Peritoneal Membrane: An Overview to Pathophysiology for Future Therapy Source: Devuyst O, Margetts PJ, Topley N. The pathophysiology of the peritoneal membrane. J Am Soc Nephrol. 2010;21:1077–1085.

Introduction and Rationale Peritoneal dialysis (PD) is a life-sustaining therapy used by 10–15% of the dialysis population, worldwide. However, infection and deleterious functional alterations in the peritoneal membrane after exposure to dialysis solutions hinder the success of long-term PD (see Table 1). Loss of dialysis capacity increases the rate of morbidity and Mortality.

Table 1: Functional alterations after peritoneal dialysis

Clinical improvements in therapy delivery and prescription have been observed (see Table 2) in reducing the incidence of peritonitis. However, infectious complications and membrane failure still remain as major problems.

Hence, this brief review was undertaken with the object of providing a more complete understanding of basic mechanisms to facilitate the development of novel, specifically targeted diagnostic and therapeutic strategies aimed at reducing infection and improvement in membrane survival and long-term outcomes in patients who are on PD (see Table 3).

Peritoneal Transport, Aquaporins and Ultrafiltration Initial studies on mouse model have confirmed the specific distribution of water channel aquaporin 1 (AQP1) and endothelial nitric oxide synthase (eNOS) in distinct vascular beds. Further, the exposure of mice to standard glucose dialysis solution yields equilibration curves for urea and glucose, sodium sieving and a net ultrafiltration (UF).

Ultrafiltration A major predictor of outcome and mortality in PD patients is the capacity for UF across the peritoneal membrane. Capillary endothelium containing ultra small pores (radius <3Å), is a major barrier to membrane transport, and accounts for about 50% of the UF and explains the sodium seiving with hypertonic dialysate.

Aquaporins Identification of aquaporins (AQPs) provides a critical insight in the molecular mechanisms involving water permeation across biological membranes. Lack of AQP1 is associated with a decrease in the osmotically driven water transport across the peritoneum, sodium sieving and UF (see Fig. 1). Hence, increasing the expression of AQP1, for example, with corticosteroid therapy, in peritoneum might be a potential approach to treat UF failure in PD patients.

Role of Nitric Oxide Synthase Isoforms in Acute Peritonitis Vasoactive substances, particulary, nitric oxide (NO), released during inflammation reaction, play a major role in molecular changes produced during acute peritonitis. Studies in rat and mouse models of acute peritonitis have identified an improvement in UF and reversal of permeability on inhibiting nitric oxide synthase (NOS) with NG-nitro-L-arginine methyl ester. Three NOS isoforms— neuronal NOS (nNOS and NOS1), inducible NOS (iNOS and NOS2) and endothelial NOS (eNOS and NOS3) are diferrentially expressed in the peritoneal membrane.

Role of Nitric Oxide Synthase Isoforms in Acute Peritonitis Deletion of eNOS attenuates the vascular proliferation and inflammatory infiltrate resulting in improved UF and reduced protein loss in the dialysate (see Fig. 2). The specific role of eNOS in mediating increased solute transport and loss of UF associated with peritonitis was further confirmed in NOS isoform deficient mice model of lipopolysaccharide (LPS)-induced peritonitis. Further, nNOS and iNOS did not show any effect. However, iNOS null mice have more inflammatory changes and an increases trend towards mortality after LPS treatment. Hence, it may be suggested that selective eNOS inhibition may improve peritoneal transport parameters and prevent vascular changes during acute peritonitis.

Regulation of Peritoneal Inflammation and Leukocyte Trafficking Acute peritonitis is well-described in PD patients and studied in murine models (see Fig. 3). Interleukin-6 (IL-6) is a key mediator involved in peritoneal inflammation regulation (see Fig. 4). A combination of clinical and in vitro investigations suggests that formation of sIL-6R/ IL-6 [soluble IL-6 receptor (sIL-6R)] complexes, resulting from proinflammatory changes, supress the release of other CXC chemokines and promotes the secretion of CC chemokines (monocyte chemoattractant protein 1 and RANTES), ensuring clearance of netrophils and trigerring the recruitment of monoluclear leukocytes, respectively. Furthermore, interferon-γ(IFN- γ) controls the initial recruitment, apoptosis and clearance of neutrophils. Hence, these studies demonstrate the facilitation of inflammation resolution and bacterial clearance in peritonium with the transition from innate to acquired immunity. Thus, therapeutic interventions to reduce inflammation and promote clearance of bacterial infections will be beneficial.

Fig. 2: Role of eNOS in transport and structural changes induced by acute peritonitis.

Cellular Studies with Transgenic Mice A major interest of transgenic mice is the possibility of harvesting cells to develop primary cultures to investigate the role of specific molecules in a given cell population. Role of toll-like receptor 4 (TLR4) in murine peritoneal mesothelial cells (MPMC) was investigated using this approach and it was identified that TLR4 is directly involved in the production of chemokines by mesothelial cells. This suggests that TLR4-mediated pathways reduce the detrimental consequences of peritoneal inflammation. Further, recent studies demonstrate that soluble form of TLR2 modulates peritoneal inflammation and leukocyte recruitment without negatively influencing the bacterial clearance in peritoneal infection. So, therapeutic interventions against inflammation can now be achieved without compromising peritoneal host defence.

Fig. 4: Regulation of Leukocyte trafficking and interleukin-6.

Fig. 5: Peritoneal mesothelial cells undergo EMT.

Fibrosis Pathways, Angiogenesis and Epithelial-to-Mesenchymal Transition Studies have demonstrated that after the exposure to injury or associated growth factors, peritoneal mesothelial cells undergo epithelial-to-mesenchymal transition (EMT) to form fibroblast (see Fig. 5). The EMT of peritoneal membrane is also associated with angiogenic stimuli and altered solute transport. Moreover, common initiating growth factors and inflammatory cytokines, and the EMT process links angiogenesis and fibrosis. Hence, development of therapeutic strategies to preserve the peritoneum as a dialysis membrane may neccesitate the understanding of mechanism of fibrosis and the interaction with angiogenesis.

Epithelial-to-Mesenchymal Transition There is an increasing evidence to suggest that treatment to prevent EMT may also ameliorate fibrosis and angiogenesis; and therefore, preserve the peritoneal membrane. Recently, biomarkers for EMT have been categorized. These include the loss of epithelial adhesion protein E-cadherin and upregulation of mesenchymal markers such as fibroblastspecific protein 1.

Peritoneal Membrane Fibrosis and Angiogenesis Increase in submesothelial thickness associated with peritoneal fibrosis and angiogenesis is the most consistent change observed in the peritoneal tissues of a PD patient (see Fig. 6). Fibrosis and angiogenesis seem to occur together in peritoneal tissues, and interventions that reduce angiogenesis also reduce fibrosis. The interaction between fibrosis and angiogenesis may occur at the level of inducing cytokines, vascular endothelial growth factor and angiogenesis.

Fig. 6: Deleterious modifications of the peritoneal membrane exposed to peritoneal dialysis.

Conclusion Transgenic mouse and cellular models have a significant impact in defining basic mechanism that operate peritoneal membrane Transport properties of the peritoneal membrane, regulation of peritoneal inflammation by cytokines and chemokines, bacterial clearance and leukocyte recruitment, and pathways involved in structural and fibrogenic alterations contribute to treatment failure

Future Perspective With the application of multiplex assay and DNA/RNA array technologies to these models, it may be possible to assess the interactive relationship of various physiologic and pathophysiologic pathways in the peritoneum; in relation to systematic parameters.

Comprehensive Basket in Anemia Management