pH and calcium regulate the water permeability of aquaporin 0

Aquaporin-0-protein interactions elucidated by crosslinking mass spectrometry

Aquaporin-0 (AQP0) constitutes 50 % of the lens membrane proteome and plays important roles in lens fiber cell adhesion, water permeability, and lens transparency. Previous work has shown that specific proteins, such as calmodulin (CaM), interact with AQP0 to modulate its water permeability; however, these studies often used AQP0 peptides, rather than full-length protein, to probe these interactions. Furthermore, the specific regions of interaction of several known AQP0 interacting partners, i.e. αA and αB-crystallins, and phakinin (CP49) remain unknown. The purpose of this study was to use crosslinking mass spectrometry (XL-MS) to identify interacting proteins with full-length AQP0 in crude lens cortical membrane fractions and to determine the specific protein regions of interaction. Our results demonstrate, for the first time, that the AQP0 N-terminus can engage in protein interactions. Specific regions of interaction are elucidated for several AQP0 interacting partners including phakinin, α-crystallin, connexin-46, and connexin-50. In addition, two new interacting partners, vimentin and connexin-46, were identified.

Synergetic Effects of Aloe Vera Extract with Trimethylglycine for Targeted Aquaporin 3 Regulation and Long-Term Skin Hydration

Aquaporin 3 (AQP3) channels are tetrameric membrane-bound channels that facilitate the transport of water and other small solutes across cell membranes in the skin. Decreased AQP3 expression is associated with skin dryness, skin aging, psoriasis, and delayed wound healing. Thus, our study focused on a novel combination based on Aloe barbadensis leaf extract and trimethylglycine for targeted AQP3 regulation in skin keratinocytes and deep skin moisturization. Firstly, a dose-finding cytotoxicity assay of the selected substances was performed with a 2,5-diphenyl-2H-tetrazolium bromide (MTT) indicator on HaCaT cells. The substances' ability to increase the amount of AQP3 in keratinocytes was evaluated in a keratinocyte cell culture by means of ELISA. Additionally, the deep skin hydration effect was confirmed in clinical research with healthy volunteers. According to the results, the maximum tolerated doses providing viability at 70% (MTDs) values for Aloe barbadensis leaf extract and trimethylglycine were 24.50% and 39.00%, respectively. Following the research and development, a complex based on Aloe barbadensis leaf extract and trimethylglycine in a 1:1 mass ratio exhibited a good cytotoxicity profile, with an MTDs value of 37.90%. Furthermore, it was shown that the combination had a clear synergetic effect and significantly increased AQP3 by up to 380% compared to the negative control and glyceryl glucoside (p < 0.001). It was clinically confirmed that the developed shower gel containing Aloe barbadensis leaf extract and trimethylglycine safely improved skin hydration after one use and over 28 days. Thus, this novel plant-based combination has promising potential for AQP3 regulation in the skin epidermis and a role in the development of dermatological drugs for the treatment of skin xerosis and atopic-related conditions.

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.

The role of mip in the development of lens in zebrafish

Major intrinsic protein (MIP) functions as a water channel and a cell-junction molecule in the vertebrate eye lens. The pathogenic mechanism behind the loss of MIP function in the lens, which leads to degraded optical quality and cataract formation, is still unclear. In this study, a zebrafish model with the mipb mutant was produced. The expression of mipb mRNA and protein was dramatically reduced in the mutant. Immunological analysis reveals that loss function of mip leads to the diffuse distribution of ZL-1 in the mutant lens. Furthermore, in situ hybridization reveals that mip knockout results in a decrease in the transcripts of beaded filament structural protein 2 (Bfsp2) in the lens. Histology study shows that lens fibers in the mutants are less uniform in shape and the fiber arrangement is disrupted. The presented data provides evidence for the essential role of mipb in the development of lens fibers. The absence of mipb during lens formation is likely to result in aberrant lens fiber formation and impaired lens function.

Cooperativity in regulation of membrane protein function: phenomenological analysis of the effects of pH and phospholipids

Interaction between membrane proteins and ligands plays a key role in governing a wide spectrum of cellular processes. These interactions can provide a cooperative-type regulation of protein function. A wide variety of proteins, including enzymes, channels, transporters, and receptors, displays cooperative behavior in their interactions with ligands. Moreover, the ligands involved encompass a vast diversity and include specific molecules or ions that bind to specific binding sites. In this review, our particular focus is on the interaction between integral membrane proteins and ligands that can present multiple "binding sites", such as protons or membrane phospholipids. The study of the interaction that protons or lipids have with membrane proteins often presents challenges for classical mechanistic modeling approaches. In this regard, we show that, like Hill's pioneering work on hemoglobin regulation, phenomenological modeling constitutes a powerful tool for capturing essential features of these systems.

Molecular dynamics simulation and structural analysis of aquaporin Z from an Antarctic Pseudomonas sp. strain AMS3

Aquaporin is a water channel protein that facilitates the movement of water across the cell membrane. Aquaporin from the Antarctic region has been noted for its psychrophilic properties and its ability to perform at a lower temperature but there remains limited understanding of the water mechanism of Antarctic Pseudomonas sp. strain AMS3 However, studies regarding aquaporin isolated from psychrophilic Pseudomonas sp. are still scattered. Recently, the genome sequence of an Antarctic Pseudomonas sp. strain AMS3 revealed a gene sequence encoding for a putative aquaporin designated as AqpZ1 AMS3. In this study, structure analysis and a molecular dynamics (MD) simulation of a predicted model of a fully hydrated aquaporin tetramer embedded in a lipid bilayer was performed at different temperatures for structural flexibility and stability analysis. The MD simulation results revealed that the structures were able to remain stable at low to medium temperatures. The protein was observed to have high flexibility in the loop region as compared to the helices region throughout the simulated temperatures. The selectivity filter and NPA motifs play a major role in solute selectivity and the pore radius of the protein. The structural and functional characterization of this psychrophilic aquaporin provides new insights for the future applications of this protein.Communicated by Ramaswamy H. Sarma.

