Microgravity significantly impacts fluid redistribution in the human body, and vasopressin (AVP) plays a crucial, yet complex, role in this process. Under chronic adaptation to microgravity, astronauts typically experience a reduction in total body water, plasma volume, and extracellular fluid volume due to several factors, including altered renal function. In the kidney collecting duct, AVP increases water reabsorption by redistribution of aquaporin-2 (AQP2) water channels. To evaluate the brain-kidney interplay in microgravity at cellular levels, mouse hypothalamic mHypoA-2/28 cells derived from the supraoptic nucleus (SON) and found to express the AQP4 water channel as in native cells, were exposed to simulated microgravity using the Random Positioning Machine (RPM) for 24 h in a hyperosmolar medium, to simulate the physiological stimulus for AVP release. This treatment resulted in a significant increase in AVP mRNA levels. Collecting duct MCD4 cells exposed to the cell culture medium collected from mHypoA-2/28, displayed a significant enrichment of AQP2 in the plasma membrane fraction, confirming AQP2 redistribution in response to actual AVP released from mHypoA-2/28 cells. In line, MCD4 cells exposed to microgravity had a significant higher rate of AQP2-expressing exosome release in the medium, suggesting that microgravity stimulates AQP2 recycling. Together these data indicate that microgravity promotes AVP release from SON thus activating the vasopressin-AQP2 axis. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Astronauts have been reported to have an unusually high rate of kidney stone formation during spaceflight, which represents a risk for the health and for the space mission success. Our studies in humans adapted to simulated microgravity (bedrest) demonstrate that exposure to microgravity results in alterations of renal function, fluid redistribution, and bone loss, which is coupled to a rise of urinary calcium excretion thus increasing the risk of renal stone formation. Vasopressin, the main hormone involved in water balance regulation, promotes urine concentration and reduces bone mass. Interestingly, measurements of vasopressin in astronauts (from 24 h to 8 days after launch), revealed concentrations considerably elevated compared to ground, suggesting a critical role for this hormone in renal stone risk in microgravity. In line, in a previous 10-days bedrest study, we provided evidence that bedrest is associated with an early increased risk of stone formation that was monitored by novel identified biomarkers. The impact of simulated microgravity in risk of stone formation is currently under investigation in a more recent 21-day bedrest campaign as part of Science for Bed Rest program of Italian Space Agency (ASI). This study involves, for the first time, men and women, thus allowing a more general understanding of sex difference in hormonal response to simulated microgravity. Preliminary results indicate a tendency toward increased vasopressin levels, evaluated as copeptin, a validated and reliable surrogate marker of vasopressin, over 21-day bedrest both in men (n=6) and women (n=6) with a peak at day 15. These preliminary data suggest that both men and women may respond to changes in blood volume or osmolality during bedrest and release higher amount of vasopressin to restore decreased plasma and extracellular volume known to occur consequently to adaptation to microgravity. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is caused by mutations in PKD1 or PKD2 genes and is characterized by abnormal cell proliferation and fluid secretion, leading to the formation and progressive enlargement of kidney cysts. Therapeutic treatments with either cinacalcet or metformin have emerged as a potential therapeutic approach to slow renal cystogenesis in ADPKD. Metformin, a drug in wide clinical use, is a pharmacological activator of AMPK, whose activity is abnormally suppressed in the context of ADPKD. On the other hand, cinacalcet, a calcium sensing receptor drug activator, has been shown to reduce cyst enlargement and cell proliferation in ADPKD cells. Here we provide evidence that combined treatment with metformin (200 µM for 3 days) and cinacalcet (1nM for 3 days) significantly reduced forskolin-induced cyst growth of human 3D cells derived from ADPKD cyst-lining epithelia. With respect to the single administration, the combined treatment had a synergistic effect in reducing S6 ribosomal protein phosphorylation (marker of mTORC1), that is early increased in ADPKD. Moreover, the dual treatment fully restored the altered bioenergetic profile observed in ADPKD cells, including improvements in mitochondrial ATP production and the multilevel impairment of oxidative phosphorylation. Together these data suggest that the combination of the two drugs has an additive beneficial effect, making the proposed study promising for therapeutic investigation. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Branching morphogenesis is a key process for constructing the tree-like architecture of multiple organs. The mechanisms regulating pancreatic ductal morphogenesis are still poorly understood, especially in the context of the particular pH dynamics of this organ. Indeed, ductal cells periodically release an alkaline juice to balance stomach acidity during digestion. This leads to a drop in extracellular pH (pHe) in the extracellular matrix (ECM) to maintain intracellular pH (pHi) homeostasis. Among the transporters involved in pH regulation, NHE1 also regulates epithelial branching morphogenesis in various tissues/organs. However, neither the effect of the changing pHe nor the role of NHE1 in branching morphogenesis has been investigated in a physiomimetic model in the human pancreas. Here, using 3D organotypic cultures of human pancreatic ductal cells (HPDE), we found that cells seeded on a Matrigel rich-ECM resembling normal ECM formed branched duct-like structures, which did not form on a more fibrotic Collagen I-rich ECM. Further, these cells overexpressed NHE1 mainly at the basolateral membrane. Ductal morphogenesis was affected by acidic pHe (pHe 6.7), which determined a hyper-branched network, and this was further increased by the inhibition of NHE1. We conclude that ECM composition and extracellular acidosis modulate branching morphogenesis in pancreatic ductal HPDE cells via NHE1 activity.

