The power of tears: how tear proteomics research could revolutionize the clinic

Abstract

A thin layer of tears covers the ocular surface. The tears are a complex fluid comprised of secretions from a number of sources including the lacrimal gland, goblet cells, cornea, and vascular sources. The tear film provides an optically smooth surface for focusing light onto the retina as well as lubrication, prevention of dehydration of the mucosal surface, protection against pathogens, and nutrition for the underlying corneal and conjunctiva epithelial cells [1]. The tears contain thousands of molecules including proteins/peptides, lipids, electrolytes, and small molecule metabolites secreted from the main and accessory lacrimal glands, meibomian glands, goblet cells, and ocular surface epithelial cells [1]. The updated three-layer model of tear film consists of an outer lipid layer (outer nonpolar lipid sub-layer and inner polar lipid sub-layer with intercalated proteins), a middle aqueous layer, and an inner mucin layer with molecular interactions with the membranes of the corneal and conjunctival epithelial cells. In healthy eyes, the thickness of the tear film is around 5 µm [2] and the volume is about 5-10 µl [1].Despite the small volume for sampling, tear fluid offers several advantages for biochemical analysis. Tears are usually collected noninvasively using Schirmer's strips, other absorbent materials, or fire-polished microcapillary tubes. However, care must be taken not to activate the corneal nerves and induce reflex tears as there is a dramatic difference in the tear protein profile between the two, with reflex tending to be more dilute than basal tears [1]. Tears can be easily obtained from healthy subjects, which is often required as a control group for 'Omics' studies, whereas other ocular fluids (i.e. aqueous humor and vitreous) are not feasible for collection from healthy people. The biggest advantage is that tears are close to the disease site (for instance, ocular surface disease, lacrimal gland disease, etc.) as compared to detecting cancer biomarkers in blood, where biomarker molecules are highly diluted. The total protein concentration of human tears ranges from 6 to 11 mg/ml. Lysozyme, the most abundant tear protein, is approximately 1-mg/ml. Unlike plasma, depletion of abundant tear proteins such as albumin is not necessary and good quality tear proteome coverage can be obtained. This simplifies sample preparation without worrying about the possible loss of tear proteins during the depletion process.Over the past decade, advances in mass spectrometry (MS) and proteomics/metabolomics/lipidomics/glycomics technology CONTACT Lei Zhou

Evaluation of lacrimal production, osmolarity, crystallization, proteomic profile, and biochemistry of capuchin monkeys’ tear film

“…The study of tear film and its components is important due to its ability to determine ocular surface health and to exhibit changes in response to several diseases. 1,17 Nonhuman primates have been used as experimental models in ocular surface dynamic studies. 7,9 This study characterized the tear film of a primate species that had not been previously evaluated for use as a possible experimental model for these tests.…”

“…It is well-known that the tear film is less complex than other body fluids, but the existence of a direct proportion between the concentration of components in the serum and in the tear film is yet unknown. 17 The concentrations of the components herein evaluated in serum samples can be considered as normal results when comparing to the previously described reference values for postprandially collected samples of CM 51,52 and humans, [53][54][55] including the parameters described by biochemistry assay. The division of the values found in tear film by those found in serum could achieved a normalization of these results, and when this proportions in both species were compared, they showed a very marked similarity, with the exception of total cholesterol.…”

“…41 One of the biggest difficulties of the study on the biochemical profile of tear films is the small volume of sample that can be obtained in each collect, even more when considering the several measures to be taken in account to avoid contamination and discomfort. 17 It must be cited that the collect of great volumes can result in exfoliative lesions of the eye surface and consequent contamination of the samples. As described by Posa et al, 42 the use of a pooled tear film sample can be an instrument for the biochemical characterization of tear films, as it has been doing for proteomic and glycomic analysis; in this way, we chose to use pooled samples for the…”

Lab on the eye: A review of tear-based wearable devices for medical use and health management

“…Since tear fluid contains thousands of proteins -the most abundant of which are lysozyme, lactoferrin, and albumin -noninvasive tear monitoring could also be used to detect protein biomarkers correlated with various diseases. Proteomic analysis of tear samples may be one approach to identify biomarkers linked to ocular diseases (21)(22)(23)(24)(25). However, the correlation between the concentration in tears and blood and its relevance to disease progression must be extensively validated before using novel tear analytes.…”

Personalized Proteomics for Precision Health: Identifying Biomarkers of Vitreoretinal Disease

“…Absorbent materials (e.g., Schirmer's strips) and microcapillary tubes can be used to sample tear fluid noninvasively (5–10 μL on average) from patients. 96 Despite the small sample volume, proteomic analysis has identified close to 2000 unique proteins in human tear fluid and numerous studies identified biomarkers for ocular surface and lacrimal gland diseases. 97 We anticipate that advances in small-volume proteomics and safe surgical acquisition of ocular fluids from different anatomical sites will give new insight into the pathophysiology and treatment of eye disease.…”