Alterations of intercellular junctions in peritoneal mesothelial cells from patients undergoing dialysis: effect of retinoic Acid

2-Deoxy-glucose ameliorates the peritoneal mesothelial and endothelial barrier function perturbation occurring due to Peritoneal Dialysis fluids exposure

Peritoneal dialysis (PD) and prolonged exposure to PD fluids (PDF) induce peritoneal membrane (PM) fibrosis and hypervascularity, leading to functional PM degeneration. 2-deoxy-glucose (2-DG) has shown potential as PM antifibrotic by inhibiting hyper-glycolysis induced mesothelial-to-mesenchymal transition (MMT). We investigated whether administration of 2-DG with several PDF affects the permeability of mesothelial and endothelial barrier of the PM. The antifibrotic effect of 2-DG was confirmed by the gel contraction assay with embedded mesothelial (MeT-5A) or endothelial (EA.hy926) cells cultured in Dianeal® 2.5 % (CPDF), BicaVera® 2.3 % (BPDF), Balance® 2.3 % (LPDF) with/without 2-DG addition (0.2 mM), and qPCR for αSMA, CDH2 genes. Moreover, 2-DG effect was tested on the permeability of monolayers of mesothelial and endothelial cells by monitoring the transmembrane resistance (RTM), FITC-dextran (10, 70 kDa) diffusion and mRNA expression levels of CLDN-1 to -5, ZO1, SGLT1, and SGLT2 genes. Contractility of MeT-5A cells in CPDF/2-DG was decreased, accompanied by αSMA (0.17 ± 0.03) and CDH2 (2.92 ± 0.29) gene expression fold changes. Changes in αSMA, CDH2 were found in EA.hy926 cells, though αSMA also decreased under LPDF/2-DG incubation (0.42 ± 0.02). Overall, 2-DG mitigated the PDF-induced alterations in mesothelial and endothelial barrier function as shown by RTM, dextran transport and expression levels of the CLDN-1 to -5, ZO1, and SGLT2. Thus, supplementation of PDF with 2-DG not only reduces MMT but also improves functional permeability characteristics of the PM mesothelial and endothelial barrier.

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.

Human peritoneal tight junction, transporter and channel expression in health and kidney failure, and associated solute transport

Next to the skin, the peritoneum is the largest human organ, essentially involved in abdominal health and disease states, but information on peritoneal paracellular tight junctions and transcellular channels and transporters relative to peritoneal transmembrane transport is scant. We studied their peritoneal localization and quantity by immunohistochemistry and confocal microscopy in health, in chronic kidney disease (CKD) and on peritoneal dialysis (PD), with the latter allowing for functional characterizations, in a total of 93 individuals (0-75 years). Claudin-1 to -5, and -15, zonula occludens-1, occludin and tricellulin, SGLT1, PiT1/SLC20A1 and ENaC were consistently detected in mesothelial and arteriolar endothelial cells, with age dependent differences for mesothelial claudin-1 and arteriolar claudin-2/3. In CKD mesothelial claudin-1 and arteriolar claudin-2 and -3 were more abundant. Peritonea from PD patients exhibited increased mesothelial and arteriolar claudin-1 and mesothelial claudin-2 abundance and reduced mesothelial and arteriolar claudin-3 and arteriolar ENaC. Transperitoneal creatinine and glucose transport correlated with pore forming arteriolar claudin-2 and mesothelial claudin-4/-15, and creatinine transport with mesothelial sodium/phosphate cotransporter PiT1/SLC20A1. In multivariable analysis, claudin-2 independently predicted the peritoneal transport rates. In conclusion, tight junction, transcellular transporter and channel proteins are consistently expressed in peritoneal mesothelial and endothelial cells with minor variations across age groups, specific modifications by CKD and PD and distinct associations with transperitoneal creatinine and glucose transport rates. The latter deserve experimental studies to demonstrate mechanistic links.Clinical Trial registration: The study was performed according to the Declaration of Helsinki and is registered at www.clinicaltrials.gov (NCT01893710).