The hormone vasopressin (AVP) controls renal water reabsorption by modulating the expression and trafficking of the water channel aquaporin-2 (AQP2) through the activation of the cAMP/PKA signal transduction pathway. Previous studies revealed that Olive Leaf Extract (OLE) counteracts the vasopressin-dependent AQP2 functions by stimulating the calcium-sensing receptor (CaSR). Here, the biological activities of p-Coumaric acid, a selective polyphenol in OLE, were investigated. Stimulation of renal collecting duct MCD4 cells with p-Coumaric acid at a concentration of 1 nM caused a significant intracellular calcium release. NPS-2143, a selective CaSR antagonist, abolished this increase. Molecular docking analysis revealed that p-Coumaric acid can form binding interactions with the binding pocket of Tecalcet, a known CaSR activator, likely suggesting that p-Coumaric acid may stimulate the CaSR. Confocal analysis and immunoblotting experiments showed that p-Coumaric acid impaired the DDAVP-dependent membrane expression of AQP2 and the consequent increase of the osmotic water permeability (Pf). Additionally, Fluorescence Resonance Energy Transfer (FRET) experiments demonstrated that p-Coumaric acid prevented the DDAVP-induced cAMP generation, consequently attenuating the AQP2 phosphorylation at serine 256. Together, these findings suggest that p-Coumaric acid may antagonize the effects of vasopressin, possibly by binding to and stimulating the CaSR.

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is caused by mutations in PKD1 or PKD2 genes, encoding polycystin-1 (PC1) or polycystin-2 (PC2), respectively, characterized by excessive cell proliferation and fluid secretion, resulting in renal cyst formation and growth. PC1 and PC2 form a complex localized on the plasma membrane, endoplasmic reticulum, and primary cilia. PC2 is a non-selective cation channel which, in renal epithelial cells, contributes to calcium transport and signaling. It has been previously shown in renal cells that high external calcium increases whole-cell currents likely mediated by PC2. In this study, we explored the possibility that the Calcium Sensing Receptor (CaSR) is involved in the functional regulation of PC2. To test this hypothesis, human conditionally immortalized Proximal Tubular Epithelial cells, isolated from urine sediments, wt or with stably downregulated PKD1 (PC1KD) or PKD2 (PC2KD) were used. Interestingly, CaSR and PC2 co-immunoprecipitated and Proximity Ligation Assay demonstrated a direct physical interaction at endogenous protein levels. Membrane potential measurements demonstrated that selective CaSR activation, elicited by the calcimimetic R568, caused plasma membrane depolarization, consistent with the modulation of PC2-mediated cation currents, which was significantly lower in PC2KD with respect to wt and PC1KD cells. To conclude, this study provides evidence for a functional coupling of CaSR and PC2, which might be relevant for therapeutic strategies to correct dysregulations occurring in ADPKD.

