Cell senescence reduced the mechanotransduction sensitivity of porcine angular aqueous plexus cells to elevation of pressure

Comparative analysis of traction forces in normal and glaucomatous trabecular meshwork cells within a 3D, active fluid-structure interaction culture environment

This study presents a 3D in vitro cell culture model, meticulously 3D printed to replicate the conventional aqueous outflow pathway anatomical structure, facilitating the study of trabecular meshwork (TM) cellular responses under glaucomatous conditions. Glaucoma affects TM cell functionality, leading to extracellular matrix (ECM) stiffening, enhanced cell-ECM adhesion, and obstructed aqueous humor outflow. Our model, reconstructed from polyacrylamide gel with elastic moduli of 1.5 and 21.7 kPa, is based on serial block-face scanning electron microscopy images of the outflow pathway. It allows for quantifying 3D, depth-dependent, dynamic traction forces exerted by both normal and glaucomatous TM cells within an active fluid-structure interaction (FSI) environment. In our experimental design, we designed two scenarios: a control group with TM cells observed over 20 hours without flow (static setting), focusing on intrinsic cellular contractile forces, and a second scenario incorporating active FSI to evaluate its impact on traction forces (dynamic setting). Our observations revealed that active FSI results in higher traction forces (normal: 1.83-fold and glaucoma: 2.24-fold) and shear strains (normal: 1.81-fold and glaucoma: 2.41-fold), with stiffer substrates amplifying this effect. Glaucomatous cells consistently exhibited larger forces than normal cells. Increasing gel stiffness led to enhanced stress fiber formation in TM cells, particularly in glaucomatous cells. Exposure to active FSI dramatically altered actin organization in both normal and glaucomatous TM cells, particularly affecting cortical actin stress fiber arrangement. This model while preliminary offers a new method in understanding TM cell biomechanics and ECM stiffening in glaucoma, highlighting the importance of FSI in these processes. STATEMENT OF SIGNIFICANCE: This pioneering project presents an advanced 3D in vitro model, meticulously replicating the human trabecular meshwork's anatomy for glaucoma research. It enables precise quantification of cellular forces in a dynamic fluid-structure interaction, a leap forward from existing 2D models. This advancement promises significant insights into trabecular meshwork cell biomechanics and the stiffening of the extracellular matrix in glaucoma, offering potential pathways for innovative treatments. This research is positioned at the forefront of ocular disease study, with implications that extend to broader biomedical applications.

The Effect of Intraocular Pressure Load Boundary on the Biomechanics of the Human Conventional Aqueous Outflow Pathway

BACKGROUND

Aqueous humor outflow resistance in the trabecular meshwork (TM), juxtacanalicular connective tissue (JCT), and Schlemm's canal (SC) endothelium of the conventional outflow pathway actively contribute to intraocular pressure (IOP) regulation. Outflow resistance is actively affected by the dynamic outflow pressure gradient across the TM, JCT, and SC inner wall tissues. The resistance effect implies the presence of a fluid-structure interaction (FSI) coupling between the outflow tissues and the aqueous humor. However, the biomechanical interactions between viscoelastic outflow tissues and aqueous humor dynamics are largely unknown.

METHODS

A 3D microstructural finite element (FE) model of a healthy human eye TM/JCT/SC complex was constructed with elastic and viscoelastic material properties for the bulk extracellular matrix and embedded elastic cable elements. The FE models were subjected to both idealized and a physiologic IOP load boundary using the FSI method.

RESULTS

The elastic material model for both the idealized and physiologic IOP load boundary at equal IOPs showed similar stresses and strains in the outflow tissues as well as pressure in the aqueous humor. However, outflow tissues with viscoelastic material properties were sensitive to the IOP load rate, resulting in different mechanical and hydrodynamic responses in the tissues and aqueous humor.

CONCLUSIONS

Transient IOP fluctuations may cause a relatively large IOP difference of ~20 mmHg in a very short time frame of ~0.1 s, resulting in a rate stiffening in the outflow tissues. Rate stiffening reduces strains and causes a rate-dependent pressure gradient across the outflow tissues. Thus, the results suggest it is necessary to use a viscoelastic material model in outflow tissues that includes the important role of IOP load rate.

Viscoelastic Biomechanical Properties of the Conventional Aqueous Outflow Pathway Tissues in Healthy and Glaucoma Human Eyes

BACKGROUND

Although the tissues comprising the ocular conventional outflow pathway have shown strong viscoelastic mechanical response to aqueous humor pressure dynamics, the viscoelastic mechanical properties of the trabecular meshwork (TM), juxtacanalicular connective tissue (JCT), and Schlemm's canal (SC) inner wall are largely unknown.

