Distal Colon Motor Dysfunction in Mice with Chronic Kidney Disease: Putative Role of Uremic Toxins

Is the enteric nervous system a lost piece of the gut-kidney axis puzzle linked to chronic kidney disease?

The enteric nervous system (ENS) regulates numerous functional and immunological attributes of the gastrointestinal tract. Alterations in ENS cell function have been linked to intestinal outcomes in various metabolic, intestinal, and neurological disorders. Chronic kidney disease (CKD) is associated with a challenging intestinal environment due to gut dysbiosis, which further affects patient quality of life. Although the gut-related repercussions of CKD have been thoroughly investigated, the involvement of the ENS in this puzzle remains unclear. ENS cell dysfunction, such as glial reactivity and alterations in cholinergic signaling in the small intestine and colon, in CKD are associated with a wide range of intestinal pathways and responses in affected patients. This review discusses how the ENS is affected in CKD and how it is involved in gut-related outcomes, including intestinal permeability, inflammation, oxidative stress, and dysmotility.

Sufficient water intake maintains the gut microbiota and immune homeostasis and promotes pathogen elimination

Water is the most abundant substance in the human body and plays a pivotal role in various bodily functions. While underhydration is associated with the incidence of certain diseases, the specific role of water in gut function remains largely unexplored. Here, we show that water restriction disrupts gut homeostasis, which is accompanied by a bloom of gut microbes and decreased numbers of immune cells, especially Th17 cells, within the colon. These microbial and immunological changes in the gut are associated with an impaired ability to eliminate the enteric pathogen Citrobacter rodentium. Moreover, aquaporin 3, a water channel protein, is required for the maintenance of Th17 cell function and differentiation. Taken together, adequate water intake is critical for maintaining bacterial and immunological homeostasis in the gut, thereby enhancing host defenses against enteric pathogens.

Huang Gan Formula Alleviates Systemic Inflammation and Uremia in Adenine-Induced Chronic Kidney Disease Rats May Associate with Modification of Gut Microbiota and Colonic Microenvironment

Purpose

This study aims to investigate the effects of Huang Gan formula (HGF), a Chinese herbal prescription used for chronic kidney disease (CKD), on the regulation of the gut microbiota and colonic microenvironment of CKD.

Methods

CKD rats were induced by 150 mg/kg adenine gavage for 4 weeks, then orally treated with or without 3.6 g/kg or 7.2 g/kg of HGF for 8 weeks. The renal function and structure were analyzed by biochemical detection, hematoxylin and eosin, Masson's trichrome, Sirius red and immunochemical staining. Average fecal weight and number in the colon were recorded to assess colonic motility. Further, the changes in the gut microbiota and colonic microenvironment were evaluated by 16S rRNA sequencing, RT-PCR or immunofluorescence. The levels of inflammatory cytokines, uremic toxins, and NF-κB signaling pathway were detected by RT-PCR, ELISA, chloramine-T method or Western blotting. Redundancy analysis biplot and Spearman's rank correlation coefficient were used for correlation analysis.

Results

HGF significantly improved renal function and pathological injuries of CKD. HGF could improve gut microbial dysbiosis, protect colonic barrier and promote motility of colonic lumens. Further, HGF inhibited systemic inflammation through a reduction of TNF-α, IL-6, IL-1β, TGF-β1, and a suppression of NF-κB signaling pathway. The serum levels of the selected uremic toxins were also reduced by HGF treatment. Spearman correlation analysis suggested that high-dose HGF inhibited the overgrowth of bacteria that were positively correlated with inflammatory factors (eg, TNF-α) and uremic toxins (eg, indoxyl sulfate), whereas it promoted the proliferation of bacteria belonging to beneficial microbial groups and was positively correlated with the level of IL-10.

Conclusion

Our results suggest that HGF can improve adenine-induced CKD via suppressing systemic inflammation and uremia, which may associate with the regulations of the gut microbiota and colonic microenvironment.

