Chi-wai Do

In Vivo Assessment of Aqueous Humor Dynamics Upon Chronic Ocular Hypertension and Hypotensive Drug Treatment Using Gadolinium-Enhanced MRI

PURPOSE Although glaucoma treatments alter aqueous humor (AH) dynamics to lower intraocular pressure, the regulatory mechanisms of AH circulation and their contributions to the pathogenesis of ocular hypertension and glaucoma remain unclear. We hypothesized that gadolinium-enhanced magnetic resonance imaging (Gd-MRI) can visualize and assess AH dynamics upon sustained intraocular pressure elevation and pharmacologic interventions. METHODS Gadolinium contrast agent was systemically administered to adult rats to mimic soluble AH components entering the anterior chamber (AC) via blood-aqueous barrier. Dynamic Gd-MRI was applied to examine the signal enhancement in AC and vitreous body upon microbead-induced ocular hypertension and unilateral topical applications of latanoprost, timolol maleate, and brimonidine tartrate to healthy eyes. RESULTS Gadolinium signal time courses in microbead-induced hypertensive eyes possessed faster initial gadolinium uptake and higher peak signals in AC than control eyes, reflective of reduced gadolinium clearance upon microbead occlusion. Opposite trends were observed in latanoprost- and timolol-treated eyes, indicative of their respective drug actions on increased uveoscleral outflow and reduced AH production. The slowest initial gadolinium uptake but strongest peak signals were found in AC of both brimonidine-treated and untreated fellow eyes. These findings drew attention to the systemic effects of topical hypotensive drug treatment. Gadolinium leaked into the vitreous of microbead-induced hypertensive eyes and brimonidine-treated and untreated fellow eyes, suggestive of a compromise of aqueous-vitreous or blood-ocular barrier integrity. CONCLUSIONS Gadolinium-enhanced MRI allows spatiotemporal and quantitative evaluation of altered AH dynamics and ocular tissue permeability for better understanding the physiological mechanisms of ocular hypertension and the efficacy of antiglaucoma drug treatments.

Mechanisms of Aqueous Humor Formation

Glaucoma is a leading cause of irreversible blindness in the world (1). Primary openangle glaucoma (POAG) is the most common form of glaucoma and its onset, as well as progression, is often insidious, leading to significant visual loss before being clinically diagnosed. POAG is frequently associated with elevated intraocular pressure (IOP) of the eye. It is incurable at present, although its progression can be retarded by lowering the IOP by medication and/or surgery. If pharmacological agents fail to attain a targeted hypotensive response within the framework of the clinical characteristics and course, surgery will be indicated. The IOP is physiologically maintained within a relatively narrow range bracketing a mean of approximately 15 mmHg. The level of IOP reflects a dynamic balance between secretion (inflow) and drainage (outflow) of aqueous humor. The aqueous humor is a transparent fluid that is formed by the ciliary processes (epithelium) of the eye. After its production, aqueous humor flows from the posterior chamber to the anterior chamber of the eye via the pupil (Fig. 1). In the anterior chamber, the temperature difference between the warmer iris and the cooler cornea results in a convectional fluid circulation.

cAMP Stimulates Transepithelial Short-Circuit Current and Fluid Transport Across Porcine Ciliary Epithelium

Purpose To investigate the effects of cAMP on transepithelial electrical parameters and fluid transport across porcine ciliary epithelium. Methods Transepithelial electrical parameters were determined by mounting freshly isolated porcine ciliary epithelium in a modified Ussing chamber. Similarly, fluid movement across intact ciliary body was measured with a custom-made fluid flow chamber. Results Addition of 1, 10, and 100 μM 8-Br-cAMP (cAMP) to the aqueous side (nonpigmented ciliary epithelium, NPE) induced a sustained increase in short-circuit current (Isc). Addition of niflumic acid (NFA) to the aqueous surface effectively blocked the cAMP-induced Isc stimulation. The administration of cAMP to the stromal side (pigmented ciliary epithelium, PE) triggered a significant stimulation of Isc only at 100 μM. No additive effect was observed with bilateral application of cAMP. Likewise, forskolin caused a significant stimulation of Isc when applied to the aqueous side. Concomitantly, cAMP and forskolin increased fluid transport across porcine ciliary epithelium, and this stimulation was effectively inhibited by aqueous NFA. Depleting Cl- in the bathing solution abolished the baseline Isc and inhibited the subsequent stimulation by cAMP. Pretreatment with protein kinase A (PKA) blockers (H89/KT5720) significantly inhibited the cAMP- and forskolin-induced Isc responses. Conclusions Our results suggest that cAMP triggers a sustained stimulation of Cl- and fluid transport across porcine ciliary epithelium; Cl- channels in the NPE cells are potentially a cellular site for this PKA-sensitive cAMP-mediated response.

