Advances in glaucoma diagnosis and therapy for the next millennium: new drugs for trabecular and uveoscleral outflow

The Effects of Cannabidiol on Aqueous Humor Outflow and Trabecular Meshwork Cell Signaling

Intraocular pressure (IOP) is regulated primarily through aqueous humor production by ciliary body and drainage through uveoscleral and trabecular meshwork (TM) tissues. The goal of this study was to measure the effect of non-psychotropic cannabidiol (CBD) on aqueous humor outflow through TM and assess the effect of CBD on the TM cell signaling pathways that are important for regulating outflow. Perfused porcine eye anterior segment explants were used to investigate the effects of CBD on aqueous humor outflow. Cultured porcine TM cells were used to study the effects of CBD on TM cell contractility, myosin light chain (MLC) and myosin phosphatase targeting subunit 1 (MYPT1) phosphorylation, and RhoA activation. In the anterior segment perfusion experiments, aqueous humor outflow was increased significantly within 1 h after adding 1 µM CBD and the effect was sustained over the 5 h of measurement. Treatment of TM cells with 1 µM CBD significantly decreased TM cell-mediated collagen contraction, inhibited phosphorylation of MLC and MYPT1, and reduced RhoA activation. Our data demonstrate, for the first time, that as a potential therapeutic agent for lowering intraocular pressure, CBD can enhance aqueous humor outflow and modify TM cell signaling.

Genetic Determinants of Intraocular Pressure

Intraocular pressure (IOP) is the cardinal and only modifiable risk factor for glaucoma, the leading cause of irreparable blindness worldwide. Twin and family studies estimate the heritability of IOP to be 40-70%, and linkage studies for IOP have identified numerous loci. Mutations in MYOC can cause markedly elevated IOP and aggressive glaucoma often requiring surgical intervention. However, the majority of the genetic basis for raised IOP and glaucoma in populations is complex, and recent large genome-wide association studies (GWASs) have identified over 100 common variants that contribute to IOP variation. In combination, these loci are predictive for primary open-angle glaucoma in independent populations, achieving an area under the receiver operating characteristic curve of 76% for high-pressure primary open-angle glaucoma; this suggests the possibility of targeted screening in the future. Additionally, GWAS findings have identified important biological pathways underlying IOP regulation, including lymphangiogenesis and lipid metabolism, providing novel targets for new therapies. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Rho kinase inhibitors for treatment of glaucoma

Affecting 60 million patients, glaucoma is the second leading cause of blindness worldwide. Despite the availability of multiple medical and surgical treatments with effective intraocular pressure lowering, many patients still progress to become visually handicapped from glaucoma due to therapeutic failure. There is therefore a great need for novel therapies to improve the standard of care, and Rho kinase (ROCK) inhibitors represent a promising new class of drugs for treatment of glaucoma. ROCK inhibitors act by increasing facility of fluid outflow from the eye, thereby reducing intraocular pressure. ROCK inhibitors also have a vasodilatory effect on conjunctival vessels, which can lead to eye redness, a less than desirable cosmetic side effect for patients that would use this medication. Although there is promising data to support the clinical potential of this class of drug, the occurrence of conjunctival hyperemia remains a potential deterrent for use by patients. Studies are underway to assess alternative dosing strategies, delivery methods and prodrug formulations that may circumvent this unwanted side effect. This review provides an up-to-date account of the basic scientific data, as well as nonclinical and clinical studies to support use of ROCK inhibitors for treatment of glaucoma.

Control of outflow resistance by soluble adenylyl cyclase

Abstract Glaucoma is a leading cause of blindness in the United States affecting as many as 2.2 million Americans. All current glaucoma treatment strategies aim to reduce intraocular pressure, even in patients with normal tension glaucoma. Typically, this is accomplished by reducing the rate of aqueous flow by limiting aqueous production or enhancing drainage using drugs and surgery. Whereas these strategies are effective in diminishing vision loss, some patients continue to lose vision and many discontinue use of their medications because of undesirable side effects. Drugs known to be effective in altering conventional outflow have for the most part been abandoned from modern clinical practice due to undesirable side effects. Identification of new drugs that could enhance conventional outflow, would offer additional options in the treatment of glaucoma and ocular hypertension. To this end, our laboratory has recently uncovered a novel pathway for regulation of conventional outflow by the ciliary body. This pathway is dependent on soluble adenylyl cyclase, an enzyme that catalyzes the generation of cyclic adenosine 3',5' monophosphate (cAMP) in response to bicarbonate.

