Renal clearance and intestinal generation of p-cresyl sulfate and indoxyl sulfate in CKD

Predictive value of protein-bound uremic toxins for heart failure in patients with chronic kidney disease

AIMS

This retrospective cohort study aimed to be the first to evaluate the association between plasma protein-bound uremic toxins (PBUTs) concentrations, echocardiographic parameters of heart failure (HF), and incident HF events in patients with chronic kidney disease (CKD) not on dialysis.

METHODS AND RESULTS

Retrospective, single-centre, cohort study at the Ghent University Hospital, Belgium. Adults with CKD stages G1-G5, not on dialysis, could be included. Exclusion criteria were ongoing pregnancy, age <18 years, active acute infection, active malignancy, history of transplantation, or a cardiovascular event within 3 months prior to inclusion. Free and total concentrations of five PBUTs were quantified at baseline: indoxyl sulfate (IxS), p-cresyl sulfate (pCS), p-cresyl glucuronide (pCG), indole-3 acetic acid (IAA), and hippuric acid (HA). Patients were grouped into three echocardiographic categories: normal left ventricular ejection fraction (LVEF) and normal left ventricular end-diastolic pressure (LVEDP), normal LVEF and increased LVEDP, and reduced LVEF, based on available echocardiographic data in a time interval of ±6 months around the plasma sample collection. A total of 523 patients were included between January 2011 and January 2014. Echocardiographic data within the predefined timeframe were available for 210 patients (40% of patients). Levels of pCG and pCS were significantly higher in patients with reduced (<50%) versus normal LVEF (P < 0.05). After a median follow-up 5.5 years, 43 (8.4%) patients reached the composite endpoint of hospitalization or mortality due to HF. Free fractions of IxS, pCS, and pCG showed the strongest association with clinical outcome: free IxS: HR 1.71 (95% CI 1.11-2.63; P = 0.015), free pCS: HR 1.82 (95% CI 1.11-3.01; P = 0.019), and free pCG: HR 1.67 (95% CI 1.08-2.58; P = 0.020), and these results were independent of age, gender, body mass index, diabetes, and systolic blood pressure. In models that were also adjusted for serum creatinine, the free fractions of these PBUTs remained significant.

CONCLUSIONS

Elevated free concentrations of IxS, pCG, and pCS were independently associated with an increased risk of HF events in non-dialysed CKD patients. Further research is necessary to confirm these findings and investigate the potential impact of PBUT-lowering interventions on HF events in this patient group.

Impact of sacubitril/valsartan on cardiac and systemic hypoxia in chronic heart failure

In heart failure patients with reduced ejection fraction, Sacubitril/valsartan (S/V) increased proBNP T71 glycosylation, which is regulated negatively by hypoxia via miR-30a in vitro. Using a cohort of 73 HFrEF patients who were transitioned from standard HF medication to S/V, we found that the increase in proBNP T71 glycosylation after S/V was associated with a decrease in cardiac hypoxia. We further found that plasma levels of K709-acteylated HIF1α, HIF-regulated and HIF-independent biomarkers also evolved consistently with a decrease in hypoxia. We further confirmed that biomarker changes were related to hypoxia, in a rat model subjected to isobaric hypoxia. We measured them in rats subjected to isobaric hypoxia. Overall, these data strongly suggest that optimally treated HFrEF patients exhibited subclinical hypoxia that is improved by S/V. The data also posit proBNP T71 glycosylation as a biomarker of cardiac hypoxia.

SLC22A11 Inserts the Uremic Toxins Indoxyl Sulfate and P-Cresol Sulfate into the Plasma Membrane

Chronic kidney disease (CKD) is a global health concern affecting millions worldwide. One of the critical challenges in CKD is the accumulation of uremic toxins such as p-cresol sulfate (pCS) and indoxyl sulfate (IS), which contribute to systemic damage and CKD progression. Understanding the transport mechanisms of these prominent toxins is essential for developing effective treatments. Here, we investigated whether pCS and IS are routed to the plasma membrane or to the cytosol by two key transporters, SLC22A11 and OAT1. To distinguish between cytosolic transport and plasma membrane insertion, we used a hyperosmolarity assay in which the accumulation of substrates into HEK-293 cells in isotonic and hypertonic buffers was measured in parallel using LC-MS/MS. Judging from the efficiency of transport (TE), pCS is a relevant substrate of SLC22A11 at 7.8 ± 1.4 µL min-1 mg protein-1 but not as good as estrone-3-sulfate; OAT1 translocates pCS less efficiently. The TE of SLC22A11 for IS was similar to pCS. For OAT1, however, IS is an excellent substrate. With OAT1 and p-aminohippuric acid, our study revealed an influence of transporter abundance on the outcomes of the hyperosmolarity assay; very high transport activity confounded results. SLC22A11 was found to insert both pCS and IS into the plasma membrane, whereas OAT1 conveys these toxins to the cytosol. These disparate transport mechanisms bear profound ramifications for toxicity. Membrane insertion might promote membrane damage and microvesicle release. Our results underscore the imperative for detailed structural inquiries into the translocation of small molecules.

Effect of Low Protein Diet Supplemented with Ketoanalogs on Endothelial Function and Protein-Bound Uremic Toxins in Patients with Chronic Kidney Disease

Studies have demonstrated that a low-protein diet supplemented with ketoanalogs (KAs) could significantly retard progression of renal function in patients with chronic kidney disease (CKD) stages 3-5. However, its effects on endothelial function and serum levels of protein-bound uremic toxins remain elusive. Therefore, this study explored whether a low-protein diet (LPD) supplemented with KAs affects kidney function, endothelial function, and serum uremic toxin levels in a CKD-based cohort. In this retrospective cohort, we enrolled 22 stable CKD stage 3b-4 patients on LPD (0.6-0.8 g/day). Patients were categorized into control (LPD only) and study groups (LPD + KAs 6 tab/day). Serum biochemistry, total/free indoxyl sulfate (TIS/FIS), total/free p-cresyl sulfate (TPCS/FPCS), and flow-mediated dilation (FMD) were measured before and after 6 months of KA supplementation. Before the trial, there were no significant differences in kidney function, FMD, or uremic toxin levels between the control and study groups. When compared with the control group, the paired t-test showed a significant decrease in TIS and FIS (all p < 0.05) and a significant increase in FMD, eGFR, and bicarbonate (all p < 0.05). In multivariate regression analysis, an increase in FMD (p < 0.001) and a decrease in FPCS (p = 0.012) and TIS (p < 0.001) remained persistent findings when adjusted for age, systolic blood pressure (SBP), sodium, albumin, and diastolic blood pressure (DBP). LPD supplemented with KAs significantly preserves kidney function and provides additional benefits on endothelial function and protein-bound uremic toxins in patients with CKD.

