The Aquaporin Splice Variant NbXIP1;1α Is Permeable to Boric Acid and Is Phosphorylated in the N-terminal Domain

Aquaporin splice variation differentially modulates channel function during marine teleost egg hydration

Aquaporin-mediated oocyte hydration is a developmentally regulated adaptive mechanism that co-occurs with meiosis resumption in marine teleosts. It provides the early embryos with vital water until osmoregulatory systems develop, and in the majority of marine teleosts causes their eggs to float. Recent studies have shown that the subdomains of two water channels (Aqp1ab1 and Aqp1ab2) encoded in a teleost-specific aquaporin-1 cluster (TSA1C) co-evolved with duplicated Ywhaz-like (14-3-3ζ-like) binding proteins to differentially control their membrane trafficking for maximal egg hydration. Here, we report that in species that encode the full TSA1C, in-frame intronic splice variants of Aqp1ab1 result in truncated proteins that cause dominant-negative inhibition of the canonical channel trafficking to the plasma membrane. The inhibition likely occurs through hetero-oligomerization and retention in the endoplasmic reticulum (ER) and ultimate degradation. Conversely, in species that only encode the Aqp1ab2 channel we found an in-frame intronic splice variant that results in an intact protein with an extended extracellular loop E, and an out-of frame intronic splice variant with exon readthrough that results in a truncated protein. Both isoforms cause dominant-negative enhancement of the degradation pathway. However, the extended and truncated Aqp1ab2-type variants can also partially escape from the ER to reach the oocyte plasma membrane, where they dominantly-negatively inhibit water flux. The ovarian follicular expression ratios of the Aqp1ab2 isoforms in relation to the canonical channel are lowest during oocyte hydration, but subsequently highest when the canonical channel is recycled, thus leaving the eggs endowed with >90% water. These findings suggest that the expression of inhibitory isoforms of Aqp1ab1 and Aqp1ab2 may represent a new regulatory mechanism through which the cell-surface expression and the activity of the canonical channels can be physiologically modulated during oocyte hydration in marine teleosts.

High-resolution structure of a fish aquaporin reveals a novel extracellular fold

The structural and functional characterization of a fish AQP reveals a water-specific AQP with unique structural features that may have implications for channel gating in response to osmotic changes.

Aquaporins are protein channels embedded in the lipid bilayer in cells from all organisms on earth that are crucial for water homeostasis. In fish, aquaporins are believed to be important for osmoregulation; however, the molecular mechanism behind this is poorly understood. Here, we present the first structural and functional characterization of a fish aquaporin; cpAQP1aa from the fresh water fish climbing perch (Anabas testudineus), a species that is of high osmoregulatory interest because of its ability to spend time in seawater and on land. These studies show that cpAQP1aa is a water-specific aquaporin with a unique fold on the extracellular side that results in a constriction region. Functional analysis combined with molecular dynamic simulations suggests that phosphorylation at two sites causes structural perturbations in this region that may have implications for channel gating from the extracellular side.

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

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

Boric acid transport activity of human aquaporins expressed in Xenopus oocytes

Boric acid is a vital micronutrient that is toxic at high concentrations in animals. However, the mechanisms underlying boric acid transport in animal cells remain unclear. To identify the plasma membrane boric acid channels in animals, we analyzed the function of human aquaporins (AQPs), which are homologous to the nodulin-like intrinsic protein family of plant boric acid channels. When human AQPs were expressed in Xenopus laevis oocytes, the results of the swelling assay showed that boric acid permeability significantly increased in oocytes expressing AQP3, 7, 8, 9, and 10, but not in those expressing AQP1, 2, 4, and 5. The boric acid influxes of these oocytes were also confirmed by elemental quantification. Electrophysiological analysis using a pH microelectrode showed that these AQPs transported boric acid (B(OH)3 ) but not borate ions (B(OH)4- ). These results indicate that AQP3, 7, 8, 9, and 10 act as boric acid transport systems, likely as channels in humans.

