Molecular mechanisms and drug development in aquaporin water channel diseases: aquaporin superfamily (superaquaporins): expansion of aquaporins restricted to multicellular organisms

The water channels aquaporin-1 and aquaporin-3 interact with and affect the cell polarity protein Scribble in 3D in vitro models of breast cancer

Cellular changes in carcinomas include alterations in cell proliferation, cell migration, cell-cell adhesion, and cellular polarity. In vitro studies have revealed that the water channels, aquaporin-1 (AQP1) and AQP3, can influence cell migration and cell-cell adhesion. Of note, we previously showed that AQP1 overexpression reduced levels of cell-cell adhesion proteins, whereas AQP3 increased levels when overexpressed in normal epithelial cells. Expression of AQP1 and AQP3 in breast carcinoma is associated with lymph node metastasis, recurrence, and poor survival of patients with breast cancer. In this study, we investigated if AQP1 and AQP3 affected cell polarity in breast cancer by studying the relationship between the major polarity protein Scribble and AQP1 and AQP3. In breast cancer tissue samples, the protein expression of AQP1, AQP3, and Scribble did not show an obvious correlation. However, in a GST pull-down assay, AQP1 and AQP3 interacted with Scribble. AQP1 overexpression reduced the size of 3D spheroids as well as reduced Scribble levels at cell-cell contacts, whereas AQP3 overexpression showed no significant change in spheroid size compared with control, AQP3 overexpression also reduced Scribble levels at cell-cell contacts. Of note, AQP1 overexpression increased cell migration and induced cell detachment and dissemination from migrating breast cancer cell sheets, whereas AQP3 overexpression did not. Thus, AQP1 and AQP3 differentially affect 3D-grown breast cancer spheroids, and especially AQP1 may contribute to cancer development and spread via negatively affecting cellular junctions and cell polarity proteins as well as increasing cell migration and cell detachment.NEW & NOTEWORTHY Overexpression of the water channels aquaporin-1 and aquaporin-3 reduced levels of the key polarity protein Scribble at cell-cell junctions, suggesting potential implications in breast cancer progression and metastasis.

From Homeostasis to Pathology: Decoding the Multifaceted Impact of Aquaporins in the Central Nervous System

Aquaporins (AQPs), integral membrane proteins facilitating selective water and solute transport across cell membranes, have been the focus of extensive research over the past few decades. Particularly noteworthy is their role in maintaining cellular homeostasis and fluid balance in neural compartments, as dysregulated AQP expression is implicated in various degenerative and acute brain pathologies. This article provides an exhaustive review on the evolutionary history, molecular classification, and physiological relevance of aquaporins, emphasizing their significance in the central nervous system (CNS). The paper journeys through the early studies of water transport to the groundbreaking discovery of Aquaporin 1, charting the molecular intricacies that make AQPs unique. It delves into AQP distribution in mammalian systems, detailing their selective permeability through permeability assays. The article provides an in-depth exploration of AQP4 and AQP1 in the brain, examining their contribution to fluid homeostasis. Furthermore, it elucidates the interplay between AQPs and the glymphatic system, a critical framework for waste clearance and fluid balance in the brain. The dysregulation of AQP-mediated processes in this system hints at a strong association with neurodegenerative disorders such as Parkinson's Disease, idiopathic normal pressure hydrocephalus, and Alzheimer's Disease. This relationship is further explored in the context of acute cerebral events such as stroke and autoimmune conditions such as neuromyelitis optica (NMO). Moreover, the article scrutinizes AQPs at the intersection of oncology and neurology, exploring their role in tumorigenesis, cell migration, invasiveness, and angiogenesis. Lastly, the article outlines emerging aquaporin-targeted therapies, offering a glimpse into future directions in combatting CNS malignancies and neurodegenerative diseases.

Aquaporin water channels: roles beyond renal water handling

Aquaporin (AQP) water channels are pivotal to renal water handling and therefore in the regulation of body water homeostasis. However, beyond the kidney, AQPs facilitate water reabsorption and secretion in other cells and tissues, including sweat and salivary glands and the gastrointestinal tract. A growing body of evidence has also revealed that AQPs not only facilitate the transport of water but also the transport of several small molecules and gases such as glycerol, H2O2, ions and CO2. Moreover, AQPs are increasingly understood to contribute to various cellular processes, including cellular migration, adhesion and polarity, and to act upstream of several intracellular and intercellular signalling pathways to regulate processes such as cell proliferation, apoptosis and cell invasiveness. Of note, several AQPs are highly expressed in multiple cancers, where their expression can correlate with the spread of cancerous cells to lymph nodes and alter the response of cancers to conventional chemotherapeutics. These data suggest that AQPs have diverse roles in various homeostatic and physiological systems and may be exploited for prognostics and therapeutic interventions.

Aquaporins and Ion Channels as Dual Targets in the Design of Novel Glioblastoma Therapeutics to Limit Invasiveness

Current therapies for Glioblastoma multiforme (GBM) focus on eradicating primary tumors using radiotherapy, chemotherapy and surgical resection, but have limited success in controlling the invasive spread of glioma cells into a healthy brain, the major factor driving short survival times for patients post-diagnosis. Transcriptomic analyses of GBM biopsies reveal clusters of membrane signaling proteins that in combination serve as robust prognostic indicators, including aquaporins and ion channels, which are upregulated in GBM and implicated in enhanced glioblastoma motility. Accumulating evidence supports our proposal that the concurrent pharmacological targeting of selected subclasses of aquaporins and ion channels could impede glioblastoma invasiveness by impairing key cellular motility pathways. Optimal sets of channels to be selected as targets for combined therapies could be tailored to the GBM cancer subtype, taking advantage of differences in patterns of expression between channels that are characteristic of GBM subtypes, as well as distinguishing them from non-cancerous brain cells such as neurons and glia. Focusing agents on a unique channel fingerprint in GBM would further allow combined agents to be administered at near threshold doses, potentially reducing off-target toxicity. Adjunct therapies which confine GBM tumors to their primary sites during clinical treatments would offer profound advantages for treatment efficacy.