Enhancer of zeste homolog 2 promotes renal fibrosis after acute kidney injury by inducing epithelial-mesenchymal transition and activation of M2 macrophage polarization

Long-term follow-up data indicates that 1/4 patients with acute kidney injury (AKI) will develop to chronic kidney disease (CKD). Our previous studies have demonstrated that enhancer of zeste homolog 2 (EZH2) played an important role in AKI and CKD. However, the role and mechanisms of EZH2 in AKI-to-CKD transition are still unclear. Here, we demonstrated EZH2 and H3K27me3 highly upregulated in kidney from patients with ANCA-associated glomerulonephritis, and expressed positively with fibrotic lesion and negatively with renal function. Conditional EZH2 deletion or pharmacological inhibition with 3-DZNeP significantly improved renal function and attenuated pathological lesion in ischemia/reperfusion (I/R) or folic acid (FA) mice models (two models of AKI-to-CKD transition). Mechanistically, we used CUT & Tag technology to verify that EZH2 binding to the PTEN promoter and regulating its transcription, thus regulating its downstream signaling pathways. Genetic or pharmacological depletion of EZH2 upregulated PTEN expression and suppressed the phosphorylation of EGFR and its downstream signaling ERK1/2 and STAT3, consequently alleviating the partial epithelial-mesenchymal transition (EMT), G2/M arrest, and the aberrant secretion of profibrogenic and proinflammatory factors in vivo and vitro experiments. In addition, EZH2 promoted the EMT program induced loss of renal tubular epithelial cell transporters (OAT1, ATPase, and AQP1), and blockade of EZH2 prevented it. We further co-cultured macrophages with the medium of human renal tubular epithelial cells treated with H₂O₂ and found macrophages transferred to M2 phenotype, and EZH2 could regulate M2 macrophage polarization through STAT6 and PI3K/AKT pathways. These results were further verified in two mice models. Thus, targeted inhibition of EZH2 might be a novel therapy for ameliorating renal fibrosis after acute kidney injury by counteracting partial EMT and blockade of M2 macrophage polarization.

Traditional Chinese Medicine: An Exogenous Regulator of Crosstalk between the Gut Microbial Ecosystem and CKD

Chronic kidney disease (CKD) is often accompanied by an imbalance in the gut microbial ecosystem. Notably, the imbalanced gut microbiota and impaired intestinal barrier are the keys to the crosstalk between the gut microbial ecosystem and CKD, which was the central point of previous studies. Traditional Chinese medicine (TCM) has shown considerable efficacy in the treatment of CKD. However, the therapeutic mechanisms have not been fully elucidated. In this review, we explored therapeutic mechanisms by which TCM improved CKD via the gut microbial ecosystem. In particular, we focused on the restored gut microbiota (i.e., short-chain fatty acid- and uremic toxin-producing bacteria), improved gut-derived metabolites (i.e., short-chain fatty acid, indoxyl sulfate, p-Cresyl sulfate, and trimethylamine-N-oxide), and intestinal barrier (i.e., permeability and microbial translocation) as therapeutic mechanisms. The results found that the metabolic pattern of gut microbiota and the intestinal barrier were improved through TCM treatment. Moreover, the microbiota-transfer study confirmed that part of the protective effect of TCM was dependent on gut microbiota, especially SCFA-producing bacteria. In conclusion, TCM may be an important exogenous regulator of crosstalk between the gut microbial ecosystem and CKD, which was partly attributable to the mediation of microbiota-targeted intervention.

Inflammation in Children with CKD Linked to Gut Dysbiosis and Metabolite Imbalance

BACKGROUND

CKD is characterized by a sustained proinflammatory response of the immune system, promoting hypertension and cardiovascular disease. The underlying mechanisms are incompletely understood but may be linked to gut dysbiosis. Dysbiosis has been described in adults with CKD; however, comorbidities limit CKD-specific conclusions.

METHODS

We analyzed the fecal microbiome, metabolites, and immune phenotypes in 48 children (with normal kidney function, CKD stage G3-G4, G5 treated by hemodialysis [HD], or kidney transplantation) with a mean±SD age of 10.6±3.8 years.

RESULTS

Serum TNF-α and sCD14 were stage-dependently elevated, indicating inflammation, gut barrier dysfunction, and endotoxemia. We observed compositional and functional alterations of the microbiome, including diminished production of short-chain fatty acids. Plasma metabolite analysis revealed a stage-dependent increase of tryptophan metabolites of bacterial origin. Serum from patients on HD activated the aryl hydrocarbon receptor and stimulated TNF-α production in monocytes, corresponding to a proinflammatory shift from classic to nonclassic and intermediate monocytes. Unsupervised analysis of T cells revealed a loss of mucosa-associated invariant T (MAIT) cells and regulatory T cell subtypes in patients on HD.

CONCLUSIONS

Gut barrier dysfunction and microbial metabolite imbalance apparently mediate the proinflammatory immune phenotype, thereby driving the susceptibility to cardiovascular disease. The data highlight the importance of the microbiota-immune axis in CKD, irrespective of confounding comorbidities.

Epigallocatechin-3-Gallate Decreases Plasma and Urinary Levels of p-Cresol by Modulating Gut Microbiota in Mice

p-Cresol (PC), a gut bacterial product of tyrosine catabolism, is recognized as a uremic toxin that has negative biological effects. Lowering the plasma PC level by manipulating the gut bacterial composition represents a promising therapeutic strategy in chronic kidney disease. This study was conducted to reveal whether epigallocatechin-3-gallate (EGCG) decreases plasma PC levels by limiting its bacterial production in a mouse model. The PC concentration in the samples was measured by high-performance liquid chromatography (HPLC) after treatments with sulfatase and β-glucuronidase. The results showed that the addition of EGCG to the diet decreased the plasma and urinary concentrations of PC in a dose-dependent manner, with a statistically significant difference between the control group and the 0.2% EGCG group. However, once EGCG was enzymatically hydrolyzed to epigallocatechin (EGC) and gallic acid, such effects were lost almost completely. The addition of 0.2% EGCG in the diet was accompanied by a decreased abundance of Firmicutes at the phylum level and Clostridiales at the order level, which constitute a large part of PC produced from tyrosine. In conclusion, EGCG, not EGC, reduced plasma and urinary concentrations of PC in mice by suppressing its bacterial production with accompanying alteration of the relative abundance of PC producers.

Altered gut microbiota and gut-derived p-cresyl sulfate serum levels in peritoneal dialysis patients

Peritoneal dialysis (PD) is a renal replacement therapy for end-stage renal disease. Gut microbiota-derived uremic solutes, indoxyl sulfate (IS), p-cresyl sulfate (PCS), and trimethylamine-N-oxide (TMAO) accumulate in PD patients. The objective was to explore the gut microbiota and their influence on uremic toxins in PD patients and healthy controls (HC). Fecal samples were collected from PD patients (n = 105) and HC (n = 102). 16S rRNA gene regions were sequenced for gut microbiota analysis. IS, PCS, and TMAO levels were measured using HPLC-MS. PD patients exhibited lower alpha diversity and altered gut microbiota composition compared to HC. At the genus level, PD patients showed increased abundance of opportunistic pathogenic bacteria, and decreased abundance of beneficial bacteria. Three Operational Taxonomic Units discriminated PD patients from HC. Phenylalanine metabolism increased in PD, whereas tryptophan metabolism was unaltered. Low serum PCS did not necessarily mean healthier due to the loss of alpha diversity, increased Proteobacteria and opportunistic pathogenic bacteria. High serum PCS was mainly caused by elevated p-cresol-producing bacteria, enriched amino acid related enzymes, and enhanced sulfur metabolism, rather than declined residual renal function. In patients with different urine volumes, the gut microbiota alpha diversity and composition were unaltered, but serum IS and TMAO were significantly elevated in anuric patients. In conclusion, the gut microbiota abundance, composition, and function were altered in PD patients, which increased the PCS levels. We provided a better understanding of the microbiota-metabolite-kidney axis in PD patients. Targeting certain bacteria could decrease the PCS levels, whereas preserving the residual renal function could reduce the IS and TMAO levels.

miR-29c Inhibits Renal Interstitial Fibrotic Proliferative Properties through PI3K-AKT Pathway

Renal fibrosis, in particular tubulointerstitial fibrosis, which is characterized by an increased extracellular matrix (ECM) formation and development in the interstitium, is the common end pathway for nearly all progressive kidney disorders. One of the sources for this matrix is the epithelial to mesenchymal transition (EMT) from the tabular epithelium. The driving force behind it is some profibrotic growth factors such as transforming growth factor-β (TGF-β) which is responsible for the formation of collagen in renal fibrosis. miR-29c, which is an antifibrotic microRNA, downregulates renal interstitial fibrosis by downregulating the TGF-β and collagen. However, it is not known whether miR-29c mediates the TGF-β1-driven PI3K-Akt pathway and Col-1 triggering within NRK-52E cultures. The main objective of this investigation was to examine the influence of miR-29c on the downregulation of the TGF-β1-driven PI3K-Akt pathway and Col-1 triggering in NRK-52E cultures. This study revealed that miR-29c inhibited TGF-β1 expression in NRK-52E cell cultures. Overexpression of miR-29c significantly inhibits NRK-52E culture proliferation mediated by TGF-β1. miR-29c inhibited the expression of Col-1 and decreased PI3K/Akt phosphorylation. These findings revealed a novel mechanism by which miR29c inhibits the proliferation of renal interstitial fibrotic cultures by downregulating the PI3k-Akt pathway, which is controlled by TGF-β1.

