Dry eye and designer ophthalmics

Dry eye disease and proteomics

Dry eye disease (DED) is a highly prevalent disease worldwide mostly associated with age, though other factors such as screen use and contact lens wear explain why it is increasingly diagnosed in younger people. DED also disproportionately affects women. Symptoms include eye dryness, burning, pain and sensitivity to light that can significantly affect quality of life. This condition may progress to cause lasting damage to the ocular surface if left untreated. Currently, diagnosis is through assessment of signs and symptoms determined by clinical tests and questionnaires. However, there is considerable overlap between normal and DED result distributions of currently available metrics as signs and symptoms fluctuate over time and with disease severity. Importantly, the non-targeted approach of proteomics means that significant changes in novel proteins may be discovered. Proteomics is a powerful tool that has been applied to the field of DED to understand changes at a biochemical level, uncover new disease biomarkers and determine the success of clinical interventions. While individual proteins may not be sensitive enough when used as single biomarkers, proteomics opens the possibility to uncover several relevant proteins that may be combined in a panel to provide more accurate diagnostic value i.e. parallel testing. In this review we discuss the use of proteomics in DED research and the potential for application of proteomic results in the clinic. We also identify shortcomings and future avenues for research.

Biosynthesized Multivalent Lacritin Peptides Stimulate Exosome Production in Human Corneal Epithelium

Lacripep is a therapeutic peptide derived from the human tear protein, Lacritin. Lacripep interacts with syndecan-1 and induces mitogenesis upon the removal of heparan sulfates (HS) that are attached at the extracellular domain of syndecan-1. The presence of HS is a prerequisite for the syndecan-1 clustering that stimulates exosome biogenesis and release. Therefore, syndecan-1-mediated mitogenesis versus HS-mediated exosome biogenesis are assumed to be mutually exclusive. This study introduces a biosynthesized fusion between Lacripep and an elastin-like polypeptide named LP-A96, and evaluates its activity on cell motility enhancement versus exosome biogenesis. LP-A96 activates both downstream pathways in a dose-dependent manner. HCE-T cells at high confluence treated with 1 μM LP-A96 enhanced cell motility equipotent to Lacripep. However, cells at low density treated with 1 μM LP-A96 generated a 210-fold higher number of exosomes compared to those treated at low density with Lacripep. As monovalent Lacripep is capable of enhancing cell motility but not exosome biogenesis, activation of exosome biogenesis by LP-A96 not only suggests its utility as a novel molecular tool to study the Lacritin biology in the corneal epithelium but also implies activity as a potential therapeutic peptide that can further improve ocular surface health through the induction of exosomes.

The Anti-Inflammatory Effects of Angiogenin in an Endotoxin Induced Uveitis in Rats

Angiogenin (ANG) is involved in the innate immune system and inflammatory disease. The aim of this study is to evaluate the anti-inflammatory effects of ANG in an endotoxin induced uveitis (EIU) rat model and the pathways involved. EIU rats were treated with balanced salt solution (BSS), a non-functional mutant ANG (mANG), or wild-type ANG (ANG). The integrity of the blood-aqueous barrier was evaluated by the infiltrating cell and protein concentrations in aqueous humor. Histopathology, Western blot, and real-time qRT-PCR of aqueous humor and ocular tissue were performed to analyze inflammatory cytokines and transcription factors. EIU treated with ANG had decreased inflammatory cells and protein concentrations in the anterior chamber. Compared to BSS and mANG, ANG treatment showed reduced expression of IL-1β, IL-8, TNF-α, and Myd88, while the expression of IL-4 and IL-10 was increased. Western blot of ANG treatment showed decreased expression of IL-6, inducible nitric oxide synthase (iNOS), IL-1β, TNF-α, and phosphorylated NF-κB and increased expression of IL-10. In conclusion, ANG seems to reduce effectively immune mediated inflammation in the EIU rat model by reducing the expression of proinflammatory cytokines, while increasing the expression of anti-inflammatory cytokines through pathways related to NF-κB. Therefore, ANG shows potential for effectively suppressing immune-inflammatory responses in vivo.

Integrated Tear Proteome and Metabolome Reveal Panels of Inflammatory-Related Molecules via Key Regulatory Pathways in Dry Eye Syndrome

Dry eye syndrome (DES) is a growing public health concern with a high global prevalence; however, the fundamental processes involved in its pathogenic mechanisms remain poorly understood. In the present study, we applied nanoscale liquid chromatography and quadrupole time-of-flight tandem mass spectrometry (nanoLC/Q-TOF-MS/MS) and ultraperformance LC/Q-TOF-MS/MS technologies on tear samples obtained from 18 dry eye patients and 19 healthy controls for integrated proteomic and metabolomic analyses. Overall, 1031 tear proteins were detected, while 190 proteins were determined to be significantly expressed in dry eye patients. Further functional analysis suggested that various biological processes were highly expressed and involved in the pathogenesis of DES, especially immune and inflammatory processes. In total, 156 named metabolites were identified, among which 34 were found to be significantly changed in dry eye patients. The results highlighted the key elements, especially inflammatory-related proteins and metabolites that played important roles in the development of DES. Further, the regulatory roles of primary pathways, including complement and coagulation cascades, glycolysis/gluconeogenesis, and amino acid metabolism, were also identified as processes involved in DES. Collectively, our work not only provided insight into the potential biomarkers of DES for diagnostic and prognostic purposes but extended our knowledge of the physiopathology of this syndrome.

