Dynamical modeling of liver Aquaporin-9 expression and glycerol permeability in hepatic glucose metabolism

Insights into the Function of Aquaporins in Gastrointestinal Fluid Absorption and Secretion in Health and Disease

Aquaporins (AQPs), transmembrane proteins permeable to water, are involved in gastrointestinal secretion. The secretory products of the glands are delivered either to some organ cavities for exocrine glands or to the bloodstream for endocrine glands. The main secretory glands being part of the gastrointestinal system are salivary glands, gastric glands, duodenal Brunner's gland, liver, bile ducts, gallbladder, intestinal goblet cells, exocrine and endocrine pancreas. Due to their expression in gastrointestinal exocrine and endocrine glands, AQPs fulfill important roles in the secretion of various fluids involved in food handling. This review summarizes the contribution of AQPs in physiological and pathophysiological stages related to gastrointestinal secretion.

Hepatocyte Aquaporins AQP8 and AQP9 Are Engaged in the Hepatic Lipid and Glucose Metabolism Modulating the Inflammatory and Redox State in Milk-Supplemented Rats

Milk is an important source of nutrients and energy, but there are still many uncertainties regarding the health effects of milk and dairy products consumption. Milk from different species varies in physicochemical and nutritional properties. We previously showed that dietary supplements with different milks in rats trigger significant differences in metabolic and inflammatory states, modulating mitochondrial functions in metabolically active organs such as the liver and skeletal muscle. Here, we have deepened the effects of isoenergetic supplementation of milk (82 kJ) from cow (CM), donkey (DM) or human (HM) on hepatic metabolism to understand the interlink between mitochondrial metabolic flexibility, lipid storage and redox state and to highlight the possible role of two hepatocyte aquaporins (AQPs) of metabolic relevance, AQP8 and AQP9, in this crosstalk. Compared with rats with no milk supplementation, DM- and HM-fed rats had reduced hepatic lipid content with enhanced mitochondrial function and decreased oxidative stress. A marked reduction in AQP8, a hydrogen peroxide channel, was seen in the liver mitochondria of DM-fed rats compared with HM-fed, CM-fed and control animals. DM-fed or HM-fed rats also showed reduced hepatic inflammatory markers and less collagen and Kupffer cells. CM-fed rats showed higher hepatic fat content and increased AQP9 and glycerol permeability. A role of liver AQP8 and AQP9 is suggested in the different metabolic profiles resulting from milk supplementation.

Aquaporins in Glandular Secretion

Exocrine and endocrine glands deliver their secretory product, respectively, at the surface of the target organs or within the bloodstream. The release of their products has been shown to rely on secretory mechanisms often involving aquaporins (AQPs). This chapter will provide insight into the role of AQPs in secretory glands located within the gastrointestinal tract, including salivary glands, gastric glands, duodenal Brunner's glands, liver, gallbladder, intestinal goblets cells, and pancreas, as well and in other parts of the body, including airway submucosal glands, lacrimal glands, mammary glands, and eccrine sweat glands. The involvement of AQPs in both physiological and pathophysiological conditions will also be highlighted.

Characterization of the Aquaporin-9 Inhibitor RG100204 In Vitro and in db/db Mice

Aquaporin-9 (AQP9) is a facilitator of glycerol and other small neutral solute transmembrane diffusion. Identification of specific inhibitors for aquaporin family proteins has been difficult, due to high sequence similarity between the 13 human isoforms, and due to the limited channel surface areas that permit inhibitor binding. The few AQP9 inhibitor molecules described to date were not suitable for in vivo experiments. We now describe the characterization of a new small molecule AQP9 inhibitor, RG100204 in cell-based calcein-quenching assays, and by stopped-flow light-scattering recordings of AQP9 permeability in proteoliposomes. Moreover, we investigated the effects of RG100204 on glycerol metabolism in mice. In cell-based assays, RG100204 blocked AQP9 water permeability and glycerol permeability with similar, high potency (~5 × 10-8 M). AQP9 channel blocking by RG100204 was confirmed in proteoliposomes. After oral gavage of db/db mice with RG100204, a dose-dependent elevation of plasma glycerol was observed. A blood glucose-lowering effect was not statistically significant. These experiments establish RG100204 as a direct blocker of the AQP9 channel, and suggest its use as an experimental tool for in vivo experiments on AQP9 function.

