Aquaporin Protein-Protein Interactions

Aquaporin Modulation by Cations, a Review

Aquaporins (AQPs) are transmembrane channels initially discovered for their role in water flux facilitation through biological membranes. Over the years, a much more complex and subtle picture of these channels appeared, highlighting many other solutes accommodated by AQPs and a dense regulatory network finely tuning cell membranes' water permeability. At the intersection between several transduction pathways (e.g., cell volume regulation, calcium signaling, potassium cycling, etc.), this wide and ancient protein family is considered an important therapeutic target for cancer treatment and many other pathophysiologies. However, a precise and isoform-specific modulation of these channels function is still challenging. Among the modulators of AQPs functions, cations have been shown to play a significant contribution, starting with mercury being historically associated with the inhibition of AQPs since their discovery. While the comprehension of AQPs modulation by cations has improved, a unifying molecular mechanism integrating all current knowledge is still lacking. In an effort to extract general trends, we reviewed all known modulations of AQPs by cations to capture a first glimpse of this regulatory network. We paid particular attention to the associated molecular mechanisms and pinpointed the residues involved in cation binding and in conformational changes tied up to the modulation of the channel function.

Inhibition of aquaporin-4 and its subcellular localization attenuates below-level central neuropathic pain by regulating astrocyte activation in a rat spinal cord injury model

The mechanisms of central neuropathic pain (CNP) caused by spinal cord injury have not been sufficiently studied. We have found that the upregulation of astrocytic aquaporin-4 (AQP4) aggravated peripheral neuropathic pain after spinal nerve ligation in rats. Using a T13 spinal cord hemisection model, we showed that spinal AQP4 was markedly upregulated after SCI and mainly expressed in astrocytes in the spinal dorsal horn (SDH). Inhibition of AQP4 with TGN020 suppressed astrocyte activation, attenuated the development and maintenance of below-level CNP and promoted motor function recovery in vivo. In primary astrocyte cultures, TGN020 also changed cell morphology, diminished cell proliferation and suppressed astrocyte activation. Moreover, T13 spinal cord hemisection induced cell-surface abundance of the AQP4 channel and perivascular localization in the SDH. Targeted inhibition of AQP4 subcellular localization with trifluoperazine effectively diminished astrocyte activation in vitro and further ablated astrocyte activation, attenuated the development and maintenance of below-level CNP, and accelerated functional recovery in vivo. Together, these results provide mechanistic insights into the roles of AQP4 in the development and maintenance of below-level CNP. Intervening with AQP4, including targeting AQP4 subcellular localization, might emerge as a promising agent to prevent chronic CNP after SCI.

Functional regulation of aquaporin dynamics by lipid bilayer composition

With the diversity of lipid-protein interactions, any observed membrane protein dynamics or functions directly depend on the lipid bilayer selection. However, the implications of lipid bilayer choice are seldom considered unless characteristic lipid-protein interactions have been previously reported. Using molecular dynamics simulation, we characterize the effects of membrane embedding on plant aquaporin SoPIP2;1, which has no reported high-affinity lipid interactions. The regulatory impacts of a realistic lipid bilayer, and nine different homogeneous bilayers, on varying SoPIP2;1 dynamics are examined. We demonstrate that SoPIP2;1's structure, thermodynamics, kinetics, and water transport are altered as a function of each membrane construct's ensemble properties. Notably, the realistic bilayer provides stabilization of non-functional SoPIP2;1 metastable states. Hydrophobic mismatch and lipid order parameter calculations further explain how lipid ensemble properties manipulate SoPIP2;1 behavior. Our results illustrate the importance of careful bilayer selection when studying membrane proteins. To this end, we advise cautionary measures when performing membrane protein molecular dynamics simulations.

Deciphering Molecular Mechanisms Involved in the Modulation of Human Aquaporins' Water Permeability by Zinc Cations: A Molecular Dynamics Approach

Aquaporins (AQPs) constitute a wide family of water channels implicated in all kind of physiological processes. Zinc is the second most abundant trace element in the human body and a few studies have highlighted regulation of AQP0 and AQP4 by zinc. In the present work, we addressed the putative regulation of AQPs by zinc cations in silico through molecular dynamics simulations of human AQP0, AQP2, AQP4, and AQP5. Our results align with other scales of study and several in vitro techniques, hence strengthening the reliability of this regulation by zinc. We also described two distinct putative molecular mechanisms associated with the increase or decrease in AQPs' water permeability after zinc binding. In association with other studies, our work will help deciphering the interaction networks existing between zinc and channel proteins.

Thermodynamics and S-Palmitoylation Dependence of Interactions between Human Aquaporin-4 M1 Tetramers in Model Membranes

Aquaporin-4 (AQP4) is a water channel protein found primarily in the central nervous system (CNS) that helps to regulate water-ion homeostasis. AQP4 exists in two major isoforms: M1 and M23. While both isoforms have a homotetrameric quaternary structure and are functionally identical when transporting water, the M23 isoform forms large protein aggregates known as orthogonal arrays of particles (OAPs). In contrast, the M1 isoform creates a peripheral layer around the outside of these OAPs, suggesting a thermodynamically stable interaction between the two. Structurally, the M1 isoform has an N-terminal tail that is 22 amino acids longer than the M23 isoform and contains two solvent-accessible cysteines available for S-palmitoylation at cysteine-13 (Cys-13) and cysteine-17 (Cys-17) in the amino acid sequence. Earlier work suggests that the palmitoylation of these cysteines might aid in regulating AQP4 assemblies. This work discusses the thermodynamic driving forces for M1 protein-protein interactions and how the palmitoylation state of M1 affects them. Using temperature-dependent single-particle tracking, the standard state free energies, enthalpies, and entropies were measured for these interactions. Furthermore, we present a binding model based on measured thermodynamics and a structural modeling study. The results of this study demonstrate that the M1 isoform will associate with itself according to the following expressions: 2[AQP4-M1]4 ↔ [[AQP4-M1]4]2 when palmitoylated and 3[AQP4-M1]4 ↔ [AQP4-M1]4 + [[AQP4-M1]4]2 ↔ [[AQP4-M1]4]3 when depalmitoylated. This is primarily due to a conformational change induced by adding the palmitic acid groups at Cys-13 and Cys-17 in the N-terminal tails of the homotetramers. In addition, a statistical mechanical model was developed to estimate the Gibbs free energy, enthalpy, and entropy for forming dimers and trimers. These results were in good agreement with experimental values.

The Versatile Role of Uromodulin in Renal Homeostasis and Its Relevance in Chronic Kidney Disease

Uromodulin, also known as the Tamm-Horsfall protein, is predominantly expressed in epithelial cells of the kidney. It is secreted mainly in the urine, although small amounts are also found in serum. Uromodulin plays an important role in maintaining renal homeostasis, particularly in salt/water transport mechanisms and is associated with salt-sensitive hypertension. It also regulates urinary tract infections, kidney stones, and the immune response in the kidneys or extrarenal organs. Uromodulin has been shown to be associated with the renal function, age, nephron volume, and metabolic abnormalities and has been proposed as a novel biomarker for the tubular function or injury. These findings suggest that uromodulin is a key molecule underlying the mechanisms or therapeutic approaches of chronic kidney disease, particularly nephrosclerosis and diabetic nephropathy, which are causes of end-stage renal disease. This review focuses on the current understanding of the role of uromodulin from a biological, physiological, and pathological standpoint.