Aquaporin Gating: A New Twist to Unravel Permeation through Water Channels

Aquaporins (AQPs) are small transmembrane tetrameric proteins that facilitate water, solute and gas exchange. Their presence has been extensively reported in the biological membranes of almost all living organisms. Although their discovery is much more recent than ion transport systems, different biophysical approaches have contributed to confirm that permeation through each monomer is consistent with closed and open states, introducing the term gating mechanism into the field. The study of AQPs in their native membrane or overexpressed in heterologous systems have experimentally demonstrated that water membrane permeability can be reversibly modified in response to specific modulators. For some regulation mechanisms, such as pH changes, evidence for gating is also supported by high-resolution structures of the water channel in different configurations as well as molecular dynamics simulation. Both experimental and simulation approaches sustain that the rearrangement of conserved residues contributes to occlude the cavity of the channel restricting water permeation. Interestingly, specific charged and conserved residues are present in the environment of the pore and, thus, the tetrameric structure can be subjected to alter the positions of these charges to sustain gating. Thus, is it possible to explore whether the displacement of these charges (gating current) leads to conformational changes? To our knowledge, this question has not yet been addressed at all. In this review, we intend to analyze the suitability of this proposal for the first time.

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.

Abnormal expression and the significant prognostic value of aquaporins in clear cell renal cell carcinoma

Aquaporins (AQPs) are a kind of transmembrane proteins that exist in various organs of the human body. AQPs play an important role in regulating water transport, lipid metabolism and glycolysis of cells. Clear cell renal cell carcinoma (ccRCC) is a common malignant tumor of the kidney, and the prognosis is worse than other types of renal cell cancer (RCC). The impact of AQPs on the prognosis of ccRCC and the potential relationship between AQPs and the occurrence and development of ccRCC are demanded to be investigated. In this study, we first explored the expression pattern of AQPs by using Oncomine, UALCAN, and HPA databases. Secondly, we constructed protein-protein interaction (PPI) network and performed function enrichment analysis through STRING, GeneMANIA, and Metascape. Then a comprehensive analysis of the genetic mutant frequency of AQPs in ccRCC was carried out using the cBioPortal database. In addition, we also analyzed the main enriched biological functions of AQPs and the correlation with seven main immune cells. Finally, we confirmed the prognostic value of AQPs throughGEPIA and Cox regression analysis. We found that the mRNA expression levels of AQP0/8/9/10 were up-regulated in patients with ccRCC, while those of AQP1/2/3/4/5/6/7/11 showed the opposite. Among them, the expression differences of AQP1/2/3/4/5/6/7/8/9/11 were statistically significant. The differences in protein expression levels of AQP1/2/3/4/5/6 in ccRCC and normal renal tissues were consistent with the change trends of mRNA. The biological functions of AQPs were mainly concentrated in water transport, homeostasis maintenance, glycerol transport, and intracellular movement of sugar transporters. The high mRNA expression levels of AQP0/8/9 were significantly correlated with worse overall survival (OS), while those of AQP1/4/7 were correlated with better OS. AQP0/1/4/9 were prognostic-related factors, and AQP1/9 were independent prognostic factors. In general, this research has investigated the values of AQPs in ccRCC, which could become new survival markers for ccRCC targeted therapy.

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.

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.

Effects of proton pumping on the structural rigidity of cristae in mitochondria

Mitochondria change their morphology and inner membrane structure depending on their activity. Since mitochondrial activity also depends on their structure, it is important to elucidate the interrelationship between the activity and structure of mitochondria. However, the mechanism by which mitochondrial activity affects the structure of cristae, the folded structure of the inner membrane, is not well understood. In this study, the effect of the mitochondrial activity on the cristae structure was investigated by examining the structural rigidity of cristae. Taking advantage of the fact that unfolding of cristae induces mitochondrial swelling, we investigated the relationship between mitochondrial activity and the susceptibility to swelling. The swelling of individual isolated mitochondria exposed to a hypotonic solution was observed with an optical microscope. The presence of respiratory substrates (malate and glutamate) increased the percentage of mitochondria that underwent swelling, and the further addition of rotenone or KCN (inhibitors of proton pumps) reversed the increase. In the absence of respiratory substrates, acidification of the buffer surrounding the mitochondria also increased the percentage of swollen mitochondria. These observations suggest that acidification of the outer surface of inner membranes, especially intracristal space, by proton translocation from the matrix to the intracristal space, decreases the structural rigidity of the cristae. This interpretation was verified by the observation that ADP or CCCP, which induces proton re-entry to the matrix, suppressed the mitochondrial swelling in the presence of respiratory substrates. The addition of CCCP to the cells induced a morphological change in mitochondria from an initial elongated structure to a largely curved structure at pH 7.4, but there were no morphological changes when the pH of the cytosol dropped to 6.2. These results suggest that a low pH in the intracristal space may be helpful in maintaining the elongated structure of mitochondria. The present study shows that proton pumping by the electron transfer chain is the mechanism underlying mitochondrial morphology and the flexibility of cristae structure.

The role of water coordination in the pH-dependent gating of hAQP10

Human aquaporin 10 (hAQP10) is an aquaglyceroporin that assists in maintaining glycerol flux in adipocytes during lipolysis at low pH. Hence, a molecular understanding of the pH-sensitive glycerol conductance may open up for drug development in obesity and metabolically related disorders. Control of hAQP10-mediated glycerol flux has been linked to the cytoplasmic end of the channel, where a unique loop is regulated by the protonation status of histidine 80 (H80). Here, we performed unbiased molecular dynamics simulations of three protonation states of H80 to unravel channel gating. Strikingly, at neutral pH, we identified a water coordination pattern with an inverted orientation of the water molecules in vicinity of the loop. Protonation of H80 results in a more hydrophobic loop conformation, causing loss of water coordination and leaving the pore often dehydrated. Our results indicate that the loss of such water interaction network may be integral for the destabilization of the loop in the closed configuration at low pH. Additionally, a residue unique to hAQP10 (F85) reveals structural importance by flipping into the channel in correlation with loop movements, indicating a loop-stabilizing role in the closed configuration. Taken together, our simulations suggest a unique gating mechanism combining complex interaction networks between water molecules and protein residues at the loop interface. Considering the role of hAQP10 in adipocytes, the detailed molecular insights of pH-regulation presented here will help to understand glycerol pathways in these cells and may assist in drug discovery for better management of human adiposity and obesity.