Proteome-Wide Differential Effects of Peritoneal Dialysis Fluid Properties in an In Vitro Human Endothelial Cell Model

To replace kidney function, peritoneal dialysis (PD) utilizes hyperosmotic PD fluids with specific physico-chemical properties. Their composition induces progressive damage of the peritoneum, leading to vasculopathies, decline of membrane function, and PD technique failure. Clinically used PD fluids differ in their composition but still remain bioincompatible. We mapped the molecular pathomechanisms in human endothelial cells induced by the different characteristics of widely used PD fluids by proteomics. Of 7894 identified proteins, 3871 were regulated at least by 1 and 49 by all tested PD fluids. The latter subset was enriched for cell junction-associated proteins. The different PD fluids individually perturbed proteins commonly related to cell stress, survival, and immune function pathways. Modeling two major bioincompatibility factors of PD fluids, acidosis, and glucose degradation products (GDPs) revealed distinct effects on endothelial cell function and regulation of cellular stress responses. Proteins and pathways most strongly affected were members of the oxidative stress response. Addition of the antioxidant and cytoprotective additive, alanyl-glutamine (AlaGln), to PD fluids led to upregulation of thioredoxin reductase-1, an antioxidant protein, potentially explaining the cytoprotective effect of AlaGln. In conclusion, we mapped out the molecular response of endothelial cells to PD fluids, and provided new evidence for their specific pathomechanisms, crucial for improvement of PD therapies.

Proteomic study of mesothelial and endothelial cross-talk: key lessons

INTRODUCTION

The peritoneum, pleura, and pericardium are yet understudied multicellular systems where mesothelial cells (MCs) and endothelial cells (ECs) are in close proximity. Crosstalk between these cell types likely plays role in molecular transport, immunological reactions, and metabolic processes in health, disease, and therapeutic intervention.

AREAS COVERED

In this review, we discuss recent proteomic efforts to characterize the crosstalk between MC and EC. We describe the proteomic methods necessary for investigation of crosstalk between MC and EC, as well as the in-vitro models that can be employed. Potential experimental approaches range from conditioned medium, via co-culture on semi-permeable membranes, to 3D cell culture based organoid models. While the biological and clinical relevance of the models may increase with their ability to mimic close cell communication, the practicality of these complex experiments corresponds vice versa, making standardization more difficult and expensive.

EXPERT OPINION

Currently, data and reports on mesothelial-to-endothelial crosstalk are still very scarce. In our opinion, the in-vitro model using semi-permeable cell culture inserts will allow to establish a basic understanding of cellular crosstalk that may occur between those cell types. Later-on, more sophisticated 3D cell cultures may be better able to simulate the transport dynamics within the peritoneal membrane.

Tight junctions: from molecules to gastrointestinal diseases

Intestinal epithelium functions as a tissue barrier to prevent interaction between the internal compartment and the external milieu. Intestinal barrier function also determines epithelial polarity for the absorption of nutrients and the secretion of waste products. These vital functions require strong integrity of tight junction proteins. In fact, intestinal tight junctions that seal the paracellular space can restrict mucosal-to-serosal transport of hostile luminal contents. Tight junctions can form both an absolute barrier and a paracellular ion channel. Although defective tight junctions potentially lead to compromised intestinal barrier and the development and progression of gastrointestinal (GI) diseases, no FDA-approved therapies that recover the epithelial tight junction barrier are currently available in clinical practice. Here, we discuss the impacts and regulatory mechanisms of tight junction disruption in the gut and related diseases. We also provide an overview of potential therapeutic targets to restore the epithelial tight junction barrier in the GI tract.

Micronutrient Improvement of Epithelial Barrier Function in Various Disease States: A Case for Adjuvant Therapy

The published literature makes a very strong case that a wide range of disease morbidity associates with and may in part be due to epithelial barrier leak. An equally large body of published literature substantiates that a diverse group of micronutrients can reduce barrier leak across a wide array of epithelial tissue types, stemming from both cell culture as well as animal and human tissue models. Conversely, micronutrient deficiencies can exacerbate both barrier leak and morbidity. Focusing on zinc, Vitamin A and Vitamin D, this review shows that at concentrations above RDA levels but well below toxicity limits, these micronutrients can induce cell- and tissue-specific molecular-level changes in tight junctional complexes (and by other mechanisms) that reduce barrier leak. An opportunity now exists in critical care—but also medical prophylactic and therapeutic care in general—to consider implementation of select micronutrients at elevated dosages as adjuvant therapeutics in a variety of disease management. This consideration is particularly pointed amidst the COVID-19 pandemic.

