A Historical Perspective on Uremia and Uremic Toxins.

Uremia, also known as uremic syndrome, refers to the clinical symptoms in the final stage of renal failure. The definition of the term has changed over time due to an improved comprehension of the kidney's function and the advancement of dialysis technology. Here, we aim to present an overview of the various concepts that have developed regarding uremia throughout the years. We provide a comprehensive review of the historical progression starting from the early days of Kolff and his predecessors, continuing with the initial research conducted by Niwa et al., and culminating in the remote sensing hypothesis of Nigam. Additionally, we explore the subsequent investigation into the function of these toxins as signaling molecules in various somatic cells.

Role of Organic Anion Transporter NPT4 in Renal Handling of Uremic Toxin 3-indoxyl Sulfate.

Sodium-phosphate transporter NPT4 (SLC17A3) is a membrane transporter for organic anionic compounds localized on the apical membranes of kidney proximal tubular epithelial cells and plays a role in the urinary excretion of organic anionic compounds. However, its physiological role has not been sufficiently elucidated because its substrate specificity is yet to be determined. The present study aimed to comprehensively explore the physiological substrates of NPT4 in newly developed Slc17a3-/- mice using a metabolomic approach. Metabolomic analysis showed that the plasma concentrations of 11 biological substances, including 3-indoxyl sulfate, were more than two-fold higher in Slc17a3-/- mice than in wild-type mice. Moreover, urinary excretion of 3-indoxyl sulfate was reduced in Slc17a3-/- mice compared to that in wild-type mice. The uptake of 3-indoxyl sulfate by NPT4-expressing Xenopus oocytes was significantly higher than that by water-injected oocytes. The calculated Km and Vmax values for NPT4-mediated 3-indoxyl sulfate uptake were 4.52 ± 1.18 mM and 1.45 ± 0.14 nmol/oocyte/90 min, respectively. In conclusion, the present study revealed that 3-indoxyl sulfate is a novel substrate of NPT4 based on the metabolomic analysis of Slc17a3-/- mice, suggesting that NPT4 regulates systemic exposure to 3-indoxyl sulfate by regulating its urinary excretion.

Sweet, bloody consumption - what we eat and how it affects vascular ageing, the BBB and kidney health in CKD.

In today's industrialized society food consumption has changed immensely toward heightened red meat intake and use of artificial sweeteners instead of grains and vegetables or sugar, respectively. These dietary changes affect public health in general through an increased incidence of metabolic diseases like diabetes and obesity, with a further elevated risk for cardiorenal complications. Research shows that high red meat intake and artificial sweeteners ingestion can alter the microbial composition and further intestinal wall barrier permeability allowing increased transmission of uremic toxins like p-cresyl sulfate, indoxyl sulfate, trimethylamine n-oxide and phenylacetylglutamine into the blood stream causing an array of pathophysiological effects especially as a strain on the kidneys, since they are responsible for clearing out the toxins. In this review, we address how the burden of the Western diet affects the gut microbiome in altering the microbial composition and increasing the gut permeability for uremic toxins and the detrimental effects thereof on early vascular aging, the kidney per se and the blood-brain barrier, in addition to the potential implications for dietary changes/interventions to preserve the health issues related to chronic diseases in future.

Gut Microbiota as a Source of Uremic Toxins.

Uremic retention solutes are the compounds that accumulate in the blood when kidney excretory function is impaired. Some of these compounds are toxic at high concentrations and are usually known as "uremic toxins". The cumulative detrimental effect of uremic toxins results in numerous health problems and eventually mortality during acute or chronic uremia, especially in end-stage renal disease. More than 100 different solutes increase during uremia; however, the exact origin for most of them is still debatable. There are three main sources for such compounds: exogenous ones are consumed with food, whereas endogenous ones are produced by the host metabolism or by symbiotic microbiota metabolism. In this article, we identify uremic retention solutes presumably of gut microbiota origin. We used database analysis to obtain data on the enzymatic reactions in bacteria and human organisms that potentially yield uremic retention solutes and hence to determine what toxins could be synthesized in bacteria residing in the human gut. We selected biochemical pathways resulting in uremic retention solutes synthesis related to specific bacterial strains and revealed links between toxin concentration in uremia and the proportion of different bacteria species which can synthesize the toxin. The detected bacterial species essential for the synthesis of uremic retention solutes were then verified using the Human Microbiome Project database. Moreover, we defined the relative abundance of human toxin-generating enzymes as well as the possibility of the synthesis of a particular toxin by the human metabolism. Our study presents a novel bioinformatics approach for the elucidation of the origin of both uremic retention solutes and uremic toxins and for searching for the most likely human microbiome producers of toxins that can be targeted and used for the therapy of adverse consequences of uremia.

