A primary culture model of rabbit conjunctival epithelial cells exhibiting tight barrier properties

Nanomicellar eye drops: a review of recent advances

INTRODUCTION

Research on nanotechnology in medicine has also involved the ocular field and nanomicelles are among the applications developed. This approach is used to increase both the water solubility of hydrophobic drugs and their penetration/permeation within/through the ocular tissues since nanomicelles are able to encapsulate insoluble drug into their core and their small size allows them to penetrate and/or diffuse through the aqueous pores of ocular tissues.

AREAS COVERED

The present review reports the most significant and recent literature on the use of nanomicelles, made up of both surfactants and amphiphilic polymers, to overcome limitations imposed by the physiology of the eye in achieving a high bioavailability of drugs intended for the therapeutic areas of greatest commercial interest: dry eye, inflammation, and glaucoma.

EXPERT OPINION

The results of the numerous studies in this field are encouraging and demonstrate that nanomicelles may be the answer to some of the challenges of ocular therapy. In the future, new molecules self-assembling into micelles will be able to meet the regulatory requirements for marketing authorization for their use in ophthalmic formulations.

Therapeutic Ultrasound for Enhanced Corneal Permeability to Macromolecules

OBJECTIVES

Topically applied macromolecules have the potential to provide vision-saving treatments for many of the leading causes of blindness in the United States. The aim of this study was to determine if ultrasound can be applied to increase transcorneal drug delivery of macromolecules without dangerously overheating surrounding ocular tissues.

METHODS

Dissected corneas of adult rabbits were placed in a diffusion cell between a donor compartment filled with a solution of macromolecules (40, 70 kDa, or 150 kDa) and a receiver compartment. Each cornea was exposed to the drug solution for 60 minutes, with the experimental group receiving 5 minutes of continuous ultrasound or 10 minutes of pulsed ultrasound at a 50% duty cycle (pulse repetition frequency of 500 ms on, 500 ms off) at the beginning of treatment. Unfocused circular ultrasound transducers were operated at 0.5 to 1 W/cm2 intensity and at 600 kHz frequency.

RESULTS

The greatest increase in transcorneal drug delivery seen was 1.2 times (P < .05) with the application of pulsed ultrasound at 0.5 W/cm2 and 600 kHz for 10 minutes with 40 kDa macromolecules. Histological analysis revealed structural damage mostly in the corneal epithelium, with most damage occurring at the epithelial surface.

CONCLUSIONS

This study suggests that ultrasound may be used for enhancing transcorneal delivery of macromolecules of lower molecular weights. Further research is needed on the long-term effects of ultrasound parameters used in this study on human ocular tissues.

Advanced Technologies of Drug Delivery to the Posterior Eye Segment Targeting Angiogenesis and Ocular Cancer

Retinoblastoma (RB), a childhood retinal cancer is caused due to RB1 gene mutation which affects the child below 5 years of age. Angiogenesis has been proven its role in RB metastasis due to the presence of vascular endothelial growth factor (VEGF) in RB cells. Therefore, exploring angiogenic pathway by inhibiting VEGF in treating RB would pave the way for future treatment. In preclinical studies, anti-VEGF molecule have shown their efficacy in treating RB. However, treatment requires recurrent intra-vitreal injections causing various side effects along with patient nonadherence. As a result, delivery of anti-VEGF agent to retina requires an ocular delivery system that can transport it in a non-invasive manner to achieve patient compliance. Moreover, development of these type of systems are challenging due to the complicated physiological barriers of eye. Adopting a non-invasive or minimally invasive approach for delivery of anti-VEGF agents would not only address the bioavailability issues but also improve patient adherence to therapy overcoming the side effects associated with invasive approach. The present review focuses on the eye cancer, angiogenesis and various novel ocular drug delivery systems that can facilitate inhibition of VEGF in the posterior eye segment by overcoming the eye barriers.

Noninvasive Framework Nucleic Acid Eye Drops for Retinal Administration

Intravitreal injection is widely employed for the treatment of retinal diseases. However, it suffers from various drawbacks, including ocular trauma, risk of infection, and poor patient compliance due to frequent administrations. Due to the presence of barriers such as the cornea, it has been a challenge to develop efficient noninvasive ophthalmic eye drops that can reach the retina. Framework nucleic acids (FNAs), known for their excellent biocompatibility and precise, controllable shape and size, have been extensively utilized in drug delivery application. Here, we report the development of size- and shape-resolved fluorescent DNA frameworks for noninvasive retinal administration. Results show that tetrahedral DNA nanostructures (TDNs) with an edge length of 20 bp can reach the retina within 6 h with the highest efficiency. Moreover, this delivery method exhibits excellent biocompatibility. Our findings provide an approach for the development of localized treatment strategies for retinal diseases using FNA-based nanocarriers.

An Update on Strategies to Deliver Protein and Peptide Drugs to the Eye

In the past few decades, advancements in protein engineering, biotechnology, and structural biochemistry have resulted in the discovery of various techniques that enhanced the production yield of proteins, targetability, circulating half-life, product purity, and functionality of proteins and peptides. As a result, the utilization of proteins and peptides has increased in the treatment of many conditions, including ocular diseases. Ocular delivery of large molecules poses several challenges due to their high molecular weight, hydrophilicity, unstable nature, and poor permeation through cellular and enzymatic barriers. The use of novel strategies for delivering protein and peptides such as glycoengineering, PEGylation, Fc-fusion, chitosan nanoparticles, and liposomes have improved the efficacy, safety, and stability, which consequently expanded the therapeutic potential of proteins. This review article highlights various proteins and peptides that are useful in ocular disorders, challenges in their delivery to the eye, and strategies to enhance ocular bioavailability using novel delivery approaches. In addition, a few futuristic approaches that will assist in the ocular delivery of proteins and peptides were also discussed.

