Indoxyl sulfate induces skeletal resistance to parathyroid hormone in cultured osteoblastic cells

Prolonged secondary hyperparathyroidism in adenine-induced CKD leads to skeletal changes consistent with skeletal hyporesponsiveness to PTH

High circulating parathyroid hormone (PTH) leading to secondary hyperparathyroidism is proposed to be a key driver of the skeletal phenotype of chronic kidney disease-mineral bone disorder (CKD-MBD) leading to high bone turnover and cortical bone deterioration. The association between high PTH and the skeletal phenotype is typically clearly demonstrated in preclinical models of CKD; however, clinical studies show the relationship between PTH and skeletal outcomes is not as clear. The clinical data have led to a proposed hyporesponsiveness to PTH in the CKD setting with unclear causes. In the current study, we assessed skeletally mature male C57BL/6J mice at 12-weeks and 21-weeks of adenine-induced CKD (Ad) with the second timepoint seven weeks longer than we have previously assessed. We found that serum BUN was high in Ad mice in both groups indicating the presence of kidney disease while PTH was higher in 21-wk Ad vs. 12-wk Ad. Despite the higher PTH, bone formation rate in 21-wk Ad mice was lower than 21-wk Ad mice. Additionally, immunohistochemical assessment of the PTH receptor, PTHR1, and RANKL, a key factor upregulated by PTH, showed a lower percentage of osteocytes positive for the proteins in 21-wk Ad vs. 12-wk Ad. Furthermore, regression analyses demonstrated a positive relationship between serum PTH and PTHR1 and RANKL at 12-weeks, but this relationship was lost by 21-weeks. Overall, these data indicate that prolonged exposure to continuously elevated PTH in adenine-induced CKD mice eventually led to an altered skeletal response indicating lower responsiveness of bone, particularly osteocytes, to the chronic PTH signal. This has implications for using PTH as a surrogate marker of bone outcomes in CKD as well as pointing to the need to better understand the time-based relationship between PTH and skeletal outcomes in CKD.

Benefits and Pitfalls of Uraemic Toxin Measurement in Peritoneal Dialysis

Chronic kidney disease is a global health burden with a rising incidence and prevalence in developed and developing nations. Once established, it results in a progressive accumulation of a myriad of uraemic toxins. Peritoneal dialysis (PD) uses the body's peritoneal membrane to remove these toxins across a semipermeable membrane to restore and maintain homeostasis. Traditionally, dialysis adequacy has been measured through clearance of urea and creatinine. However, numerous studies have shown marginal links comparing the clearance of urea and creatinine with clinical outcomes reflected in the recent changes to the ISPD guidelines on dialysis adequacy. Instead, attention has focused on protein-bound uraemic toxins (PBTs). Produced by gut bacteria, these molecules are highly protein-bound and poorly removed by either dialysis or absorptive agents. Elevated concentrations of molecules such as p-cresyl sulfate and indoxyl sulfate have been associated with abnormal cellular function and poor patient outcomes. However, widespread use of these measures to determine dialysis adequacy has been limited by the need for specialized techniques required for measurement. Altering the gut microbiome to reduce generation of PBTs through increased dietary fiber might be an alternate approach to better patient outcomes, with some initial positive reports. This report explores advantages and limitations of measuring uraemic toxins in PD, now and in the foreseeable future.

The comparison of serum bone-turnover markers in different stage of chronic kidney disease and the associated impact of intradialytic cycling in patients with end-stage renal disease

INTRODUCTION

Bone turnover markers reflected the bone remodeling process and bone health in clinical studies. Studies on variation of bone remodeling markers in different stage CKD were scant, and this study investigated the role of bedside intradialytic cycling in altering concentrations of bone-remodeling markers in patients with end-stage renal disease (ESRD).

MATERIALS AND METHODS

Participants were segmented into four groups: a group with eGFR >60 ml/min/1.73 m2, a chronic kidney disease group with eGFR 15-60 mL/min/1.73 m2), an ESRD group with an exercise intervention, and an ESRD group with standard care. Comparison of bone turnover markers was performed among groups. The intervention consisting of 12 weeks of intradialytic cycling was performed during dialysis. The variation of bone-remodeling markers was compared between the ESRD with exercise along with the ESRD with standard care after 12-week monitoring.

RESULTS

Bone-formative marker levels (bone-specific alkaline phosphatase and procollagen type 1 amino-terminal propeptide, P1NP) were higher in ESRD patients than in non-ESRD patients and were correlated with indoxyl sulfate and intact parathyroid hormone concentrations (p < 0.05). Postexercise concentrations of tartrate-resistant acid phosphatase-5b (p = 0.003) and N-terminal telopeptide-1 (p = 0.001) had increased in the ESRD patients after 12 weeks of bedside cycling. Bone-formative marker concentration was not altered in the exercise group after cycling.

CONCLUSION

Bone-formative marker concentrations increased with the severity of chronic kidney disease. Bone formative markers concentration increased along with CKD severity. We demonstrated the bone resorptive markers tartrate-resistant acid phosphatase-5b and N-terminal telopeptide-1 increased after intradialytic cycling in ESRD patients.

The uremic toxin indoxyl sulfate decreases osteocyte RANKL/OPG and increases Wnt inhibitor RNA expression that is reversed by PTH

Renal osteodystrophy (ROD) leads to increased fractures, potentially due to underlying low bone turnover in chronic kidney disease (CKD). We hypothesized that indoxyl sulfate (IS), a circulating toxin elevated in CKD and a ligand for the aryl hydrocarbon receptor (AhR), may target the osteocytes leading to bone cell uncoupling in ROD. The IDG-SW3 osteocytes were cultured for 14 days (early) and 35 days (mature osteocytes) and incubated with 500 μM of IS after dose finding studies to confirm AhR activation. Long-term incubation of IS for 14 days led to decreased expression of Tnfsf11/Tnfrsf11b ratio (RANKL/OPG), which would increase osteoclast activity, and increased expression of Wnt inhibitors Sost and Dkk1, which would decrease bone formation in addition to decreased mineralization and alkaline phosphatase (ALP) activity. When osteocytes were incubated with IS and the AhR translocation inhibitor CH223191, mineralization and ALP activity were restored. However, the Tnfsf11/Tnfrsf11b ratio and Sost, Dkk1 expression were not altered compared with IS alone, suggesting more complex signaling. In both early and mature osteocytes, co-culture with parathyroid hormone (PTH) and IS reversed the IS-induced upregulation of Sost and Dkk1, and IS enhanced the PTH-induced increase of the Tnfsf11/Tnfrsf11b ratio. Co-culture of IS with PTH additively enhanced the AhR activity assessed by Cyp1a1 and Cyp1b1 expression. In summary, IS in the absence of PTH increased osteocyte messenger RNA (mRNA) Wnt inhibitor expression in both early and mature osteocytes, decreased mRNA expression ofTnfsf11/Tnfrsf11b ratio and decreased mineralization in early osteocytes. These changes would lead to decreased resorption and formation resulting in low bone remodeling. These data suggest IS may be important in the underlying low turnover bone disease observed in CKD when PTH is not elevated. In addition, when PTH is elevated, IS interacts to further increase Tnfsf11/Tnfrsf11b ratio for osteoclast activity in both early and mature osteocytes, which would worsen bone resorption.

