Molecular mechanisms and drug development in aquaporin water channel diseases: water channel aquaporin-2 of kidney collecting duct cells

Qili Qiangxin (QLQX) capsule as a multi-functional traditional Chinese medicine in treating chronic heart failure (CHF): A review of ingredients, molecular, cellular, and pharmacological mechanisms

Chronic heart failure (CHF) is a key part of cardiovascular continuum. Under the guidance of the theory of vessel-collateral doctrine, the present study proposes therapeutic benefits of Qili Qiangxin (QLQX) capsules, an innovative Chinese medicine, on chronic heart failure. The studies show that multiple targets of the drug on CHF, including enhancing myocardial systole, promoting urine excretion, inhibiting excessive activation of the neuroendocrine system, preventing ventricular remodeling by inhibiting inflammatory response, myocardial fibrosis, apoptosis and autophagy, enhancing myocardial energy metabolism, promoting angiogenesis, and improving endothelial function. Investigation on the effects and mechanism of the drug is beneficial to the treatment of chronic heart failure (CHF) through multiple targets and/or signaling pathways. Meanwhile, it provides new insights to further understand other refractory diseases in the cardiovascular continuum, and it also has an important theoretical and practical significance in enhancing prevention and therapeutic effect of traditional Chinese medicine for these diseases.

Integrin linked kinase regulates the transcription of AQP2 by NFATC3

Two processes are associated with progressive loss of renal function: 1) decreased aquaporin-2 (AQP2) expression and urinary concentrating capacity (Nephrogenic Diabetes Insipidus, NDI); and 2) changes in extracellular matrix (ECM) composition, e.g. increased collagen I (Col I) deposition, characteristic of tubule-interstitial fibrosis. AQP2 expression is regulated by both the ECM-to-intracellular scaffold protein integrin-linked kinase (ILK) by NFATc/AP1 and other transcription factors. In the present work, we used in vivo and in vitro approaches to examine ILK participation in NFATc3/AP-1-mediated increases in AQP2 gene expression. Both NFATc3 knock-out mice and ILK conditional-knockdown mice (cKD-ILK) display symptoms of NDI (polyuria and reduced AQP2 expression). NFATc3 is upregulated in the renal medulla tubular cells of cKD-ILK mice but with reduced nuclear localization. Inner medullary collecting duct mIMCD3 cells were subjected to ILK depletion and transfected with reporter plasmids. Pharmacological activators or inhibitors determined the effect of ILK activity on NFATc/AP-1-dependent increases in transcription of AQP2. Finally, mIMCD3 cultured on Col I showed reduced activity of the ILK/GSK3β/NFATc/AQP2 axis, suggesting this pathway is a potential target for therapeutic treatment of NDI.

Global contraction or local growth, bleb shape depends on more than just cell structure

When the plasma membrane of a cell locally delaminates from its actin cortex the membrane is pushed outwards due to the cell׳s internal fluid pressure. The resulting spherical protrusion is known as a bleb. A cell׳s ability to function correctly is highly dependent on the production of such protrusions with the correct size and shape. Here, we investigate the nucleation of large blebs from small, local neck regions. A mathematical model of a cell׳s membrane, cortex and interconnecting adhesions demonstrates that these three components are unable to capture experimentally observed bleb shapes without the addition of further assumptions. We have identified that combinations of global cortex contraction and localised membrane growth are the most promising methods for generating prototypical blebs. Currently, neither proposed mechanism has been fully tested experimentally and, thus, we propose experiments that will distinguish between the two methods of bleb production.

