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TRPV6 is a membrane calcium (Ca2+) channel protein which is particularly involved in the first step in Ca2+absorption in the
intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans an ...
.


Classification

Transient Receptor Potential Vanilloid subfamily member 6 (TRPV6) is an epithelial Ca2+ channel that belongs to the transient receptor potential family (TRP) of proteins. The TRP family is a group of channel proteins critical for ionic homeostasis and the perception of various physical and chemical stimuli. TRP channels can detect
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
,
osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in a pure ...
,
olfaction The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, ...
,
taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
, and mechanical forces. The
human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the ...
encodes for 28 TRP channels, which include six
TRPV TRPV is a family of transient receptor potential cation channels (TRP channels) in animals. All TRPVs are highly calcium selective. TRP channels are a large group of ion channels consisting of six protein families, located mostly on the plasma ...
channels. The high Ca2+-selectivity of TRPV5 and TRPV6 makes these channels distinct from the other four TRPV channels (TRPV1-TRPV4). TRPV5 and TRPV6 are involved in Ca2+ transport, whereas
TRPV1 The transient receptor potential cation channel subfamily V member 1 (TrpV1), also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the ''TRPV1'' gene. It was the first isolated member of th ...
through TRPV3 are heat sensors with different temperature threshold for activation, and TRPV4 is involved in sensing
osmolarity Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/L ...
. Genetic defects in TRPV6 gene are linked to transient neonatal hyperparathyroidism and early-onset
chronic pancreatitis Chronic pancreatitis is a long-standing inflammation of the pancreas that alters the organ's normal structure and functions. It can present as episodes of acute inflammation in a previously injured pancreas, or as chronic damage with persistent pa ...
. Dysregulation of TRPV6 is also involved in
hypercalciuria Hypercalciuria is the condition of elevated calcium in the urine. Chronic hypercalciuria may lead to impairment of renal function, nephrocalcinosis, and chronic kidney disease. Patients with hypercalciuria have kidneys that put out higher levels o ...
, kidney stone formation, bone disorders, defects in
keratinocyte Keratinocytes are the primary type of cell found in the epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the basal layer (''stratum basale'') of the skin are sometimes referre ...
differentiation, skeletal deformities,
osteoarthritis Osteoarthritis (OA) is a type of degenerative joint disease that results from breakdown of joint cartilage and underlying bone which affects 1 in 7 adults in the United States. It is believed to be the fourth leading cause of disability in the ...
, male sterility, Pendred syndrome, and certain sub-types of
Cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
.


Identification

Peng ''et al'' identified TRPV6 in 1999 from rat
duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine m ...
in an effort to search for Ca2+ transporting proteins involved in Ca2+absorption. TRPV6 was also called calcium transport protein 1 (CaT1) initially although the names epithelial calcium channel 2 (ECaC2) and CaT1-like (CaT-L) were also used in early studies to describe the channel. The human and mouse
orthologs Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a sp ...
of TRPV6 were cloned by Peng et al and Weber et al, respectively. The name TRPV6 was confirmed in 2005.


Gene location, chromosomal location, and phylogeny

The human ''TRPV6''
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 b ...
is located on
chromosomal A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...
locus Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * ''Locus'' (magazine), science fiction and fantasy magazine ** ''Locus Award' ...
7q33-34 in close proximity to its homolog ''TRPV5'' on 7q35. The ''TRPV6'' gene in human encodes for 2906 bp-long
mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein. mRNA is created during the ...
. In contrast to most other proteins, which initiate translation with an AUG
codon The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accomplished by the ribosome, which links ...
, TRPV6 translation is initiated by non-AUG-codon-mediated reading. TRPV6 protein bears a 40-a.a-long N-terminal extension in
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 mate ...
and in some physiological settings in comparison to the annotated version of the protein used in biological studies. However, it is still to be determined whether the long version of the TRPV6 protein is the dominant form in different tissues. aTo be verified in different tissues. It has been hypothesized that ''Trpv5'' and ''Trpv6'' genes were generated from a single ancestral gene by
gene duplication Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through which new genetic material is generated during molecular evolution. It can be defined as any duplication of a region of DNA that contains a gene. ...
events.
Phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...
analysis has shown that TRPV6
paralogs Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a sp ...
in
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s,
sauropsids Sauropsida ("lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early syn ...
,
amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s, and
chondrichthyes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
arose out of independent duplication events in the ancestor of each group. It is speculated that two specialized Ca2+-selective ''Trpv'' homologs arose as an adaptation to achieve a greater degree of functional specialization for navigating distinct renal challenges of terrestrial animals. Two
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chrom ...
s of the ''TRPV6'' gene have been identified in humans (originally noted as CaT-La and CaT-Lb). These alleles exhibit coupled polymorphisms generating two versions of the same gene. The polymorphisms give rise to an “ancestral variant” and a “derived variant” that differ in five bases and three amino acids. The ancestral allele codes for C197(157, in parentheses are annotated amino acid numbering), M418(378), and M721(681) whereas the derived allele codes for R197(157), V418 (378) and T721(681). The frequency of the ancestral ''TRPV6'' allele varies across different population groups. It is hypothesized that selection pressures that could have changed ''TRPV6'' allele distribution include changes in patterns of milk consumption, domestication of animals, change in
ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiatio ...
exposure due to trans-equatorial migration, genomic adaptations providing immune advantages to populations encountering new pathogens.


