Combined effects of H-7 and cytochalasin B on outflow facility in monkeys

Deconstructing aqueous humor outflow - The last 50 years

Herein partially summarizes one scientist-clinician's wanderings through the jungles of primate aqueous humor outflow over the past ~45 years. Totally removing the iris has no effect on outflow facility or its response to pilocarpine, whereas disinserting the ciliary muscle (CM) from the scleral spur/trabecular meshwork (TM) completely abolishes pilocarpine's effect. Epinephrine increases facility in CM disinserted eyes. Cytochalasins and latrunculins increase outflow facility, subthreshold doses of cytochalasins and epinephrine given together increase facility, and phalloidin, which has no effect on facility, partially blocks the effect of both cytochalasins and epinephrine. H-7, ML7, Y27632 and nitric oxide - donating compounds all increase facility, consistent with a mechanosensitive TM/SC. Adenosine A1 agonists increase and angiotensin II decrease facility. OCT and optical imaging techniques now permit visualization and digital recording of the distal outflow pathways in real time. Prostaglandin (PG) F2α analogues increase the synthesis and release of matrix metalloproteinases by the CM cells, causing remodeling and thinning of the interbundle extracellular matrix (ECM), thereby increasing uveoscleral outflow and reducing IOP. Combination molecules (one molecule, two or more effects) and fixed combination products (two molecules in one bottle) simplify drug regimens for patients. Gene and stem cell therapies to enhance aqueous outflow have been successful in laboratory models and may fill an unmet need in terms of patient compliance, taking the patient out of the delivery system. Functional transfer of genes inhibiting the rho cascade or decoupling actin from myosin increase facility, while genes preferentially expressed in the glaucomatous TM decrease facility. In live NHP, reporter genes are expressed for 2+ years in the TM after a single intracameral injection, with no adverse reaction. However, except for one recent report, injection of facility-effective genes in monkey organ cultured anterior segments (MOCAS) have no effect in live NHP. While intracameral injection of an FIV. BOVPGFS-myc.GFP PGF synthase vector construct reproducibly induces an ~2 mmHg reduction in IOP, the effect is much less than that of topical PGF2⍺ analogue eyedrops, and dissipates after 5 months. The turnoff mechanism has yet to be defeated, although proteasome inhibition enhances reporter gene expression in MOCAS. Intracanalicular injection might minimize off-target effects that activate turn-off mechanisms. An AD-P21 vector injected sub-tenon is effective in 'right-timing' wound healing after trabeculectomy in live laser-induced glaucomatous monkeys. In human (H)OCAS, depletion of TM cells by saponification eliminates the aqueous flow response to pressure elevation, which can be restored by either cultured TM cells or by IPSC-derived TM cells. There were many other steps along the way, but much was accomplished, biologically and therapeutically over the past half century of research and development focused on one very small but complex ocular apparatus. I am deeply grateful for this award, named for a giant in our field that none of us can live up to.

Latrunculin B Reduces Intraocular Pressure in Human Ocular Hypertension and Primary Open-Angle Glaucoma

PURPOSE

To evaluate the safety, tolerability, and intraocular pressure (IOP)-lowering effect of Latrunculin-B (Lat-B), a marine macrolide that disrupts the actin cytoskeleton, in patients with ocular hypertension (OHT) or early primary open-angle glaucoma (POAG).

METHODS

In this Phase I, multicenter, double-masked, randomized, placebo-controlled, ascending-dose study, subjects with bilateral OHT or early POAG (>22 mm Hg) received one of four concentrations of INS115644 (Lat-B ophthalmic solutions, 0.005%, 0.01%, 0.02%, or 0.05%) in one eye over 3 days (5 single-dose instillations, separated by 12 hours). One eye was randomly assigned to active drug, the other to placebo. IOP was measured prior to treatment initiation (day 0) and on days 1 and 3.

