Counteracting vasopressin-mediated water reabsorption by ATP, dopamine, and phorbol esters: mechanisms of action

Water homeostasis is regulated by a wide variety of hormones. When in need for water conservation, vasopressin, released from the brain, binds renal principal cells and initiates a signaling cascade resulting in the insertion of aquaporin-2 (AQP2) water channels in the apical membrane and water reabsorption. Conversely, hormones, including extracellular purines and dopamine, antagonize AVP-induced water permeability, but their mechanism of action is largely unknown, which was investigated here. Addition of these hormones to mpkCCD cells decreased total and plasma membrane abundance of AVP-induced AQP2, partly by increasing its internalization to vesicles and lysosomal degradation. This internalization was ubiquitin dependent, because the hormones increased AQP2 ubiquitination, and the plasma membrane localization of AQP2-K270R, which cannot be monoubiquitinated, was unaffected by these hormones. Both hormones also increased AQP2 phosphorylation at S261, which followed ubiquitination, but was not essential for hormone-induced AQP2 degradation. A similar process occurs in vivo, as incubation of dDAVP-treated kidney slices with both hormones also resulted in the internalization and S261 phosphorylation of AQP2. Both hormones also reduced cAMP and AQP2 mRNA levels, suggesting an additional effect on AQP2 gene transcription. Interestingly, phorbol esters only reduced AQP2 through the first pathway. Together, our results indicate that ATP and dopamine counteract AVP-induced water permeability by increasing AQP2 degradation in lysosomes, preceded by ubiquitin-dependent internalization, and by decreasing AQP2 gene transcription by reducing the AVP-induced cAMP levels.

Regulation of AQP2 localization by S256 and S261 phosphorylation and ubiquitination

Vasopressin-induced water reabsorption coincides with phosphorylation of aquaporin-2 (AQP2) at S256 (pS256), dephosphorylation at S261, and its translocation to the apical membrane, whereas treatment with the phorbol ester 12-tetradecanoylphorbol-13-acetate (TPA) induces AQP2 ubiquitination at K270, its internalization, and lysosomal degradation. In this study we investigated the relationship between S256 and S261 phosphorylation in AQP2 and its ubiquitination and trafficking in MDCK cells. Forskolin stimulation associated with increased pS256 and decreased pS261 AQP2, indicating that MDCK cells are a good model. After forskolin stimulation, TPA-induced ubiquitination of AQP2 preceded phosphorylation of AQP2 at S261, which in the first instance occurred predominantly on ubiquitinated AQP2. Forskolin-induced changes in pS261 were also observed for AQP2-S256A and AQP2-S256D, which constitutively localize in vesicles and the apical membrane, respectively. Although pS261 varies with forskolin as with wild-type AQP2, AQP2-S256A is not increased in its ubiquitination. Our data reveal that pS261 occurred independently of AQP2 localization and suggest that pS261 follows ubiquitination and endocytosis and may stabilize AQP2 ubiquitination and intracellular localization. The absence of increased ubiquitination of AQP2-S256A indicates that its intracellular location is due to the lack of pS256. Furthermore, AQP2-S261A and AQP2-S261D localized to vesicles, which was due to their increased ubiquitination, because changing K270 into Arg in both mutants resulted in their localization in the apical membrane. Although still increased in its ubiquitination, AQP2-S256D-S261D localized in the apical membrane. AQP2-S256D-K270R-Ub, however, localized to intracellular vesicles. Although our localization of AQP2-S261A/D is different from that of others, these data indicate that constitutive S256 phosphorylation counterbalances S261D-induced ubiquitination and internalization or changes its structure to allow distribution to the apical membrane. The vesicular localization of AQP2-S256D-K270R-Ub, however, indicates that the dominant apical sorting of S256D can again be overruled by constitutive ubiquitination. These data indicate that the membrane localization of AQP2 is determined by the balance of the extents of phosphorylation and ubiquitination.

A LabVIEW® based generic CT scanner control software platform

UGCT, the Centre for X-ray tomography at Ghent University (Belgium) does research on X-ray tomography and its applications. This includes the development and construction of state-of-the-art CT scanners for scientific research. Because these scanners are built for very different purposes they differ considerably in their physical implementations. However, they all share common principle functionality. In this context a generic software platform was developed using LabVIEW® in order to provide the same interface and functionality on all scanners. This article describes the concept and features of this software, and its potential for tomography in a research setting. The core concept is to rigorously separate the abstract operation of a CT scanner from its actual physical configuration. This separation is achieved by implementing a sender-listener architecture. The advantages are that the resulting software platform is generic, scalable, highly efficient, easy to develop and to extend, and that it can be deployed on future scanners with minimal effort.

