The formation of cortical actin arrays in human trabecular meshwork cells in response to cytoskeletal disruption

Cross-linked actin networks (CLANs) affect stiffness and/or actin dynamics in transgenic transformed and primary human trabecular meshwork cells

Cross-linked actin networks (CLANs) in trabecular meshwork (TM) cells may contribute to increased IOP by altering TM cell function and stiffness. However, there is a lack of direct evidence. Here, we developed transformed TM cells that form spontaneous fluorescently labelled CLANs. The stable cells were constructed by transducing transformed glaucomatous TM (GTM3) cells with the pLenti-LifeAct-EGFP-BlastR lentiviral vector and selection with blasticidin. The stiffness of the GTM3-LifeAct-GFP cells were studied using atomic force microscopy. Elastic moduli of CLANs in primary human TM cells treated with/without dexamethasone/TGFβ2 were also measured to validate findings in GTM3-LifeAct-GFP cells. Live-cell imaging was performed on GTM3-LifeAct-GFP cells treated with 1 μM latrunculin B or pHrodo bioparticles to determine actin stability and phagocytosis, respectively. The GTM3-LifeAct-GFP cells formed spontaneous CLANs without the induction of TGFβ2 or dexamethasone. The CLAN containing cells showed elevated cell stiffness, resistance to latrunculin B-induced actin depolymerization, as well as compromised phagocytosis, compared to the cells without CLANs. Primary human TM cells with dexamethasone or TGFβ2-induced CLANs were also stiffer and less phagocytic. The GTM3-LifeAct-GFP cells are a novel tool for studying the mechanobiology and pathology of CLANs in the TM. Initial characterization of these cells showed that CLANs contribute to at least some glaucomatous phenotypes of TM cells.

The Dual Effect of Rho-Kinase Inhibition on Trabecular Meshwork Cells Cytoskeleton and Extracellular Matrix in an In Vitro Model of Glaucoma

The trabecular meshwork (TM) is the main site of drainage of the aqueous humor, and its dysfunction leads to intraocular pressure elevation, which is one of the main risk factors of glaucoma. We aimed to compare the effects on cytoskeleton organization and extracellular matrix (ECM) of latanoprost (LT) and a Rho-kinase inhibitor (ROCKi) on a transforming growth factor beta2 (TGF-β2)-induced glaucoma-like model developed from primary culture of human TM cells (pHTMC). The TGF-β2 stimulated pHTMC were grown and incubated with LT or a ROCKi (Y-27632) for 24 h. The expression of alpha-smooth muscle actin (αSMA) and fibronectin (FN), and phosphorylation of the myosin light chain (MLC-P) and Cofilin (Cofilin-P) were evaluated using immunofluorescence and Western blot. The architectural modifications were studied in a Matrigel^TM 3D culture. TGF-β2 increased the expression of αSMA and FN in pHTMC and modified the cytoskeleton with cross-linked actin network formation. LT did not alter the expression of αSMA but decreased FN deposition. The ROCKi decreased TGF-β2-induced αSMA and FN expression, as well as MLC-P and Cofilin-P, and stimulated the cells to recover a basal cytoskeletal arrangement. In the preliminary 3D study, pHTMC organized in a mesh conformation showed the widening of the TM under the effect of Y-27632. By simultaneously modifying the organization of the cytoskeleton and the ECM, with fibronectin deposition and overexpression, TGF-β2 reproduced the trabecular degeneration described in glaucoma. The ROCKi was able to reverse the TGF-β2-induced cytoskeletal and ECM rearrangements. LT loosened the extracellular matrix but had no action on the stress fibers.

Ripasudil in a Model of Pigmentary Glaucoma

Purpose

To investigate the effects of Ripasudil (K-115), a Rho-kinase inhibitor, in a porcine model of pigmentary glaucoma.

Methods

In vitro trabecular meshwork (TM) cells and ex vivo perfused eyes were subjected to pigment dispersion followed by K-115 treatment (PK115). PK115 was compared to controls (C) and pigment (P). Cytoskeletal alterations were assessed by F-actin labeling. TM cell phagocytosis of fluorescent targets was evaluated by flow cytometry. Cell migration was studied with a wound-healing assay. Intraocular pressure was continuously monitored and compared to after the establishment of the pigmentary glaucoma model and after treatment with K-115.