Simultaneous binding of the N- and C-terminal cytoplasmic domains of aquaporin 4 to calmodulin

Aquaporin 4 (AQP4) is a water transporting, transmembrane channel protein that has important regulatory roles in maintaining cellular water homeostasis. Several other AQP proteins exhibit calmodulin (CaM)-binding properties, and CaM has recently been implicated in the cell surface localization of AQP4. The objective of the present study was to assess the CaM-binding properties of AQP4 in detail. Inspection of AQP4 revealed two putative CaM-binding domains (CBDs) in the cytoplasmic N- and C-terminal regions, respectively. The Ca²⁺-dependent CaM-binding properties of AQP4 CBD peptides were assessed using fluorescence spectroscopy, isothermal titration calorimetry, and two-dimensional ¹H, ¹⁵N-HSQC NMR with ¹⁵N-labeled CaM. The N-terminal CBD of AQP4 predominantly interacted with the N-lobe of CaM with a 1:1 binding ratio and a K_d of 3.4 μM. The C-terminal AQP4 peptide interacted with both the C- and N-lobes of CaM (2:1 binding ratio; K_d1: 3.6 μM, K_d2: 113.6 μM, respectively). A recombinant AQP4 protein domain (recAQP4^CT, containing the entire cytosolic C-terminal sequence) bound CaM in a 1:1 binding mode with a K_d of 6.1 μM. A ternary bridging complex could be generated with the N- and C-lobes of CaM interacting simultaneously with the N- and C-terminal CBD peptides. These data support a unique adapter protein binding mode for CaM with AQP4.

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.

Calmodulin-Connexin Partnership in Gap Junction Channel Regulation-Calmodulin-Cork Gating Model

In the past four decades numerous findings have indicated that gap junction channel gating is mediated by intracellular calcium concentrations ([Ca²⁺_i]) in the high nanomolar range via calmodulin (CaM). We have proposed a CaM-based gating model based on evidence for a direct CaM role in gating. This model is based on the following: CaM inhibitors and the inhibition of CaM expression to prevent chemical gating. A CaM mutant with higher Ca²⁺ sensitivity greatly increases gating sensitivity. CaM co-localizes with connexins. Connexins have high-affinity CaM-binding sites. Connexin mutants paired to wild type connexins have a higher gating sensitivity, which is eliminated by the inhibition of CaM expression. Repeated trans-junctional voltage (Vj) pulses progressively close channels by the chemical/slow gate (CaM’s N-lobe). At the single channel level, the gate closes and opens slowly with on-off fluctuations. Internally perfused crayfish axons lose gating competency but recover it by the addition of Ca-CaM to the internal perfusion solution. X-ray diffraction data demonstrate that isolated gap junctions are gated at the cytoplasmic end by a particle of the size of a CaM lobe. We have proposed two types of CaM-driven gating: “Ca-CaM-Cork” and “CaM-Cork”. In the first, the gating involves Ca²⁺-induced CaM activation. In the second, the gating occurs without a [Ca²⁺]_i rise.

Exploring cell membrane water exchange in aquaporin-4-deficient ischemic mouse brain using diffusion-weighted MRI

BACKGROUND

Aquaporin-4 is a membrane channel protein that is highly expressed in brain astrocytes and facilitates the transport of water molecules. It has been suggested that suppression of aquaporin-4 function may be an effective treatment for reducing cellular edema after cerebral infarction. It is therefore important to develop clinically applicable measurement systems to evaluate and better understand the effects of aquaporin-4 suppression on the living body.

METHODS

Animal models of focal cerebral ischemia were created by surgically occluding the middle cerebral artery of wild-type and aquaporin-4 knockout mice, after which multi-b-value multi-diffusion-time diffusion-weighted imaging measurements were performed. Data were analyzed with both the apparent diffusion coefficient (ADC) model and a compartmental water-exchange model.

RESULTS

ADCs were estimated for five different b value ranges. The ADC of aquaporin-4 knockout mice in the contralateral region was significantly higher than that of wild-type mice for each range. In contrast, aquaporin-4 knockout mice had significantly lower ADC than wild-type mice in ischemic tissue for each b-value range. Genotype-dependent differences in the ADC were particularly significant for the lowest ranges in normal tissue and for the highest ranges in ischemic tissue. The ADCs measured at different diffusion times were significantly different for both genotypes. Fitting of the water-exchange model to the ischemic region data found that the water-exchange time in aquaporin-4 knockout mice was approximately 2.5 times longer than that in wild-type mice.

CONCLUSIONS

Multi-b-value multi-diffusion-time diffusion-weighted imaging may be useful for in vivo research and clinical diagnosis of aquaporin-4-related diseases.

Gap Junction Channelopathies and Calmodulinopathies. Do Disease-Causing Calmodulin Mutants Affect Direct Cell-Cell Communication?

The cloning of connexins cDNA opened the way to the field of gap junction channelopathies. Thus far, at least 35 genetic diseases, resulting from mutations of 11 different connexin genes, are known to cause numerous structural and functional defects in the central and peripheral nervous system as well as in the heart, skin, eyes, teeth, ears, bone, hair, nails and lymphatic system. While all of these diseases are due to connexin mutations, minimal attention has been paid to the potential diseases of cell–cell communication caused by mutations of Cx-associated molecules. An important Cx accessory protein is calmodulin (CaM), which is the major regulator of gap junction channel gating and a molecule relevant to gap junction formation. Recently, diseases caused by CaM mutations (calmodulinopathies) have been identified, but thus far calmodulinopathy studies have not considered the potential effect of CaM mutations on gap junction function. The major goal of this review is to raise awareness on the likely role of CaM mutations in defects of gap junction mediated cell communication. Our studies have demonstrated that certain CaM mutants affect gap junction channel gating or expression, so it would not be surprising to learn that CaM mutations known to cause diseases also affect cell communication mediated by gap junction channels.

Molecular mechanisms of brain water transport

Our brains consist of 80% water, which is continuously shifted between different compartments and cell types during physiological and pathophysiological processes. Disturbances in brain water homeostasis occur with pathologies such as brain oedema and hydrocephalus, in which fluid accumulation leads to elevated intracranial pressure. Targeted pharmacological treatments do not exist for these conditions owing to our incomplete understanding of the molecular mechanisms governing brain water transport. Historically, the transmembrane movement of brain water was assumed to occur as passive movement of water along the osmotic gradient, greatly accelerated by water channels termed aquaporins. Although aquaporins govern the majority of fluid handling in the kidney, they do not suffice to explain the overall brain water movement: either they are not present in the membranes across which water flows or they appear not to be required for the observed flow of water. Notably, brain fluid can be secreted against an osmotic gradient, suggesting that conventional osmotic water flow may not describe all transmembrane fluid transport in the brain. The cotransport of water is an unconventional molecular mechanism that is introduced in this Review as a missing link to bridge the gap in our understanding of cellular and barrier brain water transport.

