Mesothelial Cells

♦ Background

The introduction of peritoneal dialysis (PD) as a modality of renal replacement therapy has provoked much interest in the biology of the peritoneal mesothelial cell. Mesothelial cells isolated from omental tissue have immunohistochemical markers that are identical to those of mesothelial stem cells, and omental mesothelial cells can be cultivated in vitro to study changes to their biologic functions in the setting of PD.

♦ Method

The present article describes the structure and function of mesothelial cells in the normal peritoneum and details the morphologic changes that occur after the introduction of PD. Furthermore, this article reviews the literature of mesothelial cell culture and the limitations of in vitro studies.

♦ Results

The mesothelium is now considered to be a dynamic membrane that plays a pivotal role in the homeostasis of the peritoneal cavity, contributing to the control of fluid and solute transport, inflammation, and wound healing. These functional properties of the mesothelium are compromised in the setting of PD. Cultures of peritoneal mesothelial cells from omental tissue provide a relevant in vitro model that allows researchers to assess specific molecular pathways of disease in a distinct population of cells. Structural and functional attributes of mesothelial cells are discussed in relation to long-term culture, proliferation potential, age of tissue donor, use of human or animal in vitro models, and how the foregoing factors may influence in vitro data.

♦ Conclusions

The ability to propagate mesothelial cells in culture has resulted, over the past two decades, in an explosion of mesothelial cell research pertaining to PD and peritoneal disorders. Independent researchers have highlighted the potential use of mesothelial cells as targets for gene therapy or transplantation in the search to provide therapeutic strategies for the preservation of the mesothelium during chemical or bacterial injury.

Feasibility of Mesothelial Transplantation during Experimental Peritoneal Dialysis and Peritonitis

The mesothelial cell layer lining the peritoneum orchestrates peritoneal homeostasis. Continuous exposure to peritoneal dialysis fluids and episodes of peritonitis may damage the monolayer irreversibly, eventually leading to adhesion formation and fibrosis/sclerosis of the peritoneum. Autologous mesothelial cell transplantation is thought to be one of the options to reduce dysfunction of the peritoneal membrane. In this article we will review the mesothelial cell transplantation experiments performed in the field of peritoneal dialysis and peritonitis. In addition we will focus on the trouble shooting using cultured autologous mesothelial cells for transplantation.

Paracrine effects of transplanted mesothelial cells isolated from temperature-sensitive SV40 large T-antigen gene transgenic rats during peritoneal repair

BACKGROUND

The prevention and restoration of peritoneal damage is a critical mission in peritoneal dialysis (PD). Transplantation of mesothelial cells has been suggested to suppress peritoneal injury during PD. Few studies have examined the efficacy and safety of cell transplantation. We evaluated the paracrine effects of mesothelial transplantation during peritoneal repair using immortalized temperature-sensitive mesothelial cells (TSMCs) in chlorhexidine gluconate (CG)-induced peritoneal fibrosis rats.

METHODS

Continuous-infusion pumps containing 8% CG were placed into the abdominal cavity for 21 days. After the removal of the pumps, the TSMCs were injected into the peritoneal cavity at Day 22 (Tx-1 group) or 29 (Tx-2 group). Morphological findings and mRNA expressions of regeneration-related factors were examined at Days 22, 29 and 35.

RESULTS

Peritoneal thickness was aggravated in the Tx-1 group. Levels of transforming growth factor (TGF)-β, vascular endothelial growth factor (VEGF) and matrix metalloproteinase-2 mRNA in the Tx-1 group at Day 35 were comparable with those at Day 22. The levels of Snail, B-Raf and ERK-1, markers of epithelial to mesenchymal transition and of the RAS/MAPK pathway in the Tx-1 group, were significantly higher than those in the Tx-2 group. TGF-β and VEGF were produced from the transplanted mesothelial cells and the surrounding cells in the Tx-1 group.

CONCLUSION

It appears that the paracrine effect of transplanted mesothelial cells during peritoneal repair is associated with its surrounding condition. It is important to determine the most appropriate time for developing peritoneal repair through mesothelial transplantation.

Intrinsic Cells: Mesothelial Cells — Central Players in Regulating Inflammation and Resolution

Background

Preservation of the structural and functional integrity of the peritoneum is essential to maintain the dialytic efficacy of the peritoneal membrane. Although much improvement has been made to peritoneal dialysis (PD) fluids, they remain bioincompatible, and together with peritonitis, they continue to induce peritoneal inflammation and fibrosis.

Method

This article reviews the putative factors that mediate mesothelial cell inflammation during PD, and the mechanisms by which mesothelial cells attempt to regulate and resolve peritoneal inflammation.

