Effects of sitagliptin on peritoneal membrane: The potential role of mesothelial cell tight junction proteins

BACKGROUND

The roles of tight junction (TJ) proteins in peritoneal membrane transport and peritoneal dialysis (PD) require further characterisation. Dipeptidyl peptidase-4 is expressed in mesothelial cells, and its activity may affect peritoneal membrane function and morphology.

METHODS

Human peritoneal mesothelial cells (HPMCs) were isolated and cultured from omentum obtained during abdominal surgery, and paracellular transport functions were evaluated by measuring transmesothelial electrical resistance (TMER) and dextran flux. Sprague-Dawley rats were infused daily with 4.25% peritoneal dialysate with and without sitagliptin administration for 8 weeks. At the end of this period, rat peritoneal mesothelial cells (RPMCs) were isolated to evaluate TJ protein expression.

RESULTS

In HPMCs, the protein expression of claudin-1, claudin-15, occludin and E-cadherin was decreased by TGF-β treatment but reversed by sitagliptin co-treatment. TMER was decreased by TGF-β treatment but improved by sitagliptin co-treatment. Consistent with this, dextran flux was increased by TGF-β treatment and reversed by sitagliptin co-treatment. In the animal experiment, sitagliptin-treated rats had a lower D2/D0 glucose ratio and a higher D2/P2 creatinine ratio than PD controls during the peritoneal equilibration test. Protein expression of claudin-1, claudin-15 and E-cadherin decreased in RPMCs from PD controls but was not affected in those from sitagliptin-treated rats. Peritoneal fibrosis was induced in PD controls but ameliorated in sitagliptin-treated rats.

CONCLUSION

The expression of TJ proteins including claudin-1 and claudin-15 was associated with transport function both in HPMCs and in a rat model of PD. Sitagliptin prevents peritoneal fibrosis in PD and can potentially restore peritoneal mesothelial cell TJ proteins.

Establishment of a novel mouse peritoneal dialysis-associated peritoneal injury model

BACKGROUND

Peritoneal fibrosis induced by various factors during peritoneal dialysis (PD) can eventually lead to ultrafiltration failure and termination of PD treatment. The existing animal models are caused by a single stimulus, and cannot accurately simulate complex pathogenesis of peritoneal injury and fibrosis. We aim to develop an efficient and realistic mouse model of PD-associated peritoneal injury using daily intraperitoneal injection (I.P.) of human peritonitis PD effluent.

METHODS

Eight-week-old male C57BL/6 mice were classified into six groups: saline control; 2.5% PD fluid; 2.5% PD fluid + lipopolysaccharide (LPS); 4.25% PD fluid; 4.25% PD fluid + LPS; and peritonitis effluent. Mice received daily I.P. for 6 weeks, and were sacrificed to determine peritoneal structural and functional damage, inflammation, and fibrosis.

RESULTS

Mice in the peritonitis effluent group had low mortality. The submesothelial thickness in the peritonitis effluent group was significantly greater than that in the 2.5% PD fluid group. The peritonitis effluent group had increased expression of fibrosis markers (α-SMA, Collagen I, etc.), neutrophil granulocytes (MPO), and macrophages (CD68, F4/80) in the peritoneum based on immunohistochemical staining; and significantly higher expression of inflammation markers (IL-1β, IL-6, etc.) and fibrosis markers (TGF-β1, α-SMA, etc.) based on real-time qPCR. Modified peritoneal equilibration tests (PET) demonstrated that I.P. of peritonitis effluent reduced peritoneal ultrafiltration.

CONCLUSION

Our novel animal model of PD-associated peritoneal injury faithfully simulates the clinical pathophysiological process. This animal model may be useful for study of the pathogenesis of PD-associated peritoneal injury and identification of novel treatments.

Current techniques in peritoneal dialysis

OBJECTIVE

To provide a current overview of the technique of peritoneal dialysis in dogs and cats.

