Lacritin is a 12.3 kDa
glycoproteinGlycoproteins are proteins that contain oligosaccharide chains covalently attached to their polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
encoded in humans by the
LACRT geneA gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...
. Lacritin is a secreted protein found in
tearTear may refer to:*Tears, a type of eye secretion*Tearing, the ripping apart of something by force*Robert Tear, a Welsh singer-Elements in fiction:*Tear, a character Tear Grants in video game Tales of the Abyss...
s and
salivaSaliva is the watery and usually frothy substance produced in the mouths of humans and most other animals. Saliva is produced in and secreted from the salivary glands...
. Lacritin also promotes tear secretion and
proliferationThe word proliferation literally means the process of "giving life to" something, either an abstract or a concrete. Depending on the context used, proliferation can refer to a number of different topics, such as:*Nuclear proliferation...
of some epithelial cells. Lacritin is thus a prosecretory
mitogenA mitogen is a chemical substance that encourages a cell to commence cell division, triggering mitosis. A mitogen is usually some form of a protein....
.
Most lacritin is produced by the
lacrimal glandThe lacrimal glands are paired almond-shaped glands, one for each eye, that secrete the aqueous layer of the tear film. They are situated in the upper, outer portion of each orbit...
. Some lacritin is produced by the
meibomian glandThe meibomian glands are a special kind of sebaceous glands at the rim of the eyelids, responsible for the supply of sebum, an oily substance that prevents evaporation of the eye's tear film, prevents tear spillage onto the cheek, and makes the closed lids airtight...
, and also by epithelial cells of the
conjunctivaThe conjunctiva is a clear mucous membrane consisting of cells and underlying basement membrane that covers the sclera and lines the inside of the eyelids...
and
corneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43...
. Together these epithelia comprise much of the lacrimal functional unit (LFU).
Lacritin is a 12.3 kDa
glycoproteinGlycoproteins are proteins that contain oligosaccharide chains covalently attached to their polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
encoded in humans by the
LACRT geneA gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...
. Lacritin is a secreted protein found in
tearTear may refer to:*Tears, a type of eye secretion*Tearing, the ripping apart of something by force*Robert Tear, a Welsh singer-Elements in fiction:*Tear, a character Tear Grants in video game Tales of the Abyss...
s and
salivaSaliva is the watery and usually frothy substance produced in the mouths of humans and most other animals. Saliva is produced in and secreted from the salivary glands...
. Lacritin also promotes tear secretion and
proliferationThe word proliferation literally means the process of "giving life to" something, either an abstract or a concrete. Depending on the context used, proliferation can refer to a number of different topics, such as:*Nuclear proliferation...
of some epithelial cells. Lacritin is thus a prosecretory
mitogenA mitogen is a chemical substance that encourages a cell to commence cell division, triggering mitosis. A mitogen is usually some form of a protein....
.
Most lacritin is produced by the
lacrimal glandThe lacrimal glands are paired almond-shaped glands, one for each eye, that secrete the aqueous layer of the tear film. They are situated in the upper, outer portion of each orbit...
. Some lacritin is produced by the
meibomian glandThe meibomian glands are a special kind of sebaceous glands at the rim of the eyelids, responsible for the supply of sebum, an oily substance that prevents evaporation of the eye's tear film, prevents tear spillage onto the cheek, and makes the closed lids airtight...
, and also by epithelial cells of the
conjunctivaThe conjunctiva is a clear mucous membrane consisting of cells and underlying basement membrane that covers the sclera and lines the inside of the eyelids...
and
corneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43...
. 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. Topical lacritin promotes tearing in rabbit preclinical studies.
