Leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5) also known as G-protein coupled receptor 49 (GPR49) or G-protein coupled receptor 67 (GPR67) is a
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
that in humans is encoded by the LGR5
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
.
It is a member of
GPCR class A receptor proteins. R-spondin proteins are the biological ligands of LGR5. LGR5 is expressed across a diverse range of tissue such as in the muscle,
placenta
The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mater ...
, spinal cord and brain and particularly as a
biomarker
In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, ...
of adult
stem cell
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
s in certain tissues.
Gene
Prior to its current naming designation, LGR5 was also known as FEX, HG38, GPR49, and GPR67.
The Human LGR5 gene is 144,810
bases long and located at
chromosome 12
Chromosome 12 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 12 spans about 133 million base pairs (the building material of DNA) and represents between 4 and 4.5 percent of the to ...
at position 12q22-q23.
Both human, rat and mouse homologs contain 907
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s and seven
transmembrane domain
A transmembrane domain (TMD) is a membrane-spanning protein domain. TMDs generally adopt an alpha helix topological conformation, although some TMDs such as those in porins can adopt a different conformation. Because the interior of the lipid bil ...
s.
After
translation
Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
, the
signal peptide
A signal peptide (sometimes referred to as signal sequence, targeting signal, localization signal, localization sequence, transit peptide, leader sequence or leader peptide) is a short peptide (usually 16-30 amino acids long) present at the N-ter ...
(amino acids 1-21) is cleaved off and the mature
peptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A ...
(amino acids 22-907) inserts its transmembrane domain into the
translocon The translocon (also known as a translocator or translocation channel) is a complex of proteins associated with the translocation of polypeptides across membranes. In eukaryotes the term translocon most commonly refers to the complex that transport ...
membrane prior to packaging towards the plasma membrane.
Protein structure
LGR5 is highly conserved within the
mammalian clade.
Sequence analyses showed that the transmembrane regions and
cysteine
Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile.
When present as a deprotonated catalytic residue, sometime ...
-flanked junction between TM1 and the
extracellular domain
An ectodomain is the domain of a membrane protein that extends into the extracellular space (the space outside a cell). Ectodomains are usually the parts of proteins that initiate contact with surfaces, which leads to signal transduction.A notable ...
were highly conserved in sea anemone (''
Anthopleura elegantissima
The aggregating anemone (''Anthopleura elegantissima''), or clonal anemone, is the most abundant species of sea anemone found on rocky, tide swept shores along the Pacific coast of North America. This cnidarian hosts endosymbiotic algae called ...
''), fly (''
Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...
''), worm (''
Caenorhabditis elegans
''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (ro ...
''), snail (''
Lymnaea stagnalis
''Lymnaea stagnalis'', better known as the great pond snail, is a species of large air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae. The great pond snail is a model organism to study parasitology, ...
''), rat (''
Rattus rattus
The black rat (''Rattus rattus''), also known as the roof rat, ship rat, or house rat, is a common long-tailed rodent of the stereotypical rat genus ''Rattus'', in the subfamily Murinae. It likely originated in the Indian subcontinent, but is n ...
'') and human (''
Homo sapiens
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
'').
Homology amongst the metazoan suggests that it has been conserved across animals and was hypothesised as a chimeric fusion of an ancestral GPCR and a leucine-rich repeat motif.
Sheau Hsu, Shan Liang and Aaron Hsueh first identified LGR5, together with LGR4, in 1998 at the University Medical School Stanford, California using
expression sequence tags based on putative
glycoprotein
Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...
hormone receptors in ''Drosophila''.
Experimental evidence show that the mature receptor protein contains up to 17 leucine-rich repeats, each composed of 24 amino acids spanning the extracellular domain flanked by the cysteine-rich
N-terminal
The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
and
C-terminal
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
regions. In contrast, other glycoprotein hormone receptors such as
Luteinizing hormone
Luteinizing hormone (LH, also known as luteinising hormone, lutropin and sometimes lutrophin) is a hormone produced by gonadotropic cells in the anterior pituitary gland. The production of LH is regulated by gonadotropin-releasing hormone (GnRH) ...
,
Follicle-stimulating hormone
Follicle-stimulating hormone (FSH) is a gonadotropin, a glycoprotein polypeptide hormone. FSH is synthesized and secreted by the gonadotropic cells of the anterior pituitary gland and regulates the development, growth, pubertal maturation, an ...
and
Thyroid-stimulating hormone
Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, or abbreviated TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3) which stimulates the metabolism of ...
contain only 9 repeats.
Sequence alignment showed that the second
N-glycosylation
''N''-linked glycosylation, is the attachment of an oligosaccharide, a carbohydrate consisting of several sugar molecules, sometimes also referred to as glycan, to a nitrogen atom (the amide nitrogen of an asparagine (Asn) residue of a protein), ...
site in LGR5 (
Asn 208) aligns with that on the sixth repeat of
gonadotropin
Gonadotropins are glycoprotein hormones secreted by gonadotropic cells of the anterior pituitary of vertebrates. This family includes the mammalian hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH), the placental/ chorioni ...
and TSH receptors. The cysteine residues flanking the ectodomain form stabilising
disulfide bond
In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...
s that support the secondary structure of the leucine-rich repeats.
