The role of tear proteins in tear film stability in the dry eye patient and in the rabbit

Advances in Functional Restoration of the Lacrimal Glands

The lacrimal glands produce tears to support a healthy homeostatic environment on the ocular surface. The lacrimal gland dysfunction characteristic of dry eye disease causes ocular discomfort and visual disturbances and in severe cases can result in a loss of vision. The demand for adequate restoration of lacrimal gland function has been intensified due to advances in stem cell biology, developmental biology, and bioengineering technologies. In addition to conventional therapies, including artificial tears, tear alternatives (such as autologous serum eye drops) and salivary gland transplantation, a regenerative medicine approach has been identified as a novel strategy to restore the function of the lacrimal gland. Recent studies have demonstrated the potential of progenitor cell injection therapy to repair the tissue of the lacrimal glands. A current three-dimensional (3D) tissue engineering technique has been shown to regenerate a secretory gland structure by reproducing reciprocal epithelial-mesenchymal interactions during ontogenesis in vitro and in vivo. A novel direct reprogramming method has suggested a possibility to induce markers in the lacrimal gland developmental process from human pluripotent stem cells. The development of this method is supported by advances in our understanding of gene expression and regulatory networks involved in the development and differentiation of the lacrimal glands. Engineering science has proposed a medical device to stimulate tearing and a bio-hybrid scaffold to reconstruct the 3D lacrimal gland structure. In this review, we will summarize recent bioengineering advances in lacrimal gland regeneration toward the functional restoration of the lacrimal glands as a future dry eye therapy.

Comparison of non-invasive tear film stability measurement techniques

BACKGROUND

Measurement of tear film stability is commonly used to give an indication of tear film quality but a number of non-invasive techniques exists within the clinical setting. This study sought to compare three non-invasive tear film stability measurement techniques: instrument-mounted wide-field white light clinical interferometry, instrument-mounted keratoscopy and hand-held keratoscopy.

METHODS

Twenty-two subjects were recruited in a prospective, randomised, masked, cross-over study. Tear film break-up or thinning time was measured non-invasively by independent experienced examiners, with each of the three devices, in a randomised order, within an hour.

RESULTS

Significant correlation was observed between instrument-mounted interferometric and keratoscopic measurements (p < 0.001) but not between the hand-held device and the instrument-mounted techniques (all p > 0.05). Tear film stability values obtained from the hand-held device were significantly shorter and demonstrated narrower spread than the other two instruments (all p < 0.05), while no significant differences were observed between the two instrument-mounted devices (all p > 0.05).

CONCLUSION

Good clinical agreement exists between the instrument-mounted interferometric and keratoscopic measurements but not between the hand-held device and either of the instrument-mounted techniques. The results highlight the importance of specifying the instrument employed to record non-invasive tear film stability.

Bioengineered Lacrimal Gland Organ Regeneration in Vivo

The lacrimal gland plays an important role in maintaining a homeostatic environment for healthy ocular surfaces via tear secretion. Dry eye disease, which is caused by lacrimal gland dysfunction, is one of the most prevalent eye disorders and causes ocular discomfort, significant visual disturbances, and a reduced quality of life. Current therapies for dry eye disease, including artificial tear eye drops, are transient and palliative. The lacrimal gland, which consists of acini, ducts, and myoepithelial cells, develops from its organ germ via reciprocal epithelial-mesenchymal interactions during embryogenesis. Lacrimal tissue stem cells have been identified for use in regenerative therapeutic approaches aimed at restoring lacrimal gland functions. Fully functional organ replacement, such as for tooth and hair follicles, has also been developed via a novel three-dimensional stem cell manipulation, designated the Organ Germ Method, as a next-generation regenerative medicine. Recently, we successfully developed fully functional bioengineered lacrimal gland replacements after transplanting a bioengineered organ germ using this method. This study represented a significant advance in potential lacrimal gland organ replacement as a novel regenerative therapy for dry eye disease. In this review, we will summarize recent progress in lacrimal regeneration research and the development of bioengineered lacrimal gland organ replacement therapy.

