Tear film mucins: front line defenders of the ocular surface; comparison with airway and gastrointestinal tract mucins

Impact of inflammasomes on the ocular surface

PURPOSE OF REVIEW

The ocular surface is prone to inflammation due to exposure to environmental irritants and pathogens. Inflammasomes are intracellular, multiprotein complexes that communicate potentially dangerous signals to the immune system. The identification of inflammasomes in various inflammatory ocular surface conditions can aid in the development of therapeutics to treat these chronic inflammatory conditions.

RECENT FINDINGS

Several inflammasomes have been associated with ocular surface disorders including dry eye disease, keratitis, and allergies. Mechanisms for activation of these inflammasomes with regards to specific disorders have been explored in models to aid in the development of targeted treatments.

SUMMARY

Research efforts continue to characterize the types of inflammasomes and activators of these in inflammatory ocular surface conditions. Various therapies targeting specific inflammasome types or pyroptosis are being tested preclinically to assess effects on decreasing the associated chronic inflammation.

Sex-dependent differential increase of specialized pro-resolving mediators in extracellular vesicles secreted by human primary conjunctival goblet cells during allergic inflammation

AIMS

Conjunctival epithelium lines the inside of the eyelids and covers the sclera, thus providing stability to the eye surface. Goblet cells in conjunctival epithelium (CjGCs) are well known for their mucin-secretion function, which wet and protect the ocular surface, but other aspects are still not well understood. To expand our understanding beyond their mucin-secreting function, we investigated CjGC-secreted extracellular vesicles (EVs) and lipid mediators therein.

MATERIALS AND METHODS

Using histamine-mediated allergic inflammation in human primary CjGCs (HCjGCs) as a disease model, we quantified using ELISA a proinflammatory mediator PGE2 and two specialized pro-resolving mediators (SPMs) LXA4 and RvD1 in EVs secreted during allergic inflammation.

KEY FINDINGS

At 18 h post histamine stimulation, the amount of LXA4 and RvD1 in EVs was notably higher compared to those in unstimulated. Interestingly, this increase was only observed in female EVs but not in males. The mean fold increase of LXA4 and RvD1 in female EVs was 3.9 and 3.4, respectively, but it was only 0.9 and 1.0 in male EVs. Supplying docosahexaenoic acid (DHA, the source of RvD1 and other SPMs) to the culture medium during the allergic inflammation resulted in even higher mean fold increase of 5.3 and 6.9 for LXA4 and RvD1 in female EVs, respectively, but it was only 0.5 and 0.8 in male EVs.

SIGNIFICANCE

We conclude that HCjGCs show a clear sex difference in allergic response. Our results may also provide a new insight into the male predisposition to severe forms of allergic conjunctivitis and potential improvement in disease care in the clinic.

Polystyrene Sulfonate Resin as an Ophthalmic Carrier for Enhanced Bioavailability of Ligustrazine Phosphate Controlled Release System

Topical ocular sustained-release drug delivery systems represent an effective strategy for the treatment of ocular diseases, for which a suitable carrier has yet to be sufficiently developed. Herein, an eye-compatible sodium polystyrene sulfonate resin (SPSR) was synthesized with a uniform particle size of about 3 μm. Ligustrazine phosphate (LP) was adsorbed to SPSR by cation exchange to form LP@SPSR. LP@SPSR suspension eye drops were further developed using the combination of Carbopol 934P and xanthan gum as suspending agents. The LP@SPSR suspension showed a sustained release in vitro, which was consistent with the observed porcine corneal penetration ex vivo. Pharmacokinetics in tear fluid of rabits indicated that LP@SPSR suspension led to prolonged ocular retention of LP and a 2-fold improved the area under the drug concentration-time curve (AUC0-t). Pharmacokinetics in the aqueous humor of rabbits showed 2.8-fold enhancement in the AUC0-t compared to LP solution. The LP@SPSR suspension exhibited no cytotoxicity to human corneal epithelial cells, nor irritation was observed in rabbit eyes. Thus, the LP@SPSR suspension has been validated as a safe and sustained release system leading to enhanced ophthalmic bioavailability for treating ocular diseases.

Exploring Mucoadhesive and Toxicological Characteristics Following Modification of Linear Polyethylenimine with Various Anhydrides

Linear polyethylenimine (L-PEI) has numerous applications, such as in pharmaceutical formulations, gene delivery, and water treatment. However, due to the presence of secondary amine groups, L-PEI shows a relatively high toxicity and low biocompatibility. Here, various organic anhydrides were used to modify L-PEI to reduce its toxicity and enhance its functionality. We selected methacrylic anhydride, crotonic anhydride, maleic anhydride, and succinic anhydride to modify L-PEI. The structure of the resulting derivatives was characterized using 1H NMR and FTIR spectroscopies, and their behavior in aqueous solutions was studied using turbidimetric and electrophoretic mobility measurements over a broad range of pHs. A fluorescence flow through method determined the mucoadhesive properties of the polymers to the bovine palpebral conjunctiva. Methacrylated L-PEI and crotonylated L-PEI showed strong mucoadhesive properties at pH 7.4, likely due to covalent bonding with mucin thiol groups. In contrast, maleylated and succinylated L-PEI were poorly mucoadhesive as the pH was above their isoelectric point, resulting in electrostatic repulsion between the polymers and mucin. The toxicity of these polymers was evaluated using in vivo assays with planaria and the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) cell viability assay in human alveolar epithelial cells. Moreover, the irritancy of polymers was assessed using a slug mucosa irritation assay. The results demonstrated that anhydride modification mitigated the adverse toxicity effects seen for parent L-PEI.

Tamarind seed polysaccharide-metformin insert: Higher ocular retention, slow-release, and efficacy against corneal burn

Metformin (MET) can be an alternative therapeutic strategy for managing ocular burn primarily because of its pleiotropic mechanism. Longer retention on the ocular surface and sustained release are necessary to ensure the efficacy of MET for ocular application. Although the high aqueous solubility of MET is good for formulation and biocompatibility, it makes MET prone to high nasolacrimal drainage. This limits ocular residence and may be a challenge in its application. To address this, polymers approved for ophthalmic application with natural origin were analyzed through in silico methods to determine their ability to bind to mucin and interact with MET. An ocular insert of MET (3 mg/6 mm) was developed using a scalable solvent casting method without using preservatives. The relative composition of the insert was 58 ± 2.06 %w/w MET with approximately 14 %w/w tamarind seed polysaccharide (TSP), and 28 %w/w propylene glycol (PG). Its stability was demonstrated as per the ICH Q1A (R2) guidelines. Compatibility, ocular retention, drug release, and other functional parameters were evaluated. In rabbits, efficacy was demonstrated in the 'corneal alkali burn preclinical model'. TSP showed potential for mucoadhesion and interaction with MET. With adequate stability and sterility, the insert contributed to adequate retention of MET (10-12 h) in vivo and slow release (30 h) in vitro. This resulted in significant efficacy in vivo.

Bioinspired Lubricity from Surface Gel Layers

Surface gel layers on commercially available contact lenses have been shown to reduce frictional shear stresses and mitigate damage during sliding contact with fragile epithelial cell layers in vitro. Spencer and co-workers recently demonstrated that surface gel layers could arise from oxygen-inhibited free-radical polymerization. In this study, polyacrylamide hydrogel shell probes (7.5 wt % acrylamide, 0.3 wt % N,N'-methylenebisacrylamide) were polymerized in three hemispherical molds listed in order of decreasing surface energy and increasing oxygen permeability: borosilicate glass, polyether ether ketone (PEEK), and polytetrafluoroethylene (PTFE). Hydrogel probes polymerized in PEEK and PTFE molds exhibited 100× lower elastic moduli at the surface (E PEEK * = 80 ± 31 and E PTFE * = 106 ± 26 Pa, respectively) than those polymerized in glass molds (E glass * = 31,560 ± 1,570 Pa), in agreement with previous investigations by Spencer and co-workers. Biotribological experiments revealed that hydrogel probes with surface gel layers reduced frictional shear stresses against cells (τPEEK = 35 ± 15 and τPTFE = 22 ± 16 Pa) more than those without (τglass = 68 ± 15 Pa) and offered greater protection against cell damage when sliding against human telomerase-immortalized corneal epithelial (hTCEpi) cell monolayers. Our work demonstrates that the "mold effect" resulting in oxygen-inhibition polymerization creates hydrogels with surface gel layers that reduce shear stresses in sliding contact with cell monolayers, similar to the protection offered by gradient mucin gel networks across epithelial cell layers.

