Lacritin

Template:PBB Lacritin is a 12.3 kDa secreted glycoprotein encoded in humans by the LACRT gene.Lacritin is detected in tears and saliva. Although a tear protein, lacritin promotes tear secretion. It also promotes the proliferation of some epithelial cells. Lacritin is thus a prosecretory mitogen.

The greatest source of lacritin is the lacrimal gland. Some lacritin also is produced by the meibomian gland, and also by epithelial cells of the conjunctiva and cornea. Together these epithelia comprise much of the lacrimal functional unit (LFU). Dry eye is the most common disease or group of diseases of the LFU. Preliminary studies with small trials suggest that lacritin may be differentially downregulated in dry eye. Topical lacritin promotes tearing in rabbit preclinical studies.

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 the N-terminal region of syndecan-1's core protein. A G-protein coupled receptor (GPCR) then appears to be ligated. Targeted cells signal to NFAT and mTOR.

LACRT
Identifiers
Aliases	LACRT, entrez:90070, lacritin
External IDs	OMIM: 607360; HomoloGene: 88949; GeneCards: LACRT; OMA:LACRT - orthologs
Gene location (Human) Chr. Chromosome 12 (human) Band 12q13.2 Start 54,630,811 bp End 54,634,895 bp
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in lacrimal gland thymus stromal cell of endometrium mouth salivary gland mammary gland breast skin of abdomen minor salivary glands blood n/a More reference expression data BioGPS n/a
Gene ontology Molecular function laminin-1 binding fibronectin binding growth factor activity collagen binding protein N-terminus binding protein binding Cellular component extracellular region secretory granule extracellular space Biological process positive regulation of peptidyl-tyrosine phosphorylation calcineurin-NFAT signaling cascade positive regulation of release of sequestered calcium ion into cytosol calcium-mediated signaling positive regulation of epithelial cell proliferation protein localization to Golgi apparatus epithelial structure maintenance protein acetylation positive regulation of cell population proliferation positive regulation of epithelial cell proliferation involved in wound healing positive regulation of macroautophagy negative regulation of apoptotic process tear secretion negative regulation of lipopolysaccharide-mediated signaling pathway positive regulation of secretion positive regulation of calcium-mediated signaling defense response to bacterium regulation of signaling receptor activity positive regulation of calcineurin-NFAT signaling cascade 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: 54.63 – 54.63 Mb n/a PubMed search n/a
Wikidata
View/Edit Human

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 hydrophillic residues on opposite faces. The hydrophobic face likely forms the binding site for syndecan-1. PONDR (Predictor of Naturally Disordered Regions) predicts that the C-terminal and N-terminal halves are respectively predicted to be '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 predicted N-glycosylation site. Predicted pI of lacritin's core protein is 5.

Genomic sequencing assembled by Ensembl reveals the existence of putative lacritin orthologues in other species including: 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.

Expression and Splice Variants

Lacritin expression is poorly reflected in the Novartis Gene Atlas view at top right that depicts values at background levels. More representative are Unigene EST data [1], and the GeneSapiens LACRT display[2]. Also representative are RNA dot blot and tissue array data.

Lacritin is largely restricted to epithelial cells producing and bathed in tears, that largely comprise the LFU. The polarized lacrimal acinar cell appears to be the most prolific producer, as evidenced by strong staining of secretory granules in keeping with lacritin release after carbachol stimulation. A secondary expression site is the salivary gland, particularly a discrete group of unidentified ductal-like cells. Minute amounts of lacritin may be expressed in the thyroid gland. Some lacritin was reported in lung bronchoalveolar lavage (Human Proteinpedia: HuPA_00022). 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 [3], but some breast cancers appear to display elevated expression [4] or LACRT gene amplification.

Several lacritin splice variants have been detected. 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.

Receptor and Signaling

Lacritin targets a restricted group of epithelial cells (including human corneal epithelia), and not fibroblastic, glioma or lymphoblastic cells. To understand how, a classical biochemistry approach was used in which biotinylated cell surface proteins that bound lacritin at physiological levels of salt were sequenced by mass spectrometry. 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. 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. Heparanase cleavage of heparan sulfate side chains that also occupy the N-terminus expose a protein core binding site. This novel mechanism appears at first glance to be poor for ocular health. Heparanase release from invading lymphocytes in the corneal stroma is inflammatory. Oddly, heparanase is a normal secretory product of the corneal epithelium. Likely it is largely latent and activation may be local.

Lacritin mitogenic signaling follows two pathways: (i) Galpha_i or Galpha_o/PKCalpha/PLCgamma2/Ca²⁺/calcineurin/NFATC1, and (ii) Galpha_i or Galpha_o/PKCalpha/PLCgamma2/PLD1/mTOR. The upstream Galpha_i or Galpha_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.

Function

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. Higher human lacritin concentrations are optimal on rat or mouse cells or on rabbit eyes.

Further reading

• Green-Church KB, Nichols JJ (2008). "Mass spectrometry-based proteomic analyses of contact lens deposition". Molecular vision. 14: 291–7. PMC 2254969. PMID 18334948.
• Tsai PS, Evans JE, Green KM, Sullivan RM, Schaumberg DA, Richards SM, Dana MR, Sullivan DA (2006). "Proteomic analysis of human meibomian gland secretions". The British journal of ophthalmology. 90 (3): 372–7. doi:10.1136/bjo.2005.080846. PMID 16488965. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
• Zhou L, Beuerman RW, Foo Y, Liu S, Ang LP, Tan DT (2006). "Characterisation of human tear proteins using high-resolution mass spectrometry" (PDF). Annals of the Academy of Medicine, Singapore. 35 (6): 400–7. PMID 16865190. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
• Koo BS, Lee DY, Ha HS, Kim JC, Kim CW (2005). "Comparative analysis of the tear protein expression in blepharitis patients using two-dimensional electrophoresis". Journal of proteome research. 4 (3): 719–24. doi:10.1021/pr0498133. PMID 15952718.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Kumar R, Huebner A, Laurie GW (2002). "Genetic separation of the human lacritin gene ("LACRT") and triple A (Allgrove) syndrome on 12q13". Advances in experimental medicine and biology. 506 (Pt A): 167–74. PMID 12613904.{{cite journal}}: CS1 maint: multiple names: authors list (link)

External links

• LACRT protein, human at the U.S. National Library of Medicine Medical Subject Headings (MeSH)