Influence of Growth Differentiation Factor 15 on Intraocular Pressure in Mice

Growth differentiation factor 15 (GDF15), a stress-sensitive cytokine, and a distant member of the transforming growth factor β superfamily, has been shown to exhibit increased levels with aging, and in various age-related pathologies. Although GDF15 levels are elevated in the aqueous humor (AH) of glaucoma (optic nerve atrophy) patients, the possible role of this cytokine in the modulation of intraocular pressure (IOP) or AH outflow is unknown. The current study addresses this question using transgenic mice expressing human GDF15 and GDF15 null mice, and by perfusing enucleated mouse eyes with recombinant human GDF15 (rhGDF15). Treatment of primary cultures of human trabecular meshwork cells with a telomerase inhibitor, an endoplasmic reticulum stress-inducing agent, hydrogen peroxide, or an autophagy inhibitor resulted in significant elevation in GDF15 levels relative to the respective control cells. rhGDF15 stimulated modest but significant increases in the expression of genes encoding the extracellular matrix, cell adhesion proteins, and chemokine receptors (C-C chemokine receptor type 2) in human trabecular meshwork cells compared with controls, as deduced from the differential transcriptional profiles using RNA-sequencing analysis. There was a significant increase in IOP in transgenic mice expressing human GDF15, but not in GDF15 null mice, compared with the respective wild-type control mice. The AH outflow facility was decreased in enucleated wild-type mouse eyes perfused with rhGDF15. Light microcopy-based histologic examination of the conventional AH outflow pathway tissues did not reveal identifiable differences between the GDF15-targeted and control mice. Taken together, these results reveal the modest elevation of IOP in mice expressing human GDF15 possibly stemming from decreased AH outflow through the trabecular pathway.

Influence of the calcium voltage-gated channel auxiliary subunit (CACNA2D1) absence on intraocular pressure in mice

The etiology of elevated intraocular pressure (IOP), a major risk factor for glaucoma (optic nerve atrophy), is poorly understood despite continued efforts. Although the gene variant of CACNA2D1 (encoding α2δ1), a calcium voltage-gated channel auxiliary subunit, has been reported to be associated with primary open-angle glaucoma, and the pharmacological mitigation of α2δ1 activity by pregabalin lowers IOP, the cellular basis for α2δ1 role in the modulation of IOP remains unclear. Our recent findings reveled readily detectable levels of α2δ1 and its ligand thrombospondin in the cytoskeletome fraction of human trabecular meshwork (TM) cells. To understand the direct role of α2δ1 in the modulation of IOP, we evaluated α2δ1 null mice for changes in IOP and found a moderate (∼10%) but significant decrease in IOP compared to littermate wild type control mice. Additionally, to gain cellular insights into α2δ1 antagonist (pregabalin) induced IOP changes, we assessed pregabalin's effects on human TM cell actin cytoskeletal organization and cell adhesive interactions in comparison with a Rho kinase inhibitor (Y27632), a known ocular hypotensive agent. Unlike Y27632, pregabalin did not have overt effects on cell morphology, actin cytoskeletal organization, or cell adhesion in human TM cells. These results reveal a modest but significant decrease in IOP in α2δ1 deficient mice, and this response appears to be not associated with the contractile and cell adhesive characteristics of TM cells based on the findings of pregabalin effects on isolated TM cells. Therefore, the mechanism by which pregabalin lowers IOP remains elusive.

Glypican-4 regulated actin cytoskeletal reorganization in glucocorticoid treated trabecular meshwork cells and involvement of Wnt/PCP signaling

A common adverse response to the clinical use of glucocorticoids (GCs) is elevated intraocular pressure (IOP) which is a major risk factor for glaucoma. Elevated IOP arises due to impaired outflow of aqueous humor (AH) through the trabecular meshwork (TM). Although GC-induced changes in actin cytoskeletal dynamics, contractile characteristics, and cell adhesive interactions of TM cells are believed to influence AH outflow and IOP, the molecular mechanisms mediating changes in these cellular characteristics are poorly understood. Our studies focused on evaluating changes in the cytoskeletal and cytoskeletal-associated protein (cytoskeletome) profile of human TM cells treated with dexamethasone (Dex) using label-free mass spectrometric quantification, identified elevated levels of specific proteins known to regulate actin stress fiber formation, contraction, actin networks crosslinking, cell adhesion, and Wnt signaling, including LIMCH1, ArgBP2, CNN3, ITGBL1, CTGF, palladin, FAT1, DIAPH2, EPHA4, SIPA1L1, and GPC4. Several of these proteins colocalized with the actin cytoskeleton and underwent alterations in distribution profile in TM cells treated with Dex, and an inhibitor of Abl/Src kinases. Wnt/Planar Cell Polarity (PCP) signaling agonists-Wnt5a and 5b were detected prominently in the cytoskeletome fraction of TM cells, and studies using siRNA to suppress expression of glypican-4 (GPC4), a known modulator of the Wnt/PCP pathway revealed that GPC4 deficiency impairs Dex induced actin stress fiber formation, and activation of c-Jun N-terminal Kinase (JNK) and Rho kinase. Additionally, while Dex augmented, GPC4 deficiency suppressed the formation of actin stress fibers in TM cells in the presence of Dex and Wnt5a. Taken together, these results identify the GPC4-dependent Wnt/PCP signaling pathway as one of the crucial upstream regulators of Dex induced actin cytoskeletal reorganization and cell adhesion in TM cells, opening an opportunity to target the GPC4/Wnt/PCP pathway for treatment of ocular hypertension in glaucoma.

Glucocorticoids Preferentially Influence Expression of Nucleoskeletal Actin Network and Cell Adhesive Proteins in Human Trabecular Meshwork Cells

Clinical use of glucocorticoids is associated with increased intraocular pressure (IOP), a major risk factor for glaucoma. Glucocorticoids have been reported to induce changes in actin cytoskeletal organization, cell adhesion, extracellular matrix, fibrogenic activity, and mechanical properties of trabecular meshwork (TM) tissue, which plays a crucial role in aqueous humor dynamics and IOP homeostasis. However, we have a limited understanding of the molecular underpinnings regulating these myriad processes in TM cells. To understand how proteins, including cytoskeletal and cell adhesion proteins that are recognized to shuttle between the cytosolic and nuclear regions, influence gene expression and other cellular activities, we used proteomic analysis to characterize the nuclear protein fraction of dexamethasone (Dex) treated human TM cells. Treatment of human TM cells with Dex for 1, 5, or 7 days led to consistent increases (by ≥ two-fold) in the levels of various actin cytoskeletal regulatory, cell adhesive, and vesicle trafficking proteins. Increases (≥two-fold) were also observed in levels of Wnt signaling regulator (glypican-4), actin-binding chromatin modulator (BRG1) and nuclear actin filament depolymerizing protein (MICAL2; microtubule-associated monooxygenase, calponin and LIM domain containing), together with a decrease in tissue plasminogen activator. These changes were independently further confirmed by immunoblotting analysis. Interestingly, deficiency of BRG1 expression blunted the Dex-induced increases in the levels of some of these proteins in TM cells. In summary, these findings indicate that the widely recognized changes in actin cytoskeletal and cell adhesive attributes of TM cells by glucocorticoids involve actin regulated BRG1 chromatin remodeling, nuclear MICAL2, and glypican-4 regulated Wnt signaling upstream of the serum response factor/myocardin controlled transcriptional activity.

