New members of the lipocalin family in human tear fluid

[Dry eye and qualitative tear alteration in essential blepharospasm]

PURPOSE

We aimed to analyze the tears of patients with Benin Essential Blepharospasm (BEB) to explore possible corneo-conjunctival mechanisms for photophobia, tear deficiency and ocular pain.

METHODS

Using an observational cohort of 42 patients diagnosed with BEB, we performed Schirmer's testing, measurement of tear film pH, tear film protein electrophoresis and conjunctival impression cytology.

RESULTS

Schirmer's testing showed dry eye (8.4±9.7mm) with 71.3% eyes having a Schirmer's score<10mm. Surprisingly, the pH was basic (8.4±0.4) and was improved in patients receiving the standard treatment of quarterly botulinum toxin injections (8.32±0.36 for treated patients vs. 8.74±0.53 for untreated patients; P=0.045). Together, tear protein electrophoresis and conjunctival impression cytology showed conjunctival inflammation associated with BEB.

CONCLUSION

For the first time, this study emphasizes corneo-conjunctival mechanisms to explain the photophobia, dry eye and pain in Benign Essential Blepharospasm. While encouraging, complementary studies remain necessary to evaluate the effect of correcting tear film pH and inflammation on the quality of life of blepharospasm patients.

Tear Lipocalin and Lipocalin-Interacting Membrane Receptor

Tear lipocalin is a primate protein that was recognized as a lipocalin from the homology of the primary sequence. The protein is most concentrated in tears and produced by lacrimal glands. Tear lipocalin is also produced in the tongue, pituitary, prostate, and the tracheobronchial tree. Tear lipocalin has been assigned a multitude of functions. The functions of tear lipocalin are inexorably linked to structural characteristics that are often shared by the lipocalin family. These characteristics result in the binding and or transport of a wide range of small hydrophobic molecules. The cavity of tear lipocalin is formed by eight strands (A-H) that are arranged in a β-barrel and are joined by loops between the β-strands. Recently, studies of the solution structure of tear lipocalin have unveiled new structural features such as cation-π interactions, which are extant throughout the lipocalin family. Lipocalin has many unique features that affect ligand specificity. These include a capacious and a flexible cavity with mobile and short overhanging loops. Specific features that confer promiscuity for ligand binding in tear lipocalin will be analyzed. The functions of tear lipocalin include the following: antimicrobial activities, scavenger of toxic and tear disruptive compounds, endonuclease activity, and inhibition of cysteine proteases. In addition, tear lipocalin binds and may modulate lipids in the tears. Such actions support roles as an acceptor for phospholipid transfer protein, heteropolymer formation to alter viscosity, and tear surface interactions. The promiscuous lipid-binding properties of tear lipocalin have created opportunities for its use as a drug carrier. Mutant analogs have been created to bind other molecules such as vascular endothelial growth factor for medicinal use. Tear lipocalin has been touted as a useful biomarker for several diseases including breast cancer, chronic obstructive pulmonary disease, diabetic retinopathy, and keratoconus. The functional possibilities of tear lipocalin dramatically expanded when a putative receptor, lipocalin-interacting membrane receptor was identified. However, opposing studies claim that lipocalin-interacting membrane receptor is not specific for lipocalin. A recent study even suggests a different function for the membrane protein. This controversy will be reviewed in light of gene expression data, which suggest that tear lipocalin has a different tissue distribution than the putative receptor. But the data show lipocalin-interacting membrane receptor is expressed on ocular surface epithelium and that a receptor function here would be rational.

TFOS DEWS II pain and sensation report

Pain associated with mechanical, chemical, and thermal heat stimulation of the ocular surface is mediated by trigeminal ganglion neurons, while cold thermoreceptors detect wetness and reflexly maintain basal tear production and blinking rate. These neurons project into two regions of the trigeminal brain stem nuclear complex: ViVc, activated by changes in the moisture of the ocular surface and VcC1, mediating sensory-discriminative aspects of ocular pain and reflex blinking. ViVc ocular neurons project to brain regions that control lacrimation and spontaneous blinking and to the sensory thalamus. Secretion of the main lacrimal gland is regulated dominantly by autonomic parasympathetic nerves, reflexly activated by eye surface sensory nerves. These also evoke goblet cell secretion through unidentified efferent fibers. Neural pathways involved in the regulation of meibomian gland secretion or mucin release have not been identified. In dry eye disease, reduced tear secretion leads to inflammation and peripheral nerve damage. Inflammation causes sensitization of polymodal and mechano-nociceptor nerve endings and an abnormal increase in cold thermoreceptor activity, altogether evoking dryness sensations and pain. Long-term inflammation and nerve injury alter gene expression of ion channels and receptors at terminals and cell bodies of trigeminal ganglion and brainstem neurons, changing their excitability, connectivity and impulse firing. Perpetuation of molecular, structural and functional disturbances in ocular sensory pathways ultimately leads to dysestesias and neuropathic pain referred to the eye surface. Pain can be assessed with a variety of questionaires while the status of corneal nerves is evaluated with esthesiometry and with in vivo confocal microscopy.

