Phenotypes of aquaporin mutants in genetically altered mice

The ability to move water across lipid membranes is crucial for nutrient intake, energy generation, waste excretion, and a myriad of other functions associated with life. Aquaporins, a family of integral membrane proteins, are now recognized as the channels responsible for transporting hydrophilic molecules, including water, across relatively impervious, hydrophobic cell membranes. A tremendous amount of work has been published on characterizing these proteins, which have been found in all bacteria, yeast, plants, and animals examined to date. In addition, an increasing number of mouse models with genetically altered aquaporin expression are being reported. This article will briefly review the basic biochemistry of aquaporins and then evaluate the use (and misuse) of mice in the quest for understanding the comparative pathophysiology of aquaporins in humans.     

Regulation of plant aquaporin activity

Accumulating evidence indicates that aquaporins play a key role in plant water relations. Plant aquaporins are part of a large and highly divergent protein family that can be divided into four subfamilies according to amino acid sequence similarity. As in other organisms, plant aquaporins facilitate the transcellular movement of water, but, in some cases, also the flux of small neutral solutes across a cellular membrane. Plant cell membranes are characterized by a large range of osmotic water permeabilities, and recent data indicate that plant aquaporin activity might be regulated by gating mechanisms. The factors affecting the gating behaviour possibly involve phosphorylation, heteromerization, pH, Ca2+, pressure, solute gradients and temperature. Regulation of aquaporin trafficking may also represent a way to modulate membrane water permeability. The aim of this review is to integrate recent molecular and biophysical data on the mechanisms regulating aquaporin activity in plant membranes and to relate them to putative changes in protein structure.     

水孔蛋白在细胞延长、盐胁迫和光合作用中的作用

水孔蛋白属于一个高度保守的、能够进行跨生物膜水分运输的通道蛋白MIP家族。水孔蛋白作为膜水通道,在控制细胞和组织的水含量中扮演重要角色。本研究的重点是属于PIP亚家族的GhPIP1;2和属于TIP亚家族的γTIP1在植物细胞延长中的作用。使用特异基因探针的Northern杂交和实时荧光PCR技术证明GhPIP1;2和GhγTIP1主要在棉花纤维延长过程中显著表达,且最高表达量在开花后5d。在细胞延长过程中,GhPIP1;2和GhγTIP1表达显著,表明它们在促使水流迅速进入液泡这一过程中扮演重要角色。而且也研究了盐胁迫植物中钙离子对水孔蛋白的影响。分别或一起用NaCl或CaCl2处理原生质体或细胞质膜。结果发现在盐胁迫条件下,水渗透率值在原生质体和质膜颗粒中都下降了,同时PIP1水孔蛋白的含量也下降了,表明NaCl对水孔蛋白的功能和含量有抑制作用。同时也观察了Ca2+的两种不同的作用。感知胁迫的胞质中游离钙离子浓度的增加可能导致水孔蛋白的关闭。而过剩的钙离子将导致水孔蛋白的上游调控。同时实验已经证明大麦的一类水孔蛋白-HvPIP2;1有更高的水和CO2转移率。本研究的目标是确定负责转运水和CO2的关键水孔蛋白...     

Water structure changes induced by ceramics can be detected by increased permeability through aquaporin

Aquporins are intrinsic membrane proteins that function as water channel to transport water and/or mineral nutrients across biological membranes. In this study, we aimed to clarify whether water structure can be changed by the presence of ceramics and whether such a change can be determined by aquaporin. First, we confirmed that ceramics could transform tap water into active tap water by increasing water permeability through aquaporin. We also found that this change in water permeability by treatment with ceramics occurred in distilled water. The distilled water was determined to exhibit the same aquaporin permeability as the original tap water. Our data indicate that the aquaporin permeability of water can be changed by severe physical shocks, such as slapping and sonication, which is consistent with the implication that the aquaporin permeability is closely related to the structure of the water. In this study, using aquaporins, we first reported that the treatment of water with ceramics can affect the structure of water, and the water can retain the structure for a given period under certain condition     