Indoxyl Sulfate and Autism Spectrum Disorder: A Literature Review

Indoxyl sulfate-a bacterially derived metabolite-has been identified as a toxin that is elevated in children with autism spectrum disorder (ASD). As a neurotoxin, uremic toxin, nephrotoxin, cardiotoxin, osteotoxin, and myotoxin, indoxyl sulfate has been associated with several other conditions, including chronic kidney disease, acute kidney injury, Parkinson's disease, cognitive disorders, and mood disorders such as anxiety and depression. Indoxyl sulfate is derived from bacterial modification of host tryptophan, and elevated levels of indoxyl sulfate are associated with decreased levels of important neurotransmitters including serotonin, dopamine, and norepinephrine. This article will review what is currently known about indoxyl sulfate in relation to ASD and its comorbidities. A systematic review identified six studies of levels of indoxyl sulfate in children with ASD. All six studies found that indoxyl sulfate was significantly elevated in the urine of children with ASD compared to typically developing children. Through this review, indoxyl sulfate was identified as a toxic microbially derived metabolite that is significantly increased in a subset of children with ASD and may contribute to both core and co-morbid ASD symptoms.

The gut microbiome, chronic kidney disease, and sarcopenia

Sarcopenia is a prevalent condition in patients with chronic kidney disease (CKD), intricately linked to adverse prognoses, heightened cardiovascular risks, and increased mortality rates. Extensive studies have found a close and complex association between gut microbiota, kidney and muscle. On one front, patients with CKD manifest disturbances in gut microbiota and alterations in serum metabolites. These abnormal microbiota composition and metabolites in turn participate in the development of CKD. On another front, altered gut microbiota and its metabolites may lead to significant changes in metabolic homeostasis and inflammation, ultimately contributing to the onset of sarcopenia. The disturbance of gut microbial homeostasis, coupled with the accumulation of toxic metabolites, exerts deleterious effects on skeletal muscles in CKD patients with sarcopenia. This review meticulously describes the alterations observed in gut microbiota and its serum metabolites in CKD and sarcopenia patients, providing a comprehensive overview of pertinent studies. By delving into the intricate interplay of gut microbiota and serum metabolites in CKD-associated sarcopenia, we aim to unveil novel treatment strategies for ameliorating their symptoms and prognosis.

Improvement of Adsorption Capacity by Refined Encapsulating Method of Activated Carbon into the Hollow-Type Spherical Bacterial Cellulose Gels for Oral Absorbent

To reduce the risk of adsorption of granular activated carbon (AC) in the gastrointestinal tract, we successfully prepared a hollow-type spherical bacterial cellulose gel encapsulated with AC (ACEG) and evaluated its pH tolerance and adsorption capacity. The bacterial cellulose gel membrane of ACEG features a three-dimensional mesh structure of cellulose fibers, allowing the selective permeation of substances based on their size. In this study, the preparation method of ACEGs was investigated, and the indole saturation adsorption capacity of the obtained gel was measured. We modified the gel culture nucleus gel from calcium alginate gel to agar gel, facilitating the encapsulation of previously challenging particles. The new preparation method used sodium hydroxide solution for sterilization and dissolution to remove the debris of Komagataeibacter xylinus, which was feared to remain in the bacterial cellulose membrane. This treatment was also confirmed to have no effect on the adsorption capacity of the AC powder. Therefore, this new preparation method is expected not only to improve the performance of ACEGs but also to be applied to a wide range of adsorbent-encapsulated hollow-type bacterial cellulose gels.

Is denosumab an efficient and safe drug for osteoporosis in dialysis patients? Considerations and state of the art about its use in this setting

PURPOSE

In patients with chronic kidney disease (CKD), renal osteodystrophy may be associated with a progressive bone mass loss that increases fracture risk. Denosumab, a monoclonal antibody inhibiting osteoclast activity, is an antiresorptive medication used for the treatment osteoporosis.

METHODS

Its efficacy and safety were initially established in the FREEDOM study, showing a significant reduction in incident fractures in osteoporotic women treated with denosumab. Subsequent post hoc analyses showed its efficacy in patients stratified by kidney function, but these analyses did not include patients with advanced stages of CKD. The capability of denosumab in improving bone mineral density in uremic patients was evaluated in 12 studies including 461 dialysis patients with low bone mineral density. The improvement of bone mineral density was the final end point in these studies assessed during a follow-up of 6-60 months. Nine of these studies did not have hyperparathyroidism among criteria for patient inclusion and their participants may have low-turnover bone disease. Despite current recommendations, no patients underwent bone biopsy before denosumab therapy.

RESULTS

Overall, findings in these studies suggest that denosumab is a viable option for promoting bone mass recovery in patients with advanced stages of CKD having either high or low serum levels of PTH. However, the increase of bone mineral density was lower in patients with low serum markers of bone turnover at baseline. These studies also highlighted the need for calcium and vitamin D supplementation to prevent hypocalcemia that remains a serious concern.

CONCLUSIONS

Denosumab emerges as a potentially safe and effective option for enhancing bone health in CKD patients.

Gut Microbiota in Patients Receiving Dialysis: A Review

The human gut microbiota constitutes a complex community of microorganisms residing within the gastrointestinal tract, encompassing a vast array of species that play crucial roles in health and disease. The disease processes involved in chronic kidney disease (CKD) and end-stage kidney disease (ESKD) are now increasingly established to result in dysregulation of gut microbiota composition and function. Gut microbiota dysbiosis has been associated with poor clinical outcomes and all-cause mortality in patients with ESKD, particularly individuals receiving dialysis. Prior studies highlighted various factors that affect gut microbiota dysbiosis in CKD and ESKD. These include, but are not limited to, uraemic toxin accumulation, chronic inflammation, immune dysfunction, medications, and dietary restrictions and nutritional status. There is a lack of studies at present that focus on the evaluation of gut microbiota dysbiosis in the context of dialysis. Knowledge on gut microbiota changes in this context is important for determining their impact on dialysis-specific and overall outcomes for this patient cohort. More importantly, evaluating gut microbiota composition can provide information into potential targets for therapeutic intervention. Identification of specific microbial signatures may result in further development of personalised treatments to improve patient outcomes and mitigate complications during dialysis. Optimising gut microbiota through various therapeutic approaches, including dietary adjustments, probiotics, prebiotics, medications, and faecal transplantation, have previously demonstrated potential in multiple medical conditions. It remains to be seen whether these therapeutic approaches are effective within the dialysis setting. Our review aims to evaluate evidence relating to alterations in the gut microbiota of patients undergoing dialysis. A growing body of evidence pointing to the complex yet significant relationship which surrounds gut microbiota and kidney health emphasises the importance of gut microbial balance to improve outcomes for individuals receiving dialysis.

Kidney function and bone mineral density in chronic kidney disease patients

Background

Bone mineral density (BMD) predicts fracture risk in patients with chronic kidney disease (CKD) and in the general population. However, few studies have investigated risk factors for bone loss in patients with CKD. The aim of this study was to investigate whether renal function is associated with the rate of BMD decline.

Methods

A prospective cohort study included 1006 patients with CKD stages 2-4 between 2011 and 2016. BMD was measured using dual-energy X-ray absorptiometry at baseline and 4 years. The eGFR was measured 2-6 times during the 4-year follow-up. We analyzed the decline in bone mineral density according to CKD stage and further compared the rate of BMD decline according to eGFR trajectories at each stage.

Results

Advanced CKD stage was associated with a faster rate of decline in total hip BMD [stage 2 -0.23, stage 3A -0.39, stage 3B -0.80, stage 4 -1.23% change/year in men (P < .001); stage 2 -0.86, stage 3A -1.19, stage 3B -1.20, stage 4 -1.58% change/year in women (P < .03)]. Two distinct eGFR trajectories (Class 1 stable group; Class 2 rapid decline group) were observed. The rapid decline group showed a trend toward an increased rate of decline in total hip BMD. Subgroup analysis according to eGFR trajectories revealed a significant difference in BMD decline rate between stable and rapid decline groups.

