The gut microbiota and its relationship with chronic kidney disease

Can curcumin supplementation reduce plasma levels of gut-derived uremic toxins in hemodialysis patients? A pilot randomized, double-blind, controlled study

BACKGROUND

Gut dysbiosis is common in patients with chronic kidney disease (CKD) and is closely related to inflammatory processes. Some nutritional strategies, such as bioactive compounds present in curcumin, have been proposed as an option to modulate the gut microbiota and decrease the production of uremic toxins such as indoxyl sulfate (IS), p-cresyl sulfate (pCS) and indole-3 acetic acid (IAA).

OBJECTIVE

To evaluate the effects of curcumin supplementation on uremic toxins plasma levels produced by gut microbiota in patients with CKD on hemodialysis (HD).

METHODS

Randomized, double-blind trial in 28 patients [53.6 ± 13.4 years, fourteen men, BMI 26.7 ± 3.7 kg/m², dialysis vintage 37.5 (12-193) months]. Fourteen patients were randomly allocated to the curcumin group and received 100 mL of orange juice with 12 g carrot and 2.5 g of turmeric and 14 patients to the control group who received the same juice but without turmeric three times per week after HD sessions for three months. IS, pCS, IAA plasma levels were measured by reverse-phase high-performance liquid chromatography RESULTS: After three months of supplementation, the curcumin group showed a significant decrease in pCS plasma levels [from 32.4 (22.1-45.9) to 25.2 (17.9-37.9) mg/L, p = 0.009], which did not occur in the control group. No statistical difference was observed in IS and IAA levels in both groups.

CONCLUSION

The oral supplementation of curcumin for three months seems to reduce p-CS plasma levels in HD patients, suggesting a gut microbiota modulation.

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.

p-Cresol Sulfate Caused Behavior Disorders and Neurodegeneration in Mice with Unilateral Nephrectomy Involving Oxidative Stress and Neuroinflammation

Protein-bound uremic toxins, such as p-cresol sulfate (PCS), can be accumulated with declined renal function and aging and is closely linked with central nervous system (CNS) diseases. In the periphery, PCS has effects on oxidative stress and inflammation. Since oxidative stress and inflammation have substantial roles in the pathogenesis of neurological disorders, the CNS effects of PCS were investigated in unilateral nephrectomized C57/BL/6 mice. Unlike intact mice, unilateral nephrectomized mice showed increased circulating levels of PCS after exogenous administration. Upon PCS exposure, the unilateral nephrectomized mice developed depression-like, anxiety-like, and cognitive impairment behaviors with brain PCS accumulation in comparison with the nephrectomy-only group. In the prefrontal cortical tissues, neuronal cell survival and neurogenesis were impaired along with increased apoptosis, oxidative stress, and neuroinflammation. Circulating brain-derived neurotrophic factors (BDNF) and serotonin were decreased in association with increased corticosterone and repressor element-1 silencing transcription factor (REST), regulators involved in neurological disorders. On the contrary, these PCS-induced changes were alleviated by uremic toxin absorbent AST-120. Taken together, PCS administration in mice with nephrectomy contributed to neurological disorders with increased oxidative stress and neuroinflammation, which were alleviated by PCS chelation. It is suggested that PCS may be a therapeutic target for chronic kidney disease-associated CNS diseases.

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.

Role of S-Equol, Indoxyl Sulfate, and Trimethylamine N-Oxide on Vascular Function

Gut microbiota have been emerging as important contributors to the regulation of host homeostasis. Accordingly, several substances converted by gut microbiota can have beneficial or adverse effects on human health. Among them, S-equol, which is produced from the isoflavone daidzein in the human and animal gut by certain microbiota, exerts estrogenic and antioxidant activities. Indoxyl sulfate, which is metabolized in the liver from indole converted from dietary tryptophan by bacterial tryptophanases in the colon, is known as a protein-bound uremic toxin. Trimethylamine N-oxide, which is generated via the oxidization of gut microbiota-derived trimethylamine by hepatic flavin monooxygenases, is known as an accelerator of atherosclerosis. The aforementioned gut-derived substances could be potential regulators of systematic tissue/organ function, including the vascular system. Macro- and microvascular complications of cardiovascular and metabolic diseases, including atherosclerosis, hypertension, and diabetes, occur systemically and represent the principal cause of morbidity and mortality. Vascular endothelial and smooth muscle dysfunction play pivotal roles in the development and progression of vasculopathies. We herein review the link between the aforementioned gut-derived substances and endothelial and vascular smooth muscle cell function. This information will provide a conceptual framework that would allow the development of novel preventive and/or therapeutic approaches against vasculopathies.