Gutted: constipation in children with chronic kidney disease and on dialysis

Functional constipation is a common problem in otherwise healthy children. Children with chronic kidney disease (CKD) and on dialysis have additional disease-related risk factors including the uremic milieu, fluid and dietary restrictions, and decreased physical activity, as well as treatment-related risk factors such as dialysis therapy and polypharmacy that contribute to and compound the problem. Constipation causes significant distress for children and their caregivers. In children on peritoneal dialysis, severe constipation can impede catheter function and ultrafiltration. Accumulating evidence points to a possible bidirectional relationship between constipation and CKD, potentially mediated by gut dysbiosis with consequent increased generation of gut-derived uremic toxins and disruption of intestinal epithelium integrity leading to translocation of noxious luminal contents into the circulation inducing systemic inflammation. Effective management of constipation is required but there is little published data on the safety and effectiveness of treatments in adults or children with CKD. In this review, we discuss the diagnosis and epidemiology of functional constipation, provide an overview of its pathophysiology, summarize the therapeutic management, and reflect on the challenges in children with CKD.

A novel model of adenine-induced chronic kidney disease-associated gastrointestinal dysfunction in mice: The gut-kidney axis

Although constipation is a common complication of chronic kidney disease (CKD), there is no animal model that can be used to study the association between renal impairment and gastrointestinal function without interfering with the gastrointestinal tract of the model. Therefore, we determined whether adenine could induce CKD in association with gastrointestinal dysfunction. Six-week-old ICR mice were intraperitoneally injected with saline, 25, 50, or 75 mg adenine/kg body weight for 21 days. Blood urea nitrogen (BUN), plasma creatinine, and renal histopathology were evaluated. Defecation status was evaluated from defecation frequency and fecal water content. Colonic smooth muscle contraction was measured by the organ bath technique, and transepithelial electrical resistance (TEER) was measured using an Ussing chamber. In the 50 mg/kg treatment group, BUN and creatinine were significantly increased compared with control, and inflammatory cell infiltration, glomerular necrosis, tubular dilatation, and interstitial fibrosis were observed in renal tissues. Mice in this group also showed a significant decrease in defecation frequency, fecal water content, colonic motility index, and TEER. Overall, 50 mg/kg of adenine was the best dose to induce CKD with associated constipation and intestinal barrier impairment. Therefore, this adenine administration model can be recommended for CKD-associated gastrointestinal dysfunction research.

Examining enteric nervous system function in rat and mouse: an interspecies comparison of colonic motility

Gastrointestinal motility is crucial to gut health and has been associated with different disorders such as inflammatory bowel diseases and postoperative ileus. Despite rat and mouse being the two animal models most widely used in gastrointestinal research, minimal studies in rats have investigated gastrointestinal motility. Therefore, our study provides a comparison of colonic motility in the mouse and rat to clarify species differences and assess the relative effectiveness of each animal model for colonic motility research. We describe the protocol modifications and optimization undertaken to enable video imaging of colonic motility in the rat. Apart from the broad difference in terms of gastrointestinal diameter and length, we identified differences in the fundamental histology of the proximal colon such that the rat had larger villus height-to-width and villus height-to-crypt depth ratios compared with mouse. Since gut motility is tightly regulated by the enteric nervous system (ENS), we investigated how colonic contractile activity within each rodent species responds to modulation of the ENS inhibitory neuronal network. Here we used N^ω-nitro-l-arginine (l-NNA), an inhibitor of nitric oxide synthase (NOS) to assess proximal colon responses to the stimulatory effect of blocking the major inhibitory neurotransmitter, nitric oxide (NO). In rats, the frequency of proximal colonic contractions increased in the presence of l-NNA (vs. control levels) to a greater extent than in mice. This is despite a similar number of NOS-expressing neurons in the myenteric plexus across species. Given this increase in colonic contraction frequency, the rat represents another relevant animal model for investigating how gastrointestinal motility is regulated by the inhibitory neuronal network of the ENS.NEW & NOTEWORTHY Mice and rats are widely used in gastrointestinal research but have fundamental differences that make them important as different models for different questions. We found that mice have a higher villi length-to-width and villi length-to-crypt depth ratio than rat in proximal colon. Using the ex vivo video imaging technique, we observed that rat colon has more prominent response to blockade of major inhibitory neurotransmitter (nitric oxide) in myenteric plexus than mouse colon.

