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

Structural Basis for the Interaction between the Ezrin FERM-Domain and Human Aquaporins

The Ezrin/Radixin/Moesin (ERM) family of proteins act as cross-linkers between the plasma membrane and the actin cytoskeleton. This mechanism plays an essential role in processes related to membrane remodeling and organization, such as cell polarization, morphogenesis and adhesion, as well as in membrane protein trafficking and signaling pathways. For several human aquaporin (AQP) isoforms, an interaction between the ezrin band Four-point-one, Ezrin, Radixin, Moesin (FERM)-domain and the AQP C-terminus has been demonstrated, and this is believed to be important for AQP localization in the plasma membrane. Here, we investigate the structural basis for the interaction between ezrin and two human AQPs: AQP2 and AQP5. Using microscale thermophoresis, we show that full-length AQP2 and AQP5 as well as peptides corresponding to their C-termini interact with the ezrin FERM-domain with affinities in the low micromolar range. Modelling of the AQP2 and AQP5 FERM complexes using ColabFold reveals a common mode of binding in which the proximal and distal parts of the AQP C-termini bind simultaneously to distinct binding sites of FERM. While the interaction at each site closely resembles other FERM-complexes, the concurrent interaction with both sites has only been observed in the complex between moesin and its C-terminus which causes auto-inhibition. The proposed interaction between AQP2/AQP5 and FERM thus represents a novel binding mode for extrinsic ERM-interacting partners.

The role of phosphorylation in calmodulin-mediated gating of human AQP0

Aquaporin-0 (AQP0) is the main water channel in the mammalian lens and is involved in accommodation and maintaining lens transparency. AQP0 binds the Ca2+-sensing protein calmodulin (CaM) and this interaction is believed to gate its water permeability by closing the water-conducting pore. Here, we express recombinant and functional human AQP0 in Pichia pastoris and investigate how phosphorylation affects the interaction with CaM in vitro as well as the CaM-dependent water permeability of AQP0 in proteoliposomes. Using microscale thermophoresis and surface plasmon resonance technology we show that the introduction of the single phospho-mimicking mutations S229D and S235D in AQP0 reduces CaM binding. In contrast, CaM interacts with S231D with similar affinity as wild type, but in a different manner. Permeability studies of wild-type AQP0 showed that the water conductance was significantly reduced by CaM in a Ca2+-dependent manner, whereas AQP0 S229D, S231D and S235D were all locked in an open state, insensitive to CaM. We propose a model in which phosphorylation of AQP0 control CaM-mediated gating in two different ways (1) phosphorylation of S229 or S235 abolishes binding (the pore remains open) and (2) phosphorylation of S231 results in CaM binding without causing pore closure, the functional role of which remains to be elucidated. Our results suggest that site-dependent phosphorylation of AQP0 dynamically controls its CaM-mediated gating. Since the level of phosphorylation increases towards the lens inner cortex, AQP0 may become insensitive to CaM-dependent gating along this axis.

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.

Effect of controlled hypotensive hemorrhage on plasma sodium levels in anesthetized pigs: An exploratory study

Perioperative hyponatremia, due to non-osmotic release of the antidiuretic hormone arginine vasopressin, is a serious electrolyte disorder observed in connection with many types of surgery. Since blood loss during surgery contributes to the pathogenesis of hyponatremia, we explored the effect of bleeding on plasma sodium using a controlled hypotensive hemorrhage pig model. After 30-min baseline period, hemorrhage was induced by aspiration of blood during 30 min at mean arterial pressure <50 mmHg. Thereafter, the animals were resuscitated with retransfused blood and a near-isotonic balanced crystalloid solution and monitored for 180 min. Electrolyte and water balances, cardiovascular response, renal hemodynamics, and markers of volume regulation and osmoregulation were investigated. All pigs (n = 10) developed hyponatremia. All animals retained hypotonic fluid, and none could excrete net-free water. Urinary excretion of aquaporin 2, a surrogate marker of collecting duct responsiveness to antidiuretic hormone, was significantly reduced at the end of the study, whereas lysine vasopressin, i.e., the pig antidiuretic hormone remained high. In this animal model, hyponatremia developed due to net positive fluid balance and generation of electrolyte-free water by the kidneys. A decreased urinary aquaporin 2 excretion may indicate an escape from antidiuresis.

