Glutathionylation of the aquaporin-2 water channel: a novel post-translational modification modulated by the oxidative stress

Combining different ion-selective channelrhodopsins to control water flux by light

Water transport through water channels, aquaporins (AQPs), is vital for many physiological processes including epithelial fluid secretion, cell migration and adipocyte metabolism. Water flux through AQPs is driven by the osmotic gradient that results from concentration differences of solutes including ions. Here, we developed a novel optogenetic toolkit that combines the light-gated anion channel GtACR1 either with the light-gated K+ channel HcKCR1 or the new Na+ channelrhodopsin HcNCR1 with high Na+ permeability, to manipulate water transport in Xenopus oocytes non-invasively. Water efflux through AQP was achieved by light-activating K+ and Cl- efflux through HcKCR1 and GtACR1. Contrarily, when GtACR1 was co-expressed with HcNCR1, inward movement of Na+ and Cl- was light-triggered, and the resulting osmotic gradient led to water influx through AQP1. In sum, we demonstrate a novel optogenetic strategy to manipulate water movement into or out of Xenopus oocytes non-invasively. This approach provides a new avenue to interfere with water homeostasis as a means to study related biological phenomena across cell types and organisms.

Protein Structure and Modification of Aquaporins

Aquaporins (AQPs) allow water molecules and other small, neutral solutes to quickly pass through membrane. The protein structures of AQPs solved by crystallographic methods or cryo-electron microscopy technology show that AQP monomer consists of six membrane-spanning alpha-helices that form the central water-transporting pore. AQP monomers assemble to form tetramers, forming the functional units in the membrane, to transport water or other small molecules. The biological functions of AQPs are regulated by posttranslational modifications, e.g., phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation and protein interactions. Modifications of AQP combined with structural properties contribute to a better functional mechanism of AQPs. Insight into the molecular mechanisms responsible for AQP modifications as well as gating and transport properties proved to be fundamental to the development of new therapeutic targets or reliable diagnostic and prognostic biomarkers.

Role of microRNAs in aquaporin 2 regulation

PURPOSE OF REVIEW

The current review aims to present the most recent achievements on the role of microRNAs (miRNAs) on the kidney function to stimulate research in the field and to expand new emerging concepts.

RECENT FINDINGS

The focus is on the role of miRNAs in intercellular communication along the segments of the nephron and on the epi-miRNAs, namely the possibility of some miRNAs to modulate the epigenetic machinery and so gene expression. Indeed, recent evidence showed that miRNAs included in exosomes and released by proximal tubule cells can modulate ENaC activity on cells of collecting duct. These data, although, from in-vitro models open to a novel role for miRNAs to participate in paracrine signaling pathways. In addition, the role of miRNAs as epigenetic modulators is expanding not only in the cancer field, but also in the other kidney diseases. Recent evidence identified three miRNAs able to modulate the AQP2 promoter metilation and showing an additional level of regulation for the AQP2.

SUMMARY

These evidence can inspire novel area of research both for renal physiology and drug discovery. The diseases involving the collecting duct are still missing disease modifying agents and the expanding miRNAs field could represent an opportunity.

AQP2 Promotes Astrocyte Activation by Modulating the TLR4/NFκB-p65 Pathway Following Intracerebral Hemorrhage

Microglial and astrocyte activation and related cytokine secretion play key roles in secondary brain injury following intracerebral hemorrhage (ICH). We assessed the role of aquaporin (AQP)2 in immune response after ICH. We prospectively collected data from 33 patients with ICH and analyzed the serum AQP2 levels in these patients and age-matched healthy controls. A correlation analysis was also performed between patient serum AQP2 levels and clinical factors. In the rat ICH model, double-fluorescence staining for glial fibrillary acidic protein (GFAP) and AQP2 was performed to investigate the relationship between astrocytes and AQP2. Relative mRNA expression levels of GFAP and AQP2 were also measured. In the rat astrocyte cell line CTX-TNA2, toll-like receptor (TLR)4/nuclear factor kappa B (NFκB)-p65 pathway activation and GFAP levels were measured. The indirect influence of AQP2 on microglial polarization was assessed following exposure to the medium of astrocytes treated with AQP2-overexpression plasmid or silencing RNA. We found that the serum AQP2 expression was lower in patients with ICH. Sex and blood neutrophil count influenced serum AQP2 concentrations in patients with ICH on admission. Lower serum AQP2 levels were inversely correlated with 90-day Modified Rankin Scale scores after ICH, but were not correlated with National Institute of Health stroke scale (NIHSS) scores on admission. AQP2 overexpression and localization in GFAP-labeled astrocytes were observed in rats. AQP2 overexpression induced astrocyte activation with GFAP upregulation via TLR/NFκB-p65 signaling pathway activation in the rat astrocyte cell line CTX-TNA2. Astrocyte activation promoted interleukin-1β secretion. The medium of AQP2-overexpression astrocytes promoted the pro-inflammatory M1 phenotype in the immortal rat (HAPI) microglial cell line. Therefore, serum AQP2 is negatively correlated with post-ICH prognosis and may be a marker of inflammation in early-stage ICH. AQP2 overexpression promotes astrocyte activation and pro-inflammatory secretion, affects astrocyte-microglia crosstalk, and indirectly induces microglial polarization, which may augment inflammation after ICH.