Physical and functional interaction between CB1 cannabinoid receptors and beta2-adrenoceptors

BACKGROUND AND PURPOSE

The CB(1) cannabinoid receptor and the beta(2)-adrenoceptor are G protein-coupled receptors (GPCRs) co-expressed in many tissues. The present study examined physical and functional interactions between these receptors in a heterologous expression system and in primary human ocular cells.

EXPERIMENTAL APPROACH

Physical interactions between CB(1) receptors and beta(2)-adrenoceptors were assessed using bioluminescence resonance energy transfer (BRET). Functional interactions between these receptors were evaluated by examining receptor trafficking, as well as extracellular signal-regulated kinase (ERK) and cyclic AMP response element binding protein (CREB) signalling.

KEY RESULTS

Physical interactions between CB(1) receptors and beta(2)-adrenoceptors were demonstrated using BRET. In human embryonic kidney (HEK) 293H cells, co-expression of beta(2)-adrenoceptors tempered the constitutive activity and increased cell surface expression of CB(1) receptors. Co-expression altered the signalling properties of CB(1 )receptors, resulting in increased Galpha(i)-dependent ERK phosphorylation, but decreased non-Galpha(i)-mediated CREB phosphorylation. The CB(1) receptor inverse agonist AM251 (N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide) attenuated beta(2)-adrenoceptor-pERK signalling in cells expressing both receptors, while the CB(1) receptor neutral antagonist O-2050 ((6aR,10aR)-3-(1-methanesulfonylamino-4-hexyn-6-yl)-6a,7,10,10a-tetrahydro-6,6,9-trimethyl-6H-dibenzo[b,d]pyran) did not. The actions of AM251 and O-2050 were further examined in primary human trabecular meshwork (HTM) cells, which are ocular cells endogenously co-expressing CB(1) receptors and beta(2)-adrenoceptors. In HTM cells, as in HEK 293H cells, AM251 but not O-2050, altered the beta(2)-adrenoceptor-pERK response.

CONCLUSION AND IMPLICATIONS

A complex interaction was demonstrated between CB(1) receptors and beta(2)-adrenoceptors in HEK 293H cells. As similar functional interactions were also observed in HTM cells, such interactions may affect the pharmacology of these receptors in tissues where they are endogenously co-expressed.

Effects of MISA A on actin cytoskeleton of cultured HTM cells and intraocular pressure of rats and glaucomatous monkeys

PURPOSE

To determine the effects of misakinolide (MISA) A, which leads to the disassembly of actin filaments, on the actin cytoskeleton of cultured human trabecular meshwork (HTM) cells and on intraocular pressure (IOP) in living rats and monkeys.

METHODS

Cultured HTM cells were treated with MISA A, and the changes in the actin cytoskeleton were determined by immunofluorescence microscopy. Elevated IOP was induced in cynomolgus monkeys by unilateral laser photocoagulation of the trabecular meshwork (TM). The IOP response after topical administration of MISA A was determined in normotensive rats (Tonopen) and glaucomatous monkeys (pneumotonometer and Tonopen) at 0.5, 1, 2, 3, 4, 5, and 6 hr.

RESULTS

MISA A caused dose- and time-dependent disruption of actin stress fibers in cultured HTM cells. Actin microfilaments and vinculin containing focal contacts deteriorated after 2 hr, 30 and 10 min of incubation with 5 nM, 10 nM, and 25 nM MISA A, respectively. Recovery was also dose- and time-dependent. The actin-disrupting effects were not reversible when the cells were incubated with MISA A at a low dose (10 nM) for 24 hr or a high dose (25 nM) for 30 min. Topical administration of MISA A significantly decreased IOP in rats by 5.8 +/- 0.5 (mean +/- SEM) (p < 0.05) Tonopen rat units. In monkeys, IOP was decreased by 3.8 +/- 0.5 mmHg (p < 0.001) in the normotensive eye and by 9.2 +/- 1.2 mmHg (p < 0.001) in the glaucomatous eye.

CONCLUSIONS

MISA A greatly altered the actin cytoskeleton and cellular adhesions and reduced IOP, suggesting that MISA A may be a useful antiglaucoma strategy.