Murine hindlimb unloading (HU) is considered a model of choice for simulating the physiological effects of microgravity on several functions, including fluid and electrolyte homeostasis. Microgravity causes changes in blood redistribution, modulating vasopressin secretion, a major hormone controlling water reabsorption through the vasopressin-sensitive water channel AQP2. In this study, mice were hindlimb suspended over 4 weeks or rested in the ground as controls, and vasopressin levels, along with renal aquaporins expression were investigated. Copeptin, a stable precursor of the hormone vasopressin, significantly increased as early as 1 week of unloading which correlated with a significant increase in AQP2 total protein expression and decrease in serum osmolality, suggesting early activation of the vasopressin/AQP2 axis in this model. Conversely, in 4 weeks HU suspended mice, copeptin decreased significantly and both AQP2 mRNA and AQP2 total protein expression were significantly reduced. Consistent with a downregulation of the vasopressin/AQP2 axis an increase in serum osmolality was observed at 4 weeks HU. The basolateral water channels AQP3 and AQP4 were, on the other hand, unaffected. Immunolocalization studies confirmed reduced expression of AQP2 in renal collecting ducts of HU mice at 4 weeks. A significantly increased amount of the expressed AQP2 was found phosphorylated at Ser261, a site regulating AQP2 protein stability and degradation. In line, p38-MAPK, committed to phosphorylate Ser261 and to increase miR137 expression, an AQP2 mRNA-targeted microRNA, was significantly increased in HU, suggesting that reduced AQP2 expression was mainly due to increased protein degradation and downregulation of AQP2-mRNA translation. Our results suggest that vasopressin/AQP2 axis is upregulated as early as 1 week and may be involved in the antidiuretic response also observed in early spaceflight period in astronauts. Contrariwise, the vasopressin-AQP2 system is downregulated after 4 weeks HU, likely to counteract the persistent central venous pressure due to cephalic shift of fluids.

The main goal of this study is the identification of existing drugs that could be repurposed as antagonists of the V2R, a GPCR controlling renal water balance and involved in abnormal cell proliferation, cancer, and renal cyst enlargement. Given its clinical importance, we carried out the reverse screening of a collection of 1882 existing drugs to repurpose them towards V2R by employing PLATO, a home-built target fishing AI-based platform. Five drugs were shortlisted as promising candidates for V2R: cabergoline, clopidogrel, cloxacillin, perphenazine, and zafirlukast. Renal collecting duct MCD4 cells, stably expressing human V2R and AQP2, were used for experimentally testing the effects of the prioritized drugs on V2R responses. FRET studies were conducted to assess whether these drugs affect the DDAVP-induced cAMP responses. Interestingly, zafirlukast, at single-digit nanomolar concentration significantly reduced the DDAVP-dependent cAMP production and water reabsorption, with effects comparable to tolvaptan, a well-known selective V2R antagonist. The molecular rationale behind the observed binding was explained by mapping on the V2R a molecular cleft superimposable to CysLTR1 binding site of zafirlukast. Induced-fit docking simulations demonstrated that zafirlukast engages V2R by adopting a binding conformation closely resembling that of X-ray solved vasopressin. Taken together, our results support the repurposing of zafirlukast as a promising V2R antagonist candidate.

The water channel AQP3 is an aquaglyceroporin expressed in villus epithelial cells, and it plays a role in water transport across human colonic surface cells. Beyond water, AQP3 can mediate glycerol and H2O2 transport. Abnormal expression and function of AQP3 have been found in various diseases often characterized by altered cell growth and proliferation. Here, the beneficial effects of MOMAST® have been evaluated. MOMAST® is an antioxidant-patented natural phenolic complex obtained from olive wastewater (OWW) of the Coratina cultivar. Treatment of human colon HCT8 cells with MOMAST® reduced cell viability. Confocal studies and Western Blotting analysis demonstrated that treatment with MOMAST® significantly decreased the staining and the expression of AQP3. Importantly, functional studies revealed that the reduction of AQP3 abundance correlates with a significant decrease in glycerol and H2O2 uptake. Indeed, the H2O2 transport was partially but significantly reduced in the presence of MOMAST® or DFP00173, a selective inhibitor of AQP3. In addition, the MOMAST®-induced AQP3 decrease was associated with reduced epithelial-mesenchymal transition (EMT)-related proteins such as vimentin and β-catenin. Together, these findings propose MOMAST® as a potential adjuvant in colon diseases associated with abnormal cell growth by targeting AQP3.