Results

The mesothelium is the first line of defense to foreign particles and chemicals in the peritoneal cavity. Constant exposure of the mesothelium to the bioincompatible constituents of PD solutions results in denudation of the mesothelium and loss of the peritoneal cavity's protective layer. Detached mesothelial cells in PD solutions have the capacity to replenish the mesothelial layer through their ability to migrate and attach to areas of denudation. Mesothelial cells synthesize a plethora of growth factors, matrix proteins, and proteoglycans that aid in the reparative process and regulate the formation of chemotactic gradients that are essential for infiltration of leukocytes to sites of injury.

Conclusions

Far from being bystanders in peritoneal function, mesothelial cells have been shown to play a dynamic role in peritoneal homeostasis and immunoregulation. Studies have highlighted the potential use of mesothelial cells in gene therapy and cell transplantation, both of which may provide novel therapeutic strategies for the preservation of the peritoneum during PD.

Technology insight: Innovative options for end-stage renal disease--from kidney refurbishment to artificial kidney

The steadily growing number of patients with chronic kidney disease who will eventually develop end-stage renal disease, together with the qualitative limitations of currently available renal replacement therapies, have triggered the exploration of innovative strategies for renal replacement therapy and for salvage of renal function. Currently, new hemodialysis modalities and membranes are being used with the aim of increasing clearance of uremic toxins to afford better metabolic control. In addition to these conventional approaches, there are four innovative potential solutions to the problem of replacing renal function when kidneys fail. The first is a small, implantable device with the potential to be supplemented with human cells ('artificial kidney'). The second involves restoration of the damaged kidney by harnessing recent advances in stem-cell technology and knowledge of developmental programing ('refurbished kidney'). The third is (partially) growing a kidney in vitro with the use of therapeutic cloning ('cultured kidney'). The fourth innovative solution involves the use of other organs to replace various renal functions ('distributed kidney'). In this article we review the efforts that have been made to improve renal replacement therapies, and explore innovative approaches. We will not cover all potential solutions in detail. Rather, we aim to indicate directions of future endeavor and arouse enthusiasm in clinicians and scientists for exploration of these exciting avenues.

Peritoneal fibrosing syndrome: pathogenetic mechanism and current therapeutic strategies

Peritoneal dialysis (PD) has been established as a main renal replacement therapy for approximately 20 years. However, long-term peritoneal exposure to high glucose and other unphysiologic contents in the PD solution may potentiate the development of peritoneal fibrosing syndrome (PFS) in PD patients. PFS is composed of a wide spectrum of peritoneal alterations, which has been observed in PD patients. Molecular studies have shown that the fibrogenic effect of peritoneal mesothelial cells and the accompanying accumulation of extracellular matrix in the peritoneum are key events leading to PFS. In this review, we highlight the impact of PFS and its pathogenetic factors, including bioincompatible PD solution, multidisciplinary inflammatory mediators, and stimulatory cytokines in the peritoneal cavity. Current therapeutic strategies based on both clinical and basic evidence for the prevention or treatment of PFS are also reviewed.

Functional and molecular characterization of a peritoneal dialysis model in the C57BL/6J mouse

BACKGROUND

Animal models are important for understanding the physiology and pathophysiology of peritoneal transport during peritoneal dialysis (PD). Mechanistic investigations of rat and rabbit models of PD are mostly based on intervention studies using pharmacologic agents or blocking antibodies. These models may be limited by the time-course, lack of specificity, or side effects of such interventions. Genetically modified mice could provide an attractive alternative to the above models. In this study, we have characterized PD parameters and tested the effect of gender and dialysate volume and/or osmolality in the C57BL/6J mouse.

METHODS

Mice were submitted to a 2-hour peritoneal equilibration test in order to obtain permeability parameters. The expression of the water channel aquaporin-1 (AQP1) and endothelial NO synthase (eNOS) was investigated at the protein (immunoblotting, immunostaining) and mRNA [real-time reverse-transcription-polymerase chain reaction (RT-PCR)] levels. The potential effect of gender on these parameters was also studied.

RESULTS

Exposure of mice to 2 mL of 3.86% glucose dialysate yielded equilibration curves for urea and glucose, a sodium sieving, and a net ultrafiltration (UF) that were remarkably similar to those obtained in rats. The increase in dialysate volume (from 2 mL to 3 mL and 6 mL) resulted in a higher ultrafiltration and, for the highest volume, an increase in the diffusive mass transport coefficient (MTAC) for urea. The increase in dialysate glucose concentration (from 1.36% to 3.86% and 7%) resulted in increased sodium sieving and higher UF, whereas the MTAC for urea was unchanged. In comparison with males, females had a similar peritoneal transport rate for small solutes but a significantly lower sodium sieving, reflecting a lower AQP1 mRNA and protein expression in the peritoneum.

CONCLUSION

These data demonstrate the structural and functional similarity between mouse and rat models of PD, and further emphasize the relevance of mouse models to understand PD in humans. They also suggest that gender may influence water transport and AQP1 expression in the peritoneum.