Bioengineered Lacrimal Gland Organ Regeneration in Vivo

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

Human tear lipocalin

Human tear prealbumin, now called tear lipocalin, was originally described as a major protein of human tear fluid, which was thought to be tear specific. However, recent investigations demonstrated that it is identical with lingual von Ebner's gland protein, and is also produced in prostate, nasal mucosa and tracheal mucosa. Homologous proteins have been found in rat, pig and probably dog and horse. Tear lipocalin is an unusual lipocalin member, because of its high promiscuity for relative insoluble lipids and binding characteristics that differ from other members. In addition, it shows inhibitory activity on cysteine proteinases similar to cystatins, a feature unique among lipocalins. Although it acts as the principal lipid binding protein in tear fluid, a more general physiological function has to be proposed due to its wide distribution and properties. It would be ideally suited for scavenging of lipophilic, potentially harmful substances and thus might act as a general protection factor of epithelia.

The influence of tear proteins on the film stability of rabbit tear extracts

This study was undertaken to gain an understanding of the significance of tear proteins in stabilizing the tear film. Either a sigma agonist, N,N-dimethyl-2-phenylethylamine HCl (AF2975), or a sigma antagonist, haloperidol, was administered to rabbit eyes in order to increase or decrease protein secretion, respectively. At 0, 10 and 60 minutes after instillation, tear proteins were extracted from Schirmer strips and measured for total protein. A portion of the extract was used for separating five major protein fractions using size-exclusion HPLC. Total protein extract or individual protein fractions were measured for surface tension by the horizontal capillary method and for in vitro break up time (in vitro BUT), a newly designed procedure. A statistically significant decrease was measured for surface tension and a concomitant increase was measured for in vitro BUT for the total protein samples at 10 and 60 minutes after instillation of AF2975 compared to the vehicle treated eye. The results for haloperidol yielded an increase in surface tension and an decrease in in vitro BUT. When the tear proteins were separated into five major fractions, only the 23 minute protein fraction was found to decrease surface tension and increase in vitro BUT following AF2975 administration. Haloperidol, a sigma antagonist, showed an exact opposite effect for the total protein and the 23 minute protein fraction.

Establishment of the human reflex tear two-dimensional polyacrylamide gel electrophoresis reference map: new proteins of potential diagnostic value

To understand the changes in protein expression associated with various physiological states as well as the development of pathological eye disease, we have begun to map the protein components of normal human reflex tears. An analytical reference map of normal human reflex tears was created using two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) with pH 3.5-10 immobilized pH gradients (IPGs). Micropreparatively loaded gels were transferred to polyvinylidene difluoride (PVDF) and analysed by a combination of N-terminal sequence tagging and amino acid compositional analysis. Thirty spots were sequence tagged, resulting in identification of six different proteins (lipocalin, lysozyme, lactotransferrin, zinc-alpha-2 glycoprotein, cystatin S, cystatin SN) that matched to entries in the SWISS-PROT database. A group of N-terminally blocked proteins was clearly identified from SWISS-PROT by amino acid analysis, isoelectric point (pI) and molecular weight (Mr). A number of highly expressed protein components remain unidentified despite being subjected to amino acid analysis and Edman sequencing. A majority of the abundant proteins showed varying degrees of charge heterogeneity attributed to post-translational processing such as glycosylation and N-terminal truncation. We have identified a previously undescribed protein that we have named lacryglobin. This protein displays strong homology with mammaglobin, a protein overexpressed in breast cancer. The discovery of this homologue in tears offers the potential for disease diagnosis by screening tear fluid proteins.

