARC level in aqueous humor is a prognostic factor for surgical results of trabeculectomy. Modulation of SPARC expression may have potential clinical applications after filtration surgery.

Visual Defects and Ageing

Many diseases are related to age, among these neurodegeneration is particularly important. Alzheimer's disease Parkinson's and Glaucoma have many common pathogenic events including oxidative damage, Mitochondrial dysfunction, endothelial alterations and changes in the visual field. These are well known in the case of glaucoma, less in the case of neurodegeneration of the brain. Many other molecular aspects are common, such as the role of endoplasmic reticulum autophagy and neuronal apoptosis while others have been neglected due to lack of space such as inflammatory cytokine or miRNA. Moreover, the loss of specific neuronal populations, the induction of similar mechanisms of cell injury and the deposition of protein aggregates in specific anatomical areas are very similar events between these diseases. Intracellular and/or extracellular accumulation of protein aggregates is a key feature of many neurodegenerative disorders. The existence of abnormal protein aggregates has been documented in the RGCs of glaucomatous patients such as the anomalous Tau protein or the β-amyloid accumulations. Intra-cell catabolic processes also appear to be common in both glaucoma and neurodegeneration. They also help us to understand how the basis between these diseases is common and how the visual aspects can be a serious problem for those who are affected.

Histologic Analysis of Trabecular Meshwork Obtained From Kahook Dual Blade Goniotomy

PURPOSE

To determine whether there are identifiable, reproducible findings in the trabecular meshwork (TM) of patients with primary open-angle glaucoma (POAG) who underwent Kahook Dual Blade (KDB) goniotomy.

DESIGN

Noncomparative retrospective case series.

METHODS

Tertiary academic referral center, Veterans Affairs Medical Center. Thirteen patients (14 eyes) with POAG (100%) were treated with KDB goniotomy from May to December 2017. Isolated TM tissue was collected from 9 patients (10 eyes) and submitted for histologic analysis. Hematoxylin-eosin, periodic acid-Schiff, and elastin Van Gieson stains were completed, in addition to immunohistochemistry for collagen IV.

RESULTS

Mean age of patients was 74.2 ± 6.7 years. Trabecular beams were identified in all 10 specimens, although distorted in 4 samples, of which 3 had a history of laser trabeculoplasty. Collagen IV staining was present in 10 of 10 samples, coating the trabecular beams. Elastin was present in 8 of 10 samples along the trabecular beams. Intraocular pressure and number of glaucoma medications decreased significantly in all cases postoperatively (P < .0001, P = .035, respectively).

CONCLUSIONS

This pilot study demonstrates that tissue obtained during KDB goniotomy has a high yield of containing TM compared to reported yield of TM in specimens collected from traditional ab externo trabeculectomy (71% vs 20%, respectively). These goniotomy specimens possess sufficient anatomic preservation to be studied histologically. Trabecular meshwork obtained with this procedure may provide a novel modality to study TM dysfunction in open-angle glaucomas.

Matricellular Proteins Play a Potential Role in Acute Primary Angle Closure

PURPOSE

To quantify levels of matricellular proteins in aqueous humor samples from acute primary angle closure (APAC) and non-glaucomatous cataract eyes and investigate their correlation with intraocular pressure (IOP) fluctuation.

MATERIALS AND METHODS

Aqueous humor samples were collected from 63 eyes including 29 current APAC eyes, 12 previous APAC eyes, and 22 cataract eyes. Concentrations of four main matricellular proteins (SPARC, tenascin-C, thrombospondin-2, and osteopontin) were measured using multiplexed immunoassay kits. Correlations between matricellular proteins and age, sex, and IOP were then detected using Spearman's rank correlation coefficient.

RESULTS

The levels of SPARC, thrombospondin-2, and osteopontin were significantly elevated in the APAC group as compared to the cataract group (p < 0.001, p < 0.001, and p = 0.009, respectively). Further separation of the APAC group into current and previous APAC groups showed that only the differences of SPARC and thrombospondin-2 between the current APAC and cataract groups were significant (both p < 0.001). All four matricellular proteins were found to have a positive correlation with IOP in the current APAC group but no correlation was found in the previous APAC or cataract groups.

CONCLUSIONS

The levels of matricellular proteins were significantly elevated in the current APAC eyes and positively correlated to IOP. Further studies are necessary to investigate the molecular mechanisms and histological evidence of pathogenesis in matricellular proteins in APAC.

Vertebrate Lonesome Kinase Regulated Extracellular Matrix Protein Phosphorylation, Cell Shape, and Adhesion in Trabecular Meshwork Cells

Glaucoma, a leading cause of irreversible blindness, is commonly associated with elevated intraocular pressure (IOP) due to impaired aqueous humor (AH) drainage through the trabecular meshwork (TM). Although dysregulated production and organization of extracellular matrix (ECM) is presumed to increase resistance to AH outflow and elevate IOP by altering TM cell contractile and adhesive properties, it is not known whether regulation of ECM protein phosphorylation via the secretory vertebrate lonesome kinase (VLK) influences TM cellular characteristics. Here, we tested this possibility. Experiments carried out in this study reveal that the 32 kDa protein is a prominent VLK isoform detectable in lysates and conditioned media (CM) of human TM cells. Increased levels of VLK were observed in CM of TM cells subjected to cyclic mechanical stretch, or treated with dexamethasone, TGF-β2, and TM cells expressing constitutively active RhoA GTPase. Downregulation of VLK expression in TM cells using siRNA decreased tyrosine phosphorylation (TyrP) of ECM proteins and focal adhesions, and induced changes in cell shape in association with reduced levels of actin stress fibers and phospho-paxillin. VLK was also demonstrated to regulate TGF-β2-induced TyrP of ECM proteins. Taken together, these results suggest that VLK secretion can be regulated by external cues, intracellular signal proteins, and mechanical stretch, and VLK can in turn regulate TyrP of ECM proteins secreted by TM cells and control cell shape, actin stress fibers, and focal adhesions. These observations indicate a potential role for VLK in homeostasis of AH outflow and IOP, and in the pathobiology of glaucoma. J. Cell. Physiol. 232: 2447-2460, 2017. © 2016 Wiley Periodicals, Inc.

