Amino Acid profiles in human tear fluids analyzed by high-performance liquid chromatography and electrospray ionization tandem mass spectrometry

Tear biomarkers

An extensive exploration of lacrimal fluid molecular biomarkers in understanding and diagnosing a spectrum of ocular and systemic diseases is presented. The chapter provides an overview of lacrimal fluid composition, elucidating the roles of proteins, lipids, metabolites, and nucleic acids within the tear film. Pooled versus single-tear analysis is discussed to underline the benefits and challenges associated with both approaches, offering insights into optimal strategies for tear sample analysis. Subsequently, an in-depth analysis of tear collection methods is presented, with a focus on Schirmer's test strips and microcapillary tubes methods. Alternative tear collection techniques are also explored, shedding light on their applicability and advantages. Variability factors, including age, sex, and diurnal fluctuations, are examined in the context of their impact on tear biomarker analysis. The main body of the chapter is dedicated to discussing specific biomarkers associated with ocular discomfort and a wide array of ocular diseases. From dry eye disease and thyroid-associated ophthalmopathy to keratoconus, age-related macular degeneration, diabetic retinopathy, and glaucoma, the intricate relationship between molecular biomarkers and these conditions is thoroughly dissected. Expanding beyond ocular pathologies, the chapter explores the applicability of tear biomarkers in diagnosing systemic diseases such as multiple sclerosis, amyotrophic lateral sclerosis, Alzheimer's disease, Parkinson's disease, and cancer. This broader perspective underscores the potential of lacrimal fluid analysis in offering non-invasive diagnostic tools for conditions with far-reaching implications.

Normative Data on Serum and Plasma Tryptophan and Kynurenine Concentrations from 8089 Individuals Across 120 Studies: A Systematic Review and Meta-Analysis

In this systematic review and meta-analysis, a normative dataset is generated from the published literature on the kynurenine pathway in control participants extracted from case-control and methodological validation studies. Study characteristics were mapped, and studies were evaluated in terms of analytical rigour and methodological validation. Meta-analyses of variance between types of instruments, sample matrices and metabolites were conducted. Regression analyses were applied to determine the relationship between metabolite, sample matrix, biological sex, participant age and study age. The grand mean concentrations of tryptophan in the serum and plasma were 60.52 ± 15.38 μM and 51.45 ± 10.47 μM, respectively. The grand mean concentrations of kynurenine in the serum and plasma were 1.96 ± 0.51 μM and 1.82 ± 0.54 μM, respectively. Regional differences in metabolite concentrations were observed across America, Asia, Australia, Europe and the Middle East. Of the total variance within the data, mode of detection (MOD) accounted for up to 2.96%, sample matrix up to 3.23%, and their interaction explained up to 1.53%; the latter of which was determined to be negligible. This review was intended to inform future empirical research and method development studies and successfully synthesised pilot data. The pilot data reported in this study will inform future precision medicine initiatives aimed at targeting the kynurenine pathway by improving the availability and quality of normative data.

Slug-Flow Microextraction Mass Spectrometry for Enhanced Detection of Analytes in Human Tear Fluids using Noninvasive Microsampling and Nanoelectrospray Ionization via a Capillary

In vivo noninvasive sampling and sensitive analysis of human tear fluids at the microliter level is an important but challenging task in investigating eye health. In this work, capillary microsampling coupled with slug-flow microextraction mass spectrometry (SFME-MS) was developed for enhanced detection of analytes in human tear fluids. As low as 1.0 μL of human tear fluid could be directly sampled using a capillary, and extraction/spray solvent was then loaded into the capillary to perform slug-flow microextraction and direct nanoelectrospray ionization (nESI) of analytes. All analytical procedures, including tear microsampling, microextraction, and ionization of analytes, were performed using a capillary. Enhanced detection of therapeutic drugs and disease biomarkers in human tear fluids was successfully demonstrated. Acceptable analytical performances including sensitivity, reproducibility, and quantitation were obtained. It is found that the use of SFME could improve the nESI-MS detection of trace analytes over 100-fold that depends on the chemical properties of analytes. Overall, this study showed that SFME-nESI-MS is a highly effective method for enhanced detection of trace analytes in tear fluids and is expected to be a potentially powerful tool in significant biological and clinical applications.

Taurine, a Component of the Tear Film, Exacerbates the Pathogenic Mechanisms of Acanthamoeba castellanii in the Ex Vivo Amoebic Keratitis Model

Acanthamoeba spp. is the etiological agent of amoebic keratitis. In this study, the effect of taurine in physiological concentrations in tears (195 μM) on trophozoites of Acanthamoeba castellanii through the ex vivo amoebic keratitis model was evaluated. Trophozoites were coincubated with the Syrian golden hamster cornea (Mesocricetus auratus) for 3 and 6 h. Group 1: Control (-). Corneas coincubated with amoebic culture medium and taurine. Group 2: Control (+). Corneas coincubated with trophozoites without taurine. Group 3: Corneas coincubated with taurine 15 min before adding trophozoites. Group 4: Trophozoites coincubated 15 min with taurine before placing them on the cornea. Group 5: Corneas coincubated for 15 min with trophozoites; subsequently, taurine was added. Results are similar for both times, as evaluated by scanning electron microscopy. As expected, in the corneas of Group 1, no alterations were observed in the corneal epithelium. In the corneas of Group 2, few adhered trophozoites were observed on the corneal surface initiating migrations through cell junctions as previously described; however, in corneas of Groups 3, 4 and 5, abundant trophozoites were observed, penetrating through different corneal cell areas, emitting food cups and destabilizing corneal surface in areas far from cell junctions. Significant differences were confirmed in trophozoites adherence coincubated with taurine (p < 0.05). Taurine does not prevent the adhesion and invasion of the amoebae, nor does it favor its detachment once these have adhered to the cornea, suggesting that taurine in the physiological concentrations found in tears stimulates pathogenic mechanisms of A. castellanii.