Aquaporins and cell growth

This review covers the data concerning the relationship between cell growth and aquaporins in the cell membranes, the plasmalemma and tonoplast. Genes of aquaporins, water channel-forming proteins, are actively expressed before the onset and during cell elongation, thus providing accumulation of aquaporin protein and higher membrane hydraulic conductivity. As a result, an additional water uptake favors cell vacuolation and elongation. The review gives information on all growing plant organs. In actively dividing plant cells, only plasmalemma aquaporins are synthesized, whereas in elongating cells, tonoplast aquaporins are synthesized as well. The review includes also the findings of aquaporin research after growth completion.     

Insights into structural mechanisms of gating induced regulation of aquaporins

Aquaporin family comprises of transmembrane channels that are specialized in conducting water and certain small, uncharged molecules across cell membranes. Essential roles of aquaporins in various physiological and pathophysiological conditions have attracted great scientific interest. Pioneering structural studies on aquaporins have almost solved the basic question of mechanism of selective water transport through these channels. Another important structural aspect of aquaporins which seeks attention is that how the flow of water through the channel is regulated by the mechanism of gating. Aquaporins are also regulated at the protein level, i.e. by trafficking which includes changes in their expression levels in the membrane. Availability of high resolution structures along with numerous molecular dynamics simulation studies have helped to gain an understanding of the structural mechanisms by which water flux through aquaporins is controlled. This review will summarize the highlights regarding structural features of aquaporins, mechanisms governing water permeation, proton exclusion and substrate specificity, and describe the structural insights into the mechanisms of aquaporin gating whereby water conduction is regulated by post translational modifications, such as phosphorylation.     

The aquaporins

Water is the major component of all living cells, and efficient regulation of water homeostasis is essential for many biological processes. The mechanism by which water passes through biological membranes was a matter of debate until the discovery of the aquaporin water channels. Aquaporins are intrinsic membrane proteins characterized by six transmembrane helices that selectively allow water or other small uncharged molecules to pass along the osmotic gradient. In addition, recent observations show that some aquaporins also facilitate the transport of volatile substances, such as carbon dioxide (CO₂) and ammonia (NH₃), across membranes. Aquaporins usually form tetramers, with each monomer defining a single pore. Aquaporin-related proteins are found in all organisms, from archaea to mammals. In both uni- and multicellular organisms, numerous isoforms have been identified that are differentially expressed and modified by post-translational processes, thus allowing fine-tuned tissue-specific osmoregulation. In mammals, aquaporins are involved in multiple physiological processes, including kidney and salivary gland function. They are associated with several clinical disorders, such as kidney dysfunction, loss of vision and brain edema.     

Aquaporin water channels: molecular mechanisms for human diseases

Although water is the major component of all biological fluids, the molecular pathways for water transport across cell membranes eluded identification until the discovery of the aquaporin family of water channels. The atomic structure of mammalian AQP1 illustrates how this family of proteins is freely permeated by water but not protons (hydronium ions, H3O+). Definition of the subcellular sites of expression predicted their physiological functions and potential clinical disorders. Analysis of several human disease states has confirmed that aquaporins are involved in multiple different illnesses including abnormalities of kidney function, loss of vision, onset of brain edema, starvation, and arsenic toxicity.     

Does CO2 permeate through aquaporin-1?

Aquaporins facilitate water permeation across biological membranes. Additionally, glycerol and other small neutral solutes are permeated by related aquaglyceroporins. The role of aquaporins in gas permeation has been a long-standing and controversially discussed issue. We present an extensive set of atomistic molecular dynamics simulations that address the question of CO(2) permeation through human aquaporin-1. Free energy profiles derived from the simulations display a barrier of approximately 23 kJ/mol in the aromatic/arginine constriction region of the water pore, whereas a barrier of approximately 4 kJ/mol was observed for a palmitoyloleoylphosphatidylethanolamine lipid bilayer membrane. The results indicate that significant aquaporin-1-mediated CO(2) permeation is to be expected only in membranes with a low intrinsic CO(2) permeability.     

