*3.2. The Gut–Kidney Axis*

The pathogenic interconnection between the gut microbiome and kidney diseases is termed the gut–kidney axis [14], which is implicated in CKD and its comorbidities. A paucity of data exists regarding how the gut–kidney axis functions in the pediatric population with CKD and what the impact of the gut microbiota is in this process. However, a great deal of work on the impact of the gut–kidney axis in established CKD has been conducted, including gut barrier dysfunction, inflammation, immune response, alterations of microbiota compositions, dysregulated SCFAs and their receptors, uremic toxins, etc. (Figure 1). Each of them are discussed.

**Figure 1.** Schematic diagram summarizing the proposed mechanisms related to the gut–kidney axis involved in the pathogenesis of chronic kidney disease and its comorbidities. LPS = lipopolysaccharide; Th17 = T-helper 17 cell; Th1 = T-helper 1 cell; TLR4 = toll-like receptor 4; NF-κB = nuclear factor kappa B; SCFA = short-chain fatty acid; IS = indoxyl sulfate; PCS = p-cresyl sulfate; AHR = aryl hydrocarbon receptor; TMA = trimethylamine; TMAO = trimethylamine-N-oxide.

First, CKD can impair the intestinal barrier by disrupting the epithelial tight junction in a 5/6 nephrectomy rat model [64]. An apparent reduction of the tight junction proteins was reported in the gut mucosa of CKD animals, possibly attributed to uremic toxins [44]. As a result, an increased intestinal permeability and translocation of lipopolysaccharide (LPS) and bacteria across the intestinal barrier were reported. In CKD rats, gut bacteria could activate a T-helper 17 (Th17)/Th1 T-cell response and increase the production of inflammatory cytokines, and LPS could initiate innate immune cells through nuclear factor kappa B (NF-κB) and toll-like receptor 4 (TLR4) pathways, all triggering inflammation and immune response [65].

Second, changes in the composition of the gut microbiota are relevant to CKD. Uremia profoundly alters 190 and 175 bacterial operational taxonomic units (OTUs) of the gut microbiome in CKD humans [66] and rats [67], respectively. Specifically, the presence of aerobic bacteria such as those belonging to the phyla *Firmicutes*, *Actinobacteria*, and *Proteobacteria* in higher numbers, but fewer anaerobic bacteria, such as *Sutterellaceae*, *Bacteroidaceae*, and *Lactobacillaceae*, were observed in end stage kidney disease (ESKD) [45–47]. Notably, most research has consistently reported that animals and adult patients with CKD had low abundance of genus *Lactobacillus*, whereas the proportion of family *Enterobacteriaceae* were increased [14,66–69]. A systemic review recruiting 25 studies with 1436 CKD patients revealed that the α-diversity was decreased, and β-diversity of gut microbiota was significantly more distinct in ESKD patients than in healthy controls [69].

Third, the gut microbiota produces diverse metabolites, which are involved in multiple physiological processes, such as immunity and host energy metabolism [14]. Following dietary exposures to certain nutrients, particular microbiota-derived metabolites could be altered in ESKD patients [70]. Carbohydrates are fermented to generate SCFAs which signal the host to increase energy expenditure, enhance G protein-coupled receptor (GPCR) signaling, and act as an inhibitor for histone deacetylase (HDAC) [70–72]. SCFAs are made up of one to six carbon atoms (C1–C6), mainly consisting of acetic acid (C2), propionic acid (C3), and butyric acid (C4) [71]. In adult CKD patients, butyrate-producing microbes and butyric acid production reduced with disease severity [73].

Indoxyl sulfate (IS) and p-cresyl sulfate (PCS), both end-products of protein fermentation, and TMAO, an end-product of microbial carnitine/choline metabolism, are wellknown microbiota-derived uremic toxins. Urinary excretion of several microbial tryptophan metabolites such as IS and PCS is decreased in patients with CKD. These tryptophan metabolites mainly from the indole metabolic pathway are accumulated as uremic toxins, which are ligands for AHR [74]. Activation of AHR is able to trigger inflammation, induce oxidative stress, and modulate the Th17 axis, contributing to CKD progression in vivo and in vitro [75,76]. The level of another uremic toxin, TMAO, is high in patients with ESKD and associated with increased risk of cardiovascular disease [77,78]. TMAO generation results from the fermentation by the gut microbiota of dietary carnitine/choline, which is converted to trimethylamine (TMA) and transformed into TMAO by flavin-containing monooxygenase (FMO) in the liver. Conversely, selective targeting of gut-microbiotadependent TMAO generation has been reported to protect CKD progression in a murine model of CKD [79]. Although the uses of prebiotics, probiotics, postbiotics, and synbiotics have shown potential positive effects against uremic toxin generation, their evidence is still limited for the treatment and prevention of human CKD [80–82].

Together, the interaction between gut microbiota and CKD is bidirectional: CKD may affect the structure of the gut microbiota and contribute to gut dysbiosis, while dysbiosis in CKD patients may increase uremic toxin levels that in turn contribute to CKD progression. Considering the gut is a potential cause of CKD-related complications, gut microbiota-targeted therapeutic strategies in CKD will have a considerable impact on CKD management.