Aberrant Collagen Composition of the Trabecular Meshwork Results in Reduced Aqueous Humor Drainage and Elevated IOP in MMP-9 Null Mice

Purpose

Homeostatic turnover of the trabecular meshwork extracellular matrix (ECM) is essential to regulate aqueous humor outflow and to maintain intraocular pressure homeostasis. In this study, we evaluated aqueous humor turnover, intraocular pressure, and trabecular meshwork organization in MMP-9 null mice.

Methods

Intraocular pressure and aqueous humor turnover were measured in MMP-9 null versus wild-type mice. Morphology of the anterior segment of the eye, with special attention to the structural organization of the trabecular meshwork, was investigated by means of optical coherence tomography, light microscopy, and transmission electron microscopy. Furthermore, using quantitative real-time polymerase chain reaction and immunostainings, we evaluated the ECM composition of the trabecular meshwork. Finally, the integrity and function of the retina and optic nerve were assessed, via optical coherence tomography, histologic techniques, and optomotor testing.

Results

MMP-9 null mice displayed early-onset ocular hypertension and reduced aqueous humor turnover. While transmission electron microscopic analysis did not reveal any abnormalities in the cellular organization of the trabecular meshwork, detailed investigation of collagen expression indicated that there is an aberrant trabecular meshwork ECM composition in MMP-9 null mice. Notably, at the age of 13 months, no glaucomatous neurodegeneration was seen in MMP-9 null mice.

Conclusions

Our observations corroborate MMP-9 as an important remodeler of the collagenous composition of the trabecular meshwork and provide evidence for a causal link between MMP-9 deficiency, trabecular meshwork ultrastructure, and ocular hypertension.

Segmental outflow of aqueous humor in mouse and human

The main and only modifiable risk factor in glaucoma, the group of usually late onset progressive and irreversible blinding optic neuropathies, is elevated intraocular pressure (IOP). The increase in IOP is due to impeded aqueous humor (AH) outflow through the conventional pathway. The aberrant increased resistance at the trabecular meshwork (TM), the filter-like region in the anterior eye chamber is the major contributory factor in causing the impeded outflow. In normal as well as in glaucoma eyes the regions of the TM are divided into areas of high and low flow. The collector channels and distal outflow regions are now increasingly being recognized as potential players in contributing to impede AH outflow. Structural and molecular make-up contributing to the segmental blockage to outflow is likely to provide greater insight. Establishing segmental blockage to outflow in model systems of glaucoma such as the mouse in parallel to human eyes will expand our repertoire of tools for investigation. Further study into this area of interest has the potential to ultimately lead to the development of new therapeutics focused on lowering IOP by targeting the various components of segmental blockage of outflow in the TM and in the distal outflow region.

The potential impact of recent insights into proteomic changes associated with glaucoma

INTRODUCTION

Glaucoma, a major ocular neuropathy, is still far from being understood on a molecular scale. Proteomic workflows revealed glaucoma associated alterations in different eye components. By using state-of-the-art mass spectrometric (MS) based discovery approaches large proteome datasets providing important information about glaucoma related proteins and pathways could be generated. Corresponding proteomic information could be retrieved from various ocular sample species derived from glaucoma experimental models or from original human material (e.g. optic nerve head or aqueous humor). However, particular eye tissues with the potential for understanding the disease's molecular pathomechanism remains underrepresented. Areas covered: The present review provides an overview of the analysis depth achieved for the glaucomatous eye proteome. With respect to different eye regions and biofluids, proteomics related literature was found using PubMed, Scholar and UniProtKB. Thereby, the review explores the potential of clinical proteomics for glaucoma research. Expert commentary: Proteomics will provide important contributions to understanding the molecular processes associated with glaucoma. Sensitive discovery and targeted MS approaches will assist understanding of the molecular interplay of different eye components and biofluids in glaucoma. Proteomic results will drive the comprehension of glaucoma, allowing a more stringent disease hypothesis within the coming years.

Transcriptome analysis of adult and fetal trabecular meshwork, cornea, and ciliary body tissues by RNA sequencing

PURPOSE

To characterize the transcriptional landscape of human adult and fetal trabecular meshwork (TM), cornea, and ciliary body (CB) tissues, and to evaluate the expression level of candidate genes selected from genetic association studies of primary-open angle glaucoma, central corneal thickness, intraocular pressure, vertical cup to disc ratio, and optic nerve parameters.

METHODS

Deep RNA sequencing was performed on the selected human tissues. Transcriptome analyses were performed to 1) characterize the total number of expressed genes, 2) identify the most highly expressed genes, 3) estimate the number of novel transcripts, and 4) evaluate the expression of candidate genes in each tissue. Finally, a differential gene expression analysis was conducted to compare the adult and fetal ocular tissues.

