Lacritin

Lacritin
Identifiers
Symbols	LACRT; MGC71934
External IDs	OMIM: 607360 HomoloGene: 88949 GeneCards: LACRT Gene
Gene Ontology Molecular function • glycoprotein binding • fibronectin binding • collagen binding • growth factor activity • laminin-1 binding • protein N-terminus binding Cellular component • extracellular region • stored secretory granule Biological process • positive regulation of cell proliferation • calcium-mediated signaling • calcineurin-NFAT signaling pathway • protein localization in Golgi apparatus • positive regulation of epithelial cell proliferation • positive regulation of peptidyl-tyrosine phosphorylation • positive regulation of secretion • positive regulation of release of sequestered calcium ion into cytosol • positive regulation of NFAT protein import into nucleus • tear secretion Sources: Amigo / QuickGO
Orthologs
Species	Human	Mouse
Entrez	90070	n/a
Ensembl	ENSG00000135413	n/a
UniProt	Q9GZZ8	n/a
RefSeq (mRNA)	NM_033277	n/a
RefSeq (protein)	NP_150593	n/a
Location (UCSC)	Chr 12: 55.02 – 55.03 Mb	n/a
PubMed search	[1]	n/a

Lacritin is a 12.3 kDa glycoprotein encoded in humans by the LACRT gene.^[1][2] Lacritin is a secreted protein found in tears and saliva. Lacritin also promotes tear secretion and proliferation of some epithelial cells.^[2] Lacritin is thus a prosecretory mitogen.

Most lacritin is produced by the lacrimal gland.^[2] Some lacritin is produced by the meibomian gland, and also by epithelial cells of the conjunctiva and cornea.^[3] Together these epithelia comprise much of the lacrimal functional unit (LFU). Dry eye is the most common disease of the LFU. Preliminary studies with small trials suggest that lacritin may be differentially downregulated in dry eye,^[4] including contact lens-related dry eye.^[5] Topical lacritin promotes tearing in rabbit preclinical studies.^[6]

Lacritin cell targeting is dependent on the cell surface heparan sulfate proteoglycan syndecan-1 (SDC1). Binding utilizes an enzyme-regulated 'off-on' switch in which active epithelial heparanase (HPSE) cleaves off heparan sulfate to expose a binding site in the N-terminal region of syndecan-1's core protein.^[7] A G-protein-coupled receptor (GPCR) then appears to be ligated. Targeted cells signal to NFAT and mTOR.^[8]

Lacritin
Abbreviation	LACRT
Sources	lacrimal, meibomian and salivary glands; corneal and conjunctival epithelia
Targets	epithelial specificity
Receptors	syndecan-1 (SDC1 but not SDC2 or SDC4) as coreceptor; requires active heparanase (HPSE) to expose a protein core binding site; Signaling receptor: a GPCR is suspected (lacritin mitogenic signaling is pertussis toxin inhibitable); Site of Binding: between N-terminus of SDC1 and C-terminus of lacritin
preclinical studies suggest utility in increasing ocular tearing

Contents 1 Structure 2 Function 3 Signaling 4 Distribution 4.1 Species 4.2 Tissue 5 References 6 Further reading 7 External links

Structure

Lacritin consists of 119 amino acids after cleavage of the N-terminal signal peptide and displays several predicted alpha helices, mostly in the C-terminal half. At least one appears to be amphipathic with hydrophobic and hydrophilic residues on opposite faces. The hydrophobic face likely forms the binding site for syndecan-1. PONDR (Predictor of Naturally Disordered Regions)^[9] predicts that the C-terminal and N-terminal halves are respectively 'ordered' and 'disordered'. 11 - 12 predicted O-glycosylation sites populate the N-terminal half. One putative amphipathic alpha helix near the C-terminus may be responsible for binding the N-terminus of syndecan-1, and is the site of lacritin's only N-glycosylation site. In 'climatic droplet keratopathy' this site appears to be unglycosylated.^[10] Predicted pI of lacritin's core protein is 5.^[4]

Several lacritin splice variants have been detected in Aceview,^[11] from NEIBank EST data.^[12] Lacritin-b (11.1 kDa; pI 5.3) lacks the sequence SIVEKSILTE. Lacritin-c (10.7 kDa; pI 4.6) displays a novel C-terminus that should be incapable of binding syndecan-1. Both may be null forms of lacritin.^[4]

Function

Lacritin is a glycoprotein of the human tear film. It is mainly produced by the lacrimal gland.^[2] Some lacritin also is produced by the meibomian gland, and also by epithelial cells of the conjunctiva and cornea.^[3] The lacritin gene (LACRT) is one of the most transcriptionally regulated genes in the human eye.^[13] Functional studies suggest a role in epithelial renewal of some nongermative epithelia. By flowing downstream through ducts, it may generate a 'proliferative field'.^[8] Lacritin also promotes secretion.^[2] This raises the possibility that lacritin may have clinical applications in the treatment of dry eye, the most common eye disease. Recent studies suggest that lacritin is differentially downregulated in blepharitis.^[14]

Lacritin is an LFU prosecretory mitogen with a biphasic dose response that is optimal at 1 - 10 nM for human recombinant lacritin on human cells.^[8] Higher human lacritin concentrations are optimal on rat or mouse cells^[2] or on rabbit eyes. Lacritin flows downstream from the lacrimal gland through ducts onto the eye.