Metabolomics in Corneal Diseases: A Narrative Review from Clinical Aspects

Corneal pathologies may have subtle manifestations in the initial stages, delaying diagnosis and timely treatment. This can lead to irreversible visual loss. Metabolomics is a rapidly developing field that allows the study of metabolites in a system, providing a complementary tool in the early diagnosis and management of corneal diseases. Early identification of biomarkers is key to prevent disease progression. The advancement of nuclear magnetic resonance and mass spectrometry allows the identification of new biomarkers in the analysis of tear, cornea, and aqueous humor. Novel perspectives on disease mechanisms are identified, which provide vital information for potential targeted therapies in the future. Current treatments are analyzed at a molecular level to offer further information regarding their efficacy. In this article, we provide a comprehensive review of the metabolomic studies undertaken in the cornea and various pathologies such as dry eye disease, Sjogren’s syndrome, keratoconus, post-refractive surgery, contact lens wearers, and diabetic corneas. Lastly, we discuss the exciting future that metabolomics plays in cornea research.

Investigating the Ocular Surface Microbiome: What Can It Tell Us?

While pathogens of the eye have been studied for a very long time, the existence of resident microbes on the surface of healthy eyes has gained interest only recently. It appears that commensal microbes are a normal feature of the healthy eye, whose role and properties are currently the subject of extensive research. This review provides an overview of studies that have used 16s rRNA gene sequencing and whole metagenome shotgun sequencing to characterize microbial communities associated with the healthy ocular surface from kingdom to genus level. Bacteria are the primary colonizers of the healthy ocular surface, with three predominant phyla: Proteobacteria, Actinobacteria, and Firmicutes, regardless of the host, environment, and method used. Refining the microbial classification to the genus level reveals a highly variable distribution from one individual and study to another. Factors accounting for this variability are intriguing - it is currently unknown to what extent this is attributable to the individuals and their environment and how much is artifactual. Clearly, it is technically challenging to accurately describe the microorganisms of the ocular surface because their abundance is relatively low, thus, permitting substantial contaminations. More research is needed, including better experimental standards to prevent biases, and the exploration of the ocular surface microbiome's role in a spectrum of healthy to pathological states. Outcomes from such research include the opportunity for therapeutic interventions targeting the microbiome.

Development of lacrimal gland organoids from iPSC derived multizonal ocular cells

Lacrimal gland plays a vital role in maintaining the health and function of the ocular surface. Dysfunction of the gland leads to disruption of ocular surface homeostasis and can lead to severe outcomes. Approaches evolving through regenerative medicine have recently gained importance to restore the function of the gland. Using human induced pluripotent stem cells (iPSCs), we generated functional in vitro lacrimal gland organoids by adopting the multi zonal ocular differentiation approach. We differentiated human iPSCs and confirmed commitment to neuro ectodermal lineage. Then we identified emergence of mesenchymal and epithelial lacrimal gland progenitor cells by the third week of differentiation. Differentiated progenitors underwent branching morphogenesis in the following weeks, typical of lacrimal gland development. We were able to confirm the presence of lacrimal gland specific acinar, ductal, and myoepithelial cells and structures during weeks 4-7. Further on, we demonstrated the role of miR-205 in regulation of the lacrimal gland organoid development by monitoring miR-205 and FGF10 mRNA levels throughout the differentiation process. In addition, we assessed the functionality of the organoids using the β-Hexosaminidase assay, confirming the secretory function of lacrimal organoids. Finally, metabolomics analysis revealed a shift from amino acid metabolism to lipid metabolism in differentiated organoids. These functional, tear proteins secreting human lacrimal gland organoids harbor a great potential for the improvement of existing treatment options of lacrimal gland dysfunction and can serve as a platform to study human lacrimal gland development and morphogenesis.

Fast and Sensitive Quantification of AccQ-Tag Derivatized Amino Acids and Biogenic Amines by UHPLC-UV Analysis from Complex Biological Samples

Metabolomic analysis of different body fluids bears high importance in medical sciences. Our aim was to develop and validate a fast UHPLC-UV method for the analysis of 33 amino acids and biogenic amines from complex biological samples. AccQ-Tag derivatization was conducted on target molecules and the derivatized targets were analyzed by UHPLC-UV. The detection of the analytes was carried out with UV analysis and by Selected Reaction Monitoring (SRM)-based targeted mass spectrometry. The method was validated according to the FDA guidelines. Serum and non-stimulated tear samples were collected from five healthy individuals and the samples were analyzed by the method. The method was successfully validated with appropriate accuracy and precision for all 33 biomolecules. A total of 29 analytes were detected in serum samples and 26 of them were quantified. In the tears, 30 amino acids and biogenic amines were identified and 20 of them were quantified. The developed and validated UHPLC-UV method enables the fast and precise analysis of amino acids and biogenic amines from complex biological samples.

Metabolic phenotyping of tear fluid as a prognostic tool for personalised medicine exemplified by T2DM patients

Background/aims

Concerning healthcare approaches, a paradigm change from reactive medicine to predictive approaches, targeted prevention, and personalisation of medical services is highly desirable. This raises demand for biomarker signatures that support the prediction and diagnosis of diseases, as well as monitoring strategies regarding therapeutic efficacy and supporting individualised treatments. New methodological developments should preferably rely on non-invasively sampled biofluids like sweat and tears in order to provide optimal compliance, reduce costs, and ensure availability of the biomaterial. Here, we have thus investigated the metabolic composition of human tears in comparison to finger sweat in order to find biofluid-specific marker molecules derived from distinct secretory glands. The comprehensive investigation of numerous biofluids may lead to the identification of novel biomarker signatures. Moreover, tear fluid analysis may not only provide insight into eye pathologies but may also be relevant for the prediction and monitoring of disease progression and/ or treatment of systemic disorders such as type 2 diabetes mellitus.

Methods

Sweat and tear fluid were sampled from 20 healthy volunteers using filter paper and commercially available Schirmer strips, respectively. Finger sweat analysis has already been successfully established in our laboratory. In this study, we set up and evaluated methods for tear fluid extraction and analysis using high-resolution mass spectrometry hyphenated with liquid chromatography, using optimised gradients each for metabolites and eicosanoids. Sweat and tears were systematically compared using statistical analysis. As second approach, we performed a clinical pilot study with 8 diabetic patients and compared them to 19 healthy subjects.