Mucoadhesive, antioxidant, and lubricant catechol-functionalized poly(phosphobetaine) as biomaterial nanotherapeutics for treating ocular dryness

Dry eye disease (DED) is associated with ocular hyperosmolarity and inflammation. The marketed topical eye drops for DED treatment often lack bioavailability and precorneal residence time. In this study, we investigated catechol-functionalized polyzwitterion p(MPC-co-DMA), composed of 2-methacryloyloxyethyl phosphorylcholine (MPC) and dopamine methacrylamide (DMA) monomers, as potential topical nanotherapeutics for DED. The copolymers were synthesized via random free-radical copolymerization, producing different proportions of catecholic functionalization. All as-prepared polymer compositions displayed good ocular biocompatibility. At a feeding ratio of 1:1, p(MPC1-co-DMA1) can facilitate a robust mucoadhesion via Michael addition and/or Schiff base reaction, thus prolonging ocular residence time after 4 days of topical instillation. The hydration lubrication of MPC and radical-scavenging DMA endow the nano-agent to ease tear-film hyperosmolarity and corneal inflammation. A single dose of p(MPC1-co-DMA1) (1 mg/mL) after 4 days post-instillation can protect the cornea against reactive oxygen species, inhibiting cell apoptosis and the over-expression of pro-inflammatory factors (IL-6 and TNF-α). In clinical assessment, DED-induced rabbit eyes receiving p(MPC1-co-DMA1) could increase lacrimal fluid secretion by 5-fold higher than cyclosporine A. The catechol-functionalized polyzwitterion with enhanced lubricity, mucoadhesion, and anti-oxidation/anti-inflammation properties has shown high promise as a bioactive eye drop formulation for treating DED.

Rutin alleviates Sjogren's syndrome via CaR/NLRP3/NF-κB signal pathway

Sjogren's syndrome (SS) is an autoimmune disease. Its mechanism and treatment methods are unclear. The purpose of this study was to investigate the effects of rutin (Ru) on SS. Proteomics was used to detect differential proteins in the submandibular glands of normal mice and SS mice. Salivary secretion (SAS) and salivary gland index (SGI) were detected. Oxidative stress and inflammatory cytokine in submandibular glands were detected. The levels of NLRP3, ASC, Caspase-1, IL-1β, and p-NF-κBp65 in submandibular gland tissues and submandibular gland cells of overexpressed calcium-sensing receptor (over-CaR) mice and overexpressed CaR primary submandibular gland cells (over-CaR-PSGs) were detected. In total, 327 differential proteins were identified in the submandibular gland tissues of SS mice compared to control mice. CaR was one of the most differential proteins and significantly increased compared to control mice. Ru could significantly increase SGI and SGI, and inhibit oxidative stress and inflammatory cytokine in submandibular glands. In addition, Ru was shown to further improve SS via regulation of the CaR/NOD-like receptor thermal protein domain associated protein 3 (NLRP3)/nuclear factor kappa-B (NF-κB) signal pathway. Overexpression of CaR counteracted partial activity of Ru. CaR may be an important target for the treatment of SS. In addition, Ru improved the SS via the CaR/NLRP3/NF-κB signal pathway. This study provides a basis for the treatments for SS.

Keratan sulfate, an electrosensory neurosentient bioresponsive cell instructive glycosaminoglycan

The roles of keratan sulfate (KS) as a proton detection glycosaminoglycan in neurosensory processes in the central and peripheral nervous systems is reviewed. The functional properties of the KS-proteoglycans aggrecan, phosphacan, podocalyxcin as components of perineuronal nets in neurosensory processes in neuronal plasticity, cognitive learning and memory are also discussed. KS-glycoconjugate neurosensory gels used in electrolocation in elasmobranch fish species and KS substituted mucin like conjugates in some tissue contexts in mammals need to be considered in sensory signalling. Parallels are drawn between KS's roles in elasmobranch fish neurosensory processes and its roles in mammalian electro mechanical transduction of acoustic liquid displacement signals in the cochlea by the tectorial membrane and stereocilia of sensory inner and outer hair cells into neural signals for sound interpretation. The sophisticated structural and functional proteins which maintain the unique high precision physical properties of stereocilia in the detection, transmittance and interpretation of acoustic signals in the hearing process are important. The maintenance of the material properties of stereocilia are essential in sound transmission processes. Specific, emerging roles for low sulfation KS in sensory bioregulation are contrasted with the properties of high charge density KS isoforms. Some speculations are made on how the molecular and electrical properties of KS may be of potential application in futuristic nanoelectronic, memristor technology in advanced ultrafast computing devices with low energy requirements in nanomachines, nanobots or molecular switches which could be potentially useful in artificial synapse development. Application of KS in such innovative areas in bioregulation are eagerly awaited.

Next generation therapeutics for retinal neurodegenerative diseases

Neurodegenerative diseases affecting the visual system encompass glaucoma, macular degeneration, retinopathies, and inherited genetic disorders such as retinitis pigmentosa. These ocular pathologies pose a serious burden of visual impairment and blindness worldwide. Current treatment modalities include small molecule drugs, biologics, or gene therapies, most of which are administered topically as eye drops or as injectables. However, the topical route of administration faces challenges in effectively reaching the posterior segment and achieving desired concentrations at the target site, while injections and implants risk severe complications, such as retinal detachment and endophthalmitis. This necessitates the development of innovative therapeutic strategies that can prolong drug release, deliver effective concentrations to the back of the eye with minimal systemic exposure, and improve patient compliance and safety. In this review, we introduce retinal degenerative diseases, followed by a discussion of the existing clinical standard of care. We then delve into detail about drug and gene delivery systems currently in preclinical and clinical development, including formulation and delivery advantages/drawbacks, with a special emphasis on potential for clinical translation.

The yielding behaviour of human mucus

Mucus is a viscoelastic material with non-linear rheological properties such as a yield stress of the order of a few hundreds of millipascals to a few tens of pascals, due to a complex network of mucins in water along with non-mucin proteins, DNA and cell debris. In this review, we discuss the origin of the yield stress in human mucus, the changes in the rheology of mucus with the occurrence of diseases, and possible clinical applications in disease detection as well as cure. We delve into the domain of mucus rheology, examining both macro- and microrheology. Macrorheology involves investigations conducted at larger length scales (∼ a few hundreds of μm or higher) using traditional rheometers, which probe properties on a bulk scale. It is significant in elucidating various mucosal functions within the human body. This includes rejecting unwanted irritants out of lungs through mucociliary and cough clearance, protecting the stomach wall from the acidic environment as well as biological entities, safeguarding cervical canal from infections and providing a swimming medium for sperms. Additionally, we explore microrheology, which encompasses studies performed at length scales ranging from a few tens of nm to a μm. These microscale studies find various applications, including the context of drug delivery. Finally, we employ scaling analysis to elucidate a few examples in lung, cervical, and gastric mucus, including settling of irritants in lung mucus, yielding of lung mucus in cough clearance and cilial beating, spreading of exogenous surfactants over yielding mucus, swimming of Helicobacter pylori through gastric mucus, and lining of protective mucus in the stomach. The scaling analyses employed on the applications mentioned above provide us with a deeper understanding of the link between the rheology and the physiology of mucus.