Expression and distribution of a vacuolar aquaporin in young and mature leaf tissues of Brassica napus in relation to water fluxes

 Recently, it has been shown that water fluxes across biological membranes occur not only through the lipid bilayer but also through specialized water-conducting proteins, the so called aquaporins. In the present study, we investigated in young and mature leaves of Brassica napus L. the expression and localization of a vacuolar aquaporin homologous to radish γ-tonoplast intrinsic protein/vacuolar-membrane integral protein of 23 kDa (TIP/VM 23). In-situ hybridization showed that these tonoplast aquaporins are highly expressed not only in developing but also in mature leaves, which export photosynthates. No substantial differences could be observed between different tissues of young and mature leaves. However, independent of the developmental stage, an immunohistochemical approach revealed that the vacuolar membrane of bundle-sheath cells contained more protein cross-reacting with antibodies raised against radish γ-TIP/VM 23 than the mesophyll cells. The lowest labeling was detected in phloem cells. We compared these results with the distribution of plasma-membrane aquaporins cross-reacting with antibodies detecting a domain conserved among members of the plasma-membrane intrinsic protein 1 (PIP1) subfamily. We observed the same picture as for the vacuolar aquaporins. Furthermore, a high density of gold particles labeling proteins of the PIP1 group could be observed in plasmalemmasomes of the vascular parenchyma. Our results indicate that γ-TIP/VM 23 and PIP1 homologous proteins show a similar expression pattern. Based on these results it is tempting to speculate that bundle-sheath cells play an important role in facilitating water fluxes between the apoplastic and symplastic compartments in close proximity to the vascular tissue. Received: 23 December 1999 / Accepted: 3 June 2000     

Structural insights into eukaryotic aquaporin regulation

Aquaporin-mediated water transport across cellular membranes is an ancient, ubiquitous mechanism within cell biology. This family of integral membrane proteins includes both water selective pores (aquaporins) and transport facilitators of other small molecules such as glycerol and urea (aquaglyceroporins). Eukaryotic aquaporins are frequently regulated post-translationally by gating, whereby the rate of flux through the channel is controlled, or by trafficking, whereby aquaporins are shuttled from intracellular storage sites to the plasma membrane. A number of high-resolution X-ray structures of eukaryotic aquaporins have recently been reported and the new structural insights into gating and trafficking that emerged from these studies are described. Basic structural themes reoccur, illustrating how the problem of regulation in diverse biological contexts builds upon a limited set of possible solutions.     

Membrane lipid, not polarized water, is responsible for the semipermeable properties of living cells.

It has recently been proposed that the semipermeable surface barrier of living cells is in fact provided by water polarized in multilayers by the cellular protein, and not by the lipids of the cell membrane as had been widely supposed. This review paper summarizes a large and diverse body of recent experimental work, concerned with the structure and function of biological membranes in general and with the passive permeability properties of cell surface membranes in particular, which does not support the polarized water hypothesis. Indeed, there exists a great deal of experimental evidence which supports the concept that the lipid bilayer is a central structural feature of cell surface membranes and that one of the major functions of this lipid bilayer is to provide the selective, semipermeable barrier found at the surface of living cells.     

Plant aquaporins: Roles in plant physiology

Background

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms.

Scope of review

Here, we present comprehensive insights made on plant aquaporins in recent years, pointing to their molecular and physiological specificities with respect to animal or microbial counterparts.

Major conclusions

In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations and various physiological substrates in addition to water. Of particular relevance for plants is the transport by aquaporins of dissolved gases such as carbon dioxide or metalloids such as boric or silicic acid. The mechanisms that determine the gating and subcellular localization of plant aquaporins are extensively studied. They allow aquaporin regulation in response to multiple environmental and hormonal stimuli. Thus, aquaporins play key roles in hydraulic regulation and nutrient transport in roots and leaves. They contribute to several plant growth and developmental processes such as seed germination or emergence of lateral roots.

