The cellular basis of aqueous outflow regulation

Expression of 14-3-3 Zeta Protein in Dexamethasone-Treated Mice and Human TM-1 Cells

PURPOSE

14-3-3 zeta protein plays a potential protective role in neurodegenerative disease. Given that glaucoma and neurodegenerative diseases share a similar pathogenesis, it is possible that 14-3-3 zeta may have a similar protective effect in the glaucomatous process. In the present study, we measured the expression of 14-3-3 zeta in vivo (mouse eyes) and in vitro in a transformed human trabecular meshwork (HTM) cell line, TM-1, and assessed the possible roles of this protein in dexamethasone (DEX)-treated eyes and HTM cells.

METHODS

Mouse eyes were randomly treated with 0.1% dexamethasone (DEX) eye drops or phosphate-buffered solution (PBS) for 28 days. The expression and distribution of 14-3-3 zeta protein in mouse eyes were examined using immunofluorescence. TM-1 cells were treated with DEX (10^-6 or 10^-7 M) or PBS for 1, 4, or 7 days, and the mRNA and protein expression of 14-3-3 zeta were detected by real-time RT-PCR and Western blotting.

RESULTS

14-3-3 zeta protein was highly expressed in the mouse cornea, trabecular meshwork (TM), and ciliary body. Intraocular pressure (IOP) was significantly elevated, whereas the 14-3-3 zeta expression was significantly decreased in mouse TM after 0.1% DEX treatment for 28 days. In vitro, treatment with 10^-7 M DEX mildly increased 14-3-3 zeta mRNA and protein expression (p > 0.05), whereas 10^-6 M DEX significantly decreased expression of 14-3-3 zeta mRNA and protein (p < 0.05) compared to the control (Ctrl) group at the seventh day.

CONCLUSIONS

DEX can increase IOP in mouse eyes and concurrently downregulate 14-3-3 zeta protein expression in mouse TM. The effects of DEX on 14-3-3 zeta expression in vitro were both dose- and time-related. Our results suggest that alterations in 14-3-3 zeta protein may be implicated in DEX-induced pathological elevated IOP.

Emerging functions of pannexin 1 in the eye

Pannexin 1 (Panx1) is a high-conductance, voltage-gated channel protein found in vertebrates. Panx1 is widely expressed in many organs and tissues, including sensory systems. In the eye, Panx1 is expressed in major divisions including the retina, lens and cornea. Panx1 is found in different neuronal and non-neuronal cell types. The channel is mechanosensitive and responds to changes in extracellular ATP, intracellular calcium, pH, or ROS/nitric oxide. Since Panx1 channels operate at the crossroad of major signaling pathways, physiological functions in important autocrine and paracrine feedback signaling mechanisms were hypothesized. This review starts with describing in depth the initial Panx1 expression and localization studies fostering functional studies that uncovered distinct roles in processing visual information in subsets of neurons in the rodent and fish retina. Panx1 is expressed along the entire anatomical axis from optical nerve to retina and cornea in glia, epithelial and endothelial cells as well as in neurons. The expression and diverse localizations throughout the eye points towards versatile functions of Panx1 in neuronal and non-neuronal cells, implicating Panx1 in the crosstalk between immune and neural cells, pressure related pathological conditions like glaucoma, wound repair or neuronal cell death caused by ischemia. Summarizing the literature on Panx1 in the eye highlights the diversity of emerging Panx1 channel functions in health and disease.

Rho-kinase inhibitors offer a new approach in the treatment of glaucoma

INTRODUCTION

Primary open-angle glaucoma (POAG) is a leading cause for worldwide blindness and is characterized by progressive optic nerve damage. The etiology of POAG is unknown, but elevated intraocular pressure (IOP) and advanced age have been identified as risk factors. IOP reduction is the only known treatment for glaucoma. Recently, drugs that inhibit rho-associated protein kinase (ROCK) have been studied in animals and people for their ability to lower IOP and potentially treat POAG. ROCK inhibitors lower IOP through a trabecular mechanism and may represent a new therapeutic paradigm for the treatment of POAG.

AREAS COVERED

Exploring the place that ROCK inhibitors may occupy in our treatment of POAG requires a thorough understanding of pathophysiology and treatment. This article summarizes current research on the incidence, proposed etiologies and mechanisms of action for this drug class. ROCK inhibitor research is presented and considered in light of the current standard of pharmacologic care.

EXPERT OPINION

ROCK inhibitors alter the cell shape and extracellular matrix (ECM) of the trabecular meshwork. Preclinical studies demonstrate that these drugs have the potential to become a new therapy for glaucoma. However, ROCK inhibitors can affect multiple cell types, and their utility can be proven only after clinical studies in patients.

Immunolocalization of prostanoid EP receptor isotypes in human trabecular meshwork

PURPOSE

To assess the localization of the EP-type prostanoid receptors in the human trabecular meshwork (TM) and to determine their spatial distribution in relation to the contractile a-smooth muscle actin fibres.