Analysis of cross-population differentiation between Thoroughbred and Jeju horses

Objective

This study was intended to identify genes positively selected in Thoroughbred horses (THBs) that potentially contribute to their running performances.

Methods

The genomes of THB and Jeju horses (JH, Korean native horse) were compared to identify genes positively selected in THB. We performed cross-population extended haplotype homozygosity (XP-EHH) and cross-population composite likelihood ratio test (XP-CLR) statistical methods for our analysis using whole genome resequencing data of 14 THB and 6 JH.

Results

We identified 98 (XP-EHH) and 200 (XP-CLR) genes that are under positive selection in THB. Gene enrichment analysis identified 72 gene ontology biological process (GO BP) terms. The genes and GO BP terms explained some of THB’s characteristics such as immunity, energy metabolism and eye size and function related to running performances. GO BP terms that play key roles in several cell signaling mechanisms, which affected ocular size and visual functions were identified. GO BP term Eye photoreceptor cell differentiation is among the terms annotated presumed to affect eye size.

Conclusion

Our analysis revealed some positively selected candidate genes in THB related to their racing performances. The genes detected are related to the immunity, ocular size and function, and energy metabolism.

A thermo-responsive protein treatment for dry eyes

Millions of Americans suffer from dry eye disease, and there are few effective therapies capable of treating these patients. A decade ago, an abundant protein component of human tears was discovered and named lacritin (Lacrt). Lacrt has prosecretory activity in the lacrimal gland and mitogenic activity at the corneal epithelium. Similar to other proteins placed on the ocular surface, the durability of its effect is limited by rapid tear turnover. Motivated by the rationale that a thermo-responsive coacervate containing Lacrt would have better retention upon administration, we have constructed and tested the activity of a thermo-responsive Lacrt fused to an elastin-like polypeptide (ELP). Inspired from the human tropoelastin protein, ELP protein polymers reversibly phase separate into viscous coacervates above a tunable transition temperature. This fusion construct exhibited the prosecretory function of native Lacrt as illustrated by its ability to stimulate β-hexosaminidase secretion from primary rabbit lacrimal gland acinar cells. It also increased tear secretion from non-obese diabetic (NOD) mice, a model of autoimmune dacryoadenitis, when administered via intra-lacrimal injection. Lacrt ELP fusion proteins undergo temperature-mediated assembly to form a depot inside the lacrimal gland. We propose that these Lacrt ELP fusion proteins represent a potential therapy for dry eye disease and the strategy of ELP-mediated phase separation may have applicability to other diseases of the ocular surface.

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.

Lacritin rescues stressed epithelia via rapid forkhead box O3 (FOXO3)-associated autophagy that restores metabolism

Homeostasis is essential for cell survival. However, homeostatic regulation of surface epithelia is poorly understood. The eye surface, lacking the cornified barrier of skin, provides an excellent model. Tears cover the surface of the eye and are deficient in dry eye, the most common eye disease affecting at least 5% of the world's population. Only a tiny fraction of the tear proteome appears to be affected, including lacritin, an epithelium-selective mitogen that promotes basal tearing when topically applied to rabbit eyes. Here we show that homeostasis of cultured corneal epithelia is entirely lacritin-dependent and elucidate the mechanism as a rapid autophagic flux to promptly restore cellular metabolism and mitochondrial fusion in keeping with the short residence time of lacritin on the eye. Accelerated flux appears to be derived from lacritin-stimulated acetylation of FOXO3 as a novel ligand for ATG101 and coupling of stress-acetylated FOXO1 with ATG7 (which remains uncoupled without lacritin) and be sufficient to selectively divert huntingtin mutant Htt103Q aggregates largely without affecting non-aggregated Htt25Q. This is in keeping with stress as a prerequisite for lacritin-stimulated autophagy. Lacritin targets the cell surface proteoglycan syndecan-1 via its C-terminal amino acids Leu(108)-Leu(109)-Phe(112) and is also available in saliva, plasma, and lung lavage. Thus, lacritin may promote epithelial homeostasis widely.