Gut–Kidney Axis Investigations in Animal Models of Chronic Kidney Disease

Chronic kidney disease (CKD) is an incurable disease in which renal function gradually declines, resulting in no noticeable symptoms during the early stages and a life-threatening disorder in the latest stage. The changes that accompany renal failure are likely to influence the gut microbiota, or the ecosystem of micro-organisms resident in the intestine. Altered gut microbiota can display metabolic changes and become harmful to the host. To study the gut–kidney axis in vivo, animal models should ideally reproduce the disorders affecting both the host and the gut microbiota. Murine models of CKD, but not dog, manifest slowed gut transit, similarly to patient. Animal models of CKD also reproduce altered intestinal barrier function, as well as the resulting leaky gut syndrome and bacterial translocation. CKD animal models replicate metabolic but not compositional changes in the gut microbiota. Researchers investigating the gut–kidney axis should pay attention to the selection of the animal model (disease induction method, species) and the setting of the experimental design (control group, sterilization method, individually ventilated cages) that have been shown to influence gut microbiota.

Dual Role of Indoles Derived From Intestinal Microbiota on Human Health

Endogenous indole and its derivatives (indoles), considered as promising N-substituted heterocyclic compounds, are tryptophan metabolites derived from intestinal microbiota and exhibit a range of biological activities. Recent studies indicate that indoles contribute to maintaining the biological barrier of the human intestine, which exert the anti-inflammatory activities mainly through activating AhR and PXR receptors to affect the immune system’s function, significantly improving intestinal health (inflammatory bowel disease, hemorrhagic colitis, colorectal cancer) and further promote human health (diabetes mellitus, central system inflammation, and vascular regulation). However, the revealed toxic influences cannot be ignored. Indoxyl sulfate, an indole derivative, performs nephrotoxicity and cardiovascular toxicity. We addressed the interaction between indoles and intestinal microbiota and the indoles’ effects on human health as double-edged swords. This review provides scientific bases for the correlation of indoles with diseases moreover highlights several directions for subsequent indoles-related studies.

A tissue-like neurotransmitter sensor for the brain and gut

Neurotransmitters play essential roles in regulating neural circuit dynamics both in the central nervous system as well as at the peripheral, including the gastrointestinal tract^1-3. Their real-time monitoring will offer critical information for understanding neural function and diagnosing disease^1-3. However, bioelectronic tools to monitor the dynamics of neurotransmitters in vivo, especially in the enteric nervous systems, are underdeveloped. This is mainly owing to the limited availability of biosensing tools that are capable of examining soft, complex and actively moving organs. Here we introduce a tissue-mimicking, stretchable, neurochemical biological interface termed NeuroString, which is prepared by laser patterning of a metal-complexed polyimide into an interconnected graphene/nanoparticle network embedded in an elastomer. NeuroString sensors allow chronic in vivo real-time, multichannel and multiplexed monoamine sensing in the brain of behaving mouse, as well as measuring serotonin dynamics in the gut without undesired stimulations and perturbing peristaltic movements. The described elastic and conformable biosensing interface has broad potential for studying the impact of neurotransmitters on gut microbes, brain-gut communication and may ultimately be extended to biomolecular sensing in other soft organs across the body.

Tamoxifen administration alters gastrointestinal motility in mice

BACKGROUND

Tamoxifen is widely used for Cre-estrogen receptor-mediated genomic recombination in transgenic mouse models to mark cells for lineage tracing and to study gene function. However, recent studies have highlighted off-target effects of tamoxifen in various tissues and cell types when used for induction of Cre recombination. Despite the widespread use of these transgenic Cre models to assess gastrointestinal (GI) function, the effect of tamoxifen exposure on GI motility has not been described.

METHODS

We examined the effects of tamoxifen on GI motility by measuring total GI transit, gastric emptying, small intestinal transit, and colonic contractility in wild-type adult mice.

KEY RESULTS

We observed a significant delay in total GI transit in tamoxifen-treated mice, with unaltered gastric emptying, accelerated small intestinal transit, and abnormal colonic motility.

CONCLUSION

Our findings highlight the importance of considering GI motility alterations induced by tamoxifen when designing protocols that utilize tamoxifen as a Cre-driver for studying GI function.