RESULTS

Baseline IOPs were 22.9 ± 2.4 mm Hg and 23.5 + 3.1 mm Hg in the 0.02% and 0.05% dose groups, respectively. At 4 hours post instillation of the first dose, 0.02% INS115644 reduced IOP from baseline (mean ± SE) by 3.8 ± 0.7 mm Hg (P = 0.002) and 0.05% by 3.9 ± 1.0 mm Hg (P = 0.004). A maximum IOP decrease of 24% was noted at 4 hours after the fifth instillation of 0.02%. Adjusting for diurnal baseline and IOP in the contralateral, placebo-treated eye, the maximal 12-hour hypotensive effect was 4.0 ± 0.5 mm Hg (adjusted mean ± SE), a 17% decrease, following the fifth instillation of 0.02% (day 3). Adverse events were few and consisted mainly of mild redness, irritation, and a transient, clinically insignificant increase (≤2.5%) in central corneal thickness.

CONCLUSIONS

In OHT or POAG patients, twice daily Lat-B significantly lowered IOP compared with contralateral, placebo-treated eyes, with few and mild ocular adverse events.

TRANSLATIONAL RELEVANCE

Lat-B may be a potential therapeutic agent for glaucoma.

Effects of Y27632 on aqueous humor outflow facility with changes in hydrodynamic pattern and morphology in human eyes

PURPOSE

To determine the effect of Y27632, a Rho-kinase inhibitor on aqueous outflow facility, flow pattern, and juxtacanalicular tissue (JCT)/trabecular meshwork (TM) morphology in human eyes.

METHODS

Sixteen enucleated human eyes were perfused with PBS plus glucose (GPBS) at 15 mm Hg to establish the baseline outflow facility. Six eyes were perfused for short-duration (30 minute) with either 50 μM Y27632 or GPBS (n = 3 per group). Ten eyes were perfused for long duration (3 hours) with either 50 μM Y27632 or GPBS (n = 5 per group). Outflow pattern was labeled using fluorescent microspheres, and effective filtration length (EFL) was measured. Morphologic changes and their relationship to EFL and facility were analyzed.

RESULTS

Outflow facility significantly increased after short-duration perfusion with Y27632 compared with its own baseline (P = 0.03), but did not reach statistical significance compared with its controls (P = 0.07). Outflow facility (P = 0.01) and EFL (P < 0.05) were significantly increased after long-duration perfusion with Y27632 compared with its controls. Increases in outflow facility and EFL demonstrated a positive correlation. Morphologically, the TM and JCT of high-tracer regions were more expanded compared with low-tracer regions. A significant increase in JCT thickness was found in the long-duration Y27632 group compared with its control group (10.0 vs. 8.0 μm, P < 0.01).

CONCLUSIONS

Y27632 increases outflow facility in human eyes. This increase correlates positively with an increase in EFL, which is associated with an increased expansion in the JCT. Our data suggest that EFL could serve as a novel parameter to correlate with outflow facility.

Galectin-8 promotes cytoskeletal rearrangement in trabecular meshwork cells through activation of Rho signaling

Purpose

The trabecular meshwork (TM) cell-matrix interactions and factors that influence Rho signaling in TM cells are thought to play a pivotal role in the regulation of aqueous outflow. The current study was designed to evaluate the role of a carbohydrate-binding protein, galectin-8 (Gal8), in TM cell adhesion and Rho signaling.

Methods

Normal human TM cells were assayed for Gal8 expression by immunohistochemistry and Western blot analysis. To assess the role of Gal8 in TM cell adhesion and Rho signaling, the cell adhesion and spreading assays were performed on Gal8-coated culture plates in the presence and the absence of anti-β₁ integrin antibody and Rho and Rho-kinase inhibitors. In addition, the effect of Gal8-mediated cell-matrix interactions on TM cell cytoskeleton arrangement and myosin light chain 2 (MLC2) phosphorylation was examined.

Principal Findings

We demonstrate here that Gal8 is expressed in the TM and a function-blocking anti-β₁ integrin antibody inhibits the adhesion and spreading of TM cells to Gal8-coated wells. Cell spreading on Gal8 substratum was associated with the accumulation of phosphorylated myosin light chain and the formation of stress fibers that was inhibited by the Rho inhibitor, C3 transferase, as well as by the Rho-kinase inhibitor, Y27632.

Conclusions/Significance

The above findings present a novel function for Gal8 in activating Rho signaling in TM cells. This function may allow Gal8 to participate in the regulation of aqueous outflow.

