Effects of Unoprostone and Endothelin 1 on L-Type Channel Currents in Human Trabecular Meshwork Cells

Optogenetic stimulation of phosphoinositides reveals a critical role of primary cilia in eye pressure regulation

Glaucoma is a group of progressive optic neuropathies that cause irreversible vision loss. Although elevated intraocular pressure (IOP) is associated with the development and progression of glaucoma, the mechanisms for its regulation are not well understood. Here, we have designed CIBN/CRY2-based optogenetic constructs to study phosphoinositide regulation within distinct subcellular compartments. We show that stimulation of CRY2-OCRL, an inositol 5-phosphatase, increases aqueous humor outflow and lowers IOP in vivo, which is caused by a calcium-dependent actin rearrangement of the trabecular meshwork cells. Phosphoinositide stimulation also rescues defective aqueous outflow and IOP in a Lowe syndrome mouse model but not in IFT88^fl/fl mice that lack functional cilia. Thus, our study is the first to use optogenetics to regulate eye pressure and demonstrate that tight regulation of phosphoinositides is critical for aqueous humor homeostasis in both normal and diseased eyes.

Intraocular pressure reduction with once-a-day application of a new prostaglandin eye drop: a pilot placebo-controlled study in 12 patients

PURPOSE

To assess the ocular hypotensive effect of 15-keto fluprostenol, the oxidized metabolite of travoprost, on glaucoma patients, through a randomized double-masked placebo-controlled study.

METHODS

Twelve patients with ocular normal tension glaucoma (NTG) (intraocular pressure [IOP] < 22 mmHg) were enrolled. In order to ensure patient compliance to treatment, all study subjects were hospitalized. In each patient, the eye to be submitted to the treatments was randomly chosen. After hospital admission (day 1), those patients received for 5 days at 8 P.M. either one drop of 15-keto fluprostenol (35 μg/ml) or one drop of placebo. IOP evaluation was performed within 8 A.M. and 8 P.M. for 6 days. Furthermore, we performed a determination of cardiovascular parameters before and after the treatments.

RESULTS

Starting with the first IOP measurement after the first treatment (8 A.M. on day 2), IOP was reduced of about 14% in the eyes treated 15-keto fluprostenol, in comparison with baseline IOP values of 15-keto fluprostenol-treated patients. The IOP reduction in the 15-keto fluprostenol-treated group was significantly compared to placebo group (p < 0.05) starting from day 3 till day 6 of the study. Except for mild hyperemia in one 15-keto fluprostenol-treated eye, no other side effects were observed or reported by the enrolled patients.

CONCLUSIONS

The travoprost metabolite 15-keto fluprostenol was effective in decrease IOP and maintained IOP reduction along 5 days of treatment. The 15-keto fluprostenol can be developed as a good candidate for once-a-day NTG patients' treatment.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

An evidence-based review of unoprostone isopropyl ophthalmic solution 0.15% for glaucoma: place in therapy

Glaucoma is a progressive, neurodegenerative optic nerve disease that can cause significant visual morbidity and affects over 60 million people worldwide. The only known modifiable risk factor for glaucoma at this time is elevated intraocular pressure (IOP), which may be treated with medications, laser therapy, and/or incisional surgery. Topical ocular medications are commonly used as first-line therapy for glaucoma, although side effects may limit their use. Unoprostone is a novel 22-carbon ocular hypotensive agent that may be advantageous in treating some patients with open angle glaucoma or ocular hypertension. Unlike the 20-carbon prostanoids, such as latanoprost, that lower IOP primarily through an increase in uveoscleral outflow, unoprostone may lower IOP through increased aqueous outflow via the conventional trabecular meshwork pathway. Although not as efficacious as other prostanoids, unoprostone is effective for IOP reduction both as monotherapy and adjunctive therapy with timolol. Unoprostone has decreased affinity for the prostaglandin F2α receptor, which may explain its well tolerated ocular and systemic side effect profile compared with other prostanoids.

