Tear film stability of protein extracts from dry eye patients administered a sigma agonist

Changes in Human Tear Proteome Following Topical Treatment of Dry Eye Disease: Cyclosporine A Versus Diquafosol Tetrasodium

Purpose

To compare the changes in human tear proteome and clinical effects following topical cyclosporine A (CsA) 0.05% or diquafosol tetrasodium (DQS) 3% treatment of dry eye disease (DED), and to identify biomarkers for determining disease severity and treatment effectiveness in DED.

Methods

A total of 18 patients were diagnosed with non-Sjögren DED. Nine patients in each group were treated with topical CsA 0.05% or DQS 3% for 4 weeks. Tear samples were collected after evaluation of tear breakup time, corneal and conjunctival erosion staining, and results of Schirmer's test 1 before and after treatment. Proteomes were characterized using liquid chromatography mass spectrometry, and proteins exhibiting a fold change >1.5 or <0.67 (P < 0.05) were considered differentially expressed (DEP).

Results

A total of 794 proteins were identified, with no significant difference observed between pretreatment and posttreatment conditions. Proteomic analysis identified 54 and 106 DEPs between treatment groups (CsA and DQS, respectively), with gene ontology analysis indicating that both treatments enhanced innate and adaptive immune responses and cellular detoxification. Protein-network analysis showed that inflammation associated with the immune response was primarily responsible for the therapeutic process in both groups.

Conclusions

These results provide insight into the broad scope of changes at the ocular surface in DED and indicated that although both drugs improved the clinical parameters, the activated tear-specific biomarkers differed significantly between treatments. Our findings suggest that the DEPs identified here and those correlated with the clinical parameters might represent candidate biomarkers for DED.

Preparation and Evaluation of Topically Applied Azithromycin Based on Sodium Hyaluronate in Treatment of Conjunctivitis

Azithromycin (AZI) eye drops containing sodium hyaluronate (SH) were developed to improve the bioavailability of AZI. Interaction between AZI and SH in the AZI-SH formulation was investigated by differential scanning calorimetry, X-ray diffraction, and ¹H-nuclear magnetic resonance spectroscopy analyses. Moreover, advantages of using SH as an excipient were investigated by comparing physiological properties and pharmacokinetic behaviors of SH-containing AZI eye drops with that of hydroxypropyl methylcellulose (HPMC)-containing formulation. In addition, safety of the developed AZI-SH eye drops was evaluated by in vitro 3-(4,5-dimethyl-2-Thiazyl)-2, 5-diphenyl-2H-tetrazolium bromide assay (MTT assay) and neutral red uptake assay as well as in vivo eye irritation test and acute toxicity test. The results indicated that AZI formed a complex with SH under a slightly acidic condition. The area under the curve (AUC) of AZI in SH-containing formulation was 1.58-fold higher (P < 0.01) than that in HPMC-containing formulation due to the interaction between the amine group of AZI and the carboxyl group of SH, despite of the higher viscosity of HPMC-containing formulation. Safety evaluation showed that AZI-SH eye drops caused no obvious eye irritation and acute toxicity. In conclusion, the developed SH-containing AZI formulation possessing advantages of longer retention time and higher drug availability was a promising drug formulation for topical ocular therapy.

Sigma 1 receptor: A novel therapeutic target in retinal disease

Retinal degenerative diseases are major causes of untreatable blindness worldwide and efficacious treatments for these diseases are sorely needed. A novel target for treatment of retinal disease is the transmembrane protein Sigma 1 Receptor (Sig1R). This enigmatic protein is an evolutionary isolate with no known homology to any other protein. Sig1R was originally thought to be an opioid receptor. That notion has been dispelled and more recent pharmacological and molecular studies suggest that it is a pluripotent modulator with a number of biological functions, many of which are relevant to retinal disease. This review provides an overview of the discovery of Sig1R and early pharmacologic studies that led to the cloning of the Sig1R gene and eventual elucidation of its crystal structure. Studies of Sig1R in the eye were not reported until the late 1990s, but since that time there has been increasing interest in the potential role of Sig1R as a target for retinal disease. Studies have focused on elucidating the mechanism(s) of Sig1R function in retina including calcium regulation, modulation of oxidative stress, ion channel regulation and molecular chaperone activity. Mechanistic studies have been performed in isolated retinal cells, such as Müller glial cells, microglial cells, optic nerve head astrocytes and retinal ganglion cells as well as in the intact retina. Several compelling studies have provided evidence of powerful in vivo neuroprotective effects against ganglion cell loss as well as photoreceptor cell loss. Also described are studies that have examined retinal structure/function in various models of retinal disease in which Sig1R is absent and reveal that these phenotypes are accelerated compared to retinas of animals that express Sig1R. The collective evidence from analysis of studies over the past 20 years is that Sig1R plays a key role in modulating retinal cellular stress and that it holds great promise as a target in retinal neurodegenerative disease.