Effect of the cGMP pathway on AQP2 expression and translocation: potential implications for nephrogenic diabetes insipidus

BACKGROUND

Arginine vasopressin (AVP) binding to the V2 receptor (V2R) in renal collecting duct principal cells induces a cAMP signalling cascade resulting in the activation of protein kinase A (PKA), translocation of aquaporin-2 (AQP2) to the apical membrane and an increase in AQP2 expression. Consequently, concentration of urine is initiated. X-linked nephrogenic diabetes insipidus (NDI), characterized by the inability to concentrate urine in response to AVP, is caused by mutations in the V2R gene. Initiation of AQP2 translocation, while circumventing the V2R-cAMP-PKA pathway has been suggested as a putative therapy for these patients. In this respect, the activation of a cAMP-independent and cGMP-dependent pathway for AQP2 membrane insertion by different cyclic guanosine monophosphate (cGMP) pathway activators, such as atrial natriuretic peptide (ANP), l-arginine and 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), has been put forward. However, it is unclear whether they can increase AQP2 expression.

METHODS

Mouse cortical collecting duct (mpkCCD) cells were incubated with ANP, l-arginine and 8-Br-cGMP for 2 h and subjected to immunocytochemistry and cell surface biotinylation assays to examine their effect on AQP2 translocation. To test the effect of cGMP pathway activators on AQP2 expression, the mpkCCD cells were treated with dDAVP, ANP and l-arginine for 4 days, or with 8-Br-cGMP for the last day. AQP2 protein levels were determined by immunoblotting.

RESULTS

ANP, l-arginine and 8-Br-cGMP induced the translocation of AQP2 in the mpkCCD cells. However, in contrast to dDAVP, ANP, l-arginine and 8-Br-cGMP did not increase the expression of AQP2.

CONCLUSIONS

Our results suggest that while activators of the cGMP pathway are likely beneficial in the treatment of X-linked NDI, their ability to relieve NDI in the patients may be improved when combined with agents stimulating AQP2 expression.

Realistic expectations: the changing role of paraprofessional health workers in First Nation communities in Canada

CONTEXT

First Nation communities in Canada rely on a mix of non-indigenous professionals and large numbers of indigenous paraprofessionals to deliver healthcare. Formerly allowed to perform controlled acts in emergencies, the direct care role of paraprofessionals is now restricted because of concerns about liability and accountability. As such, they are limited to health promotion and prevention activities.

OBJECTIVES

Focusing on the largest group of indigenous health workers, viz. Community Health Representatives (CHRs), for illustration purposes, this paper (1) examines the evolving role of First Nation health workers, and (2) discusses the proposed introduction of competency-based standards for their education, certification and regulation.

METHODS

The paper is informed by findings from open ended, semi-structured and focus group interviews, as well as qualitative survey data, derived from seven studies done in Ontario, Canada.

OUTCOMES

Paraprofessionals face conflicting and sometimes unrealistic expectations. Past practices have accustomed community members to hands-on care; however, professionals will no longer delegate tasks requiring clinical skills to them. Moreover, First Nation leaders are concerned about liability for their paraprofessional employees' actions. The paper discusses issues related to paraprofessional competence, preparation for practice, and continuing health education. It then presents the National Indian and Inuit Community Health Representatives Organization's proposal to establish a scope of practice and set of competencies that can form the basis for national practice and training standards, accreditation and regulation.

CONCLUSIONS

In Canada or elsewhere, changing practice environments may require adjustments in the roles played by indigenous health workers. The case of First Nation Community Health Representatives illustrates a strategy for role transformation.