Similar hydrodynamic and morphological changes in the aqueous humor outflow pathway after washout and Y27632 treatment in monkey eyes

Our previous studies in bovine eyes demonstrated that the structural correlate to the increase in outflow facility after either Rho-kinase inhibitor Y-27632 (Y27) treatment or washout appeared to be separation between the juxtacanalicular tissue (JCT) and inner wall (IW) of the aqueous plexus, the bovine equivalent of Schlemm's canal (SC). While these findings suggest that Y27 and washout may increase outflow facility through a similar mechanism, the anatomy of bovine outflow pathway differs considerably from both the human and monkey outflow pathway; however, only the human eye does not exhibit washout. In light of this, we compared the effects of Y27 and washout on outflow facility, hydrodynamic patterns of outflow, and the morphology of the IW and JCT in monkey eyes, given that their anatomy is closer to human eyes. Twelve freshly enucleated monkey eyes were used in this study. Eyes were perfused with Dulbecco's PBS containing 5.5 mM glucose (GPBS) to establish a baseline facility at 15 mmHg. Four eyes were perfused for a short-duration (30 min) as a control, 4 eyes for a long-duration (180 min) to induce washout, and 4 eyes with GPBS+50 μM Y27 for 30 min. All eyes were then perfused with fluorescent microspheres (0.5 μm; 0.002%) to label the hydrodynamic patterns of outflow and then perfusion-fixed. Confocal images of frontal sections were taken along the IW of SC. The total length (TL) and the tracer-decorated length (FL) of the IW were measured to calculate the average percent effective filtration length (PEFL = FL/TL). Sections with SC were examined by light and electron microscopy. The TL of the IW and the length exhibiting separation (SL) in the JCT were measured to calculate the average percent separation length (PSL = SL/TL). Outflow facility increased 149.2% (p < 0.01) from baseline after washout during long-duration perfusion, and 114.9% (p = 0.004) after Y27 treatment, but did not change significantly after short-duration perfusion in control eyes (p = 0.46). Distribution of the tracer labeling appeared punctate along the IW of control eyes, while a more uniform pattern was observed after washout and Y27 treatment. PEFL in washout (83.4 ± 2.1%) and Y27 treated eyes (82.5 ± 1.6%) was 3.4-fold larger compared to controls (24.2 ± 4.2%, P < 0.001). The JCT appeared distended with loss of connections between JCT cells and between JCT cells and their extracelluar matrix in eyes with washout or after Y-27 treatment. PSL in the JCT was 2.3-fold larger in washout eyes (77.4 ± 3.3%) and 2.2-fold larger in Y27 treated eyes (75.2 ± 5.3%) versus controls (33.5 ± 5.3%, p = 0.001). Significant positive correlations were found between outflow facility and PEFL, facility and PSL and between PEFL and PSL. Our data demonstrated that similar hydrodynamic and morphological changes occurred in the aqueous humor outflow pathway of monkey eyes after induction of washout and Y27 treatment. Both Y27 and washout increase outflow facility by redistributing aqueous outflow through a larger area in the JCT. These hydrodynamic changes are likely driven by morphologic changes associated with a decrease in cell-cell and cell-matrix connections in the JCT.

The management of exfoliative glaucoma

Exfoliation syndrome (XFS) is an age-related, generalized disorder of the extracellular matrix characterized by the production and progressive accumulation of a fibrillar extracellular material in many ocular tissues and is the most common identifiable cause of open-angle glaucoma worldwide. XFS plays an etiologic role in open-angle glaucoma, angle-closure glaucoma, cataract, and retinal vein occlusion. It is accompanied by an increase in serious complications at the time of cataract extraction, such as zonular dialysis, capsular rupture, and vitreous loss. It is associated systemically with an increasing number of vascular disorders, hearing loss, and Alzheimer's disease. XFS appears to be a disease of elastic tissue microfibrils. The characteristic fibrils, composed of microfibrillar subunits surrounded by an amorphous matrix comprising various glycoconjugates, contain predominantly epitopes of elastic fibers, such as elastin, tropoelastin, amyloid P, vitronectin, and components of elastic microfibrils, such as fibrillin-1, fibulin-2, vitronectin, microfibril-associated glycoprotein (MAGP-1), and latent TGF-beta binding proteins (LTBP-1 and LTBP-2), the proteoglycans syndecan and versican, the extracellular chaperone clusterin, the cross-linking enzyme lysyl oxidase, and other proteins. A recent milestone study showed that two common single nucleotide polymorphisms in the coding region of the lysyl oxidase-like 1 (LOXL1) gene located on chromosome 15 were specifically associated with XFS and XFG. LOXL1 is a member of the lysyl oxidase family of enzymes, which are essential for the formation, stabilization, maintenance, and remodeling of elastic fibers and prevent age-related loss of elasticity of tissues. LOXL1 protein is a major component of exfoliation deposits and appears to play a role in its accumulation and in concomitant elastotic processes in intra- and extraocular tissues of XFS patients. This discovery should open the way to new approaches and directions of therapy for this protein disorder.