The vasopressin-aquaporin-2 pathway syndromes

Vasopressin is the key hormone involved in water conservation and regulation of water balance, essential for life. In the renal collecting duct, vasopressin binds to the V2 receptor, increasing water permeability through activation of aquaporin-2 redistribution to the luminal membrane. This mechanism promotes rapid water reabsorption, important for immediate survival; however, only recently it has become clear that long-term adverse effects are associated with alterations of the vasopressin-aquaporin-2 pathway, leading to several syndromes associated with water balance disorders. The kidney resistance to the vasopressin action may cause severe dehydration for patients and, conversely, nonosmotic release of vasopressin is associated with water retention and increasing the circulatory blood volume. This chapter discusses the relevance of the altered vasopressin-aquaporin-2 pathway in some diseases associated with water balance disorders, including congenital nephrogenic diabetes insipidus, syndrome of inappropriate secretion of antidiuretic hormone, nephrogenic syndrome of inappropriate antidiuresis, and autosomal dominant polycystic kidney disease. The emerging picture suggests that targeting the vasopressin-AQP2 axis can provide therapeutic benefits in those patients.

Olive Leaf Extract (OLE) impaired vasopressin-induced aquaporin-2 trafficking through the activation of the calcium-sensing receptor

Vasopressin (AVP) increases water permeability in the renal collecting duct through the regulation of aquaporin-2 (AQP2) trafficking. Several disorders, including hypertension and inappropriate antidiuretic hormone secretion (SIADH), are associated with abnormalities in water homeostasis. It has been shown that certain phytocompounds are beneficial to human health. Here, the effects of the Olive Leaf Extract (OLE) have been evaluated using in vitro and in vivo models. Confocal studies showed that OLE prevents the vasopressin induced AQP2 translocation to the plasma membrane in MCD4 cells and rat kidneys. Incubation with OLE decreases the AVP-dependent increase of the osmotic water permeability coefficient (Pf). To elucidate the possible effectors of OLE, intracellular calcium was evaluated. OLE increases the intracellular calcium through the activation of the Calcium Sensing Receptor (CaSR). NPS2143, a selective CaSR inhibitor, abolished the inhibitory effect of OLE on AVP-dependent water permeability. In vivo experiments revealed that treatment with OLE increases the expression of the CaSR mRNA and decreases AQP2 mRNA paralleled by an increase of the AQP2-targeting miRNA-137. Together, these findings suggest that OLE antagonizes vasopressin action through stimulation of the CaSR indicating that this extract may be beneficial to attenuate disorders characterized by abnormal CaSR signaling and affecting renal water reabsorption.

In Vitro and In Vivo Nutraceutical Characterization of Two Chickpea Accessions: Differential Effects on Hepatic Lipid Over-Accumulation

Dietary habits are crucially important to prevent the development of lifestyle-associated diseases. Diets supplemented with chickpeas have numerous benefits and are known to improve body fat composition. The present study was undertaken to characterize two genetically and phenotypically distinct accessions, MG_13 and PI358934, selected from a global chickpea collection. Rat hepatoma FaO cells treated with a mixture of free fatty acids (FFAs) (O/P) were used as an in vitro model of hepatic steatosis. In parallel, a high-fat diet (HFD) animal model was also established. In vitro and in vivo studies revealed that both chickpea accessions showed a significant antioxidant ability. However, only MG_13 reduced the lipid over-accumulation in steatotic FaO cells and in the liver of HFD fed mice. Moreover, mice fed with HFD + MG_13 displayed a lower level of glycemia and aspartate aminotransferase (AST) than HFD mice. Interestingly, exposure to MG_13 prevented the phosphorylation of the inflammatory nuclear factor kappa beta (NF-kB) which is upregulated during HFD and known to be linked to obesity. To conclude, the comparison of the two distinct chickpea accessions revealed a beneficial effect only for the MG_13. These findings highlight the importance of studies addressing the functional characterization of chickpea biodiversity and nutraceutical properties.