5/6 nephrectomy affects enteric glial cells and promotes impaired antioxidant defense in the colonic neuromuscular layer

AIMS

Chronic kidney disease (CKD) produces multiple repercussions in the gastrointestinal tract (GIT), such as alterations in motility, gut microbiota, intestinal permeability, and increased oxidative stress. However, despite enteric glial cells (EGC) having important neural and immune features in GIT physiology, their function in CKD remains unknown. The present study investigates colonic glial markers, inflammation, and antioxidant parameters in a CKD model.

MAIN METHODS

A 5/6 nephrectomized rat model was used to induce CKD in rats and Sham-operated animals as a control to suppress. Biochemical measures in plasma and neuromuscular layer such as glutathione peroxidase (GPx) and superoxide dismutase (SOD) activity were carried out. Kidney histopathology was evaluated. Colon morphology analysis and glial fibrillary acid protein (GFAP), connexin-43 (Cx43), nuclear factor-kappa B (NF-κB) p65, and GPx protein expression were performed.

KEY FINDINGS

The CKD group exhibited dilated tubules and tubulointerstitial fibrosis in the reminiscent kidney (p = 0.0002). CKD rats showed higher SOD activity (p = 0.004) in plasma, with no differences in neuromuscular layer (p = 0.9833). However, GPx activity was decreased in the CKD group in plasma (p = 0.013) and neuromuscular layer (p = 0.0338). Morphological analysis revealed alterations in colonic morphometry with inflammatory foci in the submucosal layer and neuromuscular layer straightness in CKD rats (p = 0.0291). In addition, GFAP, Cx43, NF-κBp65 protein expression were increased, and GPx decreased in the neuromuscular layer of the CKD group (p < 0.05).

SIGNIFICANCE

CKD animals present alterations in colonic cytoarchitecture and decreased layer thickness. Moreover, CKD affects the enteric glial network of the neuromuscular layer, associated with decreased antioxidant activity and inflammation.

Microplastic consumption induces inflammatory signatures in the colon and prolongs a viral arthritis

Global microplastic (MP) contamination and the effects on the environment are well described. However, the potential for MP consumption to affect human health remains controversial. Mice consuming ≈80 μg/kg/day of 1 μm polystyrene MPs via their drinking water showed no weight loss, nor were MPs detected in internal organs. The microbiome was also not significantly changed. MP consumption did lead to small transcriptional changes in the colon suggesting plasma membrane perturbations and mild inflammation. Mice were challenged with the arthritogenic chikungunya virus, with MP consumption leading to a significantly prolonged arthritic foot swelling that was associated with elevated Th1, NK cell and neutrophil signatures. Immunohistochemistry also showed a significant increase in the ratio of neutrophils to monocyte/macrophages. The picture that emerges is reminiscent of enteropathic arthritis, whereby perturbations in the colon are thought to activate innate lymphoid cells that can inter alia migrate to joint tissues to promote inflammation.

Survey of defecation habits in apparently healthy and chronic kidney disease cats

OBJECTIVES

Changes in bowel movements (BMs) are an important clinical sign in many diseases, including chronic kidney disease (CKD), and the purpose of this study was to collect information on BMs and fecal scores in both apparently healthy and CKD cats. A secondary aim was to assess owner awareness of BM frequency.

METHODS

Owners were asked to complete an initial online questionnaire about their cat's health and litter box habits (including predicted BM frequency and fecal scores) and were then asked to clean the box daily for 7 days and report results (observed frequency of BMs and fecal scores) daily. Differences in BM frequency and fecal scores between apparently normal and CKD cats were compared using the Mann-Whitney test, and predicted vs observed data were compared using the Wilcoxon signed rank test. Difference in percentage of cats defecating more or less than once daily were assessed with Fisher's exact test.

RESULTS

Survey data from 124 owners of apparently healthy cats and 43 owners of CKD cats who submitted two or more days of daily observations (in addition to the initial questionnaire) were analyzed. Eighty-five percent of apparently healthy cats were observed to defecate one or more times per day and 15% defecated less than once per day. Fifty-eight percent of CKD cats defecated one or more times per day and 42% defecated less than once per day. A significantly higher percentage of CKD cats defecated less than once per day in comparison with apparently healthy cats (P <0.0001). Observed BM frequency was significantly less in CKD cats compared with healthy cats (P = 0.02). Observed fecal scores were not significantly different between healthy and CKD cats.