The salivary lipocalin von Ebner's gland protein is a cysteine proteinase inhibitor

The lipocalins make up a heterogeneous superfamily of proteins. Although showing almost no sequence homology, they share very similar secondary and tertiary structures. Their ability to bind hydrophobic ligands is well established, but the physiological function of most lipocalins remains unclear. The lipocalin from the human Von Ebner's Gland of the tongue (VEGh) contains three sequence motifs corresponding with the papain-binding domains of cystatins, a family of naturally occurring cysteine proteinase inhibitors. We found that VEGh inhibited papain activity to a similar extent as salivary cystatin S. Furthermore, synthetic peptides derived from VEGh and cystatin C, comprising these three motifs, inhibited papain, too. We conclude that VEGh is a physiological inhibitor of cysteine proteinases and therefore can play a role in the control of inflammatory processes in oral and ocular tissues.

cDNA cloning of an abundant human lacrimal gland mRNA encoding a novel tear protein

An abundant 1.05 kb human lacrimal gland mRNA has been characterized by cDNA cloning. It encodes a predicted 180 residue, 20546 Da secreted protein, with a charge of +11 at ph 7 and 24.5% proline, designated as Basic Proline-rich Lacrimal Protein (BPLP), Southern blot analysis is consistent with a single BPLP gene. BPLP lacks any distinct repetitive structure, and is unrelated to the salivary proline-rich protein super-family. The pre-proprotein shows modest overall similarity to a superfamily comprising human PRPb, the mouse MSG proteins, and rat VCS-alpha 1, VCS-beta 1 and submandibular apomucin. BPLP also contains a domain with similarity to the Zp2 protein domain found in several otherwise unrelated proteins. Northern blot analysis indicated that the BPLP gene is also expressed at modest levels in the human submandibular gland, and in situ hybridization demonstrated expression of BPLP in the secretory endpieces of the human lacrimal gland. The BPLP cDNA clone defines a new human tear protein, and should provide a useful phenotypic marker of differentiation in in vitro studies of lacrimal gland function.

The human lacrimal gland synthesizes apolipoprotein D mRNA in addition to tear prealbumin mRNA, both species encoding members of the lipocalin superfamily

Apolipoprotein D (apoD), a glycoprotein originally characterized as a component of the high density lipoprotein fraction of human plasma and known to be a member of the lipocalin protein superfamily, has been found in human tear fluid by Western blot analysis. Unlike serum it seems that in the tear fluid apoD exists mainly as a disulphide linked homodimer which is not associated with lecithin/cholesterol acyltransferase (LCAT) or apolipoprotein A-I (apo A-I). By reverse-transcription-PCR (RT-PCR) of mRNA extracted from a human lacrimal gland and use of specific primers we could demonstrate expression of the apoD gene in this tissue. The amplified cDNA was cloned and a subsequent sequence analysis confirmed the identity of apoD mRNA in the human lacrimal gland. These investigations indicate that the lacrimal gland is the site of synthesis of the tear fluid apoD. Although the physiological function of apoD is unknown, it has the ability to bind phospholipids, cholesterol and other small hydrophobic molecules. Therefore, this protein might interact with meibomian lipids present in human tear fluid and probably contribute to the surface spreading of these lipids or it may function as a clearance factor, protecting the cornea from harmful lipophilic molecules.

Tear lipocalins bind a broad array of lipid ligands

To identify the native ligands of tear lipocalins, tear proteins were separated by size exclusion chromatography and the lipid content in the major protein fractions identified. Lipids extracted from native tears and purified tear lipocalins comigrated with fatty acids, fatty alcohols, phospholipids, glycolipids, and cholesterol on thin layer chromatograms. Abundant stearic and palmitic acids as well as cholesterol, and lesser amounts of lauric acid were specifically identified in extracts of purified lipocalins by gas chromatography-mass spectroscopy. A preliminary study of the ligand-protein interaction was carried out using nitroxide spin-labeled lipids.

Assignment of tear lipocalin gene to human chromosome 9q34-9qter

We assigned the gene for tear lipocalin to the long arm of human chromosome 9. Polyadenylated RNA was extracted from lacrimal gland. The coding region for tear lipocalin was amplified, sequenced and used to probe a panel of somatic cell hybrid DNA by Southern blot analysis. Regional mapping was accomplished by probing a panel of subfragments of the indicated chromosome. Restriction of genomic DNA with EcoRI failed to reveal any bands corresponding to the human tear lipocalin gene in mouse-human hybrids all of which lack chromosome 9. Southern blot analysis of human-hamster hybrids demonstrated a human 5.6 kb TaqI restriction fragment that segregated to the q34-qter region of chromosome 9 and assigned the gene for tear lipocalin to this region. Structurally homologous proteins of the lipocalin family, human placental protein 14, human alpha 1 microglobulin, and human brain prostaglandin synthase, have been mapped to this region. We suggest that the gene for tear lipocalin is part of an important lipocalin superfamily gene cluster on chromosome 9 within band q34.