The mechanism of increasing outflow facility by rho-kinase inhibition with Y-27632 in bovine eyes

Rho-kinase inhibitor Y-27632 (Y-27) affects actomyosin cytoskeletal networks and has been shown to significantly increase outflow facility (C) in enucleated porcine and rabbit eyes, as well as in vivo monkey eyes without obvious toxicity. The mechanisms underlying these responses remain largely unknown. In this study, we investigate how Y-27 affects aqueous humor C, the hydrodynamic patterns of outflow, and the morphology of the inner wall (IW) and juxtacanalicular connective tissue (JCT). 12 bovine eyes were perfused at 15 mmHg with Dulbecco's PBS containing 5.5 mM glucose (DPBS) to establish stable baseline C. The anterior chamber was exchanged and perfused with DPBS containing 50 microM Y-27 in 7 eyes, while 5 eyes received DPBS alone. Eyes were then perfused with DPBS containing fluorescent microspheres (0.5 microm; 0.002% v/v) at a fixed volume to deliver equivalent amounts of tracer to label the hydrodynamic filtration patterns. All eyes were perfusion-fixed with Karnovsky's fixative. Radial and frontal sections were prepared in all quadrants and confocal images were taken along the IW of the aqueous plexus (AP). The total length (TL) and filtration length (FL) of the IW were measured in > or =16 images/eye, and the average percent effective filtration length (PEFL=FL/TL) was calculated. Sections with AP were processed and examined by light and electron microscopy. The TL of the IW and length exhibiting JCT/IW separation (SL) were measured in > or =16 micrographs/eye, and the average percent separation length (PSL=SL/TL) was also calculated. After Y-27 treatment, C increased from 1.54+/-0.34 (+/-SEM) to 2.36+/-0.54 microL/min per mmHg (58.2+/-18.9%) while control eyes changed from 1.67+/-0.41 to 1.71+/-0.39 microl/min per mmHg (6.0+/-9.3%) and the percent changes between the Y-27-treated and control eyes were significant (p=0.03). Control eyes showed segmental distribution of tracer in the trabecular meshwork tending to cluster near collector channel ostia, whereas Y-27-treated eyes showed a more uniform pattern and extensive labeling along the IW. In Y-27-treated eyes, PEFL was 3-fold larger (57.6+/-3.7%) compared to control eyes (18.2+/-4.5%; p<0.001). Light microscopic examination revealed that, with Y-27, the IW and JCT were significantly distended compared to control eyes, with discernible separation between the IW and JCT. PSL was 2.8-fold larger in Y-27-treated eyes (59.3+/-3.6%) than in controls (20.8+/-2.0%; p<0.001). A significant positive correlation was found between PEFL and PSL (p=0.003) suggesting that as connectivity between the JCT and IW decreases the available area for aqueous humor drainage increases along the AP. Our study also demonstrates a significant positive correlation between C and the PSL (p=0.01), a finding identical to what we reported to occur during the "washout" effect in bovine eyes. Our data suggests the structural correlate to the increase in C and PEFL after Y-27-treatment is physical separation between the JCT and IW. The increase in C after Y-27-treatment may share a similar mechanism comparable with the washout effect that occurs in bovine eyes. Overall, these findings support our hypothesis that JCT/IW connectivity influences local outflow hydrodynamics that regulate C, and suggest that strategies targeting JCT/IW connectivity are promising anti-glaucoma therapies to reduce IOP.

