In vivo inhibition of transcellular water channels (aquaporin-1) during acute peritoneal dialysis in rats

AQP1-Containing Exosomes in Peritoneal Dialysis Effluent As Biomarker of Dialysis Efficiency

The water channel Aquaporin 1 (AQP1) plays a fundamental role in water ultrafiltration during peritoneal dialysis (PD) and its reduced expression or function may be responsible for ultrafiltration failure (UFF). In humans, AQP1 is expressed in the endothelium of the peritoneal capillaries but its expression in mesothelial cells (MC) and its functional role in PD is still being debated. Here, we studied a cohort of 30 patients using PD in order to determine the presence of AQP1 in peritoneal biopsies, AQP1 release in the PD effluent through exosomes and the correlation of AQP1 abundance with the efficiency of peritoneal ultrafiltration. The experiments using immunofluorescence showed a strong expression of AQP1 in MCs. Immunoblotting analysis on vesicles isolated from PD effluents showed a consistent presence of AQP1, mesothelin and Alix and the absence of the CD31. Thus, this suggests that they have an exclusive mesothelial origin. The immunoTEM analysis showed a homogeneous population of nanovesicles and confirmed the immunoblotting results. Interestingly, the quantitative analysis by ELISA showed a positive correlation between AQP1 in the PD effluent and ultrafiltration (UF), free water transport (FWT) and Na-sieving. This evidence opens the discussion on the functional role of mesothelial AQP1 during PD and suggests that it may represent a potential non-invasive biomarker of peritoneal barrier integrity, with predictive potential of UFF in PD patients.

Novel Endothelial Cell-Specific AQP1 Knockout Mice Confirm the Crucial Role of Endothelial AQP1 in Ultrafiltration during Peritoneal Dialysis

The water channel aquaporin-1 (AQP1) mediates about 50% ultrafiltration during a 2-hour hypertonic dwell in global AQP1 knockout (AQP1^-/-) mice. Although AQP1 is widely expressed in various cell types including mesothelial cells, the ultrafiltration has been assumed to be mediated via endothelial AQP1 of the peritoneum. The partial embryonic lethality and reduced body weight in AQP1^-/- mice may reflect potential confounding phenotypic effects evoked by ubiquitous AQP1 deletion, which may interfere with functional analysis of endothelial AQP1. Using a Cre/loxP approach, we generated and characterised endothelial cell- and time-specific AQP1 knockout (AQP1^fl/fl; Cdh5-Cre⁺) mice. Compared to controls, AQP1^fl/fl; Cdh5-Cre⁺ mice showed no difference in an initial clinical and biological analysis at baseline, including body weight and survival. During a 1-hour 3.86% mini-peritoneal equilibration test (mini-PET), AQP1^fl/fl; Cdh5-Cre⁺ mice exhibited strongly decreased indices for AQP1-related transcellular water transport (43.0% in net ultrafiltration, 93.0% in sodium sieving and 57.9% in free water transport) compared to controls. The transport rates for small solutes of urea and glucose were not significantly altered. Our data provide the first direct experimental evidence for the functional relevance of endothelial AQP1 to the fluid transport in peritoneal dialysis and thereby further validate essential predictions of the three-pore model of peritoneal transport.

Can Free Water Transport Be Used as a Clinical Parameter for Peritoneal Fibrosis in Long-Term PD Patients?

Sodium sieving in peritoneal dialysis (PD) occurs in a situation with high osmotically-driven ultrafiltration rates. This dilutional phenomenon is caused by free water transport through the water channel aquaporin-1. It has recently been described that encapsulating peritoneal fibrosis is associated with impaired free water transport, despite normal expression of aquaporin-1. In this review, it will be argued that free water transport can be used for assessment of fibrotic peritoneal alterations, due to the water-binding capacity of collagen. Finally, the consequences for clinical practice will be discussed.