Agonist-dependent cannabinoid receptor signalling in human trabecular meshwork cells

BACKGROUND AND PURPOSE

Trabecular meshwork (TM) is an ocular tissue involved in the regulation of aqueous humour outflow and intraocular pressure (IOP). CB1 receptors (CB1) are present in TM and cannabinoid administration decreases IOP. CB1 signalling was investigated in a cell line derived from human TM (hTM).

EXPERIMENTAL APPROACH

CB1 signalling was investigated using ratiometric Ca2+ imaging, western blotting and infrared In-Cell Western analysis.

KEY RESULTS

WIN55212-2, a synthetic aminoalkylindole cannabinoid receptor agonist (10-100 microM) increased intracellular Ca2+ in hTM cells. WIN55,212-2-mediated Ca2+ increases were blocked by AM251, a CB1 antagonist, but were unaffected by the CB2 antagonist, AM630. The WIN55,212-2-mediated increase in [Ca2+]i was pertussis toxin (PTX)-insensitive, therefore, independent of Gi/o coupling, but was attenuated by a dominant negative Galpha(q/11) subunit, implicating a Gq/11 signalling pathway. The increase in [Ca2+]i was dependent upon PLC activation and mobilization of intracellular Ca2+ stores. A PTX-sensitive increase in extracellular signal-regulated kinase (ERK1/2) phosphorylation was also observed in response to WIN55,212-2, indicative of a Gi/o signalling pathway. CB1-Gq/11 coupling to activate PLC-dependent increases in Ca2+ appeared to be specific to WIN55,212-2 and were not observed with other CB1 agonists, including CP55,940 and methanandamide. CP55940 produced PTX-sensitive increases in [Ca2+]i at concentrations>or=15 microM, and PTX-sensitive increases in ERK1/2 phosphorylation.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates that endogenous CB1 couples to both Gq/11 and Gi/o in hTM cells in an agonist-dependent manner. Cannabinoid activation of multiple CB1 signalling pathways in TM tissue could lead to differential changes in aqueous humour outflow and IOP.

Effects of novel ethacrynic acid derivatives on human trabecular meshwork cell shape, actin cytoskeletal organization, and transcellular fluid flow

To determine efficacy and therapeutic index in the context of ocular hypotensive activity of the new ethacrynic acid (ECA) derivatives of the series (SA8,248 and SA8,389), 9,000 series (SA9,000, SA9,622 and SA9,995) and ticrynafen, we undertook a comparative evaluation of the dose-dependent effects of these compounds on human trabecular meshwork (HTM) cell shape, actin cytoskeletal organization, focal adhesions and transcellular fluid flow. Responses were either scored using an arbitrary scale of 1-5 or quantified. Compounds of the 9000 series (SA9,995>SA9,000>SA9,622) were found to be 14- to 20-fold more potent than ECA, ticrynafen or analogs from the 8,000 series (SA8,389>SA8,248) in terms of ability to induce cell shape alterations in HTM cells. Similarly, compounds of the 9,000 series (SA9,995>SA9,622>SA9,000) were found to be much stronger (2 to 20 fold) than ECA, ticrynafen or analogs of the 8000 series in terms of affecting decreases in actin stress fiber content in HTM cells. Analogs of the 9000 series (SA9,622>SA9,995>SA9,000) were also observed to be 8 to 10 fold more potent than ECA (SA8,389>ECA>SA8,248>ticrynafen) at eliciting decreases in cellular focal adhesions. Interestingly, analogs of the 9000 series (SA9,000>SA9,622>SA9,995) and SA8,248 demonstrated a huge increase (by many folds) in transcellular fluid flow of HTM cell monolayers as compared to ECA and ticrynafen. Collectively, these analyses revealed that the structural modification of ECA improves its ocular hypotensive efficacy, indicating that the SA9,000 series compounds might be promising novel ocular hypotensive drugs.

T-cell-based vaccination for morphological and functional neuroprotection in a rat model of chronically elevated intraocular pressure