Mechanical Load and Piezo1 Channel Regulated Myosin II Activity in Mouse Lenses

The cytoarchitecture and tensile characteristics of ocular lenses play a crucial role in maintaining their transparency and deformability, respectively, which are properties required for the light focusing function of ocular lens. Calcium-dependent myosin-II-regulated contractile characteristics and mechanosensitive ion channel activities are presumed to influence lens shape change and clarity. Here, we investigated the effects of load-induced force and the activity of Piezo channels on mouse lens myosin II activity. Expression of the Piezo1 channel was evident in the mouse lens based on immunoblot and immufluorescence analyses and with the use of a Piezo1-tdT transgenic mouse model. Under ex vivo conditions, change in lens shape induced by the load decreased myosin light chain (MLC) phosphorylation. While the activation of Piezo1 by Yoda1 for one hour led to an increase in the levels of phosphorylated MLC, Yoda1 treatment for an extended period led to opacification in association with increased calpain activity and degradation of membrane proteins in ex vivo mouse lenses. In contrast, inhibition of Piezo1 by GsMTx4 decreased MLC phosphorylation but did not affect the lens tensile properties. This exploratory study reveals a role for the mechanical load and Piezo1 channel activity in the regulation of myosin II activity in lens, which could be relevant to lens shape change during accommodation.

Role of vasorin, an anti-apoptotic, anti-TGF-β and hypoxia-induced glycoprotein in the trabecular meshwork cells and glaucoma

Glaucoma, one of the leading causes of irreversible blindness, is commonly associated with elevated intraocular pressure due to impaired aqueous humour (AH) drainage through the trabecular meshwork. The aetiological mechanisms contributing to impaired AH outflow, however, are poorly understood. Here, we identified the secreted form of vasorin, a transmembrane glycoprotein, as a common constituent of human AH by mass spectrometry and immunoblotting analysis. ELISA assay revealed a significant but marginal decrease in vasorin levels in the AH of primary open‐angle glaucoma patients compared to non‐glaucoma cataract patients. Human trabecular meshwork (HTM) cells were confirmed to express vasorin, which has been shown to possess anti‐apoptotic and anti‐TGF‐β activities. Treatment of HTM cells with vasorin induced actin stress fibres and focal adhesions and suppressed TGF‐β2‐induced SMAD2/3 activation in HTM cells. Additionally, cobalt chloride‐induced hypoxia stimulated a robust elevation in vasorin expression, and vasorin suppressed TNF‐α‐induced cell death in HTM cells. Taken together, these findings reveal the importance of vasorin in maintenance of cell survival, inhibition of TGF‐β induced biological responses in TM cells, and the decreasing trend in vasorin levels in the AH of glaucoma patients suggests a plausible role for vasorin in the pathobiology of ocular hypertension and glaucoma.

Elevated Levels of Growth/Differentiation Factor-15 in the Aqueous Humor and Serum of Glaucoma Patients

Dysregulated levels of growth/differentiation factor-15 (GDF15), a divergent member of the transforming growth factor-beta super family, have been found to be associated with the pathology of various diseases. In this study, we evaluated the levels of GDF15 in aqueous humor (AH) and serum samples derived from primary open-angle glaucoma (POAG) and age- and gender-matched non-glaucoma (cataract) patients to assess the plausible association between GDF15 and POAG. GDF15 levels were determined using an enzyme-linked immunosorbent assay, and data analysis was performed using the Wilcoxon rank sum test, or the Kruskal–Wallis test and linear regression. GDF15 levels in the AH (n = 105) of POAG patients were significantly elevated (by 7.4-fold) compared to cataract patients (n = 117). Serum samples obtained from a subgroup of POAG patients (n = 41) also showed a significant increase in GDF15 levels (by 50%) compared to cataract patients. GDF15 levels were elevated in male, female, African American, and Caucasian POAG patients. This study reveals a significant and marked elevation of GDF15 levels in the AH of POAG patients compared to non-glaucoma cataract control patients. Although serum GDF15 levels were also elevated in POAG patients, the magnitude of difference was much smaller relative to that found in the AH.

Drebrin, an actin-binding protein, is required for lens morphogenesis and growth

BACKGROUND

Lens morphogenesis, architecture, and clarity are known to be critically dependent on actin cytoskeleton organization and cell adhesive interactions. There is limited knowledge, however regarding the identity and role of key proteins regulating actin cytoskeletal organization in the lens. This study investigated the role of drebrin, a developmentally regulated actin-binding protein, in mouse lens development by generating and characterizing a conditional knockout (cKO) mouse model using the Cre-LoxP recombination approach.

RESULTS

Drebrin E, a splice variant of DBN1 is a predominant isoform expressed in the mouse lens and exhibits a maturation-dependent downregulation. Drebrin co-distributes with actin in both epithelium and fibers. Conditional deficiency (both haploinsufficiency and complete absence) of drebrin results in disrupted lens morphogenesis leading to cataract and microphthalmia. The drebrin cKO lens reveals a dramatic decrease in epithelial height and width, E-cadherin, and proliferation, and increased apoptotic cell death and expression of α-smooth muscle actin, together with severely impaired fiber cell organization, polarity, and cell-cell adhesion.

CONCLUSIONS

This study demonstrates the requirement of drebrin in lens development and growth, with drebrin deficiency leading to impaired lens morphogenesis and microphthalmia.

Absence of S100A4 in the mouse lens induces an aberrant retina-specific differentiation program and cataract

S100A4, a member of the S100 family of multifunctional calcium-binding proteins, participates in several physiological and pathological processes. In this study, we demonstrate that S100A4 expression is robustly induced in differentiating fiber cells of the ocular lens and that S100A4 (-/-) knockout mice develop late-onset cortical cataracts. Transcriptome profiling of lenses from S100A4 (-/-) mice revealed a robust increase in the expression of multiple photoreceptor- and Müller glia-specific genes, as well as the olfactory sensory neuron-specific gene, S100A5. This aberrant transcriptional profile is characterized by corresponding increases in the levels of proteins encoded by the aberrantly upregulated genes. Ingenuity pathway network and curated pathway analyses of differentially expressed genes in S100A4 (-/-) lenses identified Crx and Nrl transcription factors as the most significant upstream regulators, and revealed that many of the upregulated genes possess promoters containing a high-density of CpG islands bearing trimethylation marks at histone H3K27 and/or H3K4, respectively. In support of this finding, we further documented that S100A4 (-/-) knockout lenses have altered levels of trimethylated H3K27 and H3K4. Taken together, our findings suggest that S100A4 suppresses the expression of retinal genes during lens differentiation plausibly via a mechanism involving changes in histone methylation.