Blood cells and endothelial barrier function

The barrier properties of endothelial cells are critical for the maintenance of water and protein balance between the intravascular and extravascular compartments. An impairment of endothelial barrier function has been implicated in the genesis and/or progression of a variety of pathological conditions, including pulmonary edema, ischemic stroke, neurodegenerative disorders, angioedema, sepsis and cancer. The altered barrier function in these conditions is often linked to the release of soluble mediators from resident cells (e.g., mast cells, macrophages) and/or recruited blood cells. The interaction of the mediators with receptors expressed on the surface of endothelial cells diminishes barrier function either by altering the expression of adhesive proteins in the inter-endothelial junctions, by altering the organization of the cytoskeleton, or both. Reactive oxygen species (ROS), proteolytic enzymes (e.g., matrix metalloproteinase, elastase), oncostatin M, and VEGF are part of a long list of mediators that have been implicated in endothelial barrier failure. In this review, we address the role of blood borne cells, including, neutrophils, lymphocytes, monocytes, and platelets, in the regulation of endothelial barrier function in health and disease. Attention is also devoted to new targets for therapeutic intervention in disease states with morbidity and mortality related to endothelial barrier dysfunction.

Regulation of macrophage motility by the water channel aquaporin-1: crucial role of M0/M2 phenotype switch

The water channel aquaporin-1 (AQP1) promotes migration of many cell types. Although AQP1 is expressed in macrophages, its potential role in macrophage motility, particularly in relation with phenotype polarization, remains unknown. We here addressed these issues in peritoneal macrophages isolated from AQP1-deficient mice, either undifferentiated (M0) or stimulated with LPS to orientate towards pro-inflammatory phenotype (classical macrophage activation; M1). In non-stimulated macrophages, ablation of AQP1 (like inhibition by HgCl2) increased by 2-3 fold spontaneous migration in a Src/PI3K/Rac-dependent manner. This correlated with cell elongation and formation of lamellipodia/ruffles, resulting in membrane lipid and F4/80 recruitment to the leading edge. This indicated that AQP1 normally suppresses migration of resting macrophages, as opposed to other cell types. Resting Aqp1-/- macrophages exhibited CD206 redistribution into ruffles and increased arginase activity like IL4/IL13 (alternative macrophage activation; M2), indicating a M0-M2 shift. In contrast, upon M1 orientation by LPS in vitro or peritoneal inflammation in vivo, migration of Aqp1-/- macrophages was reduced. Taken together, these data indicate that AQP1 oppositely regulates macrophage migration, depending on stimulation or not by LPS, and that macrophage phenotypic and migratory changes may be regulated independently of external cues.

Cardiovascular sex differences influencing microvascular exchange

The vital role of the cardiovascular (CV) system is maintenance of body functions via the matching of exchange to tissue metabolic demand. Sex-specific differences in the regulatory mechanisms of CV function and the metabolic requirements of men and women, respectively, have been identified and appreciated. This review focuses on sex differences of parameters influencing exchange at the point of union between blood and tissue, the microvasculature. Microvascular architecture, blood pressure (hydrostatic and oncotic), and vascular permeability, therefore, are discussed in the specific context of sex in health and disorders. It is notable that when sex differences exist, they are generally subtle but significant. In the aggregate, though, they can give rise to profoundly different phenotypes. The postulated mechanisms responsible for sex differences are attributed to genomics, epigenetics, and sex hormones. Depending on specific circumstances, the effect of the combined factors can range from insignificant to lethal. Identifying and understanding key signalling mechanisms bridging genomics/sex hormones and microvascular exchange properties within the scope of this review holds significant promise for sex-specific prevention and treatment of vascular barrier dysfunction.

Dissecting the roles of aquaporins in renal pathophysiology using transgenic mice