Grapevine aquaporins: Diversity, cellular functions, and ecophysiological perspectives

High-scored premium wines are typically produced under moderate drought stress, suggesting that the water status of grapevine is crucial for wine quality. Aquaporins greatly influence the plant water status by facilitating water diffusion across the plasma membrane in a tightly regulated manner. They adjust the hydraulic conductance of the plasma membrane rapidly and reversibly, which is essential in specific physiological events, including adaptation to soil water scarcity. The comprehension of the sophisticated plant-water relations at the molecular level are thus important to optimize agricultural practices or to assist plant breeding programs. This review explores the recent progresses in understanding the water transport in grapevine at the cellular level through aquaporins and its regulation. Important aspects, including aquaporin structure, diversity, cellular localization, transport properties, and regulation at the cellular and whole plant level are addressed. An ecophysiological perspective about the roles of grapevine aquaporins in plant response to drought stress is also provided.

Understanding aquaporin transport system, silicon and other metalloids uptake and deposition in bottle gourd (Lagenaria siceraria)

Aquaporins (AQPs) facilitates the transport of small solutes like water, urea, carbon dioxide, boron, and silicon (Si) and plays a critical role in important physiological processes. In this study, genome-wide characterization of AQPs was performed in bottle gourd. A total of 36 AQPs were identified in the bottle gourd, which were subsequently analyzed to understand the pore-morphology, exon-intron structure, subcellular-localization. In addition, available transcriptome data was used to study the tissue-specific expression. Several AQPs showed tissue-specific expression, more notably the LsiTIP3-1 having a high level of expression in flowers and fruits. Based on the in-silico prediction of solute specificity, LsiNIP2-1 was predicted to be a Si transporter. Silicon was quantified in different tissues, including root, young leaves, mature leaves, tendrils, and fruits of bottle gourd plants. More than 1.3% Si (d.w.) was observed in bottle gourd leaves, testified the in-silico predictions. Silicon deposition evaluated with an energy-dispersive X-ray coupled with a scanning electron microscope showed a high Si accumulation in the shaft of leaf trichomes. Similarly, co-localization of Si with arsenic and antimony was observed. Expression profiling performed with real-time quantitative PCR showed differential expression of AQPs in response to Si supplementation. The information provided in the present study will be helpful to better understand the AQP transport mechanism, particularly Si and other metalloids transport and localization in plants.

Fungal X-Intrinsic Protein Aquaporin from Trichoderma atroviride: Structural and Functional Considerations

The major intrinsic protein (MIP) superfamily is a key part of the fungal transmembrane transport network. It facilitates the transport of water and low molecular weight solutes across biomembranes. The fungal uncharacterized X-Intrinsic Protein (XIP) subfamily includes the full protein diversity of MIP. Their biological functions still remain fully hypothetical. The aim of this study is still to deepen the diversity and the structure of the XIP subfamily in light of the MIP counterparts—the aquaporins (AQPs) and aquaglyceroporins (AQGPs)—and to describe for the first time their function in the development, biomass accumulation, and mycoparasitic aptitudes of the fungal bioagent Trichoderma atroviride. The fungus-XIP clade, with one member (TriatXIP), is one of the three clades of MIPs that make up the diversity of T. atroviride MIPs, along with the AQPs (three members) and the AQGPs (three members). TriatXIP resembles those of strict aquaporins, predicting water diffusion and possibly other small polar solutes due to particularly wider ar/R constriction with a Lysine substitution at the LE2 position. The XIP loss of function in ∆TriatXIP mutants slightly delays biomass accumulation but does not impact mycoparasitic activities. ∆TriatMIP forms colonies similar to wild type; however, the hyphae are slightly thinner and colonies produce rare chlamydospores in PDA and specific media, most of which are relatively small and exhibit abnormal morphologies. To better understand the molecular causes of these deviant phenotypes, a wide-metabolic survey of the ∆TriatXIPs demonstrates that the delayed growth kinetic, correlated to a decrease in respiration rate, is caused by perturbations in the pentose phosphate pathway. Furthermore, the null expression of the XIP gene strongly impacts the expression of four expressed MIP-encoding genes of T. atroviride, a plausible compensating effect which safeguards the physiological integrity and life cycle of the fungus. This paper offers an overview of the fungal XIP family in the biocontrol agent T. atroviride which will be useful for further functional analysis of this particular MIP subfamily in vegetative growth and the environmental stress response in fungi. Ultimately, these findings have implications for the ecophysiology of Trichoderma spp. in natural, agronomic, and industrial systems.