Calcium sensing receptor exerts a negative regulatory action toward vasopressin-induced aquaporin-2 expression and trafficking in renal collecting duct

Vasopressin (AVP) plays a major role in the regulation of water homeostasis by its antidiuretic action on the kidney, mediated by V2 receptors. An increase in plasma sodium concentration stimulates AVP release, which in turn promotes water reabsorption. Upon binding to the V2 receptors in the renal collecting duct, AVP induces the expression and apical membrane insertion of the aquaporin-2 (AQP2) water channels and subsequent water reabsorption. AVP regulates two independent mechanisms: the short-term regulation of AQP2 trafficking and long-term regulation of the total abundance of the AQP2 protein in the cells. On the other hand, several hormones, acting through specific receptors, have been reported to antagonize AVP-mediated water transport in kidney. In this respect, we previously described that high luminal Ca²⁺ in the renal collecting duct attenuates short-term AVP-induced AQP2 trafficking through activation of the Ca²⁺-sensing receptor (CaSR). This effect is due to reduction of AVP-dependent cAMP generation and possibly hydrolysis. Moreover, CaSR signaling reduces AQP2 abundance both via AQP2-targeting miRNA-137 and the proteasomal degradation pathway. This chapter summarizes recent data elucidating the molecular mechanisms underlying the physiological role of the CaSR-dependent regulation of AQP2 expression and trafficking.

Green olive leaf extract (OLE) provides cytoprotection in renal cells exposed to low doses of cadmium

Cadmium (Cd) is a heavy and highly toxic metal that contaminates air, food and water. Cadmium accumulates in several organs altering normal functions. The kidney is the major organ at risk of damage from chronic exposure to cadmium as a contaminant in food and water. This study aims to investigate the beneficial effects of OLE in renal collecting duct MCD4 cells exposed to a low dose cadmium (1 μM). In MCD4 cells cadmium caused an increase in ROS production, as well as generation of lipid droplets and reduced cell viability. Moreover, cadmium exposure led to a remarkable increase in the frequency of micronuclei and DNA double-strand breaks, assessed using the alkaline comet assay. In addition, cadmium dramatically altered cell cytoskeleton architecture and caused S-glutathionylation of actin. Notably, all cadmium-induced cellular deregulations were prevented by co-treatment with OLE, possibly due to its antioxidant action and to the presence of bioactive phytocompounds. Indeed, OLE treatment attenuated Cd-induced actin S-glutathionylation, thereby stabilizing actin filaments. Taken together, these observations provide a novel insight into the biological action of OLE in renal cells and support the notion that OLE may serve as a potential adjuvant against cadmium-induced nephrotoxicity.

Vasopressin-aquaporin-2 pathway: recent advances in understanding water balance disorders

The alteration of water balance and related disorders has emerged as being strictly linked to the state of activation of the vasopressin–aquaporin-2 (vasopressin–AQP2) pathway. The lack of responsiveness of the kidney to the vasopressin action impairs its ability to concentrate the urine, resulting in polyuria, polydipsia, and risk of severe dehydration for patients. Conversely, non-osmotic release of vasopressin is associated with an increase in water permeability in the renal collecting duct, producing water retention and increasing the circulatory blood volume. This review highlights some of the new insights and recent advances in therapeutic intervention targeting the dysfunctions in the vasopressin–AQP2 pathway causing diseases characterized by water balance disorders such as congenital nephrogenic diabetes insipidus, syndrome of inappropriate antidiuretic hormone secretion, nephrogenic syndrome of inappropriate antidiuresis, and autosomal dominant polycystic kidney disease. The recent clinical data suggest that targeting the vasopressin–AQP2 axis can provide therapeutic benefits in patients with water balance disorders.