Endogenous Bioactive Lipids and the Regulation of Conventional Outflow Facility

Perturbation of paracrine signaling within the human conventional outflow pathway influences tissue homeostasis and outflow function. For example, exogenous introduction of the bioactive lipids, sphingosine-1-phosphate, anandamide or prostaglandin F(2α), to conventional outflow tissues alters the rate of drainage of aqueous humor through the trabecular meshwork, and into Schlemm's canal. This review summarizes recent data that characterizes endogenous bioactive lipids, their receptors and associated signaling partners in the conventional outflow tract. We also discuss the potential of targeting such signaling pathways as a strategy for the development of therapeutics to treat ocular hypertension and glaucoma.

Calcium channels and their blockers in intraocular pressure and glaucoma

Several factors besides high intraocular pressure assumed to be associated with the development and progression of glaucoma, and calcium channel blockers (CCBs) have been an anticipated option for glaucoma treatment by improving ocular perfusion and/or exerting neuroprotective effects on retinal ganglion cells with safety established in wide and long-term usage. Decrease in IOP has been reported after topical application of CCBs, however, the effect is much smaller and almost negligible after systemic application. Various CCBs have been reported to increase posterior ocular blood flow in vivo and to exert direct neuroprotection in neurons in vitro. Distribution of the drug at a pharmacologically active concentration in the posterior ocular tissues across the blood-brain barrier or blood-retina barrier, especially in the optic nerve head and retina where the ganglion cells mainly suffer from glaucomatous damage, is essential for clinical treatment of glaucoma. Improved visual functions such as sensitivity in the visual field test have been reported after administration of CCBs, but evidences from the randomized studies have been limited and effects of CCBs on blood flow and direct neuroprotection are hardly distinguished from each other.

Role of prostaglandins and specific place in therapy of bimatoprost in the treatment of elevated intraocular pressure and ocular hypertension: A closer look at the agonist properties of bimatoprost and the prostamides

Bimatoprost is the only representative of a novel class of prostaglandin ethanolamide (prostamide) compounds used therapeutically as an efficacious treatment for glaucoma. The pathways through which bimatoprost works to improve uveoscleral outflow to relieve elevated intraocular pressure are similar to those of the conventional prostaglandins used in glaucoma therapy, with some evidence of a preferential action at the trabecular meshwork. The pharmacology of bimatoprost is however, unclear. Pharmacological evidence supports a specific and distinct receptor-mediated agonist activity of bimatoprost at ‘prostamide’ receptors, which is selective to the prostamides as a class. However, other studies have reported either activity of bimatoprost at additional prostanoid and nonprostanoid receptors, or a conversion of bimatoprost to metabolites with agonist activity at prostaglandin FP receptors in the human eye. The formation of endogenous prostamides has been demonstrated in vivo, by a novel pathway involving the cyclooxygenase-2-mediated conversion of endogenous cannabinoid (endocannabinoid) substrates. Irrespective of the pharmacology of bimatoprost and the prostamides in general, further studies are needed to determine the biological role and biochemical pathology of prostamides in the human eye, particularly in glaucoma. Such studies may improve our understanding of uveoscleral flow and may offer new treatments for controlling intraocular pressure.

Effects of Topical Travoprost and Unoprostone on Optic Nerve Head Circulation in Normal Rabbits

PURPOSE

To evaluate the effect, and the duration of the effect, of topically administrated travoprost and unoprostone on optic nerve head (ONH) circulation in Dutch rabbits.

METHODS

First, travoprost (0.004% solution) or unoprostone (0.12% solution) was unilaterally instilled once, or once daily (travoprost) or twice daily (unoprostone) for 7 days in Dutch rabbits. The ONH tissue blood velocity (NB(ONH)) was measured using the laser speckle method at 30 and 60 min after a single instillation of travoprost or unoprostone, and before and at 1, 6, and 12 hr (travoprost or unoprostone) and 24 hr (travoprost only) after the last instillation of the aforementioned 7-day instillation regimen. Second, similar experiments were conducted with indomethacin (5 mg/kg) pretreatment.