Aquaporin-1 Facilitates Transmesothelial Water Permeability: In Vitro and Ex Vivo Evidence and Possible Implications in Peritoneal Dialysis

We previously showed that mesothelial cells in human peritoneum express the water channel aquaporin 1 (AQP1) at the plasma membrane, suggesting that, although in a non-physiological context, it may facilitate osmotic water exchange during peritoneal dialysis (PD). According to the three-pore model that predicts the transport of water during PD, the endothelium of peritoneal capillaries is the major limiting barrier to water transport across peritoneum, assuming the functional role of the mesothelium, as a semipermeable barrier, to be negligible. We hypothesized that an intact mesothelial layer is poorly permeable to water unless AQP1 is expressed at the plasma membrane. To demonstrate that, we characterized an immortalized cell line of human mesothelium (HMC) and measured the osmotically-driven transmesothelial water flux in the absence or in the presence of AQP1. The presence of tight junctions between HMC was investigated by immunofluorescence. Bioelectrical parameters of HMC monolayers were studied by Ussing Chambers and transepithelial water transport was investigated by an electrophysiological approach based on measurements of TEA+ dilution in the apical bathing solution, through TEA+-sensitive microelectrodes. HMCs express Zo-1 and occludin at the tight junctions and a transepithelial vectorial Na+ transport. Real-time transmesothelial water flux, in response to an increase of osmolarity in the apical solution, indicated that, in the presence of AQP1, the rate of TEA+ dilution was up to four-fold higher than in its absence. Of note, we confirmed our data in isolated mouse mesentery patches, where we measured an AQP1-dependent transmesothelial osmotic water transport. These results suggest that the mesothelium may represent an additional selective barrier regulating water transport in PD through functional expression of the water channel AQP1.

Alanyl-Glutamine Restores Tight Junction Organization after Disruption by a Conventional Peritoneal Dialysis Fluid

Understanding and targeting the molecular basis of peritoneal solute and protein transport is essential to improve peritoneal dialysis (PD) efficacy and patient outcome. Supplementation of PD fluids (PDF) with alanyl-glutamine (AlaGln) increased small solute transport and reduced peritoneal protein loss in a recent clinical trial. Transepithelial resistance and 10 kDa and 70 kDa dextran transport were measured in primary human endothelial cells (HUVEC) exposed to conventional acidic, glucose degradation products (GDP) containing PDF (CPDF) and to low GDP containing PDF (LPDF) with and without AlaGln. Zonula occludens-1 (ZO-1) and claudin-5 were quantified by Western blot and immunofluorescence and in mice exposed to saline and CPDF for 7 weeks by digital imaging analyses. Spatial clustering of ZO-1 molecules was assessed by single molecule localization microscopy. AlaGln increased transepithelial resistance, and in CPDF exposed HUVEC decreased dextran transport rates and preserved claudin-5 and ZO-1 abundance. Endothelial clustering of membrane bound ZO-1 was higher in CPDF supplemented with AlaGln. In mice, arteriolar endothelial claudin-5 was reduced in CPDF, but restored with AlaGln, while mesothelial claudin-5 abundance was unchanged. AlaGln supplementation seals the peritoneal endothelial barrier, and when supplemented to conventional PD fluid increases claudin-5 and ZO-1 abundance and clustering of ZO-1 in the endothelial cell membrane.

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.

Tight junction protein expression from peritoneal dialysis Effluent

Background: We hypothesized that tight junction (TJ) proteins may have a role in paracellular transport of solute and water in peritoneal dialysis (PD) patients. Previous studies on TJ proteins in PD patients have used only cultured human peritoneal mesothelial cells (HPMCs). This study was undertaken to test whether TJ proteins are directly identified from PD effluent and whether their expressions are associated with functional parameters of PD.Methods: Dialysis effluents were collected from 40 patients undergoing PD, after the peritoneal equilibration test (PET). Different molecular sizes of Amicon Ultra-15 Centrifugal Filter Units were used to concentrate and purify proteins in PD effluents, and immunoblot analyses for occludin, ZO-1, and claudins were carried out to test for their existence and relationships with peritoneal clearance or results of the PET.Results: Immunoblotting from PD effluents revealed discrete bands of occludin (∼65 kDa), ZO-1 (∼215 kDa), claudin-1 (∼22 kDa), and claudin-15 (∼22 kDa) in all 40 patients. The peritoneal creatinine clearance inversely correlated with the protein expression of claudin-1 (r= -0.369, p= .019), and the dialysate-to-plasma creatinine ratio at 4 h PET correlated with occludin (r = 0.396, p= .011) and inversely correlated with claudin-15 (r= -0.393, p= .012).Conclusion: In PD patients, expression of peritoneal TJ proteins can be estimated from the dialysis effluent and may be used as novel peritoneal biomarkers.