Results

Tear fluid was found to be a rich source for metabolic phenotyping. Remarkably, several molecules previously identified by us in sweat were found significantly enriched in tear fluid, including creatine or taurine. Furthermore, other metabolites such as kahweol and various eicosanoids were exclusively detectable in tears, demonstrating the orthogonal power for biofluid analysis in order to gain information on individual health states. The clinical pilot study revealed that many endogenous metabolites that have previously been linked to type 2 diabetes such as carnitine, tyrosine, uric acid, and valine were indeed found significantly up-regulated in tears of diabetic patients. Nicotinic acid and taurine were elevated in the diabetic cohort as well and may represent new biomarkers for diabetes specifically identified in tear fluid. Additionally, systemic medications, like metformin, bisoprolol, and gabapentin, were readily detectable in tears of patients.

Conclusions

The high number of identified marker molecules found in tear fluid apparently supports disease development prediction, developing preventive approaches as well as tailoring individual patients’ treatments and monitoring treatment efficacy. Tear fluid analysis may also support pharmacokinetic studies and patient compliance control.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13167-022-00272-7.

Metabolomics and lipidomics approaches in human tears: A systematic review

The human tear film is at the interface between the ocular surface and the external environment. Although investigation has been hindered by its small volume, improvements in preanalytical and analytical methods have allowed the omics approach to represent an innovative biomarker search strategy. There is still a significant lack of standardization, representing a barrier for performing between-studies comparisons and transferring experimental findings into clinical use and trials. We summarize the preanalytical and analytical procedures, describe the biomarkers that can be found using the metabo-lipidomics approach, and provide our expert opinion for omics investigations in human tears. For this systematic review of 38 studies, we searched PubMed by combining Boolean operators with the following keywords: tear, metabolomic, lipidomic, -omics. The human tear metabo-lipidome has been well-characterized in normal individuals using high-resolution liquid chromatography coupled with mass spectrometry. Lipid and metabolite profiles were influenced by ocular (e.g. dry eye disorders; Meibomian gland dysfunction; contact lens wear; glaucoma; keratoconus; pterygium) and systemic conditions (e.g. multiple sclerosis). Investigating the tear metabo-lipidome could improve our understanding of the pathogenesis of both ocular and systemic diseases, but also provide diagnostic as well as prognostic biomarkers.

In situ metabolic profile and spatial distribution of ocular tissues: New insights into dry eye disease

PURPOSE

Dry eye disease (DED) is a chronic multifactorial disorder affecting millions of people, yet the pathogenesis mechanisms still remain unclear. Matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) is a novel in situ visualization approach combined high-throughput mass spectrometry and molecular imaging. We aimed to explore the in situ ocular metabolic changes via MALDI-MSI to accelerate the recognition of DED pathogenesis.

METHODS

Experimental dry eye was established in Wistar rats by subcutaneous injection of scopolamine. The induction of DED was assessed by tear film breakup time, sodium fluorescein, histopathological staining and cell apoptosis. MALDI-MSI was applied to explore in situ ocular metabolomic in DED rats, and histopathological staining from same sections were used for side-by-side comparison with MALDI to annotate different tissue structures in the eye.

RESULTS

Considering the complexity of ocular tissue, we visualized the metabolites in specific ocular regions (central cornea, peripheral cornea, fornix conjunctiva, eyelid conjunctiva and aqueous humor), and identified metabolites related to DED, with information of relative abundance and spatial signatures. In addition, integrative pathway analysis illustrated that, several metabolic pathways such as glycerophospholipid, sphingolipid phenylalanine, and metabolism of glycine, serine and threonine were significantly altered in certain regions in the dry eye tissue. Moreover, we discussed how the metabolic pathways with spatiotemporal signatures might be involved in the DED process.

CONCLUSIONS

Our data exploit the advantages of in situ analysis of MALDI-MSI to accurately analyze the region-specific metabolic behaviors in DED, and provide new clues to uncover DED pathogenesis.

Anion-controlled Zn(II) coordination polymers with 1-(tetrazo-5-yl)-3-(triazo-1-yl) benzene as an assembling ligand: synthesis, characterization, and efficient detection of tryptophan in water

Tryptophan regulates and participates in various physiological systems in the human body, and its excessive intake has harmful effects. Therefore, detecting and monitoring tryptophan in water and distinguishing it from other amino acids are necessary. In addition to their excellent luminescence, coordination polymer-based sensors have good stability and high sensitivity and selectivity for sensing applications. In this work, two luminescent coordination polymers (CPs), [Zn(ttb)Cl]_n (1) and [Zn₂(ttb)₂(OH)(NO₃)]_n (2), were obtained through the solvothermal reaction of different Zn(II) salts with a rarely studied multidentate N-donor ligand, 1-(tetrazo-5-yl)-3-(triazo-1-yl) benzene (Httb). Crystallographic investigations revealed that the structure of 1 exhibits a 2D fes net with Cl^- anions acting as terminal charge balancers, and that of 2 features a 3D ant net with NO₃^- anions in a rare monodentate bridging (μ₂-O-η¹:η¹) mode. In terms of stability tests, 2 has better thermal and water stability than 1. Although both show good fluorescence performance, specific tryptophan detection, and excellent anti-interference ability, 2 has higher K_SV (111 852.6 M^-1), a lower limit of detection (LOD = 23.6 nM), and a better recovery rate than 1. Cytotoxicity experiments proved that 2 has extremely low toxicity and thus has great potential for in vivo detection. Therefore, CP 2 is a suitable candidate for advanced practical applications for the efficient sensing of tryptophan in water. The luminescence of the ligands was also calculated using DFT theory and further discussed through experiments. The quenching mechanism that occurs after tryptophan addition was explored through Hirshfeld surface analysis.