A novel missense mutation in the gene encoding major intrinsic protein (MIP) in a Giant panda with unilateral cataract formation

Background

Cataracts are defects of the lens that cause progressive visual impairment and ultimately blindness in many vertebrate species. Most cataracts are age-related, but up to one third have an underlying genetic cause. Cataracts are common in captive zoo animals, but it is often unclear whether these are congenital or acquired (age-related) lesions.

Results

Here we used a functional candidate gene screening approach to identify mutations associated with cataracts in a captive giant panda (Ailuropoda melanoleuca). We screened 11 genes often associated with human cataracts and identified a novel missense mutation (c.686G > A) in the MIP gene encoding major intrinsic protein. This is expressed in the lens and normally accumulates in the plasma membrane of lens fiber cells, where it plays an important role in fluid transport and cell adhesion. The mutation causes the replacement of serine with asparagine (p.S229N) in the C-terminal tail of the protein, and modeling predicts that the mutation induces conformational changes that may interfere with lens permeability and cell–cell interactions.

Conclusion

The c.686G > A mutation was found in a captive giant panda with a unilateral cataract but not in 18 controls from diverse regions in China, suggesting it is most likely a genuine disease-associated mutation rather than a single-nucleotide polymorphism. The mutation could therefore serve as a new genetic marker to predict the risk of congenital cataracts in captive giant pandas.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07386-8.

A microfluidic strategy for the detection of membrane protein interactions

Membrane proteins perform a vast range of vital biological functions and are the gatekeepers for exchange of information and matter between the intracellular and extracellular environment. However, membrane protein interactions can be challenging to characterise in a quantitative manner due to the low solubility and large size of the membrane protein complex with associated lipid or detergent molecules. Here, we show that measurements of the changes in charge and diffusivity on the micron scale allow for non-disruptive studies of membrane protein interactions in solution. The approach presented here uses measurements of key physical properties of membrane proteins and their ligands to characterise the binding equilibrium parameters. We demonstrate this approach for human aquaporins (AQPs), key membrane proteins in the regulation of water homeostasis in cells. We perform quantitative measurements to characterise the interactions between two full-length AQP isoforms and the regulatory protein, calmodulin (CaM), and show that CaM selectively binds AQP0. Through direct measurements of the diffusivity and mobility in an external electric field, the diffusion coefficients and electrophoretic mobilities are determined for the individual components and the resulting AQP0-CaM complex. Furthermore, we obtain directly the binding equilibrium parameters and effective charge of each component. These results open up a route towards the use of microfluidics as a general platform in protein science and open up new possibilities for the characterisation of membrane protein interactions in solution.

Calmodulin-Mediated Regulation of Gap Junction Channels

Evidence that neighboring cells uncouple from each other as one dies surfaced in the late 19th century, but it took almost a century for scientists to start understanding the uncoupling mechanism (chemical gating). The role of cytosolic free calcium (Ca2+i) in cell-cell channel gating was first reported in the mid-sixties. In these studies, only micromolar [Ca2+]i were believed to affect gating-concentrations reachable only in cell death, which would discard Ca2+i as a fine modulator of cell coupling. More recently, however, numerous researchers, including us, have reported the effectiveness of nanomolar [Ca2+]i. Since connexins do not have high-affinity calcium sites, the effectiveness of nanomolar [Ca2+]i suggests the role of Ca-modulated proteins, with calmodulin (CaM) being most obvious. Indeed, in 1981 we first reported that a CaM-inhibitor prevents chemical gating. Since then, the CaM role in gating has been confirmed by studies that tested it with a variety of approaches such as treatments with CaM-inhibitors, inhibition of CaM expression, expression of CaM mutants, immunofluorescent co-localization of CaM and gap junctions, and binding of CaM to peptides mimicking connexin domains identified as CaM targets. Our gating model envisions Ca2+-CaM to directly gate the channels by acting as a plug ("Cork" gating model), and probably also by affecting connexin conformation.

Aquaporin-1 plays a key role in erythropoietin-induced endothelial cell migration

Water influx through aquaporin-1 (AQP-1) has been linked to the ability of different cell types to migrate, and therefore plays an important part in processes like metastasis and angiogenesis. Since the erythroid growth factor erythropoietin (Epo) is now recognized as an angiogenesis promoter, we investigated the participation of AQP-1 as a downstream effector of this cytokine in the migration of endothelial cells. Inhibition of AQP-1 with either mercury ions (Hg²⁺) or a specific siRNA led to an impaired migration of EA.hy926 endothelial cells exposed to Epo (wound-healing assays). Epo also induced the expression of AQP-1 at mRNA and protein levels, an effect which was dependent on the influx of extracellular calcium through L-type calcium channels as well as TRPC3 channels. The relationship between Epo and AQP-1 was further confirmed at shorter exposure times, as the cytokine was unable to trigger calcium influxes in cells where AQP-1 had previously been knocked down. Moreover, Epo promoted changes in the subcellular localization of AQP-1 as well as rearrangements in the actin cytoskeleton, which are consistent with a migratory phenotype. Worthy of note, carbamylated erythropoietin (cEpo), the non-erythropoietic and non-promigratory derivative of Epo, was incapable of AQP-1 modulation. The therapeutical implications of aquaporin targeting in angiogenesis-related diseases highlight the importance of the present results in the context of the relationship between AQP-1 and Epo.