Integrin-linked kinase regulates tubular aquaporin-2 content and intracellular location: a link between the extracellular matrix and water reabsorption

One of the clinical alterations observed in chronic renal disease (CRD) is the impaired urine concentration, known as diabetes insipidus (DI). Tubulointerstitial fibrosis of the kidney is also a pathological finding observed in CRD and involves composition of extracellular matrix (ECM). However, an association between these two events has not been elucidated. In this study, we showed that the extracellular-to-intracellular scaffold protein integrin-linked kinase (ILK) regulates expression of tubular water channel aquaporin-2 (AQP2) and its apical membrane presence in the renal tubule. Basally, polyuria and decreased urine osmolality were present in ILK conditional-knockdown (cKD-ILK) adult mice compared with nondepleted ILK littermates. No changes were observed in arginine-vasopressin (AVP) blood levels, renal receptor (V2R), or AQP3 expression. However, tubular AQP2 was decreased in expression and apical membrane presence in cKD-ILK mice, where the canonical V2R/cAMP axis activation is still functional, but independent of the absence of ILK. Thus, cKD-ILK constitutes a nephrogenic diabetes insipidus (NDI) model. AQP2 and ILK colocalize in cultured inner medullary collecting duct (mIMCD3) cells. Specific ILK siRNAs and collagen I (Col) decrease ILK and AQP2 levels and AQP2 presence on the membrane of tubular mIMCD3 cells, which impairs the capacity of the cells to transport water under hypotonic stress. The present work points to ILK as a therapeutic target in NDI.

AQP4 KO exacerbating renal dysfunction is mediated by endoplasmic reticulum stress and p66Shc and is attenuated by apocynin and endothelin antagonist CPU0213

Aquaporin 4 (AQP4) is essential in normal kidney. We hypothesized that AQP4 knockout (KO) may exacerbate pro-inflammatory factors in the stress induced renal insufficiency. Mechanisms underlying are likely due to activating renal oxidative stress adaptor p66Shc and endoplasmic reticulum (ER) stress that could be mediated by endothelin (ET)-NADPH oxidase (NOX) pathway. AQP4 KO and wild type (WT) mice were randomly divided into 4 groups: control, isoproterenol (1mg/kg, s.c., 5d), and interventions in the last 3 days with either apocynin (NADPH oxidase inhibitor, 100mg/kg, p.o.) or CPU0213 (a dual endothelin receptor antagonist 200mg/kg, p.o.). In addition, HK2 cells were cultured in 4 groups: control, isoproterenol (10(-6)M), intervened with apocynin (10(-6)M) or CPU0213 (10(-6)M). In AQP4 KO mice elevated creatinine levels were further increased by isoproterenol compared to AQP4 KO alone. In RT-PCR, western blot and immunohistochemical assay p66Shc and PERK were significantly increased in the kidney of AQP4 KO mice, associated with pro-inflammatory factors CX40, CX43, MMP-9 and ETA compared to the WT mice. Expression of AQP4 was escalated in isoproterenol incubated HK2 cells, and the enhanced protein of PERK and p-PERK/PERK, and p66shc in vivo and in vitro were significantly attenuated by either apocynin or CPU0213. In conclusion, AQP4 KO deteriorates renal dysfunction due to exacerbating ER stress and p66Shc in the kidney. Either endothelin antagonism or NADPH oxidase blockade partly relieves renal dysfunction through suppressing abnormal biomarkers by APQ4 KO and isoproterenol in the kidney.

Regulation of aquaporin-2 in the kidney: A molecular mechanism of body-water homeostasis

The kidneys play a key role in the homeostasis of body water and electrolyte balance. Aquaporin-2 (AQP2) is the vasopressin-regulated water-channel protein expressed at the connecting tubule and collecting duct, and plays a key role in urine concentration and body-water homeostasis through short-term and long-term regulation of collecting duct water permeability. The signaling transduction pathways resulting in the AQP2 trafficking to the apical plasma membrane of the collecting duct principal cells, including AQP2 phosphorylation, RhoA phosphorylation, actin depolymerization, and calcium mobilization, and the changes of AQP2 abundance in water-balance disorders have been extensively studied. Dysregulation of AQP2 has been shown to be importantly associated with a number of clinical conditions characterized by body-water balance disturbances, including hereditary nephrogenic diabetes insipidus (NDI), lithium-induced NDI, electrolytes disturbance, acute and chronic renal failure, ureteral obstruction, nephrotic syndrome, congestive heart failure, and hepatic cirrhosis. Recent studies exploiting omics technology further demonstrated the comprehensive vasopressin signaling pathways in the collecting ducts. Taken together, these studies elucidate the underlying molecular mechanisms of body-water homeostasis and provide the basis for the treatment of body-water balance disorders.