Recyclable europium functionalized metal-organic fluorescent probe for detection of tryptophan in biological fluids and food products

An europium functionalized metal-organic fluorescent probe, Eu³⁺@UiO-66-FDC was constructed by post-synthetic modification through coordination interactions. Eu³⁺@UiO-66-FDC displayed high selectivity and sensitivity toward Tryptophan (Trp) among all the 20 natural amino acids and other general compounds in food and biological samples, with a wide linear concentration range (0-1000 μM), low detection limit (0.29 μM), and a rapid response (<1 min). Besides, this probe was utilized to detect Trp in rabbit blood serum and milk samples with good recoveries, which were verified by high-performance liquid chromatography (HPLC). Notably, this fluorescent probe proved to be a recyclable material. Hence, this work provides a reliable and recyclable fluorescent probe applicable toward the detection of Trp in biological fluids and/or food products.

Development of a HPLC-MS/MS method for assessment of thiol redox status in human tear fluids

Oxidative stress is reported to be part of the pathology of many ocular diseases. For the diagnosis of ocular diseases, tear fluid has unique advantages. Although numerous analytical methods exist for the measurement of different types of biomolecules in tear fluid, few have been reported for comprehensive understanding of oxidative stress-related thiol redox signaling. In this study, a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method was developed to determine a panel of twelve metabolites that systematically covered several thiol metabolic pathways. With optimization of MS/MS parameters and HPLC mobile phases, this method was sensitive (LOQ as low as 0.01 ng/ml), accurate (80-125% spike recovery) and precise (<10% RSD). This LC-MS/MS method combined with a simple tear fluid collection with Schirmer test strip followed by ultrafiltration allowed the high-throughput analysis for efficient determination of metabolites associated with thiol redox signaling in human tear fluids. The method was then applied to a small cohort of tear fluids obtained from healthy individuals. The method presented here provides a new technique to facilitate future work aiming to determine the complex thiol redox signaling in tear fluids for accurate assessment and diagnosis of ocular diseases.

Metabolomics comparison of cord and peripheral blood-derived serum eye drops for the treatment of dry eye disease

Allogeneic peripheral blood-derived (PBS) serum eye drops have been largely used in the treatment of dry eye disease (DED). Recently, cord blood has emerged as an effective alternative serum source (cord blood serum, CBS), containing a higher amount of growth factors than PBS, it holds the promise of a better capability to stimulate corneal healing. However, the lack of a standardized method for preparation, dispensation, storage and a poor biochemical characterization still hamper the establishment of a clinical consensus. Here the metabolomes of the two different serum eye drop preparations were compared using proton nuclear magnetic resonance spectroscopy. We found that both PBS and CBS contained several organic compounds, the majority of them already detected in human tears and may be thereby considered lacrimal substitutes. Metabolites having in the multivariate statistical analysis Partial least squares discriminant analysis (PLS-DA) a VIP scores > 1.0 were considered to be significantly different. All the metabolites identified were found to have a p < 0.05 in the univariate analysis. CBS, in particular, showed the highest amount of choline, myo-inositol, glutamine, creatine and β-hydroxybutyrate. These evidences constitute relevant advances towards serum eye drops characterization and confirm that cord blood is a valid alternative source of serum eye drops.

Metabolomics approach to biomarkers of dry eye disease using 1H-NMR in rats

Dry eye disease (DED) is a chronic and progressive lesion on the ocular surface and induces symptoms, such as burning sensation, itchy eyes, heavy eyes, tired eyes, dry feeling, facial flushing, and blurred vision. The present study was performed to develop DED biomarkers using metabolomics in a rat model. DED was induced by injecting scopolamine and exposing rats to a dry condition. Scopolamine (12 mg/kg/day for 7 days) was subcutaneously injected to male Sprague-Dawley rats. The rats were placed in dry condition with air-flow and dehumidifier. Tear volume and tear breakup time (TBUT) were measured, and eyes were examined through fluorescein staining to assess DED. Mucosal damage and immune reactions were also determined. Plasma and urinary endogenous metabolites were determined using ¹H-NMR analysis. Compared with control tear and TBUT levels were significantly decreased in the DED group whereas corneal damage was significantly increased. The levels of interleukins (IL-6) and IL-1β significantly elevated in the cornea and lacrimal glands in the DED group. TNF-α was numerically increased but not significantly different between groups. Pattern recognition using principal component analysis (PCA) and orthogonal projections to latent structure-discriminant analysis (OPLS-DA) of the NMR spectra in global profiling revealed different clusters between DED and control groups. Target profiling demonstrated that PCA and OPLS-DA score plots were separated between DED and controls in plasma and urine. Subsequently, 9 plasma metabolites were selected to examine different clustering between groups, and 26 urinary metabolites were also selected. Plasma metabolites showed a non-significant rising tendency in the DED group. Urinary phenylalanine, phenylacetate, pantothenate, glycine, succinate, methanol, valine, propylene glycol, histidine, threonine, lactate, and acetate were significantly different between control and DED rats. These results may contribute to understanding the metabolic regulation that is involved in DED and might be useful for potential biomarkers related to DED in rats.

Tear fluid collection methods: Review of current techniques

Tear fluid, composed of lipid, aqueous, and mucin layers, contains electrolytes, water, proteins, peptides, and glycoproteins. Its components may serve as diagnostic indicators of local and systemic diseases. The aim of the study was to conduct literature review in order to identify the current methods of tear collection. The most commonly used method which was relatively easy to perform and allowed to obtain sufficient tear volume for further chemical and physical analysis was selected through PubMed database search for the following keywords: tear sampling, human tears, chemical analysis of tears, physical tear analysis, animal tear sampling. Final criteria of articles selection were: human tears, tear sample collection, chemical and physical analysis of tears. Time of publication of the articles not older than 1995. The analysis of 70 articles revealed that the most common tear fluid collection methods are Schirmer tear strips and capillary tubes. Thus, we recommend the use of Schirmer strips and microcapillary tubes as the cheapest and easiest methods for sampling of tear fluid for further chemical analysis.

