The gut–kidney axis

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Abstract

The host-gut microbiota interaction has been the focus of increasing interest in recent years. It has been determined that this complex interaction is not only essential to many aspects of normal "mammalian" physiology but that it may also contribute to a multitude of ailments, from the obvious case of inflammatory bowel disease to (complex) diseases residing in organs outside the gut. An increasing body of evidence indicates that crosstalk between host and microbiota is pathophysiologically relevant in patients with chronic kidney disease (CKD). Interactions are bidirectional; on the one hand, uremia affects both the composition and metabolism of the gut microbiota and, on the other hand, important uremic toxins originate from microbial metabolism. In addition, gut dysbiosis may induce a disruption of the epithelial barrier, ultimately resulting in increased exposure of the host to endotoxins. Due to dietary restrictions and gastrointestinal dysfunctions, microbial metabolism shifts to a predominantly proteolytic fermentation pattern in CKD. Indoxyl sulfate and p-cresyl sulfate, both end-products of protein fermentation, and trimethylamine-N-oxide, an end-product of microbial choline and carnitine metabolism, are prototypes of uremic toxins originating from microbial metabolism. The vascular and renal toxicity of these co-metabolites has been demonstrated extensively in experimental and clinical studies. These co-metabolites are an appealing target for adjuvant therapy in CKD. Treatment options include dietary therapy, prebiotics, probiotics and host and bacterial enzyme inhibitors. Final proof of the concept should come from randomized controlled and adequately powered intervention studies.

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Strong association between malnutrition, inflammation, and atherosclerosis in chronic renal failure.

P Stenvinkel, O Heimbürger, F Paultre … (1999)

Atherosclerotic cardiovascular disease and malnutrition are widely recognized as leading causes of the increased morbidity and mortality observed in uremic patients. C-reactive protein (CRP), an acute-phase protein, is a predictor of cardiovascular mortality in nonrenal patient populations. In chronic renal failure (CRF), the prevalence of an acute-phase response has been associated with an increased mortality. One hundred and nine predialysis patients (age 52 +/- 1 years) with terminal CRF (glomerular filtration rate 7 +/- 1 ml/min) were studied. By using noninvasive B-mode ultrasonography, the cross-sectional carotid intima-media area was calculated, and the presence or absence of carotid plaques was determined. Nutritional status was assessed by subjective global assessment (SGA), dual-energy x-ray absorptiometry (DXA), serum albumin, serum creatinine, serum urea, and 24-hour urine urea excretion. The presence of an inflammatory reaction was assessed by CRP, fibrinogen (N = 46), and tumor necrosis factor-alpha (TNF-alpha; N = 87). Lipid parameters, including Lp(a) and apo(a)-isoforms, as well as markers of oxidative stress (autoantibodies against oxidized low-density lipoprotein and vitamin E), were also determined. Compared with healthy controls, CRF patients had an increased mean carotid intima-media area (18.3 +/- 0.6 vs. 13.2 +/- 0.7 mm2, P or = 10 mg/liter). Malnourished patients had higher CRP levels (23 +/- 3 vs. 13 +/- 2 mg/liter, P < 0.01), elevated calculated intima-media area (20.2 +/- 0.8 vs. 16.9 +/- 0.7 mm2, P < 0.01) and a higher prevalence of carotid plaques (90 vs. 60%, P < 0.0001) compared with well-nourished patients. During stepwise multivariate analysis adjusting for age and gender, vitamin E (P < 0.05) and CRP (P < 0.05) remained associated with an increased intima-media area. The presence of carotid plaques was significantly associated with age (P < 0.001), log oxidized low-density lipoprotein (oxLDL; P < 0.01), and small apo(a) isoform size (P < 0.05) in a multivariate logistic regression model. These results indicate that the rapidly developing atherosclerosis in advanced CRF appears to be caused by a synergism of different mechanisms, such as malnutrition, inflammation, oxidative stress, and genetic components. Apart from classic risk factors, low vitamin E levels and elevated CRP levels are associated with an increased intima-media area, whereas small molecular weight apo(a) isoforms and increased levels of oxLDL are associated with the presence of carotid plaques.

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Serum indoxyl sulfate is associated with vascular disease and mortality in chronic kidney disease patients.

Fellype C. Barreto, Daniela Barreto, Sophie Liabeuf … (2009)

As a major component of uremic syndrome, cardiovascular disease is largely responsible for the high mortality observed in chronic kidney disease (CKD). Preclinical studies have evidenced an association between serum levels of indoxyl sulfate (IS, a protein-bound uremic toxin) and vascular alterations. The aim of this study is to investigate the association between serum IS, vascular calcification, vascular stiffness, and mortality in a cohort of CKD patients. One-hundred and thirty-nine patients (mean +/- SD age: 67 +/- 12; 60% male) at different stages of CKD (8% at stage 2, 26.5% at stage 3, 26.5% at stage 4, 7% at stage 5, and 32% at stage 5D) were enrolled. Baseline IS levels presented an inverse relationship with renal function and a direct relationship with aortic calcification and pulse wave velocity. During the follow-up period (605 +/- 217 d), 25 patients died, mostly because of cardiovascular events (n = 18). In crude survival analyses, the highest IS tertile was a powerful predictor of overall and cardiovascular mortality (P = 0.001 and 0.012, respectively). The predictive power of IS for death was maintained after adjustment for age, gender, diabetes, albumin, hemoglobin, phosphate, and aortic calcification. The study presented here indicates that IS may have a significant role in the vascular disease and higher mortality observed in CKD patients.

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Expansion of Urease- and Uricase-Containing, Indole- and p-Cresol-Forming and Contraction of Short-Chain Fatty Acid-Producing Intestinal Microbiota in ESRD

Jakk Wong, Yvette Piceno, Todd Z. DeSantis … (2014)

Background: Intestinal microbiome constitutes a symbiotic ecosystem that is essential for health, and changes in its composition/function cause various illnesses. Biochemical milieu shapes the structure and function of the microbiome. Recently, we found marked differences in the abundance of numerous bacterial taxa between ESRD and healthy individuals. Influx of urea and uric acid and dietary restriction of fruits and vegetables to prevent hyperkalemia alter ESRD patients' intestinal milieu. We hypothesized that relative abundances of bacteria possessing urease, uricase, and p-cresol- and indole-producing enzymes is increased, while abundance of bacteria containing enzymes converting dietary fiber to short-chain fatty acids (SCFA) is reduced in ESRD. Methods: Reference sets of bacteria containing genes of interest were compiled to family, and sets of intestinal bacterial families showing differential abundances between 12 healthy and 24 ESRD individuals enrolled in our original study were compiled. Overlap between sets was assessed using hypergeometric distribution tests. Results: Among 19 microbial families that were dominant in ESRD patients, 12 possessed urease, 5 possessed uricase, and 4 possessed indole and p-cresol-forming enzymes. Among 4 microbial families that were diminished in ESRD patients, 2 possessed butyrate-forming enzymes. Probabilities of these overlapping distributions were <0.05. Conclusions: ESRD patients exhibited significant expansion of bacterial families possessing urease, uricase, and indole and p-cresol forming enzymes, and contraction of families possessing butyrate-forming enzymes. Given the deleterious effects of indoxyl sulfate, p-cresol sulfate, and urea-derived ammonia, and beneficial actions of SCFA, these changes in intestinal microbial metabolism contribute to uremic toxicity and inflammation.