The effects of the gut microbiome on the host serotonin system have been demonstrated in some studies. The gut microbiota-serotonin system axis is involved in the pathogenesis of several GI disorders, but little is known about this interaction in gut disorder etiology (9).
While serotonin synthesizes from Trp and most of Trp is absorbed in the small intestine, focusing on the serotonin system in the small intestine ecosystem may have an essential role in digestive health. That is why we looked closely at the expression of serotonin system-related genes in the small intestine. In addition, there is significant interaction between gut microbiota and the serotonin system. The interaction between serotonin signaling and microbiota composition in the small intestine is less well characterized because of challenges in sampling this segment of the digestive tract. In this study, we focused on one abundant gut microbiota as a promising probiotic, like A. muciniphila, to explain its relationship with the serotonin system in the small intestine.
The serotonin system homeostasis plays an essential role in maintaining intestinal health and GI homeostasis, so any disturbance in this system is associated with gastrointestinal disorders. Most of the bioavailability of serotonin in the gastrointestinal tract is modulated by SERT; therefore, the SERT function is critical in maintaining the serotonin system's homeostasis. Previous studies have shown that the dysfunction of SERT is associated with the pathogenesis of several gastrointestinal disorders like IBS, celiac disease, and diarrheal disease (21, 22). In the present study, our results indicated that the expression of Slc6a4 mRNA was seemingly upregulated in the EV group. Dissimilar to the EVs, the bacterium did not affect the expression of Slc6a4, and we observed the independent effects of the bacterium and its EVs in the small intestine of mice. Meanwhile, this difference could be accounted for by harsh conditions in the small intestine, where the effect of live bacteria is diminished. Similarly, we observed this effect in our previous study in the colon of mice, and it can be concluded that the EVs had a similar impact on the expression of the Slc6a4 gene in the different conditions of the GI tract. SERT function is correlated with metabolic disorders, while Singhal et al. study revealed that the mice with the deletion of SERT, metabolic syndromes, especially diabetes, occurred. They also showed that the composition of microbiota was changed, and the abundance of A. muciniphila as a beneficial bacterium was decreased, so there is a correlation between SERT function and gut microbial homeostasis (22).
Since SERT is highly expressed on the surface of nearly all IECs, in our previous laboratory studies, we investigated the effect of A. muciniphila and its EVs in the colorectal epithelial adenocarcinoma (Caco-2) cell line as a model to represent intestinal cells (5). In line with the abovementioned results, we showed that both A. muciniphila and its EVs led to the induction of Slc6a4 mRNA expression in the Caco-2 cell line (5). In the Caco-2 cell, we showed that both A. muciniphila and its EVs led to the induction of Slc6a4 mRNA expression. Despite the differences between small intestine (in vivo) and Caco-2 cells (in vitro) conditions, EVs groups showed similar results; it can be assumed that EVs as non-viable microbial metabolites may have a direct effect on the Slc6a4 gene expression. In contrast, our observations showed that A. muciniphila did not affect Slc6a4 in mice's duodenum or ileum, which indicates that bacteria may not have similar functions in different conditions. In terms of enhancing the SERT function, we worked on another next-generation probiotic, Faecalibacterium prausnitzii (F. prausnitzii), and its EVs in our previous study. As such, our results indicated that F. prausnitzii and its EVs showed the same effect as A. muciniphila, and its EVs could impact the expression of the Slc6a4 gene in the Caco-2 cell line (5). Inconsistent with our research, several studies showed that probiotics like Lactobacillus acidophilus and Bifidobacterium longum enhanced the Slc6a4 gene expression in Intestinal epithelial cells and mice intestinal tissues (23, 24). Moreover, previous in vivo studies reported the induction of SERT function after supernatant F. prausnitzii administration in mice with chronic low-grade inflammation (25). Besides, in our last experiment, we carried out a comparative study that compared the effects of A. muciniphila and F. prausnitzii supernatant in the gene expression of Slc6a4 in the Caco-2 cell line (5). Altogether, our present study and previous studies revealed that microbial metabolites or supernatants substantially affect the SERT function in addition to bacteria.
One of the genes involved in peripheral serotonin that system affected by gut microbiota is Tph1.
Our results showed that oral administration of A. muciniphila and its EVs had a significant effect on the induction of the Tph1 gene expression in the duodenum of mice. Our results showed that oral administration of A. muciniphila and its EVs had a significant effect on the induction of the Tph1 gene expression in the duodenum of mice. Meanwhile, EVs had better results in comparison with the bacterium.
Of interest, only EVs could improve Tph1 gene expression in the ileum, and A. muciniphila did not significantly affect on expression of the Tph1 gene. Given that the Tph1 enzyme is intracellular, it can be hypothesized that EVs may access the cells by passing through the inner mucus layer and could affect the expression of this gene. MAO is another intracellular enzyme of the serotonin system that regulates the catalysis of oxidative serotonin degradation. In the same vein, our results showed that EVs decreased the MaoA expression compared to A. muciniphila in the ileum of mice. It is noteworthy that A. muciniphila affected Tph1 gene expression in the duodenum compared to the ileum; the histological differences in the ileum and duodenum sections of the small intestine may explain this difference. Overall, these initial data suggest that enteric serotonin-related genes could be affected by commensal bacteria and their metabolites in the small intestine. However, it remains unknown whether these changes occur throughout the complex microenvironment of the small intestine, which requires further study.
Serotonin in the GI mediates its effects by binding to various serotonergic receptors (5-HTR1 to 5-HTR7) on intestinal epithelial cells and enteric neurons (6). To explore the impact of A. muciniphila and its EVs on the serotonin system function, we also evaluated the expression of Htr3B, Htr4, and Htr7 receptors in the duodenum and ileum of mice. The 5- Htr3 and 5- Htr4 are implicated in various physiological functions in GI, and they have been studied more in the literature compared to other serotonin receptors (6). In the duodenum, the expression of Htr3B and Htr7 genes decreased, whiles the expression of the Htr4 gene increased following EVs treatment.
Our findings also showed that the gene expression of Htr3B, Htr4, and Htr7 receptors decreased in the ileum of mice. The possible explanation for our conclusion is that the changes in the expression of these receptors can be explained by feedback regulation.
Altogether, probiotics-host crosstalk, through complex interactions along the intestinal tract, can maintain intestinal homeostasis and improve a wide range of disorders, possibly benefiting the host's health. In the context described above, here we carried out preliminarily in vivo studies of the effect of the next-generation strain on the serotonin system-related genes through a pilot study performed in different intestinal sites like the duodenum and ileum of mice. Our study provided the basis for subsequent studies to determine the signal transduction pathway.