Transgenic mice lacking renal aquaporins (AQPs), or containing mutated AQPs, have been useful in confirming anticipated AQP functions in renal physiology and in discovering new functions. Mice lacking AQPs 1-4 manifest defects in urinary concentrating ability to different extents. Mechanistic studies have confirmed the involvement of AQP1 in near-isosmolar fluid absorption in the proximal tubule, and in countercurrent multiplication and exchange mechanisms that produce medullary hypertonicity in the antidiuretic kidney. Deletion of AQPs 2-4 impairs urinary concentrating ability by reduction of transcellular water permeability in the collecting duct. Recently created transgenic mouse models of nephrogenic diabetes insipidus produced by AQP2 gene mutation offer exciting possibilities to test new drug therapies. Several unanticipated AQP functions in kidney have been discovered recently that are unrelated to their role in transcellular water transport. There is evidence for involvement of AQP1 in kidney cell migration after renal injury, of AQP7 in renal glycerol clearance, of AQP11 in prevention of renal cystic disease, and possibly of AQP3 in regulation of collecting duct cell proliferation. Future work in renal AQPs will focus on mechanisms responsible for these non-fluid-transporting functions, and on the development of small-molecule AQP inhibitors for use as aquaretic-type diuretics.

Monitoring of the peritoneal membrane

Background. Indirect methods can be used to provide valuable information about peritoneal structure and function for the indirect analysis of peritoneal membrane. Methods. The focus of this paper will be on the commonly available tools for this purpose. First, the value and clinical relevance of CA125 as a marker of mesothelial cell mass in peritoneal effluent will be evaluated. Thereafter, monitoring the peritoneal membrane by using its properties to transport solutes and water will be discussed. Results. The data obtained can be useful for tailoring dialysis adequacy, analysis of clinical problems such as ultrafiltration failure or to predict the development of peritoneal sclerosis.

Clinical application of aquaporin research: aquaporin-1 in the peritoneal membrane

Peritoneal dialysis (PD) is an established mode of renal replacement therapy based on the exchange of fluid and solutes between blood and a dialysate that has been instilled in the peritoneal cavity. The dialysis process involves osmosis, as well as diffusive and convective transports through the highly vascularized peritoneal membrane. Computer simulations predicted that the membrane contains ultrasmall pores responsible for the selective transport of water across the capillary endothelium during crystalloid osmosis. The distribution of the water channel aquaporin-1 (AQP1), as well as its molecular structure ensuring an exquisite selectivity for water, fit with the characteristics of the ultrasmall pore. Peritoneal transport studies using AQP1 knockout mice demonstrated that the osmotic water flux across the peritoneal membrane is mediated by AQP1. This water transport accounts for 50% of the ultrafiltration during PD. Treatment with high-dose corticosteroids upregulates the expression of AQP1 in peritoneal capillaries, resulting in increased water transport and ultrafiltration in rats. AQP1 may also play a role during inflammation, as vascular proliferation and leukocyte recruitment are both decreased in mice lacking AQP1. These data illustrate the potential of the peritoneal membrane as an experimental model in the investigation of the role of AQP1 in the endothelium at baseline and during inflammation. They emphasize the critical role of AQP1 during PD and suggest that manipulating AQP1 expression could be clinically useful in PD patients.

Transvascular protein transport in mice lacking endothelial caveolae

Caveolae are Ω-shaped vesicular structures postulated to play a role in transvascular protein transport. Studies on mice lacking endothelial caveolae, caveolin-1 knockout (Cav-1-KO) mice, indicate increased macromolecular transport rates. This was postulated to be due to the appearance of an alternative pathway. The present study tested whether an alternative pathway had appeared in Cav-1-KO mice. Male Cav-1-KO ( n = 12) and male control mice ( n = 13) were intubated and anesthetized using 2% isoflurane.125I-labeled albumin,131I-labeled immunoglobulin M (IgM), and polydisperse FITC-Ficoll were administered intravenously. During tracer administration, a 90-min peritoneal dialysis dwell was performed. Clearance of tracers to dialysate and permeability-surface area product for glucose were assessed. Transvascular protein transport was higher in Cav-1-KO compared with control mice. Albumin clearance from plasma to peritoneum was 0.088 ± 0.008 μl/min in control and 0.179 ± 0.012 μl/min in Cav-1-KO ( P = 0.001) mice. IgM clearance was 0.049 ± 0.003 and 0.083 ± 0.010 μl/min in control and Cav-1-KO mice, respectively ( P = 0.016). Ficoll clearance was increased in Cav-1-KO mice. In conclusion, the lack of caveolae in Cav-1-KO mice resulted in a marked increase in macromolecular transport. A two-pore analysis of the Ficoll clearance data revealed that the higher transport rate in Cav-1-KO mice was not compatible with the appearance of an alternative pathway for macromolecular transport. In contrast, the higher transperitoneal protein and Ficoll clearance is consistent with passive porous transport through an unperturbed two-pore system, presumably at an elevated capillary hydraulic pressure. Alternatively, the data may be explained by reductions in the selectivity of the endothelial glycocalyx, leading to an increased capillary hydraulic conductivity and large solute filtration.