Aquaporins: New players in breast cancer progression and treatment response

Aquaporins (AQPs) are a family of small transmembrane proteins that selectively transport water and other small molecules and ions following an osmotic gradient across cell plasma membranes. This enables them to regulate numerous functions including water homeostasis, fat metabolism, proliferation, migration, and adhesion. Previous structural and functional studies highlight a strong biological relationship between AQP protein expression, localization, and key biological functions in normal and cancer tissues, where aberrant AQP expression correlates with tumorigenesis and metastasis. In this review, we discuss the roles of AQP1, AQP3, AQP4, AQP5, and AQP7 in breast cancer progression and metastasis, including the role of AQPs in the tumor microenvironment, to highlight potential contributions of stromal-derived to epithelial-derived AQPs to breast cancer. Emerging evidence identifies AQPs as predictors of response to cancer therapy and as targets for increasing their sensitivity to treatment. However, these studies have not evaluated the requirements for protein structure on AQP function within the context of breast cancer. We also examine how AQPs contribute to a patient's response to cancer treatment, existing AQP inhibitors and how AQPs could serve as novel predictive biomarkers of therapy response in breast cancer. Future studies also should evaluate AQP redundancy and compensation as mechanisms used to overcome aberrant AQP function. This review highlights the need for additional research into how AQPs contribute molecularly to therapeutic resistance and by altering the tumor microenvironment.

Relevance of Aquaporins for Gamete Function and Cryopreservation

Simple Summary

The interaction between cells and the extracellular medium is of great importance; changes in medium composition can drive water movement across plasma membranes. Aquaporins (AQPs) are membrane channels involved in the transport of water and some solutes across membranes. When sperm enter the female reproductive tract after ejaculation, they encounter a drastic change in extracellular composition, which leads to water flowing across the plasma membrane. This triggers a series of events that are crucial to allowing fertilization to take place, such as regulation of sperm motility. In the context of assisted reproduction techniques (ART), long-term storage of gametes is sometimes required, and, during cryopreservation, these cells undergo drastic changes in extracellular medium composition. As a result, AQPs are crucial in both sperm and oocytes during this process. Cryopreservation is of considerable importance for fertility preservation in livestock, endangered species and for individuals undergoing certain medical treatments that compromise their fertility. Further research to fully elucidate the roles and underlying mechanisms of AQPs in mammalian sperm is therefore warranted.

Abstract

The interaction between cells and the extracellular medium is of great importance, and drastic changes in extracellular solute concentrations drive water movement across the plasma membrane. Aquaporins (AQPs) are a family of transmembrane channels that allow the transport of water and small solutes across cell membranes. Different members of this family have been identified in gametes. In sperm, they are relevant to osmoadaptation after entering the female reproductive tract, which is crucial for sperm motility activation and capacitation and, thus, for their fertilizing ability. In addition, they are relevant during the cryopreservation process, since some members of this family are also permeable to glycerol, one of the most frequently used cryoprotective agents in livestock. Regarding oocytes, AQPs are very important in their maturation but also during cryopreservation. Further research to define the exact sets of AQPs that are present in oocytes from different species is needed, since the available literature envisages certain AQPs and their roles but does not provide complete information on the whole set of AQPs. This is of considerable importance because, in sperm, specific AQPs are known to compensate the role of non-functional members.

Native Mass Spectrometry and Surface Induced Dissociation Provide Insight into the Post-Translational Modifications of Tetrameric AQP0 Isolated from Bovine Eye Lens

Aquaporin-0 (AQP0) is a tetrameric membrane protein and the most abundant membrane protein in the eye lens. Interestingly, there is little to no cellular turnover once mature lens fiber cells are formed, and hence, age-related modifications accumulate with time. While bottom-up mass spectrometry-based approaches can provide identification of post-translational modifications, they cannot provide information on how these modifications coexist in a single chain or complex. Native mass spectrometry, however, enables the transfer of the intact complex into the gas-phase allowing modifications to be identified at the tetramer level. Here, we present the use of native mass spectrometry and surface-induced dissociation to study the post-translational modifications of AQP0 isolated and purified from bovine eye lens, existing as multiple forms due to the different modification states naturally present.

Genome-Wide Characterization of Aquaporins (aqps) in Lateolabrax maculatus: Evolution and Expression Patterns During Freshwater Acclimation

Aquaporin (aqp) proteins are a group of small integral membrane proteins that play crucial roles as pore channels for the transport of water and other small solutes across the cell membrane. In our study, we identified 17 aqp genes from the spotted sea bass (Lateolabrax maculatus) genomic database. Gene organization, motif distribution, and selection pressure analyses were performed to investigate their evolutionary characteristics. The aqp mRNA displayed tissue-specific expression pattern in ten selected tissues of healthy spotted sea bass. To investigate the potential involvement of spotted sea bass aqps in osmoregulation, the expression profiles of aqp genes in gills were examined during freshwater (FW) acclimation using qRT-PCR. The mRNA level of aqp3a was dramatically induced during 1-3 day of the FW transition period (77-fold and 15-fold upregulated on 1 day and 3 day than in the control group), indicating that aqp3a may play an important hypo-osmoregulatory role in spotted sea bass. In addition, the expression levels of aqp1aa, aqp1ab, aqp3b, aqp7, and aqp9b increased to various degrees at 1 day after transferring to FW, suggesting their potential involvement in the FW acclimation process. Our study provides a valuable foundation for future studies aimed at uncovering the specific roles of aqp genes during salinity acclimation in spotted sea bass and other teleost species.

Autophagy induction regulates aquaporin 3-mediated skin fibroblasts aging

BACKGROUND

Long- and short-term ultraviolet (UV) exposure have distinct biological effects on human fibroblasts.

OBJECTIVES

This study aimed to elucidate the underlying mechanisms of the biological effects of UV exposure on human skin fibroblasts.

METHOD

We subjected human skin fibroblast cells with or without AQP3, DEDD, or Beclin1 manipulation to UVA treatment and evaluated autophagy and senescence/aging in them.

RESULTS

Short-term UVA irradiation induced autophagy and upregulated AQP3 but not senescence, whereas long-term UVA irradiation inhibited autophagy, AQP3, and senescence/aging in vitro and in vivo. Silencing AQP3 abolished short-term UVA irradiation-induced autophagy and led to cellular senescence, whereas AQP3 overexpression partially rescued the senescence and autophagy inhibition induced by long-term UVA exposure in vitro. Mechanistically, the transcription factor JUN was found to bind to the AQP3 promoter to activate its transcription following short-term UVA exposure. Subsequently, AQP3 interacted with DEDD to induce its ubiquitination-mediated degradation and promote autophagy, and bound to Beclin1 to directly activate autophagy. Finally, autophagy induced by AQP3 overexpression robustly prevented UVA-induced senescence/aging in vitro and in vivo.

CONCLUSIONS

Thus, our study indicates that AQP3 controls skin fibroblasts photoaging by regulating autophagy and represents a potential target for future interventions against skin aging.