RESULTS

There was an average of 12,362 protein coding genes and 3725 novel transcripts expressed in each tissue. The top most expressed genes in each tissue included SPARC (fetal cornea and TM), APOD (adult TM), CLU (adult cornea), and PTGDS (adult and fetal CB). Twenty-nine candidate genes selected from genetic association studies primarily showed high expression levels in the trabecular meshwork and cornea. Comparison of adult and fetal samples identified 2012 and 1261 differentially expressed protein-coding genes within the cornea and trabecular meshwork, respectively.

CONCLUSIONS

This study has provided an unbiased glimpse into the transcriptome of three essential anterior ocular tissues, resulting in the development of several novel hypotheses. These data can be used in the future to better guide ocular research questions.

From DNA damage to functional changes of the trabecular meshwork in aging and glaucoma

Glaucoma is a degenerative disease of the eye. Both the anterior and posterior segments of the eye are affected, extensive damage being detectable in the trabecular meshwork and the inner retina-central visual pathway complex. Oxidative stress is claimed to be mainly responsible for molecular damage in the anterior chamber. Indeed, oxidation harms the trabecular meshwork, leading eventually to endothelial cell decay, tissue malfunction, subclinical inflammation, changes in the extracellular matrix and cytoskeleton, altered motility, reduced outflow facility and (ultimately) increased IOP. Moreover, free radicals are involved in aging and can be produced in the brain (as well as in the eye) as a result of ischemia, leading to oxidation of the surrounding neurons. Glaucoma-related cell death occurs by means of apoptosis, and apoptosis is triggered by oxidative stress via (a) mitochondrial damage, (b) inflammation, (c) endothelial dysregulation and dysfunction, and (d) hypoxia. The proteomics of the aqueous humor is significantly altered in glaucoma as a result of oxidation-induced trabecular damage. Those proteins whose aqueous humor levels are increased in glaucoma are biomarkers of trabecular meshwork impairment. Their diffusion from the anterior to the posterior segment of the eye may be relevant in the cascade of events triggering apoptosis in the inner retinal layers, including the ganglion cells.

Gene transfer to the outflow tract

Elevated intraocular pressure is the primary cause of open angle glaucoma. Outflow resistance exists within the trabecular meshwork but also at the level of Schlemm's canal and further downstream within the outflow system. Viral vectors allow to take advantage of naturally evolved, highly efficient mechanisms of gene transfer, a process that is termed transduction. They can be produced at biosafety level 2 in the lab using protocols that have evolved considerably over the last 15-20 years. Applied by an intracameral bolus, vectors follow conventional as well as uveoscleral outflow pathways. They may affect other structures in the anterior chamber depending on their transduction kinetics which can vary among species when using the same vector. Not all vectors can express long-term, a desirable feature to address the chronicity of glaucoma. Vectors that integrate into the genome of the target cell can achieve transgene function for the life of the transduced cell but are mutagenic by definition. The most prominent long-term expressing vector systems are based on lentiviruses that are derived from HIV, FIV, or EIAV. Safety considerations make non-primate lentiviral vector systems easier to work with as they are not derived from human pathogens. Non-integrating vectors are subject to degradation and attritional dilution during cell division. Lentiviral vectors have to integrate in order to express while adeno-associated viral vectors (AAV) often persist as intracellular concatemers but may also integrate. Adeno- and herpes viral vectors do not integrate and earlier generation systems might be relatively immunogenic. Nonviral methods of gene transfer are termed transfection with few restrictions of transgene size and type but often a much less efficient gene transfer that is also short-lived. Traditional gene transfer delivers exons while some vectors (lentiviral, herpes and adenoviral) allow transfer of entire genes that include introns. Recent insights have highlighted the role of non-coding RNA, most prominently, siRNA, miRNA and lncRNA. SiRNA is highly specific, miRNA is less specific, while lncRNA uses highly complex mechanisms that involve secondary structures and intergenic, intronic, overlapping, antisense, and bidirectional location. Several promising preclinical studies have targeted the RhoA or the prostaglandin pathway or modified the extracellular matrix. TGF-β and glaucoma myocilin mutants have been transduced to elevate the intraocular pressure in glaucoma models. Cell based therapies have started to show first promise. Past approaches have focused on the trabecular meshwork and the inner wall of Schlemm's canal while new strategies are concerned with modification of outflow tract elements that are downstream of the trabecular meshwork.

Tissue-based multiphoton analysis of actomyosin and structural responses in human trabecular meshwork

The contractile trabecular meshwork (TM) modulates aqueous humor outflow resistance and intraocular pressure. The primary goal was to visualize and quantify human TM contractile state by analyzing actin polymerization (F-actin) by 2-photon excitation fluorescence imaging (TPEF) in situ. A secondary goal was to ascertain if structural extracellular matrix (ECM) configuration changed with contractility. Viable ex vivo human TM was incubated with latrunculin-A (Lat-A) or vehicle prior to Alexa-568-phalloidin labeling and TPEF. Quantitative image analysis was applied to 2-dimensional (2D) optical sections and 3D image reconstructions. After Lat-A exposure, (a) the F-actin network reorganized as aggregates; (b) F-actin-associated fluorescence intensity was reduced by 48.6% (mean; p = 0.007; n = 8); (c) F-actin 3D distribution was reduced by 68.9% (p = 0.040); (d) ECM pore cross-sectional area and volume were larger by 36% (p = 0.032) and 65% (p = 0.059) respectively and pores appeared more interconnected; (e) expression of type I collagen and elastin, key TM structural ECM proteins, were unaltered (p = 0.54); and (f) tissue viability was unchanged (p = 0.39) relative to vehicle controls. Thus Lat-A-induced reduction of actomyosin contractility was associated with TM porous expansion without evidence of reduced structural ECM protein expression or cellular viability. These important subcellular-level dynamics could be visualized and quantified within human tissue by TPEF.

