Regulation of cyclic AMP production in adult human ciliary processes

Ocular surface disease: a known yet overlooked side effect of topical glaucoma therapy

Ocular surface disease (OSD), a disorder affecting the lacrimal and meibomian glands and the corneal and conjunctival epithelium, is a well-known complication of topical glaucoma therapy. OSD can present as a new or pre-existing condition that virtually any anti-glaucoma formulation can exacerbate. As such, both glaucoma and OSD frequently coexist. Typical OSD symptoms include ocular discomfort, redness, burning, and dryness, whereas signs include periorbital and eyelid skin pigmentation, conjunctival scarring, and superficial punctate keratitis. Pressure-lowering eyedrops can cause toxic, allergic, and inflammatory reactions on the ocular surface. The latter can result from either preservatives or direct toxicity from the active molecule. Although usually mild, OSD can cause significant symptoms that lead to poor quality of life, decreased compliance to therapy, glaucoma progression, and worse visual outcomes. Given the chronic nature of glaucoma, lack of curative therapy, and subsequent lifelong treatment, addressing OSD is necessary. This manuscript aims to provide an up-to-date overview of OSD's signs, symptoms, and pathogenic mechanisms from glaucoma therapy toxicity.

Role of cyclic AMP in the eye with glaucoma

Glaucoma is characterized by a slow and progressive degeneration of the optic nerve, including retinal ganglion cell (RGC) axons in the optic nerve head (ONH), leading to visual impairment. Despite its high prevalence, the biological basis of glaucoma pathogenesis still is not yet fully understood, and the factors contributing to its progression are currently not well characterized. Intraocular pressure (IOP) is the only modifiable risk factor, and reduction of IOP is the standard treatment for glaucoma. However, lowering IOP itself is not always effective for preserving visual function in patients with primary open-angle glaucoma. The second messenger cyclic adenosine 3′,5′-monophosphate (cAMP) regulates numerous biological processes in the central nervous system including the retina and the optic nerve. Although recent studies revealed that cAMP generated by adenylyl cyclases (ACs) is important in regulating aqueous humor dynamics in ocular tissues, such as the ciliary body and trabecular meshwork, as well as cell death and growth in the retina and optic nerve, the functional role and significance of cAMP in glaucoma remain to be elucidated. In this review, we will discuss the functional role of cAMP in aqueous humor dynamics and IOP regulation, and review the current medications, which are related to the cAMP signaling pathway, for glaucoma treatment. Also, we will further focus on cAMP signaling in RGC growth and regeneration by soluble AC as well as ONH astrocytes by transmembrane ACs to understand its potential role in the pathogenesis of glaucoma neurodegeneration

The glial cell modulator ibudilast attenuates neuroinflammation and enhances retinal ganglion cell viability in glaucoma through protein kinase A signaling

Glaucoma is a neurodegenerative disease and the leading cause of irreversible blindness worldwide. Vision deficits in glaucoma result from the selective loss of retinal ganglion cells (RGC). Glial cell-mediated neuroinflammation has been proposed to contribute to disease pathophysiology, but whether this response is harmful or beneficial for RGC survival is not well understood. To test this, we characterized the role of ibudilast, a clinically approved cAMP phosphodiesterase (PDE) inhibitor with preferential affinity for PDE type 4 (PDE4). Here, we demonstrate that intraocular administration of ibudilast dampened macroglia and microglia reactivity in the retina and optic nerve hence decreasing production of proinflammatory cytokines in a rat model of ocular hypertension. Importantly, ibudilast promoted robust RGC soma survival, prevented axonal degeneration, and improved anterograde axonal transport in glaucomatous eyes without altering intraocular pressure. Intriguingly, ocular hypertension triggered upregulation of PDE4 subtype A in Müller glia, and ibudilast stimulated cAMP accumulation in these cells. Co-administration of ibudilast with Rp-cAMPS, a cell-permeable and non-hydrolysable cAMP analog that inhibits protein kinase A (PKA), completely blocked ibudilast-induced neuroprotection. Collectively, these data demonstrate that ibudilast, a safe and well-tolerated glial cell modulator, attenuates gliosis, decreases levels of proinflammatory mediators, and enhances neuronal viability in glaucoma through activation of the cAMP/PKA pathway. This study provides insight into PDE4 signaling as a potential target to counter the harmful effects associated with chronic gliosis and neuroinflammation in glaucoma.