Inhibition of microRNA-328 Increases Ocular Mucin Expression and Conjunctival Goblet Cells

We previously reported anti-miR-328 therapy for dry eye disease (DED). Since decreased mucin secretion is a risk factor for DED, we aimed to explore whether anti-miR-328 affects mucin expression and goblet cells. MiR-328 was increased in goblet cells when they were under desiccating stress or treated with benzalkonium chloride (BAC), both of which are risk factors for DED. Based on bioinformatics tool results, miR-328 was predicted to directly target the transcription factor CREB1 that has been known to promote the expression of mucin5AC. The inhibitory effect of miR-328 on CREB1 was confirmed by the transfection assay. A miR-328 binding site on the CREB1 gene was confirmed by the luciferase assay. Furthermore, anti-miR-328 increased CREB1 and mucin5AC in cultured goblet cells according to qPCR, Western blot, and IF staining experiments. Anti-miR-328 increased mucin5AC secretion from the cultured goblet cells based on an ELISA assay for the cultured medium. Finally, impression cytology data revealed anti-miR-328 increased conjunctival goblet cells in the DED rabbits induced by BAC. In conclusion, anti-miR-328 increases CREB1 expression leading to an increase in mucin5AC production and secretion. Furthermore, anti-miR-328 also increases conjunctival goblet cells. These results warrant the further development of anti-miR-328 therapy for DED.

Looking into the Eyes—In Vitro Models for Ocular Research

Animal research undoubtedly provides scientists with virtually unlimited data but inflicts pain and suffering on animals. Currently, legislators and scientists alike are promoting alternative in vitro approaches allowing for an accurate evaluation of processes occurring in the body without animal sacrifice. Historically, one of the most infamous animal tests is the Draize test, mainly performed on rabbits. Even though this test was considered the gold standard for around 50 years, the Draize test fails to mimic human response mainly due to human and rabbit eye physiological differences. Therefore, many alternative assays were developed to evaluate ocular toxicity and drug effectiveness accurately. Here we review recent achievements in tissue engineering of in vitro 2D, 2.5D, 3D, organoid and organ-on-chip ocular models, as well as in vivo and ex vivo models in terms of their advantages and limitations.

Recent advances in ophthalmic preparations: Ocular barriers, dosage forms and routes of administration

The human eye is a complex organ with unique anatomy and physiology that restricts the delivery of drugs to target ocular tissues/sites. Recent advances in the field of pharmacy, biotechnology and material science have led to development of novel ophthalmic dosage forms which can provide sustained drug delivery, reduce dosing frequency and improve the ocular bioavailability of drugs. This review highlights the different anatomical and physiological factors which affect ocular bioavailability of drugs and explores advancements from 2016 to 2020 in various ophthalmic preparations. Different routes of drug administration such as topical, intravitreal, intraocular, juxtascleral, subconjunctival, intracameral and retrobulbar are discussed with their advances and limitations.

Three-Dimensional Human Cell Culture Models to Study the Pathophysiology of the Anterior Eye

In recent decades, the establishment of complex three-dimensional (3D) models of tissues has allowed researchers to perform high-quality studies and to not only advance knowledge of the physiology of these tissues but also mimic pathological conditions to test novel therapeutic strategies. The main advantage of 3D models is that they recapitulate the spatial architecture of tissues and thereby provide more physiologically relevant information. The eye is an extremely complex organ that comprises a large variety of highly heterogeneous tissues that are divided into two asymmetrical portions: the anterior and posterior segments. The anterior segment consists of the cornea, conjunctiva, iris, ciliary body, sclera, aqueous humor, and the lens. Different diseases in these tissues can have devastating effects. To study these pathologies and develop new treatments, the use of cell culture models is instrumental, and the better the model, the more relevant the results. Thus, the development of sophisticated 3D models of ocular tissues is a significant challenge with enormous potential. In this review, we present a comprehensive overview of the latest advances in the development of 3D in vitro models of the anterior segment of the eye, with a special focus on those that use human primary cells.

In Vitro Cell Models for Ophthalmic Drug Development Applications

Tissue engineering is a rapidly expanding field that aims to establish feasible techniques to fabricate biologically equivalent replacements for diseased and damaged tissues/organs. Emerging from this prospect is the development of in vitro representations of organs for drug toxicity assessment. Due to the ever-increasing interest in ocular drug delivery as a route for administration as well as the rise of new ophthalmic therapeutics, there is a demand for physiologically accurate in vitro models of the eye to assess drug delivery and safety of new ocular medicines. This review summarizes current existing ocular models and highlights the important factors and limitations that need to be considered during their use.

Effects of various freezing containers for vitrification freezing on mouse oogenesis

Background

In the present study, various freezing containers were tested for mouse embryos of respective developmental stages; embryos were vitrified and then their survival rate and developmental rate were monitored. Mouse two cell, 8 cell, and blastula stage embryos underwent vitrification freezing-thawing and then their recovery rate, survival rate, development rate, and hatching rate were investigated.

Methods

EM-grid, OPS, and cryo-loop were utilized for vitrification freezing-thawing of mouse embryos.