Corneal epithelium models for safety assessment in drug development: Present and future directions

The human corneal epithelial barrier plays a crucial role in drug testing studies, including drug absorption, distribution, metabolism, and excretion (ADME), as well as toxicity testing during the preclinical stages of drug development. However, despite the valuable insights gained from animal and current in vitro models, there remains a significant discrepancy between preclinical drug predictions and actual clinical outcomes. Additionally, there is a growing emphasis on adhering to the 3R principles (refine, reduce, replace) to minimize the use of animals in testing. To tackle these challenges, there is a rising demand for alternative in vitro models that closely mimic the human corneal epithelium. Recently, remarkable advancements have been made in two key areas: microphysiological systems (MPS) or organs-on-chips (OoCs), and stem cell-derived organoids. These cutting-edge platforms integrate four major disciplines: stem cells, microfluidics, bioprinting, and biosensing technologies. This integration holds great promise in developing powerful and biomimetic models of the human cornea.

A nerve-goblet cell association promotes allergic conjunctivitis through rapid antigen passage

The penetration of allergens through the epithelial layer is the initial step in the development of allergic conjunctivitis. Although pollinosis patients manifest symptoms within minutes after pollen exposure, the mechanisms of the rapid transport of the allergens remain unclear. In the present study, we found that the instillation of pollen shells rapidly induces a large number of goblet cell-associated antigen passages (GAPs) in the conjunctiva. Antigen acquisition by stromal cells, including macrophages and CD11b+ dendritic cells, correlated with surface GAP formation. Furthermore, a substantial amount of antigen was transported to the stroma during the first 10 minutes of pollen exposure, which was sufficient for the full induction of an allergic conjunctivitis mouse model. This inducible, rapid GAP formation and antigen acquisition were suppressed by topical lidocaine or trigeminal nerve ablation, indicating that the sensory nervous system plays an essential role. Interestingly, pollen shell-stimulated GAP formation was not suppressed by topical atropine, suggesting that the conjunctival GAPs and intestinal GAPs are differentially regulated. These results identify pollen shell-induced GAP as a therapeutic target for allergic conjunctivitis.

Dry eye disease in astronauts: a narrative review

Long-duration spaceflight can have adverse effects on human health. One of the most common ocular conditions experienced by astronauts is dry eye disease (DED). Symptoms of DED include feelings of eye irritation, eye strain, foreign body sensation and blurred vision. Over 30% of International Space Station expedition crew members reported irritation and foreign body sensation. We reviewed the current literature on the prevalence and mechanisms of DED in astronauts and its potential implications for long-duration spaceflight, including the influence of environmental factors, such as microgravity and fluid shift on tear film physiology in space. DED has negative effects on astronaut performance, which is why there is a need for further research into the pathophysiology and countermeasures. As an in-flight countermeasure, neurostimulation seems to be among the most promising options.

Purinergic 2X 4 (P2X4), but not P2X7, receptors increase cytosolic [Ca2+] and stimulate mucin secretion in rat conjunctival goblet cells to maintain ocular surface health

Ionotropic purinergic receptors (P2XRs) are activated by ATP and ATP analogs. ATP can be released through ATP-permeable channels such as the pannexin hemichannels. Upon activation, the P2XRs become permeable to Ca2+, a potent stimulator of mucin secretion in conjunctival goblet cells (CGCs). The purpose of this study was to investigate the presence and function of P2XRs in CGCs. We also examined the presence of pannexin hemichannels. Rat first passage CGCs were stained with the goblet cell marker anti-cytokeratin 7 antibody and specific antibodies to P2X1-7 receptors and pannexin 1-3. mRNA expression was determined by RT-PCR using primers specific to P2XRs and pannexins. Proteins were identified with Western blotting (WB) using the same antibodies as for immunofluorescence (IF) microscopy. To study receptor function, CGCs were incubated with Fura 2-AM, exposed to agonists and antagonists, and intracellular [Ca2+] ([Ca2+]i) measured. [Ca2+]i was also measured after knock down of P2X4 and P2X7 receptor expression, and when exploiting P2XR specific characteristics. Lastly, mucin secretion was measured after the addition of several P2XR agonists. All P2XRs and pannexins were visualized with IF microscopy, and identified with RT-PCR and WB. [Ca2+]i was significantly increased when stimulated with ATP (10-7-10-4 M). Suramin, a non-selective P2XR antagonist at 10-4 M did not reduce ATP-induced peak [Ca2+]i. The potent P2X7 agonist, BzATP (10-7-10-4 M) increased the [Ca2+]i, although to a lesser extent than ATP. When measuring [Ca2+]i the effect of repeated applications of ATP at 10-5 or 10-6 M the response "desensitized" after 30-60 s. The P2X4 specific antagonist 5-BDBD decreased the P2X4 agonist, 2MeSATP,-induced [Ca2+]i increase. Furthermore, siRNA against the P2X4R, but not the P2X7R, decreased agonist-induced peak [Ca2+]i. ATP (10-5 M), BzATP (10-4 M) and 2MeSATP (10-5 M) induced mucin secretion. We conclude that all seven P2XRs are present in cultured rat CGCs. Of the P2XRs, only activation of the homotrimeric P2X4R appears to increase [Ca2+]i and induce mucin secretion. The P2X4R in CGCs offers a new therapeutic target for protective mucin secretion.

Molecular nature of ocular surface barrier function, diseases that affect it, and its relevance for ocular drug delivery

The structural and functional integrity of the ocular surface, a continuous epithelial structure comprised of the cornea, the conjunctiva, and the ductal surface of the lacrimal as well as meibomian glands, is crucial for proper vision. The ocular surface barrier function (OSBF), sum of the different types of protective mechanisms that exist at the ocular surface, is essential to protect the rest of the eye from vision-threatening physical, chemical, and biological insults. OSBF helps maintain the immune privileged nature of the cornea and the aqueous humor by preventing entry of infectious agents, allergens, and noxious chemicals. Disruption of OSBF exposes the dense nerve endings of the cornea to these stimuli, resulting in discomfort and pain. This review summarizes the status of our knowledge related to the molecular nature of OSBF, describes the effect of different ocular surface disorders on OSBF, and examines the relevance of this knowledge for ocular drug delivery.

Fungal dysbiosis and decreased tear mucin at the conjunctiva in patients with conjunctival mucosa-associated lymphoid tissue lymphoma

OBJECTIVE

This study aimed to examine the differences in the fungal microbiome between patients with conjunctival mucosa-associated lymphoid tissue (MALT) and healthy controls using metagenomic analysis.

METHODS AND ANALYSIS

This case-control study was conducted at Osaka University Hospital in Osaka, Japan, from April 2015 to March 2022. Twenty-five consecutive patients with conjunctival MALT lymphoma and 25 healthy volunteers were included. Metagenomic analysis using Internal Transcribed Spacer (ITS)1 deep sequencing and hierarchical clustering was performed to investigate differences in the fungal microbiome. To assess tear environmental change, we measured tear mucin concentrations using ELISA.

RESULTS

Detailed analyses showed fungal dysbiosis and changes in β-diversity within the conjunctiva of patients with conjunctival MALT lymphoma. Hierarchical clustering revealed that the participants could be divided into three clusters according to the Malassezia abundance: cluster I (Malassezia abundance above 70%), cluster II (Malassezia abundance 25%-70%) and cluster II (Malassezia abundance below 25%). Most patients were included in cluster I, whereas most of healthy controls were included in cluster III. The differences were significant. Tear mucin concentrations were significantly lower in patients with MALT compared with healthy controls.

CONCLUSION

The metagenomic analysis using ITS1 deep sequencing was useful for identifying the differences in commensal fungi between patients with MALT lymphoma and healthy individuals. The increased prevalence of the Malassezia genus and the decreased levels of tear mucin can lead to an allergic response of the conjunctiva, resulting in the pathogenesis associated with conjunctival MALT lymphoma. Therefore, it may be beneficial to initiate treatment when a high abundance Malassezia is detected.