General significance

Plants with genetically altered aquaporin functions are now tested for their ability to improve plant resistance to stresses. This article is part of a Special Issue entitled Aquaporins.     

The influence of natural mineral water on aquaporin water permeability and human natural killer cell activity

Aquaporins are the intrinsic membrane proteins functioning as water channel to transport water and/or mineral nutrients across the biological membrane systems. In this research, we aimed to clarify if the selected mineral water can affect aquaporin functions in vitro and the assumption of the mineral water can modify aquaporin expression and activate natural killer cell activity in human body. First, we expressed six human and eight plant aquaporin genes in oocytes and compared the effect of different kinds of natural mineral water on aquaporin activity. The oocyte assay data show that Hita tenryosui water could promote water permeability of almost all human and plant aquaporins in varying degrees, and freeze-dry and organic solvent extraction could reduce AQP2 activity but pH change and boiling could not. Second, each volunteer in two groups (10 in one group) received an oral Hita tenryosui or tap water load of 1000 ml/day for total four weeks. We found that these two kinds of water did not directly affect the relative expression levels of AQP1 and AQP9 in the blood cells, but intriguingly, the natural killer cell activities of the volunteers drinking Hita tenryosui water were significantly improved, suggesting that Hita tenryosui water has obvious health function, which opens a new and interesting field of investigation related to the link between mineral water consumption and human health and the therapies for some chronic diseases.     

Major intrinsic proteins (MIPs) in plants: a complex gene family with major impacts on plant phenotype

The ubiquitous cell membrane proteins called aquaporins are now firmly established as channel proteins that control the specific transport of water molecules across cell membranes in all living organisms. The aquaporins are thus likely to be of fundamental significance to all facets of plant growth and development affected by plant–water relations. A majority of plant aquaporins have been found to share essential structural features with the human aquaporin and exhibit water-transporting ability in various functional assays, and some have been shown experimentally to be of critical importance to plant survival. Furthermore, substantial evidence is now available from a number of plant species that shows differential gene expression of aquaporins in response to abiotic stresses such as salinity, drought, or cold and clearly establishes the aquaporins as major players in the response of plants to conditions that affect water availability. This review summarizes the function and regulation of these genes to develop a greater understanding of the response of plants to water insufficiency, and particularly, to identify tolerant genotypes of major crop species including wheat and rice and plants that are important in agroforestry. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Molecular dynamics of water in the neighborhood of aquaporins

Present knowledge obtained by molecular dynamics (MD) simulation studies regarding the dynamics of water, both in the vicinity of biological membranes and within the proteinaceous water channels, also known as aquaporins (AQPs), is reviewed. A brief general summary of the water models most extensively employed in MD simulations (SPC, SPC/E, TIP3P, TIP4P), indicating their most relevant pros and cons, is likewise provided. Structural considerations of water are also discussed, based on different order parameters, which can be extracted from MD simulations as well as from experiments. Secondly, the behaviour of water in the neighbourhood of membranes by means of molecular dynamics simulations is addressed. Consequently, the comparison with previous experimental evidence is pointed out. In living cells, water is transported across the plasma membrane through the lipid bilayer and the aforementioned AQPs, which motivates this review to focus mostly on MD simulation studies of water within AQPs. Relevant contributions explaining peculiar properties of these channels are discussed, such as selectivity and gating. Water models used in these studies are also summarised. Finally, based on the information presented here, further MD studies are encouraged.     

From Membrane Pores to Aquaporins: 50 Years Measuring Water Fluxes

This review focuses on studies of water movement across biological membranes performed over the last 50 years. Different scientific approaches had tried to elucidate such intriguing mechanism, from hypotheses emphasizing the role of the lipid bilayer to the cloning of aquaporins, the ubiquitous proteins described as specific water channels. Pioneering and clarifying biophysical work are reviewed beside results obtained with the help of recent sophisticated techniques, to conclude that great advances in the subject live together with old questions without definitive answers.