SPARC/osteonectin in mineralized tissue

Secreted protein acidic and rich in cysteine (SPARC/osteonectin/BM40) is one of the most abundant non-collagenous protein expressed in mineralized tissues. This review will focus on elucidating functional roles of SPARC in bone formation building upon results from non-mineralized cells and tissues, the phenotype of SPARC-null bones, and recent discoveries of human diseases with either dysregulated expression of SPARC or mutations in the gene encoding SPARC that give rise to bone pathologies. The capacity of SPARC to influence pathways involved in extracellular matrix assembly such as procollagen processing and collagen fibril formation as well as the capacity to influence osteoblast differentiation and osteoclast activity will be addressed. In addition, the potential for SPARC to regulate cross-linking of extracellular matrix proteins by members of the transglutaminase family of enzymes is explored. Elucidating defined biological functions of SPARC in terms of bone formation and turnover are critical. Further insight into specific cellular mechanisms involved in the formation and homeostasis of mineralized tissues will lead to a better understanding of disease progression.

The Role of the IL-20 Subfamily in Glaucoma

Glaucoma is a common disease that leads to loss of peripheral vision and, if left untreated, ultimately to blindness. While the exact cause(s) of glaucoma is still unknown, two leading risk factors are age and elevated intraocular pressure. Several studies suggest a possible link between glaucoma and inflammation in humans and animal models. In particular, our lab recently identified a T104M mutation in IL-20 receptor-B (IL-20RB) in primary open angle glaucoma patients from a large pedigree. Several of the interleukin- (IL-) 20 family of cytokines and receptors are expressed in ocular tissues including the trabecular meshwork, optic nerve head, and retinal ganglion cells. The DBA/2J mouse develops high intraocular pressures with age and has characteristic optic nerve defects that make it a useful glaucoma model. IL-24 expression is significantly upregulated in the retina of these mice, while IL-20RA expression in the optic nerve is downregulated following pressure-induced damage. The identification of a mutation in the IL-20RB gene in a glaucoma pedigree and changes in expression levels of IL-20 family members in the DBA/2J mouse suggest that disruption of normal IL-20 signaling in the eye may contribute to degenerative processes associated with glaucoma.

Secreted Protein Acidic and Rich in Cysteine in Ocular Tissue

Secreted protein acidic and rich in cysteine (SPARC), also known as osteonectin or BM-40, is the prototypical matricellular protein. Matricellular proteins are nonstructural secreted proteins that provide an integration between cells and their surrounding extracellular matrix (ECM). Regulation of the ECM is important in maintaining the physiologic function of tissues. Elevated levels of SPARC have been identified in a variety of diseases involving pathologic tissue remodeling, such as hepatic fibrosis, systemic sclerosis, and certain carcinomas. Within the eye, SPARC has been identified in the trabecular meshwork, lens, and retina. Studies have begun to show the role of SPARC in these tissues and its possible role, specifically in primary open-angle glaucoma, cataracts, and proliferative vitreoretinopathy. SPARC may, therefore, be a therapeutic target in the treatment of certain ocular diseases. Further investigation into the mechanism of action of SPARC will be necessary in the development of SPARC-targeted therapy.

The intrinsic stiffness of human trabecular meshwork cells increases with senescence

Dysfunction of the human trabecular meshwork (HTM) plays a central role in the age-associated disease glaucoma, a leading cause of irreversible blindness. The etiology remains poorly understood but cellular senescence, increased stiffness of the tissue, and the expression of Wnt antagonists such as secreted frizzled related protein-1 (SFRP1) have been implicated. However, it is not known if senescence is causally linked to either stiffness or SFRP1 expression. In this study, we utilized in vitro HTM senescence to determine the effect on cellular stiffening and SFRP1 expression. Stiffness of cultured cells was measured using atomic force microscopy and the morphology of the cytoskeleton was determined using immunofluorescent analysis. SFRP1 expression was measured using qPCR and immunofluorescent analysis. Senescent cell stiffness increased 1.88±0.14 or 2.57±0.14 fold in the presence or absence of serum, respectively. This was accompanied by increased vimentin expression, stress fiber formation, and SFRP1 expression. In aggregate, these data demonstrate that senescence may be a causal factor in HTM stiffening and elevated SFRP1 expression, and contribute towards disease progression. These findings provide insight into the etiology of glaucoma and, more broadly, suggest a causal link between senescence and altered tissue biomechanics in aging-associated diseases.