Tissue distribution

The TRPV6 protein is expressed in
epithelial 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 intercellu ...
tissues such as the
intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans an ...
,
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; blo ...
,
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 mate ...
,
epididymis The epididymis (; plural: epididymides or ) is a tube that connects a testicle to a vas deferens in the male reproductive system. It is a single, narrow, tightly-coiled tube in adult humans, in length. It serves as an interconnection between the ...
, and
exocrine gland Exocrine glands are glands that secrete substances on to an epithelial surface by way of a duct. Examples of exocrine glands include sweat, salivary, mammary, ceruminous, lacrimal, sebaceous, prostate and mucous. Exocrine glands are one ...
s such as
pancreas The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an ...
,
prostate The prostate is both an accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found only in some mammals. It differs between species anatomically, chemically, and phys ...
and
salivary The salivary glands in mammals are exocrine glands that produce saliva through a system of ducts. Humans have three paired major salivary glands (parotid, submandibular, and sublingual), as well as hundreds of minor salivary glands. Salivary glan ...
,
sweat Perspiration, also known as sweating, is the production of fluids secreted by the sweat glands in the skin of mammals. Two types of sweat glands can be found in humans: eccrine glands and apocrine glands. The eccrine sweat glands are distrib ...
, and
mammary gland A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in ...
s. TRPV6 protein expression in humans has been demonstrated in the
esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...
,
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 ...
,
small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...
, colon, pancreas, mammary glands,
ovary The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...
,
thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The ...
, and prostate by
immunohistochemistry Immunohistochemistry (IHC) is the most common application of immunostaining. It involves the process of selectively identifying antigens (proteins) in cells of a tissue section by exploiting the principle of antibodies binding specifically to ant ...
approaches. TRPV6 expression mainly confines on the apical membrane of epithelial cells. In the intestine, the protein is expressed on the brush-border membrane of
enterocyte Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells which line the inner surface of the small and large intestines. A glycocalyx surface coat contains digestive enzymes. Microvilli on the apical surface increase i ...
. Differences in the TRPV6 expression profile have been reported possibly due to variation in assay-dependent such primer design,
hybridization probe In molecular biology, a hybridization probe (HP) is a fragment of DNA or RNA of usually 15–10000 nucleotide long which can be radioactively or fluorescently labeled. HP can be used to detect the presence of nucleotide sequences in analyzed RNA ...
s, PCR vs.
northern blot The northern blot, or RNA blot,Gilbert, S. F. (2000) Developmental Biology, 6th Ed. Sunderland MA, Sinauer Associates. is a technique used in molecular biology research to study gene expression by detection of RNA (or isolated mRNA) in a sampl ...
ting, semi-quantitative PCR vs.
RT-PCR Reverse transcription polymerase chain reaction (RT-PCR) is a laboratory technique combining reverse transcription of RNA into DNA (in this context called complementary DNA or cDNA) and amplification of specific DNA targets using polymerase chai ...
, and
antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of ...
used for immunodetection. TRPV6 expression profile is also influenced by age, gender, Ca2+ and vitamin D3 levels in food, hormonal status, location within the tissue, cellular location, reproductive status, and weaning status (see Section Regulation). In humans, TRPV6 transcripts have been detected in the placenta, pancreas, prostate cancer, and duodenum and the prostate by northern blotting; and in duodenum, jejunum, placenta, pancreas, testis, kidney, brain, and colon by semi-quantitative PCR. In rodents, TRPV6 expression has been validated in the duodenum, cecum, small intestine, colon, placenta, pancreas, prostate, and epididymis by Northern Blotting. In mouse, TRPV6 transcript abundance measured by RT-PCR is as follows: prostate > stomach, brain > lung > duodenum, cecum, heart, kidney, bone > colon > skeletal muscle > pancreas. Data from Human Protein Atlas and RNA-Seq based suggest TRPV6 mRNA is low in most tissues except for the placenta, salivary gland, pancreas, and prostate. TRPV6 mRNA is expressed in the apical domain of murine osteoclasts of cortical bone. Cortical and trabecular osteocytes do not express TRPV6 mRNA whereas osteoblasts show weak expression.


Structure and biophysical properties


Primary and secondary structure

Overall, four subunits of TRPV6 arrange to form a tetrameric channel displaying a four-fold
symmetry Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...
. Beginning from
N-terminus 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 moving towards the
C-terminus 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 i ...
of the protein, each TRPV6
polypeptide 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. ...
contains: an N-terminal helix, an
ankyrin repeat The ankyrin repeat is a 33-residue motif in proteins consisting of two alpha helices separated by loops, first discovered in signaling proteins in yeast Cdc10 and ''Drosophila'' Notch. Domains consisting of ankyrin tandem repeats mediate prot ...
domain (ARD) containing six ankyrin repeats, a β-hairpin structure linker domain made up two β-strands, a helix-turn-helix motif, a pre-SI helix, TM domain made up of six TM helices (S1 through S6), a pore-loop (also called P-loop),
amphipathic An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
TRP helix, C-terminal hook, and a six-residue β-strand (β3) (Figure 1).


Tertiary and quaternary structure

The TRPV6 channel protein displays four-fold symmetry and contains two main compartments: a 30 Å-tall transmembrane domain with a central ion channel pore and a ~70 Å-tall and a ~110 Å-wide intracellular skirt enclosing a 50 Å × 50 Å cavity wide cavity underneath the ion channel. The clustering of four TRPV6 subunits forms an aqueous pore exhibiting a fourfold symmetry (Figure 2). A pre-SI helix links the intracellular portion of the protein to the TM domain through a linker domain made up of β-hairpin structure and a helix-turn-helix motif. Helices S1 through S4 form a transmembrane helical bundle or TM domain that is inserted almost perpendicularly to the plane of the plasma membrane. The pore module elements are made up of S5, S6, and the P-loop in TM domains. The pore module from each TRPV6 polypeptide participates in inter-subunit interactions to form a central ion pore (Figure 1). The pore-forming elements of each TRPV6 subunit also interact with S1-S4 domains of the adjacent polypeptide in a domain-swapped arrangement. Intersubunit interactions also occur between S1-S2 extracellular loops and S5-P and S6-P loops of the neighboring TRPV6 subunits. The conserved N-linked glycosylation site on the S1-S2 loop is required for by the Klotho-mediated activation. The intracellular skirt portion of the TRPV6 protein is mainly made up of the ankyrin repeats. The TRP domain is oriented parallel to the membrane and participates in
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, ...
interactions with the TM domain and the
hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are n ...
interactions in the intracellular skirt. The N-terminal helix, C-terminal hook, and
β-sheets The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gen ...
(formed by the β-hairpin structure in the linker domain) in the channel participates in intersubunit interactions with the ARDs to provides a framework for holding the elements of the intracellular skirt together.


Pore architecture and cation binding sites

The TRPV6 pore has four main elements, namely, the extracellular vestibule, a selectivity filter, a hydrophobic cavity, and a lower gate. Facing the central lumen of the channel, a four-residue selectivity filter (538TIID541) containing four
Aspartate Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...
541 (D541) side chains (one from each protomer) is critical for Ca2+ selectivity and other biophysical properties of the channel. This filter forms a negatively charged ring that discriminates between ions based on their size and charge.
Mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...
s in the critical pore-forming residue of TRPV6 blocks Ca2+uptake, a strategy has been used to generate TRPV6 loss-of-function models to examine the role of the channel in animal physiology. Four different types of cation binding sites are thought to exist in the TRPV6 channel. Site 1 is located in the central pore and shares the same plane that is occupied by the key selective residues D541. Site 2 is thought to be present about 6-8 Å below Site 1 followed by Site 3 which is located in the central pore axis about 6.8 Å below Site 2. Site 2 and 3 are thought to interact with partially-hydrated to equatorially-hydrated Ca2+ ions. Finally, four symmetrical cation binding sites in the extracellular vestibule mediate the recruitment of cations towards the extracellular vestibule of TRPV6 and are referred to as recruitment sites.


Ion permeation

The conductance of TRPV6 for
divalent In chemistry, the valence (US spelling) or valency (British spelling) of an chemical element, element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Description The combining capacity, ...
cations An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by con ...
follows the preference: Ca2+ > Ba2+ > Sr2+ > Mn2. Intra-cellular Mg2+ inhibits TRPV6 and contributes to the strong inward rectification exhibited by the channel. TRPV6 uptake activity is inhibited by divalent Pb2, Cu2+, Cd2+, Zn2, Co2+, Fe2+, and trivalent cations La3+, Fe3+, Gd3+. The concentration of ions to achieve the inhibition ranges from 1 to 10 μM. The TRPV6 protein is constitutive with a single-channel conductance of 42-58 ps. At low Ca2+ concentrations, a single Ca2+ ion binds in the selectivity filter formed by D541 and permits Na+ permeation. At high Ca2+ concentration, Ca2+ permeation occurs by a knock-off mechanism that involves the formation of short-lived conformations involving binding of three Ca2+ ions to residue D541.