Cooperativity and allostery in aquaporin 0 regulation by Ca2

Aquaporin 0 (AQP0) is essential for eye lens homeostasis as is regulation of its water permeability by Ca²⁺, which occurs through interactions with calmodulin (CaM), but the underlying molecular mechanisms are not well understood. Here, we use molecular dynamics (MD) simulations on the microsecond timescale under an osmotic gradient to explicitly model water permeation through the AQP0 channel. To identify any structural features that are specific to water permeation through AQP0, we also performed simulations of aquaporin 1 (AQP1) and a pure mixed lipid bilayer under the same conditions. The relative single-channel water osmotic permeability coefficients (p_f) calculated from all of our simulations are in reasonable agreement with experiment. Our simulations allowed us to characterize the dynamics of the key structural elements that modulate the diffusion of water single-files through the AQP0 and AQP1 pores. We find that CaM binding influences the collective dynamics of the whole AQP0 tetramer, promoting the closing of both the extracellular and intracellular gates by inducing cooperativity between neighboring subunits.

Experimental and Simulation Studies of Aquaporin 0 Water Permeability and Regulation

We begin with the history of aquaporin zero (AQP0), the most prevalent membrane protein in the eye lens, from the early days when AQP0 was a protein of unknown function known as Major Intrinsic Protein 26. We progress through its joining the aquaporin family as a water channel in its own right and discuss how regulation of its water permeability by pH and calcium came to be discovered experimentally and linked to lens homeostasis and development. We review the development of molecular dynamics (MD) simulations of lipid bilayers and membrane proteins, including aquaporins, with an emphasis on simulation studies that have elucidated the mechanisms of water conduction, selectivity, and proton exclusion by aquaporins in general. We also review experimental and theoretical progress toward understanding why mammalian AQP0 has a lower water permeability than other aquaporins and the evolution of our present understanding of how its water permeability is regulated by pH and calcium. Finally, we discuss how MD simulations have elucidated the nature of lipid interactions with AQP0.

BFSP1 C-terminal domains released by post-translational processing events can alter significantly the calcium regulation of AQP0 water permeability

BFSP1 (beaded filament structural protein 1, filensin) is a cytoskeletal protein expressed in the eye lens. It binds AQP0 in vitro and its C-terminal sequences have been suggested to regulate the water channel activity of AQP0. A myristoylated fragment from the C-terminus of BFSP1 was found in AQP0 enriched fractions. Here we identify BFSP1 as a substrate for caspase-mediated cleavage at several C-terminal sites including D433. Cleavage at D433 exposes a cryptic myristoylation sequence (434–440). We confirm that this sequence is an excellent substrate for both NMT1 and 2 (N-myristoyl transferase). Thus caspase cleavage may promote formation of myristoylated fragments derived from the BFSP1 C-terminus (G434-S665). Myristoylation at G434 is not required for membrane association. Biochemical fractionation and immunogold labeling confirmed that C-terminal BFSP1 fragments containing the myristoylation sequence colocalized with AQP0 in the same plasma membrane compartments of lens fibre cells. To determine the functional significance of the association of BFSP1 G434-S665 sequences with AQP0, we measured AQP0 water permeability in Xenopus oocytes co-transfected with transcripts expressing both AQP0 and various C-terminal domain fragments of BFSP1 generated by caspase cleavage. We found that different fragments dramatically alter the response of AQP0 to different concentrations of Ca²⁺. The complete C-terminal fragment (G434-S665) eliminates calcium regulation altogether. Shorter fragments can enhance regulation by elevated calcium or reverse the response, indicative of the regulatory potential of BFSP1 with respect to AQP0. In particular, elimination of the myristoylation site by the mutation G434A reverses the order of water permeability sensitivity to different Ca²⁺ concentrations.

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.

Cooperativity in proton sensing by PIP aquaporins

One of the most intriguing properties of plasma membrane intrinsic protein (PIP) aquaporins (AQPs) is their ability to modulate water transport by sensing different levels of intracellular pH through the assembly of homo- and heterotetrameric molecular species in the plasma membrane. In this work, using a phenomenological modeling approach, we demonstrate that cooperativity in PIP biological response cannot be directly attributed to a cooperative proton binding, as it is usually considered, since it could also be the consequence of a cooperative conformation transition between open and closed states of the channel. Moreover, our results show that, when mixed populations of homo- and heterotetrameric PIP channels are coexpressed in the plasma membrane of the same cell, the observed decrease in the degree of positive cooperativity would result from the simultaneous presence of molecular species with different levels of proton sensing. Indeed, the random mixing between different PIP paralogues as subunits in a single tetramer, plus the possibility of mixed populations of homo- and heterotetrameric PIP channels widen the spectrum of cooperative responses of a cell membrane. Our approach offers a deep understanding of cooperative transport of AQP channels, as members of a multiprotein family where the relevant proton binding sites of each member have not been clearly elucidated yet.

Molecular Basis of Aquaporin-7 Permeability Regulation by pH

The aquaglyceroporin AQP7, a family member of aquaporin membrane channels, facilitates the permeation of water and glycerol through cell membranes and is crucial for body lipid and energy homeostasis. Regulation of glycerol permeability via AQP7 is considered a promising therapeutic strategy towards fat-related metabolic complications. Here, we used a yeast aqy-null strain for heterologous expression and functional analysis of human AQP7 and investigated its regulation by pH. Using a combination of in vitro and in silico approaches, we found that AQP7 changes from fully permeable to virtually closed at acidic pH, and that Tyr135 and His165 facing the extracellular environment are crucial residues for channel permeability. Moreover, instead of reducing the pore size, the protonation of key residues changes AQP7’s protein surface electrostatic charges, which, in turn, may decrease glycerol’s binding affinity to the pore, resulting in decreased permeability. In addition, since some pH-sensitive residues are located at the monomer-monomer interface, decreased permeability may result from cooperativity between AQP7’s monomers. Considering the importance of glycerol permeation via AQP7 in multiple pathophysiological conditions, this mechanism of hAQP7 pH-regulation may help the design of selective modulators targeting aquaglyceroporin-related disorders.

Single-file transport of water through membrane channels

Water at interfaces governs many processes on the molecular scale from electrochemical and enzymatic reactions to protein folding. Here we focus on water transport through proteinaceous pores that are so narrow that the water molecules cannot overtake each other in the pore. After a short introduction into the single-file transport theory, we analyze experiments in which the unitary water permeability, pf, of water channel proteins (aquaporins, AQPs), potassium channels (KcsA), and antibiotics (gramicidin-A derivatives) has been obtained. A short outline of the underlying methods (scanning electrochemical microscopy, fluorescence correlation spectroscopy, measurements of vesicle light scattering) is also provided. We conclude that pf increases exponentially with a decreasing number NH of hydrogen bond donating or accepting residues in the channel wall. The variance in NH is responsible for a more than hundredfold change in pf. The dehydration penalty at the channel mouth has a smaller effect on pf. The intricate link between pf and the Gibbs activation energy barrier, ΔG‡t, for water flow suggests that conformational transitions of water channels act as a third determinant of pf.