CONCLUSIONS AND RELEVANCE

The observed BM frequency of cats with CKD was less than apparently healthy cats and represents a clinically important variation from normal.

Hand-Assisted Laparoscopic Subtotal Colectomy for Ogilvie Syndrome Associated With Idiopathic Fibrosis of Colon After Simultaneous Pancreas Kidney Transplant

Ogilvie syndrome (acute colonic pseudo-obstruction) is a rare, acquired, life-threatening disorder for which treatment plans vary from simple observation to surgical intervention. Ogilvie syndrome has been reported in patients after renal or liver transplant, but its occurrence after simultaneous pancreas-kidney transplant is rare. Herein, we present the case of a 45-year-old female recipient of a deceased donor simultaneous pancreas-kidney transplant who developed Ogilvie syndrome 10 days after a previous fecal ileus that had resolved at posttransplant week 3. She demonstrated Ogilvie syndrome with obstructive colitis features (severe abdominal pain and high-grade fever), which we immediately treated with colonic decompensation by placement of a transanal ileus tube. After several screening examinations and discontinuation of unnecessary medicines, we were not able to confirm the cause of Ogilvie syndrome in our patient. After 2 weeks, the patient remained unresponsive to the conservative treatment, and so hand-assisted laparoscopic subtotal colectomy was performed to remove the dilated colon. Her symptoms gradually resolved after surgery. Histologically, we confirmed submucosal fibrotic changes, especially at the distal end of the resected colon, without evidence of amyloidosis, and the number of Auerbach plexus ganglia had decreased. Nevertheless, we observed no degenerated appearance of ganglion cells in the Auerbach plexus or the Meissner plexus. After exclusion of several collagen diseases, including systemic sclerosis, we determined that idiopathic colonic fibrosis was the likely cause of Ogilvie syndrome in our patient. When surgery is indicated in transplant patients with Ogilvie syndrome with obstructive colitis features, colectomy should be considered.

A Physiological Approach to Pharmacokinetics in Chronic Kidney Disease

The conventional approach to approximating the pharmacokinetics of drugs in patients with chronic kidney disease (CKD) only accounts for changes in the estimated glomerular filtration rate. However, CKD is a systemic and multifaceted disease that alters many body systems. Therefore, the objective of this exercise was to develop and evaluate a whole-body mechanistic approach to predicting pharmacokinetics in patients with CKD. Physiologically based pharmacokinetic models were developed in PK-Sim v8.0 (www.open-systems-pharmacology.org) to mechanistically represent the disposition of 7 compounds in healthy human adults. The 7 compounds selected were eliminated by glomerular filtration and active tubular secretion by the organic cation transport system to varying degrees. After a literature search, the healthy adult models were adapted to patients with CKD by numerically accounting for changes in glomerular filtration rate, kidney volume, renal perfusion, hematocrit, plasma protein concentrations, and gastrointestinal transit. Literature-informed interindividual variability was applied to the physiological parameters to facilitate a population approach. Model performance in CKD was evaluated against pharmacokinetic data from 8 clinical trials in the literature. Overall, integration of the CKD parameterization enabled exposure predictions that were within 1.5-fold error across all compounds and patients with varying stages of renal impairment. Notable improvement was observed over the conventional approach to scaling exposure, which failed in all but 1 scenario in patients with advanced CKD. Further research is required to qualify its use for first-in-CKD dose selection and clinical trial planning for a wider selection of renally eliminated compounds, including those subject to anion transport.

The microbiome and host mucosal interactions in urinary tract diseases

The urinary tract consists of the bladder, ureters, and kidneys, and is an essential organ system for filtration and excretion of waste products and maintaining systemic homeostasis. In this capacity, the urinary tract is impacted by its interactions with other mucosal sites, including the genitourinary and gastrointestinal systems. Each of these sites harbors diverse ecosystems of microbes termed the microbiota, that regulates complex interactions with the local and systemic immune system. It remains unclear whether changes in the microbiota and associated metabolites may be a consequence or a driver of urinary tract diseases. Here, we review the current literature, investigating the impact of the microbiota on the urinary tract in homeostasis and disease including urinary stones, acute kidney injury, chronic kidney disease, and urinary tract infection. We propose new avenues for exploration of the urinary microbiome using emerging technology and discuss the potential of microbiome-based medicine for urinary tract conditions.

