The arginine vasopressin (AVP) gene is a gene whose product is proteolytically cleaved to produce vasopressin (also known as antidiuretic hormone or ADH), neurophysin II, and a glycoprotein called copeptin. AVP and other AVP-like peptides are found in mammals, as well as mollusks, arthropods, nematodes, and other invertebrate species. [5] In humans, AVP is present on chromosome 20 and plays a role in homeostatic regulation. The products of AVP have many functions that include vasoconstriction, regulating the balance of water in the body, and regulating responses to stress. [6] Expression of AVP is regulated by the transcription translation feedback loop (TTFL), which is an important part of the circadian system that controls the expression of clock genes. AVP has important implications in the medical field as its products have significant roles throughout body.
preproAVP | |||||||
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Identifiers | |||||||
Symbol | preproAVP | ||||||
OMIM | 192340 | ||||||
UniProt | P01185 | ||||||
Other data | |||||||
Locus | Chr. 20 p13 | ||||||
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The discovery of the AVP gene first required the discovery of one of its key products: vasopressin. In 1895, G. Oliver and E.A. Schäfer found that a substance released by the pituitary gland could elevate blood pressure. The researchers noted that intravenous injection of extracts from the pituitary gland, thyroid gland, and spleen all influence blood pressure, however the effect from the pituitary had the most significant impact. [7] Almost thirty years later, Kamm and colleagues separated the components within the pituitary gland. Using a unique, five-step separation technique, Kamm revealed one substance associated with uterine contractions – oxytocin – and another substance associated with blood pressure – vasopressin. [8]
Once the discovery and separation of vasopressin occurred, subsequent research on the product's structure, function, and mode of generation could take place. In 1951, Turner and colleagues uncovered the amino acid sequence behind the hormone. The amino acid structure was composed of eight amino acids, including phenylalanine, tyrosine, proline, glutamic acid, aspartic acid, glycine, arginine, and cystine. The structure was also found to include ammonia. [9] Following this discovery, Vincent du Vigneaud was able to synthesize a synthetic form of vasopressin in a laboratory setting. Du Vigneaud specifically noted that his final product had the same activity and composition ratios as that of naturally occurring vasopressin. [10]
The work on vasopressin ultimately allowed researchers to work backwards and identify the gene responsible for this product's generation. This final stage of research began when Gainer and colleagues found a precursor protein to vasopressin in 1977. [11] The structure of the protein was subsequently discovered by Land in 1982. By sequencing complementary DNA strands that encoded for the hormone's mRNA, Land outlined the amino acid sequence of the precursor protein. [12] Finally, one year later, Schmale, Heinsohn, and Richter isolated the AVP precursor gene in rats from their genomic library. The researchers used restriction mapping and nucleotide sequence analysis to uncover the gene's three distinct exons and the products (vasopressin, neurophysin, and glycoprotein) that each was responsible for. [13]
The 1.85 kilobase-long AVP gene, located on chromosome 20 (20p13) contains three functional domains, including AVP, neurophysin II (NP) and a C-terminal glycopeptide called copeptin. Using restriction mapping and sequencing, the gene was found to have these domains which spanned over three exons with two intronic sequences. Exon A encodes a putative signal peptide, the arginine vasopressin hormone, and the N terminus of the NP carrier protein. Exon B, which is separated from exon A with a 1 kilobase-long intron, encodes the conserved middle portion of NP. A 227 kilobase intron separates exon B from exon C, which encodes the final domain, including the C terminus of NP and the glycoprotein. The structure of this gene has been found to be generally conserved across species, including chimpanzees, Rhesus monkeys, dogs, cows, mice, rats, chicken, zebrafish, and frogs. [13]