The Role of Membrane-Tethered Mucins in Axial Epithelial Adhesion in Controlled Normal Stress Environments

The collective adhesive behavior of epithelial cell layers mediated by complex macromolecular fluid environments plays a vital role in many biological processes. Mucins, a family of highly glycosylated proteins, are known to lubricate cell-on-cell contacts in the shear direction. However, the role of mucins mediating axial epithelial adhesion in the direction perpendicular to the plane of the cell sheet has received less attention. This article subjects cell-on-cell layers of live ocular epithelia that express mucins on their apical surfaces to compression/decompression cycles and tensile loading using a customized instrument. In addition to providing compressive moduli of native cell-on-cell layers, it is found that the mucin layer between the epithelia acts as a soft cushion between the epithelial cell layers. Decompression experiments reveal mucin layers act as soft, nonlinear springs in the axial direction. The cell-on-cell layers withstand decompression before fracturing by a cohesive failure within the mucin layer. When mucin deficiency is induced via a protease treatment, it is found that the axial adhesion between the cell layers is increased. The findings which correlate changes in biological factors with changes in mechanical properties might be of interest to challenges in ophthalmology, vision care, and mucus research.

Potential Clinical Applications of Pro-Resolving Lipids Mediators from Docosahexaenoic Acid

Docosahexaenoic acid (C22:6n-3, DHA) is the precursor of specialized pro-resolving lipid mediators (SPMs), such as resolvin, protectin, and maresin families which have been considered therapeutic bioactive compounds for human health. Growing evidence indicates that DHA and SPMs are beneficial strategies in the amelioration, regulation, and duration of inflammatory processes through different biological actions. The present review discusses the reported therapeutic benefits of SPMs on various diseases and their potential clinical applications.

Comparative hydrodynamic and nanoscale imaging study on the interactions of teicoplanin-A2 and bovine submaxillary mucin as a model ocular mucin

Glycopeptide antibiotics are regularly used in ophthalmology to treat infections of Gram-positive bacteria. Aggregative interactions of antibiotics with mucins however can lead to long exposure and increases the risk of resistant species. This study focuses on the evaluation of potential interactions of the last line of defence glycopeptide antibiotic teicoplanin with an ocular mucin model using precision matrix free hydrodynamic and microscopic techniques: sedimentation velocity in the analytical ultracentrifuge (SV-AUC), dynamic light scattering (DLS) and atomic force microscopy (AFM). For the mixtures of teicoplanin at higher doses (1.25 mg/mL and 12.5 mg/mL), it was shown to interact and aggregate with bovine submaxillary mucin (BSM) in the distributions of both sedimentation coefficients by SV-AUC and hydrodynamic radii by DLS. The presence of aggregates was confirmed by AFM for higher concentrations. We suggest that teicoplanin eye drop formulations should be delivered at concentrations of < 1.25 mg/mL to avoid potentially harmful aggregations.

Ocular graft-versus-host disease (oGVHD): From A to Z

Allogeneic hematopoietic stem cell transplantation is a definitive therapy for a variety of disorders. One of the complications is acute graft-versus-host disease (aGVHD), which has a high mortality rate. Patients can also develop chronic graft-versus-host disease (cGVHD), a more indolent yet afflicting condition that affects up to 70% of patients. Ocular involvement (oGVHD) is one of the most prevalent presentations of cGVHD and can manifest as dry eye disease, meibomian gland dysfunction, keratitis, and conjunctivitis. Early recognition of ocular involvement using regular clinical assessments as well as robust biomarkers can aid in better management and prevention. Currently, the therapeutic strategies for the management of cGVHD, and oGVHD in particular, have mainly focused on the control of symptoms. There is an unmet need for translating the preclinical and molecular understandings of oGVHD into clinical practice. Herein, we have comprehensively reviewed the pathophysiology, pathologic features, and clinical characteristics of oGVHD and summarized the therapeutic landscape available to combat it. We also discuss the direction of future research regarding a more directed delineation of pathophysiologic underpinnings of oGVHD and the development of preventive interventions.

Dendrimers in Corneal Drug Delivery: Recent Developments and Translational Opportunities

Dendrimers are biocompatible organic nanomaterials with unique physicochemical properties, making them the focus of recent research in drug delivery. The cornea of the human eye presents a challenge for drug transit due to its inherently impenetrable nature, requiring nanocarrier-mediated targeted drug delivery. This review intends to examine recent advancements in the use of dendrimers for corneal drug delivery, including their properties and their potential for treating various ocular diseases. The review will also highlight the benefit of the novel technologies that have been developed and applied in the field, such as corneal targeting, drug release kinetics, treatments for dry eye disease, antibacterial drug delivery, corneal inflammation, and corneal tissue engineering. The review seeks to provide a comprehensive overview of the current state of research in this field, along with the translational developments in the field of dendrimer-based therapeutics and imaging agents and inspire the potential for future developments and translational opportunities in dendrimers based corneal drug delivery.

Hydrogels for Mucosal Drug Delivery

Mucosal tissues are often a desirable site of drug action to treat disease and engage the immune system. However, systemically administered drugs suffer from limited bioavailability in mucosal tissues where technologies to enable direct, local delivery to these sites would prove useful. In this Spotlight on Applications article, we discuss hydrogels as an attractive means for local delivery of therapeutics to address a range of conditions affecting the eye, nose, oral cavity, gastrointestinal, urinary bladder, and vaginal tracts. Considering the barriers to effective mucosal delivery, we provide an overview of the key parameters in the use of hydrogels for these applications. Finally, we highlight recent work demonstrating their use for inflammatory and infectious diseases affecting these tissues.

Characterization of Porcine Ocular Surface Epithelial Microenvironment

The porcine ocular surface is used as a model of the human ocular surface; however, a detailed characterization of the porcine ocular surface has not been documented. This is due, in part, to the scarcity of antibodies produced specifically against the porcine ocular surface cell types or structures. We performed a histological and immunohistochemical investigation on frozen and formalin-fixed, paraffin-embedded ocular surface tissue from domestic pigs using a panel of 41 different antibodies related to epithelial progenitor/differentiation phenotypes, extracellular matrix and associated molecules, and various niche cell types. Our observations suggested that the Bowman's layer is not evident in the cornea; the deep invaginations of the limbal epithelium in the limbal zone are analogous to the limbal interpalisade crypts of human limbal tissue; and the presence of goblet cells in the bulbar conjunctiva. Immunohistochemistry analysis revealed that the epithelial progenitor markers cytokeratin (CK)15, CK14, p63α, and P-cadherin were expressed in both the limbal and conjunctival basal epithelium, whereas the basal cells of the limbal and conjunctival epithelium did not stain for CK3, CK12, E-cadherin, and CK13. Antibodies detecting marker proteins related to the extracellular matrix (collagen IV, Tenascin-C), cell-matrix adhesion (β-dystroglycan, integrin α3 and α6), mesenchymal cells (vimentin, CD90, CD44), neurons (neurofilament), immune cells (HLA-ABC; HLA-DR, CD1, CD4, CD14), vasculature (von Willebrand factor), and melanocytes (SRY-homeobox-10, human melanoma black-45, Tyrosinase) on the normal human ocular surface demonstrated similar immunoreactivity on the normal porcine ocular surface. Only a few antibodies (directed against N-cadherin, fibronectin, agrin, laminin α3 and α5, melan-A) appeared unreactive on porcine tissues. Our findings characterize the main immunohistochemical properties of the porcine ocular surface and provide a morphological and immunohistochemical basis useful to research using porcine models. Furthermore, the analyzed porcine ocular structures are similar to those of humans, confirming the potential usefulness of pig eyes to study ocular surface physiology and pathophysiology.