A detailed analysis of sodium removal by peritoneal dialysis: comparison with predictions from the three-pore model of membrane function

BACKGROUND

The development of fluid and salt retention is a potential problem for all peritoneal dialysis (PD) patients. Sodium removal by the peritoneum is predominantly determined by convective fluid loss but influenced by diffusion and sieving due to free water transport as predicted by the three-pore model (TPM). The aim of the study was to establish the effect of transport status, dwell length and glucose concentration on observed ultrafiltration (UF), dialysate sodium concentration ([Na(+)](D)) and removal, and compare this with that predicted by a computer program based on the principles of the TPM.

METHODS

This was a cross-sectional study of UF and [Na(+)](D) collected prospectively from dwells classified by length, glucose concentration and membrane transport characteristics. Solute transport, converted to area parameter and UF capacity, was measured on each occasion by the peritoneal equilibration test. These parameters, along with plasma [Na(+)], were entered into the computer model. Fixed values for other parameters, e.g. hydraulic conductance and lymphatic absorption and sump volume, were used.

RESULTS

A total of 1853 dwells from 182 patients [10% were on automated PD (APD)] were analysed. There was a high degree of correlation (r = 0.83-95, P<0.001) between the observed and predicted values for UF, [Na(+)](D) and sodium removal across the full range of dwell categories. The model overpredicted UF as the net volume increased with increasing glucose concentration, independently of solute transport. This bias was not fully explained by the preferential use of hypertonic dialysate by patients with reduced UF capacity. The prediction of [Na(+)](D) described sodium sieving, which was overestimated in a small number of patients with UF failure. There were no discrepancies between continous ambulatory PD (CAPD) and APD patients.

CONCLUSION

This analysis endorses the TPM as a description of membrane function, particularly in relation to sodium sieving and removal. The relationship between dialysate glucose concentration and achieved UF appears to be more complex; even accounting for extended time on treatment and reduction in the osmotic conductance in patients preferentially using hypertonic exchanges, further adjustments may be needed to account for the tendency to overestimate UF.

The transport barrier in intraperitoneal therapy

The peritoneal cavity is important in clinical medicine because of its use as a portal of entry for drugs utilized in regional chemotherapy and as a means of dialysis for anephric patients. The barrier between the therapeutic solution in the cavity and the plasma does not correspond to the classic semipermeable membrane but instead is a complex structure of cells, extracellular matrix, and blood microvessels in the surrounding tissue. New research on the nature of the capillary barrier and on the orderly array of extracellular matrix molecules has provided insights into the physiological basis of osmosis and the alterations in transport that result from infusion of large volumes of fluid. The anatomic peritoneum is highly permeable to water, small solutes, and proteins and therefore is not a physical barrier. However, the cells of the mesothelium play an essential role in the immune response in the cavity and produce cytokines and chemokines in response to contact with noncompatible solutions. The process of inflammation, which depends on the interaction of mesothelial, interstitial, and endothelial cells, ultimately leads to angiogenesis and fibrosis and the functional alteration of the barrier. New animal models, such as the transgenic mouse, will accelerate the discovery of methods to preserve the functional peritoneal barrier.

Expression of aquaporin-3 in human peritoneal mesothelial cells and its up-regulation by glucose in vitro

BACKGROUND

Aquaporin-3 (AQP3) is a member of the water channel family that is selective for the passage of not only water, but also glycerol and urea. Our recent study demonstrated the presence of aquaporin-1 in human peritoneal mesothelial cells (HPMC). Although transcripts encoding for AQP3 has been detected by reverse transcription-polymerase chain reaction (RT-PCR) in murine peritoneal mesothelium, to date there is no documentation of protein expression on peritoneal mesothelial cells.

