The Role of Vasopressin V2 Receptor in Drug-Induced Hyponatremia

Hyponatremia is frequently encountered in clinical practice and usually induced by renal water retention. Many medications are considered to be among the various causes of hyponatremia, because they either stimulate the release of arginine vasopressin (AVP) or potentiate its action in the kidney. Antidepressants, anticonvulsants, antipsychotics, diuretics, and cytotoxic agents are the major causes of drug-induced hyponatremia. However, studies addressing the potential of these drugs to increase AVP release from the posterior pituitary gland or enhance urine concentration through intrarenal mechanisms are lacking. We previously showed that in the absence of AVP, sertraline, carbamazepine, haloperidol, and cyclophosphamide each increased vasopressin V2 receptor (V2R) mRNA and aquaporin-2 (AQP2) protein and mRNA expression in primary cultured inner medullary collecting duct cells. The upregulation of AQP2 was blocked by the V2R antagonist tolvaptan or protein kinase A (PKA) inhibitors. These findings led us to conclude that the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is the main mechanism of drug-induced hyponatremia. Previous studies have also shown that the V2R has a role in chlorpropamide-induced hyponatremia. Several other agents, including metformin and statins, have been found to induce antidiuresis and AQP2 upregulation through various V2R-independent pathways in animal experiments but are not associated with hyponatremia despite being frequently used clinically. In brief, drug-induced hyponatremia can be largely explained by AQP2 upregulation from V2R-cAMP-PKA signaling in the absence of AVP stimulation. This paper reviews the central and nephrogenic mechanisms of drug-induced hyponatremia and discusses the importance of the canonical pathway of AQP2 upregulation in drug-induced NSIAD.

Updates and Perspectives on Aquaporin-2 and Water Balance Disorders

Ensuring the proper amount of water inside the body is essential for survival. One of the key factors in the maintenance of body water balance is water reabsorption in the collecting ducts of the kidney, a process that is regulated by aquaporin-2 (AQP2). AQP2 is a channel that is exclusively selective for water molecules and impermeable to ions or other small molecules. Impairments of AQP2 result in various water balance disorders, including nephrogenic diabetes insipidus (NDI), which is a disease characterized by a massive loss of water through the kidney and consequent severe dehydration. Dysregulation of AQP2 is also a cause of water retention with hyponatremia in heart failure, hepatic cirrhosis, and syndrome of inappropriate antidiuretic hormone secretion (SIADH). Antidiuretic hormone vasopressin is an upstream regulator of AQP2. Its binding to the vasopressin V2 receptor promotes AQP2 targeting to the apical membrane and thus enables water reabsorption. Tolvaptan, a vasopressin V2 receptor antagonist, is effective and widely used for water retention with hyponatremia. However, there are no studies showing improvement in hard outcomes or long-term prognosis. A possible reason is that vasopressin receptors have many downstream effects other than AQP2 function. It is expected that the development of drugs that directly target AQP2 may result in increased treatment specificity and effectiveness for water balance disorders. This review summarizes recent progress in studies of AQP2 and drug development challenges for water balance disorders.

Systems Biology of the Vasopressin V2 Receptor: New Tools for Discovery of Molecular Actions of a GPCR

Systems biology can be defined as the study of a biological process in which all of the relevant components are investigated together in parallel to discover the mechanism. Although the approach is not new, it has come to the forefront as a result of genome sequencing projects completed in the first few years of the current century. It has elements of large-scale data acquisition (chiefly next-generation sequencing–based methods and protein mass spectrometry) and large-scale data analysis (big data integration and Bayesian modeling). Here we discuss these methodologies and show how they can be applied to understand the downstream effects of GPCR signaling, specifically looking at how the neurohypophyseal peptide hormone vasopressin, working through the V2 receptor and PKA activation, regulates the water channel aquaporin-2. The emerging picture provides a detailed framework for understanding the molecular mechanisms involved in water balance disorders, pointing the way to improved treatment of both polyuric disorders and water-retention disorders causing dilutional hyponatremia. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Prediction of Molecular Interactions And Physicochemical Properties Relevant For Vasopressin V2 Receptor Antagonism