Function
LGR5 is a member of the
Wnt signaling pathway
The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling p ...
. Although its ligand remains elusive, it has been shown that costimulation with
R-spondin 1 and
Wnt-3a induce increased internalization of LGR5. LGR5 also cointernalizes with
LRP6
Low-density lipoprotein receptor-related protein 6 is a protein that in humans is encoded by the ''LRP6'' gene. LRP6 is a key component of the LRP5/LRP6/Frizzled co-receptor group that is involved in canonical Wnt pathway.
Structure
LRP6 is a tr ...
and
FZD5
Frizzled-5 (Fz-5) is a protein that in humans is encoded by the ''FZD5'' gene.
Members of the 'frizzled
Frizzled is a family of atypical G protein-coupled receptors that serve as receptors in the Wnt signaling pathway and other signaling pathwa ...
via a
clathrin
Clathrin is a protein that plays a major role in the formation of coated vesicles. Clathrin was first isolated and named by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. Whe ...
-dependent pathway to form a ternary complex upon Wnt ligand binding. Moreover, the rapid cointernalization of LRP6 by LGR5 induces faster rates of degradation for the former. It has been shown that the
C-terminal
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
region of LGR5 is crucial for both dynamic internalization and degradation to occur, although C-terminal truncation does not inhibit LRP6 interaction and internalization, but rather, heightens receptor activity. Thus, only the initial interaction with its unknown ligand and other membrane bound receptors is crucial in its role in Wnt signalling and not the internalization itself.
LGR5 is crucial during embryogenesis as LGR null studies in mice incurred 100% neonatal mortality accompanied by several craniofacial distortions such as ankyloglossia and gastrointestinal dilation.
Ligand
LGR5 belongs to a class of class A GPCR
orphan receptor
In biochemistry, an orphan receptor is a protein that has a similar structure to other identified receptors but whose endogenous ligand has not yet been identified. If a ligand for an orphan receptor is later discovered, the receptor is referred t ...
s. Thus its ligands remain elusive. However, it has been shown that Lgr2, the fly orthologue of mammalian LGR5, binds with "high affinity and specificity" with
bursicon Bursicon (from the Greek ''bursikos'', pertaining to tanning) is an insect hormone which mediates tanning in the cuticle of adult flies.
Structure
The molecular structure of the hormone has been characterized rather recently. Bursicon is a 30 kDa n ...
, an insect heterodimeric, neurohormone that belongs in the same class as FSH, LH and TSH, which in turn are homologous to mammalian bone morphogenetic factors (BMPs) such as
gremlin
A gremlin is a mischievous folkloric creature invented at the beginning of the 20th century to originally explain malfunctions in aircraft and later in other machinery and processes and their operators. Depictions of these creatures vary widely ...
and
cerberus
In Greek mythology, Cerberus (; grc-gre, Κέρβερος ''Kérberos'' ), often referred to as the hound of Hades, is a multi-headed dog that guards the gates of the Underworld to prevent the dead from leaving. He was the offspring of the mo ...
. Therefore, LGR5 might be a receptor for a member of the large family of
bone morphogenetic protein
Bone morphogenetic proteins (BMPs) are a group of growth factors also known as cytokines and as metabologens. Originally discovered by their ability to induce the formation of bone and cartilage, BMPs are now considered to constitute a group of piv ...
antagonists.
Moreover, R-spondin proteins were shown to interact with the extracellular domain of LRG5.
The LGR5 / R-spondin complex acts by binding and subsequently internalizing
RNF43
Ring finger protein 43 is a protein that in humans is encoded by the RNF43 gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''bir ...
and
ZNRF3. RNF43 and ZNRF3 are transmembrane E3 ligases that negatively regulate wnt signaling by ubiquitinating frizzled receptors. Thereby, R-spondin binding to LGR5 potentiates wnt signaling.
Clinical relevance
LGR5 are well-established stem cell markers in certain types of tissue, wholly due to the fact that they are highly enriched in truly,
multipotent stem cells Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta.
According to Latin terms, Pluripotentia means the ability for many thin ...
compared to their immediate progeny, the transit-amplifying cells.
Intestines
Tracing
Tracing may refer to:
Computer graphics
* Image tracing, digital image processing to convert raster graphics into vector graphics
* Path tracing, a method of rendering images of three-dimensional scenes such that the global illumination is faithf ...
has revealed that LGR5 is a marker of adult intestinal stem cells. The high turnover rate of the intestinal lining is due to a dedicated population of stem cells found at the base of the intestinal crypt. In the small intestines, these LGR5
+ve crypt base columnar cells (CBC cells) have broad basal surfaces and very little cytoplasm and organelles and are located interspersed among the terminally differentiated Paneth cells.