METHOD

Our present study was designed to explore the gene and protein expression of AQP3 in HPMC and its regulation under different concentrations of glucose.

RESULTS

AQP3 protein was detected in the human peritoneal tissue by immunohistological staining using specific, affinity-purified polyclonal anti-AQP3 antibodies. AQ3 transcripts and protein expression in cultured HPMC were investigated by RT-PCR and immunoblotting analysis respectively. Cell permeability to glycerol (flux) was measured using [(14)C]glycerol incorporation. AQP3 transcript and protein were weakly expressed in HPMC constitutively. The gene expression of AQP3 and its protein biosynthesis in HPMC were inducible following exposure to glucose in a dose- and time-dependent manner (P < 0.0001). Glucose at a concentration of 200 mmol induced glycerol flux by 4.82-fold above the control value (P < 0.0001) and its effect was significantly inhibited by mercuric chloride (P < 0.01).

CONCLUSION

Our novel observation demonstrated the AQP3 expression and biosynthesis in HPMC and in vitro studies revealed that glycerol permeability in HPMC was up-regulated by glucose. Further study is warranted to elucidate the role of AQP3 in HPMC for maintaining the ultrafiltration of the peritoneal membrane.

Aquaporin water channels and endothelial cell function

The aquaporins (AQP) are a family of homologous water channels expressed in many epithelial and endothelial cell types involved in fluid transport. AQP1 protein is strongly expressed in most microvascular endothelia outside of the brain, as well as in endothelial cells in cornea, intestinal lacteals, and other tissues. AQP4 is expressed in astroglial foot processes adjacent to endothelial cells in the central nervous system. Transgenic mice lacking aquaporins have been useful in defining their role in mammalian physiology. Mice lacking AQP1 manifest defective urinary concentrating ability, in part because of decreased water permeability in renal vasa recta microvessels. These mice also show a defect in dietary fat processing that may involve chylomicron absorption by intestinal lacteals, as well as defective active fluid transport across the corneal endothelium. AQP1 might also play a role in tumour angiogenesis and in renal microvessel structural adaptation. However, AQP1 in most endothelial tissues does not appear to have a physiological function despite its role in osmotically driven water transport. For example, mice lacking AQP1 have low alveolar-capillary water permeability but unimpaired lung fluid absorption, as well as unimpaired saliva and tear secretion, aqueous fluid outflow, and pleural and peritoneal fluid transport. In the central nervous system mice lacking AQP4 are partially protected from brain oedema in water intoxication and ischaemic models of brain injury. Therefore, although the role of aquaporins in epithelial fluid transport is in most cases well-understood, there remain many questions about the role of aquaporins in endothelial cell function. It is unclear why many leaky microvessels strongly express AQP1 without apparent functional significance. Improved understanding of aquaporin-endothelial biology may lead to novel therapies for human disease, such as pharmacological modulation of corneal fluid transport, renal fluid clearance and intestinal absorption.