Rutin ameliorate PFOA induced renal damage by reducing oxidative stress and improving lipid metabolism

Perfluorooctanoic acid (PFOA) is a persistent environmental pollutant that can accumulate in the kidneys and eventually cause kidney damage. Rutin (RUTIN) is a natural flavonoid with multiple biological activities, and its use in against kidney damage has been widely studied in recent years. It is not yet known whether rutin protects against kidney damage caused by PFOA. In this study, 30 ICR mice were randomly divided into three groups: CTRL group, PFOA group and PFOA+RUTIN group. The mice were fed continuously by gavage for 28 days. Renal pathological changes were assessed by HE and PASM staining, and serum renal function and lipid indicators were measured. RNA-seq and enrichment analysis using GO, KEGG and PPI to detect differential expression of genes in treatment groups. Kidney tissue protein expression was determined by Western blot. Research has shown that rutin can improve glomerular and tubular structural damage, and increase serum CREA, HDL-C levels and decrease LDH, LDL-C levels. The expression of AQP1 and ACOT1 was up-regulated after rutin treatment. Transcriptomic analysis indicated that PFOA and rutin affect the transcriptional expression of genes related to lipid metabolism and oxidative stress, and may affected by PI3K-Akt, PPAR, NRF2/KEAP1 signaling pathways. In conclusion, rutin ameliorated renal damage caused by PFOA exposure, and this protective effect may be exerted by ameliorating oxidative stress and regulating lipid metabolism.

Methods for studying mammalian aquaporin biology

Aquaporins (AQPs), transmembrane water-conducting channels, have earned a great deal of scrutiny for their critical physiological roles in healthy and disease cell states, especially in the biomedical field. Numerous methods have been implemented to elucidate the involvement of AQP-mediated water transport and downstream signaling activation in eliciting whole cell, tissue, and organ functional responses. To modulate these responses, other methods have been employed to investigate AQP druggability. This review discusses standard in vitro, in vivo, and in silico methods for studying AQPs, especially for biomedical and mammalian cell biology applications. We also propose some new techniques and approaches for future AQP research to address current gaps in methodology.

Co-evolution at protein-protein interfaces guides inference of stoichiometry of oligomeric protein complexes by de novo structure prediction

The quaternary structure with specific stoichiometry is pivotal to the specific function of protein complexes. However, determining the structure of many protein complexes experimentally remains a major bottleneck. Structural bioinformatics approaches, such as the deep learning algorithm Alphafold2-multimer (AF2-multimer), leverage the co-evolution of amino acids and sequence-structure relationships for accurate de novo structure and contact prediction. Pseudo-likelihood maximization direct coupling analysis (plmDCA) has been used to detect co-evolving residue pairs by statistical modeling. Here, we provide evidence that combining both methods can be used for de novo prediction of the quaternary structure and stoichiometry of a protein complex. We achieve this by augmenting the existing AF2-multimer confidence metrics with an interpretable score to identify the complex with an optimal fraction of native contacts of co-evolving residue pairs at intermolecular interfaces. We use this strategy to predict the quaternary structure and non-trivial stoichiometries of Bacillus subtilis spore germination protein complexes with unknown structures. Co-evolution at intermolecular interfaces may therefore synergize with AI-based de novo quaternary structure prediction of structurally uncharacterized bacterial protein complexes.

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.

Aquaglyceroporins in Human Breast Cancer

Aquaporins are water channels that facilitate passive water transport across cellular membranes following an osmotic gradient and are essential in the regulation of body water homeostasis. Several aquaporins are overexpressed in breast cancer, and AQP1, AQP3 and AQP5 have been linked to spread to lymph nodes and poor prognosis. The subgroup aquaglyceroporins also facilitate the transport of glycerol and are thus involved in cellular metabolism. Transcriptomic analysis revealed that the three aquaglyceroporins, AQP3, AQP7 and AQP9, but not AQP10, are overexpressed in human breast cancer. It is, however, unknown if they are all expressed in the same cells or have a heterogeneous expression pattern. To investigate this, we employed immunohistochemical analysis of serial sections from human invasive ductal and lobular breast cancers. We found that AQP3, AQP7 and AQP9 are homogeneously expressed in almost all cells in both premalignant in situ lesions and invasive lesions. Thus, potential intervention strategies targeting cellular metabolism via the aquaglyceroporins should consider all three expressed aquaglyceroporins, namely AQP3, AQP7 and AQP9.

Aquaporin-5 Dynamic Regulation

Aquaporin-5 (AQP5), belonging to the aquaporins (AQPs) family of transmembrane water channels, facilitates osmotically driven water flux across biological membranes and the movement of hydrogen peroxide and CO₂. Various mechanisms have been shown to dynamically regulate AQP5 expression, trafficking, and function. Besides fulfilling its primary water permeability function, AQP5 has been shown to regulate downstream effectors playing roles in various cellular processes. This review provides a comprehensive overview of the current knowledge of the upstream and downstream effectors of AQP5 to gain an in-depth understanding of the physiological and pathophysiological processes involving AQP5.

Cortisol Interaction with Aquaporin-2 Modulates Its Water Permeability: Perspectives for Non-Genomic Effects of Corticosteroids

Aquaporins (AQPs) are water channels widely distributed in living organisms and involved in many pathophysiologies as well as in cell volume regulations (CVR). In the present study, based on the structural homology existing between mineralocorticoid receptors (MRs), glucocorticoid receptors (GRs), cholesterol consensus motif (CCM) and the extra-cellular vestibules of AQPs, we investigated the binding of corticosteroids on the AQP family through in silico molecular dynamics simulations of AQP2 interactions with cortisol. We propose, for the first time, a putative AQPs corticosteroid binding site (ACBS) and discussed its conservation through structural alignment. Corticosteroids can mediate non-genomic effects; nonetheless, the transduction pathways involved are still misunderstood. Moreover, a growing body of evidence is pointing toward the existence of a novel membrane receptor mediating part of these rapid corticosteroids' effects. Our results suggest that the naturally produced glucocorticoid cortisol inhibits channel water permeability. Based on these results, we propose a detailed description of a putative underlying molecular mechanism. In this process, we also bring new insights on the regulatory function of AQPs extra-cellular loops and on the role of ions in tuning the water permeability. Altogether, this work brings new insights into the non-genomic effects of corticosteroids through the proposition of AQPs as the membrane receptor of this family of regulatory molecules. This original result is the starting point for future investigations to define more in-depth and in vivo the validity of this functional model.