Nucleotides downregulate aquaporin 2 via activation of apical P2 receptors

Vasopressin regulates water reabsorption in the collecting duct, but extracellular nucleotides modulate this regulation through incompletely understood mechanisms. We investigated these mechanisms using immortalized mouse collecting duct (mpkCCD) cells. Basolateral exposure to dDAVP induced AQP2 localization to the apical membrane, but co-treatment with ATP internalized AQP2. Because plasma membrane-bound P2 receptors (P2R) mediate the effects of extracellular nucleotides, we examined the abundance and localization of P2R in mpkCCD cells. In the absence of dDAVP, P2Y(1) and P2Y(4) receptors localized to the apical membrane, whereas P2X(2), P2X(4), P2X(5), P2X(7), P2Y(2), P2Y(11), and P2Y(12) receptors localized to the cytoplasm. dDAVP induced gene expression of P2X(1), which localized to the apical domain, and led to translocation of P2X(2) and P2Y(2) to the apical and basolateral membranes, respectively. In co-expression experiments, P2R activation decreased membrane AQP2 and AQP2-mediated water permeability in Xenopus oocytes expressing P2X(2), P2Y(2,) or P2Y(4) receptors, but not in oocytes expressing other P2R subtypes. In summary, these data suggest that AQP2-mediated water transport is downregulated not only by basolateral nucleotides, mediated by P2Y(2) receptors, but also by luminal nucleotides, mediated by P2X(2) and/or P2Y(4) receptors.

LIP5 interacts with aquaporin 2 and facilitates its lysosomal degradation

Vasopressin binding to the V2 receptor in renal principal cells leads to activation of protein kinase A, phosphorylation of aquaporin 2 (AQP2) at Ser256, and the translocation of AQP2 to the apical membrane, resulting in concentration of the urine. In contrast, phorbol ester-induced activation of protein kinase C pathway leads to ubiquitination of AQP2 at Lys270 and its internalization to multivesicular bodies, where it is targeted for lysosomal degradation or stored for recycling. Because little is known about the regulation of AQP2 trafficking, we used the carboxy-terminal tail of constitutively nonphosphorylated AQP2 (S256A) as a bait for interacting proteins in a yeast two-hybrid assay. We isolated lysosomal trafficking regulator-interacting protein 5 (LIP5) and found that LIP5 interacted with the proximal carboxy-terminal tail (L230-D243) of AQP2 in vitro but not with AQP3 or AQP4, which are also expressed in principal cells. Immunohistochemistry revealed that LIP5 co-localized with AQP2 in principal cells. LIP5 binding occurred independent of the state of Ser256 phosphorylation or Lys270 ubiquitination. LIP5 has been shown to facilitate degradation of the EGF receptor; here, LIP5 seemed to bind this receptor. Knockdown of LIP5 in mouse renal cells (mpkCCD) reduced the phorbol ester-induced degradation of AQP2 approximately two-fold. In summary, LIP5 binds cargo proteins and, considering the role of LIP5 in protein sorting to multivesicular bodies, plays a role in the degradation of AQP2, possibly by reducing the formation of late endosomes.

Congenital nephrogenic diabetes insipidus: what can we learn from mouse models?

Aquaporins (AQPs) are central players in mammalian physiology, allowing efficient water transport through cellular membranes. To date, 13 different aquaporins have been identified in mammals (AQP0-AQP12). Knocking out genes in mice and identification of mutations in the human genes provided important information on the role of AQPs in normal physiology. While the physiological role of many AQPs only becomes clear when the putative function is challenged, the lack of AQP2 directly results in a disease phenotype. Aquaporin 2 is highly expressed in the principal cells of the renal collecting duct, where it shuttles between intracellular storage vesicles and the apical membrane. Upon hypernatraemia or hypovolaemia, the antidiuretic hormone vasopressin (AVP) is released from the pituitary into blood and binds to its type 2 receptor on renal principal cells. This initiates a cAMP signalling cascade resulting in the translocation of AQP2-bearing vesicles to the apical membrane. Subsequently, pro-urinary water reabsorption and urine concentration occurs. This process is reversed by a reduction in circulating AVP levels, which is obtained with the establishment of isotonicity. In humans, mutations in the AQP2 gene cause congenital nephrogenic diabetes insipidus (NDI), a disorder characterized by an inability to concentrate urine in response to vasopressin. Until the recent development of several congenital NDI mouse models, our knowledge on AQP2 regulation was primarily based on in vitro studies. This review focuses on the similarities between the in vitro and in vivo studies and discusses new insights into congenital NDI obtained from the mouse models.

