Enteropathogenic infections modulate intestinal serotonin transporter (SERT) function by activating Toll-like receptor 2 (TLR-2) in Crohn's disease.

Serotonin (5-hydroxytryptamine [5-HT]) is an intestinal neuromodulator that regulates several essential enteric physiological functions such as absorption or secretion of fluids, and peristaltic reflexes. Availability of the intestinal 5-HT is dependent on serotonin transporter (SERT), which uptakes 5-HT and facilitates its degradation. Interestingly, Toll-like receptor 2 (TLR-2) is co-localized with 5-HT, which suggests a possible impact of neuroendocrine cells in the inflammatory response through TLR-2 activation. Serum 5-HT levels were measured in 80 Crohn's disease (CD) patients and 40 healthy control subjects. Additionally, fully differentiated Caco-2 monolayers were infected with Mycobacteria paratuberculosis (MAP), L. monocytogenes, or M. smegmatis in the presence of exogenous 5-HT at different concentrations. Cells were subsequently harvested and used for measuring SERT activity, RNA isolation followed by RT-PCR, protein quantification, and tissue damage markers (DHE, LDH, GSH and MDA). TLR-2 intracellular signaling pathways were assessed by pre-incubating Caco-2 monolayers with selective blockers of ERK, cAMP/PKA, p38 MAPK, and 5-HT3 receptors. MAP-infected CD patients (N = 40) had higher serum 5-HT levels (462.95 ± 8.55 ng/mL, N = 40) than those without MAP infection (385.33 ± 10.3 ng/mL, N = 40). TLR-2 activation by enteropathogenic bacteria inhibited SERT activity in the presence of exogenous 5-HT by up to 50%. These effects were increasing gradually over 72 h, and MAP infection had the greatest effect on SERT inhibition when cells were exposed to 5-HT in a concentration dependent manner. Additionally, inhibition of SERT activity was accompanied with higher levels of pro-inflammatory cytokines (TNF-α, IL-6, IL-8) and oxidative stress markers (DHE, LDH and MDA), whereas SERT expression and protein level were downregulated. Consequently, inhibition of TLR-2 and p38 MAPK signaling or blocking 5-HT3 receptors restored SERT activity and reduced the production of pro-inflammatory cytokines, as reflected by the downregulation of oxidative stress and tissue damage markers. The involvement of TLR-2 in the intestinal pathology might be concluded not only from its innate immune role, but also from its effect on modulating the intestinal serotonergic response. Ultimately, regulating the intestinal serotonergic system can be therapeutically exploited to mitigate other enteropathogenic infections, which will help in understanding the gut-microbiome-brain connection.

The alternate effects of anti-TNFα therapeutics and their role in mycobacterial granulomatous infection in Crohn's disease.

INTRODUCTION: Crohn's disease is an inflammatory bowel disease that has been debated to be associated with bacterial triggers such as Mycobacterium avium subspecies paratuberculosis (MAP). Standard treatment of Crohn's disease (CD) patients includes a family of immunomodulators and biologics such as Anti-Tumor Necrosis Factor alpha (Anti-TNFα). This cytokine in particular has been known to play vital roles in fighting microbial infections through formation and maintenance of granulomas. Areas covered: This perspective is focused on elucidating the negative effects of using Anti-TNFα therapeutic agents as a treatment option in CD patients who are more likely suspected to have MAP infection, and the role of other immunomodulators in MAP infection. Expert commentary: While treatment with Anti-TNFα is beneficial to reduce inflammation and to provide short term relief to the patients, it also compromises the immune system causing susceptibility to microbial infection. More than 50% of CD patients have shown no response to Anti-TNFα treatment which indicates a demand for introducing novel CD treatment in combination with antibiotics as a future CD treatment plan.