Protein-protein interactions in AQP regulation - biophysical characterization of AQP0-CaM and AQP2-LIP5 complex formation

Protein-protein interactions play important roles in regulating human aquaporins (AQP) by gating as well as trafficking. While structural and functional studies have provided detailed knowledge of AQP transport mechanisms, selectivity as well as gating by conformational changes of loops or termini, the mechanism behind how protein-protein interactions control AQP-mediated water transport through cellular membranes remains poorly characterized. Here we explore the interaction between two human AQPs and regulatory proteins: the interaction between AQP0 and calmodulin, which mediates AQP0 gating, as well as the interaction between AQP2 and LIP5, which is involved in trafficking. Using microscale thermophoresis (MST) and fluorescence anisotropy, two methods that have the advantage of low sample consumption and detergent compatibility, we show that the interactions can be studied using both full-length AQPs and AQP peptides corresponding to the regulatory protein binding sites. However, full-length AQPs gave better reproducibility between methods and for the first time revealed that AQP0 binds CaM in a cooperative manner, which was not seen in experiments using peptides. Our study highlights that, while peptides are great tools for locating binding sites and pinpointing interacting residues, full-length proteins may give additional insights, such as binding mechanism, allostery and cooperativity, important parameters for understanding protein-protein mediated regulation in the cellular context. Our work provides a platform for further studies of AQP regulation that may be of interest for designing drugs that target AQP complexes as well as the development of artificial bio-mimetic water channels for water-purification purposes.

Overexpression of X Intrinsic Protein 1;1 in Nicotiana tabacum and Arabidopsis reduces boron allocation to shoot sink tissues

Major Intrinsic Proteins (MIP) are a family of channels facilitating the diffusion of water and/or small solutes across cellular membranes. X Intrinsic Proteins (XIP) form the least characterized MIP subfamily in vascular plants. XIPs are mostly impermeable to water but facilitate the diffusion of hydrogen peroxide, urea and boric acid when expressed in heterologous expression systems. However, their transport capabilities in planta and their impact on plant physiology are still unknown. Here, we demonstrated that overexpression of NtXIP1;1 in Nicotiana tabacum by the En2pPMA4 or the 35S CaMV promoter and in Arabidopsis, which does not contain any XIP gene, by the 35S CaMV promoter, resulted in boron (B)-deficiency symptoms such as death of the shoot apical meristem, infertile flowers, and puckered leaves. Leaf B concentrations in symptomatic tissues and B xylem sap concentrations were lower in the overexpressors than in control plants. Importantly, expression of NtXIP1;1 under the control of the AtNIP5;1 promoter complemented the B deficiency phenotype of the Atnip5;1 knockout mutant, defining its ability to act as a boric acid channel in planta. Protein quantification analysis revealed that NtXIP1;1 was predominantly expressed in young B-demanding tissues and induced under B-deficient conditions. Our results strongly suggest that NtXIP1;1 plays a role in B homeostasis and its tissue-specific expression critically contributes to the distribution of B within tobacco.

Genome-wide comparison reveals divergence of cassava and rubber aquaporin family genes after the recent whole-genome duplication

Background

Aquaporins (AQPs) are a class of integral membrane proteins that facilitate the passive transport of water and other small solutes across biological membranes. Despite their importance, little information is available in cassava (Manihot esculenta), a perennial shrub of the Euphorbiaceae family that serves the sixth major staple crop in the world.

Results

This study presents a genome-wide analysis of the AQP gene family in cassava. The family of 42 members in this species could be divided into five subfamilies based on phylogenetic analysis, i.e., 14 plasma membrane intrinsic proteins (PIPs), 13 tonoplast intrinsic proteins (TIPs), nine NOD26-like intrinsic proteins (NIPs), four X intrinsic proteins (XIPs), and two small basic intrinsic proteins (SIPs). Best-reciprocal-hit-based sequence comparison and synteny analysis revealed 34 orthologous groups (OGs) present in the Euphorbiaceae ancestor, and nearly one-to-one or two-to-one orthologous relationships were observed between cassava with rubber/physic nut, reflecting the occurrence of one so-called ρ recent whole-genome duplication (WGD) in the last common ancestor of cassava and rubber. In contrast to a predominant role of the ρ WGD on family expansion in rubber, cassava AQP duplicates were derived from the WGD as well as local duplication. Species-specific gene loss was also observed in cassava, which includes the entire NIP4 group and/or six OGs. Comparison of conserved motifs and gene expression profiles revealed divergence of paralogs in cassava as observed in rubber.

