Water channel proteins in the inner ear and their link to hearing impairment and deafness

The inner ear is a fluid-filled sensory organ that transforms mechanical stimuli into the senses of hearing and balance. These neurosensory functions depend on the strict regulation of the volume of the two major extracellular fluid domains of the inner ear, the perilymph and the endolymph. Water channel proteins, or aquaporins (AQPs), are molecular candidates for the precise regulation of perilymph and endolymph volume. Eight AQP subtypes have been identified in the membranous labyrinth of the inner ear. Similar AQP subtypes are also expressed in the kidney, where they function in whole-body water regulation. In the inner ear, AQP subtypes are ubiquitously expressed in distinct cell types, suggesting that AQPs have an important physiological role in the volume regulation of perilymph and endolymph. Furthermore, disturbed AQP function may have pathophysiological relevance and may turn AQPs into therapeutic targets for the treatment of inner ear diseases. In this review, we present the currently available knowledge regarding the expression and function of AQPs in the inner ear. We give special consideration to AQP subtypes AQP2, AQP4 and AQP5, which have been studied most extensively. The potential functions of AQP2 and AQP5 in the resorption and secretion of endolymph and of AQP4 in the equilibration of cell volume are described. The pathophysiological implications of these AQP subtypes for inner ear diseases, that appear to involve impaired fluid regulation, such as Menière's disease and Sj?gren's syndrome, are discussed.     

Biological significance and topological basis of aquaporin-partnering protein-protein interactions

Aquaporins (AQPs) are intramolecular channels essential for transport of H₂O, CO₂, and other small substrates across membranes. Through this function, AQPs can modulate CO₂ uptake and assimilation in plants and regulate water relations and many other physiological processes in all living organisms. To execute their physiological roles, AQPs may experience 3 types of hetero-molecular interaction, between AQPs and their kinases; between AQP isoforms; and between AQPs and other proteins that are neither AQPs nor kinases. Interacting with non-AQP non-kinase proteins may enable AQPs to extend their functions beyond substrate transport, and most fascinatingly, to serve as a gateway control for translocation of virulence effectors from pathogenic bacteria into the cytosol of eukaryotic cells. In this mini review, we will summarize the latter 2 types of interaction and discuss the physiological and/or pathological significance. We will also discuss a research angle to elucidate the structural basis of AQP-partnering protein interactions.     

植物水通道蛋白的干旱应答机制研究进展

干旱胁迫是严重影响全球作物生产的非生物胁迫之一,研究植物耐旱机制已成为一个重要领域。水通道蛋白是一类特异、高效转运水及其它小分子底物的膜通道蛋白,在植物中具有丰富的亚型,参与调节植物的水分吸收和运输。近10年来,水通道蛋白在植物不同生理过程中的作用,一直受到研究人员的关注,特别是在非生物胁迫方面,而研究表明水通道蛋白在干旱胁迫下对植物的耐旱性起着至关重要的作用,能维持细胞水分稳态和调控环境胁迫快速响应。水通道蛋白在植物耐旱过程中的调控机制及功能较复杂,而关于其应答机制和不同亚型功能性研究的报道甚少。该文综述了植物水通道蛋白的分类、结构、表达调控和活性调节,分别从植物水通道蛋白响应干旱表达调控机制、水通道蛋白基因表达的时空特异性、水通道蛋白基因的表达与蛋白丰度,水通道蛋白基因的耐旱转化四个方面阐明干旱胁迫下植物水通道蛋白的表达,重点阐述其参与植物干旱胁迫应答的作用机制,并提出水通道蛋白研究的主要方向。     

Aquaporins as targets of pharmacological plant-derived compounds

Aquaporins (AQPs) are integral membrane proteins that serve as selective pores through which water and small solutes cross the plasma membranes of many human tissue and cell types. They have been identified in epithelia and endothelia involved in fluid transport, such as kidney tubules and glandular epithelia, glial cells, epidermis, and adipocytes. The pathophysiological roles of these proteins and the primary and secondary involvement of AQPs are becoming apparent in diverse clinical disorders, from diabetes insipidus to various forms of edema. The advanced understanding of aquaporin biology, from the structural determinants of channel permeability to the assignment of their physiological function in different organs, will allow the use of AQPs as targets for the therapy of a wide array of diseases. In this review, the mode of action of clinically-effective plant formulae on human AQPs-related diseases at the molecular, cellular, and organism levels is explored. The use of pharmacological plant-derived compounds as a possible strategy in the therapy of diseases related to altered water homeostasis should stimulate debate and further research objectives.     