Results

It was found that recovery rate and survival rate were higher in the group of cryo-loop compared to those of EM-grid (p < 0.05). Embryonic development rate, two cell embryos to blastocyst, as well as hatching rate were higher in the control group compared to the EM-grid group and OPS group (p < 0.05), yet no difference was noted between the control group and cryo-loop group. Development rate and hatching rate of eight cell morulae and blastocysts were all lower in the treatment groups than the control group whilst hatching rate of blastocysts was higher in the control group compared to the groups of EM-grid and OPS (p < 0.05); although the cryo-loop group was shown to be slightly higher than other groups, it was not statistically significant.

Conclusions

In the study, we investigate effects of freezing containers on vitrified embryos of respective developmental stages; it was demonstrated that higher developmental rate was shown in more progressed (or developed) embryos with more blastomeres. There was however, no difference in embryonic development rate was shown amongst containers. Taken together, further additional studies are warranted with regards to 1) manipulation techniques of embryos for various vitrification freezing containers and 2) preventive measures against contamination via liquid nitrogen.

Concise Review: Comparison of Culture Membranes Used for Tissue Engineered Conjunctival Epithelial Equivalents

The conjunctival epithelium plays an important role in ensuring the optical clarity of the cornea by providing lubrication to maintain a smooth, refractive surface, by producing mucins critical for tear film stability and by protecting against mechanical stress and infectious agents. A large number of disorders can lead to scarring of the conjunctiva through chronic conjunctival inflammation. For controlling complications of conjunctival scarring, surgery can be considered. Surgical treatment of symblepharon includes removal of the scar tissue to reestablish the deep fornix. The surgical defect is then covered by the application of a tissue substitute. One obvious limiting factor when using autografts is the size of the defect to be covered, as the amount of healthy conjunctiva is scarce. These limitations have led scientists to develop tissue engineered conjunctival equivalents. A tissue engineered conjunctival epithelial equivalent needs to be easily manipulated surgically, not cause an inflammatory reaction and be biocompatible. This review summarizes the various substrates and membranes that have been used to culture conjunctival epithelial cells during the last three decades. Future avenues for developing tissue engineered conjunctiva are discussed.

The vectorial transport of salts and water is crucial for respiratory epithelial cell lines

Primary culture of respiratory epithelial cells is useful to study the pathophysiology of respiratory diseases. However, such primary culture has been very limited because of its high dependence on the availability of biopsies and the long time required to reach confluence. Therefore, cell lines are an alternative to primary cultures because they reach confluence faster and some can maintain their differentiation abilities. However, unlike primary cultures and native tissues just some cell lines are able to polarize, with normal channel functionality and transepithelial ionic flux.

Toward the practical implementation of eye-related bioavailability prediction models

The development and registration of reformulated ophthalmic products (OPs) requires eye-related bioavailability (BA) assessments. Common BA algorithms associated with other routes of application, such as the oral route, cannot be easily applied to eye-related BA testing. Here, we provide an analysis of the current literature and suggestions for further directions in the development of high-capacity, cost-effective, and highly predictive nonclinical models of eye-related drug BA. One, or a combination of these models, has the potential for routine use in research laboratories and/or the pharmaceutical industry to overcome various obstacles in reformulated OP development and registration.

Novel Nanomicellar Formulation Approaches for Anterior and Posterior Segment Ocular Drug Delivery

One of the most challenging areas of pharmaceutical research is ocular drug delivery. The unique anatomy and physiology of the eye impedes drug permeation to deeper ocular tissues. Nanosized carrier systems such as nanoparticles, liposomes, suspensions, dendrimers, and nanomicelles are being explored for ocular drug delivery. In this review, we have focused on application of emerging nanomicellar carrier systems in ocular drug delivery. Nanomicelles are nanosized vesicular carriers formed from amphiphilic monomer units. Surfactant and polymeric micellar nanocarriers provide an amenable means to improve drug solubilization, develop clear aqueous formulations and deliver drugs to anterior and posterior ocular tissues. Nanomicelles due to their amphiphilic nature encapsulate hydrophobic drugs and aid in drug delivery. Various methods are employed to develop nanosized micellar formulations depending upon the physicochemical properties of the drug. Nanomicellar carriers appear to be promising vehicles with potential applications in ocular drug delivery. In this review, we attempted to discuss about the progress in ocular drug delivery research using nanomicelles as carriers from the published literature and issued patents. Also, with regards to ocular static and dynamic barriers which prevent drug permeation, a brief discussion about nanomicelles, types of nanomicelles, their methods of preparation and micellar strategy to overcome ocular barriers, delivering therapeutic levels of drugs to anterior and posterior ocular tissues are discussed.

Ocular drug delivery

Ocular drug delivery has been a major challenge to pharmacologists and drug delivery scientists due to its unique anatomy and physiology. Static barriers (different layers of cornea, sclera, and retina including blood aqueous and blood-retinal barriers), dynamic barriers (choroidal and conjunctival blood flow, lymphatic clearance, and tear dilution), and efflux pumps in conjunction pose a significant challenge for delivery of a drug alone or in a dosage form, especially to the posterior segment. Identification of influx transporters on various ocular tissues and designing a transporter-targeted delivery of a parent drug has gathered momentum in recent years. Parallelly, colloidal dosage forms such as nanoparticles, nanomicelles, liposomes, and microemulsions have been widely explored to overcome various static and dynamic barriers. Novel drug delivery strategies such as bioadhesive gels and fibrin sealant-based approaches were developed to sustain drug levels at the target site. Designing noninvasive sustained drug delivery systems and exploring the feasibility of topical application to deliver drugs to the posterior segment may drastically improve drug delivery in the years to come. Current developments in the field of ophthalmic drug delivery promise a significant improvement in overcoming the challenges posed by various anterior and posterior segment diseases.