Protein-protein interactions in plant antioxidant defense

The regulation of reactive oxygen species (ROS) levels in plants is ensured by mechanisms preventing their over accumulation, and by diverse antioxidants, including enzymes and nonenzymatic compounds. These are affected by redox conditions, posttranslational modifications, transcriptional and posttranscriptional modifications, Ca²⁺, nitric oxide (NO) and mitogen-activated protein kinase signaling pathways. Recent knowledge about protein-protein interactions (PPIs) of antioxidant enzymes advanced during last decade. The best-known examples are interactions mediated by redox buffering proteins such as thioredoxins and glutaredoxins. This review summarizes interactions of major antioxidant enzymes with regulatory and signaling proteins and their diverse functions. Such interactions are important for stability, degradation and activation of interacting partners. Moreover, PPIs of antioxidant enzymes may connect diverse metabolic processes with ROS scavenging. Proteins like receptor for activated C kinase 1 may ensure coordination of antioxidant enzymes to ensure efficient ROS regulation. Nevertheless, PPIs in antioxidant defense are understudied, and intensive research is required to define their role in complex regulation of ROS scavenging.

Evaluation of the Relationship Between Aquaporin-1, Hepcidin, Zinc, Copper, and İron Levels and Oxidative Stress in the Serum of Critically Ill Patients with COVID-19

Our study aims to determine the relationship between hepcidin, aquaporin (AQP-1), copper (Cu), zinc (Zn), iron (Fe) levels, and oxidative stress in the sera of seriously ill COVID-19 patients with invasive mechanical ventilation. Ninety persons with and without COVID-19 were taken up and separated into two groups. The first group included seriously COVID-19 inpatients having endotracheal intubation in the intensive care unit (n = 45). The second group included individuals who had negative PCR tests and had no chronic disease (the healthy control group n = 45). AQP-1, hepcidin, Zn, Cu, Fe, total antioxidant status (TAS), and total oxidant status (TOS) were studied in the sera of both groups, and the relations of these levels with oxidative stress were determined. When the COVID-19 patient and the control groups were compared, all studied parameters were found to be statistically significant (p < 0.01). Total oxidant status (TOS), oxidative stress index (OSI), and AQP-1, hepcidin, and Cu levels were increased in patients with COVID-19 compared to healthy people. Serum TAC, Zn, and Fe levels were found to be lower in the patient group than in the control group. Significant correlations were detected between the studied parameters in COVID-19 patients. Results indicated that oxidative stress may play an important role in viral infection due to SARS-CoV-2. We think that oxidative stress parameters as well as some trace elements at the onset of COVID-19 disease will provide a better triage in terms of disease severity.

Proteomic Analysis of Roots Response to Potassium Deficiency and the Effect of TaHAK1-4A on K+ Uptake in Wheat

Potassium (K+) is essential for plant growth and stress responses. A deficiency in soil K+ contents can result in decreased wheat quality and productivity. Thus, clarifying the molecular mechanism underlying wheat responses to low-K+ (LK) stress is critical. In this study, a tandem mass tag (TMT)-based quantitative proteomic analysis was performed to investigate the differentially abundant proteins (DAPs) in roots of the LK-tolerant wheat cultivar “KN9204” at the seedling stage after exposure to LK stress. A total of 104 DAPs were identified in the LK-treated roots. The DAPs related to carbohydrate and energy metabolism, transport, stress responses and defense, and post-translational modifications under LK conditions were highlighted. We identified a high-affinity potassium transporter (TaHAK1-4A) that was significantly up-regulated after the LK treatment. Additionally, TaHAK1-4A was mainly expressed in roots, and the encoded protein was localized in the plasma membrane. The complementation assay in yeast suggested that TaHAK1-4A mediates K+ uptake under extreme LK conditions. The overexpression of TaHAK1-4A increased the fresh weight and root length of Arabidopsis under LK conditions and improved the growth of Arabidopsis athak5 mutant seedlings, which grow poorly under LK conditions. Moreover, silencing of TaHAK1-4A in wheat roots treated with LK stress decreased the root length, dry weight, K+ concentration, and K+ influx. Accordingly, TaHAK1-4A is important for the uptake of K+ by roots exposed to LK stress. Our results reveal the protein metabolic changes in wheat induced by LK stress. Furthermore, we identified a candidate gene potentially relevant for developing wheat lines with increased K+ use efficiency.

FocA and its central role in fine-tuning pH homeostasis of enterobacterial formate metabolism

During enterobacterial mixed-acid fermentation, formate is generated from pyruvate by the glycyl-radical enzyme pyruvate formate-lyase (PflB). In Escherichia coli, especially at low pH, formate is then disproportionated to CO₂ and H₂ by the cytoplasmically oriented, membrane-associated formate hydrogenlyase (FHL) complex. If electron acceptors are available, however, formate is oxidized by periplasmically oriented, respiratory formate dehydrogenases. Formate translocation across the cytoplasmic membrane is controlled by the formate channel, FocA, a member of the formate-nitrite transporter (FNT) family of homopentameric anion channels. This review highlights recent advances in our understanding of how FocA helps to maintain intracellular formate and pH homeostasis during fermentation. Efflux and influx of formate/formic acid are distinct processes performed by FocA and both are controlled through protein interaction between FocA's N-terminal domain with PflB. Formic acid efflux by FocA helps to maintain cytoplasmic pH balance during exponential-phase growth. Uptake of formate against the electrochemical gradient (inside negative) is energetically and mechanistically challenging for a fermenting bacterium unless coupled with proton/cation symport. Translocation of formate/formic acid into the cytoplasm necessitates an active FHL complex, whose synthesis also depends on formate. Thus, FocA, FHL and PflB function together to govern formate homeostasis. We explain how FocA achieves efflux of formic acid and propose mechanisms for pH-dependent uptake of formate both with and without proton symport. We propose that FocA displays both channel- and transporter-like behaviour. Whether this translocation behaviour is shared by other members of the FNT family is also discussed.

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.

Insight into the Mammalian Aquaporin Interactome

Aquaporins (AQPs) are a family of transmembrane water channels expressed in all living organisms. AQPs facilitate osmotically driven water flux across biological membranes and, in some cases, the movement of small molecules (such as glycerol, urea, CO₂, NH₃, H₂O₂). Protein-protein interactions play essential roles in protein regulation and function. This review provides a comprehensive overview of the current knowledge of the AQP interactomes and addresses the molecular basis and functional significance of these protein-protein interactions in health and diseases. Targeting AQP interactomes may offer new therapeutic avenues as targeting individual AQPs remains challenging despite intense efforts.

Uromodulin Regulates Murine Aquaporin-2 Activity via Thick Ascending Limb-Collecting Duct Cross-Talk during Water Deprivation

Uromodulin, a urinary protein synthesized and secreted from the thick ascending limb (TAL) of the loop of Henle, is associated with hypertension through the activation of sodium reabsorption in the TAL. Uromodulin is a potential target for hypertension treatment via natriuresis. However, its biological function in epithelial cells of the distal nephron segment, particularly the collecting duct, remains unknown. Herein, we examined the regulation of uromodulin production during water deprivation in vivo as well as the effect of uromodulin on the activity of the water channel aquaporin−2 (AQP2) in vitro and in vivo using transgenic mice. Water deprivation upregulated uromodulin production; immunofluorescence experiments revealed uromodulin adhesion on the apical surface of the collecting duct. Furthermore, the activation of AQP2 was attenuated in mice lacking uromodulin. Uromodulin enhanced the phosphorylation and apical trafficking of AQP2 in mouse collecting duct cells treated with the vasopressin analog dDAVP. The uromodulin-induced apical trafficking of AQP2 was attenuated via endocytosis inhibitor treatment, suggesting that uromodulin activates AQP2 through the suppression of endocytosis. This study provides novel insights into the cross−talk between TAL and the collecting duct, and indicates that the modulation of uromodulin is a promising approach for diuresis and hypertension treatment.