Good to Know: Baseline Data on Feed Intake, Fecal Pellet Output and Intestinal Transit Time in Guinea Pig as a Frequently Used Model in Gastrointestinal Research

Guinea pigs are a traditional and frequently used species in gastrointestinal research. Comprehensive knowledge of basic parameters connected with their intestinal function, such as feed intake, fecal pellet output and gastrointestinal transit time, is important for evaluating results from basic gastrointestinal research that may be applied to practical problems in human and veterinary medicine, for example, when establishing diagnostic tools. Our study revealed that over a 24-h period, single-housed guinea pigs showed a continual but day-accentuated feeding activity, consuming 57% of the total feed during the light period, with pronounced peaks of feed intake during the beginning and end of the light period. This was mirrored by fecal pellet output during the light period and almost no defecation during the dark period, while potential coprophagy not measured in this study needs to be considered. A highly comparable feeding activity was recorded in pair-housed guinea pigs, with 60% of overall feed intake within the light period, indicating that such differences in housing conditions did not influence guinea pigs' feeding behavior. Intestinal transit time was successfully recorded by oral administration of carmine red and counted 5 h on average. Hence, this study provides important information on the basic functional parameters of guinea pigs' gastrointestinal tract physiology.

A Network Pharmacology-Based Approach to Investigating the Mechanisms of Fushen Granule Effects on Intestinal Barrier Injury in Chronic Renal Failure

Purpose

Fushen Granule (FSG) is a Chinese medicine prepared by doctors for treating patients with chronic renal failure, which is usually accompanied by gastrointestinal dysfunction. Here, we explore the protective effect of FSG on intestinal barrier injury in chronic renal failure through bioinformatic analysis and experimental verification.

Methods

In this study, information on the components and targets of FSG related to CRF is collected to construct and visualize protein-protein interaction networks and drug-compound-target networks using network pharmacological methods. DAVID is used to conduct gene ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. Then, it is validated by in vitro experiments. In this study, the human intestinal epithelial (T84) cells are used and divided into four groups: control group, model group, FSG low-dose group, and FSG high-dose group. After the experiment, the activity of T84 cells is detected by a MTT assay, and the expressions of tight junction protein ZO-1, claudin-1, nuclear factor erythroid 2-related factor (Nrf2), heme oxygenase-1 (HO-1), malondialdehyde (MDA), and cyclooxygenase-2 (COX-2) are examined by immunofluorescence and/or western blotting.

Results

Eighty-six potential chronic renal failure-related targets are identified by FSG; among them, nine core genes are screened. Furthermore, GO enrichment analysis shows that the cancer-related signaling pathway, the PI3K-Akt signaling pathway, the HIF1 signaling pathway, and the TNF signaling pathway may play key roles in the treatment of CRF by FSG. The MTT method showed that FSG is not cytotoxic to uremic toxin-induced injured T84 cells. The results of immunofluorescence and WB indicate that compared with the control group, protein expressions level of ZO-1, claudin-1, and Nrf2 in T84 cells is decreased and protein expressions level of HO-1, MDA, and COX-2 is increased after urinary toxin treatment. Instead, compared with the model group, protein expressions level of ZO-1, claudin-1, and Nrf2 in T84 cells is increased and protein expressions level of HO-1, MDA, and COX-2 is decreased after FSG treatment.

Conclusion

FSG had a protective effect on urinary toxin-induced intestinal epithelial barrier injury in chronic renal failure, and its mechanism may be related to the upregulation of Nrf2/HO-1 signal transduction and the inhibition of tissue oxidative stress and inflammatory responses. Screening CRF targets and identifying the corresponding FSG components by network pharmacological methods is a practical strategy to explain the mechanism of FSG in improving gastrointestinal dysfunction in CRF.