Polyhydroxyethylaspartamide-based micelles for ocular drug delivery

In this paper three copolymers of polyhydroxyethylaspartamide (PHEA), bearing in the side chains polyethylene glycol (PEG) and/or hexadecylamine (C(16)) (PHEA-PEG, PHEA-PEG-C(16) and PHEA-C(16) respectively) have been studied as potential colloidal drug carriers for ocular drug delivery. The physical characterization of all three PHEA derivatives, using the Langmuir trough (LT) and micellar affinity capillary electrophoresis (MACE) techniques allowed to assume that whereas alone PHEA backbone is an inert polymer with respect to the interactions with lipid membranes and drug complexation, when PHEA chains are grafted with long alkyl chains like C(16) or in combination C(16) chains and hydrophilic chains like PEG, copolymers with lipid membrane interaction ability and drug complexation capability are obtained. In vitro permeability studies performed on primary cultured rabbit conjunctival and corneal epithelia cells, using PHEA-C(16) and PHEA-PEG-C(16) as micelle carriers for netilmicin sulphate, dexamethasone alcohol and dexamethasone phosphate, demonstrated that in all cases drug loaded PHEA-C(16) and PHEA-PEG-C(16) micelles provide a drug permeation across ocular epithelia greater than simple drug solutions or suspensions. In particular PHEA-PEG-C(16) acts as the best permeability enhancer in our experimental model. In vivo bioavailability studies conducted with PHEA-PEG-C(16) micelles loaded with dexamethasone alcohol, confirmed that this system also provides a drug bioavailability greater in comparison with that obtained with water suspension of the same drug after ocular administration to rabbits.

Ocular novel drug delivery: impacts of membranes and barriers

BACKGROUND

Ocular drug delivery is an extremely challenging area due to its restrictive barrier functionalities.

OBJECTIVE

Drug transport via corneal/non-corneal routes involves several intricate biological processes such as drug penetration across the ocular barriers and transfer to the anterior or posterior chambers, thus the influence of these processes on the pharmacotherapy of the eye should be fully addressed.

METHODS

To pursue the impacts of such impediments in novel drug therapy, recent publications were reviewed regarding advanced strategies such as nanomedicines.

CONCLUSION

The ocular barriers are highly specialized and selectively control the inward/outward traverse of compounds, hence a better understanding of these biological obstacles would provide a platform to advance ophthalmic drug therapy towards specified delivery/targeting with minimal adverse consequences.

Development of immortalized rat conjunctival epithelial cell lines: an in vitro model to examine transepithelial antigen delivery

The objective of these studies was to develop conjunctival epithelial cell lines for investigation of antigen translocation across a mucosal barrier. Conjunctival epithelial cells from Fischer 344 rats were immortalized with pSV3(neo) resulting in two cell lines--CJ4.1A and CJ4.3C. Each formed confluent cell layers with epithelial morphology when grown on permeable membrane filters. They expressed the SV40 T antigen, the conjunctiva-specific cytokeratin 4, the goblet cell-specific cytokeratin 7 and were negative for the corneal epithelial cell-specific cytokeratin 12. The cell lines have been in culture for over 60 passages, and the population doubling times were 22+/-7h for CJ4.1A and 23+/-9h for CJ4.3C. When grown on Transwell membranes, each cell line achieved a transepithelial electrical resistance of 600-800 Omega cm2 by 3-4 days and maintained a high resistance for several days. Both cell lines expressed zona occludens-1 at confluence. At 24h following addition of 250 microg of FITC-labeled ovalbumin to the apical chambers, 15+/-6 microg could be detected in the basal chamber of CJ4.1A and 6+/-1 microg in the basal medium of CJ4.3C. In contrast, 82+/-6 microg was detected in the lower chambers of cell-free Transwells. Similarly, Transwells containing confluent CJ4.1A or CJ4.3C cells impeded passage of 0.1 microm diameter polystyrene microspheres (5+/-1% and 4+/-1%, respectively, of the apical input), compared to 26+/-6% of the input microspheres recovered from the basal chambers of cell-free Transwells. Pretreatment with 4mM EGTA for 10 min caused an increase in OVA-FITC translocation across CJ4.3C cells. Incubation in the presence of 4mM EGTA significantly increased OVA-FITC translocation across both cell lines, relative to untreated cell layers. Morphological and functional characterization indicates that these cells provide a useful experimental tool to assess strategies for enhancing transepithelial antigen uptake.

Cell culture models of the ocular barriers

The presence of tight barriers, which regulate the environment of ocular tissues in the anterior and posterior part of the eye, is essential for normal visual function. The development of strategies to overcome these barriers for the targeted ocular delivery of drugs, e.g. to the retina, remains a major challenge. During the last years numerous cell culture models of the ocular barriers (cornea, conjunctiva, blood-retinal barrier) have been established. They are considered to be promising tools for studying the drug transport into ocular tissues, and for numerous other purposes, such as the investigation of pathological ocular conditions, and the toxicological screening of compounds as alternative to in vivo toxicity tests. The further development of these in vitro models will require more detailed investigations of the barrier properties of both the cell culture models and the in vivo ocular barriers. It is the aim of this review to describe the current status in the development of cell culture models of the ocular barriers, and to discuss the applicability of these models in pharmaceutical research.