Reno-Protective Effect of Low Protein Diet Supplemented With α-Ketoacid Through Gut Microbiota and Fecal Metabolism in 5/6 Nephrectomized Mice

Background

Low protein supplemented with α-ketoacid diet (LKD) was recommended to be an essential intervention to delay the progression of chronic kidney disease (CKD) in patients who were not yet on dialysis. Aberrant gut microbiota and metabolism have been reported to be highly associated with CKD. However, the effect of LKD on gut microbiota and related fecal metabolism in CKD remains unclear.

Methods

Mice were fed with normal protein diet (NPD group), low protein diet (LPD group), and low protein diet supplemented with α-ketoacid (LKD group) after 5/6 nephrectomy. At the end of the study, blood, kidney tissues, and feces were collected for biochemical analyses, histological, 16S rRNA sequence of gut microbiome, and untargeted fecal metabolomic analyses.

Results

Both LKD and LPD alleviate renal failure and fibrosis, and inflammatory statement in 5/6 nephrectomized mice, especially the LKD. In terms of gut microbiome, LKD significantly improved the dysbiosis induced by 5/6Nx, representing increased α-diversity and decreased F/B ratio. Compared with NPD, LKD significantly increased the abundance of g_Parasutterella, s_Parabacteroides_sp_CT06, f_Erysipelotrichaceae, g_Akkermansia, g_Gordonibacter, g_Faecalitalea, and s_Mucispirillum_sp_69, and decreased s_Lachnospiraceae_bacterium_28-4 and g_Lachnoclostridium. Moreover, 5/6Nx and LKD significantly altered fecal metabolome. Then, multi-omics analysis revealed that specific metabolites involved in glycerophospholipid, purine, vitamin B6, sphingolipid, phenylalanine, tyrosine and tryptophan biosynthesis, and microbes associated with LKD were correlated with the amelioration of CKD.

Conclusion

LKD had a better effect than LPD on delaying renal failure in 5/6 nephrectomy-induced CKD, which may be due to the regulation of affecting the gut microbiome and fecal metabolic profiles.

Novel intestinal dialysis interventions and microbiome modulation to control uremia

PURPOSE OF REVIEW

In patients with chronic kidney disease (CKD), the gut plays a key role in the homeostasis of fluid and electrolyte balance and the production and disposal of uremic toxins. This review summarizes the current evidence on the gut-targeted interventions to control uremia, fluid overload, hyperkalemia and hyperphosphatemia in CKD.

RECENT FINDINGS

Studies have emerged that support the concept of intestinal dialysis, such as colonic perfusion with a Malone antegrade continence enema stoma or colonic irrigation with a rectal catheter, as a promising adjuvant approach to control uremia in CKD, although most findings are preliminary. The use of AST-120, an oral adsorbent, has been shown to reduce circulating levels of indoxyl sulfate and p-cresol sulfate and have potential renoprotective benefits in patients with advanced CKD. Diarrhea or inducing watery stools may modulate fluid retention and potassium and phosphorus load. Accumulating evidence indicates that plant-based diets, low-protein diets, and pre-, pro-, and synbiotic supplementation may lead to favorable alterations of the gut microbiota, contributing to reduce uremic toxin generation. The effects of these gut-targeted interventions on kidney and cardiovascular outcomes are still limited and need to be tested in future studies including clinical trials.

SUMMARY

Interventions aimed at enhancing bowel elimination of uremic toxins, fluid and electrolytes and at modulating gut microbiota may represent novel therapeutic strategies for the management of uremia in patients with CKD.

Indoxyl Sulfate Contributes to mTORC1-Induced Renal Fibrosis via The OAT/NADPH Oxidase/ROS Pathway

Activation of mTORC1 (mechanistic target of rapamycin complex 1) in renal tissue has been reported in chronic kidney disease (CKD)-induced renal fibrosis. However, the molecular mechanisms responsible for activating mTORC1 in CKD pathology are not well understood. The purpose of this study was to identify the uremic toxin involved in mTORC1-induced renal fibrosis. Among the seven protein-bound uremic toxins, only indoxyl sulfate (IS) caused significant activation of mTORC1 in human kidney 2 cells (HK-2 cells). This IS-induced mTORC1 activation was inhibited in the presence of an organic anion transporter inhibitor, a NADPH oxidase inhibitor, and an antioxidant. IS also induced epithelial–mesenchymal transition of tubular epithelial cells (HK-2 cells), differentiation of fibroblasts into myofibroblasts (NRK-49F cells), and inflammatory response of macrophages (THP-1 cells), which are associated with renal fibrosis, and these effects were inhibited in the presence of rapamycin (mTORC1 inhibitor). In in vivo experiments, IS overload was found to activate mTORC1 in the mouse kidney. The administration of AST-120 or rapamycin targeted to IS or mTORC1 ameliorated renal fibrosis in Adenine-induced CKD mice. The findings reported herein indicate that IS activates mTORC1, which then contributes to renal fibrosis. Therapeutic interventions targeting IS and mTORC1 could be effective against renal fibrosis in CKD.

Higher dietary fibre intake is associated with lower CVD mortality risk among maintenance haemodialysis patients: a multicentre prospective cohort study

High fibre intake is associated with reduced mortality risk in both general and chronic kidney disease populations. However, in dialysis patients, such data are limited. Therefore, the association between dietary fibre intake (DFI) and the risk of all-cause and CVD mortality was examined in this study. A total of 1044 maintenance haemodialysis (MHD) patients from eight outpatient dialysis centres in China were included in this study. Data on DFI were collected using 24-h dietary recalls for 3 d in a week and were normalised to actual dry weight. The study outcomes included all-cause and CVD mortality. Over a median of 46 months of follow-up, 354 deaths were recorded, of which 210 (59 %) were due to CVD. On assessing DFI as tertiles, the CVD mortality risk was significantly lower in patients in tertiles 2-3 (≥0·13 g/kg per d; hazard ratio (HR) 0·71; 95 % CI 0·51, 0·97) compared with those in tertile 1 (<0·13 g/kg per d). A similar but non-significant trend was found for the association between DFI (tertiles 2-3 v. tertile 1; HR 0·83; 95 % CI 0·64, 1·07) and all-cause mortality. In summary, higher DFI was associated with lower CVD mortality risk among Chinese MHD patients. This study emphasises the significance of DFI in MHD patients and provides information that is critical for the improvement of dietary guidelines for dialysis patients.

Bacterial metabolites and cardiovascular risk in children with chronic kidney disease

Cardiovascular complications are the major cause of the marked morbidity and mortality associated with chronic kidney disease (CKD). The classical cardiovascular risk factors such as diabetes and hypertension undoubtedly play a role in the development of cardiovascular disease (CVD) in adult CKD patients; however, CVD is just as prominent in children with CKD who do not have these risk factors. Hence, the CKD-specific pathophysiology of CVD remains incompletely understood. In light of this, studying children with CKD presents a unique opportunity to analyze CKD-associated mechanisms of CVD more specifically and could help to unveil novel therapeutic targets.