Aquaporin Membrane Channels in Oxidative Stress, Cell Signaling, and Aging: Recent Advances and Research Trends

Reactive oxygen species (ROS) are produced as a result of aerobic metabolism and as by-products through numerous physiological and biochemical processes. While ROS-dependent modifications are fundamental in transducing intracellular signals controlling pleiotropic functions, imbalanced ROS can cause oxidative damage, eventually leading to many chronic diseases. Moreover, increased ROS and reduced nitric oxide (NO) bioavailability are main key factors in dysfunctions underlying aging, frailty, hypertension, and atherosclerosis. Extensive investigation aims to elucidate the beneficial effects of ROS and NO, providing novel insights into the current medical treatment of oxidative stress-related diseases of high epidemiological impact. This review focuses on emerging topics encompassing the functional involvement of aquaporin channel proteins (AQPs) and membrane transport systems, also allowing permeation of NO and hydrogen peroxide, a major ROS, in oxidative stress physiology and pathophysiology. The most recent advances regarding the modulation exerted by food phytocompounds with antioxidant action on AQPs are also reviewed.

CaSR signaling down-regulates AQP2 expression via a novel microRNA pathway in pendrin and NaCl cotransporter knockout mice

High concentrations of urinary calcium counteract vasopressin action via the activation of the calcium-sensing receptor (CaSR) that is expressed in the luminal membrane of collecting duct cells, which impairs the trafficking of aquaporin-2 (AQP2). Pendrin/NaCl cotransporter double-knockout (dKO) mice display significant calcium wasting and develop severe volume depletion, despite increased circulating vasopressin levels. We hypothesized that the CaSR-mediated impairment of AQP2 expression/trafficking underlies vasopressin resistance in dKO mice. Compared with wild-type mice, in renal inner medulla, dKO mice had reduced total AQP2 sensitive to proteasome inhibitors, higher levels of AQP2-pS261, ubiquitinated AQP2, and p38-MAPK, an enzyme that is activated by CaSR signaling and known to phosphorylate AQP2 at Ser261. CaSR inhibition with the calcilytic NPS2143 reversed these effects, which indicates that CaSR mediates the up-regulation of AQP2-pS261, ubiquitination, and degradation. Of note, dKO mice demonstrated significantly higher AQP2-targeting miRNA-137 that was reduced upon CaSR inhibition, supporting a critical role for CaSR in the down-regulation of AQP2 expression. Our data indicate that CaSR signaling reduces AQP2 abundance both via AQP2-targeting miRNA-137 and the p38-MAPK/AQP2-pS261/ubiquitination/proteasomal axis. These effects may contribute to the reduced renal concentrating ability that has been observed in dKO mice and underscore a physiologic mechanism of the CaSR-dependent regulation of AQP2 abundance via a novel microRNA pathway.-Ranieri, M., Zahedi, K., Tamma, G., Centrone, M., Di Mise, A., Soleimani, M., Valenti, G. CaSR signaling down-regulates AQP2 expression via a novel microRNA pathway in pendrin and NaCl cotransporter knockout mice.

Physiological and molecular mechanisms mediating xylem Na+ loading in barley in the context of salinity stress tolerance

Time-dependent kinetics of xylem Na⁺ loading was investigated using a large number of barley genotypes contrasting in their salinity tolerance. Salt-sensitive varieties were less efficient in controlling xylem Na⁺ loading and showed a gradual increase in the xylem Na⁺ content over the time. To understand underlying ionic and molecular mechanisms, net fluxes of Ca²⁺ , K⁺ and Na⁺ were measured from the xylem parenchyma tissue in response to H₂ O₂ and ABA; both of them associated with salinity stress signalling. Our results indicate that NADPH oxidase-mediated apoplastic H₂ O₂ production acts upstream of the xylem Na⁺ loading and is causally related to ROS-inducible Ca²⁺ uptake systems in the root stelar tissue. It was also found that ABA regulates (directly or indirectly) the process of Na⁺ retrieval from the xylem and the significant reduction of Na⁺ and K⁺ fluxes induced by bumetanide are indicative of a major role of chloride cation co-transporter (CCC) on xylem ion loading. Transcript levels of HvHKT1;5_like and HvSOS1_like genes in the root stele were observed to decrease after salt stress, while there was an increase in HvSKOR_like gene, indicating that these ion transporters are involved in primary Na⁺ /K⁺ movement into/out of xylem.