The AVP gene promoter region consists of an E-box element located 150 residues upstreams of the transcription start site, which binds mammalian clock proteins CLOCK and BMAL1 involved in generating circadian rhythms in the suprachiasmatic nucleus (SCN). [14] BMAL1 and CLOCK gene knockouts in the SCN (Bmal-/- and clk-/-) eliminate rhythmicity in AVP mRNA expression, confirming that binding of the protein heterodimers to the E-box element is necessary for the intrinsic circadian pattern of the AVP gene. [15] In addition to the E-box element, the promoter region of the AVP gene also contains a cyclic AMP (cAMP) response element (CRE) site that is involved in gene expression regulation. Daily rhythms in the phosphorylation of the CRE-binding protein (CREB) support a contribution of this element to circadian rhythmicity of the gene's expression. [14] [16] CRE/CREB-mediated regulation of the AVP gene is activated through the cAMP activation of Ras signaling pathways, culminating in the MAP kinase phosphorylation of the CREB transcription factor. [14]
Transcription of the AVP gene to produce AVP mRNA has daily rhythms, with mRNA levels peaking during the subjective day and reaching their lowest point in the subject night. This rhythm is regulated by the binding of circadian proteins to the E-box, along with transcriptional regulation of other elements, including the CRE in the promoter region. [14]
AVP is primarily known for its role as a mammalian circadian output. [15] [17] The most common product of AVP is vasopressin, which is a neurohypophysial hormone that is important in homeostatic mechanisms and processes, like cortical EEG rhythms. Its other products are neurophysin II and copeptin. AVP is produced in a specific type of neuron called magnocellular neurons (MCNs), which are located in the hypothalamus. [17] In mammals, the AVP gene is also transcribed in the suprachiasmatic nucleus where its expression is under the control of the circadian TTFL. [18] It is well established that the finished vasopressin product is transported from the cell body to the terminals in the posterior pituitary gland where it is released into the bloodstream as a result of environmental stressors like dehydration. [19]
AVP gene regulation is controlled by the TTFL. In this system, per2 protein is transcribed and phosphorylated by the CK1E enzyme. Accumulation of this protein inhibits the transcription factors Clock and BMAL1, so that no additional per product is created. [20] At the same time, per2 inhibits the transcription factors driving the AVP gene so that its expression and products are reduced. [19] It can be noted that AVP expression is regulated by the TTFL in the SCN but not in the paraventricular nucleus of the hypothalamus (PVH) and supraoptic nucleus (SON). [21]
Vasopressin is able to bind to one of three vasopressin receptors: AVPR1A, AVPR1B, and AVPR2. When vasopressin binds to AVPR1A, a G protein-coupled receptor (GPCR), phospholipase C (PLC) becomes activated. [22] [23] This pathway typically regulates vasoconstriction. When vasopressin binds to AVPR1B, a GPCR, the phosphatidylinositol-calcium second messenger system is stimulated. AVPR1A and AVPR1B are GPCRs that stimulate PLC to promote the production of DAG which activates PKC and IP3 and stimulates calcium ion release from the endoplasmic reticulum. This signaling pathway is important in regulating homeostasis and the amount of water, glucose, and salts within the blood via ACTH release and storage. [24] When vasopressin binds to AVPR2, another GPCR, adenylyl cyclase is stimulated. This second messenger pathway involves the regulation of ADH, or vasopressin, in the kidneys, which has an important diuretic purpose of retaining water and manipulating the concentrations of soluble toxic wastes and urea in urine. [25]
Within rats, the AVP gene is important for the regulation of various processes associated with the excretory system and smooth muscle cells. The AVP gene and arginine vasopressin are commonly colocalized with oxytocin because synaptic transmission of oxytocin regulates the expression of AVP mRNA. [26]
In one study conducted by Greenwood and colleagues, researchers found that AVP gene expression in rats is regulated by the cAMP responsive element-binding protein-3 like-1 (CREB3L1). CREB3L1 is activated when it is cleaved and when the AVP gene is translocated from the Golgi to the nucleus. [27] Additionally, CREB3L1 mRNA levels correspond with increased amounts of transcription of the AVP gene in the hypothalamus following a deficiency of sodium and as a consequence of diurnal rhythm in the SCN. [27] Both full-length and constitutively active forms of CREB3L1 (CREB3L1CA) induce the expression of rat AVP promoter-luciferase reporter constructs whereas a dominant-negative mutant reduces expression. From this study, the researchers concluded that CREB3L1 is a regulator of AVP gene transcription in the hypothalamus and specifically within the PVH and SON.