Global phosphotyrosinylated protein profile of cell-matrix adhesion complexes of trabecular meshwork cells

Dysregulation of the mechanical properties and cell adhesive interactions of trabecular meshwork (TM) are known to impair aqueous humor drainage and elevate intraocular pressure in glaucoma patients. The identity of regulatory mechanisms underlying TM mechanotransduction, however, remains elusive. Here we analyzed the phosphotyrosine proteome of human TM cell-extracellular matrix (ECM) adhesion complexes, which play a key role in sensing and transducing extracellular chemical and mechanical cues into intracellular activities, using a two-level affinity pull-down (phosphotyrosine antibody and titanium dioxide beads) method and mass spectrometry. This analysis identified ~1,000 tyrosine-phosphorylated proteins of TM cell-ECM adhesion complexes. Many consensus adhesome proteins were found to be tyrosine phosphorylated. Interestingly, several of the phosphotyrosinylated proteins found in TM cell-ECM adhesion complexes are known to be required for podocyte glomerular filtration, indicating the existence of molecular parallels that are likely relevant to the shared fluid barrier and filtration functions of the two mechanosensitive cell types.

Calponin-3 deficiency augments contractile activity, plasticity, fibrogenic response and Yap/Taz transcriptional activation in lens epithelial cells and explants

The transparent ocular lens plays a crucial role in vision by focusing light on to the retina with loss of lens transparency leading to impairment of vision. While maintenance of epithelial phenotype is recognized to be essential for lens development and function, knowledge of the identity of different molecular mechanisms regulating lens epithelial characteristics remains incomplete. This study reports that CNN-3, the acidic isoform of calponin, an actin binding contractile protein, is expressed preferentially and abundantly relative to the basic and neutral isoforms of calponin in the ocular lens, and distributes predominantly to the epithelium in both mouse and human lenses. Expression and MEKK1-mediated threonine 288 phosphorylation of CNN-3 is induced by extracellular cues including TGF-β2 and lysophosphatidic acid. Importantly, siRNA-induced deficiency of CNN3 in lens epithelial cell cultures and explants results in actin stress fiber reorganization, stimulation of focal adhesion formation, Yap activation, increases in the levels of α-smooth muscle actin, connective tissue growth factor and fibronectin, and decreases in E-cadherin expression. These results reveal that CNN3 plays a crucial role in regulating lens epithelial contractile activity and provide supporting evidence that CNN-3 deficiency is associated with the induction of epithelial plasticity, fibrogenic activity and mechanosensitive Yap/Taz transcriptional activation.

Switching of α-Catenin From Epithelial to Neuronal Type During Lens Epithelial Cell Differentiation

Purpose

Ocular lens fiber cell elongation, differentiation, and compaction are associated with extensive reorganization of cell adhesive interactions and cytoskeleton; however, our knowledge of proteins critical to these events is still evolving. This study characterizes the distribution pattern of neuronal-specific α-catenin (αN-catenin) and its interaction with the N-cadherin–associated adherens junctions (AJs) and their stability in the mouse lens fibers.

Methods

Expression and distribution of αN-catenin in developing mouse and adult human lenses was determined by RT-PCR, immunoblot, and immunofluorescence analyses. Characterization of αN-catenin and N-cadherin interacting proteins and colocalization analyses were performed using immunoprecipitation, mass spectrometry, and confocal imaging. Effects of periaxin deficiency on the stability of lens fiber cell AJs were evaluated using perixin-null mice.

Results

αN-catenin exhibits discrete distribution to lens fibers in both mouse and human lenses, undergoing a robust up-regulation during fiber cell differentiation and maturation. Epithelial-specific α-catenin (αE-catenin), in contrast, distributes primarily to the lens epithelium. αN-catenin and N-cadherin reciprocally coimmunoprecipitate and colocalize along with β-catenin, actin, spectrin, vinculin, Armadillo repeat protein deleted in velo-cardio-facial syndrome homolog, periaxin, and ankyrin-B in lens fibers. Fiber cells from periaxin-null mouse lenses revealed disrupted N-cadherin/αN-catenin–based AJs.

Conclusions

These results suggest that the discrete shift in α-catenin expression from αE-catenin to αN-catenin subtype that occurs during lens epithelial cell differentiation may play a key role in fiber cell cytoarchitecture by regulating the assembly and stability of N-cadherin–based AJs. This study also provides evidence for the importance of the fiber cell–specific cytoskeletal interacting periaxin, in the stability of N-cadherin/αN-catenin–based AJs in lens fibers.

Growth Differentiation Factor-15-Induced Contractile Activity and Extracellular Matrix Production in Human Trabecular Meshwork Cells

Purpose

To determine the role and regulation of growth differentiation factor-15 (GDF-15), a TGF-β-related cytokine in human trabecular meshwork (TM) cells in the context of aqueous humor (AH) outflow and IOP.

Methods

Regulation of expression by external cues, and the distribution and secretion of GDF-15 by human TM primary cell cultures, and the effects of recombinant (r) GDF-15 on TM cell contractile characteristics, actin cytoskeleton, cell adhesion, extracellular matrix (ECM), α-smooth muscle actin (αSMA), SMAD signaling, and gene expression were determined by immunoblot, immunofluorescence, mass spectrometry, cDNA microarray, and real-time quantitative PCR (RT-qPCR) analyses.

Results

Growth differentiation factor-15, a common constituent of ECM derived from the human TM cells, was confirmed to be distributed throughout the conventional aqueous humor outflow pathway of the human eye. Growth differentiation factor-15 protein levels were significantly increased in human TM cells in response to TGF-β2, dexamethasone, endothelin-1, lysophosphatidic acid, TNF-α, IL-1β treatment, and by cyclic mechanical stretch. Stimulation of human TM cells with rGDF-15 caused a significant increase in the formation of actin stress fibers and focal adhesions, myosin light chain phosphorylation, SMAD signaling, gene expression, and the levels of αSMA and ECM proteins.

Conclusions

The results of this study, including a robust induction of GDF-15 expression by several external factors known to elevate IOP, and rGDF-15-induced increase in contractility, cell adhesion, and the levels of ECM proteins and αSMA in TM cells, collectively suggest a potential role for GDF-15 in homeostasis and dysregulation of AH outflow and IOP in normal and glaucomatous eyes, respectively.

Corneal Endothelial Cell Migration and Proliferation Enhanced by Rho Kinase (ROCK) Inhibitors in In Vitro and In Vivo Models

Purpose

To explore the role of Rho-associated kinases (ROCK) in corneal physiology and regeneration, and the effects of suppressing its activity in stimulating corneal endothelial cell proliferation and migration in vitro and in vivo.

Methods

Immunohistochemistry was performed to detect RhoA and ROCK-1 and ROCK-2 in human corneal tissue. Adult porcine corneal endothelial cells (CECs) were isolated, grown to confluence, and further characterized. Under the treatment of ROCK inhibitors, changes in the cellular distribution profile of ZO-1 and F-actin were examined by immunofluorescence staining. Corneal endothelial cells migration was evaluated by scratch assay and analyzed with Axiovision software. Cell proliferation was quantified using Click-iT EdU HCS Assay. In vivo, the corneal endothelia of rabbits were surgically injured and H-1152 was topically applied for 10 days. Progress of wound healing was evaluated daily by monitoring corneal edema, inflammation, and thickness using slit-lamp examination, photography, and pachymetry. Rabbits were euthanized and enucleated for further evaluation.

