Wiring the ocular surface: A focus on the comparative anatomy and molecular regulation of sensory innervation of the cornea

Comparative fluorophotometric evaluation of the ocular surface retention time of cross-linked and linear hyaluronic acid ocular eye drops on healthy dogs

PURPOSE

Evaluate and compare the retention time on the canine ocular surface of crosslinked hyaluronic acid (X-HA), linear hyaluronic acid (L-HA) and saline solution using fluorescent compounds (fluorescein sodium salt, Alexa Fluor 488 cadaverine and Alexa Fluor 488 maleimide).

METHODS

0.9% saline solution (SAL) was combined with fluorescein sodium salt. X-HA and L-HA were covalently modified using Alexa Fluor 488 reactive moieties. Eye drops were applied to 70 eyes of 35 dogs that were previously assessed and determined to have a normal ocular surface. Employing a blue light filter (450-490 nm), digital images were captured from instillation to 180 min. Images were analyzed to assess the percent of the total ocular area covered with green fluorescence at various time points.

RESULTS

X-HA exhibited a dual phase behavior: A broad microgel coverage first, followed by accumulation in tear film meniscus and medial canthus in the second phase, remaining in contact with the ocular surface up to 180 min. Coverage with L-HA and SAL eye drops quickly migrated to the tear meniscus. No traces of the fluorescent compounds were observed by 45 min in eyes treated with SAL solution compound and, by 120 min, eyes treated with L-HA.

CONCLUSIONS

X-HA exhibited a significantly increased ocular surface contact time with the ocular surface compared with L-HA and SAL. Not only could this indicate extended lubrication time but also supports the potential use of this compound as a method for topical sustained-release drug application.

In vitro and ex vivo models of microbial keratitis: Present and future

Microbial keratitis (MK) is an infection of the cornea, caused by bacteria, fungi, parasites, or viruses. MK leads to significant morbidity, being the fifth leading cause of blindness worldwide. There is an urgent requirement to better understand pathogenesis in order to develop novel diagnostic and therapeutic approaches to improve patient outcomes. Many in vitro, ex vivo and in vivo MK models have been developed and implemented to meet this aim. Here, we present current in vitro and ex vivo MK model systems, examining their varied design, outputs, reporting standards, and strengths and limitations. Major limitations include their relative simplicity and the perceived inability to study the immune response in these MK models, an aspect widely accepted to play a significant role in MK pathogenesis. Consequently, there remains a dependence on in vivo models to study this aspect of MK. However, looking to the future, we draw from the broader field of corneal disease modelling, which utilises, for example, three-dimensional co-culture models and dynamic environments observed in bioreactors and organ-on-a-chip scenarios. These remain unexplored in MK research, but incorporation of these approaches will offer further advances in the field of MK corneal modelling, in particular with the focus of incorporation of immune components which we anticipate will better recapitulate pathogenesis and yield novel findings, therefore contributing to the enhancement of MK outcomes.

Corneal fibrosis: From in vitro models to current and upcoming drug and gene medicines

Fibrotic diseases are characterised by myofibroblast differentiation, uncontrolled pathological extracellular matrix accumulation, tissue contraction, scar formation and, ultimately tissue / organ dysfunction. The cornea, the transparent tissue located on the anterior chamber of the eye, is extremely susceptible to fibrotic diseases, which cause loss of corneal transparency and are often associated with blindness. Although topical corticosteroids and antimetabolites are extensively used in the management of corneal fibrosis, they are associated with glaucoma, cataract formation, corneoscleral melting and infection, imposing the need of far more effective therapies. Herein, we summarise and discuss shortfalls and recent advances in in vitro models (e.g. transforming growth factor-β (TGF-β) / ascorbic acid / interleukin (IL) induced) and drug (e.g. TGF-β inhibitors, epigenetic modulators) and gene (e.g. gene editing, gene silencing) therapeutic strategies in the corneal fibrosis context. Emerging therapeutical agents (e.g. neutralising antibodies, ligand traps, receptor kinase inhibitors, antisense oligonucleotides) that have shown promise in clinical setting but have not yet assessed in corneal fibrosis context are also discussed.

Clearing the light path: proteoglycans and their important roles in the lens and cornea

Some of the earliest studies of glycans were performed on mammalian corneas and lenses with many of the key concepts we currently recognize as being fundamental to our understanding of basic cell biology arising from these studies. Proteoglycans and their GAG side chains are essential components of the ECM of the lens capsule. They also are present in the anterior corneal epithelial basement membrane and the posterior (Decemet's) basement membrane, and they organize collagen fiber diameters and spacing in the corneal stroma to maintain stromal clarity. Studies using genetically engineered mice and characterization of spontaneously arising mutations in genes controlling proteoglycan synthesis have generated new insight into the roles played by proteoglycans in signal transduction. We now know that proteoglycans and GAGs can regulate cell signaling and the maintenance of avascularity and immune privilege that are hallmarks of these tissues. In addition, proteoglycan-rich matrices provide the pathways for immune cells to populate the surface of the lens as a response to corneal wounding and in a model of Experimental Autoimmune Uveitis. Here we describe what is known about proteoglycans and GAGs in the cornea and lens. This knowledge has begun to provide promising leads into new proteoglycan-based treatments aimed at restoring and maintaining homeostasis in the cornea. Future studies are needed to determine how these new drugs impact the recruitment of immune cells to the lens for functions in restoring/maintaining homeostasis in the eye.