Effects of Reactive Oxygen Species on Tubular Transport along the Nephron

Reactive oxygen species (ROS) are oxygen-containing molecules naturally occurring in both inorganic and biological chemical systems. Due to their high reactivity and potentially damaging effects to biomolecules, cells express a battery of enzymes to rapidly metabolize them to innocuous intermediaries. Initially, ROS were considered by biologists as dangerous byproducts of respiration capable of causing oxidative stress, a condition in which overproduction of ROS leads to a reduction in protective molecules and enzymes and consequent damage to lipids, proteins, and DNA. In fact, ROS are used by immune systems to kill virus and bacteria, causing inflammation and local tissue damage. Today, we know that the functions of ROS are not so limited, and that they also act as signaling molecules mediating processes as diverse as gene expression, mechanosensation, and epithelial transport. In the kidney, ROS such as nitric oxide (NO), superoxide (O₂^-), and their derivative molecules hydrogen peroxide (H₂O₂) and peroxynitrite (ONO₂^-) regulate solute and water reabsorption, which is vital to maintain electrolyte homeostasis and extracellular fluid volume. This article reviews the effects of NO, O₂^-, ONO₂^-, and H₂O₂ on water and electrolyte reabsorption in proximal tubules, thick ascending limbs, and collecting ducts, and the effects of NO and O₂^- in the macula densa on tubuloglomerular feedback.

Impact of posttranslational modifications of engineered cysteines on the substituted cysteine accessibility method: evidence for glutathionylation

The substituted cysteine accessibility method (SCAM) is widely used to study the structure and function of channels, receptors and transporters. In its usual application, a cysteine residue is introduced into a protein which lacks native cysteines following which the accessibility of the residue to the aqueous compartment is assessed. Implicit, and generally assumed, is that if the cysteine-substituted residue is not available to react with sulfhydryl reagents it is not exposed to the extracellular compartment or within the aqueous translocation pathway. We demonstrate here, in a Hela-derived cell line, that some cysteine-substituted residues of the proton-coupled folate transporter (PCFT, SLC46A1) that are inaccessible to 2-((biotinoyl)amino)ethyl methanethiosulfonate are glutathionylated by biotinylated glutathione ethyl ester in the absence of an oxidizing agent. Intramolecular disulfide formation involving cysteine-substituted residues was also identified in some instances. These posttranslational modifications limit the accessibility of the cysteine residues to sulfhydryl-reactive reagents and can have a profound impact on the interpretation of SCAM but may not alter function. When a posttranslationally modified residue is used as a reference extracellular control, the high level of exposure required for detection on Western blot results in erroneous detection of otherwise inaccessible intracellular cysteine-substituted residues. The data indicate that in the application of SCAM, when a cysteine-substituted residue does not appear to be accessible to sulfhydryl-reactive reagents, the possibility of a posttranslational modification should be excluded. The data explain the discrepancies in the assessment, and confirm the localization, of the first intracellular loop of PCFT.

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.

From 20th century metabolic wall charts to 21st century systems biology: database of mammalian metabolic enzymes

The organization of the mammalian genome into gene subsets corresponding to specific functional classes has provided key tools for systems biology research. Here, we have created a web-accessible resource called the Mammalian Metabolic Enzyme Database (https://hpcwebapps.cit.nih.gov/ESBL/Database/MetabolicEnzymes/MetabolicEnzymeDatabase.html) keyed to the biochemical reactions represented on iconic metabolic pathway wall charts created in the previous century. Overall, we have mapped 1,647 genes to these pathways, representing ~7 percent of the protein-coding genome. To illustrate the use of the database, we apply it to the area of kidney physiology. In so doing, we have created an additional database (Database of Metabolic Enzymes in Kidney Tubule Segments: https://hpcwebapps.cit.nih.gov/ESBL/Database/MetabolicEnzymes/), mapping mRNA abundance measurements (mined from RNA-Seq studies) for all metabolic enzymes to each of 14 renal tubule segments. We carry out bioinformatics analysis of the enzyme expression pattern among renal tubule segments and mine various data sources to identify vasopressin-regulated metabolic enzymes in the renal collecting duct.

Evaluating the Oxidative Stress in Renal Diseases: What Is the Role for S-Glutathionylation?