Identification of a novel MIP frameshift mutation associated with congenital cataract in a Chinese family by whole-exome sequencing and functional analysis

Purpose

To detect the underlying pathogenesis of congenital cataract in a four-generation Chinese family.

Methods

Whole-exome sequencing (WES) of family members (III:4, IV:4, and IV:6) was performed. Sanger sequencing and bioinformatics analysis were subsequently conducted. Full-length WT-MIP or K228fs-MIP fused to HA markers at the N-terminal was transfected into HeLa cells. Next, quantitative real-time PCR, western blotting and immunofluorescence confocal laser scanning were performed.

Results

The age of onset for nonsyndromic cataracts in male patients was by 1-year old, earlier than for female patients, who exhibited onset at adulthood. A novel c.682_683delAA (p.K228fs230X) mutation in main intrinsic protein (MIP) cosegregated with the cataract phenotype. The instability index and unfolded states for truncated MIP were predicted to increase by bioinformatics analysis. The mRNA transcription level of K228fs-MIP was reduced compared with that of WT-MIP, and K228fs-MIP protein expression was also lower than that of WT-MIP. Immunofluorescence images showed that WT-MIP principally localized to the plasma membrane, whereas the mutant protein was trapped in the cytoplasm.

Conclusions

Our study generated genetic and primary functional evidence for a novel c.682_683delAA mutation in MIP that expands the variant spectrum of MIP and help us better understand the molecular basis of cataract.

Fundamental structural and functional properties of Aquaporin ion channels found across the kingdoms of life

Aquaporin (AQP) channels in the major intrinsic protein (MIP) family are known to facilitate transmembrane water fluxes in prokaryotes and eukaryotes. Some classes of AQPs also conduct ions, glycerol, urea, CO₂ , nitric oxide, and other small solutes. Ion channel activity has been demonstrated for mammalian AQPs 0, 1, 6, Drosophila Big Brain (BIB), soybean nodulin 26, and rockcress AtPIP2;1. More classes are likely to be discovered. Newly identified blockers are providing essential tools for establishing physiological roles of some of the AQP dual water and ion channels. For example, the arylsulfonamide AqB011 which selectively blocks the central ion pore of mammalian AQP1 has been shown to impair migration of HT29 colon cancer cells. Traditional herbal medicines are sources of selective AQP1 inhibitors that also slow cancer cell migration. The finding that plant AtPIP2;1 expressed in root epidermal cells mediates an ion conductance regulated by calcium and protons provided insight into molecular mechanisms of environmental stress responses. Expression of lens MIP (AQP0) is essential for maintaining the structure, integrity and transparency of the lens, and Drosophila BIB contributes to neurogenic signalling pathways to control the developmental fate of fly neuroblast cells; however, the ion channel roles remain to be defined for MIP and BIB. A broader portfolio of pharmacological agents is needed to investigate diverse AQP ion channel functions in situ. Understanding the dual water and ion channel roles of AQPs could inform the development of novel agents for rational interventions in diverse challenges from agriculture to human health.

The Role of Aquaporins in Ocular Lens Homeostasis

Abstract: Aquaporins (AQPs), by playing essential roles in the maintenance of ocular lens homeostasis, contribute to the establishment and maintenance of the overall optical properties of the lens over many decades of life. Three aquaporins, AQP0, AQP1 and AQP5, each with distinctly different functional properties, are abundantly and differentially expressed in the different regions of the ocular lens. Furthermore, the diversity of AQP functionality is increased in the absence of protein turnover by age-related modifications to lens AQPs that are proposed to alter AQP function in the different regions of the lens. These regional differences in AQP functionality are proposed to contribute to the generation and directionality of the lens internal microcirculation; a system of circulating ionic and fluid fluxes that delivers nutrients to and removes wastes from the lens faster than could be achieved by passive diffusion alone. In this review, we present how regional differences in lens AQP isoforms potentially contribute to this microcirculation system by highlighting current areas of investigation and emphasizing areas where future work is required.

Aquaporin Protein-Protein Interactions

Aquaporins are tetrameric membrane-bound channels that facilitate transport of water and other small solutes across cell membranes. In eukaryotes, they are frequently regulated by gating or trafficking, allowing for the cell to control membrane permeability in a specific manner. Protein–protein interactions play crucial roles in both regulatory processes and also mediate alternative functions such as cell adhesion. In this review, we summarize recent knowledge about aquaporin protein–protein interactions; dividing the interactions into three types: (1) interactions between aquaporin tetramers; (2) interactions between aquaporin monomers within a tetramer (hetero-tetramerization); and (3) transient interactions with regulatory proteins. We particularly focus on the structural aspects of the interactions, discussing the small differences within a conserved overall fold that allow for aquaporins to be differentially regulated in an organism-, tissue- and trigger-specific manner. A deep knowledge about these differences is needed to fully understand aquaporin function and regulation in many physiological processes, and may enable design of compounds targeting specific aquaporins for treatment of human disease.

Plant and animal aquaporins crosstalk: what can be revealed from distinct perspectives

Aquaporins (AQPs) can be revisited from a distinct and complementary perspective: the outcome from analyzing them from both plant and animal studies. (1) The approach in the study. Diversity found in both kingdoms contrasts with the limited number of crystal structures determined within each group. While the structure of almost half of mammal AQPs was resolved, only a few were resolved in plants. Strikingly, the animal structures resolved are mainly derived from the AQP2-lineage, due to their important roles in water homeostasis regulation in humans. The difference could be attributed to the approach: relevance in animal research is emphasized on pathology and in consequence drug screening that can lead to potential inhibitors, enhancers and/or regulators. By contrast, studies on plants have been mainly focused on the physiological role that AQPs play in growth, development and stress tolerance. (2) The transport capacity. Besides the well-described AQPs with high water transport capacity, large amount of evidence confirms that certain plant AQPs can carry a large list of small solutes. So far, animal AQP list is more restricted. In both kingdoms, there is a great amount of evidence on gas transport, although there is still an unsolved controversy around gas translocation as well as the role of the central pore of the tetramer. (3) More roles than expected. We found it remarkable that the view of AQPs as specific channels has evolved first toward simple transporters to molecules that can experience conformational changes triggered by biochemical and/or mechanical signals, turning them also into signaling components and/or behave as osmosensor molecules.