New insight into the evolution of aquaporins from flowering plants and vertebrates: orthologous identification and functional transfer is possible

Aquaporins (AQPs) represent a family of channel proteins that transport water and/or small solutes across cell membranes in the three domains of life. In all previous phylogenetic analysis of aquaporin, trees constructed using proteins with very low amino acid identity (<15%) were incongruent with rRNA data. In this work, restricting the evolutionary study of aquaporins to proteins with high amino acid identity (>25%), we showed congruence between AQPs and organismal trees. On the basis of this analysis, we defined 19 orthologous gene clusters in flowering plant species (3 PIP-like, 7 TIP-like, 6 NIP-like and 3 SIP-like). We described specific conserved motifs for each subfamily and each cluster, which were used to develop a method for automatic classification. Analysis of amino acid identity between orthologous monocotyledon and dicotyledon AQPs from each cluster, suggested that PIPs are under high evolutionary constraint. The phylogenetic analysis allowed us the assignment of orthologous aquaporins for very distant animal lineages (tetrapods-fishes). We also demonstrated that the location of all vertebrate AQPs in the ortholog clusters could be predicted by comparing their amino acid identity with human AQPs. We defined four AQP subfamilies in animals: AQP1-like, AQP8-like, AQP3-like and AQP11-like. Phylogenetic analysis showed that the four animal AQPs subfamilies are related with PIP-like, TIP-like, NIP-like and SIP-like subfamilies, respectively. Thus, this analysis would allow the prediction of individual AQPs function on the basis of orthologous genes from Arabidopsis thaliana and Homo sapiens.     

Aquaporins and (in)fertility: More than just water transport

Aquaporins (AQPs) are a family of channel proteins that facilitate the transport of water and small solutes across biological membranes. They are widely distributed throughout the organism, having a number of key functions, some of them unexpected, both in health and disease. Among the various diseases in which AQPs are involved, infertility has been overlooked. According to the World Health Organization (WHO) infertility is a global public health problem with one third of the couples suffering from subfertility or even infertility due to male or female factors alone or combined. Thus, there is an urgent need to unveil the molecular mechanisms that control gametes production, maturation and fertilization-related events, to more specifically determine infertility causes. In addition, as more couples seek for fertility treatment through assisted reproductive technologies (ART), it is pivotal to understand how these techniques can be improved. AQPs are heterogeneously expressed throughout the male and female reproductive tracts, highlighting a possible regulatory role for these proteins in conception. In fact, their function, far beyond water transport, highlights potential intervention points to enhance ART. In this review we discuss AQPs distribution and structural organization, functions, and modulation throughout the male and female reproductive tracts and their relevance to the reproductive success. We also highlight the most recent advances and research trends regarding how the different AQPs are involved and regulated in specific mechanisms underlying (in)fertility. Finally, we discuss the involvement of AQPs in ART-related processes and how their handling can lead to improvement of infertility treatment.     

Aquaporin-11 containing a divergent NPA motif has normal water channel activity

Recently, two novel mammalian aquaporins (AQPs), AQPs 11 and 12, have been identified and classified as members of a new AQP subfamily, the "subcellular AQPs". In members of this subfamily one of the two asparagine-proline-alanine (NPA) motifs, which play a crucial role in selective water conduction, are not completely conserved. Mouse AQP11 (mAQP11) was expressed in Sf9 cells and purified using the detergent Fos-choline 10. The protein was reconstituted into liposomes, which were used for water conduction studies with a stopped-flow device. Single water permeability (pf) of AQP11 was measured to be 1.72+/-0.03x10(-13) cm(3)/s, suggesting that other members of the subfamily with incompletely conserved NPA motifs may also function as water channels.     