Extracellular matrix in the trabecular meshwork: intraocular pressure regulation and dysregulation in glaucoma

The trabecular meshwork (TM) is located in the anterior segment of the eye and is responsible for regulating the outflow of aqueous humor. Increased resistance to aqueous outflow causes intraocular pressure to increase, which is the primary risk factor for glaucoma. TM cells reside on a series of fenestrated beams and sheets through which the aqueous humor flows to exit the anterior chamber via Schlemm's canal. The outer trabecular cells are phagocytic and are thought to function as a pre-filter. However, most of the outflow resistance is thought to be from the extracellular matrix (ECM) of the juxtacanalicular region, the deepest portion of the TM, and from the inner wall basement membrane of Schlemm's canal. It is becoming increasingly evident that the extracellular milieu is important in maintaining the integrity of the TM. In glaucoma, not only have ultrastructural changes been observed in the ECM of the TM, and a significant number of mutations in ECM genes been noted, but the stiffness of glaucomatous TM appears to be greater than that of normal tissue. Additionally, TGFβ2 has been found to be elevated in the aqueous humor of glaucoma patients and is assumed to be involved in ECM changes deep with the juxtacanalicular region of the TM. This review summarizes the current literature on trabecular ECM as well as the development and function of the TM. Animal models and organ culture models targeting specific ECM molecules to investigate the mechanisms of glaucoma are described. Finally, the growing number of mutations that have been identified in ECM genes and genes that modulate ECM in humans with glaucoma are documented.

Recent Developments in Understanding the Role of Aqueous Humor Outflow in Normal and Primary Open Angle Glaucoma

Primary open angle glaucoma (POAG) is the second leading cause of blindness in the world's rapidly aging population. POAG is characterized by progressive degeneration of neural structures in the posterior segment, often associated with a concomitant elevation of intraocular pressure. Changes in IOP are believed to be caused by a disruption in the normal outflow of aqueous humor. This article reviews recent research associated with normal and POAG aqueous humor outflow. Novel findings elucidating biochemical and pathological changes in the ocular tissues affected in POAG are presented. Stem cell research, identification of lymphatic markers, and increased use of mouse models give researchers exciting new tools to understand aqueous humor outflow, changes associated with POAG and identify underlying causes of the disease.

The Function of Matricellular Proteins in the Lamina Cribrosa and Trabecular Meshwork in Glaucoma

PURPOSE

To review the current literature regarding the role of matricellular proteins in glaucoma, specifically in the lamina cribrosa (LC) region of the optic nerve head (ONH) and the trabecular meshwork (TM).

METHODS

A literature search was performed for published articles describing the expression and function of matricellular proteins such as thrombospondin (TSP), connective tissue growth factor (CTGF), secreted protein acidic and rich in cysteine (SPARC), and periostin in glaucoma.

RESULTS

In glaucoma, there are characteristic extracellular matrix (ECM) changes associated with optic disc cupping in the ONH and subsequent visual field defects. Matricellular proteins are a family of nonstructural secreted glycoproteins, which enable cells to communicate with their surrounding ECM, including CTGF, also known as CCN2, TSPs, SPARC, periostin, osteonectin, and tenascin-C and -X, and other ECM proteins. Such proteins appear to play a role in fibrosis and increased ECM deposition. Importantly, most are widely expressed in tissues particularly in the TM and ONH, and deficiency of TSP1 and SPARC has been shown to lower intraocular pressure in mouse models of glaucoma through enhanced outflow facility.

CONCLUSION

This article highlights the role of matricellular proteins in glaucoma pathology. The potential role of these proteins in glaucoma is emerging as some have an association with the pathophysiology of the TM and LC region and might therefore be potential targets for therapeutic intervention in glaucoma.

Mapping molecular differences and extracellular matrix gene expression in segmental outflow pathways of the human ocular trabecular meshwork

Elevated intraocular pressure (IOP) is the primary risk factor for glaucoma, and lowering IOP remains the only effective treatment for glaucoma. The trabecular meshwork (TM) in the anterior chamber of the eye regulates IOP by generating resistance to aqueous humor outflow. Aqueous humor outflow is segmental, but molecular differences between high and low outflow regions of the TM are poorly understood. In this study, flow regions of the TM were characterized using fluorescent tracers and PCR arrays. Anterior segments from human donor eyes were perfused at physiological pressure in an ex vivo organ culture system. Fluorescently-labeled microspheres of various sizes were perfused into anterior segments to label flow regions. Actively perfused microspheres were segmentally distributed, whereas microspheres soaked passively into anterior segments uniformly labeled the TM and surrounding tissues with no apparent segmentation. Cell-tracker quantum dots (20 nm) were localized to the outer uveal and corneoscleral TM, whereas larger, modified microspheres (200 nm) localized throughout the TM layers and Schlemm’s canal. Distribution of fluorescent tracers demonstrated a variable labeling pattern on both a macro- and micro-scale. Quantitative PCR arrays allowed identification of a variety of extracellular matrix genes differentially expressed in high and low flow regions of the TM. Several collagen genes (COL16A1, COL4A2, COL6A1 and 2) and MMPs (1, 2, 3) were enriched in high, whereas COL15A1, and MMP16 were enriched in low flow regions. Matrix metalloproteinase activity was similar in high and low regions using a quantitative FRET peptide assay, whereas protein levels in tissues showed modest regional differences. These gene and protein differences across regions of the TM provide further evidence for a molecular basis of segmental flow routes within the aqueous outflow pathway. New insight into the molecular mechanisms of segmental aqueous outflow may aid in the design and delivery of improved treatments for glaucoma patients.