Senescence-related deterioration of intercellular junctions in the peritoneal mesothelium promotes the transmesothelial invasion of ovarian cancer cells

Mechanisms of transmesothelial invasion of ovarian cancer are still poorly understood. Here we examined whether this phenomenon may be determined by an expression of intercellular junctions in peritoneal mesothelial cells (PMCs). Analysis of ovarian tumors showed that cancer cells are localized below an intact layer of PMCs. The PMCs located near the invaded cancer cells displayed low expression of connexin 43, E-cadherin, occludin, and desmoglein, as well as expressed SA-β-Gal, a marker of senescence. Experiments in vitro showed that senescent PMCs exhibited decreased levels of the four tested intercellular junctions, and that the invasion of ovarian cancer cells through the PMCs increased proportionally to the admixture of senescent cells. Intervention studies showed that the expression of connexin 43, E-cadherin, occludin, and desmoglein in senescent PMCs could be restored upon the blockade of p38 MAPK, NF-κB, AKT, JNK, HGF, and TGF-β1. When these molecules were neutralized, the efficiency of the transmesothelial cancer cell invasion was diminished. Collectively, our findings show that the integrity of the peritoneal mesothelium, which is determined by the expression of junctional proteins, is critical for the invasion of ovarian cancer. They also indicate a mechanism by which senescent PMCs may promote the invasive potential of cancer cells.

Protective effects and mechanisms of omega-3 polyunsaturated fatty acid on intestinal injury and macrophage polarization in peritoneal dialysis rats

AIM

This study was conducted to investigate the chronic injury of peritoneal glucose injection on the peritoneum and intestine and the protective effects of omega-3 polyunsaturated fatty acid (ω-3PUFA) during peritoneal dialysis (PD).

METHODS

Peritoneal dialysis animal models were established by intraperitoneal injection of 4.25% glucose for 28 days. Protein expression in ileum and peritoneum was measured by immunofloresence and immunohistochemistry. Protein expression in macrophages was measured by Western blot. Fibrosis was analyzed by Masson staining.

RESULTS

Peritoneal dialysis significantly increased the structural injury and decreased junction-related protein ZO-1 and occludin expression in ileum, the expression of proteins relating to the activation of M2 (Erg2, IRF4), but not M1 (CD38, IRF5) macrophages. PD significantly increased the expression of TGF-β1, VEGF and ALK5 protein in peritoneal tissues. PD significantly increased fibrosis (Masson staining) and the expression of fibroblast marker α-SMA in peritoneal tissues. Injection of macrophage clean reagent and ω-3PUFA significantly inhibited M2 activation, and decreased Masson staining, α-SMA, TGF-β1, VEGF and ALK5 protein expression in peritoneal tissues in PD treated rats. ω-3PUFA injection significantly decreased PD-induced injury in ileum and normalized the expression of ZO-1 and occludin in the ileum of PD rats.

CONCLUSION

Omega-3 fatty acids can provide a protective role on PD-induced peritoneal fibrosis and injury of the intestine.

Melatonin inhibits the migration of human gastric carcinoma cells at least in part by remodeling tight junction

The recurrence and metastasis is one of the major reasons for malignant tumor treatment failure. Melatonin, a naturally occuring hormone, could reduce the recurrence and metastasis of various tumors. However, the underlying molecular mechanisms of melatonin on tumor metastasis inhibition have not been fully elucidated. In the present study, we explored the impact of melatonin on the migratory capability of human gastric carcinoma cells using wound healing assay, and further investigated if the inhibition on migration ability of melatonin was embodied by relocating tight junction proteins zo-1 and occludin onto the cells surface to remodel tight junction structure. Immunofluorescence assay and Western blot analysis were performed to detect the expression and cell location of the tight junction proteins. The migration distance was decreased as the cells were treated with melatonin. And melatonin increased the membrane location of tight junction proteins, zo-1 and occludin, showed by immunofluorescence staining and Western blot analysis. The results we got show that melatonin makes tight junction proteins anchored more on the cells membrane to remodel cells tight junction, which increase cells adhesion and decrease motility, resulting in the inhibition of gastric cancer cells migration and metastasis ability.