Conclusions

Advanced CKD stage and accelerated decline in renal function were associated with rapid BMD decline in non-dialysis patients with CKD.

Metabolomic profile of secondary hyperparathyroidism in patients with chronic kidney disease stages 3-5 not receiving dialysis

Introduction

Secondary hyperparathyroidism (SHPT) is a common and serious complication of chronic kidney disease (CKD). Elucidating the metabolic characteristics of SHPT may provide a new theoretical basis for its prevention and treatment. This study aimed to perform a metabolomic analysis of SHPT in patients with CKD stages 3-5 not receiving dialysis.

Methods

A total of 76 patients with CKD, 85 patients with CKD-SHPT, and 67 healthy controls were enrolled in this study. CKD was diagnosed according to the criteria specified in the Kidney Disease Improving Global Outcomes 2012 guidelines. SHPT was diagnosed by experienced clinicians according to the Renal Disease Outcomes Quality Initiative Clinical Practice Guidelines. Serum renal function markers and the lipid profile were analyzed. Untargeted ultra performance liquid chromatography-tandem mass spectrometry was used to analyze the serum metabolites of patients with CKD and SHPT. Multivariate analysis of the data was performed using principal component analysis and partial least square discriminant analysis. Serum differential metabolites were identified and further characterized using databases. Pathway enrichment analysis was performed using the Kyoto Encyclopedia of Genes and Genomes database. Correlations between differential metabolites and clinical parameters were determined using the Spearman correlation.

Results

The serum metabolomic profiles of patients with CKD with and without SHPT differed significantly. Differential metabolites were mainly enriched in the top four Kyoto Encyclopedia of Genes and Genomes pathways: phenylalanine, tyrosine, and tryptophan biosynthesis; sphingolipid metabolism; glycerophospholipid metabolism; and phenylalanine metabolism. In total, 31 differential metabolites were identified; of these, L-tryptophan and (R)-(+)-1-phenylethylamine were decreased, while other amino acids and their derivatives, uremia toxins, carnitine, and lipids, were increased significantly in patients with SHPT compared to those without. The 14 lipid metabolites were positively correlated with levels of Urea, serum creatinine, cystatin C, and triglycerides and negatively correlated with the estimated glomerular filtration rate and levels of total and high- and low-density lipoprotein cholesterol.

Discussion

Disturbed amino acid and lipid metabolism were more apparent in patients with SHPT than in those without. This metabolomic profile of SHPT may provide a therapeutic foundation for its future clinical management.

Skeletal parathyroid hormone hyporesponsiveness: a neglected, but clinically relevant reality in chronic kidney disease

PURPOSE OF REVIEW

Defining the optimal parathyroid hormone (PTH) target in chronic kidney disease (CKD) is challenging, especially for bone outcomes, due to the substantial variability in the skeleton's response to PTH. Although PTH hyporesponsiveness is as integral a component of CKD-mineral bone disorder as elevated PTH levels, clinical awareness of this condition is limited. In this review, we will discuss factors and mechanisms contributing to PTH hyporesponsiveness in CKD. This knowledge may provide clues towards a personalized approach to treating secondary hyperparathyroidism in CKD.

RECENT FINDINGS

Indicates a link between disturbed phosphate metabolism and impaired skeletal calcium sensing receptor signaling as an important mediator of PTH hyporesponsiveness in CKD. Further, cohort studies with diverse populations point towards differences in mineral metabolism control, rather than genetic or environmental factors, as drivers of the variability of PTH responsiveness.

IN SUMMARY

Skeletal PTH hyporesponsiveness in CKD has a multifactorial origin, shows important interindividual variability, and is challenging to estimate in clinical practice. The variability in skeletal responsiveness compromises PTH as a biomarker of bone turnover, especially when considering populations that are heterogeneous in ethnicity, demography, kidney function, primary kidney disease and mineral metabolism control, and in patients treated with bone targeting drugs.

Indoxyl Sulfate-Induced Macrophage Toxicity and Therapeutic Strategies in Uremic Atherosclerosis

Cardiovascular disease (CVD) frequently occurs in patients with chronic kidney disease (CKD), particularly those undergoing dialysis. The mechanisms behind this may be related to traditional risk factors and CKD-specific factors that accelerate atherosclerosis and vascular calcification in CKD patients. The accumulation of uremic toxins is a significant factor in CKD-related systemic disorders. Basic research suggests that indoxyl sulfate (IS), a small protein-bound uremic toxin, is associated with macrophage dysfunctions, including increased oxidative stress, exacerbation of chronic inflammation, and abnormalities in lipid metabolism. Strategies to mitigate the toxicity of IS include optimizing gut microbiota, intervening against the abnormality of intracellular signal transduction, and using blood purification therapy with higher efficiency. Further research is needed to examine whether lowering protein-bound uremic toxins through intervention leads to a reduction in CVD in patients with CKD.

AST-120 improved uremic pruritus by lowering indoxyl sulfate and inflammatory cytokines in hemodialysis patients

BACKGROUND AND HYPOTHESIS

Pruritus is a common and distressing symptom that affects patients with chronic kidney disease. The concentration of protein bounded uremic toxin was associated with the uremic pruritus. The aim is to assess the efficacy of AST-120 for uremic pruritus in hemodialysis patients.

MATERIALS AND METHODS

The participants were enrolled and then divided into the AST-120 treatment group and control group with a ratio of 2:1. All participants underwent pre-observation screenings two weeks before the study with three visits. In the treatment phase (week 1 to week 4), the treatment group added 6g/day of AST-120 along with routine anti-pruritic treatment. Visual analog scale (VAS) and biochemical parameters were measured.

RESULTS

The VAS score began to be lower in the AST-120 treatment group after the 5th visiting (p < 0.05). The reduction in indoxyl sulfate (IS) at 5th week along with TNF-alpha. The reduction ratio of indoxyl sulfate correlated with reduction of parathyroid hormone.

CONCLUSION

This study has demonstrated that the four-week treatment of AST-120 decreased the severity of uremic pruritus in patients with ESRD. The concentration of IS and TNF-alpha decreased in the AST-120 treatment group. The reduction of iPTH correlated with the reduction of IS in the AST-120 treatment.

The Dietary Fiber Inulin Slows Progression of Chronic Kidney Disease-Mineral Bone Disorder (CKD-MBD) in a Rat Model of CKD

Chronic kidney disease (CKD)-mineral bone disorder (CKD-MBD) leads to fractures and cardiovascular disease. Observational studies suggest beneficial effects of dietary fiber on both bone and cardiovascular outcomes, but the effect of fiber on CKD-MBD is unknown. To determine the effect of fiber on CKD-MBD, we fed the Cy/+ rat with progressive CKD a casein-based diet of 0.7% phosphate with 10% inulin (fermentable fiber) or cellulose (non-fermentable fiber) from 22 weeks to either 30 or 32 weeks of age (~30% and ~15% of normal kidney function; CKD 4 and 5). We assessed CKD-MBD end points of biochemistry, bone quantity and quality, cardiovascular health, and cecal microbiota and serum gut-derived uremic toxins. Results were analyzed by two-way analysis of variance (ANOVA) to evaluate the main effects of CKD stage and inulin, and their interaction. The results showed that in CKD animals, inulin did not alter kidney function but reduced the increase from stage 4 to 5 in serum levels of phosphate and parathyroid hormone, but not fibroblast growth factor-23 (FGF23). Bone turnover and cortical bone parameters were similarly improved but mechanical properties were not altered. Inulin slowed progression of aorta and cardiac calcification, left ventricular mass index, and fibrosis. To understand the mechanism, we assessed intestinal microbiota and found changes in alpha and beta diversity and significant changes in several taxa with inulin, together with a reduction in circulating gut derived uremic toxins such as indoxyl sulfate and short-chain fatty acids. In conclusion, the addition of the fermentable fiber inulin to the diet of CKD rats led to a slowed progression of CKD-MBD without affecting kidney function, likely mediated by changes in the gut microbiota composition and lowered gut-derived uremic toxins. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Inflammation and gut dysbiosis as drivers of CKD-MBD