Chronic Kidney Disease, Gut Dysbiosis, and Constipation: A Burdensome Triplet

Gut dysbiosis has been implicated in the progression of chronic kidney disease (CKD). Alterations in the gut environment induced by uremic toxins, the dietary restriction of fiber-rich foods, and multiple drugs may be involved in CKD-related gut dysbiosis. CKD-related gut dysbiosis is considered to be characterized by the expansion of bacterial species producing precursors of harmful uremic toxins, such as indoxyl sulfate and p-cresyl sulfate, and the contraction of species generating beneficial short-chain fatty acids, such as butyrate. Gut-derived uremic toxins cause oxidative stress and pro-inflammatory responses, whereas butyrate exerts anti-inflammatory effects and contributes to gut epithelial integrity. Gut dysbiosis is associated with the disruption of the gut epithelial barrier, which leads to the translocation of endotoxins. Research on CKD-related gut dysbiosis has mainly focused on chronic inflammation and consequent cardiovascular and renal damage. The pathogenic relationship between CKD-related gut dysbiosis and constipation has not yet been investigated in detail. Constipation is highly prevalent in CKD and affects the quality of life of these patients. Under the pathophysiological state of gut dysbiosis, altered bacterial fermentation products may play a prominent role in intestinal dysmotility. In this review, we outline the factors contributing to constipation, such as the gut microbiota and bacterial fermentation; introduce recent findings on the pathogenic link between CKD-related gut dysbiosis and constipation; and discuss potential interventions. This pathogenic link needs to be elucidated in more detail and may contribute to the development of novel treatment options not only for constipation, but also cardiovascular disease in CKD.

Constipation in CKD

Constipation is one of the most common gastrointestinal disorders among patients with chronic kidney disease (CKD) partly because of their sedentary lifestyle, low fiber and fluid intake, concomitant medications (e.g., phosphate binders), and multiple comorbidities (e.g., diabetes). Although constipation is usually perceived as a benign, often self-limited condition, recent evidence has challenged this most common perception of constipation. The chronic symptoms of constipation negatively affect patients' quality of life and impose a considerable social and economic burden. Furthermore, recent epidemiological studies have revealed that constipation is independently associated with adverse clinical outcomes, such as end-stage renal disease (ESRD), cardiovascular (CV) disease, and mortality, potentially mediated by the alteration of gut microbiota and the increased production of fecal metabolites. Given the importance of the gut in the disposal of uremic toxins and in acid-base and mineral homeostasis with declining kidney function, the presence of constipation in CKD may limit or even preclude these ancillary gastrointestinal roles, potentially contributing to excess morbidity and mortality. With the advent of new drug classes for constipation, some of which showing unique renoprotective properties, the adequate management of constipation in CKD may provide additional therapeutic benefits beyond its conventional defecation control. Nevertheless, the problem of constipation in CKD has long been underrecognized and its management strategies have scarcely been documented. This review outlines the current understanding of the diagnosis, prevalence, etiology, outcome, and treatment of constipation in CKD, and aims to discuss its novel clinical and therapeutic implications.

Constipation in chronic kidney disease: it is time to reconsider

Constipation is highly prevalent in patients with chronic kidney disease (CKD) and is primarily characterized by decreased intestinal motility. This chronic disorder affects the quality of life of patients. However, nephrologist and dialysis clinicians have long had a disproportionately limited understanding of constipation. Accumulating evidence has revealed a relationship between constipation and cardiovascular disease and CKD. The pathogenesis of constipation in CKD patients is multifactorial: decreased physical activity, comorbidities affecting bowel movement, such as diabetes mellitus, cerebrovascular disease, and hyperparathyroidism, a restricted dietary intake of plant-based fiber-rich foods, and multiple medications, including phosphate binders and potassium-binding resins, have all been implicated. CKD is associated with alterations in the composition and function of the gut microbiota, so-called gut dysbiosis. Recent studies showed that CKD-related gut dysbiosis decreased intestinal motility via intestinal inflammation or the increased generation of gut-derived uremic toxins, such as indoxyl sulfate and p-cresyl sulfate. Furthermore, the gastrointestinal secretion of mucin was found to be decreased in CKD animal models, which may delay colonic transit by diminished lubrication in the alimentary tract. Thus, CKD-related gut dysbiosis may play a role in constipation, but limited information is currently available. Since constipation is often intractable, particularly in CKD patients, every available means needs to be employed in its treatment. The effects of probiotics, prebiotics, and synbiotics on the composition of the gut microbiota and gut-derived uremic toxins have been increasingly reported. However, their effects on stool consistency or frequency in CKD patients remain unclear. Some laxatives may be beneficial for improving not only bowel habits but also gut dysbiosis. Further studies are required to elucidate the CKD-specific pathogenesis of constipation and develop novel effective treatment options.