RESULTS

Both travoprost and unoprostone significantly increased NB(ONH) only in the treated eyes after a single instillation (p = 0.011 to 0.038); this effect was abolished by indomethacin pretreatment. In the 7-day instillation regimen, NB(ONH) was increased by 11%, 40%, 17%, 16%, and 12% only in the treated eyes just before and at 1, 6, 12, and 24 hr after the final instillation of travoprost, respectively, and increased by 10%, 25%, 13%, and 14% only in the treated eyes just before and at 1, 6, 12 hr after the final instillation of the unoprostone, respectively.

CONCLUSIONS

Topical travoprost or unoprostone significantly increased the ONH blood velocity with a single instillation and the effect persisted for 24 hr after a 7-day instillation. The effects of these drugs against retinal and ONH circulation are probably associated with the production of endogenous prostaglandins.

Local effect of topical FP-receptor agonists on retinal vessels of the ipsilateral posterior retina in normal rabbit eyes

PURPOSE

To determine whether a topically instilled prostaglandin analogue inhibits endothelin-1 (ET-1)-induced vasoconstrictive effects in the posterior retina by its local effects, and the duration of the effect in normal rabbit eyes.

METHODS

Travoprost, a potent selective FP-agonist, or unoprostone, a prostone that also has a weak non-selective FP-agonistic activity, solution was instilled once, or once daily (travoprost) or twice daily (unoprostone) for 7 days in one randomly chosen Dutch rabbit eye, and vehicle in the contralateral eye. ET-1 was intravitreously injected in both eyes 30 min after a single instillation of a test drug or its vehicle, or just after, 30, 60, 90 or 180 min after the final instillation of a 7-day instillation, and fundus photographed before, 30 and 60 min after the injection to study whether difference was seen in the ET-1-induced constriction of retinal vessels between the drug- and vehicle-instilled eyes. The same experimental procedures were conducted with indomethacin pretreatment.

RESULTS

In the rabbit eyes where travoprost was instilled for 7 days, the ET-1-induced constriction of retinal vessels was significantly inhibited only on the drug-treated side, when ET-1 was injected 30 or 60 min after the final instillation (P = 0.026-0.005), which was abolished by indomethacin pretreatment. A single instillation of travoprost or unoprostone and 7-day instillation of unoprostone showed no effect.

CONCLUSIONS

After a 7-day instillation in normal rabbit eyes, topical travoprost suppressed ET-1-induced vasoconstrictive effects only in the ipsilateral posterior retina by its local effect; this effect was maintained at least for 30 min and mediated by endogenous prostaglandins.

Update on the mechanism of action of topical prostaglandins for intraocular pressure reduction

A decade has passed since the first topical prostaglandin analog was prescribed to reduce intraocular pressure (IOP) for the treatment of glaucoma. Now four prostaglandin analogs are available for clinical use around the world and more are in development. The three most efficacious of these drugs are latanoprost, travoprost, and bimatoprost, and their effects on IOP and aqueous humor dynamics are similar. A consistent finding is a substantial increase in uveoscleral outflow and a less consistent finding is an increase in trabecular outflow facility. Aqueous flow appears to be slightly stimulated as well. Prostaglandin receptors and their associated mRNAs have been located in the trabecular meshwork, ciliary muscle, and sclera, providing evidence that endogenous prostaglandins have a functional role in aqueous humor drainage. Earlier evidence found that topical PG analogs release endogenous prostaglandins. One well-studied mechanism for the enhancement of outflow by prostaglandins is the regulation of matrix metalloproteinases and remodeling of extracellular matrix. Other proposed mechanisms include widening of the connective tissue-filled spaces and changes in the shape of cells. All of these mechanisms alter the permeability of tissues of the outflow pathways leading to changes in outflow resistance and/or outflow rates. This review summarizes recent (since 2000) animal and clinical studies of the effects of topical prostaglandin analogs on aqueous humor dynamics and recent cellular and molecular studies designed to clarify the outflow effects.