Channel gating

The conformational changes involved in channel opening are hinged around the residue
Alanine Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group side ...
566 (A566) and occur in the pore-lining helix S6 (Figure 3). The upper portion of S6 helix undergoes an α-to-π helical transition which forces the lower portion of the helix to turn by 100 degrees and tilt away from the pore axis by 11 degrees. This conformational change moves the lower portion of the helix gating the pore and thereby widens the pore size. The conformational change alters the residues facing the pore axis and triggers the formation of new electrostatic bonds subunit and salt bridges that offset the high energetic cost of unfavorable α-to-π helical transition that occurs during channel opening.


Regulation by phosphatidylinositol 4,5-bisphosphate (PIP2) and calmodulin (CaM)

The influx of Ca2+ inside the cell triggers negative feedback mechanisms to suppress TRPV6 activity and prevent Ca2+ overload. TRPV6 channel activity is regulated by the intracellular level of
phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...
phosphatidylinositol 4,5-bisphosphate Phosphatidylinositol 4,5-bisphosphate or PtdIns(4,5)''P''2, also known simply as PIP2 or PI(4,5)P2, is a minor phospholipid component of cell membranes. PtdIns(4,5)''P''2 is enriched at the plasma membrane where it is a substrate for a number of ...
(PIP2) and interactions with Ca2+-
Calmodulin Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bin ...
(CaM) complex. The depletion of PIP2 or CaM-binding inactivates TRPV6. The influx of Ca2+ in TRPV6 expressing cells activates
phospholipase C Phospholipase C (PLC) is a class of membrane-associated enzymes that cleave phospholipids just before the phosphate group (see figure). It is most commonly taken to be synonymous with the human forms of this enzyme, which play an important role ...
(PLC) which in turn hydrolyzes PIP2. Depletion in PIP2 levels results in a decline in channel activity since most TRP channels require this lipid for activation. The lipid PIP2 can override Ca2+-CaM-mediated inhibition of TRPV6. Overall, TRPV6 inactivation by calmodulin is orchestrated by a balance of intracellular Ca2+ and PIP2 concentration.


Interacting proteins

Among 20+ TRPV6 interactors identified so far, the functional consequences of Ca2+-binding protein Calmodulin (CaM) and Glucuronidase Klotho have been most extensively characterized 6, 37, 41, 42 Functional consequences of TRPV6 channel activation are summarized in the table below). Abbreviations Protein Interactor BSPRY: B-Box and Spry Domain Containing Protein; FYN: Fyn Kinase Belonging Src Family of Kinases; I-MFA: Myo D Family Inhibitor; NHERF: Na Exchanger Regulatory Factor; NIPSNAP14-Nitrophenylphosphatase Domain and Non-Neuronal SNAP25-Like Protein Homolog 1; Numb: Drosophila mutation that removes most of the sensory neurons in the developing peripheral nervous system; PTP: Protein Tyrosine Phosphatase; Rab11a: Member RAS Oncogene Family; RGS2: Regulator Of G-Protein Signaling 2; RyR1: Ryanodine Receptor 1; TRPC1: Transient receptor potential canonical 1; TRPML3: Transient receptor potential Mucolipin-3.


Physiological functions

The Ca2+-selective channel proteins TRPV6 and TRPV5 cooperate to maintain calcium concentration in specific organs. TRPV6 functions as apical Ca2+ entry channels mediating
transcellular transport Transcellular transport involves the transportation of solutes by a cell ''through'' a cell. Transcellular transport can occur in three different ways active transport, passive transport, and transcytosis. Active Transport Main article: Active ...
of this ion in the intestine, placenta, and possibly some other exocrine organs. TRPV6 also plays important roles in maternal-fetal calcium transport, keratinocyte differentiation, and Ca2+ homeostasis in the endolymphatic system of the
vestibular The Vestibular (from pt, vestíbulo, "entrance hall") is a competitive examination and is the primary and widespread entrance system used by Brazilian universities to select the students admitted. The Vestibular usually takes place from Nove ...
system, and maintenance of male fertility.


''C''a2+ absorption in intestine

Two routes of Ca2+ absorption are recognized:
paracellular transport Paracellular transport refers to the transfer of substances across an epithelium by passing through the intercellular space ''between'' the cells. It is in contrast to transcellular transport, where the substances travel ''through'' the cell, pass ...
and
transcellular transport Transcellular transport involves the transportation of solutes by a cell ''through'' a cell. Transcellular transport can occur in three different ways active transport, passive transport, and transcytosis. Active Transport Main article: Active ...
(see Figure 4). A high-Ca2+-diet favors paracellular transport of the ion across the length of the intestine allowing them to pass between the intercellular
tight junction Tight junctions, also known as occluding junctions or ''zonulae occludentes'' (singular, ''zonula occludens''), are multiprotein junctional complexes whose canonical function is to prevent leakage of solutes and water and seals between the epith ...
s that connect epithelial cells. In contrast under conditions when a2+in the lumen of the intestine is lower in comparison to its concentration in the plasma (e.g. during low dietary Ca2+), the transcellular pathway is required for adequate Ca2+ absorption. Three important steps in transcellular Ca2+ transport are recognized: cellular entry of Ca2+ ion on the apical side via TRPV6 (Step-1), the binding of Ca2+ ion with
calbindin Calbindins are three different calcium-binding proteins: calbindin, calretinin and S100G. They were originally described as vitamin D-dependent calcium-binding proteins in the intestine and kidney in the chick and mammals. They are now classi ...
-D9k (Step-2), and exit of Ca2+ from the basolateral side via the plasma membrane Ca2+ ATPase (PMCA1b). The hormone Vitamin D3 (or 1,25(OH)2D3) plays an important role in TRPV6-mediated intestinal Ca2+ absorption).


Ca2+ reabsorption in the kidney

In contrast to the intestine, where TRPV6 is the gatekeeper of Ca2+ absorption, the transcellular reabsorption of this ion in the kidney occurs through TRPV5. Although TRPV5 is a recognized gatekeeper for transcellular reabsorption of Ca2+ ion in the kidney, TRPV6
knockout A knockout (abbreviated to KO or K.O.) is a fight-ending, winning criterion in several full-contact combat sports, such as boxing, kickboxing, muay thai, mixed martial arts, karate, some forms of taekwondo and other sports involving strikin ...
(KO) mice also struggle to concentrate their urine and display hypercalciuria. TRPV6 is known to co-localize with TRPV5 Calbindin-D28K in apical domains of distal convoluted tubules and connecting tubules 0 TRPV5 KO mice compensate for Ca2+ loss by increasing TRPV6 expression in the duodenum. Moreover, a recent study analyzing vitamin D responsive genes in
ovine Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus '' Ovis'', in everyday usage it almost always refers to domesticated ...
,
canine Canine may refer to: Zoology and anatomy * a dog-like Canid animal in the subfamily Caninae ** ''Canis'', a genus including dogs, wolves, coyotes, and jackals ** Dog, the domestic dog * Canine tooth, in mammalian oral anatomy People with the surn ...
and,
equine Equinae is a subfamily of the family Equidae, which have lived worldwide (except Indonesia and Australia) from the Hemingfordian stage of the Early Miocene (16 million years ago) onwards. They are thought to be a monophyletic grouping.B. J. Ma ...
kidney suggested that TRPV6, calD9k/calD28k, and PMCA could be the main pathways orchestrating transcellular Ca2+ transport in the kidney of sheep, dogs, and horses.