Lacritin and the tear proteome as natural replacement therapy for dry eye

Tear proteins are potential biomarkers, drug targets, and even biotherapeutics. As a biotherapeutic, a recombinant tear protein might physiologically rescue the ocular surface when a deficiency is detected. Such a strategy pays more attention to the natural prosecretory and protective properties of the tear film and seeks to alleviate symptoms by addressing cause, rather than the current palliative, non-specific and temporary approaches. Only a handful of tear proteins appear to be selectively downregulated in dry eye, the most common eye disease. Lacritin and lipocalin-1 are two tear proteins selectively deficient in dry eye. Both proteins influence ocular surface health. Lacritin is a prosecretory mitogen that promotes basal tearing when applied topically. Levels of active monomeric lacritin are negatively regulated by tear tissue transglutaminase, whose expression is elevated in dry eye with ocular surface inflammation. Lipocalin-1 is the master lipid sponge of the ocular surface, without which residual lipids could interfere with epithelial wetting. It also is a carrier for vitamins and steroid hormones, and is a key endonuclease. Accumulation of DNA in tears is thought to be proinflammatory. Functions of these and other tear proteins may be influenced by protein-protein interactions. Here we discuss new advances in lacritin biology and provide an overview on lipocalin-1, and newly identified members of the tear proteome.

Tissue transglutaminase is a negative regulator of monomeric lacritin bioactivity

PURPOSE

Molar accounting of bioactive fluids can expose new regulatory mechanisms in the growing proteomic focus on epithelial biology. Essential for the viability of the surface epithelium of the eye and for normal vision is the thin, but protein-rich, tear film in which the small tear glycoprotein lacritin appears to play a prominent prosecretory, cytoprotective, and mitogenic role. Although optimal bioactive levels in cell culture are 1 to 10 nM over a biphasic dose optimum, ELISA suggests a sustained tear lacritin concentration in the midmicromolar range in healthy adults. Here we identify a reconciling mechanism.

METHODS

Monoclonal anti-lacritin 1F5 antibody was generated, and applied together with a new anti-C-terminal polyclonal antibody to tear and tissue Western blotting. In vitro tissue transglutaminase (Tgm2) cross-linking was monitored and characterized by mass spectrometry.

RESULTS

Blotting for lacritin in human tears or saliva surprisingly detected immunoreactive material with a higher molecular weight and prominence equal or exceeding the ∼23 to 25 kDa band of monomeric glycosylated lacritin. Exogenous Tgm2 initiated lacritin cross-linking within 1 minute and was complete by 90 minutes-even with as little as 0.1 nM lacritin, and involved the donors lysine 82 and 85 and the acceptor glutamine 106 in the syndecan-1 binding domain. Lacritin spiked into lacritin-depleted tears formed multimers, in keeping with ∼0.6 μM TGM2 in tears. Cross-linking was absent when Tgm2 was inactive, and cross-linked lacritin, unlike recombinant monomer, bound syndecan-1 poorly.

CONCLUSIONS

Since syndecan-1 binding is necessary for lacritin mitogenic and cytoprotective activities, TGM2 cross-linking negatively regulates lacritin bioactivity.

In-depth analysis of the human tear proteome

The tears, a critical body fluid of the surface of the eye, contain an unknown number of molecules including proteins/peptides, lipids, small molecule metabolites, and electrolytes. There have been continued efforts for exploring the human tear proteome to develop biomarkers of disease. In this study, we used the high speed TripleTOF 5600 system as the platform to analyze the human tear proteome from healthy subjects (3 females and 1 male, average age: 36±14). We have identified 1543 proteins in the tears with less than 1% false discovery rate, which represents the largest number of human tear proteins reported to date. The data set was analyzed for gene ontology (GO) and compared with the human plasma proteome, NEIBank lacrimal gland gene dataset and NEIBank cornea gene dataset. This comprehensive tear protein list may serve as a reference list of human tear proteome for biomarker research of ocular diseases or establishment of MRM (Multiple Reaction Monitoring) assays for targeted analysis. Tear fluid is a useful and an accessible source not only for evaluating ocular surface tissues (cornea and conjunctiva), inflammation, lacrimal gland function and a number of disease conditions, such as dry eye as well as response to treatment.

Comparison of three lubricant eye drop solutions in dry eye patients

PURPOSE

Lubricant eye drops that restore physiological osmolarity represent a promising strategy for dry eye syndrome as hyperosmolarity plays a central role in this disease. This preliminary study compared three lubricant eye drop solutions with different osmolarities and compositions in subjects with this condition.

METHODS

Subjects with dry eye syndrome undergoing treatment with benzalkonium chloride-containing lubricant eye drops were randomized to Carnidrop (n = 9), Optive (n = 9), or Blu Sal (n = 9). Fluorescein break-up time (FBUT) and Ocular Protection Index (OPI) were measured at baseline, 15 min, and 60 min after instillation to evaluate the stability and quality of the tear film.