Nonclinical Development of Biologics: Integrating Safety, Pharmacokinetics, and Pharmacodynamics to Create Smarter and More Flexible Nonclinical Safety Programs Optimizing Animal Use

Safety assessment of biological drugs has its challenges due to the multiple new different modalities, for example, antibody-drug conjugates, bispecifics, nanobodies, fusion proteins and advanced therapy medicinal products (ATMPs), their different pharmacokinetic and pharmacodynamic properties, and their ability to trigger immunogenicity and toxicity. In the public and in the pharmaceutical industry, there is a strong and general desire to reduce the number of animals used in research and development of drugs and in particular reducing the use of nonhuman primates. Important discussions and activities are ongoing investigating the smarter designs of early research and dose range finding studies, reuse of animals, and replacing animal experiments with in vitro studies. Other important challenges include absence of a relevant species and design of studies and developing genetically modified animals for special investigative toxicology studies. Then, the learnings and challenges from the development of the first ATMPs are available providing valuable insights in the development path for these new potentially transformative treatments. Finally, development of strategies for assessment of immunogenicity and prediction of translation of immunogenicity and associated findings to the clinic. On this, the eighth meeting for the European BioSafe members, these challenges served as the basis for the presentations and discussions during the meeting. This article serves as the workshop report reviewing the presentations and discussions at the meeting.

Tryptophan as a Safe Compound in Topical Ophthalmic Medications: In Vitro and In Vivo Studies

Background: To evaluate the effects of tryptophan (TRP) on normal human corneal and conjunctival epithelium in vitro and the re-epithelization of corneal erosion in rabbits.Materials and methods: Corneal epithelial cell (10.014 pRSV-T) and conjunctival epithelial cell (HC0597) cultures were used. The cellular metabolism, viability, secretion of IL-1β, IL-6, IL-10, cytoskeleton organization, transwell migration were determined. Cells were incubated in the presence of TRP at 1-100 μM. After corneal de-epithelization rabbits received TRP drops (100 μM), 5 times a day.Results: TRP increased conjunctival epithelium metabolism at 50 μM and increased the viability of corneal epithelium at 100 μM. TRP (10 μM) enhanced the production of IL-6 by the corneal epithelium and had no effect on IL-1β and IL-10.Conclusions: TRP had no influence on the cellular cytoskeleton but induced a significant pseudopodia projection in both epithelia. TRP did not influence corneal re-epithelization in vivo. TRP was not toxic for corneal and conjunctival epithelia.

Quantifying Sample Collection and Processing Impacts on Fiber-Based Tear Fluid Chemical Analysis

Purpose

Noninvasive analyses of tear fluid from humans and animal models in clinical and research settings most commonly use absorbent material for collection and processing. Still, the impact of these analytical techniques on tear chemical analyses remains largely unknown. The purpose of this study was to quantify the impacts of phenol red thread fiber-based tear sample collection and processing on the primary amine content.

Methods

Human tears were collected by placing the folded end of phenol red thread on the palpebral conjunctiva of the right eye for 20 seconds. The wetted thread was then processed using elution or extraction, and capillary electrophoresis with light-emitting diode-induced fluorescence detection was used for analysis and quantitation.

Results

Distinct processing methods impacted tear analysis differently. Primary amines adsorbed onto the thread partitioned in a chromatographic manner and thus any single portion of the wetted thread might not be representative of the whole sample. Quantitative assessment of five small molecule standards after on-thread processing showed significant overestimation of the actual concentration, with increased accuracy for larger volume samples. Yet collection of larger tear volumes introduced error in volume determination owing to evaporation and reduced small molecule separation resolution.

Conclusions

These results indicated that absorption-based tear fluid collection and processing significantly alter chemical content analysis, suggesting that the impacts of methods used should be regularly evaluated to standardize results drawn from different studies.

Translational Relevance

This study identifies potential inconsistencies and inaccuracies in tear analyses that are widespread across the published literature and clinical care.

Physicochemical Analysis of Sediments Formed on the Surface of Hydrophilic Intraocular Lens after Descemet's Stripping Endothelial Keratoplasty

An intraocular lens (IOL) is a synthetic, artificial lens placed inside the eye that replaces a natural lens that is surgically removed, usually as part of cataract surgery. The opacification of the artificial lens can be related to the formation of the sediments on its surface and could seriously impair vision. The physicochemical analysis was performed on an explanted hydrophilic IOL and compared to the unused one, considered as a reference IOL. The studies were carried out using surface sensitive techniques, which can contribute to a better understanding of the sedimentation process on hydrophilic IOLs’ surfaces. The microscopic studies allowed us to determine the morphology of sediments observed on explanted IOL. The photoelectron spectroscopy measurements revealed the presence of organic and inorganic compounds at the lens surface. Mass spectroscopy measurements confirmed the chemical composition of deposits and allowed for chemical imaging of the IOL surface. Applied techniques allowed to obtain a new set of information approximating the origin of the sediments’ formation on the surface of the hydrophilic IOLs after Descemet’s stripping endothelial keratoplasty.

Tear 1H Nuclear Magnetic Resonance-Based Metabolomics Application to the Molecular Diagnosis of Aqueous Tear Deficiency and Meibomian Gland Dysfunction

PURPOSE

Meibomian gland dysfunction (MGD) is a major cause of signs and symptoms related to dry eyes (DE) and eyelid inflammation. We investigated the composition of human tears by metabolomic approaches in patients with aqueous tear deficiency and MGD.

METHODS

Participants in this prospective, case-control pilot study were split into patients with aqueous tear deficiency and MGD (DE-MGD [n = 15]) and healthy controls (CG; n = 20). Personal interviews, ocular surface disease index (OSDI), and ophthalmic examinations were performed. Reflex tears collected by capillarity were processed to 1H nuclear magnetic resonance (NMR) spectroscopy and quantitative data analysis to identify molecules by spectra comparison to library entries of purified standards and/or unknown entities. Statistical analyses were made by the SPSS 22.0 program.

RESULTS

Chemometric analysis and 1H NMR spectra comparison revealed the presence of 60 metabolites in tears. Differentiating features were evident in the NMR spectra of the 2 clinical groups, characterized by significant upregulation of phenylalanine, glycerol, and isoleucine, and downregulation of glycoproteins, leucine, and -CH3 lipids, as compared to the CG. The 1H NMR metabolomic analyses of human tears confirmed the applicability of this platform with high predictive accuracy/reliability.