Aquaporins Are Differentially Regulated in Canine Cryptorchid Efferent Ductules and Epididymis

Simple Summary

The distribution and expression of aquaporins (AQPs) in the testes and spermatozoa of several animal species play important roles in spermatogenesis and spermatozoon transit in this region. The aim of this study was to evaluate AQP7, AQP8, and AQP9 localization and expression in the efferent ductules and epididymal regions (the caput, corpus, and cauda) of normal and cryptorchid dogs. The results from immunohistochemistry, Western blotting, and real-time reverse transcription polymerase chain reaction (RT-PCR) show regional tissue distributions, particularly at the level of the epithelium of efferent ductules and both the regions caput and cauda of the canine cryptorchid epididymis. These findings support the hypothesis that these channel proteins respond differently to multiple stimuli that cause cryptorchidism (hormones, heat, osmolarity, etc.) and participate in the mechanisms of cell “resilience” or apoptosis taking place in the epididymis.

Abstract

The efferent ductules and the epididymis are parts of the male reproductive system where spermatozoa mature. Specialized epithelial cells in these ducts contribute to the transport of fluids produced by spermatozoa’s metabolic activity. Aquaporins (AQPs) have been demonstrated to be expressed in the spermatozoan membrane and testis epithelial cells, where they contribute to regulating spermatozoan volume and transit through environments of differing osmolality. Due to the lack of detailed literature regarding AQP expression in the canine male genital tract, the aim of this study was to investigate both the distribution and expression of AQP7, AQP8, and AQP9 in the efferent ductules and epididymal regions (caput, corpus, and cauda) from normal and cryptorchid dogs by using immunohistochemistry, Western blotting, and real-time reverse transcription polymerase chain reaction (RT-PCR). Our results show different patterns for the distribution and expression of the examined AQPs, with particular evidence of their upregulation in the caput and downregulation in the cauda region of the canine cryptorchid epididymis. These findings are associated with a modulation of Hsp70 and caspase-3 expression, suggesting the participation of AQPs in the luminal microenvironment modifications that are peculiar characteristics of this pathophysiological condition.

AQP3 and AQP5-Potential Regulators of Redox Status in Breast Cancer

Breast cancer is still one of the leading causes of mortality in the female population. Despite the campaigns for early detection, the improvement in procedures and treatment, drastic improvement in survival rate is omitted. Discovery of aquaporins, at first described as cellular plumbing system, opened new insights in processes which contribute to cancer cell motility and proliferation. As we discover new pathways activated by aquaporins, the more we realize the complexity of biological processes and the necessity to fully understand the pathways affected by specific aquaporin in order to gain the desired outcome-remission of the disease. Among the 13 human aquaporins, AQP3 and AQP5 were shown to be significantly upregulated in breast cancer indicating their role in the development of this malignancy. Therefore, these two aquaporins will be discussed for their involvement in breast cancer development, regulation of oxidative stress and redox signalling pathways leading to possibly targeting them for new therapies.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

The role of mammalian superaquaporins inside the cell: An update

The progress on mammalian superaquaporin (sAQP), AQP11 and AQP12, in the past seven years is brought up to date from the previous review. This subfamily is separated because of the very low homology with other AQP subfamilies and it is present only in multicellular organisms excluding fungi and plants. Its unique intracellular localization, specifically in the ER has made its functional studies challenging, but it may function as glyceroporin, aquaporin and peroxiporin, H₂O₂ transporter. Knowledge on AQP11 has been expanded by tissue specific conditional knockout mice and by the identification of a SNP associated with kidney diseases. Moreover, the functional identification of AQP11 as a peroxiporin has expanded the role of AQP11 to the regulation of intracellular H₂O₂ homeostasis to prevent ER stress, which awaits further in vivo studies. As kidney-specific AQP11 knockout of developed kidney has produced little phenotype, AQP11 is critical for kidney development but its physiological significance remains to be clarified. On the other hand, little has been known on pancreas-specific AQP12. To move this field forward, the results of sAQP in lower animals will be necessary to obtain the insights into the role of mammalian sAQP, which hopefully will lead to the discovery of therapeutic targets.

AQP8 participates in oestrogen-mediated buffalo follicular development by regulating apoptosis of granulosa cells

Aquaporins (AQPs), a family of small membrane-spanning proteins, are involved in fluid transport, cell signalling and reproduction. Regulating AQP8 expression influences apoptosis of granulosa cells (GCs), ovarian folliculogenesis, oogenesis and early embryonic development in mice, but its role has never been investigated in other species. The aim of the present study was to characterize the AQP8 function in buffalo follicular development. The expression pattern of AQP8 in buffalo follicle was analysed by immunohistochemistry method. 17β-Estradiol (E2) or oestrogen receptor antagonist ICI182780 was used to treat GCs cultured in vitro, and the expression of AQP8 was detected using qRT-PCR. Its roles in apoptosis of buffalo GCs were investigated by shRNA technology. AQP8 was found to be expressed higher in secondary follicles (p < .05), and its mRNA level in GCs was upregulated by E2 via receptor-mediated mechanism in a dose-dependent manner. A 732-bp buffalo AQP8 coding region was obtained, which was highly conserved at the amino acid level among different species. AQP8-shRNA2 had more effective inhibition on target gene than AQP8-shRNA1 (66.49% vs. 58.31%) (p < .05). Knockdown of AQP8 induced GCs arrested at G2/M stage and occurred apoptosis. Compared with the control group, higher Caspase9 expression were observed in AQP8-shRNA2 lentivirus infected GCs (p < .05), while Bcl-2 and Bax expression levels had no obvious change (p > .05). Altogether, the above results indicate that AQP8 is involved in oestrogen-mediated regulation of buffalo follicular development by regulating cell cycle progression and apoptosis of GCs.

Di-lysine motif-like sequences formed by deleting the C-terminal domain of aquaporin-4 prevent its trafficking to the plasma membrane

Aquaporin-4 is a transmembrane water channel protein, the C-terminal domain of which is facing the cytosol. In the process of investigating the role of the C-terminal domain of aquaporin-4 with regard to intracellular trafficking, we observed that a derivative of aquaporin-4, in which the C-terminal 53 amino acids had been removed (Δ271-323), was localized to intracellular compartments, including the endoplasmic reticulum, but was not expressed on the plasma membranes. This was determined by immunofluorescence staining and labeling of the cells with monoclonal antibody specifically recognizing the extracellular domain of aquaporin-4, followed by confocal microscopy and flow cytometry. Deletion of additional amino acids in the C-terminal domain of aquaporin-4 led to its redistribution to the plasma membrane. This suggests that the effect of the 53-amino acid deletion on the subcellular localization of aquaporin-4 could be attributed to the formation of a signal at the C terminus that retained aquaporin-4 in intracellular compartments, rather than the loss of a signal required for plasma membrane targeting. Substitution of the lysine at position 268 with alanine could rescue the Δ271-323-associated retention in the cytosol, suggesting that the C-terminal sequence of the mutant served as a signal similar to a di-lysine motif.