Conclusions

Our findings will not only improve our knowledge on family evolution in Euphorbiaceae, but also provide valuable information for further functional analysis of AQP genes in cassava and rubber.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5780-4) contains supplementary material, which is available to authorized users.

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.

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.

Toward understanding of the high number of plant aquaporin isoforms and multiple regulation mechanisms

Since the discovery of the first plant aquaporin (AQP) in 1993, our conception of the way plants control cell water homeostasis as well as their global water balance has been revisited. Plant AQPs constitute a large family of evolutionarily related channels that, in addition to water, can also facilitate the membrane diffusion of a number of small solutes, such as urea, CO₂, H₂O₂, ammonia, metalloids, and even ions, indicating a wide range of cellular functions. At the cellular level, AQPs are subject to various regulation mechanisms leading to active/inactive channels in their target membranes. In this review, we discuss several specific questions that need to be addressed in future research. Why are so many different AQPs simultaneously expressed in specific cellular types? How is their selectivity to different solutes controlled (in particular in the case of multiple permeation properties)? What does the molecular interaction between AQPs and other molecules tell us about their regulation and their involvement in specific cellular and physiological processes? Resolving these questions will definitely help us better understand the physiological advantages that plants have to express and regulate so many AQP isoforms.

Phosphorylation of human aquaporin 2 (AQP2) allosterically controls its interaction with the lysosomal trafficking protein LIP5

The interaction between the renal water channel aquaporin-2 (AQP2) and the lysosomal trafficking regulator-interacting protein LIP5 targets AQP2 to multivesicular bodies and facilitates lysosomal degradation. This interaction is part of a process that controls AQP2 apical membrane abundance in a vasopressin-dependent manner, allowing for urine volume adjustment. Vasopressin regulates phosphorylation at four sites within the AQP2 C terminus (Ser²⁵⁶, Ser²⁶¹, Ser²⁶⁴, and Thr²⁶⁹), of which Ser²⁵⁶ is crucial and sufficient for AQP2 translocation from storage vesicles to the apical membrane. However, whether AQP2 phosphorylation modulates AQP2-LIP5 complex affinity is unknown. Here we used far-Western blot analysis and microscale thermophoresis to show that the AQP2 binds LIP5 in a phosphorylation-dependent manner. We constructed five phospho-mimicking mutants (S256E, S261E, S264E, T269E, and S256E/T269E) and a C-terminal truncation mutant (ΔP242) that lacked all phosphorylation sites but retained a previously suggested LIP5-binding site. CD spectroscopy indicated that wild-type AQP2 and the phospho-mimicking mutants had similar overall structure but displayed differences in melting temperatures possibly arising from C-terminal conformational changes. Non-phosphorylated AQP2 bound LIP5 with the highest affinity, whereas AQP2-ΔP242 had 20-fold lower affinity as determined by microscale thermophoresis. AQP2-S256E, S261E, T269E, and S256E/T269E all had reduced affinity. This effect was most prominent for AQP2-S256E, which fits well with its role in apical membrane targeting. AQP2-S264E had affinity similar to non-phosphorylated AQP2, possibly indicating a role in exosome excretion. Our data suggest that AQP2 phosphorylation allosterically controls its interaction with LIP5, illustrating how altered affinities to interacting proteins form the basis for regulation of AQP2 trafficking by post-translational modifications.

Single amino acid substitutions in the selectivity filter render NbXIP1;1α aquaporin water permeable

BACKGROUND

Aquaporins (AQPs) are integral membrane proteins that facilitate transport of water and/or other small neutral solutes across membranes in all forms of life. The X Intrinsic Proteins (XIPs) are the most recently recognized and the least characterized aquaporin subfamily in higher plants. XIP1s have been shown to be impermeable to water but permeable to boric acid, glycerol, hydrogen peroxide and urea. However, uncertainty regarding the determinants for selectivity and lack of an activity that is easy to quantify have hindered functional investigations. In an effort to resolve these issues, we set out to introduce water permeability in Nicotiana benthamiana XIP1;1α (NbXIP1;1α), by exchanging amino acid residues of predicted alternative aromatic/arginine (ar/R) selectivity filters of NbXIP1;1α for residues constituting the water permeable ar/R selectivity filter of AtTIP2;1.