Retinoic Acid, Leaky Gut, and Autoimmune Diseases

A leaky gut has been observed in a number of autoimmune diseases including type 1 diabetes, multiple sclerosis, inflammatory bowel disease, and systemic lupus erythematosus. Previous studies from our laboratory have shown that lupus mice also bear a leaky gut and that the intestinal barrier function can be enhanced by gut colonization of probiotics such as Lactobacillus spp. Retinoic acid (RA) can increase the relative abundance of Lactobacillus spp. in the gut. Interestingly, RA has also been shown to strengthen the barrier function of epithelial cells in vitro and in the absence of probiotic bacteria. These reports bring up an interesting question of whether RA exerts protective effects on the intestinal barrier directly or through regulating the microbiota colonization. In this review, we will discuss the roles of RA in immunomodulation, recent literature on the involvement of a leaky gut in different autoimmune diseases, and how RA shapes the outcomes of these diseases.

Vitamin A supplementation improves the intestinal mucosal barrier and facilitates the expression of tight junction proteins in rats with diarrhea

OBJECTIVES

The aim of this study is to investigate the specific effects of vitamin A (VA) on diarrhea in rats and its potential targets to protect the intestinal mucosa.

METHODS

Specific pathogen-free Sprague Dawley rats were fed a VA deficient (VAD) or VA normal (VAN) diet for 4 wk. Then, half of the VAN rats were treated with a VAN diet and the other half with a lactose VAN diet. VAD rats were randomly assigned to one of four groups and fed a VAD diet, lactose VAD diet, VAN diet with VA supplementation (VAS) via daily intragastric administration, or a lactose VAN diet with daily VAS. Rat weight and degree of diarrhea were evaluated daily. After 15 d, the serum retinol level was measured by high-performance liquid chromatography, and the serum diamine oxidase (DAO) and zonulin concentrations were analyzed by enzyme-linked immunosorbent assays. The small intestine mucosal pathology was observed by hematoxylin and eosin staining. Western blotting was performed to detect the protein expression levels of occludin and claudin-1 in the intestinal mucosa, and the zonula-occludens 1 expression was assessed using immunohistochemistry.

RESULTS

VAD limited weight gain in rats and increased the degree of diarrhea. The serum retinol levels and the level of tight junction (TJ) proteins claudin-1 and occludin and grip strength were affected by the interaction between lactose-induced diarrhea and the VA diet. Diarrhea, independent of VAD, significantly decreased rat weight, increased serum DAO levels, damaged small intestine villi, and impaired zonula-occludens 1 protein expression. VAD significantly increased the concentration of zonulin independently of diarrhea, but VAS increased the serum retinol level, reduced the severity of diarrhea, increased the expression levels of the TJ proteins, facilitated the restoration of the small intestine villi that were damaged by the diarrhea, and decreased the concentrations of serum DAO and zonulin.

CONCLUSIONS

VAD may aggravate the degree of diarrhea and intestinal mucosal damage during the duration of diarrhea, and VAS helps relieve diarrhea and improves intestinal damage likely by regulating the expression of TJ proteins. Therefore, VA plays a pivotal role in the protection of the intestinal mucosa during instances of diarrhea.

Functional molecules in mesothelial-to-mesenchymal transition revealed by transcriptome analyses