Aqueous outflow: segmental and distal flow

The elevated intraocular pressure (IOP) of primary open-angle glaucoma is caused by impaired outflow of aqueous humor through the trabecular meshwork. Within the juxtacanalicular region, alterations of both extracellular matrix homeostasis and the cellular tone of trabecular meshwork endothelial and the inner wall of Schlemm canal cells affect outflow. Newer pharmacologic agents that target trabecular meshwork and Schlemm canal cell cytoskeleton lower IOP. Aqueous drainage occurs nonhomogenously with greater flow going through certain portions of the TM and less going through other portions-a concept known as segmental flow, which is theoretically the result of outflow being dependent on the presence of discrete pores within Schlemm canal. The limited long-term success of trabecular meshwork bypass surgeries implicates the potential impact of resistance in Schlemm canal itself and collector channels. Additionally, others have observed that outflow occurs preferentially near collector channels. These distal structures may be more important to aqueous outflow than previously believed.

FINANCIAL DISCLOSURE

Dr. Rhee is a consultant to Aerie Pharmaceuticals, Alcon Laboratories, Inc., Allegan, Inc., Aquesys, Inc., Glaukos Corp., Ivantis, Inc., Johnson & Johnson, Merck Sharp & Dohme Corp. and Santen, Inc., and has received research funding from Alcon Laboratories, Inc., Merck Sharp & Dohme Corp., and Ivantis, Inc. No other author has a financial or proprietary interest in any material or method mentioned.

Cross-talk between ciliary epithelium and trabecular meshwork cells in-vitro: a new insight into glaucoma

PURPOSE

It is assumed that the non-pigmented ciliary epithelium plays a role in regulating intraocular pressure via its neuroendocrine activities. To test this hypothesis, we investigated the effect on a human trabecular meshwork (TM) cell line (NTM) of co-culture with a human non-pigmented ciliary epithelium cell line (ODM-2).

METHODS

The cellular cross-talk between ODM-2 and NTM cells was studied in a co-culture system in which the two cell types were co-cultured for 5 to 60 min or 2, 4 and 8h and then removed from the co-culture and analyzed. Analyses of the ERK and p38 mitogen-activated protein kinase (MAPK) pathways and of the activity of TM phosphatases and matrix metalloproteins (MMPs) were performed. Acid and alkaline phosphatase activity was determined by the DiFMUP (6, 8-difluoro-4-methylumbelliferyl phosphate) assay. MMP levels were determined by gelatin zymography.

RESULTS

Exposure of NTM cells to ODM-2 cells led to the activation of the MAPK signal transduction pathways in NTM cells within 5 min of co-culture. Phosphorylation of ERK1/ERK2 and p38 peaked at 10 and 15 min and then decreased over time. Interaction between ODM-2 and NTM cells promoted the expression of MMP-9 in the NTM cells after 4h of co-culture.

CONCLUSIONS

Our findings provide support for the hypothesis that crosstalk does indeed take place between ODM-2 and NTM cells. Future studies should be designed to determine the relationship between the MMP system, MAPK kinases and phosphatases. Manipulation of these signaling molecules and the related NTM signal transduction pathways may provide targets for developing improved treatments for glaucoma.

Canonical wnt signaling regulates extracellular matrix expression in the trabecular meshwork

PURPOSE

Canonical Wnt signaling has emerged as a critical regulator of aqueous outflow facility and intraocular pressure (IOP). In this study, we examine the role of canonical Wnt signaling on extracellular matrix (ECM) expression in the trabecular meshwork (TM) and explore the molecular mechanisms involved.

METHODS

β-catenin localization in human TM tissue was examined using immunofluorescent staining. Primary human TM cells were incubated with lithium chloride (LiCl) and the effect on active β-catenin expression was assessed by immunoblot. Adenovirus expressing a dominant-negative TCF4 mutant that lacks a β-catenin binding domain was used. Changes in the levels of the microRNA-29 (miR-29) family and ECM proteins were determined by real-time quantitative PCR and immunoblot analysis, respectively.

RESULTS

β-catenin was expressed throughout the TM, with localization primarily to the plasma membrane. Incubation of TM cells with lithium chloride increased the expression of active β-catenin. Lithium chloride treatment upregulated miR-29b expression, and suppressed the levels of various ECM proteins under both basal and TGF-β2 stimulatory conditions. Infection of TM cells with a dominant-negative TCF4 mutant induced ECM levels without a significant change in the expression of the miR-29 family.

CONCLUSIONS

Collectively, our data identify the canonical Wnt signaling pathway as an important modulator of ECM expression in the TM and provide a mechanistic framework for its regulation of outflow facility and IOP.

A laser-induced mouse model with long-term intraocular pressure elevation

Purpose

To develop and characterize a mouse model with intraocular pressure (IOP) elevation after laser photocoagulation on the trabecular meshwork (TM), which may serve as a model to investigate the potential of stem cell-based therapies for glaucoma.

Methods

IOP was measured in 281 adult C57BL/6 mice to determine normal IOP range. IOP elevation was induced unilaterally in 50 adult mice, by targeting the TM through the limbus with a 532-nm diode laser. IOP was measured up to 24 weeks post-treatment. The optic nerve damage was detected by electroretinography and assessed by semiautomatic counting of optic nerve axons. Effects of laser treatment on the TM were evaluated by histology, immunofluorescence staining, optical coherence tomography (OCT) and transmission electron microscopy (TEM).

