A new study reveals how your gut bacteria can directly tell your brain when to stop eating, opening fresh avenues for appetite and metabolic research.
Study: A gut sense for a microbial pattern regulates feeding. Image Credit: Pormezz / Shutterstock
A recent study published in the journal Nature found that feeding behavior is regulated by a previously unknown gut-brain sensory modality for microbial patterns.
All organisms interpret the world through their senses. A growing body of evidence has established a neural basis of gut sense, which constantly assesses gut luminal stimuli. Epithelial neuropod cells in the small intestine detect nutrients and relay the information via the vagus nerve to influence real-time appetitive choices.
Evidence suggests that gut microbes can modulate feeding behavior through immune signals, vagal pathways, and neuromodulators. Nonetheless, a direct neural circuit sensing microbial signals to regulate feeding is unknown. Vagal neurons form neuroepithelial circuits with neuropod cells labelled by the satiety-inducing protein, peptide YY (PYY). These PYY-neuropod cells specifically express TLR5, unlike serotonin-producing enteroendocrine cells.
The neuropod cells and other colonic cells are constantly exposed to microbes that can be recognized by microbial molecular patterns, including flagellin. Flagellin, the structural component of flagella, is conserved across bacterial phyla and activates the Toll-like receptor 5 (TLR5), a pattern-recognition receptor (PRR). Notably, flagellin levels physiologically increase in stool upon feeding.
The study and findings
In the present study, researchers determined that a gut-brain sensory modality for microbial patterns termed the “neurobiotic sense” regulates feeding behavior. First, the intestinal epithelial cell transcriptomes were sequenced using reporter mice that expressed the green fluorescent protein (GFP) under the promoters of PYY and cholecystokinin (CCK). The colon and distal ileum are enriched with PYY-labeled cells, while CCK prevails in the proximal intestinal cells.
The transcriptomes of sensory epithelial cells were compared to those of neighboring epithelial cells to identify microbial signal receptors. The team found significant enrichment of genes encoding microbial byproduct receptors, including G protein-coupled receptor 119 (Gpr119), G protein-coupled bile acid receptor 1 (Gpbar1), free fatty acid receptor 1 (Ffar1), and Ffar2. However, only PYY-GFP cells were significantly enriched for PRRs, with Tlr5 being the most prominent.
In situ hybridization was performed to verify the expression of Tlr5 in PYY-labeled cells. Colocalization increased from 24% in the ileum to 57% in the distal colon, where PYY-labeled cells have the highest density. Next, Tlr5 was knocked out from PYY-labeled cells in mice. Tlr5 ablation in PYY-labeled cells resulted in mice eating more (increased meal size in both sexes and longer meal duration in females) and gaining more weight than controls, independent of canonical immune signaling (MyD88), metabolic dysfunction, or inflammation.
Further, the researchers found that the relative levels of flagellin (a TLR5 ligand) in the stool were higher in fed mice than in fasted mice, and this response was unaltered in Tlr5-ablated mice. This indicated that colonic flagellin levels were independent of Tlr5 expression in PYY-labeled cells and that feeding correlated with higher colonic flagellin levels. In addition, the team found that PYY-labeled cells utilize TLR5 to sense flagellin, but not other TLR ligands, such as Poly(I:C), and transduce this signal by releasing PYY. Vagal neurons themselves lack TLR5 and show no direct response to flagellin.
PYY-labeled cells were found to be significantly enriched for genes involved in synaptic formation, signaling, and neurotransmission. Besides, one-fifth of PYY-labeled cells contact peripheral neurons in the colon and ileum. As such, the researchers sought to confirm whether these cells are connected to the vagus nerve using luminal optogenetics and whole-nerve electrophysiology recordings of the cervical vagus nerve, with intralipid serving as a positive control.
This showed that PYY-labeled cells directly activate the vagus nerve, establishing a direct signaling circuit between the colon and hindbrain. Further experiments indicated that PYY-labeled neuropod cells sense luminal flagellin using TLR5 and transduce this microbial signal to the vagus nerve. Next, the team investigated whether PYY release from neuropod cells was necessary for vagal activity in response to flagellin stimulus.
Blocking the neuropeptide Y receptor type 2 (Y2R), the PYY receptor on colonic vagal neurons, ablated cervical activity in response to flagellin. Calcium imaging of vagal nodose neurons revealed that 60.6% of neurons exclusively responded to flagellin, while 27.7% responded to both flagellin and nutrients, indicating the existence of a unique neuroepithelial circuit for flagellin sensing, alongside potential nutrient integration pathways. This discovery prompted investigations into how flagellin influences feeding behavior in real time.
To this end, mice were fasted overnight to induce hunger and were administered flagellin or phosphate-buffered saline (PBS) by enema. Flagellin enema resulted in a significant decrease in food intake within 20 minutes in littermate controls but had no effect on mice with TLR5 ablation in PYY-labeled cells.
Pharmacological inhibition of Y2R or TLR5 prevented flagellin-induced decrease in food consumption, suggesting that flagellin reversibly and rapidly suppresses food intake. Finally, the flagellin enema also reduced food intake in germ-free mice, suggesting that flagellin sensing is sufficient to suppress food intake, regardless of microbial signals.
Conclusions
In sum, PYY-labeled colonic neuropod cells use TLR5 to detect flagellin and rapidly signal to the brain via the vagus nerve to regulate feeding behavior through dedicated NPY2R receptors. This gut-brain neural circuit forms a neurobiotic sense, enabling the host to adjust behavior by monitoring gut microbial patterns. Notably, the study utilized Salmonella typhimurium flagellin, warranting investigations into other molecular variants of flagellin, as bacteria can be commensal or pathogenic depending on the specific variant expressed.