Expression profiles of aquaporins in rat conjunctiva, cornea, lacrimal gland and Meibomian gland

The aim of the study was to elucidate aquaporin (AQP) family member mRNA expression and protein expression/localization in the rat lacrimal functional unit. The mRNA expression of all rat AQPs (AQP0-9, 11-12) in palpebral, fornical, and bulbar conjunctiva, cornea, lacrimal gland, and Meibomian gland was measured by Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) and real time RT-PCR. Antibodies against AQP1, 3, 4, 5, 9, and 11 were used in Western blotting and immunohistochemistry to determine protein expression and distribution. Our study demonstrated characteristic AQP expression profiles in rat ocular tissues. AQP1, 3, 4, 5, 8, 9, 11, and 12 mRNA were detected in conjunctiva. AQP0, 1, 2, 3, 4, 5, 6, 11, and 12 mRNA were expressed in cornea. AQP0, 1, 2, 3, 4, 5, 7, 8, and 11 mRNA were detected in lacrimal gland. AQP1, 3, 4, 5, 7, 8, 9, 11, and 12 mRNA were identified in Meibomian gland. By Western blot, AQP1, 3, 5, and 11 were detected in conjunctiva; AQP1, 3, 5, and 11 were identified in cornea; AQP1, 3, 4, 5, and 11 were detected in lacrimal gland; and AQP1, 3, 4, 5, 9, and 11 were present in Meibomian gland. Immunohistochemistry localized AQPs to distinct sites in the various tissues. This study rigorously analyzed AQPs expression and localization in rat conjunctiva, cornea, lacrimal gland, and Meibomian gland tissues. Our findings provide a comprehensive platform for further investigation into the physiological or pathophysiological relevance of AQPs in ocular surface.

Single eye analysis and contralateral eye comparison of tear proteins in normal and dry eye model rabbits by MALDI-ToF mass spectrometry using wax-coated target plates

A study of rabbit tear protein expression in a dry eye rabbit model was performed to determine if a pattern in expressed proteins could be identified. The uniqueness of the model allows the comparison of normal (control) eye tear protein expression with surgically induced dry eye tear protein expression in individual animals. The sensitivity of the method allows for single eye analysis. One-dimensional mini-gel electrophoresis of the tear proteins did not show substantial differences between band patterns of the normal versus the dry eye, but was used to assess the molecular weight ranges of the major proteins. Specific assignments of some of the predominant proteins were obtained by tandem mass spectrometry (MS) which showed that the lower molecular weight lipid-binding proteins (approximately 10 kDa to 36 kDa) constitute a considerable amount of the observed protein, followed in lesser quantities by the transferrins which have higher molecular weights ranging from 70 kDa to 85 kDa. Enhancement of matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) MS linear mode analysis of intact proteins in tear fluid was demonstrated through the use of wax-coated MALDI plates and spot washing. MALDI-ToF MS analysis of the expressed tear proteins illustrates that differences between normal eye tear and dry eye tear protein content are manifested in changes in the lower molecular weight lipid-binding proteins such as lipophilin which exhibits an increase in concentration in the dry eye, and beta-2 microglobulin which undergoes a decrease.