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.

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.

Characterization of human aquaporin protein-protein interactions using microscale thermophoresis (MST)

Aquaporin water channels (AQPs) are membrane proteins that maintain cellular water homeostasis. The interactions between human AQPs and other proteins play crucial roles in AQP regulation by both gating and trafficking. Here, we describe a protocol for characterizing the interaction between a human AQP and a soluble interaction partner using microscale thermophoresis (MST). MST has the advantage of low sample consumption and high detergent compatibility enabling AQP protein-protein interaction investigation with a high level of control of components and environment. For complete details on the use and execution of this protocol, please refer to Kitchen et al. (2020) and Roche et al. (2017).

Molecular mechanisms governing aquaporin relocalisation

The aquaporins (AQPs) form a family of integral membrane proteins that facilitate the movement of water across biological membrane by osmosis, as well as facilitating the diffusion of small polar solutes. AQPs have been recognised as drug targets for a variety of disorders associated with disrupted water or solute transport, including brain oedema following stroke or trauma, epilepsy, cancer cell migration and tumour angiogenesis, metabolic disorders, and inflammation. Despite this, drug discovery for AQPs has made little progress due to a lack of reproducible high-throughput assays and difficulties with the druggability of AQP proteins. However, recent studies have suggested that targetting the trafficking of AQP proteins to the plasma membrane is a viable alternative drug target to direct inhibition of the water-conducting pore. Here we review the literature on the trafficking of mammalian AQPs with a view to highlighting potential new drug targets for a variety of conditions associated with disrupted water and solute homeostasis.

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.

The In Vitro Effect of Steroid Hormones, Arachidonic Acid, and Kinases Inhibitors on Aquaporin 1, 2, 5, and 7 Gene Expression in the Porcine Uterine Luminal Epithelial Cells during the Estrous Cycle

Aquaporins (AQPs) are integral membrane proteins, which play an important role in water homeostasis in the uterus. According to the literature, the expression of aquaporins in reproductive structures depends on the local hormonal milieu. The current study investigated the effect of selected PKA kinase inhibitor H89 and MAPK kinase inhibitor PD98059, on the expression of AQP1, 2, 5, and 7, and steroid hormones (E₂), progesterone (P₄), and arachidonic acid (AA) in the porcine endometrium on days 18–20 and 2–4 of the estrous cycle (the follicular phase where estrogen and follicle-stimulating hormone (FSH) are secreted increasingly in preparation for estrus and the luteal phase where the ovarian follicles begin the process of luteinization with the formation of the corpus luteum and progesterone secretion, respectively). The luminal epithelial cells were incubated in vitro in the presence of the aforementioned factors. The expression of mRNA was determined by the quantitative real-time PCR technique. In general, in Experiment 1, steroid hormones significantly increased expression of AQP1, 2, and 5 while arachidonic acid increased expression of AQP2 and AQP7. On the other hand, MAPK kinase inhibitor significantly decreased the expression of AQP1 and 5. In Experiment 2, E₂, P₄, or AA combined with kinase inhibitors differentially affected on AQPs expression. E₂ in combination with PKA inhibitor significantly decreased expression of AQP1 but E₂ or P₄ combined with this inhibitor increased the expression of AQP5 and 7. On the contrary, E₂ with PD98059 significantly increased AQP5 and AQP7 expression. Progesterone in combination with MAPK kinase inhibitor significantly downregulated the expression of AQP5 and upregulated AQP7. Arachidonic acid mixed with H89 or PD98059 caused a decrease in the expression of AQP5 and an increase of AQP7. The obtained results indicate that estradiol, progesterone, and arachidonic acid through PKA and MAPK signaling pathways regulate the expression of AQP1 and AQP5 in the porcine luminal epithelial cells in the periovulatory period.

