Our data revealed the detrimental effects of a WD, which simultaneously affects brain and liver health. Gut microbiota and their metabolites, which includes BAs, contributed to such an impact. It was also revealed through our data that inulin can reverse the pathological and molecular changes even while mice continue to consume a WD. Inulin reduced lipogenic gene such as Fasn, Cd36, Srebp1c, and Pparγ. It also induced the expression of genes responsible for fatty acid oxidation and mobilization. Those metabolic benefits likely contribute to improved liver and brain phenotypes.
Recent study has shown that inulin induced cholestatic liver cancer in TLR5 knockout mice [19]. However, such a negative impact was not noted in the current study although long-term inulin supplementation was employed. These conflicting findings could be attributed to differences in animal models, diet, or environment, emphasizing the importance of precision nutritional supplementation [20]. However, both studies revealed the significant roles of BAs and the gut microbiota in liver disease development. Moreover, the current study suggests that hepatic metabolic status is an indicator for cognitive function or vice versa. Such a linkage is likely mediated through gut microbiota-driven inflammatory signaling.
IL-17, a cytokine produced by T helper 17 (Th17) cells plays a significant role in hepatic inflammation by inducing neutrophil infiltration and fatty acid release [41]. Increased IL-17A is also found in mouse models of Alzheimer’s diseases and hepatic fibrosis [42, 43]. Moreover, our recent publication shows that short-term feeding of another WD leads to T helper type 1-/T helper type 17-biased skin inflammation before significant body weight gain is noted [31]. WD intake for 4 weeks promotes mild dermatitis and accumulation of IL-17A-producing γδ T cells in the skin. Furthermore, supplementation with cholestyramine, a BA sequestrant, can prevent skin inflammation along with a reduction in the infiltration of γδ T cells and the expression of proinflammatory mediators [31]. The differentiation of Th17 cells relies on a variety of cytokines, and transcriptional factors such as RORγt and TGFβ in synergy with IL6 are critical for generating Th17 cells [44]. Additionally, BAs have been found to be able to modulate RORγt signaling [56]. The simultaneous induction of microglial and hepatic Rorγt and Il-17a along with other inflammatory cytokines in FPC diet-fed mice, as well as their reductions in inulin-supplemented mice, suggest the significance of IL-17A in diet-associated inflammatory signaling at the systemic level.
A reduction in PSD-95 is associated with postsynaptic degeneration, altered synaptic plasticity, psychiatric diseases, dementia, and Alzheimer’s disease pathology [45]. BDNF is a vital neurotransmitter modulator required for memory and learning [46]. Our data revealed that FPC diet intake reduced LTP, BDNF, and PSD-5; however, inulin supplementation increased all of them. WD intake and obesity are associated with increased anxiety in humans and mice [47, 48]. Here, we showed that inulin supplementation can modulate anxiety as well as exploratory behavior, revealing its beneficial effects. Together, the inflammatory signaling regulated by diet or inulin supplementation not only affects disease development but also has a significant impact on neuroplasticity and behavior.
Regarding the gut microbiota, FPC diet increased Erysipelotrichaceae and Coriobacteriaceae, which are associated with the dyslipidemia phenotypes found in obese individuals and animal models [49, 50]. In contrast, inulin supplementation reduced Erysipelotrichaceae and Coriobacteriaceae, suggesting their roles in host lipid metabolism. It is likely that a diet rich in fat and cholesterol increased the abundances of Allobaculum, Holdemanella, and Olsenella, which obese mice and NAFLD patients also have increased abundance [51–53]. Our data revealed that inulin could prevent these changes along with reducing gut cholesterol and zymosterol. In consistency, APP/PS1 mouse model of Alzheimer’s disease also had decreased Allobaculum [54]. Probiotic Lactobacillus has beneficial effects in improving insulin sensitivity as well as memory and cognition [55, 56]. Our data showed that FPC diet intake reduced Lactobacillus and butyrate producing Butyrivibrio, and inulin supplementation increased their abundances. Barnesiella is an effective immunomodulator, which prohibits the colonization of pathogenic antibiotic-resistant bacteria in the gut [57]; Barnesiella was also increased in inulin-supplemented mice. Together, fermentable fiber inulin markedly shifted the gut microbiota as well as the metabolic phenotype, thereby generating health benefits.
BAs are generated by both host and microbial enzymes via metabolizing cholesterol. BA receptor FXR is pivotal for regulating metabolism and inflammation. In consistency, FXR knockout mice spontaneously develop NASH and liver cancer [58]. Our data suggest that FPC diet reduced FXR and FXR-regulated downstream effects. Additionally, HNF4α, a master regulator for hepatic gene expression, is also down regulated in FPC-fed mice. These compromised signaling pathways may account for NAFLD development. However, inulin supplementation restored them. In addition to regulating hepatic metabolism, FXR has a role in altering motor activity, cognitive function, and mood [10]. FXR agonist is currently in clinical trials to treat metabolic liver diseases [59]. Whether FXR agonists can be used to improve neuroplasticity warrants further investigation.
In addition to FXR, systemic inflammation is also accompanied by reduced TGR5 signaling. In consistency, the neuroprotective effect of TGR5 has also been shown in Aβ1-42-induced cognitive impairment mouse models [12]. Additionally, TGR5 activation prohibits hepatic encephalopathy [11, 60]. In the current study, the neuroprotective effect of TGR5 was further supported by the findings that inulin-improved neuroplasticity was accompanied by increased TGR-5-regualted signaling genes such as Dio2 and Nos1 in the microglia. Thus, via improving metabolism and reducing inflammation, TGR5 can be a potential target for increasing neuroplasticity as well.
Metabolomics data showed that 2, 8-DHQ, a gut microbe-generated metabolite, was markedly reduced by FPC but increased by inulin. 2, 8-DHQ is a species-specific aryl hydrocarbon receptor agonist and has known benefits in reducing glucose intolerance and fighting obesity [13, 61]. Additionally, glucose 6-phosphate, fructose 6-phosphate, galactose 6-phosphate, and sugar alcohols xylose and lyxose were reduced due to FPC diet intake. Conversely, inulin supplementation increased the abundance of sugar alcohols, which have lower caloric value, clearly indicating increased fermentation. Moreover, inulin increased the production of butyrate, which can be an energy source for gut epithelial cell renewal. ChemRICH analysis showed FPC diet-fed mice had decreased hexose phosphates, purine nucleosides, and pyrimidinones, which were all increased with inulin supplementation. Hexose phosphate molecules are key molecules in the pentose phosphate pathway which generates NADPH used in fatty acid biosynthesis [62]. Moreover, inulin-supplemented mice also had increased trans-4-hydroxyproline, which is implicated in wound healing [63] and β-Sitosterol, which boosts immunity [64]. Furthermore, isothreonic acid, a metabolite of vitamin C, and adenine, known as vitamin B4, were all elevated in the cecum of inulin-fed mice.
Metabolites such as palmitic acid are increased in neurodegenerative diseases such as Parkinson’s disease and Alzheimer’s disease [65]. Certain fatty acids such as monostearin, palmitic acid, arachidonic acid (phospholipids), stearic acid, zymosterol, and D-Erythrosphingosine, which clustered together, were positively correlated with FPC diet-increased microbiota Dorea, Parasutterella, Enterococcus, Allobaculum, Olsenella, and Holdemanella and negatively associated with inulin-increased Lactobacillus and Eisenbergiella. Thus, gut microbes likely altered the production of neurotransmitter-related metabolites and affected neural function.