AVP gene
Arginine Vasopressin (AVP) is a gene that encodes vasopressin, neurophysin, and glycoprotein. AVP is present on chromosome 20 in humans and plays a role in homeostatic regulation. Expression of AVP is regulated by the TTFL, which is an important part of the circadian system. AVP has important implications in the medical field due to the roles its products have in the body.
Discovery
The discovery of the AVP gene began with 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 [1]. 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 [2].
The discovery and separation of vasopressin allowed for subsequent research on its structure and function. In 1951, Turner and colleagues uncovered the amino acid sequence behind the hormone. The nine amino acid structure was comprised of phenylalanine, tyrosine, proline, glutamic acid, aspartic acid, glycine, arginine, cystine, and ammonia [3]. Following this discovery, de Vigneaud was able to synthesize a synthetic form of vasopressin in a laboratory setting. De Vigneaud specifically noted that his final product had the same activity and composition ratios as that of naturally occurring vasopressin [4].
The final stage of research leading to the discovery of the AVP gene began when Gainer and colleagues found a precursor protein to vasopressin in 1977 [5]. 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 [6]. 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) each was responsible for [7].
Structure
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 three domains spanning 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[Schmale].
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 SCN[Hiroshi]. 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[Michihiro]. In addition to the E-box element, the promoter region of the AVP gene also contains a cAMP response element (CRE) site that is involved in gene expression regulation. Daily rhythms in the phosphorylation of the CRE binding protein (CREB) supports that these elements also contribute to circadian rhythmicity of the gene expression. 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[Hiroshi].
Transcription of the AVP gene to produce AVP mRNA has daily rhythms, with mRNA levels peaking during the subjective day and reaching its 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[Hiroshi].
Function
AVP, or arginine vasopressin, is primarily known for its role as a mammalian molecular output [1,2]. The most common product of AVP is vasopressin which is a neurohypophysial hormone that is important in homeostatic mechanisms and processes and its other products are neurophysin and glycoprotein. AVP is produced in a specific type of neuron called magnocellular neurons (MCNs), which are located in the hypothalamus [2]. In mammals, the AVP gene is transcribed in the SCN, which is also in the hypothalamus, under the regulation of the genetic transcription-translation feedback loop (TTFL). The TTFL is an essential part of circadian clocks since it is the molecular machinery that controls the expression of clock genes [3]. The AVP mRNA transcript travels from the hypothalamus to the posterior pituitary where it is stored and released into the bloodstream as a result of environmental stressors, like dehydration [4].
A component of this circadian clock mechanism to note is that the AVP gene, and resulting AVP protein, do not need a PAS or BHLH domain, which mediate the various interactions that occur between transcription factors. This means that the AVP gene and resulting protein are structurally stable and can self-sustain binding processes and molecular transportation.
The transcription of the AVP gene commonly results in the vasopressin peptide that can bind to one of three vasopressin receptors: AVPR1A, AVPR1B, and AVPR2. When vasopressin binds to AVPR1A, a G-protein coupled receptor (GPCR), phospholipase C becomes activated [5][6]. This pathway typically involves regulating vasoconstriction. When vasopressin binds to AVPR1B, a GPCR, the phosphatidylinositol-calcium second messenger system is stimulated. This signaling pathway is important in regulating homeostasis and the amount of water, glucose, and salts within the blood via ACTH release and storage [7]. When vasopressin binds to AVPR2, a 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 concentrating liquid toxins in urine [8].
Rats
Within rats, the AVP gene is important for the regulation of various processes within the excretory system and smooth muscle cells. The AVP gene and arginine vasopressin are commonly colocalized with oxytocin due to how synaptic transmission of oxytocin influences the AVP mRNA expression [9].
In a clinical study, the AVP gene expression in rats is regulated by the cAMP responsive element-binding protein-3 like-1 (CREB3L1). The CREB3L1 is activated when the N-terminal of the AVP gene is cleaved during translocation from the Golgi to the nucleus [10]. Additionally, the 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 [10]. 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.
The arginine vasopressin stimulates the process of phosphorylation of aquaporin 2 (AQP2) at renal tissue, which contributes to the overall increased permeability of water in the collecting duct cells of the tissue [11]. 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.
Medical Applications
Vasopressin, a product of the AVP gene, has a variety of important medical applications. These applications include treatment of nocturnal enuresis, diabetes insipidus, and hemophilia A [1]. Additionally, it is used to treat some forms of shock, such as septic shock and vasoplegic shock [2]. It is also used during surgery to decrease blood loss [3].