The PDZ-interaction of the intestinal anion exchanger downregulated in adenoma (DRA; SLC26A3) facilitates its movement into Rab11a-positive recycling endosomes

Electroneutral NaCl absorption in the ileum and colon is mediated by downregulated in adenoma (DRA) (Cl⁻/HCO₃⁻ exchanger; SLC26A3) and Na⁺/H⁺ exchanger 3 (NHE3, SLC9A3). Surface expression of transport proteins undergoes basal and regulated recycling by endo- and exocytosis. Expression and activity of DRA in the plasma membrane depend on intact lipid rafts, phosphatidylinositol 3-kinase (PI3-kinase), and the PDZ interaction of DRA. However, it is unknown how the PDZ interaction of DRA affects its trafficking to the cell surface. Therefore, the (re)cycling pathway of DRA was investigated in HEK cells stably expressing enhanced green fluorescent protein (EGFP)-DRA or EGFP-DRA-ETKFminus (a mutant lacking the PDZ interaction motif). Early, late, and recycling endosomes were immunoisolated by precipitating stably transfected mCherry-hemagglutinin (HA)-Rab5a, -7a, or -11a. EGFP-DRA and EGFP-DRA-ETKFminus were equally present in early endosomes. In recycling endosomes, wild-type DRA was preferentially present, whereas, in late endosomes, DRA-ETKF-minus dominated. Correspondingly, EGFP-DRA colocalized with mCherry-HA-Rab11a in recycling endosomes, whereas EGFP-DRA-ETKFminus colocalized with mCherry-HA-Rab7a in late endosomes. Functionally, this different distribution was reflected by a shorter half-life of the mutant DRA. Transient expression of dominant-negative Rab11a(S25N) inhibited the activity (-17%, P < 0.05) and the cell surface expression of DRA (-30%, P < 0.05). Transient transfection of Rab4a or its dominant-negative mutant Rab4a(S22N) was without effect and thus excluded participation of the rapid recycling pathway. Taken together, the PDZ interaction of DRA facilitates its movement into Rab11a-positive recycling endosomes, from where it is inserted in the plasma membrane. A scenario emerges where specific PDZ adaptor proteins are present along several compartments of the endocytosis-recycling pathway.

Function of the membrane water channel aquaporin-5 in the salivary gland

The process of saliva production in the salivary glands requires transepithelial water transfer from the interstitium to the acinar lumen. There are two transepithelial pathways: the transcellular and paracellular. In the transcellular pathway, the aquaporin water channels induce passive water diffusion across the membrane lipid bilayer. It is well known that aquaporin-5 (AQP5) is expressed in the salivary glands, in which it is mainly localized at the apical membrane of the acinar cells. This suggests the physiological importance of AQP5 in transcellular water transfer. Reduced saliva secretion under pilocarpine stimulation in AQP5-null mice compared with normal mice further indicates the importance of AQP5 in this process, at least in stimulated saliva secretion. Questions remain therefore regarding the role and importance of AQP5 in basal saliva secretion. It has been speculated that there would be some short-term regulation of AQP5 such as a trafficking mechanism to regulate saliva secretion. However, no histochemical evidence of AQP5-trafficking has been found, although some of biochemical analyses suggested that it may occur. There are no reports of human disease caused by AQP5 mutations, but some studies have revealed an abnormal subcellular distribution of AQP5 in patients or animals with xerostomia caused by Sjögren’s syndrome and X-irradiation. These findings suggest the possible pathophysiological importance of AQP5 in the salivary glands.