CONCLUSIONS

Our key distinctive findings support that DE-MGD induces tear metabolomics profile changes. Metabolites contributing to a higher separation from the CG can presumably be used, in the foreseeable future, as DE-MGD biomarkers for better managing the diagnosis and therapy of this disease.

A Set of Global Metabolomic Biomarker Candidates to Predict the Risk of Dry Eye Disease

Purpose

We used ultraperformance liquid chromatography coupled with quadrupole/time-of-flight tandem mass spectrometry (UPLC-Q/TOF-MS/MS) to analyze the metabolic profile of reflex tears obtained from patients with dry eye disorders.

Methods

We performed a cross-sectional study involving 113 subjects: 85 patients diagnosed with dry eye syndrome (dry eye group) and 28 healthy volunteers (control group). Reflex tears (20–30 μl) were collected from the tear meniscus of both eyes of each subject using a Schirmer I test strip. MS data were acquired with a standard workflow by UPLC-Q/TOF-MS/MS. Metabolites were quantitatively analyzed and matched with entries in the Metlin, Massbank, and HMDB databases. Least absolute shrinkage and selection operator (LASSO) regression was conducted to detect important metabolites. Multiple logistic regression was used to identify the significant metabolic biomarker candidates for dry eye syndrome. Open database sources, including the Kyoto Encyclopedia of Genes and Genomes and MetaboAnalyst, were used to identify metabolic pathways.

Results

After the LASSO regression and multiple logistic regression analysis, 4 of 20 metabolic biomarker candidates were significantly correlated with Ocular Surface Disease Index score, 42 of 57 with fluorescein breakup time, and 26 of 57 with fluorescein staining. By focusing on the overlap of these three sets, 48 of 51 metabolites contributed to the incidence of dry eye and there were obvious changes in different age groups. Metabolic pathway analysis revealed that the main pathways were glucose metabolism, amino acid metabolism, and glutathione metabolism.

Conclusion

Dry eye syndrome induces changes in the metabolic profile of tears, and the trend differs with age. This evidence reveals the relationship between changes in metabolites, symptoms of dry eye syndrome, and age.

Schirmer Paper Noninvasive Microsampling for Direct Mass Spectrometry Analysis of Human Tears

Rapid and sensitive detection of metabolites and chemical residues in human tears is highly beneficial for understanding eye health. In this study, Schirmer paper was used for noninvasive microsampling of human tears, and then paper spray mass spectrometry (PSMS) was performed for direct analysis of human tears. Schirmer PSMS was successfully used for rapid diagnosis of dry-eye syndrome by detecting the volume and metabolites of human tears. Drugs of abuse, therapeutic drugs, and pharmacodynamics in human tears were also investigated by Schirmer PSMS. Furthermore, specific markers of environmental exposures in the air to human eyes, including volatile organic compounds, aerosol, and smoke, were unambiguously sampled and detected in human tears using Schirmer PSMS. Excellent analytical performances were achieved, including single-use, low-sample consumption (1.0 μL), rapid analysis (the whole analytical procedure completed within 3 min), high sensitivity (absolute limit of detection less than or equal to 0.5 pg, signal-to-noise ratio greater than or equal to 3), good reproducibility (relative standard deviation less than 10%, n = 3), and accurate quantitation (average deviation less than 3%, n = 3). Overall, our results showed that Schirmer PSMS is a highly effective method for direct tear analysis and is expected to be a convenient tool for human tear analysis in significant clinical applications.

Flexible potentiometric pH sensors for wearable systems

There is a growing demand for developing wearable sensors that can non-invasively detect the signs of chronic diseases early on to possibly enable self-health management. Among these the flexible and stretchable electrochemical pH sensors are particularly important as the pH levels influence most chemical and biological reactions in materials, life and environmental sciences. In this review, we discuss the most recent developments in wearable electrochemical potentiometric pH sensors, covering the key topics such as (i) suitability of potentiometric pH sensors in wearable systems; (ii) designs of flexible potentiometric pH sensors, which may vary with target applications; (iii) materials for various components of the sensor such as substrates, reference and sensitive electrode; (iv) applications of flexible potentiometric pH sensors, and (v) the challenges relating to flexible potentiometric pH sensors.

Tear Metabolomics in Dry Eye Disease: A Review

Dry eye disease (DED) is a multifactorial syndrome that can be caused by alteration in the quality or quantity of the precorneal tear film. It is considered one of the most common ocular conditions leading patients to seek eye care. The current method for diagnostic evaluations and follow-up examinations of DED is a combination of clinical signs and symptoms determined by clinical tests and questionnaires, respectively. The application of powerful omics technologies has opened new avenues toward analysis of subjects in health and disease. Metabolomics is a new emerging and complementary research discipline to all modern omics in the comprehensive analysis of biological systems. The identification of distinct metabolites and integrated metabolic profiles in patients can potentially inform clinicians at an early stage or during monitoring of disease progression, enhancing diagnosis, prognosis, and the choice of therapy. In ophthalmology, metabolomics has gained considerable attention over the past decade but very limited such studies have been reported on DED. This paper aims to review the application of tear metabolomics in DED.

Capillary electrophoresis of small ions and molecules in less conventional human body fluid samples: A review

In recent years, advances in sensitive analytical techniques have encouraged the analysis of various compounds in biological fluids. While blood serum, blood plasma and urine still remain the golden standards in clinical, toxicological and forensic science, analyses of other body fluids, such as breast milk, exhaled breath condensate, sweat, saliva, amniotic fluid, cerebrospinal fluid, or capillary blood in form of dried blood spots are becoming more popular. This review article focuses on capillary electrophoresis and microchip electrophoresis of small ions and molecules (e.g. inorganic cations/anions, basic/acidic drugs, small acids/bases, amino acids, peptides and other low molecular weight analytes) in various less conventional human body fluids and hopes to stimulate further interest in the field.