The Multifaceted Role of Aquaporin-9 in Health and Its Potential as a Clinical Biomarker

Aquaporins (AQPs) are transmembrane channels essential for water, energy, and redox homeostasis, with proven involvement in a variety of pathophysiological conditions such as edema, glaucoma, nephrogenic diabetes insipidus, oxidative stress, sepsis, cancer, and metabolic dysfunctions. The 13 AQPs present in humans are widely distributed in all body districts, drawing cell lineage-specific expression patterns closely related to cell native functions. Compelling evidence indicates that AQPs are proteins with great potential as biomarkers and targets for therapeutic intervention. Aquaporin-9 (AQP9) is the most expressed in the liver, with implications in general metabolic and redox balance due to its aquaglyceroporin and peroxiporin activities, facilitating glycerol and hydrogen peroxide (H₂O₂) diffusion across membranes. AQP9 is also expressed in other tissues, and their altered expression is described in several human diseases, such as liver injury, inflammation, cancer, infertility, and immune disorders. The present review compiles the current knowledge of AQP9 implication in diseases and highlights its potential as a new biomarker for diagnosis and prognosis in clinical medicine.

The Implication of Aquaporin-9 in the Pathogenesis of Preterm Premature Rupture of Membranes

OBJECTIVE

This study aimed to detect aquaporin-9 (AQP9) concentrations in the serum of patients with preterm premature rupture of membranes (PPROM) and compare them with the healthy control group with intact membranes.

MATERIAL AND METHODS

We conducted this prospective case-control study from January 2020 to June 2020. Of the 80 pregnant patients included in the study, we enrolled 42 singleton pregnant patients with PPROM as the study group and 43 healthy gestational age-, and body mass index (BMI)-matched healthy pregnant women with intact fetal membranes as the control group. We compared demographic and clinical characteristics, complete blood count and biochemical parameters, and serum AQP9 concentrations of the participants. We constructed an ROC curve to illustrate the sensitivity and specificity performance characteristics of AQP9 and calculated a cutoff value by using the Youden index.

RESULTS

Maternal serum AQP-9 concentrations were significantly higher in patients with PPROM (804.46±195.63 pg/mL) compared to the healthy pregnant women in the control group (505.97±68.89 pg/mL, p<0.001). When we examine the area under the ROC curve (AUC), the AQP-9 value can be reflected as a statistically significant parameter for diagnosing PPROM. According to the Youden index, a 654.78 pg/mL cut-off value of AQP-9 can be utilized to diagnose PPROM with 80.5% sensitivity and 100% specificity.

CONCLUSION

Maternal serum AQP9 concentrations were significantly higher in PPROM patients than healthy pregnant women with an intact membrane. We suggest that AQP9 might be an essential biomarker of the inflammatory process and energy homeostasis in PPROM.

Molecular Liver Fingerprint Reflects the Seasonal Physiology of the Grey Mouse Lemur (Microcebus murinus) during Winter

Grey mouse lemurs (Microcebus murinus) are primates that respond to environmental energetic constraints through strong physiological seasonality. They notably fatten during early winter (EW), and mobilize their lipid reserves while developing glucose intolerance during late winter (LW), when food availability is low. To decipher how the hepatic mechanisms may support such metabolic flexibility, we analyzed the liver proteome of adult captive male mouse lemurs, whose seasonal regulations are comparable to their wild counterparts. We highlight profound hepatic changes that reflect fat accretion in EW at the whole-body level, without triggering an ectopic storage of fat in the liver, however. Moreover, molecular regulations are consistent with the decrease in liver glucose utilization in LW, and therefore with reduced tolerance to glucose. However, no major regulation was seen in insulin signaling/resistance pathways. Fat mobilization in LW appeared possibly linked to the reactivation of the reproductive system while enhanced liver detoxification may reflect an anticipation to return to summer levels of food intake. Overall, these results show that the physiology of mouse lemurs during winter relies on solid molecular foundations in liver processes to adapt fuel partitioning while opposing the development of a pathological state despite large lipid fluxes.