Maternal-fetal Ca2+ transport

TRPV6 plays an indispensable role in placental Ca2+ transport. Fetal bone mineralization peaks during late pregnancy. At this stage, fetal blood has a higher concentration of Ca2+ in comparison to maternal blood thereby creating conditions that require active transcellular transport of Ca2+ from mother to the fetus. This process is very important since defects in placental transport of calcium can be precursors for Ca2+ deficiency syndromes and
intrauterine growth restriction Intrauterine growth restriction (IUGR), or fetal growth restriction, refers to poor growth of a fetus while in the womb during pregnancy. IUGR is defined by clinical features of malnutrition and evidence of reduced growth regardless of an infant's ...
s. The expression of TRPV6 increases 14-fold during the last 4 days of the
murine The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. Members of this subfamily are called murines. In terms of species richness, this subfamily is larger than all mammal families ex ...
gestational period and coincides with the peak phase of fetal bone mineralization. The protein TRPV6 is abundantly expressed in the mammalian placental tissues. Indeed, TRPV6 expression is ~1000-fold higher in comparison to TRPV5. In the placenta, TRPV6 is expressed in
trophoblast The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the p ...
s and
syncytiotrophoblast Syncytiotrophoblast (from the Greek 'syn'- "together"; 'cytio'- "of cells"; 'tropho'- "nutrition"; 'blast'- "bud") is the epithelial covering of the highly vascular embryonic placental villi, which invades the wall of the uterus to establish nu ...
s. In mice, TRPV6 mRNA and protein are expressed in the intraplacental
yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...
and the visceral layer of the extraplacental yolk sac. Most importantly, TRPV6 KO fetuses exhibit a 40% reduction in 45Ca2+ transport activity and a dramatic decrease in the ash weight (a measure of fetal bone health). In humans, trophoblasts fluid shear stress (FSS) is known to induce a TRPV6-mediated Ca2+ influx and promote
microvilli Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, ...
formation through a mechanism involving Ezrin and Akt-phosphorylation.


Epididymal Ca2+ regulation and implications on male fertility

The regulation of calcium concentration in the epididymal lumen is critical for
sperm motility Sperm motility describes the ability of sperm to move properly through the female reproductive tract (internal fertilization) or through water (external fertilization) to reach the egg. Sperm motility can also be thought of as the ''quality'', wh ...
. TRPV6-mediated reduction of luminal Ca2+ concentration in the epididymis is critical for male fertility in mice. TRPV6 KO mice or mice expressing loss-of-function version of TRPV6 channel (''Trpv6D541A''
homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
mice) have a severely impaired
fertility Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Ferti ...
. Mice expressing nonfunctional TRPV6 have a 10-fold higher concentration of Ca2+ in the epididymal lumen and Ca2+ uptake in this space is reduced by 7-to-8 folds. The increases Ca2+ ion in epididymal lumen concentration leads to significant defects in motility, fertilization capacity, and viability of sperms in ''TRPV6D541A'' mice. It appears TRPV6 and chloride channel transmembrane manner 16 A ( TMEM16A) act cooperatively to reduce the luminal concentration of Ca2+ in the epididymal lumen.


Bone health

Under conditions of sub-optimal dietary Ca2+, normal serum calcium levels in TRPV6 KO mice are maintained at the expense of bone. TRPV6 plays an important role in osteoclasts but not in osteoblasts. In mice, TRPV6 depletion results in increased osteoclasts differentiation whereas TRPV5 is essential for proper osteoclastic bone resorption.


Keratinocyte differentiation

Keratinocytes differentiation is orchestrated by calcium switch, a process that entails an influx of Ca2+ in keratinocyte which induces broad transcriptional changes necessary for
desmosome A desmosome (; "binding body"), also known as a macula adherens (plural: maculae adherentes) (Latin for ''adhering spot''), is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like adh ...
formation, stratification, and cornification. TRPV6 KO mice display thinner layers of
stratum corneum The stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis. The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the ''stratum disjunctum'' and ''stratum compa ...
and 20% of the mice also show
alopecia Hair loss, also known as alopecia or baldness, refers to a loss of hair from part of the head or body. Typically at least the head is involved. The severity of hair loss can vary from a small area to the entire body. Inflammation or scar ...
and
dermatitis Dermatitis is inflammation of the skin, typically characterized by itchiness, redness and a rash. In cases of short duration, there may be small blisters, while in long-term cases the skin may become thickened. The area of skin involved c ...
. The silencing of TRPV6 impairs Ca2+-mediated differentiation of human primary keratinocytes and downregulates differentiation markers such as
involucrin Involucrin is a protein component of human skin and in humans is encoded by the ''IVL'' gene. In binding the protein loricrin, involucrin contributes to the formation of a cell envelope that protects corneocytes in the skin. Gene This gene i ...
, transglutaminase-1, and cytokeratin-10. The hormone 1,25-dihydroxyvitamin-D3 upregulates TRPV6 in keratinocytes and triggers a Ca2+ influx. This in turn induces the expression of keratinocyte differentiation-specific pathways.


Role in the inner ear

The proteins TRPV5 and TRPV6 are expressed in several regions of the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...
as well as in primary cultures of semicircular canal duct (SCCD) epithelium. Some studies have indicated that TRPV5 and TRPV6 are needed for lowering the Ca2+ concentration in the lumen of mammalian
endolymph Endolymph is the fluid contained in the membranous labyrinth of the inner ear. The major cation in endolymph is potassium, with the values of sodium and potassium concentration in the endolymph being 0.91  mM and 154  mM, respectively. I ...
, a requirement that is essential for normal
hearing Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is audit ...
and balance.


Uterine and placental expression of TRPV6 and implications in pregnancy

The endometrial and
uterine The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...
expression of TRPV6 has been reported in mammals. The expression of TRPV6 in the uterus is thought to be hormonally regulated by
17β-estradiol Estradiol (E2), also spelled oestradiol, is an estrogen steroid hormone and the major female sex hormone. It is involved in the regulation of the estrous and menstrual female reproductive cycles. Estradiol is responsible for the development of ...
and
progesterone Progesterone (P4) is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species. It belongs to a group of steroid hormones called the progestogens and is the majo ...
in rodents. In rodents, TRPV6 mRNA is expressed in the labyrinth and spongy zone as well as placenta-unattached areas of the uterus. The stage of pregnancy is an important regulator of TRPV6 expression. The downregulation of TRPV5/6 expression and a resulting decline in Ca2+ transport is thought to change the proliferative profile of human trophoblasts; a process which in turn is linked to the development of
pre-eclampsia Pre-eclampsia is a disorder of pregnancy characterized by the onset of high blood pressure and often a significant amount of protein in the urine. When it arises, the condition begins after 20 weeks of pregnancy. In severe cases of the disease ...
. This juxtaposition of TRPV6 expression and its stringent regulation by sex hormones during pregnancy suggest that the protein may be important for embryo implantation, however conclusive evidence for this connection does not exist.