METHODS

A quadrant of the anterior segment from two human donor eyes at low- and high-flow (LF and HF) outflow regions was pressurized and imaged using optical coherence tomography (OCT). A finite element (FE) model of the TM, the adjacent JCT, and the SC inner wall was constructed and viscoelastic beam elements were distributed in the extracellular matrix (ECM) of the TM and JCT to represent anisotropic collagen. An inverse FE-optimization algorithm was used to calculate the viscoelastic properties of the ECM/beam elements such that the TM/JCT/SC model and OCT imaging data best matched over time.

RESULTS

The ECM of the glaucoma tissues showed significantly larger time-dependent shear moduli compared to the heathy tissues. Significantly larger shear moduli were also observed in the LF regions of both the healthy and glaucoma eyes compared to the HF regions.

CONCLUSIONS

The outflow tissues in both glaucoma eyes and HF regions are stiffer and less able to respond to dynamic IOP.

Modeling the biomechanics of the conventional aqueous outflow pathway microstructure in the human eye

BACKGROUND AND OBJECTIVE

Intraocular pressure (IOP) is determined by aqueous humor outflow resistance, which is a function of the combined resistance of Schlemm's canal (SC) endothelium and the trabecular meshwork (TM) and their interactions in the juxtacanalicular connective tissue (JCT) region. Aqueous outflow in the conventional outflow pathway results in pressure gradient across the TM, JCT, and SC inner wall, and induces mechanical stresses and strains that influence the geometry and homeostasis of the outflow system. The outflow resistance is affected by alteration in tissues' geometry, so there is potential for active, two-way, fluid-structure interaction (FSI) coupling between the aqueous humor (fluid) and the TM, JCT, and SC inner wall (structure). However, our understanding of the biomechanical interactions of the aqueous humor with the outflow connective tissues and its contribution to the outflow resistance regulation is incomplete.

METHODS

In this study, a microstructural finite element (FE) model of a human eye TM, JCT, and SC inner wall was constructed from a segmented, high-resolution histologic 3D reconstruction of the human outflow system. Three different elastic moduli (0.004, 0.128, and 51.5 MPa based on prior reports) were assigned to the TM/JCT complex while the elastic modulus of the SC inner wall was kept constant at 0.00748 MPa. The hydraulic conductivity was programmed separately for the TM, JCT, and SC inner wall using a custom subroutine. Cable elements were embedded into the TM and JCT extracellular matrix to represent the directional stiffness imparted by anisotropic collagen fibril orientation. The resultant stresses and strains in the outflow system were calculated using fluid-structure interaction method.

RESULTS

The higher TM/JCT stiffness resulted in larger stresses, but smaller strains in the outflow connective tissues, and resulted in a 4- and 5-fold larger pressure drop across the SC inner wall, respectively, compared to the most compliant model. Funneling through µm-sized SC endothelial pores was evident in the models at lower tissue stiffness, but aqueous flow was more turbulent in models with higher TM/JCT stiffness.

CONCLUSIONS

The mechanical properties of the outflow tissues play a crucial role in the hydrodynamics of the aqueous humor in the conventional outflow system.

The Application of Nitric Oxide for Ocular Hypertension Treatment

Despite of various therapeutic methods for treating ocular hypertension and glaucoma, it still remains the leading cause of irreversible blindness. Intraocular pressure (IOP) lowering is the most effective way to slow disease progression and prevent blindness. Among the ocular hypotensive drugs currently in use, only a couple act on the conventional outflow system, which is the main pathway for aqueous humor outflow and the major lesion site resulting in ocular hypertension. Nitric oxide (NO) is a commendable new class of glaucoma drugs that acts on the conventional outflow pathway. An increasing number of nitric oxide donors have been developed for glaucoma and ocular hypertension treatment. Here, we will review how NO lowers IOP and the types of nitric oxide donors that have been developed. And a brief analysis of the advantages and challenges associated with the application will be made. The literature used in this review is based on Pubmed database search using ‘nitric oxide’ and ‘glaucoma’ as key words.

Endogenous dual stimuli-activated NO generation in the conventional outflow pathway for precision glaucoma therapy

High intraocular pressure (IOP) has been regarded as a predominant risk factor for glaucoma. Nitric oxide (NO) is shown to lower IOP, but the magnitude and duration of IOP reduction are not satisfying due to the poor cornea penetration of NO drugs and limited NO generation in the trabecular meshwork (TM)/Schlemm's canal (SC) area. Herein, we introduce deep cornea penetrating biodegradable hollow mesoporous organosilica (HOS) nanocapsules for the efficient co-delivery of hydrophobic JS-K (J_R) and hydrophilic l-Arginine (L_O). The resulting HOS-J_RL_O can be reduced and oxidized by the ascorbic acid (AA) and catalysis of endothelial nitric oxide synthase (eNOS) in the TM/SC microenvironment to release NO for inducing appreciable IOP reduction in various glaucoma mouse models. In addition to developing an endogenous stimuli-responsive NO nanotherapeutic, this study is also expected to establish a versatile, non-invasive, and efficacious treatment paradigm for precision glaucoma therapy.