Temporal changes in the tear menisci following a blink

This study investigated tear meniscus formation after a blink. Multiple images of the inferior and superior menisci, viewed en-face, were sequentially captured using a video-slitlamp that permitted the measurement of tear meniscus height (TMH). In a similar manner, tangentially viewed images were obtained of the inferior meniscus so changes in cross-sectional radius of curvature (TMR) with time could be determined. Additionally, the relationship between inferior TMH and tear volume was quantified by the successive instillation of small aliquots of fluid. In this latter experiment the inflow and outflow of tears were controlled by anaesthetizing the ocular surface and inner nose, and use of punctal plugs. The TMH of both menisci rose after a blink (p<0.01) by a similar amount (p=0.59). This suggests that capillarity is dominant over gravity in tear drainage, however, it may simply reflect that these influences are small compared to the large flow resistances that occur in thin films, hydraulic connectivity between the two menisci, or differences in their anterior-posterior width leading to unequal volume-height relationships. TMR increased with time after a blink (p<0.0001); immediately on eye opening the curvature of the inferior meniscus was approximately circular but this soon became eccentric, with the radius of its upper half exceeding that of its lower half. The growth rate of the two menisci slowed with time and this, based on theoretical considerations, is predicted to have occurred primarily because localized thinning of the tear film adjacent to the menisci markedly raised flow resistance. Inferior TMH increased linearly with the cumulative volume of instilled fluid (r2=0.98), supporting its use in the diagnosis and classification of tear anomalies, although the gradient of this relationship varied between subjects.

Laboratory findings in tear fluid analysis

The tear film, composed of the lipid, aqueous and mucin layers, has many functions including defending the ocular surface. The tear film covering the ocular surface presents a mechanical and antimicrobial barrier and ensures an optical refractive surface. The lipid component originates from the meibomian glands of the tarsus and forms the superficial layer of the tear film. The aqueous component contains electrolytes, water, and a large variety of proteins, peptides and glycoproteins, and is primarily secreted by the lacrimal gland. Mucins are glycoproteins expressed by epithelial tissues of mucous surfaces. They protect tissues by functioning as antioxidants, providing lubrication, and inhibiting bacterial adherence. Quantitatively and qualitatively, its composition must be maintained within the fairly narrow limits to maintain a healthy and functional visual system. Abnormalities of the tear film, affecting the constituents or the volume, can rapidly result in serious dysfunction of the eyelids and conjunctiva and ultimately affect the transparency of the cornea. Many ocular surface tests have been developed for the clinical diagnosis of dry eye syndromes. This paper provides an overview on laboratory methods for the analysis of the tear film. Understanding the components of the tear film will aid in the treatment of dry eye syndromes and the ocular surface diseases.

Surface pressure measurements of human tears and individual tear film components indicate that proteins are major contributors to the surface pressure

PURPOSE

Tear film stability has been associated with a low surface tension (high surface pressure), which has been attributed to a variety of tear film components. In this study, we examined the contribution of various tear proteins, mucin, and meibomian lipids to the surface pressure of human tears.

METHODS

A Langmuir trough was used to measure and compare the surface activities of albumin, lipocalin, beta-lactoglobulin, lactoferrin, lysozyme, secretory IgA, mucin, meibomian lipid, and tears.

RESULTS

All proteins exhibited surface activity. The surface pressure-area (Pi-A) profiles of most protein films at equilibrium surface pressure (Pieq) were sigmoidal and showed hysteresis between the expansion and compression phases of the cycle. Pieq of most proteins took 4-9 hours to occur. By contrast, the Pi-A profiles for meibomian lipid films were hyperbolic rather than sigmoidal and had little hysteresis, and Pieq was attained within 1 hour. The Pi-A profiles of mucin films showed mostly hyperbolic characteristics with small hysteresis. The Pi-A profiles of films of tears were sigmoidal, showed strong hysteresis, and reached Pieq at about 5 hours. Partitioning of the proteins and whole tears into the subphase also occurred.

CONCLUSION

Comparison between the dynamic Pi-A profiles of tears and those of individual tear film components shows that tear film proteins not only are capable of surface activity but also are major contributors to the surface activity of the tear film.

Ocular Rosacea

Rosacea is a common skin disease that frequently involves the eye. Although the pathogenesis of the disease remains undefined, recent findings suggest that an altered inflammatory response plays an important role in both cutaneous and ocular rosacea. Ocular manifestations include lid and ocular surface alterations. Chronic inflammation can lead to corneal vascularization, which may compromise vision. Treatment of ocular rosacea is aimed at preventing irritation of the ocular surface (e.g., lubricants, lid hygiene) and controlling inflammation with topical and systemic anti-inflammatory drugs. Systemic tetracyclines are the mainstay of treatment. These drugs act multifactorially by decreasing bacterial flora and the expression of matrix metalloproteinases, altering meibum secretion, inhibiting the production of bacterial lipases, and providing an immunomodulatory effect.