The Hidden Intricacies of Aquaporins: Remarkable Details in a Common Structural Scaffold

Evolution turned aquaporins (AQPs) into the most efficient facilitators of passive water flow through cell membranes at no expense of solute discrimination. In spite of a plethora of solved AQP structures, many structural details remain hidden. Here, by combining extensive sequence- and structural-based analysis of a unique set of 20 non-redundant high-resolution structures and molecular dynamics simulations of four representatives, key aspects of AQP stability, gating, selectivity, pore geometry, and oligomerization, with a potential impact on channel functionality, are identified. The general view of AQPs possessing a continuous open water pore is challenged and it is depicted that AQPs' selectivity is not exclusively shaped by pore-lining residues but also by the relative arrangement of transmembrane helices. Moreover, this analysis reveals that hydrophobic interactions constitute the main determinant of protein thermal stability. Finally, a numbering scheme of the conserved AQP scaffold is established, facilitating direct comparison of, for example, disease-causing mutations and prediction of potential structural consequences. Additionally, the results pave the way for the design of optimized AQP water channels to be utilized in biotechnological applications.

OsHIPL1, a hedgehog-interacting protein-like 1 protein, increases seed vigour in rice

The cultivation of rice varieties with high seed vigour is vital for the direct seeding of rice, and the molecular basis of regulation of seed vigour remains elusive. Here, we cloned a new gene OsHIPL1, which encodes hedgehog-interacting protein-like 1 protein as a causal gene of the major QTL qSV3 for rice seed vigour. OsHIPL1 was mainly localized in the plasma membrane and nucleus. RNA sequencing (RNA-seq) revealed that the ABA-related genes were involved in the OsHIPL1 regulation of seed vigour in rice. The higher levels of endogenous ABA were measured in germinating seeds of OsHIPL1 mutants and NIL-qsv3 line compared to IR26 plants, with two up-regulated ABA biosynthesis genes (OsZEP and OsNCED4) and one down-regulated ABA catabolism gene OsABA8ox3. The expression of abscisic acid-insensitive 3 (OsABI3), OsABI4 and OsABI5 was significantly up-regulated in germinating seeds of OsHIPL1 mutants and NIL-qsv3 line compared to IR26 plants. These results indicate that the regulation of seed vigour of OsHIPL1 may be through modulating endogenous ABA levels and altering OsABIs expression during seed germination in rice. Meanwhile, we found that OsHIPL1 interacted with the aquaporin OsPIP1;1, then affected water uptake to promote rice seed germination. Based on analysis of single-nucleotide polymorphism data of rice accessions, we identified a Hap1 haplotype of OsHIPL1 that was positively correlated with seed germination. Our findings showed novel insights into the molecular mechanism of OsHIPL1 on seed vigour.

Out of the blue: Phototropins of the leaf vascular bundle sheath mediate the regulation of leaf hydraulic conductance by blue light

The Arabidopsis (Arabidopsis thaliana) leaf veins bundle-sheath cells (BSCs)-a selective barrier to water and solutes entering the mesophyll-increase the leaf radial hydraulic conductance (Kleaf) by acidifying the xylem sap by their plasma membrane H+-ATPase,  AHA2. Based on this and on the BSCs' expression of phototropins PHOT1 and PHOT2, and the known blue light (BL)-induced Kleaf increase, we hypothesized that, resembling the guard cells, BL perception by the BSCs' phots activates its H+-ATPase, which, consequently, upregulates Kleaf. Indeed, under BL, the Kleaf of the knockout mutant lines phot1-5, phot2-1, phot1-5 phot2-1, and aha2-4 was lower than that of the wild-type (WT). BSC-only-directed complementation of phot1-5 or aha2-4 by PHOT1 or AHA2, respectively, restored the BL-induced Kleaf increase. BSC-specific silencing of PHOT1 or PHOT2 prevented such Kleaf increase. A xylem-fed kinase inhibitor (tyrphostin 9) replicated this also in WT plants. White light-ineffective in the phot1-5 mutant-acidified the xylem sap (relative to darkness) in WT and in the PHOT1-complemented phot1-5. These results, supported by BL increase of BSC protoplasts' water permeability and cytosolic pH and their hyperpolarization by BL, identify the BSCs as a second phot-controlled water conductance element in leaves, in series with stomatal conductance. Through both, BL regulates the leaf water balance.

Beyond the Channels: Adhesion Functions of Aquaporin 0 and Connexin 50 in Lens Development

Lens, an avascular tissue involved in light transmission, generates an internal microcirculatory system to promote ion and fluid circulation, thus providing nutrients to internal lens cells and excreting the waste. This unique system makes up for the lack of vasculature and distinctively maintains lens homeostasis and lens fiber cell survival through channels of connexins and other transporters. Aquaporins (AQP) and connexins (Cx) comprise the majority of channels in the lens microcirculation system and are, thus, essential for lens development and transparency. Mutations of AQPs and Cxs result in abnormal channel function and cataract formation. Interestingly, in the last decade or so, increasing evidence has emerged suggesting that in addition to their well-established channel functions, AQP0 and Cx50 play pivotal roles through channel-independent actions in lens development and transparency. Specifically, AQP0 and Cx50 have been shown to have a unique cell adhesion function that mediates lens development and transparency. Precise regulation of cell-matrix and cell-cell adhesion is necessary for cell migration, a critical process during lens development. This review will provide recent advances in basic research of cell adhesion mediated by AQP0 and Cx50.

Signaling Mechanisms and Pharmacological Modulators Governing Diverse Aquaporin Functions in Human Health and Disease

The aquaporins (AQPs) are a family of small integral membrane proteins that facilitate the bidirectional transport of water across biological membranes in response to osmotic pressure gradients as well as enable the transmembrane diffusion of small neutral solutes (such as urea, glycerol, and hydrogen peroxide) and ions. AQPs are expressed throughout the human body. Here, we review their key roles in fluid homeostasis, glandular secretions, signal transduction and sensation, barrier function, immunity and inflammation, cell migration, and angiogenesis. Evidence from a wide variety of studies now supports a view of the functions of AQPs being much more complex than simply mediating the passive flow of water across biological membranes. The discovery and development of small-molecule AQP inhibitors for research use and therapeutic development will lead to new insights into the basic biology of and novel treatments for the wide range of AQP-associated disorders.

