Loosening of the mesothelial barrier as an early therapeutic target to preserve peritoneal function in peritoneal dialysis

Phenotype transition of peritoneal mesothelial cells (MCs) including the epithelial-to-mesenchymal transition (EMT) is regarded as an early mechanism of peritoneal dysfunction and fibrosis in peritoneal dialysis (PD), producing proinflammatory and pro-fibrotic milieu in the intra-peritoneal cavity. Loosening of intercellular tight adhesion between adjacent MCs as an initial process of EMT creates the environment where mesothelium and submesothelial tissue are more vulnerable to the composition of bio-incompatible dialysates, reactive oxygen species, and inflammatory cytokines. In addition, down-regulation of epithelial cell markers such as E-cadherin facilitates de novo acquisition of mesenchymal phenotypes in MCs and production of extracellular matrices. Major mechanisms underlying the EMT of MCs include induction of oxidative stress, pro-inflammatory cytokines, endoplasmic reticulum stress and activation of the local renin-angiotensin system. Another mechanism of peritoneal EMT is mitigation of intrinsic defense mechanisms such as the peritoneal antioxidant system and anti-fibrotic peptide production in the peritoneal cavity. In addition to use of less bio-incompatible dialysates and optimum treatment of peritonitis in PD, therapies to prevent or alleviate peritoneal EMT have demonstrated a favorable effect on peritoneal function and structure, suggesting that EMT can be an early interventional target to preserve peritoneal integrity.

Syndecan-1 (CD138) deficiency increases Staphylococcus aureus infection but has no effect on pathology in a mouse model of peritoneal dialysis

Background

Technique failure in peritoneal dialysis (PD) due to fibrosis and angiogenesis is complicated by peritonitis. Staphylococcus aureus infection is one of the most common causes of peritonitis in PD. The heparan sulfate proteoglycan, syndecan-1 (CD138), was reported to regulate fibrosis, angiogenesis, inflammation and S. aureus infection. The objectives of this study were to examine the effects of syndecan-1 on S. aureus infection and histopathology in a PD model.

Results

Syndecan-1^-/- and wild type mice were dialyzed for 4 weeks and infected intraperitoneally with S. aureus. Tissues were collected after 4 h for histomorphometric analysis. Intravital microscopy was used to observe leukocyte recruitment and to quantify syndecan-1 in the parietal peritoneum microcirculation. The dialyzed syndecan-1^-/- mice were more susceptible to S. aureus infection than undialyzed syndecan-1^-/- controls and wild type animals. However, peritoneal fibrosis and neovascularization due to PD did not differ between syndecan-1^-/- and wild type mice. Intravital microscopy showed that in S. aureus infection, syndecan-1 was removed from the subendothelial layer of peritoneal venules but syndecan-1 deficiency did not affect leukocyte recruitment.

Conclusions

This study indicates that, while syndecan-1 is important for providing a barrier to acute S. aureus infection in PD, it does not affect peritoneal fibrosis and angiogenesis.

Proteoglycans of uterine fibroids and keloid scars: similarity in their proteoglycan composition

Fibrosis is the formation of excess and abnormal fibrous connective tissue as a result of either a reparative or reactive process. A defining feature of connective tissue is its extracellular matrix, which provides structural support and also influences cellular activity. Two common human conditions that result from fibrosis are uterine fibroids (leiomyomas) and keloid scars. Because these conditions share a number of similarities and because their growth is due primarily to excessive extracellular matrix deposition, we compared the proteoglycans of uterine fibroids and keloid scars with corresponding normal tissues. Our analysis indicates that uterine fibroids and keloid scars contain higher amounts of glycosaminoglycans relative to normal myometrium and normal adult skin respectively. Proteoglycan composition is also different in the fibrotic tissues. Compared with unaffected tissues, uterine fibroids and keloid scars contain higher relative amounts of versican and lower relative amounts of decorin. There is also evidence for a higher level of versican catabolism in the fibrotic tissues compared with unaffected tissues. These qualitative and quantitative proteoglycan differences may play a role in the expansion of these fibroses and in their excessive matrix deposition and matrix disorganization, due to effects on cell proliferation, TGF (transforming growth factor)-β signalling and/or collagen fibril formation.