miR-21-5p: A viable therapeutic strategy for regulating intraocular pressure

Lowering intraocular pressure (IOP) is the most effective treatment of glaucoma, however most of the current available glaucoma drugs target a single molecule. MicroRNAs (miRNAs) are noncoding RNAs that target a network of molecules. This study aims to investigate the role of miR-21-5p in regulating IOP and the mechanism of function. miR-21-5p mimics was topically applied to C57/BL6 mouse eyes, which significantly increased miR-21-5p expression in the conventional outflow tissue and reduced IOP by a maximum of 17.77% at 24 h after treatment. The conventional outflow facility measured by ex vivo moue eye perfusion of miR-21-5p was significantly increased by 60.14%. Moreover, miR-21-5p overexpression significantly reduced the transendothelial electrical resistance in porcine angular aqueous plexus cells. Transcriptome analysis and further quantification by Western blot and PCR revealed that SMAD7 and FGF18 might be the downstream target of miR-21-5p in regulating aqueous humor outflow. The predicted functional pathways PTEN/eNOS, RhoB/pMLC and TIMP3/MMP9 were significantly altered after miR-21-5p transfection. Dual luciferase assay verified the direct targets of miR-21-5p. In conclusion, miR-21-5p seems to regulate IOP by modulating multiple genes that are associated with aqueous humor outflow, including genes those regulating cell adhesion, cytoskeletal dynamics and extracellular matrix turnover. Thus, miR-21-5p represents a new therapeutic strategy for glaucoma and a viable alternative to existing multidrug regimens.

Cell senescence altered the miRNA expression profile in porcine angular aqueous plexus cells

Purpose

This study investigates the impact of aging on the miRNA expression profile in porcine angular aqueous plexus (AAP) cells, which are the porcine equivalent of human Schlemm's canal endothelial cells.

Methods

AAP endothelial cells were isolated and cultured in physiologic (5% O₂) or hyperoxic condition (40% O₂) for 14 days to induce cell senescence. miRNA and protein expression profiles of control and senescent cells were analyzed with miRNA microarray and isobaric tags for relative and absolute quantification (iTRAQ), respectively.

Results

The miRNA microarray identified 33 differentially expressed miRNAs in senescent cells compared with controls (p<0.05), and quantitative real-time PCR (qRT-PCR) confirmed 12 of them (p<0.05). iTRAQ analysis identified 148 upregulated and 222 downregulated proteins (p<0.05, fold change>1.2). Bioinformatics analysis of miRNA microarray and proteomics data predicted that six out of seven miRNAs are associated with aqueous humor outflow by targeting integrin and the downstream pathways (Src/Rho kinase, focal adhesion kinase (FAK)/NO-cGMP), and one miRNA might influence gap junction by targeting the Inositol trisphosphate receptor (IP3R) /Protein kinase C (PKC) pathway.

Conclusions

This study identified miRNAs in senescent AAP cells that might regulate aqueous humor outflow by targeting proteins involved in focal adhesion, cytoskeleton, NO-cGMP signaling, and gap junction.

Regulation of Epithelial Cell Functions by the Osmolality and Hydrostatic Pressure Gradients: A Possible Role of the Tight Junction as a Sensor

Epithelia act as a barrier to the external environment. The extracellular environment constantly changes, and the epithelia are required to regulate their function in accordance with the changes in the environment. It has been reported that a difference of the environment between the apical and basal sides of epithelia such as osmolality and hydrostatic pressure affects various epithelial functions including transepithelial transport, cytoskeleton, and cell proliferation. In this paper, we review the regulation of epithelial functions by the gradients of osmolality and hydrostatic pressure. We also examine the significance of this regulation in pathological conditions especially focusing on the role of the hydrostatic pressure gradient in the pathogenesis of carcinomas. Furthermore, we discuss the mechanism by which epithelia sense the osmotic and hydrostatic pressure gradients and the possible role of the tight junction as a sensor of the extracellular environment to regulate epithelial functions.

Outflow enhancement by three different ab interno trabeculectomy procedures in a porcine anterior segment model

PURPOSE

To evaluate three different microincisional ab interno trabeculectomy procedures in a porcine eye perfusion model.

METHODS

In perfused porcine anterior segments, 90° of trabecular meshwork (TM) was ablated using the Trabectome (T; n = 8), Goniotome (G; n = 8), or Kahook device (K; n = 8). After 24 h, additional 90° of TM was removed. Intraocular pressure (IOP) and outflow facility were measured at 5 and 10 μl/min perfusion to simulate an elevated IOP. Structure and function were assessed with canalograms and histology.