Two decades ago, Kidney Disease: Improving Global Outcomes coined the term chronic kidney disease-mineral and bone disorder (CKD-MBD) to describe the syndrome of biochemical, bone and extra-skeletal calcification abnormalities that occur in patients with CKD. CKD-MBD is a prevalent complication and contributes to the excessively high burden of fractures and cardiovascular disease, loss of quality of life and premature mortality in patients with CKD. Thus far, therapy has focused primarily on phosphate retention, abnormal vitamin D metabolism and parathyroid hormone disturbances, but these strategies have largely proved unsuccessful, thus calling for paradigm-shifting concepts and innovative therapeutic approaches. Interorgan crosstalk is increasingly acknowledged to have an important role in health and disease. Accordingly, mounting evidence suggests a role for both the immune system and the gut microbiome in bone and vascular biology. Gut dysbiosis, compromised gut epithelial barrier and immune cell dysfunction are prominent features of the uraemic milieu. These alterations might contribute to the inflammatory state observed in CKD and could have a central role in the pathogenesis of CKD-MBD. The emerging fields of osteoimmunology and osteomicrobiology add another level of complexity to the pathogenesis of CKD-MBD, but also create novel therapeutic opportunities.

Clinical Perspectives of Gut Microbiota in Patients with Chronic Kidney Disease and End-Stage Kidney Disease: Where Do We Stand?

The gut microbiota (GM) plays a vital role in human health, with increasing evidence linking its imbalance to chronic kidney disease and end-stage kidney disease. Although the exact methods underlying kidney-GM crosstalk are not fully understood, interventions targeting GM were made and lay in three aspects: diagnostic, predictive, and therapeutic interventions. While these interventions show promising results in reducing uremic toxins and inflammation, challenges remain in the form of patient-specific GM variability, potential side effects, and safety concerns. Our understanding of GMs role in kidney disease is still evolving, necessitating further research to elucidate the causal relationship and mechanistic interactions. Personalized interventions focusing on specific GM signatures could enhance patient outcomes. However, comprehensive clinical trials are needed to validate these approaches' safety, efficacy, and feasibility.

The Effect of Synbiotic Supplementation on Uremic Toxins, Oxidative Stress, and Inflammation in Hemodialysis Patients-Results of an Uncontrolled Prospective Single-Arm Study

Introduction: Numerous studies to date have shown that the development of dysbiotic gut microbiota is a characteristic finding in chronic kidney disease (CKD). A number of uremic toxins progressively accumulate in the course of CKD, some of them generated by the intestinal microbiome, such as indoxyl sulfate (IS) and p-cresyl sulfate (p-CS). They are found to be involved in the pathogenesis of certain complications of uremic syndrome, including low-grade chronic inflammation and oxidative stress. The aim of the present study is to research the serum concentration of IS and p-CS in end stage renal disease (ESRD) patients undergoing conventional hemodialysis, as well as to study the possibilities of influencing some markers of inflammation and oxidative stress after taking a synbiotic. Materials and Methods: Thirty patients with end-stage renal disease (ESRD) undergoing hemodialysis treatment who were taking a synbiotic in the form of Lactobacillus acidophilus La-14 2 × 1011 (CFU)/g and prebiotic fructooligosaccharides were included in the study. Serum levels of total IS, total p-CS, Interleukin-6 (IL-6), and Malondialdehyde (MDA) were measured at baseline and after 8 weeks. Results. The baseline values of the four investigated indicators in the patients were significantly higher-p-CS (29.26 ± 58.32 pg/mL), IS (212.89 ± 208.59 ng/mL), IL-6 (13.84 ± 2.02 pg/mL), and MDA (1430.33 ± 583.42 pg/mL), compared to the results obtained after 8 weeks of intake, as we found a significant decrease in the parameters compared to the baseline-p-CS (6.40 ± 0.79 pg/mL, p = 0.041), IS (47.08 ± 3.24 ng/mL, p < 0.001), IL-6 (9.14 ± 1.67 pg/mL, p < 0.001), and MDA (1003.47 ± 518.37 pg/mL, p < 0.001). Conclusions: The current study found that the restoration of the intestinal microbiota in patients with CKD significantly decreases the level of certain uremic toxins. It is likely that this favorably affects certain aspects of CKD, such as persistent low-grade inflammation and oxidative stress.

The Potential Influence of Uremic Toxins on the Homeostasis of Bones and Muscles in Chronic Kidney Disease

Patients with chronic kidney disease (CKD) often experience a high accumulation of protein-bound uremic toxins (PBUTs), specifically indoxyl sulfate (IS) and p-cresyl sulfate (pCS). In the early stages of CKD, the buildup of PBUTs inhibits bone and muscle function. As CKD progresses, elevated PBUT levels further hinder bone turnover and exacerbate muscle wasting. In the late stage of CKD, hyperparathyroidism worsens PBUT-induced muscle damage but can improve low bone turnover. PBUTs play a significant role in reducing both the quantity and quality of bone by affecting osteoblast and osteoclast lineage. IS, in particular, interferes with osteoblastogenesis by activating aryl hydrocarbon receptor (AhR) signaling, which reduces the expression of Runx2 and impedes osteoblast differentiation. High PBUT levels can also reduce calcitriol production, increase the expression of Wnt antagonists (SOST, DKK1), and decrease klotho expression, all of which contribute to low bone turnover disorders. Furthermore, PBUT accumulation leads to continuous muscle protein breakdown through the excessive production of reactive oxygen species (ROS) and inflammatory cytokines. Interactions between muscles and bones, mediated by various factors released from individual tissues, play a crucial role in the mutual modulation of bone and muscle in CKD. Exercise and nutritional therapy have the potential to yield favorable outcomes. Understanding the underlying mechanisms of bone and muscle loss in CKD can aid in developing new therapies for musculoskeletal diseases, particularly those related to bone loss and muscle wasting.

The Dietary Fermentable Fiber Inulin Alters the Intestinal Microbiome and Improves Chronic Kidney Disease Mineral-Bone Disorder in a Rat Model of CKD

Background

Dietary fiber is important for a healthy diet, but intake is low in CKD patients and the impact this has on the manifestations of CKD-Mineral Bone Disorder (MBD) is unknown.

Methods

The Cy/+ rat with progressive CKD was fed a casein-based diet of 0.7% phosphate with 10% inulin (fermentable fiber) or cellulose (non-fermentable fiber) from 22 weeks to either 30 or 32 weeks of age (~30 and ~15 % of normal kidney function). We assessed CKD-MBD, cecal microbiota, and serum gut-derived uremic toxins. Two-way ANOVA was used to evaluate the effect of age and inulin diet, and their interaction.

Results

In CKD animals, dietary inulin led to changes in microbiota alpha and beta diversity at 30 and 32 weeks, with higher relative abundance of several taxa, including Bifidobacterium and Bacteroides , and lower Lactobacillus . Inulin reduced serum levels of gut-derived uremic toxins, phosphate, and parathyroid hormone, but not fibroblast growth factor-23. Dietary inulin decreased aorta and cardiac calcification and reduced left ventricular mass index and cardiac fibrosis. Bone turnover and cortical bone parameters were improved with inulin; however, bone mechanical properties were not altered.