Peritoneal fibrosis is a common complication of abdominal and pelvic surgery, and can also be triggered by peritoneal dialysis, resulting in treatment failure. In these settings, fibrosis is driven by activated myofibroblasts that are considered to be partly derived by mesothelial‐to‐mesenchymal transition (MMT). We hypothesized that, if the molecular signature of MMT could be better defined, these insights could be exploited to block this pathological cellular transition. Rat peritoneal mesothelial cells were purified by the use of an antibody against HBME1, a protein present on mesothelial cell microvilli, and streptavidin nanobead technology. After exposure of sorted cells to a well‐known mediator of MMT, transforming growth factor (TGF)‐β1, RNA sequencing was undertaken to define the transcriptomes of mesothelial cells before and during early‐phase MMT. MMT was associated with dysregulation of transcripts encoding molecules involved in insulin‐like growth factor (IGF) and bone morphogenetic protein (BMP) signalling. The application of either recombinant BMP4 or IGF‐binding protein 4 (IGFBP4) ameliorated TGF‐β1‐induced MMT in culture, as judged from the retention of epithelial morphological and molecular phenotypes, and reduced migration. Furthermore, peritoneal tissue from peritoneal dialysis patients showed less prominent immunostaining than control tissue for IGFBP4 and BMP4 on the peritoneal surface. In a mouse model of TGF‐β1‐induced peritoneal thickening, BMP4 immunostaining on the peritoneal surface was attenuated as compared with healthy controls. Finally, genetic lineage tracing of mesothelial cells was used in mice with peritoneal injury. In this model, administration of BMP4 ameliorated the injury‐induced shape change and migration of mesothelial cells. Our findings demonstrate a distinctive MMT signature, and highlight the therapeutic potential for BMP4, and possibly IGFBP4, to reduce MMT. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

The Peritoneum: Beyond the Tissue - A Review

Background: Despite its complexity, the peritoneum is usually underestimated in classical medical texts simply as the surrounding tissue (serous membrane) of the gut. Novel findings on physiology and morphology of the peritoneum and mesothelial cell exist but they are usually focused or limited to Continuous Ambulatory Peritoneal Dialysis research and practice. This review aims to expose, describe and analyze the most recent evidence on the peritoneum’s morphology, embryology and physiology.

Materials and Methods: A literature review was performed on Pubmed and MEDLINE. With no limit of publication date, original papers and literature reviews about the peritoneum, the peritoneal cavity, peritoneal fluid, and mesothelial cells were included (n = 72).

Results: Peritoneum develops in close relationship to the gut from an early period in embryogenesis. Analyzing together the development of the primitive gut and the surrounding mesothelium helps understanding that the peritoneal cavity, the mesenteries and other structures can be considered parts of the peritoneum. However, some authors consider that structures like the mesenteries are different to the peritoneum. The mesothelial cell has a complex ultrastructural organization with intercellular junctions and apical microvilli. This complexity is further proven by the large array of functions like selective fluid and cell transport; physiological protective barrier; immune induction, modulation, and inhibition; tissue repair and scarring; preventing adhesion and tumoral dissemination; cellular migration; and the epithelial-mesenchymal transition capacity.

Conclusion: Recent evidence on the anatomy, histology, and physiology of the peritoneum, shows that this structure is more complex than a simple serous membrane. These results call for a new conceptualization of peritoneum, and highlight the need of adequate research for identifying clinical relevance of this knowledge.

Calcium mediates cell shape change in human peritoneal mesothelial cells

Mast cells in the peritoneal membrane (PM) may degranulate to release preformed inflammatory mediators including histamine which is capable of diffusing into the surrounding interstitium, modulating cells in their vicinity including, human peritoneal mesothelial cells (hPMC). The present study aimed to investigate the quorum intracellular calcium ([Ca²⁺_i]) response to histamine compared to the membrane soluble ionophore, A23187, in adherent cultured hPMC. To examine [Ca²⁺_i] handling, Fura - 2 loaded cells were exposed to histamine and A23187. Agonist induced transient [Ca²⁺_i] event(s) (TCE) were defined and compared including, resting calcium, peak height, recovery and transient kinetics. Changes in cell shape were examined with immunocytochemistry of the cortical actin (CA) and microtubule (MT) cytoskeleton. To investigate whether histamine induced changes in cell shape were mediated by [Ca²⁺_i], mobilization of [Ca²⁺_i] was prevented with 20 μmol/l of the calcium chelator 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM). Histamine produced a dose dependent increase of [Ca²⁺_i], maximal at 1.0 mmol/l which recovered to the pre-challenge resting value. Transient multiplicity with repeated challenge was evident below a histamine threshold of 10^-2 mmol/l. Morphometric analysis of MTs and CA showed significant cell elongation plus histamine and A23187. The histamine induced cell elongation was eliminated with [Ca²⁺_i] clamping. This data indicated that increased [Ca²⁺_i] was essential for cell elongation and the formation of stress fibres and therefore has a pivotal role in the regulation of the PM barrier.