RESULTS

At 5 μl/min infusion rate, T resulted in a greater IOP reduction than G or K from baseline (76.12% decrease versus 48.19% and 47.96%, P = 0.013). IOP reduction between G and K was similar (P = 0.420). Removing another 90° of TM caused an additional IOP reduction only in T and G but not in K. Similarly, T resulted in the largest increase in outflow facility at 5 μl/min compared with G and K (first ablation, 3.41 times increase versus 1.95 and 1.87; second ablation, 4.60 versus 2.50 and 1.74) with similar results at 10 μl/min (first ablation, 3.28 versus 2.29 and 1.90 (P = 0.001); second ablation, 4.10 versus 3.01 and 2.01 (P = 0.001)). Canalograms indicated circumferential flow beyond the ablation endpoints.

CONCLUSIONS

T, G, and K significantly increased the outflow facility. In this model, T had a larger effect than G and K.

Model systems for the study of steroid-induced IOP elevation

Steroid-induced IOP elevation affects a significant number of patients. It results from a decrease in outflow facility of the aqueous humor. To understand the pathophysiology of this condition a number of model systems have been created. These include ex-vivo cell and organ cultures as well as in-vivo animal models in organisms ranging from rodents to primates. These model systems can be used to investigate specific aspects of steroid-induced IOP elevation. This brief review summarizes the strengths and limitations of the various model systems and provides examples of where these systems have been successfully used to advance our understanding of steroid-induced IOP elevation.

The Outflow Pathway: A Tissue With Morphological and Functional Unity

The trabecular meshwork (TM) plays an important role in high-tension glaucomas. Indeed, the TM is a true organ, through which the aqueous humor flows from the anterior chamber to Schlemm's canal (SC). Until recently, the TM, which is constituted by endothelial-like cells, was described as a kind of passive filter. In reality, it is much more. The cells delineating the structures of the collagen framework of the TM are endowed with a cytoskeleton, and are thus able to change their shape. These cells also have the ability to secrete the extracellular matrix, which expresses proteins and cytokines, and are capable of phagocytosis and autophagy. The cytoskeleton is attached to the nuclear membrane and can, in millionths of a second, send signals to the nucleus in order to alter the expression of genes in an attempt to adapt to biomechanical insult. Oxidative stress, as happens in aging, has a deleterious effect on the TM, leading eventually to cell decay, tissue malfunction, subclinical inflammation, changes in the extracellular matrix and cytoskeleton, altered motility, reduced outflow facility, and (ultimately) increased IOP. TM failure is the most relevant factor in the cascade of events triggering apoptosis in the inner retinal layers, including ganglion cells. J. Cell. Physiol. 231: 1876-1893, 2016. © 2016 Wiley Periodicals, Inc.

Endothelial nitric oxide synthase-related mechanotransduction changes in aged porcine angular aqueous plexus cells

PURPOSE

To investigate effects of aging on endothelial nitric oxide synthase (eNOS) expression and signaling in angular aqueous plexus (AAP) (functional equivalent to human Schlemm's canal) cells subjected to shear stress.

METHODS

The AAP cells were isolated differentially from porcine outflow tissues using puromycin selection. Cell aging was induced by culturing cells in hyperoxia condition (40% oxygen and 5% carbon dioxide) for 14 days. The AAP cells grown in chamber slides were exposed to a shear stress of 8 dynes/cm(2) for 24 hours. Expression of eNOS, eNOS-phospho Thr495, eNOS-phospho Ser1177, and Akt-phospho was tested by Western blot analysis and immunofluorescence staining. Nitric oxide (NO) levels were measured using the Griess assay.

RESULTS

Compared with control, eNOS levels in aged cells were significantly reduced by 60% (P < 0.05; n = 6). Phosphorylation of eNOS at Ser1177 and Akt at Ser473 was 63% and 80% lower in aged cells, respectively, whereas phosphorylation of the eNOS inhibition site (Thr495) increased by 6.1-fold (P < 0.05; n = 6). Shear stress (8 dynes/cm(2) for 24 hours) increased eNOS abundance (total protein and at cell borders) and phosphorylation at Ser1177 by 1.7-fold and 1.8-fold, respectively (P < 0.05; n = 6), whereas aged cells were unresponsive. In control cells exposed to shear stress, the NO concentration was 1.8-fold higher than in the static group (P < 0.05; n = 4); however, aged cells were unresponsive to shear stress (mean ± SD, 4.3 ± 1.3 vs. 4.1 ± 1.4 μM).

CONCLUSIONS

Aged AAP cells appear compromised in their mechanotransduction machinery involving eNOS, the protein product of the gene, NOS3, polymorphisms of which impart a risk for the development of glaucoma.