Changes in the tear film and ocular surface from dry eye syndrome

Dry eye syndrome (DES) refers to a spectrum of ocular surface diseases with diverse and frequently multiple aetiologies. The common feature of the various manifestations of DES is an abnormal tear film. Tear film abnormalities associated with DES are tear deficiency, owing to insufficient supply or excessive loss, and anomalous tear composition. These categorizations are artificial, as in reality both often coexist. DES disrupts the homeostasis of the tear film with its adjacent structures, and adversely affects its ability to perform essential functions such as supporting the ocular surface epithelium and preventing microbial invasion. In addition, whatever the initial trigger, moderate and severe DES is characterized by ocular surface inflammation, which in turn becomes the cause and consequence of cell damage, creating a self-perpetuating cycle of deterioration. Progress has been made in our understanding of the aetiology and pathogenesis of DES, and these advances have encouraged a proliferation of therapeutic options. This article aims to amalgamate prevailing ideas of DES development, and to assist in that, relevant aspects of the structure, function, and production of the tear film are reviewed. Additionally, a synopsis of therapeutic strategies for DES is presented, detailing treatments currently available, and those in development.

Is the cystatin-like domain of TSL functionally active in external ocular infections and during the normal diurnal cycle?

PURPOSE

To test whether the cystatin-like functional domain in tear specific lipocalin (TSL) is functionally active in tears during the normal diurnal cycle and during external ocular infections.

METHODS

Capillary tube collected reflex (RTF), open (OTF) and closed (CTF) eye tear samples were recovered from six normals and semi-quantitatively western blot assayed for cystatin C and TSL. CTF samples were immunoprecipitated with antibodies raised against TSL, cystatin C and other antiproteases and screened for the co-precipitation of proteases by casein and gelatin zymography. OTF samples recovered from individuals with viral, fungal and bacterial keratitis were similarly screened for TSL-bound proteases. Human tissue was subjected to immunohistochemical study.

RESULTS

Western blot analysis reveals a progressive increase in cystatin C in going from RTF to OTF to CTF samples (approximately 3, 7 and 30 ng microl(-1), respectively). In contrast, the concentration of TSL remains constant (approximately 1500 ng microl(-1)). Immunocytochemistry data show staining of the apical surface of the human conjunctiva and some intra-cellular staining for cystatin C, but not for cystatin A. Zymography confirms earlier data that CTF contains exceptionally high levels of proteases bound to a wide range of specific inhibitors. However, only trace amounts of proteases are complexed with cystatin C and no protease can be detected bound to TSL in either the pathological or CTF samples.

CONCLUSION

Although TSL contains a functional cystatin-like domain, it is not physiologically active during the normal diurnal cycle or during external ocular infections. Reactive proteases in CTF are most likely controlled by the presence of excess levels of more reactive cystatins, especially cystatin C, which accumulates during prolonged eye closure. Immunohistochemical data suggest that the apical conjunctiva may be a contributing source for the accumulating cystatin C.

Tears in health and disease

PURPOSE

A brief review of normal tear function and changes resulting from disease.

METHODS

The factors contributing to normal tear physiological function are considered, and the effect of changes in composition, as reported in the literature, is surveyed, with emphasis on the physical performance of the tears. Major classes of tear tests which would reveal functional changes are listed.

RESULTS

Where possible, changes of measurable functions in disease are described. Gaps in our current knowledge are indicated.

CONCLUSIONS

Many techniques exist for examination and assessment of both normal and disease tears, but further development is needed to adapt some of these to clinical situations, and to make them more specific diagnostic tests.

Oral antioxidant therapy for marginal dry eye

OBJECTIVE

To assess the efficacy of an orally administered antioxidant dietary supplement for managing marginal dry eye.

DESIGN

A prospective, randomised, placebo controlled trial with cross-over.