These CBC cells generate the plethora of functional cells in the intestinal tissue: Paneth cells, enteroendocrine cells, goblet cells, tuft cells, columnar cells and the M cells over an adult's entire lifetime. Similarly, LGR5 expression in the colon resembles faithfully that of the small intestine.
Liver
The normal liver has very low expression of LGR5, but when this organ is damaged a LGR5-positive compartment emerges that is instrumental in hepatic
regeneration
Regeneration may refer to:
Science and technology
* Regeneration (biology), the ability to recreate lost or damaged cells, tissues, organs and limbs
* Regeneration (ecology), the ability of ecosystems to regenerate biomass, using photosynthesis
...
,
probably as a consequence of increased
Wnt-signaling
.The liver cancer process appears dependent on LGR5 expressing cancer stem cells. Human
hepatocellular carcinoma
Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer in adults and is currently the most common cause of death in people with cirrhosis. HCC is the third leading cause of cancer-related deaths worldwide.
It occurs in t ...
and also murine liver cancer is characterised by the presentce of a LGR5-positive compartment not present in healthy liver. These LGR5 expressing are superior in initiating organoids and forming tumors in experimental liver cancer, while LGR5 lineage ablation significantly inhibits organoid initiation and tumor growth.
Kidney
In vivo lineage tracing showed that LGR5 is expressed in nascent
nephron
The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure ca ...
cell cluster within the developing
kidney
The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...
. Specifically, the LGR5
+ve stem cells contribute into the formation of the thick ascending limb of
Henle's loop and the
distal convoluted tubule
The distal convoluted tubule (DCT) is a portion of kidney nephron between the loop of Henle and the collecting tubule.
Physiology
It is partly responsible for the regulation of potassium, sodium, calcium, and pH.
On its apical surface (lumen ...
. However, expression is eventually truncated after postnatal day 7, a stark contrast to the facultative expression of LGR5 in actively renewing tissues such as in the intestines.
Stomach
The
stomach
The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...
lining also possess populations of LGR5
+ve stem cells, although there are two conflicting theories: one is that LGR5
+ve stem cells reside in the isthmus, the region between the pit cells and gland cells, where most cellular proliferation takes place. However, lineage tracing had revealed LGR5
+ve stem cells at the bottom of the gland,
architecture reminiscent to that of the intestinal arrangement. This suggests that LGR5 stem cells give rise to transit-amplifying cells, which migrate towards the isthmus where they proliferate and maintain the stomach
epithelium
Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
.
Ear
LGR5
+ve stem cells were pinpointed as the precursor for sensory hair cells that line the
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
.
Hair follicle
Hair follicle
The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...
renewal is governed by Wnt signalling that act upon hair follicle stem cells located in the follicle bulge. Although these cells are well characterised by CD34 and cytokeratin markers, there is a growing body of agreement that LGR5 is a putative hair follicle stem cell marker.
LGR5 in conjunction with LRG6, is expressed in a remarkable fashion: LRG6
+ve stem cells maintain the upper sebaceous gland whilst LRG5
+ve stem cells fuel the actual hair follicle shaft upon migration of transit-amplifying cells into the dermal papilla. In between these two distinct populations of stem cells are the multipotent LRG5/6
+ve stem cells that ultimately maintain the epidermal hair follicle in adults.
Cancer
The
cancer stem cell hypothesis states that a dedicated small population of cancerous stem cells
that manages to evade anti-cancer therapy maintains benign and malignant tumours. This explains recurring malignancies even after surgical removal of the
tumour
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
s.
LGR5
+ve stem cells were identified to fuel stem cell activity in murine intestinal adenomas via erroneous activation of the pro-cell cycle Wnt signalling pathway as a result of successive mutations, such as formation of adenoma via
Adenomatous polyposis coli
Adenomatous polyposis coli (APC) also known as deleted in polyposis 2.5 (DP2.5) is a protein that in humans is encoded by the ''APC'' gene. The APC protein is a negative regulator that controls beta-catenin concentrations and interacts with E-c ...
(APC) mutation.
Studies on LGR5 in colorectal cancer revealed a rather perplexing mechanism: loss of LGR5 actually increased tumourigenicity and invasion whereas overexpression results a reduction in tumourigenicity and clonogenicity. This implies that LGR5 is not an
oncogene
An oncogene is a gene that has the potential to cause cancer. In tumor cells, these genes are often mutated, or expressed at high levels. but a
tumor suppressor gene
A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results in a loss or red ...
, and that its main role is delimiting stem cell expansion in their respective niches.
Varying expression profile of LGR5 was also observed in different stages of gastrointestinal cancers, which suggests that the histoanatomical distribution of LGR5
+ve stem cells determine how the cancer advances.
Densitometry results of LGR5 expression by western blotting in the different cell lines showed that high LGR5 expression levels were apparent in BHK, AGS, VERO and NIH3T3 cell lines compared with the other cell lines.
References
Further reading
*
*
{{G protein-coupled receptors
G protein-coupled receptors