Adaptation to alkalosis induces cell cycle delay and apoptosis in cortical collecting duct cells: role of Aquaporin-2

Non-Transport Functions of Aquaporins

Although it has been more than 20 years since the first aquaporin was discovered, the specific functions of many aquaporins are still under investigation, because various mice lacking aquaporins have no significant phenotypes. And in many studies, the function of aquaporin is not directly related to its transport function. Therefore, this chapter will focus on some unexpected functions of aquaporins, such the decreased tumor angiogenesis in AQP1 knockout mice, and AQP1 promotes cell migration, possibly by accelerating the water transport in lamellipodia of migrating cells. AQP transports glycerol, and water regulates glycerol content in epidermis and fat, thereby regulating skin hydration/biosynthesis and fat metabolism. AQPs may also be involved in neural signal transduction, cell volume regulation, and organelle physiology. AQP1, AQP3, and AQP5 are also involved in cell proliferation. In addition, AQPs have also been reported to play roles in inflammation in various tissues and organs. The functions of these AQPs may not depend on the permeability of small molecules such as water and glycerol, suggesting AQPs may play more roles in different biological processes in the body.

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.

Role of Aquaporins in the Physiological Functions of Mesenchymal Stem Cells

Aquaporins (AQPs) are a family of membrane water channel proteins that control osmotically-driven water transport across cell membranes. Recent studies have focused on the assessment of fluid flux regulation in relation to the biological processes that maintain mesenchymal stem cell (MSC) physiology. In particular, AQPs seem to regulate MSC proliferation through rapid regulation of the cell volume. Furthermore, several reports have shown that AQPs play a crucial role in modulating MSC attachment to the extracellular matrix, their spread, and migration. Shedding light on how AQPs are able to regulate MSC physiological functions can increase our knowledge of their biological behaviours and improve their application in regenerative and reparative medicine.

Na+/H+ exchanger isoform 1 activity in AQP2-expressing cells can be either proliferative or anti-proliferative depending on extracellular pH

We have previously shown in renal cells that expression of the water channel Aquaporin-2 increases cell proliferation by a regulatory volume mechanism involving Na⁺/H⁺ exchanger isoform 2. Here, we investigated if Aquaporin-2 (AQP2) also modulates Na⁺/H⁺ exchanger isoform 1-dependent cell proliferation. We use two AQP2-expressing cortical collecting duct models: one constitutive (WT or AQP2-transfected RCCD₁ cell line) and one inducible (control or vasopressin-induced mpkCCD_c14 cell line). We found that Aquaporin-2 modifies Na⁺/H⁺ exchanger isoform 1 (NHE1) contribution to cell proliferation. In Aquaporin-2-expressing cells, Na⁺/H⁺ exchanger isoform 1 is anti-proliferative at physiological pH. In acid media, Na⁺/H⁺ exchanger isoform 1 contribution turned from anti-proliferative to proliferative only in AQP2-expressing cells. We also found that, in AQP2-expressing cells, NHE1-dependent proliferation changes parallel changes in stress fiber levels: at pH 7.4, Na⁺/H⁺ exchanger isoform 1 would favor stress fiber disassembly and, under acidosis, NHE1 would favor stress fiber assembly. Moreover, we found that Na⁺/H⁺ exchanger-dependent effects on proliferation linked to Aquaporin-2 relied on Transient Receptor Potential Subfamily V calcium channel activity. In conclusion, our data show that, in collecting duct cells, the water channel Aquaporin-2 modulates NHE1-dependent cell proliferation. In AQP2-expressing cells, at physiological pH, the Na⁺/H⁺ exchanger isoform 1 function is anti-proliferative and, at acidic pH, Na⁺/H⁺ exchanger isoform 1 function is proliferative. We propose that Na⁺/H⁺ exchanger isoform 1 modulates proliferation through an interplay with stress fiber formation.

miR-137 Affects Vaginal Lubrication in Female Sexual Dysfunction by Targeting Aquaporin-2

Introduction

Female sexual dysfunction (FSD) is a common disease with serious potential hazards, but it has not received much attention. The pathogenesis of FSD is urgently needed for the diagnosis and treatment of FSD.

Aim

To investigate the role of microribonucleic acid (mRNA, miR)-137 in FSD.

Methods

Vaginal epithelium tissues from 15 women with lubrication disorder and 15 women with normal function were collected for this study. The expression level of miR-137 in lubrication disorder and normal function women were measured by microarray analysis and Real-time Quantitative Polymerase Chain Reaction (PCR, qPCR). miR-137 was overexpressed in vaginal epithelial cells VK2/E6E7 by lentivirus infection. The cell water permeability was measured using the calcein-quenching method. Cell apoptosis was analyzed by flow cytometry. The potential target of miR-137 was predicted by bioinformatic analysis, then verified by luciferase reporter assays.

Main Outcome Measure

The expression level of miR-137 and aquaporin-2 (AQP2), cell water permeability, cell apoptosis, and luciferase reporter assays were examined.