Teamwork to Survive in Hostile Soils: Use of Plant Growth-Promoting Bacteria to Ameliorate Soil Salinity Stress in Crops

Plants and their microbiomes, including plant growth-promoting bacteria (PGPB), can work as a team to reduce the adverse effects of different types of stress, including drought, heat, cold, and heavy metals stresses, as well as salinity in soils. These abiotic stresses are reviewed here, with an emphasis on salinity and its negative consequences on crops, due to their wide presence in cultivable soils around the world. Likewise, the factors that stimulate the salinity of soils and their impact on microbial diversity and plant physiology were also analyzed. In addition, the saline soils that exist in Mexico were analyzed as a case study. We also made some proposals for a more extensive use of bacterial bioinoculants in agriculture, particularly in developing countries. Finally, PGPB are highly relevant and extremely helpful in counteracting the toxic effects of soil salinity and improving crop growth and production; therefore, their use should be intensively promoted.

Aquaporin 2 regulation: implications for water balance and polycystic kidney diseases

Targeting the collecting duct water channel aquaporin 2 (AQP2) to the plasma membrane is essential for the maintenance of mammalian water homeostasis. The vasopressin V2 receptor (V2R), which is a G_S protein-coupled receptor that increases intracellular cAMP levels, has a major role in this targeting process. Although a rise in cAMP levels and activation of protein kinase A are involved in facilitating the actions of V2R, studies in knockout mice and cell models have suggested that cAMP signalling pathways are not an absolute requirement for V2R-mediated AQP2 trafficking to the plasma membrane. In addition, although AQP2 phosphorylation is a known prerequisite for V2R-mediated plasma membrane targeting, none of the known AQP2 phosphorylation events appears to be rate-limiting in this process, which suggests the involvement of other factors; cytoskeletal remodelling has also been implicated. Notably, several regulatory processes and signalling pathways involved in AQP2 trafficking also have a role in the pathophysiology of autosomal dominant polycystic kidney disease, although the role of AQP2 in cyst progression is unknown. Here, we highlight advances in the field of AQP2 regulation that might be exploited for the treatment of water balance disorders and provide a rationale for targeting these pathways in autosomal dominant polycystic kidney disease.

Ezrin Is a Novel Protein Partner of Aquaporin-5 in Human Salivary Glands and Shows Altered Expression and Cellular Localization in Sjögren's Syndrome

Sjögren’s syndrome (SS) is an exocrinopathy characterized by the hypofunction of salivary glands (SGs). Aquaporin-5 (AQP5); a water channel involved in saliva formation; is aberrantly distributed in SS SG acini and contributes to glandular dysfunction. We aimed to investigate the role of ezrin in AQP5 mislocalization in SS SGs. The AQP5–ezrin interaction was assessed by immunoprecipitation and proteome analysis and by proximity ligation assay in immortalized human SG cells. We demonstrated, for the first time, an interaction between ezrin and AQP5. A model of the complex was derived by computer modeling and in silico docking; suggesting that AQP5 interacts with the ezrin FERM-domain via its C-terminus. The interaction was also investigated in human minor salivary gland (hMSG) acini from SS patients (SICCA-SS); showing that AQP5–ezrin complexes were absent or mislocalized to the basolateral side of SG acini rather than the apical region compared to controls (SICCA-NS). Furthermore, in SICCA-SS hMSG acinar cells, ezrin immunoreactivity was decreased at the acinar apical region and higher at basal or lateral regions, accounting for altered AQP5–ezrin co-localization. Our data reveal that AQP5–ezrin interactions in human SGs could be involved in the regulation of AQP5 trafficking and may contribute to AQP5-altered localization in SS patients

Genome-wide identification and expression analysis of aquaporin family in Canavalia rosea and their roles in the adaptation to saline-alkaline soils and drought stress

BACKGROUND

Canavalia rosea (Sw.) DC. (bay bean) is an extremophile halophyte that is widely distributed in coastal areas of the tropics and subtropics. Seawater and drought tolerance in this species may be facilitated by aquaporins (AQPs), channel proteins that transport water and small molecules across cell membranes and thereby maintain cellular water homeostasis in the face of abiotic stress. In C. rosea, AQP diversity, protein features, and their biological functions are still largely unknown.

RESULTS

We describe the action of AQPs in C. rosea using evolutionary analyses coupled with promoter and expression analyses. A total of 37 AQPs were identified in the C. rosea genome and classified into five subgroups: 11 plasma membrane intrinsic proteins, 10 tonoplast intrinsic proteins, 11 Nod26-like intrinsic proteins, 4 small and basic intrinsic proteins, and 1 X-intrinsic protein. Analysis of RNA-Seq data and targeted qPCR revealed organ-specific expression of aquaporin genes and the involvement of some AQP members in adaptation of C. rosea to extreme coral reef environments. We also analyzed C. rosea sequences for phylogeny reconstruction, protein modeling, cellular localizations, and promoter analysis. Furthermore, one of PIP1 gene, CrPIP1;5, was identified as functional using a yeast expression system and transgenic overexpression in Arabidopsis.

CONCLUSIONS

Our results indicate that AQPs play an important role in C. rosea responses to saline-alkaline soils and drought stress. These findings not only increase our understanding of the role AQPs play in mediating C. rosea adaptation to extreme environments, but also improve our knowledge of plant aquaporin evolution more generally.

High Levels of Tumor Necrosis Factor-Alpha Reduce Placental Aquaporin 3 Expression and Impair in vitro Trophoblastic Cell Migration

Placentas from preeclamptic women display augmented tumor necrosis factor-alpha (TNF-α) levels with reduced expression of aquaporin 3 (AQP3). However, whether TNF-α modulates AQP3 expression remains to be elucidated. We hypothesize that elevated levels of TNF-α reduce AQP3 expression and negatively impact trophoblastic cell migration. Spontaneously hypertensive rats (SHRs) and Wistar rats (14-16 weeks) were divided into hypertensive and normotensive groups, respectively. Systolic blood pressure (SBP) was measured, and animals mated. In a third group, pregnant SHRs were treated with a TNF-α antagonist, etanercept (0.8 mg/kg, subcutaneously) on days 0, 6, 12, and 18 of pregnancy. Placentas were collected on the 20th day of pregnancy. Human placental explants, from normotensive pregnancies, were incubated with TNF-α (5, 10, and 20 ng/ml) and/or etanercept (1 μg/ml). Swan 71 cells were incubated with TNF-α (10 ng/ml) and/or etanercept (1 μg/ml) and subjected to the wound healing assay. AQP3 expression was assessed by Western blot and TNF-α levels by ELISA. SBP (mmHg) was elevated in the hypertensive group, and etanercept treatment reduced this parameter. Placental TNF-α levels (pg/ml) were higher in the hypertensive group. AQP3 expression was reduced in the hypertensive group, and etanercept treatment reversed this parameter. Explants submitted to TNF-α exposition displayed reduced expression of AQP3, and etanercept incubation reversed it. Trophoblastic cells incubated with TNF-α showed decreased cell migration and reduced AQP3 expression, and etanercept incubation ameliorated it. Altogether, these data demonstrate that high TNF-α levels negatively modulate AQP3 in placental tissue, impairing cell migration, and its relationship in a pregnancy affected by hypertension.