A Phytophthora sojae CRN effector mediates phosphorylation and degradation of plant aquaporin proteins to suppress host immune signaling

Phytophthora genomes encode a myriad of Crinkler (CRN) effectors, some of which contain putative kinase domains. Little is known about the host targets of these kinase-domain-containing CRNs and their infection-promoting mechanisms. Here, we report the host target and functional mechanism of a conserved kinase CRN effector named CRN78 in a notorious oomycete pathogen, Phytophthora sojae. CRN78 promotes Phytophthora capsici infection in Nicotiana benthamiana and enhances P. sojae virulence on the host plant Glycine max by inhibiting plant H₂O₂ accumulation and immunity-related gene expression. Further investigation reveals that CRN78 interacts with PIP2-family aquaporin proteins including NbPIP2;2 from N. benthamiana and GmPIP2-13 from soybean on the plant plasma membrane, and membrane localization is necessary for virulence of CRN78. Next, CRN78 promotes phosphorylation of NbPIP2;2 or GmPIP2-13 using its kinase domain in vivo, leading to their subsequent protein degradation in a 26S-dependent pathway. Our data also demonstrates that NbPIP2;2 acts as a H₂O₂ transporter to positively regulate plant immunity and reactive oxygen species (ROS) accumulation. Phylogenetic analysis suggests that the phosphorylation sites of PIP2 proteins and the kinase domains of CRN78 homologs are highly conserved among higher plants and oomycete pathogens, respectively. Therefore, this study elucidates a conserved and novel pathway used by effector proteins to inhibit host cellular defenses by targeting and hijacking phosphorylation of plant aquaporin proteins.

CRN effectors are conserved in diverse pathogens of plants, animals, and insects, and highly expanded in Phytophthora species. Nevertheless, little is known about their functions, targets, and action mechanisms. Here, we characterized a kinase-domain-containing CRN effector (CRN78) in a notorious oomycete pathogen, P. sojae. CRN78 is a virulence-essential effector of P. sojae infection, and acts via suppression of plant H₂O₂ accumulation and defense gene expressions. We demonstrated that CRN78 might interact with plant PIP2-family aquaporin proteins, including N. benthamiana NbPIP2;2 and soybean GmPIP2-13, and regulate their phosphorylation, resulting in subsequent 26S-dependent protein degradation. Furthermore, we revealed that NbPIP2;2 was an apoplast-to-cytoplast H₂O₂ transporter and positively regulated plant immunity and ROS accumulation. Importantly, this phosphorylation may be highly conserved in many plant aquaporin proteins. Thus, this study identifies a virulence-related effector from P. sojae and a novel plant immunity-related gene, and reveals a detailed mechanism of effector-mediated phosphorylation and degradation of plant aquaporin proteins.

Intracellular location of aquaporin-2 serine 269 phosphorylation and dephosphorylation in kidney collecting duct cells

Aquaporin-2 (AQP2) is a vasopressin-regulated water channel protein responsible for water reabsorption by the kidney collecting ducts. Under control conditions, most AQP2 resides in the recycling endosomes of principal cells, where it answers to vasopressin with trafficking to the apical plasma membrane to increase water reabsorption. Upon vasopressin withdrawal, apical AQP2 retreats to the early endosomes before joining the recycling endosomes for the next vasopressin stimulation. Prior studies have demonstrated a role of AQP2 S269 phosphorylation in reducing AQP2 endocytosis, thereby prolonging apical AQP2 retention. Here, we studied where in the cells S269 was phosphorylated and dephosphorylated in response to vasopressin versus withdrawal. In mpkCCD collecting cells, vacuolar protein sorting 35 knockdown slowed vasopressin-induced apical AQP2 trafficking, resulting in AQP2 accumulation in the recycling endosomes where S269 was phosphorylated. Rab7 knockdown, which impaired AQP2 trafficking from the early to recycling endosomes, reduced vasopressin-induced S269 phosphorylation. Rab5 knockdown, which impaired AQP2 endocytosis, did not affect vasopressin-induced S269 phosphorylation. Upon vasopressin withdrawal, S269 was not dephosphorylated in Rab5 knockdown cells. In contrast, S269 dephosphorylation upon vasopressin withdrawal was completed in Rab7 or vacuolar protein sorting 35 knockdown cells. We conclude that S269 is dephosphorylated during Rab5-mediated AQP2 endocytosis before AQP2 joins the recycling endosomes upon vasopressin withdrawal. While in the recycling endosomes, AQP2 can be phosphorylated at S269 in response to vasopressin before apical trafficking.