SIGNIFICANCE

Reactive oxygen species (ROS) have long been considered as toxic derivatives of aerobic metabolism displaying a harmful effect to living cells. Deregulation of redox homeostasis and production of excessive free radicals may contribute to the pathogenesis of kidney diseases. In line, oxidative stress increases in patients with renal dysfunctions due to a general increase of ROS paralleled by impaired antioxidant ability.

RECENT ADVANCES

Emerging evidence revealed that physiologically, ROS can act as signaling molecules interplaying with several transduction pathways such as proliferation, differentiation, and apoptosis. ROS can exert signaling functions by modulating, at different layers, protein oxidation since proteins have "cysteine switches" that can be reversibly reduced or oxidized, supporting the dynamic signaling regulation function. In this scenario, S-glutathionylation is a posttranslational modification involved in oxidative cellular response.

CRITICAL ISSUES

Although it is widely accepted that renal dysfunctions are often associated with altered redox signaling, the relative role of S-glutathionylation on the pathogenesis of specific renal diseases remains unclear and needs further investigations. In this review, we discuss the impact of ROS in renal health and diseases and the role of selective S-glutathionylation proteins potentially relevant to renal physiology.

FUTURE DIRECTIONS

The paucity of studies linking the reversible protein glutathionylation with specific renal disorders remains unmet. The growing number of S-glutathionylated proteins indicates that this is a fascinating area of research. In this respect, further studies on the association of reversible glutathionylation with renal diseases, characterized by oxidative stress, may be useful to develop new pharmacological molecules targeting protein S-glutathionylation. Antioxid. Redox Signal. 25, 147-164.

Trypanosome Lytic Factor-1 Initiates Oxidation-stimulated Osmotic Lysis of Trypanosoma brucei brucei

Human innate immunity against the veterinary pathogen Trypanosoma brucei brucei is conferred by trypanosome lytic factors (TLFs), against which human-infective T. brucei gambiense and T. brucei rhodesiense have evolved resistance. TLF-1 is a subclass of high density lipoprotein particles defined by two primate-specific apolipoproteins: the ion channel-forming toxin ApoL1 (apolipoprotein L1) and the hemoglobin (Hb) scavenger Hpr (haptoglobin-related protein). The role of oxidative stress in the TLF-1 lytic mechanism has been controversial. Here we show that oxidative processes are involved in TLF-1 killing of T. brucei brucei. The lipophilic antioxidant N,N'-diphenyl-p-phenylenediamine protected TLF-1-treated T. brucei brucei from lysis. Conversely, lysis of TLF-1-treated T. brucei brucei was increased by the addition of peroxides or thiol-conjugating agents. Previously, the Hpr-Hb complex was postulated to be a source of free radicals during TLF-1 lysis. However, we found that the iron-containing heme of the Hpr-Hb complex was not involved in TLF-1 lysis. Furthermore, neither high concentrations of transferrin nor knock-out of cytosolic lipid peroxidases prevented TLF-1 lysis. Instead, purified ApoL1 was sufficient to induce lysis, and ApoL1 lysis was inhibited by the antioxidant DPPD. Swelling of TLF-1-treated T. brucei brucei was reminiscent of swelling under hypotonic stress. Moreover, TLF-1-treated T. brucei brucei became rapidly susceptible to hypotonic lysis. T. brucei brucei cells exposed to peroxides or thiol-binding agents were also sensitized to hypotonic lysis in the absence of TLF-1. We postulate that ApoL1 initiates osmotic stress at the plasma membrane, which sensitizes T. brucei brucei to oxidation-stimulated osmotic lysis.

Targeted silencing of the Aquaporin 2 gene of Rhipicephalus (Boophilus) microplus reduces tick fitness

BACKGROUND

Ticks are blood-feeding arthropods that can affect human and animal health both directly by blood-feeding and indirectly by transmitting pathogens. The cattle tick Rhipicephalus (Boophilus) microplus is one of the most economically important ectoparasites of bovines worldwide and it is responsible for the transmission of the protozoan Babesia bovis, the etiological agent of bovine babesiosis. Aquaporins (AQPs) are water channel proteins implicated in physiological mechanisms of osmoregulation. Members of the AQP family are critical for blood-feeding arthropods considering the extreme osmoregulatory changes that occur during their feeding. We investigated the pattern of expression of a newly identified AQP2 gene of R. microplus (RmAQP2) in different tick tissues and stages. We also examined in vivo the biological implications of silencing expression of RmAQP2 silencing during tick feeding on either uninfected or B. bovis-infected cattle.