Implications in Human Diseases


Transient Neonatal Hyperparathyroidism

Loss of TRPV6 in murine placenta severely impairs Ca2+ transport across trophoblast and reduces embryo growth, induces bone calcification, and impairs bone development. In humans, the insufficient maternal-fetal transport caused by pathogenic genomic variants of TRPV6 is thought to be a cause for skeletal defects observed in selected case reports o
transient neonatal hyperparathyroidism
(TNHP) cases. These variants are believed to compromise the plasma membrane localization of the protein.
Exome sequencing Exome sequencing, also known as whole exome sequencing (WES), is a genomic technique for sequencing all of the protein-coding regions of genes in a genome (known as the exome). It consists of two steps: the first step is to select only the sub ...
of an infant with severe
antenatal Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal devel ...
onset thoracic insufficiency with accompanying fetal skeletal abnormalities indicates the critical role of TRPV6 in maternal-fetal transport. The study indicated that compound
heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. ...
variants of TRPV6 result in severe undermineralization and severe
dysplasia Dysplasia is any of various types of abnormal growth or development of cells (microscopic scale) or organs ( macroscopic scale), and the abnormal histology or anatomical structure(s) resulting from such growth. Dysplasias on a mainly microscopi ...
of the fetal skeleton.


Chronic Pancreatitis

Recent evidence indicates that naturally occurring TRPV6 loss of function variants predisposes certain
demographics Demography () is the statistical study of populations, especially human beings. Demographic analysis examines and measures the dimensions and dynamics of populations; it can cover whole societies or groups defined by criteria such as ed ...
to
chronic pancreatitis Chronic pancreatitis is a long-standing inflammation of the pancreas that alters the organ's normal structure and functions. It can present as episodes of acute inflammation in a previously injured pancreas, or as chronic damage with persistent pa ...
(CP) by dysregulating calcium homeostasis in the pancreatic cells.
Sequencing In genetics and biochemistry, sequencing means to determine the primary structure (sometimes incorrectly called the primary sequence) of an unbranched biopolymer. Sequencing results in a symbolic linear depiction known as a sequence which suc ...
studies among chronic pancreatitis patients revealed the presence of 33
missense In genetics, a missense mutation is a point mutation in which a single nucleotide change results in a codon that codes for a different amino acid. It is a type of nonsynonymous substitution. Substitution of protein from DNA mutations Missense mu ...
and 2
nonsense Nonsense is a communication, via speech, writing, or any other symbolic system, that lacks any coherent meaning. Sometimes in ordinary usage, nonsense is synonymous with absurdity or the ridiculous. Many poets, novelists and songwriters have u ...
variants predisposed
Japanese Japanese may refer to: * Something from or related to Japan, an island country in East Asia * Japanese language, spoken mainly in Japan * Japanese people, the ethnic group that identifies with Japan through ancestry or culture ** Japanese diaspor ...
,
German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...
, and
French French (french: français(e), link=no) may refer to: * Something of, from, or related to France ** French language, which originated in France, and its various dialects and accents ** French people, a nation and ethnic group identified with Franc ...
patients to a higher risk of CP. Overall, these studies have shown that disease-inducing TRPV6 loss-of-function genomic variants are over-represented in German, French, Chinese, and Japanese CP patients in comparison to controls in their respective groups. The loss-of-function variants are believed to compromise calcium transport in the pancreas by act by either reducing the total protein level and/or compromising Ca2+ uptake activity by the channel.


Kidney Stone Formation

The role of TRPV6 in renal stone formation has been suggested through sequencing studies conducted on a
cohort Cohort or cohortes may refer to: * Cohort (educational group), a group of students working together through the same academic curriculum * Cohort (floating point), a set of different encodings of the same numerical value * Cohort (military unit) ...
of 170 patients in
Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
. The studies revealed that the frequency of TRPV6 gain-of-function haplotype is significantly higher in Ca2+-stone formers ( nephrolithiasis) in comparison to non-formers. The observed hypercalciuria
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (biology), morphology or physical form and structure, its Developmental biology, developmental proc ...
s from animal studies and studies on TRPV6
single nucleotide polymorphisms In genetics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently lar ...
(SNPs) suggest that TRPV6 haplotype could be an important risk factor for absorptive and renal hypercalciuria (kidney stones due to impaired intestinal absorption and renal re-absorption respectively). The lower incidence of kidney stone diseases in
African-Americans African Americans (also referred to as Black Americans and Afro-Americans) are an ethnic group consisting of Americans with partial or total ancestry from sub-Saharan Africa. The term "African American" generally denotes descendants of enslav ...
and a relatively higher prevalence of ancestral haplotype suggest theory according to which this haplotype endows an advantage of increased Ca2+ reabsorption in this demographic and reduces the incidence of kidney stones.


Bone Resorptive Diseases

TRPV6 KO mice exhibit osteoporosis-like symptoms such as reduced
bone mineral density Bone density, or bone mineral density, is the amount of bone mineral in bone tissue. The concept is of mass of mineral per volume of bone (relating to density in the physics sense), although clinically it is measured by proxy according to optical ...
and hypercalciuria. The hormone
estrogen Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal ac ...
, the deficiency of which is linked to post-menopausal osteoporosis, also regulates the expression of TRPV6 in humans. Indeed, a lower calcium absorption seen in older
postmenopausal Menopause, also known as the climacteric, is the time in women's lives when menstrual periods stop permanently, and they are no longer able to bear children. Menopause usually occurs between the age of 47 and 54. Medical professionals often d ...
women is attributed to reduced TRPV6. The C-terminal portion of
Soricidin Soricidin is a paralytic oligopeptide found in the venomous saliva of the northern short-tailed shrew (''Blarina brevicauda'');Preclinical studies of this drug have shown great promise in the treatment of bone resorptive diseases. The high degree of similarity between Hereditary Vitamin D–Resistant Rickets (HVDRR) disease symptoms and observed phenotypes in TRPV6 KO mice has led some experts to postulate pathological connections between the disease and TRPV6 dysfunction. TRPV6 plays an important chondroprotective role by regulating multiple aspects of
chondrocyte Chondrocytes (, from Greek χόνδρος, ''chondros'' = cartilage + κύτος, ''kytos'' = cell) are the only cells found in healthy cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and prote ...
function, such as
extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...
secretion, the release of matrix-degrading enzymes,
cell proliferation Cell proliferation is the process by which ''a cell grows and divides to produce two daughter cells''. Cell proliferation leads to an exponential increase in cell number and is therefore a rapid mechanism of tissue growth. Cell proliferation r ...
, and
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes in ...
. Furthermore, TRPV6 knockout mice display multiple
osteoarthritis Osteoarthritis (OA) is a type of degenerative joint disease that results from breakdown of joint cartilage and underlying bone which affects 1 in 7 adults in the United States. It is believed to be the fourth leading cause of disability in the ...
(OA) phenotypes such as
cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck ...
fibrillation,
eburnation Eburnation is a degenerative process of bone commonly found in patients with osteoarthritis or non-union of fractures. Friction in the joint causes the reactive conversion of the sub-chondral bone to an ivory-like surface at the site of the cartilag ...
, and loss of
proteoglycan Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan (GAG) chain(s). The point of attachment is a serine (Ser) residue to whic ...
s.