Phosphorylation-Dependent Regulation of Mammalian Aquaporins

Water homeostasis is fundamental for cell survival. Transport of water across cellular membranes is governed by aquaporins—tetrameric integral membrane channels that are highly conserved throughout the prokaryotic and eukaryotic kingdoms. In eukaryotes, specific regulation of these channels is required and is most commonly carried out by shuttling the protein between cellular compartments (trafficking) or by opening and closing the channel (gating). Structural and functional studies have revealed phosphorylation as a ubiquitous mechanism in aquaporin regulation by both regulatory processes. In this review we summarize what is currently known about the phosphorylation-dependent regulation of mammalian aquaporins. Focusing on the water-specific aquaporins (AQP0–AQP5), we discuss how gating and trafficking are controlled by phosphorylation and how phosphorylation affects the binding of aquaporins to regulatory proteins, thereby highlighting structural details and dissecting the contribution of individual phosphorylated residues when possible. Our aim is to provide an overview of the mechanisms behind how aquaporin phosphorylation controls cellular water balance and to identify key areas where further studies are needed.

Structural Basis for Mutations of Human Aquaporins Associated to Genetic Diseases

Aquaporins (AQPs) are among the best structural-characterized membrane proteins, fulfilling the role of allowing water flux across cellular membranes. Thus far, 34 single amino acid polymorphisms have been reported in HUMSAVAR for human aquaporins as disease-related. They affect AQP2, AQP5 and AQP8, where they are associated with nephrogenic diabetes insipidus, keratoderma and colorectal cancer, respectively. For half of these mutations, although they are mostly experimentally characterized in their dysfunctional phenotypes, a structural characterization at a molecular level is still missing. In this work, we focus on such mutations and discuss what the structural defects are that they appear to cause. To achieve this aim, we built a 3D molecular model for each mutant and explored the effect of the mutation on all of their structural features. Based on these analyses, we could collect the structural defects of all the pathogenic mutations (here or previously analysed) under few main categories, that we found to nicely correlate with the experimental phenotypes reported for several of the analysed mutants. Some of the structural analyses we present here provide a rationale for previously experimentally observed phenotypes. Furthermore, our comprehensive overview can be used as a reference frame for the interpretation, on a structural basis, of defective phenotypes of other aquaporin pathogenic mutants.

Plant and Mammal Aquaporins: Same but Different

Aquaporins (AQPs) constitute an ancient and diverse protein family present in all living organisms, indicating a common ancient ancestor. However, during evolution, these organisms appear and evolve differently, leading to different cell organizations and physiological processes. Amongst the eukaryotes, an important distinction between plants and animals is evident, the most conspicuous difference being that plants are sessile organisms facing ever-changing environmental conditions. In addition, plants are mostly autotrophic, being able to synthesize carbohydrates molecules from the carbon dioxide in the air during the process of photosynthesis, using sunlight as an energy source. It is therefore interesting to analyze how, in these different contexts specific to both kingdoms of life, AQP function and regulation evolved. This review aims at highlighting similarities and differences between plant and mammal AQPs. Emphasis is given to the comparison of isoform numbers, their substrate selectivity, the regulation of the subcellular localization, and the channel activity.

Is Aquaporin-3 a Determinant Factor of Intrinsic and Extrinsic Aging? An Immunohistochemical and Morphometric Study

Aquaporin-3 (AQP3) is an aquaglyceroporin that plays a role in skin hydration, cell proliferation, and migration. The aim of this work was to investigate the expression of AQP3 in sun-exposed and sun-protected human skin from different age groups to understand the relationship between AQP3 and skin aging. Using standard immunohistochemical techniques, sun-exposed and sun-protected skin biopsies were taken from 60 normal individuals. AQP3 was expressed in the basal and the suprabasal layers, sparing the stratum corneum, in all specimens. Dermal expression was detected in fibroblasts, endothelial cells, and adnexa. Sun-protected skin showed a significantly higher epidermal H-score and percentage of expression (P=0.002 and <0.001, respectively) compared with sun-exposed skin. The AQP3 expression intensity showed a gradual decrease from the 20 to 35-year-old group to the 35 to 50-year-old group, with the least immunoreactivity in the above 50-year-old group. A significant difference was detected in the H-score in favor of the 20 to 35-year-old group in sun-exposed and sun-protected skin (P<0.001 for both). A significant negative correlation was noted between the AQP3 expression percentage and the age in sun-exposed (r=-0.64, P<0.001) and sun-protected skin (r=-0.53, P<0.001). In conclusion, the skin dryness observed in intrinsic and extrinsic aged skin may be explained, at least in part, by AQP3 downregulation. This may open new avenues sufficient to control skin texture and beauty. Its interaction in skin protein organization and gene polymorphism can also be tackled in future research. In addition, clinical trials using AQP3 topical applications should be carried out to evaluate its effectiveness in the reversal of age-related skin changes.

An Aversive Response to Osmotic Upshift in Caenorhabditis elegans

Environmental osmolarity presents a common type of sensory stimulus to animals. While behavioral responses to osmotic changes are important for maintaining a stable intracellular osmolarity, the underlying mechanisms are not fully understood. In the natural habitat of Caenorhabditis elegans, changes in environmental osmolarity are commonplace. It is known that the nematode acutely avoids shocks of extremely high osmolarity. Here, we show that C. elegans also generates gradually increased aversion of mild upshifts in environmental osmolarity. Different from an acute avoidance of osmotic shocks that depends on the function of a transient receptor potential vanilloid channel, the slow aversion to osmotic upshifts requires the cGMP-gated sensory channel subunit TAX-2. TAX-2 acts in several sensory neurons that are exposed to body fluid to generate the aversive response through a motor network that underlies navigation. Osmotic upshifts activate the body cavity sensory neuron URX, which is known to induce aversion upon activation. Together, our results characterize the molecular and cellular mechanisms underlying a novel sensorimotor response to osmotic stimuli and reveal that C. elegans engages different behaviors and the underlying mechanisms to regulate responses to extracellular osmolarity.