Practical issues concerning tear protein assays in dry eye

Dry eye is a common clinical condition diagnosed by cumulative evidence of symptoms and signs. Many new treatments in dry eye are either expensive, invasive, have potential for side effects, or are not easily accessible. In severe dry eye, the ideal modality of treatment to begin with is often not clear as specific molecular disturbances are not evident from just examination of clinical manifestations. Assessing the effects of ongoing treatment is not straight forward since there is lack of agreement between clinical signs and symptoms. There is a need to have more objective methods of selecting treatment for dry eye and monitoring the effect of treatment. Recently, there are many new technologies applied to the discovery of tear biomarkers, for e.g., mass spectrometry based proteomics techniques and multiplex assays such as the bead-based sandwich indirect immunofluorescent assays. Tear proteins assays have even been made available as point-of-care devices. This review focuses on the evidence for the involvements of tear proteins in dry eye, possible changes in tear concentrations with therapy and the strength of evidence regarding dry eye pathology. Much remains to be done in terms of developing office-based assays and ascertaining their reliability, but current evidence suggests that tear proteins have a role in the clinical practice of dry eye.

Tear lipocalin captures exogenous lipid from abnormal corneal surfaces

Purpose. The cornea is protected by apical hydrophilic transmembrane mucins and tears. In pathologic states the mucin barrier is disrupted, creating potential for meibomian lipids to adhere more strongly. Undisplaced lipids create an unwettable surface. The hypothesis that pathologic ocular surfaces alter lipid binding and the ability of tear proteins to remove lipids was tested. Methods. Corneas with pathologic surfaces were studied for lipid adhesion and removal by tears. Capture of fluorescence-labeled phospholipids by human tears was assessed by steady state fluorometry. Tear proteins were separated by gel filtration chromatography and analyzed for bound lipids. Results. Contact angle measurements revealed strong lipid adherence to corneas submerged in buffer. Lower contact angles are observed for lipids on completely de-epithelialized corneas compared with intact corneas (P = 0.04). Lipid removal from these surfaces is greater with whole tears than with tears depleted of tear lipocalin (P < 0.0005). Significantly fewer lipids are captured by tears from Bowman's layer than from epithelial-bearing surfaces (P < 0.025). The only tear component to bind the fluorescence-tagged lipid is tear lipocalin. The histology of a rare case of dry eye disease demonstrates the dominant features of contemporaneous bullous keratopathy. Lipid sequestration from this cornea by tear lipocalin was robust. Conclusions. Lipid is captured by tear lipocalin from corneas with bullous keratopathy and dry eye. Lipid removal is slightly abrogated by greater lipid adhesion to Bowman's layer. Reduced secretion of tear lipocalin documented in dry eye disease could hamper lipid removal and exacerbate ocular surface pathology.

Human tear viscosity: An interactive role for proteins and lipids

Human tear viscosity is poorly understood. Tears need to remain on the ocular surface for lubrication without causing damage to the surface epithelia due to drag when blinking. Whole tears are shear-thinning (non-Newtonian), which cannot be explained by the amount of mucin present, nor by individual proteins. Whole tears minus lipids become Newtonian. Though no free lipids had previously been found in collected tears, tear lipocalin (TL), a major tear protein, is known to bind lipids. In this study, we aimed to confirm whether there are any free lipids in collected tears, and to clarify the combined contribution of tear proteins to viscosity, including experiments on recombinant TL, both without (apo-TL) and with (holo-TL) bound lipid. We also investigated possible oligomer formation by holo- and apo-TL as a mechanism for viscosity using SDS-PAGE and analytical ultracentrifugation (AU). For comparison, we have included results for beta-lactoglobulin, a well-characterised lipocalin protein. No free lipids were detected in whole tears. Rheology showed that any protein combination that included lysozyme or lactoferrin was shear-thinning, as was apo-TL, though holo-TL was Newtonian (linear). Results from SDS-PAGE and AU showed apo-TL to be entirely monomeric, but holo-TL showed some dimerization. Both apo- and holo-beta-lactoglobulin exhibited a monomer-dimer equilibrium. We conclude that hetero-protein interactions, possibly electrostatic, involving lipid-binding-induced structural changes to TL, significantly contribute to the viscosity of human tears.