Coupling/Uncoupling Reversibility in Isolated Mitochondria from Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae uses fermentation as the preferred pathway to obtain ATP and requires the respiratory chain to re-oxidize the NADH needed for activity of Glyceraldehyde-3-phosphate. This process is favored by uncoupling of oxidative phosphorylation (OxPhos), which is at least partially controlled by the mitochondrial unspecific pore (_ScMUC). When mitochondrial ATP synthesis is needed as in the diauxic phase or during mating, a large rise in Ca²⁺ concentration ([Ca²⁺]) closes _ScMUC, coupling OxPhos. In addition, _ScMUC opening/closing is mediated by the ATP/ADP ratio, which indicates cellular energy needs. Here, opening and closing of _ScMUC was evaluated in isolated mitochondria from S. cerevisiae at different incubation times and in the presence of different ATP/ADP ratios or varying [Ca²⁺]. Measurements of the rate of O₂ consumption, mitochondrial swelling, transmembrane potential and ROS generation were conducted. It was observed that _ScMUC opening was reversible, a high ATP/ADP ratio promoted opening and [Ca²⁺] closed _ScMUC even after several minutes of incubation in the open state. In the absence of ATP synthesis, closure of _ScMUC resulted in an increase in ROS.

Aquaporins and diseases pathogenesis: From trivial to undeniable involvements, a disease-based point of view

Aquaporins (AQPs), as transmembrane proteins, were primarily identified as water channels with the ability of regulating the transmission of water, glycerol, urea, and other small-sized molecules. The classic view of AQPs involvement in therapeutic plan restricted them and their regulators into managing only a narrow spectrum of the diseases such as diabetes insipidus and the syndrome of inappropriate ADH secretion. However, further investigations performed, especially in the third millennium, has found that their cooperation in water transmission control can be manipulated to handle other burden-imposing diseases such as cirrhosis, heart failure, Meniere's disease, cancer, bullous pemphigoid, eczema, and Sjögren's syndrome.

Aquaporin regulation in metabolic organs

Aquaporins (AQPs) are a family of 13 small trans-membrane proteins, which facilitate shuttling of glycerol, water and urea. The peculiar role of AQPs in glycerol transport makes them attractive targets in metabolic organs since glycerol represents the backbone of triglyceride synthesis. Importantly, AQPs are known to be regulated by various nuclear receptors which in turn govern lipid and glucose metabolism as well as inflammatory cascades. Here, we review the role of AQPs regulation in metabolic organs exploring their physiological impact in health and disease.

Involvement of aquaglyceroporins in energy metabolism in health and disease

Aquaglyceroporins are a group of the aquaporin (AQP) family of transmembrane water channels. While AQPs facilitate the passage of water, small solutes, and gases across biological membranes, aquaglyceroporins allow passage of water, glycerol, urea and some other solutes. Thanks to their glycerol permeability, aquaglyceroporins are involved in energy homeostasis. This review provides an overview of what is currently known concerning the functional implication and control of aquaglyceroporins in tissues involved in energy metabolism, i.e. liver, adipose tissue and endocrine pancreas. The expression, role and (dys)regulation of aquaglyceroporins in disorders affecting energy metabolism, and the potential relevance of aquaglyceroporins as drug targets to treat the alterations of the energy balance is also addressed.