Lacritin cell targeting is dependent on the cell surface
heparan sulfateHeparan sulfate is a linear polysaccharide found in all animal tissues. It occurs as a proteoglycan in which two or three HS chains are attached in close proximity to cell surface or extracellular matrix proteins...
proteoglycanProteoglycans are glycoproteins that are heavily glycosylated. They have a core protein with one or more covalently attached glycosaminoglycan chain. The chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions, due to the occurrence of sulfate and...
syndecan-1Syndecan 1 is a protein which in humans is encoded by the SDC1 gene.- Function :The protein encoded by this gene is a transmembrane heparan sulfate proteoglycan and is a member of the syndecan proteoglycan family...
(SDC1). Binding utilizes an enzyme-regulated 'off-on' switch in which active epithelial
heparanaseHeparanase, also known as HPSE, is an enzyme that acts both at the cell-surface and within the extracellular matrix to degrade polymeric heparan sulfate molecules into shorter chain length oligosaccharides.-Synthesis and structure:...
(HPSE) cleaves off heparan sulfate to expose a binding site in 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
NFATNuclear factor of activated T-cells is a general name applied to a family of transcription factors shown to be important in immune response. One or more members of the NFAT family is expressed in most cells of the immune system...
and mTOR.
Structure
Lacritin consists of 119 amino acids after cleavage of the N-terminal
signal peptideA signal peptide is a short peptide chain that directs the transport of a protein.Signal peptides may also be called targeting signals, signal sequences, transit peptides, or localization signals....
and displays several predicted alpha helices, mostly in the C-terminal half. At least one appears to be amphipathic with
hydrophobicIn chemistry, hydrophobicity is the physical property of a molecule that is repelled from a mass of water....
and
hydrophilicHydrophile, from the Greek "water" and φιλια "friendship," refers to a physical property of a molecule that can transiently bond with water through hydrogen bonding. This is thermodynamically favorable, and makes these molecules soluble not only in water, but also in other polar solvents...
residues on opposite faces. The hydrophobic face likely forms the
binding siteIn biochemistry, a binding site is a region on a protein, DNA, or RNA to which specific other molecules and ions—in this context collectively called ligands, or more specifically, protein ligands—form a chemical bond....
for syndecan-1. PONDR (Predictor of Naturally Disordered Regions) predicts that the C-terminal and N-terminal halves are respectively '
orderedProtein folding is the physical process by which a polypeptide folds into its characteristic and functional three-dimensional structure from random coil....
' 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. Predicted p
I of lacritin's core protein is 5.
Several lacritin
splice variantsAlternative splicing is a process by which the exons of the RNA produced by transcription of a gene are reconnected in multiple ways during RNA splicing...
have been detected in Aceview, from NEIBank
EST-Places:* Estonia - shorthand for the country of ** Estonian language, the Estonian language in ISO 639.2 or ISO 639-3 language codes* Est , a town in Gelderland.* Est Region , one of Burkina Faso's 13 administrative regions...
data. Lacritin-b (11.1 kDa; p
I 5.3) lacks the sequence SIVEKSILTE. Lacritin-c (10.7 kDa; p
I 4.6) displays a novel C-terminus that should be incapable of binding syndecan-1. Both may be null forms of lacritin.
Function
Lacritin is a
glycoproteinGlycoproteins are proteins that contain oligosaccharide chains covalently attached to their polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
of the human tear film. It is mainly produced by the
lacrimal glandThe lacrimal glands are paired almond-shaped glands, one for each eye, that secrete the aqueous layer of the tear film. They are situated in the upper, outer portion of each orbit...
. Some lacritin also is produced by the
meibomian glandThe meibomian glands are a special kind of sebaceous glands at the rim of the eyelids, responsible for the supply of sebum, an oily substance that prevents evaporation of the eye's tear film, prevents tear spillage onto the cheek, and makes the closed lids airtight...
, and also by epithelial cells of the
conjunctivaThe conjunctiva is a clear mucous membrane consisting of cells and underlying basement membrane that covers the sclera and lines the inside of the eyelids...
and
corneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43...