Results

miR-137 was found to be highly expressed in vaginal epithelial tissues of women with lubrication disorder. Additionally, functional in vitro studies suggested that overexpression of miR-137 leads to a decrease in cell permeability. By combining target prediction and examination, we identified AQP2 as the direct mechanistic target of miR-137 that affected the water permeability of vaginal epithelial cells.

Conclusion

Our results point to a novel role for miR-137 and its downstream effector AQP2 in vaginal lubrication, which can be manipulated as therapeutic targets against lubrication disorder and its related disorders.

Zhang H, Liu T, Zhou Z. miR-137 affects vaginal lubrication in female sexual dysfunction by targeting Aquaporin-2. Sex Med 2018;6:339–347.

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.

Role of aquaporins in cell proliferation: What else beyond water permeability?

In addition to the extensive data demonstrating the importance of mammalian AQPs for the movement of water and some small solutes across the cell membrane, there is now a growing body of evidence indicating the involvement of these proteins in numerous cellular processes seemingly unrelated, at least some of them in a direct way, to their canonical function of water permeation. Here, we have presented a broad range of evidence demonstrating that these proteins have a role in cell proliferation by various different mechanisms, namely, by allowing fast cell volume regulation during cell division; by affecting progression of cell cycle and helping maintain the balance between proliferation and apoptosis, and by crosstalk with other cell membrane proteins or transcription factors that, in turn, modulate progression of the cell cycle or regulate biosynthesis pathways of cell structural components. In the end, however, after discussing all these data that strongly support a role for AQPs in the cell proliferation process, it remains impossible to conclude that all these other functions attributed to AQPs occur completely independently of their water permeability, and there is a need for new experiments designed specifically to address this interesting issue.

Overexpression of AQP3 Modifies the Cell Cycle and the Proliferation Rate of Mammalian Cells in Culture

Abnormal AQP3 overexpression in tumor cells of different origins has been reported and a role for this enhanced AQP3 expression in cell proliferation and tumor processess has been indicated. To further understand the role AQP3 plays in cell proliferation we explore the effect that stable over expression of AQP3 produces over the proliferation rate and cell cycle of mammalian cells. The cell cycle was analyzed by flow cytometry with propidium iodide (PI) and the cell proliferation rate measured through cell counting and BrdU staining. Cells with overexpression of AQP3 (AQP3-o) showed higher proliferation rate and larger percentage of cells in phases S and G2/M, than wild type cells (wt). Evaluation of the cell response against arresting the cell cycle with Nocodazole showed that AQP3-o exhibited a less modified cell cycle pattern and lower Annexin V specific staining than wt, consistently with a higher resistance to apoptosis of AQP3-overexpressing cells. The cell volume and complexity were also larger in AQP3-o compared to wt cells. After transcriptomic analysis, RT-qPCR was performed to highlight key molecules implicated in cell proliferation which expression may be altered by overexpression of AQP3 and the comparative analysis between both type of cells showed significant changes in the expression of Zeb2, Jun, JunB, NF-kβ, Cxcl9, Cxcl10, TNF, and TNF receptors. We conclude that the role of AQP3 in cell proliferation seems to be connected to increments in the cell cycle turnover and changes in the expression levels of relevant genes for this process. Larger expression of AQP3 may confer to the cell a more tumor like phenotype and contributes to explain the presence of this protein in many different tumors.

Aquaporin 2-increased renal cell proliferation is associated with cell volume regulation

We have previously demonstrated that in renal cortical collecting duct cells (RCCD(1)) the expression of the water channel Aquaporin 2 (AQP2) raises the rate of cell proliferation. In this study, we investigated the mechanisms involved in this process, focusing on the putative link between AQP2 expression, cell volume changes, and regulatory volume decrease activity (RVD). Two renal cell lines were used: WT-RCCD(1) (not expressing aquaporins) and AQP2-RCCD(1) (transfected with AQP2). Our results showed that when most RCCD(1) cells are in the G(1)-phase (unsynchronized), the blockage of barium-sensitive K(+) channels implicated in rapid RVD inhibits cell proliferation only in AQP2-RCCD(1) cells. Though cells in the S-phase (synchronized) had a remarkable increase in size, this enhancement was higher and was accompanied by a significant down-regulation in the rapid RVD response only in AQP2-RCCD(1) cells. This decrease in the RVD activity did not correlate with changes in AQP2 function or expression, demonstrating that AQP2-besides increasing water permeability-would play some other role. These observations together with evidence implying a cell-sizing mechanism that shortens the cell cycle of large cells, let us to propose that during nutrient uptake, in early G(1), volume tends to increase but it may be efficiently regulated by an AQP2-dependent mechanism, inducing the rapid activation of RVD channels. This mechanism would be down-regulated when volume needs to be increased in order to proceed into the S-phase. Therefore, during cell cycle, a coordinated modulation of the RVD activity may contribute to accelerate proliferation of cells expressing AQP2.