Aquaporins - Expression, purification and characterization

Aquaporin water channels facilitate the bi-directional flow of water and small, neutral solutes down an osmotic gradient in all kingdoms of life. Over the last two decades, the availability of high-quality protein has underpinned progress in the structural and functional characterization of these water channels. In particular, recombinant protein technology has guaranteed the supply of aquaporin samples that were of sufficient quality and quantity for further study. Here we review the features of successful expression, purification and characterization strategies that have underpinned these successes and that will drive further breakthroughs in the field. Overall, Escherichia coli is a suitable host for prokaryotic isoforms, while Pichia pastoris is the most commonly-used recombinant host for eukaryotic variants. Generally, a two-step purification procedure is suitable after solubilization in glucopyranosides and most structures are determined by X-ray following crystallization.

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.

Degradation of connexin 50 protein causes waterclefts in human lens

Cataracts are mainly classified into three types: cortical cataracts, nuclear cataracts, and posterior subcapsular cataracts. In addition, retrodots and waterclefts are cataract subtypes that cause decreased visual function. To maintain an orderly and tightly packed arrangement to minimize light scattering, adhesion molecules such as connexins and aquaporin 0 (AQP0) are highly expressed in the lens. We hypothesized that some main and/or subcataract type(s) are correlated with adhesion molecule degradation. Lens samples were collected from cataract patients during cataract surgery, and mRNA and protein expression levels were measured by real-time RT-PCR and western blotting, respectively. The mRNA levels of adhesion molecules were not significantly different among any cataract types. Moreover, AQP0 and connexin 46 protein expressions were unchanged among patients. However, connexin 50 protein level was significantly decreased in the lens of patients with WC cataract subtype. P62 and LC3B proteins were detected in the WC patients' lenses, but not in other patients' lenses. These results suggest that more research is needed on the subtypes of cataracts besides the three major types of cataract for tailor-made cataract therapy.

Plasma membrane aquaporins interact with the endoplasmic reticulum resident VAP27 proteins at ER-PM contact sites and endocytic structures

Plasma membrane (PM) intrinsic proteins (PIPs) are aquaporins facilitating the diffusion of water and small solutes. The functional importance of the PM organisation of PIPs in the interaction with other cellular structures is not completely understood. We performed a pull-down assay using maize (Zea mays) suspension cells expressing YFP-ZmPIP2;5 and validated the protein interactions by yeast split-ubiquitin and bimolecular fluorescence complementation assays. We expressed interacting proteins tagged with fluorescent proteins in Nicotiana benthamiana leaves and performed water transport assays in oocytes. Finally, a phylogenetic analysis was conducted. The PM-located ZmPIP2;5 physically interacts with the endoplasmic reticulum (ER) resident ZmVAP27-1. This interaction requires the ZmVAP27-1 cytoplasmic major sperm domain. ZmPIP2;5 and ZmVAP27-1 localise in close vicinity in ER-PM contact sites (EPCSs) and endocytic structures upon exposure to salt stress conditions. This interaction enhances PM water permeability in oocytes. Similarly, the Arabidopsis ZmVAP27-1 paralogue, AtVAP27-1, interacts with the AtPIP2;7 aquaporin. Together, these data indicate that the PIP2-VAP27 interaction in EPCSs is evolutionarily conserved, and suggest that VAP27 might stabilise the aquaporins and guide their endocytosis in response to salt stress.

Voltage-gating of aquaporins, a putative conserved safety mechanism during ionic stresses

Aquaporins are transmembrane water channels found in almost every living organism. Numerous studies have brought a good understanding of both water transport through their pores and the regulations taking place at the molecular level, but subtleties remain to be clarified. Recently, a voltage-related gating mechanism involving the conserved arginine of the channel's main constriction was captured for human aquaporins through molecular dynamics studies. With a similar approach, we show that this voltage-gating could be conserved among this family and that the underlying mechanism could explain part of plant AQPs diversity when contextualized to high ionic concentrations provoked by drought. Finally, we identified residues as adaptive traits which constitute good targets for drought resistance plant breeding research.

Atrogin1-induced loss of aquaporin 4 in myocytes leads to skeletal muscle atrophy

The water channel aquaporin 4 (AQP4) regulates the flux of water across the cell membrane, maintaining cellular homeostasis. Since AQP4 is enriched in the sarcolemma of skeletal muscle, a functional defect in AQP4 may cause skeletal muscle dysfunction. To investigate a novel mechanism underlying skeletal muscle atrophy, we examined AQP4 expression and its regulation in muscle using the rotator cuff tear (RCT) model. Human and mouse AQP4 expression was significantly decreased in atrophied muscle resulting from RCT. The size and the number of myotubes were reduced following AQP4 knockdown. Atrogin 1-mediated ubiquitination of AQP4 was verified with an ubiquitination assay after immunoprecipitation of AQP4 with an anti-AQP4 antibody. In this study, we identified high mobility group box 1 (HMGB1) as a potent upstream regulator of atrogin 1 expression. Atrogin 1 expression was increased by recombinant mouse HMGB1 protein, and the HMGB1-induced atrogin 1 expression was mediated via NF-κB signaling. Our study suggests that loss of AQP4 appears to be involved in myocyte shrinkage after RCT, and its degradation is mediated by atrogin 1-dependent ubiquitination. HMGB1, in its function as a signaling molecule upstream of the ubiquitin ligase atrogin 1, was found to be a novel regulator of muscle atrophy.

Aqp9 Gene Deletion Enhances Retinal Ganglion Cell (RGC) Death and Dysfunction Induced by Optic Nerve Crush: Evidence that Aquaporin 9 Acts as an Astrocyte-to-Neuron Lactate Shuttle in Concert with Monocarboxylate Transporters To Support RGC Function and Survival

Aquaporin 9 (AQP9) is an aquaglyceroporin that can transport lactate. Accumulating evidence suggests that astrocyte-to-neuron lactate shuttle (ANLS) plays a critical role in energy metabolism in neurons, including retinal ganglion cells (RGCs). To test the hypothesis that AQP9, in concert with monocarboxylate transporters (MCTs), participates in ANLS to maintain function and survival of RGCs, Aqp9-null mice and wild-type (WT) littermates were subjected to optic nerve crush (ONC) with or without intravitreal injection of an MCT2 inhibitor. RGC density was similar between the Aqp9-null mice and WT mice without ONC, while ONC resulted in significantly more RGC density reduction in the Aqp9-null mice than in the WT mice at day 7. Positive scotopic threshold response (pSTR) amplitude values were similar between the two groups without ONC, but were significantly more reduced in the Aqp9-null mice than in the WT mice 7days after ONC. MCT2 inhibitor injection accelerated RGC death and pSTR amplitude reduction only in the WT mice with ONC. Immunolabeling revealed that both RGCs and astrocytes expressed AQP9, that ONC predominantly reduced astrocytic AQP9 expression, and that MCTs 1, 2, and 4 were co-localized with AQP9 at the ganglion cell layer. These retinal MCTs were also co-immunoprecipitated with AQP9 in the WT mice. ONC decreased the co-immunoprecipitation of MCTs 1 and 4, but did not impact co-immunoprecipitation of MCT2. Retinal glucose transporter 1 expression was increased in Aqp9-null mice. Aqp9 gene deletion reduced and increased the intraretinal L-lactate and D-glucose concentrations, respectively. Results suggest that AQP9 acts as the ANLS to maintain function and survival of RGCs.