Roles of the conjunctiva in ocular drug delivery: a review of conjunctival transport mechanisms and their regulation

Conjunctiva plays many roles including protection of ocular surface, production of tear film, and a conduit for drug clearance (depending on drug properties) into the systemic circulation or for drug transport to the deep tissues of the eye. The conjunctiva, which is a moderately tight epithelium, endowed with various transport processes for the homeostasis of ions, solutes, and water in the conjunctival surface and tear film. Modulation of ion transport in the conjunctiva leads to alterations in transconjunctival fluid flow that may become useful for treatment of dry-eye state in the eye. As a possible drug delivery route to the posterior portion of the eye, conjunctiva is an attractive route due to both larger surface area than that of cornea and expression of several key transport processes. Tear contains D-glucose and many amino acids, in addition to the usual ions in the body fluids. Several ion-coupled solute transport processes for absorption of amino acids, D-glucose, monocarboxylate, nucleosides, and dipeptides are expressed in the conjunctiva. Thanks to the rich endowment of these transport processes, drug transport across the conjunctiva into the intraocular tissues may become quite feasible. Subconjunctival injection of microparticles and matrix materials (which allows sustained release of drugs) is shown to maintain reasonable levels of various drugs in the vitreous, perhaps attesting to the fact that conjunctiva per se may contribute as a part of multiple transport barrier(s) in ocular drug delivery. In addition, several conjunctival approaches have been investigated to optimize treatment of dry-eye syndrome and intraocular diseases, and more can be accomplished in the coming years.

Lectins as endocytic ligands: an assessment of lectin binding and uptake to rabbit conjunctival epithelial cells

PURPOSE

To investigate the binding and uptake pattern of three plant lectins in rabbit conjunctival epithelial cells (RCECs) with respect to their potential for enhancing cellular macromolecular uptake.

METHODS

Three fluorescein-labeled plant lectins (Lycoperison esculentum, TL; Solanum tuberosum, STL; and Ulex europaeus 1, UEA-1) were screened with respect to time-, concentration-, and temperature-dependent binding and uptake. Chitin (30 mg/ml) and L-alpha-fucose (10 mM) were used as inhibitory sugars to correct for nonspecific binding of TL or STL and UEA-1, respectively. Confocal microscopy was used to confirm internalization of STL.

RESULTS

The binding and uptake of all three lectins in RCECs was time-dependent (reaching a plateau at 1-2 h period) and saturable at 1-h period. The rank order of affinity constants (km) was STL>TL>UEA-1 with values of 0.39>0.48>4.81 microM, respectively. However, maximal, specific binding/uptake potential was in the order UEA-1>STL>TL with values of 53.7, 52.3, and 15.0 nM/mg of cell protein, respectively. Lectins showed temperature dependence in their uptake, with STL exhibiting the highest endocytic capacity. Internalized STL was visualized by confocal microscopy to be localized to the cell membrane and cytoplasm.

CONCLUSION

Based on favorable binding and uptake characteristics, potato lectin appears to be a useful candidate for further investigation as an ocular drug delivery system.

Ex vivo produced human conjunctiva and oral mucosa equivalents grown in a serum-free culture system

PURPOSE

We sought to develop full-thickness ex vivo produced human conjunctiva and oral mucosa equivalents using a serum-free culture system without a feeder layer and to compare conjunctiva and oral mucosa equivalents to assess their suitability as graft materials for eyelid reconstruction.

MATERIALS AND METHODS

Human conjunctival and oral mucosal keratinocytes were cultured, expanded, and seeded onto AlloDerm (LifeCell Corp, Branchburg, NJ), a cadaveric, acellular dermis, to produce ex vivo produced full-thickness mucosa equivalents. Histology of equivalents and their expression of immunoreactive Ki-67, a proliferation marker, and GLUT1, a membrane antigen seen in barrier tissues, were examined at 4, 11, and 18 days after seeding onto AlloDerm.

RESULTS

Progressive epithelial stratification was observed on day 4, 11, and 18 conjunctiva and oral mucosa equivalents. Ki-67 immunoreactivity progressively increased with cultured time in both types of equivalent, indicating the continued presence of actively proliferating cells. GLUT1 immunoreactivity, concentrated in the basal keratinocytes of stratified epithelia of both types of equivalents, mimicked native tissue and indicated a high glycolytic state of the basal cells.

CONCLUSIONS

Conjunctival and oral mucosal equivalents are similar to native tissue and demonstrate high proliferative and glycolytic states. Due to the similarity to conjunctiva, oral mucosal equivalents may be useful for eyelid reconstruction. Their advantages for surgical reconstruction include 1) ease of obtaining autogenous oral epithelium for expansion in vitro without the possibility of contaminating cellular- or serum-borne biologic agents, 2) growth of intact, confluent epithelia on rigid, transplantable human allogeneic dermis that may be surgically transplanted, and 3) reduced donor site morbidity and surgical time.

Impairment of conjunctival glutathione secretion and ion transport by oxidative stress in an adenovirus type 5 ocular infection model of pigmented rabbits