Here, we comprehensively review the interaction of the human gut microbiome and the microbial metabolism of nutrients with host immunity and cardiovascular end-organ damage. The human gut microbiome is evolutionary conditioned and modified throughout life by endogenous factors as well as environmental factors. Chronic diseases, such as CKD, cause significant disruption to the composition and function of the gut microbiome and lead to disease-associated dysbiosis. This dysbiosis and the accompanying loss of biochemical homeostasis in the epithelial cells of the colon can be the result of poor diet (e.g., low-fiber intake), medications, and underlying disease. As a result of dysbiosis, bacteria promoting proteolytic fermentation increase and those for saccharolytic fermentation decrease and the integrity of the gut barrier is perturbed (leaky gut). These changes disrupt local metabolite homeostasis in the gut and decrease productions of the beneficial short-chain fatty acids (SCFAs). Moreover, the enhanced proteolytic fermentation generates unhealthy levels of microbially derived toxic metabolites, which further accumulate in the systemic circulation as a consequence of impaired kidney function. We describe possible mechanisms involved in the increased systemic inflammation in CKD that is associated with the combined effect of SCFA deficiency and accumulation of uremic toxins. In the future, a more comprehensive and mechanistic understanding of the gut–kidney–heart interaction, mediated largely by immune dysregulation and inflammation, might allow us to target the gut microbiome more specifically in order to attenuate CKD-associated comorbidities.

A Link between Chronic Kidney Disease and Gut Microbiota in Immunological and Nutritional Aspects

Chronic kidney disease (CKD) is generally progressive and irreversible, structural or functional renal impairment for 3 or more months affecting multiple metabolic pathways. Recently, the composition, dynamics, and stability of a patient's microbiota has been noted to play a significant role during disease onset or progression. Increasing urea concentration during CKD can lead to an acceleration of the process of kidney injury leading to alterations in the intestinal microbiota that can increase the production of gut-derived toxins and alter the intestinal epithelial barrier. A detailed analysis of the relationship between the role of intestinal microbiota and the development of inflammation within the symbiotic and dysbiotic intestinal microbiota showed significant changes in kidney dysfunction. Several recent studies have determined that dietary factors can significantly influence the activation of immune cells and their mediators. Moreover, dietary changes can profoundly affect the balance of gut microbiota. The aim of this review is to present the importance and factors influencing the differentiation of the human microbiota in the progression of kidney diseases, such as CKD, IgA nephropathy, idiopatic nephropathy, and diabetic kidney disease, with particular emphasis on the role of the immune system. Moreover, the effects of nutrients, bioactive compounds on the immune system in development of chronic kidney disease were reviewed.

Gut Microbiome, Functional Food, Atherosclerosis, and Vascular Calcifications-Is There a Missing Link?

The gut microbiome is represented by the genome of all microorganisms (symbiotic, potential pathogens, or pathogens) residing in the intestine. These ecological communities are involved in almost all metabolic diseases and cardiovascular diseases are not excluded. Atherosclerosis, with a continuously increasing incidence in recent years, is the leading cause of coronary heart disease and stroke by plaque rupture and intraplaque hemorrhage. Vascular calcification, a process very much alike with osteogenesis, is considered to be a marker of advanced atherosclerosis. New evidence, suggesting the role of dietary intake influence on the diversity of the gut microbiome in the development of vascular calcifications, is highly debated. Gut microbiota can metabolize choline, phosphatidylcholine, and L-carnitine and produce vasculotoxic metabolites, such as trimethylamine-N-oxide (TMAO), a proatherogenic metabolite. This review article aims to discuss the latest research about how probiotics and the correction of diet is impacting the gut microbiota and its metabolites in the atherosclerotic process and vascular calcification. Further studies could create the premises for interventions in the microbiome as future primary tools in the prevention of atherosclerotic plaque and vascular calcifications.

Effects of the L-tyrosine-derived bacterial metabolite p-cresol on colonic and peripheral cells

Specific families of bacteria present within the intestinal luminal content produce p-cresol from L-tyrosine. Although the hosts do not synthesize p-cresol, they can metabolize this compound within their colonic mucosa and liver leading to the production of co-metabolites including p-cresyl sulfate (p-CS) and p-cresyl glucuronide (p-CG). p-Cresol and its co-metabolites are recovered in the circulation mainly conjugated to albumin, but also in their free forms that are excreted in the urine. An increased dietary protein intake raises the amount of p-cresol recovered in the feces and urine, while fecal excretion of p-cresol is diminished by a diet containing undigestible polysaccharides. p-Cresol in excess is genotoxic for colonocytes. In addition, in these cells, this bacterial metabolite decreases mitochondrial oxygen consumption, while increasing the anion superoxide production. In chronic kidney disease (CKD), marked accumulation of p-cresol and p-CS in plasma is measured, and in renal tubular cells, p-cresol and p-CS increase oxidative stress, affect mitochondrial function, and lead to cell death, strongly suggesting that these 2 compounds act as uremic toxins that aggravate CKD progression. p-Cresol and p-CS are also suspected to play a role in the CKD-associated adverse cardiovascular events, since they affect endothelial cell proliferation and migration, decrease the capacity of endothelial wound repair, and increase the senescence of endothelial cells. Finally, the fact that concentration of p-cresol is transiently increased in young autistic children biological fluids, and that intraperitoneal injection of p-cresol in animal models induces some behavioral characteristics observed in the autism spectrum disorders (ASD), raise the view that p-cresol may possibly represent one of the components involved in ASD etiology. Further pre-clinical and clinical studies are obviously needed to determine if the lowering of p-cresol and/or p-CS circulating concentrations, by dietary and/or pharmacological means, would allow, by itself or in combination with other interventions, to improve CKD progression and associated cardiovascular outcomes, as well as some neurological outcomes in children with an early diagnosis of autism.

Chronic Kidney Disease-Associated Itch (CKD-aI) in Children-A Narrative Review

Chronic kidney disease (CKD) is a condition of widespread epidemiology and serious consequences affecting all organs of the organism and associated with significant mortality. The knowledge on CKD is rapidly evolving, especially concerning adults. Recently, more data is also appearing regarding CKD in children. Chronic itch (CI) is a common symptom appearing due to various underlying dermatological and systemic conditions. CI may also appear in association with CKD and is termed chronic kidney disease-associated itch (CKD-aI). CKD-aI is relatively well-described in the literature concerning adults, yet it also affects children. Unfortunately, the data on paediatric CKD-aI is particularly scarce. This narrative review aims to describe various aspects of CKD-aI with an emphasis on children, based on the available data in this population and the data extrapolated from adults. Its pathogenesis is described in details, focusing on the growing role of uraemic toxins (UTs), as well as immune dysfunction, altered opioid transmission, infectious agents, xerosis, neuropathy and dialysis-associated aspects. Moreover, epidemiological and clinical aspects are reviewed based on the few data on CKD-aI in children, whereas treatment recommendations are proposed as well, based on the literature on CKD-aI in adults and own experience in managing CI in children.