RESULTS

Here, we present functional results regarding the amino acid substitutions in the putative filters as well as deletions in loops C and D of NbXIP1;1α. In addition, homology models were created based on the high resolution X-ray structure of AtTIP2;1 to rationalize the functional properties of wild-type and mutant NbXIP1;1α. Our results favour Thr 246 rather than Val 242 as the residue at the helix 5 position in the ar/R filter of NbXIP1;1α and indicate that the pore is not occluded by the loops when heterologously expressed in Pichia pastoris. Moreover, our results show that a single amino acid substitution in helix 1 (L79G) or in helix 2 (I102H) is sufficient to render NbXIP1;1α water permeable. Most of the functional results can be rationalized from the models based on a combination of aperture and hydrophobicity of the ar/R filter.

CONCLUSION

The water permeable NbXIP1;1α mutants imply that the heterologously expressed proteins are correctly folded and offer means to explore the structural and functional properties of NbXIP1;1α. Our results support that Thr 246 is part of the ar/R filter. Furthermore, we suggest that a salt bridge to an acidic residue in helix 1, conserved among the XIPs in clade B, directs the orientation of the arginine in the ar/R selectivity filter and provides a novel approach to tune the selectivity of AQPs.

Aquaglyceroporins Are the Entry Pathway of Boric Acid in Trypanosoma brucei

The boron element possesses a range of different effects on living beings. It is essential to beneficial at low concentrations, but toxic at excessive concentrations. Recently, some boron-based compounds have been identified as promising molecules against Trypanosoma brucei, the causative agent of sleeping sickness. However, until now, the boron metabolism and its access route into the parasite remained elusive. The present study addressed the permeability of T. brucei aquaglyceroporins (TbAQPs) for boric acid, the main natural boron species. To this end, the three TbAQPs were expressed in Saccharomyces cerevisiae and Xenopus laevis oocytes. Our findings in both expression systems showed that all three TbAQPs are permeable for boric acid. Especially TbAQP2 is highly permeable for this compound, displaying one of the highest conductances reported for a solute in these channels. Additionally, T. brucei aquaglyceroporin activities were sensitive to pH. Taken together, these results establish that TbAQPs are channels for boric acid and are highly efficient entry pathways for boron into the parasite. Our findings stress the importance of studying the physiological functions of boron and their derivatives in T. brucei, as well as the pharmacological implications of their uptake by trypanosome aquaglyceroporins.

Understanding Aquaporin Transport System in Eelgrass (Zostera marina L.), an Aquatic Plant Species

Aquaporins (AQPs) are a class of integral membrane proteins involved in the transport of water and many other small solutes. The AQPs have been extensively studied in many land species obtaining water and nutrients from the soil, but their distribution and evolution have never been investigated in aquatic plant species, where solute assimilation is mostly through the leaves. In this regard, identification of AQPs in the genome of Zostera marina L. (eelgrass), an aquatic ecological model species could reveal important differences underlying solute uptake between land and aquatic species. In the present study, genome-wide analysis led to the identification of 25 AQPs belonging to four subfamilies, plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin 26-like intrinsic proteins (NIPs), small basic intrinsic proteins (SIPs) in eelgrass. As in other monocots, the XIP subfamily was found to be absent from the eelgrass genome. Further classification of subfamilies revealed a unique distribution pattern, namely the loss of the NIP2 (NIP-III) subgroup, which is known for silicon (Si) transport activity and ubiquitously present in monocot species. This finding has great importance, since the eelgrass population stability in natural niche is reported to be associated with Si concentrations in water. In addition, analysis of available RNA-seq data showed evidence of expression in 24 out of the 25 AQPs across four different tissues such as root, vegetative tissue, male flower and female flower. In contrast to land plants, higher expression of PIPs was observed in shoot compared to root tissues. This is likely explained by the unique plant architecture of eelgrass where most of the nutrients and water are absorbed by shoot rather than root tissues. Similarly, higher expression of the TIP1 and TIP5 families was observed specifically in male flowers suggesting a role in pollen maturation. This genome-wide analysis of AQP distribution, evolution and expression dynamics can find relevance in understanding the adaptation of aquatic and land species to their respective environments.