Functional characterization of an aquaporin from a microsporidium, Nosema bombycis

Microsporidia are a diverse group of eukaryotic organisms, capable of causing parasitic infections in both vertebrates and invertebrates. During the germination process, there is an increase in the osmotic pressure of microsporidian spores. As part of this study, we cloned a homologous aquaporin gene in Nosema bombycis, and named it Nosema bombycis aquaporin (NbAQP). Sequence analysis revealed that the NbAQP contains an open reading frame with a length of 750 bp and encodes a polypeptide of 249 amino acids. Amino acid sequence homology was greater than 50% that of five aquaporins from other microsporidian species. Indirect immunofluorescence (IFA) and immunogold electron microscopy showed NbAQP to be located predominantly in the spore wall of N. bombycis spores. The results of qRT-PCR analysis revealed that NbAQP expression remained high 0 h after inoculation and decreased sharply to 24 h, increased gradually from 2 days and peaked at 6 days. After expression of NbAQP in Xenopus laevis oocytes, it was observed that NbAQP can promote rapid penetration of water into oocytes. The associated permeation rate was 2–3 times that of the water-injected and uninjected oocytes. Antibody blocking experiments showed that the inhibition rate of spore germination was approximately 28% after antibody blocking. The difference in germination rate between the control group and the NbAQP group was significant (P < 0.05). This study shows for the first time that N. bombycis contains functional water channel proteins and provides a platform suitable for further research into the mechanisms underlying the regulation of NbAQP protein expression. Further study of NbAQP and their inhibitors may have significance for prevention of microsporidiosis.

Role of Pore-Lining Residues in Defining the Rate of Water Conduction by Aquaporin-0

Compared to other aquaporins (AQPs), lens-specific AQP0 is a poor water channel, and its permeability was reported to be pH-dependent. To date, most water conduction studies on AQP0 were performed on protein expressed in Xenopus oocytes, and the results may therefore also reflect effects introduced by the oocytes themselves. Experiments with purified AQP0 reconstituted into liposomes are challenging because the water permeability of AQP0 is only slightly higher than that of pure lipid bilayers. By reconstituting high amounts of AQP0 and using high concentrations of cholesterol to reduce the permeability of the lipid bilayer, we improved the signal-to-noise ratio of water permeability measurements on AQP0 proteoliposomes. Our measurements show that mutation of two pore-lining tyrosine residues, Tyr-23 and Tyr-149 in sheep AQP0, to the corresponding residues in the high-permeability water channel AQP1 have additive effects and together increase the water permeability of AQP0 40-fold to a level comparable to that of AQP1. Molecular dynamics simulations qualitatively support these experimental findings and suggest that mutation of Tyr-23 changes the pore profile at the gate formed by residue Arg-187.

Molecular Biology of Aquaporins

Aquaporins (AQPs ) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, thirteen AQPs , which are distributed widely in specific cell types in various organs and tissues, have been characterized in humans. Four AQP monomers, each of which consists of six membrane-spanning alpha-helices that have a central water-transporting pore, assemble to form tetramers, forming the functional units in the membrane. AQP facilitates osmotic water transport across plasma membranes and thus transcellular fluid movement. The cellular functions of aquaporins are regulated by posttranslational modifications , e.g. phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation, and protein interactions. Insight into the molecular mechanisms responsible for regulated aquaporin trafficking and synthesis is proving to be fundamental for development of novel therapeutic targets or reliable diagnostic and prognostic biomarkers.

Rat Aquaporin-5 Is pH-Gated Induced by Phosphorylation and Is Implicated in Oxidative Stress

Aquaporin-5 (AQP5) is a membrane water channel widely distributed in human tissues that was found up-regulated in different tumors and considered implicated in carcinogenesis in different organs and systems. Despite its wide distribution pattern and physiological importance, AQP5 short-term regulation was not reported and mechanisms underlying its involvement in cancer are not well defined. In this work, we expressed rat AQP5 in yeast and investigated mechanisms of gating, as well as AQP5’s ability to facilitate H₂O₂ plasma membrane diffusion. We found that AQP5 can be gated by extracellular pH in a phosphorylation-dependent manner, with higher activity at physiological pH 7.4. Moreover, similar to other mammalian AQPs, AQP5 is able to increase extracellular H₂O₂ influx and to affect oxidative cell response with dual effects: whereas in acute oxidative stress conditions AQP5 induces an initial higher sensitivity, in chronic stress AQP5 expressing cells show improved cell survival and resistance. Our findings support the involvement of AQP5 in oxidative stress and suggest AQP5 modulation by phosphorylation as a novel tool for therapeutics.

A novel MIP mutation in familial congenital nuclear cataracts

We screened 60 known genes which are involved in inherited cataract in a pregnant woman with a four-generation family history of autosomal dominant congenital nuclear cataract through next-generation sequencing (NGS) and identified a heterozygous mutation, c.508dupC (p.L170fs), in the major intrinsic protein (MIP) gene. This mutation results in a frame-shift in MIP and has not been previously reported. The correlation of the mutation with disease was validated by Sanger sequencing of DNA from the other affected or unaffected members of the family. Therefore, our data expand the mutation spectrum of MIP mutation, and suggest that NGS is an accurate, rapid, and cost-effective method in the genetic diagnosis of congenital nuclear cataract.

Spatial distributions of phosphorylated membrane proteins aquaporin 0 and MP20 across young and aged human lenses

In the human ocular lens it is now realized that post-translational modifications can alter protein function and/or localization in fiber cells that no longer synthesize proteins. The specific sites of post-translational modification to the abundant ocular lens membrane proteins AQP0 and MP20 have been previously identified and their functional effects are emerging. To further understand how changes in protein function and/or localization induced by these modifications alter lens homeostasis, it is necessary to determine the spatial distributions of these modifications across the lens. In this study, a quantitative LC-MS approach was used to determine the spatial distributions of phosphorylated AQP0 and MP20 peptides from manually dissected, concentric layers of fiber cells from young and aged human lenses. The absolute amounts of phosphorylation were determined for AQP0 Ser235 and Ser229 and for MP20 Ser170 in fiber cells from the lens periphery to the lens center. Phosphorylation of AQP0 Ser229 represented a minor portion of the total phosphorylated AQP0. Changes in spatial distributions of phosphorylated APQ0 Ser235 and MP20 Ser170 correlated with regions of physiological interest in aged lenses, specifically, where barriers to water transport and extracellular diffusion form.