The Evolutionary Aspects of Aquaporin Family

Aquaporins (AQPs ) are a family of transmembrane proteins present in almost all species including virus. They are grossly divided into three subfamilies based on the sequence around a highly conserved pore-forming NPA motif: (1) classical water -selective AQP (CAQP), (2) glycerol -permeable aquaglyceroporin (AQGP) and (3) AQP super-gene channel, superaquaporin (SAQP). AQP is composed of two tandem repeats of conserved three transmembrane domains and a NPA motif. AQP ancestors probably started in prokaryotes by the duplication of half AQP genes to be diversified into CAQPs or AQGPs by evolving a subfamily-specific carboxyl-terminal NPA motif. Both AQP subfamilies may have been carried over to unicellular eukaryotic ancestors, protists and further to multicellular organisms. Although fungus lineage has kept both AQP subfamilies, the plant lineage has lost AQGP after algal ancestors with extensive diversifications of CAQPs into PIP, TIP, SIP, XIP, HIP and LIP with a possible horizontal transfer of NIP from bacteria. Interestingly, the animal lineage has obtained new SAQP subfamily with highly deviated NPA motifs, especially at the amino-terminal halves in both prostomial and deuterostomial animals. The prostomial lineage has lost AQGP after hymenoptera, while the deuterostomial lineage has kept all three subfamilies up to the vertebrate with diversified CAQPs (AQP0, 1, 2, 4, 5, 6, 8) and AQGPs (AQP3, 7, 9, 10) with limited SAQPs (AQP11, 12) in mammals. Whole-genome duplications, local gene duplications and horizontal gene transfers may have produced the AQP diversity with adaptive selections and functional alternations in response to environment changes. With the above evolutionary perspective in mind, the function of each AQP could be speculated by comparison among species to get new insights into physiological roles of AQPs . This evolutionary guidance in AQP research will lead to deeper understandings of water and solute homeostasis.

Expression and localization of aqua-glyceroporins AQP3 and AQP9 in rat oral epithelia

Aquaporins (AQPs) are a family of small integral membrane proteins made up of 6 hydrophobic, a-helical, membrane-spanning domains surrounding a highly selective aqueous pore. AQP3, AQP7, and AQP9, termed aqua-glyceroporins, are known to be involved in the transport of water, glycerol, and other small molecules. In this study, we investigated the expression and localization of aqua-glyceroporins in rat oral stratified squamous epithelia of the palate, the buccal mucosa, the inferior aspect of the tongue, and the oral floor by using RT-PCR, immunofluorescence, and immunogold electron microscopy. AQP3 and AQP9 mRNAs were expressed in whole oral epithelium. Immunostaining for AQP3 was recognized in each type of epithelium. The results suggest that AQP3 synthesis begins predominantly in the cytoplasm of the basal cells. During the process of epithelial cell differentiation, AQP3 protein appears to accumulate and be transported to the plasma membrane, from where it is incorporated into the cornified or surface layers. The intracellular localization of AQP3 appears to correlate with the differentiation of keratinocytes, suggesting that it acts as an enhancer of the physiological permeability barrier together with membrane coating granules. The distribution pattern of AQP9 was limited to the marginal areas of the basal and suprabasal layers, which was different from that of AQP3. This difference in distribution between AQP3 and AQP9 suggests that AQP9 in rat oral epithelia acts as a channel by facilitating glycerol uptake from the blood through the endothelial cells of the capillary vessels to the oral stratified squamous epithelium. AQP3 and AQP9 facilitate both transcellular osmotic water flow and glycerol transport as pore-like passive transporters in the keratinocytes of oral epithelia, and may play a key role in not only hydration and the permeability barrier, but also cell proliferation, differentiation, migration, development, and wound healing by generating ATP.

Hepatocyte and Sertoli Cell Aquaporins, Recent Advances and Research Trends

Aquaporins (AQPs) are proteinaceous channels widespread in nature where they allow facilitated permeation of water and uncharged through cellular membranes. AQPs play a number of important roles in both health and disease. This review focuses on the most recent advances and research trends regarding the expression and modulation, as well as physiological and pathophysiological functions of AQPs in hepatocytes and Sertoli cells (SCs). Besides their involvement in bile formation, hepatocyte AQPs are involved in maintaining energy balance acting in hepatic gluconeogenesis and lipid metabolism, and in critical processes such as ammonia detoxification and mitochondrial output of hydrogen peroxide. Roles are played in clinical disorders including fatty liver disease, diabetes, obesity, cholestasis, hepatic cirrhosis and hepatocarcinoma. In the seminiferous tubules, particularly in SCs, AQPs are also widely expressed and seem to be implicated in the various stages of spermatogenesis. Like in hepatocytes, AQPs may be involved in maintaining energy homeostasis in these cells and have a major role in the metabolic cooperation established in the testicular tissue. Altogether, this information represents the mainstay of current and future investigation in an expanding field.

Aquaporin 3 is regulated by estrogen in the chicken oviduct and is involved in progression of epithelial cell-derived ovarian carcinomas

Aquaporins (AQPs) are membrane proteins that passively deliver water across the plasma membrane to play an important role in maintaining cell shape. Members of the AQP family are distributed in most of the tissues in the human body and perform a variety of functions based on the water homeostasis suitable for each organ. However, there is little known about the expression and regulation of AQP family members in chickens. Therefore, we determined the expression of AQPs in various tissues of chickens. Among 13 isotypes, AQP3 was highly expressed in the chicken oviduct. Expression of AQP3 messenger RNA (mRNA) increased in the magnum (P < 0.001) and isthmus (P < 0.001) of chick oviducts treated with diethylstilbestrol. Consistent with these results, the localization of AQP3 was detected in the glandular and luminal epithelia of the magnum and isthmus of oviducts of diethylstilbestrol-treated chicks. In addition, the pattern of expression of AQP3 changed in an estrogen-dependent manner during the molting period. During the regenerative period of the oviduct after molting, expression of AQP3 mRNA increased coordinately with increasing concentrations of estradiol (P < 0.001), whereas expression of AQP3 mRNA decreased as concentrations of estradiol in plasma decreased in response to induced molting (P < 0.001). Also, expression of the AQP3 increased (P < 0.001) in cancerous ovaries of laying hens. In conclusion, AQP3 does not simply function to transport water into and out of cells but also appears to be closely involved in development of the chicken oviduct, which is regulated by estrogens. Furthermore, our results suggest AQP3 as a new diagnostic for early detection and treatment of epithelial cell-derived ovarian carcinomas.

The Trafficking of the Water Channel Aquaporin-2 in Renal Principal Cells-a Potential Target for Pharmacological Intervention in Cardiovascular Diseases

Arginine-vasopressin (AVP) stimulates the redistribution of water channels, aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. By this AVP directs 10% of the water reabsorption from the 170 L of primary urine that the human kidneys produce each day. This review discusses molecular mechanisms underlying the AVP-induced redistribution of AQP2; in particular, it provides an overview over the proteins participating in the control of its localization. Defects preventing the insertion of AQP2 into the plasma membrane cause diabetes insipidus. The disease can be acquired or inherited, and is characterized by polyuria and polydipsia. Vice versa, up-regulation of the system causing a predominant localization of AQP2 in the plasma membrane leads to excessive water retention and hyponatremia as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), late stage heart failure or liver cirrhosis. This article briefly summarizes the currently available pharmacotherapies for the treatment of such water balance disorders, and discusses the value of newly identified mechanisms controlling AQP2 for developing novel pharmacological strategies. Innovative concepts for the therapy of water balance disorders are required as there is a medical need due to the lack of causal treatments.