Pathophysiological changes to the peritoneal membrane during PD-related peritonitis: the role of mesothelial cells

The success of peritoneal dialysis (PD) is dependent on the structural and functional integrity of the peritoneal membrane. The mesothelium lines the peritoneal membrane and is the first line of defense against chemical and/or bacterial insult. Peritonitis remains a major complication of PD and is a predominant cause of technique failure, morbidity and mortality amongst PD patients. With appropriate antibiotic treatment, peritonitis resolves without further complications, but in some PD patients excessive peritoneal inflammatory responses lead to mesothelial cell exfoliation and thickening of the submesothelium, resulting in peritoneal fibrosis and sclerosis. The detrimental changes in the peritoneal membrane structure and function correlate with the number and severity of peritonitis episodes and the need for catheter removal. There is evidence that despite clinical resolution of peritonitis, increased levels of inflammatory and fibrotic mediators may persist in the peritoneal cavity, signifying persistent injury to the mesothelial cells. This review will describe the structural and functional changes that occur in the peritoneal membrane during peritonitis and how mesothelial cells contribute to these changes and respond to infection. The latter part of the review discusses the potential of mesothelial cell transplantation and genetic manipulation in the preservation of the peritoneal membrane.

Pathophysiology of the peritoneal membrane during peritoneal dialysis: the role of hyaluronan

During peritoneal dialysis (PD), constant exposure of mesothelial cells to bioincompatible PD solutions results in the denudation of the mesothelial monolayer and impairment of mesothelial cell function. Hyaluronan, a major component of extracellular matrices, is synthesized by mesothelial cells and contributes to remesothelialization, maintenance of cell phenotype, and tissue remodeling and provides structural support to the peritoneal membrane. Chronic peritoneal inflammation is observed in long-term PD patients and is associated with increased hyaluronan synthesis. During inflammation, depolymerization of hyaluronan may occur with the generation of hyaluronan fragments. In contrast to native hyaluronan which offers a protective role to the peritoneum, hyaluronan fragments exacerbate inflammatory and fibrotic processes and therefore assist in the destruction of the tissue. This paper will discuss the contribution of mesothelial cells to peritoneal membrane alterations that are induced by PD and the putative role of hyaluronan in these processes.

Profiles of carbohydrate ligands associated with adsorbed proteins on self-assembled monolayers of defined chemistries

Conserved protein-carbohydrate-lipid pathogen-associated molecular patterns (PAMPs) interact with cells of the innate immune system to mediate antigen recognition and internalization and activation of immune cells. We examined if analogous "biomaterial-associated molecular patterns" composed of proteins, specifically their carbohydrate modifications, existed on biomaterials, which can play a role in mediating the innate immune response to biomaterials. To probe for these carbohydrates in the adsorbed protein layer, as directed by the underlying biomaterial chemistry, self-assembled monolayers (SAMs) presenting -CH(3), -OH, -COOH, or -NH(2) were preincubated with serum/plasma, and the presence of carbohydrate ligands of C-type lectin receptors (CLRs) was investigated using lectin probes in an enzyme-linked lectin assay (ELLA). Presentation of CLR ligands was detected on control tissue culture polystyrene (TCPS). Absorbances of mannose or N-acetylglucosamine increased with decreasing incubating serum concentration, whereas absorbances of sialylated epitopes or fucose remained unchanged. Absorbances of alpha-galactose or N-acetylgalactosamine decreased with decreasing incubating serum concentration; beta-galactose was undetectable. Among SAM endgroups, preincubation with 10% serum resulted in differential presentation of CLR ligands: higher alpha-galactose on COOH SAMs than NH(2) or CH(3) SAMs, highest complex mannose on NH(2) SAMs, and higher complex mannose on OH SAMs than CH(3) SAMs. Least sialylated groups were detected on CH(3) SAMs. In summary, biomaterial chemistry may regulate protein adsorption and hence unique presentation of associated carbohydrates. The ultimate goal is to identify the effects of protein glycosylations associated with biomaterials in stimulating innate immune responses.