Pendred Syndrome

The dysfunction gene
Slc26a4 Pendrin is an anion exchange protein that in humans is encoded by the ''SLC26A4'' gene (solute carrier family 26, member 4). Pendrin was initially identified as a sodium-independent chloride-iodide exchanger with subsequent studies showing that ...
has been linked to Pendred syndrome – a genetic disorder that results in syndromic deafness in children. The disease is caused by mutations in which compromise the function of the encoded protein pendrin - an anion Cl/HCO3 exchanger expressed in the inner ear. The loss of function in this gene is thought to reduce the pH value of mammalian endolymph and impair Ca2+ absorption via TRPV5 and TRPV6. This in turn could prevent the uptake of Ca2+ and impairs the luminal reduction in Ca2+ concentration within the endolymphatic system of the ear.


Cancer

The overexpression of TRPV6 has been validated in the colon,
parathyroid Parathyroid glands are small endocrine glands in the neck of humans and other tetrapods. Humans usually have four parathyroid glands, located on the back of the thyroid gland in variable locations. The parathyroid gland produces and secretes p ...
, pancreatic, and
thyroid cancer Thyroid cancer is cancer that develops from the tissues of the thyroid gland. It is a disease in which cells grow abnormally and have the potential to spread to other parts of the body. Symptoms can include swelling or a lump in the neck. Ca ...
whereas its expression is reportedly downregulated in
esophageal cancer Esophageal cancer is cancer arising from the esophagus—the food pipe that runs between the throat and the stomach. Symptoms often include difficulty in swallowing and weight loss. Other symptoms may include pain when swallowing, a hoarse voi ...
,
non-small cell lung cancer Non-small-cell lung cancer (NSCLC) is any type of epithelial lung cancer other than small-cell lung carcinoma (SCLC). NSCLC accounts for about 85% of all lung cancers. As a class, NSCLCs are relatively insensitive to chemotherapy, compared to s ...
, and
renal cancer Kidney cancer, also known as renal cancer, is a group of cancers that starts in the kidney. Symptoms may include blood in the urine, lump in the abdomen, or back pain. Fever, weight loss, and tiredness may also occur. Complications can include spr ...
. TRPV6 is considered to be an oncochannel that is hypothesized to mediate cancer progression by triggering Ca2+-entry induced aberrations in molecular drivers regulating processes such as
cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and sub ...
,
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes in ...
, and
migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
; thereby conferring proliferative and survival advantages to cancer cells. Overexpression of TRPV6 correlates strongly with pathological stage, tumor grade, extra-prostatic invasion,
lymph node A lymph node, or lymph gland, is a kidney-shaped organ of the lymphatic system and the adaptive immune system. A large number of lymph nodes are linked throughout the body by the lymphatic vessels. They are major sites of lymphocytes that includ ...
metastasis Metastasis is a pathogenic agent's spread from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, the ...
, and resistance to
androgen An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This in ...
-targeted therapies in
prostate cancer Prostate cancer is cancer of the prostate. Prostate cancer is the second most common cancerous tumor worldwide and is the fifth leading cause of cancer-related mortality among men. The prostate is a gland in the male reproductive system that su ...
. The expression TRPV6 has been touted as a prognostic marker for advanced prostate cancer since its expression is strongly dependent on the grade of the tumor. Expression of TRPV6 is significantly elevated in breast
adenocarcinoma Adenocarcinoma (; plural adenocarcinomas or adenocarcinomata ) (AC) is a type of cancerous tumor that can occur in several parts of the body. It is defined as neoplasia of epithelial tissue that has glandular origin, glandular characteristics, o ...
tissue in comparison to normal breast tissue. TRPV6 expression has been reported multiple breast cancer cell lines and prostate cancer cell lines. The prostate cancer cell lines PC-3 and LnCAP overexpress TRPV6 relative to benign epithelial cells PrEC and BPH-1. The silencing of TRPV6 in prostate cancer cells decreases proliferation rate,
S-phase S phase (Synthesis Phase) is the phase of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Since accurate duplication of the genome is critical to successful cell division, the processes that occur during ...
accumulation, and expression of tumor marker
proliferating cell nuclear antigen Proliferating cell nuclear antigen (PCNA) is a DNA clamp that acts as a processivity factor for DNA polymerase δ in eukaryotic cells and is essential for replication. PCNA is a homotrimer and achieves its processivity by encircling the DNA, wher ...
(PCNA) expression. TRPV6 overexpression is believed to induce aberrant Ca2+-uptake in prostate cancer line and activate transcription factor Nuclear Factor of Activated T cells (NFAT). Expression of TRPV6 is upregulated by estrogen, progesterone, and estradiol in breast cancer cell line T47D. In agreement with these observations, the
estrogen receptor Estrogen receptors (ERs) are a group of proteins found inside cells. They are receptors that are activated by the hormone estrogen (17β-estradiol). Two classes of ER exist: nuclear estrogen receptors ( ERα and ERβ), which are members of the ...
antagonist An antagonist is a character in a story who is presented as the chief foe of the protagonist. Etymology The English word antagonist comes from the Greek ἀνταγωνιστής – ''antagonistēs'', "opponent, competitor, villain, enemy, ri ...
Tamoxifen Tamoxifen, sold under the brand name Nolvadex among others, is a selective estrogen receptor modulator used to prevent breast cancer in women and treat breast cancer in women and men. It is also being studied for other types of cancer. It has b ...
reduces TRPV6 expression in T47D cells and suppresses Ca2+-uptake of the channel in both ER-positive and ER-negative breast cancer cell lines. The overexpression of TRPV6 is associated with early-stage
colon cancer Colorectal cancer (CRC), also known as bowel cancer, colon cancer, or rectal cancer, is the development of cancer from the colon or rectum (parts of the large intestine). Signs and symptoms may include blood in the stool, a change in bowe ...
and its silencing in colon cancer induces apoptosis and inhibits cancer cell proliferation. In terms of mechanism, mutations within the calmodulin-binding domains of TRPV6 channels confers invasive properties to colon adenocarcinoma cells. The proteins p38α and GADD45α are believed to upregulate TRPV6 expression signaling in SW480 colon cancer cells by enhancing vitamin D signaling. TRPV6 has been reported to amplify
Insulin-like growth factor The insulin-like growth factors (IGFs) are proteins with high sequence similarity to insulin. IGFs are part of a complex system that cells use to communicate with their physiologic environment. This complex system (often referred to as the IGF " ...
s (IGF)-induced
PI3K Phosphoinositide 3-kinases (PI3Ks), also called phosphatidylinositol 3-kinases, are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking, which i ...
- PDK1-
Akt Protein kinase B (PKB), also known as Akt, is the collective name of a set of three serine/threonine-specific protein kinases that play key roles in multiple cellular processes such as glucose metabolism, apoptosis, cell proliferation, tran ...
signaling in human colon cancer and promote colon cancer. TRPV6 is up-regulated in primary cancer tissues from
pancreatic cancer Pancreatic cancer arises when cells in the pancreas, a glandular organ behind the stomach, begin to multiply out of control and form a mass. These cancerous cells have the ability to invade other parts of the body. A number of types of pancr ...
patients and promotes the proliferation of pancreatic
neuroendocrine tumor Neuroendocrine tumors (NETs) are neoplasms that arise from cells of the endocrine ( hormonal) and nervous systems. They most commonly occur in the intestine, where they are often called carcinoid tumors, but they are also found in the pancreas, lu ...
s NFAT-dependent mechanisms. Silencing of TRPV6 induces apoptosis and cell cycle arrest in pancreatic cancer cells and inhibits their invasion, proliferation, and migration. Forced expression of TRPV6 in
gastric cancer Stomach cancer, also known as gastric cancer, is a cancer that develops from the lining of the stomach. Most cases of stomach cancers are gastric carcinomas, which can be divided into a number of subtypes, including gastric adenocarcinomas. Lym ...
cells increases their sensitivity to
capsaicin Capsaicin (8-methyl-''N''-vanillyl-6-nonenamide) ( or ) is an active component of chili peppers, which are plants belonging to the genus ''Capsicum''. It is a chemical irritant for mammals, including humans, and produces a sensation of burnin ...
-induced apoptosis whereas the
siRNA Small interfering RNA (siRNA), sometimes known as short interfering RNA or silencing RNA, is a class of double-stranded RNA at first non-coding RNA molecules, typically 20-24 (normally 21) base pairs in length, similar to miRNA, and operating ...
-mediated silencing of the channel suppresses this sensitivity. TRPV6 downregulation in
esophageal carcinoma Esophageal cancer is cancer arising from the esophagus—the food pipe that runs between the throat and the stomach. Symptoms often include difficulty in swallowing and weight loss. Other symptoms may include pain when swallowing, a hoarse vo ...
has been suggested to be a prognostic marker of disease-specific survival in patients suffering from
esophageal cancer Esophageal cancer is cancer arising from the esophagus—the food pipe that runs between the throat and the stomach. Symptoms often include difficulty in swallowing and weight loss. Other symptoms may include pain when swallowing, a hoarse voi ...
. Low TRPV5 and TRPV6 co-expression have suggested as predictive markers for poor recurrence-free survival in
non-small cell lung cancer Non-small-cell lung cancer (NSCLC) is any type of epithelial lung cancer other than small-cell lung carcinoma (SCLC). NSCLC accounts for about 85% of all lung cancers. As a class, NSCLCs are relatively insensitive to chemotherapy, compared to s ...
.