Gut Microbiota and Their Derived Metabolites, a Search for Potential Targets to Limit Accumulation of Protein-Bound Uremic Toxins in Chronic Kidney Disease.

Chronic kidney disease (CKD) is characterized by gut dysbiosis with a decrease in short-chain fatty acid (SCFA)-producing bacteria. Levels of protein-bound uremic toxins (PBUTs) and post-translational modifications (PTMs) of albumin increase with CKD, both risk factors for cardiovascular morbidity and mortality. The relationship between fecal metabolites and plasma concentrations of PBUTs in different stages of CKD (n = 103) was explored. Estimated GFR tends to correlate with fecal butyric acid (BA) concentrations (rs = 0.212; p = 0.032), which, in its turn, correlates with the abundance of SCFA-producing bacteria. Specific SCFAs correlate with concentrations of PBUT precursors in feces. Fecal levels of p-cresol correlate with its derived plasma UTs (p-cresyl sulfate: rs = 0.342, p < 0.001; p-cresyl glucuronide: rs = 0.268, p = 0.006), whereas an association was found between fecal and plasma levels of indole acetic acid (rs = 0.306; p = 0.002). Finally, the albumin symmetry factor correlates positively with eGFR (rs = 0.274; p = 0.005). The decreased abundance of SCFA-producing gut bacteria in parallel with the fecal concentration of BA and indole could compromise the intestinal barrier function in CKD. It is currently not known if this contributes to increased plasma levels of PBUTs, potentially playing a role in the PTMs of albumin. Further evaluation of SCFA-producing bacteria and SCFAs as potential targets to restore both gut dysbiosis and uremia is needed.

Extracellular Vesicles and Their Relationship with the Heart-Kidney Axis, Uremia and Peritoneal Dialysis.

Cardiorenal syndrome (CRS) is described as primary dysfunction in the heart culminating in renal injury or vice versa. CRS can be classified into five groups, and uremic toxin (UT) accumulation is observed in all types of CRS. Protein-bound uremic toxin (PBUT) accumulation is responsible for permanent damage to the renal tissue, and mainly occurs in CRS types 3 and 4, thus compromising renal function directly leading to a reduction in the glomerular filtration rate (GFR) and/or subsequent proteinuria. With this decrease in GFR, patients may need renal replacement therapy (RRT), such as peritoneal dialysis (PD). PD is a high-quality and home-based dialysis therapy for patients with end-stage renal disease (ESRD) and is based on the semi-permeable characteristics of the peritoneum. These patients are exposed to factors which may cause several modifications on the peritoneal membrane. The presence of UT may harm the peritoneum membrane, which in turn can lead to the formation of extracellular vesicles (EVs). EVs are released by almost all cell types and contain lipids, nucleic acids, metabolites, membrane proteins, and cytosolic components from their cell origin. Our research group previously demonstrated that the EVs can be related to endothelial dysfunction and are formed when UTs are in contact with the endothelial monolayer. In this scenario, this review explores the mechanisms of EV formation in CRS, uremia, the peritoneum, and as potential biomarkers in peritoneal dialysis.

A low aromatic amino-acid diet improves renal function and prevent kidney fibrosis in mice with chronic kidney disease.