Conjunctival epithelial cells of pigmented rabbits secrete reduced glutathione (GSH) into the apical (mucosal) fluid. The aim of the current study was to determine the effect of oxidative stress resulting from viral infection and that of GSH supplementation on redox status, GSH, and ion transport in freshly excised conjunctival tissues and epithelial cell layers in primary culture (RCEC) of adenovirus type 5 (Ad5)-infected rabbits. Lipid peroxidation (LPO) products, nitric oxide (NO), and expression of nitric oxide synthase (NOS2) were quantitated as a function of time after viral inoculation. Unidirectional fluxes of [3H]GSH and changes in short-circuit current (Isc) from mucosal supplementation of Ad5-inoculated conjunctival tissues with GSH and glutathione monoethyl ester (GSH-MEE) were also measured. Ad5 inoculation significantly decreased conjunctival GSH level by 19, 45, 48, and 50% at 8, 24, 48, and 72 h postinfection, respectively. LPO product and NO levels increased significantly (2- and 100-fold, respectively) above that of uninfected controls on Day 3 post-Ad5 inoculation, and co-treatment with GSH-MEE and tocopherol succinate abolished this effect. NO levels showed a progressive increase post-Ad5 inoculation, reaching 0.22 +/- 0.06, 8.12 +/- 0.91, and 2.05 +/- 0.65 microM on Days 1, 3, and 5, respectively, and the highest level was observed on the day of maximal viral replication (Day 3). A very significant induction of the expression of NOS2 on Days 1, 3, and 5 post-Ad5 inoculation was observed. Uninfected control conjunctival tissues displayed a net serosal-to-mucosal GSH flux (Jsm), where the mucosal-to-serosal flux (Jms) was approximately 14 pmol h(-1) cm(-2) and the Jsm was approximately 22 pmol h(-1) cm(-2). In Ad5-inoculated rabbits similar GSH flux was observed in both the sm and ms directions, and the net GSH flux was negligible. Isc and potential difference (PD) across conjunctival tissues of Ad5-inoculated rabbits decreased by > or = 50% compared with control, while the transepithelial electrical resistance (TEER) remained unchanged. Mucosal, but not serosal, superfusion of GSH or GSH-MEE in Ad5-inoculated conjunctival tissues increased the Isc by up to 40% in approximately 100 min. Our results show that net secretion of GSH across rabbit conjunctiva is totally blocked after Ad5 inoculation and active ion transport rate decreased by approximately 50%. Decreased net GSH secretion into mucosal fluid after Ad5 infection may have resulted from a decreased intracellular GSH pool due to oxyradical-induced changes in redox status and lower active ion transport. Mucosal treatment of Ad5-infected conjunctival tissues with pharmacological levels of GSH appears to transstimulate mucosal GSH secretion and restore active ion transport activity, suggesting a potentially useful therapeutic regimen for ocular infections.

Control of mucin production by ocular surface epithelial cells

Multiple species of mucins are synthesized and secreted by corneal and conjunctival epithelial cells. These mucins are vital components of the tear film protecting the ocular surface from the external environment by providing a physical and chemical barrier. The release of mucins must be tightly regulated as both mucin overproduction and underproduction cause ocular surface disorders. Mucin production can be regulated by controlling mucin synthesis, mucin release, or proliferation of the cells that produce the mucin. This review will focus on the evidence demonstrating the control of the mechanisms responsible for production of mucins, their secretion, and corneal and conjunctival epithelia cell proliferation. By understanding these mechanisms under normal conditions, treatments can be designed for diseases of the mucous production of the ocular surface.

Comparison of morphological and functional characteristics of primary-cultured human conjunctival epithelium and of Wong–Kilbourne derivative of Chang conjunctival cell line

PURPOSE

To analyze the relevance of a human conjunctival cell line in a study of conjunctival epithelium. We investigated and compared the effects of IFNgamma and TNFalpha in a primary culture of human conjunctiva and in a human conjunctival cell line.

METHODS

A primary-cultured human conjunctival epithelium and a human conjunctival cell line (Chang cells) were treated for 72 hr with 20, 200, 400 and 600 U ml(-1) IFNgamma or with 1100 and 11,000 U ml(-1) TNFalpha. Then, the expression of HLA DR, CD40, CD44, CD63, CD80, CD86, Fas receptor, E-cadherin, ICAM-1, MUC1, cytokeratins and vimentin were investigated by flow cytometry. Cell morphology was studied with phalloidin staining. Apoptosis was detected by flow cytometry with Annexin V and via cell cycle analysis.

RESULTS

The primary culture of human conjunctival epithelium expressed cytokeratin K4, non-keratinized squamous epithelial marker. Chang cells presented a more dedifferentiated phenotype and were cytokeratin K4 negative. In primary-cultured cells, IFNgamma (600 U ml(-1)) induced only a low level of apoptosis and a significant upregulation of most tested proteins such as HLA DR, Fas, ICAM-1, CD40 and CD63. In the Chang cell line, IFNgamma induced a significant level of apoptosis at concentrations of 200, 400 and 600 U ml(-1). HLA DR and CD63 were induced at lower levels than in primary-cultured cells. Other proteins were modified in a similar manner after IFNgamma treatment in both systems. In the primary-cultured cells, TNFalpha induced an important upregulation of ICAM-1, Fas and CD40 whereas CD44 and CD63 were significantly decreased. Conversely, only a very weak alteration of CD63 and ICAM-1 was observed in the Chang cell line after TNFalpha treatment.

CONCLUSIONS

A primary culture of a human conjunctival epithelium demonstrated well-defined epithelial features. TNFalpha and IFNgamma, two inflammatory cytokines, induced different effects in both cellular systems, in a primary-cultured conjunctival epithelium and a human conjunctival cell line. Inflammation-related molecules were highly upregulated in the primary culture and, to a lesser extent, in the Chang cell line. Thus, the Chang cell line differs in certain features from a primary culture of human conjunctival epithelium, a fact which emphasizes the complexity of interpretation of in vitro data and this should be taken into consideration in in vitro studies of human conjunctival epithelium.

Regulation of mucin and fluid secretion by conjunctival epithelial cells

Tears play a vital role in the health and protection of the cornea and conjunctiva. The tear film consists of multiple layers and different glands secrete each layer. Because of many and varied requirements of the ocular surface cells, the volume, composition and structure of the tear film must be exquisitely controlled. If any layer of the tear film is disrupted or altered, the entire tear film is affected, often with deleterious effects. This chapter reviews the current knowledge of the neural and growth factor regulation of electrolyte, water and protein secretion from the goblet and stratified squamous cells of the conjunctiva as well as the mechanisms used for fluid secretion. The evidence presented in this review suggests that parasympathetic nerves stimulate goblet, but not stratified squamous, cell secretion. Sympathetic nerves stimulate stratified squamous, but not goblet, cell secretion, while P2Y(2) agonists stimulate secretion from both cell types. Growth factors regulate goblet cell secretion, but their effects on stratified squamous cell secretion are unknown.