Arginine vasopressin stimulates the process of phosphorylation of aquaporin 2 (AQP2) of renal tissue which contributes to the overall increased permeability of water in the collecting duct cells of the tissue. The phosphorylation of AQP2 leads to activation of the protein kinase A signaling pathway which amplifies the permeability of water by stimulating the rat equivalent of the urea transporter 1 protein. [28]
The functions of vasopressin make it useful for a variety of important medical applications. Since it plays a role in the regulation of many physiological functions, like regulation of water and sodium excretion, blood volume, vasoconstriction, and response to stress, vasopressin can be helpful in the treatment of conditions related to these functions. [29] These applications include treatment of nocturnal enuresis, diabetes insipidus, and hemophilia A. [30] Additionally, it is used to treat some forms of shock including septic shock and vasodilatory shock. It is also used during surgery to decrease blood loss. [31]
The hypothalamus is a small part of the vertebrate brain that contains a number of 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 is located below the thalamus and is part of the limbic system. It forms the basal part of the diencephalon. All vertebrate brains contain a hypothalamus. In humans, it is about the size of an almond.
Human vasopressin, also called antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin, is a hormone synthesized from the AVP gene as a peptide prohormone in neurons in the hypothalamus, and is converted to AVP. It then travels down the axon terminating in the posterior pituitary, and is released from vesicles into the circulation in response to extracellular fluid hypertonicity (hyperosmolality). AVP has two primary functions. First, it increases the amount of solute-free water reabsorbed back into the circulation from the filtrate in the kidney tubules of the nephrons. Second, AVP constricts arterioles, which increases peripheral vascular resistance and raises arterial blood pressure.
The paraventricular nucleus of hypothalamus is a nucleus in the hypothalamus, that lies next to the third ventricle. Many of its neurons project to the posterior pituitary where they secrete oxytocin, and a smaller amount of vasopressin. Other secretions are corticotropin-releasing hormone (CRH) and thyrotropin-releasing hormone (TRH). CRH and TRH are secreted into the hypophyseal portal system, and target different neurons in the anterior pituitary. Dysfunctions of the PVN can cause hypersomnia in mice. In humans, the dysfunction of the PVN and the other nuclei around it can lead to drowsiness for up to 20 hours per day. The PVN is thought to mediate many diverse functions through different hormones, including osmoregulation, appetite, wakefulness, and the response of the body to stress.
The suprachiasmatic nucleus or nuclei (SCN) is a small region of the brain in the hypothalamus, situated directly above the optic chiasm. It is the principal circadian pacemaker in mammals, responsible for generating circadian rhythms. Reception of light inputs from photosensitive retinal ganglion cells allow it to coordinate the subordinate cellular clocks of the body and entrain to the environment. The neuronal and hormonal activities it generates regulate many different body functions in an approximately 24-hour cycle.
CREB-TF is a cellular transcription factor. It binds to certain DNA sequences called cAMP response elements (CRE), thereby increasing or decreasing the transcription of the genes. CREB was first described in 1987 as a cAMP-responsive transcription factor regulating the somatostatin gene.
A circadian clock, or circadian oscillator, also known as one’s internal alarm clock is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time.
Vasoactive intestinal peptide, also known as vasoactive intestinal polypeptide or VIP, is a peptide hormone that is vasoactive in the intestine. VIP is a peptide of 28 amino acid residues that belongs to a glucagon/secretin superfamily, the ligand of class II G protein–coupled receptors. VIP is produced in many tissues of vertebrates including the gut, pancreas, cortex, and suprachiasmatic nuclei of the hypothalamus in the brain. VIP stimulates contractility in the heart, causes vasodilation, increases glycogenolysis, lowers arterial blood pressure and relaxes the smooth muscle of trachea, stomach and gallbladder. In humans, the vasoactive intestinal peptide is encoded by the VIP gene.