Conclusions

The addition of the fermentable fiber inulin to the diet of CKD rats led to changes in the gut microbiota composition, lowered gut-derived uremic toxins, and improved most parameters of CKD-MBD. Future studies should assess this fiber as an additive therapy to other pharmacologic and diet interventions in CKD.

Significance Statement

Dietary fiber has well established beneficial health effects. However, the impact of fermentable dietary fiber on the intestinal microbiome and CKD-MBD is poorly understood. We used an animal model of progressive CKD and demonstrated that the addition of 10% of the fermentable fiber inulin to the diet altered the intestinal microbiota and lowered circulating gut-derived uremic toxins, phosphorus, and parathyroid hormone. These changes were associated with improved cortical bone parameters, lower vascular calcification, and reduced cardiac hypertrophy, fibrosis and calcification. Taken together, dietary fermentable fiber may be a novel additive intervention to traditional therapies of CKD-MBD.

Effects of the uremic toxin indoxyl sulphate on human microvascular endothelial cells

Indoxyl sulphate (IS) is a uremic toxin accumulating in the plasma of chronic kidney disease (CKD) patients. IS accumulation induces side effects in the kidneys, bones and cardiovascular system. Most studies assessed IS effects on cell lines by testing higher concentrations than those measured in CKD patients. Differently, we exposed a human microvascular endothelial cell line (HMEC-1) to the IS concentrations measured in the plasma of healthy subjects (physiological) or CKD patients (pathological). Pathological concentrations reduced cell proliferation rate but did not increase long-term oxidative stress level. Indeed, total protein thiols decreased only after 24 h of exposure in parallel with an increased Nrf-2 protein expression. IS induced actin cytoskeleton rearrangement with formation of stress fibres. Proteomic analysis supported this hypothesis as many deregulated proteins are related to actin filaments organization or involved in the endothelial to mesenchymal transition. Interestingly, two proteins directly linked to cardiovascular diseases (CVD) in in vitro and in vivo studies underwent deregulation: COP9 signalosome complex subunit 9 and thrombomodulin. Future experiments will be needed to investigate the role of these proteins and the signalling pathways in which they are involved to clarify the possible link between CKD and CVD.

High-Resolution Mass Spectrometry for the Measurement of PTH and PTH Fragments: Insights into PTH Physiology and Bioactivity

Full-length parathyroid hormone (PTH 1-84) is crucial for the regulation of calcium and phosphate homeostasis and bone remodeling. PTH 1-84 is metabolized into various PTH fragments, which are measured with varying levels of efficiency by PTH immunoassays. These PTH fragments, which increase in serum as CKD progresses, could potentially modulate the effects of PTH 1-84 and contribute to CKD-associated bone disorders. To obtain a true biologic representation of total PTH bioactivity, it is necessary to measure not only PTH 1-84 but also PTH fragments that are present in circulation. Traditional second-generation PTH immunoassays collectively measure PTH 1-84, PTH fragments, and post-translationally modified PTH 1-84, making it difficult to accurately predict the character of underlying renal osteodystrophy. This review highlights current advances in methods available for PTH measurement and the clinical relevance of PTH fragments in CKD. We emphasize the usefulness of mass spectrometry as a potential reference method for PTH measurement.

Role of Calcimimetics in Treating Bone and Mineral Disorders Related to Chronic Kidney Disease

Renal osteodystrophy is common in patients with chronic kidney disease and end-stage renal disease and leads to the risks of fracture and extraosseous vascular calcification. Secondary hyperparathyroidism (SHPT) is characterized by a compensatory increase in parathyroid hormone (PTH) secretion in response to decreased renal phosphate excretion, resulting in potentiating bone resorption and decreased bone quantity and quality. Calcium-sensing receptors (CaSRs) are group C G-proteins and negatively regulate the parathyroid glands through (1) increasing CaSR insertion within the plasma membrane, (2) increasing 1,25-dihydroxy vitamin D3 within the kidney and parathyroid glands, (3) inhibiting fibroblast growth factor 23 (FGF23) in osteocytes, and (4) attenuating intestinal calcium absorption through Transient Receptor Potential Vanilloid subfamily member 6 (TRPV6). Calcimimetics (CaMs) decrease PTH concentrations without elevating the serum calcium levels or extraosseous calcification through direct interaction with cell membrane CaSRs. CaMs reduce osteoclast activity by reducing stress-induced oxidative autophagy and improving Wnt-10b release, which promotes the growth of osteoblasts and subsequent mineralization. CaMs also directly promote osteoblast proliferation and survival. Consequently, bone quality may improve due to decreased bone resorption and improved bone formation. CaMs modulate cardiovascular fibrosis, calcification, and renal fibrosis through different mechanisms. Therefore, CaMs assist in treating SHPT. This narrative review focuses on the role of CaMs in renal osteodystrophy, including their mechanisms and clinical efficacy.

Interaction of Human Serum Albumin with Uremic Toxins: The Need of New Strategies Aiming at Uremic Toxins Removal

Chronic kidney disease (CKD) is acknowledged worldwide to be a grave threat to public health, with the number of US end-stage kidney disease (ESKD) patients increasing steeply from 10,000 in 1973 to 703,243 in 2015. Protein-bound uremic toxins (PBUTs) are excreted by renal tubular secretion in healthy humans, but hardly removed by traditional haemodialysis (HD) in ESKD patients. The accumulation of these toxins is a major contributor to these sufferers’ morbidity and mortality. As a result, some improvements to dialytic removal have been proposed, each with their own upsides and drawbacks. Longer dialysis sessions and hemodiafiltration, though, have not performed especially well, while larger dialyzers, coupled with a higher dialysate flow, proved to have some efficiency in indoxyl sulfate (IS) clearance, but with reduced impact on patients’ quality of life. More efficient in removing PBUTs was fractionated plasma separation and adsorption, but the risk of occlusive thrombosis was worryingly high. A promising technique for the removal of PBUTs is binding competition, which holds great hopes for future HD. This short review starts by presenting the PBUTs chemistry with emphasis on the chemical interactions with the transport protein, human serum albumin (HSA). Recent membrane-based strategies targeting PBUTs removal are also presented, and their efficiency is discussed.

Serum concentrations of free indoxyl and p-cresyl sulfate are associated with mineral metabolism variables and cardiovascular risk in hemodialysis patients

BACKGROUND

Indoxyl sulfate (IS) and p-cresyl sulfate (PCS) are uremic toxins associated with cardiovascular outcome in CKD patients. The present work is an analysis of the association of serum free, total IS and PCS with cardiovascular events and calcium-phosphate metabolism variables in hemodialysis patients.

METHODS

Serum levels of total and free IS and PCS were measured in 139 hemodialysis patients. Their relationship with calcium-phosphate metabolism variables were tested in an observational cohort study. In addition, their association with cardiovascular events was investigated during a 4-year follow-up.

RESULTS

Patients in the highest tertile (T3) of serum free IS showed lower serum 1,25(OH)₂D compared to patients in the middle (T2) and lowest tertile (T1); in addition to this, T3 patients showed lower serum irisin than T1 patients and lower serum PTH than all the other subjects (T1 + T2) combined. Serum PTH was also measured during the two years after the baseline measurement and was higher in patients in the T1 than in those in the T3 of serum free IS. Cox regression analysis showed that cardiovascular risk was lower in T1 patients than in those in the T3 of serum free PCS, both using a univariate (OR 2.55, 95% CI 1.2-5.43; p = 0.015) or multivariate model (OR 2.48, 95% CI 1.12-5.51; p = 0.003).