Collagen-binding peptidoglycans: a biomimetic approach to modulate collagen fibrillogenesis for tissue engineering applications

The small leucine-rich proteoglycans (SLRPs), prevalent in collagenous tissues, regulate collagen fibrillogenesis and provide a host of biochemical cues critical to tissue function and homeostasis. Incorporating SLRPs may enhance tissue engineering designs that mimic the native extracellular matrix, although SLRPs purified from animal sources bear low yields and lack design control. Consequently, we have designed synthetic peptidoglycans, inspired by the native SLRP decorin, that contain a collagen-binding peptide attached to a glycosaminoglycan (GAG) chain. These peptidoglycans modulate collagen fibrillogenesis and decrease fibril diameter in vitro, similarly to decorin, while maintaining the characteristic D-banded fibrils. Application for tissue engineering is demonstrated as these peptidoglycans are incorporated into collagen gels seeded with smooth muscle cells. Gels formed with peptidoglycans and decorin show a faster rate of gel compaction, and one peptidoglycan uniquely increases elastin production. The peptidoglycan design can be tailored with respect to the peptide sequence and GAG identity and is expected to have versatile application in tissue engineering.

Connective tissue growth factor (CTGF/CCN2) is increased in peritoneal dialysis patients with high peritoneal solute transport rate

Peritoneal fibrosis (PF) is an important complication of peritoneal dialysis (PD) therapy that often occurs in association with peritoneal high transport rate and ultrafiltration failure (UFF). To study the possible pathogenic role of connective tissue growth factor (CTGF) in the relationship of PF and UFF, dialysate CTGF contents (n = 178) and tissue CTGF expression (n = 61) were investigated by ELISA, real-time PCR, immunohistochemistry, and in situ hybridization. CTGF production with and without TGF-beta1 stimulation in human peritoneal mesothelial cells (HPMC) from the spent patients' peritoneal dialysate (n = 32) was studied in vitro. The dialysate-to-plasma ratio for creatinine (D/P Cr) was positively correlated to dialysate CTGF concentration and estimated local peritoneal production of CTGF. CTGF mRNA expression was 11.4-fold higher in peritoneal membranes with UFF than in pre-PD renal failure peritoneum and was correlated with thickness of the peritoneum. CTGF protein and mRNA were detected in mesothelium and in fibroblast-like cells. In cultured HPMC, TGF-beta(1)-induced expression of CTGF mRNA was increased at 12 and 24 h and was correlated with D/P Cr. In contrast, bone morphogenic protein-4 mRNA expression was inversely correlated with D/P Cr. Our results suggest that high peritoneal transport state is associated with fibrosis and increased peritoneal CTGF expression and production by mesothelial cells, which can be stimulated by TGF-beta1. Dialysate CTGF concentration could be a biomarker for both peritoneal fibrosis and membrane function. Functional alteration of mesothelial cells may be involved in progression of peritoneal fibrosis in high transport state.

Inflammatory ascites formation induced by macromolecules in mice and rats

Different macromolecules were administered intraperitoneally to stimulate formation of protein-rich ascitic fluid in rodents. Stimulatory effect of plant lectins depended on the attachment to cell surface carbohydrates, Canavalia ensiformis (ConA) lectin was used in the majority of experiments. The time course of ConA-induced ascites was divided into an early (up to 4 h) and a late (from 6 h on) phase, with a transitional period between the two. Water and protein accumulation showed parallel time courses: volume of the ascitic fluid peaked at around 3 h, and fibrin threads appeared after 6 h. Viscosity of the ascitic fluid and its supernatant increased with time, reaching maximal fibrinogen concentration at around 16 h. Peritoneal permeability, followed by pleural and pericardial effusions, was elicited only by lectins that form soluble complexes with serum glycoproteins, whereas the effect of serum-precipitating lectins was restricted to the peritoneum. Macromolecules with serial positive charges (e.g., polylysine or polyethyleneimine) enhanced peritoneal permeability by ionic interactions with cell surface molecules. Viscosity of the polycation-induced ascitic fluid did not tend to increase with time and corresponded to the early phase of the ConA-induced ascites. Polyglutamate, a polyanionic macromolecule, inhibited the effect of polycations, but not that of ConA. The most efficient stimulatory macromolecules appear to induce ascites by noncovalent cross-linking of cell surface glycoproteins or glycosaminoglycans or both. A similar mechanism may operate in the maintenance of basal secretion to prevent eventual desiccation. Noncovalent cross-linking appears to be a common denominator of both basal and enhanced permeability.

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.