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.

Rab7 involves Vps35 to mediate AQP2 sorting and apical trafficking in collecting duct cells

Aquaporin-2 (AQP2) is a vasopressin-regulated water channel protein responsible for osmotic water reabsorption by kidney collecting ducts. In response to vasopressin, AQP2 traffics from intracellular vesicles to the apical plasma membrane of collecting duct principal cells, where it increases water permeability and, hence, water reabsorption. Despite continuing efforts, gaps remain in our knowledge of vasopressin-regulated AQP2 trafficking. Here, we studied the functions of two retromer complex proteins, small GTPase Rab7 and vacuolar protein sorting 35 (Vps35), in vasopressin-induced AQP2 trafficking in a collecting duct cell model (mpkCCD cells). We showed that upon vasopressin removal, apical AQP2 returned to Rab5-positive early endosomes before joining Rab11-positive recycling endosomes. In response to vasopressin, Rab11-associated AQP2 trafficked to the apical plasma membrane before Rab5-associated AQP2 did so. Rab7 knockdown resulted in AQP2 accumulation in early endosomes and impaired vasopressin-induced apical AQP2 trafficking. In response to vasopressin, Rab7 transiently colocalized with Rab5, indicative of a role of Rab7 in AQP2 sorting in early endosomes before trafficking to the apical membrane. Rab7-mediated apical AQP2 trafficking in response to vasopressin required GTPase activity. When Vps35 was knocked down, AQP2 accumulated in recycling endosomes under vehicle conditions and did not traffic to the apical plasma membrane in response to vasopressin. We conclude that Rab7 and Vps35 participate in AQP2 sorting in early endosomes under vehicle conditions and apical membrane trafficking in response to vasopressin.

Quantifying the Interaction of Phosphite with ABC Transporters: MicroScale Thermophoresis and a Novel His-Tag Labeling Approach

The combination of MicroScale Thermophoresis (MST) and near-native site-specific His-tag labeling enables simple, robust, and reliable determination of the binding affinity between proteins and ligands. To demonstrate its applicability for periplasmic proteins, we provide a detailed protocol for determination of the binding affinity of phosphite to three ABC transporter periplasmic-binding proteins from environmental microorganisms. ABC transporters are central to many important biomedical phenomena, including resistance of cancers and pathogenic microbes to drugs. The protocol described here can be used to quantify protein-ligand and protein-protein interactions for other soluble, membrane-associated and integral membrane proteins.

Structural Insights into AQP2 Targeting to Multivesicular Bodies

Vasopressin-dependent trafficking of AQP2 in the renal collecting duct is crucial for the regulation of water homeostasis. This process involves the targeting of AQP2 to the apical membrane during dehydration as well as its removal when hydration levels have been restored. The latter involves AQP2 endocytosis and sorting into multivesicular bodies (MVB), from where it may be recycled, degraded in lysosomes, or released into urine via exosomes. The lysosomal trafficking regulator-interacting protein 5 (LIP5) plays a crucial role in this by coordinating the actions of the endosomal sorting complex required for transport III (ESCRT-III) and vacuolar protein sorting 4 (Vps4) ATPase, resulting in the insertion of AQP2 into MVB inner vesicles. While the interaction between LIP5 and the ESCRT-III complex and Vps4 is well characterized, very little is known about how LIP5 interacts with AQP2 or any other membrane protein cargo. Here, we use a combination of fluorescence spectroscopy and computer modeling to provide a structural model of how LIP5 interacts with human AQP2. We demonstrate that, the AQP2 tetramer binds up to two LIP5 molecules and that the interaction is similar to that seen in the complex between LIP5 and the ESCRT-III component, charged multivesicular body protein 1B (CHMP1B). These studies give the very first structural insights into how LIP5 enables membrane protein insertion into MVB inner vesicles and significantly increase our understanding of the AQP2 trafficking mechanism.