Discovery of Molecular Therapeutics for Glaucoma: Challenges, Successes, and Promising Directions

Glaucoma, a heterogeneous ocular disorder affecting ∼60 million people worldwide, is characterized by painless neurodegeneration of retinal ganglion cells (RGCs), resulting in irreversible vision loss. Available therapies, which decrease the common causal risk factor of elevated intraocular pressure, delay, but cannot prevent, RGC death and blindness. Notably, it is changes in the anterior segment of the eye, particularly in the drainage of aqueous humor fluid, which are believed to bring about changes in pressure. Thus, it is primarily this region whose properties are manipulated in current and emerging therapies for glaucoma. Here, we focus on the challenges associated with developing treatments, review the available experimental methods to evaluate the therapeutic potential of new drugs, describe the development and evaluation of emerging Rho-kinase inhibitors and adenosine receptor ligands that offer the potential to improve aqueous humor outflow and protect RGCs simultaneously, and present new targets and approaches on the horizon.

Elevation of intraocular pressure in glaucoma patients after automated visual field testing

PURPOSE

To examine whether automated visual field (VF) testing may exert a short-term influence on subsequent intraocular pressure (IOP) measurement during the same visit.

METHODS

We reviewed patients with primary open-angle glaucoma whose most recent visit occurred at a major academic institution from July to December 2009 and who had 3 visits without intervening changes in glaucoma management within the previous 5 years. Exclusion criteria were patient admittance of nonadherence with medical therapy and documented difficulty of IOP measurement. One hundred nine right eyes from 109 patients were included. IOP obtained within 30 minutes after VF testing was compared with IOP from the previous and next visits without VF testing. Subgroup analyses included the role of reliability of VF test performance, surgical versus medical IOP control, and different topical medications.

RESULTS

The average IOP measured after VF testing was 14.9 ± 4.7 mm Hg, higher than both the previous (13.7 ± 4.4 mm Hg, P < 0.001) and next visits without VF examination (13.8 ± 4.4 mm Hg, P < 0.001). A total of 22.9% of patients experienced a more than 20% increase of IOP. Eyes with surgical control had less IOP elevation than eyes with medical control (3.1% ± 15.9% vs. 11.7% ± 17.4%, P = 0.009). Users of β-blockers or α-2-agonists had less IOP elevation than eyes controlled with prostaglandins or carbonic anhydrase inhibitors (0.9% ± 15.1% vs. 9.0% ± 12.3%, P = 0.030).

CONCLUSIONS

This retrospective study suggests that patients with primary open-angle glaucoma experience a small and transient increase in IOP after VF testing and that this effect may be lower after surgical pressure control.

Recurrent serous macular detachment after topical ocular hypotensive medication

We document the recurrence of a CSMD in a patient subsequent to the instillation of topical ocular hypotensive medications and its resolution on discontinuation of therapy. An independent cause or causes contributing to the development of CSMD other than the use of topical ocular hypotensive medications cannot be ruled out in this case, neither can it be considered dissociated from the use of these drugs. We recommend that patients with VTS, those with a history of CSMD or having developed recurrent episodes of CSMD requiring management with topical ocular hypotensive medications, be cautiously monitored for the possible occurrence or exacerbation of CSMD.

Requirement of intact sympathetic transmission for the ocular hypotensive effects of melatonin and 5-MCA-NAT

Melatonin and its analogue, 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT), potently reduce intraocular pressure, and may be good candidates for the treatment of ocular hypertension and glaucoma. After chemical sympathectomy by reserpine or 6-hydroxydopamine, the hypotensive effects of melatonin and 5-MCA-NAT are severely inhibited. This indicates that the sympathetic nervous system is involved in the production and drainage of aqueous humour by the ciliary body and trabecular meshwork, and that it mediates the effects of melatonin and its analogue, 5-MCA-NAT.

Eyeing endothelins: a cellular perspective

Endothelin is an endogenous vasoactive peptide that is considered among the most potent vasoconstrictor substances known. In addition to its vascular effects, endothelins and their receptors have been shown to be present in the eye and to have a number of ocular actions that may be important for ocular homeostasis, but, in excess can be a potential contributor to ocular neuropathy in glaucoma. The current review focuses on the cellular and molecular aspects of endothelins and its receptors in the eye with an emphasis on its relationship to ocular function and its potential role in the etiology of glaucoma pathophysiology.