Results

The average IOP of C57BL/6 mice was 14.5±2.6 mmHg (Mean ±SD). After laser treatment, IOP averaged above 20 mmHg throughout the follow-up period of 24 weeks. At 24 weeks, 57% of treated eyes had elevated IOP with the mean IOP of 22.5±2.5 mmHg (Mean ±SED). The difference of average axon count (59.0%) between laser treated and untreated eyes was statistically significant. Photopic negative response (PhNR) by electroretinography was significantly decreased. CD45+ inflammatory cells invaded the TM within 1 week. The expression of SPARC was increased in the TM from 1 to 12 weeks. Histology showed the anterior chamber angle open after laser treatment. OCT indicated that most of the eyes with laser treatment had no synechia in the anterior chamber angles. TEM demonstrated disorganized and compacted extracellular matrix in the TM.

Conclusions

An experimental murine ocular hypertension model with an open angle and optic nerve axon loss was produced with laser photocoagulation, which could be used to investigate stem cell-based therapies for restoration of the outflow pathway integrity for ocular hypertension or glaucoma.

TGF-β2-mediated ocular hypertension is attenuated in SPARC-null mice

PURPOSE

Transforming growth factor-β2 (TGF-β2) has been implicated in the pathogenesis of primary open-angle glaucoma through extracellular matrix (ECM) alteration among various mechanisms. Secreted protein acidic and rich in cysteine (SPARC) is a matricellular protein that regulates ECM within the trabecular meshwork (TM), and is highly upregulated by TGF-β2. We hypothesized that, in vivo, SPARC is a critical regulatory node in TGF-β2-mediated ocular hypertension.

METHODS

Empty (Ad.empty) or TGF-β2-containing adenovirus (Ad.TGF-β2) was injected intravitreally into C57BL6-SV129 WT and SPARC-null mice. An initial study was performed to identify a stable period for IOP measurement under isoflurane. The IOP was measured before injection and every other day for two weeks using rebound tonometry. Additional mice were euthanized at peak IOP for immunohistochemistry.

RESULTS

The IOP was stable under isoflurane during minutes 5 to 8. The IOP was significantly elevated in Ad.TGF-β2-injected (n = 8) versus Ad.empty-injected WT (n = 8) mice and contralateral uninjected eyes during days 4 to 11 (P < 0.03). The IOPs were not significantly elevated in Ad.TGF-β2-injected versus Ad.empty-injected SPARC-null mice. However, on day 8, the IOP of Ad.TGF-β2-injected SPARC-null eyes was elevated compared to that of contralateral uninjected eyes (P = 0.0385). Immunohistochemistry demonstrated that TGF-β2 stimulated increases in collagen IV, fibronectin, plasminogen activator inhibitor-1 (PAI-1), connective tissue growth factor (CTGF), and SPARC in WT mice, but only PAI-1 and CTGF in SPARC-null mice (P < 0.05).

CONCLUSIONS

SPARC is essential to the regulation of TGF-β2-mediated ocular hypertension. Deletion of SPARC significantly attenuates the effects of TGF-β2 by restricting collagen IV and fibronectin expression. These data provide further evidence that SPARC may have an important role in IOP regulation and possibly glaucoma pathogenesis.

Matricellular proteins in the trabecular meshwork: review and update

Abstract Primary open-angle glaucoma (POAG) is a leading cause of blindness worldwide, and intraocular pressure (IOP) is an important modifiable risk factor. IOP is a function of aqueous humor production and aqueous humor outflow, and it is thought that prolonged IOP elevation leads to optic nerve damage over time. Within the trabecular meshwork (TM), the eye's primary drainage system for aqueous humor, matricellular proteins generally allow cells to modulate their attachments with and alter the characteristics of their surrounding extracellular matrix (ECM). It is now well established that ECM turnover in the TM affects outflow facility, and matricellular proteins are emerging as significant players in IOP regulation. The formalized study of matricellular proteins in TM has gained increased attention. Secreted protein acidic and rich in cysteine (SPARC), myocilin, connective tissue growth factor (CTGF), and thrombospondin-1 and -2 (TSP-1 and -2) have been localized to the TM, and a growing body of evidence suggests that these matricellular proteins play an important role in IOP regulation and possibly the pathophysiology of POAG. As evidence continues to emerge, these proteins are now seen as potential therapeutic targets. Further study is warranted to assess their utility in treating glaucoma in humans.

The role of matricellular proteins in glaucoma

Glaucoma is an optic neuropathy affecting approximately 60million people worldwide and is the second most common cause of irreversible blindness. Elevated intraocular pressure (IOP) is the main risk factor for developing glaucoma and is caused by impaired aqueous humor drainage through the trabecular meshwork (TM) and Schlemm's canal (SC). In primary open angle glaucoma (POAG), this elevation in IOP in turn leads to deformation at the optic nerve head (ONH) specifically at the lamina cribrosa (LC) region where there is also a deposition of extracellular matrix (ECM) molecules such as collagen and fibronectin. Matricellular proteins are non-structural secreted glycoproteins that help cells communicate with their surrounding ECM. This family of proteins includes connective tissue growth factor (CTGF), also known as CCN2, thrombospondins (TSPs), secreted protein acidic and rich in cysteine (SPARC), periostin, osteonectin, and Tenascin-C and -X and other ECM proteins. All members appear to play a role in fibrosis and increased ECM deposition. Most are widely expressed in tissues particularly in the TM and ONH and deficiency of TSP1 and SPARC have been shown to lower IOP in mouse models of glaucoma through enhanced outflow facility. The role of these proteins in glaucoma is emerging as some have an association with the pathophysiology of the TM and LC regions and might therefore be potential targets for therapeutic intervention in glaucoma.