Tear lipocalin: evidence for a scavenging function to remove lipids from the human corneal surface

PURPOSE

Lipid contamination of the cornea may create an unwettable surface and result in desiccation of the corneal epithelium. Tear lipocalin (TL), also known as lipocalin-1, is the principal lipid-binding protein in tears. TL has been shown to scavenge lipids from hydrophobic surfaces. The hypothesis that TL can remove contaminating fatty acids and phospholipids from the human corneal surface was tested.

METHODS

TL was purified from pooled human tear samples by size exclusion and ion exchange chromatographies. Tears depleted of TL were reconstituted from fractions eluted by size exclusion chromatography that did not contain TL. Fresh and formalin-fixed human corneas were obtained from exenteration specimens. Fluorescent analogs of either palmitic acid or phosphatidylcholine were applied to the corneal epithelial surface. Tears, TL, or tears depleted of TL were applied over the corneas, and spectrofluorometry and fluorescent stereomicroscopy were used to monitor the removal of fluorescent lipids. Tears used in the experiments were then fractionated by size exclusion chromatography to determine the component of tears associated with fluorescent lipids.

RESULTS

Significant enhancement of fluorescence for 16AP and NBD C(6)-HPC was evident in solutions incubated with whole tears and purified TL but not with tears depleted of TL for fixed and unfixed corneas. After the experiment, size exclusion fractions of tears showed that the fluorescence component coeluted with TL.

CONCLUSIONS

TL scavenges lipids from the human corneal surface and delivers them into the aqueous phase of tears. TL may have an important role in removing lipids from the corneal surface to maintain the wettability and integrity of the ocular surface.

Decreased tear lipocalin concentration in patients with meibomian gland dysfunction

BACKGROUND/AIM

Recent studies have demonstrated that tear lipocalin (TL) and phospholipids have a crucial role in maintaining tear film stability. The level of TL in patients with meibomian gland dysfunction (MGD) was examined and these data were correlated with the severity of their clinical disorder.

METHODS

12 patients with obstructive MGD, 12 patients with seborrhoeic MGD, and 12 age matched normal control subjects participated in this study. 3 mul of unstimulated tears were collected with a micropipette from the inferior tear meniscus in the right eye of all subjects. Tear samples were fractionated by high performance liquid chromatography, and TL concentrations were assayed with a bicinchoninic acid technique.

RESULTS

The mean concentration of TL in patients with obstructive and seborrhoeic MGD was significantly lower than in normal controls. TL concentration correlated positively with tear film break up time and negatively with fluorescein staining scores.

CONCLUSION

These results suggest that TL deficiency may be a predisposing factor for the manifestation of symptoms in MGD.

The Economic Burden of Dry Eye

PURPOSE

To develop a conceptual framework for analyzing the economic burden of dry eye and a preliminary assessment of key factors that contribute to that burden.

METHODS

The MEDLINE database was searched from 1966 to May 2003 combining the term "dry eye" with various economic terms. In addition, individual interviews with a panel of clinicians were conducted to provide additional insight on resource use.

RESULTS

Direct resource utilization among dry eye sufferers includes healthcare professional visits, nonpharmacological therapies, pharmacological treatments, and surgical procedures, with the latter 2 categories being the major cost drivers. Complementary and alternative medicine (CAM) therapies are a newly recognized component of the dry eye economic burden. There is wide variation in patterns of diagnosis and treatment, but current therapies are not universally effective. Given the prevalence of the condition, indirect costs may be large. Utilization of pharmacological therapies, especially those other than tear replacements, the extent of CAM use, cost of complications of surgical procedures, and indirect costs are unknown. The natural history and probability that patients will transition between therapies, based on underlying disease severity, need to be elucidated.

CONCLUSIONS

Dry eye is a prevalent condition with the potential for a high economic burden; additional studies are needed to further characterize the economic impact.

Functional cavity dimensions of tear lipocalin

PURPOSE

We calibrated the cavity of tear lipocalin with a series of fluorescent labeled lipids of increasing chain length and varying diameter.

METHODS

Cavity length was assessed with competitive fluorescent assays in which DAUDA was displaced from apo-tear lipocalin with ligands of increasing carbon chain lengths from C12-C24. The concentrations of competitors that inhibit 50% of the binding of DAUDA (IC(50)) were compared. Functional diameters of tear lipocalin and beta-lactoglobulin were estimated with fatty acids bearing fluorescent labels of various diameters. The cavity dimensions of other lipocalins were derived from their published crystal structure coordinates.