Regulation of the epithelial sodium channel (ENaC) by membrane trafficking

The epithelial Na(+) channel (ENaC) is a major regulator of salt and water reabsorption in a number of epithelial tissues. Abnormalities in ENaC function have been directly linked to several human disease states including Liddle syndrome, psuedohypoaldosteronism, and cystic fibrosis and may be implicated in salt-sensitive hypertension. ENaC activity in epithelial cells is regulated both by open probability and channel number. This review focuses on the regulation of ENaC in the cells of the kidney cortical collecting duct by trafficking and recycling. The trafficking of ENaC is discussed in the broader context of epithelial cell vesicle trafficking. Well-characterized pathways and protein interactions elucidated using epithelial model cells are discussed, and the known overlap with ENaC regulation is highlighted. In following the life of ENaC in CCD epithelial cells the apical delivery, internalization, recycling, and destruction of the channel will be discussed. While a number of pathways presented still need to be linked to ENaC regulation and many details of the regulation of ENaC trafficking remain to be elucidated, knowledge of these mechanisms may provide further insights into ENaC activity in normal and disease states.

Nucleotides downregulate aquaporin 2 via activation of apical P2 receptors

Vasopressin regulates water reabsorption in the collecting duct, but extracellular nucleotides modulate this regulation through incompletely understood mechanisms. We investigated these mechanisms using immortalized mouse collecting duct (mpkCCD) cells. Basolateral exposure to dDAVP induced AQP2 localization to the apical membrane, but co-treatment with ATP internalized AQP2. Because plasma membrane-bound P2 receptors (P2R) mediate the effects of extracellular nucleotides, we examined the abundance and localization of P2R in mpkCCD cells. In the absence of dDAVP, P2Y(1) and P2Y(4) receptors localized to the apical membrane, whereas P2X(2), P2X(4), P2X(5), P2X(7), P2Y(2), P2Y(11), and P2Y(12) receptors localized to the cytoplasm. dDAVP induced gene expression of P2X(1), which localized to the apical domain, and led to translocation of P2X(2) and P2Y(2) to the apical and basolateral membranes, respectively. In co-expression experiments, P2R activation decreased membrane AQP2 and AQP2-mediated water permeability in Xenopus oocytes expressing P2X(2), P2Y(2,) or P2Y(4) receptors, but not in oocytes expressing other P2R subtypes. In summary, these data suggest that AQP2-mediated water transport is downregulated not only by basolateral nucleotides, mediated by P2Y(2) receptors, but also by luminal nucleotides, mediated by P2X(2) and/or P2Y(4) receptors.

Expression of the Water Channel Protein Aquaporin-9 in Human Astrocytic Tumours: Correlation with Pathological Grade

This study investigated the expression of the water channel protein aquaporin-9 (AQP9) in 100 cases of human astrocytic tumour compared with normal brain tissue. AQP9 mRNA and protein were assessed using reverse transcription-polymerase chain reaction and Western blot, respectively. The expression of both AQP9 mRNA and protein in astrocytic tumours was significantly greater than in normal brain tissue and was positively correlated with pathological grade. No significant correlations were found between the level of AQP9 mRNA or protein expression and gender, age, tumour size or tumour site. This study indicates that AQP9 may play an important role in the malignant progression of brain astrocytic tumours and may, therefore, be useful as a biomarker in its diagnosis and as a new target for gene therapy. The molecular mechanisms by which AQP9 affects the proliferation and apoptosis of astrocytic tumours need to be studied further.