. The lacritin gene (
LACRT) is one of the most transcriptionally regulated
genesGênes is the name of a département of the First French Empire in present Italy. It was named after the city Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa. It was divided into the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
in the human
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
. Functional studies suggest a role in epithelial renewal of some nongermative epithelia. By flowing downstream through ducts, it may generate a 'proliferative field'. Lacritin also promotes
secretionSecretion is the process of elaborating, releasing, and oozing chemicals from a cell, a secreted chemical substance or amount of substance. In contrast to excretion, the substance may have a certain function, rather than being a waste product....
. 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
blepharitisBlepharitis is an ocular disease characterized by inflammation of the eyelid margins. Blepharitis may cause redness of the eyes, itching and irritation of the eyelids in one or both eyes. Its appearance is often confused with conjunctivitis and due to its recurring nature it is the most common...
.
Lacritin is an LFU prosecretory mitogen with a
biphasicA biphasic system is one which has two phases.* In chemistry, when oil and water are poured into a beaker, they form a biphasic system. See mixtures.* In chronobiology, biphasic sleep refers to a nap or siesta in addition to the usual sleep episode at night...
dose response that is optimal at 1 - 10 nM for human
recombinantRecombinant may refer to :* A recombinant organism - an organism that contains a different combination of alleles from either of its parents. See Recombination.* Recombinant DNA - a form of artificial DNA...
lacritin on human cells. Higher human lacritin concentrations are optimal on rat or mouse cells or on rabbit eyes. Lacritin flows downstream from the
lacrimal glandThe lacrimal glands are paired almond-shaped glands, one for each eye, that secrete the aqueous layer of the tear film. They are situated in the upper, outer portion of each orbit...
through ducts onto the
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
.
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. New studies suggest that the cell surface
proteoglycanProteoglycans are glycoproteins that are heavily glycosylated. They have a core protein with one or more covalently attached glycosaminoglycan chain. The chains are long, linear carbohydrate polymers that are negatively charged under physiological conditions, due to the occurrence of sulfate and...
syndecan-1 is partly responsible. Syndecan-1 binds many growth factors through its
heparan sulfateHeparan sulfate is a linear polysaccharide found in all animal tissues. It occurs as a proteoglycan in which two or three HS chains are attached in close proximity to cell surface or extracellular matrix proteins...
side chains. It also binds lacritin, but interestingly heparan sulfate interferes with lacritin binding. Since syndecans are always decorated with heparan sulfate, this means that
heparanaseHeparanase, also known as HPSE, is an enzyme that acts both at the cell-surface and within the extracellular matrix to degrade polymeric heparan sulfate molecules into shorter chain length oligosaccharides.-Synthesis and structure:...
must be available to partially or completely cleave off heparan sulfate allowing lacritin to bind. Indeed,
siRNASírna Sáeglach , son of Dian, son of Demal, son of Rothechtaid mac Main, was, according to medieval Irish legend and historical tradition, a High King of Ireland...
studies without and with rescue demonstrated that heparanase regulates lacritin function.
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 spectrometryMass spectrometry is an analytical technique for the determination of the elemental composition of a sample or molecule. It is also used for elucidating the chemical structures of molecules, such as peptides and other chemical compounds...
. Few bound. The most prominent was syndecan-1 (SDC1). In confirmatory pull-down assays, binding was not shared with family members
syndecan-2Syndecan-2 is a protein that in humans is encoded by the SDC2 gene.-Interactions:SDC2 has been shown to interact with Laminin, alpha 3, VIL2 and CASK.-Further reading:...
or
syndecan-4Syndecan-4 is a protein that in humans is encoded by the SDC4 gene.-Further reading:...
, 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
heparanaseHeparanase, also known as HPSE, is an enzyme that acts both at the cell-surface and within the extracellular matrix to degrade polymeric heparan sulfate molecules into shorter chain length oligosaccharides.-Synthesis and structure:...
(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. Likely it is largely latent and activation may be local.