Transmembrane Mucin Response in Conjunctival Epithelial Cells Exposed to Wall Shear Stresses

Human conjunctival epithelium cells (HCEC) line the inner surface of the eyelid and cover the sclera and are continuously subjected to wall shear stresses (WSS). The effects of external forces on the conjunctival epithelium are not fully known. The conjunctival epithelium contains stratified squamous cells that synthesize the membrane-spanning mucins MUC1 and MUC16, which play important roles in protecting the ocular surface. Alterations in both gel-forming and membrane-tethered mucins occur in drying ocular surface diseases. The aim of this study was to explore the mechanobiological characteristics of transmembrane mucin secretion and cellular alterations of primary HCEC exposed to airflow-induced WSS perturbations. We exposed the HCEC to a steady WSS of 0.5 dyne/cm2 for durations of 15 and 30 min. Cytoskeletal alterations and MUC1 secretions were studied using immunohistochemically fluorescent staining with specific antibodies. We investigated for the first time an in vitro model of membrane-tethered mucin secretion by HCEC in response to WSS. The exposure of HCEC to WSS increased the polymerization of F-actin, altered the cytoskeletal shape and reduced the secretion of membrane-tethered MUC1.

The protective role of conjunctival goblet cell mucin sialylation

Gel-forming mucins secreted by conjunctival goblet cells have been implicated in the clearance of allergens, pathogens, and debris. However, their roles remain incompletely understood. Here we show that human and mouse conjunctival goblet cell mucins have Alcian blue-detectable sialic acids, but not sulfates in the steady state. Interestingly, Balb/c mouse strain lacks this sialylation due to a point mutation in a sialyltransferase gene, St6galnac1, which is responsible for sialyl-Tn synthesis. Introduction of intact St6galnac1 to Balb/c restores the sialylation of conjunctival goblet cell mucus. Sialylated mucus efficiently captures and encapsulates the allergen particles in an impenetrable layer, leading to the protection of mice from the development of allergic conjunctivitis. Expression of ST6GALNAC1 and sialyl-Tn is upregulated in humans under conditions with chronic stimuli. These results indicate that the sialylated glycans on the ocular mucins play an essential role in maintaining the conjunctival mucosa by protecting from the incoming foreign bodies such as allergen particles.

Epigenetic Regulation of Corneal Epithelial Differentiation by TET2

Epigenetic DNA modification by 5-hydroxymethylcytosine (5hmC), generated by the Ten-eleven translocation (TET) dioxygenases, regulates diverse biological functions in many organ tissues, including the mammalian eye. For example, 5hmC has been shown to be involved in epigenetic regulation of retinal gene expression. However, a functional role of 5hmC in corneal differentiation has not been investigated to date. Here, we examined 5hmC and TET function in the human cornea. We found 5hmC highly expressed in MUC16-positive terminally differentiated cells that also co-expressed the 5hmC-generating enzyme TET2. TET2 knockdown (KD) in cultured corneal epithelial cells led to significant reductions of 5hmC peak distributions and resulted in transcriptional repression of molecular pathways involved in corneal differentiation, as evidenced by downregulation of MUC4, MUC16, and Keratin 12. Additionally, integrated TET2 KD RNA-seq and genome-wide Reduced Representation Hydroxymethylation Profiling revealed novel epigenetically regulated genes expressed by terminally differentiated cells, including KRT78, MYEOV, and MAL. In aggregate, our findings reveal a novel function of TET2 in the epigenetic regulation of corneal epithelial gene expression and identify novel TET2-controlled genes expressed in differentiated corneal epithelial cells. These results point to potential roles for TET2 induction strategies to enhance treatment of corneal diseases associated with abnormal epithelial maturation.

Multifunctional glycoprotein coatings improve the surface properties of highly oxygen permeable contact lenses

To achieve and maintain good operability of medical devices while reducing putative side effects for the patient, a promising strategy is to tailor the surface properties of such devices as they critically dictate the tissue compatibility and the biofouling behavior. Indeed, those properties can be strongly improved by generating mucin coatings on such medical devices. However, using coatings on optical systems, e.g., contact lenses, comes with various challenges: here, the geometrical and optical characteristics of the lens may not be compromised by either the coating process or the coating itself. In this study, we show how mucin macromolecules can be attached onto the surfaces of rigid, gas permeable contact lenses while maintaining all critical lens parameters. We demonstrate that the generated coatings improve the surface wettability (contact angles are reduced from 105° to 40° and liquid film break-up times are increased from <1 s to 31 s) and prevent tribological damage to corneal tissue. Additionally, such coatings are highly transparent (transmission values above 98 % compared to an uncoated sample are reached) and efficiently reduce lipid deposition to the lens surface by 90 % but fully maintain the geometrical and mechanical properties of the lenses. Thus, such mucin coatings could also be highly beneficial for other optical systems that are used in direct contact with tissues or body fluids.

Immunocytochemical characterization of ex vivo cultured conjunctival explants; marker validation for the identification of squamous epithelial cells and goblet cells

Tissue-engineered products are at the cutting edge of innovation considering their potential to functionally and structurally repair various tissue defects when the body's own regenerative capacity is exhausted. At the ocular surface, the wound healing response to extensive conjunctival damage results in tissue repair with structural alterations or permanent scar formation rather than regeneration of the physiological conjunctiva. Conjunctival tissue engineering therefore represents a promising therapeutic option to reconstruct the ocular surface in severe cicatrizing pathologies. During the rapid race to be a pioneer, it seems that one of the fundamental steps of tissue engineering has been neglected; a proper cellular characterization of the tissue-engineered equivalents, both morphologically and functionally. Currently, no consensus has been reached on an identification strategy and/or markers for the characterization of cultured squamous epithelial and goblet cells. This study therefore evaluated the accuracy of promising markers to identify differentiated conjunctival-derived cells in human primary explant cultures through immunocytochemistry, including keratins (i.e., K7, K13, and K19) and mucins (i.e., MUC1, MUC5AC, and PAS-positivity). Comparison of the in vivo and in vitro cellular profiles revealed that the widely used goblet cell marker K7 does not function adequately in an in vitro setting. The other investigated markers offer a powerful tool to distinguish cultured squamous epithelial cells (i.e., MUC1 and K13), goblet cells (i.e., MUC5AC and PAS-staining), and conjunctival-derived cells in general (i.e., K19). In conclusion, this study emphasizes the power alongside potential pitfalls of conjunctival markers to assess the clinical safety and efficacy of conjunctival tissue-engineered products.

Transcorneal but not transpalpebral electrical stimulation disrupts mucin homeostasis of the ocular surface

PURPOSE

Transcorneal electrical stimulation (TcES) is increasingly applied as a therapy for preserving and improving vision in retinal neurodegenerative and ischemic disorders. However, a common complaint about TcES is its induction of eye pain and dryness in the clinic, while the mechanisms remain unknown.

METHOD

TcES or transpalpebral ES (TpES) was conducted in C57BL6j mice for 14 days. The contralateral eyes were used as non-stimulated controls. Levels of intracellular [Ca2+] ([Ca2+]i) were assessed by Fura-2AM. The conductance resistances of the eye under various ES conditions were measured in vivo by an oscilloscope.

RESULTS

Although TcES did not affect tear production, it significantly induced damage to the ocular surface, as revealed by corneal fluorescein staining that was accompanied by significantly decreased mucin (MUC) 4 expression compared to the control. Similar effects of ES were detected in cultured primary corneal epithelium cells, showing decreased MUC4 and ZO-1 levels after the ES in vitro. In addition, TcES decreased secretion of MUC5AC from the conjunctiva in vivo, which was also corroborated in goblet cell cultures, where ES significantly attenuated carbachol-induced [Ca2+]i increase. In contrast to TcES, transpalpebral ES (TpES) did not induce corneal fluorescein staining while significantly increasing tear production. Importantly, the conductive resistance from orbital skin to the TpES was significantly smaller than that from the cornea to the retina in TcES.

CONCLUSION

TcES, but not TpES, induces corneal epithelial damage in mice by disrupting mucin homeostasis. TpES thus may represent a safer and more effective ES approach for treating retinal neurodegeneration clinically.

Maleimide-Decorated PEGylated Mucoadhesive Liposomes for Ocular Drug Delivery

Liposomes are promising spherical vesicles for topical drug delivery to the eye. Several types of vesicles were formulated in this study, including conventional, PEGylated, and maleimide-decorated PEGylated liposomes. The physicochemical characteristics of these liposomes, including their size, zeta potential, ciprofloxacin encapsulation efficiency, loading capacity, and release, were evaluated. The structure of these liposomes was examined using dynamic light scattering, transmission electron microscopy, and small angle neutron scattering. The ex vivo corneal and conjunctival retention of these liposomes were examined using the fluorescence flow-through method. Maleimide-decorated liposomes exhibited the best retention performance on bovine conjunctiva compared to other types of liposomes studied. Poor retention of all liposomal formulations was observed on bovine cornea.