Physiology and pathophysiology of the vasopressin-regulated renal water reabsorption

To prevent dehydration, terrestrial animals and humans have developed a sensitive and versatile system to maintain their water homeostasis. In states of hypernatremia or hypovolemia, the antidiuretic hormone vasopressin (AVP) is released from the pituitary and binds its type-2 receptor in renal principal cells. This triggers an intracellular cAMP signaling cascade, which phosphorylates aquaporin-2 (AQP2) and targets the channel to the apical plasma membrane. Driven by an osmotic gradient, pro-urinary water then passes the membrane through AQP2 and leaves the cell on the basolateral side via AQP3 and AQP4 water channels. When water homeostasis is restored, AVP levels decline, and AQP2 is internalized from the plasma membrane, leaving the plasma membrane watertight again. The action of AVP is counterbalanced by several hormones like prostaglandin E2, bradykinin, dopamine, endothelin-1, acetylcholine, epidermal growth factor, and purines. Moreover, AQP2 is strongly involved in the pathophysiology of disorders characterized by renal concentrating defects, as well as conditions associated with severe water retention. This review focuses on our recent increase in understanding of the molecular mechanisms underlying AVP-regulated renal water transport in both health and disease.

Short-chain ubiquitination mediates the regulated endocytosis of the aquaporin-2 water channel

To regulate mammalian water homeostasis, arginine-vasopressin (AVP) induces phosphorylation and thereby redistribution of renal aquaporin-2 (AQP2) water channels from vesicles to the apical membrane. Vice versa, AVP (or forskolin) removal and hormones activating PKC cause AQP2 internalization, but the mechanism is unknown. Here, we show that a fraction of AQP2 is modified with two to three ubiquitin moieties in vitro and in vivo. Mutagenesis revealed that AQP2 is ubiquitinated with one K63-linked chain at K270 only. In Madin-Darby canine kidney cells, AQP2 ubiquitination occurs preferentially when present in the apical membrane, is transiently increased with forskolin removal or PKC activation, and precedes its internalization. Internalization kinetics assays with wild type (wt) and ubiquitination-deficient (K270R) AQP2 revealed that ubiquitination enhances AQP2 endocytosis. Electron microscopy showed that a translational fusion of AQP2 with ubiquitin (AQP2-Ub) localized particularly to internal vesicles of multivesicular bodies (MVBs), whereas AQP2-K270R largely localized to the apical membrane, early endosomes, and the limiting membrane of MVBs. Consistent with this distribution pattern, lysosomal degradation was extensive for AQP2-Ub, low for AQP2-K270R, and intermediate for wt-AQP2. Our data show that short-chain ubiquitination is involved in the regulated endocytosis, MVB sorting, and degradation of AQP2 and may be the mechanism used by AVP removal and PKC-activating hormones to reduce renal water reabsorption. Moreover, because several other channels are also (short-chain) ubiquitinated, our data suggest that ubiquitination may be a general mediator for the regulated endocytosis and degradation of channels in higher eukaryotes.

Fibronectin is a hypoxia-independent target of the tumor suppressor VHL

The von Hippel-Lindau (VHL) tumor suppressor gene regulates the extracellular matrix by controlling fibronectin deposition. To identify novel VHL target genes, we subjected mRNA from VHL-deficient RCC cells (786-0-pRC) and a transfectant re-expressing wildtype VHL (786-0-VHL) to differential expression profiling. Among the differentially expressed genes, we detected that fibronectin is upregulated in the presence of VHL, while it is not affected by hypoxia. Thus regulation of fibronectin deposition by VHL occurs at the transcriptional level, irrespective of oxygen levels.

Two cases of cross-sensitivity in subjects allergic to paraphenylenediamine following ingestion of Polaronil

We report the cases of 2 women presenting allergy to paraphenylenediamine (PPD). Both patients had a history of eczema that worsened following the ingestion of the antihistamine Polaronil (dexamethasone/dexchlorpheniramine). This clinical presentation could be explained by cross-sensitivity to sulfanilic acid (4-aminobenzene sulfonic acid), a metabolite of sunset yellow (FD&C No. 6). Sunset yellow is an azo dye present in this tablet. Indeed, PPD-allergic subjects may suffer from cross-sensitivity to related compounds, especially to those that can be ingested such as azo dyes. Such compounds are used in some instances by the food and pharmaceutical industries, but their presence is often undisclosed.

BLAST Search Updater: a notification system for new database matches

BLAST Search Updater (BSU) can help molecular biologists perform large numbers of BLAST searches routinely and screen the results for novel matches. The search result files (HTML format) and delta files (novel matches) are WWW viewable and BSU also emails users alignments of new matches.

AVAILABILITY

BSU runs on UNIX machines and is freely available to academic users at http://athena.bioc.uvic.ca/genomes/. Additional figures and description are also available.