Signaling

One particularly remarkable feature about lacritin is its cell targeting specificity. Lacritin targets a restricted group of epithelial cells (including human corneal epithelia), and not fibroblastic, glioma or lymphoblastic cells.^[8] New studies suggest that the cell surface proteoglycan syndecan-1 is partly responsible. Syndecan-1 binds many growth factors through its heparan sulfate side-chains. It also binds lacritin, but heparan sulfate interferes with lacritin binding. Since syndecans are always decorated with heparan sulfate, this means that heparanase must be available to partially or completely cleave off heparan sulfate, allowing lacritin to bind. Indeed, siRNA studies without and with rescue demonstrated that heparanase regulates lacritin function.^[7]

Biotinylated cell surface proteins from a lacritin-responsive cell were incubated with lacritin under conditions of physiological salt. Those that bound lacritin were sequenced by mass spectrometry. Few bound. The most prominent was syndecan-1 (SDC1). In confirmatory pull-down assays, binding was not shared with family members syndecan-2 or syndecan-4, indicating that the protein core (and not the negatively charged heparan sulfate side-chains) was the main site of binding. Further analysis narrowed the site to syndecan-1's N-terminal 51 amino acids.^[7] Syndecan-1 is widely expressed on epithelial and other cell types. How is this mechanism cell-specific? The answer appears to be the restricted availability of active epithelial heparanase (HPSE) that serves as an 'off-on' switch for lacritin binding. This novel mechanism appears at first glance to be poor for ocular health, since heparanase release from invading lymphocytes in the corneal stroma is inflammatory. Yet heparanase is a normal secretory product of the corneal epithelium.^[15] Likely it is largely latent and activation may be local.

Lacritin mitogenic signaling^[8] follows two pathways:

• Gα_i or Gα_o → PKCα/PLCγ2 → Ca²⁺ → calcineurin → NFATC1
• Gα_i or Gα_o → PKCα/PLCγ2/PLD1 → mTOR

Rapid dephosphorylation of PKCα causes it to transiently move from the cytoplasm to the area of the Golgi apparatus and peripheral nucleus. Here, it forms a complex with PKCα and PLCγ2 from which downstream mTOR and NFAT signaling is initiated.^[8]

The upstream Gα_i or Gα_o signaling suggests the involvement of a G-protein-coupled receptor (GPCR). A candidate GPCR is under study. Syndecan-1 likely serves as a co-receptor. Binding lacritin may improve its GPCR affinity.

Distribution

Species

Genomic sequencing assembled by Ensembl reveals the existence of putative lacritin orthologues in other species including:^[16] Dasypus novemcinctus (nine-banded armadillo), Echinops telfairi (lesser hedgehog), Felis silvestris catus (house cat), Macaca mulatta (rhesus macaque or monkey), Myotis lucifugus (little brown bat), Pan troglodytes (chimpanzee), Sorex araneus (common shrew), and Tupaia belangeri (northern tree shrew). PONDR and O-glycosylation patterns are similar.^[4]

Tissue

Tissue distribution has been examined in humans and monkeys. Lacritin is most highly expressed in the lacrimal gland, moderately in salivary glands and slightly in mammary (cancer but not or rarely normal), and thyroid glands.^{[2][13][17][18]} The salivary gland expression can be attributed to a discrete group of unidentified ductal-like cells.^[2] Some lacritin was reported in lung bronchoalveolar lavage.^[19] In lacrimal gland, polarized lacrimal acinar cells appear to be the most prolific lacritin producers, as evidenced by strong staining of secretory granules^[2] in keeping with lacritin release after carbachol stimulation.^[3] Carbachol-dependent release involves PKC and calcium signaling.^[20] Some lacritin is produced by the meibomian gland, and also by epithelial cells of the conjunctiva and cornea ^[3] that together with lacrimal gland comprise much of the lacrimal functional unit (LFU). Viewed collectively, the LFU is the primary source of lacritin in the body, and the eye the main target.^[2]

Some changes have been noted in disease. Dry eye is the most common eye disease. Preliminary studies with small trials suggest that lacritin may be differentially downregulated in dry eye.^[4] In climatic droplet keratopathy, N119 appears to be unglycosylated.^[10] Also a normal breast cancer localization reported by some has not been replicated in Unigene (the 'mammary gland' hit is for breast cancer) and gene array studies,^[18] but some breast cancers appear to display elevated expression^[18] or LACRT gene amplification.^[21]

References

Further reading

External links

• MeSH LACRT protein, human