The evolutionary aspects of aquaporin family

Aquaporins (AQPs) were originally identified as channels facilitating water transport across the plasma membrane. They have a pair of highly conserved signature sequences, asparagine-proline-alanine (NPA) boxes, to form a pore. However, some have little conserved amino acid sequences around the NPA boxes unclassifiable to two previous AQP subfamilies, classical AQPs and aquaglyceroporins. These will be called unorthodox AQPs in this review. Interestingly, these unorthodox AQPs have a highly conserved cysteine residue downstream of the second NPA box. AQPs also have a diversity of functions: some related to water transport such as fluid secretion, fluid absorption, and cell volume regulation, and the others not directly related to water transport such as cell adhesion, cell migration, cell proliferation, and cell differentiation. Some AQPs even permeate nonionic small molecules, ions, metals, and possibly gasses. AQP gene disruption studies have revealed their physiological roles: water transport in the kidney and exocrine glands, glycerol transport in fat metabolism and in skin moisture, and nutrient uptakes in plants. Furthermore, AQPs are also present at intracellular organelles, including tonoplasts, mitochondria, and the endoplasmic reticulum. This review focuses on the evolutionary aspects of AQPs from bacteria to humans in view of the structural and functional diversities of AQPs.     

皂苷类成分对血管内皮细胞功能的调节作用研究进展

血管内皮细胞在维持血管生理稳态中发挥了重要的作用,其功能障碍是动脉粥样硬化、冠心病、脑卒中、肿瘤等多种重大疾病发生发展的病理基础,调节血管内皮细胞功能是防治上述疾病的主要途径之一。大量研究表明,皂苷类成分可通过改善血管内皮功能达到治疗疾病的目的。综述了近年来报道的皂苷类成分调节血管内皮功能的研究进展,旨在为皂苷类成分作用机制的阐明和相关重大疾病的防治提供一定参考。     

Aquaporin-3 functions as a glycerol transporter in mammalian skin

The AQPs (aquaporins) are a family of homologous water transporting proteins expressed in many mammalian epithelial, endothelial and other cell types. Phenotype analysis of mice lacking individual AQPs has been informative in elucidating their role in mammalian physiology. For example, phenotype analysis has indicated an important role of AQPs in the renal urinary concentrating mechanism (AQP1-AQP4), brain water balance and neural signal transduction (AQP4), exocrine gland secretion (AQP5) and ocular fluid balance (AQP1, AQP5). In skin, the aquaglyceroporin AQP3 is expressed in the basal layer of epidermal keratinocytes. Mice deficient in AQP3 have dry skin with reduced SC (stratum corneum) hydration, decreased elasticity and impaired biosynthesis. Mechanistic analysis of the altered skin phenotype in AQP3 deficiency suggested that the glycerol rather than the water transporting function of AQP3 is important in skin physiology. The glycerol content of SC and epidermis of AQP3 deficient mice is reduced, whereas that of dermis and serum is normal. The dry, relatively inelastic skin in AQP3 null mice is probably related to the humectant properties of glycerol, and the impaired SC repair to impaired epidermal biosynthetic function. The key role of AQP3 in epidermal physiology might be exploited in the development of improved cosmetics and new therapies for skin diseases associated with altered skin water content.     

What Are Aquaporins For?

The prime function of aquaporins (AQPs) is generally believed to be that of increasing water flow rates across membranes by raising their osmotic or hydraulic permeability. In addition, this applies to other small solutes of physiological importance. Notable applications of this simple permeability hypothesis (SPH) have been epithelial fluid transport in animals, water exchanges associated with transpiration, growth and stress in plants, and osmoregulation in microbes. We first analyze the need for such increased permeabilities and conclude that in a range of situations at the cellular, subcellular and tissue levels the SPH cannot satisfactorily account for the presence of AQPs. The analysis includes an examination of the effects of the genetic elimination or reduction of AQPs (knockouts, antisense transgenics and null mutants). These either have no effect, or a partial effect that is difficult to explain, and we argue that they do not support the hypothesis beyond showing that AQPs are involved in the process under examination. We assume that since AQPs are ubiquitous, they must have an important function and suggest that this is the detection of osmotic and turgor pressure gradients. A mechanistic model is proposed—in terms of monomer structure and changes in the tetrameric configuration of AQPs in the membrane—for how AQPs might function as sensors. Sensors then signal within the cell to control diverse processes, probably as part of feedback loops. Finally, we examine how AQPs as sensors may serve animal, plant and microbial cells and show that this sensor hypothesis can provide an explanation of many basic processes in which AQPs are already implicated. Aquaporins are molecules in search of a function; osmotic and turgor sensors are functions in search of a molecule.     