METHODS

Cryosections of human anterior segments were obtained from 17 different donors and immunostained with different EP receptor subtype specific antibodies. Double staining for the EP2 receptor and smooth muscle actin was carried out. Western blots of TM protein samples were studied.

RESULTS

No specific staining for the EP1 receptor was observed. The antibodies against the EP2 receptor revealed in all donors intense staining of human trabecular cells throughout the meshwork. EP3 receptor specific staining was not detected. EP4 immunostaining was confined to the corneoscleral region near Schwalbe's line. On western blots, the EP2 receptor was detected. In the posterior TM, the EP2 receptor staining was associated with the dense network of actin fibres.

CONCLUSIONS

These immunocytochemical results present evidence that the EP2 receptor is the most abundantly expressed isotype of the PGE receptors in the human TM. This conclusion is in agreement with our previous findings at the transcript level. The relaxant responses of the TM to application of EP2 receptor agonists, and flow enhancement evoked by prostaglandin PGE1, may be explained by the close spatial association of the EP2 receptor with actin fibres.

Aqueous humor outflow effects of 2-arachidonylglycerol

This study was conducted to test the effects of 2-arachidonylglycerol (2-AG), an endocannabinoid, on aqueous humor outflow facility, to study the cellular mechanisms of 2-AG, and to investigate the possible existence and activity of monoacylgylcerol lipase (MGL), a 2-AG metabolic enzyme, in the trabecular meshwork (TM). The effects of 2-AG on aqueous humor outflow facility were measured using an anterior segment perfused organ culture model. The expression and activity of MGL in TM tissues were assessed using Western blot analysis and an enzyme activity assay respectively. 2-AG induced activation of p42/44 mitogen-activated protein (MAP) kinase was determined by Western blot analysis using an anti-phospho p42/44 MAP kinase antibody. AlexaFluor 488-labeled phalloidin staining was used to examine actin filament in cultured TM cells. Administration of 10nM of 2-AG caused a transient enhancement of aqueous humor outflow. In the presence of 100nM of LY2183240, an inhibitor of MGL, the effect of 10nM of 2-AG on outflow was prolonged by at least 4h. The 2-AG-induced enhancement of outflow was blocked by SR141716A, a CB1 antagonist, and SR144528, a CB2 antagonist. In Western blot studies, a 35kDa band representing MGL was detected on TM tissues with an anti-MGL antibody. The 2-AG enzymatic hydrolysis activity was detected in TM tissues and this activity was reduced by 70.1+/-5.3% with the addition of 100 nM of LY2183240. Treatment of trabecular meshwork cells with 10nM of 2-AG plus 100 nM LY2183240 for 5h evoked phosphorylation of p42/44 MAP kinase. The 2-AG-induced enhancement of p42/44 MAP kinase phosphorylation was blocked by pretreatment with SR141716A, SR144528, as well as PD98059, an inhibitor of the p42/44 MAP kinase pathway. In addition, the outflow-enhancing effect of 2-AG was blocked by pretreatment with PD98059. Furthermore, treatment with 2-AG plus LY2183240 caused rounding of TM cells and a reduction of actin stress fibers in TM cells. Pretreatment with SR141716A, SR144528, and PD98059 blocked these 2-AG-induced morphology and cytoskeleton changes in TM cells. In conclusion, the results from this study demonstrate that administration of 2-AG increases aqueous humor outflow facility and this effect of 2-AG is mediated through both the CB1 and CB2 cannabinoid receptors. In addition, this study reveals the existence and the activity of MGL, a 2-AG metabolizing enzyme, in the TM tissues. Furthermore, this study suggests that 2-AG-induced enhancement of outflow facility involves the p42/44 MAP kinase signaling pathway and changes in actin cytoskeletons in TM cells.

Rho GTPase/Rho kinase inhibition as a novel target for the treatment of glaucoma

Rho kinase (ROCK1 and ROCK2) is a serine/threonine kinase that serves as an important downstream effector of Rho GTPase, and plays a critical role in regulating the contractile tone of smooth muscle tissues in a calcium-independent manner. Several lines of experimental evidence indicate that modulating ROCK activity within the aqueous humor outflow pathway using selective inhibitors could achieve very significant benefits for the treatment of increased intraocular pressure in patients with glaucoma. The rationale for such an approach stems from experimental data suggesting that both ROCK and Rho GTPase inhibitors can increase aqueous humor drainage through the trabecular meshwork, leading to a decrease in intraocular pressure. In addition to their ocular hypotensive properties, inhibitors of both ROCK and Rho GTPase have been shown to enhance ocular blood flow, retinal ganglion cell survival and axon regeneration. These properties of the ROCK and Rho GTPase inhibitors indicate that targeting the Rho GTPase/ROCK pathway with selective inhibitors represents a novel therapeutic approach aimed at lowering increased intraocular pressure in glaucoma patients.