Indoxyl-Sulfate-Induced Redox Imbalance in Chronic Kidney Disease

The accumulation of the uremic toxin indoxyl sulfate (IS) induces target organ damage in chronic kidney disease (CKD) patients, and causes complications including cardiovascular diseases, renal osteodystrophy, muscle wasting, and anemia. IS stimulates reactive oxygen species (ROS) production in CKD, which impairs glomerular filtration by a direct cytotoxic effect on the mesangial cells. IS further reduces antioxidant capacity in renal proximal tubular cells and contributes to tubulointerstitial injury. IS-induced ROS formation triggers the switching of vascular smooth muscular cells to the osteoblastic phenotype, which induces cardiovascular risk. Low-turnover bone disease seen in early CKD relies on the inhibitory effects of IS on osteoblast viability and differentiation, and osteoblastic signaling via the parathyroid hormone. Excessive ROS and inflammatory cytokine releases caused by IS directly inhibit myocyte growth in muscle wasting via myokines’ effects. Moreover, IS triggers eryptosis via ROS-mediated oxidative stress, and elevates hepcidin levels in order to prevent iron flux in circulation in renal anemia. Thus, IS-induced oxidative stress underlies the mechanisms in CKD-related complications. This review summarizes the underlying mechanisms of how IS mediates oxidative stress in the pathogenesis of CKD’s complications. Furthermore, we also discuss the potential role of oral AST-120 in attenuating IS-mediated oxidative stress after gastrointestinal adsorption of the IS precursor indole.

Unconjugated p-cresol activates macrophage macropinocytosis leading to increased LDL uptake

Patients with chronic kidney disease (CKD) and end-stage renal disease suffer from increased cardiovascular events and cardiac mortality. Prior studies have demonstrated that a portion of this enhanced risk can be attributed to the accumulation of microbiota-derived toxic metabolites, with most studies focusing on the sulfonated form of p-cresol (PCS). However, unconjugated p-cresol (uPC) itself was never assessed due to rapid and extensive first-pass metabolism that results in negligible serum concentrations of uPC. These reports thus failed to consider the host exposure to uPC prior to hepatic metabolism. In the current study, not only did we measure the effect of altering the intestinal microbiota on lipid accumulation in coronary arteries, but we also examined macrophage lipid uptake and handling pathways in response to uPC. We found that atherosclerosis-prone mice fed a high-fat diet exhibited significantly higher coronary artery lipid deposits upon receiving fecal material from CKD mice. Furthermore, treatment with uPC increased total cholesterol, triglycerides, and hepatic and aortic fatty deposits in non-CKD mice. Studies employing an in vitro macrophage model demonstrated that uPC exposure increased apoptosis whereas PCS did not. Additionally, uPC exhibited higher potency than PCS to stimulate LDL uptake and only uPC induced endocytosis- and pinocytosis-related genes. Pharmacological inhibition of varying cholesterol influx and efflux systems indicated that uPC increased macrophage LDL uptake by activating macropinocytosis. Overall, these findings indicate that uPC itself had a distinct effect on macrophage biology that might have contributed to increased cardiovascular risk in patients with CKD.

Gut microbiome, parathyroid hormone, and bone

PURPOSE OF REVIEW

Microorganisms in the gut (the 'microbiome') and the metabolites they produce (the 'metabolome') regulate bone mass through interactions between parathyroid hormone (PTH), the immune system, and bone. This review summarizes these data and details how this physiology may relate to CKD-mediated bone disease.

RECENT FINDINGS

The actions of PTH on bone require microbial metabolite activation of immune cells. Butyrate is necessary for CD4+ T-cell differentiation, T-reg cell expansion and CD8+ T-cell secretion of the bone-forming factor Wnt10b ligand. By contrast, mice colonized with segmented filamentous bacteria exhibit an expansion of gut Th17 cells and continuous PTH infusion increases the migration of Th17 cells to the bone marrow, contributing to bone resorption. In the context of CKD, a modified diet, frequent antibiotic therapy, altered intestinal mobility, and exposure to multiple medications together contribute to dysbiosis; the implications for an altered microbiome and metabolome on the pathogenesis of renal osteodystrophy and its treatment have not been explored.

SUMMARY

As dysregulated interactions between PTH and bone ('skeletal resistance') characterize CKD, the time is ripe for detailed, mechanistic studies into the role that gut metabolites may play in the pathogenesis of CKD-mediated bone disease.

The Impact of CKD on Uremic Toxins and Gut Microbiota

Numerous studies have indicated that the progression of chronic kidney disease (CKD) to end-stage renal disease (ESRD) is strictly associated with the accumulation of toxic metabolites in blood and other metabolic compartments. This accumulation was suggested to be related to enhanced generation of toxins from the dysbiotic microbiome accompanied by their reduced elimination by impaired kidneys. Intestinal microbiota play a key role in the accumulation of uremic toxins due to the fact that numerous uremic solutes are generated in the process of protein fermentation by colonic microbiota. Some disease states, including CKD, are associated with the presence of dysbiosis, which can be defined as an "imbalanced intestinal microbial community with quantitative and qualitative changes in the composition and metabolic activities of the gut microbiota". The results of studies have confirmed the altered composition and functions of gut microbial community in chronic kidney disease. In the course of CKD protein-bound uremic toxins, including indoxyl sulfate, p-cresyl glucuronide, p-cresyl sulfate and indole-3-acetic acid are progressively accumulated. The presence of chronic kidney disease may be accompanied by the development of intestinal inflammation and epithelial barrier impairment leading to hastened systemic translocation of bacterial-derived uremic toxins and consequent oxidative stress injury to the kidney, cardiovascular and endocrine systems. These findings offer new therapeutic possibilities for the management of uremia, inflammation and kidney disease progression and the prevention of adverse outcomes in CKD patients. It seems that dietary interventions comprising prebiotics, probiotics, and synbiotics could pose a promising strategy in the management of uremic toxins in CKD.

Challenges of reducing protein-bound uremic toxin levels in chronic kidney disease and end stage renal disease

The prevalence of chronic kidney disease (CKD) in the worldwide population is currently estimated between 11% and 13%. Adequate renal clearance is compromised in these patients and the accumulation of a large number of uremic retention solutes results in an irreversible worsening of renal function which can lead to end stage renal disease (ESRD). Approximately three million ESRD patients currently receive renal replacement therapies (RRTs), such as hemodialysis, which only partially restore kidney function, as they are only efficient in removing mainly small, unbound solutes from the circulation while leaving larger and protein-bound uremic toxins (PBUTs) untouched. The accumulation of PBUTs in patients highly increases the risk of cardiovascular events and is associated with higher mortality and morbidity in CKD and ESRD. In this review, we address several strategies currently being explored toward reducing PBUT concentrations, including clinical and medical approaches, therapeutic techniques, and recent developments in RRT technology. These include preservation of renal function, limitation of colon derived PBUTs, oral sorbents, adsorbent RRT technology, and use of albumin displacers. Despite the promising results of the different approaches to promote enhanced removal of a small percentage of the more than 30 identified PBUTs, on their own, none of them provide a treatment with the required efficiency, safety and cost-effectiveness to prevent CKD-related complications and decrease mortality and morbidity in ESRD.

Contribution of uremic dysbiosis to insulin resistance and sarcopenia

BACKGROUND

Chronic kidney disease (CKD) leads to insulin resistance (IR) and sarcopenia, which are associated with a high mortality risk in CKD patients; however, their pathophysiologies remain unclear. Recently, alterations in gut microbiota have been reported to be associated with CKD. We aimed to determine whether uremic dysbiosis contributes to CKD-associated IR and sarcopenia.

METHODS

CKD was induced in specific pathogen-free mice via an adenine-containing diet; control animals were fed a normal diet. Fecal microbiota transplantation (FMT) was performed by oral gavage in healthy germ-free mice using cecal bacterial samples obtained from either control mice (control-FMT) or CKD mice (CKD-FMT). Vehicle mice were gavaged with sterile phosphate-buffered saline. Two weeks after inoculation, mice phenotypes, including IR and sarcopenia, were evaluated.