Auto-Adhesion Potential of Extraocular Aqp0 during Teleost Development

AQP0 water channels are the most abundant proteins expressed in the mammalian lens fiber membranes where they are essential for lens development and transparency. Unlike other aquaporin paralogs, mammalian AQP0 has a low intrinsic water permeability, but can form cell-to-cell junctions between the lens fibers. It is not known whether the adhesive properties of AQP0 is a derived feature found only in mammals, or exists as a conserved ancestral trait in non-mammalian vertebrates. Here we show that a tetraploid teleost, the Atlantic salmon, expresses four Aqp0 paralogs in the developing lens, but also expresses significant levels of aqp0 mRNAs and proteins in the epithelia of the pronephros, presumptive enterocytes, gill filament and epidermis. Quantitative PCR reveals that aqp0 mRNA titres increase by three orders of magnitude between the onset of somitogenesis and pigmentation of the eye. Using in situ hybridization and specific antisera, we show that at least two of the channels (Aqp0a1, -0b1 and/or -0b2) are localized in the extraocular basolateral and apical membranes, while Aqp0a2 is lens-specific. Heterologous expression of the Aqp0 paralogs in adhesion-deficient mouse fibolast L-cells reveals that, as for human AQP0, each intact salmon channel retains cell-to-cell adhesive properties. The strongest Aqp0 interactions are auto-adhesion, suggesting that homo-octamers likely form the intercellular junctions of the developing lens and epithelial tissues. The present data are thus the first to show the adhesion potential of Aqp0 channels in a non-mammalian vertebrate, and further uncover a novel extraocular role of the channels during vertebrate development.

The Role of Aquaporins in pH-Dependent Germination of Rhizopus delemar Spores

Rhizopus delemar and associated species attack a wide range of fruit and vegetables after harvest. Host nutrients and acidic pH are required for optimal germination of R. delemar, and we studied how this process is triggered. Glucose induced spore swelling in an acidic environment, expressed by an up to 3-fold increase in spore diameter, whereas spore diameter was smaller in a neutral environment. When suspended in an acidic environment, the spores started to float, indicating a change in their density. Treatment of the spores with HgCl2, an aquaporin blocker, prevented floating and inhibited spore swelling and germ-tube emergence, indicating the importance of water uptake at the early stages of germination. Two putative candidate aquaporin-encoding genes-RdAQP1 and RdAQP2-were identified in the R. delemar genome. Both presented the conserved NPA motif and six-transmembrane domain topology. Expressing RdAQP1 and RdAQP2 in Arabidopsis protoplasts increased the cells' osmotic water permeability coefficient (Pf) compared to controls, indicating their role as water channels. A decrease in R. delemar aquaporin activity with increasing external pH suggested pH regulation of these proteins. Substitution of two histidine (His) residues, positioned on two loops facing the outer side of the cell, with alanine eliminated the pH sensing resulting in similar Pf values under acidic and basic conditions. Since hydration is critical for spore switching from the resting to activate state, we suggest that pH regulation of the aquaporins can regulate the initial phase of R. delemar spore germination, followed by germ-tube elongation and host-tissue infection.

MALDI Imaging Mass Spectrometry Spatially Maps Age-Related Deamidation and Truncation of Human Lens Aquaporin-0

PURPOSE

To spatially map human lens Aquaporin-0 (AQP0) protein modifications, including lipidation, truncation, and deamidation, from birth through middle age using matrix-assisted laser desorption ionization (MALDI) imaging mass spectrometry (IMS).

METHODS

Human lens sections were water-washed to facilitate detection of membrane protein AQP0. We acquired MALDI images from eight human lenses ranging in age from 2 months to 63 years. In situ tryptic digestion was used to generate peptides of AQP0 and peptide images were acquired on a 15T Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Peptide extracts were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and database searched to identify peptides observed in MALDI imaging experiments.

RESULTS

Unmodified, truncated, and fatty acid-acylated forms of AQP0 were detected in protein imaging experiments. Full-length AQP0 was fatty acid acylated in the core and cortex of young (2- and 4-month) lenses. Acylated and unmodified AQP0 were C-terminally truncated in older lens cores. Deamidated tryptic peptides (+0.9847 Da) were mass resolved from unmodified peptides by FTICR MS. Peptide images revealed differential localization of un-, singly-, and doubly-deamidated AQP0 C-terminal peptide (239-263). Deamidation was present at 4 months and increases with age. Liquid chromatography-MS/MS results indicated N246 undergoes deamidation more rapidly than N259.

CONCLUSIONS

Results indicated AQP0 fatty acid acylation and deamidation occur during early development. Progressive age-related AQP0 processing, including deamidation and truncation, was mapped in human lenses as a function of age. The localization of these modified AQP0 forms suggests where AQP0 functions may change throughout lens development and aging.

Detecting Aquaporin Function and Regulation

Water is the major component of cells and tissues throughout all forms of life. Fluxes of water and solutes through cell membranes and epithelia are essential for osmoregulation and energy homeostasis. Aquaporins are membrane channels expressed in almost every organism and involved in the bidirectional transfer of water and small solutes across cell membranes. Aquaporins have important biological roles and have been implicated in several pathophysiological conditions suggesting a great translational potential in aquaporin-based diagnostics and therapeutics. Detecting aquaporin function is critical for assessing regulation and screening for new activity modulators that can prompt the development of efficient medicines. Appropriate methods for functional analysis comprising suitable cell models and techniques to accurately evaluate water and solute membrane permeability are essential to validate aquaporin function and assess short-term regulation. The present review describes established assays commonly used to assess aquaporin function in cells and tissues, as well as the experimental biophysical strategies required to reveal functional regulation and identify modulators, the first step for aquaporin drug discovery.