Lacritin mitogenic signaling follows two pathways:
- Gαi
Gi alpha subunit is a heterotrimeric G protein subunit that inhibits the production of cAMP from ATP.- Receptors :...
or Gαo → PKCαProtein kinase C alpha , also known as PRKCA, refers to both a human gene and the protein that is encoded by it.-Background:...
/PLCγ21-phosphatidylinositol-4,5-bisphosphate phosphodiesterase gamma-2 is an enzyme that in humans is encoded by the PLCG2 gene.-Interactions:PLCG2 has been shown to interact with PTPN11, LYN, Bruton's tyrosine kinase, SHC1 and GAB2.-Further reading:...
→ Ca2+ → calcineurinCalcineurin is a protein phosphatase also known as protein phosphatase 3, PPP3CA, and formerly known as protein phosphatase 2B . It activates the T cells of the immune system and can be blocked by drugs. Calcineurin activates NFATc , a transcription factor by dephosphorylating it...
→ NFATC1Nuclear factor of activated T-cells, cytoplasmic 1 is a protein that in humans is encoded by the NFATC1 gene.-Further reading:...
- Gαi or Gαo → PKCα/PLCγ2/PLD1
Phospholipase D1 is an enzyme that in humans is encoded by the PLD1 gene.-Interactions:Phospholipase D1 has been shown to interact with RALA, BIN1, Amphiphysin, Alpha-synuclein, PEA15, RHOA, Protein kinase N1 and CDC42.-Further reading:...
→ 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.
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.
Species
Genomic sequencing assembled by
EnsemblEnsembl is a joint scientific project between the European Bioinformatics Institute and the Wellcome Trust Sanger Institute, which was launched in 1999 in response to the imminent completion of the Human Genome Project with scientists in the United States, the international consortium comprised...
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.
Tissue
Tissue distribution has been examined in humans and monkeys. Lacritin is most highly expressed in the
lacrimal glandThe lacrimal glands are paired almond-shaped glands, one for each eye, that secrete the aqueous layer of the tear film. They are situated in the upper, outer portion of each orbit...
, moderately in
salivary glandThe salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into glucose...
s and slightly in
mammaryMammary glands are the organs that, in mammals, produce milk for the sustenance of the young. These exocrine glands are enlarged and modified sweat glands and give mammals their name...
(cancer but not or rarely normal) and
thyroid glandThe thyroid is one of the largest endocrine glands in the body. This gland is found in the neck inferior to the thyroid cartilage and at approximately the same level as the cricoid cartilage...
s.The
salivary glandThe salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into glucose...
expression can be attributed to a discrete group of unidentified ductal-like cells.Some lacritin was reported in lung bronchoalveolar lavage. In lacrimal gland, polarized lacrimal acinar cells appear to be the most prolific lacritin producers, as evidenced by strong staining of secretory granules in keeping with lacritin release after carbachol stimulation.Some lacritin is produced by the
meibomian glandThe meibomian glands are a special kind of sebaceous glands at the rim of the eyelids, responsible for the supply of sebum, an oily substance that prevents evaporation of the eye's tear film, prevents tear spillage onto the cheek, and makes the closed lids airtight...
, and also by epithelial cells of the
conjunctivaThe conjunctiva is a clear mucous membrane consisting of cells and underlying basement membrane that covers the sclera and lines the inside of the eyelids...
and
corneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43...
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
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
the main target.
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.In 'climatic droplet keratopathy' N119 appears to be unglycosylated.Also a normal
breast cancerBreast cancer is a cancer that starts in the breast, usually in the inner lining of the milk ducts or lobules. There are different types of breast cancer, with different stages , aggressiveness, and genetic makeup. With best treatment, 10-year disease-free survival varies from 98% to 10%...
localization reported by some has not been replicated in
Unigene (the '
mammary glandMammary glands are the organs that, in mammals, produce milk for the sustenance of the young. These exocrine glands are enlarged and modified sweat glands and give mammals their name...
' hit is for breast cancer) and gene array studies, but some breast cancers appear to display elevated expression or LACRT gene amplification.