CONCLUSIONS

Serum free IS may be associated with PTH and 1,25(OH)₂D secretion, whereas free PCS may predict cardiovascular risk in hemodialysis patients.

Peripheral Blood Mononuclear Cells (PBMCs) to Dissect the Underlying Mechanisms of Bone Disease in Chronic Kidney Disease and Rare Renal Diseases

PURPOSE OF REVIEW

To describe the methods that can be used to obtain functional and mature osteoclasts from peripheral blood mononuclear cells (PBMCs) and report the data obtained with this model in two peculiar diseases, namely pediatric chronic kidney disease-associated mineral and bone disorders (CKD-MBD) and nephropathic cystinosis. To discuss future research possibilities in the field.

RECENT FINDINGS

Bone tissue undergoes continuous remodeling throughout life to maintain bone architecture; it involves two processes: bone formation and bone resorption with the coordinated activity of osteoblasts, osteoclasts, and osteocytes. Animal models fail to fully explain human bone pathophysiology during chronic kidney disease, mainly due to interspecies differences. The development of in vitro models has permitted to mimic human bone-related diseases as an alternative to in vivo models. Since 1997, osteoclasts have been generated in cell cultures, notably when culturing PBMCs with specific growth factors and cytokines (i.e., M-CSF and RANK-L), without the need for osteoblasts or stromal cells. These models may improve the global understanding of bone pathophysiology. They can be been used not only to evaluate the direct effects of cytokines, hormones, cells, or drugs on bone remodeling during CKD-MBD, but also in peculiar genetic renal diseases inducing specific bone impairment.

Dietary strategies for gut-derived protein-bound uremic toxins and cardio-metabolic risk factors in chronic kidney disease: A focus on dietary fibers

Chronic kidney disease (CKD) is associated with altered composition and function of gut microbiota. The cause of gut dysbiosis in CKD is multifactorial and encompasses the following: uremic state, metabolic acidosis, slow colonic transit, dietary restrictions of plant-based fiber-rich foods, and pharmacological therapies. Dietary restriction of potassium-rich fruits and vegetables, which are common sources of fermentable dietary fibers, inhibits the conversion of dietary fibers to short-chain fatty acids (SCFA), which are the primary nutrient source for the symbiotic gut microbiota. Reduced consumption of fermentable dietary fibers limits the population of SCFA-forming bacteria and causes dysbiosis of gut microbiota. Gut dysbiosis induces colonic fermentation of protein and formation of gut-derived uremic toxins. In this review, we discuss the roles and benefits of dietary fiber on gut-derived protein-bound uremic toxins and plant-based dietary patterns that could be recommended to decrease uremic toxin formation in CKD patients. Recent studies have indicated that dietary fiber supplementation may be useful to decrease gut-derived uremic toxin formation and slow CKD progression. However, research on associations between adherence of healthy dietary patterns and gut-derived uremic toxins formation in patients with CKD is lacking.

Toxic Effects of Indoxyl Sulfate on Osteoclastogenesis and Osteoblastogenesis

Uremic toxins, such as indoxyl sulfate (IS) and kynurenine, accumulate in the blood in the event of kidney failure and contribute to further bone damage. To maintain the homeostasis of the skeletal system, bone remodeling is a persistent process of bone formation and bone resorption that depends on a dynamic balance of osteoblasts and osteoclasts. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates the toxic effects of uremic toxins. IS is an endogenous AhR ligand and is metabolized from tryptophan. In osteoclastogenesis, IS affects the expression of the osteoclast precursor nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) through AhR signaling. It is possible to increase osteoclast differentiation with short-term and low-dose IS exposure and to decrease differentiation with long-term and/or high-dose IS exposure. Coincidentally, during osteoblastogenesis, through the AhR signaling pathway, IS inhibits the phosphorylation of ERK, and p38 reduces the expression of the transcription factor 2 (Runx2), disturbing osteoblastogenesis. The AhR antagonist resveratrol has a protective effect on the IS/AhR pathway. Therefore, it is necessary to understand the multifaceted role of AhR in CKD, as knowledge of these transcription signals could provide a safe and effective method to prevent and treat CKD mineral bone disease.

Interplay between gut microbiota, bone health and vascular calcification in chronic kidney disease

Deregulations in gut microbiota may play a role in vascular and bone disease in chronic kidney disease (CKD). As glomerular filtration rate declines, the colon becomes more important as a site of excretion of urea and uric acid, and an increased bacterial proteolytic fermentation alters the gut microbial balance. A diet with limited amounts of fibre, as well as certain medications (eg phosphate binders, iron supplementation, antibiotics) further contribute to changes in gut microbiota composition among CKD patients. At the same time, both vascular calcification and bone disease are common in patients with advanced kidney disease. This narrative review describes emerging evidence on gut dysbiosis, vascular calcification, bone demineralization and their interrelationship termed the 'gut-bone-vascular axis' in progressive CKD. The role of diet, gut microbial metabolites (ie indoxyl sulphate, p-cresyl sulphate, trimethylamine N-oxide (TMAO) and short-chain fatty acids (SCFA)), vitamin K deficiency, inflammatory cytokines and their impact on both bone health and vascular calcification are discussed. This framework may open up novel preventive and therapeutic approaches targeting the microbiome in an attempt to improve cardiovascular and bone health in CKD.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

A phosphate and calcium-enriched diet promotes progression of 5/6-nephrectomy-induced chronic kidney disease in C57BL/6 mice

C57BL/6 mice are known to be rather resistant to the induction of experimental chronic kidney disease (CKD) by 5/6-nephrectomy (5/6-Nx). Here, we sought to characterize the development of CKD and its cardiac and skeletal sequelae during the first three months after 5/6-Nx in C57BL/6 mice fed a calcium- and phosphate enriched diet (CPD) with a balanced calcium/phosphate ratio. 5/6-NX mice on CPD showed increased renal fibrosis and a more pronounced decrease in glomerular filtration rate when compared to 5/6-Nx mice on normal diet (ND). Interestingly, despite comparable levels of serum calcium, phosphate, and parathyroid hormone (PTH), circulating intact fibroblast growth factor-23 (FGF23) was 5 times higher in 5/6-Nx mice on CPD, relative to 5/6-Nx mice on ND. A time course experiment revealed that 5/6-Nx mice on CPD developed progressive renal functional decline, renal fibrosis, cortical bone loss, impaired bone mineralization as well as hypertension, but not left ventricular hypertrophy. Collectively, our data show that the resistance of C57BL/6 mice to 5/6-Nx can be partially overcome by feeding the CPD, and that the CPD induces a profound, PTH-independent increase in FGF23 in 5/6-Nx mice, making it an interesting tool to assess the pathophysiological significance of FGF23 in CKD.

Circulating p-Cresyl Sulfate, Non-Hepatic Alkaline Phosphatase and Risk of Bone Fracture Events in Chronic Kidney Disease-Mineral Bone Disease

Patients with chronic kidney disease (CKD), especially those undergoing hemodialysis, are at a considerably high risk of bone fracture events. Experimental data indicate that uremic toxins intricately involved in bone-related proteins exert multi-faced toxicity on bone cells and tissues, leading to chronic kidney disease–mineral and bone disorder (CKD-MBD). Nonetheless, information regarding the association between p-cresyl sulfate (PCS), non-hepatic alkaline phosphatase (NHALP) and skeletal events remains elusive. We aim to explore the association between PCS, NHALP and risk of bone fracture (BF) in patients with hemodialysis. Plasma concentrations of PCS and NHALP were ascertained at study entry. Cox proportional hazard regression analyses were used to determine unadjusted and adjusted hazard ratios (aHRs) of PCS for BF risk. In multivariable analysis, NHALP was associated with incremental risks of BFs [aHR: 1.06 (95% CI: 1.01–1.11)]. The association between the highest PCS tertile and BF risk remained robust [aHR: 2.87 (95% CI: 1.02–8.09)]. With respect to BF events, the interaction between NHALP and PCS was statistically significant (p value for the interaction term < 0.05). In addition to mineral dysregulation and hyperparathyroidism in hemodialysis patients, higher circulating levels of PCS and NHALP are intricately associated with incremental risk of BF events, indicating that a joint evaluation is more comprehensive than single marker. In light of the extremely high prevalence of CKD-MBD in the hemodialysis population, PCS may act as a pro-osteoporotic toxin and serve as a potential surrogate marker for skeletal events.