Histochemical Comparison of Human and Rat Lacrimal Glands: Implications for Bio-Engineering Studies

Purpose

The purpose of this study was to determine whether rodent lacrimal glands (LGs) represent a suitable surrogate for human tissue in bio-engineering research, we undertook a meticulous histological and histochemical comparison of these two tissues.

Methods

Histological techniques and immunohistochemistry were used to compare the structure of adult human and rat LG tissues and the expression of key functional tissue elements.

Results

Compared with humans, the rat LG is comprised of much more densely packed acini which are devoid of an obvious central lumen. Myoepithelial, fibroblasts, dendritic cells, T cells, and putative progenitor cells are present in both tissues. However, human LG is replete with epithelium expressing cytokeratins 8 and 18, whereas rat LG epithelium does not express cytokeratin 8. Furthermore, human LG expresses aquaporins (AQPs) 1, 3, and 5, whereas rat LG expresses AQPs 1, 4, and 5. Additionally, mast cells were identified in the rat but not the human LGs and large numbers of plasma cells were detected in the human LGs but only limited numbers were present in the rat LGs.

Conclusions

The cellular composition of the human and rat LGs is similar, although there is a marked difference in the actual histo-architectural arrangement of the tissue. Further variances in the epithelial cytokeratin profile, in tissue expression of AQPs and in mast cell and plasma cell infiltration, may prove significant.

Translational Relevance

The rat LG can serve as a useful surrogate for the human equivalent, but there exist specific tissue differences meaning that caution must be observed when translating results to patients.

Sex-based differences in conjunctival goblet cell responses to pro-inflammatory and pro-resolving mediators

Many conjunctival inflammatory diseases differ between the sexes and altered conjunctival goblet cells (CGCs) response is often involved. Inflammation is initiated by the release of pro-inflammatory mediators and terminated by the biosynthesis of specialized pro-resolution mediators (SPMs). Herein, we determined the sex-based difference in the responses of CGCs to inflammatory stimuli or pro-resolving lipid SPMs and their interaction with sex hormones. GCs were cultured from pieces of human conjunctiva in RPMI media. CGCs were transferred 24 h before the start of experiments to phenol red-free and FBS-free media to minimize exogenous hormones. RT-PCR, immunofluorescence microscopy (IF), and Western Blot (WB) were performed to determine the presence of sex hormone receptors. Cellular response to pro-inflammatory stimuli or SPMs was studied by measuring the increase in intracellular [Ca²⁺] ([Ca²⁺]_i) using fura 2/AM microscopy. Use of RT-PCR demonstrated estrogen receptor (ER) α in 4/5 males and 3/3 females; ERβ in 2/4 males and 2/3 females; and androgen receptors (AR) in 3/3 male and 3/3 female CGCs. Positive immunoreactivity by IF and protein expression by WB was detected using antibodies for the ERα and ERβ in 3/3 males and 3/3 females, while AR were only present in males. Significantly different Ca²⁺ responses between sexes were found with carbachol only at 10^–3 M, but not with histamine or leukotriene (LT) B₄ at any concentration used. Incubation with dihydrotestosterone (DHT), estrone (E1), or estradiol (E2) at 10^–7 M for 30 min significantly inhibited the LTB₄-stimulated [Ca²⁺]_i increase in male and female CGCs. Incubation with DHT, E1, and E2 overnight significantly inhibited the LTB₄ response in females, while DHT and E2 significantly inhibited the LTB₄ response in males. The SPM lipoxin A₄ (LXA₄) (10^–9–10⁻⁸ M), but not the resolvins D1 or D2, induced an [Ca²⁺]_i increase that was significantly higher in males compared to females. We conclude that male and female CGCs showed differences in the expression of sex hormone receptors. Treatment with sex hormones altered pro-inflammatory mediator LTB₄-induced response. Males compared to females have a higher response to the ω-6-fatty acid derived SPM LXA₄, indicating males may terminate inflammation in conjunctival goblet cells faster than females.

Effect of Recombinant Human Lubricin on Model Tear Film Stability

Purpose

To investigate and quantify the effect of recombinant human lubricin (rh-lubricin) on model tear film stability.

Methods

A custom-built, interferometry-based instrument called the Interfacial Dewetting and Drainage Optical Platform was used to create and record the spatiotemporal evolution of model acellular tear films. Image segmentation and analysis was performed in MATLAB to extract the most essential features from the wet area fraction versus time curve, namely the evaporative break-up time and the final wet area fraction (A10). These two parameters indicate the tear film stability in the presence of rh-lubricin in its unstressed and stressed forms.

Results

Our parameters successfully captured the trend of increasing tear film stability with increasing rh-lubricin concentration, and captured differences in rh-lubricin efficacy after various industrially relevant stresses. Specifically, aggregation and fragmentation caused by a 4-week, high temperature stress condition negatively impacted rh-lubricin's ability to maintain model tear film stability. Adsorbed rh-lubricin alone was not sufficient to resist break-up and maintain full area coverage of the model tear film surface.

Conclusions

Our results demonstrate that fragmentation and aggregation can negatively impact rh-lubricin's ability to maintain a stable tear film. In addition, the ability of rh-lubricin to maintain wetted area coverage is due to both freely dispersed and adsorbed rh-lubricin.

Translational Relevance

Our platform and analysis method provide a facile, intuitive, and clinically relevant means to quantify the effect of ophthalmic drugs and formulations intended for improving tear film stability, as well as capture differences between variants related to drug stability and efficacy.

Automation of dry eye disease quantitative assessment: A review

Dry eye disease (DED) is a common eye condition worldwide and a primary reason for visits to the ophthalmologist. DED diagnosis is performed through a combination of tests, some of which are unfortunately invasive, non‐reproducible and lack accuracy. The following review describes methods that diagnose and measure the extent of eye dryness, enabling clinicians to quantify its severity. Our aim with this paper is to review classical methods as well as those that incorporate automation. For only four ways of quantifying DED, we take a deeper look into what main elements can benefit from automation and the different ways studies have incorporated it. Like numerous medical fields, Artificial Intelligence (AI) appears to be the path towards quality DED diagnosis. This review categorises diagnostic methods into the following: classical, semi‐automated and promising AI‐based automated methods.

Mucosal vaccine delivery: A focus on the breakthrough of specific barriers

Mucosal vaccines can effectively induce an immune response at the mucosal site and form the first line of defense against microbial invasion. The induced mucosal immunity includes the proliferation of effector T cells and the production of IgG and IgA antibodies, thereby effectively blocking microbial infection and transmission. However, after a long period of development, the transformation of mucosal vaccines into clinical use is still relatively slow. To date, fewer than ten mucosal vaccines have been approved. Only seven mucosal vaccines against coronavirus disease 2019 (COVID-19) are under investigation in clinical trials. A representative vaccine is the adenovirus type-5 vectored COVID-19 vaccine (Ad5-nCoV) developed by Chen and coworkers, which is currently in phase III clinical trials. The reason for the limited progress of mucosal vaccines may be the complicated mucosal barriers. Therefore, this review summarizes the characteristics of mucosal barriers and highlights strategies to overcome these barriers for effective mucosal vaccine delivery.