CLINICAL IMPLICATION

Peritoneal dialysis is the process by which water and solutes move between blood in the peritoneal capillaries and fluid (dialysate) instilled into the peritoneal cavity, across the semipermeable membrane of the peritoneum. The primary indication for peritoneal dialysis (PD) in animals is for treatment of renal failure to correct water, solute, and acid-base abnormalities and to remove uremic toxins.

SUMMARY

Peritoneal dialysis is a modality of renal replacement therapy commonly used in human medicine for the treatment of chronic kidney disease and end-stage kidney failure. Peritoneal dialysis utilizes the peritoneum as a membrane across which fluids and uremic solutes are exchanged. Dialysate is instilled into the peritoneal cavity and, through the process of diffusion and osmosis, water, toxins, electrolytes, and other small molecules are allowed to equilibrate.

Aquaporin-1 in the peritoneal membrane: implications for peritoneal dialysis and endothelial cell function

PD (peritoneal dialysis) is an established mode of renal replacement therapy, based on the exchange of fluid and solutes between blood in peritoneal capillaries and a dialysate that has been introduced into the peritoneal cavity. The dialysis process involves diffusive and convective transports and osmosis through the PM (peritoneal membrane). Computer simulations predicted that the PM contains ultrasmall pores (radius <3 A, 1 A=10(-10) m), responsible for up to 50% of UF (ultrafiltration), i.e. the osmotically driven water movement during PD. Several lines of evidence suggest that AQP1 (aquaporin-1) is the ultrasmall pore responsible for transcellular water permeability during PD. Treatment with corticosteroids induces the expression of AQP1 in the PM and improves water permeability and UF in rats without affecting the osmotic gradient and permeability for small solutes. Studies in knockout mice provided further evidence that osmotically driven water transport across the PM is mediated by AQP1. AQP1 and eNOS (endothelial nitric oxide synthase) show a distinct regulation within the endothelium lining the peritoneal capillaries. In acute peritonitis, the up-regulation of eNOS and increased release of nitric oxide dissipate the osmotic gradient and prevent UF, whereas AQP1 expression is unchanged. These results illustrate the usefulness of the PM to investigate the role and regulation of AQP1 in the endothelium. The results also emphasize the critical role of AQP1 during PD and suggest that manipulation of AQP1 expression may be used to increase water permeability across the PM.

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.

Aquaporin-1 in the peritoneal membrane: Implications for water transport across capillaries and peritoneal dialysis

Peritoneal dialysis (PD) is an established mode of renal replacement therapy, based on the exchange of fluid and solutes between blood in peritoneal capillaries and a dialysate that has been introduced in the peritoneal cavity. The dialysis involves diffusive and convective transports and osmosis through the highly vascularized peritoneal membrane. Computer simulations predicted that the membrane contains ultrasmall pores (radius < 3 A) responsible for the transport of solute-free 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 perfectly fit with the characteristics of the ultrasmall pore. Treatment with corticosteroids induces the expression of AQP1 in peritoneal capillaries and increases water permeability and ultrafiltration in rats, without affecting the osmotic gradient and the permeability for small solutes. Studies in knockout mice provided further evidence that osmotically-driven water transport across the peritoneal membrane is mediated by AQP1. AQP1 and endothelial NO synthase (eNOS) show a distinct regulation within the endothelium lining peritoneal capillaries. In acute peritonitis, the upregulation of eNOS and increased release of NO dissipate the osmotic gradient and result in ultrafiltration failure, despite the unchanged expression of AQP1. These data illustrate the potential of the peritoneal membrane to investigate the role and regulation of AQP1 in the endothelium. They also emphasize the critical role of AQP1 during peritoneal dialysis and suggest that manipulating AQP1 expression may be used to increase water permeability across the peritoneal membrane.