TGFβ signaling and cardiovascular diseases

Transforming growth factor β (TGFβ) family members are involved in a wide range of diverse functions and play key roles in embryogenesis, development and tissue homeostasis. Perturbation of TGFβ signaling may lead to vascular and other diseases. In vitro studies have provided evidence that TGFβ family members have a wide range of diverse effects on vascular cells, which are highly dependent on cellular context. Consistent with these observations genetic studies in mice and humans showed that TGFβ family members have ambiguous effects on the function of the cardiovascular system. In this review we discuss the recent advances on TGFβ signaling in (cardio)vascular diseases, and describe the value of TGFβ signaling as both a disease marker and therapeutic target for (cardio)vascular diseases.     

Roles of aquaporins in the brain

It is now over 10 years ago that aquaporin 1 (AQP1) was discovered and cloned from the red blood cells, and in 2003 the Nobel price in Chemistry was awarded to Pr. Peter Agre for his work on AQPs, highlighting the importance of these proteins in life sciences. AQPs are water channels. To date this protein family is composed of 11 sub-types in mammalians. Three main AQPs described in the mammalian brain are AQP1, AQP4 and AQP9. Several recent studies have shown that these channels are implicated in numerous physiological functions. AQP1 has a role in cerebrospinal fluid formation, whereas AQP4 is involved in water homeostasis and extracellular osmotic pressure in brain parenchyma. AQP4 seems also to have an important function in oedema formation after brain trauma or brain ischemia. AQP9 is implicated in brain energy metabolism. The level of expression of each AQP is highly regulated. After a trauma or an ischemia perturbation of the central nervous system, the level of expression of each AQP is differentially modified, resulting in facilitating oedema formation. At present, the exact role of each AQP is not yet determined. A better understanding of the mechanisms of AQP regulation should permit the development of new pharmacological strategies to prevent oedema formation. AQP9 has been recently specifically detected in the catecholaminergic neurons of the brain. This new result strengthens the hypothesis that the AQPs are not only water channels, but that some AQPs may play a role in energy metabolism as metabolite channels.     

Structural insights into the Aedes aegypti aquaporins and aquaglyceroporins – an in silico study

There has been a fair bit of understanding on the structure–function relationship of Aquaporins (AQPs) from plants and vertebrates obtained from available X-ray crystallography data. However, there is a lacuna in understanding the structure of AQPs from sanguinivorous insects like the mosquito where it plays a crucial role in survival. In this study, we have built homology models for the Aedes aegypti AQPs, identified key channel lining residues and compared the structure and sequence with orthodox AQPs. Although Ar/R filter residues of AaAQP1 were exactly similar to orthodox AQPs, AaAQP2 has a substitution at LE1position possibly making it less efficient in high capacity water transport. The huge difference in the selectivity filter region of AaAQP3 suggests a different transport property for this channel. The changes observed in the H5 position of the filter of AaAQP4 and AaAQP5 may explain the presence of a larger pore aperture to permit the passage of larger solute molecules. AaAQP6 possesses a completely hydrophobic filter like that in mammalian super aquaporins. The identified key residues are pivotal in understanding the mechanism of action and gating of these channels.     

Human aquaporins: Regulators of transcellular water flow

Background

Emerging evidence supports the view that (AQP) aquaporin water channels are regulators of transcellular water flow. Consistent with their expression in most tissues, AQPs are associated with diverse physiological and pathophysiological processes.

Scope of review

AQP knockout studies suggest that the regulatory role of AQPs, rather than their action as passive channels, is their critical function. Transport through all AQPs occurs by a common passive mechanism, but their regulation and cellular distribution varies significantly depending on cell and tissue type; the role of AQPs in cell volume regulation (CVR) is particularly notable. This review examines the regulatory role of AQPs in transcellular water flow, especially in CVR. We focus on key systems of the human body, encompassing processes as diverse as urine concentration in the kidney to clearance of brain oedema.

Major conclusions

AQPs are crucial for the regulation of water homeostasis, providing selective pores for the rapid movement of water across diverse cell membranes and playing regulatory roles in CVR. Gating mechanisms have been proposed for human AQPs, but have only been reported for plant and microbial AQPs. Consequently, it is likely that the distribution and abundance of AQPs in a particular membrane is the determinant of membrane water permeability and a regulator of transcellular water flow.