Trimethylamine N-Oxide in Relation to Cardiometabolic Health-Cause or Effect?

Trimethylamine-N-oxide (TMAO) is generated in a microbial-mammalian co-metabolic pathway mainly from the digestion of meat-containing food and dietary quaternary amines such as phosphatidylcholine, choline, betaine, or L-carnitine. Fish intake provides a direct significant source of TMAO. Human observational studies previously reported a positive relationship between plasma TMAO concentrations and cardiometabolic diseases. Discrepancies and inconsistencies of recent investigations and previous studies questioned the role of TMAO in these diseases. Several animal studies reported neutral or even beneficial effects of TMAO or its precursors in cardiovascular disease model systems, supporting the clinically proven beneficial effects of its precursor, L-carnitine, or a sea-food rich diet (naturally containing TMAO) on cardiometabolic health. In this review, we summarize recent preclinical and epidemiological evidence on the effects of TMAO, in order to shed some light on the role of TMAO in cardiometabolic diseases, particularly as related to the microbiome.

Toxicity of Microplastics and Nanoplastics in Mammalian Systems

Fragmented or otherwise miniaturized plastic materials in the form of micro- or nanoplastics have been of nagging environmental concern. Perturbation of organismal physiology and behavior by micro- and nanoplastics have been widely documented for marine invertebrates. Some of these effects are also manifested by larger marine vertebrates such as fishes. More recently, possible effects of micro- and nanoplastics on mammalian gut microbiota as well as host cellular and metabolic toxicity have been reported in mouse models. Human exposure to micro- and nanoplastics occurs largely through ingestion, as these are found in food or derived from food packaging, but also in a less well-defined manner though inhalation. The pathophysiological consequences of acute and chronic micro- and nanoplastics exposure in the mammalian system, particularly humans, are yet unclear. In this review, we focus on the recent findings related to the potential toxicity and detrimental effects of micro- and nanoplastics as demonstrated in mouse models as well as human cell lines. The prevailing data suggest that micro- and nanoplastics accumulation in mammalian and human tissues would likely have negative, yet unclear long-term consequences. There is a need for cellular and systemic toxicity due to micro- and nanoplastics to be better illuminated, and the underlying mechanisms defined by further work.

Gut microbiota and chronic kidney disease: evidences and mechanisms that mediate a new communication in the gastrointestinal-renal axis

Chronic kidney disease (CKD) represents a growing public health problem associated with loss of kidney function and cardiovascular disease (CVD), the main leading cause of morbidity and mortality in CKD. It is well established that CKD is associated with gut dysbiosis. Over the past few years, there has been a growing interest in studying the composition of the gut microbiota in patients with CKD as well as the mechanisms by which gut dysbiosis contributes to CKD progression, in order to identify possible therapeutic targets to improve the morbidity and survival in CKD. The purpose of this review is to explore the clinical evidence and the mechanisms involved in the gut-kidney crosstalk as well as the possible interventions to restore a normal balance of the gut microbiota in CKD. It is well known that the influence of the gut microbiota on the gut-kidney axis acts in a reciprocal way: on the one hand, CKD significantly modifies the composition and functions of the gut microbiota. On the other hand, gut microbiota is able to manipulate the processes leading to CKD onset and progression through inflammatory, endocrine, and neurologic pathways. Understanding the complex interaction between these two organs (gut microbiota and kidney) may provide novel nephroprotective interventions to prevent the progression of CKD by targeting the gut microbiota. The review is divided into three main sections: evidences from clinical studies about the existence of a gut microbiota dysbiosis in CKD; the complex mechanisms that explain the bidirectional relationship between CKD and gut dysbiosis; and reports regarding the effects of prebiotic, probiotic, and synbiotic supplementation to restore gut microbiota balance in CKD.