Pharmacological Targeting

Several chemical inhibitors are known to inhibit TRPV6. Some compounds that have demonstrated inhibitory activity towards TRPV6 include TH-1177,
2-Aminoethoxydiphenyl borate 2-Aminoethoxydiphenyl borate (2-APB) is a chemical that acts to inhibit both IP3 receptors and TRP channels (although it activates TRPV1, TRPV2, & TRPV3 at higher concentrations). In research it is used to manipulate intracellular release of ca ...
(2-APB), 2-APB derivative 22b,
Econazole Econazole is an antifungal medication of the imidazole class. It was patented in 1968, and approved for medical use in 1974. Medical uses Econazole is used as a cream to treat skin infections such as athlete's foot, tinea, pityriasis versicol ...
, Miconazole,
Piperazine Piperazine () is an organic compound that consists of a six-membered ring containing two nitrogen atoms at opposite positions in the ring. Piperazine exists as small alkaline deliquescent crystals with a saline taste. The piperazines are a broad ...
derivative Cis-22a,
Capsaicin Capsaicin (8-methyl-''N''-vanillyl-6-nonenamide) ( or ) is an active component of chili peppers, which are plants belonging to the genus ''Capsicum''. It is a chemical irritant for mammals, including humans, and produces a sensation of burnin ...
, Δ9-
tetrahydrocannabivarin Tetrahydrocannabivarin (THCV, THV, O-4394, GWP42004) is a homologue of tetrahydrocannabinol (THC) having a propyl (3-carbon) side chain instead of a pentyl (5-carbon) group on the molecule, which makes it produce very different effects from THC. ...
, Xestospongin C,
Lidocaine Lidocaine, also known as lignocaine and sold under the brand name Xylocaine among others, is a local anesthetic of the amino amide type. It is also used to treat ventricular tachycardia. When used for local anaesthesia or in nerve blocks, lid ...
, gold-caged nanoparticle (PTX-PP@Au NPs) and Sorcidin C-13 (SOR-C13) synthetic peptide. Among different inhibition strategies tested so far, the 13-amino acid peptide SOR-C13 has shown the most promise. This 13-amino acid peptide derived from 54-amino acid peptide found in the paralytic venom of the
northern short-tailed shrew The northern short-tailed shrew (''Blarina brevicauda'') is the largest shrew in the genus ''Blarina'', and occurs in the northeastern region of North America. It is a semifossorial, highly active, and voracious insectivore and is present in a v ...
(''Blarina brevicauda'') reduces cancer growth in cell and animal models. This anti-cancer agent has recently completed a Phase I clinical safety trial that had enrolled 23 patients with advanced
solid tumors 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 ...
of epithelial origin non-responsive to all standard-of-care treatments.


Regulation

The regulation of TRPV6 can be examined mainly in the context of its physiological, hormonal, and molecular factors. The hormonal regulation of TRPV6 has been characterized most extensively. In this regard, its regulation by the hormone vitamin D3 and sex hormones has been examined in considerable detail. Rodent studies suggest that the TRPV6 channel is regulated by a wide range of physiological factors such as diet, age, gender, pregnancy, lactation, sex hormones, exercise, age, and gender. Some biological and pharmacological agents known to regulate TRPV6 include glucocorticoids,
immunosuppressive drug Immunosuppressive drugs, also known as immunosuppressive agents, immunosuppressants and antirejection medications, are drugs that inhibit or prevent activity of the immune system. Classification Immunosuppressive drugs can be classified int ...
s, and
diuretic A diuretic () is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. A diuretic tablet is sometimes colloquially called a water tablet. There are several categories of diuretics. All diuretics i ...
s.