Localization and trafficking of aquaporin 2 in the kidney

Aquaporins (AQPs) are membrane proteins serving in the transfer of water and small solutes across cellular membranes. AQPs play a variety of roles in the body such as urine formation, prevention from dehydration in covering epithelia, water handling in the blood–brain barrier, secretion, conditioning of the sensory system, cell motility and metastasis, formation of cell junctions, and fat metabolism. The kidney plays a central role in water homeostasis in the body. At least seven isoforms, namely AQP1, AQP2, AQP3, AQP4, AQP6, AQP7, and AQP11, are expressed. Among them, AQP2, the anti-diuretic hormone (ADH)-regulated water channel, plays a critical role in water reabsorption. AQP2 is expressed in principal cells of connecting tubules and collecting ducts, where it is stored in Rab11-positive storage vesicles in the basal state. Upon ADH stimulation, AQP2 is translocated to the apical plasma membrane, where it serves in the influx of water. The translocation process is regulated through the phosphorylation of AQP2 by protein kinase A. As soon as the stimulation is terminated, AQP2 is retrieved to early endosomes, and then transferred back to the Rab 11-positive storage compartment. Some AQP2 is secreted via multivesicular bodies into the urine as exosomes. Actin plays an important role in the intracellular trafficking of AQP2. Recent findings have shed light on the molecular basis that controls the trafficking of AQP2.

Curcumin attenuates EGF-induced AQP3 up-regulation and cell migration in human ovarian cancer cells

OBJECTIVE

Aquaporin (AQP) water channels are expressed in high-grade tumor cells of different tissue origins. Based on the involvement of AQPs in angiogenesis and cell migration as well as our previous studies which show that AQP3 is involved in human skin fibroblasts cell migration, in this study, we investigated whether AQP3 is expressed in cultured human ovarian cancer cell line CaOV3 cells, and whether AQP3 expression in these cells enhances cell migration and metastatic potential.

METHODS

Cultured CaOV3 cells were treated with EGF and/or various reagents and subjected to cell migration assay by phagokinetic track mobility assay or biochemical analysis for expression or activation of proteins by SDS-PAGE/Western blot analysis.

RESULTS

In this study, we demonstrate that AQP3 is expressed in CaOV3 cells. EGF induces CaOV3 migration and up-regulates AQP3 expression. EGF-induced cell migration is inhibited by specific AQP3 siRNA knockdown or AQP3 water transport inhibitor CuSO4 and NiCl2. We also find that curcumin, a well known anti-ovarian cancer drug, down-regulates AQP3 expression and reduces cell migration in CaOV3, and the effects of curcumin are mediated, at least in part, by its inhibitory effects on EGFR and downstream AKT/ERK activation.

CONCLUSIONS

Collectively, our results provide evidence for AQP3-facilitated ovarian cancer cell migration, suggesting a novel function for AQP3 expression in high-grade tumors. The results that curcumin inhibits EGF-induced up-regulation of AQP3 and cell migration, provide a new explanation for the anticancer potential of curcumin.

Involvement of Intracellular Ca2+ in the Regulatory Volume Decrease After Hyposmotic Swelling in MDCK Cells

We examined the source of Ca(2+) involved in the volume regulation of Madin-Darby canine kidney (MDCK) cells with confocal microscopy and fluoroprobes. Hyposmosis induced a transient increase in cell volume, as well as cytoplasmic Ca(2+), which peaked at 3 to 5 min and gradually decreased to reach the initial value within about 30 min. This late decrease in cell volume, as well as the transient rise in cytoplasmic Ca(2+), was reduced in Ca(2+)-free solution and was abolished by pretreatment with thapsigargin. In conclusion, Ca(2+) released from the intracellular store contributes to the regulatory volume decrease following hyposmotic swelling in MDCK cells.