The Influence of Dietary Interventions on Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD)

Chronic kidney disease is a health problem whose prevalence is increasing worldwide. The kidney plays an important role in the metabolism of minerals and bone health and therefore, even at the early stages of CKD, disturbances in bone metabolism are observed. In the course of CKD, various bone turnover or mineralization disturbances can develop including adynamic hyperparathyroid, mixed renal bone disease, osteomalacia. The increased risk of fragility fractures is present at any age in these patients. Nutritional treatment of patients with advanced stages of CKD is aiming at prevention or correction of signs, symptoms of renal failure, avoidance of protein-energy wasting (PEW), delaying or prevention of the occurrence of mineral/bone disturbances, and delaying the start of dialysis. The results of studies suggest that progressive protein restriction is beneficial with the progression of renal insufficiency; however, other aspects of dietary management of CKD patients, including changes in sodium, phosphorus, and energy intake, as well as the source of protein and lipids (animal or plant origin) should also be considered carefully. Energy intake must cover patients' energy requirement, in order to enable correct metabolic adaptation in the course of protein-restricted regimens and prevent negative nitrogen balance and protein-energy wasting.

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.

Differentiating the causes of adynamic bone in advanced chronic kidney disease informs osteoporosis treatment

Patients with chronic kidney disease (CKD) have an increased fracture risk because of impaired bone quality and quantity. Low bone mineral density predicts fracture risk in all CKD stages, including advanced CKD (CKD G4-5D). Pharmacological therapy improves bone mineral density and reduces fracture risk in moderate CKD. Its efficacy in advanced CKD remains to be determined, although pilot studies suggest a positive effect on bone mineral density. Currently, antiresorptive agents are the most commonly prescribed drugs for the prevention and therapy of osteoporosis. Their use in advanced CKD has been limited by the lack of large clinical trials and fear of causing kidney dysfunction and adynamic bone disease. In recent decades, adynamic bone disease has evolved as the most predominant form of renal osteodystrophy, commonly associated with poor outcomes, including premature mortality and progression of vascular calcification. Evolving evidence indicates that reduction of bone turnover by parathyroidectomy or pharmacological therapies, such as calcimimetics and antiresorptive agents, are not associated with premature mortality or accelerated vascular calcification in CKD. In contrast, chronic inflammation, oxidative stress, malnutrition, and diabetes can induce low bone turnover and associate with poor prognosis. Thus, the conditions causing suppression of bone turnover rather than the low bone turnover per se may account for the perceived association with outcomes. Anabolic treatment, in contrast, has been suggested to improve turnover and bone mass in patients with advanced CKD and low bone turnover; however, uncertainty about safety even exceeds that of antiresorptive agents. Here, we critically review the pathophysiological concept of adynamic bone disease and discuss the effect of low bone turnover on the safety and efficacy of anti-osteoporosis pharmacotherapy in advanced CKD.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia, or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in 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.

Inflammation, Oxidative Stress, and Bone in Chronic Kidney Disease in the Osteoimmunology Era

Bone is not only a mineralized and apparently non-vital structure that provides support for locomotion and protection to inner organs. An increasing number of studies are unveiling new biologic functions and connections to other systems, giving the rise to new fields of research, such as osteoimmunology. The bone marrow niche, a new entity in bone physiology, seems to represent the site where a complex crosstalk between bone and immune/inflammatory responses takes place. An impressive interplay with the immune system is realized in bone marrow, with reciprocal influences between bone cells and haematopoietic cells. In this way, systemic chronic inflammatory diseases realize a crosstalk with bone, resulting in bone disease. Thus, pathogenetic links between chronic kidney disease-mineral bone disorders and osteoporosis, cardiovascular disease, and ageing are common. The aim of this narrative review is to provide a general view of the progresses in the field of bone research and their potential clinical implications, with emphasis on the links with inflammation and the connections to osteoimmunology and chemokines.

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.

Type I Angiotensin II Receptor Blockade Reduces Uremia-Induced Deterioration of Bone Material Properties

Chronic kidney disease (CKD) is associated with a high incidence of fractures. However, the pathophysiology of this disease is not fully understood, and limited therapeutic interventions are available. This study aimed to determine the impact of type 1 angiotensin II receptor blockade (AT-1RB) on preventing CKD-related fragility fractures and elucidate its pharmacological mechanisms. AT-1RB use was associated with a lower risk of hospitalization due to fractures in 3276 patients undergoing maintenance hemodialysis. In nephrectomized rats, administration of olmesartan suppressed osteocyte apoptosis, skeletal pentosidine accumulation, and apatite disorientation, and partially inhibited the progression of the bone elastic mechanical properties, while the bone mass was unchanged. Olmesartan suppressed angiotensin II-dependent oxidation stress and apoptosis in primary cultured osteocytes in vitro. In conclusion, angiotensin II-dependent intraskeletal oxidation stress deteriorated the bone elastic mechanical properties by promoting osteocyte apoptosis and pentosidine accumulation. Thus, AT-1RB contributes to the underlying pathogenesis of abnormal bone quality in the setting of CKD, possibly by oxidative stress. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Serum biomarkers of the calcium-deficient rats identified by metabolomics based on UPLC/Q-TOF MS/MS

BACKGROUND

We previously identified the urinary biomarkers to diagnose calcium deficiency and nutritional rickets by ultra-performance liquid chromatography/quadrupole time-of-flight tandem mass spectrometry (UPLC/Q-TOF MS/MS). To find biomarkers of calcium deficiency and further confirm these biomarkers in serum, we performed serum metabolomics analysis of calcium-deficient rats.

METHODS

A calcium-deficient rat model was established with a low-calcium diet for 12 weeks. Serum metabolomics based UPLC/Q-TOF MS/MS and multivariate statistical analysis was performed to identify the alterations in metabolites associated with calcium deficiency in rats.

RESULTS

Bone mineral density, serum parathyroid hormone and alkaline phosphatase were significantly decreased in the low-calcium diet group (LCG) compared to the normal calcium diet group (NCG). Serum metabolic-profiling analysis could definitively distinguish between the LCG and NCG and identified 24 calcium-deficient biomarkers. Three metabolites (indoxyl sulfate, phosphate, and taurine) of the 24 biomarkers were found in our previous urinary metabolomics study of rats with a calcium deficiency and nutritional rickets. The areas under the curve (AUCs) of these three biomarkers were greater than 0.8, and the combination of any two biomarkers was higher than 0.95.

CONCLUSION

Dietary calcium deficiency induced the alterations of metabolites in the serum of rats, and the three identified biomarkers had relatively high diagnostic values for calcium deficiency in rats.