Despite decades of use of low protein diets (LPD) in the management of chronic kidney disease (CKD), their mechanisms of action are unclear. A reduced production of uremic toxins could contribute to the benefits of LPDs. Aromatic amino-acids (AA) are precursors of major uremic toxins such as p-cresyl sulfate (PCS) and indoxyl sulfate (IS). We hypothesize that a low aromatic amino acid diet (LA-AAD, namely a low intake of tyrosine, tryptophan and phenylalanine) while being normoproteic, could be as effective as a LPD, through the decreased production of uremic toxins. Kidney failure was chemically induced in mice with a diet containing 0.25% (w/w) of adenine. Mice received three different diets for six weeks: normoproteic diet (NPD: 14.7% proteins, aromatic AAs 0.019%), LPD (5% proteins, aromatic AAs 0.007%) and LA-AAD (14% proteins, aromatic AAs 0.007%). Both LPD and LA-AAD significantly reduced proteinuria, kidney fibrosis and inflammation. While LPD only slightly decreased plasma free PCS and free IS compared to NPD; free fractions of both compounds were significantly decreased by LA-AAD. These results suggest that a LA-AAD confers similar benefits of a LPD in delaying the progression of CKD through a reduction in some key uremic toxins production (such as PCS and IS), with a lower risk of malnutrition.

Impact of kidney dysfunction on hepatic and intestinal drug transporters.

Emerging information suggests that pathology of the kidney may not only affect expression and function of membrane transporters in the organ, but also in the gastrointestinal tract and the liver. Transporter dysfunction may cause effects on handling of drug as well as endogenous compounds with subsequent clinical consequences. A literature search was conducted on Ovid and PubMed databases to select relevant in vitro, animal and human studies that have reported expression, protein abundance and function of the gastrointestinal and liver localized ABC transporters and SLC carriers in kidney dysfunction or uremia states. The altered function of drug transporters in the liver and intestines in kidney failure subjects may provide compensatory activity in handling endogenous compounds (e.g. uremic toxins), which is expected to affect drug pharmacokinetics and local drug actions.

Impaired Tubular Secretion of Organic Solutes in Advanced Chronic Kidney Disease.

BACKGROUND: The clearance of solutes removed by tubular secretion may be altered out of proportion to the GFR in CKD. Recent studies have described considerable variability in the secretory clearance of waste solutes relative to the GFR in patients with CKD. METHODS: To test the hypothesis that secretory clearance relative to GFR is reduced in patients approaching dialysis, we used metabolomic analysis to identify solutes in simultaneous urine and plasma samples from 16 patients with CKD and an eGFR of 7±2 ml/min per 1.73 m2 and 16 control participants. Fractional clearances were calculated as the ratios of urine to plasma levels of each solute relative to those of creatinine and urea in patients with CKD and to those of creatinine in controls. RESULTS: Metabolomic analysis identified 39 secreted solutes with fractional clearance >3.0 in control participants. Fractional clearance values in patients with CKD were reduced on average to 65%±27% of those in controls. These values were significantly lower for 18 of 39 individual solutes and significantly higher for only one. Assays of the secreted anions phenylacetyl glutamine, p-cresol sulfate, indoxyl sulfate, and hippurate confirmed variable impairment of secretory clearances in advanced CKD. Fractional clearances were markedly reduced for phenylacetylglutamine (4.2±0.6 for controls versus 2.3±0.6 for patients with CKD; P<0.001), p-cresol sulfate (8.6±2.6 for controls versus 4.1±1.5 for patients with CKD; P<0.001), and indoxyl sulfate (23.0±7.3 versus 7.5±2.8; P<0.001) but not for hippurate (10.2±3.8 versus 8.4±2.6; P=0.13). CONCLUSIONS: Secretory clearances for many solutes are reduced more than the GFR in advanced CKD. Impaired secretion of these solutes might contribute to uremic symptoms as patients approach dialysis.

Association of Sarcopenia and Gut Microbiota Composition in Older Patients with Advanced Chronic Kidney Disease, Investigation of the Interactions with Uremic Toxins, Inflammation and Oxidative Stress.