Current State of Knowledge on Primary Sjögren's Syndrome, an Autoimmune Exocrinopathy

Primary Sjögren's syndrome (pSS) is a chronic systemic autoimmune rheumatic disease characterized by lymphoplasmacytic infiltration of the salivary and lacrimal glands, whereby sicca syndrome and/or systemic manifestations are the clinical hallmarks, associated with a particular autoantibody profile. pSS is the most frequent connective tissue disease after rheumatoid arthritis, affecting 0.3-3% of the population. Women are more prone to develop pSS than men, with a sex ratio of 9:1. Considered in the past as innocent collateral passive victims of autoimmunity, the epithelial cells of the salivary glands are now known to play an active role in the pathogenesis of the disease. The aetiology of the "autoimmune epithelitis" still remains unknown, but certainly involves genetic, environmental and hormonal factors. Later during the disease evolution, the subsequent chronic activation of B cells can lead to the development of systemic manifestations or non-Hodgkin's lymphoma. The aim of the present comprehensive review is to provide the current state of knowledge on pSS. The review addresses the clinical manifestations and complications of the disease, the diagnostic workup, the pathogenic mechanisms and the therapeutic approaches.

Advances in aquaporin-2 trafficking mechanisms and their implications for treatment of water balance disorders

In mammals, conservation of body water is critical for survival and is dependent on the kidneys' ability to minimize water loss in the urine during periods of water deprivation. The collecting duct water channel aquaporin-2 (AQP2) plays an essential role in this homeostatic response by facilitating water reabsorption along osmotic gradients. The ability to increase the levels of AQP2 in the apical plasma membrane following an increase in plasma osmolality is a rate-limiting step in water reabsorption, a process that is tightly regulated by the antidiuretic hormone arginine vasopressin (AVP). In this review, the focus is on the role of the carboxyl-terminus of AQP2 as a key regulatory point for AQP2 trafficking. We provide an overview of AQP2 structure, disease-causing mutations in the AQP2 carboxyl-terminus, the role of posttranslational modifications such as phosphorylation and ubiquitylation in the tail domain, and their implications for balanced trafficking of AQP2. Finally, we discuss how various modifications of the AQP2 tail facilitate selective protein-protein interactions that modulate the AQP2 trafficking mechanism.

Mapping of CaM, S100A1 and PIP2-Binding Epitopes in the Intracellular N- and C-Termini of TRPM4

Molecular determinants of the binding of various endogenous modulators to transient receptor potential (TRP) channels are crucial for the understanding of necessary cellular pathways, as well as new paths for rational drug designs. The aim of this study was to characterise interactions between the TRP cation channel subfamily melastatin member 4 (TRPM4) and endogenous intracellular modulators-calcium-binding proteins (calmodulin (CaM) and S100A1) and phosphatidylinositol 4, 5-bisphosphate (PIP2). We have found binding epitopes at the N- and C-termini of TRPM4 shared by CaM, S100A1 and PIP2. The binding affinities of short peptides representing the binding epitopes of N- and C-termini were measured by means of fluorescence anisotropy (FA). The importance of representative basic amino acids and their combinations from both peptides for the binding of endogenous TRPM4 modulators was proved using point alanine-scanning mutagenesis. In silico protein-protein docking of both peptides to CaM and S100A1 and extensive molecular dynamics (MD) simulations enabled the description of key stabilising interactions at the atomic level. Recently solved cryo-Electron Microscopy (EM) structures made it possible to put our findings into the context of the entire TRPM4 channel and to deduce how the binding of these endogenous modulators could allosterically affect the gating of TRPM4. Moreover, both identified binding epitopes seem to be ideally positioned to mediate the involvement of TRPM4 in higher-order hetero-multimeric complexes with important physiological functions.

Genome-wide identification and characterisation of Aquaporins in Nicotiana tabacum and their relationships with other Solanaceae species

BACKGROUND

Cellular membranes are dynamic structures, continuously adjusting their composition, allowing plants to respond to developmental signals, stresses, and changing environments. To facilitate transmembrane transport of substrates, plant membranes are embedded with both active and passive transporters. Aquaporins (AQPs) constitute a major family of membrane spanning channel proteins that selectively facilitate the passive bidirectional passage of substrates across biological membranes at an astonishing 108 molecules per second. AQPs are the most diversified in the plant kingdom, comprising of five major subfamilies that differ in temporal and spatial gene expression, subcellular protein localisation, substrate specificity, and post-translational regulatory mechanisms; collectively providing a dynamic transportation network spanning the entire plant. Plant AQPs can transport a range of solutes essential for numerous plant processes including, water relations, growth and development, stress responses, root nutrient uptake, and photosynthesis. The ability to manipulate AQPs towards improving plant productivity, is reliant on expanding our insight into the diversity and functional roles of AQPs.

RESULTS

We characterised the AQP family from Nicotiana tabacum (NtAQPs; tobacco), a popular model system capable of scaling from the laboratory to the field. Tobacco is closely related to major economic crops (e.g. tomato, potato, eggplant and peppers) and itself has new commercial applications. Tobacco harbours 76 AQPs making it the second largest characterised AQP family. These fall into five distinct subfamilies, for which we characterised phylogenetic relationships, gene structures, protein sequences, selectivity filter compositions, sub-cellular localisation, and tissue-specific expression. We also identified the AQPs from tobacco's parental genomes (N. sylvestris and N. tomentosiformis), allowing us to characterise the evolutionary history of the NtAQP family. Assigning orthology to tomato and potato AQPs allowed for cross-species comparisons of conservation in protein structures, gene expression, and potential physiological roles.

CONCLUSIONS

This study provides a comprehensive characterisation of the tobacco AQP family, and strengthens the current knowledge of AQP biology. The refined gene/protein models, tissue-specific expression analysis, and cross-species comparisons, provide valuable insight into the evolutionary history and likely physiological roles of NtAQPs and their Solanaceae orthologs. Collectively, these results will support future functional studies and help transfer basic research to applied agriculture.

Prokaryotic Aquaporins

Aquaporins are integral membrane proteins that facilitate the diffusion of water and other small, uncharged solutes across the cellular membrane and are widely distributed in organisms from humans to bacteria. However, the characteristics of prokaryotic aquaporins remain largely unknown. We investigated the distribution and sequence characterization of aquaporins in prokaryotic organisms and summarized the transport characteristics, physiological functions, and regulatory mechanisms of prokaryotic aquaporins. Aquaporin homologues were identified in 3315 prokaryotic genomes retrieved from the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, but the protein clustering pattern is not completely congruent with the phylogeny of the species that carry them. Moreover, prokaryotic aquaporins display diversified aromatic/arginine constriction region (ar/R) amino acid compositions, implying multiple functions. The typical water and glycerol transport characterization, physiological functions, and regulations have been extensively studied in Escherichia coli AqpZ and GlpF. A Streptococcus aquaporin has recently been verified to facilitate the efflux of endogenous H₂O₂, which not only contributes to detoxification but also to species competitiveness, improving our understanding of prokaryotic aquaporins. Furthermore, recent studies revealed novel regulatory mechanisms of prokaryotic aquaporins at post-translational level. Thus, we propose that intensive investigation on prokaryotic aquaporins would extend the functional categories and working mechanisms of these ubiquitous, intrinsic membrane proteins.