RESULTS

At 15 min, a significant increase in FBUT vs. baseline was reported with Carnidrop (from 2.0 ± 0.8 to 4.8 ± 2.0; p = 0.004) but not in patients who received Optive or Blu Sal. At 60 min, FBUT was significantly increased vs. baseline with Carnidrop (from 2.0 ± 0.8 to 6.0 ± 2.8, p = 0.001) and Optive (from 2.9 ± 2.8 to 4.3 ± 2.9, p = 0.004) but not with Blu Sal. At 15 min, OPI was significantly increased from baseline in only the Carnidrop group (from 0.4 ± 0.2 to 1.0 ± 0.4, p = 0.003). This increase was significantly greater with Carnidrop than with Blu Sal (p = 0.003). At the 60 min evaluation, OPI remained significantly increased from baseline in only the Carnidrop group (p = 0.003).

CONCLUSIONS

Carnidrop produces a larger increase in FBUT and OPI than Optive and Blu Sal in subjects with dry eye syndrome over a 1 h period, possibly because of its hypo-osmolarity and high osmolyte (in particular L-carnitine) content. The instillation of compounds that improve the quality and stability of the tear film, which are impaired in dry eye syndrome, could be effective in the treatment of this condition.

Lacritin, a novel human tear glycoprotein, promotes sustained basal tearing and is well tolerated

PURPOSE

Lacritin is a novel human tear glycoprotein that promotes basal tear peroxidase secretion by rat lacrimal acinar cells in vitro. This study investigates whether lacritin is prosecretory when added topically to the ocular surface of normal living rabbits, and if so, what is its efficacy and tolerability versus cyclosporine and artificial tears.

METHODS

Purified recombinant human lacritin (1, 10, 50, or 100 μg/mL), inactive lacritin truncation mutant C-25 (10 μg/mL), cyclosporine (0.05%), or artificial tears were topically administered to eyes of normal New Zealand White rabbits either as a single dose or three times daily for 14 days with monitoring of basal tear production. Basal tearing under proparacaine anesthesia was repeatedly assessed throughout and 1 week after chronic treatment ceased. Eyes were examined weekly by slit-lamp biomicroscopy.

RESULTS

Lacritin acutely increased basal tearing to 30% over vehicle at 240 minutes. Three times daily treatment with 10-100 μg/mL lacritin was well tolerated. Basal tearing became progressively elevated 4, 7, and 14 days later and was 50% over baseline (50 μg/mL lacritin) 1 week after treatment had ceased. Cyclosporine elevated tearing to a similar level on days 4 and 7 but had little or no effect on day 14 and had returned to baseline 1 week after ending treatment. C-25 and artificial tears had no effect.

CONCLUSIONS

Lacritin acutely stimulates basal tear flow that is sustained for at least 240 minutes. Two weeks of lacritin treatment three times daily was well tolerated and progressively elevated the basal tear flow. One week after treatment ended, basal tearing was still 50% over baseline. In contrast, cyclosporine triggered mild to moderate corneal irritation and a temporary elevation in tearing.

Lacritin and other new proteins of the lacrimal functional unit

The lacrimal functional unit (LFU) is defined by the 2007 International Dry Eye WorkShop as 'an integrated system comprising the lacrimal glands, ocular surface (cornea, conjunctiva and meibomian glands) and lids, and the sensory and motor nerves that connect them'. The LFU maintains a healthy ocular surface primarily through a properly functioning tear film that provides protection, lubrication, and an environment for corneal epithelial cell renewal. LFU cells express thousands of proteins. Over 200 new LFU proteins have been discovered in the last decade. Lacritin is a new LFU-specific growth factor in human tears that flows through ducts to target corneal epithelial cells on the ocular surface. When applied topically in rabbits, lacritin appears to increase the volume of basal tear secretion. Lacritin is one of only a handful of tear proteins preliminarily reported to be downregulated in blepharitis and in two dry eye syndromes. Computational analysis predicts an ordered C-terminal domain that binds the corneal epithelial cell surface proteoglycan syndecan-1 (SDC1) and is required for lacritin's low nanomolar mitogenic activity. The lacritin-binding site on the N-terminus of SDC1 is exposed by heparanase. Heparanase is constitutively expressed by the corneal epithelium and appears to be a normal constituent of tears. Binding triggers rapid signaling to downstream NFAT and mTOR. A wealth of other new proteins, originally designated as hypothetical when first identified by genomic sequencing, are expressed by the human LFU including: ALS2CL, ARHGEF19, KIAA1109, PLXNA1, POLG, WIPI1 and ZMIZ2. Their demonstrated or implied roles in human genetic disease or basic cellular functions are fuel for new investigation. Addressing topical areas in ocular surface physiology with new LFU proteins may reveal interesting new biological mechanisms and help get to the heart of ocular surface dysfunction.