Results

The percentage of cells with stress fibers increased in response to pigment but declined sharply after treatment with K-115 (P: 32.8% ± 2.9%; PK115: 11.6% ± 3.3%, P < 0.001). Phagocytosis first declined but recovered after K-115 (P: 25.7% ± 2.1%, PK115: 33.4% ± 0.8%, P <0.01). Migration recuperated at 12 hours with K-115 treatment (P: 19.1 ± 4.6 cells/high-power field, PK115: 42.5 ± 1.6 cells/high-power field, P < 0.001). Ex vivo, eyes became hypertensive from pigment dispersion but were normotensive after treatment with K-115 (P: 20.3 ± 1.2 mm Hg, PK115: 8.9 ± 1.7 mm Hg; P < 0.005).

Conclusions

In vitro, K-115 reduced TM stress fibers, restored phagocytosis, and restored migration of TM cells. Ex vivo, K-115 normalized intraocular pressure.

Translational Relevance

This ex vivo pigmentary glaucoma model provides a readily available basis to investigate new drugs such as the rho-kinase inhibitor studied here.

Establishment of human trabecular meshwork cell cultures using nontransplantable corneoscleral rims

Human trabecular meshwork (hTM) cell isolation in academic settings utilizes the motile nature of these cells, allowing them to migrate away from the explant and proliferate on distal regions of the culture substrate. Corneoscleral rims used for transplantation are a potential source of explants for the establishment of hTM cell cultures. However, cell isolation and the initiation of primary cell cultures from ocular tissues stored in Optisol-GS medium for an extended period of time (>6 days) has proven difficult, since Optisol-GS remarkably reduces cell viability and cellularity. Therefore, explants obtained from ocular tissues stored in Optisol-GS do not often provide adequate cell yield to initiate primary cell cultures if conventional culture techniques are used. Therefore, the majority of the research on primary hTM cell isolation has been accomplished using donor tissue obtained within 72 h postmortem. The goal of this study was to develop an hTM cell isolation procedure from nontransplantable ocular materials, utilizing the anchorage dependency of TM cells. This procedure yielded functionally viable cells, eficiently dissociated from the trabecular meshwork. Isolated cells demonstrated typical hTM cell characteristics including monolayer formation, contact inhibition, phagocytosis, and responses to glucocorticoid exposure. To the best of our knowledge, this is the first time an expired explant has been utilized in the successful isolation of hTM cells. Our results clearly demonstrate the advantage of increasing the anchor points of hTM cells for enhanced cell migration out from the explants, which have limited cell proliferative capacity.

Impact of pigment dispersion on trabecular meshwork cells

PURPOSE

Dysfunction of the trabecular meshwork (TM) in pigmentary glaucoma contributes to increased aqueous humor outflow resistance and intraocular pressure. In this study, we investigated the effect of pigment dispersion on trabecular meshwork cells.

METHODS

Porcine TM cells from ab interno trabeculectomy specimens were exposed to pigment dispersion, then, analyzed for changes in morphology, immunostaining, and ultrastructure. Their abilities to phagocytose migrate, and contraction was quantified. An expression microarray, using 23,937 probes, and a pathway analysis were performed.

RESULTS

Stress fiber formation was increased in the pigment dispersion group (P) (60.1 ± 0.3%, n = 10) compared to control (C) (38.4 ± 2.5%, n = 11, p < 0.001). Phagocytosis declined (number of cells with microspheres in P = 37.0 ± 1.1% and in C = 68.7 ± 1.3%, n = 3, p < 0.001) and migration was reduced after 6 h (cells within the visual field over 6 h in P = 28.0.1 ± 2.3 (n = 12) and in C = 40.6 ± 3.3 (n = 13), p < 0.01). Pigment induced contraction at 24 h onwards (p < 0.01). Microarray analysis revealed that Rho signaling was central to these responses.

CONCLUSION

Exposure of TM cells to pigment dispersion resulted in reduced phagocytosis and migration, as well as increased stress fiber formation and cell contraction. The Rho signaling pathway played a central and early role, suggesting that its inhibitors could be used as a specific intervention in treatment of pigmentary glaucoma.