Aquaporin 5 Plays a Role in Estrogen-Induced Ectopic Implantation of Endometrial Stromal Cells in Endometriosis

Aquaporin 5 (AQP5) participates in the migration of endometrial cells. Elucidation of the molecular mechanisms associated with AQP5-mediated, migration of endometrial cells may contribute to a better understanding of endometriosis. Our objectives included identifying the estrogen-response element (ERE) in the promoter region of the AQP5 gene, and, investigating the effects of AQP5 on ectopic implantation of endometrial cells. Luciferase reporter assays and electrophoretic mobility shift assay (EMSA) identified the ERE-like motif in the promoter region of the AQP5 gene. After blocking and up-regulating estradiol (E₂) levels, we analysed the expression of AQP5 in endometrial stromal (ES) cells. After blocking E₂ /or phosphatidylinositol 3 kinase(PI3K), we analysed the role of AQP5 in signaling pathways. We constructed an AQP5, shRNA, lentiviral vector to knock out the AQP5 gene in ES cells. After knock-out of the AQP5 gene, we studied the role of AQP5 in cell invasion, proliferation, and the formation of ectopic endometrial implants in female mice. We identified an estrogen-response element in the promoter region of the AQP5 gene. Estradiol (E₂) increased AQP5 expression in a dose-dependent fashion, that was blocked by ICI182,780(an estrogen receptor inhibitor). E₂ activated PI3K /protein kinase B(AKT) pathway (PI3K/AKT), that, in turn, increased AQP5 expression. LY294002(PI3K inhibitor) attenuated estrogen-enhanced, AQP5 expression. Knock-out of the AQP5 gene with AQP5 shRNA lentiviral vector significantly inhibited E₂-enhanced invasion, proliferation of ES cells and formation of ectopic implants. Estrogen induces AQP5 expression by activating ERE in the promoter region of the AQP5gene, activates the PI3K/AKT pathway, and, promotes endometrial cell invasion and proliferation. These results provide new insights into some of the mechanisms that may underpin the development of deposits of ectopic endometrium.

Expression of aquaporins in intestine after heat stroke

Heat stroke (HS) has been shown to induce intestinal barrier dysfunction during whole body hyperthermia. HS-induced intestinal permeability change may result from modulation of aquaporin (AQP) expression, which subsequently regulates water homeostasis. This study aimed to evaluate AQP expression in the intestine of rats with HS at different recovery time points. Sprague-Dawley (SD) rats were exposed to an ambient temperature of 40 ± 0.5°C until a maximum core temperature of 40.5°C was attained. The small intestine was surgically removed and histologically examined, and AQP expression was determined by reverse transcription polymerase chain reaction and immunohistochemical staining. H&E staining revealed those intestinal villi were destroyed from HS0 to HS1 and rebuilt from HS3 to HS12. We further stain with activated caspase 3 found expressed at HS0 and back to normal at HS3. Investigation of AQP mRNA expression identified 10 genes. PCR results of AQP1, 3, 7, 8, and 11 transcripts were significantly higher in the HS group than in the sham group. Immunohistochemical staining showed a more than 11-fold increase in AQP3 and 11 expressions at HS0. AQP1 and 8 increased at HS1 and AQP7 increased at HS3 compared with those in the sham group. In this study, we found HS induced jejunum damage and cell apoptosis. AQPs were upregulation/downregulation after HS in different time point suggested that water/glycerol transport was important when hyperthermia occurred. Furthermore, the biological function of the AQP needs more exploration in response to HS.

EPA attenuates ultraviolet radiation-induced downregulation of aquaporin-3 in human keratinocytes

Eicosapentaenoic acid (EPA) is an omega-3 polyunsaturated fatty acid (ω-3 PUFA) that protects against photodamage and photocarcinogenesis in mammals. Aquaporin-3 (AQP3) is a water/glycerol transport protein that is found in basal layer keratinocytes. In this study, we have investigated the protective effect of EPA against ultraviolet B (UVB)-induced AQP3 downregulation in human keratinocytes. EPA treatment was found to increase AQP3 gene and protein expression in human epidermal keratinocytes (HaCaT). Using a specific inhibitor, we observed that the effect of EPA on AQP3 expression was mediated by extracellular signal-regulated kinase (ERK) activation. UVB radiation induced AQP3 downregulation in HaCaT cells, and it was found that EPA treatment attenuated UVB-induced AQP3 reduction and the associated cell death. UVB-induced downregulation of AQP3 was blocked by EPA and p38 inhibitor SB203580. Collectively, the present results show that EPA increased AQP3 expression and that this led to a reduction UVB-induced photodamage.

Reciprocity in the developmental regulation of aquaporins 1, 3 and 5 during pregnancy and lactation in the rat

Milk secretion involves significant flux of water, driven largely by synthesis of lactose within the Golgi apparatus. It has not been determined whether this flux is simply a passive consequence of the osmotic potential between cytosol and Golgi, or whether it involves regulated flow. Aquaporins (AQPs) are membrane water channels that regulate water flux. AQP1, AQP3 and AQP5 have previously been detected in mammary tissue, but evidence of developmental regulation (altered expression according to the developmental and physiological state of the mammary gland) is lacking and their cellular/subcellular location is not well understood. In this paper we present evidence of developmental regulation of all three of these AQPs. Further, there was evidence of reciprocity since expression of the rather abundant AQP3 and less abundant AQP1 increased significantly from pregnancy into lactation, whereas expression of the least abundant AQP5 decreased. It would be tempting to suggest that AQP3 and AQP1 are involved in the secretion of water into milk. Paradoxically, however, it was AQP5 that demonstrated most evidence of expression located at the apical (secretory) membrane. The possibility is discussed that AQP5 is synthesized during pregnancy as a stable protein that functions to regulate water secretion during lactation. AQP3 was identified primarily at the basal and lateral membranes of the secretory cells, suggesting a possible involvement in regulated uptake of water and glycerol. AQP1 was identified primarily at the capillary and secretory cell cytoplasmic level and may again be more concerned with uptake and hence milk synthesis, rather than secretion. The fact that expression was developmentally regulated supports, but does not prove, a regulatory involvement of AQPs in water flux through the milk secretory cell.