Results

H-1152 exhibited significant stimulatory effect on CEC migration and proliferation in vitro compared with both untreated and Y-27632–treated cells. Furthermore, topical administration of H-1152 led to marked reduction in corneal edema and formation of multinucleate CECs in vivo suggestive of proliferation associated with healing.

Conclusions

H-1152 exhibited a better stimulatory effect on CEC migration and proliferation in vitro than Y-27632. Our findings suggest that topical administration of H-1152 promotes healing of injured corneal endothelium in vivo. These results demonstrate the efficacy of ROCK inhibitors as a potential topical therapy for patients with corneal endothelial disease.

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.

Ankyrin-B in lens architecture and biomechanics: Just not tethering but more

The ankyrins are a family of well-characterized metazoan adaptor proteins that play a key role in linking various membrane-spanning proteins to the underlying spectrin-actin cytoskeleton; a mechanistic understanding of their role in tissue architecture and mechanics, however, remains elusive. Here we comment on a recent study demonstrating a key role for ankyrin-B in maintaining the hexagonal shape and radial alignment of ocular lens fiber cells by regulating the membrane organization of periaxin, dystrophins/dystroglycan, NrCAM and spectrin-actin network of proteins, and revealing that ankyrin-B deficiency impairs fiber cell shape and mechanical properties of the ocular lens. These observations indicate that ankyrin-B plays an important role in maintaining tissue cytoarchitecture, cell shape and biomechanical properties via engaging in key protein: protein interactions required for membrane anchoring and organization of the spectrin-actin skeleton, scaffolding proteins and cell adhesive proteins.

Ankyrin-B directs membrane tethering of periaxin and is required for maintenance of lens fiber cell hexagonal shape and mechanics

Periaxin (Prx), a PDZ domain protein expressed preferentially in myelinating Schwann cells and lens fibers, plays a key role in membrane scaffolding and cytoarchitecture. Little is known, however, about how Prx is anchored to the plasma membrane. Here we report that ankyrin-B (AnkB), a well-characterized adaptor protein involved in linking the spectrin-actin cytoskeleton to integral membrane proteins, is required for membrane association of Prx in lens fibers and colocalizes with Prx in hexagonal fiber cells. Under AnkB haploinsufficiency, Prx accumulates in the soluble fraction with a concomitant loss from the membrane-enriched fraction of mouse lenses. Moreover, AnkB haploinsufficiency induced age-dependent disruptions in fiber cell hexagonal geometry and radial alignment and decreased compressive stiffness in mouse lenses parallel to the changes observed in Prx null mouse lens. Both AnkB- and Prx-deficient mice exhibit disruptions in membrane organization of the spectrin-actin network and the dystrophin-glycoprotein complex in lens fiber cells. Taken together, these observations reveal that AnkB is required for Prx membrane anchoring and for maintenance of lens fiber cell hexagonal geometry, membrane skeleton organization, and biomechanics.

Rap1 GTPase is required for mouse lens epithelial maintenance and morphogenesis

Rap1, a Ras-like small GTPase, plays a crucial role in cell-matrix adhesive interactions, cell-cell junction formation, cell polarity and migration. The role of Rap1 in vertebrate organ development and tissue architecture, however, remains elusive. We addressed this question in a mouse lens model system using a conditional gene targeting approach. While individual germline deficiency of either Rap1a or Rap1b did not cause overt defects in mouse lens, conditional double deficiency (Rap1 cKO) prior to lens placode formation led to an ocular phenotype including microphthalmia and lens opacification in embryonic mice. The embryonic Rap1 cKO mouse lens exhibited striking defects including loss of E-cadherin- and ZO-1-based cell-cell junctions, disruption of paxillin and β1-integrin-based cell adhesive interactions along with abnormalities in cell shape and apical-basal polarity of epithelium. These epithelial changes were accompanied by increased levels of α-smooth muscle actin, vimentin and N-cadherin, and expression of transcriptional suppressors of E-cadherin (Snai1, Slug and Zeb2), and a mesenchymal metabolic protein (Dihydropyrimidine dehydrogenase). Additionally, while lens differentiation was not overtly affected, increased apoptosis and dysregulated cell cycle progression were noted in epithelium and fibers in Rap1 cKO mice. Collectively these observations uncover a requirement for Rap1 in maintenance of lens epithelial phenotype and morphogenesis.

Regulation of plasticity and fibrogenic activity of trabecular meshwork cells by Rho GTPase signaling

Glaucoma, a prevalent blinding disease is commonly associated with increased intraocular pressure due to impaired aqueous humor (AH) drainage through the trabecular meshwork (TM). Although increased TM tissue contraction and stiffness in association with accumulation of extracellular matrix (ECM) are believed to be partly responsible for increased resistance to AH outflow, the extracellular cues and intracellular mechanisms regulating TM cell contraction and ECM production are not well defined. This study tested the hypothesis that sustained activation of Rho GTPase signaling induced by lysophosphatidic acid (LPA), TGF-β, and connective tissue growth factor (CTGF) influences TM cell plasticity and fibrogenic activity which may eventually impact resistance to AH outflow. Various experiments performed using human TM cells revealed that constitutively active RhoA (RhoAV14), TGF-β2, LPA, and CTGF significantly increase the levels and expression of Fibroblast Specific Protein-1 (FSP-1), α-smooth muscle actin (αSMA), collagen-1A1 and secretory total collagen, as determined by q-RT-PCR, immunofluorescence, immunoblot, flow cytometry and the Sircol assay. Significantly, these changes appear to be mediated by Serum Response Factor (SRF), myocardin-related transcription factor (MRTF-A), Slug, and Twist-1, which are transcriptional regulators known to control cell plasticity, myofibroblast generation/activation and fibrogenic activity. Additionally, the Rho kinase inhibitor-Y27632 and anti-fibrotic agent-pirfenidone were both found to suppress the TGF-β2-induced expression of αSMA, FSP-1, and collagen-1A1. Taken together, these observations demonstrate the significance of RhoA/Rho kinase signaling in regulation of TM cell plasticity, fibrogenic activity, and myofibroblast activation, events with potential implications for the pathobiology of elevated intraocular pressure in glaucoma patients.

L-type calcium channels play a critical role in maintaining lens transparency by regulating phosphorylation of aquaporin-0 and myosin light chain and expression of connexins

Homeostasis of intracellular calcium is crucial for lens cytoarchitecture and transparency, however, the identity of specific channel proteins regulating calcium influx within the lens is not completely understood. Here we examined the expression and distribution profiles of L-type calcium channels (LTCCs) and explored their role in morphological integrity and transparency of the mouse lens, using cDNA microarray, RT-PCR, immunoblot, pharmacological inhibitors and immunofluorescence analyses. The results revealed that Ca (V) 1.2 and 1.3 channels are expressed and distributed in both the epithelium and cortical fiber cells in mouse lens. Inhibition of LTCCs with felodipine or nifedipine induces progressive cortical cataract formation with time, in association with decreased lens weight in ex-vivo mouse lenses. Histological analyses of felodipine treated lenses revealed extensive disorganization and swelling of cortical fiber cells resembling the phenotype reported for altered aquaporin-0 activity without detectable cytotoxic effects. Analysis of both soluble and membrane rich fractions from felodipine treated lenses by SDS-PAGE in conjunction with mass spectrometry and immunoblot analyses revealed decreases in β-B1-crystallin, Hsp-90, spectrin and filensin. Significantly, loss of transparency in the felodipine treated lenses was preceded by an increase in aquaporin-0 serine-235 phosphorylation and levels of connexin-50, together with decreases in myosin light chain phosphorylation and the levels of 14-3-3ε, a phosphoprotein-binding regulatory protein. Felodipine treatment led to a significant increase in gene expression of connexin-50 and 46 in the mouse lens. Additionally, felodipine inhibition of LTCCs in primary cultures of mouse lens epithelial cells resulted in decreased intracellular calcium, and decreased actin stress fibers and myosin light chain phosphorylation, without detectable cytotoxic response. Taken together, these observations reveal a crucial role for LTCCs in regulation of expression, activity and stability of aquaporin-0, connexins, cytoskeletal proteins, and the mechanical properties of lens, all of which have a vital role in maintaining lens function and cytoarchitecture.