EGFR-mediated expression of aquaporin-3 is involved in human skin fibroblast migration

AQP3 (aquaporin-3), known as an integral membrane channel in epidermal keratinocytes, facilitates water and glycerol movement into and out of the skin. Here, we demonstrate that AQP3 is also expressed in cultured human skin fibroblasts, which under normal wound healing processes migrate from surrounding tissues to close the wound. EGF (epidermal growth factor), which induced fibroblast migration, also induced AQP3 expression in a time- and dose-dependent manner. CuSO4 and NiCl2, previously known as AQP3 water transport inhibitors, as well as two other bivalent heavy metals Mn2+ and Co2+, inhibited EGF-induced cell migration in human skin fibroblasts. AQP3 knockdown by small interfering RNA inhibited EGF-induced AQP3 expression and cell migration. Furthermore, an EGFR (EGF receptor) kinase inhibitor, PD153035, blocked EGF-induced AQP3 expression and cell migration. MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase]/ERK inhibitor U0126 and PI3K (phosphoinositide 3-kinase) inhibitor LY294002 also inhibited EGF-induced AQP3 expression and cell migration. Collectively, our findings show for the first time that AQP3 is expressed in human skin fibroblasts and that EGF induces AQP3 expression via EGFR, PI3K and ERK signal transduction pathways. We have provided evidence for a novel role of AQP3 in human skin fibroblast cell migration, which occurs during normal wound healing.

Placental and membrane aquaporin water channels: correlation with amniotic fluid volume and composition

OBJECTIVES

To assess the role of aquaporins (AQPs) in the regulation of amniotic fluid (AF) volume, we determined AF volume and composition and placental and fetal membrane AQP expression throughout the second half of murine gestation.

METHODS

Pregnant CD1 mice were sacrificed at e10-19 and AF volume and composition determined. Placenta and fetal membranes were screened for AQP gene expression. AQP gene expression was quantified by real-time RT PCR and protein location determined by immunohistochemistry. Changes in AF volume were correlated with AQP expression.

RESULTS

Both membranes and placenta demonstrated expression of AQP1, -3, -8 and -9. Advancing gestation was associated with increased AF volume from e10 to e16, with a marked decrease in AF volume from e16 to e19. By immunohistochemistry, AQP1 was localized to placental vessels and AQP3 to trophoblast. AF volume was negatively correlated with fetal membrane AQP1 and placental AQP1 and AQP9 expression, and positively correlated with placental AQP3 expression.

CONCLUSION

Changes in AQPs with advancing gestation, and their correlation with AF volume, suggest a role in mediating placental and membrane water flow and ultimately AF volume. AQP1 appears to regulate fetal membrane water flow, and AQP3 is a likely candidate for the regulation of placental water flow.

Aquaporin-5 water channel in lipid rafts of rat parotid glands

Aquaporin-5 (AQP5), an apical plasma membrane (APM) water channel in salivary glands, lacrimal glands, and airway epithelium, has an important role in fluid secretion. The activation of M3 muscarinic acetylcholine receptors (mAChRs) or alpha1-adrenoceptors on the salivary glands induces salivary fluid secretion. AQP5 localizes in lipid rafts and activation of the M3 mAChRs or alpha1-adrenoceptors induced its translocation together with the lipid rafts to the APM in the interlobular ducts of rat parotid glands. This review focuses on the mechanisms of AQP5 translocation together with lipid rafts to the APM in the interlobular duct cells of parotid glands of normal rats and the impairment of AQP5 translocation in diabetes and senescence.

Regulation of aquaporin-2 trafficking and its binding protein complex

Trafficking of water channel aquaporin-2 (AQP2) to the apical membrane is critical to water reabsorption in renal collecting ducts and its regulation maintains body water homeostasis. However, exact molecular mechanisms which recruit AQP2 are unknown. Recent studies highlighted a key role for spatial and temporal regulation of actin dynamics in AQP2 trafficking. We have recently identified AQP2-binding proteins which directly regulate this trafficking: SPA-1, a GTPase-activating protein (GAP) for Rap1, and cytoskeletal protein actin. In addition, a multiprotein "force generator" complex which directly binds to AQP2 has been discovered. This review summarizes recent advances related to the mechanism for AQP2 trafficking.