Functional importance of the actin cytoskeleton in contraction of bovine iris sphincter muscle

1. The contractile capacity of smooth muscle cells depends on the cytoskeletal framework of the cell. The aim of this study was to determine the functional importance of both the actin and the tubulin components of the cytoskeleton in contractile responses of the bovine isolated iris sphincter muscle. 2. In each preparation, two contractions to the muscarinic agonist carbachol were obtained. The maximum responses of the first contractions were taken as 100%. The second contractions to carbachol were elicited in the presence of either cytochalasin B (50 and 5 microm), an inhibitor of the actin cytoskeleton, or colchicine (100 microm), an inhibitor of the tubulin cytoskeleton (30 min incubation). 3. Cytochalasin B, at a concentration of 50 microm, significantly decreased the contractions induced by carbachol, with the maximum response reduced to 21.8 +/- 6.6% (n = 12) of the initial maximum. The maximal contractions to carbachol in the presence of colchicine reached 96.2 +/- 7.9% (n = 9) of the initial contraction, which was not significantly different from control second responses to carbachol with neither drug present, which reached 113.3 +/- 7.6% (n = 7). 4. The effect of cytochalasin B was dose-dependent, since at a lower concentration of 5 microm, the drug decreased the maximum contraction to carbachol to 60.3 +/- 8.8% (n = 6). The effect of cytochalasin B was at least partially reversible, since after the use of the higher concentration of 50 microm, contractions to carbachol increased to 62.3 +/- 15.5% (n = 4) of the maximal response, after 1 h repeated washing of the preparations. 5. Cytochalasin D, at a concentration of 50 microm, completely abolished the contractions induced by carbachol (n = 4). 6. These findings suggest that in bovine iris sphincter muscle, contractions to carbachol are highly dependent, from a functional point of view, on actin polymerization, and not, to any important degree, on the polymerization of tubulin.

ML-7, chelerythrine and phorbol ester increase outflow facility in the monkey Eye

Baseline or post-drug outflow facility was measured by two-level constant pressure perfusion of the anterior chamber (AC). The AC of one eye of cynomolgus monkeys was exchanged with the myosin light chain kinase (MLCK) inhibitor ML-7, the protein kinase (PK) C inhibitor chelerythrine (CHEL), or the PKC activator phorbol myristate acetate (PMA), followed by continuous AC infusion of the drug. The opposite eye similarly received the corresponding vehicle solution. The facility-effectiveness of subthreshold doses of ML-7 or CHEL + a subthreshold dose of the serine-threonine kinase inhibitor H-7, and of facility-effective doses of CHEL + a subthreshold or effective dose of PMA, were also determined. In 45 min post-exchange perfusions, 100 and 500 microM ML-7 increased outflow facility by 32 and 76%, while 100 and 500 microM CHEL increased facility by 68 and 101%, respectively, adjusted for baseline and contralateral control eye resistance washout. In 90 min post-exchange perfusions, 100 microM ML-7 or CHEL time-dependently increased outflow facility by 23, 49 and 69%, or by 44, 108 and 125% in the first, second and third 30 min periods, respectively. At 50 microM, ML-7 was ineffective, but CHEL increased outflow facility by 36% in the third 30 min period. Ten microM H-7 potentiated the outflow facility effect of 50 microM ML-7 or 20 microM CHEL by 36 and 28%, respectively, in the second 30 min period, and that of 50 microM CHEL by 44% in the overall 60 min post-exchange perfusion, compared to the H-7 only-treated contralateral eye. Ten, 50 or 100 n M PMA dose-dependently increased outflow facility by 23, 62 or 174%. Ten n M PMA + 50 microM CHEL did not induce any additional significant changes in outflow facility compared to 50 n M CHEL alone, while the effect of 50 n M PMA and 100 microM CHEL together was 63% more than that of 100 microM CHEL alone. In conclusion, ML-7/CHEL may increase outflow facility by a cytoskeletal mechanism. Separate or combined treatment with CHEL and PMA increases outflow facility, suggesting that PKC inhibition may not be involved in the facility-increase with either drug.