Vasopressin receptor 1A (V1AR), or arginine vasopressin receptor 1A is one of the three major receptor types for vasopressin, and is present throughout the brain, as well as in the periphery in the liver, kidney, and vasculature.
Vasopressin V1b receptor (V1BR) also known as vasopressin 3 receptor (VPR3) or antidiuretic hormone receptor 1B is a protein that in humans is encoded by the AVPR1B gene.
Vasotocin is an oligopeptide homologous to oxytocin and vasopressin found in all non-mammalian vertebrates and possibly in mammals during the fetal stage of development. Arginine vasotocin (AVT), a hormone produced by neurosecretory cells within the posterior pituitary gland (neurohypophysis) of the brain, is a major endocrine regulator of water balance and osmotic homoeostasis and is involved in social and sexual behavior in non-mammalian vertebrates. In mammals, it appears to have biological properties similar to those of oxytocin and vasopressin. It has been found to have effects on the regulation of REM sleep. Evidence for the existence of endogenous vasotocin in mammals is limited and no mammalian gene encoding vasotocin has been confirmed.
Timeless (tim) is a gene in multiple species but is most notable for its role in Drosophila for encoding TIM, an essential protein that regulates circadian rhythm. Timeless mRNA and protein oscillate rhythmically with time as part of a transcription-translation negative feedback loop involving the period (per) gene and its protein.
Neuronal PAS domain protein 2 (NPAS2) also known as member of PAS protein 4 (MOP4) is a transcription factor protein that in humans is encoded by the NPAS2 gene. NPAS2 is paralogous to CLOCK, and both are key proteins involved in the maintenance of circadian rhythms in mammals. In the brain, NPAS2 functions as a generator and maintainer of mammalian circadian rhythms. More specifically, NPAS2 is an activator of transcription and translation of core clock and clock-controlled genes through its role in a negative feedback loop in the suprachiasmatic nucleus (SCN), the brain region responsible for the control of circadian rhythms.
The PER3 gene encodes the period circadian protein homolog 3 protein in humans. PER3 is a paralog to the PER1 and PER2 genes. It is a circadian gene associated with delayed sleep phase syndrome in humans.
Rev-Erb alpha (Rev-Erbɑ), also known as nuclear receptor subfamily 1 group D member 1 (NR1D1), is one of two Rev-Erb proteins in the nuclear receptor (NR) family of intracellular transcription factors. In humans, REV-ERBɑ is encoded by the NR1D1 gene, which is highly conserved across animal species.
Period circadian protein homolog 1 is a protein in humans that is encoded by the PER1 gene.
Basic helix-loop-helix ARNT-like protein 1 or aryl hydrocarbon receptor nuclear translocator-like protein 1 (ARNTL), or brain and muscle ARNT-like 1 is a protein that in humans is encoded by the BMAL1 gene on chromosome 11, region p15.3. It's also known as MOP3, and, less commonly, bHLHe5, BMAL, BMAL1C, JAP3, PASD3, and TIC.
Hitoshi Okamura is a Japanese scientist who specializes in chronobiology. He is currently a professor of Systems Biology at Kyoto University Graduate School of Pharmaceutical Sciences and the Research Director of the Japan Science Technology Institute, CREST. Okamura's research group cloned mammalian Period genes, visualized clock oscillation at the single cell level in the central clock of the SCN, and proposed a time-signal neuronal pathway to the adrenal gland. He received a Medal of Honor with Purple Ribbon in 2007 for his research and was awarded Aschoff's Ruler for his work on circadian rhythms in rodents. His lab recently revealed the effects of m6A mRNA methylation on the circadian clock, neuronal communications in jet lag, and the role of dysregulated clocks in salt-induced hypertension.
Transcription-translation feedback loop (TTFL) is a cellular model for explaining circadian rhythms in behavior and physiology. Widely conserved across species, the TTFL is auto-regulatory, in which transcription of clock genes is regulated by their own protein products.
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Elizabeth Maywood is an English researcher who studies circadian rhythms and sleep in mice. Her studies are focused on the suprachiasmatic nucleus (SCN), a small region of the brain that controls circadian rhythms.