Sarcopenia is a prevalent condition in chronic kidney disease (CKD). We determined gut microbiota (gMB) composition in CKD patients with or without sarcopenia. Furthermore, we investigated whether in these patients, there was any association between gMB, uremic toxins, inflammation and oxidative stress. We analyzed gMB composition, uremic toxins (indoxyl sulphate and p-cresyl sulphate), inflammatory cytokines (interleukin 10, tumor necrosis factor α, interleukin 6, interleukin 17, interleukin 12 p70, monocyte chemoattractant protein-1 and fetuin-A) and oxidative stress (malondialdehyde) of 64 elderly CKD patients (10 < eGFR < 45 mL/min/1.73 m2, not on dialysis) categorized as sarcopenic and not-sarcopenic. Sarcopenia was defined according to European Working Group on Sarcopenia in Older People 2 criteria. Sarcopenic patients had a greater abundance of the Micrococcaceae and Verrucomicrobiaceae families and of Megasphaera, Rothia, Veillonella, Akkermansia and Coprobacillus genera. They had a lower abundance of the Gemellaceae and Veillonellaceae families and of Acidaminococcus and Gemella genera. GMB was associated with uremic toxins, inflammatory cytokines and MDA. However, uremic toxins, inflammatory cytokines and MDA were not different in sarcopenic compared with not-sarcopenic individuals, except for interleukin 10, which was higher in not-sarcopenic patients. In older CKD patients, gMB was different in sarcopenic than in not-sarcopenic ones. Several bacterial families and genera were associated with uremic toxins and inflammatory cytokines, although none of these latter substantially different in sarcopenic versus not-sarcopenic patients.

Acute Kidney Injury and Organ Dysfunction: What Is the Role of Uremic Toxins?

Acute kidney injury (AKI), defined as an abrupt increase in serum creatinine, a reduced urinary output, or both, is experiencing considerable evolution in terms of our understanding of the pathophysiological mechanisms and its impact on other organs. Oxidative stress and reactive oxygen species (ROS) are main contributors to organ dysfunction in AKI, but they are not alone. The precise mechanisms behind multi-organ dysfunction are not yet fully accounted for. The building up of uremic toxins specific to AKI might be a plausible explanation for these disturbances. However, controversies have arisen around their effects in organs other than the kidney, because animal models usually depict AKI as a kidney-specific injury. Meanwhile, humans present AKI frequently in association with multi-organ failure (MOF). Until now, medium-molecular-weight molecules, such as inflammatory cytokines, have been proven to play a role in endothelial and epithelial injury, leading to increased permeability and capillary leakage, mainly in pulmonary and intestinal tissues.

Transfer of uremic solutes across the human term placenta: An ex vivo study in the dual-side perfused cotyledon.

INTRODUCTION: An increasing number of women becomes pregnant while suffering from chronic kidney disease (CKD). As a result of decreased renal function, uremic solutes circulate at high levels in the maternal circulation. This study aimed to acquire more knowledge about the placental transfer of uremic solutes across the human placenta. METHODS: Placental transfer was studied in healthy term placentas, via the ex vivo dual-side human cotyledon perfusion technique (closed-closed set-up for both maternal and fetal circulations). Uremic solute concentrations in maternal and fetal perfusates were measured via LC-MS/MS over 180 min of perfusion. RESULTS: We found that the studied compounds demonstrated different degrees of placental transfer. Fetal-to-maternal perfusate ratios at t = 180 min were for anthranilic acid 1.00 ± 0.02, indole-3-acetic acid 0.47 ± 0.08, hippuric acid 0.36 ± 0.18, l-arabinitol 0.33 ± 0.04, indoxyl sulfate 0.33 ± 0.11, neopterin 0.28 ± 0.14 and kynurenic acid 0.13 ± 0.03. All uremic solutes studied also emerged in the perfusates when cotyledons were perfused in the absence of uremic solute concentrations added to the maternal reservoir. For kynurenin these concentrations were so high, it complicated the calculation of a transfer ratio for the exogenously administered compound. DISCUSSION: After 180 min of exposure the extent of placental transfer differs substantially for the solutes studied, reflecting different transfer rates. Future studies should investigate to what extent specific uremic solutes reach the fetal circulation in vivo and how they may interfere with organ function and development of the unborn child.

Uremic Toxins and Protein-Bound Therapeutics in AKI and CKD: Up-to-Date Evidence.

Uremic toxins are defined as harmful metabolites that accumulate in the human body of patients whose renal function declines, especially chronic kidney disease (CKD) patients. Growing evidence demonstrates the deteriorating effect of uremic toxins on CKD progression and CKD-related complications, and removing uremic toxins in CKD has become the conventional treatment in the clinic. However, studies rarely pay attention to uremic toxin clearance in the early stage of acute kidney injury (AKI) to prevent progression to CKD despite increasing reports demonstrating that uremic toxins are correlated with the severity of injury or mortality. This review highlights the current evidence of uremic toxin accumulation in AKI and the therapeutic value to prevent CKD progression specific to protein-bound uremic toxins (PBUTs).