Acute or chronic glaucoma is often associated with an increase in intraocular pressure (IOP). In many patients, however, therapeutic pressure reduction does not halt disease progression. Neuroprotection has been proposed as a complementary therapeutic approach. We previously demonstrated effective T-cell-based neuroprotection in experimental animals vaccinated with the synthetic copolymer glatiramer acetate (copolymer-1, Cop-1), a weak agonist of self-antigens. This study was undertaken to test different routes and modes of vaccination with Cop-1 as treatment modalities for protection against retinal ganglion cell (RGC) death caused by chronic elevation of IOP in rats, and to determine whether anatomical neuroprotection is accompanied by functional neuroprotection. In a chronic model of unilaterally high IOP, Cop-1 vaccination, with or without an adjuvant, protected rats against IOP-induced loss of RGCs by eliciting a systemic T-cell-mediated response capable of cross-reacting with self-antigens residing in the eye. In rats deprived of T cells, Cop-1 (unlike treatment with alpha2-adrenoreceptor agonists) was not protective of RGCs, substantiating the contention that its beneficial effect is not conferred directly but is T-cell-mediated. Pattern electroretinography provided evidence of functional protection. Thus, vaccination with adjuvant-free Cop-1 can protect RGCs from the consequences of elevated IOP in rats. This protection is manifested both morphologically and functionally. These findings can be readily implemented for the development of a therapeutic vaccination to arrest the progression of glaucoma.

Regulation of myosin light chain phosphorylation in the trabecular meshwork: role in aqueous humour outflow facility

Cellular contraction and relaxation and integrity of the actin cytoskeleton in trabecular meshwork (TM) tissue have been thought to influence aqueous humour outflow. However, the cellular pathways that regulate these events in TM cells are not well understood. In this study, we investigated physiological agonist-mediated regulation of myosin light chain (MLC) phosphorylation in the TM, and correlated such effects with alterations in aqueous outflow facility, since MLC phosphorylation is a critical biochemical determinant of cellular contraction in TM cells. Treatment of serum starved human TM cells with endothelin-1 (0.1 microM), thromboxane A2 mimetic U-46619 (1.0 microM), or angiotensin II (1 microM), all of which are agonists of G-protein coupled receptors, triggered activation of MLC phosphorylation, as determined by urea/glycerol-based Western blot analysis. Agonist-stimulated increase in MLC phosphorylation was associated with activation of Rho GTPase in TM cells, as determined in pull-down assays. In contrast, treatment of human TM cells with a novel Rho-kinase inhibitor H-1152 (0.1-2 microM), in the presence of serum reduced basal MLC phosphorylation. H-1152 also increased aqueous outflow facility significantly in a dose-dependent fashion, in perfusion studies with cadaver porcine eyes. This effect of H-1152 on outflow facility was associated with decreased MLC phosphorylation in TM tissue of drug-perfused eyes. Collectively, this study identifies potential physiological regulators of MLC phosphorylation in human TM cells and demonstrates the significance of Rho/Rho-kinase pathway-mediated MLC phosphorylation in modulation of aqueous outflow facility through TM.

The role of protein kinase C in modulation of aqueous humor outflow facility

The elevated intraocular pressure that is commonly associated with glaucoma is believed to arise due to impairment of trabecular meshwork (TM) function. Although the TM and Schlemm's canal (SC) comprise the major route for aqueous humor outflow, little is known about the potential signaling mechanisms involved in the regulation of aqueous outflow. Based on knowledge regarding the role of protein kinase C (PKC) in vascular biology, we sought to understand the contribution of the PKC pathway towards outflow function by studying the modulation of contractile and morphological characteristics of TM and SC cells. We investigated the involvement of PKC in regulation of myosin light chain (MLC) phosphorylation, formation of actin stress fibers and integrin-ECM adhesions (focal adhesions) in human TM and SC cells and correlated these changes with aqueous outflow facility measured in an enucleated porcine whole eye perfusion model. Expression and distribution of PKC isoforms (alpha and epsilon ) in TM and SC cells and tissues was confirmed by Western blot and immunohistochemical analysis, respectively. Both, pharmacological activators (phorbol-12-myristate 13-acetate (PMA) and phorbol-12,13-dibutyrate (PDBu)) and inhibitors (staurosporine and GF109203X) of PKC were found to induce changes in cell shape (retraction and rounding up) and cytoskeletal organization in human TM and SC cells. While PMA and PDBu produced an increase in formation of actin stress fibers and focal adhesions and in MLC phosphorylation, PKC inhibitors were observed to induce contrasting effects in these cells. Intriguingly, both PDBU and GF109203X caused increases in aqueous outflow facility in the perfusion model. The PKC inhibitor (GF109203X) increased outflow by 46% while the PKC activator (PDBu) only increased outflow by 27%. These results suggest that PKC might play an important role in modulation of aqueous outflow facility by regulating MLC phosphorylation and thereby, the morphological and cytoskeletal characteristics of TM and SC cells.