Ablation of Aquaporin-9 Ameliorates the Systemic Inflammatory Response of LPS-Induced Endotoxic Shock in Mouse

Septic shock is the most severe complication of sepsis, being characterized by a systemic inflammatory response following bacterial infection, leading to multiple organ failure and dramatically high mortality. Aquaporin-9 (AQP9), a membrane channel protein mainly expressed in hepatocytes and leukocytes, has been recently associated with inflammatory and infectious responses, thus triggering strong interest as a potential target for reducing septic shock-dependent mortality. Here, we evaluated whether AQP9 contributes to murine systemic inflammation during endotoxic shock. Wild type (Aqp9^+/+; WT) and Aqp9 gene knockout (Aqp9^−/−; KO) male mice were submitted to endotoxic shock by i.p. injection of lipopolysaccharide (LPS; 40 mg/kg) and the related survival times were followed during 72 h. The electronic paramagnetic resonance and confocal microscopy were employed to analyze the nitric oxide (NO) and superoxide anion (O₂⁻) production, and the expression of inducible NO-synthase (iNOS) and cyclooxigenase-2 (COX-2), respectively, in the liver, kidney, aorta, heart and lung of the mouse specimens. LPS-treated KO mice survived significantly longer than corresponding WT mice, and 25% of the KO mice fully recovered from the endotoxin treatment. The LPS-injected KO mice showed lower inflammatory NO and O₂⁻ productions and reduced iNOS and COX-2 levels through impaired NF-κB p65 activation in the liver, kidney, aorta, and heart as compared to the LPS-treated WT mice. Consistent with these results, the treatment of FaO cells, a rodent hepatoma cell line, with the AQP9 blocker HTS13268 prevented the LPS-induced increase of inflammatory NO and O₂⁻. A role for AQP9 is suggested in the early acute phase of LPS-induced endotoxic shock involving NF-κB signaling. The modulation of AQP9 expression/function may reveal to be useful in developing novel endotoxemia therapeutics.

Stopped-flow Light Scattering Analysis of Red Blood Cell Glycerol Permeability

Stopped-Flow Light Scattering (SFLS) is a method devised to analyze the kinetics of fast chemical reactions that result in a significant change of the average molecular weight and/or in the shape of the reaction substrates. Several modifications of the original stopped-flow system have been made leading to a significant extension of its technical applications. One of these modifications allows the biophysical characterization of the water and solute permeability of biological and artificial membranes. Here, we describe a protocol of SFLS to measure the glycerol permeability of isolated human red blood cells (RBCs) and evaluate the pharmacokinetics properties (selectivity and potency) of isoform-specific inhibitors of AQP3, AQP7 and AQP9, three mammalian aquaglyceroporins allowing transport of glycerol across membranes. Suspensions of RBCs (1% hematocrit) are exposed to an inwardly directed gradient of 100 mM glycerol in a SFLS apparatus at 20 °C and the resulting changes in scattered light intensity are recorded at a monochromatic wavelength of 530 nm for 120 s. The SFLS apparatus is set up to have a dead time of 1.6-ms and 99% mixing efficiency in less than 1 ms. Data are fitted to a single exponential function and the related time constant (τ, seconds) of the cell-swelling phase of light scattering corresponding to the osmotic movement of water that accompanies the entry of glycerol into erythrocytes is measured. The coefficient of glycerol permeability ( Pgly , cm/s) of RBCs is calculated with the following equation: Pgly = 1/[(S/V)τ] where τ (s) is the fitted exponential time constant and S/V is the surface-to-volume ratio (cm-1) of the analyzed RBC specimen. Pharmacokinetics of the isoform-specific inhibitors of AQP3, AQP7 and AQP9 are assessed by evaluating the extent of RBC Pgly values resulting after the exposure to serial concentrations of the blockers.

Aquaporin-9 is involved in the lipid-lowering activity of the nutraceutical silybin on hepatocytes through modulation of autophagy and lipid droplets composition