Lens-derived Semaphorin3A regulates sensory innervation of the cornea

The cornea, one of the most highly innervated tissues of the body, is innervated by trigeminal sensory afferents. During development, axons are initially repelled at the corneal margin, resulting in the formation of a circumferential nerve ring. The nature and source of guidance molecules that regulate this process remain a mystery. Here, we show that the lens, which immediately underlies the cornea, repels trigeminal axons in vivo and in vitro. Lens ablation results in premature, disorganized corneal innervation and disruption of the nerve ring and ventral plexus. We show that Semaphorin3A (Sema3A) is expressed in the lens epithelium and its receptor Neuropilin-1 (Npn1) is expressed in the trigeminal ganglion during cornea development. Inhibition of Sema3A signaling abrogates axon repulsion by the lens and cornea in vitro and phenocopies lens removal in vivo. These results demonstrate that lens-derived Sema3A mediates initial repulsion of trigeminal sensory axons from the cornea and is necessary for the proper formation of the nerve ring and positioning of the ventral plexus in the choroid fissure.

Roles of cell-adhesion molecules nectin 1 and nectin 3 in ciliary body development

Nectins are Ca2+-independent immunoglobulin-like cell-cell-adhesion molecules consisting of four members. Nectins homophilically and heterophilically trans-interact to form a variety of cell-cell junctions, including cadherin-based adherens junctions in epithelial cells and fibroblasts in culture, synaptic junctions in neurons, and Sertoli cell-spermatid junctions in the testis, in cooperation with, or independently of, cadherins. To further explore the function of nectins, we generated nectin 1–/– and nectin 3–/– mice. Both nectin 1–/– and nectin 3–/– mice showed a virtually identical ocular phenotype, microphthalmia, accompanied by a separation of the apex-apex contact between the pigment and non-pigment cell layers of the ciliary epithelia. Immunofluorescence and immunoelectron microscopy revealed that nectin 1 and nectin 3, but not nectin 2, localized at the apex-apex junctions between the pigment and non-pigment cell layers of the ciliary epithelia. However, nectin 1–/– and nectin 3–/– mice showed no impairment of the apicolateral junctions between the pigment epithelia where nectin 1, nectin 2 and nectin 3 localized, or of the apicolateral junctions between the non-pigment epithelia where nectin 2 and nectin 3, but not nectin 1, localized. These results indicate that the heterophilic trans-interaction between nectin 1 and nectin 3 plays a sentinel role in establishing the apex-apex adhesion between the pigment and non-pigment cell layers of the ciliary epithelia that is essential for the morphogenesis of the ciliary body.

Proteasome inhibition by chronic oxidative stress in human trabecular meshwork cells

The pathophysiologic mechanisms leading to the malfunction of the trabecular meshwork (TM)-Schlemm's canal (SC) outflow pathway in glaucoma are still unclear. We hypothesize that chronic oxidative stress may contribute to the malfunction of the outflow pathway by impairing the intracellular proteasome system of the cells, decreasing the ability of the tissue to modulate outflow resistance. To study the effects of chronic oxidative stress on proteasome function, primary cultures of human TM cells were incubated under 40% oxygen and proteasome activity was analyzed by measuring the accumulation of enhanced green fluorescent protein fused to a PEST motif. Changes in proteasome content, cellular senescence, and cell viability were also monitored. After 10 days of exposure to chronic oxidative stress, TM cells showed a marked decline in proteasome activity that was associated with premature senescence and decreased cell viability. These results suggest that proteasome failure may be involved in glaucoma pathophysiology.

In vivo gene transfer to the mouse eye using an HIV-based lentiviral vector; efficient long-term transduction of corneal endothelium and retinal pigment epithelium

We have evaluated the transduction profiles of an HIV-based lentiviral vector delivered regionally to ocular tissues in vivo. Following subretinal injection, a green fluorescent protein (GFP) reporter gene was efficiently and stably expressed in retinal pigment epithelial (RPE) cells. Limited transduction of adjacent photoreceptors occurred in newborn mice, but was inefficient in adult animals. Injection of the vector into the anterior chamber resulted in efficient and stable transduction of corneal endothelial cells. Efficient in vivo gene transfer into cells of the corneal endothelium and retinal pigment epithelium by lentiviral vectors may therefore offer a valuable approach to the treatment of disorders of the cornea and outer retina.

A large-scale in situ screen provides molecular evidence for the induction of eye anterior segment structures by the developing lens

The anterior segment of the vertebrate eye includes the cornea, iris, ciliary body, trabecular meshwork, and lens. Although malformations of these structures have been implicated in many human eye diseases, little is known about the molecular mechanisms that control their development. To identify genes involved in anterior segment formation, we developed a large-scale in situ hybridization screen and examined the spatial and temporal expression of over 1000 genes during eye development. This screen identified 62 genes with distinct expression patterns in specific eye structures, including several expressed in novel patterns in the anterior segment. Using these genes as developmental markers, we tested for the presence of inductive signals that control the differentiation of anterior segment tissues. Organ culture recombination experiments showed that a chick lens is capable of inducing the expression of markers of the presumptive iris and ciliary body in the developing mouse neural retina. The inducing activity from the lens acts only over short ranges and is present at multiple stages of eye development. These studies provide molecular evidence that an evolutionarily conserved signal from the lens controls tissue specification in the developing optic cup.