Preliminary evaluation of fecal fatty acid concentrations in cats with chronic kidney disease and correlation with indoxyl sulfate and p-cresol sulfate

BACKGROUND

Straight- and branched-chain (BCFA) short-chain fatty acids (SCFAs) are produced by colonic microbiota and have both beneficial and deleterious effects in humans with chronic kidney disease (CKD). Fecal SCFAs in cats with CKD have not been described.

OBJECTIVE

To characterize fecal SCFA concentrations in cats with CKD as compared to healthy geriatric cats and correlate SCFA to serum indoxyl sulfate (IS) and p-cresol sulfate (pCS) concentrations.

ANIMALS

Twenty-eight cats with CKD (International Renal Interest Society [IRIS] stages 2, 3, and 4) and 11 older (≥ 8 years) healthy geriatric cats.

METHODS

Prospective, cross-sectional study. Voided feces were analyzed using stable isotope dilution gas chromatography-mass spectrometry to determine fecal concentrations of SCFAs. Serum concentrations of IS and pCS were measured using liquid chromatography tandem mass spectrometry.

RESULTS

Fecal isovaleric acid concentrations were significantly higher in CKD cats(P = .02) Cats with IRIS CKD stage 3 and 4 had significantly higher fecal isovaleric acid concentrations compared to healthy geriatric cats (P = .03), but not compared to IRIS CKD stage 2 cats. Total fecal concentrations of BCFAs were found to correlate weakly with serum creatinine concentration (rho, 0.33; P = .05), blood urea nitrogen concentration (rho, 0.40; P = .01), and pCS concentration (rho, 0.35; P = .04).

CONCLUSIONS AND CLINICAL IMPORTANCE

Fecal isovaleric acid concentrations were higher in CKD cats, particularly in late stage disease, compared to healthy geriatric cats. Fecal BCFA concentrations correlated with pCS and were higher in cats with muscle wasting, providing evidence for malassimilation of protein in CKD cats.

Chronic kidney disease exacerbates ischemic limb myopathy in mice via altered mitochondrial energetics

Chronic kidney disease (CKD) substantially increases the severity of peripheral arterial disease (PAD) symptomology, however, the biological mechanisms remain unclear. The objective herein was to determine the impact of CKD on PAD pathology in mice. C57BL6/J mice were subjected to a diet-induced model of CKD by delivery of adenine for six weeks. CKD was confirmed by measurements of glomerular filtration rate, blood urea nitrogen, and kidney histopathology. Mice with CKD displayed lower muscle force production and greater ischemic lesions in the tibialis anterior muscle (78.1 ± 14.5% vs. 2.5 ± 0.5% in control mice, P < 0.0001, N = 5-10/group) and decreased myofiber size (1661 ± 134 μm2 vs. 2221 ± 100 μm2 in control mice, P < 0.01, N = 5-10/group). This skeletal myopathy occurred despite normal capillary density (516 ± 59 vs. 466 ± 45 capillaries/20x field of view) and limb perfusion. CKD mice displayed a ~50-65% reduction in muscle mitochondrial respiratory capacity in ischemic muscle, whereas control mice had normal mitochondrial function. Hydrogen peroxide emission was modestly higher in the ischemic muscle of CKD mice, which coincided with decreased oxidant buffering. Exposure of cultured myotubes to CKD serum resulted in myotube atrophy and elevated oxidative stress, which were attenuated by mitochondrial-targeted therapies. Taken together, these findings suggest that mitochondrial impairments caused by CKD contribute to the exacerbation of ischemic pathology.