Diet-induced gut/brain dysregulation as a driver for obesity: a role for gut microbiota

Background: Obesity is a challenge for global public health and is characterized by a status of low-grade inflammation secondary to an abnormal release of cytokines, adipokines, and reactive oxygen species. In the digestive tract, high fat diet (HFD) promotes an increment of intestinal permeability, due to an impairment of the intestinal epithelial barrier (IEB), and inflammation. In this respect, it has been proposed that changes in gut microbiota composition may determine alterations in the IEB, facilitating the translocation of immunogenic products in the bloodstream, contributing to the induction and maintenance of the metainflammation. Indeed, this phenomenon is not limited to the IEB, but can spread also to the blood-brain barrier (BBB), with an increased translocation of microbiome-derived products in the central nervous system (CNS). This gives rise to the onset of inflammatory responses that could lead to an increase in food consumption, further sustaining the progression of the disease. The aim of this work was, in the first phase to characterize the sequence of the intestinal, systemic, and central alterations in the early stages of obesity, and, subsequently, in the second phase, to modify the gut microbiota composition with the supplementation of probiotics to dam the instauration of the vicious circle that characterizes this pathology. Methods: In the Phase 1, male C57BL/6J mice were fed with standard diet (SD) or HFD for 1, 2, 4, 6 and 8 weeks and a time-course assessment of the changes in the intestine and central nervous system was performed. In the Phase 2 male C57BL/6J mice, SD- or HFD-fed, were treated with SF68 (10^8 CFU/day) starting concomitantly to the beginning of the diet or 4 weeks after. At the sacrifice plasma and intestinal and brain samples were collected for the subsequent analyses. Results: Phase 1. HFD determined a significant alteration in IEB permeability, with reduced levels of Occludin and ZO-1, which occured already after 1 week. In the early stages of HFD, there was also an increase in colonic myeloperoxidase and IL-1β levels, concomitant with an increased TLR4 expression. HFD settled a rearrangement of the enteric neuromuscular compartment, with a significant reduction in cholinergic stimuli starting from week 4 and, in parallel, an increment in the tachykininergic component from week 6. Even if systemic IL-1β levels raised only after the establishment of obese condition, a BBB disruption has been observed starting from 2 weeks of HFD, with a parallel increase in CNS IL-1β levels. In the brains of HFD-fed mice there was also a rearrangement of the neuroinflammatory pathway signalling, with an increase in HDAC2, p-STAT3 and TLR4 expression in the first week and an overexpression of NF-κB that perpetuates in HFD mice from week 1 to 8. Phase 2. After 8 weeks, SF68 administration counteracted the body weight gain in HFD mice. In parallel, SF68 treatment acted against intestinal inflammation in HFD-fed animals reducing plasma IL-1β and LBP and colonic MPO, MDA and eosinophils, and improved IEB integrity normalising the ZO-1, occludin and claudin expression as well as the acidic mucin content, altered in obese mice. SF68 also normalised the faecal concentration of butyrate and ameliorated its bioavailability via the increase in intestinal butyrate transporter (SMCT1) expression and the reduction of intestinal HDAC1. Conclusions: HFD intake determines an increased activity of the immune system in the colon and an initial IEB impairment, along with alterations in the butyrate transportation mechanisms. These changes are followed by pre-symptomatic BBB disruption, highlighting an early increase in CNS permeability that leads to neuroinflammatory processes associated with the obese condition. In this scenario, the supplementation with SF68 reduces intestinal inflammation and reinforces the enteric epithelial barrier in obese mice, improving the transport and utilization of butyrate and could be a valuable integration to diet and exercise for the management of the obese condition and also other pathological scenarios characterized by an altered intestinal permeability, resulting from an unsettlement of tissue butyrate bioavailability, such as IBDs, IBS, or visceral pain.