Hepatic steatosis is the hallmark of non-alcoholic fatty liver disease (NAFLD), the hepatic manifestation of the metabolic syndrome and insulin resistance with potential evolution towards non-alcoholic steatohepatitis (NASH), cirrhosis and hepatocellular carcinoma. Key roles of autophagy and oxidative stress in hepatic lipid accumulation and NAFLD progression are recognized. Here, we employed a rat hepatoma cell model of NAFLD progression made of FaO cells exposed to oleate/palmitate followed or not by TNFα treatment to investigate the molecular mechanisms through which silybin, a lipid-lowering nutraceutical, may improve hepatic lipid dyshomeostasis. The beneficial effect of silybin was found to involve amelioration of the fatty acids profile of lipid droplets, stimulation of the mitochondrial oxidation and upregulation of a microRNA of pivotal relevance in hepatic fat metabolism, miR-122. Silybin was also found to restore the levels of Aquaporin-9 (AQP9) and glycerol permeability while reducing the activation of the oxidative stress-dependent transcription factor NF-κB, and autophagy turnover. In conclusion, silybin was shown to have molecular effects on signaling pathways that were previously unknown and potentially protect the hepatocyte. These actions intersect TG metabolism, fat-induced autophagy and AQP9-mediated glycerol transport in hepatocytes.

Aquaglyceroporins: Drug Targets for Metabolic Diseases?

Aquaporins (AQPs) are a family of transmembrane channel proteins facilitating the transport of water, small solutes, and gasses across biological membranes. AQPs are expressed in all tissues and ensure multiple roles under normal and pathophysiological conditions. Aquaglyceroporins are a subfamily of AQPs permeable to glycerol in addition to water and participate thereby to energy metabolism. This review focalizes on the present knowledge of the expression, regulation and physiological roles of AQPs in adipose tissue, liver and endocrine pancreas, that are involved in energy metabolism. In addition, the review aims at summarizing the involvement of AQPs in metabolic disorders, such as obesity, diabetes and liver diseases. Finally, challenges and recent advances related to pharmacological modulation of AQPs expression and function to control and treat metabolic diseases are discussed.

Nuclear Receptor Regulation of Aquaglyceroporins in Metabolic Organs

Nuclear receptors, such as the farnesoid X receptor (FXR) and the peroxisome proliferator-activated receptors gamma and alpha (PPAR-γ, -α), are major metabolic regulators in adipose tissue and the liver, where they govern lipid, glucose, and bile acid homeostasis, as well as inflammatory cascades. Glycerol and free fatty acids are the end products of lipid droplet catabolism driven by PPARs. Aquaporins (AQPs), a family of 13 small transmembrane proteins, facilitate the shuttling of water, urea, and/or glycerol. The peculiar role of AQPs in glycerol transport makes them pivotal targets in lipid metabolism, especially considering their tissue-specific regulation by the nuclear receptors PPARγ and PPARα. Here, we review the role of nuclear receptors in the regulation of glycerol shuttling in liver and adipose tissue through the function and expression of AQPs.

Lipopolysaccharide Modifies Glycerol Permeability and Metabolism in 3T3-L1 Adipocytes

Aquaglyceroporins-aquaporin membrane channels (AQP) that conduct glycerol and other small neutral solutes in addition to water-play major roles in obesity. In adipocytes, aquaglyceroporins mediate glycerol uptake and release across the plasma membrane, which are two key steps for triacylglycerols (TAGs) synthesis (lipogenesis) and hydrolysis (lipolysis). The aim of this study was to assess both glycerol permeability and metabolism in undifferentiated 3T3-L1 cells (UDCs) as well as in untreated (CTL-DCs) versus lipopolysaccharide (LPS-DCs)-treated differentiated 3T3-L1 adipocytes. Glycerol release, TAGs content and whole membrane glycerol permeability were significantly increased in DCs as compared to UDCs. Moreover, in DCs, LPS treatment significantly increased TAGs content and decreased glycerol permeability. In addition, a significant reduction in whole membrane glycerol permeability was observed in LPS-DCs as compared to CTL-DCs. The relative contributions of AQP3, AQP7 and AQP9 (facilitated diffusion), as well as that of the phospholipid bilayer (simple diffusion), to the whole membrane glycerol permeability, were estimated biophysically in UDCs, CTL-DCs and LPS-DCs, using selective AQP inhibitors. Further studies will be required to determine if modifications in either subcellular localization and/or activity of aquaglyceroporins could account for the data herein. Nevertheless, our findings provide novel insights in understanding the LPS-induced adipocyte hypertrophy that accompanies obesity.