Pilocarpine permeability across ocular tissues and cell cultures: influence of formulation parameters

In vitro permeation studies of drugs across biological barriers are promising tools for estimating the quality and quantity of drug transport in vivo. The objective of this work was to compare the permeability of the hydrophilic model drug pilocarpine-HCl (P-HCl) through different ocular tissues and cell cultures: isolated pig cornea (PCr) and sclera (PSc), rabbit conjunctiva (RCo), and rabbit conjunctival (RCoEC) or corneal epithelial cell culture (RCrEC). Furthermore, the study included investigations about the influence of the excipients benzalkonium chloride (BAC) and ethylene diamine tetra acetic acid disodium salt (EDTA) on the permeability of the small drug. In general, BAC caused a facilitated drug transport, while EDTA hardly influenced the P-HCl concentration on the acceptor side, except for RCoEC. Additionally, the impact of variation in buffer solution pH and tonicity on drug transport in both cell cultures was tested. The higher the tonicity of the buffer solution (80, 300, and 600 mOsm/kg) the lower the permeability coefficient (P(eff)). At different pH values (6.4, 7.4, and 8.4) the P(eff) showed a directly proportional demeanor. In summary, a good correlation between the isolated tissues and cell cultures with regard to P-HCl transport could be observed.

Characterization of cyclic AMP-regulated chloride conductance in the pigmented rabbit conjunctival epithelial cells

We have previously reported that the pigmented rabbit conjunctiva is a Cl- secreting tissue, subject to cAMP, Ca2+, and PKC modulation. The present study was conducted to characterize, at the cellular and molecular levels, cAMP-regulated Cl- channels in rabbit conjunctival epithelial cells. cAMP-inducible Cl- channel properties were evaluated by monitoring the whole-cell currents using patch clamp techniques. Results showed that 10 microM forskolin significantly stimulated a glibenclamide-inhibitable whole-cell conductance by approximately five-fold. Furthermore, reduction of the Cl- concentration in the bathing solution through partial substitution of NaCl with Na-isethionate resulted in a rightward shift of the reversal potential for both baseline and forskolin-stimulated whole-cell currents from 0 to values close to the theoretical Cl- reversal potential predicted by the Nernst equation. Western blot analysis with a monoclonal antibody recognizing the epitope in the C-terminus of the cystic fibrosis transmembrane conductance regulator (CFTR) showed a positive band at its molecular weight, approximately 170 kD. Immunostaining under confocal microscopy revealed a CFTR specific signal in the apical sections of primary conjunctival epithelial cells. In addition, RT-PCR detection amplified a cDNA fragment 100% identical to the predicted portion of the cloned rabbit CFTR message. The stage is thus set for determining the extent of CFTR contribution to cAMP-regulated Cl- conductance in pigmented rabbit conjunctival epithelial cells.

Identification and localization of acid-base transporters in the conjunctival epithelium

Acid-base transporters of rabbit and porcine conjunctival epithelia were identified and localized with immunoblotting and immunohistochemical techniques using specific antibodies against carriers commonly found in epithelia, i.e. the Cl(-)/HCO3(-)exchanger (AE2), Na(+)/H(+) exchanger (NHE-1, -2, -3) and the electrogenic Na(+)-(n)HCO3(-) cotransporter (NBC). Western blot analysis demonstrated that anti-AE2 reacted with an approximate 170 kDa protein in both rabbit and pig cell membranes prepared from separately isolated bulbar and palpebral conjunctivae. NHE1 was similarly identified in these distinct conjunctival regions but results with anti-NBC were ambiguous. Histochemical examinations indicated that the AE2 and NHE1 proteins reside on the basolateral surfaces of the plasma membrane throughout the multilayered tissue. The immunostaining of porcine cryosections for AE2 and rabbit sections for NHE1 was specific, because of its abolishment following either pre-absorption with the corresponding peptide or omission of the primary antibody. Screening with anti-NBC produced weak staining of the sections that appeared to be non-specific. For confirmation of these results, the acid-base transporters present in rabbit cell cultures of conjunctival epithelia were ascertained from the changes in intracellular pH (pH(i)) evoked upon sequential superfusion with media of altered composition. This approach readily obtained Na(+)- and Cl(-)-dependent pH(i)effects consistent with the existence of Cl(-)/HCO3(-) and Na(+)/H(+)exchange activities. Evidence for the presence of NBC could not be acquired, thereby substantiating the observations from the immunodetection techniques. The identity and location of the antiporters that were found suggested that these elements could contribute to transcellular Cl(-)transport in the basolateral-to-apical direction. To test this possibility, the effects of AE and/or NHE inhibition were determined on the bumetanide-insensitive Cl(-)-dependent short-circuit current across rabbit conjunctivae freshly isolated in Ussing-type chambers. These experiments revealed that such current is indeed sustained by the antiporters. Results with acetazolamide further suggested that the contribution of the acid-base transporters towards transepithelial Cl(-)secretion is variable and dependent upon individual rates of metabolic CO(2)production. Overall, the present study provides an initial identification of the acid-base transporters present in the conjunctiva. Besides their likely role in intracellular pH regulation, the parallel, basolateral expression of AE2 and NHE1 indicates that these elements do not directly contribute to the pH of the tear film but may complement the Na(+)-2Cl(-)-K(+)cotransporter in effectuating Cl(-)secretion.