METHODS

In silico gene analyses were performed by multiple alignments of amino acid sequences and topology prediction. Levels of RmAQP2 transcripts in different tick tissues and stages were analyzed by reverse transcriptase quantitative PCR. Patterns of expression of RmAQP2 protein were investigated by immunoblots. Gene silencing was performed by RNA interference and in vivo functional analyses carried out by feeding ticks on either uninfected or B. bovis-infected cattle.

RESULTS

RmAQP2 transcripts were found in unfed larvae, engorged nymphs, and salivary glands and guts of partially engorged females; however, of all tick tissues and stages examined, RmAQP2 protein was found only in salivary glands of partially engorged females. RmAQP2 silencing significantly reduced tick fitness and completely abrogated protein expression. The effect of RmAQP2 silencing on fitness was more pronounced in females fed on a B. bovis-infected calf than in ticks fed on an uninfected calf and none of their larval progeny survived.

CONCLUSIONS

Collectively, considering the gene expression and tick fitness data, we conclude that RmAQP2 is critical for tick blood feeding and may be a suitable candidate target for the development of novel strategies to control R. microplus and tick-borne parasites.

A novel variant of aquaporin 3 is expressed in killifish (Fundulus heteroclitus) intestine

Killifish (Fundulus heteroclitus) are euryhaline teleosts that are widely used in environmental and toxicological studies, and they are tolerant to arsenic, in part due to very low assimilation of arsenic from the environment. The mechanism of arsenic uptake by the intestine, a major route of arsenic uptake in humans is unknown. Thus, the goal of this study was to determine if aquaglyceroporins (AQPs), which transport water and other small molecules including arsenite across cell membranes, are expressed in the killifish intestine, and whether AQP expression is affected by osmotic stress. Through RT-PCR and sequence analysis of PCR amplicons, we demonstrated that the intestine expresses kfAQP3a and kfAQP3b, two previously identified variants, and also identified a novel variant of killifish AQP3 (kfAQP3c) in the intestine. The variants likely represent alternate splice forms. A BLAST search of the F. heteroclitus reference genome revealed that the AQP3 gene resides on a single locus, while an alignment of the AQP3 sequence among 384 individuals from eight population ranging from Rhode Island to North Carolina revealed that its coding sequence was remarkably conserved with no fixed polymorphism residing in the region that distinguishes these variants. We further demonstrate that the novel variant transports arsenite into HEK293T cells. Whereas kfAQP3a, which does not transport arsenite, was expressed in both freshwater (FW) and saltwater (SW) acclimated fish, kfAQP3b, an arsenic transporter, was expressed only in FW acclimated fish, and kfAQP3c was expressed only in SW acclimated fish. Thus, we have identified a novel, putative splice variant of kfAQP3, kfAQP3c, which transports arsenic and is expressed only in SW acclimated fish.

Aquaporins, vasopressin, and aging: current perspectives

Functioning of the hypothalamic-neurohypophyseal-vasopressin axis is altered in aging, and the pathway may represent a plausible target to slow the process of aging. Arginine vasopressin, a nine-amino acid peptide that is secreted from the posterior pituitary in response to high plasma osmolality and hypotension, is central in this pathway. Vasopressin has important roles in circulatory and water homoeostasis mediated by vasopressin receptor subtypes V1a (vascular), V1b (pituitary), and V2 (vascular, renal). A dysfunction in this pathway as a result of aging can result in multiple abnormalities in several physiological systems. In addition, vasopressin plasma concentration is significantly higher in males than in females and vasopressin-mediated effects on renal and vascular targets are more pronounced in males than in females. These findings may be caused by sex differences in vasopressin secretion and action, making men more susceptible than females to diseases like hypertension, cardiovascular and chronic kidney diseases, and urolithiasis. Recently the availability of new, potent, orally active vasopressin receptor antagonists, the vaptans, has strongly increased the interest on vasopressin and its receptors as a new target for prevention of age-related diseases associated with its receptor-altered signaling. This review summarizes the recent literature in the field of vasopressin signaling in age-dependent abnormalities in kidney, cardiovascular function, and bone function.