RESULTS

IR and sarcopenia were evident in CKD mice compared with control mice. These features were reproduced in CKD-FMT mice compared with control-FMT and vehicle mice with attenuated insulin-induced signal transduction and mitochondrial dysfunction in skeletal muscles. Intestinal tight junction protein expression and adipocyte sizes were lower in CKD-FMT mice than in control-FMT mice. Furthermore, CKD-FMT mice showed systemic microinflammation, increased concentrations of serum uremic solutes, fecal bacterial fermentation products and elevated lipid content in skeletal muscle. The differences in gut microbiota between CKD and control mice were mostly consistent between CKD-FMT and control-FMT mice.

CONCLUSIONS

Uremic dysbiosis induces IR and sarcopenia, leaky gut and lipodystrophy.

Cognitive Function and Uremic Toxins after Kidney Transplantation: An Exploratory Study

BACKGROUND

Cognitive functions are altered in patients with CKD. However, it is suggested that cognitive functions improve after kidney transplantation, at least partially. A possible cause for this improvement could be the reduction of uremic retention solutes after transplantation. This study assessed the association between the changes in uremic toxin concentration with the changes in cognitive function in patients after kidney transplantation.

METHODS

Ten recipients of kidney transplants were compared with 18 controls (nine patients on hemodialysis, and nine patients with CKD stage 4 or 5 [eGFR <30 ml/min per 1.73 m²] who were not on dialysis). An extensive neuropsychological assessment, covering the five major cognitive domains (i.e., memory, attention and concentration, information processing speed, abstract reasoning, and executive function), was done before transplantation, at 1 week post-transplant, and 3 months after transplantation. Similarly, assessments of the 18 matched, control patients were performed longitudinally over a period of 3-5 months. Concentrations of 16 uremic retention solutes (indoxyl glucuronide, p-cresyl glucuronide, phenylglucuronide, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, p-cresyl sulfate, hippuric acid, phenyl sulfate, kynurenine, tryptophan, kynurenic acid, tyrosine, indole-3-acetic acid, phenylalanine, trimethylamine N-oxide, and phenylacetylglutamine) were measured in serum samples collected at the time of the neuropsychological assessments.

RESULTS

A significant improvement in cognitive function was only found in the processing-speed domain, and this was observed in both patients who received a transplant and patients with CKD. No significant differences between patients who received a transplant and the control groups were seen in the other cognitive domains. As expected, the serum concentration of most uremic toxins decreased significantly within 1 week after kidney transplantation.

CONCLUSIONS

There was no significant improvement in cognitive function that could be specifically related to kidney transplantation in the first 3 months after the procedure. These data do not support the notion that uremic toxins exert an immediate effect on cognitive function.

Targeting the gut microbial metabolic pathway with small molecules decreases uremic toxin production

Uremic toxins are a class of toxins that accumulate in patients with chronic kidney disease (CKD). Indoxyl sulfate (IS), a typical uremic toxin, is not efficiently removed by hemodialysis. Modulation of IS production in the gut microbiota may be a promising strategy for decreasing IS concentration, thus, delaying CKD progression. In the present study, we identified isoquercitrin (ISO) as a natural product that can perturb microbiota-mediated indole production without directly inhibiting the growth of microbes or the indole-synthesizing enzyme TnaA. ISO inhibits the establishment of H proton potential by regulating the gut bacteria electron transport chain, thereby inhibiting the transport of tryptophan and further reducing indole biosynthesis. This non-microbiocidal mechanism may enable ISO to be used as a therapeutic tool, specifically against pathologies triggered by the accumulation of the microbial-produced toxin IS, as in CKD. Herein, we have shown that it is possible to inhibit gut microbial indole production using natural components. Therefore, targeting the uremic toxin metabolic pathway in gut bacteria may be a promising strategy to control host uremic toxin production.

Indoxyl Sulfate, a Tubular Toxin, Contributes to the Development of Chronic Kidney Disease

Indoxyl sulfate (IS), a uremic toxin, causes chronic kidney disease (CKD) progression via its tubulotoxicity. After cellular uptake, IS directly induces apoptotic and necrotic cell death of tubular cells. Additionally, IS increases oxidative stress and decreases antioxidant capacity, which are associated with tubulointerstitial injury. Injured tubular cells are a major source of transforming growth factor-β1 (TGF-β1), which induces myofibroblast transition from residual renal cells in damaged kidney, recruits inflammatory cells and thereby promotes extracellular matrix deposition in renal fibrosis. Moreover, IS upregulates signal transducers and activators of transcription 3 phosphorylation, followed by increases in TGF-β1, monocyte chemotactic protein-1 and α-smooth muscle actin production, which participate in interstitial inflammation, renal fibrosis and, consequently, CKD progression. Clinically, higher serum IS levels are independently associated with renal function decline and predict all-cause mortality in CKD. The poor removal of serum IS in conventional hemodialysis is also significantly associated with all-cause mortality and heart failure incidence in end-stage renal disease patients. Scavenging the IS precursor by AST-120 can markedly reduce tubular IS staining that attenuates renal tubular injury, ameliorates IS-induced oxidative stress and rescues antioxidant glutathione activity in tubular epithelial cells, thereby providing a protective role against tubular injury and ultimately retarding renal function decline.

Exploring the Parameter Space of p-Cresyl Sulfate Adsorption in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have high porosity and surface area, making them ideal candidates for adsorption-mediated applications. One high-value application is the removal of uremic toxins from solution for dialysis. Previous studies have reported adsorptive removal of the uremic toxin p-cresyl sulfate from solution via zirconium-based MOFs, but a specific analysis of parameters contributing to adsorptive uptake is needed to clarify differences in uptake performance between MOFs. We synthesized zirconium 1,3,5-benzenetricarboxylate (MOF-808) and an iron-based analog, MIL-100(Fe), and compared their adsorptive uptake with previously reported values of other zirconium-based MOFs. MIL-100(Fe) adsorbed three times more p-cresyl sulfate from solution on a per mass basis than MOF-808 and had a greater adsorption efficiency than 75% of previously reported Zr-based MOFs. We compared p-cresyl sulfate uptake by MOFs as a function of BET surface area, number of aromatic carbons in the organic linker, internal cage diameter, and pore window diameter. There is poor correlation between p-cresyl sulfate uptake and each of the variables considered, but the number of aromatic carbons of the MOF linker was a better predictor of uptake than BET surface area (R² = 0.7034 and 0.1430, respectively), and pore window aperture was a better predictor of uptake than the pore cage diameter (R² = 0.4780 and 0.0383, respectively). We hypothesize that the greater adsorptive capacity of MIL-100(Fe) compared to MOF-808 results from direct coordination of p-cresyl sulfate to vacant metal sites in the MOF, and the total adsorption may be accounted for by some combination of adsorptive interactions occurring at both metal and organic linker sites near to the exterior particle surface. The adsorptive uptake of p-cresyl sulfate by MIL-100(Fe) was observed to increase with p-cresyl sulfate content, mass of MIL-100(Fe), and volume of p-cresyl sulfate solution; the mass of MIL-100(Fe) had the greatest effect on total adsorption.

The Role of Gut Dysbiosis in the Bone-Vascular Axis in Chronic Kidney Disease

Patients with chronic kidney disease (CKD) are at increased risk of bone mineral density loss and vascular calcification. Bone demineralization and vascular mineralization often concur in CKD, similar to what observed in the general population. This contradictory association is commonly referred to as the 'calcification paradox' or the bone-vascular axis. Mounting evidence indicates that CKD-associated gut dysbiosis may be involved in the pathogenesis of the bone-vascular axis. A disrupted intestinal barrier function, a metabolic shift from a predominant saccharolytic to a proteolytic fermentation pattern, and a decreased generation of vitamin K may, alone or in concert, drive a vascular and skeletal pathobiology in CKD patients. A better understanding of the role of gut dysbiosis in the bone-vascular axis may open avenues for novel therapeutics, including nutriceuticals.