Aquaporins in the kidney: from molecules to medicine

The discovery of aquaporin-1 (AQP1) answered the long-standing biophysical question of how water specifically crosses biological membranes. In the kidney, at least seven aquaporins are expressed at distinct sites. AQP1 is extremely abundant in the proximal tubule and descending thin limb and is essential for urinary concentration. AQP2 is exclusively expressed in the principal cells of the connecting tubule and collecting duct and is the predominant vasopressin-regulated water channel. AQP3 and AQP4 are both present in the basolateral plasma membrane of collecting duct principal cells and represent exit pathways for water reabsorbed apically via AQP2. Studies in patients and transgenic mice have demonstrated that both AQP2 and AQP3 are essential for urinary concentration. Three additional aquaporins are present in the kidney. AQP6 is present in intracellular vesicles in collecting duct intercalated cells, and AQP8 is present intracellularly at low abundance in proximal tubules and collecting duct principal cells, but the physiological function of these two channels remains undefined. AQP7 is abundant in the brush border of proximal tubule cells and is likely to be involved in proximal tubule water reabsorption. Body water balance is tightly regulated by vasopressin, and multiple studies now have underscored the essential roles of AQP2 in this. Vasopressin regulates acutely the water permeability of the kidney collecting duct by trafficking of AQP2 from intracellular vesicles to the apical plasma membrane. The long-term adaptational changes in body water balance are controlled in part by regulated changes in AQP2 and AQP3 expression levels. Lack of functional AQP2 is seen in primary forms of diabetes insipidus, and reduced expression and targeting are seen in several diseases associated with urinary concentrating defects such as acquired nephrogenic diabetes insipidus, postobstructive polyuria, as well as acute and chronic renal failure. In contrast, in conditions with water retention such as severe congestive heart failure, pregnancy, and syndrome of inappropriate antidiuretic hormone secretion, both AQP2 expression levels and apical plasma membrane targetting are increased, suggesting a role for AQP2 in the development of water retention. Continued analysis of the aquaporins is providing detailed molecular insight into the fundamental physiology and pathophysiology of water balance and water balance disorders.

Expression of aquaporin-1 in human peritoneal mesothelial cells and its upregulation by glucose in vitro

Aquaporin (AQP) is a family of water channels that are highly selective for the passage of water and occasionally glycerol. In previous studies, only AQP1 was found in human peritoneal endothelial cells in both control subjects and patients on peritoneal dialysis. As human peritoneal mesothelial cells (HPMC) play an important role in dialysis adequacy and fluid balance in continuous ambulatory peritoneal dialysis patients, this study examined whether AQP1 is present in HPMC. It was found that AQP1 mRNA and protein are present in HPMC constitutively. The localization of AQP1 protein in peritoneal mesothelial cells was confirmed by double immunohistochemical staining of the mesothelial lining of human peritoneal membrane. More important, the expression of AQP1 in HPMC is not constitutive and the transcription and biosynthesis of AQP1 in HPMC is inducible by osmotic agents such as glucose and mannitol. There was significant enhancement of AQP1 biosynthesis upon exposure to glucose in a time- and dose-dependent manner (P < 0.0001). Similar findings were observed in the AQP1 biosynthesis by an endothelial cell line, EA.hy 926. Of particular interest, the upregulation in AQP1 mRNA or biosynthesis in mesothelial cells was always significantly higher than that of endothelial cells when the experiments were conducted under identical settings (P < 0.001). AQP1 expression in HPMC was demonstrated for the first time. Osmotic agents upregulate both mRNA and protein expression of this aquaporin. The role of AQP1 in HPMC in maintaining the ultrafiltration of the peritoneal membrane is potentially of clinical interest.

Transport asymmetry in peritoneal dialysis: application of a serial heteroporous peritoneal membrane model

The transport of macromolecules during peritoneal dialysis is highly selective when they move from blood to dialysate but nearly completely unselective in the opposite direction. Aiming at describing this asymmetry, we modeled the peritoneal barrier as a series arrangement of two heteroporous membranes. First a three-pore membrane was considered, crossed by small [radius of the small pore (r(s)) approximately 45 A], large [radius of the large pore (r(L)) approximately 250 A], and transcellular pores accounting for 90, 8, and 2% to the hydraulic conductance, respectively, and with a corresponding pore area over diffusion distance (A(0)/Delta x) set to 50,000 cm. We calculated the second membrane parameters by fitting simultaneously the bidirectional clearance of molecules ranging from sucrose [molecular weight = 360, permeating solute radius (a(e)) approximately 5 A] to alpha(2)-macroglobulin (molecular weight = 820,000, a(e) approximately 90 A). The results describe a second two-pore membrane with very large pores (r(L) approximately 2,300 A) accounting for 95% of the hydraulic conductance, minor populations of small (r(s) approximately 67 A) and transcellular pores (3 and 2%, respectively), and an A(0)/Delta x approximately 65,000 cm. The estimated peritoneal lymph flow is approximately 0.3 ml/min. The two membranes can be identified as the capillary endothelium and an extracellular interstitium lumped with the peritoneal mesothelium.