Elsevier

Experimental Eye Research

Volume 117, December 2013, Pages 39-52
Experimental Eye Research

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

https://doi.org/10.1016/j.exer.2013.05.020Get rights and content

Highlights

  • We review lacritin and tear lipocalin, and update the extracellular tear proteome.

  • We propose that advantage be taken of tear proteins as potential biomarkers.

  • Tear proteins might also serve as drug targets or therapeutics in dry eye.

  • With such an approach, causes of ocular surface diseases may be addressable.

Abstract

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.

Introduction

Tears accumulate on the avascular corneal epithelium, and vascularized conjunctiva, as a translucent film rich in proteins, lipids and metabolites. The tear proteome is estimated to comprise 1543 proteins (Zhou et al., 2012), with over half designated as ‘intracellular’ (Table 1) by Gene Ontology, implying that cell death from normal epithelial renewal may be a contributor. Beyond its capacity to lubricate the lid, tears are essential for the refraction of light (Montés-Micó, 2007). Equally important and irreplaceable by drugs or drops is the role of tears in promoting corneal epithelial health for when tears are chronically insufficient, the epithelium becomes stressed and releases inflammatory cytokines that further exacerbate the situation (Massingale et al., 2009). Dry eye affects 5–6% of the general population, rising to 6–9.8% and as high as 34%, respectively in postmenopausal women (Schaumberg et al., 2003) and the elderly (Lin et al., 2005).

Relatively few tear proteins appear to be selectively down- or upregulated in dry eye (Table 1). Appreciating which are bioactive and at what molar levels would be insightful. The only growth factor-like molecule downregulated in mild to severe aqueous deficiency was lacritin (Srinivasan et al., 2012). Lacritin promotes basal tearing when added topically in rabbits (Samudre et al., 2011). Also decreased was lipocalin-1 (Srinivasan et al., 2012), that cleanses the ocular surface of lipids that would otherwise interfere with ocular surface wetting (Glasgow and Gasymov, 2011). Lacritin was the most severely downregulated protein in contact lens-related dry eye (Nichols and Green-Church, 2009) – perhaps in part because it is readily adsorbed on contact lenses (Green-Church and Nichols, 2008). It is also deficient in blepharitis (Koo et al., 2005), a common inflammation of the eyelid, associated with evaporative dry eye (Mathers, 1993). Two studies did not note any lacritin change in dry eye using mass spectrometry coupled with liquid chromatographic cationic separation followed by reverse phase separation (Zhou et al., 2009, Boehm et al., 2013), although one observed a decrease of lipocalin-1 (Zhou et al., 2009). 2-D SDS PAGE prior to mass spectrometry (Koo et al., 2005, Nichols and Green-Church, 2009, Srinivasan et al., 2012) is necessary to distinguish monomeric lacritin from inactive multimeric lacritin (Velez et al., 2013), and likely also the inactive lacritin-c splice variant (McKown et al., 2009). Exploration of lacritin cell targeting and signaling mechanisms has revealed a network of interdependent molecules, each necessary for lacritin activity. New evidence is suggesting that some of these are also decreased in dry eye. Here we review recent advances in our understanding of lacritin, and provide an overview of lipocalin-1 whose eye specific expression parallels that of lacritin. We also update our current understanding of the tear proteome.

Section snippets

Structure and expression

The discovery of lacritin indirectly emerged from a screen for novel factors capable of promoting tear protein secretion, with cDNA cloning out of a human lacrimal gland library (Sanghi et al., 2001). The lacritin gene, LACRT, is one of the most eye specific (Sanghi et al., 2001) and resides on 12q13, within ∼1.24 Mb of the AAAS gene associated with alacrima (Kumar et al., 2002). Human lacritin is coded by a 417 bp open reading frame that translates as a 14.3 kDa hydrophilic protein with a 19

Structure and expression

Tear lipocalin (LCN1; recently reviewed by Glasgow and Gasymov, 2011, Dartt, 2011) was originally noted as an unknown band in early electrophoretic separations of human tears and named tear pre-albumin (Erickson et al., 1956), based on its paper electrophoretic mobility near serum albumin proximal to the anode. Lack of immunological cross-reactivity in normal human tears suggested that the two were distinct (Josephson and Lockwood, 1964), in keeping with earlier studies by Fleming suggesting

New additions to the tear proteome

We previously assembled all tear proteomic data into a single table, restricting entry to proteins designated as ‘extracellular’ or ‘plasma membrane’ in their primary or alternative location (Laurie et al., 2008). Now updated with 139 new entries from Zhou et al. (2012), the additions supplement tears with proangiogenic, anti-angiogenic, retinal survival, epithelial repair, cysteine protease inhibitor, immunosuppressive, and immunostimulatory activities (Table 1). Thirteen are highlighted below.

Conclusions

Advantage should be taken of tear proteins as potential biomarkers, drug targets, and biotherapeutics. Tear-based biotherapeutics have considerable potential, particularly with the relatively small number of tear proteins that appear to be selectively downregulated in dry eye. Rather than simply alleviating symptoms, causes of ocular surface diseases may be addressable. Lacritin- and lipocalin-1-based therapeutics offer a platform to initiate this approach. Newly identified members of the tear

Notes added in proof

  • 1.

    One tear proteomic article was overlooked. Aluru et al (PLoS One 7, e51979, 2012) compared tears from 73 normals to 129 individuals suffering from aqueous deficient dry eye. 2-D SDS PAGE with mass spectrometry identified seven downregulated proteins: AZGP1, CST_ [cystatin type not specified], IGJ, LACRT, LTF, PRR4 and SCGB2A2. As per Table 1, all except IGJ have been previously noted as downregulated in dry eye. Lacritin was downregulated in 95% of cases.

  • 2.

    Low et al, 2013 (J. Proteomics [Epub

Acknowledgments

GWL is supported by R01 EY013143 and EY018222. RK is supported by SR/FT/LS-157/2012 (RK). The authors acknowledge the multi-institutional Lacritin Consortium for help with much of the lacritin work reviewed, particularly the development of lacritin and syndecan-1 constructs by Ron Raab and Robert McKown at James Madison University, the supply of human tears by Denise Ryan (Walter Reed Army Medical Center), animal studies by Pat Williams' group (Eastern Virginia Medical School), and mechanistic

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    Grant information: NIH RO1EY013143, RO1EY018222 (GWL); SR/FT/LS-157/2012 RK.

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