Connective tissue growth factor-mediated upregulation of neuromedin U expression in trabecular meshwork cells and its role in homeostasis of aqueous humor outflow

PURPOSE

Connective tissue growth factor (CTGF) is a matricellular protein presumed to be involved in the pathobiology of various fibrotic diseases, including glaucoma. We investigated the effects of Rho GTPase-dependent actin cytoskeletal integrity on CTGF expression and CTGF-induced changes in gene expression profile in human trabecular meshwork (HTM) cells.

METHODS

CTGF levels were quantified by immunoblotting and ELISA. CTGF-induced changes in gene expression, actin cytoskeleton, myosin light chain (MLC) phosphorylation, and extracellular matrix (ECM) proteins were evaluated in trabecular meshwork (TM) cells by cDNA microarray, q-PCR, fluorescence microscopy, and immunoblot analyses. The effects of neuromedin U (NMU) on aqueous humor (AH) outflow were determined in enucleated porcine eyes.

RESULTS

Expression of a constitutively active form of RhoA (RhoAV14), activation of Rho GTPase by bacterial toxin, or inhibition of Rho kinase by Y-27632 in HTM cells led to significant but contrasting changes in CTGF protein levels that were detectable in cell lysates and cell culture medium. Stimulation of HTM cells with CTGF for 24 hours induced actin stress fiber formation, and increased MLC phosphorylation, fibronectin, and laminin levels, and NMU expression. NMU independently induced actin stress fibers and MLC phosphorylation in TM cells, and decreased AH outflow facility in perfused porcine eyes.

CONCLUSIONS

These data revealed that CTGF influences ECM synthesis, actin cytoskeletal dynamics, and contractile properties in TM cells, and that the expression of CTGF is regulated closely by Rho GTPase. Moreover, NMU, whose expression is induced in response to CTGF, partially mimics the effects of CTGF on actomyosin organization in TM cells, and decreases AH outflow facility, revealing a potentially important role for this neuropeptide in the homeostasis of AH drainage.

Rac1 GTPase-deficient mouse lens exhibits defects in shape, suture formation, fiber cell migration and survival

Morphogenesis and shape of the ocular lens depend on epithelial cell elongation and differentiation into fiber cells, followed by the symmetric and compact organization of fiber cells within an enclosed extracellular matrix-enriched elastic capsule. The cellular mechanisms orchestrating these different events however, remain obscure. We investigated the role of the Rac1 GTPase in these processes by targeted deletion of expression using the conditional gene knockout (cKO) approach. Rac1 cKO mice were derived from two different Cre (Le-Cre and MLR-10) transgenic mice in which lens-specific Cre expression starts at embryonic day 8.75 and 10.5, respectively, in both the lens epithelium and fiber cells. The Le-Cre/Rac1 cKO mice exhibited an early-onset (E12.5) and severe lens phenotype compared to the MLR-10/Rac1 cKO (E15.5) mice. While the Le-Cre/Rac1 cKO lenses displayed delayed primary fiber cell elongation, lenses from both Rac1 cKO strains were characterized by abnormal shape, impaired secondary fiber cell migration, sutural defects and thinning of the posterior capsule which often led to rupture. Lens fiber cell N-cadherin/β-catenin/Rap1/Nectin-based cell-cell junction formation and WAVE-2/Abi-2/Nap1-regulated actin polymerization were impaired in the Rac1 deficient mice. Additionally, the Rac1 cKO lenses were characterized by a shortened epithelial sheet, reduced levels of extracellular matrix (ECM) proteins and increased apoptosis. Taken together, these data uncover the essential role of Rac1 GTPase activity in establishment and maintenance of lens shape, suture formation and capsule integrity, and in fiber cell migration, adhesion and survival, via regulation of actin cytoskeletal dynamics, cell adhesive interactions and ECM turnover.

Characterization of cytoskeleton-enriched protein fraction of the trabecular meshwork and ciliary muscle cells

PURPOSE

To understand the molecular basis for the known distinct contractile characteristics of trabecular meshwork (TM) and ciliary muscle (CM) cells, the cytoskeleton-enriched protein fractions of the TM and CM cells were isolated and characterized.

METHODS

The nonionic surfactant insoluble fraction enriched for cytoskeletal proteins was isolated from human and porcine TM tissue and cells and from CM cells and was characterized by SDS-PAGE, mass spectrometry, and immunoblotting techniques.

RESULTS

The cytoskeleton-enriched protein fraction derived from both human and porcine TM cells contained Plectin 1, Filamin A, non-muscle myosin IIA, clathrin, α-actinin, vimentin, actin, caldesmon, myosin IC, and annexin A2 as major proteins and was noted to exhibit compositional similarity with the cytoskeletal protein fraction isolated from TM tissue. Importantly, the cytoskeletal protein composition of the TM cells was also found to be similar to that noted for CM and vascular endothelial cells. Although the activity of myosin II, a crucial regulator of cellular contraction and a major component of the cytoskeletal protein fraction in TM and CM cells, was regulated predominantly by Rho kinase in both cell types, myosin light chain kinase (MLCK) also appeared to control myosin II activity in CM cells.

CONCLUSIONS

These data reveal that the activity of non-muscle myosin II, a critical molecule of cellular contraction, was found to be regulated differentially in TM and CM cells by the Rho kinase and the MLCK pathways despite their compositional similarity in cytoskeletal protein profile.

Abundant expression of ponsin, a focal adhesion protein, in lens and downregulation of its expression by impaired cytoskeletal signaling

PURPOSE

This study was undertaken to improve understanding of the defective lens developmental changes induced by the transgenic overexpression of the Rho GDP dissociation inhibitor RhoGDIalpha. The study was focused on a single differentially expressed gene encoding ponsin, a cell adhesion interacting signaling adaptor protein.

METHODS

Total RNA extracted from the P7 lenses of Rho GDIalpha transgenic mice was subjected to cDNA microarray analysis. Ponsin distribution in the mouse lenses was determined by immunofluorescence and immunoblot analyses. Interactions among ponsin, actin, and Rho GTPase signaling pathways were explored in lens epithelial cells.