We have developed two ligand and receptor-based computational approaches to study the physicochemical properties relevant to the biological activity of vasopressin V2 receptor (V2R) antagonist and eventually to predict the expected binding mode to V2R. The obtained Quantitative Structure Activity Relationship (QSAR) model showed a correlation of the antagonist activity with the hydration energy (EH2O) , the polarizability (P) and the calculated partial charge on atom N7 (q6) of the common substructure. The first two descriptors showed a positive contribution to antagonist activity, while the third one had a negative contribution. V2R was modeled and further relaxed on a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocoline (POPC) membrane by molecular dynamics simulations. The receptor antagonist complexes were guessed by molecular docking, and the stability of the most relevant structures were also evaluated by molecular dynamics simulations. As a result, amino acid residues Q96, W99, F105, K116, F178, A194, F307, and M311 were identified with the probably most relevant antagonist-receptor interactions on the studied complexes. The proposed QSAR model could explain the molecular properties relevant to the antagonist activity. The contributions to the antagonist-receptor interaction appeared also in agreement with the binding mode of the complexes obtained by molecular docking and Molecular Dynamics. These models will be used in further studies to look for new V2R potential antagonist molecules.

Case Report: Long-Term Tolvaptan Treatment in a Child With SIADH and Suprasellar Arachnoid Cyst

Suprasellar arachnoid cysts represent a rare occurrence in the pediatric population and usually cause symptoms related to mass effect and can occasionally cause endocrine dysfunctions. The association between SAC and the syndrome of inappropriate antidiuretic hormone (SIADH) in the pediatric population has rarely been described previously. In most cases, SIADH is temporary and resolves by treating the underlying cause. The first-line treatment consists of fluid restriction in asymptomatic children. Oral urea and demeclocycline are other effective treatment options. Vaptans are a new class of medication for the management of SIADH. These agents are a nonpeptide vasopressin V2 receptor antagonist that selectively antagonizes the antidiuretic effect of AVP, resulting in excretion of diluted urine or “aquaresis.” Their efficacy has been shown in adult patients with euvolemic or hypervolemic hyponatremia. However, evidence is lacking in pediatric patients with SIADH. We report the case of a 9-year-old female child with a SAC, who underwent endoscopic fenestration at the age of 2 years. After surgery she developed chronic hyponatremia due to SIADH. Hyponatremia was refractory to treatment with fluid restriction, oral sodium, and urea. In order to normalize serum sodium levels, tolvaptan treatment was started on a compassionate-use basis; 24–48 h later serum sodium levels returned to normal. To date, tolvaptan has been used regularly for 6 years with no side effects occurring during the treatment period. This is the first case of a child with chronic SIADH secondary to SAC successfully treated with tolvaptan. Further studies are needed to demonstrate its usefulness on a broader case series.

A new Kunitz-type snake toxin family associated with an original mode of interaction with the vasopressin 2 receptor.

Background and purpose. Venomous animals express numerous Kunitz-type peptides. The mambaquaretin-1 (MQ1) recently identified from the Dendroaspis angusticeps venom is the most selective antagonist of the arginine-vasopressin V2 receptor (V2R) and the unique Kunitz-type peptide active on a GPCR. We aimed to exploit other mamba venoms to enlarge the V2R-Kunitz peptide family and get insight into the MQ1 molecular mode of action. Experimental approach. We used a bio-guided screening assay to identify novel MQs and placed them phylogenetically. Several newly identified MQs were produced by solid phase peptide synthesis. They were characterized in vitro by binding and functional tests andin vivo by diuresis measurement in rats. Key results. Eight additional MQs were identified with nanomolar affinities for the V2R, all antagonists. MQs form a new subgroup in the Kunitz family, close to the V2R non-active dendrotoxins and to 2 V2R active cobra toxins. Sequence comparison between active and non-active V2R Kunitz peptides highlighted 5 specific V2R positions. Four of them are involved in V2R activity and belong to the 2 large MQ1 loops. We finally determined that 8 positions, part of these 2 loops, interact with the V2R. The variant MQ1-K39A showed specificity for the human versus the rat V2R . Conclusions and implications. A third function and mode of action is now associated with the Kunitz-peptides. The number of MQ1 residues involved in V2R binding is large and may explain its absolute selectivity. MQ1-K39A represents the first step in the improvement of the MQ1 design for medicinal perspective.

Tolvaptan Response in a Hyponatremic Newborn with Syndrome of Inappropriate Secretion of Antidiuretic Hormone

The use of tolvaptan to treat both euvolemic and hypervolemic hyponatremia has rapidly increased in recent years. However, data on its effects on children, especially newborns and infants, are limited. Here, we present a newborn who developed syndrome of inappropriate secretion of antidiuretic hormone following an intracranial hematoma drainage operation who was unresponsive to conventional treatments. The infant was successfully treated with tolvaptan, a competitive inhibitor of the vasopressin V2 receptor.