Increased aquaporin-1 expression in choroid plexus epithelium after systemic hyponatremia

The expression of aquaporin-1 (AQP1) protein, a water channel, in the choroid plexus epithelium was examined in the rat to determine the role of AQP1 in the pathophysiology of systemic hyponatremia. Systemic hyponatremia was produced by coadministration of 30 ml hypotonic dextrose solution (140 mM) intraperitoneally and 3 microg 1-deamino-8-d-arginine vasopressin (dDAVP) subcutaneously. This hydration protocol produced severe systemic hyponatremia: 2h after drug administration, normonatremic serum osmolarity and Na(+) concentration (296+/-5mOsm/L and 140+/-5 mM) were reduced to 252+/-5mOsm/L and 117+/-2 mM, respectively. AQP1 expression in the third ventricle choroid plexus epithelium was determined by using immunohistochemistry and quantitative immunoblot analysis. After 2h of systemic hyponatremia, AQP1 expression was significantly increased by 28.2+/-2.7% (n=6, P<0.05), whereas it was attenuated to 15.7+/-2.2% (n=6, P<0.05) after 6h of systemic hyponatremia. In addition, TUNEL staining revealed apoptotic granule cells in the dentate gyrus (DG) adjacent to the third ventricle with a peak measured 2h after the start of the systemic hyponatremia protocol, while virtually no apoptotic granule cells left 6h after the start of the hyponatremic treatment. These findings suggest that AQP1-mediated excessive water influx into the brain through the blood-cerebrospinal fluid barrier during systemic hyponatremia may elevate the intracranial pressure and result in the apoptotic brain cell death. Thus, AQP1 in the choroid plexus may play an important role in the pathogenesis of systemic hyponatremia.

Acute oxidative stress induces peritoneal hyperpermeability, mesothelial loss, and fibrosis

We explored the acute and long-term effects of short-lived, intense oxidative stress on peritoneal permeability and structure, induced with intraperitoneal injection of the oxidant agent deoxycholate, in rats. Ten minutes after the experimental intervention, peritoneal dialysis, performed over an exposure time of 60 minutes, revealed an increased urea dialysate/plasma ratio, greater glucose absorption, increased albumin losses in the effluent dialysate, and a reduced ultrafiltration rate. Mesothelial-cell imprints taken from the anterior liver surface indicated a substantially decreased density in the cell population. After the recovery period of 30 days, all alterations were still evident. Additionally, macroscopic and histologic observations made at this time interval detected peritoneal fibrosis and sclerosis, characterized by peritoneal adhesions, wrapping of intestinal loops, and the presence of a layer of fibrous tissue dressing the cavitary aspect of the liver peritoneal envelope. This report describes a reproducible experimental model of peritoneal fibrosis induced by acute oxidative injury. On the basis of these findings, it may be speculated that functional and structural alterations observed in patients are related to long-term continuous exposure of the monolayer to oxidative injury resulting from the high concentrations of d-glucose present in peritoneal dialysis solutions.

Regulation of brain aquaporins

Emerging evidence suggests that brain aquaporins (AQP) play important roles for the dynamic regulation of brain water homeostasis and for the regulation of cerebrospinal fluid production. This review deals with the short- and long-term regulation of AQP4 and AQP9, both expressed in astrocytes, and of AQP1, expressed in the choroid plexus. AQP1 and 4 have in other cell types been shown to be regulated by phosphorylation. Phosphorylation affects the gating of AQP4 and the trafficking and insertion into membrane of AQP1. Mercury inhibits the water permeability of AQP1 and AQP9, but not AQP4. The permeability of AQP4 is increased by lead. AQP4 is also regulated by protein-protein interaction. The assembly between AQP4 and syntrophin is required for the proper localization of AQP4 in the astrocyte plasma membrane that faces capillaries. There is evidence from studies on peripheral tissues that steroid hormones regulate the expression of AQP1, AQP4 and AQP9. There is also evidence that the expression of AQP1 can be regulated by ubiquitination, and that osmolality can regulate the expression of AQP1, AQP4 and AQP9. Further insight into the mechanisms by which brain AQPs are regulated will be of utmost clinical importance, since perturbed water flow via brain AQPs has been implicated in many neurological diseases and since, in brain edema, water flow via AQP4 may have a harmful effect.