Selective Transport of Protein-Bound Uremic Toxins in Erythrocytes

Indoxyl sulfate impairs erythropoiesis at BFU-E stage in chronic kidney disease

Chronic kidney disease (CKD) is a global health condition characterized by a progressive deterioration of kidney function. It is associated with high serum levels of uremic toxins (UT), such as Indoxyl Sulfate (IS), which may participate in the genesis of several uremic complications. Anemia is one of the major complications in CKD patients that contribute to cardiovascular disease, increase morbi-mortality, and is associated with a deterioration of kidney failure in these patients. Our study aimed to characterize the impact of IS on CKD-related erythropoiesis. Using cellular and pre-clinical models, we studied cellular and molecular effects of IS on the growth and differentiation of erythroid cells. First, we examined the effect of clinically relevant concentrations of IS (up to 250 μM) in the UT7/EPO cell line. IS at 250 μM increased apoptosis of UT7/EPO cells at 48 h compared to the control condition. We confirmed this apoptotic effect of IS in erythropoiesis in human primary CD34⁺ cells during the later stages of erythropoiesis. Then, in IS-treated human primary CD34⁺ cells and in a (5/6 Nx) mice model, a blockage at the burst-forming unit-erythroid (BFU-E) stage of erythropoiesis was also observed. Finally, IS deregulates a number of erythropoietic related genes such as GATA-1, Erythropoietin-Receptor (EPO-R), and β-globin. Our findings suggest that IS could affect cell viability and differentiation of erythroid progenitors by altering erythropoiesis and contributing to the development of anemia in CKD.

Contribution of Hypoalbuminemia and Anemia to the Prognostic Value of Plasma p-Cresyl Sulfate and p-Cresyl Glucuronide for Cardiovascular Outcome in Chronic Kidney Disease

Free plasma concentrations of protein-bound uremic toxins (PBUTs) may be influenced by serum albumin and hemoglobin. The potential association of serum albumin and hemoglobin with free levels of p-cresyl sulfate (pCS) and p-cresyl glucuronide (pCG) and their predictive value for cardiovascular morbidity and mortality were explored. A total of 523 non-dialysis chronic kidney disease (CKD) stages G1–G5 patients were prospectively followed for the occurrence of fatal or non-fatal cardiovascular events over a 5.5-year period. A negative correlation was found between albumin and between hemoglobin, and both total and free pCS and pCG. In multiple linear regression, PBUTs were negatively associated with eGFR (estimated glomerular filtration rate) and hemoglobin but not albumin. In multivariate Cox regression analysis, albumin was a predictor of outcome, independent of pCS and pCG, without interactions between albumin and pCS or pCG. The relation of low hemoglobin with adverse outcome was lost when albumin was entered into the model. Lower concentrations of pCS and pCG are associated with higher serum albumin and hemoglobin. This may indicate that there are two pathways in the blood that potentially contribute to attenuating the vasculotoxic effects of these PBUTs. The association of PBUTs with cardiovascular risk is not explained by albumin levels, which remains a strong and independent predictor for adverse outcome.

Indoxyl Sulfate Induces Oxidative Changes in Plasma and Hemolysate

The deteriorating function of the kidneys in chronic kidney disease (CKD) is associated, among other things, with the retention of many unnecessary metabolic products in the body. Indoxyl sulfate (IS) belongs to the group of uremic toxins with a high protein binding affinity. Moreover, this compound can generate oxidative stress. We hypothesized that a high concentration of IS might induce oxidative changes in erythrocytes and plasma components, and could therefore contribute to CKD progression. In this study, we evaluated the influence of IS on the oxidative stress parameters in plasma and hemolysate. Moreover, as a result of the action of IS, we observed a decrease in the total antioxidant capacity and a change in the activity of catalase and superoxide dismutase in hemolysate and plasma. The obtained results indicate that IS induces oxidative damage to hemolysate and plasma components. Greater changes in the parameters of oxidative stress were observed in hemolysate than in plasma treated with indoxyl sulfate. The obtained results suggest that the increased concentration of IS in patients with chronic kidney disease may lead to a decrease in the lifespan of erythrocytes in their bloodstream.

Research progress on the relationship between IS and kidney disease and its complications

Indoxyl sulphate (IS) a representative uraemic toxin in the blood of patients with chronic kidney disease (CKD). Its accumulation may be closely related to CKD and the increasing morbidity and mortality of the disease's related complications. Timely and effective detection of the IS level and efficient clearance of IS may effectively prevent the progression of CKD and its related complications. Therefore, this article summarizes the research progress of IS related, including IS in CKD and its associated complications including chronic kidney disease, chronic kidney disease with cardiovascular disease, renal anemia, bone mineral metabolic disease and neuropsychiatric disorders, looking for IS accurate rapid detection methods, and explore the efficient treatment to reduce blood levels of indole phenol sulphate.

In vitro study of dual layer mixed matrix hollow fiber membranes for outside-in filtration of human blood plasma

