Highlights d GZMA mRNA expression correlates with inflammation in human CRC patients d GzmA deficiency reduces gut inflammation and colorectal cancer development d Therapeutic GzmA inhibition in WT mice reduces colorectal cancer development d GzmA induces IL-6 production in macrophages activating pSTAT3 in colon cancer cells

TLR2 is a microbiota recognition receptor that has been described to contribute to intestinal homeostasis and to ameliorate inflammatory intestinal injury. In this context, serotonin (5-HT) has shown to be an essential intestinal physiological neuromodulator that is also involved in intestinal inflammatory diseases. Since the interaction between TLR2 activation and the intestinal serotoninergic system remains non-investigated, our main aim was to analyze the effect of TLR2 on intestinal serotonin transporter (SERT) activity and expression and the intracellular pathways involved. Caco-2/TC7 cells were used to analyze SERT and TLR2 molecular expression and SERT activity by measuring 5-HT uptake. The results showed that apical TLR2 activation inhibits SERT activity in Caco-2/TC7 cells mainly by reducing SERT protein level either in the plasma membrane, after short-term TLR2 activation or in both the plasma membrane and cell lysate, after long-term activation. cAMP/PKA pathway appears to mediate short-term inhibitory effect of TLR2 on SERT; however, p38 MAPK pathway has been shown to be involved in both short- and long-term TLR2 effect. Reciprocally, 5-HT long-term treatment yielded TLR2 down regulation in Caco-2/TC7 cells. Finally, results from in vivo showed an augmented intestinal SERT expression in mice Tlr2-/-, thus confirming our inhibitory effect of TLR2 on intestinal SERT in vitro. The present work infers that TLR2 may act in intestinal pathophysiology, not only by its inherent innate immune role, but also by regulating the intestinal serotoninergic system.

Intestinal inflammatory diseases are the result of multiple processes, including mucosal oxidative stress and perturbed homeostasis between commensal bacteria and mucosal immunity. Toll-like receptors (TLRs) recognize molecular-associated microorganisms' patterns and trigger innate immunity responses contributing to intestinal homeostasis and inflammatory responses. However, TLRs effects on redox balance in intestinal mucosa remain unknown. Therefore, the present study analyzes the effect of TLR2, TLR3, and TLR4 on both oxidative damage of lipids and proteins, and the activity of antioxidant enzymes in enterocyte-like Caco-2 cells. The results show that the activation of these TLRs increased lipid and protein oxidation levels; however, the effect on the antioxidant enzymes activity is different depending on the TLR activated. These results suggest that the activation of TLR2, TLR3, and TLR4 might affect intestinal inflammation by not only their inherent innate immunity responses, but also their pro-oxidative effects on intestinal epithelial cells.

Highlights d Multi-omics analysis identifies MUC17 as a major membrane mucin in ileum d MUC17 localizes to apical glycocalyx covering enterocytic brush-border membranes d IL-22 signaling during weaning upregulates MUC17 in enterocytes d MUC17 guards enterocytes from exogenous molecules and bacteria

Oxidative stress is thought to play a key role in the development of intestinal damage in intestinal inflammatory diseases. Several molecules are involved in the intestinal inflammation, either as pro- or anti-inflammatory factors; however, their effects on intestinal oxidative stress seem to be controversial. This work analyzes the contribution of pro- and anti-inflammatory molecules to the balance of oxidative damage in intestinal epithelial cells, as well as their effects on cellular antioxidant enzyme activity. With this purpose, the lipid and protein oxidation, together with the activity of catalase, superoxide dismutase, and glutathione peroxidase, were determined in the Caco-2 cells treated with serotonin, adenosine, melatonin, and TNFα, as proinflammatory factors, and IL-10, as an anti-inflammatory cytokine. The results have shown that all the proinflammatory factors assayed increased oxidative damage. In addition, these factors also inhibited the activity of antioxidant enzymes in the cells, except melatonin. In contrast, IL-10 did not alter these parameters but was able to reduce the prooxidant effects yielded by serotonin, adenosine, melatonin, or TNFα, in part by restoring the antioxidant enzymes activities. In summary, proinflammatory factors may induce oxidative damage in intestinal epithelial cells, whereas IL-10 seems to be able to restore the altered redox equilibrium in Caco-2 cells.

Intestinal epithelial cells could release IL-10 in response to TLR activation, playing an intestinal tissue-dependent and critical intestinal immune role.

Disruption of the microbiota–gut–brain axis results in a wide range of pathologies that are affected, from the brain to the intestine. Gut hormones released by enteroendocrine cells to the gastrointestinal (GI) tract are important signaling molecules within this axis. In the search for the language that allows microbiota to communicate with the gut and the brain, serotonin seems to be the most important mediator. In recent years, serotonin has emerged as a key neurotransmitter in the gut–brain axis because it largely contributes to both GI and brain physiology. In addition, intestinal microbiota are crucial in serotonin signaling, which gives more relevance to the role of the serotonin as an important mediator in microbiota–host interactions. Despite the numerous investigations focused on the gut–brain axis and the pathologies associated, little is known regarding how serotonin can mediate in the microbiota–gut–brain axis. In this review, we will mainly discuss serotonergic system modulation by microbiota as a pathway of communication between intestinal microbes and the body on the microbiota–gut–brain axis, and we explore novel therapeutic approaches for GI diseases and mental disorders.

Serotonin (5-HT) is an essential gastrointestinal modulator whose effects regulate the intestinal physiology. 5-HT effects depend on extracellular 5-HT bioavailability, which is controlled by the serotonin transporter (SERT) expressed in both the apical and basolateral membranes of enterocytes. SERT is a critical target for regulating 5-HT levels and consequently, modulating the intestinal physiology. The deregulation of innate immune receptors has been extensively studied in inflammatory bowel diseases (IBD), where an exacerbated defense response to commensal microbiota is observed. Interestingly, many innate immune receptors seem to affect the serotonergic system, demonstrating a new way in which microbiota could modulate the intestinal physiology. Therefore, our aim was to analyze the effects of NOD1 activation on SERT function, as well as NOD1's interaction with other immune receptors such as TLR2 and TLR4. Our results showed that NOD1 activation inhibits SERT activity and expression in Caco-2/TC7 cells through the extracellular signal-regulated kinase (ERK) signaling pathway. A negative feedback between 5-HT and NOD1 expression was also described. The results showed that TLR2 and TLR4 activation seems to regulate NOD1 expression in Caco-2/TC7 cells. To assess the extend of cross-talk between NOD1 and TLRs, NOD1 expression was measured in the intestinal tract (ileum and colon) of wild type mice and mice with individual knockouts of TLR2, and TLR4 as well as double knockout TLR2/TLR4 mice. Hence, we demonstrate that NOD1 acts on the serotonergic system decreasing SERT activity and molecular expression. Additionally, NOD1 expression seems to be modulated by 5-HT and other immune receptors as TLR2 and TLR4. This study could clarify the relation between both the intestinal serotonergic system and innate immune system, and their implications in intestinal inflammation.