Integrated Lipidomics and Metabolomics Analysis of Tears in Multiple Sclerosis: An Insight into Diagnostic Potential of Lacrimal Fluid

Metabolomics based on mass spectrometry represents an innovative approach to characterize multifactorial diseases, such as multiple sclerosis (MuS). To date, the most important biomarker source for MuS diagnosis is the cerebrospinal fluid. However, an important goal for research is to identify new molecules in more easily accessible biological fluids. A very interesting biofluid in MuS is represented by tears, considered as an intermediate fluid between the cerebrospinal fluid and serum. In this work, we developed a merged strategy for the analysis of lipids containing choline by Liquid Chromatography coupled to Tandem Mass Spectrometry (LC-MS/MS), as well as for the targeted analysis of free carnitine, acylcarnitines and aminoacids by direct infusion mass spectrometry. Samples for both metabolomics and lipidomics approaches were obtained in a single extraction procedure from tears of patients affected by MuS and healthy controls. Tear lipidomics showed 30 phospholipids significantly modulated and, notably, many sphingomyelins resulted lower in MuS. Moreover, the metabolomics approach carried out both on tears and serum highlighted the diagnostic potential of specific aminoacids and acylcarnitines. In conclusion, the metabolic profiling of tears appears to reflect the pathological conditions of the central nervous system, suggesting that the molecular repository of tears can be considered as a source of potential biomarkers for MuS.

Analysis of amino acids in human tears by hydrophilic interaction liquid chromatography and quadrupole orbitrap mass spectrometry

Amino acids in human tears play certain physiological roles and their determination is challenging due to complicated chemical properties.

Chiral Analysis of Non-Protein Amino Acids by Capillary Electrophoresis

A high number of non-protein amino acids are chiral compounds that have demonstrated to be relevant in different fields. Their determination enables to obtain valuable information related to food quality and safety and has also a high interest from a biological point of view since many of them are key compounds in metabolic pathways or are related with different pathologies.In the development of analytical methodologies to perform chiral separations, capillary electrophoresis (CE) is well-established and one of the most powerful separation techniques as a consequence of its high efficiency, short analysis time, and versatility.This chapter shows, by means of three interesting examples, the application of different CE methodologies to the chiral analysis of non-protein amino acids. The first example describes different electrokinetic chromatography (EKC)-UV methodologies based on the use of negatively charged cyclodextrins as chiral selectors to carry out the stereoselective separation of ten different non-protein amino acids of relevance from a biological or food analysis point of view. The second method illustrates the EKC-UV analysis of L-citrulline and its enantiomeric impurity in food supplements using sulfated-γ-cyclodextrin as chiral selector. The last example shows the simultaneous enantiomeric separation of 3,4-dihydroxy-DL-phenylalanine and all the other chiral constituents involved in the phenylalanine-tyrosine metabolic pathway by using an EKC-MS methodology.

Combined Targeted Analysis of Metabolites and Proteins in Tear Fluid With Regard to Clinical Applications

Purpose

To establish a robust workflow for combined mass spectrometry–based analysis of metabolites and proteins in tear fluid with regard to clinical applicability.

Methods

Tear fluid was taken from 12 healthy volunteers at different time points using specially designed Schirmer strips. Following the liquid extraction of metabolites from standardized punches, the remaining material was processed for bottom-up proteomics. Targeted metabolite profiling was performed adapting a metabolomics kit, which targets 188 metabolites from four different analyte classes. Proteomics was performed of the identical samples targeting 15 tear proteins relevant to ocular health.

Results

Sixty metabolites could be consistently determined in all tear samples (98 metabolites were detectable in average) covering acylcarnitines, amino acids, biogenic amines, and glycerophospholipids. Following normalization, the majority of metabolites exhibited intraindividual variances of less than 20%, both regarding different times of sampling, and the individual eye. The targeted analysis of tear proteins revealed a mean intraindividual variation of 23% for the three most abundant proteins. Even extreme differences in tear secretion rates resulted in interindividual variability below 30% for 65 metabolites and two proteins.

Conclusions

The newly established workflow can be used for combined targeted detection of metabolites and proteins in one punch of a Schirmer strip in a clinical setting.

Translational Relevance

Our data about intra- and interindividual as well as intereye variation provide a valuable basis for the design of clinical studies, and for the applicability of multiplexed “omics” to well accessible tear fluid with regard to future routine use.

GADD34 Function in Protein Trafficking Promotes Adaptation to Hyperosmotic Stress in Human Corneal Cells

GADD34, a stress-induced regulatory subunit of the phosphatase PP1, is known to function in hyperosmotic stress through its well-known role in the integrated stress response (ISR) pathway. Adaptation to hyperosmotic stress is important for the health of corneal epithelial cells exposed to changes in extracellular osmolarity, with maladaptation leading to dry eye syndrome. This adaptation includes induction of SNAT2, an endoplasmic reticulum (ER)-Golgi-processed protein, which helps to reverse the stress-induced loss of cell volume and promote homeostasis through amino acid uptake. Here, we show that GADD34 promotes the processing of proteins synthesized on the ER during hyperosmotic stress independent of its action in the ISR. We show that GADD34/PP1 phosphatase activity reverses hyperosmotic-stress-induced Golgi fragmentation and is important for cis- to trans-Golgi trafficking of SNAT2, thereby promoting SNAT2 plasma membrane localization and function. These results suggest that GADD34 is a protective molecule for ocular diseases such as dry eye syndrome.

Prospective, Randomized, Single Masked, Parallel Study Exploring the Effects of a Preservative-Free Ophthalmic Solution Containing Hyaluronic Acid 0.4% and Taurine 0.5% on the Ocular Surface of Glaucoma Patients Under Multiple Long-Term Topical Hypotensive Therapy

INTRODUCTION

To compare the effects of a preservative-free (PF) ophthalmic solution containing hyaluronic acid (HA) 0.4% and taurine (TAU) 0.5% with those of a PF ophthalmic solution containing HA 0.2% on ocular surface signs, symptoms, and morphological parameters in glaucoma patients under multiple long-term topical hypotensive therapy.