Hemodialysis mainly removes small water-soluble uremic toxins but cannot effectively remove middle molecules and protein-bound uremic toxins. Besides, the therapy is intermittent leading to fluctuating blood values and fluid status which adversely impacts patients' health. Prolonged hemodialysis (with adequate anticoagulation) could improve the removal of toxins and the development of portable and wearable artificial kidneys could offer more flexibility in the dialysis scheme. This would enhance patients' overall health, autonomy, mobility and flexibility, allowing patients to participate in social and economic life. However, the time that patients' blood is exposed to the dialyzer material is longer during prolonged hemodialysis, and blood clots could obstruct the fiber lumen, resulting in a decrease of the effective membrane surface area available for toxin removal. The outside-in filtration (OIF) mode, wherein blood flows through the inter-fiber space instead of through the fiber lumina, has been applied widely in blood oxygenators to prevent fiber clotting, but not in hemodialysis. In this study, we present for the first time the development of a mixed matrix membrane (MMM) for OIF of human blood plasma. This MMM combines diffusion and adsorption and consists of a polymeric membrane matrix with activated carbon (AC) particles on the inside layer, and a polymeric particle-free layer on the outer fiber layer. Our results show that in vitro MMM fibers for OIF demonstrate superior removal of the protein-bound uremic toxins, indoxyl sulfate and hippuric acid, compared to both earlier MMM fibers designed for inside-out filtration mode and commercial high-flux fibers. STATEMENT OF SIGNIFICANCE: Current hemodialysis therapy cannot effectively remove protein-bound toxins. Prolonged hemodialysis could improve toxin removal. However, during prolonged hemodialysis, blood clots could obstruct the fiber lumen, resulting in decreased effective membrane surface area available for toxin removal. We have prepared, for the first time, dual layer mixed matrix hollow fiber membranes (MMM) for outside-in filtration (OIF). The OIF mode wherein blood would flow through the inter-fiber space instead of through the fiber lumina could prevent fiber clotting. Moreover, the MMMs combine diffusion and adsorption to improve (protein-bound) toxin removal. We believe that the new design of our MMM fibers is an important contribution concerning the development of artificial kidney systems and the improvement of the health and well-being of patients with renal failure.

Structural component changes of erythrocytes caused by oxidative stress generated by indoxyl sulfate

Indoxyl sulfate (IS) belongs to groups of uremic toxins binding to proteins. This compound may contribute to the generation of oxidative stress in chronic kidney disease (CKD) patients. We hypothesized that a high concentration of IS in the blood may induce structural changes of erythrocyte components and thus may contribute to CKD progression. In the present study, we evaluated the influence of IS on hemolysate and membrane proteins' conformational state, lipid membrane fluidity, and internal viscosity in erythrocytes. We examined thiols, carbonyl groups, peroxides, and TBARS levels in erythrocyte incubated with IS. The treatment of erythrocytes with IS led to increase in lipid membrane fluidity, decrease in the internal viscosity of the cells and the motion of the spin labels attached to hemolysate proteins. We did not observe conformational changes in plasma membrane proteins; however, in the plasma membranes of erythrocytes incubated with IS, a decrease in the content of thiol groups and increase in the carbonyls levels and peroxides and TBARS in comparison with the control was observed. The obtained results indicate that IS induces the oxidative damage of erythrocyte components. This may be an important factor that affects the functional properties of erythrocytes in CKD patients.

The impact of intradialytic cycling on the removal of protein-bound uraemic toxins: A randomised cross-over study

The evidence on impact of intradialytic exercise on the removal of urea, is conflictive. Impact of exercise on kinetics of serum levels of protein-bound uraemic toxins, known to exert toxicity and to have kinetics dissimilar of those of urea, has so far not been explored. Furthermore, if any effect, the most optimal intensity, time point and/or required duration of intradialytic exercise to maximise removal remain obscure. We therefore studied the impact of different intradialytic cycling schedules on the removal of protein-bound uraemic toxins during haemodialysis (HD).This randomised cross-over study included seven stable patients who were dialysed with an FX800 dialyser during three consecutive midweek HD sessions of 240 min: (A) without cycling; (B) cycling for 60 min between 60th and 120th minutes of dialysis; and (C) cycling for 60 min between 150th and 210th minutes, with the same cycling load as in session B. Blood and dialysate flows were respectively 300 and 500 mL/min. Blood was sampled from the blood inlet at different time points, and dialysate was partially collected (300 mL/h). Small water soluble solutes and protein-bound toxins were quantified and intradialytic reduction ratios (RR) and overall removal were calculated per solute.Total solute removal and reduction ratios were not different between the three test sessions, except for the reduction ratios RR_60-120 and RR_150-210 for potassium.In conclusion, we add evidence to the existing literature that, regardless of the timing within the dialysis session, intradialytic exercise has no impact on small solute clearance, and demonstrated also a lack of impact for protein-bound solutes.

Gut microbiota generation of protein-bound uremic toxins and related metabolites is not altered at different stages of chronic kidney disease

Chronic kidney disease (CKD) is characterized by accumulation of protein-bound uremic toxins such as p-cresyl sulfate, p-cresyl glucuronide, indoxyl sulfate and indole-3-acetic acid, which originate in the gut. Intestinal bacteria metabolize aromatic amino acids into p-cresol and indole, (further conjugated in the colon mucosa and liver) and indole-3-acetic acid. Here we measured fecal, plasma and urine metabolite concentrations; the contribution of gut bacterial generation to plasma protein-bound uremic toxins accumulation; and influx into the gut of circulating protein-bound uremic toxins at different stages of CKD. Feces, blood and urine were collected from 14 control individuals and 141 patients with CKD. Solutes were quantified by ultra-high performance liquid chromatography. To assess the rate of bacterial generation of p-cresol, indole and indole-3-acetic acid, fecal samples were cultured ex vivo. With CKD progression, an increase in protein-bound uremic toxins levels was observed in plasma, whereas the levels of these toxins and their precursors remained the same in feces and urine. Anaerobic culture of fecal samples showed no difference in ex vivo p-cresol, indole and indole-3-acetic acid generation. Therefore, differences in plasma protein-bound uremic toxins levels between different CKD stages cannot be explained by differences in bacterial generation rates in the gut, suggesting retention due to impaired kidney function as the main contributor to their increased plasma levels. Thus, as fractional clearance decreased with the progression of CKD, tubular clearance appeared to be more affected than the glomerular filtration rate, and there was no net increase in protein-bound uremic toxins influx into the gut lumen with increased plasma levels.