The small Rho GTP-binding proteins are important cell morphology, function, and apoptosis regulators. Unlike other Rho proteins, RhoB can be subjected to either geranylgeranylation (RhoB-GG) or farnesylation (RhoB-F), making that the only target of the farnesyltransferase inhibitor (FTI). Fluorescence resonance energy transfer experiments revealed that RhoB is activated by hyperosmolarity. By contrast, hyposmolarity did not affect RhoB activity. Interestingly, treatment with farnesyltransferase inhibitor-277 (FTI-277) decreased the cell size. To evaluate whether RhoB plays a role in volume reduction, renal collecting duct MCD4 cells and Human Kidney, HK-2 were transiently transfected with RhoB-wildtype-Enhance Green Fluorescence Protein (RhoB-wt-EGFP) and RhoB-CLLL-EGFP which cannot undergo farnesylation. A calcein-based fluorescent assay revealed that hyperosmolarity caused a significant reduction of cell volume in mock and RhoB-wt-EGFP-expressing cells. By contrast, cells treated with FTI-277 or expressing the RhoB-CLLL-EGFP mutant did not properly respond to hyperosmolarity with respect to mock and RhoB-wt-EGFP expressing cells. These findings were further confirmed by 3D-LSCM showing that RhoB-CLLL-EGFP cells displayed a significant reduction in cell size compared to cells expressing RhoB-wt-EGFP. Moreover, flow cytometry analysis revealed that RhoB-CLLL-EGFP expressing cells as well as FTI-277-treated cells showed a significant increase in cell apoptosis. Together, these data suggested that: (i) RhoB is sensitive to hyperosmolarity and not to hyposmolarity; (ii) inhibition of RhoB farnesylation associates with an increase in cell apoptosis, likely suggesting that RhoB might be a paramount player controlling apoptosis by interfering with responses to cell volume change.

High concentrations of urinary calcium counteract vasopressin action via the activation of the Calcium-Sensing Receptor (CaSR) expressed in the luminal membrane of the collecting duct cells, which impairs the trafficking of aquaporin-2 (AQP2). In line with these findings, we provide evidence that, with respect to wild-type mice, CaSR knock-in (KI) mice mimicking autosomal dominant hypocalcaemia, display a significant decrease in the total content of AQP2 associated with significantly higher levels of AQP2 phosphorylation at Ser261, a phosphorylation site involved in AQP2 degradation. Interestingly, KI mice also had significantly higher levels of phosphorylated p38MAPK, a downstream effector of CaSR and known to phosphorylate AQP2 at Ser261. Moreover, ATF1 phosphorylated at Ser63, a transcription factor downstream of p38MAPK, was significantly higher in KI. In addition, KI mice had significantly higher levels of AQP2-targeting miRNA137 consistent with a post-transcriptional downregulation of AQP2. In vivo treatment of KI mice with the calcilytic JTT-305, a CaSR antagonist, increased AQP2 expression and reduced AQP2-targeting miRNA137 levels in KI mice. Together, these results provide direct evidence for a critical role of CaSR in impairing both short-term vasopressin response by increasing AQP2-pS261, as well as AQP2 abundance, via the p38MAPK-ATF1-miR137 pathway. KEY POINTS: Calcium-Sensing Receptor (CaSR) activating mutations are the main cause of autosomal dominant hypocalcaemia (ADH) characterized by inappropriate renal calcium excretion leading to hypocalcaemia and hypercalciuria. Current treatments of ADH patients with parathyroid hormone, although improving hypocalcaemia, do not improve hypercalciuria or nephrocalcinosis. In vivo treatment with calcilytic JTT-305/MK-5442 ameliorates most of the ADH phenotypes of the CaSR knock-in mice including hypercalciuria or nephrocalcinosis and reverses the downregulation of the vasopressin-sensitive aquaporin-2 (AQP2) expression, providing direct evidence for a critical role of CaSR in impairing vasopressin response. The beneficial effect of calcilytic in reducing the risk of renal calcification may occur in a parathyroid hormone-independent action through vasopressin-dependent inhibition of cAMP synthesis in the thick ascending limb and in the collecting duct. The amelioration of most of the abnormalities in calcium metabolism including hypercalciuria, renal calcification, and AQP2-mediated osmotic water reabsorption makes calcilytic a good candidate as a novel therapeutic agent for ADH.