General significance

Elucidating the mechanisms that regulate transcellular water flow will improve our understanding of the human body in health and disease. The central role of specific AQPs in regulating water homeostasis will provide routes to a range of novel therapies. This article is part of a Special Issue entitled Aquaporins.     

自噬对血管功能的影响及与相关疾病的关联研究进展

自噬(autophagy)是细胞利用溶酶体降解自身受损的细胞器和大分子物质的过程,在稳定细胞内环境中发挥着重要作用．研究发现,自噬影响血管功能,与血管疾病的病理生理进程密切相关．本文从自噬对血管功能的影响,与血管相关疾病(如动脉粥样硬化、腹主动脉瘤、肺动脉高压、糖尿病血管并发症等)的关系及药物对血管壁细胞自噬的调控进行综述,希望从自噬的角度来了解血管的功能和病变及一些疾病的发生发展进程,为治疗血管相关疾病提供新的思路．     

Expression and function of aquaporins in human skin: Is aquaporin-3 just a glycerol transporter?

The aquaporins (AQPs) are a family of transmembrane proteins forming water channels. In mammals, water transport through AQPs is important in kidney and other tissues involved in water transport. Some AQPs (aquaglyceroporins) also exhibit glycerol and urea permeability. Skin is the limiting tissue of the body and within skin, the stratum corneum (SC) of the epidermis is the limiting barrier to water loss by evaporation. The aquaglyceroporin AQP3 is abundantly expressed in keratinocytes of mammalian skin epidermis. Mice lacking AQP3 have dry skin and reduced SC hydration. Interestingly, however, results suggested that impaired glycerol, rather than water transport was responsible for this phenotype. In the present work, we examined the overall expression of AQPs in cells from human skin and we reviewed data on the functional role of AQPs in skin, particularly in the epidermis. By RT-PCR on primary cell cultures, we found that up to 6 different AQPs (AQP1, 3, 5, 7, 9 and 10) may be selectively expressed in various cells from human skin. AQP1, 5 are strictly water channels. But in keratinocytes, the major cell type of the epidermis, only the aquaglyceroporins AQP3, 10 were found. To understand the role of aquaglyceroporins in skin, we examined the relevance to human skin of the conclusion, from studies on mice, that skin AQP3 is only important for glycerol transport. In particular, we find a correlation between the absence of AQP3 and intercellular edema in the epidermis in two different experimental models: eczema and hyperplastic epidermis. In conclusion, we suggest that in addition to glycerol, AQP3 may be important for water transport and hydration in human skin epidermis.     

Expression and function of aquaporins in human skin: Is aquaporin-3 just a glycerol transporter?

The aquaporins (AQPs) are a family of transmembrane proteins forming water channels. In mammals, water transport through AQPs is important in kidney and other tissues involved in water transport. Some AQPs (aquaglyceroporins) also exhibit glycerol and urea permeability. Skin is the limiting tissue of the body and within skin, the stratum corneum (SC) of the epidermis is the limiting barrier to water loss by evaporation. The aquaglyceroporin AQP3 is abundantly expressed in keratinocytes of mammalian skin epidermis. Mice lacking AQP3 have dry skin and reduced SC hydration. Interestingly, however, results suggested that impaired glycerol, rather than water transport was responsible for this phenotype. In the present work, we examined the overall expression of AQPs in cells from human skin and we reviewed data on the functional role of AQPs in skin, particularly in the epidermis. By RT-PCR on primary cell cultures, we found that up to 6 different AQPs (AQP1, 3, 5, 7, 9 and 10) may be selectively expressed in various cells from human skin. AQP1, 5 are strictly water channels. But in keratinocytes, the major cell type of the epidermis, only the aquaglyceroporins AQP3, 10 were found. To understand the role of aquaglyceroporins in skin, we examined the relevance to human skin of the conclusion, from studies on mice, that skin AQP3 is only important for glycerol transport. In particular, we find a correlation between the absence of AQP3 and intercellular edema in the epidermis in two different experimental models: eczema and hyperplastic epidermis. In conclusion, we suggest that in addition to glycerol, AQP3 may be important for water transport and hydration in human skin epidermis.