Vitreal pathogenic role in optic pit foveolar retinoschisis and central serous retinopathy

PURPOSE

To expand on current theories concerning the vitreal-induced mechanism underlying the development of foveolar retinoschisis and macular sensory detachments associated with optic nerve head pits. To propose the notion that vitreal traction may contribute to the pathogenesis of serous detachments in central serous chorioretinopathy (CSC).

REPORTS

We describe two patients, one with macular retinoschisis and the other with central serous detachment. The first patient, a 45-year-old Hispanic female, presented with a temporally located optic nerve head pit, foveolar retinoschisis and schisis retinal spaces extending to the surrounding macula and to the disc. The second patient, a 43-year-old Haitian male, developed a central serous retinal detachment OS with decreased visual acuity one day following in-office administration of Apraclonidine (0.5 per cent Iopidine, Alcon) and Dorzolamide-Timolol Maleate (Cosopt, Merck) to lower elevated intraocular pressure (IOP). Macular retinal pigment mottling and epiretinal membrane sheen OU had been observed on his initial visit. Visual acuity improved within a three-day period with resolution of the serous detachment.

CONCLUSION

We suggest that the persistence of Cloquet's canal may permit fluid leakage into the proximal vitreous in cases of congenital optic nerve head pits. Tangential vitreal traction may promote the opening of a fistula at the optic pit and additionally thrust vitreal fluid into the pit and retinal space inducing the formation of schisis spaces, foveolar-schisis and underlying sensory serous detachment. We question whether a reduction in vitreous volume, induced by initial administration of anti-glaucoma medications, may contribute to the development and/or recurrence of central serous choroidopathy in predisposed individuals.

TGF-beta2 inhibits growth of uveal melanocytes at physiological concentrations

The effect of TGF-beta2 on growth of uveal melanocytes in vitro was studied and the dose-dependent inhibitory effect of TGF-beta2 was compared with the known concentration of TGF-beta2 in aqueous humor. Uveal melanocytes were isolated and cultured with medium supplemented with cAMP elevating agents and basic fibroblast growth factor. The uveal melanocytes were plated into multi-well plates. After 24 hr, TGF-beta2 was added to the medium in various concentrations. After 5 days, the cells were detached, counted and compared to the controls. The effect of TGF-beta2 on DNA synthesis (as evaluated by uptake of bromodeoxyuridine) were also tested. TGF-beta2 inhibited growth and DNA synthesis of cultured uveal melanocytes in a dose-dependent manner at concentrations from 0.03-10.0 ng ml-1. The growth-inhibition of TGF-beta2 was present even in serum-free medium. TGF-beta2 had little or no effect on melanogenesis of cultured uveal melanocytes. The serum used for cultivation did not contain active TGF-beta1 or TGF-beta2 as measured by immunoassay. The known amount of active TGF-beta2 in aqueous humor (0.2-0.4 ng ml-1) is sufficient to inhibit the growth of uveal melanocytes. It indicates that TGF-beta2 is a potent growth inhibit factor of uveal melanocytes and may play an important role in maintaining the non-proliferative, relatively quiescence status of uveal melanocytes in vivo.

Identification of endothelin receptor subtypes in rat ciliary body using subtype-selective ligands

The endothelins are important vasoactive ocular peptides and there is some evidence that they may modulate intraocular pressure. We investigated the existence and localization of endothelin receptor subtypes using subtype selective ligands in rat ciliary body. Scatchard transformation of saturation binding experiments revealed that the KD and Bmax for [125I]ET-1 and [125I]ET-3 to membranes from ciliary body were 41.7+/-9 pM and 236+/-20 fmol mg-1 protein and 37. 8+/-0.4 pM and 160+/-2.0 fmol mg-1 protein, respectively. Competitive experiments in the presence of cyclic pentapeptide BQ123 (selective for ETA receptors) and BQ3020 (selective for ETB receptors), demonstrated the existence of ETA and ETB receptors in a ratio of 35:65. Cross-linking of [125I]ET-1 and [125I]ET-3 to ciliary body membranes resulted in the labeling of two bands with apparent molecular masses of 52 and 34 kDa, suggesting that ETA and ETB receptors have similar molecular mass. The 34 Kda band is a proteolytic degradation product of the 52 Kda band. Autoradiographic results show that specific [125I]ET-1 binding sites, displaced by BQ123 and BQ3020, are localized to the ciliary epithelium, supporting the idea that ETA and ETB subtype receptors exist in this tissue.