Human aquaporin-11 is a water and glycerol channel and localizes in the vicinity of lipid droplets in human adipocytes

OBJECTIVE

For a long time Aquaporin-7 has been the only aquaporin associated with the adipose tissue, and its dysregulation has been linked to the underlying mechanisms of obesity. However, the presence of alternative glycerol channels within the adipose tissue has been postulated, which has prompted us to the search of alternate glycerol transport routes in adipocytes. In view of this, it is hypothesized that Aquaporin-11 (AQP11) would have a role in adipocyte cell biology.

METHODS

The expression, the localization and the function of human AQP11 (hAQP11) in cultured differentiated adipocytes were investigated.

RESULTS

Gene expression analysis revealed the presence of AQP11 in both subcutaneous and visceral human mature adipocytes. It is found that hAQP11 is primarily located intracellularly in human adipocytes and partially colocalizes with perilipin, pointing towards AQP11 preferential location in the vicinity of lipid droplets. Overexpression of hAQP11 in 3T3-L1 adipocytes enabled to validate its function as a water channel and reveal its glycerol permeation activity.

CONCLUSIONS

hAQP11 permeates both water and glycerol, localizing in the vicinity of lipid droplets in human adipocytes.

Aquaporin-11 control of testicular fertility markers in Syrian hamsters

The present study sought novel changes to the hamster testicular transcriptome during modulation of fertility by well-characterized photoperiodic stimuli. Transition from long days (LD, 14 h light/day) to short days (SD, 10h light/day) triggered testicular regression (61% reduction of testis weight, relative to LD) in SD-sensitive (SD-S) hamsters within 16 weeks. After 22 weeks of SD exposure, a third cohort of hamsters became SD-refractory (SD-R), and exhibited testicular recrudescence (137% testis weight gain, relative to SD-S). Partial interrogation of the testicular transcriptome by annealing-control-primer-modified differential display PCR provided several candidates for regulation of testicular functions. Multiple linear regression modeling indicated the best correlation for aquaporin 11 (Aqp11) with changes in testis weight. Correlations were also strongest for Aqp11 with expression levels of reference cDNAs that control spermatogenesis (Hspa2 and Tnp2), steroidogenesis (Cox2, 3βHsd, and Srebp2), sperm motility (Catsper1, Pgk2, and Tnp2), inflammation (Cox2), and apoptosis (Bax and Bcl2). Moreover, siRNA-mediated knockdown of testicular Aqp11 mRNA and protein reduced Hspa2 and Tnp2 mRNA levels, and it increased 3βHsd mRNA levels. It also reduced mRNA levels for Sept12, which is a testis-specific inducer of spermatogenesis. These results suggest a central role for testicular Aqp11 signaling in the coordinate regulation of crucial components of fertility.

Molecular and functional characterization of multiple aquaporin water channel proteins from the western tarnished plant bug, Lygus hesperus

Aquaporins (AQPs) are integral membrane channel proteins that facilitate the bidirectional transfer of water or other small solutes across biological membranes involved in numerous essential physiological processes. In arthropods, AQPs belong to several subfamilies, which contribute to osmoregulation, respiration, cryoprotection, anhydrobiosis, and excretion. We cloned and characterized five novel AQPs from the western tarnished plant bug, Lygus hesperus, a polyphagous insect pest of food and fiber crops throughout western North America. The L. hesperus AQPs (LhAQP1-5) belong to different phylogenetic subfamilies, have unique transcription profiles and cellular localizations, and all transport water (but not glycerol) when heterologously expressed in Xenopus laevis oocytes. Our results demonstrate that multiple AQPs with possible compensatory functions are produced in L. hesperus that likely play important roles in maintaining water homeostasis in this important insect pest.

The role of mammalian superaquaporins inside the cell

BACKGROUND

The mammalian two superaquaporins, AQP11 and AQP12, are present inside the cell and their null phenotypes in mice suggest their unusual functions.

SCOPE OF REVIEW

The surveyed literature on these superaquaporins and our unpublished data has been incorporated to speculate their roles.

MAJOR CONCLUSIONS

AQP11 and AQP12 have unique NPA boxes with a signature cysteine residue. Although some water permeability of AQP11 was demonstrated in liposomes and cultured cells, its permeability to glycerol is unknown. The function of AQP12 still remains to be clarified. AQP11 null mice develop polycystic kidneys following large intracellular vacuoles in the proximal tubule, which may be caused by ER stress or vesicle fusion failure. The role of AQP11 in the kidney and liver seems to alleviate the tissue damage and facilitate the recovery. Its expression in the sperm, thymus and brain suggests its potential roles in these organs in spite of the apparently normal null phenotype. Although AQP12 null mice appear normal, they suffer from severe pancreatitis, suggesting its role in the fusion of zymogen granules.

GENERAL SIGNIFICANCE

As many issues are unsolved, the clarification of the function and roles of the superaquaporin may lead to the identification of new roles of AQPs. This article is part of a Special Issue entitled Aquaporins.

In silico study of human aquaporin AQP11 and AQP12 channels

AQP11 and AQP12 are the most distantly related paralogs of the aquaporin family in human. They share indeed a low sequence similarity with other aquaporins and exhibit a modified N-terminal NPA signature motif. Furthermore, they have an anomalous subcellular localization. The AQP11 and AQP12 biological role remains to be fully clarified and their ability to allow transport of water is still debated. We have built accurate 3D-models for AQP11 and AQP12 and comprehensively compared their sequence and structure to other known aquaporins. In order to investigate whether they appear compatible or not with water permeability, we especially focused on the amino acid composition and electrostatics of their channels, keeping the structure of the low-water efficiency AQP0 as a reference system. Our analysis points out a possible alternative ar/R site and shows that these aquaporins feature unique residues at key pore-lining positions that make the shape, composition and electrostatics of their channel peculiar. Such residues can represent pivotal hints to study and explain the AQP11 and AQP12 biological and molecular function.

Knockdown of hepatic aquaglyceroporin-9 alleviates high fat diet-induced non-alcoholic fatty liver disease in rats

Non-alcoholic fatty liver disease (NAFLD) has emerged as a common public health problem that can progress to end-stage liver disease. Although the function of aquaglyceroporin-9 (AQP9) as a glycerol channel has been proven, the mechanism of AQP9 in the development of NAFLD is poorly understood. To investigate the effect of hepatic glycerol uptake inhibition, we utilized a lentivirus-associated RNA interference technique to knock down the expression of hepatic AQP9 in high fat diet (HFD)-induced NAFLD in rodents. Male Sprague-Dawley rats were fed with a standard chow diet or HFD for 8 weeks. Rats fed a HFD were separated into 3 groups: HFD, control and transfection group, which were injected with equivalent amounts of PBS, lentivirus-scramble and lentivirus-shRNA-AQP9 through the hepatic portal vein. The results showed a remarkable decrease in the expression of AQP9 mRNA and protein in liver for the transfection group. Furthermore, less hepatic steatosis occurred when compared to the HFD and control groups after 8weeks. Our results indicate a potential role for AQP9 in the pathogenesis of hepatic steatosis.