Phosphorylation of human AQP2 and its role in trafficking

Human Aquaporin 2 (AQP2) is a membrane-bound water channel found in the kidney collecting duct whose regulation by trafficking plays a key role in regulating urine volume. AQP2 trafficking is tightly controlled by the pituitary hormone arginine vasopressin (AVP), which stimulates translocation of AQP2 residing in storage vesicles to the apical membrane. The AVP-dependent translocation of AQP2 to and from the apical membrane is controlled by multiple phosphorylation sites in the AQP2 C-terminus, the phosphorylation of which alters its affinity to proteins within the cellular membrane protein trafficking machinery. The aim of this chapter is to provide a summary of what is currently known about AVP-mediated AQP2 trafficking, dissecting the roles of individual phosphorylation sites, kinases and phosphatases and interacting proteins. From this, the picture of an immensely complex process emerges, of which many structural and molecular details remains to be elucidated.

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.

Retinal and optic nerve degeneration in liver X receptor β knockout mice

The retina is an extension of the brain. Like the brain, neurodegeneration of the retina occurs with age and is the cause of several retinal diseases including optic neuritis, macular degeneration, and glaucoma. Liver X receptors (LXRs) are expressed in the brain where they play a key role in maintenance of cerebrospinal fluid and the health of dopaminergic neurons. Herein, we report that LXRs are expressed in the retina and optic nerve and that loss of LXRβ, but not LXRα, leads to loss of ganglion cells in the retina. In the retina of LXRβ^-/- mice, there is an increase in amyloid A4 and deposition of beta-amyloid (Aβ) aggregates but no change in the level of apoptosis or autophagy in the ganglion cells and no activation of microglia or astrocytes. However, in the optic nerve there is a loss of aquaporin 4 (AQP4) in astrocytes and an increase in activation of microglia. Since loss of AQP4 and microglial activation in the optic nerve precedes the loss of ganglion cells, and accumulation of Aβ in the retina, the cause of the neuronal loss appears to be optic nerve degeneration. In patients with optic neuritis there are frequently AQP4 autoantibodies which block the function of AQP4. LXRβ^-/- mouse is another model of optic neuritis in which AQP4 antibodies are not detectable, but AQP4 function is lost because of reduction in its expression.

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.

In vitro expression and functional characterization of NPA motifs in aquaporins of Nosema bombycis

Nosema bombycis contains functional aquaporins (NbAQPs), which are key targets for exploring the mechanism of N. bombycis infection; however, the regulation of these NbAQPs remains unknown. The two highly conserved asparagine-proline-alanine sequences (NPA motifs) play important roles in AQP biogenesis. As part of this study, we constructed a series of NbAQP mutants (NbAQP_NPA1, NbAQP_NPA2, and NbAQP_NPA1,2) and expressed them in BmN cells. The results showed that mutations in either NPA motif or in both NPA motifs did not affect NbAQP expression in vitro. After expression in Xenopus laevis oocytes, those injected with wild-type NbAQP rapidly expanded, whereas oocytes injected with NbAQP_NPAs did not significantly change in size. The associated water permeability (pf) of NbAQP_NPAs was significantly reduced five-six times compared to that of wild-type NbAQP. These results indicated that NPA motifs are necessary for the water channel function of AQPs in N. bombycis. The present study shows for the first time that the NbAQP NPA motif has an impact on the water permeability of aquaporin in N. bombycis, thereby providing a platform for further research into the mechanisms underlying the regulation of NbAQP expression.

Thermophoresis for characterizing biomolecular interaction

The study of biomolecular interactions is crucial to get more insight into the biological system. The interactions of protein-protein, protein-nucleic acids, protein-sugars, nucleic acid-nucleic acids and protein-small molecules are supporting therapeutics and technological developments. Recently, the development in a large number of analytical techniques for characterizing biomolecular interactions reflect the promising research investments in this field. In this review, microscale thermophoresis technology (MST) is presented as an analytical technique for characterizing biomolecular interactions. Recent years have seen much progress and several applications established. MST is a powerful technique in quantitation of binding events based on the movement of molecules in microscopic temperature gradient. Simplicity, free solutions analysis, low sample volume, short analysis time, and immobilization free are the MST advantages over other competitive techniques. A wide range of studies in biomolecular interactions have been successfully carried out using MST, which tend to the versatility of the technique to use in screening binding events in order to save time, cost and obtained high data quality.

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.