RESULTS

The RhoGDIalpha transgenic mouse lenses revealed a marked downregulation of expression of multiple splice variants of ponsin. Expression of one of the ponsins (U58883) was found to be abundant in normal mouse lenses. Although ponsin was localized predominantly to the focal adhesions in lens epithelial cells, it was distributed to both the epithelium and fibers, with some isoforms being enriched primarily in the Triton X-100-insoluble fraction in lens tissue. Further, whereas constitutively active RhoA induced ponsin clustering at the leading edges, inhibition of Rho kinase and latrunculin treatment were noted to lead to decreases in ponsin protein levels in lens epithelial cells.

CONCLUSIONS

Abundant expression of ponsin, a focal adhesion protein in the lens tissue indicates a potential role for this protein in lens fiber cell migration and adhesion. Ponsin expression appears to be closely dependent on Rho GTPase-regulated integrity of actin cytoskeletal organization.

Novel molecular insights into RhoA GTPase-induced resistance to aqueous humor outflow through the trabecular meshwork

Impaired drainage of aqueous humor through the trabecular meshwork (TM) culminating in increased intraocular pressure is a major risk factor for glaucoma, a leading cause of blindness worldwide. Regulation of aqueous humor drainage through the TM, however, is poorly understood. The role of RhoA GTPase-mediated actomyosin organization, cell adhesive interactions, and gene expression in regulation of aqueous humor outflow was investigated using adenoviral vector-driven expression of constitutively active mutant of RhoA (RhoAV14). Organ-cultured anterior segments from porcine eyes expressing RhoAV14 exhibited significant reduction of aqueous humor outflow. Cultured TM cells expressing RhoAV14 exhibited a pronounced contractile morphology, increased actin stress fibers, and focal adhesions and increased levels of phosphorylated myosin light chain (MLC), collagen IV, fibronectin, and laminin. cDNA microarray analysis of RNA extracted from RhoAV14-expressing human TM cells revealed a significant increase in the expression of genes encoding extracellular matrix (ECM) proteins, cytokines, integrins, cytoskeletal proteins, and signaling proteins. Conversely, various ECM proteins stimulated robust increases in phosphorylation of MLC, paxillin, and focal adhesion kinase and activated Rho GTPase and actin stress fiber formation in TM cells, indicating a potential regulatory feedback interaction between ECM-induced mechanical strain and Rho GTPase-induced isometric tension in TM cells. Collectively, these data demonstrate that sustained activation of Rho GTPase signaling in the aqueous humor outflow pathway increases resistance to aqueous humor outflow through the trabecular pathway by influencing the actomyosin assembly, cell adhesive interactions, and the expression of ECM proteins and cytokines in TM cells.

Characterization of lens fiber cell triton insoluble fraction reveals ERM (ezrin, radixin, moesin) proteins as major cytoskeletal-associated proteins

To understand lens fiber cell elongation- and differentiation-associated cytoskeletal remodeling, here we identified and characterized the major protein components of lens fiber cell Triton X-100 insoluble fraction by mass spectrometry and immunoblot analysis. This analysis identified spectrin, filensin, vimentin, tubulin, phakinin, and beta-actin as major cytoskeletal proteins in the lens fibers. Importantly, ezrin, radixin, and moesin (ERM), heat-shock cognate protein 70, and beta/gamma-crystallins were identified as major cytoskeletal-associated proteins. ERM proteins were confirmed to exist as active phosphorylated forms that exhibited intense distribution in the organelle free-zone fibers. Furthermore, ERM protein phosphorylation was found to be dramatically reduced in Rho GTPase-targeted transgenic mouse lenses. These data identify the ERM proteins, which cross-link the plasma membrane and actin, as major and stable cytoskeletal-associated proteins in lens fibers, and indicate a potential role(s) for the ERMs in fiber cell actin cytoskeletal and membrane organization.

Rho GDP dissociation inhibitor-mediated disruption of Rho GTPase activity impairs lens fiber cell migration, elongation and survival

To explore the role of the Rho GTPases in lens morphogenesis, we overexpressed bovine Rho GDP dissociation inhibitor (Rho GDI alpha), which serves as a negative regulator of Rho, Rac and Cdc42 GTPase activity, in a lens-specific manner in transgenic mice. This was achieved using a chimeric promoter of delta-crystallin enhancer and alpha A-crystallin, which is active at embryonic day 12. Several individual transgenic (Tg) lines were obtained, and exhibited ocular specific phenotype comprised of microphthalmic eyes with lens opacity. The overexpression of bovine Rho GDI alpha disrupted membrane translocation of Rho, Rac and Cdc42 GTPases in Tg lenses. Transgenic lenses also revealed abnormalities in the migration pattern, elongation and organization of lens fibers. These changes appeared to be associated with impaired organization of the actin cytoskeleton and cell-cell adhesions. At E14.5, the size of the Rho GDI alpha Tg lenses was larger compared to wild type (WT) and the central lens epithelium and differentiating fibers exhibited an abnormal increase of bromo-deoxy-uridine incorporation. Postnatal Tg eyes, however, were much smaller in size compared to WT eyes, revealing increased apoptosis in the disrupted lens fibers. Taken together, these data demonstrate a critical role for Rho GTPase-dependent signaling pathways in processes underlying morphogenesis, fiber cell migration, elongation and survival in the developing lens.

Influence of actin cytoskeletal integrity on matrix metalloproteinase-2 activation in cultured human trabecular meshwork cells

PURPOSE

The goal of this study was to investigate the possible link between actin cytoskeletal integrity and the activation of matrix metalloproteinases (MMPs) in trabecular meshwork (TM) cells.

METHODS

Primary human TM (HTM) cells treated with different actin cytoskeleton-interfering agents, including cytochalasin D, latrunculin A, ethacrynic acid (ECA), a Rho kinase inhibitor (Y-27632), and H-7 (serine/threonine kinase inhibitor), were examined for changes in actin cytoskeletal organization by phalloidin staining, MMP-2 activation by gelatin zymography, expression of MT1-MMP by quantitative real-time PCR analysis, levels of tissue inhibitor of metalloproteinases (TIMP-1 and TIMP-2), and activation of p38 mitogen-activated protein kinase (p38 MAPK) and extracellular signal-regulated protein kinase (ERK) by immunoblotting.

RESULTS

Treatment of HTM cells with cytochalasin D and latrunculin A led to significant activation of MMP-2, p38 MAPK, and ERK1/2, which appeared to correlate with changes in cell morphology and actin depolymerization. Additionally, treatment with these cytoskeleton-disrupting agents elicited increased expression of MT1-MMP in HTM cells, concomitant with a decrease in the levels of secreted TIMP-1 and TIMP-2. In contrast, treatment with ECA, Y-27632, or H-7 triggered changes in cell shape and reduced actin stress fibers in HTM cells but did not exert significant effects on MMP-2 activation or MT1-MMP expression.

CONCLUSIONS

These studies indicate that cytochalasin D- and latrunculin A-induced alteration of actin cytoskeletal integrity in HTM cells is associated with MMP-2 activation, most likely through the upregulation of its activator, MT1-MMP. These data provide a mechanistic connection between actin cytoskeletal organization and MMP-2 activation in TM cells and offer new insights into extracellular matrix remodeling in the aqueous outflow pathway.