Merging the Professional with the Layperson: Optometric Services for the Community

In this subject, we aim to integrate community services with students’ learning opportunities. Students work in groups and organize and implement a vision screening project by themselves under supervision. Through this subject, students are given opportunities to act upon their roles as community service providers, care, work, and communicate with various disciplines to bridge together knowledge they have acquired during their university studies and then share and apply their special interests with others. The project not only benefits service users, but also provides a very structured learning environment for students to gain experience working with a number of people in different levels of the community and to better understand societal needs, especially in this major public health concern of blindness prevention.

Characterization and Regulation of Gap Junctions in Porcine Ciliary Epithelium

Purpose Gap junctions provide a conduit between the intracellular fluids of the pigmented (PE) and non-pigmented (NPE) ciliary epithelial cells, and are therefore critical in the secretion of the aqueous humor (AH). However, opinions differ concerning the connexin (Cx) composition of the gap junctions. Therefore, we aimed to characterize the expression of Cx in the porcine ciliary epithelium (CE), a favorable model for humans; and determine the contribution of the highest expressed Cx to AH secretion. Methods Freshly-harvested porcine CE cells were used. The mRNA and protein expressions of gap junctions were assessed by reverse transcription polymerase chain reaction (RT-PCR) and Western blotting (WB), respectively. The relative gene expressions of various Cx were determined by quantitative PCR. The gap junction permeability of isolated PE-NPE cell couplets was evaluated by Lucifer Yellow dye transfer. Results Using RT-PCR and WB, Cx43, Cx45, Cx47, Cx50, and Cx60 were present in porcine CE, with Cx43 being the most abundant isoform, having over 200-fold higher expression than other Cx. Cx43 was primarily localized in the PE-NPE interface and the basolateral membranes of PE cells. Knockdown of Cx43 by siRNA significantly reduced gene and protein expressions, resulting in reduction of transcellular fluid flow by 90%. Conclusions Cx43 was found to be the major component of gap junctions in porcine CE. Consistent with results from a bovine model, our results support the important role of Cx43 in mediating AH secretion. This finding may shed light on the development of a novel ocular hypotensive agent.

Methods to Induce Chronic Ocular Hypertension: Reliable Rodent Models as a Platform for Cell Transplantation and Other Therapies

Glaucoma, a form of progressive optic neuropathy, is the second leading cause of blindness worldwide. Being a prominent disease affecting vision, substantial efforts are being made to better understand glaucoma pathogenesis and to develop novel treatment options including neuroprotective and neuroregenerative approaches. Cell transplantation has the potential to play a neuroprotective and/or neuroregenerative role for various ocular cell types (e.g., retinal cells, trabecular meshwork). Notably, glaucoma is often associated with elevated intraocular pressure, and over the past 2 decades, several rodent models of chronic ocular hypertension (COH) have been developed that reflect these changes in pressure. However, the underlying pathophysiology of glaucoma in these models and how they compare to the human condition remains unclear. This limitation is the primary barrier for using rodent models to develop novel therapies to manage glaucoma and glaucoma-related blindness. Here, we review the current techniques used to induce COH-related glaucoma in various rodent models, focusing on the strengths and weaknesses of the each, in order to provide a more complete understanding of how these models can be best utilized. To so do, we have separated them based on the target tissue (pre-trabecular, trabecular, and post-trabecular) in order to provide the reader with an encompassing reference describing the most appropriate rodent COH models for their research. We begin with an initial overview of the current use of these models in the evaluation of cell transplantation therapies.

AB002. Cone rescue in retinitis pigmentosa by the treatment of Lycium barbarum (Random Clinical Trial)

Retinitis pigmentosa (RP) is a group of heredofamilial retinal diseases which is characterized by night blindness and progressive visual field loss. This study aims to study the treatment effect of Lycium barbarum (LB) on retinal functions and structure of RP patients. The study is a double-masked randomized controlled trial. RP subjects received scheduled eye examination including visual acuity (VA), Humphrey field analysis (HFA), ganzfeld flash electroretinogram (ffERG) and optical coherence tomography (OCT). The suitable subjects were randomly allocated to either LB-treatment or placebo groups with the supply of LB or placebo for 12 months. There were total 41 RP subjects (22 in LB group and 19 in placebo group) completed the 12 months intervention. The compliance rates for LB and placebo groups were 89.8%±12.5% and 85.3%±7.7% respectively. As compared with placebo group, there were no deteriorations of both high and low contrast VA in LB group (P<0.01). In addition, certain improvements of scotopic rod response and photopic cone response of ffERG were obtained in LB group (P<0.05). In the OCT measurement, an obvious thinning of macular thickness was observed in placebo group but not found in LB group (P<0.05). However, there were no changes found in the sensitivity of central visual field between two groups. Our results confirm that the 12-month LB treatment for RP patients had neuroprotective effect on retina and is believed to delay or minimize the deterioration of visual function in RP. Funding: Health and Medical Research Fund (01121876) and PolyU Internal Grants (G-YBBS, G-YBGS, Z-0GF).