Angelini, Ines; Centrone, Mariangela; D'Agostino, Mariagrazia; Semeraro, Daniela; Mastrodonato, Maria; Caponio, Giusy Rita; Di Mise, Annarita; Ranieri, Marianna; Valenti, Giovanna; Tamma, Grazia Author Information

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a rare X-linked disease caused by gain-of-function mutations of arginine vasopressin receptor 2 (V2R). Patients with NSIAD are characterized by the inability to excrete a free water load and by inappropriately increased urinary osmolality despite very low levels of plasma vasopressin, resulting in euvolaemic hyponatraemia. To dissect the signalling downstream V2R constitutively active variants, Flp-In T-REx Madin-Darby canine kidney (FTM) cells, stably transfected with V2R mutants (R137L, R137C and F229V) and AQP2-wt or non-phosphorylatable AQP2-S269A/AQP2-S256A, were used as cellular models. All three activating V2R mutations presented constitutive plasma membrane expression of AQP2-wt and significantly higher basal water permeability. In addition, V2R-R137L/C showed significantly higher activity of Rho-associated kinase (ROCK), a serine/threonine kinase previously suggested to be involved in S269-AQP2 phosphorylation downstream of these V2R mutants. Interestingly, FTM cells expressing V2R-R137L/C mutants and AQP2-S269A showed a significant reduction in AQP2 membrane abundance and a significant reduction in ROCK activity, indicating the crucial importance of S269-AQP2 phosphorylation in the gain-of-function phenotype. Conversely, V2R-R137L/C mutants retained the gain-of-function phenotype when AQP2-S256A was co-expressed. In contrast, cells expressing the F229V mutant and the non-phosphorylatable AQP2-S256A had a significant reduction in AQP2 membrane abundance along with a significant reduction in basal osmotic water permeability, indicating a crucial role of Ser256 for this mutant. These data indicate that the constitutive AQP2 trafficking associated with the gain-of-function V2R-R137L/C mutants causing NSIAD is protein kinase A independent and requires an intact Ser269 in AQP2 under the control of ROCK phosphorylation. KEY POINTS: Nephrogenic syndrome of inappropriate antidiuresis is caused by two constitutively active variant phenotypes of AVPR2, one sensitive to vaptans (V2R-F229V) and the other vaptan resistant (V2R-R137C/L). In renal cells, all three activating arginine vasopressin receptor 2 (V2R) variants display constitutive AQP2 plasma membrane expression and high basal water permeability. In cells expressing V2R-R137L/C mutants, disruption of the AQP2-S269 phosphorylation site caused the loss of the gain-of-function phenotype, which, in contrast, was retained in V2R-F229V-expressing cells. Cells expressing the V2R-F229V mutant were instead sensitive to disruption of the AQP2-S256 phosphorylation site. The serine/threonine kinase Rho-associated kinase (ROCK) was found to be involved in AQP2-S269 phosphorylation downstream of the V2R-R137L/C mutants. These findings might have clinical relevance for patients with nephrogenic syndrome of inappropriate antidiuresis.

Renal collecting duct principal cells play a key role in controlling body water balance. Principal cells express the water channels AQP2, AQP3, and AQP4 that mediate renal water reabsorption. AQP3 and AQP4 are expressed at the basolateral membrane constitutively. Conversely, AQP2 is localized in intracellular vesicles and translocates to the plasma membrane under vasopressin action. Stimulation with vasopressin activates the cAMP/PKA signal transduction pathway that induces the redistribution of AQP2 from an intracellular pool to the apical plasma membrane. AQP2 trafficking and function depend on multiple post-translational modifications. Moreover, several proteins control different steps activated by the vasopressin stimulation that triggers the redistribution of the AQP2 vesicles. A-kinase anchoring proteins (AKAPs) together with phosphodiesterases and adenylate cyclases play crucial roles in modulating local changes of cAMP. Soluble N-ethylmaleimide sensitive fusion factor attachment protein receptors (SNARE), cytoskeletal proteins, and the small GTPases of the Rho family regulate the fusion and the endocytotic retrieval of AQP2 vesicles. Abnormal vasopressin signaling and altered AQP2 expression or trafficking can lead to disorders characterized by deregulated mechanisms controlling water homeostasis. This review provides updated data on the molecular signals regulating vasopressin-induced AQP2 trafficking in health and disease.