The promiscuous binding of pharmaceutical drugs and their transporter-mediated uptake into cells: what we (need to) know and how we can do so

A recent paper in this journal sought to counter evidence for the role of transport proteins in effecting drug uptake into cells, and questions that transporters can recognize drug molecules in addition to their endogenous substrates. However, there is abundant evidence that both drugs and proteins are highly promiscuous. Most proteins bind to many drugs and most drugs bind to multiple proteins (on average more than six), including transporters (mutations in these can determine resistance); most drugs are known to recognise at least one transporter. In this response, we alert readers to the relevant evidence that exists or is required. This needs to be acquired in cells that contain the relevant proteins, and we highlight an experimental system for simultaneous genome-wide assessment of carrier-mediated uptake in a eukaryotic cell (yeast).

Water channel proteins in the gastrointestinal tract

Water transport through the human digestive system is physiologically crucial for maintaining body water homeostasis and ensure digestive and absorptive functions. Within the gastrointestinal tract, water recirculates, being secreted with the digestive juices and then almost entirely absorbed by the small and large intestine. The importance of aquaporins (AQPs), transmembrane water channel proteins, in the rapid passage of water across plasma membranes in the gastrointestinal tract appears immediately evident. Several AQP isoforms are found in gastrointestinal epithelia, with AQP1, 3, 7, 10 and 11 being the most abundantly expressed in the whole gut. On the other hand, AQP4 and 8 are located selectively in the stomach and colon, respectively. Here we review AQP expression and localization at the tissue, cellular and subcellular level in gastrointestinal epithelia, and their modification in various gut diseases.

Effect of acetazolamide on aquaporin-1 and fluid flow in cultured choroid plexus

Acetazolamide (AZA), used in treatment of early or infantile hydrocephalus, is effective in some cases, while its effect on the choroid plexus (CP) remains ill-defined. The drug reversibly inhibits aquaporin-4 (AQP4), the most ubiquitous "water pore" in the brain, and perhaps modulation of AQP1 (located apically on CP cells) by AZA may reduce cerebrospinal fluid (CSF) production. We sought to elucidate the effect of AZA on AQP1 and fluid flow in CP cell cultures.CP tissue culture from 10-day Sprague-Dawley rats and a TRCSF-B cell line were grown on Transwell permeable supports and treated with 100 μM AZA. Fluid assays to assess direction and extent of fluid flow, and AQP1 expression patterns by immunoblot, Immuncytochemistry (ICC), and quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) were performed.Immunoblots and ICC analyses showed a decrease in AQP1 protein shortly after AZA treatment (lowest at 12 h), with transient AQP1 reduction mediated by mRNA expression (lowest at 6 h). Transwell fluid assays indicated a fluid shift at 2 h, before significant changes in AQP1 mRNA or protein levels.Timing of AZA effect on AQP1 suggests the drug alters protein transcription, while affecting fluid flow by a concomitant method. It is plausible that other mechanisms account for these phenomena, as the processes may occur independently.

MIPModDB: a central resource for the superfamily of major intrinsic proteins

The channel proteins belonging to the major intrinsic proteins (MIP) superfamily are diverse and are found in all forms of life. Water-transporting aquaporin and glycerol-specific aquaglyceroporin are the prototype members of the MIP superfamily. MIPs have also been shown to transport other neutral molecules and gases across the membrane. They have internal homology and possess conserved sequence motifs. By analyzing a large number of publicly available genome sequences, we have identified more than 1000 MIPs from diverse organisms. We have developed a database MIPModDB which will be a unified resource for all MIPs. For each MIP entry, this database contains information about the source, gene structure, sequence features, substitutions in the conserved NPA motifs, structural model, the residues forming the selectivity filter and channel radius profile. For selected set of MIPs, it is possible to derive structure-based sequence alignment and evolutionary relationship. Sequences and structures of selected MIPs can be downloaded from MIPModDB database which is freely available at http://bioinfo.iitk.ac.in/MIPModDB.

Dual functional characteristic of human aquaporin 10 for solute transport

BACKGROUND/AIMS

Although aquaglyceroporins have been generally believed to operate in a channel mode, which is of nonsaturable nature, for glycerol as well as for water, we recently found that human aquaporin 9 (hAQP9) operates in a carrier-mediated mode, which is of saturable nature, for glycerol. Based on the finding, we assumed that such a characteristic might be shared by the other aquaglyceroporins and examined the functional characteristics of hAQP10, which is an intestine-specific aquaglyceroporin.

METHODS

Transport assays were conducted using Xenopus laevis oocytes expressing hAQP10 derived from the microinjected cRNA.

RESULTS

The transport of glycerol by hAQP10 was found to be highly saturable with a Michaelis constant of 10.4 μM and specifically inhibited by several glycerol analogs such as monoacetin. Furthermore, when glycerol was preloaded in hAQP10-expressing oocytes, its efflux was trans-stimulated by extracellular glycerol. These results indicate the involvement of a carrier-mediated mechanism in glycerol transport by hAQP10. Interestingly, a channel mechanism was also found to be involved in part in hAQP10-mediated glycerol transport.

CONCLUSION

The present study unveiled the uniquely dual functional characteristic of hAQP10 as a carrier/channel for solute transport, providing a novel insight into its operation mechanism, which would help further elucidate its physiological role.

Immunolocalization of aquaporins in rat brain

The aquaporins (AQPs) are a family of homologous water channels expressed in many tissues. In this study, the expression and immunolocalization of different AQP subtypes in rat brains were investigated by RT-PCR, immunohistochemistry and immunofluorescence. The data showed that AQP1 was expressed in the subpial processes of astrocytes, choroid plexus and ependyma. AQP3, AQP5 and AQP8 had similar distribution patterns in piriform cortex, choroid plexus, hippocampus and dorsal thalamus. AQP4 and AQP9 were widely expressed in the rat brain and distributed in the subpial processes of astrocytes, ependyma, dorsal thalamus, hippocampus, white matter, suprachiasmatic nucleus (SCN) and supraoptic nucleus. AQP3, AQP4, AQP5, AQP8 and AQP9 were found in the Bergmann glial cells of cerebellum, cochlear nucleus and trapezoid nuclei. The distinct localization of various AQPs in cerebrum and the similarities of distribution patterns within cerebellum, cochlear nucleus and trapezoid nuclei suggest that AQPs may play an important role in maintaining the specific microenvironments of the brain.