Cross-linked actin networks (CLANs) in glaucoma

One of the major causes of decreased vision, irreversible vision loss and blindness worldwide is glaucoma. Increased intraocular pressure (IOP) is a major risk factor associated with glaucoma and its molecular mechanisms are not fully understood. The trabecular meshwork (TM) is the primary site of injury in glaucoma, and its dysfunction results in elevated IOP. The glaucomatous TM has increased extracellular matrix deposition as well as cytoskeletal rearrangements referred to as cross-linked actin networks (CLANs) that consist of dome like structures consisting of hubs and spokes. CLANs are thought to play a role in increased aqueous humor outflow resistance and increased IOP by creating stiffer TM cells and tissue. CLANs are inducible by glucocorticoids (GCs) and TGFβ2 in confluent TM cells and TM tissues. The signaling pathways of these induction agents give insight into the possible mechanisms of CLAN formation, but to date, the mechanism of CLANs regulation by these pathways has yet to be determined. Understanding the role CLANs play in IOP elevation and their mechanisms of induction and regulation may lead to novel treatment options to help prevent or intervene in glaucomatous damage to the trabecular meshwork.

RhoA/ROCK signaling regulates TGFβ-induced epithelial-mesenchymal transition of lens epithelial cells through MRTF-A

Transforming growth factor (TGF)-β-induced epithelial-mesenchymal transition (EMT) leads to the formation of ocular fibrotic pathologies, such as anterior subcapsular cataract and posterior capsule opacification. Remodeling of the actin cytoskeleton, mediated by the Rho family of GTPases, plays a key role in EMT, however, how actin dynamics affect downstream markers of EMT has not been fully determined. Our previous work suggests that myocardin related transcription factor A (MRTF-A), an actin-binding protein, might be an important mediator of TGFβ-induced EMT in lens epithelial cells. The aim of the current study was to determine the requirement of RhoA/ROCK signaling in mediating TGFβ-induced nuclear accumulation of MRTF-A, and ultimate expression of α-smooth muscle actin (αSMA), a marker of a contractile, myofibroblast phenotype. Using rat lens epithelial explants, we demonstrate that ROCK inhibition using Y-27632 prevents TGFβ-induced nuclear accumulation of MRTF-A, E-cadherin/β-catenin complex disassembly, and αSMA expression. Using a novel inhibitor specifically targeting MRTF-A signaling, CCG-203971, we further demonstrate the requirement of MRTF-A nuclear localization and activity in the induction of αSMA expression. Overall, our findings suggest that TGFβ-induced cytoskeletal reorganization through RhoA/ROCK/MRTF-A signaling is critical to EMT of lens epithelial cells.

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.

Wnt inhibition induces persistent increases in intrinsic stiffness of human trabecular meshwork cells

Wnt antagonism has been linked to glaucoma and intraocular pressure regulation, as has increased stiffness of human trabecular meshwork (HTM) tissue. We have shown culturing HTM cells on substrates that mimic the elevated stiffness of glaucomatous tissue leads to elevated expression of the Wnt antagonist secreted frizzled related protein 1 (SFRP1), suggesting a linkage between SFRP1 and HTM mechanobiology. In this study, we document biomechanical consequences of Wnt antagonism on HTM cells. Cells were treated with the Wnt antagonists (SFRP1, KY02111, and LGK-974) for 8 days and allowed to recover for 4 days. After recovery, intrinsic cell stiffness and activation of the Wnt pathway via β-catenin staining and blotting were assayed. Basal cell stiffness values were 3.71 ± 0.37, 4.33 ± 3.07, and 3.07 ± kPa (median ± S.D.) for cells derived from 3 donors. Cell stiffness increased after 0.25 μg/mL (4.32 ± 5.12, 8.86 ± 8.51, 4.84 ± 3.15 kPa) and 0.5 μg/mL (16.75 ± 5.59, 13.18 ± 7.99, and 8.54 ± 5.77 kPa) SFRP1 treatment. Stiffening was observed after 10 μM KY02111 (10.72 ± 5.63 and 6.57 ± 5.53 kPa) as well as LGK-974 (9.60 ± 7.41 and 11.40 ± 9.24 kPa) treatment compared with controls (3.79 ± 1.01 and 5.16 ± 2.14 kPa). Additionally, Wnt inhibition resulted in decreased β-catenin staining and increased phosphorylation at threonine 41 after recovery. In conclusion, this work demonstrates a causal relationship between Wnt inhibition and cell stiffening. Additionally, these findings suggest transient Wnt inhibition resulted in durable modulation of the mechanical phenotype of HTM cells. When placed in context with previous results, these findings provide a causal link between Wnt antagonism and cell stiffness and suggest a feedback loop contributing to glaucoma progression.