CONTACT

cupton@uvic.ca

Viral genome organizer: a system for analyzing complete viral genomes

The viral genome organizer (VGO) is designed to simplify the characterization and annotation of complete viral genomes (particularly those of large poxviruses) and to help researchers discover new genes and detect gene fragmentation. VGO is based on Genotator [Harris, N.L., 1997. Genome Res. 7, 754-762], an annotation workbench designed for the analysis of eukaryotic genomic sequences. VGO automates a number of database search routines (FASTA, BLASTP, PSI-BLAST and TBLASTN), processes the results through a multiple-alignment viewer (MView; [Brown, N.P., Leroy, C., Sander, C. , 1998. Bioinformatics 14, 380-381]) and serves to manage the hundreds of DNA, protein and database search results files that must be organized when dealing with large complete poxviral genomes. It also directs the generation a self-dotplot of the genome by Dotter [Sonnhammer, E.L.L., Durbin, R., 1995. A dot-matrix program with dynamic threshold control suited for genomic DNA and protein sequence analysis. Gene 167: GC1-10. http://www.sanger.ac. uk/Software/Dotter/] to uncover repeated genes and sequences and provides Internet links to programs for generation of restriction maps and analysis of potential PCR primers. The user-friendly graphical interface displays DNA and protein sequences, links to search results, ORFs, stop-start codons, restriction sites and flags of database searches. Currently, VGO and associated programs run in an X-windows environment on commonly available UNIX machines.

Adhesion molecule profiles in atopic dermatitis vs. allergic contact dermatitis: pharmacological modulation by cetirizine

BACKGROUND

Experimental data suggest that there is an imbalance between Th1 and Th2 cells in atopic dermatitis (AD) skin compared to allergic contact dermatitis (ACD). This imbalance (Th2 and Th1 predominance, respectively) implies the production of different cytokines in these two conditions leading to different expression of adhesion molecules on skin endothelial cells.

OBJECTIVE

The expression of VCAM-1 (IL-4/Th2-dependent) and ICAM-1 (INF-gamma/IL-1) on dermal vessels was compared in six patients with AD and six patients with ACD. The effect of cetirizine, a highly selective H1-receptor antagonist on the expressions was studied.

METHODS

Six patients with AD were challenged with Dermatophagoides pteronyssimus (DPT patch tests applied to clinically normal skin) and six patients with ACD challenged in the same way with allergens of the European standard series. Skin biopsies at challenged sites were performed before and 6, 24 and 48 h after challenge. The experiment was carried out under double-blind cross-over conditions during a 4-day treatment with a placebo and cetirizine.

RESULTS

In AD patients, the scores for both VCAM-1 and ICAM-1 were high before and after challenge. In ACD patients, the ICAM-1 score was high at each experimental time, but the VCAM-1 score, which was significantly lower before challenge, increased at 6, 24 and 48 h after challenge. The administration of cetirizine significantly reduced the VCAM-1 expression in AD patients at each experimental time.

CONCLUSION

It is concluded that the increased VCAM-1 expression in AD patients compared to ACD may reflect greater IL-4 and/or IL-13 production in situ. The study also confirms the existence of a modulating effect of cetirizine in vivo on adhesion molecule expression.

Strength through sharing: interdisciplinary teamwork in providing health and social services to northern native communities

The delivery of health and social services in Canada's northern First Nations is undermined by the fact that professionals from outside and para-professionals from the communities often fail to respect one another's capabilities or to understand one another's roles and, consequently, do not work well together. This paper explores the personal, professional, and situational causes, using examples of mental health care in the Sioux Lookout Zone of northwestern Ontario. Arguing that an interdisciplinary team approach is the ideal and, perhaps, the only real way in which essential services can be delivered, the authors suggest ways to achieve more effective collaboration.

Digital image analysis of the effect of topically applied linoleic acid on acne microcomedones

A major pathogenic factor of acne is the disturbed keratinization of the follicular infundibulum. It has been hypothesized that a relative decrease in linoleic acid in the sebum could be responsible, in part, for this. The aim of the present study was objectively to evaluate the effects of topically applied linoleic acid on the size of microcomedones in patients with mild acne. The design was a double-blind placebo-controlled randomized cross-over study. Evaluations were performed by digital image analysis of cyanoacrylate follicular biopsies. There was a significant effect of topically applied linoleic acid on the size of follicular casts and microcomedones, an almost 25% reduction in their overall size being achieved over a 1-month treatment period. In contrast, no change was found at placebo-treated sites. It is concluded that topical linoleic acid might play a role as a comedolytic agent in acne-prone patients.