Association between Uremic Toxin Concentrations and Bone Mineral Density after Kidney Transplantation

Although uremic osteoporosis is a component of mineral and bone disorder in chronic kidney disease, uremic toxin (UT) concentrations in patients with end-stage kidney disease and bone mineral density (BMD) changes after kidney transplantation have not previously been described. We hypothesized that elevated UT concentrations at the time of transplantation could have a negative impact on bone during the early post-transplantation period. Hence, we sought to determine whether concentrations of UTs (trimethylamine-N-oxide, indoxylsulfate, p-cresylsulfate, p-cresylglucuronide, indole-3-acetic acid, hippuric acid, and 3-carboxy-4-methyl-5-propyl-furanpropionic acid) upon transplantation are predictive markers for (i) osteoporosis one month after transplantation, and (ii) a BMD decrease and the occurrence of fractures 12 and 24 months after kidney transplantation. Between 2012 and 2018, 310 kidney transplant recipients were included, and dual-energy X-ray absorptiometry was performed 1, 12, and 24 months after transplantation. The UT concentrations upon transplantation were determined by reverse-phase high-performance liquid chromatography. Indoxylsulfate concentrations upon transplantation were positively correlated with BMD one month after transplantation for the femoral neck but were not associated with osteoporosis status upon transplantation. Concentrations of the other UTs upon transplantation were not associated with osteoporosis or BMD one month after transplantation. None of the UT concentrations were associated with BMD changes and the occurrence of osteoporotic fractures 12 and 24 months after transplantation. Hence, UT concentrations at the time of kidney transplantation were not predictive markers of osteoporosis or fractures.

The clinical impact of gut microbiota in chronic kidney disease

Gut microorganisms play critical roles in both maintaining host homeostasis and the development of diverse diseases. Gut dysbiosis, an alteration of the composition and function of gut microorganisms, is commonly seen in patients with chronic kidney disease (CKD). CKD itself contributes to a disruption of the symbiotic relationship between the gut microbiota and the host, while the resulting gut dysbiosis may play a part in stage progression of CKD. This bidirectional relationship supports the concept that the gut microbiota is considered a novel focus for the pathogenesis and management of CKD. This article examines the interaction between the gut microbiota and the kidney, the mutual effects of dysbiosis and CKD, and possible treatment options to restore gut eubiosis, and reduce CKD progression and its related complications.

Resveratrol Rescue Indoxyl Sulfate-Induced Deterioration of Osteoblastogenesis via the Aryl Hydrocarbon Receptor /MAPK Pathway

Indoxyl sulfate (IS), a uremic toxin derived from dietary tryptophan metabolism by the gut microbiota, is an endogenous aryl hydrocarbon receptor (AhR) agonist and a key player in bone remodeling. Resveratrol (RSV), an AhR antagonist, plays a protective role in shielding against AhR ligands. Our study explored the impact of IS on osteoblast differentiation and examined the possible mechanism of IS in controlling the expression of osteoblastogenesis markers through an in-depth investigation of AhR signaling. In vivo, we found histological architectural disruption of the femoral bones in 5/6 nephrectomies of young adult IS exposed mice, including reduced Runx2 antigen expression. RSV improved the diaphysis architecture, Runx2 expression, and trabecular quality. In vitro data suggest that IS at 500 and 1000 μM disturbed osteoblastogenesis through suppression of the ERK and p38 mitogen-activated protein kinase (MAPK) pathways, which were found to be downstream of AhR. RSV proved to ameliorate the anti-osteoblastogenic effects of IS through the inhibition of AhR and downstream signaling. Taken together, we demonstrated that the IS/AhR/MAPK signaling pathway plays a crucial role in the inhibition of osteoblastogenesis, and RSV has a potential therapeutic role in reversing the IS-induced decline in osteoblast development and suppressing abnormal bone turnover in chronic kidney disease patients.

Gut-Derived Protein-Bound Uremic Toxins

Chronic kidney disease (CKD) afflicts more than 500 million people worldwide and is one of the fastest growing global causes of mortality. When glomerular filtration rate begins to fall, uremic toxins accumulate in the serum and significantly increase the risk of death from cardiovascular disease and other causes. Several of the most harmful uremic toxins are produced by the gut microbiota. Furthermore, many such toxins are protein-bound and are therefore recalcitrant to removal by dialysis. We review the derivation and pathological mechanisms of gut-derived, protein-bound uremic toxins (PBUTs). We further outline the emerging relationship between kidney disease and gut dysbiosis, including the bacterial taxa altered, the regulation of microbial uremic toxin-producing genes, and their downstream physiological and neurological consequences. Finally, we discuss gut-targeted therapeutic strategies employed to reduce PBUTs. We conclude that targeting the gut microbiota is a promising approach for the treatment of CKD by blocking the serum accumulation of PBUTs that cannot be eliminated by dialysis.

Osteoblastic differentiation of bone marrow mesenchymal stem cells in uremic rats

Severe secondary hyperparathyroidism (SHPT) represents a high turnover bone disease, osteitis fibrosa, but the pathogenesis of osteitis fibrosa remains to be fully elucidated. We examined the characteristics of the differentiation of bone marrow mesenchymal stem cells (BMSCs) into osteoblasts in uremic rats. We bred 5/6 nephrectomized (Nx) rats with a high phosphorus (P) diet to induce SHPT (Nx + HP), or Nx (Nx + ND) and normal rats (Nc + ND) fed a standard diet (ND). After 8 weeks, BMSCs were isolated from the femur and serum were analyzed. BMSCs underwent flow cytometric examination for the expression patterns of cell surface markers (CD90⁺, CD29⁺, CD45^-, and CD31^-). Serum creatinine (Cre) levels were significantly elevated in the Nx + NP rats compared with the Nc + NP rats. Cre levels in the Nx + HP rats were levels to those in the Nx + ND rats. Serum P and PTH levels were significantly elevated in the Nx + HP rats compared with the Nx + ND rats. Bone morphometrical analysis showed increases in both osteoid volume and eroded surfaces in the Nx + HP but not in the Nx + ND rats. The populations of harvested BMSCs were similar between all three groups. Alp, Runx2, Pth1r and Cyclin D1 mRNA expression in the BMSCs from the Nx + ND rats were significantly suppressed compared with those isolated from the Nc + ND groups. Alizarin red staining tended to be similar to the expression of these mRNA. These results suggest that the BMSCs differentiation into osteoblasts was disturbed in the uremic rats.

Indoxyl Sulfate Contributes to Adipose Tissue Inflammation through the Activation of NADPH Oxidase

Adipose tissue inflammation appears to be a risk factor for the progression of chronic kidney disease (CKD), but the effect of CKD on adipose tissue inflammation is poorly understood. The purpose of this study was to clarify the involvement of uremic toxins (indoxyl sulfate (IS), 3-indoleacetic acid, p-cresyl sulfate and kynurenic acid) on CKD-induced adipose tissue inflammation. IS induces monocyte chemoattractant protein-1 (MCP-1) expression and reactive oxygen species (ROS) production in the differentiated 3T3L-1 adipocyte. An organic anion transporter (OAT) inhibitor, an NADPH oxidase inhibitor or an antioxidant suppresses the IS-induced MCP-1 expression and ROS production, suggesting the OAT/NADPH oxidase/ROS pathway is involved in the action of IS. Co-culturing 3T3L-1 adipocytes and mouse macrophage cells showed incubating adipocytes with IS increased macrophage infiltration. An IS-overload in healthy mice increased IS levels, oxidative stress and MCP-1 expression in epididymal adipose tissue compared to unloaded mice. Using 5/6-nephrectomized mice, the administration of AST-120 suppressed oxidative stress and the expression of MCP-1, F4/80 and TNF-α in epididymal adipose tissue. These collective data suggest IS could be a therapeutic target for the CKD-related inflammatory response in adipose tissue, and that AST-120 could be useful for the treatment of IS-induced adipose tissue inflammation.