Effect of prebiotic (fructooligosaccharide) on uremic toxins of chronic kidney disease patients: a randomized controlled trial

BACKGROUND

Microbial-derived uremic toxins, p-cresyl sulfate (PCS), indoxyl sulfate (IS) and indole 3-acetic acid (IAA), have been associated with the burden of chronic kidney disease (CKD). Prebiotics have emerged as an alternative to modulate the gut environment and to attenuate toxin production. This trial aims to investigate the effect of a prebiotic fructooligosaccharide (FOS) on uremic toxins of non-dialysis-dependent CKD (NDD-CKD) patients.

METHODS

A double-blind, placebo-controlled, randomized trial was conducted for 3 months. In all, 50 nondiabetic NDD-CKD patients [estimated glomerular filtration rate (eGFR) <45 mL/min/1.73 m2], aged 18-80 years, were allocated to prebiotic (FOS, 12 g/day) or placebo (maltodextrin, 12 g/day) groups. Primary outcomes were changes in serum (total and free) and urinary (total) PCS. Secondary outcomes included changes in IS, IAA, serum markers of intestinal permeability (zonulin), gut-trophic factors (epidermal growth factor and glucagon-like peptide-2), eGFR, inflammation (high sensitive c-reactive protein and interleukin-6), homeostatic model assessment-insulin resistance, lipid profile and gastrointestinal symptoms.

RESULTS

From 50 participants (54% men, 57.3 ± 14.6 years and eGFR 21.4 ± 7.6 mL/min/1.73 m2), 46 completed the follow-up. No changes in dietary intake or gastrointestinal symptoms were observed. There was a trend in the difference of serum total ΔPCS (treatment effect adjusted for baseline levels: -12.4 mg/L; 95% confidence interval (-5.6 to 0.9 mg/L; P = 0.07) and serum-free Δ%PCS [intervention -8.6 (-41.5 to 13.9%) versus placebo 3.5 (-28.8 to 85.5%); P = 0.07] between the groups. The trend in the difference of serum total ΔPCS was independent of eGFR and dietary protein:fiber ratio intake. No difference was found in urinary PCS. Aside from the decreased high-density lipoprotein cholesterol in the intervention, no differences were observed in the change of IS, IAA or other secondary outcome between the groups.

CONCLUSIONS

Our result suggests the potential of FOS in reducing serum total and free PCS in nondiabetic NDD-CKD patients.

Protein-Bound Uremic Toxins in Hemodialysis Patients Relate to Residual Kidney Function, Are Not Influenced by Convective Transport, and Do Not Relate to Outcome

Protein-bound uremic toxins (PBUTs) are predominantly excreted by renal tubular secretion and hardly removed by traditional hemodialysis (HD). Accumulation of PBUTs is proposed to contribute to the increased morbidity and mortality of patients with end-stage kidney disease (ESKD). Preserved PBUT excretion in patients with residual kidney function (RKF) and/or increased PBUT clearance with improved dialysis techniques might improve the prognosis of patients with ESKD. The aims of this study are to explore determinants of PBUTs in HD patients, and investigate whether hemodiafiltration (HDF) lowers PBUT plasma concentrations, and whether PBUTs are related to the outcome. Predialysis total plasma concentrations of kynurenine, kynurenic acid, indoxyl sulfate, indole-3-acetic acid, p-cresyl sulfate, p-cresyl glucuronide, and hippuric acid were measured by UHPLC-MS at baseline and after 6 months of follow-up in the first 80 patients participating in the CONvective TRAnsport Study (CONTRAST), a randomized controlled trial that compared the effects of online HDF versus low-flux HD on all-cause mortality and new cardiovascular events. RKF was inversely related to kynurenic acid (p < 0.001), indoxyl sulfate (p = 0.001), indole-3-acetic acid (p = 0.024), p-cresyl glucuronide (p = 0.004) and hippuric acid (p < 0.001) plasma concentrations. Only indoxyl sulfate decreased by 8.0% (−15.3 to 34.6) in patients treated with HDF and increased by 11.9% (−15.4 to 31.9) in HD patients after 6 months of follow-up (HDF vs. HD: p = 0.045). No independent associations were found between PBUT plasma concentrations and either risk of all-cause mortality or new cardiovascular events. In summary, in the current population, RKF is an important determinant of PBUT plasma concentrations in HD patients. The addition of convective transport did not consistently decrease PBUT plasma concentrations and no relation was found between PBUTs and cardiovascular endpoints.

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.

A Renal Clinician's Guide to the Gut Microbiota

It is increasingly recognized that the gut microbiota plays a role in the progression of chronic diseases and that diet may confer health benefits by altering the gut microbiota composition. This is of particular relevance for chronic kidney disease (CKD), as the gut is a source of uremic retention solutes, which accumulate as a result of impaired kidney function and can exert nephrotoxic and other harmful effects. Kidney dysfunction is also associated with changes in the composition of the gut microbiota and the gastrointestinal tract. Diet modulates the gut microbiota, and there is much interest in the use of prebiotics, probiotics, and synbiotics as dietary therapies in CKD, as well as dietary patterns that beneficially alter the microbiota. This review provides an overview of the gut microbiota and its measurement, its relevance in the context of CKD, and the current state of knowledge regarding dietary manipulation of the microbiota.

Indoxyl sulfate associates with cardiovascular phenotype in children with chronic kidney disease

BACKGROUND

Cardiovascular disease is the leading cause of death in children with chronic kidney disease (CKD). Serum levels of gut-derived uremic toxins increase with deterioration of kidney function and are associated with cardiac comorbidities in adult CKD patients.

METHODS

Indoxyl sulfate (IS) and p-cresyl sulfate (pCS) were measured by high-performance liquid chromatography in serum of children participating in the Cardiovascular Comorbidity in Children with CKD (4C) Study. Results were correlated with measurements of the carotid intima-media thickness (cIMT), central pulse wave velocity (PWV), and left ventricular mass index (LVMI) in children aged 6-17 years with initial eGFR of 10-60 ml/min per 1.73 m².

RESULTS

The median serum levels of total IS and of pCS, measured in 609 patients, were 5.3 μmol/l (8.7) and 17.0 μmol/l (21.6), respectively. In a multivariable regression model, IS and pCS showed significant positive associations with urea and negative associations with eGFR and uric acid. Furthermore, positive associations of pCS with age, serum albumin, and non-Mediterranean residency and a negative association with glomerular disease were observed. By multivariable regression analysis, only IS was significantly associated with a higher cIMT SDS at baseline and progression of PWV SDS within 12 months, independent of other risk factors.

CONCLUSIONS

Serum levels of gut-derived uremic toxins IS and pCS correlated inversely with eGFR in children. Only IS was significantly associated with surrogate markers of cardiovascular disease in this large pediatric CKD cohort.