Role of P-glycoprotein in restricting propranolol transport in cultured rabbit conjunctival epithelial cell layers

PURPOSE

To determine the role of P-glycoprotein (P-gp) in propranolol transport in cultured rabbit conjunctival epithelial cell layers (RCEC).

METHODS

The localization of P-gp in the cultured RCEC as well as in the excised conjunctiva was determined by immunofluorescence technique. The role of P-gp in transepithelial transport and uptake of propranolol in conjunctival epithelial cells cultured on Transwell filters was evaluated in the presence and absence of P-gp competing substrates, an anti-P-gp monoclonal antibody (4E3 mAb), or a metabolic inhibitor, 2,4-dinitrophenol (2,4-DNP).

RESULTS

Immunofluorescence studies revealed positive staining in the apical membrane of cultured RCEC and in the apical surface of the superficial cell layers in the excised conjunctiva, but not the basolateral membrane of cultured RCEC. Transport of propranolol showed preference in the basolateral-to-apical direction. The net secretory flux was saturable with a Km of 71.5 +/- 24.0 nM and a Jmax of 1.45 +/- 0.17 pmol/cm2/hr. Cyclosporin A, progesterone, rhodamine 123, verapamil, 4E3 mAb and 2,4-DNP all increased apical 50 nM propranolol uptake by 43% to 66%. On the other hand, neither beta-blockers (atenolol, metoprolol, and alprenolol) nor organic cation transporter substrates (tetraethylammonium (TEA) and guanidine), affected apical 50 nM propranolol uptake.

CONCLUSIONS

The energy-dependent efflux pump P-gp appears to be predominantly located on the apical plasma membrane of the conjunctival epithelium. It may play an important role in restricting the conjunctival absorption of some lipophilic drugs.

Meeting future challenges in topical ocular drug delivery: development of an air-interfaced primary culture of rabbit conjunctival epithelial cells on a permeable support for drug transport studies

The purpose of this study was to develop and characterize a functional air-interfaced primary culture of rabbit conjunctival epithelial cells grown on a permeable support for drug transport studies. Conjunctival epithelial cells from the pigmented rabbit were isolated, seeded at 1.2 x 10(6) cells cm(-2) on permeable Transwell filters, and cultured at the air interface using a modified PC-1 medium. Conjunctival epithelial cell layers showed a transepithelial resistance of 1.1+/-0.1 kOmega cm(2), a potential difference of 17.0+/-0.5 mV, and an equivalent short-circuit current (I(eq)) of 16.1+/-0.4 microA cm(-2). The I(eq) was reduced by 35% using 0.01 mM bumetanide, 66% using 0.1 mM ouabain, 46% using 2 mM barium chloride (all three in the basolateral fluid), and 63% using 0.3 mM NPAA in the apical fluid, consistent with active Cl(-)-secretion across the conjunctival epithelial barrier. Amiloride-sensitive Na(+) channels were absent. The permeability of the cell layers to polar solutes decreased with increased solute size, and the calculated equivalent pore size was about 8.0 nm. The Papp of beta-blockers varied with lipophilicity in a sigmoidal fashion. Uridine transport showed temperature sensitivity and directionality, favoring transport in the apical-to-basolateral direction. Apical L-carnosine uptake was reduced by 46% in the absence of an inwardly directed proton gradient, and lowering the temperature to 4 degrees C abolished direction-dependent L-carnosine uptake. Furthermore, uptake was inhibited by 73% using apical 10 mM glycyl sarcosine (a dipeptide transporter substrate) and by 60% using 1 mM L-valacyclovir (a dipeptide prodrug). In conclusion, a functional air-interfaced primary culture of rabbit conjunctival epithelial cell layers was established. This air-interfaced primary culture model may be useful for studying passive and active transport processes for ion and solute translocation in the mammalian conjunctival epithelial barrier in a defined experimental setting.

Beta adrenergic antagonist permeation across cultured rabbit corneal epithelial cells grown on permeable supports

PURPOSE

To determine whether cultured rabbit corneal epithelial cells (RCEC), grown on permeable supports, provide a suitable in vivo model for characterizing transcellular drug permeation and metabolism.

METHODS

Primary rabbit corneal epithelial cells grown in DMEM-F12 were seeded on Transwell-COL inserts coated with fibronectin. The epithelial barrier integrity was evaluated, based on measurements of 14C-mannitol and 3H-PEG900, and their transepithelial electrical resistance (TEER). Ultrastructure evaluation was based on scanning electron microscopy and transmission electron microscopy, which were performed 8 days after seeding. Measurements of beta adrenergic antagonist permeability were performed to assess transcellular permeability.

RESULTS

Eight days after seeding, the TEER reached a peak of 144 omega.cm2 and the 14C-mannitol and 3H-PEG900 permeabilities were 6.8 x 10(-6) and 2.9 x 10(-6) cm/sec, respectively. Ultrastructural analysis revealed a multilayered structure with numerous microplicae and typical cytoplasmic organelles along with desmosomes. The relationship between permeation of beta-blockers and lipophilicity resembled the intact isolated cornea.

CONCLUSIONS

This is the first description of cultured RCEC grown on permeable support. Many of its properties mimic those described in the intact corneal epithelium. Even though its electrical tightness is less than that of the intact cornea, the transcellular permeability to lipophilic beta-antagonists is comparable to the isolated preparation. Therefore, this model will facilitate characterization of ocular permeation mechanisms of hydrophobic drugs whose route of permeation is transcellular.