Lens fiber cell elongation and differentiation is associated with a robust increase in myosin light chain phosphorylation in the developing mouse

Myosin II, a molecular motor, plays a critical role in cell migration, cell shape changes, cell adhesion, and cytokinesis. To understand the role of myosin II in lens fiber cell elongation and differentiation, we determined the distribution pattern of nonmuscle myosin IIA, IIB, and phosphorylated regulatory myosin light chain-2 (phospho-MLC) in frozen sections of the developing mouse lens by immunofluorescence analysis. While myosin IIA was distributed uniformly throughout the differentiating lens, including the epithelium and fibers, myosin IIB was localized predominantly to the epithelium and the posterior tips of the lens fibers. In contrast, immunostaining with a di-phospho-MLC antibody localized intensely and precisely to the elongating and differentiating primary and secondary lens fibers, co-localizing with actin filaments. An in situ analysis of Rho GTPase activation revealed that Rho-GTP was distributed uniformly throughout the embryonic lens, including epithelium and fibers. Inhibition of myosin light chain kinase (MLCK) activity by ML-7 in organ cultured mouse lenses led to development of nuclear lens opacity in association with abnormal fiber cell organization. Taken together, these data reveal a distinct spatial distribution pattern of myosin II isoforms in the developing lens and a robust activation of MLC phosphorylation in the differentiating lens fibers. Moreover, the regulation of MLC phosphorylation by MLCK appears to be critical for crystallin organization and for maintenance of lens transparency and lens membrane function.

The role of the lens actin cytoskeleton in fiber cell elongation and differentiation

The vertebrate ocular lens is a fascinating and unique transparent tissue that grows continuously throughout life. During the process of differentiation into fiber cells, lens epithelial cells undergo dramatic morphological changes, membrane remodeling, polarization, transcriptional activation and elimination of cellular organelles including nuclei, concomitant with migration towards the lens interior. Most of these events are presumed to be influenced in large part, by dynamic reorganization of the cellular actin cytoskeleton and by intercellular and cell: extracellular matrix interactions. In light of recent and unprecedented advancement in our understanding of the mechanistic bases underlying regulation of actin cytoskeletal dynamics and the role of the actin cytoskeleton in cell function, this review attempts to summarize current knowledge regarding the role of the cellular actin cytoskeleton, in lens fiber cell elongation and differentiation, and regulation of actin cytoskeletal organization in the lens.

α-Crystallin localizes to the leading edges of migrating lens epithelial cells

alpha-crystallin (alphaA and alphaB) is a major lens protein, which belongs to the small heat-shock family of proteins and binds to various cytoskeletal proteins including actin, vimentin and desmin. In this study, we investigated the cellular localization of alphaA and alphaB-crystallins in migrating epithelial cells isolated from porcine lens. Immunofluorescence localization and confocal imaging of alphaB-crystallin in confluent and in migrating subconfluent cell cultures revealed a distinct pattern of subcellular distribution. While alphaB-crystallin localization was predominantly cytoplasmic in confluent cultures, it was strongly localized to the leading edges of cell membrane or the lamellipodia in migrating cells. In accordance with this pattern, we found abundant levels of alphaB-crystallin in membrane fractions compared to cytosolic and nuclear fractions in migrating lens epithelial cells. alphaA-crystallin, which has 60% sequence identity to alphaB-crystallin, also exhibited a distribution profile localizing to the leading edge of the cell membrane in migrating lens epithelial cells. Localization of alphaB-crystallin to the lamellipodia appears to be dependent on phosphorylation of residue serine-59. An inhibitor of p38 MAP kinase (SB202190), but not the ERK kinase inhibitor PD98059, was found to diminish localization of alphaB-crystallin to the lamellipodia, and this effect was found to be associated with reduced levels of Serine-59 phosphorylated alphaB-crystallin in SB202190-treated migrating lens epithelial cells. alphaB-crystallin localization to the lamellipodia was also altered by the treatment with RGD (Arg-Ala-Asp) peptide, dominant negative N17 Rac1 GTPase, cytochalasin D and Src kinase inhibitor (PP2), but not by the Rho kinase inhibitor Y-27632 or the myosin II inhibitor, blebbistatin. Additionally, in migrating lens epithelial cells, alphaB-crystallin exhibited a clear co-localization with the actin meshwork, beta-catenin, WAVE-1, a promoter of actin nucleation, Abi-2, a component of WAVE-1 protein complex and Arp3, a protein of the actin nucleation complex, suggesting potential interactions between alphaB-crystallin and regulatory proteins involved in actin dynamics and cell adhesion. This is the first report demonstrating specific localization of alphaA and alphaB-crystallins to the lamellipodia in migrating lens epithelial cells and our findings indicate a potential role for alpha-crystallin in actin dynamics during cell migration.

Expression of dominant negative Rho-binding domain of Rho-kinase in organ cultured human eye anterior segments increases aqueous humor outflow

PURPOSE

Based on pharmacological inhibition of activity, a role has been proposed for Rho-kinase in the modulation of aqueous outflow and intraocular pressure (IOP). This study employed a molecular approach to specifically address the role of Rho-kinase in the modulation of aqueous humor outflow.

METHODS

Adenoviral vectors expressing green fluorescent protein alone (Ad-GFP) or the dominant negative Rho-binding domain of Rho-kinase and GFP (Ad-DNRK-GFP) were utilized in these experiments. Human and porcine primary trabecular meshwork (TM) cells were infected with 30 MOI (multiplicity of infection) of Ad-GFP alone or with Ad-DNRK-GFP. Changes in cell shape, actomyosin cytoskeletal integrity, and the status of myosin light chain (MLC) phosphorylation were evaluated. The aqueous outflow facility was determined in organ cultured anterior segments of human cadaver eyes infected with 10(7) pfu adenoviral vectors (Ad-GFP or Ad-DNRK-GFP) using a constant flow perfusion system.

RESULTS

Expression of DNRK resulted in changes in cell shape (cell rounding, cell-cell detachment) and decreased actin stress fiber and focal adhesion staining in TM cells. These cellular changes were associated with substantially reduced myosin light chain phosphorylation. Additionally, organ cultured human eye anterior segments infected with Ad-DNRK-GFP exhibited a significant increase in the outflow facility (80%, n=9) compared to control anterior segments infected with Ad-GFP (5%).

CONCLUSIONS

This study demonstrated that specific inhibition of Rho-kinase activity in trabecular meshwork cells led to alterations in cell shape and presumed contractile properties, and we hypothesize that these morphological and contractile events underlie the observed effects of dominant negative Rho-kinase on the aqueous humor outflow facility. These data provide molecular evidence for the hypothesis of Rho-kinase being a potential cellular target involved in the regulation of aqueous humor outflow resistance, with implications for novel glaucoma therapy.