Quantitative Translation of Microfluidic Transporter in Vitro Data to in Vivo Reveals Impaired Albumin-Facilitated Indoxyl Sulfate Secretion in Chronic Kidney Disease

Indoxyl sulfate (IxS), a highly albumin-bound uremic solute, accumulates in chronic kidney disease (CKD) due to reduced renal clearance. This study was designed to specifically investigate the role of human serum albumin (HSA) in IxS renal secretion via organic anion transporter 1 (OAT1) in a microfluidic system and subsequently apply quantitative translation of in vitro data to predict extent of change in IxS renal clearance in CKD stage IV relative to healthy. Conditionally immortalized human proximal tubule epithelial cells overexpressing OAT1 were incubated with IxS (5-200 μM) in the HSA-free medium or in the presence of either HSA or CKD-modified HSA. IxS uptake in the presence of HSA resulted in more than 20-fold decrease in OAT1 affinity (K_m,u) and 37-fold greater in vitro unbound intrinsic clearance (CL_int,u) versus albumin-free condition. In the presence of CKD-modified albumin, K_m,u increased four-fold and IxS CL_int,u decreased almost seven-fold relative to HSA. Fold-change in parameters exceeded differences in IxS binding between albumin conditions, indicating additional mechanism and facilitating role of albumin in IxS OAT1-mediated uptake. Quantitative translation of IxS in vitro OAT1-mediated CL_int,u predicted a 60% decrease in IxS renal elimination as a result of CKD, in agreement with the observed data (80%). The findings of the current study emphasize the role of albumin in IxS transport via OAT1 and explored the impact of modifications in albumin on renal excretion via active secretion in CKD. For the first time, this study performed quantitative translation of transporter kinetic data generated in a novel microfluidic in vitro system to a clinically relevant setting. Knowledge gaps and future directions in quantitative translation of renal drug disposition from microphysiological systems are discussed.

The gut–kidney–heart axis in chronic kidney disease

The recent explosion of scientific interest in the gut microbiota has dramatically advanced our understanding of the complex pathophysiological interactions between the gut and multiple organs in health and disease. Emerging evidence has revealed that the gut microbiota is significantly altered in patients with chronic kidney disease (CKD), along with impaired intestinal barrier function. These alterations allow translocation of various gut-derived products into the systemic circulation, contributing to the development and progression of CKD and cardiovascular disease (CVD), partly mediated by chronic inflammation. Among potentially toxic gut-derived products identifiable in the systemic circulation, bacterial endotoxin and gut metabolites (e.g., p-cresyl sulfate and trimethylamine-N-oxide) have been extensively studied for their immunostimulatory and atherogenic properties. Recent studies have also suggested similar biological properties of bacterial DNA fragments circulating in the blood of patients with CKD, even in the absence of overt infections. Despite the accumulating evidence of the gut microbiota in CKD and its therapeutic potential for CVD, the precise mechanisms for multidirectional interactions between the gut, kidney, and heart remain poorly understood. This review aims to provide recent evidence on the associations between the gut microbiota, CKD, and CVD, and summarize current understanding of the potential pathophysiological mechanisms underlying the “gut–kidney–heart” axis in CKD.

1,25(OH)2 D3 attenuates indoxyl sulfate-induced epithelial-to-mesenchymal cell transition via inactivation of PI3K/Akt/β-catenin signaling in renal tubular epithelial cells

OBJECTIVES

Indoxyl sulfate (IS), a uremic toxin, has been shown to promote the epithelial-to-mesenchymal transition (EMT) of human proximal tubular cells and to accelerate the progression of chronic kidney disease (CKD). Despite the well-known protective role of 1,25-dihydroxyvitamin D₃ [1,25(OH)₂ D₃] in EMT, the effect of 1,25(OH)₂ D₃ on IS-induced EMT in human proximal tubular epithelial cells and the underlying mechanism remain unclear. The aim of this study was to determine whether IS (0-1 mM) dose-dependently inhibited the protein expression of E-cadherin and increased the protein expression of alpha-smooth muscle actin, N-cadherin, and fibronectin.

METHODS

This study investigated the molecular mechanism by which 1,25(OH)₂ D₃ attenuates IS-induced EMT. HK-2 human renal tubular epithelial cells was used as the study model, and the MTT assay, Western Blotting, siRNA knockdown technique were used to explore the effects of 1,25(OH)₂ D₃ on EMT in the presence of IS.

RESULTS

Pretreatment with 1,25(OH)₂ D₃ inhibited the IS-induced EMT-associated protein expression in HK-2 cells. IS induced the phosphorylation of Akt (S473) and β-catenin (S552) and subsequently increased the nuclear accumulation of β-catenin. Pretreatment with 1,25(OH)₂ D₃ and LY294002 (phosphoinositide 3-kinase [PIK3] inhibitor) significantly inhibited the IS-induced phosphorylation of Akt and β-catenin, nuclear β-catenin accumulation, and EMT-associated protein expression.

CONCLUSIONS

Results from the present study revealed that the anti-EMT effect of 1,25(OH)₂ D₃ is likely through inhibition of the PI3K/Akt/β-catenin pathway, which leads to down-regulation of IS-driven EMT-associated protein expression in HK-2 human renal tubular epithelial cells.

The role of chronic kidney disease-associated dysbiosis in cardiovascular disease

IMPACT STATEMENT

Negative alterations, or dysbiosis, in the intestinal microbial community balance in response to chronic kidney disease is emerging as a substantial and important factor in inducing and exacerbating multiple comorbid conditions. Patients with renal insufficiency experience a substantial increase in cardiovascular risk, and recent evidence is shedding light on the close interaction between microbiome dysbiosis and increased cardiovascular events in this population. Previous association and recent causality studies utilizing experimental animal models have enriched our understanding and confirmed the impact of microbial community imbalance on cardiac health in both the general population and in patients with renal impairment.

Evolution of protein-bound uremic toxins indoxyl sulphate and p-cresyl sulphate in acute kidney injury

BACKGROUND

There is a gradual increase in serum concentrations of protein-bound colon-derived uremic toxins indoxyl sulphate (IxS) and p-cresyl sulphate (pCS) as chronic kidney disease (CKD) progresses. In acute kidney injury (AKI), up till now, the retention pattern has not been studied.

METHODS

In this study, 194 adult patients admitted with sepsis to the intensive care unit were included. IxS, pCS and serum creatinine (sCrea) were quantified at inclusion (D0) and at day 4, unless follow-up ended earlier (D_end).

RESULTS

Serum levels of sCrea (P < 0.001), IxS (P < 0.001) and pCS (P < 0.05) were higher in patients with AKI according to RIFLE classification at D0. In contrast with sCrea, IxS and pCS levels only increased from stage I (IxS) and F (pCS) on. When grouped according to evolution in RIFLE class from D0 to D_end, all solute concentrations were higher (P < 0.001) in the group with unfavourable evolution. In this group, there was a marked rise in sCrea (P < 0.001), a moderate one for pCS (P < 0.05), but no change for IxS (P = 0.112). There was a decrease (P < 0.001) of all solute concentrations in the group with favourable evolution. Comparing AKI with CKD patients matched for sCrea, total levels of both IxS and pCS were higher (P < 0.01) in patients with CKD.

CONCLUSIONS

Although concentrations of IxS and pCS both tend to rise in sepsis patients with AKI, their evolution does not conform with that of sCrea. For the same level of sCrea, IxS and pCS concentrations are lower in AKI compared with CKD.

Intestinal Barrier Function in Chronic Kidney Disease

The kidneys are key contributors to body homeostasis, by virtue of controlled excretion of excessive fluid, electrolytes, and toxic waste products. The syndrome of uremia equals the altered physiology due to irreversible loss of kidney function that is left uncorrected for, despite therapeutic intervention(s). The intestines and its microbial content are prime contributors to this syndrome. The intestinal barrier separates the self (or the so-called “milieu intérior”) from the environment. In the large intestine, the intestinal barrier keeps apart human physiology and the microbiota. The enterocytes and the extracellular mucin layer functions form a complex multilayered structure, facilitating complex bidirectional metabolic and immunological crosstalk. The current review focuses on the intestinal barrier in chronic kidney disease (CKD). Loss of kidney function results in structural and functional alterations of the intestinal barrier, contribution to the syndrome of uremia.