RESULTS

In tear lipocalin, the binding affinities of fatty acids increased up to a carbon chain length of 18 (22.5 A) but remained constant from C18-C24. The cavity length of other lipocalins in crystal form were similar to tear lipocalin in solution. Tear lipocalin showed decreased binding affinities with progressively increasing ring dimensions of the ligand. In contrast to beta-lactoglobulin and retinol binding protein, tear lipocalin bound DAUDA and cholesterol in the calyx. Neither tear lipocalin nor beta-lactoglobulin bound P646 in their respective cavities. The calculated inter-sheet distances at the mouth of the crystallized lipocalins ranged from 16-22A.

CONCLUSIONS

Tear lipocalin is more promiscuous than beta-lactoglobulin or retinol binding protein because of a greater functional diameter. Differences in ligand specificity of the various lipocalins can not be explained simply by variation in cavity length or the intersheet distances at the calyx mouths as determined by crystal structure. Other factors may influence ligand specificity such as size and/or dynamic motion of loops between the beta strands.

Structural changes in human tear lipocalins associated with lipid binding

Structural and conformational changes in tear lipocalins were detected in association with ligand binding and release. Circular dichroism measurements demonstrated that ligand binding induces beta structure formation, aromatic side chain asymmetry, and a more rigid state in tear lipocalins (TL). The exposure of the tyrosyl component is less in apo-TL than in holo-TL. The sole tryptophan residue, Trp17, is buried in both holo- and apo-TL. The steady state exposure of Trp17 is the same in holo- and apo-TL, but the dynamic exposure is two-fold greater in apo-TL. Maneuvers to unfold the protein with urea or incubation in an acidic environment resulted in increased exposure of aromatic amino acids. Electron paramagnetic resonance studies verified that lipids are liberated from TL in an acidic environment. Acidic pH promotes conformational changes in TL involving aromatic residues, particularly the conserved residue Trp17. These changes are associated with lipid release. The liberation of lipid from the cavity of TL under acidic conditions involves a molten globule state of the protein. We postulate that TL, exposed to the steep surface pH gradient that exists at lipid-aqueous interfaces, would release lipid in association with a molten globule transition. The data suggest a plausible regulatory mechanism for lipid delivery from lipocalins at the tear film surface.

Uptake of N,N-dimethyl-2-phenylethylene HCl into acini cells removed from rabbit lacrimal glands

Acini cells were obtained from the lacrimal gland of the white New Zealand rabbit. Following isolation and purification, the cells were used to study the uptake of N,N'-dimethyl-2-phenylethylamine HCl (AF2975), which was found to be sodium- and proton-independent, but energy-dependent. Uptake was mainly accomplished via a carrier-mediated transport system for which a Km of 8.72+/-0.96 mM, a Vmax of 602.6+/-41.3 nmol/mg of protein/min, and an exponential coefficient of 2.55+/-0.46 were obtained following a least squares nonlinear fit to the Hill equation. With the addition of the metabolic inhibitors, sodium azide or 2,4-dinitrophenol, the initial uptake rates were reduced from the control experiments by 35.7% and 26.2%, respectively.

The influence of tear proteins on the film stability of rabbit tear extracts

This study was undertaken to gain an understanding of the significance of tear proteins in stabilizing the tear film. Either a sigma agonist, N,N-dimethyl-2-phenylethylamine HCl (AF2975), or a sigma antagonist, haloperidol, was administered to rabbit eyes in order to increase or decrease protein secretion, respectively. At 0, 10 and 60 minutes after instillation, tear proteins were extracted from Schirmer strips and measured for total protein. A portion of the extract was used for separating five major protein fractions using size-exclusion HPLC. Total protein extract or individual protein fractions were measured for surface tension by the horizontal capillary method and for in vitro break up time (in vitro BUT), a newly designed procedure. A statistically significant decrease was measured for surface tension and a concomitant increase was measured for in vitro BUT for the total protein samples at 10 and 60 minutes after instillation of AF2975 compared to the vehicle treated eye. The results for haloperidol yielded an increase in surface tension and an decrease in in vitro BUT. When the tear proteins were separated into five major fractions, only the 23 minute protein fraction was found to decrease surface tension and increase in vitro BUT following AF2975 administration. Haloperidol, a sigma antagonist, showed an exact opposite effect for the total protein and the 23 minute protein fraction.