Mechanism of unoprostone-induced cytosolic Ca2+ mobility in cultured porcine corneal endothelial cells

The effect of unoprostone isopropyl on the intracellular free Ca(2+) signaling ([Ca(2+)](i)) in cultured porcine corneal endothelial cells was evaluated by using fura-2-AM as a Ca(2+) probe. In Ca(2+)-containing buffer and Ca(2+)-free buffer, unoprostone increased [Ca(2+)](i) concentration dependently from 28.2 to 0.282microM, diluted from original concentration to 1/100, 1/1000 and 1/10,000. The response was inhibited on extracellular Ca(2+) removal. In Ca(2+)-free buffer, pretreatment of the cells with unoprostone inhibited the endoplasmic reticulum Ca(2+) pump inhibitor thapsigargin-induced [Ca(2+)](i) increase and the mitochondria uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP)-induced Ca(2+) release by 96 and 95%, respectively. Pretreatment of the cells with thapsigargin or CCCP also inhibited unoprostone-1-induced [Ca(2+)](i) rise by 84 and 57%, respectively. The intracellular calcium release caused by unoprostone was partially inhibited by phospholipase C inhibitor (U73122) and by phospholipase A(2) inhibitor aristolochic acid. After incubation of the cells with unoprostone in Ca(2+)-free buffer, the addition of 5mM CaCl(2) increased Ca(2+) influx, implying that release of Ca(2+) from internal stores caused by unoprostone further induced capacitative Ca(2+) entry. These results suggest that unoprostone-induced [Ca(2+)](i) increase in corneal endothelial cells results from Ca(2+) influx from external buffer and release of Ca(2+) from intracellular stores from the endoplasmic reticulum and mitochondria Ca(2+) stores followed by capacitative Ca(2+) entry. Unoprostone-induced intracellular Ca(2+) release was also modulated by phospholipase A(2) and C-coupled events.

Cellular and molecular effects of unoprostone as a BK channel activator

Effects of unoprostone isopropyl (unoprostone), a prostaglandin metabolite analog; latanoprost, a PGF(2alpha) analog; and PGF(2alpha) were examined in HCN-1A cells, a model system for studies of large conductance Ca(2+) activated K(+)(BK) channel activator-based neuroprotective agents. Unoprostone and latanoprost, both used as anti-glaucoma agents, have been suggested to act through FP receptors and have neuroprotective effects. Ion channel activation, plasma membrane polarization, [Ca(2+)](i) changes and protection against long-term irreversible glutamate-induced [Ca(2+)](i) increases were studied. Unoprostone activated iberiotoxin (IbTX)-sensitive BK channels in HCN-1A cells with an EC(50) of 0.6+/-0.2 nM and had no effect on Cl(-) currents. Unoprostone caused IbTX-sensitive plasma membrane hyperpolarization that was insensitive to AL8810, an FP receptor antagonist. In contrast, latanoprost and PGF(2alpha) activated a Cl(-) current sensitive to [Ca(2+)](i) chelation, tamoxifen and AL8810, and caused IbTX-insensitive, AL8810-sensitive membrane depolarization consistent with FP receptor-mediated Ca(2+) signaling Cl(-) current activation. Latanoprost and PGF(2alpha), but not unoprostone, increased [Ca(2+)](i). Unoprostone, PGF(2alpha) only partially, but not latanoprost protected HCN-1A cells against glutamate-induced Ca(2+) deregulation. These findings show that unoprostone has a distinctly different mechanism of action from latanoprost and PGF(2alpha). Whether unoprostone affects the BK channel directly or an unidentified signaling mechanism has not been determined.