AMP-activated protein kinase regulates intraocular pressure, extracellular matrix, and cytoskeleton in trabecular meshwork

PURPOSE

In this study, we investigate how adenosine monophosphate-activated protein kinase (AMPK) affects extracellular matrix (ECM) and cellular tone in the trabecular meshwork (TM), and examine how deletion of its catalytic α2 subunit affects IOP and aqueous humor clearance in mice.

METHODS

Human TM tissue was examined for expression of AMPKα1 and AMPKα2, genomically distinct isoforms of the AMPK catalytic subunit. Primary cultured human TM cells were treated for 24 hours with the AMPK activator 5-amino-1-β-Dffff-ribofuranosyl-imidazole-4-carboxamide (AICAR), under basal or TGF-β2 stimulatory conditions. Conditioned media (CM) was probed for secreted protein acidic and rich in cysteine (SPARC), thrombospondin-1 (TSP-1), and ECM proteins, and cells were stained for F-actin. Cells underwent adenoviral infection with a dominant negative AMPKα subunit (ad.DN.AMPKα) and were similarly analyzed. Intraocular pressure, central corneal thickness (CCT), and aqueous clearance were measured in AMPKα2-null and wild-type (WT) mice.

RESULTS

Both AMPKα1 and AMPKα2 are expressed in TM. AICAR activated AMPKα and suppressed the expression of various ECM proteins under basal and TGF-β2 stimulatory conditions. AICAR decreased F-actin staining and increased the phospho-total RhoA ratio (Ser188). Transforming growth factor-β2 transiently dephosphorylated AMPKα. Infection with ad.DN.AMPKα upregulated various ECM proteins, decreased the phospho-total RhoA ratio, and increased F-actin staining. AMPKα2-null mice exhibited 6% higher IOP and decreased aqueous clearance compared with WT mice, without significant differences in CCT or angle morphology.

CONCLUSIONS

Collectively, our data identify AMPK as a critical regulator of ECM homeostasis and cytoskeletal arrangement in the TM. Mice that are AMPKα2-null exhibit higher IOPs and decreased aqueous clearance than their WT counterparts.

Pharmacological regulation of SPARC by lovastatin in human trabecular meshwork cells

PURPOSE

Statins have been shown to increase aqueous outflow facility. The matricellular protein SPARC (secreted protein acidic and rich in cysteine) is a critical mediator of aqueous outflow and intraocular pressure (IOP). Here, we examine the effects of lovastatin on SPARC expression in trabecular meshwork (TM) cells, exploring the molecular mechanisms involved.

METHODS

Primary cultured human TM cells were incubated for 24, 48, and 72 hours with 10 μM lovastatin. In separate cultures, media was supplemented with either farnesyl pyrophosphate (FPP) or geranylgeranyl pyrophosphate (GGPP) for the duration of the 72-hour time point experiment. Trabecular meshwork cells were also pretreated for 24 hours with lovastatin followed by 24-hour stimulation with 3 ng/mL TGF-β2. Cell lysates and media were harvested and relative mRNA and protein level changes were determined. Krüppel-like factor 4 (KLF4) localization in normal human anterior segments was examined by immunofluorescence. Adenovirus expressing human KLF4 was used and relative changes in SPARC mRNA and protein levels were assessed.

RESULTS

Incubating TM cells with lovastatin suppressed SPARC mRNA and protein levels. This effect was reversed upon media supplementation with GGPP but not FPP. Pretreating cells with lovastatin inhibited TGF-β2 induction of SPARC. The KLF4 transcription factor was expressed throughout the TM and the inner and outer walls of Schlemm's canal. Lovastatin treatment upregulated KLF4 mRNA and protein levels. Overexpression of KLF4 downregulated SPARC expression.

CONCLUSIONS

Collectively, our data identify lovastatin as an important pharmacological suppressor of SPARC expression in TM cells, and provide further insight into the molecular mechanisms mediating statin enhancement of aqueous outflow facility.

The effects of tenascin C knockdown on trabecular meshwork outflow resistance

PURPOSE

Tenascin C (TNC) is a matricellular glycoprotein whose expression in adult tissue is indicative of tissue remodeling. The purpose of the current study was to determine the localization of TNC in trabecular meshwork (TM) tissue and to analyze the effects of TNC on intraocular pressure (IOP).

METHODS

Human TM frontal sections were immunostained with anti-TNC and imaged by confocal microscopy. TNC mRNA and protein levels were quantitated in anterior segments perfused at physiological and elevated pressure. Short, hairpin RNA (shRNA) silencing lentivirus targeting full-length TNC (shTNC) was applied to anterior segment perfusion organ cultures. The IOPs and central corneal thickness (CCT) of wild-type, TNC(-/-), and tenascin X (TNX(-/-)) knockout mice were measured.

RESULTS

TNC was distributed in the juxtacanalicular (JCT) region of adult human TM, predominantly in the basement membrane underlying the inner wall of Schlemm's canal. Application of shTNC lentivirus to human and porcine anterior segments in perfusion culture did not significantly affect outflow rate. Although TNC was upregulated in response to pressure, there was no difference in outflow rate when shTNC-silenced anterior segments were subjected to elevated pressure. Furthermore, IOPs and CCTs were not significantly different between TNC(-/-) or TNX(-/-) and wild-type mice.

CONCLUSIONS

TNC does not appear to contribute directly to outflow resistance. However, TNC immunolocalization in the JCT of adult human eyes suggests that certain areas of the TM are being continuously remodeled with or without an IOP increase.