Signaling Pathways Used by the Specialized Pro-Resolving Mediator Maresin 2 Regulate Goblet Cell Function: Comparison with Maresin 1

Specialized pro-resolving mediators (SPMs), including Maresins (MaR)-1 and 2, contribute to tear film homeostasis and resolve conjunctival inflammation. We investigated MaR2's signaling pathways in goblet cells (GC) from rat conjunctiva. Agonist-induced [Ca2+]i and high-molecular weight glycoconjugate secretion were measured. MaR2 increased [Ca2+]i and stimulated secretion. MaR2 and MaR1 stimulate conjunctival goblet cell function, especially secretion, by activating different but overlapping GPCR and signaling pathways, and furthermore counter-regulate histamine stimulated increase in [Ca2+]i. Thus, MaR2 and MaR1 play a role in maintaining the ocular surface and tear film homeostasis in health and disease. As MaR2 and MaR1 modulate conjunctival goblet cell function, they each may have potential as novel, but differing, options for the treatment of ocular surface inflammatory diseases including allergic conjunctivitis and dry eye disease. We conclude that in conjunctival GC MaR2 and MaR1, both increase the [Ca2+]i and stimulate secretion to maintain homeostasis by using one set of different, but overlapping, signaling pathways to increase [Ca2+]i and another set to stimulate secretion. MaR2 also resolves ocular allergy.

Novel vision restoration techniques: 3D bioprinting, gene and stem cell therapy, optogenetics, and the bionic eye

BACKGROUND

Vision restoration has been one of the most sought-after goals of ophthalmology because of its inception. Despite these problems being tackled from numerous different perspectives, a concrete solution has not yet been achieved. An optimal solution will have significant implications on the patient's quality of life, socioeconomic status, and mental health.

METHODS

This article will explore new and innovative approaches with one common aim-to restore functional vision for the visually impaired. These novel techniques include 3D bioprinting, stem cell therapy, gene therapy, implantable devices, and optogenetics.

RESULTS

While the techniques mentioned above show significant promise, they are currently in various stages of development ranging from clinical trials to commercial availability. Restoration of minimal vision in specific cases has already been achieved by the different methods but optimization of different parameters like biocompatibility, spatiotemporal resolution, and minimizing the costs are essential for widespread use.

CONCLUSION

The developments over the past decade have resulted in multiple milestones in each of the techniques with many solutions getting approved by the FDA. This article will compare these novel techniques and highlight the major advantages and drawbacks of each of them.

The Functional Anatomy of the Cornea and Anterior Chamber in Lampreys: Insights From the Pouched Lamprey, Geotria australis (Geotriidae, Agnatha)

Extant lampreys (Petromyzontiformes) are one of two lineages of surviving jawless fishes or agnathans, and are therefore of critical importance to our understanding of vertebrate evolution. Anadromous lampreys undergo a protracted lifecycle, which includes metamorphosis from a larval ammocoete stage to an adult that moves between freshwater and saltwater with exposure to a range of lighting conditions. Previous studies have revealed that photoreception differs radically across the three extant families with the Pouched lamprey Geotria australis possessing a complex retina with the potential for pentachromacy. This study investigates the functional morphology of the cornea and anterior chamber of G. australis, which is specialised compared to its northern hemisphere counterparts. Using light microscopy, scanning and transmission electron microscopy and microcomputed tomography, the cornea is found to be split into a primary spectacle (dermal cornea) and a scleral cornea (continuous with the scleral eyecup), separated by a mucoid layer bounded on each side by a basement membrane. A number of other specialisations are described including mucin-secreting epithelial cells and microholes, four types of stromal sutures for the inhibition of stromal swelling, abundant anastomosing and branching of collagen lamellae, and a scleral endothelium bounded by basement membranes. The structure and function of the cornea including an annular and possibly a pectinate ligament and iris are discussed in the context of the evolution of the eye in vertebrates.

Chambered warm moist air eyelid warming devices - a review

BACKGROUND

Eyelid warming is an important treatment for meibomian gland dysfunction (MGD). Specialized chambered devices, using warm moist air have been developed.

PURPOSE

To critically evaluate the literature on the safety and efficacy of chambered warm moist air devices in MGD treatment and pinpoint areas of future research.

METHODS

PubMed and Embase were searched on 06 June 2021. The search term was '(warm OR heat OR steam OR goggle OR spectacle OR moist air) AND (meibomian OR MGD OR blepharitis OR eyelid OR dry eye OR DED)'. All relevant articles with available English full text were included.

RESULTS

Eighteen articles assessing the application of chambered warm moist air eyelid warming devices were identified. In single-application studies, steam-based eyelid warming increased the eyelid temperature and improved symptoms, lipid layer thickness, and tear film breakup time (TBUT). In treatment studies, the steam-based devices improved TBUT and symptom scores. However, in the only randomized controlled trial (RCT) comparing chambered steam-based heat to hot towel treatment, there was no difference between groups for the primary outcome measure; the proportion of subjects noting symptom improvement after 4 weeks.

CONCLUSION

Currently available chambered warm moist air eyelid warming devices are safe and effective at raising eyelid temperature to therapeutic levels and improving signs and symptoms of dry eye. However, it is not clear if they provide a greater benefit than other eyelid warming therapies. Further well-conducted RCTs comparing moist and dry heat devices should be conducted on patients across the range of DED severities and subtype spectrum.

Role of Neutrophils on the Ocular Surface

The ocular surface is a gateway that contacts the outside and receives stimulation from the outside. The corneal innate immune system is composed of many types of cells, including epithelial cells, fibroblasts, natural killer cells, macrophages, neutrophils, dendritic cells, mast cells, basophils, eosinophils, mucin, and lysozyme. Neutrophil infiltration and degranulation occur on the ocular surface. Degranulation, neutrophil extracellular traps formation, called NETosis, and autophagy in neutrophils are involved in the pathogenesis of ocular surface diseases. It is necessary to understand the role of neutrophils on the ocular surface. Furthermore, there is a need for research on therapeutic agents targeting neutrophils and neutrophil extracellular trap formation for ocular surface diseases.

Cationic self-assembled peptide-based molecular hydrogels for extended ocular drug delivery

The physiological barriers and clearance mechanism of the eye challenge the therapeutic delivery for treating various ocular disorders effectively. Here, we show the use of a cationic peptide (i.e., Nap-FFKK) as the molecular hydrogelator for generating supramolecular hydrogels spontaneously in a pH value of 5-7 which allows it to function as a promising ocular drug vehicle. The cationic peptide-based hydrogel hardly exhibited the cytotoxicity against human corneal epithelial cell (i.e., HCEC) from the in vitro cytotoxicity assay. Moreover, the single topical instillation of the hydrogel resulted in high ocular tolerance and biocompatibility. In vivo corneal distribution of the cationic peptide-based hydrogel showed that it dramatically increased the retention and the adhesion on the surface of cornea, compared to the anionic peptide-based analogue, owing to the ionic interactions with mucin on the ocular surface. In addition, we also synthesized environment-sensitive fluorophore-conjugated analogues (i.e., NBD-FFKK and NBD-FFD) to visualize the uptake of hydrogels in HCEC cells, revealing that the cationic peptide-based hydrogel displayed the better in vitro cellular uptake than the anionic peptide-based hydrogel. More importantly, the resulting cationic Nap-FFKK supramolecular hydrogel displayed a superior ocular bioavailability over that of anionic Nap-FFD supramolecular hydrogel, as indicated by in vivo pharmacokinetics study. This work, as a systematic investigation of ionic peptide-based molecular hydrogels in the ocular application, illustrates a new and powerful supramolecular approach for antagonizing clinically difficult ocular drug delivery. STATEMENT OF SIGNIFICANCE: Here we show the use of a cationic peptide as the molecular hydrogelator for generating supramolecular hydrogels, which allows it to function as a promising ocular drug vehicle for antagonizing the therapeutic delivery difficulties associated with the physiological barriers and clearance mechanism of the eye. The in vitro and in vivo studies of the hydrogel both show high ocular tolerance and biocompatibility. Moreover, the in vivo corneal distribution of the hydrogel exhibits the increased retention and adhesion on the surface of cornea. This work, as an investigation of cationic peptide-based molecular hydrogels in the ocular application, illustrates a powerful supramolecular approach for overcoming clinically difficult ocular drug delivery.