Protein-protein interactions of drug uptake transporters that are important for liver and kidney

Drug uptake transporters are membrane proteins responsible for the trans-membrane transport of endo- and xenobiotics, including numerous drugs. They are important for the uptake of drugs into target tissues or into organs for metabolism and excretion. Many drug uptake transporters have a broad spectrum of structural-independent substrates, which make them vulnerable to drug-drug interactions. Recent studies have shown more and more complex pharmacokinetics involving transporters, and regulatory agencies now require studies to be performed to measure the involvement of transporters in drug development. A better understanding of the factors affecting the expression of transporters is needed. Despite many efforts devoted to the functional characterization of different drug uptake transporters, transporter in vitro to in vivo extrapolations are far from predicting the behavior under physiological conditions. There is an increasing number of uptake transporters demonstrated to form protein-protein interactions or to oligomerize. This raises the possibility that these interactions between or among transporters could help explaining the gap between in vitro and in vivo measurement of drug transporters. In this review, we summarized protein-protein interactions of drug uptake transporters that are important for pharmacokinetics, especially those in the liver and the kidneys.

Rice aquaporin PIP1;3 and harpin Hpa1 of bacterial blight pathogen cooperate in a type III effector translocation

Varieties of Gram-negative bacterial pathogens infect their eukaryotic hosts by deploying the type III translocon to deliver effector proteins into the cytosol of eukaryotic cells in which effectors execute their pathological functions. The translocon is hypothetically assembled by bacterial translocators in association with the assumed receptors situated on eukaryotic plasma membranes. This hypothesis is partially verified in the present study with genetic, biochemical, and pathological evidence for the role of a rice aquaporin, plasma membrane intrinsic protein PIP1;3, in the cytosolic import of the transcription activator-like effector PthXo1 from the bacterial blight pathogen. PIP1;3 interacts with the bacterial translocator Hpa1 at rice plasma membranes to control PthXo1 translocation from cells of a well-characterized strain of the bacterial blight pathogen into the cytosol of cells of a susceptible rice variety. An extracellular loop sequence of PIP1;3 and the α-helix motif of Hpa1 determine both the molecular interaction and its consequences with respect to the effector translocation and the bacterial virulence on the susceptible rice variety. Overall, these results provide multiple experimental avenues to support the hypothesis that interactions between bacterial translocators and their interactors at the target membrane are essential for bacterial effector translocation.

Plant Aquaporins in Infection by and Immunity Against Pathogens - A Critical Review

Plant aquaporins (AQPs) of the plasma membrane intrinsic protein (PIP) family face constant risk of hijack by pathogens aiming to infect plants. PIPs can also be involved in plant immunity against infection. This review will utilize two case studies to discuss biochemical and structural mechanisms that govern the functions of PIPs in the regulation of plant infection and immunity. The first example concerns the interaction between rice Oryza sativa and the bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo). To infect rice, Xoo uses the type III (T3) secretion system to secrete the proteic translocator Hpa1, and Hpa1 subsequently mediates the translocation of T3 effectors secreted by this system. Once shifted from bacteria into rice cells, effectors exert virulent or avirulent effects depending on the susceptibility of the rice varieties. The translocator function of Hpa1 requires cooperation with OsPIP1;3, the rice interactor of Hpa1. This role of OsPIP1;3 is related to regulatory models of effector translocation. The regulatory models have been proposed as, translocon-dependent delivery, translocon-independent pore formation, and effector endocytosis with membrane protein/lipid trafficking. The second case study includes the interaction of Hpa1 with the H2O2 transport channel AtPIP1;4, and the associated consequence for H2O2 signal transduction of immunity pathways in Arabidopsis thaliana, a non-host of Xoo. H2O2 is generated in the apoplast upon induction by a pathogen or microbial pattern. H2O2 from this source translocates quickly into Arabidopsis cells, where it interacts with pathways of intracellular immunity to confer plant resistance against diseases. To expedite H2O2 transport, AtPIP1;4 must adopt a specific conformation in a number of ways, including channel width extension through amino acid interactions and selectivity for H2O2 through amino acid protonation and tautomeric reactions. Both topics will reference relevant studies, conducted on other organisms and AQPs, to highlight possible mechanisms of T3 effector translocation currently under debate, and highlight the structural basis of AtPIP1;4 in H2O2 transport facilitated by gating and trafficking regulation.

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.

Aquaporins: More Than Functional Monomers in a Tetrameric Arrangement

Aquaporins (AQPs) function as tetrameric structures in which each monomer has its own permeable pathway. The combination of structural biology, molecular dynamics simulations, and experimental approaches has contributed to improve our knowledge of how protein conformational changes can challenge its transport capacity, rapidly altering the membrane permeability. This review is focused on evidence that highlights the functional relationship between the monomers and the tetramer. In this sense, we address AQP permeation capacity as well as regulatory mechanisms that affect the monomer, the tetramer, or tetramers combined in complex structures. We therefore explore: (i) water permeation and recent evidence on ion permeation, including the permeation pathway controversy-each monomer versus the central pore of the tetramer-and (ii) regulatory mechanisms that cannot be attributed to independent monomers. In particular, we discuss channel gating and AQPs that sense membrane tension. For the latter we propose a possible mechanism that includes the monomer (slight changes of pore shape, the number of possible H-bonds between water molecules and pore-lining residues) and the tetramer (interactions among monomers and a positive cooperative effect).

Nodulin Intrinsic Protein 7;1 Is a Tapetal Boric Acid Channel Involved in Pollen Cell Wall Formation

Boron is an essential plant micronutrient that plays a structural role in the rhamnogalacturonan II component of the pectic cell wall. To prevent boron deficiency under limiting conditions, its uptake, distribution, and homeostasis are mediated by boric acid transporters and channel proteins. Among the membrane channels that facilitate boric acid uptake are the type II nodulin intrinsic protein (NIP) subfamily of aquaporin-like proteins. Arabidopsis (Arabidopsis thaliana) possesses three NIP II genes (NIP5;1, NIP6;1, and NIP7;1) that show distinct tissue expression profiles (predominantly expressed in roots, stem nodes, and developing flowers, respectively). Orthologs of each are represented in all dicots. Here, we show that purified and reconstituted NIP7;1 is a boric acid facilitator. By using native promoter-reporter fusions, we show that NIP7;1 is expressed predominantly in anthers of young flowers in a narrow developmental window, floral stages 9 and 10, with protein accumulation solely within tapetum cells, where it is localized to the plasma membrane. Under limiting boric acid conditions, loss-of-function T-DNA mutants (nip7;1-1 and nip7;1-2) show reduced fertility, including shorter siliques and an increase in aborted seeds, compared with the wild type. Under these conditions, nip7;1 mutant pollen grains show morphological defects, increased aggregation, defective exine cell wall formation, reduced germination frequency, and decreased viability. During stages 9 and 10, the tapetum is essential for supplying materials to the pollen microspore cell wall. We propose that NIP7;1 serves as a gated boric acid channel in developing anthers that aids in the uptake of this critical micronutrient by tapetal cells.