METHODS

Eligible patients underwent evaluation of ocular surface parameters by ocular surface disease index (OSDI) and glaucoma symptom scale (GSS) questionnaires, breakup time test (BUT), Schirmer I test, corneal and conjunctival staining (Oxford scale), and conjunctival in vivo confocal microscopy (Heidelberg Retina Tomograph 3, Heidelberg Engineering GmbH, Heidelberg, Germany). After the baseline visit, patients were randomized to use a PF ophthalmic solution containing HA 0.4% and TAU 0.5%, QID, in both eyes (group 1) or to use a PF ophthalmic solution containing HA 0.2%, QID (group 2) in addition to the ongoing preserved hypotensive treatment. Follow-up visits were scheduled at 30 and 90 days.

RESULTS

Thirty-nine eyes of 39 glaucoma patients were included in the study. At baseline, results of study tests of both groups were similar. After 90 days in group 1 the BUT (p = 0.01), the Oxford score (p = 0.03), the conjunctival goblet cells (CGC) density (p = 0.0005) ,and the two questionnaires score significantly improved (OSDI, p = 0.003; GSS, p = 0.003) compared to baseline values, while in group 2 all these parameters did not differ from baseline (BUT, p = 0.39; Oxford score, p = 0.54; CGC density, p = 0.33, OSDI p = 0.65, GSS, p = 0.25). The BUT and the CGC density were statistically different between groups both at 30 and 90 days (p = 0.04 and p = 0.04, respectively). The Schirmer I test did not statistically change after 90 days in both groups.

CONCLUSIONS

The PF ophthalmic solution with HA 0.4% and TAU 0.5% seems to improve CGC density and reduce signs and symptoms of dry eye in glaucoma patients under long-term multiple preserved hypotensive therapy.

TRIAL REGISTRATION

ClinicalTrials.gov identifier, NCT03480295.

Sleep deprivation disrupts the lacrimal system and induces dry eye disease

Sleep deficiency is a common public health problem associated with many diseases, such as obesity and cardiovascular disease. In this study, we established a sleep deprivation (SD) mouse model using a 'stick over water' method and observed the effect of sleep deficiency on ocular surface health. We found that SD decreased aqueous tear secretion; increased corneal epithelial cell defects, corneal sensitivity, and apoptosis; and induced squamous metaplasia of the corneal epithelium. These pathological changes mimic the typical features of dry eye. However, there was no obvious corneal inflammation and conjunctival goblet cell change after SD for 10 days. Meanwhile, lacrimal gland hypertrophy along with abnormal lipid metabolites, secretory proteins and free amino-acid profiles became apparent as the SD duration increased. Furthermore, the ocular surface changes induced by SD for 10 days were largely reversed after 14 days of rest. We conclude that SD compromises lacrimal system function and induces dry eye. These findings will benefit the clinical diagnosis and treatment of sleep-disorder-related ocular surface diseases.

Recent advances in the applications of metabolomics in eye research

Metabolomics, the identification and quantitation of metabolites in a system, have been applied to identify new biomarkers or elucidate disease mechanism. In this review, we discussed the application of metabolomics in several ocular diseases and recent developments in metabolomics regarding tear fluids analysis, data acquisition and processing.

A luminescent turn-up metal–organic framework sensor for tryptophan based on singlet–singlet Förster energy transfer

A highly stable MOF, ZJU-108, was synthesized with Zn²⁺ and 6-(4-pyridyl)-terephthalic acid (H₂pta) as construction units, and it exhibits an impressive turn-on luminescence enhancement response to tryptophan.

TFOS DEWS II Tear Film Report

The members of the Tear Film Subcommittee reviewed the role of the tear film in dry eye disease (DED). The Subcommittee reviewed biophysical and biochemical aspects of tears and how these change in DED. Clinically, DED is characterized by loss of tear volume, more rapid breakup of the tear film and increased evaporation of tears from the ocular surface. The tear film is composed of many substances including lipids, proteins, mucins and electrolytes. All of these contribute to the integrity of the tear film but exactly how they interact is still an area of active research. Tear film osmolarity increases in DED. Changes to other components such as proteins and mucins can be used as biomarkers for DED. The Subcommittee recommended areas for future research to advance our understanding of the tear film and how this changes with DED. The final report was written after review by all Subcommittee members and the entire TFOS DEWS II membership.

Regional differences in glutathione accumulation pathways in the rat cornea: Mapping of amino acid transporters involved in glutathione synthesis

In this study we have sought to complete the identification and localisation of uptake pathways involved in accumulating precursor amino acids involved in GSH synthesis in the rat cornea. To do this, we performed reverse transcription PCR (RT-PCR) to identify the Excitatory Amino Acid Transporters (EAAT 1-5) responsible for glutamate uptake, and glycine transporters (GLYT 1-2) at the transcript level. Western blotting was used to verify protein expression, while immunolabelling of sagittal sections was used to localise transporters to the different layers of the cornea. Immunolabelling of en face sections was used to examine the subcellular distribution of proteins in the corneal endothelium. Our findings revealed EAAT 1-5 and GLYT 1-2 to be expressed at the transcript and protein level in the rat cornea. Immunohistochemistry revealed all amino acid transporters to be localised to the epithelium. In the majority of cases, labelling was restricted to the epithelium, and labelling absent from the stroma or endothelium. However, EAAT 4 and GLYT 2 labelling was detected in the stroma with EAAT 4 labelling also present in the endothelium. Overall, the identification of amino acid transporters strongly supports the existence of an intracellular GSH synthesis pathway in the rat corneal epithelium. This suggests that regional differences in GSH accumulation pathways exist, with direct uptake of GSH and intracellular synthesis of GSH restricted to the endothelial and epithelial cell layers, respectively. This information is important in the design of targeted strategies to enhance GSH levels in specific layers of the cornea to prevent against oxidative damage, corneal swelling and loss of corneal transparency.

CodY, a master integrator of metabolism and virulence in Gram-positive bacteria

A growing body of evidence points to CodY, a global regulator in Gram-positive bacteria, as a critical link between microbial physiology and pathogenesis in diverse environments. Recent studies uncovering graded regulation of CodY gene targets reflect the true nature of this transcription factor controlled by ligands and reveal nutrient availability as a potentially critical factor in modulating pathogenesis. This review will serve to update the status of the field and raise new questions to be answered.