Impaired cytoskeletal organization and membrane integrity in lens fibers of a Rho GTPase functional knockout transgenic mouse

To investigate the effects of Rho GTPase inactivation on lens fiber cell cytoskeletal and morphological integrity, a transgenic mouse model expressing C3-exoenzyme (a bacterial toxin) in a lens-specific manner was utilized. Cryosections of whole eyes from C3 transgenic mice and littermate controls were stained for F-actin with rhodamine-phalloidin or immunostained for beta-catenin, aquaporin-0 or connexin-50, and confocal images were recorded. Lens fiber cell morphology was examined at both light and electron microscopic levels. To investigate the influence of Rho GTPase inactivation on the profiles of gene expression, cDNA libraries generated from transgenic and littermate control mouse lenses were screened by cDNA microarray analysis. In contrast to the wild-type lens, fiber cells of the transgenic lens were grossly swollen and disorganized, with abnormal membrane architecture. Staining of F-actin, beta-catenin, aquaporin-0 and connexin-50 was reduced dramatically in the C3 transgenic lens as compared to controls. Western blot analysis and cDNA microarray analysis did not reveal any noticeable decreases in actin, beta-catenin and aquaporin-0 protein levels or expression in C3 transgenic lenses, indicating that altered cytoskeletal organization in response to Rho GTPase inactivation might underlie the noted changes in staining for these proteins. Additionally, cDNA microarray analysis of C3 lens revealed altered expression (at least two-fold, compared to littermate controls) of 44 genes. These include genes encoding extracellular matrix and basement membrane proteins, cell survival and apoptotic pathways, and ion and protein transport. These data indicate that disruption of Rho GTPase function in the developing mouse lens results in abnormal cytoskeletal organization, fiber cell interactions, impaired lens fiber cell morphology and altered gene expression of cellular proteins involved in diverse functions. This work reveals that the morphological and cytoskeletal abnormalities triggered upon Rho GTPase inactivation in lens could be one of the important insults associated with cataract formation in C3 transgenic mouse lens.

Expression of nonphagocytic NADPH oxidase system in the ocular lens

PURPOSE

The primary goal of this study was to characterize the Rac GTPase associated, NADPH oxidase-mediated Reactive Oxygen Species (ROS)-generating system in the lens tissue.

METHODS

NADPH oxidase activity in lens tissue was determined by quantifying superoxide-induced lucigenin photoemission. Immunological and PCR/RT-PCR techniques were utilized to determine expression of different components of the NADPH oxidase system in lens tissue. Growth factor stimulated ROS production was determined quantitatively in human lens epithelial cells using dichlorofluorescein diacetate.

RESULTS

Lens homogenates from different species showed generation of superoxide in a lucigenin-enhanced chemiluminescence assay in the presence of NADPH. This activity was found to be lens protein concentration dependent, heat sensitive, and inhibitable by superoxide dismutase and the flavoprotein inhibitor, diphenyleneiodonium (DPI). The distribution of superoxide generating activity in lens was confined predominantly to the lens epithelium, with very low levels in cortex and none in the nucleus. Immunological assays have demonstrated the presence of p67phox and p47phox in lens tissue, while PCR and RT-PCR reactions amplified DNA products corresponding to the p67phox, p40phox, p22phox, gp91phox, and Rac1 components of the NADPH oxidase complex from human and mouse lens cDNA libraries. Serum starved human lens epithelial cells stimulated with different growth factors including EGF, b-FGF, PDGF, TGF-beta, and LPA demonstrated increased production of ROS, a response which was blocked by inhibitors of NADPH oxidase, such as DPI and the antioxidant-N-acetyl cysteine (NAC). RT-PCR analysis of human lens RNA confirmed readily detectable levels of expression of low molecular weight protein tyrosine phosphatase (LMW-PTP), which is a well-characterized target of redox signaling pathway(s).

CONCLUSIONS

These data demonstrate the presence of a functional nonphagocytic NADPH oxidase system in lens that is predominantly localized to the lens epithelium. Several growth factors appear to stimulate the activity of lens NADPH oxidase, resulting in increased production of ROS in lens epithelial cells, indicating that redox signaling may have an important role in growth factor effects on lens growth and development.

Growth factor induced activation of Rho and Rac GTPases and actin cytoskeletal reorganization in human lens epithelial cells

PURPOSE

To determine the involvement of the Rho GTPases-mediated signaling pathway in growth factor-stimulated actomyosin cytoskeletal organization and focal adhesion formation in lens epithelial cells.

METHODS

Serum starved human lens epithelial cells (SRA01/04) were treated with different growth factors including epidermal growth factor (EGF), basic-fibroblast growth factor (b-FGF), platelet derived growth factor (PDGF), transforming growth factor beta (TGF-beta), insulin-like growth factor 1 (IGF-1), lysophosphatidic acid (LPA), and thrombin. Growth factor stimulated activation of Rho and Rac GTPases were evaluated by GTP-loading pull-down assays. Changes in actin cytoskeletal organization and focal adhesions were determined by fluorescence staining using FITC-phalloidin and anti-vinculin antibody/rhodamine-conjugated secondary antibody, respectively. Fluorescence images were recorded using either confocal or fluorescence microscopy.

RESULTS

Rho GTPase activity was significantly augmented in human lens epithelial cells treated with EGF, b-FGF, TGF-beta, IGF-1, and LPA. Rac GTPase activation, in contrast, was significantly enhanced in response to only EGF or b-FGF. Serum starved human lens epithelial cells exhibited a strong increase in cortical actin stress fibers and integrin-mediated focal adhesions in response to b-FGF, PDGF, TGF-beta, thrombin, and LPA. While EGF induced a striking increase in membrane ruffling and a marginal increase on focal adhesion formation, IGF-1 had no effect on either. Pretreatment of lens epithelial cells with C3-exoenzyme (an irreversible inhibitor of Rho-GTPase), lovastatin (an isoprenylation inhibitor), or the Rho kinase inhibitor Y-27632 abolished the ability of the different growth factors to elicit actin stress fiber and focal adhesion formation. EGF induced membrane ruffling, however, was not suppressed by Y-27632 and C3-exoenzyme.

CONCLUSIONS

These results demonstrate that different growth factors induce actin cytoskeleton reorganization and formation of cell-ECM interactions in lens epithelial cells and this response of growth factors appears to be mediated, at least in part, through the Rho/Rho kinase-mediated signaling pathway. The ability of growth factors to trigger activation of Rho and Rac GTPases along with actomyosin cytoskeletal reorganization and formation of focal adhesions might well play a crucial role in lens epithelial cell proliferation, migration, elongation and survival.

Selective expression of the small GTPase RhoB in the early developing mouse lens

This report describes the expression and distribution pattern of RhoB GTPase in the developing mouse lens. RhoB expression was confirmed by sequencing an reverse transcriptase-polymerase chain reaction-generated DNA fragment of RhoB. Immunohistochemical analysis of RhoB revealed expression in the lens vesicle (both anterior and posterior vesicle) at embryonic day (E) 11.5, and in the epithelium and primary fibers of the E14.5 lens. Compared with the neonatal stage (day 1), where RhoB is detected in the entire lens (epithelium, primary, and secondary fibers), expression of this protein is restricted to the epithelial and outer cortical secondary fibers in postnatal lenses (from day 7 to day18). Interestingly, in E11.5 and E14.5 lenses, RhoB is localized predominantly in the lens, but not detectable in the retina, cornea, or other ocular tissues. RhoB expression appears to be down-regulated in the postnatal lens with concomitant up-regulation in the retina and cornea, compared with earlier stages of development (eyes of E11.5, E14.5, and neonatal mice). This study reveals the selective expression of RhoB in the lens during early eye development and suggests a potential role for this small GTPase in cytoskeletal reorganization associated with lens epithelial cell elongation and differentiation.