Physiological fluid interfaces: Functional microenvironments, drug delivery targets, and first line of defense

Fluid interfaces, i.e. the boundary layer of two liquids or a liquid and a gas, play a vital role in physiological processes as diverse as visual perception, oral health and taste, lipid metabolism, and pulmonary breathing. These fluid interfaces exhibit a complex composition, structure, and rheology tailored to their individual physiological functions. Advances in interfacial thin film techniques have facilitated the analysis of such complex interfaces under physiologically relevant conditions. This allowed new insights on the origin of their physiological functionality, how deviations may cause disease, and has revealed new therapy strategies. Furthermore, the interactions of physiological fluid interfaces with exogenous substances is crucial for understanding certain disorders and exploiting drug delivery routes to or across fluid interfaces. Here, we provide an overview on fluid interfaces with physiological relevance, namely tear films, interfacial aspects of saliva, lipid droplet digestion and storage in the cell, and the functioning of lung surfactant. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe therapies and drug delivery approaches targeted at fluid interfaces. STATEMENT OF SIGNIFICANCE: Fluid interfaces are inherent to all living organisms and play a vital role in various physiological processes. Examples are the eye tear film, saliva, lipid digestion & storage in cells, and pulmonary breathing. These fluid interfaces exhibit complex interfacial compositions and structures to meet their specific physiological function. We provide an overview on physiological fluid interfaces with a focus on interfacial phenomena. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe novel therapies and drug delivery approaches targeted at fluid interfaces. This sets the scene for ocular, oral, or pulmonary surface engineering and drug delivery approaches.

Mechanobiology of conjunctival epithelial cells exposed to wall shear stresses

The human conjunctival epithelial cells (HCEC) line the inner sides of the eyelids and the anterior part of the sclera. They include goblet cells that secret mucus into the tear film that protects the ocular surface. The conjunctival epithelium is subjected to mechano-physical stimuli due to eyelid movement during blinking, during wiping and rubbing the eyes, and when exposed to wind and air currents. We cultured primary HCEC under air-liquid interface (ALI) conditions in custom-designed wells that can be disassembled for installation of the in vitro model in a flow chamber. We exposed the HCEC after ALI culture of 8-10 days to steady and oscillatory airflows. The in vitro model of HCEC was exposed to steady wall shear stresses (sWSS) of 0.5 and 1.0 dyne/cm² for lengths of 30 and 60 min and to oscillatory wall shear stresses (oWSS) of 0.5 and 0.77 dyne/cm² amplitudes for a length of 10 min. Cytoskeletal alterations and MUC5AC mucin secretion in response to WSS were investigated using immunohistochemically fluorescent staining and enzyme-linked lectin assay (ELLA), respectively. The results revealed that both exposure times and sWSS values increased the polymerization of F-actin filaments while mucin secretion decreased. However, after a recovery of 24 h in the incubator we observed a decrease of F-actin fibers and mucin secretion only for exposure of 30 min. The length of exposure was more influential on cytoskeletal alterations than the level of sWSS. The very small effect of sWSS on mucin secretion is most likely related to the much smaller amount of goblet cell than in other mucus-secreting tissue. The results for both oWSS amplitudes revealed similar trends regarding F-actin and mucin secretion. Immediately post-exposure we observed an increase in polymerization of F-actin filaments while mucin secretion decreased. However, after 24-h recovery we observed that both F-actin and mucin secretion returned to the same values as for unexposed cultures. The results of this study suggest that WSS should be considered while exploring the physiological characteristics of HCEC.

Characterization of the rabbit conjunctiva: Effects of sulfur mustard

Sulfur mustard (SM; bis (2-chloroethyl) sulfide) is a potent vesicant which causes irritation of the conjunctiva and damage to the cornea. In the present studies, we characterized the ocular effects of SM in New Zealand white rabbits. Within one day of exposure to SM, edema and hazing of the cornea were observed, followed by neovascularization which persisted for at least 28 days. This was associated with upper and lower eyelid edema and conjunctival inflammation. The conjunctiva is composed of a proliferating epithelium largely consisting of stratified columnar epithelial cells overlying a well-defined dermis. Superficial layers of the conjunctival epithelium were found to express keratin 1, a marker of differentiating squamous epithelium, while in cells overlying the basement membrane expressed keratin 17, a marker of stratified squamous epithelium. SM exposure upregulated keratin 17 expression. Mucin 5 ac producing goblet cells were interspersed within the conjunctiva. These cells generated both acidic and neutral mucins. Increased numbers of goblet cells producing neutral mucins were evident after SM exposure; upregulation of expression of membrane-associated mucin 1 and mucin 4 in the superficial layers of the conjunctival epithelium were also noted. These data demonstrate that ocular exposure of rabbits to SM causes significant damage not only to the cornea, but to the eyelid and conjunctiva, suggesting multiple targets within the eye that should be assessed when evaluating the efficacy of potential countermeasures.

Alterations in Mucin-Associated Gene Expression on the Ocular Surface in Active and Stable Stages of Atopic and Vernal Keratoconjunctivitis

Purpose

To evaluate the presence of ocular surface mucin in patients with atopic and vernal keratoconjunctivitis (AKC/VKC), we investigated the mRNA expression levels of SAM-pointed domain-containing ETS-like factor (SPDEF) and mucin-related genes on the ocular surface.

Methods

Nineteen patients with AKC or VKC were divided into two groups based on the severity of the disease as determined by their clinical scores for AKC/VKC: the stable group and the active group. Impression cytology was performed in all patients using filter paper, and the expression levels of SPDEF, MUC1, MUC4, MUC5AC, MUC16, and eotaxin-2 mRNA were determined by real-time reverse-transcription polymerase chain reaction.

Results

The results showed that the expression levels of SPDEF and MUC5AC mRNA in the active group were significantly decreased compared with those in the stable group. Furthermore, clinical scores were significantly negatively correlated with the expression levels of SPDEF mRNA and significantly positively correlated with the expression levels of eotaxin-2, which is a biomarker for eosinophilic inflammation on the ocular surface. Cluster analysis classified the patients with AKC/VKC into three clusters, and the stable group was divided into two clusters according to the condition of ocular surface mucin.

Conclusions

Ocular surface mucin in patients with AKC/VKC is altered in accordance with the clinical severity of the disease.

Evaluation of the Presence and Functional Importance of Nucleoside Transporters in Lacrimal Gland for Tear Disposition of Intravenously Injected Substrate in Rabbits

Purpose: Purpose of the current study was to assess the presence and functionality of the nucleoside transporters in the lacrimal gland for the tear disposition of its substrate given intravenously in rabbits.Materials and Methods: Rabbits were divided into two groups - control and blocker pretreated. The blocker pretreated group received 5 mg/kg of dipyridamole 30 min before ribavirin (substrate), which was given at a dose of 2.5 mg/kg. All the treatments were given intravenously. Blood and tear samples were collected at 5, 15, 30, 60, 90, 120, 180, 240, 300 and 360 min (n = 4; each time point) after substrate administration. Tear samples were collected on Schirmer's strips, and plasma was separated immediately after blood collection. All the samples were stored at -80°C until analysis by LC-MS/MS.Results: Plasma ribavirin concentration for blocker pretreated group showed significantly (p < .05) higher levels at 5, 15, 30, 60, 120, 180 and 300 min as compared to the control group. Similarly, tear ribavirin concentration for blocker pretreated group also showed a significant (p < .05) increase at 5, 15, 60, 90, 180, 240 and 300 min compared to the control group. Plasma and tear AUC_(0-6) for blocker pretreated group was 1.7 (p < .001) and 2.42 (p < .001) folds higher in a significant manner as compared to the control group, respectively. Percentage penetration of ribavirin from plasma to tears was also different between control and blocker pretreated group. Permeation ratio of ribavirin from plasma to tear for blocker pretreated group was found to be 1.4-folds higher in a significant (p < .05) manner.Conclusion: It is evident from the results that nucleoside transporters are present in lacrimal gland. The blocker treatment induced increase in tear transport of ribavirin indicates the possibility of the presence of nucleoside transporters on the apical side of lacrimal acinar cells in the uptake position.