Vitamin D

Multiple dose-response and time-course experiments in rodents and colon cancer cell lines have conclusively shown TRPV6 mRNA is robustly induced by this vitamin D at extremely low concentrations. At least five vitamin D response elements (VDREs) at positions −1.2, −2.1, −3.5, −4.3, and −5.5 kb relative to transcriptional start site (TSS) have been identified on TRPV6 transcripts. Among these five sites, VDREs at positions −1.2, −2.1, and −4.3 kb are significantly more responsive to 1,25-(OH)2D3 in comparison to VDREs located at −3.5 and −5.5 kb which do not appear to contribute substantially to vitamin D mediated transcriptional regulation in the intestine. Mechanism wise, TRPV6 transcription is initiated in response to
vitamin D Receptor The vitamin D receptor (VDR also known as the calcitriol receptor) is a member of the nuclear receptor family of transcription factors. Calcitriol (the active form of vitamin D, 1,25-(OH)2vitamin D3) binds to VDR, which then forms a heterod ...
(VDR)-mediated signaling, although other non-direct mechanisms cannot be ruled out. Important steps in vitamin D mediated transcriptional regulation include 1) binding of vitamin D on its cognate vitamin D receptor (VDR), 2) the translocation of vitamin D receptor (VDR)- retinoid X receptor heterodimer complex in the
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
, 3) binding VDR-RXR complex on the TRPV6
gene promoter In genetics, a promoter is a sequence of DNA to which proteins bind to initiate transcription of a single RNA transcript from the DNA downstream of the promoter. The RNA transcript may encode a protein ( mRNA), or can have a function in and ...
, 4) recruitment of steroid receptor coactivator 1 and
RNA polymerase II RNA polymerase II (RNAP II and Pol II) is a multiprotein complex that transcribes DNA into precursors of messenger RNA (mRNA) and most small nuclear RNA (snRNA) and microRNA. It is one of the three RNAP enzymes found in the nucleus of euka ...
on the promoter, and 5) transcriptional activation mediated through
histone H4 Histone H4 is one of the five main histone proteins involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, H4 is involved with the structure of the nucleosome of the 'beads on ...
acetylation : In organic chemistry, acetylation is an organic esterification reaction with acetic acid. It introduces an acetyl group into a chemical compound. Such compounds are termed ''acetate esters'' or simply '' acetates''. Deacetylation is the oppos ...
events.


Diet

The level of Ca2+and vitamin D in the diet are the most important regulators of TRPV6 expression. The expression of TRPV6 is thought to be modulated strongly to fine-tune Ca2+ absorption from the diet, especially under conditions when dietary Ca2+ availability is low. In rodents, restricting Ca2+ availability in the diet induces dramatic up-regulation in the duodenal expression of TRPV6. Calcium influx from the diet and its subsequent binding to calbindin-D9k could be the rate-limiting step that modulates vitamin D-dependent regulation TRPV6. When dietary Ca2+ is insufficient, normal blood calcium levels in TRPV6 KO mice are maintained at the expense of bone. In many rodent lines, genetic variations in TRPV6, calbindin-D9k, PMCA1b mRNA influence intestinal Ca absorption and its impact on bone marrow density.


Pregnancy and lactation

Duodenal expression of TRPV6 transcripts is upregulated in WT and VDR KO mice during
pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ...
and
lactation Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...
. The hormone
prolactin Prolactin (PRL), also known as lactotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secreted from the pi ...
upregulates TRPV6 transcription and facilitates an increase in intestinal Ca2+ absorption in lactating and pregnant rats, possibly as a adaptive mechanism for overcoming the loss in bone mineralization content during lactation.


Aging

The intestinal expression of TRPV6 in mice varies dramatically by age and relative tissue location. The duodenal expression of TRPV6 is undetectable at P1 and increases 6-fold as mice age to P14. Similarly, the expression also varies with age in the
jejunum The jejunum is the second part of the small intestine in humans and most higher vertebrates, including mammals, reptiles, and birds. Its lining is specialised for the absorption by enterocytes of small nutrient molecules which have been previou ...
, where TRPV6 levels increases from P1 to P14, become weak at 1-month age and becomes undetectable in older mice. The expression of TRPV6 in older rats (12-months) is at least 50% lower in comparison to younger counterparts (2-months old). In both WT and VDR KO mice, the age-associated decline in intestinal absorption of Ca2+ is accompanied by a decline in duodenal expression of TRPV6.


Sex hormones

Sex hormones play an important role in the regulation of TRPV6. In comparison to male mice, female mice exhibit a 2-fold higher increase in duodenal expression of TRPV6 mRNA following vitamin D treatment. Sex hormone-associated differential regulation of TRPV6 across genders is believed to be correlated to differences in relative risk to osteoporosis in older postmenopausal women which have been reported to have lower TRPV6 and VDR expression in comparison to males. Estrogen treatment upregulates TRPV6 transcripts by 8-fold in VDR KO mice and by 4-fold in ovariectomized mice. Greater than 50% reduction in TRPV6 mRNA has been observed in estrogen receptor α KO mice. It is believed that estrogen could be differentially regulating Ca2+ absorption in the duodenum by increasing TRPV6 expression through ERα. Anti-progesterone agent
RU486 Mifepristone, also known as RU-486, is a medication typically used in combination with misoprostol to bring about a medical abortion during pregnancy and manage early miscarriage. This combination is 97% effective during the first 63 days of p ...
and anti-estrogen agent ICI 182,780 suppress TRPV6 expression in rodents by their respective antagonist action on progesterone and
estrogen receptor Estrogen receptors (ERs) are a group of proteins found inside cells. They are receptors that are activated by the hormone estrogen (17β-estradiol). Two classes of ER exist: nuclear estrogen receptors ( ERα and ERβ), which are members of the ...
s. Estrogen, progesterone, and
dexamethasone Dexamethasone is a glucocorticoid medication used to treat rheumatic problems, a number of skin diseases, severe allergies, asthma, chronic obstructive lung disease, croup, brain swelling, eye pain following eye surgery, superior vena ...
are known to upregulate TRPV6 expression in the
cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting o ...
and
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamus ...
of mice suggesting a potential involvement of TRPV6 in calcium absorption in the
brain A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...
.


Glucocorticoids

Subcutaneous administration of glucocorticoids dexamethasone induces both renal and intestinal expression of TRPV6 in mice within 24 hours of whereas oral application of
prednisolone Prednisolone is a steroid medication used to treat certain types of allergies, inflammatory conditions, autoimmune disorders, and cancers. Some of these conditions include adrenocortical insufficiency, high blood calcium, rheumatoid arth ...
reduction in TRPV6 which is also accompanied by reduced Ca2+ absorption in duodenum. Intestinal regulation of TRPV6 in response to glucocorticoids appears to be VDR-dependent. The enzyme serum and glucocorticoid-regulated kinase 1 ( SKG1) regulates TRPV6 expression by enhancing phosphatidylinositol-3-phosphate-5-kinase PIKfyve (PIP5K3). This kinase is critical for the generation of secondary messenger PIP2, a known lipid activator of TRPV6. ---- *
TRPV TRPV is a family of transient receptor potential cation channels (TRP channels) in animals. All TRPVs are highly calcium selective. TRP channels are a large group of ion channels consisting of six protein families, located mostly on the plasma ...
* TRPV5 *
calcium channel A calcium channel is an ion channel which shows selective permeability to calcium ions. It is sometimes synonymous with voltage-gated calcium channel, although there are also ligand-gated calcium channels. Comparison tables The following tables e ...
s * calcium absorption * transcellular pathway * gating mechanism *
calmodulin Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bin ...
* maternal-fetal transport * transient neonatal hyperparathyroidism *
chronic pancreatitis Chronic pancreatitis is a long-standing inflammation of the pancreas that alters the organ's normal structure and functions. It can present as episodes of acute inflammation in a previously injured pancreas, or as chronic damage with persistent pa ...
*
kidney stones Kidney stone disease, also known as nephrolithiasis or urolithiasis, is a crystallopathy where a calculus (medicine), solid piece of material (kidney stone) develops in the urinary tract. Kidney stones typically form in the kidney and leave the ...
*
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...


Notes


References


Further reading

* * * *


External links

* * {{Transient receptor potential channel modulators Ion channels