SETTING

Eye Clinic, Department of Vision Sciences, Glasgow Caledonian University.

SUBJECTS

Forty marginal dry eye sufferers composed of 30 females and 10 males (median age 53 y; range 38-69 y).

INTERVENTIONS

Baseline assessments were made of tear volume sufficiency (thread test), tear quality (stability), ocular surface status (conjunctival impression cytology) and dry eye symptoms (questionnaire). Each subject was administered courses of active treatment, placebo and no treatment, in random order for 1 month each and results compared to baseline.

RESULTS

Tear stability and ocular surface status were significantly improved following active treatment (P<0.05). No changes from baseline were detected following administration of placebo and no treatment (P>0.05). Absolute increase in tear stability correlated with absolute change in goblet cell population density. Tear volume was not improved following any treatment period and dry eye symptom responses were subject to placebo effect.

CONCLUSIONS

Oral antioxidants improved both tear stability and conjunctival health, although it is not yet understood whether increased ocular surface health mediates increased tear stability or vice versa.

SPONSORS

This study was supported by a PhD scholarship funded by the Department of Vision Sciences, Glasgow Caledonian University, Scotland. Antioxidant supplements and placebos were kindly donated by Vitabiotics.

Towards a closed eye model of the pre-ocular tear layer

Although the tear film has been extensively studied as it exists in the open eye state, until recently very little was known as to what happens to the tear film on eye closure. Recent studies have shown that eye closure results in a profound change in the composition, origins, turnover and physiological functions of the tear film. These changes include a shift from an inducible, neurologically controlled, lacrimal secretion containing among other proteins primarily lysozyme, lactoferrin and tear specific lipocalin, to a much slower, constitutive-type of secretion, composed almost exclusively of sIgA. This change is accompanied by the build-up of sialoglycoproteins of epithelial and goblet cell origin, the build-up and activation of complement and the build-up of serum proteins. In addition, various cytokines and proinflammatory mediators accumulate, including some which are potent inducers of angiogenesis and leukochemotaxis. The closed eye also exhibits the recruitment and activation of massive numbers of PMN cells. This results in a stagnant, closed eye layer, which is extremely rich in reactive complement products, PMN cell proteases including protease-3, elastase, capthepsin G, MMP-9 and urokinase. We have postulated that this shift represents a fundamental change in host-defense strategies from a passive-barrier defense to an active immune, inflammatory, phagocyte-mediated process and that this shift is necessitated in order to protect the cornea from entrapped microorganisms. Studies have shown that autologous cell damage is avoided in closed eye tear fluid, by the accumulation of several modulators of complement activation, which shift activation towards opsonization of entrapped microorganisms and the build-up of a wide array of antiproteases. Some of the latter are likely to arise from the ocular surface tissues. Corneal neovascularization may be avoided in part by the build-up of alpha2-macroglobulin and the conversion of plasminogen to angiostatin. It is highly probable that other bioactive protein fragments are produced in the closed eye, which contribute to homeostasis. Areas of future study are indicated.

Modulation of tear film protein secretion with phosphodiesterase inhibitors

A double-blind randomized clinical study was conducted to determine whether nicardipine hydrochloride was a useful treatment for dry eye.We examined its effect on the tear film, ocular surface and ocular comfort. Nicardipine hydrochloride, 3-isobutyl-1-methylxanthine and pilocarpine hydrochloride were dissolved in an artificial tear vehicle and applied topically to one eye of 12 subjects on separate days. Ocular physiology, ocular comfort and tear volume were assessed. The trial was repeated with nicardipine in an aqueous gel vehicle. Tears were collected and assessed for protein concentration and protein profile, using electrophoresis and mass spectrometry. Nicardipine induced conjunctival redness and symptoms of dryness and irritation. There was no change in total tear protein concentration or volume. An increase in a 68 kDa protein was observed, this was probably due to conjunctival vessel dilation and leakage of albumin. The adverse symptomatology and increased conjunctival redness experienced with nicardipine make it an undesirable treatment for dry eye.